Older blog entries for tbenedict (starting at number 3)

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

X
Share this page