Neal Y. Lii

Experimental study on impedance control for the five-finger dexterous robot hand DLR-HIT II

This paper presents experimental results on the five-finger dexterous robot hand DLR-HIT II, with Cartesian impedance control based on joint torque and nonlinearity compensation for elastic dexterous robot joints. To improve the performence of the impedance controller, system parameter estimations with extended kalman filter and gravity compensation have been investigated on the robot hand. Experimental results show that, for the harmonic drive robot hand with joint toruqe feedback, accurate position tracking and stable torque/force response can be achieved with cartesian and joint impedance controller. In addition, a FPGA-based control architecture with flexible communication is proposed to perform the designed impedance controller.

Toward a task space framework for gesture commanded telemanipulation

This paper introduces a new framework for task space telemanipulation. The TASK space grasping and MANipulation (TaskMan) concept utilizes a library of tasks based on gesture commands, which replaces the conventional mapping required between the human hand and the end effector. Task communication between the human machine interface (HMI) and the robot end effector requires two symbiotic but nonidentical state machines on the master and slave side. The task states on two sides are synchronized via a single channel communication, as opposed to multi-channel joint space or Cartesian mapped information. HAND gesture command for grasping and MANipulation (HandyMan) HMI command algorithm is proposed for the recognition of hand gestures, which incorporates a library of intuitive task gestures to be used by the teleoperator wearing a CyberGlove. The task gestures are used to drive the states of the TaskMan state machines. With the proposed concepts, this work has realized teleoperated grasp and manipulation with a 15-DoF robot hand in task space. Full 6-DoF of object manipulation was achieved with different grasp combinations, and demonstrated higher repeatability, success rate and easier operation compared to conventional joint space teleoperation methods.

Toward understanding the effects of visual- and force-feedback on robotic hand grasping performance for space teleoperation

This paper introduces a study aimed to help quantify the benefits of limited-performance force-feedback user input devices for space telemanipulation with a dexterous robotic arm. A teleoperated robotic hand has been developed for the European Space Agency by the German Aerospace Center (DLR) for a lunar rover prototype. Studies carried out on this telerobotic system investigated several criteria critical to telemanipulation in space: 1) grasping task completion time, 2) grasping task difficulty, 3) grasp quality, and 4) difficulty level for the operator to assess the grasp quality. Several test subjects were allocated to remotely grasp regular and irregular shaped objects, under different combinations of visual- and force-feedback conditions. This work categorized the benefits of visual- and force-feedback in teleoperated grasping through several performance metrics. Furthermore, it has been shown that, with local joint-level impedance control, good grasping performance with rigid hard objects can be achieved, even with limited force-feedback information and low communication bandwidth. On the other hand, a performance ceiling was also found when grasping deformable objects, where the limited force-feedback setup cannot sufficiently reflect the object boundary to the teleoperator.

EXPERIMENTAL EVALUATION OF CARTESIAN AND JOINT IMPEDANCE CONTROL WITH ADAPTIVE FRICTION COMPENSATION FOR THE DEXTEROUS ROBOT HAND DLR-HIT II

This paper presents impedance controllers with adaptive friction compensation for the five-finger dexterous robot hand DLR-HIT II in both joint and Cartesian space. An FPGA-based control hardware and software architecture with real-time communication is designed to fulfill the requirements of the impedance controller. Modeling of the robot finger with flexible joints and mechanical couplings in the differential gear-box are described in this paper. In order to address the friction due to the complex transmission system and joint coupling, an adaptive model-based friction estimation method is carried out with an extended Kalman filter. The performance of the impedance controller with both adaptive and parameter-fixed friction compensations for the robot hand DLR-HIT II are analyzed and compared in this paper. Furthermore, gravity estimation is implemented with Least Squares technique to address uncertainties in gravity compensation due to the close proximity and complexity of robot hand components. Experimental results prove that accurate position tracking and stable torque/force response can be achieved with the proposed impedance controller with friction compensation on five-finger dexterous robot hand DLR-HIT II.

An adaptive compliant multi-finger approach-to-grasp strategy for objects with position uncertainties

This paper presents an adaptive and compliant approach-to-grasp strategy for multi-finger robotic hands, to improve the performance of autonomous grasping when encountering object position uncertainties. With the proposed approach-to-grasp strategy, the first robot finger to experience unexpected impact would pause its movement in a compliant manner, and remains in contact with the object to minimize the unplanned motion of the target object. At the same time, the remainder of the fingers continuously, adaptively move toward re-adjusted grasping positions with respect to the first finger in contact with the object, without the need for on-line re-planning or re-grasping. An adaptive grasp control strategy based on spatial virtual spring framework is proposed to achieve local (e.g. not resorting to the robotic arm) in-hand adjustments of the fingers not yet in contact. As such, these fingers can be adaptively driven to the adjusted desired position to accomplish the grasp. Experimental results demonstrate that significantly larger position errors with respect to the hand workspace can be accommodated with the proposed adaptive compliant grasp control strategy. As much as 391% increase in position error area coverage has been achieved. Finally, beyond the quantitative analysis, additional observations during the extensive experiment trials are discussed qualitatively, to help examine several open issues, and further understand the approach-to-grasp phases of the robot hand tasks.

