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:
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
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:
An externally hosted image should be here but it was not working when we last tested it.
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
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)
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)
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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) 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.
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
) 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.
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.
............U...................................................................... finger
------------------------------------------------------------------------------------ string
==================== glass..........................T solenoid
0................0..............0 piezos
Ignore the dots, they're only there to avoid formatting issues.
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
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
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.
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.
No, in opposite direction. I mean, you push the piezo down, the solenoid strikes up.
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.
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.
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
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.
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...
Well, I'm using that in my non-hammered prototype, but mechanical amplification sounds really cool as a concept.
No disrespect meant, at all
But you show only the basic theoretical concept, maybe somebody can contribute some actual construction ideas.
Good luck
No worries, I was only afraid you had misread me. To avoid any further confusions, I went on to explain my instrument only so that the context would be clear, but the only thing I'm asking for here is if anyone knows how to wire a piezo to the gate of a transistor so that it acts as a 'proportional switch'.
Thank you all, by the way.
I don't think you're going to be happy with piezo going to the base of a single transistor. You would be better off (IMHO) using an opamp circuit with high input impedance to buffer the voltage from the piezo, and then doing something with it. However we need to step back a long way...
I'm not sure what your output is to be. You show an LED - is that an indicator? Part of an optocoupler intended to show a digital "hit" signal? Are you wanting to drive ferrous strings with your solenoids with an AC signal of the right note? Or are you using a pulse on the solenoid to pull a hammer and hit the string? They all have quite different signal conditioning and drive requirements.
In speaking of delays, the problem is not (so much) electrons getting into your solenoid, but if it's pulling a hammer there can easily be tens of ms delay before the hammer strikes something. That little delay could I think become very painful. If you're driving ferrous strings with the solenoid, there are phase issues to consider if the driven string is the same as the sensed string or in any way coupled to it. Not to mention stability, unless I'm really misunderstanding your configuration.
I suggest you draw out a big functional (not physical) diagram with specifying of the relevant things that need to happen in your design, and the manner of interface between those functions, e.g. digital on/off, AC waveform, sound wave, impact, etc.
So your system might have a signal chain like this:
0) impact surface, e.g. metal string
1) impact sensor, output is note waveform
2) signal conditioning from sensor, output is amplified note waveform
3) power amplification
4) transducer, output is electromagnetic note waveform
5) metal string, output is sound and feedback back to input
Or do you mean this?
1) key accepts finger-strike
2) piezo sensor produces pulse proportional to strike force
3) signal conditioning, produces proportional pulse
4) power amplifier
5) solenoid, produces force proportional to input pulse
6) hammer
7) string
?
I'm not sure what your output is to be. You show an LED - is that an indicator? Part of an optocoupler intended to show a digital "hit" signal? Are you wanting to drive ferrous strings with your solenoids with an AC signal of the right note? Or are you using a pulse on the solenoid to pull a hammer and hit the string? They all have quite different signal conditioning and drive requirements.
In speaking of delays, the problem is not (so much) electrons getting into your solenoid, but if it's pulling a hammer there can easily be tens of ms delay before the hammer strikes something. That little delay could I think become very painful. If you're driving ferrous strings with the solenoid, there are phase issues to consider if the driven string is the same as the sensed string or in any way coupled to it. Not to mention stability, unless I'm really misunderstanding your configuration.
I suggest you draw out a big functional (not physical) diagram with specifying of the relevant things that need to happen in your design, and the manner of interface between those functions, e.g. digital on/off, AC waveform, sound wave, impact, etc.
So your system might have a signal chain like this:
0) impact surface, e.g. metal string
1) impact sensor, output is note waveform
2) signal conditioning from sensor, output is amplified note waveform
3) power amplification
4) transducer, output is electromagnetic note waveform
5) metal string, output is sound and feedback back to input
Or do you mean this?
1) key accepts finger-strike
2) piezo sensor produces pulse proportional to strike force
3) signal conditioning, produces proportional pulse
4) power amplifier
5) solenoid, produces force proportional to input pulse
6) hammer
7) string
?
Thanks again for the general interest and help!
This is exactly what I mean. Only the hammer is the solenoid itself.
Do you think there would be a noticeable delay for the solenoid to travel 0.5 cm? There's lots of solenoid pianos using an AD converter to read velocity, then PWM to the solenoid, and they don't seem to suffer from significant delay, even with all the digital processing.
This is exactly what I mean. Only the hammer is the solenoid itself.
Do you think there would be a noticeable delay for the solenoid to travel 0.5 cm? There's lots of solenoid pianos using an AD converter to read velocity, then PWM to the solenoid, and they don't seem to suffer from significant delay, even with all the digital processing.
