CIS009-2, Mechatronics - Sensors & Signal Conditioning · CIS009-2, Mechatronics Sensors & Signal Conditioning David Goodwin Department of Computer Science and Technology University

CIS009-2, MechatronicsSensors & Signal Conditioning

David Goodwin

Department of Computer Science and TechnologyUniversity of Bedfordshire

29th November 2012

31

Mechatronics

David Goodwin

Sensors

Position sensor

Potentiometer

LVDT

Proximity Sensors

Rotary Encoders

Heat

Thermostat

Thermistors

RTD

Thermocouple

Optical

LDR

Photodiode

Phototransistor

Department ofComputer Science and

TechnologyUniversity ofBedfordshire

Outline

1 SensorsPosition sensor

PotentiometerLinear variable differentialtransformerProximity SensorsRotary Encoders

Temperature Sensors

ThermostatThermistorsResistance TemperatureDetectorThermocouple

OpticalLDRPhotodiodePhototransistor

31

Mechatronics

David Goodwin

3Sensors

Position sensor

Potentiometer

LVDT

Proximity Sensors

Rotary Encoders

Heat

Thermostat

Thermistors

RTD

Thermocouple

Optical

LDR

Photodiode

Phototransistor



Sensors

31

Mechatronics

David Goodwin

4Sensors

Position sensor

Potentiometer

LVDT

Proximity Sensors

Rotary Encoders

Heat

Thermostat

Thermistors

RTD

Thermocouple

Optical

LDR

Photodiode

Phototransistor



Sensors

Sensors are “Input” devices which convert one type of energyor quantity into an electrical analog signal.

Detect Position, Temperature, Light, Pressure and Velocity.

Some sensors called “Self-generating” sensors generate outputvoltages or currents relative to the quantity being measured,such as thermocouples and photo-voltaic solar cells and theiroutput bandwidth equals that of the quantity being measured.

Some sensors called “Modulating” sensors change theirphysical properties, such as inductance or resistance relativeto the quantity being measured such as inductive sensors,LDR’s and potentiometers and need to be biased to providean output voltage or current.

Not all sensors produce a straight linear output andlinearization circuitry may be required.

31

Mechatronics

David Goodwin

5Sensors

Position sensor

Potentiometer

LVDT

Proximity Sensors

Rotary Encoders

Heat

Thermostat

Thermistors

RTD

Thermocouple

Optical

LDR

Photodiode

Phototransistor



Signal Conditioning

Op-amps can be used to provide amplification of signals Very small analogue signal voltages produced by sensors (few

milli-volts) Voltage gain up to 10,000 and a current gain up to 1,000,000

with the amplification of the signal being linear with theoutput signal being an exact reproduction of the input, justchanged in amplitude.

Generally some form of amplification, impedance matching,isolation between the input and output or filtering may berequired before the signal can be used and this is convenientlyperformed by Operational Amplifiers.

31

Mechatronics

David Goodwin

Sensors

6Position sensor

Potentiometer

LVDT

Proximity Sensors

Rotary Encoders

Heat

Thermostat

Thermistors

RTD

Thermocouple

Optical

LDR

Photodiode

Phototransistor



Position sensors

A position sensor is any device that permits positionmeasurement.

It can either be an absolute position sensor or a relative one(displacement sensor).

Position sensors can be linear, angular, or multi-axis.

31

Mechatronics

David Goodwin

Sensors

Position sensor

7Potentiometer

LVDT

Proximity Sensors

Rotary Encoders

Heat

Thermostat

Thermistors

RTD

Thermocouple

Optical

LDR

Photodiode

Phototransistor



Potentiometer

inexpensive and easy to use position sensor. either angular (rotational) or linear (slider type) in its

movement resistance value between the wiper/slider and the two end

connections changes giving an electrical signal output resistance is proportional to position. can be used as position feedback devices in order to create

“closed loop” control, such as in a servomechanism.

31

Mechatronics

David Goodwin

Sensors

Position sensor

8Potentiometer

LVDT

Proximity Sensors

Rotary Encoders

Heat

Thermostat

Thermistors

RTD

Thermocouple

Optical

LDR

Photodiode

Phototransistor



Potentiometer

DC reference voltage is applied across the two outer fixedconnections. The output voltage signal is taken from thewiper terminal of the sliding contact as shown below.

configuration below produces a potential divider type circuit. apply a 10v across the resistive element of the potentiometer

the maximum output voltage 10 volts, with the minimum of0 volts.

