|servo-driven ratchet and pawl|
I've been working on a project that requires some precision motion control, rotating an object 360 degrees in 12 steps. At first, I tried using a stepper motor but it drew too much power, ran hot, and was too heavy. Worst of all, it wasn't strong enough to turn my load. A servo would be a better solution but most servos are only capable of 180 degrees of motion. How do you get 360 degrees of rotation from the limited throw of a servo? A ratchet and pawl looked like it might work.
|drafting the ratchet|
|the finished design|
With the design finished, I printed the parts in ABS at 230°C with a brim. Since the appearance isn't critical, I printed with a 0.2 mm layer height at 80 mm/s. I mounted the pieces to a scrap piece of lexan and used some small springs to tension the pawl and arm against the ratchet. Using an online calculator I determined that for a 30° arc, the servo arm needed to be about 19 mm long to give me the necessary linear motion from the ratchet arm.
|servo connected to arduino|
I used an arduino to test the mechanism. I simply modified the Sweep code from the Servo library and shortened the sweep from 180° to 30°. To my surprise, it worked on the first try. I modified the code a few degrees to dial the servo motion in perfectly. This is a nifty little design that should come in handy in the future. Next I'll print a hub to mount this to the part I need to rotate.
Files are available on Thingiverse and YouMagine.