Mostly-Used Old-Fashion PID Tuning Methods:
For tuning of PID controllers, industries still use methods like
Trial-and-Error, Ziegler Nichols, Cohen Coon, IMC and Lambda which are
quite obsolete in today’s world. These methods mostly rely on trial and
error steps for questing PID tuning parameters or on some old-fashion
empirical rules and equations set back around 1940’s when processes were
simple, production grades & rates were not as changing as it is today,
product and environmental specifications were not so tight as today. To
cope up with these problems PID tuning should be appropriate to achieve
the specification, production rate which is required. All these
old-fashion PID tuning methods rely on time consuming and large
step-test changes by forcing the PID controller to manual mode and
producing either too aggressive or sluggish PID control action. Many
times, their performance and success rely on the experience of a process
control engineer whose knowledge is based on many years of PID tuning
and process understanding. Therefore, all these methods will never
produce optimal PID tuning parameter values which are required for good
PID control in auto or in cascade mode when typical and frequent SP
changes and the effect of process disturbances are present. The
procedure followed during the old-fashion technique is described below:
- A typical PID tuning session begins when a plant operator requests
help from a process control engineer to tackle with an oscillatory or
sluggish response of PID controller to achieve its SP after a
condition change or a load upset.
- Based on past experience; he starts to work on the PID tuning
parameters using one of several old-fashion PID tuning methods. To
tune a PID controller often process control engineer first needs to
put the PID controller in manual mode.
- In manual mode, for finding the process dynamics parameters for PID
tuning parameters, he will do some step-tests. Many times, due to
presence of process disturbances and noise these step-tests need to be
large enough for distinguishing the real process dynamics from typical
process upsets which is not recommended in steady state.
- Once the PID parameters are calculated, after putting them in PID
controller they need to be tested by online changing a PID controller
SP. If new PID tuning parameters are not performing well the job of
process control engineer is to fine tune them. Fine tuning method is
purely based on his experience and trial-and-error procedure. As seen
from all above steps tuning of a PID controller by old-fashion
approach require an experience process control engineer, uses a lot of
time, produces high uncertainty of results and upsets the plant
seriously due to needful step-tests.
A large number of manufacturing plants still use these methods. The
reasons which account for usage of these methods are absence of
engineering knowledge and understanding, unavailability of robust
process control software tools for closed-loop system identification,
PID tuning and optimization, and PID tuning parameters simulation and
testing without conducting serious plant step-tests due to fear of
causing shutdowns and plant problems.