The difference between the reading of an instrument and the true value of what is being measured, expressed as a percent of full instrument scale.
Action
The function of a controller. Specifically, what is done to regulate the final control element to effect control. Types of action include ON-OFF, proportional, integral and derivative.
Active Device
A device capable of producing gain; for example, transistors, and ICs.
Alarm
A condition, generated by a controller, indicating that the process has exceeded or fallen below the limit point.
Ambient Temperature
The temperature of the immediate surroundings in which a controller must operate.
Analog Setpoint Indication
A dial scale to indicate setpoint as opposed to digital setpoint indication. The traditional clock face is a good example of analog indication.
Automatic Tuning
Sometimes referred to as "self-tuning." The ability of a control to select and adjust the three control parameters (Proportional, Integral, and Derivative) automatically via a complex algorithm. Generally no operator input is required.
Bandwidth
See "Proportional Band."
Bumpless Transfer
When transferring from auto to manual operation, the control output(s) will not change ("bumpless"- a smooth transition).
Closed Loop
A signal path which includes a forward path, a feedback path and a summing point, and forms a closed circuit.
Cold Junction Compensation
Measurement of temperature at thermocouple connections to controller and compensation for the "cold end" junction millivoltage generated here.
Common Mode
The noise signal that is common to all sensor wires.
Common Mode Rejection
The ability of an instrument to reject interference from a common voltage at its input terminals with relation to ground, usually expressed in dB.
Compensation
See "Cold Junction Compensation."
Control Point
See "Setpoint."
Cool Gain
In Athena microprocessor-based temperature controllers, a reference Gain value that is expressed in terms of the controller¹s Span, divided by the cooling proportional band, in degrees.
Current Proportioning
An output from a controller which provides current proportional to the amount of power required.
Cycle Time
The time necessary to complete a full ON-through-OFF period in a time proportioning control system.
Current Alarm
Provides an alarm signal when a current level is detected below or above a preselected level.
DV/DT
Rate of change of voltage over time. A rapidly rising voltage waveform could induce false firing of an SCR. MOV¹s or R-C Snubber Circuits are used to prevent this false firing.
Dead Band
The range through which an input can be varied without initiating observable response.
Derivative
The process by which a controller senses the rate of temperature change and alters output.
Deviation Alarm
An alarm referenced at a fixed number of degrees, plus or minus, from setpoint.
DIN
Deutsche Industrial Norms, a widely-recognized German standard for engineering units.
Differential
The temperature difference between the points at which the controller turns the heater on and off. Typically used when discussing an on/off controller.
Direct Acting
Increase in value of output as the measured value increases.
Drift
A deviation of the system from setpoint that typically occurs over a long period of time. Drift may be caused by such factors as changes in ambient temperature or line voltage.
Droop
Occurs when the actual system temperature stabilizes at some value below the desired setpoint. If system droop is unacceptable, a common solution is the use of a control incorporating an automatic or manual reset feature.
Duty Cycle
Percentage of load "ON" time relative to total cycle time.
Feedback Controller
A mechanism that measures the value of the controlled variable, compares with the desired value and as a result of this comparison, manipulates the controlled system to minimize the size of the error.
Frequency Response
The response of a component, instrument, or control system to input signals at varying frequencies.
Gain
Amount of increase in a signal as it passes through any part of a control system. If a signal gets smaller, it is attenuated. If it gets larger, it is amplified.
Guaranteed Soak
On a ramp and soak controller, a feature that stops the clock if the temperature drops below a preset value, then continues the timing when the temperature recovers.
Heat Gain
In Athena microprocessor-based temperature controllers, a reference Gain value that is expressed in terms of the controller¹s Span, divided by the heating proportional band, in degrees.
Hysteresis
Temperature sensitivity between turn on and turn off points on on-off control. Prevents chattering.
I2T
A measure of maximum one time overcurrent capability for a very short duration. Value used for fuse sizing to protect SCRs.
Impedance
The total opposition to electrical flow in an ac circuit.
Integral Function
This automatically adjusts the position of the proportional band to eliminate offset.
Isolation
Electrical separation of sensor from high voltage and output circuitry. Allows for application of grounded or ungrounded sensing element.
Lag
The time delay between the output of a signal and the response of the instrument to which the signal is sent.
Latching Alarm
Requires operator intervention to reset even though the alarm condition on the input may have disappeared.
Metal Oxide Varistor (MOV)
A semiconductor device that acts as a safety valve to absorb high voltage transients harmlessly, thereby protecting the SCRs and preventing false firing.
Noise
An unwanted electrical interference.
Normal Mode Rejection
The ability of an instrument to reject interference; usually of line frequency across the input terminals (common mode).
Offset
A sustained deviation of the controlled variable from setpoint (this characteristic is inherent in proportional controllers that do not incorporate reset action). Also referred to as Droop.
On/Off Control
Control of temperature about a setpoint by turning the output full ON below setpoint and full OFF above setpoint in the heat mode.
Open Loop
Control system with no sensory feedback.
Output
Action in response to difference between setpoint and process variable.
Overshoot
Condition where temperature exceeds setpoint due to initial power up.
Parameter
A physical property whose value determines the response of an electronic control to given inputs.
PD Control
Proportioning control with rate action.
Phase
The time-based relationship between two alternating waveforms.
Phase-angle Firing
A form of power control where the power supplied to the process is controlled by limiting the phase angle of the line voltage as opposed to burst firing.
PI Control
Proportioning control with auto reset.
PID
Proportional, integral and derivative control action.
Positive Temperature Coefficient
A characteristic of sensors whose output increases with increasing temperature.
Process Variable
System element to be regulated, such as pressure, temperature, relative humidity, etc.
Proportional Action
Continuously adjusts the manipulated variable to balance the demand.
Proportional Band
The amount of deviation of the controlled variable required to move through the full range (expressed in % of span or degrees of temperature). An expression of Gain of an instrument (the wider the band, the lower the gain).
Proportioning Control Plus Derivative Function
A controller incorporating both proportional and derivative action senses the rate temperature change and adjusts controller output to minimize overshoot.
Proportioning Control Plus Integral
A controller incorporating both proportional and integral action.
Proportional, Integral and Derivative Control
A PID controller is a three-mode controller incorporating proportional, integral, and derivative actions.
Ramp
Automatic adjustment for the setpoint for the temperature increase or decrease from process temperature. The target value can be either above or below the current measured value. The ramp value is a combination of time and temperature.
Ramp To Setpoint
Allows the operator to enter a target time for the controller to reach setpoint.
Range
The difference between the maximum and the minimum values of output over which an instrument is designed to operate normally.
Rate(Action)
Control function that produces a corrective signal proportional to the rate at which the controlled variable is changing. Rate action produces a faster corrective action than proportional action alone. Also referred to as Derivative Action. Useful in eliminating overshoot and undershoot.
Resistor-Capacitor (RC) Snubber Circuit
Controls the maximum rate of change of voltage and limits the peak voltage across the switching device. Used to prevent false firing of SCRs.
Reference Junction
See "Cold Junction Compensation."
Reproducibility
The ability of an instrument to duplicate with exactness, measurements of a given value. Usually expressed as a % of span of the instrument.
Reset Action
Control function that produces a corrective signal proportional to the length of time and magnitude the controlled variable has been away from the setpoint. Accommodates load changes. Also called Integral Action.
Reverse Acting
Reduces the output as the measured value increases.
Radio Frequency Interference (RFI)
RFI is commonly generated by devices that switch the output power at some voltage other than zero. Typically, phase-angle fired SCRs may generate RFI while zero-cross fired SCRs virtually eliminate RFI.
Resistance Temperature Detector (RTD)
Typically a wire wound device that displays a linear change in resistance for a corresponding temperature change. An RTD has a positive temperature coefficient.
SCR
This term has two separate and distinct meanings: 1) A solid-state semiconductor component that conducts or resists current flow depending upon whether a trigger voltage is present at the gate terminal. 2) A complete power controller that utilizes SCRs or TRIACs as the switching devices to control current flow.
Segment
In a ramp and soak controller, one part of a profile.
Soak
One segment with no setpoint change.
Solid-State Relay (SSR)
Semiconductor device that switches electrical current on and off in response to an electrical signal at the control terminals.
Sensitivity
The minimum change in input signal required to produce an output change in the controller.
Series Mode
A condition in which a noise signal appears in series with a sensor signal. setpoint: The position to which the control point setting mechanism is set, which is the same as the desired value of the controlled variable.
Span
The difference between the top and bottom scale values of an instrument. On instruments starting at zero, the span is equal to the range.
Standby
Method of putting controller into the idle mode.
Surge Current
A high current of short duration that generally occurs when the power is first applied to inductive loads. The surge generally lasts no more than several ac cycles.
Thermistor
A bead-like temperature sensing device consisting of metallic oxides encapsulated in epoxy or glass. The resistance of a thermistor typically falls off sharply with increasing temperature, making it a particularly good sensing device. A thermistor has a negative temperature coefficient.
Thermocouple
The junction of two dissimilar metals. A small voltage is generated at this junction, increasing as its temperature rises.
Thermocouple Break Protection
Fail-safe operation that ensures output shutdown upon an open thermocouple condition.
Three-mode Control
Proportioning control with reset and rate.
Thyristor
Any of a group of solid-state controlling devices. These devices are referred to as TRIACs, SCRs and DIACs.
Time Proportioning Control Mode
In this mode, the amount of controller "on" time depends upon the system temperature. At the beginning of each time base interval, the signal from the sensor is analyzed and the controller is kept "ON" for a percentage of the time base.
Triac
A device, similar to a controlled rectifier, in which both the forward and reverse characteristics can be triggered from blocking to conducting (Also see Thyristor).
Zero Switching
Action that provides output switching only at the zero voltage crossing point of the ac sine wave.
