A method for measuring the center of gravity of a backpack

Abstract

This invention discloses a method for measuring the center of gravity of a backpack, comprising the following steps: establishing a local coordinate system A based on the backpack; establishing a global coordinate system B based on the laboratory, and establishing a relationship between the local coordinate system A and the global coordinate system B; measuring the projected coordinates of the backpack's center of gravity C0 in the global coordinate system B in two different backpack postures; calculating the three-dimensional coordinates of the backpack's center of gravity C0 in the global coordinate system B; and calculating the three-dimensional coordinates of the backpack's center of gravity C0 in the local coordinate system A. Compared with traditional center of gravity measurement methods such as the suspension method, the data obtained by this invention is not only closer to reality, but also more stable and easier to operate. Furthermore, by combining dynamics and kinematics, this method completely solves the problem of determining and dynamically tracking the backpack's center of gravity, providing practical methodological support for scientific research related to backpacks.

Description

Technical Field

[0001] The present invention relates to the research field of human biomechanics mechanism, and in particular to a method for measuring the center of gravity of a backpack. Background Art

[0002] Backpacks are the most common way to carry items and are widely used by soldiers, students, and climbers. The biomechanical outcome variables studied in the field of backpack loading can be roughly categorized into four groups: (1) kinetics (e.g., joint torques); (2) kinematics (e.g., joint angles); (3) electromyography (EMG) (e.g., integrated electromyography (IEMG)); and (4) spatiotemporal (e.g., rhythm). Accordingly, these biomechanical variables provide insights into the potential mechanisms of injury and physical degradation in backpackers. For example, carrying a backpack with a lower center of gravity is beneficial for increasing trunk stability and preventing fall injuries during mountaineering activities, and the optimal weight of a backpack for students should be less than 15% of their body weight.

[0003] Although there are a large number of studies on the kinematics, dynamics, and muscle activity of backpack loading, the researchers in these studies have focused on the performance of the human body after carrying a backpack, and the effects of backpacks of different weights and designs on the human body. There are not many studies on the biomechanical mechanisms of the human body's compensatory posture after carrying a backpack.

[0004] Dynamic analysis based on motion capture allows us to understand the exact position of the human body's center of gravity and its changes during exercise. If we also obtain the position of the backpack's center of gravity during exercise, then the relative positional relationship between the human body's center of gravity and the backpack's center of gravity during exercise becomes clear. The strategic mechanism of how the human body controls the backpack's position will also become clearer, allowing us to provide more data references for backpack design and improvements to carrying strategies.

[0005] Without adjusting the body posture, carrying a loaded backpack will cause the center of mass of the entire system to move backward and higher in the sagittal plane. Therefore, in order to maintain the normal vertical trajectory of the body's center of gravity, the body begins to lean forward, and the amplitude of the forward lean changes linearly with the change of the load. The change of the center of gravity during movement can be used as an effective reference indicator for balance and neural control. However, although the movement of the center of gravity of the human body can be estimated using a human body model, the movement of the center of gravity of the backpack (trajectory, speed, etc.) has not been reported in current studies. The main reason is that researchers cannot well determine the time series of the position coordinates of the center of gravity of the backpack, and therefore cannot deterministically give the mechanical load (acceleration and inertia moment of the trunk) of the backpack on the human body. This may lead to our inability to fully understand the mechanical mechanisms of various posture compensations generated by the human body under load.

[0006] Therefore, whether more accurate information on the external mechanical conditions of human motion can be provided for subsequent dynamic analysis depends on whether the real-time three-dimensional spatial coordinates of the backpack's center of gravity in motion can be determined. Summary of the Invention

[0007] In response to the above-mentioned problems existing in the prior art, the present invention provides a method for measuring the center of gravity of a backpack, which can determine the real-time three-dimensional spatial coordinates of the center of gravity of the backpack in a moving state, and can provide a curve of the change of the center of gravity of the backpack over time, which helps to better understand the mechanical mechanisms of various posture compensations produced by the human body under a load state.