Then the potentiometer wiper will vary the output signalfrom 0 to 10 volts, with 5 volts indicating that the wiper orslider is at its half-way or centre position.

31

Mechatronics

David Goodwin

Sensors

Position sensor

Potentiometer

9LVDT

Proximity Sensors

Rotary Encoders

Heat

Thermostat

Thermistors

RTD

Thermocouple

Optical

LDR

Photodiode

Phototransistor



Linear variable differential transformerLVDT

One type of positional sensor that does not suffer frommechanical wear problems is the “Linear Variable DifferentialTransformer”.

an inductive type position sensor which works on the sameprinciple as the AC transformer that is used to measuremovement.

It is a very accurate device for measuring linear displacementand whose output is proportional to the position of itsmoveable core.

consists of three coils wound on a hollow tube former, oneforming the primary coil and the other two coils formingidentical secondaries connected electrically together in seriesbut 180 out of phase either side of the primary coil.

A moveable soft iron ferromagnetic core is connected to theobject being measured

A small AC reference voltage is applied to the primarywinding which induces an EMF signal into the two adjacentsecondary windings.

31

Mechatronics

David Goodwin

Sensors

Position sensor

Potentiometer

10LVDT

Proximity Sensors

Rotary Encoders

Heat

Thermostat

Thermistors

RTD

Thermocouple

Optical

LDR

Photodiode

Phototransistor




31

Mechatronics

David Goodwin

Sensors

Position sensor

Potentiometer

11LVDT

Proximity Sensors

Rotary Encoders

Heat

Thermostat

Thermistors

RTD

Thermocouple

Optical

LDR

Photodiode

Phototransistor




in the centre of the tube and the windings the two inducedemf’s in the secondary windings cancel each other out so theresultant output voltage is zero.

As the core is displaced slightly to one side the inducedvoltage in one of the secondaries will be become greater thanthe other and an output will be produced.

The polarity of the output signal depends upon the directionand displacement of the moving core.

The result is a differential voltage output which varies linearlywith the cores position.

typical application as a pressure transducer.

Advantages of the LVDT linearity; that is its voltage output to displacement is

excellent; very good accuracy; good resolution; highsensitivity as well as frictionless operation. They are alsosealed for use in hostile environments.

31

Mechatronics

David Goodwin

Sensors

Position sensor

Potentiometer

LVDT

12Proximity Sensors

Rotary Encoders

Heat

Thermostat

Thermistors

RTD

Thermocouple

Optical

LDR

Photodiode

Phototransistor



Proximity sensors

A proximity sensor is a sensor able to detect the presence ofnearby objects without any physical contact.

Often emits an electromagnetic field or a beam ofelectromagnetic radiation, and looks for changes in the fieldor return signal.

Different proximity sensor targets demand different sensors.For example,

capacitive photoelectric sensor might be suitable for a plastictarget

inductive proximity sensor always requires a metal target.

A proximity sensor adjusted to a very short range is oftenused as a touch switch.

31

Mechatronics

David Goodwin

Sensors

Position sensor

Potentiometer

LVDT

13Proximity Sensors

Rotary Encoders

Heat

Thermostat

Thermistors

RTD

Thermocouple

Optical

LDR

Photodiode

Phototransistor



Proximity Sensors

Proximity sensors do not actually measure displacement orangular rotation they are mainly used to detect the presenceof an object within a close proximity.

Non-contact devices that use a magnetic field for detection. A coil is wound around an iron core within an electromagnetic

field to form an inductive loop. When a ferromagnetic material is placed within the “eddy

current field” generated around the inductive sensor theinductance of the coil changes significantly.

31

Mechatronics

David Goodwin

Sensors

Position sensor

Potentiometer

LVDT

14Proximity Sensors

Rotary Encoders

Heat

Thermostat

Thermistors

RTD

Thermocouple

Optical

LDR

Photodiode

Phototransistor



Proximity Sensors

Ideal for use in dirty or wet environments.

The range of proximity sensors is very small, typically 0.1mmto 12mm.

One main disadvantage of these types of sensors is that theyare “Omni-directional” (can sense a metallic object eitherabove, below or to the side of it).

