Development and implementation of an MR-safe encoder for pneumatic actuators of the MR-safe catheter robot for endovascular applications

Endovascular procedures can be a challenging and delicate task for surgeons. Surgeons have to rely on 2d-imaging and relative small forces an torques to manoeuvre a catheter through the blood vessel. Also fluoroscopy is used as an imaging technique which can be harmful for medical personnel which performs these operations frequently. To circumvent these problems a tele-manipulation robotic prototype has been developed, which enables surgeons to operate more remotely, without being exposed to health risks due to fluoroscopy. The platform also features a navigation system that provides the surgeons with visual and haptic guidance during the procedure.

The actuators of the secondary robot, which move the catheter and guidewire, are pneumatically driven and fully made of MR-safe materials, enabling the secondary robot to be MR-safe. Both the rotary and linear actuators move the catheter in distinct steps. A limitation of the current prototype is that it lacks any positional feedback from the actuators. Occasionally there is a loss of pressure in the piston chamber of the actuator, which causes a reduced force output, therefore it may occur that the actuator misses steps. To compensate for these errors, a feedback system is required to improve the reliability and robustness of the system.

To achieve feedback from the actuators, an MR-safe sinusoidal encoding method is proposed. In the proposed method optic fibers are used together with a grating pattern from MR-safe material to create sinusoidal-like analog signals. The method is split into two basic designs to achieve the desired resolution. In the first design, the linear movement of the actuator is translated to an angular movement such that the size of the grating pattern can be increased. Subsequently, two optic fibers are used to create an analog encoding signal. In the second design, three analog signals are created by three cables and a grating pattern of which the grating period is six times the desired resolution. Three optic fibers and a grating pattern create three analog signals such that one grating period can be divided into six sections with the length of the desired resolution.

The tests proved that the accuracy of the encoders was satisfactory. The precision of both encoders was relatively low, which caused measurement deficiencies. The first design clearly had more measurement complications, caused by a too low sampling frequency and mechanical problems such as slack. The second design has still a too low precision, which is caused by a poor analog signal. In future works it is recommended to further develop the second design, which includes Improving the analog signal of the second design, and create an adequate alignment of the encoder with respect to the static motor locations.

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