ATI
Q46D-ODO Optical D.O. System
Part 7 –PID Controller Details
55
O&M Manual
Rev-J (8/17)
úû
ù
êë
é
+
+
=
ò
dt
t
de
D
t
d
t
e
I
t
e
P
output
)
(
)
(
)
(
1
)
(
Where:
output =
controller output
P =
proportional gain
I =
integral gain
D =
derivative gain
t =
time
e(t) =
controller error (e=measured variable – setpoint)
Figure 20 - Q46 ISA (Ideal) Equation
The most notable feature of the algorithm is the fact the proportional gain term
affects all components directly (unlike some other algorithms - like the “series”
form.) If a pre-existing controller utilizes the same form of the algorithm shown
above, it is likely similar settings can for made if the units on the settings are
exactly the same. Be careful of this, as many times the units are the reciprocals
of each other (i.e. reps-per-min, sec-per-rep.)
PID stands for “proportional, integral, derivative.” These terms describe the three
elements of the complete controller action, and each contributes a specific
reaction in the control process. The PID controller is designed to be primarily
used in a “closed-loop” control scheme, where the output of the controller directly
affects the input through some control device, such as a pump, valve, etc.
Although the three components of the PID are described in the setting area
(section 6.25), here are more general descriptions of what each of the PID
elements contribute to the overall action of the controller.
P
Proportional gain. With no “I” or “D” contribution, the controller output is
simply a factor of the proportional gain multiplied by the input error
(difference between the measured input and the controller setpoint.)
Because a typical chemical control loop cannot react instantaneously to a
correction signal, proportional gain is typically not efficient by itself – it
must be combined with some integral action to be useful. Set the P term to
a number between 2-4 to start. Higher numbers will cause the controller
action to be quicker.
I
Integral gain. Integral gain is what allows the controller to eventually drive
the input error to zero – providing accuracy to the control loop. It must be
used to affect the accuracy in the servo action of the controller. Like
proportional gain, increasing integral gain results in the control action
happening quicker. Set the I term to a number between 3-5 to start (1-2
more than P). Like proportional gain, increasing the integral term will
cause the controller action to be quicker.
