Mohsen Mahvash

High-fidelity haptic synthesis of contact with deformable bodies

A method for synthesizing the haptic response of nonlinear deformable objects from data obtained by offline simulation helps create surgical simulators with high-fidelity haptic feedback. Haptic displays provide users with artificially created tactile sensations. One important use of these displays is to recreate the experience caused by contact between a tool and an object. This capability can be useful in several applications, such as surgical simulators, because users experience an enhanced sense of realism when a haptic simulation is combined with a graphic simulation. Haptic displays require two essential subsystems: a haptic device, which typically has a handle connected to sensors and actuators, and a computational system that interfaces with the device.

High-fidelity passive force-reflecting virtual environments

Passivity theory is applied to the creation of synthetic, complex multidimensional haptic environments. It can be shown that under appropriate conditions, sufficiently high rendering rates can guarantee the passivity of a simulation produced by a haptic device coupled to a discrete-time realization of a nominally passive environment. The creation of a passive, globally defined, virtual environment is either analytically complex or computationally costly. A method is described whereby a passive environment is created from transitions between locally defined force models that encode static conservative force fields. This is applied to the haptic rendering of tool contact with deformable bodies, in which sparse force-deflection responses are used to define local models. Passivity, continuity, and fidelity are provided by response-function interpolation rather than by interpolation of forces, as in previous methods. The work also includes an illustrative example.

Force-Feedback Surgical Teleoperator: Controller Design and Palpation Experiments

In this paper, we develop and test a 6-degree-of-freedom surgical teleoperator that has four possible modes of operation: (1) direct force feedback, (2) graphical force feedback, (3) direct and graphical force feedback together, and (4) no force feedback. In all cases, visual feedback of the: environment is provided via a head-mounted display. A position-position controller with local dynamic compensators provides the direct force feedback. The graphical force feedback is overlaid on the environment image, and displays a bar whose height and color is related to the environment force estimated using the current applied to the actuators of the patient-side arm. We evaluate the performance of the teleoperator modes in assisting a user to find the location of stiff objects hidden inside a soft material, similar to a calcified artery hidden in heart tissue and a tumor in the prostate. Seven people used the teleoperator t:o perform palpation in these materials. Results showed that direct force feedback mode minimizes palpation task error for the heart model.

Friction Compensation for Enhancing Transparency of a Teleoperator With Compliant Transmission

This paper presents a model-based compensator for canceling friction in the tendon-driven joints of a haptic-feedback teleoperator. Unlike position-tracking systems, a teleoperator involves an unknown environment force that prevents the use of tracking position error as a feedback to the compensator. Thus, we use a model-based feedforward friction compensator to cancel the friction forces. We provide conditions for selecting compensator parameters to ensure passivity of the teleoperator and demonstrate performance experimentally.

Modeling the Forces of Cutting With Scissors

Modeling forces applied to scissors during cutting of biological materials is useful for surgical simulation. Previous approaches to haptic display of scissor cutting are based on recording and replaying measured data. This paper presents an analytical model based on the concepts of contact mechanics and fracture mechanics to calculate forces applied to scissors during cutting of a slab of material. The model considers the process of cutting as a sequence of deformation and fracture phases. During deformation phases, forces applied to the scissors are calculated from a torque-angle response model synthesized from measurement data multiplied by a ratio that depends on the position of the cutting crack edge and the curve of the blades. Using the principle of conservation of energy, the forces of fracture are related to the fracture toughness of the material and the geometry of the blades of the scissors. The forces applied to scissors generally include high-frequency fluctuations. We show that the analytical model accurately predicts the average applied force. The cutting model is computationally efficient, so it can be used for real-time computations such as haptic rendering. Experimental results from cutting samples of paper, plastic, cloth, and chicken skin confirm the model, and the model is rendered in a haptic virtual environment.

Friction compensation for a force-feedback telerobotic system

This paper presents a model-based approach to cancel friction in the joints of the manipulators of a force-feedback telerobotic system. Friction compensation can improve the transparency of telerobotic systems, where transparency is quantified in terms of a match between the impedance of the environment and the impedance transmitted to the user. We used Dahl friction models to compensate for physical friction in the device. Experiments performed on a telerobotic system demonstrated that teleoperation transparency is improved by using these models. Further, the stability of the teleoperation is analyzed using passivity theory, and it is shown that the master-slave system remains stable up to a certain level of friction compensation

Haptics for Robot-Assisted Minimally Invasive Surgery

Robot-assisted minimally invasive surgery (RMIS) holds great promise for improving the accuracy and dexterity of a surgeon while minimizing trauma to the patient. However, widespread clinical success with RMIS has been marginal and it is hypothesized by engineers and surgeons alike that the lack of haptic feedback presented to the surgeon is a limiting factor. The objective of our research is to acquire, display, and determine the utility of haptic information during RMIS. This overview paper examines the design, analysis, practicality, and effectiveness of various force estimation and display methods. In particular, we describe our experience in adding force feedback to an experimental version of the da Vinci surgical system, a commercially available teleoperated RMIS system.

Enhancing Transparency of a Position-Exchange Teleoperator

Dynamic properties of robotic manipulators, including inertia, damping, and friction, limit the transparency of a haptic-feedback teleoperator. In this paper, we develop a position-exchange controller to provide haptic-feedback for a surgical teleoperator. The controller consists of proportional controllers and model-based feedforward terms that cancel the dynamic properties of the manipulators. We show that the teleoperator transmits the impedance of a soft environment to the operator when the gains of the proportional controllers are very high and dynamic terms of the manipulators are canceled. However, the high gains and complete cancellation of the dynamic terms of the manipulators can make the teleoperator unstable. We use IlewellynJs criteria for absolute stability to limit the controller parameters to values that keep the teleoperator stable during interactions with any passive user and environment. Experimental results using a custom version of the da Vinci Surgical System show that 70% of the inertia of the slave manipulators during low-frequency motion and almost all static friction of the manipulators during sliding motion can be canceled

Passivity-based high-fidelity haptic rendering of contact

A method is described whereby the virtual haptic interaction with deformable elastic objects is created in terms of two processes: a slow process which carries on the simulation, and a fast process to render forces. Passivity theory is used to design an update strategy which reproduces exactly pre-computed responses between a tool and an object. This yields a design procedure for adjustable local models which guarantee the passivity of the interaction while preserving fidelity. Two examples of local models are given and some experimental results are reported.

Novel Approach for Modeling Separation Forces Between Deformable Bodies

Many minimally invasive surgeries (MISs) involve removing whole organs or tumors that are connected to other organs. Development of haptic simulators that reproduce separation forces between organs can help surgeons learn MIS procedures. Powerful computational approaches such as finite-element methods generally cannot simulate separation in real time. This paper presents a novel approach for real-time computation of separation forces between deformable bodies. Separation occurs either due to fracture when a tool applies extensive forces to the bodies or due to evaporation when a laser beam burns the connection between the bodies. The separation forces are generated online from precalculated force-displacement functions that depend on the local adhesion/separation states between bodies. The precalculated functions are accurately synthesized from a large number of force responses obtained through either offline simulation, measurement, or analytical approximation during the preprocessing step. The approach does not require online computation of force versus global deformation to obtain separation forces. Only online interpolation of precalculated responses is required. The state of adhesion/separation during fracture and evaporation are updated by computationally simple models, which are derived based on the law of conservation of energy. An implementation of the approach for the haptic simulation of the removal of a diseased organ is presented, showing the fidelity of the simulation