Stringed Musical Robot

Research project on the development of new tools for musical expression at the Ghent University

<Zi>

a zither playing robot or automated Qanun

dr.Godfried-Willem RAES

2009-2016

Robot: <Zi>

In our quite large collection of musical instruments at Logos Foundation, we had since long a bunch of different Zithers of different kinds: German made ones with some 48 steel strings, a Han-Koto as well as a few zithers stemming from 19th century musical automatons. For many years we had thoughts about finding a way to turn at least one of these into a musical robot. The problems, as soon as we started designing it on the drawing table seemed very unsurmountable. The main reason being the too close spacing of the strings. No matter what kind of plucking mechanism we imagined, it either took too much physical space (using bidirectional solenoids) or it would be way too slow and monophonic (using a sledge mechanism with a single plectrum) to allow automation of all of them. So the idea was dropped for many years.

In 2013 we were asked by Osama Abdulrassol to consider the automation of an arabic Qanun. The same problems we had analysed already reappearing and some new ones in top: the Qanun uses microtonal pitchchanges using a mechanically pretty simple system (mandalar) , but again due to size/force constraints, very difficult to automate well.

In may 2013 we decided to have a throw at it, and started making a first prototype for a plucking mechanism. The perspective being to also make the zither itself rather than trying to automate an existing instrument. For the first time in our carreer as a robot designer, we decided to construct the automation mechanism prior to and fully independently of the actual sounding instrument. This entails that we designed the actual instrument only after the mechanism for the plucking was fully up and running. Thus, first a prototype plucker was made using a solenoid assembly from Syndyne:These components are normally used as register knobs on pipe organs with electromagnetic registration. We contacted the factory in order to obtain these components with a straight anchor, as this would be much easier to attach the plectra. If we use four rows of pluckers, and using a somewhat weird order of strings it appeared possible to design the zither with a string distance of 15 mm. So for a 3 octave instrument, the width could be limited to ca. 60 cm. We made a plucker assembly for a maximum of 38 strings, welded from stainless steel. The electronics require twice as many pulse/hold circuits and thus we needed no less than six driver boards and microprocessors. These boards are the same as the ones we developed for <Qt>, <Bomi> and a few more robots where velocity control combined with a hold function was needed. The boards have a maximum of 14 outputs each and taking into acount that here we need two outputs for each string, we need one board for every seven strings. The strange order of the strings was caused by the design rule dictating the strings to be plucked at about 1/7th of their lenght. The picture shows the plucker mechanism with associated electronics before wiring.

The circuit for the control of this prototype bidirectional solenoid assembly for each of the strings is given below:

As the pluckers at all times had to be in a deterministic position, it was impossible to implement a true power off for this robot. Only on cold boot, the pluckers will all go to a left position and thus pluck quite a few strings and sounding a cluster of notes. If we would implement a true power-off, this unwanted noise would be produced on any power on command. This implies that at any time when the robot is switched on, now allways 38 solenoids will be active in their position-hold state.

It took us more than two months to finish this prototype plucking mechanism. As soon as this was ready we started experimenting and designing the actual instrument in function of the possibilities of the mechanism. Parameters that were fixed are the distance between strings (ca. 15mm), the shortest possible string and therewith the highest possible note and last but not least, the maximum thickness and force on the strings as this is limited by the maximum force the pluckers can deliver. This drove our designs into the direction of using carbon fiber or nylon strings as used on the qanun and using expanded polystyrene as soundboard material. The <Rodo> robot, designed earlier was our first attempt to use this material for a soundboard and was proven to be pretty succesfull. Moreover, as the traditional qanun uses a bridge resting at 4 to 5 points on stretched membranes (in this respect it shows some acoustic similarity with the banjo), the idea to go for a styrofoam soundboard is not at all alien to the instrument.

We forsake the use of double or triple strings for each note as we could not make this to work properly with our pivoting plucking mechanism. So we went for 38 single strings, however, tuned chromatically. (Qanuns have a diatonic tuning). For the tuning mechanism, after long experiments using mandolin tuning pegs in rows of four which came out to be unworkable, we designed a simple yet very effective mechanism. The advantage of our mechanism is that the strings are tensioned by pulling only and thus we can avoid any torsional forces in the strings as would have been the case if we rolled them on a cylinder as usually done. Another advantage of our mechanism is that close spacing of the pegs is easy to achieve. So, all tuning pegs could be arranged in a single row. Here is a picture of the prototype of the mechanism:


Users have to be carefull in tensioning the small M4 nut, if the tension is too high the string will be cut off! Tension should be just such as to prevent the string from sliding out. As an alternative to using the M4 nut, one can also make a knot around the tuning peg. This mechanism makes it perfectly possible to implement mandalar, by inserting small fork inserts between the long hexagonal tuning nut and the holder plate. However, we did not consider automation of such devices in this robot, mainly because it would make the whole construction way to heavy.

