In particular the invention relates to a method and an arrangement for determining sedimentation conditions in MR fluid dampers.

BACKGROUND OF THE INVENTION MR fluid dampers are used in a variety of applications today. Applications include for example training equipment, brakes, clutches, mounts and valves. For example seismic structures utilise MR fluid dampers as shock absorbers and structures that can provide support to buildings in areas where earthquakes are common. MR fluid dampers are also used as semi-active vibration and shock absorbers in the vehicle and aircraft industry. The most important application of MR fluids today is in semi- active dampers for cars and motorcycles, whereas civil engineering structures probably provide the greatest technical challenges.

MR fluids consist of micron size ferromagnetic particles in a viscous media, i. e. in a carrier fluid. In the absence of a magnetic field the MR fluid flows freely similar to that of a Newtonian fluid. Upon application of a magnetic field, the particles within the suspension become magnetised and align themselves to the direction of the magnetic field. The formation of these particle chains restricts the movement of the fluid within the suspension thereby increasing its viscosity.

Especially MR dampers that are passive for long periods of time are susceptible to sedimentation. Due to sedimentation the ferromagnetic particles eventually leave the suspension. Sedimentation is mainly due to density difference between the fluid and the particles. Sedimentation is also affected by particle size, shape, fluid viscosity, as well as wetting of particle surface by the fluid. Despite surfactants and improved fluid design, sedimentation cannot be completely eliminated.

It is important to know the sedimentation state of an MR fluid damper. A sedimented fluid does not operate properly and can cause unexpected damage in its area of application by failure. Prior art methods for inspecting the sedimentation state of an MR fluid are virtually non-existent; the applicant is unaware of any methods that would allow the sedimentation state of the fluid to be observed without taking a direct sample from the fluid.

Taking a sample from the MR fluid includes an important drawback; it is both tedious to arrange and mechanically compromises the MR fluid damper, which dampers typically are very hermetic structures. Measurement from MR fluid samples by a microscope or chemical tests, for example, is not well applicable to practical field measurements.

There is also known a method for determining particle contents without chemical tests or microscopes. International patent application WO 00/50883 discloses a method and device for determining metallic contaminant particles in lubricating grease, where the particle content in a sample is measured using the so-called eddy current effect.

In above said known method a sample pole of the measuring sensor is a small flat substrate, on whose surface a sample i. e. a thin layer of lubricating grease is applied.

The sample pole is disposed at the end of the core of the electric magnet. Once the sample has been spread on the sample pole, the determination of the particle content can be performed. Sample of lubricating grease is exposed to a first alternating magnetic field, which is produced with an electric magnet having a coil. The content of contaminant particles in the lubricating grease sample is determined on the basis of the influence of second magnetic field resisting alternations in the first magnetic field. Eddy currents induced in the contaminant particles have produced the second magnetic field. Upon more detailed analysis found in WO 00/50883 the used circuit will reduce to a state, when the conductivity, the permeability and the frequency have been selected, the impedance of the coil will depend exclusively on the distance between the coil and the object, i. e. the contaminant particle sample pole N.

However, the prior art eddy current measurement methods have serious disadvantages, one of which is their poor temporal sampling ability. The sample on the pole dries fast, and virtually every sample is disposable and usable only for a few seconds, or a very limited time. A further major hindrance is also the collection of the fluid to the sampling pole, as physical contact by wetting is required for every measurement.

The tedious and poor temporal sampling ability results also to virtually nonexistent subsequent data manipulation possibilities. As the prior method always requires manual work of collecting samples, it is also impossible to use the prior art method as an automatic continuous monitoring tool. Due to the aforementioned reasons the prior art is not applicable for measurement sets that form data profiles, such as sedimentation profiles.

