CN110549338B

CN110549338B - Robot automatic assembly method for round-rectangular composite hole parts

Info

Publication number: CN110549338B
Application number: CN201910853702.7A
Authority: CN
Inventors: 吴伟国; 高力扬; 王通
Original assignee: Harbin Institute of Technology
Current assignee: Harbin Institute of Technology
Priority date: 2019-09-10
Filing date: 2019-09-10
Publication date: 2022-06-21
Anticipated expiration: 2039-09-10
Also published as: CN110549338A

Abstract

A robot automatic assembly method for round-rectangular composite hole parts relates to the technical field of robot assembly control. The invention discloses an automatic assembly method of round-rectangular composite hole parts, which is used for judging the contact state according to force feedback information and correspondingly adjusting different contact states in order to solve the problems that the automatic assembly of the round-rectangular composite hole parts is in a large number of contact states and the assembly strategy is difficult to determine. The assembling process is divided into an approaching stage, a hole searching stage and an inserting stage, wherein the assembling part is enabled to rapidly approach the assembled part by using a 5-time spline track planning method in the approaching stage, 4 contact states are in the hole searching stage, the contact states in the inserting stage are divided into 41 types of 7 types according to the number and relative positions of contact points, stress analysis is carried out on each contact state, and a corresponding hole searching or inserting assembling strategy is provided. The assembly simulation of the round-rectangular composite hole type parts is carried out, and the result shows that the assembly method can complete assembly on the premise of preventing overlarge contact force.

Description

Robot automatic assembly method for round-rectangular composite hole parts

Technical Field

The invention relates to a robot automatic assembly strategy and method for irregular (round-rectangular composite hole type) parts, and relates to the technical field of robot assembly control.

Background

Assembly is an important link in the product manufacturing process, and the assembly quality often directly affects the final product quality. At present, a large number of assembly tasks in the industrial production process still need to be completed by adopting a manual assembly operation mode. However, the manual assembly has many problems, such as high work intensity, low work efficiency, high error rate, high cost, etc. In response to these problems, there is an increasing need in the manufacturing industry to perform assembly operations using robots instead of human labor. The robot assembly can improve the production efficiency, and the application range of the robot assembly is wider, such as in dangerous environments of high temperature, radiation, vacuum and the like.

The round-rectangular composite hole type part is called a cylindrical-rectangular composite shaft hole type part, is a composite hole type part formed by combining cylindrical geometric characteristics and rectangular geometric characteristics, and the assembly of the parts is very common in the manufacturing industry, such as the assembly of a flat key, the assembly of a spline, the assembly of a round shaft part with a key and the like.

At present, a plurality of patents and academic papers have been published for the automatic assembly problem of parts with only round holes or only square holes, for example, the invention patent with the publication number of CN109382828A and the patent number of ZL201811275792.8 provides a teaching and learning assembly method for the assembly of round holes and shaft parts, a robot uses an RCC flexible wrist to compensate errors in the assembly process, and the obtained adjustment motion is taught to reduce the posture error between the shaft part and the hole part so as to keep the position error in the range which can be compensated by the RCC wrist; the invention patent with the publication number of CN109531560A and the patent number of ZL201910018179.6 provides an automatic assembling method for matching of small-gap circular shaft holes, the pose error of a shaft relative to a hole piece is accurately measured through a micro-displacement measuring device, and then the assembling is completed through one-time action. For the assembly of square-hole parts, no patent is published at present, the assembly process of the square shaft hole is divided into six stages in an academic paper published in 2012 by Park et al, the contact condition of each stage is analyzed, and finally an assembly strategy is formulated on the basis to gradually realize assembly operation; kim et al in 2018 published academic papers designed a clamp with an angle measurement system for square shaft hole part assembly, and multiple experiments show that the clamp can accurately sense the pose error between a square column and a hole piece in cooperation with a six-dimensional force sensor.

Different from the situation that the contact points in the circular hole assembly are both in the cylindrical part and the square hole assembly and are both in the square column part, in the assembly of the circular-rectangular composite hole part, the contact points between the hole part and the shaft part matched with the hole part can be simultaneously distributed on the intersection line of the cylindrical part and the square column part and the intersection line of the cylindrical part and the square column part, so that the contact state is more complicated than the situation of the circular hole assembly or the square hole assembly, and the mechanical balance equation corresponding to the contact state is completely different. From the analysis, it can be known that the assembling operation of the circular-rectangular composite hole type part cannot be completed by simply applying the methods related to the circular hole assembling and the square hole assembling in the above patents and academic papers.

In summary, for the problem of robot automatic assembly of circular-rectangular composite hole parts, there is no complete contact state analysis result in the prior published patents and academic researches, and no contact state determination, assembly strategy corresponding to the contact state determination and concrete implementation control method are given.

Disclosure of Invention

The assembling method aims at solving the problem of automatic assembly of hole parts with circular arcs, rectangular straight edges and right angles in geometric elements, and avoids damage to assembling parts and robots caused by overlarge assembling contact force due to improper pose among shaft holes in the assembling process. In order to solve the problems that automatic assembly of circular-rectangular composite hole parts is in a large contact state and an assembly strategy is difficult to determine, the hole parts are called as the circular-rectangular composite hole parts, shaft parts matched with the hole parts are called as the circular-rectangular composite shaft parts, and the common forms of the hole parts comprise: flat key hubs, splined hubs, a-type flat key holes, C-type flat key holes, contoured round-rectangular holes, and the like.

The technical scheme adopted by the invention for solving the technical problems is as follows:

a robot automatic assembly method for round-rectangular composite hole parts is characterized in that the assembly objects of the assembly method are hole parts and shaft parts which have round and rectangular straight sides and right angles in geometric elements, wherein the parts with the composite round holes and square holes are called round-rectangular composite hole parts, the parts with the composite round columns and square columns are called round-rectangular composite shaft parts (common parts in engineering practice, belonging to the round-rectangular composite hole/shaft parts, are provided with splines, A-type flat keys, C-type flat keys and the like),

the automatic assembly system for the round-rectangular composite hole type parts by applying the assembly method comprises the following steps: the robot comprises an upper computer, a base, a six-degree-of-freedom robot, a six-dimensional force/torque sensor, an assembly part (a shaft part matched with a hole part), a clamping fixing seat and an assembly part (a round-rectangular composite hole part). The six-degree-of-freedom robot is any robot which can realize spatial six-degree-of-freedom motion and has position control precision meeting assembly operation, and is not limited to a joint type serial six-degree-of-freedom operating arm; the upper computer communicates with the servo driver/controller and the six-dimensional force/torque sensor of the six-degree-of-freedom robot through buses, and the used buses CAN be Ethernet (Ethernet), RS485 networks, CAN buses and the like.

The assembly part is clamped on a tool side interface of the six-dimensional force/torque sensor, a robot side interface of the six-dimensional force/torque sensor is fixedly connected with a tail end interface of the six-degree-of-freedom robot, the assembly part with the round-rectangular composite hole is fixed in an interface of the clamping fixed seat, and the clamping fixed seat and the six-degree-of-freedom robot are fixed on the base. Coordinate system Σ O-xyz is a base coordinate system with its origin fixed to the center of the robot interface of the base, coordinate system Σ O_P-xyz and Σ O_SThe origin of xyz is located in the center of the circle of the lower surface of the fitting and the force-measuring center of the six-dimensional force/torque sensor, respectively, and these two coordinate systems are kept relatively stationary during the fitting operation. In sigma O_PIn the system, the direction of a z axis is parallel to the inserting direction of the assembly member into the assembled member, the direction of an x axis is parallel to the symmetrical plane of the geometric outline of the assembly member and is vertical to the z axis, and the direction of a y axis is determined by the vector direction obtained by multiplying the unit vector of the z axis by the unit vector of the x axis in a crossed mode.

The relative position of the circular-rectangular composite hole of the assembly object with respect to the six-degree-of-freedom robot can be calculated according to the design drawing dimensions of the base, the clamping fixing seat and the assembly object, or according to the actual measurement result after the system is assembled, the relative position is defined as the estimated position of the assembly object and is influenced by uncertain factors such as machining errors, measurement errors and the like, and the estimated position is not equal to the actual position of the assembly object.

In the assembling method of the round-rectangular composite hole parts, the six-degree-of-freedom robot needs to adjust the pose of the assembling part according to force feedback information measured by a six-dimensional force/torque sensor, so that the assembling part is finally installed in the round-rectangular composite hole of the assembled part. The assembly process is divided into three stages: an approach stage, a hole searching stage and an insertion stage.

The assembling operation is started to enter an approaching stage firstly, the assembling part needs to rapidly approach the assembled part from an initial position under the control of a six-freedom-degree robot to an assembling preparation position which is positioned above an estimated position of the assembled part, and the preparation position can be added with an increment vector [0,0, delta z ] from the estimated position]^TAnd (4) calculating. The trajectory planning of the above-described motion applies a sub-spline method in which the velocity and acceleration at the starting point (initial position of the assembly) and the ending point (assembly ready position) are both 0.

After the assembly part 4 reaches the assembly preparation position, the hole searching stage is started, after the stage is started, the six-freedom-degree robot 2 enables the assembly part 4 to slowly move towards the assembled part 6, the trial is carried out, and when the contact force in the z-axis direction is larger than the threshold value F_maxThe control system of the assembly operation judges the contact state between the assembly part 4 and the assembled part 6 according to the feedback of the later six-dimensional force/torque sensor 3, then lifts the assembly part 4 for a distance and adjusts the pose thereof according to the previous contact state, and the trial process of the assembly part 4 to the assembled part 6 is repeated after the adjustment is finished until the conditions are met:

F_z＜F_max&h≥h_d (1)

wherein F_zRepresenting the z-axis force component (transformed into the Σ OP system) measured by the six-dimensional force/torque sensor, h representing the distance along the z-axis that the fitting is inserted into the fitted part, h_dIs the insertion depth discrimination threshold for completion of hole search, F_maxIs the threshold value of the contact force.

When the condition in the formula (1) is met, the tail end of the assembly part is inserted into the round-rectangular composite hole of the assembled part, and the insertion depth reaches h_dThe insertion phase can be switched to, considering that the hole searching is completed. During the insertion phase the insert continues to advance in the negative z-axis direction, whenever F_z(corresponding to the resistance to insertion) reaches a threshold F_maxIn the process, the clamping possibly occurs in the insertion process, the contact state of the insertion needs to be judged at the moment, and then the device is adjusted according to the contact stateThe position of the fitting in the hole is maintained, and the insertion is continued until the conditions are met:

F_z＜F_max&h≥h_max (2)

wherein h is_maxIs the insertion depth discrimination threshold for completed assembly. When the condition in the formula (2) is met, the depth of the assembly part inserted into the assembled part reaches h_maxAnd considering that the assembly parts are successfully assembled into the assembled parts, and finishing the assembly operation.

In the automatic assembly system applying the assembly method of the round-rectangular composite hole parts, a force/position mixed control system is required to be used for controlling, and X is defined as [ X, y, z, theta ]_P1,θ_P2,θ_P3]^TIs the pose vector of the assembly member, wherein x, y and z are respectively Sigma O_PAt a position coordinate, theta, within a base coordinate system sigma O_P1、θ_P2、θ_P3Are respectively sigma O_P3 Euler angles relative to the Σ O system; theta ═ theta₁,θ₂,θ₃,θ₄,θ₅,θ₆]^TAnd τ ═ τ [ τ ]₁,τ₂,τ₃,τ₄,τ₅,τ₆]^TAre respectively defined as a joint angle and a joint driving moment vector of the six-degree-of-freedom robot, wherein theta_iAnd τ_i(i-1, 2, …,6) represents the joint angle and drive torque of the ith joint, respectively;^SF_eis a six-dimensional force/torque sensor 3 in sigma O_SThe original data of the force rotation (comprising three force components and three moment components) measured in the system,^PF_eis to be^SF_eConversion to Σ O_PThe amount of rotation of the force in the system; x_d、F_d、θ_dTarget values X, F and theta in the control process, respectively, and δ X and δ F are the adjustment amount of X and the deviation of the F theoretical value from the actual value in the control process, respectively.

For the force position hybrid control system, the control flow in each control period is divided into the following steps:

step one, feedback sampling. Reading a servo motor encoder and a six-dimensional force/torque sensor of the six-degree-of-freedom robot to respectively obtain joints of the robotAngular vectors θ and Σ O_SRotation of force in the system^SF_eThe position and orientation vector X, Sigma O of the assembly part can be obtained from theta according to the positive kinematic simulation of the six-freedom-degree robot_PRotation of force in the system^PF_eCan be calculated according to equation (3).

In formula (3), R is Σ O_SIs tied to sigma O_PRotational transformation matrix of the system, P_SIs O_SPoint-in-Sigma O_PPosition vector within the system, 0_3×3Is a 3 < th > order all-zero square matrix.

Step two, planning the track. Planning the pose of the assembly part according to the assembly flow, and if the assembly process is in the approaching stage, calculating the target pose X of the assembly part at the current moment according to a 5-time spline function_d(ii) a If the assembly process is in the hole searching stage or the inserting stage, the z-axis component F of the contact force needs to be judged firstly_zWhether or not the threshold value F is reached_maxIf F is_z<F_maxThen X is generated by continuing to approach or insert the assembly into the assembled part_dOtherwise, the pose adjustment quantity of the assembly part is obtained according to the contact state, and then X is generated according to the adjustment quantity_d。

And step three, impedance control. According to the target force screw quantity F_dAnd force feedback^PF_eAnd (3) calculating the adjustment quantity delta X of the pose X of the assembly part by using the deviation delta F, wherein the used impedance control model is shown as a formula (4).

M, B, K are the inertial, damping and stiffness arrays of the virtual impedance model between the fitting and the fitted piece. For the model in equation (4), the transfer function from δ F to δ X is shown in equation (5), and δ X can also be calculated from δ F and this transfer function in actual control.

Where s represents the laplace variable in the transfer function.

And step four, position control. Planning the track to obtain the target pose X_dSuperposed with the adjustment delta X obtained by impedance control, and calculated according to the inverse kinematics equation of the six-degree-of-freedom robot to obtain the joint angle target vector theta_dAnd then, calculating joint driving torque tau according to a feedforward + PD feedback control law of an equation (6), sending the joint driving torque tau to a servo driving/controller of each joint of the robot, and finishing command output of the current control period.

Wherein M is_RIs a generalized inertial array estimation value, C, of a six-degree-of-freedom robot_RAnd G_RRespectively an estimate of the centrifugal/Coriolis force term and an estimate of the gravity term, K_vAnd K_pThe coefficient matrix of differential terms and the coefficient matrix of proportional terms in the PD feedback control are respectively.

In the hole searching stage of the assembling method for the circular-rectangular composite hole type parts, the hole searching movement of the assembling parts is planned according to the contact state when the assembling parts are contacted with the assembling parts each time, and the contact state is divided into 4 types according to the number of contact points and the positions on the bottom surfaces (the planes contacted with the assembling parts in the hole searching stage) of the assembling parts. Definitions l and θ_CRespectively indicating contact points at Σ O_PxO of a series_PThe phase angle and the radial length in the y plane, a, b, c and r are the geometric dimensions of the bottom surface contour of the assembly part, r is the radius of the circular arc, a is the width of the rectangular part, and b is the intersection line of the circular arc part and the rectangular part to O_PThe distance of the points, c, is the length of the rectangular portion.

In the first contact state of the hole searching stage, a single contact point is positioned inside an incomplete arc of the bottom surface contour of the assembly part, which indicates that a point in the bottom surface contour of the assembly part interferes with the edge of the round-rectangular composite hole of the assembled part, and the assembly part moves in a direction away from the contact point.

In the second contact state of the hole searching stage, a single contact point is positioned on the incomplete arc boundary of the bottom surface contour of the assembly part, which indicates that the assembly part and the assembled part are in a sharp-angled contact state of the edge, the contact state is a critical state before the hole searching is completed, and the assembly part should move a small distance in a direction away from the contact point.

In the third contact state of the hole searching stage, two contact points are positioned on the incomplete circular arc boundary of the bottom surface outline of the assembly part, which indicates that the fox hunting section determined by the two contact points is suspended out of the boundary of the circular-rectangular composite hole of the assembled part, so that the assembly part should move in the direction away from the connecting line of the contact points, the contact state can be equivalent to the first contact state, and the two contact points are regarded as one contact point at the middle point of the connecting line, so that the adjustment strategies are completely the same.

