PID TUNING
Tuning PID control loops for fast response
When choosing a tuning strategy for a specific control
loop, it is important to match the technique to the needs of that loop and the
larger process. It is also important to have more than one approach in your
repertoire, and the Cohen-Coon method can be a handy addition in the right
situation.
The well-known Ziegler-Nichols tuning rules work well only
on processes with very long time constants relative to their dead times, and on
level control loops. However, its performance is not good on flow, liquid
pressure, and many other loops that require fast adjustment. In contrast, the
Cohen-Coon tuning rules work well on virtually all self-regulating processes
and were designed to give a very fast response.
The method's original design resulted in loops with too much
oscillatory response and consequently fell into disuse. However, with some
modification, Cohen-Coon tuning rules proved their value for control loops that
need to respond quickly while being much less prone to oscillations.
Applicable process types
The Cohen-Coon tuning method isn't suitable for every
application. For starters, it can be used only on self-regulating processes.
Most control loops, e.g., flow, temperature, pressure, speed, and composition,
are, at least to some extent, self-regulating processes. (On the other hand,
the most common integrating process is a level control loop.)
A self-regulating process always stabilizes at some point of
equilibrium, which depends on the process design and the controller output. If
the controller output is set to a different value, the process will respond and
stabilize at a new point of equilibrium.
Target controller algorithm
Cohen-Coon tuning rules have been designed for use on a
non-interactive controller algorithm such as that provided by the Dataforth MAQ
20 industrial data acquisition and control system. There are controllers with
similar characteristics available from other suppliers.
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