Series Elastic Actuator Design Research Articles

A convex optimization framework for robust-feasible series elastic actuators

Kinematic and kinetic requirements for robotic actuators are subject to uncertainty in the motion of the load. Safety factors account for uncertainty in the design stage, but defining factors that translate to reliable systems without over-designing is a challenge. Bulky or heavy actuators resulting from overdesign are undesirable in wearable or mobile robots, which are prone to uncertainty in the load due to human–robot or robot–environment interaction. In this paper, we use robust optimization to account for uncertainty in the design of series elastic actuators. We formulate a robust-feasible convex optimization program to select the optimal compliance–elongation profile of the series spring that minimizes one or multiple of the following objectives: spring elongation, motor energy consumption, motor torque, or motor velocity. To preserve convexity when minimizing energy consumption, we lump the energy losses in the transmission as viscous friction losses, which is a viable approximation for series elastic actuators powered by direct or quasi-direct drives. Our formulation guarantees that the motor torque, winding temperature, and speed are feasible despite uncertainty in the load kinematics, kinetics, or manufacturing of the spring. The globally optimal spring could be linear or nonlinear. As simulation case studies, we design the optimal compliance–elongation profiles for multiple series springs for a robotic prosthetic ankle. The simulation case studies illustrate examples of our methodology, evaluate the performance of robust feasible designs against optimal solutions that neglect uncertainty, and provide insight into the selection of different objective functions. With this framework the designer specifies uncertainty directly in the optimization and over the specific kinematics, kinetics, or manufacturing parameters, aiming for reliable robots that reduce overdesign.

A SERIES ELASTIC ACTUATOR DESIGN AND CONTROL IN A LINKAGE BASED HAND EXOSKELETON.

This paper presents the design of a series elastic actuator and a higher level controller for said actuator to assist the motion of a user's hand in a linkage based hand exoskeleton. While recent trends in the development of exoskeleton gloves has been to exploit the advantages of soft actuators, their size and power requirements limit their adoption. On the other hand, a series elastic actuator can provide compliant assistance to the wearer while remaining compact and lightweight. Furthermore, the linkage based mechanism integrated with the SEA offers repeatability and accuracy to the hand exoskeleton. By measuring the user's motion intention through compression of the elastic elements in the actuator, a virtual dynamic system can be utilized that assists the users in performing the desired motion while ensuring the motion stability of the overall system. This work describes the detailed design of the actuator followed by performance tests using a simple PD controller on the integrated robotic exoskeleton prototype. The performance of the proposed high level controller is tested using the integrated exoskeleton glove mechanism for a single finger, using two types of input motion. Preliminary results are discussed as well as plans for integrating the proposed actuator and high level controller into a complete hand exoskeleton prototype to perform intelligent grasping.

Robust Optimal Design of Energy Efficient Series Elastic Actuators: Application to a Powered Prosthetic Ankle.

Design of rehabilitation and physical assistance robots that work safely and efficiently despite uncertain operational conditions remains an important challenge. Current methods for the design of energy efficient series elastic actuators use an optimization formulation that typically assumes known operational requirements. This approach could lead to actuators that cannot satisfy elongation, speed, or torque requirements when the operation deviates from nominal conditions. Addressing this gap, we propose a convex optimization formulation to design the stiffness of series elastic actuators to minimize energy consumption and satisfy actuator constraints despite uncertainty due to manufacturing of the spring, unmodeled dynamics, efficiency of the transmission, and the kinematics and kinetics of the load. To achieve convexity, we write energy consumption as a scalar convex-quadratic function of compliance. As actuator constraints, we consider peak motor torque, peak motor velocity, limitations due to the speed-torque relationship of DC motors, and peak elongation of the spring. We apply our formulation to the robust design of a series elastic actuator for a powered prosthetic ankle. Our simulation results indicate that a small trade-off between energy efficiency and robustness is justified to design actuators that can operate with uncertainty.

Comparison of Two Series Elastic Actuator Designs Incorporated into a Shoulder Exoskeleton.

Low impedance and torque control are critical for movement rehabilitation using robotic exoskeletons. A grounded 3 degree of freedom shoulder exoskeleton was designed for movement assistance in shoulder abduction/adduction, flexion/extension, and shoulder internal/external rotation. Two series elastic actuators designs were developed using a linear spring arrangement with a global nonlinear stiffness behavior. RMS errors during application of constant torque were less than.06 Nm in shoulder add/abd and less than.04 Nm in arm rotation as the limb was moved in sinusoidal trajectories up to 3.5 Hz. For abd/adduction, the step response rise time was.05 s, and free mode impedance peaked at.007 Nm/deg during 3.5 Hz oscillations. For arm rotation, the step response rise time was.03 s, and impedance peaked at.023 Nm/deg during 3.5 Hz oscillations. Both SEA designs had performance measurements that were similar to other SEA designs in terms of torque tracking, but with much lower impedance than previously reported.

Variable Stiffness Series Elastic Actuator Design for Active Suspension

Variable Stiffness Series Elastic Actuator Design for Active Suspension

Biomimetic compliant lower limb exoskeleton (BioComEx) and its experimental evaluation

Exoskeleton robots are generally used for rehabilitation and load-carrying applications. Stable and flexible walking with minimal energy consumption of the human body is achieved by the compliant operation of the human joints. Essentially, the stiffness of the human ankle joint varies continuously, while the stiffness of the knee and hip joints remains nearly constant during loading phases of the walking cycles. With inspiration from the human leg biomechanics, a new biomimetic compliant lower limb exoskeleton robot (BioComEx) was constructed and its preliminary experimental tests were carried out in this paper. A variable stiffness actuator was developed for the ankle joint of BioComEx, and series elastic actuator designs were employed in the knee and hip joints of the robot. The joint actuators of BioComEx were designed longitudinally in order to be embedded inside of the thigh and shank leg segments for the compactness. Separate force sensors were employed in each segment of the robot for the modularity of the control strategies. Since the exoskeletons can be used for both load-carrying and rehabilitation, the developed robot was tested in both human-in-charge and robot-in-charge modes, respectively. The robot needs to mimic movement of healthy user joints in the human-in-charge mode and maximize the compliance between the user and robot; thus, the interaction forces should be reduced as possible. Hence, a modular closed-loop impedance control algorithm was developed for this mode. On the other hand, in the robot-in-charge mode, the robot joints need to track predefined gait references to make the patient walk. PID position control algorithm was chosen for preliminary test of the robot in this mode. Experimental results show that BioComEx is sufficiently satisfactory for walking applications of healthy and paralyzed users.