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Research Papers

Combined Temperature and Force Control for Robotic Friction Stir Welding

[+] Author and Article Information
Axel Fehrenbacher, Neil A. Duffie, Nicola J. Ferrier, Frank E. Pfefferkorn

Department of Mechanical Engineering,
University of Wisconsin,
Madison, WI 53706

Christopher B. Smith

Friction Stir Link, Inc.,
Brookfield, WI 53045

Michael R. Zinn

Department of Mechanical Engineering,
University of Wisconsin,
Madison, WI 53706
e-mail: mzinn@wisc.edu

1Corresponding author.

Manuscript received December 4, 2012; final manuscript received October 31, 2013; published online January 15, 2014. Assoc. Editor: Robert Landers.

J. Manuf. Sci. Eng 136(2), 021007 (Jan 15, 2014) (15 pages) Paper No: MANU-12-1357; doi: 10.1115/1.4025912 History: Received December 04, 2012; Revised October 31, 2013

Use of robotic friction stir welding (FSW) has gained in popularity as robotic systems can accommodate more complex part geometries while providing high applied tool forces required for proper weld formation. However, even the largest robotic FSW systems suffer from high compliance as compared to most custom engineered FSW machines or modified computer numerical control (CNC) mills. The increased compliance of robotic FSW systems can significantly alter the process dynamics such that control of traditional weld parameters, including plunge depth, is more difficult. To address this, closed-loop control of plunge force has been proposed and implemented on a number of systems. However, due to process parameter and condition variations commonly found in a production environment, force control can lead to oscillatory or unstable conditions and can, in extreme cases, cause the tool to plunge through the workpiece. To address the issues associated with robotic force control, the use of simultaneous tool interface temperature control has been proposed. In this paper, we describe the development and evaluation of a closed-loop control system for robotic friction stir welding that simultaneously controls plunge force and tool interface temperature by varying spindle speed and commanded vertical tool position. The controller was implemented on an industrial robotic FSW system. The system is equipped with a custom real-time wireless temperature measurement system and a force dynamometer. In support of controller development, a linear process model has been developed that captures the dynamic relations between the process inputs and outputs. Process validation identification experiments were performed and it was found that the interface temperature is affected by both spindle speed and commanded vertical tool position while axial force is affected primarily by commanded vertical tool position. The combined control system was shown to possess good command tracking and disturbance rejection characteristics. Axial force and interface temperature was successfully maintained during both thermal and geometric disturbances, and thus weld quality can be maintained for a variety of conditions in which each control strategy applied independently could fail. Finally, it was shown through the use of the control process model, that the attainable closed-loop bandwidth is primarily limited by the inherent compliance of the robotic system, as compared to most custom engineered FSW machines, where instrumentation delay is the primary limiting factor. These limitations did not prevent the implementation of the control system, but are merely observations that we were able to work around.

Copyright © 2014 by ASME
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References

Figures

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Fig. 1

Schematic of the FSW process

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Fig. 2

Schematics of (a) varying workpiece properties, (b) varying workpiece geometry, and (c) varying workpiece thickness

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Fig. 3

Schematic of welding around a workpiece corner

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Fig. 4

Schematic of through hole locations for the thermocouples on the FSW tool (not to scale, section view). The thermocouples are exposed at the tool-workpiece interface.

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Fig. 5

Schematic illustrating the main components of the wireless DAQ system used for FSW

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Fig. 6

Photograph of assembled instrumented tool holder for FSW

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Fig. 7

Close-up view of FSW tool showing the exposed thermocouples at the shoulder-workpiece and probe-workpiece interfaces

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Fig. 8

Signal flow schematic of experimental testbed

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Fig. 9

Schematic of simple analytical 1-DOF lumped parameter model

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Fig. 10

Block diagram of multi-input-multi-output dynamic process model with dynamic cross-coupling

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Fig. 11

Example welds used for model parameter estimation for (a) changing the spindle speed from 1900 rpm to 1700 rpm and (b) changing the vertical tool position from 9 mm to 10.5 mm

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Fig. 12

Frequency response plot of open-loop process model of (a) output interface temperature relative to commanded spindle speed and (b) output axial force relative to commanded vertical tool position

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Fig. 13

Block diagram of closed-loop interface temperature control system adjusting spindle speed

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Fig. 14

Simulated compensated open-loop frequency response of interface temperature control system

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Fig. 15

Simulated closed-loop frequency response of interface temperature control system

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Fig. 16

Block diagram of closed-loop axial force control system with vertical tool position as control input

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Fig. 17

Simulated compensated open-loop frequency response of axial force control system

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Fig. 18

Simulated closed-loop frequency response of axial force control system

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Fig. 19

Block diagram of combined closed-loop interface temperature and axial force control system using both spindle speed and vertical tool position as control inputs

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Fig. 20

Measured interface temperatures and forces when welding 6.35 mm thick 6061-T6 with a constant spindle speed of 2000 rpm, constant travel speed of 300 mm/min and a step in commanded vertical tool position from 10.5 mm to 11.0 mm (at 48 s)

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Fig. 21

Command tracking of closed-loop interface temperature control system using a sinusoidal command with 10 °C amplitude and frequency of 0.2 Hz. Constant commanded vertical tool position 10.5 mm. Constant travel speed of 360 mm/min. Feedback of probe interface temperature.

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Fig. 22

Command tracking of closed-loop axial force control system using step commands of 500 N magnitude. Constant spindle speed 2300 rpm. Constant travel speed of 360 mm/min.

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Fig. 23

Disturbance rejection of closed-loop axial force control system using an (a) 3 mm positive and (b) 5 mm negative ramp disturbance. Constant spindle speed 2300 rpm. Constant travel speed of 360 mm/min.

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Fig. 24

Photographs of weld surfaces related to Fig. 23. Top: No control, bottom: With axial force control. (a) 3 mm positive ramp disturbance and (b) 5 mm negative ramp disturbance.

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Fig. 25

Command tracking of combined closed-loop interface temperature and axial force control system. (a) Temperature part, (b) force part (same weld). Constant travel speed of 300 mm/min. Feedback of probe interface temperature.

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Fig. 26

Simultaneous command tracking of combined closed-loop interface temperature (a) and axial force (b) control system (same weld). 0.1 Hz sinusoidal commands for both interface temperature (15 °C amplitude) and axial force (500 N amplitude). Constant travel speed of 300 mm/min. Feedback of probe interface temperature.

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Fig. 27

Command tracking of temperature (a) and disturbance rejection of force (b) of combined closed-loop interface temperature and axial force control system (same weld). Step commands in interface temperature of 10 °C magnitude. 5 mm negative ramp disturbance for axial force. Constant travel speed of 300 mm/min. Feedback of probe interface temperature.

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Fig. 28

Schematic of different backing plates to affect thermal boundary conditions

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Fig. 29

Simultaneous disturbance rejection of combined closed-loop interface temperature (a) and axial force (b) control system (same weld). Welding over different backing plates (step disturbance for temperature) and 3 mm negative ramp disturbance for axial force. Constant travel speed of 300 mm/min. Feedback of probe interface temperature.

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Fig. 30

Weld surfaces when welding over different backing plates and applying a 3 mm negative ramp disturbance for axial force. (a) No control—arrow indicates area where shallow plunge depth and low temperature result in voids (b) with combined temperature and force control.

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Fig. 31

Frequency response plot of open-loop process model of axial force relating to commanded vertical tool position when (a) varying the time delay and (b) varying the stiffness

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Fig. 32

Maximum achievable crossover frequency (Hz) when varying the stiffness (i.e., natural frequency) and time delay

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