[0008] The technical solutions of the present invention are as follows:

[0009] A method for measuring the center of gravity of a backpack comprises the following steps:

[0010] S1. Establish a local coordinate system A based on the backpack;

[0011] S2. Establish a global coordinate system B based on the laboratory, and establish the relationship between the local coordinate system A and the global coordinate system B;

[0012] S3, measuring the projection coordinates of the backpack's center of gravity C0 in the global coordinate system B at two different postures of the backpack;

[0013] S4, calculate the three-dimensional coordinates of the backpack's center of gravity C0 in the global coordinate system B;

[0014] S5. Calculate the three-dimensional coordinates of the backpack's center of gravity C0 in the local coordinate system A.

[0015] Furthermore, the specific steps of step S1 are as follows:

[0016] S1-1. Set three markers P0, P1, and P2 on the backpack, and set P0 as the origin of the local coordinate system A; the three-dimensional coordinates of the backpack's center of gravity C0 in the local coordinate system A are (x, y, z), and the three-dimensional coordinates in the global coordinate system B are (X, Y, Z); the vector of the backpack's center of gravity C0 and P0 is It is expressed as a constant in the local coordinate system A and as a variable in the global coordinate system B;

[0017] S1-2, take vector Construct the coordinate axis vectors of the local coordinate system A respectively:

[0018] x-axis:

[0019] y-axis:

[0020] z-axis:

[0021] The three coordinate axis vectors of the local coordinate system A are obtained using the vector product formula as follows:

[0022]

[0023] The meaning of each variable is as follows: is the x-axis basis vector of the global coordinate system B, is the y-axis basis vector of the global coordinate system B, is the z-axis basis vector of the global coordinate system B;

[0024] is the x-axis vector of the local coordinate system A The expression in the global coordinate system B, A 11 yes The component in the x-axis direction of the global coordinate system B, A 12 yes The component in the y-axis direction of the global coordinate system B, A 13 yes The component in the z-axis direction of the global coordinate system B;

[0025] is the y-axis vector of the local coordinate system A The expression in the global coordinate system B, A 21 yes The component in the x-axis direction of the global coordinate system B, A 22 yes The component in the y-axis direction of the global coordinate system B, A 23 yes The component in the z-axis direction of the global coordinate system B;

[0026] is the z-axis vector of the local coordinate system A The expression in the global coordinate system B, A 31 yes The component in the x-axis direction of the global coordinate system B, A 32 yes The component in the y-axis direction of the global coordinate system B, A 33 yes The component in the z-axis direction of the global coordinate system B;

[0027] Normalizing the above three coordinate axis vectors, we get the three basis vectors of the local coordinate system A as follows:

[0028]

[0029] Furthermore, the specific method of step S2 is as follows:

[0030] The projection of the three basis vectors of the local coordinate system A on the global coordinate system B constitutes the direction cosine matrix:

[0031]

[0032] Furthermore, the specific steps of step S3 are as follows:

[0033] S3-1, according to the direction cosine matrix [A br ]Establish the following relationship:

[0034] [a b ]=[A br ]1[a r ] 1 =[A br ]2[a r ] 2 (1)

[0035] where [a b ] is the coordinate matrix of the backpack's center of gravity C0 in the local coordinate system A, [a r ] is the coordinate matrix of the backpack's center of gravity C0 in the global coordinate system B, and the subscripts 1 and 2 represent two different postures of the backpack;

[0036] S3-2. Select a weight W for fixing and carrying the backpack. m The backpack carrying object is placed on the force measuring platform and the coordinates of the pressure center COP of the backpack carrying object in the global coordinate system B are measured and recorded as (x m ,y m );

[0037] S3-3, a weight of W b The backpack is fixed on the backpack load, and the COP data is measured again. The combined pressure center COP of the backpack and the backpack load is (x c1 ,y c1 );

[0038] S3-4, loosen the backpack straps, adjust the backpack posture, and measure the COP data again. The combined pressure center COP of the backpack and the backpack load is (x c2 ,y c2 ).

