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ABSTRACT
During the design of a suspension system, a number of conflicting requirements has to be met. The suspension setup has to ensure a comfortable ride and good cornering characteristics at the same time. Also, optimal contact between wheels and road surface is needed in various driving conditions in order to maximize safety. Instead of a passive suspension, present in most of today’s cars, an active suspension can be used in order to better resolve the trade-off between these conflicts.
Furthermore, the active suspension’s potential for improving comfort is examined using a quarter-car model in combination with the skyhook damping principle. Performing simulations with an unrestricted actuator shows that comfort can slightly be improved with little actuator action and without deteriorating road holding and suspension travel. Further improving the comfort level requires more actuator action and results in considerable degradation of road holding and suspension travel. However, improving comfort with the active suspension does not require but actually produces a small amount of energy as it functions as a skyhook damper.
. Introduction
The main task of the suspension system is to provide a comfortable and safe ride. Usually the suspension consists of passive force elements which are designed to optimize the trade-off between ride comfort, suspension travel and wheel load variations. Thanks to the suspension, the human body is protected against uncomfortable bumps in the road. By selecting the right spring and damper characteristics, the suspension functions as a filter and only passes those frequencies which are less uncomfortable for the human body. However, the characteristics should also guarantee a safe ride. Therefore, the wheel load variation should be small in order to prevent the wheels from losing contact with the road.
All this should be achieved in the available suspension travel. The only contacts between car and road are the four contact patches of the tires and it is very important to exploit these areas optimally. Therefore, usually a system of links connects the unsprung mass (wheel, brake, steering hub) to the sprung mass (car body). The geometry of these links is a tradeoff between optimal orientation of the wheels with respect to the road in case of suspension travel caused by bumps in the road and suspension travel caused by cornering. This will be explained in the next section. Another trade off comes to light when the car is cornering.
The springs should be stiff enough to avoid exaggerate rolling of the car’s body, because otherwise the suspension geometry should be able to extremely compensate for this situation. Often, an anti-roll bar is used in order to prevent exaggerate rolling. Then, the spring stiffness can decrease in order to improve comfort. However, the anti-roll bars stiffness is limited because it is undesirable to transmit vibrations due to road irregularities from one wheel to the other.
2. Suspension design conflicts:
During cornering the car’s tires produce so-called slip forces in lateral direction. These forces, displayed as horizontal arrows in Figure 1, result in an unfavorable deformation of the contact patch and a counter clockwise torque around a horizontal axis through the car’s centre of gravity.
Additional vertical reaction forces, the vertical arrows in Figure 1, counteract the torque and prevent the car from rolling over. In case of a passive suspension system, these reaction forces will cause the springs on the left side of the car to further compress and on the right side of the car to expand which causes some roll of the car’s body. Depending on the geometry of the suspension links, the orientation of the wheels with respect to the car’s body will change during suspension travel.
Active suspension system:
In this chapter, an active suspension system is considered which can prevent suspension travel under a varying load, theoretically without consuming energy. Therefore, it is extremely suitable for levelling a car during accelerating, braking and cornering, or for taking care of static load variations. Furthermore, a mathematical model of the system will be derived which can be used by a controller in order to level the car or improve the comfort level for example.
3.1 Active suspension system design:
The average force in the suspension struts differs from the static value during cornering and in order to eliminate body roll the length of the struts should remain the same. By using the principle of a lever, the varying load can be counterbalanced by a constant force at a varying distance from the fulcrum. This is visualized in Figure 3. In case the system perfectly eliminates body roll, the lever will not rotate. Because all relevant forces are perpendicular to the direction of adjustment (horizontal arrows) of the fulcrum and constant force (hollow arrow), the adjustment will not cost any
A possible application of this concept is presented in Figure 3.1. The load, coming from the wheel, is applied to the lever at the top left corner of the picture. The primary spring (represented by the upper cylinder) takes care of this initial load, caused by the vehicle’s mass. The system also contains a pretension secondary spring (represented by the lower cylinder). It is initially positioned such that it points to the fulcrum and therefore does not produce any torque with respect to the fulcrum. The triangular frame which connects all the elements represents the car’s chassis. If the load changes, caused by cornering for example, the secondary spring is repositioned such that it neutralizes the load variance. As a result, the lever does not rotate and body roll is prevented during cornering. Because the shape of the lever is a circular arc which radius is equal to the compressed length of the secondary spring, this spring is always be oriented perpendicularly to the lever and its length will not change. Repositioning the secondary spring will therefore not require any energy.
Figure 3.4
The working principle of this final concept has been used before in the Delft Active Suspension (DAS), but here the active part has been integrated in the suspension system whereas DAS is an add-on system. Initially, its goal has been to suppress the roll- and pitch motion of the vehicle in an energy efficient manner.