<Zi> being a string instrument, it will be clear that it cannot be used without proper maintenance. Not only users will have to tune the instrument properly, but also the bridges and the plectrums may need regular attention. This is because in tuning, the bridges may move slightly and may cause the plectrums to get out of adjustment. Bowed string instrument players will certainly be familiar with this problem. The need for regular tuning arises from the use of nylon strings. Had we used steel, the tuning would be a lot more stable, but the tone quality would be far off the gentle qanun sound we had in mind in the design.

The original string order and lay out looked like: The string gages used as well as the sounding string lenghts are given in the table.

After many months of experimenting and trying to get the plectrums adjusted properly, we had to confess that the design of the plucker mechanism was a complete failure. We gave it up altogether and decided to design a second prototype. This time using longitudinal bidirectional solenoids with permanent magnets. These solenoids are stable in either of their end positions and they only require a pulse of changing polarity to make them change position. As this type of solenoid could not be obtained with an anti-rotation shaft, we decided to design round plectra with a 2 mm central hole for plucking the strings. The plectra are mounted on the shafts with two stainless steel M2 nuts.

Not only the plucker assembly had to be completely build again and redesigned, but also the electronics, including the power supply. This type of solenoid requires pulses of alternating polarity, thus requiring H-bridges to drive them. We used an old and proven H-bridge in IC form, the L298N. With a single 18F2525 microprocessor chip, we can steer a group of 8 solenoids. Here is the circuit:

Five of these boards are required for the complete qanun. The power requirements are a lot more relaxed as compared to the first prototype design. This mainly because of the pulse-only operation of the solenoids. However, these solenoids having a DC resistance of only 4.2 Ohms, draw a pulse current of 2.8A each, which is at the limit of what the L298 drivers can cope with. The data sheet specifies a maximum of 3A, non repetitive pulse. The pulses being limited to maximum 50ms with a 50% duty cycle relaxes the limits though. Unfortunately there is as yet no integrated mosfet H-bridge on the market with a wider range. As to the power supply, a 12V / 500VA transformer and some parallelled LT1024-12V regulators seemed adequate. Here is a picture of these solenoids:

 

Only when integrated in the context of our M&M robot orchestra with its wealth of varied sensor systems allowing full interactivity with gesture and audio, this automate will become a true robot. That's after all were its destination is to be sought.


Midi Mapping and implementation:

note range This is the ambitus for the instrument when using a complete set of Qanun strings. This also is the ambitus as implemented in MIDI.

However, this ambitus was at the original base of our design: original note range To reach this ambitus, guitar strings can be used. Obviously, if other strings are used, other tunings are certainly possible. The instrument would than behave as a transposing instrument and should be treated accordingly. The lowest note in the original design (40), low E corresponds to the lowest note on the guitar and will apply if the instrument is strung with guitar strings.  As to the Qanun, there is no real standard for the ambitus of the traditional Qanun, although midi 50 (D) to 93 (a''), diatonic, is commonly found in Turkey.

Midi channel: fixed to 4 (counting 0-15).


Note Off: Implemented for all notes in the range. Note Off does not reset the repetition rate.

Note On: Implemented for notes in the range. Velo-byte is used for the striking force. The range is rather limited. The lights are also mapped on notes, but make use of a range outside the normal range of the zither.They are mapped on notes 120,121.

Key pressure: can be used to let notes repeat automatically. The pressure value sets the repeat frequency. The command can be sent even prior to note-on commands. The value send will be preserved until reset with a key pressure command for the corresponding note with value zero. Controller 30 will override individual key pressure commands.

Controller 14: Sets the minimum velocity level required to pluck the string. The setting for this controller has to be carefully examined as it depends on the string material used, the allignment of the pluckers and the tuning of the strings. It should be adjusted to such a value that with velocity value 1 all strings are guaranteed to be plucked. If this condition is not met, the instrument will behave erroneously.

