idea for a low-cost alternative to servos

Servos really aren't the ideal shape for making a robotic joint, as was recently discussed in another thread. You really want something with a
through-shaft, like the Tamiya Worm Gear Box HE. But then you have the problem of providing an absolute position encoder, without doubling the cost of the joint.
I had an idea last night for how to do this using a quadrature encoder plus one additional optical sensor. No, it's not a reference notch, but it's similar: it tells you whether you are ahead of or behind the neutral position. That way, when your robot starts up, it can safely move all its joints to the neutral position. From that point on, you can use the quadrature encoder for position control. And those things are available both tiny and cheap these days (in the $1 range).
Read more about it here: <http://www.strout.net/info/robotics/ideas/cheap-servo-alternative.html>
Please let me know what you think.
Best, - Joe
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Joe Strout wrote:

That's a good idea. We did something like that on our DARPA Grand Challenge vehicle. The steering servo system had an encoder on the motor and a pot on the steering shaft, so that we could center the steering at powerup using the pot, then set the zero for the encoder. We did it that way because you can get hollow shaft pots designed for steering shafts (they're used in car stability control systems) but a switch and cam would have worked as well.
The switch has the nice property that you know at power up which way you can safely run the motor, so you can go for home position at high speed. Most industrial systems use limit switches, and home by slowly running in one direction until a limit switch trips. So startup is slow.
                John Nagle
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
http://www.openservo.org
John Nagle wrote:

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

This is a link to a group that's making replacement electronics for existing servos. That has little relevance to the topic of this thread, which is about replacing the motors, gears, and encoders.
Thanks, - Joe
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Joe Strout wrote:

the important bit is the servo loop controller code, you can use it to control whatever hardware you want.
dan
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Well, I have to respectfully disagree. Controller code is easy; it's getting the power, speed, and weight you need out of the mechanics, without spending a fortune on it, that's the hard part.
Best, - Joe
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
interesting, someone with the 'other piece'....
dan
Joe Strout wrote:

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.