Tachometers (velocity measurement)


#1

Tachometers measure velocity, and typically output a voltage (0-10V) proportional to the velocity. For use with an RMC, a tach should output voltage within the +/-10V range. Tachometers are not commonly used for motion control. Typically, a position encoder works better.

Tachometer versus Encoder
A tachometer measures velocity. An encoder measures position. A motion controller can obtain both position and velocity from an encoder, but only velocity from a tach. The RMC can usually control motion better with a position encoder than with a tachometer. This is because the encoder provides more information for control than just the velocity from a tachometer. The RMC can control velocity very well with an encoder.
Historically, tachometers were useful because they provide a direct velocity signal, with no quantization error (velocites calculated from a position encoder will have quantization error). The tachometer has only a small amount of noise inherent in analag signals. However, the high resolution of modern encoders allows very accurate velocites with minimal qunatization noise, making tachometers less useful.
Since tachometers are analog, it is impossible to measure zero speed exactly (the axis standing still). If your application needs to be able to hold the axis still, you should use a position encoder instead of a tachometer.
Usually, the only reason to use a tach is if you have an extra analog input on the controller anyway and don’t want to add an extra input for an encoder. Again, if you need to control the axis at a stop, you will definitely need a position encoder.

Tachometer Recommendations
Delta is aware of the following tachometers: