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PX4 mixer definitions |
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===================== |
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Files in this directory implement example mixers that can be used as a basis |
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for customisation, or for general testing purposes. |
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Mixer basics |
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------------ |
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Mixers combine control values from various sources (control tasks, user inputs, |
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etc.) and produce output values suitable for controlling actuators; servos, |
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motors, switches and so on. |
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An actuator derives its value from the combination of one or more control |
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values. Each of the control values is scaled according to the actuator's |
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configuration and then combined to produce the actuator value, which may then be |
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further scaled to suit the specific output type. |
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Internally, all scaling is performed using floating point values. Inputs and |
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outputs are clamped to the range -1.0 to 1.0. |
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control control control |
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v v v |
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scale scale scale |
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| v | |
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+-------> mix <------+ |
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scale |
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v |
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out |
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Scaling |
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------- |
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Basic scalers provide linear scaling of the input to the output. |
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Each scaler allows the input value to be scaled independently for inputs |
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greater/less than zero. An offset can be applied to the output, and lower and |
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upper boundary constraints can be applied. Negative scaling factors cause the |
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output to be inverted (negative input produces positive output). |
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Scaler pseudocode: |
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if (input < 0) |
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output = (input * NEGATIVE_SCALE) + OFFSET |
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else |
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output = (input * POSITIVE_SCALE) + OFFSET |
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if (output < LOWER_LIMIT) |
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output = LOWER_LIMIT |
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if (output > UPPER_LIMIT) |
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output = UPPER_LIMIT |
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Syntax |
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------ |
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Mixer definitions are text files; lines beginning with a single capital letter |
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followed by a colon are significant. All other lines are ignored, meaning that |
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explanatory text can be freely mixed with the definitions. |
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Each file may define more than one mixer; the allocation of mixers to actuators |
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is specific to the device reading the mixer definition, and the number of |
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actuator outputs generated by a mixer is specific to the mixer. |
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A mixer begins with a line of the form |
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<tag>: <mixer arguments> |
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The tag selects the mixer type; 'M' for a simple summing mixer, 'R' for a |
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multirotor mixer, etc. |
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Null Mixer |
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.......... |
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A null mixer consumes no controls and generates a single actuator output whose |
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value is always zero. Typically a null mixer is used as a placeholder in a |
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collection of mixers in order to achieve a specific pattern of actuator outputs. |
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The null mixer definition has the form: |
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Z: |
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Simple Mixer |
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............ |
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A simple mixer combines zero or more control inputs into a single actuator |
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output. Inputs are scaled, and the mixing function sums the result before |
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applying an output scaler. |
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A simple mixer definition begins with: |
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M: <control count> |
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O: <-ve scale> <+ve scale> <offset> <lower limit> <upper limit> |
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If <control count> is zero, the sum is effectively zero and the mixer will |
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output a fixed value that is <offset> constrained by <lower limit> and <upper |
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limit>. |
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The second line defines the output scaler with scaler parameters as discussed |
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above. Whilst the calculations are performed as floating-point operations, the |
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values stored in the definition file are scaled by a factor of 10000; i.e. an |
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offset of -0.5 is encoded as -5000. |
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The definition continues with <control count> entries describing the control |
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inputs and their scaling, in the form: |
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S: <group> <index> <-ve scale> <+ve scale> <offset> <lower limit> <upper limit> |
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The <group> value identifies the control group from which the scaler will read, |
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and the <index> value an offset within that group. These values are specific to |
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the device reading the mixer definition. |
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When used to mix vehicle controls, mixer group zero is the vehicle attitude |
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control group, and index values zero through three are normally roll, pitch, |
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yaw and thrust respectively. |
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The remaining fields on the line configure the control scaler with parameters as |
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discussed above. Whilst the calculations are performed as floating-point |
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operations, the values stored in the definition file are scaled by a factor of |
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10000; i.e. an offset of -0.5 is encoded as -5000. |
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Multirotor Mixer |
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................ |
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The multirotor mixer combines four control inputs (roll, pitch, yaw, thrust) |
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into a set of actuator outputs intended to drive motor speed controllers. |
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The mixer definition is a single line of the form: |
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R: <geometry> <roll scale> <pitch scale> <yaw scale> <deadband> |
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The supported geometries include: |
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4x - quadrotor in X configuration |
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4+ - quadrotor in + configuration |
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6x - hexcopter in X configuration |
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6+ - hexcopter in + configuration |
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8x - octocopter in X configuration |
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8+ - octocopter in + configuration |
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Each of the roll, pitch and yaw scale values determine scaling of the roll, |
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pitch and yaw controls relative to the thrust control. Whilst the calculations |
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are performed as floating-point operations, the values stored in the definition |
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file are scaled by a factor of 10000; i.e. an factor of 0.5 is encoded as 5000. |
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Roll, pitch and yaw inputs are expected to range from -1.0 to 1.0, whilst the |
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thrust input ranges from 0.0 to 1.0. Output for each actuator is in the |
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range -1.0 to 1.0. |
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In the case where an actuator saturates, all actuator values are rescaled so that |
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the saturating actuator is limited to 1.0. |
Lorenz Meier
11 years ago