Chapter 4 Vehicle Testing The purpose of this chapter is to describe the field testing of the controllable dampers on a Volvo VN heavy truck. The first part of this chapter describes the test vehicle used in the damper field testing. The second part provides a background on how the dampers were mounted on the test vehicle. Next, the configuration of sensors used to control the dampers and acquire the test data, as well as the data acquisition system are described. Finally, this chapter describes the field tests that were performed along with an analysis of the results. 4.1 Test Vehicle Description The test vehicle used in this study was a Volvo VN series, heavy truck with L4 cab, as shown in Figure 4.1. Figure 4.1. Volvo VN Heavy Truck, Model 77 with the Test Trailer The test truck includes a 48 ft box trailer that was unladen for our tests. The tractor weighs 44, pounds and the legal limit on the gross vehicle weight for this vehicle is 8, pounds. 37
4.2 Damper Installation on Test Vehicle The Volvo VN heavy truck has six dampers at the primary suspension, as shown in Figure 4.2. Figure 4.2. Location of Primary Suspension Dampers on Test Vehicle Of the six dampers, four were replaced with controllable MR Dampers, as shown in Figure 4.3. Figure 4.3. Location of MR Dampers on Test Vehicle We selected to place four dampers on the truck, because we did not have enough dampers for all six locations, and our past experience had shown that we can get most of the benefits of 38
semiactive dampers by placing them on only two of the four locations at the drive axle. The four MR dampers are shown installed on the front and rear axles in Figures 4.4 and 4.5, respectively. front driver passenger Figure 4.4. MR Dampers Installed on the Front Axle rear driver passenger Figure 4.5. MR Dampers Installed on the Rear Axle The four MR dampers were wired to the system controller, which was located in the sleeper cab of the test vehicle. The wiring between the controller MR dampers was installed such that they can be used for repeated tests, while the MR dampers were installed for quick exchange with the truck s stock dampers. 39
4.3 Sensors and Data Acquisition System In order to control the dampers according to the skyhook policy described in section 2.1.4, it is necessary to sense the velocities at each end of the controllable dampers. The accelerometers used for this, shown in Figure 4.6, were manufactured by PCB Piezotronics, and had a sensitivity of 1 mv/g and were used in conjunction with a PCB 584 series 16 channel signal conditioner. Figure 4.6. PCB Accelerometers Used for Field Testing Eight accelerometers, one at each end of each damper, were used to capture the data needed to control the four dampers, as shown in Figure 4.7. Four of the accelerometers were mounted on the truck s frame rail, near the top of each of the four MR dampers and four were located on the front and rear axles, approximately below the accelerometers on the frame rail. frame rail mounted accelerometer axle mounted accelerometer Figure 4.7. Rear Passenger-Side Accelerometers 4
Additionly, three accelerometers were used to capture data for evaluating vehicle ride quality. These accelerometers were arranged in a triax configuration, as shown in Figure 4.8. Figure 4.8. Accelerometer Triax The accelerometer triax was located at the B-post, directly behind the driver, 35 inches above the cab floor, as shown in Figure 4.9, in order to measure the vibration transmissions to the cab in the vertical, lateral, and fore and aft directions. Figure 4.9. Accelerometer Triax Location The signals from the eight accelerometers located on the frame and axles of the truck were multiplexed to allow the signal to be simultaneously recorded and used in the control of the dampers. The acceleration signals were integrated by the controller to find the velocity at each 41
end of the dampers. Based on the sign of the relative velocity across each damper, the controller supplies either a zero or a three amp current to the damper, according to the on-off skyhook control policy that was discussed earlier. All eleven channels of accelerometer data were sampled at 6 Hz and recorded for the duration of the test using a sixteen channel SON D recorder model PC216Ax, shown in Figure 4.1. Figure 4.1. SON D recorder All channels of the D recorder were tested with known waveforms before and after the tests, to ensure proper functioning of the recorder channels. 4.4 Field Testing The effect of the dampers was investigated with respect to both transient and steady state dynamics. In the transient dynamic tests, the truck was driven over the speed bump shown in figure 4.11 at 6-7 mph. Figure 4.11. Speed Bump Test 42
