Terms of Process Control

Process control is the automatic control of an output variable by the sensing of the amplitude of the output parameter from the process and comparing it to the desired or set level and feeding an error signal back to control an input variable. The processor uses the error signal to generate a correction signal to control the actuator and the input variable.

A feedback loop is the signal path from the output back to the input for any correction by comparing it to the desired or set level.

A controlled or measured variable is the monitored output variable from a process.
 
A manipulated variable is the input variable or parameter to a process that is varied by a control signal from the processor to an actuator.
 
A set point is the desired value of the output parameter or variable being monitored by a sensor. Any deviation from this value will generate an error signal.
 
An instrument is a device for measuring physical quantities.
 
A sensor is a device that can detect physical variables.
 
A transducer is a device that can change one form of energy to another form of energy.
 
A converter is a device that can be used to change the format of a signal without changing the energy form.
 
An actuator is a device that is used to control an input variable in response to a signal from a controller.
 
A controller is a device that monitor signals from transducers and take the necessary action to keep the process within specified limits according to a pre defined program by activating and controlling the necessary actuators.
 
Ladder networks are normally used to program the controllers.
 
Programmable Logic Controllers are microprocessor based systems, that have the ability to monitor several variables and control several actuators.
 
An error signal is the difference between the set point and the measured variable's amplitude.
 
A correction signal is the signal used to control power to the actuator to set the level of the input variable.
 
A transmitter is a device used to amplify and format signals so that they are suitable for transmission over long distances with zero or minimal loss of information. The transmitted signal can be in pneumatic, digital or analog.
 
Reproductibility is the ability of an instrument to repeatedly read the signal over time and give the same output under the same conditions.
 
Sensitivity is a measure of the change in the output of an instrument for a change in the measured variable (transfer function).
 
Offset is the reading of an instrument with zero input.
 
Drift is the change in the reading of an instrument of a fixed variable with time.
 
Hysteresis is the difference in readings obtained when an instrument approaches a signal from the opposite direction.
 
Resolution is the smallest change in a variable to which the instrument will respond.
 
Linearity is a measure of the proportionality between the actual value of a variable being measured and the output of the instrument over its operating range.