Concept Of Moving Centre of Gravity for Improved Directional Stability for Automobiles - Simulation

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1 IJIRST International Journal for Innovative Research in Science & Technology Volume 2 Issue 11 April 2016 ISSN (online): Concept Of Moving Centre of Gravity for Improved Directional Stability for Automobiles - Simulation Joseph Sebastian UG Student Department of Mechanical Engineering Saintgits College of Engineering Subin Antony Jose UG Student Department of Mechanical Engineering Saintgits College of Engineering Siyad S UG Student Department of Mechanical Engineering Saintgits College of Engineering Ton Devasia UG Student Department of Mechanical Engineering Saintgits College of Engineering Prof. Sajan Thomas Professor Department of Mechanical Engineering Saintgits College of Engineering Abstract This paper is a study based on the implementation of a new concept in AUTOMOBILE which will help in improving the directional stability and handling characteristics of vehicle. The purpose of this study is to analyze the influence of position of CENTRE OF GRAVITY of a vehicle in its stability in accordance with YAW MOTION, ROLL MOTION, UNDER STEER and OVER STEER. The concept is to implement a mechanism which can bring SHIFTING OF C.G in automobile, as per the conditions. The influence of position of C.G is much bigger for the balancing of forces in dynamic stability of the vehicle. The concept of moving C.G will help to acquire this added stability to the vehicle even in the worst conditions.the directional stability of a vehicle is influenced by the steering angle and slip angle of the tire to an extent. It is possible to have a variation in these values by the shifting of CG. The designing of a convenient mechanism which helps in achieving the movement of the mass (either by pumping a high density fluid or my movement of a solid block by mechanical linkage) is to be done and have to be tested in a real time vehicle. Before proceeding to the practical level of the concept, validation of the idea based on theoretical aspect has to be done. For this, a dynamic simulation is to be done with the software MATLAB. Keywords: Shifting of CG, directional stability, handling characteristics Background I. INTRODUCTION The development achieved so far in the field of automobile industry is remarkable. Among the various factors such as performance, fuel efficiency, stability etc, the vehicle stability is having the highest importance in the current scenario. There are a lot of factors that affect the stability of a vehicle, like aerodynamics, geometry, mass specific, moment specific, tire specific, roadway specific, driving techniques and so on. In the present situation, the Centre of mass, which have a significant effect in the stability of the vehicle, is considered to be fixed. Slight variation in the position of CG may arise due to the change in load because of the weight of passengers. A shift in the position of CG could in effect control the handling characteristics of a vehicle. Properties such as understeer, oversteer, and neutral steer are explaining the stability of a vehicle during its dynamics. These mentioned properties are found to have direct relationship with the position of CG, tyre properties and slip angle. Hence a variation in the position of CG could in effect bring a controlled vehicle movement in any maneuver and at any speed. Idea Generation and Screening Centre of gravity can be assumed as a point where whole of the mass of the body may be assumed to be concentrated. By the implementation of the Concept of Moving Centre of Gravity in vehicle, we are trying to improve the directional stability and the handling characteristics of the vehicle. There are a lot of forces acting on a vehicle during its motion. The force balancing is the phenomenon which brings adequate stability during driving. Handling characteristics of a road vehicle are concerned with its response to steering commands and to environmental inputs affecting the direction of motion of the vehicle such as wind and road disturbances. There are two basic problems in vehicle handling: one is the control of the vehicle to a desired path, the other is the All rights reserved by 556

2 stabilization of the direction of motion against external disturbances. In most of the extreme cases, the balancing of forces is done by control of the brake force. If a moving centre of gravity is possible to be implemented in a vehicle, the braking constraints can be held apart and better stability can be achieved. It is made possible by altering the slip angle and stiffness coefficient of tyre which directly involves the directional stability and are parameters that determines whether the vehicle is in oversteer, understeer or neutral steer condition. Concept Testing In this project, the effect of the C.G location in a vehicle during its dynamics is analyzed by dynamic simulation. The required hand calculations that define the effect of position of CG will be done by the available standard equations regarding vehicle handling and dynamic stability. The simulation will be carried out by using the dynamic analyzing software MATLAB, which is a powerful tool that can be effectively used for this purpose. The shift of an additional weight and its effect in bringing a change to the position of CG is analyzed. A mechanism which can impart a change in the position of C.G by the movement of a mass is to be designed which can be an effective way to improve the stability of vehicle by the concept of moving Centre of gravity. II. OBJECTIVE Automobile is one of the fast developing and highly advanced field in the world and automobile manufacturing giants are in an urge to capitalize the field with hi tech inventions that aids the performance and safety of the vehicle. Having a distinguish with the performance and safety of the vehicle, it is always the safety that leads in front. The main objective of this project is to introduce a new concept in automobiles which may have a great influence in the vehicle handling characteristics and directional stability. The various phenomenon such as yaw motion, roll, pitch, understeer, oversteer that defines the stability of a vehicle are in direct relation regarding the position of CG of the vehicle. As far as concerned, no recognizable studies have been conducted with a concept of a moving centre of gravity in a vehicle. This project aims in analyzing the effect of CG and the various advantages that can be accomplished by implementing a moving centre of gravity which can be controlled as the required conditions and terrain. By incorporating a real time monitoring and controlling system, the proposed method, if successful, can be implemented in the future super vehicles. Traction Control System (TCS) III. EXISTING TECHNIQUES A traction control system (TCS), is typically (but not necessarily) a secondary function of the electronic stability control (ESC) on production motor vehicles, designed to prevent loss of traction of driven road wheels. TCS is activated when throttle input and engine torque are mismatched to road surface conditions. Intervention consists of one or more of the following: Brake force applied to one or more wheels Reduction or suppression of spark sequence to one or more cylinders Reduction of fuel supply to one or more cylinders Closing the throttle, if the vehicle is fitted with drive by wire throttle In turbocharged vehicles, a boost control solenoid is actuated to reduce boost and therefore engine power. Typically, traction control systems share the electro hydraulic brake actuator (which does not use the conventional master cylinder and servo) and wheel speed sensors with ABS. Pneumatic Suspension System If we look at the different suspension systems used in motor vehicles today, the most apparent difference between them is that they are either mechanical or air suspension systems. Both types are, of course, incapable of meeting all technical requirements. If they are, however, directly compared, it soon becomes apparent that air suspension offers major benefits compared with mechanical suspension systems. As a result air suspension systems are used to an increasing extent in commercial vehicles. Benefits of Air Suspension Systems 1) By changing the bellows pressure, depending on the load carried on the vehicle, the distance between the road surface and the vehicle s superstructure addresses the same level. This means that the boarding or loading height, and the headlight settings, remains constant. 2) Spring comfort remains almost unchanged across the whole of the loading range; again this is achieved by changing the bellows pressure. The passenger on a motor coach will always perceive the same pleasant type of oscillations. Sensitive loads can thus be carried without being severely damaged. The well-known jumping of an unlade or partially laden trailer no longer occurs if an air suspension system is used. 3) The stability of the steering system and the transfer of the braking forces are improved since all wheels always have good adhesion to the road surface. 4) The pressure in the air bellows, depending on the load the vehicle carries, is ideal for use in controlling automatic load-sensitive braking. All rights reserved by 557

