Design of constant temperature control system of circuit aging test box

Design of constant temperature control system of circuit aging test box

0 Preface

At present, various electronic products are used in all aspects of our daily life, but all electronic products have a certain service life, and will gradually age with the passage of time. The circuit test experiment box is widely used in the temperature control system of the thermostat It is generally designed based on single chip microcomputer, and its actuator generally adopts thyristor module, and the control algorithm adopts two-bit control method, fuzzy control method, traditional: PID control method, fuzzy PID control method, etc. The implementation of the thyristor module as the actuator circuit is relatively complicated, and the system reliability is difficult to guarantee. The double-bit control method is used to control the accuracy of rough control, and the simple fuzzy control method to control the intensive reading is also not guaranteed. For temperature control systems with large time-delay characteristics, both traditional PID control and fuzzy PID control methods have the disadvantage that parameters are difficult to adjust. Based on the above analysis, this paper proposes a single-chip constant temperature control system based on solid-state relay and self-tuning PID algorithm, which has the characteristics of simple and reliable circuit and high control accuracy.

1 Classification and working principle of solid state relays

1.1 Classification of solid state relays

Solid state relay (SSR) is a non-contact electronic switch with relay characteristics developed by combining separate electronic components, integrated circuits or chips, and hybrid microcircuit technology. SSR has the characteristics of low input control voltage, small drive current, no contact, small electromagnetic interference, high insulation withstand voltage, corrosion resistance, strong anti-interference ability, long life and high reliability.

According to the type of load power supply, SSR can be divided into AC solid state relay (AC-SSR) and DC solid state relay (DC-SSR). According to the different AC-SSR control trigger mode, it can be divided into two types: random conduction (P) type and zero-crossing trigger (Z) type. P-type AC-SSR can be turned on immediately after the control signal is input, and turned off when the load current crosses zero, so it may cause greater interference at the moment of turning on. The Z-type AC-SSR is turned on after the control signal is input, and the AC power passes near zero voltage, and its turn-off conditions are the same as the P-type, so the interference is very small.

1.2 Working principle of solid state relay

Because the Z-type AC-SSR has the characteristics of little interference, the Z-type AC-SSR is selected as the actuator of the temperature control system of the constant temperature box, and its working principle is introduced by taking FIG. 1 as an example.

The working principle of solid state relay

The circuit is composed of a signal input circuit, a zero voltage monitoring system, a work instruction circuit, a bidirectional thyristor control circuit and an absorption circuit. The photocoupler GD serves as an isolation element between the input circuit and the output circuit, and VD prevents Vin from being burned by positive and negative connections. VT is an inverter, SCR is a unidirectional thyristor, BR is a bidirectional rectifier bridge, and TR is a bidirectional thyristor. R7 and C1 form a surge absorption network to absorb the peak voltage or surge current contained in the power supply to prevent impact or interference on the switching circuit.

The working process of the circuit is: when there is no input signal, the phototransistor in GD is cut off, VT ​​is the AC voltage zero point monitor, the base current is obtained through R3 and saturated, and the SCR gate is locked at a low potential and turned off Broken state. When there is an input signal, the phototransistor is turned on, and the state of the SCR is determined by the VT at this time. If the power supply voltage is greater than the zero-crossing voltage, the voltage divider P of the voltage divider R2, R3 is greater than VBE, VT is saturated and turned on, the SCR gate is cut off due to the low voltage, and the TR gate is not triggered It is in shutdown state. Only when the power supply voltage is less than the zero-crossing voltage and the voltage at point P is less than VBE, the VT is turned off, and the SCR gate is turned on by obtaining the trigger signal through R4. Trigger pulses in the positive and negative directions of R6 → BR → SCR → BR → R5 and R5 → BR → SCR → BR → R6 are obtained at the gate of TR, TR is turned on, thus turning on the load power supply. When the input signal is turned off, the phototransistor in GD is turned off, and the VT is saturated and turned on, so that the SCR gate is clamped at a low potential and turned off, but at this time TR remains on, and there is still current flowing through the load. Until the load current decreases with the VAC to less than the maintenance current of the triac TR, it will automatically shut down and cut off the load power supply.

It should be noted that the so-called zero-crossing voltage does not really have to be the zero point of the power supply voltage waveform, but refers to triggering in the range of 10 to 20 V or-(10 to 20) V.

