Calibrating your servos in OpenTx

Mike Shellim 4 Dec 2013
updated 18 July 2020

Introduction

In this article, I will explain about servo calibration, where it fits in the workflow, and how to do it correctly. 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 it is

Calibration refers to the adjustments you make in the OUTPUTS menu. This is where mixer values are mapped to actual control surface deflections. In other words, where your 'idealised' mixing scheme is adjusted for the actual model.

 

With careful adjustment, you can

• achieve precise tracking of ailerons, flaps and V-tail, regardless of linkage differences
• limit control surface movement to safe levels
• linearise control surface movements

 

The OUTPUTS menu

The OUTPUTS menu is where you'll make the adjustments.

The columns are as follows (key adjustments in bold):

• 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 - subtrim added/not-added to end points

 

Setting servo end points

To visualise your servo end points, you need to generate mixer values of -100 and +100 for each servo channel. This is easily done, on demand, using my Calibration mode.

• Set the end points for maximum possible travel, as limited by the model's hinges and linkages.
• Refine the adjustment, by equalising the travel on the left and right sides. (If a non-linear response is required, for example for expo or aileron differential, then this should be done in the inputs or mixers, not in the outputs!)

The calibrated end points will almost certainly be greater than needed - you can use the INPUTS menu to reduce the travel for each individual stick input.

Setting the servo centre

To visualise your servo centre position (1500 mS pulse), you need to generate a mixer value of zero for each servo channel. Again this can be done using my Calibration mode. The servo centres should be adjusted so the control surfaces are at their neutral position.

Calibration using Min/Max/Subtrim or Curves

There are two methods of calibration. The first, simpler, method is to adjust Min/Max and Subtrim. Alternatively you can specify a curve.

The two methods in more detail:

• Min, max and subtrim
  This method of calibration is pretty simple, and suitable for ailerons, elevator and rudder. It's equivalent to using a 3-point curve.
• Curve
  This method is useful for balancing up flaps. A curve in OpenTx may have between 2 and 17 points. This method offers finer control.

Note that OpenTX applies Min, Max and Subtrim regardless of whether a curve is specified. To avoid confusion when using a curve, leave Min, Max and Subtrim at their 'pass thru' values. These are -100, 100 and 0 respectively (or -150, 150 and 0 if using extended limits).

Preparing for calibration

Here are a few things to check before your first calibration:

'Calibration mode'

Consider adding a 'Calibration Mode' to your setup - it allows you to generate calibration reference values on demand.

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.

Set 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.

Doing 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. 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 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 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 large, and it's important that they track each other precisely.

The solution is to:

• Calibrate one flap with a 2-point curve (for a linear response).
• Calibrate the second flap with a 5-point curve (for match the first flap)

Using this method, the flap neutral is 'floating'. Once calibration is complete, the neutral position is finalised 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. Adjust the end points to maximum possible travel (limited by linkage).
   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 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 finalise the control surface travel. The approach that I recommend is:

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

A closer look at Subtrim Mode and PPM Centre

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. By entering CAL mode you can easily see if the centres have drifted. If the drift is small, it's not necessary to do a full recalibration - simply adjust PPM Centre (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

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.