Excercise AS-4

(1)

Institute of Automatic Control and Robotics Faculty of Mechatronics

Automation Systems laboratory

Excercise AS-4

„

Identification of process in control system of level in tank with free outflow of liquid

”

Autors : PhD. Eng. Danuta Holejko, PhD. Eng. Jakub Możaryn, MSc. Eng. Kacper Malinowski,

BSc. Eng. Michał Bezler

Warsaw, 2016

(2)

Automation Systems

2

Identification of process in control system of level in tank with free outflow of liquid

The aim of the exercise is to determine a mathematical model of the process of the changes in the liquid level in the open tank, on the basis of experimentally determined static and dynamic characteristics,. The process of obtaining a mathematical model is called the identification of the object.

1.Introduction

The control object is the technological process that takes place in the device and the desired course of which is obtained by external control (control). The control object is subject to interferences.

The processes of automated technological processes are controlled on the basis of measurements of quantities characterizing a given process, and whose desired course is determined in the control task. These are usually physical quantities such as temperature, pressure, viscosity, content of ingredients etc. . It is said that these quantities are the controlled output quantities of the control object (process) (y1 , y2 , .... yn).

In order for a given technological process to be implemented, appropriate materials streams (eg appropriate quantities of reacting components) or energy streams (eg fuel, electricity) must be supplied to it. The desired course of controlled quantities will depend on the size of these streams and their parameters.

Thus, the amounts of energy or matter supplied are input quantities (x1, x2,...xm) of the control object (process). Other input quantities are values that adversely affect the course of regulated quantities. They are all kinds of disturbances (z1, z2,... zk). These disturbances may directly affect the process, eg in a temperature control system such disturbances are changes in the ambient temperature, or distort the energy or matter streams fed to the object, eg in a temperature control system such disturbances are changes in the amount of fuel. The relationship between output (controlled) and input variables creates a description of the object in the process sense (Figure 1a).

Devices in which technological processes are carried out are equipped with executive execution units (ZW), which are, for example, control valves, variable capacity pumps, motors, contactors, etc., enabling delivery of energy or process streams. Another elements are measuring transducers (PP) , providing information on the course of changes in controlled quantities. The executive units, as a result of their influence on control signals determined using technical terminology with the symbols CV1,CV2, ... CVm,, and produced by controllers, shape the intensity of material or energy streams. These signals have the control vales of the control object in the apparatus sense as a component of the control system. The output values of the control object so understood are the output signals of the PV1, PV2, ... PVn measuring transducers, called process variables. The dependence between the object's output signals (process variables) and its input signals (control variables and disturbances) is a description of the object in the apparatus sense (Fig. 1b).

(3)

„Identification of process in control system of level in tank with free outflow of liquid”

Automation Systems

3

a) b)

Fig. 1. Schematic diagram of the control object with one controlled variable: a) - the object of regulation in the process sense, b) - the object of regulation in the apparatus sense of

apparatus; symbols: x, y - input, output variable of the object in the process sense, CV - control signal, PV - measurement transducer output signal (process variable), z1 , z2 ,….. zk –

disturbances.

In the simplest cases, the control object can have one output signal (one controlled variable), one control signal and many disturbaces (Figure 1a). Its mathematical description is the following dependence of the output signal on the input signals

𝑃𝑉 = 𝑓(𝐶𝑉, 𝑧 ,𝑧 ,….𝑧 ) (1)

which, depending on the properties of the object, can be an algebraic equation or a linear or non-linear differential equation with fixed or variable coefficients.

The correct assessment of the properties of the control objects is a basic condition enabling the design of control systems. In general, the analysis of the object's properties takes place in two stages. The first stage is a process analysis, the effect of which is to establish process relationships between quantities regulated as physical variables and process input quantities, which are usually the parameters of energy streams or materials supplied to the process. The results of this analysis are the basis for the proper selection of the measuring transducer and the executive unit, that is, for the correct design of the control object in the apparatus sense. General process relationships should be determined by a technologist who understands the physical side of the process. However, it is often necessary to use control engineer to describe the object's properties in a form that is useful for adjustment purposes.

The second stage of the analysis is to determine the mathematical model designed in the apparatus sense of the object as a relationship between the signals (process variables) PV and CV (control variables) and interference. Created models due to their application features can be models: global or local (parametric).

