Crystal Instruments Spider - Order Tracks and Order Spectrum

Spider DSA User’s Manual
266
This special hardware circuitry allows the dedicated tachometer channel to
sample at the highest possible sampling rate. This technique has several obvious
advantages:
◼ The time domain signal of the tachometer input is transformed by
A/D converter into a digital signal. The user can observe the pulse
trains of the tachometer signal and set threshold arbitrarily.
◼ Accurate phase information can be obtained relative to each data
channel because the tachometer channel, which is fed by high
frequency sampling oscillator, is synchronized with data channels.
◼ The RPM estimation is not influenced by the current data sampling
rate.
◼ Two independent tachometer input channels provide independent
RPM signals simultaneously.
◼ A reconstructed series of pulses at instants when the tachometer
generated a pulse is created and can be used for analysis.
Pulse per Rev is defined as the number of tachometer input pulses per shaft
revolution. Pulse per Rev must be defined by the user so that the instrument can
calculate the shaft frequency from the tachometer frequency. The relationship is:
Tachometer Reference freq = Tachometer freq / Pulses per Rev
Revolution Period = 1/(Tacho Ref Freq)
Pulse Period = 1/(Tacho Frequency)
In most tests, the Pulses per Rev will be 1. (Sensing a keyway or other once/turn
target is relatively simple.) However, in other situations, such as viewing a
toothed flywheel or reference gear, the Pulses per Rev can be as high as hundreds.
In general, the higher the Pulse per Rev, the more resolute an order analysis
becomes.
Pulse Detection
A good tachometer processing instrument allows the user to see the tachometer
signal in its original time waveform. This aids setting the threshold of pulse
detection, simplifying tachometer setup.
Order Tracks and Order Spectrum
Synchronizing the sampling to the rotating speed allows presentation of
measurement results in the angle and order domains in lieu of the time and
frequency domains. An order is simply a frequency divided by a reference
frequency, normally a machine’s shaft-turning frequency. This means that the