Series: LEGO bagpiper

A little progress has been made. And a lot have been learnt.

So my fingertips worked… sort of. But the ‘microphones’ were not large enough to close the holes. And rigid plastic is not good, rubber or something else soft is better to cover the holes. I am now using the 1×1 rubber pads used on LEGO Technic tracks (like in the BOOST set):

Still not perfect but better.

Also found out that rubber balloons have some non-linear properties. After reaching some internal pressure level, if I keep inflating it the pressure drops drastically. So my idea of using a pressure sensor to regulate air volume is not so easy to achieve. Elastic hysteresis doesn’t help either…

I increased the LEGO air compressor air flow by changing the 2x Power Function ‘L’ motors with ‘XL’ motors and gearing it faster. But still not enough for long tests and still too noisy. So for now I am using a small air pump used on Arduino and other projects. At 5V it’s almost strong enough to ditch the “bag” (the balloon).

This is now the current state of the project:

1. LEGO MINDSTORMS EV3
2. Wi-Fi Power Plug
3. Mini air pump
4. Mindsensors pressure sensor for EV3
5. Party Balloon
6. MINDSTORMS EV3 medium motor
7. LEGO custom air valve
8. LEGO Powered Up Technic Hub (2x)
9. Practice chanter
10. LEGO Powered Up Large Angular motor (8x)

The air pump is powered by a power wall adapter connected to a Wi-Fi Power Plug. The plug state (ON/OFF) is controlled through UDP messages sent from the EV3.

The EV3 also reads the pressure inside the pneumatic circuit with a Mindsensore pressure sensor and tries to keep pressure steady by turning the air pump ON whenever pressure drops bellow a threshold.

The air valve is used only at start, when we need to inflate the balloon. The EV3 medium motor is used to close the valve (squeezing the aquarium air tube between the balloon and the chanter) and when there is enough air in the balloon the valve is opened.

Each Technic Hub is responsible for controlling 4 LEGO fingers with 4 Powered Up Large Angular motors. Probably smaller and faster motors would be better since they have much more strength than needed (if I don’t take care, they bend the “fingers” easily)… but that’s what I have with internal zero reference, it helps coding (but I did I had a problem with two of them, looks like they came with a large offset, near 100º… had to replace them).

The whole setup can now be described as an IoT project. I don’t have everything integrated yet so the EV3 functions are still independent of the Technic Hub functions:

• the EV3 can be controlled manually and/or with MQTT messages
• the Technic Hub can be controlled with Jupyter Notebook (using pybricksdev) or with two Chrome browsers running the Pybricks IDE

I have a video of current stage in action – please be warned, terrible noise:

In this video the reed has an elastic ring applied to it (I ordered a bag with the chanter). It does reduce the noise a bit but it also changes the required air pressure to play it so I ended up with a very large balloon… and that also changes the reed’s sound. Will have to make further tests to choose the best balloon size and try to adjust my code for it.

In the meantime, wainting for two orders to arrive:

• some other practice reeeds to check if that makes any difference in the sound (I feel that my reed sounds like a cracked cane… my coding is bad, my setup is not the best but it does sound odd)
• a pair of USB adapters for the Technic Hubs, I am tired of replacing batteries

After some attempts, decided to gave up (at least for now) using LEGO parts as fingertips. Even the LEGO rubber pads were too hard and narrow to close enough the holes for the chanter to play something that reminds music – for instance I could not notice any difference between a G (all holes closed) and a A (opening just the hole near the chanter endpoint).

I tried Technic rubber links and even rubber tires… no luck.

So I started with small circles of EVA foam glued to LEGO 2×2 dishes attached to the same Technic connectors I was using with the LEGO rubber pads. It looked much like those ear pads used on headphones… and then I remembered that Neil Fraser used exactly that for his Robotic Bagpipe Chanter:

These new fingertips improved sound quality but only for the higher notes.. still having problems with G, A, B…

Adding adhesive felt pads didn’t make it better.

But replacing EVA foam with packaging protective foam (not sure what material it is made off) made a big difference:

Great, I can finally produce distinguishable notes! Still far from perfection because air pressure inside the balloon has a strong influence on note pitch and sometimes the reed sounds very bad (and sometimes doesn’t sound at all)… but it’s still a major milestone:

So what’s next? Oh yes… programming.