In the fourth contact state of the hole searching stage, a single contact point is positioned in a rectangular part of the bottom surface outline of the assembly part, which indicates that the assembly part and the assembled part generate interference of the rectangular part due to the fact that the symmetry planes are not parallel, and the assembly part is rotated around the z axis so that the contact point is rotated out of the rectangular part.

In the above four contact states, Sigma O_PThe force balance equations within the system can all be written uniformly as:

wherein F_x、F_y、F_zRespectively converting the force feedback measured by the six-dimensional force/torque sensor 3 into sigma-delta O_PThree force components in the system, M_x、M_y、M_zRespectively converting the force feedback measured by the six-dimensional force/torque sensor 3 into sigma-delta O_PThe three force components in the system, f, are the magnitude of the normal force at the contact point (equivalent contact point for the third contact state) at the hole search stage. Solving equation (7) can obtain:

according to the definitions of various types of contact states and the solution result in the equation (8), the contact state discriminant and the assembly adjustment strategy corresponding to each contact state in the hole searching stage can be summarized in table 1.

TABLE 1 discriminant of contact status in hole-searching stage and adjustment strategy for each assembly corresponding to contact status

In the hole searching stage of the automatic assembly operation, the hole searching track after the assembly part 4 and the assembled part 6 are tentatively contacted each time can be planned according to the following steps:

step one, according to the force feedback transformation, the force rotation quantity is obtained^PF_eThe force balance equation in column 2 of Table 1 was solved according to equation (8) to obtain f, l, and θ_C；

Step two, determining the contact state type of the hole searching stage according to the discriminant in the 3 rd column of the table 1;

and step three, obtaining the pose adjustment amount of the assembly part in the next trial by the adjustment strategy in the 4 th column of the table 1, and superposing the pose adjustment amount to the normal adjustment motion track to obtain the hole searching motion track of the assembly part 4 in the next trial.

In the insertion stage of the assembling method of the round-rectangular composite hole parts, every time the z-axis force measured by the six-dimensional force/torque sensor 3 is fed back F_zReaches the threshold value F_maxIn the process, the pose of the assembly part 4 needs to be adjusted according to the current contact state, so that the assembly process can be continued without being stuck. The lateral and bottom edges of the fitting part 4 may come into contact with the fitting part 6 during the insertion phase, and for identifying the contact state, four basic contact points are defined, in each case:

the first type of contact point is located on the side of the cylindrical part of the assembly member 4, the mechanical balance equation of which is shown in formula (9), and mu is the friction coefficient between the assembly member 4 and the assembled member 6.

The second type of contact point is located at the bottom edge of the cylindrical part, and the mechanical balance equation is shown as the formula (10).

The third type of contact point is located on the side of the square column part, and the mechanical balance equation of the third type of contact point is shown as the formula (11).

The third type of contact point is located on the bottom edge of the square column part, and the mechanical balance equation of the third type of contact point is shown as the formula (12).

According to the number of contact points in the actual contact state, the types of the basic contact points and the relative positions, the contact states in the insertion stage are divided into 7 types and 41 types, wherein the seven types are respectively: a single point first type contact (figure 8) having a first or second type of basic contact point, a single point second type contact (figure 9) having a third or fourth type of basic contact point, a two point first type contact (figure 10) having a first type of basic contact point and a second type of basic contact point, a two point second type contact (figure 11) having a first or second type of basic contact point and a third or fourth type of basic contact point, two-point third type contacts (fig. 12) having a third type of base contact point and a fourth type of base contact point, three-point first type contacts (fig. 13) having a first or second type of base contact point and two third or fourth type of base contact points, and three-point second type contacts (fig. 14) having two first or second type of base contact points and a third or fourth type of base contact point. The 7 major categories respectively contain 2, 8, 1, 16, 4, 2 and 8 contact states, and for the convenience of later representation, P-i-j is taken as the code of the contact major category in the insertion stage, wherein i represents the number of the contact points, and j represents the serial number of the contact major category after the number of the contact points is determined; p-i-j-k is taken as the contact state number belonging to the P-i-j large class in the insertion stage, k represents the contact state serial number in the P-i-j large class, for example, two-point second-class contact is represented by P2-2, and the 10 th contact state is written as P2-2-10.

For the contact states in fig. 8 to 14, the mechanical equilibrium equations are obtained by combining the mechanical equations of the basic contact points in the equations (9) to (12), the distinguishing conditions of the major classes of the contact states can be classified according to the characteristics of the mechanical equations of the major classes, and the derivation results of the mechanical equilibrium equations and the distinguishing conditions are shown in table 2, wherein c_iAnd s_i(i ═ 1,2,3) are cos θ, respectively_CiAnd sin θ_CiAbbreviation of (a), theta_CiAnd f_iRespectively, the ith contact point is in Sigma O_PPolar angle and normal contact force in the system, k_mn(m is 1,2, …, 7; n is 1,2,3,4) is the nth discrimination parameter for the mth major category of contact states for discriminating the specific contact state within the major category, k is_mnAll values of (A) are in a closed range of [ -1,1 [ ]]And (4) the following steps.

TABLE 2 mechanics equilibrium equations and discriminating conditions for the large class of contact states at the insertion stage

For the broad categories of contact states in table 2, the criteria for each contact state in each broad category and the corresponding adjustment movements are given in table 3, wherein the total number of adjustment movements of the assembly is 5, i.e. two translations along the x-axis and the y-axis and three rotations around the x-axis, the y-axis and the z-axis in the Σ OP system, in x, y, θ_x、θ_y、θ_zRespectively showing the above 5 adjustment movements; each adjustment movement has a positive direction and a negative direction, which are respectively indicated by "+" and "-"; the adjusting movement is abbreviated to a complex form of the sign and sign of the variable, e.g. negative rotation of the assembly 4 about the x-axis is abbreviated to "θ" in table 3_x- ". Since some contact states correspond to the same fitting adjustment movement and it is not necessary to distinguish them in the control of the insertion stage, the judgment conditions of the contact states corresponding to the same adjustment movement are merged and written as one row in table 3.

TABLE 3 Distinguishing Condition for contact State type in insertion phase and Assembly pose adjustment strategy

Assembly force F for each assembly down-fitting during the insertion phase of an automatic assembly operation_zReaches the threshold value F_maxThe adjustment movement of the fitting can then be planned as follows:

step one, according to the force feedback transformation, the force rotation quantity is obtained^PF_eJudging the contact large class to which the current contact state belongs according to the large class judgment conditions in the fourth column in the table 2 by the force and moment components in the table;

step two, solving the mechanical equilibrium equation of the third column in the table 2 to obtain the position theta of the reaction contact point_C1And a base class discrimination parameter k_mn；

And step three, determining a specific contact state according to the judgment conditions in the third column of the table 3, and checking the adjustment motion corresponding to the current contact state in the fourth column of the table 3.

The invention has the beneficial effects that:

according to the assembling theory and method of the round-rectangular composite hole part, the assembling motion process is divided into three stages of approaching, searching and inserting, PD feedback control based on dynamics feedforward is adopted in the approaching stage, so that the mechanical arm can approach the hole part quickly, and the dynamic control performance of the mechanical arm is improved; in the searching stage, a plane hole searching method based on a six-dimensional force sensor is adopted, a hole searching strategy is formulated according to the contact state, and the method is suitable for a hole searching task of a composite hole part; in the insertion stage, the contact state in the extreme insertion process is identified by utilizing force information and joint angle information fed back by the sensor, a track planner of the control system generates a corresponding pose adjustment scheme on the basis of the identification of the contact state, and combines a compliance control strategy to realize the robot assembly task of the circular-rectangular composite hole part, thereby filling the blank in the aspects of assembly theory and method for robot assembly of the composite hole part at home and abroad.

The round-rectangular composite hole part is a common part, but the automatic assembly of the part has the characteristics of round holes and square holes, so that the problems of various contact states and difficulty in determining an assembly strategy are solved.

