1 Transducers EKT 451 CHAPTER 6

2 Definition of a Transducers   Transducer is any device that converts energy in one form to energy in another.   Also known as sensor.   Convert electrical energy to mechanical displacement.   Convert non-electrical physical quantity to electrical signal. e.g. temperature, sound or light to an electrical signal.

3 Definition of a Transducers   Function of transducer   To sense the presence, magnitude, change in and frequency of some measurand.   To provide an electrical output, when appropriately processed and applied to a readout device. measurand electrical output Excitation

4 Classification of Transducers   Transducer can be classified according to their application, based primarily on their physical quantity, property or condition to be measured. 1) Passive transducers require an external power. Their output is a measure of some variation, such resistance and capacitance. 2) Self generating transducer do not require an external power. they produce analog voltage or current when stimulated by some physical form of energy

5 Selecting a Transducers   Transducer has to be compatible with its application 1) Operating range – should maintain range requirements and good resolution. 2) Sensitivity – must be sensitive enough to allow sufficient output. 3) Frequency response and resonant frequency – is the transducer flat over the needed range. 4) Environment compatibility - corrosive fluids, the pressures, shocks and interactions.

6 Selecting a Transducers 5) Minimum sensitivity – minimally sensitive to expected stimuli other than measurand. 6) Accuracy – subject to repeatability and calibration errors. 7) Usage and ruggedness - mechanical and electrical intensities versus size and weight. 8) Electrical – what are signal-to-noise ratios when combined with amplifiers and frequency response limitations.

7 Resistive Transducers   Also called displacement transducer.   to detect and transmit the physical position of mechanical parts via electrical signals

8 Strain Gauge Transducers a passive transducer Uses electric resistance variation in wires to sense the strain produced by a force on wires. Measuring weight, pressure, mechanical force, or displacement. A tensile stress tends to elongate the wire and thereby increase its length and decrease its cross sectional area

9 Capacitive Transducers   capacitive transducers can be made to be self-contained units.   Some transducers work by making one of the capacitor plates movable, either in such a way as to vary the overlapping area or the distance between the plates.   Other transducers work by moving a dielectric material in and out between two fixed plates to detect and transmit the physical position of mechanical parts via electrical signals

10 Differential Capacitive Transducers  have 3 wire connections: one wire for each of the "end" plates and one for the "common" plate.  implementation in a bridge circuit

11 Differential Capacitive Transducers  This bridge circuit is similar in function with strain gauges: it is not intended to be in a "balanced" condition all the time, but rather the degree of imbalance represents the magnitude of the quantity being measured.

12 Inductive Transducers Inductive transducers may be either of the self generating or passive type. The self generating type utilises the basic electrical generator principle, a motion between a conductor and magnetic field induces a voltage in the conductor (generator action). A tachometer is an inductive transducer that directly converts speed or velocity into electrical signal.

13 Variable Inductive Transducers  this device is a transformer:  primary winding powered by an external source of AC voltage, and two secondary windings connected in series-bucking fashion.  It is variable because the core is free to move between the windings.  When the core is centered and both windings are outputting the same voltage, the net result at the output terminals will be zero volts.

14 Thermocouple Transducers  The Thermocouple is a thermoelectric temperature sensor which consists of two dissimilar metallic wires  These two wires are connected at two different junctions, one for temperature measurement and the other for reference  The temperature difference between the two junctions is detected by measuring the change in voltage across the dissimilar metals at the temperature measurement junction

15 Thermocouple Transducers  Low cost.  No moving parts, less likely to be broken.  Wide temperature range.  Reasonably short response time.  Reasonable repeatability and accuracy.

16 Thermistor Transducers  Have great resistance at low temperatures but when they warm up their resistance decreases rapidly.  Current can then flow through them.  When the thermistor is warmed up by the hair drier its resistance drops.  As its resistance drops, current flows into the base of the transistors allowing the LED to light.

17 Piezoelectric Transducers When a mechanical pressure is to a crystal, a displacement of the crystal causes a potential difference occur. The thickness, length, and width directions can be stretched or compressed to generate electrical output

18 Test #2 28 March March 2006 Chapter ac/dc Meters and Chapter ac/dc Bridges Chapter ac/dc Meters and Chapter ac/dc Bridges 3.30 – – 4.30