Do not detect non-metallic objects Although Capacitive Proximity Sensors (suitable for

non-metalic target) and Ultrasonic Proximity Sensors (usessound) are available.

31

Mechatronics

David Goodwin

Sensors

Position sensor

Potentiometer

LVDT

Proximity Sensors

15Rotary Encoders

Heat

Thermostat

Thermistors

RTD

Thermocouple

Optical

LDR

Photodiode

Phototransistor



Rotary Encoders

Rotary Encoders resemble potentiometersmentioned earlier but are non-contactoptical devices used for converting theangular position of a rotating shaft intoan analogue or digital data code.

Light source is passed through a rotating high-resolutionencoded disk that contains the required code patterns.

Photo detectors scan the disk as it rotates and information ina stream of binary output pulses is fed to counters orcontrollers which determine the actual angular position of theshaft.

31

Mechatronics

David Goodwin

Sensors

Position sensor

Potentiometer

LVDT

Proximity Sensors

16Rotary Encoders

Heat

Thermostat

Thermistors

RTD

Thermocouple

Optical

LDR

Photodiode

Phototransistor



Rotary Encoders

Simple Incremental Encoder

Absolute Position Encoder

31

Mechatronics

David Goodwin

Sensors

Position sensor

Potentiometer

LVDT

Proximity Sensors

Rotary Encoders

17Heat

Thermostat

Thermistors

RTD

Thermocouple

Optical

LDR

Photodiode

Phototransistor



Temperature Sensors

Temperature sensors vary from simple ON/OFF thermostaticdevices to highly sensitive semiconductor types that cancontrol complex process control plants.

Moving molecules/atoms produces heat (kinetic energy) andthe greater the movement, the more heat that is generated.

Temperature sensors consist of two basic physical types:

Contact Temperature Sensors Required to be in physicalcontact with the object being sensed and useconduction to monitor changes in temperature.They can be used to detect solids, liquids or gasesover a wide range of temperatures.

Non-contact Temperature Sensors Use convection and radiationto monitor changes in temperature. They can beused to detect liquids and gases that emit radiantenergy as heat rises and cold settles to the bottomin convection currents or detect the radiant energybeing transmitted from an object in the form ofinfra-red radiation (the sun).

31

Mechatronics

David Goodwin

Sensors

Position sensor

Potentiometer

LVDT

Proximity Sensors

Rotary Encoders

Heat

18Thermostat

Thermistors

RTD

Thermocouple

Optical

LDR

Photodiode

Phototransistor



ThermostatBi-metallic strip

The Thermostat is a contact type electro-mechanicaltemperature sensor or switch,

consists of two different metals such as nickel, copper,tungsten or aluminium etc, that are bonded together to forma Bi-metallic strip.

The different linear expansion rates of the two dissimilarmetals produces a mechanical bending movement when thestrip is subjected to heat.

31

Mechatronics

David Goodwin

Sensors

Position sensor

Potentiometer

LVDT

Proximity Sensors

Rotary Encoders

Heat

Thermostat

19Thermistors

RTD

Thermocouple

Optical

LDR

Photodiode

Phototransistor



ThermistorsThermally sensitive resistor

The Thermistor is a thermally sensitive resistor, a resistorwhich changes its resistance with changes in temperature.

Thermistors are generally made from ceramic typesemiconductor materials such as oxides of nickel, manganeseor cobalt coated in glass which makes them easily damaged.

Their main advantage over snap-action types is their speed ofresponse to any changes in temperature, accuracy andrepeatability.

Most types of thermistor’s resistance value goes DOWN withan increase in the temperature.

Thermistors are rated by their resistive value at roomtemperature (usually at 25 C), their time constant (the timeto react to the temperature change) and their power ratingwith respect to the current flowing through them.

For sensing purposes resistances in the kilo-ohms are generallyused.

31

Mechatronics

David Goodwin

Sensors

Position sensor

Potentiometer

LVDT

Proximity Sensors

Rotary Encoders

Heat

Thermostat

20Thermistors

RTD

Thermocouple

Optical

LDR

Photodiode

Phototransistor




Thermistors are passive resistive devices which means weneed to pass a current through it to produce a measurablevoltage output. Then thermistors are generally connected inseries with a suitable biasing resistor to form a potentialdivider network and the choice of resistor gives a voltageoutput at some pre-determined temperature point or value.

