Design of automatic resistance furnace temperature controller based on single chip microcomputer PID control

This resistance furnace temperature controller is mainly based on the single-chip microcomputer STC89C52. It collects the hot end temperature through the S-type thermocouple and performs cold end temperature compensation to obtain the actual temperature T. Then it is converted into digital by the amplifier circuit and ADC0832, and linearized. After that, it is controlled by the single-chip microcomputer STC89C52, and the adjustment of PID parameters is added. Finally, the temperature value, PID output value and parameter value are displayed through the LCD1602. It uses the simplest hardware unit and software programming to realize the functions of temperature acquisition, temperature PID control, temperature display, upper and lower limit alarms, PID parameter display, PID parameter modification, etc.

Contents Abstract Contents
Chapter 1 Introduction 1.1 Thermocouple 1.1.1 Thermocouple and its working principle 1.1.2 Cold end temperature compensation of thermocouple 1.1.3 Classification of thermocouple 1.2 PID 1.2.1 Introduction to PID 1.2.2 Introduction to PID algorithm 1.2.3 Position PID algorithm Chapter 2 Overall design of resistance furnace temperature controller 2.1 Functional requirements 2.2 Hardware design 2.3 Software design Chapter 3 Hardware design of resistance furnace temperature controller 3.1 S-type thermocouple 3.1.1 S-type thermocouple and its graduation table 3.1.2 S-type thermocouple linearization processing 3.2 Single chip microcomputer STC89C52 and its peripheral interface circuit 3.2.1 Single chip microcomputer STC89C52 3.2.2 Single chip microcomputer STC89C52 peripheral interface circuit 3.3 ADC0832 3.4 LCD1602 display Chapter 4 Software design 4.1 Main program of resistance furnace temperature controller 4.2 ADC0832 data acquisition subroutine 4.3 Thermocouple linearization scale conversion program 4.4 PID control program 4.5 PID parameter modification subroutine 4.6 LCD display subroutine Chapter 5 Summary

Chapter 1 Preface

Before using the resistance furnace temperature controller, we need to understand its manufacturing process and the overview of some key components so that we can deal with some minor problems that arise by ourselves or provide help for enthusiasts to learn. Therefore, in this chapter, we will introduce the basic knowledge about the application of the resistance furnace temperature controller.

1.1 Thermocouple 1.1.1 Thermocouple and its working principle

Thermocouples are key components in temperature measuring instruments. They have been used in various industries due to their simple structure, stable properties, wide temperature measurement range, high measurement accuracy, good dynamic performance, easy use and maintenance. In particular, due to their stable properties, thermocouple temperature measurement plays a very important role in the measurement of high temperature environments.

The working principle of thermocouples is mainly based on the thermoelectric effect. The so-called thermoelectric effect refers to the two ends of two conductors of different components (called thermocouple wires or hot electrodes) connected to form a loop. When the temperatures of the two junctions are different, an electromotive force will be generated in the loop. The end that directly measures the medium is called the working end, and the other end is called the cold end.

1.1.2 Cold junction temperature compensation of thermocouple

From the temperature measurement principle of thermocouples, we can know that the size of the thermoelectric potential is not only related to the hot end temperature, but also to the cold end temperature. Only when the cold end temperature is constant can the hot end temperature be judged by the size of the thermoelectric potential.

The cold end temperature compensation methods include ice point method, constant temperature migration method, calculation correction method, bridge compensation method, and software compensation method. Here we use the calculation correction method:

The calculation correction method is based on the intermediate temperature law, and its calculation formula is as follows:

 EAB（T，0）=EAB（T，T0）+EAB（T0，0）                （1.1）

1.1.3 Classification of thermocouples

According to the requirements of industrial standardization, thermocouples can be divided into standardized thermocouples and non-standardized thermocouples.

Standardized thermocouples refer to thermocouples with mature technology, mass production, stable performance, wide application, unified scale and listed in international and national standard documents. Non-standardized thermocouples refer to thermocouples that are not yet mature enough in research. Although there are products and they can be used, there is no unified scale and they need to be calibrated individually, which brings inconvenience to our use. Therefore, we do not use and research them on a daily basis.

