OneStopGate.Com
OnestopGate   OnestopGate
   Friday, March 29, 2024 Login  
OnestopGate
Home | Overview | Syllabus | Tutorials | FAQs | Downloads | Recommended Websites | Advertise | Payments | Contact Us | Forum
OneStopGate

GATE Resources
Gate Articles
Gate Books
Gate Colleges 
Gate Downloads 
Gate Faqs
Gate Jobs
Gate News 
Gate Sample Papers
Training Institutes

GATE Overview
Overview
GATE Eligibility
Structure Of GATE
GATE Coaching Centers
Colleges Providing M.Tech/M.E.
GATE Score
GATE Results
PG with Scholarships
Article On GATE
Admission Process For M.Tech/ MCP-PhD
GATE Topper 2012-13
GATE Forum




GATE 2025 Exclusive
Organizing Institute
Important Dates
How to Apply
Discipline Codes
GATE 2025 Exam Structure

GATE 2025 Syllabus
Aerospace Engg..
Agricultural Engg..
Architecture and Planning
Chemical Engg..
Chemistry
Civil Engg..
Computer Science / IT
Electronics & Communication Engg..
Electrical Engg..
Engineering Sciences
Geology and Geophysics
Instrumentation Engineering
Life Sciences
Mathematics
Mechanical Engg..
Metallurgical Engg..
Mining Engg..
Physics
Production & Industrial Engg..
Pharmaceutical Sciences
Textile Engineering and Fibre Science

GATE Study Material
Aerospace Engg..
Agricultural Engg..
Chemical Engg..
Chemistry
Civil Engg..
Computer Science / IT
Electronics & Communication Engg..
Electrical Engg..
Engineering Sciences
Instrumentation Engg..
Life Sciences
Mathematics
Mechanical Engg..
Physics
Pharmaceutical Sciences
Textile Engineering  and Fibre Science

GATE Preparation
GATE Pattern
GATE Tips N Tricks
Compare Evaluation
Sample Papers 
Gate Downloads 
Experts View

CEED 2013
CEED Exams
Eligibility
Application Forms
Important Dates
Contact Address
Examination Centres
CEED Sample Papers

Discuss GATE
GATE Forum
Exam Cities
Contact Details
Bank Details

Miscellaneous
Advertisment
Contact Us


Home » GATE Study Material » Electrical Engineering » Electrical Elements » Strain Gages

Strain Gages

Looking for GATE Preparation Material? Join & Get here now!

** Gate 2013 Question Papers.. ** CEED 2013 Results.. ** Gate 2013 Question Papers With Solutions.. ** GATE 2013 CUT-OFFs.. ** GATE 2013 Results.. **

<<Previous
Strain Gages

  • V1 = Vin*350.03/(350+350.03) = 0.50002142*Vin.

If the supply voltage is 5v, that change becomes:

  • .00002142*Vin = 0.0001071v.

  • That is about one tenth of a millivolt.  If we read that change with a voltmeter that goes to 3 v - which we would need since V1 is around 2.5 volts - we would need a 5-1/2 digit meter just to see the first significant figure in the voltage change.


        Now we can define what the problem really is.

  • If we use a voltage divider, the voltage change is very small and occurs out in the fifth decimal place in a typical example.

  • We need something that will improve this situation.


Using A Bridge Circuit

        A bridge circuit can help with our problem.  Here's a bridge circuit.

We will choose Ra and Rb to have the same value.  That will produce 2.5 volts at the middle of the left branch.

  • Since both Rc and Rs are 350W, the voltage at the mid-point of Rc and Rs is also 2.5 volts.

  • That means Vout = 0 volts when the strain gage is unstrained!

        There are some implications of this result with the bridge circuit.

  • If the voltage is zero when the gage is unstrained - the bridge is balanced - and the voltage becomes 0.0001071v when the gage is strained, then the change is large percentage-wise.

  • That voltage may be small, but we can amplify it - and we won't be amplifying it embedded in a DC voltage.  We'll need a differential amplifier - something like this.  Here the bridge output can be amplified to a usable level - depending upon the gain of the amplifier - and the output can be made to be zero at zero strain.

There are some other alternatives also.

