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Basilio Bona – Politecnico di Torino · 2012. 5. 21. · Shot noise is distinct from current...

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ROBOTICA ROBOTICA ROBOTICA ROBOTICA 03CFIOR 03CFIOR 03CFIOR 03CFIOR Basilio Bona Basilio Bona DAUIN DAUIN Politecnico di Torino Politecnico di Torino
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Page 1: Basilio Bona – Politecnico di Torino · 2012. 5. 21. · Shot noise is distinct from current fluctuations in thermal equilibrium, which happen without any applied voltage and with

ROBOTICAROBOTICAROBOTICAROBOTICA03CFIOR03CFIOR03CFIOR03CFIOR

Basilio BonaBasilio BonaDAUIN DAUIN –– Politecnico di TorinoPolitecnico di Torino

Page 2: Basilio Bona – Politecnico di Torino · 2012. 5. 21. · Shot noise is distinct from current fluctuations in thermal equilibrium, which happen without any applied voltage and with

Mobile & Service RoboticsMobile & Service Robotics

Sensors forSensors for RoboticsRobotics –– 11Sensors for Sensors for Robotics Robotics  11

Page 3: Basilio Bona – Politecnico di Torino · 2012. 5. 21. · Shot noise is distinct from current fluctuations in thermal equilibrium, which happen without any applied voltage and with

An Example of robots with their sensors

3ROBOTICA 03CFIORBasilio Bona

Page 4: Basilio Bona – Politecnico di Torino · 2012. 5. 21. · Shot noise is distinct from current fluctuations in thermal equilibrium, which happen without any applied voltage and with

Another example

Omnivision Camera (360°)

Pan-Tilt-Zoom (PTZ) camera

Sonars

IMU=Inertial Measurement Unit

Laser Scanner

Encoders inside differential wheels

Bumpers

4

Passive support wheel

ROBOTICA 03CFIORBasilio Bona

Page 5: Basilio Bona – Politecnico di Torino · 2012. 5. 21. · Shot noise is distinct from current fluctuations in thermal equilibrium, which happen without any applied voltage and with

Definition

A sensor is a device that produces a measurable response to a change in a physical condition (such as temperature) or to a change in a chemical concentration

Usually commonly used sensors convert the physical Usually commonly used sensors convert the physical quantity into a signal which can be measured electrically

The sensors are classified according to the following The sensors are classified according to the following criteria:1 Primary Input quantity (aka measurand)1. Primary Input quantity (aka measurand)2. Transduction principles 3 Measured property (as temperature flow displacement3. Measured property (as temperature, flow, displacement, 

proximity, acceleration, etc.)4. Material and technology4. Material and technology5. Application

5ROBOTICA 03CFIORBasilio Bona

Page 6: Basilio Bona – Politecnico di Torino · 2012. 5. 21. · Shot noise is distinct from current fluctuations in thermal equilibrium, which happen without any applied voltage and with

Sensors types

Proprioceptive sensors (PC) They measure quantities coming from the robot itself, e.g., 

d h l l d b h di b hmotor speed, wheel loads, robot heading, battery charge status, etc.

Exteroceptive sensors (EC) Exteroceptive sensors (EC) They measure quantities coming from the environment: e.g., 

walls distance, earth magnetic fields, intensity of the , g , yambient light, obstacle positions, etc.

Passive sensors (SP) They use the energy coming from the environment

Active sensors (SA) They use the energy they produce and measure the reaction 

of the environment (better performance, but may influence the environment)the environment)

6ROBOTICA 03CFIORBasilio Bona

Page 7: Basilio Bona – Politecnico di Torino · 2012. 5. 21. · Shot noise is distinct from current fluctuations in thermal equilibrium, which happen without any applied voltage and with

A l S th ti i bl d id th

Sensors types

Analog Sensors: they measure continuous variables and provide the information as a physical reading (mercury thermometers and old style voltmeters are good examples of analog sensors)style voltmeters are good examples of analog sensors)

Digital Sensors: they measure continuous or discrete variables, but the provided information is always digital, i.e., discretizedp y g , ,

Continuous Sensors: although the name is somehow misleading, Continuous Sensors: although the name is somehow misleading, continuous sensors (analog or digital) provide a reading that is on a continuous range, as opposite to ON/OFF sensors

Binary Sensors : they give only two levels of information ON/OFF or YES/NO: a lamp that switches on when a certain temperature level is attained, is an analog binary sensor