The invention relates to an automatic assembly method of round-rectangular composite hole parts, which judges the contact state according to force feedback information and correspondingly adjusts the contact state according to different contact states. The assembling process is divided into an approaching stage, a hole searching stage and an inserting stage, wherein the assembling part is enabled to rapidly approach the assembled part by using a 5-time spline track planning method in the approaching stage, 4 contact states are totally adopted in the hole searching stage, 7 large contact states and 41 types are totally adopted in the inserting stage according to the number and relative positions of contact points, stress analysis is carried out on each contact state, and a corresponding hole searching or inserting assembling strategy is provided. The assembly simulation of the round-rectangular composite hole type parts is carried out, and the result shows that the assembly method can complete assembly on the premise of preventing overlarge contact force.

Drawings

Fig. 1 is a schematic geometric configuration diagram of an assembly object (round-rectangular composite hole type part) of the assembly method, in which: a) a special-shaped round and rectangular composite hole, b) an open special-shaped round and rectangular composite hole, c) a hub with a key, d) a c-shaped flat key groove, e) a-shaped flat key groove, and f) a spline hub;

FIG. 2 is a view showing a configuration of an automatic assembling system to which the assembling method of the present invention is applied; FIG. 3 is a virtual prototype three-dimensional entity of the automated assembly system; FIG. 4 is a flow chart of the assembly of the round-rectangular composite hole parts according to the present invention; FIG. 5 is a block diagram of a control system of the automated assembly system; FIG. 6 is a schematic diagram of four contact states in the hole searching stage;

FIG. 7 is a schematic illustration of four basic contact points during an insertion phase; FIG. 8 is a schematic view of a single point first type contact state at the insertion stage; FIG. 9 is a schematic view of a single point second type contact state at the insertion stage; FIG. 10 is a schematic illustration of a two-point first-type contact state at the insertion stage (two-point first-type contact state P-2-1-1); FIG. 11 is a schematic diagram of a two-point second type contact state at the insertion stage; FIG. 12 is a schematic view of the third type of contact at two points during the insertion phase; FIG. 13 is a schematic illustration of a three point first type contact state during an insertion phase; FIG. 14 is a schematic illustration of a three point contact state of the second type at the insertion stage;

FIG. 15 is a drawing showing dimensional parameters of an assembly object in a simulation of assembling a circular-rectangular composite hole-like part (in the drawing, the upper left is a front view of an assembly member, the lower left is a plan view, the upper right is a front view of an assembly member, and the lower right is a plan view);

FIG. 16 is a graph showing contact force of hole search simulation and moment generated by the contact force and pose error; FIG. 17 is a contact force curve of a contact state identification simulation at the insertion stage; FIG. 18 is a video screenshot of a circle-rectangle composite hole type part assembly simulation; FIG. 19 is a graph showing the contact force and pose error curves of the assembly simulation of the round-rectangular composite hole type part.

Detailed Description

The first specific implementation way is as follows: the assembling method aims at solving the problem of automatic assembly of hole parts with circular arcs, rectangular straight edges and right angles in geometric elements, and avoids damage to assembling parts and robots caused by overlarge assembling contact force due to improper pose between shaft holes in the assembling process. The hole parts are called as round-rectangular composite hole parts, the shaft parts matched with the hole parts are called as round-rectangular composite shaft parts, and fig. 1 shows a plurality of common forms of the hole parts, wherein the common forms comprise flat key hubs, spline hubs, A-shaped flat key holes, C-shaped flat key holes, special round-rectangular holes and the like.

Among the common parts of the engineering practice, the parts belonging to the circular-rectangular composite hole/shaft type are: splines, a-type flat keys, C-type flat keys, and the like.

As shown in fig. 2, the automatic assembly system for circular-rectangular composite hole type parts using the assembly method comprises: the robot comprises an upper computer, a base 1, a six-degree-of-freedom robot 2, a six-dimensional force/torque sensor 3, an assembly part 4 (a shaft part matched with a hole part), a clamping and fixing seat 5 and an assembled part 6 (a round-rectangular composite hole part). The six-degree-of-freedom robot 2 is any robot which can realize spatial six-degree-of-freedom motion and has position control precision meeting assembly operation, and is not limited to a joint type serial six-degree-of-freedom operating arm; the upper computer communicates with 6 servo drivers/controllers of the six-degree-of-freedom robot 2 and the six-dimensional force/torque sensor 3 through buses, and the used buses CAN be Ethernet (Ethernet), RS485 networks, CAN buses and the like.

Fig. 3 shows a virtual prototype picture of the automatic assembly system, wherein the assembly part 4 is clamped on a tool side interface of the six-dimensional force/torque sensor 3, a robot side interface of the six-dimensional force/torque sensor 3 is fixedly connected with an end interface of the six-degree-of-freedom robot 2, the assembled part 6 with the round-rectangular composite hole is fixed in an interface of the clamping fixed seat 5, and the clamping fixed seat 5 and the six-degree-of-freedom robot 2 are both fixed on the base 1.

In fig. 3, a coordinate system Σ O-xyz is a base coordinate system whose origin is fixed to the center of the robot interface of the base 1, and the coordinate system Σ O_P-xyz and Σ O_SThe origin of xyz is located at the center of the circle of the lower surface of the fitting 4 and the force-measuring center of the six-dimensional force/torque sensor 3, respectively, and these two coordinate systems are kept relatively stationary during the fitting operation. In sigma O_PIn the system, the z-axis direction is parallel to the inserting direction of the assembly member 4 into the assembled member 6, the x-axis direction is parallel to the symmetry plane of the geometric outline of the assembly member 4 and is perpendicular to the z-axis, and the y-axis direction is determined by the vector direction obtained by multiplying the unit vector of the z-axis by the unit vector of the x-axis.

Since the relative position of the circular-rectangular composite hole of the assembly object 6 with respect to the six-degree-of-freedom robot 2 can be calculated based on the design drawing dimensions of the base 1, the holder fixing base 5, and the assembly object 6, or based on the actual measurement result after the system is assembled, the relative position is defined as the estimated position of the assembly object 6, and is affected by uncertainty factors such as machining and manufacturing errors and measurement errors, and the estimated position is not equal to the actual position of the assembly object 6, if the movement of the assembly object 6 is controlled directly by pressing the estimated position of the assembly object 6 during the assembly process, interference is likely to occur between the assembly object 6 and the assembly object 4, and it is necessary to adjust the assembly object 4 based on the feedback of the six-dimensional force/torque sensor 3.

The second embodiment is as follows: in the assembling method of the round-rectangular composite hole parts, the six-degree-of-freedom robot 2 needs to adjust the pose of the assembling part 4 according to the force feedback information measured by the six-dimensional force/torque sensor 3, so that the assembling part is finally installed in the round-rectangular composite hole of the assembled part 6. The assembly process is divided into three stages: the flow chart of the approach phase, the hole searching phase, the insertion phase and the assembly is shown in FIG. 4, wherein F_zRepresents the z-axis force component (transformed into the Σ OP system) measured by the six-dimensional force/torque sensor 3, h represents the distance along the z-axis that the fitting 4 is inserted into the to-be-fitted part 6_dAnd h_maxThe insertion depth discrimination thresholds, F, of completion of hole search and completion of assembly, respectively_maxIs the threshold value of the contact force.

The assembly operation starts with the approach stage, and the assembly part 4 needs to rapidly approach the assembled part 6 from the initial position under the control of the six-degree-of-freedom robot 2 to an assembly preparation position above the estimated position of the assembled part 6, wherein the preparation position can be added with an incremental vector [0,0, delta z ] from the estimated position]^TAnd (4) calculating. The trajectory planning of the above-described movement applies a 5-th-order spline method in which the velocity and acceleration at the starting point (initial position of the fitting 4) and the ending point (fitting preparation position) are both 0.

After the assembly part 4 reaches the assembly preparation position, the hole searching stage is started, after the stage is started, the six-freedom-degree robot 2 enables the assembly part 4 to slowly move towards the assembled part 6, the trial is carried out, and when the contact force in the z-axis direction is larger than the threshold value F_maxThe control system of the assembly operation will be based on the latter six-dimensional forceThe feedback of the torque sensor 3 judges the contact state between the assembly part 4 and the assembled part 6, then the assembly part 4 is lifted for a certain distance and the posture of the assembly part is adjusted according to the previous contact state, and the trial process of the assembly part 4 to the assembled part 6 is repeated after the adjustment is finished until the conditions are met:

F_z＜F_max&h≥h_d (1)

when the condition in the formula (1) is satisfied, the end of the assembly member 4 is inserted into the circular-rectangular composite hole of the assembled member 6, and the insertion depth reaches h_dAnd considering that the hole searching is finished, switching to an insertion stage. During the insertion phase the insert 4 is constantly advanced in the negative z-axis direction, each time F_z(corresponding to the resistance to insertion) reaches a threshold F_maxAnd then, the insertion process is considered to be possibly blocked, the inserted contact state needs to be judged, the position of the assembly part 4 in the hole is adjusted according to the contact state, and the insertion is continued until the conditions are met:

F_z＜F_max&h≥h_max (2)

when the condition in the formula (2) is met, the depth of the assembly part 4 inserted into the assembled part 6 reaches h_maxThe assembled member 6 is considered to have been successfully assembled with the assembled member 4, and the assembling operation is completed. Other steps are the same as in the first embodiment.

The third concrete implementation mode: in the automatic assembling system using the assembling method for the circular-rectangular composite hole parts, a force/position hybrid control system as shown in fig. 5 needs to be constructed, and X is defined as [ X, y, z, θ ═ X, y, z, θ_P1,θ_P2,θ_P3]^TIs the pose vector of the assembly 4, where x, y, z are respectively Σ O_PAt a position coordinate, theta, within a base coordinate system sigma O_P1、θ_P2、θ_P3Are respectively sigma O_P3 Euler angles relative to the Σ O system; theta is ═ theta₁,θ₂,θ₃,θ₄,θ₅,θ₆]^TAnd τ ═ τ [ τ ]₁,τ₂,τ₃,τ₄,τ₅,τ₆]^TAre respectively defined as a joint angle and a joint driving moment vector of the six-degree-of-freedom robot 2, where θ_iAnd τ_i(i ═ 1,2, …,6) respectively represent joint angles and drive moments for the ith joint;^SF_eis a six-dimensional force/torque sensor 3 in sigma O_SThe original data of the force rotation (comprising three force components and three moment components) measured in the system,^PF_eis to be^SF_eConversion to Σ O_PThe amount of internal rotation of the system; x_d、F_d、θ_dTarget values X, F and theta in the control process, respectively, and δ X and δ F are the adjustment amount of X and the deviation of the F theoretical value from the actual value in the control process, respectively.

step one, feedback sampling. The servo motor encoder of the six-degree-of-freedom robot 2 and the six-dimensional force/torque sensor 3 are read to obtain a joint angle vector theta and a sigma-delta O of the robot respectively_SAmount of rotation of force in the system^SF_eThe pose vector X, Sigma O of the assembly part can be obtained from theta according to the positive kinematic simulation of the six-degree-of-freedom robot 2_PRotation of force in the system^PF_eCan be calculated according to equation (3).

Step two, planning the track. Planning the position and posture of the assembly part 4 according to the described assembly flow (figure 4), if the assembly process is in the approaching stage, calculating the target position and posture X of the assembly part 4 at the current moment according to a 5-time spline function_d(ii) a If the assembly process is in the hole searching stage or the inserting stage, the z-axis component F of the contact force needs to be judged firstly_zWhether or not the threshold value F is reached_maxIf F is_z<F_maxX is generated by continuing to approach or insert the assembly 4 into the assembled part 6_dOtherwise, the fitting 4 is determined according to the contact stateA pose adjustment amount, and then X is generated based on the adjustment amount_d。

M, B, K are the inertial, damping and stiffness arrays of the virtual impedance model between the fitting 4 and the fitted part 6. For the model in equation (4), the transfer function from δ F to δ X is shown in equation (5), and δ X can also be calculated from δ F and this transfer function in actual control.

Where s represents the laplace variable in the transfer function.

And step four, position control. Planning the track to obtain the target pose X_dSuperposed with the adjustment quantity delta X obtained by impedance control, and calculated according to the inverse kinematics equation of the six-degree-of-freedom robot 2 to obtain a joint angle target vector theta_dAnd then, calculating joint driving torque tau according to a feedforward + PD feedback control law of an equation (6), sending the joint driving torque tau to a servo driving/controller of each joint of the robot, and finishing command output of the current control period.

Wherein M is_RIs a generalized inertial array estimation value, C, of a six-degree-of-freedom robot 2_RAnd G_RRespectively an estimate of the centrifugal/Coriolis force term and an estimate of the gravity term, K_vAnd K_pThe coefficient matrix of differential terms and the coefficient matrix of proportional terms in the PD feedback control are respectively. The other steps are the same as those in the second embodiment.

The fourth concrete implementation mode: in the hole searching stage of the method for assembling the circular-rectangular composite hole type part, the hole searching movement of the assembling part 4 is planned according to the contact state when the assembling part is contacted with the assembled part 6 every time, the contact state is divided into 4 types as shown in figure 6 according to the number of contact points and the positions on the bottom surface (the plane contacted with the assembled part in the hole searching stage) of the assembling part, wherein the contact points are represented by red round dots, and l and theta are_CRespectively indicating contact points at Σ O_PxO of a series_PThe phase angle and the radial length in the y plane, a, b, c and r are the geometric dimensions of the bottom surface contour of the assembly part, r is the radius of the circular arc, a is the width of the rectangular part, and b is the intersection line of the circular arc part and the rectangular part to O_PThe distance of the points, c, is the length of the rectangular portion.

Fig. 6a) shows the first contact state in the hole searching stage, wherein a single contact point is positioned inside a broken arc of the bottom surface contour of the assembly member, which shows that a point in the bottom surface contour of the assembly member interferes with the edge of the circular-rectangular composite hole of the assembled member, and the assembly member should move away from the contact point.

Fig. 6b) shows a second contact state in the hole searching stage, in which a single contact point is located on the incomplete arc boundary of the profile of the bottom surface of the assembly member, and illustrates a contact state of the assembly member 4 and the assembled member 6 in a sharp corner of the edge, which is a critical state before the hole searching is completed, and the assembly member should move a small distance away from the contact point.

Fig. 6c) shows a third contact state in the hole searching stage, in which two contact points are located on the incomplete arc boundary of the bottom profile of the assembly member, which illustrates that the fox-hunting segment defined by the two contact points overhangs the boundary of the circular-rectangular composite hole of the assembled member 6, so that the assembled member should move away from the connecting line of the contact points, and this contact state can be equivalent to the first contact state in fig. 6a), and the two contact points are regarded as one contact point at the midpoint of the connecting line, so that the adjustment strategies are completely the same.

Fig. 6d) shows a fourth contact state in the hole searching stage, in which a single contact point is located in the rectangular portion of the profile of the bottom surface of the assembly, illustrating that the interference of the rectangular portion is generated by the non-parallel symmetry planes of the assembly 4 and the assembled part 6, and the assembly is rotated about the z-axis (the direction of the axis perpendicular to the plane of the paper in fig. 5) so that the contact point is rotated out of the rectangular portion.

wherein F_x、F_y、F_zRespectively converting the force feedback measured by the six-dimensional force/torque sensor 3 into sigma-delta O_PThree force components in the system (here F)_zF in FIG. 3_zSame definition), M_x、M_y、M_zRespectively converting the force feedback measured by the six-dimensional force/torque sensor 3 into sigma-delta O_PThe three force components in the system, f, are the magnitude of the normal force at the contact point (equivalent contact point for the third contact state) at the hole search stage. Solving equation (7) can obtain:

step one, according to the force feedback transformation, the force momentum is obtained^PF_eSolving the force balance equation in column 2 of Table 1 according to equation (8)To obtain f, l, theta_C；

and step three, obtaining the pose adjustment amount of the assembly part in the next trial by the adjustment strategy in the 4 th column of the table 1, and superposing the pose adjustment amount to the normal adjustment motion track to obtain the hole searching motion track of the assembly part 4 in the next trial. The other steps are the same as those in the third embodiment.

