Gaetan, I'm a little confused in regards to your application. We have a potentionmeter that you have told us sets the acceleration. What sets the speed?

Assuming your motor is from 0 to 6000rpm which equates to your pwm value of 0 to 255. If your pot is set to maximum I gather you want the motor to accelerate from 0 to 6000 over 10 seconds, or 3000 to 6000 rpm in 5 seconds (assuming linear accel). Is this what you want?

Re-read what I posted previously, at no point did I suggest you go to a 16 bit pwm - 8 bit pwm is most likely more than adequate for your application. What I believe you need is an integrator function to give you the accel/decel feature. Time to re-read your high school maths!

If you post a snippet of your code, collectively we may be able to lead you in the right direction.

I spent some time in 2000 staying in Weert. Cold it was!