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An autopilot is a mechanical, electrical, or hydraulic system used to guide an aerial vehicle without assistance from a human being. It also maintains the orientation of the plane by monitoring the relevant flight data from inertial measurement instruments and then using that data to cause corrective actions. In this project an attempt has been made to design, implement and develop an autopilot for a glider plane. The required corrective measures are affected by a set of servo motors which helps the flight path and orientation to be maintained at the desired levels.

In the early days of aviation, aircraft required the continuous attention of a pilot in order to fly safely. As aircraft range increased allowing flights of many hours, the constant attention led to serious fatigue. An autopilot is designed to perform some of the tasks of the pilot. Along the flight path the vehicle is under the influence of various accelerating forces in all directions and these factors cause it to deviate from its desired path. So the plane loses its heading as well as orientation. This is where autopilot comes into picture.
There are three levels of control in autopilots for smaller aircrafts. A single-axis autopilot controls an aircraft in the roll axis only. A two-axis autopilot controls an aircraft in the pitch axis as well as roll axis with pitch-oscillation-correcting ability. A three-axis autopilot adds control in the yaw axis and is not required in many small aircraft. The flight may also receive inputs from on-board radio navigation systems to provide true automatic flight guidance once the aircraft has taken off until shortly before landing.



The first aircraft autopilot was developed by Sperry Corporation in 1912. The autopilot connected a gyroscopic heading indicator and attitude indicator to hydraulically operated elevators and rudders. It permitted the aircraft to fly straight and level on a compass course without a pilot's attention, greatly reducing the pilot's workload.
The autopilot control systems have evolved drastically since the turn of the century. Modern autopilots use computer software to control the aircraft. The software reads the aircraft's current position, and controls a flight control system to guide the aircraft. In such a system, besides classic flight controls, many autopilots incorporate thrust control capabilities that can control throttles to optimize the air-speed, and move fuel to different tanks to balance the aircraft in an optimal attitude in the air. Although autopilots handle new or dangerous situations inflexibly, they generally fly an aircraft with a lower fuel-consumption than a human pilot.

An autopilot is an example of a control system. Control systems apply an action based on a measurement and almost always have an impact on the value they are measuring. It's called a negative feedback loop because the result of a certain action (the air conditioning unit clicking on) inhibits further performance of that action. All negative feedback loops require a receptor, a control center and an effector.
autopilot feedback loop
Let's consider the example of a pilot who has activated a single-axis autopilot.

  1. The pilot sets a control mode to maintain the wings in a level position.
  2. However, even in the smoothest air, a wing will eventually dip.
  3. Position sensors on the wing detect this deflection and send a signal to the autopilot computer.
  4. The autopilot computer processes the input data and determines that the wings are no longer level.
  5. The autopilot computer sends a signal to the servos that control the aircraft's ailerons. The signal is a very specific command telling the servo to make a precise adjustment.
  6. Each servo has a small electric motor fitted with a slip clutch that, through a bridle cable, grips the aileron cable. When the cable moves, the control surfaces move accordingly.
  7. As the ailerons are adjusted based on the input data, the wings move back toward level.
  8. The autopilot computer removes the command when the position sensor on the wing detects that the wings are once again level.
  9. The servos cease to apply pressure on the aileron cables.

This loop, shown above in the block diagram, works continuously, many times a second, much more quickly and smoothly than a human pilot could. Two- and three-axis autopilots obey the same principles, employing multiple processors that control multiple surfaces. Some airplanes even have auto-thrust computers to control engine thrust. Autopilot and auto-thrust systems can work together to perform very complex manoeuvres.
The three basic control surfaces that affect an airplane's attitude are as follows:

  1. The elevators: devices on the tail of a plane that control pitch (the swaying of an aircraft around a horizontal axis perpendicular to the direction of motion).
  2. The rudder is also located on the tail of a plane. When the rudder is tilted to starboard (right), the aircraft yaws -- twists on a vertical axis -- in that direction. When the rudder is tilted to port (left), the craft yaws in the opposite direction.
  3. Ailerons:  on the rear edge of each wing roll the plane from side to side.

Autopilots can control any or all of these surfaces. A single-axis autopilot manages just one set of controls, usually the ailerons. This simple type of autopilot is known as a "wing leveller" because, by controlling roll, it keeps the aircraft wings on an even keel. A two-axis autopilot manages elevators and ailerons. Finally, a three-axis autopilot manages all three basic control systems: ailerons, elevators and rudder.
The heart of a modern automatic flight control system is a computer with several high-speed processors. To gather the intelligence required to control the plane, the processors communicate with sensors located on the major control surfaces. They can also collect data from other airplane systems and equipment, including gyroscopes, accelerometers, altimeters, compasses and airspeed indicators.
The processors in the AFCS then take the input data and, using complex calculations, compare it to a set of control modes. A control mode is a setting entered by the pilot that defines a specific detail of the flight. For example, there is a control mode that defines how an aircraft's altitude will be maintained. There are also control modes that maintain airspeed, heading and flight path.
These calculations determine if the plane is obeying the commands set up in the control modes. The processors then send signals to various servomechanism units. A servomechanism, or servo for short, is a device that provides mechanical control at a distance. One servo exists for each control surface included in the autopilot system. The servos take the computer's instructions and use motors or hydraulics to move the craft's control surfaces, making sure the plane maintains its proper course and attitude.
The above illustration shows how the basic elements of an autopilot system are related. For simplicity, only one control surface -- the rudder -- is shown, although each control surface would have a similar arrangement. Notice that the basic schematic of an autopilot looks like a loop, with sensors sending data to the autopilot computer, which processes the information and transmits signals to the servo, which moves the control surface, which changes the attitude of the plane, which creates a new data set in the sensors, which starts the whole process again. This type of feedback loop is central to the operation of autopilot systems. It's so important that we're going to examine how feedback loops work in the next section.

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