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Title:
ROBOTIC ASSEMBLY WITH PARTIAL FIXTURING
Document Type and Number:
WIPO Patent Application WO/2011/153156
Kind Code:
A2
Abstract:
A method of mating a first part to a second part is disclosed using an automated robot having a gripper. The method includes positioning the first part on a partially constraining fixture. A second part is grasped with the robot gripper. The second part is moved into contact with the first part using the robot, causing the first part to slide on the partially constraining fixture to bring the first part into alignment with the second part. The first part is then mated to the second part.

Inventors:
WANG JIANJUN (US)
VITTOR TIMOTHY (DE)
ROSSANO GREGORY (US)
EAKINS WILLIAM (US)
FUHLBRIGGE THOMAS A (US)
Application Number:
PCT/US2011/038591
Publication Date:
December 08, 2011
Filing Date:
May 31, 2011
Export Citation:
Click for automatic bibliography generation   Help
Assignee:
ABB RESEARCH LTD (CH)
WANG JIANJUN (US)
VITTOR TIMOTHY (DE)
ROSSANO GREGORY (US)
EAKINS WILLIAM (US)
FUHLBRIGGE THOMAS A (US)
International Classes:
B29C65/00
Foreign References:
EP0383336A21990-08-22
JPS6076930A1985-05-01
Other References:
None
Attorney, Agent or Firm:
RICKIN, Michael, M. (29801 Euclid AvenueWickliffe, OH, US)
Download PDF:
Claims:
Claims

What is claimed is:

1 . A method of mating a first part to a second part using an automated robot having a gripper, the method comprising: positioning the first part on a partially constraining fixture; grasping a second part with the robot gripper; moving the second part into contact with the first part using the robot, causing the first part to slide on the partially constraining fixture to bring the first part into alignment with the second part; and mating the first part to the second part.

2. The method according to claim 1 wherein the partially constraining fixture includes a first and a second layer wherein the first layer is rigid and smooth relative to the second layer.

3. The method according to claim 2 wherein the second layer is a foam material.

4. The method according to claim 2 wherein the first part is fee to slide in two axis on the first layer of the partially constraining fixture.

5. The method according to claim 2 wherein the first part is free to slide in one axis on the first layer of the partially constraining fixture.

6. The method according to claim 1 wherein the automated robot is an articulated 6- axis robot.

7. The method according to claim 1 wherein the automated robot is a Cartesian gantry robot.

8. The method according to claim 1 wherein the automated robot is a multi-arm robot.

9. The method according to claim 1 wherein the second part remains in constant contact with the first part during and between the step of causing the first part to slide on the partially constraining fixture to bring the first part into alignment with the second part and the step of mating the first part to the second part.

10. A method of mating a first component to a second component, the method comprising: constraining a first component in a first fixture or gripper attached to a first motion device; constraining a second component in a second fixture or gripper, said first or said second component having motion freedom in at least one direction relative to its corresponding fixture or gripper; causing a relative motion between said first and said second component so that said first component contacts said second component and, driven by the contact and the relative motion between said first and said second component, said component having motion freedom in at least one direction relative to it's corresponding fixture or gripper moves within its fixture or gripper to correct any misalignment between said first and said second components; and after any misalignment is corrected, completing the mating of said first and said second component.

1 1 . The method according to claim 10 wherein said second fixture or gripper is stationary.

12. The method according to claim 10 wherein said second fixture or gripper is attached to a second motion device.

13. The method according to claim 12 wherein said relative motion between said first and said second component is then provided by said first and said second motion device.

14. The method according to claim 10 wherein said relative motion between said first and said second component is provided by said first motion device.

15. The method according to claim 10 wherein said step of completing mating of said first and said second component includes continuing said relative motion between said first and said second component.

16. The method according to claim 10 wherein said step of completing mating of said first and said second component includes a finishing motion other than said relative motion to correct misalignment between said first and said second components.

Description:
Robotic Assembly with Partial Fixturing

[0001] The assembly of palm size consumer electronics such as cell phones, GPS devices and PDAs is predominately a manual task. The difficulty in automating this task is largely due to the unique characteristics of the components involved in the assembly. The components in the palm size consumer electronics may be very small, typically in the range of 1 mm to 10mm. Also, these components are made of plastics, aluminum, rubber, or titanium. Thus, they are generally lightweight (often less than 10 grams) and flexible/deformable. The components commonly have non-symmetric, nonstandard shapes. Finally, securing these components together is predominantly through tight tolerance assembly techniques such as press and snap fits.

[0002] Conventional assembly automation solutions are primarily directed toward large and heavy components, and generally apply three different strategies to meet the assembly tolerance requirements. The first is to use a very accurate and repeatable robot, fixture and gripper. This strategy is limited by the cost and the achievable accuracy of the robots, the fixture and gripper and is thus often found in loose-fit assembly applications. The second approach is to implement compliance in the robot, gripper or fixture. The compliance could be passive, as in a remote center compliant (RCC) device, or active as in a force sensor based force control robot. A good design of compliance is often task dependent and should be able to automatically correct the part alignment error caused by the part variation and the inaccuracies in the robot, the fixture and gripper. In this regard, the compliance strategy lowers the accuracy requirement on the robot, the fixture and gripper. To accommodate larger alignment errors, the compliance strategy is often augmented with an active search motion. This motion moves the mating components to all or a portion of possible configurations in order to find the mating configuration. The third strategy is to apply sensors, particularly vision, to accurately locate the parts before the assembly. With the part location known accurately, the final assembly is accomplished by the accurate robot motion. The sensing strategy therefore loosens the accuracy requirement on the gripper and the fixture, but not on the robot unless the vision is also employed during the assembly.

[0003] Conventional assembly automation solutions directly or indirectly assume the parts are reliably fixed in the gripper or the fixture. There might be compliance or motion freedom built in the gripper or fixture so that the part could move together with the gripper or the fixture. Nevertheless, no motion freedom exists at the interface between the part and the fixture, or between the part and the gripper. Because of the size and weight, large and heavy components can be easily and reliably secured in the gripper or fixture either directly by the gravity induced friction force, external force generated by a vacuum, compressed air or the electric motors. With the proper fixture or gripper design, the contact force that occurs during the assembly will not displace the parts in the gripper or fixture. Unfortunately for the parts found in the palm size consumer electronics, reliable and secure fixturing/gripping that can withstand the assembly force is difficult to realize.

[0004] Automated Printed Circuit Board (PCB) assembly is currently the state of the art in the area of small electronic component assembly, but its automation solution still follows the conventional strategies. Highly accurate general purpose robots or dedicated pick and place machines guarantee the accurate assembly motion. Ubiquitous machine vision locates/inspects the part at nearly every step. Sophisticated and compliant grippers ensure reliable picking and placement. Various feeders present the components accurately and repeatedly.

[0005] Compared to a typical consumer electronics assembly, the PCB assembly is relatively more simple, in that the assembly is primarily a 2D task and the majority of components have a standard geometry and size. Thus, the most difficult task in PCB assembly automation, the so called odd-form component assembly, is relatively easy for consumer electronics assembly automation. However, the problem that remains is that all these strategies are very costly to implement.

[0006] Thus, there is a need in the art for an automated assembly method capable of assembling small components at an economical cost. mary of the Invention

[0007] According to one aspect of the present invention, a method of mating a first part to a second part is disclosed using an automated robot having a gripper. The method includes positioning the first part on a partially constraining fixture. A second part is grasped with the robot gripper. The second part is moved into contact with the first part using the robot, causing the first part to slide on the partially constraining fixture to bring the first part into alignment with the second part. The first part is then mated to the second part.

[0008] According to another aspect of the present invention, a method of mating a component part to a second component is provided. The method includes

constraining a first component in a first fixture or gripper attached to a first motion device. A second component is constrained in a second fixture or gripper. The first or second component has motion freedom in at least one direction relative to its

corresponding fixture or gripper. A relative motion is caused between the first and the second component so that the first component contacts the second component and, driven by the contact and the relative motion between the first and the second

component, the component having motion freedom in at least one direction relative to its corresponding fixture or gripper moves within its fixture or gripper to correct any misalignment between the first and the second components. After any misalignment is corrected, the mating of the first and second components is completed.

Brief Description of the Drawings

[0009] Figure 1 shows a robot system according to the present invention.

[00010] Figure 2 shows an isometric view of an exemplary printed circuit board (PCB).

[00011] Figure 3 shows an isometric view of an exemplary shield can to be mounted to the PCB.

[00012] Figure 4a shows a side view of the PCB and shield can.

[00013] Figure 4b shows a top view of the PCB and shield can. [00014] Figure 5a shows a side view of the PCB and shield can wherein the PCB part is tilted with respect to the shield can.

[00015] Figure 5b shows a top view of the PCB and shield can wherein the PCB part is tilted with respect to the shield can.

[00016] Figure 6a shows a side view of the PCB and shield can wherein the PCB part is moved downwardly with respect to the shield can.

[00017] Figure 6b shows a top view of the PCB and shield can wherein the PCB part is moved downwardly with respect to the shield can.

[00018] Figure 7a shows a side view of the PCB and shield can wherein the PCB part contacts the shield can in a sweeping motion.

[00019] Figure 7b shows a top view of the PCB and shield can wherein the PCB part contacts the shield can in a sweeping motion.

[00020] Figure 8a shows a side view of the assembled PCB and shield can.

[00021] Figure 8b shows a top view of the assembled PCB and shield can.

[00022] Figure 9 shows an isometric view of an exemplary frame.

[00023] Figure 10 shows an isometric view of an exemplary button to be mounted to the frame.

[00024] Figure 1 1 a shows a side view of the frame and button wherein the frame is roughly aligned over the button.

[00025] Figure 1 1 b shows a top view of the frame and button wherein the frame is roughly aligned over the button.

[00026] Figure 12a shows a side view of the frame and button wherein the frame is lowered to tap the button into alignment.

[00027] Figure 12b shows a top view of the frame and button wherein the frame is lowered to tap the button into alignment.

[00028] Figure 13a shows a side view of the assembled frame and button.

[00029] Figure 13b shows a top view of the assembled frame and button.

[00030] Figure 14 is a flowchart generally describing the steps of the present invention.

Detailed Description of the Invention

[00031] The present invention provides a method for assembling small

components typically found in consumer electronics such as cell phones and PDAs. Unlike traditional automation solutions that require the parts be fully fixed, the method of the present invention allows the parts to move in a partially constrained manner relative to the fixture to achieve accurate alignment for the final assembly. Thus, the fixturing is partial. In other words, the part may move in one or more axis relative to the fixture itself. Due to the elimination of the accurate fixture, the present invention provides an inexpensive and robust method for assembling small parts. It should be appreciated that the fixtures may be stationary or may be movable, including fixtures/grippers mounted on any type of motion device such as a robot, a conveyor or a turn table. In particular, the fixtures or grippers can be attached to one or more than one arm of a multi-arm robot.

[00032] Generally, the present invention enables the assembling of two

components by using an alignment motion in combination with partial part fixturing. Accordingly, a first component is constrained in a first fixture or gripper attached to a first motion device. A second component is constrained in a second fixture or gripper that is either stationary or attached to a second motion device. At least one component has motion freedom in at least one direction relative to its corresponding fixture or gripper. A relative motion between the first and second component is then provided by one or both of the motion devices such that the first component engages and contacts the second component. Driven by the contact and the relative motion, the partially constrained component moves within its fixture or gripper to self correct any

misalignment between the two components. After the alignment, the assembly is accomplished either with a continuation of the alignment motion or another finishing motion. It should be appreciated that the first and second motion device is only for the convenience of the description. They could be two separate devices providing different motions, or the same device that can generate two separate motions. One example of the latter case is a multi-arm robot that has independent motions for each arm.

[00033] With reference now to Fig. 1 , an exemplary embodiment of the present invention is shown. As can be seen, the first motion device is a robot 10. Robot 10 may be, for example, an articulated 6-axis robot, a Cartesian gantry robot, a robot having less than 6 axes such as a scara robot, or a robot having more than 6 axes such as a multi-arm robot. Robot 10 is controlled by a controller 12 that is programmable to perform the steps described herein below. Robot 10 includes one or more arms, one of the arms including a gripper 14 that carries or grasps a first part 16. A second part 18 is carried on a fixture 20 that is stationary in the workspace. In yet another embodiment, second part 18 may be carried in a gripper or fixture mounted on a second robot or another arm if robot 10 is a multi-arm robot.

[00034] As will be appreciated by one of ordinary skill in the art, the controller 12 of the present invention may include a computer readable medium having computer- readable instructions stored thereon which, when executed by a processor, carry out the operations of the present inventions as herein described. The computer-readable medium may be any medium that can contain, store, communicate, propagate, or transport the user-interface program instruction for use by or in connection with the instruction execution system, apparatus, or device and may by way of example but without limitation, be an electronic, magnetic, optical, electromagnetic, infrared, or semiconductor system, apparatus, device, or propagation medium or other suitable medium upon which the program is printed. More specific examples (a non-exhaustive list) of the computer-readable medium would include: a portable computer diskette, a hard disk, a random access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM or Flash memory), an optical fiber, a portable compact disc read-only memory (CD-ROM), an optical storage device, a transmission media such as those supporting the Internet or an intranet, or a magnetic storage device. Computer program code or instructions for carrying out operations of the present invention may be written in any suitable programming language provided it allows achieving the described technical results. [00035] With reference now to Fig. 2, the PCB 24 is shown in greater detail. As can be seen, PCB 24 includes a plurality of electrical elements. In the present embodiment, a shield base 30 surrounds a portion of the PCB requiring a shield. Shield base 30 is generally rectangular and includes four raised side-walls 32a-d. Side-walls 32 form four corners 34a-d. With reference to Fig. 3, the shield can 28 is shown. As can be seen, shield can 28 includes a generally rectangular shaped planar face 36. Side-walls 38a-d extend upwardly from each edge and form corners 40a-d. Shield can 28 is sized to be snap-fit over shield base 30.

[00036] With reference now to Figs. 4a and 4b, the method according to the present invention is shown in greater detail. As can be seen, robot gripper 14 includes a pair of opposed fingers 22 that together rigidly clamp PCB 24 therebetween.

According to one embodiment, fixture 20 includes three stacked layers. A bottom layer 25 is a rigid backing that may be affixed to a stationary base or robot gripper. A mid- layer 26 is a resilient/complaint material such as, for example, a foam. Finally, a top layer 27 is provided that is relatively rigid and smooth. As can be seen, the top layer 27 provides the backing for the shield can 28 so that it will not bend under contact force. Also, as will be described below, the smooth surface of top layer 27 enables the shield can 28 to slide thereon. Shield can 28 is positioned on top layer 27 with side walls 38a- d extending upwardly toward PCB 24.

[00037] The installation of the shield can 28 onto the PCB 24 begins with the PCB 24 being generally parallel to planar face 36 of shield can 28. Further, a corner 34a of shield base 30 is positioned approximately above the shield can corner 40a. Next, as shown in Fig. 5a and 5b, the robot 10 tilts the PCB 24 so that PCB 24 is disposed at an angle relative to planar face 36. In the present embodiment, the PCB 24 is tilted at two axis, (around the x and y axis) though it should be appreciated that the tilting can be along one axis or all three. Next, as shown in Fig. 6a and 6b, the robot 10 moves PCB downward to position corner 34a of shield base 30 proximate to corner 40a of shield can 28. Shield base 30 preferably does not contact planar face 36, but is interior to side walls 38 of shield can 28. In other words, corner 34a is lowered to a position proximate, but interior to (relative to side walls 38) corner 40a. Next, as shown in Fig. 7a and 7b, the robot 10 moves corner 34a, in a motion coplanar with top layer 27, towards corner 40a of shield can 28. Corner 34a is moved a sufficient amount so that it engages corner 40a of shield can 28. Because shield can 28 in not firmly attached to fixture 20, once corner 34a engages corner 40a, shield can 28 moves with PCB 24 and aligns therewith.

[00038] Next, the robot 10 orients the PCB 24 to again be parallel to the top layer 27 of fixture 20. At the end of the sweeping motion and flattening motions, the walls 32 of shield base 30 are aligned with, and interior to, walls 38 of shield can 28. Robot 10 then presses PCB 24 directly down toward fixture 20. Because the shield base 30 and shield can 28 are already aligned, the snap fit is assured. Thereafter, as shown in Fig. 8a and 8b, robot 10 raises the PCB 24 with the shield can 28 affixed to shield base 30.

[00039] It should be appreciated that, though the above described embodiment allows free motion in both the x and y directions on fixture 20, other embodiments are contemplated. For example, one or two walls may extend upwardly from top layer 27 such that the sweeping motion drags the shield can 28 towards one or both walls.

According to this embodiment, when shield can 28 reaches one or both walls it would be further constrained by the PCB 24 and the walls, forcing the edges of the PCB and the shield can 28 to be aligned.

[00040] With reference now to Fig. 9, a frame 100 is shown. As can be seen, frame 100 includes a plurality of spanning links 102 which connect a pair of opposed side walls 104. Side wall 104 includes an opening 1 12 which receives an input button 1 14. With reference now to Fig. 10, an exemplary button 1 14 is shown. Button 1 14 includes a body portion 1 15 and a projection 1 16 which includes a generally curved front wall and a generally tapered side wall 1 17 sized to extend through opening 1 12 of frame 100. Button 1 14 is secured in hole 1 12 when a pin 1 18 is received in a

corresponding hole 1 19 on the interior surface of side wall 104.

