bode plot for servo motor

[edmondchoi]

hi help,

I am new to Bode plot. i understand what it means, but if i apply to real application, i messed up.

for servo system(servo driver to drive motor with typical A,B,C incremental encoder),

what i get from bode plot is a Gain vs frequency graph.

for example  -20db at 200Hz.

i get it means , at 200Hz, where the motor rotate at certain rpm. the  feedback amplitude/ input amplitude < 1. 

but how can feedback varies in Amplitude if it is ABC encoder?  

isn't it only vary in frequency? 

what is the use of amplitude in this case? 

[LennartM]

I don't know what you mean with 'feedback varies in Amplitude if it is ABC encoder'?

The amplitude is a measure how good your system can track your reference.

[edmondchoi]

amplitude in the gain plot = 20log(feedback / input).

if input is frequency (which physically, excite the motor and result in rotation speed), then i get Feedback from encoder. where the encoder is in 5V incremental pulse.

i am confuse that feedback is always 5V?  or the feedback is referring to something else in Servo Motor case?

[LennartM]

If the feedback is always 5V then there is something wrong with your encoder. Or you are missing signals.

But your description is vague. To be able to help we should know which modules you are using and how the program looks like. You referring to the 'Servo Motor case' is this a Labview example or something?