Daniela Constantinescu

Haptic rendering of rigid contacts using impulsive and penalty forces

A new simulation approach is proposed to improve the stability and the perceived rigidity of contacts during haptic interaction with multirigid body virtual environments. The approach computes impulsive forces upon contact and penalty and friction forces during contact. The impulsive forces are derived using a new multiple collision resolution method that never increases the kinetic energy of the system. When new contacts arise, the impulsive forces generate large hand accelerations without requiring increased contact stiffness and damping. Virtual objects and linkages are regarded as points in the configuration space, and no distinction is made between them in the proposed approach.

Haptic rendering of planar rigid-body motion using a redundant parallel mechanism

We present a system for rendering planar rigid-body motion by means of a redundant parallel mechanism. The device design, the control architecture and the passive virtual environment simulation are presented. The system is used to compare various virtual walls and friction models proposed for haptic applications. In addition, the reset-integrator dry friction model proposed by Haessig and Friedland (1991) is implemented in a haptic interface for the first time.

Distributed model predictive control of constrained weakly coupled nonlinear systems

Abstract This paper proposes a distributed model predictive control (MPC) strategy for a large-scale system that consists of several dynamically coupled nonlinear systems with decoupled control constraints and disturbances. In the proposed strategy, all subsystems compute their control signals by solving local optimizations constrained by their nominal decoupled dynamics. The dynamic couplings and the disturbances are accommodated through new robustness constraints in the local optimizations. The paper derives relationships among, and designs procedures for, the parameters involved in the proposed distributed MPC strategy based on the analysis of the recursive feasibility and the robust stability of the overall system. The paper shows that, for a given bound on the disturbances, the recursive feasibility is guaranteed if the sampling interval is properly chosen. Moreover, it establishes sufficient conditions for the overall system state to converge to a robust positively invariant set. The paper illustrates the effectiveness of the proposed distributed MPC strategy by applying it to three coupled cart-(nonlinear) spring–damper subsystems.

Passive Multirate Wave Communications for Haptic Interaction in Slow Virtual Environments

Haptic interaction in slow virtual environments (VEs) can become unstable due to the phase lag introduced in the control loop by the slow update rate of the VE. Increasing the physical damping and/or limiting the contact stiffness rendered to users can mitigate the destabilizing effect of the low VE update rate. However, large physical damping and compliant virtual contacts decrease the sense of presence in VEs, especially during interaction with rigid virtual objects. To increase the maximum virtual contact stiffness that can be rendered to users without increasing the interface damping, this paper proposes a control strategy based on multirate wave communications between a haptic interface and a VE updated at a slow and fixed rate. The multirate wave communications are shown to be guaranteed passive only if the decrease of the wave sampling rate at the connection between the haptic interface and the VE does not cause aliasing. Therefore, an antialiasing low-pass filter is placed before the wave rate drop in the communications. The passivity condition is verified analytically and numerically for multirate haptic interaction in VEs with various contact stiffnesses and update rates. The transparency of haptic interaction in slow VEs to which users connect via passive multirate wave communications is investigated analytically in the frequency domain. Experiments validate that passive multirate wave communications can render stiffer contact in slow VEs than conventional direct coupling, and illustrate the destabilizing effect of the aliasing caused by the sampling rate drop in the communications.

Centralized multi-user multi-rate haptic cooperation using wave transformation

This paper proposes a multi-rate control strategy for multi-user haptic cooperation. In the proposed architecture, the virtual environment simulation runs on a central server to which all users are connected as clients. Users send position commands to and receive forces from the centralized virtual object that they cooperatively manipulate via wave-based controllers. After deriving the passivity condition for the multi-rate wave transformation, the paper investigates the stability of the cooperation between two users within a multi-rate discrete-time state space framework. The analysis predicts that clients can cooperatively manipulate much stiffer centralized virtual objects when connected to the server using wave transformations than when connected using virtual coupling. Experiments performed in a 1 degree of freedom (DOF) virtual environment confirm the analytical results.

Haptic rendering of rigid body collisions

This paper addresses the haptic rendering of rigid body collisions. A new method is proposed in which collision rendering is achieved in two steps. First, the haptic simulation uses a contact model whose stiffness is infinite during collisions and finite during sustained contact. This model is combined with a passive collision resolution scheme to compute collision impulses when new contacts arise. Second, the impulses are applied to the users hand by a controller that coordinates forces and positions between the virtual environment and the haptic interface. Haptic rendering of rigid body collisions imparts forces that generate large hand accelerations when new contacts arise without requiring increased contact stiffness and damping. Experiments with a planar rigid virtual world validate the proposed approach.

Networked Haptic Cooperation Using Remote Dynamic Proxies

Networked haptic cooperation entails direct interaction among users as well as joint manipulation of virtual objects. To increase the realism of both types of interactions, this paper introduces remote dynamic proxies. Remote dynamic proxies are second order dynamic representations of users at the remote peer sites. They are generated according to dynamics laws and are controlled by the user whom they represent through a virtual coupler. Hence, they move in a physically intuitive manner and do not suffer from position discontinuities due to network packet transmission limitations. The remote dynamic proxies are integrated into a distributed control architecture for networked haptic cooperation. An experimental comparison of the new controller to two recently proposed controllers demonstrates smoother rendering of contact between users, as well as stable cooperation for larger network delays.

Haptic feedback using local models of interaction

A local model of interaction proposed for haptic rendering of rigid body motion is extended to the rigid body manipulation of articulated structures. In the proposed local model, the motion constraints imposed on the user by the articulated structure are rendered through the local dynamics, while those imposed by other virtual objects are rendered through impulsive, penalty, and friction contact forces. The proposed local model is used to interface a planar haptic device to a virtual world comprised of both rigid objects and articulated structures moving within an enclosure of rigid walls.

Extending the Z-Width of a Haptic Device Using Acceleration Feedback

This paper proposes a nonlinear controller to extend the Z-width of a haptic device. A time-domain passivity analysis of the Z-width diagram leads to the new haptic controller, which employs acceleration feedback. The passivity condition for one degree of freedom (1DOF) haptic interaction with a virtual wall via the proposed controller is derived using passivity theory in the frequency domain. The perfomance of the proposed controller is validated experimentally on a PHANTOM Omni haptic device. The experiments illustrate that the new controller considerably extends the Z-witdh of the haptic interface.

Passive shared virtual environment for distributed haptic cooperation

For distributed haptic cooperation systems, this paper develops a framework for virtual environments such that the design of the coordinating controllers is decoupled from the network topology and the communication issues. The discrete-time n-port passivity of the shared virtual object (SVO) is presented when n SVO copies are distributed on an undirected and connected communication topology with unreliable data transmission. Wave nodes as passive network elements can be implemented on multilateral wave-based communication architecture to passively distribute power across the network. In this note, the wave node scheme introduced in [16] is employed to construct a passive wave-based network architecture in order to passively interconnect multiple discrete-time port-Hamiltonian local SVO copies alongside their coordinating controllers. The performance analysis shows that the proposed network architecture: (i) possesses n-port passivity over a network with time-varying delay and packet-loss; (ii) is lossless when subjected to communications with no time delay; and (iii) offers less dissipation comparing to the network structures built based on the node scheme proposed in [18]. Simulations in which a VO is shared among four peers across a network with constant and varying time-delay validate the analysis.