Vibration control of a single-link flexible manipulator using an array of fiber optic curvature sensors and PZT actuators

This paper presents a novel distributed sensing and actuation approach for actively suppressing vibrations within flexible link manipulators. Through vibration suppression, the method acts to regulate the shape of flexible links and, consequently, improves the performance of any independent trajectory controller being employed over the manipulator joints. To demonstrate the approach, a series of piezoceramic actuators (PZTs) are bonded to the surface of a single-link flexible manipulator. Slewing of the flexible link induces vibrations in the link that persist long after the hub stops rotating. The vibration suppression is achieved through a combined scheme of PD-based hub motion control and a PZT actuator controller that is a composite of linear and angular velocity feedback controllers. A Lyapunov approach is used to synthesize the composite controller, and a unique, commercially-available sensor, called ShapeTape^?, that provides the linear and angular velocity feedback. The sensor array is comprised of a series of fiber optic curvature sensors that are laminated on a long, thin ribbon tape which can be embedded into the flexible link and measures the bend and twist of the link’s centerline. Simulation and experimental results show the effectiveness of the proposed approach and the ability of the new sensor to provide the requisite feedback.     

Enhancing kinematic accuracy of redundant wheeled mobile manipulators via adaptive motion planning

An increasing demand for wheeled mobile manipulators (WMMs) in fields that require high precision tasks has introduced new requirements as far as their kinematic accuracy. A WMM consists of a manipulator mounted on a mobile base. This configuration usually makes the WMM a redundant robotic system. One limiting factor is that the kinematic accuracy of the mobile base is often lower than that of the manipulator because of modeling errors and disturbances such as slippage. Thus, it makes sense to distribute more of a given set of motion requirements for the entire mobile manipulator to the manipulator when possible — this is usually done using a weighting matrix that sums the mobile base motion and the manipulator motion. In this paper, considering a redundant WMM, a novel adaptive motion planning method is proposed as a secondary objective in the null space of the primary objective (improvement of the WMM’s kinematic accuracy) for the WMM. Also, a tertiary objective to move the manipulator away from its singularity is proposed. This objective is activated if the secondary objective is feasible and the WMM still has remaining redundant degrees of freedom. The proposed method is implemented at the acceleration level to circumvent the discontinuity of the commanded joint velocity due to the adaptive motion planning algorithm. The advantages and effectiveness of the proposed approach are demonstrated with a traditional motion planning approach through experiments.     

Position control of a Stewart platform using inverse dynamics control with approximate dynamics

Configuration-dependent nonlinear coefficient matrices in the dynamic equation of a robot manipulator impose computational burden in real-time implementation of tracking control based on the inverse dynamics controller (IDC). However, parallel manipulators such as a Stewart platform have relatively small workspace compared to serial manipulators. Based on the characteristics of small motion range, nonlinear coefficient matrices can be approximated to constant ones. The modeling errors caused by such approximation are compensated for by H_∞ controller that treats the error as disturbance. The proposed IDC with approximate dynamics combined with H_∞ control shows good tracking performance even for fast tracking control in which computation of full dynamics is not easy to implement.     

A decentralized kinematic control architecture for collaborative and cooperative multi-arm systems

In this paper a two-layer decentralized framework for kinematic control of cooperative and collaborative multi-robot systems is developed. The motion of the system is specified at the workpiece level, by adopting a task-oriented formulation for cooperative tasks. The first layer computes the motion of the single arms in the system. In detail, the control unit of each robot computes the end-effector motion references in a decentralized fashion on the basis of the knowledge of the assigned cooperative task and the motion references computed by its neighbors. Then, in the second layer, each control unit computes the reference joint motion of the corresponding manipulator from the end-effector reference motion. The approach is, then, tested in simulation on a work-cell composed by several manipulators, and experimentally on a dual-arm kinematically redundant work-cell composed by industrial manipulators.     

Robust learning control of a high precision planar parallel manipulator

End-point positioning accuracy and fast settling time are essential in the motion system aimed at semiconductor packaging applications. In this paper, a novel robust learning control method for a direct-drive planar parallel manipulator is presented. A frequency-domain system identification approach is used to identify the high frequency dynamic of the manipulator. A robust control design method is employed to design a stable, fast tracking response feedback controller with less sensitivity to high frequency disturbance and the control parameters are determined using genetic algorithm. A Fourier-series-based iterative learning controller is designed and used on the feedforward path of the controller to further improve the settling time by reducing the dynamic tracking error of the manipulator. Experimental results demonstrate that the planar parallel manipulator has significant improvements on motion performance in terms of positioning accuracy, settling time and stability when compared with traditional XY-stages. This shows that the proposed manipulator provides a superior alternative to XY-motion stages for high precision positioning.     

Steerability for planar dissipative passive haptic interfaces

This research addresses the ability of dissipative passive actuators to generate control effects on a passive haptic interface. The ability is first identified as the steerability, i.e., the ability to redirect motions of a manipulator. The force-generation analysis of each individual actuator is then selected as an approach to evaluate the steerability. Passive steerability angle is defined to quantify the steerability for a planar manipulator. Two-sided steerability is defined as a minimum requirement for a planar robot to redirect all motions. A nonredundant 2R manipulator does not have two-sided steerability in all situations. Redundancy brought by serial or parallel structures could improve the steerability of a manipulator. The steerability theorem is developed to determine two-sided steerability for a multijoint manipulator. The research focuses on using brakes, a common type of dissipative passive actuators.     

A Novel Manipulator for Percutaneous Needle Insertion: Design and Experimentation

In this paper, we present the design of a novel 5-DOF manipulator for percutaneous needle insertion. The requirements of the manipulator have been instigated by a relatively common medical procedure: low-dose rate brachytherapy of the prostate. The manipulator can perform orientation, insertion, and rotation of the needle and linear motion of the stylet to drop radioactive seeds contained in a thin hollow needle (cannula) at targeted locations. The key features of the manipulator such as backdrivable joints, a fault-tolerant needle driver, stationary actuators, and redundant sensors enhance overall safety and reliability of the mechanism, a critical requirement for surgical manipulators. The manipulator is an integral part of a system utilizing a mechanically rotated side-firing transducer to create 3-D ultrasound images of the organ and utilizing 3-D SLICER software to visualize those images. Experimental results in agar phantoms prove that the manipulator is capable of positioning the needle tip at the targeted locations with good accuracy.      

Quantitative analysis and evaluation of bilateral control schemes applied to electro-hydrostatic actuators

A bilateral teleoperator allows operators to use a master manipulator to interact with the environment via a slave manipulator. It can be used in remote, hazardous or inaccessible situations. Although numerous bilateral teleoperation studies have been conducted on electrical actuators, research on hydraulic actuators, especially research on electro-hydrostatic actuators (EHAs), a class of pump-controlled systems, is currently limited. In bilateral teleoperation, one issue concerns how to tune the controller parameters with regard to stability and transparency in the presence of uncertainty. In this paper, an approach based on quantitative feedback theory (QFT) that provides guidance on controller parameter selection during the bilateral teleoperation of EHAs is introduced. Parametric uncertainties in dynamics of human operators, master manipulators and environments are described in templates to compute bounds quantitatively, and trade-offs between stability and transparency are visualized. Four commonly used bilateral control schemes are exemplified using this method: force reflection (FR), position error (PE), shared compliant control (SCC) and force reflection with passivity (FRP). The bilateral controllers are tuned through simulations and are validated through contact experiments involving both soft and hard environments. Given its simple structure and excellent level of transparency, FR is found to be the most suitable scheme of the four for teleoperations of EHAs.     

Modulation of dynamic behavior of anthropomorphic robot: A biomimetic approach with force redundancy

The biomimetic approach to robotics is an attempt to apply solutions created by the evolution of biological systems to technical systems. We investigate the feedforward modulation methodology of the dynamic behavior of an anthropomorphic manipulator, which inherently possess abundant actuators. In this work, we explain that for complete modulation of the dynamic behavior of such a system, redundant actuation is essential. Redundant actuation enables the system to modulate the stiffness or weighted stiffness properties such as motion frequency, and even modulates the damping ratio of the system. Thus, the modulation capability of the stiffness property plays an important role. We also address the need for the incorporation of active damper for complete modulation of the dynamic behavior. An illustrative example of a biomimetic, 2-degrees of freedom manipulator resembling the musculoskeletal structure of the human upper-extremity and driven by six actuators, is given. The modulation capability of motion frequencies and damping ratios is verified through a virtual trajectory planning that represents a point-to-point motion accomplished by the progressive movement of the equilibrium posture. To show the effectiveness of the proposed algorithms, several simulation results are illustrated.     

