A/D: Analog-to-digital.
Actuator: A device which accepts the output from a control system and moves a final control element (usually a valve) to change a process condition. See also "Final Control Element."
Adaptive Control: Method of control whereby tuning (response) of the control system is varied with the process conditions, unlike other control where tuning is manual and remains constant.
Algorithm: Mathematical representation of the action performed by a controller such as proportional, integral, derivative,or combinations of those modes.
Alphanumeric: A character set that contains both letters and digits and perhaps other characters such as punctuation marks.
Analog Computer: A computing device comprised of functional modules such as amplifiers, multipliers, dividers, etc., interconnected in such a way as to facilitate the solution of a set of mathematical expressions or to implement some control strategy. The input to and the output from an analog computer are continuous signals as contrasted with a digital computer which updates an output every scan.
Attenuation: An increase or decrease in signal magnitude between two points or between two frequencies.
Baud Rate: The number of bits or discrete pieces of information transmitted per second.
Bit: Abbreviation for "binary digit." A single character in a binary number, represented by zero (0) or one (1).
Byte: The number of adjacent binary digits operated upon as a unit.
Cascade Control: Controllers arranged such that the output of one controller manipulates the setpoint input of a second controller instead of manipulating a process variable directly.
Control Action, Derivative (Rate): Control action in which the controller output is proportional to the rate of change of the input.
Control Action, Integral (Reset): Control action in which the controller output is proportional to the time integral of the error signal.
Control Action, Proportional: Control action in which the controller output has a linear relationship to the error signal.
Controller: A device which receives a measurement of the process variable, compares that measurement with a set point representing the desired control point, and adjusts its output based on the selected control algorithm to minimize the error between the measurement and the setpoint. If an increase in the measured process variable above the setpoint causes an increase in the magnitude of the controller output, the controller is said to be "direct acting." If a process variable increase above the setpoint causes a decrease in the magnitude of the controller output, the controller is "reverse acting."
Data Base: A collection of values for process variables, setpoints, scaling factors, control parameters, limits, constants, identifiers, etc. for access by the application programs in a computer-based control system. Data Highway: A high-speed serial or parallel data path which connects several units of a distributed control or data collection system.
DDC: Direct Digital Control. A control technique in which a digital computer is used as the sole controller and its output is used to set the final control element. This is in contrast to supervisory control.
Dead Band: The range through which an input may vary without changing the output. In a mechanical instrument such as a meter movement or strip-chart recorder, the dead band is caused by friction and slack or "play" in the readout mechanism. In a controller, dead band is a zone around the setpoint in which the measurement may vary without initiating a compensating controller response.
Dead Time: The interval of time lag between the initiation of a controller output or stimulus and the start of the resulting observable process response.
Dedicated Control: Using one controller to control one process variable.
Derivative Control: A mode of control using an algorithm which anticipates when a process variable will reach its desired control point by sensing its rate of change. This allows a control change to take place before the process variable overshoots the desired control point. See also "Control Action, Derivative (Rate)."
Derivative Time: The time difference by which the output of a proportional-derivative (PD) controller leads the controller input when the input changes linearly with time.
Digital Computer: An electronic machine for performing calculations on discrete quantities of data. Usually includes bulk storage devices such as disks, tape units, etc., in addition to internal memory. Also includes devices for printing and/or displaying output data.
Distributed Control System: Any control system in which the degradation or failure of any single element will affect only the control loop, or related loops, in which it operates.
EPROM (Erasable Programmable Read-Only Memory):
A memory device with information placed into it during manufacture that cannot be altered by the computer. It can only be erased and reprogrammed with special equipment.
Error Signal: The signal resulting from the difference between the setpoint reference signal and the process variable feedback signal in a controller.
Feedback Control: A type of control whereby the controller receives a feedback signal representing the condition of the controlled process variable, compares it to the setpoint, and adjusts the controller output accordingly.
Feedforward Control: A type of control which takes corrective action based on disturbances before the process variable is upset.
Final Control Element: That component of a control system (such as a valve) which directly changes the manipulated variable.
Gain: The ratio of change in output divided by the change in input that caused it. Both input and output must be in the same units; hence gain is a dimensionless number.
Hierarchy: The ranking or precedence of the elements in a supervisory system. For example, a lower ranking element such as a local controller affects only one variable while a higher ranking element such as a computer might affect many variables.
