ATI
Q45D-ODO Optical D.O. System
Part 7 –PID Controller
O & M Manual
Revision C (7/09)
57 -
The easiest process’ to control with closed-loop schemes are generally linear,
and symmetrical, in nature. For example, controlling level in tank where the
opening of valve for a fixed period of time corresponds linearly to the amount that
flows into a tank. Chemical control process’ can be more problematic when the
nature of the setpoint value is non-linear relative to the input of chemical added.
For example, D.O. control of a process may appear linear only in a certain range
of operation, and become highly exponential at the extreme ranges of the
measuring scale. In addition, if a chemical process is not symmetrical, that
means it responds differentially to the addition and subtraction of chemical. It is
important in these applications to study steady-state impact as well as step-
change impact to process changes. In other words, once the process has
apparently been tuned under normal operating conditions, the user should
attempt to force a dramatic change to the input to study how the output reacts. If
this is difficult to do with the actual process input (the recommended method), the
user can place the control in manual at an extreme control point such as 5% or
95%, and release it in manual. The recovery should not be overly oscillatory. If
so, the loop needs to be de-tuned to deal with that condition (reduce P and/or I.)
