Rockwell Automation 1771-PD PID MODULE (+DU) - Manual (Page 126)

Programming
Chapter 3
3Ć83
Finally, the lead/lag ratio determines the initial response of the filter to a
step input. For example, a lag dominated filter of 2/4 will respond with an
initial step change of half the step input value. A lead dominated filter of
4/2 will respond with an initial step change of twice the step input value.
The filter response to the step change cannot exceed eight times the step
input value if the lead/lag ratio should exceed 8:1 (Figure 3.27).
To select the lead/lag feature, W30(W50)B05 must be set to 1. The lead
value time constant TB and the lag value time constant TC must be set in
the appropriate word. A value of zero entered for either the lead or lag
value disables that function
Figure 3.27
Lead Filter (Lag Function Disabled)
0 50 100 150 200 250 300 350 400 450 500
100
800
Step Input = 100
10
0
999.9
0
100
0
TC = 0
Lead
Lag TC
= TB
Response in seconds
Percent of step input
11132
750
700
650
600
550
500
450
400
350
300
250
200
150
Closed Loop Control
Standard PID closed loop control is shown in Figure 3.28. the module
compares the process variable input with the desired setpoint. The
resultant error is operated upon by any selected combination of
proportional, integral or derivative functions. The PID function adjusts
the output to bring the process variable equal to the setpoint.