romfs: Added back mixer readme, now gets filtered out

ROMFS/px4fmu_common/mixers/README

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+PX4 mixer definitions
+=====================
+
+Files in this directory implement example mixers that can be used as a basis
+for customisation, or for general testing purposes.
+
+Mixer basics
+------------
+
+Mixers combine control values from various sources (control tasks, user inputs,
+etc.) and produce output values suitable for controlling actuators; servos,
+motors, switches and so on.
+
+An actuator derives its value from the combination of one or more control
+values. Each of the control values is scaled according to the actuator's
+configuration and then combined to produce the actuator value, which may then be
+further scaled to suit the specific output type.
+
+Internally, all scaling is performed using floating point values. Inputs and
+outputs are clamped to the range -1.0 to 1.0.
+
+control    control   control
+   |          |         |
+   v          v         v
+ scale      scale     scale
+   |          |         |
+   |          v         |
+   +-------> mix <------+
+              |
+            scale
+              |
+              v
+             out
+
+Scaling
+-------
+
+Basic scalers provide linear scaling of the input to the output.
+
+Each scaler allows the input value to be scaled independently for inputs
+greater/less than zero. An offset can be applied to the output, and lower and
+upper boundary constraints can be applied. Negative scaling factors cause the
+output to be inverted (negative input produces positive output).
+
+Scaler pseudocode:
+
+if (input < 0)
+    output = (input * NEGATIVE_SCALE) + OFFSET
+else
+    output = (input * POSITIVE_SCALE) + OFFSET
+
+if (output < LOWER_LIMIT)
+    output = LOWER_LIMIT
+if (output > UPPER_LIMIT)
+    output = UPPER_LIMIT
+
+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.
+
+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 <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.
+
+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> <deadband>
+
+The supported geometries include:
+
+  4x - quadrotor in X configuration
+  4+ - quadrotor in + configuration
+  6x - hexcopter in X configuration
+  6+ - hexcopter 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.
+
+In the case where an actuator saturates, all actuator values are rescaled so that 
+the saturating actuator is limited to 1.0.