Tag Archives: 3D printing

Glass Print Bed for Printrbot Simple Metal. Part III

Posted 11/24/2015

A few days ago I posted a long account of my attempt to add a glass plate printing surface to my Printrbot Simple Metal, to address printing problems due to non-planar warping issues with the original print bed. As described in the post, I was able to satisfy myself that the glass plate was much flatter than the original bed, but I still couldn’t get consistent prints except in a very small area – printing the same part at the same Z-axis offset resulted in either a print that wouldn’t adhere, or gouges in the painter’s tape. My conclusion at the time was that maybe the vaunted ‘auto-leveling’ feature (actually just 3-point tilt correction) has been inadvertently disabled in my firmware version, or maybe it wasn’t ever functional in the first place.

So, in this post I describe my efforts to manually level the glass plate print bed. Based on the results from the previous post, I know that the bed must slope downward from the back to the front, as the print was more or less OK at the back, but became separated from the bed as the print positions progressed toward the front. So, I put down a layer of painter’s tape underneath the glass plate at the front edge of the original print bed, and then ran the same series of prints, starting at the rear right corner as before.

The following two photos show the painter’s tape shim installation

Painter’s tape shim installation

With this setup, I started by printing the 20mm cal cube at the extreme left rear corner of the print area, what I’m calling ‘Position 1’. The first print failed partway through when the piece detached from the bed, indicating the Z-axis offset was too high, at Z = -1.1. This was interesting all by itself, as Z = -1.1 was the offset I used for the entire first experiment a few days ago; I speculate that the addition of the painter’s tape shim may have lowered the plate very slightly at the back corner. I changed the offset to Z = -1.2, and this gave me a good Position 1 print, as shown below.

Position 1 print with Z = -1.1. Note the detachment

Position 1 print with Z = -1.2

Then I moved on to Position 2 (still at extreme left edge, but midway from back to front) and tried again. As shown in the following photo, I got a good print here as well.

Position 2 print with Z = -1.2

From there I moved on to Position 3 (extreme front left corner of the print area), and Position 4 (front edge, midway from left to right). As shown in the following photo, I got a very good print at Position 3, but the Position 4 print detached immediately.

Positions 3 and 4. Position 3 printed OK, but Position 4 detached immediately

So, it appears at the moment like I have the plate leveled in the front-to-back direction, but not in the left-to-right direction. Next I’m going to try adding a layer of tape from front to back at the extreme right edge and see what this does. Note in all this that Printrbot’s ‘Auto-level’ feature should already be compensating, but it appears to be AWOL.

With one layer of tape added, I was still getting very poor results in the X (left to right) direction, so I added a second layer of tape. After this, I was able to get all 5 prints to stick, although as the photo below shows, the last two were ‘iffy’

With two layers of tape front to back at the extreme right edge

So, I added a third layer of tape, and ran another print series, and got almost identical results! How can this be? I’ve added three layers of tape, and I’m still not able to print in the near right-hand corner – this just doesn’t make any sense.

OK, the only possible way it could make any sense at all, is if the Printrbot ’tilt-correction’ algorithm IS enabled, but not functioning correctly, either due to the algorithm itself, or due to incorrect Z-axis sensor readings from the probe. So did another test print, but this time I concentrated solely on the Z-axis servo worm gear. At first I simply put my fingers on the worm gear so I could feel if there was any Z-axis movement during lateral head moves, and sure-nuff, there was some! Then I put a painter’s tape ‘flag’ on the worm gear and I can easily see Z-axis movement during even small lateral head moves. The clear indication is that the tilt-correction algorithm is working, but for prints near the right-hand edge of the print bed, the correction is wrong. I wonder if the errors are due to scaling based on inaccurate print bed dimensions in the printer setup dialogs; this would seem inconceivable, but what do I know?

On a recent print at the near right edge, with 3 layers of tape, the probe measurements were:

(10,143,1.46), (10,10,1.79), (143,10,2.32)

with the tape removed, the measurements are

(10,143,1.35), (10,10,1.59), (143,10,1.94).

This seems to indicate that positive Z is ‘up’ and that lower positive Z values indicate a surface that is actually closer to the bottom of the printrbot. This, in turn, should mean that I could achieve what I want by putting tape under the left edge, not the right, even though the print performance indicated just the opposite! Oh, I have a headache!

with the right-edge 3-layer tape shim removed entirely, I re-ran the original test print series. Instead of the mess I got the first time, printing at all 5 positions succeeded as shown in the following photo – huh?????

All three tape shim layers removed from the right edge

OK, so this just should not be happening! When I started this post, with no shimming at all, I could print to position 1, 2 & 3 (along the left edge, from rear to front), but not positions 4 or 5 (along the front edge, from left to right). This behavior is what led me to shim along the right edge, but although I thought I was making the situation better, increasing the shim height from 2 to 3 layers didn’t help at all, and now I find that with no shim layers I get really nice prints at all 5 positions – what gives?

