added gain interfaces to constants to standardize

hardware_interface/include/hardware_interface/types/hardware_interface_type_values.hpp

@@ -35,6 +35,20 @@ constexpr char HW_IF_FORCE[] = "force";
 constexpr char HW_IF_CURRENT[] = "current";
 /// Constant defining temperature interface name
 constexpr char HW_IF_TEMPERATURE[] = "temperature";
+
+/// Gains interface constants
+/// Constant defining proportional gain interface name
+constexpr char HW_IF_PROPORTIONAL_GAIN[] = "proportional";
+/// Constant defining integral gain interface name
+constexpr char HW_IF_INTEGRAL_GAIN[] = "integral";
+/// Constant defining derivative gain interface name
+constexpr char HW_IF_DERIVATIVE_GAIN[] = "derivative";
+/// Constant defining integral clamp interface name
+constexpr char HW_IF_INTEGRAL_CLAMP_MAX[] = "integral_clamp_max";
+/// Constant defining integral clamp interface name
+constexpr char HW_IF_INTEGRAL_CLAMP_MIN[] = "integral_clamp_min";
+/// Constant defining the feedforward interface name
+constexpr char HW_IF_FEEDFORWARD[] = "feedforward";
 }  // namespace hardware_interface
 
 #endif  // HARDWARE_INTERFACE__TYPES__HARDWARE_INTERFACE_TYPE_VALUES_HPP_