Pitched Percussion Robot



Godfried-Willem RAES

2019 / 2020

[Nederlandstalige versie]



Aluminum tubes were used in earlier robots such as our <Tubi>, a quartertone instrument. In 2019, we did build the Plus-Minus project, using aluminum tube of greater thickness. Extending the register on the low side, appeared impossible using the same type of tube. When the tubes are made longer, the second overtone becomes dominant. Hence the idea to apply mensuring on the proportions. Thus this instrument uses aluminum pipes of different diameters. The tube lenghts can be calculated using following formula, valid as long as the tube length is at least 10 times larger than the diameter. The frequency for the first overtone, given by Olson as 2.758 times the fundamental frequency, after our measurements, it ought to be 2.705 times the fundamental. This is the pitch perceived on the attack of the tube. It can be measured exactly by suspending the tube exactly in the centre, the first overtone having a node there.

Note that the modulus of elasticity (Youngs modulus) is temperature dependent. This curve illustrates this dependency for aluminum: The variation is quite small, but when tuning, one has to be carefull to cool the tubes to ambient temperature before checking the pitch. Grinding and sanding makes the tubes very hot... A bucket of cold water next to the sander is of great help here.

The alloy used for the tubes is AlMgSi 0.5 F22 (Werkstof nummer: 3.3206, International 6063)

The composition is: 0.3-0.6% Si , 0.1-0.3% Fe, 0.1% Cu, 0.1% Mn, 0.35-0.6% Mg, 0.05% Cr, 0.15% Zn, 0.15% Ti, 0.15% other elements, saldo to make 100% is Al

An alternative alloy is AlMgSi 1 (Werkstof nummer: 3.2315, international 6082)

Composition: 0.7-1.3% Si , 0.5% Fe, 0.1% Cu, 0.4-1% Mn, 0.6-1.2% Mg, 0.05% Cr, 0.25% Zn, 0.1% Ti, 0.15% other elements, saldo to make 100% is Al. However this alloy is only available in tubes 57/47. Moreover, we couldn't obtain this alloy as it appears not to be a stock item.

Tube sizing:

60mm / 50mm tube: (2.331 kg/m)

midi note frequency (Hz) length (mm) suspension m constant weigth kg
48 130.8 1627.5 365 1.1172  
49 138.6 1581 354.5 1.1169  
50 146.8 1536 344 1.1169  
51 155.6 1491 334 1.115  
52 164.8 1449 325 1.1157  
53 174.6 1407 315.5 1.1145  

55mm / 45mm tube: (2.200 kg/m)

midi note frequency (Hz) length (mm) suspension m constant weigth kg
54 184 1298 291 1.1045  
55 195 1261 283 1.1044  
56 207.6 1225 274.5 1.1033  
57 220 1190 267 1.104  
58 233.1 1156 259 1.1037  
59 246.9 1123 252 1.1036  

50mm / 40mm tube: (1.908 kg/m)

midi note frequency (Hz) length (mm) suspension m constant weigth kg
60 261.6 1040 233 1.1129 1.955
61 277 1010 226.5 1.112 1.906
62 293 980 220 1.1092 1.848
63 311 952 213.5 1.1089 1.801
64 329 925 207 1.1092 1.746
65 349 898 201 1.1075 1.698

40mm / 30mm tube: (1.484 kg/m)

midi note frequency (Hz) length (mm) suspension m constant weigth kg
66 369 773 173 1.1135 1.136
67 391 751.5 168.5 1.115 1.108
68 415 730 163.7 1.1146 1.076
69 440 709 159 1.1139 1.048
70 466 689 154.5 1.1145 1.018
71 493 668.5 150 1.1116 0.988

35mm / 25mm tube: (1.272 kg/m)
midi note frequency (Hz) length (mm) suspension m constant weigth kg
72 523 602.5 135 1.1121 0.765
73 554 585 131 1.1107 0.743
74 587 567 127 1.1055 0.721
75 622 551 123.5 1.106 0.701
76 659 535 120 1.1047 0.680
77 698 519 116 1.1015 0.660

