Abstract: This article introduces and analyzes the principles of the digital intelligent aircraft weighing system and torque balance, and proposes a method for accurately measuring the aircraft wheel track (i.e. the actual force application point position of the aircraft on the weighing platform) using the digital intelligent aircraft weighing system and torque balance principle. It also provides the precautions that should be paid attention to in this method, thereby ensuring accurate, fast, and convenient measurement of the aircraft wheel track.
Keywords: digital; Aircraft weighing; Torque balance; Track measurement
At present, there are many methods to measure the Barycentric coordinate system of objects, such as gravity method, suspension method, moment balance method, conservation of mass moment method, and so on. The moment balance method is the most commonly used one. In recent years, with the development of force load cells from analog to digital, research and practice have continuously improved digital intelligent circuits, digital compensation technology, and digital compensation processes. The development of integrated digital intelligent force load cells and digital intelligent weighing system technology has become mature; Similarly, aircraft weighing systems have also developed with the development of digital load cells and intelligent digital weighing systems. Although the measurement of aircraft wheel track has changed from the traditional tape measure to the current high-precision and costly laser three-dimensional measurement, no matter how accurate the measured track is, the result is only the line length of the Centroid point of the aircraft wheel, not the line length of the actual force action point when the aircraft wheel acts on the ground (or the scale platform), which inevitably affects the measurement result of the aircraft center of gravity. This article introduces and analyzes the principles of the digital intelligent aircraft weighing system and the structure of the weighing platform, and proposes a method for accurately measuring the aircraft wheel track (i.e. the actual force application point position of the aircraft on the weighing platform) using the structural characteristics of the digital intelligent aircraft weighing system and the principle of torque balance. It also provides precautions for this method to ensure accurate, fast, and convenient measurement of the aircraft wheel track.
1:The Current Situation of Aircraft Wheel Track Measurement and the Characteristics of Digital Intelligent Aircraft Weighing System
1.1. Current Situation of Aircraft Wheel Track Measurement
The work of measuring the length of the connecting line between the two points at the center of an aircraft wheel axle usually requires measuring its horizontal distance, which is the length of the projection of the connecting line between the two points at the center of the aircraft wheel axle on a certain horizontal reference plane. At present, the wheelbase measurement of aircraft wheels has developed from the earliest steel tape, tape and other traditional distance measuring tools to optical, magnetic wave Distance measuring equipment, electronic Total station, electronic Dumpy level, as well as all-purpose and laser intelligence, and the measurement accuracy is also getting higher and higher. However, the highest measurement accuracy is only the geometric position and size of the aircraft wheel base, rather than the actual position and size of the force acting on the ground or weighing platform by the aircraft wheel. The torque balance principle used in the weight and center of gravity measurement of the aircraft requires
The current measurement methods affect the accuracy of aircraft weight and center of gravity measurement due to the location and size of the actual force application point.
1.2:Principle of torque balance
Moment can cause objects to rotate in different directions. If these two moments are of equal magnitude, the lever will maintain balance. This is the lever balance condition in junior high school textbooks and the simplest case of torque balance; If the moment of rotation of an object in the counterclockwise direction is specified as a positive moment and the moment of rotation in the clockwise direction is specified as a negative moment, then the equilibrium condition of an object with a fixed axis of rotation is that the algebraic sum of the moments is zero, that is, the situation where the combined moments of multiple forces acting on the object are zero is called moment equilibrium.
In engineering practice, people generally use the torque balance principle equation to support an object in a balanced state through multiple force load cells to determine the magnitude of the force at each support point. The direction of the force is determined based on the direction of the torque (counterclockwise or clockwise), and the position of the force action line at each support point is determined through measurement. The measurement of aircraft weight and center of gravity is carried out using the principle of torque balance. This article will also discuss the method of measuring aircraft wheel track using the principle of torque balance and the weighing platform of the digital intelligent aircraft weighing system in practical applications. The measured wheel track is the relative position size of the actual point of force acting on each weighing platform of the aircraft wheel, which meets the requirements of aircraft weight and center of gravity measurement.
