A 3 DoF parallel cable driven body weight support (BWS) system has been developed for the University of Utah's Treadport Locomotion Interface, for purposes of rehabilitation, simulation of steep slopes, and display of reduced gravity environments. The Treadport's large belt (6 by 10 feet) requires a multi-cable support system to ensure that the unloading forces are close to vertical. This paper presents the design and experimental validation, including the system model and force control.

This paper presents an automated routine for calibrating a fingernail imaging system, with the intent of predicting fingerpad forces. The system uses a magnetic levitation haptic device to apply forces to the human fingerpad while recording images of the nail and surrounding skin. A novel force controller is implemented to interact stably with the human fingerpad. The data are used to calibrate a principal component regression model relating pixel intensity to 3-D force. Using data from this automated routine, this model simultaneously predicts 3-D force with an RMS error of 0.56 ± 0.03 N (7.7% of the full range of forces).

This paper presents the Treadport Active Wind Tunnel (TPAWT)-a full-body immersive virtual environment for the Treadport locomotion interface designed for generating wind on a user from any frontal direction at speeds up to 20 kph. The goal is to simulate the experience of realistic wind while walking in an outdoor virtual environment. A recirculating-type wind tunnel was created around the pre-existing Treadport installation by adding a large fan, ducting, and enclosure walls. Two sheets of air in a non-intrusive design flow along the side screens of the back-projection CAVE-like visual display, where they impinge and mix at the front screen to redirect towards the user in a full-body cross-section. By varying the flow conditions of the air sheets, the direction and speed of wind at the user are controlled. Design challenges to fit the wind tunnel in the pre-existing facility, and to manage turbulence to achieve stable and steerable flow, were overcome. The controller performance for wind speed and direction is demonstrated experimentally.

This paper investigates the effect of two different parameters on the registration and force prediction accuracy of using Active Appearance Models (AAM) to align fingernail images. First, the color channel used to form the AAM is varied between (1) an averaged grayscale image, (2) the red channel, (3) the green channel and (4) the blue channel. Second, the number of landmark points used to create the AAM is varied between 6 and 75. The color channel is found to have an effect on the registration accuracy and the force prediction error. The green, blue and grayscale images are approximately equivalent, while the red images have a larger error across all metrics used. The number of landmark points may be reduced to 25 with no significant effect on either the registration accuracy or the force prediction error, though further reduction has shown some effect. With this information, a simpler registration model can be used that requires fewer calculations.

This paper demonstrates the effectiveness of registering fingernail images using Active Appearance Models (AAM). A shape model is formed using two contours, one to trace the edge of the fingernail and a second to outline the edge of the finger. This model is used to create an AAM, which is then used to register several hundred images. Using the images and associated force measurements, three different models relating pixel intensity to finger contact force are formed. This process is repeated for eight subjects. These image-force models could then be used to measure the finger contact force without obstructing the haptic sense or instrumenting the object being touched, allowing for more natural interaction between human and robot. Two of the models are found to predict force with an RMS error of 0.3N normal force (4% of the full range) and 0.23N shear force (3% of the full range). These two models are also shown to be nearly independent of image resolution, with no significant difference in the results for resolutions between 10-by-10 and 50-by-50. The simpler of the two models relies on the Texture Parameters generated by the AAM. Since these parameters are already calculated as part of the registration process, this removes an additional calculation step and could lead to improved real-time processing.

This paper presents a method of imaging the coloration pattern in the fingernail and surrounding skin to infer fingertip force direction (which includes four major shear-force directions plus normal force) during planar contact. Nail images from 15 subjects were registered to reference images with random sample consensus (RANSAC) and then warped to an atlas with elastic registration. With linear discriminant analysis, common linear features corresponding to force directions, but irrelevant to subjects, are automatically extracted. The common feature regions in the fingernail and surrounding skin are consistent with observation and previous studies. Without any individual calibration, the overall recognition accuracy on test images of 15 subjects was 90%. With individual training, the overall recognition accuracy on test images of 15 subjects was 94%. The lowest imaging resolution, without sacrificing classification accuracy, was found to be between 10-by-10 and 20-by-20 pixels.

This research creates a steady headwind at a user position in the scaled Treadport Active Wind Tunnel (TPAWT). The TPAWT adds a wind display system to the previously developed Treadport virtual environment, and this research builds upon prior work to provide improved control of headwind angle at the user position. Key to this research is the addition of a negative pressure plenum at the rear of the treadport to improve nominal flow stability. The previous controller based upon the small gain theorem with a dynamic extension is then modified to provide wind angle feedback control. A conditional angular rate-switching controller is added to reduce wind angle oscillations at the user. A vorticity-meter is developed to assure that the wind flow is centered at the user position. As a result, this research reduces wind angle error by 75% compared to previous work.

This paper relates 5 observability indexes for robot calibration to the "alphabet optimalities" from the experimental design literature. These 5 observability indexes are shown to be the upper and lower bounds of one another. All observability indexes are proved to be equivalent when the design is optimal after a perfect column scaling. It is shown that when the goal is to minimize the variance of the parameters, D-optimality is the best criterion. When the goal is to minimize the uncertainty of the end-effector position, E-optimality is the best criterion. It is proved that G-optimality is equivalent to E-optimality for exact design.

This paper presents a new updating algorithm to reduce the complexity of computing an observability index for kinematic calibration of robots. An active calibration algorithm is developed to include an updating algorithm in the pose selection process. Simulations on a 6-DOF PUMA robot with 27 unknown parameters shows that the proposed algorithm performs more than 50,000 times better than exhaustive search based on randomly generated designs.

This paper develops a static observer for estimating wind speed in order to control wind speed and angle control at a user position in a submersive virtual environment. Addition of wind display evolves the Treadport Virtual environment into a highly immersive virtual environment called Treadport Active Wind Tunnel (TPAWT). Experiments on a scaled model of the TPAWT show that headwind flow stream diverges at the user. Pitot tube sensors placed at a particular region of converged flow in the scaled TPAWT provide measurements with lower noise. Open loop experiments on a scaled model of TPAWT show that there exists a relationship between speed measured at this region of converged flow and the wind speed at the user position. Using this relationship, the wind speed at the user can be estimated. We use this relation and combine previously used speed and angle controllers based upon the small gain theorem with a dynamic extension and conditional angular rate-switching control. Finally, we simultaneously control wind speed and headwind angle.

This paper presents a method of imaging the coloration pattern in the fingernail and surrounding skin to infer fingertip force direction during planar contact. Nail images from 7 subjects were registered to reference images with RANSAC and then warped to an atlas with elastic registration. Recognition of fingertip force direction, based on linear discriminant analysis, shows that there are common color pattern features in the fingernail and surrounding skin for different subjects. Based on the common features, the overall recognition accuracy is 92%.

Accurate models of the feel of physical objects are essential to improving the realism of haptic simulations. This paper presents a method for automatically obtaining experimentally based models of general passive, nonlinear devices for use in haptic playback applications, with specific emphasis on modeling switches and buttons. The method, based on the exponentially weighted least-squares (EWLS), allows estimation of position- and direction-dependent parameters of a general nonlinear model. Results are presented for two push-button switches.

This paper presents a technique termed eigennails to classify fingertip force during contact based on the coloration patterns in the fingernail and surrounding skin. Fingertip force is classified into six directions: no force, normal force only, two directions (left/right) of lateral shear force, and two directions (forward/backward) of longitudinal shear forces. Based on the face recognition technique eigenfaces, a small number of eigennails are sufficient to express the color pattern features for shear force direction classification. Results show that 98% of 960 fingernail images of 8 different subjects are correctly classified. The lowest imaging resolution without sacrificing classification accuracy is found to be 10-by-10.

As an extension of our previous work on estimating fingertip forces by imaging the fingernail (Y. Sun, et al., 2006), the dynamic features of the coloration response of different parts of the fingernail and surrounding skin to different force levels are studied. The effect of the cardiovascular state on measurable coloration is also characterized. The accuracy of normal force estimated by generalized least squares is presented. A time compensation method for fast force estimation is presented, based on a knowledge of the time constants from the dynamic response of individual fingernail regions

Describes the simulation of stairs on a treadmill style locomotion interface using torso force feedback. The active mechanical tether of the Sarcos Treadport locomotion interface applies a specialized force profile to simulate the forces of stair walking. The biomechanics of subjects walking on real stairs versus walking under the specialized force profile were compared. It was found that the tether force was able to adjust the subject's motion from standard slope walking towards that of stairs.

