Design and statics model of an extensible hybrid-driven continuum robot with variable stiffness

Large workspace, variable stiffness, and accurate positioning are crucial indicators for enhancing the adaptability and reliability of continuum manipulators in operation tasks. Many structural design approaches only consider one of the extensible or variable stiffness features of continuum manipulators. The statics model needs to be constantly improved in accordance with the design structure to enhance position precision. This paper designed an extensible and variable stiffness continuum manipulator based on a helical structure combined with soft pneumatic actuators (SPAs) to improve both the ability of flexibility and payload capacity in confined environments. It is significant to remark that we have established a hybrid-driven statics model that considers the influence of tendon path, friction, and stiffness gain on the precision of the endpoint. Using the fourth-order Runge-Kutta method in conjunction with the shooting method, we have determined the shape of the manipulator based on a known value of the force sensor. The stiffness of the continuum manipulator is altered by generating antagonistic forces through pressurization of the soft actuators under the constraint of the driving tendons. Then, we constructed a prototype of an extensible and variable stiffness continuum manipulator to verify the accuracy of the statics model. Experimental results demonstrate a stiffness variation factor of 9 times, with a length range of 230 mm similar to 340 mm. The relative accuracy of the endpoint is 2.29 % calculated by the established statics model. The proposed design can lay the foundation for the execution of diverse tasks in a confined environment.