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  #21  
Old 06-24-2012, 07:36 PM
Jeff Jeff is offline
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A big Thank You to John for his help in resolving the servo problems I had been having.

I have uploaded the two final music compositions of a Halloween 2012 spectacular we have been working on.

The first is a Dead Zombie Ballerina segment, that will be performed by a small girl.
http://youtu.be/hcg9TlOhSw8


And the second is a choreographed dancing skeleton segment, that will be performed by four small children.
http://youtu.be/_SrYhGLFemA

Unfortunately the audio is a mess. The camcorder I'm using is sensitive to high frequencies, and the Tesla Coil audio badly overdrives the microphones.
Hopefully I'll be able to improve the recordings later.


Regards, Jeff
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  #22  
Old 06-25-2012, 07:30 PM
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Jeff, thanks for uploading the videos!

Would you care to post your parts list? It would be interesting to know what servos, solenoids, and power supply you have used.

Does the latency of servo arm travel add any challenge as far as sequencing is concerned?
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  #23  
Old 06-25-2012, 08:26 PM
Jeff Jeff is offline
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Hi John, I'd be happy to.

The system is composed of three MD24's; two for the 37 note scale, and the third as a servo only output to drive the pedal. Everything was assembled on plated through prototyping board, using point to point wiring.

The solenoids themselves are Ledex P/N 195207-228

The Ledex solenoid plungers were bored to hollow-out the solenoid shaft nearly through it's length, removing approximately 50% of the total plunger weight. I needed to reduce the mass to speed up the note response. The full mass plunger was too heavy for a fast roll. This was a completely empirically derived modification. I began boring extra plungers with increasingly larger end mills, and measuring the force using a digital force gauge. The voltage and timing were precisely controlled during the tests. I stopped when I began to detect the force rolling off. Evidently the magnetic field with half the drive voltage rating does not saturate through to the center of the plunger steel shaft.

An .080" thick felt circle pad is punched in the center, and pushed onto the tapered shoulder of the solenoid plunger to eliminate any noise if the plunger "bottoms out" inside the solenoid cavity. It's needed mostly for the lightweight higher notes.
Mounted on the end of the external shaft is a .380" long x .500" diameter delrin pad, pushed on by an interference fit. A .125" thick x .500" diameter soft leather disc pad is glued on to the delrin mount as the "mallet" with automotive grade silicone adhesive.

The solenoids are mounted to an aluminum "L" rail. The striker spacing is centered to the bars. Another "L" rail is inverted, and serves to support the plunger return, with a strip of 1/2" foam weatherstrip material dampening them.
The solenoid rail is attached to the Vibraphone frame at three locations along it's length. The rail is spaced critically from the wood Vibraphone frame to ensure the striker pads contact the bar at the outside edge. Inward mounting would result in a dampened note response.

I am using an approximately 20ms fixed-length drive pulse to each solenoid. This appears to work the best, based on the power supply voltage range.

The solenoid drivers are the standard TIP122 Darlington transistors using John's published output circuit, with the blocking diode mounted at each solenoid. There are white LED's for each of the 37 output channels from the MD24's.

The sustain pedal is controlled by a Chinese POS servo. It was advertised as a 35kg/cm high torque / fast moving servo. I purchased it here on ebay for $100 It immediately overheated at an 8kg holding load.
I DO NOT recommend buying this servo. The spec's are fake, actually delivering a fraction of the advertized force. I had to remove the H-bridge output IC and fabricate a large copper finned heatsink soldered between the IC and the board, and cool it with a fan. Without it, the servo would overheat and shut down within a few seconds of cycling the pedal. I would have purchased one of these servos if I were to do it again.
On the plus side, the Chinese servo was very fast, and included a trim pot to vary the throw speed. I didn't need to compensate for servo latency because the arm travel was very fast by default.

I used note output for servo control. C6 to be specific, as it was beyond the range of the instruments. I adjusted the servo throw values in the sysex to fit the application. You'll have to ask John about this unreleased note on sysex if you prefer to go the same route. For me, adding a pedal track during a dub was much easier than building a CC message.

An LM338K based variable voltage power supply in the controller functions as a precision volume control. It ranges from 12-22vdc at 6amps. An 8k uF capacitor bank stiffens the output voltage enough to drive ten solenoids together in parallel. And everything fits inside a 7" x 10" plastic project box beneath the tremelo motor.

The main power supply is an external 24vdc 15A output switching supply, required because the servo can consume 150w during a pedal depression cycle.

Regards, Jeff

Last edited by Jeff; 06-25-2012 at 09:21 PM. Reason: typo
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  #24  
Old 08-29-2012, 01:23 PM
Jeff Jeff is offline
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Since completing the Vibraphone, I've moved on to complete a Glockenspiel, two bass drums, and a Cymbal controlled via the MD24.

Another part of the automated orchestra was a MIDI driven Tesla Coil, and that finally came together last Saturday. It's big; standing over 2m tall.

I've been able to play two notes in polyphony so far. I'm hoping to achieve three note chords before the Halloween extravaganza.

Below is a link to a youtube video of the "first light". Portions of the sugar plum fairy, and Bach's Fugue are performed. Hope you folks are amused.

http://youtu.be/Fcb_M_t3XTk

Regards, Jeff
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  #25  
Old 09-04-2012, 11:17 AM
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Pretty amazing! Thanks for posting the video.
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  #26  
Old 06-28-2013, 10:07 PM
J.D. J.D. is offline
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I'm bumping this because it's an extraordinarily helpful reference. Subjective data is really nice to have when you're about to buy a few hudred dollars worth of parts. Thanks Jeff.
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