Vivek Sangwan

Assessment of Motion of a Swing Leg and Gait Rehabilitation With a Gravity Balancing Exoskeleton

The gravity balancing exoskeleton, designed at University of Delaware, Newark, consists of rigid links, joints and springs, which are adjustable to the geometry and inertia of the leg of a human subject wearing it. This passive exoskeleton does not use any motors but is designed to unload the human leg joints from the gravity load over its range-of-motion. The underlying principle of gravity balancing is to make the potential energy of the combined leg-machine system invariant with configuration of the leg. Additionally, parameters of the exoskeleton can be changed to achieve a prescribed level of gravity assistance, from 0% to 100%. The goal of the results reported in this paper is to provide preliminary quantitative assessment of the changes in kinematics and kinetics of the walking gait when a human subject wears such an exoskeleton. The data on kinematics and kinetics were collected on four healthy and three stroke patients who wore this exoskeleton. These data were computed from the joint encoders and interface torque sensors mounted on the exoskeleton. This exoskeleton was also recently used for a six-week training of a chronic stroke patient, where the gravity assistance was progressively reduced from 100% to 0%. The results show a significant improvement in gait of the stroke patient in terms of range-of-motion of the hip and knee, weight bearing on the hemiparetic leg, and speed of walking. Currently, training studies are underway to assess the long-term effects of such a device on gait rehabilitation of hemiparetic stroke patients.

Design and Optimization of a Cable Driven Upper Arm Exoskeleton

This paper outlines the design of a wearable upper arm exoskeleton that can be potentially used to assist and train arm movements of stroke survivors or subjects with weak musculature. In the last 10 years, a number of upper arm training devices have emerged. However, due to their size and weight, their use is restricted to clinics and research laboratories. Our proposed wearable exoskeleton builds upon our research experience in wire driven manipulators and design of rehabilitative systems. The exoskeleton consists of three main parts: (i) an inverted U-shaped cuff that rests on the shoulder, (ii) a cuff on the upper arm, and (iii) a cuff on the forearm. Six motors mounted on the shoulder cuff drive the cuffs on the upper arm and forearm with the use of cables. In order to assess the performance of this exoskeleton prior to use on humans, a laboratory test-bed has been developed where this exoskeleton is mounted on a model skeleton, instrumented with sensors to measure joint angles. This paper describes the design details of the exoskeleton and addresses the key issue of parameter optimization to achieve a useful workspace based on kinematic and kinetic models. The optimization results have also been motivated from activities of daily living.

Differentially Flat Designs of Underactuated Open-Chain Planar Robots

A fully actuated system can execute any joint trajectory. However, if the system is underactuated, not all joint trajectories are attainable. For such systems, it is difficult to characterize attainable joint trajectories analytically. Numerical methods are generally used to characterize these. This paper investigates the property of differential flatness for underactuated planar open-chain robots and studies dependence on inertia distribution within the system. It is shown that certain choices of inertia distributions make an underactuated open-chain planar robot with revolute joints feedback linearizable, i.e., also differentially flat. Once this property is established, trajectory between any two points in the state space can be planned, and a controller can be developed to correct for errors. To demonstrate the proposed methodology in hardware, experiments with an underactuated 3-DOF planar robot are also presented.

Gait training after stroke: a pilot study combining a gravity-balanced orthosis, functional electrical stimulation, and visual feedback.

Rationale: This case report describes the application of a novel gait retraining approach to an individual with poststroke hemiparesis. The rehabilitation protocol combined a specially designed leg orthosis (the gravity-balanced orthosis), treadmill walking, and functional electrical stimulation to the ankle muscles with the application of motor learning principles. Case: The participant was a 58-year-old man who had a stroke more than three years before the intervention. He underwent gait retraining over a period of five weeks for a total of 15 sessions during which the gravity compensation provided by the gravity-balanced orthosis and visual feedback about walking performance was gradually reduced. Outcomes: At the end of five weeks, he decreased the time required to complete the Timed Up and Go test; his gait speed increased during overground walking; gait was more symmetrical; stride length, hip and knee joint excursions on the affected side increased. Except for gait symmetry, all other improvements were maintained one month post-intervention. Conclusions: This case report describes possible advantages of judiciously combining different treatment techniques in improving the gait of chronic stroke survivors. Further studies are planned to evaluate the effectiveness of different components of this training in both the subacute and chronic stages of stroke recovery.

Design of a Novel Two Degree-of-Freedom Ankle-Foot Orthosis

An ankle-foot orthosis (AFO) is commonly used to help subjects with weakness of ankle dorsiflexor muscles due to peripheral or central nervous system disorders. Both these disorders are due to the weakness of the tibialis anterior muscle, which results in the lack of dorsiflexion assist moment. The deformity and muscle, weakness of one joint in the lower extremity influences the stability of the adjacent joints, thereby requiring compensatory adaptations. We present an innovative ankle-foot orthosis (AFO). The prototype AFO would introduce greater functionality over currently marketed devices by means of its pronation-supination degree of freedom in addition to flexion/extension. This orthosis can be used to measure joint forces and moments applied by the human at both joints. In the future, by incorporation of actuators in the device, it will be used as a training device to restore a normal walking pattern.

