Comment by zwieback | Hacker News Reader

Comment by zwieback | Hacker News Reader

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Our nomenclature for servos in machine controls is a little different, for example a traditional 3phase servo with a 3phase hall sensor is still called "servo" even if it doesn't have an optical encoder. With this setup you can commutate the motor and get smooth motion and variable speed control but not precise position control. We then add on a optical shaft encoder or a optical linear encoder for position control.

In the video he has what looks like a magnetic absolute encoder, I have honestly never seen that in industrial applications although I have seen "absolute" optical encoders that have a backup battery to store the home point, kind of weird imo.

I was surprised when I first encountered servos with just magnetic hall effect sensors but there are actually lots of applications where you want speed control but not accurate position control.

Absolute encoders can be really good for position critical applications that you don't want to re-home all the time. Linear stages, winches - if you don't want to re home it and it needs accurate position control you start looking at an absolute encoder.

Even for speed control, hall effect sensors are kind of a poor way to track position. What is nice about hall effect sensors is that you can use them as a signal to perform brushless commutation in your motor controller, and then also use them as a poor-man's encoder. Very useful if you don't need that much accuracy in your application, but you do need brushless motors for some reason. But one of the first things I would go to as a application engineer was recommend customers get an encoder mounted.

It seems like an absolute encoder would immediately lose its advantages if a reduction gear were used though? Then you would still need to rehome the number of revolutions.

Wouldn't you mount the encoder after the reductor then, where the position actually matters?

Yes, we are discussing servos packaged with an absolute encoder

You can get multi-turn absolute encoders for these applications.

Yeah, we have one application like that, a big gantry that would be a pain to home. Interestingly, the high res absolute encoder is also used to commutate the servo (it's sine) but the teaching of the commutation is a little nerve wracking. Maybe I'm getting old but the whole setup stresses me out

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