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Old 28th February 2014, 02:58 PM   #1
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Default Piezo driving solenoid

Hi there,

I'm attempting to design some sort of solenoid piano (not exactly, but it's the shortest description I can think of) where the impact on a piezo sensor would drive a solenoid to hit x times stronger than the impact on the piezo.

The strength of the solenoid, as with any other electromagnet, depends on current, not voltage. At least, that's what I've been told.

So I can think of two ways of doing this: with a current amplifier, or with a transistor. The second seems much easier.
I'm thinking something as easy as this:
Click the image to open in full size.

The charge from the piezo opens more or less the gate of the transistor, letting more or less amps from the battery to flow into the solenoid bobbin. Would this work?

Using a current amplifier instead I could have some trouble since the piezo generates AC pulses, which would make the solenoid go back and forth, but I guess that could be easily solved too.

Anyway, I happen to be a total noob for this stuff, so any help/advise/similar experiences will be very appreciated
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Old 28th February 2014, 04:27 PM   #2
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not quite sure what your trying to accomplish sounds like your trying to increase / amplify finger pressure to increase the striking force on the piano string?
why?

Last edited by turk 182; 28th February 2014 at 04:32 PM.
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Old 28th February 2014, 05:50 PM   #3
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Uhm, that'd be long to explain actually, I wish I had a video of the prototype...
But yeah, basically it's just what you say, I want to amplify finger pressure to increase the striking force on the string,
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Old 28th February 2014, 06:45 PM   #4
jcx is offline jcx  United States
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there are lots of details to the sensors, signal conditioning, drive and actuator that need to get greatly expanded on

and the task itself could use a more complete description - there are many options, limitations, have to sort "needs" from "wants" and test both against physics and practicality

for instance which piezo ?– pressure or bender, PZT or polymer – on a compliant or hard surface – any required motion feedback or “touch or feel” to the input?

the easiest and most common in engineering is to find some products doing what you want and steal the design ideas (or even cheaper in almost all cases – just buy them)

Last edited by jcx; 28th February 2014 at 06:49 PM.
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Old 28th February 2014, 07:19 PM   #5
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Ok, then I'll try to explain as easy as possible.

Take a quick look at this video only to get a general idea how it works:
Kelstone: NAMM 2012 Product Showcase - YouTube

My design is quite similar in many ways. The differences are, mine is fretless (I'm working in microtonal stuff), the strings are very close (1mm) to the surface against which they are tapped, that surface is glass (improves sustain for fretless) and it is acoustic, it has a body, a rectangular wooden box. I've built a few of prototypes like that and I'm really really happy with it.

Now, the only thing, as an acoustic tapping instruments, it is not very loud and I'd like to use it for busking. Of course I could use any common form of amplification (piezo, magnetic pickups -> battery amp), but I find the idea of mechanical amplification incredibly cool and it sounds perfectly natural.

So I've built some solenoids and tried them with a 12v battery and a simple switch. They produce a very cool and loud sound (much like a dulcimer).

So I thought, what if instead of a glass as a fretboard for all strings, we use a 1cm wide sheet of glass for each string, resting on top of two piezos (or two pieces that rest on piezos :P) so that each time I tap a string against the glass, a current is generated, and that current is amplified to drive a solenoid, hitting the string five times harder than my tapping.

Little ASCII drawing:

U <- finger
--------------------------------------------------------------------------------------------- string
============================= glass II solenoid
0 0 0 piezos (or things that rest on piezos)

There's a very small chance that the piezos get excited not by your finger tapping, but the vibration of the string as the solenoid strikes it, starting a feedback loop. I doubt it would happen, but even in that case it would be easy to correct, so nothing to worry there.
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Old 28th February 2014, 08:27 PM   #6
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Oops, the ASCII drawing is screwed. I'll try again:

............U..................................... ................................. finger
------------------------------------------------------------------------------------ string
==================== glass..........................T solenoid
0................0..............0 piezos

Ignore the dots, they're only there to avoid formatting issues.
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Old 4th March 2014, 03:59 PM   #7
jcx is offline jcx  United States
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the hammer/solenoid is at one end, striking in the same direction as the piezo sense?

seems like lots of places for delay to enter the system - how quickly must the hammer strike after the finger before it becomes annoying? are their phase issues that change with note played?

a stiff narrow fingerboard for each wire may have flex/bounce issues

not saying it can't be done - but lots of system issues

since you are looking at transients a modern multichannel digital storage 'scope or equivalent usb/soundcard + 'scope sw, deep one shot pretrigger buffer, good triggers would be real helpful
doesn't have to have high bandwidth though so can be pretty cheap
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Old 4th March 2014, 10:12 PM   #8
JMFahey is online now JMFahey  Argentina
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It can be done, of course, but nobody will design and develop it for you, for free.
And before you jump and say "I'll pay all expenses" ..... think again.
Developing this for "somebody else" will quickly surpass Thousands of Dollars.

