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Basilio BonaBasilio BonaDAUIN DAUIN –– Politecnico di TorinoPolitecnico di Torino
Mobile & Service RoboticsMobile & Service Robotics
Sensors forSensors for RoboticsRobotics –– 11Sensors for Sensors for Robotics Robotics 11
An Example of robots with their sensors
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Another example
Omnivision Camera (360°)
Pan-Tilt-Zoom (PTZ) camera
Sonars
IMU=Inertial Measurement Unit
Laser Scanner
Encoders inside differential wheels
Bumpers
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Passive support wheel
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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
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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)
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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
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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
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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
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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
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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
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Accuracy and precision
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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
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NoiseNoise
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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
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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
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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
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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
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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
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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
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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
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