压力传感器：使用皮托静态管进行校准
English
Condividere
Panoramica
资料来源：北卡罗来纳州立大学机械和航空航天工程系，北卡罗来纳州罗利市，史雷亚斯·纳西普尔
流体压力是确定系统空气动力学特性所需的重要流动特性。由于压力计的准确性和操作简单性，是历史最悠久且仍然存在的压力测量系统之一。压力计通常是一个U形玻璃管，部分充满液体，如图1所示。U-tube 压力计不需要校准，因为它没有任何运动部件，并且其测量值是重力函数和液体的密度。因此，操纵仪是一个简单而精确的测量系统。
图 1.U 管操纵仪的原理图。
通过将平静态探头的停滞和静态压力端口连接到压力测量装置的端口，在飞机上获得实时压力测量。这允许飞行员获取现有的飞行条件，并在飞行条件发生任何变化时发出警告。虽然压力计提供非常精确的压力读数，但它们本质上是笨重的。测量飞机压力需要更现实的解决方案，因为主要设计目标之一是尽可能降低飞机的整体重量。如今，将施加压力转换为电信号的机电压力传感器由于体积小、重量轻、几乎可以放置在机身的任何地方而广泛用于飞机压力感应应用。上述特性不仅有助于减少重量，还减少了将皮托静态探头连接到传感器所需的管道量，从而减少了数据响应时间。此外，在实验飞机飞行测试中，微型压力传感器派在位，因为它们使研究人员能够最大限度地收集压力数据，而不会显著增加飞机的重量。虽然存在不同测量技术的不同类型的压力传感器，但更常见的传感器类型之一是电容式压力传感器。由于传感器只能发送电压和电流方面的信号，因此需要对传感器进行校准，以将特定信号的强度与导致传感器产生信号的压力相关联。将传感器电流或电压与物理测量（在我们的案例压力）相关联的最终曲线拟合通常称为传感器校准曲线。
在本实验中，在亚音速风洞中放置了一个平视静态探头，其停滞和静态压力端口连接到U管电力计和压力传感器的总端口和静态端口。然后，风洞在不同的动态压力设置下运行，并记录U管压力计的相应压力读数以及传感器产生的电流读数。然后，这些数据用于生成压力传感器的校准曲线。
Principi
Procedura
Risultati
The following constants were used in the analysis: water density, ρwater: 61.04 lb/ft3; acceleration due to gravity, g: 32.15 ft/s2; and manometer off-set, hoff = 0.8 in. The variation in manometer data for increasing and decreasing dynamic pressures (with and without correcting for the instrument off-set) is shown in Figure 7. Figure 8 shows a plot of the transducer current readings against the manometer pressure, which was calculated using Equation 3.
In order to obtain the calibration curve for the pressure transducer, two linear curves are fitted through the increasing and decreasing data points, respectively. The corresponding linear fit equations are:
(5)
(6)
The equations for the increasing and decreasing curves are almost similar, and the two curves align with each other, as observed in Figure 8. Therefore, it can be deduced that the pressure transducer does not have any hysteresis. A single calibration equation relating the current to the pressure (Equations 5 or 6) can be used for the transducer, thereby removing the necessity of using the bulky U-tube manometer system for all future pressure measurements.
Figure 7. Variation of manometer fluid height with wind tunnel dynamic pressure. Please click here to view a larger version of this figure.
Figure 8. Calibration curves for the pressure transducer. Please click here to view a larger version of this figure.
Applications and Summary
Electromechanical transducers are popular replacements for some of the bulkier measurement systems. However, transducers need to be calibrated regularly using standardized measuring devices in order to be effective experimental tools. In this experiment, an off-the-shelf capacitive type electromechanical pressure transducer was calibrated by comparing the current signals generated by the transducer for a range of dynamic pressure conditions in a subsonic wind tunnel to the pressure measurements from a U-tube manometer. Results showed that a linear relationship exists between the transducer's current signal and pressure with negligible sensor hysteresis. A single calibration equation relating the transducer current output to pressure was obtained.
Modern electromechanical measurement systems provide a path to automating experimental data acquisition and can be used in real-time static and dynamic systems for data monitoring and analysis. However, proper calibration practices, like the one demonstrated in this experiment, are necessary to help users obtain accurate and repeatable data using said sensors.
Trascrizione
All airplanes use pressure measurements in order to make real-time calculations of wind speed. In an airplane, these pressure measurements are obtained using a pitot-static tube.
A pitot-static tube has openings that measure the stagnation pressure and the static pressure. Recall that stagnation pressure is the sum total of the static pressure and the dynamic pressure, so the pitot-static tube is used to measure the dynamic pressure and therefore the flow velocity.One method to correlate wind speed to pressure using the pitot-static tube is by using a fluid manometer.