30mm / 20mm tube: (1.060 kg/m)

midi note frequency (Hz) length (mm) suspension m constant weigth kg
78 739.99 466.5 104.6 1.1247  
79 783.99 453.6 101.7 1.1266  
80 830.61 440 98.65 1.1231  
81 880 426.5 95.6 1.118  
82 932.33 414.5 92.93 1.1187  
83 987.77 402 90.13 1.1149  
84 1046.5 391.5 87.77 1.1203  
85 1108.7 380 85.2 1.1182  
86 1174.66 369 82.7 1.1171  
87 1244.51 357 80 1.1078  
88 1318.5 347.5 77.9 1.112  
89 1396.9 336 75.3 1.1014  
90 1479 326.5 73.2
91 1567 316.8 71.03

The tubes are suspended on their nodal point at both ends. Thus we could avoid the swinging of the tubes and as a consequence, unreliable operation on repeated notes. This is a problem we had on the <Tubi> robot.

This robot uses seven fast PIC microprocessors, five 8-bit controllers and two 16-bit controllers, handling following tasks:


The beaters for the tubes were selected such that the mass of the moving anchor is in the order of one fifth of mass of the tube to be struck. This condition made the use of pretty hefty solenoids for the low notes mandatory. Hence our selection of Kuhnke solenoids, type HD8286-RF for the notes 48 to 65 and type HD6286-RF for 66 to 71. For the higher notes, Tremba solenoids were used. The beaters have adjustable bakelite knobs (threaded) covered with a layer of felt. These solenoids are specified to operate at 24V (100% duty cycle). As we wanted to implement a wide range of dynamic, we operate them on a 100V voltage. At that voltage, the pulse durations required to activate the coils range from 3.5ms to 30ms. Microcontrollers (PIC 18F4620) are used to generate these pulses proportional to the required velocity. As this robot uses many and heavy solenoids, it became a pretty expensive robot to construct.

It appeared of utmost importance to build the instrument such that the beaters strike the tubes exactly at the point halfway their length, thus eliminating as much as possible the sounding of the first (inharmonic) overtone, having a nodal point at that place. The circuit controlling the operation of the beaters is: Two of these boards are used for the beaters. The MOSFET's used here are IRL640 types and the clamping diodes MUR620C.


The dampers make this robot very different than the earlier <Tubi> robot, where we did not use any kind of damping. With the dampers -the damping can be controlled by the users- the robot becomes a valid alternative for our <Vibi> robot, an automated vibraphone.

To reduce costs a bit, we used cheap Chinese long-stroke magnets for the dampers on the lowest 2 octaves. At Banggood they go for ca. 17$ a piece. They are rated for 12 V at a duty cycle of 5%. The DC resistance of the winding is 1.5 Ohm. Applying the handy formula given in the Kuhnke catalog: we can derive that the nominal voltage for the coils is 5.8 V. However, if we leave them activated for a long time (> 5 minutes) they become very hot and their resistance goes up to 2.2 Ohm. Thus, to keep things safe, the continuous voltage should be kept below 5 V. At that voltage, the coils still draw 3.33 A of current and consume 16.6 W. With such figures in mind, it's easy to understand why we limited the polyphony of this robot to four notes...

To protect the damper solenoids, we applied an old trick: wiring a 35 W - 12 V Halogen bulb in series with the coils, automatically generate a high striking force, going down quickly as the bulb starts glowing, thus increasing its resistance by a factor 10. The time the dampers are active can be controlled by the user, as we mapped it on the release byte accompanying the note-off commands. If note-off with release can not be used (most commercial sequencer software seems to ommit its implementation), we provided two controllers to set the amount of damping.

