autotest: add test for minimum altitude avoidance fence

Tools/autotest/arducopter.py

@@ -1693,6 +1693,59 @@ def MinAltFence(self):
 
         self.zero_throttle()
 
+    # MinAltFenceAvoid - fly down and make sure fence action does not trigger
+    # Also check that the vehicle will not try and ascend too fast when trying to backup from a min alt fence due to avoidance
+    def MinAltFenceAvoid(self):
+        '''Test Min Alt Fence Avoidance'''
+        self.takeoff(30, mode="LOITER")
+        """Hold loiter position."""
+
+        # enable fence, only min altitude
+        # No action, rely on avoidance to prevent the breach
+        self.set_parameters({
+            "FENCE_ENABLE": 1,
+            "FENCE_TYPE": 8,
+            "FENCE_ALT_MIN": 20,
+            "FENCE_ACTION": 0,
+        })
+
+        # Try and fly past the fence
+        self.set_rc(3, 1120)
+
+        # Avoid should prevent the vehicle flying past the fence, so the altitude wait should timeouts
+        try:
+            self.wait_altitude(10, 15, timeout=90, relative=True)
+            raise NotAchievedException("Avoid should prevent reaching altitude")
+        except AutoTestTimeoutException:
+            pass
+        except Exception as e:
+            raise e
+
+        # Check ascent is not too fast, allow 10% above the configured backup speed
+        max_ascent_rate = self.get_parameter("AVOID_BACKUP_SPD") * 1.1
+
+        def get_climb_rate(mav, m):
+            m_type = m.get_type()
+            if m_type != 'VFR_HUD':
+                return
+            if m.climb > max_ascent_rate:
+                raise NotAchievedException("Ascending too fast want %f got %f" % (max_ascent_rate, m.climb))
+
+        self.context_push()
+        self.install_message_hook_context(get_climb_rate)
+
+        # Reduce fence alt, this will result in a fence breach, but there is no action.
+        # Avoid should then backup the vehicle to be over the new fence alt.
+        self.set_parameters({
+            "FENCE_ALT_MIN": 30,
+        })
+        self.wait_altitude(30, 40, timeout=90, relative=True)
+
+        self.context_pop()
+
+        self.set_rc(3, 1500)
+        self.do_RTL()
+
     def FenceFloorEnabledLanding(self):
         """Ensures we can initiate and complete an RTL while the fence is
         enabled.
@@ -10633,6 +10686,7 @@ def tests1d(self):
              self.MaxAltFence,
              self.MaxAltFenceAvoid,
              self.MinAltFence,
+             self.MinAltFenceAvoid,
              self.FenceFloorEnabledLanding,
              self.FenceFloorAutoDisableLanding,
              self.FenceFloorAutoEnableOnArming,