31

Mechatronics

David Goodwin

Sensors

Position sensor

Potentiometer

LVDT

Proximity Sensors

Rotary Encoders

Heat

Thermostat

21Thermistors

RTD

Thermocouple

Optical

LDR

Photodiode

Phototransistor




Thermistors are non-linear devices and their standardresistance values at room temperature is different betweendifferent thermistors.

have an exponential change with temperature which can beused to calculate its resistance for any given temperaturepoint.

However, when used with a series resistor such as in a voltagedivider network or Whetstone Bridge type arrangement, thecurrent obtained in response to a voltage applied to thedivider/bridge network is linear with temperature.

Then, the output voltage across the resistor becomes linearwith temperature.

31

Mechatronics

David Goodwin

Sensors

Position sensor

Potentiometer

LVDT

Proximity Sensors

Rotary Encoders

Heat

Thermostat

Thermistors

22RTD

Thermocouple

Optical

LDR

Photodiode

Phototransistor



Electrical resistance temperature sensorRTD

The Resistance Temperature Detector (RTD) is a precisiontemperature sensors made from high-purity conducting metalssuch as platinum, copper or nickel wound into a coil andwhose electrical resistance changes as a function oftemperature, similar to that of the thermistor.

Unlike the thermistor their output is extremely linearproducing very accurate measurements of temperature.

However, they have poor sensitivity, that is a change intemperature only produces a very small output change forexample, 1Ω per 1 C.

Like the thermistor, RTD’s are passive resistive devices andby passing a constant current through the temperature sensorit is possible to obtain an output voltage that increaseslinearly with temperature.

The RTD is usually connected into a Whetstone Bridgenetwork which has additional connecting wires forlead-compensation and/or connection to a constant currentsource to avoid errors from self heat of the resistive wires.

31

Mechatronics

David Goodwin

Sensors

Position sensor

Potentiometer

LVDT

Proximity Sensors

Rotary Encoders

Heat

Thermostat

Thermistors

RTD

23Thermocouple

Optical

LDR

Photodiode

Phototransistor



Thermocouple

Thermocouples have the widest temperature range of alltemperature sensors, from below −200 C to over 2000 C.

Thermocouples are thermoelectric sensors that consists of twojunctions of dissimilar metals, such as copper and constantanthat are welded or crimped together.

One junction is kept at a constant temperature called thereference (Cold) junction, while the other the measuring(Hot) junction.

When the two junctions are at different temperatures, avoltage is developed across the junction which is used tomeasure the temperature sensor as shown below.

31

Mechatronics

David Goodwin

Sensors

Position sensor

Potentiometer

LVDT

Proximity Sensors

Rotary Encoders

Heat

Thermostat

Thermistors

RTD

24Thermocouple

Optical

LDR

Photodiode

Phototransistor



Thermocouple

When fused together the junction of the two dissimilar metalsproduces a “thermo-electric” effect which gives a constantpotential difference of a few millivolts (mV) between them(Seebeck effect).

If both the junctions are at the same temperature thepotential difference across the two junctions is zero.

When the junctions are connected within a circuit and areboth at different temperatures a voltage output will bedetected relative to the difference in temperature between thetwo junctions.

31

Mechatronics

David Goodwin

Sensors

Position sensor

Potentiometer

LVDT

Proximity Sensors

Rotary Encoders

Heat

Thermostat

Thermistors

RTD

Thermocouple

25Optical

LDR

Photodiode

Phototransistor



Light Sensors

Light Sensors generate an output signal indicating theintensity of light by measuring the radiant energy that existsin a very narrow range of frequencies (light).

The light sensor is a passive devices that convert “lightenergy” into an electrical signal output.

Photo-conductive Cells Vary their electrical resistance whensubjected to light. Photoconductivity results fromlight hitting a semiconductor material whichcontrols the current flow through it. More lightincreases the current for a given applied voltage.

Photo-junction Devices Mainly true semiconductor devices suchas the photodiode or phototransistor which uselight to control the flow of electrons and holesacross their PN-junction. Photojunction devicesare specifically designed for detector applicationand light penetration with their spectral responsetuned to the wavelength of incident light.

