6. Bicycle Experiment

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1 ME 105 Mechanical Engineering Lab Page 1 2ME 105 Mechanical Engineering Laboratory Spring Quarter Bicycle Experiment David Bothman, 6 May 2002 Introduction The bicycle experiment has three goals: to introduce some of the tools and problems associated with performing a field experiment that is an experiment conducted away from the laboratory; to perform time series analysis on accelerometer and odometer readings; and to practice programming skills employed in data analysis. Pre-Lab Reading Read this document thoroughly and review programming notes. Pre-Lab Work Write a code in the programming language of your choice the can read in data from the data logger file format. Retrieve a sample data file from the URL: Notice that data are in comma separated format that can be read by Matlab and Excel. Have your program numerically integrate the acceleration data over the interval appearing between the two mark points. Acceleration is reported to Channel 2 as a percent of 5 volts. Take the accelerometer calibration to approximately 1G = 0.5V. Therefore, a change of 10.0 appearing on Channel 2 is interpreted as 0.5V or 1G. There is an unknown offset (of about 2.5V) to the acceleration reading that can be determined from the time series measurement using the knowledge that the average acceleration is zero. Use your program to evaluate speed as a function of time from the sample data and find the average speed of the bike. Required Equipment Bicycle instrumented with wheel rotation sensor and accelerometer Battery powered portable data collection unit 1.1 Experimental Set-up The experiment consists of a bicycle instrumented with an off-the-shelf wheel rotation sensor and an accelerometer. A battery powered portable data logger records

2 ME 105 Mechanical Engineering Lab Page 2 distance and acceleration measurements over a programmable time interval. A box mounted on the handlebar has three switches: system power, data collection on/off, and a switch that records a marker pulse. Distance is measured by wheel rotation which is detected by a magnetic switch. The rotation signal is relayed to the data logger through a signal conditioning board. A small, padded, plastic instrument case mounted on the rear rack of the bicycle contains the data logger, signal conditioning board and accelerometer. The accelerometer output is also relayed to the data logger through the signal conditioning board. Figure 1 shows an instrumentation diagram. Control box on handlebar Instrument case on the rear rack Mark Switch 100k Data Start Switch Power Switch (white) 0.1µ 100k (black) (yellow) (red) (orange) Cateye Computer 5 9V Out 6 9V In 2 (black) C 1 C SIG (white) COM (black) ADXL105EM-1 Accelerometer SIG (white) COM (blk) +5 (red) Signal Conditioning Card Accel Sig (white) Dist Sig (blue) 2 3 Pace Data Logger Wheelmounted Magnet Wheel Rotation Switch (black) 2 1 Distance sensor on wheel 9V Battery Figure 1: Instrumentation diagram Distance sensor The distance sensor is comprised of a magnet attached to the wheel and a reed switch attached to the fork of the bicycle. When the magnet passes the switch its field causes the switch to close. The reed switch comes with an off-the-shelf bicycle computer a Cat Eye Enduro 2. The reed switch signal is split off from the connection to the Cat Eye computer, and is relayed through the signal conditioning board to Channel 3 of the data logger.

ME 105 Mechanical Engineering Lab Page 3 1.1.2 Accelerometer The accelerometer is an Analog Devices ADXL105-EM-1 single axis MEMS accelerometer. It can measure ± 4g with a bandwidth of 0 100 Hz.

3 ME 105 Mechanical Engineering Lab Page Accelerometer The accelerometer is an Analog Devices ADXL105-EM-1 single axis MEMS accelerometer. It can measure ± 4g with a bandwidth of Hz. Supplied with 5V it generates a ±2.5V signal about a 2.5V mean, so the range is 0-5V. This is a ratiometric sensor, meaning that the output is scaled to the supply voltage. It is a good quality inexpensive sensor. However, its main drawback is a rather large zero drift. Clever signal analysis can compensate for this drift. The accelerometer output is connected to Channel 2 of the data logger. The 5V battery supply is measured on channel Switch box A small chassis box mounted on the handlebar holds control switches and the Cat Eye cycle computer. Two slide switches on the box are used to turn on/off the 9V battery supply and to turn on/off the data logger. The data logger will be programmed to start recording when a finite resistance bridges Channel 1 to ground. A mark button provides a momentary change in resistance to Channel 1 for the purpose of marking events in the data record. If several data runs are conducted, the start and stop times can be identified with the mark switch Pace XR440M data logger Data is collected and stored on an XR440-M data logger manufactured by Pace Scientific. The unit is shown in Fig. 2. It is battery powered and can store about 2 minutes worth of data on 4 channels at 200 Hz. The unit can be connected to a PC using a serial connection. This connection is used to configure the data logger and to download data at the end of an experiment. The unit can store 128,024 measurements at 8-bit resolution and 86,016 at 12-bit resolution. :70 2: XR440 Pocket Data Logger

