Low Cost Scanning Laser Rangefinder
by
Ben Johannesen
School of Information Technology and Electrical Engineering,
University of Queensland
Submitted for the Degree of
Bachelor of Engineering
in the Division of Electrical Engineering
October 2001
34 Payne St
Indooroopilly 4068
Tel. (07) 3720 2918
October 19, 2001
The Head
School of Information Technology and Electrical Engineering
University of Queensland
St Lucia, QLD 4072
Dear Professor Kaplan,
In accordance with the requirements of the Bachelor of Engineering in the division of
Electrical Engineering. I present the following thesis entitled “Low-Cost Scanning
Laser Rangefinder”. This work was conducted under the supervision of Dr. Brian
Lovell.
I declare that the work submitted in this thesis is my own, except as acknowledged, and
has not previously submitted as a degree at the University of Queensland or any other
institution.
Yours sincerely,
Ben Johannesen
To Mum and Dad,
for their love, encouragement and the sacrifices they have made to make my
Engineering Degree and this Thesis possible.
iv
Acknowledgments
The completion of this thesis would not have been at all possible without the assistance
of others. Many have offered their guidance, time, assistance and a listening ear and I
would like to show my immense appreciation to for this to the following persons.
The SIP Lab Gurus (Peter Waldeck, Carlos Leung, Chris O’Brien, Ben Appleton and
others) for the assistance, encouragement, and all the laughs shared, throughout this
year.
Associate Professor Brian Lovell for giving me the opportunity to work on such an
interesting topic and for the guidance provided throughout the year.
Luke “Pooky” Blanch for taking the time proofread my draft and correct the numerous
errors that I had missed.
Friends and Family who listened to me and supported me when things were tough.
v
Abstract
This thesis addresses the design of a low cost Scanning Laser Rangefinder for use in
Human Computer Interaction applications. The use of a Scanning Laser Rangefinder is
a very effective way of providing an interactive environment between a human and a
computer. A Scanning Laser Rangefinder creates a non-contact two-dimensional
interactive surface. It is small and portable and is flexible in its possible application.
The design of the Scanning Laser Rangefinder has been broken down into its
subsections and the theory and design considerations needed to implement the Scanning
Laser Rangefinder have been discussed in length. The design and implementation of a
Time-of-Flight range finding system suitable for integration into the Scanning Laser
Rangefinder has been attempted. Two different Time-to-Amplitude Converter methods
of design have been implemented and tested along with a laser driver and receiver. One
type of Time to Amplitude Converter has been tested successfully but is not suitable for
integration into the Scanning Laser Rangefinder. The second type of Time to Amplitude
Converter, while suitable for use in a Scanning Laser Rangefinder, has not been
implemented successfully.
No working prototypes have been developed but the work done forms the backbone for
work to continue on the project. An approach to successfully completing the
development of a Scanning Laser Rangefinder has been outlined. By breaking the
project down into three smaller individual projects it has been suggested that a working
prototype along with software could be finished in the next two years.
vi
Contents
ACKNOWLEDGMENTS............................................................................................IV
ABSTRACT.................................................................................................................... V
CONTENTS ..................................................................................................................VI
LIST OF FIGURES......................................................................................................IX
CHAPTER 1.................................................................................................................... 1
INTRODUCTION .......................................................................................................... 1
1.1 A SCANNING LASER RANGEFINDER................................................................... 1
1.2 THESIS OVERVIEW............................................................................................. 3
CHAPTER 2.................................................................................................................... 4
CURRENT HCI DEVICES ........................................................................................... 4
2.1 DIFFERENT HCI DEVICES .................................................................................. 4
2.1.1 Softboard................................................................................................... 4
2.1.2 Gesture wall.............................................................................................. 5
2.1.3 Smart desk................................................................................................. 6
2.2 THE MIT SCANNING LASER RANGEFINDER....................................................... 7
2.2.1 Operation .................................................................................................. 8
2.2.2 Applications .............................................................................................. 9
2.3 CHAPTER SUMMARY........................................................................................ 10
CHAPTER 3.................................................................................................................. 11
DESIGN SPECIFICATIONS AND THEORY .......................................................... 11
3.1 DESIGN SPECIFICATIONS.................................................................................. 11
3.2 DESIGN BREAKDOWN ...................................................................................... 12
3.3 THEORY OF DESIGN ......................................................................................... 13
vii
3.3.1 Range finding.......................................................................................... 13
3.3.2 Calculating the range ............................................................................. 16
3.3.3 Laser Driver Electronics ........................................................................ 19
3.3.4 Receiver Electronics ............................................................................... 19
3.3.5 Scanning Electronics and Angle Calculation ......................................... 20
3.3.6 Microcontroller....................................................................................... 21
3.4 CHAPTER SUMMARY........................................................................................ 22
CHAPTER 4.................................................................................................................. 24
DESIGN IMPLEMENTATION.................................................................................. 24
4.1 LASER DRIVER ELECTRONICS.......................................................................... 24
4.2 RECEIVER ELECTRONICS.................................................................................. 26
4.3 SCANNER HARDWARE ..................................................................................... 27
4.4 AVERAGING MODE TIME-TO-AMPLITUDE CONVERTER................................... 28
4.5 CURRENT MODE TIME-TO-AMPLITUDE CONVERTER........................................ 30
4.6 MICROCONTROLLER ........................................................................................ 32
4.7 CHAPTER SUMMARY........................................................................................ 32
CHAPTER 5.................................................................................................................. 34
DISCUSSION OF RESULTS ...................................................................................... 34
5.1 OVERVIEW OF THE DEVELOPMENT .................................................................. 34
5.2 THE DEVELOPMENT AND TESTING OF THE RANGEFINDER ............................... 35
5.2.1 Performance of the Laser Driver and Receiver...................................... 36
5.2.2 Performance of the Time-to-Amplitude Converters................................ 36
5.3 CHAPTER SUMMARY........................................................................................ 38
CHAPTER 6.................................................................................................................. 39
FUTURE WORK.......................................................................................................... 39
6.1 THE WORK REMAINING ................................................................................... 39
6.2 COMPLETING THE DESIGN ............................................................................... 40
6.3 CHAPTER SUMMARY........................................................................................ 41
viii
CHAPTER 7.................................................................................................................. 42
CONCLUSIONS ........................................................................................................... 42
REFERENCES ............................................................................................................. 44
APPENDIX A................................................................................................................ 46
ix
List of Figures
FIGURE 1.1 OPERATION OF THE SCANNING LASER RANGEFINDER ........................................ 2
FIGURE 2.1 THE MICROFIELD SOFTBOARD.......................................................................... 5
FIGURE 2.2 THE GESTURE WALL......................................................................................... 6
FIGURE2.3 THE SMART DESK.............................................................................................. 7
FIGURE 2.4 THE MIT SCANNING LASER RANGEFINDER........................................................ 8
FIGURE 3.1 BLOCK DIAGRAM OF SCANNING LASER RANGEFINDER..................................... 12
FIGURE 3.2 TRIANGULATION METHOD .............................................................................. 14
FIGURE 3.3 PHASE MEASUREMENT ................................................................................... 15
FIGURE 3.4 AVERAGING MODE WAVEFORMS..................................................................... 17
FIGURE 4.1 PULSING LASER CIRCUIT ................................................................................ 25
FIGURE4.2 RECEIVER BLOCK DIAGRAM ............................................................................ 26
FIGURE4.3 PRINTER SCAN HEAD FOR SCANNER................................................................. 27
FIGURE 4.4 CURRENT MODE TAC .................................................................................... 31
1
Chapter 1
Introduction
As computers and software become more powerful and the area of their application
expands, the traditional mouse and keyboard, become more restrictive. Many different
devices have been developed to increase Human Computer Interaction (HCI) and this
development continues today. The most commonplace Human Computer Interactive
device on the commercial market today is the Touch Screen. However, even the touch
screen restricts HCI to some extent. These shortcomings and the search to find devices
that overcome them were the driving force behind the development of a Scanning Laser
Rangefinder for this thesis project
1.1 A Scanning Laser Rangefinder
The Media Labs at MIT [1] have already developed a Scanning Laser Rangefinder for
HCI applications [2]. A Scanning Laser Rangefinder allows the user greater freedom to
interact with a computer than other devices developed for this purpose. Using a
Scanning Laser Rangefinder allows the user to interact with a large interactive surface
in real time.
