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APPLICATION DATASIEMENS

AD353-118Rev 1October 2005

Procidia Control SolutionsCoarse/Fine Control

This application data sheet describes imple-menting a coarse/fine control strategy in aSiemens 353 controller1.

A coarse/fine control strategy uses two finalcontrol elements (FCEs), one large and onesmall, connected for an additive affect on theprocess. The large FCE supports large changes

in the manipulated variable, but often lacksresolution. To improve resolution and increaseturndown, the small FCE, which has betterresolution, is used to trim the large FCE.

Coarse/Fine control is sometimes called bigvalve/little valve control. Figure 1 shows asystem that employs this strategy for pH control.

AIC

AT

pH

Effluent

M

Reagent

Influent

Figure 1 Coarse/Fine pH Control

1The Siemens family of 353 controllers includes Model353 Process Automation Controller, Model 353R RackMount Process Automation Controller, Model 352PlusSingle-Loop Digital Controller, Model 354 UniversalControl Station, and Model 354N Universal LoopController

There are several coarse/fine control strategies:split-range control, floating control, and center-seeking control. Each will enable precise flowcontrol which will improve product consistency.

A coarse/fine control strategy can be easilyimplemented in a 353 controller. The followingsections discuss each method, the theory of

operation, advantages and disadvantages, andhow it can be configured in a 353.

Split-Range Control

The split-range control strategy, shown in Figure2, involves closing the coarse valve andmanipulating only the fine valve when the flowdemand is low. As flow demand increases, thefine valve is gradually opened until fully open. Itthen remains open as the coarse valve ismanipulated.

FIC

f(x)

SP

f(x)

Open

Close

Valves

x

CoarseF

ine

Controller Output

1x

A

A*1

1-xx( ) ( ) A( )*

A

FT

0 1

Figure 2 Split Range Control

This scheme features immediate response todemand changes and fine resolution for low flowdemands. However, resolution becomes coarsewhen the demand exceeds the capacity of the finevalve, but this is a simple method to provide highturndown flow control.

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Figures 5 and 6 show a floating controlconfiguration. In this configuration the fine PIDfunction block located in loop 1 determines thefine valve signal. The A/M transfer function blockallows manual control of the valve positions. TheID function block located in loop 2 is used as afloating or integral only controller.

The Limit function block LMT1 in loop 2 limits thefine valve signal to between 25% and 75%. Thisallows a difference to develop between the P andS inputs to the ID controller when the fine valvesignal moves outside these limits. This errorcauses the coarse flow controller to move thecoarse valve.

The fine PID controller in loop 1 responds tochanges produced by the coarse valve byreturning the fine valve to a position within thelimits. This causes the error at the coarse flowcontroller to become zero, which holds the coarsevalve at its new position.

Center-Seeking Control

The center-seeking control strategy, shown inFigure 7, is similar to the floating strategy. Itinvolves manipulating only the fine valve when thevalve signal is between certain trip points, butwhen a trip point is reached, the coarse valve isadjusted until the fine valve is returned to 50%,midstroke. This differs from the floating control,

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which returns the fine valve to the trip point ratherthan 50%.

Like floating control, this strategy responds quicklyto small changes in flow demand over the entirerange by directly adjusting the fine valve.However, it responds slowly to a large demandthat saturates the fine valve. The advantage of

center-seeking control is that it returns the finevalve to where it can respond alone to largerchanges in either direction.

FICSP

FT

ID

Fine Coarse

50%

P S

>75%

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Appl ications

In addition to the pH control system shown inFigure 1, a coarse/fine control strategy can beimplemented on any process requiring preciseflow control. One of the above strategies can beimplemented based on the characteristics of theflow such as frequency of changes in flowdemand and the range of these changes.

Appl ication Support

OtherApplication Data sheets in this series canbe found at the Siemens public Internet site.

Detailed information about the FCO and functionblocks mentioned in this publication can be found

in the Users Manual for each controller.

Siemens sales representatives in the UnitedStates and globally are available to provideapplication support.

The configurations shown in this publication werecreated in Siemens i|config GraphicalConfiguration Software. Those with CF353 inparenthesis in the figure title are available on theSiemens public Internet site.

Procidia, i|ware, i|config, and 352Plus are trademarks of Siemens Energy & Automation, Inc. Other trademarks are the property oftheir respective owners. All product designations may be trademarks or product names of Siemens Energy & Automation, Inc. orother supplier companies whose use by third parties for their own purposes could violate the rights of the owners.

Siemens Energy & Automation, Inc. assumes no liability for errors or omissions in this document or for the application and use ofinformation in this document. The information herein is subject to change without notice.

Siemens Energy & Automation, Inc. is not responsible for changes to product functionality after the publication of this document.Customers are urged to consult with a Siemens Energy & Automation, Inc. sales representative to confirm the applicability of theinformation in this document to the product they purchased.

Control circuits are provided only to assist customers in developing individual applications. Before implementing any control circuit,it should be thoroughly tested under all process conditions.

Copyright 2005, Siemens Energy & Automation, Inc.

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