A Novel Magnetostrictive Injection Actuator Based on ... - IEEE Xplore

A Novel Magnetostrictive Injection Actuator Based on New Giant Magnetostrictive Materials L. Pîslaru-Dănescu1, A.M. Morega2,3, Senior Member, IEEE, and M. Morega2, Member, IEEE National Institute for Electrical Engineering, ICPE-CA, Microelectromechanical Department, Bucharest, ROMANIA 2 University POLITEHNICA of Bucharest, Faculty of Electrical Engineering, Bucharest, ROMANIA 3 “Gheorghe Mihoc – Caius Iacob” Institute of Statistical Mathematics and Applied Mathematics, Romanian Academy [email protected], [email protected], [email protected] 1

Abstract—Compliance with EU regulations for environmental protection sets new performance criteria to fuel injection technology in modern combustion engines for passenger and commercial vehicles. According to EURO-5 norm regulation the quality of the air/fuel mixture combustion depends mainly on an adequate spraying of the fuel, with as fine as possible droplets. The paper introduces the construction, validates the design, and evaluates the working conditions of a fuel injector prototype, activated by a magnetostrictive actuator. Prototype performance is analyzed both experimentally and by numerical simulation. The results are evaluated and compared, aiming to the optimization of the prototypes' design. Keywords: injection actuator, giant magnetostrictive material, numerical simulation, finite element analysis.

I.

INTRODUCTION

M

agnetostriction may be described as the deformation of a magnetic core in response to a change in its magnetization. Magnetostrictive materials are subject to significant dimensional change when subject to magnetic fields (Joule effect) and, reciprocally, the variation of state of stress produces a change in magnetization (Villari effect). The Joule effect makes them usable in actuation while the Villari effect is useful for sensing [1]. Magnetostrictive transduction is used in sonars, acoustic devices, active vibration and position control and fuel injection systems, to name only several applications. The magnetostrictive coefficient of a certain material (i.e. the relative deformation λ) represents the change in length as the magnetization increases from zero to its saturation value. It is usually expressed in parts per million (ppm), as the length change multiplied by a million and divided by the initial length. The relative deformation depends on parameters such as temperature T, applied external stress σ0 (pre-stress), external magnetic field strength, and mechanical load. In conventional magnetostrictive materials, e.g. iron, nickel, cobalt, λ may increase to approx. 50 ppm. The socalled “Giant magnetostrictive materials” (GMM) exhibit much higher magnetostriction. In such materials, λ may rise up to approx. 2000 ppm at room temperature. Terfenol-D1 is one of the widely known GMMs [3]. It has a saturation magnetostriction of approx. 2000 ppm, which 1

Terfenol is (Tb0.3Dy0.7Fe1.95). The name TERFENOL-D comes from the metallic elements, and the acronym of its first manufacturer – terbium (TER), iron (FE), Naval Ordinance Labs (NOL) –, and Dysprosium-D [2].

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makes it usable in applications involving uniaxial compression and high working frequency that facilitate simpler designs for devices such as the injector actuators for the internal combustion engines. Although GMM is an enabling technology for the new generation of actuators used in injection applications, magnetostrictive actuators are not used in automotive industry, especially for injection, yet. This work is concerned with reporting a GMM solution to actuation for automobile industry. According to EURO-5 norm regulation, the quality of the air/fuel mixture combustion depends mainly on an adequate spraying of the fuel, with as fine as possible droplets. Te usage of such a device is a decisive factor in reducing the pollution of the environment. Injection actuators for the automobile industry may be enabling technologies for a cleaner environment. Their usage is a decisive factor in reducing the pollution of environment due to the contemporary heat engines. In this paper we present a magnetostrictive actuator for the injection systems of high power (500 kW and beyond) internal combustion engines. Such motors are part of large generating sets for locomotive, ships and heavy machineries. In this area, GMM compete with piezoceramic materials in specific micro-actuation applications mainly because they provide for larger forces and higher relative frequency at lower voltages. GMM allows operation beyond 100 kHz while providing for precise displacements. We propose here the prototype of a fuel injector activated by a magnetostrictive actuator. A special feature of the research is the usage of the Pulse Width Modulation (PWM) principle for the construction of the electronic control system. The magnetic bias can be produced either with permanent magnets or by using electromagnetic coils. The magnetic field can be switched faster by using lower inductance coils. Numerical simulations based on the development of a fully coupled magnetoelastic finite element (FEM) model that implements the bidirectional coupling between the magnetic field and structural problems specific to the magnetostrictive injection actuator (MIA) provides for valuable information on the insights of MIA and offers great potential for enhancing and optimizing designs for sensors and actuators. II. CONSTRUCTING A MAGNETOSTRICTIVE ACTUATOR FOR FUEL INJECTION Fig. 1 provides a qualitative interpretation of the working

principle for a GMM: direct and reverse magnetostriction may be used in actuator and transducer designs. Basically, the conversion between magnetic and mechanical energies facilitates the magnetostrictive injection actuator (MIA) construction. This dual function recommends the magnetostrictive material for application in automotive mechatronics systems.

A. An Overview Of Common-Rail Injection At international level, emission regulations for thermal engines are more and more restrictive, demanding a rapid development of injection equipment with increased performances. In less than a decade, diesel engines reached the fourth generation of common-rail systems, and gasoline engines progressed from single-point injection to multipoint injection, and the latest, to direct injection. The common-rail injection system progressed along two main directions: increasing the injection pressure, from 1,350 bar (first generation) to 2,500 bar (fourth generation), and to the improvement of the injector control system so that it could control the needed number of injections per cycle and the amount of injected fuel [3], [4]. The current work is concerned with the second goal that may be reached by designing fast acting, reliable actuators (depending on engine power and speed and on the engine application) aimed at producing the new type of injection equipment for the engine manufacturers that must meet the requirements of emission regulations.

Fig. 1. A first principle interpretation of the duty cycle in actuation applications using GMM.

B. Experimental Verification Of The Magnetostrictive Injection Actuator The MIA linearity and the PWM electronic power modulator are crucial to the design validation. First, a power electronic module using pulse width modulation (PWM) was designed and built.

Fig. 2 shows the schematic diagram of the GMM magnetostrictive injection actuator of concern in this paper.

Fig. 2. Schematic diagram of “GMM” injection magnetostrictive actuator: 1 – the flex pivot; 2 – preload washer; 3 – upper housing; 4 – magnetostrictive material; 5 – electromagnetic coil; 6 – housing pivot; 7 – bottom housing; 8 – permanent magnet.

An important role in the construction of the GMM injection actuator is played by the applied external mechanical stress (pre-stress), and by the bias magnetic field. Magnetic bias may be provided either by a permanent magnet or a second coil [1]. However, the electromagnetic solution allows for easily changeable magnetic operating point. A Hall sensor may be used to measure the variation of magnetic flux in the magnetostrictive material. For instance, the AD 22151 Hall effect all purpose sensor can be integrated into the casing of the magnetostrictive actuator.

Fig. 3. The MIA command duty cycle, KU [%], as function of the input voltage, Uc.

The pulse duty factor2 (PDF) of its output voltage, which commands the MIA, KU [%] (Fig. 3), is adjusted via the input voltage Uc (pin 3 in Fig. 4). Next, a linear electromagnetic actuator (concerning the mobile device motion) controlled by the same input voltage Uc was designed and built. The motion of the mobile part must be between 0 mm, relative to a minimal and 0.5 mm, relative to a maximal. The cue voltage is within the range 0.75 – 3.4 V. The control signal of the electronic system is PWM, i.e. a rectangular waveform of constant frequency and variable pulse duration. The RMS value of the current through the actuator, for a fixed frequency of the PWM cue voltage, depends mainly on 2 For a periodic pulse train, PDF is the ratio of the pulse duration to the pulse period.

KU [5]. Consequently, the excursion of the mobile part of MIA depends on the ratio of the impulse duration to the PWM signal period [4]. A short duration impulse leads to a low current through the actuator, respectively a small excursion of the mobile device, while a long duration impulse determines a higher current as well as a linear displacement directly proportional to its value. The PWM generator discharges on the MIA impedance (Fig. 4). A probe of a FLUKE 190 oscilloscope is connected in parallel with MIA. The oscilloscope, with digital capabilities, is connected through an optical isolated serial port USB to the computer for data acquisition and processing. b. KU = 80%. Fig. 5. Control pulse wave forms for two different duty cycles.

MIA mobile part makes then an oscillatory, periodic, and linear motion of the same frequency. The whole application is using the PWM waveform controller DRV101T [5]. Fig. 5 presents the output rectangular pulse for an input rectangular pulse applied on pin 1 (Fig. 4), with UC = 5 V, f = 10 Hz, and KU = 60% and 80%, respectively. Finally, the thermal map of the pilot MIA obtained with the FLUKE Ti 20 Thermal Imager is presented in Fig. 6. The thermo-vision picture was made after the actuator was activated for 30 min with the waveform shown in Fig. 5,b.

Fig. 4. The electronic design of the MIA testing stand.

Our pilot MIA provides for a linear and alternative periodic motion, with a frequency between 1 – 250 Hz and amplitudes of 0 – 1 mm. When a rectangular pulse of 5V amplitude and 1 – 250 Hz variable frequency (from an external generator) is applied on pin 1, MIA enters in oscillatory mode.

Fig. 6. Thermo-vision picture of the MIA – after 30 min of duty.

The process was monitored for 30 minutes and the thermal stress may be estimated from the thermal image provided at the end by the FLUKE Ti 20 Thermal Analyser. Numerical simulation to assess the heat transfer performance of MIA is not reported here. III. THE MATHEMATICAL MODEL

a. KU = 60%.

The numerical model of MIA is based on the axial symmetry of the device. This approach leads to smaller numerical models and reduced computational times. MIA has a steel housing enclosing a drive coil. The computational domain is presented in Fig. 7. The magnetostrictive material, placed in the core, works as

actuator when a current passing through the drive coil produces a magnetic field. The magnetic field bias is provided by a cylindrical permanent magnet, placed on the central column. Other technical solutions may be available, and this is the object of a future research. Although not shown here explicitly, the action of a pre-stressing spring is accounted for. Infinite elements

magnetic fields. The coil is modeled as an equivalent (homogenized) electroconductive domain, where the individual wires are not discernable. • The equivalent current density varies “quasi-statically” so that there is no inductive effect and no skin-effect. The magnetic field part of the problem is described by the azymuthal, induction currents model, as follows [6] For the magnetostrictive material •

,

(1)

Flex pivot For all other parts of the computational domain , where A is the magnetic vector potential,

Soft iron core

GMM

a. The computational domain.

is the azymuthal

electrical current density (in the coil), H is the magnetic field strength. The permanent magnet has the remanent magnetization, Brem = 0.8 T and the relative permeability µr = 1.1. The magnetic core is made of soft iron (see the B-H curve in Fig. 8). The GMM core is made of Terfenol-D (see the H-B curve in Fig. 9).

Coil

Permanent magnet

(2)

b. The FEM mesh.

Fig. 7. The axial-symmetric model and the FEM mesh made of approx. 10,500 Lagrange, quadratic elements.

Fig. 9. The H-B magnetization curve of Terfenol-D [2].

Fig. 8. The B-H magnetization curve of the soft iron core [6].

In what follows several assumptions are made: The material is assumed to be in a pre-stressed state that would yield maximum magnetostriction. • The constitutive relation between magnetostriction and magnetic field is non-linear. • A non-linear B-H curve is used to model realistic magnetic behavior including saturation effect at high •

The boundary conditions that close the magnetic field problem are: symmetry, for the Oz axis, and magnetic insulation on the rest. The free strain displayed by the GMM is usually modeled using the linear constitutive relation λ = dH, where d is the piezo-magnetic strain coefficient. However, the magnetostrictive coefficient has a non-linear dependence on the applied magnetic field and on the mechanical stress in the material (Fig. 10) ,

(3)

where λr,z is the magnetostriction along the directions r, z,

which depends on the magnetostriction constant, λS, and the magnetization direction cosine. The direction cosine is the ratio of magnetization along the required direction (Mi) and the saturation magnetization (Ms) of the material. Here, λS = 2,000 ppm and Ms = 1.5×106 A/m = 18,849.556 Oe.

Fig. 11. The field current in the coil for KU = 70%, and f = 15 Hz. The interval preceding the cyclic performance is discarded. Fig. 10. Magnetostrictive coefficient as function of the applied magnetic field and mechanical stress for Terfenol-D [2], [7]. [KSI] is kilo-pound [force] per square inch.

