I. Introduction
Automatic control systems have a wide range of applications in various fields, especially in the industrial field. Temperature control is one of the most common types of control in control systems. With the rapid development of single-chip technology, the control of controlled objects through single-chip microcomputer has become an important development direction in the field of automatic control in the future. This design requires the design of an electric furnace temperature control system using a single chip microcomputer.
Second, the characteristics of the electric furnace temperature control system
The temperature control system is mainly composed of four parts: temperature sensor, temperature regulator, actuator and controlled object. The system structure diagram is shown in Figure 1. The controlled object is a large-capacity, large-inertia electric heating furnace temperature object, which is a typical multi-order volume late-time characteristic. It is often approximated as a second-order volume containing pure hysteresis in engineering; due to the large capacitance of the controlled object, The actuator of the thyristor is usually used as a regulator, and the specific circuit diagram is shown in FIG. 2 .
Characteristics of the actuator: The temperature regulation of the electric furnace is to change the ratio of the electric wire closing time Tb to the breaking time Tk by the intermittent action of the regulating agent (power supply energy), α=Tb/Tk.
Adjusting the temperature of the heating furnace, in the industry, by adjusting the load to two cycles within a set period, then turning off several cycles and changing the ratio of the thyristor switching time in the set period, The average AC voltage is the load power, which is commonly referred to as the power regulator or the cycle controller; the power regulator triggers the thyristor conduction when the power supply voltage crosses zero, so the complete sine wave is obtained on the load. It is only the voltage cycle that is turned on in the set period Tc. As shown in FIG. 3, it is assumed that the wave number of the period in which the period Tc is turned on is n, and the period of each cycle is T, and the output power of the power regulator is P=n×T×Pn/Tc, and Pn is the set period. The output power of the device when the voltage in Tc is fully passed.
Third, the electric heating principle of the electric furnace
When a current flows through a conductor, because any conductor has a resistance, the electrical energy forms a loss in the conductor and is converted into thermal energy. According to the Joule's law:
Q=0.2412 Rt Q—heat energy, card;
I-current, An 9
R a resistor, ohmic,
t a time, seconds.
According to the above formula, when 1 kWh of electric energy is converted into thermal energy, Q=(0.24×1000×36000)/1000=864 kcal.
In the electrothermal technology, it is calculated according to 1 kWh = 860 kcal. An electric furnace is structurally a device that converts electrical energy into thermal energy, which is effective
Used to heat specified workpieces and maintain high efficiency.
Fourth, the classification of electric furnace heating methods
Resistance furnaces are produced in different ways according to heat, and can be divided into two types: indirect heating and direct heating. Indirect heating type resistance furnace is a heating element made of a special resistance material inside the furnace. The heat is generated when the current passes through the heating element, and the charge placed in the furnace is heated by heat conduction, convection, and radiation. Direct heating type resistance furnace, the power supply is directly connected to the material to be heated, making it strong
A large current flows directly through the material to be heated to cause the material to heat itself up to achieve a heating effect. Most of the industrial resistance furnaces are indirectly heated, and only a part of them are directly heated by the special needs of the heating process.
Fifth, the hardware part of the electric furnace control system
The temperature regulator is the core part of the temperature control system. It is controlled by single-chip microcomputer and realizes intelligence. It is mainly composed of input channel, output channel, man-machine dialogue channel and some peripheral circuits. The block diagram is shown in Figure 4. Specifically, it consists of 8031 ​​MCU, 16K Erasing program memory, keyboard and display interface circuit, and parallel I/O chip 8255. It amplifies, compares, and calculates the temperature signal sent by the sensor, outputs a control signal, triggers the execution device, realizes automatic temperature control, and realizes functions of conversion, zero adjustment, and soft adjustment of amplitude modulation of various temperature sensors. In order to improve the anti-interference ability of the system, the temperature sensor signal should be grounded separately by a shielded wire. In addition, electromagnetic shielding measures are also applied to the host to prevent other electromagnetic interference.
(1) 8031 ​​chip: MCS-51 series MCU is an 8-bit MCU developed by Intel Corporation of the United States, and can be divided into multiple sub-series. The MCS-51 series MCU has 40 pins, including 32 I/O interface pins, 4 control pins, 2 power pins, and 2 clock pins. Pin description: P0.0～P0.7: P0 port 8-bit port line, the first function is used as general-purpose I/O interface, and the second function is used as address/data multiplex port P1.0~P1.7 when memory expansion : P1 port 8-bit line, the general I/O interface has no second function. P2.0~P2.7: P2 port 8-bit port line, the first function is used as a general-purpose I/O interface, and the second function is used as a memory extension to transmit a high 8-bit address. P3.0～P3.7: P3 port 8-bit port line, the first function is used as general-purpose I/O interface, and the second function is used as control signal for single-chip microcomputer. ALE/PROG: Address Latch Enable/Program Pulse Input Signal Line (Output Signal) PSEN: Off-chip Program Memory Development Signal Pin (Output Signal)>. EA/Vpp: Off-chip program memory uses the signal pin/program power input pin. RST/VPD: Reset/alternate power supply pin.
