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Nov 13th, 2017 - Torque and Diameter Sensor Bugs [ARCHIVE]


Hi everyone,

We're taking a break from production this week since there's not much to report there - we're moving along slowly but surely, shipping units consistently but still slower than we'd hoped. See the previous update if you missed it for any news here.

Instead, we'd like to address another issue that we've noticed - a huge thank you to our existing customers for their help in tracking things down, and in their patience and understanding given they never signed up to be debuggers! For those that just want to know what's wrong, and what we're going to do about it, read everything until the line break - after that, we're going to get technical.

The first issue has to do with minor variations in our torque sensor, which can affect user experience and may also have a minor performance affect in terms of throughput. We're still looking into exactly what it affects, as it is admittedly pretty minor. So far what we know is that some units have been skipping auger steps based on improper torque readings - which should never happen, of course! - and others have slightly less throughput than we'd like to see. If you notice your unit skipping (a loud, violent "click" from the auger during extrusion) please let us know immediately and we'll work with you to tweak your settings so that your unit performs as it should. Long term, we will likely address this issue by calibrating the torque arms more rigorously before they leave - so we don't expect any lasting issues here.

The second issue, slightly more upsetting, is clear filament. Despite our best efforts, some pretty thorough investigation has revealed that our diameter sensors aren't tracking clear filament as accurately as they could be, resulting in fake noise that can confuse the control system and decrease performance, at times considerably. This is of course not acceptable at all, so we'll be throwing some extra weight behind it until we've figured out how to best manage this (almost for sure, it will be a software upgrade - we will keep anyone posted if we have to change any physical parts to fix things, and will send replacement parts to any units affected if it comes to that, free of charge). In the meantime, we think it's fair to just give everyone that already has a unit a packet of Masterbatch and some more pellets so that they can extrude anything other than clear filament (white is fine), so that they can use ProtoCycler until we've gotten this one bug sorted out without issue. If you really, really demand clear filament, see the technical section below...it's not totally useless, just not the quality we've promised you. We'll be e-mailing everyone who's already got a unit or will be getting one shortly on their color preference, and should have gotten stock and supply sorted out within a week or so. For everyone who's placed an order but has yet to get a unit, your pellets will come pre-coloured - we'll reach out to ask preference, but will not hold your order until we hear like we do with address confirmation (you'll just get blue...ReDeTec blue, of course!)

We understand this can be a bit upsetting to hear after such an incredibly long wait - shouldn't they work flawlessly by now?! - but unfortunately this is one of the realities of bringing a brand new product to market. We're doing everything we can to keep you all honestly informed, to remedy the situation in the short term with a bit of a band-aid in terms of free pellets / masterbatch, and to get it sorted out as quickly as possible to minimize any disruptions. While it would be much easier to either call it "insignificant" or just add a disclaimer that performance is only confirmed with opaque filament...that just wouldn't be fair to all of you, especially after waiting so long for the product you were promised.

That wraps up the high level update, but if you want to really dive down into the specifics, see below - and apologies again for this less than stellar news.

-The ReDeTec Team

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Tech overview - Torque Sensor

As you may already know from our previous updates, calibrating the torque sensor is important - the orientation of the lever arm that rides along the chain at "0 torque" can vary enough to change the scale of readings. The obvious solution and one we've implemented on all units to date is to allow some adjust-ability in the chain tension, by moving the stepper motor in it's sliding mount, to ensure that the torque arm is always at the same angle at rest. Here's four extruders, showing exactly what we mean...and the new issue, if you can spot it!

Note the angle of the torque arms (bottom left) - the "long" leg should be ~5 degrees off of vertical, like the right three extruders - the left most is exactly vertical, which would affect the torque readings, and is usually fixed by moving the stepper ever so slightly to adjust the chain tension until the lever just - and only just - rests right on the main spur at zero torque (the left most extruder here will never reach that point, meaning the torque sensor is always "pre loaded" - totally messing up our measurements).

Did you spot the new issue? If not, don't worry - we didn't either until we started digging pretty deeply. Look closer at the screw head that the right side of the chain passes over. The left most is covered - because the chain tension is a bit too tight. The second from the left is just uncovered where the hex head would go - a tiny bit too loose, as we could take up some slack and still have the torque idler rest against the spur. The 3rd one over is perfect - the torque arm is at the right angle, but there's no excess chain tension. The 4th one (right most) is the kicker! While it's very hard to see, the angle of the right most torque idler is actually a few degrees closer to vertical than the middle two torque arms. This would suggest the chain is a little too tight...but there's already too much slack in it, as you can see from the un-covered hex head of the screw. This means our torque sensor has a constant ~2-3 degree offset.

Does this seem incredibly minor? It is. Are we really splitting hairs over 2-3 degrees? We are. While it might not seem like much, across a max range of ~40 degrees, that's 5-7 of our value%!All it does is change the 0 offset of the torque function in code - so your machine might always run a little higher pressure than normal, or a little lower. It still responds quickly and actively to any changes in torque, the readings are equally accurate and the function is still relatively linear. The only change is that, instead of reading "0-50" for torques of 0-50, now it might read "2-52", or "-2 - 48". In the first case, the reading would always be artificially high - so your unit would slow down slightly, and end up being a bit slower than normal. In the second case, the readings are always a bit low, and it might try and take a step with the auger when it actually shouldn't, and skip.

Of course, the solution here is to just calibrate the torque sensor for each one operationally, instead of adjusting chain slack when they're put together. The cause, most likely, are slight variations in the sprockets we've sourced - their spec isn't overly tight as it generally doesn't matter within ANSI chain standards, but for us it turned out to be a minor inconvenience. We will of course continue to monitor and test this issue to ensure there aren't any further negative effects we don't yet know, but as of right now it's thankfully extremely minor, and should be "already solved" going forward.

