Role of Speed Regulation and Speed Modulation in Velocity-Field Based Control

The velocity vector field (flow) controller is a well-established control strategy for lower limb exoskeletons. In this paper, we analyze this controller and propose modifications to improve its performance. We demonstrate that flow control acts as a variable proportional-derivative error regulator, where the parameter {Gamma} represents the desired norm of the hip-knee joint velocity vector (path speed). Based on this, we introduce two modifications to {Gamma}: (1) a constant {Gamma} set to the mean desired path speed, and (2) a variable {Gamma} that mimics natural path speed during unassisted walking. We compared the modified flow controllers with a slow-{Gamma} version in experiments involving seven participants walking on a treadmill at 0.6m/s, 0.8m/s, and 1.0m/s. Compared to the slow-{Gamma} controller, the RMS tracking error decreased by 30.7{+/-} 11.3% and the range of motion of the knee increased by 48.2 {+/-} 5.5% for the mean-{Gamma} controller, while the variable-{Gamma} controller had 32.4 {+/-} 14.7% smaller RMS error and 50.5 {+/-} 6.5% larger range of motion of the knee. Additionally, the slow-{Gamma} controller consistently applied resistive power, whereas participants reported more comfortable and natural gait with the modified controllers. We also compared them with the original tuning of flow controller, with results indicating superior performance from the proposed modifications. These findings demonstrate effectiveness across different walking speeds and offer a tuning strategy for future flow controller use.