Thinking back through everything I did this evening, I realized there was one thing I did that could maybe have affected the situation – I changed the printer extents via the Repetier ‘Printer Settings’ dialog. This should not affect tilt correction calculations at all, since the Printrbot already knows the absolute coordinates for all three Z-axis probe points, so why would it care (or even know) what extents have been set in Repetier?

As a test of this wild and zany idea, I set the Y-Max value in Repetier to 350, basically twice what it should be, and set up for another print series. When the probe results came in, they were:

(10,143,-1.06), (10,10,-0.85), (143,10,-0.5) – whoa! How can that possibly be? Clearly there is some coupling from the values set in Repetier’s Printer Settings dialog, but why?

For position 2, the Z-axis probe reports (10,143,1.42), (10,10,1.56), (143,10,1.90)

– Oh, now I have an even bigger headache! How can this be? Negative values one time, and positive values the next – with exactly the same hardware and software configuration (including the same – bogus – Y Max value)??

For position 3, I get: (10,143,1.46), (10,10,1.72), (143,10,2.03) – print was fully successful

For position 4, I get: (10,143,1.44), (10,10,1.70), (143,10,2.02) – print was successful, but marginal

For position 5, I get: (10,143,1.44), (10,10,1.71), (143,10,2.02) – print fully successful

All three tape shim layers removed from the right edge

OK, I give up – I’m officially baffled. Everything I have done has had either no effect, or an effect opposite to what I expected. Then returning the experimental setup to its baseline condition did not restore baseline results – WTFO@!!@#$%^&*(

Frank

3D Printed Terminal Strip Cover/LED Bracket

Glass Print Bed for Printrbot Simple Metal – Part II

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Posted 11/19/2015

A week or so ago I posted an account of my attempt to add a glass plate to my Printrbot Simple Metal print bed. I thought I had a great idea – place patches of copper foil tape on top of the glass plate at the three points where the Z-axis probe sensor checks for print bed tilt (it’s called auto-leveling, but it is actually more like auto-tilt correction), and everything should be groovy. Unfortunately, what I thought would happen and what actually happened were two different things. It appeared that either tilt correction wasn’t being applied at all, or it was being applied, but in the wrong direction (which doesn’t make any sense either, because I didn’t change anything except adding the glass plate).

So, I decided to make another run at the problem. My new approach was three-fold; first, I took some time to thoroughly investigate the flatness of the original Printrbot print bed, and the flatness of the glass plate. Second, I decided to cover the entire glass plate with copper foil, not just the small areas used by the Z-axis sensor. Third, I figured out a way (the ‘G30’ command) to take Z-axis sensor reading at multiple spots on the print plate, rather than just the 3 points used in normal printing, so I could really determine how well the Z-axis sensor itself behaved.

Printrbot print bed and glass plate flatness investigation:

I used two different techniques to evaluate the flatness of both the original Printerbot print bed and the glass plate addition. The first technique was to place a light source so that it illuminated the bed from the side, and look for light leakage under a straight-edge placed on the bed. The second technique was to use a piece of printer paper (approx 0.09 mm thickness) to see if there were any areas where the paper would slide freely under the straight edge. The photos below show the result for both the original print bed and the glass plate. The first image shows the setup, with an LED flashlight placed so it shines light parallel to the surface, with a steel straight-edge blocking the beam except where the bed is warped. Subsequent photos show warped areas.

Flashlight shining along surface of print bed, with steel straight-edge blocking light except where the bed is warped.

Next I did the same thing with the glass plate, as shown in the following images.

The glass plate is clearly much flatter than the original print bed. I verified this using the paper technique – I was able to easily slide a piece of 0.09 mm paper under the straight-edge on the original print bed, but not on the glass plate. So, as far as flatness is concerned, the glass plate is a clear winner.

Z-Axis Probe Measurements

Since I was now convinced that the glass plate was much flatter than the original print bed, this issue could not be the reason I was having so much trouble trying to get reliable prints at different points on the print bed. So, I turned my attention to the only other possible factor, the Z-axis probe itself. If it was somehow producing aberrant readings, maybe that would explain why the Z-axis offset required for printing at the near right corner was wildly inappropriate for the same print in the back left corner. To acquire the data I made use of the G-code ‘G30’ command, which causes a Z-axis probe operation at the current X/Y location. I recorded Z-axis probe readings at 12 locations on the glass plate, and plotted the results using Excel’s ‘Surface’ plot feature

Z-axis probe sensor readings for glass plate

11/20/15 Glass Plate Z-axis probe sensor results

As can be seen in both the data and the surface plot, the glass plate is pretty flat – with only a 0.42 mm deviation from one extreme from the other.

The same set of measurements were performed on the original Printrbot metal print bed, with the following results:

11/20/15 Original print bed Z-axis probe results

The data shows that the original print bed is pretty flat too, even though the illumination experiment showed that it exhibits significant warping. I suspect that the probe measurement locations were too widely spaced to capture the low spots. In any case, all that can be said for sure at this point is that both surfaces (original print bed and glass plate) are pretty flat, and both exhibit some tilt, with Z-axis probe measurements consistently rising from the left rear corner (0,153) to the front right corner (130,3).

