Lecture Outline

1. Input Transducers
1. Definitions
2. Factors in Choice
3. Types of Transducers
1. Position
• Synchro
• Displacement (contacting)
• Displacement (noncontacting)
2. Temperature
• Bimetal Strip
• Thermocouple
• Thermistor
• Resistance Temperature Detector (RTD)
• Platinum resistance
• Pyrometer
3. Light
• Phototube
• Photoresistor
• Photodiode
• Phototransistor
• Solar Cell
4. Other
• Strain Gauge
• Pressure
4. Rules for Using
5. Resistive Transducers
2. References & Links

A. Definitions

Transducer

A transducer is a sensor that changes energy from one form to another. More technically a transducer converts a physical parameter into another form. Transducers can be used at the input (a microphone) or the output (a speaker) of a system. With electronic-measuring systems, the input transducer converts a quantity to be measured (temperature, humidity, flow rate, weight) into an electrical parameter (voltage, current, resistance, capacitance) that can be processed by an electronic instrument or system.

Electronic Transducer

A transducer which provides output as an electrical signal:

• voltage
• current
• or a change in resistance, capacitance, or inductance.

Passive Transducer

A transducer which requires no energy to operate, for example a solar cell.

Active Transducer

A transducer which requires energy to be added, for example a photo resistor.

B. Factors in Choice

• sensitivity
• range
• temperature coefficient
• linearity
• size
• cost

C. Types of Transducers

• Synchro

A synchro is an electromechanical transducer used for shaft angle measurement and positioning. There are several different types of synchros, but all can be thought of basically as transformers. In physical appearance, a synchro resembles a small AC motor with a diameter ranging from a little over 1cm to about 10cm.
• Displacement (contacting)

Contacting transducers typically use a sensing shaft with a coupling device to follow the position of the measured device. A contacting type displacement sensor that relates a change in inductance to displacement is the linear variable differential transformer (LVDT). The sensing shaft is connected to a moving magnetic core inside a specially wound transformer. The primary of the transformer is in line and located between two identical secondaries. The primary winding is excited with AC (usually in the range of 1 to 5 kHz). When the core is centered, the voltage induced in each secondary is equal. As the core moves off center, one secondary will be greater than the other. This transducer has excellent sensitivity, linearity and repeatability.
• Displacement (noncontacting)

Noncontacting displacement transducers include optical and capacitive transducers. Photocells can be arranged to observe light through holes in an encoding disk, or to count fringes painted on the surface to be measured. Optical systems are fast; but noise, including background light sources, can produce spurious signals in optical sensors.
• Fibre-optic sensors also make excellent proximity detectors for close ranges. Reflective sensors use two fibre bundles, one for transmitting light and the other for receiving light from a reflective surface. Light is transmitted in the fibre bundle without any significant attenuation. When it leaves the transmitting fibre bundle, it forms a spot on the target that is inversely proportional to the square of the distance. The receiving bundle is aimed at the spot and collects the reflected light to an optical sensor. The light intensity detected by the receiving bundle depends on the physical size and arrangement of the fibres as well as the distance to the spot and the reflecting surface, but the technique can respond to distances less then 1µm. The major disadvantage is limited dynamic range.
• Capacitive sensors can be made into very sensitive displacement and proximity transducers. The capacitance is varied by moving one of the plates with respect to the second plate. The moving plate can be any metallic surface such as the diaphragm of a capacitive microphone or a surface that is being measured. The capacitor can be used to control the frequency of a resonant circuit to convert the capacitive change into a useable electrical output.

• Bimetal Strip

A bimetal strip is made of two pieces of metal in thin lengths bonded along their long faces. With temperature change the bimetal strip will flex towards one metal or the other depending on the physical properties of the individual metals. The metal which expands more under heat will bend towards the one which doesn't react as much, causing a physical deformation of the strip. By taking quantitative observations of the bending, and given the properties of each metal an accurate temperature can be taken.
• Thermocouple


The thermocouple is formed by joining two dissimilar metals. A small voltage, called the Seebeck Voltage, is produced across the junction of the two metals when heated. The amount of voltage produced is dependent on the types of metals and is directly proportional to the temperature of the junction (positive temperature coefficient); however, this voltage is generally much less than 100mV. The voltage versus temperature characteristic of thermocouples is somewhat nonlinear, but the amount of nonlinearity is predictable. Thermocouples are widely used in certain industries because they have a wide temperature range and can be used to measure up to very high temperatures.

Some common metal combinations used in commercial thermocouples are chromel-alumel (chromel is a nickel-chromium alloy and alumel is a nickel-aluminum alloy), iron-constantan (constantan is copper-nickel alloy), chromel-aluminum, tungsten-rhunium alloys, and platinum-10% Rh/Pt. Each of these types of thermocouple has a different temperature range, coefficient, and voltage characteristic and is designated by the letters E, J, K, W, and S, respectively. The overall temperature range covered by thermocouples is from -250ºC to 2000ºC.