SUMMARY OF THE INVENTION The object of the invention is to provide a new, simple and reliable method and arrangement for determining sedimentation of an MR fluid. A further object of the invention is to provide a new, simple and reliable method and arrangement for determining the sedimentation state of an MR fluid in an MR fluid damper without mechanically compromising the damper that contains it. An even further object of the invention is to provide a new method and arrangement for determining the sedimentation state of an MR fluid and monitoring the development of the sedimentation state with respect to a physical variable such as time or position in the damper.

The invention is characterised by the features defined in the independent claim 1 and 6. The dependent claims describe preferred embodiments of the invention.

The invention is based on an inductive measurement, especially a developed application of eddy current phenomenon. When alternating current is conducted to a coil of the inductive measuring sensor, a magnetic field with alternating directions will be produced, which is perpendicular to the coil wires and parallel with the coil core. When the coil is placed in the vicinity of a metal body, circular eddy currents are induced on the metal surface, and these currents proceed perpendicularly to the direction of the coil core. The eddy currents, in turn, produce a second magnetic field, a"secondary field", which resists the original first, i. e. primary magnetic field.

The secondary field partly weakens the intensity of the primary field, and this appears as a measurable variation of the impedance or voltage of the coil. The coil and an electrically conductive body in its vicinity, in this case a plurality of sedimenting particles, thus form a system configuration that allows the degree of sedimentation of the particles in the vicinity of the coil to be determined with respect to one or several physical variables such as time or position. As the invention is based solely on the determination of EMF's (Electro Motive Force), it is possible to perform measurements without physical contact to the particles or the fluid containing them.

The method of the invention is applicable to the determination of the total content of both ferromagnetic and non-magnetic metal particles, such as for instance aluminium, titanium and brass particles. Particles determined by the method must be electrically conductive. However, in MR fluids the particles in the suspension are typically both electrically conductive and magnetically permeable.

In one preferred embodiment of the method of the invention, the sedimentation state and profile of a magnetorheological fluid is determined by making a measurement of a first known MR fluid, and the measurement value of a second MR fluid to be determined is compared with reference values in order to determine the sedimentation profile of the latter.

In another preferred embodiment of the apparatus of the invention, the measuring unit comprises means for determining the impedance of an inductive measuring sensor, which determines the influence of a second magnetic field produced by eddy currents on the sensor impedance, the sedimentation profile of particles in the MR fluid being further determined on the basis of the determined impedance and as a function of a physical variable, such as time, height or position in the MR fluid.

Another advantage of the invention is that it is possible to determine the sedimentation state and/or profile by non-contacting inductive measurements. The outcome of the determination of the sedimentation of the suspension particles yielded by the method and the arrangement of the invention will thus represent the sedimentation profile in the original, untouched MR fluid in a container. Thus, the invention has the benefit of rapid and reliable real-time determination of the sedimentation profile of particles in suspension, especially in field measurements. In one aspect of the invention sedimentation profile data is collected continuously or predetermined time intervals and communicated by means of wireless (cellular network, radio or satellite) or wire connections.

The invention also has the advantage of allowing repeated analysis of the MR fluid in order to ensure the content value, and/or any other kind of analysis.

The invention also has the advantage of automatable determination and easy measurement data manipulation. The results can be collected and processed to standard computing devices such as a PC (Personal Computer). A further subsequent and preceding advantage of the invention is the easy communication of measurement data; by standard prior art data communication methods. The invention is applicable as a method and a network of devices for continuous,

centralised monitoring of vast arrays of MR fluid structures. Thus, the invention also has the advantage of the apparatus or a network of apparatuses for determining the sedimentation of suspension particles being easily manageable.

The invention also has the advantage that the arrangement can be accomplished as a small-sized, portable, programmable device. The device is thus applicable also to random dynamic field measurements.

DESCRIPTION OF DRAWINGS AND EMBODIMENTS The invention will be explained in detail below with reference to the accompanying drawings, in which Figure 1 illustrates an arrangement for measuring sedimentation of an MR fluid according to the invention; Figure 2 illustrates a method for determining the sedimentation of an MR fluid in accordance with the invention as a flow diagram; Figure 3 is a schematic diagram of another arrangement for measuring sedimentation of an MR fluid according to the invention; Figure 4 shows an exemplary calibration curve for an arrangement of the invention; Figure 5 shows a diagram representing the sedimentation profile of a typical MR fluid as a function of time; Figure 6 is a diagram representing a multidimensional, exemplary, theoretical sedimentation profile of the invention.

