overheads for acs111 system modelling 1

ACS111 Systems Modelling 1

Overheads for ACS111 Systems Modelling

The following resources are taken from the 2009/2010 ‘introduction to modelling’ lecture as forming a part of Systems Modelling for first year engineering undergraduate. For supporting and other documentation for this lecture and others on the course please see http://controleducation.group.shef.ac.uk/OER_index.htm.

The main focus is on electrical and mechanical systems, but there is also some discussion of dc motors, fluids and heat as well as an introduction to time series modelling. The main emphasis is on why modelling is important and how to go about doing this from first principles (e.g. Kirchhoff's laws, Newton's Laws, etc.). Given the focus is on new students arriving at University, there is no attempt to develop models beyond second order.

The resources here include the lecture hand out (pdf) which includes embedded tutorial questions, some powerpoints for structuring lectures , flash animations to step through modelling process for electrical circuits and a large data base of CAA developed on WebCT (here provided in a zip file). The lecture notes also contains a brief overview on usage for lecturing staff.

These were developed at the University of Sheffield and authored by J A Rossiter from the Department of Automatic Control and Systems Engineering.

© University of Sheffield 2009. This work is licensed under a Creative Commons Attribution 2.0 License.

http://controleducation.group.shef.ac.uk/OER_index.htm

http://www.shef.ac.uk/

http://www.shef.ac.uk/acse

Acs111 Systems Modelling 2

Systems Modelling

Week 1

Anthony RossiterDepartment of Automatic Control and Systems EngineeringUniversity of Sheffieldwww.shef.ac.uk/acse


Contents• Introduction

• Why is modelling important for an engineer?

• Understanding behaviour

• What is a model?

• Types of system

• System attributes

• Environment

• Attributes

• What are models used for?

• What is modelling?

• UNITS IN MODELLING GET A DATA BOOK

• Dimensional consistency

• Possible case studies

• Summary

• Credits & notices


Student responsibility

• You are adults entirely responsible for your own progress!

• You must work regularly, a minimum of 5hrs private study per week per module.

• Get plenty of sleep – you cannot learn effectively if tired.

• See me asap if genuine problems.


MOLE*

• This module has many resources on MOLE* vista.

• All the courseworks can be accessed and submitted only via MOLE*.

• If you cannot find ACS111 on your MOLE* login page, check your module registration.

*MOLE is University of Sheffield’s Virtual Learning Environment and access is permissible only through the University.


Courseworks

1. Essay – due by end week 2 and 3 10%

2. CAA 1 – due by end week 5 10%

3. CAA 2 – due by end week 12 10%

4. Case study – due by Wed. week 10 10%

The exam counts for 60%.NOTE: DON’T book flights/holidays early. You will score zero for any missing assignments due to absence.


Why is modelling important for an engineer?

Split into groups of 5 and consider this question.

We will collate group answers in 5-10minutes.


Why is modelling important?

• Expensive to build and test prototypes. • Use models to deduce the expected behaviour of a

real system. • Use models in design stage.

Example: Choose the size of struts for building a single lane bridge for cars to span a river width 10m!

Answer on Board!


Understanding behaviour

Models allow us to understand why a system behaves the way it does. One can then propose alterations which would be expected to improve behaviour.

1.Why does a mass spring damper system oscillate ? How can one reduce oscillation ?

2.You need a new computer. It must be capable of running a given piece of software and should not become outdated to quickly. How would you decide which computer to buy ?

3.You need to buy a radiator to fit in a room. Explain how you would decide which radiator to buy.


What is a model?

You have a pogo stick and a ruler.

Find the mass of the lecturer.


Types of system

1. Physical (focus of this module)2. Chemical3. Biological4. Ecological5. Economic6. Computer7. Control8. Etc.Systems involve interconnections of different subsystems.

Broad based knowledge is essential.This module focuses on understanding the smallest parts.


Examples

1. List some systems and identify the type.

2. Consider a simple heating system in your house. What information would you need to model this ?

3. What information would you need to construct a model for filling and emptying a bath tub ?

4. Revisit the examples of the central heating and bath tub and list all the attributes.


System attributes

1. Entities or components

2. Interconnections

3. Inputs (adjustable or uncontrolled)

4. Dependent variables

5. Initial conditions

6. Outputs and states

7. Reasons for existence

8. Control (e.g. adjust 3 so 6 achieves 7)


Environment

1. Everything outside the system which affects the system but is unaffected by the system.

2. Where to separate the system from the environment is not always clear and requires judgement.


Environment examples

• A spacecraft is affected by the gravitational pull of a nearby planet but its pull on the planet, although real, is sufficiently small to be ignored for spacecraft trajectory prediction.

• A short-circuit to earth of a mains cable from an electricity generator may be considered to have negligible effect on the potential of the earth itself.

• Power consumption of a freezer is effected by the environment, the freezer system can not be treated in isolation from the external temperature. Although a freezer does heat up its environment, this effect is usually negligible.


Attributes

List the components, inputs, outputs, states, purpose, connections and environment for the following systems.

1. Kettle

2. Car

3. Robot arm

4. Keyboard


What are models used for?

1. To predict behaviour of real system2. To avoid risk and expense

Models take many forms: Scaled physical models, laboratory models, pilot plant, computer models, mathematical models, etc.

ACS111 focuses on mathematical models.


What is modelling?

1. Fundamental laws of physics apply to ideal entities.

2. Real system usual comprise physical entities but are imperfect.

3. Approx. is essential to decide on numbers of interconnections, entities, etc. that might represent the system.

4. Formulate all the equations describing the ‘ideal’ system and manipulate into a suitable form.


Computer modelling

Increasingly computers are used for modelling and model simulation.

SIMULINK: construct a simple model (as a block diagram) and then parameter design by repeated simulation.

MATLAB to simulate a suspension system and parameter design by repeated simulation.

There is a demand for:

1. Engineers with sufficient modelling skills to code models into computer software and sufficient insight to do design.

2. Computer programmers with sufficient engineering awareness and mathematical skills to develop modelling and simulation software that is easy to use.


Do a critique of the essays and ‘critiques’

You can do this in groups of about 3.

1. What makes a good essay?

2. What makes a good critique?


UNITS IN MODELLING

GET A DATA BOOKIt is essential that you place units against every physical quantity. This is so you can ensure dimensional compatibility. For instance:

1. Which is heavier, an object weighing 3kg, an object weighing 3002grammes or an object weighing 6.4pounds ?

2. Which distance is longest ? 200 inches, 5metres, 50000mm, 5.2 yards or 17 feet

It is conventional to use metric units. You are recommended to always change all units to metric before you start and then no errors will occur.


Units

Stiffness Newtons per metre N/mDamping Newton seconds per metre Ns/mMass Kilogrammes KgForce Newton NEnergy Joules JPower Energy per second (Watt) J/s WResistance OhmsCurrent Ampere AVoltage volts VCharge coulomb CCapacitance farads FInductance Henrys H


Dimensional consistency

When you have an equation with various parts adding together, you must ensure that the dimensions of all the components are identical, otherwise the quantities cannot add.

For instance you cannot add seconds to kilogrammes as that is a nonsense.

1. What are the units in the mass-spring-damper model ?

2. What are the units in the RLC circuit model ?


Possible case studiesHere are some case study items for you

to consider through this course.

• What are the design criteria?• What behaviour do I want?• How do I choose the parameters?

• Lego vehicle• Scalextric• Bicycle pump and foot pump.• Hair dryer• Pogo stick• Zoom lens on a camera.• Electric kettle.• Car suspension (single wheel

model).• Bicycle gearing.• Noise filtering (time series)• Pattern recognition.• Modelling for control.• Water pistol

Any more suggestions ?


Summary

1. By next week you should have completed part of coursework 1.

2. This is to submit a brief essay on why modelling is important.

3. Submissions by discussions board only!


This resource was created by the University of Sheffield and released as an open educational resource through the Open Engineering Resources project of the HE Academy Engineering Subject Centre. The Open Engineering Resources project was funded by HEFCE and part of the JISC/HE Academy UKOER programme.

© University of Sheffield 2009

This work is licensed under a Creative Commons Attribution 2.0 License.

Where Matlab® screenshots are included, they appear courtesy of The MathWorks, Inc.

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overheads for acs111 system modelling 1

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