View Full Version : Making a cheaper Poppy and other questions

11-11-2013, 06:25 PM
First time poster looking for advice. I run an annual Halloween animatronics show for our neighborhood association. I have 5 "Mars Attacks" aliens that entertain the crowd with various skits. While their heads have X, Y, Z & jaw action (plus two-axis eyes for some of them), the arm & leg movements are very simple puppet mechanisms. I'd like to upgrade them and am fascinated by the Poppy Project's body (http://www.poppy-project.org).

However, that's *way* too expensive for me. Most of the cost is in the Dynamixel servos they use (21x MX-28s, 2x MX-64 and 2x AX-12). I've never used the Dynamixel line before (but have seven years of controlling hobby servos using various micro-controllers). My question to the experienced group: How can I dramatically reduce the cost of the Poppy, given I don't need it's full capabilities in my application?

My animatronics do not have to be self-standing and don't really have much of a payload (the head is about 22 ounces, though, but again, I can support the body via shoulder wires or by pinning the pelvis to a support rod). I really just need to create "motorized marionettes". I'm willing to modify their CAD models to accommodate different servos (once they publish the models, that is). This could open the door to AX-12 / AX-18s. Also, I've found the SpringRC SR-518 knock-off of the AX-18s on ebay (e.g.: http://bit.ly/1j0nDPL ). Has anyone used these servos? They are supposed to be "Dynamixel compatible". Could this be a cheap option?

I could also save a ton of money by just using RX-24s and wouldn't need to make any changes to the parts files. I could save a little money by going with RX-28s all around. What's the downside of RX-28s vs MX-28s? Specs are mostly similar -- just looks like comms differences, which shouldn't matter in my application.

I do have to keep in mind the availability of controllers and related "posing/recording" software. I want to easily be able to program certain motions and play them back in a recorded sequence. Does the Artbotix-M work with the RX series out of the box? Any other guidance/suggestions? I really don't know much about the "best" controller/software combos, as this particular world is quite new to me.

Thanks in advance for any help you're willing to give!

11-11-2013, 08:12 PM
Main mechanical difference: The RX series has a 1024 quanta resolution over a 300 degree range of motion; the MX series has a 4096 quanta resolution over a full 360 degree range of motion and can also serve as (expensive) wheel motors.

Another option is to take your "mechanical marionettes" description literally, and put some nylon string on some spools and winding/unwinding those spools with DC gearmotors and limit switches to prevent over-winding. If you reset by going to the limit switch, you can then dead reckon movement pretty well; even better if you use gearmotors with encoders (and a controller capable of reading them.)

A pair of Pololu 25D or 37D gear motors with encoders and a 2x5A RoboClaw controller is a lot cheaper than two equivalent strength/speed Dynamixels...

11-12-2013, 09:00 PM
If you support the body and are not actually using the arms/legs to lift anything, you can probably replace most of the servo's with ax-12's which would save you a lot of money. It all depends on how heavy the joints you are moving at that point. For example, the head tilt joint or the shoulder joint would probably be seeing the most load if the central body was supporting all the weight. You would need to do the calculations to determine if the ax-12 is appropriate or not torque wise (They typically can run at about 1/3 of their maximum torque continuously without overheating).