The system has been developed towards a working prototype. However, DAS has mainly been used for improving the ride comfort instead of maximizing lateral acceleration due to leveling the vehicle. It is remarked that isolating the vehicle from vibrations due the road irregularities requires suspension deflection and because of this energy consumption is inevitable.
Therefore the goal of implementing DAS into a vehicle differs from the goal of this report, i.e. maximizing lateral acceleration by levelling the car in an energy efficient manner. More information on DAS can be found in [Auto Technology; Suspension Systems: Optimising the Tyre Contact Patch, pp. 66-68, Edition 4, 2001 Van der Knaap ,1995].
4. Active roll control:
In this chapter, roll control in literature will be studied first. Safety systems installed on most modern cars, like ABS and ESP, are used to limit the amount of roll for vehicle rollover prevention. An active suspension system has been implemented in several state-of-the-art cars, mainly with the goal to reduce the amount of body roll. The result is a higher maximal cornering velocity and an improved directional stability especially when taking emergency evasive actions.
This increase in safety will generally come at the cost of additional consumption of energy however. The active suspension system designed in the previous chapter, together with a controller to level the car, will be used in a simulation to examine the effect of body roll elimination on the maximum lateral acceleration of the car.
4.1 Roll control in literature:
Roll control in literature mainly focuses on vehicle rollover prevention instead of complete body roll elimination. The goal of rollover prevention is to avoid this very specific type of accidents, whereas the goal of body roll elimination is to optimize the contact between tire and road surface and thus to improve the cornering characteristics of the vehicle. In general, two different situations in which rollover may arise can be distinguished. Rollover caused by extreme manoeuvring at high speeds on a plane surface, is the first case.
The second one is called tripped rollover. In this case the vehicle has already started skidding and rollover occurs because the wheels hit an obstacle or encounter soft soil. Modern vehicle dynamics control systems like the Electronic Stabilization Program (ESP) use individual wheel braking to avoid skidding and thus help to prevent tripped rollover. Their primary task is stabilization of the vehicle’s yaw motion.
Martens [2005] investigates the influence of a number of active safety systems, such as ESP, active steering, active suspension and active roll control on vehicle stability. These systems react faster, more accurate and flexible than the driver if an unexpected deviation from the vehicle’s desired yaw rate occurs. The ESP system requires little additional hardware as it makes use of the Anti-lock Braking System (ABS) which is present on most of today’s vehicles. Roll dynamics is much faster affected by steering than by braking. A steering/braking control system supports the driver in case of emergency as it allows larger obstacle avoidance manoeuvres. If steer-by-wire technology is present in the vehicle, the potential of active steering can be utilized. Vehicle stability may be improved by implementing a suspension system with active roll control which is capable of reducing sudden load transfer in roll direction due to slaloming for example. A vehicle equipped with all these systems is best capable of avoiding rollover.
4.2 Roll control in practice:
The suspension of a luxury car is usually designed with emphasis on comfort. Because of the accompanying soft suspension settings the vehicle will roll excessively during cornering, resulting in a decrease in comfort and an inferior contact between tires and road. Therefore, most high-end car manufacturers have developed a suspension system which is able to reduce or even eliminated body roll. This system can also be used to eliminate pitch in order to increase the comfort level of the car.
4.2.1 Hydraulic and pneumatic suspension systems:
It is based on the company’s traditional “Hydro pneumatic suspension”, in which a large sphere is positioned on top of each of the four suspension struts. Each sphere, see Figure 4.1, contains two compartments separated by an elastic membrane. One compartment is filled with compressed gas, the other with high-pressure fluid. Shocks, caused by road irregularities, are transmitted via the fluid and the membrane into the gas. The gas compartment functions as a spring, thus absorbing the energy and releasing it back into the fluid. Restrictions in the fluid compartment smooth the reaction and consequently provide damping.
Comfort improvement by active suspension system:
The suspension system is also responsible for providing a comfortable ride. In contrast to the previous chapter, where suspension travel is prevented in order to improve cornering characteristics, suspension travel is now required in order to isolate the passengers from uncomfortable vibrations. The active suspension system was designed such that it does not consume any energy when producing forces which prevent suspension travel.
When the system produces forces during suspension travel, in order to damp vibrations, energy is being consumed or produced however. In this chapter the results of implementing a comfort oriented performance improvement controller, frequently used in literature, will be investigated. A short analysis will be carried out using a quarter-car model with an unrestricted actuator. Hereafter, a quarter-car model will be used which includes actuator dynamics and force limitation. This model can also be used to investigate the suspension’s energy consumption.
