Proportional/Integral/Derivative (Pid) Loop Instruction - Siemens SIMATIC S7-200 System Manual

Proportional/Integral/Derivative (PID) Loop Instruction

The PID Loop instruction (PID) executes a PID loop
calculation on the referenced LOOP based on the input and
configuration information in Table (TBL).
Error conditions that set ENO = 0:
H SM1.1 (overflow)
H 0006 (indirect address)
Special Memory bits affected:
H SM1.1 (overflow)
The PID loop instruction (Proportional, Integral, Derivative
Loop) is provided to perform the PID calculation. The top of
the logic stack (TOS) must be ON (power flow) to enable the
PID calculation. The instruction has two operands: a TABLE
address which is the starting address of the loop table and a
LOOP number which is a constant from 0 to 7.
Eight PID instructions can be used in a program. If two or more PID instructions are used with the
same loop number (even if they have different table addresses), the PID calculations will interfere
with one another and the output will be unpredictable.
The loop table stores nine parameters used for controlling and monitoring the loop operation and
includes the current and previous value of the process variable, the setpoint, output, gain, sample
time, integral time (reset), derivative time (rate), and the integral sum (bias).
To perform the PID calculation at the desired sample rate, the PID instruction must be executed
either from within a timed interrupt routine or from within the main program at a rate controlled by a
timer. The sample time must be supplied as an input to the PID instruction via the loop table.
Auto-Tune capability has been incorporated into the PID instruction. Refer to Chapter 15 for a
detailed description of auto-tuning. The PID Tuning Control Panel only works with PID loops
created by the PID wizard..
Table 6-43 Valid Operands for the PID Loop Instruction
Inputs/Outputs
TBL
LOOP
STEP 7-Micro/WIN offers the PID Wizard to guide you in defining a PID algorithm for a
closed-loop control process. Select the Tools > Instruction Wizard menu command and then
select PID from the Instruction Wizard window.
Instruction
Wizard
Tip
The setpoint of the low range and the setpoint of the high range should correspond to the
process variable low range and high range.
Data Types Operands
BYTE VB
BYTE Constant (0 to 7)
S7-200 Instruction Set Chapter 6
145