Reference Array For The Pid Function; Scaling Input And Outputs; Reference Array Parameters - GE VersaMax PLC User Manual

14.2 Reference Array for the PID Function

Words
1 Loop Number
(Address + 0) Optional number of the PID block. It provides a common
identification in the CPU with the loop number defined by an
operator interface device.
2 Algorithm
(Address + 1) 1 = ISA algorithm
2 = independent algorithm
PID Built-in Function Block
For public disclosure
This parameter block for the PID function occupies 40 words of memory, located at the
starting Address specified in the PID function block operands. Some of the words are
configurable. Other words are used by the CPU for internal PID storage and are normally
not changed.
Every PID function call must use a different 40-word memory area, even if all the
configurable parameters are the same.
The configurable words of the reference array must be specified before executing the PID
function. Zeros can be used for most default values. Once suitable PID values have been
chosen, they can be defined as constants in BLKMOV functions so the program can set
and change them as needed.
The PID function does not pass power flow if there is an error in the configurable
parameters. The function can be monitored using a temporary coil while modifying data.
Note If you set the Override bit (bit 0) of the Control Word (word 15 of the reference
array) to 1, the next four bits of the control word must be set to control the PID function
block input contacts, and the Internal SP (word 16) and Internal PV (word 18) must be set
because you have taken control of the PID function block away from the ladder logic.

14.2.1 Scaling Input and Outputs

All parameters of the PID function are 16 bit integer words for compatibility with 16 bit
analog process variables. Some parameters must be defined in either PV counts or units or
in CV counts or units.
The SP input must be scaled over the same range as the PV, because the PID function
calculates error by subtracting these two inputs.
The process PV and control CV counts do not have to use the same scaling. Either may be
-32000 or 0 to 32000 to match analog scaling, or from 0 to 10000 to display variables as
0.00% to 100.00%. If the process PV and control CV do not use the same scaling, scale
factors are included in the PID gains.

14.2.2 Reference Array Parameters

The reference array parameters are described on the following table. The Machine Edition
software allows you to modify the configurable parameters for a PID instruction in real
time in online programmer mode. To customize PID parameters, right click the PID
function and select Tuning.
Parameter/Description
Low Bit Range
Units
Integer 0 to 255 (for user
display only)
-
Set by the CPU
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