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Process control is an engineering discipline that deals
with architectures, mechanisms, and algorithms for controlling the output of a
specific process. See also control theory.
For example, heating up the temperature in a room is a process that has the specific, desired outcome to reach and maintain a
defined temperature (e.g. 20°C), kept constant over time. Here, the
temperature is the controlled variable. At the same time, it is the input variable since it is measured by a
thermometer and used to decide whether to heat or not to heat. The desired temperature (20°C) is the set point. The
state of the heater (e.g. the setting of the valve allowing hot water to through it) is called the manipulated variable
since it is subject to control actions.
Control Loops
Schematic figure of an open control loop
Process control uses so-called control loops to achieve its task. Basically, two different kinds of control loops
exist: open loops and closed loops.
An open control loop simply performs a function on the input variable to determine the appropriate setting for the
manipulated variable in order to achieve the desired value of the controlled variable.
Schematic figure of a feedback closed control loop
A feedback closed control loop feeds back the value of the controlled variable into the controller and uses them to
change the manipulated variables. Thus, the value of the controlled variable itself is used to determine its future value.
A third variant is the feedforward closed control loop, using
intermediate variables as an indication for the state of the controlled variables. This scheme is used if measuring the
controlled variables is not possible directly.
Examples
A thermostat is a simple example for a closed control loop: It constantly
measures the current temperature and controls the heater's valve setting to increase or decrease the room temperature according
the user-defined setting. An anti-lock braking
system (ABS) is a more complex example.
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