A variable speed cross search method for shaft hole posture alignment in assembly

Abstract

A variable-speed cross-search method for aligning the posture of a shaft and hole during assembly belongs to the field of assembly robot technology. It includes the following steps: establishing a hole coordinate system, performing a preliminary search for the posture of the hole, and after preliminarily estimating the posture angle of the hole, fixing the posture angle of the shaft in the current search direction to the estimated posture angle of the hole, while recording the positive and negative data of the shaft during the search; estimating the posture angle of the hole, and using the positive and negative data to estimate the positive clearance and negative clearance of the shaft and hole respectively; when stuck, estimating the angular motion range of the shaft in the positive and negative directions to the torque threshold at the current depth; constructing a variable-speed search function; searching along the current search direction according to the search speed, while recording the data; if the shaft and hole are stuck, repeating the steps; otherwise, continuing to insert the shaft into the hole until assembly is complete. The present invention can align the shaft with the hole more quickly, thereby reducing assembly time and average contact torque, overcoming the limitations of existing methods.

Description

Technical Field

[0001] The invention belongs to the technical field of assembly robots, and in particular relates to a speed-changing cross search method for aligning the posture of shaft holes in assembly. Background Art

[0002] In industrial production, robots are often used for assembly tasks such as circuit board assembly, automobile production, high-voltage line maintenance, and plug assembly. These tasks are equivalent to shaft-and-hole assembly. However, practical applications are limited by insufficient measurement accuracy of the shaft and hole pose, as well as insufficient robot motion precision. Consequently, in shaft-and-hole assembly with small clearances, robots cannot simply move directly to the desired pose to complete the assembly.

[0003] By visually determining the hole pose, the shaft can be directly moved into the hole even when the clearance between the shaft and the hole is large. Improving equipment precision to achieve tight clearance assembly is difficult and costly. By designing a search trajectory based on vision and incorporating force information, tight clearance assembly can be achieved without increasing the accuracy of the camera or robot. Existing search methods are primarily categorized into two types: planar search and spatial search.

[0004] The plane search method traverses the area surrounding the hole and uses force information to correct the search direction, allowing the shaft to fit into the hole. However, to achieve this traversal, the shaft must move along a dense spiral search trajectory. The smaller the shaft-hole clearance, the denser the spiral trajectory required, making the assembly process slow and time-consuming. When the shaft-hole clearance is smaller than the robot's motion accuracy and there is a significant deviation in posture, this method is almost impossible to complete the assembly.

[0005] Subsequently developed spatial search methods can improve search speed and enable close-clearance shaft-hole assembly and alignment. By tilting the shaft, the shaft end face is positioned at a distinct lowest point relative to the hole end face. This differential positional deviation between the shaft and hole produces significantly different contact forces and moments. However, in practical applications, the hole position is often difficult to accurately measure, impacting assembly accuracy and cycle time.

[0006] In summary, existing methods essentially use passive compliance to achieve posture alignment between the shaft and hole. Some literature uses active oscillation based on compliance to free the shaft from jamming and improve assembly success rates. Due to interference from friction and other forces, and because torque information can only determine whether the shaft and hole are jammed, it is impossible to estimate the magnitude of the posture deviation between the shaft and hole. Since the posture of the hole cannot be actively estimated, the shaft is truly aligned with the hole near the end of assembly. This increases the duration of jamming, thereby increasing assembly time and average contact torque. In the assembly of deep holes and shafts, posture deviations can cause jamming and contact torque between the shaft and hole. This can prolong assembly time and increase wear on the shaft and hole. In shaft-hole assembly, assembly time and contact torque are key metrics for evaluating algorithm performance. Summary of the Invention

[0007] In order to overcome the existing technology, the present invention proposes a variable speed cross search method for the alignment of the shaft hole posture in assembly, so as to solve the problem that the existing method cannot quickly and accurately estimate the posture of the hole, thereby prolonging the blocking time and assembly time and increasing the average contact torque.

[0008] A variable speed cross search method for aligning the posture of the shaft hole in the assembly includes the following steps:

[0009] S1. Establish the three coordinate axes of the hole coordinate system as x, y and z, perform a preliminary search for the hole's posture, and search in the x and y directions respectively. Drive the shaft to rotate along the x and y directions to search. After preliminarily estimating the hole's posture angle, fix the shaft's posture angle in the current search direction to the estimated hole's posture angle. At the same time, record the positive and negative data of the shaft during the search process: angle, torque and weight;

[0010] S2, estimate the attitude angle of the hole, and use the positive and negative data to estimate the positive clearance and negative clearance of the shaft hole respectively;

[0011] S3, when stuck, estimate the angular motion range of the axis in the positive and negative directions to the torque threshold at the current assembly depth along the z direction;

[0012] S4, constructing a variable speed search function;

[0013] S5, searching along the current search direction at the search speed while recording data, the recorded data being the same as in step S1;

[0014] S6. If the shaft is stuck in the hole, repeat steps S2-S5; otherwise, continue to insert the shaft into the hole until the assembly is completed.

[0015] The beneficial effects of the present invention compared to the prior art are:

[0016] 1. The method of this application can reduce the influence of data noise and actively estimate the hole posture based on the data;

[0017] 2. This method designs a variable speed search function based on the estimated attitude angle and the estimated shaft-hole clearance, which can improve the speed of attitude search;

[0018] 3. The method of the present application significantly shortens the assembly time and reduces the average contact torque in the assembly of square, circular and triangular shaft holes.

[0019] The present invention will be further described below with reference to the accompanying drawings and embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 This is a flow chart of a variable speed cross search method for aligning the posture of the shaft hole in the assembly of the present application;

[0021] Figure 2 This is the diagram of the axis hole assembly process in hole posture estimation;

[0022] Figure 3 It is a graph showing the relationship between attitude angle and search speed using a function curve;

[0023] Figure 4 It is a graph showing the relationship between contact torque and search speed using a function curve;

[0024] Figure 5 is a diagram of an experimental platform constructed in accordance with an embodiment;

[0025] Figure 6 Schematic diagram of the holes for assembling square, circular and triangular shaft holes in the embodiment;

[0026] Figure 7 The figure is a curve of the torque on the shaft along the x and y directions obtained by assembling the square shaft hole using the existing method in the embodiment;

[0027] Figure 8 The figure is a curve diagram of the torque applied to the shaft along the x and y directions obtained by assembling the square shaft hole using this method in the embodiment. DETAILED DESCRIPTION

[0028] The embodiments of the technical solution of the present invention will be described in detail below with reference to the accompanying drawings. Unless otherwise specified, the technical terms or scientific terms used in this application have the common meanings understood by those skilled in the art to which the present invention belongs.

[0029] Before estimating the hole pose, the previous process needs to be completed first. The position of the hole is measured by the camera, and the hole coordinate system O is established at the center of the hole according to the measured value. H , the three coordinate axes of the coordinate system are x, y and z. Then, through position search and attitude correction, the axis is made ready for attitude search, and then the hole attitude estimation is performed:

[0030] Figure 1 A flow chart showing a variable speed cross search method for alignment of shaft holes in assembly is presented.

[0031] In the variable speed cross search method constructed in this application, the shaft searches alternately along the x and y directions, and the rotation speed is continuously adjusted as the shaft changes during rotation. The x and y directions are perpendicular to each other and form a cross shape. The method includes the following steps:

[0032] S1. Perform a preliminary search for the hole's posture in the x and y directions. Drive the shaft to rotate along the x and y directions to search. After preliminarily estimating the hole's posture angle, fix the shaft's posture angle in the current search direction to the estimated hole's posture angle. Simultaneously, record the shaft's positive and negative data during the search: angle, torque, and weight. It is a set of data recorded after the shaft hole contacts in the forward search; It is a set of data recorded after the shaft-hole contacts in the negative search;

[0033] S2, estimate the attitude angle of the hole, and use the positive and negative data to estimate the positive clearance and negative clearance of the shaft hole respectively;

[0034] S3, when stuck, estimate the angular motion range of the axis in the positive and negative directions to the torque threshold at the current assembly depth along the z direction;

[0035] S4, constructing a variable speed search function;

[0036] S5, searching along the current search direction at the search speed while recording data, the recorded data being the same as in step S1;

[0037] S6. If the shaft is stuck in the hole, repeat steps S2-S5; otherwise, continue to insert the shaft into the hole until the assembly is completed.

[0038] The proposed method can quickly and accurately estimate the posture of the hole, which can align the shaft to the hole more quickly, thereby reducing the assembly time and the average contact torque, overcoming the limitations of existing methods.

[0039] Specifically, in step S2, after the shaft and hole contact, there is a piecewise linear relationship between the change in contact torque and the change in rotation angle. To obtain this piecewise linear relationship, the θ recorded in S1 is used. H 、T H 、w H and θ L 、T L 、w L , combined with the weighted least squares method to perform a polynomial fitting, and obtain the positive and negative polynomial parameters k respectively H and k L The relationship between the shaft-hole clearance δ, the assembly depth Δh, and the shaft-to-hole offset angle Δθ is δ = Δh·Δθ.

[0040] The position of the shaft in the z direction when it contacts the upper surface of the hole is h0, and the positions after contact in the positive and negative directions are h H 、h L ; Using k H and k LCalculate the angle θ when the contact force between the shaft and the hole reaches the threshold H1 and θ L1 ; Then, estimate the hole attitude angle θ M The positive clearance δ of the shaft and hole is estimated by using the data recorded after the shaft and hole contact in the positive and negative searches. H and the negative clearance between the shaft and the hole δ L ;

[0041] k H =wls(θ H ,T H ,w H 0,k L =wls(θ L ,T L ,w L )

[0042] k H and k L Respectively represent the first-order polynomial parameters obtained by fitting using the weighted least squares method in the positive and negative directions, It is a set of data recorded after the shaft hole contacts in the forward search; It is a set of data recorded after the shaft-hole contacts in the negative search;

[0043] θ H1 =(-T limit -k H (2)) / k H (1),θ L1 =(T limit -k L (2)) / k L (1)

[0044] k H (2) represents the coefficient of the first-order term in the first-order polynomial obtained by forward fitting, k H (1) represents the coefficient of the 0th degree term in the linear polynomial obtained by forward fitting; T limit is the contact torque threshold: k L (2) represents the coefficient of the first-order term in the first-order polynomial obtained by negative fitting, k L (1) represents the coefficient of the 0th degree term in the linear polynomial obtained by negative fitting;

[0045] θ M =((h0-h H )·θ H1 +(h0-h L )·θ L1 ) / (2·h0-h H -h L )

[0046] δ H =(h0-h H )·(θ H1 -θ M )

[0047] δ L =(h0-h L )·θ M -θ L1 ).

[0048] Furthermore, when stuck, it is estimated that at the current depth, the angular motion ranges of the axis moving in the positive and negative directions to the torque threshold are δθ respectively. H and δθ L ;

[0049] δθ H =δ H / (h0-h C )

[0050] δθ L =δ L / (h0-h C )

[0051] Among them, δ represents the shaft-hole clearance, θ M Indicates the estimated hole attitude angle, defines the position when the shaft contacts the upper surface of the hole as h0, and defines the position along the assembly depth direction at the current moment during assembly as h C , the direction of coordinate axis z is defined as the assembly depth direction.

[0052] Furthermore, a variable speed search function is constructed, and the search speed is defined as v, where v includes the positive search (P-search), negative search (N-search) and estimated θ along the search direction. M Then return to the hole posture position θ from the forward position M The rotation speed (forward return P-back) is determined by the angle v θ and v related to the torque F composition;

[0053]

[0054]

[0055] Among them, v θ Indicates the search speed adjusted according to the attitude angle, v F represents the search speed adjusted according to the contact torque; v max 、v end and v back are the maximum speed during the search, the speed during the contact phase, and the speed during the return phase, T limitis the contact torque threshold, T is the current direction contact torque fed back by the sensor, where θ is the rotation angle of the axis, k is a coefficient greater than zero, and v r =v max -v end ,θ H =θ M +δθ H ,θ L =θ M -δθ L θ H In the forward search, θ represents the angular position where the contact torque between the shaft and the hole is expected to reach the threshold. L Indicates the angular position where the contact torque between the shaft and the hole is expected to reach the threshold in the negative search.

[0056] According to the estimated attitude angle and the estimated shaft-hole clearance, a variable speed search function is designed to improve the speed of attitude search; according to the torque information, a speed limit function v is designed. F , ensuring the safety of assembly.

[0057] For example, in step S1, the drive shaft is rotated and searched along the x and y directions using admittance control, and the external force is converted into a deviation displacement relative to the desired displacement through admittance control.

[0058] Assembly process such as Figure 2 As shown in the figure, the lowest point is generated after the axis rotates along the y direction, and the position search is performed through the admittance control until the three-point contact state is reached (black dot position), as shown in the figure. Figure 2 As shown in (c), the rotation angle along y is then reset to zero, completing the hole position search. Then, the posture search is performed.

