Some musings on measuring bellows activity in electronic Anglo concertinas

[wearyhacker]

For a number of years now a friend on mine has been building an experimental electronic Anglo concertina. He has now arrived at a reasonable working prototype. One of the features of this concertina is volume modulation using a twin port differential pressure sensor. One port of the sensor is connected to the bellows enclosure and the other the outside world. The actual differential pressure variation measured in the bellows during playing proves to be quite small, that is, less than 3 kilo pascals. Finding affordable sensors with that low pressure range has proved difficult. However, using a Chinese manufactured CFSensor XGZP190 3kPa is currently under investigation.

 

The has led me to think about other ways to measure this activity that do not involve pressure sensors. Fundamentally, the energy expended by the player in moving the bellows end plates is transferred to the reeds and converted into sound. The force being exerted on each end plate can be measured using a load cell sensor of some sort placed where the heel of the hand meets the end plate. This force will be divided equally between all the currently pressed buttons. If the additional force applied by fingertips holding buttons down can be ignored, then a simple calculation to provide a numeric value that is applied to all the “voices” currently playing can be made. if the hand grips were hinged at the top with limited movement at the bottom then push and pull could be differentiated as well as measured. Otherwise a differential pressure sensor may still be needed purely to detect pushing or pulling.

 

My friend says that this may have to wait for a future version!

 

Apologies if this has been tried before. But any comments will be appreciated.

Edited May 12 by wearyhacker

[Frank Edgley]

I admire the knowledge to be able to do this, and understand the functioning etc. But I am not sure why anyone would want to make such an instrument. I know you could make it sound like anything you wanted with such an instrument, but there are other platforms that you can do that already eg. keyboards, Would it only be for the appearance? A concertina that can be made to sound like a trumpet, for example? What's the point? Do that on a keyboard. Sorry if this sounds disrespectful.. That is not my intention.

[wearyhacker]

I too hope that you don't find this disrespectful. However I do think that your post of is somewhat off topic. I am looking for comments on the measurement technique outlined in the second paragraph. I apologise for not making this clear.

Edited May 12 by wearyhacker

[ttonon]

7 hours ago, wearyhacker said:

The force being exerted on each end plate can be measured using a load cell sensor of some sort placed where the heel of the hand meets the end plate. This force will be divided equally between all the currently pressed buttons.

Weary, are you proposing to build a digital concertina that attempts to duplicate the effect of bellows pressure (both positive and negative) on volume using a force sensor rather than a pressure sensor? That's not a bad idea because it saves you from making a kind of a bellows chamber. To do it, I'd suggest the best way is with a single differential force transducer that is sensitive to the entire force that the musician would apply in a squeezing fashion between his two hands. 

 

But it would be a mistake to "divide the force equally among all the pressed buttons." In the conventional instrument, all the pressed buttons see the same driving pressure difference. That pressure is basically the force between the two hands divided by the cross sectional area of the bellows, which is not divided equally among the pressed notes. All such notes see the same pressure, and in your case, should see the same measured force. 

 

Incidentally, differential pressure transducers are highly accurate, cheap, and available for any range of pressure suitable in bellows driven free reed instruments. 

 

 

 

 

 

[Chris Ghent]

Years ago I connected a magnehelic gauge to a bellows and was amazed to find the operating pressure inside was minimal. By memory it was .025” water column. 

The force applied to fingertips should be no more than about 60 grams times the number of buttons pushed and is separate to the pushing or pulling action.  It will be there whether pushing the bellows or pulling.  I think I would ignore it.  
 

You would only need to fit your gadget to the active end of the concertina.  However there are many different ways people hold an anglo and simple pressure in and out on the handrest leads to waggle in the bellows leading to more complex ways of holding and bracing the movement.  
 

 

 

 

[ttonon]

2 minutes ago, Chris Ghent said:

By memory it was .025” water column. 

Hi Chris, good to hear from you. 

 

In my experience, a typical bellows differential pressure for free reeds is from about something less than an inch of water to up around six inches or a little larger for loud players. An inch of water is equal to 0.249 kilopascal. 

 

Since pressure difference is the driver, that's approximately true for all free reed instruments. The arm force on the bellows required for that depends on the bellows cross sectional area. Since concertinas have a relatively small cross-sectional area, less force is required than for full size accordions to produce the same volume. 

[Clive Thorne]

What about a dashpot connected to a sprung centre return displacement measuring device, so that when pushing/pulling the sensor is displaced, but at rest it returns to it's central position.

 

This would have the advantage of allowing some movement in the "bellows", more like a real concertina, although there might be a bit of a lag in the sensor returning to zero.

 

Actually, think about it as I type, have a dash pot with a pressure sensor, which would allow movement, but return to zero instantly when you stopped pushing/pulling. The amount of movement under pressure could be adjusted by altering the bleed rate on the dashpot (you could even simulate running out of air!).

 

You could choose the dashpot diameter to give a realistic total force between the ends, but obviously the pressure in there would be much higher than in full bellows area, and so, possibly easier/cheaper to measure.

 

Addendum:

 

Obviously you'd need some sort of bellows framework to make the two ends move relative to each other in the expected way.

 

OR  you could just use a real concertina! No batteries to go flat, no ampliiers to lug around!, but reeds to go out of tune, and no "Dial a key/layout" feature.

 

 

 

Edited May 12 by Clive Thorne

[Paul_Hardy]

4 hours ago, wearyhacker said:

However I do think that your post of is somewhat off topic.

 

I've started a new topic as 

 

[wearyhacker]

When I find out how the quote function works I will use it.

 

@ttonon Regarding the division of pressure. The target machine has a working bellows but no reeds. The idea is to make the "feel" of the instrument as real as possible.

