The ride comfort vs. handling compromise for off-road vehicles

This thesis examines the classic ride comfort vs. handling compromise when designing a vehicle suspension system. A controllable suspension system, that can, through the use of suitable control algorithms, eliminate this compromise, is proposed and implemented. It is a well known fact that if a vehicle suspension system is designed for best ride comfort, then handling performance will suffer and vice versa. This is especially true for the class of vehicle that need to perform well both on- and off-road such as Sports Utility Vehicles (SUV’s) and wheeled military vehicles. These vehicles form the focus of this investigation. The ride comfort and handling of a Land Rover Defender 110 Sports Utility Vehicle is investigated using mathematical modelling and field tests. The full vehicle, non-linear mathematical model, built in MSC ADAMS software, is verified against test data, with favourable correlation between modelled and measured results. The model is subsequently modified to incorporate hydropneumatic springs and used to obtain optimised spring and damper characteristics for ride comfort and handling respectively. Ride comfort is optimised by minimising vertical acceleration when driving in a straight line over a rough, off-road terrain profile. Handling is optimised by minimising the body roll angle through a double lane change manoeuvre. It is found that these optimised results are at opposite corners of the design space, i.e. ride comfort requires a soft suspension while handling requires a stiff suspension. It is shown that the ride comfort vs. handling compromise can only be eliminated by having an active suspension system, or a controllable suspension system that can switch between a soft and a stiff spring, as well as low and high damping. This switching must occur rapidly and automatically without driver intervention. A prototype 4 State Semi-active Suspension System (4S4) is designed, manufactured, tested and modelled mathematically. This system enables switching between low and high damping, as well as between soft and stiff springs in less than 100 milliseconds. A control strategy to switch the suspension system between the “ride” mode and the “handling” mode is proposed, implemented on a test vehicle and evaluated during vehicle tests over various on- and off-road terrains and for various handling manoeuvres. The control strategy is found to be simple and cost effective to implement and works extremely well. Improvements of the order of 50% can be achieved for both ride comfort and handling. AFRIKAANS : In hierdie proefskrif word die klassieke kompromie wat getref moet word tussen ritgemak en hantering, tydens die ontwerp van ‘n voertuig suspensiestelsel ondersoek. ‘n Beheerbare suspensiestelsel, wat die kompromie kan elimineer deur gebruik te maak van toepaslike beheeralgoritmes, word voorgestel en geïmplementeer. Dit is ‘n bekende feit dat, wanneer die karakteristieke van ‘n voertuigsuspensiestelsel ontwerp word vir die beste moontlike ritgemak, die hantering nie na wense is nie, en ook omgekeerd. Dit is veral waar vir ‘n spesifieke kategorie van voertuie, soos veldvoertuie en militêre wielvoertuie, wat oor goeie ritgemak en hantering, beide op paaie en in die veld, moet beskik. Die fokus van die huidige studie val op hierdie kategorie voertuie. Die ritgemak en hantering van ‘n Land Rover Defender 110 veldvoertuig is ondersoek deur gebruik te maak van wiskundige modellering en veldtoetse. Die volvoertuig, nielineêre wiskundige model, soos ontwikkel met behulp van MSC ADAMS sagteware, is geverifieer teen eksperimentele data en goeie korrelasie is verkry. Die model is verander ten einde ‘n hidropneumatiese veer-en-demperstelsel te inkorporeer en verder gebruik om optimale veer- en demperkarakteristieke vir onderskeidelik ritgemak en hantering te verkry. Ritgemak is geoptimeer deur in ‘n reguit lyn oor ‘n rowwe veldterreinprofiel te ry, terwyl hantering geoptimeer is deur ‘n dubbelbaanveranderingsmaneuver uit te voer. Die resultaat is dat die geoptimeerde karakteristieke op die twee uiterstes van die ontwerpsgebied lê. Beste ritgemak benodig ‘n sagte suspensie terwyl beste hantering ‘n harde suspensie benodig. Daar word aangedui dat die ritgemak vs. hantering kompromie slegs elimineer kan word deur gebruik van ‘n aktiewe suspensiestelsel, of ‘n beheerbare suspensiestelsel wat kan skakel tussen ‘n sagte en stywe veer, asook hoë en lae demping. Dié oorskakeling moet vinnig en outomaties geskied sonder enige ingryping van die voertuigbestuurder. ‘n Prototipe 4 Stadium Semi-aktiewe Suspensie Stelsel (4S4) is ontwerp, vervaardig,getoets en wiskundig gemodelleer. Die stelsel skakel tussen hoë en lae demping, asook tussen ‘n stywe en sagte veer binne 100 millisekondes. ‘n Beheerstrategie wat die suspensiestelsel skakel tussen die “ritgemak” en “hantering” modes is voorgestel, op ‘n toetsvoertuig geïmplementeer en evalueer tydens voertuigtoetse oor verskeie pad- en veldry toestande, asook tydens omrol- en hanteringstoetse. Die beheerstrategie is koste-effektief en maklik om te implementeer en werk besonder goed. Verbeterings in die orde van 50% kan behaal word vir beide ritgemak en hantering.Thesis (PhD (Mechanical Engineering))--University of Pretoria, 2011.Mechanical and Aeronautical Engineeringunrestricte

