Setting function block parameters is initially performed by the pure PID parameters, i.e. the proportional action coefficient gain, the reset time ti and the rate time td.
The D component is delayed by the time td_lag. The td/td_lag ratio is termed the differential gain, and is generally selected between 3 and 10. The D component can either be based upon the system deviation ERR (d_on_pv = "0") or the controlled variable PV (d_on_pv = "1"). Should the D component be determined by the controlled variable PV, then the D component will not be able to cause jumps when reference variable fluctuations (changes in input SP) take place. In principle the D component only influences disturbances and process changes.
Control direction reversal
A reversed behavior of the controller can be achieved by reversing the sign of gain. A positive value on gain causes the increase of the output value, for a positive error variable. A negative value on gain causes the increase of the output value, for a positive error variable.
Manipulated variable limiting
The limits ymax and ymin limit the output within the prescribed range. Therefore ymin ≤ Y ≤ ymax.
The outputs qmax and qmin signal that the limit value has been reached, i.e. that the output signal is limited.
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qmax = 1 if Y ≥ ymax
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qmin = 1 if Y ≤ ymin
The upper limit ymax for limiting the manipulated variable must be greater than the lower limit ymin, otherwise the function block reports an error and does not function.
Should limiting of the manipulated variable take place, the anti-windup reset should ensure that the I component "cannot go berserk". Anti-windup measures are only taken if the controller I component is not switched off. Anti-windup limits are identical to those for the manipulated variable. The anti-windup measures disregard D component values, to avoid being falsely triggered by D component peaks.
The anti-windup measures correct the I component in such a way that:
Selecting the control types
There are four different control types, which are selected via the elements en_p, en_i and en_d:
|
Controller type
|
en_p
|
en_i
|
en_d
|
|
P controller
|
1
|
0
|
0
|
|
PI controller
|
1
|
1
|
0
|
|
PD controller
|
1
|
0
|
1
|
|
PID controller
|
1
|
1
|
1
|
|
I controller
|
0
|
1
|
0
|
The I component can also be disabled with ti = 0.