3 5) In the area of control for interchangeable platforms, air suspension systems are an excellent basis for cost-effective loading and unloading of containers. Tyre Pressure Monitoring System (TPMS) A tire-pressure monitoring system (TPMS) is an electronic system designed to monitor the air pressure inside the pneumatic tires on various types of vehicles. TPMS report real-time tire-pressure information to the driver of the vehicle, either via a gauge, a pictogram display, or a simple low-pressure warning light. TPMS can be divided into two different types direct (dtpms) and indirect (itpms). TPMS are provided both at an OEM (factory) level as well as an aftermarket solution. The target of a TPMS is avoiding traffic accidents, poor fuel economy, and increased tire wear due to under-inflated tires through early recognition of a hazardous state of the tires. Due to the influence tire pressure has on vehicle safety and efficiency, tire-pressure monitoring (TPM) was first adopted by the European market as an optional feature for luxury passenger vehicles in the 1980s Indirect Tpms Indirect TPMS do not use physical pressure sensors but measure air pressures by monitoring individual wheel rotational speeds and other signals available outside of the tire itself. First generation itpms systems are based on the principle that under-inflated tires have a slightly smaller diameter (and hence higher angular velocity) than a correctly inflated one. These differences are measurable through the wheel speed sensors of ABS/ESC systems. Second generation itpms can also detect simultaneous underinflation in up to all four tires using spectrum analysis of individual wheels, which can be realized in software using advanced signal processing techniques. The spectrum analysis is based on the principle that certain eigenforms and frequencies of the tire/wheel assembly are highly sensitive to the inflation pressure. These oscillations can hence be monitored through advanced signal processing of the wheel speed signals. Current itpms consist of software modules being integrated into the ABS/ESC units. itpms cannot measure or display absolute pressure values, they are relative by nature and have to be reset by the driver once the tires are checked and all pressures adjusted correctly. The reset is normally done either by a physical button or in a menu of the on-board computer. itpms are, compared to dtpms, more sensitive to the influences of different tires and external influences like road surfaces and driving speed or style. The reset procedure, followed by an automatic learning phase of typically 20 to 60 minutes of driving under which the itpms learns and stores the reference parameters before it becomes fully active, cancels out many, but not all of these. As itpms do not involve any additional hardware, spare parts, electronic or toxic waste as well as service whatsoever (beyond the regular reset), they are regarded as easy to handle and very customer friendly. Since factory installation of TPMS became mandatory in November 2014 for all new passenger vehicles in the EU, various itpms have been type-approved according to UN Regulation R64. Examples for this are most of the VW group models, but also numerous Volvo, Opel, Ford, Mazda, PSA, FIAT and Renault models. itpms are quickly gaining market shares in the EU and are expected to become the dominating TPMS technology in the near future. itpms are regarded as inaccurate by some due to their nature, but given that simple ambient temperature variations can lead to pressure variations of the same magnitude as the legal detection thresholds, many vehicle manufacturers and customers value the ease of use and tire/wheel change higher than the theoretical accuracy of dtpms. Direct Tpms Direct TPMS employ pressure sensors on each wheel, either internal or external. The sensors physically measure the tire pressure in each tire and report it to the vehicle's instrument cluster or a corresponding monitor. Some units also measure and alert temperatures of the tire as well. These systems can identify under-inflation in any combination, be it one tire or all, simultaneously. Although the systems vary in transmitting options, many TPMS products (both OEM and aftermarket) can display real time tire pressures at each location monitored whether the vehicle is moving or parked. There are many different solutions, but all of them have to face the problems of exposure to hostile environments. The majority are powered by batteries which limit their useful life. Some sensors utilize a wireless power system similar to that used in RFID tag reading which solves the problem of limited battery life by electromagnetic induction. This also increases the frequency of data transmission up to 40 Hz and reduces the sensor weight which can be important in motorsport applications. If the sensors are mounted on the outside of the wheel, as are some aftermarket systems, they are subject to mechanical damage, aggressive fluids, as well as theft. When mounted on the inside of the rim, they are no longer easily accessible for battery change and the RF link must overcome the attenuating effects of the tire which increases the energy need. IV. STEADY STATE CORNERING Handling is the responsiveness of vehicle to driver inputs or ease of control. Handling is a measure of the driver vehicle closed loop system. Vehicle only must be characterized as an open loop system. Vehicle response to steering input or directional response. The most common measure of open loop response is the Under steer Gradient. Under steer Gradient is only valid for steady state. Steady-state handling performance is concerned with the directional behaviour of a vehicle during a turn under non-time-varying conditions. An example of a steady-state turn is a vehicle negotiating a curve with constant radius at constant forward speed. In the analysis of steady-state handling behaviour, the inertia properties of the vehicle are not involved. When a vehicle is negotiating a turn at moderate or high, speeds, the effect of the centrifugal force acting at the center of gravity can no longer be neglected. To balance the centrifugal force the tires must develop appropriate cornering forces. A side force acting on a tire produces a side slip angle. All rights reserved by 558