Cartesian impedance control on five-finger dexterous robot hand DLR-HIT II with flexible joint

This paper presents an impedance controller for five-finger dexterous robot hand DLR-HIT II, which is derived in Cartesian space. By considering flexibility in finger joints and strong mechanical couplings in differential gear-box, modeling and control of the robot hand are described in this paper. The model-based friction estimation and velocity observer are carried out with an extended Kalman filter, which is implemented with parameters estimated by Least Squares Method. The designed estimator demonstrates good prediction performance, as shown in the experimental results. Stability analysis of the proposed impedance controller is carried out and described in this paper. Impedance control experiments are conducted with the five-finger dexterous robot hand DLR-HIT II in Cartesian coordinates system to help study the effectiveness of the proposed controller with friction compensation and hardware architecture.

Flexible FPGA-based controller architecture for five-fingered dexterous robot hand with effective impedance control

Several practical issues associated with achieving effective impedance performance in the finger joint space and stable grasping on a five-fingered dexterous hand are investigated in this work. A Multiprocessor structure based on field programming gate array (FPGA) is proposed to realize the high-level hand impedance controller. The key feature of the hardware system is a dual-processor architecture based controller, one of which is used for data communication control and the other for joint and object level control. High speed (200μs cycle time) multipoint low-voltage differential signaling (LVDS) serial data bus communication between each finger and the controller, Ethernet communication between monitor PC and controller, are all implemented on a single FPGA chip. Experimental results and simulation with a five-fingered dexterous robot hand demonstrate that the controller architecture is able to achieve the desired robot hand impedance control performance and effective stable grasping.

Driver-Input Sensor Selection and Topologies for Fault-Tolerant Drive-By-Wire Applications

Drive-by-wire applications, such as brake-by-wire, steer-bywire and throttle-by-wire, are becoming increasingly popular in the automotive industry. This work examines different sensors for driver-input systems for Drive-by-wire, as well as the integration of these sensors to achieve the fault-tolerance necessary for such safety-critical applications. A pedal test bed is constructed for studying the integration of similar and dissimilar sensors. Two sensor integration topologies involving multiple sensors, similar and dissimilar, are proposed to demonstrate different possibilities of fault-tolerant systems. The merits and limitations of both systems when coping with sensor and mechanical faults and failures are presented.

EXPERIMENTAL ANALYSIS ON SPATIAL AND CARTESIAN IMPEDANCE CONTROL FOR THE DEXTEROUS DLR/HIT II HAND

This paper presents an experimental study on impedance control in both Cartesian and object level with adaptive friction compensation for dexterous robot hand based on joint torque feedback. To adaptively decrease the effects of high friction caused by complex transmission systems and joint coupling, a friction observer is proposed based on the extended Kalman filter (EKF) in this paper. A Cartesian impedance controller is implemented on a multi-fingered dexterous robot hand with identical fingers, based on the modelling of each modular finger. In addition, a flexible n-fingered object frame is proposed in this paper, applicable to any finger configuration with three or more fingers (n ≥ 3). This enables the design of a 6-DoF spatial impedance controller. Stability of the closedloop system with friction observer is analysed. A position error of less than 0.16 ◦ is achieved using joint impedance control with adaptive friction compensation, which shows significant improvement in performance, as compared to 1.5 ◦ without compensation, and 0.5 ◦ with fixed-parameters friction compensation. Experimental results confirm the improvement in performance for the robot hand with Cartesian impedance control and adaptive joint friction compensation, demonstrating the effectiveness of spatial impedance controller with the proposed object frame and estimation strategy.

A Study on Sensor Fusion for Fault Tolerant Brake-by-Wire Driver Input Design with Dissimilar Sensors

Recently the automotive industry has witnessed a significant interest in brake-by-wire applications for purposes of brake systems [1][2], hybrid powertrain [3], etc. To ensure acceptable dependability of brake-bywire, redundancy of components such as sensors, actuators, ECUs and communication bus should be implemented. Algorithms for managing redundant components are critical to the proper function of such systems. This paper focuses on the sensor fusion aspects of dissimilar sensors to achieve fault tolerance. Measurement conversion accuracy issues for dissimilar sensors, such as the driver input unit’s physical characteristics, are discussed. Over 200 pedal actuation samples have been collected to examine several redundancy management algorithms in a drive input device under conditions of sensor and mechanical faults.