4.5:add fuel level sensor linearity params to upcoming

common/source/docs/common-fuel-sensors.rst

@@ -91,4 +91,16 @@ Advanced Parameter Setup
 
 Fuel level is normally calculated using a linear relationship between fuel level and reported analog voltage from the sensor. However, some sensors have a non-linear output versus fuel level. ArduPilot provides up to a third order polynomial curve fit by changing the following parameters from their default values (which is a linear fit):
 
+example shown for first battery monitor
+- :ref:`BATT_FL_FF<BATT_FL_FF>` :First order polynomial fit term
+- :ref:`BATT_FL_FS<BATT_FL_FS>` :Second order polynomial fit term
+- :ref:`BATT_FL_FT<BATT_FL_FT>` :Third order polynomial fit term
+- :ref:`BATT_FL_OFF<BATT_FL_OFF>` :offset term
 
+every voltage sample from the sensor is modified as:
+
+measured sensor voltage = vm
+
+modified voltage = (BATT_FL_FT * vm^3 + BATT_FL_FS * vm^2 + BATT_FL_FF * vm) + BATT_FL_OFF
+
+This aalows the non-linearity to be compensated. The values for these parameters should be obtained from the sensor manufacturer, if applicable.