Summer 2007 EE100 EE43 Lab 7 EECS 100 43 Lab 7 Project Sensor Strain Gauge 1 Objective In this lab you will build an op amp amplifier circuit for a strain gauge You will then interface this circuit to a PIC board labs 8 and 9 2 Equipment a Breadboard b Wire cutters c Wires d Oscilloscope e Function Generator f Power supply g LMC6482 op amp h Strain gauge bridge on your lab bench i Various connectors for the power supply function generator and oscilloscope j Resistors you will pick their values depending on your strain gauge 3 Theory a System Block Diagram In this lab you will design a strain gauge system that you can interface to your PIC microcontroller the last two labs of the summer Figure 1 shows the expected block diagram of your strain gauge system Figure 1 Strain Gauge block diagram YOU NEED TO READ THIS LAB AHEAD OF TIME SO THAT YOU CAN COME PREPARED IN LAB MY SUGGESTION IS TO GET TOGETHER WITH YOUR LAB PARTNER S WELL BEFORE LAB STARTS AND MAKE SURE YOU UNDERSTAND THE SYSTEM IF YOU DON T YOU WILL NOT BE ABLE TO FINISH THE LAB IN 3 HOURS ALSO NOTE THIS LAB IS WORTH 7 OF YOUR TOTAL CLASS POINTS IT IS THE CRUX OF THE PROJECT DIGITAL PART IS 3 Let us examine the blocks in figure 1 excluding the scope University of California Berkeley Department of EECS Summer 2007 EE100 EE43 Lab 7 b Strain Gauge A nice description of strain gauges is in Chapter 5 of your book Thus READ 1 p 155 in your book Practical Perspective 2 pp 165 166 in your book Section 5 6 The Difference Amplifier Circuit We will not be using the 4 pair strain gauge model in your book Rather for simplicity we will be using only one strain gauge and build a Wheatstone bridge network out of discrete resistors Figure 2 shows the first circuit you will build in lab this is the Strain Gauge block in figure 1 Figure 2 The circuit inside the Strain Gauge block The actual strain gauge is modeled by a variable resistor RStrain The Wheatstone bridge is also described in your book Hence READ pp 71 72 Section 3 6 Measuring Resistance The Wheatstone Bridge in your book You will understand that the Wheatstone Bridge in figure 2 is balanced 1 Therefore in figure 2 Va Vb When you deflect the strain gauge RStrain changes by a very small amount around 0 5 to 1 This deflection causes a very small change in Vab Thus you need to amplify Vab this is the purpose of your Sensor Interface block in figure 1 c Sensor Circuit Figure 3 below shows a MultiSim screen shot of your sensor circuit It contains two amplifiers2 a difference amplifier followed by a non inverting amplifier for additional gain 1 A more general condition for a balanced Wheatstone Bridge is in your book Here we chose all the resistors to be equal this is a special case of the general condition 2 Note that I have logged into iserver1 eecs berkeley edu for the LMC6482 model in MultiSim Please log into iserver1 or iserver2 or iserver3 and use the LMC6482 model Also notice that a real LMC6482 has two op amps in every IC So one IC is enough for this lab University of California Berkeley Department of EECS Summer 2007 EE100 EE43 Lab 7 Figure 3 The sensor interface circuit Figure 4 shows a screen shot of the completed system in MultiSim NOTICE THAT THE POWER SUPPLIES IN THIS LAB ARE 0 and 5 V So you should not use the 25 V supply on the bench for this lab I also assumed a theoretical rest resistance of the strain gauge to be 119 4 ohms The actual resistance in lab will vary depending on your strain gauge But AGAIN PLEASE MAKE SURE YOU UNDERSTAND THIS LAB REALLY WELL BEFORE THE ACTUAL LAB SESSION OR YOU WILL NOT BE ABLE TO FINISH THIS IN LAB IN 3 HOURS You should now answer the question s in your prelab Figure 4 The completed Strain Gauge interface Rstrain is the resistance of the strain gauge For the simulation above I have assumed the strain gauge is at rest University of California Berkeley Department of EECS Summer 2007 EE100 EE43 4 PRELAB Lab 7 NAME SECTION a TASK 1 Derive the gains of the two op amp circuit in figure 3 Fill table 1 Op amp U1A Difference amplifier U1B Noninverting Amplifier Gain Expression in terms of output voltages and input voltages G1 G2 Table 1 Theoretical Gain Calculations Show your work here The total gain is G V7 Vb Va G1 x G2 b TASK 2 Log into the remote server and design the circuit in figure 4 using MultiSim Record the values of Vb Va and the output voltage V7 of the non inverting amplifier from figure 4 in table 2 below use a simulated Multimeter to measure the voltages You are simulating push and pull on the strain gauge by changing the value of Rstrain Rstrain W Vb Va V from Multsim V7 V from Multsim Simulated Theoretical Gain Gain G V7 Vb Va From TASK 1 120 1 119 4 118 7 Table 2 Simulated vs Theoretical Gain Do you have a nice dynamic range for V7 i e does V7 go from approx 5 V down to approx 1 5 V PRELAB COMPLETE TA CHECKOFF University of California Berkeley Department of EECS Summer 2007 EE100 EE43 5 REPORT Lab 7 NAME S SECTION a TASK 1 Make sure that there is a strain gauge apparatus on your workbench Out of the 4 strain gauges on the apparatus pick one Measure the resistance of the strain gauge at rest at the maximum pull and maximum push I recommend that one partner hold the strain gauge on the bench while the other pushes or pulls3 Record your resistance measurements in table 3 Position Maximum Pull Rest Maximum Push Rstrain W Table 3 Rstrain values that indicate the position b TASK 2 Based on your measurements pick a value for R6 R7 and R8 R6 R7 R8 in your Wheatstone bridge from figure 4 Get this value checked off by the TA R for Wheatstone bridge c TASK 3 Build the circuit shown in figure 4 You may have to change the values of the resistors in the op amp circuits depending on your strain gauge The design criterion is that your output voltage at node 7 V7 in figure 4 should swing from approx 1 0 V to 5 0 V as you push and pull the strain gauge TURN IN ONE REPORT PER GROUP AT THE END OF YOUR LAB SESSION THERE IS NO TAKE HOME REPORT 6 REVISION HISTORY Date Author Summer 2007 Bharathwaj Muthuswamy Revision Comments Typed up source documentation organized lab report typed up solutions 3 DO NOT APPLY SUPERHUMAN STRENGTH ON THE STRAIN GAUGE If the strain gauge breaks the person who broke it PAYS for buying a new strain gauge University of California Berkeley Department of EECS
View Full Document
Unlocking...