Master of Science Thesis in Electrical Engineering Department of Electrical Engineering, Linköping University, 2020 Strategies for road profile in adaptive suspension control Fredrik Skoglund Master of Science Thesis in Electrical Engineering Strategies for road profile in adaptive suspension control: Fredrik Skoglund LiTH-ISY-EX--20/5317--SE Supervisor: Victor Fors isy, Linköpings universitet Anton Albinsson Volvo Cars Examiner: Jan Åslund isy, Linköpings universitet Division of Vehicular Systems Department of Electrical Engineering Linköping University SE-581 83 Linköping, Sweden Copyright © 2020 Fredrik Skoglund Abstract Semi-active suspension systems has become an increasingly popular alternative to the passive suspensions in recent time. By varying the spring stiffness and damping coefficient in the system, different characteristics can be achieved de- pending on driver preferability and road disturbances. The difference in damp- ing coefficient will however lead to a trade-off between comfort and road holding, which means that in order to improve one of the areas, performance in the other will need to be reduced. This trade-off will also be different depending on the underlying road disturbances. This thesis is conducted on behalf of Volvo Cars, who are looking for a strategy in order to analyze the trade-off for different ISO classified roads which has been done through a literature study, theoretical analysis and comparison of control methods for different roads. A skyhook based controller was built with added modifications in order to reduce the jerk in the vehicle. Tests with the controller were carried out on a quarter car model as well as a full car model in IPG Car- maker. The simulations with the quarter car model showed that the controller improved the peak jerk values as well as the comfort, compared to both skyhook control and passive suspension. The full car simulations consisted of test runs on four roads, all with different road profiles and frequency content. For each road, tests were conducted with two different damper control methods as well as with a varying spring stiffness. The tested damper control methods were force gain control, which modulates the force request from the skyhook controller, as well as rate limitation control, which modulates the rate at which current is applied to the damper. The results showed that the trade-off between road holding and comfort appeared differently when the road profile changed. All results were compared to Janeway’s comfort criterion in order to validate the comfort of the vehicle. The outcome of the thesis was a strategy that involved various tuning for the semi-active damper and air sus- pension depending on the primary and secondary ride nature of the road. More specifically, a stiff damping is preferred during general primary ride conditions and a soft damping is preferred during general secondary ride conditions. For cases when the low frequency disturbances are small, the trade-off will be small and comfort can be improved with little to no cost of road holding. When the high frequency disturbances increase in amplitude, the damper should be tuned depending on preferability. For improved road holding ability, a stiff damping is preferred and for improved comfort, a soft damping is preferred. A soft spring will, for most cases, be preferred in terms of both road holding and comfort. iii Acknowledgments I would like to begin by expressing my most sincere gratitude towards all of the employees and fellow thesis workers at Volvo Cars who has shown great support and encouragement. A special thank you to Anton Albinsson, who has provided excellent knowledge and guidance over the course of the semester. I would also like to thank my supervisor at Linköping University, Victor Fors, who has provided continuous guidance and invaluable discussions. Gothenburg, May 2020 Fredrik Skoglund v Contents 1 Introduction 1 1.1 Description . 1 1.2 Problem formulation and objective . 2 1.3 Related research . 3 1.3.1 Semi-active suspension control . 3 1.3.2 Ride comfort . 3 1.4 Outline . 4 2 Theory 5 2.1 Vehicle theory . 5 2.2 Vibrations . 6 2.2.1 Ride comfort . 6 2.2.2 Road holding . 7 2.2.3 Power spectral density analysis . 7 2.2.4 Root mean square . 8 2.2.5 Performance index . 8 2.3 Suspension system . 9 2.3.1 Linkages . 9 2.3.2 Dampers . 10 2.3.3 Springs . 12 2.4 Control methods . 14 2.4.1 Skyhook control . 14 2.4.2 Acceleration driven damper control . 