Andrew Burks

The Problem

I need to be able to turn 30 identical stepper motors on and off individually.  I can only afford to have one unique wire going to each stepper unit because I only have ~40 digital outputs to work with.  I can afford to have a few common outputs that are jumped from board to board.  basically I need to turn four inputs that go in a pattern into one input.

The four lines on the motor driver (H-Bridge) basically take turns going high when I want the motor to turn.  When I want it to stand still, only one of the lines should be high.  This is called “wave driving” a stepper motor.  Here is what happens when a bipolar stepper motor is wave driven.

t=0:  A=1  B=0  C=0  D=0

t=1:  A=0  B=1  C=0  D=0

t=2:  A=0  B=0  C=1  D=0

t=3:  A=0  B=0  C=0  D=1

t=4=0

The Solution

A Serial in Parallel out (SIPO) Shift Register does basically exactly what I’m looking for.  If I have one common clock (a line that goes high every 1/4 step) and connect the 4th output to the data input, then the four parallel outputs will shift through my 4 states like a champ.  The only catch is that I need to seed the circuit with the initial “1″ so that the “1″ can move along the shift register.

Luckily, because a shift register is just 4 flip-flops lined up in a row, I could build my own shift register out of flip flops, and access the set/reset abilities of the individual flip-flops.  So in the final setup, I had a single clock coming from the Arduino (pulsing at 100ms intervals) which controlled the speed of the motor, and a “stop” pin coming from the Arduino to control whether or not the motor was turning.

The “stop” pin was tied to the reset pin on the first flip-flop and the set pins on the other 3.  This means that when the “stop” pin was driven low, it would force the shift register into the “1-0-0-0″ state, and when it was released the “1″ would shift sequentially at the speed of the clock to drive the motor.  Here is a view of the protoboard layout (the center IC is the motor driver, and the other two each contain two flip-flops):

Pros

This is a huge improvement over controlling all 120 lines individually.  An Arduino mega can easily output a single clock and 30 control lines.  The cost of each circuit is about $4 in parts (three Integrated Circuits, or ICs), more if you PCB it.  It works, and it lets you do a powered brake as well.

Cons

The two IC’s with flip-flops are about $2.50 0f the total parts cost.  For this price ($2.50×30=$75) it would technically be cheaper to buy some other board that can take serial from the Arduino and control the 120 outputs.  Also, the wiring is a bit complex and uninsulated because each flip-flop’s output feeds into the next one’s input.

More Photos

A completely axially symmetric vibraphone robot would be awesome.  We decided to move away from a big row of keys and towards a round plate of keys.  Here is a quick render of the key mounting structure and how it incorporates the ball retrieval and distribution system:

Structure

The large round plate is actually a 30-gon not a circle.  It is inscribed in a 32″ circle, and is 1/4″ thick.  There are 60 unique (thank you design tables!) plastic supports that slide onto notches in the aluminum.  Each plastic support has to be unique because of the awkward hole spacing in the individual keys.

There are already notches in the plastic for clips that should hold it into the aluminum plate (aka “Megaplate”).  However, depending on the design of the ball deployment mechanism, the retaining clips for the plastic plates should be incorporated into the support for the mechanism.  Here is a close up of the plastic supports:

Distribution

Finally, here is a close up of Mike Ornstein’s ball collection and sorting mechanism.  It uses brushes from the bottom of doors to pull balls up an archimedes screw into a paintball-style hopper.