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Pressure sensitive paint surface pressure measurement technology is an aerodynamic optical measurement technology developed in the late 1980s. Compared with the measurement technology based on discrete pressure measuring holes, PSP, as a non-contact pressure measurement technology, can obtain the full-field pressure distribution of the measurement surface at a long distance, avoiding damage to the model and interference with the flow field.
It also has the characteristics of high spatial resolution and data acquisition rate. It has a wide range of application prospects in aerospace, automobile manufacturing and impeller machinery, and is regarded as one of the most promising wind tunnel test technologies in the 21st century.
The performance evaluation of pressure sensitive paint or coating is divided into two methods: static and dynamic:
(1) Static characteristic test: This refers to the performance test of the pressure sensitive coating under static or very slowly changing pressure conditions. This test is usually used to evaluate the sensitivity of the coating, that is, the degree of response of the coating after pressure is applied. Static characteristic test also includes testing the sensitivity of the coating at different temperatures.
(2) Dynamic characteristic test: This refers to the performance test of the pressure sensitive coating under dynamic or rapidly changing pressure conditions. This test is usually used to evaluate the response speed of the coating, that is, the ability of the coating to respond to rapidly changing pressure.
Recently, a user has proposed the need for static property testing of pressure-sensitive coatings, requiring precise control of vacuum pressure and temperature on the static property testing instrument, and providing a controllable vacuum pressure, oxygen concentration and temperature environment for the pressure-sensitive coating. The indicators are as follows:
(1) Single-sided heating of a square metal sheet coated with a pressure-sensitive coating. The entire sheet sample is placed in a closed cavity with an optical window on the top, and the vacuum pressure in the cavity is required to be accurately controlled.
(2) Sample size: 50mm×50mm×5mm.
(3) Sample temperature: -10~80℃, temperature control accuracy ±0.1℃.
(4) Vacuum pressure: absolute pressure 1Pa~600kPa, accuracy ±1% of reading.
(5) Oxygen concentration: 0~80%, accuracy ±1%.
This article will propose a precise control solution for temperature, air pressure and atmosphere environment of the static property testing device of pressure-sensitive coatings in response to the technical requirements proposed by the above users, providing accurate control of various temperatures and variable vacuum pressures for the testing device.
From the above technical requirements, it can be seen that the environmental control variables required for the static characteristic test of the pressure-sensitive coating are temperature, vacuum pressure (positive and negative pressure) and oxygen concentration, and these three variables are required to have different adjustable values.
To this end, this solution will use the following three independent technologies to reach precise control of these three variables:
(1) Temperature control: Using TEC semiconductor refrigeration technology based on the Peltier principle, this temperature control technology is currently more suitable for heating and cooling technology in the temperature range of -10~80℃, with high accuracy, fast response speed, easy implementation and simple structure.
(2) Vacuum pressure control: Using dynamic balance method technology, by controlling the gas flow entering and exiting the test chamber, the intake and exhaust flow rates are dynamically balanced, thereby achieving accurate and constant control of the vacuum pressure set arbitrarily in the wide range of 1Pa~600kPa (absolute pressure).
(3) Oxygen concentration control: Using gas mass flow control technology, the flow rates of oxygen and other environmental gases are controlled separately, thereby reaching precise control of the oxygen concentration in the mixed gas.
It should be noted here that in the dynamic balance method vacuum pressure control process, for the higher air pressure range with an absolute pressure in the range of 1kPa~600kPa, the downstream control mode is required to obtain higher control accuracy, that is, the opening of the fixed intake electric control needle valve is kept constant to keep the intake flow constant, and the vacuum pressure control is performed by quickly and automatically adjusting the opening of the downstream exhaust Proportional Pressure Regulator.
For the low-pressure and high-vacuum range with an absolute pressure below 1kPa, the upstream control mode is required to achieve higher-precision control, that is, the exhaust Proportional Pressure Regulator is fully opened to allow the vacuum pump to extract the gas in calibration chamber at full speed, and vacuum degree is controlled by quickly and automatically adjusting the opening of upstream intake Proportional Pressure Regulator.
In order to reach separate adjustment of the two Proportional Pressure Regulators, the solution is equipped with KaoLu’s Proportional Pressure Regulator. The two independent control channels can be used to operate the upstream and downstream control modes respectively, and perform independent PID automatic control or manual control. This controller also comes with computer software, which can be remotely set and operated through the host computer.
To accurately control the oxygen concentration or oxygen proportion. By mixing the various working gases after precise measurement in the mixing tank and then entering the calibration chamber, the oxygen partial pressure in the calibration chamber can be accurately controlled. In the process of oxygen concentration control, the following two points should be paid special attention to:
(1) For a certain single working gas, a gas mass flow controller of the corresponding gas is required.
(2) The pressure of the mixing tank should be constantly controlled or a pressure reducing valve should be added at the outlet of the mixing tank to keep the outlet pressure of the mixing tank stable. This is very important for accurately controlling the vacuum pressure in the calibration chamber.
In summary, this solution can well meet the various technical requirements proposed by users, and has high control accuracy and automatic control capabilities. In addition, this solution also has the following characteristics:
(1) This solution has strong applicability. By changing the parameter indicators of the relevant components, it can be applied to the static characteristics test needs of pressure-sensitive coatings with different control ranges.
(2) The temperature and vacuum pressure controllers used in the solution come with computer software, which can be used to debug and operate the entire control system directly through the computer screen, and the various process parameter change curves in the control process are automatically stored. In this way, there is no need to write any control software to quickly build a temperature and vacuum pressure control system, which greatly facilitates the calibration of the static characteristics of pressure-sensitive coatings.