Abstract:
A magnetorheological (MR) fluid device including a pressurized MR liquid with an improved performance is provided. Also provided is a method for minimizing cavitation of a common magnetorheological device, comprising providing an MR fluid within the device with a pressure of at least 100 psi. The device as provided minimizes cavitation in the device, and can be broadly used in the railway vehicle suspension system with excellent performance.

Description:
[0001]     This application claims the benefit of U.S. provisional patent application No. 60/703,428 filed on Jul. 29, 2005 which is explicitly incorporated by reference in its entirety. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     This invention relates to a magnetorheological (MR) fluid device, and more particularly to a magnetorheological (MR) fluid damper having a pressurized MR fluid.  
         [0004]     2. Description of Prior Art  
         [0005]     Magnetorheological fluid devices that employ an MR fluid as the working medium to create controllable viscous damping forces are quite promising for vibration reduction applications. Compared to the conventional semi-active device such as variable orifice dampers, MR fluid dampers are fast responding and have less moving parts (only the piston assembly), which makes them simple and reliable.  
         [0006]     The good adaptability of MR devices also provides them with novel applications in promising flexibility. A variety of MR devices have been developed for different applications, such as MR rotary devices used in exercise equipments, clutches and brakes; and linear MR devices used in suspension systems of automobiles or railway vehicles.  
         [0007]     MR fluids commonly used in an MR device are one kind of controllable fluids that are able to reversibly change from a viscous liquid to a semi-solid (rheological change) with a controllable yield strength in milliseconds when exposed to a magnetic field. A common MR fluid comprises three major components: dispersed ferromagnetic particles, a carrier liquid and a stabilizer. When no magnetic field is applied (off-state), the MR fluid flows freely like a common liquid. When a sufficient strength of a magnetic field is applied (on-state), the ferromagnetic particles acquire dipole moments aligned along with the direction of the magnetic field to form linear chains parallel to the applied field. Consequently, this phenomenon solidifies the MR fluid to result in an increase of the MR fluid yield strength and restricts the movement of the MR fluid. The yield strength of the fluid increases as the strength of the applied magnetic field increases. Once the applied magnetic field is removed, the MR fluid goes back to the freely flowing liquid again within milliseconds.  
         [0008]     A common MR damper may include a piston assembly with a piston rod sliding in an interior portion of a closed damper body that is fully filled with MR fluids. The piston rod has at least one end attached to the piston assembly within the damper body and has at least one end outside the damper body.  
         [0009]     The damper body and at least one end of the piston rod are attached to separate structures in order to provide a damping force along the direction of the piston rod according to the relative motion between these two separate structures. When the piston is displaced, the MR fluids are forced to move from a compression chamber to an expansion chamber in the MR damper via an orifice. Then, the MR fluids inside the orifice are exposed to an applied magnetic field with different magnitudes upon applications. The magnetic field is generated by an electromagnetic circuit that is commonly located at a staging area of the piston core.  
         [0010]     U.S. Pat. Nos. 5,277,281 and 5,878,851 to Carlson et al. and U.S. Pat. No. 6,427,813 to Carlson disclose different MR damper designs.  
         [0011]     However, the MR fluid damper suffers from force lag phenomenon. Force lag phenomenon is, firstly, due to air pockets that are trapped inside the MR damper during the MR fluid-filling process. Secondly, it is due to the relatively high viscosity of the MR fluids. Both of these two factors will cause cavitation during the damper operation and degrade the performance of the MR damper. It would, therefore, be desirable to provide an MR fluid damper with the minimum cavitation.  
         [0012]     Carlson&#39;s patent (U.S. Pat. No. 6,427,813) discloses an MR damper with an accumulator which includes an external compensator chamber for expansion and extraction of an MR liquid and a gas charge chamber. Though Carlson mentions that the accumulator can pressurize the MR liquid such that any cavitation is minimized, Calson keeps silent to how to minimize cavitation.  
         [0013]     The references cited herein are explicitly incorporated by reference in its entirety.  
       SUMMARY OF THE INVENTION  
       [0014]     In order to overcome the above problems in the prior art, the present invention provides a magnetorheological fluid device which comprises a pressurized MR liquid at least 100 psi.  
         [0015]     One aspect of the present invention is to provide a magnetorheological fluid device, comprising:  
         [0016]     a) a housing including a hollow;  
         [0017]     b) a moving mechanism within the hollow, the housing and the moving mechanism positioned to define at least one working portion and at least one chamber within the hollow;  
         [0018]     c) a magnetorheological fluid within the at least one working portion and the at least one chamber, which has a pressure at least 100 psi; and  
         [0019]     d) means for generating a magnetic field to act upon the MR fluid within the working portion to cause a rheology change therein.  
