My first build and an intro

[stef_d]

Hello everybody.
Stef, from France. Finally posting my first message, after years of silent observations !

I am almost 2 months into my first build, a smallish desktop CNC dremel contraption.
I am using some salvaged Offanengo HY100 steppers (2 phase), and a home made break board/power board based on L297 and L298.
so far, progress is steady but good.
I've got the x and y -axis in place. the gantry moves smootly on the sledges.

First hurdle is a little play in the gantry on the x axis when in the middle of the machine. i put this down to slight flex in the tubes. i plan to fix this by filling them with resin, and a massive 20mm threaded bar. that should hopefully stiffen things up a bit !


Second problem, which is one that really i dont know how to solve is the noise comming from the motors.
as soon as the power is on them, they buzz. and it's pretty loud.
I tried adjusting the L297 chopper frequency above 22KHz, but the noise is still audible, even well above this frequency. I am using a 1nF cap, and a 10K to 220K resistor (pot), so the max theorical freq is well above 140KHz... but it is still annoyingly loud. i can hear the frequency going up as i turn the potentiometer, but ones in a while, it drops down again.
any idea on this one ?

(I tried various oscillator caps (10nF - 0.8nF), i tried different decoupling on the 5V supply, i tried adjusting the current by tuning Vref... actually, above a fairly high Vref, the motor noise stops... but that value of Vref gives me a current that's way too high.. my motor almost burnt last night !)

[nophead00]

It might be a beat frequency between two drives. Try running just one axis to see if the noise goes away. I.e. if you mix two ultrasonic frequencies the result is the sum and difference and the difference may be audible.

[stef_d]

Originally Posted by nophead00 It might be a beat frequency between two drives. Try running just one axis to see if the noise goes away. I.e. if you mix two ultrasonic frequencies the result is the sum and difference and the difference may be audible.

you tell me !
(i am an RF engineer, by the way !)
no it's definitely not cross modulation, as i get the noise from just one motor/power board.
it's probably some sub-harmonics, you are right, but i cant seem to be able to suppress it.

[stef_d]

for info, i just came accross this:
http://www.st.com/stonline/products/...re/an/1675.pdf

it's an application note that describes some of the issues related to the 297/298N pair.
Including the audio noise.
and some fix.
it's mostly parasitic spikes on Vsens fooling the chopper flip-flop. This is why a higher Vref stops the noise.
the result is sub harmonic of the oscillator frequency, in the audio range.
I'll try to get my head around some fix, but it looks like a better decoupling/grounding strategy is required.