Figure 7 is a schematic diagram of another arrangement for measuring sedimentation of an MR fluid, especially in an MR fluid damper; Figure 8A and 8B are schematic diagrams showing front-and back views of a portable arrangement for measuring the sedimentation profile of MR fluids in dynamic field environments ; and Figure 9 shows a network of sedimentation profile observing devices fitted to various MR fluid structures.

DETAILED DESCRIPTION OF EMBODIMENTS An arrangement for measuring sedimentation of an MR fluid according to the invention is schematically presented in Figure 1. Respectively, a method for measuring sedimentation is presented as a flow diagram in Figure 2. We refer to both of the figures in the following when we explain the basic idea of the invention.

An MR fluid damper 100 typically consists of a container 102 such as a hydraulic cylinder containing MR fluid 101 i. e. micron-sized, magnetically polarisable particles suspended within a fluid, usually oil. A piston head 103 is arranged in the container 102 and so the container is divided into two sections 102a, 102b. As the piston 103 is moved, the MR fluid within the container 102 is forced to pass from one section 102a (or 102b) to another section 102b (or 102a) through one or several small orifices 104 at high speeds. The orifices 104 are arranged in this case in the piston head 103, but alternatively they can be arranged in the connection of the container 102. An electromagnet 105 located in connection with the orifices 104 is utilized to generate the magnetic field. Behavior of the MR fluid in the orifices 104 is controlled by subjecting the magnetic field. In the absence of a magnetic field, the MR fluid flows freely in the orifices 4 while in the presence of a magnetic field, the fluid behaves as a semi-solid. The damping force is developed by a pressure difference across the piston head 103. We refer to an article"Semi-active control systems for seismic protection of structures: a state-of-art review"Michael D.

Symans and Michael C. Constantinou, Engineering Structures 21 (1999) pp. 469- 487.

An arrangement for measuring sedimentation state of an MR fluid 101 comprises a sensor unit 110 with inductive measuring sensor 106 and further a processing unit 111. The measuring sensor 106 includes an electric magnet 107 having a core 108 and a coil 109. The inductive sensor 106 is fixed in the connection with the container 102 and it has magnetic contact with the MR fluid 101. In the Figure 1 the inductive sensor 106 is arranged at the bottom end 1021 of the container 102.

Especially the inductive sensor 106 is fixed so that the end of the core 108 is against the lower or bottom end 1021, as shown schematically in Figure 1. The sensor unit 110 includes also a high frequency alternating current source 110a for energising electric magnet 107.

When the inductive measuring sensor 106 is set on its place, the determination of the particle content in the container 102, especially at the bottom end 1021 in the

vicinity of the sensor 106, is performed according to the method presented as a flow diagram in Figure 2.

In phase 200 the MR fluid 101 is exposed to a first alternating magnetic field H1, which is produced with an electric magnet 102 of the inductive measuring sensor 106. The particle content in the MR fluid 101 is determined on the basis of the influence of second magnetic field H2 resisting alternations in the first magnetic field H1. In phase 210 eddy currents are induced in the suspension particles. The suspension particles are typically ferromagnetic or otherwise conductive and permeable particles as explained above. The eddy currents in the suspension particles induce a secondary magnetic field H2 in phase 220. This field H2 opposes any changes in the primary field H1 in phase 230, (Lenz's Law). This opposing field results in a change in impedance of the sensor coil 109 in phase 240, which is measured in the processing unit 111.

