Mehrdad R. Kermani

On the Feasibility and Suitability of MR Fluid Clutches in Human-Friendly Manipulators

An investigation into the suitability of magneto-rheological (MR) clutches in the context of developing feasible actuation solutions for physical human-robot interaction is presented. Contact and collision forces pose great danger to humans, and thus, the primary criteria for actuator development is safety. While the majority of existing solutions make use of mechanical compliance in some form, in this paper, we will approach the problem by considering the use of MR clutches for coupling the motor drive to the joint. The suitability of MR actuators to provide an intrinsically safe actuation platform is investigated by modeling the torque to mass, and torque to inertia ratios, as well as output impedance of the MR clutch. These figures are compared to commercially available servo motors as well as mechanically compliant based human-safe actuator models. The MR clutch is analytically shown to have superior mass and inertia characteristics over servo motors while either matching or surpassing the intrinsic safety characteristics of the modeled compliant actuator. The implementation of MR-clutch-based actuation systems is investigated by examining the distributed active semiactive approach. The proposed approach is discussed in terms of mechanical as well controller complexity and relates the investigation to the feasibility of practical implementations. Performance characteristics are empirically investigated by experimentation with a prototype MR clutch constructed for this purpose. The prototype MR clutch can transmit torque up to 75 Nm and has a bandwidth of 30 Hz. Torque to mass and torque to inertia ratios of the prototype MR clutch are substantially greater than that of comparable servo motors. Conclusions drawn from this investigation indicate that indeed MR clutch actuation approaches can be developed to balance safety and performance while maintaining reasonable system complexity.

Friction Identification and Compensation in Robotic Manipulators

In this paper, friction identification and friction compensation in the joints of a robotic manipulator are studied. The friction force is modeled using a single-state dynamic model that is utilized in the friction compensation algorithm. A new method for identifying the parameters of the aforementioned model in a robotic manipulator is presented. To evaluate the performance of this method, two different manipulators with different friction characteristics are examined. A 2-DOF manipulator that is used for high-speed micrometer precision manipulation and a 4-DOF macromanipulator that is used for long-reach positioning tasks are considered. It is shown that despite the different nature of the two manipulators, the same method can effectively improve the speed, accuracy, and smoothness of the manipulation in both cases.

Flexure control of a positioning system using piezoelectric transducers

This brief addresses the control problem of a structurally flexible positioning system by utilizing transducers made of Lead (Pb) Zirconate Titanate (PZT), commonly referred to as piezoelectric transducers, to act as actuator and sensor elements. The system possesses nonlinear dynamics and undesirable characteristics such as friction. The placement of PZT actuators on the flexible structure is studied and a nonlinear tracking control scheme augmented with a friction compensation term is developed. Experimental results are presented which illustrate the effectiveness of the control algorithm and improved performance of the controller as compared to the case where no secondary actuation is used.

Design and validation of a Magneto-Rheological clutch for practical control applications in human-friendly manipulation

In this paper we present the design and experimental validation of a Magneto-Rheological (MR) clutch developed for servo control applications in human friendly robotic systems. The clutch is developed as a practical prototype for potential industrial applications in the low to mid torque range. Conventional servo motors have been shown to lack the necessary properties to deliver intrinsically safe actuation for physical interaction with humans. Several human safe actuation paradigms have been proposed, however, tend to increase mechanical complexity as well as require more sophisticated control strategies. Furthermore, many of the proposed solutions have failed to demonstrate a practical implementation that provides the necessary performance capabilities required for practical applications in automation, as well as more general servo control applications. MR clutch devices have been shown to provide favorable characteristics for such tasks, however have focused primarily on haptic systems or low torque applications. The developed prototype MR clutch is capable of transmitting up to 75 Nm, and has a control bandwidth of approximately 30 Hz.

A Novel Force Modeling Scheme for Needle Insertion Using Multiple Kalman Filters

In this paper, the interaction force between a surgical needle and soft tissue is studied. The force is modeled using a novel nonlinear dynamic model. Encouraged by the LuGre model for representing friction forces, the proposed model captures all stages of needle-tissue interaction, including puncture, cutting, and friction forces. An estimation algorithm for identifying the parameters of the model is presented. This online approach, which is based on sequential extended Kalman filtering, enables us to characterize the total contact force using an efficient mathematical model. The algorithm compares the axial force measured at the needle base with its expected value and then adapts the model parameters to represent the actual interaction force. While the nature of this problem is very complex, the use of multiple Kalman filters makes the system highly adaptable for capturing the force evolution during an interventional procedure in standard operating conditions. To evaluate the performance of our model, experiments were performed on artificial phantoms.