These solenoids drive circular felt covered plates mounted at the underside of the tubes. The felt here is 10 mm thick, a requirement for silent operation. Fall back of the anchors is also dampened with thick felt.

A sustain controller (#64) is implemented in the firmware to disable the damper mechanism alltogether.

The circuit to control the dampers (two boards are used) is identical to the one described above for the beaters. However components are different. The MOSFET's here are IRLZ34NPBF and here we did not mount clamping diodes, but MOV's. (Epcos B72210S0140K101)


In <Tubo> pipe resonators are coupled to all tubes up to note 91. This highly reinforces the fundamental pitch of the tubes. Due to the high mass of the tubes, the sustain of the tones is very long. In order to limit building height of the instrument, we used closed pipe resonators for the lowest nine notes and open pipes for the higher notes. All resonators were made from PVC tube material, except for the highest octave, where we used brass and copper. In orchestrations for the logos robotorchestra of orchestral music, <Tubo> makes a better alternative for the celesta.


The resonators were mounted on an assembly that can move over a limited trajectory. By automating this movement using two DC-motor with an excentric shaft, we succeeded in building a quite novel vibrato mechanism. Modulating the distance between the open ends of the resonators and the aluminum pipes leads to a vibrato quite simular to that implemented on the vibraphone. The speed of the vibrato can be controlled. Due to the high mass of the resonator assemblies, we had to limit the maximum vibrato speed to slightly over 2.5 Hz. Extreme slow vibrato is perfectly possible. When the vibrato mechanism is switched off, the motors will slow down and return the resonators to their optimum coupled position automatically. One motor steers the vibrato for the notes 48 to 65 and another one the notes 66 to 91. Hence we implemented two separate controllers (#20 and #21) for the vibrato speed. Controller #22 is implemented as well, and will set the vibrato speed for both motors together.


The circuit used to drive the vibrato mechanism is: Here a 16-bit PIC microcontroller is used.

MIDI-hub board:

All circuits shown above get their 5V power from a MIDI-hub board, controlling the lights and taking care of MIDI parsing. This board also houses a high current relay used to switch the high power supplies on. Here is the circuit:

Power Supply:

This robot is still under construction right now. It ought to be ready around january 2020.



Midi note range: 48-91

Note Off commands with release are required to activate the dampers. Velocity is implemented and has a wide controll range. If noteoff with release cannot be used (many sequencers do not implement it), the release values used can be set with controller 25. This value will also be used when noteon commands with velo=0 are used to switch notes off. It is permissible to only send note-on commands, if dampers are not used. It is also permissible to only send noteoff+release commands, causing the tubes to be played with the dampers only. This gives a rather soft tone and sounds the pitch of the air column in the tubes together with their fundamental note.

Lights are mapped on midi-notes as follows:

The key pressure command controls the repetition rate for the lights. It is not implemented for the beaters.


The midi listen channel is 4 (counting 0-15)..

Techical specifications:

Design and construction: dr.Godfried-Willem Raes

Collaborators on the construction of this robot:



Music composed for <Tubo>:


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Omdat ons vaak wordt gevraagd hoeveel werk en tijd kruipt in, en nodig is voor, het bouwen van een muzikale robot, hebben we ook voor <Tubo> een uitgebreid en gedetailleerd bouw- en onderhoudsdagboek bijgehouden: Dit bouwdagboek is uitgebreider dan dat voor vele vroegere robots omdat we -wetend dat ons einde stilaan naderbij komt- jongeren die in ons voetspoor zouden willen treden zoveel mogelijk tegemoet wilden komen.


iAfmetingen & andere technische specifikaties:

Belangrijke nota voor gebruikers:

