Process control
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 flow through it) is called the manipulated variable since it is subject to control actions.
In practice, process control systems can be characterized as one or more of the following forms:
- Discrete – Found in many manufacturing, motion and packaging applications, discrete process control systems use a device called a ‘’’programmable logic controller’’ to read a set of digital and analog inputs, apply a set of logic statements, and generate a set of outputs. Robotic assembly, such as that found in automotive production, can also be characterized as discrete process control.
- Batch – Some applications require that specific quantities of raw materials be combined in specific ways for particular durations to produce an intermediate or end result. One example is the production of adhesives and glues, which normally require the mixing of raw materials in a heated vessel for a period of time to form a quantity of end product.
- Continuous – Often, a physical system is represented though variables that are smooth and uninterrupted in time. The control of the water temperature in a heating jacket, for example, is an example of continuous process control.
Application has elements of discrete, batch and continuous process control are often called hybrid applications.
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. A simple method switches the heater either completely on, or completely off, and an overshoot and undershoot of the controlled temperature must be expected. A more expensive method varies the amount of heat provided by the heater depending on the difference between the required temperature (the "setpoint") and the actual temperature. This minimizes over/undershoot.
An anti-lock braking system (ABS) is a more complex example.
See also
- Actuator
- Automation
- Automatic control
- Closed-loop controller
- Control engineering
- Control panel
- Control system
- Control theory
- Controllability
- Controller (control theory)
- Cruise control
- Current loop
- Digital control
- Distributed control system
- Feedback
- Feed-forward
- Fieldbus control system
- Flow control valve
- Fuzzy control system
- Laplace transform
- Measurement instruments
- Negative feedback
- Non-linear control
- Open-loop controller
- PID controller
- Positive feedback
- Regulator (automatic control)
- Setpoint
- Sliding mode control
- Temperature control
- Transducer
- Valve
- Watt governor