Transducers. ME 3251 Thermal Fluid Systems

Transcription

1 Transducers ME 3251 Thermal Fluid Systems 1

2 Transducers Transform values of physical variables into equivalent electrical signals Converts a signal from one form to another form 2

3 Types of Transducers Variable-Resistance Transducer Differential Transformer (LVDT) Capacitive Transducer Piezoelectric Transducer Photoelectric Transducer Photoconductive Transducer Photovoltaic Transducer Ionization Transducer Magnetometer Search Coil Hall-Effect Transducer Digital Displacement Transducer 3

4 Variable-Resistance Transducer Resistance Potentiometer, rheostat Converts linear or angular displacement into an electric signal Can be used to convert force and pressure to a displacement through mechanical methods Moving contact on a slide-wire or moving contact that moves through an angular displacement on a solid conductor (e.g. graphite) Fairly cheap some can be expensive, though 4

5 Differential Transformer (LVDT) Coverts displacement into electric signal Three coils placed in a linear relationship with a magnetic core free to move inside the coils Alternating input voltage impressed in center coil, and output voltage from two end coils depends on magnetic coupling between the core and coils The coupling is dependent on the position of the core If the core remains near the center of the coils the output is very nearly linear. Most differential transformers are only operated in this linear range, and these are called linear variable differential transducers (LVDT) 5

6 LVDT Continued 6

7 Capacitive Transducer Measures a change in capacitance through a change in distance between 2 plates, d Measures capacitance change through a change in overlapping area, A, resulting from plate movement or a change in dielectric constant, ε C A = ε d is use for ε = 1 for air ε = 1 for plastics in 2 cm 2 7

8 Capacitive Transducer cont. For liquid level measurements two electrodes are placed in the liquid, and the dielectric constant varies between them according to the liquid level Thus, capacitance between electrodes is a direct indication of liquid level 8

9 Piezoelectric Transducer A force applied to a piezoelectric crystal between two electrodes produces a stress and corresponding deformation in the crystal Deformation produces potential difference at crystal surface The stress on the crystal produces an output voltage: E = gtp where t is crystal thickness in m p is impressed force in N 2 m g is voltage sensitivity g = d ε Piezoelectric crystals used as pressure transducers for dynamic measurements 9

10 Photoelectric Transducer Converts a light beam into a usable electric signal Light strikes cathode and releases electrons Electrons are attracted toward anode, thereby producing an electric current in external circuit Cathode and anode enclosed in glass or quartz, which is evacuated or filled with inert gas Photoelectric tubes useful for measuring light intensity 10

11 Photoconductive Transducers Voltage impressed on semiconductor material Light strikes semiconductor material and a decrease in resistance occurs, thereby producing an increase in current Useful for measurement of radiation at all wavelengths 11

12 Photovoltaic Transducers Sandwich design of a metal base plate and a thin transparent metallic layer with a semiconductor material in between Light strikes barrier between transparent metal layer and semiconductor material, and a voltage is generated Most widely used application of photovoltaic cell is the light exposure meter in photographic work 12

13 Ionization Transducer Converts displacement to a voltage through a capacitance change As tube moves horizontally between the two plates, a dc potential difference is created between electrodes 1 and 2 13

14 Magnetometer Search Coil Transforms a magnetic field signal into a voltage A flat coil with a specified number of turns is placed in a magnetic field, and a voltage is generated To measure a steady magnetic field, the search coil must be moving. Typically a rotating coil is used in this situation. 14

15 Hall-Effect Transducers An external current passes through a semiconductor plate. When a magnetic field is impressed on a plate in a direction perpendicular to the plate surface, a voltage is generated This voltage is the Hall voltage E H = K H K H IB t where is the Hall coefficient 15

16 Digital Displacement Transducers Used to measure angular and linear displacements For an angular measurement, a wheel rotates and light is alternately transmitted and stopped through small cutouts The signal is sent to the photodetector, amplified, then sent to a counter. The number of counts is proportional to angular displacement. Sensitivity can be improved by increasing the number of cutouts 16

17 Digital Displacement Transducers For a linear measurement, small reflecting strips are installed on a motion device. Light is then alternately reflected and absorbed with linear motion presenting a signal to the photodetector Calibration with a known displacement standard must be performed 17