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Miscellaneous Topics

Using Solar Cells

Q: Does anybody have any experience with the solderless type solar cells found in some pocket calculators? I opened up three old calculators, and two had these. If I clip them into aligator clips, they give me voltage, but a little wiggle can interupt the current. How do I get reliable power out of these puppies?

A: There are 3 optional methods:

Physically Connecting Motors to Legs

Q: The gear assembly I've got is the 'guts' of an oven timer. I'm worried about soldering directly to the gear as it my eventually wind up melting the assembly. How does everyone else do it?

A: I use "Dubro Wheel Collars" from a hobby store. Available in different sizes, these cinch down onto shafts and can be soldered to. Makes for easy, removable leg holder-downers.

Winding your own Coils

I've been winding the coils with an old hand driven drill (the crank-type).

Connecting Gearheads to Motors

Q: I just tried to attach a pager motor to the gearbox from the motors described. I was able to remove the very tiny gear from the original motor, but it split when I attempted to force it on the pager motor shaft. To do it, I would have to either enlarge the hole in the gear (very tiny and brittle), or shorten and grind down the diameter of the pager motor shaft. Also very hard. I would then have to mount the pager motor to the rear plate of the gearbox. Any suggestions?

A: Did you try to sweat fit the gear to the shaft? It has been a while--- so I will just outline the idea! The gear should be reamed to be just under size of the shaft at room temperature. Say 0.002 to 0.003 inches. You may want to ask a good machinist about these numbers. I do not have my references handy. Put the motor (with the shaft) into your freezer for an hour. Warm the gear up with your hand or a hairdryer just about the time you remove the mortor from the freezer. You should be able to just hand press fit the gear on the shaft. Leave to get to room temperature.

Removing Pager Weights

Q: I recently aquired several pager motors. The only problem is that they have these pesky little weights on them. Does anyone have any tips on removing the weights?

A: Did you save the motor shaft? Good. Now, grip your next motor BY THE WEIGHT in a vise, and use this shaft & a small hammer to punch the motor down and out of the weight from above.

Servo Motor Disection

Q: How do I utilize servos in a walker?

A: (Excerpted from the Solarbotics "ScoutWalker 1.1" guide)

"Start by removing the bottom plate from the servo, and be careful not to lose the screws (sorry to sound like your mom, but these are dang-near impossible to replace).

Gently pry the servos electronic guts out with a electronic-guts puller tool (if that's not handy, use a screwdriver). The PCB is connected to the servo motor and feedback pot, so expect to see these as you pull the whole assembly out.

Remove the motor from the PCB by desoldering it from the solder mounting pads indicated. As for the leftover PCB and pot, you can feed it to your cat or dog, but not your hampster - that would be silly...

Gently push the motor back into place. Then solder the full length of wire onto the motor and put the bottom plate back on. Then measure only 2" from where the wire leaves the servo and clip off the extra wire.

Repeat the whole thing over again on the other servo, but keep all the remaining wire connected to the servo."

You don't need to remove the stops for most walker applications, as the travel extents are usually less than the 270 degrees the servo stops allow.

fractional DOF

Q: I understand what DOF (degrees of freedom) means but where does 1.25 or 2.5 come from?

A: The fractional degrees of freedom are a single degree of freedom that is then shared among several limbs. As an example: A four legged robot with a motor per leg and a waist motor could be said to have (a) "...four 1-DOF legs and a 1-DOF torso," (b) "...four 1.25-DOF legs."

Scaling BEAM robots

From: "John A. de Vries II" : Very generally speaking, the answer is: length scales linearly (what luck!), area goes up as the square and volume (hence mass) goes up as the cube. It makes for some interesting effects: little robots that work just fine tear themselves apart when made bigger. One of the biggies is glue strength (or solder strength) vs. momentum. Materials used make a big difference.

From: Dave Hrynkiw : Only problem with that is the volume effect. When you double the size of something, it's volume goes up by the cube. That's why tiny ants can lift (what is it?) 100X their mass, but a whale will be crushed to death by it's own weight if lifted out of the water.

Copyleft 1996-1998, Brian O. Bush

Brian O. Bush / bushbo@mediaone.net
Updated: Nov. 21st, 1997