Differentially Flat Design of Bipeds Ensuring Limit Cycles

For bipedal walking, a set of joint trajectories is acceptable as long as it satisfies certain overall motion requirements, such as: 1) it is repetitive (limit cycles); 2) it allows the foot to clear ground; and 3) it allows the biped to move forward. Since the actual trajectory followed by a biped is not as important, a biped having some unactuated joints can also meet these motion requirements. Furthermore, due to physical constraints, a biped cannot have an actuator between the foot and the ground. Hence, it is underactuated during the phase when the foot is rolling on the ground. Besides underactuation, a bipedal robot has nonlinear dynamics and impacts. In general, it is difficult to prove existence of limit cycles for such systems. In this paper, a design methodology that renders planar bipedal robots differentially flat is presented. Differential flatness allows generation of parameterized limit cycles for this class of planar nonlinear underactuated bipeds. Sequential quadratic-programming-based numerical optimization routines are used to optimize these limit cycles while satisfying the motion constraints. The planning and control methodology is illustrated by a two-link biped.

Exoskeletons for Gait Assistance and Training of the Motor-Impaired

Robotics is emerging as a promising tool for training of human functional movement. The current research in this area is focused primarily on upper extremity movements. This paper describes novel designs of three lower extremity exoskeletons, intended for gait assistance and training of motor-impaired patients. The design of each of these exoskeletons is novel and different. Force and position sensors on the exoskeleton provide feedback to the user during training. The exoskeletons have undergone limited tests on healthy and stroke survivors to assess their potential for treadmill walking. GBO is a gravity balancing un-motorized orthosis which can alter the gravity acting at the hip and knee joints during swing. ALEX is an actively driven leg exoskeleton which can modulate the foot trajectory using motors at the joints. SUE is a bilateral swing-assist un-motorized exoskeleton to propel the leg during gait.

Differential Flatness of a Class of

A fully actuated system can execute any joint trajectory. However, if a system is under-actuated, not all joint trajectories are attainable. The authors have actively pursued novel designs of under-actuated robotic arms which are both controllable and feedback linearizable. These robots can perform point-to-point motions in the state space, but potentially can be designed to work with fewer actuators, hence with lower cost. With this same spirit, the technical note investigates the property of differential flatness for a class of planar under-actuated open-chain robots having a specific inertia distribution, but driven by only one or two actuators. This technical note addresses the following theoretical question: what placement of one or two actuators will make an n-DOF planar robot differentially flat if it is designed so that its center of mass always lies at joint 2?

n

This paper presents the design of a wearable upper arm exoskeleton that can be used to assist and train arm movements of stroke survivors or subjects with weak musculature. In the last ten years, a number of upper-arm training devices have emerged. However, due to their size and weight, their use is restricted to clinics and research laboratories. Our proposed wearable exoskeleton builds upon our extensive research experience in wire driven manipulators and design of rehabilitative systems. The exoskeleton consists of three main parts: (i) an inverted U-shaped cuff that rests on the shoulder, (ii) a cuff on the upper arm, and (iii) a cuff on the forearm. Six motors, mounted on the shoulder cuff, drive the cuffs on the upper arm and forearm, using cables. In order to assess the performance of this exoskeleton, prior to use on humans, a laboratory test-bed has been developed where this exoskeleton is mounted on a model skeleton, instrumented with sensors to measure joint angles and transmitted forces to the shoulder. This paper describes design details of the exoskeleton and addresses the key issue of parameter optimization to achieve useful workspace based on kinematic and kinetic models.Copyright

-DOF Planar Manipulators Driven by 1 or 2 Actuators

For bipedal walking, a set of joint trajectories is acceptable as long as it satisfies certain overall motion requirements, such as: 1) it is repetitive (limit cycles); 2) it allows the foot to clear ground; and 3) it allows the biped to move forward. Since the actual trajectory followed by a biped is not as important, a biped having some unactuated joints can also meet these motion requirements. Furthermore, due to physical constraints, a biped cannot have an actuator between the foot and the ground. Hence, it is underactuated during the phase when the foot is rolling on the ground. Besides underactuation, a bipedal robot has nonlinear dynamics and impacts. In general, it is difficult to prove existence of limit cycles for such systems. In this paper, a design methodology that renders planar bipedal robots differentially flat is presented. Differential flatness allows generation of parameterized limit cycles for this class of planar nonlinear underactuated bipeds. Sequential quadratic-programming-based numerical optimization routines are used to optimize these limit cycles while satisfying the motion constraints. The planning and control methodology is illustrated by a two-link biped.