H-infinity Controlled Actuators in Automotive Active Suspension System

In this paper active suspension with linear electric motor has been designed. The active suspension is often hardly rejected for its big energy consumption. This contribution deals with direct real-time energy control in suspension system. The linear electric motor is used as an actuator. This motor can in specific situation generate the energy for next use. The H-infinity controller has been developed to improve car behavior, in particular passenger comfort and car stability. The second objective of the controller is to control energy distribution. Our submission is mainly focused on vehicle suspension, but it can be generalized to any kind of active suspension. INTRODUCTION Nowadays increased attention has been focused on suspension innovations. This paper objective is to describe active suspension design with attention to energy distribution control. In the time of growing interest of renewable energy resources the minimization of energy consumption can be small contribution to better utilization of energy resources. Especially for the car application the energy consumption play important role of the design process. MOTIVATION In this paper, active suspension with linear electric motor and H∞ controller has been designed. The biggest disadvantage in most active suspension systems is energy demands, which hardly increase together with increasing interference with system. This drawback can be eliminated in case of linear electric motor, because there is a possibility to recuperate energy during special suspension movements and consequently store this energy and use it later, when it is necessary. In the next chapters the way how to control energy distribution has been described. Naturally, any kind of suspension systems are designed to meet certain requirements. Two important requirements should be improved in suspension systems – disturbance absorbing (in case of vehicle: passenger comfort) and attenuation of disturbance transfer to floor (in case of vehicle: car handling). The first requirement can be characterized as attenuation of damped mass acceleration or as peak minimization of damped mass vertical displacement. Second requirement is characterized as attenuation of force acting on the floor or in the simple model car as attenuation of unsprung mass acceleration. Effort devoted to passive suspension design is ineffective,because there is a contradiction between both requirements. The best result (in sense of requirements improvement) can be achieved by active suspension, which means that some additional force can act on system. SUSPENSION SYNTHESIS MODEL The H-infinity controller synthesis has been done using simple basic active suspension model. Such model consists of suspension spring, damper, linear electric motor as actuator, sprung mass, unsprung mass and spring connecting unsprung mass tofloor (road). See Fig. 1. More complex model has been usedfor analysis and validation. The complex model description is behind the scope of this article and interested reader can find it in the [1]. 1 Copyright © 2008 by ASME Fig. 1. Simplified suspension model LINEAR ELECTRIC MOTOR Linear electric motor can transform electrical energy to straight motion of the rotor. In fact, linear motor works on the same basis as conventional electric motor, the only difference is that linear motor has unfolded windings to straight direction. It can be imagined as common rotary motor with diameter equal to infinity. Basic principle is shown in Fig. 2 (figure has been adapted from manufacturer spreadsheets). The same principle in larger scale is used for example in magnetic levitation trains In this paper another property of motor has been utilized. The motor is able to recuperate energy, thus motor can transform energy from straight motion to electrical energy. That means linear motor use vertical motions to produce electrical energy. The objective of controller design is to find the balance between energy demands and supply. In other words to find the trade-off between performance and energy consumption. All experiments have been done using tubulus linear electric motor TBX3810 (ThrustTube). Its important properties are: