Automotive MR Actuators State of Art

Automotive MR Actuators State of Art J. Gołdasz 1 Technical Center, BWI Group, ul. Podgórki Tynieckie 2, 30-399 Kraków, PL Abstract: Magnetically responsive fluids have been designed to undergo a transition from a fluid to a pseudo-solid material in the presence of magnetic field. So far the controllable property of magnetorheological (MR) fluids has been utilized by the automotive industry in the form of semi-active vehicle dampers and powertrain mounts. Since the introduction in 2002 in the North American market the technology has been proven on many vehicle platforms from compact sedans to sport-utility-vehicles and supercars. The most recent incarnation of this technology has revealed major progress in controllability and dynamics. In the review paper the author discusses the recent developments and provides and in-depth overview of the state-of-the-art of MR actuator technologies in several key areas related to performance range, authority and dynamics. Keywords: magnetorheological actuator, MR damper, automotive suspension, review Introduction Magnetorheological (MR) fluids are a suspension of fine, non-colloidal, low-coercivity ferromagnetic particles in a carrier fluid. The material is a representative of the class of fluids whose physical properties can be programmed to achieve particular goals. Specifically, MR fluids have shown the ability to transition from a liquid to a semi-solid with a yield stress when subjected to external magnetic fields [1]. When exposed to magnetic fields the material develops a yield stress resulting in an increased resistance-to-flow and force build-up. The individual characteristics have made them suitable for use in semi-active applications and vehicle suspension platforms in particular. By using the technology real-time benefits in vibration damping and isolation were already recognized in the 1990s [2, 3]. The inventors of the technology claimed a method of controlling the characteristics of an MR device through an on-board electronic control unit (ECU). In the 1990s the progress that has been made in the smart fluid formulation, hardware, control algorithms and power electronics has resulted in the world s first MR damper-based vehicle suspension system [2]. The MagneRide TM technology has remained the first and only high-volume produced semi-active suspension system that is based on smart fluids. In 2009 BWI Group expanded its MR product line with controlled powertrain mounts. To begin with, the MR damper system required four position sensors, yaw sensor and lateral acceleration sensor as well as the information on vehicle speed and 1 Faculty of Electrical Engineering and Computer Science, Cracow University of Technology, ul. Warszawska 24, 31-155 Kraków, Poland steering, four controlled MR dampers and an ECU (Electronic Control Unit). Connecting the system to on-board CAN network ensured the flawless exchange of information between the MR system and the others present on the car, namely, traction control, air suspension and ABS. Since the introduction on the North American Cadillac Seville STS 2002 vehicle platform the system has undergone three major series of improvements (generations). In the actuator area they were focused on improving the dynamic range, low-speed response and dynamics. The automotive MR actuators are unique devices in several aspects. Alternative semi-active vehicle suspension systems are valve-based. In other words, they require the action of continuously variable electromechanical actuators to vary the damping force level. For comparison, valveless MR dampers have the ability to influence the damping force by modifying the apparent viscosity of smart fluids exposed to magnetic fields of sufficient magnitude. From the engineering perspective, benefits over other competitive systems are numerous. The control valve is rid of moving mechanical parts (simplicity). The high range of damping forces results in a major turn-up ratio advantage in the low and medium speed regimes of damper operation. The system is fast and noiseless (no moving parts in the control valve), and it draws less than 30 W of electrical power per damper under most driving conditions. For example, MR system total power consumption on a high performance car was found to be 10-15 W (typical road conditions) and average 38 W (the Nürburgring Nordschleife loop). Moreover, damper measurements of the recent generation have shown

the response time of the system to be below 10 milliseconds, and its durability equals that which is required for the semi-active valve-based dampers. Since its introduction, significant performance improvements have been achieved due to better fluid formulations, control algorithm development and the hardware evolution. In sections that follow below the author discusses the advances in the damper hardware which gave rise to improved dynamics and performance gains. Control algorithm, sensors, and on-line processing details are beyond the scope of the review paper. MR System Performance gains and dynamics in the MR semiactive chassis system are defined by the damper device parameters and the power driver (control) unit. As such, both damper configurations incl. control valves and power drivers are described in sections that follow below. By far the most common configuration of automotive MR dampers is a single-tube gascharged long-stroke structure. The damper structures developed so far allow for applications in various vehicle chassis configuration, namely, double wishbone suspensions as well as MacPherson struts and the like. a) damper b) strut Fig. 1. Automotive MR dampers cavitation-free operation. The piston assembly incorporates flow passages for the fluid to pass through while in motion. In a typical configuration shown in Fig. 1 the piston rod is attached to the vehicle s body and the cylinder to the wheel. In a Mac Pherson strut (see Fig. 1b) the single-tube cartridge is utilized in an upside-down position. Although twin-tube structures have been developed [4], the single-tube dampers are still the configuration of choice in all implemented vehicle platforms so far. Control Valve In general, MR dampers produce low damping forces in the non-energized condition and high forces when energized. The variation range is limited by both geometrical constraints and magnetic saturation. The control valve that is located in the piston assembly as in Fig. 2 provides means for altering the MR fluid flow resistance (via magnetic field induced yield stress change) and the resulting damping force adjustment. Specifically, annular flow channel geometry, magnetic circuit characteristics incl. material magnetisation characteristics of the control valve components are modified in order to meet particular application needs. Existing control valve designs can be categorized in terms of primary (annular) and secondary (bypass) flow path geometry, coil arrangement, core details as well as performance-related criteria [5]. The performance metrics include, e.g. the dynamic range, turn-up ratio, low-speed tuning and response time. The dynamic range that is defined as the difference between the min. damping force in the off-state and the on-state force at maximum magnetic field strength (current), and is usually altered through changes in the geometry of the flow path and magnetic circuit modifications at a design stage. The so-called turn-up is the ratio of the maximum onstate force to minimum damping force at a given velocity. Next, since the first generation, engineering attention has been given to tuning low-speed performance through hardware as well as controlled means. On the hardware side, the low-speed control was initially achieved by a semi-circular bypass located inside the annular flow path [6]. Subsequent inventions included so-called thru-core bypass (see Fig. 3) and flux on-the-core features [7, 8]. Finally, the response time is the time required by the force output of the actuator to reach 90% of the steadystate force level during current rise (and fall below 10% during current decay). On the hardware side, progress in each of the important performance metrics was achieved through changes in the flow geometry of the control valve as well as redesigning of the magnetic circuit For example, the cylinder shown in Fig. 1a incorporates the piston and rod assembly separating the fluid volume into the rebound (upper) volume and the compression (lower) volume. Within the compression volume there is a floating piston that separates the fluid from a pressurized gas for ACTUATOR 2016, MESSE BREMEN 2/4

