Derby Talk

A recent emailer related a problem encountered at a race he attended: A car containing a magnet "fried" (permanently damaged) the finish line electronics.

I have not been able to acquire details as to the type of magnet, the length of the aluminum track, nor the brand and model of the electronics.

Finish line electronics typically have a pair of wires serving each (photodiode) sensor positioned just below the underside of the track and running perpendicular to the lanes. If the wires are not shielded adequately, a sufficiently powerful magnet in a car moving at typical PWD car speeds across the finish line should be able to induce a current in those wires which then creates a voltage in the sensitive input circuits sufficient to "fry" low power components. Protection circuitry (e.g. clamping diodes) is common in some circuits, especially if the input is from external (uncontrolled) sources, but I don't know if it is typical in commercial finish line equipment.

I know that there are some small but very powerful magnets that can be easily salvaged from scrapped personal computers.

I wonder if one of these magnets were used in the suspect car in hopes of getting a "kick start" from the start gate when it opens. Commercial start gates use stainless steel which is only lightly magnetic, so an ordinary magnet would probably not provide a meaningful effect.

Our district race inspection procedure includes a test using a pair of (steel) paper clips dangled in the front of the car to detect magnets, but I don't know for sure that it is applied consistently. The procedure would miss a magnet placed in rear of the car by someone who was deliverately aiming for damage rather than advantage. Lack of detection places the electronics in the tracks (which we borrow) at risk.

Has anyone encountered this problem? What do you do to enforce the usual "no magnets" rule? Does anyone have inspection gear to detect magnets?

Stan Pope wrote:Has anyone encountered this problem?

This is the first that I have ever heard of that issue.

Stan Pope wrote:What do you do to enforce the usual "no magnets" rule?

We hadn't worried about it for any of our races, since our start pins are stainless steel.

gpraceman wrote:

Stan Pope wrote:What do you do to enforce the usual "no magnets" rule?

We hadn't worried about it for any of our races, since our start pins are stainless steel.

The stainless steel rod in my supply bin is lightly attracted to a common permanent magnet, but not nearly as strongly as the same size steel rod. This was the basis for my statement that "Commercial start gates use stainless steel which is only lightly magnetic, so an ordinary magnet would probably not provide a meaningful effect." However, perhaps I should pull one of the bolts from my MW start gate and see how/if it reacts. Also, I should tear down one of my "on the way to recycling" PC's and grab some of its magnets to repeat the test.

According to this article in Scientific American, my SS rod may be different alloy than the SS hardware in the start gate. Also, it may have been subject to different stresses since its manufacture.

However, the start gate pins are really not the point of this thread ... rather I am concerned about possibly damage to one (or more) of the finish line units on tracks that we borrow for our district derby by a "science geek" dad who didn't read the rules carefully!

Stan Pope wrote:However, perhaps I should pull one of the bolts from my MW start gate and see how/if it reacts.

Our stainless MicroWizard pins (bolt, with a vinyl cover) did not seem to attract a magnet; however, I didn't try one of those rare-earth "super" magnets.

FatSebastian wrote:

Stan Pope wrote:However, perhaps I should pull one of the bolts from my MW start gate and see how/if it reacts.

Our stainless MicroWizard pins (bolt, with a vinyl cover) did not seem to attract a magnet; however, I didn't try one of those rare-earth "super" magnets.

Same as my gate.

Wonder if a technique such as suspending the bolt (or the magnet) on a long dark thread and watching for thread deflection from vertical when the magnet (or the bolt) is brought near might be required.

The attraction to my SS rod is very slight (barely perceptible with a handheld common magnet), and the attraction to the smaller bolt may be even less.

Wonder how much attractive force would be needed to gain 0.001 sec on a 4-section Freedom track? Not much, I suspect.

I tried a small rare earth magnet on the starting pin of my freedom track, and there was no discernible attraction.

Perhaps thy compass could guide you to a suitable solution?

Lessemf.com/dcgauss.html

Good contributions to related "kick start" issue.

I've inquired of my correspondant regarding specifics of the finish line electronics.

I have looked at my MicroWizard "Fast Track" and see separate, closely spaced pairs of wires serving each photodiode. The pairs are not "twisted", but, even so, there should be little difference in the currents induced in each wire of the pairs. This should protect the photodiodes.

There is still the possibility of "common mode" current which could affect upstream semiconductors. Maybe I can organize a test which does not put my timer at risk...

MaxV wrote:I tried a small rare earth magnet on the starting pin of my freedom track, and there was no discernible attraction.

I pulled two rare earth magnets from a decommissioned PC component and tried one of them against my MicroWizard start gate bolt. Holding the magnet between my fingers and approaching the start gate bolt with it, I could not feel any attraction. Next, I balanced the 1.5" long magnet across my index finger and approached the bolt from above with one end of the magnet above the bolt. Beginning at a separation of about 1/8", the magnet tilted on my finger toward the bolt.

In absolute terms, the attractive force to the stainless steel bolt seems miniscule when compared to its attraction to my file cabinet, for instance! (Prying it off the file cabinet is a challenge.) The question in my mind is whether such a small attraction would cut a meaningful amount of time off one's time. (Grandson's 3-run time was 1 ms behind the next faster car ... gaining 1/2 ms per run would have moved him up one spot! Gaining 1 ms per run would have moved him up 3 spots!).

I also tried to see if I could measure any current or voltage induced in wires by a moving rare earth magnet. My 'scope has long since succumbed to a lightning transient in my radio tower, so I have no way to measure short duration spikes. The impedance and inertia of my VOM was simply not up to the job of showing the presence of any transients when the magnet was dropped through a loop in the leads from 1 foot above. So ... help is needed for this part!