Supernumerary Robotic Limbs (SRLs) have been successfully applied in bracing and as an assistive technology for people with disabilities. These tasks only require perception internal to the SRL-human system. However, SRLs show promise in applications requiring external perception such as opening a door when one’s hands are full. One path toward developing SRLs that accomplish these tasks is to use human-in-the-loop control, thus leveraging the human’s superior perception system to help the SRLs. However, the effects on the user of controlling additional limbs are unclear. This article presents an experimental study where humans, wearing two single degree of freedom SRLs, were instructed to minimize the position error between the subject’s natural and robotic limbs and the corresponding targets, one for each limb. First, subjects performed worse with their natural limbs when asked to perform the task with two natural and two robotic limbs as opposed to with just their natural limbs, suggesting that shared control could help. Second, subjects moved their natural limbs together followed by moving their SRLs together. This informs both the choice of control scheme for the SRLs and the division of labor within a task. Third, subjects showed significant concurrent use of the natural and robotic limbs.

Most of the wearable robots today assist their users by acting in parallel or in series to their natural limbs. We propose a different approach to wearable robotics, consisting of devices that provide users with additional, independent robotic limbs. We present a wearable robot prototype that can achieve these goals with an extremely light weight apparatus. In order to control additional robotic limbs as if they were part of the user's body, we need voluntary signals that are independent of natural limb motions and comfortable to measure. One suitable solution - explored in this study - is the use of muscle activation signals generated by the torso. We hypothesize that a human is competent to move the extra limbs voluntarily and independently without interfering with the natural arms and legs. We developed a wearable suit to measure these signals, and we tested three possible real-time control strategies linking torso muscle contraction to the motions of two simulated extra limbs. The experimental data show that the velocity control strategy yields the highest motion accuracy, minimum muscular effort, maximum independence from the natural limbs and the fastest learning rate. This control strategy has then been applied to the control of the physical robot prototype, worn by human subjects. All of the subjects achieved accurate (normalized tracking error <; 0.5), independent (normalized natural arm motions <; 0.15) control of the extra limbs.

A robot attached to a human body can support the human when performing tasks in dangerous environments or when taking postures that are fatiguing and uncomfortable. This paper presents a new type of supernumerary robotic limbs (SRL) that supports the human body against floors, walls, and surrounding structures so that the human can perform a task safely, comfortably, and stably. First, the use of wearable robots for securing and supporting a human body is discussed, followed by an SRL design concept. The SRL is worn around the waist and can brace the human body by making contact with a wall, grasping a rail, or being anchored to the floor. Quasi-static stability and compliance with which the body is supported are analyzed. Two control methods for stabilizing the body support system are considered: one is null-space stabilization using Hessian matrices, and the other is joint servo stiffness, based on the Jacobian. A prototype robot is designed and tested. Potential applications of the SRL in diverse fields are discussed.

This paper presents a novel approach to balance assistance and joint load reduction for human bipedal walking. We introduce a new type of wearable robot, called Supernumerary Robotic Limbs (SRL), that provides two additional legs for augmenting stability and reducing the loads on human leg joints. Unlike exoskeletons, the SRL is kinematically independent of the human skeletal structure, and can therefore take an arbitrary posture to provide optimal assistance in coordination with human motions. Furthermore, unlike crutches, canes, and other balance assistance equipment, the SRL can provide balancing support autonomously and thereby free the human arms from holding those tools. First, the new design concept and balance assistance strategy are described, followed by kinematic and static modeling. Two gate patterns of the combined human and SRL are discussed. Optimal gate synthesis that maximizes the area of support polygon is discussed. Finally, the gate control strategies are implemented on a prototype SRL, using body motion sensors to enable real-time, seamless coordination between the user and the robot.

A new type of wearable robot that assists the wearer with extra arms secured around the hips is presented. Supernumerary Robotic Limbs (SRL) can hold objects, clamp them to a fixture, guide and support human hands, and assist the wearer in performing a task as a close co-worker. This paper focuses on a class of tasks where SRL physically interacts with the environment through contact. SRL makes contact with a wall and thereby braces the human body against the environment. SRL also guides the human hands by placing a drill jig over the drilling location. Bracing the human body and guiding the hands, SRL can enhance the drilling task stability and accuracy. The SRL technology is applied to aircraft assembly, where conventional industrial robots failed to perform effectively. First, the basic design concept of SRL is summarized, and task strategies using SRL and their functional requirements are described. Kinematic and static properties resulting from the structural closed loops formed around the SRL, the human, and the environment are analyzed, and effective strategies for physical disturbance rejection and fine positioning are discussed. A prototype robotic arm grasps the aircraft fuselage structure. Another robotic arm places a drill jig precisely on the fuselage structure, and guides and stabilizes a hand drill held by the human user. An optimization method is developed in order to identify the SRL kinematic configuration and joint torques that stabilize the drill and at the same time minimize the human workload.

