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:

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

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

Now some of you guys are thinking that’s irresponsible. Agreed. But hey, ever thought you may get to be 90 years old and wish you’d been a little irresponsible in years prior?

So the title suggests that things are going to go poorly. Yes, in the end we have trouble with the driver, but we’ll see that perhaps the problem was user error.

Tweaking the Code

So let’s talk about the changes we made to the Arduino code to run Rover 2. You’ll find all the code for Rover 2 available on GitHub. Here is the full source for the Arduino sketch that we will now discuss.

First I want to call your attention to the pin mappings:

  // M60 to Arduino
  // right motor
  const int pb = 10; //pb should be 9 or 10 if you plan to call setPwmFrequency(pb, 8) for noise reduction below
  const int b1 = 8;
  const int b2 = 13;

  // M60 to Arduino
  // left motor 
  const int pa = 9; //pa should be 9 or 10 if you plan to call setPwmFrequency(pa, 8) for noise reduction below
  const int a1 = 12; 
  const int a2 = 11;

Note that the section above is slightly changed from the L298N code. We moved the pwm output (pa and pb) to pins 9 and 10. The reason for this is that when I drove the M60⁰ with the Arduino, there was an obnoxious buzzing sound. A quick google suggested that the noise could be reduced by adjusting the PWM frequency for the PWM pins. Well, it turns out that adjusting PWM frequencies on pins other than 9 or 10 disrupts the Arduino timing functions that we rely on. So can you adjust the frequency of pins 9 and 10 with no consequences? Nope, this messes with the functionality of the Servo library. Score for us, we’re not using the Servo library.¹

  leftMotorSpeed = constrain(leftMotorSpeed, -254, 254); //reduce to +-254 for M60
  rightMotorSpeed = constrain(rightMotorSpeed, -254, 254); //reduce to +-254 for M60

While the L298N considers a pwm value of +-255 as a maximum, those values cause the M60 to stop. So we adjusted the max to +-254 and everything seemed fine, until…

I can’t remember the exact sequence of events the first time it happened, but I was coding up the Arduino M60 driver and wiring up Rover 2 and all of a sudden I hear something like a spark and I saw smoke come off the driver. I really don’t remember this time if I was uploading code to the Arduino or not (more on this later)…

So, after tinkering with that Arduino code with Rover 2’s gears in neutral, it seemed that the M60 was powering the motors quite well. But then I noticed something odd.  Here are a few pictures to show you how the M60’s LED lights behave when you go forward or reverse:

M60 Full Throttle Forward

Full Throttle Forward

M60 Full Throttle Reverse

Full Throttle Reverse

So there’s a problem! For some reason the “B” channel light is not turning on. Also, regrettably, channel B is not sending voltage to the right motor in reverse. Remember that reverse was working when we ran Rover 1 with the M60. Therefore, after the smoke incident mentioned above, I assume that I must have somehow fried M60’s ability to do reverse on channel B. Bummer!

Hard Test Run

OK, so we only have reverse on 1 wheel. Hey, who says you can’t have a little fun going forward? We took Rover 2 outside and drove it around for ~45 minutes. It performed flawlessly (minus irreversible right wheel). I drove it as hard as I could and put my hand on the M60’s heat sink — it was only a little warm — quite impressive. I drove Rover 2 down the little side road next to our house (not the big highway you see in the video), probably 200 meters away (it was quite blurry for a near-sighted fellow who eschews glasses) and only turned it around because Mrs. Roby was nervous about the robot being so far away. I was curious if the Turnigy radio would lose range, but it performed perfectly.

Now that we had tested Rover 2 and the M60, it was time to come back inside and make a few code tweaks…

Burning Out Channel “A” Reverse: Eureka!?

Here is the fateful order of events I now want to present into the public domain of M60 driver knowledge:

1. M60 motor driver is turned on and powered by 24v power source
2. M60 motor driver is connected to Arduino as indicated in the pin mapping above
3. Mac is connected to Arduino via USB
4. Code is changed in Arduino IDE
5. Code is uploaded from Mac to Arduino
6. !!!!!!M60 SPARKS, SIZZLES AND SMOKES!!!!!

More smoking and sparking than last time! It happened right after I uploaded the Arduino code (think less than 1 second) and it lasted for a couple of seconds. Could it be that the Arduino is putting garbage output on the pins and the M60 doesn’t have protection from errant control pin input?

Wrapping Up

The M60 driver turned out to be a real adventure! You can tell from the videos above that it effortlessly runs the rover. With the exception of the unfortunate reverse blowouts, it performed quite well.

Let’s end both our M60 experiment and this blog post on on this note.  Next time we’re going to make the move to the time honored Sabertooth driver. Why do all the cool kids rave about the Sabertooth controllers? I think you’ll see.

Until then,

Sincerely yours,

Roby

Notes

0. Keep in mind that “M60” is the alias we’ve given to the mysterious driver who elsewhere goes by the full name “60A High Power MOS Dual Channel H-bridge DC Motor Driver Module”
1. The considerations for adjusting pin frequencies are outlined here in the documentation for the setPwmFrequency(..) function.
2. When discussing this “M60” driver, there are so many unknowns (and hence, caveats) it’s comical. For instance, if you look at the different M60 drivers on eBay, you’ll see there are slight variations between some of them. My M60 has 2 capacitors. Some of the new ones (like this one) have 3 obvious capacitors and a blue cylindrical component labelled “221” which may also be a capacitor — either way my M60 doesn’t have it. A manufacturer spec sheet for this driver would be wonderful: if some kind reader either knows Mandarin or has ninja Google skills, that would be a great find.

Wheelchair Frames Dream of The Day They Are Retrofitted for Robotics Glory

Today we introduce Rover 2! Rover 2 is one small step for our project, one giant leap in terms of utility. This robot frame is made to carry people, and in the USA, that’s often an ambitious proposition.

Enough talk, let’s jump into a photo essay showing the migration from lowly Rover 1 to mighty Rover 2.

We welded a stainless plate to the wheelchair’s seat mount

This base will come in handy before long when we want to ramp up the platform size

We removed the electronics platform from Rover 1 and added a clear 24v JD2914 relay

Parallel ATC Fuses — Didn’t have a Maxi fuse holder laying around, so we’re using 2 30a fuses in parallel on the battery. This is clearly a hack.

Question: what to do when you don’t have a 60 amp fuse holder laying around the shop and you need to fuse for 60 amps of current? We went for a hack: use two 30 amp fuses in parallel. I fully agree if you’re thinking that this is not an ideal move, but our alternatives were: 1. Wait for Maxi Fuse holder in the mail, or 2. Use unfused power source. Option #1 is out because our project must steam ahead, and option #2 is clearly a sin which could prove deadly. We went for the dual 30a fuse solution and it worked out fine.

Playing with Heat

We’ve previously talked about these curious heat shrink solder connectors. The last experiment with Rover 1 was so short-lived, I decided to give them another go with a little more current on the line.

XT60 Heat Shrink Solder (before heat gun)

XT60 Heat Shrink Solder (before heat gun)

XT60 Heat Shrink Solder (after heat gun)

XT60 Connectors Are Great

If you’re planning numerous missions with your big robot, you’ll be charging it every day. You want to be able to easily plug up the charger. Here’s a simple solution: wire an XT60 connector in series to the power supply. I love these little connectors and I’ve never seen them have a failure of any kind. If you don’t have a few male/female sets in your bag, you can snag them reasonably off Amazon or eBay. Keep in mind 2 things when ordering:

1. Double check that you’re ordering both male and female ends. You don’t want to purchase 5 pigtails, lose sleep eagerly anticipating their arrival in the mail, then receive the package and with horror find that they’re all male adapters.
2. Keep an eye on the gauge (thickness) of wiring. For instance, in the Amazon link above you’ll see the first/cheapest pigtail results are for 14AWG (2.5mm) wiring. That may work for your application, but you may need 12AWG (4mm) pigtails if you’re pumping a ton of current — we specified 12AWG (4mm) in the eBay link, just to make the point about wire gauge.

Clarifying XT60 Nomenclature (fancy way of saying I won’t remember either)

The picture above is presented in an effort to clear up any confusion about which end is male or female. I’m sympathetic to this confusion because the plastic shroud of either connector is the gender opposite of the inner metal connectors. Wow, this is creating more confusion. Let’s just stick with the picture above.

More Love Letters to the Electric Wheelchair

XT60 Wired to Charger

Charging Rover 2

If you’ve been following this blog you know that I can’t stop raving about these wheelchair platforms. Here is another fun trick: cut the factory 3 pronged end off your wheelchair charger and solder/crimp a male XT60 adapter in it’s place. BAM! Just like that you’ve got a great charging system for your robot.

Mrs. Roby would really appreciate this kind of effusive praise directed her way a little more often. I know a lot of you engineer men out there are with me, right? Why is it so easy to talk at length about wheelchairs but writing 2 paragraphs to your sweetheart in a Valentines Day card seems like an insurmountable mountain?

Wow, things just got way too deep. Let’s sign off with another series of pictures:

Notice the rubber anti-vibration mounts aren’t really necessary for this application (sans IMU)

If you look closely you’ll see an 80 amp rating on the relay — wouldn’t bet on that if I were you

Notice the green relay ground wire sliding down from the platform connects to the on-off switch

The location of the on-off switch is a little odd due to interference from the frame

Here’s a look at how things appear from below

View from above (before wiring up motors)

Wrapping Up

Let’s stop there and save the software and tuning for the next post.

Here’s a hint: sparks are going to fly.

Sincerely,

Roby

Electric Wheelchair Base: The Pinnacle of Large-Scale Skid-Steer Robotics Testing Platforms

So why are we so madly in love appreciative of the electric wheelchair? Here are some of the top reasons:

1. Payload Capacity — How much can this frame carry, you ask? I’ve routinely seen numbers in the 500 lb (227 kg) range for “heavy duty” models. If you need a test robot platform exceeding those specs, then you really do have a fascinating project.

2. Tried & True Power Source — Lead-acid batteries have been around for 150+ years and while they are far inferior to the new lithium batteries on many points, with a little care¹ they are reliable, rugged and cheap.

3. Battle Tested Gear Motors — Yes, they call the drive assembly a Gear Motor. It’s a DC motor coupled to a reduction gearbox, a built-in shaft for mounting the wheel, and usually fitted with an electric brake (that we often disable). If you’re not interested in grabbing them cheap off Craigslist or eBay, you can pick them up new in the US for ~$600/pair. You’ll often see these motors on various combat robots.

4. Flooded Subsidized Market — Somehow apparently everyone’s grandparent is able to secure one of these $2000 wheelchairs for free that they either don’t use, or have left to the family when they passed on to another life, and the wheelchair is of no use to the family. I suppose we could devolve into a long philosophical complainypants discussion on appropriate government spending, but I’d rather just be thrilled that those of us in the USA benefit by buying such an amazing item in a soft second-hand market.

5. Huge User Community  — If you’ve never checked out Burgerman at WheelChairDriver.com, you’re in for a treat. That guy is a genius.

24v Lead-Acid Battery Chargers

6. Charger Included — Your friendly Craigslist seller will nearly always spring for including the charger with purchase. This charger is specially made to charge 2 12v lead-acid batteries wired together in series. You’ll notice from the picture that the form factor of the two charges is quite different, but their function is the same: they’ll supply roughly 28 volts and charge up your batteries.

You’ll notice that I’ve made no distinction between brands in this discussion. It appears to me that all the brands source basically all their parts from the same suppliers.

Note that the wheelchair above has the large 12″ (30.5 cm) wheels. Some models have smaller wheels, but for our application the big wheels are likely the better choice.

Another Warning

This robot platform weighs 152.8 lbs (69.3 kg). It’s approximately 24″ (61 cm) wide, 29″ (74 cm) long and 14″ (36 cm) tall. Wait, let’s repeat that:

THIS ROBOT WEIGHS 152.8 POUNDS (69.3 kg). THAT’S 7x THE WEIGHT OF THE AVERAGE 1 YEAR OLD CHILD.

Ok, I’m back. But friends, let’s sober up for a moment. Remember, we’re talking about unloaded weight. Our computing/sensor apparatus will add to that weight and size. We have to commit to spending considerable mental energy (and financial resources) on safety.

Until a teenager is afraid of a car, they shouldn’t drive. If you aren’t a little afraid of the unchecked potential of an electric wheelchair based robot, I don’t think you’re ready to build this kind of robot.

Let’s rest on that thought for now.

Sincerely yours,

Roby

Notes

1. It’s very important not to discharge lead-acid batteries below somewhere between 10.8v and 11.6v. Both the Sabertooth controller, and the stock wheelchair controller (that we won’t use), have built in functionally to prevent over-discharging the lead-acid batteries. Here’s a nice article with specific floor voltages.