Linear electromagnetic actuation system for active vehicle suspension

Lead Research Organisation: University of Sheffield
Department Name: Electronic and Electrical Engineering

Abstract

The basic function of a car suspension is to support the weight of the vehicle, maximise the friction between the tyres and the road surface, provide steering stability with good handling, and ensure the comfort of the passengers. The dynamics of a moving car are generally considered from two perspectives, viz. ride and handling, three important issues being vibration isolation, road holding and cornering. The car suspension system attempts to solve the challenges unique to each, by (i) absorbing energy while travelling over rough roads and dissipating it without causing undue oscillation of the vehicle, (ii) maintaining the wheel geometry to maximise tyre contact with the road, (iii) reacting the weight of the car during cornering, so as to minimise body roll. Although car suspensions have evolved and improved over the years, the three fundamental components remain springs, dampers (shock absorbers) and anti-roll bars. In essence, the springs absorb the oscillatory motion of the wheels; the shock absorbers control unwanted spring motion by damping vibratory motions, the kinetic energy of the suspension movement being converted into heat energy which is dissipated by hydraulic fluid; and the anti-roll bars provide additional stability, combatting the roll of a car on its suspension as it corners, by resisting the vertical movement of one wheel relative to the other, which results in a more level ride. There are, of course, numerous variations and different configurations of suspension, and a car usually has a different design on the front and back. However, whilst suspension systems are a fundamental element of any vehicle and may appear to be relatively simple, designing and implementing them to balance passenger comfort with handling is a complex task. Soft suspensions provide a smooth ride, but result in body roll or pitch during braking, acceleration and cornering, whilst stiff suspensions minimise body motion and allow cars to be driven more aggressively, albeit at the expense of ride quality. To overcome the limitations of conventional suspension systems, over the years, various alternative suspension technologies have been developed. For example, hydrostatic, hydrogas, hydropneumatic and hydraulic - an innovation which has previously been exploited in motorsport. However, these also have their limitations and/or are too expensive for production cars. Recent advances in linear electromagnetic machines, facilitated by advances in magnetic materials, power electronics and digital control systems, may, however, make it possible to introduce a totally new suspension technology. This is the subject of the proposed research, which envisages using a single linear motor at each wheel in place of the conventional shock absorber and spring system. The main benefit of employing linear motors is that they can move much faster than conventional fluid-based damper suspension systems, and can, therefore, respond quickly enough to virtually eliminate all movement and vibration of the body of a car under all driving and road conditions, and to counter body roll by automatically stiffening the suspension when cornering, thereby giving the driver a greater sense of control and, hence, improving safety. The research programme will address the design optimisation of force-dense, energy-efficient linear electrical motors and the associated mathematical algorithms which will be necessary to provide the required active control of the suspension system. The utility of the developed suspension technology will be demonstrated on a quarter car rig, and the resulting vehicle performance improvements will also be quantified by simulations over the full range of ride, handling and stability.

Publications

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Jiabin Wang (2008) Design Considerations for Tubular Flux-Switching Permanent Magnet Machines in IEEE Transactions on Magnetics

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Tuplin S (2008) Proc. International Symposium on Advanced Vehicle Control (AVEC2008) in Design and control of a linear electromagnetic actuation system for active vehicle suspensions

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Wang J (2008) A tubular flux-switching permanent magnet machine in Journal of Applied Physics

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Wang J (2009) Proc. 7th International Symposium on Linear Drives for Industry Applications in A Linear Electromagnetic Actuation System for Active Vehicle Suspensions

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Wang J (2011) A Linear Permanent-Magnet Motor for Active Vehicle Suspension in IEEE Transactions on Vehicular Technology

 
Description Active suspension systems have long been a desired goal of automotive companies. They offer the potential for consummate refinement of vehicle ride without compromising the handling and load carrying capacity. Previous incarnations of systems included hydro-pneumatic and hydro-gas units, capable of offering the necessary control options, but were expensive to maintain and run with consumers unlikely to pay 15% extra at the pumps for a smoother ride.
The development of a Linear Electromagnetic Actuator (LEA) for suspension control offers the improvement in vehicle ride without the associated power consumption. On a typical drive cycle the mean demand is 50W per unit; comparable to an air conditioning unit. The so called 'Green' credentials of such a unit have also been simulated, demonstrating that when operated in a damping only mode it can harvest the previously unrecovered energy from the suspension system. This, with due consideration to efficiencies, equates to a 1.4 gCO2/km reduction for a typical vehicle on a country drive cycle. Another of the main advantages of an LEA system is that it can combine several suspension control systems into one unit. Currently automotive companies use separate systems to control ride, roll and ride-height. This adds complexity, weight and expense. The design realised here incorporates ride and roll control into a single unit, with height adjustment easily added through an internal ball screw at the top mount of the integral coil spring.
Simulation of the system has demonstrated that a significant improvement of ride control can be achieved using a Linear Quadratic Gaussian (LQG) regulator. This regulator combines a state feedback gain (obtained using a Linear Quadratic Regulator) and a Kalman estimator to provide a tuneable optimal control for the suspension. The improvement in ride, characterised by the decrease of body acceleration, is of the order of 32%. Roll control is implemented using open loop control with the measured lateral acceleration of the vehicle and the known roll stiffness. This allows any level of roll elimination to be implemented within the force limits of the motor. Simulation suggests that the unit developed is capable of delivering 100% elimination at 4.60 vehicle roll angle for a single steer event, and 82.5% roll reduction on a country road. Similar strategies are easily implemented for squat/dive control.
A 42V LEA unit has been designed, built and incorporated successfully into an active suspension unit. A novel configuration has been adopted, with the passive load carrying coil spring mounted within the motors moving core. This allowed the spatial constraints imposed (Jaguar XJ air spring dimensions) to be met. Initial static testing revealed a 6% reduction in the force capability of the unit compared to the simulated prediction. This equates to a peak force of 3.5KN at 152A. Following this the active unit was installed on a quarter vehicle rig, where the mass and length of the rig are equivalent to the quarter car's mass and track of a jaguar XJ. Initial results from the dynamics tests demonstrated that the motor, and low level control, are capable of replicating a passive suspension system.
Exploitation Route An active LEA unit has been successfully designed and implemented, providing a realistic opportunity for mass production of an active suspension system for road vehicles. With the advent of Hybrid technology and power electronics it is hoped that this cutting edge technology will become a reality on customer vehicles. Further development of the unit is necessary to bring it to a production implementable state; namely upgrading to a 300V system with liquid cooling, replicating the standard voltage on hybrid vehicles. This will facilitate a 50% reduction in both mass and volume, without increasing the power demands or heat generation of the unit.
Sectors Transport

 
Description No. The recent price increases in rare-earth magnets lead to relatively high cost for commercial exploitations.
Sector Transport
Impact Types Societal,Economic

 
Description Jaguar Cars Ltd 
Organisation Jaguar Land Rover Automotive PLC
Department Jaguar Land Rover
Country United Kingdom 
Sector Private 
Start Year 2007