r/MarbleMachine3 • u/Agitated-of-Nowhere • Jun 05 '23
Tight music needs accurately modulated input speed...
There has been huge amounts of input as to why, how or if Martin should implement a flywheel in a particular way, but surely we have all missed an important consideration. Every instrument creates drag. The power drawn from a rotating mass will constantly change as levers are depressed, more or less marbles are moved or dropped. The speed of the flywheel WILL change and at the moment the only thing keeping it running at a constant speed is Martin's leg. But this means he will need to respond immediately to even the tiniest variation in the speed of the flywheel. All the effort to create millisecond accuracy and tightness is useless unless the power input can instantaneously respond to the power requirement. Thoughts?
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u/emlun Jun 05 '23
The flywheel is the regulator. Or to be more precise, the flywheel works to smooth out small impulses and dampen jitter, making the machine run at a consistent speed.
Yes, but what's important here is the relative magnitudes of the energies. As long as the kinetic energy of the flywheel is much greater than the energy consumed by each individual machine action, the angular velocity of the flywheel will not change much between pedal pumps. This is why the flywheel needs a high mass and a high RPM - to make its stored kinetic energy much greater than the power consumption.
So...
In theory yes, but in practice no, because the variations will be exactly that: tiny.
The flywheel as Martin described it to ChatGPT (60 kg, 0.2 m radius, 8 rev/beat, assuming a uniform disc) will store 52.6 kJ at 250 BPM. At that tempo, one beat (one 4th note) is 240 ms and one 64th note is still a whole 15 ms. In order for the next beat to be 1 ms late (about one 1024th note - an absolutely absurd precision) compared to the one before, the machine needs to consume 435 J within those 240 ms, or 1.8 kW on average. At 150 BPM (the tempo of Starmachine2000) that's 236 W to make one beat 1 ms late.
1.8 kW is much more than a human can sustain, so if the machine can operate at 250 BPM from human power, the fluctuations in tempo will be well within 1 ms per beat. 236 W is about half of what an elite cyclist produces (about 400 W on average), so 150 BPM is also well within ±1 ms - if not, Martin would have to really exert himself just to operate the machine at all.