Zhaopeng Chen

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.

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.

Analysis of the multi-finger dynamics for robot hand system based on EtherCAT

A multi-finger dynamics model has been presented in this study, which contains a single finger dynamics model equation and a restraint equation between fingers based on Lagrangian multiplier controller. To validate the model, an EtherCAT master and slave platform has been developed based on FPGA. Meanwhile, the multi-finger dynamics algorithm has been designed in the TwinCAT. Finally, the experiments demonstrate this strategy can be implemented and operated by online grasping object.

Research of a novel miniature tactile sensor for five-finger dexterous robot hand

This paper presents a novel miniature tactile sensor of DLR/HIT II dexterous robot hand and outlines the calibration experiments performed. The sensor is based on a piezoresistive mechanism with a highly integrated and miniaturized design. The operating principle of the proposed sensor is explained. The elastic architecture of the proposed sensor is also described. Multiple electrodes are mounted on the the sensor, with rows-column scanning for the acquisition sensor data. The sensor has a large resistance range from 16kΩ to 920kΩ. The experimental results have demonstrated a measurement range of up to approximately 600kPa. The calibration results indicate clear linearity within the measured range.

Towards a Functional Evaluation of Manipulation Performance in Dexterous Robotic Hand Design

Dexterous multifingered hands are the most complex and versatile variants of robotic end effectors. Compared to simpler grippers and underactuated hands, they should be more capable of grasping and, especially, manipulating different objects. This paper explores the relationship between kinematic design and manipulation performance of robotic hands. Some evaluation criteria frequently used by hand designers to verify kinematic configurations are revisited. The results from these criteria are scrutinized and compared with the evaluation of the manipulation workspace and the ranges of motion of inhand manipulation for a set of predefined objects. Simulations and actual manipulation experiments are carried out with different kinematic configurations on a modular dexterous hand. The results show some disconnection between perceived good designs through common evaluation criteria and their actual, realizable manipulation performance. This work finally gives some insight toward a more holistic approach to design hands that better address grasp and manipulation for the intended tasks and applications.

A Compliant Multi-finger Grasp Approach Control Strategy Based on the Virtual Spring Framework

This paper presents an adaptive compliant multi-finger grasp approach control strategy based on based on a new interpretation of the virtual spatial spring framework, to improve the grasp performance for target objects with position errors. An n-finger virtual spatial spring frame is proposed to achieve the adaptive compliant grasp control. Two-finger grasp control based on a single virtual spring is tackled, and then extended to multi-finger grasp control. Virtual springs for self-collision avoidance among digits are constructed to form the complete adaptive compliant grasp control law. With the virtual-spring based adaptive compliant grasp approach control strategy, the first robot finger to experience unexpected impact remains in contact with the object, while the rest of the fingers are continuously, adaptively driven toward re-adjusted grasping positions by the virtual springs without the need for on-line replanning. Experimental results demonstrate effectiveness of the virtual-spring based grasp controller, and significantly larger position errors of the target object can be accommodated with the proposed adaptive compliant grasp control strategy.