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Research Papers

The “α-Invariant”: An Energy-Based Nonlinear Minimal Damping Model for Robotic Joints With Friction

[+] Author and Article Information
Bart Milne

Department of Mechanical Engineering,
University of Canterbury,
Christchurch 8140, New Zealand
e-mail: milne.bje@gmail.com

Chris Hann

Department of Electrical Engineering,
University of Canterbury,
Christchurch 8140, New Zealand
e-mail: chris.hann@canterbury.ac.nz

XiaoQi Chen

Professor,
Department of Mechanical Engineering,
University of Canterbury,
Christchurch 8140, New Zealand
e-mail: xiaoqi.chen@canterbury.ac.nz

1Corresponding author.

Contributed by the Design Engineering Division of ASME for publication in the JOURNAL OF COMPUTATIONAL AND NONLINEAR DYNAMICS. Manuscript received October 15, 2015; final manuscript received November 3, 2016; published online January 25, 2017. Assoc. Editor: Arend L. Schwab.

J. Comput. Nonlinear Dynam 12(4), 041011 (Jan 25, 2017) (10 pages) Paper No: CND-15-1340; doi: 10.1115/1.4035192 History: Received October 15, 2015; Revised November 03, 2016

System identification of the sinusoidal steady-state response of the Phantom Omni using a local linear model revealed that friction has a non-negligible effect on the accuracy of a global linear model, particularly at low frequencies. Some of the obvious errors observed with the global linear model at low frequencies were (i) the response amplitude was lower; (ii) local linear model coefficients became physically impossible (e.g., negative) at low frequencies; and (iii) low frequency inputs resulted in a greater nonlinearity in the response compared to higher frequency inputs. While standard friction models such as Coulomb friction could be used to model the nonlinearity, there is a desire to create a friction model that is not only accurate for sinusoidal steady-state responses, but can also be generalized to any input response. One measure that is universally present in dynamical systems is energy, and in this paper the relationship between a generalized measure of energy and damping for modeling the effect of friction is developed. This paper introduces the “α-invariant” as a means of generalizing the friction behavior observed with sinusoidal steady-state responses to other waveforms. For periodic waveforms, the α-invariant is shown to be equivalent to the energy dissipated in each cycle, which demonstrates the physical significance of this quantity. The α-invariant nonlinear model formulation significantly outperforms the linear model for both sinusoidal steady state and step responses, demonstrating that this method accurately represents the physical mechanisms in the Phantom Omni. Overall, the α-invariant provides an efficient way of capturing nonlinear dynamics with a small number of parameters and experiments.

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References

Figures

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Fig. 1

Simplified diagram of the Phantom Omni showing center of masses

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Fig. 2

Illustrative nonlinear damping curves as a function of α for various values of inertia

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Fig. 3

Illustrative nonlinear damping model as a function of inertia for different values of α

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Fig. 4

C as a function of inertia

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Fig. 5

β̃linear as a function of inertia

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Fig. 6

Nonlinear C model as a function of inertia I with global linear model for comparison

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Fig. 7

Nonlinear β̃ model as a function of inertia I with global linear model for comparison

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Fig. 8

α-invariant as a function of frequency

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Fig. 9

C as a function of α-invariant at γ1 = 65 deg, γ2 = 90 deg

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Fig. 10

β̃ as a function of α-invariant at γ1 = 65 deg, γ2 = 90 deg

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Fig. 11

Fundamental frequency magnitude response for γ1 = 65 deg, γ2 = 90 deg

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Fig. 12

Fundamental frequency phase response for γ1 = 65 deg, γ2 = 90 deg

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Fig. 13

Time domain response comparison for γ1 = 65 deg, γ2 = 90 deg at f = 2.5 Hz

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Fig. 14

Comparison of time domain responses to a step input for γ1 = 65 deg, γ2 = 90 deg

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