Todays vehicles rely on a number of electronic control systems. Some of them are self-contained, stand-alone controllers fulfilling a particular function while others are co-ordinated by higher-level supervisory logics. Examples of such vehicle control systems include braking control, traction control, acceleration control, lateral stability control, suspension control and so forth. Such systems aim to enhance ride and handling, safety, driving comfort and driving pleasure.
This report focuses on semi-active suspension control. The thrust of this work is to provide a comprehensive overview of theoretical and design aspects of vehicle semi-active systems based on IPG CarMaker virtual Driving Simulator Software and Test Rig.
The job of a car suspension system is to maximise the friction between the tires and the road surface, to provide steering stability with good handling and to ensure the comfort of the passengers. There are also other basic functions, which suspension system is design to perform, such as, Support vehicle Weight, Maintain correct vehicle ride height, Prevent or reduce damage to chassis from force of impacts with obstacles (including landing after jumping, in case of rally Racing) and also to maintain correct wheel alignment.
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When a tire hits a bump the wheel moves up. Without an intervening structure, all of wheel's vertical energy is transferred to the frame, causing the vehicle and its occupants to moves in the same direction. When gravity takes over, the vehicle comes back down, and the wheels can slam back into the road surface, the force of the impact with the ground would again be transferred directly to the chassis and its occupants. Depending on the size of the bump, without suspension the tires could lose contact with the ground (road), traction is lost, it would be uncomfortable for passengers, the chassis would be subjected to damaging shock loads, and directional / steering control could be lost. In order to Isolation from the forces transmitted by external excitation needs a system that will absorbs the energy of the vertical accelerated wheel, allowing the frame and body to ride undisturbed while the wheels follows bumps in the road.
Suspensions are employed in mobile applications, such as terrain vehicles, or in non-mobile applications, such as vibrating machinery or civil structures. In the case of a vehicle, a classical car suspension aims to achieve isolation from the road by means of spring-type elements and viscous dampers (shock absorbers) and contemporarily to improve road holding and handling.
The elastic element of a suspension is constituted by a spring (coil springs but also air springs and leaf springs), whereas the damping element is typically of the viscous type. In such a device the damping action is obtained by throttling a viscous fluid through orifices; depending on the physical properties of the fluid (mainly its viscosity), the geometry of the orifices and of the damper, a variety of force versus velocity characteristics can be obtained. This technology is very reliable and has been used since the beginning of the last century (Bastow, 1993).
The main purpose of this project is to investigate and design the testing, on a test rig how semi-active suspension control can improve vehicle ride from road profile.
Main objective of this project is to construct and design a quarter car model of Semi-active suspension system on a test rig and control the suspension system subject to excitation from a road profile and system using an improved sliding mode control. The proportional-integral sliding mode was chosen as a control strategy, and the road profile is estimated by using an observer design. The performance of the system will be compared with virtual computer simulation model (IPG Carmaker) and with the real life simulation using test rig.
The parameters to investigate and observed in this study is;
Detailed analysis of performance of the system using frequency analysis and impulse response characteristic
The performance of this controller will be determined by performing computer simulation using IPG CARMAKER, MATLAB, PWM circuit and real life simulation on TEST RIG.
2.1 The Role of the Suspension System
Traditionally automotive suspension designs have been a compromise between the three conflicting criteria of road holding, load carrying and passenger comfort.
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The suspension system must support the weight of the vehicle, provide directional control during handling manoeuvres and provide effective isolation of passengers or payload from road disturbances [Wright 84]. Good ride comfort requires a soft suspension, whereas insensitivity to applied loads requires stiff suspension. Good handling requires a suspension setting somewhere between the two.
Due to these conflicting demands, suspension design has had to be something of a compromise, largely determined by the type of use for which the vehicle was designed. Active suspensions are considered to be a way of increasing the freedom one has to specify independently the characteristics of load carrying, handling and ride quality.
A passive suspension system has the ability to store energy via a spring and to dissipate it via a damper. Its parameters are generally fixed, being chosen to achieve a certain level of compromise between road holding, load carrying and comfort.
An active suspension system has the ability to store, dissipate and to introduce energy to the system. It may vary its parameters depending upon operating conditions and can have knowledge other than the strut deflection the passive system is limited to.
2.2 Components of the Suspension System
Control Arm:Â A movable lever that fastens the steering knuckle to the frame of the vehicle.
Control Arm Bushing:Â This is a sleeve which allows the control arm to move up and down on the frame.
Strut Rod:Â Prevents the control arm from swinging forward and backwards.
Ball Joints:Â A joint that allows the control arm and steering knuckle to move up and down and sideways as well
Shock absorbers or Struts:Â prevents the suspension from bounce after spring compression and extension
Stabilizer Bar:Â Limits body roll of the vehicle during cornering
Spring:Â Supports the weight of the vehicleÂ
2.3 Suspension fundamental principles
2.3.1 Principles of suspension
The suspension system isolates the body from road shocks and vibrations which would otherwise be transferred to the passengers and load.