Hysteresis: Difference between upscale and downscale output in instrument response when subjected to the same input approached from opposite directions.
Integral (Reset) Control: A control algorithm which attempts to eliminate the offset (caused by proportional control) between the measurement and setpoint of the controlled process variable. See also "Control Action, Integral (Reset)."
Integral (Reset) Time: The proportionality constant in the equation relating the controller output to the error for integral control CO = Kiò(PV – SP) dt. , where: Ki = Kp/Ti. Kp is the integral gain of the controller. Ti is the time required to produce a change in controller output equal to the change in error input.
Integral Windup/Windown: Also called "controller windup/windown" or "reset windup." Saturation of the controller output at its maximum positive or negative value due to an error signal existing for an excessive period of time. Can be caused by the controller being left on "automatic" when the measurement transmitter is out of service.
Intrinsically Safe: Refers to equipment or wiring which is incapable of releasing sufficient electrical or thermal energy under either abnormal or normal conditions to cause ignition of a specific hazardous atmospheric mixture in its most easily ignited concentration.
I/P Transducer: (Current-to-pneumatic). A device which converts an electrical current signal to a proportional pneumatic signal for the purpose of interfacing electronic and pneumatic parts of a control system. A typical I/P transducer might convert a 4-20 mA signal from an electronic controller to a 3-15 psig signal to actuate a pneumatic valve.
I/O Devices: Input/output devices used to enter data into and receive data from a computer or control system. Examples are analog and digital input and output devices for handling process measurements and conditions as well as "business" type devices such as terminals, printers, plotters, etc.
Noise: In process instrumentation, an unwanted component of a signal or variable. Noise may be expressed in units of the output or in percent of output span.
Offset: The steady-state deviation of the controlled variable from the set-point, usually caused by a disturbance or a load change in a system employing a proportional-only controller such as a level controller. Offset will eventually be reduced to zero by the integral action in a PI or PID controller.
P Controller: A controller which produces proportional control action only.
Parallel Data: Data transmission where all data bits of a data word are processed at once.
PD Controller: A controller which produces proportional plus derivative (rate) control action.
PI Controller: A controller which produces proportional plus integral (reset) control action. PID Controller: A controller which produces proportional plus integral (reset) plus derivative (rate) control action.
PROM (Programmable Read-Only Memory): A device with information placed into it during manufacture that cannot be altered by the computer. It can, however, be reprogrammed using special equipment.
Proportional Band: The change in the controller error signal required to produce a full range change in output due to proportional control action. It is the reciprocal of gain expressed as a percentage: PB(%) = 100/k.
Proportional Control: A mode of control using an algorithm which causes the output of a controller to change in a linear fashion as the error signal (process variable – setpoint difference) changes. See also "Control Action, Proportional."
RAM (Random Access Memory): Memory which contains no pre-programmed information but is loaded and/or altered by the computer system. It is of a "volatile" nature in that all the contents are lost when electrical power is removed. RAM memories are usually provided with battery backup power systems, making it "non volatile."
Ramp: An increase or decrease of a variable at a constant rate of change with respect to time. Reset Rate: The inverse of integral time; usually expressed as "repeats per minute."
ROM (Read-Only Memory): Memory with information placed into it during manufacture that cannot be altered. Serial Communications: Sending bits of information in succession along a single circuit (pair of wires).
Setpoint: The desired value at which a process variable is to be controlled.
Software: A set of programs and associated data tables which causes the hardware components of a computer system to perform the desired tasks.
Split-Ranging: Action in which two or more final control elements are actuated by a single controller output. For example, in a heating circuit, 0-50% of the controller output operates a primary heat source and the 50-100% portion of the controller output operates a secondary heat source.
Steady-State: The condition when all process properties are constant with time, transient responses having died out. Supervisory Control: A method of computer control whereby a computer or master station provides setpoints to individual controllers which independently perform the actual control algorithms.
Telemetry: A technique which permits a measured quantity to be transmitted and interpreted at a distance from the measuring location. Form, or types of telemetry include analog, digital, frequency, and pulse.
Transmitter: A device that converts a process measurement (pressure, flow, level, temperature, etc.) into an electrical or pneumatic signal suitable for use by an indicating or control system.
Word, Computer: A group of bits treated as a unit and capable of being stored in one computer location. Some common word lengths are 8 bits, 16 bits, and 32 bits.
Actuator: A device which accepts the output from a control system and moves a final control element (usually a valve) to change a process condition. See also "Final Control Element."
Adaptive Control: Method of control whereby tuning (response) of the control system is varied with the process conditions, unlike other control where tuning is manual and remains constant.
Algorithm: Mathematical representation of the action performed by a controller such as proportional, integral, derivative,or combinations of those modes.
Alphanumeric: A character set that contains both letters and digits and perhaps other characters such as punctuation marks.
Analog Computer: A computing device comprised of functional modules such as amplifiers, multipliers, dividers, etc., interconnected in such a way as to facilitate the solution of a set of mathematical expressions or to implement some control strategy. The input to and the output from an analog computer are continuous signals as contrasted with a digital computer which updates an output every scan.
Attenuation: An increase or decrease in signal magnitude between two points or between two frequencies.
Baud Rate: The number of bits or discrete pieces of information transmitted per second.
Bit: Abbreviation for "binary digit." A single character in a binary number, represented by zero (0) or one (1).
Byte: The number of adjacent binary digits operated upon as a unit.
Cascade Control: Controllers arranged such that the output of one controller manipulates the setpoint input of a second controller instead of manipulating a process variable directly.
Control Action, Derivative (Rate): Control action in which the controller output is proportional to the rate of change of the input.
Control Action, Integral (Reset): Control action in which the controller output is proportional to the time integral of the error signal.
Control Action, Proportional: Control action in which the controller output has a linear relationship to the error signal.
Controller: A device which receives a measurement of the process variable, compares that measurement with a set point representing the desired control point, and adjusts its output based on the selected control algorithm to minimize the error between the measurement and the setpoint. If an increase in the measured process variable above the setpoint causes an increase in the magnitude of the controller output, the controller is said to be "direct acting." If a process variable increase above the setpoint causes a decrease in the magnitude of the controller output, the controller is "reverse acting."
Data Base: A collection of values for process variables, setpoints, scaling factors, control parameters, limits, constants, identifiers, etc. for access by the application programs in a computer-based control system. Data Highway: A high-speed serial or parallel data path which connects several units of a distributed control or data collection system.
DDC: Direct Digital Control. A control technique in which a digital computer is used as the sole controller and its output is used to set the final control element. This is in contrast to supervisory control.
Dead Band: The range through which an input may vary without changing the output. In a mechanical instrument such as a meter movement or strip-chart recorder, the dead band is caused by friction and slack or "play" in the readout mechanism. In a controller, dead band is a zone around the setpoint in which the measurement may vary without initiating a compensating controller response.
Dead Time: The interval of time lag between the initiation of a controller output or stimulus and the start of the resulting observable process response.
Dedicated Control: Using one controller to control one process variable.
Derivative Control: A mode of control using an algorithm which anticipates when a process variable will reach its desired control point by sensing its rate of change. This allows a control change to take place before the process variable overshoots the desired control point. See also "Control Action, Derivative (Rate)."
Derivative Time: The time difference by which the output of a proportional-derivative (PD) controller leads the controller input when the input changes linearly with time.
Digital Computer: An electronic machine for performing calculations on discrete quantities of data. Usually includes bulk storage devices such as disks, tape units, etc., in addition to internal memory. Also includes devices for printing and/or displaying output data.
Distributed Control System: Any control system in which the degradation or failure of any single element will affect only the control loop, or related loops, in which it operates.
EPROM (Erasable Programmable Read-Only Memory):
A memory device with information placed into it during manufacture that cannot be altered by the computer. It can only be erased and reprogrammed with special equipment.
Error Signal: The signal resulting from the difference between the setpoint reference signal and the process variable feedback signal in a controller.
Feedback Control: A type of control whereby the controller receives a feedback signal representing the condition of the controlled process variable, compares it to the setpoint, and adjusts the controller output accordingly.
Feedforward Control: A type of control which takes corrective action based on disturbances before the process variable is upset.
Final Control Element: That component of a control system (such as a valve) which directly changes the manipulated variable.
Gain: The ratio of change in output divided by the change in input that caused it. Both input and output must be in the same units; hence gain is a dimensionless number.
Hierarchy: The ranking or precedence of the elements in a supervisory system. For example, a lower ranking element such as a local controller affects only one variable while a higher ranking element such as a computer might affect many variables.
