[sdiy] Magnetic String Actuation

[Spiros Makris]

Well, it has been 2 years(!) since my last update. Time flies fast!
I have not abandoned the project, but I do move really slowly due to my
lack of electromagnetics knowledge and woodwork skills. I made a spring
reverb driver following the circuits on eliot sound products (which worked
like a charm), and after observing the construction of the tank actuators I
couldn't help but fall back into the string actuation rabbit hole; I pulled
out the Chinese electromagnets, and went over the MRP papers yet again;
it's amazing how many things you miss when you're clueless on a topic.

So I knew the chinese magnets I have do *something* but I was at a loss
when it came to driving power into it, picking up the signal, etc.
Things I've learned so far:
1. The MRP uses a 5Ohm electromagnet wound with 28AWG and a total radius of
20mm or so (actual coil radius seems like 10mm; they don't have any
mechanical drawings on the supplier's website). Its cost is 67$ per piece,
which is too much for my project, so on to other solutions, I guess.
2. The low impedance is important to make the task of driving power into it
easier. The magnetic field is only dependent on current and turns; thus a
thinner wire will dissipate more power and require greater voltage for the
same current and turns. This is quite obvious I guess, but I hadn't
understood the implications  until now.
3. The cheap chinese magnets I can easily source don't come close to 8
ohms. The small ones I have are 70ohm, rated at "1W". They come with the
suggestion to use them at 1W but at 50% duty cycle, so I guess their real
rating is even smaller. Other options could handle more power but are
substantially larger; I don't seem to need that much for now.
4. My DIY coil does work if you put some effort into finding the sweet
spot, but I finally figured out why. Using a simple rod (screw) as a core
makes the magnetic field lines spread, and combined with the miniscule
cross section posed by the string, the resulting force is minimal. The kind
of "E core" those electromagnets use (and MRP also uses) make the field
lines start and end on the same side of the magnet, making the field much
more concentrated, producing better results. This was pointed out by a
fellow synth-diy member, but I was too clueless to understand at the time.
5. The spring reverb actuator uses a horseshoe core with a coil wrapped on
one of the legs. A tiny magnet sits between the spring and the mounting
point, making the produced force strong enough to induce movement. Again,
this was also pointed out by another member, but it didn't click until I
saw it in person. A similar design that sits around a string might be
better suited than the cylindrical electromagnets, and possibly have less
EMI emissions since less field lines exit the assembly.
6. Using a +15V supply you can drive just enough current on the 70 Ohm
coil, assuming some headroom loss on the driver stage. Based on experiments
run thus far, this should be plenty to induce movement reliably, so now I
need to figure out a good way of driving the coil.
7. The circuits by eliot sound products aimed at the spring reverb can
actually drive it (I tried), but are optimised for the much different
current-impedance requirements of the reverb tank. Furthermore, they don't
offer the possibility for a trimmable DC offset out of the box, which is
essential for the application (the field must be biased). MRP presents
a power amplifier as well as a smart approach for a bipolar to unipolar
voltage converter, but the parts cost is not negligible.
8. My DIY coil is a horrible actuator but actually works fine as a single
string pickup. So does one of the actuator electromagnets I got from china.
Getting rid of the EMI is another story, but MRP mentions a cancellation
approach which might actually work (measure the current going through the
magnet and subtract it from the pickup. it cannot account for core
saturation and hysteresis, but it's better than nothing.)
9. All examples I've seen so far use a single actuator and bias the field
either with a permanent magnet or with some DC bias current through the
actuator. Since you cannot produce a pushing force, it is critical to have
some biasing so that the produced force is actually the same shape as the
input AC signal. I think it is possible to have two magnets placed on
opposite sides of the spring, operated on the positive and negative half
cycles respectively. This would get rid of the DC biasing requirement,
effectively reducing the power dissipation and doubling the potential power
output of the assembly, while keeping the actuator power rating (and thus,
its size) the same. Based on my understanding so far, the absolute polarity
of the field does not matter, as long as only one of the actuators is
magnetised at any given time the force will change directions. This might
simplify the driver design and get rid of some signal conditioning stages.
The horseshoe core approach used with a coil wrapped around each of the
legs seems ideal for this.