It also must keep the tires in contact with the road. When a tire hits an obstruction, there is a reaction force. The size of this reaction force depends on the unsprung mass at each wheel assembly.
The sprung mass is that part of the vehicle supported by the springs - such as the body, the frame, the engine, and associated parts.
Unsprung mass includes the components that follow the road contours, such as wheels, tires, brake assemblies, and any part of the steering and suspension not supported by the springs.
Vehicle ride and handling can be improved by keeping unsprung mass as low as possible. When large and heavy wheel assemblies encounter a bump or pothole, they experience a larger reaction force, sometimes large enough to make the tire lose contact with the road surface.
Wheel and brake units that are small, and light, follow road contours without a large effect on the rest of the vehicle.
At the same time, a suspension system must be strong enough to withstand loads imposed by vehicle mass during cornering, accelerating, braking, and uneven road surfaces.
2.3.2 High bandwidth systems
In a high bandwidth (or ``fully active'') suspension system we generally consider an actuator connected between the sprung and unsprung masses of the vehicle. A fully active system aims to control the suspension over the full bandwidth of the system. In particular this means that we aim to improve the suspension response around both the ``rattle-space'' frequency (10-12 Hz) and ``tyre-hop'' frequency (3-4Hz). The terms rattle-space and tyre-hop may be regarded as resonant frequencies of the system. A fully active system will consume a significant amount of power and will require actuators with a relatively wide bandwidth. These have been successfully implemented in Formula One cars and by, for example, Lotus [Wright 84]
2.3.3 Low bandwidth systems
Low bandwidth system is also known as slow-active or band-limited system. In this class the actuator will be placed in series with a road spring and/or a damper. A low bandwidth system aims to control the suspension over the lower frequency range, and specifically around the rattle space frequency. At higher frequencies the actuator effectively locks-up and hence the wheel-hop motion is controlled passively. With these systems we can achieve a significant reduction in body roll and pitch during manoeuvres such as cornering and braking, with lower energy consumption than a high bandwidth system.
2.3.4 Preview Systems
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These aim to increase the bandwidth of a band-limited system by using feed-forward or knowledge of future road inputs. Some systems aim to measure road disturbances ahead of the car, and then use both standard feedback control and feed-forward from the sensor to achieve a superior response.
2.4 Suspension system types
In general, based on the damper used vehicle suspension systems can be classified into three types. They are
2. Semi Active
3. Active suspension systems.
Each suspension system has its own advantages and disadvantages. However, in practical applications semi-active and active models are most commonly chosen over passive system. The details of the three types are described in the following sections.
2.4.1 Passive suspension system
Passive suspension system has damper as the energy dissipating element, and an energy-storing element as the spring. The two elements can only decapitate energy and they cannot add energy to the system, therefore this kind of suspension systems are called passive. Figure 1.4 shows Passive suspension system.
Figure 0 Passive suspension system
2.4.2 Semi Active Suspension System
Semi- Active suspension system has been introduced in order to replace complexity and cost while improving ride and handling. In this suspension system, the passive suspension spring is retained, while the damping force in the damper can be modulated (adjusted) in accordance with operating conditions. Figure 1.3 shows the schematic view of a semi active suspension system.
The regulating of the damping force can be achieved by adjusting the orifice area in the damper, thus changing the resistance of fluid flow. Most recently the possible application of electro-rheological and magneto-rheological fluids to the development of controllable dampers has also attracted considerable interest.
2.4.2 Active Suspension System
Below Figure 1.6 shows an active suspension system, in which a force actuator is placed in parallel to passive system. In active suspension systems, sensors are used to measure the accelerations of sprung mass and unsprung mass and the analogue signals from the sensors are sent to a controller. The controller is designed to take necessary actions to improve the performance abilities already set. The controller amplifies the signals which are fed to the actuator to generate the required forces to form closed loop system (active suspension system). The performance of this system is then compared with that of the open loop system (passive suspension system).
It should be noted that an active suspension system requires external power to function, and that there is also a considerable penalty in complexity, reliability, cost and weight.
2.5 Suspension Technologies Overview
The development of the Active Shock technology ends the typical suspension compromise between high performance and ride quality in all conditions. Over the years numerous types of shock absorber technologies have been created, however most can be placed in one of three categories:
Cost Performance Graph
2.5.1 Semi Active
Control the damping actively in real time. Very low power is required from the vehicle since it is only being used for control, not to support the vehicle.Â Semi-active systems provide a huge improvement in performance for a modest increase in cost and complexity compared to passive systems. The spring can be either adaptive or passive depending on the application.