2
Figure 1.1 Operation of the Scanning Laser Rangefinder
A Scanning Laser Rangefinder operates by scanning a laser beam across the front of a
two dimensional surface creating a two dimensional interactive plane. The typical set up
of a Scanning Laser Rangefinder is shown in Figure 1.1 [3]. Here the device is placed in
front of a rear projection screen so that its scan plane is parallel to the screen. When a
user cuts the scan plane with their hand the position of their hand is recorded and sent to
the PC that is controlling what is being projected onto the screen. This allows a user to
essentially interact with the projector screen.
Two-dimensional surfaces can be transformed into an interactive surface by the use of
this technology. Small and easily portable, a Scanning Laser Rangefinder is low cost
[2], it creates a non-contact interactive surface and is flexible in its possible
applications. These features are not prominent in many other HCI devices making a
Scanning Laser Rangefinder a very viable device to enhance Human Computer
Interaction.
3
1.2 Thesis Overview
This thesis documents the development of a Scanning Laser Rangefinder for HCI
applications. The development of such a device is a significantly large task and a final
prototype was not completed. This thesis however does detail the design method for
developing a Scanning Laser Rangefinder and implementation and testing of several
subsections of the Scanning Laser Rangefinder design.
Many devices have already been developed for HCI and several of these, including a
Scanning Laser Rangefinder developed by MIT, are described and reviewed in Chapter
2.
The specifications of the Scanning Laser Rangefinder were derived from the
information presented in Chapter 2 and presented in Chapter 3. This chapter also breaks
down the design into relevant subsections and describes the theory and design
considerations that need to be taken into account when implementing the Scanning
Laser Rangefinder.
Chapter 4 describes the implementation of several system subsections and chapter 5
discusses the results that were obtained as well as the progression of the project.
Chapter 6 outlines the future work that needs to be done in order to complete the
Scanning Laser Rangefinder and the final conclusions from this project are then
presented in Chapter 7.
The final schematics of the implemented subsections are included in the Appendices.
4
Chapter 2
Current HCI Devices
This chapter reviews several of the current devices used to allow Human Computer
Interaction. This review will give a better understanding of what Human Computer
Interaction requires, and allow the derivation of a set of specifications for the Scanning
Laser Rangefinder that is to be developed. The MIT Media labs developed many of the
devices reviewed, this includes a review of the operation and application of the
Scanning Laser Rangefinder developed by the Media Labs.
2.1 Different HCI Devices
2.1.1 Softboard
Microfield Graphics [4] developed a smart white board application that they named the
Softboard. This application uses two scanning lasers to track and record the information
that is written on a white board. The pens and eraser that are used on the white board
have special tags that allow the lasers to track them as well as identify their function
(pen or eraser); if it is a pen it also identifies the colour. The position of the pen being
tracked is determined using two scanning lasers, one in each top corner of the board.
Figure 2.1 [5] shows the Softboard as well as the scanner set-up. When the scanning
laser detects the tag identifying the object to be tracked, the angle of the laser is
recorded. The two angles can then be used to triangulate the position of the object.
5
Figure 2.1 The Microfield Softboard
The Softboard can be used to printout what has been written on the board, record an
entire presentation and then play it back later on a PC or teleconference the presentation
that is being presented on the board. The scanners on the board have a scan rate of 416
Hz and it can record 80 data points per second [5]. The board is limited to only tracking
one object at a time and this object must be specially tagged. The Softboard costs
around $3000 [5] and lacks portability due to its size and its weight of 27 Kg [5]. While
the Softboard is very effective as an interactive white board it is limited to performing
only this function.
2.1.2 Gesture wall
Electromagnetic field sensing is a very popular way of allowing a user to interact with a
screen or object. One such device using this method is the Gesture Wall [3] that was
6
developed by the MIT Media Labs for the Brain Opera. The system is shown in Figure
2.2 [3] and uses electromagnetic fields to track the movement of a person’s body. The
Figure 2.2 The Gesture Wall
user stands upon a brass transmitter plate that drives their body with a small
electromagnetic field. A screen stands in front of the user and has four pickup antennas,
one at each corner of the screen. These antennae measure the distance a person’s body
is from the screen using their electromagnetic field, thus allowing the user to interact
with the screen. While this system does allow the user to interact with a large screen it
cannot distinguish between the movements of different parts of the users body. In order
for a user to interact with the screen using nothing but their hand, very rigid postural
restraints were placed upon them so as the rest of the body did not move and effect the
measurements [3]. The system also places the constraint on the user of having to stand
in a particular spot in order to interact with the screen.
2.1.3 Smart desk
The Smart Desk shown in Figure 2.3 [2] is another human movement tracking system.
It utilises video cameras to track the movement of an individual. Systems that use video
cameras to track a person’s movement require a large amount of processing by the host
computer. Computers that are capable of processing the data from these cameras in real
7
Figure2.3 The Smart Desk
time can often be quite expensive. The use of video cameras in itself makes the system
an expensive option when developing a human computer interaction device. This kind
of device is relatively portable and can produce quite reasonable results when tracking
movement. An advantage of a system such as this is that it can distinguish between
different parts of a person such as their hand and their head.
2.2 The MIT Scanning Laser Rangefinder
The Media Labs at MIT have already developed a working Scanning Laser Rangefinder
for real time human computer interaction [2]. This rangefinder was tested in several
different HCI applications and found to be very successful. The operation and the
applications of this device are reviewed in the next two sections.
8
2.2.1 Operation
Figure 2.4 The MIT Scanning Laser Rangefinder
The device (Figure 2.4 [3]) operates by scanning a laser beam with a rotating mirrored
polygon. When an object such as a users hand is detected in the scan plane, the
Scanning Laser Rangefinder records the range and the angle of the object and then
transmits this to a PC. This data is then be used in HCI software.
The Scanning Laser Rangefinder use a 5mW laser diode as the light source and a high
gain Avalanche Photodiode as the receiver. The light reflected off of the users hand is
reflected off the rotating polygon and into the receiver. The rotation of the mirror
between when the light reflects and when the light is received becomes insignificant due
to the high speed of light. A scan rate of 30 Hz was uses to scan the mirror. A
maximum range of 6m was achieved. The measured range was a linear measurement
and used a continuous wave quadrature phase measuring range-finding method. The
measured range had 10-bit resolution while the angle had a resolution of 16 bits; this
gave an accuracy of less than a centimetre. The device was also quite cost effective
being produced for less than $500 US. [2]
9
2.2.2 Applications
MIT developed several applications
for their Scanning Laser Rangefinder.
These applications were used in
conjunction with a rear projection
screen. The rangefinder was placed
in front of the screen so that its
scanning plane was parallel to the
screen as shown in Figure 1.1. The
first application that was developed
placed rotating multicoloured squares
at the position of the person’s hands
(Figure 2.5 [3]). This was then used
as a music controller. By moving
their hands to different sections of the
screen, the user was able to control
different elements of a musical piece.
A second application was a mouse pointer program using the Scanning Laser
Rangefinder to drive the windows system. Using a projection screen as described
earlier, the program was only capable of allowing the user to single click on an object.
This enabled the user to insert their hand into the plane and either click on an object or
click and drag an object. This application does not replace the mouse but creates a
touch screen like environment.
The third and arguably most interesting of the applications developed by the Media
Labs was the Stretchable Music application [2]. The Stretchable Music program was an
interactive graphical music program allowing a user to change the parameters of a piece
of music by stretching and pulling various objects that are projected onto a screen.