1.3:Characteristics of digital intelligent aircraft weighing system
The measurement of aircraft weight and center of gravity is based on the principle of torque balance. There are currently three methods for measuring aircraft weight and center of gravity using the principle of torque balance: platform measurement, suspension measurement, and jack measurement. Regardless of the measurement method, the magnitude of the force value is sensed by force load cells with 3 points (see Figure 1), 4 points, or multiple support points. Then, the force value data signal sensed by the load cells is collected through the collection system, and the coordinate value of the aircraft center of gravity is obtained by solving the signal through computer software.
Figure 1 Schematic diagram of platform aircraft weighing
The platform measurement system measures the force on the landing gear or support jack of an aircraft to weigh the aircraft and determine its center of gravity; The platform type measurement system is an independent weighing system that measures the weight of each wheel or jack, and each weighing platform is supported by four load cells. The most common structural form is the platform platform.
The digital aircraft weighing system uses a highly integrated and intelligent processing unit inside the load cell to preprocess the analog signal through A/D conversion, filtering, and digital compensation before outputting a digital signal. The digital weighing instrument or the computer used collects real-time output data from each load cell and processes it before displaying it; Its main characteristics are: (1) The digital aircraft weighing system can address each load cell in the system separately, allowing for monitoring, fault identification, and separate processing of each load cell,The weighing information of each load cell greatly improves the control ability, flexibility, and intelligence of the weighing system. This feature creates conditions for the measurement of aircraft wheel tread using the principle of torque balance and the use of actual aircraft weighing systems; (2) After the calibration of the weighing platform, the zero point value and full load (net output) value of each load cell are stored in the instrument or computer software. When the weighing platform is calibrated again, it can be compared with the previous calibration values. This feature simplifies the stability monitoring of load cells in the aircraft weighing system; (3) The digital weighing system can directly process the raw weighing data provided by each load cell, and each load cell can provide a resolution of over 20 bits, equivalent to 1000000 counts. A system with four load cells can provide 4 × 1000000 counts for analysis. In terms of aircraft weighing systems, such high resolution load cells, combined with high-precision compensation for load cells (such as temperature compensation, linear compensation, etc.), are very advantageous for measuring multiple types of aircraft using a weighing system.
Figure 2 Schematic diagram of two-dimensional coordinate system for aircraft weighing platform plane.
2:Principles and precautions for measuring the wheelbase of aircraft wheels
2.1:Principles of Aircraft Wheel Track Measurement
The measurement principle of aircraft wheel track is based on the characteristics of the digital intelligent aircraft weighing system’s weighing platform. The static balance equation and torque balance principle can be used to conveniently and accurately measure the position of the force points on the weighing platform.
Taking the three-point aircraft weight and center of gravity measurement system as an example (see Figure 1), the measurement of aircraft wheel track is explained. Taking the plane two-dimensional coordinate system of the aircraft weighing platform (see Figure 2), it is easy to transform this coordinate system from the coordinate system used by the aircraft weighing system in the horizontal plane, creating conditions for the integration of the aircraft weight and center of gravity measurement system and the aircraft wheel track measurement software.
Assuming that the force applied to the wheels of the A-scale aircraft is FA and the coordinates are A (X1, Y1), the forces on the four load cells are FAi (i=1, 2, 3, 4) and the coordinates are Ai (X1i, Y1i) (i=1, 2, 3, 4); The force applied to the wheels of the B-scale aircraft is FB, with coordinates B (X2, Y2). The forces on the four load cells are FBi (i=1, 2, 3, 4), and the coordinates are Bi (X2i, Y2i) (i=1, 2, 3, 4); The force applied to the wheels of the C-scale aircraft is FC, with coordinates C (X3, Y3). The forces on the four load cells are FCi (i=1, 2, 3, 4), and the coordinates are Ci (X3i, Y3i) (i=1, 2, 3, 4);
For the A-scale platform, based on the principle of static balance:
FA = FA1 + FA2 + FA3 + FA4
