Building a Delta Robot Part 1: Defining Success

What is a delta robot?

My goal is to make a delta robot that draws pictures.  Later I’d like to 3D print in ABS plastic.  So what am I looking to achieve?

  1. Repeatable accuracy: 0.25mm.  Say the robot is holding a pen pointing at a grid on a piece of paper.  It starts at (0,0,0), moves around, comes back and is within 0.25mm of (0,0,0).  It might be right on.  It might stop at (0,0.125,0).
  2. Envelope size: 15cm*15cm*15cm.  That’s the maximum range of motion for the tool the robot is holding.  So from (0,0,0) it can move from (-7.5,-7.5,0) to (7.5,7.5,15).  This is easy to test
  3. Weight limit: 50 grams.  Delta robots aren’t known for carrying a lot of weight, and all I’m moving is a pen.  I’ll probably have to use stronger stepper motors if I ever try 3D printing.
  4. Play/Slop: Does the tool undercut or overshoot the target line?
  5. Maximum velocity: 20cm/s?
  6. Maximum acceleration: 20cm/s/s?
  7. Automatic calibration: I want the robot to know where (0,0,0) the moment I turn it on.  No guess work.

I plan to test each of these conditions as follows:

  1. I’ll mount a 3D digital touch probe to the table and a metal cone to the tool tip.  After each movement the computer can record the amount of push on the probe.  After a few hundred samples a pretty clear picture should form.  The tests would then be repeated near the corners of the envelope.  To make life easier our math is going to try to do better than 0.25mm.
  2. Using the same cone, and a ruler taped to the table.  Test that the cone can travel from (0,0,0) to the desired distance on the rule.  Repeat in every direction.
  3. The metal cone should weigh 50 grams so that the previous tests are done at maximum load, when the machine is being strained to the limit.
  4. I’ll print a test pattern on a normal printer, tape it to the table, and then have the machine draw the same design and see if they match.  This will depend a lot on the physics of momentum.
  5. I’m not sure.  The cone could touch a switch at either end of the envelope a few dozen times and then I’d have a measure of top speed.
  6. See #5
  7. The same touch switches could be used to tell when the robot has re-centered itself.

In the next installment I’m going to cover the math and look at how it will help us design to spec build a computer model and then adjust it to fit the math.