Driving over the speed bump induced large amplitude oscillations in the test vehicle, which were then damped out by the suspension system. This test was performed repeatedly to increase the accuracy of the data, which was collected for four cases: 1. The MR dampers on the truck, operated according to the on-off skyhook control policy outlined previously. 2. The MR dampers on the truck, continuously operated in their off (zero current) state. 3. The MR dampers on the truck, continuously operated in their on (three amp current) state. 4. The original passive dampers in place (i.e., stock dampers). The above represent one semiactive case and three passive cases. The three passive cases represent hard damping (MR dampers in their on state), soft damping (MR dampers in their off state), and medium damping (stock dampers). The steady state portion of the test consisted of driving the test vehicle along a straight, level road at a sustained highway speed of 55 mph. In this case, the input to the suspensions is the road input at the tires. The steady state tests were conducted for cases 1, 2, and 4 of the transient tests. Case 3 was not performed with a steady state input as the expected performance can be extrapolated from the measured performance of cases 2 and 4. The data resulting from the field tests of the dampers consists of ten-second segments of eleven channels of data sampled at 6 Hz, resulting in approximately 66, data points for each data segment. Each data set needed to be heavily processed in order to extract the necessary information. Each of the data points in the data set is a voltage, which from the sensitivity of the accelerometers and the gains of the signal conditioner, can be converted into acceleration. Each data channel maps to an accelerometer in one of the eleven positions previously outlined. In order to discuss the separate accelerometers, it is necessary to label each of the accelerometer positions. To facilitate this, each wheel of the truck was given a letter as shown in Figure 4.12. 43
Figure 4.12. Convention As shown in Table 4.1, accelerometers are then referred to by pair of letters referring first to the wheel letter shown above, and second to either T of B corresponding to either frame-mounted or axle-mounted accelerometers, respectively. The triax accelerometers are referred to by the direction in which they are measuring. Table 4.1. Accelerometer- Channel Assignments for Field Testing Channel 1 accelerometer 2 accelerometer 3 accelerometer 4 accelerometer 5 accelerometer 6 accelerometer 7 accelerometer 8 accelerometer 9 1 11 4.5 Transient Data Analysis The transient or speed bump data was looked at in both the time and frequency domains, but the main analysis was carried out in the frequency domain. In each ten-second data set, the truck hits the speed bump with the front wheels at about two seconds into the set. The ten second data set is long enough for the vehicle oscillations to damp out by the end of the data set. 44
4.5.1 Time Domain Analysis of the Transient Data The time domain analysis of the transient data was performed with respect to both acceleration and displacement, with the first steps of the data processing being the same for both. The first steps included decimating the data by first passing it through a lowpass filter, and then resampling it at a lower frequency. The low pass filter used for the decimation was a 3 point finite impulse response (FIR) filter, shown in Figure 4.13. Figure 4.13. Frequency Response for 3 Point FIR Filter Used in Decimation This filter was chosen because of its low pass-band ripple, and steep attenuation at higher frequencies. The data was then resampled with a decimation factor of 6 (i.e., every 6 th point was used), moving the new Nyquist frequency to 5 Hz. The next step was to apply a digital filter to the decimated data in order to eliminate both high frequency noise and low frequency drift. A Chebyshev bandpass filter was created with a bandpass of 1 to 15 Hz, steep attenuation on either side of the passband, and unity magnitude within the passband. The low end of the bandpass was chosen to be 1 Hz to match the low end of the useful range of the accelerometers. Figure 4.14 shows the ideal filter in red and the actual filter used in blue. 45
1.9.8.7.6 magnitude.5.4.3.2.1 5 1 15 2 25 3 35 4 45 5 frequency (Hz) Figure 4.14. Frequency Response for Chebyshev Filter Used The filter was applied to the data in first the forward direction and then the data reversed and the filter reapplied. This eliminated phase distortion and modified the magnitude by the square of the filter magnitude. The passband magnitude of the filter used is unity with small ripples to eliminate the effect of the filter magnitude. This filtering was accomplished using the ML.m file filtfilt. The effect of applying this type of filter was experimentally verified by testing a known signal. The known test signal, shown in Figure 4.15, was a decaying 4 Hz sine wave. 2 4 Hz test signal 1.5 1.5 signal -.5-1 -1.5-2 1 2 3 4 5 6 7 8 9 1 time Figure 4.15. Test Signal Used for Validating Filters 46
