Calibration Mode

Mike Shellim 4 Dec 2013

1. Introduction to Calibration Mode

Calibration Mode is a special flight mode which you can add to any model. It's an essential aid for calibrating your servos. When Calibration Mode is active, all mixers and trims are disabled, allowing end points and centres to be visualised.

heading image


2. Implementation

Implementation is straightforward, a single REPL mix is appended to each servo channel.

Servo Channel

Mix 1 ...

Mix 2 ...


Mix N: Src=[stick] wt=+/-100 Multiplex=REPL Trim=NO_TRIM

Looking at the extra line:

When the last line is active (we'll see later how to activate it), OpenTx will pass the raw stick value in the range -100 to 100 directly to the Outputs where it's scaled according to the channel's Min/Max/Subtrim values. This results in the following behaviour:

3. How to integrate Calibration Mode in your setup

Step 1 - append REPL mix to servo channels

For each of the servo channels, add the following mixer-line as the last line:

Src is the stick or switch to control the calibration on that servo. You can choose any convenient control - it need not be the same as the control used in flight. Typically, a single control will drive a pair of related surfaces. Here's a typical scheme for a sailplane:

Set weight to -100 or +100 so that both surfaces to move together in the same sense. This allows you to match their travel by eye.

Step 2. Activate using a switch

The CAL lines may be activated directly via a switch, or indirectly via a dedicated flight mode. The flight mode method has the advantage that the flight mode name e.g. 'CAL' is displayed on the screen. Use FM1 (the highest priority mode). If this is inconvenient (maybe you already have flight modes assigned), then the direct switch approach is fine.

Either way, do please use a virtual latch like this one, for safety reasons - the last thing you want is to accidentally activate calibration mode in flight!

Step 3. Set servo rotation and mixer weights

To make calibration as intuitive as possible, set the DIRECTION of each servo as follows:

The easiest way is to try it and see: go into the servos menu; for each servo, adjust SUBTRIM back and forth, and see which way the control surface is moving.

If you change a servo's DIRECTION, remember that all the mixes for that channel will be reversed. To correct this, reverse the sign of the wt parameter of all the mixers affecting that channel (except the REPL line).

As mentioned, the weights in the REPL lines should be set to +/-100, the sign should be chosen so that the surfaces move together in the same direction in Calibration mode, to allow easy matching by eye.

4. Stick calibration

For servo calibration to work, your sticks must also be properly calibrated. This is done via an OpenTx menu (Long press Menu -> Page x 8)

5. Screenshot

Below is a screenshot from OpenTx Companion, showing two aileron channels and the extra CAL lines. The 'R' at the beginning denotes a REPL directive.

C9X screenshot

6. Demo EEPROM image

  1. Download EEPROM image (right click and save as)
  2. Open EEPROM image in Companion9X - or, better, transfer to your transmitter and monitor the effect on the channel monitor screen.
  3. Calibration mode is entered via switch SF.