1. Dan O. Popa, Intro to EE, Freshman Practicum, Spring 2015 EE 1106 : Introduction to EE Freshman Practicum Lab-Lecture 7: Strain gage and Wheatstone Bridge

2. Dan O. Popa, Intro to EE, Freshman Practicum, Spring 2015 2 Pressure Sensor

3. Dan O. Popa, Intro to EE, Freshman Practicum, Spring 2015 Circuit for pressure sensor measurements: Wheatstone Bridge Balance Condition If Bridge is balanced at R gi 3

4. Dan O. Popa, Intro to EE, Freshman Practicum, Spring 2015 4 Strain Gage (Gauge)

5. Dan O. Popa, Intro to EE, Freshman Practicum, Spring 2015 The Strain Gauge Measures the effects of very small displacements (lengthening or shortening) of a resistive sensor Is an isometric device with conductor strain gauges having a maximum resistive change of 1 to 3 % Resistive change based on both dimensional changes of conductor and strain placed on the sensor (piezoresistive effect) Sensors are small, but glued (bonded) to larger structures to increase range of forces measured 5

6. Dan O. Popa, Intro to EE, Freshman Practicum, Spring 2015 6

7. Basic Theory 7

8. Dan O. Popa, Intro to EE, Freshman Practicum, Spring 2015 Different Conductive Materials 8

9. Dan O. Popa, Intro to EE, Freshman Practicum, Spring 2015 Selection of Type Metal foil gauges have a linear resistance change with strain but are less sensitive Semiconductor gauges based on piezoresistive effect in germanium and silicon and are much more temperature sensitive Resistance change is nonlinear for S-C strain gauges and are not as commonly used as metal foil 9

10. Dan O. Popa, Intro to EE, Freshman Practicum, Spring 2015 FlexiForce Sensor Strain Gauge Alternative FlexiForce sensors provide a flexible, paper- thin solution at only 0.008" (0.2mm) thick. The sensors are constructed of two layers of substrate (polyester film). A conductive material (silver) is applied on each substrate, followed by a layer of pressure-sensitive ink. Adhesive is then used to laminate the two layers of substrate together to form the sensor. www.tekscan.com

11. Dan O. Popa, Intro to EE, Freshman Practicum, Spring 2015 11

12. Dan O. Popa, Intro to EE, Freshman Practicum, Spring 2015 Types of Foil Strain Gauges 12

13. Dan O. Popa, Intro to EE, Freshman Practicum, Spring 2015 Measurement Circuit: Wheatstone Bridge Base resistance for bonded gauges between 30 Ω and 3 kΩ (typically 350 Ω) R1 R2 and R3 are balancing resistors (e.g. 350 Ω so bridge balanced at 0 strain and V = 0 Temperature changes cause change in dimensions of structure and gauge and resistivity of gauge resulting in baseline drift 13

14. Dan O. Popa, Intro to EE, Freshman Practicum, Spring 2015 Beam with Two Gauges Beam example 14 Two stressed gauges

15. Dan O. Popa, Intro to EE, Freshman Practicum, Spring 2015 Next Time: Maximum Power Theorem, Solar Panel Lab Prelab 8, Lab 7 report due 15