         [0020]     Another aspect of the present invention is directed to a method for minimizing cavitation of a magnetorheological device which comprises providing an MR fluid within the device with a pressure at least 100 psi.  
         [0021]     Still another aspect of the present invention is to provide a suspension system of a railway vehicle comprising at least one magnetorheological damper defined according to the present invention between a truck and a car body of the railway vehicle.  
         [0022]     In an example embodiment of the invention, the MR fluid has a pressure between 100 psi and 400 psi. In another example embodiment, the MR fluid has a pressure between 100 psi and 200 psi.  
         [0023]     The MR device as provided in the present invention has an improved performance because it can significantly minimize cavitation compared to those in the art. While applied to in a railway vehicle system, it may increase the damping force at the lower sway mode without degrading the performance of the railway vehicle at the higher frequency upper sway mode. Furthermore, the device according to the invention can cope with various vibration motions under different situations. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0024]     The foregoing features and other advantages of the invention will be better understood from the accompanying drawings together with a description thereof given below, which serve to illustrate example embodiments of the invention. In the drawings,  
         [0025]      FIG. 1  illustrates a partial cross-sectional side view of an MR damper according to the present invention;  
         [0026]      FIG. 2  is a graph that shows the effect of force-lag phenomenon under different pressurized MR fluids; and  
         [0027]      FIGS. 3-5  are a bottom view, a side view and a front view of a schematic railway vehicle utilizing MR fluid dampers of the invention, respectively. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0028]     Now referring to the drawings, in which like reference numerals represent like elements throughout, some example embodiments of the invention are illustrated.  
         [0029]     An MR device  10 , particularly an MR damper, according to an example embodiment of the present invention is shown in  FIG. 1 .  
         [0030]     The MR damper  10  includes a housing or body  14  which is normally made from a magnetically-soft material, such as low-carbon steel. In this embodiment, the housing  14  provides a cylindrical hollow  140 .  
         [0031]     The housing  14  is closed by two covers  16  and  16 ′ at its two ends, which are tied by tie rod nuts  18 ,  18 ′,  18 ″ and  18 ′″ on tie rods  20  and  20 ′ (in this embodiment, there being 8 rod nuts and 4 tie rods in total that are not fully shown in  FIG. 1 ). They are assembled together to form a partially closed compartment.  
         [0032]     Two circular apertures  24  and  24 ′ are formed at the center of the rod covers  16  and  16 ′, respectively. The apertures  24  and  24 ′ respectively receive two piston rods  30  and  30 ′ which are axially slidable. The apertures  24  and  24 ′ preferably include two bearings and seals  44  and  44 ′, which allow the piton rods to axially move and prevent escape of fluids inside from the compartment  22 .  
         [0033]     A piston assembly  12  is provided to embrace the two piston rods to axially slide synchronously with the piston rods within the housing  14 . The piston assembly  12  comprises a piston head sleeve  26 , which is attached to the two piston rods  30  and  30 ′ by means of screws or welding.  
         [0034]     In an example embodiment of the present invention, the piston rods  30  and  30 ′ have the same diameter, which are axially extended out of the housing  14 .  
         [0035]     Since there is no change in volume within the closed interior compartment  22  as the piston rods move, this arrangement has an advantage that a rod-volume compensator, accumulator or other similar devices are not needed to be incorporated into the damper.  
         [0036]     The piston head sleeve  26  is preferably manufactured by a magnetically-soft material with at least one spool and three spools  28 ,  28 ′ and  28 ″ in this embodiment. Having a separate piston head sleeve  26  attached to the piston rods  30  and  30 ′ to form the piston assembly  12  allows a more expensive whole piece piston assembly to be replaced. It also allows a simple and cost-effective way of modifying a conventional piston damper to an MR damper while reducing complexity and problems of center alignment, which will be described in detail later. In addition, it has a particularly simple geometry in which the outer cylindrical housing is a part of the magnetic circuit.  
         [0037]     The piston assembly  12  divides the compartment  22  into a first fluid chamber  32  and a second fluid chamber  34 .  
         [0038]     In the invention, cushion rings  36  and  36 ′ are provided, which are attached to the two piston rods  30  and  30 ′ and axially extended along the piston rods from the piston head sleeve  26  respectively. The cushion rings are configured in such a shape that hydromechanically provides a smoother movement and reduces the resistance between the piston assembly  12  and the MR fluid  48  caused by the relatively high viscosity of the fluid during damper operation.  
         [0039]     A gap between the inner wall (diameter)  38  of the cylindrical housing and the outer diameter  40  of the piston sleeve  26  forms a working portion, a fluid orifice  42 .  