A new torque control strategy for brushless motors with minimum torque ripple and minimum copper losses while maintaining the phase currents under magnetic (or current) saturation is presented. The optimal control, a nonlinear map from desired torque and position to the motor's phase currents, is the closed form solution to a quadratic programming problem. Experimental results demonstrate virtually ripple-free torque and improved tracking accuracy. In case of magnetic or current saturation, the maximum torque capability of the motor is considerably increased.

Electromagnetic trackers have many favorable characteristics but are notorious for their sensitivity to magnetic field distortions resulting from metal and electronic equipment in the environment. We categorize existing tracker calibration methods and present an improved technique for reducing the static position and orientation errors that are inherent to these devices. A quaternion-based formulation provides a simple and fast computational framework for representing orientation errors. Our experimental apparatus consists of a 6-DOF mobile platform and an optical position measurement system, allowing the collection of full-pose data at nearly arbitrary orientations of the receiver. A polynomial correction technique is applied and evaluated using a Polhemus Fastrak resulting in a substantial improvement of tracking accuracy. Finally, we apply advanced visualization algorithms to give new insight into the nature of the magnetic distortion field.

This is a survey of research issues for haptic and locomotion interfaces. A key issue is the workspace and dexterity of haptic devices. In the simulation of real environments, interactive geometric computations on complex models are a fundamental problem. Haptic rendering of forces of contact are increasingly based on measurements on real environments. Scientific visualization is another promising area for haptic interfaces, but the issue of how to usefully display data haptically needs to be worked out. Locomotion rendering issues include the display of inertial forces, slope, unilateral constraints, and turning. Examples of approaches to these issues are presented.

This paper reports on advances in the design and development of a high performance direct-drive joint for robotics and automation. The key design aspects of the mechanical components, sensors, control architecture, and a hydraulically actuated dynamometer testbed are described. We also present a number of advanced implementations in control, motor torque control, and motion control using positive joint torque feedback. Experimental results illustrate outstanding performance regarding thermal response, torque ripple, reference trajectory tracking, torque disturbance rejection, and joint stiffness.

Positive joint torque feedback can compensate the detrimental effects of load torques on position tracking performance. However, with (real world) nonideal torque sources, simple unity gain positive torque feedback can actually deteriorate the performance, or even result in instability. An H/sup /spl infin// joint torque feedback is proposed which takes the dynamics and uncertainty of the actuator into account and minimizes the system's sensitivity to load torques. In our experimental direct-drive system, the H/sup /spl infin// positive torque feedback inner loop drastically improved the disturbance attenuation (25 dB) and load decoupling properties of a simple PID position controller.

A haptic display for grasping and manipulating virtual objects in a CAD environment is investigated for the development of rapid prototyping technology. The operator receives the sensation of contacting and tracing of the surface of the virtual object, and of grasping and manipulating the object from the haptic display. After the control of the haptic display is formulated, it is implemented on the Sarcos Dexterous Arm Master. The proposed haptic display is experimentally confirmed to provide realistic sensation that enables the operator to grasp and manipulate the virtual object easily as intended.

A haptic display system is presented for manipulating virtual mechanisms derived from a mechanical CAD design. Links are designed and assembled into mechanisms using Utah's Alpha-1 CAD system, and are then manipulated with a Sarcos Dextrous Arm Master. Based on the mechanism's kinematics and the virtual grasp, the motion of the master is divided into motion of the mechanism and constraint violation. The operator experiences the dynamic forces from the mechanism plus constraint forces.

The open-loop torque control of synchronous motors with minimum torque ripple and minimum copper losses is presented. The control problem is viewed as an optimization problem for the nonlinear mapping from desired torque and position to the motor's phase currents. The performance of the controller to produce virtually ripple-free torque and to enhance the precision of position tracking is demonstrated experimentally.

In analytical and experimental investigation of a jet pipe controlled electropneumatic servoactuator designed for use in the Utah/MIT dextrous hand has been performed. An accurate dynamic model has been developed and verified which includes detailed representation of the jet pipe element, orifice areas and fluid flows. The effects and modeling of hysteresis in the jet pipe are discussed. Actuator saturation is shown to be an obstacle in achieving high bandwidth. Theoretical and experimental results are presented for frequency response and time-domain force tracking.

This paper addresses the dynamics and control of direct drive robots with positive joint torque feedback. We analytically derive the system dynamics in closed form. Even though it is coupled and nonlinear in general, it is substantially simpler than the robot link dynamics. We also derive conditions on the robot configuration which result in linear dynamics. Motion control laws for both cases are proposed.

This work deals with closed-loop calibration methods where the robot endpoint is constrained to lie on a plane. Previously published calibration approaches are shown to have certain weaknesses. A new solution is given using DH and Hayati notations and standard nonlinear least squares optimization. The procedure is extended via the implicit loop method, which takes input noise into account. Pose selection is guided by the noise amplification index. Simulation and experimental results are presented for a PUMA 560 industrial manipulator and are compared to those obtained from an open-loop calibration procedure.

Applying positive joint torque feedback in direct drive motor systems reduces the effect of external load torques. This is most beneficial when accurate position tracking is required in the presence of large disturbances or for complex loads as found in robotic manipulators. We formulate the problem of torque feedback in the presence of actuator dynamics. The problem of finding the optimal torque feedback which minimizes the maximum disturbance sensitivity is equivalent to the model-matching problem. The effect of the load on the rotor dynamics appears as a perturbation minimized by the proposed torque feedback. To synthesize the torque filter, only the actuator dynamics and a very rough knowledge about the mechanical load is required.

This paper presents a new model reference adaptive control method to control motor torque in direct drive reluctance motors in an accurate and ripple-free fashion. It is based on the motor's voltage equation and requires only measurements of motor angle, winding voltages and currents. In addition, we introduce a new optimal torque-to-current conversion which minimizes copper losses. Simulations of a reluctance motor with magnetically coupled phases demonstrate fast parameter convergence and accurate torque tracking.

A full tactile sensing suite for the finger segments and palm of the Utah/MIT dextrous hand is presented. The rubber-based sensors employ capacitance sensing and floating electrodes in the top layer, and contain local electronics for excitation, filtering, analog-to-digital conversion, and serial communication. Experimental results on static, dynamic, and spatial properties are presented. Use of the tactile sensor in contact force control is demonstrated.

This paper presents a new and simple procedure to identify link mass parameters used for gravity compensation and on-line automatic torque sensor calibration for a robot manipulator. The approach employs single-joint rotation and a recursive procedure that proceeds distally to proximally to identify composite mass moments. Experimental results on presented for the Sarcos Dextrous Arm.

This paper presents a new observability index to quantify the selection of best pose set in robot calibration. This noise amplification index is considerably more sensitive to calibration error than previously published observability indices. Support for the proposed index as provided analytically and geometrically, and also through comparison against previous indices by a simulation for a 3-link planar robot and by an experiment for a 3-DOF redundant parallel-drive robot.

A method is presented for autonomous kinematic calibration of the RSI 6-DOF hand controller, a two-loop parallel mechanism comprised of three 6-DOF arms with potentiometers on the first three joints of each arm. This double closed-loop kinematic calibration method is an adaptation of a previously developed single closed-loop method. The kinematic parameters identified are the joint angle offsets and the joint angle gains. Experimental results are presented and compared to results using a special calibration fixture.>

A unified formulation for the calibration of both serial-link robots and robotic mechanisms having kinematic closed-loops is presented and applied experimentally to two 6-degree-of-freedom devices: the RSI 6-DOF hand controller and the MEL "modified Stewart platform". The unification is based on an equivalence between end-effector measurements and constraints imposed by the closure of kinematic loops. Errors are allocated to the joints such that the loop equations are satisfied exactly, which eliminates the issue of equation scaling and simplifies the treatment of multi-loop mechanisms. For the experiments reported here, no external measuring devices are used; instead we rely on measurements of displacements in some of the passive joints of the devices. Using a priori estimates of the statistics of the measurement errors and the parameter errors, the method estimates the parameters and their accuracy, and tests for unmodeled factors.>

A new method for kinematic calibration of a robot is presented, based on triaxial measurement of acceleration by a sensor fixed to the robot end-point. The kinematic parameters are evaluated by means of a series of simple tests (two for each joint). In these tests each joint, except the one under consideration, is kept fixed. Acceleration and encoder output are then acquired. Simulations and experimental results applied on Sarcos dextrous arm are presented to verify this method.>

