Vehicle with pivoting front axle: correlation between axle load and wheel hub

[pietro82]

Hi all,

For a project I have to perform some fatigue analysis of a commercial vehicle with a fpivoting front axle with no suspension system (i.e. the axle can freely rotates along the vehicle longitudinal axle). The aim of the analysis is to rank the most common tasks according to their pseudo-damage. Since we have a tight schedule, we need to install sensors on several vehicle samples, therefore we have in mind to perform a fast analysis using accelerometers instead of strain gauges. So in first instance, we need to check the methodology, therefore I have installed 2 accelerometers at the wheel hubs to measure the axle vertical accelerations and 2 strain gauges to measure the axle vertical load and then I drove the vehicle on some hard surfaces. The verical load signal on the left side almost overlaps the one on the right side due to the type of suspension. I have double integrated the accelerations to get the displacement and after that I have removed any signal drift due to the numerical integration. As stated in other papers there is a high correlation between the displacement and vertical load, upper than 0.9.
What surprised me is that in the left side the correlations is positive (i.e. the displacement increases with the increasing of the load) and in the other one is negative (i.e. the displacement decrease with the increasing of the load). How is that possible?
Any suggestion is welcome.

Thanks

cheers

 

[GregLocock]

I don't know about your particular problem, but using accelerometers to predict fatigue loads from road inputs has never been satisfactory in my experience. Good luck.

Cheers

Greg Locock


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[pietro82]

Hi Greg,

I have never tested it and I would believe with you. I found many papers from SAE and some whitepapers from Ncode where it was used with some degree so I wanna give it a try. What I find so far is:
[ul]
[li]Using a high pass filter for removing the drift of the integrated displacement arises a wrong damage computation due to the filter roll-off e phase-lag, so it's necessary to use other methods for removing the drift.[/li]
[li]With non stationary loads, such as varying vehicle weight condition, there is an almost null correlation between the displacement and the vertical load[/li]
[/ul]

But I will explore more the method in the next days

Regarding the suspension: for the kind of suspension: the load between the two side are identical and the two acceleration signals have a phase-lag of around 180°. When a tire (e.g. the right one) would hit a bump, it moves up and the other one would move down, while the load increases in both sides. Keep in mind this, I would expect high correlation between the displacement and load on the right side only if the most of road "events" (bumps, patholes, ecc) happen of that side. Could it be the case?

 

[GregLocock]

Well you've certainly identified one problem, using an inertial reference for your displacements is wrong.

So, imagine a one wheel bump on the RHS. The left hand wheel sees no displacement, yet sees just as much force as the right hand wheel as it rises over the bump.

Here's another issue. My guess is that first bending of your beam axle is around 40 Hz (say). Above that the accelerometers on each hub are no longer connected via a rigid beam, but by a filter.

Yet for fatigue you need data to rather more than 40Hz.



Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[pietro82]

Hi Greg,

you're on right about the filter...I will explore more deeply the problem. I think for the kind of vehicle the first frequency could be upper than 40 Hz, for this reason at first istance I dind't take care that much about the bending first frequency.
I'll keep you updated.

Thanks

Cheers