Modeling of a closed loop cable-conduit transmission system

Many surgical robots use cable-conduit pairs in a pull-pull configuration to actuate the instruments and transmit power into the patient´s body. Friction between the cable and the conduit makes the system nonlinear and accounts for major losses in tension transmission across the cable. This paper proposes an analytical model for a similar cable-conduit system and formulates the load transmission characteristics. The dynamic model uses discrete elements with friction losses and cable stretch calculated for each of the segments. The simulations predict backlash, cable slacking, and other nonlinear behavior. These results are verified with an experiment using two DC motors coupled with a cable-conduit pair. The drive motor is run in position control mode, while the load motor simulates a passive environment torsional spring. Experimental results are compared with the simulation and various implications are discussed.