[0059] Figure 2 (a) indicates the initial position for the shaft hole to be assembled. Figure 2 (b) indicates the axis along O H -y rotates to the lowest point, Figure 2 F in (b) m and F z denote the desired forces driving the shaft in the x and z directions, respectively, Figure 2 (d) indicates that the rotation angle of the axis along y is reset to zero and the search continues along x and y rotations. Figure 2 (e) indicates that the hole position search is completed.

[0060] v in step S4 θ like Figure 3 As shown in the figure, the relationship between the attitude angle and the search speed is represented by a function curve (positive return P-back; negative return N-back); v F like Figure 4As shown in Figure 1, the relationship between contact torque and search speed is represented by a function curve. When contact torque is generated between the shaft and the hole, reducing the search speed can reduce the disturbance effect.

[0061] Example

[0062] Build as Figure 5 The experimental platform shown is used to carry out the shaft-hole assembly experiment.

[0063] In the experiment, the square, circular and triangular shaft holes were assembled (such as Figure 6 The performance of the proposed method was verified (as shown). The square and triangular holes have a side length of 30mm, the circular hole has a diameter of 30mm, the shaft-hole clearance is 0.1mm, and the hole depth is 35mm. The material is aluminum alloy. The proposed method was experimentally verified against the "Flexible Shaft-Hole Assembly Using Partial Helical Force Trajectory and Inclined Shaft Attitude" method, and their performance was compared.

[0064] The flexible shaft-hole assembly method using partial helical force trajectories and tilted shaft posture is denoted as Method 1, and the proposed hole posture estimation method based on variable-speed cross search is denoted as Method 2. Ten assembly runs were conducted for each shaft-hole configuration using both methods. Data related to time and contact torque were collected for the assembly of square, circular, and triangular shaft-hole configurations.

[0065] Taking square hole search as an example, Method 1 averaged a total assembly time of 79.26 seconds, while Method 2 averaged a total time of 71.02 seconds. Method 1 averaged a pose search time of 59.98 seconds, while Method 2 averaged a time of 52.03 seconds. Method 1 averaged a preliminary pose search time of 25.6 seconds and a precise search time of 35.285 seconds, while Method 2 averaged 19.55 seconds and 32.31 seconds, respectively. Method 1's average contact torque was 0.1562 Nm, exceeding Method 2's 0.1109 Nm. These data are shown in Table 1. Assembly performance data for circular and triangular holes is also shown in Table 1. Figure 7 and Figure 8 The torque curves of the shaft along the x and y directions when method 1 and method 2 are used in the square shaft-hole assembly are shown respectively. Comparing the curves, the shaft is subjected to a larger torque for a longer time when method 1 is used, while the shaft is subjected to a larger torque for a shorter time when method 2 is used.

[0066] In the initial posture search stage, method2 estimates the current direction hole posture θ M After that, it takes less time. This shows that using position control to move the pile quickly to θ M, which saves time compared to using force control to continue searching. In the posture precision search phase, method 2 takes less time, indicating that position control is used and θ is aligned. M Compared with torque control, method 2 saves time. In the assembly of the three shapes of shaft holes, method 2 has a smaller contact torque, which means less wear on the shaft hole during the assembly process.

[0067] The proposed method demonstrates superior performance in the pose search process for shaft-hole assembly. During the pose search phase for three shaft-hole shapes, assembly time was shortened by 13% to 40%, and average contact torque was reduced by 15% to 69%. This method achieves rapid assembly and minimizes wear between the shaft and the hole.

[0068] Table 1. Data table of hole posture estimation

[0069]

[0070] The present invention has been fully demonstrated through the preferred embodiments, but this does not limit the present invention. Any technician with relevant professional knowledge can make appropriate adjustments or optimizations to the above-disclosed structures and technical contents without violating the core technical solutions of the present invention to form equivalent implementation cases. These adjusted or optimized implementation cases still fall within the scope of the technical solutions of the present invention.