[0039] Furthermore, the specific method of step S4 is as follows:

[0040] Through the plane force balance equation we get:

[0041]

[0042] According to equation (2), we can get (x i ,yi ), which is the coordinate matrix [a r ] in the x and y coordinates;

[0043] According to the relationship (1), we can get:

[0044] [A br ]1[a r ]1=[A br ]2[a r ] 2 (3)

[0045] Expands to:

[0046]

[0047] Move items to organize:

[0048]

[0049] Merge and we get:

[0050]

[0051] The right side of equation (4) is a known quantity, C i Representation; the matrix equations are combined in pairs to produce three equation groups:

[0052]

[0053] Solve the above equations and average the Z values ​​obtained from the three combinations to form the coordinate matrix [a r ] is the optimal solution for the z coordinate in .

[0054] Furthermore, the specific method of step S5 is as follows:

[0055] According to the relationship (1), the equations are listed respectively:

[0056] [a b ]=[A br ]1[a r ] 1 (8)

[0057] [a b ]=[A br ]2[a r ] 2 (9)

[0058] Calculate [a b ], take the average result as [a b ], thus obtaining the three-dimensional coordinates of the backpack's center of gravity C0 in the local coordinate system A [a b ].

[0059] The beneficial technical effects of the present invention are:

[0060] Compared to traditional center of gravity measurement methods such as the suspension method, direct measurements using a mannequin with a backpack on its back yield data that is not only closer to reality but also more stable and easier to use. Furthermore, in most applications, three-dimensional center of gravity measurement of irregular geometric objects only requires determining the static center of gravity of the object. However, the dynamic changes in the center of gravity of a backpack as it moves with the human body are a more important input parameter for human force analysis. This invention constructs a fixed local coordinate system on the outside of the backpack and establishes a transformation relationship with the laboratory's global coordinate system. By combining dynamics and kinematics, this method completely solves the problem of determining and dynamically tracking the backpack's center of gravity, providing practical methodological support for scientific research related to backpacks. BRIEF DESCRIPTION OF THE DRAWINGS

[0061] Figure 1 It is a schematic diagram of the backpack landmark points;

[0062] Figure 2 It is a schematic diagram of measuring the center of pressure COP of a backpack and a mannequin using a force plate;

[0063] Figure 3 This is a schematic diagram of using Visual3D to simulate the movement of the backpack's center of gravity;

[0064] Figure 4 It is the motion curve of the center of gravity of the backpack. DETAILED DESCRIPTION

[0065] The present invention is described in detail below with reference to the accompanying drawings and embodiments. It is apparent that the embodiments described are only a portion of the embodiments of the present invention, rather than all of them. All other embodiments derived by persons of ordinary skill in the art based on the embodiments of the present invention without inventive effort are intended to fall within the scope of protection of the present invention.

[0066] This embodiment primarily determines the relative coordinates of the backpack's center of gravity relative to the local coordinate system, assuming that these relative coordinates do not change (or change within an acceptable range) during motion. This allows for coordinate transformation to determine the changes in the global coordinates of the backpack's center of gravity during motion, enabling motion tracking of the backpack's center of gravity. Therefore, the experimental backpack requires that the internal filling be compacted, meaning that it exhibits minimal deformability. Once the backpack is properly loaded, small sandbags can be used to fill gaps between items.

[0067] The example uses 5 steps to measure the center of gravity of a backpack:

[0068] S1. Establish a local coordinate system A based on the backpack;

[0069] S2. Establish a global coordinate system B based on the laboratory, and establish the relationship between the local coordinate system A and the global coordinate system B;

[0070] S3, measuring the projection coordinates of the backpack's center of gravity C0 in the global coordinate system B at two different postures of the backpack;

[0071] S4, calculate the three-dimensional coordinates of the backpack's center of gravity C0 in the global coordinate system B;

[0072] S5. Calculate the three-dimensional coordinates of the backpack's center of gravity C0 in the local coordinate system A.

[0073] The specific methods of the above 5 steps are as follows:

[0074] 1. Specific method of step S1:

[0075] S1-1. Cut a thermoplastic plate of appropriate size, shape it through heat treatment, and then process it into an appropriate curvature so that it fits well with the surface of the backpack. Fix the two with double-sided tape (or tie them together with foam skin film) so that they are integrated with the backpack and no longer have relative movement.

