Rockwell Automation 1771-QB Linear Pos. - Manual (Page 95)

Formatting Module Data (WRITES)
Chapter 7
7Ć16
Excess Following Error (words 14 and 43)
The excess following error is the maximum allowable axis error above the
expected following error at the programmed velocity for the current move. The
expected following error for a given velocity equals the velocity divided by the
proportional gain.
When the following error reaches the maximum value permitted, as specified by
the excess following error parameter, the module initiates an immediate stop
(loop fault). To disable excess following error checking, specify an excess
following error of zero.
Figure 7.20
Excess Following Error Word
50022
Excess following error,
BCD or binary
9.999 inch or 99.99 mm max
15 14 13 12 11 10 09 08 07 06 05 04 03 02 01 00
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Example: If axis movement is 5 ips and proportional gain (K
P
) at that speed is
0.05 ips/mil (where 1 mil = .001 inch), then
Expected Following Error = (5)/(0.05) = 100 mil
If you specify an excess following error of 50 mil, then an immediate stop will
occur if the following error reaches 150 mil (the expected following error plus
50 mil).
Maximum PID Error (words 15 and 44)
The maximum PID error is the maximum position error when the integral and
derivative components are enabled for final axis positioning (i.e., when the
desired velocity is zero and the axis is within the PID band).
When the maximum PID error is exceeded, the module initiates an immediate
stop (loop fault).