Abstract: This paper introduces the working principle of AT89C52 microcontroller as the core of resistance furnace temperature control system of industrial components, the new detail element, analyzes the hardware structure of the system, finally the system flow chart are given.
Key words: single chip microcomputer; industrial resistance furnace; temperature measurement; control system

1 Introduction
Industrial resistance furnace is widely used in chemical industry and metallurgy industry, so temperature control is very important in industrial production and scientific research. The control system belongs to the first order pure lag, which has the characteristics of large inertia, pure lag and nonlinearity, which leads to the overshoot of the traditional control mode, long regulating time and low control precision. SCM temperature control, has the advantages of simple circuit design, high accuracy, good control effect, has important practical significance to improve the production efficiency and promoting the progress of science and technology. The main technical indexes of the temperature control system introduced in this paper are: temperature control range: 300 to 1000 degrees Celsius; constant temperature time: 0 ~ 24 hours; control precision: + 1 C; overshoot <1%.

2 overall design and system principles
The system is composed of single-chip microcomputer AT89C52, temperature detection circuit, keyboard display and alarm circuit, clock circuit, temperature control circuit, etc.. The system uses a new type of components, strong function, high precision, simple hardware circuit. Its hardware schematic diagram shown in figure 1.
Fig. 1 Schematic diagram of hardware
In the system, making use of the thermocouple to measure the actual temperature of industrial resistance furnace and converted into millivolt voltage signal. The voltage signal through the temperature detection circuit is converted into digital signal corresponding to the furnace temperature into single-chip, single-chip microcomputer for data processing, through the LCD display temperature and determine whether the alarm at the same time, temperature and the set temperature, according to the PID algorithm set to calculate the control quantity, according to the amount of control by controlling the conduction of solid state relay on and off so as to control the resistance wire of the conduction time in order to achieve temperature control. The clock circuit of the system can be timed according to the requirement.

3 hardware design

3.1 temperature detecting circuit
This system uses K (nickel chromium nickel silicon thermocouple), its can measure the temperature of 1312 DEG C, the linearity is better, and the price is cheap. The output of K type thermocouple is millivolt voltage signal, to convert it to digital signal and CPU communication. The traditional temperature detection circuit adopts the "sensor - Filter - amplifier - Cold - end compensation - linear processing - A/D conversion" mode, the conversion of many links, the circuit is complex, low precision. In this system, the high precision integrated chip MAX6675 is used to complete the conversion of "thermocouple potential temperature", which is simple, high precision and low cost.
MAX6675 is a type K thermocouple converter developed by MAXIM company, integrated filter, amplifier, and a thermocouple disconnection detection circuit, with the cold end compensation, the potential of a type K thermocouple output directly into the 12 digit volume, resolution 0.25 C. Temperature data through the SPI port output to the microcontroller, its cold end compensation range is -20 ~ 80 degrees Celsius, the measurement range is 0 ~ 1023.75. Table 1 pin function for MAX6675.

Design of industrial resistance furnace temperature control system

Table 1MAX6675 pin function chart
Figure 2 for the system temperature detection circuit.
Design of industrial resistance furnace temperature control system

Fig. 2 temperature detecting circuit
When the P2.5 is low and the P2.4 port generates a clock pulse, the SO MAX6675 pin outputs the conversion data. In each pulse falling edge of the output of a pulse signal data, 16 complete a complete data output, the first output of high potential D15, the final output is low potential D0, D14-D3 for the corresponding temperature data conversion. When the P2.5 is high, the MAX6675 starts a new temperature conversion. In the application of MAX6675, should pay attention to its layout in the I/O chip away from other places, in order to reduce the impact of power supply noise; MAX6675 T- must be grounded, and the power of the chip are simulated, and don't confuse and affect the accuracy of digital readout chip.

3.2 clock circuit
In the system need to accurately display the temperature rise time, and so on, so choose a clock chip DS12887 constitute the timing circuit to complete the accurate timing of the time. DS12887 with the clock, alarm clock, 12/24 hours and automatic leap year compensation function; general RAM contains 10B clock control register and 4B status register and 114B; with programmable square wave output function; alarm interrupt, periodic interrupt, interrupt clock update by software or shielding test. When in use, the peripheral circuit is not required, and the peripheral interface is good. In this system, the DS12887 address / data multiplex bus is connected with the P0 port of the single chip microcomputer. Through the timer interrupt, CPU every 0.4 seconds to read a DS12887 internal time standard registers, get the current time, and sent to the LCD display. Whenever the industrial resistance furnace from one state to another state, DS12887 through the CPU to clear the time, restart the time. In addition, through the DS12887, you can also set the heating time and the constant temperature time of industrial resistance furnace. The circuit is shown in figure 3.
Design of industrial resistance furnace temperature control system
Figure 3 keyboard, clock, alarm and temperature control circuit

3.3 keyboard display and alarm circuit
This system uses the 3*3 keyboard, by the monolithic computer I/O port control, can pass the key to set the temperature and the time, some keys can realize different function under the different situation. The dot matrix character LCD display TC1602, the system will be extended chip 8155 P0 port, PC.0 ~ PC.2 port is connected to the TC1602 interface, TC1602 display format is 16*2, can display the temperature, setting time, real time etc.. The alarm circuit is connected with the I/O port of the single chip microcomputer and the driving chip MC1413, and the buzzer is driven by the MC1413. Keyboard circuit and clock circuit as shown in figure 3.

3.4 temperature control circuit
The temperature control circuit comprises a driving chip MC1413, an over zero AC solid state relay (SSR type Z). Alarm and temperature control circuit as shown in figure 3.
SSR type Z contains zero crossing detection circuit, when the control signal is added, and the load power supply voltage over zero, SSR can be switched on; and the control signal is disconnected, SSR at the junction of the positive and negative half cycle at the junction. That is, when the SSR type Z in 1 second for the full state, the trigger frequency is 100HZ; when the SSR type Z in a second conduction time of 0.5 seconds, the trigger frequency is 50HZ. In this system, the PID control algorithm is adopted to change the heating power of the industrial resistance furnace and the purpose of adjusting the temperature of the furnace by changing the conduction time of the SSR type Z in the unit time.

4 software design
In the software system, the main program to complete the initialization of the system and the electric stove wire turn-on and turn off; temperature measurement, keyboard input, display, timing and control algorithm by the subroutine to complete; interrupt service program timing measurement and reading time. The flow chart is shown in figure 4.
Design of industrial resistance furnace temperature control system
Fig. 4 program flow chart of control system

5 Conclusion
The system uses a K type thermocouple signal processing chip MAX6675, has changed the traditional temperature measurement circuit is complex, complicated procedures, low accuracy; the use of the clock chip can on time accurate timing; the use of advanced PID control algorithm control, high precision, small overshoot; a simple circuit design, setting multiple functions, simple operation. After repeated experiments, it is proved that it has strong stability, high precision, strong practicability, good control effect and wide application prospect.
The author of this article innovation: changing the temperature detection circuit is used in the traditional "sensor - Filter - amplifier - cold end compensation linearization A/D conversion mode, using the high precision MAX6675 chip to complete the conversion of thermocouple temperature", without external circuit, simple wiring, high precision; the use of the clock chip to facilitate accurate timing, reduce the burden of MCU; the use of advanced PID control algorithm control, good control effect, small overshoot.