         [0040]     Each piston rod  30  or  30 ′ has a threaded rod end  46  or  46 ′, respectively. A first structure that needs a vibration control is attached to at least one end of the piston rods  30  and  30 ′ by means of welding or fastening of at least one of threaded rod ends  46  and  46 ′. A second structure related to the first structure is attached to the MR damper housing or body  14  by means of welding of the covers  16  and  16 ′ or fastening the tie rod  20  or  20 ′.  
         [0041]     When the piston rods  30  and  30 ′ are displaced (says from right to left in  FIG. 1 ) due to a vibration-induced movement from the structure that is attached to the MR damper body  14 . Then the MR fluid  48  is forced to flow from a compression chamber (the first fluid chamber  32 ) to an expansion chamber (the second fluid chamber  34 ) through the annular fluid orifice  42 .  
         [0042]     A magnetic field is generated when an electric current is applied to the preferably three spools of wound coils  50 ,  50 ′ and  50 ″, then a yield strength of the MR fluid  48  is increased in response to the magnetic field generated. The flow of the MR fluid  48  between the fluid chambers  32  and  34  can be controlled by the magnitude of the induced magnetic field via modulation of the electrical current applied to the wound coils  50 ,  50 ′ and  50 ″. In this way, the desired damping rate of the MR damper  10  is modulated so as to reduce the vibration of the attached structures.  
         [0043]     Spaces between pole pieces  52 ,  52 ′,  52 ″ and  52 ′″ and the inner diameter  38  of the cylindrical body  14  form an active fluid region where the MR fluid  48  is being polarized. In this example embodiment of the present invention, the wound coils  50 ,  50 ′ and  50 ″ are wrapped in an alternate fashion in order to minimize inductance and allow an addictive magnetic field at the pole pieces  52 ′ and  52 ″. Electrical wires  54  that are connected to the wound coils  50 ,  50 ′ and  50 ″ are preferably sealed by using a hermetic seal  56  that is placed in a pilot hole  58 . Then the electrical wires  54  exit from the piston head sleeve  26  via a wire tunnel  60  to the threaded rod end  46 ′. Epoxy-resin pastes  62 ,  62 ′ and  62 ″ are coated on the outer diameter of the wound coils  50 ,  50 ′ and  50 ″ in order to avoid the direct contact of the wound coils  50 ,  50 ′ and  50 ″ with the MR fluid  48  to prevent them from being worn and short-circuited.  
         [0044]     Referring to  FIG. 1 , one or more sensors  74  are arranged at the above structure to collect signals which are transmitted to a controller  72  which controls a current to be applied to the wires  54 . The controller  72  can be any of those in the art.  
         [0045]     Now referring again to  FIG. 1 , during the on-state of MR fluid damper  10 , the MR fluid  48  will be polarized to a high yield stress level by the high magnetic field induced through the electromagnetic circuit, so that it acts like a plug at the fluid orifice  42  between the two fluid chambers  32  and  34 , which are divided by the piston assembly  12 . As a result, the MR fluid in the annular fluid orifice  42  acts like an O-ring seal and slides with the piston assembly  12  in a direction of the inner diameter of the cylindrical housing  14 , not allowing any fluid to pass from the compression chamber to the expansion chamber through the fluid orifice  42  during the damper operation cycle and vice versa. This situation causes cavitation in the expansion chamber and then initiates the force-lag phenomenon of the MR damper.  
         [0046]     Due to the relatively high viscosity of the MR fluid, it is very difficult to eliminate all the air pockets and dissolved air therein, even though special care is taken to do so in the art.  
         [0047]     The inventors have developed an inventive method and device using an appropriate pressure of the MR liquid to obviate the above drawbacks.  
         [0048]     The inventors have determined that a successful solution is to increase the pressure of the MR fluid in the closed interior compartment  22  so as to reduce the effect of the trapped air and overcome the seal plug effect due to the relatively high yield stress of the MR fluid  48 .  
         [0049]     The inventors have conducted experiments to identify the effect of force-lag phenomenon against pressures of the MR fluid in the device. An MR damper with different pressurized fluids according to the invention is tested under a 20 mm, 0.1 Hz triangular displacement excitation with operation current at 1.5 A. The result is shown in  FIG. 2 .  
         [0050]     Referring to  FIG. 2 , which shows the effect of pressurized MR fluids at 0, 25, 50, 75 and 100 psi on the force-lag phenomenon, it can be seen that the force-lag phenomenon can be reduced as the MR fluid pressure is increased. When the pressure of the MR fluid within the damper is raised to 100 psi, the force-lag phenomenon is nearly eliminated.  
         [0051]     It is expected that the performance of the MR damper will be fine where the MR fluid keeps a pressure from 100 psi to 400 psi, preferably from 100 psi to 200 psi.  
         [0052]     The inventors have also discovered that in order to prevent the force-lag phenomenon of the MR damper  10 , special care is needed in filling of the MR fluid to minimize the trapped air pockets. In this example embodiment as shown in  FIG. 1 , an inlet  64  and an outlet  64 ′ are respectively provided at the covers  16  and  16 ′ so as to keep the fluid being filled in the device in one direction, which will help solve this problem.  
         [0053]     In a preferable embodiment, an inlet is configured to connect a directional valve. In another embodiment, a directional valve is fit to the housing  14  as an inlet, which is readily understood for one of ordinary skill in the art.  
         [0054]     The directional valve that is used in the invention can be any of those well-known to ordinary skill in the art.  
         [0055]     An exemplary MR fluid filling setup including a hand pump (for example, ENERPAC® P-142), two pressure gauges, two quick-release couplers (for example, FASTER® ANV 14 GAS), etc. is used in the invention to pressurize the fluid chamber in order to prevent the force-lag phenomenon of the MR damper. The MR fluid will be pumped into the MR damper by using the hand pump. One pressure gauge is used to monitor the outlet pressure of the hand pump, and the other pressure gauge is used to monitor the internal pressure of the MR damper. The quick couplers are used in a hydraulic system to quickly connect lines without losing fluids or fluid pressure. The quick coupler consists of two mating halves: the plug (male) half and the coupler (female) half. The female coupler itself acts as a directional valve, which can withstand a working pressure as high as 5,000 psi.  
         [0056]     An MR fluid  48  is first introduced into the MR damper  10  via the inlet/outlet  64  or  64 ′ through a passageway  66  or  66 ′ to the compartment  22 . When the compartment  22  is fully filled with the MR fluid  48 , a hydraulic directional valve  68  and a hydraulic fastener  70  are fastened to the inlet/outlet  64 ,  64 ′ respectively or vice versa. In order to minimize the trapped air pockets inside the MR damper  10 , the MR damper  10  is pre-run for several cycles and kept stable for several hours. Then the MR fluid filling process as aforementioned is repeated until no more refills can be done. The above can help minimize the air pocket inside the MR damper. Finally, the compartment  22  of the MR damper  10  is pressurized in order to prevent the force-lag effect by pressuring the MR fluid in the MR damper  10  via the directional valve  68 . The use of the directional valve  68  provides a compact and alternate solution to the use of an accumulator to solve the force-lag effect.  
         [0057]     The MR damper according to the present invention is broadly applied to the vibration reduction system, in particular to a railway vehicle suspension system. The MR damper  10  can be used to replace conventional dampers to provide an excellent performance in the railway suspension system. In practice, the MR damper body is attached to a first structure of the railway vehicle (says the truck) through the covers  16  and  16 ′ or the tie rod  20  or  20 ′. Then the at least one end of the piston rods  30  and  30 ′ is attached to a second structure of the railway vehicle (says the car body) through the at least one end of the threaded rod ends  46  and  46 ′. The controller  72  may be used to control the MR damper  10  via controlling an input current according to the information from the sensor  74 .  
         [0058]      FIGS. 3, 4  and  5  illustrate a railway vehicle  76  utilizing MR dampers  78 ,  78 ′,  78 ″ and  78 ′″, according to an example embodiment of the present invention.  
         [0059]     MR dampers  78  and  78 ′ are attached in a secondary suspension system between the car body  80  and leading truck  82 . MR dampers  78 ″ and  78 ′″ are attached in the secondary suspension system between the car body  80  and trailing truck  84 . Numerals  86 ,  86 ′ and  86 ″ represent the longitudinal (x), lateral (y), vertical (z) directions of the railway vehicle, respectively; and numerals  88 ,  88 ′ and  88 ″ represent the yaw, roll, and pitch directions of the railway vehicle, respectively.  
         [0060]     A control strategy adopted based on the measurement of the absolute lateral velocity of the car body and compared with a predetermined threshold velocity can be found in “Semi-Active Suspension Improves Rail Vehicle Ride” by O&#39;Neill and Wale. In this embodiment of the present invention, the absolute lateral velocities of a car body center  90  above the leading truck  82  and a car body center  92  above the trailing truck  84  will be measured individually by different sensors. Then, the damping forces of those two sets of the MR dampers  78 ,  78 ′ and  78 ″,  78 ′″ will be controlled individually according to the comparison of the measurement of each sensor with the predetermined threshold velocity.  
         [0061]     Although the above example embodiments of the present invention have been described herein for illustrative purpose, one of ordinary skill in the art will appreciate that various modifications, additions and substitutions, without departing from the spirit of the invention can be made, which will fall within the scope of the appended claims.