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SCHOOL OF ENGINEERING Sensors and Actuators Part 2 Pr. Nazim Mir-Nasiri

02. Sensors and Actuators

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SCHOOL OF ENGINEERING

Sensors and Actuators

Part 2

Pr. Nazim Mir-Nasiri

Type of Sensors

Pr. Nazim Mir-Nasiri

• Displacement Sensors are concerned with measurement of the amount by which some object has been moved

• Position sensor are concerned with the determination of the position of some object with respect to some reference points

• Proximity sensors are a form of position sensor but used to determine when an object has moved to within some particular critical distance of the sensor. These are essentially devices which gives on-off outputs only

Displacement and Position Sensors

Pr. Nazim Mir-Nasiri

Displacement and position sensors can be grouped into two basic types:• contact sensors in which the measurement object comes

into mechanical contact with the sensorFor example, sensing shaft of the sensor can be in direct contact with the object being monitored and its either linear or rotary movement can be used to cause changes in electrical voltage, resistance, capacitance or mutual inductance• non-contacting where there is no physical contact between

the measured object and the sensor For example, the presence in the vicinity of the measured object can cause changes in air pressure in the sensor or change in inductance or capacitance

Potentiometer Sensors

Pr. Nazim Mir-Nasiri

• A potentiometer consists of a resistance element with sliding contact which can move over the length of the element.

• Such element can be used for liner and rotary displacement detection that is then converted into the potential difference.

• The rotary potentiometer consists of a circular wire-wound track or a film of conductive plastic over which a rotatable sliding contact can be rotated

Potentiometer Sensors

Pr. Nazim Mir-Nasiri

• With a constant input voltage VS between terminals 1 and 3, the output voltage Vout between terminals 2 and 3 is a fraction of the input voltage which depends on the ratio of the resistances R23 and R13 : Vout / Vs = R23 // R13

• If the track has a constant resistance per unit of length or per unit of angle then : Vout / Vs = R23 // R13 = L23 /L13 = 23 /13

Potentiometer Characteristics

Pr. Nazim Mir-Nasiri

• With a wire-wound track the slider moving from one turn to the other will change the voltage output in steps, each step being a movement of one turn

• If the potentiometer has N turns then the resolution in percentage is 100%/N. The resolution then depends on the diameter of the wire that is ranging from 1.5 mm for a coarsely wound track to 0.5 mm for a finely wound one

• Non-linearity error ranges from less than 0.1% to about 1%• The track resistance ranges from 20 to 200 k. • Conductive plastic has ideally infinite resolution • Non-linearity error is about 0.05%• The track resistance ranges from 500 to 80 k The conductive plastic has a higher temperature coefficient of resistance than the wire and so temperature changes have a greater effect on accuracy

Effect of a Load on Potentiometer Readings

Pr. Nazim Mir-Nasiri

• The potential difference across the load VL is only directly proportional to Vout if the load is infinite

• If the load is finite then the relations between the output voltage and the angle become non-linear

Effect of a Load on Potentiometer Readings

Pr. Nazim Mir-Nasiri

• The resistance RL is in parallel with the fraction x of the potentiometer resistance RP . The combined resistance then

• The total resistance (in series) across the source is

Effect of a Load on Potentiometer Readings

Pr. Nazim Mir-Nasiri

• The voltage across the load VL is the voltage across the combined load and it is a fraction of the applied voltage VS across the total resistance (voltage divider concept)

• If the load is of infinite resistance

Effect of a Load on Potentiometer Readings

Pr. Nazim Mir-Nasiri

• If the load is of finite resistance, the error introduced by the load is

Stain-Gauged Elements/Sensors

Pr. Nazim Mir-Nasiri

• There are three types of electrical resistance stain gauges: metal wire (a), metal foil strip (b), or a strip of semiconductor materials (c, which is wafer-like and can be stuck onto surface like a postage stamp

• When subject to strain, its resistance R changes and its relative change is proportional to strain =l/l

• G is the coefficient proportionality called gauge factor• Thus, the resistance change R is the basis for the measurement of the change in length l

Stain-Gauged Elements/Sensors

Pr. Nazim Mir-Nasiri

• The gauge factor G of metal wire or foil strain gauge is about 2

• Silicon p- and n- type semiconductors stain gauges have gauge factors of about +100 or more for p-type silicon and -100

Or more for n-type silicon

• Gauge factor is supplied by the manufacturer from the calibration made on samples of stain gauges

• The calibration involves subjecting the sample gauge to known strain and measuring their changes in resistance

Stain-Gauged Elements/Sensors

Pr. Nazim Mir-Nasiri

• The problem with all stain gauges is that their resistance not only changes with stain but with temperature as well

• Although semiconductor stain gauges have higher gauge factor they also have much greater sensitivity to temperature than metal strain gauges. However, this dependency can be eliminated by a special interfacing circuit

Example,

Stain-Gauged Elements/Sensors

Pr. Nazim Mir-Nasiri

• Stain gauges are normally attached to flexible elements in the form of cantilevers (a), rings (b), or U-shaped elements (c)

• When the flexible element is bent or deformed (typically of order 1 mm to 30 mm) as result of force being applied and contact point is displaced, then the electrical resistance stain gauge is stained and so give a resistance change which can be monitored• The non-linearity error for this range of deformation is about 1% of full range

Capacitive Elements/Sensors

Pr. Nazim Mir-Nasiri

• The capacitance of a parallel plate capacitor is given by

• Measurement of linear distance can take one of the forms

Capacitive Elements/Sensors

Pr. Nazim Mir-Nasiri

• If the separation distance d is increased by a displacement x then the capacitance reduces and become

• The relative change in capacitance

• There is a non-linear relations between ΔC and displacement x

Capacitive Elements/Sensors

Pr. Nazim Mir-Nasiri

• The nonlinearity can be overcome by using push-pull displacement sensor that has three plates and the central

plate is the sensing element that moves between the other two

• When C1 is located in one arm of an a.c. bridge and C2 is in another one, thenthe resulting out-of-balance voltage from the bridge is directly proportional to x

• This sensor can monitor displacements from a few millimeters to hundred millimeters

• Nonlinearity and hysteresis error are About ± 0.01% of full range

Capacitive Elements/Sensors

Pr. Nazim Mir-Nasiri

• One popular form of capacity proximity sensor consists of a single capacitor probe with other plate being formed by the object, which has to be metallic and earthed

• As the object approaches so the “plate separation” of the capacitor changes, becoming significant and detectible when the object is close to the probe

Capacitive Sensors -Applications

Pr. Nazim Mir-Nasiri

Position Measurement/Sensing, Displacement Measurement

• Their outputs always indicate the size of the gap between the sensor's sensing surface and the target

• This is useful in:

- Automation requiring precise location

- Semiconductor processing

- Final assembly of precision equipment such as disk drives

- Precision stage positioning

Capacitive Sensors -Applications

Pr. Nazim Mir-Nasiri

Dynamic Motion

• Measuring the dynamics of a continuously moving target, such as a rotating spindle or vibrating element (up to 15kHz), requires some form of noncontact measurement

• This is useful in:

- Precision machine tool spindles

- Disk drive spindles

- High-speed drill spindles

- Ultrasonic welders

- Vibration measurements

Capacitive Sensors -Applications

Pr. Nazim Mir-Nasiri

Thickness Measurement

• Measuring material thickness in a noncontact fashion is a common application for capacitive sensors. The most useful application is a two-channel differential system in which a separate sensor is used for each side of the piece being measured

• This is useful in:

- Silicon wafer thickness

- Brake rotor thickness

- Disk drive platter thickness

Capacitive Sensors -Applications

Pr. Nazim Mir-Nasiri

Assembly testing

• Capacitive sensors have a much higher sensitivity to conductors than to nonconductors. Therefore, they can be used to detect the presence/absence of metallic subassemblies in completed assemblies.

• An example is a connector assembly requiring an internal metallic snap ring which is not visible in the final assembly. Online capacitive sensing can detect the defective part and signal the system to remove it from the line.

Differential Transformer - Sensors

Pr. Nazim Mir-Nasiri

• Linear Variable Differential Transformer (LVDT) consists of three

coils symmetrically spaced along an insulated tube

• The central coil is the primary coil and the other two are identical secondary coils which are connected in series in such a way that their outputs oppose each other

• A magnetic core is moved through the central tube as a result of the displacement being monitored

Differential Transformer - Sensors

Pr. Nazim Mir-Nasiri

• Linear Variable Differential Transformer (LVDT) consists of three

coils symmetrically spaced along an insulated tube

Differential Transformer - Sensors

Pr. Nazim Mir-Nasiri

• When there is an alternating voltage input to the primary coil, alternating e.m.f.s are induced in the secondary coils

• With the magnetic core central, the amount of magnetic material in each of the secondary coils is the same and the e.m.f.s induced in each coil are the same. Since they are so connected that their outputs oppose each other, the net result is zero output

Differential Transformer - Sensors

Pr. Nazim Mir-Nasiri

• However, when the core is displaced from the central position

there is a greater amount of magnetic core in one coil than the other

• The result is that a greater e.m.f. is induced in one coil than the other. Therefore, there is now a net output from the two coils

• The greater displacement the more difference between two e.m.s. and the more reading value of the sensor

Differential Transformer - Sensors

Pr. Nazim Mir-Nasiri

The e.m.f. induced in a secondary coil by a changing current i in the primary coil is given by:

where M is the mutual inductance which depends on the number of turns on the coils and the ferromagnetic core. • The input current is • The e.m.f. in the secondary windings is then

• where the values of k1, k2 and φ depend on the degree of coupling between the primary and secondary coils for a partial position

• φ is the phase difference between the primary and the secondary alternating voltages

Differential Transformer - Sensors

Pr. Nazim Mir-Nasiri

Two outputs are connected in serious but opposite to each other, the resulting output then

• When the core is equally in both coils, k1 equals k2 and the output voltage is zero

• When the core is more in 1 that 2 k1 > k2 and the resulting voltage is in phase with the input voltage

• When the core is more in 2 than 1 k1 < k2 and the output voltage is 180° out of phase with input voltage

Differential Transformer - Sensors

Pr. Nazim Mir-Nasiri

The output provides the same amplitude regardless for two different displacement up or down

•To differentiate the outputs for different positions of the core we need to take into the account a phase difference in two different positions

• A phase sensitive demodulators (low pass filter) is used together with LVDT to convert the output into a d.c. voltage which then gives a unique value for each displacement

Differential Transformer - Sensors

Pr. Nazim Mir-Nasiri

• Operating ranges of LVDT is from ±2 mm to ±400 mm• The non-linearity error is about ±0.25%.

• LVDT is used as primary transducer for monitoring displacement with free end may be spring loaded for contact with the surface being monitored

• LVDT are also used as secondary transducer in the measurement of force, weight and pressure; these variables are transformed into displacements which can then be monitored by LVDT

• Rotary variable differential transformers

Differential Transformer - Sensors

Pr. Nazim Mir-Nasiri

• Rotary variable differential transformers (RVDT) can be used for the measurement of rotation

• The core is a cardioid-shaped piece of magnet and rotation causes more of it pass into one secondary coil than the other

• The range is typically ±40°• The linearity error is about ±0.5% of the range

Aircraft and Helicopter Throttle Position RVDT

Eddy Current Proximity Sensors

Pr. Nazim Mir-Nasiri

• Alternating current in the (reference or transmitting) coil produces an alternating magnetic filed

• If there is a non-magnetic but metal object in close proximity to the field, then eddy currents are induced in it. They produce by themselves a magnetic field

• This distorts the original magnetic field (in sensing coil)• It changes the impedance of the sensing coil and as a result in

amplitude of the alternating current• This can be used to trigger a switch at preset level of a.c.

current

Eddy Current Proximity Sensors

Pr. Nazim Mir-Nasiri

• The sensor can be used to trigger a switch at preset level of a.c. current

• They are inexpensive, small, highly reliable, very sensitive to small displacements

Inductive Proximity Sensors

Pr. Nazim Mir-Nasiri

• It consists of coil wound round the core.• When the end of the coil is close to a metal object its

inductance changes• This change can be monitored by its effect on a resonant circuit

and the change used to trigger a switch• It can only be used for the detection of metal objects