CN115685872A - A robot assembly algorithm based on compliance control - Google Patents

Abstract

The invention provides a robot assembly algorithm based on compliance control, which belongs to the technical field of industrial intelligent assembly. Firstly, the method of oblique spiral search is used to guide the robot to search outside the hole. Second, guide the robot to adjust within the hole. Finally, guide the robot to jack in the hole. The invention can improve the success rate of shaft hole assembly through the process of robot hole finding, adjustment and hole insertion based on the compliance control, and achieve certain adaptability to machining errors of various shaft hole parts. At the same time, the method of compliance control is also in the assembly process to ensure that the contact stress does not exceed the allowable value. In addition, the 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 low cost, and has higher assembly efficiency and accuracy compared with traditional methods.

Description

A robot assembly algorithm based on compliance 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 compliance control method.

Background technique

The robot precision shaft hole assembly operation refers to the use of robots to assemble the shaft parts into the matching hole parts. The assembly process is greatly affected by the assembly system, assembly objects and environmental factors. There may be pose deviations between the part holes and shafts. Even if deformation occurs, an appropriate assembly strategy is required to solve the large assembly force when the shaft end faces are in contact, and avoid damage to parts and robots.

Compliant control is a method to solve the large stress in robot assembly, but this kind of method needs to cooperate with certain assembly strategies to better complete the assembly task. Therefore, the robot assembly algorithm based on compliance control also came into being.

Contents of the invention

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

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

A robot assembly algorithm based on compliant control, including 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 and the position is deviated, the butt joint of the shaft hole will be jammed. 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 oblique 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 can find hole positions faster than other paths. In the hole-seeking stage, force control is carried out in the Z direction of the axial feed, and the robot adopts admittance control to maintain a constant pressure at the contact point of the shaft hole so that the hole can be found. The position control is carried out in the X and Y directions of the coordinates of the radial plane. The position trajectory is a search method of a spiral line. The search starts from the contact point and is carried out from the inside to the outside at a constant linear velocity and angular velocity away from the trajectory formed by this point. ,As shown in Figure 1.

Its equation of motion is:

In the formula: t is the search time, w is the search angular frequency, which is reflected in the density of the helix, r is the search radius, and v is the search speed.

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

Align the center of the shaft hole in the Y-axis direction: When the hole is searched, there will be two contact states known in the industry between the shaft and the hole: one-point contact and two-point contact. As shown in Figure 2, due to the small inclination angle, there will be no three-point contact. When the moment M _x in the X-axis direction is 0, there is two-point contact, and the center of the shaft hole in the Y-axis direction is already aligned. When M _x ≠ 0, it is in one-point contact, and it needs to be adjusted along the Y-axis to align the Y-axis direction of the two-point contact. Adjustment method: when M _x >0, move to the negative direction of the Y-axis coordinate system; when M _x <0, move to the positive direction of the Y-axis.

One-point contact of the shaft hole in the X-axis direction: when the shaft hole is aligned in the Y-axis direction and then adjust the shaft to translate along the X-axis direction, when the moment M _y in the Y-axis direction is greater than 0.5Nm, it can be judged as a one-point contact, as shown in Figure 3 Show.

Axis tilt back to normal: Replace the tool coordinate point of the robot from the center point o of the original axis with the contact point s of the axis hole, and 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 just. According to the geometric relationship, the axial distance h between the contact point s and the axis center point o is:

In the formula, m represents the vertical height between point _o1 in the first contact state of the shaft hole and point o in the current contact state; d represents the diameter of the shaft.

In the process of the axis returning to alignment, in order to ensure that no jamming occurs, the inclination angle α cannot be too large during the initial helical search. When the two points of the axis hole are in contact, that is, the edge position P ₁ of the axis coincides with the edge position P ₂ of the hole , a critical jam will occur. At this time, the axis inclination α _max is:

In the formula, D represents the diameter of the hole, and d represents the diameter of the shaft.

Fine-tuning the shaft hole. When the shaft just enters the hole, there may be a little contact in the hole. According to the torque of the X and Y axes at this time, fine-tune the torque to minimize the torque.

Step 3: Guide the robot to plug in the hole;

At this point, the center point of the shaft hole has 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 positioning pin is inserted into the positioning slot. Not done, locating pin not inserted into locating slot. At this time, the gripping cabin of the robot 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 slot, and the docking is completed; otherwise, it continues to rotate around the Z-axis. When it detects that the Z-axis contact force becomes smaller, Then insert again along the Z-axis direction, and when F _Z is detected to increase again, it is considered that the docking is completed and the insertion is stopped.

Effect and benefit of the present invention are:

The invention improves the success rate of shaft hole assembly through the process of robot hole finding, adjustment, and hole insertion based on compliance control, and achieves certain adaptability to machining errors of various shaft hole parts. At the same time, the method of compliance control is also in the assembly process to ensure that the contact stress does not exceed the allowable value. In addition, the 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 low cost, and has higher assembly efficiency and accuracy compared with traditional methods.

Description of drawings

Fig. 1 is a helical trajectory figure in the first stage in the embodiment of the present invention;

Figure 2 is a state diagram of one-point contact (a) and two-point contact (b) during the adjustment process in the hole. The Z coordinate in the figure indicates the axial feed direction, and X, Y indicate the radial direction.

Figure 3 is a diagram of the point contact state of the shaft 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, and s represents the contact between the shaft and the hole point, h represents the distance the axis is inserted when a point touches.

Detailed ways

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

In the embodiment of the present invention, the active compliance control method is used to judge the contact state of the parts by using the position and the sensor data of the six-dimensional force sensor. According to the different contact states of shaft and hole parts, the assembly process can be divided into three processes: finding holes outside the hole, adjusting inside the hole, and inserting holes. 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 holes outside the hole. First of all, in the hole-seeking stage, the robot performs force control in the Z direction, maintains a constant force of 10N, and keeps the shaft hole at the contact point with a constant pressure pointing to the axis within the allowable contact force range, so that the hole can be found. Position control is carried out in the X and Y directions, and the position trajectory is a helical search method. The search starts from the contact point and is carried out from the inside to the outside at a constant linear velocity and angular velocity away from the trajectory formed by this point, as shown in Figure 1. Show.

Its equation of motion is:

In the formula: t is the search time, w is the search angular frequency, which is reflected in the density of the helix, _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 searching for the hole, judge the contact state between the shaft and the hole. As shown in FIG. 2 , when the X-axis moment M _x =0, there is a two-point contact, and the center of the shaft hole in the Y-axis direction is already aligned. When M _x ≠ 0, it is in one-point contact, and it is adjusted along the Y-axis translation to achieve the alignment of the Y-axis direction of the two-point contact. Adjustment method: when M _x >0, move to the negative direction of the Y axis; when M _x <0, move to the positive direction of the Y axis. After the shaft holes are aligned in the Y-axis direction, adjust the shaft to translate along the X-axis direction. When M _y 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 axis to the contact point s, rotate the workpiece around the point s by -α, that is, rotate around the Y axis by -α, and then the axis can be returned to positive. According to the geometric relationship, the axial distance h between the contact point s and the axis center point o is:

In the formula, m represents the vertical height between point _o1 in the first contact state of the shaft hole and point o in the current contact state.

In the process of the axis returning to the alignment, in order to ensure that no jamming occurs, the inclination angle α cannot be too large during the initial spiral search. When the two points of the axis hole are in contact, that is, when P ₁ of the axis in the figure coincides with P ₂ of the hole, A critical jam will occur. At this time, the axis inclination α _max is:

When the shaft just enters the hole, there may be a little contact in the hole. According to the moment of the X and Y axes at this time, fine-tune the moment to minimize the moment.

Step 3: Guide the robot to plug in the hole;

So far, the center point of the shaft hole has been aligned, and inserted along the Z direction. When the F _Z detected by the six-dimensional force sensor suddenly increases, it rotates around the Z axis. If it is detected to become smaller, it enters and exits along the Z axis again. When F _Z is detected to increase, it is considered that the docking is completed and the insertion is stopped.

The descriptions presented above of the exemplary embodiments are for illustration only and are not intended to be exhaustive or to limit the invention to the precise forms described. Obviously, many modifications and variations are possible to those skilled in the art based on the above teaching. The exemplary embodiments were chosen and described in order to explain certain principles of the invention and its practical application, thereby enabling others skilled in the art to understand, implement and utilize the various exemplary embodiments of the invention and various alternatives thereof and modified form. It is intended that the scope of the invention be defined by the appended claims and their equivalents.

Claims (2)

1. A robot assembly algorithm based on compliance control, comprising the steps of:

step 1: guiding the robot to search holes outside the holes by adopting an inclined spiral searching method;

step 2: guiding the robot to adjust in the hole;

the Y-axis direction of the center of the shaft hole is aligned: when a hole is searched, two known contact states occur between the shaft and the hole: one point contact and two point contact are realized, and the condition of three point contact can not occur; moment M in the X-axis direction _x The contact is carried out at two points when the contact is 0, and the Y-axis direction of the center of the shaft hole is aligned; when M is _x When the contact is not equal to 0, the contact is in one point contact, and the Y-axis direction of the contact of two points is aligned by translation adjustment along the Y-axis; the adjusting method comprises the following steps: when M is _x If the direction is more than 0, moving towards the negative direction of the Y-axis coordinate system; when M is _x If the angle is less than 0, moving to the positive direction of the Y axis;

the shaft holes are contacted at one point in the X-axis direction: when the shaft hole is aligned in the Y-axis direction, the shaft is adjusted to translate along the X-axis direction, and when the moment M in the Y-axis direction _y If the number is more than 0.5N-m, the contact can be judged to be one-point contact;

shaft inclination aligning: replacing a tool coordinate point of the robot with a shaft hole contact point s from a central point o of an original shaft, and rotating a workpiece around the point s clockwise by-alpha, namely rotating the workpiece around the Y shaft by-alpha to return the shaft to the right; from the geometric relationship, the axial distance h between the contact point s and the shaft center point o is:

wherein m represents the first contact state of the shaft hole o ₁ Vertical height between the point and the existing contact state point o; d represents the diameter of the shaft;

during the shaft aligning process, in order to ensure that no jamming occurs, the inclination angle α cannot be too large during the initial spiral search, and the two points of the shaft hole are contacted, i.e., the edge position P of the shaft ₁ And the edge position P of the hole ₂ When coincidence occurs, critical sticking will occur; the tilt angle of the time axis alpha _max Comprises the following steps:

wherein D represents the diameter of the hole and D represents the diameter of the shaft;

fine adjustment is carried out on the shaft hole, a point of contact may occur in the hole when the shaft is just inserted into the hole, and fine adjustment is carried out according to the moment of the X and Y axes at the moment so as to minimize the moment;

and step 3: guiding the robot to insert the hole in the hole;

until the center point of the axle hole is aligned, and inserted along Z direction, when the six-dimensional force sensor detects the contact force F _Z At sudden increases, there are two possibilities: firstly, after the butt joint is completed, the positioning pin is inserted into the positioning groove; secondly, the butt joint is not completed, and the positioning pin is not inserted into the positioning groove; at the moment, the robot clamping cabin rotates around the Z axis, and if the sensor detects that the Z axis moment is increased, the positioning pin is considered to be inserted into the positioning groove, and the butt joint is completed; otherwise, continuing to rotate around the Z axis, when the Z axis contact force is detected to be reduced, inserting along the Z axis direction again, and when the Z axis contact force is detected to be reduced again, inserting along the Z axis direction again, and when F is detected again _Z And when the size is increased, the butting is considered to be finished, and the insertion is stopped.

2. A compliance control based robot assembly algorithm according to claim 1, wherein in step 1, the tilted spiral search is an optimized search strategy for a two-dimensional environment, and can contain all possible hole positions within a specific search radius and find the hole positions faster than other paths; in the hole searching stage, force control is carried out in the axial feeding Z direction, and the robot adopts admittance control to keep a constant pressure at the contact point of the shaft hole so as to find the hole; the position of the radial plane is controlled in the direction of a coordinate X, Y, the track of the position is a searching mode of a spiral line, and the searching is carried out by taking a contact point as a starting point and keeping away from the track formed by the point from inside to outside at constant linear speed and angular speed;

the motion equation is as follows:

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

Images