  • The output voltage from the bridge can be amplified by a differential amplifier in a data acquisition board.  You don't necessarily have to build your own amplifier.  Most currently available data acquisition boards have differential amplifiers that will amplify the difference between two input voltages.

  • You could measure the output voltage with a good voltmeter with an ungrounded input.

        An important consideration when using bridge circuits is choice of values for those resistors that have values you can choose.  In the bridge we just considered, only the strain gage resistance was fixed.  That leads to a question.

  • How do you choose resistors in a bridge circuit so that performance is optimized?

Let's imagine that you have a strain gage.  Let's also assume that you have measured the thermistor, and you know the following.

  • Resistance at zero strain = 350W.

  • You want to build a bridge that has a zero output voltage at zero strain.  In other words, you want to build a balanced bridge.

The question is "How to build the bridge?".  We'll work on an answer to that question starting next.  We will assume that the supply voltage is five (5) volts.

  • The strain gage is Rs. That means that Rs is 350W.  There are many ways that we can build a balanced bridge.  Here are a few.

  • Circuit 1:

    • Ra = Rb = 10,000W.

    • Rc = Rs = 350W.

    • Both voltages out of the bridge (Vout,+ and  Vout,-) are half of the supply voltage, so, since they are equal, the output of the bridge is zero volts.

  • Circuit 2:

    • Ra = 1,000W,  Rb = 10,000W.

    • Rc = 3500W,  Rs = 350W.

    • Both voltages out of the bridge (Vout,+ and  Vout,-) are (1/11) of the supply voltage, so, since they are equal, the output of the bridge is zero volts.

  • Circuit 3:

    • Ra = 10,000W,  Rb = 1,000W.

    • Rc = 35W,  Rs = 350W.

    • Both voltages out of the bridge (Vout,+ and  Vout,-) are (10/11) of the supply voltage, so, since they are equal, the output of the bridge is zero volts.

        Let's look at the implications of one choice.  We'll look at Circuit 1.

  • Unstrained, both sides form voltage dividers with 350Wand 10,000W- equal values, on both sides of the divider, so that the output from both is 2.5v with a 5v supply.

  • Now, compute the changed voltage from the sensor side of the bridge when the strain gage is strained.  We will assume that we have the load we discussed above, and that the strain gage sensor's resistance changes to 350.03W.

    • When the strain gage sensor changes to 350.03W, the output on the sensor side of the bridge changes to:

      • Vout,- = 5*350.03/(350+350.03) = 0.50002142*5 = 2.500107.

    • With that voltage, the output of the bridge becomes:

      • 2.5 - 2.500107 = .000107 = 107mv.

What we have demonstrated is that we get a very small voltage change with this choice of resistors for the bridge.  There is always the possibility that a different choice of resistors would produce better results.  Let's check that out.  Let's look at Circuit 2.  Here is what we noted above for Circuit 2.

  • Circuit 2:

    • Ra = 1,000W,  Rb = 10,000W.

    • Rc = 3500W,  Rs = 350W.

    • Both voltages out of the bridge (Vout,+ and  Vout,-) are (1/11) of the supply voltage, so, since they are equal, the output of the bridge is zero volts.

        Now, with no strain the bridge is balance, and with a 5 volt supply, we would have 10/11 of five volts or 0.4545454 volts.  When the load is applied and the sensor resistance changes, the voltage from the sensor side of the bridge is going to be:

  • Vout,- =  5*350.03/(350.03+3500) = 0.4545808734

  • The voltage has changed by 35.4mv, so that is the output voltage from the bridge.

  • The output voltage here is much smaller than the output voltage from Circuit 1.


Problem

        Compute the output voltage from the loaded bridge for Circuit 3.



        What can we conclude from this?

  • When we chose resistors that placed Vs and Vd near the "rails" - i.e. near zero/ground or near the power supply voltage - the voltage didn't change very much when the strain gage was strained.  Maybe we should have expected that!

  • The best sensitivity - in terms of voltage change for the same resistance change - seemed to come when all the resistors were equal when unstrained.

What might we think about now?

  • Investigating the sensitivity mathematically is one thing we should do.  That's another topic, and there is a section in the lesson on bridge circuits that covers senstivity.