I was weighing my options moving forward; I find the solution of
1.50$/piece from China very appealing, but it's hardly optimal. Wrapping 20
meters of wire around a core is very easy (that will bring me to 8ohms with
32AWG), but getting an off the shelf core of just the right shape is very
hard, and probably more than 1.5$. I found a ferromagnetic 3D printer
filament, which might do the trick and allow me to experiment with other
configurations that might be better suited (i.e. horseshoe core). That's
still a bit involved so it will have to wait a few months.

I figured I will insist on the off the shelf parts, to get the ball rolling
and arrive at a functional prototype as soon as possible. I am playing
around with some simple class A BJT amplifier that will use the pickup as
collector resistance. This will allow me to have a bias current through the
pickup, and is very easy to breadboard for evaluation. The horrible power
efficiency is not that important in my case, since I'm looking at 4-5
magnets (as opposed to dozens of them used in other more substantial
projects). I will replicate my DIY coil design to make single string
pickups, since it seems to do the trick just fine, and deal with that part
later (if ever). The woodworking part seems daunting, especially the tuning
pegs. I've looked at the various options and I think wooden tuning pegs
used in instruments like the oud would work in my case and are simple to
install on a wooden plank.


On Wed, 14 Sept 2022 at 00:52, Spiros Makris <spirosmakris92 at gmail.com>
wrote:

