Magnetically detecting crossing of finish line

[Arto Sarle]

Hi all,

I'm looking for advice on what would be the best method for a
home-built transponder to detect when it has crossed over a certain
point (the finish line, of course) on a kart car track.

After passing over the finish line, the transponder needs to transmit
timing information (via RF) to a PC. The reason I want an "active"
transponder is that at some point in the future it can be used to
transmit other telemetry as well.

At the moment I'm thinking of creating a localized EM field at the
finish line by burying a wire loop or permanent magnet there, and then
including a magnetic (Hall effect) sensor or switch in the
transponder.

Provided that the transponder would be mounted in the car 10-20 cm
above the track surface and the car may be moving up to 80 km/h, do
you think this method would work reliably? Most uses for Hall sensors
seem to be in the millimeter range.

I'd much appreciate your thoughts or any suggestions for a better
transponder activation method!

Best regards,
Arto

 

[R.Legg]

Hi all,

I'm looking for advice on what would be the best method for a
home-built transponder to detect when it has crossed over a certain
point (the finish line, of course) on a kart car track.

After passing over the finish line, the transponder needs to transmit
timing information (via RF) to a PC. The reason I want an "active"
transponder is that at some point in the future it can be used to
transmit other telemetry as well.

At the moment I'm thinking of creating a localized EM field at the
finish line by burying a wire loop or permanent magnet there, and then
including a magnetic (Hall effect) sensor or switch in the
transponder.

Provided that the transponder would be mounted in the car 10-20 cm
above the track surface and the car may be moving up to 80 km/h, do
you think this method would work reliably? Most uses for Hall sensors
seem to be in the millimeter range.

I'd much appreciate your thoughts or any suggestions for a better
transponder activation method!

Best regards,
Arto

Think optical.

RL

 

[Arto Sarle]

Think optical.

RL

I've given some thought to optical solutions, but couldn't come up
with a satisfactory trackside solution that would work under all
circumstances (such as when there are multiple cars crossing the
finish line simultaneously) ?

Arto

 

[legg]

I've given some thought to optical solutions, but couldn't come up
with a satisfactory trackside solution that would work under all
circumstances (such as when there are multiple cars crossing the
finish line simultaneously) ?

Arto

Detection would have to be vertical. Either place emitter to shine
through track, detector to sense through track, or use reflective tape
on track to bounce back info to integrated emitter/receiver.

Magnetic solutions have to be immune to noise (relatively massive near
these crude motors and brushes) and variations in the devices being
detected.

If you want to be archaic, then use a video/flash camera with
playback, as might be the case in the real world. This could enhance
whatever sort of vroom vroom effect you're getting out of this
activity.

RL

 

[John Smith]

On 3 Nov 2003 06:11:04 -0800, [email protected] (Arto Sarle) wrote:

Detection would have to be vertical. Either place emitter to shine
through track, detector to sense through track, or use reflective tape
on track to bounce back info to integrated emitter/receiver.

Magnetic solutions have to be immune to noise (relatively massive near
these crude motors and brushes) and variations in the devices being
detected.

If you want to be archaic, then use a video/flash camera with
playback, as might be the case in the real world. This could enhance
whatever sort of vroom vroom effect you're getting out of this
activity.

RL

Thanks for answering! I'm very intrigued about your suggestion to use
reflective tape on the track, bouncing a beam of light back into a detector
in the transponder. If this would work, it would be exactly the kind of
low-budget, simple-yet-beautiful design that I'm interested in. It would
also allow the detection point/line itself to be totally passive.

I think, though, that instead of tape I'd probably use some sort of
reflective white paint, as that would make maintenance easier (tape might be
short-lived on the kart track).

Could you offer any suggestion as to what kind of emitter/receiver might
best work in this configuration, and how close to the track would the
transponder have to be mounted? Do you think there might be problems because
of false positives (other reflective surfaces such as a coin on the track)?

Arto

 

[Arto Sarle]

Thanks for answering! I'm very intrigued about your suggestion to use
reflective tape on the track, bouncing a beam of light back into a detector
in the transponder. If this would work, it would be exactly the kind of
low-budget, simple-yet-beautiful design that I'm interested in. It would
also allow the detection point/line itself to be totally passive.

I think, though, that instead of tape I'd probably use some sort of
reflective white paint, as that would make maintenance easier (tape might be
short-lived on the kart track).

Could you offer any suggestion as to what kind of emitter/receiver might
best work in this configuration, and how close to the track would the
transponder have to be mounted? Do you think there might be problems because
of false positives (other reflective surfaces such as a coin on the track)?

Arto

Oops! The above was posted by me but with an incorrect name ;-)

Arto

 

[R.Legg]

Could you offer any suggestion as to what kind of emitter/receiver might

best work in this configuration, and how close to the track would the
transponder have to be mounted? Do you think there might be problems because
of false positives (other reflective surfaces such as a coin on the track)?

Reflective sensing is the most dificult and prone to interference,
especially if three or four are operating in close viscinity.

You'll find IR emmitter reciever packages with angled and focused
devices. For the distance involved you'd probably have to modify them.

As to reflectivity of target, I suggest useing the best possible to
begin with. Reflective tape in .0025 thickness(including adhesive) is
a commodity item.

Simplest is reciever below track, just detecting ambient light - gets
blocked off by traffic, to give your signal.

RL

 

[Wong Sy Ming]

The commercial units for R/C racing can handle multiple "triggers" past a
loop buried in the ground. The transponders are intelligent, having their
own power source and such...

If cost is not an object, you might want to have a look at
http://www.amb-it.com

 

[Arto Sarle]

Hi!

Thanks for the suggestion! I am familiar with AMB, as they seem to dominate
this field.

However, going for an existing system is out of the question as there are
eventual special needs to be addressed. Right now I just need to figure out
what is the best, most reliable way to actually implement the finish line
crossing with an active transponder.

Arto

 

[Arto Sarle]

Simplest is reciever below track, just detecting ambient light - gets

blocked off by traffic, to give your signal.

I agree it would likely be the simplest method, but I need to send out the
ID from the transponder, so I need to figure out a way to trigger the
transponder's RF transmitter instead of activating the detection loop itself
(as most existing systems seem to do).

Arto