(2) The 8255A chip is a programmable I/O interface chip produced by Intel Corporation. It has three 8-bit parallel I/O ports. It has three working modes and can be changed by program. Therefore, it is flexible and versatile. It can be used as an intermediate interface circuit when the microcontroller is connected to various peripheral devices. The 8255 has three basic working modes. The three working modes are determined by the working mode control word. The mode control word is provided by the CPU through input/output commands. The PC port of the three ports is divided into two parts, and the upper part is called with the PA port. For group A, the lower part is called group B with the PB port. The PA port can work with modes 0, 1, and 2, while the PB port can only work in modes 0 and 1. The 8255 has 40 pins and is double-row straight. Plug-in package, the function of each pin is as follows: D0--D7: Three-state bidirectional data line, connected with the data bus of the MCU, used to transmit data information. CS: Chip select signal line, active low, indicating that the chip is selected. RD: Read signal line, active low, control data readout. WR: Write signal line, active low, control data write. Vcc: +5V power supply. A0--PA7: A port input/output line. PB0--PB7: B port input/output line. PC0--PC7: C port input/output line. RESET: Reset signal line. A1, A0: Address line, used to select the 8255 internal port. GND: Ground.
(3) 74LS373 chip
74LS373 is an eight-D latch with a three-state gate. When the enable signal line OE is low, the tri-state gate is in the on state, allowing 1Q-8Q output to OUT1-OUT8. When the OE terminal is high, The output tristate gate is open and the output lines OUT1-OUT8 are in a floating state. G is called the data entry line. When 74LS373 is used as the address latch, the enable signal OE of the tri-state gate should be made low first. At this time, when the G terminal is high, the latch is used. The output (1Q-8Q) state is the same as the input (1D-8D) state; when the G terminal returns from high level to low level (falling edge), the data of the input terminal (1D-8D) is locked into 1Q-8Q. In the eight-bit latch. When 74LS373 is used as the address latch, their G terminal can be directly connected to the latch control signal terminal ALE of the microcontroller, and the address latch is performed on the falling edge of ALE.
The pin description is as follows: D0~D7: Latch 8-bit data input line Q0~Q7: Latch 8-bit data output line GND: Ground pin Vcc: Power pin, +5V effective OE: Chip select signal pin G : Latch control signal input pin.
(4) 6116 chip
6116 is a 2K*8-bit static random access memory chip, manufactured by CMOS process, single +5V power supply, rated power consumption 160mW, typical access time 200ns, 24-wire dual in-line package, its pin function description is as follows: A0 ~ A10: Address input line O0~O7: bidirectional tri-state data line, sometimes represented by D0~D7: chip select signal input terminal, active low: read strobe signal input line, active low
: Write strobe signal input line, active low
Vcc: working power input pin, +5V GND: line ground
(5) 2746 chip
The 2764 is an 8K*8-byte UV-removal, electrically programmable read-only memory with a single +5V supply, an operating current of 75mA, a holding current of 35mA, and a readout time of up to 250nS in a 28-pin dual in-line package. The meaning of each pin is: A0 ~ A12: 13 address lines, addressable 8K bytes;
: chip select line; O0-O7: data output line;: data output strobe line; PGM: programming pulse input terminal; Vpp: programming power supply; Vcc: main power supply, generally +5V. GND: Ground pin
(6) ADC0809 converter
ADC0809 is a typical 8-bit 8-channel successive approximation A/D converter, CMOS technology, which can realize time-division acquisition of 8 analog signals, 8 analog strobe switches on the chip, and corresponding channel address locks. The decoder circuit is stored with a conversion time of about 100μs. It is packaged in a dual-row 28-pin package. The pin descriptions are as follows: IN0~IN7: 8 analog input channels; ADDA~ADDC: address lines are used to select analog inputs. Channel; ALE: address latch enable signal; START: conversion start signal; D0 ~ D7: data output line; OE: output enable signal, low level allows conversion result output; CLOCK: clock signal input pin, usually used 500KHz; EOC: Conversion end signal, 0 means converting, 1 means end of conversion; Vcc: +5V voltage; VREF(+), VREF(-): reference voltage.
(7) Selection of temperature detecting components, transmitters and ADCs
The temperature sensing component and transmitter should be selected to take into account the temperature control range and accuracy requirements. For the measurement range of 0~1000°C, thermocouple, such as nickel-chromium thermocouple, indexing number EU, output signal is 0~41.32mV, output 0~10mA through millivolt transmitter, then pass current ——The voltage conversion circuit converts to a 0~5V voltage signal. In order to improve the measurement accuracy, the transmitter can be zero-shifted. For example, the temperature measurement range is changed to 400~1000°C. When the thermocouple gives 16.4~41.32mV, the transmitter outputs 0~10mV, so 8-bit A/ is used. The D converter enables the quantization error to reach ±2.34 °C.
(8) Expansion of the interface chip
Since the system has to display, alarm, keyboard input and control, the system expands a 8155 in the 8031 ​​system. It has three 8-bit I/O ports and 256 bytes of RAM, which can be used as external data memory. The system uses, P2.1 of 8031 ​​is connected to CE of 8155, P2.0 is connected with IO/M of 8155. When P2.1=0, P2.0=1, three I/O ports in 8155 are selected. The port address is as follows: 0100H 〖〗 Command status register 0101H〗 〖A port 0102H 〖B〗 port 0103H 〖C〗 port or control port register 0104H, the count value is lower eight bits 0105H, the count value is higher eight bits and mode register. When P2.2=0, ADC0809 is selected (allowing each channel to be converted and reading the corresponding conversion result). The conversion end signal EOC is inverted and connected to the external interrupt INT1 (P3.3) of the microcontroller. When P3.3=0, the conversion is completed. We use 0 channel as input and 0809 as an external data storage unit with address 03F8H. When writing data, the WR signal of 8031 ​​makes ALE and START valid, and the lower three bits of the 74LS373 latched address are stored in 0809. And start ADC0809, D 9EOC is low, A / D conversion is in progress, when EOC is high, it means the end of the conversion, 8031 ​​can read the converted data.
(9) Temperature control circuit
The temperature control circuit uses a thyristor power adjustment method. The bidirectional thyristor string is in a 50 Hz AC power supply and heating wire circuit, as long as the pulse signal of the thyristor switch on time is changed within a given period. This can be controlled by a program output pulse using an I/O line. In order to achieve the purpose of zero-crossing triggering, an AC zero-crossing detection circuit is required. The circuit outputs a pulse corresponding to the zero-crossing time of the 50 Hz AC voltage as a synchronous pulse for triggering the triac, so that the thyristor is turned on when the AC voltage crosses zero. The voltage comparator LM311 turns the 50 Hz sinusoidal alternating voltage into a square wave. The rising edge and the falling edge of the square wave are respectively used as the trigger signals of the monostable trigger. The narrow pulse of the one-shot trigger is mixed by the diode or the gate, and the synchronous pulse corresponding to the 220V mains zero-crossing moment is obtained. This pulse is applied as a trigger sync pulse to the temperature control circuit, and is applied as a count pulse to the P3.4 and P3.5 inputs of the microcontroller 8031.
Sixth, the software part of the electric furnace control system
The system software is programmed by interrupt mode. The main part is the clock interrupt program, which is mainly composed of input processing program, control algorithm program, display processing, output processing and self-diagnosis program. The flowchart is shown in Figure 5. After the instrument is powered on, the initialization program performs the time setting, and the clock is interrupted every 500ms. After the interrupt, the clock interrupt processing is entered. For the pure lag, the large inertia link control object generally adopts the integral separation PID control algorithm. In the general PID control, when the system has a large disturbance or a large increase in the set value, due to the large deviation and the inertia and hysteresis of the system, under the action of the integral term, a large overshoot will occur. Long-term fluctuations, this phenomenon is particularly serious in the process of slowly changing temperature. For this reason, the integral separation measures are adopted, that is, when the deviation is large, the integral action is cancelled, and when the deviation is small, the integral action is input. The integral separation PID control algorithm is as follows:
Seven, furnace temperature automatic control principle
According to the deviation of the furnace temperature from the given temperature, the heat source energy supplied to the furnace is automatically turned on or off, or the heat source energy is continuously changed, so that the furnace temperature is stabilized to a given temperature range to meet the needs of the heat treatment process. There are several kinds of commonly used adjustment rules for temperature automatic control, such as two-position, three-position, proportional, proportional integral and proportional integral differential. The resistance furnace temperature control is such a feedback adjustment process. The actual furnace temperature and the required furnace temperature are deviated. The control signal is obtained by the treatment of the deviation to adjust the thermal power of the resistance furnace, thereby realizing the control of the furnace temperature. According to the proportional, integral and differential control of the deviation (PID control), it is the most widely used control method in process control.
