i don't think that reversing the wires would do something good.. i don't want the motor to work in reverse. i want that each time the gcode ask Z motor to go down, then after that, a command ask Z motor to go back from the same height..
if you look at standard start gcode for 3d printer, we work with absolute coordinate... so when you slice your model, the gcode ask to the printer to go at coordinate of the 1st layer, coordinate of the second layer, coordinate of the third layer.. etc etc..
if i put a "gcode at start of each layer" that you can do in slic3r, i can only ask to the printer to go back at layer one height.. i don'T know the command to ask the z motor to go back for [layer height] mm .
answer my own question
i think if i add this gcode to "each layer gcode" of RH, i would work..
G91: switch to relative positioning so all further commands are relative to the current position (that's what we want)
G1 F150: slow movement rate down to 2.5mm/sec for Z axis (must be done BEFORE moving, otherwise will interpolate the slowdown and the movement together, grinding the Z motor)
G1 Z-[layer_height]: move Z down by layer height, leaving others alone
need to test it after work.. not sure if i can use '-' to ask for a reverse movement..