[0076] S1-2, stick three reflective marker balls with a diameter of 14 mm on the thermoplastic board (such as Figure 1 ) is used for spatial positioning and motion capture by an infrared optical capture system (brand is British Vicon).

[0077] S1-3. Set the names of the three marker balls as P0, P1, and P2 respectively, and use them to establish the local coordinate system A, with P0 as the origin; set the coordinates of the backpack center of gravity C0 in the local coordinate system A as (x, y, z), and the coordinates in the global coordinate system B as (X, Y, Z); the vector of the backpack center of gravity C0 and P0 is It is expressed as a constant in the local coordinate system A and as a variable in the global coordinate system B;

[0078] S1-4, take vector Construct the coordinate axis vectors of the local coordinate system A respectively:

[0079] x-axis:

[0080] y-axis:

[0081] z-axis:

[0082] Using the vector product formula (with vector and Take multiplication as an example, where the subscripts are coordinate components):

[0083]

[0084] In this way, the three coordinate axis vectors of the local coordinate system A can be obtained and can be expressed as follows (in vector form): For example):

[0085]

[0086] The three coordinate axis vectors of the local coordinate system A are as follows:

[0087]

[0088] The meaning of each variable is as follows: is the x-axis basis vector of the global coordinate system B, is the y-axis basis vector of the global coordinate system B, is the z-axis basis vector of the global coordinate system B;

[0089] is the x-axis vector of the local coordinate system A The expression in the global coordinate system B, A 11 yes The component in the x-axis direction of the global coordinate system B, A 12 yes The component in the y-axis direction of the global coordinate system B, A 13 yes The component in the z-axis direction of the global coordinate system B;

[0090] is the y-axis vector of the local coordinate system A The expression in the global coordinate system B, A 21 yes The component in the x-axis direction of the global coordinate system B, A 22 yes The component in the y-axis direction of the global coordinate system B, A 23 yes The component in the z-axis direction of the global coordinate system B;

[0091] is the z-axis vector of the local coordinate system A The expression in the global coordinate system B, A 31 yes The component in the x-axis direction of the global coordinate system B, A 32 yes The component in the y-axis direction of the global coordinate system B, A 33 yes The component in the z-axis direction of the global coordinate system B.

[0092] By normalizing the above three-axis vectors, we can obtain three basis vectors (unit vectors):

[0093] (r is the vector modulus)

[0094] The three basis vectors of the local coordinate system A are obtained as follows:

[0095]

[0096] 2. Specific method of step S2:

[0097] The projection of the three basis vectors of the local coordinate system A on the global coordinate system B constitutes the direction cosine matrix:

[0098]

[0099] Through this direction cosine matrix, the coordinate transformation between the local coordinate system A and the global coordinate system B can be achieved.

[0100] 3. Specific method of step S3:

[0101] Assume [a b ] is the coordinate matrix of the backpack's center of gravity C0 in the local coordinate system A, [a r ] is the coordinate matrix of the backpack's center of gravity C0 in the global coordinate system B. Then the direction cosine matrix [A br ], the following relationship can be established:

[0102] [a b ]=[A br ]1[a r ] 1 =[A br ]2[a r ] 2 (1)

[0103] The subscripts 1 and 2 represent two different postures of the backpack. The meaning of the above formula is: no matter how the coordinates of the backpack's center of gravity C0 in the global coordinate system B change, it can always be transformed into the local coordinate system A through the coordinate transformation matrix. Since the backpack's center of gravity C0 is fixed relative to the local coordinate system A [a b ], so through the global coordinate equations of two different positions given by the above formula, we can solve the expression of the backpack's center of gravity C0 in the local coordinate system A.

