AC_PosControl: add get_z_accel_cmss helper function

libraries/AC_AttitudeControl/AC_PosControl.cpp

@@ -629,11 +629,10 @@ void AC_PosControl::run_z_controller()
     _accel_target.z += _accel_desired.z;
 
 
-    // the following section calculates a desired throttle needed to achieve the acceleration target
-    float z_accel_meas;         // actual acceleration
 
     // Calculate Earth Frame Z acceleration
-    z_accel_meas = -(_ahrs.get_accel_ef_blended().z + GRAVITY_MSS) * 100.0f;
+    const float z_accel_meas = get_z_accel_cmss();
+
 
     // ensure imax is always large enough to overpower hover throttle
     if (_motors.get_throttle_hover() * 1000.0f > _pid_accel_z.imax()) {

libraries/AC_AttitudeControl/AC_PosControl.h

@@ -342,6 +342,9 @@ protected:
     //          init_takeoff
     void run_z_controller();
 
+    // get earth-frame Z-axis acceleration with gravity removed in cm/s/s with +ve being up
+    float get_z_accel_cmss() const { return -(_ahrs.get_accel_ef_blended().z + GRAVITY_MSS) * 100.0f; }
+
     ///
     /// xy controller private methods
     ///