Figure 2.6 [2] shows several users interacting with the Scanning Laser Rangefinder and
the stretchable music program. Each object on the screen represents a layer or
Figure 2.5 Rotating Squares
10
Figure2.6 Stretchable Music
section of the music being played. The program allowed for multiple users but only one
object could be manipulated at any one time. Other users that weren’t manipulating an
object added extra graphics and sound by interacting with the scanner.
2.3 Chapter Summary
This chapter reviewed several devices that can be used for human computer interaction
including a Scanning Laser Rangefinder that was developed by MIT. It is evident from
the reviews that previously developed devices for HCI are limited by things such as
position of the user, the objects that can be tracked, the flexibility of the application of
the device, the cost and also the portability of the device. The use of a Scanning Laser
Rangefinder overcomes many of these limitations. The Scanning Laser Rangefinder
only required the user to cut the scan plane with their hand in order to interact. It was
relatively cheap, small for portability and could be used for many different applications.
The following chapter will use these evaluations to determine a set of specifications for
the device to be developed.
11
Chapter 3
Design Specifications and Theory
This chapter derives a set of design specifications for the Scanning Laser Rangefinder
using the information obtained from the reviews of other HCI devices in the previous
chapter. The chapter then goes on to breakdown the system into its relevant subsystems
and detail the theory of operation of each subsystem as well as the design considerations
that need to be taken into account when designing each subsystem.
3.1 Design Specifications
This Thesis outlines the development of a Scanning Laser Rangefinder for applications
in Human Computer Interaction. The final product must be able to track the position of
a person’s or several persons hands in a two dimensional plane. The people interacting
with the Scanning Laser Rangefinder should not be required to wear any special devices
to allow their hands to be detected. In order for the Scanning Laser Rangefinder to
operate correctly no constraints pertaining to position or posture should be placed upon
the user. The resolution of the Scanning Laser Rangefinder should be approximately 1-
2 cm. A smaller resolution is not required as the device is to be used in relation to large
interactive surfaces. A change of a few millimetres within an area of several metres is
insignificant in this situation. The final product should be a small, easily portable
device to allow for flexibility in its possible applications. It is also required that the
12
Scanning Laser Rangefinder be a low cost device. The final design should cost less
than $500 US, the cost of the Scanning Laser Rangefinder developed by MIT.
3.2 Design Breakdown
This section breaks down the design of the Scanning Laser Rangefinder into its relevant
subsections and outlines what each subsection must do. In order to breakdown the
design of the Scanning Laser Rangefinder into subsections, the method of determining
the position of a person’s hand had to be decided upon. It was decided that the
Scanning Laser Rangefinder would operate in a similar fashion to the one that was
designed by the Media Labs at MIT [2]. This method was chosen because of its
simplicity.
This method uses polar co-ordinates to determine the position of a person’s hands in a
two dimensional plane. The laser beam is scanned across the front of a two-
dimensional surface that a user is interacting with. When the users hand cuts the scan
plane, the laser beam is reflected back allowing the range to be calculated as well as the
angle of the beam at the time the person’s hand is detected. This information, along
with software to interpret it, would then allow a user to interact with a computer.
uC
RangeFinding
Electronics
LaserDriver
ReceiverAmp
Scan AngleElectronics
RotatingPolygonDriver
ADCLaser
Photodiode
AngleReceiver
RotatingPolygon
SerialCommsto PC
Figure 3.1 Block Diagram of Scanning Laser Rangefinder
13
The design of the Scanning Laser Rangefinder was broken down into several sections.
The Scanning Laser Rangefinder needs to calculate two things, the range and the angle
of the users hand or hands. In order to calculate the range, laser light must be
transmitted using a laser and laser driver. This light is required to be received by a
photodiode and then amplified. Electronics is then needed to find the range using the
transmitted and received light. Once the range has been calculated it must be sampled
and transmitted to the PC where it can be processed, most likely by a microcontroller.
To calculate the angle, electronics is first needed to control the mirror that scans the
laser. Electronics is then needed to calculate the angle and then this data needs to be
transferred to the PC. These sections make up the block diagram shown in Figure 3.1.
The sections that are shown in the block diagram will be discussed in more detail in the
following section of this chapter.
3.3 Theory of Design
This section of the chapter outlines the theory required to implement each of the
subsections detailed in the previous section. It also discusses the different
considerations that need to be taken into account and the methods to be used to design
each subsection.
3.3.1 Range finding
The core of the design is contained in the calculation of the range, of which there are
three main approaches. How each of these approaches works as well as the advantages
and disadvantages will be discussed in the following sections. A method to calculate the
range will then be decided upon and the options in implementing this method of range
finding will be investigated.
3.3.1.1 Triangulation
Calculating distance-using triangulation employs the simple principle of trigonometry.
Figure 3.2 [6] below shows how trigonometry is used to calculate the range. If the
14
distance Y is known and the angles θ and φ are known then the distance Z can be found
using the law of Sines.
P3
P1
P2
φ
α
θ
Y
Z
Figure 3.2 Triangulation Method
Implementing a laser-based system that employs this method of range finding is fairly
simple. A laser would be placed at position P1 at a fixed angle φ. A Position Sensitive
Photodiode would then be placed a known distance from P1 at position P2. The output
of the Position Sensitive Photodiode would determine the angle θ and the distance Z
would then be easily calculated.
The dynamic range of a triangulation-based system is limited by whether the reflected
light hits the Position Sensitive Photodiode. When the angle α is too small or too great
the reflected light will miss the photodiode and no range will be recorded. These
minimum and maximum angles determine the minimum and maximum range of a
triangulation system. Triangulation based systems experience reduced accuracy with
increasing range [6] and the output of the position sensitive photodiode is a non-linear
measurement [2].
The Media Labs at MIT developed a Scanning Laser Rangefinder that used the
triangulation method. The device that was developed provided a range of less than one-
meter. The range was limited mostly by the non-linearity of the Position Sensitive
Photodiode. [2]
15
3.3.1.2 Continuous Wave Phase Measurement
Continuous Wave Phase measurement uses the phase difference between the transmitted
signal and the returned signal to calculate the distance. The formula below shows the
relationship between the distance and phase [6].
fcd
πφ
πλφ
44== …(1)
Figure 3.3 Phase Measurement
Figure 3.3 [6] shows the relationship between transmitted and reflected waveforms,
where x is the distance corresponding to the differential phase. The maximum distance
of a rangefinder using this method is limited by the wavelength of the transmitted light.
In a system where the receiver and the transmitter are located at the same position the
maximum range is limited to half the wavelength of the transmitted signal. This is due
to phase wrap that occurs after one wavelength; this is then halved because the reflected
signal is inverted. In a laser based system where the wavelength of the laser light is
very small, the light is modulated first and the phase difference between the modulating
waves is measured.
16
The Media Labs at MIT constructed a second scanning laser rangefinder using this
method of range-finding and achieved very good results [2]. The range finding circuitry
though was quite complex and expensive in comparison to the Time of Flight method.
3.3.1.3 Time-of-flight
Time of flight is the third method of measuring distance. It works on the very simple
principle that from knowing the speed of light we can calculate the distance to an object
by measuring the time it takes for light to travel to an object and reflect back.
To obtain centimetre resolution using time of flight measurement, timing differences in
the sub nanosecond range must be measured. This makes the use of a microcontroller
or a PC to measure the time impossible since they would require a clock speed of
approximately 30 GHz to achieve centimetre resolution.
An analogue circuit to measure the time of flight called a Time to Amplitude Converter
is required. This device will give a linear output for the range [6]. A time of flight
system requires the least analogue hardware but in order to maintain the timing
information, very fast and accurate receiver and Time to Amplitude Converter circuitry
is required. A disadvantage of using time of flight is that it is affected by background
light, with the receiver needing to able to distinguish the returning laser light from the
room light. This can be difficult since the returning laser light is many magnitudes less
than the background light.
3.3.2 Calculating the range
It was decided that time of flight would be the method used to determine the range. The
deciding factors in choosing time of flight were the simplicity of the hardware and the
low cost. Even though time of flight requires high-speed components the cost should
still be low due to the small amount of hardware needed. Triangulation was given little
consideration due to the non-linearity of the range measurement and the results that
MIT obtained from their design of a Scanning Laser Rangefinder using this method.
Although Continuous Wave phase measurement is quite accurate and the MIT Scanning
17
Laser Rangefinder design achieved good results using this method, it was decided
against due to the complex circuitry required and its cost.
The two following sections outline two different methods of measuring the time of
flight of a laser. Both methods require the laser to be pulsed and a receiver to detect the
reflected pulses.
3.3.2.1 Averaging Mode Time-to-Amplitude Converter
This section outlines the operation and performance of an Averaging Mode Time to
Amplitude Converter. This type of Time to Amplitude Converter has no dead time and
a resolution of 1 picosecond when a pulse rate of 5 MHz is used [7].
Start
Stop
U
T
t
Figure 3.4 Averaging Mode Waveforms
The operation and construction of this kind of Time to Amplitude Converter is very
simple and inexpensive [7]. The time interval measurement is done by measuring the
duty cycle of a waveform that is formed by the start and stop pulses. The start and stop
pulses are used to form a square wave that has a pulse width t equal to the time of flight.
Figure 3.4 shows the waveform diagram. The duty cycle is a linear measurement and it
can be easily seen that when the duty cycle is 0% the time of flight will be 0 and when
the duty cycle is 100% the time of flight will be at its maximum measurable amount T.
The maximum measurable distance will be determined by the period of the transmitted
waveform. The waveform U can be formed simply by using digital logic.
18
Time
Volta
ge
Figure 3.5 Averaging using Low Pass Filter
The duty cycle of the waveform U can be found by averaging the waveform to get a dc
output. The waveform can be averaged using a resistor capacitor low pass filter. This
works on the simple principle that the rate of charge does not equal the rate of
discharge. The capacitor will charge and discharge until it reaches a point where the
amount of current charged equals the amount of current discharged. This is shown in
Figure 3.5. It can be seen that the output will have some ripple and that average is not
reached instantaneously. The amount of ripple and the time it takes for the average to
be reached is dependent on the value of the resistor and capacitor.
3.3.2.2 Current Mode Time-to-Amplitude Converter
This section describes the operation of a Current Mode Time to Amplitude Converter.
This type of Time to Amplitude Converter requires only one pulse to get an accurate
distance measurement although it does have some dead time. A resolution in the
picosecond range can be achieved with these circuits [8].
This circuit measures the time interval by charging a capacitor for the duration of time
that it takes the pulse to travel from the transmitter to the object and reflect back to the
receiver. From when the start pulse leaves the transmitter until the pulse is received a
19
constant current source charges a capacitor. Once this is done the voltage needs to be
sampled and then reset before the next pulse can be sent. This leads to dead time. The
accuracy of this method will be determined by the accuracy and stability of the
components used to construct the circuit.
3.3.3 Laser Driver Electronics
The laser driver is a relatively simple circuit but some considerations need to be taken
into account when designing it due to the highly sensitive nature of laser diodes. The
laser will need to be pulsed and depending on what Time-to-Amplitude Converter is
used the laser may need to be pulsed at up to tens of megahertz. A 5 mW laser diode
would be sufficient to generate enough light to ensure that some is reflected back off the
users hand, as was the case with the MIT design [2].
Laser Drivers require precise control of the current and this is usually done using a
feedback circuit. They also need a slow starter circuit to protect the laser diode against
transients when the laser is initially turned on. Manufacturers of laser diodes usually
supply details for driver circuits for their lasers.
A driver for a laser diode is a very simple circuit and is fairly cheap to construct. Laser
Diodes are also relatively cheap starting at a cost of approximately $15 - $20 [9]. An
added cost that can be quite expensive is the collimating lens and housing that is
required for the operation of laser diodes. The cost of these can range from $50 - $200
[9] [10].
3.3.4 Receiver Electronics
Since time of flight is being used to calculate the range, the time-to-amplitude converter
only needs a stop pulse from the receiver when a pulse is received. This means that the
receiver electronics are relatively simple. The receiver needs to detect the reflected
pulses, amplify the received signal and then convert this received signal into a string of
pulses that the Time to Amplitude Converter can use.
20
The main consideration that needs to be taken into account when designing the receiver
is the delay that it will be added between when the receiver receives the pulse and when
the Time-to-Amplitude Converter receives the pulse. A delay of just one nanosecond
will cause the calculated distance to be out by 15 centimetres. While a delay between
the input and output of the receiver is unavoidable it should be kept to a minimum. This
is so the output of the Time-to-Amplitude Converter is not overly affected by the delay
and the measurement can be calibrated in the software.
Another consideration when designing the receiver will be the effect of background
light. The ratio of received laser light to background light will be considerably small.
The receiver will be required to distinguish between the background light and the laser
light so it can determine when a pulse is received. A partial solution to the ratio of
background light to laser light would be to use an optical filter to filter out a large
portion of the background light while letting the laser light pass through.
The Scanning Laser Rangefinder that was built by MIT used an avalanche Photodiode
to detect the laser light. Avalanche Photodiodes have extremely high gain and very
low-rise times. Avalanche photodiodes are very expensive, in the price range of $300 -
$400 [11]. A silicon photodiode could be used for a considerably lower price but for a
loss in performance.
3.3.5 Scanning Electronics and Angle Calculation
The scanning electronics and angle calculation for the Scanning Laser Rangefinder are
relatively straightforward. The approach that was used by MIT [2] is a simple and easy
way of calculating the angle and can be applied to this system even though a different
method of range calculation is being used.
The laser will be scanned using a rotating mirrored polygon. When the laser beam is
reflected from a users hand and received, the angle that the laser is at will need to be
recorded along with the range. The angle can be calculated simply by counting the time
between when a scan starts and when the scan stops. When a reflection is received the
21
time will be recorded. This time, along with the scan rate, can then be used to calculate
the angle of the laser beam relative to the angle of the beam at the start of a scan.
Figure 1.1 helps to understand the operation of the scanning part of the Scanning Laser
Rangefinder. Here, a cube is shown as the scanning polygon. It shows that as the cube
rotates the laser will be scanned. When the cube rotates through to its next side a new
scan will begin. When a cube is used to scan the laser the total scan will be 180
degrees. If the Scanning Laser Rangefinder is being placed in the corner of the plane it
is scanning, only 90 degrees of the scan is being used. This results in only half of the
time for one scan being used to record information. The other half of the scan can be
used to send and process the recorded data before the next scan begins.
The Scanning Laser Rangefinder is required to be used in human interaction so changes
in the position of the person’s hands once projected onto a screen need to appear
smooth. This means the position of the object being tracked needs to be updated at least
once every 25 seconds [12]. Since the data is being transmitted at the end of each scan
then the laser must be scanned at a minimum of 25 Hz. The maximum scan rate will be
limited by how much time is needed to send the data and the number of pulses that need
to be sent each scan. As the scan rate is increased the number of pulses sent per scan
decreases, this causes the distance between each pulse to increase which means that
when this distance gets too large, the persons hand may fall in between two pulses and
no pulse will be reflected back. The distance between each pulse also increases as the
distance from the Scanning Laser Rangefinder increases.
3.3.6 Microcontroller
This section outlines the function of the microcontroller in the operation of the Scanning
Laser Rangefinder. The microcontroller is required to record the range and angle of the
object being detected and then transmit this data to a PC.
To calculate the angle the microcontroller must count between when a start scan signal
is received and when a stop scan signal is received. When a scan is in progress and a
pulse is received, the microcontroller must record the value of the counter and store it in
22
the memory. When a pulse is received the microcontroller must also sample the output
of the Time to Amplitude Converter in order to get the range of the object.
