Conventional Quarter-Car Rig - Report Example

Quarter-car test Rigs The quarter-car test rig having simple test beds is seen as an alternative compared to other complex rig. The rig has a reduction in complexity as there is only focus on a single corner of the vehicle and the system may be taken as being a single-input/single output (SISO). With this set up there is reduction in computation time and level of complexity with chances of achieving closed form solution. This allows for better understanding of the problem as well as the results. But with its simplicity there are many shortcomings that come with the traditional quarter car test rig. In this rig there is simplification of the suspension components in the vehicle if not total removal. Also the simple rig usually lack the incorporation of multiple suspension designs which is a desirable thing in terms of reduction of costs. Companies such as Servotest and MTS have the knowhow of coming up with the high performance rigs but implementation of search project is found to be very expensive as well as being highly time consuming. In addressing this issues of expenses and time usually there is omission of the tire dynamics and as a replacement there is use of a simple linear spring. With the removal of the tire dynamics the analysis is made to be simple where there is reduction in the computation requirement and ease of implementation with regards to simulation of the quarter-car as hardwarein-the-loop. Even though this approach makes the understanding of the problem to be much simple the approach makes it difficult to have good correlation results obtained from the quarter-car test and the data originating from the car that is being represented. The problem comes about because of the choice made with regard to sprung and unsprung mass that are represented as linear moving objects. In case we have the bearings having highly non-linear friction, it will present a challenge when it comes to dynamics of the dynamics being represented. For suspension and tire, equivalent springs will normally be used and these are unlikely to have equivalent non-linear characteristics or the linear characteristics may not be represented in the entire range of the test operation. There is likely to be gross error in representing of the actual vehicle due to these deficiencies together with the suspension geometry being either simplified or all together eliminated. Conventional quarter-car rig The simplified quarter-car test rig is shown in figure 1where it can be observed that the suspension is represented wholly by having a set of air springs. In this rig electromagnetic mounts represent the tire while the sprung mass representation is through use of sliding carriage that carry lead weights. Figure 1: Simplified Quarter-car Test Rig In the conventional rig we have bearings being the roller wheels and Teflon bushings running in grooves that are on aluminum uprights (Goncalves, 2001). The other issue that is likely to come up is with regards to the rigidness of the load frame as there are many bolted joints and aluminium extrusion construction. In the conventional quarter-car rigs there is also the limitation not being able to introduce dynamics such as he the lateral forces and weight transfer of vehicle which come to focus when we have acceleration, cornering, braking and aerodynamic loading. There is assertion that when a car is negotiating a corner, for the front the wheel on the outer side would carry almost all the front load with the inner wheel being almost load less (Milliken,1995). This means that there are drastic changes with regards to vertical loads carried by the vehicle when the listed events come to play. There are also chances of rolling resulting to a change in geometry orientation when there is occurrence of the cornering. This in effect brings about altering of the kinematics as well as dynamics of the suspension. In conventional rigs where there is no incorporation of the actual or at least equivalent suspension geometry there is no accommodation of such dynamics. Functional Requirements that need to be addressed in conventional quarter-car rigs On reviewing the operational functions that incorporated in the improved quarter-car rigs an improved quarter-car test rig has been designed. The requirement addressed by the new improved quarter car rig are The design is such that there can be accommodation of various actual suspension hardware that can be mounted and tested and this makes it possible to include kinematics and dynamics of the suspension geometry of a vehicle. The design is able to accommodate a wider range of corner weights In the design accommodates sprung mass external forces including aerodynamic loading and weight transfer There is flexibility in the design making it possible the addition of more functionalities such as rolling, rotation and steering of the tire With these functionalities addressed , the new state-of-the art test rig comes out as being much more flexible, it allows for the car to be represented more accurately and realistically with all this being achieved with the lowest cost possible. Functional Requirements Fulfilled in the new state-of –art rig With the new quarter-car test rig adapting a modular design it has the ability of accommodating a variety of suspension design. The figure 2 shows the fully assembled new state-of art rig where we have installation of Porsche 996 suspension. With the design being such that it receives an adapter plate as opposed to a single specific suspension, the rig is not restricted to serving one suspension design and one corresponding car mass. The configuration that comes from the base plate design coupled with the single-piece design for the reaction load makes it possible to have a large freedom in the placement of tire in relation to the sprung mass. When it comes to the moving the new state of art rig is designed such that there can be addition of mass and means that vehicles with a wider range of weights may be replicated. The rig has additional functionality provisions