A fluid manometer is generally a U-shaped glass tube that is partially filled with liquid. One arm of the manometer is connected to the stagnation pressure port on the pitot-static tube, and the other to the static pressure port. In stagnant air, where this is no difference between the static pressure and stagnation pressure, the manometer fluid height difference is zero.
When the manometer experiences a pressure differential, it is visualized by a change in fluid height. The pressure differential, or dynamic pressure, is calculated from delta H using this equation. Here, rho L is the density of fluid in the manometer and G is gravitational acceleration. This relationship is used to calculate the wind speed by substituting it into the velocity equation. We can then solve for the free-stream velocity, V infinity, using the free-stream density, rho infinity.
However, fluid manometers are bulky, and require manual reading onboard the aircraft. Thus, a more convenient method to measure the pressure differential is to use a pressure transducer in place of the manometer. This enables us to convert the pressure differential into an electrical signal.
A capacitance pressure transducer is based on the working principle of a capacitor, which consists of two conductive plates separated by an insulator. Capacitance is measured by the following equation, where mu is the dielectric constant of the insulator material, A is the area of plates, and D is the spacing between the plates.
To make the capacitance pressure transducer, one of the conductive plates is replaced by a flexible conducting diaphragm. When pressure is applied, the diaphragm deflects causing a change in the spacing between the plates D, resulting in a change in capacitance. The electronics in the transducer are calibrated to generate specific current changes for corresponding deviations in capacitance. Thus, a current reading corresponds to a given applied pressure.
Like the manometer, the pressure transducer is connected to the pitot-tube and is calibrated in a wind tunnel with known wind speeds. This enables us to generate a mathematical relationship between current and pressure, and by extension, current and wind speed.
In this lab demonstration, we will use a pitot-static tube in a wind tunnel connected to a pressure transducer. We will then calibrate the pressure transducer at various wind speeds and determine the relationship between voltage and speed.
For this experiment, you’ll need to use a wind tunnel with its own calibrated pressure transducer and ability to reach a dynamic pressure of 25 psf. You will also use a standard pitot-static tube and a differential U-tube manometer with colored water to calibrate this differential pressure transducer.
To begin, mount the pitot-static tube inside of the wind tunnel on the top of the test section using a vertical sting mount. Ensure that the probe is at the center of the test section. Align the pitot tube with the direction of flow, so that the primary port faces directly into the air flow.
Next, align the top of the manometer fluid to the double O-ring marker on the glass tube. If the reading on the main scale does not correspond to zero, align the fluid to a different reference point, and record the offset height.
Use a T-connector to split the flow from one tube to two, then connect the stagnation and static pressure outlets on the pitot-static tube, to the corresponding ports on the U-tube manometer. Mount the pressure transducer outside of the wind tunnel test section on a vertical surface. Set up a standard voltage supply to power the pressure transducer and a multimeter to read the output current. Then, connect the stagnation and static pressure outlets to the corresponding pressure ports on the transducer.
Now, secure the wind tunnel doors and switch on all of the systems. Then, take readings of the wind tunnel transducer pressure, the manometer height, and the differential pressure transducer current. Record the measurements for the no airflow condition as the base line zero reading. Now turn on the wind tunnel, and set the dynamic pressure in the test section to one psf.
Once the flow has stabilized, record the transducer pressure, the manometer height difference, and transducer current. Increase the dynamic pressure setting in the wind tunnel in steps of one psf, up to a maximum setting of 20 psf, recording the data at each step. In order to check for hysteresis, decrease the dynamic pressure in steps of one psf, back down to zero psf, again recording data at each step. When all of the measurements have been collected, shut down all systems.
Now, lets take a look at the results. First, we look at a plot of the manometer height readings with increasing and decreasing dynamic pressure. Two measurements are shown here for each trace. One is the actual manometer reading, and the other has been corrected with the offset height of 0.8 inches. We can calculate the manometer pressure from the manometer height, using the simple equation shown. Here, we use the density of the liquid in the manometer, which is in this case water, gravitational acceleration, and the manometer offset and height measurements.
Now that we have calculated the pressure from the manometer reading, we’ll plot it against the pressure transducer current readings. To obtain the calibration curve for the pressure transducer, we’ll fit the increasing and decreasing data separately, resulting in two linear best fit equations.
However, we see that the increasing and decreasing data line up. So we can conclude that the pressure transducer does not exhibit hysteresis. Thus, we can simplify to a single calibration equation, thereby enabling us to measure pressure using the current reading from pressure transducer, rather than the bulky fluid manometer. By connecting the pitot-static probe to the calibrated transducer, we can directly measure the dynamic pressure and therefore, wind speeds.
In summary, we learned how pressure differentials measured during flight correlate to flow velocity. We then calibrated a pressure transducer by subjecting a pitot-static tube to varying wind speeds, and determined the relationship between voltage and wind speed.