Many industrial processes require information on and control of both linear and angular position, and rate of motion. Some transducers use position-sensing devices to convert temperature, level, and/or pressure into electrical units, and controllers can use position-sensing devices to monitor the position of an adjustable valve for feedback control.
Potentiometers:
The wiper or slider arm of a linear potentiometer can be mechanically connected to the moving section of a sensor. Where angular displacement is involved, a single or multiturn (up to 10 turns) rotational type of potentiometer can be used.
Linear variable differential transformers (LVDTs):
LVDTs are devices used for measuring small distances and are an alternative to the potentiometer. The device consists of a primary coil with two secondary windings, one on either side of the primary. A movable core, when centrally placed in the primary, will give equal coupling to each of the secondary coils. When an ac voltage is applied to the primary, equal voltages will be obtained from the secondary windings. when the core is centrally positioned.
An output voltage proportional to displacement is obtained for limited travel. These devices are not as cost-effective as potentiometers but have the advantage of being non-contact. The non-contact electrically isolated, accurate, and have better longevity than potentiometers
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Capacitive:
Capacitive devices can be used to measure angular or linear displacement. There are three methods of changing the capacitance.
- Changing the distance between the plates,
- Moving one plate with respect to the other plate to reduce the overlapping area between the plates,
- Moving a dielectric material between fixed plates.
All the three are shown in below picture:
Capacitance variation is a very accurate method of measuring displacement and is used extensively in micromechanical devices where the distance is small, giving high capacitance per unit area. Capacitive devices are used in the measurement of pressure, acceleration, and level.
Light interference lasers:
Monochromatic (single frequency) light can be generated with a laser and collimated into a narrow beam. The light from the laser beam is split and a percentage goes to a detector, but the main beam goes to a mirror attached to an object whose change in distance is being measured, as shown below:
The reflected beam is then directed to the detector via the beam splitter and a mirror. A change in the position of the object of one-quarter wavelength increases both the incident and reflected beam length one-quarter wavelength, giving a change at the detector of one-half wavelength. When the reflected beam is in phase with the incident beam, (d) is N times an even number of quarter-wavelengths of the laser beam, the light amplitudes add, and an output is obtained from the detector.
The movement of the object generates interference fringes between the incident light and the reflected light. These fringes can be counted to give the distance the object moves. The wavelength of the light generated by a laser is about 5 × 10−7m, so that relative positioning to one-quarter wavelength (0.125 m) over a distance of 50 cm to 1m is achievable.
Accelerometers:
Accelerometers sense speed changes by measuring the force produced by the change in the velocity of a known mass. The accelerometer is a piezoelectric device.
The seismic mass produces a force on the piezoelectric element during acceleration, which causes the lattice structure to be strained. The strain produces an electric charge on the edges of the crystal.An amplifier can be integrated into the package to buffer the high output impedance of the crystal.
Accelerometers are used in industry for the measurement of changes in velocity of moving equipment, in the automotive industry as crash sensors for air bag deployment, and in shipping crates to measure shock during the shipment of expensive and fragile equipment.