Process Control and Common Terms

Common terms in the process control terminology

Analog Signal

Analog signals are like voltage or electric current signal, representing temperature, pressure, level etc. Usually the electrical current signal is of magnitude 4-20 mA where 4 mA is the minimum point of span and 20 mA is the maximum point of span.

Analog to Digital Converting, A-D Converting

Electronic hardware converts analog signal like voltage, electric current, temperature, or pressure into digital data a computer can process and interpret.

Auto Mode

In auto mode the output is calculated by the controller using the error signal - the difference between set point and the process variable.

Closed Loop

Controller in automatic mode.

Cascade

Two or more controllers working together. The output of the master controller is the set point for the "slave" controller.

Controller Output - CO

Output signal from the controller.

DDE Windows Dynamic Data Exchange

A standard Microsoft operating system method for communicating between applications. Replaced by OLE for process control - OPC.

Dead Band

The range through which an input can be varied without initiating a response.

Dead Time

Dead time is the amount of time it takes for the process variable to start changing after changing output as a control valve, variable frequency drive etc.

Derivative - D

The derivative - D - part of a PID controller. With derivative action the controller output is proportional to the rate of change of the process variable or process error.

Delay

A term commonly used in stead of dead time.

Deviation

Any departure from a desired or expected process value.

Digital Signal

A discrete value at which an action is performed. A digital signal is a binary signal with two distinct states - 1 or 0, often used as an on - off indication.

Digital Control System - DCS

Digital Control System - DCS refers to larger digital control systems.

Discrete Logic

Refers to digital "on - off" logic.

Discrete I/O

On or off signals sent or received to the field.

Distributed Control System - DCS

A control system where the controller elements are not central in location but distributed throughout the system with each component sub-system controlled by one or more controllers.

Dominant Lag Process

Most processes consist of both dead time and lag. If the lag time is larger than the dead time, the process is a dominant lag process. Most process plant loops are dominant lag types. This includes most temperature, level, flow and pressure loops.

Error

In the control loop the error = set point - process value.

Gain

Gain = 100 / Proportional Band. More gain in the controller gives a faster loop response and a more oscillatory (unstable) process.

Gain in the process is defined as the change in input divided by the change in output. A process with high gain will react more to the controller output changing.

Gain Margin

The difference in the logarithms of the amplitude ratios at the frequency where the combined phase angle is 180 degrees lag is the gain margin.

Hysteresis

The signal change before the output unit (valve or similar) moves.

Input/Output - I/O

Electronic hardware where the field devices are wired.

Integral Action - I

The integral part of the PID controller. With integral action, the controller output is proportional to the amount and duration of the error signal. If there is more integral action, the controller output will change more when error is present.

Load Upset

An upset to the process not from changing the set-point.

Lag Time

Lag time is the amount of time after the dead time that the process variable takes to move 63.3% of its final value after a step change in valve position.

Measurement

Measurement is the same as the process value.

Manual Mode

In manual mode the output is set manual.

Mode

The controller can be set in auto, manual, or remote mode.

Man Machine Interface - MMI

Refers to the software that the process operator operates the process with.

Output

Output of the controller.

Overshoot

The amount a process exceed the set point during a change in the system load or change in the set point.

PID Controller

Controller including Proportional, Integrating and Derivative controller functions. Cfr. ANSI/IEE Standard 100-1977.

Process Value - PV

The actual value in the control loop, temperature, pressure, flow, composition, pH, etc

Programmable Logic Controller - PLC

Controllers replacing relay logic, usually with PID controllers.

Process Variable - PV

The actual value in the control loop, temperature, pressure, flow, composition, pH, etc. See Process Value.

Proportional Band - P

With proportional band the controller output is proportional to the error or a change in process variable. Proportional Band = 100/Gain

Rate

Same as the derivative or "D" part of PID controllers.

Register

A data storage location in a PLC.

Regulator

A controller changing the a output variable to move the process variable back to the set point

Repeatability

The variation in outputs for the same change of input.

Reset

Same as the integral or "I" part of PID controllers.

Reset Windup

Integral action continuing to change the controller output value after the actual output reaches a physical limit.

Response Time

The rate of interrogating a transmitter.

Sample Interval

The rate at which a controller samples the process variable and calculates a new output.

Set Point

The set point is the desired value of the process variable.

Time Constant

Same as lag time.

Transmitter

A transmitter sense the actual value of a system and transforms the value to a standardized signal - 4-20 mA is common for analog signals - as input for the control system.

Related Topics

  • Process Control - Instrumentation and process control systems - engineering and documentation

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