Print Trials:

After convincing myself that the glass plate was indeed flat, and that any residual tilt should be well within the Printrbot’s correction range, I decided to do a series of test prints – starting at the rear left corner and moving toward the near right corner. For the first print, I adjusted the Z-offset for optimum printing – not too close, not too far away. Then I kept this same Z-offset for all the other prints. The following photos tell the story.

20mm cal cube printed at the rear left corner (0,153)

20mm cal cube printed at (0,103)

First try at printing 20mm cal cube at the third position (0,53). Note the blob attached to the extruder!

2nd try at printing 20mm cal cube at the third position (0,53). Note the raised corner

20mm cal cube printed attempt at the near right corner. Part is separated from the bed and ‘blobbed’ to extruder and

20mm cal cube printed at the rear edge, midway toward the right

20mm cal cube print attempt at the rear right corner. Part has separated from bed and is ‘blobbed’ to extruder.

So, at this point I’m pretty sure of three things:

The glass plate print bed is pretty darned flat
The Z-axis probe seems to be working correctly
The Printrbot tilt correction feature does not appear to be working properly.

Stay tuned,

Frank

Wall-E goes back to basics; Ping sensors and fixed forward-facing LIDAR

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Posted November 14, 2015

Back in the spring of this year I ran a series of experiments that convinced me that acoustic multipath problems made it impossible to reliably navigate and recover from ‘stuck’ conditions using only acoustic ‘ping’ sensors (see “Adventures with Wall-E’s EEPROM, Part VI“). At the end of this post I described some alternative sensors, including the LIDAR-lite unit from Pulsed Light.

In this same article, I also hypothesized that I might be able to replace all the acoustic sensors with a single spinning-LIDAR system using another motor and a cool slip-ring part from AdaFruit. In the intervening months between April and now, I have been working on implementing and testing this spinning LIDAR idea, but just recently arrived at the conclusion that this idea too is a dead end (to paraphrase Thomas Edison – “I have not failed. I’ve just found 2 ways that won’t work.”). I simply couldn’t make the system work. In order to acquire ranging data fast enough to keep Wall-E from crashing into things, I had to get the rotation rate up to around 300 rpm (i.e. 5 rev/sec). However, when I did that, I couldn’t process the data fast enough with my Arduino Uno processor and the data itself became suspect because the LIDAR was moving while the measurement was being taken, and the higher the rpm got, the faster the battery ran down. In the end, I realized I was throwing most of the LIDAR data away anyway, and was paying too high of a price in terms of battery drain and processing for the data I did keep.

So, in my last post on the spinning-LIDAR configuration I summarized my findings to date, and described my plan to ‘go back to basics’; return to acoustic ‘ping’ sensors for left/right wall ranging, and replace the spinning-LIDAR system with a fixed forward-facing LIDAR. Having just two ‘ping’ sensors pointed in opposite directions should suppress or eliminate inter-sensor interference problems, and the fixed forward-facing LIDAR system should be much more effective than an acoustic sensor for obstacle and ‘stuck’ detection situations.

Over the last few weeks I have been reworking Wall-E into the new configuration. First I had to disassemble the spinning LIDAR system and all its support elements (Adafruit slip-ring, motors and pulleys, speed control tachometer, etc). Then I re-mounted left and right ‘ping’ sensors (fortunately I had kept the appropriate 3D-printed mounting brackets), and then designed and 3D-printed a front-facing bracket for the LIDAR unit. While I was at it, I 3D-printed a new left-side bumper to match the right-side one, and I also decided to retain the laser pointer from the previous version. The result is shown in the pictures below.

Wall-E after being stripped down for remodeling.

Oblique view showing the new fixed front-facing LIDAR installation, complete with laser pointer.

Side view showing both ‘ping’ sensors. Note the new left-side bumper

After getting all the physical and software rework done, I ran a series of bench tests to test the feasibility of using the LIDAR for ‘stuck’ detection. I was able to determine that by continuously calculating the mathematical variance of the last 50 LIDAR distance, I could reliably detect the ‘stuck’ condition; while Wall-E was actually moving, this variance remained quite large, but rapidly decreased to near zero when Wall-E stopped making progress. In addition, the instantaneous LIDAR measurements were found to be fast enough and accurate enough for obstacle detection (note here that I am currently using the much faster ‘Blue Label’ version of the LIDAR-Lite unit).

Finally, I set Wall-E loose on the world (well, on the cats anyway) with some ‘field’ testing, with very encouraging results. The ‘stuck’ detection algorithm seems to work very well, with very few ‘false positives’, and the real-time obstacle detection scheme also seems to be very effective. Shown below is a video clip of one of the ‘field test’ runs. The significant events in the video are:

14 sec – 30 sec: Wall-E gets stuck on the coat rack, but gets away again. I *think* the reason it took so long (16 seconds) to figure out it was stuck was because the 50-element diagnostic array wasn’t fully populated with valid distance data in the 14 seconds from the start of the run to the point were it hit the coat rack.
1:14: Wall-E approaches the dreaded ‘stealth slippers’ and laughs them off. Apparently the ‘stealth slippers’ aren’t so stealthy to LIDAR ;-).
1:32: Wall-E backs up and turns around for no apparent reason. This may be an instance of a ‘false positive’ ‘stuck’ declaration, but I don’t really know one way or the other.
2:57: Wall-E gets stuck on a cat tree, but gets away again no problem. This time the ‘stuck’ declaration only took 5-6 seconds – a much more reasonable number.
3:29: Wall-E gets stuck on a pair of shoes. This one is significant because the LIDAR unit is shooting over the toe of the shoe, and Wall-E is wriggling around a bit. But, while it took a little longer (approx 20 sec), Wall-E did manage to get away successfully!

So, it appears that at least some of my original goals for a wall-following robot have been met. In my first post on the idea of a wall-following robot back in January of this year, I laid out the following ‘system requirements’:

Follow walls and not get stuck
Find and utilize a recharging station
Act like a cat prey animal (i.e. a mouse or similar creature)
Generate lots of fun and waste lots of time for the humans (me and my grandson) involved

So now Wall-E does seem to follow walls and not get stuck – check. It still cannot find/utilized a charging station, so that one has definitely not been met. With the laser pointer left over from the spinning-LIDAR version, it is definitely interesting to the cats, and one of them has had a grand time chasing the laser dot – check. Lastly, the Wall-E project has been hugely successful in generating lots of fun and wasting lots of time, so that’s a definite CHECK!! ;-).

Next Steps:

While I’d love to make some progress on the idea of getting Wall-E to find and utilize a charging station, I’m not sure that’s within my reach. However, I do plan to see if I can get my new(er) 4WD chassis up and running with the same sort of ping/LIDAR sensor setup, to see if it does a better job of navigating on carpet. Stay tuned!

Frank

Glass Print Bed for Printrbot Simple Metal

2 Replies

Posted 11/08/2015

I’ve been having some problems lately getting good prints on my Printrbot Simple Metal 3D printer, and after a lot of inet research I concluded the problem was a warped print bed. The bed that comes with the Simple is pretty nice, but just not thick enough IMHO to avoid some warping. And once the bed becomes even a little bit non-planar, then the very nice ‘auto-leveling’ (more accurately ‘bed tilt compensation’) ceases to be very useful.

There was some discussion about putting a glass plate down over the original bed, which takes care of the non-planar issue, but then the Hall-effect metal bed sensor can’t ‘see’ the metal print bed through the glass plate, unless the glass plate is too thin to do any good. What to do?

I’m a long-time Electrical Engineer and antenna researcher, and so it occurred to me that I might be able to fake out the bed sensor by applying some adhesive-backed metal tape to the top of a glass plate, thereby establishing a new ‘zero’ reference at the top surface of the glass plate. I ran some simple experiments, and found that the bed sensor worked fine with even very thin strips of just about any metal, including copper. Since I knew I could get copper tape in various thicknesses and widths, I thought this idea might be worth a try.

The glass plate:

I remembered seeing a post somewhere that someone had found that a popular picture frame size worked just great for the Printrbot Simple Metal print bed, so I headed over to my local JoAnne’s fabric to see what was available. I found lots of cheap picture frames in various sizes, but nothing that even remotely approached a good size for my print bed – bummer. So, I decided to go the custom route, and, after carefully measuring my bed, got a piece of 3/32″ (~2.3mm) glass cut to 9.5 x 6.5″. I brought the piece home and carefully smoothed the edges with my belt sander (a sander drum attachment for a drill would work also). Unfortunately, when I laid the piece on my print bed, I discovered I had screwed up – the 9.5″ dimension was slightly too large, and the pieced didn’t quite fit between the two sets of mounting screws. I was really bummed out, until I realized I might be able to recover from this disaster by simply grinding cutouts for the screwheads, thereby converting a ‘bug’ into a feature! ;-). Indeed I was able to do this with a Dremel tool and a small grinding bit, and when I was finished I had a nice, self-registering glass plate for my Printrbot!

Screw head cutouts ground with Dremel tool and small grinding bit

The Sensor Tape:

I didn’t have any adhesive backed copper tape handy (I used this stuff by the roll in my prior life as a research scientist, but didn’t think to take any with me into retirement), so I tolled the net for a while for sources. I finally wound up ordering a 10′ roll of 1/4″ adhesive-backed copper tape from eBay

Assembling the new Print Bed:

I used heavy-duty document clips to hold the glass plate down on the original print bed, and then placed copper tape patches at the X/Y home location and the two ‘slope compensation’ sensing points, and then covered the entire thing with blue painter’s tape to provide the same first-layer adhesion as before.

