User manual

PID Loop Operation
(DL450 Only)
Maintenance
and Troubleshooting
8--32
PID Loop Operation (DL450 only)
DL405 User Manual, 4th Edition, Rev. A
Enter PID Tuning Parameters
Another PID setup dialog, Tuning, is for entering the PID parameters shown as: Gain
(Proportional Gain), Reset (Integral Gain) and Rate (Derivative Gain)
Recall the position and velocity forms of the PID loop equations which were
introduced earlier. The equations basically show the three components of the PID
calculation: Proportional Gain (P), Integral Gain (I) and Derivative Gain (D). The
following diagram shows a form of the PID calculation in which the control output is
the sum of the proportional gain, integral gain and derivative gain. With each
calculation of the loop, each term receives the same error signal value.
Process Variable
Σ
Error Term
+
--
Control OutputSetpoint
Σ
+
P
I
D
Loop Calculation
+
+
The P, I and D gains are 4--digit BCD numbers with values from 0000 to 9999. They
contain an implied decimal point in the middle, so the values are actually 00.00 to
99.99. Some gain values have units y Proportional gain has no unit, Integral gain
may be selected in seconds or in minutes, and Derivative gain i s in seconds.
Gain (Proportional Gain) y This is the most basic gain of the three. Values range
from 0000 to 9999, but they are used internally as xx.xx. An entry of “0000“
effectively removes the proportional term from the PID equation. This
accommodates applications which need integral--only loops.
Reset (Integral Gain) y Values range from 0001 to 9998, but they are used
internally as xx.xx. An entry of “0000“ or “9999“ causes the integral gain to be “
∞”,
effectively removing the integrator term from the PID equation. This accommodates
applications which need proportional --only loops. The units of integral gain may be
either seconds or minutes, as shown in the above dialog.
Rate (Derivative Gain) y Values which can be entered range from 0001 to 9999,
but they are used internally as xx.xx. An entry of “0000“ allows removal of the
derivative term from the PID equation (a common practice). This accommodates
applications which require only proportional and/or integral loops. Most control
loops will operate as a PI loop.