Abbas Fattah

Gravity-Balancing Leg Orthosis and Its Performance Evaluation

In this paper, we propose a device to assist persons with hemiparesis to walk by reducing or eliminating the effects of gravity. The design of the device includes the following features: 1) it is passive, i.e., it does not include motors or actuators, but is only composed of links and springs; 2) it is safe and has a simple patient-machine interface to accommodate variability in geometry and inertia of the subjects. A number of methods have been proposed in the literature to gravity-balance a machine. Here, we use a hybrid method to achieve gravity balancing of a human leg over its range of motion. In the hybrid method, a mechanism is used to first locate the center of mass of the human limb and the orthosis. Springs are then added so that the system is gravity-balanced in every configuration. For a quantitative evaluation of the performance of the device, electromyographic (EMG) data of the key muscles, involved in the motion of the leg, were collected and analyzed. Further experiments involving leg-raising and walking tasks were performed, where data from encoders and force-torque sensors were used to compute joint torques. These experiments were performed on five healthy subjects and a stroke patient. The results showed that the EMG activity from the rectus femoris and hamstring muscles with the device was reduced by 75%, during static hip and knee flexion, respectively. For leg-raising tasks, the average torque for static positioning was reduced by 66.8% at the hip joint and 47.3% at the knee joint; however, if we include the transient portion of the leg-raising task, the average torque at the hip was reduced by 61.3%, and at the knee was increased by 2.7% at the knee joints. In the walking experiment, there was a positive impact on the range of movement at the hip and knee joints, especially for the stroke patient: the range of movement increased by 45% at the hip joint and by 85% at the knee joint. We believe that this orthosis can be potentially used to design rehabilitation protocols for patients with stroke

On the design of cable-suspended planar parallel robots

In this paper we present a workspace analysis methodology that can be applied for optimal design of cable-suspended planar parallel robots. The significant difference between regular parallel robots and cable-suspended parallel robots is that the cables in cable-suspended robots can only carry tension forces. The workspace of a planar cable robot is characterized as the set of points where a reference point of moving platform can reach with tensions in all suspension cables. In the design of cable-suspended parallel robots, the suspension points of the cables, size and shape of the moving platform are the design variables. The workspace area and global condition index are used as the objective functions to optimize the design parameters. The global condition index is a measure of isotropicity of the manipulator. The design variables are determined for different numbers of cables using both objective functions at a specified orientation and also at different orientations of moving platform. Experimental results to measure the workspace area demonstrate the effectiveness of this method.

Theory and design of an orthotic device for full or partial gravity-balancing of a human leg during motion

Gravity balancing is often used in industrial machines to decrease the required actuator efforts during motion. In the literature, a number of methods have been proposed for gravity balancing that include counterweights, springs, and auxiliary parallelograms that determine the center of mass. However, these concepts have not yet been seriously applied to rehabilitation machines. This paper presents the underlying theory and design of an orthosis for the human leg that can fully or partially balance the human leg over the range of its motion. This design combines the use of auxiliary parallelograms to determine the center of mass along with springs to achieve a full or partial gravity balanced orthosis design. A first prototype has been constructed to demonstrate the effectiveness of the idea. Future prototypes will have parameters that will be tuned to the geometry and inertia of a human subject and be tailored to an individuals needs.

Design of a Passive Gravity-Balanced Assistive Device for Sit-to-Stand Tasks

A sit-to-stand assist device can serve the needs of people suffering from muscle weakness due to age or disabilities that make sit-to-stand a difficult functional task. The objective of this paper is to design a passive gravity-balancing assist device for sit-to-stand motion. In our study, it has been shown that the contribution to the joint torques by the gravitational torque is dominant during sit-to-stand motion. On the basis of this result, a gravity balanced assistive device is proposed. This passive device uses a hybrid method to identify the center-of-mass of the system using auxiliary parallelograms first. Next, appropriate springs are connected to the device to make the total potential energy of the system due to the gravity and the springs constant during standing up. A demonstration prototype with the underlying principles was fabricated to test the feasibility of the proposed design. The prototype showed gravity balancing and was tested by the authors. This prototype will be modified appropriately for clinical testing.

