UC Berkeley EECS Department EECS 40 Lab B Boser UID LAB2 Electronic Scale 1 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 measure this length change How can we do this with an electronic device 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 R L GF Ro Lo 1 where Lo and Ro are the nominal length and resistance respectively and L and R are the changes due to applied force The nominal length and value of the resistor Lo and Ro can be measured If we further determine R we can calculate L and with a bit of physics determine the applied force Assuming you can measure resistance with a resolution of 0 1 what is the minimum length change that you can detect for Ro 218 and Lo 82 mm Use GF 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 Ro without any extra weight applied to the band Then determine R for one three and six weights Report your results in the table below 1 pt Ro R 1 weight R 3 weights R 6 weights 1 1 pt 1 1 pt 1 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 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 and leave the microcontroller interface for later 1 February 12 2009 LAB2 v644 Figure 1 Strain gage in a half bridge circuit configuration Resistors in combination with a voltage source come to the rescue here also Figure 1 shows a so called half bridge configuration where the strain gage resistor Rstrain is connected to a reference resistor Rref and a balanced supply An important objective is to achieve nominally zero output voltage vo when no strain weights are applied to the scale This is achieved in the circuit when Rref is set equal to the nominal value Ro of the strain gage resistor and the supply voltages Vdd and Vss are equal Under these conditions and for Vdd Vss 4 V what is the value of Vo for a 1 increase of Rstrain from its nominal value 1 pt 1 Build the circuit in the laboratory using a solderless breadboard download the guide from the manual section of the website Choose Rref as close as possible to Ro Do not just rely on the color code but measure the actual resistor values Use more than one resistor to synthesize the value to within a few percent of the correct value Set up the laboratory supply for Vdd Vss 5 V Then take the following measurements of vo no weights no weights no weights 1 weight 6 weights 1 pt 2 adjust Vss such that vo 0 V 1 pt 2 1 pt 2 1 pt 2 Ask the teaching assistant to verify the circuit operation Full Bridge Circuit The half bridge circuits has several drawbacks While doing the measurements you may have noticed how difficult it is to accurately set the null point and keep it stable Any change of the supply voltage directly affects the output of the circuit In practice such changes occur frequently e g as the result of a sudden increased current consumption of a different part of the circuit such as an amplifier or microcontroller The need for balanced supply Vdd Vss is a further drawback of the half bridge circuit The full bridge configuration shown in Figure 2 on the next page results in significant improvements Four resistors are used all with nominally equal value Ro The output voltage vo is the difference of v a and vb and only a single supply Vs is needed Let s investigate the full bridge s ability to reject supply voltage variations For our analysis let s assume that the bridge is balanced i e vo 0 V Now let s say the supply voltage is initially Vs 4 1 V but then drops suddenly by 10 Calculate the resulting change of vo 1 pt 2 This property of the full bridge significantly reduces its offset voltage in practical situations where e g supply variations are common In practice of course the reference and nominal strain resistance will not be exactly equal As a consequence vo 2 February 12 2009 LAB2 v644 Figure 2 Full bridge configuration Figure 3 Potentiometer Equivalent circuit diagram left and symbol right has an offset i e its value is not zero when no load is applied to the scale Rather than tweaking the reference resistor values we use a potentiometer for offset adjustment Figure 3 shows the circuit diagram and symbol for a potentiometer The sum R T R a Rb of the values of resistors R a and Rb is constant e g 1 k potentiometers are available with many different values A knob or screw terminal is used to adjust values of R a and Rb such that Ra RT 2 R b 1 R T 3 with 0 1 depending on the setting of the adjustment knob Calculate the values of R a and Rb for a potentiometer with …
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