Hysteresis: Difference between upscale and downscale output in instrument response when subjected to the same input approached from opposite directions.
Integral (Reset) Control: A control algorithm which attempts to eliminate the offset (caused by proportional control) between the measurement and setpoint of the controlled process variable. See also "Control Action, Integral (Reset)."
Integral (Reset) Time: The proportionality constant in the equation relating the controller output to the error for integral control CO = Kiò(PV – SP) dt. , where: Ki = Kp/Ti. Kp is the integral gain of the controller. Ti is the time required to produce a change in controller output equal to the change in error input.
Integral Windup/Windown: Also called "controller windup/windown" or "reset windup." Saturation of the controller output at its maximum positive or negative value due to an error signal existing for an excessive period of time. Can be caused by the controller being left on "automatic" when the measurement transmitter is out of service.
Intrinsically Safe: Refers to equipment or wiring which is incapable of releasing sufficient electrical or thermal energy under either abnormal or normal conditions to cause ignition of a specific hazardous atmospheric mixture in its most easily ignited concentration.
I/P Transducer: (Current-to-pneumatic). A device which converts an electrical current signal to a proportional pneumatic signal for the purpose of interfacing electronic and pneumatic parts of a control system. A typical I/P transducer might convert a 4-20 mA signal from an electronic controller to a 3-15 psig signal to actuate a pneumatic valve.
I/O Devices: Input/output devices used to enter data into and receive data from a computer or control system. Examples are analog and digital input and output devices for handling process measurements and conditions as well as "business" type devices such as terminals, printers, plotters, etc.
Noise: In process instrumentation, an unwanted component of a signal or variable. Noise may be expressed in units of the output or in percent of output span.
Offset: The steady-state deviation of the controlled variable from the set-point, usually caused by a disturbance or a load change in a system employing a proportional-only controller such as a level controller. Offset will eventually be reduced to zero by the integral action in a PI or PID controller.
P Controller: A controller which produces proportional control action only.
Parallel Data: Data transmission where all data bits of a data word are processed at once.
PD Controller: A controller which produces proportional plus derivative (rate) control action.
PI Controller: A controller which produces proportional plus integral (reset) control action. PID Controller: A controller which produces proportional plus integral (reset) plus derivative (rate) control action.
PROM (Programmable Read-Only Memory): A device with information placed into it during manufacture that cannot be altered by the computer. It can, however, be reprogrammed using special equipment.
Proportional Band: The change in the controller error signal required to produce a full range change in output due to proportional control action. It is the reciprocal of gain expressed as a percentage: PB(%) = 100/k.
Proportional Control: A mode of control using an algorithm which causes the output of a controller to change in a linear fashion as the error signal (process variable – setpoint difference) changes. See also "Control Action, Proportional."
RAM (Random Access Memory): Memory which contains no pre-programmed information but is loaded and/or altered by the computer system. It is of a "volatile" nature in that all the contents are lost when electrical power is removed. RAM memories are usually provided with battery backup power systems, making it "non volatile."
Ramp: An increase or decrease of a variable at a constant rate of change with respect to time. Reset Rate: The inverse of integral time; usually expressed as "repeats per minute."
ROM (Read-Only Memory): Memory with information placed into it during manufacture that cannot be altered. Serial Communications: Sending bits of information in succession along a single circuit (pair of wires).
Setpoint: The desired value at which a process variable is to be controlled.
Software: A set of programs and associated data tables which causes the hardware components of a computer system to perform the desired tasks.
Split-Ranging: Action in which two or more final control elements are actuated by a single controller output. For example, in a heating circuit, 0-50% of the controller output operates a primary heat source and the 50-100% portion of the controller output operates a secondary heat source.
Steady-State: The condition when all process properties are constant with time, transient responses having died out. Supervisory Control: A method of computer control whereby a computer or master station provides setpoints to individual controllers which independently perform the actual control algorithms.
Telemetry: A technique which permits a measured quantity to be transmitted and interpreted at a distance from the measuring location. Form, or types of telemetry include analog, digital, frequency, and pulse.
Transmitter: A device that converts a process measurement (pressure, flow, level, temperature, etc.) into an electrical or pneumatic signal suitable for use by an indicating or control system.
Word, Computer: A group of bits treated as a unit and capable of being stored in one computer location. Some common word lengths are 8 bits, 16 bits, and 32 bits.