Scribbler Project

This project began when I stopped by the hobby store to load up on servo motors. I tried to buy ten but they only had two of the cheap ones in stock. So I am thinking, what can I build with two motors. Mayby something that draws.


I started the scribbler project by sketching a motor driven platform with a leaver moving over it like the tone arm of a record player.


I wanted to sketch on post-it notes. I liked the idea that my scribbler would be small and travel well and wouldn't need a mechanism to hold the paper. That got me thinking of a hand with a wrist. I elevated one motor so that I could use the pen itself as one leaver. Then the tone arm became the only other moving part which could be driven by an inverted motor and the platter became a baseboard that didn't have to move.


I was worried about motor cable length. I though I could put a cybord on the moving arm, but that wasn't necessary. I started thinking about dimentions, range of motion, and part placement in my final sketch, several versions drawn on top of each other.


This was going to be easy.


I decided to work freehand, cutting slightly off square shapes to hide any imprecision. I started with the arm. The large round hole is where the moving part of the servo will fit and be glued to the baseboard. I cut small holes through the baseboard to access the servo drive screw. This lets me adjust angular positions after gluing.


I cut a V notch across the bottom of the arm leaving the top-side paper to become a hinge. This will let the pen motor move freely up and down as the geometry of the moving pen requires.


It took me three tries to get a pen holder that I liked. Each one was simpler than the previous. You can see number two in the photo below. Again I cut an access hole so that I can reach the servo mounting screw to change or adjust pen holders.


For testing I used AnalogKnobConverter hooked straight to ServoMotor which I repeated for both channels. This made the scribbler work like an Etch-a-Sketch.

    Knob -> Servo (arm)
    Knob -> Servo (pen)

Here is my first drawing, a range of motion study.


The scribbler is scribbling after half day and without making a single measurement or writing a line of code.


I spent the rest of the day playing with the scribbler. I decided I wanted a single knob version where the pen movement followed the arm movement only delayed by a second or two. I added a fifo (first-in, first-out) queue as an option to the FilterTransform and inserted it in the path to the pen motor.

    Knob -+-> Servo (arm)
          +-> Fifo -> Servo (pen)

Another option that I'll try someday is to use a very slow ramp to move the arm so that I can use the scribbler as a chart recorder, seismograph style.


Last edited March 28, 2006
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