How exactly do (hobby) servo motors work?

Gordon McComb wrote:


But how easily do they back-drive?
Measure the minimum force that will back-drive the servo. That gives you a lower limit on the force that can be sensed. If your multi-legged robot's legs won't back-drive under the weight of the robot, force feedback to control contact isn't going to work.
Ball bearings are a good thing.
                John Nagle
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John Nagle wrote:

For the test I posted no backdriving was involved. A static shaft against a load is a common application for force feedback. It is not a given that the force or weight to equal or exceed the backdrive torque to get a useful reading. All you need is a resistance to the drive of the motor -- which can come from a leg touching the ground and lifting the robot -- and you have an increase in current consumption. Therefore you have something to read as force feedback.
I suppose if the robot is light enough, and the gear raio of the servo is high enough, that there will be very little current demand either way, including lifting the robot and making it walk. I think this is an obvious misapplication of the concept, though, plus somewhat unlikely. Most of the time we're hitting against the upper torque limits of these servos for this type of application.
-- Gordon
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Gordon McComb wrote:

[snip]
Hence the name "squirrel cage motor".
:-/
--
Paul Hovnanian mailto: snipped-for-privacy@Hovnanian.com
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