The next test I carried out on my desk was a motion repeatability test. For this test, I loaded the desktop with 20 lbs. of dead weight to simulate motion under a typical loading configuration. I then indicated the tip of the desktop to track the vertical motion of this point while I ran the desktop up and down.
To stay safely within the 1″ measurement range of the indicator, I restricted the desktop motion to back-and-forth moves of 5 mm and 10 mm. The desktop reliably repeated to within 0.001″ (0.025 mm) on each move. From the footage I got of the tests, the actual positional uncertainty is likely less (in the tenths-of-thousandths). This is consistent with my expectations, given the step size of the stepper motors and the lead on my leadscrews. For the 200 steps/rev. motors and the 2 mm pitch leadscrews, I expected the positional “float” to be within a single step, i.e. 2/200 = 0.01 mm. This is approximately 0.0005″, which is consistent with the test results.
After doing these tests and filming the videos, I realized I could have done the test across a much wider range of motion simply by allowing the desktop to come off of contact with the probe for part of the move. I don’t expect the results of this test to differ significantly, since the error motions due to actuation is almost entirely due to this within-step float in the motors (unless the motors skip steps somewhere along a longer move). I may repeat this test if I have time next week.
The following two videos show the test in action: