Older blog entries for tbenedict (starting at number 16)

I'm curious about something for most of the "bug style" walking robots. (This question really doesn't apply to the RoboOne style biped robots.) Almost all of the bug-style walkers have their legs articulated the same way: The entire leg pivots around a vertical rotary axis at the hip joint. Also on the hip joint is a horizontal rotary axis that lets the entire leg move up and down. Past this there's typically at least one other knee joint, also usually oriented horizontally.

Earlier when someone posted about the Boston Dynamics "Big Dog" robot, someone made a comment about how odd the leg joint arrangement was. It's odd for a robot, but not for a dog. The primary joint is a horizontal rotary axis, oriented parallel to the length of the body. This lets the leg swing in closer to the body or out, farther away. Mounted on this is another rotary axis that pivots with the rest of the leg. It's neither always vertical or always horizontal, but with the leg directly under the robot it's mostly a horizontal rotary axis that lets the leg swing forward and back.

It's this hip design (a very good analog to what you see on a cat or a dog) that lets the Boston Dynamics robot have such nimble feet and respond so well to a disturbing force (e.g. a swift kick to the ribs, as you can see on the videos on their web site.)

The only other place I've seen that close an analog between nature and robot was the study done at MIT on the mechanics of cockroach leg joints. It's not nearly as obvious as one might think, and bears little to no resemblance to the hexapod arrangements available from most robotic supply houses. Among other things, the hip joints more closely resemble those of BigDog.

So I'm curious about two things: One, why haven't more people tried this hip orientation? For accelerating from a dead-stop, it places more of the loads along the leg rather than across it (much easier on the joints!)

Second, and more important, did I just ask the "but why?" question that's going to get me out of mini-sumo and into my next phase of exploration in robotics?

In a way I hope the answer to the second question is no. I love building mini-sumo robots. In another way I hope it's yes. Walking robots pose REALLY neat questions I'd like to play with.

I guess I really hope the answer is "both"! It means I get to make walking mini-sumo robots! Though hitting the 500g limit with three or more servos per leg is pushing it...

Tom

Robotics projects have been put on hold while we recover from the earthquake out here in Hawaii. My robots survived fine (some books fell on one of them, but that's it.) The shop more or less survived fine. Mill and lathe are still on the bench, though I did have a hand scraper and a couple of other tools fall. My wife had more tool avalanches than me, including some torches (thank goodness we never leave tanks attached!)

Most of my time has been spent at work trying to get things operational there. It's amazing how violent an earthquake can be. It's even more amazing to see how fast our facility has been brought back online. It's entirely due to the efforts of some phenomenal people. With any luck we'll be back on the sky tonight, maybe even before the bad weather clears.

Back to work!

Tom

I got a nice lesson in infrared optics, the necessity of reading a datasheet before using a product, and the concept of paranoia in a robot.

I've been adding sensors to my mini-sumo robot, Shallow Blue. I finally installed the last of them, a rear-facing Sharp IR proximity switch set to trigger at 40cm and closer. For now any time the sensor trips, the robot is set to spin around 180 degrees. So far so good.

I loaded the code, verified, plopped it in a ring, and turned it on. It immediately set to spinning around and wouldn't stop! Paranoia!

Of course it's simpler than that. The vertical spread of the beam was impinging on the floor, giving the sensor a reading closer than 40cm. It was doing exactly what it was told to do. I was able to verify this in a dimly lit room using an older CCD camera (Nikon 950), which could image the robot as well as the beam spot from the sensor. A quick modification to wedge the sensor at an angle to the floor solved the problem.

Tom

Our club had its third meeting last week. Two mini sumo robots and two line-followers (with one doing double-duty). There was a lot of enthusiasm for the mini sumo competition, and it looks like we'll have more active builders over the next month. Four people have declared an intention to, and two have already purchased hardware. I'm hoping we can hook a fifth.

I'm building a second mini-sumo, a stock Mark III, which I'll be using to tune up new changes to Shallow Blue. Once that's done the sensors and boards will be ripped off the new 'bot so the 4WD platform I'm building out can be finished. Not sure if that'll happen by the next meeting, but I can hope.

The rest of the stripped Mark III, a chassis two servos and two wheels, will be outfitted with an R/C receiver for remote control ala Battlebots. This isn't in an effort to develop an R/C mini sumo class, but as a way to get people interested in the club. We've got two public appearances coming up in December, and being able to hand someone a transmitter and say, "Want to try your hand against an autonomous robot?" may possibly draw new people into the club. Have to see.

The rest of the meeting went well. It looks like we may have a FIRST Robotics team forming up in town, and there was a call for mentors. There should also be several Ballbot teams forming up before January, and possibly at least one underwater ROV team as well. In addition it looks like we're getting a fair number of "big kids" interested in tackling some larger projects. Should be a busy year!

In response to svo's post about the steam engine (nice engine, by the way!) I'd say a good, solid confirmed on that one! My first steam engine was made entirely on the lathe, not using a mill at all, even for all the little rectangly bits and drill patterns. So it's nice to see the opposite situation holds true as well.

So here's a challenge... First, some background:

A recent series of articles in Machinist's Workshop (or Home Shop Machinist... I take both and don't distinguish much between them) described a 4-cycle gas engine called the EVIC Twin. What sets this thing apart is that it uses electronic solenoids for the valves, and is entirely controlled (throttle and all) from a PIC.

Now for the challenge:

An oscillating steam engine isn't the most efficient piece of equipment on the planet. (My first steam engine was an oscillator, too.) This is why oscillating engines were almost never used on locomotives. This is also why all manner of valve gear was developed over the years. So if the idea of the EVIC was extended to steam, and a steam engine was put entirely under microprocessor control, what would such a beastie be like?

Any takers? Anyone actually want to build one?

In answer to bear on building cheap robots: It always depends on where the bits and pieces come from. Good example: I've got about fifteen steppers lying around on my bench. They came from scrapped floppy drives. Cost? $0 monetary outlay, though as you point out a fair bit of time was spent removing them. Another example: Last time I took trash to the transfer station, someone had dumped a bag full of R/C toys. I picked 'em up. One just needed a battery charge and some contact cleaner on the power switch. (Free toy for the kids!) Another had a fried transmitter. Question is, do I fix or replace the transmitter, or just gut the car and stick a microcontroller in it? $0 monetary outlay, and not much more than that for time invested so far. Offroad chassis for a RoboMagellan project? Possibly so. (Though my kids would kill me if they heard me say that about "their" toy.)

Problem with this approach is it doesn't scale. If you're doing commercial robots, you can't rely on surplus or salvage. This is one reason why commercial robots tend to be a lot more expensive than what can be built at home, even given similar construction methods. It also doesn't give any gurantee of identical spares. In that stack of steppers on my bench, only two are of identical make and model.

One more question to ask when estimating cost: Do you count the cost of items previously purchased? For example, the line follower I'm building is getting its entire drive train out of spares from my ROV and mini-sumo. Total cost? I could say zero since the parts are just lying around. Total cost for someone trying to duplicate what I'm building? Considerably more, especially since I didn't buy the motors as single units.

I guess what all this boils down to is this: Do people estimate cost based on their out-of-pocket expenses, or do people estimate cost based on what someone else would have to spend in order to reproduce what they built?

Even putting it so simply still leaves you with hairy questions. For example, do you add in tooling costs? I've got a complete machine shop at home and a decent electronics shop as well. I don't add in machining or tooling costs, but someone trying to reproduce what I do would have to.

I'd be curious to hear what other people's practices are when they estimate cost on a project.

Tom

Our club had its second meeting. Bigger turnout, and three mini sumo bots (including mine). Shallow Blue got its butt thoroughly kicked. I've got a list of changes to make before the next meeting: one software, one hardware, and one electronic. Good mix.

Turns out there aren't rules for the line following event yet, so I'm planning for right angle turns on the high speed line follower I'm making. People laughed when I said I was after two meters per second. Fine by me. I doubt I'll reach that. But it's good to have a goal, and it'd be a riot if I could hit that, even if it's only on the straightaways.

The club issued two challenges to its members: Make a robot that can track a straight black line across a white field, and/or make a robot that'll stay on a table. One, of course, is in preparation for building a line-follower. The other, of course, is to get people thinking about how to keep a mini-sumo bot in the ring.

The next meeting should be fun!

Tom

In reply to andycamach's question, I use an OOPic (obviously), and I've got a 68HC11 controller I built years ago that I'm about to put back into service. The next flavor I'd like to get into is the Atmel AVR series. They're inexpensive, powerful, and have nice development tools. Can't beat it. But I'm not going to get rid of either of the other two.

On another topic, I ran face-first into the whole idea of a wire gauge limiting the amount of current you can move through it. I replaced the tether on my ROV with a smaller gauge, more flexible tether. Voila I lost half the available power. DOH! I swapped back to the beefier but stiffer tether.

I think I'm running into a fundamental limitation of small ROVs. A small motor can still draw a lot of current, so the wiring still needs to be beefy. So you're still dragging around a thick tether. So you need beefier motors. So... There's a point of diminishing returns.

In an ideal world I'd put the batteries on-board and not run a powered tether at all. An RS-232 or RS-485 connection would make for an extremely lightweight tether, but it would require brains-on-board as well. At that point I'm out of the micro ROV end of things and would do better to go ahead and build a larger platform. That's for the next ROV project.

Tom

My copy of Dennis Clark's OOPic book finally came in. Yaaay! Only four days later than Amazon thought, but that's one of the prices for living on an island: slow mail. I'm about halfway through it. Good information for anyone using OOPic processors. Lots of info that's not readily available online.

I had to re-arrange the ballast on my mini-ROV. Given the density of stainless, it took a LOT of washers and nuts to make up enough mass for ballast. I swapped it all out for a slab of brass I had lying around, which was drilled to screw into the threaded holes in the motor mount. It wound up being heavier than I needed, so I added more buoyancy. The extra mass makes it a little easier to maneuver, given the overly stiff tether. Still need a better tether...

I'm starting design work on the line follower. Propulsion will be the same, regardless of the rules. So I'm starting with that. I've got several 9kRPM motors lying around I'd like to use. I want to try building an o-ring belt drive system since changing pulley ratios is a simple matter of making a new set of pulleys on the lathe. I'll report on that as it happens.

For the sensor assembly and chassis I've got some 1/2" white nylon sheet (aka $3 cutting board from the local grocery store). It's fairly low friction, so a sensor pad that would double as a "third wheel" on a two-wheel robot makes sense. For that matter, the entire chassis could be cut out of a single piece of nylon. It's not my favorite material to cut (Delrin would be way better) but it's cheap, it's locally available, it's got nice friction characteristics, and it would block ambient light from the line sensors. I'll report on this as it happens, too.

The layout of the line follower sensors will depend heavily on the rules for the competition. Right angle turns would necessitate having sensors forward and outboard that could detect the turn in time for the wheel motors to turn the bot. I won't start on that until the rules have been clarified.

Next Tuesday is the second meeting of our local robot club. In addition to seeing the rules for the competition in Hilo, I'm also excited to see who shows up. School is now in session, so I hope we get a good percentage of the teachers and kids who will be participating in academic competitions this year. Time will tell. This will also be the first time my mini-sumo gets in the ring with another mini-sumo bot. It's funny... I know I'll lose, but I'm excited anyway. At least now I'll have some idea of what I need to do to make it competitive.

Should be fun.

Tom

The ROV is done. I installed a two AA battery holder to supply power, and everything ran fine. Buoyancy was added in the form of foam pipe insulation and zip-ties (though it takes on some water during use, and changes the slight positive buoyancy to slight negative buoyancy... not so good.) Ballast was added by tapping two of the holes in the plexi and installing two #8 screws and enough washers to balance it out. If I do change out the buoyancy for something else, changing ballast to match is dead-easy.

The only real gotcha is the tether: It's too stiff for the mass of the ROV. Since the current levels are so low (3V at 1.6A) I can switch to lighter wire and get more flexibility out of it. This is still in the R&D phase, though I've got an old serial mouse cord I'm going to try out.

The motors are quite strong, and can propel the little thing plenty fast. I can't wait to replace the tether with lighter wire and take it out for a spin in some open water. (Well, more open than the five gallon bucket I'm using.)

I need to step up my efforts on my mini-sumo. I found out there's a competition in Hilo in December, and one of the events is mini-sumo. (The other is line following, which I also want to build a bot for, preferably using the aging 68HC11 MiniBoard I've got on my bench. But I need to see the rules first to see if I'm dealing with right angle turns.)

The big mods to the mini sumo will be side and back-looking IR proximity sensors, and to move the center edge sensor on the front scoop to the back of the bot. This will require a fair bit of re-coding on my part, but should make it survive a little longer in the ring.

Can't wait!

Tom

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