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Research Papers

Improved Haptic Fidelity Via Reduced Sampling Period With an FPGA-Based Real-Time Hardware Platform

[+] Author and Article Information
Marcia K. O’Malley

Department of Mechanical Engineering and Materials Science, Rice University, Houston, TX 77005omalleym@rice.edu

Kevin S. Sevcik

Department of Mechanical Engineering and Materials Science, Rice University, Houston, TX 77005

Emilie Kopp

 National Instruments, 11500 N Mopac Expwy, Austin, TX 78759-3504emilie.kopp@ni.com

J. Comput. Inf. Sci. Eng 9(1), 011002 (Feb 09, 2009) (7 pages) doi:10.1115/1.3072904 History: Received November 06, 2007; Revised December 19, 2008; Published February 09, 2009

A haptic virtual environment is considered to be high-fidelity when the environment is perceived by the user to be realistic. For environments featuring rigid objects, perception of a high degree of realism often occurs when the free space of the simulated environment feels free and when surfaces intended to be rigid are perceived as such. Because virtual surfaces (often called virtual walls) are typically modeled as simple unilateral springs, the rigidity of the virtual surface depends on the stiffness of the spring model. For impedance-based haptic interfaces, the stiffness of the virtual surface is limited by the damping and friction inherent in the device, the sampling rate of the control loop, and the quantization of sensor data. If stiffnesses greater than the limit for a particular device are exceeded, the interaction between the human user and the virtual surface via the haptic device becomes nonpassive. We propose a computational platform that increases the sampling rate of the system, thereby increasing the maximum achievable virtual surface stiffness, and subsequently the fidelity of the rendered virtual surfaces. We describe the modification of a PHANToM Premium 1.0 commercial haptic interface to enable computation by a real-time operating system (RTOS) that utilizes a field programmable gate array (FPGA) for data acquisition between the haptic interface hardware and computer. Furthermore, we explore the performance of the FPGA serving as a standalone system for communication and computation. The RTOS system enables a sampling rate for the PHANToM that is 20 times greater than that achieved using the “out of the box” commercial hardware system, increasing the maximum achievable surface stiffness twofold. The FPGA platform enables sampling rates of up to 400 times greater, and stiffnesses over 6 times greater than those achieved with the commercial system. The proposed computational platforms will enable faster sampling rates for any haptic device, thereby improving the fidelity of virtual environments.

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Copyright © 2009 by American Society of Mechanical Engineers
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Figures

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Figure 1

PHANToM commercial package components

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Figure 2

Custom hardware platform setup with real-time operating system and FPGA

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Figure 3

PHANToM experiment hardware orientations. A solenoid release was triggered to initiate the stylus dropping to the virtual surface. Orientations were varied to leverage gravitational forces, and adjustable weights ensured consistent applied forces on the virtual surfaces.

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Figure 4

RTOS-20 passive versus nonpassive behavior comparison. The virtual wall is located at −2 cm in the y-direction. The lower-stiffness virtual wall (500 N/m) allows deeper penetration into the virtual surface, and at steady state, the interaction is passive. The higher-stiffness virtual wall (2500 N/m) exhibits nonpassive behavior at steady state (beyond approximately 1000 ms).

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Figure 5

y-axis rms values for RTOS and FPGA. Steady-state RMS values above 0.003 cm are considered as nonpassive interactions. FPGA systems are able to display higher stiffnesses before reaching the defined limit for nonpassive behavior than the RTOS systems. Increasing the sampling rate of the FPGA from 100 kHz to 400 kHz does not result in any further improvements in maximum stiffness.

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