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Controls for hydraulic cylinders in construction machines

Controls for hydraulic cylinders in construction machines

Controls for hydraulic cylinders in construction machines

(OP)
I'm modeling a few construction machines for simulation and I need to know how the actuators are normally controlled. Are PID controllers typical---in backhoes, excavators, and aerial lifts, say? How do they keep the actuators stable against loads without constantly adjusting the force produced by the actuator? I've heard that pilot-operated check valves are one strategy, as they allow actuation in a single direction while locking in the opposite direction until the pilot is energized (for retraction, say). Can someone confirm? Thanks in advance for any bits you can share! :)

RE: Controls for hydraulic cylinders in construction machines

In construction machines the only controls I ever see are PID where the human provides the PID.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Controls for hydraulic cylinders in construction machines

(OP)
Hahaha. Thanks! Sorry for the show of ignorance. I've dealt more with robotic systems with automatic control systems and I guess I extrapolated to construction machines, about which I clearly don't know much.

The human provides the PID using a joystick or lever or some other mechanism, and then a proportional valve translates that into a hydraulic force to drive the actuator and whatever the actuator connects to. I guess it's that simple :D

RE: Controls for hydraulic cylinders in construction machines

You got it. It is simple but a good operator comes out of a lot practice tuning that PID.

I suspect the machines with active control are machines like horizontal boring machines and farming machinery like pickers and processors.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Controls for hydraulic cylinders in construction machines

"proportional valve translates that into a hydraulic force to drive the actuator".
That is not really correct either. The joystick controls how much the valve opens. The human only has visual feedback of what motion occurs as a result. Force is a result of the load's reaction to the flow of oil to the actuator. There is a limit to the oil pressure. Also, hoses and oil have some degree of "springiness".

RE: Controls for hydraulic cylinders in construction machines

Yeah there's no "proportional valve" (of the fancy electronically regulated type). Just a plain spring-centered manually-operated spool valve.

It's possible that some functions might have PO checks or counterbalance valves in the circuit to help ensure that the function in question moves in the commanded direction only and doesn't come crashing down uncommanded. But they'll be totally mechanically/hydraulically controlled.

RE: Controls for hydraulic cylinders in construction machines

(OP)
Thank you all for the helpful posts! I know it's basic stuff, but when you lack basic intuition, basic's the best info you can get. Also thanks for the link to examples of valve-solenoid-joystick kits! Cheers :)

RE: Controls for hydraulic cylinders in construction machines

Well, there are electrohydraulic controls as well. Danfoss PVG32 is pretty advanced with its OBE and spool position feedback and it's also quite common in truckmounted cranes, backhoes etc.
Eaton CMA is another advanced valve with spool position feedback.

Parker has mobile valves with EHC, typically used in forestry machines.

RE: Controls for hydraulic cylinders in construction machines

Proportional control is a broad term. In mobile hydraulics, we dream about proportional motion control in the way that the actuator speed is proportional to the control lever position. But we can never get that since pump pressure and load pressure varies during a motion cycle. To get perfectly proportional actuator speed we need a closed-loop system with velocity feedback from the actuator. In agricultural tractors with 3-point hitch, and airplanes with rodder controls we want the actuator to get a position proportional to the control lever position. That way the operator can see on the lever what position the 3-point hitch or the rodder has. This type of proportional control always requires closed-loop feedback. Usually, this is by a mechanical linkage between the control lever, the control valve, and the actuator. Modern agricultural tractors accomplish the feedback electronically. I assume the same applies to modern airplanes.
Most construction machines (like excavators) use simple hydraulic pilot control. Scandinavian CTL (CutToLength) machines use as mentioned by Jacc EHC, ElectroHydraulicControl, which basically is an electric joystick that controls the hydraulic pilot control of the main spool valve. The last few years CTL equipment has developed more advanced cranes with (knuckle boom) crane tip control (John Deere Forestry IBC, Intelligent Boom Control) which includes position sensors in all crane actuators. I know the construction/terminal equipment industry has some of that development too.

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