Friday, February 19, 2010

Barbot 2010--DNA Lounge in San Francisco

I took R2D2 down to San Francisco last night for an event at the DNA Lounge called Barbots.  Dave Caulkins put together a bunch of robot builders with entertaining drink delivery contraptions.  R2 spent the night carousing with patrons.  He was very popular.  Chris James went with me and provided much needed droid wrangling.  Thanks Chris.  He also brought along his R2 drink tray (Return of the Jedi--Jabba's barge.)  But we couldn't quite get it to work.  Here are a couple of Chris' pictures:

Lots more pictures at Chris' blog here:

The new drive system worked great.  It's very zippy, maybe too much.  After talking to Chris, some tweaks might be in order.  He suggests that Dimension Engineering might be able to provide some ideas on how to curve out the performance specs on the Saber Tooth speed controller so he slows to stop and accelerates a b it more slowly.  I think I'll pursue that.  But I can also try some different gearing to bring down his top speed.  I've got an 18 tooth gear on the wheel and a 14 on the motor now.  If I dropped that down to a 13 or a twelve on the motor, I'd go from 3.3 mph, to 3.1 mph, to 2.8 mph respectively.  I don't know enough about performance curves and engineering to know if this would actually change the way he starts and stops, or whether it would just cap his top speed.  Seems like if there's a smaller gear on there, he won't jump off the line so quick.  It will be pretty easy to check.  I think I bought a couple of these other gears from The Surplus Center before, or I can order them for $2-3 a piece.  The chains would need to be shortened a bit, but it wouldn't take more than an hour to do the switch given the way the drive is set up.  

Friday, February 5, 2010

Drive System 7

Several cool developments today.  First, I finally got a technique figured out for bending expanded cell PVC board to the diameter curve I want.  With a bunch of trial and error, I was able to heat up some squares of the board with a heat gun.

This is tricky  because the heat gun doesn't have a wide spread, so you've got to keep turning the piece over and trying to get it all heated up evenly.  It will warp and buckle as the front and backside expand at different rates, but once it's all hot, the piece will lay flat and it will be pretty floppy and soft.  This is 1/8" pvc board.  Thicker pieces would be harder to manage, I think.  A lot of the Boba Fett and Jango Fett guys use this same technique to make armor pieces for their costumes.  Once it's evenly heated up, then I quickly wrapped it around a piece of PVC pipe that's 2.85" O.D. or so.

I pulled the edges down around the pipe and let it cool off.  The resulting piece has a nice even curve to it and it's 3 3/16" in diameter on the outside, which is exactly what I want.

The official size of the battery boxes is 3" wide.  I want to go a little bit bigger than that to make the fit over the NPC motors easy, but not so big that it is noticeable.  This should be just right.  So with some cuts and cleaning up, I got these:

I cut a the side wall pieces and did some gluing and got these:

I should be able finish these up in a few days.  Rather than tear my old hoses out of my original battery boxes and the knurled hose fittings, I order some new fittings from a guy on Ebay, and some hoses from Mike Velcheck.  Both very cheap.  That way I'll have another complete set of battery boxes for later.  

I also worked on the drive system to tune it up today.  I found the spec sheet on the Sabertooth 2 x 25 speed controller here at Dimension Engineering.  And I set the dip switches for a linear throttle after following their guidelines.  That seems to have smoothed out and improved the performance of my motors.  

Then I took another look at the front foot shell wheels.  I've always thought that having two caster wheels in there would be better.  They would distribute the weight and keep the caster from crabbing if the shell gets out of level.  And I want to make the ride as even and smooth as possible.  I had two caster wheels that I picked up in the hardware store near Mike Senna's house from R2LA.  But there's not enough room in there for both of them to swing freely all the way around.  They intersect this much with the top one in the down position and the bottom one in the up position:
Now I've always assumed that this kind of configuration would be a no go.  I assumed that if the two wheel intersected, then when you drive it around and made a turn, you could have them bind up on each other, or one would interfere with the turning of the other and they'd start sliding sideways.  But I thought I'd just give it a try to see what happened since it was just a matter of screwing a few screws in to test.  Much to my surprise, they worked fine.  I ran the R2 forward and back, side to side, and tried everything I could to get them to bind up or interfere and there was no problem at all.  First, whenever you change direction, both casters tend to pivot in unison immediately.  You never seem to have one going one direction with the other turning the other way.  Second, if they are colliding, they seem to be aligning and straightening out from it so fast it's not detectable.  So I'm all set with a double wheel in front now.  

I also took out some of the spacers that the wheels attach to--I think about 3/8".  This had and interesting effect.  First it lowered the front end a bit and helped to hide the front wheels, so that's good.  But it looks like the drive motor shells in the back legs are a bit out of level--the front wheel is engaged and the rear wheel is engaged and touching, but not as heavily.  This unweighting a bit of the rear wheel made the skittering problem stop in tight turns.  But the wheels are still engaged and when they need to dig in, they do.  Combine that improvement with the dip switch change on the speed controller and the handling now is just great.  It's exactly what I wanted.  He's responsive and smooth with lots of torque.  So I'm psyched about the drive system.  

Monday, February 1, 2010

Drive System 6--Parts List

Don't know if you can read it, but what I've tried to do here is import a spread sheet from Excel (as a jpeg) that gives the full list of all the parts I used on the new drive train and the costs.  The total cost is around $535.  That's compared to $1k or so for the JAG drives, or about the same for the Senna drive system.  I was able to cut a lot of the costs by getting all the gears, chains, and other stuff for a fraction of the price at The Surplus Center.  A scooter motor set up will cost you less--you could probably get it all done for under $200 if you had the tools and the determination to do it all yourself.  Email me for more details if you need them.  

I've also been looking at some of the documentation for the Sabertooth 2 x 25 speed controller I'm using from Dimension Engineering.  It looks like you can get different sorts of response curves with different dip switch settings.  So I'm going to tinker with those a bit and see if I can get those high torque motors to start up and stop a bit more gently.  Anyone have any experience setting that up with NPC motors and the Sabertooth?