Axial piston motors and oscillation 3

[hydroman247]

Hi guys, another question for you.

4 wheel drive mobile machine.
Main pump is a gear pump
Electro proportional spool control
4 axial piston motors. (2 motors on each side hosed in series).
Please note, there is no flow divider, just the drive spool direct to the wheel motors with a cross line anti-cavitation valve in between.

When driving normally it is fine, but when I select low speed which is ~20L/min through the drive valve and am at full lock I get lurching oscillations. This also seems to only happens when going forwards at full lock, either left or right.

The only other little issue is at very low speed, I mean <0.5km/h, the whole chassis shakes and the wheels twitch a lot. Like something is opening and closing very rapidly causing pressure spikes. When I checked the pressures between both side pairs of wheel motors, I could see this oscillation clearly.

Does anyone have any idea what could be causing this? The fact that it only happens going forward has to be a big clue. The steering angle is very sharp but this problem did not occur to the same extent with the previous wheel motors from a different brand, although they were also axial piston.

Thanks,

Hydroman

 

[hydromech]

Interesting...

Does the system have closed loop control?

If so, how are you closing the loop?

Does the proportional valve have spool position feedback?

Piping the motors in series means that the first pump in line has a higher pressure because the added delta P across the second motor. This is a minor problem until you change the direction of the steering wheels. This creates a different vector on the motors in series, possibly creating a conflict in the flow and pressure between the motors. In low flow conditions, this effect may be exacerbated by hysteresis in the proportional valve. The valve may become unstable as it tries to find a control position.

If you have PID control, you could back the gain off to reduce the valve response.

There may be no feedback on the spool position and this called be all wrong..? Please advise...

What made you pipe the motors in series? I think parallel would be better. The difference in steering vector is a major contributor to the problem...I think. Assuming that the lead motor changes depending on direction of travel, the additional pressure could lessen or increase the instability you are seeing.

As always...a schematic diagram will be most useful.

Cheers

Adrian

 

[hydroman247]

I am not working on the electronic side but I can suggest that but I am not 100% sure on the design work. It was not done by myself.

There is no spool position feedback as far as I am aware.

The motors are hosed in series to increased efficiency and lower cost I imagine.

I cannot post a schematic unfortunately but I can post a pressure log. This shows the pressures in between the wheel motors. One gauge on each side of the machine.

 

[hydroman247]

You can the 1st and 3rd parts are driving in high displacement at max speed ~20l/min. The 2nd and 4th parts show high displacement at lowest possible speed with the small oscillations.

 

[hydromech]

The frequency looks to be about 2Hz.

This tends to indicate an unstable control valve. With low flow and low pressure, there is less resistance to the spool movement.

Your statements on the pressure conflict each other. First you say the low flow is approx 20 l/min, then you say the high flow is 20 l/min. Which is correct?

What is the make and size of the proportional valve? It may be too big for the application. Proportional directional control valves and flow control valves work better if they have higher flow rates. They control better. Equally, working them with lower flow can lead to instability.

worth checking the proportional valve I think.

Cheers

Adrian

 

[hydroman247]

The first issue is separate from the second. (I think)

1st = 20l/min at full lock induces violent jerking of the machine.
2nd = Lowest flow possible, even in a straight line it oscillates. That is the 2hz oscillation you can see in the picture. 3:33-3:47 and 4:09 onwards. As soon as I speed up even a tiny bit, this tiny low pressure oscillation disappears.

I mentioned high displacement rather than high flow. The wheel motors have high and low displacement. Both of these issues are in high displacement.

The proportional drive valve is made by Bucher and has a 40l spool in it. The unstable spool makes sense for the high frequency oscillation but not necessarily the more violent full lock steering issue which as you said, could just be the angle which is very sharp.

I have just been out for more testing and seen that the shaking gets progressively worse the larger the steer angle and the faster you drive. It is strange that the only thing that has changed is the manufacturer of the wheel motor. Same displacement etc.

Thanks for all your help!

 

[hydromech]

Where are you...geographically speaking?

 

[hydroman247]

UK

 

[hydromech]

I got that from the SI units you are using...