Global (balance) models created for the purpose of technological process analysis, its optimization and start-up are determined on the basis of dependencies between process variables binding eg energy, mass, location and condition of individual elements making up the process in the full range of their variability and based on balances of these values for the whole object. Such a model usually has the form of non-linear differential-integral relations. It can be used both in the design of the control system and work point optimization.

Local (parametric) models describe the properties of an object in the vicinity of a given working point, which is generally sufficient to select the parameters installed in the element control system, to analyze the stability of the system with the controller and to select the control algorithm and structure of the control system. Such a model is usually in the form of a predetermined mathematical description, eg in the form of transfer function: the object

(4)

Automation Systems

4

and disturbances transfer functions. whose unknown parameters are determined during the identification process. The mathematical model of the object can also be presented in the form of a block diagram that provides information about the structure of the object, which is helpful when designing the structure of the control system. An exemplary block diagram of an object with one controlled variable and two disturbance inputs z1, z2 developed for incremental variables, is shown in Fig.2.

a) b)

Fig. 2. Block diagram of the control object: a) detailed diagram, b) simplified diagram As mentioned before, the control object in the apparatus sense is not only the process taking place in the device (Gproc(s) transfer function) but also the ZW executive unit (GZW(s) transfer function) controlled by a CV signal and a PP measurement transducer (GPP(s) transfer function) generating a PV signal (Fig. 2a). The product of these transfer functions represents the operator-determined dependence of the process variable PV of the CVi control is the transfer function of the object defined by the symbol 𝐺 (𝑠) (Fig. 2 b). The nature of changes in the controlled variable due to disturbances is defined by disturbance transfer functions 𝐺 (𝑠), 𝐺 (𝑠) … 𝐺 ( 𝑠) (these transfer functions due to non-measurable disturbancese can be estimated in a qualitative rather than quantitative way). The block scheme from Fig. 2a is obtained and verified in the design and selection phase of individual object installation units, while the diagram in Fig. 2b is obtained in a running and operating control system.

The disturbances acting on the object, of which there are most often, are unmeasurable and act randomly at various places in the object, but as a result they always disrupt the desired course of the process, and their operation manifests itself by changing the control variable and thus the process variable causing it to increase in value or its decrease. Also, depending on the construction of the execution unit, physical nature and process properties and static characteristics of the measuring transducer used, the increase in the CV output signal value of the process control controller should cause an increase or decrease in the value of the controlled variable. These interactions were shown in the block diagram from Fig. 2 through the summation node. Characters in the summation node show possible directions of control and interference interactions.

Acquiring the model can be carried out analytically based on the knowledge of equations describing the physical-chemical relations of the object or experimentally. The experimental method can be an active or passive experiment.

The active experiment consists stimulating the object with determined extortion. This is most often step, pulse or sinusoidal input signal. The obtained response to this extortion allows to determine the parameters of the assumed mathematical model on the basis of appropriate graphic structures, which for practical and design reasons is in the form of a not

(5)

„Identification of process in control system of level in tank

very complex substitute transfer function

of the actual tested object in the environment of the selected work

the experiment, the object must be in a steady state. Accuracy of identification depends on the amplitude of the extortion, which should be high enough to minimize the inf

disturbances and small enough to not cause the

extortion should be long enough to reveal the nature of the response.

Objects, like other automation elements, are classified according to their dynamic properties. The most general classification is the division due to the ability to achieve or not achieve a permanent balance after the introduction of the

view, the objects are divided into:

 static ,

 astatic.

Example answers of static and astatic objects for stepwise CV control in incremental coordinates is shown in Fig.3.

a)

Figure. 3. Examples of general responses to the stepping force a) static object, b) astatic

For static objects whose step response

mathematical models, presented in the form of the transfer function, are most commonly used:

G

or

G

where: kob - amplification of the object (in the sense of the apparatus, the quantity is unaltered), Tz - the substitute

[min].

Parameters of the model of the control object defined by the formula (2) can be determined from the response to the step excitation using the

(Fig.3a) or secant (Fig.4).

Automation Systems

5

substitute transfer function. This transfer function approximates the properties tual tested object in the environment of the selected working

the experiment, the object must be in a steady state. Accuracy of identification depends on the amplitude of the extortion, which should be high enough to minimize the inf

enough to not cause the saturation of the object extortion should be long enough to reveal the nature of the response.

