The strong magnetic fields and limited space make it challenging to design the actuation for mechatronic systems intended to work in MRI environments. Hydraulic and pneumatic actuators can be made MRI-compatible and are promising solutions to drive robotic devices inside MRI environments. In this paper, two comparable haptic interface devices, one with hydrodynamic and another with pneumatic actuation, were developed to control one-degree-of-freedom translational movements of a user performing functional MRI (fMRI) tasks. The cylinders were made of MRI-compatible materials. Pressure sensors and control valves were placed far away from the end-effector in the scanner, connected via long transmission lines. It has been demonstrated that both manipulandum systems were MRI-compatible and yielded no artifacts to fMRI images in a 3-T scanner. Position and impedance controllers achieved passive as well as active subject movements. With the hydrodynamic system we have 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 more favorable for fast or force-controlled MRI-compatible applications, whereas hydrodynamic actuation is recommended for applications that require higher position accuracy, or slow and smooth movements.
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