Program instructions
7.9 PID
S7-200 SMART
System Manual, V2.3, 07/2017, A5E03822230-AF
319
7.9.5
Forward- or reverse-acting loops
The loop is forward-acting if the gain is positive and reverse-acting if the gain is negative.
(For I or ID control, where the gain value is 0.0, specifying positive values for integral and
derivative time will result in a forward-acting loop, and specifying negative values will result
in a reverse-acting loop.)
Variables and ranges
The process variable and setpoint are inputs to the PID calculation. Therefore the loop table
fields for these variables are read but not altered by the PID instruction.
The output value is generated by the PID calculation, so the output value field in the loop
table is updated at the completion of each PID calculation. The output value is clamped
between 0.0 and 1.0. The output value field can be used as an input by the user to specify
an initial output value when making the transition from manual control to PID instruction
(auto) control of the output. (See the discussion in the "Modes" section below.)
If integral control is being used, then the bias value is updated by the PID calculation and the
updated value is used as an input in the next PID calculation. When the calculated output
value goes out of range (output would be less than 0.0 or greater than 1.0), the bias is
adjusted according to the following formulas:
●
when the calculated output M
n
> 1.0
●
when the calculated output M
n
< 0
MX is the value of the adjusted bias
MP
n
is the value of the proportional term of the loop output at sample time n
MD
n
is the value of the differential term of the loop output at sample time n
M
n
is the value of the loop output at sample time n
By adjusting the bias as described, an improvement in system responsiveness is achieved
once the calculated output comes back into the proper range. The calculated bias is also
clamped between 0.0 and 1.0 and then is written to the bias field of the loop table at the
completion of each PID calculation. The value stored in the loop table is used in the next PID
calculation.
The bias value in the loop table can be modified by the user prior to execution of the PID
instruction in order to address bias value problems in certain application situations. Care
must be taken when manually adjusting the bias, and any bias value written into the loop
table must be a real number between 0.0 and 1.0.
A comparison value of the process variable is maintained in the loop table for use in the
derivative action part of the PID calculation. You should not modify this value.
