Abstract:
A simple and robust suspension arrangement is provided for taking into account different suspension modes without the need for excessive sensoring or electronics. The novel suspension arrangement includes a first suspension element, which directly suspends the object to the frame, and a second suspension element, which suspends the object to the frame through a magnetic coupling between the object and the second suspension element. The magnetic coupling provides a magnetic coupling force (F h ) to act as a threshold such that the suspension arrangement is designed to magnetically decouple the second suspen-sion element from the object when the excitation force (F e ) transmitted be-tween the frame and the object exceeds the magnetic coupling force (F h ).

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to suspension arrangements. More specifically, the invention relates to a suspension arrangement according to the preamble portion of claim  1 . 
       BACKGROUND ART 
       [0002]    Suspending an object to a frame is a compromise between stability and comfort. In order to protect equipment, personnel or a structure, such as a vehicle against shocks, the suspension should be soft enough to absorb impacts between the frame and the suspended object. However, in most applications soft isolation alone cannot be used because maximised isolation will typically make the object unstable thus preventing the normal use thereof. On the other hand, the suspension between the object and frame should be rigid during normal operation in order to keep the stability and also prevent the isolated or suspended personnel from feeling sickness from the low frequency oscillation. Let us consider the suspension of a vehicle for example. During driving on a smooth surface, it is preferred that the suspension is rigid for stability of the vehicle, whereas the suspension should be soft during shocks caused by bumps or holes in the road. Such a passive suspension is therefore not ideal. 
         [0003]    There have been numerous attempts to optimise suspension arrangements between a frame and the suspended object. Such attempts typically feature a regulator in the suspension designed to sensor the dynamics of the object, such as acceleration, and to alter the damping characteristics of the suspension elements accordingly. Alternatively the suspension characteristics are altered by the user. One example of such a suspension arrangement is presented in US 2010/0276906 A1, which discloses suspension system for a vehicle, which suspension system with a damping assembly operatively connected to an actuator and a controller for controlling movement of the actuator thus regulating the damping rate of the damping assembly. The suspension system makes use of a signal generating device, which provides an output electric signal representing a desired user adjustment to the damping rate of the damping assembly. 
         [0004]    Whether the adjustment is made by the user or automatically by a sensor arrangement, such systems have a tendency to be quite complicated and therefore expensive and delicate. 
         [0005]    It is therefore an aim of the present invention to provide a simple and robust suspension arrangement capable of taking into account different suspension modes without the need for excessive sensoring or electronics. 
       SUMMARY 
       [0006]    The aim of the present invention is achieved with aid of a novel suspension arrangement for suspending an object to a frame for protection against excessive excitation forces transmitted from between the frame or and the object. The arrangement includes a first suspension element, which directly suspends the object to the frame, and a second suspension element, which suspends the object to the frame through a magnetic coupling between the object and the second suspension element. The magnetic coupling provides a magnetic coupling force to act as a threshold such that the suspension arrangement is designed to magnetically decouple the second suspension element from the object when the excitation force transmitted between the frame and the object exceeds the magnetic coupling force. 
         [0007]    More specifically, the suspension arrangement according to the present invention is characterized by the characterizing portion of claim  1 . 
         [0008]    Considerable benefits are gained with aid of the present invention. 
         [0009]    Compared to conventional passive suspension arrangements, the proposed solution provides a remarkable improvement to the isolation properties of the suspension. For example, where an impact is of an order of 10-200 G, the proposed suspension arrangement may be able to reduce the impact transmitted to the isolated object to less than one G. This is a considerable reduction, which is beneficial in protecting for example delicate measuring apparatuses or personnel of sea vessels or land vehicles against sudden unanticipated shocks, or personnel in vehicles or sea vessels designed for rough sea or terrain. 
