AP_Logger: Log D gain limit cycle gain scaler

Used by Plane roll and pitch loops in APM_Control

libraries/AP_Logger/AP_Logger.h

@@ -305,6 +305,7 @@ public:
         float I;
         float D;
         float FF;
+        float Dmod;
     };
 
     void Write_PID(uint8_t msg_type, const PID_Info &info);

libraries/AP_Logger/LogFile.cpp

@@ -831,7 +831,8 @@ void AP_Logger::Write_PID(uint8_t msg_type, const PID_Info &info)
         P               : info.P,
         I               : info.I,
         D               : info.D,
-        FF              : info.FF
+        FF              : info.FF,
+        Dmod            : info.Dmod
     };
     WriteBlock(&pkt, sizeof(pkt));
 }

libraries/AP_Logger/LogStructure.h

@@ -756,6 +756,7 @@ struct PACKED log_PID {
     float   I;
     float   D;
     float   FF;
+    float   Dmod;
 };
 
 struct PACKED log_Current {
@@ -1275,10 +1276,10 @@ struct PACKED log_Arm_Disarm {
 #define MAG_UNITS "sGGGGGGGGG-s"
 #define MAG_MULTS "FCCCCCCCCC-F"
 
-#define PID_LABELS "TimeUS,Tar,Act,Err,P,I,D,FF"
-#define PID_FMT    "Qfffffff"
-#define PID_UNITS  "s-------"
-#define PID_MULTS  "F-------"
+#define PID_LABELS "TimeUS,Tar,Act,Err,P,I,D,FF,Dmod"
+#define PID_FMT    "Qffffffff"
+#define PID_UNITS  "s--------"
+#define PID_MULTS  "F--------"
 
 #define QUAT_LABELS "TimeUS,C,Q1,Q2,Q3,Q4"
 #define QUAT_FMT    "QBffff"
@@ -1897,6 +1898,7 @@ struct PACKED log_Arm_Disarm {
 // @Field: I: integral part of PID
 // @Field: D: derivative part of PID
 // @Field: FF: controller feed-forward portion of response
+// @Field: Dmod: scaler applied to D gain to reduce limit cycling
 
 // @LoggerMessage: PM
 // @Description: autopilot system performance and general data dumping ground