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Research Papers

Dynamic Weight-Shifting for Improved Maneuverability and Rollover Prevention in High-Speed Mobile Manipulators

[+] Author and Article Information
Justin Storms

Mechanical Engineering,
University of Michigan,
Ann Arbor, MI 48109
e-mail: jgstorms@umich.edu

Dawn Tilbury

Professor
Fellow ASME
Mechanical Engineering,
University of Michigan,
Ann Arbor, MI 48109
e-mail: tilbury@umich.edu

1Corresponding author.

Contributed by the Dynamic Systems Division of ASME for publication in the JOURNAL OF DYNAMIC SYSTEMS, MEASUREMENT, AND CONTROL. Manuscript received June 8, 2015; final manuscript received May 27, 2016; published online July 11, 2016. Assoc. Editor: Azim Eskandarian.

J. Dyn. Sys., Meas., Control 138(10), 101007 (Jul 11, 2016) (11 pages) Paper No: DS-15-1262; doi: 10.1115/1.4033841 History: Received June 08, 2015; Revised May 27, 2016

Mobile manipulators have reduced maneuverability and risk rolling over when operated at high speeds. One of the main contributing factors is the higher center of gravity (CG) due to the manipulator arm. This paper proposes a new dynamic weight-shifting method that uses the manipulator arm on the mobile robot to improve maneuverability and reduce rollover risk. A control law is developed such that the manipulator arm keeps a low CG and the contribution of the reaction moments from its inertia is small in comparison to the reaction moments due to gravity. A linear dynamic model is used to analyze the effect of the arm design (link length, mass, etc.) on the roll dynamics. A higher fidelity nonlinear simulation is used to evaluate roll reduction and the impact on handling dynamics. Last, the dynamic weight-shifting method is implemented in hardware. With regard to reducing rollover risk, simulation results from the nonlinear model (NLM) show a 29% reduction in wheel normal load transfer by using the proposed method. In terms of improving maneuverability, experimental results with hardware demonstrate a 13% increase in lateral acceleration when using dynamic weight-shifting. By reducing the vehicle's roll motion, dynamic weight-shifting can increase safe operating speeds and maneuverability.

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Figures

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Fig. 1

Modified 1:10 scale RC car with a two-link manipulator arm used for experimental validation

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Fig. 2

Handling dynamics 2DOF bicycle model

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Fig. 3

Diagram of 1DOF model used for roll dynamics

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Fig. 4

Functional diagram of nonlinear vehicle and manipulator arm model (adapted from Ref. [16])

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Fig. 5

LM and NLM steady-state comparison for various steering inputs and forward velocities. The handling model comparison is shown for Arm Stat. in (a) and Arm Mov. in (b). The roll model comparison for Arm Stat. is in (c) and Arm Mov. is in (d).

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Fig. 6

Transient responses of the LM and NLM with Arm Stat. and Arm Mov. Simulations were run with forward velocity Vx=8 m/s and steplike steering input δ=3 deg. Solid lines represent simulation results from the NLM. Dashed lines represent simulation results from the linear handling and roll models. Dashed–dotted lines represent simulation results using the NLM ay to simulate the linear roll model.

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Fig. 7

Sensitivity of normalized roll reduction factor ρ¯ with respect to (a) link length L, (b) end effector mass mee, and (c) control gain Kϕ for the LM and NLM

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Fig. 8

Transient responses of the NLM with constant forward velocity Vx = 8 m/s and steplike steering input δ=4 deg. Plot (a) shows the X–Y path traveled by the vehicle, (b) shows the resulting lateral acceleration transient, (c) shows the roll angle response, and (d) shows the tire normal load transients during the maneuver.

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Fig. 9

Transient responses of the NLM with constant forward velocity Vx = 8 m/s and steplike steering input δ=6 deg for Arm Stat. and δ=3 deg for Arm Mov. Plot (a) is the X–Y path traveled by the vehicle, (b) is the lateral acceleration transient, (c) is the roll angle response, and (d) is the tire normal load transients during the maneuver.

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Fig. 10

Graph of the rollover stability region over a range of forward speeds and steering inputs for Arm Stat. (a) and Arm Mov. (b) cases. The rollover stability regions in plots (a) and (b) for the NTLO conditions are shown in terms of lateral accelerations for a given forward speed in plots (c) and (d).

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Fig. 11

Graph showing the effect of weight-shifting on handling dynamics. The percent increase in lateral acceleration is shown at each forward speed-steering input combination that resulted in NTLO conditions for both the Arm Stat. and Arm Mov. cases.

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Fig. 12

Transient responses of the experimental platform with forward velocity Vx=5 m/s and steplike steering input δ=20 deg. Plot (a) shows the X–Y path traveled by the vehicle with markers at 0.5, 1, 1.5, and 2 s, (b) shows the resulting lateral acceleration transients, (c) shows the roll angle response, and (d) shows the vertical acceleration transients during the maneuver.

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Fig. 13

Transient responses of the experimental platform with constant throttle and steering input δ=15 deg. Plot (a) shows the X–Y path traveled by the vehicle, (b) shows the resulting lateral acceleration transients steady-state values indicated, (c) shows the roll angle response, and (d) shows the vertical acceleration transients during the maneuver.

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