NASA Center for Intelligent Robotic Systems for Space Exploration

NASA's program for the civilian exploration of space is a challenge to scientists and engineers to help maintain and further develop the United States' position of leadership in a focused sphere of space activity. Such an ambitious plan requires the contribution and further development of many scientific and technological fields. One research area essential for the success of these space exploration programs is Intelligent Robotic Systems. These systems represent a class of autonomous and semi-autonomous machines that can perform human-like functions with or without human interaction. They are fundamental for activities too hazardous for humans or too distant or complex for remote telemanipulation. To meet this challenge, Rensselaer Polytechnic Institute (RPI) has established an Engineering Research Center for Intelligent Robotic Systems for Space Exploration (CIRSSE). The Center was created with a five year $5.5 million grant from NASA submitted by a team of the Robotics and Automation Laboratories. The Robotics and Automation Laboratories of RPI are the result of the merger of the Robotics and Automation Laboratory of the Department of Electrical, Computer, and Systems Engineering (ECSE) and the Research Laboratory for Kinematics and Robotic Mechanisms of the Department of Mechanical Engineering, Aeronautical Engineering, and Mechanics (ME,AE,&M), in 1987. This report is an examination of the activities that are centered at CIRSSE

On Sensorless Collision Detection and Measurement of External Forces in Presence of Modeling Inaccuracies

The field of human-robot interaction has garnered significant interest in the last decade. Every form of human-robot coexistence must guarantee the safety of the user. Safety in human-robot interaction is being vigorously studied, in areas such as collision avoidance, soft actuators, light-weight robots, computer vision techniques, soft tissue modeling, collision detection, etc. Despite the safety provisions, unwanted collisions can occur in case of system faults. In such cases, before post-collision strategies are triggered, it is imperative to effectively detect the collisions. Implementation of tactile sensors, vision systems, sonar and Lidar sensors, etc., allows for detection of collisions. However, due to the cost of such methods, more practical approaches are being investigated. A general goal remains to develop methods for fast detection of external contacts using minimal sensory information. Availability of position data and command torques in manipulators permits development of observer-based techniques to measure external forces/torques. The presence of disturbances and inaccuracies in the model of the robot presents challenges in the efficacy of observers in the context of collision detection. The purpose of this thesis is to develop methods that reduce the effects of modeling inaccuracies in external force/torque estimation and increase the efficacy of collision detection. It is comprised of the following four parts: 1. The KUKA Light-Weight Robot IV+ is commonly employed for research purposes. The regressor matrix, minimal inertial parameters and the friction model of this robot are identified and presented in detail. To develop the model, relative weight analysis is employed for identification. 2. Modeling inaccuracies and robot state approximation errors are considered simultaneously to develop model-based time-varying thresholds for collision detection. A metric is formulated to compare trajectories realizing the same task in terms of their collision detection and external force/torque estimation capabilities. A method for determining optimal trajectories with regards to accurate external force/torque estimation is also developed. 3. The effects of velocity on external force/torque estimation errors are studied with and without the use of joint force/torque sensors. Velocity-based thresholds are developed and implemented to improve collision detection. The results are compared with the collision detection module integrated in the KUKA Light-Weight Robot IV+. 4. An alternative joint-by-joint heuristic method is proposed to identify the effects of modeling inaccuracies on external force/torque estimation. Time-varying collision detection thresholds associated with the heuristic method are developed and compared with constant thresholds. In this work, the KUKA Light-Weight Robot IV+ is used for obtaining the experimental results. This robot is controlled via the Fast Research Interface and Visual C++ 2008. The experimental results confirm the efficacy of the proposed methodologies

Robust high-performance control for robotic manipulators

Model-based and performance-based control techniques are combined for an electrical robotic control system. Thus, two distinct and separate design philosophies have been merged into a single control system having a control law formulation including two distinct and separate components, each of which yields a respective signal component that is combined into a total command signal for the system. Those two separate system components include a feedforward controller and a feedback controller. The feedforward controller is model-based and contains any known part of the manipulator dynamics that can be used for on-line control to produce a nominal feedforward component of the system's control signal. The feedback controller is performance-based and consists of a simple adaptive PID controller which generates an adaptive control signal to complement the nominal feedforward signal

