Adaptive Control for Improved Transparency in Haptic Interaction with Virtual Environments ADAPTIVE CONTROL FOR IMPROVED TRANSPARENCY IN HAPTIC INTERACTION WITH VIRTUAL ENVIRONMENTS

Impedance-type haptic devices are being developed for applications requiring high output forces and large workspaces. The natural dynamics of such haptic interfaces can be significant enough to interfere with the user’s perception of the virtual environment both in free motion and in contact. In such cases, the haptic control algorithm must be able to reduce /alter the apparent dynamics of the interface. In addition, due to stability constraints, conventional spring-damper controllers are unable to render highly stiff environments. In this thesis, adaptive nonlinear controllers have been proposed to improve the stability and transparency in haptic rendering. Through a separation of control and dynamic simulation, the proposed controllers can couple impedance-type haptic devices with impedance and admittance-type virtual environment simulators. The intervening dynamics of the interface, subject to stability constraints, can be replaced with an adjustable mass-damper tool within the proposed framework. Nonlinear dynamics for haptic device and parametric uncertainty in user’s arm dynamics are considered in the design of controllers which require position, velocity and force measurements. The transparency and stability of the proposed haptic control systems are investigated using a Lyapunov analysis. The effect of one sample time computation delay in a discrete-time implementation of the controller on the closed-loop stability of iv the control system is studied analytically. It is shown that the use of a discretetime low-pass filter on selected terms in the control action can significantly improve the system stability margin for reducing the device perceived inertia. The formulation of an optimization problem allows for automatic selection of the filter parameters to balance impedance reduction against noise amplification in the loop. The controllers are implemented on a two-axis impedance-type haptic device for interacting with impedance and admittance-type virtual environments. In the impedance-type environment, interaction with a virtual wall is modeled by a spring-damper coupler. This model along with an alternative constraint-based rigid wall model are employed in the admittance-type simulations. Although the two controllers behave similarly in free motion, the controller for admittance-type environments is capable of rendering significantly stiffer rigid contacts. Moreover, the proposed controllers demonstrate a highly improved transparency compared to that achievable by conventional spring-damper virtual couplers.