A new approach to physically assisting the human with a wearable robot is presented. Supernumerary Robotic Limbs (SRLs) attached to the human waist support the body efficiently when the human is taking fatiguing postures, e.g. hunching over, squatting, or reaching the ceiling. Unlike a leg exoskeleton, where powered joints are attached to the human joints and are constrained to move together with the human limb, the SRL can take an arbitrary posture to maximize the load bearing efficiency. Taking a near-singular configuration, the SRL can bear a large load with small power consumption. First, the “bracing” strategy for supporting the human body is described, followed by a mathematical analysis of the load bearing efficiency. The optimal SRL posture and joint torques are then obtained in order to minimize the human load. Numerical and experimental results using a prototype of the SRL demonstrate the effectiveness of the method.

We present the Supernumerary Robotic Limbs (SRL), a wearable robot designed to assist human workers with additional arms and legs attached to the wearer's body. The SRL can work closely with the wearer by holding an object, positioning a workpiece, operating a powered tool, securing the human body, and more. Although the SRL has the potential to provide the wearer with greater strength, higher accuracy, flexibility, and dexterity, its control performance is hindered by unpredictable disturbances due to involuntary motions of the wearer, which include postural sway and physiological tremor. This paper presents 1) a Kalman filter approach to estimate the state of the SRL despite the involuntary wearer's motion, and 2) a method for improving the accuracy and stabilizing the human body and the SRL. The dynamics of the human-SRL system are analyzed, including human-induced disturbance models based on biomechanics literature. A discrete Kalman filter is constructed and its performance is evaluated in terms of error covariance. A “bracing” technique is then introduced to suppress the human-induced disturbances; one robotic limb grasps an environment structure and uses it as a support to attenuate the disturbances. We show how bracing can be used to shape the stiffness parameters at the robot base. This in turn allows to enhance state estimation accuracy in the areas of the workspace where the user needs assistance.

The body representation in the human mind is dynamic, and illusions or traumatic events can modify it to include additional limbs. This remarkable adaptability of the central nervous system to different body configurations opens new possibilities in the field of human augmentation. In order to fully exploit this potential, we developed a new type of wearable co-robot that can perform tasks in close coordination with the human user. The system, named Supernumerary Robotic Limbs (SRL), consists of two additional robotic arms worn through a backpack-like harness. The SRL can assist the user by holding objects, lifting weights and streamlining the execution of a task. If the SRL perform movements closely coordinated with the user and exhibit human-like dynamics, they might be incorporated into the body representation and perceived as parts of the user's body. As a result, the human would be able to extend the range of available skills and manipulation possibilities, performing tasks more effectively and with less effort. This paper presents a communication, estimation and control method for the SRL, aimed to perform tasks in tight coordination with the wearer. The SRL observes the user motion, and actively assists the human by employing a coordinated control algorithm. In particular, skills involving the direct cooperation of two human workers are transferred to the SRL and a single user. Demonstration data of the two humans - a leader and an assistant - are analyzed and a state estimation algorithm is extracted from them. This can be used to control the SRL accordingly with the used end effectors. A causal relationship relating the assistant's motion to the leader's motion is identified based on System Identification methods. This approach is applied to a drilling operation performed by two workers. An effective coordination skill is identified and transferred to the SRL, to make them act like the human follower.

Dynamic balance depends on proper foot placement in legged locomotion and corresponding placement strategies have mainly been developed using the linear inverted pendulum model as theoretical framework. While this model can identify single leg strategies for balance control, it does not consider the double support that is common to bipedal locomotion, indicating that current strategies do not fully exploit the theoretical potential of balance control in bipedal systems. Here we extend the linear inverted pendulum model to a bipedal system which includes double support dynamics, and derive a reactive balance controller based on foot placement and double stance length. We show that this controller enables the model to stand and walk at user-defined target speeds, to transition between these behaviors by acceleration and deceleration, and to react to intermittent disturbances and compensate for permanent ones, as long as they are compatible with the swing leg dynamics placing the feet. Finally, we discuss how the versatility of this balance controller depends on including double support and suggest further steps to improve dynamic balance control in bipedal systems.