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 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.

Sawppy the Backyard Rover

The Google AIY Vision box installed atop the camera mast is mostly for looks and, once powered up, will run its own “Joy Detector” demo independently of the rest of the rover. Once physical mounting and electrical power for the box was done, attention returned to the webcam.

As a first step, the easiest thing to do is to attach it to the Raspberry Pi 3 sitting in the rover chassis via USB. This is never going to be the long-term solution. For remote operation, a Raspberry Pi 3 doesn’t have the grunt to do a good job at real-time video compression and streaming. For autonomous operation, a Raspberry Pi 3 doesn’t have the grunt to do a good job at image recognition and analysis. We hope the AIY Vision box can handle the latter role in the future, but that’s not where things are now. At the moment Sawppy is strictly a real-time remotely operated rover and what we want is live video.

One solution is to use a webcam that has onboard h.264 video compression hardware. Such cameras exist, but they are pricy, and getting the video stream out of the camera without its dedicated Windows software can get tricky.

The alternative is to sacrifice bandwidth efficiency to get a quick solution. It is relatively straightforward to pull individual frames out of a webcam in the form of JPEG still images, and it is computationally trivial to send them one after another in rapid succession to create a Motion-JPEG video stream.

It took less than an hour to get basic camera operation up and running on mjpg-streamer thanks to these instructions. We have a video stream that is crystal clear and we are not straining the Raspberry Pi 3 processor to do it. We are, however, putting a load on the wireless network because the 1280×720 Motion-JPEG video stream takes up about 30 megabits per second (mbps) of bandwidth. This is a tremendous bandwidth hog when considering Netflix can deliver full 1920×1080 high-definition video in less than 5 mbps.

But that’s something to be addressed later. In the meantime, it’s time for Sawppy to roam the backyard!

Backyard Explorer

Here’s the rover’s eye view:

Backyard Explorer

Sawppy the Rover’s Mast Cameras

With a crude but functional folding base for the mast installed, attention turns to the camera to be installed at the top of that mast. Mars rover Curiosity has an extensive sensor array on top of its mast, but at the moment we only have a HP HD 4310 webcam and frankly, it looks a little sad and lonely atop that camera mast. It certainly fails to present a profile reminiscent of Curiosity’s array. What to do?

Add another camera! And not just any camera, one of the most intriguing device to be widely available: the Google AIY Vision kit. I picked it up because it seems like a cool way to get started playing with Google’s neural network software development resources. It’s pretty amazing that something I can pick up at my local Target incorporates a programmable neural net processor. What an exciting time to play with technology.

But that’s still on the to-do list. Until them, this AIY Vision box is only running the default out of the box “Joy Detector” demo. It’s not clear if the capabilities of this vision processor would even overlap with anything that’ll actually be useful for an autonomous rover. In the meantime I shall express my optimism by bolting the box onto the camera mast alongside the webcam. Now the top of the mast looks better, with proportions more closely analogous to Curiosity.

Camera platform - front

Although the vision processor only has the default demo, I still thought it would be fun to have it up and running. Even if it’s not helping the rover understand its world (yet). Since the general-purpose computing portion of the kit is a commodity Raspberry Pi Zero WH device, we could build another microUSB 5V regulator module to power it.

Camera platform - rear

Now Sawppy the Rover can join the ranks of projects that incorporate an AIY Vision kit, with plans to make it do more, but just running the default “Joy Detector” demo for now. Like this cool owl perched on your shoulder and this robot dog.

Sawppy the Rover’s Folding Camera Mast

The tallest object on Mars rover Curiosity is a camera and sensor array mounted on top of a mast. During Curiosity’s journey from Earth to Mars, this mast was folded down for transport. As a motorized model of the rover, Sawppy will also feature a sensor-topped mast. And while packaging constraints aren’t as strict for a hobbyist rover model, it would still be very convenient to fold the mast for transport.

As a simple first draft, we’ll have a fixed non-folding mount for the camera mast, which is a piece of PVC pipe. This will serve as the fallback solution. If Sawppy doesn’t get an elegant folding solution, I could at least pull the PVC out of this non-folding mount for transport.

