ni.com Sensor Measurement Fundamentals Series

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1 Sensor Measurement Fundamentals Series

2 Strain Gage Measurements Doug Farrell Product Manager National Instruments

3 Key Takeaways Strain gage fundamentals Bridge-based measurement fundamentals Measurement error reduction Fiber-optic strain measurement

4 Strain Applications Structural Fatigue Monitoring Hydro Turbine Test Oil Pipeline Monitoring

5 Strain Gage Gage backing (nonconducting) Test specimen Force Force Foil conductor Leads

6 Foil Strain Gage Operation Strain gages are variable resistive sensors Resistance changes with strain How much the resistance changes is dependent on the sensors gage factor

7 Voltage Divider Divides the voltage across a linear circuit R 1 + V EX - V IN R 2

8 The Wheatstone Bridge Used to measure resistive devices V IN = 0 if R 1 /R 2 = R 3 /R 4 V IN 0 if change in resistance (strain) Requires voltage excitation R 3 R V IN - V EX - R 4 R 2

9 Quarter-Bridge Strain Gage One gage R 1, R 2, and R 3 are fixed resistors R 3 is called the quarter-bridge completion resistor Specimen Grid R 3 R 1 + V IN - R 2

10 Quarter-Bridge Type I R 3 R 1 + V IN - R 4 R 2

11 Quarter-Bridge Type II R 3 R 1 + V IN - R 4 R 2

12 Half-Bridge Strain Gage Two gages R 1 and R 2 are fixed resistors Specimen Grid R 1 + V IN - R 2

13 Half-Bridge Type I R 3 R 1 + V IN - R 4 R 2

14 Half-Bridge Type II R 3 R 1 + V IN - R 4 R 2

15 Full-Bridge Strain Gage Four gages All resistors in the Wheatstone Bridge are strain gages Generally two sensors on top, two on bottom Specimen V EX + + V IN - V EX -

16 Full-Bridge Type I R 3 R 1 + V IN - R 4 R 2

17 Full-Bridge Type II R 3 R 1 + V IN - R 4 R 2

18 Full-Bridge Type III R 3 R 1 + V IN - R 4 R 2

19 Bridge Configurations Measurement Type Quarter-Bridge Half-Bridge Full-Bridge Type I Type II Type I Type II Type I Type II Type III Axial Strain Yes Yes Yes No No No Yes Bending Strain Yes Yes Yes Yes Yes Yes No Compensation Temperature No Yes Yes Yes Yes Yes Yes Transverse Sensitivity No No Yes No No Yes Yes Sensitivity ß Sensitivity at 1,000 ue ~0.5 mv/v ~0.5 mv/v ~0.65 mv/v ~1.0 mv/v ~2.0 mv/v ~1.3 mv/v ~1.3 mv/v Installation Number of Bonded Gages 1 1* Mounting Location Single Side Single Side Single Side Opposite Sides Opposite Sides Opposite Sides Number of Wires 2 or Bridge Completion Resistors * A second strain gage is placed in close thermal contact with the structure but not bonded. Opposite Sides

20 Rosette Specific layout used in plane strain applications 45 45

21 Mounting Strain Gages Surface Preparation Gage Bonding Lead Wire Attachment Protective Coating

22 Sources of Error High-frequency noise Filtering, excitation level Sensor self-heating Gage type, excitation level, structure Lead wire and bridge arm resistance Remote sensing, shunt calibration Improper calibration Offset nulling, shunt calibration Excitation source stability Compensation, ratiometric architecture

23 Filtering Lowpass Filter Time Domain Time Domain Lowpass Filter Frequency Domain Frequency Domain

24 Excitation Level Signal-to-noise ratio (SNR) Higher excitation delivers better SNR Impacts gage self-heating Higher excitation leads to self-heating o Introduces thermocouple effects o Changes the adhesive s ability to transfer strain o Changes gage resistivity and sensitivity Bridge configuration dependent

25 Remote Sensing NI

26 Null Calibration Removes offsets Ensures that ~0 V are measured when gage is unstrained Compensates for inherent bridge imbalance Can be performed in hardware or software

27 Shunt Calibration + V IN - + _ V EX

28 Null and Shunt Calibration ε Offset Error Uncalibrated Null calibrated Gain Error Shunt and null calibrated σ

29 Hardware Demonstration

30 Software Demonstration

Stability of Excitation Voltage ratio is used in all equations to compute strain (V strained V unstrained ) / V excitation Unstable excitation causes errors in

31 Stability of Excitation Voltage ratio is used in all equations to compute strain (V strained V unstrained ) / V excitation Unstable excitation causes errors in reading Advantages High accuracy and low susceptibility to excitation temperature drift Reduced regulation design requirements allow for increased channel count

32 Fiber Bragg Gratings (FBGs) λ Input Signal λ B λ Fiber Bragg Grating Transmitted Signal Optical Fiber Core Reflected Signal Λ B = Grating Period B = 2n e Λ B λ B

33 Temperature and Strain Strain Change Λ b 2n

34 Analogous Measurement: Strain Gages Resistive (Foil) Gage Force Force Strain: e DL L DR R GF e GF 2 Optical FBG Gage Force Force Strain: e DL L D λ λ K e K 0.8

35 Wavelength-Division Multiplexing λ 1 λ 2 λ 3 Λ 1 Λ 2 Λ 3 Multiplex multiple FBG sensors at unique wavelengths on a single optical fiber λ λ 5 λ 6 λ 4 n-1 λ n λ 3 λ 1 λ 2 Optical Sensor Interrogator (NI PXIe-4844) P Reflected Signal λ 1 λ 2 λ 3 λ 4 λ 5 λ 6 λ n-1 λ n λ

media Multiplexed sensors Dozens of sensors per fiber reduces cabling Allows for long

36 Benefits of FBG Optical Sensing Electrically nonconductive and passive Immune to high voltages, EMI Nonexplosive Environmentally stable Insensitive to corrosive and caustic media Multiplexed sensors Dozens of sensors per fiber reduces cabling Allows for long distance signal transmission (km) Very small and lightweight Reduced weight/drag effect

37 Acquiring the Measurement SC Express High Channel Count High Performance NI CompactDAQ Portable Measurements USB, Ethernet, WiFi Rugged NI CompactRIO Headless Operation

Measuring the Effects of Earthquakes on Bridges

The Solution Measuring strain, force, and

38 Measuring the Effects of Earthquakes on Bridges The Challenge Building a test that best measures how a vehicle s suspension interacts with a bridge during an earthquake. The Solution Measuring strain, force, and displacements to characterize the behavior of the vehicles and bridges.

39 /strain

Strain Gauge Measurement A Tutorial

Application Note 078 Strain Gauge Measurement A Tutorial What is Strain? Strain is the amount of deformation of a body due to an applied force. More specifically, strain (ε) is defined as the fractional