The test signal was hidden by combining it with both a decaying 2 Hz sine wave and a decaying.8 Hz sine wave, as shown in Figure 4.16. 2.5 test signal "hidden" with.8 and 2 Hz signals 2 1.5 1.5 signal -.5-1 -1.5-2 -2.5 1 2 3 4 5 6 7 8 9 1 time Figure 4.16. Test Signal Hidden The combination of these three waves was put through the filter shown in Figure 4.14, using both standard filter techniques and the zero-phase forward and reverse filtering that filtfilt applies. The results of putting the hidden test signal through the filter in Figure 4.14 using both standard filtering and zero-phase forward and reverse filtering are shown in Figure 4.17. The ML.m file for this purpose is included in Appendix 1b. 47
Figure 4.17. Effect of Applied Filters to a Known Signal Though the zero phase forward and reverse filtering induces greater discrepancies at the start of the data set than standard filtering, it is more effective at preserving the transient character of the data. At this point the data processing differs depending on whether it is acceleration or displacement that is of interest. 4.5.1.1 Acceleration Data Analysis The acceleration data was mean-zeroed and plotted versus time to obtain the summary information. The summary information consists of: the global acceleration maximum during the ten second data block the local acceleration maximum immediately following the global maximum the corresponding times of the two acceleration maximums the slope of the decay between the first and second acceleration maximums the RMS acceleration for the time period of one second before the first acceleration maximum to two seconds after. 48
The ML.m file that was used to compute this information is included in Appendix 1c. A sample plot of the acceleration data for channel 1 (cab acceleration in the vertical direction) is shown in Figure 4.18 with a line connecting the first and second peaks used in the summary information. 4 filtered acceleration of channel 1 3 2 2 ) acceleration (m/s 1-1 -2-3 -4-5 1 2 3 4 5 6 7 8 9 1 Figure 4.18. Sample Plot of Acceleration Data for Channel 1 Plots of this for the all channels tested in each of the four test scenarios (MR dampers with skyhook control, MR continuously on, MR continuously off, and original dampers) are included in Appendix 2. Values of both the maximum and RMS acceleration were averaged across like data sets for each channel. There were nine data sets taken in which the test truck, equipped with MR dampers and skyhook control, was driven over the same speed bump. The average peak acceleration amplitude and average RMS acceleration for each of these nine sets of data were averaged together. The results of this are shown in Figures 4.19 and 2. 49
Average Peak Acceleration Amplitude 14 Acceleration 12 1 8 6 4 2 Figure 4.19. Acceleration Results: Average Peak Acceleration Amplitude for the Test Vehicle w/mr Dampers and Skyhook Control Policy Average RMS Acceleration 2.5 RMS Acceleration 2 1.5 1.5 Figure 4.2. Acceleration Results: Average RMS Acceleration for the Test Vehicle w/ MR Dampers and Skyhook Control Policy The test in which the truck with the MR dampers being operated continuously in the on or three amp state was driven over the speed bump was repeated five times. The peak acceleration amplitude and RMS acceleration from each of these five data sets were averaged together. Since the quantities to be compared from one test case to another are averaged across data sets, the 5
number of data sets from test to test does not need to be the same. The results for the five data sets are shown in Figures 4.21 and 22. Average Peak Acceleration Amplitude 14 12 1 Acceleration 8 6 4 2 Figure 4.21. Acceleration Results: Average Peak Acceleration Amplitude for the Test Vehicle w/mr Dampers Operated in the On State Average RMS Acceleration 2.5 2 RMS Acceleration 1.5 1.5 Figure 4.22. Acceleration Results: Average RMS Acceleration for the Test Vehicle w/mr Dampers Operated in the On State There were four data sets in which the test vehicle was driven over the same speed bump with the MR dampers on the truck and being operated continuously in the off or zero amp state. The results for the four data sets are shown in Figures 4.23 and 24. 51
Average Peak Acceleration Amplitude 1 9 8 7 Acceleration 6 5 4 3 2 1 Figure 4.23. Acceleration Results: Average Peak Acceleration Amplitude for the Test Vehicle w/mr Dampers Operated in the Off State Average RMS Acceleration 2.5 2 RMS Acceleration 1.5 1.5 Figure 4.24. Acceleration Results: Average RMS Acceleration for the Test Vehicle w/mr Dampers Operated in the Off State There were six data sets in which the test vehicle was driven over the same speed bump with the truck s original dampers in place. These data sets serve as a baseline with which to judge the 52
