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Application for Process Automation

Control Performance Monitoring (CPM)for monitoring Control Loops

Application Note

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Warranty, Liability and Support

Control Performance Monitoring (CPM) 32486166

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008Allrightsreserved

Note The application examples are not binding and do not claim to be com-

plete regarding the circuits shown, equipment and possibilities. The soft-ware samples do not represent a customer-specific solution. They onlyserve as a support for typical applications. You are responsible for ensur-ing that the described products are used correctly. These application ex-amples do not release you from your own responsibility regarding profes-sional usage, installation, operation and maintenance of the plant. Whenusing these application examples, you acknowledge that Siemens cannotbe made liable for any damage/claims beyond the scope described in theliability clause. We reserve the right to make changes to these applica-tion examples at any time without prior notice. If there are any deviationsbetween the recommendations provided in these application examples

and other Siemens publications e.g. catalogs then the contents of theother documents have priority.

Warranty, Liability and Support

We accept no liability for information contained in this document.

Any claims against us based on whatever legal reason resulting fromthe use of the examples, information, programs, engineering and perform-ance data etc., described in this application example shall be excludedSuch an exclusion shall not apply in the case of mandatory liability, e.g. un-der the German Product Liability Act (Produkthaftungsgesetz), in case of

intent, gross negligence, or injury of life, body or health, guarantee for thequality of a product, fraudulent concealment of a deficiency or breach of acondition which goes to the root of the contract (wesentliche Ver-tragspflichten). The damages for a breach of a substantial contractual obli-gation are, however, limited to the foreseeable damage, typical for the typeof contract, except in the event of intent or gross negligence or injury to life,body or health. The above provisions do not imply a change in the burdenof proof to the detriment of the orderer.

Copyright 2008 Siemens A&D. These application examples or ex-tracts from them must not be transferred or copied without the ap-proval of Siemens A&D.

For questions about this document please use the following e-mail address:

mailto:[email protected]
mailto:[email protected]:[email protected]

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Preface


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PrefaceObjective of the application

Control Performance Monitoring (CPM) as a function of SIMATIC PCS 7Advanced Process Control (APC) enables the permanent automatic moni-toring of the performance of control loops of a plant. This gives the plantoperator the opportunity to counteract reduced performance with specificmeasures at an early stage.

The configuration example on hand shows the integration of the CPM func-tionality into a PCS 7 project and the operator view in process mode.

Main contents of this applicationThe following main points are discussed in this application:

Configuring the CPM block

Initialization

Monitoring and interpretation of the results

Validity

valid for PCS 7 V7.0 SP1

Reference to the Automation and Drives Service & Support

This article is from the Internet application portal of the Automation andDrives Service & Support. Clicking the link below directly displays thedownload page of this document.

http://support.automation.siemens.com/WW/view/de/32486166
http://support.automation.siemens.com/WW/view/de/32486166http://support.automation.siemens.com/WW/view/de/32486166

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Table of Contents


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Table of Contents

Table of Contents ......................................................................................................... 41 Control Performance Monitoring (CPM) ....................................................... 51.1 CPM in SIMATIC PCS 7................................................................................... 51.2 What is the purpose of CPM?........................................................................... 71.3 Functionality...................................................................................................... 92 Implementing the Monitoring Function ...................................................... 102.1 Installing.......................................................................................................... 102.2 Configuration .................................................................................................. 102.2.1 CFC configuration........................................................................................... 102.2.2 OS configuration............................................................................................. 133 Startup and Evaluation................................................................................. 143.1 Activate the monitoring function...................................................................... 173.1.1 Setting an operating point............................................................................... 173.1.2 Initializing the CPM......................................................................................... 193.2 Mode of operation........................................................................................... 213.2.1 Setpoint modification ...................................................................................... 213.2.2 Deterioration of the control performance ........................................................ 243.3 Evaluation of monitoring parameters .............................................................. 273.3.1 Remaining control deviation ........................................................................... 273.3.2 Changing or reducing the CPI ........................................................................ 273.3.3 Changing the process gain............................................................................. 273.3.4 Overshoot too large ........................................................................................ 283.3.5 Settling ratio too small .................................................................................... 283.3.6 Scatter plot...................................................................................................... 293.4 Benefits for the plant operator ........................................................................ 304 Control Loop Monitoring for meshed Control Loop Structures............... 314.1 Cascaded control............................................................................................ 314.2 Split range control........................................................................................... 314.3 PID controller with gain scheduler .................................................................. 314.4 Override control .............................................................................................. 324.5 Feedforward control........................................................................................ 324.6 Smith predictor................................................................................................ 324.7 Ratio control.................................................................................................... 324.8 Multi-variable control....................................................................................... 325 History ........................................................................................................... 34

