Andrew Burks

Archive for September, 2010

Gate-Style Ball Dispenser

by on Sep.30, 2010, under RobOrchestra, Robotics Club, Vibratron

The Problem

After several iterations of the wheel-style ball dispensing approach, I decided to try a gating mechanism instead.  The main drawback of the wheel approach was the low throughput.  Only about 4 balls per second could be dispensed.  Xylobot, however, can play over 20 notes per second.  While Xylobot’s speed isn’t fully necessary, at least 8 balls per second is a reasonable goal.  The wheel mechanisms do not meet that goal.

The Solution

The basic idea of a single gate mechanism is that the stream of balls is free to fall out the end of its containing tube whenever the gate is open.  The trick is opening the gate for a short enough period of time to consistently allow only one ball to pass through the gate per activation.  In our prototype, the time required to let one ball through was about 40ms.  With delays to account for the return motion of the gate and the settling of the balls, the prototype gate mechanism could achieve a rate of 11 balls per second.


The most important part of this design is constraining the balls to one dimensional travel.  This was achieved by basing the gate around an aluminum tube with an inner diameter of .5”.  The gate would be entering perpendicular to the direction of travel, through a hole in the aluminum tube.  When the gate is up the balls roll through, but when the gate is down the balls are unable to pass through the gate.

The gate itself is a cone of plastic.  The conical shape is important because any non-angled surface has the potential to jam by clamping on the top of the ball.

The plastic cone is attached to a pull-type solenoid with a spring return.  The built in mounting bracket on the solenoid is a great advantage, but the 3-4 amp power draw (at 12 volts) is a major disadvantage.  The gate is normally closed, but when powered the solenoid pulls the cone out of the way of the balls.

In the initial prototype the mechanism made a large amount of excess noise when releasing a ball.  Quiet actuation is important for a musical instrument because the sound of the ball hitting the vibraphone key needs to not be overwhelmed by any other noise.  The addition of an o-ring to act as a hard stop when the gate returns to the closed position should help to muffle the noise produced by the prototype.

The final integration of this mechanism (actually 30 of these mechanisms, one per key) will have the tube nearly vertical, with the gate at the bottom.  With balls feeding into the top of the tube from Mike’s hopper mechanism, the gate will dispense one ball at a time, allowing them to fall onto the key and play a note.


Side View

Gate - Cross Section

Isometric Render

Gate - Iso Opaque

Isometric Render with Transparent Tube

Gate - Iso Transparent

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by on Sep.24, 2010, under RobOrchestra, Robotics Club

As I create content for the RobOrchestra website, I figured I might as well include it here:


This robot is an autonomous xylophone capable of playing chords of up to 5 notes at a time at speeds of up to 20 notes per second. As one of the oldest and most reliable members of the RobOrchestra, Xylobot currently lives in the robotics club.

Xylobot is connected to a car battery and programmed for demo mode. When activated, the robot plays Nikolai Rimsky-Korsakov’s “Flight of the Bumblebee” at a normal speed, then repeats the song at twice the speed.


Each of the 17 keys has a small push-type solenoid mounted just beneath it. When 12 volts of electricity passes through a solenoid, it pops up and strikes the key, then falls back down, all in a fraction of a second.


When the Arduino Duemilanove receives a serial command from its master, it parses the 5-10 character string to determine what note it should play. Then, the note is transmitted in binary across five digital outputs to a demux board. This board uses the binary signals to select one of the seventeen notes, then activates the logical line for that note. The MOSFET board allows each logical line to control the higher voltage solenoids with individual MOSFET circuits.


View of Xylobot with the keys removed, exposing the solenoids and electronics:

Electronics boards from right to left: Arduino, Demux, MOSFET

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