Drive System 5

I've done a lot over the weekend. After the bench test of the first motor, I worked for a while on getting it mounted into the shell and onto the leg. That went pretty well--there were some hassles taking things apart and putting them back together to get everything just right. But it went in fine.  I've got the R2 up on a custom dolly I built for him and laying down on the work bench.  So this is looking up into the foot shells.  You can see that the clearances are really tight.  I had to trim down the overall length of the side plates a bit, probably because I used .25" thick stock to make the shells.


The next day I went to work assembling the other drive frame.  Since I had worked out all the details on the first one, this one went to together much quicker.

You can see that I've modified the sidewall on the top plate here (the whole thing is upside down) so that the bracket for the motor will sit down flush to the side plate.  This helped too because I needed to move the motor in tighter to the shell to get it to fit inside a battery box.  I did some work on this one, put keys into the keyways, cut chains to fit on this side and the other side, and so on.  Tricky work, but rewarding.  I've really enjoyed this part of the build.  

So this motor went into the shell and onto the leg pretty quickly.  There was a lot of trimming and adjusting of the shells to get it all to sit right, but they went in.  

All that crud on the foot shell is where the weathering ran down behind the battery boxes when I painted it on.  Had a brief scare when I hooked these up at first.  I didn't get one of these wire nuts on tight, and I had one of the motors wired to work the other direction from the other one, so the connection between the wires in the wire nut got really hot and burned up the wire nut.  I haven't seen that before.  But no harm was done.  I replaced the nut and wired the motor in reverse of this one and they run in tandem now.  You can see how fat and tall this thing is to fit inside the battery box.  Here too:

I also discovered when I got these in that they lift the R2 a bit higher than my old ones.  I think my old clearance was about .5" I did that deliberately, not wanting a lot of wheel exposed.  But these show quite a bit:

In fact, I had to jack up my front caster with a piece of 3/4" plywood to get it flush.  That's too much I think, but fixing it will be hard.  There is room in the drive frames for the wheel to run further up.  That is, there's .5" of clearance or so between the top of the wheel and the bottom of the frame top plate.  So the wheel could be raised in the frame effectively lowering the R2 by the same amount.  That would mean cutting 4 new drive frame side plates and redrilling all the holes in them.  Then I'd have to take these completely apart and reassemble them with the new side plates.  But making the new plates wouldn't be too hard.  

So I did quite a bit of drive testing here.  The motors are a dramatic difference. First, they have amazing amounts of torque.  Nothing will stop them.   I even was able to just drive him up the curb--this is ill advised, but I wanted to see if he could pull the load.  No problem.

That was pretty cool.  And the responsiveness is just great.  You touch the throttle and he goes, pull to the left or right and he turns instantly and proportionally to the stick.  With the scooter motors the response was awful and I just couldn't control what he did that well.  I'd have to throw the stick over all the way to get him to turn and then over compensate to straighten him out.  He also stops immediately now.  Chris James had commented on this with his NPC motors.  When you take the power off of these, the motor stop completely with no coasting.  So his stops are kind of abrupt.  It can be dealt with by driving him more gently with the radio.  I'll need to get used to it.  He also can drive backwards with great control now and turn on a dime.  

I am seeing the skittering problem in really tight turns that people had warned about.  With the four wheel drive system, when you do a tight turn, one of the wheels will not be tracking or turning at the right speed because the radius of the circle it is driving isn't the same as the radius for the other wheel.  So it will break loose and jump sideways a bit causing some jerking.  That's annoying.  Not sure what I'd do about it.  One possibility would be to cut up some new side plates for a one wheel system instead of the two wheel ones, and maybe put a small caster up front for stability, and raise the wheel in the frame to lower the R2. That would take care of a couple of problems.  The result would be close to a JAG system, but with chains instead of belts.  BTW, these motors and the chain/gear system are much quieter than the old scooter motors.  The noise isn't an issue at all.  I'm very happy about that.  And with the excellent level of control I've got now, I won't worry about taking him into a crowded environment.  

The battery box problem is next.  I think I'll try making some slightly oversized boxes to save me all the trouble of hacking and sanding and modifying my old ones or the motors.  My old boxes are a bit narrow.  So I got a piece of 3" PVC pipe yesterday for a measly $2.50.  It's 3.5" O.D.  I think I'll cut it in half down the length and then take .25" of an inch off so that the overall width of the half piece is 3.25" or so.  Then I'll build the battery boxes around that.  I'll post pictures when I do it.  I think I'll make the boxes a little taller than called for too so that the tops of the motors clear on the inside.  They will be big, but I think it won't be noticeable to anyone, probably even builders.  Should take me too long to whip those up, although I will have to paint them and then get the battery harness and the braided hoses and knurled hose fittings off of my old ones and transfer them.  

Here a picture of Sam (my three year old) checking out the new rig:

And finally here's some video of the new motors in action.  It's pretty hard to drive with the RC radio and take the movie at the same time, and I'm not used to the motors yet, but you get the idea. 

Drive System 4

Did a bunch of work on the drive system today.  I finally had all the parts in hand, so I could move forward without stalling.  Most of the day was spent modifying, cutting, trimming, drilling, and filing all the pieces to get them all to fit together right.  I got the foot shells off of the R2 and checked the fit.  It turns out that there's not much width in there, so I needed to reduce the width of the frames.  That meant moving the side plates to the inside of the lip instead of the outside of the lip on the top plate.  The wheels have wider hubs than the treads, so I used the band saw and sander to trim those down to have a narrower profile:

Once the wheels were trimmed down, I could get the axles and bushing in there and start cutting the chains and getting all the gears set up.  This all went pretty well.  It looks like I won't need an idle gear on this side.  The chain is just right for snugness here.
Getting the other side with the motor bracket, gear, chain and motor all cut and sized right was tricky.  I used some carriage head bolts on the inside there for a low profile. That way the bolt heads holding the bracket on don't interfere with the motor gear.

There are several little things to do here.  The inserts need to be tightened up and secured with a small screw.  The axles need to be cut to a miminal length.  And some of the bolts haven't been put in there.  It took me a little while to get everything lined up and get smooth rolling action on the wheels.  But it seems to work well now.

Very beefy.  Notice that I cut the lip off of the top plate on the frame so that the motor bracket would sit in there flush to the side plate.  To get the motor to line up where it should go up inside the battery boxes, I needed to get it shifted in closer to the frame.  More on that in a minute.

I still haven't cut the outside axle off here.  And I used my old drive frames to map the holes on top where the threaded rods from the legs come down.  So far it looks like all of this fits pretty well into the foot shells and onto the legs.  Some other views:

I had to do some work on the foot shell for the bracket to fit through the hole into the battery box.  Cut the holes much bigger.

The battery boxes are going to be a problem.  With the bracket, the motors stick out a lot.  The outside width of the motors is about  2.5 inches.  My battery boxes are made with PVC pipe that's about 2.9 inches wide on the outside.  So the interior space with .25" side walls is 2.4.  That is, the motors won't really fit inside the battery boxes.  For now I'm going to get these motors all assembled and installed without the battery boxes.  I can think of a couple of possible fixes for the battery boxes.  1) I could cut out the whole back side of these, and then add a pieces of 1/8" PVC cell board across the back to increase the interior volume.  Then I could build up the curved sections to blend in with the box with body putty or something.  That would take quite a bit of work.  2)  I could make some new battery boxes with the next size up of PVC pipe.  The problem is that the next size up is 3.5" O.D.  The plans call for the battery boxes to be 3" wide exactly, so that would be a pretty big fudge.  You'd be able to see how fat those were right away.  But the inside diameter would then be 3.042" which would be plenty to house the NPC motors.  I'll think about it some more.  I don't think this will be too hard to solve, but I will probable have to effectively rebuild my battery boxes.  Fortunately, since I have been selling those, I have several of them around and I can build them quickly.

And here's a video of the bench test of the motor all mounting into the drive system.  Works well.  I am going to try to take the half link out of the chain on the motor gear.

Drive System 3

I snatched a bit of time today to do some work.  Almost all of the my supplies have arrived now.  On the Senna design, the wheel is fixed to the axle and the gears are fixed to the axle to drive the wheel.  So the connection between the axle and the frame needs to tolerate some metal on metal friction or have a bearing or something.  I have these two options:

Those are bearings on the left with a .5" inside bore and an outside diameter of 1.25" or so (Can't remember.)  I picked them up at a local surplus yard (Blue Collar Supply) for $.50 each.  On the right are some impregnated bushings from McMaster-Carr.  I think these are the same ones Mike Senna recommends.  The inside bore is .5" and the outside diameter is .75"  I think I'd prefer to have the axles riding in a bearing that could turn.  But I decided that I would start with the smaller ones.  That way I can bore out the frame plates to fit, and if I don't like them, I can go bigger to the bearings.  Plus, I don't have an effective way to cut/drill an accurate hole for the bearings to mount in.
So I set up a jig on my drill press that would let me drill all eight holes in the side plates as accurately and consistently as possible.  It's important to get the holes aligned across the frames exactly right, or the wheels will sit crooked and not track right.  Earlier, after I rough cut all the trapezoidal side plates, I clamped them all together and sanded all the edges on the belt sander.  That got all four side clean, and it got the size off all of them as close as I could get them.  That helps for cuts and drilling later.  It's always worth it when you're creating multiple pieces of something to set up a jig or fence or some other method to make all the cuts as uniform as possible.  That makes fitting and every step thereafter much easier and it will produce more accurate results.  Here's the jig on the drill press.  Just a couple of blocks clamped down to hold the plates in exactly the right place:

I'm using the step drill bit here because it's so effective at drilling these larger holes.  This way, the steps each just have to take out 1/8" of material or so at a time instead of cutting the hole thing at once.  I just discovered these recently and I use them all the time now.  Don't buy one of the outrageously expensive ones at a tool store--$50-$75!!  Harbor Freight has always got some for a few bucks.  Get a couple and when it wears out, throw it out.

Drilling here gave me some trouble.  The bit was binding and heating up when I got down deep.  I have heard people say that you need to lubricate metal when you are cutting it so that the blade or bit slides.  It reduces friction, and the cutting edge does all the work.  I was skeptical about this, but I tried it here on the bit and it worked great.  The bit just slide right through these pieces once I had a good shot of WD-40 on the bit and the piece.  And it didn't catch on fire, like I thought it would.  Valuable lesson about working with metal.

That gave me some clean, uniform holes.  I think they were no more than a millimeter off across the different pieces so the jig worked pretty well:

A bit more grinding on the inside of these holes with the dremel tool, and they were ready to take the bushings for the axles.

In order to get the fat wheels into the frames, it looks like I am going to have to assemble the side plates on the outside of the lip on the top channel.
I'll have to countersink the holes so those bolt heads sit down flush.  But you can see it come together here.  That'll be a quick fix when I get some more time.  And I'll need to cut a slot down the top of the top plate to receive the threaded rods coming down from the legs.  More on that later.

Next I rough cut a couple of six inch pieces of axle.  That should be enough to accommodate the gears, the bushings, and the axle collars.  There's a bit of loose space inside between the wheels and the bushings here.  But I think a single .5" washer on either side of the wheel will be just right.

Next up, I'll get the gears and motor brackets all figured out and start assembling the rest of the pieces.  The plan is to make this a four wheel drive system.  So the motors will drive one wheel, and then that wheel will drive the other wheel with a gear and chain.  Check the pictures of his system in the Yahoo Group for comparison.  More soon.

Drive System 2

I've had several false starts on the drive system upgrade.  At one point I thought I would be able to attach a gear to my old wheels and use my old drive frames and add the NPC motors in, but after I got the parts from The Surplus Center I could see that that wouldn't work.  So I reconsidered.


I've been really impressed with Mike Senna's drive system ever since he developed it.  So I've resolved to give it a go with some modifications.  I searched around Sacramento a bit for wheels and found a place that
sold me these:

The one's that Mike lists in his plan are also Colson caster wheels, but his are 5" x 1.5".  My distributor didn't have those, but he had these for about $10 a piece.  They are 5" x 2".  What that actually means is that the hubs are 2" across, but the treads are about 1.5".  And I think the tread width on the 5" x 1.5" is 1.25".  So mine are .25" wider.  I've had a picture in my head since I started this project of wide fat wheels with lots of torque--or even tank treads--that would conquer any terrain.  So I opted for the wide ones here.  We'll see if that's realistic.  The wide wheels may cause trouble in tight turns.  The wide wheels won't work on Senna's plans, however, because he's using a piece of rectangular tubular aluminum to fashion the frames, and I think the width on those is 2" on the outside, and .125 or .187 on the side walls.  So there's only about 1.75" clearance on the inside. But I have a solution for that in a minute.

I had some 1/8" aluminum plate and some 3" wide channel left over from before.  So I have cut a couple of pieces of the channel.  I think the lip on these is 1". Then I have drilled and counter sunk holes for bolts on both sides.

Next I have cut 4 trapezoidal side plates.  I'm following the Senna planes on outside dimensions.  (I've mentioned it before, but the Senna plans are part of the "Files" attachments to the R2 Builders Yahoo group.  Go down to Senna files, then look in the Senna Drivetrain folder.)  The side plates are going to bolt onto the channel top plates above to make the frame.

  Even after building my first set of drives, I had apprehension about cutting all of this aluminum.  I got tendonitis from the hacksaw before.  So I talked to a friend at a machine shop and he said to use a fine toothed jigsaw blade in my jigsaw that is intended for soft metals.  I was very skeptical.  But h was right; the jigsaw cuts this 1/8" plate just like wood.  Very easy to work with.

So I clamped these side plates into position on the U channel to mark and drill the holes for mounting in exactly the right spot, drilled them out, and then the bolts went through no problem.  My bolts were a bit long and protruding into the space inside, so I trimmed them off with the Dremel tool.

Here's the result.  I've put the plates on the inside but they can be easily be mounted on the outside to make a little more room.  So there's a bit more versatility with the interior width here than working with the rectangular tubing.  The drawback is that the bolts can work lose.  So I'm going to add some locktite to them on final assembly or superglue.  When the rest of my materials get shipped here next week, I'll start laying out the axle locations, add some bolts across the bottom for bracing, and so on.  More details then.  I'm excited to get those NPC motors in there.

R2D2 in Sacramento

Chris James and I spent the afternoon at the California Museum here in Sacramento.


It turns out that there’s been a Lucasfilm R2D2 sitting here in Sacramento for a couple of months on display, and I didn’t know about it. The California Museum has had a set of science fiction movie costumes on display calle—out of this world. Most of them come from the collection of a private collector. Several pieces were authentic, some weren’t. The stormtrooper helmet and blaster were great to see up close. The helmet, cape, robe, and lightsaber on the Darth Vader are from the films. The placard said that the Gorn mask from the original series Star Trek episode was the real thing, but I can’t believe it. That episode was filmed 44 years ago—the mask is in perfect shape with no wear or fading. And some of the details on the neck weren’t right. By contrast, look at how trashed the Stormtrooper helmet is from the late 1970s.

The R2D2 and C3PO were actually part of another exhibit honoring George Lucas in the California Hall of Fame. So I think they will be there for a while. The R2 has a number of odd things on it. The shoulder mounts that attach the legs to the body are much too narrow so the legs are rubbing on the barrel. The legs look short, and the insides of the horseshoes are smooth. Chris James and I debated it at great length—longer that you would have thought possible—and concluded that it must be an amalgam of parts from Lucasfilm put together to make this static, 2 legged display model. It didn’t appear that the dome could turn, and there were no foot motors, and the lights probably didn’t work. Nevertheless, it was very cool to get to see it up close like this.

I suspect that they were making these horseshoes and couldn't be bothered to machine the grooves into the insides.  And Chris says there's something different about the layout on this shoulder hub on the outer ring.  But I haven't checked that yet.

Here you can see how close the legs are mounted to the body.  I don't know what's going on with that.  I don't recall seeing that variation on any of the screen used R2s, but my memory is bad for all of these little details.  I don't know if any of this droid was every screen used.  I really doubt it.

This was a great item to see in person.  I think this was an Empire Strikes Back stunt stormtrooper helmet.  It looks to be the same one reviewed on StarWarsHelmets.com here:  http://www.starwarshelmets.com/original-ESB-stormtrooper-armor-helmets.htm  I know because it had that big glued on patch that said Star Wars on the back.  Someone stuck that on there at some point and it's never been removed.










And here's the suspicious Gorn.  You can see the perfect condition and paint job.  If the original was made of latex, there's no way it would look like this 44 years later.  In fact, I can't imagine any material that would last that long.  But it's a cool item anyway.  I hope the collector who loaned all of these to the museum didn't pay a fortune for it.

I've got lots more pictures from every angle of the R2 and the Stormtrooper helmet on flickr here:
http://www.flickr.com/photos/30353667@N08/

Holding

I'm waiting on parts from The Surplus Center to get here for the upgraded drive system. Then I'll document the full process. I've been looking at Grant Imahara's (of Mythbuster fame)book about building combat robots: Kickin Bot. Dumb name, but the book is a great resource for an R2 builder. Very well written and researched. He gives all the necessary background and lots of clear explanations of the basics. You can pick it up cheap on Amazon.

Drive System Upgrade

I've been researching and planning a new drive system for the R2. Right now, I've got some E150 Razor scooter motors in there, with 5" caster wheels. I used the chains and gears from the scooter. That system has gotten me around pretty well, but I think I pushed one of the motors too hard early on and its performance is pretty poor now. I can't make quick tight turns, especially with the left motor, and he won't spin in a small radius. Overall the motors just aren't that responsive and don't have a lot of torque. So it's a bit like driving an old worn out car around.

The motors of choice for builders in recent years have become the NPC 2212s. These are in the JAG drive systems, and they are what Mike Senna uses in his drive system (See the Senna files under Drive System in the Yahoo Groups page.)
The Surplus Center has some versions of these motors much cheaper. NPC wants $150, The Surplus Center has them for $50.

Several club members have investigated and here's the short story: the SC motors have a very short shaft on them, maybe 1/2". Apparently, however, it is possible to get this short shaft to work on a Senna style drive. That could save you another $200 overall. It is possible to use a press and some machine shop tools to make a new longer drive shaft, press out the old one, and put the new one in. But the consensus seems to be that this is difficult to do yourself without the right tools, and expensive if you get a shop to do it.

I bought a set of NPC motors. They are hard to get ahold of, and their website doesn't work. But if you call them persistently, they'll help. And they will give R2 Builders a discount to $135 if you mention the club, although this hasn't worked for everyone who tries. I got it and the motors look great.

I've seriously considered just building one of those two systems. Mike's has four wheel drive so that drive system is really rugged and doesn't run into any trouble getting across door thresholds or even going on grass. That's really appealing. I also found that if you get as many matching parts as you can from The Surplus Center you can save $200-$300 on the price that Mike projects in his Bill of Materials. I like the idea of building it up out of parts I collect or build more than buying a ready made system. Even if I had the cash to get the system already made, I wouldn't prefer that.

The only draw back that I can see, and this isn't much of one, for the 4 wheel system is that when you do tight turns, there's no differential gear to proportion the power to the different wheels, so one set of wheels will turn at a slightly wrong rate since they are tracking through a different radius circle, and they will chatter or slide laterally across the ground. I don't think it's a big problem, and as Victor Franco said to me it's a perfectly acceptable trade off for the killer drive system. I find that pretty compelling. And I may still go to a Senna type drive.

The JAG system has been very reliable, lots of builders use it, and it only has one wheel, so there's no chatter problem. And it has a belt drive. My experience with scooter motors is that the chains and sprockets are noisy. You can hear the chains spinning on the gear. So having a belt drive is pretty appealing too. I have spent a lot of time lately trying to learn about gear, sprockets, and timing pulleys and trying to decide which sort of system would be the best to build. Some of the good websites for parts for these things, besides The Surplus Center, are McMaster-Carr, and sdp-si.com. I have been searching through these sites and others looking for just the right timing gear pulley to do what I want, but I can't find them. A timing gear pulley will have grooves that accept a grooved drive belt. I'd like to use them on a single wheel drive system and get the best of both systems.

That search led me to the question of just what gears do I need to run for the R2. I don't know much about this aspect of engineering, so I've been trying to educate myself.

The NPC motors turn at 285 rpm at top speed. That's pretty fast. If you hooked those straight up to the wheels, it would be 4.2 mph. Here's how I figured that out, and I hope it's right. The standard wheel is 5" in diameter. Nothing much bigger will fit in the shells. So that times pi gives you 15.7" for the circumference. Turning at 285 rpms, means 4474 inches per minute. Multiplied by 60 gives you 268470 inches per hour. Divided by 12 inches per foot, equals 22372 feet per hour. And divide that by 5,280 feet in a mile, equals 4.2 mph. I figure a very fast walk is about 4 mph.

So that suggests that the speed needs to be geared down some. If there is a gear on the motor shaft and a gear on the wheel and a chain or belt connecting them, then the way to figure out the speed is a function of the ratio of the sizes of those gears. That is, if there is an 18 tooth gear on the motor and a 10 tooth gear on the wheel, then for every full revolution of the motor, the wheel will make 1.8 revolutions. This set up would speed up the wheel faster than the motor. What I need is to slow it down.

The JAG system has a 20/32 gearing. 20 on the motor and 32 on the wheel. So the wheel turns .625 of a full revolution for a turn of the motor. That makes 285 rpm into 178 rpm on the wheels at about 2.6 mph. The Senna drive runs at 18/11 for 2.6 mph, or 18/14 for 3.3 mph.

So I figure a speed somewhere in the range of 2.6 to 3.3 mph will be good. The problem for the timing gears and belts idea is that I can't find just the right size timing gears to give me that gearing. The limitation is the .5" axle. There's a .5" shaft on the NPC drive and I want to have one on my wheels too. But there's only a certain range of gears that are typically produced for a .5" bore. That range starts up high, so if I got them I'd have these huge honking gears on the wheel and motor.

And I think that as gear size goes up, the necessary torque to turn the gear increases. So stalls are more likely for the motor if you get close to its stall torque. But I'm no engineer. No doubt it would really help to talk to one at this point.

Long story short, I'd like to get the 30/22 or 18/14 gear ratio with gears and belts, but I can't find the stock parts for that. I could have some custom made up, but that would be a lot more money and time to wait. And I don't have enough confidence at this point to launch into that.

But there are cheap readily available chains and sprockets for these gear ratios at The Surplus Center. They have sprockets with keyed .5" shafts for $2 and $3 for example. That's a deal. I can afford to experiment a bit at those kinds of costs. They also have different size chains: #25, #35, #40, and so on. I think #35 will be suitable. That's cheap from them too.

So I think I can put a whole drive system including NPC motors together using chains and sprockets from SC for less than $500. I'll have an itemized list up here shortly. Compare that to $1000 or so for for the Senna or JAG systems.

I'm going to try to reuse the 5" caster wheels I have, and use the aluminum channel and sheet that I picked up here locally. The wheels were $13 a piece, and I have spent maybe $40 on aluminum. And then I'm going to buy parts from the Surplus Center and maybe McMaster Carr, and construct these myself. I'll document the build and give a parts list, prices and sources as I settle on final options.