Wednesday, July 27, 2011

Bogdan makes a measurement suggestion...

After I got the x and y axes operating independently, Bogdan suggested that I measure the steps between the limits switches to see how much difference in measurements might be attributable to the mechanical microswitches that I am using.  I had already been recording that with the y axis, so I decided to extend the monitoring code a bit.

With the y-axis, I had been simply writing the number of pulses to an SD card whenever a limit switch was tripped.  In that I was running the axes at 65 mm/sec, I was getting a rather substantial thump whenever a switch was encountered.  When I thought about it, I began to suspect that the impact was a result of the time it was taking the write to the SD card to happen in that I was doing that immediately after the switch was tripped.  I changed the code to record a set number of triggering events for each axis and then exit the stepper loop and print the whole set of measurements at one time.  That reduced the noise of switch triggering on the x axis to almost nothing.  It also reduced the noise from triggering events on the y axis, but not as much.  Considering the y-axis is shifting the whole weight of the x axis assembly, the extra momentum generated thereby is probably causing the larger thump.

I first took a set of 50 triggering events running at 65 mm/sec.

You can see that the two limits switches on serving the x axis trigger with slightly different sensitivities, one triggering about 0.3 mm greater than the other {transition is running at 0.89 mm/step for both axes}. The both y axis limits switches trigger at the same place except that occasionally one gets moody and triggers 8 steps {~0.6-0.7 mm} longer than the first.

I then took another set of measurements at 32.5 mm/sec. The NEMA 23s were near resonance frequencies at this speed raising the noise level of the printer considerably. I will have to see about damping this.

What you can see is that the variation on the x-axis stayed about the same while the moodiness of the one limit switch on the y axis disappeared. Notice also that the steps between switches are down.

From there, I took a set of measurements at my usual printing speed for Rapman at 22 mm/sec.

Decreasing the transition velocity got us further away from the resonance frequencies of the NEMA 23s. It must be said, however, that the printer was still louder than when I was running it at 65 mm/sec. You can notice here that the variation in limit switch triggering has dropped to 1-2 steps.

This has been an interesting exercise. One thing that is obvious now is that to control noise levels I should be controlling the stepper speed both by the delays between steps and by adjusting the level of microstepping that I am using.

1 comment:

Bogdan Kecman said...

This is super low jitter taking into account how fast you are moving (3900mm/min)... And the machine looks very sturdy while doing this so .. looks like you nailed it :D

As for the SD writing time issue, you have enough ram on that 32mx to store this in ram and write only from time to time, also you could save ram by storing only a 4bit value (difference from previous run) but anyhow I don't think you will be running this test any more so this shown good enough figures :D