Strain gauges attached to the driving pulleys of the instrument of surgical robots allow estimation of the torque between the instrument and environment. Transmission of torque applied to the instrument tip is analyzed, and the result confirms that the friction and the gap existing between the driving pulleys and the torque sensor are significant factors inducing estimation error. Characteristics of the friction in each degree-of-freedom also vary.
This paper attempts to provide better estimation of the interaction force and torque between the instrument and environment in multi-degree-of-freedom motion. The gap is eliminated by incorporating the torque sensor into a single part. This torque sensor is designed base on the beam theory. Performance test of designed torque sensor such as linearity, hysteresis is carried out. The characteristics of the friction are analyzed by comparing the torque estimated from the strain gauge and the torque actually measured at the instrument tip. It can be confirmed the direction of the wrist motion should be considered to estimate grip torque in the multi-degree-of-freedom motion. A friction model is developed based on Dahl model and reflects the characteristics of the particular driving mechanism.
Contact experiments using springs that simulate the elasticity of a tissue interaction are performed to verify the accuracy of the proposed model. The each pulley is driven to motion of a sine wave in single-degree-of-freedom motion. The results show relative error of roll, wrist, right grip, left grip torque can be reduced to 8.43%, 1.89%, 3.49%, 1.37%, respectively. The pulleys corresponding to wrist and right grip motion are driven to the motion that mixed with sine, constant, linear, quadratic, and logarithmic functions in multi-degree-of-freedom motion The results show relative error of wrist and right grip torque can be reduced to 1.34%, 2.34%, respectively.