Anyone else having problems with their timing loop? Read on...

[bedsrcmcc]

We'd been having a few issues recently with our timing loop missing cars, the odd Mardave (alu chassis) and some 1:12th Associated (carbon chassis).

We were worried that our AMB Timing system or the loop was playing up, but it has become clear that our system is running fine, but I wish I could say the same for the cars mentioned above.

One of our members who's got plenty of electrical knowledge, and more to the point equipment close to hand has been running some experiments and the results are both interesting but also alarming, read on...


I've been doing some testing on transponder type and location.

• First of all I setup a loop in the lab and coupled its output to a Spectrum Analyser and Oscilloscope.
• Then I checked the received signal from both the AMB and MRT transponders when sat in the loop without any car at all.
• The AMB has a higher signal level than the MRT and its phase change (used to transmit the transponder number) is smoother and better defined.
• So the AMB should be less problematic than the MRT, although the output from the MRT is still perfectly fine.
• I then fitted both to the proper mounting point in my 1/12th car and this is where it got interesting.
• The signal from the MRT disappeared almost completely, the signal from the AMB dropped too low to be useable although it could still be seen.
• So bottom line is that where I was mounting the transponders in my 1/12th car will never work and it's absolutely not the fault of the decoding system or loop positioning at all.
• After trying many mounting positions it turns out that the best place to mount the transponder on a carbon chassis car is in a recess cut in to the front bumper so that the transponder can stick out somewhat from the front carbon edge. This is going to be a bit tricky on my Associated, but I've got a few ideas.
• The transponder runs at a frequency of 5MHz and seems to use a relatively simple Phase Shift Keying modulation.
• At this frequency the coupling is more magnetic than RF, sort of like an air-cored transformer where the transponder is the primary and the loop is the secondary.
• The carbon and aluminium chassis’ will interrupt the path of the magnetic flux lines and reduce the efficiency of the transformer coupling.
• With the transponder mounted in the centre of the chassis (as in my Associated RC12R5.2) even with an aperture cut out below it, most of the magnetic flux is interrupted.
• With the transponder mounted higher up and sticking out over the carbon edge, some of the magnetic field is still interrupted but large portions are not - which is why this tends to work.
• I've had another thought about mounting a transponder above a cut-out in a conductive chassis such as aluminium or carbon fibre. This could actually give worse results than no cut-out at all. What actually happens with a cut-out is that some flux lines go through the centre of the transponder and the cut-out in one direction, and then return around the outside of the chassis in the other direction. The effect is that the chassis actually becomes a short-circuited secondary stopping induction into the main pickup loop. This could be the reason why the signal dropped to almost nothing when mounted above the aperture.

I would be interested to know if other clubs/drivers have had the odd issue with these types of chassis not being picked up during a race.

Warmest regards

[Dombrasky]

interesting reading...the thing i found out about transponders was after many years of mounting them as low as possible in cars, i had a wire break on mine that was on the top of the servo on a losi 22,so i used a hand out Amb
as it was the new smaller ones i fitted it low down on the chassis behind the servo
next heat i was missing laps and had a weak signal,because it was brand new i new it was to do with were i mounted it
so.. i put it on top of the servo and it worked spot on,this proved to me that the signal must fan out to the side and not straight down as i had previously thought

[bedsrcmcc]

Had this response from Rob Nelson about the problems we found...

Hi Scott,

Thanks for the email. Graham has experienced what many carbon/aluminium chassis car owners have been frustrated by. Its not just an issue in RC but also full size vehicles to a lesser extent.

Very interesting and thorough analysis and a very good explanation of why carbon/aluminium is a problem.

Its not just aluminium but any conductive metal but aluminium tends to be the metal used for RC chassis.

The system uses what Mylaps call "Magnetic Induction" which is exactly what Graham deduced. The issue of carbon and aluminium is well know and is highlighted in the instruction sheet that comes with the Mylaps (and I think the older AMB) transponder. With carbon and aluminium chassis I recommend mounting above the chassis and at the edge, with some cars the steering servo is available in a suitable place. Bumper mountings also work if some distance from carbon/aluminium can be achieved. It would be helpful if the car manufacturers could take this into consideration.

My simple explanation is that the transponder is like a light that shines on the loop. Carbon and aluminium won't let the light through but other material (plastic, fiberglass) do. The light needs to illuminate the loop for as long as possible (hits) and needs to be as bright as possible (strength). Sometimes height helps with the faster cars to increase hits. The RC3/4 Decoder is very sensitive so signal strength is not such a big issue. Mylaps say strength >100 and hits >10 at speed should be OK. If there is noise then strength needs to be at least 60 above the noise.

One of the issues of side mounting is that if the cars tend to be at one side of the track as they cross the loop if the transponder is on the "wrong" side the signal can be compromised.

The difference in the signal for MRT is because they are reproducing the signal recorded from a "genuine" transponder.

If might be worth highlighting these findings. People are far more likely to read something on your website than in the sheet that comes with the transponder or I post on my website!

Thanks to Rob for this reply and I hope this helps others...