Spider DSA User’s Manual
265
angular resolution. This is the domain of the shaft encoder, an electro-optical
device that produces a train of n logic-level pulses per shaft revolution. (Be very
wary of “homemade” multi-pulse tachometers; uneven target spacing can
generate the false impression that the shaft speed is modulated.) While less
facilitating, a once-per-turn pulse signal can be used to synchronize an order
analysis. An example of a once-per-turn optical tachometer is shown in the figure
below.
Figure 173. Optical tachometer setup.
This 1/turn pulse must be used to facilitate N/turn sampling of a machine’s
dynamic signals. In the ancient days of yore, a complex accessory box called a
tracking adaptor was used to provide this sample-rate multiplication. The
tracking adaptor (tracking ratio tuner) was comprised of two components, a
phase-locked loop (PLL) and a voltage-tuned filter. The PLL accepted a variable
frequency input pulse train (i.e. 1/rev) and provided an output pulse train at a
higher frequency of exactly M/D (M and D being available integer settings) times
the input rate; this pulse train became the order-normalizing sample rate. The
second component was a voltage-tuned low-pass filter driven by the PLL phase
error (an analog voltage) that tracked the sample rate and was thus a tracking
anti-aliasing filter, expanding and contracting its passband in tune with machine
speed.
Today’s digital re-sampling algorithm does away with the need for an external
tracking adaptor. It provides better tracking because of its fundamental
assumption that the shaft changes speed with constant angular acceleration
between pulses. The phase-locked loop circuit implied a tacit assumption of
constant angular velocity between tachometer pulses.
The Spider-80x modules provides for high-fidelity sampling of the tachometer
signal to preserve the details of its waveform. Regardless of the sample rate
applied to the input channels, the tachometer is recorded as a time-stream at the
instrument’s maximum sample rate of 102.4 kHz. A dedicated high-speed counter
precisely measures the period between adjacent tachometer input pulses.
To Next Page
Loading...