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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

can someone help me with some logarithmic and exponential equations.
Jeffrey Reply
sure. what is your question?
ninjadapaul
20/(×-6^2)
Salomon
okay, so you have 6 raised to the power of 2. what is that part of your answer
ninjadapaul
I don't understand what the A with approx sign and the boxed x mean
ninjadapaul
it think it's written 20/(X-6)^2 so it's 20 divided by X-6 squared
Salomon
I'm not sure why it wrote it the other way
Salomon
I got X =-6
Salomon
ok. so take the square root of both sides, now you have plus or minus the square root of 20= x-6
ninjadapaul
oops. ignore that.
ninjadapaul
so you not have an equal sign anywhere in the original equation?
ninjadapaul
Commplementary angles
Idrissa Reply
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what is a good calculator for all algebra; would a Casio fx 260 work with all algebra equations? please name the cheapest, thanks.
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a perfect square v²+2v+_
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Abdirahman Reply
algebra 2 Inequalities:If equation 2 = 0 it is an open set?
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or infinite solutions?
Kim
The answer is neither. The function, 2 = 0 cannot exist. Hence, the function is undefined.
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Kristine 2*2*2=8
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Differences Between Laspeyres and Paasche Indices
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No. 7x -4y is simplified from 4x + (3y + 3x) -7y
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is it 3×y ?
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J, combine like terms 7x-4y
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Asali
I'm not good at math so would you help me
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what is the problem that i will help you to self with?
Asali
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Need to simplify the expresin. 3/7 (x+y)-1/7 (x-1)=
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. After 3 months on a diet, Lisa had lost 12% of her original weight. She lost 21 pounds. What was Lisa's original weight?
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I start with an easy one. carbon nanotubes woven into a long filament like a string
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Yes, Nanotechnology has a very fast field of applications and their is always something new to do with it...
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AMJAD
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AMJAD
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Stotaw
In this morden time nanotechnology used in many field . 1-Electronics-manufacturad IC ,RAM,MRAM,solar panel etc 2-Helth and Medical-Nanomedicine,Drug Dilivery for cancer treatment etc 3- Atomobile -MEMS, Coating on car etc. and may other field for details you can check at Google
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Prasenjit
after 100 year this will be not nanotechnology maybe this technology name will be change . maybe aftet 100 year . we work on electron lable practically about its properties and behaviour by the different instruments
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Prasenjit Reply
At high concentrations (>0.01 M), the relation between absorptivity coefficient and absorbance is no longer linear. This is due to the electrostatic interactions between the quantum dots in close proximity. If the concentration of the solution is high, another effect that is seen is the scattering of light from the large number of quantum dots. This assumption only works at low concentrations of the analyte. Presence of stray light.
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the Beer law works very well for dilute solutions but fails for very high concentrations. why?
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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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