Optimal vehicle suspension characteristics for increased structural fatigue life

Heavy off-road vehicle suspension systems face unique challenges. The ride comfort versus handling compromise in these vehicles has been frequently investigated using mathematical optimisation. Further challenges exist due to the large variations in vehicle sprung mass. A passive suspension system can only provide optimal isolation at a single payload. The designer of such a suspension system must therefore make a compromise between designing for a fully-laden or unladen payload state. This work deals with suspension optimisation for vehicle structural life. The paper mainly addresses two questions: (1) What are the suspension characteristics required to ensure optimal isolation of the vehicle structure from road loads? and (2) If such optimal suspension characteristics can be found, how sensitive are they to changes in vehicle payload? The study aims to answer these questions by examining a Land Rover Defender 110 as test vehicle. An experimentally validated non-linear seven degree-of-freedom mathematical model of the test vehicle is constructed for the use in sensitivity studies. Mathematical optimisation is performed using the model in order to find the suspension characteristics for optimal structural life for the vehicle under consideration. Sensitivity studies are conducted to determine the robustness of the optimal characteristics and their sensitivity to vehicle payload variation. Recommendations are made for suspension characteristic selection for optimal structural life.http://www.elsevier.com/locate/jterraai201

Longitudinal vehicle dynamics control for improved vehicle safety

The aim is to investigate the improvements in vehicle safety that can be achieved by limiting the vehicle speed based on GPS path information. The control strategy is aimed at reducing vehicle speed before a potentially dangerous situation is reached, in contrast with widely used stability control systems that only react once loss of control by the driver is imminent. An MSC.ADAMS/View simulation model of an off-road test vehicle was developed and validated experimentally. A longitudinal speed control system was developed by generating a reference speed based on the path information. This reference speed was formulated by taking into account the vehicle’s limits due to lateral acceleration, combined lateral and longitudinal acceleration and the vehicle’s performance capabilities. The model was used to evaluate the performance of the control system on various tracks. The control system was implemented on the test vehicle and the performance was evaluated by conducting field tests. Results of the field tests indicated that the control system limited the acceleration vector of the vehicle’s centre of gravity to prescribed limits, as predicted by the simulations, thereby decreasing the possibility of accidents caused by rollover or loss of directional control due to entering curves at inappropriately high speeds.http://www.elsevier.com/locate/jterrahb201

Improving the braking performance of a vehicle with ABS and a semi-active suspension system on a rough road

Rapid advances have been made in the field of vehicle dynamics in terms of improving the ride, handling and safety using actuators and control systems. Optimising a vehicle’s ride comfort or handling has led to the development of semi-active suspension systems. Anti-lock braking systems (ABS) have resulted in significant improvements in vehicle braking whilst maintaining directional control over the vehicle. These advances have improved vehicle and occupant safety in general, but there are often some trade-offs. For example, the stopping distance of a vehicle fitted with ABS on an undulating road is significantly increased compared to braking without ABS. This has severe implications, especially in the off-road vehicle industry. The effects of spring and damper characteristics on the braking performance of a sports-utility-vehicle (SUV) on hard rough terrain are investigated. The approach is simulation based, using an experimentally validated full vehicle model of the SUV, built in Adams in co-simulation with MATLAB and Simulink. The simulations were performed on measured road profiles of a Belgian paving and parallel corrugations (or a washboard road). The results indicate that the suspension system has a significant impact on the braking performance, resulting in differences in stopping distances of up to 9 m.The National Research Foundation (DAAD-NRF)http://www.elsevier.com/locate/jterrahj201

Digital image correlation techniques for measuring tyre-road interface parameters : Part 2 - Longitudinal tyre slip ratio measurement

Measurement of tyre longitudinal slip-ratio is often estimated from three independent measurements devices namely wheel rotation speed, vehicle speed and tyre rolling radius. This produces an expensive measurement system to indirectly determine the slip-ratio. This paper presents a method by which the slip-ratio is determined from a video camera using digital image correlation techniques. The camera, mounted in such a way that the contact patch region is captured, enables the system to measure the tyre tread speed and ground speed at the contact patch. The slip-ratio is then determined from these two measurements.National Research Foundation (DAAD-NRF)http://www.elsevier.com/locate/jterra2016-10-31hb201

Digital image correlation techniques for measuring tyre-road interface parameters : Part 1 - Side-slip angle measurement on rough terrain

