LAB3-ME120

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1 San Jose State University Department of Mechanical Engineering LAB 3: LOAD CELLS LAB REPORT Lab author: Binh Nguyen Lab date: 10/23/2022 I. ABSTRACT This lab will help the student to understand the concept of strain gauge load cells and how to improve signal by applying signal conditioning techniques. II. TABLE OF CONTENTS LAB 3: LOAD CELLS LAB REPORT 1 I. ABSTRACT 1 II. TABLE OF CONTENTS 1 III. SUMMARY 2 IV.INTRODUCTION 2 V.APPARATUS AND PROCEDURE 2 VI. RESULTS 8 VIII. CONCLUSION 9 IX. REFERENCE 10 X. APPENDICES 11
2 III. SUMMARY This lab report will use the data acquisition to collect the data for strain gauge load cells, by applying filtered (cutoff frequency) and moving average techniques to smooth the signal. Resulting in the data set of 0g , 500g, 1000g, 1500g, 1900g will have a linear line showing that the signal increases linearly. Using this data set we can find the theoretical Kthy for finding percent error. IV.INTRODUCTION Load Cell: Using a load cell, you can gauge mechanical force, primarily object weight. Almost all modern electronic scales that measure weight rely on load cells to do so. They are used frequently because they can measure weight accurately. Strain Gauges: A sensor known as a strain gauge measures electrical resistance that changes in relation to changes in strain. Strain is the material's displacement as well as deformation as a result of applied stress. Stress is the force exerted on a material divided by the cross-sectional area of the material. The purpose of load cells is to concentrate stress through beam components that house strain gauges. Strain gauges turn the applied force, pressure, torque, and so on. into a measurable electrical signal. Wheatstone Bridge: For the accurate measurement of low resistance, the Wheatstone bridge is employed. Physical parameters like temperature, light, and strain are measured using a Wheatstone bridge and an operational amplifier. Variations of the Wheatstone bridge can be used to measure things like impedance, inductance, and capacitance. 𝑉 . 𝐺𝐹 . 𝑑 . 𝑔 . 𝑐 We have proportionality constant : 𝐾 𝑆 𝑡ℎ𝑦 𝐸𝐼 V. APPARATUS AND PROCEDURE Build the VI: 1. Insert the load cell into the fixture and attach it to the edge of your lab table. The “10 kg” text should be upright. 2. Connect the myDAQ to the PC via USB 3. Wire the load cell to the myDAQ: a. Red Wire: 5V b. Black Wire: Ground
3 c. White Wire: AI0- d. Green Wire: AI0+ 4. Open a Blank VI in LabVIEW. 5. Build the front panel as follows: a. Place a Waveform Chart: i. Title the chart “Unfiltered Signal.” ii. Turn off auto scaling on both axes. iii. Set the minimum Y-Axis value to -2. iv. Set the maximum Y-Axis value to 4. v. Rename the Y-Axis label to “Signal Voltage (mV).” b. Place a Stop Button. c. Place a Numeric Indicator. i. Title the indicator “Unfiltered Signal (mV).” 6. Build the block diagram as follows: a. Place a While Loop and wire the stop button to the stop condition. b. Place an Input DAQ Assistant and set it up as follows: i. Select Acquire Signals » Analog Input » Voltage. ii. Choose the myDAQ and select channel ai0. iii. Click Finish. iv. Set the Signal Input Range to Max: 4m and Min: -2m. v. Set the Acquisition Mode to Continuous Samples. vi. Set the Samples to Read to 100. vii. Set the Rate to 5k. c. Place a Multiply function and create a constant at one of the input terminals with a value of 1000. This is to convert the signal from volts to millivolts. d. Wire the data terminal from the DAQ Assistant to the open input terminal of the multiply function. Wire the output of the multiply function to the Unfiltered Signal waveform chart and numeric indicator. 7. Run the VI and ensure you get a signal on the waveform chart. The signal should be within the limits of the Y-Axis, but if it is not, change the limits accordingly. Make sure to make the same change to the Signal Input Range in the DAQ Assistant settings. 8. Test the setup by pressing down on the end of the load cell. The signal voltage should increase. If it decreases, switch the green and white wires on the myDAQ Low-Pass Filter: 9. Make the following changes to the front panel: a. Copy the waveform chart and rename it “Filtered Signal.” b. Copy the numeric indicator and rename it “Filtered Signal (mV).” c. Place a Vertical Pointer Slide and title it “Cutoff Frequency (Hz).” i. Change the upper limit to 2000.