Mast - Fixed

With a fallback in place, let’s set goals of this project:

  • Use PVC pipe for mast structure.
  • Hollow center to run wires.
  • Fold and unfold without use of tools.
  • For sake of camera image, mast must be stable when raised.
  • Stability less important when mast is lowered.
  • Leverage natural flexibility of plastic to hold movable things in position.

The last bullet point turned out to be more difficult than expected. Our world is full of plastic spring clips, and familiarity lead to an incorrect impression it would be easy. It actually takes a fine control over dimension tolerances to make a plastic clip that flexes to allow movement yet clips tightly for a stable position. We also need good control over surface finish to create a mast hinge that rotates smoothly. Both are within reach of hobbyist level 3D printer but it takes effort.

The first few iterations focused on a rigid cylindrical collar over the end of the PVC pipe, and a U-shaped mounting block on the body that clips onto that collar. The basic concept is for the U-shaped mounting block to have indents that favor two collar positions. Raised mast and 90-degrees rotated lowered mast. The U should be flexible enough to allow the collar to rotate between these two positions, but rigid enough to hold collar position once it reached one of the indents, plus a smooth surface finish so collar rotates well between the positions. This proved challenging and all of these iterations were either too stiff to be moved, or didn’t hold tightly enough after mast is deployed.

Mast - U clip variations

After these failures trying to make a mounting block that can balance rigidity and flexibility requirements, the requirements were split up to separate parts so each can focus on their job. The mounting block will focus on being rigid, and springy flexibility will be delivered by a separate external clip.

This second design also had a back-up provision for a screw to hold PVC collar in place. This would fail to meet the tool-less requirement but it didn’t take much effort to add to the experiment. It turns out the screw position had too little leverage to hold things in place anyway.

Mast - External Clip 1

Since this mount block and PVC collar worked well enough to deliver rotational motion, design iteration focused on evolving the spring clip design. The clip got bulkier to help reinforce rigidity of the mounting block, but it also had a flexible clip for the PVC pipe placed so it had good leverage to hold the pipe in place without compromising the collar’s ability to reinforce the mounting block.

Mast - External Clip 2

This is still really crude but it covers the basic functional requirements. It’ll be good enough for Sawppy’s first public appearance at JPL. In fact it was good enough to elicit delighted appreciation from a JPL engineer whose day job is running the real mastcam on Curiosity. Of course, she told me I should motorize my mast mechanism because her real mastcam does. Maybe in a future iteration! I definitely intend to put more effort into polishing this mechanism.

Sawppy the Rover’s Own Wireless Network

So far all of Sawppy tests have been performed as a client of a nearby wireless network. But we won’t always have a network to connect to. Sawppy may venture where we don’t have access privileges to a nearby network or possibly go places where there is no network at all.

This problem was encountered before on the SGVHAK rover project. The Raspberry Pi 3 on SGVHAK rover were modified to become its own wireless network access point by following directions posted on Raspberry Foundation website. The downside of Pi configured to be its own network is that we lose the ability to connect to the internet. This is probably a problem that can be solved by diving deeper into Linux networking configuration but there was also the problem of limited power of Raspberry Pi’s on-board wireless hardware.

As an experiment to reduce configuration headaches and to increase wireless power range, a TP-Link TL-WR802N mini router was added to the rover chassis. Now Sawppy the Rover has its own wireless network, giving us the following advantages:

  • We can use Raspberry Pi in its default configuration – no need to reconfigure Raspbian networking stack.
  • A wireless router is already designed to work to establish both (1) a standalone wireless network and simultaneously (2) have the ability to connect to an external wired network. Plug in an Ethernet cable and Sawppy is on the internet, no reconfiguration required.
  • A wireless router is already set up to be secure. TP-Link may or may not have covered all the bases, but it is expected to be more secure than a Raspberry Pi configured by a Linux networking amateur.
  • A dedicated wireless router has a more powerful wireless antenna.
  • Additional modules can be added to Sawppy and communicate over local wireless network. For example, adding a commodity wireless IP camera.

