R.V. Patel

An inverse dynamics control strategy for tip position tracking of flexible multi‐link manipulators

In this article, we present an inverse dynamics control strategy to achieve small tracking errors for a class of multi-link structurally flexible manipulators. This is done by defining new outputs near the end points of the arms as well as by augmenting the control inputs by terms that ensure stable operation of the closed loop system under specific conditions. The controller is designed in a two-step process. First, a new output is defined such that the zero dynamics of the original system are stabilized. Next, to ensure stable asymptotic tracking, the control input is modified such that stable asymptotic tracking of the new output or approximate tracking of the actual output may be achieved. This is illustrated for the case of single- and two-link flexible manipulators. ©1997 John Wiley & Sons, Inc.

Inversion-based sliding control of a flexible-link manipulator

In this paper a control strategy based on nonlinear inversion is considered for a class of multi-link, structurally flexible manipulators to achieve small tip-position tracking errors while maintaining robust closed-loop performance. This is accomplished by defining new outputs near the end points of the arms. Motivated by the concept of a sliding surface in variable structure control (VSC), a robustifying term is developed to drive the nonlinear plants error dynamics onto a sliding surface. On this surface the error dynamics are guaranteed to be independent of parametric uncertainties. In order to avoid over-excitation of higher frequency flexural modes due to control chattering, the discontinuous functions normally used in classical VSC are replaced by saturation nonlinearities at the outset. This also facilitates analysis by the standard Lyapunov techniques. The controller performance is demonstrated by simulation on a two-link manipulator with the second link flexible and with considerable parametric...

Impact reduction for redundant manipulators using augmented impedance control

In this article, the problem of controlling redundant manipulators to reduce collision impact effects is considered, and an augmented kinematics and impedance control scheme is proposed for its solution. The proposed scheme achieves satisfactory performance by minimizing the magnitudes of impulsive forces as well as reducing rebound effects of the end-effector. In the proposed control scheme, kinematic redundancy is resolved using an augmented kinematics approach where the augmentation of the Jacobian matrix is based on an impact model derived using the Cartesian-space dynamic model of the manipulator. The proposed impact controller uses a simplified impedance control scheme aimed at reducing impulsive forces as well as rebound effects. The performance of the proposed controller is illustrated by computer simulations.

An inverse dynamics sliding control technique for flexible multi-link manipulators

In this paper, a control strategy based on nonlinear inversion is considered that results in small tip-position tracking errors while maintaining robust closed-loop performance for a class of multi-link structurally flexible manipulators. This is achieved by defining new outputs near the end points of the arms as well as by augmenting the nominal control inputs by terms that ensure stable operation of the closed loop system in the presence of considerable parametric uncertainties. Motivated by the concept of a sliding surface in variable structure control (VSC) a robust control term is developed to drive the nonlinear plants error dynamics onto a sliding surface. The discontinuous functions normally used in classical VSC are replaced by saturation nonlinearities at the outset. This also facilitates analysis by the standard Lyapunov techniques. The controller performance is demonstrated by simulation on a two-link flexible manipulator with a considerable amount of parametric uncertainty.

Robotic suturing forces in the presence of haptic feedback and sensory substitution

There has been some interest in recent years on how information about interactions happening between surgical instruments and tissue during robot-assisted surgery could improve the efficiency and reliability of a surgical task. In this paper, it is hypothesized that various modes of sensory feedback have the potential to enhance performance in robot-assisted surgery in terms of the amount of applied forces. User performance during telemanipulated suturing is compared for cases where force feedback is replaced or complemented by visual representation of the force levels. In addition to confirming the above hypothesis, the results indicate a tradeoff between the magnitudes of applied forces and the time required to complete the task

Efficient computation of manipulator inertia matrices and the direct dynamics problem

A modeling scheme that uses the concepts of generalized and augmented links is proposed for computing joint-space inertia matrices of open-chain rigid body systems. Expressions for evaluating these matrices are derived, utilizing orthogonal Cartesian tensors and the Newton-Euler equations of motion. The resulting recursive algorithm is applicable to all rigid-link manipulators having open-chain kinematic structures with revolute and/or prismatic joints. An efficient implementation of the algorithm shows that the joint-space inertia-matrix of a six degree-of-freedom manipulator with revolute joints can be computed in approximately 420 multiplications and 339 additions. For manipulators with 0 degrees to 90 degrees twist angles, the number of computations is reduced to 271 multiplications and 250 additions. >

Multi-sensory force/deformation cues for stiffness characterization in soft-tissue palpation.

In the commercially available robot-assisted surgical systems, camera vision constitutes the only flow of data from the patient side to the surgeon side. This paper studies how various modalities for feedback of interaction between a surgical tool and soft tissue can improve the efficiency of a typical surgical task. Utilizing a haptics-enabled master-slave test-bed for minimally invasive surgery, user performance during a telemanipulated soft tissue stiffness discrimination task is compared under visual, haptic, graphical, and graphical plus haptic feedback modes in terms of task success rate and completion time and the amount of energy transfer and consequently trauma to tissue. While no significant difference is found in terms of the task completion times, graphical cueing and visual cueing are found to lead to the highest success rate and the highest risk of tissue damage (proportional to energy), respectively

Tool/tissue interaction feedback modalities in robot-assisted lump localization

Providing a surgeon with information regarding contacts made between tools and tissue during robot-assisted interventions can improve task efficiency and reliability. It is hypothesized that various modalities of contact feedback have the potential to enhance performance in a robot-assisted minimally invasive environment. In this paper, (kinesthetic) haptic feedback is compared with visual feedback of haptic information in terms of several performance metrics. Using a haptics-capable master-slave test-bed for endoscopic surgery, experiments involving a lump localization task are conducted and the performance of human subjects is compared for these two modalities of contact feedback. It is shown that the two feedback modalities result in comparable localization accuracies - an advantage of visual haptic feedback due to the lower system complexity required - while the task completion times are significantly shorter with haptic feedback

An observer-based inverse dynamics control strategy for flexible multi-link manipulators

This paper presents an observer-based inverse dynamics control strategy that results in small tip-position tracking errors for a class of multi-link structurally flexible manipulators. It was shown in Moallem et al. (1996) that small tracking errors are achievable by defining new outputs near the end points of an arm and augmenting the control inputs by terms that ensure stable operation of the closed-loop system. However, the control strategy required measurements of rates of change of flexible modes with time that are not conveniently measurable. A nonlinear observer is introduced here to estimate these variables by assuming that joint positions, rates and deflection modes are measured. By a proper choice of observer gains the error dynamics are guaranteed to consist of a stable linear part plus a bounded perturbation term that results in local asymptotically stable observation of deflection rates.

Optimization of an actuated flexible arm for improved control properties

A multi-objective optimization index is introduced for improving the dynamic behavior of an actuated structurally flexible arm through geometric shape design. The improvement in the dynamic behavior is achieved by defining a vector optimization cost function. This cost function is associated with the frequency of the lowest system zero when joint angles are taken as outputs, the gain sensitivity of the open-loop system modes, and the gain sensitivity of the closed-loop system modes at its transmission zeros. The cost function is utilized to optimize the structural shape for a single-link flexible arm. The optimized link is shown to yield superior robustness and performance characteristics in a closed-loop system when compared to the non-optimized uniform link.