Sometimes you want the zero position of an actuator and the position it starts in to be different. For example, the zero position of one of mine happens to intersect with a solid object, and I don't want to move the actuator because then it wouldn't match the real world. To get around this, I have added a start_position parameter to ModelActuator. Others may also find this useful.
The attached patch includes half of my previous actuator patch in an attempt to be standalone.