Introduction to tachometers

The tachometer is a very important sensor in vibration analysis of rotating machines, since many of the important excitations and natural frequencies depend on speed. A tachometer will output either a sine wave or a square wave where the frequency (and sometimes the amplitude) will be dependent on the shaft speed. For steady-state conditions, one can get away with fairly simple tachometer processing. But for transients where the speed changes very quickly, some fairly complicated processing may be needed to get the data you want. For example, in my work I've frequently need to process changes of several thousand RPM in 1 - 2 seconds.

First, let's introduce some of the important parameters and tradeoffs in designing a tachometer algorithm.

1. Update rate. How often does the tachometer algorithm give you a new speed? Once a second, a hundred times a second?
2. Speed precision / number of significant digits. Can the tachometer algorithm distinguish between 60 RPM and 61 RPM? Between 60 RPM and 60.0001 RPM?
3. Lag time. When there is a sudden change in the speed, say from 100 RPM to 200 RPM, how long is it before the tachometer algorithm reaches the new speed (e.g. successive samples may be 100, 150, 175, 200. We want to know how long before we get 200). This is different from update rate. You may be getting a new reading a hundred times a second, but that information may be half a second behind the actual shaft speed.
4. Accuracy. If the speed is a constant 100 RPM, does the tachometer report that, or does it report 101 or 99?
5. Computational cost. For post-processing, the computational cost is usually not important, but for online applications it may be a limiting factor.

Various tachometer algorithms will be developed, and the tradeoffs in these parameters will be discussed. I will focus on algorithms for digital signal processing, but analog processing is also available (frequency to voltage converters).

For the various examples, assume a tachometer which gives 60 pulses per revolution. The performance of the algorithms will be examined on a simulated signal which starts at 10.5 RPM and then increases to 20.5 RPM in 1 second.

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This work by Daniel Kiracofe (daniel dot kiracofe at gmail dot com) is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License./' $I