Method and apparatus for diagnosing pneumatic control valve by using positioner model

1. A method comprising diagnosing a pneumatic control valve by using a positioner model,

the pneumatic control valve including a controller for outputting a control pressure, a positioner for receiving the control pressure as an input and controlling a pressure supplied to an actuator, the actuator for driving a valve based on the supplied pressure and the valve, wherein diagnosing the pneumatic control valve comprises:

establishing a relational expression for outputting a valve stem displacement according to the control pressure, the relational expression including parameters for properties of the positioner, the actuator and the valve;

determining an initial parameter value by adjusting values of the parameters until a difference between a measurement value for a normal operation of the pneumatic control valve and an output value obtained by the relational expression is smaller than a predetermined error limit;

determining a diagnostic parameter value by adjusting the values of the parameters until the difference between a measured value for a diagnostic operation of the pneumatic control valve and an output value obtained by the relational expression is smaller than a predetermined error limit; and

determining whether the pneumatic control valve has an abnormality by comparing the initial parameter value and the diagnostic parameter value, the positioner model comprising the relational expression and wherein the relational expression is an expression for outputting the valve stem displacement according to the control pressure, the relational expression including parameters for the properties of the positioner, the actuator, and the valve.

2. The method of claim 1 , wherein the parameters include a model parameter, an internal parameter and an estimated internal parameter, wherein the output value obtained by the relational expression changes according to a change of the model parameter value, wherein the internal parameter is a fixed value according to a specification of the pneumatic control valve, and wherein the estimated internal parameter is a fixed value obtained by estimating initially for the pneumatic control valve by one time.

3. The method of claim 2 , wherein the step of determining an initial parameter value determines the initial parameter value and an initial estimated internal parameter based on a predetermined internal parameter value, wherein the step of determining a diagnostic parameter value determines the diagnostic parameter value based on the predetermined internal parameter value and the initial estimated internal parameter value, and wherein the step of determining whether the pneumatic control valve has an abnormality determines whether the pneumatic control valve has an abnormality by comparing the initial model parameter and the diagnostic model parameter.

4. The method of claim 2 , wherein the model parameter includes a preload of a spring included in the positioner and a stem displacement transmissibility of a stem displacement feedback connection part included in the positioner.

5. The method of claim 2 , wherein the internal parameter includes a cross sectional area of a diaphragm included in the positioner and a cross sectional area of a piston included in the actuator.

6. The method of claim 2 , wherein the estimated internal parameter includes a pressure amplification factor of a pilot valve included in the positioner and a bulk modulus of elasticity of air within a piston included in the actuator.

7. The method of claim 1 , wherein the relational expression has a following transfer function:

x

s

=

1

1

-

H

1

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G

1

*

(

input

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⁢

1

*

H

1

)

+

1

1

+

H

2

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G

2

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input

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H

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input1: Input signal, P c *A c −F popre

H 1 : Forward Gain 1, V p *P s *A act *(1/k air )

G 1 : Backward Gain 1, G fb

input2: input signal, F stem

H 2 : Forward Gain 2, 1/k air

G 2 : Backward Gain 2, A act *V p *P s *−G fb

wherein, x s represents a stem displacement of the valve, P c represents a control pressure, A c represents a diaphragm cross sectional area of the positioner, F popre represents an internal spring preload of the positioner, V p represents a pressure amplification factor of a pilot valve of the positioner, P s represents a supply pressure of a pilot valve of the positioner, A act represents a cross sectional area of a piston of the actuator, k air represents a bulk modulus of elasticity of air within a piston, G fb represents a stem displacement transmissibility and F stem represents a load generated in the actuator or the valve.

8. An apparatus for diagnosing a pneumatic control valve by using a positioner model, the pneumatic control valve including a controller for outputting a control pressure, a positioner for receiving the control pressure as an input and controlling a pressure supplied to an actuator, the actuator being for driving a valve based on the supplied pressure and the valve, the further comprising: a model generation unit for establishing a relational expression for outputting a valve stem displacement according to the control pressure, the relational expression including parameters for properties of the positioner, the actuator and the valve; an initial value determination unit for determining an initial parameter value by adjusting the values of the parameters until a difference between a measurement value for a normal operation of the pneumatic control valve and an output value obtained by the relational expression is smaller than a predetermined error limit; a diagnostic value determination unit for determining a diagnostic parameter value by adjusting the values of the parameters until the difference between a measured value for a diagnostic operation of the pneumatic control valve and an output value obtained by the relational expression is smaller than a predetermined error limit; and a diagnostic unit for determining whether the pneumatic control valve has an abnormality based on a comparison between the initial parameter value and the diagnostic parameter value, the positioner model comprising the relational expression and wherein the relational expression is an expression for outputting the valve stem displacement according to the control pressure, the relational expression including parameters for the properties of the positioner, the actuator, and the valve.

9. The apparatus of claim 8 , wherein the parameters include a model parameter, an internal parameter and an estimated internal parameter, wherein the output value obtained by the relational expression changes according to a change of the model parameter value, wherein the internal parameter is a fixed value according to a specification of the pneumatic control valve, and wherein the estimated internal parameter is a fixed value by estimating initially for the pneumatic control valve by one time.

10. The apparatus of claim 9 , wherein the initial value determination unit determines the initial parameter value and an initial estimated internal parameter based on a predetermined internal parameter value, wherein the diagnostic value determination unit determines the diagnostic parameter value based on the predetermined internal parameter value and the initial estimated internal parameter value, and wherein the diagnostic unit determines whether the pneumatic control valve has an abnormality by comparing the initial model parameter and the diagnostic model parameter.

11. The apparatus of claim 8 , wherein the relational expression has a following transfer function:

x

s

=

1

1

-

H

1

⁢

G

1

*

(

input

⁢

⁢

1

*

H

1

)

+

1

1

+

H

2

⁢

G

2

*

(

input

⁢

⁢

2

*

H

2

)

input1: Input signal, P c *A c −F popre

H 1 : Forward Gain 1, V p *P s *A act *(1/k air )

G 1 : Backward Gain 1, G fb

input2: input signal, F stem

H 2 : Forward Gain 2, 1/k air

G 2 : Backward Gain 2, A act *V p *P s *−G fb

wherein, x s represents a stem displacement of the valve, P c represents a control pressure, A c represents a diaphragm cross sectional area of the positioner, F popre represents an internal spring preload of the positioner, V p represents a pressure amplification factor of a pilot valve of the positioner, P s represents a supply pressure of a pilot valve of the positioner, A act represents a cross sectional area of a piston of the actuator, k air represents a bulk modulus of elasticity of air within a piston, G fb represents a stem displacement transmissibility and F stem represents a load generated in the actuator or the valve.

12. A storage medium readable by a computer for diagnosing a pneumatic control valve by using a positioner model, wherein the control valve includes a controller for outputting a control pressure, a positioner for receiving the control pressure as an input and controlling a pressure supplied to an actuator, the actuator for driving a valve based on the supplied pressure and the valve, the computer comprising a processor to perform: a command for establishing a relational expression for outputting a valve stem displacement according to the control pressure, the relational expression including parameters for properties of the positioner, the actuator and the valve; a command for determining an initial parameter value by adjusting the values of the parameters until a difference between a measurement value for a normal operation of the pneumatic control valve and an output value obtained by the relational expression is smaller than a predetermined error limit; a command for determining a diagnostic parameter value by adjusting the values of the parameters until the difference between a measured value for a diagnostic operation of the pneumatic control valve and an output value obtained by the relational expression is smaller than a predetermined error limit; and a command for determining whether the pneumatic control valve has an abnormality by comparing the initial parameter value and the diagnostic parameter value, the positioner model comprising the relational expression, the relational expression outputting the valve stem displacement according to the control pressure, the relational expression including parameters for the properties of the positioner, the actuator, and the valve.

13. The method of claim 1 , wherein diagnosing a pneumatic control valve by using a positioner model is carried out non-abstractly.

14. A method comprising diagnosing a pneumatic control valve,

the pneumatic control valve including a controller for outputting a control pressure, an actuator, a valve stem having a valve, a positioner for receiving the control pressure as an input from the controller and using said control pressure to provide a supplied pressure to said actuator, thereby enabling said actuator to displace said valve stem based on said supplied pressure,

wherein, in response to said control pressure, said valve stem undergoes a displacement that depends on mechanical properties of said positioner, said actuator, and said valve,

said mechanical properties being characterized by parameters,

wherein diagnosing said pneumatic control valve comprises

obtaining a measurement value for normal operation of said pneumatic control valve,

adjusting values of the parameters for the properties of the positioner, the actuator, and the valve until a difference between said measurement value and a stem displacement that results from said values of said parameters is smaller than a predetermined error limit, thereby obtaining an initial-parameter value,

adjusting the values of the parameters for the properties of the positioner, the actuator, and the valve until the difference between a measured value for a diagnostic operation of the pneumatic control valve and an output value obtained by the relational expression is smaller than a predetermined error limit, thereby determining a diagnostic parameter value, and

comparing the initial parameter value and the diagnostic parameter value, and

based on said comparison, determining that said pneumatic control valve has an abnormality.