An analytical and experimental investigation of a jet pipe controlled electropneumatic servoactuator designed for use in the Utah/MIT Dextrous Hand (UMDH) has been performed. An accurate dynamic model has been developed and verified which includes detailed representation of the jet pipe element, orifice areas and fluid flows. The effects and modelling of hysteresis in the jet pipe are discussed. Theoretical and experimental results are presented for closed-loop force control showing that the force bandwidth can be raised as high as 80 Hz.>

This paper presents an autonomous method to calibrate joint torque sensors for advanced robots; as a by-product, the position sensors can also be calibrated. When one joint of a manipulator is rotated the gravity torque exerted on the joint varies sinusoidally with rotation angle. By means of sinusoidally curve fitting, the bias of the torque sensor and the gain of the position sensor can be found directly. The gravity vector expressed in the defined base coordinates, can also be found. We then determine the torque sensor gain as well as the rotation angle offset, by attaching a known load to robot in different end positions. Experimental results are shown for the first two joints of the Sarcos dextrous arm.>

A method is presented for autonomous kinematic calibration of a 3-DOF redundant parallel robot. Multiple closed loops are used in a least squares optimization method. Ill-conditioning, column scaling of the gradient matrix, and observability indices for the best pose set of robot calibration configurations are discussed. Experimental results are presented and compared with the results using an external calibration device.>

Precise teleoperation of dextrous robotic hands by hand masters requires an accurate human hand model. A kinematic model of a human index finger is developed as an example for human hand modeling. The parameters of the model are determined by open-loop kinematic calibration. Singular value decomposition is used as a tool for analyzing the kinematic model and the identification process. Accurate and reliable results are obtained only when the numerical condition is minimized through parameter scaling, model reduction and pose set selection. The identified kinematic parameters show the kinematic model and calibration procedure have an accuracy on the order of a few millimetres.>

Describes the use of a nonlinear proportional and derivative (NPD) controller for both non-contact transition force control (NCTFC) (zero velocity when contact occurs) and contact transition force control (CTFC) (non-zero velocity when contact occurs). The authors present a gain design method for NCTFC. Simulations and experiments on the Sarcos Dextrous Arm show that PD control becomes unstable for CTFC while NPD control is stable and reaches the steady state quickly. Experiments with human subjects on NCTFC and CTFC are also presented.>

Closed-loop kinematic calibration has been experimentally implemented on the Sarcos Dextrous Arm. The elbow joint is made mobile by adding an unsensed hinge joint at the endpoint attachment to ground. The calibrated parameters include the joint angle offsets and the hinge-related parameters.>

The transmission system of the Utah/MIT Dextrous Hand (UMDH) is investigated theoretically and experimentally. It is shown that the friction of the routing pulleys is not negligible and should be considered in the force control of the UMDH. In the frequency range of the pneumatic actuators (80 Hz), tendons act like springs and the first mode of the tendons is above that frequency range.>

A recently developed kinematic calibration technique for six degree-of-freedom (DOF) arms, the Jacobian measurement method, is generalized to include underconstrained and overconstrained arms. Using joint torque sensing and a wrist six-axis force/torque sensor to measure the Jacobian, experimental results are presented for the first five DOFs of the Sarcos Dextrous Arm. Results are compared to two standard calibration methods, i.e., open loop calibration and circle point analysis. Measurements of endpoint position and orientation were obtained with the Optotruk and the Bird sensor, two commercial motion tracking systems.

A method for kinematic calibration is presented, which directly estimates the Jacobian matrix from measurements and extracts the kinematic parameters from the Jacobian exactly and without iteration. Two different experimental approaches are presented for measuring the Jacobian. In the first approach, a six degree-of-freedom manipulator is rigidly attached to the environmental at its endpoint, in one pose only. Joint torques are systematically exerted and the resulting endpoint forces and torques are measured. In the second approach, a manipulator attains different motions through the same endpoint location, and the joint velocities are related to measured endpoint linear and angular velocities. Simulations are presented to verify the method.>

The authors present a kinematic calibration method that does not require endpoint measurements or precision points. By forming manipulators into mobile closed kinematic chains, it is shown that consistency conditions in the kinematic loop closure equations are adequate to calibrate the manipulator from joint angle readings alone. This closed-loop kinematic calibration method is an adaptation of an iterative least-squares algorithm used in calibrating open-chain manipulators. Example tasks include a fixed endpoint (zero-degree-of-freedom (DOF) task), the opening of a door (one-DOF task), and point contact (three-DOF task).>

