Mixing and Actuators

The PX4 architecture ensures that the airframe layout does not require special case handling in the core controllers.

Mixing means to take force commands (e.g. turn right) and translate them to actuator commands which control motors or servos. For a plane with one servo per aileron this means to command one of them high and the other low. The same applies for multicopters: Pitching forward requires changing the speed of all motors.

Separating the mixer logic from the actual attitude controller greatly improves reusability.

Control Pipeline

A particular controller sends a particular normalized force or torque demand (scaled from -1..+1) to the mixer, which then sets individual actuators accordingly. The output driver (e.g. UART, UAVCAN or PWM) then scales it to the actuators native units, e.g. a PWM value of 1300.

Control Groups

PX4 uses control groups (inputs) and output groups. Conceptually they are very simple: A control group is e.g. attitude, for the core flight controls, or gimbal for payload. An output group is one physical bus, e.g. the first 8 PWM outputs for servos. Each of these groups has 8 normalized (-1..+1) command ports, which can be mapped and scaled through the mixer. A mixer defines how each of these 8 signals of the controls are connected to the 8 outputs.

For a simple plane control 0 (roll) is connected straight to output 0 (aileron). For a multicopter things are a bit different: control 0 (roll) is connected to all four motors and combined with throttle.

Control Group #0 (Flight Control)

• 0: roll (-1..1)
• 1: pitch (-1..1)
• 2: yaw (-1..1)
• 3: throttle (0..1 normal range, -1..1 for variable pitch / thrust reversers)
• 4: flaps (-1..1)
• 5: spoilers (-1..1)
• 6: airbrakes (-1..1)
• 7: landing gear (-1..1)

Control Group #1 (Flight Control VTOL/Alternate)

• 0: roll ALT (-1..1)
• 1: pitch ALT (-1..1)
• 2: yaw ALT (-1..1)
• 3: throttle ALT (0..1 normal range, -1..1 for variable pitch / thrust reversers)
• 4: reserved / aux0
• 5: reserved / aux1
• 6: reserved / aux2
• 7: reserved / aux3

Control Group #2 (Gimbal)

• 0: gimbal roll
• 1: gimbal pitch
• 2: gimbal yaw
• 3: gimbal shutter
• 4: reserved
• 5: reserved
• 6: reserved
• 7: reserved (parachute, -1..1)

Control Group #3 (Manual Passthrough)

• 0: RC roll
• 1: RC pitch
• 2: RC yaw
• 3: RC throttle
• 4: RC mode switch
• 5: RC aux1
• 6: RC aux2
• 7: RC aux3

This group is only used to define mapping of RC inputs to specific outputs during normal operation (see quad_x.main.mix for an example of AUX2 being scaled in a mixer). In the event of manual IO failsafe override (if the PX4FMU stops communicating with the PX4IO board) only the mapping/mixing defined by control group 0 inputs for roll, pitch, yaw and throttle are used (other mappings are ignored).

• 0: function 0 (default: parachute)
• 1: function 1
• 2: function 2
• 3: function 3
• 4: function 4
• 5: function 5
• 6: function 6
• 7: function 7

Virtual Control Groups

These groups are NOT mixer inputs, but serve as meta-channels to feed fixed wing and multicopter controller outputs into the VTOL governor module.

Control Group #4 (Flight Control MC VIRTUAL)

• 0: roll ALT (-1..1)
• 1: pitch ALT (-1..1)
• 2: yaw ALT (-1..1)
• 3: throttle ALT (0..1 normal range, -1..1 for variable pitch / thrust reversers)
• 4: reserved / aux0
• 5: reserved / aux1
• 6: reserved / aux2
• 7: reserved / aux3

Control Group #5 (Flight Control FW VIRTUAL)

• 0: roll ALT (-1..1)
• 1: pitch ALT (-1..1)
• 2: yaw ALT (-1..1)
• 3: throttle ALT (0..1 normal range, -1..1 for variable pitch / thrust reversers)
• 4: reserved / aux0
• 5: reserved / aux1
• 6: reserved / aux2
• 7: reserved / aux3

Mapping

Since there are multiple control groups (like flight controls, payload, etc.) and multiple output groups (first 8 PWM outpus, UAVCAN, etc.), one control group can send command to multiple output groups.

PX4 Mixer Definitions

Files in ROMFS/px4fmu_common/mixers implement mixers that are used for predefined airframes. They can be used as a basis for customisation, or for general testing purposes.

Mixer File Names

A mixer file must be named XXXX.main.mix if it is responsible for the mixing of MAIN outputs or XXXX.aux.mix if it mixes AUX outputs.

Syntax

Mixer definitions are text files; lines beginning with a single capital letter followed by a colon are significant. All other lines are ignored, meaning that explanatory text can be freely mixed with the definitions.

Each file may define more than one mixer; the allocation of mixers to actuators is specific to the device reading the mixer definition, and the number of actuator outputs generated by a mixer is specific to the mixer.

For example: each simple or null mixer is assigned to outputs 1 to x in the order they appear in the mixer file.

A mixer begins with a line of the form

<tag>: <mixer arguments>


The tag selects the mixer type; 'M' for a simple summing mixer, 'R' for a multirotor mixer, etc.

Null Mixer

A null mixer consumes no controls and generates a single actuator output whose value is always zero. Typically a null mixer is used as a placeholder in a collection of mixers in order to achieve a specific pattern of actuator outputs.

The null mixer definition has the form:

Z:


Simple Mixer

A simple mixer combines zero or more control inputs into a single actuator output. Inputs are scaled, and the mixing function sums the result before applying an output scaler.

