Blocked actuation of twisted and coiled tube-based polymer actuators driven by hydraulic pressure

Twisted and coiled polymer actuators (TCPAs) offer promising capabilities for compact actuation systems, yet their dynamic performance under constrained conditions remains poorly understood. This study investigates the blocked actuation behavior of hydraulically driven polyvinyl chloride (PVC) tube-based TCPAs subjected to pre-strain. A custom-built test platform enabled high-resolution characterization over 100 actuation cycles under cyclic hydraulic loading (0–1.2 MPa), revealing significant time-dependent and nonlinear effects of the tube-based TCPAs. A viscoelastic hysteresis model based on the generalized Maxwell framework is developed to capture their dynamic response, showing good agreement with experimental results. It is found that cyclic preconditioning reduces the activation pressure threshold and improves the repeatability of actuation. The blocked torque response increases rapidly with the spring index before gradually declining, while the force output improves consistently with greater pre-strain. A lower spring index, combined with higher pre-strains, yields an enhanced actuation capacity, whereas a larger spring index with limited pre-strain suppresses performance. Additionally, higher pressurization rates mitigate hysteresis and boost actuation efficiency. These findings advance the understanding of constrained-state performance in tube-based TCPAs.