EE100 Lab2

# EE100 Lab2 - UC Berkeley, EECS -40 Lab 100 LAB2: Electronic...

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LAB2: Electronic Scale 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 crashes. In fact, similar circuits to the one we build are used in all these applications, albeit using technologies that allow much smaller size. For 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 measure this length change. 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 accompanied 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 = and L o = mm? Use = 2 for this and all subsequent calculations. 1 pt. 0 In the laboratory, attach the metal band with attached strain gage to the bench. Measure the 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. 1 Δ R , 1 weight 1 pt. 1 Δ R , 3 weights 1 pt. 1 Δ R , 6 weights 1 pt. 1 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 of resistance, and small changes in the setup (e.g. a wire moved) can result in big resistance changes. Also, as for all measurements, keep wires short. Half Bridge Circuit Using an Ohm-meter to evaluate the output of our scale is not very practical. Typically we prefer a voltage output for sensors. Voltages can easily be interfaced for example to microcontrollers (small computers), which in turn can be connected to a display or other appropriate device. In this lab we focus on getting a voltage out of our sensor

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## This note was uploaded on 02/09/2011 for the course EE 100 taught by Professor Blah during the Spring '11 term at Eastern Michigan University.

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EE100 Lab2 - UC Berkeley, EECS -40 Lab 100 LAB2: Electronic...

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