Damping rate and spring rate are fixed. Some systems provide adjustability of either damping rate or spring preload however they rely on user understanding and input and are not continuously variable or real time. Passive systems can only be optimized for a single speed and payload over a given terrain, any changes result in compromised performance.
2.5.3 Fully Active
The damping and spring forces are actively controlled in real time. Large amount of energy are required from the vehicle to achieve force and velocity targets. Fully active systems have the potential to provide a small additional increase in performance over semi-active systems; however they are much higher cost and complexity and draw significant power from the vehicle.
The Active Shock technology is continuously variable real time adjustment of the damping force. This is achieved with a variable orifice and traditional hydraulic shock fluid.
Automatically adjusts damping for varying terrain in real time
Does NOT require any user adjustment or understanding of vehicle dynamics
Actively controls vehicle dynamics, balance, roll and pitch for better handling and increased safety
Can maintain optimal damping through a large range of payloads and road surfaces
Compensates for changes in temperature maintaining optimal damping at all times
Benign failure mode, if power is lost the shock simply reverts to a passive mode
Inherently stable since power is used for control purposes only
Small power draw from the vehicle
Economical to manufacture
MR Fluid Suspension
Magneto Rheological Fluid dampers have a fixed orifice and vary the viscosity of the fluid to change the damping. MR fluid contains iron particles suspended in oil which align themselves when the fluid is subjected to a magnetic field, effectively changing the viscosity. The magnetic field is typically applied though an electrical winding in the damper piston.
Limited dynamic range, less than 1/2 damping range of Active Shock
Fluid has a limited life and thickens as it breaks down. Typically it has a shorter life than standard hydraulic fluid in passenger car applications and even less in off-road vehicles.
Loss of power leaves the shock in the minimum damping setting resulting in bottoming and poor suspension control
Temperature sensitive, large change in damping rate when temperature varies
Non-standard fluid is heavy and expensive
Fully Active Suspension
Fully active suspensions have been developed in the past with limited success however the complexity and cost have been major hurdles. The incremental increase in performance between semi-active and fully active is minimal however the increases in cost, complexity and power required are very large.
High cost, 5-10 times the cost of semi-active
High complexity & weight
Very poor failure modes
Require excessive amounts of power, up to 50 HP for HMWWV systems
Potential disconnect between drivers feeling for the road and the road surface
Potential for instability since large amounts of energy are being injected into the system. Sensor failure or inaccuracy can result in uncontrolled suspension movement
2.6 System Modelling
Generally for analysis of vehicle suspension systems various types of vehicle suspension models were taken by the researchers. The most commonly used models are
1. Quarter car model
2. Half car model
3. Full car model
The details of the three types are discussed below;
2.6.1 Quarter car model
On this particular model, only quarter of the vehicle is taken into consideration to develop a vehicle suspension low level controller. Model is a two dimensional model because only movement on z direction is taken into consideration. The general representation of a quarter car model is shown in figure 1.1. It basically consists of a single wheel which is represented in the form of a spring. However in some cases the wheel can be considered as equivalent to a parallel combination of spring and a damper. The actual shock absorber is assumed to support only one fourth of the weight of the total car body mass including passenger's weight.
The advantages of these types of models are that they are simple and easy to analyze mathematical relations involved in the model.
2.6.2 Half car model
Unlike quarter car model where only one wheel is analyzed half car model considers two wheels, viz. one front and one rear wheel. In this type of models half of the weights of the entire car including that of passengers are considered for analysis purpose. The main advantages of this type of models are
Vehicle pitch motions can be simulated.
Front and rear dampers and spring characteristics can be modelled differently which is also different on the actual vehicle.
3. Body motions and centre of gravity effect can be simulated.
Fig 1.2 shows the general representation of half car model. As shown in the figure both the front wheel and back wheel supports half of the car weight by means of separate dampers attached to them. Model is again a two dimensional model which has movement only on Z direction.
2.6.3 Full Car model
In full car model, total weights of the car body as well as the passengers are considered and four wheels were taken for analysis. The pitch and roll motions of the car were also taken into consideration. Model is a three dimensional model which has a movement on Z direction.
On full-car model
Vehicle pitch and roll motions can be simulated.
Different front and rear suspension system geometries can be modelled (for example, vehicle has independent suspension on the front and a solid dead beam on the rear and this can be geometrically modelled on a full car model).
Left and right hand side body and tire motions of the vehicle can be simulated separately.
Un-sprung and sprung mass motions can be evaluated for both front & rear right and the left hand side suspension systems.
A full-car model is based on the four identical quarter-car models, which are coupled together by solid rods with respect to pitch and roll moment of inertia. Then braking, accelerating and steering influences should be reflected, i.e. longitudinal and lateral acceleration are considered. Therefore vehicle body roll and pitch, which cause the centre of gravity movements and this is an important attribute for car stability during driving through the curves.