Process Control - Common Terms
Common terms used in the process control terminology.
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.
In auto mode the output is calculated by the controller using the error signal - the difference between set point and the process variable.
Controller in automatic mode.
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.
The range through which an input can be varied without initiating a response.
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.
A term commonly used in stead of dead time.
Any departure from a desired or expected process value.
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.
Refers to digital "on - off" logic.
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.
In the control loop the error = set point - process value.
Gain = 100 / Proportional Band. More gain in a controller gives a faster loop response - and a more oscillatory (unstable) process.
Gain in the process - Process Gain - is defined as "change in process output" divided by "change in process input". A process with high process gain reacts more to a change in the process input than a process with low process gain.
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.
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.
An upset to the process not from changing the set-point.
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 is the same as the process value.
In manual mode the output is set manual.
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 of the controller.
The amount a process exceed the set point during a change in the system load or change in the set point.
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
Same as the derivative or "D" part of PID controllers.
A data storage location in a PLC.
A controller changing the a output variable to move the process variable back to the set point
The variation in outputs for the same change of input.
Same as the integral or "I" part of PID controllers.
Integral action continuing to change the controller output value after the actual output reaches a physical limit.
The rate of interrogating a transmitter.
The rate at which a controller samples the process variable and calculates a new output.
The set point is the desired value of the process variable.
Same as lag time.
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.