CN115685872B - Robot assembly algorithm based on compliant control - Google Patents

Abstract

The invention provides a robot assembly algorithm based on compliant control, and belongs to the technical field of industrial intelligent assembly. First, a method of inclined spiral search is adopted to guide a robot to perform out-hole searching. Second, the robot is guided to adjust in the hole. Finally, the robot is guided to jack in the hole. The invention can improve the success rate of shaft hole assembly through the processes of robot hole searching, adjusting and inserting holes based on flexible control, and achieves certain adaptability to machining errors of various shaft hole parts. Meanwhile, the method of flexible control ensures that the contact stress does not exceed the allowable value in the assembly process. In addition, the invention can effectively improve the axle hole assembly success rate and reduce the single assembly time under a small amount of test data and lower cost, and has higher assembly efficiency and accuracy compared with the traditional method.

Description

A robot assembly algorithm based on compliant control

Technical Field

The invention belongs to the technical field of industrial intelligent assembly and relates to a robot precision shaft hole assembly algorithm based on a compliant control method.

Background technique

Robot precision shaft-hole assembly refers to the use of robots to assemble shaft parts into matching hole parts. The assembly process is greatly affected by the assembly system, assembly objects and environmental factors. There may be posture deviations or even deformation between the part holes and shafts. Appropriate assembly strategies are needed to solve the large assembly forces when the shaft end faces are in contact to avoid damage to parts and robots.

Compliance control is a method to solve the problem of large stress in robot assembly, but this method needs to be combined with certain assembly strategies to complete the assembly task better. Therefore, robot assembly algorithms based on compliance control have also emerged.

Summary of the invention

The main problem solved by the present invention is to overcome the shortcomings of the above-mentioned method, and to provide a robot assembly algorithm based on compliant control to solve the problems of low efficiency, high contact stress, and low success rate of current robot precision shaft hole assembly. The present invention is based on an active compliant control method, and uses the sensor data of the six-dimensional force sensor at the end flange position of the robot to judge the contact state of the shaft hole parts. According to the different contact states of the shaft hole parts, the assembly process can be divided into three processes: finding the hole outside the hole, adjusting the hole inside the hole, and inserting the hole. The assembly task is completed by adopting different assembly strategies for the above three processes.

In order to achieve the above object, the technical solution of the present invention is:

A robot assembly algorithm based on compliant control includes the following steps:

Step 1: guide the robot to search for holes outside the hole;

When the center of the shaft hole is not aligned or the position is deviated, the shaft-hole connection will be stuck. At this time, the robot must find the hole position. The present invention improves the existing method and proposes an inclined spiral search method. The inclined spiral search is an optimized search strategy for a two-dimensional environment. It can include all possible hole positions within a specific search radius and the speed of finding the hole position is faster than other paths. In the hole-finding stage, force control is performed in the Z direction of the axial feed, and the robot adopts admittance control to keep a constant pressure at the shaft-hole contact point so that the hole can be found. Position control is performed in the X and Y directions of the coordinates where the radial plane is located, and the position trajectory is a spiral search method. The search starts from the contact point and proceeds from the inside to the outside at a constant linear velocity and angular velocity away from this point to form a trajectory, as shown in Figure 1.

Its equation of motion is:

Where: t is the search time, w is the search angular frequency, which is reflected in the density of the spiral line, r is the search radius, and v is the search speed.

Step 2: Guide the robot to adjust in the hole;

Alignment of the shaft-hole center in the Y-axis direction: After the hole is searched, the shaft and the hole will have two contact states known in the industry: one-point contact and two-point contact. As shown in Figure 2, due to the small inclination angle, three-point contact will not occur. When the torque _Mx in the X-axis direction = 0, it is in two-point contact, and the shaft-hole center is aligned in the Y-axis direction. When _Mx ≠0, it is in one-point contact, and it is necessary to adjust the Y-axis translation to achieve two-point contact alignment in the Y-axis direction. Adjustment method: When _Mx > 0, move to the negative direction of the Y-axis coordinate system; when _Mx < 0, move to the positive direction of the Y-axis.

The shaft hole is in one-point contact in the X-axis direction: After the shaft hole is aligned in the Y-axis direction, the shaft is adjusted to translate along the X-axis direction. When the moment _My in the Y-axis direction is greater than 0.5Nm, it can be determined as a one-point contact, as shown in Figure 3.

Axis tilting return: Replace the robot's tool coordinate point from the original axis center point o to the axis hole contact point s, rotate the workpiece clockwise around point s by -α, that is, rotate around the Y axis by -α, and the axis can be returned to the center. According to the geometric relationship, the axial distance h between the contact point s and the axis center point o is:

Where m represents the vertical height between point _o1 when the shaft and hole are in initial contact and point o in current contact; d represents the diameter of the shaft.

In the process of shaft return, in order to ensure that there is no jamming, the inclination angle α during the initial spiral search cannot be too large. When the shaft and hole are in contact at two points, that is, when the edge position _P1 of the shaft coincides with the edge position _P2 of the hole, critical jamming will occur. At this time, the shaft inclination angle α _max is:

Where D is the diameter of the hole and d is the diameter of the shaft.

Fine-tune the shaft hole. When the shaft just enters the hole, there may be a point of contact inside the hole. Fine-tune the X and Y axis torques at this time to minimize the torque.

Step 3: Guide the robot to insert the hole;

At this point, the center point of the shaft hole has been aligned, and it is inserted along the Z direction. When the contact force F _Z detected by the six-dimensional force sensor suddenly increases, there are two possibilities: one is that the docking is completed and the positioning pin is inserted into the positioning groove, and the other is that the docking is not completed and the positioning pin is not inserted into the positioning groove. At this time, the robot clamping compartment rotates around the Z axis. If the sensor detects that the Z-axis torque increases, it is considered that the positioning pin is inserted into the positioning groove and the docking is completed; otherwise, it continues to rotate around the Z axis. When it is detected that the Z-axis contact force decreases, it is inserted again along the Z axis. When F _Z is detected to increase again, it is considered that the docking is completed and the insertion stops.

The effects and benefits of the present invention are:

The present invention improves the success rate of shaft hole assembly through the process of hole finding, adjustment, and jack insertion by a robot based on compliant control, and achieves a certain adaptability to the processing errors of various shaft hole parts. At the same time, the compliant control method also ensures that the contact stress does not exceed the allowable value during the assembly process. In addition, the present invention can effectively improve the success rate of shaft hole assembly and reduce the single assembly time with a small amount of test data and a low cost, and has higher assembly efficiency and accuracy than traditional methods.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG1 is a spiral trajectory diagram in the first stage in an embodiment of the present invention;

FIG2 is a state diagram of one-point contact (a) and two-point contact (b) during the adjustment process in the hole, in which the Z coordinate represents the axial feed direction, and X and Y represent the radial directions.

Figure 3 is a diagram of the one-point contact state of the shaft and hole in the X-axis direction during the adjustment process in the hole; in the figure, d and D represent the diameters of the shaft and the hole respectively, P1 and P2 represent the edge positions of the shaft and the hole respectively, s is the contact point of the shaft and the hole, and h represents the distance the shaft is inserted when the one-point contact occurs.

Detailed ways

The specific implementation of the present invention is further described below in conjunction with the accompanying drawings and technical solutions.

In the embodiment of the present invention, an active compliance control method is used to judge the contact state of parts using the position and six-dimensional force sensor data. According to the different contact states of the shaft hole parts, the assembly process can be divided into three processes: finding the hole outside the hole, adjusting the hole inside the hole, and inserting the hole. The assembly task is completed by adopting different assembly strategies for the above three processes.

Referring to FIG. 1 , in this embodiment, taking the assembly process of a shaft hole part as an example, the assembly algorithm includes the following steps:

Step 1: Guide the robot to search for holes outside the hole. First, during the hole-finding phase, the robot performs force control in the Z direction, maintaining a constant force of 10N, and maintaining a constant pressure pointing in the axial direction at the contact point within the allowable contact force range so that the hole can be found. Position control is performed in the X and Y directions, and the position trajectory is a spiral search method. The search starts from the contact point and moves away from this point from the inside to the outside at a constant linear velocity and angular velocity, as shown in Figure 1.

Its equation of motion is:

Where: t is the search time, w is the search angular frequency, which is reflected as the density of the spiral line, _r is the search radius, and v is the search speed.

When the contact force in the X and Y directions exceeds the threshold, the hole search phase ends.

Step 2: Guide the robot to adjust in the hole; in this example, the center of the shaft hole is aligned with the Y-axis direction: after the hole is searched, the contact state of the shaft and the hole is judged. As shown in Figure 2, when the X-axis torque _Mx = 0, it is in two-point contact, and the center of the shaft hole is aligned in the Y-axis direction. When _Mx ≠0, it is in one-point contact, and it is adjusted along the Y-axis to achieve two-point contact in the Y-axis direction. Adjustment method: When _Mx >0, move in the negative direction of the Y-axis; when _Mx <0, move in the positive direction of the Y-axis. After the shaft hole is aligned in the Y-axis direction, adjust the shaft to translate in the X-axis direction. When _My is greater than a certain threshold, it can be judged as a one-point contact, as shown in Figure 3. Set the tool coordinate point of the robot from the center o of the shaft to the contact point s, rotate the workpiece around point s by -α, that is, rotate around the Y-axis by -α, and the shaft can be returned to the center. According to the geometric relationship, the axial distance h between the contact point s and the center point o of the shaft is:

Where m represents the vertical height between point _o1 when the shaft and hole are in the initial contact state and point o in the current contact state.

