Valve positioner having adjustable gain

Valve positioner apparatus having a feedback mechanism including a feedback spring contained in a housing and a balance beam also contained within said housing and operably connected to a spool valve for supplying fluid to a valve actuator, said valve positioner apparatus having independently operable span and gain adjustment apparatus, said gain adjustment apparatus comprising:

• a hinge spring having a stiffness and including a first end connected to said valve positioner housing and a second end thereof connected to said balance beam;
• and,
• a gain adjustment mechanism connected to said hinge spring for varying the stiffness of said hinge spring.

In a valve positioner for use with a pneumatic valve actuator having a rotatable output shaft for effecting rotary positioning of a valve, said valve positioner including an elongated axially displaceable spool valve located between said pneumatic valve actuator and a supply of pneumatic pressure for controlling rotary movement of said actuator output shaft, a transducer coupled to said spool valve and responsive to a control signal for effecting axial displacement of said spool valve away from a predetermined neutral position when it is desired to effect rotation of said output shaft, said transducer including a balance beam mounted for movement in response to said control signal and being operably connected at a first end to said spool valve and at a second end to a hinge spring having a stiffness, and a feedback mechanism responsive to rotary movement of said rotatable output shaft, said feedback mechanism including a feedback spring for generating a force in opposition to a force generated in said transducer by said control signal for returning said spool valve to said neutral position when said output shaft has rotated to a desired position, wherein the improvement comprises:

• a gain adjustment mechanism operably connected to said hinge spring for varying the stiffness of said hinge spring.

A method of operating a valve positioner of a type including a hinge spring having a stiffness and being secured at a first end to a housing of said valve positioner and at a second end to a balance beam which operates a spool valve of said valve positioner, said positioner including a feedback mechanism including a feedback spring for providing a balancing force to said balance beam, said valve positioner further including a span adjustment mechanism and an independently operable gain adjustment mechanism mounted for movement along said hinge spring, the method comprising the steps of:

• calibrating said positioner at least in part by use of said span adjustment mechanism;
• and
• moving said gain adjustment mechanism along said hinge spring thereby varying the stiffness of said hinge spring and the gain of said positioner.