6. Road holding improvement by active suspension system:
The suspension system has to ensure a safe ride. The wheels may not lose contact with the road surface as these four contact surfaces are the only way of transmitting forces to the road. Wheel load variations, i.e. fluctuation of the contact force between tire and road surface, should therefore be small. An active suspension may be used to minimize these variations. In this chapter, the results of implementing a simple control law for improving contact with the road will be investigated. A short analysis will be carried out using a quarter-car model with negligible actuator dynamics to investigate the influence of restrictions on the force produced by the actuator. Thereafter, a quarter-car model will be used which includes the actuator dynamics and force limitation. This model can also be used to investigate the suspension’s energy consumption.
7. Advantages of an active suspension:
In an active suspension system, the passive force elements are replaced or assisted by active force elements. These elements are able to produce a force when required and act independent of the suspension condition. Therefore, the trade-off between ride comfort, suspension travel and wheel load variations can be better resolved. Furthermore, an active suspension system can be used in order to eliminate body roll during cornering. As a consequence, the wheels can be oriented optimally with respect to the road both in case of encountering a bump and during cornering. The mentioned trade-off disappears, and also an anti-roll bar is not needed anymore.
Thanks to this system, the complicated and space consuming suspension links can be replaced with a compact and simple trailing arm suspension. The compact suspension system allows for designing a smaller and lower car without affecting its interior space. This leads to lower air drag. Also, static load variations can be taken care of. In case of an active suspension system with variable spring stiffness, this stiffness can be adjusted proportionally to the change of mass. As a consequence, the natural vertical frequency of the car’s body will not change and can be chosen at a frequency which is less uncomfortable for the human body. In case of a passive suspension system, a significant portion of the available suspension travel is used to take care of static load variations and of body roll caused by cornering. The active suspension system can take care of these variations by adjusting its stiffness. Therefore, a lower initial stiffness can be used which is favourable for the comfort level. 9
An active suspension system introduces the possibility to adjust the suspension setup to the type of driving situation and to individualize the handling characteristics and comfort level of the vehicle. The short trip to the supermarket may well be a bit bumpy, but the long boring drive to office should rather be more comfortable. Thanks to this, a possible passenger is able to serenely check his or her e-mail, schedule a meeting or glance through the minutes of yesterday’s meeting without getting carsick. Because every single person is different, everybody puts different demands on the suspension characteristics. Because the car may be used by different persons every now and then, the suspension of the car should be adjustable such that a broad spectrum of handling-, steering- and comfort characteristics can be achieved. One could think of a system which allows the driver to switch between pre-programmed suspension conditions, from soft to hard and several conditions in between, or a manually programmed condition.
Everyone can then drive the car he or she prefers. Thanks to the development of a large number of Advanced Driver Assistance Systems (ADAS), the active safety of future cars will increase tremendously. Nevertheless, the suspension should always guarantee maximal grip on the road surface to take care of unexpected hazardous situations.
8. Conclusions:
Usually the suspension consists of passive force elements which are designed to optimize the trade-off between ride comfort, suspension travel and wheel load variations. Also, the geometry design of the suspension links is a trade off between optimal orientation of the wheels in case of bumps in the road or during cornering.
Furthermore, the springs should be stiff enough to avoid exaggerate body roll or pitch during cornering, accelerating and braking. Modern suspension systems provide possibilities for optimizing the trade-offs, but will never be able to eliminate the conflicts. Moreover, they are complex and space consuming. The additional elements of an active suspension system are able to produce forces when required and therefore the trade-off between ride comfort, suspension travel and wheel load variations can be better resolved.
Furthermore, an active suspension system can be used in order to eliminate body roll during cornering. As a result, the complicated and space consuming suspension links can be replaced with a compact and simple trailing arm suspension. Also, static load variations can be taken care of by adjusting the stiffness of the suspension. It can be adjusted to the driving situation and to individualize the handling characteristics and comfort level of the vehicle. An active suspension system is investigated which can prevent suspension travel under a varying load, theoretically without consuming energy. By using the principle of a lever, the varying load can be counterbalanced by a constant force at a varying distance from the fulcrum. This principle has been used before in the Delft Active Suspension (DAS).
However, the goal of implementing DAS into a vehicle differs from the goal of this report, maximizing lateral acceleration by levelling the car in an energy efficient manner. A performance improvement controller determines the required force the actuator has to produce in order to level the car or improve the comfort level for example. An actuator controller makes sure that the required force is produced as precise as possible by using a mathematical model of the active suspension system. This controller consists of a PD-action, assisted by a correction for the estimated disturbance experienced by the actuator.