hydraulic gas accumulator

[wangp1283]

I need an actuating system that is light/compact enough for vehicular use. Such as a small truck.

The actuator should be able to deliver a max. force of 20,000 lb (about 90,000 N). The maximum velocity is 0.1 m/s. The displacement is 0.03m. The actuator is to continuously adjust the position of a load within this 0.03m range.

Since Power = FV, this would mean a max power output of 9000 W.

But such a power demand last only 1 or 2 second, which is less than 5% of the operating time. I don't think it'll be a good idea to have a big pump that can output 9000 W.

I wish I can use a much smaller pump that can satisfy the small demand for the rest of the time, on the order of 1000 W.

Then, use an accumulator to supply the very short peak demand. Since the max power demand last only about 1 second, that would mean an energy of 9000 J need to be stored. 9000 J isn't quite a lot.

But the problem is, is it possible to integrate an accumulator into such an actuator system? so the pump can be made smaller, or turned off.

 

[btrueblood]

Yes.

What are you using 20,000lb x 12 in. for?

 

[Rob45]

btrueblood:
That travel is 1.18 inches, not 11.8 in.

 

[wangp1283]

It's for shifting a CVT.

How can it be engineered so that there will be very little power use when the actuator is not moving. Because theoretically a zero power is needed to produce a force with no displacement.

Yet on current CVTs, even if there is no shifting (no displacement), the hydraulic actuator and pump still uses up a lot of energy to supply the "clamping force" , which is the force the pulley halves must exert on the belt at all times in order to create the necessary friction drive force.

I don't get how that works.

 

[wangp1283]

How about electrics?

Are they more expensive, heavy, and less capable of heavy force than hydraulics?

 

[wangp1283]

Alright, I've gotten a better understanding of the actuators after read this:

http://www.machinedesign.com/ASP/strArticleID/56768/strSite/MDSite/viewSelectedArticle.asp

It turns out "Hydraulic power supplies run continuously to ensure there's a reservoir of pressure with enough fluid flow for any possible move. So system pressure must always be at its full rated value, and the pump is usually overrated so it will handle peak loads. Consequently, hydraulic systems are continuously consuming energy. Actuators, on the other hand, only use power when activated, and are normally rated to handle full loads at a minimum of 25% of full-rated load capacity for all duty cycles."

So basically a hydraulic actuator will take energy even when there is no movement. This is undesirable.

Looks like electric actuator might be the answer, but they don't seem able to take on load greater than 2000 lb. Anyone have familiar with them?

 

[CESSNA1]

WANGP1283:

This is an interesting problem, high force and short distance. Consider using an accumulator charged to provide 21000-22000 pounds of force. When activation is requied this pressure will provide the force to move the actuator and at the same time a pressure switch on the accumulator would engage the hyraulic pump to recharge the accumulator. The pump would be shut off at 21000-22000 psi.

Regards
Dave

 

[octavedoctor]

There is no need for the pump to run at full pressure and flow when not delivering useful power.

Accumulators are routinely used with "Offloading valves" specifically for this purpose. When the accu pressure rises above the setpoint, the pump is diverted to tank. Still there are flow losses but much less than you think - less than 1/10th of rated power for a gear pump, even less for a variable displacement pump.

Electric actuators (=servomotor and ballscrew) in fact will consume large amounts of energy when holding load. Just generating torque with a motor requires energy becuase the windings have a finite resistance. Remember 9000W at 12V is 750A. This is starter-motor territory and not the sort of drain you want to inflict on your vehicle electrical system.

Sounds like hydraulic is your best option. You need a gear pump, offloading valve (all the cartridge valve makers do them- Sterling, Integrated, Sun...), and a good proportional valve like the Sauer Danfoss PVG32 with PVE actuator. Then you need a position feedback transducer on your load and a microcontroller like the Sauer Danfoss MC50 to implement a position feedback loop. This will give you the control and power you require with minimal electrical power consumption.

 

[wangp1283]

octavedoctor, does this mean I need a pump that's rated for 9000 W?

So you are saying even if there is no useful work being done, just to "hold a load" will require power that's less than 1/10 of the rated wattage, so in this case, it's 900W. 900 W is still quite a bit since it's more than 1 HP.

But I want to know if it's possible to use a smaller pump (less than 9000 W) to satisfy the AVERAGE power demand (which is a lot less than 9000W), and only use an accumulator to satisfy the short temporary peak demand.

I'm not sure how to extract energy from such an accumulator in an accurate, useful way, to produce the desired motion needed at any given time.

 

[aviat]

Your force is equal to the pressure time the area of the actuator. You don't need 21000 psi. System pressures are often 3000 psi, so you would need aprox 3" dia actuator.
You need to know the flow rate to determine HP to drive the pump. It is the flow that gives you the speed.

As far as using an accumulator I don't think it should be a problem, but I don't know what you are trying to do.

The site below has many formulas for clyinder and accumulators under downloads. Also there is a fluid power professionals forum on this site.

http://www.hydraulicsupermarket.com/technical.html

 

[wangp1283]

Are there any difference between the following 2 applications:

1. the actuator is used to lift a "dead" load, like a steal box up a certain height. The actuator only has to control the position, and speed of the box.

2. the actuator is used to control the pulley halves of a belt CVT. Not only does the actuator has to control the position (gear ratio) and the speed of movement (rate of change in gear ratio) of the pulley halves, it ALSO has to supply a "clamping' pressure so there is enough friction.

 

[wangp1283]

On the above question, here is my own opinion, which I hope you can correct if it's wrong.

If you have shut off valve between the pump and actuation cylinder. and if you
close the shut off valve. It's true the POSITION will be hold, which
means the belt won't "force apart" the pulley apart. But I don't think the "clamping"
PRESSURE will be hold. Just depending on the belt tension against the pulley sheaves will not
give you enough friction force to drive the load. Because now there
is no pressure from the cylinders. It'll be like a fixed pulley and
just by running a belt on it will not give the necessary friction
force.


But then again, why can't they incorporate an accumulator of some
sort to provide a consistant pressure. Consider using an accumulator
charged to provide 21000-22000 pounds of force. When activation is
required this pressure will provide the force to move the actuator (which I don't know how you can control the motion)
and at the same time a pressure switch on the accumulator would
engage the hydraulic pump to recharge the accumulator. The pump
would be shut off at 21000-22000 psi.

The above approach might work if you are using it to actuate a
fixed dead load, like lifting a steel box up a certain height. If
there was no movement, then a valve can be closed that separates the
accumulator from the load, and then the cylinder can just "hold the
load" indefinately without power. So the steel box will stay up
there fine. And if movement is needed, the valve can open and the
accumulator can supply the pressure to move the box. And once the
accumulator near empties, it can be recharged by the pump, which is
not running most of the time. This is efficient. In this
application, the only thing that's important is the POSITION of the
load.

But the reason I think this approach cannot be used for a variable
pulley CVT is because not only is the POSITION important for the
pulley actuator, but it also needs to supply a constant clamping pressure to
the pulley halves. The belt itself will not supply the necessary
pressure (unlike the steel box). Additional pressure must be
supplied by the actuator to not only hold it in position, but ALSO,
supply a force that will create enough friction for the CVT to work.
You cannot use a preset accumulator to provide this pressure
because there is no way to control the position. The accumulator
might just provide too much pressure that'll shift the pulley
position unnecessararily unless there is a way to control it, which
I dont' think there is. You can try to add a valve that closes, but
then that'll won't get you any pressure anymore. You'll end up with
a fixed pulley, just like the beginning. It's hard to make an efficient design. The pump has to be working continuously even when there is no movement.


Therefore, the scenarios are not the same, even though they appear to be.

Any comments?

 

[adfka]

Thinking outside of the box, would it be possible to utilize a helix system such as a snowmobile uses, then augment it with a hydraulix assist?
I'm thinking you could have the majority of torque supplied by the system, and move your requirements down to the order of 100 psi.

 

[wangp1283]

Hi guys, I redesigned my project. The required actuator force is now only 2000 lb. Basically, I've decreased the actuation force by a factor of 10.

Do you think an electric actuator can also be considered?

I have a conception that electrics are not as good as hydraulics when the force is high, but electrics can be more efficient if the load is low.

Anyone know why electrics aren't as feasible as hydraulics for high loads?

Thanks

 

[EdDanzer]

The energy density of high-pressure hydraulic fluid is far more the possible with electricity. To actuate your cylinder and reduce the power loss, you can use several methods. octavedoctor is correct but another term is unloading valve. We use a Sun unloading valve to charge an accumulator, this reduced the system cost and engine power required. A servo valve should not be nessacary.
You could try posting this in the Fluid Power Professionals group.