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Temperature controllers accept inputs from temperature sensors or thermometers, and output a control signal to keep the temperature at the desired level. Temperature controllers use several different control techniques. Limit control establishes set points that, when reached, sends a signal to stop or start a process variable. Linear control matches a variable input signal with a correspondingly variable control signal. Feedforward control does not require a sensor and provides direct control-compensation from the reference signal.
Proportional, integral and derivative (PID) control requires real-time system feedback. PID control monitors the error between the desired variable value and the actual value, and adjusts the control accordingly. Fuzzy logic is a control technique in which variables can have imprecise values (as in partial truth) rather than a binary status (completely true or completely false). Temperature controllers that use advanced or non-linear controls such as neural networking, adaptive gain, or emerging algorithms are also available.
Specifications for temperature controllers include number of inputs, number of outputs, input types, output types, and number of zones (if applicable). The number of inputs is the total number of signals sent to the temperature controller. The number of outputs is the sum of all outputs used to control, compensate or correct the process. Input types for temperature controllers include direct current (DC) voltage, current loops, analog signals from resistors or potentiometers, frequency inputs, and switch or relay inputs. Output types include analog voltage, current loops, switch or relay outputs, and pulses or frequencies. Some temperature controllers can also send inputs or receive outputs in serial, parallel, Ethernet or other digital formats which indicate a process variable. Others can send inputs and receive outputs from information converted to an industrial fieldbus protocol such as CANbus, PROFIBUS®, or SERCOS.
Temperature controllers differ in terms of user interface features and regulatory compliance. Many temperature controls feature a digital front panel or analog components such as knobs, switches, and meters. Computer-programmable, web-enabled, and Ethernet or network-ready temperature controllers are also available. In terms of compliance, a temperature control that is destined for sale in the European marketplace should meet the requirements of the Restriction of Hazardous Substances (RoHS) and Waste Electrical and Electronics Equipment (WEEE) directives from the European Union (EU).