31

Mechatronics

David Goodwin

Sensors

Position sensor

Potentiometer

LVDT

Proximity Sensors

Rotary Encoders

Heat

Thermostat

Thermistors

RTD

Thermocouple

Optical

26LDR

Photodiode

Phototransistor



Light Dependent ResistorLDR

Light Dependent Resistor (LDR) is made from exposedsemiconductor material that changes resistance from severalthousand Ohms in the dark to only a few hundred Ohmswhen light falls upon it.

Photoresistive cells have a long response time requiring manyseconds to respond to a change in the light intensity.

31

Mechatronics

David Goodwin

Sensors

Position sensor

Potentiometer

LVDT

Proximity Sensors

Rotary Encoders

Heat

Thermostat

Thermistors

RTD

Thermocouple

Optical

27LDR

Photodiode

Phototransistor



Light Dependent ResistorLDR

Connecting a light dependantresistor in series with a standardresistor like this across a singleDC supply voltage has one majoradvantage, a different voltagewill appear at their junction fordifferent levels of light.

The amount of voltage dropacross series resistor, R2 isdetermined by the resistive valueof the light dependant resistor.

No light present, resistance ofLDR is very high, zero base biasis applied to the transistor TR1and the relay is “OFF”.

Light level increases, resistance ofLDR decreases, causing the basebias voltage at V1 to rise.

At some point determined by thepotential divider network, thebase bias voltage is high enoughto turn the transistor TR1 “ON”and activate the relay.

31

Mechatronics

David Goodwin

Sensors

Position sensor

Potentiometer

LVDT

Proximity Sensors

Rotary Encoders

Heat

Thermostat

Thermistors

RTD

Thermocouple

Optical

28LDR

Photodiode

Phototransistor



Light Dependent ResistorLight Level Sensing Circuit

The operational amplifier is configured as a DifferentialAmplifier.

As with the previous circuit the output from the operationalamplifier is used to control a relay.

When the light level sensed by the LDR falls below areference the output from the op-amp changes stateactivating the relay and switching the connected load.

The hysteresis of the two switching points is set by thefeedback resistor Rf can be chosen to give any suitablevoltage gain of the amplifier.

31

Mechatronics

David Goodwin

Sensors

Position sensor

Potentiometer

LVDT

Proximity Sensors

Rotary Encoders

Heat

Thermostat

Thermistors

RTD

Thermocouple

Optical

LDR

29Photodiode

Phototransistor



PhotodiodePhoto-diode Amplifier Circuit

Photodiode light sensor is similar to that of a conventionalPN-junction diode except that the diodes outer casing iseither transparent or has a clear lens to focus the light ontothe PN junction for increased sensitivity.

When light falls upon the junction more hole/electron pairsare formed and the leakage current increases. This leakagecurrent increases as the illumination of the junction increases.

31

Mechatronics

David Goodwin

Sensors

Position sensor

Potentiometer

LVDT

Proximity Sensors

Rotary Encoders

Heat

Thermostat

Thermistors

RTD

Thermocouple

Optical

LDR

30Photodiode

Phototransistor



Photodiode

Photo-current-to-voltage convertor circuit using anoperational amplifier as the amplifying device.

The output voltage (Vout) is given as Vout = Ip × Rf andwhich is proportional to the light intensity characteristics ofthe photodiode.

This “zero-bias” op-amp configuration gives a highimpedance loading to the photodiode resulting in lessinfluence by dark current and a wider linear range of thephotocurrent relative to the radiant light intensity.

31

Mechatronics

David Goodwin

Sensors

Position sensor

Potentiometer

LVDT

Proximity Sensors

Rotary Encoders

Heat

Thermostat

Thermistors

RTD

Thermocouple

Optical

LDR

Photodiode

31Phototransistor



Phototransistor

An alternative photo-junction device to the photodiode is thePhototransistor which is basically a photodiode withamplification.

In the NPN transistor the collector is biased positively withrespect to the emitter so that the base/collector junction isreverse biased.

When light falls on the base more electron/hole pairs areformed in this region and the current produced by this actionis amplified by the transistor.

The overall sensitivity is a function of collector current andcan be controlled by connecting a resistance between the baseand the emitter.

Documents

CIS009-2, Mechatronics - Sensors & Signal Conditioning · CIS009-2, Mechatronics Sensors & Signal Conditioning David Goodwin Department of Computer Science and Technology University