The fifth concrete implementation mode: in the insertion stage of the assembling method of the round-rectangular composite hole parts, every time the z-axis force measured by the six-dimensional force/torque sensor 3 is fed back F_zReach the threshold value F_maxIn the process, the pose of the assembly part 4 needs to be adjusted according to the current contact state, so that the assembly process can be continued without being stuck. The side and bottom edges of the fitting part 4 may come into contact with the fitting part 6 during the insertion phase, and for the purpose of identifying the contact state, four basic contact points are defined, as shown in fig. 7, depending on the position of the contact points on the fitting part 4.

In the order of fig. 7a) to d), the 4 defined basic contact points are:

According to the number of contact points in the actual contact state, the types of the basic contact points and the relative positions, the contact states in the insertion stage are divided into 7 types and 41 types, wherein the seven types are respectively: a single point first type contact (FIG. 8) having a first or second type of base contact point, a single point second type contact (FIG. 9) having a third or fourth type of base contact point, a two point first type contact (FIG. 10) having a first type of base contact point and a second type of base contact point, a two point second type contact (FIG. 11) having a first or second type of base contact point and a third or fourth type of base contact point, two-point third type contacts (fig. 12) having a third type of base contact point and a fourth type of base contact point, three-point first type contacts (fig. 13) having a first or second type of base contact point and two third or fourth type of base contact points, and three-point second type contacts (fig. 14) having two first or second type of base contact points and a third or fourth type of base contact point. The 7 major categories respectively contain 2, 8, 1, 16, 4, 2 and 8 contact states, and for the convenience of later representation, P-i-j is taken as the code of the contact major category in the insertion stage, wherein i represents the number of the contact points, and j represents the serial number of the contact major category after the number of the contact points is determined; p-i-j-k is taken as the contact state number belonging to the P-i-j large class in the insertion stage, k represents the contact state serial number in the P-i-j large class, for example, the two-point second class contact is taken as P2-2, and the 10 th contact state is written as P2-2-10.

For the contact states in fig. 8 to 14, the mechanical balance equation is obtained according to the combination of the mechanical equations of the basic contact points in the equations (9) to (12), and the judgment conditions of the major classes of the contact states can be according to the mechanical equations of the major classesThe characteristics are classified and the derivation of the above equation of mechanical equilibrium and the discrimination conditions is shown in Table 2, where c_iAnd s_i(i ═ 1,2,3) are cos θ, respectively_CiAnd sin θ_CiAbbreviation of (a), theta_CiAnd f_iRespectively, the ith contact point is at Sigma O_PPolar angle and normal contact force in the system, k_mn(m is 1,2, …, 7; n is 1,2,3,4) is the nth discrimination parameter of the mth major category of contact states for discriminating the specific contact state within the major category, k is_mnAll values of (A) are in a closed range of [ -1,1 [ ]]And (4) the following steps.

TABLE 2 mechanical equilibrium equation and discrimination condition for large class of contact state at insertion stage

For the broad categories of contact states in table 2, the criteria for each contact state in each broad category and the corresponding adjustment movements are given in table 3, wherein the total number of adjustment movements of the assembly is 5, i.e. two translations along the x-axis and the y-axis and three rotations around the x-axis, the y-axis and the z-axis in the Σ OP system, in x, y, θ_x、θ_y、θ_zRespectively showing the above 5 adjustment movements; each adjustment movement has a positive direction and a negative direction, which are respectively indicated by "+" and "-"; the adjusting movement is abbreviated to the sign of the variable and the sign is combined, for example, a negative rotation of the fitting part 4 about the x-axis is abbreviated to "θ" in table 3_x- ". Since some contact states correspond to the same fitting adjustment movement and it is not necessary to distinguish them in the control at the insertion stage, these contact states are merged and written as a row in table 3 after the judgment conditions of the contact states corresponding to the same adjustment movement are merged.

Assembly force F for each downward assembly of the assembly parts 4 during the insertion phase of an automatic assembly operation_zReaches the threshold value F_maxThe adjustment movement of the fitting can then be planned as follows:

And step three, determining a specific contact state according to the judgment conditions in the third column of the table 3, and checking the adjustment motion corresponding to the current contact state in the fourth column of the table 3. The other steps are the same as those in the fourth embodiment.

The sixth specific implementation mode is as follows: as the feasibility verification of the hole searching stage, the inserting stage and the integral automatic assembly control method provided by the invention, the automatic assembly simulation of the circular-rectangular composite hole part is carried out. In this simulation, the automated assembly system used the virtual prototype system of fig. 3, and the assembly piece and the assembled piece selected a clearance fit round-rectangular composite hole type part for the nuclear industry, the specific dimensional parameters of which are shown in fig. 15, the radius of the cylindrical part is 1.5mm, the length and width of the square column part are 28mm and 1.8mm, respectively, the thickness of the cylinder and the square column in the assembly piece is 30mm, and the clearance between the assembly piece and the shaft hole of the assembled piece is 0.03 mm.

The automatic assembly simulation performed is divided into 3 parts in total: hole searching simulation, simulation of contact state judgment in the insertion stage and integral automatic assembly simulation. The purpose of the hole searching simulation is to verify the proposed hole searching method, in an initial state, only the estimated pose of the assembled part 6 exists in the automatic assembly control system, the deviation of the pose and the true value of the pose of the assembled part 6 is called a hole searching error, wherein the hole searching error translated along the x axis and the y axis is called a hole searching position error, and the initial hole searching position error is a random number within the range of +/-1 mm; the hole searching error rotating around the z axis is called the angle error of the hole searching, and the initial hole searching angle error is a random number within a range of +/-4 degrees (the hole searching error is not set in the translation direction along the z axis and the rotation direction around the x axis and the y axis). For different initial hole searching errors within the above range, multiple sets of hole searching simulations are performed, and fig. 16 shows a contact force curve and a hole searching error curve of one set of hole searching simulations.

In the simulation process shown in fig. 16, in order to simplify the adjustment movement of the assembly member pose, the hole searching errors in three directions (translation along the x-axis and the y-axis and rotation around the z-axis) are respectively and independently compensated, so that the position errors and the angle errors of the hole searching during the hole searching process show certain fluctuation. At the moment of finishing hole searching, the positions are all smaller than 0.1mm, the angle error is smaller than 0.3 degrees, meanwhile, the contact force in the whole hole searching process is smaller than 10N, and the moment generated by the contact force is smaller than 0.1 Nm.

In order to verify the proposed method for discriminating the contact state in the insertion stage, the second part of simulation is the simulation for discriminating the contact state in the insertion stage, one contact state is randomly selected from each large class of contacts in the simulation process, each contact state lasts for 300 sampling cycles, then the contact state is switched to the contact state of the next large class, so that a 2100-point sampling sequence corresponding to 7 contact states can be obtained, and each point in the sequence is discriminated according to the discrimination conditions in table 2 and table 3, so that the discrimination result of the contact state can be obtained.

According to the method, the discrimination simulation of a plurality of groups of contact states is carried out, wherein 7 preset contact states in one group of simulation are P-1-1-1, P-1-2-2, P-2-1-1, P-2-2-9, P-2-3-3, P-3-1-2 and P-3-2-7 respectively, and figure 17 shows the discrimination result (the contact state schematic diagram and the code number on the upper part of the figure) and the contact force curve (wherein the contact state schematic diagram and the code number on the upper part of the figure) obtained in the simulation process and the contact force curve (wherein

) It can be seen that the accuracy of the recognition result is 100%.

In order to carry out complete feasibility verification on the proposed automatic assembly method, coherent assembly simulation including an approach stage, a hole searching stage and an insertion stage is carried out, and a video screenshot obtained by simulation is shown in FIG. 18, wherein 0-6 s is a long-range video screenshot of the approach stage; after 6s, the assembly part 4 is mainly subjected to fine adjustment near the assembled part 6, so that the close shot video is switched to, the hole searching process can be seen to be finished within about 20s, and the assembly part 4 is basically aligned to the assembled part 6; after 20s, the insertion phase is started, and the assembly operation is completed after the assembly part 4 is completely assembled into the assembled part 6 after 45 s.

Fig. 19 shows an error curve of the assembling force during the assembling process and the distance between the assembling part 4 and the assembling completion position, and it can be seen that the force of the assembling force is less than 16N and the moment is less than 0.04Nm during the whole assembling process, so that the proposed assembling method for the circular-rectangular composite hole type part can complete the automatic assembling operation of the circular-rectangular composite hole type part on the premise of ensuring the assembly to be flexible.