Objects, like other automation elements, are classified according to their dynamic erties. The most general classification is the division due to the ability to achieve or not achieve a permanent balance after the introduction of the step input signal

view, the objects are divided into:

ers of static and astatic objects for stepwise CV control in incremental coordinates is shown in Fig.3.

Examples of general responses to the stepping force a) static object, b) astatic object.

For static objects whose step response has a course as in Fig. 3a, the following mathematical models, presented in the form of the transfer function, are most commonly used:

s T z

ob

ob e

s T

k s CV

s s PV

G ⁰

1 )

( ) ) (

( ^

 



 

s n ob

ob e

Ts k s CV

s s PV

G ^^

 



 

) 1 ) (

( ) ) (

(

amplification of the object (in the sense of the apparatus, the quantity is the substitute lead time constant [min], T0 - the substitute

Parameters of the model of the control object defined by the formula (2) can be determined from the response to the step excitation using the following

with free outflow of liquid”

approximates the properties point. Before starting the experiment, the object must be in a steady state. Accuracy of identification depends on the amplitude of the extortion, which should be high enough to minimize the influence of of the object. The duration of Objects, like other automation elements, are classified according to their dynamic erties. The most general classification is the division due to the ability to achieve or not step input signal. From this point of

ers of static and astatic objects for stepwise CV control in incremental

b)

Examples of general responses to the stepping force a) static object, b) astatic

has a course as in Fig. 3a, the following mathematical models, presented in the form of the transfer function, are most commonly used:

(2)

(3) amplification of the object (in the sense of the apparatus, the quantity is

the substitute delay time constant Parameters of the model of the control object defined by the formula (2) can be following methods: tangent

(6)

Automation Systems

6

Figure 4. Illustration of the secant method for determining the time constants of the model (2) of the control object

In the case of the tangent method, these parameters are determined directly from the graph as shown in Fig.3a, while using the secant method (Fig.4) passing through the points P1, P2, values of time constants T0, Tz is determined from the following equation

0 2

2 0 1

2 ln 1

2 ln T t T

t T t

z  



 

(4) For astatic objects, whose step response is as shown in Fig. 3b, the mathematical model presented in the form of the transfer function is the most common

s T z

ob e

s T s CV

s s PV

G 1 ⁰

) (

) ) (

(  ^



 (5)

Parameters of the model defined by the formula (5) are read directly from the graph in Fig. 3b.

In a passive experiment, model parameters are determined based on the measurement of available signals during normal operation of the control system without the necessity of interrupting its operation and disturbing the operating conditions. In this method we have no influence on the signals given to the object and the identification of the object is difficult in this method due to the often low ability to stimulate signals. The signal analysis allows to determine the so-called stochastic, which due to the accuracy of the description of the object's properties can be used in diagnostic systems or to optimize the regulation process or to develop a different from the standard regulation algorithm.

(7)

Automation Systems

7

2. DESCRIPTION OF THE PROCESS / LABORATORY STAND Experiments were conducted on laboratory stand presented in Fig. 1.

Controlled process is a change of liquid level in connected tanks Z1, Z2. Fig.5 presents installation diagram.

Depending on the configuration \of the valves V2 and V4 and the use of specially constructed elastic tube there can be realized process with various properties (see Table 1).

Table 1. Properties of the process depending on the valves configuration.

No. Type of the process

Valve Valve setup Process

value 1.

1st order lag system without delay

V2 Closed

Level H1 V4 Liquid flows directly into tank Z1

2. 1st order lag system with delay

V2 Closed

Level H1 V4 Liquid flows into tank Z1 through

elastic tube W 3. 2nd order lag

system without delay

V2 Open

Level H2 V4 Liquid flows directly into tank Z1

4. 2nd order lag system with delay

V2 Open

Level H2 V4 Liquid flows into tank Z1 through

elastic tube W

(8)

Automation Systems

8

Figure 1. Installation scheme of the laboratory stand for designing the control system of the liquid level in a tank.

Description : Z1, Z2 – tanks,

V1, V3 - manual outflow valves

V2 - manual valve connecting tanks Z1 and Z2, V4 - threeway valve ,

W- elastic tube,

T0 - transport delay introduced by elastic tube,

VE1- electromagnetic valve to cut-off the outflow from tank Z1.

VE2 - electromagnetic valve to cutt-off the water outflow at the pump plunging.

H1 , H2 - height of the liquid column in tank Z1, Z2, PV – output signal from the liquid level sensor, LT1, LT2 – liquid level sensors in tanks Z1, Z2,

Q - output signal from the flow sensor QT (Venturi flowmeter), P- variable displacement pump,

CV - pump control signal (output signal from PLC controller),

P1, P2 - manual switches of electromagnetic valves to introduce disturbances.

(9)

Automation Systems

9

Components of the stand (Fig. 1) are: tanks Z1 and Z2, a pump P is controlled by standard current signal that corresponds to a change of the pump capacity 0 - 6.5 [l / min], LT1, LT2 are pressure transducers (measuring range 0-500 mmH20) for measuring a liquid level in each of tanks (H1, H2), W is elastic tube, through which the liquid can flow into the tank Z1 (it introduces the transport delay to the object). V1, V2, V3, V4 are manually operated valves, which are used to change the way that liquid flows. There are also two electromechanical valves (VE1, VE2), used to introduce disturbances into the process. By ZK1

we denote leakage from the tank Z1 (opening the valve VE1), and by ZK2 we denote leakage at pump outlet (opening the valve VE2).

Connection of the tanks is realized by using of shut-off valve V2. The controlled variable is the height of the column of liquid in the tank Z1 (H1) or tank Z2 (H2). Requirement for a system is to regulate it to keep at a preset constant value the height of water column despite the disturbances acting on the object.

Disturbances are changes of the flow at the inlet or outlet of the tanks. These disturbances are carried out by remote-controlled shut-off magnetoelectric valves VE1, VE2. Opening / closinf of valves is realized using the P1 and P2 buttons located on the desktop stand. The valve VE1 induces a step change in flow at the outlet from the tank Z1. It is the fault VE1.

The valve VE2 induces a step change in flow at the inlet to the tank Z1 (ie. liquid drop from the pump). It is the fault VE2.

The control value of the object is a standard signal 4 - 20 mA from the controller Simatic PLC S7-1200 (Siemens). This signal is converted by a electronic circuit to the voltage signal 0 - 10 V and operate the pump, the output of which varies in the range: 0 - 6.5 l / min at the inlet to the tank Z1. This pump acts as an actuator in the control system. In addition, a flow measurement is carried out using a Venturi flowmeter. The flow changes from 0 to 6.5 L / min causes a pressure difference across the Venturi flowmeter in the range from 0 to 500 mm H2O. Applied differential pressure transducer converts the pressure difference across the orifice to a standard signal range of 4 - 20 mA.

Information on the current value of the controlled variable (height H1 or H2 of the liquid column) is provided by electrical transducers with the range of 0 - 500 mm H2O and the output signal 4 - 20 mA (measuring the height of the liquid column is carried out using the indirect method by measuring hydrostatic pressure).

(10)

Automation Systems

10 3. EXPERIMENTS

Identification of the described process will be conducted in the control system presented in the scheme at Fig.6.

Identification of the process will be conducted on the basis of the measurement of the static characteristics in the full range of possible changes in the controlled variable and the time response of the process to introduced step input signal for a chosen operationg point selected from static characteristic. This will be therefore an experiment in which intentional impact on the process will take place through the control signal set by the operator.

Measurements of static and dynamic properties of the object will be carried out in a fixed set point open control system of liquid level (in manual mode). Static and dynamic properties would be represented by the relationship between the controlled variable, which is the height H1 of the liquid column in the tank Z1 processed to a measured signal PV, the control signal CV and between the controlled variable and the introduced disturbances. These compounds will be represented by the transfer function of a process Gob (s) and the transfer functions of disturbances GZ1 (s), GZ2 (s). These transfer functions will represent the dynamic properties od the system in the vicinity of the operating point of the process.

(11)

Automation Systems

11

Figure 2. Electrical scheme of the control system of the liquid level in a tank.

The laboratory stand include:

 tanks connected in series,

 SIMATIC S7-1200 PLC (Programmable Logic Controller) with the analog inputs / outputs modul and power supply 24V,

 HMI display with a color touch screen,

 desktop PC with TIA Portal software.

