Measurement Computing WBK17 - Manual (Page 24)

WBK17, pg. 24 987996 WBK17, Counter/Encoder Module
There are other useful encoder options; for example, analog filtering. If the encoders are known to run in a
noisy environment then some analog filtering may be in order. A low-pass filter [with 20kHz or 100kHz
cutoff frequency] can be used to reduce extraneous high-frequency [normal mode] noise that may come in
on the encoder lines. In addition, if the encoder is connected in differential mode any common-mode noise
will be reduced by the common-mode rejection of the WBK17’s front-end differential amplifier,
typically –70dB.
If the encoder stops rotating, but is vibrating [due to the machine it is mounted to], the debounce feature can
be used to eliminate false edges. An appropriate debounce time can be chosen and applied to each encoder
channel. Refer to the
Debounce Module section on page 7 for additional information regarding debounce
times.
Relative position and velocity can be obtained from each of the encoders. However, during an acquisition,
data that is relative to the Z-position can not be obtained until the encoder locates the Z-reference.
During an acquisition, data that is relative to the Z-position can not be obtained until the
encoder locates the Z-reference.
Note that the number of Z-reference crossings can be tabulated. If the encoder was turning in
only one direction, then the Z-reference crossings will equal the number of complete revolutions. This
means that the data streaming to the PC will be relative position, period = 1/velocity, and revolutions.
You can set detection setpoints on any or all channels. Detection setpoints can be used as
data markers, digital port updates, and as mapped channels.
Each of the WBK17’s eight channels has one detection signal. For measurement
purposes, the detection signals can be routed to an unused counter. The number of times
a certain detection setpoint has been met can be the output data for this counter channel.
Reference Note:
For a complete discussion of setpoints see the section,
Pattern Detection and Data Markers ,
beginning on page 31of this document module.
A typical acquisition might take 6 readings off of the WBK17 module as illustrated below. The user
determines the scan rate and the number of scans to take. Note that the channel numbers shown assume that
the WBK17 is the first expansion module.
WBK17 Acquisition of Six Readings per Scan
In general, the output of each channel’s counter is latched at the beginning of each scan period (called the
start-of-scan.) Every time the WBK17 receives a start-of-scan signal from the WaveBook, the counter
values are latched and are available to be sourced-down to the WaveBook as each WBK17 channel is
selected [during the scan period].
The WBK17 clears all counter channels at the beginning of the acquisition. This means that the values
returned during scan period 1 will always be zero. The values returned during scan period 2 reflect what
happened during scan period 1.
The scan period defines the timing resolution for the WaveBook and the WBK17.
If you need a higher timing resolution, shorten the scan period.