Design en konstruktie: dr.Godfried-Willem Raes

verdere medewerkers:


board PIC-type source code HEX-dump
Hub 18F2620



dempers 4865 18F4620



beaters 4865 18F4620 Tubo_beat4865.bas Tubo_beat4865.hex
dempers 6691 18F4620 Tubo_damp6691.bas Tubo_damp6691.hex
beaters 6691 18F4620 Tubo_beat6691.bas Tubo_beat6691.hex
vibrato 4865 24EP128MC202 Tubo_DC_motor.bas Tubo_DC_Motor4865.hex
vibrato 6691 24EP128MC202 Tubo_DC_motor.bas Tubo_DC_Motor6691.hex

Compiler: Proton+ (Proton8, version and Proton24, version

Programmer: MPLAB-IPE with Pickit3 (Microchip)


Bibliography, references and datasheets:

Onderdelen specifikaties en bron:

Witte LED strip: 12V (Farnell)

Kuhnke magneten voor noten 48 tot 65: HD8286-R-F (Distrelec), 24V - 16W, Rdc=36 Ohm.

Halogeenlampjes: Osram 12V 35W GY6.35 socket. (Conrad)

Linear, LT1084-12 datasheet

Dunkermotoren BG40x50

Epcos MOV dataheet

Linear power supply 12V - 25A, PCB ontwerp @ 200%

Beater VDR's: Philips type 2322 592 1512. ac 150Vrms, dc 200V, Iph 400A (8us), Varistor voltage 216-264V @ 1mA, clamping voltage @50A: 470V, 90pF

International Rectifier (IOR) , Mosfet IRLZ34N (mosfets op de demperboards)

Mosfet IRL640 (mosfets op de beaterboards)

Microchip manual for 18F2620 and 18F4620

MUR620C datasheet (terugslagdiodes op de beaterboards)

Crouzet motor datasheet (RS-Components)

Omron Proximity sensor E2A-M12KS04-WPC12M (Farnell , order nr. 1603084)

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Onderhoud en (de)montage instrukties:

<Tubo> is opgebouwd uit verschillende modules die kunnen worden losgenomen:

1.- Beater unit voor de noten 48 tot 65: frontaal vastgezet met 4 M12 bouten. Vooraleer de module los te maken moet de 30-polige konnektor worden losgemaakt.

2.- Demper unit voor de noten 48 tot 65: frontaal vastgezet met M12 bouten. Vooraleer de module los te maken moeten de Weidmueller konnektors op het demper 4865 PCB losgemaakt worden.

3.- Resonator unit voor de noten 48 tot 65: om die module los te nemen moet de lange as (12 mm diameter) volledig uitgetrokken worden aan de kant van noot 65.

4.- Resonator unit voor de noten 66 tot 91: los te nemen na verwijdering van de lange as (10 mm diameter). De stelringen moeten worden losgeschroefd (inbus sleuteltje gebruiken) om de as te kunnen verwijderen.

5.- Klankbuizen unit voor de noten 66 tot 91: Met bouten vastgeschroefd op het hoofdchassis. De vibrato motor met excentriek bevind zich vast gemonteerd op deze module.

6.- Beater unit voor de noten 66 tot 91: frontaal vastgezet met M12 bouten. Zijdelings verstelbaar met vier M10 bouten. Om deze unit los te nemen moeten de konnektor aan de kabelboom van de PCB worden losgenomen.

7.- Demper unit voor de noten 66 tot 91: frontaal vastgezet met vier M8 bouten en moeten aan de binnenzijde van de 100x100x5x50 kokerprofielen.

8.- Besturingsunit 1: Power supply en midihub board. Vanhieruit vertrekken alle bedradingen voor de andere boards. Het midi-hub board is moeilijk toegankelijk: om het te bereiken kan best de 12V SMPS voedingsmodule worden losgenomen van de ronde basisplaat. Deze module bevat ook de beater- en demperbesturingsboard voor de noten 48 tot 65.

