A Method of Measuring Elasticity using the Step-out Phenomenon of a Stepper Motor

Toshikazu KAWAI, Kensuke NISHIO, Shota MIZUNO, Yusuke MORITA, Yuji NISHIZAWA, Tatsuo NAKAMURA
Vol. 3 (2014) p. 14-20

Knowing the mechanical characteristics of organs during surgery ensures effective diagnosis and treatment. A surgeon’s hand can sense the characteristics of organs in open abdominal or chest surgery. However, this sensing ability is reduced in endoscopic surgery, and it is completely lacking in robot-assisted endoscopic surgery. A surgical manipulator with a sensing function should have an autoclavable mechanism, and should be simple to control. In this paper, we propose a method of measuring elasticity using the step-out phenomenon of a stepper motor. Prototype devices were constructed and tested on silicone rubber test pieces and on organs both in vitro and in vivo.The devices were then calibrated using a material testing machine. Elasticity was defined in terms of the Young’s modulus Eor the spring constant K, which was calculated based on the force applied by the motor, the length of the arm attached to it, and the angular displacement of the arm when step-out occurred. Three different prototype devices were constructed to evaluate the effectiveness of the proposed method. All devices consisted of a stepper motor, a rotary encoder, and an arm. To measure silicone rubber objects, the arm was a simple bar. To measure organs in vitro, a bevel-geared jaw was used, and to measure organs in vivo, a long linkage-type forceps was employed. Each prototype had a resolution of approximately 1 ┬Ám. The elasticity values determined using these devices were compared with those measured using a material testing machine. For silicone rubber objects and organs in vitro, a good correlation was found between the elasticity values determined using the prototype devices and the material testing machine. In the in vivo experiment, the prototype device was capable of distinguishing differences between organs. This paper demonstrates the possibility of replacing a force sensor by using the proposed stepper motor-based method for measuring organ properties.