Tech overview - Clear Filament

This one involved quite a bit of hair pulling. It's no secret that we spent a very long time on our sensor algorithms because, well, detecting filament diameter is hard! Unless you have a perfectly collimated light source or a laser micrometer, shadows, excess background light, and lensing effects with clear filament can all make things *very* difficult to get an accurate estimation.

In the ideal response, the edges are perfectly flat, and exactly at the edges of the filament. In real life though, you get a much "softer" response - which makes things much more difficult! To add to the problem, clear filament can have a "lensing" effect - where it acts like a lense and creates multiple other "high points" of light in the pattern, throwing off everything. Never mind dead pixels, poorly aligned lighting, etc...

 

Using some tricks from AI, our current algorithm effectively looks for "edge features" and weights them in terms of their sharpness, magnitude, and relative location. This allows the algorithm to be pretty robust against dead pixels, lensing effects, varrying amounts of background light, etc - and in normal situations the algorithm always achieves sub pixel accuracy, resolving to about 0.01mm across a range of sample rod gauges - from 1.5 to 3.0mm, with the lid open or closed, and even with lensing filament, we're almost always within 0.01mm of the calipers...which is all good news!

The bad news is that, relatively recently, we've discovered that the algorithm isn't actually as robust against lensing in practice, as it is in controlled tests. What we mean by this is that if you hold a piece of clear filament stationary, the sensor will resolve it perfectly. But if the lensing pattern continues to shift and change, it can confuse the algorithm (likely based on the weighting of relative positions, which change too quickly, but we're not 100% sure yet), leading to excessive noise. This noise confuses the feedback algorithm, and it ends up trying to compensate for problems that aren't there - sometimes with bad results.

Of course, discovering this issue took quite a bit of time - whenever we'd check the sensors or do a study on the algorithm, it would perform near flawlessly...but our clear filament kept having issues. We initially thought it was a calibration error - maybe the colourant somehow made it easier to extrude consistent filament? - but after really diving into the issue we realized that the filament being created was equally consistent whether it was clear or opaque - ti was just the sensor readings that could vary. We ended up logging 30 second samples across many different units, filaments, etc, and then comparing the 30 second samples with the actual filament extruded. We did this with the control systems off - to ensure that it would not in any way alter the filament from what our sensors had read - and then again with the systems on, to see how the noise affected the control system and therefore the output.  Here's what we found:

Black filament, no control system:

Here's 30 seconds of black filament if you extruded it from a system without feedback...not to mention any competitors! We happened to catch this one on a "downswing" that would normally be compensated for in feedback control, so try to ignore the general downward trend...it will climb back up the same amount over the next 30 seconds, we promise! The filament generally wavers within a 0.1mm range, though across the entire snippet our range is 1.74mm - 1.91mm - a 0.17mm spread and hitting diameters that are a little too large for some printers! The main point, however, is this - there aren't any sudden jumps in filament size. Even completely "unintelligent" extrusion isn't awful - it's just not very good. Compare this to the clear filament response below...

Clear filament, no control system:

First and foremost, please note the scale of the Y axis! It is not the same as above, and we do not want to mislead people by changing our axes...that's just how excel spit them out and given our perpetual shortage of time, we just left it with this disclaimer. That being said, the error is pretty clear - at around 2027, the filament drops .15mm in the span of 3/10ths of a second, and then jumps back up just as much about 5 seconds later. This is an exemplary failure as it's much larger and longer than most - the brief blip around 2037 is more normal. But it's very easy to see that this response pattern is nothing at all like what we got for the black filament...even though the physical measurements for both are nearly identical. That huge drop at 2027-2032? It doesn't actually exist - this is what the filament actually looks like when measured with calipers (keep in mind this is a bit of an estimate based on our measurement ability, but you get the idea):

Now our range is 1.62mm - 1.82mm, a 0.2mm spread and pretty much in line with what the black filament does.

What about the control system?

This is where things get interesting. The control system is actually designed to ignore massive jumps in filament diameter, because it presumes they are impossible, and are an error, and so it reacts to them a little slower than it otherwise would (a long, long, long time ago our sensors used to crap out and throw garbage values every once in a while, and at the time this was the quick fix...it's stayed ever since). So while it gets confused for a second, it doesn't *completely* screw things up. We've yet to find it so out of spec the control system fails outright. That being said, the quality of our clear filament is pretty much bottom of the barrel right now - +/- 0.1mm to +/-0.15mm based on extraneous factors - and that's just not good enough. So, as we mentioned, we'll be going back to the drawing board somewhat on our algorithm to give it a fighting chance with clear filament as well, and we'll keep everyone up to date on our progress in the meantime. As mentioned earlier, we'll also send some master-batch of your choice so you can extrude high quality filament. If you really need clear, please e-mail us at support@redetec.com and we'll see what we can do behind the scenes for you. Ultimately, we'll get our clear filament to the same quality all of our coloured stuff is: +/- 0.05mm throughout any perturbations.

Of course, we'd like to get things even better, and just before we started shipping we'd done some very interesting work on smith modeling and predictive feedback, to try and get rid of those short term 0.05mm oscillations you can still see in the above response. We've had to put that off while we get through production, as we already meet our performance targets but are way behind schedule in shipping! But as we switch some more effort back to our sensor algorithm to solve the clear filament issue, we'll also be getting back into our work with Smith Predictions to try and improve everything even further - and will of course release all of that as a free firmware update once it's ready!

If you've made it this far, give yourself a pat on the back - and thank you for being as interested in high performance filament extrusion as we are. That's it for this week, but we will continue to update everyone on both these two bugs and our shipment progress as we make headway on both. Thanks everyone!