4 Slip Angle (α) Concept Of Moving Centre of Gravity for Improved Directional Stability for Automobiles -Simulation Slip angle α is the angle formed between the direction of wheel travel and the line of intersection of the wheel plane with the road surface. Tire Cornering (Lateral) Stiffness (Cα ) The cornering stiffness C α is used to compare the cornering behaviour of different tires, which is defined as: The handling characteristics of the vehicle depend, to a great extent, on the relationship between the slip angles of the front and rear tires, α f and α r. Low Speed Turning At low speed (parking lot manoeuvres) tires need not develop lateral forces. Tires roll with no slip angle and the centre of turn must lie off projection of rear axle. Perpendiculars from front wheels pass through same turn centre. Fig. 1: Steady State Cornering The figure 6.1 given above shows the diagrammatic model of an automobile in steady state cornering. The steer angle for both the tyres and the turning radius R is also shown together with. High Speed Cornering High speed cornering produces different equations with respect to low speed cornering. Tires must develop significant lateral forces to counteract the lateral acceleration. Slip angles will be present at each wheel. Tire slip angle is the angle between direction of heading and direction of travel. Lateral or cornering force grows with slip angle. Cα cornering stiffness Below about 5, the slip relationship is linear. Positive slip angle produces negative force (to the left) on tire. Thus, Cα must be negative. SAE defines Cα as the negative of the slope. Cornering Stiffness Depends on Several Variables Tire size, Number of plies, Tire type (radial or bias ply), Cord angles, Wheel width, Load, Tread design, Inflation pressure. Speed not a strong influence on cornering forces produced by tire Yaw Velocity Vs Speed V. STANDARD PLOTS One of the important parameters that is used to define the vehicle handling characteristics is yaw velocity. Yaw motion is initiated along the vertical axis. The given graph (fig 7.1) shows the basic relationship between yaw velocity and speed. It shows the three cases, namely, oversteer, understeer and neutral steer. Critical speed is the maximum speed beyond which the curve shoot-up for oversteer vehicle, which indicates that the vehicle has lost its stability, and this is a highly dangerous situation. Characteristic speed for undeersteer vehicle is the speed beyond which the curve becomes flat, and it shows great effort is required at steering wheel by the driver for proper steering. All rights reserved by 559

5 Lateral Acceleration Vs Speed Fig. 2: Yaw velocity VS Speed Lateral acceleration is the force that throws car passengers sideways during a turn. Lateral acceleration is equal to the velocity squared, divided by the radius of the circle. The common unit for lateral acceleration is the g force. The acceleration created when a vehicle corners that tends to push a vehicle sideways. Because of centrifugal force, the vehicle is pushed outward. For this reason, you need to accelerate a little as you reach the apex of the curve to pull you through the curve. Lateral acceleration VS Speed is also a standard plot (shown in the fig 7.2) that indicates the handling property of vehicle. Lateral acceleration going beyond a particular value is dangerous. For oversteer vehicle, it can be seen that a sudden rise in lateral acceleration is occurring as speed increases. It has to be noted that a sufficient amount of acceleration is required for smooth handling. But in the case of understeer vehicle, the required lateral acceleration is not obtained. Fig. 3: Lateral acceleration VS Speed Curvature Response Vs Speed The Curvature response VS Speed characteristic plot is obtained by doing the constant steer angle test. In this, the steering wheel of the vehicle is locked at a particular steer angle and the speed is increased. The response of the curve is made as a plot and is shown in the fig For an understeer vehicle, the radius of the curve increases as the speed is increased. For oversteer vehicle, the radius gets reduced and it seems like the curve converges into a point. In the case of a neutral steer vehicle, the radius of the vehicle won t change even the speed is increased to a very high value. But achieving this condition is radically not possible. All rights reserved by 560

6 Fig. 4: Curvature response VS Speed VI. EXPERIMENTAL MODEL The analysis to find the effect of position of CG and the handling characteristics is to be done on a real time automobile. For that, we are choosing a vehicle with the specifications as given in the table 8.1. The vehicle with the below mentioned dimensions is given as the input to the matlab model. Initially, the analysis will be made on this particular vehicle with no change in position of CG. But, after that, what we are going to do is to add a few mass to the actual mass and analyze what effect that had brought in. Then we will shift the mass to both forward and backward direction and will observe how the understeer coefficient- which is a parameter of handling characteristics and directional stability- will get varied. Table 1 Data of Vehicle Parameters Specification Length Width Track width 54 Wheel base 58 Height of C.G 10 Kerb weight 300 kg Vehicle that is being considered is a custome single seater vehicle. The position of the driver seat is assumed to be exactly at the point of center of gravity action, so that, the addition of driver weight will not in efect affect the position of Centre of Gravity. A schematic diagram of the vehicle made in solidworks is also given in the figure 8.1. Fig. 5: Vehicle model All rights reserved by 561

7 VII. MATLAB MODEL The simulation and the dynamic analysis of the vehicle with the new parameters are done with the software, MATLAB The module used for the analysis is SIMULINK and basic inputs and models are incorporated from SIM MECHANICS module of SIMULINK. MATLAB (matrix Laboratory) is a multi-paradigm numerical computing environment and fourth-generation programming language. A proprietary programming language developed by MathWorks, MATLAB allows matrix manipulations, plotting of functions and data, implementation of algorithms, creation of user interfaces, and interfacing with programs written in other languages, including C, C++, Java, Fortran and Python. Although MATLAB is intended primarily for numerical computing, an optional toolbox uses the MuPAD symbolic engine, allowing access to symbolic computing abilities. An additional package, Simulink, adds graphical multi-domain simulation and model-based design for dynamic and embedded systems. The required plots are obtained from the software with a set of values for different positions of CG. The XY Plot and Radius VS Speed plots are the plots that are going to be generated with the software using the different inputs. Two sensors are incorporated in order to obtain the output. They are, 1) Yaw rate sensor 2) Speed sensor. The defined systems within the model are, 1) World 2) Unsprung system 3) Sprung system 4) Front wheel 1,2 5) Rear wheel 1,2 Fig. 6: the generated matlab model The figure 9.1 is the generated matlab model. As said earlier, it includes the unsprung system, sprung system, the world to mention axis system and various forces acting on it including gravitational force, and two rear and front wheels each. The various input parameters are given at this level for the need to do the analysis. Such part with parameters given and shown in block diagram are given in the figures 9.2 and 9.3. The figure 9.3 combines all the inputs given together into one single unit, so that necessary changes in the values for the parameters can be made. All rights reserved by 562

8 Fig. 7: The generated system with incorporated parameters and necessary inputs. Fig. 8: Combined car model with flow diagram prior to the simulation VIII. MAIN RESULTS Total mass = 300 kg Additional mass= 30 and 50 kg Table 2 mass shift and change in C.G Displacement(m) Effective shift in C.G(m) Effective shift in C.G(m) (30kg) (50kg) The table 10.1 shows how the position of CG is shifted when additional mass of 30 kg and 50 kg are applied to the existing system. It can be seen that, as the magnitude of mass increases, then the distance to which the CG shift occurred also increases. All rights reserved by 563

9 Table 3 Yaw velocity gain Mass added Shift of CG Time The table 10.2 is also a hand calculation done for calculating the yaw velocity gain with different CG position. The masses 30 kg and 50 kg are added. Also, the shift of CG to forward direction and backward direction are calculated for corresponding movement of extra mass. Fig. 9:Yaw velocity gain All rights reserved by 564

10 The yaw velocity gain Vs Time in seconds is plotted in the figure The values taken are 30 kg and 50 kg mass added separately and movement to forward and backward direction is considered. The graph gives a clear cut indication that, the movement of CG have influenced in varying the yaw velocity gain. Movement of mass in forward direction increases the slope of the curve, where as backward movement reduces the slope of the curve. According to the driving situation, whether a high velocity gain is required, or a low velocity gain is required, it can be achieved by the shift of the extra mass to the suitable position. Fig. 10: Understeer gradient The actual value of understeer coefficient for the considered vehicle specification is The values can be obtained from the figure Since the understeer gradient is positive, the vehicle is in understeer condition. The effect of extra mass added can be easily distinguished from the graph obtained. It can be seen that as the mass moves forward, the value of understeer coefficient gets reduced. Also, the change in value is proportional to the magnitude of added mass. As the mass is shifted to backwards, the value of understeer coefficient gets increased, which indicates that the vehicle gets further understeer property. Normally vehicle with a slight understeer is preferred, because if the vehicle is having oversteer property, by chance the speed gets passed critical speed, then sudden loss of stability occur and it may even result in drastic accidents. But, if the understeer value is high than a particular limit, then the effort that must be given by the driver on the steering wheel will be very much high. Hence, the understeer coefficient must be kept to an optimum value and, this concept, that is, shifting of CG, found to have an effect in achieving the required value of understeer coefficient. Fig. 11: XY Plot-actual condition All rights reserved by 565

11 The effect of understeer coefficient is well observed from the XY plot given in the figure This plot is obtained by conducting constant steering angle test to the vehicle. The steering wheel will be locked to a particular steer angel, and the velocity of the vehicle will be increased. For an understeer vehicle, the curve will appear to diverge, that is, the radius gets increase as shown in the graph. IX. CONCLUSION The development achieved so far in the field of automobile is remarkable and most of the giant automotive industries are coming up with new advanced techniques in order to improve the safety of the passenger. Most luxurious vehicles give their primary concern not to the performance of the vehicle, instead, how reliable the vehicle is. The concept that is being introduced in this project is to improve the handling performance and directional stability of vehicle by implementing a moving CENTRE OF GRAVITY, so that various characteristics of the vehicle during its dynamics can be controlled significantly. The steering and handling properties, that is, understeer, oversteer, yaw motion, curvature response, lateral acceleration etc are found to be controlled effectively with the introduction of this concept. With further development and more analysis and research, we can find and incorporate new innovative and promising method accompanied with this concept, such as combining TPMS together with SHIFTING OF CENTRE OF GRAVITY, a better driving condition with more confidence to the driver and also safety can be provided and hence the face of the automobile industry can be highlighted. There exist a practical difficulty in incorporating a mechanism like this, and it seems to be the biggest problems that have to be the main concern regarding this concept. The addition of extra mass will have other negative effects which are not considered in this project such as efficiency, performance etc. But incorporating all these together, it would be a heavy task to be done to arrive at a conclusion. Since the project showed a positive result in the analysis, with further studies and research in this topic, improved conditions can be achieved. ACKNOWLEDGEMENT First and foremost, we express our heartfelt gratitude to God almighty for being the guiding light throughout our project, without whose intercession this project would not have been a successful one. We thank our parents for being a guiding light and supporting me all throughout our life. We would like to extend our sincere thanks to the Principal Dr. M C Philipose and our Head of Department Dr. Sreejith C.C for rendering all the facilities and help for the successful completion of our project. We take this opportunity to express our sincere profound obligation to our guide prof. Sajan Thomas, Department of Mechanical Engineering for his helpful suggestions and overall guidance throughout this project. We are thankful to Er. Vineeth V K, Assistant Professor (Project coordinator), Er. Arun K Varghese, Assistant Professors, who gave us an opportunity to present the project successfully. We would like to extend our gratitude to our friends who have encouraged and supported me for the successful presentation of our project. REFERNCES [1] Dynamics of vehicles with high gravity centre by Dishank Bari & Ankit Wahane, University of Bielsko-Biala, Department of Mechanics, Willowa 2, Bielsko- Biala, Poland [2] A text book on Theory of Ground Vehicles by J Y WONG. [3] Fundamentals of Vehicle Dynamics by Thomas D.Gillespie SAE. [4] Numerical Effectiveness of Models and Methods of Integration of the Equations of Motion of a Car by Marek Szczotka, Szymon Tengler, and StanislawWojciech, [5] Understanding Parameters Influencing Tire Modeling by Nicholas D. Smith, Colorado State University, 2004 Formula SAE Platform All rights reserved by 566