My guess is that they use brushless Motors that in turn powers the hydraulic pump. But there must be some kind of valve that could help in the process. Still the burning question is just by adjusting the flow and pressure will it actually control precision required hydraulic equipment? Since there are lots of valves, which combination of valves can work?

I know there are many easy ways to control an electric actuator with feedback, using Arduino or simple "pwm" etc.

  • $\begingroup$ They use proportional valves with precise position sensors with feedback. Check out Mannesman Rexroth Sigma... $\endgroup$
    – Solar Mike
    Jul 18, 2019 at 16:22
  • 1
    $\begingroup$ the other thing they do is equip the piston or rod with a precision position sensor and use it to generate a feedback control signal. $\endgroup$ Jul 18, 2019 at 17:34
  • $\begingroup$ I worked with one of the engineers that specified the system for the hydraulically controlled legs for an oil rig. He told me the product spec (cahier des charges) was plus/minus 1 metre for each leg for the height control. What they provided was working to plus/minus 1cm... Funnily, the got the contract - and that was using all the standard components “off the shelf”, they did not have to design anything bespoke... $\endgroup$
    – Solar Mike
    Jul 18, 2019 at 18:09
  • $\begingroup$ thanks for the reply @SolarMike do you know what the valves are called ? Will try and search it for more info. The Mannesman Rexroth Sigma gives me a general search result. Is there only one valve involved? I got dizzy looking at all the different kinds of valves they have. Whats the normal valve for an open loop? $\endgroup$ Jul 18, 2019 at 20:35
  • $\begingroup$ A “normal” valve? That is one relevant for the task... $\endgroup$
    – Solar Mike
    Jul 18, 2019 at 20:57

1 Answer 1


Typical Hydraulic Servo Setup (as used in structural testing):

The power source is usually a pump (e.g. a swash-plate pump) arranged to provide (up to a rated flow rate) at ~constant pressure. Hydraulic accumulators may be included to smooth out transient pressure drops.

The valves are typically directional control valves, so they can drive a piston/cylinder both ways. See https://en.wikipedia.org/wiki/Directional_control_valve

For structural testing, the valves are usually piloted spool valves, electrically (coil) driven, precision ground (more often, lapped.) These valves are fairly linear (in terms of coil-current-input to flow-rate output) and impressive bandwidths (hundreds of Hz., esp. in flight controls like aircraft, missiles or booster rockets) but that linearity comes at the cost of flow leakages, on the order of a couple percent of the peak flow (per valve!) The valve drive is often dithered (strongly!) to minimize stiction (e.g. from particles in the fluid getting into the valve.)

Feedback transducers used vary, but for high-power/high-damage-if-anything-goes-wrong systems, analog sensing, especially built-right-into-the-device, ratiometric sensing (e.g. LVDTs) is useful, because such analog stroke feedback permits simple, robust (e.g. PID) control loops for position control. Load control is also common for systems with a reliable force sensor; cylinder loadcells are common.

  • $\begingroup$ For a four legged Robot max dynamic load of 6kN to be controlled precisely, what kind of valve or a manifold is best? I'm relatively new to hydraulics and there are just too many valves to look for. I know they all have their functions but isn't a check valve for max pressure, a relief valve to release pressure and a directional control valve not enough. But i do need to be able to run it efficiently as it is a mobile unit. Thank you for your help. $\endgroup$ Jul 24, 2019 at 10:36
  • $\begingroup$ My hands-on exp. is too far from a legged, mobile robot to be much help. I dealt with fixed structural testing machines w/peak forces above one megaNewton with effectively unlimited 220 VAC 3-phase power. Suggest you study what you can find re Boston Dynamics' hydraulics/control systems (e.g. for Atlas and BigDog) as closer to your goals. HTH $\endgroup$
    – Catalyst
    Jul 25, 2019 at 11:12

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