Road surface model

[JasonCSBB]

Hi all,

I've built a simple quarter car suspension model which I am using for my own education. Does anyone have, or have a link to, a road surface model, for use in a quarter car model? I've seen power (or amplitude) vs. frequency distribution plots, but these are good for linear systems, and not good enough for nonlinear models. A simple power distribution model doesn't have information regarding the probability distribution of peak accelerations of bumps and dips. IOW you can synthesize 2 different road models with the same amplitude/frequency characteristic, but have a completely different distribution of the size of the bumps/dips. Thus the reaction of a nonlinear model (e.g. in terms of RMS acceleration of the car's body), will be completely different.

My model has a PWL (non straight line, multiple breakpoints) model for the damper's F-V characteristic, so the size of the bumps matter...

TIA.

 

[JasonCSBB]

Bueller? Anyone?

Would a probability density function of instantaneous acceleration, with a spectral density function, adequately describe a given "type" of road surface?

 

[FeX32]

A couple minutes turned this up for me

http://sem-proceedings.com/20i/sem....lities-Linear-Non-linear-Vehicle-Modeling.pdf

as well as the attached paper.
It may not be exactly what you want. But there seems to not ba a standard model.
Best of luck




Fe

 

[JasonCSBB]

Thanks for the tip! I will go and digest it.

 

[FeX32]

No problem.



Fe

 

[JasonCSBB]

I digested it, <burp!>, and I learned something, but the author didn't touch on the subject of the distribution of the road displacement.

IIRC a random input variable can be described as having a given spectral density, and a distribution curve.

For example, noise with a flat spectral density, white noise, can have any distribution function. White noise doesn't imply gaussian or uniform distribution.

I believe that a quarter car model with a non-linear damper model, e.g. has a blowoff, will react differently in terms of RMS passenger vertical acceleration, with different distribution curves, for a given spectral distribution, despite the RMS displacement of the road surface model being the same.

http://en.wikipedia.org/wiki/White_noise

"It is often incorrectly assumed that Gaussian noise (i.e., noise with a Gaussian amplitude distribution — see normal distribution) is necessarily white noise, yet neither property implies the other. Gaussianity refers to the probability distribution with respect to the value, in this context the probability of the signal reaching an amplitude, while the term 'white' refers to the way the signal power is distributed over time or among frequencies.'

 

[FeX32]

I believe that a quarter car model with a non-linear damper model, e.g. has a blowoff, will react differently in terms of RMS passenger vertical acceleration, with different distribution curves

I agree with you on this. Assuming that the nonlinearity is a 'hardening' effect the larger amp. RMS input should impend harder on the passengers. Size of bumps do matter as you say


Fe