Principle of Temperature Controller
Industrial thermostats are generally divided into two categories: one is mechanical thermostat (such as electrical contact mercury thermometer, electrical contact bimetallic thermometer, temperature control switch) and the other is electronic thermostat. Mechanical thermostat is being phased out, and the application of electronic thermostat is widely introduced here in detail. The electronic thermostat consists of four main mechanisms: conversion display mechanism, setting mechanism, comparative operation mechanism and output mechanism. When the temperature sensor converts the field temperature into electric signal (_or mV) and transmits it to the thermostat, the conversion display mechanism of the thermostat converts the electric signal into digital display or analog indication, and the setting value of the internal and the setting mechanism is carried out through the comparative mechanism. After comparison (or PID operation), the controller (contactor, solid-state relay, power controller) is output through the output mechanism, and then the heater/cooler is controlled by the controller.
Simple Temperature Controller
Fixed input signal
(2) Fixed temperature display range
(3) On/OFF control (PI, PD)
(4) Generally speaking, it is necessary to point out when ordering relay output (SSR, 4-20mA).
(5) Generally, only one alarm can be selected, and only deviation alarm can be used.
Display mode: pointer indication or digital display
Setting mode: knob setting or finger setting
Intelligent PID Temperature Controller
(1) Input signal is generally universal input (individual orders need to be specified)
(2) Temperature range can be chosen freely.
(3) Free control mode: ON/OFF and PID can be freely chosen.
(4) Controlling function: Heating/cooling can freely choose internal parameter setting.
Control period, input correction and return difference can be set freely
Alarm setting value and alarm mode are optional.
_Relay, SSR and 4-20mA should be selected when ordering.
No need to point out that digital display and touch keys are fixed.
Relatively speaking, the two kinds of panel sizes are 48-times, 48, 72, 48, 96, 96 (individual 48, 24, 160, 80) are the most commonly used: 48 *48.
3. Function and Application
Because of its simple operation and application, this article does not give a detailed introduction. The following mainly introduces the intelligent function description, application and precautions, and some common sense and simple sharing.
Input refers to the graduation number of the temperature sensor matched with it (international simple English code). Generally, there are two types (more than a dozen) of thermocouples and thermal resistors. Generally, internal parameters are optional, but some manufacturers need to specify input type when ordering. Most manufacturers can also accept standard signals such as 4-20mA and 1-5VDC of various transmitters, and display values can be self-calibrated and used as humidity and pressure meters. Autolise AT48A series also has this function. In addition to choosing the correct wiring, we should also pay attention to that the thermocouple is two-wire system (positive and negative poles can not be connected back), and the thermal resistance is generally three-wire system (A, B, B). In order to ensure the accuracy of temperature measurement, the thermocouple wire should be connected to the wiring terminal of the instrument or extended to the wiring terminal of the instrument through compensating wire, while the wiring of the thermal resistance to the instrument should be as small as possible, and the resistance of the three wires should be kept as low as possible. To be the same size, if the two-wire thermal resistance, then instrument B, B short connection
Display is generally four-digit LED display, resolution can be set, and display accuracy is different from different manufacturers, generally 0.1%, 0.2%, 0.3%, 0.5%; indicating the allowable error value = +actual measurement value * accuracy level, so the accuracy has nothing to do with the instrument range.
The main output is selected according to the operator (control heater), and there are three main types of output: relay, solid state relay and 4-20mADC. Most manufacturers have only one main output.
Control mode is for the main output, internal parameters are optional, mainly ON/OFF, PID regulation two kinds. Simply speaking, ON/OFF control is the display value (PV) once higher or lower than the main set value (SV) for interruption and on-off control, and the unique thermal inertia of temperature control makes this control keep the temperature fixed in the set value, within a certain range, so the accuracy is low, while PID control. Adjustment is based on the main set value (SV) as the goal, according to the size of thermal inertia to carry out ahead, lag or heating time of the proportional, differential, integral integrated control, through self-tuning two back and forth, will be better with the set value (SV) to maintain balance, this control method has high precision, so this control method is generally chosen.
The control function is for the main output, that is, heating/cooling control, general internal parameters can be selected (only heating control in some cases), heating/cooling control output is the opposite, the wrong choice will lead to complete temperature runaway, most of the cases using heating control.
The control backlash is generally aimed at ON/OFF control mode, in order to prevent frequent action of output contacts due to the measurement input value (PV) approaching the set value (SV) and fluctuation, so as to reduce the service life of mechanical switches such as relays or contactors. The contact on/off equivalent to the main output is based on the set value (SV)+backlash, which is called bilateral backlash. There are also unilateral backlash, that is, setting value (SV) "+" or "-" backlash value is action value. The larger the backlash value, the lower the control precision and the smaller the backlash value, the higher the control precision, but the shorter the switching life, and set according to the actual situation.
The control cycle refers to the proportional period. The shorter the control cycle, the higher the control precision. But for the system where the operator is a relay or contactor, it should not be too short. Because of frequent action, the life of the operator is shortened, usually 20 seconds.