Thermostat working principle and various
types of differences
Thermostat working principle
In order to precisely control the process temperature without human intervention, a controller is required for the temperature control system. The controller receives an input signal from a temperature sensor such as a thermocouple or RTD, compares the actual temperature with the required control temperature (also referred to as a set value), and finally transmits the output signal to the control element. The controller is part of the overall control system, so when selecting the appropriate controller, the entire system should be analyzed. When selecting a controller, consider the following factors:
1. Enter the type of sensor (thermocouple, RTD) and temperature range
2. Required output type (electromechanical relay, SSR, analog output)
3. Required control algorithm (on/off, ratio, PID)
4. Type and amount of output (heating, cooling, alarm, limit)
Differences and working principles of different types of controllers
There are three basic types of controller: switch, ratio, and PID. Depending on the system being controlled, the operator can use one of the types for process control.
On/off control
The switch controller is the simplest type of temperature control device. The output of this type of device is not on or off, and there is no intermediate state. Only when the temperature crosses the set value, the switch controller will switch the output. In the heating control, the ON signal is output when the temperature is lower than the set value, and the OFF signal is output when the temperature is higher than the set value. Every time the temperature crosses the set value, the controller switches the output state, so the process temperature will continue to cycle from the set value to below, then fall back below the set value. In order to prevent damage to the contactor and the valve due to the excessively high circulation speed, a switch difference, also called “hysteresis”, should be added to the operation of the controller. With this mechanism, the output will not turn off or on again until the temperature exceeds a certain level. In this way, when the temperature fluctuates around the set value, the output can be prevented from “jittering” or switching quickly and frequently. Switching control is typically used in applications where there is no need for precise control, systems that can not handle frequent heat source switching, systems with large mass and very slow temperature changes, and temperature alarms. The limit controller is a special type of switch control for alarms. This type of controller uses a latching relay that must be manually reset to shut down the process when a certain temperature is reached.
Proportional control
Proportional control is designed to avoid repeated cycles in switch control. As the temperature approaches the set point, the proportional controller will reduce the average power provided to the heater. This delays the heating rate of the heater so that the temperature does not exceed the set value, but approaches the set point and maintains a stable temperature. This proportional control can be achieved by controlling the on and off times. "Time proportional control" can change the ratio of "on" time to "off time" to achieve temperature control. The proportional control takes place within the "proportional band" around the setpoint temperature. Outside the proportional band, the controller is equivalent to a switching device and is only fully switched on (proportional band below) or completely off (proportional band above). However, within the proportional band, the controller output will be proportionally switched on and off depending on the difference between the measuring point and the setpoint. When the set value is reached (the midpoint of the proportional band), the on-off time ratio of the output is 1:1, that is, the on-time and the off-time are equal. If the temperature is close to the set value, the on-off time will change proportionally based on the temperature difference. If the temperature is lower than the set value, the output turns on longer; if the temperature is too high, the output turns off longer.
PID control
The third controller type introduces integration and derivative control based on proportional control, ie PID control. This controller combines proportional control with two other adjustment mechanisms to help the device automatically compensate for changes in the system. Integral and derivative adjustments are expressed in units of time and are referred to as "reset" and "rate" adjustments, respectively. The proportional, integral, and derivative terms must be adjusted or “tuned” by trial and error for specific systems. Among the three controller types, the PID controller has the highest accuracy and stability and is best suited for systems with relatively low mass and systems that are sensitive to changes in energy during the process. This type of controller is recommended for systems with frequent load changes and for automatic compensation by the controller due to frequent changes in setpoints, available energy, or controlled mass. OMEGA offers a variety of controllers that automatically auto-tune, that is, self-tuning controllers.
Standard size
Since the thermostat is usually installed in the dashboard, the panel must reserve a suitable space for the thermostat. To facilitate the replacement of different types of thermostats, most thermostats use a standard DIN size design. The most commonly used DIN dimensions are as follows.
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Thermostat type
Switch Controller: The switch controller is the simplest controller that can perform switch control operations.
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Self-tuning PID controller: The PID controller can implement very strict control, but the PID algorithm needs to be set. Self-tuning controllers can achieve this function.
Multi-loop controller: Each control loop usually contains one input and at least one output. OMEGA offers a variety of multi-loop controllers that can handle multiple control loops. OMEGA's CN1507 can handle up to seven control loops.
Safety Limit Controller: The safety limit controller is a switch controller that uses a latched output. When the output status changes, manual reset is required to recover. Safety limit controllers are often used as redundant controllers to close the process when undesired limits are reached.
Temperature Switches: Adjustable temperature switches are suitable for applications that require economical temperature control solutions. Temperature switches are usually not very complicated and easier to set up than more complex electronic controls.
