darknrgy's blog

6WD Robot - Motor Work

 

I spent a bunch of time working on software in the last few weeks.

I ended up rewriting the computer->AVR protocol to be much more lightweight; it can send arbitrary length packets of bytes which can then be interpreted as different size numbers - 8 bit ints, floats, whatever. I also added a CRC-16 check which is TOTALLY UNNECESSARY but it was an interesting task and it might just keep the robot from driving off a cliff one day. 

I abstracted some of my old libraries a bit better and wrote some new ones. Most notably, a time library for checking time deltas and other things. I ended up using it quite a bit. 

I learned enough python in a couple of weeks to write a new robot server using asynchronous sockets and ended up completing some IRL work tasks with python as well. Python is very fancy and I plan to use it throughout the project on the robot's main computer and on the client side using wxpython. My plan is to have a very snazzy interface that shows motor RPM, amp draw, temperatures, fault notices, lots of buttons to turn stuff on and off, etc, etc. 

All that is pretty boring though. CHECK THIS OUT

 

The motors are officially installed and working! In the video I just have them connected to the power supply so nothing too fancy software-wise. I wrote a reactive PID controller though, more on that in a second. BUT FIRST  

It's actually doing something! Turning in place with a 6WD drive robot has one nuance that you have to account for. The corner wheels will drag and this will cause a great deal of power to overcome. The solution is easy - just mount the middle wheels lower to the ground so that the corner wheels barely touch. As I add more weight to the robot, I'll probably need to continue to lower the middle wheels even more, but this test showed that the solution does work. 

Now for the really fun stuff! Reactive PID controllers. On my previous robots I just controlled the throttle using PWM directly. This has some drawbacks. As the robot traverses different surfaces, inclines, etc, it speeds up and slows down as the load changes. It also introduces a bit of drift in direction as each side of the drive train will not always be working with equal efficiency. What I really wanted was to tell my robot STOP YOUR STUPID MOTOR WHINING AND JUST GO THE SPEED I TELL YOU. Thus, I read things like books and wiki pages.

• http://en.wikipedia.org/wiki/PID_controller
• Designing Mobile Autonomous Robots - John Holland, Chapter 5

It's not eas easy as it sounds. The idea is to write an algorithm that will automatically adjust the motor power based on feedback on it's rpm. First, you need to actually measure the RPM, which means using an encoder - like an opto-encoder or hall sensor - and using that as a hardware interrupt on the microcontroller. Then you do math, but not very good math because it involves tweaking numbers that are pretty arbitrary unless you do extremely good math. Extremely good math involves modeling your entire robot - the physics, the electrical characteristics of all the components, and probably even the temperature and the orientation of the moon and what astrology sign you are. That's what it says on the wiki page. Literally, reactive PIDs were invented so that you could just solve it the American way, with a hammer and shocking yourself repeatedly. 

Long story short, here is the core of my motor controller: WARNING HERE BE (probably bad) CODE: http://pastebin.com/4G8hWunz Note that I when I say stuff like pwm_init(), I'm calling yet another library I wrote. Apparently in arduino land this is all done for you, but I like doing things the hard way. 

It works surprising well after a bit of tweaking (burning a new image on the avr chip ten thousand times). I't pretty cool to listen to both motors spin in sync even when you load one of them down by pinching it and burning yourself. And blowing up capacitors. I have a fire extinguisher now. 

The motor driver has served me well, but it's pretty primitive. Just a basic H-Bridge with a failure mode of blowing up. I found a much nicer package, again through http://www.pololu.com/, but this one has some pretty neat features - current and temp limiting, fault indicators, and best of all, current signal, which means I can measure that on the microcontroller and show the client how many amps each motor is pulling. Pretty cool!

http://www.pololu.com/catalog/product/712

I have some prototyping boards on the way and I've got most of the circuit mapped out. Next up will be building the power bus, logic circuits, and making the robot do things without an umbilical chord. 

6WD Robot - Chassis Complete!

 

The 6WD Robot chassis is complete and I couldn't be happier about the results. But first, some lessons learned. 

 

Don't permanently weld acrylic on components you know you will have to adjust you stupid stupid idiot. 

There were a few problems with my original design. I opted to build frame rails that would hold the three wheels on each side. The frame rails were to be welded to the main platform with acrylic glue for more permanence than the universe itself. It looked like this: 

There are a few problems with this. First, because I planned to use roller chain to transmit power across the three axles, I would need to drill those god damned holes exactly the right distance from each other.

This is stupid. Even if you laser cut the holes with an electron microscope laser cutting device it would be stupid. At the length of chain I was working with, even 0.25mm would be the difference between a slack chain and a taught chain. I drilled these holes as exactly as I could; I created a template, temp glued the template to my working surface, lined up the drill bit in a drill press, clamped everything down, and repeated the process for each hole.

It did not work; the minute differences in distances resulted in inconsistent chain tension. 

Secondly, chains always need a way to be adjusted. Anyone who rides a motorcycle knows this. I know this because I ride motorcycles. But instead I decided to ignore the compendium of roller chain maintenance and just try and wing it. Over time chains will become more and more slack. 

Finally, by constructing an immovable frame configuration, I was severely limiting my options. I had permanently acrylic-welded the frame rails to the main platform. Acrylic welding cannot be undone unless you work at NASA. It becomes one piece. What if I wanted to set the axle height differently for the middle axle (turns out I did!). What if one day I wanted to make an articulating suspension? Now I can do this because I've decided to modularize the axle assemblies. 

Modularized Axle Assemblies

I cut a module out of each frame rail to make six modularized axle assemblies. As long as the bottom edge of the rails (seen above) are flat, you can weld them to a roughly cut piece. What I did was insert the bearings, shafts, and fixed them in place by temporarily putting these sprockets on each side. The sprockets are just there to make sure the axle is perpendicular to the frame rails. By doing this, I could be confident that the axle would be aligned perfectly with the frame rails. Then, I just set it on the 1/8" square piece and welded it on. Here are several in various stages of construction:

Next came assembly of the axle assemblies to the main platform. This time, I decided to use screws. Screws can be unscrewed, which is very much unlike the permanence of the universe. It also allows me to adjust the chain tension by filing the holes or just drilling new ones! Screws! 

At this point I'm so overly obsessed with alignment and lasers that I decided to use lasers for alignment. Probably not in the way you're thinking, though. I like to draw lines on acrylic by etching into it with a small screwdriver. An etched line is several micrometers thinner than a line drawn with a pen. Problem is, it's very hard to see an micrometer-thick etched line... unless you use lasers! By pointing a laser at any edge of acrylic, the other edges and small etched lines light up brilliantly. 

Arguably, this made absolutely no difference in how well I aligned my center axle assemblies, but it looks fucking cool. 

Here's my workspace, just for fun:

 

Another thing I decided to glue in was the bearings into the holes in the acrylic. My thinking was that I was cheap and lazy and superglue is easy enough to break loose when I needed to replace them. This was just yet another mistake in a stream of fuck-ups that drove me into a state of depression and a lack of progress on my robot. Bearings are remarkably intolerant of problems with alignment. In the process of putting superglue into the holes and shoving bearings into said superglued holes, I introduced tiny variations in  angle, thus boning the alignment. And, since the holes now had micrometer-thick layers of superglue, future alignment would likewise be boned. 

So, I decided to do it the right way and let the bearings "float" in their holes and provide end-stops to prevent them from relinquishing their robot duties via falling off. I've gotta hand it to Lowe's. I managed to find "special application" vinyl washers with specifications so exact that I might as well have been programming Alan's 3-D printer. Lightweight, vinyl, 13mm inner diameter, sufficient outer diameter for gluing surface area, 1/8" thick. And, I managed to find some shims for my shafts that came in handy later. 

Epoxying the bearing end-stops to the axle assemblies

Drilling the holes between the axle assemblies and the main platform was as anxiety-inducing as a reality television show about making robots. Especially since I was using a wood drill bit instead of something more proper, but driving to Lowe's to buy a $15 "SPECIAL APPLICATION YOU WILL USE THIS ONLY ONCE" pieces of metal was getting quite old. Using the drill press was not an option because of the geometry of the various clamps, laziness, and the fact that it was pretty cold in my garage (amplifies fear-induced trembling). Also, if I managed to break one of my axle assemblies, it would mean hours of table-sawing, universe permanent acrylic welding, and epoxying vinyl end-stops from Lowe's. I've become very weary of stupid mistakes costing me hours. 

Drilling acrylic is actually sort of tricky. You have to go slow or your hole will be a mess. Your working surface has to be solidly planted or it's going to a) spin wildly destroying all within the radius of your target surface or b) immediately bind on the bit upon breaking through, rocketing your target surface upward into your face/the sun/into a million fucking pieces. Make sure and clamp your surface down, wear gloves and protective eyewear, and drink exactly one beer so your hands don't shake uncontrollably from the terror. 

Once I had the axle assemblies attached to the main platform I assembled the drive components and attached the wheels. 

 

Beautiful! The chassis glides across the floor showing very little drive-train friction. It's at least tolerant to being rammed into a potted plant at a decent speed without damage. Overall, I am very happy with the result and I'm glad I spent all this time on it. I did notice something right away; turning in place requires quite a bit of force. The corner wheels grip substantially and this level of force is not going to be acceptable. The solution is easy. The center wheels need to be mounted about 1/4" lower than the corner wheels. This reduces contact patch on the corner wheels, reducing drag in a turn. The trade off in traction is negligible; I made this fucker 6WD so it could drive over small mountains, not putter around in office buildings. In any kind of off road condition, all six wheels will make contact. With any luck, this robot will be able to drive up a curb. 

Next up, I've been researching motors. I had tested a cheap handheld drill gear-motor from Harbor Freight, but there's just not enough gear reduction. It's a powerful motor, draws far more amps than I want to deal with, and would propel my robot at warp speed. Too bad. 

I've decided I want the robot to go around 2 feet per second and carry a 10 pound load straight up if need be. You know, for good measure. There are two motors; one for each side. I think this ought to do the trick: http://www.pololu.com/catalog/product/1445. Feel free to do the calculations if you want. The wheels are 4.24" and I'm hoping to do a 1:1 transmission.