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Research Papers

Design and Implementation of Nonlinear Force Controllers for Friction Stir Welding Processes

[+] Author and Article Information
Xin Zhao

 Cummins, HHP Diesel Electronic Controls, 2851 State Street, Columbus, IN 47201

Prabhanjana Kalya

Department of Mechanical and Aerospace Engineering, Missouri University of Science and Technology, 1870 Miner Circle, Rolla, MO 65409-0050pk34b@mst.edu

Robert G. Landers

Department of Mechanical and Aerospace Engineering, Missouri University of Science and Technology, 1870 Miner Circle, Rolla, MO 65409-0050landersr@mst.edu

K. Krishnamurthy

Department of Mechanical and Aerospace Engineering, Missouri University of Science and Technology, 1870 Miner Circle, Rolla, MO 65409-0050kkrishna@mst.edu

J. Manuf. Sci. Eng 130(6), 061011 (Nov 04, 2008) (10 pages) doi:10.1115/1.3006326 History: Received October 25, 2007; Revised September 04, 2008; Published November 04, 2008

In friction stir welding (FSW) processes, force control can be used to achieve good welding quality. This paper presents the systematic design and implementation of FSW force controllers. The axial and path forces are modeled as nonlinear functions of the FSW process parameters (i.e., plunge depth, tool traverse rate, and tool rotation speed). Equipment models, which include communication delays, are constructed to relate the commanded and measured actuator signals. Based on the dynamic process and equipment models, nonlinear feedback controllers for the axial and path forces are designed using the polynomial pole placement technique. The controllers are implemented in a Smith predictor-corrector structure to compensate for the inherent equipment communication delays, and the controller parameters are tuned to achieve the best closed loop response possible given equipment limitations. In the axial force controller implementation, a constant axial force is maintained, even when gaps are encountered during the welding process. In the path force controller implementation, a constant path force is maintained, even in the presence of gaps, and wormhole generation during the welding process is eliminated by regulating the path force.

Copyright © 2008 by American Society of Mechanical Engineers
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References

Figures

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Figure 1

Friction stir welding system

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Figure 2

FSW head with tool and six-axis force/moment sensor

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Figure 3

Robotic friction stir welding force control program functional block structure

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Figure 4

Commanded and measured plunge depth responses

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Figure 5

Commanded and measured tool rotation speed responses

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Figure 6

Plunge depth equipment model delays and time constants

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Figure 7

Tool rotation speed equipment model delays and time constants

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Figure 8

Plunge depth equipment modeled and measured Bode diagrams

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Figure 9

Tool rotation speed equipment modeled and measured Bode diagrams

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Figure 10

Closed loop force control system block diagram

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Figure 11

Axial and path force closed loop system sensitivity functions

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Figure 12

Axial force control system Bode diagrams and stability margins (Ga=19.4 dB and Pa=84.4 deg) and path force control system Bode diagrams and stability margins (Gp=22.6 dB and Pp=83.9 deg)

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Figure 13

Lap welding experimental setup

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Figure 14

Experimental results for step changes in reference axial force (v=2.0 mm/s and ω=1600 rpm)

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Figure 15

Four-piece lap welding experimental setup with substructure and skin-to-skin gaps

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Figure 16

Four-piece experimental results for axial force control (test 4, top and middle subplots) and constant plunge depth (test 10, bottom subplot, d=4.20 mm): Fr=2.95 kN, v=2.0 mm/s, ω=2100 rpm, and g=0.381 mm

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Figure 17

Experimental setup for welding experiments along a gap

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Figure 18

Axial force and plunge depth when welding along a gap with implementation of axial force controller (v=2.0 mm/s, ω=1600 rpm, Fr=3.00 kN, and tapered gap, g=0.381→0.762 mm)

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Figure 19

Path force and tool rotation speed for path force controller (d=4.20 mm and v=2.6 mm/s)

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Figure 20

Path force and tool rotation speed when welding along a gap with implementation of path force controller (d=4.25 mm, v=3.2 mm/s, and tapered gap, g=0.381→0.762 mm)

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Figure 21

Path force before and after path controller implementation (v=3.2 mm/s, d=4.20 mm, and Fr=0.22 kN)

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Figure 22

Nugget cross sections (a) with path force control (d=4.20 mm, v=3.2 mm/s, and Fr=0.22 kN) and (b) without path force control (d=4.20 mm, v=3.2 mm/s, and ω=900 rpm)

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