0.3 Strain gages and force measurement  (Page 2/4)

Thought Question

A half bridge could be made with two gages in tension on opposite legs. When would this be useful? What would bethe main problem with doing this?

Part 2: calibration of the strain-gaged cantilever beam

Your TA will provide an aluminum beam instrumented with strain gages.

1. Clamp the beam securely to the edge of the lab table with the gaged portion of the beam cantilevered beyondthe edge of the bench.

2. Using the junction box provided, connect a single strain gage in a quarter bridge configuration. (Refer toFigure 3 as a guide.)

3. Connect the junction box output cable to Channel 2 of the SCXI-1520 strain gage module. The SCXI-1520provides an excitation voltage to the bridge and amplifies the output voltage from the bridge to ranges that are easily observableor acceptable for data acquisition. The excitation voltage Vi is 5 V and the gain is 500. When connected to the signal conditioningboard, the amplified bridge output can be read by the data acquisition software.

4. Open the“Lab4”VI that you created in Temperature Measurement and First-Order Dynamic Response .

5. Hang weights from the end of the beam.

6. Record the voltage measured with LabVIEW in your lab book.

7. Repeat steps 5 and 6 for several different weights.

8. In Excel, plot the voltage (input) versus weight (output).

9. Find the best fit linear relationship for the data. The resulting equation can be used to calibrate thevoltage output of the strain gages.

10. Apply several loads not used for calibration to test the validity of the linear curve fit.

Part 3: comparison of theoretical strain to measured strain

In this step, we will compare a theoretically based estimateof strain for a given load to that which was measured earlier. First we will determine the strain correspondingto the voltage measurements of step 2.

1. Measure the excitation voltage for bridge circuit, Vi.

The circuit analyses from step one showed that the bridge output Vo is related to the excitation voltage by thefollowing relationship

where K equals 1/4 for a quarter bridge, 1/2 for a half bridge, and 1 for a full bridge. The voltage measured inLabVIEW is related to the bridge output by

where Kamp is the gain of the signal conditioning board. Because the bridge resistances are not balancedexactly and the weight of the beam itself produces some strain, you will observe a nonzero output voltage when there is no loadapplied.

2. Measure the output voltage with no load. Call this voltage Voffset.

3. To determine the strain induced by the applied loads, measure the changes in the display voltages relativeto this offset voltage (Vdisplay–Voffset).

For a strain gage, the gage factor is defined as

where epsilon is the strain experienced by the gage. The gages used in this lab have a gage factor of 2.12±0.8%.

4. Derive an expression for the strain in the beam using equations (2) through (4). Your empirical strainestimate should be in the range of 0 to 2000 microstrains.