pct52 issue 2 instruction manual

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Essentials of Process Control

Temperature Control Process

Instruction Manual

PCT52

ISSUE 2

December 2012



Table of ContentsCopyright and Trademarks...................................................................................... 1

General Overview ....................................................................................................... 2

Equipment Diagrams................................................................................................... 3

Important Safety Information....................................................................................... 5

Introduction.............................................................................................................. 5

Electrical Safety....................................................................................................... 5

Description .................................................................................................................. 6

Overview.................................................................................................................. 6

PCT52 Temperature Control Process ..................................................................... 7

Software .................................................................................................................. 8

Installation................................................................................................................. 10

Advisory................................................................................................................. 10

Electrical Supply.................................................................................................... 10

Installing the PC Software ..................................................................................... 10

Installing the Equipment ........................................................................................ 11

Operation .................................................................................................................. 14

Operating the PC Software.................................................................................... 14

Operating the Equipment....................................................................................... 14

Equipment Specifications.......................................................................................... 17

Overall Dimensions ............................................................................................... 17

IFD Connections.................................................................................................... 17

Other Specifications .............................................................................................. 19

Environmental Conditions...................................................................................... 19

Routine Maintenance................................................................................................ 21

Responsibility ........................................................................................................ 21

General.................................................................................................................. 21

Accessing the electrical circuits inside the electrical enclosure............................. 21

Laboratory Teaching Exercises................................................................................. 22

Index to Exercises ................................................................................................. 22

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Table of Contents

iii

Exercise A - Manual Control of Heater Temperature (Open loop) ............................ 23

Exercise B - On/Off Control of Heater Temperature (Closed loop)........................... 26

Exercise C - Proportional Control of Heater Temperature (Closed loop, P only and P+ I)............................................................................................................................. 30

Exercise D - Optimising Proportional Control of Heater Temperature (Closed loop,P+I+D)....................................................................................................................... 36

Exercise E - Manual Control of Air Temperature (Open loop) .................................. 41

Exercise F - On/Off Control of Air Temperature (Closed loop) ................................. 44

Exercise G - Proportional Control of Air Temperature (Closed loop, P only and P + I).................................................................................................................................. 48

Exercise H - Optimising Proportional Control of Air Temperature (Closed loop,

P+I+D)....................................................................................................................... 54

Exercise I - Using a PID Controller (PCT54)............................................................. 59

Exercise J - Using a PLC controller (PCT55)............................................................ 60

Contact Details for Further Information..................................................................... 61



1

Disclaimer

This document and all the information contained within it is proprietary to ArmfieldLimited. This document must not be used for any purpose other than that for which itis supplied and its contents must not be reproduced, modified, adapted, published,translated or disclosed to any third party, in whole or in part, without the prior written

permission of Armfield Limited.

Should you have any queries or comments, please contact the Armfield CustomerSupport helpdesk (Monday to Thursday: 0830 – 1730 and Friday: 0830 - 1300 UKtime). Contact details are as follows:

United Kingdom International

(0) 1425 478781(calls charged at local rate)

+44 (0) 1425 478781(international rates apply)

Email: [email protected]

Fax: +44 (0) 1425 470916

Copyright and Trademarks

Copyright © 2012 Armfield Limited. All rights reserved.

Any technical documentation made available by Armfield Limited is the copyrightwork of Armfield Limited and wholly owned by Armfield Limited.

Brands and product names mentioned in this manual may be trademarks orregistered trademarks of their respective companies and are hereby acknowledged.



General Overview

Four independent but complementary process units provide a compact, hands-onintroduction to the fundamental principles of process control engineering. Each fluid-based process provides a clear demonstration of a different single PID loop usingTemperature, Flow, Temperature or Pressure as the measured variable.

Each process is supplied complete with software that allows it to be controlled usinga suitable PC via a USB connection. The effect of making changes to the system orto the controller configuration can be quickly investigated by applying repeatabledisturbances to the process, clearly demonstrating the need for correct matching ofsystem characteristics and controller settings. The software allows all appropriatemeasurements and controller settings to be displayed continuously and recorded forlater analysis, if required by the user. Alternatively the process can be operated usingan external controller such as the optional Industrial PID controller (PCT54) orProgrammable Logic Controller (PCT55). If required, a PC can be used to record theresponses of the process when using an external controller.

A Sensor Conditioning and Calibration Trainer (PCT56) completes this range ofprocess control products.

The range of products comprises the following small-scale processes, optionalcontrollers and sensor calibration trainer:

PCT50 Level Control Process

PCT51 Flow Control Process

PCT52 Temperature Control Process

PCT53 Pressure Control Process

PCT54 Industrial PID Controller

PCT55 Programmable Logic Controller

PCT56 Sensor Conditioning and Calibration Trainer

This instruction manual describes the operation of the PCT52 Temperature ControlProcess when used with a PC to control and record the process.

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Equipment Diagrams

Figure 1: Front View of the PCT52 Temperature Contro l Apparatus

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Armfield Instruction Manual

4

Figure 2: Schematic d iagram of t he PCT52 Temperature Control Process (Direct heating)

Figure 3: Schematic diagram of the PCT52 Temperature Control Process (Indirect heating)



Important Safety Information

Introduction

All practical work areas and laboratories should be covered by local safetyregulations which must be followed at all times.

It is the responsibility of the owner to ensure that all users are made aware ofrelevant local regulations, and that the apparatus is operated in accordance withthose regulations. If requested then Armfield can supply a typical set of standardlaboratory safety rules, but these are guidelines only and should be modified asrequired. Supervision of users should be provided whenever appropriate.

Your PCT52 Temperature Control Process has been designed to be safe in usewhen installed, operated and maintained in accordance with the instructions in thismanual. As with any piece of sophisticated equipment, dangers exist if the equipmentis misused, mishandled or badly maintained.

Electrical SafetyThe equipment described in this Instruction Manual operates from a mains voltageelectrical supply via a 24 V DC adapter. It must be connected to a supply of the samefrequency and voltage as marked on the equipment or the mains lead. If in doubt,consult a qualified electrician or contact Armfield.

The equipment must not be operated with any of the panels removed.

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Description

Where necessary, refer to the drawings in the Equipment Diagrams section.

Overview

The PCT52 Temperature Control Process consists of a variable speed fan below aheater with radial fins at the base of a vertical clear acrylic duct, together with anelectrical connection box, mounted on a common base plate. It belongs to a family offour processes having common features, as follows:

All four processes are free standing, without the need for a permanent watersupply or drain connection, and suitable for locating on a work bench alongsidea PC.

Each process incorporates an electronic interface with sensor conditioning,drive circuits and integrated USB connection. This means that the user onlyneeds to fill the process with water (where appropriate), connect the mains

adaptor and connect the supplied USB cable to a suitable PC for theequipment to be operational.

The use of transparent materials gives clear visibility of the process inoperation and the location of control components such as sensors and controlelements.

The facility to vary the operating characteristics to allow optimization of the PIDsettings to suit the characteristics and to demonstrate the differing responsesof both optimum and non-optimum controller configuration.

Supplied with software and USB lead allowing control and data acquisitionusing a PC.

Disturbances (step changes) can be applied to the process, remotely from thePC, to test the stability of the system with the controller settings underinvestigation. The magnitude of the disturbance can be changed in fixed stepsfor repeatability or varied continuously to achieve a specific response in thesystem.

Manual operation via the PC will allow open loop testing of each process todetermine the system response prior to assigning appropriate values for the P,I and D terms, cycle time etc in a closed loop configuration.

In addition to control of the process and initiation of step changes, the softwaresupplied will also allow recording of the responses, graphing etc. when testingany of the control loops.

The electrical interface, mounted alongside the process, incorporatesadditional front panel connectors that allow the input and output signals to beconnected to an alternative PID controller, Programmable Logic Controller orsimilar control equipment if required by the end user.

The electrical supply is provided by a universal 24 Volt DC in-line adaptor withinterchangeable leads to suit the local electrical supply.

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Description

PCT52 Temperature Control Process

Refer to the equipment diagrams for details about the PCT52 Temperature Process.The front view shows the layout of the PCT52 and circuit diagrams showing the

process in operation with direct heating (heater temperature T1 controlled by varyingpower to the heater) or indirect heating (air temperature T2 controlled by varying

power to the heater). These alternatives ware described in detail later.

PCT52 is a temperature control process using air as the working fluid for safety andconvenience in use.

The process consists of a variable speed fan mounted below a radial array ofceramic resistors that form the heater (2) at the base of a vertical clear acrylic duct(5). Air drawn into the duct from below the support plinth, by the fan, passes over theheater fins (6) before discharging at the top of the duct.

Variation of fan motor speed (Fan Control) and variation of heater voltage (HeaterPower) results in changes in the temperature of the heater surface (T1) and the

temperature of the air after the heater (T2).

The temperature on the surface of the heater is measured using a platinumresistance temperature sensor T1 that is bonded to one of the heater fins. Thetemperature of the air after the heater is measured using a separate platinumresistance temperature sensor T2. T2 is built into a probe assembly (3) at the top ofthe duct. The sensor is surrounded by a shield (4) that is positioned vertically fornormal operation. The probe can be rotated so that the shield is horizontal to slowdown the response of the sensor by eliminating direct air flow over the sensor. Thisallows the effect of sensor response or lag to be demonstrated.

Disturbances can be applied to the process by varying the fan motor speed

continuously or in fixed steps so that system characteristics, step changes etc can berepeated precisely allowing direct comparison of different controller settings. The fanis designed to operate at a minimum operating speed to avoid excessivetemperatures that would result with no air flow. Therefore the fan will vary fromminimum to maximum speed when the speed control is varied from 0% to 100%.When using PCT54 or PCT55 to control the process the air flow can be switchedbetween low and high settings to create a step change.

The sensors measuring the heater and air temperatures, the fan and the heater areconnected to an electrical interface (1) that incorporates the necessary signalconditioning, allowing the process to be operated directly from a PC using one USBport. Alternatively the process can be operated using a controller such as PCT54

(PID Controller) or PCT55 (PLC with PID controller) connected to the electricalinterface.

The computer software supplied with PCT52 allows control of the temperatureprocess and data logging of the responses using a PC. Alternatively the softwareallows data logging only while operating the process remotely using a PID controlleror PLC.

Two process control loops are available for demonstration:

Direct control of heater surface temperature by proportionally varying heatervoltage

Indirect control of air temperature by proportionally varying heater voltage

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The PCT52 is self contained requiring only a mains electrical supply to the in-line DCadapter and connection to either a PC via the USB port or to a process controllersuch as PCT54 (PID Controller) or PCT55 (PLC with PID controller).

Software

The PCT52 is supplied with an educational software package with a wide range offacilities and functions. The computer is the primary interface between the user andthe equipment. The software displays a real time process mimic diagram withreadings of the relevant sensor outputs and controls for the system inputs. Manual,On/off, time proportioned and PID control loops can be configured using twopredetermined student exercises:

Ex 1 Direct Temperature contro l (PC)

Exercises demonstrating Direct Control of heater temperature by varying heaterpower:

Exercise A - Manual Control of Heater Temperature (Open loop)

Exercise B - On/Off Control of Heater Temperature (Closed loop)

Exercise C - Proportional Control of Heater Temperature (Closed loop, P onlyand P + I)

Exercise D - Optimising Proportional Control of Heater Temperature (Closedloop, P+I+D)

Ex 2 Indirect Temperature Control (PC)

Exercises Demonstrating Indirect control of air temperature by varying heater power:

Exercise E - Manual Control of Air Temperature (Open loop)

Exercise F - On/Off Control of Air Temperature (Closed loop)

Exercise G - Proportional Control of Air Temperature (Closed loop, P only andP + I)

Exercise H - Optimising Proportional Control of Air Temperature (Closed loop,P+I+D)

Alternative exercises are included that allow the process to be monitored and

recorded on the PC while using an external controller such as PCT54 or PCT55:

Ex 3 Direct Temperature Control (External Contro l)

Exercise I - Using a PID Controller (PCT54)

Exercise J - Using a PLC controller (PCT55)

Ex 4 Indirect Temperature Contro l (External Control)

Exercise I - Using a PID Controller (PCT54)


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Description

9

When controlling using a PC all control and sensor signals including the setpoint, andcontroller configuration can be logged continuously using the PC. Disturbances offixed magnitude can be introduced remotely using the PC to evaluate the stability ofa particular process / controller configuration.

When using the PC to monitor the process the input and output signals can bemonitored and logged. Disturbances are introduced remotely from the externalcontroller.

This eliminates the need for a separate process recorder or chart recorder foranalysis of the process control responses. This software is compatible with PCsusing Microsoft WindowsTM 2000 onwards. The computer communicates to thePCT52 using a standard universal serial bus (USB) interface. Installation instructionsare included in the section Installing the Software. The software includes acomprehensive online Help Text.

The use of a Virtual Serial Port to allow the USB port to communicate with thesoftware allows users to write their own software if they have the necessary skills and

appropriate software such as Labview. Information about the USB channelconfiguration is included in the section IFD connections.

Note: Armfield cannot provide assistance in writing alternative software to operatethe PCT52.



Installation

Adv isory

Before operating the equipment, it must be unpacked, assembled and installed asdescribed in the steps that follow. Safe use of the equipment depends on following

the correct installation procedure.

Electrical Supply

The universal adapter supplied with PCT52 requires connection to a single phase,fused electrical supply. Leads are supplied allowing connection to:

220V / 50 Hz via European style 2-pin plug

220V / 50 Hz via UK style 3-pin plug

120V / 60Hz via USA 3-pin plug (125V style)

220V / 60Hz via USA 3-pin plug (250V style)

Installing the PC Software

Before operating PCT52 it will be necessary to install the software from the CD-ROMsupplied with PCT52 onto an appropriate PC (PC not supplied).

For instructions on how to install and run the software insert the CD-ROM into theoptical drive on the PC (PC not supplied) then choose ‘Help’ from the menu.

After installing and running the software on the PC, instructions on how to operatethe software can be obtained by choosing the ‘Help’ tab in the top right hand corner

of the screen as shown below:

Note that when operating the software for the first time it will be necessary to enablethe USB virtual COM port by choosing the Red telephone icon (Start COM session).

Full instructions about enabling the port are included in the Help menus.

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Installation

Installing the Equipment

To install the equipment:

1. Carefully remove the PCT52 from its packing and remove any looseprotective materials. Retain the power supply, connecting leads and CD-

ROM in a safe place until required for use.

2. Place the PCT52 on a firm, level bench top or table where the equipment is tobe operated with access to a mains electrical supply.

3. Wipe the equipment with a soft damp cloth to remove any dust etc. beforeproceeding.

4. Switch on the PC and connect the USB lead from the PCT52 to the PC.

5. Load the PCT52 software then choose experiment ‘Ex1: Direct TemperatureControl (PC control)’ from the menu.

For information about operating the software refer to the section Operating theSoftware.

6. Enable the Virtual COM port using the red telephone icon in the top toolbar.

7. Display the mimic diagram by choosing View then Diagram or via the mimic

diagram icon .

8. The mimic diagram will be displayed as follows:

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9. Connect the lead from the 24V dc power adapter to the socket marked 24VDC IN at the rear of the electrical enclosure. Connect the mains lead withappropriate plug to the mains inlet socket on the power adapter. The fan onPCT52 will operate at maximum speed until the software is operating.

10. Click Power On (in Controls) to allow the PCT52 to be operated via the PC.

11. Each time the software starts the PID controller is reset to Off, with the heateroff and the fan operating at minimum speed. Default values for the Set pointand control parameters are restored unless a previous set-up has beensaved.

Correct operation of the PCT52 can be checked using the manual controls inthe software before using the equipment for automatic control exercises.

12. On the mimic diagram (in Controls) set the Fan Control to 50% by typing inthe value or clicking on the up arrow until 50% is displayed.

Check that the fan speed reduces and the two temperature sensors readsimilar temperatures (exact agreement is not necessary for the controldemonstrations).

13. Set the Heater Power to 50% on the mimic diagram (in Controls), thenconfirm that heater temperature T1 starts to rise immediately with a largechange in temperature followed by the air temperature more slowly with amuch smaller change in temperature. Turn off the heater by setting the

Heater Power to 0%.

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Installation

13

14. The operation of the PCT52 has been confirmed and the equipment is readyfor use.



Operation

Where necessary, refer to the drawings in the Equipment Diagrams section.

Operating the PC Software

Details about operating the software can be obtained by choosing the ‘Help’ tab inthe top right hand corner of the screen as shown below:

Operating the EquipmentNote: Before operating the equipment ensure that it has been correctly installed inaccordance with the Installation section, and that you have read the Important SafetyInformation at the beginning of this manual.

Operating the Heater (Manual operation)

The Heater Power can be varied by the operator to change the heat entering theprocess duct.

On the mimic diagram (in Controls) set the Heater Power to the required percentageof maximum power by typing in the value or clicking on the up arrow until the

required percentage is displayed. For example, to achieve half power type 50 or clickthe appropriate arrow until 50 % is displayed.

Operating the Heater (PID Control)

The Heater Power is varied via the PID window on the mimic diagram.

When set to Manual Mode the PID controller gives a fixed Heater Power that is setby the operator. The fixed Heater Power is also indicated on the mimic diagram. InManual mode the pump speed can also be varied from the mimic diagram (inControls).

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Operation

When set to Automatic Mode the PID controller varies the heater power in an attemptto maintain the required temperature in the system. The output from the controller willdepend on the current measured temperature (T1 or T2 as appropriate), the setpointin the PID controller and the P, I and D settings of the controller.

In Automatic mode the heater power is indicated on the mimic diagram but cannot bechanged on the mimic diagram.

Operating the Fan Motor (Manual operation)

The fan motor speed can be varied by the operator to change the air flow through theprocess duct.

On the mimic diagram (in Controls) set the Fan Control to the required percentage ofmaximum output by typing in the value or clicking on the up arrow until the requiredpercentage is displayed. For example, to achieve half speed type 50 or click theappropriate arrow until 50 % is displayed. Note that the fan will operate at a minimumpreset speed when Fan Control is set to 0%. The fan speed will vary from minimum

to maximum as the Fan Control is varied from 0 to 100%.

The fan motor speed is used to inject a small step change to the temperatures is thesystem to test the stability of the control loop.

As the fan speed is preset to a minimum speed, to prevent damage to the equipmentin use, step changes can be applied by alternating between settings of 0% or 100%. Alternatively, the magnitude of the step change can be reduced by settingappropriate intermediate values of percentage.

Connecting the PCT52 to an external controller

The fan motor, heater and temperature sensors on PCT52 are permanentlyconnected to the electrical enclosure.

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16

The 0-5 Volt output from temperature sensors T1 and T2, the 0-5 Volt input to controlthe voltage to the heater and a digital input to switch the fan motor between high andlow speed are available via connectors on the front of the electrical enclosure. Thisallows a PID controller such as PCT54 or a PLC such as PCT55 to operate theprocess. The connections between PCT52 and the external controller should bemade using the leads supplied with PCT54 or PCT55.

When operating the fan via an external controller variable speed operation is notavailable, the fan speed alternating between the minimum preset speed andmaximum speed depending on the digital output from the controller.

Calibration of the temperature sensors

The software includes the facility to recalibrate the temperature sensors should thisbecome necessary. To calibrate T1 or T2 choose ‘Options’ in the top toolbar,‘Calibrate IFD channels’ then choose the appropriate sensor, T1 measuring heatertemperature or T2 measuring air temperature. The procedure for calibrating thesensor is described in the help text included in the software see Operating the PC

Software.



Equipment Specifications

Overall Dimensions

Height - 250mm

Width - 230mm

Depth - 255mm

IFD Connections

The electrical interface on PCT52 incorporates a PCB that provides the USBconnection to a PC. The pin-outs of the 50 way connector on this PCB are as follows:

PinNo

Channel No PCT52 Function Signal Eng Unit

1 Channel 0 Analog Input T1 Temperature 0 – 5V

2 Channel 1 Analog Input Not used

3 Channel 2 Analog Input T2 Temperature 0 – 5V




7 Channel 6 Analog Input Drive 1 monitor (Heater) 0 – 5V


9 Channel 8 Analog Input Drive 2 monitor (Fan) 0 – 5V








17




17 Analog ground 0V

18 Amp Lo 0V

19 Analog Output 2 Not used

20 Analog Output 3 Not used

21 Power Ground 0V

22 Channel 0 Analog Output Heater power 0 – 5V 0-100%

23 Channel 0 Analog Ground 0V

24 Channel 1 Analog Output Fan motor speed 0 – 5V 0-100%

25 Channel 1 Analog Ground 0V

26 Digital Ground 0V


28 Channel 0 Digital Input Drive 1 Enable (Heater)

29 Channel 1 Digital Input Drive 2 Enable (Fan motor)

30 Channel 2 Digital Input Not used








38 Digital Output Line 0 Power enable

39 Digital Output Line 1 Watchdog

40 Digital Output Line 2 Remote / Local operation

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Equipment Specifications

41 Digital Output Line 3 Output 1 On/Off (Heater)


43 Digital Output Line 4

Output 1 On/Off (Fan

Motor)

44 Digital Output Line 5 Not used




48 Ground 0V

49 Not used 0 – 5V

50 Not used 0 – 5V

Other Specifications

Environmental Conditions

This equipment has been designed for operation in the following environmentalconditions. Operation outside of these conditions may result reduced performance,damage to the equipment or hazard to the operator.

a. Indoor use;

b. Altitude up to 2000m;

c. Temperature 5°C to 40°C;

d. Maximum relative humidity 80% for temperatures up to 31°C, decreasinglinearly to 50% relative humidity at 40°C;

19




20

e. Mains supply voltage fluctuations up to ±10% of the nominal voltage;

f. Transient over-voltages typically present on the MAINS supply;

Note: The normal level of transient over-voltages is impulse withstand (over-voltage) category II of IEC 60364-4-443;

g. Pollution degree 2.

Normally only nonconductive pollution occurs.

Temporary conductivity caused by condensation is to be expected.

Typical of an office or laboratory environment



Routine Maintenance

Responsibility

To preserve the life and efficient operation of the equipment it is important that theequipment is properly maintained. Regular maintenance of the equipment is the

responsibility of the end user and must be performed by qualified personnel whounderstand the operation of the equipment.

General

The equipment should be disconnected from the electrical supply when not in use.

Cover the equipment, particularly the process duct to prevent dust build up on theheater.

Accessing the elect rical c ircu its inside the electr ical enclosure

Maintenance of the PCT52 does not require access to the electrical circuits or

components located inside the electrical enclosure. However, in the event of anelectrical problem it may be necessary for a competent electrician to gain access tothe inside of the enclosure as follows:

Ensure that the equipment is disconnected from the electrical supply (not justswitched off).

The electrical circuits inside the enclosure are accessible after unscrewing the frontpanel then carefully sliding the loose side panel forwards, but only as far as theflexible ribbon cable will allow.

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Laboratory Teaching Exercises

Index to Exercises

Exercises demonstrating Direct Control of Heater Temperature byvarying heater power

Exercise A - Manual Control of Heater Temperature (Open loop)

Exercise B - On/Off Control of Heater Temperature (Closed loop)

Exercise C - Proportional Control of Heater Temperature (Closed loop, P only and P+ I)

Exercise D - Optimising Proportional Control of Heater Temperature (Closed loop,P+I+D)

Exercises demonstrating Indirect Control of Ai r Temperature by varyingheater power

Exercise E - Manual Control of Air Temperature (Open loop)


Exercise G - Proportional Control of Air Temperature (Closed loop, P only and P + I)

Exercise H - Optimising Proportional Control of Air Temperature (Closed loop,P+I+D)

Exercises demonstrating Direct Control or Indirect Control using anexternal controller

Exercise I - Using a PID controller (PCT54)


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Exercise A - Manual Control of Heater Temperature (Openloop)

Objective

To control the surface temperature of the heater in the process duct by manuallyvarying the power supplied to the heater.

To determine the characteristics of the temperature process with direct heating.

Method

Using a PC to operate the process, the heater power (heat input into the processduct) will be varied by the operator in an attempt to maintain a steady heatertemperature in the process duct.

Disturbances will be applied to the process by changing the fan motor speed. Thischanges the velocity of the air flowing over the heater resulting in a change in the

surface temperature of the heater. The magnitude of the disturbance is determinedby the change in motor speed resulting in a change in air flowrate.

Equipment Required

PCT52 Temperature Process

PC with PCT52 software loaded

Theory

For the surface temperature of the heater (T1) to remain constant the power suppliedto the heater must match the power lost from the heater to the air flowing over it. The

temperature of the heater depends on the power supplied to the heater and the flowof air over the heater (not controlled). The power supplied to the heater must beincreased to raise the temperature of the heater and decreased to lower thetemperature of the heater.

Equipment set up

Ensure that the apparatus has been set up according to the Installation section andthe power supply connected to the socket marked 24V IN at the rear of the electricalenclosure.

Connect the USB socket at the rear of the electrical enclosure a suitable PC onwhich the PCT52 software has been installed using the USB cable supplied. Check

that the PC is switched on then run the PCT52 software and select Experiment Ex1:Direct Control (PC Control).

Ensure that the Virtual COM port has been enabled using the red telephone icon inthe top toolbar.

Choose the icon (or View \ Diagram) to display a mimic diagram of the process.

Click Power On (in Controls).

Confirm correct operation of the equipment as follows:

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Ensure that the Fan Control is set to 0% by typing in the value or by clicking thedown arrow (in Controls). Note that fan will operate at a minimum preset speed whenset to 0%.

Confirm that the fan is operating.

Confirm that Temperature sensors T1 and T2 indicate ambient temperature.

Adjust the Heater Power to 50% by typing in the value or by clicking the up arrow (inControls).

Monitor Temperatures T1 and T2 and observe that the temperature of the heaterrises quickly followed by a small rise in the air temperature. Allow T1 to settle at asteady then increase the Fan Control to 100%.

Confirm that the speed of the air increases and the heater temperature falls slightly.

Reset the Heater Power to 0% and the Fan Control to 0% on the mimic diagram and

allow the duct to cool with the fan operating at minimum speed. The equipment isready for use.

Procedure

Gradually adjust the Heater Power until the surface temperature of the heater T1reads 50°C.

When temperature T1 is constant choose the icon to begin data logging and notethe temperature reading T1 in °C.

Increase the motor speed to 50% on the mimic diagram. Observe that the

temperature falls slightly due to the additional air flow and resulting heat loss from theheater. When T1 has settled at a new value, increase the Heater Power as requiredto restore the original temperature of 50°C.

When temperature T1 is constant increase the fan motor speed to 100% causinganother fall in heater temperature. When T1 has settled at a new value, increase theHeater Power to restore the original temperature of 50°C.

When temperature T1 is constant reduce the fan motor speed to 0 % causing theheater temperature to rise. When T1 has settled at a new value, reduce the HeaterPower to restore the original temperature of 50°C.

Set the Heater Power to 0% and allow the heater to cool.

Choose the icon to finish data logging.

Choose the icon (or View \ Graph) to display a graph of the response obtained.Observe the rising and falling temperate of the heater as the fan motor is increasedand decreased and action is taken to restore the temperature.

Return to the mimic diagram.

Create a new results sheet by selecting the icon in the tool bar of the software

then choose the icon to begin data logging.

Set the Fan Control to 0% (minimum speed).

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Exercise A

25

Set the Heater Power to 25% and observe the rise in temperature T1. When T1 issteady set the Heater Power to 0% and allow the heater to cool.

When temperature T1 is steady at ambient temperature set the Heater Power to 75%and observe the rise in temperature T1. When T1 is steady set the Heater Power to0% and allow the heater to cool.

Set the Fan Control to 100% then repeat as above with the Heater Power set to 25%then 75%.


View the graph obtained and observe the different rates of heating at differentsettings of Heater Power. Also note that the rate of cooling depends on the fan motorspeed. These characteristics will affect the control of the process.

Results

Save a copy of the results obtained so that the data can be viewed in graphical ortabular format at a later date.

Conclusion

The heater temperature in a duct will only remain constant when the Heater Powermatches the heat transferred to the air.

Excess heat entering the duct will cause the heater temperature to rise. Insufficientheat entering the duct will cause the heater temperature to fall.

When the heater temperature is stable any disturbance such as a change in airflowwill result in a change in temperature, requiring variation of the Heater Power to

restore the original temperature.

The rates of heating or cooling are likely to be different and depend on the HeaterPower and the airflow through the duct.



Exercise B - On/Off Control of Heater Temperature(Closed loop)

Objective

To control the heater temperature in the process duct using an On/Off controller toautomatically switch the heater on or off as necessary to maintain the required heatertemperature in the process duct.

To determine the variations in heater temperature due to the dead band inherent inan on/off controller.

To change the heater temperature inside the process duct by changing the set pointon the on/off controller and to determine the effect of disturbances to the process.

To investigate the effect of changes in airflow through the duct.

Method

Using a PC to operate the process, the heater (heat into the process duct) will beswitched on and off by the controller in an attempt to maintain a steady heatersurface temperature.

Disturbances can be applied to the process by increasing and decreasing the air flowby changing the speed of the fan motor. Increasing the air flow reduces the surfacetemperature of the heater because more heat is transferred from the heater to the air.

Temperature sensor measures the heater and air temperatures (T1 and T2,respectively) in the process duct and indicate the values on the PC.

Equipment RequiredPCT52 Temperature Process.


Theory

An On/Off controller is a simple and effective way of controlling many processes butdoes have disadvantages because its output can only be on or off. In the case of thetemperature process in this exercise the heater is switched on (full Heater Power) oroff.

An On/Off controller incorporates a dead band to avoid rapid switching of thecontrolled variable when at the setpoint i.e. in this case the temperature must riseabove the setpoint by a fixed amount before the heater switches off and thetemperature must fall below the setpoint by a fixed amount before the heaterswitches off again.

Note: On PCT52 an On/Off controller with fixed dead band is created by setting theProportional Band to 0% in the PID controller. In a commercial On/Off controller thedead band can be varied to suit the process. This allows the choice of less frequentswitching but larger variations in the process variable or closer control of the processvariable but more frequent switching with attendant wear etc. of the components.

26



Exercise B

Equipment set up


Connect the USB socket at the rear of the electrical enclosure a suitable PC onwhich the PCT52 software has been installed using the USB cable supplied. Checkthat the PC is switched on then run the PCT52 software and select Experiment Ex1:Direct Control (PC Control).











Reset the Heater Power to 0% and the Fan Control to 0% on the mimic diagram andallow the duct to cool with the fan operating at minimum speed. The equipment isready for use.

Procedure

Choose the PID box on the mimic diagram, set the Proportional Band to 0%, IntegralTime to 0 and Derivation time to 0 then set the Set Point to 35°C. Click Apply to enterthe changes to the settings.

Choose the Automatic Mode of Operation.

27




Select the icon to begin data logging.

The heater will switch on, the heater temperature (T1) will gradually rise. The

temperature will continue to rise until it reaches the temperature set on the controllerI.e. the setpoint of 35°C. When the T1 rises above the setpoint by a fixed amount(the dead band) the heater will switch off and T1 will start to fall. When T1 falls belowthe setpoint by a fixed amount (the dead band) the heater will switch on again andthe cycle will continue.

In the PID controller adjust the set point to 50°C, click Apply then observe the heaterremains on until T1 rises to the new set point then switches on and off as before tomaintain the new temperature.

Adjust the set point to 80°C and observe the response of the process.

Effect of Disturbances

With the set point at 80°C set the Fan Control to 100%. Observe that temperature T1falls faster due to the increase in airflow then the controller switches on the heater torestore the original temperature T1.

Set the Fan Control to 50% and observe how the process returns to the original setpoint.

Set the temperature set point to 35°C and observe how the process returns to thenew set point.


28



Exercise B

29

Results

Choose the icon (or View \ Graph) to display a graph of the responses obtained.Observe the rising and falling air temperature about each of the set points as theheater switches on and off to maintain the required air temperature. Observe thechange in the rate of heating when the fan motor speed is increased and decreased

to disturb the process.

From the graph or table of results ( icon or View\Table) determine the variation intemperature about the set point to determine the dead band of the controller. In acommercial controller the dead band would usually be adjustable to suit the processbut too small a variation would lead to wear of components switching rapidly on andoff.

From the graph also observe that the rate of heating when the heater is switched onis not necessarily the same as the rate of cooling when the heater is switched off.

Conclusion

Where a low cost control solution is required, where small variations in processvariable are acceptable and where the response of the process is slow enough thenan On\Off controller can be used to control the process. The temperature process onPCT52 is such a process. Note: a faster process such as flow control (demonstratedusing PCT51) is not suitable for on\off control.

The device controlled by an On\Off controller (in this case a heater) is simply on oroff as required to maintain the process variable.

The air temperature will alternate above and below the setpoint due to the deadbandin the On/Off controller.



Exercise C - Proportional Control of Heater Temperature(Closed loop, P only and P + I)

Objective

To control the heater temperature in the process duct using a proportional controllerto automatically vary the Heater Power (Direct heating).

To determine the response of a temperature process when using a P only controllerto vary the Heater Power.

To determine the response of a temperature process when using a P+I controller tovary the Heater Power.

To change the heater temperature inside the process duct by changing the set pointon the P or P+I controller and to determine the effect of disturbances to the process.

Method

Using a PC to operate the process, the Heater Power (heat flow into the processduct) will be varied automatically by a proportional controller in an attempt to maintaina steady heater temperature in the process duct.

A platinum resistance sensor measures the temperature T1 at the surface of theheater.

The effect of the Proportional Band setting (P) can be demonstrated by configuring aP only controller and observing the response of the process to different settings of P.

The contribution of the of the Integral Time setting (I) can be demonstrated by

configuring a P+I controller and observing the response of the process to differentsettings of I with P fixed then different combinations of P+I.

Disturbances can be applied to the process by increasing and decreasing the fanmotor speed. This changes the airflow through the process duct resulting in a changein the heater temperature inside the duct. The magnitude of the disturbance isdetermined by the size of the variation in fan motor speed.

Temperature sensors measure the heater and air temperature (T1 and T2,respectively) in the process duct and indicates the value on the PC.

Equipment Required

PCT52 Temperature Process.


Theory

Proportional term (P)

The Proportional Band, P, setting on a process controller makes a change to theoutput (Heater Power on PCT52) that is proportional to the current error value (thedifference between the measured temperature T1 and the set point on the controller).The proportional response can be adjusted by multiplying the error by a constant Kp,called the Proportional Gain. This is related to the Proportional Band setting on the

controller as follows:

30



Exercise C

Proportional Gain (Kp) = 100% / Proportional Band %

i.e. 100% P term means unity gain (change in controller output = error at input)

and 50% P term means a gain of 2 (change in controller output = 2x error at input)

A low setting of the P term (large gain) results in a large change in the output for agiven change in the error. If the P term is too low, the system can become unstable.In contrast, a large setting of the P term (low gain) results in a small output responseto a large input error, and a less responsive or less sensitive controller. If the P termis too high, the control action may be too small when responding to systemdisturbances resulting in slow response and offsets of the resulting process variablefrom the set point.

A Proportional-only controller will not always settle at the set point, but may retain asteady-state offset. Offset can be reduced in Proportional-only control by reducingthe P term setting. However, if the P-term is set too small then hunting or oscillatingwill occur. The offset can be minimised by adding a bias to the set point (setting the

set point above or below the required value to compensate for the offset) but thistechnique is only appropriate if the system characteristics are known and fixed. Abetter solution is to remove the offset by adding Integral action to the controller (P+I)as described below.

Integral term (I)

The contribution from the integral term is proportional to magnitude and duration ofthe error. The Integral term in a PID controller is the sum of the instantaneous errorover time and gives the accumulated offset that should have been correctedpreviously. The resulting controller output is the sum of the contribution from theIntegral term and the contribution from the P term.

When the I term is correctly adjusted any residual offset in the process variable dueto the P term will be gradually reduced by the Integral term until the offset iseliminated. If the time setting of the I term is too long then correction to any offset willbe very slow.

However, since the integral term responds to accumulated errors from the past, it cancause the present value to overshoot the setpoint value or to make the processcompletely unstable if the time setting of the I term is too short. If this occurs the Iterm makes adjustments to the controller output faster than the process can respond.I.e. the I term winds up the controller output so that the process overshootsconsiderably, hence the term Integral Wind-up or Integral saturation.

Careful selection of the I term in combination with the P term will give efficientresponse to changes in the system.

Derivative term (D)

The derivative term slows the rate of change of the controller output. Derivativecontrol is used to reduce the magnitude of the overshoot produced by the I term andimprove the combined controller-process stability. However, the derivative term slowsthe response of the controller. Also, differentiation of a signal amplifies noise andthus this term in the controller is highly sensitive to noise in the error term, and cancause a process to become unstable if the noise and the derivative gain aresufficiently large.

31




Equipment set up














Procedure - P only controller

Choose the PID box on the mimic diagram, set the Proportional Band P to 100%, theIntegral Time I to 0 and the Derivation time D to 0 then set the Set Point to 35°C(default at start up). Click Apply to enter the changes to the settings.


32



Exercise C


The heater will switch on and temperature T1 will gradually rise. T1 will continue torise until it reaches a steady temperature but this will not correspond with the setpointof 35°C on the controller because of the offset inherent in a P only controller.

Set the Fan Control to100 % to disturb the process. Observe that T1 falls slightly, theHeater Power then rises slightly due to the increased error but a large offset remains.When T1 has settled set the Fan Control to 0% and allow T1 to settle.

Adjust the set point to 50°C then click apply and observe the response to arequested increase in heater temperature. When T1 has settled, return the set pointto 35°C and observe the response.

Adjust the P term to 50% in the PID controller then click Apply. Observe that theHeater Power varies and the offset reduces slightly. Increase and decrease themotor speed as before to disturb the process and observe the response, the stability

of T1 and any reduction in offset.

Adjust the P term to 20% and repeat the above steps.


Adjust the P term to 5% and repeat the above steps



33




Results – P only cont roller

Choose the icon (or View \ Graph) to display a graph of the responses obtained.

Choose Format \ Graph Data and Plot Heater Temperature T1 and Set point on 1axis, and Controller Output on the second axis.

Observe the changes in T1 and the stability of the responses for each change in Psetting with a disturbance or change in set point.

Choose a setting for P that gives stable control with a reasonably small offset thatcan be used in the next part of the exercise introducing Integral action.

Procedure - P+I Control ler

Create a new results sheet for this run by selecting the icon in the tool bar of thesoftware.

Set the Fan Control to 100%.

Choose the PID box on the mimic diagram, set the Proportional Band P to the valueselected above, the Integral Time I to 0 seconds and the Derivation time D to 0 thenset the Set Point to 35°C. Click Apply.


Choose the Automatic mode of operation and allow the T1 to stabilise.

Adjust the I term to 100 seconds in the PID controller then click Apply. Observe thatthe Heater Power varies slowly to reduce the offset. When T1 is settled at the set

point increase the Fan Control to 100% to disturb the process. Observe that T1 fallsslightly, the Heater Power varies and T1 gradually returns to the set point. When T1has settled reduce the Fan Control to 0% and allow T1 to settle.

Adjust the set point to 50°C then click Apply and observe the response to arequested increase in T1. When the T1 has settled return the set point to 35°C andobserve the response.

Adjust the I term to 50 seconds in the PID controller then click Apply. Observe thattemperature T1 returns to the set point faster than before. When T1 is settled at theset point vary the Fan Control as before to disturb the process.

Adjust the set point to 80°C then click Apply and observe the response to arequested increase in T1. When the T1 has settled return the set point to 35°C andobserve the response.

Continue reducing the Integral time setting and repeating the disturbance and setpoint changes until temperature T1 becomes unstable.


Results – P+I control ler


Choose Format \ Graph Data and Plot Heater Temperature T1 and Set point on 1axis, and Controller output on the second axis.

34



Exercise C

35

Observe the changes in heater temperature and the stability of the responses foreach change in I setting with a disturbance or change in set point.

Choose a setting for I that gives stable control and reduces any offset swiftly withoutcausing the system to become unstable.

Since the effect of Integral Action is related to the Proportional Band setting (reducedP term gives greater effect for any setting of I) the above exercise should berepeated at a completely different value of P term to determine the necessary changein I for effective control of the temperature and to demonstrate the relationshipbetween P and I.

Conclusion

A large Proportional Band setting will result in very stable control but will result inslow changes and large offsets in the process variable.

Reducing the Proportional Band will improve the speed of response and give smaller

offsets but the process may become unstable if the proportional band in reduced toofar.

Optimum results are obtained with a suitable combination of Proportional Band anIntegral action to eliminate any offset from the setpoint.

Reducing the Integral Time setting increases the speed at which any offset isreduced but settings that are too short will result in instability because the process isunable to respond quickly enough to the changes from the controller. The effect iscalled integral saturation of integral wind up and must be avoided.

Increasing the Proportional Band reduces the effect of Integral Action so shorter

setting of I is required to eliminate offset swiftly.



Exercise D - Optimising Proportional Control of HeaterTemperature (Closed loop, P+I+D)

Objective

To control the heater temperature in the process duct using a proportional PIDcontroller to automatically vary the Heater Power (Direct heating).

To determine the optimum settings for the P, I and D terms appropriate to thetemperature process and to test the stability of the system by applying step changesand changing the set point.

Method

Using a PC to operate the process, the Heater Power (heat flow into the processduct) will be varied automatically by a proportional controller in an attempt to maintaina steady heater temperature (T1) in the process duct.

Appropriate settings for the Proportional Band setting (P), the Integral Time (I) andthe Derivative Time (D) will be determined by applying a step change to the processand observing the response of the system (the Reaction Curve Method).

The parameters determined using the Reaction Curve Method can be refined bytesting the response of the system with these parameters set.

Disturbances can be applied to the process by increasing and decreasing the fanmotor speed. This changes the airflow through the process duct resulting in a changein the heater temperature inside the duct. The magnitude of the disturbance isdetermined by the size of the variation in fan motor speed.

Note: This exercise assumes a basic knowledge of the equipment and the softwarefrom experience gained performing previous exercises. Refer to previous exercises iffurther information is required.

Equipment Required



Theory

Reaction Curve Method

The controller output is operated manually so that a step change is applied to theprocess without corrections being applied by the controller.

The start of the step change is determined from the graph of controller output, thesize of the step change is recorded as M % (change in controller output) and theshape of the Temperature curve is recorded.

A typical response curve is shown below:

36



Exercise D

Typical response with the Reaction Curve method

Draw a straight line through the point of maximum slope so that the line intersects thetime axis.

Measure the dead time L in minutes. (Time at which step change is applied to timewhere straight line intersects time axis.)

Calculate the maximum slope R.

Determine R1 using the equation

The optimum settings for the controller may be calculated as follows:

Ultimate Period Method (Ziegler-Nichols)

This alternative technique utilises the response of the system with the controllerconfigured for Proportional control only. It cannot be applied to relatively stableprocesses because the system must have the ability to oscillate continuously.

The Proportional Band is gradually reduced in steps and at each step a step changeis applied to test the stability of the system. This is repeated until the process variablecontinually oscillates. The correct point is achieved when the amplitude of theoscillations remains constant. If the oscillations gradually die away then the P termshould be decreased slightly. Conversely if the oscillations gradually increase thenthe P term should be increased until the amplitude remains constant.

Note the setting of the P term, PBc at which continual cycling occurs.

From the graph obtained measure the period of the oscillation, Pc, in minutes.

37




Typical response with the Ultimate Period method


Equipment set up







Ensure that the Fan Control is set to 0% by typing in the value or by clicking thedown arrow (in Controls). Note that fan will operate at a minimum preset speed when

set to 0%.






38



Exercise D


Procedure


Set the PID controller for Manual Mode of operation.


Adjust the output from the controller to give a steady heater temperature (T1)typically 50°C by varying the Heater Power.


Leave the controller in manual operation and apply a step disturbance to the process

by increasing the Fan Control to 100%. Note the magnitude of the step changeapplied (delta = M%).

The step change will result in a new T1 in the process duct. The open loop responsemay be analysed from the graph to determine the optimum settings for P, I and Dusing the equations listed in the theory above.

Having calculated the starting values apply these to the controller then click Apply.

Test the stability of the process by applying disturbances using the varying fan motorspeed or by changing the set point. (Refer to previous exercises if necessary forinformation about this).


Set the controller for Proportional control only, initially with a P term of 100%. Set thecontroller for Automatic Mode of operation.


Allow the process variable to settle then apply a step change to the process byincreasing or decreasing the motor speed.

If the process variable settles to a steady new value, decrease the P term and re-apply a step change (by increasing or decreasing the motor).

Continue adjusting the P term and applying a step change until the process variablecontinually oscillates.

Note the setting of the P term, PBc, at which continual cycling occurs.


The closed loop response may be analysed from the graph to determine the optimumsettings for P, I and D using the equations listed in the theory above.


39




40

Test the stability of the process by applying disturbances using the motor speed or bychanging the set point. (Refer to previous exercises if necessary for informationabout this).

Results

Retain graphs for each test showing the response of the system using Open Loopand Closed Loop tuning techniques.

Conclusion

Starting values for P, I and D terms on a process controller can be determined bytesting the response of the system using Open Loop or Closed Loop tuningtechniques.

The values obtained using these techniques are only a starting point and minoradjustments may be necessary to obtain optimum control of a process.



Exercise E - Manual Control of Air Temperature (Openloop)

Objective

To control the temperature of air in the process duct by manually varying the HeaterPower supplying heat to the process duct (Indirect heating).

To determine the characteristics of the temperature process with indirect heating.

Method

Using a PC to operate the process, the heater (heat input into the process duct) willbe varied by the operator in an attempt to maintain a steady air temperature in theprocess duct.

Disturbances will be applied to the process by changing the fan motor speed. Thischanges the heat transferred from the heater to the air resulting in a change in the air

temperature inside the duct. The magnitude of the disturbance is determined by thespeed of the fan motor.

Temperature sensors measure the heater and air temperature (T1 and T2,respectively) in the process duct and indicate the values on the PC.

Equipment Required



Theory

For the air temperature (T2) to remain constant the power supplied to the heatermust match the power increase required by the air The temperature of the airdepends on the power supplied to the heater and the flow of air over the heater (notcontrolled). The power supplied to the heater must be increased to raise thetemperature of the air and decreased to lower the temperature of the air.

Equipment set up







41










Reset the Heater Power to 0% and the Fan Control to 0% on the mimic diagram and

allow the duct to cool with the fan operating at minimum speed. The equipment isready for use.

Procedure

Gradually adjust the Heater Power until the air temperature T2 is 5°C above the initialambient air temperature. Observe that the rise in air temperature T2 is much smallerand much slower than the rise in temperature T1 on the heater.

When temperature T2 is constant choose the icon to begin data logging and notethe temperature reading T2 in °C.

Increase the fan motor speed to 50% on the mimic diagram. Observe that the airtemperature falls slightly due to the additional air flow and resulting heat loss from theheater. When T2 has settled at a new value, increase the Heater Power as requiredto restore the original air temperature.

When temperature T2 is constant increase the fan motor speed to 100% causinganother fall in heater temperature. When T2 has settled at a new value, increase theHeater Power to restore the original air temperature.

When temperature T2 is constant reduce the fan motor speed to 0% causing the airtemperature to rise. When T2 has settled at a new value, reduce the Heater Power torestore the original air temperature.

Set the Heater Power to 0% and allow the heater to cool.


Choose the icon (or View \ Graph) to display a graph of the response obtained.Observe the rising and falling temperate of the heater as the fan motor is increasedand decreased and action is taken to restore the temperature.

Return to the mimic diagram.

Create a new results sheet by selecting the icon in the tool bar of the software

then choose the icon to begin data logging.

42



Exercise E

43

Set the Fan Control to 0% (minimum speed).

Set the Heater Power to 25% and observe the rise in temperature T2. When T2 issteady set the Heater Power to 0% and allow the heater and air to cool.

When temperature T2 is steady at ambient temperature set the Heater Power to 75%

and observe the rise in temperature T2. When T2 is steady set the Heater Power to0% and allow the heater and air to cool.

Set the Fan Control to 100% then repeat as above with the Heater Power set to 25%then 75%.


View the graph obtained and observe the different rates of heating at differentsettings of Heater Power. Also note that the rate of cooling depends on the fan motorspeed. These characteristics will affect the control of the process.

Carefully rotate temperature sensor T2 so that the protective shroud is horizontal (toreduce the speed of response by preventing direct air flow over the sensor).

Choose the icon to begin data logging.

Repeat the above changes to determine the effect of the slow response oftemperature sensor T2.


Return the shield to the vertical position for normal use.

View the graph obtained and observe the effect of the slow response of temperaturesensor T2. This characteristic will affect the control of the process.

Results

Save a copy of the results obtained so that the data can be viewed in graphical ortabular format at a later date.

Conclusion

The air temperature in a duct will only remain constant when the heat into the ductmatches the heat increase in the air.

Excess heat entering the duct will cause the air temperature to rise. Insufficient heatentering the duct will cause the air temperature to fall.

When air temperature is stable in a duct any disturbance such as additional airflowwill result in a change in temperature, requiring variation of the heater power torestore the temperature.

The speed of response of a sensor will affect the ability to control a process.



Exercise F - On/Off Control of Air Temperature (Closedloop)

Objective

To control the temperature of air in the process duct using an On/Off controller toautomatically start and stop the heater as required to maintain the required airtemperature (T2) in the process duct (indirect heating).

To determine the variations in air temperature (T2) due to the dead band inherent inan On/Off controller.

To change the air temperature inside the process duct by changing the set point onthe On/Off controller and to determine the effect of disturbances to the process.

Method

Using a PC to operate the process, the heater (heat into the process duct) will be

switched on and off by the controller in an attempt to maintain a steady airtemperature in the process duct.

Disturbances can be applied to the process by increasing and decreasing the air flowby changing the speed of the fan motor. Increasing the air flow reduces the airtemperature because more air has to be heated.

Temperature sensors measure the heater and air temperature (T1 and T2,respectively) in the process duct and indicate the values on the PC.

Equipment Required



Theory

An On/Off controller is a simple and effective way of controlling many processes butdoes have disadvantages because its output can only be on or off. In the case of theTemperature process in this exercise the heater is started and stopped.

An on/off controller incorporates a dead band to avoid rapid switching of thecontrolled variable when at the setpoint i.e. in this case the temperature of the airmust rise above the setpoint by a fixed amount before the heater switches off and thetemperature must fall below the setpoint by a fixed amount before the heater startsagain.

Note: On PCT52 an On/Off controller with fixed dead band is created by setting theProportional Band to 0 % in the PID controller. In a commercial On/Off controller thedead band can be varied to suit the process. This allows the choice of less frequentswitching but larger variations in the process variable or closer control of the processvariable but more frequent switching with attendant wear etc. of the components.

Equipment set up

Ensure that the apparatus has been set up according to the Installation section andthe power supply connected to the socket marked 24V IN at the rear of the electrical

enclosure.

44
















Procedure

Choose the PID box on the mimic diagram, set the Proportional Band, Integral Timeand Derivation time to 0 then set the Set Point to 5°C above the initial ambient airtemperature. Click Apply to enter the changes to the settings.


45





The heater will switch on, the heater temperature (T1) will quickly rise followed by theair temperature (T2) more gradually. The air temperature T2 will continue to rise untilit reaches the temperature set on the controller. When T2 rises above the setpoint by

a fixed amount (the dead band) the heater will switch off and T2 will start to fall.When T2 falls below the setpoint by a fixed amount (the dead band) the heater willswitch on again and the cycle will continue.

In the PID controller adjust the set point to 10°C above the ambient temperature,click Apply then observe the heater remains on until T2 rises to the new set pointthen switches on and off as before to maintain the new temperature.

Adjust the set point to 20°C above the ambient temperature and observe theresponse of the process.

Effect of Disturbances

Adjust the set point to 10°C above the ambient temperature.

Set the Fan Control to 100%. Observe that temperature T2 falls faster due to theincrease in airflow then the controller switches on the heater to restore the originaltemperature T2.

Set the Fan Control to 50% and observe how the process returns to the original setpoint.

Set the temperature set point to 5°C above ambient and observe how the processreturns to the new set point.


46




47

The heater will switch on, the air temperature (T2) will gradually rise. Thetemperature will continue to rise until it reaches the temperature set on the controller.When temperature T2 rises above the setpoint by a fixed amount (the dead band) theheater will switch off and T2 will fall. When T2 falls below the setpoint by a fixedamount (the dead band) the heater will switch on again and the cycle will continue.

Results

Choose the icon (or View \ Graph) to display a graph of the responses obtained.Observe the rising and falling air temperature about each of the set points as theheater starts and stops to maintain the required temperature. Observe the change inthe rate of heating when the fan motor speed is increased and decreased disturb theprocess.