2 Realization of constant temperature control system

In the design of the electrical aging test of the circuit board of a therapeutic instrument, a constant temperature aging test box is needed. According to the principle of circuit aging, in the life cycle T of the electronic product, the failure rate in the early and late stages of the product life is much greater than the failure rate in the middle of the product life. Its use time-failure rate curve is shown in Figure 2. The purpose of designing the constant temperature aging test chamber is to make the aging degree equal to the actual product use time t1 in a short time before the product is shipped, so as to detect the products that are prone to failure in the early life of the product, so as to reduce Product repair rate improves the quality of products shipped from the factory.

Use time-failure rate curve

According to experience and actual calculations, the temperature control requirements of the aging test chamber of this electronic product are: 70 ± 0.2 ℃ constant temperature for 2 h.

2. A hardware design based on single chip microcomputer

The hardware design block diagram of constant temperature control system based on AT89C51 single-chip microcomputer is shown in Figure 3. Because the control temperature is 70 ° C, PT100 platinum resistance is selected as the detection element. The parallel interface circuit 8155A chip is used to expand the keyboard and LED display interface, and the A / D converter ADC0809 performs digital touch conversion. The alarm output is connected to a buzzer, which is used to detect the high temperature alarm. The keyboard is used for setting system parameters and given values ​​to realize system start, stop and other functions. The LED is used for display of setting parameters, given temperature, current temperature, etc.

Block diagram of hardware design

The working process of the system is as follows: After the system enters the working state, the platinum resistance first converts the temperature of the aging test chamber into an electric current, and through the temperature detection circuit and the A / D converter, it is converted into a digital signal and input into the single chip microcomputer. Then, the control output is obtained through the operation of the single chip microcomputer and sent to the PWM chip. The PWM chip sets the digital signal to a PWM signal with a certain duty cycle within a certain period, and drives the SSR to control the heating wire on and off. This cycle will eventually achieve the purpose of precise temperature control.

2.2 Software design based on self-tuning PID algorithm

The software design flow chart of the system is shown in Figure 4. The system first initializes, sets the controller parameters through the keyboard, and starts the PID auto-tuning function through the PID auto-tuning key. Then set the required temperature value through the keyboard, and then call the measurement display subroutine to display the current temperature. When the measured temperature is equal to the set temperature, the program returns to the temperature setting module. When the measured temperature is not equal to the set temperature, the PID control module is started, and then returns to the measurement display module, and so on, until the measured temperature is equal to the set temperature. Due to the large time lag and nonlinear characteristics of the temperature control system of the thermostat, it is difficult to set the conventional PID control parameters. Here, a self-tuning PID algorithm is designed to solve the difficult problem of PID tuning. The following describes the design process of the self-tuning PID algorithm.

PID algorithm design process

The discrete PID control algorithm expression is:

Discrete PID control algorithm expression

According to Ziegler-Nichle conditions, the three parameters of PID can be set by the following formula:

Formula tuning

Where: TC is the critical oscillation period.

Substituting equation (2) into equation (1), the discrete PID algorithm only has one setting parameter

KP selects the optimal criterion of integral of deviation absolute value multiplied by time (ITAE) with fast and stable characteristics:

Setting parameters

As the objective function. When it is the minimum value, the control system is in the best state. It has less consideration of the initial error of the unit step response, and focuses on the function of weighing the error that occurs later in the transient response. The characteristic of the system designed by this criterion is that its transient response has a small overshoot, that is, it has strong anti-interference ability, and has sufficient damping of oscillation, and has good selectivity and sensitivity.

Discrete formula (3) gives:

Optimal criterion

Parameter auto-tuning method: compare QITAEn-1 with the last calculation after each calculation of QITAEn, and correct the KP in the PID algorithm according to the change trend of QITAE. The correction period is generally (5 ~ 10) T. The specific correction algorithm is as follows:

Correction algorithm

When QITAE tends to be constant, the PID auto-tuning algorithm ends.

3 Conclusion

The self-tuning PID constant temperature control system based on the solid state relay designed according to the temperature control requirements of the circuit aging test box has the characteristics of simple and reliable design method, convenient use and high control accuracy. This design can be applied to the development and design of new products of the circuit aging test box of the enterprise, and it can achieve good control effects and meet the user's product needs. It is worth popularizing and applying to the majority of enterprises.



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