Wheel self leveling system?

[TeoAlfa]

I will try to make the picture first.
We have a hyraulic trailer where all the wheels are connected to the main chassis with a hydraulic cylinder. The chassis can be lifted or lowered by adjusting the hydraulic cylinder.
But what we do when the trailer have to pass over some ground irregularities? Some wheels will touch the ground and some others won't so the chassis will take extra loads.
I have seen some boat trailers where their wheels are capable to follow these irregularities keeping its wheels connected to the ground? Could anyone tell me how this can be done?
I note that there is no suspension system (like a truck). The wheel connects to the chassis only by the hydraulic cylinder.
Thanks in advance.

 

[HPost]

The wheels that move will have accumulators attached to the cylinders. So when the wheel is displaced the pressure goes up and the part of the chassis above that wheel will see the load.

 

[TeoAlfa]

I have read here (

http://www.fagioli.it/Pages/SPMTs.aspx?lang=en-US#prettyPhoto

) that it is a matter of splitting the cylinders in groups where all wheels of each group is connected to each other.

 

[HPost]

Yes, but that's for multi wheeled, multi axle machines for carrying very heavy loads. How many wheels has your trailer got?

 

[TeoAlfa]

Hi and thanks. My trailer desaign has 28 wheels for 200t load.

 

[PNachtwey]

We did something like that for this

http://www.airporttech.tc.faa.gov/Airport-Pavement/National-Airport-Pavement-Test-Facility-

Each wheel had to maintain the same force even though some wheels would go into ruts.
The goal is to test runways, not wheels or suspensions.
Large aircraft will create ruts in the runways and the runway will actually ooze up at the edges as the ruts are pushed down.

What is needed is an auto leveling algorithm too.
We can do that for presses where the platen must be kept level to avoid binding even though the loads on each corner of the press is different.
Each axis needs a position feed back and either a load cell or two pressure feedbacks to calculate force. That is a lot of I/O to coordinate. However, it can be done with off the shelf motion controllers but the person that is making the system work must have some experience at hydraulic control, using cascaded loops and programming.






Peter Nachtwey
Delta Computer Systems

http://www.deltamotion.com

http://forum.deltamotion.com/

 

[TeoAlfa]

@PNachtwey
Do you mean that a complicated algorith will be needed to control each one hydraulic cylinder? That would be too hard to achieve i suppose.
Would it be more easy to connect the cylinders in let's say 3 groups. One wheel pass over a bump, it is compressed by some amount and the rest of the cylinders of the same group get extended a small amount.
For example let's say we have 12 wheel carriers. We group the carriers in 3 groups of 4 carriers each.
Each group contains 4 wheel carriers. One wheel passes over a bump and gets compressed by let's say 50mm. Let's assume that 50mm of linear displacement equates to 1lt of oil. One litre of oil will leave from trhe cylinder of this wheel carrier and will move to the rest 3 cylinders of the same group. So each of the 3 cylinders of the group will get an extra of 1/3lt and each one will extend by 50/3 = 16.7mm.
The groups should be chosen wisely though for stability reasons.

 

[PNachtwey]

[/quote]
of oil. One litre of oil will leave from trhe cylinder of this wheel carrier and will move to the rest 3 cylinders of the same group. So each of the 3 cylinders of the group will get an extra of 1/3lt and each one will extend by 50/3 = 16.7mm.
[/quote]
Why? How? That is wishful thinking.

The other 3 cylinders will still move up relative the the rest of the 28 wheels. What if the load on the wheels is uneven? That isn't good. The controller needs a gyro to know what is level. It needs to have the ability to measure force when static so it knows what force to maintain while moving. Then the hydraulic cylinders need to extend or retract to maintain that force/ The force may need to be tweaked by the outer level loop.


Peter Nachtwey
Delta Computer Systems

http://www.deltamotion.com

http://forum.deltamotion.com/

 

[3DDave]

PNachtwey,

Putting cylinders in parallel is a workable manner to distribute loads. By definition the loads will be equal within any group. They have similar areas and similar pressures. It works well enough for the M1000 tank trailer to carry a 70 ton battle tank. There's no need to be perfectly level, which would be a definite problem on mountain roads.

 

[PNachtwey]

This is MUCH more difficult than it appears

Putting cylinders in parallel is a workable manner to distribute loads. By definition the loads will be equal within any group

How do you figure? OK, yes all the cylinders connected to the same line will have the same pressure but what if the load is different from one cylinder to another?
Then one of the cylinders will be pushed back while the others extend resulting in an uneven load.
What is necessary is to maintain level. Your trick doesn't do that.
At the risk of being called advertising , look at this

http://deltamotion.com/peter/Videos/SyncPosForce.mp4

This is called auto leveling and is used by press manufacturers to keep the platens level so they don't bind.
This is a good start but it assumes that level is where both axes are extended the same amount.
In TeoAlfa's application the cylinders will need to extend different distances and keep the sum of forces and torques constant so TeoAlfa's application is more complicated yet but this is a good start.
3DDave, your trick would not keep the platen in the video horizontal.

The techniques required are out side the scope of this forum.