The system control program is designed with two interrupt nesting methods. First, the T0 counter is generated with a timer interrupt as the sampling period of the system. Start A/D in the interrupt service routine, read in the sampled data, perform digital filtering, upper and lower limit alarm processing, PID calculation, and then output control pulse signal. The pulse width is determined by the T1 counter overflow interrupt. While waiting for the T1 interrupt, the current sampled value is converted into the corresponding temperature value and placed in the display buffer, and then the display subroutine is called. After returning from the T1 interrupt, it returns to the main program from the T0 interrupt and continues to display the current sampling temperature, waiting for the next T0 interrupt.
1) Two-position adjustment - it has only two states of opening and closing. When the furnace temperature is lower than the limit given value, the actuator is fully open; when the furnace temperature is higher than the given value, the actuator is fully closed. (actuators generally use contactors)
2) Three-position adjustment--It has two set values ​​of upper and lower limits. When the furnace temperature is lower than the lower limit given value, the entertainer is fully open; when the furnace temperature is between the upper and lower limits, the actuator part is turned on. When the furnace temperature exceeds the upper limit given value, the actuator is fully closed. (When the tubular heater is a heating element, the three-position adjustment can be used to achieve different heating and insulation power)
3) Proportional adjustment (P adjustment) - The output signal (M) of the regulator is proportional to the deviation input (e). Ie: M=ke
Where: K-----scale factor
There is a proportional relationship between the input and output of the proportional regulator at any time. Therefore, when the furnace temperature change is proportionally adjusted to reach equilibrium, the furnace temperature cannot be added to the deviation of the given value--called “static differenceâ€. â€
4) Proportional integral (PI) adjustment - In order to "static difference", add integral (I) to adjust the integral in the proportional adjustment. The adjustment means that the output signal and deviation of the regulator increase with time, until the deviation is eliminated. There is no output signal, so the combination of “static difference†proportional adjustment and integral adjustment can be eliminated.
5) Proportional integral derivative (PID) adjustment--Proportional integral adjustment will increase the adjustment process and increase the amplitude of the temperature fluctuation. For this reason, differential (D) adjustment is introduced. The differential adjustment refers to the proportional division of the output and deviation of the regulator to the time. The differential regulator has an adjustment signal output when the temperature changes. The faster the change speed and the stronger the output signal, the faster the adjustment speed can be. The combination of proportional adjustment, integral adjustment and differential adjustment is called proportional integral differential adjustment. (Generally, the thyristor regulator is used as an actuator).
According to the operation situation of the production site, this temperature control method has higher precision, stable system performance and meets the actual needs of production. Main equipment: thermocouple or thermal resistance, intelligent PID temperature controller, thyristor trigger power regulator, etc.
Eight, the main technical characteristics:
The resistance furnace consumes heat from the conversion of electrical energy. Some of the materials are lost to the space by the materials and heat transfer of the electric furnace. The other part is used to heat the workpiece in the furnace. The front part forms the electric furnace loss power, and the latter part forms the electric furnace effective power.
When the electric furnace starts to heat up, the brick body in the furnace absorbs a large amount of heat to increase the temperature of the furnace, and then dissipates this part of the heat to the space when the furnace is cooled down; this part forms the heat storage loss of the furnace body. An advanced electric furnace should have low empty furnace losses and high effective power. Less heat storage loss. The size of the empty furnace loss is an important indicator to measure the efficiency of the electric furnace. The electric furnace with small empty furnace loss can obtain high technical productivity and low unit power consumption ratio. The efficiency of general industrial resistance furnaces. Small electric stoves are lower. Large electric furnace
Higher, the efficiency of the box-type electric furnace from 10-100 kW is about 65-85%, and the loss of the empty furnace is about 35--15% of the total power. The time from the room temperature to the working temperature of the electric furnace has a significant influence on the economic index of the electric furnace. When the heating time is short, the time for the furnace to be put into normal use is longer, the daily productivity is higher, and the electric power consumption per kg of the workpiece is reduced. Therefore, it is necessary to use the furnace lining material with low thermal inertia and reduce the heat storage capacity of the furnace to accelerate the heating rate of the electric furnace: the heat storage capacity of the furnace body has a great influence on the cycle operation furnace, especially the electric furnace produced in one or two shifts per day.