Neural network-based control of balanced robotic manipulators with joint flexibility

This paper addresses an improvement on the controlled performance of balanced manipulators in a practical level by implementing neural network based controller. The mass balancing of robotic manipulators has been shown to have favorable effects on the dynamic characteristics. However, it was also pointed out that for the manipulators having a certain degree of flexibility at the joints, due to the lowered structural natural frequencies and reduced velocity related terms, mass balancing results in vibratory motion at high speed operation. Such a vibratory tendency of the balanced flexible joint manipulator limits the admissible range of servo gains of the conventional controllers, making those controllers unsuitable for controlling the manipulator at high speeds.

To avoid such difficulty, an artificial neural network (NN) controller is introduced to realize the dynamic control of the balanced flexible joint manipulators. A feedforward type of NN controller is proposed and its performance is evaluated through a series of numerical simulations. The proposed NN controller showed much better tracking performances over the conventional PD controller.     

End-point vibration sensing of planar flexible manipulators through visual servoing

A visual servoing approach to the control of planar flexible robotic manipulators is adopted in this paper, based on the composite control theory, where the camera sensor is used together with the strain gauge measurements, to estimate the tip deformation. A fast Kalman filter, built on an integral manifold approximation of the manipulator model, can be used to fuse in the most effective way the measurements coming from different sensors, each one perturbed by its own noise. As a consequence, the signal to noise ratio of the deformation measurements can be effectively improved. A difficulty however arises in deriving a linear relation between the camera output and the state variables: the specific contribution of this paper is the derivation of such a linear relation in the fast time scale. Simulation results based on a two link planar flexible manipulator show the potential of the proposed approach to gain a more effective suppression of the tip vibrations, while an experimental example demonstrates its practical feasibility.     

Motion planning of a golf swing robot

We report our studies of a new motion planning method for a two-link golf swing robot. We introduce two fundamental properties of manipulator dynamics: the time-reversal symmetry property and the pendulum property. The former is used to reverse the rest-to-point pre-impact swing (backswing and downswing) into a symmetric point-to-rest swing in time, and the latter is used to design a proportional-derivative plus gravity, joint and coupling torque compensation control algorithm to plan the reversed pre-impact swing and the point-to-rest follow-through swing. By using this method, accurate impact position and hitting speed control of the golf swing and low computational cost are achieved. The introduced properties hold true for general multi-revolute-joint planar manipulators and the proposed motion planning algorithm can be implemented on them too.     

A fractional approach for the motion planning of redundant and hyper-redundant manipulators

The trajectory planning of redundant robots through the pseudoinverse control leads to undesirable drift in the joint space. This paper presents a new technique to solve the inverse kinematics problem of redundant manipulators, which uses a fractional differential of order α to control the joint positions. Two performance measures are defined to examine the strength and weakness of the proposed method. The positional error index measures the precision of the manipulator's end-effector at the target position. The repeatability performance index is adopted to evaluate if the joint positions are repetitive when the manipulator execute repetitive trajectories in the operational workspace. Redundant and hyper-redundant planar manipulators reveal that it is possible to choose in a large range of possible values of α in order to get repetitive trajectories in the joint space.     

Continuous Path Controller for the Remote Ultrasound Diagnostic System

A master-slave type remote ultrasound diagnostic system has been developed. The impedance controller has been implemented and reported for the positions of the master and slave manipulators to display and control the contact force between the ultrasound probe and the affected area. The present paper introduces an alternative orientation controller utilizing continuous path (CP) control in the remote ultrasound diagnostic system. The major contribution of the present paper is an introduction to the velocity-control-based CP controller for the master-slave type remote medical system, which realizes the continuous motion of the slave manipulator without a reduction in the master motion tracking performance. It is difficult to communicate information for control at high sampling rates between the master and slave site because the communication network between the master and slave site (local area network, integrated services digital network, asymmetric digital subscriber line, etc.) is limited. To cope with this problem, the CP controller was introduced to the orientation controller in special remote medical systems. The CP control realizes the continuous motion of the slave manipulator under the limited sampling rate of the orientation data transmitted by the master manipulator and high master motion tracking performance of the slave manipulator. This allows the slave manipulator safety to be improved and decreases the volume of the transmitted data. The experimental results demonstrate the accuracy of the path control of the slave manipulator using the CP control in the master--slave manipulation system, compared with the conventional point-to-point control.     

