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 /
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Joe Strout wrote:

Most worm drives won't back-drive, so you're stuck with dumb positioning mode. Can't do force feedback.
If you want to get really clever, though, look up "series elastic actuators". That's a gear drive followed by a spring, with a sensor to report how much the spring is compressed. It's a trick for making worm drives back driveable by software means. You don't get any useful energy recovery; the spring is too stiff.
If you want to get really really clever, the right answer is probably a gear drive followed by a spring followed by a fast electrically-actuated brake, so you can do things like jump by setting the brake, winding up the spring, and releasing the brake. Real legged running robots need such mechanisms.
None of this advanced stuff is going to be easy to build out of standard R/C parts, but for those of you into machine shop work, stereolithography, and mechatronics, those are directions to explore.
Something to think about if you're doing R&D in this area is using a stereolithography method like SLA or SLS. SLS can be used to make nylon parts, and SLA can now make ABS parts. Costs are high, but cost is proportional to object size, so small parts aren't too bad. We're still talking about $100-$200 per run, but part complexity doesn't matter. (That's your problem; you have to send in a 3D CAD file.)
                John Nagle
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Not entirely dumb: the central controller would have access to the position of each joint. So if something's blocking it, it would know. (It could perhaps also have access to the current consumption of each motor, which would be another clue when one has stalled.)
But I agree you couldn't do manual positioning of the robot as a way of programming poses, which is a neat trick -- not one I'm willing to pay $90 per servo for, though.

That IS a neat idea. It might also put a little forgiveness into the system, so it doesn't break when my toddler tries to force it.

That sounds cool. A bit more than I want to bite off right now, but maybe something to shoot for.

Right; it also reinforces my urge to go with these gearboxes instead of standard servos.
Thanks, - Joe
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Joe Strout wrote:

These motors are a lot bigger than servos, so it goes your robot would be big, too. However, no idea shouldn't be tried. Maybe a larger Robo-One biped without the sticker shock is what people are looking for.
-- Gordon
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Well, it looks to me like the motor + gearbox is about 50% longer than a servo box, but it's also narrower. So it would tend to produce a humanoid with longer, slightly thinner (and more humanlike, IMHO) limbs. Probably wouldn't make the best Robo-One competitor, but as you say, maybe it should be tried anyway.
Best, - Joe
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Joe:
Joe Strout wrote:

[much snippage]
I've always had a soft spot for the Tamiya Worm Gear Box. I once designed a board for it, but I never populated the board with chips and debugged it. Here's the URL:
<http://gramlich.net/projects/robobricks/motorscan/index.html
I'd design it quite differently now.

Actually, I was able to design a quadrature encoder that fits inside of a Tamiya twin gear box. Here's the URL:
<http://gramlich.net/projects/robobricks/twingearsensorleft/index.html <http://gramlich.net/projects/robobricks/twingearsensorright/index.html
The concept is that the PCB fits inside of the gearbox. One of the gears has a Black and White spoke pattern painted onto it using nail polish. (Yes, Black and White nail polish can be purchased at the local drug store! I have no idea who wears it, but I'm sure the drug store wouldn't stock it unless somebody actually bought the stuff. Go figure.)
Add a microcontroller and a LS7377 (my favorite quad shaft encoder chip) and you have a full shaft servo that that can hold a position without conusuming power and is quite inexpensive.
-Wayne
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Have you considered "10 turn" potentiometers. Another is a magnetic pickup on the wheel/axle. I understand thats how ABS systems in vehicles sense wheel spin/lock. Something similar might work if scaled down. Not alot of fine motor control just rough position guessing.

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Polytechforum.com is a website by engineers for engineers. It is not affiliated with any of manufacturers or vendors discussed here. All logos and trade names are the property of their respective owners.