Research on a micro-rolling-moment six-component strain gauge balance of composite structure

Wang Yuhua, Liu Wei, Xie Bin, Zhang Yue(China Aerodynamics Research and Development Center, Mianyang Sichuan 621000, China)

Research on a micro-rolling-moment six-component strain gauge balance of composite structure

Wang Yuhua*, Liu Wei, Xie Bin, Zhang Yue
(China Aerodynamics Research and Development Center, Mianyang Sichuan 621000, China)

Rolling-moment measurement at micro level is extremely important for wind tunnel testing. Compared to conventional force tests, micro-rolling-moment measurement test is characterized by small rolling moment loading whose value of quantity is below 0.5N·m, and an obvious difference between rolling moment load and other aerodynamic loads in the value of quantity. It is a great challenge to design a six-component strain gauge balance of appropriate structure for that purpose. To solve this problem, efforts were made to optimize the balance structure and the concept of composite six-component balance was developed and realized, which has reduced the measurement deviation (Δmx0<1.1×10-6) and has addressed the configuration contradictions. In this paper, key techniques and primary technical measures for the newly-designed balance are introduced, with an account of the results of balance analysis and calculation, static calibration and test measurements.

balance;micro-rolling-moment;composite structure;wind tunnel

0 Introduction

At present, there are two ways to measure small-rolling-moment in wind tunnel tests. One is the using conventional multi-component balance or double cross elasticity of flexure structure balance[2-3], and the other is using air balance system[4-5]. As for micro-rolling-moment whose value of quantity is below 0.5N·m, the conventional multi-component balance or double cross elasticity of flexure structure balance is not able to conduct the measurement. The air balance can measurte the micro-rolling-moment, but it is a single component balance, so it can’t implement six components measurement at the same time.

In some special high-speed wind tunnel tests, the rolling moment of the balance is much lower than the loads of other components[6]. Take the ratio of normal forceYto rolling moment for example, for conventional six-component balance, the ratio ranges from 50/m to 120/m, whereas for the micro-rolling-moment six-component balance it is about 250/m. In either case, the measurement accuracy would be affected due to the fact that the interference of the other components on the rolling moment cannot be totally corrected. It’s necessary to develop a micro-rolling-moment six-component strain gauge balance that will not only help achieve the goal of simultaneous measurement of six components, but also improve the accuracy of the wind tunnel testing.

1 Technical key points and difficulties

1.1 Small rolling moment balances

In the past, HSAI of CARDC developed quite a few five-component balances, which can measure smallMxranging from 0.3N·m to 0.5N·m or above. Table 1 lists HSAI’s conventional balances withMx≤1N·m, including their specified load andMxsensitivities. As is shown in Table 1, there is just one six-component balance in the stock, whoseMxsensitivity is only 1.0mV/(N·m).

Table 1 Design roads and Mx sensitivity of balances for small-rolling-moment measurement表1 部分小滚转力矩天平设计载荷及Mx分量灵敏度

1.2 Strength vs. sensitivity for rolling moment component

The key technical problem in the balance development is the design of strength and sensitivity for the rolling moment component[7-8].

At present the component that is sensitive toMxload normally is of a “米”-shaped structure and is positioned at the geometric center of symmetry of the balance to reduce the stresses exerted by other components on theMxcomponent[9]. Figure 1 shows a typical five-component smallMxbalance. The structure has greatly reduced the stresses byYandZcomponents onMxcomponent, thus having guaranteed component strength.

Fig.1 Typical structure of a five-component with small Mx balance

Normally, to take both axial force and rolling moment into consideration, the axial force component and rolling moment component of the six-component balance for relatively small rolling moment assume a tandem structure with the two components separated from each other (Fig.2). However, in the case of the balance with micro-rolling-moment, the same structure would lead to contradiction between sensitivity and strength for the rolling moment component and an increase in the length of the balance. If the rolling moment is measured with normal force or side force component, the sensitivity requirement cannot be met.

Fig.2 Tandem structure with two components

2 Composite structured Mx component

To solve the problems, we conjured up a composite structure that integrates axial force and rolling moment components as shown in Fig.3. TheMxcomponent consists of three pairs of symmetrically arranged beams forming a“米”-shaped structure. Based on calculation and analysis, the angles between the central planes of each beam and the horizontal plane being 90°, 20° and -20° respectively(1#,2#,3#). In this case, axial force is measured by the spring and rolling moment is measured by the “米”-shaped structure in the middle of the axial force component. With this design, the sensitivity ofMxcomponent has been greatly improved, and the stresses acted on theMxcomponent by other components, especially byYandZ, have been reduced to ensure component strength and reduce interference on rolling moment component deformation which indicates greater anti-interference ability.

Fig.3 Composite structure with six components

Due to the presence of normal force and side force, with the same electrical bridge output, if the rolling moment component is a separate part, then the distance between its root and the reference center of the balance moment will be lengthened. In contrast, the bending moment stress induced by normal force or side force at the root of the rolling moment component will be reduced.

3 Application and results analysis

3.1 Major technical parameters

There is a six-component internal balance with a diameter (D) of 24mm and a component length (L) of 80mm. The balance is primarily intended for measuring test model’s rolling moment at micro level (Mx0) with the angle of attack near zero. The basic measurement requirement is Δmx0<5×10-6, and we try to achieve Δmx0<3×10-6.The specified loads for the components are shown in Table 2.

Table 2 Design roads of balance表2 天平设计载荷

3.2 Sensitivity

The material for the balance and supportis F141, whose limit of bending strength isσb=1862N/mm2, longitudinal elastic modulus isE=1.8725×105N/mm2, and shear elastic modulus isG=0.72×105N/mm2[10].

There are two independent rolling moment components in design, namely,Mx1andMx2, and the superposition of which makesMx3. The calculation formulae of the output for theMx1andMx2are traditional. The outputMx3is the sum of outputs of theMx1andMx2.

When in use, the components can either work separately or function as one measuring component by double bridges. The output of the three components can be compared and crosschecked, therefore improving the credibility of balance measurement.

BridgeMx1is composed of 16 strain gauges attached to Beam 2 and Beam 3, and BridgeMx2is composed of 8 strain gauges attached to Beam 1. The bridge voltages are 24V and 12V respectively. The full scale output voltage forMx1is approximately 7.0mV, which is a relatively higher value; whereas that forMx2is around 2.5mV, which is a bit lower. Comparatively speaking, with a value of about 9.5mV, the full scale output voltage forMx3is the highest. The sensitivity ofMx3is 47.5mV/(N·m), it is higher than that of all the other present smallMxbalances. The results comparison of calculation and calibration for different components are shown in Table 3.

Table 3 Results comparison of calculation and calibration for different components表3 各分量计算和校准结果比较

3.3 Strength

The balance is intended for use in hypersonic wind tunnel withMa=5～8. Projector is used in the test. Impulsive factorn=3, safety factorK=2, allowable stress is as follows:

The result of strength analysis is shown in Fig.4. The maximum stress of 220.1 N/mm2occurs at the lateral side, on top of Beam 1, and at the root of theMxcomponent. The max stress is below the allowable value and therefore can meet the strength requirement[11-12].

Fig.4 Balance strength analysis result

3.4 Static calibration

The static calibration of the balance was conducted with a body axis calibration rig[13-16]. A special loading device was applied in the process so that the interference of different components on Mx can be accurately corrected. Table 4 is a list of loads and calibration uncertainty for the components.

Table 4 Loads and calibration uncertainty 表4 天平载荷及校准不确定度

3.5 Analysis of test result

Four repeatability tests were performed atMa=5 to verify the performance of the new balance. Table 5 gives the limit deviations of the rolling moment components. The limit deviations of the other components are the same with that of conventional balances.

It is indicated by repetitive tests that all the technical parameters of the balance have met the design requirement. AtMa=5, the mean limit deviation forMx1andMx3is Δmx0<1×10-6; forMx2, the mean limit deviation is Δmx0<2.2×10-6due to its comparatively lower sensitivity and output signal.

Table 5 Limit deviations of the rolling moment components表5 重复性试验极限偏差

In the testing process, the balance demonstrated good anti-interference ability owing to itscomposite structure. AtMa=5, mean limit deviation for Δmx0is very small and atMa=8, Δmx0<1.1×10-6, which suggests stable performance. The value ofMx0is a little lower than that of single-component small rolling moment balances used in prior tests at HSAI, primarily because the latter cannot be deducted of the influence of normal force (side force) onMx0measurement as the former.

4 Conclusions

It is proven by test results that the newly-developed composite structured balance is a successful improvement, which has addressed the contradiction between strength and sensitivity as well as accomplishing accurate measurement of six components simultaneously. The balance boasts a rolling moment component of high sensitivity, good anti-interference, repeatability and stability.

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Author biography:

Wang Yuhua(1971-),female, born in Tianjin, senior Engineer. Research field: development and application in wind tunnel strain gage balance. Address: China Aerodynamics Research and development Center, Mianyang, Sichuan(621000). E-mail:1394577243@qq.com

(编辑：李金勇)

1672-9897(2015)03-0076-05

WangYH,LiuW,XieB,etal.Researchonamicro-rolling-momentsix-componentstraingaugebalanceofcompositestructure.JournalofExperimentsinFluidMechanics, 2015, 29(3): 76-79,98. 王玉花, 刘伟, 谢斌, 等. 复合式结构微量滚转力矩六分量天平研究. 实验流体力学, 2015, 29(3): 76-79,98.

复合式结构微量滚转力矩六分量天平研究

王玉花*, 刘 伟, 谢 斌, 张 悦

(中国空气动力研究与发展中心, 四川 绵阳 621000)

微量滚转力矩测力试验是一项非常重要的风洞试验。相对于常规测力试验，微量滚转力矩测力试验的滚转力矩载荷小，量值在0.5N·m以下，同时滚转力矩载荷与其它的气动载荷量值相差较大，采用应变天平测量微量滚转力矩，很难实现六分量天平结构设计。针对这一问题我们开展了六分量微量滚转力矩天平的研究工作，通过优化天平结构，在现有技术手段的基础上提出了复合式六分量天平结构设计解决方案，不仅实现了六分量微量滚转力矩天平结构设计，而且其滚转力矩系数mx0的测量误差Δmx0<1.1×10-6。本文主要介绍了复合式六分量微量滚转力矩天平研究的关键技术及主要技术措施，并给出了天平分析计算、静态校准和试验测量结果。

天平；微量滚转力矩；复合式结构；风洞

V211.752

A

date: 2014-07-28;Revised date:2015-01-08

10.11729/syltlx20140087

*Corresponding author E-mail: 1394577243@qq.com