Go Kart Detector

[obroni]

Hi,

I'm working on upgrading a project that acts as a drag strip for human powered go karts. I originally built it a few years ago using a old laptop and a velleman usb interface adapter. I now wish to rebuild some of it using a picaxe instead so it is more mobile and less power hungry.

The old design used large paddle switches (2 bits of wood and a hinge) for the starting sensor and a plank of wood with a vibration sensor for the end of the strip. Whilst these worked fairly well kids tended to have trouble aiming the cart correctly to hit the wood and on hard ground the vibration sensors tended to trigger too early as the carts came thundering towards them. Its also quite tough to get them to stay still.

As part of the rebuild I would like to also redesign the sensors. I have a few ideas and I was wondering if anybody could comment on them if they have tried something similar or suggest any alternatives.

1. 2 LDR's and a comparator.
One LDR is always looking up at the sky the other is in the middle of the lane also facing up with a shroud reducing its visible angle. When the cart goes over it will cause the comparator to trigger. Only problem I see is that the driver will need to drive the kart directly over the LDR unless I make several spanning the lane.

2. IR break beam
I'm not sure if this will work as some karts have a very small surface area or wheels with spokes. I'm thinking the beam will be big enough to shine around these small blockages on some karts.

3. Hose with LED and LDR at either end
Stretch a hose out across the lane with a LED one end and a LDR the other. When the kart hits the hose it should block the light assuming the LDR reacts quick enough. Getting the hose to stay in a straight line however could be tricky.


Any ideas?

 

[geoff07]

How about IR but using a reflector at the distant end
- beam size = reflector size
- no cables across the route, as with the ldr solution
- if you encode the IR you can discriminate between lanes
- range depends on strength of IR illumination but the ones across my garage door work over 17 feet very reliably (not using a reflector in this case).
- if necessary perhaps you could fit a small plate on the front of the carts at the right height to break the beam

Hoses would be a devil to keep straight and thus working.

Or maybe put the IR sender on each cart?

Sounds like a fun project!

 

[Andrew Cowan]

If I was doing this project, I would use a $2 ebay laser pointer pointing at an LDR. With an area of about 1mm square, wheels etc should easily block the beam.

Andrew

 

[obroni]

Hmmm.. interesting option. Maybe a bit tricky to get it lined up and I would have to make sure that they couldn't be knocked out of alignment.

Would a LDR be fast enough? I thought they took several milliseconds to react, would a spinning spoke wheel block the laser for long enough. I suppose I could place it higher to try and catch the body.

The carts are not designed by me. A number of Scout groups bring their own carts and pit them against each other. They will all have different ground clearances, height, width, build materials and wheels. So I know of at least one cart that the bottom is about 30cm's from the ground and another where even the top of the cart is below 30cm.

 

[Andrew Cowan]

I've done it before - it is a real pain to line up, but once done, it's great (unless someone knocks it)...

An LDR should be fast enough - if not, use a phototransistor or LED.

A

 

[Jamster]

If there was an LDR on the road pointing up so that any kart coming over would block the sun from it, that will get you a resonable length response. LDRs are cheap and reletively robust so could probably be runover.

If you were able to embed it in the road and have tubing for a cable to the side of the road then that would be better too.

 

[Dippy]

Blimey! 2 hours and the laser fraternity haven't cried Aaaargh!

If you are woried about a laser then use a powerful IRLED and have focussing/shielding on the receiver end.
That would act like a beam (think about it) and be easier to lign up than a laser.
And a laser would only need the slightest knock to mess it up.

I guess you could even put a reflector on the nose of the dragster and have a reflective sensor; laser or IRLED.

 

[techElder]

Any time you get into timing 'vehicles' running down a track to see who gets there first, you end up specifying the height, width and length of the 'vehicles' sooner or later.

Most of the time this happens because some joker figures out where the break-beam is and extends his vehicle to trip it before anyone else.

 

[obroni]

I think I may struggle to keep a laser in alignment considering in previous years half the central barrier has been taken out by bad driving.

I will give the upwards facing LDR and a IR break beam a test to see which works the best or maybe even use both of them in combination.

I will report back my findings.

 

[SAborn]

You spoke of a hose with a ir beam through ir earlier, this got me thinking why not use a rubber tube and a pressure transducer, like the old garages use to so they knew when a car pulled in or the highways for counting cars.

One cheap option for a transducer would be to use the pressure sensor out of a automatic washing machine that sensors the water level in the tub,

Most of these are just a diaphragm activating a on/off switch, and some adjustment on the screw on the back should allow for it to sense a small change in pressure.

I recently had a play with one from a Fisher and Paykel washing machine and these are very good as they use a frequency that is modified as the pressure changes.
The frequency is between 27khz with no pressure and 20khz full pressure on the diaphragm.

Here if a photo of the Fisher and Paykel sensor, but they all look much the same.