Re: Re: Servo Control--Torque vs Velocity

Jon Elson,

Nice Tach explanation!

Ballendo

P.S. I wonder the same thing re: Mariss' "pulse multiplied" drive(s)?!

>Velocity servos can use any method to obtain velocity info, including
>deriving it from the encoder. The only problem with this is there are
>no velocity updates between encoder counts. The system is literally
>open-loop between encoder counts, as any change in velocity is
>unknown until the next encoder count comes in. This lowers bandwidth,
>possibly by a LOT. Consider the case of VERY slow motion on one
>axis, perhaps while following a curve almost parallel to another
>axis.
>The encoder counts may only be coming in at a rate of 10 counts/sec.
>It would be impossible to have a real world bandwidth over 5 Hz
>under this condition, even if the servo loop on the computer is being
>processed at 1000 Hz.

>A DC tach is not troubled by this, and provides wideband velocity
>information no matter how slow the axis is moving.