Calibrating outputs/servos

Mike Shellim 8 Jan 2013
Last updated 8 September 2022

Introduction

In this article, I explain how the Outputs menu works, and how to calibrate the outputs on your model. It may sound simple, but there's a little more to it than meets the eye.

Doing a calibration will allow you to simplify your setup and keep your model in trim.

Background

You probably already know how to adjust your servo travel and centres: you go to the Outputs menu and adjust Min, Max and Subtrim. (Alternatively, you can also define a curve, however for now we'll focus on the former method.)

But what is actually happening?

In fact, the Outputs is a simple mapping layer. It converts the aggregated mixer values for each channel - representing % of available travel - into PWM commands which define actual travel.

In effect, it allows you to deal with the effects of linkage geometry at the level of each servo. Taking it a stage further, it allows you to map a perfectly symmetrical mixer design, to an imperfect real model. Localising these adjustments in the Outputs menu offers several benefits.

Mapping using Min/Max/Subtrim

Salient points:

• Min, Max and Subtrim define a 3-point curve:
  • Min defines the PWM value for a mixer value of -100%
  • Max defines the PWM value for a mixer value of +100%
  • Subtrim defines the PWM value corresponding to a mixer value of zero.
• Direction reverses the effect

On entry to the Outputs stage, mixer values are clipped to +/-100%. Min and Max therefore represent absolute travel limits. Think of them as electronic end stops.

Conversion table for % travel to PWM units

Min, Max and Subtrim represent PWM values, yet OpenTX displays them as percentages. The conversion is as follows:

• −150% = 732 μs
• −100% = 988 μs
• 0% = 1500 μs
• 100% = 2012 μs
• 150% = 2268 μs

Standard servos have a nominal range of 1000 to 2000μs, so you can think of these percentages as 'percentage of the range of a standard servo - roughly!'.

PWM values outside the range 988 - 2012 μs are only available if the'extended limits' option is enabled (MODEL menu).

Summary

Okay, so this is what we've learned so far:

• The MIXERS layer assigns a percentage value to each channel, representing % of available travel.
• The OUTPUTS layer maps the percentage value to a PWM value representing actual travel. The mapping is defined by a 3-point curve (-100, Min), (0, Subtrim), (+100, Max). Instead, or in addition, a custom curve can be specified.
• The final PWM value represents an actual servo deflection.

Calibration goals

The purpose of the calibration is to set the optimal values for Min/Max and Subtrim. For most pilots, this will mean:

• Set safe limits for servo travel
• Equalise control surface movements on left and right sides
• Linearise responses to stick movements.

You will need

• the actual model (obviously!)
• a way of generating mixer values of -100, 0 and +100. My Calibration Mode does this on demand. If that's not possible, then set all your input and mixer weights to 100%, and centre your trims.
• The subtrim mode should be left at the default 'delta' mode (this is set in the last column of the OUTPUTS menu). We'll see why later.

Calibration procedure

Okay, so let's start calibrating!

Calibrating ailerons, elevator, rudder, V-tail

Calibration is really simple, you're just setting the 'never-exceed' limits, and balancing up the left and right sides. In more detail:

1. Open the OUTPUTS menu
2. Activate Calibration Mode
3. Adjust Subtrim for correct centre position.
4. Adjust Max and Min for each servo:
   • First, adjust thee for maximum possible control surface travel
   • Next, refine so that control surface travel is equal up/down (or left/right).
   • Finally, refine so that left and right surfaces match.
5. Exit from Calibration mode

The outputs are now calibrated.

Calibrating flaps

Flaps present a greater challenge. They are characterised by grossly asymmetric movement. Also, flaps have large deflections, and it's important that they track precisely.

Fortunately OpenTX allows these to be calibrated with great precision. Instead of Min/Max and Subtrim, you can assign an output curve.

So the procedure calibrating flaps is:

1. Calibrate one flap with a 2-point curve, in other words just set the end points. This flap will be the reference for calibrating the second flap. With just the end points defined, the servo centre will be at some arbitrary position. We'll use an offset mix to deal with this (step 3).
2. Calibrate the second flap with a 5-point curve, to track the first flap.
3. Set the neutral position using an offset mix.

Here's the procedure in a more 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
   First, calibrate the left flap. The aim is to (a) set the limits of servo movement, and (b) to obtain an approximately 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, -20) and (100,20). The low point values ('20') are to avoid accidental damage to your linkages before finalising the adjustment.
   2. Enter CAL mode
   3. Adjust the two points for maximum possible travel (limited only by the linkages). The flap deflection should vary more or less linearly with the calibration input. If necessary, you can add an extra point to the curve.
   4. Exit the CURVE menu
   5. Exit Calibration mode
3. Calibrate the RIGHT flap
   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
4. Set an offset mix
   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. Exit CAL mode
   2. Create a mix in each flap servo channel.
   3. For each mix, set src = 'MAX'. This generates a fixed offset.
   4. 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 the inputs

After calibration, you can finalise the control travel in the INPUTS and MIXERS menus. Good practice is as follows:

1. For the flight controls (elevator, aileron and rudder): adjust travel in the INPUTS menu and leave mixer weights at 100%.
2. For all other interactions: adjust in MIXERS menu. If the calibration has been carried out correctly, then the mixer weights can equal on the left and right sides.

There are two options for Subtrim mode:

• 'delta' (default) - Subtrim adjustment does not affect end points
• 'equals': Subtrim adjustment shifts end points. So the end points are MIN+SUBTRIM and MAX+SUBTRIM. The output is clipped to MIN and MAX.

 

Delta mode allows independent calibration of centres and end points, and is therefore recommended.

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.

Using CAL mode to identifify problems

Most models suffer from drifting neutrals a few times during their lifetime. This may be due to drifting servos, or damaged linkages.

You can check for drift by entering CAL and seeing if the calibrated centres have changed. If the amount of drift is small, you can simply adjust PPM Centre - the effect is similar to moving the servo arm on its spline.

Once you exit CAL mode, any trim offsets will be restored to their previous settings.

By calibrating your channels and checking regularly, you can correct for servo drift while at the same time preserving your carefully adjusted trim offsets.

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 settings. 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:

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.

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 paired (left/right) mixers can 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.

Appendix

The OUTPUTS menu

Columns as follows:

• Channel label - 6 character channel name
• Subtrim - centre adjustment
• Min - end point 1
• Max - end point 2
• Dir - direction
• 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

The key fields for calibration are Min, Max, Dir, Curve and Subtrim mode.