 

In a perfect system the total "work" being put in by two hands must be equal to the total of the "work" being done by all the active "reeds". The presence of a pneumatic reservoir (bellows) means that the work must be distributed evenly.

 

My bedtime now. I will reply to other issues tomorrow.

Edited May 12 by wearyhacker

[ttonon]

11 minutes ago, wearyhacker said:

The idea is to make the "feel" of the instrument as real as possible.

Weary, I may have misunderstood what you want to do. Are you trying to make the artificial bellows move like the true bellows, where the bellows speed of compression or extension will increase with the number of keys pressed?

 

Tom

www.bluesbox.biz

[wearyhacker]

9 hours ago, ttonon said:

Weary, I may have misunderstood what you want to do. Are you trying to make the artificial bellows move like the true bellows, where the bellows speed of compression or extension will increase with the number of keys pressed?

 

Tom

www.bluesbox.biz

 

Tom,

 

This idea for using a force based model to simulate bellows activity is at a purely conceptual stage at the moment. Back in the real world my friend is building an electronic Anglo concertina that has speakers for audio output. There is no midi output but there is a headphone socket. So this is definitely not a "midi" concertina. The goal on the hardware side is to create an ultra low cost instrument (tens of pounds) that feels like a real concertina.

 

The software part uses Soundfonts and Wavetable Synthesis to emulate expensive real world concertinas. This means taking sample recordings of a real individual concertina and converting them into a soundfont file. The current iteration of the software has a default wavetable built in, this uses samples from a Jones Anglo (worth hundreds of pounds). The microcontroller used has a slot for a sd card so potentially thousands of different concertinas could be loaded in on demand.

 

[ttonon]

Weary, will you kindly answer my question? Thanks.

 

Tom

[Richard Mellish]

My preference is for a simple force sensor rather than real bellows, simply to avoid ever running out of air in either direction. At least responsiveness to the force direction, pull or push, is of course essential with an Anglo-style keyboard. There is another current thread on electronic concertinas*, and on that one Jody points out the importance of loudness reflecting pressure. I agree and have designed that into my Anglo-pattern MIDI keyboard, although it could do with some refinement. I can provide details if desired.

 

*

 

[wearyhacker]

21 hours ago, ttonon said:

Weary, will you kindly answer my question? Thanks.

 

Tom

 

Tom,

 

Sorry, I thought I had.

 

On 5/13/2024 at 9:06 AM, wearyhacker said:

The goal on the hardware side is to create an ultra low cost instrument (tens of pounds) that feels like a real concertina.

 

The current instrument has a hand made set of bellows and the buttons in the end plates operate as both a valve and a switch (edit: this is not true, the bellows are in fact just leaky and the buttons operate tac switches through a specially designed spring loaded push rod). I will be meeting my friend in the pub tonight. I hope to persuade him to make an appearance on this thread and allow me to post some pictures. He has actually been a member of this forum since 2006 but has only ever posted one message, and that was over 10 years ago! The ultimate lurker.

 

21 hours ago, Richard Mellish said:

I can provide details if desired.

 

Richard,

I would be interested in that as well.

Edited May 14 by wearyhacker

[Richard Mellish]

I've just responded to a PM from one interested person. I may as well post the same info here.

 

My skills are limited. I am very aware that the whole arrangement is not pretty. I am not proud of it at all, but it works and serves as a proof of principle.

 

I have uploaded a description of the complete system and two pictures showing how the two pcbs (greenish) with the button switches are attached to the load cell.

mellish.uk/files/RM MIDI concertina.pdf

mellish.uk/files/P2020004.jpg

mellish.uk/files/P2020005.jpg

[wearyhacker]

Mea culpa. Sticking load cells on both end caps is stupid. It just will not work! I had not thought it through. Doh, basic physics. The bit about the distribution of the overall force to individual reeds still applies though.

 

Fr = Fb/n where Fr is the force applied to an individual reed, Fb is the force applied to the bellows and n is the number of buttons currently pressed.

 

Fb will relate to the rate of change of the bellows volume.

 

Temperature changes? May be too small to detect.

 

Pantograph?

 

Phase change foam?

 

I need to go away and think!

 

[Richard Mellish]

In a real concertina or an electronic one with bellows, whatever you do with the ends while pulling and pushing, whether keeping them parallel or tilting them, results in some pressure difference between the inside of the bellows and the outside world. In the real concertina, that pressure difference applies to any and all pairs of pad and reed for which buttons are currently pressed. It is not divided between the reeds, though the resulting air flow is the sum of the flows past all the reeds. In the electronic one, the pressure difference is sensed and passed to the sound-generating system; same pressure signal for any and all notes.

 

With a concertina-style electronic keyboard using a load cell, what you can do with the ends depends on how they are mechanically attached to the load cell. I was somewhat surprised with mine that straight pulling or pushing versus some degree of twisting seems to make minimal difference to the "pressure" signal from the load cell.

[wearyhacker]

Richard,

 

I am talking about dividing the force not the "pressure".

 

When the pressure reading inside the bellows is sampled, at that instant you can consider the bellows enclosure as a rigid pressure vessel  containing a mass of air (measured in grams) with a number of venturi attached to it. All these venturi will have the same instantaneous pressure differential across them. That is as you rightly say "the same pressure signal". However the units of that measurement are force per unit area. A venturi has a cross sectional area. Provided that all the venturi are the same size then the air mass that exhausts through each one will be same and will move at the same velocity. However the mass will distributed evenly across all venturi. So if there is 100g of air inside the vessel and 5 venturi then each venturi will exhaust 20g of air at the same velocity. So the "force" is shared out evenly between the venturi.

 

My head hurts!