(12)

3.1.Visualization

During the exercise the student communicates with the PLC via the operator diplay (SIMATIC HMI KPT600).

On the Home screen, select an object tank ( structure of a control system (

the object visualization pressing the buttom "Tank" (Zbiornik)

In the next step user should go to the screen with object identification (button

"Identyfikacja obiektu", Fig.

Automation Systems

12

During the exercise the student communicates with the PLC via the operator diplay On the Home screen, select an object tank (Zbiornik) and the single close structure of a control system (Jednoobwodowy). After pressing the "Start" button, user go to the object visualization pressing the buttom "Tank" (Zbiornik) (Fig. 3).

Figure 3. Visualisation of the stand

"Identyfikacja obiektu", Fig. 4).

Figure 4. Screen for identification of a process.

During the exercise the student communicates with the PLC via the operator diplay ) and the single close-loop ing the "Start" button, user go to

Screen for identification of a process.

(13)

From this screen ("Identyfikacja obiektu", Fig. 8), it is possible to set (in control signals on both analog outputs (in this proscess do not use analog output 1).

Visualisation, in the form of plots the changes of the input and output values, is carried out on a PC using software TIA PORTAL. The home screen of this visua

Fig. 5.

The start screen (Fig.

pressing "Regulator główny" button, user will be send to the screen with monitoring of the input and output variables (Fig.

Automation Systems

13

From this screen ("Identyfikacja obiektu", Fig. 8), it is possible to set (in control signals on both analog outputs (in this proscess do not use analog output 1).

Visualisation, in the form of plots the changes of the input and output values, is carried out on a PC using software TIA PORTAL. The home screen of this visua

Figure 5. Start screen in TIA Portal simulation.

The start screen (Fig. 5) contains information about the entered structure. When pressing "Regulator główny" button, user will be send to the screen with monitoring of the

nput and output variables (Fig. 6).

From this screen ("Identyfikacja obiektu", Fig. 8), it is possible to set (in percents) control signals on both analog outputs (in this proscess do not use analog output 1).

Visualisation, in the form of plots the changes of the input and output values, is carried out on a PC using software TIA PORTAL. The home screen of this visualisation is shown in

in TIA Portal simulation.

) contains information about the entered structure. When pressing "Regulator główny" button, user will be send to the screen with monitoring of the

(14)

Figure 6. Screen with monitoring of the input and output variables

Over plots there is a graph showing the active disturbances in the form of a bar chart:

 Red rolor - valve VE1,

 Orange color - valve VE2,

 Green color - "SP step function".

1. Start / Stop button allow user to stop or resume monitoring of the input / output values.

2. The „Zwiększ przedział czasu” ("Increase the time interval") and ("Decrease time interval") buttons allow user to modify the currently displayed

15sec to 16min

3. The fields below the text

Increasing the scope is performed immediately. In the case of reducing the scope of the scale the changes will be visible after some

4. . The "skok SP" ("SP step") button allows user to generate the step function of a setpoint, together with the monitoring of it in the upper graph.

5. The text next to the "Cofnij" ("Undo") button allows user to enter the value of the SP later activated the "Skok SP" ("SP step") button.

change the value of this field before resetting the value by pressing "Cofnij"

(""Undo") button.

6. The "Cofnij" ("Undo") button resets the step function activated by "skok SP" ("SP s button.

Automation Systems

14

Screen with monitoring of the input and output variables

valve VE1, valve VE2,

"SP step function".

Start / Stop button allow user to stop or resume monitoring of the input / output values.

The „Zwiększ przedział czasu” ("Increase the time interval") and ("Decrease time interval") buttons allow user to modify the currently displayed time interval ranging from The fields below the text „Oś” ("Axis") allow scaling of the Y

Increasing the scope is performed immediately. In the case of reducing the scope of the scale the changes will be visible after some time.

. The "skok SP" ("SP step") button allows user to generate the step function of a setpoint, together with the monitoring of it in the upper graph.

The text next to the "Cofnij" ("Undo") button allows user to enter the value of the SP later the "Skok SP" ("SP step") button. After entering any of the values, do not change the value of this field before resetting the value by pressing "Cofnij"

The "Cofnij" ("Undo") button resets the step function activated by "skok SP" ("SP s Screen with monitoring of the input and output variables

Start / Stop button allow user to stop or resume monitoring of the input / output values.

