Prepared by Mike Crompton (Rev 28 May 2002)

Devices that convert different forms of energy into electrical energy.
Thermocouples
Two wires of different material, usually twisted together. When heat (or cold) is applied to the wires a small voltage is produced at the contact point of the wires. The amount of voltage is dependent on the type of wire material, and can vary from under 10

Volts to slightly over 75

Volts per degree C. Typical wire material combinations are:

Platinum/Rhodium – Lowest sensitivity.

Chromel/Constantan - Highest sensitivity.

Tungsten/Rhenium – Greatest temperature range.

Iron/Constantan – Good sensitivity, good range.

The relatively small changes in voltage produced by thermocouples are usually fed to some kind of amplifier, often an OpAmp. They can then be used to activate a device or create a display.
Resistance Temperature Detectors (RTDs)
A metal transducer that increases it’s resistance relatively linearly with temperature. The metals used to make RTDs are platinum, nickel, copper and a nickel/iron alloy. The most commonly used metal is platinum because of it’s higher resistance, constant temperature coefficient and large temperature range. (-200

C to +850

C). It is less sensitive than the others but because of it’s high resistance it gives a greater voltage drop for a given current.
Low values of current are always desirable in temperature sensing devices to prevent any heating effect generated by large current values. An RTD is often incorporated into a
Wheatstone bridge circuit with the “A” and “B” points of the bridge being connected to an
OpAmp. The OpAmp output would in turn be connected to a power amplifier and used to drive a heating (or cooling) device. Accurate control of temperature to +/- 0.1

C are easily obtained.
Thermistors.
Thermistors have a negative temperature coefficient and therefore reduce their resistance with temperature. Manufactured from semi-conductor materials, they can have a wide variety of resistance ranges. They are however non-linear devices and decrease their resistance exponentially with linear increases in temperature. A typical temp/resistance change might be:

-100

C to 0

C 10M

change.

0

C to +100

C 10k

change.

+100

C to +200

C 100

change.
Special circuits to ‘linearize’ thermistors are often used. These can vary from a simple series parallel resistor/thermistor network, to OpAmp circuits with complicated compensating networks. As with RTDs, thermistors connected as part of a bridge and feeding an
OpAmp/Driver circuit can achieve very accurate temperature control.
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Semi-Conductor Temperature Transducers.
As with most areas of electronics, transistorized integrated circuit packages (chips) are available that have self contained temperature sensing, linearizing and output circuits. The output can vary from current that increases by 1

Amp per

K, the AD590 chip, to the LM34 and LM35 chips that are three terminal devices providing 10 mV of output for each

F or each

C respectively. The latter two can operate between -50

F and +300

F or -55

C to
+150

C if a dual +/- power supply is used. With a single + only supply their range changes to +5

F to +300

F and +2

C to +150

C respectively.
Resistance Strain Gauges .
The electrical resistance of a given material is dependant on the length and cross sectional area (or thickness) of that material. Resistance strain gauges rely on this principle to measure the amount of deformation due to strain in an object. Two very common types of strain gauge are the Bonded Wire and the Foil. Both have a resistive wire or foil bonded to a flexible substrate. The substrate is then fixed to an object and a force is applied causing the object to deform and either stretch or compress the wire/foil. When stretched the wire/foil becomes longer and thinner thereby increasing it’s resistance, and when compressed the wire/foil becomes shorter and thicker reducing the resistance.
Wire/Foil.
Substrate.
Wire/Foil stretches and reduces it’s thickness when bent this way.
Wire/Foil compresses and increases
It’s thickness when bent this way.
When included as part of a bridge circuit, the changes in resistance as strain deforms the
Wire/Foil produce a voltage across the “A & B” points which can drive a calibrated
OpAmp.
Semi-Conductor Strain Gauges.
Strain gauges using Semi-Conductor materials as opposed to the metallic material in
Resistance Strain Gauges, give a far greater resistance change with deformation. This gives about 30 times more sensitivity when used in a bridge configuration. However the Semi-
Conductor Strain Gauge is much less accurate, is very temperature sensitive and is very difficult to compensate/calibrate. As a result it’s use is usually limited to applications where temperature variations are very small and a ‘torque to balance’ or ‘drive to null’ mode of operation is used.
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Piezoelectric Force/Strain Gauges.
Some materials in their crystalline form (Quartz for example) will produce a voltage when pressure is applied. (They will also physically deform when a voltage is applied). This is the
‘Piezoelectric effect’ and is used to measure pressure, force or strain. The Piezoelectric gauge is often used for small forces that change rapidly, in the order of milliseconds, and it’s output can best be represented as a capacitor in series with a DC voltage source. As the pressure/strain changes the voltage produced changes and the capacitor charges (or discharges) to the new level.
Other Pressure Transducers.
Pressure is a force spread over an area and is expressed as such. i.e. lbs./inch
2
or kiloPascals/meter
2
It can also be expressed as:

Absolute pressure. Referenced to zero (a Vacuum).

Gauge pressure.

Differential pressure.
Referenced to atmospheric pressure.
The difference in pressure at two points.
Mechanical devices are often used to measure pressure and amongst these, bellows, diaphragms, manometers and Bourdon tubes are the most common. Each produces some form of mechanical movement with changes in pressure, and this movement is translated into an electronic signal by the devices described above. i.e. Something as simple as a potentiometer, or as technical as a strain gauge or piezoelectric gauge.
Flow of liquids or gas can be measured as volume (cubic capacity per time interval
[m 3 /sec]) or as mass ( weight per time interval [kg/sec]). Volume flow is the easiest to measure, and can be converted to mass flow by correcting for temperature. (Volume changes with temp).
Hot-Wire Flow Transducers.
A hot wire will lose heat as a gas or liquid flows past it. The greater the flow volume the greater the heat loss. Changes in temperature result in changes in resistance and this change can be equated to the flow rate. A platinum RTD can also be used for this application. When connected in a bridge, the change in resistance creates an “A to B” potential difference. This potential difference is fed to an OpAmp whose output is used to increase the voltage across the bridge. The resulting increase in power (Watts = V 2 /R) heats the RTD and returns it to it’s original resistance. The voltage across the bridge is therefore directly proportional to the flow.
Vane Transducers.
Probably the simplest type, a vane connected to a potentiometer is moved by the flow of liquid/gas. The potentiometer resistance reading is directly proportional to the vane movement, which in turn is proportional to the flow. Sometimes a gear box is used to translate small movements of the vane to large movements of the potentiometer shaft.
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3.
Turbine Transducers.
The flow of liquid or gas forces a turbine to rotate. The speed of rotation is proportional to the flow rate. Counting or displaying the number of revolutions per time interval can be accomplished by a rotating magnet and a stationary magnetic sensor. Each time the magnet passes the sensor a voltage spike is induced. These spikes or pulses can be counted or used in a capacitor charging circuit. If the capacitor circuit has a relatively long CR time, the charge will increase with more pulses per second and decrease with less. The voltage across the capacitor is therefore proportional to the flow.
Level sensors can detect the actual level of a substance (usually a liquid), or when a certain level has been reached.
Float Sensor.
A float attached to a variable resistor. Resistance is proportional to float/liquid level.
RTD Level Sensor.
With a particular voltage/current applied to an RTD sensor it will heat to a certain temperature (in air) due to the power and poor heat dissipating effect of air. This will produce a certain resistance. If the RTD is placed so that it is just touching a liquid, and the level of the liquid increases, much of the heat will be conducted away through the liquid. As more of the RTD becomes submerged, even more heat is dissipated resulting in a decrease in resistance proportional to the liquid level.
Ultrasonic Level Sensor.
This type of sensor works in a similar fashion to radar. A burst of sound beyond the human hearing range (ultrasonic sound) is directed at the top of a liquid. The time taken for the reflected sound to be detected (divided by 2 because it has to travel to and from the surface) can be computed into distance. As the level rises the time becomes shorter.
The methods for sensing the presence of, or distance to, an object are very similar to level sensors, and include ultrasonic, magnetic, capacitive and optical.
Ultrasonic Detectors.
These devices function in an almost identical manner to the ultrasonic level sensor previously discussed. Reflected sound indicates the presence of, or distance to, an object.
Magnetic Detectors.
Physically contacting or non-contacting magnetic devices can be employed for presence/distance detection. Non-contacting devices are often employed for detecting metallic objects. The presence of these will disrupt the magnetic field around an inductor, creating more or less disruption with changes in distance. This change in magnetic field can influence, for example, the amplitude or frequency of an oscillator, giving a means by which presence or distance can be indicated.
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Contact magnetic detecting often uses a dual secondary transformer with a movable iron core (a Linear Variable Differential
Transformer or LVDT) as shown at right.
The position of the core affects the amount of voltage induced in each secondary winding, and therefore the total voltage across both. e.g. If the core is centered between both secondaries (diag. 1 at right) they will have the same induced voltage.
Vout 1 will equal Vout 2 and the potential difference (differential) between them is 0V.
If the core is moved until it contacts the object (as in dia. 2 at right), the upper secondary will have a large voltage induced, the lower will have 0 induced voltage (Vout 1 = large, Vout 2 = 0), and the difference between them is large. Changes in distance will change the core position, changing secondary differential voltage. The voltage change is representative of the distance change.
Capacitive Detectors.
In a similar manner to the effect on magnetic fields in magnetic detectors, objects influence the electrostatic field of capacitors. Particularly non-metallic objects with a dielectric constant greater than 1.2 (air is 1). Most non-metals fall into this category and this form of detection is particularly suited to them. As with magnetic detectors, a change in capacitance brought about by a change in dielectric constant can change amplitude, frequency or phase of an electronic signal. The amount of change is related to the distance from the introduced object.
Optical Detectors.
Most optical detectors use laser beams to emit a narrow beam of light and then measure the time or the pattern of the reflected light, or react to an interruption of the beam (presence).
Time will give distance information, and the pattern can be used to identify the object. The most obvious application of pattern use is the ‘Bar Code’ reader found everywhere from retail stores to warehouses to railway freight cars.
Velocity Sensors.
The two types of velocity are rotational and linear. Rotational velocity can be measured by a tachometer consisting of one of the following devices:

A DC generator whose output voltage is proportional to rotational speed (RPM).

A Reed Switch activated by a magnet on the rotating object. Each time the reed switch closes, a pulse is produced. The time between pulses is related to the RPM.

An optical sensor activated by light reflected from a mirror on the object. As above, the time between pulses of reflected light is indirectly proportional to rotational speed.
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
Linear velocity is often measured by:

A rotational sensor measuring the rotational velocity of a wheel of known diameter and then translating this to linear velocity.

Time taken for the object to pass two fixed points of known separation. (Any of several previously discussed sensors could be used to detect the passing of the object).

The frequency/phase shift (Doppler Effect) of a reflected radar beam bounced off the object. An obvious application of this method is the Police radar gun.
Acceleration Transducers (Accelerometers).
The rate of change of velocity is called acceleration and devices that measure this are accelerometers. Many accelerometers use a pendulous mass to sense the rate of change in velocity. Inductors or capacitors translate the pendulum movement into an electrical signal.
An OpAmp takes this signal and it’s output is used to torque the pendulum back to the zero/null position. The OpAmp output is directly proportional to the force acting on the pendulum. Devices using this method can sense acceleration/deceleration in the range of tens of microGs to several Gs. G being the acceleration force attributable to gravity of 32 Ft per Sec per Sec (32Ft/Sec
2
).
A second less expensive and less sensitive method utilizes a strain gauge attached to a known mass. The resistance change of the strain gauge being equivalent to the G force experienced by the known mass.
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