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FCS-2 Fly Control System for small UAVs

  FCS-2 system enables autonomous flying. It was projected using an experience in flying with FCS-1 autopilot. By using most technologically advanced elements (especially new RCM3400 microprocessor module) smaller size, weight and current consumption were obtained . System can control 8 servos. Four of them are used for plane control and the next 4 channels can be used by user to control the additional functions (for example camera positioning, stil camera shutter triggering, parachute release etc). Autopilot software enables to take the single or serial pictures after reaching programmed waypoint or on command from ground. FCS-2 system allows using 18 additional channels. To use them the external module (picture on the left) must be connected to the autopilot via serial port (the same as for GPS receiver connecting).

Airframe stabilization and navigation.

  The plane is stabilised by using gyro and acceleration sensors and 2 preassure sensors for altitude and airspeed measure. Navigation function can be realized in different way in accordance with the selected mode.

Automatic mode
It’s a main autopilot work mode. In this mode the plane flies along the programmed way using the GPS data. User can change the flight way by changing the current waypoint (from among points defined earlier).

Semi-automatic mode
In this mode autopilot still stabilizes the plane but navigation function is taken over by the operator, which defines the course of flight. GPS data are still used but defined waypoints aren’t valid. The airframe flies along te defined course until the operator changes it. Base station periodically sends the command with defined course. If autopilot won’t receive it the plane is turn back to the base station. Automatic mode can be turn on at any time.

Inertial mode (INS)
This mode uses elements of inertial navigation and in principle is dedicated for situations when GPS fix is lost. In this situation system calculates the angle of turn towards base station (on base the last valid data) , automaticaly turns on INS mode and carries out the turn using the data from gyro sensors. Next the FCS-1 system keeps the stright flight. In this mode the wind influence increases. System enables significant reduction of this influence. This mode can be used as well in situations when the operator is able to navigate using alternative to GPS information sources eg. camera or indirect plane observation. Flight direction is changed by means of command containing information obout turn angle.

Manual control mode
Operator can enable this mode by switching on the manual control transmiter. In this mode whole airframe control (stabilization and navigation) is returned to him. Manual control is mainly used for launch and recovery.

Base – ground station software.

The ground station software (for Windows) is an integral part of the system. Using this sofware operator can define waypoints (up to 99), flight altitude, speed and all factors needed for setting the airplane. Most of the factors and flight parameters can be changed during the flight via radiomodem link. In the same way the actual flight data can be received (for example position, course , airspeed , altitude, RPM, battery voltages and so on). Most of data are stored in the autopilot’s memory and can be sent to the ground station after landing. The data incoming to the base station are processed and displayed on the screen in text or grafical form. For example the picture to the left shows wind speed and direction. During the flight aplication collects data n airframe speed (from GPS) in 90 channels which represent different flight directions . For multi-directional trajectory the sinusoidal graph is drawn. Amplitude of this sinusoide is equal to wind speed and channel corresponding to the maximum speed informs us obout wind direction. BASE software enables using this data to correct the curse in INS mode during the flight to take off point. Other feature of BASE aplication is the mission programming e.g. setting the servos in user channels at defined positions after reaching choosen waypoints.

Autopilot construction and support functions.

The FCS-2 autopilot construction makes it’s mounting in the airframe very simple and minimizes the number of wire links between the components of the system. Low level of E-M radiation allows use the autopilot unit without the case but this one makes mounting easier and allows wrapping it with a sponge layer for vibration reduction. 2 voltages are needed for supplying the system. These voltages can be taken from one battery (using voltage regulator) but the more safe way is use 2 batteries. A very useful autopilot’s function is an automatic trimming. After the take-off operator can trim the airframe. At the moment of switching into the autonomous mode the control surfaces servos positions are stored in the autopilot memory. At this moment the transmitter sticks must be in neutral positions (except the throttle position). FCS-2 autopilot can measure the voltage on the A/D input and integrate it in time. This function can be usefull for defining the charge taken from battery supplying the electric propulsion. Current - voltage converter should be used in this case. For increasing the flying security FCS-2 system allows to block the manual control when the plane cann't be controlled by RC link from Ground station place.

The figure above shows an example of test fly route record. After travelling whole route and reaching the start point the plane flies by defined route again if manual control mode is not enabled. The route shown on the figure defined by 4 waypoints and startpoint was travelled 3 times before the RC link was activated. Data recording was started and finished at the points marked as START and END respectively.

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