In an advantageous embodiment of the invention the apparent impedance value is compared with the impedance value of a reference sensor (comp. Fig 3), and the difference in impedance can be amplified and converted to a voltage signal, and finally recorded by common data acquisition equipment. In addition to the static influence of an MR fluid on the apparent impedance value of the sensor, accumulation of the particles of the MR fluid 101, due to sedimentation at the lower end of the container 102, are reflected by an increase in the recorded signal. The sedimentation development in the container 102 typically ends up in a concentration pile up at the bottom due to gravitation.

Thus, in phase 250 the sedimentation state of the MR fluid can be deduced by measuring the suspension particle content corresponding to a specific physical state defined by the values of at least one physical variable, such as time, position in the MR fluid 101 and/or in the container 102, temperature, pressure, viscosity, or the like, of the MR fluid. The values of these physical parameters may be measured and acquired automatically from the environment or manually from the user during, before, after or in between any phase of the inventive method described in Figure 2.

By repeating the phases 200-250 the sedimentation profile is then subsequently determined as a group of sedimentation state values. The sedimentation profile can be determined as a function of time, position or any other variable associated with the MR fluid 101 and thus the sedimentation profile of the MR fluid can be extracted in phase 260. It is clear that the number of iterations and thus the number of data points can be variable from one single reading to a vast array of readings or

several sets of data. The sedimentation profile can also be measured as the particle content as a function of position such as height h of the container 102, temperature, pressure, viscosity, strain, shear, stress, concentration of foreign substance or as a function of any mechanical, chemical, physical or thermodynamic variable.

Another embodiment of a measuring arrangement 300 for measuring sedimentation state of an MR fluid is shown in Figure 3. The measuring arrangement 300 comprises a sensor unit 390 and a processing unit 395. The sensor unit 390 has, in addition to one i. e. the first inductive measuring sensor 310, a second inductive measuring sensor 360, which is preferably identical with the first inductive measuring sensor. Both of the inductive sensors 310 and 360 are same sort of inductive sensor 106 as in the embodiment in Figure 1. The actual impedance determination is performed with the first inductive measuring sensor 310. The second inductive measuring sensor 360 is adapted to act as a reference sensor when the influence of the second magnetic field on the first magnetic field is determined.

The MR fluid 301, which sedimentation state is to be measured, is in a container 302. The first measuring sensor 360 is arranged against the wall of the container 300 and preferably at the lower end of the container 300 at height h from the bottom end 302a.

The sensor unit 390 includes also an alternating electric current i. e. AC source 320 and two resistors 330,340. The second measuring sensor 360 is preferably supplied from the same alternating electric current source 320 as the first measuring sensor 310. Protective resistors 330,340 are serially connected with the measuring sensors 310, 360.

The AC source 320 has a high frequency, typically in the megahertz range. This ensures that eddy currents with adequate intensity are produced in the suspension particles of the MR fluid. Lower driving frequencies can naturally be reached by increasing the inductive sensitivity of the sensor unit 390.

The influence of the second magnetic field results in a change in impedance in the first measuring sensor 310 as explained in connection with the embodiment of Figure 1. Both the signal from the second measuring sensor 360 and the first measuring sensor 310 enter the processing unit 395. The processing unit 395 comprises a differential amplifier 350, analog/digital i. e. A/D converter 370 and data handling unit 380. The signal from the first measuring sensor 310 is fed into the differential amplifier 350 along with the signal from the second and reference measuring sensor 360. The output signal from the differential amplifier 350 is fed

into the A/D converter 370, which digitises the millivolt difference signal. The digital signal is then typically transferred to the data handling unit 380 that is a microcomputer, a PC or any other computer unit with memory units.

In an alternative embodiment the digital measuring signal is fed into a transmitter 381 such as a radio transmitter instead of the data handling unit 380 (dotted line in Figure 3). Alternatively preprocessed data from the data handling unit 380 is fed into the radio transmitter 381. The digital signal, measuring data and/or preprocessed data, is transmitted to another location e. g. data handling centre for remote processing and/or storing.

In preferable embodiment the data handling unit 380 stores the measuring results in a memory unit 380a as a database, a database table or the like, and shows the results on a display screen or any similar output device, or communicates the data to another arrangement such as data handling centre 382 through the Internet, VPN (Virtual Private Network), telephony/cellular network or any other communication channel 383.

The data handling unit 380 (or alternatively data handling centre 382) comprises vast amounts of historical or general sedimentation profile data in a preferable embodiment. Current measurement results may be compared with historical values stored in a database, or to threshold values. If the data handling unit detects abnormal or undesired results, for example very intense sedimentation, the data handling unit communicate or issue a warning signal.

In a most important embodiment the container 310 is a part of MR fluid damper as in the embodiment of Figure 1. These embodiments allow sedimentation real time testing directly from hermetic or otherwise compact MR fluid dampers.

In another embodiment the container 310 is a test tube, which is installed in vertical direction X-X. Advantage is that sedimentation properties of different MR fluids can be tested beforehand in laboratory scale.

When testing the sedimentation properties of MR fluids used in the MR fluid dampers, it is possible to define calibration curve for different MR fluids. In one embodiment of the invention an array of known calibration MR fluid samples is tested, and a calibration curve is extracted for the measuring results. In another embodiment of the invention the same sample is tested within predetermined time periods. However, the latter testing process can take time and testing material should be conserved in stable state for long times. An exemplary calibration curve,

fitted with a least squares method known form prior art is typically a straight line L, as exhibited in Figure 4.

The suspension particle sedimentation profile is determined by making a calibration measurement of such a first MR fluid, whose suspension particle content is known beforehand, and by comparing the measurement value of a second MR fluid to be determined with the reference value in order to determine the suspension particle concentration and/or sedimentation of the latter.

More complex and diverse measuring sensors can be constructed in accordance with the invention, having diverse calibration curves that are more complicated than a least squares fit, so the exemplary nature of Figure 4 should be emphasised.

In one embodiment of the invention it is possible to define the calibration curve beforehand and to store the curve in a memory unit in connection with the processing unit 380. Then the second measuring sensor 360 is not necessary anymore and it can be omitted from the measuring arrangement 300.

An exemplary sedimentation profile as a function of time is illustrated in Figure 5.

The sedimentation curve S reveals interesting characteristics: the sedimentation begins only after a short incubation period, and proceeds then quite fast to a relatively steady sedimentation level. Sedimentation behaviours of various MR fluids are naturally diverse, so the exemplary nature of Figure 5 should be emphasised. Similar profiles with few or vast numbers of data points may be measured and recorded as a function of position, pressure, temperature, viscosity or any other physical variable. Likewise multidimensional sedimentation profiles can be extracted as a function of several physical variables.

As sedimentation behaviour is quite variable from an MR fluid to another, may a multidimensional sedimentation profile exhibit quite pathological characteristics.

For example, it may be that in a certain state of temperature and pressure the 2- variable sedimentation profile soars to very sharp peak 1001, and it is vital to know when an infinitesimal change in either the temperature or pressure will push the sedimentation behaviour of the fluid to a critical state where sedimentation is accelerated. An exemplary multidimensional sedimentation profile is exhibited in Figure 6.

Another embodiment of a measuring arrangement 700 for measuring sedimentation state of an MR fluid is shown in Figure 7. The measuring arrangement 700 comprises a sensor unit 720 with an inductive measuring sensor 710 and AC source

711. The sensor unit 720 and especially the measuring sensor 710 is a similar unit and, respectively, inductive sensor as in the embodiments in Figure 1 or Figure 3. In connection with sensor unit 720 is also a processing unit 730. An exemplary fluid damper container 701 contains MR fluid 702 as the fluid-damping medium.

In this embodiment at least one measuring sensor 710 is immersed into the MR fluid 702. The measuring sensor 710 is then inside the container 701. The measuring sensor 710 is located preferably at the bottom of the container 701 or close to the bottom. The sedimentation development in the container 701 typically ends up in a concentration pile up at the bottom due to gravitation.

