Technologies

The presented dielectric elastomer system (DES) variable stiffness actuator (VSA) is designed to address the commercial challenges of a powered ankle-foot orthosis (PAFO) by optimizing peak power reduction in a compact device form factor. Fixed-stiffness, series elastic actuators have been used in PAFOs to reduce peak motor power, but their fixed stiffness values only yield optimal power reduction for one set of gait parameters. Since gait type, user weight, and gait speed are all parameters that vary during normal use of orthotic devices, VSA technology is desirable to compensate for these variations. State-of-the-art VSAs often have a bulky variable stiffness mechanism and an additional motor to control it. These extra components increase the cost, weight, and size of the actuator, and decrease its durability. Our design uses a DES to modulate actuator stiffness via changes in the electric field, thereby eliminating the requirement for a second motor and reducing the overall weight of the device. This innovation enables lighter, faster, and simpler stiffness modulation compared to traditional methods, allowing for improved portability and compliance with the user.

Figure 1: Schematic Representation of DES VSA Design.An electric motor drives a ball screw causing the input clamp to translate in the x-direction. The input clamp is elastically connected to the output clamp through two sets of dielectric elastomers (DE, represented as springs). The load (F) is applied to the output clamp.

Technical Summary:

The described design implements DE as series elastic elements, so that actuator stiffness can be modulated via changes in electric field. The prototype is designed to supplement human ankle torque during both normal and fast walking. The configuration of the DES could allow for regeneration of energy during cyclic operations, such as walking. Furthermore, the system can act as a strain sensor that measures force output of the actuator without requiring additional position or force sensors. The DES VSA actuated joint is back-drivable and capable of shock absorption, providing a safer human-machine interface than rigid orthosis and prosthesis actuators. It also provides stiffness modulation with less mechanical complexity than state-of-the-art VSAs.