4 ME 105 Mechanical Engineering Lab Page Software Data collection software is supplied with the data logger. The program Pslog32 is installed on the lab computers, and is also available as a free download from This software can be tested without a data logger connected. All of the manuals and help files are included in this download. The software allows users to set the sample rate, resolution and the recording start / stop conditions. The sensor types, input ranges and channel names are also entered. Once configured the setup file is downloaded to the data logger via the serial port. After data has been collected the download menu is used to transfer data from the recorder back to the computer. The software also allows the inspection of data without recording. The procedures for sending configuration files and transferring data are described below. 2 Experimental Procedure During a typical data run the user will: 1) turn on the power 2) download the calibration configuration file to the data logger 3) verify operation of sensors 4) download the experiment configuration file to the data logger 5) wheel the bike to the test course 6) prepare for data run and turn on the recorder 7) press the mark switch to indicate the start of the data run 8) pedal the bike over the course 9) press the mark pulse to indicate the end of the run and then either turn the recorder off or complete a second run. 10) return the bike to the lab and download the data from the portable data logger into the PC 2.1 Downloading the calibration configuration to the XR440M The procedure for downloading the calibration configuration to the data logger is as follows: Connect the data logger to the mating serial cable on the computer. Start the software double click on the Pocket Logger 32bit icon Select setup from the send menu Select load file from the Pocket Logger Setup screen and load calibration.set Verify that the screen has the same information as shown in Fig. 3. Use the send button to transfer the configuration file to the data logger.

ME 105 Mechanical Engineering Lab Page 5 Note that Channel 1 is set to measure resistance associated with the mark switch and to start data logging.

5 ME 105 Mechanical Engineering Lab Page 5 Note that Channel 1 is set to measure resistance associated with the mark switch and to start data logging. Channels 2-4 are set to measure voltages between 0-5V corresponding to the accelerometer on Channel 2, the wheel rotation sensor on Channel 3, and the 5 volt supply on Channel 4. For the purpose of verifying that all of the sensors are working, the data rate is set to a period of 2 seconds (f = 0.5 Hz.). Figure 3: Pocket Logger Calibration Setup Screen 2.2 Verifying that all of the sensors are working After loading the calibration setup into the Pace data logger, you can view data in real time. Turn on instrument power on the handlebar control box. From the main pull down menu select Receive Select Real Time You should see a screen showing the current data values and the logger settings, as shown in Fig. 4. Tilt the bicycle backwards to make sure that the accelerometer is working. Place the magnet in front of the wheel rotation switch to make sure that it is working. When you move the data recording switch to the on position the resistance on channel one should change from OPEN to about 70 kω.

6 ME 105 Mechanical Engineering Lab Page 6 Figure 4: Pocket Logger Display Screen Sensor Chan. Range Typical Values Calibration Factors Notes Start / Stop / Mark M Ohm "OPEN" when recording switch is off ~ 60 kohm when recording switch is on ~ 30 kohm when mark switch is pressed n/a resistance values are approximate mark switch only active when recording switch is on Accelerometer % of 5 V 50% FSO when level Zero-G voltage = Verify calibration factors by tilting +/- 0.5V (10% of range) at when tilted 90 deg. Volts/G =.507 bike 90 deg. at beginning of run. Wheel rotation % of 5 V.15% FSO when magnet away from switch 95% FSO when magnet is in front of switch Supply voltage % of 5 V 0% FSO when power is off % FSO when power is on Note: FSO = full scale output 1 pulse per revolution Measured circumference of wheel n/a Replace battery if below 95% FSO Table 1 Typical Data Values

7 ME 105 Mechanical Engineering Lab Page 7 Pressing the mark switch should reduce the resistance to about 35 kω. Turning the data-recording switch to off should cause channel one to again read OPEN. Channel 4 should be indicating 98.8% of its 5V range, or 4.94V Table 1 summarizes the typical values and calibration factors for each sensor. 2.3 Downloading the experiment configuration to the XR440-M Select setup from the send menu Select load file from the Pocket Logger Setup screen and load 200 Hz data run.set Use the send button to transfer the configuration file to the data logger Verify that the screen has the same information shown in Fig. 5 Use the send button to transfer the configuration file to the data logger Connect the data logger to the mating serial cable on the computer The recorder is now ready and will now begin recording data as soon as the datarecording switch is turned on. Note that data rate is set to 200 Hz for the experiment. Figure 5: Pocket Logger Display Screen