effectiveness of the MR dampers. The averaged maximum acceleration and averaged RMS acceleration are shown in Figures 4.25 and 26. Average Peak Acceleration 8 7 6 Acceleration 5 4 3 2 1 Figure 4.25. Acceleration Results: Average Peak Acceleration Amplitude for the Test Vehicle with Original Dampers in Place Average RMS Acceleration 2.5 2 RMS Acceleration 1.5 1.5 Channel Figure 4.26. Acceleration Results: Average RMS Acceleration for the Test Vehicle with Original Dampers in Place 53
4.5.1.2 Displacement Data Analysis After the data was decimated and filtered, there were transients at the start and end of the data that existed as artifacts of the digital filtering. This effect can be seen looking at the filter test signal shown earlier in Figure 4.16. While the data was being analyzed in terms of acceleration, this effect was unimportant, however since looking at the data in terms of displacement requires the data to be integrated twice which amplifies these errors, corrections must be made. In order to correct for this error, the value of the acceleration of the first and last one second of data was set to zero, and then the data was again mean zeroed. To integrate the data, each set was put through a 1/s integrator block (corresponding to multiplying each frequency component by 1/jw) using the ML command LSIM. The data, which is now velocity, was re-filtered using the filter shown in Figure 4.13, and again mean zeroed. Finally, the data was again integrated using an integrator block. The data, which is now displacement, was plotted and summary information extracted. The summary information consists of: the global displacement maximum during the eight second data block the local displacement maximum immediately following the global maximum the corresponding times of the two displacement maximums the slope of the decay between the first and second displacement maximums the RMS displacement for the time period going from one second before the first displacement maximum to two seconds after. The ML.m file that was used to do this is included in Appendix 1d. Figures 4.27-33 are sample plots for seven of the eleven measurement positions showing displacement versus time. These plots show a trend that the MR dampers operated continuously in their off state allow the highest levels of displacement, and the MR dampers operated continuously in their on state allow the lowest levels of displacement. Both the MR semiactive and original damper displacements tend to be between these two extremes. 54
.3 front passenger frame displacement.2 MR dampers operated with skyhook control displacement (m).1 -.1 -.2 -.3 1 2 3 4 5 6 7 8.3 front passenger frame displacement.2 Original passive dampers.1 -.1 -.2 -.3 1 2 3 4 5 6 7 8.3 front passenger frame displacement.2 MR dampers continuously off (soft).1 -.1 -.2 -.3 1 2 3 4 5 6 7 8.3 body displacement (pitch direction) MR dampers continuously on (hard).2.1 -.1 -.2 -.3 1 2 3 4 5 6 7 8 Figure 4.27. Front Passenger-Side Frame Displacement Sample Plots 55
.3 front passenger axle displacement.2 MR dampers operated with skyhook control.1 -.1 -.2 -.3 1 2 3 4 5 6 7 8.3 front passenger axle displacement Original passive dampers.2.1 -.1 -.2 -.3 1 2 3 4 5 6 7 8.3 front passenger axle displacement.2 MR dampers continuously off (soft).1 -.1 -.2 -.3 1 2 3 4 5 6 7 8.3 front passenger axle displacement.2 MR dampers continuously on (hard).1 -.1 -.2 -.3 1 2 3 4 5 6 7 8 Figure 4.28. Front Passenger-Side Axle Displacement Sample Plots 56
.3 front driver frame displacement.2 MR dampers operated with skyhook control.1 -.1 -.2 -.3 1 2 3 4 5 6 7 8.3 front driver frame displacement Original passive dampers displacement (m).2.1 -.1 -.2 -.3 1 2 3 4 5 6 7 8.3 front driver frame displacement.2 MR dampers continuously off (soft).1 -.1 -.2 -.3 1 2 3 4 5 6 7 8.3 front driver frame displacement MR dampers continuously on (hard).2.1 -.1 -.2 -.3 1 2 3 4 5 6 7 8 Figure 4.29. Rear Driver-Side Frame Displacement Sample Plots 57
.3 front driver axle displacement.2 MR dampers operated with skyhook control displacement (m).1 -.1 -.2 -.3 1 2 3 4 5 6 7 8.3 front driver axle displacement Original passive dampers displacement (m).2.1 -.1 -.2 -.3 1 2 3 4 5 6 7 8.3 front driver axle displacement.2 MR dampers continuously off (soft).1 -.1 -.2 -.3 1 2 3 4 5 6 7 8.3 front driver axle displacement MR dampers continuously on (hard).2.1 -.1 -.2 -.3 1 2 3 4 5 6 7 8 Figure 4.3. Rear Driver-Side Axle Displacement Sample Plots 58