Motion control of a tendon-based parallel manipulator using optimal tension distribution

This paper presents the motion control of a six degree-of-freedom tendon-based parallel manipulator, which moves a platform with high speed using seven cables. To control the motion of the platform along desired trajectories in space, nonlinear feedforward control laws in the cable length coordinates are used. Taking account of the effect of redundancy on actuation, the optimal tension distribution should be considered to the advantage of the control laws. Using a method based on the analysis of the workspace condition, tension constraints and limiting torque constraints of actuators, an analytical solution for optimum tension distribution was found and used to compute the force in each cable for compensation of dynamic errors. It is experimentally demonstrated that the proposed control laws reduce the energy consumption of the actuators and satisfy the path tracking accuracy.     

A self-adjusting and modular supervisory control algorithm for planar dexterous manipulation

Many applications require precise handling and manipulation of delicate objects. In some cases, the object must be transported to a new location following a strict travel path including time-related constraints. This paper presents a self-adjusting modular control algorithm for dexterous manipulation of planar objects using multiple manipulators with precise path and timing deliveries. The popular caging approach is simple, and usually effective when manipulating objects with multiple devices but can fail following complex paths with orientation adjustments under time-critical tracking requirements. The proposed approach exploits the dynamics of the object in real-time using tracking control and allocates the force that needs to be applied by each manipulator based on their current position around the object to maximize their capability to push in the direction of the contact angle. The new algorithm is self-adjusting and modular; It can adjust its force allocation according to configuration changes during operation, and manipulators execute the same algorithm regardless of their number. The advantages of the new approach are successfully demonstrated both with simulations and testbed experiments, including orientation tracking, which is not typically featured with the caging approach. Conditions to check when the new algorithm is most effective are also analyzed. The closed-loop stability and performance of the new algorithm are also studied and necessary conditions are identified.     

Force feedback control of robot manipulator by the accelerationtracing orientation method

The authors propose a novel approach to force and compliance control of multi-degree-of-freedom (DOF) robot manipulators. The acceleration tracing orientation method (ATOM) is applied to both controllers. The control law is described in the Cartesian space; however, the final command is the acceleration in the joint space. The interactive terms in each joint disturb and deteriorate the joint motion. The disturbance observer cancels out the total sum of these terms and enables each joint to trace the acceleration command. As a result, a robust control is possible in the force task. The testing of the proposed system in a three-DOF robot manipulator is discussed     

Motion generation for hyper dynamic manipulation

Basic concept about how to realize hyper dynamic manipulation by a robot has been given by the authors in previous works. Based on the analysis results of human hyper dynamic manipulation, two key issues concerning the realization of hyper dynamic manipulation by an anthropomorphic mechanism, namely, utilization of dynamically coupled driving and structural joint stops, have been emphasized. Therefore, unlike that for conventional manipulators, our approach is to design and control a hyper dynamic manipulator based on the utilization of dynamically coupled driving and structural joint stops. Motion generation based on this approach is critical to the utilization of dynamically coupled driving and joint stops, and presents great difficulties. In this paper, a new method for motion generation is proposed, and its superior numerical stability achieved with high accuracy is shown by simulation results. Many meaningful and interesting results for the hyper dynamic manipulation for a two-joint manipulator are obtained in simulation. Moreover, the hyper dynamic manipulation is realized successfully by a prototype of a golf swing robot, designed based on the concept presented in this paper.     

Active control to flexible manipulators

Active control law for flexible structures using nonlinear conventional motor with gear actuators has been a problem without an accepted solution with experimental evidence until now. This is due to the existence of a dead zone in torque caused by the nonlinear frictions inside the actuator. The torques needed to attenuate the vibrations, although calculated by the control law, are consumed by the frictions and do not arrive until the flexible structure. This work proposes a control strategy with friction compensation using neural networks to solve this problem. Experimental results obtained with a flexible manipulator attested to the excellent performance of the proposed control law.     

Global minimum-jerk trajectory planning of robot manipulators

A new approach based on interval analysis is developed to find the global minimum-jerk (MJ) trajectory of a robot manipulator within a joint space scheme using cubic splines. MJ trajectories are desirable for their similarity to human joint movements and for their amenability to path tracking and to limit robot vibrations. This makes them attractive choices for robotic applications, in spite of the fact that the manipulator dynamics are not taken into account. Cubic splines are used in a framework that assures overall continuity of velocities and accelerations in the robot movement. The resulting MJ trajectory planning is shown to be a global constrained minimax optimization problem. This is solved by a newly devised algorithm based on interval analysis and proof of convergence with certainty to an arbitrarily good global solution is provided. The proposed planning method is applied to an example regarding a six-joint manipulator and comparisons with an alternative MJ planner are exposed