1. Silesian University of Technology
Faculty of Transport
Department of Automotive Vehicle Construction
DSc PhD Łukasz Konieczny
PHASE ANGLE AS ADDITIONAL PARAMETER IN EVALUATION OF THE TECHNICAL CONDITION OF CAR SHOCK ABSORBER
Przestawienie tematu autora i promotora pracy
TENTH WORKSHOP ON NONSTATIONARY
SYSTEMS AND THEIR APPLICATIONS
February 5 – 9, 2017, Gródek nad Dunajcem, Poland.

2. Plan Introduction Car Suspension Subsystem
Test stand and measurement system- Shock absorber characteristic
Test stand and measurement system
Methodology of signal processing
Analysis of results
Summary

3. Introduction - Suspension
The objective of the suspension in the motor vehicle is to reduce these vertical movements.
The essential criteria defining the quality of a suspension can be listed as follows:
Suspension comfort for the passengers (Effective acceleration affecting the
passengers )
Forces affecting the load (Effective value of structure acceleration)
Wheel load fluctuations (Effective value of the dynamic wheel load), which influence the grip between tires and road (driving safety) and the transferable load on the road surface.

4. Introduction – Suspension parts
The most common spring used in passenger cars is the coil spring.
Damping elements-hydraulic shock absorbers
Arms and links - elements taking over load in certain directions

5. Introduction - Suspension
Shock absorbers serve both to guarantee driving safety of a vehicle as well as to optimise driving comfort

6. Introduction - Suspension

7. Introduction - Methods for suspension investigation

8. Introduction - Suspension

9. Introduction – technical conditions of shock absorber
Klaus Burger Maschinenbau Haldenwang 2005 CITA CONFERENCE ‘Global Perspective on Roadworthiness Enforcement’

10. Introduction – influence of technical conditions of shock absorber on safety
Fig. . Graph presents the braking distance for different dumping efficiency of shock absorber with and without system ABS from 80 km/h velocity
Sa – damping efficiency [%], S – braking distance [m]
Fig. . Graph presents the braking distance for different dumping efficiency of shock absorber with and without system ESP from 80 km/h velocity on corner
Sa – damping efficiency [%], S – braking distance [m]

11. Test stand and measurement system- Shock absorber characteristic
The indicator test stand used in researches is on the Department of Transport, Silesian University of Technology
The measurement system used in test is shown on fig. The force sensor CL 16 accuracy class of 0.5% is relative to the result of a measurement in the range of 10% to 100% range of the sensor. Displacement transducer is characterized by a basic error of 0.5% of the measuring range. The analyzer SigLab overall accuracy: ± % of full scale. Uncertainty due to the components used measurement system is therefore about 1%.

12. Shock absorber characteristic - results

13. Shock absorber characteristic - results
Force versus displacement and force versus velocity diagram for new shock absorber.

14. Shock absorber characteristic
Force versus displacement force versus velocity diagrams for shock absorber with 50% volume of oil

15. Shock absorber characteristic - Dumping characteristics for shock absorber with oil leak.
The dumping characteristic for new shock absorber is nonsymmetrical and nonlinear. For shock absorber with 70% volume of oil the dumping forces are almost the same. The greater oil leak (below 70 % volume of oil in shock absorber) causes essentially decrease of dumping force to smaller value.

16. Test stand and measurement system car service station
Eusama method, developed by The European Association of Shock Absorber Manufacturers, attempts to assess adhesive force (in %) of the wheel on the ground. Damping efficiency of the absorber is shown by Eusama factor as follows:
WE=(Qmin/Qst)x100%
where:
Q min - minimum measured dynamic tire-to-support contact force
Qst - static tire-to-support contact force (static weight)

17. Test stand and measurement system car service station
The paper presents an assessment of the technical condition of the vehicle shock absorbers built into the vehicle on the basis of the signals recorded during the vibration test. The test was performed on a test stand with kinematic harmonic forcing. The test stand can enforce vibrations in the range 0-21 Hz and amplitude of 6 mm ( typical test stand used in EUSAMA method). The shock absorbers with indicated technical condition were built into the vehicle (new shock absorber and with oil leak). The research was conducted on the twin-tube hydraulic shock with a modified structure that allows to change the amount of the fluid. The study was conducted for the shock absorber of rear suspension of passenger car Fiat Punto. Acceleration was measured using capacitive sensors of accelerations type ADXL. Acceleration was recorded on test stand platform and near the axis of the wheel and on the upper point of attachment of the rear shock absorber. Registered acceleration diagrams were subject to further treatment consisting of filtration and analysis. For selected technical states the results were compared.
Fig. View of car on the test stand and platforms of test stand

