Hello everyone, my name is JP from Victoria University of Wellington, and I'm going to tell you about a research on mechatronic chordophones
Let's start with mechatronic instruments. These are systems that use electronics and mechanical actuators to make music.
With them, we can create complex sounds that digital systems cannot offer yet
We can control them precisely, we can use MIDI controllers and sensors to interact with them, and they tend to be very expressive,
allowing us to manipulate sound parameters in a way that's much more deliberate than what we can usually do with conventional instruments
Mechatronic chordophones a a subset of these instruments, but they use strings to make music.
Their designs can be very different from each other and we can even find them in humanoid robot bands such as CompressorHead.
At Victoria University of Wellington, we have built quite a few of these chordophones, including MechBass,
a Mechatronic bass guitar, and Swivel 2, a slide guitar robot.
When we think about how we make music with these chordophones, it's important to understand how they work, which can be simple.
If we think of a guitarist, the right hand usually plucks a string --
that would be our picking mechanism -- while the left hand (the fretting hand) selects the notes -- that's gonna be our pitch shifter.
which is going to be the focus of our presentation today.
If you were not around last year, let me introduce you to ProtoChord, our mechatronic monochord prototype
that we have created for performance and composition through expressive playing.
We have also been using it to iterate through different approaches for design challenges that we have seen in previous systems
This might be a good chance for me to recommend our paper from last year that
Introduces its revolving picker for fast picking speed and a wide range of dynamics
Moving on to our pitch shifters...
These are mechanisms that are in charge of note selection as well as pitch-based techniques such as slides, bends, and vibrato
The way we select notes in chordophones is by stretching the string and applying pressure at specific points
The easiest way to do this in a robot is to place the
actuators directly on top of this point so we can trigger them immediately for virtually no delay. Although this is a fast approach,
it's somewhat restricted expressively because once fixed, we cannot move these actuators
Which is why we have opted for more expressive approaches such as linear displacement systems,
which allow that continuous movement that we would want in a more expressive system.
Normally, we have done belt driven pitch shifters, and the ones that we have selected for ProtoChord, which are robot arm pitch shifters.
Belt driven systems use timing belts and pulleys to move the bridge or carriage that applies pressure to the string along the instrument
They tend to be slower than robot arms
However, their velocity is uniform across the instrument which makes them more precise and easier to control
On the other hand, robot arms tend to be more agile and easier to reach higher speeds
However, we're not manipulating the position of the carriage directly. We're changing the angle of the articulator arm which results in this position change
This leads to a variable resolution which makes it harder to control precisely
Because when we extend the arm, you have high resolution movements and when we contract it, we have a bit of larger steps
We also want to be careful with kinematic singularities at the end of the motion range
because if we're not careful, the arm can get stuck and it might lead to damage to parts of the instrument.
A couple of other components that we need to consider are clamping mechanisms
which is how we're going to select a note. We use our actuators
to either clamp down or perhaps rotate and apply pressure to the string.
And we also have damping attachments. As a guitarist if you want to play a rest, you need to mute your string.
So traditionally these have been their own dedicated component with their own actuators like you can see on the left on MechBass.
But a more recent approach has started to integrate them into a clamping mechanism of the pitch shifter.
On the right we can see that if we spin them one way (if we spin this front disk), the brass rods engage and play the note.
If we spin them the other way, the soft materials touch the strings and we can mute them.
A few limitations that we need to work with when considering this type of system is that these are not
Instantaneous mechanisms. Moving the carriage from one point to another takes time.
So we want our pitch shifter to be as fast as possible without trading off precision
We also want to avoid structural damage like the kinematic singularities that we mentioned and we want them to be mechanically quiet
while avoiding electromagnetic interference
Let's compare a few chordophones briefly. Although Swivel2 and Strumbot have proved that their pitch shifters are very fast,
these chordophone designs are very different from ProtoChord
So it's not going to be particularly useful to compare them
On the other hand GuitarBot and MechBass are a lot more similar and although they are belt driven
We're gonna get a better result from this comparison
So our target speed is gonna be somewhere between GuitarBot's 250 milliseconds and MechBass's 360ms
This speed measurement is the time that it takes for the carriage to move from the first
position to a position at which the pitch changes to an octave above
Now for precision
A cent is 1/100 of a semitone. So we want to say at a range of plus minus six cents
Which is the just noticeable pitch difference in human beings.
Let's take a look at ProtoChord's articulator robot arm.
We can see that it's a nimble design that can perform subtle motions to clamp or damp the string
We have designed it
for smooth linear motion using the X-Rail building system and we drive it with a Dynamixel
Smart Servo motor which provides useful feedback on position and torque
The end effector carriage holds the clamping mechanism
Recent designs use a pair of metal rods to clamp the string which they can hold while sliding.
In our clamper, we use a single aluminum rod much like a bottleneck so we can apply slight pressure to the string
That way we can perform note changes and we can perform smooth slides with barely any resistance
If we rotate the clamper towards the damper, we can mute the strings
We have been experimenting with this damping attachment and we have used pourable silicone and foam.
We can perform both the clamping and damping gestures with a single micro servo motor
Now moving on to our electronics, here's our custom PCB:
We use a teensy 3.5 which is high-powered and fast enough to drive all of our actuators
however
we need to integrate a tri-state buffer to handle bi-directional communications with the DynaMixel as
well as a logic level converter to transform our
3.3 volts from the Teensy to our 5 volts logic that the Dynamixel servo motor requires
To play specific notes with our pitch shifter, we have to consider 2 factors
the Dynamixel position and the clamping servo value
We have to be very precise, so whenever we target a note, we first use the Dynamixel using built-in values and
we set the carriage at the position where it can play the right note
Then I have to consider the clamping servo value to apply the right amount of pressure and to maintain intonation
I don't want to stay short or overshoot the pitch
For the most part the clamping servo pressure tends to be uniform
But as I approach the middle of the string
It does require a bit more rotation to apply more force as you can see on that A#4 or B4.
Now we evaluated the system and
first when it comes to speed, we achieve the speed of 227 ms
to go across that first octave that we were targeting
for that standard test that we use to compare to GuitarBot and MechBass
It's not considerably faster, but it's a reasonable trade off for the expressivity this system brings
Now when it comes to pitch precision,
we managed to stay under the +/- 6 cents that we're targeting and we match Swivel 2 +/- 4 cents
To wrap things up that is our pitch shifter: a fast subtle and precise mechanism that enables expressive techniques.
This has helped ProtoChord meet both musical and technical requirements and we are pretty satisfied with the result
There are a few things that can be improved
For example, although we've managed to keep the mechanical noise under 60 decibels
(which is the level of a quiet conversation), at fast speeds,
the operation does get a bit above the level where we'd want it to be
Part of this is because the chassis ended up being a bit too lightweight
So there are things that we intend to address in the future. And in fact, let me show you something exciting
we have built a new prototype based on what we have learned from this one and
we are in the final construction stages of a six-string variation of
ProtoChord as a multi-string chordophone
Hopefully I'll get to tell you all about it next year. Thank you very much!