Introduction

1.1 Introduction

A haptic display (or force reflecting interface) is a device which lets the user touch, feel and manipulate virtual environments. Other connections between the human and the simulation can be made with audio and visual displays, and their implementation is not addressed in this thesis. A haptic display may be thought of as a device which generates mechanical impedances. "Impedance," here, should be understood to represent a dynamic (history-dependent) relationship between velocity and force. For instance, if the haptic interface is intended to represent manipulation of a point mass, it must exert on the user's hand a force proportional to acceleration; whereas if it is to represent squeezing of a spring, it must generate a force proportional to displacement.

The haptic interface consists of a real-time simulation of a virtual environment and a manipulandum which serves as the interface between the human operator and the simulation (see Figure 1). The operator grasps the manipulandum through some mechanical interface (i.e. a handle) and moves throughout the workspace. Based on information from various sensors, the simulation calculates forces to output with the actuators. These forces are felt by the operator through the manipulandum, making it seem to the operator as if he/she is actually interacting with the virtual environment.

Figure 1. Conceptual block diagram of the interaction between a human operator and a haptic interface.

In fact, this thesis will directly address only one block of the diagram shown above - the manipulandum, which includes the sensors and actuators. While we cannot divorce ourselves from the simulation entirely, we will not address the best way to implement general classes of virtual environments. Instead we choose to investigate the effects of manipulandum dynamics and sensor performance on the implementation of a particular environment, the "virtual wall." Implicit in this approach is the applicability of the results we obtain to other, more complicated, virtual environments. While we have reason to believe that the virtual wall is a true benchmark problem whose solution is broadly applicable, it must be left to further research to address the implementation of whole classes of virtual environments, such as multiple degree of freedom rigid body dynamics.

Since the human operator grabs directly onto the manipulandum, we cannot ignore his/her effect on the system either. Instead, we make conservative assumptions about his/her behavior which allow us to develop performance specifications for the haptic interface. Certainly a thorough investigation into the mechanical behavior of the human operator could be used to improve the accuracy of the results presented later in this thesis. This effort has been undertaken by several researchers [2, 11, 15, 20]. The results of these studies should prove useful in the design of haptic manipulanda.

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