• How does Tacho processing work?

Answer: Tacho processing, as performed by the Rotating machinery application, uses the traditional method for determining the frequency of an input signal. This processing method relies on monitoring the number of fixed time increments observed between a start and stop of a single cycle of the tacho waveform. This assumes that the tacho waveform is of a single sinusoid nature and that a start/stop event pair can be determined similar to the way triggering is performed on impact waveforms. Once the trigger events (start/stop) have been defined, the number of fixed time increments are counted between the start/stop events of a single tacho cycle. Knowing the frequency (i.e. delta T) of the fixed time increments we can convert the "counts" to a frequency by the simple equation:

Tacho_period = Counts/Tacho_Clock Tacho_freq = 1.0 /Tacho_period Ref_freq = Tacho_freq / Pulses_per_Rev

There are usually 2 ways that the tacho can be measured. 1) The simplest is to use a data channel as a tacho channel ("pulse" option). In this case, the Tacho_Clock is actually the sample rate of the data channels. This sample rate usually limits the tacho frequency range since the tacho range is now set by the input data frequency range requirement. In addition, due to the "frame" nature of the input sampling process (i.e. we have to acquire a certain number of data points (1024,2048,4096,...etc.) we are limited to how we acquire the tacho signal. This restriction usually means we get several tacho cycles in every data frame. The result is often an "averaged" value which is acceptable unless the tacho signal is changing frequency during the data frame event.

2) Another way tacho can be measured is to use a tacho hardware daughter card ("H/W" option) that contains its own Tacho_Clock which runs at a much higher speed; typically 100Mhz. This tacho hardware also contains special double buffered counters (i.e. to read "Counts") which maintain a continuous counter reading (from tacho cycle to tacho cycle) to avoid skipping any triggered cycles of the tacho signal. There is also an option to allow these counters to "average" several tacho periods for cases when the input tacho frequency is very high (see accuracy discussion). The advantage here is that we decouple the Tacho_Clock from the input data rate. In addition, we double-buffer the counters to avoid missing or combining tacho input cycles. This means we can have an extremely high update rate to the tacho frequency as it is based only on the input tacho frequency. This is important when dealing with any tacho source that has a varying input frequency (a.k.a. "skew").

ACCURACY Tacho measurement accuracy, in either of the two measurement methods, basically relies on how accurately we measure the "Counts" value. For a fixed frequency input, the counts are expected to vary depending on the frequency/accuracy of the trigger points and how close the Tacho_Clock frequency is to the input tacho frequency. Our internal specification for counting accuracy is 1 count in a 1000. This means that the "Count" value should be at least 1000 in order to ensure a reasonable accurate result.

A) TACHO H/W For example, the H/W tacho has a clock of:

100 x 10^6 hz (100 Mhz)

For Count equal to 1000 we have a tacho frequency of:

1.0 / (1000 / 100 x 10^6) = 100,000 hz

While the tacho H/W can measure still higher input frequencies, the accuracy will begin to degrade.

B) PULSES For the case of using the data sample clock, take a typical example of 2000 BW (10240 hz sample rate). In this case:

1.0 / (1000 / 10240) = 10 hz

Obviously, we must relax the accuracy specification for the fixed sample case in order to get a reasonable range. However, assuming that we only measure 5 points per cycle (the best due to the filter cutoff), we find that the absolute max tacho freq will be:

1.0 / ( 5 / 10240 ) = 2000 hz

This measurement of the tacho is sure to have a lot of "bounce"/variability due to the poor counts available to determine the true tacho frequency. This measurement is somewhat improved by using averaging (due to there probably being several tacho cycles in the input buffer). Note that at the max freq there will be over 200 cycles in a 1024 point waveform. The bottom line is that the "pulses" option (i.e. using a data channel) is only usable in a few cases when the tacho frequencies are on the order of the data frequencies. This can be okay when using very low pulses per tacho revolution. Since the tacho frequency being measured must include the pulses/rev effect, the maximum frequency input must still be below the filter cutoff. For our example above, if we had a pulses/rev of 100 (not unusual for flywheel or geared data), then the usable MAX tacho (reference frequency value) would be:

2000 / 100 = 20 hz

This is the maximum fundamental frequency. Your only choice in these applications is to try increasing the measurement BW which then increases the sample rate. Unfortunately this also decreases the frequency resolution of the data analysis ( delta_F = SampleRate / FrameSize ). In any event, such a (pulses) tacho source is not a recommended reference source for performing any tracking sampling measurements due to its lower accuracy and inability to track a slewed data set. OFFLINE processing of "pulses" option data may be the only way to perform tracked data sampling since the tacho waveform is completely digitized and tacho frequency estimators can be employed to better estimate the reference frequency values.

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