Standardized thermocouples include platinum-rhodium 10-platinum thermocouple (S type), platinum-rhodium 30-platinum-rhodium 6 thermocouple (B type), nickel-cadmium-nickel-silicon thermocouple (K type), nickel-cadmium-copper-nickel thermocouple (E type), and copper-copper-nickel thermocouple (T type). We focus on K-type and S-type thermocouples:

S-type thermocouple is a precious metal thermocouple, the chemical composition of its positive conductor is platinum-rhodium alloy, which contains 10% rhodium and 90% platinum. The negative conductor is pure platinum.

S-type thermocouples have the advantages of high accuracy, wide temperature measurement range, long service life and good stability. In particular, they have good anti-oxidation performance at high temperatures, so our product resistance furnace temperature controller uses thermocouples as temperature sensors.

K-type thermocouple is the cheapest metal thermocouple used at present. The chemical composition of its positive electrode is: Ni:Cr=90:10; the chemical composition of its negative electrode is: Ni:Si=97:3, and the measuring temperature is -200℃~1300℃.

1.2 PID1.2.1 Introduction to PID

PID control is the most widely used control law in engineering applications. Its main features are simple structure, good stability, reliable operation and convenient adjustment. The characteristics of its various parameters are as follows:

The role of proportional regulation is to reflect the deviation of the system in proportion. Once the system has a deviation, proportional regulation will immediately produce a regulating effect to reduce the deviation. A large proportional effect can speed up the regulation and reduce the error, but an excessively large proportional effect will reduce the stability of the system and even cause system instability. The role of integral regulation is to eliminate the steady-state error of the system, because once there is an error, integral regulation will take effect until there is no error, and the output of integral regulation remains constant. The role of differential regulation is to reflect the rate of change of the system deviation signal. It has predictiveness and can predict the trend of deviation change. Therefore, it can produce an advanced control effect, so that the deviation is eliminated by the differential regulation before it is formed. Therefore, the differential action can improve the dynamic performance of the system.

1.2.2 Introduction to PID Algorithm

PID algorithms have two types of outputs: positional and incremental. The result of the incremental PID algorithm output is an increment, that is, the control amount that needs to be increased (negative value means decrease) on the basis of the last control amount. For example, in a thyristor motor speed control system, the increment of the control amount means the amount by which the trigger phase of the thyristor needs to be advanced or delayed on the original basis. The output of the position algorithm is expressed as where the current trigger phase should be. For example, in a temperature control system, the incremental algorithm is expressed as the proportion of power-on time that needs to be increased or decreased based on the last power-on time proportion; the output of the position algorithm directly indicates how long the power will be on during this cycle.

1.2.3 Position PID algorithm

The position PID algorithm can directly indicate how long the power needs to be turned on, so it is widely used. Its calculation formula is as follows:

                  （1.2）

Where , is the basic deviation, which indicates the difference between the current measured value and the set target value, and the result can be positive or negative. When the set target is used as the minuend, a positive number indicates that the set value has not been reached. A negative number indicates that the set value has been exceeded. The cumulative deviation is the algebraic sum of each deviation value. , and are the three control parameters of the PID algorithm, which are called proportional constant, integral constant and differential constant, respectively. Different values ​​are selected for different control objects, and field adjustment is required to obtain better results.

Chapter 2 Overall Design of Resistance Furnace Temperature Controller

This resistance furnace temperature controller is mainly based on the single-chip microcomputer STC89C52. It collects the hot end temperature through the thermocouple and performs cold end temperature compensation to obtain the actual temperature T. Then it is converted into digital by the amplifier circuit and ADC0832, and linearized. After that, it is controlled by the single-chip microcomputer STC89C52, and the adjustment of PID parameters is added. Finally, the temperature value, PID output value and parameter value are displayed through the LCD1602. It uses the simplest hardware unit and software programming to realize the functions of temperature acquisition, temperature PID control, temperature display, upper and lower limit alarms, PID parameter display, PID parameter modification, etc.