airspeed: Prevent the filter from overshooting into the negative airspeed range

src/drivers/meas_airspeed/meas_airspeed.cpp

@@ -258,6 +258,12 @@ MEASAirspeed::collect()
 	report.temperature = temperature;
 	report.differential_pressure_pa = diff_press_pa;
 	report.differential_pressure_filtered_pa =  _filter.apply(diff_press_pa);
+
+	/* the dynamics of the filter can make it overshoot into the negative range */
+	if (report.differential_pressure_filtered_pa < 0.0f) {
+		report.differential_pressure_filtered_pa = _filter.reset(diff_press_pa);
+	}
+
 	report.differential_pressure_raw_pa = diff_press_pa_raw;
 	report.voltage = 0;
 	report.max_differential_pressure_pa = _max_differential_pressure_pa;

src/lib/mathlib/math/filter/LowPassFilter2p.cpp

@@ -69,7 +69,7 @@ float LowPassFilter2p::apply(float sample)
     // do the filtering
     float delay_element_0 = sample - _delay_element_1 * _a1 - _delay_element_2 * _a2;
     if (isnan(delay_element_0) || isinf(delay_element_0)) {
-        // don't allow bad values to propogate via the filter
+        // don't allow bad values to propagate via the filter
         delay_element_0 = sample;
     }
     float output = delay_element_0 * _b0 + _delay_element_1 * _b1 + _delay_element_2 * _b2;
@@ -81,5 +81,10 @@ float LowPassFilter2p::apply(float sample)
     return output;
 }
 
+float LowPassFilter2p::reset(float sample) {
+    _delay_element_1 = _delay_element_2 = sample;
+    return apply(sample);
+}
+
 } // namespace math
 

src/lib/mathlib/math/filter/LowPassFilter2p.hpp

@@ -52,18 +52,30 @@ public:
         _delay_element_1 = _delay_element_2 = 0;
     }
 
-    // change parameters
+    /**
+     * Change filter parameters
+     */
     void set_cutoff_frequency(float sample_freq, float cutoff_freq);
 
-    // apply - Add a new raw value to the filter 
-    // and retrieve the filtered result
+    /**
+     * Add a new raw value to the filter
+     *
+     * @return retrieve the filtered result
+     */
     float apply(float sample);
 
-    // return the cutoff frequency
+    /**
+     * Return the cutoff frequency
+     */
     float get_cutoff_freq(void) const {
         return _cutoff_freq;
     }
 
+    /**
+     * Reset the filter state to this value
+     */
+    float reset(float sample);
+
 private:
     float           _cutoff_freq; 
     float           _a1;