Improved Design and Performance of Haptic Two-Port Networks through Force Feedback and Passive Actuators
Tognetti, Lawrence Joseph
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Haptic systems incorporate many different components, ranging from virtual simulations, physical robotic interfaces (super joysticks), robotic slaves, signal communication, and digital control; two-port networks offer compact and modular organization of such haptic components. By establishing specific stability properties of the individual component networks, their control parameters can be tuned independently of external components or interfacing environment. This allows the development of independent haptic two-port networks for interfacing with a class of haptic components. Furthermore, by using the two-port network with virtual coupling paradigm to analyze linear haptic systems, the complete duality between an admittance controlled device with velocity (position) feedback and virtual coupling can be compared to an impedance controlled device with force feedback and virtual coupling. This research first provides background on linear haptic two-port networks and use of Llewelyn's Stability Criterion to prove their stability when interfaced with passive environments, with specific comments regarding application of these linear techniques to nonlinear systems. Furthermore, man-machine interaction dynamics are addressed, with specific attention given to the human is a passive element assumption and how to include estimated human impedance / admittance dynamic limits into the two--port design. Two--port numerical tuning algorithms and analysis techniques are presented and lay the groundwork for testing of said haptic networks on HuRBiRT (Human Robotic Bilateral Research Tool), a large scale nonlinear hybrid active / passive haptic display. First, two-port networks are numerically tuned using a linearized dynamic model of HuRBiRT. Resulting admittance and impedance limits of the respective networks are compared to add insight on the advantages / disadvantages of the two different implementations of haptic causality for the same device, with specific consideration given to the advantage of adding force feedback to the impedance network, selection of virtual coupling form, effects of varying system parameters (such as physical or EMF damping, filters, etc.), and effects of adding human dynamic limits into the network formulation. Impedance and admittance two-port network implementations are experimentally validated on HuRBiRT, adding further practical insight into network formulation. Resulting experimental networks are directly compared to those numerically formulated through use of HuRBiRT's linearized dynamic models.