Calibrating your servos in OpenTx

Mike Shellim 4 Dec 2013
updated 16 June 2019

Introduction

In this article, we'll take a look at servo calibration - what it is, the benefits, and a technique for doing it correctly. To understand this article, you should be already be familiar with the Key Concepts.

What is servo calibration?

Servo calibration is the procedure for setting up the centre and end points of your servos. It's done in the OUTPUTS menu. There are three main goals:

Calibration may be as simple as adusting Min, Max and Subtrim, or you can fine tune using curves.

Calibration (CAL) mode

During calibration, all mixers, inputs and trims must be disabled. There's no built-in method for this, but you can do it via my calibration mode (CAL).

Benefits of servo calibration

Before we go through the calibration procedure, let's look at the benefits:

1 - Mixer-level symmetry

Done correctly, calibration compensates for linkage mismatches at the channel level. After calibration your model will appear - to the mixers - to be perfectly symmetrical. The left- and right-side mixes can therefore have identical weights.

2 - Simpler design and adjustment

Since the left- and right-side mixers will have identical weights, there's no need for separate adjustments for the left and right sides. By using GVARs or cascading mixers, you can have a single adjustment point affecting both sides. This makes mixer configuration much quicker and less error prone.

3 - Separation of roles:

Calibration promotes a clear separation of roles:

This makes it easier to design, then configure, your setup.

4 - Never lose your trim settings

If you check your calibration regularly, any changes due to drifting servos or bent linkages are easily identified. Re-calibrate to compensate for any shifts - all trim offsets will be restored after exiting from CAL mode. No longer do you need to worry if that 1mm offset is due to a genuine trim offset, or a bent linkage!

Introduction to the OUTPUTS

Calibration is carried out in the Outputs menu.

servos menu

Outputs menu

Key fields as follows:

 

Two methods of calibration

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

screenshot

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

The two methods in more detail:

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:

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. Also, flap deflections are often very large, and it's important that they track each other precisely.

The solution to both these issues to

Once calibration is complete, an offset mix may be used to set the flap neutral position.

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.)
      curves
    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.

 

Adjusting travel of ailerons, elevator and rudder

After calibration, you may wish to reduce the control surface travel. The approach that I recommend is:

  1. Primary flight controls (elevator, aileron and rudder): Set rates in the Inputs menu, leaving the mixer weights at 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:

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.

Avoid Autotrim

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

Use GVARs and cascading mixers

With a calibrated setup, the weights for left- and right-side mixers will be identical. You can take advantage of this by using GVARs and cascading mixers in your design. Both methods offer the possiblity of a single menu point for your mixer adjustments, instead of having to adjust left- and right- sides individually. This is a great time saver, and will also simplify your design.

Extended LIMITS

[To be expanded.]

Calibration the easy way

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