Sawppy Post-Faire Cleanup

When I work on Sawppy, I test and run indoors. At DTLA Maker Faire Sawppy ran all over, both indoors and out. Most of the time people were playing with Sawppy on a piece of artificial turf at Maguire Gardens. This is an outdoor space where people would walk their dogs, raising obvious sanitation concerns running Sawppy on my home carpet after the event.

Well, after a long day of work, who doesn’t enjoy kicking off their shoes and soaking their feet? I could give Sawppy the same royal treatment. All six wheels were removed and soaked in a tub filled with a mixture of water and household bleach. A retired toothbrush was used to scrub off dirt particles clinging to the wheel. Hopefully this removed most of the contaminants Sawppy might have picked up during the event.

Sawppy kicks off shoes

It was also a good time to perform an inspection to see how Sawppy held up mechanically. In addition to the set screw mentioned yesterday, a few chassis mounting screws have fallen out and need to be replaced. I designed plenty of redundancy in these mounts so there was little risk of Sawppy falling apart.

Sawppy lost fasteners

After a few hours of soaking, the wheels were hung up to dry like old socks. What has six rover wheels but is not a rover? This laundry rack.

Sawppy laundry

(Cross-posted to Hackaday.io)

Sawppy at DTLA Mini Maker Faire

Yesterday Sawppy went on an adventure to the downtown Los Angeles Mini Maker Faire. There Sawppy found a receptive and appreciative audience. There were a lot of enchanted kids, interested parents, and other makers who might be building their own Sawppy rovers.

The morning started out with Sawppy sitting on a table alongside a few different builds of JPL open source rover. Eric’s build is on the left in black and white, Santa Susana High School build is on the right with purple printed parts.

Taking Sawppy around and talking to individuals about Sawppy was a lot of fun and something I’ve done in other contexts before. I have hopes for a few of the contacts to develop into something cool for Sawppy’s future. What’s new this time was that I also signed up to give a short 15-minute presentation about Sawppy and that took more work and preparation. Thanks to the 2-minute “lightning talk” opportunities at Hackaday LA the past few months I’m less nervous about public speaking than I used to be, but I still got pretty stressed about it. I’m sure it’s a matter of practice and the more I can take advantage of such opportunities the better I’ll get.

Roger Sawppy

Outside of the presentation, Sawppy and I spent most of our time on the astroturf across the walkway from the officially assigned display area. It was a hilly part of the park which meant there were no tables or booths set up there, and it was a good place to demonstrate rover suspension in action. I had a spare phone set up to be Sawppy control and handed the control to anyone who wanted to pilot Sawppy for a bit.

Sawppy on lawn.jpg

Most were content to run around the turf. Some of the little ones tried to run Sawppy into their siblings. A few ran into the bushes beyond the turf for a more rugged demonstration of Sawppy chassis. A perpetual favorite is to have Sawppy climb over shoes.

Sawppy running over feet

Thanks to refinements to improve robustness over the past few months, Sawppy came out of the experience with only a slightly wobbly left rear wheel that was easily repaired by tightening the set screw on the left rear steering servo coupler. A great improvement over earlier outings!

(Cross-posted to Hackaday.io)

Sawppy Will Be At DTLA Mini Maker Faire

The Downtown Los Angeles (DTLA) Mini Maker Faire, hosted at the Los Angeles Public Library central location, is coming up this weekend and my rover Sawppy will be among the many maker projects at the event.

DTLA Mini Maker Faire Website

Sawppy will be one of several rovers present. JPL’s Open Source Rover team should be there with their original build, SGVHAK will be there with the beta build rover I contributed to, which inspired my Sawppy and they’ll all be hanging out together.

The JPL team will also be giving a brief presentation in the KLOS Children’s Theater upstairs about their rover project, followed by an even briefer presentation by me on building Sawppy. Both of these talks are listed on the workshop schedule though (as far as I know) there is no hands-on workshop activity planned. Sawppy will be present and running for people to see up close, but no assembly (and certainly no disassembly!) is planned. I may bring an extra corner steering unit for people to play with, and they’ll be welcome to take that apart and put it back together, but not much beyond that.

(Cross-posted to Hackaday.io)

Sawppy Sees Brief Internet Fame

A few days ago I noticed a sudden spike in internet traffic to Sawppy – page views on my personal blog, Sawppy’s Hackaday.io project page, the Github repo, and YouTube video all rose dramatically. It took a little digging around various statistics reporting pages to figure out where the interest was coming from. Answer: someone had submitted Sawppy to Hacker News giving Sawppy a brief taste of internet fame.

Given the general attention span of the internet at large, the traffic disappeared just as quickly as it came. But in that brief moment in time, a few thousand people spared a few seconds (or more) of their lives to look over Sawppy and that’s more than what I had before.

Sawppy SpikeAnd this bit of exposure might lead to other interesting projects down the line. It seems to have caught the eye of someone with interest in the Pi Wars robot competition. Sawppy’s current configuration is indeed controlled by a Raspberry Pi, but according to contest rules Sawppy is too big to fit as-is. I’m not sure a six-wheeled rocker-bogie suspension would be useful for any contest objectives (challenges) in Pi Wars. But it would absolutely make my day if I see one of the competitors downscale Sawppy to fit in the size envelope, thereby creating a “Sawppy Jr.”

(Cross-posted to Hackaday.io)

Dell Inspiron 11 3000 (3180) As Robot Brain Candidate

Well, I should have seen this coming. Right after I wrote I wanted to be disciplined about buying hardware, that I wanted to wait until I know what I actually needed, a temptation rises to call for a change in plans. Now I have a Dell Inspiron 11 3000 (3180) on its way even though I don’t yet know if it’ll be a good ROS brain for Sawppy the Rover.

Dell Notebook Inspiron 11 3000 3180

The temptation was Dell’s Labor Day sale. This machine in its bare-bones configuration has a MSRP of $200 and can frequently be found on sale for $170-$180. To kick off their sale event, Dell made a number of them available for $130 and that was too much to resist.

This particular hardware chassis is also sold as a Dell Chromebook, so the hardware specs are roughly in line with the Chromebook comments in my previous post. We’ll start with the least exciting item: the heart is a low-end dual-core x86 CPU, an AMD E2-9000e that’s basically the bottom of the totem pole for Intel-compatible processors. But it is a relatively modern 64-bit chip enabling options (like WSL) not available on the 32-bit-only CPUs inside my Acer Aspire or Latitude X1.

The CPU is connected to 4GB of RAM, far more than the 1GB of a Raspberry Pi and hopefully a comfortable amount for sensor data processing. Main storage is listed as 32GB of eMMC flash memory which is better than a microSD card of a Pi, if only by a little. The more promising aspect of this chassis is the fact that it is also sometimes sold with a cheap spinning platter hard drive so the chassis can accommodate either type of storage as confirmed by the service manual. If I’m lucky (again), I might be able to swap it out with a standard solid state hard drive and put Ubuntu on it.

It has most of the peripherals expected of a modern laptop. Screen, keyboard, trackpad, and a webcam that might be repurposed for machine vision. The accessory that’s the most interesting for Sawppy is a USB 3 port necessary for a potential depth camera. As a 11″ laptop, it should easily fit within Sawppy’s equipment bay with its lid closed. The most annoying hardware tradeoff for its small size? This machine does not have a hard-wired Ethernet port, something even a Raspberry Pi has. I hope its on-board wireless networking is solid!

And lastly – while this computer has Chromebook-level hardware, this particular unit is sold with Windows 10 Home. Having the 64-bit edition installed from the factory should in theory allow Windows Subsystem for Linux. This way I have a backup option to run ROS even if I can’t replace the eMMC storage module with a SSD. (And not bold enough to outright destroy the Windows 10 installation on eMMC.)

Looking at the components in this package, this is a great deal: 4GB of DDR4 laptop memory is around $40 all on its own. A standalone license of Windows 10 Home has MSRP of $100. That puts us past the $130 price tag even before considering the rest of the laptop. If worse comes to worst, I could transfer the RAM module out to my Inspiron 15 for a memory boost.

But it shouldn’t come to that, I’m confident even if this machine proves to be insufficient as Sawppy’s ROS brain, the journey to that enlightenment will be instructive enough to be worth the cost.

Anticipating Limitations of a Raspberry Pi 3 Robot Brain

rosorg-logo1As investigation into ROS continues, it’s raising concern that a self-contained autonomous robot will likely need a brain more powerful than a Raspberry Pi. It’s a very capable little computing platform and worked well serving as Sawppy’s brain when operated as a remote-control vehicle. But when the rover needs to start thinking on its own, would a little single-board computer prove to be limiting?

CPU

raspberry-pi-logoThe most obvious point of concern is the low power ARM CPU. The raw processing capability of the chip is actually fairly respectable, and probably won’t be the biggest problem. But there are two downsides with the chip:

  1. It has a very small memory cache, so the chip will have problems working with large data sets. (Say, a detailed map of the robot’s surroundings.) Without a large cache or high bandwidth memory, the CPU will sit idle as it starves for data.
  2. It has limited heat dissipation. Under sustained load, the CPU will heat up and reach the point where it will have to slow itself down to avoid overheating.

Both of these are consistent with design objective of the chip. It is very good at quickly completing a task using its high-speed CPU, then wait for its next task. This type of workload is common to devices like cell phones. In contrast, a robot has to process sensor inputs, evaluate its current condition, and decide what to do about it in a constantly running loop. There’s no “wait for next task” downtime where the computer can cool down and clean up its memory cache.

Memory

The main memory is also a point of concern. There’s only 1GB of RAM on board a Raspberry Pi 3 with no option for expansion. This is already pretty cramped to run a modern operating system, never mind the robotic software we’d like to run on top. To mitigate limitations of small RAM, modern operating systems can page memory out to storage. But that just makes the next problem worse…

Storage

A Raspberry Pi uses commodity microSD flash memory as main storage. These devices are designed for usage scenarios like holding photos in a digital camera. Each bit of capacity is only expected to be written to a handful of times in its lifetime. But when serving as main storage of a Raspberry Pi actively running complex applications (or serving as paged memory) high traffic sections of the microSD may receive new write data several times a second, leading to premature failure.

Raspberry Pi in the TurtleBot 3

A Raspberry Pi 3 serves as the on-board brains of a TurtleBot 3 ‘Burger’ and ‘Waffle Pi’ variants. I had been curious how they got around the problems above and the answer is they’ve divided up workload of a robot brain across multiple computers. The Raspberry Pi 3 reads sensor data and outputs motor data, but performs little computation itself. Sensor data is sent over the network to a desktop computer who does the computation, evaluation, and decision making. Once an action is decided, the desktop computer sends motor commands over the network back to the Raspberry Pi.

This is a cost-effective approach because anyone doing robotics research will already have a powerful desktop computer where development is taking place. By offloading computation to said computer and keeping the robot’s on-board processing simple, it makes the robot a lot cheaper.

This is fine for development, but the fact TurtleBot 3 makers chose this approach reinforces the suspicion that an actual self-contained autonomous robot will need something more than a Raspberry Pi.

JPL Open Source Rover is Officially Official

OSR RocksBack in January of this year I joined a team of pre-release beta testers for a project out of nearby Jet Propulsion Laboratory (JPL). While not exactly a state secret, we were asked not to overtly broadcast or advertise the project until after JPL’s own publicity office started doing so. This publicity release happened two days ago so the JPL Open Source Rover is now officially public.

Our team members drew from SGVHAK, so we’ve been calling our rover SGVHAK rover instead of JPL open source rover. Past blog entries talking about SGVHAK’s customization were described as done in contrast to a vague undefined “baseline rover.” I’ve gone back and edited those references (well, at least the ones I could find) to point to JPL’s rover web site. Which had gone live a few weeks ago but that was a “soft opening” until JPL’s publicity office made everything officially public.

After SGVHAK team completed the rover beta build in March, I went off on my own to build Sawppy the Rover as a much more affordable alternative to a rover model. To hit that $500 price point, I had described the changes and trade-offs against SGVHAK rover but it was really against JPL’s open source rover. I’ve fixed up these old blog posts minimally – the references are now correct though some of the sentence structures got a little awkward.

As part of JPL’s open source rover project, they had established a public web forum where people can post information about their builds. To share our story I’ve gone ahead and created a forum thread for SGVHAK rover, and a separate one for Sawppy.

I look forward to seeing what other people will build.

Github Seems To Have Stopped Showing STL Changes

A few years ago Github courted the 3D printing crowd by offering a 3D model viewer to see STL files, and then added a feature to visualize differences between revisions of STL files.

039e6170-1c8b-11e3-8020-b3157840fcf6

One of the reasons I put Sawppy STLs on Github is for people to see parts in their browser without having to install any software. I thought it would also be cool for people to see parts as they evolved. When I first started playing with putting STL files on Github, I thought this was a great way to track changes across major revisions.

Unfortunately, the revision visualization module seems to be gone. The 3D model viewer is still there so the primary motivation for putting STLs on Github is still good. But when I try to view file changes, the changes are no longer shown. The official help documentation still talks about the feature, it just doesn’t seem to work.

I liked seeing STL diffs visually and it makes me sad the feature is now inaccessible.

Road to Sawppy is Paved with Plastic

Today our Sawppy storyline on this blog has caught up to Sawppy version 1.0. The mechanical design for the six-wheel chassis has matured enough that it is a sufficiently functional platform for future projects. We still have mechanical tasks to do ahead of us: the camera mast still needs work, and Sawppy needs a robot arm like the real rovers. But the mechanical work will take a pause, so refinements in electrical design and software can catch up.

As part of declaring version 1.0, the assembly process has been documented on Github in the hopes that other people will build their own Sawppy. I know there’s interest but I don’t know how that interest will translate into action. It would be very rewarding for me to see other rovers running around.

The version 1.0 milestone also marks a time for housecleaning. I had been keeping all iterations of parts I’ve designed and printed on this project in a big bucket of fail. This is occasionally useful when I need to refer back to what I did for comparison to see if I’m actually improving the design. It was also useful to dig up for illustrating various posts on this blog as I tell Sawppy’s story. Now that I’ve completed documentation on Github and told the story of Sawppy evolution on this blog, it’s time to discard them.

But before we do that, a big group picture of all the retired parts with Sawppy the Rover version 1.0.

Sawppy with Parts

Sawppy the Rover Receives WiFi Upgrade, Increases Range

Sawppy is now back up and running with all its 3D printed parts recreated in MatterHackers PETG plastic. While the pieces could be replaced piecemeal, I decided to take everything apart and reassemble the whole thing so I could take pictures along the way and document the assembly process. Since I was basically rebuilding the rover from scratch anyway, I performed another upgrade: the compact Netgear WiFi router previously installed has been replaced with larger dual-band unit, the Asus RT-AC1200. Test drives have proven the new router gives Sawppy significantly more range!

Sawppy can now rove a decent distance from its handler. Far enough that it’s no longer easy to see exactly which way the rover is pointed, and we need to occasionally refer to Sawppy’s on-board camera video feed to see what’s going on. In the picture below, John is holding his phone showing the video feed while Emily is on the driving controls. This picture was taken as Emily drove Sawppy back towards us. In the relatively quiet RF environment of this industrial park, Sawppy can drive about three times further away than the distance shown in the picture before wireless communication suffers occasional data dropouts.

This range test proved that Sawppy can get far enough away that driving it around becomes a team activity: one to monitor the situation and another on the controls. This is more than enough range for most of our purposes.

What’s still unknown is Sawppy’s tolerance of noisy RF environments. That test will come, eventually…

Sawppy Pilots

Reclaiming Bearings From 3D Printed Parts: Round 2

Now that PETG is up and running smoothly on my printer, it was put to work reprinting all Sawppy parts so I could replace the existing PLA parts with PETG parts. Retiring parts means throwing away things like heat-set inserts, but the bearings could be recovered. I originally designed a few holes into the parts to make bearing removal easier, but I’ve since realized those holes are unnecessary. Plastic — both PLA and PETG — are soft enough for the bearings to flex and move inside their assigned positions. This flex allows us to work bearings loose by twisting a shaft inside the bearing.

Bearing Extraction

I tried describing this with words but my vocabulary really isn’t up to the task. Here’s a video to illustrate the technique.

This works well enough I think those bearing removal assist holes I added before are now unnecessary. I’ll probably remove the holes on my next revision of rover parts so it looks less like a piece of Swiss cheese.

Problems Printing PETG With Monoprice Maker Select (Wanhao Duplicator i3)

The first experiment in PETG was printing a servo coupler. It was small, printed at 0.1mm layer height. After the success of that initial experiment, I set the printer to work on a Sawppy rover wheel overnight at 0.3mm layer height. It did not turn out well.

First Complex PETG

Little bits of extraneous PETG strings all over the place! Stringing is usually credited to poor retraction settings, but that’s not the whole story here. Once this print gets above the first 20mm and no longer printing the center hub, it no longer performs any retracts – the wheel is printed in a continuous motion without retracts.

What these strings actually demonstrate is not poor retraction, but very poor layer adhesion. As the print head circles the perimeter laying down filament, it’s not all sticking and instead dragging along little bits of PETG causing these strings. It’s not very visible from this camera angle, but there are visible gaps between layers. And the layers came apart with only minor physical handling.

The layers came apart more easily in the middle sections. This was puzzling – what problem would be worse in the middle of the night but magically recover by morning? The answer: ambient air temperature. Apparently PETG needs more time than PLA to properly bond with the previous layer, and when cooled too quickly it won’t bond. I can’t change the weather on command, but I could turn off the print cooling fan.

Turning off print cooling helped somewhat, but it was not the whole solution. PETG melts less easily than PLA, which is a desired feature when it comes to rover parts that don’t deform under heat. But that attribute also creates printing headaches. The 0.1mm layer height print bonded well but the 0.3mm print did not, leading to the hypothesis that the print nozzle couldn’t melt PETG fast enough to deliver triple the volume of plastic.

To test this hypothesis, the print speed was cut to 1/3 of previous speed. The test object worked well, but this print speed is not acceptable. It would turn a rover wheel from an 8-hour print project to an all-day 24 hour print!

Another test is to turn up the heat on the nozzle, hopefully a hotter nozzle will melt PETG more quickly. This worked… briefly. It got too hot for the liner and it deformed, jamming the print path.

Damaged PTFE liner

The liner was original so perhaps it was just time for a replacement anyway. But when the replacement liner also jammed up within a few prints, I knew this was not going to work.

Given these data points, the hypothesis of “hot end couldn’t melt PETG fast enough” has merit. We know slowing down works, but is unacceptably slow. We know heating up works, until the liner quits.

I was not willing to accept the slowdown, so the alternative is to upgrade the hardware.

First Simple PETG Print is a Success

Once the Maker Select reached “good enough” status it was back to work printing PLA parts for Sawppy in parallel with my Maker Ultimate. This allowed me to iterate through designs much more quickly and was instrumental in getting Sawppy built in time for its first public appearance at JPL’s IT Expo.

