Jump to content

Torque/acceleration/traction


Recommended Posts

I'm currently working on the automated control of a scalextric car (3yr Masters Project) and whilst we're 90% of the way there, we're having some problems with wheelspin.

the on-board microprocessor uses an optical encoder on the motor shaft to calculate displacement, obviously, if the wheel slips and spins the displacement measurement gains error but we want to accelerate as fast as possible.

I understand that once torque is greater than traction, the wheel spins, and that maximum accleration should be when the torque is equal to traction.

what happens when the wheel spins? why/how does traction recover? when?

I appreciate that this is all a bit 'in-theory' bet never having studied mechanics in detail, i'm struggling with the subtlties of this one.

any help appreciated!

Link to comment
Share on other sites

You may need to measure the speed of one of the non-driven wheels and compare it, or perhaps just limit the rate of rise of the motor speed to what you know the vehicle will actually be capable of - there are optimum levels of slip for a car tyre, I think best traction is actually achieved at something like 15% slip, or something, errr... :unsure:

A read of the MSEFI Traction Control page may help you, it's fairly basic but sounds similar to what you are trying to achieve.

Link to comment
Share on other sites

Thanks John,

That 15% ties in with our lab experiments (ish), the car accelerates faster with a degree of spin, but then asymtotes as spin increases.

Can't measure the speed of the non-driven wheels, as they don't touch the track! stooopid pick-up plate gets in the way.

Our goal is to limit the rate of acceleration so as to attain ~optimal performance, i'm after a bit of logic/theory to steer us in the right direction.

newcar2.jpg

Link to comment
Share on other sites

I believe you are in the wonderful world of coefficients of friction here.

Unfortunately it was too long ago for me to recall whether it is static, sliding, rolling or a combination (me thinks a combination, so even more fun!). This is a great Google topic, as loads of 'academic beards' have published papers on it. I started, then felt my eyeballs rolling. Over to you :).

One thing I am pretty sure you will need to establish are the frictional properties of the tyres and the track. There are some generic values here http://www.roymech.co.uk/Useful_Tables/Tri...co_of_frict.htm, but seem to relate to real tyres rather than scalextric. These will need the area of each material in contact.

EDIT: Sod it, I was bored. You will also need to establish the force applied horizontally at right angles to the radius of the wheel i.e the circumference of the tyre (ooooh - I used to know this as well!), based on the force applied at the centre by the axle/motor.

A combination of these together with the mass of the vehicle should lead you to the ooomph required to get it to spin and thus not to spin. With the small (relatively) figures you will be dealing with on a scalextric model, as opposed to a 2 tonne car, I think you will struggle to get a precise answer mathematically, so I would recommend lots of practical testing :lol:.

Good luck.

Link to comment
Share on other sites

Thanks,

The time for reading papers has passed, we've got 3 hours of lab time left and the report due-date is monday, this was a kind of last minute attempt at finding some sort of revalation that might help us out.

The project is fundamently flawed anyway, the bare car could do a 6m lap in ~3s with a fixed voltage supply, we're now fighting to get it under the 5s mark due to the weight of all the hardware we've added. :rolleyes:

Link to comment
Share on other sites

The project is fundamently flawed anyway, the bare car could do a 6m lap in ~3s with a fixed voltage supply, we're now fighting to get it under the 5s mark due to the weight of all the hardware we've added. :rolleyes:

Flawed - nah! Surely your base line is with the weight of the vehicle as it is now with the electronics off and then measuring the improvement with them switched on?

As I said above, I think you will struggle to get a consistent practical result simply due to the tiny figures you are working with.

Link to comment
Share on other sites

a valid point about the baseline, however our initial brief was along the lines of 'get it better than you could do it by hand' no mention of the baseline vehicle mass :(

FWIW, we've just got a consistant lap time of 3.6s so we're getting there. tuning from datalogged plots and guesswork is a time consuming iterative process.

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

 Share

×
×
  • Create New...

Important Information

We use cookies to ensure you get the best experience. By using our website you agree to our Cookie Policy