Controller 20: Sets the tuning of the instrument. The range is 33 to 52. The default is 50.
Controller 30: Can be used to set the repeat frequency of all notes to one and the same value. By default this controller is zero.
Controller 66: Robot on/off switch. Sending a power off command (Ctrl 66 set to 0) will cause a reset of all controllers to their default start up value. Also settings for note repetition (key pressure commands) will be reset.

Controller 127: Sending this controller will reset all pluckers to an inward position. A power off command will be performed as well, thus causing a complete reset. The command takes some 10ms and users should make sure they do not send any other midi commands to the robot during this time interval. The command should not be used in midi sequences. Also, be warned that this command likely will pluck a lot of strings, as all pluckers in an outward position on entry, will be retracted to an inward position and thus the corresponding strings will be plucked. On a cold boot of the robot, this command is issued automatically.

 

Technical specifications:

Design and construction: dr.Godfried-Willem Raes

Collaborators on the construction of this robot:

Music composed for <Zi>:

 

Pictures taken during the construction in our workshop (in chronological order):

harp-frame
bridges Zi

 

Back to composers guide to the M&M robot orchestra.

Back to Main Logos page:index.html To Godfried-Willem Raes personal home page... To Instrument catalogue Go to Godfried-Willem Raes' homepage

 


Construction & Research Diary:

Board 3: notes 66 - 73


Archival
Rejected version 1.0 documentation:

Power supplies:

 

Circuit details solenoid driver boards:

The string lengths are calculated and the string gages follow those of classical Qanun strings. We used a complete set for a 26 note diatonic Qanun (Dupont Kanun Teli), strings from Turkey.

Board 1:

string nr board output connector pin mapping remarks PIC pulse pin PIC hold pin string lenght (mm) string diameter
- 1 2 note 120 hold only - lite 4 = RA1 3 = RA2  
- 2 3 note 121 hold only - lite 2 = RA3 5 = RA0  
- 3 4 nc nc (broken) 6 = RA5 7 = RA4 (*)  
- 4 5 nc nc 9 = RE1 8 = RE0  
1 5 7 50 left 37 = RB4 10 = RE2 650 1.25
1 6 8   right 35 = RB2 36 = RB3  
2 7 9 78 left 33 = RB0 34 = RB1 201 0.80
2 8 10   right 29 = RD6 30 = RD7  
3 9 12 69 left 27 = RD4 28 = RD5 287 0.90
3 10 13   right 23 = RC4 24 = RC5  
4 11 14 60 left 21 = RD2 22 = RD3 419 1.00
4 12 15   right 16 = RC1 15 = RC0  
5 13 17 51 left 18 = RC3 17 = RC2 620 1.25
5 14 18   right 20 = RD1 19 = RD0  

This board uses BYV27 diodes in the hold-circuit. On the other 5 boards we used MUR4100 types. The double diodes are invariably BYV32 types. These are only mounted on outputs meant to switch inductive loads. The P-channel Mosfets are BSP254A types and the power Mosfets all IRL640. On this board pin RA5 is programmed to be used as a loopspeed measurement output. The pin can be accessed on the gate connection of the hold mosfet, not mounted on the board. Weidmueller connector pins 1,6,11,16, 19 are connected to the positive hold voltage on all boards (+6 V).

The source code for the 18F4620 processor on this board can be downloaded here.

Board 2:

string nr board output connector pin mapping remarks PIC hold pin PIC pulse pin string length (mm)
string diameter
6 1 2 79 left 4 = RA2 3 = RA1 194 0.80
6 2 3   right 2 = RA0 5 = RA3  
7 3 4 70 left 6 = RA4 7 = RA5 275 0.90
7 4 5   right 8 = RE0 9 = RE1  
8 5 7 61 left 10 = RE2 37 = RB4 401 1.00
8 6 8   right 36 = RB3 35 = RB2  
9 7 9 52 left 34 = RB1 33 = RB0 594 1.25
9 8 10   right 30 = RD7 29 = RD6  
10 9 12 80 left 28 = RD5 27 = RD4 187 0.80
10 10 13   right 24 = RC5 23 = RC4  
11 11 14 71 left 22 = RD3 21 = RD2 264 0.90
11 12 15   right 15 = RC0 16 = RC1  
12 13 17 62 left 17 = RC2 18 = RC3 384 1.00
12 14 18   right 19 = RD0 20 = RD1  

The source code for the 18F4620 processor on this board can be downloaded here.

