Re: Timing using Transponders

From: dmm (dmmilne_at_ozemail.com.au)
Date: 08/07/04 

Date: Sun, 08 Aug 2004 08:18:28 +1000

On Wed, 04 Aug 2004 07:45:50 GMT, "Iain Wilkie" <iain@wilkie-electronics.co.uk> wrote:

>I have been looking at race timing systems. The type of system normally used
>is an infra-red beam (at say a finish line) that is broken and a time from
>an accurate

I'd use a laser rather than IR, but that's just me.

>clock is logged. The competator number is manually allocated against that
>time.
>
>In order to eliminate the manually entered competator number you can use a
>transponder
>which is attached to the competator, and a mat is placed at the finish which
>emits a magnetic
>field which powers up the transponder which it turn transmits its ID (i.e.
>competator number) to
>an aerial in the mat, hence eliminating the need to identify the competator.
>This is very useful if you have
>a great number of competators crossing the finish line at the same time.

The AMB tags that you refer to below are self powered, or powered by the
car's on-board battery. This (presumably) is to eliminate any delay in powering
up the tag via a transmitted EM field from the loop in the track. The tag is
probably transmitting continuously during the race.

>HOWEVER, most people on this forum will understand that this can only be
>used to identify the competator
>and not to log the time since the point that the transponder actually powers
>up and transmits depends on various
>parameters like the actual physical position and height of the transponder
>as it enters the magnetic field and also
>the boundries of the magnetic field is fairly undefined, also the speed at
>which the transponder enters
>and moves through the field will affect this. . This means really a beam is
>still used to define the actual
>physical finish "line", since the transponder may transmit from either a few
>feet before or after the finish line.
>These mats are about one metre wide.

The loops in professional gear are embedded into the track, and don't have mats.
Mats would get picked up by the disturbed airflow of the car and get flung around
the track, which would be very dangerous to other drivers, pit crew, and the public.
They would also severely degrade the cars braking or cornering ability and I cannot
imagine any authoritive motorsporting body allowing mats onto a track.

Since the transponders are active transmitters, then the loop would only have to
receive the signals. If the loop construction is long (across the track) and narrow,
(say 50mm wide) then the received tag signal strength would have a bell shaped
curve to it as the car passes the loop. The point at which it is at a maximum is
where the tag is directly above the loop. If the loop is long and wide (a metre or
more), the signal strength would would be two bell curves, probably superimposed
to a degree, forming an "M" type signal. This would give you the speed (time taken
to get between the two peaks of the bell curve), and the position of the car.

The systems get calibrated before the races during practice sessions so the expected
signal strength of the tag on each car can be determined and logged.

>There is a system made by AMB that claims they have a system similar to the
>above that CAN be used for
>timing as well. I cannot understand how this can be done - remember we are
>looking for accuracies of 1 millisecond
>as a competator crosses a finish line that is effectively a thin line and
>they are moving a different speeds.
>Have a look at http://www.amb-it.com/amb.asp and help me get my head round
>this if anyone can.
>
>Thanks in Advance
>
>Iain

At 300km/hr, the distance travelled by a car in 1ms is 0.083m, much less than a metre.
That should be plenty of time for a controller to log the data. For those who are
less amored (spelling??) of the metric system, 300km/hr is about 187miles/hr,
or 273 ft/sec, or 3 1/4" per millisecond.

TAG Heuer are also the Olympic, Indy500 and F1 timekeepers, amongst others.

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