An approach to identifying the kinematic models of manipulators and their task geometry is presented. Starting with the observation that in many tasks manipulators naturally form mobile closed kinematic chains, it is shown that these closed loops can be identified by an iterative least-squares algorithm similar to that used in calibrating open chain manipulators. By merely using joint angle readings and self motions, consistency conditions can be utilized to identify the kinematic parameters. While the task of a robot opening a door is studied in detail, the method readily generalizes to a large class of robot tasks. Simulations are presented to accompany the analysis.>

A microrobot having two high-performance parallel drive limbs has been developed for manipulation, surgery, and dynamic mechanical testing of very small objects such as single living cells. The end-points of each limb move in overlapping spherical workspaces of 1 mm diameter with minimum open- and closed-loop movements of 1 nm and 10 nm, respectively. With optimal nonlinear model-based controllers the limbs can move at up to 2 m/s relative to each other. A variety of end effectors, including ferroelectric polymer microgrippers, may be attached to the limbs to permit cell manipulation. A 3D laser vision system with resolution of 50 to 100 nm has been developed to provide the microrobot with volume images containing magnitude, phase, polarization, and special information. A macro version of the microrobot has been built to enable force-reflecting teleoperation of the microrobot. The telemicrorobot system permits both microscopic objects and continuum models to be felt. A high-performance parallel computer has been designed to meet the substantial computational and control requirements of the system.>

Trajectory tracking errors resulting from the application of various controllers have been experimentally determined on the MIT Serial Link Direct Drive Arm. The controllers range from simple analog PD (proportional-derivative) control applied independently at each joint to feedforward and computed torque methods incorporating full dynamics. It was found that trajectory tracking errors decreased as more dynamic compensation terms were incorporated. There was no significant difference in trajectory tracking performance between the feedforward controller using independent digital servos and the full computed torque controller. Implementing the model-based controller highlights the need for accurate control of joint torque, accurate joint position and velocity sensing, and adequate sampling rates.>

Path planning techniques for kinematically redundant manipulators are developed which involve the optimization of integral cost criteria. For a variety of such criteria, the necessary condition consists of n+l first-order differential equations, where n is the number of manipulator degrees of freedom and l is the degree of the redundancy. This necessary condition is essentially equivalent to the system of 2n equations announced by Y. Nakamura, H. Hanafusa (Int. J. Robotics Res., vol.6, no.1, pp.32-42, 1987). Refinements to these basic necessary conditions involve various boundary conditions. Numerical examples highlight the advantages of optimally resolving kinematic redundancy according to an integral cost criterion (compared with the more common approach of optimizing an instantaneous criterion). It is seen, however, that care must be taken regarding trajectories which meet the necessary conditions yet fail to be optimal. It is concluded that any sufficiency conditions for such optimization will involve explicit consideration of the topology of liftings of workspace paths.>

The authors describe a fully implemented computational architecture (CONDOR) that controls the Utah-MIT dexterous hand and other complex robots. The architecture derives its power from the highly efficient real-time environment provided for its control processors, coupled with a development host that allows flexible program development. By mapping the memory of a dedicated group of processors into the address space of a host computer, efficient sharing of system resources between them is possible. The software is characterized by a few simple design concepts but provides the facilities out of which more powerful utilities such as a multiprocessor pseudo-terminal emulator, a transparent and fast file server, and a flexible symbolic debugger could be constructed.>

Work on building robot models to be used in designing model-based controllers is described. Various algorithms are presented estimating the kinematic, link-inertial, and load-inertial parameters. It is shown experimentally that accurate estimates can be obtained automatically using sensor data taken during movement. The algorithms have been implemented on the MIT Serial Link Direct Drive Arm.>

For pt.I see ibid., p.1374-9 (1988). Work on model-based control with the MIT Serial Link Direct Drive Arm is described. It is shown that model-based control leads to performance superior to control not based on carefully constructed models. Trajectory control, trajectory learning, and force control are treated. A new type of trajectory learning is considered in which a robot fine-tunes one particular trajectory through repetition. Various experiments with the direct drive arm are reported to validate the importance of model-based control.>