According to the principle of torque balance, take the moments along the X and Y axes (see Figure 2), and obtain:
FA . X1 = FA4 . X14 + FA3 . X13 一 FA2 . X12 一 FA1 . X11 FA .Y1 = FA4 .Y14 + FA3 .Y13 + FA2 .Y12 + FA1 .Y11
X = FA4 . X14 + FA3 . X13 一 FA2 . X12 一 FA1 . X11
FA1 + FA2 + FA3 + FA4
Y = FA4 .Y14 + FA3 .Y13 + FA2 .Y12 + FA1 .Y11
FA1 + FA2 + FA3 + FA4
Using the same method, obtain the coordinates of the force points on the B and C scales as follows:
X = FB4 . X24 一 FB3 . X23 一 FB2 . X22 + FB1 . X21
FB1 + FB2 + FB3 + FB4
Y = FB4 .Y24 + FB3 .Y23 + FB2 .Y22 + FB1 .Y21
FB1 + FB2 + FB3 + FB4
X = FC4 . X34 一 FC3 . X33 一 FC2 . X32 + FC1 . X31
FC1 + FC2 + FC3 + FC4
Y = FC4 .Y34 + FC3 .Y33 + FC2 .Y32 + FC1 .Y31
FC1 + FC2 + FC3 + FC4
After the assembly and debugging of the aircraft weighing system are completed, the FAi, FBi, FCi (i=1, 2, 3, 4), as well as the corresponding coordinates Ai (X1i, Y1i), Bi (X2i, Y2i), Ci (X3i, Y3i) (i=1, 2, 3, 4) are all known after leveling on the aircraft weighing platform;
Based on the above principles and the obtained BC and AM values, it is convenient to measure the wheelbase of aircraft wheels, that is, the distance between the actual force application points of each wheel on the aircraft weighing platform, ensuring more accurate measurement of the aircraft center of gravity.
2:Precautions for measuring the wheelbase of aircraft wheels
When designing an aircraft weighing platform, in addition to considering the stiffness of the platform and the force transmission structure of the load cells, the following points should also be noted:
2.1:The structure of the platform scale is generally that the scale platform is supported by four or more load cells. The floating support is a steel ball and ball socket or “Roly-poly toy” structure. Both structures are rolling friction. Before the aircraft is mounted on the scale platform, the friction coefficient between the wheel and the ground is greater than the rolling friction coefficient of the two structures, and the constraints of the aircraft landing gear are released on the scale platform, Its outward deformation causes displacement between the weighing platform and the load cell support point, affecting the measurement accuracy of the aircraft wheel track; For this purpose, a displacement load cell must be installed between the upper and lower tables in the X and Y directions of the scale platform plane for compensation, as shown in Figure 3, to ensure the measurement accuracy of the aircraft wheelbase.
Figure 3 Schematic diagram of displacement load cell compensation
2.2: If a jack is used to lift the aircraft on the platform scale platform for measuring the weight and center of gravity of the aircraft, the lateral force on the scale platform will directly affect the measurement accuracy of the system due to the deformation of the support of the aircraft wings and fuselage and the adjustment of the aircraft support attitude; In addition to installing a displacement load cell between the upper and lower tables of the weighing platform for compensation, a lateral force resistant sliding plate researched by China Ningbo Saintbond Intelligent Technology Co.,Ltd. can also be installed at the foot of each jack to compensate for displacement deviation caused by aircraft jacking or aircraft attitude adjustment (see Figure 4 for the appearance of the side sliding plate). The lateral force resistant sliding plate is composed of upper, lower, and elastic return components, The contact surface of the upper and lower sliding plates uses materials with very small friction coefficients. When the upper sliding plate is subjected to lateral force during use, the upper sliding plate can compress the elastic return body, achieving ± 30mm slip within a 360 º range. After the measurement is completed, it automatically returns to its original position.
With the continuous improvement of aircraft flight speed and altitude, the corresponding safe range of aircraft center of gravity is also shrinking, and the requirements for the accuracy of the measurement system for aircraft weight and center of gravity are also increasing. Therefore, to ensure higher measurement accuracy of aircraft center of gravity, accurate measurement of aircraft wheel pitch is also very important. The method of measuring aircraft wheel pitch introduced in this article is simple, convenient, low-cost, and technically feasible, Especially when the measured aircraft wheelbase is actually the position of the actual force acting on the aircraft wheels, it can ensure the accuracy and credibility of the aircraft center of gravity data. The author believes that this method can also be developed for measuring the wheelbase of other objects such as cars, trains, or the position of the force acting point. At the same time, it also makes it possible for aircraft weighing to develop towards digitization, intelligence, automation, and high-precision.