Chapter 26
PID Instruction
26–4
The PID instruction uses the following equation:
Output
+
K
C
[(
E)
)
1
ń
T
I
ŕ
(
E)dt
)
T
D
·
D(PV)
ń
dt]
)
bias
Standard Gains constants:
Term
Range (Low to High)
Reference
Controller Gain K
C
0.1 to 25.5 (dimensionless)
Proportional
Reset Term 1/T
I
25.5 to 0.1 (minutes per repeat)
Integral
Rate Term T
D
0.01 to 2.55 (minutes)
Derivative
The derivative term (rate) provides smoothing by means of a low pass filter.
The cutoff frequency of the filter is 16 times greater than the corner
frequency of the derivative term.
Normally, you place the PID instruction on a rung without conditional logic.
The output remains at its last value and the integral sum (words 17 and 18) is
cleared when the rung is false.
The PID instruction is located under CPT/MTH in the HHT instruction set
menu. After you select PID, the following display appears:
F1
F2
F3
F4
F5
ZOOM on PID –(PID)– 1/3 2.0.0.0.*
NAME: PROP INTEGRAL DERIVATIVE
CONT BLK:
PROCESS:
OUTPUT:
CONTROL BLOCK SIZE 23 WORDS
ENTER CONTROL BLK:
This is the first of three data entry displays. The prompt line asks you to
enter Control Block, then Process, and then Output.
•
Control Block – This is a file that stores the data required to operate the
instruction. The file length is fixed at 23 words and should be entered as
an integer file address. For example, an entry of N7:2 will allocate
elements N7:2 through N7:24. The control block layout is shown on page
26–8
.
Do not write to control block addresses with other instructions in your
program except as described later in this chapter. If you are re-using a
block of data which was previously allocated for some other use, it is
good practice to first zero the data.
The PID Equation
Entering Parameters
