chap7sensors_v1

Design of Mobile Robots

Self Learning Module: Chapter 7

R.Nandakumar, V.K. Agrawal, Divya Rao

D e p a r t m e n t o f E l e c t r i c a l a n d E l e c t r o n i c s E n g i n e e r i n g , P E S I T

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Self Learning Module Design of Mobile Robots:

7.0 Sensors used in Robots including Mobile Robots 7.1Aim: To help students understand different sensors used in robots with a specific reference to robots that move on ground.

7.2Learning Objective: After completing this chapter the student will be conversant with: Sensors used in mobile robot, Classification of sensors based on function. Classification of sensors based on operation Criteria followed for choice of a sensor for the requirement Principle of operation of these sensors, Eelectronics used for interfacing these sensors to microcontrollers, Points to be ensured in installation to get the required performance out of the sensors,

7.3Approach followed: As indicated earlier we take the example of a specific mobile robot, in this case a Micromouse. The selected example of micromouse operates in a well defined environment. But some of the ground based mobile robots like mine detection robot, surveillance robots etc. operate in highly complex environment. We will also briefly discuss about sensors used for such robots.

7.4Assumptions made: The following assumptions are made to choose the sensors: In this chapter we will discuss about sensors used in micromouse as well as robots that

move on ground As robot is going to be autonomous, we need to consider sizeand power requirement for

sensor selection. We need to consider working environment for sensor selection. Hence following factors

also need to be considered: o Temperature and humidity, o Altitude, o Pollution like chemical vapour, radiation, dust o Vibration, shock , acceleration etc. o AC electrical noise and interference etc for sensor selection.

7.5Classification of sensor based on function: Sensors are classified based on their function in the robot. 7.5.1 Sensors used in Primary Control System of Robot: Mostly Robots work on the principle of feedback control. It can be shown that in a feedback control system, the accuracy and sensitivity of closed-loop gain of the control system is mainly controlled by feedback factor. In control systems of robot, the sensors which come in the feedback path and are very critical for the performance of the robot come under one category.





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7.5.2 Sensors used for safety aspects of the robot: Sensors used in robot for proper and safe operation of the robot. If an industrial robot is used for transporting components from place to place, it needs to have sensors to avoid collision with other moving objects in the environment. Similarly a cleaning robot needs to avoid objects in the space, avoid steps, walls etc. in the area of work 7.5.3 Sensors related to payload of the robot: Robots carry payloads for doing the tasks for which they are designed. A mine detection robot has to navigate in the area using its sensors. At the same time it needs sensors to detect mines. A cleaning robot needs to identify the garbage or dust in an area and needs sensors for doing the task properly. A robot which needs to do surveillance needs a stabilized platform which needs to be maintained level irrespective of ground on which robot is moving. 7.6Developments in Sensor Technology: Application of semiconductor manufacturing technology to mechanica and electromechanical systems has resulted in Micro Electro Mechanical Systems (MEMS) technology. Because of the rapid developments taking place in the field of MEMS, sensors available for robotics has greatly increased. Better accuracy and better interfacing to microcontrollers are made possible at much lower costs than what was possible a decade ago. In this chapter while discussing about some of the sensors, we will discuss about the underlying principle and the MEMS-based sensor. 7.7 Sensors used in Primary Control System of Mobile Robots: As our topic of study is Mobile Robtots, we will discuss about sensors used in mobile robots for control of robot motion. We discussed about different motors used in robots motion control. We saw that Stepper, DC Servomotor and Brushless DC (BLDC) are the types of motors used. Because of the higher voltage needed and less efficient operation, we will not discuss about robots using stepper motor for their motion. Whether we use DC Servomotor or BLDC motor, we need angular position sensor for motion control of robots. Though BLDC uses sensor or other techniques to do commutation and switching for its windings, still a sensor is used for feedback control of robots motion. Magnetic pick-off based or optical encoders are the normal choice. Out of this optical encoder are more commonly used because of their light weight and sealed construction. We will discuss about this type of encoder in the following section. 7.7.1 Optical encoders for rotary motion sensing: For explanation purpose we have taken Agilent Technologys HEDS type of encoders and the Quadrature IC HCTL 2022. But the basic principle is similar for other products also. The block diagram and thewaveform from the two quadrature channels are shown in Fig. 7.7.1 As seen in the diagram, the unit contains single LED as its light source The light is collimated by a lens to theintegrated detector circuit. The code wheel passes between the LED and photo detectors The set of photo detectors with integrated electronics produce the quadrature

waveforms.





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The two waveforms produced at output of encoder are A and B which are in quadrature phase.

Some encoders provide a third output called Index pulse. This is produced for every 360-degree revolution of code wheel.

One of the important specifications is the number of pulses from each channel for 360-degree rotation of code wheel. Typically we may choose an encoder that produces 500-pulses per revolution for micromouse application.

With quadrature IC, about which we will discuss in the next section, we can obtain (4 x 500) = 2000-pulses per revolution.

There are encoders available that provide 220 pulses per revolution and even higher. The technique used in such high resolution encoders are much different from the one

we discuss in this section.

Fig. 7.7.1 Encoder sensor block diagram and quadrature waveform

Quadrature IC for Encoders: This IC accepts Channel-A and Channel-B from incremental encoder. We need to feed a clock for the encoder which can be of maximum frequency 33-MHz. This IC does the following functions: Can identify the direction of rotation of the encoder wheel Provides the total count received from encoder if we had chosen up-counting mode IC number HCTL-2022 has a 32-bit counter to store the total count and can be read out

to microcontroller through 8-pin output in byte-serial mode. Can derive rate and approximate acceleration information from the counter by sampling

counter at regular interval. The encoder pulse outputs, the state diagrams and logic to determine direction of

rotation are shown in Fig. 7.7.2





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Fig. 7.7.2 Pulse from encoder and state diagram

The arrangement of components in an optical encoder are given in Fig. 7.7.3

Fig. 7.7.3 Physical arrangement in optical encoder

7.7.2 Track Following Sensor Arrangement for Mobile Robots: Mobile Robots used in industries, hospitals and other well defined environments may use track following arrangement to move through well defined tracks. The track sensing and alignment to track forms part of the control requirement of the robot.





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One of the common methods of defining a track is by embedding on floor along the required path a narrow retro-reflective tape / black tape. Suitable way-points are marked using such tapes at pre-determined locations. This helps the robot to reset itself at the way-points. Normally a set of Infra Red (IR) Emitter and Detector pairs are arranged on the robot base to illuminate and sense the tape. The arrangement of IR-Emitter and Detector pairs is shown in Fig. 7.7.4.

Fig. 7.7.4 Five sets of emitter-detector pairs below PCB

Reason for choice of Infra-Red as against visible spectrum: When the ambient (surrounding) visible light level is high, Infra-red is preferred to

visible. Factors to be considered: The optics is provided with filter to cut-down visible wavelength and allow IR

wavelength. The emitter and detector pair is chosen to have peak response around the same

wavelength. Reasons why pulsed emission is preferred over continuous emission: When the peak current specification of infra-red emitter is much higher than average

current we prefer pulsed emission. For example, manufacturer of an IR-emitter gives average forward current value of 100-

mA and surge forward current of 2-A for 100-us. If we use this emitter and pulse 500-mA current, the power is going to be 5-times that of continuous current.

By suitable choice of duty cycle, we can still be within the emitters average power rating.

This will provide better signal to noise ratio in the detector. Just by taking the detector reading before pulsing emitter and during emitter pulsing, we

can eliminate the offset output from emitter due to ambient light. By taking the detector output when emitter is pulsed, that is in a synchronous manner,

we minimize the interference due to other systems like motor. We can use simple digital filter to eliminate effect of interference from other sources.

This is possible by taking 3-readings of detector during one emitter pulse and noting whether all three values are same.





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When to choose Retro-reflective tape and when Black tape: If the floor of the building where robot is moving around is dark in color and not

reflective, we can use retro-reflective tape for the track. Retro-reflective tape sends back a good percentage of incident infra-red radiation.

Hence detectors above the tape get good signal. By choosing high emitter peak current, proper spacing between emitter-detector pair to

floor, proper design of hoods to shield out external light, the signal received by detector can be made large. In such a case, we can get good logic-1 level signal even without an amplifier.

Rest of the emitter-detector pairs in the array, which are not above the tape will produce logic-0 level signal

If the floor is very reflective, it is better to use black tape. In this case the detectors above tape produce logic-0 output and that not on tape

produce logic-1 state because of reflection from floor. Fig. 7.7.5 shows the dimensional details of the IR-emitter and also the radiation pattern of the emitter. Fig. 7.7.6 shows the dimension details of the IR-detector transistor and also the receiver responsivity Vs angle and rise-time test arrangement (rise time typically will be around 20-30-usec)

Fig. 7.7.5 IR Emitter dimensions and radiation pattern





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Fig. 7.7.6 IR Detector dimensions and receiving pattern and rise time

Real World Navigation of Mobile Robots: Another extreme of mobile robot is one which has to move around in real world environment autonomously. Googles driverless car is such a mobile robot. It has radars, vision camera, GPS navigation system etc. The sensors used in such mobile robot are outside the scope of our module.





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differencebetween.com/difference-between-google-car-and-ordinary-car/

Fig. 7.7.7 Googles Driverless Car

Unmanned Ground Vehicles (UGV): There are some robots that are in between the Googles Driverless Car and theone which we discussed as track following robot. This category of robots is called Autonomous Ground Vehicles. These are meant for any terrain. The sensors that are made use of in this type of robotic vehicles are: Vision system for both day light and night vision conditions GPS based position determination. Magnetometers to get attitude / direction of movement Accelerometers for torque control, gradient detection, tilt correction for stabilized

platforms on the vehicle etc. Rate gyros to measure / control body rates

We are not going to deal with the vision systems and GPS based position determination system in this module. We will discuss about the principles of other sensors in the following section.





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Unmanned Ground Vehicles - Wikipedia

7.7.3 Magnetometers: Magnetometers are used in the navigation of mobile robots . Magnetometer makes use of earths magnetic field for that purpose. As magnetic field on earth has a dip component also, for proper determination of earths field, a triaxial magnetometer will be an ideal one. Such magnetometers are readily available. They comprise of three single axis magnetometers in the same housing with associated electronics. Though flux-gate magnetometers are widely used in avionic systems, we will discuss about magneto-resistive type of accelerometers which are of lower accuracy but lower cost. These magnetometers also come with hybrid electronics and are very small in size and weight. These are highly suited for robotics and we will discuss about these in this section. As with any magnetometer measurement, we need to ensure that these sensors are not placed near permanent / electromagnet. They should not be placed in housing made of magnetic materials like mild steel, which may shielld earths magnetic field and result in false reading.





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As earths magnetic field data is available for any region, the earths magnetic field itself can be used for calibration of magnetometers. For study purpose let us take Honeywells magneto resistive sensors though number of manufacturers are available offering similar sensors. Magneto-resistive Sensor: Anisotropic magnetoresistance (AMR) occurs in ferrous materials. It is a change in resistance when a magnetic field is applied perpendicular to the current

flow in a thin strip of ferrous material as shown in Fig. 7.7.8.

Fig. 7.7.8 Principle of Magneto Resistive (MR) sensor

The transducer is arranged in the form of a Wheatstone bridge as shown in Fig. 7.7.9.

Fig. 7.7.9 MR Transducer

When no magnetic field is present, the resistance, R, of all four magnetoresistors is the

same. The bridge supply, Vb, causes current to flow through the resistors. A crossed applied field, H, causes the magnetization in two of the oppositely placed

resistors to rotate towards the current, resulting in an increase in the resistance, R. In the remaining two oppositely-placed resistors magnetization rotates away from the

current resulting in a decrease in the resistance, R. For Honeywells MR sensors the sensitivity is typically 3 mV/(V/Oe) and the range of

linearity is within 2-Oe

Fig. 7.7.10 shows single and two-axis magnetometers. These provide the basic Wheatstones bridge providing differential output.





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Fig. 7.7.10 Single and Two-axes magneto-resistive sensors

Fig. 7.7.11 shows hybrid circuit having single and 2-axis sensors (forming 3-axis sensor) with Instrumentation grade amplifiers. This hybrid module provides high output as amplifier is incorporated.

Fig. 7.7.11 Triaxial magneto-resistive sensors with hybrid electronics

7.7.4 MEMS Accelerometers: Accelerometers based on MEMS technology may be working on: Strain-gauge technique Piezo-electric techniqe





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Capacitance type We are going to discuss the one based on capacitance type. Though there are many manufactures producing this type, we will discuss here about the Analog Devices ADXL series. The reason for choosing the above one is because of its use in micromouse and other mobile robots. This sensor is of low cost type and consumes low power. The operating principle can be understood by referring to Fig. 7.7.12

Fig. 7.7.12 Accelerometer Structure

Proof mass attached through springs to substrate. The proof mass can move only up / down A movable and fixed plate capacitors are formed around this The two sets of capacitors are excited using square wave of opposite polarity. A reference voltage is used for phase-sensitive demodulation Under zero acceleration, the output voltage is zero Depending on the direction of acceleration, opposite polarity signal is produced at

output of demodulator The magnitude of the voltage is directly proportional to magnitude of acceleration. This is illustrated in Fig. 7.7.13





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Fig. 7.7.13 Circuit for measuring acceleration

The physical dimension of the accelerometer with integrated electronics is shown in Fig. 7.7.14

Fig. 7.7.14 Package dimension

7.7.5 MEMS Gyroscopes: In the case of Accelerometers, force on the proof mass is F = m x a In the case of MEMS Gyroscope Coriolis force is made to act on the proof mass. Coriolis force is illustrated in Fig. 7.7.15





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Fig. 7.7.15 Coriolis Force

Two polysilicon sensing structures each contain a dither frame which is electrostatically

driven to resonance. This produces the necessary movement to create a Coriolis effect during rotation. At two of the outer extremes of each frame, orthogonal to the dither motion, are

movable fingers that are placed between fixed fingers to form a capacitive pickoff structure that senses Coriolis motion

Rest of the signal processing is similar to Accelerometer.

7.7.6 Potentiometers: You may wonder how a good old component like potetiometer finds use in the robotics! These are very much used in many Radio Controlled (RC) toys like cars, model aircrafts and helicopters for manipulating the control surfaces, throttle etc. It is incorporated in a device called servo. You may vary the width of a pulse that is sent 50-times a second from 4 to 20-msec. Depending upon the width of the pulse a shaft will turn from zero to 90, 180, 270 or nearly 360 deg. Inside the assembly called servo you will find processor electronics, DC brushed / brushless motor, the spider whose angle is to be controlled and a potentiometer for feedback. Such servos are used not only in RC toys but also in robots like humanoid to move the leg and hip joints. Hence do not be surprised to see a potentiometer being used to sense angle of rotation! 7.7.7 Tactile sensors: If a robot has to handle a glass bottle / cup, it needs to handle it with some predetermined force. It should not crush the glass nor should glass slip out of its grip. Such applications need tactile sensors. Such sensors may start from a simple micro-switch. Some of the manufacturers provide data on force to operate the switch. It can also be a simple limit switch where the force is specified. It can be strain-gauge based sensor which measures force. Such sensors may need integral amplifiers to bring the output to a few volts range.





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One may have to do survey and choose a suitable sensor and modify it for a specific application. 7.7.8 Obstacle Detection: This requirement may arise to avoid collision of robot with other moving / stationary

objects. One of the common type of sensor is Ultrasonic transmit-receive set. A pair of ultrasonic devices may be used at the specified frequency in pulse-mode. As sound travels at approximately 300 m / sec, the round-trip distance to the obstacle

decides the delay between transmit and receive pulses. Knowing the delay, the distance to obstacle may be determined. In this case, we need to get specifications on type of obstacle and reflection

characteristics of ultrasound. Maximum distance for sensing an obstacle, resolution, accuracy are parameters to be

considered for such sensors. The associated electronics for oscillator, pulsing circuit, power amplifier, detector, time

delay measurement using microcontroller are to be planned to make a complete obstacle detection system.

Obstacle detection and ranging systems are availble that work on IR pulsing, Laser pulsing etc.

The field of sensors is vast and there is steady increase in the sensors available.

7.8 Sum up: We discussed need of sensors in robots to do: Control of robot For the payload requirements For the safety aspects of the robot and its environment Different types of sensors Their operating principle Associated electronics Factors to be considered in the choice etc.

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