An analytical model for deflection of flexible needles during needle insertion

This paper presents a new needle deflection model that is an extension of prior work in our group based on the principles of beam theory. The use of a long flexible needle in percutaneous interventions necessitates accurate modeling of the generated curved trajectory when the needle interacts with soft tissue. Finding a feasible model is important in simulators with applications in training novice clinicians or in path planners used for needle guidance. Using intra-operative force measurements at the needle base, our approach relates mechanical and geometric properties of needle-tissue interaction to the net amount of deflection and estimates the needle curvature. To this end, tissue resistance is modeled by introducing virtual springs along the needle shaft, and the impact of needle-tissue friction is considered by adding a moving distributed external force to the bending equations. Cutting force is also incorporated by finding its equivalent sub-boundary conditions. Subsequently, the closed-from solution of the partial differential equations governing the planar deflection is obtained using Greens functions. To evaluate the performance of our model, experiments were carried out on artificial phantoms.

Adaptive Modeling of a Magnetorheological Clutch

In this paper, a new open-loop model for a magnetorheological-based actuator is presented. The model consists of two parts relating the output torque of the actuator to its internal magnetic field, and the internal magnetic field to the applied current. Each part possesses its own hysteretic behavior. The first part uses a novel nonlinear adaptive model that relates the internal magnetic field to the applied current. The second part uses an open-loop Bingham model to relate the output torque to an internal magnetic field. The model facilitates accurate control of the actuator using its input current. It also eliminates the need for force/torque sensors for providing feedback signals. The accuracy of the constructed model is validated through simulations. The model is assessed against a widely accepted hysteresis modeling approach, known as the Preisach model and its advantages are highlighted. Experimental results using the prototyped actuation mechanism further verify the accuracy of the model and demonstrate its effectiveness.

Robot-Assisted Needle Steering Using a Control Theoretic Approach

This paper presents a new 2D motion planner for steering flexible needles inside relatively rigid tissue. This approach uses a nonholonomic system approach, which models tissue-needle interaction, and formulates the problem as a Markov Decision Process that is solvable using infinite horizon Dynamic Programming. Unlike conventional numerical solvers such as the value iterator which inherently suffers from the curse of dimensionality for processing large-scale models, partitioned-based solvers show promising numerical performance. Given the locations of the obstacles and the targeted area, the proposed solver provides a descent solution where high spatial or angular resolution is required. As theoretically expected, it is shown how prioritized partitioning increases computational performance compared to the generic value iteration which has been used in an existing steering approach. Starting from any initial condition in the workspace, this method enables the needle to reach its target and avoid collisions with obstacles through selecting the shortest path with the least number of turning points thereby causing less trauma. In this paper, emphasis is given to the control aspects of the problem rather than to biomedical issues. Experimental results using an artificial phantom show that the method is capable of positioning the needle tip at the targeted area with an acceptable level of accuracy.

Optimizing the performance of piezoelectric actuators for active vibration control

This paper discusses the selection process for piezoelectric transducers (PZT) used as actuator elements for suppressing vibrations in a flexible beam system. The effects of changing physical parameters such as the relative thickness of the piezoelectric ceramic with respect to the beam, the optimum location of the PZT actuator, and the length of the PZT are studied based on the singular value decomposition of the controllability Grammian of the resulting system. A model for a clamped-mass cantilevered beam is developed and its frequency response is compared with that obtained experimentally. Simulation results are given to illustrate how this method can be used to determine physical properties and location of the PZT actuator. Further experimental studies are currently being performed.

Dynamics of Translational Friction in Needle–Tissue Interaction During Needle Insertion

In this study, a distributed approach to account for dynamic friction during needle insertion in soft tissue is presented. As is well known, friction is a complex nonlinear phenomenon. It appears that classical or static models are unable to capture some of the observations made in systems subjected to significant frictional effects. In needle insertion, translational friction would be a matter of importance when the needle is very flexible, or a stop-and-rotate motion profile at low insertion velocities is implemented, and thus, the system is repeatedly transitioned from a pre-sliding to a sliding mode and vice versa. In order to characterize friction components, a distributed version of the LuGre model in the state-space representation is adopted. This method also facilitates estimating cutting force in an intra-operative manner. To evaluate the performance of the proposed family of friction models, experiments were conducted on homogeneous artificial phantoms and animal tissue. The results illustrate that our approach enables us to represent the main features of friction which is a major force component in needle–tissue interaction during needle-based interventions.