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hw11soln - ME 365 Homework Set 11 Out Due December 1 2011...

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ME 365 Homework Set 11 Out November 17, 2011 Due December 1, 2011 Problem No. 1 The diagram of a typical bridge circuit is shown below: We know that the Thevenin-equivalent of any system is represented by the circuit diagram: (a) What is the Thevenin-equivalent voltage ( E th ) of the bridge? Solution: E th is V 0 when there is nothing connected across V 0 s th V R R R R R R R R E ) )( ( 3 2 4 1 4 2 3 1 (b) Calculate the output impedance ( Z out ) of the bridge? Short the V s 3 2 3 2 4 1 4 1 R R R R R R R R Z out
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ME 365 Homework Set 11 Out November 17, 2011 Due December 1, 2011 (c) A secondary measurement device is to be connected across the output terminals of the bridge. Determine the relationship between Z in of the secondary device and Z out of the bridge that will ensure that loading is below 1%. out in out in in Z Z Z Z Z 99 99 . 0  Problem No. 2 (a) Explain why we need to use bridge circuits in conjunction with strain gages. (b) Explain the temperature effect on strain gages, and how to compensate for the temperature change within a bridge circuit. (c) Explain why in a bridge circuit with variable reactive impedance elements, one cannot use a DC supply voltage. Solutions: (a) Bridge circuit provides an output voltage which is proportional to the change of the impedance rather than the absolute value of impedance. (b) Strain gages are very sensitive to the temperature change. During the experiment, if the temperature changes, we may end up measuring the resistance change due to the temperature change, rather than the change due to the stretching or compression of the structure. We can use two strain gages to remove the temperature dependency. The second gage, also called
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