The measuring sensor 710 is arranged to measure particle content readings at timed intervals or continuously and communicate them to the processing unit 730. The processing unit 720 may sound an alarm, when the sensor unit 710 signals high particle content, and thus sedimentation at the bottom of the damper.

The sensor unit 720 measures the effect of the secondary magnetic field H2 as a function of time, but also as a function of position in the container 701 of the damper, pressure of the MR fluid, temperature of the MR fluid or any other physical variable. The processing unit 730 is arranged to extract a sedimentation state and/or profile from these measurements.

In one advantageous embodiment the processing unit 730 is connected to a radio transmitterlreceiver 740, by which the processing unit communicates the measurement data to another remote processing and/or storing unit. The processing unit 730 can also receive measurement instructions from a communication connection and a remote unit, and operate the measuring sensor 710 accordingly.

In one embodiment the processing unit 730 is also arranged to correlate different sets of measurements and compose sedimentation profiles, or multidimensional sedimentation profiles from the said data.

In another embodiment the processing unit 730 may also feature information and methods with which to account for the electromagnetic effects originating from the container 701 of the MR fluid. This feature is especially desirable when the MR damper container is made of a conductive and permeable material that supports similar electromagnetic effects as the inductive reaction of the suspension particles.

However, any container of the MR fluid in the measuring arrangement of the invention is normally non-conducting and magnetic field permeable material.

Further another embodiment of a measuring arrangement 800 for measuring sedimentation state of an MR fluid is shown in Figure 8A and 8B. The measuring arrangement 800 is a portable version of the measuring arrangement in accordance with the invention. Front view, Figure 8A, shows that the arrangement comprises several measuring sensor units 801,802, 803, 804, which can be pressed against the outer surface of the container of an MR fluid damper. The backend, Figure 8B, shows that the arrangement has a display 820, such as an LCD display for instance, and a keyboard 830, such as a QWERTY keyboard. Calibration data or other data may be loaded to the portable measuring arrangement 800 to assist in field measurements. Likewise the portable measuring arrangement 800 may comprise memory means that can be used to record data arrays of performed sedimentation measurements for later processing or for later upload to another processing device.

With the portable measuring arrangement 800 of the invention the magnetic fields can be applied to various sections of a container containing an MR fluid. In this way, sedimentation measurements, for example from various heights of a column like container, can be obtained quickly and easily even in difficult field conditions.

Figure 9 shows a network of sedimentation profile recording devices 910,920, 930 in a schematic field environment. MR fluid dampers are extensively used as seismic damping structures 901 in environments where earthquakes are abundant, such as volcanic areas VA or ocean shores OS that are near to tectonic borders. It should be noticed that sedimentation profile recording devices may also be attached to MR fluid dampers in vehicles, or any other MR fluid dampers, and still form a network.

In this exemplary embodiment, the recording devices 910,920, 930 communicate their sedimentation profile data to a processing unit such as a server 940 through communications connections that are either wireless connections, such as microwave-, radio-, or satellite connections, or fibre optic connections. The server is for example a standard server such as a Unix, HP-UX, or MS/NT server that may contain a database and associated database management software in some embodiments.

The server 940 may comprise complicated and sophisticated computational logic that is synchronised with the recording devices 910,920, 930. For example, network wide sedimentation threshold values can be computed and communicated through the network in order to give an accurate overview of the status of the MR fluid structures as a whole.

We wish to point out that in the drawings the measuring sensors of the invention are emphasised by drawing it bigger form as in real life compared to the MR fluid containers.

The invention has been explained with reference to the aforementioned embodiments and several industrial and commercial advantages of the invention have been demonstrated. The inventive method allows sedimentation measurements in a fast, easy and reliable way, and thereby increases the safety of using MR fluid dampers. The invention also greatly increases the possibilities to continuously monitor and diagnose MR fluids and entire systems of several MR structures.

The invention is not restricted only to the aforementioned embodiments, as many variants are conceivable without departing from the original inventive idea and scope of the attached claims.