Sine waves

[Mjolinor]

I am just having some difficulty visualising this problem. It is related to an electronic ignition I am making.

The situation is:

I have a rotating disc with two magnets opposite each other.
I have a pick up plate with two coils opposite each other.
Manufacturing error means that these magnets and coils are not likely to be exactly 180 degrees apart.

With the coils in series will I get a single sine wave that is exactly right irrespective of the actual angle between the magnets and between the pickups provided that angle is small?

It is just baking my noodle a bit. Sometimes I conclude that there may be a slight DC offset, sometimes I think it's a sine wave and sometimes I think I may get two peaks. My brain is proper mush trying to get this straight. My gut says it is a single perfect sine wave.

[rs20]

*ASSUMING* each magnet/coil interaction would produce a pure sine individually, the result of adding the two (or four, or any number actually) sine waves of any phase will just be another simple sine wave, with no DC offset.

Now, given your description of the physical system, I would expect a pulse as each magnet went past a coil, not a nice sine wave, so that assumption above would be false.

[Mjolinor]

I don't think the sine bit is important. I will just detect a level or a level change or a peak but let's assume pulses then. If the angle is small are the pulses 180 degrees apart.

By small I mean that the magnet is still passing one coil when the second one passes.

If you assume they are 178 and 182 (that is much more than it will be) then on one pass both coils will be level at the top of the wave together, almost half a rotation later (178 degrees) one will be at the max and 4 degrees behind the other when it passes but the peak will still be 2 degrees before that. So that is exactly 180 degrees between pulse max and in fact it should be 180 degrees between any two points on the generated waveform.

I think, perhaps, maybe.

[Ian.M]

Nope. As soon as one pickup gives a non-sinusoidal waveform you risk broadening or double-peaking of the pulse and if they don't get exactly the same field strength the pulse may be assymetric.     Also if the two magnets are not exactly 180 degrees apart and exactly the same strength and radial distance, you'll get pulse to pulse jitter. 

As its a royal PITA to adjust the rotating magnets, a more sane design would be a single magnet + non-magnetic counterweight, as the pickup radial positions can easily be adjusted with mounting screws through two slotted mounting holes on the pickup and an eccentric screw in another hole, or a notch in the pickup and nearby hole in the baseplate to lever against with a 'tweaker' (made from a screwdriver with the end ground into a round pin at one edge of the blade), to adjust its position.  Use a soft O-ring belt drive from a synchronous motor and a heavy flywheel to minimise wow and flutter and adjustment should be as simple as scoping the pickups in turn with a DSO, triggered from whichever you decide is the master, while you tweak the others for equally spaced pulses.

[Mjolinor]

The more sane way to do it would be to stick one magnet and one coil on the crank instead of the cam.

I am still not convinced. I think that whatever variation there is in pulse strength / position there will be a point on the wave edge that is always 180 degrees. When both coils are together then the rising edge will be steeper than when the coils are staggered. I think that maximum will always be the point that is 180. That's just about the hardest to detect typically.

[Mjolinor]

OK. What about this. Pick two points, one rising and one falling on each pulse (or sine wave) the middle of those two points will always be 180 degrees apart.

[Ian.M]

I wouldn't bet on that.  Why not scratch-build a test jig with a record player, coils, magnets and bluetack and convince yourself of all the ways multiple magnets are a *bad* idea.

[Mjolinor]

I wouldn't bet on that.  Why not scratch-build a test jig with a record player, coils, magnets and bluetack and convince yourself of all the ways multiple magnets are a *bad* idea.

I already have it set up on my lathe. I also have a Boyer ignition that uses the setup explained above and there is absolutely no jitter and exactly 180 degrees between triggers. I think that internally they use a PIC but it is all potted so no examining what is in there.

The disc is only 50mm and does produce a sine wave as far as looking with a scope says anyway.

[mikerj]

The disc is only 50mm and does produce a sine wave as far as looking with a scope says anyway.

I'm somewhat familiar with the construction of the Boyer conversions, but I'm struggling to understand how a sinusoidal waveform would be possible given that the magnets spend the majority of their time not passing over either of the coils.  Also a sinusoidal waveform would be pretty useless for accurate ignition triggering. 

Do you mean that you get a single sinusoid-like pulse each time the magnet passes over the coil like you get from e.g. a variable reluctance sensor?

[Mjolinor]

The Boyer system uses very weak magnets but I am using rare earth ones so that is probably why the effect is much greater. I imagine that with the weaker magnets you would get pulses.

[patrick1]

how many cyl is the car ?...  because using a single magnet / pickup is dangerous, - if it missfires once, then you might blow the engine.

safer too use one pickup for each cyl

i have same issue with a homemade electric motor, - which has 4 stages of power, - north,off,south,off etc etc   ... and i use a 4017 decade counter... worth a look if your not familiar, - good for a single hall effect trigger system....    but as i say, if you can have one per cyl it will be safer.

[Mjolinor]

Two cylinder. BMW R75/6 1976. It uses wasted spark with two series six volt coils.

[patrick1]

if i were you, and the current system is still  in place and working.

i would use opamps too determine sine wave TDC,  and set the comparator pin too a lower voltage too ovoid starting issues.

and the output of those analog devices, can trigger a 2n3055 too dump a capacitor onto the ign coil.


unless you wanted too take the high voltage plasma route ?...  then just stick another capacitor across the spark gap, charged by an inverter via 20kv diodes....  always need overkill on the diode front i find.  - but simplicity is better. - counting steps between pulses is a recipe for anziety

[mikerj]

if i were you, and the current system is still  in place and working.

i would use opamps too determine sine wave TDC,  and set the comparator pin too a lower voltage too ovoid starting issues.

and the output of those analog devices, can trigger a 2n3055 too dump a capacitor onto the ign coil.

A 2N3055 is a very poor choice with it's low gain and low Vcemax  (~100v), and pretty much useless for a CDI system.

A sinusoid is also poor choice to trigger an ignition system from, the slower the trigger signal moves the greater the jitter you get on triggering.

[Mjolinor]

I am using one of these for driving the coils:
https://www.ebay.co.uk/itm/Bmw-R80-R100-R65-Mono-Ignition-Amplifier-Gs-Rt-Rs-Bosch/133153082887?hash=item1f008ace07:g:SKYAAOSw9GRdZVAc

Pretty standard Bosch trigger do dad as used on a lot of vehicles.

I do have it all working fine with a hall effect but I am having difficulty getting exactly 180 degrees apart for the two sparks per cam rev. That is why I thought I would use a pair of coils in series because manufacturing tolerances can be nulled.