.3 body displacement (roll direction).2 MR dampers operated with skyhook control.1 -.1 -.2 -.3 1 2 3 4 5 6 7 8.3 body displacement (roll direction) Original passive dampers.2.1 -.1 -.2 -.3 1 2 3 4 5 6 7 8.3 body displacement (roll direction).2 MR dampers continuously off (soft).1 -.1 -.2 -.3 1 2 3 4 5 6 7 8.3 body displacement (roll direction) MR dampers continuously on (hard).2.1 -.1 -.2 -.3 1 2 3 4 5 6 7 8 Figure 4.31. B-Post Roll Displacement Sample Plots 59
.3 body displacement (heave direction).2 MR dampers operated with skyhook control.1 -.1 -.2 -.3 1 2 3 4 5 6 7 8.3 body displacement (heave direction) Original passive dampers (m).2.1 -.1 -.2 -.3 1 2 3 4 5 6 7 8.3 body displacement (heave direction).2 MR dampers continuously off (soft) displacement (m).1 -.1 -.2 -.3 1 2 3 4 5 6 7 8.3 body displacement (heave direction) MR dampers continuously on (hard) displacement (m).2.1 -.1 -.2 -.3 1 2 3 4 5 6 7 8 Figure 4.3. B-Post Heave Displacement Sample Plots 6
.3 body displacement (pitch direction).2 MR dampers operated with skyhook control.1 -.1 -.2 -.3 1 2 3 4 5 6 7 8.3 body displacement (pitch direction) Original passive dampers displacement (m).2.1 -.1 -.2 -.3 1 2 3 4 5 6 7 8.3 body displacement (pitch direction).2 MR dampers continuously off (soft) displacement (m).1 -.1 -.2 -.3 1 2 3 4 5 6 7 8.3 body displacement (pitch direction) MR dampers continuously on (hard) displacement (m).2.1 -.1 -.2 -.3 1 2 3 4 5 6 7 8 Figure 4.33. B-Post Pitch Displacement Sample Plots 61
Plots of this for the all channels in each of the four test scenarios (MR dampers with skyhook control, MR continuously on, MR continuously off, and original dampers) are included in Appendix 3. Values of both the maximum and RMS displacement were averaged across like data sets for each channel. There averaged maximum peak and RMS displacement for the nine sets of data where the MR dampers were being controlled with the skyhook policy are shown in Figures 4.34 and 35. Average Peak Displacement Amplitude.35 Displacement.3.25.2.15.1.5 Figure 4.34. Displacement Results: Average Peak Displacement Amplitude for the Test Vehicle w/mr Dampers and Skyhook Control Policy 62
Average RMS Displacement.12.1 RMS Displacement.8.6.4.2 Figure 4.35. Displacement Results: Average RMS Displacement for the Test Vehicle w/mr Dampers and Skyhook Control Policy The result of averaging the peak and RMS displacement for the five data sets where the test vehicle was operated with the MR dampers continuously on are shown in Figures 4.36 and 37. Average Peak Displacement Amplitude.35.3.25 Displacement.2.15.1.5 Figure 4.36. Displacement Results: Average Peak Displacement Amplitude for the Test Vehicle w/mr Dampers Operated in the On State 63
Average RMS Displacement.12.1 RMS Displacement.8.6.4.2 Figure 4.37. Displacement Results: Average RMS Displacement for the Test Vehicle w/mr Dampers Operated in the On State The result of averaging the peak and RMS displacement for the four data sets where the test vehicle was operated with the MR dampers continuously off are shown in Figures 4.38 and 39. Average Peak Displacement Amplitude.4.35.3 Displacement.25.2.15.1.5 Figure 4.38. Displacement Results: Average Peak Displacement Amplitude for the Test Vehicle w/mr Dampers Operated in the Off State 64
Average RMS Displacement.16 RMS Displacement.14.12.1.8.6.4.2 Channel Figure 4.39. Displacement Results: Average RMS Displacement for the Test Vehicle w/mr Dampers Operated in the Off State The result of averaging the peak and RMS displacement for the six data sets where the test vehicle was operated with the original dampers in place are shown in Figures 4.4 and 41. Average Peak Displacement.35 Displacement.3.25.2.15.1.5 Figure 4.4. Displacement Results: Average Peak Displacement for the Test Vehicle with Original Dampers in Place 65
Average RMS Displacement.12.1 RMS Displacement.8.6.4.2 Figure 4.41. Displacement Results: Average Peak Displacement for the Test Vehicle with Original Dampers in Place 4.5.1.3 Results of Time Domain Analysis of Transient Tests There are two parts of the time domain analysis of the transient tests. The first part of the discussion will deal with the acceleration data and the second part will look at the results derived from the displacement data. A comparison of the average peak accelerations as measured at the eleven measurement points while the test vehicle is driven over the speed bump is shown in Figure 4.42. 66
