Actuator

In gliders and small airplanes the pilot’s stick movements are sent directly by cable to the control surfaces. His muscle strength is sufficient to overcome the small control moments. In larger airplanes, however, mechanical or electrical devices must boost the human power. Moreover, if you take the man out of the loop, as for instance in missiles, and replace him by low-voltage autopilot signals, considerable amplification and power input are required to move the surfaces. The devices that deliver that boost are called actuators.

An actuator is a device that actualizes steering inputs to motivators. These moti­vators can be aileron, elevator, and rudder, or could be gimbaled nozzles of rockets. Even reaction jets are grouped into this category. We distinguish accordingly be­tween actuators for aerodynamic control, thrust vector control (TVC), and reaction jet control systems (RCS). Hydraulics, pneumatics, or electromehanical devices can accomplish the power amplification. Power consumption, size, and cost are important selection criteria.

For six-DoF simulations we are mostly concerned with the accurate modeling of the dynamic characteristics of these devices. Needless to say that actuator com­panies, like Chandler Evans, invest great resources in presenting to the customer accurate performance specifications. These include mathematical models that can be used in system simulations for performance studies. The models are of high order and include all known nonlinear effects.

My purpose is less ambitious. I want to show you simple models, which nonethe­less convey the salient characteristics of actuators. Most likely, you will have to model actuators for aerodynamic and thrust vector control. The more esoteric RCS are used for precision steering in exo-atmospheric vehicles, as direct force motivators. Their response is so fast that static modeling is sufficient.