R. Olaru, C. Pal, Camelia Petrescu

Technical University of Iasi, Faculty of Electrical Engineering,

53 D.Mangeron Blvd., Iasi 6600, Romania

Magnetic fluids or ferrofluids are ultrastable colloidal suspensions of ultrafine ferromagnetic particles in various carrier liquids. Discovered over 30 years ago, these materials have both magnetic properties, usually assigned to magnetic solids, and properties characteristic to fluids, a fact that confers them an extremely wide range of applicability. Magnetic force in a ferrofluid, proportional to the field gradient and to the liquid magnetization, makes possible the control and positioning of a volume of magnetic fluid and the levitation of a magnet or non-magnetic body. The main applications of ferrofluids are: rotary seals, inertia dampers, loudspeakers, magnetogravi-metric separators, sensors and transducers [1, 2]. Another interesting application is the current to pressure transducer with self-correcting nozzle [3].

We have recently studied a magnetic fluid electromechanic device that produces small forces acting upon a lever generating small displacements [4, 5].

This paper presents a new type of magnetic fluid device with an elastic membrane that produces small current controlled displacements of the disk. These displacements can be converted in pressure by means of a nozzle flapper valve preamplifier.

The control device has a simple construction (Fig.1) its main parts being an inductor, an elastic membrane, and a magnetic fluid, based on transformer oil, having the saturation magnetization = 45kA/m.

The inductor is a ferromagnetic core coil excited by the command current I. The ferromagnetic core has an axial cavity.

In a magnetic fluid with the magnetization M placed in a magnetic field H, a magnetic force having the volume density appears:

= (M•N )H (1)

The magnetic forces produce a distortion of the elastic membrane. Producing wide range current variations, small displacements of the disk attached to the membrane may be obtained.

The pressure exerted by the magnetic fluid upon the elastic membrane may be calculated using Maxwell's magnetic tensor. Since the system has axial symmetry the problem may be analysed in the r-z plane. The expression for Maxwell's magnetic tensor is [6 ]:

(2)

where and are the radial and axial components of the magnetic flux density in air and is the ferrofluid permeability. Since an analytical solution for and is difficult to obtain, the finite element method (FEM) was used. In an axially symmetric system the numerical analysis using FEM is made in terms of the modified magnetic potential U= r. and the magnetic flux density components may be then calculated numerically using the relations:

, (3)

Since the electromagnetic converter is placed in air (unbounded system), the limits of the analysed domain were considered large enough (compared to the system itself) in order to obtain zero values of the modified magnetic potential on the boundaries.

The grid in this numerical example has 19285 nodes and 38000 elements. Fig. 2 presents the magnetic field lines and Fig.3 the pressure on the elastic membrane in radial direction.

As may be seen pressure has low values in the space corresponding to the core cavity (r <  2 mm) and increases in radial direction, reaching its highest values at the outer edge of the iron core (r = 7.5 mm). In the neighbourhood of the vessel containing the magnetic fluid, pressure has again increasing values that get however below the maximum pressure.

Experimental setup working as an electropneumatic converter is presented in Fig. 4. The nozzle situated near the circular membrane forms, together with the variable resistance and with the disk, a nozzle-flapper valve preamplifier. Its output pressure, depending on the distance between the nozzle and the disk (the initial value being  < 0.2 mm), is given by a pneumatic amplifier having the amplification factor k = 20. The pressure indicated by the manometer is expressed by the relation:

(4)

where, is the pneumatic resistance formed of the nozzle fix resistance , and the variable nozzle-disk resistance depending on the distance d, (d).

The pneumatic amplifier ensures an output pressure in the range 0.2 ?  1 bar for an input pressure in the range 0.2 ?  0.24 bar. Two nozzles with different cavity diameters (0.9 mm and 2 mm respectively) were used.

Generally the sensitivity of the electropneumatic system D p/D I is larger when the small nozzle is used as shown by the plots in Fig. 5. The current to pressure plot shows a hysteresis effect due to the elastic membrane of the electromechanic device.

If the ferromagnetic core axial cavity is filled with a cork, the system sensitivity decreases due to the supplementary elastic force generated by the decrease in the air volume confined in the cavity (Fig. 6). At same time the hysteresis effect is larger than in the case of an open cavity.

A reduction of the hysteresis effect and an improvement of the system linearity may be obtained by applying a negative reaction between output and input by means of a pressure electronic transducer.

Fig. 6. Pressure vs. current characteristics for plugged channel (small nozzle for two values of distance do)

In this paper a simple electro-mechanical device is presented. This device uses a magnetic fluid in order to generate small forces that deform an elastic membrane equipped with a central disc. The device may be used as a control element for a nozzle - flapper pneumatic preamplifier as part of an electropneumatic converter. The magnetic pressure profile on the elastic membrane was obtained numericaly using the finite element method. Using the program written in MATLAB an optimum device geometry may be obtained using, for example, the ratio F/I between the distorting equivalent force and the control current as an optimum criterion .

The experimental results obtained with an electropneumatic converter show a static characteristic pressure versus current with linear domains as well as with a hysteresis effect. In order to eliminate this effect and to improve the system linearity and precision a feedback circuit with an integrated electronic pressure transducer must be used.

In practice the solution of filling in the ferromagnetic core channel with a cork does not seem to be sufficient. A complete filling in of the channel with magnetic fluid and the application of a second elastic membrane at the top of the device seems to be a better technical solution.