Worm Gearbox as substitute for servo

I've been investigating the possibility of using a worm gearbox with
encoder as a more economical substitute for servos, even for things like
Robo-One humanoids.  It sounds nutty, but it just might work!

Let's start with the Tamiya Worm Gear Box HE [1] (about $10).  This can
be assembled with two gear ratios, 216:1 and 336:1.  It comes with a
Mabuchi RE-260 motor, which gives us roughly:

  No gearing:    15 g cm of torque at 8300 RPM
  216:1 gear:  3240 g cm of torque at 38.4 RPM
  336:1 gear:  5040 g cm of torque at 24.7 RPM

So we're over 5 kg cm of torque right out of the box.  But Mabuchi makes
a variety of stronger motors [2] which are only slightly larger; I'm
sure we could use them anyway since the motor doesn't really sit inside
this gearbox.  There's the RE-280, which is a bit faster and stronger,
and then there's the RE-360, which is over twice as strong:

  No gearing:    32 g cm of torque at 8321 RPM
  216:1 gear:  6912 g cm of torque at 38.5 RPM
  336:1 gear: 10752 g cm of torque at 24.8 RPM

Now we've got over 10 kg cm of torque, which is what I've been told is
necessary for at least some of the joints in a humanoid robot (probably
the legs and hips in particular).  Compare this to the Hitec HS-5945MG
digital servo, which generates 9200 g cm of torque, and has a speed
equivalent to 62.5 RPM.  Yes, that's over twice as fast -- but it also
costs $90.

I haven't found a source yet for the RE-360, but the RE-260 costs about
$2, and the RE-280 (the next step down from the RE-360) is $2.25, so I
doubt the RE-360 costs more than $5 or so.  We're at about $15 for the
gearbox and better motor.  (Note that this motor was the result of a
pretty quick search; more searching might turn up a faster one, to make
up that lost speed without losing torque.)

Of course, we still need an encoder scheme; this can be done with a
quadrature encoder and an additional optical interrupt switch [3], which
add a couple more bucks to the total.

It's also possible that you could ditch the quadrature encoder and use
another simple opto switch, under the assumption that you know which way
you're driving the motor, as at least one guy has done [4].  Since this
uses a worm gear, it can't be driven from the outside, and if any errors
do creep in, you can clear them every time you cross your zero point
(which you detect via the other opto switch).

You would of course still need a motor controller.  Something custom to
this application might be appropriate, especially if you're planning a
bot with 20 of these things.

So to me, it looks like for under $25 or so, you could get performance
comparable to a $90 servo, and have the added benefit of a very
convenient form factor and a complete through-shaft for making nice
balanced joints.  (In fact, I suspect that you could get by with less
torque because you're driving a joint axle, instead of only driving one
side of a joint as most servos do.)

What do you think?  Am I off my rocker?

Cheers,
- Joe

[1] http://www.tamiyausa.com/product/item.php?product-idr004
[2]
http://homepage.ntlworld.com/edward.matthews/ddmbc_files/motors/mabuchi.h
tm
[3]
http://www.strout.net/info/robotics/ideas/cheap-servo-alternative.html
[4] http://www.instructables.com/id/EOWNKDH2JMEQHO9934/