> I got a random 24V/5kg magnet to try out. It's this one, for reference:
> [image: image.png]
> It actually works! I tried a few different things it seems like a good
> starting point for what I want to achieve.
> I first tried driving the magnet with a signal from my VCO, without any
> feedback. I didn't even have to tune as precisely as we discussed earlier -
> just being in the vicinity and getting the position right resulted in very
> obvious sympathetic action. Sweeping the frequency up produced various
> harmonics by the string. I didn't get to try more complex sounds yet, but
> this is very promising.
> I added feedback from the pickups to the magnet, through my eurorack. Sure
> enough, there was sustain as long as I got the position right. This is
> tricky; moving up and down a string with your left hand means the length is
> changing and thus the position you need to be with your right hand to stay
> on that specific harmonic. Since the feedback is coming from all the
> strings, it's not possible to treat it entirely as an ebow, since moving to
> a new string while the old one is still vibrating is messing up your
> feedback and gets in the way of the new vibration building up. There is
> some sympathetic action here, though, meaning that you can take advantage
> of a string that is still sustaining to kickstart the next one. Pretty neat.
> It was mentioned that it's not easy to start the vibration from the idle
> state, and I found this to be true. I thought, most positive feedback
> circuits are kickstarted by their inherent noise, so maybe I should just
> add some noise to the mix and see what happens. For some reason, my amp
> didn't like it and acted a bit crazy at high levels; eventually I figured
> out high pass noise is ok and blasted it through. The effect is like a very
> soft bow, and I think it's less dependent on the position you choose.
> Mixing some feedback into this seems to help the oscillation start issue,
> but maybe it's just my impression. Unfortunately because the pickups also
> listen to the magnet's output (it's pretty loud, considering the guitar's
> gain) I found limited use for it for now.
> I tried impulses as well, which admittedly aren't coming through great in
> an AC coupled amp, but still I do get a thump on the string and something
> that resembles a very light, rounded pluck. Unfortunatelly the pickup can
> catch this too, so the sound is not ideal. Faster looping envelopes also a
> sort of sustain , so maybe that's another path to that effect. Turns out
> the electromagnet doesn't like this, it got very warm after a few minutes
> of thumps coming its way.
> All in all I'm very impressed by the results, considering I just bought
> random Chinese parts and threw them together in an afternoon. I'd like to
> try a few different magnets to get a better high frequency response, but in
> the context of a sympathetic string bank this might actually work just
> fine. I'll try sending a melody through and see how it reacts to it.
> Unfortunately I can't actuate all 7 strings; I'll need to make some wooden
> prototype for this to work (and get more magnets and amplifiers). I'm
> thinking of picking up the sound (not the feedback) with piezo pickups; do
> I need an actual acoustic chamber for this to work or is a simple wooden
> plank fine, to get things going?
> I'll upload some samples later. This has been in my mind for 10 years give
> or take and I can finally see a path to making it work thanks to everyone
> that chimed in!
>
>
>
> On Tue, 13 Sept 2022 at 12:25, ulfur hansson via Synth-diy <
> synth-diy at synth-diy.org> wrote:
>
>> you can also run current (signal) through the string itself - and place
>> magnets close to thw string - be careful not to zap yourself though by
>> accidentally touching the strings, also make sure the string gauge is heavy
>> enough to handle the current :)
>>
>> sent from outer space
>>
>> > On 13 Sep 2022, at 09:06, René Schmitz <synth at schmitzbits.de> wrote:
>> >
>> > On 13.09.2022 08:26, Spiros Makris via Synth-diy wrote:
>> >> Alright! Then it sounds like I'm on the right track since I got the
>> cheapest electromagnets (2.5kg) I could find on aliexpress! I'm kinda
>> itching to give this a go so I'll pay a visit to my local robotics shop and
>> get another cheap one and give it a go sooner.
>> >>
>> >> Harry that's useful, I'll try the middle of the string length for my
>> next experiment. While you are right, I do think my tuning was good enough
>> to warrant some sympathetic action. It's true that the sustainer
>> configuration will probably produce vibration more easily, since it cuts
>> out any problems with tuning.
>> >
>> > I'm thinking, you should try to drive it at half the frequency of the
>> natural frequency of the string, as with a simple plain electromagnet, and
>> an non-magnetized string, the force is attractive regardless of the
>> polarity of the current.
>> >
>> > So you get two peaks of force per cycle when you drive it with a sine
>> wave.
>> >
>> > This is a <electroboom> full bridge rectifier </electroboom> action,
>> which doubles the frequency.
>> >
>> >
>> >> My goal is to create a sympathetic string bank and excite it with
>> sound from synthesizers or other electric instruments (ie guitars, lutes
>> etc). This effect is most common on instruments like the sitar, but all
>> stringed instruments have some sympathetic effect going between the strings
>> while you play them
>> >>  I expect that by adding the right amount of positive feedback I can
>> get a sort of sustain control, and self-oscillation when cranked up. My
>> assumption here is that the actuator, string, amplifier and pickup
>> essentially form an oscillator, with the string acting as a high Q
>> resonator, akin to an LC tank.
>> >
>> >
>> > An Ebow has a magnetic bias in the form of a pre-magnetized core (both
>> in the pickup and the transducer),
>> >
>> > so that they can use the same frequency that they pick up. You might
>> want to add a DC current to your magnet to simulate this, and be able to
>> use feedback. Or stick a permanent magnet on your core.
>> >
>> >
>> > Best,
>> >
>> >  René
>> >
>> >
>> > --
>> > synth at schmitzbits.de
>> > http://schmitzbits.de
>> >
>> > _______________________________________________
>> > Synth-diy mailing list
>> > Synth-diy at synth-diy.org
>> > http://synth-diy.org/mailman/listinfo/synth-diy
>> > Selling or trading? Use marketplace at synth-diy.org
>>
>> _______________________________________________
>> Synth-diy mailing list
>> Synth-diy at synth-diy.org
>> http://synth-diy.org/mailman/listinfo/synth-diy
>> Selling or trading? Use marketplace at synth-diy.org
>>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://synth-diy.org/pipermail/synth-diy/attachments/20241105/a92571af/attachment.htm>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: image.png
Type: image/png
Size: 23330 bytes
Desc: not available
URL: <http://synth-diy.org/pipermail/synth-diy/attachments/20241105/a92571af/attachment.png>