RepRap: Blog

Someone likes "Hello World"? Ok, a compromise. Here it is with some 3, 4, and 5 hundred micron markers. The markers are done at a pitch of 15, 20, and 25 microns respectively:

For a sense of scale, the "m" in "300μm" is about the width of a hair.

Attaching the probe was harder than expected. The eventual solution was to put it on a 20mm Metriccano Square Strip using an M3 x 35mm screw, adding a spring washer, and attaching that to the end of the Z Axis Driver with a nut. Tightening the nut raises the tip. Tip can be easily swung in and out by sticking a rod into the yellow block as a lever.

I adjusted Z height for the starting dot for each pass, getting lighter each time for smaller dots. With the binocular microscope it is easy to see that the dots get deeper as the probe tracks right. Possibly the slide wasn't quite level?

The other stand-out is the curling of "ld" which I'm unsure about but suspect stage rotation. More testing needed, but I think I need to get that probe in the dead centre next, so the UV LED will cure resin properly, and get a few mm of height adjustment.

Once that's done:

Make new fine probe tip, make new prepped slide, test Z Touch, and make some small stuff! 

I have to clear up desk space, so I though I might document the progression from the V0.04 axis driver through to what I hope is the reasonably stable V0.05 Linear Axis Driver. Here's the family portrait:

 

The good news is that the V0.05 is still mostly compatible with the existing V0.04 Table and Stage hardware. The major "Gotcha" is that the old Z Tower won't hold the probe in the right place. That's printing off as I write, and all being well I'll have everything back together tonight.

There will need to be some later structural changes to the XY Table to improve rigidity on the micron scale and resist rotation at extremities, but it's good enough to get on with some fabricating for now. Ideally I'd get a new probe holder designed, but if I can bodge it reasonably tidily to the point where others can easily replicate it and get on with depositing resin, that's what'll happen.

Some family business is coming up in about a week which will drag me kicking and screaming from my workbench. That will give me no excuse not to update the documentation and code. Should manage another blog update or two by then. 

Many mods and great results from the Z Axis Driver. I'm unable to see any lateral movement at all (with USB microscopes anyway) as the probe goes up and down. So far I've driven it from the Stage all the way up to 3.5mm, and it is such a relief to have a usable range that does not require me to manually position the probe tip to within a mm of the stage and tighten a clamp without shaking. I have no probe holder yet, it's literally just bolted on. Looks like this:

 

A lot of the holes are going away now I know where I want to mount things. This should result in a more rigid structure that prints faster and is less confusing to assemble. I'm happy enough to put a "V0.05" stamp on it and prepare for an interim release. 

New features 

From the left: In black you see a U-shaped bracket bolted to the end of the framework. This is an easily adjustable Limit Switch, enabling maximum usable range and simple realignment of the contacts in the event of a mishap.

Two holes right, you will see a couple of blue vertical cylinders; "Nut Bars." These are clamped to the frame with two M3 x 50mm screws, which add rigidity. The crossbeams above and below hold M3 nuts drilled out to 3mm which significantly limit the wobble of the drive screw and make the thing much less finicky to assemble (more on that later).

One hole right there are 3-hole long lugs that more or less allow mounting to V0.04 frame parts. Not perfect, needs a bit of shim, but functional.

Next we see the rectangular pair of complementary flexures. These constrain the moving end so that it can only move up and down. Coupled with the Nut Bars, these give the probe excellent stability and repeatability.

Completely missing off the right end is any kind of adjustable probe holder. You can see the small shiny probe tip, bottom right, and for now it's just held there with a bit of Metriccano and an M3 x 40mm screw. Unfortunately the increased length of the Axis Driver means the old mount no longer holds the probe tip near the centre of the Stage, so I've had to pull the slide out and examine it with a proper microscope. The dreaded "Hello World" (I swear I will do something else shortly):

Nut Bar Advantage

On that micrograph you'll see two obvious things: Smaller dots on the top image, and random displacement on the Y axis.

Y axis error is large because the Y Axis Driver lacks the lower Nut Bar. I'm replacing that driver with one that has a Lower Nut Bar right now.

Dots are much more consistent in size because the probe no longer oscillates. The dots are smaller at the top (approx 10μm dia) because I was able to position the probe more accurately. This was done using the new video workbench. If I push too hard on the surface with the probe tip, it starts sliding off to the left and the dots get elongated.

Video Workbench Height Determination 

I've switched to using vlc to mpv. The latter is better supported and allows me to assign keys to move around the image and zoom in and out easily (plus it doesn't use the accursed Flatpak). By zooming in on the one pixel at the very tip of the (now stable) probe, I can discern a brightness change in that pixel that is caused when the probe tip hits the glass slide and thus get a really good indicator of zero height.

As the Stage isn't optimally positioned, figuring that out that was tricky. But it should get easier once I've fixed the Z Axis Driver support structure.

So, get that Lower Nut Bar on the Y Axis Driver, Relocate the Z Axis Driver so the probe fits on the Stage, and we're back on track. I'll do a release then, unless anyone contacts me asking for files. 

Ok, V0.04 tagged in github, on to V0.05

I've made a simple complementary Z Flexure that straps onto the V0.05 Axis Driver. This is needed because the probe just dangles off the end of it, while the X & Y axes have additional constraints built-in. Without this flexure, the probe could shift back and forth.  There should be a similar flexure on the other side.

I'll probably have to shorten it by 10mm to make it fit amongst all the other stuff dangling over the Stage (it's parametric, fortunately). The Axis Driver will need a few slight mods to make sure there are captive nuts in place to secure the Z Flexure. Also I'll need a new Probe assembly that bolts on to the end of the Axis Driver, and most likely a new method of supporting the Z Axis Driver because I doubt it'll fit the Z Tower very well. But I has optimisms.

Much progress with the linear axis driver as I recover, now my guts are where they should be once more. The drivers' longer arms had to be re-shaped because they collided with the Nut Bar, the limit switch can now be repositioned, an overall shape is emerging for the Axis Driver base, and the pantograph flexures no longer hang up on each other.

You can see that the blue and yellow flexures are held in slightly different cases. Those are just what developed as I was adjusting the available travel up and down the Drive Screw. The latest improvement though is adding a Lower Nut Bar that provides a bearing surface right next to the Drive Screw Coupling (only fitted on the blue one, not that it's terribly visible). This reduces the tendency of the drive screw to wobble. It might seem obvious to do that, but I've been telling myself that it shouldn't be necessary. Well, after some introspection lying in a hospital bed I think I was being stupidly stubborn and I've put one in.

As result the new driver seems more consistent, even though it looks like it's made from a kid's construction toy - which it essentially is - and I'll create a proper case design for V0.05 now I know where the bits need to go. I've managed to keep the shape compatible with the V0.04 too. That way I can move to V0.05 and still give people who want to play with the new driver on old hardware the option. I've not adapted it for the Z axis yet, so that'll sill need a V0.04 Driver for a while.

So, testing with the new X & Y axes. The "FAB" logo was done at 25μm/pixel, the "Hello World" at 30μm. A major advance here is that the slide was not manually repositioned at all - the new Axis Driver has a significantly enlarged working area and stays sufficiently linear throughout it. To me there appears to be a lot less overall variation in dot spacing. The 'o's are the same size and silly things no longer happen to the 'W'.

I'm not happy enough with the Z axis though, which I tweaked from image to image. That's why the dots are different. Originally I just wanted to get on with deposition experiments, but that'll need multiple layers. I've reached the point where I want the Z axis to be repeatable like the X & Y axes are now, rather than merely sufficient for initial experimentations. I could waste a lot of time on layers if the Z isn't being consistent.

So priority list: Everything on github, new version, new Z driver, deposition experiments.

The goal of the FPath Project is to follow the Feynman Path - in other words have machines make smaller machines which then make smaller machines. 

Up to this point I have mostly concentrated on developing the optical closed loop feedback control software to support the project. With the conclusion of Experiment 008, I have begun to make a rather modest foray into things small.