of the damper system. Key advances include higher on-state force, faster flux decay (rise) and improved flux linearity vs. applied current as shown in Fig. 4 and Fig. 5, respectively. a) single-coil piston Fig.5. Magnetic flux vs. current Fig.2. MR control valves b) dual-coil piston Other patented solutions for the automotive industry include control valves with multiple parallel flow paths, high-performance designs utilizing specific ferromagnetic materials, valves with large fluid activation area ratio, fail-safe features, asymmetric valves and the like [9-13]. Over the years significant progress has been made in sensory structures incorporating velocity-, position-, and flux-sensing mechanisms embedded into control valves and/or damper assemblies [14-16]. Power Driver Fig.3. Thru-core bypass feature: magnetic flux density distribution Fig.4.Tuning range: on-state forces As most present solenoids MR actuators are PWM (Pulse Width Modulation) driven. PWM drivers are the power supply technology that is usually exploited by the automotive industry due to low power consumption. Employing the driver in a regulated current loop as revealed in Fig. 6a clearly provided significant benefits in terms of improved response time and bandwidth [5]. The current control assumes no time delay between the current flowing in the coil circuit and the magnetic flux that is delivered to the control gap in the piston assembly. In all solenoid structures the relationship between current and flux, however, is complex incl. magnetisation characteristics, magnetic hysteresis and eddy currents. The eddy currents, for example, would slow down the flux response by creating a field that opposes the magnetic field induced by the current in the coil. Recent advances, therefore, highlighted the need for a magnetic flux driver rather than a current loop driver [16]. As shown in Fig. 6b the flux controller delivers the current driver with the commanded ACTUATOR 2016, MESSE BREMEN 3/4

value of current in order to drive the flux to the required value. Effectively, the flux-based control approach has provided a more accurate control over the damper force. By using a flux controller the influence of flux delaying factors can be minimized or excluded. As such, the system bandwidth is further augmented. This approach allows for compensation of manufacturing and material variations as well. Flux-based control approaches have resulted in a far superior control over the force output when compared to that of conventional current drivers see Figs. 7 and 8. Specifically, Fig. 7 illustrates the gap flux time history that was obtained using a flux controller. It is clear from the presented comparison that the response time has been accelerated by using the flux controller (and not the current controller). The improved dynamic behaviour of the control circuit has an immediate effect on the force output as highlighted in Fig. 8. i cmd Flux feedback ϕ cmd Current driver Flux driver Current feedback u c ic a) current control concept Current driver Current feedback u c i c b) flux control concept Fig. 6. Power drivers for MR actuators: current control vs. flux control [16] Summary The competitive automotive field has been stimulating the development of semi-active actuators based on smart (MR) fluids. Since the introduction to the challenging US market in 2002, MR actuators have undergone a rigorous development process aimed at improvements in actuator dynamics and performance range in order to keep up with recent requirements of OEMs. The purpose of the review paper was to highlight latest development in the area of automotive MR actuators and those found in controlled passenger vehicle suspension systems only. The paper reveals several promising features of the MR damper system incl. actuation, damping force range, and response time. F d F d Acknowledgment Unless stated otherwise, all figures are included here courtesy of BWI Group. Also, the author would like to thank Mr Guy Tessier, Engineering Manager, BWI Group (France), for providing the experimental data and other valuable information in the paper. ϕ, [mwb] i c, [A] 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0-0.1 Flux Response w. Flux Control Flux Response w. Current Control flux control current control residual flux -0.2 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 t, [s] 10 8 6 4 2 0-2 -4 a) measured gap flux Current Overdrive generated by Flux Control flux control current control Reverse Current generated by Flux Control -6 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 t, [s] b) measured coil current Fig. 7. Example: current control vs. flux control Fig. 8. Example: current decay flux controller vs. current controller ACTUATOR 2016, MESSE BREMEN 4/4

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