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- On bending beam 1 just one strain gauge is fixed.
- Use a multimeter to measure the resistivity change when bending the beam.
- If you had a 10mA current source like in fig. 4, what voltage change would you expect across the strain gauge for a 2‰ 1‰ strain of the beam? What voltage change would you need to measure, if you want to resolve the maximum strain of 2‰ 1‰ to 100 steps?
- Use an additional resistor to build the setup from circuit 2 (fig. 5), a voltage divider. Measure the voltage with the multimeter across the strain gauge if you connect a 5V source as Vs for both the unloaded and loaded beam.
- Now imagine the strain gauge in circuit 3 (fig. 6) as a quarterbridge with the strain gauge as R1. What do you expect?
- For this setup, give the equation for the voltage Vd. What happens if R1 changes?
- What voltage change would you measure for a 1‰ strain and 5V supply voltage?
- Would it be possible to compensate a temperature difference here?
- Use bending beam 2 which has 2 strain gauges to set up a halfbridge as shown in circuit 3 (fig. 6). The two straing gauges on the beam need to be connected to the amplifier by using the screw terminal. Then set the amplifier to half-bridge and use a supply voltage of 10V. The top turning knob should be set to 0.5mV/V at UB=10V. The lower switch should be set to 1mV/V.
- Give the equation for Vd in this setup.
- Try to balance the voltage to as close as possible to zero without any load on the beam by using the "zero balance".
- Use the amplifier manual/the settings to find the corresponding real value of Vd for 1V amplifier output voltage.
- Now put load on the beam and compare the outputs of the amplifier with and without load on the beam. Calculate the stress you applied.
Mathinline body --uriencoded--E_%7Bsteel%7D=210000 \cfrac%7BN%7D%7Bmm%5e2%7D \qquad and \qquad "Querkontraktionszahl" (poisson's \quad ratio) \quad \nu=0.28 - Could a temperature change cause a strain, would this setup be able to compensate it?
- What would happen if one strain gauge were on top and the other one on the bottom of the beam?
- Beam 3 is not a usual bending beam, but a torsional wave. The strain gauges are fixed in a 45° angle, as the strain is largest in this angle. Connect the strain gauges to the amplifier by using the screw terminal. Set the amplifier to full-bridge, use a supply voltage of 10V and set the zero balance. Keep all other settings.
- Measure the amplified voltage of Vd while applying a torque to the wave!
- Calculate the torque you have applied by using these equations:
Mathinline body --uriencoded--\epsilon= \cfrac%7BT%7D%7B2 S_P G%7D \qquad G=\cfrac%7BE%7D%7B2(1+\nu)%7D \qquad S_P=2 \cfrac%7B\pi(d_a%5e4-d_i%5e4)%7D%7B32d_a%7D \qquad with \quad E_%7Bsteel%7D=210000 \cfrac%7BN%7D%7Bmm%5e2%7D \quad and \quad \nu=0.28
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