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Research Papers

The Identification of the Key Enablers for Force Control of Robotic Friction Stir Welding

[+] Author and Article Information
William R. Longhurst

Graduate Department of Mechanical Engineering,  Vanderbilt University, Welding Automation Laboratory, VU Station B 351592,2301 Vanderbilt Place, Nashville, TN 37235-1592russ.longhurst@vanderbilt.edu

Alvin M. Strauss

Professor of Mechanical Engineering, Department of Mechanical Engineering,  Vanderbilt University, Welding Automation Laboratory, VU Station B 351592, 2301 Vanderbilt Place, Nashville, TN 37235-1592al.strauss@vanderbilt.edu

George E. Cook

Associate Dean for Research and Graduate Studies School of Engineering,  Vanderbilt University, Welding Automation Laboratory, VU Station B 351826, 2301 Vanderbilt Place, Nashville, TN 37235-1826george.e.cook@vanderbilt.edu

J. Manuf. Sci. Eng 133(3), 031008 (Jun 09, 2011) (11 pages) doi:10.1115/1.4004132 History: Received June 10, 2009; Revised April 03, 2011; Published June 09, 2011; Online June 09, 2011

Friction stir welding (FSW) is a solid state welding process that uses a rotating tool to plastically deform and then forge together materials. This process requires a large axial force to be maintained on the tool as the tool is plunged into the work piece and traversed along the weld seam. Force control is required if robots are to be used. Force control provides compensation for the compliant nature of robots. Without force control, welding flaws would continuously emerge as the robot repositioned its linkages to traverse the tool along the weld seam. Insufficient plunge depth would result and cause the welding flaws as the robot’s linkages yielded from the resulting force in welding environment. As FSW continues to emerge in manufacturing, robotic applications will be desired to establish flexible automation. The research presented here identifies the key enablers for successful and stable force control of FSW. To this end, a FSW force controller was designed and implemented on a retrofitted Milwaukee Model K milling machine. The closed loop proportional, integral plus derivative control architecture was tuned using the Ziegler–Nichols method. Welding experiments were conducted by butt welding 0.25 in. (6.35 mm) × 1.50 in. (38.1 mm) × 8.0 in. (203.2 mm) samples of aluminum 6061 with a 0.25 in. (6.35 mm) threaded tool. The experimental force control system was able to regulate to a desired force with a standard deviation of 129.4 N. From the experiments, it was determined that tool geometry and position are important parameters influencing the performance of the force controller, and four key enablers were identified for stable force control of FSW. The most important enabler is the maintaining of the position of a portion of the tool’s shoulder above the work piece surface. When the shoulder is completely submerged below the surface, an unstable system occurs. The other key enablers are a smooth motion profile, an increased lead angle, and positional constraints for the tool. These last three enablers contribute to the stability of the system by making the tool’s interaction with the nonlinear welding environment less sensitive. It is concluded that successful implementation of force control in the robotic FSW systems can be obtained by establishing and adhering to these key enablers. In addition, force control via plunge depth adjustment reduces weld flash and improves the appearance of the weld.

Copyright © 2011 by American Society of Mechanical Engineers
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References

Figures

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Figure 1

FSW machine at Vanderbilt University

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Figure 2

Block diagram of force control via plunge depth

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Figure 3

Trivex and threaded FSW tool

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Figure 4

Weld sample with no force control

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Figure 8

¼ in. FSW tool with a flat and a tapered shoulder

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Figure 10

Regulation of z force, with the Trivex tool

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Figure 11

Regulation of z force, with the threaded tool

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Figure 12

P control of step input

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Figure 13

PID control with step input, example 1

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Figure 14

PID control with step input, example 2

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Figure 15

½ mm step disturbance

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Figure 16

1 mm step disturbance

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Figure 17

Weld over a 1 mm step

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Figure 18

Marco section from a ¼ in. threaded tool, example 1

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Figure 19

Marco section from a ¼ in. threaded tool, example 2

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Figure 5

Servo motor motion profile

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Figure 7

Sensitivity due to lead angle and plunge depth

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Figure 6

Estimated work piece contact area under a flat shoulder

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