Following [6], we neglect the negative 1/3 term in (3), which indicates that in the absence of any magnetic field, the magnetic moments are randomly oriented in the material because we have assumed that the material is sufficiently prestressed such that all magnetic moments are perpendicular to the direction of magnetization at the beginning of the magnetization process. The structural stress-strain axial model is concerned with the GMM piece only: this part is an almost incompressible material that can undergo large strains (finite deformations). Magnetostriction does not produce stress in the material unless it is constrained. Therefore, modeling magnetostriction as an initial strain ensures that the material remains stress-free when the strain in the body is the same as the magnetostriction. The generalized Hooke’s law is used then ,

The algebraic system, produced by FEM discretization was solved with the generalized alpha solver (the temporal part) and the parallel PARDISO solver (the spatial part) [6]. A tight control on the time step variation had to be performed in order to keep the fast (5×10-8 sec) time rise and fall for the signal. Fig. 12 shows the magnetic field and the deformation of the GMM at an "on" instant.

(4)

where [C] is the stiffness, [ε] is the strain, [εi] is the initial strain, [σ] is the stress and [σi] is the initial stress. Hence, the GMM piece is assumed to be in a pre-stressed state that would yield maximum magnetostriction, [7], [8]. The boundary conditions for the structural model are of a. Magnetic flux density. b. Total displacement (216 times larger). symmetry type for the sides situated on the symmetry axis, Fig. 12. Numerical simulation results for “on” interval, KU = 70%, f = 15 Hz. “free” (unconstrained), and “fixed” for the bottom. Fig. 13 shows the deformation of the GMM for the IV. NUMERICAL SIMULATION RESULTS AND DISCUSSION periodic control signal in Fig. 11. Apparently, GMM The mathematical model was implemented and solved in deformation follows the control signal. The raise and fall Comsol Multiphysics, finite element software [6] that uses ramps are adjustable by tuning the control signal amplitude Galerkin formalism. Here we report the numerical simulation (hence, the field current) and the permanent magnet remanent results for the case presented in Fig. 5, but for KU = 70%, and magnetization. Another optimization degree of freedom is the f = 15 Hz. First, the control signal (voltage) was converted to geometric aspect ratio of the GMM piece. These research current (amperturns), and synthesized using delayed paths make the object of future investigation. Finally, in many applications, it is desirable to find out Heaviside functions, as shown in Fig. 11.

either the free strain of the GMM or the displacement obtained from a GMM transducer as a function of the input current or input magnetic field.

Fig. 13. The deformation of the GMM piece – numerical simulation results.

Here we report the investigation of the free strain of the magnetostrictive material or displacement obtained from the transducer as a function of the input current or input magnetic field. To this aim, we conducted a parametric analysis.

Terfenol (Tb0.27Dy0.75Fe2) may be used to design low profile magnetostrictive actuators with very good dynamic characteristics at working frequencies of up to 100 Hz. These magnetostrictive actuators can equip fuel injectors for “common rail” systems, especially for high power motors of 350 kW and above. Besides the very good dynamic properties highlighted in the paper, the magnetostrictive actuator exhibits high forces and proper displacements for the mobile equipment. The most adequate mode to regulate the magnetostrictive injection actuator is by using a DRV101T controller with a rectangular control voltage, frequency in the range 1 – 50 Hz, peak-to-peak amplitude Ucd = 5 V, and an adjustable duty cycle corresponding to the engine working conditions. The thermal regime, as experimentally evidenced, is very good and the mechanical construction is very robust. These qualities also recommend the magnetostrictive actuator for the fuel injection applications. Numerical simulation provides for accurate information valuable in understanding, developing and optimising the design of the magnetostrictive injection actuator. Future research is concerned with optimising the magnetostrictive injection actuator prototype presented in this paper. ACKNOWLEDGMENTS The MIA prototyping and the experimental part of the work were carried on at ICPE-CA. The numerical simulations were conducted in the Laboratory for Multiphysics Modeling at UPB. REFERENCES [1]

Fig. 14. The electric potential – numerical simulation results.

Fig. 14 shows the non-linear λ vs. H curve, for steady state working conditions of the GMM in the reported MIA. This behavior explains the raising and falling flanks of the deformation seen in Fig. 13. V. CONCLUSIONS The novel Giant Magnetostrictive Materials, such as

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