A fast architecture for controlling the Utah-MIT hand and other such complex robots is reviewed. These robots are characterized by a large number of joints and consequently demand powerful computer architectures to be controlled and utilized effectively. The version of the architecture derives its power from a novel bus-to-bus adaptor to couple a development host running a time-sharing operating system with a multimicroprocessor system devoted to real-time computations. The software is characterized by a few simple design concepts but provides the facilities out of which powerful utilities like a multiprocessor pseudoterminal emulator, a transparent fast file server, and a truly flexible powerful symbolic debugger could be constructed.>

Trajectory tracking errors resulting from the application of various controllers have been experimentally determined on the MIT Serial Link Direct Drive Arm. The controllers range from simple analog PD control applied independently at each joint to feedforward and computed torque methods incorporating full dynamics. It was found that trajectory tracking errors decreased as more dynamic compensation terms were incorporated. There was no significant difference in trajectory tracking performance between the feedforward controller using independent digital servos and the full computed torque controller.

Robots in force control mode often become unstable during contact with stiff environments, due mainly to the high gain nature of wrist force-sensor feedback. To achieve stable force control, we propose an essential reliance on fast open-loop joint torque control, and relegate tip force sensor feedback to a slow loop to maintain accuracy. Dynamic stability is analyzed with a simple model of the robot and its environment. The analyses are verified by single-link experiments on the MIT Serial Link Direct Drive Arm.

In active force or compliance implementations for multi-link manipulators, there is typically a kinematic coordinate transformation in the feedback path. A coordinate transformation will affect the dynamics of the closed-loop system and possibly make it unstable. It is shown that the hybrid force/position control method of Raibert and Craig (1981) exhibits such kinematically induced instabilities for revolute manipulators, whereas other force control methods, such as the stiffness control and the operational space methods, do not. Both theoretical analyses and experimental results on the MIT Serial Link Direct Drive Arm are given.

This paper compares local and global optimization methods for minimizing torque loading at the joints of a redundant manipulator in a least squares sense. Simulations show that the local methods show an unexpected instability for relatively long trajectories. The global method, however, was able to find a stable solution which outperformed the local algorithms at all times.

Methods for resolving kinematic redundancies of manipulators by the effect on joint torque are examined. When the generalized inverse is formulated in terms of accelerations and incorporated into the dynamics, the effect of redundancy resolution on joint torque can be directly reflected. One method chooses the joint acceleration null-space vector to minimize joint torque in a least squares sense; when the least squares is weighted by allowable torque range, the joint torques tend to be kept within their limits. Contrasting methods employing only the pseudoinverse with and without weighting by the inertia matrix are presented. The results show an unexpected stability problem during long trajectories for the null-space methods and for the inertia-weighted pseudoinverse method, but more seldom for the unweighted pseudoinverse method. Evidently, a whiplash action develops over time that thrusts the endpoint off the intended path, and extremely high torques are required to overcome these natural movement dynamics.

Trajectory tracking errors resulting from the application of various controllers have been experimentally determined on the MIT Serial Link Direct Drive Arm. The controllers range from simple PD control applied independently at each joint to feedforward control incorporating full dynamics followed by a separate PD control loop. It was found that trajectory tracking errors decreased as more feedforward terms were incorporated.

This paper presents a fast and efficient way to compute the mapping from a desired external force on a grasped object to the joint torques of a multi-jointed, multi-fingered dexterous hand. The efficiency derives from avoiding the use of any Jacobian. The algorithm models contacts as point contacts with friction and works for three or four-fingered dexterous hands.

This paper provides an overview of the hardware and software systems developed for the Utah-MIT dextrous hand. It builds on the issues raised in an earlier paper [Siegel et al. 1985]. An innovative task scheduling system and a new implementation of our message-passing based inter-task communication system are discussed. Finally, an application program using this system to implement hand-level primitive operations is presented.

The design, construction, and performance of a tactile sensor and a thermal sensor that can be combine into one compact device with both modalities is discussed. The tactile sensing portion is composed of an 8×8 array of force sensing cells with 1.8 mm center to center spacing. Heat transduction is measured by a 4×4 array of temperature sensors. A number of these devices will be mounted on the Utah-MIT four-fingered dextrous hand, providing a unique research environment for pursuing sensor driven control strategies.

