Industrial process control
Process Control is an engineering discipline that deals architectures, mechanisms, and algorithms for controlling the output of a specific process.
For example, heating up the temperature in a room is a process that has the specific, desired outcome to reach and keep a defined temperature (e.g. 20°C) 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
Process control uses of 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.
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 that uses 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.