Onderdelen die enig onderhoud kunnen vergen zijn in eerste plaats de viltjes op de kogels van de kloppers. De viltschijven op de dempers kunnen onderhevig zijn aan motten, al zijn ze daartegen behandeld. Die behandeling moet zeker om de vijf jaar opnieuw uitgevoerd worden. De resonators zijn in principe stabiel gestemd, maar -indien de hulzen (moffen) op hun bovenkant zouden verschuiven- kunnen ze, indien niet vastgelijmd, worden bijgestemd. De stemming van de klankbuizen is voor vijftig jaar gegarandeerd stabiel. Wanneer piepgeluiden hoorbaar zijn bij geaktiveerd vibratomechanisme, moeten de draaipunten van de lange assen waarop de resonators kunnen draaien, van een druppeltje smeerolie worden voorzien.

Debug informatie m.b.t. de motor-besturingsboards:

Betekenis van de LED's op het board:

Enkele midi controllers werden geimplementeerd voor research doeleinden. Het is niet de bedoeling dat ze bij normaal gebruik worden verstuurd:

Tentatieve kostprijsberekening:

Aluminium buizen (Demar-Lux) 200 kg
Kuhnke magneten HD8286-R-F (Distrelec) [beaters 48-65] 18
Kuhnke magneten H6286-R-F (Distrelec) [beaters 66-71] 6 96,-
Blauwe magneten (Laukhuff) 25 75,- niet gebruikt

Tubular solenoids

Tremba ZMF-3865d-15.002 - 24V (Conrad)

50 64,-  
Tremba ZMF-3258d.002 - 12V (Conrad) F9545173455 [dempers 66-77] 28 33.73-  
Banggood magneten 12V - 1.5 Ohm (Banggood)[dempers 48-65] 25 17.33-
Banggood magneten JF-0539B 12V - small - ongeschikt bevonden 36 4.50- niet gebruikt
Banggood 12V - 2A, 10mm 20N 25x25x22mm JF-0826B1   5.35 niet gebruikt
Banggood 12V -500mA   7.53    
Inox (Demar-Lux), ca. 140 kg   3.50  
Wielen en lagers (De Coene)
PVC (resonatoren) (Eriks, Audenaert)      
Vilt en rubber      
Hasberg reflektor     niet gebruikt
Hub PCB 1    
Mosfet driver PCB's 4 500,-  
LT1084-12 regulators (Farnell) 5
EREA tranformator 300VA 12V 1
Toroidal 2 x 35V 350VA (Farnell) 1
SMPS 12V 12.5A - XP Power (Farnell) 1
70HFLR20S01 diodes (4) (RS-Components) 4 14,25-
Lasercutting van de viltschijven (Polo)
Kogelknoppen M8, 40, 36, 32, 25 mm (Fabory)      
Kogelknoppen M4 20, 16 mm (Fabory)        
Kogelknoppen M6, 25 mm (Fabory)        
Lasgassen en elektroden (Air Liquide)      
aluminium plaatmateriaal en profielen (Demar-Lux)      
Varistors 150V ac Philips 2322 592 1512 (old stock)      
MOV's 14Vac Epcos B72210S0140K101 (Farnell)      
Holpijpen (Decoene)
Dunkermotor BG40X50 + PLG42S 24V / 2.1A 1   niet gebruikt  
Crouzet Motor 80835004 (24V-147rpm) (RS Components) 1
Micromotor McLennan E192-24-25, 166 rpm 24V, 21W 90Ncm (Farnell) 1
Shock absorbers M6 - MF (Farnell / Fabory) 6      
PTFE staff material (MEA), 16 mm diameter        
PTFE plate material, 3.5mm thickness        
Omron sensors E2AM12KS04WPC12M (Farnell) 2 36.00
Motor control PCB's 2 280.00
Schroefdraadadaptors M4 int, M6 ext. Thorlabs AE4M6M 30    
Stelringen 20x32x16, inox (MEA) 14      

geschat totaal: 20.000 Euro


geschat op 9 maand FT. (ca. 37.500 Euro)

Last update: 2019-11-11 by Godfried-Willem Raes