8 ME 105 Mechanical Engineering Lab Page Prepare for riding tests With the data-recording switch OFF, take the bike to your test course. You may use the courtyard or the bike path. The test course should be straight, relatively level and have a vertical wall nearby. Once you start the data logger you should be prepared to begin the riding tests immediately. Use the tape measure to determine the circumference of the wheel. Use the 100m tape measure to determine the length of your test course. Make a chalk mark at the start and finish positions. You should practice the following procedure once before turning on the data logger Calibrate the accelerometer The accelerometer can be easily calibrated by collecting data while the bike is balance against a wall in a wheelie position. Make sure that instrument power is on and hold the bike vertical on level ground. Turn the data-recording switch on to begin collecting data. After a couple of seconds, balance the bike in a wheelie position with both wheels against a vertical wall. Hold the bike in the wheelie position for a couple of seconds and return the bike to level ground, holding vertical for a couple of seconds. Immediately prepare for the riding tests described below. 2.5 Riding tests - Position the wheel at the start position Place the front tire on the start line. Lift the wheel off of the ground and position the magnet just past the switch. The first measured pulse will be one full wheel rotation down the course. - Get on the bike and ride The rider should get in position on the bike. Push the mark button once to mark the beginning of the run. Ride the bike toward the end of the test track. - Ride to the finish line and stop Stop with the front wheel falling short of the finish line. Hold the bike still and push the mark button to mark the end of the run. Mark the distance traveled to the end of the track to measure later. - Reposition the bike at the finish line Using the finish line as a starting point, repeat the preceding steps riding back to the starting line - Turn off the data-recording switch. - Return with the bike to the lab.

ME 105 Mechanical Engineering Lab Page 9 3 Transferring data from the XR440M to the computer Data are now stored on the Pace data logger and can be downloaded back to the PC.

9 ME 105 Mechanical Engineering Lab Page 9 3 Transferring data from the XR440M to the computer Data are now stored on the Pace data logger and can be downloaded back to the PC. Connect the serial cable from the computer to the data logger. Start the Pocket Logger 32bit software if it is not already running. Select Data from the Receive pull down menu. Select a file name and path for the data. Storing the data temporarily on the hard disk and then transferring it to your X-Drive on the ECI system is probably the best approach. The computer will establish communication with the logger and transfer all of the data to the computer. This will take a couple of minutes. When the transfer is complete the software will ask whether you would like to graph the data. Select yes. The computer should prepare a graph similar to the one shown in Fig. 6 below. 4 Checking the data 4.1 Does the data look reasonable? Examine the data to make sure that it looks reasonable. Run the experiment again if there is a problem with the data. Figure 6: Sample Data Display You should check the following: Are the start and end of the data runs clearly identified? Is the level-ground value of acceleration near 50%? Does the value increase by an

10 ME 105 Mechanical Engineering Lab Page 10 amount consistent with 1G when the bike is held against the wall? Are the distance counts clear? Is the number of counts correct for the wheel circumference and run length that you measured? 4.2 Save the data file in a format that other programs can import. From the Pocket Logger Graph screen select File Export Select the ASCII (text) file option Select comma separated values. Comma separated value files (CSV) can be read easily by Matlab and Excel. Enter a file name and path. 5 Analyzing the Data 5.1 Data conversion Your raw measurements must be converted into units of acceleration and distance. Use the 1-G measurement taken prior to the bike ride to calibrate the accelerometer. Find the offset from the average of the accelerometer record. Use the wheel dimensions to calibrate the distance sensor. 5.2 Integrating the accelerometer data To calculate a velocity time series from the accelerometer data it is necessary to compute the area under the data during each sample period. The discrete nature of the data makes this quite easy. Referring to the Fig. 7 below, the area under the curve during the first sample period is simply: v 1 = a 0 *dt + (a 1 -a 0 )*dt/2 (1a) and v j+1 = v j + a j *dt + (a j+1 -a j )*dt/2 (1b) a a 1 a 2 a 0 t 0 t 0 + t t 0 +2 t t Figure 7: Numerical integration

11 ME 105 Mechanical Engineering Lab Page 11 Integrate the velocity data in a similar fashion to obtain a distance record. Experiment Report Give careful thought to a list of questions that you feel are import to the results of this lab and that define a theme for your report. Annotate your report as directed. Pay attention to sources of error in this experiment and explain why these errors arise and comment (briefly) on the relative orders of magnitude. Be sure to include uncertainty analysis as appropriate. Include in your lab report a comparison of the actual distance of the bike ride with that measured by the distance sensor and calculated from the acceleration record. Calculate the average speed of the bicycle ride. Smooth the acceleration record by taking a running mean of the data. For a three point running average, the measurement at t i is averaged with t i-1 and t i+1. You will need to average a relatively large number of measurements to smooth the acceleration record. Present this graph in your report and comment on any distinguishing characteristics you observe.

DATA ACQUISITION KIT DESCRIPTION INSTALLATION LAYOUT. Wirings connections

DATA ACQUISITION KIT DESCRIPTION EVO 3 data logger (8 or 13 channels version) Interface Junction Box Aim Infrared transmitter 12 Volts power cable for infrared transmitter Infrared receiver Wirings to