15 2.4.3 Mixed Skyhook-ADD control . 15 2.5 ISO classification of road . 15 3 Controller implementation 17 3.1 Controller description . 17 3.1.1 Skyhook controller implementation . 18 3.1.2 Deadband implementation . 21 3.1.3 Rate limit implementation . 22 3.2 Continuously controlled damper model . 23 vii viii Contents 4 Quarter car model simulation 25 4.1 Quarter car model . 25 4.2 Test cases . 27 4.2.1 Low frequency response . 27 4.2.2 High frequency response . 29 4.2.3 Response of entire frequency spectra . 31 5 Full car model simulation 33 5.1 IPG Carmaker . 33 5.2 Test cases . 33 5.2.1 Road A . 35 5.2.2 Road B . 39 5.2.3 Road C . 43 5.2.4 Road D . 47 5.3 Result comparison . 51 6 Discussion 53 6.1 Conclusion . 54 6.1.1 Future scope . 55 Bibliography 57 A Road A 61 B Road B 63 C Road C 65 D Road D 67 Figures 2.1 The dynamic motions of a vehicle . 5 2.2 Transfer function from road to vertical displacement (zs) and road holding (zu zr ) for varying damping coefficients. 11 2.3 Damper working− space for active suspension (inside the blue square), not dependent on the direction of the damper velocity, and for semi-active suspension (between the red lines), dependent on the direction of the damper velocity. 12 2.4 Transfer function from road to vertical displacement (zs) and road holding (z z ) for varying spring stiffness. 13 u − r 3.1 Block diagram of the skyhook controller and added modifications together with the damper. 17 3.2 Force and torque based on the heave velocity and angular velocity of pitch and roll. 19 3.3 Modulation curve for different parameter values. 22 3.4 Plot of current over time for varying rate limitations. 23 3.5 CCD damper force as a function of damper velocity and current . 24 4.1 Quarter car model . 26 4.2 Quarter car model in Simulink . 26 4.3 RMS values of body acceleration for the low frequency response . 28 4.4 Peak jerk values for the low frequency response . 29 4.5 RMS values of body acceleration for the high frequency response . 30 4.6 Peak jerk values for the high frequency response . 30 4.7 PSD plot of body acceleration for 0.1 to 30Hz . 31 5.1 Road profile of road A . 35 5.2 PSD plot of road A for the right side of the vehicle with ISO classi- fication . 36 5.3 Trade-off between road holding and comfort with force gain set to 1 as reference . 37 5.4 Janeway’s comfort criterion for rate limit set to 1.6 . 38 ix x Figures 5.5 Road profile of road B . 39 5.6 PSD plot of road B for the right side of the vehicle with ISO classi- fication . 40 5.7 Trade-off between road holding and comfort with force gain set to 1 as reference . 41 5.8 Janeway’s comfort criterion for rate limit set to 0.5 . 42 5.9 Road profile of road C . 43 5.10 PSD plot of road C for right side of vehicle with ISO classification 44 5.11 Trade-off between road holding and comfort with force gain set to 1 as reference . 45 5.12 Janeway’s comfort criterion for rate limit set to 0.5 . 46 5.13 Road profile of road D . 47 5.14 PSD plot of primary and secondary ride parts of road D with ISO classification . 48 5.15 Trade-off between road holding and comfort with force gain set to 1 as reference for the primary ride part of road D . 50 5.16 Trade-off between road holding and comfort with force gain set to 1 as reference for the secondary ride part of road D . 51 A.1 PSD plot of body acceleration . 61 A.2 PSD plot of tire deflection for the front right wheel . 62 B.1 PSD plot of body acceleration . 63 B.2 PSD plot of tire deflection for the front right wheel . 64 C.1 PSD plot of body acceleration . 65 C.2 PSD plot of tire deflection for the front right wheel . 66 D.1 Plot of body acceleration over time . 67 D.2 Plot of tire deflection over time . 68 Tables 2.1 Road classification . 16 5.1 Comfort and road holding values for road A . 36 5.2 Comfort and road holding values for road B . 40 5.3 Comfort and road holding values for road C . 44 5.4 Comfort and road holding values for the primary ride part of road D ..................................... 49 5.5 Comfort and road holding values for the secondary ride part of road D . 49 xi xii Tables 1 Introduction This chapter consists of a problem description and problem formulation along with a literature review of related research and thesis outline. 1.1 Description Ride comfort is an important aspect in cars. To minimize the acceleration of the vehicle body, and thus increasing comfort, caused by vibrations from the road a suspension is used.