FC 153 - ISC Parameter Converter

The ISC parameter converter function block calculates optimal tuning parameters for the associated inferential smith controller (ISC controller, function code 160) using the output of the model parameter estimator (function code 152). The tuning values are the process gain, process deadtime and process lag time. The outputs describe process dynamics at one operating point.

 

Through a direct link with the ISC controller, the tuning parameters may be directly adapted. However, they will only be adapted when:

 

The appropriate adapt option is selected in S9.

        and

The quality output of the associated model parameter estimator is good (zero).

 

The ISC parameter converter also supervises an automated initialization routine for establishing initial estimates for the associated ISC and model parameter estimator. After completion of initialization, the converter tunes:

 

 

The ISC parameter converter also updates the estimated process deadtime for the associated ISC controller and model parameter estimator whenever input S3 is connected to a function block other than number five, and the initialization trigger equals zero. The initialization value of deadtime will be used as long as the initialization trigger equals one. If S3 is set to five then, the ISC parameter converter updates the model parameter estimator with the value used by the ISC controller.

 

 

Outputs:

Blk

Type

Description

N

R

Estimated process gain

N+1

R

Estimated process time constant

N+2

R

Adjusted process deadtime

N+3

R

Estimated process operating point

N+4

R

Initialization output:

0 = initialization complete, not in progress (normal)

1 = initialization failed or aborted

2 = initialization in progress

N+5

B

Pulse output from 0 to 1 and after 5 cycles, back to 0, initiated after completion of the automated initialization routine.

 

 

Specifications:

Spec

Tune

Default

Type

Range

Description

S1

N

5

I

Note 1

Block address of associated model parameter estimator

S2

N

5

I

Note 1

Block address of associated ISC controller

S3

N

5

I

Note 1

Block address for deadtime

S4

N

0

I

Note 1

Block address of hold signal

S5

Y2

-9.2E18

R

Full

Minimum allowable value for process gain

S6

Y2

9.2E18

R

Full

Maximum allowable value for process gain

S7

Y2

0.000

R

0.0 - 9.2E18

Minimum allowable value for process lag time

S8

Y2

9.2E18

R

0.0 - 9.2E18

Maximum allowable value for process lag time

S9

N

0

I

0 - 3

Adapt option:

0 = no adapt

1 = adapt process gain only

2 = adapt lag time only

3 = adapt both process gain and lag time

S10

Y

0

B

0 or 1

Initialization trigger (on 0 to 1 transition)

S11

Y

5.000

R

Full

Maximum control output change for initialization

S12

Y

0.000

R

Full

Spare real parameter

S13

N

0

I

Note 1

Spare boolean input

 

NOTES:

  1. Maximum values are:9,998 for the BRC-100, IMMFP11/12 and31,998 for the HAC

  2. Value automatically specified by the initialization routine.

 

 

153.1   Explanation

 

The ISC parameter converter function block calculates optimal tuning parameters for the associated inferential smith controller using the outputs of the model parameter estimator. Direct links between the function blocks simplify implementation.

 

The model parameter estimator generates the value for the process gain and process lag time. The outputs describe process dynamics at one operating point. This information directly converts to optimal tuning parameters for the inferential smith controller at this operating point using simple algebraic equations.

 

The tuning parameters for the ISC controller (process gain and lag time) are automatically adjusted by the ISC parameter converter as the model parameter estimator changes its estimates. However, the controller tuning time constant (S10) for the ISC controller is not automatically adjusted; this tuning time constant provides a mechanism for establishing the desired controller performance.

 

The ISC parameter converter also supervises an automated initialization routine for the self-tuning inferential smith controller. When the control station is set to automatic mode after the initialization trigger is changed from zero to one, the ISC parameter converter exercises the control output by a series of two step changes (in opposite directions) of a size previously established (typically ±5 percent), and monitors the reaction of the controlled process variable to estimate the process deadtime, gain and lag time. The automated initialization routine is immediately aborted if the control station for the ISC controller is set to manual mode.

 

After enough data has been collected to establish statistically valid estimates, the initialization routine is automatically terminated and the control station for the ISC controller is set automatically to manual mode.

 

The process deadtime estimated from the initialization routine is used by the model parameter estimator to determine online values of the process gain and lag time whenever S3 specifies block address five. If the process deadtime is externally calculated as a function of some process variable, then this value is connected to S3 and is used by the model parameter estimator and ISC controller.

 

The estimated values of process deadtime, gain and lag time from the initialization routine are used by the ISC parameter converter to automatically establish the initial values of a number of other specifications:

 

 

The initialized specifications can be manually changed after the initialization routine is complete. However, they should be valid for most applications.

 

Upon completion or failure of the automated initialization routine, the ISC parameter converter automatically returns the control station for the ISC controller to manual mode, and sets the value of the appropriate controller specifications. At this point, the initial settings can be monitored and validated before they are actually used by the controller. The initialization trigger must be manually set to zero for normal operation.

 

 

Specifications

 

S1

(Block address of associated model parameter estimator) Establishes the link between the ISC parameter converter and the associated model parameter estimator. The ISC parameter converter obtains the estimated value of the process model parameters and the status of the estimates through this link. The process deadtime for the model parameter estimator is updated through this link.

 

S2

(Block address of associated inferential smith controller) Links the ISC parameter converter with the associated ISC controller. Updating of the ISC controller tuning parameters and the process deadtime occur through this link.

 

S3

(Block address for process deadtime) Locates the value of the deadtime the ISC controller and model parameter estimator use. If the deadtime is not predicted as a function of a process variable, use the default address to permit the process deadtime setting of the ISC controller to be used by the ISC parameter converter.

 

S4

(Block address of hold signal) Identifies a hold switch for the ISC parameter converter. If the value of this switch is set to one, parameter conversion continues but the tuning parameters of the ISC controller are not automatically updated. New values for the calculated tuning parameters are available at the block outputs whenever the status of the model parameter estimator indicates parameter estimator locked on. When the estimator status indicates new parameter estimation in progress, the tuned ISC parameter will be displayed and the operator can manually tune the ISC controller. If the value of the switch is set to zero, the ISC parameter converter automatically tunes the parameter of the ISC specified by the adapt option. The operator can manually tune the ISC controller when the status from the model parameter estimator indicates new parameter estimation in progress.

 

S5 through S8

(Minimum and maximum tuning parameters) Required for commissioning of the self-tuning ISC controller and to increase the fault tolerance of ISC controller operation. Minimum and maximum values are preset by the automated initialization routine, but can be adjusted to match the process. In the event that the ISC parameter converter generates values for the tuning parameters outside of the previously specified constraints, the tuning parameters for the controller are limited to the constrained values.

 

S9

(Adapt option) Permits selection of self-tuning for either or both controller tuning parameters. If using the adaptive parameter scheduler (function code 154), the scheduled parameters should not be selected for self-tuning with this specification.

 

S10

(Initialization trigger) Provides the trigger for the automated initialization routine. When the trigger changes from zero to one and the ISC controller is in manual mode, the initialization routine is activated. As a safeguard, the station associated with the ISC controller must then be placed in automatic mode for initialization to proceed. The routine is automatically terminated when adequate data has been generated for process identification. As long as the initialization trigger equals one, the process deadtime will be that estimated by the initialization routine. This value can be changed manually by tuning the ISC controller. The model parameter estimator is automatically updated to this value. When the initialization trigger is set equal to zero, <S3> (block address for process deadtime) is utilized for process deadtime if S3 does not equal five.

 

S11

(Maximum control output change for initialization) Establishes the maximum change from the manually set valve position to be permitted during the automated initialization routine. Either a positive or negative step change can be specified.

 

 

153.2   Applications

 

The specialized function blocks required for self-tuning of the inferential smith controller are the model parameter estimator (function code 152), ISC parameter converter (function code 153) and the smith predictor (function code 160).

 

Figure 153-1 shows a basic self-tuning configuration. For more application information on self-tuning control, reference the Self Tuning Control application guide.