Calibrating your servos in OpenTx

Mike Shellim 4 Dec 2013
This article is in permanent beta.

1. Introduction

In this article, I'll take a look at servo calibration - what it is, how to do it correctly, and the considerable benefits which result. To understand this article, you should be already be familiar with the Key Concepts.

1.1 What is servo calibration?

Servo calibration is a general term for servo adjustment. However in this article, I'll use it in relation to a specific procedure to achieve the following highly desirable goals:

Some key points to note:

1.2 Support for calibration in OpenTx

OpenTx all the features needed for efficient servo calibration. Unlike many Far Eastern sets, OpenTx offers servo curves, which permit precise adjustment of servo responses right down at the servo level. And with a simple modification to each servo channel, you can:

2. Benefits of correct calibration

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

2.1 Mixer-level symmetry

Done correctly, servo calibration compensates for mismatches in linkage geometry at the level of each 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 high mixes to both reduce the mixer count, and make your setup easier to adjust.

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.

3. 'Calibration mode'

Hopefully by now I will have convinced you of the benefits of calibration. The next step is to integrate my 'Calibration Mode' into your setup, allowing you to disable mixers and trims on demand. It also makes it easy to match up control surface movements. The text which follows will assume that Calibration mode is available in your setup.

4. The Outputs menu

The Outputs menu is where you'll carry out the calibration. (Note: in OpenTx 2.0 and before, the menu is called 'Servos'.)

4.1 Menu layout

Below is a screenshot of the Outputs menu showing the first seven channels. Six are assigned to servos, while CH7 is unassigned.

servos menu

Servos menu (OpenTx v2)

The key fields which you can change are as follows


4.2 Min/max/subtrim v. curves

There are two methods of calibrating a servo:

5. Preparing for calibration

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

5.1 Servo rotation

Adjustment is easier if your servos rotate in a consistent sense when making adjustments in CAL mode. 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.

Leave SUBTRIM MODE 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 so that the flap responds linearly to the input (the servo centre will be at end up at some arbitrary position). Then calibrate the second flap to track the first. For both servos, you'll use Curves instead of Min/Max/Subtrim, but with different points:

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.)
    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 High Mixes 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/or High Mixes. 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

Any model - even the most accurately moulded F3F ship - can suffer from bent linkages and temperature-induced drift. The problem is: how do you distinguish between the this drift, and your carefully adjusted trims?

Calibration takes care of this! Recall that during calibration all trims are cancelled. When you enter Calibration mode, any deviation of the centres must be caused by temperature drift or a knocked linkage. After calibration, your trim settings will be restored as they were before.

(ASIDE: I find this feature to be of tremendous benefit - before an F3F competition I will do a quick calibration check - even a small shift in centring is immediately detectable).

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.