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May 27, 2021, 05:01 AM
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Simple Rudder Only Free Flight Controller


Why a FF controller? Because flying space is limited. The controller constrains the model to fly loosely along a circle.
Why a simple controller? Because it is cheap, easily hacked, without the irritating compass/gyro calibration.
And the absence of gyros means no vibration sensitivity in case of a gas powered model.

This controller is designed for stable free flight powered models or tow line gliders.
Only the rudder is used to control the model. We want to keep the free flight spirit.
A GPS provides the course (not the heading) and the position (latitude & longitude) of the model.
The wind speed must be less than the flying speed because the controller is not aware of the heading of the model (no compass).
The controller is loose because most GPS have a large time lag of about one second. But this gentle behavior fits with the FF spirit.

Simple Arduino controller for FF models (1 min 25 sec)

File is attached below.

- Pro Micro 3.3V/8MHz (2.6g)
- Beitian BN-180 GPS 10Hz (4.9gr)
- Small servo (4gr or less)
- 2S battery or 1S battery
- Motor + ESC (optional if glider)
- Active piezo buzzer (optional)

The SparkFun Pro Micro [ 3.3V/8MHz] is a little development board. It's an Arduino-compatible microcontroller.
The board is programmed via the user friendly Arduino IDE:
Read this page to familiarize yourself with the Pro Micro and the IDE set-up:

1- Launch phase: once the GPS position of the model at power-on is saved (the rudder wiggles) the motor spools up and the model is launched. At this point the position of the rudder is kept to zero (mechanical trim position). The flight switches to the Hover phase when the model reaches the Launch circle (distance R1 from the launch point). At that point the control loop is activated.
2- Hover phase: the model is constrained to fly along the Hover circle. The flight switches to the Hunt phase is the model wanders away from the Hover circle (if the distance to the launch point is greater than the Hover circle radius R2 plus a given radius deviation DR.)
3- Hunt phase: the model is guided back to the Hover circle by following a course tangent to the circle. The flight switches back to the Hover state once the model reaches the Hover circle radius plus the radius deviation DR.

R1, R2 and DR are parameters that can be modified at the flying field. The direction of the turn, CW or CCW, is also a selectable parameter.

The controller comprises three loops:
1- Radius control (R) : Proportional (this loop is active only during the Hover phase).
2- Course control (Khi) : Proportional-Derivative.
3- Course rate control (Omega) : Proportional-Integral.

When choosing the supply voltage, look for the compatibility between the components.
Note: the Beitian BN-180 GPS is a 3.3V device. It has its own 3.3V voltage regulator.

2S battery + ESC (electric powered model)
- The ESC is connected to the 2S battery.
- The 5V BEC supply voltage is connected to the RAW pin of the Pro Micro (3.3V voltage regulator).
- You can use a 5V servo whose + pin is connected to the 5V BEC, but whose signal pin is connected to the Pro Micro 3.3V PWM pin.
- If you want to use a 3.3V servo you must connect its + pin to the Pro Micro 3.3V VCC pin (output of the Pro Micro voltage regulator).
- The Beitian BN-180 GPS is powered by the 5V BEC. It has its own 3.3V voltage regulator. It should not be powered by the Pro Micro 3.3V because of the drop out voltage of its regulator.

2S battery (gaz powered model or glider)
- Same as above but the 5V ESC BEC is replaced by a 5V voltage regulator.

1S battery (all models)
- If existing, the ESC is connected to the 1S battery.
- The 1S battery is connected directly to the VCC pin of the Pro Micro (voltage regulator bypass).
- The 3.3V servo is powered by the 1S battery.
- The Beitian BN-180 GPS is powered by the 1S battery.

The parameters may be changed via:
- the Serial Monitor of the Arduino IDE
- a mobile phone with a serial link application and an OTG cable.

I personally use the Serial USB Terminal application with my Android mobile.
Last edited by Paul_BB; Jun 18, 2021 at 01:23 AM.
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