Claims

1. A speed-changing cross search method for aligning the posture of the shaft hole in an assembly, characterized by: The following steps are involved: S1. Establish the three coordinate axes of the hole coordinate system as x, y and z, perform a preliminary search for the hole's posture, and search in the x and y directions respectively. Drive the shaft to rotate along the x and y directions to search. After preliminarily estimating the hole's posture angle, fix the shaft's posture angle in the current search direction to the estimated hole's posture angle. At the same time, record the positive and negative data of the shaft during the search process: angle, torque and weight; S2, estimate the attitude angle of the hole, and use the positive and negative data to estimate the positive clearance and negative clearance of the shaft hole respectively; S3, when stuck, estimate the angular motion range of the axis in the positive and negative directions to the torque threshold at the current assembly depth along the z direction; S4. Construct a variable speed search function; the search speed v is: Among them, v θ Indicates the search speed adjusted according to the attitude angle, v F represents the search speed adjusted according to the contact torque; v max 、v end and v back are the maximum speed during the search, the speed during the contact phase, and the speed during the return phase, T limit is the contact torque threshold, T is the current direction contact torque fed back by the sensor, where θ is the rotation angle of the axis, k is a coefficient greater than zero, and v r =v max -v end ,θ H =θ M +δθ H ,θ L =θ M -δθ L θ M The estimated hole attitude angle is represented by the angular motion range of the axis moving in the positive and negative directions to the torque threshold, respectively: H and δθ L ,θ H In the forward search, θ represents the angular position where the contact torque between the shaft and the hole is expected to reach the threshold. L Indicates the angular position when the contact torque between the shaft and the hole is expected to reach the threshold in the negative search. P-search indicates positive search, N-search indicates negative search, and P-back indicates positive return. S5, searching along the current search direction at the search speed while recording data, the recorded data being the same as in step S1; S6. If the shaft is stuck in the hole, repeat steps S2-S5; otherwise, continue to insert the shaft into the hole until the assembly is completed.

2. The method for speed-changing cross search for alignment of shaft holes in assembly according to claim 1, characterized in that: The specific process of step S2 is: the position of the shaft in the z direction when it contacts the upper surface of the hole is defined as h0, and the positions after contact in the positive and negative directions are defined as h H 、h L ; Using k H and k L Calculate the angle θ when the contact force between the shaft and the hole reaches the threshold H1 and θ L1 ; and use the positive and negative data to estimate the positive clearance δ of the shaft and hole respectively H and the negative clearance between the shaft and the hole δ L ; k H =wls(θ H ,T H ,w H ),k L =wls(θ L ,T L ,w L ) k H and k L Respectively represent the first-order polynomial parameters obtained by fitting using the weighted least squares method in the positive and negative directions, It is a set of data recorded after the shaft hole contacts in the forward search; It is a set of data recorded after the shaft-hole contacts in the negative search; θ H1 =(-T limit -k H (2)) / k H (1),θ L1 =(T limit -k L (2)) / k L (1) k H (2) represents the coefficient of the linear term in a linear polynomial, k H (1) represents the coefficient of the 0th degree term in a linear polynomial; T limit is the contact torque threshold; k L (2) represents the coefficient of the first-order term in the first-order polynomial obtained by negative fitting, k L (1) represents the coefficient of the 0th degree term in the linear polynomial obtained by negative fitting; θ M =((h0-h H )·θ H1 +(h0-h L )·θ L1 ) / (2·h0-h H -h L ) d H =(h0-h H )(θ H1 -θ M ) d L =(h0-h L )(θ M -θ L1 )。 3. The speed-changing cross search method for aligning the posture of the shaft hole in the assembly according to claim 2, characterized in that: The specific process of step S3 is as follows: when stuck, it is estimated that at the current depth, the angular motion ranges of the axis moving in the positive and negative directions to the torque threshold are δθ respectively. H and δθ L ; dth H =d H / (h0-h C ) dth L =d L / (h0-h C ) Among them, δ represents the shaft-hole clearance, the position when the shaft contacts the upper surface of the hole is defined as h0, and the position along the assembly depth direction at the current moment during assembly is defined as h C , the direction of coordinate axis z is defined as the assembly depth direction.

4. The method for searching for a cross-shaped object in a speed-changing manner for aligning the posture of axle holes in an assembly according to claim 3, characterized in that: In step S3 , the shaft-hole clearance δ=Δh·Δθ, where Δh represents the shaft-hole assembly depth and Δθ represents the offset angle of the shaft relative to the hole.

5. The speed-changing cross search method for aligning the posture of the shaft hole in assembly according to claim 1, characterized in that: In step S1, the drive shaft is searched by rotating along the x-direction and the y-direction using admittance control.

Images