This paper presents inexpensive methods whereby the vehicle side-slip angle can be measured accurately at low speeds on any terrain using cameras. Most commercial side-slip angle sensor systems and estimation techniques rely on smooth terrain and high vehicle speeds, typically above 20 km/h, to provide accurate measurements. However, during certain in-situ tyre and vehicle testing on off-road conditions, the vehicle may be travelling at speeds slower than required for current sensors and estimation techniques to provide sufficiently accurate results. Terramechanics tests are typical case in point. Three algorithms capable of determining the side-slip angle from overlapping images are presented. The first is a simple fast planar method. The second is a more complex algorithm which can extract not only the side-slip angle but also its rotational velocities and scaled translational velocities. The last uses a calibrated stereo-rig to obtain all rotations and translational movement in world coordinates. The last two methods are aimed more at rough terrain applications, where the terrain induces motion components other than typical predominant yaw-plane motion. The study however found no discernible difference in measured side-slip angle of the methods. The system allows for accurate measurement at low and higher speeds depending on camera speed and lighting.National Research Foundation (DAAD-NRF).http://www.elsevier.com/locate/jterra2016-10-31hb201

Rough terrain profiling using digital image correlation

Road profiling is an important aspect of vehicle dynamics simulations especially over rough terrains. The accurate measurement of rough terrains allows for more accurate multi body simulations. Three dimensional road profiles are usually performed by utilising a line scan sensor which measures several points lateral to the road. The sensors range from simple road following wheels to LiDAR sensors. The obtained line scans are longitudinally stitched together using the orientation and position of the sensor to obtain a full three dimensional road profile. The sensor’s position and orientation therefore needs to be accurately determined in order to combine the line scans to create an accurate representation of the terrain. The sensor’s position and orientation is normally measured using an expensive inertial measurement unit or Inertial Navigation System (INS) with high sensitivity, low noise and low drift. This paper proposes a road profiling technique which utilises stereography, based on two inexpensive digital cameras, to obtain three-dimensional measurements of the road. The system negates the use of an expensive INS system to determine orientation and position. The data sets also require subsampling which can be computationally expensive. A simple subsampling routine is presented which takes advantage of the structure of the data sets to significantly speed up the process.National Research Foundation (DAAD-NRF).http://www.elsevier.com/locate/jterra2016-06-30hb201

Modelling of a semi-active hydropneumatic spring–damper unit

The mathematical modelling of a suspension unit is considered. The unit comprises a hydraulic cylinder connecting the vehicle body to the unsprung mass, two nitrogen-filled accumulator springs and two damper ports. The model takes the deflection rate as input and iteratively employs simple fluid dynamics theory to calculate the flow-rates from each accumulator to the cylinder. It calculates the pressure in the accumulators by time-integrating the flow rates to determine the gas volumes and then invoking ideal gas theory. This renders the dynamic force of the unit as output. Model predictions are compared with measurements.US Government through its European Research Office of the US Armyhttp://www.inderscience.com/jhome.php?jcode=ijvdhb201

Profiling of rough terrain

This study concentrates on obtaining profiles of rough terrain suitable for vehicle dynamics simulations in a cost-effective manner. Commercially available inertial profilometers are unable to profile the terrains of interest due to their severe roughness. A mechanical profilometer is developed and evaluated by profiling obstacles with known profiles, as well as rough 3–D test track profiles. A good correlation between the profiled and actual terrains is achieved. Realistic three–dimensional (3–D) terrain models are generated from the terrain profiles. The Displacement Spectral Densities (DSDs) of the profiled terrains are found to contain discrete peaks; a straight line fit would not be an accurate estimation for the specific rough terrains. Comparisons between the terrains defined in the International Roughness Index (IRI) and the present study indicate that the roughness index of the terrains profiled with the mechanical profilometer is significantly higher than the terrains normally profiled by inertial profilometers.http://www.inderscience.com/sample.php?id=31hb201

Vehicle suspension force and road profile prediction on undulating roads

Controllable suspension systems have the capability of changing suspension forces. One control approach is to define a cost function with the aim of optimising either ride comfort or handling. Such controllers are usually reactive and not pro-active. Controllers can benefit significantly by having a priori knowledge of the effect that changing the suspension settings will have on the suspension forces. This is especially true for vehicles traversing very rough terrain. This paper addresses the a priori knowledge needed by predicting what the suspension forces will be before changing the suspension setting. The proposed approach involves estimating sprung and unsprung mass acceleration, estimating the road excitation, and then predicting the suspension forces. A quarter car model is used to illustrate the concept. Thereafter, the concept is extended to a nonlinear multibody dynamics model and finally validated experimentally. Results indicate that the suspension force in a different suspension modes can be predicted before switching suspension modes.https://www.tandfonline.com/loi/nvsd202021-06-04hj2020Mechanical and Aeronautical Engineerin