Calibrating your servos in OpenTx

Mike Shellim 4 Dec 2013
updated 17 August 2017

1. Introduction

In this article, I'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.

1.1 What is servo calibration?

Servo calibration is the process of adjusting the end points and centres of each channel. The aims of calibration are three-fold:

Calibration is carried out in the OUTPUTS menu. During calibration all mixers, inputs and trims should be bypassed. There's no built-in method for this, but you can do it via my calibration mode (CAL).

1.2 Separation of roles

Done correctly, calibration promotes a clear separation of roles:

As we'll see, this separation of roles makes it much easier to design and maintain your setup. For example, if you replace a servo, it's just a couple of adjustments in the OUTPUTS menu - no need for endless fiddling with mixers.

2. Benefits of servo calibration

Before we go through the calibration procedure, let's look in more detail at the benefits.

2.1 Mixer-level symmetry

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

2.2 Simpler design and adjustment

Since the left- and right-side mixers have identical weights, you can employ GVARs and cascading mixers to reduce the mixer count. This also makes the setup much easier to adjust, with no need for separate left/right adjustments.

2.3 Never lose your trim settings

An added bonus: calibration makes it easy to identify drifting servos and bent linkages, and you can correct for these without losing your trim settings. No longer do you need to worry if that 1mm offset is due to a genuine trim offset, or a bent linkage.

3. 'Calibration mode'

Hopefully by now I will have convinced you of the benefits of calibration. The next step is to modify your setup so you can bypass mixers and trims on demand. You can do this using my 'Calibration Mode' modification. The text which follows will assume that Calibration mode is available in your setup.

4. The Outputs menu

4.1 Overview

Calibration is carried out in the Outputs menu.

servos menu

Servos menu (OpenTx v2)

The key fields are as follows:

 

4.2 Two methods of calibration

Internally OpenTx applies both the curve (if specified) and Min/Max/Subtrim. If using a curve, make sure to set Min/Max/Subtrim to -100/100/0 (or -150/150/0 if using extended limits).

5. Preparing for calibration

There are a couple of items to check before your first calibration:

5.1 Servo rotation

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

For more detailed instructions please see Calibration mode instructions.

5.2. Subtrim mode

The SUBTRIM MODE parameter determines the behaviour of the end points as you adjust subtrim. Always leave at the default setting ("^"), so adjusting subtrim does not affect the end points. More on this later...

6. Performing the calibration

OK, so now you're ready to start calibrating. Remember to activate Calibration mode before making any adjustments.

6.1 Calibrating ailerons, elevator, rudder, V-tail

Calibrating ailerons, elevator, rudder or V-tail is straightforward. The Min/Max/Subtrim method is usually sufficient, unless the linkages are highly non-linear. The goal is to (a) set 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. While still in Calibration mode, 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 paired surfaces (e.g. ailerons, flap, V-tail) have identical travel
  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.)

6.2. Calibrating Flaps

Flaps are a special case. They are characterised by grossly asymmetric movement (much more down than up); if you calibrate the servos using the technique described in 6.1 above (so servo centre => neutral flap), the response will be severely non-linear, so diff will not work correctly, and flaps will not track the ailerons.

The solution is to ignore the flap neutral position for the time being; first calibrate one flap channel for correct end points and a linear response (the servo centre will correspond to some arbitrary flap position). Then calibrate the second flap to track the first flap. For both servos, you'll use Curves instead of Min/Max/Subtrim:

Here's the procedure in detail:

  1. Set Min, Max and Subtrim to defaults
    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 operating 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 as 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 extra points to the curve as required.
    5. OK, so now you have fixed the end points, and the flap deflection varies more or less linearity with the input. Note that the flap neutral is 'floating' - we'll fix this later.
    6. Exit the CURVE menu
    7. Exit Calibration mode
  3. Calibrate the RIGHT flap servo.
    Use the left flap as a reference for setting up the right flap.
    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 as we noted above, 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.

If you've done your calibration correctly, the weight will be identical for both left and right flaps. This means you can use a GVAR to adjust both flaps from a single menu point. Which leads us neatly to the next section:

7. Use GVARs and cascading mixers to simplify your setup

With a properly calibrated setup, you'll be able to use identical weights left- and right-side mixers. This is very useful, as it permits the use of GVARs and cascading mixers. These can greatly simplify your setup by (a) reducing the number of mixers and (b) providing a single menu point for your mixer adjustments (instead of having to adjust left- and right- sides individually).

8. Adjusting travel of ailerons, elevator and rudder

The travel you've set in calibration are maximal. For actual flight, you will probably wish to reduce them. Where do you make the adjustment?

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

9. 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.

10. Correct drifting control surfaces

Even the most accurately moulded F3X ship can suffer from bent linkages or drifting servos, perhaps as a result of wear and tear or temperature changes. These faults inevitably lead to wandering neutrals which can cause undesirable trim changes. The question is: how do you distinguish between these faults, and legitimate adjustments made with the trim levers?

The answer is: you don't need to! Recall that calibration is performed with trims disabled. Therefore, if you enter CAL mode and find that your neutrals have changed, it can't be due to the adjustments made with the trim levers; it can only be due to mechanical, temperature or electronic changes. After recalibration, your trim offsets will be restored to exactly as they were before.

By checking your calibration settings before every flying session, you can ensure that your trim offsets are always consistent, regardless of mechanical or temperature issues.

11. Using Autotrim (avoid!)

OpenTx allows you to re-centre your trims, by moving the offsets to SUBTRIM. This can be done in the Model Setup menu. Unfortunately, using this feature will trash your carefully calibrated SUBTRIM values.

If you intend to recalibrate your servos from time to time as is advisable then avoid using this feature.

12. Extended LIMITS

[To be expanded.]

13. Calibration the easy way

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