The mini router is mounted to a stand bolted above the rear right corner. This location was chosen because that’s where real Mars rover Curiosity’s UHF communications antenna is mounted. Power for this little router comes from a micro-USB cable, so we could power it with another unit of the same power converter used to power the Raspberry Pi.

Mini router

The TL-WR802N is a nice compact solution that avoided having to mess with Raspbian (Linux) networking and, more importantly, the risk of doing it wrong. It was also more powerful than a Raspberry Pi’s on board wireless networking, but sadly not by as much as originally hoped.

Sometime in the future Sawppy will get a more powerful router for greater range, hopefully a dual-band unit to access the less crowded 5 GHz WiFi band. With more power and 5 GHz ability, Sawppy should have a better chance of tolerating noisy RF environments like SCaLE exhibit floor.

Laser-Cut Acrylic For Sawppy the Rover’s Equipment Bay

Up until this point, all the equipment inside Sawppy the Rover’s equipment bay has been installed onto the bay’s aluminum extrusion beams in an ad-hoc fashion. Sometimes they were zip-tied onto a beam, sometimes taped, and sometimes they sat on a small 3D-printed shelf. None of them were very robust and they all shared the problem they can be damaged by obstacles Sawppy is trying to climb over.

They were just temporary placeholder solutions until we could get access to a laser cutter to create a sheet of acrylic to serve as the bottom of Sawppy’s equipment bay. One was drawn up to match equipment bay dimensions in CAD and the file waited until laser cutter access was available.

Box bottom pre installation

Since the acrylic sheet’s dimensions came from CAD and not the actual box that has been built, it’s no surprise its installation exposed some assembly errors. This is a tale as old as engineering, made better (or worse) by CAD software. It fits in CAD!

Aside from overall dimension errors, there was also a problem caused by overlooking the room taken up by our M3 nut installation tool. Installation tool for panel wants to occupy the same physical space as installation tool for body corner. This doesn’t work very well in the real world. The real fix is to move fastener holes on the acrylic sheet. For now our workaround is to offset insertion tool and use only one of the M3 nuts.

Box bottom insert conflict

This first attempt fits well enough to proceed with Sawppy construction. Now we have a bottom panel to the equipment bay and components now have something nice to sit on.

But we have one final problem for today: laser cutting a sheet of clear acrylic means it is very hard to photograph. This picture tries to draw attention to the clear sheet with a hex wrench placed on top of the sheet. The hex wrench is not floating in mid-air!

Box bottom installed


Expandable Braided Sleeves Neaten Sawppy the Rover’s Wiring

After wheel traction was improved by a coat of Plati-Dip, focus turns to the other problem exposed by initial climb tests: serial bus servo wiring harness. First we have the functional problem of wires dangling and risk getting caught on obstacles. There are three wires in the bundle and it only takes one snagged wire to stop the fun.

We also have an aesthetic problem with the bundle. It was obviously assembled by an amateur with ill-fitting joints and unsightly transitions. The original idea was that we could tuck our wiring harness inside these aluminum extrusion beam’s channels, but the poor soldering job meant the wiring bundles are now too fat to fit.

The most obvious solution is to use more zip-ties to hold wires closer to suspension components, but there’s a downside to be aware of: zip ties grabbing tightly on wiring insulation is not good. Too tight, and it will chafe on the insulation and possibly cause short circuits long term. What we desire is a way to hold the wires in place, but not too tightly, distribute the fastening stress across all three wires (instead of whichever one happens to be ‘outside’) and give them a tiny bit of room to move around to avoid chafing.

The answer to all of the above wishes is an expandable braided sleeve. Sawppy the Rover is certainly not the first to have a need for wire management, and there are a wide assortment of sleeve materials and designs to choose from. For a serious engineering project, we’ll need to evaluate the material’s mechanical properties, thermal tolerance, and chemical resistance. But since a little hobbyist rover isn’t picky about such things, we’re going with a cheap PET variant from Amazon. In this particular instance, something cheap is far better than nothing at all.

Here are the two wiring harnesses (one for each side of the rover) in the middle of this wire management project. On top is the ‘before’ picture showing all its chaotic awkwardness, and below is the ‘after’ where braided sleeves have almost completely masked all the flaws. And with the eyesore factor eliminated, we can put them back on Sawppy’s chassis.

Braided Sleeve

Plasti-Dipping Sawppy the Rover’s Wheels

Plasti-Dip is a spray-on flexible coating that has found a niche of car customization, which was how I encountered the product and used to turn ideas into reality. One subset of Plasti-Dip users take advantage of its temporary spray-on peel-off status to customize their car’s wheels, sometimes changing them around as often as other people change fingernail colors. It’s common enough that dip is available in “wheel kits” tailored for the purpose.

For automotive wheel customization, the dip goes on the metal with the rubber tire masked off from dip. I’m going to dip Sawppy’s wheels but doing the reverse – putting dip all around the outside perimeter of the wheel and by doing so, give Sawppy the rubber tire that it current lacks. The motivation comes from the fact Sawppy’s climbing ability is currently traction-limited despite the grousers printed on the outside surface. Experimentation has proved that 3D-printed hard plastic wheels lack grip against certain obstacles that I’d like Sawppy to climb. There are several ideas on how to improve traction on these wheels, the easiest one to try first is Plasti-Dip as there are a few spray cans on hand from earlier car projects.

Again as a reversal from dipping car wheels, I’ll mask off the interior surfaces. I had hoped this will give a nifty two-tone appearance, white innards contrasting with the outer coat of black dip. To accomplish this without tedious masking tape work, wheel covers were designed and printed.

Sawppy Wheel Dip 1 - Caps

They are installed to either side of the wheel on a 8mm steel rod.

Sawppy Wheel Dip 2 - Caps Installed

Plan A was to use an old rover wheel prototype as the base for a spray stand. This did not work as well as hoped because it meant the I had to move all around the stand in order to cover all sides of the wheel.

Sawppy Wheel Dip 3 - Stand

Plan B was to hold the steel rod and rotate it by hand during spraying. This dramatically reduced the effort but the results were uneven.

Sawppy Wheel Dip 4 - Handheld

Plan C worked the best – install 8mm rod into cordless drill chuck, set the gearbox to “Low”, and use it to turn the wheel at a much more consistent rate, which directly translated to a much more consistent coating.

Sawppy Wheel Dip 5 - Motorized

Sadly, all the effort spent on masking off the insides with wheel covers failed to deliver a crisp white/black contrast.

The first problem is a nature of 3D printing: gaps between layers are great at pulling liquids along with capillary action. Many of these gaps pulled liquid dip into various crevices between layers, resulting in an uneven coloration.

The second problem is user error. After a wheel was sprayed, it was set aside to dry and cure while the next wheel is sprayed. The smart thing to do would be to keep the wheels far away from the work area, or provide a divider, or at least make sure the work area is not upwind of the drying area. None of these occurred, which meant the white parts of the wheel now look really dirty thanks to overspray. Thanks to capillary action, a small droplet soaks in to make a big mess that is impossible to clean.

These are valuable lessons to remember if I try this again.

Sawppy Wheel Dip 6 - Overspray

Sawppy the Rover Conquers Backpack

Once Sawppy received its new power system that could deliver the amperage necessary for challenging climbs, it was time to see what the rover can do. Here’s video footage of a particular test where it climbed a backpack.


The most impressive part of this particular demonstration is the height of that backpack. At its highest point, the backpack was approximately double the diameter of Sawppy’s wheels. This was a little difficult to see from the angle of the above video, here’s a picture taken from a different angle.

Sawppy and Backpack.jpg

Credit for this climbing ability goes to the rocker-bogie suspension geometry copied from real rovers JPL sent to Mars.  It allows real rovers to roam a planet with no roads, and on this planet it allows little rover models to climb obstacles better than other wheeled vehicles of similar size.

While the suspension geometry is working marvelously, this series of climbing experiments exposed other limitations. Sometimes Sawppy’s servo motors do not have enough torque, and that is an understandable weakness of these $15 serial bus servos. Like we’ve said before: it’s a trade off for hitting its price target. Also, the wiring harnesses need to be better managed, it snagged on a few obstacles that the rover otherwise would have no problem climbing.

And lastly: grousers on these wheels only help in some situations. Sawppy’s climbing ability is frequently limited by lack of traction. The current design works well when there’s something for the grousers to grip – for example the rough texture of a backpack – but sometimes the grousers can’t find a feature to grip onto. This leaves us with the very limited friction of hard PLA plastic.

As an experiment to help Sawppy get a grip, we’ll try coating the wheel surfaces to see if we can improve traction.

Distributing Power Inside Sawppy the Rover

Now that Sawppy has a big battery pack sticking out the back, the next task is to think about how to distribute that power to internal components. This is not yet a big problem in Sawppy’s current configuration because we only have two things right now: the Raspberry Pi and the serial bus servo controller. But since Sawppy has ambition to grow in sophistication, it’s worth thinking about the problem while it’s still easy.

With two components, Sawppy already has two different voltage level requirements. The Raspberry Pi 3 requires regulated 5 volts up to 2.5 amps though fortunately that’s a problem with a known solution. The serial bus servos may draw up to 1 amp each. With ten of them on board, the potential maximum draw would exceed the capability of the voltage regulator used for the Pi. Fortunately the servos are OK with direct 2-cell lithium cell power so there’s no need to find a beefy voltage regulator.

One solution for rover power is to create a master power supply module that can regulate and deliver power required by each component. But at this point in Sawppy’s evolution, it’s too early to know what components never mind what power they want. It’s better to go with a distributed system where raw battery power is sent out and each endpoint is responsible for its own power regulation. This will mean lots of duplication, but it also offers freedom to swap things in and out for experimentation.

For the moment, that means the existing solution for Raspberry Pi will be used. That solves the electrical problem but physical robustness leaves something to be desired. We can’t leave wires dangling by their solder joints!

Flappy Pi Power

A 3D-printed enclosure was designed to hold both the step-down voltage converter and the micro-USB plug used to interface with the Raspberry Pi. They are now connected by a short segment of rigid wire, eliminating movement and a potential point of failure. Power input wires for the voltage converter now have a little bit of help against physical forces from the enclosure. It’s not full and proper strain relief but it’s a start.

Converter Enclosure - open

The two halves of the enclosure are held with zip-ties. Again, not a full and proper solution but much better than dangling wires and bare circuit boards. It’ll be good enough while Sawppy evolves.

Converter Enclosure - closed

Powering Sawppy the Rover

Once we had our first powered test drive (hooray!) It was time to address the most obvious problem it unveiled: the power supply. The test had used a small 2-cell lithium polymer battery pack that was over ten years old. Originally purchased for small lightweight remote-control aircraft, its capacity has diminished over time as lithium chemistry battery cells tend to do. Sawppy will need a higher capacity battery pack.

Remote control aircraft battery packs like the old unit on hand tend to be small, lightweight, and correspondingly expensive. Fortunately there is another remote control hobby niche that is less weight-sensitive and has power consumption profile nearly identical to Sawppy: remote-control off-road vehicles. The 1/10th scale monster truck category seems to have standardized on using a pair of 2-cell lithium packs so that’s what I bought.

A few accessories were purchased alongside the battery. First a pack of power switches so Sawppy can be turned on and off more elegantly than reaching in and unplugging the battery. To protect the battery, we’ll want an always-on and visible volt meter to monitor its voltage. To protect the rover, we’ll add a fuse holder in line with the battery and a variety pack of fuses. The battery pack is advertised to deliver up to 30C which is far more than any Sawppy component would want (or can tolerate.) Hence a fuse to cut power if there should be an unfortunate short-circuit.

Once all the parts were on hand, the next decision is where to put them all. Since Sawppy is intended to be a mechanically faithful model of actual Mars rovers, the answer is obvious: we’ll put our power system in the back, sticking out at an angle just like real rovers have their radioisotope thermoelectric generator (RTG). Sometime down the line we might print a cosmetic cover that makes it look more like the actual RTG, but for now we’ll settle for matching its position and orientation.

Sawppy Power


First Time Sawppy the Rover Rolled On Its Own Power

Once the serial bus servo wiring harness was soldered (twice, once for each side) they were examined with a multi meter for obvious mistakes: First to make sure there’s continuity across all the terminals that are supposed to be connected, and make sure the bits that are supposed to be separate are indeed independent. After that was verified, it was time to hook everything up and try it for real. All we needed was a Raspberry Pi 3 to run SGVHAK rover’s software (modified to command Sawppy’s serial bus servos) and finding an old two-cell lithium ion battery pack to power it all.

Once everything powered up, the rover could move but in a haphazard uncoordinated fashion because the steerable wheels weren’t pointing in the correct directions. Before it can move well, we needed to find and record their actual center position. In theory there shouldn’t be any work necessary because a servo knows its center position and all we need to do is line up with it. In practice, each servo has a tiny bit of error, each 3D-printed shaft coupler has a little bit of error, etc. Overriding all these little errors is the fact these steel steering shafts were cut by hand on a poor man’s lathe so the set screw detents on either end of these shafts don’t line up at all. All these sources of error add up and they’re why we needed the ability to adjust steering center position.

Once center positions were established, Sawppy was able to roll around on its wheels. There was still a lot of work to do but it was exciting to cross this threshold.

Servo Rover

Sawppy the Rover Needs Wiring Harnesses

With mechanical construction complete for Sawppy the Rover, the next thing to do is work on an electrical nervous system to drive this skeleton. First up: running wires for all ten LewanSoul LX-16A serial bus servos, six driving and four steering.

From an electrical design perspective this wiring is trivial. Each servo needs three wires: ~7.4V power from the battery, electrical ground, and a data wire over which they all communicate using 5V/ground TTL serial. Electrically they are all wired in parallel, and the physical shape of implementation is up to the builder.

The first option is daisy-chaining. Each servo has two ports that are electrically parallel so a single three-wire harness can go to one servo, then another harness can connect it to the next servo, and so forth. This works fairly well for small humanoid robots where there are a lot of servos in a line on a single robot limb, but it doesn’t scale very well to a rover with rocker-bogie suspension since there isn’t a convenient line to daisy-chain along.

The second option is run a wire from each servo to a central location in the rover’s body. This approach has the advantage of allowing us to use thinner wires. In other arrangements where multiple servos are hooked to a single wire, they may all be drawing power so wire gauge has to be beefy enough to meet the worst case power draw scenario even if it is rare. Giving each servo its own wire eliminates this power variability concern.

Each servo came with a short 10cm segment of wire for daisy chaining, which is not nearly long enough for Sawppy’s chassis. For reliability and neatness the best thing to do is to have custom long wiring harnesses built, but first we’ll try a few arrangements using parts on hand to figure out what works.

For the first build, each wire that came with the servo will be cut in half and additional wires spliced in to extend to length we need. A retired ATX PC power supply with lots of 18 gauge wire was scavenged for this project. Given the generous power capacity of 18 gauge wire, there was no need to worry about multiple servos drawing too much power for the wire.

Building Sawppy Wiring Harness

The topology chosen for the first build is to mimic the topology of each rocker-bogie suspension assembly. When laid out it looks like a tree branch. This involves making a Y-shaped solder joint at each branching point, a task that’s just outside my soldering skill comfort zone. The solder joints look terrible to start but each one is a practice opportunity. By the end, the joints are halfway respectable. Or at least, not painful to look at.

From a physical functionality perspective, mimicking the physical chassis topology made sense. From a soldering task difficulty perspective, I’m not sure this was the best way to go. If I build a future revision I’ll try a different arrangement for comparison, but for now we have enough to proceed.


Differential Bar Brace Completes Mechanical Assembly of Sawppy Rover

After the assembly session at April 2018 Third Thursday SGVHAK meet-up, we could see the major outline of a rover but some parts were held only by friction and not yet properly fastened. Once everything was buttoned up, it uncovered a mechanical issue: there was a surprising amount of stress on the differential. Enough to bend its 8mm steel bar.

We need a brace to reinforce that bar.

The first version of the differential brace was made of four pieces – two pieces above the shaft, left-right and two pieces below. This was a beginner’s mistake that didn’t help at all against flexing because the stress went into narrowing/widening the gap between left/right pieces. Second version of the brace eliminated the left-right split and consisted only of two pieces – one above and one below the bar. Again, there’s some looseness in the gap between these two pieces, but a few M3 fasteners kept its flex in check so overall bending of differential bar was dramatically reduced.

Differential Bar Brace

With the differential bar enforced, mechanical assembly for initial build is complete. The rocker-bogie suspension geometry is fully active and now we can proceed to build the electric wiring harnesses we’ll need to bring Sawppy the Rover to life.

Sawppy Mechanical Assembly Complete

Sawppy Rover’s Negative Camber Traced to CAD Error

There were a few bumps in the road on the way to turn a rocker-bogie suspension idea starting from a side view in a JPL PowerPoint into a 3D-printed model. Here we’ll cover one particular error that didn’t surface until we had a physical prototype standing on its own.

When we put all the pieces together and held just by friction, we got a rough idea of how the rover will look. Since it was rough, there was no need to worry about minor alignments. Once things started getting buttoned down, though, weirdness become worrisome. Like this problem: the rover’s front wheels have a clearly visible negative camber. In this front view picture, it’s visible as a tilt to the left. In case the tilt is not visually obvious, look at where the wheel meets the table: it should be at the center of the wheel’s width, but the contact patch is actually to the left because the entire wheel assembly is tilted left.

Negative Camber

Negative camber is something occasionally seen on race cars that compete on maneuverability. It helps the tires “lean against” cornering forces at the expense of increased tire wear when going straight. There’s a whole car subculture of people who like to induce extreme negative camber into their cars for looks. And while I respect that people are free to do what they want to their own cars, I don’t care to do it to mine and I definitely didn’t set out to “stance” my rover. This is a bug, not a feature.

Tracing through CAD operations, the cause was pinned down to rotate operations performed on the suspension arm segment connecting rover body to front wheel. This aluminum extrusion beam is vertically tilted downward from the body to the joint, and also rotated horizontally to splay out away from the body. When I performed the vertical rotation followed by the horizontal rotation, I didn’t notice that the axis used for horizontal rotation was also affected by the vertical rotation. So I thought I was rotating about the vertical (Z) axis but I was actually rotating about an axis vertically tilted from Z. This oversight resulted in the camber.

Editing the transformations to make sure we rotate about the actual X, Y, and Z axis (instead of a rotated variant) resolved this particular issue. It’s just one example among many problems encountered while building a 3D printed rocker-bogie suspension.

Sawppy Rover Takes Shape at SGVHAK Meet

The April 2018 Third Thursday SGVHAK Meetup was a big day for Sawppy the Rover. It was the day a backpack full of 3D printed parts and a stack of aluminum extrusion beams met to take the shape that would become Sawppy.

At the beginning of the evening, Sawppy’s left-side rocker-bogie was fully assembled. This earlier assembly gave first hand physical feedback that its component pieces were overbuilt. The right-hand side suspension adopted the slimmed-down revision so we can compare side by side after the printed parts are assembled. The main body box and differential connecting the two sides were in a similar state – printed parts awaiting assembly.

[mle_makes] volunteered to help with assembly and the two of us started putting Sawppy together while chit-chatting with everybody else at the SGGHAK meet. Some of the discussions were motivated by people looking over the rover’s design and construction technique, and the positive feedback was a major encouragement for me to write it up for Hackaday.

Several hours of assembly later, we could definitely see the outline of a rover but the night was winding down and people started talking about leaving but not everything on the rover was buttoned up yet. It was close – we could taste it!

In order to show off to everyone who attended the meet, as well as letting [mle_makes] get some satisfaction for an evening’s worth of work, I decided to push the remaining assemblies together hoping they’ll stay together just by friction. It worked… somewhat. People at the meet got a lot more interested when they could see the rover in assembled state, even if it’s just loosely held together and has a tendency to fall apart every few minutes.

Sawppy Takes Shape

There are still a few more hours of physical assembly remaining before Sawppy can really stand on its own six wheels, but that milestone isn’t far away.