Claims

1. A robot automatic assembly method for round-rectangular composite hole parts is characterized in that the assembly objects of the assembly method are hole parts and shaft parts which have round and rectangular straight sides and right angles in geometric elements, wherein the parts with the composite round holes and square holes are called round-rectangular composite hole parts, the parts with the composite round columns and square columns are called round-rectangular composite shaft parts,

the automatic assembly system for the round-rectangular composite hole type parts, which is applied to the assembly method, is characterized by comprising the following steps: the robot comprises an upper computer, a base (1), a six-degree-of-freedom robot (2), a six-dimensional force/torque sensor (3), an assembly part (4), a clamping fixing seat (5) and an assembled part (6); wherein the six-degree-of-freedom robot 2 is any robot which can realize spatial six-degree-of-freedom motion and has position control precision meeting assembly operation, the upper computer is communicated with 6 servo driving/controllers and a six-dimensional force/torque sensor (3) of the six-degree-of-freedom robot (2) through a bus, the assembly part (4) is a round-rectangular composite shaft part, the assembled part (6) is a round-rectangular composite hole part,

the assembly part (4) is clamped on a tool side interface of the six-dimensional force/torque sensor (3), a robot side interface of the six-dimensional force/torque sensor (3) is fixedly connected with a tail end interface of the six-degree-of-freedom robot (2), the assembly part (6) with a round-rectangular composite hole is fixed in an interface of the clamping fixed seat (5), and the clamping fixed seat (5) and the six-degree-of-freedom robot (2) are both fixed on the base (1);

according to the design drawing sizes of the base (1), the clamping fixed seat (5) and the assembled part (6) or the actual measurement result after the system is assembled, the relative position of the round-rectangular composite hole of the assembled part (6) relative to the six-degree-of-freedom robot (2) can be calculated, the relative position is defined as the estimated position of the assembled part (6), and the estimated position is not equal to the actual position of the assembled part (6), so if the motion of the assembled part (4) is directly pressed against the estimated position of the assembled part (6) in the assembling process, the interference between the assembled part (6) and the assembled part (4) is easy to generate, and the assembled part (4) needs to be adjusted according to the feedback of the six-dimensional force/torque sensor (3);

the assembling process is divided into three stages, namely an approaching stage, a hole searching stage and an inserting stage;

the assembling operation is started by entering an approaching stage, the assembling part (4) needs to rapidly approach the assembled part (6) from an initial position under the control of the six-freedom-degree robot (2) to an assembling preparation position above the estimated position of the assembled part (6), and the preparation position can be added with an increment vector [0,0, delta z ] from the estimated position]^TCalculating to obtain delta z as position increment in the vertical direction; trajectory planning for the above described motion applies a 5-th order spline, where the velocity and acceleration at the start and end points are both0, the starting point refers to the initial position of the assembly (4), and the end point refers to the assembly preparation position;

after the assembly part (4) reaches the assembly preparation position, the hole searching stage is started, after the stage is started, the six-degree-of-freedom robot (2) enables the assembly part (4) to slowly move towards the assembled part (6) for trial, and when the contact force in the vertical direction is larger than a threshold value F_maxThe control system of the assembly operation judges the contact state between the assembly part (4) and the assembled part (6) according to the feedback of the later six-dimensional force/torque sensor (3), then lifts the assembly part (4) for a distance and adjusts the pose of the assembly part according to the previous contact state, and the trial process of the assembly part (4) to the assembled part (6) is repeated after the adjustment is completed until the conditions are met:

F_z＜F_max&h≥h_d (1)

wherein F_zRepresents the vertical force component, F, measured by a six-dimensional force/moment sensor (3)_maxIs a threshold value of the contact force, h represents the depth of the assembly part (4) inserted into the assembled part (6), h_dJudging a threshold value of the insertion depth after hole searching is completed;

the contact states of the assembly parts (4) and the assembled parts (6) in the hole searching stage are 4, and the hole searching movement of the assembly parts (4) can be determined according to the contact state after each trial;

in the first contact state of the hole searching stage, a single contact point is positioned inside an incomplete arc of the bottom surface contour of the assembly part, which indicates that one point in the bottom surface contour of the assembly part interferes with the edge of the round-rectangular composite hole of the assembled part, and the assembly part moves in a direction away from the contact point;

in the second contact state of the hole searching stage, a single contact point is positioned on the incomplete arc boundary of the bottom surface outline of the assembly part, which indicates that the assembly part (4) and the assembled part (6) are in a sharp-angle contact state of the edge, the state is a critical state before the hole searching is finished, and the assembly part moves a small distance in a direction away from the contact point;

in the third contact state of the hole searching stage, two contact points are positioned on the incomplete arc boundary of the bottom surface outline of the assembly part, which indicates that the fox hunting section determined by the two contact points is suspended out of the circle-rectangle composite hole boundary of the assembled part (6), so that the assembly part should move towards the direction far away from the connecting line of the contact points, the contact state can be equivalent to the first contact state of the hole searching stage, and the two contact points are regarded as one contact point at the middle point of the connecting line, so that the adjustment strategies are completely the same;

in the fourth contact state of the hole searching stage, a single contact point is positioned in a rectangular part of the bottom surface outline of the assembly part, the interference of the rectangular part is generated by the fact that the symmetry plane of the assembly part (4) and the assembled part (6) is not parallel, and the assembly part is rotated around the z axis to enable the contact point to be screwed out of the rectangular part;

wherein F_x、F_y、F_zRespectively, the force feedback measured by the six-dimensional force/torque sensor (3) is converted into sigma-delta O_PThree force components within the system;

M_x、M_y、M_zrespectively converting the force feedback measured by the six-dimensional force/torque sensor (3) into sigma-delta O_PThree force components in the system, wherein f is the size of the normal force at the contact point in the hole searching stage; solving equation (7) can obtain:

according to the definitions of various contact states and the solving result in the formula (8), the contact state discriminant and the assembly adjustment strategy corresponding to each contact state in the hole searching stage are summarized in table 1:

In the hole searching stage of the automatic assembly operation, the hole searching track after the assembly part (4) and the assembled part (6) are in trial contact each time can be planned according to the following steps:

step three, obtaining the pose adjustment amount of the assembly part in the next trial by the adjustment strategy in the 4 th column of the table 1, and superposing the pose adjustment amount to the normal adjustment motion track to obtain the hole searching motion track of the assembly part (4) in the next trial;

when the condition in the formula (1) is met, the tail end of the assembly part (4) is inserted into the round-rectangular composite hole of the assembled part (6), and the insertion depth reaches h_dWhen the hole searching is finished, switching to an insertion stage; during the insertion phase the insert (4) is continuously advanced in the negative z-axis direction, i.e. in the direction in which the insert is inserted vertically downwards into the component to be assembled, whenever F_zReaches the threshold value F_maxAnd when the plugging process is considered to be possibly blocked, judging the inserted contact state, adjusting the position of the assembly part (4) in the hole according to the contact state, and continuing to insert until the conditions are met:

F_z＜F_max&h≥h_max(2) wherein h is_maxIs an insertion depth discrimination threshold for completion of assembly;

when the condition in the formula (2) is met, the depth of the assembly part (4) inserted into the assembled part (6) reaches h_maxThe assembly member (4) is considered to be successfully assembled into the assembled member (6), and the assembly operation is finished.