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Control Performance Monitoring (CPM)


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1 Control Performance Monitoring (CPM)

Note A general overview on CPM as a function of SIMATIC PCS 7 APC (Ad-vanced Process Control) is provided by the White Paper SIMATIC PCS7 APC Portfolio

http://pcs.khe.siemens.com/efiles/pcs7/support/marktstudien/WP_PCS7_APC_EN.pdf

1.1 CPM in SIMATIC PCS 7

For Control Performance Monitoring a function block CPM and the appro-priate visualization is used for monitoring the performance of a control loop.

The division of the various CPM functions is listed below.

Functions in the AS level

CPM block in connection with PID controller block:

Abbildung 1-1

PID functionblock

CPM functionblock: Backpack

to PID block

PID functionblock

CPM functionblock: Backpack

to PID block
http://pcs.khe.siemens.com/efiles/pcs7/support/marktstudien/WP_PCS7_APC_EN.pdfhttp://pcs.khe.siemens.com/efiles/pcs7/support/marktstudien/WP_PCS7_APC_EN.pdfhttp://pcs.khe.siemens.com/efiles/pcs7/support/marktstudien/WP_PCS7_APC_EN.pdfhttp://pcs.khe.siemens.com/efiles/pcs7/support/marktstudien/WP_PCS7_APC_EN.pdf

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The CPM block fulfills the following tasks:

Short term statistics

CPI calculation (Control Performance Index)

Alarm signalling

Functions in the OS level

CTRL_PID-Faceplate with a Link to the CPM-Faceplate:

Abbildung 1-2

CPM-Faceplate:

Figure 1-3

Contol Performance Status

Contol Performance Index

Standard deviation of controlledvariable

Mean value of control error

Mean value of manipulated va-riable

Estimated stationary processgain

Opening the CPM-Faceplates

via the CTRL_PID-Faceplate

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Overview display of all control loops of a plant possible by copying the

block icons of the CPM blocks together on an OS page. Statistics on archive data and data export

Simple predefined graphical analysis

Configurable alarm limits for standard deviation and overshoot (> pre-ventive maintenance)

Additional Offline evaluation

Graphic evaluation in MS-Excel:

Histogram

Scatter plot

etc.

Analysis of alarm archives

Numeric analysis (e.g. in Matlab):

Minimal variance

Harris index

Disturbance models

Fourier transformation

1.2 What is the purpose of CPM?

The basic principle of the combined approach realized in the CPM block isusing stochastic as well as deterministic characteristics for the control per-formance, i.e. the suitable characteristics depending on the operatingmode.

Monitoring of controlled variables

CPM offers a permanent monitoring of the performance of control loops.

The CPM block calculates:

Stochastic characteristics of the control performance in stationary oper-ating modes of the process

Mean value of the controlled variable, variance and standard devia-tion of the controlled variable

Average of the manipulated variable und control deviation

Control performance index

Estimated stationary process gain

Deterministic characteristics of control performance for setpoint steps

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Response and settling time as well as the settling ratio

Overshoot absolute and relating to the jump height.

Further statistic and graphic evaluation of the signals in the control loopover longer, randomly selectable periods of time are offered in the picturemodule of the CPM block.

Localization of problems

Using an overview representation of a plant or plant unit enables a quickoverview of the state of all control loops by means of the CPM block icons(traffic-light function).

