Sensors and detectorsHow to use sensor and detectors (in robotics)

RACE PROJECT

VIGO (SPAIN) September 26-29, 2012

Definitions (1) Sensor: a device for sensing a physical variable of a physical

system or an environment A sensor is in most cases associated with electronic circuits

(converter) to generate a conditioned, normalized, amplified electric signal

A sensor plus a converter form a transducer, a device which transforms energy from one type to another (in this specific case electric energy)

Sometimes the sensor reacts by generating an electric signal itself, so it can be considered a transducer

Definitions of sensors and transducers do not always agree, so the word “sensor” alone might be enough to indicate these types of devices

Sensors are what the robots need to know the world …◦ from http://www.robotshop.com/sensors.html

Classifications 1 (physical

characteristic)Primary (sensors/transducers)

◦ Temperature◦ Light (photoelettric)◦ Strain gauge / Mechanical tension◦ Magnetic field◦ Displacement (potenziometers)◦ …

Secondary (sensors/transducers)◦ Force, acceleration, pressure (based on strain gauge)◦ Displacement (based on photoelettric, magnetic field,

capacitance, …)◦ Speed (based on displacement and time)

Classifications 2 (output electric signal)Analog

◦ The output is an electric signal which varies continuously according to the variations of the physical variables beeing measured

Digital◦ The output is an electric pulse signal which can assume

only two values: logical 0 and 1. The frequency or the code associated with the pulse sequence carries the information about the physical variables beeing measured

Classifications 3 (energetic

behavior)Active

◦ They provide an electric signal which can be directly processed without further consumption of energy: for example the photovoltaic cells and termocouples

Passive◦ The require an electrical generator in order to transduce

the physical variable in an electric signal: for example the potentiometer

Specifications (static and dynamic parameters) Transfer function (transcaratteristic) Monotone function Linearity Offset Operation range Hysteresis Sensitivity Resolution Repeatibility Stability Response time (time costant e bandwidth) Input and output impedance

Specifications (parametri caratteristici statici e

dinamici)Monotone function

Linearity

Sensitivity

Sensors (in this presentation) Thermoresistance, thermistor, PN junction, integrated

sensor (temperature) Photoresistor, photodiode, phototransistor (light) Tachometer (angolar speed) Encoder (different tipes for displacement and speed)

Temperature - Thermoresistance (1)Metallic conductors with a known “resistance vs temperature caracteristic curve”.The basic physical principle of these devices is that the electric conductivity (resistivity) decreases (increases) as the temperature increases. This holds true for materials like platinum, nickel, copper.

The value of T is in 0CIf β and γ are small compared to the value of α this relation can be considered linear (for example platinum)If not it might be necessary to perform a linearization (for example for nickel and copper for temperatures above 1000C)

Thermoresistance have a low sensitivity

Temperature - Thermoresistance (2)Example of a temperature monitoring system from 00C to 3000C, output tension between 0V and 10 V, based on PT100.

VR

V1

Temperature - Thermistor NTCUnipolar semiconductor material

The basic physical principle of these devices is that the electric conductivity (resistivity) of the pure semiconductor material increases (decreases) as the temperature increases.

NTC (Negative Temperature Coefficient); T is in 0K

Thermistors are very sensitive, but R is strongly non linear with respect to T.

With highly doped semicobductor material it is possible to obtain PTC type (Positive Temperature Coefficient) thermistors

Temperature - PN junctionIn a direct bias PN junction with costant current the direct diode tension decreases by 2,5 mV with the increase of 10C of the temperature. The exact value of the tension for a given temperature depends upon the value of the costant current of the diode.

A small signal junction diode like 1N914 or 1N4148 can be easily used as a temperature sensor.

Good time responseCalibration is required

Temperature - IC AD590 (1)Integrated sensor:

Devices which embed the sensor plus the circuits to normalize, linearize, amplify the signal, in other words a “transducer”.

The integrated T sensors are based on the linear dipendence between VD e T of the direct bias costant current diode (see prevous page). The junction is the BE junction of a BJT transistor

AD590: high impedence current generatoriT is in 0K; K is μA/0K The generated current is directly proportional to the absolute T valueIt can be located far from the measurement instrument (it works with current) and it is not very sensitiv to noiseIt can show scale and offset errorsThe output current signal is converted to tensiom through a resistance plus a I/V converter (for example based on OpAmp)

Temperature - IC AD590 (2)

Light sensor – Photoresistor (1)Devices in which the information associated

to light is converted in variation of resistance: resistivity decreases (conductivity increases) as the light increases

Made with N type semiconductore materiale (not a PN junction)

Thoughness, low priced, sensitivityThey can dissipatehigh values of power (for

example to control relays)Limited bandwidth

Light sensor – Photoresistor (2)

Light sensor – Photodiode (1)When a reversed biased PN junction is

illuminated the total reverse current is given by the sum of the typical revers current plus a component proportional to the luminous flux

These devices are very fast and are highly used as detectors in telecommunication systems based on fiber optics

Light sensor – Photodiode (2)

Speed -Tachometer generator (1)

The tachometer generator (dynamo) is a small generator that produces an output voltage that is very accurately determined by its operating speed

Tachometer generator (2) Simulation file

Speed - Incremental encoder (1)Device which measures the angolar displacement of a shaft in order to get informations about the angular speed (for example of a motor)It is made by a rotary disc and a Led/fototransistor system. On the circumference of the disc a set of holes has been set all at the same distance on from the other.

When the disc rotates the light beam either is interrupted (no hole and bjt in cut-off mode) or goes through from the led to the phototransistor (precence of the hole and bjt in saturation mode). The phototransistor generates a train pulse, one pulse for each hole crossed by the light beam.

From the number of pulses it is possible to determine the angular displacement and, in relation to time, the angular speed

With the incremental encoder it is possible to measure the speed, but it is not possible to determine the rotational direction

Speed - Incremental encoder (2)

Speed - Incremental encoder (2)INSERIRE LA FIGURA

Speed - Incremental encoder (3)Two/three phases incremental encoder to determine the rotational direction

Speed - Incremental encoder (4)Two/three phases incremental encoder

Speed – Absolute encoder (1)Used to determine the shaft angular position. Each combination of holes is coded so to provide the angular position of the disc. Normally the Gray code is used in order to prevent the transmission of errorsWith simple combinatory Exor circuit it is possible to convert Gray code in natual binary code

Motor control with encoder (1)Analog control system

Motor control with encoder (2)Digital control system

Sensors (Society of robots)

Sensors specific for robotics: Society of robots

http://www.societyofrobots.com/sensors.shtml

Sensors in robotics & applications Accelerometer Color Sensors Digital Compass Encoder (Slot, Rotary, Linear) Infrared Emitter/Detector Load and Torque Sensors Mercury Tilt Switch Photoresistor Robot Computer Vision SharpIR Rangefinder Sonar Tactile Bumper Switch

Sensors in robotics & applications Accelerometer Color Sensors Digital Compass Encoder (Slot, Rotary, Linear) Infrared Emitter/Detector Load and Torque Sensors Mercury Tilt Switch Photoresistor Robot Computer Vision SharpIR Rangefinder Sonar Tactile Bumper Switch

Robot for the contestIdeas for the final meeting