ETF Wheatstone Bridge - Engineering the Future · Engineering Activity Teacher Guide Wheatstone...

1Engineering the Future

Wheatstone Bridge /Strain GaugeS5/S6

Scottish Charity Number SC004401 Scottish Charity Number SC015263

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www.engineeringthefuture.info

Wheatstone Bridge / Strain GaugeEngineering Activity Teacher Guide


ContentsIntroduction

Outline of engineering activity

Engineering and learning principles

Learning outcomes

Structure and timing

Key resources

Activity 1: Using a purpose built strain

gauge and amplifier

Activity 2: Modelling a bridge

Activity 3: Demonstrating understanding

by presenting findings

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IntroductionThe materials in this pack form one of a series of units which promote the teaching and learning of engineering in the secondary school curriculum.

The module was developed by a school-university partnership involving Holy Cross High School, Hamilton (Brian Smith and Linda Glancy) and the University of Strathclyde Departments of Electronic and Electrical Engineering (Roy Pearson and Tony Gachagan) and Mathematics (Tony Mulholland). The purpose of this activity is to introduce Higher Physics students to a practical engineering application of the Wheatstone bridge circuit.

This school-university partnership was one of a number of collaborative networks of school and university staff in Electrical and Electronic Engineering (EEE) created by the Engineering the Future project. These enabled staff and students from schools and universities to work together to create exciting and innovative programmes for school pupils, supported by world leading engineering research groups. These materials have already been trialled successfully in secondary schools across Scotland.

In the words of pupils involved: ‘It’s more fun, so you want to know more about physics.’ – ‘A lot of work: challenging but you’ve accomplished something, it ‘clicks’ and you remember it.’ – ‘… instead of being told step by step what to do you get to vary it yourself.’ – ‘…it just made you feel really smart once you’d done it.’

Engineering the Future is a 3-year project funded by a major grant from the UK Engineering and Physical Sciences Research Council (EPSRC) which is running from October 2006 to early 2010. It involves staff from the Department of Electronic and Electrical Engineering in the University of Strathclyde and from the Departments of Electronics and Electrical Engineering and of Educational Studies in the University of Glasgow and science teachers in some 20 secondary schools in 9 education authorities in Scotland and in 2 independent schools. The financial support afforded to the project by EPSRC is gratefully acknowledged as are the advice and encouragement provided by EPSRC and by the Universities of Strathclyde and of Glasgow.



Science

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Creativity & Innovation

Leisure

Health

Wealth Creation

Society

Industry

Infrastructure

Engineering

Engineering the Future was planned in response to the general recognition that engineering in this country, in particular electrical/electronic engineering, faces serious challenges. The number of young people taking up university engineering courses is low and becoming lower. Many young people – including ambitious high achievers – have very limited or distorted ideas about what engineering involves. In particular, they do not associate a creative, inventive, problem-solving and entrepreneurial approach to life and work with their science and mathematics work – an approach essential for equipping individuals with the skills necessary to meet the needs of today and the demands of tomorrow.

This is not a matter of academic interest. Engineers use science and mathematics, in conjunction with the tools of technology, to create new systems, infrastructures, devices, products and commodities for the overall benefit of society. There is an urgent economic need to embed and highlight engineering in the school curriculum. Engineering, the application of scientific and mathematical knowledge to practical issues, needs and problems, is fundamental to the creation of new technologies and sustainable industries. Engineering requires and supports the development of high levels of scientific and mathematical competence in the service of useful design, creativity, innovative thinking and problem-solving. It requires and fosters the confidence, drive, determination to succeed, teamwork and business acumen necessary to promote economic enterprise. There is an economic need for a larger number of engineers, high level engineering graduates and very capable technician engineers, with such skills to build a strong indigenous high-tech economic base.

We hope that you find these materials useful.



Teacher GuideOutline of engineering activityThe purpose of this activity is to introduce SQA Higher Physics students to a practical engineering application of the Wheatstone bridge circuit. They will consider how an engineer can model and monitor changes in the strain on a road bridge as part of an ongoing safety assessment.

After learning the underlying physics of the Wheatstone Bridge circuit incorporating a strain gauge and the operational amplifier (in differential mode), the students are asked to investigate the relationship between the out-of-balance potential difference and the applied force on the strain gauge. Each group of students selects one of a number of bridges of different designs and builds a model for testing. They then use their scientific knowledge and findings to measure the strain (using a known load) in the middle of the bridge and at either end. The results are presented to the rest of the class and comparisons made.

Engineering and learning principlesThe activities highlight the application of electrical engineering principles to find solutions to problems in the real world. They incorporate the use by pupils of up-to-date instrumentation. The teacher has opportunities to explain the nature of engineering as the practical application of Physics principles, using existing technology and developing new technology, to address issues and needs. Pupils learn about these engineering applications in practical activities (for example, using the Wheatstone bridge equipment to measure strain on a model bridge) and in discussion in groups and with the teacher.

The characteristics of engineering involved are:

specification of application requirements•

application of scientific knowledge •

teamwork•

report writing.•



Learning outcomes The activities are related to SQA Higher Physics content statements as follows:

Section 2.1 No. 15

“State that for an initially balanced Wheatstone bridge, as the value of one resistor is changed by a small amount, the out-of-balance p.d. is directly proportional to the change in resistance.”

Students will be able to:

Learn how a strain gauge and amplifier can be used together 1. to convert a very small applied force into a measurable electrical signal.

Learn to work in a group to build and model a realistic 2. engineering scenario.

Learn to analyse and present data and reach valid 3. conclusions.

Structure and timingThere are three sets of activities:

Familiarising students with a strain gauge incorporated in 1. the Wheatstone Bridge and amplifier circuit – 1 period.

Constructing a model bridge and measuring the strain on it 2. at various places as a result of an applied load – 1 period.

Presenting their findings to the rest of the class – 1 period.3.

The work can be done in 2-3 hours. It could be extended, e.g., by engaging the pupils in research about strain gauge applications

Key resourcesStrain gauge; amplifier; digital voltmeter; Newton balance • (0-1N)

Power pack; hand-held oscilloscope(s)•

Signal generator, hand-held oscilloscope(s)•

Several K’NEX model bridge kits (1 per group).•



Activity 1Using a purpose built strain gauge and amplifier The teacher needs to ensure that the pupils understand the nature and uses of a strain gauge and an amplifier (the instructions booklets supplied by the equipment makers can be helpful) and can link the activities to previous learning. It is possible to run these activities as demonstrations – for example, the teacher can show how the strain gauge works and how a version of it can be used on a model bridge to simulate monitoring of a real one. The various types of real world application can be explained. The pupils can then undertake the strain gauge experiment individually or in groups.

Resources requiredPhilip Harris strain gauge system•

Clamp (to secure the gauge system)•

Lascelles instrument amplifier•

0-1N force meter•

a small screwdriver or similar instrument to balance the • bridge circuit

graph paper (force against out-of-balance p.d.) •

Time Required1 period

PreparationThe teacher would have to spend some time familiarising themselves with how to balance the bridge circuit and finding a suitable gain on the amplifier.



ProcedureConnect the output of the amplifier to the voltmeter•

Balance the Wheatstone bridge for a straight hacksaw blade • by adjusting the variable resistor.

Pull up the hacksaw blade with a force of 0.1N using the • Newton balance and record results.

Spring Balance

Wheatstone BridgeAmplifier



Activity 2Modelling a bridgeThe pupils should be introduced to different bridge designs (e.g. suspension, cantilever) using PowerPoint or internet resources. The K’NEX bridge kits have useful information which includes photographs and descriptions of well known world bridges such as the Forth Railway Bridge and San Francisco Bay Bridge. Each group of pupils should select a type of bridge and build it (instructions are included in the kits).

The strain gauges are normally glued (using specialised glue) to the structure but this would mean that the strain at only one position could be measured. However, if one of the beam connections on the K’NEX is loosened then it is possible to wedge the strain gauge between the beam and the socket. It is then possible to fit several strain gauges to the bridge at the same time and simply connect each to the Wheatstone Bridge/Amplifier circuit in turn.

If the bridge circuit cannot be completely balanced then an increase in the out-of-balance p.d. can be used. The bridge can then be put under strain by adding model cars or masses.

Resources requiredPurpose built Wheatstone bridge circuits (diagram included)•

Several K’NEX model bridge kits•

Strain gauges•

Lascelles instrument amplifier•

Time Required1 period



PreparationPurpose-built Wheatstone bridge circuits have to be constructed (circuit design is shown on the manufacturer’s instructions).

Connecting leads have to be attached to the strain gauges.

The teacher needs to spend some time prior to this activity finding the best gain and meter setting in order that the Wheatstone bridge circuit can be balanced.

Some practice in placing the strain gauges between the beams and sockets would be helpful.

4.7KΩ

4.3kΩ

Physically largepot for sensitivty

120Ω

120Ω

1kΩ

amplifier



ProcedurePupils need to select a suitable model bridge and build it • (possibly these could have been built before hand).

The pupils should attach the strain gauges or alternatively • use just one strain gauge and move it to various positions.

The strain gauge(s) should be connected to the bridge circuit • and amplifier as shown.

The bridge circuit should be balanced.•

The bridge should now be put under strain and the readings • recorded.

Amplifier Wheatstone bridge circuit

Strain Gauge wedged between joints



Activity 3Demonstrating understanding by presenting findings

PreparationMake sure each group has a note of their findings and access to possible resources for their presentation, e.g. computer, poster paper.

Time required1 period

ProcedureEach group should demonstrate their understanding by 1. preparing a presentation which will last for 3 minutes. The format for the presentation is flexible but it must involve every member of the group and include:

the challenges and successes they met when building the • bridge and testing it, including any adjustments they had to make

their findings, explaining how accurate they think they are•

the scientific knowledge and understanding behind • measuring the strain on the bridge.



Groups should be asked to present findings and then be 2. questioned briefly by another group chosen at random. Some possible points of discussion are:

Which bridge design is strongest? •

Why are there different bridge designs? •

What about the effects of the wind or earthquakes? •

What would be result of changing the length of the bridge? •

Can they modify the bridge design to make it stronger?•

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