Older blog entries for tbenedict (starting at number 11)

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.


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!


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.


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.


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!


Another big thanks to Botnerd for certifying me as a Journeyman. I appreciate it.

Update on the ROV: I fabricated the plexiglass motor mount, got the motors fitted, wired up the tether and the control panel, and would've finished except it was late (10pm) I had to get up early for work (6am) and I had no #8 screws to mount the terminal strip for power distribution (RATS!)

The frame fabrication went great. Seriously low-tech, but here's the deal: I drew the ROV up in Rhino3D (very nice 3D modeling and CAD package), converted the frame to a flat 2D shape, and printed it 1:1. The printout was stuck to a sheet of plexiglass using Super77 adhesive. The trick is to spray the paper with a thin coat and let it sit long enough to get tacky. The idea is to wind up with the consistancy of a Post-It. Once stuck on the plexi it was time to cut.

Holes came first. I used normal twist drills. With light pressure at the bottom of the hole you can avoid most chip-out problems, but overheating was an issue. At one point I made some spade drills for drilling plexi, but didn't have one in this size. All the holes were drilled to take a #8-32 tap. I didn't wind up tapping any, but that way if I want secure threaded anchor points for ballast, I've got them.

The shape itself was cut out on a scroll saw. I used too aggressive a blade, so I got some chipping. A finer skip-tooth blade with reversed teeth on the bottom would've been better. I even have them, so I'm kicking myself. But not hard. It's a prototype.

The 3D forming was done using a heat gun and a wooden form. My "form" consisted of a bunch of 2x4 and 2x2 lumber and some quickclamps. Cheesy, but I got nice 90-degree bends in the plexi. Only problem is the wood surface isn't smooth and now I have wood grain patterns on my plexi. Ah well.

The frame itself came out great and the motors zip-tied right into place. Wiring the umbilical was a snap, and with all the holes in the plexi there were plenty of places to anchor the motor mount into the ROV frame.

Only thing left is power. Then it's time to test!


A big thanks to marev for certifying me as an Apprentice and to steve for certifying me as a Journeyman. I appreciate your trust in me.

With the certification from steve I should probably move my ROV build out of my blog and into a project, but the thing's almost finished. I'm going to go ahead and finish it up in my blog, and put my next robot into a project.

The props came in and were mounted last night. The only real gotcha: the motors have a 2mm shaft and the props have a 1/8" bore. Making Delrin shaft adapters took about five minutes, and the newly-propellered motors run great, even in water.

Rather than focus on the ROV, I'd like to make a plug for having a home shop that includes at least a lathe and a drill press. I could try to make a case of having tools pay for themselves, but I won't. I haven't made money off my tools yet, and I've easily spent more on them than any other single hobby including photography.

The real pay-off for having good tools is the ease with which you can do other things. Making those shaft adapters without a lathe would've been painful, at best. With a lathe it was so easy I didn't even slow down. To date that lathe has made ROV parts, robot parts, rocket parts, optics mounts, toys for the kids in the neighborhood, custom tooling, even a steam engine. The list is too long to write out. In monetary terms I've never made back the $750 I spent on it six years ago. But I have to think it's payed for itself time and time again in what my shop can now do.

If you don't have a lathe and a drill press in your shop, read up on them. Price them. See if maybe they're something to consider. The online communities for home shop machinists are very active and on the whole extremely supportive. Even with zero machining knowlege, I've seen people tool up and get going in ways that are simply amazing.

I wouldn't give them up for the world.


P.S. And if you thought a lathe was fun, you're going to have a blast with a mill!

Still waiting on the props (perhaps today!) and my copy of Dennis Clark's OOPic book. But I did get a chance to do some more work on the ROV and on my Mark III.

I ran the motors while hooked up to a digital power supply and RTD for measuring temperature. Driving at 3.0V it drew 0.38A no-load and ran at 34C (base temp 25C). Stalled at 3.0V it peaked out the power supply, drawing 2.18V at 2.2V. Basically the power supply clamped and wouldn't let it go dead-short.

At 6.0V it drew 0.52A and heated up to 46C pretty quickly. My guess is closer to 50C, but I didn't leave the motor running past five minutes. Since stalling at 3V peaked out the limits on the supply, I didn't try stalling at 6V.

With the motor submerged in water, 3.0V drew 1.8A with no measurable increase in temperature (it's heat-sinked to the water, so I expected this.) A 6.0V it drew 2.2A in the water.

What I take this to mean: Motors draw current like nuts when wet. So I can't put this ROV under microprocessor control very easily. For the money I'd spend on a beefy enough H-bridge to handle the current, I could just as well build dry enclosures for the motors and run off the 1A H-bridges I've got on both my micros. I have yet to try these motors with props in the water, but the requirements are only going to go up, not down.

Running at 3V still produced plenty of torque. So long as the props don't totally stall the motors, I'm going to under-drive them and see if I can prolong the motors' lives a little longer.

On the Mark III OOPic, I scaveneged more stuff and made more diagnostic tools. I had some micro switches removed from some mice that I soldered directly to some headers to plug into the Mark III Sensor Board. The jacks are arranged just like the Handyboard digital I/O jacks (which also match the ones on the Miniboard and the 6.270 board). I have a Miniboard, so this is handy. These went in the toolbox with the diagnostic LEDs I wired up to headers. Makes troubleshooting a breeze.

I'm sending out the announcement today for our club's second meeting. This lights the fire under my butt to get this ROV finished. I'd like to demonstrate it at this club meeting and see who's interested in building one. I also need to build out a mini sumo ring so if anyone else brings their sumo bot we can throw them in and let them go at it.


No new work on the ROV, but some thoughts in case anyone is reading this and planning to duplicate what I'm doing:

First, I'm beginning to think the idea of using 30kRPM motors is a mistake. I still don't have the props, so I can't test what they do under load, but I'm beginning to suspect it'll overload the motors. 10:1 gearboxes to bring them down to 3kRPM and 10x torque (minus geartrain losses) would be good. But if I'm going to get wrapped around the axle about this, I'd rather start over with gearmotors from Lynxmotion and build pressure compensated enclosures. Which defeats the whole purpose of making it low-cost and easy to build. So I'll run with what I have and replace them with motors wound for torque when they finally burn out.

On the framework, I wound up scrapping the cable clips. Oddly enough, lining up the holes in the plexi to mate up with the cable clips was problematic. I made a new motor mount plate from 1/8" plexi and just drilled some holes so I could zip-tie it to the frame. This worked out better and made for a cleaner arrangement, believe it or not.

But that got me thinking... Why not just build the entire ROV frame out of 1/8" plexi? At the scale I'm working at (3" on a side, give or take) it's plenty rigid. And since I'm already looking at making yet another motor mount plate with a 90 degree bend in it to support the vertical motor, I'm seriously considering starting over on the entire framework.

Here's the argument in favor of re-doing the whole thing in plexi: Plexiglass is cheap. An 8"x8" sheet at the hardware store is much less than half of what I paid for all the drip irrigation fittings and poly hose. And I can make several ROV frames out of a single 8"x8" sheet. Cost is low.

Next, you can cut plexi with a jeweler's saw, a coping saw, or a scroll saw. (I've got my scroll saw set up to take jeweler's saw blades, so it can cut anything from 3/4" plywood down to thin sheet silver.) Almost anyone can work the stuff.

Next, hot-forming plexi is entirely doable in the home shop. So the whole thing can be laid out in a single sheet and bent to shape over a form. And since all the bends are 90 degree bends, the form can be a simple block.

Finally, thanks to modern printers and print drivers, making sawcut parts from CAD drawings is dead-easy. Print 1:1, spray some Super 77 adhesive on the back of the printout (LIGHTLY!) let it cure for a couple of minutes until tacky, and stick it on the plexi sheet (with the protective film left on!) Drill all the drill holes, sawcut to the lines, file the edges clean, and peel the protective film off the plexi. The drawing comes off with the film and voila, you have a made-to-order plexiglass part for your robot or ROV.

And before leaving this topic, most laser cutting services will cut plexi. So if the cost of a scroll saw or the thought of using a jeweler's saw frame for hours is too daunting, consider having it cut by places like Pololu, who charges by the inch.

Since this is a demonstration ROV, I'll use my scroll saw. If it looks like the local club wants to adopt it for a club build, we can mass-produce the plexi frames at a laser cutting service and package them with the parts OR use it as an opportunity to teach people to use a scroll saw and a drill press.

No props = no new fabrication tonight. But I'll probably spend the evening playing with layouts for a plexi frame and try to come up with a new design based around that idea.

Wish I could post pictures. I need to start a new area on my web page.


The frame of the submin ROV is complete. The 1/4" drip irrigation hardware worked out well. You can heat-form the tubing to some extent, but it does want to relax back into its original shape, so you have to tweak things to get them to go into alignment and stay there.

I got some 1/4" cable clips to attach the motor platform to the frame, and some 3/4" cable clips to hold the motors. Way high tech, I know. But it works and it was cheap.

Oddly enough, the frame itself was rather costly. There are 12 T-connectors in the frame. At $0.50 apiece that's $6 simply for the T-connectors. The clips were another $2.50 per package (one for each size). The 50' roll of poly hose was another $8. Things stacked up fast, and now it's looking like the frame for a PVC-based ROV may be close to the same cost as the one I built for the submin ROV. Go figure. That $30 figure has already slipped, and I haven't picked up the DPDT switches I'll need to drive the motors. Time to scrounge.

The motors still have their brass pinions on them. I'll press them off when the propellers get here from Tower Hobbies. Until then the next task is to make the motor platform and get the motors semi-mounted. More on that later.


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