Calibrating outputs/servos

Mike Shellim 8 Jan 2013
Updated 14 April 2022

Introduction

In this article, I explain how to calibrate the outputs (servos) on your model. It may sound simple, but there's a little more to it than meets the eye.

Doing a calibration will allow you to simplify your setup and keep your model in trim.

What is calibration?

You probably already know how to adjust your servo travel and centres: you go to the Outputs menu and adjust Min, Max and Subtrim.

You can instead define a curve, useful for calibrating the flaps as we'll see later.

screenshot

CH1 calibrated via min/max/subtrim
CH3 calibrated via curve

But what is actually happening?

Mapping

Another way to think about it:

This is how the mapping works:

On entry to the Outputs stage, mixer values are clipped to +/-100%. Min and Max therefore represent absolute travel limits. Think of them as electronic end stops.

Now here's a potential source of confusion: Min, Max and Subtrim represent PWM values, which would normally be expressed in microseconds. However, by default OpenTX displays them as percentages. The conversion is as follows:

Standard servos usually have a nominal range of 1000 to 2000μs, so you can think of these percentages as 'percentage of the range of a standard servo - roughly!'.

PWM values outside the range 988 - 2012 μs are only available if the'extended limits' option is enabled (MODEL menu).

Summary

Okay, so this is what we've learned so far:

Calibration goals

The purpose of the calibration is to set the optimal values for Min/Max and Subtrim. The precise goals are:

You will need

Performing the calibration

Okay, so let's start calibrating!

Calibrating ailerons, elevator, rudder, V-tail

Calibration is really simple, you're just setting the 'never-exceed' limits, and balancing up the left and right sides. In more detail:

  1. Open the OUTPUTS menu
  2. Activate Calibration Mode
  3. Adjust Subtrim for correct neutral.
  4. Adjust Max and Min for each servo:
    • First, adjust thee for maximum possible control surface travel
    • Next, refine so that control surface travel is equal up/down (or left/right).
    • Finally, refine so that left and right surfaces match.
  5. Exit from Calibration mode

The outputs are now calibrated.

Calibrating flaps

Flaps present a greater challenge. They are characterised by grossly asymmetric movement. Also, flaps have large deflections, and it's important that they track precisely.

Fortunately OpenTX allows these to be calibrated with great precision:

  1. Calibrate one flap with a 2-point curve, setting the end points only. This flap will be the reference for calibrating the second flap. The servo centre will be at some arbitrary camber position, and we'll use an offset mix to deal with this.
  2. Calibrate the second flap with a 5-point curve, to track the first flap.
  3. Set the neutral position using an offset mix.

Here's the procedure in detail:

  1. Set Min, Max and Subtrim to 'pass thru' values
    1. Open the OUTPUTS menu
    2. For each flap servo, set MIN, MAX and SUBTRIM to -100, +100 and 0 respectively (or -150, +150 and 0 if using extended limits).
  2. Calibrate the LEFT flap
    First, calibrate the left flap. The aim is to (a) set the limits of servo movement, and (b) to obtain an approximately linear response. The flap neutral is not considered in this step.
    1. Go to the CURVE column, and define a 2-point curve with points
      (-100, -20) and (100,20). The low point values ('20') are to avoid accidental damage to your linkages before finalising the adjustment.
    2. Enter CAL mode
    3. Adjust the two points for maximum possible travel (limited only by the linkages). The flap deflection should vary more or less linearly with the calibration input. If necessary, you can add an extra point to the curve.
    4. Exit the CURVE menu
    5. Exit Calibration mode
  3. Calibrate the RIGHT flap
    Now we adjust the right flap to match the left flap, and we do this using a multi-point curve.
    1. Go to the CURVE column and define a 5-point straight line curve
    2. Enter calibration mode
    3. Move the stick to the 0/25/50/75/100 % positions; at each position, adjust the corresponding point so that the right flap exactly matches the left flap. (Depending on the linkage geometry, it may be necessary to go back and reduce one or other end point on the left flap.)
      curves
    4. Exit the CURVE menu
    5. Exit Calibration mode
  4. Set an offset mix
    The flap servos are now calibrated, and the flaps should track perfectly. However the flap neutral is floating. To fix this we need to apply an offset at the mixer level as follows:
    1. Exit CAL mode
    2. Create a mix in each flap servo channel.
    3. For each mix, set src = 'MAX'. This generates a fixed offset.
    4. Adjust the weight of 'MAX' mix, until the flap is at the correct neutral
    Other mixes can of course be added to the flap channels, for example for roll control, camber etc.

Adjusting the inputs

After calibration, you can finalise the control travel in the INPUTS and MIXERS menus. Good practice is as follows:

  1. For the flight controls (elevator, aileron and rudder): adjust travel in the INPUTS menu and leave mixer weights at 100%.
  2. For all other interactions: adjust in MIXERS menu. If the calibration has been carried out correctly, then the mixer weights can equal on the left and right sides.

A closer look at Subtrim Mode and PPM Centre

There are two options for Subtrim mode:

 

Delta mode allows independent calibration of centres and end points, and is therefore recommended.

Changing between "equals" and "delta" modes will cause your end points to jump, so start with delta mode and stick with it.

If later on you need to shift the whole curve with a single adjustment, then use the PPM Centre adjustment. The effect is simillar to 'equals' mode, but with less aggressive clipping.

Correct drifting control surfaces

Most models suffer from drifting neutrals a few times during their lifetime. You can check for drift by entering CAL and seeing if the calibrated centres have changes. If the drift is small, simply adjust PPM Centre (do this while still in CAL mode). Once you exit CAL mode, any trim offsets will be restored.

Trims => Subtrims - AVOID!!

OpenTx allows you to re-centre your trims, by moving the offsets to SUBTRIM. Obviously, using this feature will trash your calibration. Avoid!

 

Calibration-centred design

Even greater benefits can be achieved by designing your setup with calibration in mind from the start. I call this 'calibration-centred design'. There are two main aspects:

Incorporate a CAL flight mode

The first step is to reserve FM1 as your CAL mode. That way, you can check the calibration at any time for drifting servos, bent linkages and so on.

Use GVARs and cascading mixers

A key goal of calibration is to match up responses between the left and right sides at the servo level. This means that paired (left/right) mixers can have identical weights. By using GVARs and/or cascading mixers, you can have a single menu point for each pair of adjustments. This results in:

Calibration the easy way

All the canned setups published on this site have CAL mode already built-in, protected against accidental operation.

Appendix

The OUTPUTS menu

servos menu

Outputs menu

Columns as follows:

The key fields for calibration are Min, Max, Dir, Curve and Subtrim mode.