Design of a Series Elastic Actuator for an Assistive Exoskeleton using Numerical Methods and Gait Data

Actuators are essential components of powered exoskeletons and contribute significantly to the size and weight of the device. Optimal actuator design is essential to reach the power, weight and cost required for the task. Series Elastic Actuators (SEA) are a specialized type of actuator that assures safety and comfort in robotics systems that interact with the human body. SEA’s contain an elastic element placed in series with the motor. The spring, when characterized, provides a cost effective way of sensing the actuator torque or force, by measuring its deflection. In this article, a series elastic actuator is designed for a powered hip flexion-extension exoskeleton that intends to correct the pathological gait of children with cerebral palsy (CP). A comprehensive sizing method is presented for the main actuator components: the motor, the reduction gearbox and the spring, using numerical methods and gait data. This method can be applied to any joint for which kinematic data is available, as well as general mechatronic applications. The actuator contains a novel, machined helical spring. Its design and experimental characterization are discussed.