How Foil Point Contact Bounce causes a White Light
You wire up a foil. Used a tester to check all is ok. You plug it to a machine. You beat the foil. White light! You return to the bench frustrated. Let's have a look at what is actually going on.
Lets start at the tip
The sensing tip of a foil is a normally closed switch. This means electrical contact is by default connected. The switch opens when the point is depressed, normally against the opponent’s body. We call this a “touch.” Unlike the epee, there is only one wire going to the point. The second wire is actually the body of the foil itself. This is why insulating tape is needed in the upper portions of the foil, so that an unlucky hit will not result in the lamee shorting out the tip and the body of the foil, (we can talk about the consequences of frayed tape in another blog).
Like all switches it is subject to contact bounce - a momentary intermittent opening and closing of a switch, when opening, closing and more importantly when subjected to shock or vibration. The foil is routinely beaten or parried during use. If contact bounce is extreme this can trigger the electronic device to falsely detect a touch.
Is there a way to see these intermittent opening and closing? Yes there is. Is there some way of determining if a foil is about to go bad? We think so, but it does require the use of an oscilloscope. For now we visited a couple of fencing clubs and tested the usual club foils, some new, some good, some passable some are really bad. Here is what we got:
We took a lot of foils hooked them up to an oscilloscope or a "scope" and beat them to our hearts delight. The typical trace looks like the one you see above. So let's try to break down what you see in the oscilloscope graph, usually called a "trace" by the electrical engineers. From left to right that is the time axis, each big square is 1 millisecond. The whole trace then shows 10 milliseconds. (10 squares, count them left to right). When the point contact is opening, i.e. its misbehaving, you will see the yellow line jump up high. When the contact closes it will go down low again.
The trace in figure 1, which is what we deemed to be your garden variety club foil, shows that the point intermittently opening and closing until about the 7 millisecond (mS) square. Notice also when the point is open, i.e. its up in the air, it lasts for about .2mS. Lets call that its "air time."
A millisecond (mS) is a very short time indeed. It is imperceptible to the human eye. One traditional movie video frame is about 40mS long, (the old 24 frames per second). The new modern video formats at 60 frames per second has frames lasting about 17 mS long.
At this point it might be useful to know that FIE states that for a foil hit to occur, the point has to "break contact for 14 milliseconds". The term "break contact" is a bit subject to interpretation because it doesn't say how "clean" it has to be. But for now, however you interpret this rule, neither the air time (.2 mS) nor the bouncing time (7mS) would be breaking that rule. This foil will not trigger a white light when beaten.
Here is another trace of another typical foil, this one is better than the previous "typical one" but still within the "typical" category in our opinion.
In this foil, the intermittency time is mostly confined to the first 2 milliseconds (mS) with one lone outlier at the 5mS square. The "choppy" nature of this trace shows that there is some dirt in the contacts somewhere, (possibly even on our scope probes) but it is not affecting the measurement.
Next on the line is a neglected foil, relegated to the dark corner of the armory...
The trace above is from a poorly maintained foil. This particular foil had contact bounce lasting beyond 15 mS (only the first 10mS is shown), the air time exceeds the typical .2mS as well. Whether or not this would cause a typical scoring apparatus to detect a touch will probably depend on the manufacturer of the scoring apparatus. Some manufacturers may allow contact bounce to happen within the 14mS contact break time, some may not allow it. In our opinion contact bounce should not last 14mS anyway. This is quite an one unusual of the bunch.
We felt sorry for the foil so we took some contact cleaner and cleaned the point and contacts. We beat the blade again and here is what we got:
The performance improved marginally only. There seems to be less bouncing now but the contact bounce duration still lasted until the 15mS mark (trace only shows up to 10mS). The cause of this long contact bounce time probably has more to do with the spring or mechanical issues rather than dirty contacts. The trace does show less "choppiness" indicating improvement in contact quality. Still we wouldn't use this foil in a serious competition. Unless of course the goal is to frustrate you and your opponent and probably the umpire and the spectators as well. Back to the corner it went.
Next we were able to talk the armorer to bring us out a brand new foil, fresh out of the wrapper for our beating pleasure. Here is the trace:
Except for a fleeting skip of a bounce, the whole trace was clean and noise free. And we had to beat this foil a bit hard for that even to come out. Most of the time there was no bounce at all.
It is important to know that the harder you beat the foil the long the contact bounce lasts. But no matter how hard you beat them, the typical good foils will not bounce near the 14 mS square.
Conclusion
It does seem that with an oscilloscope it is possible to determine the quality of the point and sort of predict its performance and likely deterioration. In fact some commercially available testing tools incorporate a simple oscilloscope as part of their testing equipment to detect these intermittent contact breaks.
It does look like that if you have a foil blowing a white light when beaten, it is usually a spring or a mechanical issue that is causing it, not just simple dirty contacts.