Board 3:

string nr board output connector pin mapping remarks PIC hold pin PIC pulse pin string length (mm) string diameter
13 1 2 53 left 4 = RA2 3 = RA1 571 1.10
13 2 3   right 2 = RA0 5 = RA3  
14 3 4 81 left 6 = RA4 7 = RA5 180 0.70
14 4 5   right 8 = RE0 9 = RE1  
15 5 7 72 left 10 = RE2 37 = RB4 254 0.80
15 6 8   right 36 = RB3 35 = RB2  
16 7 9 63 left 34 = RB1 33 = RB0 368 0.90
16 8 10   right 30 = RD7 29 = RD6  
17 9 12 54 left 28 = RD5 27 = RD4 545 1.10
17 10 13   right 24 = RC5 23 = RC4  
18 11 14 82 left 22 = RD3 21 = RD2 174 0.70
18 12 15   right 15 = RC0 16 = RC1  
19 13 17 73 left 17 = RC2 18 = RC3 244 0.80
19 14 18   right 19 = RD0 20 = RD1  

The source code for the 18F4620 processor on this board can be downloaded here.

Board 4:

string nr board output connector pin mapping remarks PIC hold pin PIC pulse pin string length (mm) string diameter
20 1 2 64 left 4 = RA2 3 = RA1 353 0.90
20 2 3   right 2 = RA0 5 = RA3  
21 3 4 55 left 6 = RA4 7 = RA5 518 1.10
21 4 5   right 8 = RE0 9 = RE1  
22 5 7 83 left 10 = RE2 37 = RB4 168 0.70
22 6 8   right 36 = RB3 35 = RB2  
23 7 9 74 left 34 = RB1 33 = RB0 235 0.80
23 8 10   right 30 = RD7 29 = RD6  
24 9 12 65 left 28 = RD5 27 = RD4 339 0.90
24 10 13   right 24 = RC5 23 = RC4  
25 11 14 56 left 22 = RD3 21 = RD2 498 1.00
25 12 15   right 15 = RC0 16 = RC1  
26 13 17 84 left 17 = RC2 18 = RC3 162 0.70
26 14 18   right 19 = RD0 20 = RD1  

The source code for the 18F4620 processor on this board can be downloaded here.

Board 5:

string nr board output connector pin mapping remarks PIC hold pin PIC pulse pin string length (mm) string diameter
27 1 2 75 left 4 = RA2 3 = RA1 226 0.80
27 2 3   right 2 = RA0 5 = RA3  
28 3 4 66 left 6 = RA4 7 = RA5 324 0.90
28 4 5   right 8 = RE0 9 = RE1  
29 5 7 57 left 10 = RE2 37 = RB4 476 1.00
29 6 8   right 36 = RB3 35 = RB2  
30 7 9 85 left 34 = RB1 33 = RB0 156 0.70
30 8 10   right 30 = RD7 29 = RD6  
31 9 12 76 left 28 = RD5 27 = RD4 217 0.80
31 10 13   right 24 = RC5 23 = RC4  
32 11 14 67 left 22 = RD3 21 = RD2 310 0.90
32 12 15   right 15 = RC0 16 = RC1  
33 13 17 58 left 17 = RC2 18 = RC3 457 1.00
33 14 18   right 19 = RD0 20 = RD1  

The source code for the 18F4620 processor on this board can be downloaded here.

Board 6:

string nr board output connector pin mapping remarks PIC hold pin PIC pulse pin string length string diameter
34 1 2 86 left 4 = RA2 3 = RA1 151 0.70
34 2 3   right 2 = RA0 5 = RA3  
35 3 4 77 left 6 = RA4 7 = RA5 209 0.80
35 4 5   right 8 = RE0 9 = RE1  
36 5 7 68 left 10 = RE2 37 = RB4 298 0.90
36 6 8   right 36 = RB3 35 = RB2  
37 7 9 59 left 34 = RB1 33 = RB0 437 1.00
37 8 10   right 30 = RD7 29 = RD6  
38 9 12 87 left 28 = RD5 27 = RD4 146 0.70
38 10 13   right 24 = RC5 23 = RC4  
- 11 14 nc nc 22 = RD3 21 = RD2  
- 12 15 nc nc 15 = RC0 16 = RC1  
- 13 17 122 hold only - lite 17 = RC2 18 = RC3  
- 14 18 123 hold only -lite 19 = RD0 20 = RD1  

The source code for the 18F4620 processor on this board can be downloaded here.

This board does not have the pulse Mosfets nor the double diodes in the four right-most outputs.

Schematic:

Plucking birectional solenoid assemblies: