Mike Shellim 4 Dec 2013
updated 26 April 2017
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.
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:
OpenTx has all the features needed for efficient servo calibration. OpenTx offers servo-side curves, which permit precise adjustment of servo response. And with a simple modification to your setup, you can:
Before we go through the calibration procedure, let's look at the benefits of servo calibration in more detail:
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.
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.
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.
The Outputs menu is where you'll carry out the calibration. (Note: in OpenTx 2.0 and before, the menu is called 'Servos'.)
Below is a screenshot of the Outputs menu showing the first seven channels. Six are assigned to servos, while CH7 is unassigned.
The key fields which you can change are as follows
There are two methods of calibrating a servo:
There are a couple of items to check before your first calibration:
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.
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...
OK, so now you're ready to start calibrating. Remember to activate Calibration mode before making any adjustments.
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:
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.)
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:
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:
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:
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).
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:
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.
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.
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.
[To be expanded.]
All the canned setups published on this site have Calibration mode already built-in, protected against accidental operation.