Sensors and detectors. How 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
Howtousesensor and detectors (in robotics)
VIGO (SPAIN) September 26-29, 2012
Metallicconductorswith a known “resistance vs temperature caracteristic curve”.
The basicphysicalprincipleofthesedevicesisthat the electricconductivity (resistivity) decreases (increases) as the temperature increases. Thisholdstrueformaterialslikeplatinum, nickel, copper.
The valueof T is in 0C
If β and γ are smallcomparedto the valueofαthis relation can beconsideredlinear (forexampleplatinum)
Ifnotitmightbenecessarytoperform a linearization (forexamplefor nickel and copperfortemperaturesabove 1000C)
Thermoresistancehave a low sensitivity
Exampleof a temperature monitoring system from 00C to 3000C, output tensionbetween 0V and 10 V, based on PT100.
Unipolar semiconductor material
The basicphysicalprincipleofthesedevicesisthat the electricconductivity (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 isstrongly non linearwithrespectto T.
Withhighlydopedsemicobductor material itispossibletoobtain PTC type (Positive Temperature Coefficient) thermistors
In a directbias PN junctionwithcostantcurrent the directdiodetensiondecreasesby 2,5 mVwith the increaseof 10C of the temperature.
The exactvalueof the tensionfor a given temperature dependsupon the valueof the costantcurrentof the diode.
A smallsignaljunctiondiodelike 1N914 or 1N4148 can beeasilyusedas a temperature sensor.
Deviceswhichembed the sensor plus the circuitstonormalize, linearize, amplify the signal, in otherwords a “transducer”.
The integrated T sensors are based on the lineardipendencebetween VD e T of the directbiascostantcurrentdiode (seeprevouspage). The junctionis the BE junctionof a BJT transistor
AD590: high impedencecurrent generatori
T is in 0K; K isμA/0K
The generatedcurrentisdirectlyproportionalto the absolute T value
It can belocated far from the measurementinstrument (itworkswithcurrent) and itisnotverysensitivtonoise
It can show scale and offset errors
The output currentsignalisconvertedtotensiomthrough a resistance plus a I/V converter (forexamplebased on OpAmp)
The tachometer generator (dynamo) is a small generator that produces an output voltage that is very accurately determined by its operating speed
Devicewhichmeasures the angolar displacementof a shaft in ordertogetinformationsabout the angularspeed (forexampleof a motor)
Itismadeby a rotary disc and a Led/fototransistor system.
On the circumferenceof the disc a set ofholeshasbeen set all at the samedistance on from the other.
When the disc rotates the light beameitherisinterrupted (no hole and bjt in cut-off mode) or goesthroughfrom the led to the phototransistor (precenceof the hole and bjt in saturation mode).
The phototransistorgenerates a trainpulse, onepulseforeachholecrossedby the light beam.
From the numberofpulsesitispossibletodetermine the angulardisplacement and, in relation totime, the angularspeed
With the incremental encoder itispossibletomeasure the speed, butitisnotpossibletodetermine the rotational direction
INSERIRE LA FIGURA
Two/threephasesincremental encoder todetermine the rotational direction
Usedtodetermine the shaftangular position.
Eachcombinationofholesiscoded so toprovide the angular position of the disc.
Normally the Gray code isused in ordertoprevent the transmissionoferrors
WithsimplecombinatoryExorcircuititispossibletoconvertGray code in natualbinary code