questions on incremental encoder

[BrianE22]

I'm trying to get a rotary table I bought years ago for my mill running. It uses a Yaskawa servomotor and encoder. The encoder is marked "incremental". From what I can tell from the documentation I've been able to find there is only one output from the encoder. So without quadrature outputs how does the controller know which direction the motor is running? I'm guessing that the controller just assumes if you command it to go backwards, it actually does and it subtracts counts. Does that seem right to you?

 

[3DDave]

It may only be used to generate a tachometer signal. Otherwise it would seem possible to lose a count at each reversal and I think that would not be good.

 

[IRstuff]

Incremental encoders usually come with specific limit markers so that the software knows where zero and the endpoints are, and the incremental counts keep track from thereon. Note that incremental encoders aren't necessarily direction-blind; the signals could have directionality embedded in the rising or falling edges, for example.

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[itsmoked]

Absolutely the controller sends a movement and assumes it goes in the right direction. If it doesn't the user has screwed up the wiring or mounting. There is no reason for them to feedback the direction on that.

Keith Cress
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[BrianE22]

I did find someone at Yaskawa that confirmed that the encoder is only a single output unit. There's no other outputs so I don't think there are "specific limiter markers". "Rising or falling edges" sounds interesting - I'll have to look at the pulse train with a scope. The encoder itself looks to have quite a few chips and a 16 conductor cable coming from it so I was a bit suspicious. Must be this encoder shares parts with other encoders they make.

My background is with servo controlled hydraulics so I'm used to seeing overshoot and oscillations in the control loops. Electrical servos for machine tools are probably a whole different ballgame. The rotary table uses a worm gear which is probably not back-drivable and so the load can't drive the motor.

Isn't it possible that there might be some jitter (3DDave) at some stopped positions? That, I would think would cause the position of the table to vary if the unit was run back and forth all day.