Process Variable
Physical quantity that can be measured, altered, transmitted, and recorded
Measurement
Act of reading a value at a certain time; the value of a process variable as indicated by a suitable instrument
Setpoint
Desired value of a variable
Control Point
Actual value of a variable
On/Off Control
Final control element is either fully on or fully off; has no intermediate position
PLC (Programmable Logic Controller)
Device that collects process system data and controls the process
HMI (Human Machine Interface)
Display that allows an operator to call up information about process variables to be monitored and permit changes to be made
Error
Difference between the measured signal and the setpoint
Signal Evaluation
Deciding whether to respond and in which direction
Feedback
Signal that instructs a controller to correct a variable
Open Loop Control
Regulated only by changes from outside the process; there is no feedback
Closed Loop Control
Measuring, comparing, computing, and correcting go on continually; uses feedback
Process Loop
Set of devices working together to control a process or part of a complex process
Analog Signal
Has a unique value for every value of a process variable
Transducer
Receives the output of a primary element and converts it to a standard signal
Zeroing (A Transducer)
Adjusting to produce minimum signal when process variable is zero
Spanning (A Transducer)
Adjusting to produce maximum signal when process variable is at ts maximum
Digital Signal
Can assume one of only two values
Binary Code
Uses only two digits (0 and 1) to represent any decimal number
ASCII
American Standard Code for Information Interchange; means of communicating with a computer
Proportional Action (Gain)
Control that adjusts the final element to a position between on and off that is in proportion to the signal from the process
Integral Action (Reset)
control based on the size of the error and the length of time the error persists
Derivative Action (Rate)
Control based on how fast a measured variable changes
PID
Proportional, integral, and derivative control combined
Controlled Variable
Any process variable that is maintained by a control system
Offset
The constant difference between the setpoint and the control point, when process operation is stable
Primary Element (Detector/Sensor)
A device that detects changes in the value of a process variable
Transducer (Transmitter)
The device that receives the output of a primary element and converts it into a standard signal
Controller
The device that compares the measurement of a process variable to the setpoint and sends a correcting signal to the final control element
Error Signal
The signal produced by the controller when it detects a difference between the control point and the setpoint
Final Control Element
The valve, pump, or other device that directly regulates a process
Standard Signal
Carrier of process control information
Transfer Function
Graph that shown the relationship between a device's input and output
Range
Upper and lower operating limits between which a device is designed to operate
Span
Difference between the upper and lower operating limits of a device
Hysteresis
Difference in output for the same input value depending on whether a variable is increasing or decreasing
Deadband
The part of the range through which an input change produces no output change
Noise
Electrical interference caused by unwanted voltages
Fiber Optics
Transmission of signals in the form of light through transparent fibers