Robust tuning of robot control systems

The computed torque control problem is examined for a robot arm with flexible, geared, joint drive systems which are typical in many industrial robots. The standard computed torque algorithm is not directly applicable to this class of manipulators because of the dynamics introduced by the joint drive system. The proposed approach to computed torque control combines a computed torque algorithm with torque controller at each joint. Three such control schemes are proposed. The first scheme uses the joint torque control system currently implemented on the robot arm and a novel form of the computed torque algorithm. The other two use the standard computed torque algorithm and a novel model following torque control system based on model following techniques. Standard tasks and performance indices are used to evaluate the performance of the controllers. Both numerical simulations and experiments are used in evaluation. The study shows that all three proposed systems lead to improved tracking performance over a conventional PD controller

A Base Force/Torque Sensor Approach to Robot

Experimental results presented show that an accurate estimation of inertia parameters is attainable. Since the sensor is external to the manipulator, the same sensor can be used for parameter estimation for a number of different systems

Experimental evaluation of robot controllers

In the last decade an abundance of control laws for nonlinear robotic systems was proposed. A careful evaluation of several classes of these controllers has been made, to overcome some drawbacks of previous evaluations. Experiments were done on a simple robotic system, with prismatic joints only and with low cost controller hardware. A flexible and effective real-time software environment, using object oriented programming techniques, was developed, easing the implementation and the evaluation of many control laws. The experience gained leads to the recommendation to develop as good a model as possible combined with adaptive control for tuning of the model parameter

Method and apparatus for configuration control of redundant robots

A method and apparatus to control a robot or manipulator configuration over the entire motion based on augmentation of the manipulator forward kinematics is disclosed. A set of kinematic functions is defined in Cartesian or joint space to reflect the desirable configuration that will be achieved in addition to the specified end-effector motion. The user-defined kinematic functions and the end-effector Cartesian coordinates are combined to form a set of task-related configuration variables as generalized coordinates for the manipulator. A task-based adaptive scheme is then utilized to directly control the configuration variables so as to achieve tracking of some desired reference trajectories throughout the robot motion. This accomplishes the basic task of desired end-effector motion, while utilizing the redundancy to achieve any additional task through the desired time variation of the kinematic functions. The present invention can also be used for optimization of any kinematic objective function, or for satisfaction of a set of kinematic inequality constraints, as in an obstacle avoidance problem. In contrast to pseudoinverse-based methods, the configuration control scheme ensures cyclic motion of the manipulator, which is an essential requirement for repetitive operations. The control law is simple and computationally very fast, and does not require either the complex manipulator dynamic model or the complicated inverse kinematic transformation. The configuration control scheme can alternatively be implemented in joint space