7ROBOTICA 03CFIORBasilio Bona

Page 8: Basilio Bona – Politecnico di Torino · 2012. 5. 21. · Shot noise is distinct from current fluctuations in thermal equilibrium, which happen without any applied voltage and with

Sensors classification

Category Sensors Type

Tactile senso s/p o imit

Contact sensors (on/off), bumpers EC - SPProximity sensors EC SATactile sensors/proximity

sensors

y(inductive/capacitive) EC - SA

Distance sensors (inductive/capacitive) EC - SA

Active wheel sensors

Potentiometric encoders PC - SPOptical, magnetic, Hall-effect, inductive, capacitive encoders, PC - SAinductive, capacitive encoders, syncro and resolvers

PC SA

Heading sensors with respect to Compasses EC - SPGyroscopes PC SPg p

a fixed RF Gyroscopes PC - SPInclinometers EC – SP/AGPS (outdoor only) EC – SA

Absolute cartesian sensorsOptical or RF beacons EC – SAUltrasonic beacons EC – SARefelctive beacons EC – SARefelctive beacons EC SA

8ROBOTICA 03CFIORBasilio Bona

Page 9: Basilio Bona – Politecnico di Torino · 2012. 5. 21. · Shot noise is distinct from current fluctuations in thermal equilibrium, which happen without any applied voltage and with

Sensors classification

Category Sensors Type

Reflective sensors EC - SA

Active distance sensors(active ranging)

Ultrasonic sensors EC - SA

Laser range finders, Laser scanners EC - SA(active ranging)

Optical triangulation (1D) EC - SA

Structured light (2D) EC - SA

Motion and velocity sensors (speed relative to fixed or mobile objects)

Doppler radar EC - SA

Doppler sound EC - SA

Vision sensors: distance from stereo vision, feature analysis, segmentation object

CCD and CMOS cameras EC - SA

Integrated packages for visual ranging EC - SA

segmentation, object recognition

g g

Integrated packages for object tracking EC - SA

9ROBOTICA 03CFIORBasilio Bona

Page 10: Basilio Bona – Politecnico di Torino · 2012. 5. 21. · Shot noise is distinct from current fluctuations in thermal equilibrium, which happen without any applied voltage and with

Sensor characteristics

D i Dynamic range Resolution Linearity Bandwidth or frequency Transfer function

Reproducibility/precision Accuracy Systematic errors Systematic errors Hysteresis Temperature coefficient Temperature coefficient Noise and disturbances: signal/noise ratio C t Cost

10ROBOTICA 03CFIORBasilio Bona

Page 11: Basilio Bona – Politecnico di Torino · 2012. 5. 21. · Shot noise is distinct from current fluctuations in thermal equilibrium, which happen without any applied voltage and with

Sensor characteristics

Dynamic range Ratio between lower and upper measurement limits, expressed in 

dBdB Example: voltage sensor min=1 mV, max 20V: dynamic range 86dB Range = upper limit of dynamic range Range = upper limit of dynamic range 

Resolution Minimum measurable difference between two values Resolution = lower limit of dynamic range Digital sensors: it depends on the bit number of the A/D converter Example 8 bit=25510 range 20 V ‐> 20/255 = 0.08

Bandwidth Diff b t d l f i Difference between upper and lower frequencies Large bandwidth means large transfer rate Lower bandwidth is possible in acceleration sensorsLower bandwidth is possible in acceleration sensors

11ROBOTICA 03CFIORBasilio Bona

Page 12: Basilio Bona – Politecnico di Torino · 2012. 5. 21. · Shot noise is distinct from current fluctuations in thermal equilibrium, which happen without any applied voltage and with

Basilio Bona 12ROBOTICA 03CFIOR

Page 13: Basilio Bona – Politecnico di Torino · 2012. 5. 21. · Shot noise is distinct from current fluctuations in thermal equilibrium, which happen without any applied voltage and with

Accuracy and precision

13ROBOTICA 03CFIORBasilio Bona

Page 14: Basilio Bona – Politecnico di Torino · 2012. 5. 21. · Shot noise is distinct from current fluctuations in thermal equilibrium, which happen without any applied voltage and with

Accuracy and Precision

P i i R bili R d ibiliPrecision = Repeatability = Reproducibility

Precise but Accurate butt inot accurate not precise

Precise andaccurate

Not accurate andnot precise

14ROBOTICA 03CFIORBasilio Bona

Page 15: Basilio Bona – Politecnico di Torino · 2012. 5. 21. · Shot noise is distinct from current fluctuations in thermal equilibrium, which happen without any applied voltage and with

