Design and Pilot Evaluation of an Ungrounded Force Perturbation Device for Elbow Joint Dynamics Assessment

Abstract

Stroke survivors often exhibit motor impairments, which hinder activities of daily living. While grounded robotic perturbation devices can accurately quantify joint dynamics via system identification, their size and fixed positioning limit functional assessments under realistic conditions. To address this gap, we present the design and pilot evaluation of a novel, wearable perturbation device capable of delivering ungrounded force perturbations to the user’s forearm.

The device uses a linear solenoid actuator housed in a wrist brace to generate short, pulse-type forces, thereby inducing small angular deflections (approximately 1–3°) to the arm. An inertial measurement unit (IMU) placed on the brace tracks the resulting movement, while an accelerometer on the solenoid coil measures the perturbation force. Nine healthy participants performed three tasks—relax, resist, and move. Random pulse signals were used to prevent anticipation of the perturbations. The device successfully deflected the arm in all tasks. The largest deflections was recorded during the relax task and smaller, though still measurable, deflections in the resist and move tasks.

Estimated stiffness values in each task indicated that the device could distinguish different levels of joint rigidity, although comparisons with established literature showed some over- or underestimation. Factors such as non-rigid brace attachment and off-center actuator placement contributed to these discrepancies. Despite these limitations, the prototype demonstrates the feasibility of wearable, ungrounded force perturbations for assessing elbow dynamics. Future work will focus on improving the device’s rigidity, exploring multi-degree-of-freedom perturbations, and refining stiffness estimation algorithms to better capture realistic joint behaviors.