From the graph or table of results ( icon or View\Table) determine the variation intemperature about the set point to determine the dead band of the controller. In acommercial controller the dead band would usually be adjustable to suit the processbut too small a variation would lead to wear of components switching rapidly on andoff.

From the graph observe that the rate of heating when the heater is on is notnecessarily the same as the rate of cooling when the heater is stopped.

Conclusion

Where a low cost control solution is required, where small variations in processvariable are acceptable and where the response of the process is slow enough thenan On\Off controller can be used to control the process. The temperature process onPCT52 is such a process. Note: a faster process such as flow control (demonstratedusing PCT51) is not suitable for on\off control.

The device controlled by an On\Off controller (in this case a heater) is simply on oroff as required to maintain the process variable (air temperature).

The air temperature will alternate above and below the setpoint due to the deadbandin the On/Off controller.

The maximum set point achievable will be limited by the characteristics of theprocess, namely heater power and airflow.



Exercise G - Proportional Control of Air Temperature(Closed loop, P only and P + I)

Objective

To control the temperature of air in the process duct using a proportional controller toautomatically vary the Heater Power (Indirect heating).

To determine the response of a temperature process when using a P only controllerto vary Heater Power.

To determine the response of a temperature process when using a P+I controller tovary the Heater Power.

To change the air temperature inside the process duct by changing the set point onthe P or P+I controller and to determine the effect of disturbances to the process.

Method

Using a PC to operate the process, the Heater Power (heat flow into the processduct) will be varied automatically by a proportional controller in an attempt to maintaina steady air temperature in the process duct.

The effect of the Proportional Band setting (P) can be demonstrated by configuring aP only controller and observing the response of the process to different settings of P.

The contribution of the of the Integral Time setting (I) can be demonstrated byconfiguring a P+I controller and observing the response of the process to differentsettings of I with P fixed then different combinations of P+I.

Disturbances can be applied to the process by increasing and decreasing the fanmotor speed. This changes the airflow through the process duct resulting in a changein the air temperature inside the duct. The magnitude of the disturbance isdetermined by the size of the variation in fan motor speed.

Temperature sensors measure the heater and air temperature (T1 and T2,respectively) in the process duct and indicate the value on the PC.

Equipment Required



Theory

Proportional term (P)

The Proportional Band, P, setting on a process controller makes a change to theoutput (Heater Power on PCT52) that is proportional to the current error value (thedifference between the measured air temperature and the set point on the controller).The proportional response can be adjusted by multiplying the error by a constant Kp,called the Proportional Gain. This is related to the Proportional Band setting on thecontroller as follows:

Proportional Gain (Kp) = 100% / Proportional Band %

i.e. 100% P term means unity gain (change in controller output = error at input)

48



Exercise G

and 50% P term means a gain of 2 (change in controller output = 2x error at input)

A low setting of the P term (large gain) results in a large change in the output for agiven change in the error. If the P term is too low, the system can become unstable.In contrast, a large setting of the P term (low gain) results in a small output responseto a large input error, and a less responsive or less sensitive controller. If the P termis too high, the control action may be too small when responding to systemdisturbances resulting in slow response and offsets of the resulting process variablefrom the set point.

A Proportional-only controller will not always settle at the set point, but may retain asteady-state offset. Offset can be reduced in Proportional-only control by reducingthe P term setting. However, if the P-term is set too small then hunting or oscillatingwill occur. The offset can be minimised by adding a bias to the set point (setting theset point above or below the required value to compensate for the offset) but thistechnique is only appropriate if the system characteristics are known and fixed. Abetter solution is to remove the offset by adding Integral action to the controller (P+I)as described below.

Integral term (I)

The contribution from the integral term is proportional to magnitude and duration ofthe error. The Integral term in a PID controller is the sum of the instantaneous errorover time and gives the accumulated offset that should have been correctedpreviously. The resulting controller output is the sum of the contribution from theIntegral term and the contribution from the P term.

When the I term is correctly adjusted any residual offset in the process variable dueto the P term will be gradually reduced by the Integral term until the offset iseliminated. If the time setting of the I term is too long then correction to any offset will

be very slow.

However, since the integral term responds to accumulated errors from the past, it cancause the present value to overshoot the setpoint value or to make the processcompletely unstable if the time setting of the I term is too short. If this occurs the Iterm makes adjustments to the controller output faster than the process can respond.I.e. the I term winds up the controller output so that the process overshootsconsiderably, hence the term Integral Wind-up or Integral saturation.

Careful selection of the I term in combination with the P term will give efficientresponse to changes in the system.

Derivative term (D)The derivative term slows the rate of change of the controller output. Derivativecontrol is used to reduce the magnitude of the overshoot produced by the I term andimprove the combined controller-process stability. However, the derivative term slowsthe response of the controller. Also, differentiation of a signal amplifies noise andthus this term in the controller is highly sensitive to noise in the error term, and cancause a process to become unstable if the noise and the derivative gain aresufficiently large.

Equipment set up

Ensure that the apparatus has been set up according to the Installation section and

the power supply connected to the socket marked 24V IN at the rear of the electricalenclosure.

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Procedure - P only controller

Choose the PID box on the mimic diagram, set the Proportional Band P to 100 %, theIntegral Time I to 0 and the Derivation time D to 0 then set the Set Point to 5°C abovethe initial ambient temperature. Click Apply to enter the changes to the settings.


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Exercise G


The heater will switch on and the air temperature will gradually rise. T2 will continueto rise until it reaches a steady temperature but this will not correspond with thesetpoint on the controller because of the offset inherent in a P only controller.

Set the Fan Control to100% to disturb the process. Observe that T2 falls slightly, theHeater Power then rises slightly due to the increased error but a large offset remains.When T2 has settled set the Fan Control to 0% and allow T2 to settle.

Adjust the set point to 10°C above ambient then click apply and observe theresponse to a requested increase in air temperature. When T2 has settled, return theset point to 5 °C above ambient temperature and observe the response.

Adjust the P term to 50% in the PID controller then click Apply. Observe that theHeater Power varies and the offset reduces slightly. Increase and decrease themotor speed as before to disturb the process and observe the response, the stability

of T2 and any reduction in offset.



Adjust the P term to 5% and repeat the above steps



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Results – P only cont roller


Choose Format \ Graph Data and Plot Air Temperature T2 and Set point on 1 axis,and Controller Output on the second axis.

Observe the changes in T2 and the stability of the responses for each change in Psetting with a disturbance or change in set point.

Choose a setting for P that gives stable control with a reasonably small offset thatcan be used in the next part of the exercise introducing Integral action.

Procedure - P+I Control ler

Create a new results sheet for this run by selecting the icon in the tool bar of thesoftware.

Choose the PID box on the mimic diagram, set the Proportional Band P to the valueselected above, the Integral Time I to 0 seconds and the Derivation time D to 0 thenset the Set Point to 5°C above ambient temperature. Click Apply.


Choose the Automatic mode of operation and allow the T2 to stabilise.

Adjust the I term to 100 seconds in the PID controller then click Apply. Observe thatthe Heater Power varies to reduce the offset in air temperature. When T2 is settledat the set point set the Fan Control to 100% disturb the process. Observe that T2falls slightly, the Heater Power varies and T2 gradually returns to the set point. When

T2 has settled set the Fan Control to 0% and allow T2 to settle.

Adjust the set point to 10°C above ambient then click Apply and observe theresponse to a requested increase in T2. When the T2 has settled return the set pointto 5°C above ambient and observe the response.

Adjust the I term to 50 seconds in the PID controller then click Apply. Observe thatthe T2 returns to the set point faster than before. When T2 is settled at the set pointvary the motor speed as before to disturb the process.

Continue reducing the Integral time setting and repeating the disturbance and setpoint changes until temperature T2 becomes unstable.

Results – P+I control ler


Choose Format \ Graph Data and Plot Air Temperature T2 and Set point on 1 axis,and Controller output on the second axis.

Observe the changes in Air temperature (T2) and the stability of the responses foreach change in I setting with a disturbance or change in set point.

Choose a setting for I that gives stable control and reduces any offset swiftly withoutcausing the system to become unstable.

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Exercise G

53

Since the effect of Integral Action is related to the Proportional Band setting (reducedP term gives greater effect for any setting of I) the above exercise should berepeated at a completely different value of P term to determine the necessary changein I for effective control of the temperature and to demonstrate the relationshipbetween P and I.

Conclusion

A large Proportional Band setting will result in very stable control but will result inslow changes and large offsets in the process variable.

Reducing the Proportional Band will improve the speed of response and give smalleroffsets but the process may become unstable if the proportional band in reduced toofar.

Optimum results are obtained with the lowest proportional Band setting that givesstable control but with the addition of Integral action to eliminate any offset.

Reducing the Integral Time setting increases the speed at which any offset isreduced but settings that are too short will result in instability because the process isunable to respond quickly enough to the changes from the controller. The effect iscalled integral saturation of integral wind up and must be avoided.

Increasing the Proportional Band reduces the effect of Integral Action so shortersetting of I is required to eliminate offset swiftly.



Exercise H - Optimising Proportional Control of AirTemperature (Closed loop, P+I+D)

Objective

To control the heater temperature in the process duct using a proportional PIDcontroller to automatically vary the Heater Power (Direct heating).

To determine the optimum settings for the P, I and D terms appropriate to thetemperature process and to test the stability of the system by applying step changesand changing the set point.

To investigate the effect of process lag / sensor response on the optimum PIDsettings

Method

Using a PC to operate the process, the Heater Power (heat flow into the process

duct) will be varied automatically by a proportional controller in an attempt to maintaina steady air temperature (T2) in the process duct.

Appropriate settings for the Proportional Band setting (P), the Integral Time (I) andthe Derivative Time (D) will be determined by applying a step change to the processand observing the response of the system (the Reaction Curve Method).

The parameters determined using the Reaction Curve Method can be refined bytesting the response of the system with these parameters set.

Disturbances can be applied to the process by increasing and decreasing the fanmotor speed. This changes the flow of air through the process duct resulting in a

change in the air temperature inside the duct. The magnitude of the disturbance isdetermined by the size of the variation in motor speed.

Temperature sensors measure the heater and air temperature (T1 and T2,respectively) in the process duct and indicate the value on the PC.

The shield on air temperature sensor T2 can be positioned horizontally to slow downthe response of the sensor, creating a lag and the effect on the optimum PID settingscan be investigated.

Note: This exercise assumes a basic knowledge of the equipment and the softwarefrom experience gained performing previous exercises. Refer to previous exercises if

further information is required.

Equipment Required



Theory


The controller output is operated manually so that a step change is applied to theprocess without corrections being applied by the controller.

54



Exercise H

The start of the step change is determined from the graph of controller output, thesize of the step change is recorded as M% (change in controller output) and theshape of the Temperature curve is recorded.

A typical response curve is shown below:

Typical response with the Reaction Curve method

Draw a straight line through the point of maximum slope so that the line intersects thetime axis.

Measure the dead time L in minutes. (Time at which step change is applied to timewhere straight line intersects time axis.)

Calculate the maximum slope R.

Determine R1 using the equation



This alternative technique utilises the response of the system with the controllerconfigured for Proportional control only. It cannot be applied to relatively stableprocesses because the system must have the ability to oscillate continuously.

The Proportional Band is gradually reduced in steps and at each step a step changeis applied to test the stability of the system. This is repeated until the process variablecontinually oscillates. The correct point is achieved when the amplitude of theoscillations remains constant. If the oscillations gradually die away then the P termshould be decreased slightly. Conversely if the oscillations gradually increase thenthe P term should be increased until the amplitude remains constant.

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Note the setting of the Proportional Band, PBc, at which continual cycling occurs.From the trace obtained on the recorder, measure the period of the oscillation, Pc, inminutes.

Typical response with the Ultimate Period method


Equipment set up







Ensure that the Fan Control is set to 0% by typing in the value or by clicking the uparrow (in Controls). Note that fan will operate at a minimum preset speed when set to0%.




56



Exercise H




Procedure


Set the PID controller for Manual Mode of operation.


Adjust the output from the controller to give a steady air temperature (T2) typically10°C above ambient temperature by varying the Heater Power.


Leave the controller in manual operation and apply a step disturbance to the processby increasing the Fan Control to 100%. Note the magnitude of the step changeapplied (delta = M%).

The step change will result in a new T2 in the process duct. The open loop responsemay be analysed from the graph to determine the optimum settings for P, I and Dusing the equations listed in the theory above.


Test the stability of the process by applying disturbances using the varying fan motorspeed or by changing the set point. (Refer to previous exercises if necessary forinformation about this).


Repeat the Reaction Curve method by applying a step change then test the stabilityof the process as above.

Rotate air temperature sensor T2 so that the shield is positioned vertically with airpassing directly over the sensor.


Set the controller for Proportional control only, initially with a P term of 100%. Set thecontroller for Automatic Mode of operation.


Allow the process variable to settle then apply a step change to the process byincreasing or decreasing the motor speed.

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58

If the process variable settles to a steady new value, decrease the P term and re-apply a step change (by increasing or decreasing the motor).

Continue adjusting the P term and applying a step change until the process variablecontinually oscillates.

Note the setting of the P term, PBc, at which continual cycling occurs.


The closed loop response may be analysed from the graph to determine the optimumsettings for P, I and D using the equations listed in the theory above.


Test the stability of the process by applying disturbances using the motor speed or bychanging the set point. (Refer to previous exercises if necessary for informationabout this).


Repeat the Ultimate Period method by changing the Proportional Band setting andapplying a step change then test the stability of the process as above.

Rotate air temperature sensor T2 so that the shield is positioned vertically with airpassing directly over the sensor.

Results

Retain graphs for each test showing the response of the system using Open Loop

and close Loop tuning techniques.

Conclusion

Starting values for P,I and D terms on a process controller can be determined bytesting the response of the system using Open Loop or Closed Loop tuningtechniques.

The values obtained using these techniques are only a starting point and minoradjustments may be necessary to obtain optimum control of a process.

Speed of sensor response or changes to the system may require different settings ofP, I and D to give optimum control.



Exercise I - Using a PID Control ler (PCT54)

Objective

To control the surface temperature of a heater in the process duct using aCommercial PID controller to automatically vary the power to the heater (Direct

heating, Reverse action).

To control the temperature of air in the process duct using a Commercial PIDcontroller to automatically vary the power to the heater (Indirect heating, Reverseaction).

To demonstrate the operation of a commercial controller and the use of menus toconfigure the instrument.

To configure the controller to indicate process variables in engineering units (defaultsetting 0 – 100% input).

MethodThe PCT54 PID Controller, or equivalent PID controller, can be used to control theprocess by connecting the signals from the controller to the electrical sockets on thefront of the electrical console.

Equipment Required


PCT54 PID Controller or alternative PID controller with appropriate input and outputsignals:

For Direct Control: 0 – 5 Volt input and 0 – 5 volt output

For Indirect Control: 0 – 5 Volt input and 0 – 5 volt output

PC with PCT52 software loaded (only required for logging the response of theprocess).

Theory

Any of the previous exercises performed using a PC to control the process can berepeated using a commercial PID controller, typically PCT54. A PC can be used tolog the response of the process by selecting the appropriate exercise when loadingthe software:

Choose Ex 3: Direct Control (External Control) when controlling the heatertemperature in a PID loop

Choose Ex 4: Indirect Control (External Control) when controlling the airtemperature in a PID loop

Step changes can be applied to the process using the front panel controls on PCT54.

Equipment set up / Procedure

Refer to the instruction manual supplied with PCT54 for details about connections tothe electrical console on the process and operating the PID controller itself.

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Objective

To control the heater temperature in the process duct using a CommercialProgrammable Logic controller to automatically vary the Heater Power to vary the

heat into the process duct (Direct control).

To control the air temperature in the process duct using a CommercialProgrammable Logic controller to automatically vary the Heater Power to vary theheat into the process duct (Indirect control).

To demonstrate the operation of a commercial Programmable Logic Controller withtouch panel interface.

To demonstrate how the configuration of a PLC can be changed using downloadablesoftware.

MethodThe PCT55 Programmable Logic Controller, or equivalent PLC, can be used tocontrol the process by connecting the signals from the controller to the electricalsockets on the front of the electrical console.

Equipment Required


PCT55 Programmable Logic Controller or alternative PLC with appropriate input andoutput signals:

For Direct control: 0 – 5 Volt input and 0 – 5 volt output

For Indirect control: 0 – 5 Volt input and 0 – 5 volt output

PC with PCT52 software loaded (only required for logging the response of theprocess).

Theory

Any of the previous exercises performed using a PC to control the process can berepeated using a commercial Programmable Logic Controller, typically PCT55. A PCcan be used to log the response of the process by selecting the appropriate exercisewhen loading the software:

Choose Ex 3: Direct Control (External Control) when controlling the heatertemperature in a PID loop

Choose Ex 4: Indirect Control (External Control) when controlling the airtemperature in a PID loop

Step changes can be applied to the process using the front panel controls on PCT55.

Equipment set up / Procedure

Refer to the instruction manual supplied with PCT55 for details about connections tothe electrical console on the process and operating the Programmable Logic

Controller itself.

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Contact Details for Further Information

Main Office: Armf ield Limited

Bridge HouseWest StreetRingwoodHampshireEngland BH24 1DY

Tel: +44 (0)1425 478781Fax: +44 (0)1425 470916Email: [email protected]

[email protected]: http://www.armfield.co.uk

US Office: Armf ield Inc.

9 Trenton - Lakewood RoadClarksburg, NJ 08510

Tel/Fax: (609) 208 2800Email: [email protected]

pct52 issue 2 instruction manual

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