Dynamic path control method and system for industrial robot

Abstract

The invention relates to a method and a system for dynamic path control of an industrial robot. A method for performing robotic dynamic path modification using a remote tool center point (RTCP) coordinate framework for applications such as material dispensing. The machining tool is fixedly mounted in the work unit, and the robot grips and moves the workpiece during machining. An RTCP framework is defined at an end of the tool. A nominal machining path on the workpiece is defined, and a prescribed offset distance from an end of the tool to the workpiece is defined. A sensor measures an actual offset distance from the tool tip to the workpiece. The controller applies the offset in the RTCP framework and calculates robot motion that causes the robot to move the workpiece nominal path past the tool tip at the offset distance. The controller translates the offset from the RTCP framework to a work cell coordinate framework and performs inverse kinematics calculations to generate robot articulation commands.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims the benefit of priority to U.S. Provisional Patent Application Serial No. 63 / 519,893, filed on August 16, 2023, and entitled “ROBOT REAL TIME PATHTRACKING UNDER REMOTE TOOL CENTER POINT FRAME.” Technical Field

[0003] The present disclosure relates generally to the field of real-time motion control of industrial robots, and more particularly to a method for controlling a robot in which a tool is fixed and the robot holds and moves a workpiece, wherein a nominal path and a user-specified tool offset are defined on the workpiece, and the robot moves the workpiece such that the path traced by the tool tip deviates from the nominal path by an amount specified by the user. Background Art

[0004] As is well known, industrial robots are used to perform a wide range of manufacturing, assembly and material movement operations. Many of these operations and tasks are performed by articulated robots (such as five-axis robots or six-axis robots) that have servo motors at each rotating joint. Control of such robots is provided in real time, where the end-of-arm tool center point motion program is divided into small motion increments and the robot controller performs real-time feedback control calculations to estimate the joint motor input commands that move the robot end-of-arm tool according to the specified motion program.

[0005] Some common robotic applications involve moving a tool along a continuous path on a workpiece. These applications include specific examples such as dispensing a bead of caulk or adhesive along a path on a workpiece, laser welding and cutting, and even grinding the edge or surface of a workpiece.

[0006] Traditional techniques for programming robot motion for these continuous path following applications involve holding the workpiece in a fixture and the robot moving the tool so that the tool tip follows a continuous path on the workpiece. These methods may be satisfactory for certain simple types of following paths, but they require a second robot (or person, etc.) to remove each workpiece after machining and place a new workpiece in the fixture.

[0007] A more significant limitation of existing path tracking methods is that the nominal path and any offsets on the workpiece are calculated in the "world" (work cell) coordinate frame. This not only limits the ability to control the distance and orientation of the tool end offset, but also makes it impossible or impractical to use sensors to implement real-time tracking of the offset and control the robot to make real-time fine adjustments to the offset.

[0008] In view of the above, there is a need for an improved method for real-time path tracking of a robot that simplifies the definition of a desired offset and enables real-time tracking of the actual offset while also enabling the robot to process a continuous stream of workpieces without assistance. Summary of the invention

[0009] The present disclosure describes a method and system for performing dynamic path modification of a robot using a remote tool center point (RTCP) coordinate frame, which is used for process applications such as material dispensing or laser cutting. A processing tool is mounted in a fixed position in a work cell, and the robot holds and moves a workpiece during processing. The RTCP frame is defined at the end of the tool. A nominal processing path on the workpiece is defined, and a specified offset distance (such as the distance from the end of the processing tool to the workpiece) is defined, which provides the best quality of the process performed. Sensors on or near the processing tool measure the actual offset distance from the end of the tool to the workpiece. The robot controller applies the offset in the RTCP frame and calculates the robot motion so that the robot moves the workpiece so that the nominal path is moved through the end of the processing tool while maintaining the specified offset distance. The controller converts the offset from the remote tool center point coordinate frame to the work cell (world) coordinate frame and performs inverse kinematics calculations to generate the required robot joint motion commands.