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4 ii. Change the lower limit to 1. 10. Make the following changes to the block diagram: a. Place a Filter Express VI from the Express » Signal Analysis palette. Keep the default settings, ensuring the filter type is set to low-pass. b. Wire the unfiltered signal (after the multiply function) to the Signal terminal of the Filter Express VI. Wire the Filtered Signal terminal to the Filtered Signal waveform chart and numeric indicator. c. Wire the Cutoff Frequency vertical pointer slide to the Lower Cut-Off terminal on the Filter Express VI. Moving Average: 11. The low-pass filter was useful for reducing noise, but the numeric indicator shows that the filtered signal is still somewhat unsteady. One solution is to apply a moving average to the VI. A moving average is the average of the most recent N values. We will start by setting N to 10 values with shift registers. 12. Make the following changes to the front panel: a. Copy the Filtered Signal waveform chart and rename it “Filtered Moving Average.” b. Copy the Filtered Signal numeric indicator and rename it “Filtered Moving Average (10) (mV).” 13. Make the following changes to the block diagram: a. Place a From DDT Express VI from the Express » Sig Manip palette. Set the conversion to Single Scalar. b. Add a shift register to the while loop by right-clicking the edge and selecting Add Shift Register. c. Wire the filtered signal to the input of the From DDT Express VI. Wire the output of the From DDT Express VI to the right-most shift register. d. Place a Compound Arithmetic function from the Numeric palette. Drag it down so that it has 10 inputs. e. Drag the left-most shift register down so that it has 10 outputs. Wire each output to an input on the compound arithmetic function. f. Place a Divide function and set the y terminal to a constant 10. g. Wire the output of the compound arithmetic function to the x terminal of the divide function. h. Wire the output of the divide function to the Filtered Moving Average waveform chart and numeric indicator. 14. Before running, change the limits of the X-Axis on the Filtered Moving Average waveform chart to 0 and 230. This will ensure the waveform scale matches the other two charts 15. Run the VI.
5 16. The 10-sample moving average seems to slightly smooth the signal. A larger sample will create a smoother signal, but this will be very tedious to wire with shift registers. An alternative is to use arrays. The thought process is to store the previous N signal values in a 1D array, add the latest value onto the beginning of the array (array size is now N+1), and take the average of the first N values in that array. The number of samples (N) will be specified with a control on the front panel. 17. Make the following changes to the front panel: a. Copy the vertical pointer slide and rename it “Samples in Moving Average.” i. Set the limits from 10 to 500. ii. Set the representation to I32 by right-clicking and selecting Representation. b. Copy the Filtered Moving Average numeric indicator and rename it “Filtered Moving Average (N) (mV).” c. Extend the legend above the Filtered Moving Average waveform chart so that it includes “Plot 1.” Rename the white line to “Moving Average (10)” and the red line from “Plot 1” to “Moving Average (N).” 18. Make the following changes to the block diagram: a. Place a Build Array function from the Array palette. Expand it to have two inputs. b. Place an Array Subset function from the Array palette. c. Insert a shift register on the while loop. d. Wire the top element terminal of the build array function to the output terminal of the From DDT Express VI. e. Wire the appended array output terminal of the build array function to the array input terminal of the array subset function. f. Wire the subarray output terminal from the array subset function to the right-most shift register. g. Wire the left-most shift register to the bottom element input terminal of the build array function. h. Wire the Samples in Moving Average vertical pointer slide to the length input terminal of the array subset function. i. Place a Mean function from the Mathematics » Prob & Stat palette. j. Wire the mean function input terminal to the output terminal of the array subset function. k. Wire the output terminal of the mean function to the Filtered Moving Average (N) numeric indicator. l. Place a Bundle function from the Cluster, Class, & Variant palette. m. Break the wire that leads to the Filtered Moving Average waveform chart. Attach the wire from the divide function output terminal to the top input terminal of the bundle function. Wire the Filtered Moving Average waveform chart to the output
6 terminal of the bundle function. The broken wire will be resolved with the next step. n. Wire the output terminal of the mean function to the bottom input terminal of the bundle function. 19. Run the VI and set the Samples in Moving Average vertical slide to 10. Press on the load cell a few times and observe the waveform chart to verify that the array method of calculating the moving average is equivalent to the shift register method. 20. Now change the vertical pointer slide to 500 samples. Press on the load cell and keep the load applied, observing the waveform chart. 21. Remove the load and let the moving averages stabilize Proportionality Constant: 22. With a filtered and averaged signal, we are ready to experimentally measure the proportionality constant. This will be done by measuring the load cell signal from known masses, exporting the data to Excel, and applying a linear regression. 23. Make the following changes to the front panel: a. Place a Numeric Control titled “Mass (g).” b. Place an OK Button titled “Record.” 24. Make the following changes to the block diagram: a. Place a Write to Measurement File Express VI from the File I/O palette. i. Set the File Format to Microsoft Excel (.xlsx). ii. Set the name and location for the saved file. b. Place a Merge Signals function from the Express » Sig Manip palette. c. Wire the Record OK button to the Enable terminal of the Write to Measurement File Express VI. d. Wire the Mass numeric control to the top input of the merge signals function. e. Wire the output terminal of the mean function to the bottom input of the merge signals function. f. Wire the output of the merge signals function to the Signals input of the Write to Measurement File Express VI. 25. Run the VI. Hang a known mass on the load cell, wait for the reading to stabilize, enter the mass value in grams into the Mass numeric control on the front panel, and click the Record OK button. Repeat this for four different masses and for the condition of no mass (unloaded load cell). Use a large range of masses, keeping in mind you can hook masses together (ex: 500 g, 1000 g, 1500 g, & 1900 g). 26. Stop the VI and open your Excel data file. The left column is the mass in grams and the right column is the signal in millivolts. Plot a scatter-plot and apply a linear trendline. Display both the equation and R-squared value on the chart. You may need to adjust the label options of the equation text box to show more decimal places. Display three significant figures for the slope. See Figure 16 for the location of this setting
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7 27. Record the values for the experimental proportionality constant ( 𝐾𝑒𝑥𝑝 ) and voltage offset ( 𝑉 𝑜 𝑓𝑓 𝑠 𝑒 𝑡 ) from your linear regression equation, with reference to Equation 9. Include the Excel plot with the displayed linear regression equation in your report. Calibrating the Load Cell: 28. We now have all the information necessary to calibrate the load cell. This will be done by inputting the experimental proportionality constant and offset voltage into the VI. 29. Make the following changes to the front panel: a. Make two copies of the Mass (g) numeric control and rename them “K” and “Offset Voltage (mV).” b. Copy one of the numeric indicators and rename it “Calibrated Mass (g).” 30. Make the following changes to the block diagram: a. The goal is to solve Equation 9 for mass ( 𝑚 ), since the signal voltage ( 𝑉 0) is measured, and the offset voltage ( 𝑉 𝑜 𝑓𝑓 𝑠 𝑒 𝑡 ) and proportionality constant ( 𝐾 ) are defined by the newly placed numeric controls. b. Place and wire a Subtract and Divide function to make the following computation: 𝑉 −𝑉 𝑚 = 0 𝑜𝑓𝑓𝑠𝑒𝑡 𝐾 31. Run the VI and input the experimental proportionality constant ( 𝐾 ) and the offset voltage ( 𝑉 𝑜 𝑓𝑓 𝑠 𝑒 𝑡 ) into their respective numeric controls. Hang an unknown mass from the load cell. Record the calibrated mass measurement from the VI and compare it with the real mass of the object (you can measure this with the digital scale under the window). 32. Preference all the steps above by figures below. Figure 1: Front panel
8 Figure 2: Code block diagram VI. RESULTS Figure 3: Data set for 0g,500g,1000g,1500g,1900g
𝑅 0.00037 9 Figure 4: test for unknown mass VII. DISCUSSION After reviewing my data that was used to create the form of a linear function in figure 3, I can state that the lab object was successfully archived. Since the 2 value is 0.99, or 99%, we can infer that the mass/signal relationship accounts for 99% of the variation in the data, or that the mass/signal relationship explains the majority of the variation in the data. Additionally, comparing the professor's provided unknown mass yields a result of 501.634g, which is very close to the actual weight of 500g, demonstrating my program's accuracy and perfection. Furthermore when comparing the Kthy and K from the linear function: 𝑉 . 𝐺𝐹 . 𝑑 . 𝑔 . 𝑐 (5). (2) . ( 0.08 ) . (9.81). ( 0.0127 ) 𝐾 𝑆 = 2 2 = 0. 00037 𝑉/𝑘𝑔 9 4 4 𝑡ℎ𝑦 𝐸𝐼 (70.10 )[ (0.0127) (0.0127−0.0017) ] 2 2 => percent error = 0.000483−0.00037 = 30. 5% Even the calculation giving large percent error but since it have same order of magnitude it is acceptable
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2 10 VIII. CONCLUSION With the help of this lab, I will have the opportunity to learn more about using Load Cells, Strain Gauges, and Wheatstone Bridges to gather weight data for a given mass that will have a linear function, from that calculates the result for Kthy then compare with K from the linear function. Additionally, the result of 𝑅 given by the graph is 0.99 which is 99% we can conclude that all the data points are near to the linear equation showing the accuracy of the program proves that we have accomplished the goal of the lab to understand the working principles behind a strain gauge load cell. Furthermore, even the comparison between Kthy and K gives a large percent error of 30.5% but since it has the same order of magnitude it is acceptable. However, there is some systematic error that I can’t fix, which is the unstableness of the signal from the output of the DAQ where those numbers keep going up and down while not staying, I think this happens because the DAQ is not in the correct condition. IX. REFERENCE Al-Mutlaq, S. (n.d.). Getting started with load cells. Sparkfun. Retrieved from https://learn.sparkfun.com/tutorials/getting-started-with-load-cells HTC Sensor. (n.d.). Parallel beam load cell TAL220 datasheet. Retrieved from https://cdn.sparkfun.com/datasheets/Sensors/ForceFlex/TAL220M4M5Update.pdf Kuphaldt, T. R. (n.d.). Strain gauges. All about circuits. Retrieved from https://www.allaboutcircuits.com/textbook/direct-current/chpt-9/strain-gauges/
11 X. APPENDICES Figure 5: Data set