Motors: move batt voltage lift_max calcs to parent

Moving from MotorsMatrix to parent Motors class allows them to be used
by other frame types
Also added sanity check of batt_voltage_min

libraries/AP_Motors/AP_MotorsMatrix.cpp

@@ -160,18 +160,6 @@ void AP_MotorsMatrix::output_armed()
     _rc_throttle.calc_pwm();
     _rc_yaw.calc_pwm();
 
-    if(_batt_voltage_max > 0 && _batt_voltage_min < _batt_voltage_max) {
-        float batt_voltage = _batt_voltage + _batt_current * _batt_resistance;
-        batt_voltage = constrain_float(batt_voltage, _batt_voltage_min, _batt_voltage_max);
-        // filter at 0.5 Hz
-        // todo: replace with filter object
-        _batt_voltage_filt = _batt_voltage_filt  + 0.007792f*(batt_voltage/_batt_voltage_max-_batt_voltage_filt);         // ratio of current battery voltage to maximum battery voltage
-        _lift_max = _batt_voltage_filt*(1-_thrust_curve_expo) + _thrust_curve_expo*_batt_voltage_filt*_batt_voltage_filt;
-    } else {
-        _batt_voltage_filt = 1;
-        _lift_max = 1;
-    }
-
     // if we are not sending a throttle output, we cut the motors
     if (_rc_throttle.servo_out == 0) {
         // range check spin_when_armed

libraries/AP_Motors/AP_Motors_Class.cpp

@@ -171,6 +171,9 @@ void AP_Motors::output()
     // update max throttle
     update_max_throttle();
 
+    // calc filtered battery voltage and lift_max
+    update_lift_max_from_batt_voltage();
+
     // output to motors
     if (_flags.armed ) {
         output_armed();
@@ -226,3 +229,26 @@ int16_t AP_Motors::apply_thrust_curve_and_volt_scaling(int16_t pwm_out, int16_t
     }
     return (temp_out*(_thrust_curve_max*pwm_max-pwm_min)+pwm_min);
 }
+
+// update_lift_max from battery voltage - used for voltage compensation
+void AP_Motors::update_lift_max_from_batt_voltage()
+{
+    // sanity check battery_voltage_min is not too small
+    _batt_voltage_min = max(_batt_voltage_min, _batt_voltage_max * 0.6f);
+
+    // if disabled or misconfigured exit immediately
+    if(_batt_voltage_max <= 0 && _batt_voltage_min >= _batt_voltage_max) {
+        _batt_voltage_filt = 1.0f;
+        _lift_max = 1.0f;
+        return;
+    }
+
+    // add current based voltage sag to battery voltage
+    float batt_voltage = _batt_voltage + _batt_current * _batt_resistance;
+    batt_voltage = constrain_float(batt_voltage, _batt_voltage_min, _batt_voltage_max);
+
+    // filter at 0.5 Hz
+    // todo: replace with filter object
+    _batt_voltage_filt = _batt_voltage_filt  + 0.007792f*(batt_voltage/_batt_voltage_max-_batt_voltage_filt);         // ratio of current battery voltage to maximum battery voltage
+    _lift_max = _batt_voltage_filt*(1-_thrust_curve_expo) + _thrust_curve_expo*_batt_voltage_filt*_batt_voltage_filt;
+}

libraries/AP_Motors/AP_Motors_Class.h

@@ -178,6 +178,9 @@ protected:
     // apply_thrust_curve_and_volt_scaling - thrust curve and voltage adjusted pwm value (i.e. 1000 ~ 2000)
     int16_t             apply_thrust_curve_and_volt_scaling(int16_t pwm_out, int16_t pwm_min, int16_t pwm_max) const;
 
+    // update_lift_max_from_batt_voltage - used for voltage compensation
+    void                update_lift_max_from_batt_voltage();
+
     // flag bitmask
     struct AP_Motors_flags {
         uint8_t armed               : 1;    // 1 if the motors are armed, 0 if disarmed