Gaetan -

You might want to place a small 'dead zone' around the range of the converted potentiometer signal intended to be zero acceleration. Implemented in firmware with the provision for varying its width, dead zone can eliminate sudden changes between acceleration and deceleration, and allow the speed command to to be a constant value after the acceleration signal has entered the dead zone. At the edges of the dead zone, force its output to begin at zero for smooth operation. Do not let the output jump suddenly as the result will be a sudden acceleration.

Cheers,

Bruce