Musical Robot

Research project on the development of new tools for musical expression

<Zip>

a robotic spring player

dr.Godfried-Willem RAES

2009-2026

<Zip>

This robotic instrument has a very long history as its first construction was started in 2009 and only in 2026, it reached the stage of finalization. On the way to its realization it had known very different appearances. It started off as an attempt to create an automated quanun, but all our attempts to turn it into a useful instrument failed. The main reason for the failing 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 bi-directional 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. A first version made use of pluckers using bi-directional solenoids, in this case solenoid assemblies made by 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. The picture shows the plucker mechanism with associated electronics before wiring. However, we never got this mechanism to work well as a string plucker. Later we found a good use for this mechanism in our <Tinti> robot.

For many years we gave up altogether developing a plucked string instrument until we decided to design a second prototype. This time using longitudinal bi-directional 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 were 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 were required for the complete qanun. The power requirements turned out to be 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 unortodoxically paralleled LT1024-12V regulators seemed adequate. Here is a picture of these small solenoids:

But, once more this mechanism was a complete failure when it came to plucking strings. The solenoids just did not develop enough force in the mid position of their trajectory, no matter how we tried to drive them. Thus we were forced to drop the entire project once more. Many years later, in 2026, we turned back to the failed project with an entirely different idea, dropping the plucked strings altogether. Now the new idea was to use the bistable solenoids to drive springs in longitudinal resonance. The solenoids having only very limited force (<=6 N) , we had to use quite weak springs, wound with pretty thin steelwire. The diameter of the springs was limited by the distance between the solenoids (ca. 13 mm) . As a resonator we used a circular piece of Styrofoam in a round stainless steel frame. Styrofoam was proven to be very efficient as a soundboard and resonator in previous robots such as <Rodo>.

 

Only when integrated in the context of our 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:

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 instrument.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.r 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.=ontroller 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 springs, as all pluckers in an outward position on entry, will be retracted to an inward position and thus the corresponding springs 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 <Zip>:

 

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

Version 1:

harp-frame
bridges Zi

Version 2:

Version 3 - renamed <Zip>::

 

 

 

 

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

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Construction & Research Diary:

Archival
Rejected version 1.0 documentation:

see Zi.html

Plucking birectional solenoid assemblies: