In modern industry, the processing process of many products needs to be completed in a specific temperature and humidity environment. The performance testing tests such as reliability, weather resistance and service life of many products also need to be carried out in a specific temperature and humidity environment. Taking the benchmark testing of LED lamps as an example, the testing tests related to temperature and humidity include rapid temperature cycle test, alternating damp heat test and early reliability guarantee test, Limit stress sensitivity test, etc., in which the rapid temperature cycle test requires the environment to cycle between high temperature + 85 ℃ and low temperature – 40 ℃ at a temperature change rate of 10 ℃ / min: the alternating damp heat test requires the product to be in normal working state, and the sample power supply is switched on and off at the high temperature and high humidity stage in each cycle. The power-off time is not less than 3min and not more than 6min, the upper limit temperature is 55 ℃, the lower limit temperature is – 10 ℃;
The early reliability guarantee test requires the product to work stably under the rated electrical stress under the environment of temperature 70 ℃ and relative humidity 85%, and control the power state to be on and off for 15 seconds; The limit stress sensitivity test requires the ambient temperature to start from 85 ℃ and maintain the step of 1 hour in the temperature step of 15 ℃.
In order to create a specific temperature and humidity environment, many manufacturers and scientific research institutions have studied the product technology of high and low temperature test chamber, constant temperature and humidity test chamber and so on. Although there are high and low temperature test chambers, constant temperature and humidity test chambers and other products with relatively mature technology on the market, their temperature and humidity accuracy has not yet achieved satisfactory results. Therefore, this paper focuses on the temperature and humidity control of high and low temperature test chamber and related products, in order to enlighten the product research and development of relevant manufacturers and scientific research institutions in related fields.
2 Control technology
2.1 Relationship between temperature and humidity
In a closed test chamber, temperature and humidity are not two independent control quantities. They affect each other, that is, the coupling in control. Changes in temperature can lead to changes in humidity. Generally, increasing the temperature will evaporate the condensed steam and improve the humidity of the test chamber, while reducing the temperature will condense some steam into water droplets and reduce the humidity. Similarly, the change of humidity will also affect the temperature. Increasing the humidity, the injection of cold steam will reduce the temperature of the test chamber, and the heat released by water vapor condensation in the process of reducing the humidity will increase the temperature. Therefore, in the control process, we must consider coupling, and in the separate control, we need to realize decoupling.
2.2 Temperature and humidity control mode
The control mode of the traditional temperature and humidity test chamber is the anti integral saturation PID and bthc temperature and humidity regulation control mode. The anti integral saturation PID control method mainly uses the integral separation method and PID separation method, that is, when tracking the controlled quantity, the integral effect is cancelled, and the proportional term can quickly track the change of deviation. When the controlled quantity is close to the given value, the integral function is added, so that the stability time can be shortened, the static error can be eliminated, and the effect of integral correction can be achieved. This method gives full play to the regulation function of each part of the PID regulator and can improve the dynamic characteristics of the system. From its principle, it can be seen that the switching of regulator increases the difficulty of implementation, and the coupling problem of temperature and humidity is not considered. Although the control effect is improved compared with the traditional PID control, it can not meet the control requirements of high performance.
3 Temperature and humidity double closed loop control technology
Double closed-loop DC speed regulation system is a classical method in DC speed regulation. It has the advantages of wide speed regulation range, good stability, high speed stability accuracy, fast response and strong anti-interference ability. Its basic principle is to add a current loop in the speed loop to realize speed negative feedback and current negative feedback. Current negative feedback realizes maximum current start, speeds up response speed, and speed negative feedback realizes steady-state error free.
In the temperature and humidity control of the test chamber, the control effect required is that after setting the temperature and humidity, the system can quickly and accurately reach the set value. Therefore, the double closed-loop system can be used to control the two control quantities of temperature and humidity respectively. For the above-mentioned coupling problem of temperature and humidity, the researchers pointed out that compared with the impact of temperature change on humidity, the impact of humidity change on temperature is much smaller and can be ignored. That is, we first control the temperature, and then control the humidity after the temperature reaches the set value, so that both items reach the set value. This method is used to achieve the purpose of decoupling. If double closed-loop control is adopted for temperature and humidity, satisfactory dynamic response speed and steady-state accuracy can be achieved. Both speed control and current control are PI controllers with amplitude limit. The actuators are alloy electric heater and fully enclosed compressor according to the current needs of heating and cooling after a given temperature.
High and low temperature test chamber is widely used in the fields of testing and scientific experiments. Based on the rapidity and steady-state error of the control mode of DC speed regulation system, this paper proposes to use the double closed-loop mode to control the temperature and humidity, so as to achieve the fast and accurate control effect required in the application, and further analyzes its feasibility and superiority. The double closed-loop DC speed regulation system has a very mature theoretical and application basis, which shows the superiority, stability and reliability of the double closed-loop system. Using the double closed-loop system to control the temperature and humidity of the test chamber is a new idea, which is expected to enlighten the research of relevant units. This paper does not carry out parameter calculation and simulation experiment verification for a certain type of test chamber, which will be further studied in the next work.