Using a servo motor for an elevator trim indicator

This tutorial will cover how to configure Mobiflight with a servo motor to control an elevator trim indicator gauge, like in a Cessna C172. We will not go into any details about how the elevator trim gauge internal mechanism should be built. In general, the servo will move a needle in a limited range, controlled by Mobiflight depending on the value of the elevator trim position given by the flight simulator.

Wiring connections

A 9g servo can be used for this example. In addition to the 5v and GND wires, only one Arduino digital pin is needed to connect the servo. In this tutorial we show pin 9 being used, but it can be any available pin.

Even small servo motors like the 9 gram type typically use 500 mA to 750 mA. This is a relatively high current for an Arduino, so use of an external power supply is recommended. Note the external power supply GND wire is also connected with the Arduino GND.

Mobiflight device configuration

Create the device configuration for the servo motor in Mobiflight Modules. Specify the data pin where the servo motor was connected. In this example, it is pin 9.

Mobiflight output configuration

Create an output config in Mobiflight, to display the value of the generic variable using the servo we connected.

Sim variable tab

The sim variable can be selected from the Mobiflight presets dropdown menus. For custom variables not available from the menus, you can directly enter the code into the code box (tick on the "Show preset code" opens this box).

For the generic elevator trim position, use this variable.

(A:ELEVATOR TRIM PCT, percent over 100)

Please note that this variable takes values in the range from -1.0 to +1.0.

Modify tab

Because the simulator variable range is -1.0 to +1.0, it is necessary to change this value to a positive integer scale for use in the servo movements. The servo only takes non-negative integer values.

In this case, we have chosen to use an interpolation modifier that takes as input the -1.0 and +1.0 in the left column, as received from the simulator. Here we define the output to be 255 and 0 in the right column. Please note that by specifying 255 first and 0 second, we are setting the movement to be inverted. If this inversion is not required, then specify 0 first and 255 second.

Display tab

The display tab is where the parameters for the servo motor movement are specified.

As mentioned before, the servo requires a non-negative ascending scale. This means that both Min and Max values need to be 0 or greater, and Max must always be greater than Min. As we have chosen to use a servo scale of 0 to 255, as given by the output of the interpolation table defined in the previous section, we specify the same scale in the Min value and Max value boxes.

The Max rotation parameter is the percentage of the maximum movement that the servo motor can achieve, which typically is about 180 degrees. In this example, we are setting a max rotation of 50% which will correspond to about 90 degrees rotation. In other words, the 0 percent position of the servo scale is matched to the Min value of the sim scale and the Max rotation percent position is matched to the Max value of the sim scale.

You may now set a test value at the bottom of the output config specification. Press the Test button and the servo motor should move accordingly. It is important to note that the zero position of the servo motor needs to be manually set to the minimum value in the gauge scale. This implies detaching the needle and resetting it accordingly.

Mobiflight will move the servo to the zero position on every exit and move to the current elevator trim position whenever it goes into run mode.

CONCLUSION

Congratulations.

You should now have a properly configured servo that will move according to the value of the elevator trim wheel position given by the simulator.