Multivariable Loop-Shaping in Bilateral Telemanipulation

Abstract This paper presents an architecture and control methodology for obtaining transparency and stability robustness in a multivariable bilateral teleoperator system. The work presented here extends a previously published single-input, single-output approach to accommodate multivariable systems. The extension entails the use of impedance control techniques, which are introduced to render linear the otherwise nonlinear dynamics of the master and slave manipulators, in addition to a diagonalization multivariable loop shaping technique, used to render tractable the multivariable compensator design. A multivariable measure of transparency is proposed based on the relative singular values of the environment and transmitted impedance matrices. The approach is experimentally demonstrated on a three degree-of-freedom scaled telemanipulator pair with a highly coupled environment. Using direct measurement of the power delivered to the operator to assess the system's stability robustness, along with the proposed measure of multivariable transparency, the loop-shaping compensation is shown to improve the stability robustness by a factor of two and the transparency by more than a factor of five. Fite and Goldfarb Multivariable Loop Shaping … 2 Introduction Bilateral teleoperation systems provide for human interaction with an environment while alleviating the necessity of direct contact between the two. Using a pair of robot manipulators, such a system enables dexterous human manipulation in remote, hazardous, or otherwise inaccessible environments. Bilateral telemanipulators can additionally incorporate power attenuation or amplification between the human operator and environment, allowing for human manipulation of microscopic objects (in the case of macro-micro bilateral telemanipulation) or large-scale objects (in the case of man-amplifiers). The teleoperative performance can be characterized by the transparency, which is a measure of the extent to which the telemanipulation system presents the undistorted dynamics of the environment to the human operator. A common goal in the control of bilateral telemanipulation is to provide transparent teleoperation while ensuring the robust stability of the human-telemanipulator-environment loop. Prior Work Several researchers have investigated aspects of transparency and stability in telemanipulation, primarily through the use of two-port network modeling techniques. Doyle [7], to assess the stability of a macro-micro bilateral telemanipulator interacting with a passive human operator and environment. Though the telemanipulator itself was a singledegree-of-freedom system, the human-teleoperator-environment interaction was formulated in a manner that required multivariable tools in order to assess stability robustness. Colgate did not explicitly treat transparency, but instead utilized impedance shaping to intentionally alter the dynamics as perceived by the human operator through the telemanipulator. Itoh et al. experimentally implemented a six degree-of-freedom telemanipulator using passivity theory to address stability robustness, but instead of providing transparency, the telemanipulator was controlled to exhibit a task-oriented dynamic behavior specified in order to facilitate a particular telemanipulation task. Hashstrudi-Zaad and Salcudean theoretically assessed the performance and stability robustness of a three degree-of-freedom telemanipulator by incorporating a parallel force/position control to linearize and decouple the manipulators, and by assuming the human operator and environment to be decoupled, in which case the analysis reduces to that required for three decoupled single-degree-of-freedom systems. In contrast to the combined hybrid parameter/passivity based approach, the architecture proposed by Fite et al. [8] formulates the teleoperation system as a single feedback loop to which the tools of classical control theory can then be applied to address the performance and stability robustness. In so doing, the stability robustness of the system is addressed in a non-conservative manner, and the transparency is addressed only in the bandwidth of interest. This loop shaping approach was developed in a single input, single output context; since telemanipulation Fite and Goldfarb Multivariable Loop Shaping … 4 applications generally involve systems with coupled multiple degrees of freedom, however, such a method is of limited utility without extension to the multivariable case. As such, the work presented in this paper extends this previously published approach to the multivariable case of telemanipulation. Specifically, the extension entails the use of impedance control techniques to render linear the otherwise nonlinear dynamics of the master and slave manipulators, and employs a diagonalization multivariable loop shaping technique used to render tractable the multivariable loop shaping compensator design. A multivariable measure of transparency is additionally proposed based on the relative singular values of the environment and transmitted impedance matrices. 3 Multivariable Telemanipulation Architecture Fite and Goldfarb Multivariable Loop Shaping … 6 Given the master/human and slave/environment dynamics as previously described, the loop shaping telemanipulation architecture is obtained by combining the master/human and slave/environment subsystems with the position and force scaling matrices, C 1 and C 2 , respectively, as shown in The transparency of the multivariable teleoperation loop is determined by the relative distortion between the transmitted impedance (i.e., the impedance felt by the human operator) and the actual environment impedance. The impedance transmitted to the human operator by the telemanipulation system is given by: For perfect transparency, the transmitted impedance transfer function matrix of Eq. (3) should equal the actual environment impedance, Z e . In practice, these matrices need only be similar within some frequency band of interest. Thus, within this band of interest, perfect transparency requires the singular values of the transmitted impedance transfer function matrix to equal those of the actual environment impedance transfer function matrix. As such, a measure of the desired multivariable performance can be given by the ratio of the respective singular values of the impedance transmitted to the human operator to those of the environment impedance: where n rank = ) ( e t Z , Z and i δ represents distortion in the teleoperative system. A desired bandwidth of transparency can be prescribed by ensuring that the distortion i δ in each singular Fite and Goldfarb Multivariable Loop Shaping … 7 value is less than some allowable amount of distortion ∆ for a desired bandwidth of operation. For ∆= 3 dB, a prescription for good teleoperative performance can be written as: where t Ω is a desired bandwidth of teleoperative transparency. The overall objective of the control architecture is to achieve the desired performance specified by Eq. (5) while ensuring the robust stability of the closed-loop system. With the introduction of a loop shaping compensator in the motion communication channel, th