fMRI-Compatible Robotic Interfaces with Fluidic Actuation

Actuation is a major challenge in the development of robotic systems intended to work in functional Magnetic Resonance Imaging (fMRI) procedures, due to the high magnetic fields and limited space in the scanner. Fluidic actuators can be made fMRI-compatible and are, thus, promising solutions. In this work we developed two robotic interface devices, one with hydraulic and another with pneumatic actuation, to control one degree-of-freedom translational movements of a user that performs fMRI tasks. Due to the fMRI-compatibility restrictions, special materials were used for the endeffector which works in the scanner bore, and active components such as the control valves and pressure sensors, had to be placed far away from the endeffector with long transmission lines in between. Therefore, the two fMRI-compatible setups differed from conventional fluidic actuation systems and brought control difficulties. Both systems have been proved to be fMRI-compatible and yield no image artifacts in a 3T scanner. Passive as well as active subject movements were realized by classical position and impedance controllers. With the hydraulic system we achieved smoother movements, higher position control accuracy and improved robustness against force disturbances than with the pneumatic system. In contrast, the pneumatic system was back-drivable, showed faster dynamics with relatively low pressure, and allowed force control. Furthermore, it is easier to maintain and does not cause hygienic problems after leakages. In general, pneumatic actuation is favorable for fast or force-controlled applications, whereas hydraulic actuation is recommended for applications that require high position accuracy, or slow and smooth movements.