Average Peak Acceleration Amplitude 14 12 1 8 6 4 2 MR Active MR 3A MR A Original Figure 4.42. Average Peak Acceleration Comparison A comparison of the four test cases shows that the MR dampers controlled with the skyhook control policy exhibit equal or greater levels of average peak acceleration than the original dampers on all channels. The levels of average peak acceleration were significantly higher for the MR active case than the original dampers for accelerometer positions,,, and. As these positions all represent to measurements being taken on the axles of the truck, this result was not unexpected, as the MR dampers can be softer than the stock dampers. For the measurement positions measuring frame acceleration (,,, and ), the levels of average peak acceleration were similar for the MR active case and the original dampers, with the original damper case exhibiting slightly better performance (lower acceleration). The results of the tests where the MR dampers were controlled with either zero (continuously off) or three amps (continuously on) of current tend to envelope the average peak acceleration values of the MR active case at positions measuring frame acceleration. The results of measurements made at the B-post in the y, z, and x directions (corresponding to roll, heave, and pitch respectively) show that the MR active case accentuates the acceleration seen by the cab of the vehicle. A comparison of the average RMS accelerations as measured at the eleven measurement points while the test vehicle is driven over the speed bump is shown in Figure 4.43. 67
Average RMS Acceleration 2.5 2 1.5 1.5 MR Active MR 3A MR A Original Figure 4.43. Average RMS Acceleration Comparison The comparison of the four test cases show that the MR dampers controlled with the skyhook control policy exhibit greater levels of RMS acceleration than the case with the original dampers on all channels measured on the axles of the truck. This was also found to be true for channels measured on the frame in the front of the truck. However, the levels of RMS acceleration at the frame in the rear of the truck ( and ) showed the MR active case to be better (lower levels of RMS acceleration) than the original case. The MR active case is shown to transmit less RMS acceleration to the frame of the truck than the cases where the MR dampers were either continuously on or continuously off. The results of measurements taken at the B-post in the y, z, and x directions (roll, heave, and pitch) show that the MR active case accentuates the acceleration seen by the cab of the vehicle versus the original dampers. A comparison of the average peak displacement as measured at the eleven measurement points while the test vehicle is driven over the speed bump is shown in Figure 4.44. 68
Average Peak Displacement Amplitude.4.35.3.25.2.15.1.5 MR Active MR 3A MR A Original Figure 4.44. Average Peak Displacement Comparison The comparison shows that the use of the MR dampers with the skyhook control policy increased the vertical displacement of both the axle and the frame in the front of the test vehicle (,,, and ) as compared to the stock dampers. In the rear of the truck, the application of the MR dampers with the skyhook control policy had little effect on the vertical displacement of either the axle or frame compared to the original dampers. s taken at the B-post in the y, z, and x directions (roll, heave and pitch of the cab of the truck) show increased motion with the MR dampers and skyhook control policy when compared to the original dampers. A comparison of the average RMS displacement as measured at the eleven measurement points while the test vehicle is driven over the speed bump is shown in Figure 4.45. 69
Average RMS Displacement.16.14.12.1.8.6.4.2 MR Active MR 3A MR A Original Figure 4.45. Average RMS Displacement Comparison The comparison shows that even though the MR dampers with the skyhook control policy had higher levels of peak vertical displacement of the frame, the RMS displacement of the frame was not increased. This points to higher levels of initial frame displacement as the truck passes over the speed bump, but quicker dampening of the vibration in the MR dampers and the skyhook control policy. The RMS displacement of the front axle was higher for the MR skyhook control case than it was for the original dampers. The RMS displacements of the rear of the truck, both axle and frame, were reduced in the MR damper skyhook control case. 4.5.2 Frequency Domain Analysis of the Transient Data The transient data was investigated in the frequency domain as well. The first step in this investigation mean zeroed the data. The next step involved creating an averaged fft of the acceleration data for each data set. This was done by decimating each data set twenty times. In each of the twenty sets the decimation started three elements later than the last set. This created twenty ffts of the same data, which were then averaged together frequency by frequency. The ML m file that did this is included in Appendix 1-e. These averaged ffts were then again averaged, this time across like data sets. The data sets included: 7