Peter Nachtwey
Delta Computer Systems

http://www.deltamotion.com

http://forum.deltamotion.com/

 

[TeoAlfa]

I copy from Fagioli's website:

A hydraulic cylinder supports each of the wheel bogies. The cylinders on each bogie can be linked hydraulically to the other wheel bogie cylinders to form groups (Diagram 1).
This allows free flow of hydraulic fluid between each hydraulic ram allowing the SPMT to negotiate uneven ground, cambers and gradients whilst maintaining equal loading in each hydraulic ram within that group (Diagram 2). To link all hydraulic rams on a single trailer would result in an unstable transporters bed with no control. The SPMT is normally split hydraulically into three separate hydraulic groups (A three-point suspension) that can be controlled individually from the main power unit (Diagram 3).
As the SPMT negotiates uneven ground, cambers or gradients the hydraulic fluid will free flow within each suspension to maintain equal loading within each suspension group (Diagram 4).
Addition hydraulic fluid can be can be pumped from the Power Unit (PPU) into or out of each cylinder group to adjust the level of the SPMT.
Systematic and controlled pumping of hydraulic fluid into or out of each suspension can raise or lower the transporter bed +300mm from the normal running height of 1500mm. (Diagram 5)

 

[3DDave]

PNachtwey

You keep adding a constraints that are not required.
There is no requirement for a trailer to be level. Only that the load be distributed.

To argue that the technique cannot work when there are already trailers that are configured exactly like this is odd at best.

Let me ask this. If I put 5 capacitors in parallel, what will happen if one is at higher voltage than the others? The exact same thing happens to hydraulic cylinders in parallel.

 

[TeoAlfa]

Dave, do you think we can achieve the same result by adding an accumulator to each cylinder without connecting them in groups?

 

[3DDave]

TeoAlfa - Not so much. Unless the accumulator is large compared to the cylinder volume the load will not be evenly distributed. It would cause them to act like conventional springs.

 

[PNachtwey]

It looks to me that TeoAlfa wanted to keep the bed level. The link said that connecting all the cylinders together would not be stable. I agree with that.

I can see where four wheels may be plumbed in parallel but a group of four wheels goes over a low spot they won't all extend unless more oil is applied to the group. Your suggestion does simplify the problem a bit but it doesn't eliminate the problem.

Peter Nachtwey
Delta Computer Systems

http://www.deltamotion.com

http://forum.deltamotion.com/

 

[3DDave]

There's no requirement to maintain a fixed height or fixed attitude or fixed elevation above sea level. The title is unfortunate and there is nothing in the description to indicate otherwise.

The described task is just to make sure all tires maintain contact. If one wheel in a group of 4 drops an inch, the bed above the group drops 1/3 inch and the oil is transferred from the cylinder that aren't in the hole to the one that is. The group carries the same load before and after, for small angle attitude changes.

Leveling is just another function of the SPMT system, but isn't required to maintain constant wheel loading.

 

[PNachtwey]

There's no requirement to maintain a fixed height or fixed attitude or fixed elevation above sea level.

Why did TeoAlfa say "self leveling" then? Look at the thread title. Look at the picture in the links.

BTW, if a group of wheels falls into a low spot where does the oil come from to extend the wheels? Without extra oil the group of wheels will not extend downwards.






Peter Nachtwey
Delta Computer Systems

http://www.deltamotion.com

http://forum.deltamotion.com/

 

[3DDave]

The extra oil comes from the deck descending. If a capacitor charged to 4.5 volts is attached in parallel with capacitors charged to 5V where does the charge come from to charge the new cap to match the voltage? What happens to the voltage in the other capacitors?

I worked engineering support for the M1000 and the Tunner 60K aircraft loader - they both used this system and it worked very well.

Also - probably English isn't his first language - the description talks a lot about keeping wheels loaded and nothing about keeping the deck level as well.

 

[PNachtwey]

The extra oil comes from the deck descending.

This assumes all the cylinders are connected in parallel. That won't work if the load isn't even.
I don't see why you keep disputing that. If the cylinders are plumbed in parallel we both agree that they will have the same pressure but if the loads are different the cylinders with the heavier loads will retract.
They will probably retract until they bottom out. The oil will go to the cylinders that aren't as heavily loaded.

You can see from the video I/we know how to keep a truck bed or platen level, We can also control the platen or truck bed to be at any angle. Keeping the platen or truck bed level is simpler because the there isn't a need to rotate vectors to keep the end points in a plane.

This company uses our controller to retrofit Moog flight simulators

http://deltamotion.com/peter/Videos/Automate India 6DOF.wmv

Yes, they are in India.

this is the largest 6DOF platform in North America. In is in or around Vancouver BC.
It is used to make movies. It was used in the movie 2012. Notice the concrete blocks which were used to simulate the weight of the movie sets.

http://deltamotion.com/peter/Videos/40x40test.mp4

My point is that we have experience at doing many systems where the platform, platen, truck bed, etc must be kept at a certain orientation. I can tell you that putting all the cylinders in parallel so they are all at the same pressure will not work UNLESS they all see the same load force.






Peter Nachtwey
Delta Computer Systems

http://www.deltamotion.com

http://forum.deltamotion.com/

 

[3DDave]

I think you are a sales guy and have moved into 'selling' territory.