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In this lab, you will learn about strain gages and the Wheatstone bridge circuit. You will see how they can be used for strain and force measurement. You will modify an existing program to measure the dynamic characteristics of a second-order system.

Introduction

In this lab, you will learn about strain gages and the Wheatstone bridge circuit. You will see how they can beused for strain and force measurement. You will modify an existing program to measure the dynamic characteristics of a second-ordersystem.

Teaching objectives

  • Gain practical experience with resistance strain-measurement techniques.
  • Learn about the Wheatstone bridge and how it is used in strain measurement.
  • Use a beam instrumented with strain gages as a force measurement device.
  • Use strain gages to measure the natural frequency and damping in a beam.
  • Design a force transducer for measuring thrust from a model rocket motor.

Preparatory reading:

Figliola and Beasley

Strain Measurement: pp. 425–446

Procedure

Part 1: strain gages and the wheatstone bridge

The metal foil strain gages used in this lab are resistors with a nominal (unstrained) resistance of 120 ohms. Asthey are put in tension, their resistance increases; as they are compressed, their resistance decreases. The Wheatstone bridgeprovides a way to convert these changes in resistance to changes in voltage, which are easy to work with. These voltages can beconditioned, transmitted, or stored digitally.

Wheatstone Bridge Circuit

Figure 1 shows a Wheatstone bridge configuration.

  • Four resistors are connected in an end-to-end fashion.
  • The input or excitation voltage is connected to the bridge between top and bottom nodes of the circuit.
  • The output is the difference between the voltage at the left node and the voltage at the right node.
  • An excitation voltage is required to convert the change in resistance (in the legs of the bridge) to a change in voltage atthe output of the bridge.

For the bridge shown, the output voltage is expressed as

equation (1)

When building a Wheatstone bridge with strain gages, all four resistors have the same nominal value. Bridges canbe built in the following configurations:

  • Quarter Bridge-One strain gage and three fixed resistors
  • Half Bridge- Two strain gages and two fixed resistors
  • Full Bridge- Four strain gages
Quarter Bridge Configuration

Quarter bridges

Figure 3 illustrates a quarter bridge configuration. The quarter bridge has one active leg, i.e., one legwith a changing resistance. From equation (1) above we can derive an expression for the output voltage as a function of theresistance change∆R:

equation (2)

Half and full bridges

Figure 5 and Figure 6 show half-bridge and full-bridge configurations respectively.

  • Half bridge: two active legs, one in tension and one in compression. These legs are adjacent legs in the bridge.
  • Full bridge: four active legs, two in tension and two in compression. The gages in tension are on opposite legs of thebridge.

Using equation 1 and Figure 1 as a guide, derive expressions for the output voltage of the half-bridge andfull-bridge circuits.

Questions & Answers

what is the stm
Brian Reply
is there industrial application of fullrenes. What is the method to prepare fullrene on large scale.?
Rafiq
industrial application...? mmm I think on the medical side as drug carrier, but you should go deeper on your research, I may be wrong
Damian
How we are making nano material?
LITNING Reply
what is a peer
LITNING Reply
What is meant by 'nano scale'?
LITNING Reply
What is STMs full form?
LITNING
scanning tunneling microscope
Sahil
how nano science is used for hydrophobicity
Santosh
Do u think that Graphene and Fullrene fiber can be used to make Air Plane body structure the lightest and strongest. Rafiq
Rafiq
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Mahi
what is simplest way to understand the applications of nano robots used to detect the cancer affected cell of human body.? How this robot is carried to required site of body cell.? what will be the carrier material and how can be detected that correct delivery of drug is done Rafiq
Rafiq
what is Nano technology ?
Bob Reply
write examples of Nano molecule?
Bob
The nanotechnology is as new science, to scale nanometric
brayan
nanotechnology is the study, desing, synthesis, manipulation and application of materials and functional systems through control of matter at nanoscale
Damian
Is there any normative that regulates the use of silver nanoparticles?
Damian Reply
what king of growth are you checking .?
Renato
What fields keep nano created devices from performing or assimulating ? Magnetic fields ? Are do they assimilate ?
Stoney Reply
why we need to study biomolecules, molecular biology in nanotechnology?
Adin Reply
?
Kyle
yes I'm doing my masters in nanotechnology, we are being studying all these domains as well..
Adin
why?
Adin
what school?
Kyle
biomolecules are e building blocks of every organics and inorganic materials.
Joe
anyone know any internet site where one can find nanotechnology papers?
Damian Reply
research.net
kanaga
sciencedirect big data base
Ernesto
Introduction about quantum dots in nanotechnology
Praveena Reply
what does nano mean?
Anassong Reply
nano basically means 10^(-9). nanometer is a unit to measure length.
Bharti
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Damian Reply
absolutely yes
Daniel
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Anassong
How can I make nanorobot?
Lily
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s. Reply
there is no specific books for beginners but there is book called principle of nanotechnology
NANO
how can I make nanorobot?
Lily
what is fullerene does it is used to make bukky balls
Devang Reply
are you nano engineer ?
s.
fullerene is a bucky ball aka Carbon 60 molecule. It was name by the architect Fuller. He design the geodesic dome. it resembles a soccer ball.
Tarell
what is the actual application of fullerenes nowadays?
Damian
That is a great question Damian. best way to answer that question is to Google it. there are hundreds of applications for buck minister fullerenes, from medical to aerospace. you can also find plenty of research papers that will give you great detail on the potential applications of fullerenes.
Tarell
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Source:  OpenStax, Introduction to mechanical measurements. OpenStax CNX. Oct 18, 2006 Download for free at http://cnx.org/content/col10385/1.1
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