18. Test stand and measurement system
The acceleration of vibration were measured by ADXL 204 and ADXL 321 sensors. Those are modern parametric sensors built in chip. For the data acquisition the analog-digital card was used (μDAQ 30 adapted for use with PC computers support the USB interface). The main technical parameters of the μDAQ 30 are 14-bit resolution, max 250kHz sampling.
The vibration test were conducted on passenger car Fiat Punto. In the vehicle was built in front shock absorber with simulated fault (oil leak – the volume of oil was changed in the range 100% - full oil to 35% with 10% or 15% step). The construction of this shock absorber was modified in order to make oil volume variable (fig.)
Fig. Rear shock absorber after modification and metering of oil in rear shock absorber.

19. Test stand and measurement system
The recorded acceleration of vibration in chosen measure points: vertical accelerations of body, vertical accelerations of wheel and vertical accelerations of test stand platform were analyzed. View of mounted accelerometers is presented on fig.
Fig. Scheme of measurement system

20. Methodology of signal processing
During the experiment the vibration excitation was divided into 3 stages ( increase of frequency, constant frequency and decrease frequency). The time realization of acceleration of vibration of test platform, wheel and body for unfiltered (blue)
and filtered (green) signals were presented in fig.
Fig. Time realization of vertical acceleration of vibration of platform, wheel and body
(blue-not filtering, green- after filtering).

21. Methodology of signal processing
The signals of acceleration of vibration recorded on real object are noisy. Therefore the signals have been flirted by zero phase filter in Matlab software. This filter doesn’t allow for phase changing in filtering signals. Those properties are very important during phase angle analysis. It is “off-line” filter, so it must be used after recording signals. The short part time signal of acceleration before and after filtering have been presented in fig. For low frequency the amplitude of acceleration is very difficult to filter (for used measurement system) so only the frequency in range 8 [Hz] to 21 [Hz] was analysed. For this reason also the body accelerations wasn’t analyzed.
Fig. Acceleration of vibration before and after filtering

22. Methodology of signal processing
The signal was divided into half periods. In every indexing period (i) there was determined maximum value of accelerations for signals (wheel AKi, platform APi,), period of time Ti (and frequency of oscillation) and phase angle (φi) relative to excitation. For analysis of research results on the influence of different state of technical condition of shock absorber on magnitude and phase angle the methodology described above were used.
Fig. The vibration signals of excitation test stand and wheel,
after filtering and interpolation with points of sign changes and maximums

23. Analysis of results
The results have been presented in fig. The values given in the legend on right corner on figure correspond to filling of shock absorber. The phase angle was determined relative to excitation (the oscillation period of platform)
Fig. Magnitude diagram
Fig. Phase angle diagram
The results shows that changes of phase angle from the shock absorber filling are significant after crossing the 60% of oil filling. For 35-60% phase angle change in resonant frequency is steep like in low damping. For % phase angle changes are mild.

24. Analysis of results
The nondimensional magnitude is result of divided maximum value of wheel accelerations to platform accelerations in each period.
Fig. Magnitude diagram
Changes resulting from the shock absorber filling are significant after crossing the 60%of oil volume. For % is visible peak for resonance frequency of unsprung mass (above 15 Hz). The amplitude of wheel acceleration in resonance is a few times higher than the amplitude of platform acceleration. For 35-60% is not visible peak for resonance frequency and amplitude of wheel acceleration are several times higher than platform accelerations.

25. Summary
After analysis of phase and magnitude diagram it can be concluded that changes resulting from the shock absorber filling are significant after crossing the 60% of oil volume. The graphs of phase and magnitude brings additional diagnostic information about the technical state of shock absorber, and can be used by evaluation of technical condition of car shock absorber.

26. Thank you!