diff --git a/hbp_nrp_backend/requirements.txt b/hbp_nrp_backend/requirements.txt
index a280870..3a75354 100644
--- a/hbp_nrp_backend/requirements.txt
+++ b/hbp_nrp_backend/requirements.txt
@@ -1,6 +1,6 @@
 # other requirements
 lxml==3.4.0
-PyYAML==3.11
+PyYAML==5.1
 rospkg==1.0.38
 catkin_pkg==0.2.10
 pyxb==1.2.4
diff --git a/hbp_nrp_cleserver/hbp_nrp_cleserver/server/CLEGazeboSimulationAssembly.py b/hbp_nrp_cleserver/hbp_nrp_cleserver/server/CLEGazeboSimulationAssembly.py
index 9406908..344d59b 100644
--- a/hbp_nrp_cleserver/hbp_nrp_cleserver/server/CLEGazeboSimulationAssembly.py
+++ b/hbp_nrp_cleserver/hbp_nrp_cleserver/server/CLEGazeboSimulationAssembly.py
@@ -419,7 +419,7 @@ def __load_tfs(self):
                 continue
 
             try:
-                tfm.set_transfer_function(tf.code, new_code, tf.name, tf.active)
+                tfm.set_transfer_function(tf.code, new_code, tf.name, tf.active, tf.priority)
             except tfm.TFLoadingException as loading_e:
                 logger.error(loading_e)
                 tfm.set_flawed_transfer_function(tf.code, tf.name, loading_e)
diff --git a/hbp_nrp_commons/hbp_nrp_commons/sim_config/SimConfig.py b/hbp_nrp_commons/hbp_nrp_commons/sim_config/SimConfig.py
index 794aa6b..bd72c82 100644
--- a/hbp_nrp_commons/hbp_nrp_commons/sim_config/SimConfig.py
+++ b/hbp_nrp_commons/hbp_nrp_commons/sim_config/SimConfig.py
@@ -119,18 +119,25 @@ class _TF(object):  # pragma: no cover
     """
     Model to store TF information
     """
-    def __init__(self, name, code, src=None, active=False):
+    def __init__(self, name, code, src=None, active=False, priority=0):
         """
         Initialize a transfer function object
 
         :param name: name of the transfer function
         :param code: compiled (restricted) source code of transfer function
         :param src: source code (plain text) of transfer function
+        :param active: active state of the transfer function
+        :param priority: specifies execution order of the transfer function. Transfer functions with
+        higher priority are executed first.
         """
         self.name = name
         self.code = code
         self.src = src
         self.active = active
+        try:
+            self.priority = int(priority)
+        except (ValueError, TypeError):
+            self.priority = 0
 
 
 class SimConfig(object):
@@ -331,8 +338,9 @@ def _read_dom_data(self):
             code = generate_tf(_tf, self.sim_dir)
             name = get_tf_name(code)
             src = _tf.src if _tf.src else None  # must be not None and not ""
+            priority = _tf.priority if _tf.priority else 0
             active = bool(_tf.active) if _tf.active else False
-            self._tfs.append(_TF(name, code, src, active))
+            self._tfs.append(_TF(name, code, src, active, priority))
 
     def _read_robot_models(self):
         """
diff --git a/hbp_nrp_commons/hbp_nrp_commons/tests/sim_config/test_SimConfig.py b/hbp_nrp_commons/hbp_nrp_commons/tests/sim_config/test_SimConfig.py
index 9d32ed0..a2cc604 100644
--- a/hbp_nrp_commons/hbp_nrp_commons/tests/sim_config/test_SimConfig.py
+++ b/hbp_nrp_commons/hbp_nrp_commons/tests/sim_config/test_SimConfig.py
@@ -30,7 +30,7 @@
 from mock import patch, Mock, MagicMock, ANY, mock_open
 from hbp_nrp_commons.MockUtil import MockUtil
 
-from hbp_nrp_commons.sim_config.SimConfig import SimConfig
+from hbp_nrp_commons.sim_config.SimConfig import SimConfig, _TF
 
 __author__ = 'Hossain Mahmud'
 
@@ -116,6 +116,19 @@ def test_read_data(self):
     def test_properties(self):
         pass
 
+    def test_priority_is_int(self):
+        # test _TF priority value is always initialized to an integer
+        tf_1 = _TF("dummy_name", "dummy_code",priority=None)
+        tf_2 = _TF("dummy_name", "dummy_code", priority="string_value")
+        tf_3 = _TF("dummy_name", "dummy_code", priority=False)
+        tf_4 = _TF("dummy_name", "dummy_code", priority=3.1416)
+
+        self.assertTrue(type(tf_1.priority) == int)
+        self.assertTrue(type(tf_2.priority) == int)
+        self.assertTrue(type(tf_3.priority) == int)
+        self.assertTrue(type(tf_4.priority) == int)
+
+
 
 if __name__ == '__main__':
     unittest.main()