In the process of shaft return, in order to ensure that there is no jamming, the inclination angle α during the initial spiral search cannot be too large. When the shaft and hole are in contact at two points, that is, when P ₁ of the shaft and P ₂ of the hole coincide in the figure, critical jamming will occur. At this time, the shaft inclination angle α _max is:

When the shaft just enters the hole, there may be a point of contact inside the hole. According to the X and Y axis torque at this time, fine adjustment is performed to minimize the torque.

Step 3: Guide the robot to insert the hole;

At this point, the center point of the shaft hole has been aligned, and it is inserted along the Z direction. When the F _Z detected by the six-axis force sensor suddenly increases, it rotates around the Z axis. If it is detected to be decreasing, it moves in and out along the Z axis again. When F _Z is detected to increase again, it is considered that the docking is completed and the insertion is stopped.

The description presented in the above exemplary embodiments is only used to illustrate the technical solution of the present invention, and is not intended to be exhaustive, nor is it intended to limit the present invention to the precise form described. Obviously, it is possible for a person of ordinary skill in the art to make many changes and variations based on the above teachings. The exemplary embodiments are selected and described to explain the specific principles of the present invention and its practical application, so that other technicians in the field can easily understand, implement and use the various exemplary embodiments of the present invention and its various selected forms and modified forms. The scope of protection of the present invention is intended to be defined by the attached claims and their equivalent forms.

Claims (2)

1. A robot assembly algorithm based on compliant control, characterized in that it includes the following steps: Step 1: Use the inclined spiral search method to guide the robot to find holes outside the hole; Step 2: Guide the robot to adjust in the hole; Alignment of the shaft and hole center in the Y-axis direction: After searching for a hole, there are two known contact states between the shaft and the hole: one-point contact and two-point contact, and there will be no three-point contact. When the torque _Mx in the X-axis direction is 0, it is in two-point contact, and the shaft and hole center are aligned in the Y-axis direction. When _Mx ≠0, it is in one-point contact, and it is necessary to adjust the Y-axis translation to achieve the two-point contact alignment in the Y-axis direction. Adjustment method: When _Mx >0, move to the negative direction of the Y-axis coordinate system; when _Mx <0, move to the positive direction of the Y-axis. The shaft hole is in one-point contact in the X-axis direction: After the shaft hole is aligned in the Y-axis direction, the shaft is adjusted to translate along the X-axis direction. When the moment _My in the Y-axis direction is greater than 0.5Nm, it can be determined as a one-point contact. Axis tilt return: Replace the robot's tool coordinate point from the original axis center point o to the axis hole contact point s, rotate the workpiece clockwise around point s by -α, that is, rotate around the Y axis by -α, and the axis can be returned to the center; according to the geometric relationship, the axial distance h between the contact point s and the axis center point o is: Where, m represents the vertical height between point _o1 when the shaft and hole are in the initial contact state and point o in the current contact state; d represents the diameter of the shaft; In the process of shaft return, in order to ensure that there is no jamming, the inclination angle α during the initial spiral search cannot be too large. When the shaft and hole are in contact at two points, that is, the edge position _P1 of the shaft coincides with the edge position _P2 of the hole, critical jamming will occur; at this time, the shaft inclination angle α _max is: In the formula, D represents the diameter of the hole, and d represents the diameter of the shaft; Fine-tune the shaft hole. When the shaft just enters the hole, there may be a point of contact inside the hole. Fine-tune the X and Y axis torques at this time to minimize the torque. Step 3: Guide the robot to insert the hole; At this point, the center points of the shaft holes have been aligned and inserted along the Z direction. When the contact force F _Z detected by the six-dimensional force sensor suddenly increases, there are two possibilities: one is that the docking is completed and the locating pin is inserted into the locating slot; the other is that the docking is not completed and the locating pin is not inserted into the locating slot; at this time, the robot clamping compartment rotates around the Z axis. If the sensor detects that the Z-axis torque increases, it is considered that the locating pin is inserted into the locating slot and the docking is completed; otherwise, it continues to rotate around the Z axis. When it is detected that the Z-axis contact force becomes smaller, it is inserted again along the Z axis. When F _Z is detected to increase again, it is considered that the docking is completed and the insertion is stopped. 2. A robot assembly algorithm based on compliant control according to claim 1, characterized in that, in the step 1, the inclined spiral search is an optimized search strategy for a two-dimensional environment, which can include all possible hole positions within a specific search radius and the speed of finding the hole position is faster than other paths; in the hole-finding stage, force control is performed in the Z direction of the axial feed, and the robot adopts admittance control to keep a constant pressure at the shaft-hole contact point so that the hole can be found; position control is performed in the X and Y directions of the coordinates where the radial plane is located, and the trajectory of the position is a spiral search method, and the search starts from the contact point and proceeds from the inside to the outside at a constant linear velocity and angular velocity away from the trajectory formed by this point; Its equation of motion is: Where: t is the search time, w is the search angular frequency, which is reflected in the density of the spiral line, r is the search radius, and v is the search speed.