Testing:

I used a 20mm hollow cal cube as my test object, starting at a point near the X/Y home position. As expected, I had to adjust the Z offset some, and wound up with a Z offset of about -0.8 to get a decent print.

Then I tried moving the cube around on the print bed, and found that I had to keep moving the Z offset further negative as I moved the print position away from the X/Y home position. To get the cube to print properly, I would up with a Z offset of -2.0mm, way more than I had expected, and then when I tried to move the print back to near the X/Y origin with the same Z offset, I got the extrusion drag marking shown – bummer!

After seeing the above problem occur, I tried to figure out what was going wrong, and having a heck of a time with it. Here’s what I know:

The Z offset required to get decent first-layer adhesion gets markedly more negative the further away the print position is from the X/Y home position
Trying to print near the X/Y home position with the Z offset required for a position far away from X/Y home causes extrusion head dragging.
However, in a somewhat contradictory finding, if I manually move the extrusion head position arm (the Y axis, I think) toward the outer edge of the print bed, it starts to drag about halfway across for X values near home, and this condition becomes more marked when the bed is manually moved in the positive X direction (to the left looking at the front of the printer). In other words, when moving the extrusion head in manual mode, it appears the print bed has a marked upward slope as X & Y increase, causing significant head drag. However, when printing, the opposite effect seems to occur, where the print bed appears to have a marked downward slope as X/Y increase. This makes no sense at all!

So, at the moment I’m officially baffled. In manual mode, the printrbot behaves as if the bed is sloped upward as X & Y increase, but in printing mode (where I assume the ’tilt compensation’ is in effect), the printrbot behaves as if the bed is sloped in the opposite direction. What gives!?

More to come (I hope)

Under-Counter Heat Gun Holster

1 Reply

Posted 08/18/2015

I have an old Black & Decker paint stripping gun that I have found to be perfect for shrinking heat-shrink tubing on my electronics projects. It’s old and beat up, but it is does that one thing very well. However, it is big, bulky and its metal tip stays HOT for a long time after use (note the burned rubber/plastic on the tip), so finding a place to put the bloody thing after use has always been a bit of a PITA.

Oldie-but-goody – my trusty B & D paint stripper works perfectly for heat-shrink tubing

I was recently re-arranging my office/lab work areas for better efficiency, and I once again ran into the problem of “where do I put this bloody heat gun!”, when I had an epiphany; I have not only one, but TWO 3D printers, and so I should be able to design and print some sort of under counter holster for this thing!

When our Hobbit-house (earth sheltered house) was built way back when, I had my office/lab outfitted with a built-in wrap-around work surface with NO LEGS! The work surface is supported with 5/16″ steel L-brackets built into the wall structure, so I can roll my work chair from one end to the other with nary a chance of banging my knees – yay!

Anyway, that meant that I had these steel beams in several places along the span of my work surface, and it turned out that one of them was in just the right place to be a convenient under-counter mounting location for my new holster brainstorm. And as an added bonus, when the heat gun is in the holster, the HOT metal tip would rest against the steel support, not against anything remotely flammable – double yay!

So, now all I had to do was design and print something that would capture the forward body of the gun but still allow for easy insertion & removal. After a few minutes with my trusty digital calipers and another few minutes on TinkerCad, I had a preliminary design that I thought might work. I decided to not add a mounting structure in the initial design, as I just wanted to see if I had the dimensions right, and then adjust from there. With TinkerCad and a 3D printer, you don’t have to be right the first time; if it doesn’t work or isn’t complete – it’s just a few minutes work to adjust the design, and another few minutes (or in the case of the holster – another few hours) to print another iteration; the modern version of ‘cut and try’ is ‘print and try’ ;-).

Version 1 of the heat gun under-counter holster. I didn’t include any mounting provisions on this first version – and it turned out I didn’t need any

The holster was printed in white ABS on my MicroCenter PowerSpec Pro 3D printer. I used white ABS because that was what was on the printer at the moment (and I didn’t want to use the brilliant neon magenta ABS stuff I had on the other extruder head). I assumed I was going to go through several versions (and I still may) so I didn’t really care what I used for the first version. In any case, the print took about 3 hours (most of which was spent while I was away playing bridge – hee hee). When I tried the result on my heat gun, it actually fit pretty well. The holster was just a teeny bit undersized, but the walls were thin enough (and ABS is flexible enough) so that the gun fit easily but tightly; I hadn’t really planned it this way, but I’d rather be lucky than good!

The whole thing worked so well in the initial tests that I decided to try and mount it under the counter on the steel L-bracket as planned. Instead of my non-existent mounting surface, I drilled some 6-32 holes in the front and rear upper inside corners. The rounded corners of the gun leave a bit of space there, enough so a protruding screw head won’t cause a problem. The biggest problem was drilling the mounting holes in the steel bracket – a 5/16″ thick piece of steel is not a trivial job!

After getting the first hole in, I noticed that I didn’t really need the second one – the heat gun stays pretty level, and can’t go anywhere, even with just one mounting screw. The following photos show the installation. The holster is mounted far enough under the counter that I can roll my chair past this spot without my knees hitting the gun, but still close enough so I can easily remove/insert the gun in the holster.

LIDAR-Lite Visible Laser Testing

The Duplicate Bridge ‘Blame’ Chip

LIDAR-Lite Gets its Own Motor

2 Replies

Posted June 16, 2015

In my last post (http://gfpbridge.com/2015/05/lidar-lite-rotates/ – over a month ago – wow!) I described the successful attempt to mate the Pulsed Lite LIDAR with a 6-channel slip ring to form a spinning LIDAR system for my Wall-E wall-following robot. As an expedient, I used one of Wall-E’s wheel motors as the drive for the spinning LIDAR system, but of course I can’t do that for the final system. So, I dived back into Google and, after some research, came up with a really small but quite powerful geared DC motor rated for 100 RPM at 6 VDC (In the image below, keep in mind that the shaft is just 3mm in diameter!)

Very small 100RPM geared DC motor. See http://www.ebay.com/itm/1Pcs-6V-100RPM-Micro-Torque-Gear-Box-Motor-New-/291368242712

In my previous work, I had worked out many of the conceptual challenges with the use of an offset motor, O-ring drive belt, and slip ring, so now the ‘only’ challenge was how to replace Wall-E’s temporarily-borrowed wheel motor with this little gem, and then somehow integrate the whole thing onto the robot chassis.

The first thing I did was come up with a TinkerCad design for mounting the micro-motor on the robot chassis, and the pulley assembly from the previous study onto the motor shaft. Wall-E’s wheel motor has a 6mm shaft, but the micro-motor’s shaft is only 3mm, so that was the first challenge. Without thinking it through, I decided to simply replace the center section of the previous pulley design with a center section sporting a 3mm ‘D’ hole. This worked, but turned out to be inelegant and WAY too hard. What I should have done is to print up a 3mm-to-6mm shaft adapter and then simply use all the original gear – but no, I had to do it the hard way! Instead of just adding one new design (the 3mm-to-6mm adapter), I wound up redesigning both the pulley and the tach wheel – numerous times because of course the first attempts at the 3mm ‘D’ hole were either too large or too small – UGGGGGHHHH!!

Anyway, the motor mount and re-engineered pulley/tach wheel eventually came to pass, as shown below. I started with a basic design with just a ‘cup’ for the motor and a simple drive belt pulley. Then I decided to get fancy and incorporate a tach wheel and tach sensor assembly into the design. Rather than having a separate part for the sensor assembly, I decided to integrate the tach sensor assembly right into the motor mount/cup. This seemed terribly clever, right up until the point when I realized there was no way to get the tach wheel onto the shaft and into the tach sensor slot – simultaneously :-(. So, I had to redesign the motor ‘cup’ into a motor ‘sleeve’ so the motor could be slid out of the way, the tach wheel inserted into the tach sensor slot, and then the motor shaft inserted into the tach wheel ‘D’ hole – OOPS! ;-).

Assembled Miniature DC Motor Mount

Miniature DC motor with chassis mount and belt drive wheel

Miniature DC motor partially installed in chassis mount

Miniature DC motor mount, with tach sensor attachment, side/bottom view

Miniature DC motor mount, with tach sensor attachment, side view showing motor contact cover

Miniature DC motor mount, with tach sensor attachment, top view

Completed assembly, with drive belt pulley and tach wheel

Next up – the LIDAR mount. The idea was to mount the LIDAR on the ‘big’ side of the 6-channel slip ring assembly, and do something on the ‘small’ side to allow the whole thing to slide toward/away from the motor mount to adjust the drive belt tension. As usual, I didn’t have a clue how to accomplish this, but the combination of TinkerCad and 3D printing allowed me to evolve a workable design over a series of trials. The photo below shows the LIDAR mounted on the ‘big’ side of the slip ring, along with several steps in the evolution of the lower slide chassis mount

Evolution of the slide mount for the LIDAR slip ring assembly

The last two evolutionary steps in this design are interesting in that I realized I could eliminate a lot of structure, almost all the mounting hardware, and provide much easier access to the screw that secures the slide mount to the robot chassis. This is another huge advantage to having a completely self-contained design-fabrication-test loop; a new idea can be designed, fabricated, and tested all in a matter of a half-hour or so!

Original and 2nd versions of the LIDAR slip ring assembly mount

Once I was satisfied that the miniature motor, the tach wheel and tach sensor assembly, and the spinning LIDAR mount were all usable, I assembled the entire system and tested it using an Arduino test sketch.

