Thursday, January 31, 2013

Comparison between contact and non-contact tachometers

Contact tachometer:

§  Used when rotor is accessible

§  It’s used in concentric mode. It’s always applied to shaft center. Shaft should have a centering hole, so the rotating element will not slip out

§  It can also be applied in tangential mode if the rotor has an external circular rim, but rpm recalculation must be conducted based on the diameter ratio between rotor and the rotating element

§  No need for rotary encoder

§  Transfers rotational motion of rotor and converts it internally to pulses

§  Measures instantaneous rpm (based on number of divisions of the internal-rotary encoder)

§  It must be parallel to the rotating shaft.

Non-contact (laser) tachometer:

§  Requires too much tuning before each measurement.

§  It’s –in most cases- supplied with a counter feature

§  It’s sensitive to distance between it and rotor.

§  It’s used for indoor use only.

§  Sensitive to reflectivity of rotor.

§  An external encoder is needed. Some models use a reflecting tape for generating pulses.

§  It can be used also with an external emitter (sender), so it will counts pulse receptions.