[00041] With reference now to Fig. 1 1 a and 1 1 b, in a first step, the frame 100 is held securely in the robot gripper 14 and oriented so that the side wall 104 with hole 1 12 is facing upwardly and is generally parallel to fixture 20. As with the first embodiment, fixture 20 includes a bottom layer 25, a compliant mid-layer 26, and a rigid and smooth top layer 27. Button 1 14 is positioned on top layer 27 and is roughly aligned vertically with opening 1 12. In a next step, as shown in Fig. 12a and 12b, the robot 10 tilts frame 100 relative to top layer 27 and moves frame 100 downwardly toward button 1 14. One or more edges of opening 1 12 contact the tapered or beveled edges of button 1 14. Because the button 1 14 is carried on a compliant material and further because button 1 14 is free to slide laterally, the contact will cause button 1 14 slide and align with hole 1 12. According to the present embodiment, no additional sensors (such as force sensors) are required to determine when to stop the downward tapping motion. The motion may be preprogrammed by teach pendant or a program in the controller. Force sensors are unnecessary because of the compliance/smoothness of the fixture allows the button 1 14 to be displaced without damage. Once the frame 100 reaches the limit of the downward tapping alignment motion, it is retracted upwardly. This tapping motion may be repeated one or more additional times. In one embodiment, the tapping depth is progressively increased. According to another embodiment, a sensing means, such as force sensors or the monitoring of motor current of one or more robot joints, could be used to monitor the part displacement. The sliding of the part could be assisted using techniques such as force control.

[00042] In a next step, after the one or more tapping motions is complete, the robot 10 lowers frame 100 downwardly (while still tilted) and then brings the frame 100 parallel again with the top layer 27 to cause the now aligned button 1 14 to be retained within hole 1 12 with a snap-fit. As shown in Fig. 13a and 13b, the robot 10 then raises frame 100 with the installed button 1 14.

[00043] With reference now to Fig. 14, an exemplary flowchart of one embodiment of the present invention is shown. At a first step 200, a first part is grasped by a robot gripper. Robot gripper may be any mechanism secured to the robot which can firmly grasp the first part. At a second step 202 a second part is positioned on the partially constraining fixture. As discussed above, partially constraining fixture may include a compliant surface. Further, partially constraining fixture may be flat and planar and/or may include one or more upwardly extending walls that prevent sliding movement in one or more directions. As discussed above, the partially constraining fixture may be part of a grasping mechanism on another robot or another arm of a multi-arm robot. It should of course be appreciated that steps 200 and 202 may be accomplished simultaneously. At a next step 204 a feature of the first part is roughly aligned with a feature of the second part. This may be accomplished by directly preprogramming the positions of the first and second part if their positions in the gripper and the fixture are repeatable, or using external sensors such as vision and laser scanners to locate the positions of each part. Instead of finding the positions of each part first and then moving them close, one alternative mimicking the human hand-eye coordination is to use the visual servo technique to constantly adjust the positions of the parts until the alignment is achieved in the image.

[00044] In a next step 206 an alignment motion is performed with the first part, wherein the first part contacts the second part to draw it into alignment therewith. As discussed above, the second part may slide laterally on the fixture in order to move into alignment with the first part. Further, as discussed above, the alignment motion may be in the form of a sweeping motion (i.e. generally lateral) or in the form of a tapping motion (i.e. generally vertical). According to one aspect of the invention, as discussed above, the second part may be partially fixtured on a second robot or on the other arm of a multi-arm robot. According to this embodiment, the alignment motion may be performed by one or both robots.

[00045] In a final step 208 the feature of the first part is mated with the feature of the second part. As discussed above, this is generally performed by pressing the two features together with sufficient force. However, it should be appreciated that other mating techniques might be used, such as, for example, screwing/rotation. Once the first and second parts are mated the assembly may be removed or may be mated with additional parts in a similar manner. For example, a plurality of fixtures may be provided, and part carried by the robot gripper may be moved to each fixture to mount an additional element using the techniques described above.

[00046] The present invention offers many advantages. First, the solution is considerably cheaper than conventional methods. The alignment motion, whether it be the sweeping motion or the tapping motion, is designed to handle a reasonable amount of errors in the system, for instance, the vision locating error, the grasping error, the part placement errors and the robot inaccuracies. As a result, a less accurate robot and tooling can be used with the proposed invention. Cost saving also comes from the design and manufacturing of the partially constraining fixture or gripper. Specifically, it is technically more difficult and more expensive to design a fully constraining fixture or gripper for small parts. Using the above described invention, a simple flat plane can be used as a fixture for many parts. The second advantage of the present invention is its resistance to system errors or inaccuracies due to the explicit handling of the system errors and inaccuracies using alignment motion. A slight manufacturing tolerance or installation error does not lead to catastrophic failure or degeneration of the successful rate.

[00047] It is to be understood that the description of the foregoing exemplary embodiment(s) is (are) intended to be only illustrative, rather than exhaustive, of the present invention. Those of ordinary skill will be able to make certain additions, deletions, and/or modifications to the embodiment(s) of the disclosed subject matter without departing from the spirit of the invention or its scope, as defined by the appended claims.