[0104] The problem now comes down to how to get [a r ]. The measurement steps are as follows:

[0105] S3-1. Place the mannequin without a backpack on a force platform (Kistler, Switzerland). Use a barbell plate to weight the mannequin base (e.g. Figure 2); measure the coordinates of the pressure center COP in the global coordinate system B, denoted as (x m ,y m );

[0106] S3-2, put the backpack across the model and measure the COP data again (such as Figure 2 ); The combined pressure center of the backpack and the human model in this posture is (x c1 ,y c1 );

[0107] S3-3, loosen the backpack straps to a certain extent and obtain the COP data of the backpack in another posture (such as Figure 2 ); The combined pressure center of the backpack and the human model in this posture is (x c2 ,y c2 ).

[0108] 4. Specific method of step S4:

[0109] The weight of the mannequin (including the base) is known to be W m , the backpack weight is W b , combined with the above measurement results, we can get Figure 2 The projection coordinates of the center of mass C0 of the backpack (x i ,y i )(i=1,2), as follows:

[0110] In a static state, the ground reaction force is equal to the gravity (a pair of balanced forces) and has no horizontal component. Through the plane force balance equation, it is easy to get:

[0111]

[0112] Solve (x i ,y i ), that is, to determine [a r ] The x and y coordinates in the coordinate matrix are missing, but the z coordinate is missing. In other words, the projection coordinates of the backpack's center of gravity C0 on the horizontal plane of the global coordinate system B have been obtained.

[0113] According to the relationship (1), we can get:

[0114] [A br ]1[a r ] 1 =[A br ]2[a r ] 2 (3)

[0115] Expands to:

[0116]

[0117] Move items to organize:

[0118]

[0119] Merge and we get:

[0120]

[0121] The right side of equation (4) is a known quantity, C i Representation; the matrix equations are combined in pairs to produce three equation groups:

[0122]

[0123] Solve the above equations, average the Z values ​​obtained from the three combinations, and determine Z1 and Z2 as the coordinate matrix [a r ] is the optimal solution for the z coordinate in . In this way, the three-dimensional coordinates of the backpack center of gravity C0 in the global coordinate system B under the two postures are calculated.

[0124] 5. Specific method of step S5:

[0125] According to the relationship (1), the equations are listed respectively:

[0126] [a b ]=[A br ]1[a r ] 1 (8)

[0127] [a b ]=[A br ]2[a r ] 2 (9)

[0128] Calculate [a b ], take the average result as [a b ], thus obtaining the three-dimensional coordinates of the backpack's center of gravity C0 in the local coordinate system A [a b ].

[0129] Get the three-dimensional coordinates of the backpack's center of gravity C0 in the local coordinate system A [a b ], the motion curve of the backpack's center of gravity C0 can be easily simulated using the motion analysis software Visual3D. Figure 3 As shown, the local coordinate system A is still established using the corresponding points P0, P1, and P2 on the local coordinate system A. A virtual point is established for the center of gravity C0 of the backpack, named bagcog, and its three relative coordinates are filled in with the [a b ], that is, enter [a b ]Z coordinate, enter [ab ], enter [a b ]'s X coordinate. Figure 4 This is a three-dimensional time-based curve of the backpack's center of gravity, C0, in the global coordinate system B, while a subject walks on a treadmill while wearing a backpack. From top to bottom, the X, Y, and Z coordinates represent forward, backward, left, right, and upward / downward motion, respectively. Its fluctuations are well synchronized with both the human torso center of gravity and the overall center of gravity.

[0130] Although the embodiments of the present invention have been disclosed as above, they are not limited to the applications listed in the description and implementation methods. They can be fully applied to various fields suitable for the present invention. For those familiar with the art, for those of ordinary skill in the art, various changes, modifications, substitutions and variations can be made to these embodiments without departing from the principles and spirit of the present invention. Therefore, without departing from the general concept defined by the claims and the scope of equivalents, the present invention is not limited to specific details.