Experiment 008 takes a look at some very inexpensive and commonly available stepper motor driven micro linear actuators to see if they might be suitable for the creation and manipulation of millimeter scale objects.

Turns out they are. There are, of course, some caveats. You definitely have to use closed loop feedback control with these things - they have so much slop and backlash that open loop control would simply fail.

Have a look at the Experiment 008 Video and you can see these el-cheapo linear actuators reliably tracking to an accuracy of better than 50 microns.   

Labels: Closed Loop Feedback, FPath, Micro Stepper Linear Actuators, Nanotechnology

In the photo below you can see that the new Pantograph Driver is a wee bit too long to fit on the V0.04 XY Table framework, and there's a bit of a gap. I think it would be wise at this point to draw a line under V0.04 because it's going to start to get incompatible with V0.05 printed hardware pretty soon. Not entirely sure what V0.05 will look like yet, which is why it's a selection of Metriccano holes, but the general form is coming together.

As well as the length issue, that thin, green connection between the XY Table and the Y axis can't continue to be flexible, and once I make that rigid V0.04 Axis Drivers won't work so well. I'm also going to redesign the XY flexures so that the axis pairs are crosslinked to reduce rotation - no guarantees that things will still fit the old frame after that. On the plus side I'm going to be more enthusiastic with the captive nuts, so it won't be a pain in the armpit to assemble.

I'm not anticipating any changes in the required software or electronics. Indeed, using off-the-shelf controller systems is part of the ethos behind this project. Some tweaks to the GRBL config will be required for the increased range of motion though, and I suspect I'll be able to drive the X & Y axes somewhat faster in V0.05 so the max. speed config items will change too.

If anyone - and I know there are about 12 of you - out there would like me to do anything with V0.04 before I draw the line, now would be a good time to speak up :)

All that said, I'll keep compatibility with V0.04 hardware as long as I can, because I have other deposition experiments to do that I'd like to get on with before V0.05 is finalised. They really need the extended range of motion that V0.05 will bring to the table (ahah) though.

I'm going under the knife tomorrow, so I'll give you until I get out of hospital to make your peace with V0.04  - I'll properly tag it in github if/when everyone is happy. [Edit: surgery went well, but won't be doing much for a week]

As the probe tip is shot to heck, I have little compunction about dragging it over the slide. So before I replaced it I thought I'd do a little test. I simply drew an X=Y line from (-2000,-2000) to (1500,1500) μm. The X axis is driven with the Pantograph Driver, the Y with the V0.03/4 driver. The track makes for interesting viewing.

This shows that towards the extremes of the Y axis the drive is oscillating to the point where sometimes it is going backwards! However there is a small patch in the middle that is reasonably linear, and that's my actual work area.

The X axis is demonstrating reasonable linearity throughout. There's a bit of a curve, and I suspect that's our stage rotation issue rearing its head - and I'm confident of reducing that significantly in V0.05

My hypothesis here is that as the Y axis is driven by a pivoting arm, and that at the extremes of travel the horizontal distance between the Drive Screw and the arm's pivot is reduced. This then forces the drive screw out of alignment and the wobbling begins. The X axis does not suffer from this as it is linear rather than rotational and the horizontal distance between the central pivot linkages and the Drive Screw does not change (I hope).

To test this, and simultaneously end up with a machine that works better, I suspect the most productive thing to do would be to fit the Y axis with a Pantograph Driver. The problem here is that the Z axis suffers from exactly the same problem and I would be tempted to replace that as well, which would delay resin experiments.

So I shall restrain myself and only do the Y axis, try some resin stuff, and if that's unhappy replace the Z axis as well. This is all possible now due to the presence of a limit switch on the Pantograph Driver. This is much easier to fiddle with than the one on the V0.03/4 Driver and I'll probably incorporate it in the final design.

 

Hmm, that's a bit of a warts-and-all shot under the workbench. Anyway, please remember that I've only got a day before surgery (don't worry about it) and may not be able to do much for a week afterwards if things are not as straightforward as expected. Here's hoping all goes to the original plan.