Monday, December 27, 2010

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...

Saturday, December 25, 2010

TOBL: XPWMShield V1.0

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

Gerber minus the drill holes.


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.

Monday, December 20, 2010

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 ;)