Sadly what you're describing isn't as easy as it looks. I was involved in research to do this sort of thing about 25 years ago and the results weren't pretty. We were using pneumatic actuators and valves coupled with microprocessor controllers but the problems with hydraulics are the same, pretty much.
Positioning using a hydraulic cylinder and a proportional valve gets messy when you consider inertia, mass flow through the valve and into the cylinder, the differential characteristics of the two cylinder piston faces (the rod face has less active area than the non-rod face hence the same pressure on both sides of the piston causes differential force and, absent a braking system will cause creep when the system is supposedly static) and a whole load of other bugger factors.
We got it to work, sort of, eventually developing a cheap industrial robotic arm but precision was not its strong point and hunting and overshoot were a fact of life. It was not a commercial success.
If you are seeking rotary motion at the joints rather than linear extension/retraction you might want to consider using vane-type rotary actuators rather than cylinders. They do have the advantage that the vanes have the same pressure/force on each side as the rotating shaft comes out the side of the vane chamber, not through one end of a cylinder. They also simplify the actuation at the joint. Rotary encoders are also cheaper and more robust than linear ones; we got decent results from using wirewound potentiometers to measure angular movement, building a resistance/angle lookup table in the MPU controller's memory for each joint but we ended up with 8-bit Gray encoders in the production version for engineering reasons.
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Positioning using a hydraulic cylinder and a proportional valve gets messy when you consider inertia, mass flow through the valve and into the cylinder, the differential characteristics of the two cylinder piston faces (the rod face has less active area than the non-rod face hence the same pressure on both sides of the piston causes differential force and, absent a braking system will cause creep when the system is supposedly static) and a whole load of other bugger factors.
We got it to work, sort of, eventually developing a cheap industrial robotic arm but precision was not its strong point and hunting and overshoot were a fact of life. It was not a commercial success.
If you are seeking rotary motion at the joints rather than linear extension/retraction you might want to consider using vane-type rotary actuators rather than cylinders. They do have the advantage that the vanes have the same pressure/force on each side as the rotating shaft comes out the side of the vane chamber, not through one end of a cylinder. They also simplify the actuation at the joint. Rotary encoders are also cheaper and more robust than linear ones; we got decent results from using wirewound potentiometers to measure angular movement, building a resistance/angle lookup table in the MPU controller's memory for each joint but we ended up with 8-bit Gray encoders in the production version for engineering reasons.