To sample the output of the Time to Amplitude Converter the microcontroller must
have an analogue to digital converter (ADC). The ADC should have the appropriate
resolution so as to meet the accuracy specification of one centimetre. In the current
mode Time to Amplitude Converter the ADC will have to sample the output of the
Time to Amplitude Converter before the next pulse is sent. Therefore the ADC will
have to have a sampling rate that allows the pulse repetition rate of the laser to be
sufficient to give the required results. For the averaging mode Time to Amplitude
Converter the ADC will have to sample the output of the Time to Amplitude Converter
before the laser passes the reflecting object because the output of the Time to Amplitude
Converter does not hold the value of the range like the current mode Time to Amplitude
Converter. The value of the Time to Amplitude Converter will also have to be saved to
the memory.
Once the microcontroller has obtained the values of the range and angle it must transmit
them to a PC. The microcontroller will transmit the data when it receives the stop scan
signal and this must be completed before the start scan signal is received. The amount
of data that must be transmitted will depend on how many samples of data are taken.
This will be varied depending on how fast the microcontroller can transmit the data.
The more samples taken will lead to more accurate the results. So the faster the
microcontroller can transmit the data the more accurate the results can be.
3.4 Chapter Summary
This chapter derived a set of specifications for the Scanning Laser Rangefinder. These
specifications are:
• It must be easily portable
• It must have a resolution of 1-2 cm
• It must cost under US$500
23
• Users must not be required to wear or hold special tagged objects or be
constrained by position or posture
The chapter then went on to breakdown the design of the Scanning Laser Rangefinder
into its subsections. The theory behind each of these subsections was discussed as well
as the design considerations that need to be taken into account when designing each
system.
It was decided that Time of Flight would be used to calculate the range. Two methods
of calculating the range using Time of Flight were discussed as well the design
considerations for the laser driver, the receiver, the scanner and the microcontroller.
24
Chapter 4
Design Implementation
The implementation of each of the subsections of the system discussed in the previous
chapter is described in this chapter of the Thesis. The theory and design considerations
discussed in the previous chapter, along with the design specifications, are used to
decide on how each subsection will be implemented. The chapter discusses the
selection of components and details the practical operation of each subsection taking
into account how each subsection needs to interact with the other sections.
4.1 Laser Driver Electronics
The Sanyo DL3147-021 Red 5mW laser diode was chosen as the laser diode to be used.
It was chosen for its low cost, just under $20 [9], and its availability. The Laser diode
was purchased from Thor Labs [9] as was the collimating lens and housing. The
collimating lens was much more expensive than anticipated. It cost approximately $200
[9]. Much cheaper collimating lenses are available but not able to be purchased in time
for this project.
An Automatic Current Control driver that was recommended by Sanyo [13] was used to
drive the laser diode. This circuit [13] did not have the ability to pulse the laser diode
but it was modified so that it could. The driver included soft start circuitry and
feedback to control the current. The op amp and transistors that were used in the Sanyo
25
circuit could not be purchased so other components were chosen. The transistors were
replaced with their general-purpose counterparts and the op amp was replaced by the
National rail-to-rail LM6132.
A
I2
-12 V
I3
I1
Figure 4.1 Pulsing Laser Circuit
Figure 4.1 shows the principle of how the laser diode was pulsed. The constant current
source represents the function of the laser driver. The driver is drawing a constant
current I1 = I3 through the laser diode. The diode can be pulsed by stopping and starting
the current I3. Instead of turning the constant current driver on and off to pulse the
diode, a current equal to I3 can be added at node A to turn the diode off. Kirchoff’s
current law states that if the current I2 is equal to I1 then the current I3 will be zero. So
the pulsing circuit is quite simple, by switching the current I2, which is equal to I1, on
and off the laser diode will be pulsed.
The pulsing current I2 was generated using an N channel MOSFET and a resistor. When
the MOSFET is switched on, a current flows causing the laser diode to turn off. When
the switch is off, no current flows so the laser diode is switched on. The current I2 is set
using a resistor and the knowledge of the voltage drop across the laser diode. The
26
BSN10 was chosen as the MOSFET. It is a small signal MOSFET and was chosen
because of its fast switching time of approximately 5 nanoseconds [14]. The final
schematic of the laser driver is shown in Appendix A.
4.2 Receiver Electronics
The Receiver was required to detect a very small amount of laser light that was reflected
off of a users hand. The main factor in the performance of the receiver lies in the
performance of the photodiode used. If the photodiode cannot detect the reflected light
then no matter how good the rest of the amplification is, the receiver will be useless.
The MIT Scanning Laser Rangefinder used an Avalanche Photodiode to detect the
reflected light but this was far too costly for this prototype. Instead, a silicon
photodiode was used.
The BPW34S was used as the photodiode in the receiver. It has a very fast rise time of
20 nanoseconds and a reasonable gain for a silicon diode [15]. An advantage that this
photodiode has over many others is its large active area. It has an active area of 7mm2.
This would allow more of the reflected light to be caught and therefore increasing the
range. The BPW34S cost approximately $3 [11].
Photodiode Op AmpAmplifier
Low PassFilter
CE TransistorAmplifier Comparator
Figure4.2 Receiver Block Diagram
Once the Photodiode has detected the laser light, the received signal needs to be
amplified and converted to a square wave that can act as the stop signal for the Time to
Amplitude Converter. Figure 4.2 shows the block diagram for the receiver. The output
of the photodiode is initially amplified so that the signal can be processed easier. Red
cellophane was placed over the photodiode in order to eliminate a lot of the background
light. This allowed the first amplification stage to have a higher gain, therefore
27
amplifying the laser light more, before the effect of the background light caused the
amplifier to saturate. This amplification was done with a simple op-amp circuit [16].
This amplified signal contains the received signal as well as all of the background light.
This background light dominates the output of the amplifier and needs to be removed
from the signal so that the laser light can be fully amplified. The background light is
removed from the amplified signal using a resistor capacitor low pass filter. This low
pass filter also eliminates the 100 Hz oscillations of any fluorescent lights that may be
affecting the received signal. The cut-off of the low pass filter must be lower than that
of the pulse repetition rate of the laser so that it is not also removed from the signal.
After the background light was removed from the signal the signal was again amplified,
this time using a common emitter transistor amplifier and then converted to a square
wave using a MAX902 fast comparator.
4.3 Scanner Hardware
Figure4.3 Printer Scan Head for Scanner
28
The scanner is required to scan the laser beam and send start and stop signals to a
microprocessor. To scan the laser beam a mirrored polygon is required as well as a
motor and driver to rotate the polygon. The construction of this scanner could become
quite expensive and could be a very tedious task getting the polygon to rotate smoothly.
A very inexpensive solution to this problem was found. The Scanning Laser Head from
an old laser printer was obtained before the printer was thrown out. Figure 4.3 shows
the top view of the scanning head. The scan head contains everything that is needed to
scan the laser beam. No documentation on the operation of the scan head could be
acquired but with only four input pins its operation could be easily determined.
To get the start and stop scan signals two fast phototransistors were chosen. Both were
the BPW85B. These transistors would be placed at positions A and B so that the laser
beam would strike the transistor as it passed. These phototransistors would be used as
switches. When the laser beam passed the transistor at position A it would activate the
switch sending a start scan signal to the microprocessor. When the beam passed the
transistor at position B it would activate the switch sending a stop scan signal.
The rotating polygon is a hexagon; this means the total possible scan angle is 120
degrees. This will result in less time between each scan. This affects the amount of
data that can be sent before the next scan begins.
4.4 Averaging Mode Time-to-Amplitude Converter
An averaging mode Time to Amplitude Converter was constructed using the design by
Koskinen and Kostamovaara [7]. The article by Koskinen and Kostamovaara gave
results for the resolution of this Time to Amplitude Converter that were far beyond the
need of the 1cm resolution that is required. This circuit is also very simple and cheap to
construct making it ideal for this application.
