1.Overview
High and Low Temperature Humidity Test Chamber is a kind of equipment that can be used for the reliability test of lithium batteries, small electronic products, electronic components and materials, or for the climate test of other products. The temperature adjustment range of high and low temperature damp heat laboratory is generally +60 ℃ ~-50 ℃, and the relative humidity adjustment range: within +20 ℃ ~+40 ℃, the humidity is 50%~95% RH; Within +40 ℃ ~+60 ℃, the humidity is 30%~95% RH. Because the temperature range is large and the negative temperature is low, the refrigeration system in the laboratory is relatively complex, so using the conventional relay control and manual operation methods is not only complex, but also inconvenient to operate. The high and low temperature laboratory control system designed by using the dry and wet bulb temperature control table and PLC can be operated manually and automatically. It is not only simple in circuit, but also convenient in operation. This control system is introduced below.
2.Chamber Structure and Working Principle
The laboratory is mainly composed of chamber body, heater, humidifier, refrigeration system and air supply system. The method of balancing temperature and cooling capacity is adopted to adjust and control the air dry and wet bulb temperature, and the air distribution form of return air under the full orifice air supply, so as to obtain the air temperature and humidity required by the test and meet the accuracy required by the test.
3.Control Strategy
The heater in the laboratory is divided into two groups for control. One group is the main heating, which is PID adjusted by the dry bulb temperature control instrument, and the output power is automatically controlled, with an output of 0~100% (electric heating power); The other group is auxiliary heating, which is started and stopped manually or automatically controlled by the program. It is mainly used when the heat provided by the main heating cannot meet the test requirements. Like the heater, the humidifier is also divided into two groups. One group is the main humidifier, which is PID adjusted by the wet bulb temperature control instrument, and the output power is automatically controlled, with an output of 0~100% (electric humidification power); The other group is auxiliary humidification, which is started and stopped manually or automatically controlled by the program. It is mainly used when the amount of humidity provided by the main humidification cannot meet the test requirements. The refrigeration system in the laboratory is a cascade refrigeration system composed of two low-temperature compressors. There are nine solenoid valves in the system. Different combinations of the refrigerator and the solenoid valve produce different refrigerating capacity. The refrigeration system can be divided into five levels according to the refrigerating capacity.
3.1 Automatic Control Strategy
According to the working principle of the laboratory, usually the refrigeration system works at a certain refrigeration level, and the relatively stable cooling capacity is input. The input of main heating and main humidification is automatically adjusted by the dry and wet bulb temperature control table. When the power of main heating or main humidification is insufficient, the auxiliary heating or auxiliary humidification is started. When the cooling capacity is reasonably matched with the heating capacity and humidification capacity, the system will quickly achieve dynamic balance, and the temperature and humidity in the laboratory will soon stabilize. Is the cooling capacity invested in the refrigeration system more or less Indirect analysis can be made by observing the output parameter out of the temperature and humidity control table. If the value of the output parameter out is very large (100%), the temperature and humidity in the laboratory is still lower than the set value, indicating that the cooling input is too much. If the out value is very small (0%), the temperature and humidity in the laboratory is still higher than the set value, indicating that the cooling input is too little. It can be seen that the output parameter out of the temperature and humidity control table can be used as the basis for the automatic control of the refrigeration stage.
4.System Functions
(1)Manual and automatic switching functions, select on the control panel. When manual operation is selected, each switch button can be operated directly on the panel. However, due to the different input quantities of heating, humidification and refrigeration under different working conditions, experienced personnel are required to operate. When automatic is selected, all equipment switches are turned on, and the actual input of heating, humidification and refrigeration is controlled by program, which is simple to operate.
In operation, if the automatic control temperature and humidity stability time is long, it can be interfered through the switch of the panel. For example, when changing from low temperature to high temperature, according to the automatic control strategy, the refrigeration level will drop level by level, which requires a certain delay time. At this time, you can directly turn off all refrigerators and heat them up. When the temperature is close to the set value, turn on the refrigerator switch again, which is controlled by the program.
(2) The test time can be set manually. After the test, turn off the machine in sequence. After the chiller, heater and humidifier stop, the fan will continue to operate for 2 minutes and then stop, blowing out the hot and humid gas in the air-conditioning cabinet to prevent the equipment in the air-conditioning cabinet from rusting.
(3) Safety protection and alarm function.
The system control circuit has conventional safety protection functions such as overload, under-voltage, open circuit and lack of power. The chiller is a chiller, and a water flow protector is set in the cooling water pipeline. When the water flow is lower than a certain set value, the water flow protector acts and the chiller stops. In addition, the chiller has oil pressure protection, temperature protection, high and low pressure protection, etc. The laboratory has the function of high temperature protection. If the temperature is higher than a certain temperature, all equipment will be automatically cut off to prevent fire. The humidifier has the function of preventing dry burning. The whole system has complete protection functions, with sound and light prompts at the same time.
5.Summary
The above control system has been applied in the equipment, and the automatic control effect is good. The temperature and humidity stabilization time during automatic control is slightly longer than that during manual control, but it meets the stabilization time required by the contract. The fluctuation range after temperature and humidity stabilization meets the test requirements and reaches the design accuracy. Because the operation is simple, it is more popular with operators, and the utilization rate of automatic control is relatively high.