Programming the LEGO Technic Hubs with Pybricks is not difficult (much, much more easier than using assembly with PIC controllers in the 90’s). But one thing is making motors move the way you want and other is making a choreography that sounds like music.

The chanter has 8 holes – 7 are placed above, the other one is downside. There are several sites on the web showing the proper fingertips positions and more than a few adopted a notation of black and white circles (representing closed and open holes). Much like binary notation.

So I chose to use ‘0’ for motor in rest position (closing the hole) and ‘1’ for motor in some other position that opens the hole. So a single byte can represent any possible note (and of course also lots of other combinations).

Using the most significant bit for the higher pitch hole (the one downside) and the less significant bit for the low pitch hole (the one near the end tip of the chanter) this is the result:

   G = 0 0 0 0 0 0 0 0
   A = 0 0 0 0 0 0 0 1 
   B = 0 0 0 0 0 0 1 1
   C = 0 0 0 0 0 1 1 0
   D = 0 0 0 0 1 1 1 0
   E = 0 0 0 1 0 0 0 1
   F = 0 0 1 1 0 0 0 1
HI_G = 0 1 1 1 0 0 0 1
HI_A = 1 1 1 0 0 0 0 1

As I am (was) using two hubs, it seemed logic to group each representation in two nibbles of 4 bits so the same program running on both Hubs could position its 4 fingers just by receiving a char with a value between ‘0’ (all holes closed) and ‘F’ (all holes open):

   G = 0 0 0 0 | 0 0 0 0 = 0 | 0
   A = 0 0 0 0 | 0 0 0 1 = 0 | 1
   B = 0 0 0 0 | 0 0 1 1 = 0 | 3
   C = 0 0 0 0 | 0 1 1 0 = 0 | 6
   D = 0 0 0 0 | 1 1 1 0 = 0 | E
   E = 0 0 0 1 | 0 0 0 1 = 1 | 1
   F = 0 0 1 1 | 0 0 0 1 = 3 | 1
HI_G = 0 1 1 1 | 0 0 0 1 = 7 | 1
HI_A = 1 1 1 0 | 0 0 0 1 = E | 1

The main program loop would be in fact just a bunch of if-then-elif’s based on the key pressed (if using Pybricks Chrome-based IDE) or the character received (if using pybricksdev or accessing the Nordic UART Service directly):

while True:
    if keyboard.poll(0):
        char = stdin.read(1)
        if char == '0': #0000
                fingerA_down()
                fingerB_down()
                fingerC_down()
                fingerD_down()
        elif char == '1': #0001
                fingerA_down()
                fingerB_down()
                fingerC_down()
                fingerD_up()
        elif...

and this really worked – pressing some keys on Chrome made the fingers move.

But that’s not enough for playing music. What movements should be done and for how long for this apparently random fingering to produce something that doesn’t sound like me stepping on a cat’s tail?

Ardu McDuino for the rescue.

XenonJohn was gentle enough to include his Arduino code with the Instructables for his Bagpipe Playing Robot. He used a 3-char notation for his music where the first char represents an embellishment (like a gracenote or a doubling), the second char represents the main note and the last char represents the duration of the note. For instance:

-GR

means playin a ‘G’ for 1/4 of the time of a full duration note (I believe this means a ‘quarter note’ or a ‘crotchet’) with no embellishment.

And he also included two songs in his source code: ‘Amazing Grace’ and ‘Scotland The Brave’.

Now this is something I can use.

Instead of implementing his whole code with Pybricks I just adapted his song sequence. I opted for not representing embellishments since they can be represented by (fast) sequences of finger positions (assuming that there is enough bandwidth between the “controller” and each hub to send everything and the hub processes everything fast enough… if I find that is not the case I might return to XenonJohn’s notation).