Just as a free sample: think that "something" will drive your solenoids, that the piezos will be mere "triggers" and at best, there will be some proportionality between force hitting the Piezos and solenoid force driving the hammers.
That in a pure electro-mechanical system as you envision.

Or you can go "Plan B" where Piezos are actual pickups and their signal is power amplified straight into speakers, no use for solenoids, hammers or tuned strings or bars.

Or Plan C, where piano keys are just switches and actual sound generators are tuned bars or strings.

Your basic idea about piezos straight (or with some amplification) driving solenoids is interesting as an idea (although it could be considered some kind of Carillon) but needs quite a lot of practical development.
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Old 4th March 2014, 11:31 PM   #9
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Quote:
Originally Posted by jcx View Post
the hammer/solenoid is at one end, striking in the same direction as the piezo sense?
No, in opposite direction. I mean, you push the piezo down, the solenoid strikes up.

Quote:
seems like lots of places for delay to enter the system - how quickly must the hammer strike after the finger before it becomes annoying?
I really can't see any significant delay, it's only amplifying current and then solenoid, what will it take for the electrons to get through the solenoid's bobbin? You don't notice any lag when you're playing guitar with a piezo and an amp.

Quote:
are their phase issues that change with note played?
I think you're thinking of something else. The piezo will of course catch some of the vibration from the string (the note), but the main burst will be the impact of finger and string against the glass, independently of the note. If you have a guitar amplified with a piezo, try tapping with your finger against the piezo: it's so much louder than any note you can produce with any of the strings.

So it's much simpler. Actually, a way I could easily do it would be using an AD converter, like Arduino, to read the piezo from the analogPins and then using PWM to drive the solenoids proportionally. That is so easy even I could do it. But I'd really prefer an analog solution, as it would have a higher resolution and less latency.

Quote:
a stiff narrow fingerboard for each wire may have flex/bounce issues
Nah, I've taken that already into consideration and it shouldn't be a big problem. What I fear is accidentally triggering the nearby piezos, but one worry at a time :P
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Old 4th March 2014, 11:58 PM   #10
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Quote:
Originally Posted by JMFahey View Post
It can be done, of course, but nobody will design and develop it for you, for free.
And before you jump and say "I'll pay all expenses" ..... think again.
Developing this for "somebody else" will quickly surpass Thousands of Dollars.
Wowow, slow down, I wasn't expecting anybody to develop it for me for free. And I won't jump and say "I'll pay expenses", if I had any money I wouldn't be on the diy side of things.

Quote:
Just as a free sample: think that "something" will drive your solenoids, that the piezos will be mere "triggers" and at best, there will be some proportionality between force hitting the Piezos and solenoid force driving the hammers.
That in a pure electro-mechanical system as you envision.
That's pretty much what I've been saying since the first post.

But it's not that hard, I think. I've built solenoids which did work as expected. Now, from what I understand the strength of a electromagnetic field is proportional to the current that goes through the bobbin (and the number of turns, the area, etc...).
A piezo generates a voltage, which we can either convert to current and then amplify, or we can use that voltage to control the gate of a transistor, which I think would be so much easier. So at one end of the transistor we have two 12v batteries (for instance) and at the other end the solenoid. Now say my piezos generate between 0 and 5 volts. Say the gate of the transistor is fully open at 5 volts. So, for instance, if I strike the glass fretboard at 3 newtons, the piezo generates 2.5 volts and 1/2 of the battery current goes through, etc...

Quote:
Or you can go "Plan B" where Piezos are actual pickups and their signal is power amplified straight into speakers, no use for solenoids, hammers or tuned strings or bars.
Well, I'm using that in my non-hammered prototype, but mechanical amplification sounds really cool as a concept.
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