FACTS Engineering Direct Logic 405, Manual

Page: 58 / 62

PID FUNDAMENTALS 5.4
Reverse Acting Loop (Cooling)
The Temperature Controller allows a loop to be programmed as reverse acting. With a reverse acting
loop, the output is driven in the opposite direction of the error. For example, if SP>PV then a reverse
acting controller will decrease output to increase the PV.
Mx Ki e Mx
n n
= − +
−
*
1
Mx Kc e Kr PV PV Mx
n n n
= − + − +
−
* ( )
1
RESET WINDUP PROTECTION
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 n
= − +
−
*
1
For example, assume the output is controlling a heater and the temperature remains at some value
greater than the Setpoint. The negative error (e
n
) will cause the bias term (Mx) to constantly decrease
until the output M goes to 0 turning off the heater. However, since the error term is still negative, the
bias will continue to decrease becoming ever more negative. When the temperature finally does come
back down below the SP, the heater will stay off until the error is positive for long enough to cause the
bias to become positive again. This will cause the controller to overshoot the SetPoint.
One way to solve the problem is to simply clamp the bias between limits. The Temperature Controller
does this. However, if this is the only thing that is done then the output will not move off 0.0 (thus turning
on the heater) until the temperature has become less than the SetPoint. This will also cause the
controller to undershoot.
The Temperature Controller solves the overshoot problem by freezing the bias term.
Freeze Bias
Freeze Bias means the Temperature Controller simply stops changing the bias (Mx) whenever the
computed normalized output (M) goes outside the interval 0.0 to 1.0.
Mx = Ki * e
n
+ Mx
n-1
M = Kc * e
n
- Kr(PV
n
- PV
n-1
) + Mx
M
n
= 0 if M < 0
M
n
= M if 0
#
M
#
1
M
n
= 1 if M > 1
Mx
n
= Mx if 0
#
M
#
1
Mx
n
= Mx
n-1
otherwise
Thus in our example, the bias will probably not go all the way to zero so that, when the temperature does
begin to come down, the loop will begin to turn on the heater sooner than it would have if the bias had
been allowed to go all the way to zero. This action has the effect of reducing the amount of overshoot.