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Author Topic: Analog variable pulse timing circuit for a pulse motor of your choice  (Read 4184 times)

Offline MileHigh

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I thought this might be interesting for those looking for a new way of timing the firing of their pulse motors:

I figure the desire is to have a coil firing pulse with as sharp an edge as possible (in both directions) with full control over the pulse width and the starting angle relative to top-dead-center.  And you want to do it cheaply and easily and in the analog domain.

There must me a little $2 op-amp chip out there.   Power it from the source 12-volt battery.  I would have an independent pick up coil on a stand that you could move around.  That gives you the freedom to experiment with different configurations of pick-up coil.  Moving the stand around changes the start timing of the pulse and the width of the pulse (there is another way to change the pulse width.)  So you fire the pulse from an alternate magnet on the rotor.  Suppose you have a rotor with four magnets 90 degrees apart.  You can use either magnet that's 90 degrees away from the magnet that is currently facing the drive coil to govern the pulse timing.  Your pick-up coil output connects to the + input of the op-amp and you have a 100K potentiometer to dial the voltage on the - input of the op-amp.  That's the second way to change the pulse width.  (Tie one side of the pick-up coil to a simple resistor voltage divider and a filtering cap at about +6 volts.)  The output from the op-amp is going to either be very close to ground or very close to +12 volts.  So you use that to fire your IGBT or MOSFET or transistor.  There may be little tiny analog tweaks that are required, typically by adding tiny decoupling capacitors to key voltage nodes to ensure that you have clean and glitch-free switching.

I would also run the op-amp output to a simple transistor voltage-follower (or a simple NPN switch with the LED tied high) to power a bright LED that strobes the rotor.  So you paint some white lines in the right places on the rotor and that's your rotary strobe to observe the pulse timing.  So you don't even need a scope.

So ultra clean fast switching and fully variable pulse timing to hunt for the sweet spot and throw in a strobe for free.  All this for less than $8.  You get variable pulse width control with the potentiometer but it will change depending on the speed of the rotor.  That is all part of the tweaking.

Perhaps instead of running off the source battery voltage you run the whole thing off a regulator chip at say 9 volts.  Just to stay away from the battery when the voltage output starts to choke.

This is open source!!!

Offline TinselKoala

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Re: Analog variable pulse timing circuit for a pulse motor of your choice
« Reply #1 on: October 01, 2013, 03:56:28 AM »
It's an interesting idea, using a comparator/op amp that way. I might try it. I suppose the slew rate of even something like a 741 would be fast enough for a mechanical device. Let's see.... I have some OP07s I think, maybe I'll try putting a circuit together.

Offline MileHigh

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Re: Analog variable pulse timing circuit for a pulse motor of your choice
« Reply #2 on: October 01, 2013, 05:04:57 AM »

Yup, even a retro LM741 should have a slew rate that is very very good, and very damn fast for the all-important switch-off.   Plus you can't forget that the IGBT/MOSFET/transistor itself will increase the switching speed.  I am going to guess that the switching of the actual coil driving switch will be as fast or nearly as fast as the datasheet spec for the part.

For op-amps, I know the LM301 took over but I don't know what "today's" general purpose op-amp is.  The beauty of the LM741 of course is that the "lower" performance means that it won't be prone to going into spontaneous oscillation in conventional amplifier circuits.  I am just not sure if you lose a lot of voltage operating room powering it at "+/- 6 volts."  You might have to use an op-amp that is "more comfortable" with a 12-volt total potential difference.  I don't know, but the data sheets are just a magic click away.

The problem with the conventional setup of a pick-up coil driving a base resistor is that you have limited and awkward control over the switch-off time.  There is not much means to ensure that it is sharp.  If you are a newbie and have no scope, it's a parameter/issue that you basically have no control over and are probably not even aware of.  When you switch to an op-amp comparator, that "problem" goes away.

And of course with this configuration the pick-up coil itself causes absolutely zero Lenz drag on the rotor.

My additional thought is that if you use say two LEDs from an LED light bulb, they are ridiculously bright.  So perhaps just a single white dot at the center of each rotor magnet would do the trick for a strobe illumination system.  They are so bright that you would not have to dim the lights to see the strobe effect.