ATI
Q46H/64 Dissolved Ozone System
Part 8
– PID Controller Details
56
O&M Manual
Rev-J (11/18)
If it appears that even large amounts of integral gain (>20) don’t appreciably increase the desired
response, drop I back to about 1.0, and increase P by 1.00, and start increasing I again. In most
chemical control schemes, I will be approximately 3 times the value of P.
8.3
Classical PID Tuning
Unlike many high speed position applications where PID loops are commonly used, the chemical
feed application employed by this instrument does not require intense mathematical exercise to
determine tuning parameters for the PID. In fact, the risk of instability is far greater with overly
tuned PID control schemes. In addition, many of the classical mathematical exercises can be
damaging or wasteful in the use of chemicals when the process is bumped with large amounts of
input error to seek a response curve. Because of this, the general adjustment guidelines
described in section 8.2 PID Algorithm are sufficient for almost all application tuning for this
instrument. Beyond this, many sources are available for classical tuning methods.
8.4
Manual PID Override Control
The Q46 PID output function allows the user to take manual control of the PID control signal.
This is often useful when starting up a control loop, or in the event that you wish to bump the
system manually to measure system response time
.
To access the manual PID control, you must be in the MEASURE mode of operation and you
must have the PID output displayed on the lower line. This line will indicate “XX.X% XX.X mA”
with the X values simply indicating the current values. With this display on the screen, press and
hold the ENTER key for a
bout 5 seconds. You will see a small “m” show up between the % value
and the mA value. This indicates you are now in manual mode.
Once in manual, you may increase the PID output by pressing the UP arrow or you may decrease
the output by pressing the LEFT arrow. This will allow you to drive the PID output to any desired
setting.
To revert to normal PID control, press and hold the ENTER key again until the “m” indicator
disappears.
8.5
Common PID Pitfalls
The most common problem occurring in PID control applications involves the false belief that
proper settings on only the PID controller can balance any process to an efficient level.
Close-loop control can only be effective if all elements in the loop are properly selected for the
application, and the process behavior is properly understood. Luckily, the nature of simple
chemical control process’ are generally slow in nature. Therefore, even a de-tuned controller
(one that responds somewhat slowly) can still provide substantial improvements to setpoint
control. In fact, damaging oscillatory behavior is far more likely in tightly tuned controllers where
the user attempted to increase response too much.
When deciding on a PID control scheme, it is important to initially review all elements of the
process. Sticking valves, undersized pumps, or delays in reaction times associated with chemical
addition can have a dramatic effect on the stability of the control loop. When controlling a
chemical mix or reaction, the sensor should be placed in a location that ensures proper mixing or
reaction time has occurred.
