User manual

PID Loop Operation

(DL450 Only)

Maintenance

and Troubleshooting

8--10

PID Loop Operation (DL450 only)

DL405 User Manual, 4th Edition, Rev. A

The DL450 also combines the integral sum and the initial output into a single term

called the bias (Mx). This results in the following set of equations:

Mx = Ki * e

+Mx

n--1

=Kc*e

-- Kr(PV

-- P V

n--1

)+Mx

The DL450 by default will keep the normalized output M in the range of 0.0 to 1.0.

This is done by clamping M to the nearer of 0.0 or 1.0 whenever the calculated output

falls outside this range. The DL450 also allows you to specify the minimum and

maximum output limit values (within the range 0 to 4095 in binary if using 12 bit

unipolar).

NOTE: The equations and algorithms, or parts of, in this chapter are only for

references. Analysis of these equations can be found in most good text books about

process control.

Reset windup can occur if reset action (integral term) is specified and the

computation of the bias term Mx is:

Mx = Ki * e

+Mx

n--1

For example, assume the output is controlling a valve and the PV remains at some

value greater than the setpoint. The negative error (en) will cause the bias term (Mx)

to constantly decrease until the output M goes to 0 closing the valve. However, since

the error term is still negative, the bias will continue to decrease becoming ever more

negative. When the PV finally does come back down below the SP, the valve will stay

closed until the error is positive for long enough to cause the bias to become positive

again. This will cause the process variable to undershoot.

One way to solve the problem is to simply clamp the normalized bias between 0.0

and 1.0. The DL450 CPU does this.However, if this is the only thing that is done, then

the output will not move off 0.0 (thus opening the valve) until the PV has become less

than the SP. This will also cause the process variable to undershoot.

The DL450 CPU is programmed to solve the overshoot problem by either freezing

the bias term, or by adjusting the bias term.

Reset Windup

Protection