Specification
What is the aim of the FCS ?
Basically, nothing else than conducting the flight to move safely the airplane from a Point A to a point B minimizing the cost (Fuel consumption), minimizing the time and maximizing the passenger comfort… Yep quite a lot of things to do ! CS Design is not something very easy.

Actuators
Actuators act on the system. For a mechanical system actuators apply forces and torques on the system.
For our FCS, actuators are:

• Engines, they generate the thrust
• Ailerons, they allow tilting the aircraft towards the left or right (along the roll axis, that is one of wings is higher than the other one) and thus they allow the aircraft to change its heading)
• Elevator, it allows tilting the aircraft towards the up or down (along the pitch axis, that is the nose will be higher than the queue or the opposite)
• Rudder, it allows tilting the aircraft (along the yaw axis, towards the left or the right in the plane of the aircraft)
• Spoilers, slats and flaps to better act on the lift

Actuators receive their orders from the controller.

Some other things that fall into the actuators category: action mean, levers, muscles.

Sensors
The sensors observe the state of the system. They measure critical parameters. For our FCS, sensors are:

• Gyroscopes, they measure the angular velocities of the aircraft (roll, pitch, yaw rates)
• Accelerometers, they measure linear accelerations.
• Angle of attack probe, it gives the angle between the velocity vector and the pitch attitude angle.
• Pitot tube measures the velocity with respect to the air.
• Barometer allows estimating the altitude through the air pressure.
• GPS to give the aircraft position

The controllers gets information about the system from the sensors.

Some other things that fall into the sensor category: feedback, detectors, observers, measures, references.

Controllers
The controller gets the high level orders from the pilot (where do you want to go, heading, altitude, etc ?). Then, using the control laws it converts the order to low level electric orders sent to the actuators. Next, it checks that the aircraft does what it told it to do. The small discrepancy between the order and the measure is analyzed by the controller which sends an update to the actuators and the control loop goes on. The frequency of the whole process is typically 8 Hz.
The controller is made of computers and softwares. Softwares themselves are made of quite a lot of different parts, the core part being the implementation of control theory algorithms.
For sake of simplicity I am including the estimators, these algorithms that use several data measurements to filter the measurements or compute not measured data, inside the controller.

Some other things that fall into the controller category: guidance, navigation, control,stability, GNC, brain, think, computer, estimators, software, automatics, algorithms, methods, processes, regulator, commander, boss, manager.

Conclusion
In controls, we often speak about loop and feedback. The scheme at the top of this article is the representation of the order in which the parameters are sent and obtained. The pilot enters the parameters into the flight computer. The computer sends the first orders to the actuators. Actuators act on the aircraft. Sensors measure the state of the system and feed them back to the computer. Then, the computer verifies if everything behaves accordingly with what it expects, otherwise (for instance if there were external perturbations or modelling errors) it sends new orders. And the cycle goes on.
Finally, you should basically understand what a control system, controllers, actuators and sensors are. In future posts, I will explain the differences between Guidance, Navigation, and Control. I will give you more example of CS and what their controllers, actuators and sensors are. Some controlled (or not!) systems I have in mind are Economy, Climate, Politics, Society, Company, etc.
In the mean time, you can play the following game. Whatever you see, ask yourself if it is more a controller, a sensor or an actuator and for which CS. Or just do that for the following things: justice, police, lawyer, economist, trader, SEC, president, soldier, medical doctor, clothes, volcano, and ThinkoSphere.
Playing this game, you will quickly understand this motto I often have in mind: “It’s all about controls”!