Distal End Force Estimation of Tendon-sheath Mechanism Using a Spring Sheath

Tendon-sheath mechanism (TSM) is often used as a power transmission system in many applications, such as rehabilitation, wearables, and endoscopic surgery robots. It comprises a tendon for transmitting the power and a sheath to guide the tendon. The tendon passes through a narrow and harsh pathway to actuate an end-effector. Consequently, TSM exhibits highly nonlinear force transmission characteristics, resulting in poor control performance. Additionally, attaching and using a force feedback sensor at its distal end is virtually impossible in most applications owing to space constraints and safety concerns. To address these drawbacks, this study proposes a novel sensing spring wire to estimate the distal end force in TSM. Unlike existing methods, the proposed technique does not require complex hyperparameters or modeling and uses only a mechanical part to predict the force. The sensing wire is composed of the sheath. The biocompatibility, simple structure, and small size of the spring sheath facilitate convenient interaction with the existing TSM. Additionally, the spring sheath exhibits the characteristics of a spring, which ensures a linear relationship between the displacement and force. The results indicate that after calibrating the spring sensor, the distal end force can be easily determined using the proximal force despite the absence of an electrical sensor. In situations where the friction is different for each shape, when the force at the distal end was estimated using only the force at the proximal end measured by the sensor, the average RMSE was 75.91 mN and the maximum error was 176.52 mN . The proposed method verifies that the proximal tension and displacement data are sufficient to estimate the force.