Give me a big enough lever...

The real servos finally arrived! here is the Hi-Tech HS-805BB Mega Giant Scale Servo being tested. I simply attached one of the many great horns and plates that it comes with to my fish-line spool, using its own screw and voilà! 19.8 / 24.7 kg*cm (4.8V/6V) of torque! It is temporarily held in place with straps as you can see because as you can imagine the square holes of the Vex plates don't line up with anything but their own stuff (I promised I would stop bitching about it!)

For the rest of the controls I am using the HS-645MG UltraTorque Metal Gear Servos that have 7.7 / 9.6 kg*cm (4.8V/6V) of torque, very much on the safe side.

On the left of the picture you see Vex's red optical encoder which I planned to use to determine the position of the continuous rotation motors that lift the arms, but again, not even the extremely versatile Make controller could recognize the simple signal. I am sure it can be done but why waste time. I ordered a set of high-res 10 turn potentiometers that are not only more precise but can "hold" the last position even when the power is turned off.

Command and Control

The recent integration of the Make controller with Flash, its scripting language and image processing capabilities, solves many of the problems I was dealing with, from the choice of microcontroller to the controlling software.

The program currently reads the position of the performer on stage and even the distance from head to ground, all with only one cheap webcam as an input device. There is so much information that can be acquired through image analysis that effectively eliminates the need for a variety of sensors that I had planned to use.

The only limitation is the amount of servo ports in the controller. I will have to daisy-chain at least two, to handle all the servos I think I need.

The servo that supports the weight of the marionette most of the time needs much higher torque than the ones I have. I have not calculated the minimum torque yet, but need to do that ASAP to order it and start testing the software that Philip and I are designing. I am very excited and happy with the progress so far. At first I had my doubts that the solution he proposed would work, but now I see it can, and in a very simple and elegant way.

The two sizes of spools we currently use (5 and 8 cm diameter) happen to be just perfect to provide most of the travel that I need without using continuous rotation motors instead of standard servos. This simplifies programing. We found out by testing a variety of "identical" modified servo motors that the speed of rotation, given equal parameters, is quite different, which implies writing routines to address each one of them to synchronize, a royal pain!

The fish-line spools with their bushings in place

The only continuous rotation motors I need are for the arms since they travel a much greater distance.

What was I thinking...

Just to finish the pulley rant, I created a template to drill the round wood pieces that I bought (99c/bag) at the local craft store. They seemed perfectly round in the bag...



Much to my surprise (am I new? or what!) they were far from round, I found out when I tried to fit them in my template. To make it short, I sanded them enough to fit, drilled and glued them together. As I was venting my frustration for not finding proper pulleys, my wife who works next to me in the studio, asked why I did not use the spools that she uses for her beading or the ones that I use for stringing (fish line) the marionette.

DUH!

I must be very tired, been sleeping 4 hours max for months now. why didn't I think of the spools which were in front of me? So, here, I have a steady supply of the perfect size/weight spools.


I only needed to create bushings to adapt to the servos, real servos that is, not the modified ones by Vex which of course have a "proprietary" square shaft that only fits their kit stuff, I still cannot get over their lack of foresight and integration with the rest of the world. So much for Open Anything! Dean Kamen, you know much better than that!

I created the bushings with an aluminum rod that happened to have the right diameter.
I drilled the shaft hole to attach the Hi-Tech servos I will probably use, notched them to hold the adhesive better and epoxied them to the spools.