Digital tachometer, General Digital tachometers are particularly suitable for the precision measurement and monitoring of all time related quantities, which are able to be converted into a proportional frequency using appropriate sensors.
Time-related quantities include rotational and linear velocity, flow rate and related quantities. The instrument can be programmed to measure absolute values ratio, or proportional difference. The proportional frequency is generally produced by a magnetic wheel on the shaft, which is scanned by a radial mounted impulse sensor For control applications, a high resolution pick-up can be coupled directly to the motor shaft Calculating digital tachometer using the period measurement method The digital tachometer implements the period measurement method, with the subsequent calculation of a reciprocal value. Two absolute values, with independent setup parameters, or their ratio or proportional difference (selectable) can be measured. The measurement is carried out automatically and repetitively, or externally through a contact.
In ratio and proportional difference modes, the values used in the calculation are acquired simultaneously, These values are taken at the same point in time, without a delay, whereby a higher accuracy can be attained. In contemporary implementations, the values are taken successively. Measurement at the same point in time can therefore not be guaranteed. The number of periods measured is selectable. 1 period for a fast reaction time (approx, 40 ms), 10 periods if the drive shaft rotation is uneven.
The display update cycle time can be set to 1, 2 or 3 seconds Setting the parameters is very simple on the front mounted key panel, An input frequency and the corresponding display value can -be entered separately for channel 1 and 2, All other parameters are calculated automatically by the instrument. The programming function can be disabled using the switch on the rear of the instrument. A freely programmable, isolated analog output (12-bit resolution) is available for recording measurement data. A particular frequency range can be magnified to the full resolution of the output. E.g. Range 1. 749 - 1.752 rpm 0 - 20 mA (To monitor the control accuracy of a drive using a chart recorder). 2 Alarm limits can be set to anywhere within the overall range of the instrument. The hysteresis is freely programmable, and the relay can be set for active-energized or fail-safe mode.
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