47
O&M Manual
Rev-H (6/19)
Part 9 – PID Controller Details
9.1
PID Description
PID control, like many other control schemes, are used in chemical control to improve the efficiency
of chemical addition or control. By properly tuning the control loop that controls chemical addition,
only the amount of chemical that is truly required is added to the system, saving money. The
savings can be substantial when compared to a system which may be simply adding chemical at a
constant rate to maintain some minimal addition under even the worst case conditions. The PID
output controller is highly advantageous over simple control schemes that just utilize direct
(proportional only) 4-20 mA output connections for control, since the PID controller can
automatically adjust the “rate” of recovery based on the error between the setpoint and the
measured value
– which can be a substantial efficiency improvement..
The PID controller is basically designed to provide a “servo” action on the 4-20 mA output to control
a process. If the user requires that a measured process stay as close as possible to a specific
setpoint value, the controller output will change from 0% to 100% in an effort to keep the process
at the setpoint. To affect this control, the controller must be used with properly selected control
elements (valves, proper chemicals, etc.) that enable the controller to add or subtract chemical
rapidly enough. This is not only specific to pumps and valves, but also to line sizes, delays in the
system, etc.
This section is included to give a brief description of tuning details for the PID controller, and is not
intended to be an exhaustive analysis of the complexities of PID loop tuning. Numerous sources
are available for specialized methods of tuning that are appropriate for a specific application.
9.2
PID Algorithm
As most users of PID controllers realize, the terminology for the actual algorithm terms and even
the algorithms themselves can vary between different manufacturers. This is important to recognize
as early as possible, since just plugging in similar values from one controller into another can result
in dramatically different results. There are various basic forms of PID algorithms that are commonly
seen, and the implementation here is the most common version; The ISA algorithm (commonly
referred to as the “ideal” algorithm.)
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 23 - Q46 ISA (Ideal) Equation
