Webot

The diagonal legs need to move together.

In the Turtle bot, this is accomplished by sensors.

My initial idea was to use a fan gear and only run leg at one time on one side.

I had sensors built in to synchronize the action. 

Unfortunately, the sensors were limit switches that added load to the motion.  I could never get the motion synchronized.

"If only I could link the diagonal legs!" 

I tried to do it with a laser cut belt and cogs but it was not working well.

Then I came up with the idler concept seen now with the dual driveshaft concept.

This positively links the motion of the diagonal motors.

The timing to run each diagonal is determined by the half relative and half absolute

code wheel  that is on the shaft between the walls.  This runs through a ITR-9608, 

possibly the least expensive and most forgiving photo sensor in the world. 

The diagonals alternate taking steps for straight forward motion. Depending on the position of the inactive leg, moving the other diagonal continuously will turn in one direction or the other. This is the reason for the relative encoder with about a 10 degree resolution. I have not yet quite determined the best place to stop to turn each way.

Again,  I will drop in some pictures to show all these steps when I get a chance.

Square shaft Round hole

Getting the gears to press fit on the shaft was a 6 month endeavor.  I was using 2 and 3mm round shafts because I wanted to use an off the shelf 0.5M gear to dock with the gear on the motor. 

Unfortunately, if I glued it, it came loose. If I press fit it, it would crack or come loose.

I finally realized that I actually could cut the 0.5M gear on the laser cutter. It just took some seam compensation of 0.1mm to make sure the kerf of the laser cutter did not make the teeth too small.

Once I found this out, I could make square shafts and square holes in the gears. I could make a stepped shaft so it would sit axially in the correct position. I made some clips to hold the gears on the outside of the gearbox although they could use a little optimization.

Now I have very little friction and have completely eliminated the slippage issue.

Pictures to come.

One of the biggest struggles I have had was regarding what kind of power to use. 

To meet the $10 target,  I could not do anything more than provide a battery holder.

This was good in the sense that it  ran 6V which gave the motors a little extra pep.

The downside was the weight. On the recent versions the the battery pack did not 

fit under the bottom drive shaft. I had recently extended the back.

I made a version which would attempt to doc with a BL-4C battery. The Rev 12  acrylic 

pattern has an attempt at holding this battery in place.  I did not have good luck in 

keeping it in place while walking.  Adding the additional battery boost circuit was also cumbersome. In any case, here is a link to the acrylic. https://cad.onshape.com/documents/b10aa98723fb8ce297ce7b2f/w/d4d43d764bc67b14d7e563de/e/fad4e69b9bd54361983d38ae

I will put the DXF in the files as Rev 12.