Antagonistic twisted string actuation for disposable flexible medical robots

For flexible cable-driven disposable medical robots (endorobots) to be controlled efficiently, quickly, and safely, an actuation unit that is simple, cost-effective, and space-saving is required. A twisted string actuator (TSA) meets these requirements since it is a small, strong, lightweight, and low-cost gear that converts a rotary motion into a linear pulling motion. As the TSA only provides tensile forces, two antagonistically operating TSAs are required for each degree of freedom to bend and stretch an endorobot. This paper presents a first prototype of a cable-driven endorobot with a gripper to retract tissue for creating a sufficient workspace. In two vertical planes, the instrument is bent using four TSAs. A first control approach for moving the instrument shows a sufficient antagonistic principle. The position of the TSA slider is measured with a potentiometer. This position information is used to set a P-type controller so that two TSAs move in opposite directions and the instrument can be moved. The nominal torque of the motor is 15.3 mNm and the initial length is 300 mm of the Ø 0.25 mm Dyneema strings. Therefore, the TSA is capable of applying a force between 318 N and 462 N in the intended working area of a contraction of 10 mm to 20 mm. To allow a complete movement of the instrument, the TSA contracts 10 mm in approx. 4 seconds. According to the first setup, antagonistic twisted string actuation is acceptable for use in endorobotics. However, in the future more sophisticated control approaches will be necessary to account for the TSA's nonlinear behavior and miniaturization of the actuator unit.