Force limited valve actuator and method therefor

A force limited valve actuator for a light duty valve adapted to be actuated with less than 1,100 N, said valve having a linearly actuated valve stem operative between a closed position and an open position, the valve actuator comprising:

• an electric motor controlled by a valve control signal providing a rotational output;
• a geared transmission coupled to said motor and providing a rotational drive based upon said rotational output;
• an actuator body;
• a two piece threaded drive having a first threaded body threadably interconnected to a complementary second threaded body, said first body having either a threaded nut element or a threaded screw element and the second body having either the complementary threaded screw element or the complementary threaded nut element, said threaded drive disposed within said actuator body;
• said first threaded body linearly coupled to said valve stem and adapted to operate between said open and closed valve positions;
• said second threaded body coupled to said geared transmission converting said rotational drive to a linear movement output and screwed rotation between said first and second threaded bodies;
• an actuation surface on said second threaded body;
• a spring interposed between said actuation surface and said actuator body;
• a positional stem on said second body indicating a position of said actuation surface with respect to said actuator body;
• said first and second threaded bodies moving between first, second and third linear positions with respect to said actuator body,
• in said first linear position said first threaded body linearly positioning said valve stem in a first predetermined position and said actuation surface disposed at a neutral position with respect to said actuator body;
• in said second linear position said first threaded body protruding outboard of said second threaded body and said valve stem being in a second predetermined position while said actuation surface is in said neutral position;
• and in said third linear position said actuation surface is moved away from said valve stem to a control position while said valve stem is in said second predetermined position thereby providing a force sensor for said valve stem;
• said first and second threaded bodies moved between said first, second and third linear positions by one-way screw rotation between said first and second threaded bodies;
• whereby said positional stem indicates said actuation surface at said control position and indicates a force level on said valve stem thereby effecting said valve control signal.