Claims

1. A method for measuring the center of gravity of a backpack, characterized in that: The following steps are involved: S1. Establish a local coordinate system A based on the backpack and a global coordinate system B based on the laboratory. S2. Establish the relationship between the local coordinate system A and the global coordinate system B. The specific steps are as follows: S2-1. Set three markers P0, P1, and P2 on the backpack, and set P0 as the origin of the local coordinate system A; the three-dimensional coordinates of the backpack's center of gravity C0 in the local coordinate system A are (x, y, z), and the three-dimensional coordinates in the global coordinate system B are (X, Y, Z); the vector of the backpack's center of gravity C0 and P0 is It is expressed as a constant in the local coordinate system A and as a variable in the global coordinate system B; S2-2, take vector Construct the coordinate axis vectors of the local coordinate system A respectively: x-axis: y-axis: z-axis: The three coordinate axis vectors of the local coordinate system A are obtained using the vector product formula as follows: The meaning of each variable is as follows: is the x-axis basis vector of the global coordinate system B, is the y-axis basis vector of the global coordinate system B, is the z-axis basis vector of the global coordinate system B; is the x-axis vector of the local coordinate system A The expression in the global coordinate system B, A 11 yes The component in the x-axis direction of the global coordinate system B, A 12 yes The component in the y-axis direction of the global coordinate system B, A 13 yes The component in the z-axis direction of the global coordinate system B; is the y-axis vector of the local coordinate system A The expression in the global coordinate system B, A 21 yes The component in the x-axis direction of the global coordinate system B, A 22 yes The component in the y-axis direction of the global coordinate system B, A 23 yes The component in the z-axis direction of the global coordinate system B; is the z-axis vector of the local coordinate system A The expression in the global coordinate system B, A 31 yes The component in the x-axis direction of the global coordinate system B, A 32 yes The component in the y-axis direction of the global coordinate system B, A 33 yes The component in the z-axis direction of the global coordinate system B; S2-3. Normalize the three coordinate axis vectors mentioned above to obtain the three basis vectors of the local coordinate system A as follows: S2-4. The projections of the three basis vectors of the local coordinate system A on the global coordinate system B form the direction cosine matrix: S3. Measure the projection coordinates of the backpack's center of gravity C0 in the global coordinate system B at two different backpack postures. The specific steps are as follows: S3-1, according to the direction cosine matrix [A br ]Establish the following relationship: [a b ]=[A br ]1[a r ]1=[A br ]2[a r ]2 (1) where [a b ] is the coordinate matrix of the backpack's center of gravity C0 in the local coordinate system A, [a r ] is the coordinate matrix of the backpack's center of gravity C0 in the global coordinate system B, and the subscripts 1 and 2 represent two different postures of the backpack; S3-2. Select a weight W for fixing and carrying the backpack. m The backpack carrying object is placed on the force measuring platform and the coordinates of the pressure center COP of the backpack carrying object in the global coordinate system B are measured and recorded as (x m ,y m ); S3-3, a weight of W b The backpack is fixed on the backpack load, and the COP data is measured again. The combined pressure center COP of the backpack and the backpack load is (x c1 ,y c1 ); S3-4, loosen the backpack straps, adjust the backpack posture, and measure the COP data again. The combined pressure center COP of the backpack and the backpack load is (x c2 ,y c2 ); S4. Calculate the three-dimensional coordinates of the backpack's center of gravity C0 in the global coordinate system B. The specific method is as follows: The plane force balance equation is obtained: According to equation (2), we can get (x i ,y i ), which is the coordinate matrix [a r ] in the x and y coordinates; According to the relationship (1), we can get: [A br ]1[a r ]1=[A br ]2[a r ]2 (3) Expands to: Move items to organize: Merge and we get: The right side of equation (4) is a known quantity, C i Representation; the matrix equations are combined in pairs to produce three equation groups: Solve the above equations and average the Z values ​​obtained from the three combinations to form the coordinate matrix [a r ] in the optimal solution of the z coordinate; S5. Calculate the three-dimensional coordinates of the backpack's center of gravity C0 in the local coordinate system A.

2. The method for measuring the center of gravity of a backpack according to claim 1, wherein: The specific method of step S5 is as follows: According to the relationship (1), the equations are listed respectively: [a b ]=[A br ]1[a r ]1 (8) [a b ]=[A br ]2[a r ]2 (9) Calculate[a b ], take the average result as [a b ], thus obtaining the three-dimensional coordinates of the backpack's center of gravity C0 in the local coordinate system A [a b ].

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