Maple Systems MAPware-7000 - Manual (Page 158)

MAPware-7000 Ladder Logic Guide 150
1010-1041 rev. 00
Instruction 95- PID4
Expression:
Space Requirement: 1 line x 6 column Location Requirement: Middle, Right rail
Function:
This function performs PID (Proportional, Integral, and Derivative) control which is a
fundamental method of feed-back control.
The basic idea behind the PID controller is to read a sensor, then compute the desired actuator
output by calculating proportional, integral, and derivative responses, then sum those three
components to derive an output value.
PID4 is based upon the following formula:
--P--|------------------I---------------------|---------D----------|
M= M
-1
+ K
P
* (e – e
-1
+ (e/T
I
) * (T
S
+1)) + K
D
(e – 2e
-1
+ e
-2
)
where:
M= manipulation value (range: 0 to 4095)
M
-1
= previous calculated manipulation value (range: 0 to 4095)
Kp = the proportional gain constant (range: -32768 to +32767)
e = deviation or error value;
if Action Type = reverse action then e=S-P;
if Action Type = forward action then e=P-S
A = Action Type (range: 0 for forward, 1 for reverse); this determines if the manipulation
value increases/decreases as the present value increases. For example, if you have an
electric heater, you would want the manipulation value to increase as temperature goes
down (i.e. reverse action so e=S-P). On the other hand, if you have a control valve used
to put cold air into a system, you would want the manipulation value to decrease as
temperature goes down (i.e. forward action so e=P-S).
e
-1
= deviation of prior computation (range: -32768 to +32767)
e
-2
= deviation of second to last computation (range: -32768 to +32767)
T
I
= integral time value (range: 0 to +32767)