I have a Gumstix/Robostix in a biped with 21 servos.  The Robostix controls two servos, the other 19 are attached to a PicoPic servo controller, see http://www.picobotics.com/PicoPic.html for details.  I actually have an SD21 but haven't used it yet - the PicoPic is smaller and I found that it fits better.   I've also got a hexapod with the same servo controller, but no Gumstix (it uses bluetooth to connect w/ my laptop).

It's worth investigating the serial protocol of whatever controller you want to buy - find out how many bytes per command, then multiply that by the number of channels you need, and make sure the servo controller can accept data fast enough.  Most will accept 115200bps, which is plenty, but some have limits as low as 9600.  Most servos only expect 50-60hz, so there's no point going higher than that, but my hexapod's 'brain' only works at about 15-20hz and the motion is noticeably notchy.  In that particular case the serial line isn't the limiting factor, but with a 9600bps servo controller, it would be.  The SD21's I2C interface is probably fast enough, but I haven't done the math.

On 10/14/06, tc6 <tc6@free.fr> wrote:
Hello all,

I plan to build a small hexapod robot using Gumstix and Robostix cards,
and I would like to know if someone has already tried to control several
servomotors (at least 18) using those two cards. Is it possible to
control them using all available ports of Robostix card or would it be
better to use a specific i2c servo controller card (like SD21 card for

Any advice would be appreciated a lot.

Best regards,

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Nate Waddoups
Redmond WA USA
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