An unloading valve is not the same as a relief valve or a regulating valve. From my training: An unloading valve is used to direct all flow from the pump to the tank at low pressure to reduce heating, and to allow the electric motor / pump to start under no-load. A relief valve is used to control maximum system pressure. When the maximum pressure is reached, the valve opens to direct flow to the tank without reducing the pressure. A pressure reducing valve, also called a regulating valve, is used to reduce pressure to a segment of the system. Not everyone that works in hydraulics agrees with the nomenclature that I learned.
Instead of a variable speed motor, I have seen pressure and flow compensating pumps that have a swash plate that is adjusted to reduce the flow from the pump without affecting the pressure, at a constant electric motor speed.
No you still need an "unloading valve" or Bypass valve, no different from an electric forklift. The PTO is variable speed and the unloading valve is still necessary when the load is too great, or if the cylinder has reached it's maximum travel. Otherwise you would burn out the PTO or motor.
Hydraulic vs. Electric is not a winner-take-all competition. Each has roles in which it is better suited. One of my customers offers products that use either one or the other as the prime mover, depending on application. It means more engineering work for my customer, but their end users are happy with a solution that fits their needs, rather than adjusting their needs to fit a single solution.
Hydraulic robots have become few and far between (in my experience). The rationale that I was told was because of the automotive industry. Automotive is a big user of robots. When robots were hydraulic, 3 (sometimes 4) trades were required to work on them. The electrician for the controller, the millwright for the hardware, or for tooling issues (sometimes a tool-maker also), and a plumber for the hydraulics. The all-electric robot eliminated one trade from that group.
The manipulator arm on the Space Shuttle was a 'hydraulic' application = high torque and low speed. Clean-room issues i.e. hydraulic oil leaks would have been a problem. A broken wire does not 'leak' electricity in the same way a broken hose would leak oil.
One thing that I think is missing from your hydraulic system is the unloading valve. Many of the machines that I have worked on had an unloading valve to direct flow to the tank at low pressure to reduce heating when the system was in standby.
Festo's BionicKangaroo combines pneumatic and electrical drive technology, plus very precise controls and condition monitoring. Like a real kangaroo, the BionicKangaroo robot harvests the kinetic energy of each takeoff and immediately uses it to power the next jump.
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