The circuit has three main parts. The formation of the pulse U1 has a pulse width that is
equal to the time of flight. The averaging of the signal U1 and the averaging of the
29
inverse of U1, called U2. The calculation of the difference between the average of U1
and the average of U2 in order to find the voltage corresponding to the time of flight,
called U3. The average of the signal U1 could be used as the voltage representing the
time-of-flight but the article states that finding the difference between U1 and U2 gives
better accuracy.
The output U3 can be represented by the equation [7]
( )UtfUUU 12213 −=−= (2)
U = max voltage of U1
t = time of flight
f = pulse repetition rate
The sensitivity of the output of the Time to Amplitude Converter can be given by [7]
fUS 2= (V/s) (3)
The maximum range of the Time to Amplitude Converter is given by
fRangeMax
2103 8×= (4)
Therefore since the range we require is approximately 5m, the pulse repetition rate will
need to be 30 MHz (4). This will result in a sensitivity of 300 MV/s (3). This equates to
10mV/cm.
Digital logic was used to build the first part of the circuit that forms the pulse U1 using
the start and stop signals from the laser driver and the receiver. Only two D flip-flops
and four XOR gates were required to form U1 and U2. The circuit outlined in the article
[7] used ECL logic but due to the high cost, ECL logic this was not considered as an
option for this design. Instead, Fast TTL logic was opted for. Fast TTL logic is several
30
times slower than ECL but is still very fast and is still sufficient for this application.
Fast TTL is also a lot lower in cost than ECL and the trade off between cost and speed
that is made when choosing Fast TTL is justified.
The signals U1 and U2 are averaged using a simple resistor capacitor low pass filter. The
time constant of the low pass filter will determine how much ripple is in the averaged
voltage. The fewer ripples in the waveform the more accurate the results will be.
Once the signals U1 and U2 have been averaged, a standard three op-amp differential
amplifier circuit is used to ascertain the difference between these two voltages. This
circuit needs to have high precision, as a small change in the voltage equals a significant
change in the time of flight. The OPA27, a low noise precision op amp was chosen to
implement the differential amplifier in order to keep the output as accurate as possible.
The output of the differential amplifier will range from –5 V to 5V and will represent a
5m range.
4.5 Current mode Time-to-Amplitude Converter
There are many varying ways of constructing a Current Mode Time to Amplitude
Converter, each giving slightly different results. The simplest method of constructing
the Time to Amplitude Converter was attempted first [8]. If this method was successful
then it could be integrated into other more accurate circuits.
Figure 4.4 shows the simple operation of the Time to Amplitude Converter [8]. The
two switches begin in the closed position with constant current I flowing through them
to ground. The capacitor at this time has zero charge since it is connected to ground
through S1. When a pulse is sent switch S1 opens and the current flows into the
capacitor charging it at a constant rate. When the pulse arrives at the receiver switch S2
opens stopping the current from flowing. The voltage of the capacitor is held at this
point until it can be sampled. Once the voltage is sampled both switches are closed,
discharging the capacitor, and the circuit is ready for another pulse to be sent.
31
VDD
I
S1
S2
Figure 4.4 Current Mode TAC
The circuit in Figure 4.4 was constructed using an LM334 constant current source, a
capacitor and two fast switching small signal n channel mosfets. The mosfets were
BSN10’s. The LM334 has a range of current output from 1uA to 10mA. The voltage
range the cap would be charged over would be 5 volts thus giving an output of
10mV/cm over a 5m range. The capacitor would have to be charged over a time
interval of 33.3 nanoseconds (10m time of flight). A 33 picofarads capacitor was used
and the current required from the current source was calculated using formula (5) [16].
The required current was calculated to be 33 picoamps.
tCIV = (5)
A buffer was also required on the output of the capacitor because an ADC would sink a
small amount of current. Due to the capacitance being so small even a tiny current
drawn form the capacitor would cause it to discharge. Using formula (5) it can be
shown that a current of 1 nanoamp being drawn from the capacitor will cause the
capacitor, when charged to 5 volts, to discharge in just 0.165 seconds. The op amp used
to make the buffer would have to have a very low input bias current. FET op amps
typically have a very low input bias current in the nanoamp region [16] but this would
32
not be sufficient in this case. The LM6482 was chosen because it has a maximum input
bias current of 4 picoamps. Over a period of one second this would only cause the
voltage of the cap to drop by 0.12 volts. This op amp is more than sufficient since an
ADC would be able to sample this voltage in a much quicker time.
4.6 Microcontroller
The microcontroller chosen for this application was the PIC16F877. This
microcontroller was chosen because it met the specifications outlined in Chapter 3. The
PIC16F877 has a 5Mhz internal clock speed and requires 1 clock cycle per instruction
[17]. It has a 10 Bit ADC with a sampling time of approximately 20 microseconds [17].
It also has a 16-bit timer.
It was believed that the internal clock and its ability to perform 1 instruction per second
would allow it to communicate serially with the PC at the maximum rate. The 16-bit
timer was believed good enough as a 16-bit timer was used by MIT in their Scanning
Laser Rangefinder. The averaging mode Time to Amplitude Converter requires
10mV/cm over a 10 Volt range thus requiring 1000 divisions. The current mode Time
to Amplitude Converter required 10mV/cm over a 5 Volt range therefore requiring 500
divisions. The 10-bit ADC would therefore be sufficient for both Time-to-Amplitude
Converters since it offers 1024 divisions of accuracy.
This microcontroller was also chosen because of the availability of programming
software and evaluation hardware. As well as this there is quite a large amount of
applications information available for this chip on the Internet making it very appealing.
4.7 Chapter Summary
Having identified the requirements of each subsection of the system in the previous
chapter this chapter outlined the implementation of each subsection. Design details for
the Laser Driver, Receiver and both Time to Amplitude Converters were presented.
The practical design considerations of the Scanning Hardware and the microcontroller
33
were discussed and some components were selected. The design of these two sections
was not completed due to time constraints. The full schematics for the finished designs
can be found in Appendix A. The performance and suitability of each of these sections
is discussed in detail in the following chapter along with the difficulties encountered
when implementing each of them.
34
Chapter 5
Discussion of Results
Having implemented the laser driver, receiver and both Time to Amplitude Converters
their performance was evaluated with respect to the specifications of the system and
their appropriateness for use in the system. This chapter discusses the performance of
the different subsections that were implemented as well as discussing the overall
development of the system and how this went.
5.1 Overview of the Development
The goal of this thesis project was to develop a low cost scanning laser rangefinder
similar to the one developed by the MIT media labs. This scanning laser rangefinder
was to have a 1-2cm resolution and a 5m range as well as being low in cost. These
goals were not met. While designs of several subsections of the Scanning Laser
Rangefinder were developed and tested no working prototypes were completed.
The task of developing this device turned out to be more difficult and time consuming
than first thought. It was thought that the thesis paper on the development of the MIT
Scanning Laser Rangefinder would make the task of developing a similar one relatively
simple. While the paper from MIT was valuable in learning how such a device would
function it provided no information on how to develop the rangefinder since the method
35
of range finding was different. Changing the method of range finding meant that a
rangefinder had to be developed from scratch.
A lot of time was spent researching certain elements of the design that one would not
expect to require much time. The two most significant were the selection of a laser
diode and an appropriate microcontroller. The selection of an appropriate laser diode
was made difficult due to the lack of data supplied on the performance of the diodes and
also their availability. Manufacturers provide very little data on their laser diodes and a
lot of time was spent searching for information and contacting manufacturers about the
performance of their diodes. The main performance characteristic that needed to be
considered was how fast the laser diode could be pulsed. While finding a
microcontroller that would meet the specifications was relatively simple, finding
software and evaluation boards was a more difficult task. A significant amount of time
was spent trying to find a microcontroller that would meet the specifications as well as
have a readily available evaluation board design and programming software.