Eight sets of data with the MR dampers were in place and controlled according to the sky hook control policy Four data sets with the MR dampers were in place and powered continuously at three amps Four data sets with the MR dampers were in place and not powered Five data sets with the stock dampers on the truck A set of data (eleven measurement positions) for each of these four test cases is included as Appendix 3. In order to facilitate comparison between the four different test cases, the results of the frequency domain analysis were looked at in terms of average peak intensity in four frequency bands. The four frequency bands that were chosen for analysis are: 1-4 Hz 4-9 Hz 9-14 Hz 14-19 Hz. These bands were chosen based on a study by M. Ahmadian [13], who correlates these bands to different aspects of the truck dynamics. These correlations are summarized in Table 4.2. Table 4.2. Summary of Frequency/Truck Dynamics Correlations Frequency Band Truck Dynamics 1-4 Hz rigid body modes of the truck frame (heave and pitch) 4-9 Hz first bending mode of the truck frame 9-14 Hz wheel hop frequencies of the three tractor axles 14-19 Hz second bending mode of the truck frame For each of the four test cases, the average peak intensity was calculated for each of the for frequency bands. The average peak intensity was defined as the sum over the frequency band of the product of the magnitude of the acceleration at each frequency times the frequency width. The ML.m file used to calculate this information is included in Appendix 1f. This was performed for each of the eleven channels of acceleration data captured. The average acceleration peak intensity results in the 1-4 Hz frequency band, corresponding to the rigid body modes of the truck frame, is shown in Figure 4.46 for the four cases tested. 71
Average Peak Intensity (m/s^2-hz).5.45.4.35.3.25.2.15.1.5. Average Peak Intensity for Frequency Band 1-4 Hz MR Active MR 3A MR A Original Figure 4.46. Average Peak Intensity in the 1-4 Hz Frequency Band The average acceleration peak intensity results in the 4-9 Hz frequency band, corresponding to the first bending mode of the truck frame is shown in Figure 4.47 for the four cases tested..1.9.8.7 Average Peak Intensity for Frequency Band 4-9 Hz.6.5.4 MR Active MR 3A MR A Original.3.2.1. Figure 4.47. Average Peak Intensity in the 4-9 Hz Frequency Band The average acceleration peak intensity results in the 9-14 Hz frequency band, corresponding to the wheel hop frequencies of the three tractor axles is shown in Figure 4.48 for the four cases tested. 72
.7.6.5 Average Peak Intensity for Frequency Band 9-14 Hz.4.3 MR Active MR 3A MR A Original.2.1. Figure 4.48. Average Peak Intensity in the 9-14 Hz Frequency Band The average acceleration peak intensity results in the 14-19 Hz frequency band, corresponding to the second bending mode of the truck frame is shown in Figure 4.49 for the four cases tested. Average Peak Intensity (m/s^2-hz).4.35.3.25.2.15.1.5. Average Peak Intensity for Frequency Band 14-19 Hz MR Active MR 3A MR A Original Figure 4.49. Average Peak Intensity in the 14-19 Hz Frequency Band The frequency domain analysis points to the effectiveness of the MR dampers and the skyhook control policy. The average peak intensity of the measured acceleration is broken down into four frequency bands. When these results are shown in terms of percent increase versus the case where the truck was equipped with the original dampers, it is evident that there is a positive 73
effect of using the MR dampers with the skyhook control policy. A decrease in the average peak intensity is shown by a negative percent increase. The results in the four frequency bands are shown in Figure 4.5. Average Peak Intensity of MR Active Case vs. Original Case (Transient) Percent Increase 1. 8. 6. 4. 2.. -2. -4. -6. 1-4 Hz 4-9 Hz 9-14 Hz 14-19 Hz Figure 4.5. Percent Increase of Average Peak Intensity: MR Active vs. Original (Transient) In the front of the truck (,,, and ) the use of the MR dampers with the skyhook control policy significantly increased the average peak intensity of the acceleration of the axle in both the 1-4 Hz and 14-19 Hz bands. The 4-9 Hz and 9-14 Hz bands showed a decrease in the average peak intensity at these same positions. The rear of the truck (,,, and ) showed a significant reduction in the average peak intensity of the acceleration as measured on both the axle and the frame of the truck. The acceleration measured in the cab of the roll, pith and heave directions showed significant decreases in the average peak intensity in the frequency bands 1-4 Hz, 4-9 Hz, and 14-19 Hz. The 4-9 Hz results in particular point to increased operator comfort as the human body resonance typically falls in a 5-7 Hz range [12]. 74