Workspace and Design Analysis of Cable-Suspended Planar Parallel Robots

In this paper, we present a method to obtain the workspace of cable-suspended planar parallel robots. The significant difference between regular parallel robots and cable-suspended parallel robots is that the cables in cable-suspended robots can only carry tension forces. Hence, static equilibrium equations must be used to compute the forces in the cable robot. The workspace is characterized as the set of points where the reference point on the end-effector platform can reach with tensions in the cables. In this paper, we address this problem for robots with two, three, or more cables. Then, we study the question of design of the cable robot to determine the number of cables, location of the suspension points of the cables, and size of the moving platform to attain the largest workspace.Copyright

Design and workspace analysis of a 6-6 cable-suspended parallel robot

In this paper, we study the design and workspace of a 6-6 cable-suspended parallel robot. The workspace volume is characterized as the set of points where the centroid of the MP (MP) can reach with tensions in all suspension cables at a constant orientation. This paper attempts to tackle some aspects of optimal design of a 6DOF cable robot by addressing the variations of the workspace volume and the accuracy of the robot using different geometric configurations, different sizes and orientations of the MP. The global condition index is used as a performance index of a robot with respect to the force and velocity transmission over the whole workspace. The results are used for design analysis of the cable-robot for a specific motion of the MP.

A gravity balancing leg orthosis for robotic rehabilitation

A number of methods can be found in the literature to gravity balance a machine. In this paper we use hybrid method to achieve gravity balancing. Hybrid method employs a mechanism to locate center of mass of the robot in conjunction with springs. This method is used to develop a rehabilitation device, which compensates the effect of gravity on a human leg. For a quantitative evaluation of the performance of the device, electromyograph data of the muscles involved in the motion of leg were collected and analyzed. This data showed that the machine could be used for gravity balancing of the leg and could be potentially used for rehabilitation of patients.

Gravity-balancing of classes of industrial robots

Gravity balancing of industrial robots is an important issue because these robots may have massive links in order to manipulate large payloads. In this paper, we present techniques to gravity balance the weight of moving links in industrial robots with various arrangements of joints. In a typical industrial robot, the first joint axis is parallel to the gravity vector, hence, the weight of the first moving link does not have any effect on gravity balancing of the robot. Our approach for gravity balancing of industrial robots uses two steps: (i) we locate the center of mass of distal segments of the robot using auxiliary parallelograms; (ii) we connect springs between the center of mass and other members in the robot such that the total potential energy of the system is invariant with configuration. In this paper, we present designs for gravity balancing of classes of industrial robots with revolute and prismatic joints

Reactionless space and ground robots: novel designs and concept studies

Abstract For conventional designs of space and earth robots, manipulator motions result in forces and moments on the base. These forces and moments cause translation and rotation of the free-floating base of a space robot. For earth robots, the same forces and moments get transmitted to the base and may cause undesirable base excitations. The objective of this paper is to systematically analyze the fundamentals of reactionless space and earth robots. Based on this analysis, design of two distinct classes of planar robots is proposed, with appropriate choices of geometric and inertial parameters. Due to underlying principle of conservation of angular momentum for these special robots, the trajectory must satisfy additional constraints. We illustrate the reactionless feature of these robots and trajectory planning of these robots through computer simulations. Currently, we are fabricating such reactionless robots to illustrate the underlying concepts.

Design of differentially flat planar space robots: a step forward in their planning and control

The motion of free-floating space robots is characterized by nonholonomic, i.e., non-integrable rate constraint equations. These constraints originate from principles of conservation of linear and angular momentum. It is well known that these rate constraints can also be written as input-affine drift-less control systems. Trajectory planning of these systems is extremely challenging and computation intensive since the motion must satisfy differential constraints. However, under certain conditions, these drift-less control systems can be shown to be differentially flat. The property of flatness allows a computationally in-expensive way to plan trajectories for the dynamic system between two configurations as well as develop feedback controllers. Nonholonomic rate constraints for free-floating planar open-chain robots are systematically studied to determine the design conditions under which the system exhibits differential flatness. Under these design conditions, the property of flatness is used for trajectory planning and feedback control under perturbations in the initial state.