So this is my way of representing a song:

sequence = [
A, R,
D, Z,
F, R,
E, N,
D, N,
F, Z,
]

where the first character is the note and the second character is the duration (still using Ardu McDuino, too lazy to change that for now):

j = 1 = whole note (semibreve)
Z = 1/2 = half note (minim)
R = 1/4 = quarter note (crotchet)
N = 1/8 = eight note (quaver)
L = 1/16 = sixteenth note (semiquaver)
K = 1/32 = thirty-second note (demisemiquaver)

But for gracenotes this doesn’t work good since a grace note is played for 1/32 and the next note should be reduced from this value. So I had three timing notations for the 3 cases where a gracenote is used in Ardu McDuino songs (before a note with a timing of R, Z or N) reducing these timing by 1/32:

x = R – 1/32 = 1/4 – 1/32 = 1/4.57
y = N – 1/32 = 1/8 – 1/32 = 1/10.67
z = Z – 1/32 = 1/2 – 1/32 = 1/2.13

these look strange but are easy to use in my code since I define a tempo for my song and each timing is just the multiplication of this tempo by this numbers.

First attempt looked promising but not quite right, then I find a silly math error and now I finally have something that sounds familiar:

I found out that everytime I tested Tuxie McPython the two Technic Hubs were getting hot. Very hot.

Also batteries were draining fast. I hate changing batteries on stationary robots/automata. For my MINDSTORMS EV3 I tend to use the Li-Ion battery as a PSU, leaving it connected to a 12V/1.5A wall power adapter.

So I ordered 2x USB Power Box from PV Productions. They fit on the Technic Hub in the place of the 6xAA battery holder and they can withstand 9V/2A so it seamed a good and clean method.

Less than 3 days after they arrived I melted one 🙁

The 4 Large Angular motors were consuming too much current. To proper close the holes I was forcing the motors to strongly press the fingers against the chanter but never thought that would require 2A (probably even more).

So I changed the code to reduce the fingers strength and opted for other fingertips also made with protective foam but a different one, apparently less “foamy” and a bit thinner. I glued over the EVA foam fingertips I already tried before and cut them a bit larger so the hole would get more covered (no fotos but can be seen used on last video).

Even so, rechargeable NiMH batteries were still draining too fast (and the hubs were still getting hot). So I picked some alligator cables and 2.1 mm barrel adapters and used 2 of my wall power adapters to power the Hubs. I set them for 9V (measured 9.1 with no load) and connected them – it worked:

but while playing I got a blinking LED and very often one of the hubs just powered off. The adapters are rated 1.5A… so the motors were still demanding too much current.

As I said before… I don’t have other proper motors – the smaller version of this motors, sold with the 51515 Robot Inventor, are not easy to find and it would be very expensive to buy 8 of them on Bricklink. I could use the BOOST external motors but I don’t have 8 of them and I am not so sure they have enough power to properly close the holes. Same for Technic motors (except maybe the larger ones) and none of them have internal zero reference (not a major blocker but its something that helps).

So I have 8 motors that demand too much current and two hubs that get too hot and shutdown… I have no other proper motors… but I do have other two identical hubs.

So last version of Tuxie McPython now uses 4 Technic Hubs, each controlling two fingers. Code is essentially the same, just less “if…then…elif” and a shorter “command vocabulary”:

   G = ['0', '0', '0', '0'] # 00 00 00 00
   A = ['0', '0', '0', '1'] # 00 00 00 01
   B = ['0', '0', '0', '3'] # 00 00 00 11
   C = ['0', '0', '1', '2'] # 00 00 01 10
   D = ['0', '0', '3', '2'] # 00 00 11 10
   E = ['0', '1', '0', '1'] # 00 01 00 01
   F = ['0', '3', '0', '1'] # 00 11 00 01
Hi_G = ['1', '3', '0', '1'] # 01 11 00 01
Hi_A = ['3', '2', '0', '1'] # 11 10 00 01

So I now send just 4 different commands (‘0’, ‘1’, ‘2’ or ‘3’) to each hub instead of previous 16 but from the controller side have to send those messages to 4 hubs instead of just 2. My only concern was that my laptop couldn’t proper handle 4 simultaneous BLE sessions but thankfully it works fine.

With 2 motors per hub I can now use NiMH batteries without blinking LEDs. The hubs still get hot (not so hot as before but still very warm) and the batteries will probably run down after a few songs but for that I already ordered 9V/2A power adapters and more alligator cables.