PID for PLCs 350

auto mode

A system that is set for closed-loop control.

automatic feedback

Feedback that is controlled by an electronic or electromechanical mechanism rather than a person.

autotune

A feature in some motor control devices that automatically adjusts the PID control system at start-up.

bias

A value between 0 and 100% that is added to the output after the gain calculation brings the controller to the steady state mode. Bias is sometimes used to stabilize a control system.

closed-loop system

A type of control system that automatically changes the output based on the difference between the feedback signal to the input signal.

constant value

A value that does not change.

control mechanism

A system or device that operates or manipulates another system or device.

control system

A manual or automatic mechanism used to manage dynamic processes by adjusting or maintaining physical variables such as temperature, speed, or flow rate.

controller

A processor driven device that uses logic based software to provide electrical control to machines. Also known as controller or PLC.

controller tuning

A system of adjusting the controller's response to error by adjusting values in the PID algorithm.

cruise control

A control mechanism for automatically regulating the speed of a vehicle. Cruise control allows the driver to maintain the speed of a car without continually holding down the accelerator pedal.

deadtime

The delay between a change in the PV and a change in the output.

delta

A mathematical operator used to indicate a change in a value. It is represented by the Greek letter Δ.

derivative control

A variable that allows a controller to anticipate whether the PV will overshoot the SP, and reduces the output as needed.

deviation alarm

An alarm that alerts the operator that the output is nearing its minimum or maximum allowed value.

error

The difference between the SP and the PV. When errors are detected, the controller sends instructions to the control system to adjust the output to compensate.

feedback signal

The actual value detected by a sensor as a process is taking place. Also known as the process variable.

gain

A type of closed-loop feedback control in which a constant value is multiplied with the error in order to adjust the output. Also known as proportional gain and proportional band.

gain-only control

A type of closed-loop system control that uses only the proportional aspect of the PID algorithm.

inner alarm band

An alarm system configuration that allows for an early warning as the temperature approaches a minimum or maximum allowed value.

integral

A small part of a larger whole. In PID, the integral is a variable time period used to adjust an output.

integral action

The time it takes a control system to adjust output once it reaches the setpoint. Adding the integral causes the output to change more rapidly or more slowly based on the integral time specified.

integration

The process of dividing an area into small parts and using the sum of those parts to determine the total value.

manual mode

A system that is set for open-loop control.

maximum error signal

The highest or strongest possible error signal.

open-loop system

A type of control system that uses only an input signal to actuate an output. There is no automatic feedback to adjust the process, so adjustments must be made manually by the operator.

oscillate

To waver back and forth. Excessive oscillation in a signal can cause a control system to become unstable.

outer alarm band

An alarm system configuration that alerts the operator when the output has exceeded its mimimum or maximum allowed value.

overshoot

To exceed a set value.

PID

One of the most common types of process control. Proportional, integral, derivative control adjusts system outputs when there is a difference between the setpoint and process variable.

PID algorithm

A mathematical formula used to control and stablize the outputs of closed-loop control systems. The equation for PID is: Output = Gain (Error + l/Integral Time ∫ Error x deadtime + Derivative Time Constant x ΔError/ ΔTime).

process variable

The actual value detected by a sensor as a process is taking place. Also known as the feedback signal.

proportional band

A type of closed-loop feedback control in which a constant value is multiplied with the error in order to adjust the output. Also known as gain and proportional gain.

proportional gain

A type of closed-loop feedback control in which a constant value is multiplied with the error in order to adjust the output. Also known as gain and proportional band.

proportional, integral, derivative control

One of the most common types of process control. PID adjusts system outputs when there is a difference between the setpoint and process variable.

PV

Process variable.The actual value detected by a sensor as a process is taking place.

rate

A variable that allows a controller to anticipate whether the PV will overshoot the SP, and reduces the output as needed.

reset

The time it takes a control system to adjust output once it reaches the setpoint. Reset causes the output to change more rapidly or more slowly based on the integral time specified.

setpoint

A preset value such as a specific temperature, speed, or flow rate that the control system is supposed to reach. Also known as the target value.

SP

Setpoint. A preset value such as a specific temperature, speed, or flow rate that the control system is supposed to reach.

summing junction

The part of a PID control system in which the SP and PV are compared to each other. The difference between the SP and PV is called error.

target value

A preset value such as a specific temperature, speed, or flow rate that the control system is supposed to reach. Also known as the setpoint.

undershoot

To fall short of a set value.