[0010] Additional features of the presently disclosed systems and methods will become apparent from the following description and appended claims, taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Figure 1 is an illustration of an industrial robot performing a machining operation on a workpiece by moving a machining tool along a prescribed path while the workpiece is held in a fixed position as is known in the art;

[0012] Figure 2A , Figure 2B and Figure 2C is an illustration of an industrial robot performing a machining operation by moving a workpiece relative to a fixed machining tool with dynamic path modification using a remote tool center point coordinate frame in accordance with an embodiment of the present disclosure;

[0013] Figure 3is a diagrammatic representation of movement of a workpiece relative to a fixed machining tool with dynamic path modification using a remote tool center point coordinate frame in accordance with an embodiment of the present disclosure;

[0014] Figure 4 is a flow chart of a method for dynamic path modification using a remote tool center point coordinate frame according to an embodiment of the present disclosure;

[0015] Figure 5 is a graphical representation of movement of a workpiece relative to a fixed machining tool with dynamic path modification (including variable offset distances) using a remote tool center point coordinate frame in accordance with an embodiment of the present disclosure;

[0016] Figure 6 is a graphical representation of movement of a workpiece relative to a fixed machining tool with dynamic path modification (including static tracking) using a remote tool center point coordinate frame in accordance with an embodiment of the present disclosure; and

[0017] Figure 7 is an illustration of movement of a workpiece relative to a fixed machining tool with dynamic path modification using a remote tool center point coordinate frame according to another embodiment of the present disclosure. DETAILED DESCRIPTION

[0018] The following discussion of embodiments of the present disclosure directed to dynamic path modification using a remote tool center point coordinate frame is merely exemplary in nature and is in no way intended to limit the disclosed apparatus and techniques or their applications or uses.

[0019] Industrial robots are used in a variety of manufacturing, assembly, and material movement operations. In one common type of application, robots are used to perform machining operations—such as dispensing a bead of caulk, cutting material with a welding gun, or performing a laser welding operation. These machining operations all involve a machining tool that follows a continuous path along a workpiece.

[0020] Figure 1 1 is an illustration of an industrial robot performing a machining operation on a workpiece by moving a machining tool along a prescribed path while the workpiece is held in a fixed position, as is known in the art. As will be appreciated by those skilled in the art, the robot 100 (only partially shown) is a multi-axis articulated robot having a fixed base, a plurality of arm links 110, and a tool. The machining tool 120 is coupled to an external arm link at a wrist joint or in a similar configuration, wherein the robot 100 can control the joint angular position so that the end of the tool 120 is moved to any desired position and orientation.

[0021] The workpiece 130 is held in a fixed position in the robotic work cell by a fixture 140, which is shown as a simple rod or bar. Figure 1 In the prior art technique, the robot 100 is controlled by a controller (not shown) to move the end of the processing tool 120 along a prescribed path 132 on a fixed workpiece 130. For example, consider that the processing tool 120 is a material dispenser, and the processing operation is to apply a bead of adhesive along the path 132 on the workpiece 130. Then, the workpiece 130 needs to be removed from the fixture 140 (performed by another robot or machine, or by a person) and assembled with a mating part, and a new workpiece needs to be attached to the fixture 140 (perhaps by yet another robot or another person).

[0022] Figure 1 The prior art techniques shown have been used effectively for numerous applications, but additional machinery or manpower is required to treat the workpiece before and after processing. Another disadvantage of the prior art processing techniques occurs if it is necessary to maintain an offset between the end of the tool 120 and the path 132 on the workpiece 130. In such applications, it is usually necessary to measure the offset perpendicular to the path 132, which may include complex three-dimensional wiring when following the surface of the workpiece 130. If the tool is relatively light, this method of the robot carrying the tool works well, but there are several reasons that can improve the application of the tool being fixed and the robot carrying the workpiece. For example, the workpiece is relatively light, but the tool has a complex layout (requiring heavy or fixed piping, such as certain plasma cutting, water jet cutting or welding applications), or the tool is sensitive to the mechanical plasticity of the arm (such as grinding) and is best kept in a firm fixture, or the process is difficult to control, so it is best to make the layout short (such as dispensing with heat sealants, which may be difficult to keep hot and accurately control the dispensing rate through a long hose), or it is not possible to install sensors associated with the tool on the robot, so it is better for the application to mount the tool in a fixed position.

[0023] like Figure 1 As shown, the techniques of the present disclosure have been developed to overcome the shortcomings of prior art methods of continuous path machining operations.

[0024] Figure 2A , Figure 2B and Figure 2Cis an illustration of an industrial robot performing a machining operation by moving a workpiece relative to a fixed machining tool with dynamic path modification using a remote tool center point coordinate frame in accordance with an embodiment of the present disclosure. The robot 200 (mostly to the right off the page) is a multi-axis articulated robot with a plurality of arm links 210, as discussed above. In this case, the robot 200 holds a workpiece 220 and moves the workpiece 220 relative to a fixed machining tool 230. A sensor 240 is fixedly mounted to the tool 230 or mounted in a position close to the tool 230.

[0025] A nominal path 222 is defined on the workpiece 220. The nominal path 222 may be defined in a 3D computer-aided design (CAD) system in a manner understood by one skilled in the art, wherein a solid model of the workpiece 220 is available. The nominal path 222 (which is three-dimensional in nature due to some height changes of the mating surface of the workpiece 220) is typically defined directly on the workpiece surface (the mating surface, facing upward in this view). In the examples shown throughout this disclosure, the nominal path 222 is along the outer edge on the "flange" or mating surface of the workpiece 220.

[0026] exist Figure 2A 2 , the tip of tool 230 (defined as the remote tool center point) is located at the start point along nominal path 222. The direction in which the tool center point will track path 222 is indicated by arrow 224. Figure 2B and Figure 2C The tool 230 is shown in the same position (because the tool 230 is fixed) while the robot 200 moves the workpiece 220 so that the path 222 is tracked across the tool center point. Figure 2B The robotic arm 210 is shown having moved the workpiece 220 a short distance along a path 222 relative to the tool 230 , as indicated by arrow 224 . Figure 2C The robot arm 210 is shown having moved the workpiece 220 so that a large portion of the path 222 has been traced across the end of the tool 230. This is the basic operating principle of the presently disclosed technology; the machining tool is fixed and the robot moves the workpiece while performing machining operations on the workpiece.

[0027] In the simplest machining application example, the robot 200 can manipulate the workpiece 220 so that the end of the tool 230 tracks the nominal path 222 directly on the surface of the workpiece 220. However, in many real-world examples, it is desired that the end of the tool 230 (the remote tool center point) be offset from the nominal path 222 by some user-specified distance. This offset distance and how it is handled in the technology of the present disclosure is discussed further below.

[0028] Figure 3is an illustration of movement of a workpiece relative to a fixed machining tool with dynamic path modification using a remote tool center point coordinate frame in accordance with an embodiment of the present disclosure. Figure 3 The same artifact 220 as in FIG. 2 is depicted, along with other hardware components and principles involved in the techniques of this disclosure.

[0029] Figure 3 The robot 200 and robot arm 210 are not shown in the figure, but the operation of the robot and robot arm is as described above; the workpiece 220 is coupled to the end-of-arm gripper or tool on the robot arm 210. The robot 200 is controlled to move the workpiece 220 so that the end 232 of the cross tool 230 tracks the offset path on the workpiece 220. Figure 3 Only a portion of the workpiece 220 is shown, as well as a portion of the nominal path 222 described above. A sensor 240 is also shown fixed to the tool 230 or close to the tool. The sensor 240 can be any type of sensor suitable for determining the distance between the end 232 of the tool 230 and the surface of the workpiece 220 along the nominal path 222. Non-limiting examples of the sensor 240 include a laser distance sensor, an ultrasonic sensor, and a 3D camera. The sensor 240 can also be a combination of the listed sensor types and can be configured to sense parameters of the machining operation itself - such as surface finish quality, the size of the dispensed material bead, or some other parameter. These sensor measurements (including offset distance and machining operation parameters) are used to estimate dynamic adjustments to the commanded offset distance in real time during the machining operation.

[0030] A remote tool center point coordinate frame (RTCP frame) 350 is defined at the end 232 of the tool 230 , the RTCP frame having a fixed position and orientation known to the robotic controller. Figure 3 The RTCP frame 350 shown in FIG. 3 (slightly offset from the end 232 for visual clarity) has one axis pointing upward and the other two axes lying in a horizontal plane; however, any RTCP frame orientation may be used to suit a particular application. In a preferred embodiment, the local Z axis of the RTCP frame 350 is oriented parallel to the axis of the tool 230, with the positive Z direction pointing "downward" toward the workpiece 220.

[0031] The offset path 370 is offset from the nominal path 222 by an offset vector 360. The actual offset distance is measured by the sensor 240 and the calculation in the RTCP frame 350 is performed by the controller, which then controls the robot 200 to move the workpiece 220 so that the offset path 370 is tracked across the end 232 of the processing tool 230. The purpose of the offset is to position the end 232 of the processing tool 230 at the optimal distance from the surface of the workpiece 220 in order to perform a specific processing operation. For example, a 5 mm offset may be used to obtain the best quality caulking bead from the applicator tool, or a 7 mm offset may be used relative to the end of the laser welding tool. The offset is defined by the user and can be in any vector direction or orientation in the RTCP frame 350. The offset can vary along the length of the nominal path 222. This will be discussed further below.

[0032] Figure 4 4 is a flow chart of a method for dynamic path modification using a remote tool center point coordinate frame according to an embodiment of the present disclosure. Figure 4 Prior to the method, the workpiece is mounted on the robot arm, the machining tool and the sensor are fixed in the work cell, a remote tool center point coordinate frame (RTCP frame) is defined at the tool tip, a nominal path for machining operations on the workpiece is defined, and a desired offset distance between the tool tip and the nominal path is defined. The steps of flowchart 400 are then executed continuously and in real time in the robot controller.

[0033] At block 402, a current position along the nominal path in an RTCP frame is determined. The current position along the nominal path is known to the robot controller. An expected offset distance corresponding to the current position along the nominal path is also determined. As previously discussed, the expected offset distance may be constant for the entire nominal path, or the expected offset distance may vary with the position along the nominal path.

[0034] At box 404, measurements are taken by the sensor and provided to the robot controller. At box 406, the sensor measurements are processed to determine the actual offset in the RTCP frame. Due to small variations in workpiece shape, process dynamics (e.g., inconsistent sealant viscosity), workpiece-to-robot mounting configuration, and other factors, the actual offset may be slightly different from the offset obtained by applying the desired (theoretical) offset to the nominal path. Taking measurements at box 404 and determining the actual offset at box 406 provides a real-time measured feedback signal for precise control of the robot. That is, if the actual measured offset is less than the desired offset at the current position along the nominal path, the difference (desired offset minus measured offset) will be added to the desired offset distance to be used in the next step.

[0035] At summing node 408, the actual offset from block 406 is applied to the nominal path and the desired offset to calculate the deviation that needs to be corrected in the next robot control cycle. For example, if the actual offset distance is 0.1 mm less than the desired offset distance relative to the nominal path, then at the next control cycle, the controller will command a robot target position that increases the desired offset by 0.1 mm while moving along the nominal path to the next incremental position.

[0036] At box 410, the robot target position described above is converted from the RTCP frame to the world (work cell) coordinate frame. The world or work cell coordinate frame is the coordinate frame in which the robot motion is programmed. At box 412, inverse kinematics calculations are performed to determine the robot joint motions that will move the target position "on" the workpiece (actually offset from the workpiece, at a location in a known position and orientation relative to the robot gripper) to the location of the tool end fixed in the work cell. At box 414, the set of joint angles calculated at box 412 is output; that is, the robot is moved to the calculated joint angles. From box 414, the process continuously loops back to boxes 402 and 404 for the next robot control cycle.

[0037] Figure 5 is an illustration of movement of a workpiece relative to a fixed machining tool with dynamic path modification (including variable offset distances) using a remote tool center point coordinate frame in accordance with an embodiment of the present disclosure. Figure 5 The same workpiece 220 and nominal path 222 are depicted as in Figure 2, along with the robotic arm 210 and processing tool 230 as previously discussed. The sensor 240 is also shown as before.

[0038] exist Figure 5 , the offset distance varies depending on the position along the nominal path 222. One reason to vary the offset distance is due to applications where the speed of the workpiece relative to the tool also varies along the nominal path - such as when a larger or smaller bead of gap filler is desired in one location or another of the workpiece.

[0039] The offset path 570 is different from the offset path 370 described previously. The desired offset distance is defined as zero at the start / end position on the nominal path 222. At point 570A, as the nominal path moves along the left edge of the workpiece 220, the desired offset distance begins to increase gradually and then stabilizes at a constant offset value. The desired offset distance then begins to drop back and reaches zero at point 570B. The shape of the nominal path 222 and / or the desired offset may be defined using a spline function. Figure 5The example of illustrates the flexibility of the disclosed technique for adjusting the offset according to any specific application, which can be achieved by making the desired offset distance a parameter that varies based on the position along the nominal path 222.

[0040] Figure 6 is an illustration of a workpiece being moved relative to a fixed machining tool with dynamic path modification (including static tracking) using a remote tool center point coordinate frame in accordance with an embodiment of the present disclosure. Figure 6 The same workpiece 220 and nominal path 222 as in FIG. 2 are depicted, as well as the robotic arm 210 , processing tool 230 , and sensor 240 as previously discussed.

[0041] exist Figure 6 , a set of offsets 670 are applied in a static tracking mode in which the position along the nominal path 222 remains fixed and the offsets in the principle direction of the RTCP frame are applied. For example, static tracking with multiple offsets may be advantageous for laser welding a bracket to a master workpiece. Figure 6 The example of again illustrates the flexibility of the disclosed technique for adjusting the offset vector according to specific application requirements.

[0042] Figure 7 is an illustration of movement of a workpiece relative to a fixed machining tool with dynamic path modification using a remote tool center point coordinate frame according to another embodiment of the present disclosure. Figure 7 A robot 700 having a robot arm 710 is depicted moving a workpiece 720 in an application different from the previously discussed applications. A machining tool 730 is shown, which is fixed to the ground with the tool tip (tool center point) located at the top of the tool 730. For clarity of drawing, Figure 7 Sensors that would otherwise be mounted on or near the tool 730 are omitted.

[0043] exist Figure 7 In the embodiment, the machining tool 730 may be a grinding tool, and the robot 700 moves the workpiece 720 around in various spatial orientations so that the grinding tool 730 grinds the edges and surfaces of the workpiece 720 . Figure 7 Examples are shown in which, for example, a nominal path on workpiece 720 may be moved to different spatial locations relative to the end of tool 730 and, therefore, varying vector components may be employed to define the offset, or the offset may be calculated in 3D vector space rather than simply along the Z axis of the RTCP frame.

[0044] Throughout the foregoing discussion, various computers and controllers are described and implied. It should be understood that the software applications and modules of these computers and controllers are executed on one or more electronic computing devices having processors and memory modules. Specifically, this includes the processors in the robot controllers discussed above. Specifically, the processors in these devices are configured to perform robot dynamic path modification using the above-mentioned remote tool center point coordinate frame.

[0045] Although several exemplary aspects and embodiments of the method and system for dynamic path modification of a robot using a remote tool center point coordinate frame have been discussed above, those skilled in the art will recognize modifications, permutations, additions and sub-combinations thereof. It is therefore intended that the following appended claims and claims introduced thereafter be interpreted to include all such modifications, permutations, additions and sub-combinations as being within their true spirit and scope.

Claims

1. A method for dynamic path control of an industrial robot, the method comprising: Mounting the workpiece on the robot arm; providing a machining tool having a tool tip at a fixed position and a sensor at a fixed position proximate to the tool tip; defining a remote tool center point coordinate frame called the RTCP frame having an origin at the tool tip; defining a nominal path for a machining operation on the workpiece; determining a desired offset distance between the tool tip and the nominal path; as well as The robot arm is controlled using a computer having a processor and a memory to move the workpiece such that the nominal path moves past the tool tip at the desired offset distance.

2. The method according to claim 1, wherein: Controlling the robot arm comprises: Determine the target location on the offset path; Converting the target location from the RTCP frame to a world coordinate frame; performing inverse kinematics calculations on the target location in the world coordinate frame to estimate robot joint commands for moving the workpiece so that the target location on the offset path is located at the origin of the RTCP frame; and The robot arm is controlled by the computer using the robot joint commands.

3. The method according to claim 2, wherein: Determining your target positioning includes: determining a current position of the machining operation along the nominal path; determining the expected offset distance based on the current location; measuring the actual offset distance between the tool tip and the workpiece using the sensor; determining an adjusted offset distance by subtracting the actual offset distance from the desired offset distance and then adding the desired offset distance; The target position on the offset path is estimated by offsetting the current position of the machining operation along the nominal path by the adjusted offset distance.

4. The method according to claim 3, wherein: The offset distance is defined in coordinates of the RTCP frame and includes at least an offset component along a z-axis of the RTCP frame, the offset component being aligned with an axial length of the processing tool.

5. The method according to claim 2, further comprising: Static tracking is performed wherein the robot arm moves the workpiece so as to track a set of offset vectors by positioning the tool tip relative to the target on the offset path.

6. The method according to claim 1, wherein: The desired offset distance varies based on the position along the nominal path.

7. The method according to claim 1, wherein: The processing tool is a material dispenser, a cutting or welding tool with a laser or a welding torch, or a grinding tool.

8. The method of claim 1, wherein: The industrial robot is a multi-axis articulated robot, and the computer is a robot controller that sends joint motion commands to the industrial robot and receives joint state feedback used in closed-loop robot control.

9. A method for dynamic path control of an industrial robot, the method comprising: Mounting the workpiece on the robot arm; providing a machining tool having a tool tip at a fixed position and a sensor at a fixed position proximate to the tool tip; defining a remote tool center point coordinate frame called the RTCP frame having an origin at the tool tip; defining a nominal path for a machining operation on the workpiece; determining a desired offset distance between the tool tip and the nominal path; as well as Using a computer having a processor and a memory to control the robot arm to move the workpiece so that the nominal path moves through the tool end at the expected offset distance, including: determining a target location on the offset path based on the expected offset distance and an actual offset distance measured by the sensor, converting the target location from the RTCP frame to a world coordinate frame, performing inverse kinematics calculations on the target location in the world coordinate frame to estimate robot joint commands for moving the workpiece so that the target location on the offset path is located at the origin of the RTCP frame, and controlling the robot arm using the robot joint commands by the computer.

10. A dynamic path control system for an industrial robot, the dynamic path control system comprising: a robot arm on which a workpiece is mounted; a machining tool having a tool tip mounted at a fixed position; a sensor mounted at a fixed position near the end of the tool; as well as A controller having a processor and a memory, configuring the controller with input data including a remote tool center point coordinate frame, referred to as an RTCP frame, defined at the tool tip, a nominal path for a machining operation defined on the workpiece, and a desired offset distance between the tool tip and the nominal path, The controller is further configured to control the robotic arm to move the workpiece such that the nominal path moves past the tool tip by the desired offset distance.

Images