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Subject: Re: Detecting moment when a fast moving object passes over a line
Date: Sat, 23 Nov 2002 23:27:38 GMT
NNTP-Posting-Date: Sat, 23 Nov 2002 18:27:38 EST
Organization: Road Runner
Richard Sim wrote:
> Hi, I'm working on a project currently that's for automatic lap scoring in
> radio controlled car races, where the cars may be travelling in excess of
> 40-50 MPH at the point where they cross over the start/finish line, and
> several cars may pass over the line within close proximity of eachother.
> The cars will have "transponders" in them, which detect when they cross over
> the start/finish line, and send a signal to a reciever/decoder that's
> connected to a PC.
> I'm having trouble finding a way to implement the start/finish line
> reliably; the biggest problems are that the transponder may be anything up
> to 15cm (6") above the ground with upto 3mm of graphite, plastic or
> aluminium between it and the ground as they will be inside the cars, and
> must be as small and light as possible (1.5"x1.5" is pretty much stretching
> Anything that works on line-of-sight (such as IR) is out of the question
> unfortunately, as the cars may very well be completely covered in mud, dust,
> oil, etc.
> So far I'm thinking of using 2 linear hall effect sensors connected to an
> op-amp to create a very sensitive magnetic sensor, with some kind of wire
> loop or electro-magnet under the start/finish line.
> I'm not very confidant that this will work however, as firstly I think the
> wire loop/electro-magnet wouldn't have a strong enough magnetic field to
> trigger the hall effect sensors, and secondly the hall effect sensors may be
> triggered by the car's motor as depending on the type of R/C car, the motor
> could be very close to the transponder (and the motors have very powerful
> magnets as they can go in excess of 50,000 RPM).
> The tranponders have to transmit their unique ID to the reciever/decoder
> when they pass over the start/finish line, and to do this I'm probably going
> to be using the Linx ES series of RF modules (but the LC series for testing
> however as they're much cheaper).
> I'm definately a beginner at electronics, and this is the first complex
> circuit I've worked on, so any help in finding a reliable way to implement
> the start/finish line & transponders would be greatly appreciated (and
> please be detailed as, like I say, I'm a beginner).
There are a number of things that come to mind here. I have never done
such a thing, but it seems to me that perhaps what you may want is to
have the transponders on all the time, and only detect it when they pass
over the line. For this I would use some directional antenna, like a
small dish or cannon type. The issue with this idea is how long it takes
for the transponder to repeat the signal. To take care of this, I would
simply use a steady tone instead of a digital ID, but now this is
probably going to require more involved decoding than off-the-shelf
stuff. I guess before going any further, I would suggest finding out
what is normally done for this type of thing. Perhaps they use a dish to
send an initialization signal over the line, so when a car passes, it
picks up the signal, and transmits the ID. In this way, proximity is no
issue, as long as it passes through the signal, which is pointed out
across the track where the line is. The problem with this MAY be that
some cars might pick up the signal earlier than others. It depends on
beam width. Surely there is some data out on the internet about this.