Novel Transmission Mechanism (Shaft-following Gear Mechanism) Applied to a Puncture Robot for Three Dimensional Punctures

Computed Tomography (CT)-guided intervention requires an expertized skill to physicians and a numerous of robot-assisted needle insertion system has been developed. As a challenging issue developing the CT-guided needle insertion robot system, the inside of a CT gantry is a special environment where there is no room for movement and no metal can be inserted because of causing an artifact on the acquired image. The gantry entry portion of the robot therefore requires miniaturization and non-metallization. The most difficult part of the non-metallization of a robot is the motor element. While many of the robot's structural materials can be made non-metallic thanks to advances in plastic materials, the actuator cannot be made non-metallic. In some cases, the actuator can be attached directly to the moving part, but this limits the angle of motion of the robot to where the actuator is outside the imaging range. In this report, we introduce a unique mechanism that enables the needle manipulation with a miniaturized configuration inside the CT gantry. The mechanism satisfies the requirements of power transmission between axes whose relative positions are indeterminate, miniaturization to prevent interference with robot motion, and non-metallization. This mechanism combines a single-node link and gears to transmit rotational power while passively responding to changes in the positional relationship between axes. The hinge fixation of the basic single-node link is aligned with the axis of rotation of the first gear, and the subsequent gears can be freely arranged to mesh with each other. The minimum configuration is 3 gears, one at the hinge, and two at each end. Theoretically, the number of gears can be increased, but it is desirable to use as few gears as possible because transmission accuracy will decrease due to backlash and other factors. The movement of the mechanism does not necessarily affect the state of the gears at all. If the absolute angle of the linkage changes, the positional relationship between adjacent gears changes while the distance between the axes remains constant. If the positional relationship of the meshing gears changes, the gear will rotate. If rotation is simply transmitted, this angular change does not affect the operation of the machine, but if rotation is transmitted for the purpose of angle determination, the angle input side must be adjusted. This depends on the changing tip position coordinates. Therefore, we developed an algorithm that outputs adjustment parameters upon input of information on the mechanism to be used. The algorithm derives an approximate equation from numerical analyses that limits the error from backlash. We applied this mechanism to our robot with a 25G needle and inserted the needle into the target from various angles. The accuracy was 2.5 mm, which satisfied our requirements.