Pneumatic-Delay-Compensated Stiffness Rendering Using a Fusion Hybrid Linear Actuator for Light-Contact Oscillation Reduction
Mechanical compliance is essential for stable and safe human–machine interaction in force-rendering interfaces. Instability is particularly problematic when a low-inertia, highly backdrivable actuator with intrinsic delay interacts with a human or compliant environment. Pneumatic actuators offer a high power-to-weight ratio, low moving mass, and inherent backdrivability; however, their pressure dynamics introduce delays that can destabilize low-damping, compliant contacts. This study examines delay-induced oscillations during virtual-spring rendering using a pneumatic–electromagnetic fusion hybrid linear actuator (FHLA) with distributed macro–mini actuation. We demonstrate that, even under light-contact conditions, cross-coupling between the rendered virtual spring and physical compliance forms a feedback loop susceptible to oscillation in delayed, low-impedance systems. To enable this actuation in the integrated FHLA—where only the combined output force is measurable—the pneumatic force component is estimated from pressure measurements. Experiments with 10mm displacement and 2–4N/mm virtual stiffness under light-contact conditions demonstrate a reduction of more than 99% in oscillation amplitude compared with pneumatic-only actuation. These results highlight enhanced force-rendering behavior through delay-compensated hybrid actuation.