Multi-Rate Control Architectures for Network-Based Multi-User Haptics Interaction

Abstract

<p> Cooperative haptics enables multiple users to manipulate computer simulated objects in a shared virtual environment and to feel the presence of other users. Prior research in the literature has mainly addressed single user haptic interaction. This thesis is concerned with haptic simulation in multi-user virtual environments in which the users can interact in a shared virtual world from separate workstations over Ethernet-based Local Area Networks (LANs) or Metropolitan Area Networks (MANs). In practice, the achievable real-time communication rate using a typical implementation of network protocols such as the UDP and TCP/IP can be well below the 1kHz update rate that is suggested in the literature for high fidelity haptic rendering. However by adopting a multi-rate control strategy as proposed in this work, the local control loops can be executed at 1kHz while the data packet transmission between the user workstations occur at a lower rate. Within such a framework, two control architectures, namely centralized and distributed are presented. In the centralized controller a central workstation simulates the virtual environment, whereas in the distributed controller each user workstation simulates its own copy of the virtual environment. Two different approaches have been proposed for mathematical modeling of the controllers and have been used in a comparative analysis of their stability and performance. The results of such analysis demonstrate that the distributed control architecture has greater stability margins and outperforms the centralized controller. They also reveal that the limited network transmission rate can degrade the haptic fidelity by introducing viscous damping into the virtual object perceived impedance. This extra damping is compensated by active control based on the damping values obtained from the analytical results. Experimental results conducted with a dual-user/dual-finger haptic platform are presented for each of the proposed controller under various scenarios in which the user workstations communicate with UDP protocol subjected to a limited transmission rate. The results demonstrate the effectiveness of the proposed distributed architecture in providing a stable and transparent haptic simulation in free motion and in contact with rigid environments.</p>