4.6 Steady State Data Analysis The steady state data, like the transient data, was examined in both the time and frequency domains. The time domain analysis of the transient data consisted of calculating the RMS acceleration for three cases. The first of the three cases that was investigated was the truck equipped with the MR dampers, and controlled according to the sky hook control policy. The second of the three cases was the truck equipped with the MR dampers and operated with the dampers continuously in the on or three amp state. The third case was the truck operated with the original dampers in place. In each of the cases, the full ten second data block was used in the RMS acceleration calculation. The result of this analysis is shown in Figure 4.51. RMS Acceleration 2.5 2. 1.5 1..5. RMS Acceleration for Steady State Data driver side front, driver side front, passenger side front, passenger side front, MR Active MR 3A Original passenger side rear, passenger side rear, Figure 4.51. RMS Acceleration Results for Steady State Data The steady state data was analyzed in the frequency domain in the same manner as the transient data, with the exception that only three cases were investigated, the three cases being MR active, MR 3A, and original dampers respectively. Another difference between the frequency domain analysis carried out for the steady state and transient data is that in the steady state analysis multiple data sets representing the same operating condition were not investigated as was the case in the transient data analysis. A set of data (eleven measurement positions) for each of three cases (MR active, MR 3A, and original) is included as Appendix 4. The average acceleration 75
peak intensity results in the 1-4 Hz frequency band, corresponding to the rigid body modes of the truck frame, is shown in Figure 4.52 for the three cases tested. 1.2E-2 1.E-2 8.E-3 6.E-3 4.E-3 2.E-3.E+ Average Peak Intensity for Frequency Band 1-4 Hz MR Active MR 3A Original Figure 4.52. Average Peak Intensity in the 1-4 Hz Frequency Band The average acceleration peak intensity results in the 4-9 Hz frequency band, corresponding to the first bending mode of the truck frame is shown in Figure 4.53 for the three cases tested. Average Peak Intensity for Frequency Band 4-9 Hz Average Peak Intensity (m/s^2-hz) 1.E-2 9.E-3 8.E-3 7.E-3 6.E-3 5.E-3 4.E-3 3.E-3 2.E-3 1.E-3.E+ MR Active MR 3A Original Figure 4.53. Average Peak Intensity in the 4-9 Hz Frequency Band 76
The average acceleration peak intensity results in the 9-14 Hz frequency band, corresponding to the wheel hop frequencies of the three tractor axles is shown in Figure 4.54 for the three cases tested. 2.5E-2 2.E-2 Average Peak Intensity for frequency Band 9-14 Hz 1.5E-2 1.E-2 MR Active MR 3A Original 5.E-3.E+ Figure 4.54. Average Peak Intensity in the 9-14 Hz Frequency Band The average acceleration peak intensity results in the 14-19 Hz frequency band, corresponding to the second bending mode of the truck frame is shown in Figure 4.55 for the three cases tested. 77
1.4E-2 1.2E-2 1.E-2 8.E-3 6.E-3 4.E-3 2.E-3.E+ Average Peak Intensity for Frequency Band 14-19 Hz MR Active MR 3A Original Figure 4.55. Average Peak Intensity in the 14-19 Hz Frequency Band 4.6.1 Results of Time Domain Analysis of Steady State Tests In order to clearly show the effect that the MR dampers had with the skyhook control policy as compared to the original dampers, the percent increase in RMS acceleration is plotted in Figure 4.56. Since this is a percent increase, a negative number represents a decrease in the RMS value of the measured acceleration. Percent Increase 2. 1.. -1. -2. -3. -4. -5. -6. RMS Acceleration of MR Active Case vs. Original Case Figure 4.56. Percent Increase of RMS Acceleration: MR Active vs. Original (Steady State) Channels 1 and 2, corresponding to the frame of the truck in the front show that the use of the MR dampers with the skyhook control policy has reduced the RMS acceleration at these positions by close to 5% on both sides of the truck. Channels 6 and 8 show that the RMS value 78
of the acceleration as measured on the rear axle was also significantly reduced. Further, channels 9,1, and 11 show that the use of the MR dampers with the skyhook control policy on the primary suspension was effective at reducing the RMS value of the acceleration seen by the cab of the truck. 4.6.2 Results of Frequency Domain Analysis of Steady State Tests In order to show the effectiveness of the MR dampers with the skyhook control policy versus the original dampers, the percent increase in the average peak intensity was plotted. This is shown in Figure 4.57. Average Peak Intensit of MR Active Case vs. Original Case (Steady State) Percent Increase 2. 15. 1. 5.. -5. 1-4 Hz 4-9 Hz 9-14 Hz 14-19 Hz -1. Figure 4.57. Percent Increase of Average Peak Intensity: MR Active vs. Original (Steady State) At most of the measurement locations, the MR dampers with the skyhook control policy showed an increase in the average peak intensity of the measured acceleration. This points to larger amplitude accelerations with shorter duration than with the original dampers. 79