Robotic systems assess joint dynamics objectively by perturbing the limb and estimating properties such as impedance. Position perturbations constrain the limb to a target trajectory, reducing variability in task execution but obstructing voluntary motion. Force perturbations allow voluntary movement but elicit orientation-dependent responses, increasing the number of trials needed for accurate estimates. To overcome these limitations, we combined the flexibility of admittance control with the repeatability of position perturbations. A minimum-jerk trajectory ensures smooth transitions. The experiment with six healthy participants was performed to demonstrate the reliability, accuracy and smoothness of applying such perturbations during voluntary movement. Reliability was the proportion of perturbations that reached the target velocity within one millisecond of the acceleration time window. Accuracy was measured as the RMSE between the target and measured velocity during the constant velocity. Smoothness was assessed as perceivability: the fraction of trials in which participants correctly detected a perturbation. The controller allows continuous voluntary movement, switching only during perturbations to impose a precise, specified perturbation. All perturbations reached the target velocity within one millisecond of the acceleration time window; thus, the method is reliable. Under the most demanding condition— an increase to 200 deg/s in 0.01 s—the RMSE between target and measured velocity was 1.1 deg/s (0.55%), indicating a high accuracy. Specially designed perturbations had a perceivability accuracy of 22.1%, indicating smooth transitions between control modes. Together, these results indicate a promising approach for assessing joint dynamics during voluntary elbow movement, enabling assessment during activities of daily living.

To increase the quality of life of stroke patients, better diagnostics with the ability to identify the cause of motor impairment are needed. Robotic diagnostics increases the resolution of measurements, allows for tracking progress over a longer period, and can be used to evaluate new treatments. The Shoulder Elbow Perturbator (SEP) was developed to improve the diagnostics of post-stroke motor impairment. The SEP has already been tested on patients, showing promising results in identifying the cause of motor impairment, but no SEP system performance analysis has been published. To identify the joint properties of the elbow accurately, the SEP should have a bandwidth of at least 12 Hz. Furthermore, admittance and velocity control are required for various possible experimental tasks. This paper shows that the SEP performs adequately for the desired perturbations and experimental conditions for system identification of the human elbow. The SEP's performance is analysed with multisine signals to determine the bandwidth and endpoint dynamics. The velocity controller bandwidth is 50 Hz, and the admittance controller bandwidth is 65 Hz. Furthermore, the controller is stable. Thus, the SEP meets all the requirements and should be able to provide the desired perturbations and experimental conditions needed for system identification of the human elbow.