Methods for resolving kinematic redundancies of manipulators by the effect on joint torque are examined. When the generalized inverse is formulated in terms of accelerations and incorporated into the dynamics, the effect of redundancy resolution on joint torque can be directly reflected. One method chooses the joint acceleration null-space vector to minimize joint torque in a least squares sense; when the least squares is weighted by allowable torque range, the joint torques tend to be kept within their limits. Contrasting methods employing only the pseudoinverse with and without weighting by the inertia matrix are presented. The results show an unexpected stability problem during long trajectories for the null-space methods and for the inertia-weighted pseudoinverse method, but rarely for the unweighted pseudoinverse method. Evidently a whiplash action develops over time that thrusts the endpoint off the intended path, and extremely high torques are required to overcome these natural movement dynamics.

This paper presents the computational architecture for the Utah-MIT hand, and discusses design issues encountered in its hardware and software development. The large number of linkages, actuators, and sensors offers a potentially severe computational burden for control; a multiprocessor hardware configuration has been developed to distribute this computation. In the interests of efficiency, minimal operating systems were devised for each processor which nevertheless were made sufficiently general for task scheduling, intertask communication, and debugging. This computational architecture is a general system which is potentially useful for other robotics applications.

The minimum-time path for a robot arm has been a long-standing and unsolved problem of considerable interest. We present a general solution to this problem that involves joint-space tesselation, a dynamic time-scaling algorithm, and graph search. The solution incorporates full dynamics of movement and actuator constraints, and can be easily extended for joint limits and workspace obstacles, but is subject to the particular tesselation scheme used. The results presented show that, in general, the optimal paths are not straight lines, but rather curves in joint-space that utilize the dynamics of the arm and gravity to help in moving the arm faster to its destination. Implementation difficulties due to the tesselation and to combinatorial proliferation of paths are discussed.

In order to stably grasp objects without using object models, tactile feedback from the fingers is sometimes necessary. This feedback can be used to adjust grasping forces to prevent a part from slipping from a hand. If the angle of force at the object finger contact can be determined, slip can be prevented by the proper adjustment of finger forces. Another important tactile sensing task is finding the edges and corners of an object, since they are usually feasible grasping locations. This paper describes how this information can be extracted from the finger-object contact using strain sensors beneath a compliant skin. For determining contact forces, strain measurements are easier to use than the surface deformation profile. The finger is modelled as an infinite linear elastic half plane to predict the measured strain for several contact types and forces. The number of sensors required is less than has been proposed for other tactile recognition tasks. A rough upper bound on sensor density requirements for a specific depth is presented that is based on the frequency response of the elastic medium. The effects of different sensor stiffnesses on sensor performance are discussed.

An efficient algorithm is presented for the calculation of the inverse kinematic accelerations for a 6 degree-of-freedom manipulator with a spherical wrist. The inverse kinematic calculation is shown to work synergistically with the inverse dynamic calculation, producing kinematic parameters needed in the recursive Newton-Euler dynamics formulation. Additional savings in the dynamics computation are noted for a class of kinematically well-structured manipulators such as spherical-wrist arms and for manipulators with simply-structured inertial parameters.

The essential grammatical information of American Sign Language (ASL) is conveyed through changes in the movement and spatial contouring of the hands and arms. An interactive computer graphic system is described for the analysis and modeling of sign language movement. This system consists of four components. The first component reconstructs actual movements in three dimensions and allows the user to interactively segment and transform the data for later analysis. The second component allows a user to interactively create synthetic signs by specifying angle functions in a jointed model. The third component provides a novel technique for manipulating movement quality independently of spatial path. The fourth component allows the building of complex stimuli and real-time stimulus sequencing for psycholinguistic experiments. The emphasis is on interactive techniques and data structures that allow analysis and modeling of the complex hand and arm movements of American Sign Language.

An efficent Lagangian formulation of manipulator dynamics has been developed. The efficiency derives from recurrence relatons for the velocities, accelerations, and generalized forces. The number of additons and multiplicatins varies linearly with the number of joint, as opposed to past Lagrangian dynamics formulations with an n4 dependence. Wih this formulation it should be possible in principle to compute the Lagrangian dynamics in real time. The computational complexities of this and other dynamics formulations including rect Newton-Euler formulations and tabular formulations are compared. It Is concluded that recursive formultions based either on the Lagrangian or Newton-Euler dynamics offer the best method of dynamns calculation.