Calibrating your servos in OpenTx

Mike Shellim 4 Dec 2013
updated 22 Feb 2020

Introduction

Servo calibration is an essential part of setting up a model. In this article, I'll explain what it is, and how to do it correctly for different types of control surface. I'll also explain the benefits of calibration-centred design.

To get the best out of this article, you should be already be familiar with the Key Concepts.

What is servo calibration?

Recall that OpenTX has three programming 'layers':

• INPUTS and MIXERS define the control logic.
• OUTPUTS converts commands to actual control surface deflections.

Calibration is concerned with the adjustments in the OUTPUTS layer.

Goals of calibration

The goals of calibration are

• Limit servo travel (to avoid damage to linkages)
• Equalise left/right control surface responses
• Linearise control surface movement (provides a baseline for aileron diff, expo etc.)

After calibration, all your model will appear - to the mixers - to be perfectly symmetrical. Left- and right-side mixers will therefore have identical weights. We'll see why this is useful for calibration-centred design.

CAL Mode

In order to visualise the calibration adjustments, we must disable all inputs, mixers and trims. An easy way to do this is to add a calibration mode to your setup.

Introduction to the OUTPUTS menu

Calibration is carried out in the Outputs menu.

Key fields as follows:

• Channel label - 6 character channel identifier
• Subtrim - servo centre adjustment
• Min - end point 1
• Max - end point 2
• Dir - direction of rotation
• Curve - response curve (alternative to Min/Max/Subtrim)
• PPM centre - pulse width (microseconds) corresponding to centre command
• Subtrim mode - end points follow/ignore subtrim

 

Two methods of calibration

There are two ways of calibrating an output. You can either adjust min/max and subtrim. Alternatively you can specify a curve.

The two methods in more detail:

• Using min, max and subtrim
  This method of calibration is suitable for ailerons, elevator and rudder.
  
• Specifying a curve
  This method is especially useful for balancing up flaps. A curve in OpenTx may have between 2 and 17 points. The first and last points define the servo end points. .

If using the curve method, set Min/Max/Subtrim to their 'pass thru' values. That is, set min/max/subtrim to -100/100/0, (or -150/150/0 if using extended limits).

 

Preparing for calibration

'Calibration mode'

If you haven't already done so, add a 'Calibration Mode' to your setup.

Set the servo direction

Calibration is easier if your servos rotate in a consistent direction. The convention I use is:

• As Min/Max/Subtrim are increased (+ button), the control surface moves up or right.
• As Min/Max/Subtrim are decreased (- button), the control surface moves down or left.

More info in calibration mode.

Choose the subtrim mode

The SUBTRIM MODE parameter determines the behaviour of the end points as subtrim is adjusted. Leave at the default ("^"), so adjusting subtrim will not affect the end points. More on this later.

Performing the calibration

So now you're ready to start calibrating your servos. The method you use will depend on the particular control surfaces:

Calibrating ailerons, elevator, rudder, V-tail

Min/Max/Subtrim method is usually sufficient for these. The goal is to (a) set the neutrals, (b) maximise travel and (c) achieve a linear response.

Here's the procedure:

1. Open the Outputs menu
2. Activate Calibration Mode
3. Adjust SUBTRIM so that the control surface is at the correct 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 servos are now calibrated.

(NOTE: While the Min/Max/Subtrim method is very accurate, it can be painfully slow in OpenTx 2.0. I have therefore started experimenting with 3-point curves. Curves use coarser increments so is a whole lot faster, and resolution is sufficient.)

Calibrating flaps

Flaps are characterised by grossly asymmetric movement which means that they cannot be calibrated using the method described in the previous section (which assumes symmetrical movement). Also, flap deflections are often large, and it's important that they track each other precisely.

The solution to both these issues to

• Use a 2-point curve for one flap (for a linear response).
• Use a 5-point curve on the other (for accurate tracking of the other flap)

Using this method, the flap neutral is 'floating'. Once calibration is complete, the neutral position is set 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 servo:
   The aim is to (a) set the travel limits, and (b) to obtain a 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, -100) and (100,100).
   2. Enter Calibration mode
   3. Move the stick back and forth, and adjust the points to provide maximum possible travel (limited by the flap linkage).
   4. If the linkage geometry is good, the flap deflection will vary approximately linearly with the calibration input. This is what you want! If necessary, you can improve linearity by adding an extra point to the curve.
   5. OK, so now you have fixed the end points, and the flap response is roughly linear.
   6. Exit the CURVE menu
   7. Exit Calibration mode
3. Calibrate the RIGHT flap servo.
   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

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 of ailerons, elevator and rudder

After calibration, you can set the actual control surface travel. The approach that I recommend is:

1. Primary flight controls (elevator, aileron and rudder): do all the travel adjustment in the INPUTS menu. Set the downstream mixer weights to 100%.
2. All other interactions: adjust in Mixers menu

Subtrim Mode, PPM Centre

In this section, I'll go into a little more detail about Subtrim Mode.

As we've seen, the Servos menu has a column for 'Subtrim Mode'. This can be either '^' or '='. There are some significant differences:

• '^' (default) - servo end points are not affected by SUBTRIM adjustment
• '=' - servo end points are shifted by SUBTRIM (in effect, the servo end points are MIN + SUBTRIM and MAX + SUBTRIM).

If you change modes, the end points will jump, so you once you choose a mode you should stick with it.

So... which mode should you use? I would strongly recommend using the default option ('^'). Subsequently, if you to need to correct a drifting control surface (see below), then it's quicker to adjust PPM Centre which offsets the whole servo response. The adjustment to PPM Centre should also be done in Calibration mode.

Correct drifting control surfaces

All models will suffer from bent linkages or drifting servos during their lifetime. On other systems, these slight errors can often go unnoticed or be confused with trim settings.

With OpenTx and a properly CAL'd setup, you can easily check for problems - go into CAL mode and see if the calibrated neutrals have changed. If the drift is small, it's not necessary to do a full recalibration - simply adjust PPM Centre for the affected outputs (do this while still in CAL mode). This will offset the whole servo response. Once you exit CAL mode, any trim offsets will be restored.

By doing a quick CAL check before every flying session, you can ensure that your trim offsets are consistent, regardless of mechanical or temperature issues.

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

Calibration is useful on its own. However, 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.

2. Use GVARs and cascading mixers

After calibration, paired mixers will have identical weights. By designing your setup to use 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.