  • For now, you have enough information to do some interesting things in the lab, and you have some idea of how to choose resistors when you use the bridge circuit with a resistive sensor.

<<Previous



Discussion Center

Discuss/
Query

Papers/
Syllabus

Feedback/
Suggestion

Yahoo
Groups

Sirfdosti
Groups

Contact
Us

MEMBERS LOGIN
  
Email ID:
Password:

  Forgot Password?
 New User? Register!

INTERVIEW EBOOK
Get 9,000+ Interview Questions & Answers in an eBook. Interview Question & Answer Guide
  • 9,000+ Interview Questions
  • All Questions Answered
  • 5 FREE Bonuses
  • Free Upgrades
GATE RESOURCES
 
  • Gate Books
  • Training Institutes
  • Gate FAQs
  • GATE BOOKS
     
  • Mechanical Engineeering Books
  • Robotics Automations Engineering Books
  • Civil Engineering Books
  • Chemical Engineering Books
  • Environmental Engineering Books
  • Electrical Engineering Books
  • Electronics Engineering Books
  • Information Technology Books
  • Software Engineering Books
  • GATE Preparation Books
  • Exciting Offers



    GATE Exam, Gate 2009, Gate Papers, Gate Preparation & Related Pages


    GATE Overview | GATE Eligibility | Structure Of GATE | GATE Training Institutes | Colleges Providing M.Tech/M.E. | GATE Score | GATE Results | PG with Scholarships | Article On GATE | GATE Forum | GATE 2009 Exclusive | GATE 2009 Syllabus | GATE Organizing Institute | Important Dates for GATE Exam | How to Apply for GATE | Discipline / Branch Codes | GATE Syllabus for Aerospace Engineering | GATE Syllabus for Agricultural Engineering | GATE Syllabus for Architecture and Planning | GATE Syllabus for Chemical Engineering | GATE Syllabus for Chemistry | GATE Syllabus for Civil Engineering | GATE Syllabus for Computer Science / IT | GATE Syllabus for Electronics and Communication Engineering | GATE Syllabus for Engineering Sciences | GATE Syllabus for Geology and Geophysics | GATE Syllabus for Instrumentation Engineering | GATE Syllabus for Life Sciences | GATE Syllabus for Mathematics | GATE Syllabus for Mechanical Engineering | GATE Syllabus for Metallurgical Engineering | GATE Syllabus for Mining Engineering | GATE Syllabus for Physics | GATE Syllabus for Production and Industrial Engineering | GATE Syllabus for Pharmaceutical Sciences | GATE Syllabus for Textile Engineering and Fibre Science | GATE Preparation | GATE Pattern | GATE Tips & Tricks | GATE Compare Evaluation | GATE Sample Papers | GATE Downloads | Experts View on GATE | CEED 2009 | CEED 2009 Exam | Eligibility for CEED Exam | Application forms of CEED Exam | Important Dates of CEED Exam | Contact Address for CEED Exam | CEED Examination Centres | CEED Sample Papers | Discuss GATE | GATE Forum of OneStopGATE.com | GATE Exam Cities | Contact Details for GATE | Bank Details for GATE | GATE Miscellaneous Info | GATE FAQs | Advertisement on GATE | Contact Us on OneStopGATE |
    Copyright © 2024. One Stop Gate.com. All rights reserved Testimonials |Link To Us |Sitemap |Privacy Policy | Terms and Conditions|About Us
    Our Portals : Academic Tutorials | Best eBooksworld | Beyond Stats | City Details | Interview Questions | India Job Forum | Excellent Mobiles | Free Bangalore | Give Me The Code | Gog Logo | Free Classifieds | Jobs Assist | Interview Questions | One Stop FAQs | One Stop GATE | One Stop GRE | One Stop IAS | One Stop MBA | One Stop SAP | One Stop Testing | Web Hosting | Quick Site Kit | Sirf Dosti | Source Codes World | Tasty Food | Tech Archive | Software Testing Interview Questions | Free Online Exams | The Galz | Top Masala | Vyom | Vyom eBooks | Vyom International | Vyom Links | Vyoms | Vyom World
    C Interview Questions | C++ Interview Questions | Send Free SMS | Placement Papers | SMS Jokes | Cool Forwards | Romantic Shayari