Mike Shellim 8 Jan 2013
updated 17 December 2020
In this article, I will explain about output calibration: where it fits in the workflow, and how to do it correctly. I'll also explain the benefits of calibration-centred design.
First a bit of quick revision on the data flow from mixers to the outputs:
The MIXER menu is where you design the control logic, assuming a perfectly symmetrical model with perfect linkages.
The OUTPUTS menu is where you marry up your 'perfect' design to a real model, in other words, calibrate the outputs.
Let's look at the Outputs layer in more detail, in particular what OpenTX does, and what you'll do ('calibration').
First, OpenTX clips the incoming mixer values to -100 and +100. These therefore represent the maximum and minimum commanded positions. You have no control over this step.
Next, OpenTX applies Min, Max and Subtrim adjustments. These map the commanded positions to actual servo movements.
During calibration, for each channel, you'll adjust Min and Max to set the control surface limits (mixer values of -100 and +100). Likewise, you'll adjust Subtrim to set the control surface neutral (mixer values of zero). The process of adjustment is called calibration. Let's now look at this in more detail.
To carry out a calibration you'll first need
The aims of calibration are:
When calibration is complete, the model will behave symmetrically, regardless of the imperfections in the linkages, servos etc.
To perform a calibration, you'll be using the OUTPUTS menu:
Outputs menu
Columns as follows:
The key fields for calibration are Min, Max, Dir, Curve and Subtrim mode.
There are two methods of calibration.
CH1 calibrated via min/max/subtrim
CH3 calibrated via curve
Let's look at these in more detail:
This method is suitable for ailerons, elevator and rudder. Min and Max adjust the end points, while Subtrim adjusts the centre. They are equivalent to using a 3-point curve.
Extended limits
By default, Min and Max have default maxima of -100% and 100%. However, you can extend these limits to -150% and 150% by setting the 'extended limits' option in the radio settings menu. This offers a 50% increase in maximum servo movement, if your servos allow.
Curves allows full control over the ouput response. They're particularly useful for flaps. If using a curve, it's good practice to leave Min, Max and Subtrim at their 'pass thru' values of -100, 100 and 0 respectively (or -150, 150 and 0 if using extended limits).
Before you perform a calibration:
Calibration relies on generating mixer values of -100, 0 and +100 via the sticks. You can do this on demand, by adding a 'Calibration Mode' to your setup. If that's not possible, then set all your input and mixer weights to 100%, and centre your trims.
Subtrim mode determines whether or not subtrim adjustments also affect the end points. Use the default (Delta) mode. More on this later.
So now you're ready to start the calibration. Let's go!
The procedure is straightforward:
The outputs 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.
Flaps present a greater challenge, fortunately OpenTX allows these to be calibrated with great precision.
Flaps are characterised by grossly asymmetric movement which means that the servo centre will not correspond to the control surface neutral. Also, flaps have large deflections, and it's important that they track precisely.
A method which deals with both these issues is as follows:
Here's the procedure in detail:
After calibration, you can finalise the control surface travel by adjusting the stick rates in the INPUTS menu. Good practice is as follows:
There are two options for Subtrim mode:
Delta mode is recommended as it allows independent calibration of centres and end points!! Changing between "equals" and "delta" modes will cause your end points to jump, so start with delta mode and stick with it.
If later on you need to shift the whole curve with a single adjustment, then use the PPM Centre adjustment. The effect is simillar to 'equals' mode, but with less aggressive clipping.
Most models suffer from drifting neutrals a few times during their lifetime. You can check for drift by entering CAL and seeing if the calibrated centres have changes. If the drift is small, simply adjust PPM Centre (do this while still in CAL mode). Once you exit CAL mode, any trim offsets will be restored.
OpenTx allows you to re-centre your trims, by moving the offsets to SUBTRIM. Obviously, using this feature will trash your calibration. Avoid!
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:
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.
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:
All the canned setups published on this site have CAL mode already built-in, protected against accidental operation.