Instructions for collecting data with force plates

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2314

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Mechanical Engineering

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Feb 20, 2024

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APA 2314- Laboratory Techniques in Exercise Physiology and Biomechanics University of Ottawa 1 APA 2314 Instructions for collecting data with force plates Equipment: Kistler 9286AA force platform, tray with sandpaper bottom, tray with rubber bottom, two masses (4.5 kg each), computer equipped with Vicon Nexus software, Vicon data acquisition system, and Microsoft Excel software. Methodology: 1. Open Vicon Nexus on the desktop. 2. Click on Go Live . 3. You should see the word “ LIVE ” in the View pane. 4. Select 5 MX cameras + 4 force plates + 4 NewEMG from the drop down menu in the System tab of the Resources pane. This is your system configuration. 5. When you expand the Devices node (click on ), you should see 4 force plates and an EMG system lit up in green. For this lab, you will be using the Kistler 9286AA force plate located in the raised walkway, which is plate #3 in the Nexus software. 6. Expand the node for force plate #3 Kistler 9286AA . Single-click on Force . In the View pane (where you can see a visualization of the lab), click on 3D Perspective at the top of the screen and select Graph . Ensure that Components is selected in the second drop down menu. The image should change to a real-time feed of your force plate data in the x, y and z axes. 7. To prepare the force plate, you will need to complete a hardware zero and a software zero. *** IMPORTANT - Make sure nothing is on or near the force plate when you zero it. a. For the hardware zero: On the Kistler Control Unit, press Operate until the light above it turns green. b. For the software zero: Right click on force plate #3 Kistler 9286AA under Devices in the Resources pane and select zero level . All three data channels should change to zero. Note: The force plate must be zeroed before every trial. 8. When you are analyzing your data, you will need to know which axis to evaluate. Determine the axis system you will be working with by exploring what causes changes to occur on the graphs (e.g. If you add a weight to the force plate, what happens? Which graph is most affected? If you slide your hand across the force plate, which graph is most affected? If you slide your hand in a direction perpendicular to your previous motion, which graph is most affected?). Ultimately, you will be pulling the trays across the force plate; you need to know which axis that motion is taking place in. ** You will now prepare the system to collect your first trial. The first trial that you will be collecting is a static one. This trial will give you the weight of the tray-mass condition you are analyzing first. This value represents your normal force. Note: All trials MUST begin with NOTHING on the force plate (the tray and mass will be placed on the force plate after data collection has begun).

APA 2314- Laboratory Techniques in Exercise Physiology and Biomechanics University of Ottawa 2 9. In the Communications pane at the bottom of the screen select your course by double-clicking on APA 2314 – APA 2714 (green circle). Select Friction-Frottement 2024 (yellow circle) then create and name your lab section (gray circle) – e.g.: Monday 8am. 10. In the Tools pane, select the Capture tab ( ). Under Next Trial Setup name your trial based on the condition you are investigating (e.g. rubber 10lbs static). Select Dynamic from the Trial Type drop down menu. 11. To begin data collection click on Start . Have someone place the tray and the mass on the force plate. 12. Wait a few seconds (3-5s) for the force plate to stabilize and then press Stop to end the trial. 13. Once the static trial has been collected, load your data file by double-clicking on it in the Communications pane. To view your data, once again single-click on Force in the expanded #3 Kistler 9286AA node under Devices . Graphs of your forces in the x, y, and z axes will appear. 14. Confirm that the weight of the set-up has a plateau value of approximately 48N for the 4.5kg conditions or approximately 92N for the 9kg conditions. **You will now analyze the data to determine the exact weight of the system. 15. You may notice that your data looks “noisy” or “choppy”. This is because we haven’t filtered the data yet. A pipeline has been set up to filter your data and export it to Excel for analysis. To run this pipeline, select the Pipeline tab ( ) in the Tools pane . Ensure that the “Current Pipeline” selected is called Friction Lab . Press to execute the pipeline. Once complete, you should see a green checkmark beside both operations in the pipeline (“Filter Analog Data – Butterworth” and “Export ASCII”). 16. To view your filtered data, again single-click on Force in the expanded #3 Kistler 9286AA node under Devices. You should notice that your data looks much smoother. Save your trial by clicking on . 17. To view your data file, load your data file from the directory in which it is saved (remember you can access this folder by clicking on the directory path link in blue in the Communications window). This time you will be opening the static trial (e.g. rubber 10lbs static.csv). 18. Locate the data associated with the force plate you used for the experiment (should be columns U to AC in Excel). Determine which channel to evaluate in order to get the normal force (F Normal ) of your trial (Fx, Fy or Fz?). 19. Scroll down to the bottom of the data and take the average of the last ~20 frames of data from that channel. a. To do this, type =average( in the cell below the last data cell in the column. b. Select your data by clicking and dragging the mouse over the data of interest (i.e. the last ~20 frames of data). c. Close the bracket by typing ) d. Press Enter on the keyboard. 20. The new value that appears is your F Normal for that condition. Record this value. Remember to keep an additional significant figure as this number will be used in calculations later.

APA 2314- Laboratory Techniques in Exercise Physiology and Biomechanics University of Ottawa 3 **You will now collect your dynamic (moving) trials. Five good dynamic trials must be collected for each condition, and each trial must be collected with a different person controlling the computer so that each student has the opportunity to become familiar with the computer software (the same person should pull the tray for all trials). REMEMBER: All trials MUST begin with NOTHING on the force platform! 21. Go back to the Vicon Nexus software. Click on Go Live . In the Tools pane, select the Capture tab ( ). Under Next Trial Setup name your trial based on the condition you are investigating (e.g. rubber 10lbs dynamic1). Click Start . 22. Once data collection has started, the tray-mass system may be added to a far edge of the force plate. Allow it to remain there, untouched, for a second so it can stabilize. Then, have someone in your group very slowly apply horizontal force to the rope, gradually and gently increasing the applied force until the tray starts to slide across the force platform. The pulling should continue in a fluid motion (ideally with a constant force) for a second or two longer, until the person controlling the computer presses Stop . It doesn’t matter which direction you pull the tray, as long as you know your axis system. 23. Once the dynamic trial has been collected, load your data file by double-clicking on it in the Communications pane. To view your data, once again single-click on Force in the expanded #3 Kistler 9286AA node under Devices . Graphs of your forces in the x, y, and z axes will appear. 24. One of your graphs should resemble the “ideal curve” below. *Your graph may be the inverse of this depending on which direction the tray was pulled (positive versus negative direction). The point immediately before the tray “slips” and begins its state of motion should be at the peak of the incline in the graph; this should also be the maximum force value for your trial. If it is not, you have a few choices: a. Repeat the trial OR b. Manually locate the value at the slipping point by matching the time it occurred with the force data in the Excel spreadsheet you are about to create. NOTE: You collected the force plate data at 200 Hz. Therefore, if “slipping” occurred in frame 2000 (for example), that’s equivalent to 10 s in your trial. OR c. Crop the trial (as was done during the Motion Analysis Lab) such that the slipping point is the max. static kinetic Applied Force (N) Frictional Force (N) starts to move

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APA 2314- Laboratory Techniques in Exercise Physiology and Biomechanics University of Ottawa 4 25. If your curve does not resemble the ideal curve, disregard the trial and collect a new trial ensuring that the rope on the tray is pulled very gradually and gently. 26. Once you have a good dynamic trial, run the same pipeline as before to filter your data and export it to Excel for analysis (select the Pipeline tab in the Tools pane, e nsure that the “Current Pipeline” selected is called Friction Lab and press to execute the pipeline. Once complete, you should see a green checkmark beside both operations in the pipeline). **This time, you will analyze the data to determine the maximum horizontal force applied to the tray before it began its state of motion. The instructions below assume that the maximum force value in your graph occurs at the “slipping” point, immediately before motion of the tray began. If this is not the case, you will have to manually find it or crop the trial (as described in Step 24b and 24c above). 27. To view your filtered data, again single-click on Force in the expanded #3 Kistler 9286AA node under Devices. You should notice that your data looks much smoother. Save your trial by clicking on . 28. Load your data file from the directory in which it is saved (remember you can access this folder by clicking on the directory path link in the Communications window). This time you will be opening the dynamic trial (e.g. rubber 10lbs dynamic1.csv). 29. Locate the data associated with the force plate you used for the experiment (should be columns U to AC in Excel). Determine which channel to evaluate in order to get the frictional force (F Friction ) of your trial (Fx, Fy or Fz?). 30. You need to locate the maximum force in this column of data. To do this: a. Scroll down to the bottom of the data and type =max( in the cell below the last data cell in the column. b. Select all of the data in this column. i. A shortcut way to do this is to click on the last data cell then press and hold “Shift”, “Ctrl” and “↑” on the keyboard. Then release each of these keys. It’s ok that the headings are included in the selected data. They won’t affect the maximum force value. c. Close the bracket by typing ) d. Press Enter on the keyboard. 31. The new value that appears is your F friction for that trial. It represents the point in the trial immediately preceding motion and is used to calculate the coefficient of static friction (μ static ). Record this value, again keeping an additional significant figure. You do not need to save your currently open Excel file as long as you have recorded the force value in your table provided below but you may do so if you would like. 32. Repeat steps 21-31 until you have 5 good dynamic trials analyzed. Ensure that a different person operates the computer for each trial. Once you have completed all the trials, compute the coefficient of static friction for each condition. You only need to calculate it once per condition, based on the mean of the five dynamic trials.