5mm is a long way to travel, and it depends on how hard you want to drive the solenoid. You will need to do some calculations involving:
- how hard you want to hit the string
- how heavy is the moving metal
- therefore what speed you want it to achieve
- therefore how much solenoid force is required
- and how long it will take to travel
- how much inductance does the coil have, and therefore
- how much voltage is required to get to the desired current fast enough
Don't forget to take into account damping and/or a return spring and how much force it exerts. And make sure you run the numbers across some meaningful dynamic range of impact forces, e.g. at least 10dB. The metal will move nice and fast for the higher levels, but will it move unacceptably slowly at minimum strike speed? You don't want to be applying force with the solenoid while the metal is hitting the string because that will cause it to buzz, bounce or damp the string. The solenoid force has to be an impulse to the moving metal hammer which is over before the hammer strikes.
(the hammer is the moving metal core. the solenoid is just the wire coil. You're thinking of both together as "the solenoid" but really the hammer is a separate item and must be analysed on its own terms, particularly its inertia)
PS: your diode is backwards if it's the switchoff catch-diode for a solenoid.
- how hard you want to hit the string
- how heavy is the moving metal
- therefore what speed you want it to achieve
- therefore how much solenoid force is required
- and how long it will take to travel
- how much inductance does the coil have, and therefore
- how much voltage is required to get to the desired current fast enough
Don't forget to take into account damping and/or a return spring and how much force it exerts. And make sure you run the numbers across some meaningful dynamic range of impact forces, e.g. at least 10dB. The metal will move nice and fast for the higher levels, but will it move unacceptably slowly at minimum strike speed? You don't want to be applying force with the solenoid while the metal is hitting the string because that will cause it to buzz, bounce or damp the string. The solenoid force has to be an impulse to the moving metal hammer which is over before the hammer strikes.
(the hammer is the moving metal core. the solenoid is just the wire coil. You're thinking of both together as "the solenoid" but really the hammer is a separate item and must be analysed on its own terms, particularly its inertia)
PS: your diode is backwards if it's the switchoff catch-diode for a solenoid.
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Ah, ok then.
I meant there's nothing like a piano mechanism,.
As I said earlier, I built a solenoid with a big screw with alnico magnets. Trying it with a 12v battery and an on/off switch, it was immediate (for human perception), so I suppose moving from that range I should be ok. I might try a heavier screw also (since f=ma).
What do you mean by damping here? If in a traditional piano-like sense, the open strings are muted at the nut with cloth and foam so that when you stop pressing them against the fretboard they stop sounding. This has worked fine for the non-amplified prototype so far.
As I don't fully trust my math I think my way of running the numbers will be using some pots. If the hammer is unacceptably slow at minimum strike speed, I'll adjust the range, make the minimum strike speed higher, try with different screws, solenoid diameter, magnets, etc... No big worries here.
Now THIS is a major concern. But I expect (perhaps wishful thinking) that the nature of the piezo will be on my side, as they send a clear pulse-like burst when you hit them.
But I'm curious, why is the piezo to transistor a bad idea?
Thanks for your interest.
By damping, I mean of the hammer not the string. Just a matter of ensuring you get a single clean strike. Having a return spring is probably enough.
In terms of "why not just a transistor?", basically because it's not sufficient. The input impedance is too low, there is nowhere near enough gain and there is insufficient control of the signal. The signal you get from a piezo is tiny (mV/uA) and you need to turn it into possibly a few amps to drive the solenoid.
Since it sounds like you just want to hack on it instead of designing anything, get your piezo sensor and measure the output. Google up "instrument amplifier" and use that to get a clean signal, and then figure out how you're going to do power amplification for the solenoid.
In terms of "why not just a transistor?", basically because it's not sufficient. The input impedance is too low, there is nowhere near enough gain and there is insufficient control of the signal. The signal you get from a piezo is tiny (mV/uA) and you need to turn it into possibly a few amps to drive the solenoid.
Since it sounds like you just want to hack on it instead of designing anything, get your piezo sensor and measure the output. Google up "instrument amplifier" and use that to get a clean signal, and then figure out how you're going to do power amplification for the solenoid.
Bad?
No, not bad by itself, just incomplete.
You need some , say, 15 to 50 parts between them.
Including a couple Op Amps , plus some other elements, active and passive.
Just a simple block diagram, without delving into what each of them needs inside:
Piezo > high input impedance preamp > halfwave or fullwave active rectifier > *probably* some kind of threshold detector or Schmitt trigger to avoid low level noise/hum/mechanical noise (your fingers/clothing/floor vibration) triggering the solenoid or making it chatter (you might call it signal processing) > some controllable current source driving the solenoid , which might be as simple as a transistor or a Darlington or a MosFet and a couple resistors to a somewhat more complex circuit.
Somewhere in the path there should be some gain/sensitivity adjustment so the solenoid hit matches the force hitting the piezo.
That said, this might be simplified somewhat if accurate results aren't *that* important, but the point here is that "the Piezo signal into the solenoid" is not enough, it needs some kind of interface or signal processing.
All your worries about chatter, AC signal hitting the coils, etc. won't happen because from the second stage on there's no more AC, but rectified DC control voltage.
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