Calibrating your outputs in OpenTx

Mike Shellim 8 Jan 2013
Updated 27 August 2021

Introduction

In this article, I will explain about calibration of the outputs: where it fits in the workflow, and how to do it correctly. I'll also explain the benefits of calibration-centred design.

Why calibrate?

Calibration is about the adjustments related to linkages and servos etc. These adjustments are made n the Outputs menu. The procedure for adjustment is called 'calibration'.

Localising these adjustments in the Outputs means that you can design your mixers without regard to your model's geometry. Conversely, if you swap out a servo or knock a linkage, you can recalibrate quickly on the field, without touching your mixers.

Key concepts revision

To understand where calibraton fits in, let's look at the data flow from mixers to the outputs:

Summary:

• The MIXERS layer aggregates the effect of all the mixers, and assigns a percentage value to each channel.
• The OUTPUTS maps converts the % values to servo deflections.

A key point to understand: OpenTX clips the values coming from the mixer layer so they cannot exceed limits of +/-100%, regardless of stick positions.

Mixer values to servo deflections

The job of the outputs layer is to map mixer values (in range +/-100%) to actual control surface deflections. You control the process, by adjusting Min, Max and Subtrim in the OUTPUTS menu. In essence, Min, Max and Subtrim define a 3-point curve mapping mixer values to servo deflections.

• Min maps -100% to lower servo deflection limit
• Max maps +100% to upper servo deflection limit
• Subtrim maps zero to servo deflection centre
• Direction reverses the effect

You can also define your own curve with up to 17 points, and we'll see why that's useful for calibrating flaps.

Calibration prerequisites

To carry out a calibration you'll first need

• the actual model
• a way of generating mixer values of -100, 0 and +100. My Calibration Mode does this on demand.

The Outputs menu

The Outputs menu, showing the Min/Max/Subtrim adjustments and the curve option:

Preparation

Add a 'Calibration mode'

You will need a way to generate mixer values of +/-100%. You can do this on demand, by adding a 'Calibration Mode' to your setup. If that's not possible, then set all your input and mixer weights to 100%, and centre your trims.

Set subtrim mode

'Subtrim mode' is the last column in the OUTPUTS menu. Use the default (Delta) mode, as this allows independent adjustment of subtrim and end points. More on this later.

Performing the calibration

So now you're ready to start the calibration. Your goals will be:

• Set safe limits to servo travel
• Equalise control surface movements on left and right sides
• Linearise resposes

Okay, so let's get calibrating!

Calibrating ailerons, elevator, rudder, V-tail

The procedure is straightforward:

1. Open the OUTPUTS menu
2. Activate Calibration Mode
3. Adjust SUBTRIM so that the control surface is at the neutral position.
4. Adjust MAX and MIN for each servo:
   1. First, adjust for max possible control surface travel
   2. Next, refine so that control surface travel is equal up/down (or left/right).
   3. Finally, refine so that left and right surfaces match (paired surfaces only).
5. Exit from Calibration mode

The outputs are now calibrated.

Note: While the Min/Max/Subtrim method provides very fine adjustment, it can be painfully slow. I therefore now use 3-point curves instead. The are much faster to adjust, and provide useful visual feedback.

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.)
      
   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

The limits you set in calibration are just that - limits! These limits will normally be greater than what's needed for flight. 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.
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.

Extended limits

By default, Min and Max have default maxima of -100% and 100%. However, you can extend these limits to -150% and 150% by setting the 'extended limits' option in the radio settings menu. This offers a 50% increase in maximum servo movement, if your servos allow.

A closer look at Subtrim Mode and PPM Centre

There are two options for Subtrim mode:

• 'delta' (default) - Subtrim adjustment does not affect end points
• 'equals': Subtrim adjustment shifts end points. So the end points are MIN+SUBTRIM and MAX+SUBTRIM. The output is clipped to MIN and MAX.

 

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:

• cleaner logic
• quicker configuration
• reduced data entry errors

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

Columns as follows:

• Channel label - 6 character channel name
• Subtrim - centre adjustment
• Min - end point 1
• Max - end point 2
• Dir - direction
• Curve - response curve (alternative to Min/Max/Subtrim)
• PPM centre - pulse width (microseconds) corresponding to centre command
• Subtrim mode - subtrim added/not-added to end points

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