Sabertooth – Deep South Robotics

Rover 2 Photo Essay

Roby — Tue, 02 May 2017 13:05:06 +0000

Welcome back guys!

I think the last post touched a nerve — it seems that many of you guys have had difficulty breaking into the precision GPS market. If you’re not yet inspired to take the precision RTK GPS plunge, just wait — we’re going to have some fun in upcoming posts pitting the ComNav K501G in an Epic Showdown with some of the competitors we mentioned last time. I don’t know about you, but I can hardly wait.

Rover 2

We now present the Rover 2 build as a photo essay:

Notice that we’ve labelled the main components to give you a high-level overview of what’s where. Let’s start digging into the build by working our way from the base up to the top…

Two on/off switches (like these) — one dedicated to the drivetrain (Sabertooth 2×32), the other dedicated to everything else.

Right up front, my eager sons, we need to have an honest discussion about on/off switches. So go ahead and strike up the fireplace, grab a cup of coffee, come over here and jump up close while ‘ole Dad Roby has a heart-to-heart discussion with ya’ about the temptations you will soon face. If you’ve built a robot before, you’ve likely experienced the brutal temptation that I must now warn you about…

Of course we’re talking about the temptation to mount an on/off switch in a non-standard posture.

Young sons, heed my words: OFF MUST BE DOWN. I really don’t care about which way you make ON, but you MUST make the down position of the switch correspond to OFF.

The only exception to this whatsoever is if what you’re turning ON/OFF doesn’t matter. For instance, if the switch controls your secret Justin Bieber loudspeaker, then you may get away with it. Otherwise, please dear sons, don’t attempt to retrain an entire lifetime’s worth of muscle memory just because you think a sideways switch will look cool, or because it will fit more cleanly in your enclosure.

If you refuse to make OFF correspond to down, and if one day things go wrong and you’re not able to instantly turn off your robot via muscle memory, then you’ll deserve the bruises you’ll receive. I pray it’s only bruises, my son.

Notice that’s a Raspberry Pi 3 in the black case sitting on top of the center case. Pixhawk autopilot is in the enclosure below the black case.

Cover pulled off of Pixhawk case reveals Pixhawk sitting all alone in center enclosure.

Power box contains Sabertooth 2×32 controller, terminal blocks, 24v-to-12v DC-DC converter, 12v-to-5v DC-DC converter, relays, and USB power supply. I know what you’re thinking — those wires look out of control. Agreed.

That long ribbon cable on the left snaking out of the Raspberry Pi is going up to the Pi camera.

Confession time: I can’t stop buying these waterproof enclosures or these waterproof cable glands. An orange ear plug fills in the air gap on a big cable gland.

Astute readers will note that a Raspberry Pi 3 has WiFi built in. So what’s up with the ethernet cable? My experience is that a single onboard router with devices connected to it via ethernet is a preferable solution.

Notice the small slot cut in the side of the Pi case. This allows us to easily remove the case without removing the video ribbon (and fiddling with the Pi’s regretfully brittle plastic camera ribbon lock).

Another picture to show the little slot cut in the side of the Pi case, enabling us to remove the case without pulling the video ribbon. Little tweaks like this will make you much happier after removing the case 100x.

View of the Raspberry Pi 3 with lid removed.

Notice that the Pixhawk is mounted on a vibration reducing mount within the enclosure. The Pixhawk’s gyros and accels would like to say “thank you”.

Can you guess why the Pi has a distinctive sheen?

Did you guess? It’s conformal coating! I like to leave the Pi exposed for testing — the reason for this is that it seems you’re always plugging in another cable or pin to the Pi, and enclosing it inside a case creates future time drains. Here we have a problem, though: what happens when the unexpected April shower pops up in the middle of your mission? If your Pi’s unprotected, you may have just lost $35. Our answer to this situation is a good coating of MG Chemicals conformal coating.

The coating glows when you illuminate it with a UV light, revealing how much coverage you achieved. Man, stuff that glows is timelessly cool.

Tip: insert the video ribbon cable, the HDMI cable, and the USB cable BEFORE applying conformal coating. Otherwise the coating has a sneaky tendency to get inside those connectors. You will have nightmares getting a good connection if you let this happen. Yes, I learned the hard way.

They put warnings all over this stuff like it’s pure Uranium. Probably best not to consume.

Conformal coating coverage test in process.

If Industrial Velcro ever ceases production I will quit robotics. Velcro, please don’t make me back that statement up. Seriously though, guys, think about how much it hurt to let go of that fav pair of Superman shoes at age thirteen eight with the Velcro straps. And here you’ve spent all these years thinking that you couldn’t look forward to Velcro again ’till you hit 75 and went shoe shopping at Wal-Mart. Velcro, we missed you. Welcome back.

Well, who do we have here? I see a Pixhawk up there at the top and that’s the power box over there on the left. Now, what is that little card at the bottom….

It’s the much hyped ComNav K501G L1/L2 GPS/GLONASS 10hz RTK receiver card!! You know, the card I talked so much about last time that Mrs. Roby started acting weird. You’ll observe that I make notes on the card about baud rates and port assignments — this method proves more reliable than my memory.

Robot building pop quiz: what in the world is that red arrow pointing at?

Do you know the answer?

It’s a Gore vent! Technically, this vent is a knock-off that you can buy here or here. If you’ve never read field scientists rave about Gore vents, you’re in for a treat. You’re likely familiar with GORE-TEX waterproof jackets, shoes, gloves, etc. It turns out that the company behind GORE-TEX is GORE Industries, and they also make GORE vents. These guys fired the Product Name Consultant 30 years ago and never looked back. GORE products are based off this fascinating synthetic material that’s somehow impermeable to water molecules, but it allows air to pass through. So, for the vents, the result is that the enclosure can breath, but it’s waterproof. Breathability is important because pressure inside the enclosure is equalized, greatly reducing stress on the waterproof seal and increasing the seal’s longevity.

Notice the dual nuts anchoring the center enclosure underneath the platform. This dual-nut trick allows us to level up the Pixhawk by adjusting the nuts. You’ll observe that both plastic enclosures pictured are floating on anti-vibration bushings. You’ll also note the blue ethernet cable winding out of the GNSS receiver’s enclosure on the left. If you’re not a network person, you may not have known that ethernet cables are just 4 pair of copper/aluminum wires — they’re often the perfect hack for low amperage power and signal wires.

Moving up from the base to the center section you see, starting at the top/back, the wireless router (running dd-wrt as a repeater bridge), the Odroid-C2 (barely visible inside the center case), and the GoPro camera case (like this) modified to hold a Pi Camera.

That mass of wires in the enclosure above is mostly related to the 6 ultrasonic sensors on the Rover. A little Arduino (impossible to see in this picture) is sitting in the upper right corner, processing all the readings from the sensors and feeding obstacle information back down to the Raspberry Pi. We also found room in that box for a 3DR telemetry radio that’s dedicated to receiving RTK corrections.

Here we see an Odroid C2 sitting all alone in the center enclosure. Note that the Odroid doesn’t have WiFi built in (in contrast the Pi 3 has WiFi) so having the onboard router is already paying off. The Odroid is an amazingly powerful little computer that we’ll have much more to say about in upcoming posts. We’re currently running an armbian distro of linux (highly recommended) on the oDroid. The oDroid is there to take care of heavy lifting; for instance, the code generating missions is a bit of resource-heavy java that the oDroid smoothly runs in a Tomcat Servlet container.

Here you can see the front ultrasonic sensor array mounted to a sheet of Lexan polycarbonate. Notice that the sensors on the left and right are not intended for outdoor use while the sensor in the middle is like the sensors you’ve seen on the rear bumper of vehicles for the past 15 years. The indoor ultrasonic sensors have always outperformed the waterproof ones in my experience.

If you look closely at the camera’s mount over on the right hand side, you’ll notice that we thread nylon cable ties through the rubber bushings, under the metal platform, and back up through a tiny hole beside each rubber bushing. This little trick serves the dual purpose of securing our camera and enabling us to make precision leveling adjustments by tightening the cable ties. If you’re looking to spend much time building robots, you won’t regret the decision to purchase a variety pack of cable ties.

I’ve got to give the TP-LINK router credit, it’s been a reliable wireless bridge and I’m quite sure the manufacturer didn’t intend for it to run around on a robot.

If you look closely at the camera case, you’ll see my hot glue crush springing up all around. The truth is, I could let go of Velcro before letting go of hot glue — but the thought of losing either knots up a strong man’s gut.

All of the enclosures are floating on some form of vibration dampening bushing. If you run a rover long enough off-road, you’ll likely develop similar habits.

Here’s a view of the rear of the Pi camera enclosure. There may be a good, standard weather-resistant pi camera enclosure nowadays, but I couldn’t find one when building this rover. These GoPro enclosures are readily available, their tilt is easily adjustable, and they will only set you back ~ $6USD.

Here you see the compass enclosure (holding a HMC5883L compass) mounted on top, as far away as possible from magnetic friends who would have a bad influence on her. If you look over at the left side of the compass enclosure, you can barely make out an arrow pointing North. I’ve never regretted making any little note like that which mitigated future confusion when the inevitable day comes that you will remove the component.

Every sensor will eventually be removed, replaced, changed, upgraded, etc. We need to push on this a little more: EVERY COMPONENT WILL BE REMOVED. You will not be eternally happy with the location you chose for any component. You will eventually add more sensors, add more servos, add more radios, add more antennas, add another computer, then wake up at 2 a.m. and realize your whole arrangement is sub-optimal.

When you are building your robot, always think about making your future self happy when you rearrange every component. The Good Lord, it seems, did not grant unto us roboticists the ability to perfectly place any part.

We’ve identified these components on top before, but let’s repeat: L1/L2 GNSS antenna (upper left), compass (lower left), 915mhz waterproof antenna (lower right) for 3DR radio. Here are links for purchasing the 900 and 433 version of that antenna (different frequencies are allowed in different countries, you likely know) — I think you’ll be impressed with the antenna’s build quality, especially for ~$8USD.

Wait a second, what kind of potting compound are we using to insulate the compass/magnetometer?

Hot glue, of course! Now, let’s be clear: hot glue is not an appropriate long-term electronic potting compound. If you’re wanting a permanent potting compound, you’ll want to check out something like the MG Chemicals offering, or even the random China stuff.

FAIR WARNING: the entire field of potting and potting compounds is wonderfully fascinating! If you’re not careful, your family won’t see you again tonight while you’re lost somewhere on the internet in a geek-binge of potting compound reviews and information.

If you look at the magnetometer’s clear lid above, you can almost make out another hack we pulled: Aluminum foil inside the lid (secured in place by a huge glob of hot glue, of course) to reflect sunlight. The objective of all this insulating is to shield the compass from sudden changes in temperature which could seriously degrade it’s performance.

One more thing: notice that we use our nylon cable tie threading trick to secure and level the compass enclosure. It’s very important both to mount a compass level relative to earth on your rover, and, later on when calibrating your autopilot, it’s important to calibrate the “level” reading on your autopilot to be true relative to earth. Look, I know if you’re new to this stuff that the last part of that sentence doesn’t mean much, but later on we’ll talk a lot more about this.

It’s worth keeping in mind, if you’re the kind of person who likes to keep random useful robot knowledge floating around in your mind, that a magnetometer’s performance, if used alone, severely degrades when you introduce much roll or pitch. Fortunately for us, however, we’ll be using the ArduPilot software which uses the Pixhawk’s accels and gyros, along with slick and impressive math, to give good heading readings even in steep roll/pitch situations.

One last note: rubber grommets will keep sharp metal corners from cutting into your sensitive wires. As an added bonus, I think the black grommets contrast favorably on the silver aluminum body

Wrapping Up

Man that was a big set of pictures! Hey, if anything wasn’t clear, just shoot a question in the comments and we’ll try to clarify.

I have a feeling that next time we’re going to stir up a little more GPS excitement.

Until we meet again,

Sincerely yours,

Roby

Ignore At Your Shin’s Peril

Roby — Sat, 15 Apr 2017 13:29:11 +0000

Testing rover 2 testing with wheels elevated is significantly less stressful

Regular readers know that I occasionally assume the role of ‘ole Dad Roby and hand down what I believe to be hard-won wisdom. When you’re testing out your wheelchair robot — especially in the early phases before you know it like the back of your hand — you have to develop a pre-flight ritual BEFORE YOU FLIP THE “ON” SWITH for ensuring that the motors are either 1. in neutral or 2. off the ground.

Failure to heed this advise, especially in my early days as a roboticist, cost your author SIGNIFICANT AND ENTIRELY AVOIDABLE PAIN IN THE FORM OF CUT AND BRUISED SHINS!

Perhaps you’re thinking: Yeah that’s a great little tidbit of advise, but what kind of super fancy expensive specialized mount can you use for the task of holding up a big robot?

Side view of elevated testing platform

Now it’s clear: I’m just using the sturdy metal charger to hold up the robot. Hey, it’s not a perfect holder, but it’s far better than testing out a new motor driver on a grounded platform.

Silver “safe lines” are drawn with a metallic Sharpie

Here’s another cheap trick that makes it easy to tell if Rover 2 is in gear. Notice in the picture above that either motor is engaged by turning the metal knob to be parallel with the Rover. When the metal knob is perpendicular to the wheels, it’s in neutral. Perhaps most people naturally remember that kind of thing, but I don’t. So I like to take a silver Sharpie and draw a line indicating which way the knob will be when the gears are engaged. Now I know when I look at the robot, as long as the knobs aren’t inline with the silver line, that my shins are safe.

So I hope, dear ambitious roboticists, that you’ll heed this warning and spare your shins the pain that mine have endured. Let us close with the words of Valiant-for-Truth from Pilgrim’s Progress:

“I give my sword to the one who follows me in my pilgrimage…

My marks and scars I carry with me”

Sincerely yours,

Roby

Sabertooth Tricks

Roby — Fri, 14 Apr 2017 10:00:16 +0000

Roby riding on yet-to-be-formally-introduced Rover 3

Last time we spoke at length about the Sabertooth controller and mentioned it’s ability to record flight data. Here is a sample recording from a run of a big rover (specifically, the big robot pictured above):

Sabertooth raw data

It turns out this data is quite useful. For instance, you can create a chart of the amp draw for both motors, i.e.:

Big robot amp draw over ~15 minute (900 second) run

Monitor Sabertooth Wirelessly with VirtualHere

Perhaps you think that amp draw chart is great, but how do you connect a computer to your rover’s Sabertooth when your rover is running a mission 200 meters away? I’ve had good luck with the help of a Raspberry Pi 3 with a little software app called VirtualHere¹ that allows you to connect to USB devices over the internet.

Here are the steps to get going:

Install the VirtualHere server on your Raspberry Pi
Install the VirtualHere client on your PC
Connect the Sabertooth to the Raspberry Pi via a USB cable
Start VirtualHere server on Raspberry Pi via:
```
sudo /usr/sbin/vhusbdarm -b
```
Open the VirtualHere client on your PC. You should see the server (called “Raspberry Hub” in the image below) (you may have to specify the server’s IP if it doesn’t automatically populate). You should see a list of USB devices connected to your Raspberry Pi — notice below that Sabertooth 2×32 is the only device connected to the Raspberry Pi. So we click on the Sabertooth 2×32 and select “Use”.
VirtualHere Windows client screen
Next open up DEScribe, click “Connect and Download Settings” and you should see an option for “Serial Port” like “COM3”:
DESCribe connect screen — note that COM3 is the virtual Sabertooth device on my machine
Now click “OK” and DEScribe should connect to the Sabertooth. Voila! You’re wirelessly connected to a remote Sabertooth controller!

Monitoring live Sabertooth data on remote robot

Let’s wrap up this brief post on that note. Next time we’ll present a few tricks to hopefully keep your shins healthy while building these big robots. Here’s looking forward to seeing you again soon!

Sincerely,
Roby

Notes

VirtualHere is free to connect to 1 device. For more devices, you’ll need to purchase a license.

Sabertooth Standing Tall

Roby — Thu, 13 Apr 2017 02:46:10 +0000

Sabertooth 2×32 compared to the M60¹ — You may find it worthwhile to kick down an extra Benjamin and pick up the Sabertooth

Welcome back fellow, future, and potential robotocists! In our last post we played with a little fire while programming the M60. While taking the photo above I became curious what was beneath the heat-sink for the M60, so here we present State Exhibit A37:

State Exhibit A37: Do you see the smoking gun?

Burned M60 Close-Up

You can probably see the burned component in the upper middle of the board below the green terminal bank. I assume that’s a MOSFET transistor — if any reader with an electrical background would like to inspect the image and report back, I’d love to hear from ya!²

You likely realize that this enigmatic M60 has now found it’s way into the last 3 posts of this blog: like a bad girlfriend, we just can’t let her go! Now listen here my sons: you don’t want to marry the M60. I know she’s mysterious and wild, but you’ve got to trust ‘ole Dad Roby on this one: the Sabertooth is the girl for you.

Sabertooth & Robot: A Well Arranged Marriage

Exchanging the M60 for the Sabertooth means eliminating the Arduino: we wire the Radio receiver directly to the Sabertooth and configure with a relatively friendly Windows application called DEScribe — more on this shortly. Let’s check some pics from the Sabertooth retrofit:

Brass Offsets Mounted to Poly Platform

Arduino Remains On-Board Unused (For Now)

Rover 2 with Sabertooth, Relay, and Radio

Assembled Rear View

Assembled Front View

A few comments/clarifications. First, if you look closely at the pics and follow the green/brown wires from Sabertooth M1A/M1B, you’ll notice that those wires trace down to the right motor. Accordingly, if you follow the blue/yellow wires from Sabertooth M2A/M2B you’ll find them connecting to the left motor. For a simple man like me, this is a headache; specifically, I wish the right motor’s wires went to right side of the sabertooth and the left motor’s wires went to the left side. Perhaps there’s a way to configure this in the DEScribe software, but I haven’t found it.

You’ll notice 4 breadboard wires connecting the Sabertooth to the radio receiver. These are 5v, Ground, Steer and Speed. The Sabertooth (like the M60) magically anticipates you might like to have a 5v power supply and provides an auxiliary 5v and ground. Remember that Steer and Speed come across the air as two independent channels — we’re going to instruct the Sabertooth to take care of mixing these into appropriate left and right motor current.

How Do I Love Thee?

So what’s so great about the Sabertooth? Several things. First, look again at the picture at the top comparing the drivers and notice the difference in the terminal block size. The Sabertooth easily accommodates 10 AWG wire. Good luck with that on the M60. Here are photos with both drivers referenced against red 10AWG wire:

10AWG in M60 Not Recommended

Sabertooth 10AWG Room to Spare

The M60 uses 5.08mm pitch connectors like these. Max wire size rating is 12AWG. The big luxurious terminal bank in the Sabertooth feels like flying first class.

Aesthetically the Sabertooth is easy on the eyes. You know a Sabertooth when you see that beautiful aluminum CNC milled chassis with integrated heat-sinks flexing on either side.

What’s not to love about that beautiful chassis?

Firing up Windows to Configure the Sabertooth

Yes, I’m a Mac guy. If you still own a laptop/desktop, there a big chance you’re on Windows. In a perfect world Dimension Engineering would provide configuration software native to Mac, but alas, they must not have an unlimited budget. Never fear: it’s easy to run Windows on a Mac via bootcamp.

So let’s connect our computer to the Sabertooth via USB and open up the DEScribe software.

DEScribe RC tab — Much to like but one complaint: Servo Center entered is not honored

Notice in the screenshot we’ve selected the “RC” tab in DEScribe. For the “Calibration” field we selected “Fixed”. There are various other options offering automatic calibration — I prefer to find out the stick’s min/max/center and manually enter them. As we’ll see in a moment, we can find the raw servo values when we click the “Diagnostics” tab.

However, before we get to that, I want to point out what I believe is a bug with the Sabertooth: Servo Center is not honored. Specifically, I’ve observed that the Sabertooth treats the first radio values it sees when it boots up as the center. Stated another way, when the Sabertooth is powered on, regardless of what value you set for Servo Center, the Sabertooth is going to take the first radio PWM values and say that those values correspond to 0% throttle and 0% steering. Perhaps this is a safety measure (think accidentally booting up your robot with your radio controller at full throttle), but I wish there was a way to override this behavior. At a minimum, DEScribe should at least inform us what PWM values the sabertooth is currently treating as the de facto “Servo Center”.

Notice that you get to decide if you want either exponential or linear mapping for throttle/steering. I strongly prefer the exponential map as it gives you a lot more control at lower speeds. Keep in mind if you want this kind of functionality with the M60 you’d better find an Arduino library (or roll your own code).

DEScribe Motor Outputs

Next up we click on the “Motor Outputs” tab. Notice I set Deadband to +- 5%. I like to have a fairly large zone that the Sabertooth considers as neutral input — otherwise when you stop your robot, if you don’t get the radio controller sticks perfectly centered, you’ll be pushing a tiny bit of current to the motor when you intend for it to be at rest — this isn’t what we want.

You’ll notice that I don’t cover the “User Mode” tab as I’ve never taken time to figure out exactly how to make custom modes work. Perhaps a Dimension rep would point out that my limited gripes could be easily solved by creating a custom “User Mode”.

While we’re speaking of Dimension reps, I have spoken with them (yes, actual people who work at the place where they make the part) in the past and they are refreshingly friendly/helpful/knowledgeable.

DEScribe Diagnostics Tab

Click “Show Inputs/Ouputs” on Diagnostics tab

You’ll notice in the “Live” view above that you get an informative peek into the current state of the controller. Of particular interest is the “Record…” button at the bottom. It turns out the Sabertooth allows you to record the controller state during a run for review — this is very handy, in fact, we’re going to explore this a little more in an upcoming post.

Enough talk! Let’s check out a run (that’s little guy laughing in the background):

If you look carefully at the last few seconds of the run above, when Rover 2 crashes down from the last wheelie, it throws off the radio receiver, crashing to a halt. Fortunately the robot escaped harm.

Until next time,

Sincerely yours,

Roby

Notes

Remember that “M60” is more commonly known as “60A High Power MOS Dual Channel H-bridge DC Motor Driver Module”
Inquisitive readers may like a picture of the back of the fried M60 for further forensic analysis.