NoiseNoise

15ROBOTICA 03CFIOR

Page 16: Basilio Bona – Politecnico di Torino · 2012. 5. 21. · Shot noise is distinct from current fluctuations in thermal equilibrium, which happen without any applied voltage and with

Noise

All sensors are subject to noise since due to random All sensors are subject to noise, since, due to random fluctuations or electromagnetic interference, they add to the measured signal an undesired component that cannot bemeasured signal an undesired component that cannot be precisely known

If th i i ll th th t fl t ti d If the noise is smaller than the measurement fluctuations and the noise introduced by the electronic components, it is not influentinfluent

On the contrary it can degrade the entire chain plant‐sensor‐controller and make it unusable

16ROBOTICA 03CFIORBasilio Bona

Page 17: Basilio Bona – Politecnico di Torino · 2012. 5. 21. · Shot noise is distinct from current fluctuations in thermal equilibrium, which happen without any applied voltage and with

Noise

Noise is often spread on a large frequency spectrum and many noise sources produce the so‐called white noise, where the power spectral density is equal at every frequency

The noise is often characterized by the spectral density of the noise Root Mean Square (RMS), given as

/V Hz

Since it is a density, to obtain the RMS value one shall integrate the spectrum density in the frequency band of interest. This type of distribution adds to the measure an error term that is proportional to the square root of the bandwidth of the measuring system

17ROBOTICA 03CFIORBasilio Bona

Page 18: Basilio Bona – Politecnico di Torino · 2012. 5. 21. · Shot noise is distinct from current fluctuations in thermal equilibrium, which happen without any applied voltage and with

Noise types

Noise are of many types; these includeNoise are of many types; these include Shot noise Thermal noise Thermal noise Flicker noise Burst noise Avalanche noiseTo know the noise type is important for modeling purposes

18ROBOTICA 03CFIORBasilio Bona

Page 19: Basilio Bona – Politecnico di Torino · 2012. 5. 21. · Shot noise is distinct from current fluctuations in thermal equilibrium, which happen without any applied voltage and with

Shot noise

Shot noise, often called quantum noise, is always associated to random fluctuations of the electric current in electrical conductors, due to the current being carried by discrete charges (electrons) whose number per unit time fl d lfluctuates randomly

This is often an issue in p‐n junctions. In metal wires this is h l l b d d lmuch less important, since correlation between individual 

electrons remove these random fluctuations Shot noise is distinct from current fluctuations in thermal 

equilibrium, which happen without any applied voltage and ith t t fl i Th th lwithout any average current flowing. These thermal 

equilibrium current fluctuations are known as thermal noisenoise

The shot noise spectrum is flat 19ROBOTICA 03CFIORBasilio Bona

Page 20: Basilio Bona – Politecnico di Torino · 2012. 5. 21. · Shot noise is distinct from current fluctuations in thermal equilibrium, which happen without any applied voltage and with

Thermal noise

Thermal noise, also called Johnson–Nyquist noise, is the electronic noise generated by the thermal agitation of the h i ( ll h l ) i id l i lcharge carriers (usually the electrons) inside an electrical conductor at equilibrium, which happens regardless of any applied voltageapplied voltage

Thermal noise is approximately white pp y

With good approximation the amplitude of the signal has a b b l d fGaussian probability density function

20ROBOTICA 03CFIORBasilio Bona

Page 21: Basilio Bona – Politecnico di Torino · 2012. 5. 21. · Shot noise is distinct from current fluctuations in thermal equilibrium, which happen without any applied voltage and with

Flicker noise

Flicker noise, also called 1/f noise or pink noise  is characterized by a frequency spectrum such that the power spectral density is inversely proportional to the frequency

It is always present in active components of electronic circuits and in many passive ones

It is proportional to the current amplitude, so if the current is sufficiently low, the thermal noise will predominate

21ROBOTICA 03CFIORBasilio Bona

Page 22: Basilio Bona – Politecnico di Torino · 2012. 5. 21. · Shot noise is distinct from current fluctuations in thermal equilibrium, which happen without any applied voltage and with

White noise

White noise is a random signal (or process) with a flat power spectral density

The signal contains equal power within a fixed bandwidth at any center frequency

An infinite‐bandwidth white noise signal is a purely theoretical construction

The bandwidth of white noise is limited in practice by the mechanism of noise generation, by the transmission medium and by finite observation capabilities

A random signal is considered “white noise” if it is observed to have a flat spectrum over the widest possible bandwidth

White noise is often used for modeling purposesg p p

22ROBOTICA 03CFIORBasilio Bona


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