In the field of industrial automation, temperature controllers are a very common type of control device. For example, they are used in high-speed PID control in the packaging machinery industry, heating and cooling control in plastic film extrusion machines, and solenoid valve control in continuous combustion furnaces, among other applications. Temperature controllers play a key role in all of these applications.
How a Thermostat Works
A thermostat works primarily by collecting temperature readings from a temperature sensor, determining the difference between the measured temperature and the setpoint, and then sending a signal to a relay to initiate heating or cooling, thereby keeping the system at a constant temperature. Below is a simple schematic diagram of a thermostat.
Thermostat Parameters
PV: The ambient temperature detected by the thermostat
SV: The desired ambient temperature
PID: Proportional (P), Integral (I), and Derivative (D) time constants.
P: In PID control, this represents the proportional relationship between the magnitude of the control output and the deviation between the measured value and the setpoint. The greater the deviation, the larger the output. A higher setting for the instrument's proportional parameter P results in lower control sensitivity and higher stability; a lower setting for P results in higher sensitivity and lower stability.
I: The primary purpose of the integral operation is to eliminate steady-state error. As long as a deviation exists, the integral action will move the control variable in the direction that eliminates the deviation. The integral time is the unit representing the strength of the integral action; the shorter the integral time set on the instrument, the stronger the integral action.
D: The primary function of the derivative action is to correct the output based on the rate of change of the deviation, enabling the control process to return to its original state as quickly as possible. The derivative time is the unit representing the intensity of the derivative action; the longer the derivative time set on the instrument, the stronger the instrument's derivative correction on the controlled variable.
Relay Output: Relay outputs can drive contactors or heaters
Voltage Output: Voltage output can drive solid-state relays.
Linear Current Output: Analog output can directly drive a variable-frequency drive or provide feedback to a controller, which then drives the load.
Event Input: Event input refers to digital input.
Heater Open-Circuit Alarm: A heater open-circuit alarm is a device that monitors the heating status of a temperature control system, automatically monitors the heater current to determine whether the heater is open-circuited, and outputs a digital signal.