After a few tests, and some adjustments to the tach sensor setup, I felt I had a workable spinning LIDAR setup, and was optimistic that I would have Wall-E ‘back on the road again’ within a few days, proudly going where no robot had gone before. However, I soon realized that a significant fly had appeared in the ointment – I wasn’t going to be able to accurately determine where the LIDAR was pointed – yikes! The problem was this; in order to tell Wall-E which way to move, I had to be able to map the surroundings with the LIDAR. In order to do that, I had to be able to associate a relative angle (i.e. 30 degrees to the right of Wall-E’s nose) with a distance measurement. Getting the distance was easy – just ask the LIDAR for a distance measurement. However, coming up with the relative angle was a problem, because all I really know is how fast the motor shaft is turning, and how long it has been since the index plug on the tach wheel was last ‘seen’. Ordinarily this information would be sufficient to calculate a relative angle, but in this case it was complicated by the fact that the LIDAR turntable is rotating at a different rate than the motor, due to the difference in pulley diameters – the drive ratio. For every 24mm diameter drive pulley rotation, the 26mm diameter LIDAR pulley only rotates 24/26 times, meaning that the LIDAR ‘falls behind’ more and more each rotation. So, in order to determine the current LIDAR pointing angle, I would have to know not only how long it has been since the last index plug sighting, but also how many times the drive pulley has rotated since the start of the run, and the exact relative positions of the drive pulley and the LIDAR at the start of the run. Even worse, the effect of any calculation errors (inaccurate pulley ratio, roundoff errors, etc) is cumulative, to the point where after a few dozen revolutions the angle calculation could be wildly inaccurate. And this all works only if the drive belt has never slipped at all during the entire run. Clearly this was not going to work in my very non-ideal universe :-(.

After obsessing over this issue for several days and nights, I finally came to the realization that there was only one real solution to this problem. The tach wheel and sensor had to be moved from the motor drive side of the drive belt to the LIDAR side. With the tach wheel/sensor on the LIDAR side, all of the above problems immediately and completely disappear – calculation of the LIDAR pointing angle becomes a simple matter of measuring the time since the last index plug detection, and calculation errors don’t accumulate. Each time the index plug is detected, the LIDAR’s pointing angle is known precisely; pointing angle calculation errors might accumulate during each rotation, but all errors are zeroed out at the next index plug detection. Moreover, the pointing angle calculation can be made arbitrarily accurate (to the limit of the Arduino’s computation capability and timer resolution) by using any per-revolution error term to adjust the time-to-angle conversion factor. As a bonus, the motor no longer has to be speed-controlled – I can run it open-loop and just measure the RPM using the tach wheel/sensor. As long as the motor speed doesn’t change significantly over multiple-revolution time scales, everything will still work.

So, back to TinkerCad for major design change number 1,246,0025 :-). This time I decided to flip the 6-channel slip ring so the ‘big’ half was on the chassis side, and the ‘small’ half was on the LIDAR side, thereby allowing more room for the new tach wheel on the spinning side, and allowing for a smaller pulley diameter (meaning the LIDAR will rotate faster for a given motor speed). The result of the redesign is shown in the following photo.

Revised LIDAR and DC motor mounting scheme, with Tach wheel and sensor moved to LIDAR mount

In the above photo, note the tach sensor assembly is still on the motor mount (didn’t see any good reason to remove it. The ‘big’ (non-spinning) half of the slip ring module is mounted in a ‘cup’ and secured with two 6-32 set screws, and the tach wheel and LIDAR belt pulley is similarly mounted to the ‘small’ (spinning) half. The tach sensor assembly is a separate peice that attaches to the non-spinning ‘cup’ via a slotted bracket (not shown in the photo). The ‘big’ slip ring half is secured in the cup in such a way that one of the holes in the mounting flange (the black disk in the photo) lines up with the IR LED channel in the tach sensor assembly. The tach wheel spins just above the flange, making and breaking the LED/photo-diode circuit. Note also how the slip ring ‘cup’ was mounted on the ‘back’ side of the drive belt tension slide mount, allowing much better access to the slide mount friction screw. The right-angle LIDAR bracket was printed separately from the rest of the LIDAR assembly to get a ‘cleaner’ print, and then press-fit onto the pulley/tach wheel assembly via an appropriately sized hole in the LIDAR bracket. The following movie shows the whole thing in action

In the above movie, note the quality of the tach sensor signal; it is clamped to the voltage rails on both the upper and lower excursions, and the longer ‘high’ signal of the index plug is clearly visible at the far right of the oscilloscope screen.

Details of the Tach Wheel:

Wall-E’s spinning LIDAR system features a tachometer wheel with an ‘index plug’, as shown below. Instead of a series of regularly spaced gaps, the gap in one section is missing, forming a triple-width ‘plug’ that allows me to detect the ‘index’ location. However, this means that instead of 20 equally spaced open-to-opaque or opaque-to-open transitions, there are only 18, as two of the transitions are missing. In addition, the LIDAR pointing angle isn’t as straightforward to calculate. If the trailing edge of the ‘index plug’ is taken as 0 degrees, then the next transition takes place at 18 degrees, then 36, 54, 72, 90, … to 306 degrees. However, after 306, the next transition isn’t 324 degrees – it is 360 (or 0) as the 324 and 342 degree transitions (shown in red in the diagram below) are missing. So, when assigning pointing angles to the interrupt number, I have to remember to multiply the interrupt index number (0 – 17) by 18 degrees (17*18 = 306). This also means that there is no ability to ‘see’ obstacles in the 54 degree arc from 306 to 360 degrees.

Diagram of the tach wheel for Wall-E’s spinning LIDAR system

Next Steps: Now that I have a LIDAR rotation system that works, the next step is to design, implement and test the program to acquire LIDAR distance data and accurately associate a distance measurement with a relative angle. With the new tach wheel/sensor arrangement, this should be relatively straightforward – but how to test? The LIDAR will be spinning at approximately 100 RPM, so it will be basically impossible to simply look at it and know where it is/was pointing when a particular measurement was taken, so how do I determine if the relative angle calculations are correct?

I could put the whole think in a large cardboard box like I did with the XV-11 NEATO spinning LIDAR system (see ‘Fun with the NEATO XV-11 LIDAR module‘); If the (angle, distance) data pairs acquired accurately depict the walls of the container over multiple runs, that would go a long way toward convincing myself that the system is working correctly. I think that if the angle calculation was off, the lines plotted in one run wouldn’t line up with the ones from subsequent runs – in other words the walls of the box would blur or ‘creep’ as more data was acquired. I could also modify the box with a near-distance feature at a known relative angle to Wall-E’s nose, so it would be easy to tell if the LIDAR’s depiction of the feature was in the right orientation.
I could mount a visible laser (similar to a common AV pointer) to the LIDAR so I could see where it is pointing. This would be a bit problematic, because this would simply paint a circle on the walls of the room as the LIDAR spins. In order to use a visible laser as a calibration device, I’d need to ‘blink’ it on and off in synchronism with the angle calculation algorithm so I could tell if the algorithm was operating correctly. For instance, if I calculated the required time delay (from the index plug detection time) for 0 degrees relative to the nose, and blinked the laser at that time, I should see a series of on/off dots directly in front of Wall-E’s nose. If the dots appear at a different angle but are stationary, then I have a constant error term somewhere. It they drift left or right, then I have a multiplicative factor error somewhere.

I think I’ll try both techniques; the box idea sounds good, but it may take a pretty large box to get good results (and I might even be better off using my entire office), and it might be difficult to really assess the accuracy of the system. I already have a visible laser diode, so implementing the second idea would only require mounting the laser diode on top of the LIDAR and using two of the 6 slip ring channels to control it. I have plenty of digital I/O lines available on the Arduino Uno, so that wouldn’t be a problem.

RobotGeek Laser, Item# ASM-RG-LASER available from Trossen Robotics (TrossenRobotics.com)

Stay tuned!

Frank

LIDAR-Lite Rotates!

1 Reply

Posted 5/29/15

In previous posts I described a couple of LIDAR alternatives to my original ultrasonic ping sensor system for Wall-E’s navigation capabilities, and the challenges I faced in implementing them. The LIDAR-Lite module is easily interfaced to an Arduino controller via the I2C interface and there is plenty of example code for doing this. However, in order to use it as the primary navigation sensor, it needs to spin at a controlled 1-5 RPS (60-300 RPM) and there has to be a way to determine the rotational angle associated with each distance measurement. In a previous post (http://gfpbridge.com/2015/05/dc-motor-speed-control-using-optical-tachometer/) I described my experiments with one of Wall-E’s wheel motors to implement speed control using an IR LED/photodiode/toothed-wheel tachometer.

After successfully implementing speed control on Wall-E using the right wheel motor, I next turned my attention to implementing a drive train to connect the wheel motor to the LIDAR Lite unit. I couldn’t just connect the LIDAR module to the motor shaft, as the LIDAR wiring would simply wrap itself to death as soon as the motor started turning. I had previously acquired the slip ring module (described in http://gfpbridge.com/2015/04/robot-of-the-future-lidar-and-4wd/) shown below

Adafruit Slip Ring with 6 contacts

So I needed a way to connect the rotating part of the slip ring to the LIDAR module, and the non-rotating part to the robot chassis and (via a drive belt) to the motor shaft.

In TinkerCad, I designed a grooved pulley with a rectangular cross-section axial hole to fit over the motor shaft, an adapter from the rectangular LIDAR mounting plate to the cylindrical slip ring rotating side, and a chassis mounting bracket that would allow the non-rotating side of the slip ring to be adjusted toward and away from the motor shaft pulley to properly tension the drive belt. The drive belt is a standard rubber O-ring from McMaster Carr.

Now that I have motor speed control working and the LIDAR spinning, I need to connect the LIDAR electrically to the Arduino Uno controller and see if I can actually collect angle-specific LIDAR distance data (distance from the LIDAR, angle from the wheel speed tachometer control software). Stay Tuned!

Frank

Spinning LIDAR drive assembly and LIDAR unit. Note O-ring drive belt.