cleanflight/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.

Custom Motor Mixing

Custom motor mixing allows for completely customized motor configurations. Each motor must be defined with a custom mixing table for that motor. The mix must reflect how close each motor is with reference to the CG (Center of Gravity) of the flight controller. A motor closer to the CG of the flight controller will need to travel less distance than a motor further away.

Steps to configure custom mixer in the CLI:

1. Use mixer custom to enable the custom mixing.
2. Use cmix reset to erase the any existing custom mixing.
3. Issue a cmix statement for each motor.

The cmix statement has the following syntax: cmix n THROTTLE ROLL PITCH YAW

Mixing table parameter	Definition
n	Motor ordering number
THROTTLE	All motors that are used in this configuration are set to 1.0. Unused set to 0.0.
ROLL	Indicates how much roll authority this motor imparts to the roll of the flight controller. Accepts values nominally from 1.0 to -1.0.
PITCH	Indicates the pitch authority this motor has over the flight controller. Also accepts values nominally from 1.0 to -1.0.
YAW	Indicates the direction of the motor rotation in relationship with the flight controller. 1.0 = CCW -1.0 = CW.

Example 1: A KK2.0 wired motor setup

Here's an example of a X configuration quad, but the motors are still wired using the KK board motor numbering scheme.

KK2.0 Motor Layout

  1CW      2CCW
     \    /
       KK
     /    \
  4CCW     3CW

1. Use mixer custom
2. Use cmix reset
3. Use cmix 1 1.0, 1.0, -1.0, -1.0 for the Front Left motor. It tells the flight controller the #1 motor is used, provides positive roll, provides negative pitch and is turning CW.
4. Use cmix 2 1.0, -1.0, -1.0, 1.0 for the Front Right motor. It still provides a negative pitch authority, but unlike the front left, it provides negative roll authority and turns CCW.
5. Use cmix 3 1.0, -1.0, 1.0, -1.0 for the Rear Right motor. It has negative roll, provides positive pitch when the speed is increased and turns CW.
6. Use cmix 4 1.0, 1.0, 1.0, 1.0 for the Rear Left motor. Increasing motor speed imparts positive roll, positive pitch and turns CCW.

Example 2: A HEX-U Copter

Here is an example of a U-shaped hex; probably good for herding giraffes in the Sahara. Because the 1 and 6 motors are closer to the roll axis, they impart much more force than the motors mounted twice as far from the FC CG. The effect they have on pitch is the same as the forward motors because they are the same distance from the FC CG. The 2 and 5 motors do not contribute anything to pitch because speeding them up and slowing them down has no effect on the forward/back pitch of the FC.

HEX6-U

.4........3. 
............
.5...FC...2. 
............
...6....1...
  

Command	Roll	Pitch	Yaw
Use cmix 1 1.0, -0.5, 1.0, -1.0	half negative	full positive	CW
Use cmix 2 1.0, -1.0, 0.0, 1.0	full negative	none	CCW
Use cmix 3 1.0, -1.0, -1.0, -1.0	full negative	full negative	CW
Use cmix 4 1.0, 1.0, -1.0, 1.0	full positive	full negative	CCW
Use cmix 5 1.0, 1.0, 0.0, -1.0	full positive	none	CW
Use cmix 6 1.0, 0.5, 1.0, 1.0	half positive	full positive	CCW