Variable tensile stiffness pneumatic actuators with adjustable stick-slip friction of soft-tooth structures

• Axial tensile stiffness adjustment with stick–slip friction of two soft-tooth structures. • Integration of actuation, resistance, and sensing. • Capability of the pneumatic actuator for whole-stage muscle strength rehabilitation. Patients with hemiplegia, neurologic injuries, and sports trauma require muscle strength rehabilitation training, the existing rigid equipment is uncomfortable and bulky, and wearable training devices are not suitable for the whole-stage of passive, assistant, active and resistance rehabilitation training, and are difficult to provide comprehensive and real-time feedback on the current rehabilitation level. This paper proposes a multi-layer cylindric soft actuator with axial tensile stiffness variation, and pneumatic control is utilized to achieve axial elongation and resistance adjustment. The resistance is changed through the stick–slip friction of soft structure during tooth structures engagement and sliding, and the tensile stiffness of the soft actuator can be changed by 7.2 times with air pressure increased by only 20 kPa. Capacitive strain sensing is used to obtain elongation and stiffness feedback, and the closed-loop control errors for axial elongation and stiffness variation are only 2.02 % and 1.20 %, respectively. Finally, an application of elbow joint rehabilitation training demonstrates that the proposed variable stiffness actuator is feasible for whole-stage training and providing feedback on joint angle amplitude and strength.