A few problems surfaced at this event, but the one that prompted a complete reprint of Sawppy was PLA deformation under Southern California summer heat. This is where the current 3D printer story line rejoins the rover construction story line. With this experience of plastic deformation, I now have motivation to try using a different material. There are a few options, and PETG presented the best tradeoff between temperature tolerance, ease of printing, and cost.

The first object to be reprinted in PETG were the steering servo couplers. This proved to be a weak point that needed to be addressed. The design was printed at 0.1mm layer height so the sideways hole for the M3 thread heat-set insert would have clear definition. (This turned out to be unnecessary – later couplers were printed at 0.3mm layer height and functioned adequately.)

I knew PETG had different requirement for printing, starting with print nozzle temperature. I started with my PLA print profile and dialed up the heat. In order to test layer bond in the print, and also to get a feel of PETG failure mode, I put the result in a vise and cranked the handle. I was happy to see PETG deformed rather than shattered as PLA would. Examination of the deformed object showed layer bonding is good. This is a good start for printing PETG.

PETG crush test

Rovers Gonna Rove at SGVLUG/SGVHAK BBQ

The annual summer BBQ for San Gabriel Valley Linux User’s Group (SGVLUG + offshoot subgroups like SGVHAK) was this past weekend, so obviously a few projects from the group made a showing too. This included our SGVHAK rover and my own follow-up Sawppy the Rover.

Sawppy joined the socializing first, hanging out next to some of the seats attracting attention from members who haven’t seen the motorized rover model before. (And some who have but just wanted another look.)

SGVLUG BBQ 1

Later in the evening, SGVHAK rover joined on the party. The rovers would occasionally run around sometimes at the same time and sometimes into each other. They sort of got into the way of people at times, getting underfoot like a big cat might. Fortunately nobody tripped and fell over a rover.

SGVLUG BBQ 2

This was Sawppy’s first public outing since conversion to PETG, a conversion motivated by earlier PLA parts that deformed under heat. This weekend turned out to be a great test since the entire region is in the middle of a record-breaking heat wave. And this time, Sawppy did not melt under the scorching sun.

Sawppy also got a few different pilots, as anyone who expressed interest was offered a chance to take the helm. Partially to share the toy, but also to get some time seeing Sawppy driven by people who don’t drive the same way I do. The intent was to probe for unknown areas of weakness – after all, this type of activity is how we exposed the weakness in SGVHAK rover’s original steering mechanism.

Fortunately no such flaws were found today. Sawppy survived the evening driving by multiple people (and running over things, and running into things) without any apparent damage. This is a very good endorsement of Sawppy’s design maturity.

Sawppy at Hackaday LA

Sawppy at Hackaday LA

I brought Sawppy the Rover to the June 2018 meet for Hackaday Los Angeles. As is typical of these events, there were two scheduled speakers who gave half-hour talks then the floor opens for anyone present to do a quick 2-minute “Lightning Talk”. I presented Sawppy the Rover to the Hackaday LA audience.

I remember the first time I signed up to do a Lightning Talk months ago and was horribly nervous. I still get nervous when it came time to hold the microphone and speak to the crowd, but it gets a little bit easier every time I do it. I really appreciate the opportunity to practice public speaking in little 2-minute chunks.

The audience seemed to really like Sawppy. When I started driving my rover down the center isle between the chairs, a lot of cell phone cameras came out to take quick snaps. Several people came up to ask for more details afterwards during the socializing session, including Jacob who just bought a book that’s all about Curiosity and is at least as big of a fan of the Mars rovers as I am. Jacob says he wants to build a Sawppy of his own, and I said I’m happy to help his efforts. There was also a little boy who seemed to be around 7 years old, asking questions about Sawppy that were far more sophisticated than I had expected of that age. I was definitely impressed.

At least one of those cell phone pictures has already made it online. It’s not exactly being internet famous but I’ll happily take it!

Monoprice Maker Select (Wanhao Duplicator i3) As Rover Workhorse

The story told so far of Sawppy the Rover is about its development and construction, and that story has reached the point where Sawppy is running into shortcomings of PLA plastic. The next step in that story is to look into rebuilding Sawppy in PETG plastic, but before we take that step, I’m going to rewind a bit and tell a different aspect of the story.

The story of the 3D printers behind the rover parts.

A very deliberate design goal for Sawppy is for it to be accessible to interested builders who don’t have a high end expensive 3D printer. Designing to be printable on low-end machines means avoiding printing parts that demand tight tolerances, so Sawppy uses pre-made gears and bearings instead of printing them. I’ve been pretty happy with my Monoprice Maker Ultimate (Wanhao Duplicator i6) but I know it is a few steps up from entry level. To make sure Sawppy can be printed on entry level machines, the best thing is to buy one and use it to print Sawppy.

So off we go to Monoprice.com to buy an open-box Maker Select (Wanhao Duplicator i3) which is one of the most affordable Prusa i3 clones available on the market. There are cheaper printer kits, but ready-built printers don’t come much cheaper than these either in price or quality.

This is actually my second go at one of these printers. I had purchased a new (not open-box) Maker Select before as a relative 3D printing beginner and it printed well for a month. But a few days out of the 30-day satisfaction guarantee period, the control box started filling the room with smoke of burnt electronics and it never powered on again. I returned that machine under the 1-year warranty and used the store credit towards the Maker Ultimate I have now.

This time around, in exchange for the open-box discount, I fully expect a problematic printer. People don’t return 3D printers for no reason, so it’ll be a complete luck of the draw on the particular…. personality… of an open-box 3D printer. And this time around, with a year of 3D printing under my belt, I hope to better able to deal with printer issues.

I expected an adventure, and I got one! This is the story of an open-box printer that was (eventually) able to print PETG parts for Sawppy the Rover version 1.0.Monoprice218641

Sawppy the Rover’s PLA Wheels Deformed Under Heat

Sawppy tearing up its right-front steering coupler was the most obvious attrition from spending a day showing off to JPL employees. There was another problem less visible to casual onlookers: Sawppy’s wheels were slowly deforming under the southern California sun. By the end of the day, some of the wheels were decidedly out of round.

Out of round PLA rover wheel

The 3D printers that printed Sawppy’s parts were loaded with Monoprice’s PLA plastic filament. PLA plastic is relatively cheap, easy to print, and rigid which makes the material good for prototyping mechanical components. Part of why it is easy to print is that it is easy for the print head to melt the material for extrusion then quickly solidifies into the proper shape. The downside of this “easy to melt” behavior is that it doesn’t take a lot of heat before PLA plastic softens. Sawppy’s thin delicate wheels couldn’t hold their shape after spending a few hours on sun-heated concrete and asphalt.

Earlier tests indicated the rigidity – and corresponding brittleness – was a weakness of printing these wheels in PLA. Now we can add lack of temperature tolerance to the list. It was not surprising but still mildly disappointing to see PLA distort.

Given this experience with brittleness and deformation under summer sun, we have motivation to look into a more ductile plastic material that is less likely to deform in the summer. The first candidate is ABS plastic, which I’ve tried to print before without much success. The second candidate is MatterHackers PETG, which is a favorite among local makers.

Now I have motivation to give it a try.

The Curse of the Right-Front Rover Wheel

During JPL’s IT Expo, Sawppy was on display and roaming around. Partway during the day, when setting up for a photo shoot, a shaft coupler tore out of Sawppy’s right-front steering servo. This was embarrassing and Sawppy limped along with some field repairs until the end of the day when we could analyze its failure and devise a path forward for all of Sawppy’s servos, not just the front-right wheel steering that failed.

On that day, the baseline JPL Open Source Rover was on display but was not driving because one of the steering couplers had failed on an earlier JPL outreach event and has yet to be repaired. Which wheel failed? The right-front wheel.

Even earlier – when the SGVHAK rover was just about to make its debut at SCaLE, a test drive session damaged the gearbox in one of the steering mechanisms. This required a short-term servo hack to get the rover back up and running in time for SCaLE. Which wheel was damaged and needed the hack? The right-front wheel.

Rover Servo Steering Hack

Spirit, one of the actual rovers on Mars, has a recurring mechanical issue with one of its wheels. Which wheel was it? The right-front wheel.

Four rovers, same problematic wheel.

There are likely some perfectly logical explanations for why this might happen, but to the irrational human mind, such stories are where superstitions are born.

Upgrade Servo Horn Fastener To Address Coupler Weakness

Sawppy the Rover’s steel shaft upgrade for bigger flatter detents, along with applying thread locking compound to set screws, was the final task completed before it was time to go visit JPL. Since Sawppy will be running throughout the day, I expected that the next weakest point in the system will surface. And indeed one did! The winner of “next weakest link in the chain” is the shaft coupler holding the servo horn to the steel shaft.

The coupler was 3D-printed and designed around the LewanSoul LX-16A servo and the servo horn mounting screws that came in its package. LewanSoul products using these servos are made of thin sheet metal, so these screws are very short. In fact they are exactly as long as the servo horn is thick, possibly to ensure that it’s impossible to accidentally fasten the servo horn into the servo body.

Unfortunately, what this also means is that when used to fasten a 3D-printed plastic part, there is not enough thread engagement for a good grip. So when one of Sawppy’s corner steering is over-stressed, the screws tore out of the servo horn.

This particular coupler design only used three screws. I tried to perform a field repair by fastening into alternate holes on the horn but that quickly tore out as well. By the end of the day we have this mess: a servo horn where every hole is damaged.

Chewed up servo horn

It’s clear that to accomodate a 3D-printed shaft coupler, we can’t rely on the short screws that came in the servo package. We’ll have to upgrade to something longer and these screws (McMaster-Carr catalogue item# 98685A220) look like good candidates.

Longer screw for servo horn

Improve Motor Shafts With Larger Flatter Detents, And Apply Loctite to Set Screws

After roaming the backyard and other terrain over several days, the weakest part of Sawppy has been uncovered: the set screws holding couplers onto our steel motor shafts are losing their grip. A few of them are sliding off their designated positions, some of them are backing out.

The set screw detents on the shaft were made by passing a Dremel cutting wheel over the shaft as described earlier, and now we have first hand evidence that’s not good enough. We need to make the detents wider and flatter. Something closer to what’s described here. This isn’t a job for a Dremel cutting wheel, as a wheel has a circular cutting profile. What we need is something nice and flat and can bite into metal, like a file.

The first attempt to make bigger detents were holding the shaft by hand and running it back and forth over the file. This was not working. Finger strength isn’t good enough to hold shaft at constant angle, so instead of a flat surface we ended up with rounded surfaces.

Handheld filing no good

The second attempt brought a vise into the picture and flipped the arrangement around. Vise jaws hold the shaft, and hands hold the file. This delivered much better results.

File and vise.jpg

Here’s a set of before/after pictures. First the “before” image showing shallow and inconsistent detents cut with Dremel tool. Visible within them are circular indentations made by set screws – this particular pair of set screws had not yet started slipping and now we’re making sure they never will.

Detent - Before

And the same shaft, after using the metal file to cut flatter and deeper indents:

Detent - After

That was part 1 of the solution. Part 2 is to apply thread locking compound to our set screws. Loctite 242 was available at the local home improvement store.

Apply Loctite 242

If this isn’t good enough, we’ll upgrade to the more expensive Loctite 243 available at the local automotive parts store. According to the Loctite thread locker guide, 243 has a higher breakaway torque of 180 lb-in as compared to 110 lb-in for Loctite 242.

Before we do that, though, it’s worth running Sawppy around for a while to see what the next weakest point is. After bigger flatter detents and applying thread locker, the weakest point is probably not the set screws anymore.