atbetaflight/docs/Mixer.md

# Mixer

Cleanflight supports a number of mixing configurations as well as custom mixing.  Mixer configurations determine how the servos and motors work together to control the aircraft.

## Configuration

To use a built-in mixing configuration, you can use the Chrome configuration GUI.  It includes images of the various mixer types to assist in making the proper connections.  See the Configuration section of the documentation for more information on the GUI.

You can also use the Command Line Interface (CLI) to set the mixer type:

1. Use `mixer list` to see a list of supported mixes
2. Select a mixer.  For example, to select TRI, use `mixer TRI`
3. You must use `save` to preserve your changes

## Supported Mixer Types

| Name          | Description               | Motors         | Servos           |
| ------------- | ------------------------- | -------------- | ---------------- |
| TRI           | Tricopter                 | M1-M3          | S1               |
| QUADP         | Quadcopter-Plus           | M1-M4          | None             |
| QUADX         | Quadcopter-X              | M1-M4          | None             |
| BI            | Bicopter (left/right)     | M1-M2          | S1, S2           |
| GIMBAL        | Gimbal control            | N/A            | S1, S2           |
| Y6            | Y6-copter                 | M1-M6          | None             |
| HEX6          | Hexacopter-Plus           | M1-M6          | None             |
| FLYING_WING   | Fixed wing; elevons       | M1             | S1, S2           |
| Y4            | Y4-copter                 | M1-M4          | None             |
| HEX6X         | Hexacopter-X              | M1-M6          | None             |
| OCTOX8        | Octocopter-X (over/under) | M1-M8          | None             |
| OCTOFLATP     | Octocopter-FlatPlus       | M1-M8          | None             |
| OCTOFLATX     | Octocopter-FlatX          | M1-M8          | None             |
| AIRPLANE      | Fixed wing; Ax2, R, E     | M1             | S1, S2, S3, S4   |
| HELI_120_CCPM |                           |                |                  |
| HELI_90_DEG   |                           |                |                  |
| VTAIL4        | Quadcopter with V-Tail    | M1-M4          | N/A              |
| HEX6H         | Hexacopter-H              | M1-M6          | None             |
| PPM_TO_SERVO  |                           |                |                  |
| DUALCOPTER    | Dualcopter                | M1-M2          | S1, S2           |
| SINGLECOPTER  | Conventional helicopter   | M1             | S1               |
| ATAIL4        | Quadcopter with A-Tail    | M1-M4          | N/A              |
| CUSTOM        | User-defined              |                |                  |


## Servo filtering

A low-pass filter can be enabled for the servos.  It may be useful for avoiding structural modes in the airframe, for example.  

### Configuration 

Currently it can only be configured via the CLI:

1. Use `set servo_lowpass_freq = nnn` to select the cutoff frequency.  Valid values range from 10 to 400.  This is a fraction of the loop frequency in 1/1000ths. For example, `40` means `0.040`.
2. Use `set servo_lowpass_enable = 1` to enable filtering.

The cutoff frequency can be determined by the following formula:
`Frequency = 1000 * servo_lowpass_freq / looptime`

For example, if `servo_lowpass_freq` is set to 40, and looptime is set to the default of 3500 us, the cutoff frequency will be 11.43 Hz.

### Tuning

One method for tuning the filter cutoff is as follows:

1. Ensure your vehicle can move at least somewhat freely in the troublesome axis.  For example, if you are having yaw oscillations on a tricopter, ensure that the copter is supported in a way that allows it to rotate left and right to at least some degree.  Suspension near the CG is ideal.  Alternatively, you can just fly the vehicle and trigger the problematic condition you are trying to eliminate, although tuning will be more tedious.

2. Tap the vehicle at its end in the axis under evaluation.  Directly commanding the servo in question to move may also be used.  In the tricopter example, tap the end of the tail boom from the side, or command a yaw using your transmitter.

3. If your vehicle oscillates for several seconds or even continues oscillating indefinitely, then the filter cutoff frequency should be reduced. Reduce the value of `servo_lowpass_freq` by half its current value and repeat the previous step.

4. If the oscillations are dampened within roughly a second or are no longer present, then you are done.  Be sure to run `save`.