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Tachometer won't Display RPM Value


PROBLEM

My tachometer does not give an RPM reading when the reflective tape is on the rotor OD.

SOLUTION

If the rotor is large or spinning quickly, you may be exceeding the limits of the response time of the tach sensor.First ensure that your tach is setup according to the instructions in section 2 of the Instrument Reference Guide. In particular, ensure that the tach is aimed at a slight angle to the rotor surface. If you are using an optical tachometer, adjust the sensitivity (SENS) with a small screwdriver if necessary but leave the other screw on 'LON'.If you still get no tach signal, consider the following example:A single 20 mm-wide strip of reflective tape is fitted to the outer surface of a 0.4 m diameter rotor which is spinning at 3600 RPM. The surface of the rotor will be moving at about 75 m/s, so the strip of tape will flick past in 0.00026 seconds = 0.26 msThe GSM2RSN Photo Sensor has a response time of 0.35 ms, so the tape is flicking past just a little too quickly.

How do I fix this?

Either attach the tape to the shaft instead of the rotor OD, or fit a longer strip of tape, in-line with the rotation.

How do I know if this is the Problem?

Perform this calculation and then check the answer below:

  • Calculate 20 * W / (R * D)

W is the reflective strip width in mm (e.g. 20)

R is the rotor RPM (e.g. 3600)

D is the rotor diameter in meters (e.g. 0.4)

  • If the answer is lower than 0.35 (ms), then you should apply one of the fixes above.