A temperature controller is part of a measurement and control system and is responsible for controlling the temperature in an installation. Temperature controllers are often used together with temperature sensors. Besides the standard on/off thermostat function, temperature controllers have a PID function. Thus, heating processes can not only be started or stopped, but also maintained and controlled according to preset values.
A temperature controller regulates temperature. As part of a dynamic system, a temperature controller is responsible for maintaining or adjusting the temperature.
The temperature controller controls the difference between the measured value and the set value until the measured value equals the set value. They are often used in combination with Pt100 temperature sensors. The universal input of the temperature controller must be configured for this purpose. The Pt100 sensor is linearised in the controller. Apart from Pt100 (Pt500 or Pt1000) sensors, thermocouples and unit signals (0..+10V, 4..+20 mA) can be configured.
The classic digital output of the temperature controller is the relay output, as a make contact or changeover contact. The mechanical output is used for control tasks requiring low switching frequency (contactors, solenoid valves, etc.).
When using relay outputs, the lifetime of the contacts must be taken into account.
Example: the technical specifications of a temperature controller may contain the following information: contact life 350,000 trips at nominal load or 750,000 trips at 1A. The faster a process reacts over time, the higher the required switching frequencies.
A proportional temperature controller (P controller) forms the control deviation based on the set point and the actual value and increases this deviation by the proportional band Xp (Pb). The result is output as the output level.
Example: for a control deviation of 5K, a P controller for a temperature control path with an Xp of 10d igits produces an output level of 50%.
An integrating temperature controller (I-temperature controller) controls the magnitude of the controller's output based on the difference between the measured and set value (setpoint).
Example: If the setpoint value is set to 60 s and the control deviation is 2 K, the output level increases by 2% per 60 s.
An I-temperature controller with a relatively large I-time (tn, rt) relative to the process responds slowly. The controller builds up the output level slowly. The actual value moves very slowly towards the desired value.
An I-temperature controller with a small I-time (tn, rt) relative to the process builds up the output level quickly. If the actual value reaches the setpoint level, the output level of the controller has assumed too high a value. The power delivered to the process is too high and the actual value exceeds the target value.
PI controllers combine the advantages of both components: gain (P) and offload time (I). If a control deviation occurs with a PI controller, the P component provides the magnitude of the output change. The I component provides the speed between output changes until the control deviation is 0.
The P-component is the proportional band PB or XP and the i-component is the integration time RT or TN. ( parameters in English/German).
PI temperature controllers are used in a wide range of applications, such as HVAC systems, furnaces, refrigerators and laboratory equipment.
The D-time of a PD temperature controller responds to the change of the measured value.
When the measured value changes in the direction of the set point value and thus there is a risk of exceeding the measured value, the output change is slowed down with the d-action.
When the measured value moves away from the set point value, the output change is accelerated by the d-action.
A PID temperature controller works Proportional, Integrating and Differentiating. Here, the intensity of the individual parameters is adapted to the system to be controlled.
Example Industrial Furnace Construction:
The engineer sets a setpoint for the temperature on the PID controller. With this setpoint, the product should be heated in the oven. A resistance temperature sensor in the oven then measures the temperature and transmits it to the controller. The PID controller then independently controls the heating output. Such that the desired temperature in the oven is reached and maintained for a certain period of time.
By using a PID temperature controller, the oven has more extensive functionality than the standard electronic thermostat on/off function.