Tutorials : How to Extract a Pure Stick Signal

Mike Shellim  23 November 2003


When the Aileron control is used to drive a servo, or as an input to a mixer, the servo movement will take into account any Differential or Exp which the user has dialled in via the control settings. However, sometimes you just want a servo to follow the stick position linearly, i.e. ignoring any Diff or Expo which may be dialled in.

There is no built-in way of doing this, but we can achieve this by making use analogue switches.

The example below demonstrates how to drive a servo so that the position is dependent solely on the position of the aileron stick. All Diff, Expo, trim offsets etc. are ignored.


1 - Assign the aileron stick


Stick Control Switch  DIFF      Remarks
C Aileron ON -50 Just set an arbitrary differential value for testing (it should be ignored by servo 1 below).

2 - Assign a servo


Servo Control Remarks
1 Servomix Just one servo for now

3 - Assign Mixer

Servomix set up

Input Control/Mixer Switch  Switch Output
ON A3-C* 0 to 100
ON A3-C -100 to  0

How does it work?

The key is the servomix. You'll notice that the inputs are FIXED VALs - these appear after AUX4 in the menu.

So how does a FIXED VAL input provide a variable output depending on the aileron stick position? The answer is in the analogue switch which is set in the Servo Travel screen. Switch A3-C is an analogue switch which attenuates between 0 - 100% depending on the position of stick C, i.e. the aileron stick. The attenuation depends solely on the stick position, i.e. the attenuation is not affected by the aileron control settings like Diff and Expo.

Why do we need two mixer inputs? The analogue switch only provides an output from 0 to +100 (if FIXED VAL = 100) or 0 to -100 (if FIXED VAL=-100). In order to achieve the full range of -100 to +100 required by the servo, we use two inputs with opposite settings for the analogue switch and opposite values for FIXED VAL. The mixer sums the two outputs to provide a output from -100 and +100.