After it was realised that the full development of the Scanning Laser Rangefinder was
not possible in the time frame required the focus was shifted to developing a laser
rangefinder that could be easily integrated into a Scanning Laser Rangefinder at a later
date. Much of the time spent investigating areas of the development of the Scanning
Laser Rangefinder was not relevant to the design of the rangefinder. While some of the
information that was collected has been included in the thesis a lot of it simply could not
be included since the research was not completed and to complete it would have
affected the development of the rangefinder.
5.2 The Development and Testing of the Rangefinder
Design and testing was done for the laser driver, receiver and the two different Time to
Amplitude Converters. This section of the thesis discusses the performance of these
designs and states whether they are suitable for use in the scanning laser rangefinder.
36
5.2.1 Performance of the Laser Driver and Receiver
The laser driver that was designed in section 4.1 of chapter four was tested but the
testing was not completed because the laser diode blew up before any final tests could
be completed. The highly sensitive nature of laser diodes did prove to be a significant
problem when trying to test the laser. Two laser diodes were blown up before they
could even be connected to the driver. This was due to static electricity. While all the
precautions such as antistatic mats and wristbands were taken they did not prevent the
laser diodes form blowing up. The non-pulsing laser driver was tested with a laser
diode and it performed well. The pulsing laser driver was only tested with a high
intensity LED. These tests performed as expected but could only be made at
frequencies up to 50 KHz since the LED did not work at higher frequencies.
The laser receiver was not fully tested either due to the fact that the laser diode had been
destroyed. It was tested though with the high intensity LED and the laser driver. The
receiver only worked over a very small range due to the lack of light being emitted from
the LED. It could be seen though that the background light and the 100Hz oscillations
from the fluorescent lights could be completely filtered out and that the use of
cellophane paper did increase the range by filtering out some of the background light. It
was hoped to increase the range by connecting several photodiodes in parallel thus
increasing the receiving area but this was not tested due to time constraints.
5.2.2 Performance of the Time-to-Amplitude Converters
The majority of the time was spent testing the two Time to Amplitude Converter
designs. The first design completed and tested was the Averaging Mode Time to
Amplitude Converter. At the time that this design was completed, the laser driver and
receiver had not been designed. This design was tested using a string of logic inverters
to create a time delay. Changing the number of inverters used could test several
different time delays. Several different time delays were tested and the output of the
Time to Amplitude Converter appeared to be relatively linear and it was believed it
37
could obtain the required accuracy. Before more tests could be done it was realised that
the averaging method would not be sufficient to be used in a scanning device.
It was discussed earlier in chapter two that the Averaging Mode Time to Amplitude
Converter would take a short time before the average was reached. It was believed that
the time it took to reach this average would be quick enough but after some simulations
using PSPICE it was found that this would not be the case. When the laser beam is
scanning it will only be reflected off of the user for a small amount of time. It was
calculated that at a range of 5 m, scan rate of 30Hz and pulse repetition rate of 10 MHz
only several pulses would be reflected back form the users hand. PSPICE analysis
showed that this would not be long enough to generate an average value without
significant ripple. A PSPICE simulation was performed and found that when the
average was reached after only 5 pulses the ripple was approximately 1 Volt peak to
peak. This was totally inappropriate for generating the required accuracy.
At this point the testing on the averaging mode Time to Amplitude Converter was
abandoned and testing on the current mode Time to Amplitude Converter was started.
The results from the Current Mode Time to Amplitude Converter were even more
disappointing. The implementation described in the previous chapter did not function.
When the circuit was analysed on an oscilloscope it showed that the capacitor was not
charging at all. The time interval that was being measured was changed from the
nanosecond range to the millisecond range and the capacitor was changed for a larger
one and the circuit was again tested. This time the circuit performed as expected giving
relatively good results for the limited amount of testing done on it.
From these results it is easily deduced that the problem was most likely due to the very
small time interval over which the capacitor was charged. Testing showed that the
switches were switching and that the time interval between them was accurate.
Therefore the likely problem was the current source. Unfortunately no further testing
could be done on the circuit and the current source to determine the exact problem.
There are many other variations of Current Mode Time to Amplitude Converters that
could have been tested if time had permitted.
38
5.3 Chapter Summary
This chapter discussed the results of the thesis project. The performance of the
implemented designs of the laser driver, receiver and both Time to Amplitude
Converters were evaluated. The overall design process was also discussed and
evaluated.
It was discovered during the project after much research that the implementation of the
Scanning Laser Rangefinder was much more difficult than first anticipated and that it
was not possible to complete a prototype of a Scanning Laser Rangefinder. The focus at
this point was shifted to developing a laser rangefinder that could be integrated into a
Scanning Laser Rangefinder.
Thorough testing was not completed on any of the implemented subsystems and this
resulted in no working prototypes being completed. The testing of the final subsystems
was hindered by the destruction of the laser diode resulting in the laser driver, receiver
and Time to Amplitude Converters not being fully tested. The laser driver, receiver and
Averaging Mode Time to Amplitude Converter did function but the Current Mode Time
to Amplitude Converter did not work at all. It is presumed from the limited results
obtained that with further testing it would be found that the laser driver, receiver and
Averaging Mode Time to Amplitude Converter would function fully and as expected. It
was also found that the Averaging Mode Time to Amplitude Converter would not be
suitable for use in a Scanning Laser Rangefinder.
39
Chapter 6
Future Work
Having come to the end of the project it is appropriate to consider the future work that
can be carried out on the Scanning Laser Rangefinder. This chapter discusses the work
that needs to be completed in order to develop a working prototype of a Scanning Laser
Rangefinder. The task of finishing the Scanning Laser Rangefinder is broken down into
several smaller projects and a time to finish these tasks is completed. Suggestions of
ways to demonstrate these tasks in an interesting manner are also made.
6.1 The Work Remaining
This thesis did not produce any working prototypes for the Scanning Laser Rangefinder.
This leaves a significant amount of work still to be done. The laser driver and receiver
electronics were tested to a reasonable extent and only minor testing and alterations
should be required in order to complete them fully. The development for the Current
Mode Time to Amplitude Converter has still got quite a way to go since the reasons for
its failure to work were not discovered before the end of the project. While little
practical work was done on the scanning electronics the completion of this part of the
project would not be too difficult. This is because the development of the scanning
electronics has already been performed by MIT and it would just be a matter of using
the information from the MIT project to get a prototype working.
40
6.2 Completing the Design
The design of the rangefinder could be completed by working on the independent
subsections while keeping in mind the requirements of the Scanning Laser Rangefinder
design. These requirements have already been discussed in Chapter 3. The project
could be broken down into two individual projects, the laser rangefinder and the
Scanning Angle calculation.
A lot of work has already been done on a laser rangefinder in this thesis. Many of the
design considerations have been discussed and tested. The development of a laser
rangefinder using time of flight should be a relatively simple task using the information
provided in this thesis. This laser rangefinder could be used in a one dimensional HCI
application to make an interesting demonstration. One option for demonstrating this
would be the use of the rangefinder to control the horizontal movement of the bat in the
computer game Pong. Pong is an incredibly simple game. It involves two bats hitting a
ball back and forth from top to bottom of the computer screen. Each of these bats can
only move in one dimension. The computer game could be projected upon a screen and
a user could play the game without having contact with any physical objects.
The Scanning Angle project would involve more background research and consideration
than the laser rangefinder, as it has not been considered as much in this thesis. The
calculation of the scan angle though is much simpler than the calculation of the range.
This project would use the same laser driver and scanner as that in the laser rangefinder
project. In this project two identical scanners could be produced and used to form a two
dimensional interactive plane similar to that used with the Softboard described in
Chapter 2. Interesting software could then be written to demonstrate the function of the
scanners.
