Hydraulic Accumulator Sizing 1

[Frankbroadanddeep]

I am intersted in how to specify an accumulator design to damp the oscillations of arm attached to a single acting cylinder. The cylinder is controlled by a bang bang solenoid.
I know the pump delivery and pressure, pipe sizes, cylinder diameter and stroke. I know also that the oscillation frequency is about 5 Hz.
I would like the system to achieve full speed in a second or so but without the kick ?

Any help would be appreciated.

 

[kcj]

Adding an accumulator (if between valve and load) might make things worse, as it lowers the natural frequency.

I am betting the issue is that the bang/bang valve puts a pressure rise into the system too fast and you are seeing the natural result of exciting the system faster than it is capable of responding.

Search on Jack Johnson's materials on closed loop servo control, or other sources of proportional and servovalve theory.

the fact that you know the natural frequency is 5 hz is a big help. Its hard to calculate because of many assumptions.

I'd suggest a proportional valve with ramping set slower to eliminate the ringing. You open the spool at a controlled rate and avoid exciting the system at its natural frequency.

a cheap approach might be a soft shift bang/bang valve. Same as regular solenoid bang/bang, but has some damping in the spool to slow it's response down. I have used Continental Hydraulics, but most solenoid valve mfrs offer them.

kcj

 

[Frankbroadanddeep]

Thanks kcj

I should have said I am looking for a solution for the machines already in service which at the moment may cause my measuring system problems as the oscillations interfere with the averaging window function. The next generation of the machine will reputedly have proportional control to overcome the oscillation problem. In other words I do need a low frequency performing system. If I can spec a accumulator which just does this I can reccomend this to problematic installations until the MkII design comes along next year.

Many Thanks (I know now that I'm on the right track)

 

[PUMPDESIGNER]

kcj may be correct about making the thing worse, but am not sure. The net effect of an accumulator can be at times equated to that of a spring, compresses (or stretches) then returns that same energy into the system.

We were able one time to use an accumulator to fix a problem, but the only way it worked was to add a check valve with a hole drilled into the check valve. Free flows one way into the accumulator, then returns the pressure into the system slowly.

What you have then is a shock absorber, fast one way, slow the other.

PUMPDESIGNER

 

[EdDanzer]

First calculate how much oil volume needs to be displaced to slow down the acceleration. Then find the pressure needed to start the system moving. This pressure will be the precharge pressure for the accumulator. With the system pressure , the precharge pressure and volume to displace known you should be able to size the accumulator.
The difficulty will be finding a correct size, it may take several small units.

 

[kcj]

pumpdesigner has a good comment to make the accumulator into a damper. I have done that with an inline, micrometer knob type of flow control with reverse flow check (Parker, Bruning, Deltrol, etc.) It needs to be quite large to minimize the flow restriction or else it prevents the accumulator from doing its job. The result is free flow into the accumulator and controlled (adjustable) reverse flow out to control the discharge rate.

You didn't say you were doing a field fix of existing problem, but I just assumed it because that's what I deal with mostly. A good reminder of my own unrecognized bias.

As far as sizing, if you go too small it may not have the effect you want. If you go too large, it should work just extra space and cost.

If fixing one or two machines, I'd try the accumulator and adjustable flow control. If making a kit for many machines, I'd try the check valve and work out the drilling. Less fiddle factor for the operators to play with that way.

But either way, I'd try a soft shift D03 or D05 valve first to try and avoid the shock in the first place.
k