[klowrey]

Hydraulics shouldn't have any give, as the working fluid is considered "incompressible". Of course in the real world the tubing can expand slightly and there are friction losses, but the reason they went with hydraulics in the first place is they can set a position and not have to use more energy to hold it there (since the cylinders are pressurized).

If the gear ratio on these motors is high, then there can only be faked compliance in the tuned force-torque controllers you mentioned. MIT's little cheetah robot, on the other hand, deliberately used low-gear ratios to keep things naturally squishy if needed. This is the way to go; putting elastic tendons or spring elements seems like a good idea but then you can't actually model the non-linearity well (the 1st order motor becomes a 2nd or higher order system).

[jvanderbot]

Ah, thank you. I understand