2. The method for the robotic automatic assembly of circular-rectangular composite hole parts as claimed in claim 1, wherein the six-degree-of-freedom robot (2) during the assembly operation is controlled by a force/position hybrid control system,

firstly, a coordinate system is defined, the origin of a base coordinate system sigma-xyz is fixed at the center of a robot interface of a base (1), and an assembly coordinate system sigma-O_P-xyz sum of measurements Σ O_S-the origin of xyz is located at the centre of the circle of the lower surface of the fitting (4) and the force measuring center of the six-dimensional force/torque sensor (3), respectively; in sigma O_PIn the system, the z-axis direction is parallel to the insertion direction of the assembly part (4) into the assembled part (6), the x-axis direction is parallel to the symmetry plane of the geometric outline of the assembly part (4) and is vertical to the z-axis, and the y-axis direction is determined by the vector direction obtained by multiplying the unit vector of the z-axis by the unit vector of the x-axis;

the variables of the control system are defined as follows: x ═ X, y, z, θ_P1,θ_P2,θ_P3]^TIs the pose vector of the assembly member (4), wherein x, y and z are respectively sigma O_PAt a position coordinate, theta, within a base coordinate system sigma O_P1、θ_P2、θ_P3Are respectively sigma O_P3 Euler angles relative to the Σ O system; theta is ═ theta₁,θ₂,θ₃,θ₄,θ₅,θ₆]^TAnd τ ═ τ [ τ ]₁,τ₂,τ₃,τ₄,τ₅,τ₆]^TAre respectively defined as a joint angle and a joint driving moment vector of the six-degree-of-freedom robot (2), wherein theta_iAnd τ_i(i-1, 2, …,6) represents the joint angle and drive torque of the ith joint, respectively; SF_eIs a six-dimensional force/torque sensor (3) in sigma-delta O_SRaw data of the torque measured in the system, PF_eIs to be^SF_eConversion to Σ O_PThe amount of internal rotation of the system; x_d、F_d、θ_dX, F and theta target values in the control process respectively, and δ X and δ F are the adjustment quantity of X and the deviation of F theoretical value and actual value in the control process respectively;

for a force/position hybrid control system for automatically assembling circular-rectangular composite hole parts, the control flow in each control period comprises the following steps:

step one, feedback sampling: servo motor encoder and six-dimensional force/torque transmission for six-degree-of-freedom robot (2)The sensors 3 read the data to respectively obtain the joint angle vectors theta and Sigma O of the robot_SAmount of rotation of force in the system^SF_eThe pose vector X, Sigma O of the assembly part can be obtained from theta according to the positive kinematic simulation of the six-degree-of-freedom robot 2_PIntrinsic torque PF_eCan be calculated according to the formula (3);

in formula (3), R is Σ O_SIs tied to sigma O_PRotational transformation matrix of the system, P_SIs O_SPoint-in-Sigma O_PPosition vector within the system, 0_3×3Is a 3-order all-zero square matrix;

step two, planning a track: planning the pose track of the assembly part (4) according to the assembly flow, and if the assembly process is in an approaching stage, calculating the target pose X of the assembly part (4) at the current moment according to a 5-time spline function_d(ii) a If the assembly process is in the hole searching stage or the inserting stage, the z-axis component F of the contact force needs to be judged firstly_zWhether or not the threshold value F is reached_maxIf F is_z<F_maxX is generated by the assembly part (4) continuously approaching or inserting the assembly part (6)_dOtherwise, the pose adjustment amount of the assembly part 4 is obtained according to the contact state, and then X is generated according to the adjustment amount_d；

Step three, impedance control: according to the target force screw quantity F_dAnd force feedback PF_eThe deviation delta F of the assembly part calculates the adjustment quantity delta X of the pose X of the assembly part, the used impedance control model is shown as a formula (4),

m, B, K are respectively an inertia array, a damping array and a rigidity array of the virtual impedance model between the assembly part (4) and the assembled part (6); for the model in the formula (4), the transfer function from δ F to δ X is shown in the formula (5), δ X is calculated according to δ F and the transfer function during actual control,

wherein s represents the laplace variable in the transfer function;

step four, position control: planning the track to obtain the target pose X_dSuperposed with the adjustment delta X obtained by impedance control, and calculated according to the inverse kinematics equation of the six-freedom-degree robot (2) to obtain a joint angle target vector theta_dThen calculating the joint driving torque tau according to the feedforward + PD feedback control law of the formula (6), sending the joint driving torque tau to a servo driving/controlling device of each joint of the robot, finishing the instruction output of the current control period,

wherein M is_RIs a generalized inertial array estimation value, C, of a six-degree-of-freedom robot 2_RAnd G_RRespectively an estimate of the centrifugal/Coriolis force term and an estimate of the gravity term, K_vAnd K_pThe coefficient matrix of differential terms and the coefficient matrix of proportional terms in the PD feedback control are respectively.

3. The method for robotic assembly of circular-rectangular composite hole-like parts according to claim 2, wherein there are four main types of 41 contact states in the insertion stage, and each contact state corresponds to an independent assembly adjustment strategy,

z-axis force feedback F measured by the six-dimensional force/torque sensor (3) every time during the insertion phase_zReaches the threshold value F_maxIn the process, the pose of the assembly part (4) needs to be adjusted according to the current contact state, so that the assembly process can be continued without being stuck; the lateral and bottom edges of the interior fitting (4) may come into contact with the fitting part (6) during the insertion phase, and for the purpose of identifying the contact state, four basic contact points are defined, differing from the positions of the contact points on the fitting part (4), in each case:

the first type of contact point is positioned on the side surface of the cylindrical part of the assembly part (4), the mechanical balance equation is shown as a formula (9), mu is the friction coefficient between the assembly part (4) and the assembled part (6),

the second type of contact point is positioned at the edge of the bottom surface of the cylindrical part, the mechanical balance equation of the second type of contact point is shown as a formula (10),

the third type of contact point is positioned on the side surface of the square column part, the mechanical balance equation of the third type of contact point is shown as the formula (11),

the third type of contact point is positioned on the edge of the bottom surface of the square column part, the mechanical balance equation is shown as the formula (12),

according to the number of contact points in the actual contact state, the types of the basic contact points and the relative positions, the contact states in the insertion stage are divided into 7 types of 41 types, and the seven types are respectively: a single point first type contact comprising a first or second type of base contact point, a single point second type contact comprising a third or fourth type of base contact point, a two point first type contact comprising a first type of base contact point and a second type of base contact point, a two point second type contact comprising a first or second type of base contact point and a third or fourth type of base contact point, a two point third type contact comprising a third type of base contact point and a fourth type of base contact point, a three point first type contact comprising a first or second type of base contact point and two third or fourth type of base contact point, a three point second type contact comprising two first or second type of base contact point and a third or fourth type of base contact point; the 7 major categories respectively contain 2, 8, 1, 16, 4, 2 and 8 contact states, and for the convenience of later representation, P-i-j is taken as the code of the contact major category in the insertion stage, wherein i represents the number of the contact points, and j represents the serial number of the contact major category after the number of the contact points is determined; p-i-j-k is taken as the contact state number belonging to the P-i-j large class in the insertion stage, k represents the contact state serial number in the P-i-j large class, for example, the second-class contact with two points is taken as P2-2, wherein the 10 th contact state is written as P2-2-10;

for the above 7 types of contact states, the mechanical balance equations are obtained according to the combination of the mechanical equations of the basic contact points in the equations (9) to (12), the mechanical balance equations and the corresponding judgment conditions for the types of contact states are given in table 2,

C in Table 2_iAnd s_i(i ═ 1,2,3) are cos θ, respectively_CiAnd sin θ_CiAbbreviation of (a), theta_CiAnd f_iRespectively, the ith contact point is at Sigma O_PPolar angle and normal contact force in the system, k_mn(m is 1,2, …, 7; n is 1,2,3,4) is the nth discrimination parameter for the mth major category of contact states for discriminating the specific contact state within the major category, k is_mnAll values of (A) are in a closed range of [ -1,1 [ ]]Internal;

for the broad categories of contact conditions in Table 2, the criteria and correspondence for each contact condition in each broad category are given in Table 3Wherein the adjusting movement of the assembly member has a total of 5, namely two translation movements along the x-axis and the y-axis and three rotation movements around the x-axis, the y-axis and the z-axis in the Σ OP system, and the two translation movements are equal to each other in x, y and θ_x、θ_y、θ_zRespectively showing the above 5 adjustment movements; each adjustment movement has a positive direction and a negative direction, which are respectively indicated by a plus sign and a minus sign; the adjustment movement is abbreviated to a combination of the sign of the variables and the sign, so that a negative rotation of the assembly 4 about the x-axis is abbreviated in table 3 to "θ_x- "; after the judgment conditions of the contact states corresponding to the same adjustment motion are merged together, these contact states are merged and written as one row in table 3,

Assembly force F for each downward assembly of the assembly part (4) during the insertion phase of an automatic assembly operation_zReaches the threshold value F_maxThe adjustment movement of the fitting can then be planned as follows: