Older blog entries for tbenedict (starting at number 5)

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.

Tom

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.

Tom

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.

Tom

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.

Tom

I'm starting a new project: a sub-miniature ROV. I can't call it a nano ROV since I'm using Mabuchi 130 motors on it, and not something smaller like a pager motor. But it's certainly smaller than the PVC-framed ROVs most people are making.

The idea behind the project is simple: I want to make something for under $30 US that anyone else with some pocket change could make. I got the idea after visiting the Robert Gordon University web site and seeing the RGU ROV kit picture on this page:

http://www.rgu.ac.uk/eng/robotics/page.cfm?pge=14748

They use larger motors than I'm looking at, and I'll probably use styrofoam fishing floats instead of film cans, but the framework idea is brilliant. It's made from 1/4" irrigation parts from the hardware store.

I ordered the propellers from Tower Hobbies for $1.05 apiece (I ordered six, but only plan to use three). The motors are 1st Gen Stage 2 XMods motors from Radio Shack. They're being discontinued and are discounted. Basically it's four 130-frame sized 30kRPM 6v motors for ten bucks.

I'd describe the rest of it, but there's really not much to it once the motors and props are mounted. The one real catch is the props have a 1/8" bore and the motors have a 2mm shaft. I've got a lathe at home and plenty of Delrin rod so making shaft adapters is a no-brainer. I can't offer an easy source for them to people who read this, but for the robot club in town I'll offer sets of three to anyone willing to build one of these beasties.

Initially I was looking at building dry enclosures for the motors, or building oil-filled pressure compensated enclosures. But there's a fellow in the UK who did saltwater testing on bare Mabuchi motors, and found that if you're careful to spray them out with WD-40 to displace the water after use, they really don't wear all that fast. For the sake of simplicity and the sake of not scaring people off from doing this build, I'm going with bare motors as well, just like the RGU ROV.

As I said earlier, this is just a first step. It's a demonstration project to get people in our local club into the idea of building ROVs (and other robots!) on the cheap. I don't plan on mounting any cameras or lights on the thing. I expect the biggest adventure it'll ever go on will be in a small aquarium (without the fish) or a bathtub.

But once I find enough people willing to build one of these I'm going to spring the idea of a larger ROV on them. Bilge pump motors are sealed, and the impeller assemblies can be cut away to expose the motor shaft for mounting larger propellers on. There's a wealth of information available on ROVs at this scale, so building a platform to take cameras, lights, manipulators, thermometers, depth gauges, etc. should be entirely doable.

I'll post more once the ROV starts to take shape.

Tom

I recently gutted an old Microsoft serial mouse and pulled out two discrete ALPS shaft encoders. (All the guts of the mouse were made by ALPS, but I was after the encoders.) They're three-wire devices, and appear to be mechanical contact rotary encoders. Twenty minutes of soldering gave them leads, jacks for plugging them into the Mark III, and away I went.

Using the encoders on the Mark III is dead-simple. the oQEncode object pretty much takes care of everything. You tell it what two I/O pins you're using and it sets up a counter and does the rest.

I haven't put them to use yet (there are some mounting concerns I haven't worked through, and it's apparent there aren't even bushings to keep the shaft concentric with the encoder body) but as far as the controller's concerned they're good to go.

This is one thing I like about the OOPic: It makes scavenging dead-easy since it seems like whatever you plug in can be read, used, and put into service.

Tom

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