It is the aim to recognize approaching errors early on and to focus the at-

tention of the user on those control loops of a plant which are not workingproperly anymore.

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For example, in a main plant with several hundreds of control loops:

Which area deserves a closer look?

Which control loops have errors approaching?

Which control loops should be optimized with the highest priority?

1.3 Functionality

The CPM block evaluates signals setpoint value, actual value and the ma-nipulated variable of the PID controller in a sliding time frame. The ControlPerformance Index (CPI) is calculated for this.

Abbildung 1-4

The operating mode of the controller is also taken into consideration here.In stationary operating modes of the process the determined stochasticcharacteristics are compared with the reference values which were deter-mined during commissioning. For setpoint steps the stochastic characteris-tics are irrelevant by definition and are temporarily frozen. The monitoringof the deterministic characteristics is automatically activated.

When falling short of a defined limit of the control performance a messageis generated, as well as for exceeding a defined limit for the overshoot forsetpoint steps.

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Implementing the Monitoring Function


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2 Implementing the Monitoring Function

2.1 Installing

The installation of the PCS 7 APC Library V70 occurs via the PCS 7 framesetup. For the individual installation of the PCS 7 APC Library V70 it is re-quired that the PCS 7 Library V70 SP1 has already been installed on theEngineering System.

2.2 Configuration

For a PID controller an assigned CPM block is installed into the same CFCplan and interconnected with the controller. The CPM block is connected

with the assigned controller block via a name convention: The name of theCPM instance corresponds entirely to the name of the controller instanceplus the ending "_cpm".

Note As an example, the configuration occurs directly in the CFC plan of theS7 program. If in practice all controllers of a type shall be monitored, it isappropriate to make the expansions at the respective process tag typehence to update the plans in the program. As a sample solution for this aprocess tag type PIDCTRL_ConPerMon is offered in the APC Library. Itcorresponds to the previous process tag type PIDCTRL, expanded by the

control loop monitoring as well as a logic for treatment of measured value.

2.2.1 CFC configuration

To configure the CPM block, proceed as follows:

Table 2-1

Instruction Notes / Screenshot

1. Open the plan with theCTRL_PID in the CFC editor.

2. Declare the CTRL_PID as thepredecessor for installing thenext block.

To do this click with the rightmouse-button and select "Prede-cessor for Insertion Position"from the submenu.

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3. In the dialog box "Predecessorfor Insert Position" you select towhich OB the subsequent blocksare installed (in this exampleOB35, i.e. the 100ms cycle).

4. The field with the runtime proper-ties in the block header is thenshaded in green.

5. Draw the CPM block from theblock catalog next to theCTRL_PID via Drag&Drop. You

find it in the PCS 7 APC V70library in the CONTROL blocks.

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6. Open the properties dialog of theCPM block and change its labelto the name of the CTRL_PIDfollowed by _cpm (here in thisexample: CTRL_TEMP_cpm).

7. Interconnect the CPM block withthe CTRL_PID.

The block connections to beinterconnected are listed in thetable below.

8. Compile the S7 program andload it into the automating sys-tem.

Table 2-2 Interconnecting CTRL_PID with CPM

Connection of the CTRL_PID Connection of the CPM for the monitoring

PV_IN (actual value) PV_Mon

SP (active setpoint) SP_Mon

LMN (manipulated value output) MV_Mon

QMAN_AUT (manual/automatic) PID_AutAct

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2.2.2 OS configuration

Apart from the OS compilation no special configuration steps are neces-sary.

Apart from the block icon for the PID controller a separate block icon isgenerated for the CPM. This can be moved on demand into a higher-leveloverview screen. The respective faceplate can also be opened from thefaceplate of the CTRL_PID.

Figure 2-1

Note As long as the CPM has not been initialized the CPI (Control Perform-ance Index) is represented in gay in block icon and faceplate.

The initialization is described in chapter 3.1.1 "Setting an operatingpoint".

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Startup and Evaluation


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3 Startup and Evaluation

Mainly after commissioning and optimizing the PID controller to be moni-tored, the CPM block is initialized in a stationary operating mode of theprocess. Thereby it saves the appropriate parameters as reference values.

This chapter describes the handling of the CPM functionalities via the face-plate.

Vies in the faceplate of the CPM

Figure 3-1 Overview CPM faceplate

In the individual screens the various operator control and monitoring proc-esses can be controlled/viewed.

Standard view

The standard view provides the user with an overview of the following pa-rameters:

Control performance index (CPI)

Standard deviation of the actual value (PV_StdDev)

Mean value of the control deviation in the time frame (ER_Mean)

Mean value of the manipulated value in the time frame (MV_Mean)

Standard view Message view Parameter view

Statistics viewSetpoints viewLimits view

Scatterplot-view

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Stationary process gain (StatGain)

Message view

Here the warning and alarm messages or notes generated by the processappear. They can be acknowledged by the user.

Parameter overview

In the parameter overview the following parameter values can be viewed:

Reference of the variance of the process value (RefVariance)

Reference of the standard deviation of the actual value (RefStdDev)

Furthermore, requirements can be specified for the following parameters:

Reference of the actual value (PV_Ref)

Reference of the manipulated value (MV_Ref)

The user can initialize the CPM via the "Initialize" button.

Limits view

Here the warning and alarm limits can be defined for the process.

Visualizing the views:

Control performance index (CPI)

Value for measured overshoot

Limit values for warnings and alarms

Setpoint view

In the setpoint view the evaluation of the step response is a striking featurefor the user. The "StepPhase" field informs of the current phase of the stepresponse:

Phase 0 > ready

Phase 1 > rising

Phase 2 > overshoot Phase 3 > steady state

The setpoint view illustrates the following parameters:

Absolute overshoot of the step response (OverAbs)

Relative overshoot of the step response (Overshoot)

Phase of the step response (StepPhase)

Settling time of the step response (SettliTime)

Settling ratio of the step response (SettlRatio)

Status display for the evaluation of the step response

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In the normal case the transition from phase 3 to phase 0 occurs automati-

cally. In order to shorten the process, this transition can also be forced ondemand by pressing the "Break Eval." button.

Statistic view

In this statistic view of the picture module the most important measuredvalues of the monitored control loop are represented, which are automati-cally saved in the process value archive:

Setpoint value

Actual value with 3 tolerance band

Manipulated variable

CPI

Automatic mode (binary)

Upper or lower manipulated variable limit active (binary)

Scatter plot view

A scatter plot is a two-dimensional representation of all measured valuepairs actual value (PV) over manipulated value (MV).

For well set control loops the data points should form an elliptical cloud.

Note Further information on the individual views is available in the online helpat "APC Faceplates > Picture modules > CPM > ...".

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3.1 Activate the monitoring function

Note For this instruction the PCS 7 example project was used for AdvancedProcess Control. It is contained in the PCS 7 Installation and can beopened in the SIMATIC Manager via "File > Open > Example Projects >APC_ExaSP".

3.1.1 Setting an operating point

Prior to initializing the CPM the controller in manual mode is set to a sensi-ble operating point.

Note The actual value can also settle to a new operating mode in automaticmode via setpoint change at the controller.

Reading and calculating stochastic data (e.g. variance) is not necessaryyet.

Setting the operating point in manual mode

Table 3-1 PID standard view

Procedure Screenshot

1. Open the faceplate of the PIDcontroller in Runtime. Set theoperating mode to manual viathe operating faceplate of thePID controller.

Select the mode from the drop-down menu for the operatingmode by activating "Manual".

Press the "Ausf" button.

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2. The controlled variable (e.g. tem-perature) shall be set to a de-sired operating point.

To do this you enter a manipu-lated value via the "MAN" con-nection of e.g. "15%" (in thisexample relatively small in orderto provide a relatively large oper-ating area around the operatingpoint for the later following set-point step).

Then you wait until the actual

value has reached a stationarystate.

Switching over to automatic mode

The PID controller to be monitored is taken in automatic mode and the set-point is set to a new operating point.

Table 3-2 PID standard view


1. Set the operating mode via thedrop-down menu. Select the"Auto" option.

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2. The PID controller is now in au-tomatic mode.

3.1.2 Initializing the CPM

If the process (i.e. the controlled variable) is in a stationary operating mode,the CPM block is initialized. This saves reference values for control per-formance, manipulated variable and actual value.

Table 3-3 CPM parameter view and standard view


1. Select the length of the timeframe (TimeWin) large enough.

Hinweis:

As a start value the time frameshould be 10 times as long asthe longest process constant or10 times as long as the resettime of the PID controller.

2. Press the "Initialize" button toinitialize the block.

The button can only be selectedif the controller curve is in the"StepPhase = 0" state and thePID controller is in "Auto" mode.

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3. The control performance index isnow in the "CPI valid" state, i.e.the control loop is in steady stateand the CPM has been initial-ized.

Note:

After the initialization the qualitycode for the estimated processgain (ProcGain) can change to"unsafe, process-related". Theprocess gain is calculated from

differences with the referenceoperating point. At the operatingpoint itself it can therefore not bedetermined.

The control performance index (CPI) should now be approx. 100% andhence show that the control loop works appropriately. Stochastic variationsmay cause the 100% to be exceeded temporarily.

Note Further information on the interpretation of the calculations of the blockare available in the online help at "APC Library V7.0 > CPM ControlPerformance Monitor > Functions of CPM" and "APC Faceplates > Face-plates > CPM > CPM: Parameter View".

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3.2 Mode of operation

The following section describes the monitoring of the stochastic and deter-ministic characteristics of the control loop.

3.2.1 Setpoint modification

After changing he setpoint the PID controller starts adjusting the actualvalue to the new sepoint. It determines a manipulated variable from thecontrol deviation which affects the control process in such a way that thecontrol deviation amounts to only a minimum in the steady state.

Setpoint step

Table 3-4 PID standard view and trend view


1. Changing the setpoint by select-ing the input field "SP" and thereyou enter the value "150".

2. Directly after changing the set-point the step response is visiblein the curve diagram.

The curve is composed of thefollowing phases:

0 - ready

1 - rising

2 - overshoot

3 steady state

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3. After a relatively short period oftime the actual value has leveledwith the setpoint (phase 3).

Consequences of the setpoint step in CPM

Table 3-5 CPM standard view


1. After the setpoint step the CPIand the process gain becomeinvalid.

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2. If the PID controller has adjustedthe actual value accordingly, theCPI changes back to "CPI valid".It moves around 100%.

Note Further information on the setpoint change are available in the onlinehelp at "APC Library V7.0 > CPM Control Performance Monitor > Func-tions of CPM > Monitoring deterministic characteristics of the control per-formance" and "APC Faceplates > Faceplates > CPM > CPM: Setpointview".

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3.2.2 Deterioration of the control performance

If the control performance deteriorates slowly during runtime, this may bean indication that controller setting and control process behavior do notharmonize anymore.

ProcTimeLag Faceplate

In the APC example project this behavior is modeled by extending theprocess time constant after operating the simulation block ProcTimeLag .

Table 3-6 ProcTimeLag faceplate: Process time constant


1. In the runtime you select thepicture module "ProcTime" whichopens the faceplate.

2. In the input field you enter thevalue "8", for example, for thenew time constant and acknowl-edge the "Ausf" button.

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Effects in the CPM

Table 3-7 CPM faceplate standard view


1. After changing the time constantit can be observed that the CPIslowly drops and hence displaysa deterioration of the controlperformance.

At an appropriate parameterization the drop of the CPI causes the trigger-ing of a message.

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Figure 3-2 Warning message after a drop of the CPI in the message view

Reasons for the deterioration can be changed controller parameters andcontrol process parameters as well as a change in the final control elementcurve. Wrongly adjusted controllers make the control loop too slow, cause alarge control deviation or undampened oscillations of the controlled vari-able. As a solution all parameters should be checked and reset if neces-sary.

Furthermore a changed time frame during runtime may cause the CPI todeviate from the original value. Here it helps to reinitialize the CPM.

Note During the setpoint step the alarms are suppressed.

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3.3 Evaluation of monitoring parameters

3.3.1 Remaining control deviation

The mean value of the control deviation in the sliding time frame is dis-played in the standard view of the CPM faceplate. For a controller with I-part (PI or PID controller) is should be almost equal zero. If not the follow-ing problem causes are possible:

The performance of the final control element is insufficient. As a resultthe controller keeps going to the limit with its actuating signal. This maybe due to unfavorably dimensioned final control elements, changed op-erating conditions or also due to wear.

The manipulated variable required by the controller will not be effectivein the process as the final control element is defective or the communi-cation is disconnected, for example.

3.3.2 Changing or reducing the CPI

Deterioration of the control performance due to changed control pa-rameters, changed final control element curve or changed control proc-ess parameters

Stochastic fluctuations may lead to a temporary exceeding of the 100%mark. This however is unproblematic.

In the following cases the CPI changes naturally without causing any prob-lems in the control loop:

The time frame length was changed.

The distorted CPI signal was smoothed

3.3.3 Changing the process gain

Signs of wear in the process lead to a gradual change in process gain.In many cases this affects the control performance.

Sudden and temporary change indicates an external disturbance im-pact. Such disturbances can be avoided by improved controller settings.Should they occur more often, it may pay off to search for the causes.

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3.3.4 Overshoot too large

An important parameter for monitoring the control performance is the over-shoot. Too large overshoots of the step response are due to a wrong gainsetting of the controller or changed control process. If the overshoot is toolarge, it helps in many cases to reduce the gain of the controller.

3.3.5 Settling ratio too small

The settling ration, quotient of rise time and settling time, must be largerthan 25%. If this is not the case, the reset time at the PID controller is set

too slow, which causes a slow leveling of the actual value with the setpointvalue.

Figure 3-3 Rise and settling time after a setpoint step

0 20 40 60 80 100 120 140 160 180 2000

0.5

1

1.5

2

2.5

3

SettlingTime

RiseTime

OverAbso PV_TolMax

PV_TolMin

StepHeight

PV

SP

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3.3.6 Scatter plot

For well set control loops the data points in the scatter plot should form anelliptical cloud.

Figure 3-4 Scatter plot for a well set control loop

Non-linearities such as friction losses in valves (which cause a parallelo-gram-type distorted contour), but also quantization effects and other un-usual statistical distributions can be recognized.

Figure 3-5 Scatter plot for a valve with adhesion problems

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3.4 Benefits for the plant operator

Main advantages

Simple configuration and automatic initialization of the CPM functional-ities

Permanent, automatic monitoring of the control loops of the plant

Possibility of counteracting reduced performance early on with directedmeasures.

Application experiences

Long-term effects of Control Performance Monitoring on the control per-formance:

Abbildung 3-6

Control Performance,Process Performance

Online Monitoring (+10%)Good Support

No Monitoring (-25%)Support

No Monitoring (-40%)Poor Support

No Monitoring (-40%)No Support

Time

2-3 yearscommissioning

Control Performance,Process Performance

Online Monitoring (+10%)Good Support

No Monitoring (-25%)Support

No Monitoring (-40%)Poor Support

No Monitoring (-40%)No Support

Time

2-3 yearscommissioning

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Control Loop Monitoring for meshed Control Loop

Structures


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4 Control Loop Monitoring for meshed Control Loop

Structures

4.1 Cascaded control

For a cascaded control you should only apply the CPM block to the mastercontrol but not to the slave control. The CPM block cannot make any sensi-ble statements on the control performance of the slave control loop, since:

the variance of the actual value in the slave control loop depends onthe variance of the setpoint which is given as manipulated value by themaster control.

there are neither operating phases with constant setpoint, neither de-fined setpoint steps.

Furthermore, from a process point of view, the mater control loop is natu-rally the one whose control performance should be monitored while thecontrol performance of the auxiliary loop (slave loop) is of secondary sig-nificance. Nevertheless, it is advised to set the slave loop carefully beforestarting the optimization and monitoring of the master controller, since abad behavior of the slave controller cannot be compensated by the mastercontroller.

4.2 Split range control

The split range function block contains two individual (static) curves for bothactuators. If both actuators have clear differences in their performance ca-pability (can be interpreted as different process gain for heating/cooling)this should be compensated by different inclinations of both curvebranches, so the controller finds a process behavior as linear as possible(i.e. in dependent of positive or negative sign). If this is not possible, thecontrol performance will be slightly different in both areas. The initializationof the CPM block should then be performed in the worse area in order toavoid false alarms.

4.3 PID controller with gain scheduler

The objective of gain scheduling is to reach a uniform control performanceover the entire operating range. If this is not successful the initialization ofthe CPM block must be performed in an operating point with worse controlperformance in order to avoid false alarms. It is recommended to widen thealarm boundaries at the CPM block: admit smaller CPIs and larger over-shoots.

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4.4 Override control

Depending on the process state, various controls are active for overridecontrol whose control performance varies naturally. It is recommended toonly apply the control loop monitoring to the main controller and to sup-press it via the input parameter ManSuprCPI if the limiting controller be-comes active.

4.5 Feedforward control

The task of the feedforward control is to avoid or at least reduce deteriora-tion of the control performance due to changes at a measurable distur-bance variable. Principally the control loop monitoring can therefore beused as for the single control loop. If the disturbance variable is temporarilyvary calm and temporarily very agitated, according fluctuations of the con-trol performance cannot be excluded, since the feedforward control repre-sents a model-based intervention, and a model is never a perfect represen-tation of reality.

4.6 Smith predictor

For processes with dead times the smith predictor enables a higher controlperformance than a simple PI controller. Principally the control loop moni-toring can therefore be used as for the single control loop. If the dead time

changes during runtime, deterioration of the control quality must be ex-pected.

4.7 Ratio control

For a ratio control the control loop monitoring can only be applied to themain control loop, if the sepoints for added components are determinedfrom the actual value of the main component, since then permanent set-point value changes can be expected in the control loops for the addedcomponents similar as for the slave control loop of a cascade. If the set-points for added components are determined from the setpoint of the maincomponent the subordinate control loops can also be monitored.

4.8 Multi-variable control

The mathematical concept of the CPM block has been designed for single-variable control loops. If an increased variance is detected in a control loop,the block cannot determine whether the actual cause is within this controlloop or whether other influences through interactions were brought in fromthe field. If you observe a strong interconnection between various controlloops of your plant, or even employ multi-variable controllers, the state-ments of the CPM block must be viewed with caution.

Nevertheless it is sensible to equip a multi-variable controller such as the

ModPreCon block with a control loop monitoring in order to determine

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whether the control performance that was achieved during commissioning

of the controller is maintained during runtime. In this case each controlchannel of the multi-variable controller receives a separate CPM block. It isnecessary to configure some additional logic functions prior to the input pa-rameter "ManSuprCPI":

If one or several other channels of the multi-variable controller are in annon-stationary state (e.g. setpoint step) indicated by the output parame-ter "CPI_SupRoot = 1", then the temporarily increased variance cannotbe avoided in this control channel and should not cause a CPU mes-sage.

If one or several other channels of the multi-variable controller show in-creased variances (bad control performance), indicated by the respec-

tive output "CPI_WrnAct = 1", then these variances cause an increasedvariance in this control channel via the interconnection, which cannot beprevented and hence should not lead to a CPI warning. In this way itmay be possible to detect the actual disturbance cause in a multi-variable system: The channel which first detects an increased variancesets the alarm while follow-up alarms in neighboring channels are sup-pressed.

Note In the multi-variable case the process gains estimated from the single-variable consideration are irrelevant. Setting the input parameter"StGainValid = 0" this is also displayed accordingly in the operator screenas status "unsafe, process-related"

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History


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5 History

Table 5-1 History

Version Date Modification

V1.0 13.11.2008 First issue

Documents

32486166 AppNote CPM En