Design of an external gravity compensator for a tendon-based serial mechanism

In this paper, the external gravitational compensation device of a tendon-based serial instrument was studied. Typically, a spring-based gravitational compensation device produces a compensation force that is in proportion to the angle of the link to which it wishes to perform the gravitational compensation. However, if several links are connected in a series, it is difficult to generate adequate compensation forces on all links because the orientation of gravity varies depending on the angle of the previous link, thereby rendering the necessary compensation forces variable. Therefore, in the case of a multi-axis serial mechanism, a gravitational compensation device using a parallelogram mechanism capable of transmitting a reference plane with respect to the direction of gravity has been widely used. A serial mechanism using a parallelogram mechanism for the gravitational compensation device is advantageous because gravitational compensation is possible by transmitting the reference plane to all links in the direction of gravity, regardless of the angle of each link. The only disadvantage is that it is difficult to manufacture due to its small size. To compensate for these drawbacks, we propose an external gravitational compensation device that is applicable to tendon-based series mechanisms. The proposed external gravitational compensation device can mediate reference plane transfer and gravitational compensation through tendons outside the serial mechanism, and it is advantageous in that the size and weight of the link can be reduced because it is installed outside the link. In this paper, we designed and verified the reference plane control mechanism that could transfer the reference plane similar to the parallelogram mechanism from the outside and the gravitational compensation mechanism that could mediate gravitational compensation according to the angle of each link. In addition, the prototype was assembled and experimentally verified.