in the left button
i added code on action

if(x<=0){
x=0;
}
else {
x=x-increment;
}

initialization
i added
  increment = dxm/(1/unit);

it work beautifully, and i even added scaffolds to suggest the relationship

if ( error == 0 ) {
l_x="d="+l_x;
}
else if ( vs*unit+(-error*unit) >= 1 ){
l_x="d=" + dvalue((int)(x/dxm-1),unit)+ "+" +dvalue(vs*unit,unit)+ "+(" + dvalue(-error*unit,(unit))+ ") = " + l_x;
// l_x="d=" + dvalue((int)(x/dxm-1),unit)+ "+" +dvalue(vs*unit,unit)+ "+(" + dvalue(-error*unit,(unit)) + ")+ "=" ;
}
else if ( vs*unit+(-error*unit) <= 0 ){
l_x="d=" + dvalue((int)(x/dxm+1),unit)+ "+" +dvalue(vs*unit,unit)+ "+(" + dvalue(-error*unit,(unit))+ ") = " + l_x;
}
else {
    l_x="d="+l_x;
    }

it is a work of art and definitely a great java for sense making!

thanks!
http://www.phy.ntnu.edu.tw/ntnujava/index.php?topic=684.msg2452#msg2452
to see this great applet!