After the completion of these two projects and third project could involve the
integration of the two hardware schemes to construct the Scanning Laser Rangefinder
that was initially the aim of this thesis. If the previous two projects were done well the
41
integration of the hardware would be very simple and this would allow quite significant
software to be written for HCI applications.
6.3 Chapter Summary
This chapter discussed the work that remains before a working prototype of the
Scanning Laser Rangefinder can be completed. It was suggested that the design of the
Scanning Laser Rangefinder be broken down into three smaller projects. Each of these
projects could be an undergraduate thesis project. The first two projects could run in
conjunction with one another. The first project would involve the development of a
laser rangefinder and the second would involve the development of the Scanning Angle
electronics. These two projects would take into account the requirements of the
Scanning Laser Rangefinder. Project three would then be to integrate these two designs
and construct the Scanning Laser Rangefinder. Suggestions were also made on how to
demonstrate each of theses projects in a human computer interaction application.
42
Chapter 7
Conclusions
This thesis addressed the design and implementation of a Scanning Laser Rangefinder.
The Scanning Laser Rangefinder was to be similar to the one developed by the Media
Labs at MIT. The Scanning Laser Rangefinder was intended for use in Human
Computer Interaction applications. It was to track the position of a person’s hands in a
two dimensional plane which could then be used to allow a user to interact with
graphical software running on a PC.
The theory and design considerations of building such a device were discussed in detail.
Subsections of a time of flight laser rangefinder were designed, implemented and tested.
To calculate the range two different Time to Amplitude Converters were implemented
and tested. A design for an Averaging Mode Time to Amplitude Converter was
implemented and tested. The testing was not fully completed once it was found that this
method of range finding is not sufficient to be used in a scanning application. The
implementation of a Current Mode Time to Amplitude Converter was not successful
and no explanation for its non-operation could be given. A laser driver and receiver
were developed and tested. These sections of the design performed as expected but
more testing is required before their design is finalised.
While a working prototype was not developed, this thesis provides a solid basis for
continued work on the implementation of a Scanning Laser Rangefinder. The thesis
discussed many of the important issues involved with designing such a device and
43
narrowed down the design options through decisions based on the relevant theory and
results from the testing of possible circuits. The results of this thesis will make future
work on development of a Scanning Laser Rangefinder much easier since many dead
ends in the design process have already been eliminated.
44
References
[1] “Media Labs,” MIT, http://www.media.mit.edu (current Oct.18, 2001).
[2] J.A. Strickon, Design and HCI Applications of a Low-Cost Scanning Laser
Rangefinder, masters thesis, Massachusetts Institute of Technology,
Massachusetts, Department of Electrical Engineering and Computer Science,
1999.
[3] J.A. Paradiso et al. “Sensor Systems for Interactive Surfaces,” IBM Systems
Journal, Vol. 39, Nos. 3&4, Oct. 2000, pp. 892-914.
[4] “Microfield Graphics,” http://www.microfield.com (current Oct.18, 2001).
[5] H. Eglowstien, “Almost as Good as Being There,” BYTE Magazine, Vol. 19, No.
4, Apr. 1994, pp. 173-4, 176.
[6] H.R. Everett, Sensors for Mobile Robots: Theory and Application, A K Peters,
Wellesly, Massachusetts, 1995.
[7] M. Koskinen, J. Kostamovaara, “An Averaging Mode Time-to Amplitude
Converter with Picosecond Resolution,” IEEE Transactions on Instrumentation
and Measurement, Vol. 42, No. 4, Aug. 1993, pp. 866-870.
[8] J.M. Rochelle, M.L. Simpson, “Current-Mode Time-to-Amplitude Converter for
Precision Sub-Nanosecond Measurement,” Proc. Conf. IEEE Nuclear Science
Symposium and Medical Imaging, IEEE, New York, NY, 1992, pp. 439-449.
45
[9] “Thorlabs,” http://www.thorlabs.com (current Oct.18, 2001).
[10] “Meredith Instruments,” http://www.mi-lasers.com (current Oct.18, 2001).
[11] “Farnell,” http://www.farnell.com/australia (current Oct.18, 2001).
[12] “Tom’s Hardware Guide,” The Meaning of ‘Flip at Vertical Retrace’ or
'VSYNC', http://www.tomshardware.com/graphic/98q4/981007/index-02.html
(current Oct, 2001).
[13] “Sanyo LCD & LED Website,” http://www.sanyo.co.jp/lcd_led (current Oct.18,
2001).
[14] “BSN10; BSN10A: N-Channel Enhancement Mode Vertical D-MOS
Transistors,” Phillips Semiconductors Datasheet, SC13b, Apr. 1995,
http://www.semiconductors.philips.com (current Oct.18, 2001).
[15] “BPW 34 S: Silicon PIN Photodiode,” Infineon Technologies Datasheet, 2001-
02-21, http://www.infineon.com (current Oct.18, 2001).
[16] P. Horowitz, W. Hill, The Art of Electronics, Cambridge University Press,
Cambridge, 1980.
[17] “PIC16F87x Datasheet,” Microchip Datasheet, DS30292C,
http://www.microchip.com (current Oct.18, 2001).
46
Appendix A
Schematics
47
12
34
56
ABCD
65
43
21
D C B A
Title
Num
ber
Revi
sion
Size B Dat
e:19
-Oct
-200
1Sh
eet
of
File
:H
:\PRO
TEL
\The
sis.d
dbDr
awn
By:
R2 390
C1 1u R3 39
R5 100K
R1 10
R4 1.2K
R7 47K
R10
1K
R11
10K
R9 20K
R8 1.2K
C6 100n
C7 100n
Q1
BC33
8
Q3
BC32
8
Q2
BC32
8
Q4
BSN
10
R6 RES2
C5 10u
C2 47u
C3 47u
C4 22u
-12
MO
D
1 2 3J1
MO
D
-12
U1 LM61
34
LD1
DL
ZD1
5V
POT1
5K
Figure A.1 Laser Rangefinder Schematic
48
12
34
56
ABCD
65
43
21
D C B A
Title
Num
ber
Revi
sion
Size B Dat
e:19
-Oct
-200
1Sh
eet
of
File
:H
:\PRO
TEL
\The
sis.d
dbDr
awn
By:
U1
OPA
MP
U2
Com
para
tor
PD BPW
34S
Q1
NP
N
C1
C2
C3
C4
R3
R1R4
R7R6
R5R2
VC
CV
CC
VC
CV
CC
REF
OU
T
Figure A.2 Receiver Schematic
49
12
34
56
ABCD
65
43
21
D C B A
Title
Num
ber
Revi
sion
Size B Dat
e:19
-Oct
-200
1Sh
eet
of
File
:H
:\PRO
TEL
\The
sis.d
dbDr
awn
By:
C1C2
VC
C
VCC
VC
C
VC
C
VCC
CLK
3
D2
SD4 CD 1
Q5
Q6
714
U1A
CLK
11
D12
SD10 CD13
Q9
Q8
U1B
4 56
U2B
89
10
U2C
1112 13
U2D
1 23
714
U2A
Stop
R1 R2
R3
R6
R5R7
R4
R8
R9
C3 C4C5
C6
U3
OP
AMP
U4
OP
AMP
U5 OPA
MP
OUT
Figure A.3 Averaging Mode TAC Schematic
50
12
34
56
ABCD
65
43
21
D C B A
Title
Num
ber
Revi
sion
Size B Dat
e:19
-Oct
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1Sh
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of
File
:H
:\PRO
TEL
\The
sis.d
dbDr
awn
By:
U3 OPA
MP
Q1
BSN
10
Q2
BSN
10
C1 33p
V-
3V
+2
R1
U4
LM33
4
R1
C1
VC
C
VCC
VC
C
CLK
3
D2
SD4 CD 1
Q5
Q6
714
U1A
CLK
11
D12
SD10 CD13
Q9
Q8
U1B
VC
CV
CC
Stop
Star
t
Out
Figure A.4 Current Mode TAC Schematic
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