1. Home
  2. General Robotics Forum
  3. idea for a low-cost alternative to servos

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:

Please let me know what you think.

Best,

- Joe

Reply to
Joe Strout
Loading thread data ...

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

Reply to
John Nagle

formatting link

John Nagle wrote:

Reply to
rue_mohr

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

Reply to
Joe Strout

the important bit is the servo loop controller code, you can use it to control whatever hardware you want.

dan

Reply to
rue_mohr

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

Reply to
Joe Strout

interesting, someone with the 'other piece'....

dan

Joe Strout wrote:

Reply to
rue_mohr

PolyTech Forum website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.