Process Control - Common Terms
A comprehensive reference guide for common terminology used in process control systems, including definitions of analog/digital signals, controller modes, PID components, and key process control concepts.
| Term | Definition |
|---|---|
| Analog Signal | Electrical signals like voltage or electric current representing temperature, pressure, or level. Typically 4-20 mA where 4 mA is minimum and 20 mA is maximum span. |
| Analog to Digital Converting (A-D Converting) | Electronic hardware that converts analog signals like voltage, current, temperature, or pressure into digital data for computer processing. |
| Auto Mode | Controller mode where output is calculated using the error signal - the difference between set point and process variable. |
| Closed Loop | Controller operating in automatic mode. |
| Cascade | Two or more controllers working together where the master controller's output becomes the set point for the slave controller. |
| Controller Output (CO) | Output signal from the controller. |
| DDE Windows Dynamic Data Exchange | Standard Microsoft OS method for communication between applications. Replaced by OLE/OPC for process control. |
| Dead Band | Range through which an input can be varied without initiating a response. |
| Dead Time | Time duration for the process variable to start changing after changing an output (control valve, VFD, etc.). |
| Derivative (D) | Derivative part of PID controller where output is proportional to rate of change of process variable or error. |
| Delay | Term commonly used instead of dead time. |
| Deviation | Any departure from a desired or expected process value. |
| Digital Signal | Discrete binary value with two states (1 or 0), often used for on-off indication. |
| Digital Control System (DCS) | Larger digital control systems for process management. |
| Discrete Logic | Digital on-off logic operations. |
| Discrete I/O | On or off signals sent or received to the field. |
| Distributed Control System (DCS) | Control system where controller elements are distributed throughout rather than centrally located. |
| Dominant Lag Process | Process where lag time is larger than dead time. Most process plant loops are dominant lag types including temperature, level, flow, and pressure loops. |
| Error | In control loop: Error = Set Point - Process Value. |
| Gain | Calculated as 100 / Proportional Band. Higher gain provides faster loop response but more oscillatory/unstable process. Process Gain = change in output / change in input. |
| Gain Margin | Difference in logarithms of amplitude ratios at frequency where combined phase angle is 180 degrees lag. |
| Hysteresis | Signal change required before output unit (valve or similar) moves. |
| Input/Output (I/O) | Electronic hardware where field devices are wired. |
| Integral Action (I) | Integral part of PID controller where output is proportional to amount and duration of error signal. |
| Load Upset | Process upset not caused by changing the set-point. |
| Lag Time | Time after dead time that process variable takes to move 63.3% of final value after step change in valve position. |
| Measurement | Same as process value. |
| Manual Mode | Controller mode where output is set manually. |
| Mode | Controller setting: auto, manual, or remote mode. |
| Man Machine Interface (MMI) | Software that process operator uses to operate the process. |
| Output | Output signal from the controller. |
| Overshoot | Amount a process exceeds the set point during system load change or set point change. |
| PID Controller | Controller including Proportional, Integrating, and Derivative functions per ANSI/IEEE Standard 100-1977. |
| Process Value (PV) | Actual value in control loop: temperature, pressure, flow, composition, pH, etc. |
| Programmable Logic Controller (PLC) | Controllers replacing relay logic, usually with PID controller functions. |
| Process Variable (PV) | Actual value in control loop including temperature, pressure, flow, composition, pH, etc. |
| Proportional Band (P) | Controller output proportional to error or process variable change. Proportional Band = 100 / Gain. |
| Rate | Same as derivative or D part of PID controllers. |
| Register | Data storage location in a PLC. |
| Regulator | Controller that changes output variable to move process variable back to set point. |
| Repeat-ability | Variation in outputs for the same change of input. |
| Reset | Same as integral or I part of PID controllers. |
| Reset Windup | Integral action continuing to change controller output after actual output reaches physical limit. |
| Response Time | Rate of interrogating a transmitter. |
| Sample Interval | Rate at which controller samples process variable and calculates new output. |
| Set Point | Desired value of the process variable. |
| Time Constant | Same as lag time. |
| Transmitter | Senses actual system value and transforms it to standardized signal (typically 4-20 mA for analog signals) as input to control system. |