More Sand Table Updates- Making It Run Quietly

The Spice Must Flow sand table was working well and I liked running it at 500 mm/sec because that speed was fast enough to make it interesting to watch.  The problem was running it at 500 mm/sec was noisy.  Not vacuum cleaner noisy, but loud enough that you wouldn't want to have it running in your living room on a regular basis.

I decided to figure out where the noise was coming from and how to reduce it to an acceptable level.

I identified several sources of noise (in order from loudest to quietest):

1. Motors
2. Belt teeth hitting smooth pulleys
3. Pulley bearings
4. Sliding bearings in X and Y axes

I originally built the table with NEMA-23 motors because I had them on hand.  I tried switching to NEMA-17 motors but they turned out to be only about 1 dB quieter, as measured with a sound meter app running on my phone.

That was when I was driving the motors at 16:1 ustepping using the smoothieboard that was originally installed in the table.

Higher Microstepping Ratios

The next thing I did was switch to a Duet WiFi controller board to use high microstepping ratios to try to quiet the motor noise.  The result was mixed, but I was able to figure out from my tests what needed to be done.

Here's video of the table with the Duet WiFi board driving the NEMA-17 motors.  Notice that at 100 mm/sec, any microstepping ratio above 32:1 is pretty quiet.  In this drive configuration I could only get to 175 mm/sec at 256:1, so I ran it at 128:1 so I could at least get to 350 mm/sec, but it was still noisy.  It occurred to me that at 100 mm/sec, the motors were turning at 1.25 revs per sec and at 350 mm/sec they were turning at almost 4.5 revs/sec.  So the key to quiet motor operation seems to be keep the motors turning slowly even as the mechanism moves fast.



I ordered a set of loop belts and pulleys that would give a 1:5 step up so that when the motors were turning at 1.25 revs/sec, the mechanism would be moving at 500 mm/sec.  Stepping up the speed by 5x divides the torque by the same amount and I found the NEMA-17 motors I had weren't up to the task.  I also found that at high microstepping ratios, the NEMA-17 motors hissed loudly and playing with the driver parameters wouldn't fix it.

I redesigned the motor mounts for the NEMA-23 motors- the hissing noise went away, and now the mechanism was able to run at 500 mm/sec again.

NEMA-23 1:5 motor mount and corexy drive assembly.  There's an 80 tooth pulley on the motor shaft and a 16 tooth pulley on the corexy drive shaft.  The original 40 tooth pulley drives the corexy mechanism.  Two printed green spacers ensure proper position of the 40 tooth pulley for corexy stacked belt configuration.  The motor mount and drive pulley blocks are kept in place using t-nuts.

With the 1:5 step up, I am able to run the motors at 256:1 ustepping to well beyond 500 mm/sec (I took it up to 750 without any issues).  The mechanism ran relatively quietly but there was still a lot of zip-zip sound that seemed to be due to the belt teeth hitting the smooth pulleys.  I redesigned the Y axis pulley blocks and enlarged the belt pass-through holes so I could put a twist in the belts and keep the smooth back sides of the belts against the smooth pulleys.  The zipping noise disappeared completely.

This is the previous Y axis pulley block design with a relatively small belt pass-through hole that would not allow the belt to be twisted.

The new Y axis pulley block (yellow) design with the large belt pass-through hole that allows for a twist in the corexy drive belt.  The other side of the corexy mechanism is identical.

One other change I made was to remove the magnetic endstop switches and replace them with standard type microswitches with levers.  The Duet board motor drivers can detect when a motor stalls, so in theory it should be possible to home the machine without using any endstop switches, but I'll have to get the friction in the mechanism down before I start messing around with that.  I may be running the motors too slowly for stall detection to work.

The Milwaukee MakerFaire was coming up fast so a lot of this work was done in a couple late nights and there was no time to test the mechanism fully assembled with the sand box, so that had to be done at the MakerFaire.  The result was disappointing.  The Y axis moves very easily but there's a lot of friction in the X axis motion.  I tried sanding down the UHMW bearings to give them a little looser fit on the X axis tube, but UHMW doesn't sand well.  I applied some silicone lube to the X axis tube and it worked fine for a few hours, but eventually, the friction came back and the patterns started shifting.

The MakerFaire wasn't really a quiet environment, but most of the noise I heard coming from the assembled table was the quiet grinding sound of the ball moving through the sand (!) so I think I'm finally getting close to the end of the design process.

I'm going to rework the bearings for the X axis and see if I can get the friction down to an acceptable level.  I'm also working on a new design for the sandbox that will have a thinner bottom and will allow an air gap between the magnet and the bottom of the box to minimize any noise that might come from dragging the magnet against it.  That will also reduce friction a bit, which can't hurt.

More on the Duet WiFi Board

I would ultimately like to make this table into a piece of living room acceptable furniture.  One of the things I disliked about using the SmoothieBoard was having the control panel on the table in a place where I could easily (by crawling under the table) access it.  That's great for displaying it at a MakerFaire, but not so great when it's at home.  For a piece of furniture, I want to hide the controller completely and even try to minimize visibility of the power cord.

When I was home working on the mechanism, the Duet WiFi board was perfect.  I was able to tweak all the configuration settings, and generate and wirelessly upload pattern files to run.  However, when I was at the MakerFaire, the wifi at the venue was flaky and I was unable to set the board or even my laptop to communicate on the network.  After some panicked digging around the reprap firmware wiki, I found that the firmware includes an access point mode in which the wifi radio on the Duet board will act as a host and I was able to connect to it with my laptop and get the whole thing up and running.  The only problem with that is that you have to start the access point mode by connecting to the board via USB, so I found myself crawling under the table again to get the thing running.

Fortunately, you only have to connect to the board using USB once to set up access point mode.  The wifi module will remember the settings (SSID, IP address, password) you used and will restart access point mode if you put an M552 S2 command in the config.g file that is run each time the board powers up.  Now when the machine powers up, I can connect my laptop by manually switching it to use the "network" that the Duet board is broadcasting.  No more crawling under the table to connect to the USB port!

Summary

256:1 microstepping, running the motors slowly using 1:5 drive step-up, and twisting the belts all contributed to reduced operating noise of the mechanism.  If you're going to build a sand table, and you want to run it fast, I recommend all the above to keep operation quiet.

Update!  Video, or it didn't happen!

More of The Spice Must Flow from Mark Rehorst on Vimeo.

The Spice Must Flow from Mark Rehorst on Vimeo.

The Spice Must Flow (again) from Mark Rehorst on Vimeo.