• Thermistor
  A thermistor is a resistive device made from a semi-conductive material such as nickel oxide or cobalt oxide. The resistance of a thermistor changes inversely with temperature (negative temperature coefficient). The temperature characteristic is more nonlinear for thermistors than for thermocouples or RTDs; in fact, a thermistor's temperature characteristic in essentially logarithmic. Also, like the RTD, the temperature range of the thermistor is more limited than that of a thermocouple. Thermistors have the advantage of a greater sensitivity than either thermocouples or RTDs and are generally less expensive. This means that their change in resistance per degree change in temperature is greater. Since they are both variable-resistance devices, the thermistor and the RTD can be used in similar circuits.
• Resistance Temperature Detector (RTD)

The RTD is a resistive device in which the resistance changes directly with temperature (positive temperature coefficient). The RTD id more nearly linear then the thermocouple. RTDs are constructed in either a wire-wound configuration or by metal film technique. The most common RTDs are made of platinum, nickel or nickel alloys.
• Platinum resistance

A distinct and unique property of platinum is the linearity of it's resistance change with temperature, this predictability makes it and ideal transducer for temperature change detection and accurate measurement.
• Pyrometer

A pyrometer in an instrument used for measuring temperatures by using the thermal radiation emitted by a heated object. In a radiation pyrometer, the emitted radiation is detected by a sensor such as a thermocouple. In an optical pyrometer, the colour of an electrically heated filament is matched visually to that of the emitted radiation. Pyrometers are useful for measuring the temperature of distant, moving or inaccessible objects.

• Phototube

Phototubes (Photomultipliers) consist of a photocathode coated with alkali metals. A single photon of light striking the photocathode ejects an electron from the metal via the photoelectric effect. These electrons are guided and accelerated by an electric potential to a positively charged secondary-emission electrode (dynode), where they free up still more electrons by direct collision and energy loss. These electrons are similarly accelerated to the next dynode where they free up still more electrons. Typically, at each stage, about 4 secondary electrons are emitted for each incident electron. Therefore, large amplification or gain factors are possible with just a few stages of dynodes. Gains as high as 10^8 can be achieved with 14 dynode stages. Phototubes are widely used in astronomy for precision measurements of the brightness of stars and galaxies since they provide a pulse for each incident photon. The number of photons detected in a specific time is a measure of the optical targets apparent brightness.
• Photoresistor
  Photoresistors consist of a photosensitive material with resistive properties changing light energy into resistance. The resistance of the photoresistor varies inversely with the intensity of light energy it is exposed to. A photoresistor exposed to less light will provide higher resistance.
• Photodiode

The simplest photodiode is a reverse biased p-n (diode) junction. When no light falls on the device only a small amount of current flows (the dark current). When light falls on the device, additional carriers are generated, and more current flows. Photodiodes typically work in the visible light - near infrared region of the spectrum. They are high impedance devices, and operate at relatively low currents (typically 10uA dark current, rising to 100uA when illuminated). They have fairly linear responses to increasing illumination, and generally have very fast response times.
• Phototransistor

A phototransistor is a transistor with the current for the base connection of the transistor supplied by a photoelectric cell; it is similar to a photodiode supplying the base current to a transistor. The phototransistor has a much higher current output than a photodiode for comparable illumination levels. However, it does not operate as fast as photodiodes (about 100kHz being the top limit), and also has higher dark current.
• Solar Cell

A solar cell is a passive transducer which turns light energy into an electrical current. The more light the cell is exposed to the more electrical energy is produced. A solar cell can be used in many devices such as light-level switches and power generators.

• Strain Gauge
  
  A strain gauge is basically a long very thin strip of resistive material that is bonded to the surface of an object on which strain is to be measured, such as the wing or tail section of a plane under test. When a force acts on the object to cause a slight elongation, the strain gauge also lengthens proportionally and its resistance increases. Most strain gauges are formed in the pattern shown to the left to achieve enough length for a sufficient resistance value in a small area. Strain gauge characteristics follow the formulae shown, Metals have K values between 2 and 4.5, and semiconductors are upwards of 150.
  
  
• Pressure


Pressure transducers are devices that exhibit a change in resistance proportional to a change in pressure. Basically, pressure sensing is accomplished using a strain gauge bonded to a flexible diaphragm, as shown to the right. When a net positive pressure exists on one side of the diaphragm, the diaphragm is pushed upward and its surface expands. This expansion causes the strain gauge to lengthen and its resistance to increase.

D. Rules for Using

1. Calibrate
2. Monitor Environment
3. Control Environment

E. Resistive Transducers

• Voltage divider
  If Zout = 0 then Vout = Vrt = V((RT)/(R+RT)). This is nonlinear unless Rt << R when Vout << V. If Rt >> R, then current through Rt changes a lot and current sensitivity becomes a potential problem.

• Bridge Circuits
  Wheatstone bridges are used for strain measurements. The diagram above shows a Quarter Bridge Wheatstone bridge. It consist of 4 resistors arranged in a diamond orientation. The DC voltage runs to the top and bottom of the diamond and the output voltage is measured across the middle. If the output voltage is zero, the bridge is said to be balanced. One or more of the bridge maybe a resistive transducer (i.e. strain gage). Then the other bridges are resistors with resistance that is the same as the strain gage(s). By changing the strain from a resistive strain gage, the balanced bridge will be unbalanced. This will create a voltage to appear across the middle of the bridge (induced voltage). To rebalance it the resistors in the opposite bridges can be adjusted. The change in resistance that caused the induced voltage can be measured and converted to units of strain. Vout = V{(dR/R)/(4+2dR/R)} if dR/R << 1 then Vout = V/4(dR/R) For a 1/2 bridge - the sensitivity is doubled, and we get twice the output that a 1/4 V/2(dR/R).

Text Reference:
Basic Operational Amplifiers and Linear Integrated Circuits, Second Edition; Floyd and Buchla. ©1999 by Prentice-Hall Inc. Simon & Schuster/A Viacom Company: Upper Saddle River, New Jersey 07458
Electronic Devices and Circuit Theory, Sixth Edition; Robert L Boylestad and Louis Nashelsky. ©1996 by Prentice-Hall Inc. Simon & Schuster/A Viacom Company: Upper Saddle River, New Jersey 07458

Internet Device Reference:

• see course web site for list