AP_RangeFinder: added SONAR_RMETRIC option

this allows correct support for non-ratiometric rangefinders such as
the SF/02

libraries/AP_RangeFinder/AP_RangeFinder_analog.cpp

@@ -72,7 +72,11 @@ void AP_RangeFinder_analog::update_voltage(void)
    source->set_pin(ranger._pin[state.instance]);
    source->set_stop_pin((uint8_t)ranger._stop_pin[state.instance]);
    source->set_settle_time((uint16_t)ranger._settle_time_ms[state.instance]);
-   state.voltage_mv = source->voltage_average_ratiometric() * 1000U;
+   if (ranger._ratiometric[state.instance]) {
+       state.voltage_mv = source->voltage_average_ratiometric() * 1000U;
+   } else {
+       state.voltage_mv = source->voltage_average() * 1000U;
+   }
 }
 
 /*

libraries/AP_RangeFinder/RangeFinder.cpp

@@ -76,7 +76,13 @@ const AP_Param::GroupInfo RangeFinder::var_info[] PROGMEM = {
     // @Increment: 1
     AP_GROUPINFO("_SETTLE_MS", 8, RangeFinder, _settle_time_ms[0], 0),
 
-    // 9..12 left for future expansion
+    // @Param: _RMETRIC
+    // @DisplayName: Ratiometric
+    // @Description: This parameter sets whether an analog rangefinder is ratiometric. Most analog sonars are ratiometric, meaning that their output voltage is influenced by the supply voltage. Some analog rangefinders (such as the SF/02) have their own internal voltage regulators so they are not ratiometric
+    // @Values: 0:No,1:Yes
+    AP_GROUPINFO("_RMETRIC", 9, RangeFinder, _ratiometric[0], 1),
+
+    // 10..12 left for future expansion
 
 #if RANGEFINDER_MAX_INSTANCES > 1
     // @Param: 2_PIN
@@ -129,6 +135,12 @@ const AP_Param::GroupInfo RangeFinder::var_info[] PROGMEM = {
     // @Units: milliseconds
     // @Increment: 1
     AP_GROUPINFO("2_SETTLE_MS", 20, RangeFinder, _settle_time_ms[1], 0),
+
+    // @Param: 2_RMETRIC
+    // @DisplayName: Ratiometric
+    // @Description: This parameter sets whether an analog rangefinder is ratiometric. Most analog sonars are ratiometric, meaning that their output voltage is influenced by the supply voltage. Some analog rangefinders (such as the SF/02) have their own internal voltage regulators so they are not ratiometric
+    // @Values: 0:No,1:Yes
+    AP_GROUPINFO("2_RMETRIC", 21, RangeFinder, _ratiometric[1], 1),
 #endif
 
     AP_GROUPEND

libraries/AP_RangeFinder/RangeFinder.h

@@ -68,6 +68,7 @@ public:
     // parameters for each instance
     AP_Int8  _type[RANGEFINDER_MAX_INSTANCES];
     AP_Int8  _pin[RANGEFINDER_MAX_INSTANCES];
+    AP_Int8  _ratiometric[RANGEFINDER_MAX_INSTANCES];
     AP_Int8  _stop_pin[RANGEFINDER_MAX_INSTANCES];
     AP_Int16 _settle_time_ms[RANGEFINDER_MAX_INSTANCES];
     AP_Float _scaling[RANGEFINDER_MAX_INSTANCES];