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Unformatted text preview: UC Berkeley, EECS Department B. E. Boser EECS 40 Lab LAB2: Electronic Scale UID: Enter the names and SIDs for you and your lab partner into the boxes below. Name 1 SID 1 Name 2 SID 2 Strain Gages In this lab we design an electronic scale. The device could equally well be used as an orientation sensor for an electronic camera or display, or as an acceleration sensor, e.g. to detect car crashed. In fact, similar circuits to the one we build are used in all these applications, albeit using technologies that result in much smaller size. In our scale we use the fact that metal bends if subjected to a force. In the lab we use an aluminum band with one end attached to the lab bench. If we load the other side, the band bends down. As a result, one side of the band gets slightly longer and the other one correspondingly shorter. All we need to do to build a scale is to measure this length change. Attention: The setup and gage are fragile. Load it only with the supplied brass weights and do not manually force or bend the aluminum band. How can we do this with an electronic device? It turns out we need to look no farther than to simple resistors. A resistor is similar to a road constriction, such as a bridge or tunnel. The longer the constriction, the higher the “resistance”. Cars (or electrons) will back up. Increasing the width on the other hand reduces the resistance. If we glue a resistor to our metal band its value will increase and decrease proportional to the length change. The percent change is called the “gage factor” GF and is approximately two (since an increase in length is accom- panied by a corresponding decrease in width due to conservation of volume): a 1 % change in length results in a 2 % change in resistance. Mathematically we can express this relationship as Δ R R o = GF Δ L L o (1) where L o and R o are the nominal length and resistance, respectively, and Δ L and Δ R are the changes due to applied force. The nominal length and value of the resistor, L o and R o , can be measured. If we further determine Δ R we can calculate Δ L , and, with a bit of physics, determine the applied force. Assuming you can measure resistance with a resolution of 0.1 Ω , what is the minimum length change that you can detect for R o = Ω and L o = mm? Use GF = 2 for this and all subsequent calculations. 1 pt. In the laboratory, attach the metal band with attached strain gage to the bench. Measure the nominal resistance R o without any extra weight applied to the band. Then determine Δ R for one, three, and six weights. Report your results in the table below: R o 1 pt. 4 Δ R , 1 weight 1 pt. 5 Δ R , 3 weights 1 pt. 6 Δ R , 6 weights 1 pt. 7 The small changes may be difficult to resolve if the display of the meter flickers. Use the bench top meter (not handheld device), and make sure the connections are reliable. Poor connections can contribute several Ohms resistance, and small changes in the setup (e.g. a wire moved) can result in big resistance changes. Also, as for allresistance, and small changes in the setup (e....
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This note was uploaded on 09/25/2010 for the course EECS 25248 taught by Professor Boser during the Spring '10 term at University of California, Berkeley.
- Spring '10