XPWMShield V1.1: ...??

According to UPS tracking my Advanced Circuits order was delivered yesterday at 12:47pm and signed by a fellow named "WILSON." It's true, I could see the truck out my window! Regrettably, somewhere along the 100ft path from that truck to the front desk of South Campus Commons 1, the package was vaporized. It was never logged and nobody knows where it is. All I can say now is WIILLLSSSOOOOOOOOOOOOOOONNNNN!! And some four letter words which I already shared with the elevator and don't need to be shared here. I'll wait a few days before re-ordering, ugh.

TOBL: Teaser

What happened to TOBL?? Well it's alive, better than alive. In an effort to actually finish the thing before returning to school I focused more on the building and less on the documenting. It was close but I had to take it back with me to school:

What's up TSA?

I used 1-1/2" Dubro tail wheels for the drivetrain, mainly because they have ridiculous grip and aluminum hubs. To my surprise, the smallest outer diameter of the hubs was ~0.401". To support the steel standoffs I bored them out to 3/8", so that's like 0.013" wall thickness or less at the very center. We're good though, the two halves didn't torque themselves apart on the lathe. The hub diameter just barely supported the gear-mounting holes as well, but they don't post these dimensions online so not my fault.

This just looks goofy.

Small snack for the mill.

After making those aluminum plates I thought lasering some out of acrylic would be cooler. It was good milling experience still, I guess? It took a few tries to adjust from the gears' theoretical to actual pitch diameter -in the end an extra 0.02" radius. The servo-gears are also not properly supported i.e.they are cantilevered off of the spline gear. So they wobble, just a little. Enough though that the servos stalled and stripped some teeth internally. This was actually a good wakeup call because the stronger HS-82MG motor wasn't designed with the HS-81 plastic gears in mind. New gears are on the way, enough to bring two HS-81s to life.

Not even including the set that actually made it to production.

Look for test footage next post! (waiting on parts in the meantime :/)

XPWMShield V1.0

After a long wait (it says 2010 on the board because it was sent out last year!) XPWMShield has arrived from AdvancedCircuits. And with a bag of microwave popcorn in the packaging? Awesome company.

XPWMShield V1.0

This revision of the board lived a short life. I only got it yesterday, soldered on all the components today, and just broke it five minutes ago. Before that though here's V1.0 in all its glory, running off a 1300mAh, 7.4V, 2-cell, LiPo battery I got off HobbyKing:

She lives.

Need a better camera.

Just as I was going to test the wireless, one of the leads from the battery touched the bottom of the board, contacted a pin/via and blew up a trace coming out of the voltage regulator. Great. But at least the $34 Arduino Nano and $24 XBee are alright.

As it turns out the diode I put in between the XBee DOUT and Arduino RX was disabling radio communication. I put it there so that the software could be updated wirelessly without having to remove the XBee. So, I removed the diode and soldered a wire in its place -the same for the busted trace.

Ghetto wiring aside I can't complain, it works really well. With some minor revisions it's time to order more for future projects!

TOBL: Quick Software Update

There was an issue controlling both servos at once. With one fader value being sent over to the Arduino one servo would work fine. With both, that same servo would tweak out. After isolating the error to the Processing side, it turns out the float defining the fader values was preventing the second fader from registering. With that cleared up the code finally works as intended*!

* The interface needs some work to be more user friendly. For example, a deadband at the neutral position so you don't have to land the fader on the exact pixel that corresponds to STOP. Also, there's a point - about halfway through the range - where there's no noticeable change in speed. Playing with the fader ranges in the touchOSC editor should fix that. My only other complaint is a little lag in feedback due to the speed of wifi. On the other hand, it can be controlled from anywhere in range of the network aka college campus, so we'll let that slide.

TOBL: HS-82MG-360

A key ingredient for this project is a solid drive system. I opted to take the servo route as opposed to straight motors. Servos come pre-geared-down, they're easy to mount, and easy to control. What's not to love? Well they're perhaps less interesting from a controls standpoint, and in certain applications they scream rookie. If you've been following this project though, you know they're not trivial to work with, especially micro servos.

I've encountered plenty of problems with these little guys. Finally though, after buying two HS-81s, two HS-82MGs, and three HS-81 gearsets, then breaking two spline gears and a potentiometer, I have created the perfect (for my purposes) servo. Behold, the HS-82MG-360:

They said it couldn't be done. Seriously:

That's right it is spinning 360 degrees proving that impossibility is no match for being both stubborn and patient. If you're really interested in building one of these miniature torque monsters then you should see this post for details. The quick summary of how to make an HS-82MG-360 is as follows:

1. Replace the spline gear with the plastic one from an HS-81 gearset.

2. Cut out the first mechanical restrictor, found in the top of the gearcase with a pocket knife.

3. Cut out the second mechanical restrictor, found in the potentiometer. This holds the gears in place so be careful not to cut too much.

4. Cut the three wires connecting the potentiometer to the control board.

5. Solder small low-watt resistors to the loose wires keeping in mind you should:

a. Pick resistors of the same value such that their sum is the value indicated on the potentiometer. In this case, the pot read 5k and used two 3kohm resistors, so it doesn't have to be exact. I'm pretty sure the control board is just looking for a value somewhere in the designated vicinity.

b. Connect them such that the two outside wires initially from the potentiometer are individually connected to the central wire. Red-Yellow and Green-Yellow in this case.

6. Cover the exposed leads of the resistors with your choice of hot glue, heat shrink, or electrical tape.

7. Reassemble and test.

Control is handled the same way as before only this time the delay corresponds to a speed instead of a an angle. With this particular servo that's 600usec to 2400usec, 1500usec is neutral. Until next time...

TOBL: XPWMShield V1.0

From the people who brought you Gerber Baby Food, I give you Gerber files:

Gerber minus the drill holes.

EAGLE.

I sent out XPWMShield V1.0 and the final final design hasn't changed much since the final design. Mainly, the EAGLE Drc had me make the vias a bit bigger and space out a few traces so, with those minor edits, my fingers are crossed that it will work. I learned the secret of Advanced Circuits, which is to type "Student" into the additional information box at the bottom to order Qty one. Maybe that's not a secret, but I was all ready to order the minimum four and blow ~$150. Also, Santa came through on the Weller so look out, SMD Soldering.

TOBL: Back in Business

I'm back in Massachusetts finally! To my great surprise I arrived at Logan Airport with all my luggage. Part of me was anticipating a little note from the TSA to the effect of: "Err sorry...we had to blow up your bag." Not the case though, my suitcase, featuring a square foot of aluminum, batteries, and lot's of suspect wires made it on and off the plane. So did a loaded gun though, so maybe next time I'll get more creative. Anyways, the University of Maryland was kind enough to grace us with a ~five week winter recess. It's time to exploit that and get back into this project.

Cutting out the wheel plates on the mill.

The finished pieces.

Not a perfect job on the band saw, but I took my sweet time on the mill so hopefully all the critical dimensions are in tact, which are the three outermost holes that align the gears. I'll be going into the shop as often as possible to machine the plates for the servos, the board mounts, and the rods which string all the plates together. The gears, wheels, and other little mechanical pieces are on order too.

Now, the board, or XPWMShield as it has been dubbed ("XPeeWMShield" would have been cool but I'm not that immature). It's finally done and time to send it out to Advanced Circuits! I'm pretty excited...and a little anxious since this will be my first custom board. It should be a good wireless platform for future projects. As you can tell I very much value compact-ness. This project would have been a whole mess easier to do at a bigger scale, but I like the idea of keeping things small. Final dimensions are 1.8"x3.2", which is is pretty close to the target 2"x3". I can deal with the extra length, more importantly its narrow enough to stay out of harms way when rolling.

Click to enlarge.

Fortunately I discovered that the old (HS-81) spline gears can replace those of the HS-82MG, so that means no custom gear-making. I ordered a few sets of that gearset to finish off the servos and that's all for now. I'll do my best to keep this pace through the holidays...that new soldering iron would help, Santa ;)