diff --git a/xarm_gripper/urdf/xarm_gripper_model.xacro b/xarm_gripper/urdf/xarm_gripper_model.xacro
index 61854f24..11b70e15 100644
--- a/xarm_gripper/urdf/xarm_gripper_model.xacro
+++ b/xarm_gripper/urdf/xarm_gripper_model.xacro
@@ -16,7 +16,7 @@
 
     <xacro:xarm_gripper_urdf prefix="${prefix}" attach_to="${attach_to}" xyz="${xyz}" rpy="${rpy}" />
 
-    <xacro:xarm_gripper_transmission prefix="${prefix}" hard_interface="$(eval 'EffortJointInterface' if effort_control else 'VelocityJointInterface' if velocity_control else 'PositionJointInterface')" />
+    <xacro:xarm_gripper_transmission prefix="${prefix}" hard_interface="$(eval 'EffortJointInterface' if ${effort_control} else 'VelocityJointInterface' if ${velocity_control} else 'PositionJointInterface')" />
 
     <xacro:xarm_gripper_gazebo prefix="${prefix}" />