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help - ringing at 5V of uC board of servo driver

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Full Member level 4
Jan 10, 2009
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Istanbul , Turkey
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hello forum ,

I have designed a servo driver for brushless servo motor
**broken link removed** <-- the photo is here

the uC board ( STM32 ) is optically isolated from IPM of power board
and it has its own power supply ( LM2575 )

I have finished software and I could run and speed control the servo motor

however I have following problem
there is ringing at the 5V supply of uC board

this ringing makes the encoder counter ( STM32 timer ) miscount 1 - 2 times / minute sporadically when I run the motor

I have painted an osciloscope image at the attachment

as you can see there is a corelation between PWM frequency and ringing frequency ( both 10 KHz )


can you help me how can I eliminate this ringing ?

I'd begin with figuring out the current loop for the servo
drive currents. Since it's (by intent) isolated, you should
be able to separately power the servo driver and its
motor load; divorce the grounds, power, all of it. It may
be that there is some ground loop issue.

You should also not assume that the ringing you see, is
the ringing perceived by the uC. That can easily be a
'scope probe ground-location artifact unless your probe
is directly across the uC and no other probe is attached
elsewhere - ground offset transients on one probe can
corrupt others through the common chassis.

Without a detailed schematic, it is very difficult to suggest solutions. But as dick_freebird suggests, the ringing you see may be "fake," and is just there because of EMI coupling into the probe.

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