A simple mixer definition begins with:

M: <control count>
O: <-ve scale> <+ve scale> <offset> <lower limit> <upper limit>


If <control count> is zero, the sum is effectively zero and the mixer will output a fixed value that is <offset> constrained by <lower limit> and <upper limit>.

The second line defines the output scaler with scaler parameters as discussed above. Whilst the calculations are performed as floating-point operations, the values stored in the definition file are scaled by a factor of 10000; i.e. an offset of -0.5 is encoded as -5000.

The definition continues with <control count> entries describing the control inputs and their scaling, in the form:

S: <group> <index> <-ve scale> <+ve scale> <offset> <lower limit> <upper limit>


The S: lines must be below the O: line.

The <group> value identifies the control group from which the scaler will read, and the <index> value an offset within that group.
These values are specific to the device reading the mixer definition.

When used to mix vehicle controls, mixer group zero is the vehicle attitude control group, and index values zero through three are normally roll, pitch, yaw and thrust respectively.

The remaining fields on the line configure the control scaler with parameters as discussed above. Whilst the calculations are performed as floating-point operations, the values stored in the definition file are scaled by a factor of 10000; i.e. an offset of -0.5 is encoded as -5000.

An example of a typical mixer file is explained here.

Multirotor Mixer

The multirotor mixer combines four control inputs (roll, pitch, yaw, thrust) into a set of actuator outputs intended to drive motor speed controllers.

The mixer definition is a single line of the form:

R: <geometry> <roll scale> <pitch scale> <yaw scale> <idlespeed>


The supported geometries include:

• 4x - quadrotor in X configuration
• 4+ - quadrotor in + configuration
• 6x - hexacopter in X configuration
• 6+ - hexacopter in + configuration
• 8x - octocopter in X configuration
• 8+ - octocopter in + configuration

Each of the roll, pitch and yaw scale values determine scaling of the roll, pitch and yaw controls relative to the thrust control. Whilst the calculations are performed as floating-point operations, the values stored in the definition file are scaled by a factor of 10000; i.e. an factor of 0.5 is encoded as 5000.

Roll, pitch and yaw inputs are expected to range from -1.0 to 1.0, whilst the thrust input ranges from 0.0 to 1.0. Output for each actuator is in the range -1.0 to 1.0.

Idlespeed can range from 0.0 to 1.0. Idlespeed is relative to the maximum speed of motors and it is the speed at which the motors are commanded to rotate when all control inputs are zero.

In the case where an actuator saturates, all actuator values are rescaled so that the saturating actuator is limited to 1.0.

Helicopter Mixer

The helicopter mixer combines three control inputs (roll, pitch, thrust) into four outputs ( swash-plate servos and main motor ESC setting). The first output of the helicopter mixer is the throttle setting for the main motor. The subsequent outputs are the swash-plate servos. The tail-rotor can be controlled by adding a simple mixer.

The thrust control input is used for both the main motor setting as well as the collective pitch for the swash-plate. It uses a throttle-curve and a pitch-curve, both consisting of five points.

The throttle- and pitch- curves map the "thrust" stick input position to a throttle value and a pitch value (separately). This allows the flight characteristics to be tuned for different types of flying. An explanation of how curves might be tuned can be found in this guide (search on Programmable Throttle Curves and Programmable Pitch Curves).

The mixer definition begins with:

H: <number of swash-plate servos, either 3 or 4>
T: <throttle setting at thrust: 0%> <25%> <50%> <75%> <100%>
P: <collective pitch at thrust: 0%> <25%> <50%> <75%> <100%>


T: defines the points for the throttle-curve. P: defines the points for the pitch-curve. Both curves contain five points in the range between 0 and 10000. For simple linear behavior, the five values for a curve should be 0 2500 5000 7500 10000.

This is followed by lines for each of the swash-plate servos (either 3 or 4) in the following form:

S: <angle> <arm length> <scale> <offset> <lower limit> <upper limit>


The <angle> is in degrees, with 0 degrees being in the direction of the nose. Viewed from above, a positive angle is clock-wise. The <arm length> is a normalized length with 10000 being equal to 1. If all servo-arms are the same length, the values should al be 10000. A bigger arm length reduces the amount of servo deflection and a shorter arm will increase the servo deflection.

The servo output is scaled by <scale> / 10000. After the scaling, the <offset> is applied, which should be between -10000 and +10000. The <lower limit> and <upper limit> should be -10000 and +10000 for full servo range.

The tail rotor can be controller by adding a simple mixer:

M: 1
S: 0 2  10000  10000      0 -10000  10000


By doing so, the tail rotor setting is directly mapped to the yaw command. This works for both servo-controlled tail-rotors, as well as for tail rotors with a dedicated motor.

The blade 130 helicopter mixer can be viewed as an example. The throttle-curve starts with a slightly steeper slope to reach 6000 (0.6) at 50% thrust. It continues with a less steep slope to reach 10000 (1.0) at 100% thrust. The pitch-curve is linear, but does not use the entire range. At 0% throttle, the collective pitch setting is already at 500 (0.05). At maximum throttle, the collective pitch is only 4500 (0.45). Using higher values for this type of helicopter would stall the blades. The swash-plate servos for this helicopter are located at angles of 0, 140 and 220 degrees. The servo arm-lenghts are not equal. The second and third servo have a longer arm, by a ratio of 1.3054 compared to the first servo. The servos are limited at -8000 and 8000 because they are mechanically constrained.