Signal conditioning techniques make it possible to derive all motion measurements displacement, velocity, or acceleration—from a measurement of any one of the three. Nevertheless, it is sometimes advantageous to measure velocity directly, particularly in the cases of short-stroke rectilinear motion or high-speed shaft rotation.
The analog transducers frequently used to meet these two requirements are
- Magnet-and-coil velocity transducers
- Tachometer generators
A third category of velocity transducers, Counter-type velocity transducers, is simple to implement and is directly compatible with digital controllers.
The operation of magnet-and-coil velocity transducers is based on Faraday’s law of induction. For a solenoidal coil with a high length-to-diameter ratio made of closely spaced turns of fine wire, the voltage induced into the coil is proportional to the velocity of the magnet.
Magnet-and-coil velocity transducers are available with strokes ranging from less than 10 mm to approximately 0.5 m.
A tachometer generator is, as the name implies, a small AC or DC generator whose output voltage is directly proportional to the angular velocity of its rotor, which is driven by the controlled output shaft. Tachometer generators are available for shaft speeds of 5000 r/min, or greater, but the output may be nonlinear and there may be an unacceptable output voltage ripple at low speeds.
AC tachometer generators are less expensive and easier to maintain thanDC tachometer generators, but DC tachometer generators are directly compatible with analog controllers and the polarity of the output is a direct indication of the direction of rotation.
The output of an AC tachometer generator must be demodulated (i.e., rectified and filtered), and the demodulatormust be phase sensitive in order to indicate direction of rotation. Counter-type velocity transducers operate on the principle of counting electrical pulses for a fixed amount of time, then converting the count per unit time to velocity.
Counter-type velocity transducers rely on the use of a proximity sensor (pickup) or an incremental encoder. Proximity sensors may be one of the following types:
- Electro-optic
- Variable reluctance
- Hall effect
- Inductance
- Capacitance
Since a digital controller necessarily includes a very accurate electronic clock, both pulse counting and conversion to velocity can be implemented in software (i.e., made a part of the controller program). Hardware implementation of pulse counting may be necessary if time-intensive counting would divert the controller from other necessary control functions.
A special-purpose IC, known as a quadrature decoder/counter interface, can perform the decoding and counting functions and transmit the count to the controller as a data word.
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