call a controller method from javascript ruby on rails

I'm coding a scrabble application and I want to call an update method when the player drops a tile so I change the position in the database

 function update_position(ev, posx, posy)
  {

    new Ajax.Request('/tiles_controller/update', {
    method: 'post',
    parameters: {
posx: posx
posy: posy}
                 });}`

and my controller looks like

  def update
    @tile = Tile.find(params[:id])
 @tile.update_attributes(params[:tile])
      end
    end
  end

I know the controller doesn't work I'm newbie coding in rails and I'm not sure how to pass the parameters from the javascript.

Please help