PID Control
The PID controller is the most common form of feedback. It was an essential element of early governors and it became the standard tool when process control emerged in the 1940s. In process control today, more than 95% of the control loops are of PID type, most loops are actually PI control. PID controllers are today found in all areas where control is used. The controllers come in many different forms.
There are stand-alone systems
in boxes for one or a few loops, which are manufactured by the
hundred thousands yearly. PID control is an important ingredient of a
distributed control system. The controllers are also embedded in many
special-purpose control systems. PID control is often combined with logic,
sequential functions, selectors, and simple function blocks to build the
complicated automation systems used for energy production, transportation,
and manufacturing. Many sophisticated control strategies, such as
model predictive control, are also organized hierarchically. PID control is
used at the lowest level; the multivariable controller gives the setpoints
to the controllers at the lower level. The PID controller can thus be said
to be the “bread and buttert’t of control engineering. It is an important ’
component in every control engineer’s tool box.
PID controllers have survived many changes in technology, from mechanics
and pneumatics to microprocessors via electronic tubes, transistors,
integrated circuits. The microprocessor has had a dramatic influence
on the PID controller. Practically all PID controllers made today are based
on microprocessors. This has given opportunities to provide additional features
like automatic tuning, gain scheduling, and continuous adaptation.
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