The effect on the continuous operation furnace is not obvious. The heating capacity is the main technical index of an electric furnace. The heating capacity refers to the effective power of the electric furnace. The theoretical calculation can be used to calculate the maximum weight of the specified material to the rated temperature within one hour, in kilograms per hour.
(5) Basic structure and type of resistance heating furnace
Resistance furnaces have been developed with the development of the mechanical industry. Due to the needs of various heating processes and smelting processes, resistance furnaces are a wide variety of products. The furnace structure of the electric resistance furnace is divided into two types: periodic type and continuous type. Periodic work furnace. Such as box type electric furnace, trolley type electric furnace, well type electric furnace and other box type electric furnace, the outer casing is generally welded by steel and steel plates, and the small electric furnace is generally made with a bracket because it needs to maintain a certain height of the working surface. Under the box-shaped housing, there are legs or brackets that support the furnace body. The medium-sized electric furnace is large in weight and the weight of the workpiece added to the furnace is also large, so the furnace body and the brickwork are generally welded directly on the chassis. The large electric furnace can be designed as a steel-free chassis on a specific dedicated foundation, and the bricks are welded in situ, but the electric furnace cannot be lifted and moved after installation.
The furnace door of small and medium-sized electric furnace can be opened and closed by counterweight and manual device. The lower part generally has sand sealing groove. Some furnace doors are also provided with sand sealing groove to ensure good sealing. When the furnace door is closed, it is pressed. The device allows the door to be in close contact with the door frame to reduce air leaks. Large electric furnaces can be electrically or pneumatically and hydraulically opened and closed. The electric heating elements can generally be arranged on the left and right side walls and the bottom surface of the furnace. In order to obtain a good thermal field, it is better to arrange electric heating elements on the roof. Because the height of the workpiece in the furnace generally does not exceed the width, the heating of the above two aspects is more effective than the two aspects. Large and medium-sized electric furnaces can be equipped with some electric heating elements on the guard door and the rear wall to reduce the temperature difference in the furnace. In order to ensure better balance of heat loss at the furnace door, it can be used in larger box-type electric furnaces. The length of the furnace at the door of the furnace is 1/3 as a control area.
The furnace that protects the gas should be equipped with the necessary equipment to ensure safe operation and good tightness. The well type electric furnace is generally a cylindrical furnace, the inner diameter is generally 600 mm, the size is small, and it is inconvenient to install and repair; the furnace shell is welded with steel as the skeleton, and the small furnace cover can be opened and closed by manual mechanism, large and medium The electric or hydraulic mechanism can be opened and closed, and the height and diameter of the electric furnace workpiece is generally at the bottom of the furnace at a height of 1 to 1.5, and the height to diameter ratio is 2 or more. Most of the workpiece is suspended in the furnace mouth or on a special hanger outside the furnace. The setting of the control zone is generally 1 to 1.5 times the diameter. When the temperature control requirements are not high. Sometimes a control zone is twice as long as the diameter.
Controlled atmosphere box-shaped multi-purpose furnace. Generally, it is divided into an anterior chamber and a furnace and a cooling tank. The front chamber is connected to the cold-sense tank by a welded space formed by a section steel and a steel plate, and the upper side is an air cooling chamber provided with a water-cooled wall. In the middle, there is a rack that passes through the workpiece and the rack that rises up and down. When the cylinder is operated by the top cylinder (electrically or hydraulically), the workpiece enters the cooling tank, and is rapidly cooled or austempered, and the workpiece is lifted in the upper atmosphere when rising. cold.
The furnace is located behind the front chamber, with a furnace door in the middle, when feeding and discharging. The furnace door is opened by the upper cylinder. The furnace is made of carburized carbon. Electric heaters are two types, one is made of high-resistance alloy sheet; in order to eliminate surface area carbon short circuit, the heater surface is coated with special high-temperature insulating glaze; the other is radiant tube type, horizontal or vertical insertion In the furnace betting, the metal heater is composed of a large-section circular electric resistance wire in the tube. The furnace has a strong fan. The front of the furnace is a pushing device for feeding and discharging into the furnace. The electric furnace is equipped with a gas generator or directly injects organic liquid and carbon potential control equipment into the protection.
Nine, use
The main purpose of industrial resistance furnaces is for the mechanical industry to heat raw materials, blanks and mechanical parts. For example, the billet is heated before the sheet is rolled, and the forging is heated. Heat treatment of mechanical parts and semi-finished products to improve their mechanical properties, such as rod quenching, tempering, annealing, normalizing, gas carburizing, nitriding, etc. It is also used for sintering and brazing. Some resistance furnaces are used for the melting of low-melting metals and the heating of the ceramic glass industry.
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