The „Zwiększ przedział czasu” ("Increase the time interval") and ("Decrease time time interval ranging from

"Axis") allow scaling of the Y-axis of the graph.

Increasing the scope is performed immediately. In the case of reducing the scope of . The "skok SP" ("SP step") button allows user to generate the step function of a setpoint, The text next to the "Cofnij" ("Undo") button allows user to enter the value of the SP later After entering any of the values, do not change the value of this field before resetting the value by pressing "Cofnij"

The "Cofnij" ("Undo") button resets the step function activated by "skok SP" ("SP step")

(15)

Automation Systems

15

To print the chart, the course of the changing values should be stopped by START / STOP button, the user should press the button on a computer keyboard PrtSc and then paste saved screen into graphical editor.

3.2. Experimental identification of the static properties of controlled process.

Identification of the static properties of the process will be done by measuring the static characteristics of the process as a relation between steady state of process variable and the control variable in the full range of operating disturbances. These characteristics are important to determine the possible operating points of the object and limit control signal. Static characteristics will be executed for the three separate cases:

1. The valves VE1 and VE2 closed => no disturbance, 2. The valve VE1 - open, VE2 - closed => disturbance VE1 3. The valve VE1 - closed VE2 - open => disturbance VE2.

The results of measurements should be included in the appropriate cells in Table 2.

Each of these cases requires the same procedure as given below:

a) Check the position of the valves VE1 and VE2 by checking the state of the switches P1 and P2, or indicators on the computer visualization.

b) Set at the HMI panel the control signal CV to 0%

c) Read and record the current value of the water level (PV).

d) read the water level H1 using the scale provided on the the tank wall.

e) Repeat the points c) to e) changing the value of the control signal CV according to Table 2.

Repeat the procedure for another two cases with disturbances.

Obtained results write in Table 1.

Table 1. The results of measurements of static characteristics of the controlled process

CV[%] 0 20 30 40 50 60 70 80 100

1.

H1[cm]

PV[%]

2.

H1[cm]

PV[%]

3.

H1[cm]

PV[%]

(16)

Automation Systems

16

3.3. Experimental identification of the dynamic properties of controlled process.

The purpose of identification is to determine the parameters of following equivalent transmittances: process (Gob.(s)) and disturbances (G z1.(s), G z2.(s)). This identification will be carried out by active experiment which uses a method of the step response by generating abrupt changes in the control signal CV and the disturbances. This method allows to determine the parameters of the given transmittance of a process based on a simple graphic design. From the obtained during the test responses of the process to the abrupt disruptions are parameters of these trasmittances to be to be determined.

3.3.1. Determination of the step response of the control object to the change of control CV (increase in pump performance) in an open system. Calculation of the parameters of transmittance Gob (s).

Tests should be carried out using the following procedure:

a) check that the valves VE1 and VE2 are closed.

a) set the CV = 50% in the HMI panel.

b) wait until PV the reaches the steady value c) change the value CV from 50% to 60%

d) wait until the water level will determine, ie. PV ≈ const

e) Record the entire course of a transition on PC by choosing the appropriate time interval. For the process in laboratory it is recommended to take approx. 300 sec.

f) After stopping the plot on the screen using the STOP button, press the button PrtSc on a computer keyboard, paste saved screen in graphical editor and print two copies of the transient response (for later processing of the chart using the tangent and secant methods).

(17)

According to Fig.7 described by the transmittance (

From the plot there should be read parameters values according to transfer function:

,T0, kob. These parameters should be read using the tnagent

The calculated transfer functions parameters can be applied to the tuning of the controller parameters in the control system of the liquid level in the tank.

3.3.2. Determination of the step response of the control object to

opening of the VE1 valve (increase of the liquid outflow). Calculation of the parameters of transmittance G

a) set the CV = 50% in the HMI panel.

d) wait until the water level will determine, ie. PV e) using switch P1 (fig. 1) introduce the disturbance VE1.

f) wait until the water level will determine, ie. PV g) Record the entire course of a transition on PC interval.

h) After stopping the plot on the screen using the STOP button, press the button PrtSc on a computer keyboard, paste saved screen in graphical editor and print two copies of the transient response (for later proc

secant methods).

Automation Systems

17

Figure

graphic design to calculate the parameters of the transfer function using tangent method.

7, for the process there can be assumed mathematical model described by the transmittance (1):

s e T zs T

kob s

CV s s PV

Gob ⁰

1



 

 ( )

) ) (

( 



From the plot there should be read parameters values according to transfer function:

. These parameters should be read using the tnagent method and the secant method.

Determination of the step response of the control object to

opening of the VE1 valve (increase of the liquid outflow). Calculation of the parameters of transmittance Gz1 (s).

= 50% in the HMI panel.

d) wait until the water level will determine, ie. PV ≈ const e) using switch P1 (fig. 1) introduce the disturbance VE1.

f) wait until the water level will determine, ie. PV ≈ const

g) Record the entire course of a transition on PC by choosing the appropriate time h) After stopping the plot on the screen using the STOP button, press the button PrtSc on a computer keyboard, paste saved screen in graphical editor and print two copies of the transient response (for later processing of the chart using the tangent and

Figure 7. An example of graphic design to calculate the parameters of the transfer function using tangent method.

there can be assumed mathematical model

(1) From the plot there should be read parameters values according to transfer function: Tz

method and the secant method.

Determination of the step response of the control object to the distirbance - opening of the VE1 valve (increase of the liquid outflow). Calculation of the

by choosing the appropriate time h) After stopping the plot on the screen using the STOP button, press the button PrtSc on a computer keyboard, paste saved screen in graphical editor and print two copies

essing of the chart using the tangent and

(18)

Automation Systems

18

For the process and disturbace VE1 there can be assumed mathematical model described by the transmittance (2):

s e T s T

k s

f s s PV

Gz ⁰

1 1

1 1 

 

 ( ) ) ) (

( 

 (2)

where: f- change of the flow surface of the valve VE1, accordig to the manufacturer data sheets of the element f = 30.4 %.

3.3.4. Determination of the step response of the process to the distirbance - opening of the VE2 valve ("dumping" the pump). Calculation of the parameters of transmittance Gz2 (s).

b) set the CV = 50% in the HMI panel.

c) wait until the water level will determine, ie. PV ≈ const d) using switch P2 (fig. 1) introduce the disturbance VE2.

e) wait until the water level will determine, ie. PV ≈ const

f) Record the entire course of a transition on PC by choosing the appropriate time interval.

g) After stopping the plot on the screen using the STOP button, press the button PrtSc on a computer keyboard, paste saved screen in graphical editor and print two copies of the transient response (for later processing of the chart using the tangent and secant methods).

For the process and disturbace VE2 there can be assumed mathematical model described by the transmittance (3):

s e T s T

k s

f s s PV

Gz ⁰

2 1

2 2 

 

 ( )

) ) (

( 

 (3)

where f- change of the flow surface of the valve VE2, accordig to the manufacturer data sheets of the element f = 30.4 %.

(19)

Automation Systems

19 4. REPORT

The report shall contain the following elements: description of the exercises, diagrams, registered plots with data processing, graphs made on the basis of measurements (static characteristics),etc. Following questions should be answered:

1) Determine the parameters of the transfer fuctions of the facility using the tangent and secant methods.

2) Determine the course of the step response object calculated using calculated transmittance and make a comparison with the real response.

3) Draw a block diagram of the i, and perform the analysis.

4) Draw the statc characteristics of the process and perform an analysis of it's static properties,

5) Determine, based on the static characteristics, possible points of operation of the control system,

6) Compare the value of the gain of the object kob obtained from the static characteristics of the dynamic step response characteristics. Comment on the results.

7) Determine based on the block diagram what should be controller working mode (NORMAL or REVERSE) in a closed loop control system.

5. BIBLIOGRAPHY

1.Kościelny W.J.: Materiały pomocnicze do nauczania podstaw automatyki dla studiów wieczorowych, WPW, 1997, 2001.

2. Węgrzyn S.: Podstawy automatyki. PWN, 1980.

3. Żelazny M.: Podstawy automatyki . PWN, 1976.

4. Kościelny W.J., Holejko D.: Automatyka Procesów Ciągłych, OWPW, 2012.