diff --git a/rcb4/armh7interface.py b/rcb4/armh7interface.py
index 91263a0d..d1ac7ddc 100644
--- a/rcb4/armh7interface.py
+++ b/rcb4/armh7interface.py
@@ -705,6 +705,8 @@ def search_servo_ids(self):
                 self.servo_on_states_dict[index] = True
             else:
                 self.servo_on_states_dict[index] = False
+        # internally call actuator_to_joint_matrix to initialize it.
+        self.actuator_to_joint_matrix  # NOQA
         return servo_indices
 
     def search_air_board_ids(self):