that can be exploited when the need arises. Figure 2: New State of art quarter rig Need for improvements in the new state of art rig In the new state of art-quarter rig there can be additions of functionality that is expected to be in steps. The first of this functionality is to be achieved by increasing the force loading capability of the sprung mass. Improvement on the performance of the rig can be improved through replication of inertial and aerodynamic loading on the sprung mass. The design will be such that the sprung mass can be handle Aerotech electro-magnetic motors with one motor being placed on each side. This design is innovative bearing in mind that in most 7-post systems there is application of electro-hydraulic having force feedback. In the design we will have magnets of the motors being bolted on the load frame directly. In the new state of art quarter-cat rig we already have the sprung mass being designed in such a way that the armatures will be bolted to the mass directly. With such a combination there is application of both the desired aerodynamic and inertial load. The other improvement is thinking of demonstrating the level of flexibility in the test bed through implementation of the suspension of a totally different vehicle type. The plan is to have suspension of AM General HMMWV being replicated. The vehicle is very different having a corner weight that is in the tune of 2000 lbs above that that of Porsche and in addition the vehicle experiences vibrations of much larger amplitude is sprung mass. The other issue is thinking of a NASCAR Nextel Cup suspension being installed in the rig. This case will involve dealing with racecars having a short-long arm suspension that is not in any way comparable to that of both Porsche and HMMWV in terms of design. The rig should be such that at any given time there can be reconfiguration of the rig from testing one type of suspension to a different type of suspension in just a few hours. We can also have dynamically controllable roll degree of freedom being installed on the rig. With this additional df it will be possible to have the suspension geometry being changed through simulation of a body roll with low frequency equivalent to what would be expected when a cornering event is undertaken. A body roll will result to changes in suspension geometry and this result to replication of non-linearity that are not accounted for by majority of single post rigs. Complex Shakers The shakers can pass as some of the most complex equipment and they include the 4-post, 7-post and 8-post shakers and kinematic and compliance rigs. In a typical 4-post rig we have 4 servo actuators. In a case where we have tire-coupled test rig, each of actuator will support vehicle under each tire. In the case where there is spindle coupling there is mounting of each of the spindle directly on the actuator. This makes it possible for the test rig to input signals into the vehicle with the responses being measured. The 7-post and the 8-post have similar working where in the former there is addition of 3 more actuators while in the later 4 more actuators are to be added, the actuators are attached between ground and the sprung mass of the vehicle under test. With the additional actuators there is increased capability in terms of simulation of vehicle response from inputs such as acceleration, cornering, braking as well as aerodynamic loading. The shakers have high abilities but they have the shortcoming of being expensive in terms of building and maintenance. The rigs are highly sophisticated and are described as being muli-input/multi-output (MIMO) systems. In order to be used one should have high degree of control knowledge and understanding about the system. In these rigs there is encounter of multivariable problems which need multi-step iteration in order for suitable drive files to be obtained which in turn is used as the input commands for the actuators. Upon the extraction of the covered data from the test run on the systems, interpretation and correlation to the real world presents a lot of difficulty. What leads to this situation in dealing with the complex test rigs is the lack of literature and other relevant documentation [Laser,2006, Solderling,1999]. There are very few papers in which there is exhaustive discussion of multi-post test (Kelly, 2002; Vetturi,2002; Kowalczyk,2002 and Vaes,2003). The improvement on the complex test rigs is through ensuring that there is more information detailing how the rigs work. There is also need to develop tools that can help in the interpretation of output data. Figure 3: Image of ServoTest 7-post Test Rig References Kelly J., Kowalczyk H., Oral H. A., “Track Simulation and Vehicle Characterization with 7 Post Testing,” SAE Technical Paper Series, No. 2002-01- 3307, Warrendale, PA, 2002. Vetturi D., Magalini A., “Road Profile Excitation on a Vehicle Measurements and Indoor Testing Using a Four-post Rig,” Dipartimento di Ingengeria Meccanica – Universita degli Studi di Brescia, 2002. Dr.Christoph Leser Personal communication on 7-post test road-profile generation methods with , MTS Systems Corporation, December 5, 2006. Solderling S., Sharp M., Leser C., “Servo Controller Compensation Methods Selection of the Correct Technique for Test Applications,” SAE Technical Paper Series, No. 1999-01-3000, Warrendale, PA, 1999. Kowalczyk, H., “Damper Tuning with the Use of a Seven Post Shaker Rig,” SAE Technical Paper Series, No. 2002-01-0804, Warrendale, PA, 2002. Vaes D., Souverijns W., De Cuyper J., Swevers J., Sas P., “Optimal Decoupling for Improved Multivariable controller Design, Applied on an Automotive Vibration Test Rig,” Proceedings of the American Control Conference, v 1, p. 785-790, 2003. Goncalves F., “Dynamic Analysis of Semi-Active Control Techniques for Vehicle Applications,” Master’s Thesis, Virginia Tech, Blacksburg, VA, August 2001. Milliken W. F., Milliken D. L., Race Car Vehicle Dynamics, Society of Automotive Engineers Inc., Warrendale, PA, 1995. 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