Where in the UK?

 

[hydroman247]

I would rather not identify my location. I work in the south though.

 

[hydromech]

I didn't want you post code...just approx location.

If you were in the middle, I would have come for a look...down south is too far.

Never mind, we will have to help solve it remotely.

 

[hydroman247]

Thanks for the offer! I have people here I will be asking next week if I don't work it out, but everyone is away at the moment. Half of our engineers are in Germany and the rest seem to be on holiday.

I am going to do more testing with the previous motors and see the extent of the symptoms with that set up and compare. I am thinking of increasing the crossline relief orifice between the two sides from 0.5mm to 1mm and seeing if that makes a difference. I don't want to affect the off road capabilities though.

 

[MikeHalloran]

If the machine steers only two wheels, as I suspect, then when you turn, the wheels on each side (f/r) are at different radii, and are fighting each other since they are plumbed in series but trying to turn at different rpm.

I'm guessing the prior motors were plumbed differently, or were 'looser', or had some kind of internal relief valves.


Mike Halloran
Pembroke Pines, FL, USA

 

[hydroman247]

Yes the front wheels are at different angles when turning. At full lock it is very noticeable and with very non forgiving tyres and a rigid chassis this is probably what it going on. It is just strange that the previous motor which is a very very similar design, does not suffer from the same issues.

The older motors were plumbed in exactly the same and work almost identically apart from these 2 issues.

 

[ccfowler]

The problem with the lurching during sharp turns seems to be exactly the common "wind-up" that happens with a classic all-mechanical 4 wheel drive where a non-differential transfer case drives both the front and rear axles that each have center differentials. When all four wheels have very good traction, the lurching gets very violent, and broken components can be an unhappy result. Usually, one or another of the tires slips just enough resulting in the lurching with the release of some of the "wind-up" sparing whichever weakest part (usually U-joints) from breaking. A differential in the transfer case cleans up this mess. This violent lurching is not usually experienced where traction is poor such as on snow or ice. The lurching is usually worse at lower speeds, and I have always attributed that to more variability in traction at the different wheels due to more "bouncing" at the different tire contact patches at greater speeds.

My guess is that Mike's speculation that the old motors were looser is right, and their leakage provided the needed defacto front to rear differential functionality.

Valuable advice from a professor many years ago: First, design for graceful failure. Everything we build will eventually fail, so we must strive to avoid injuries or secondary damage when that failure occurs. Only then can practicality and economics be properly considered.

 

[hydroman247]

We ended up swapping flow direction through the motors with no change so pretty sure you are right CCfowler. Only thing that makes sense at the moment. The lurching only comes on when the motors are in high displacement so I am going to get the motors swapped back to originals to confirm this it is the geometry of the machine.

Thank for all your help.

 

[hydroman247]

Does anyone have any recommendations for this issue? I have tried a larger cross line restrictor from 0.5mm to 1mm but don't really want to go any larger otherwise it will affect performance when there is low traction on one wheel.

 

[hydroman247]

After trying almost everything we can think of, including bypass restrictors and adjusting the anti cavitation valve, it sitll judders. The fact that the bypass restrictor (A to B 0.5mm on the 2 front wheel motors) did not work is now pointing to a spool issue.

If you look at the picture below, you will see a pressure log. This is monitoring one side of this series parralel setup. The pink line is a gauge before the rear wheel motor, the green line is inbetween the 2 motors and the blue line is on the outlet of the front wheel motor.

At the beginning the machine was driving straight and as the wheels are turned to full lock, the oscillations begin. What is interesting is that pressure drop on the pink line and then the green line actually peaks before the pink line picks up fully.

It is very strange how these motors are reacting with our drive system.

 

[Compositepro]

Similar lurching will happen when you turn any vehicle that doesn't have differential action between the wheels. Something has to give and it is usually wheel slip or a broken axle. A pressure relief valve may help in your case.

 

[MikeHalloran]

I'd be inclined to plumb the fore/aft motors on a side in parallel, and provide each with an excess flow fuse so a slipping wheel can't consume all the pump flow.


Mike Halloran
Pembroke Pines, FL, USA