I have a CVT transmission, which consists of a metal belt and four circular toroid discs, that form two pulleys. I need to be able to move the two toroids further and closer together. This movement has to be separate for the two pulleys, so when the drive toroids (connected to the engine) get further apart, decreasing the diameter, the driven toroids (connected to the driveshaft) have to get closer together as to tense the belt. This all has to be controlled by a raspberry pi. I thought about maybe using a hydraulic piston, but I'm not very familiar with hydraulics.
How should I design the system to move the toroids closer and further apart?
mechanical : perhaps threads - would need to be fine and you need to think about the disadvantages, but stepper motors may give suitable control
pneumatic pistons, however the inherent "flexibility" due to air compressibility may not give sufficiently fine control
hydraulics: used in many control systems as liquids are only slightly compressible, and at the pressures you will use they are assumed incompressible. Hydraulics are used from small agricultural implements up to controlling the legs of drilling platforms in the North Sea - and they can do that to a tolerance of better than 10mm for a leg that supports many tons.