Calibrating your outputs in OpenTx

Mike Shellim 8 Jan 2013
updated 17 December 2020

Introduction

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

Key concepts revision

First a bit of quick revision on the data flow from mixers to the outputs:

The MIXER menu is where you design the control logic, assuming a perfectly symmetrical model with perfect linkages.

The OUTPUTS menu is where you marry up your 'perfect' design to a real model, in other words, calibrate the outputs.

The Outputs layer

Let's look at the Outputs layer in more detail, in particular what OpenTX does, and what you'll do ('calibration').

First, OpenTX clips the incoming mixer values to -100 and +100. These therefore represent the maximum and minimum commanded positions. You have no control over this step.

Next, OpenTX applies Min, Max and Subtrim adjustments. These map the commanded positions to actual servo movements.

During calibration, for each channel, you'll adjust Min and Max to set the control surface limits (mixer values of -100 and +100). Likewise, you'll adjust Subtrim to set the control surface neutral (mixer values of zero). The process of adjustment is called calibration. Let's now look at this in more detail.

Calibration prerequisites

To carry out a calibration you'll first need

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

 

Aims of calibration

The aims of calibration are:

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

When calibration is complete, the model will behave symmetrically, regardless of the imperfections in the linkages, servos etc.

Exploring the OUTPUTS menu

To perform a calibration, you'll be using 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.

Calibration methods

There are two methods of calibration.

Let's look at these in more detail:

Method 1: Min, max and subtrim

This method is suitable for ailerons, elevator and rudder. Min and Max adjust the end points, while Subtrim adjusts the centre. They are equivalent to using a 3-point curve.

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.

Method 2: Curves

Curves allows full control over the ouput response. They're particularly useful for flaps. If using a curve, it's good practice to leave Min, Max and Subtrim at their 'pass thru' values of -100, 100 and 0 respectively (or -150, 150 and 0 if using extended limits).

Preparation

Before you perform a calibration:

Add a 'Calibration mode'

Calibration relies on generating mixer values of -100, 0 and +100 via the sticks. 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 determines whether or not subtrim adjustments also affect the end points. Use the default (Delta) mode. More on this later.

Performing the calibration

So now you're ready to start the calibration. Let's go!

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, fortunately OpenTX allows these to be calibrated with great precision.

Flaps are characterised by grossly asymmetric movement which means that the servo centre will not correspond to the control surface neutral. Also, flaps have large deflections, and it's important that they track precisely.

A method which deals with both these issues is as follows:

1. Calibrate the first flap with a 2- or 3-point curve (to achieve a linear response). This flap will be the reference for the second flap.
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
   Now 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.
   2. Enter CAL mode
   3. Adjust curve 2 points for maximum possible travel (limited only by the linkages).
   4. The flap deflection should vary more or less linearly with the calibration input. If necessary, you can add an extra point to the curve.
   5. Exit the CURVE menu
   6. 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. Create a mix in each flap servo channel.
   2. For each mix, set src = 'MAX'. This generates a fixed offset.
   3. 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 travel (stick rates) of ailerons, elevator and rudder

After calibration, you can finalise the control surface travel by adjusting the stick rates in the INPUTS menu. Good practice is as follows:

1. For the flight controls (elevator, aileron and rudder): adjust weights 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 be set to equal values, on the left and right sides.

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 is recommended as it allows independent calibration of centres and end points!! 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:

1. 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.

2. 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 mixers can be designed assuming an 'ideal' model, in other words left and right mixer pairs 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.