Measurement Computing WBK17 - Manual (Page 33)

WBK17, Counter/Encoder Module 987996 WBK17, pg. 33
Example Detection: Channel 1 in Totalizing Counter Mode
The detection circuit works on data that is latched out of the counters and put into the acquisition stream at
the scan rate. This data is acquired according to the pre-acquisition setup (scan group, scan period, etc.)
and returned to the PC. The actual counters may be counting much faster than the scan rate and therefore
only every 10
th , 100 th , or n th count will show up in the acquisition data. Therefore it is possible to set a
small detection window on a totalizing counter channel and have the detection setpoint “stepped over” since
the scan period was too long. Even though the counter value stepped into and out of the detection window,
the actual values going back to the PC may not. This is true no matter what mode the counter channel is in.
The setting of a detection window must be done with a scan period in mind.
Two possible solutions for overcoming this problem. The scan period could be shortened to give more
timing resolution on the counter values or the setpoint window (High Limit and Low Limit) could be
widened. Perhaps a combination of both solutions is the best compromise.
Example: Setpoint Detection on Encoder Position and Velocity.
The figure below shows how setpoints can be used on two channels. Channel 4 is in encoder mode using
channel 1 as the Z-index, therefore relative to Z position information will be acquired on channel 4.
Channel 5 is the encoder’s B signal input. Channel 5 is in period mode, the data being acquired will be
inversely proportional to the encoder’s velocity.
Two setpoints have been programmed, one on channel 4 and the other on channel 5. Channel 4’s setpoint is
a window setpoint, Detect 4 will go high whenever the encoder’s relative position is within the setpoint
window. Channel 5’s setpoint is a “less than value” setpoint, Detect 5 will go high whenever the encoder’s
velocity is greater than the setpoint.