         [0010]    The benefits of the proposed suspension arrangement compared conventional passive methods include outstanding protection against shocks, which leads to increased durability, and stability. A particular advantage compared to fuse-like safety systems is that the novel arrangement is reversible, whereby the arrangement may be used over and over again. Compared to active or semi-active suspension arrangements, the proposed solution is significantly simpler in construction, which makes it reliable and affordable. Furthermore the novel suspension arrangement can react instantly to shock, transient or high vibration loading. It is to be noted that active systems will always have some internal delay built in because sensors etc. must notice the shock loading before it can deliver order to move from stiff to soft state. The proposed suspension arrangement is reversible which is great advantage compared to many one-shot arrangements that are used in military applications. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0011]    In the following, exemplary embodiments of the invention are described in greater detail with reference to the accompanying drawings in which: 
           [0012]      FIG. 1  presents a schematic illustration of a suspension arrangement according to a first embodiment before and after a shock, 
           [0013]      FIG. 2  presents a schematic illustration of the suspension arrangement of  FIG. 1  during a shock, 
           [0014]      FIGS. 3 a  to 3 c    present graphs showing measurements of three tests of an arrangement of  FIG. 1 , 
           [0015]      FIG. 4 a    presents a schematic illustration of a suspension arrangement according to a second embodiment during a shock, 
           [0016]      FIG. 4 b    presents a schematic illustration of the suspension arrangement of  FIG. 4 a    during a shock, where the frame has moved upwards, 
           [0017]      FIG. 4 c    presents a schematic illustration of the suspension arrangement of  FIG. 4 a    during a shock, where the frame has moved downwards, 
           [0018]      FIG. 5 a    presents a schematic illustration of a suspension arrangement according to a third embodiment before and after a shock, 
           [0019]      FIG. 5 b    presents a schematic illustration of the suspension arrangement of  FIG. 5 a    during a shock, where the frame has moved upwards, 
           [0020]      FIG. 5 c    presents a schematic illustration of the suspension arrangement of  FIG. 5 a    during a shock, where the frame has moved downwards, 
           [0021]      FIG. 6 a    presents a schematic illustration of a suspension arrangement according to a fourth embodiment before and after a shock, 
           [0022]      FIG. 6 b    presents a schematic illustration of the suspension arrangement of  FIG. 6 a    during a shock, where the frame has moved upwards, 
           [0023]      FIG. 6 c    presents a schematic illustration of the suspension arrangement of  FIG. 6 a    during a shock, where the frame has moved downwards, and 
           [0024]      FIG. 7  presents a diagram illustrating test results of an excitation test performed with a suspension arrangement according to  FIGS. 5 a  to 5 c   , where an excitation curve is shown in a dashed thick line and the response curve of the isolated object is shown a solid thin line. 
       
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       [0025]    An exemplary embodiment is described with reference to  FIGS. 1 and 2 , wherein a simple suspension arrangement  100  is illustrated. In the shown example, an object  130  is suspended to a frame  200  by means of two suspension elements, namely a first and second suspension element  110 ,  120 , respectively. The first and second suspension elements  110 ,  120  are coupled to the object  130  via a coupling interface  132 , which in the FIGS. has been depicted as a mere platform connecting the suspension elements  110 ,  120  to the object  130  in parallel. It would also be possible to arrange the suspension elements  110 ,  120  on opposite sides of the object  130 , whereby the frame  200  would surround the object  130  (not shown). The coupling interface  132  connects to or forms part of a mass  131 , which is to be suspended to the frame  200 . In the illustrated example, the mass  131  is approximated to form all of the mass of the object  130 , whereas the coupling interface  132  is assumed weightless. Instead of a suspension arrangement, the novel concept could also be referred to as a suspension apparatus, which includes the disclosed elements. The expression suspension arrangement is nevertheless used throughout this disclosure. 
         [0026]    In the shown embodiment, both suspension elements  110 ,  120  include a spring and a damper. In this context it is to be understood that the damper may employ any suitable method for damping, for example viscosity, friction, electrical, pneumatic etc. In the illustrated embodiments, a conventional viscose damper is depicted. The spring  111  and damper  112  of the first suspension element  110  are coupled in parallel and permanently to the coupling interface  132  and ultimately to the mass  131  and on the other hand to the frame  200 . In other words, a first suspension element  110  is configured to directly suspend the object  130  to the frame  200 . In this context the expression direct suspension means that the object is resiliently fixed to the frame by the first suspension element in a permanent fashion. Such permanent attachment is to be understood as lacking a connection through a clutch or similar detachable coupling, which is configured to automatically disengage in a reaction to load (cf. connection of the second suspension element  120  through a magnetic coupling  140 ). It is to be noted that the shown example shows a spring and a damper in both suspension elements. However as is shown hereafter in connection with  FIGS. 6 a  to 6 c   , a suspension element may also be arranged with a spring or damper only. The spring  121  and damper  122  of the second suspension  120  are also arranged in parallel to connect the frame  200  to the object  130 , but said spring  121  and damper  122  are connected to the coupling interface  132  via a magnetic coupling  140 . The magnetic coupling  140  includes two major components. Firstly, the magnetic coupling  140  features a magnet  141 , which in the shown embodiment is a permanent magnet connected to the second suspension element  120  by combining the spring  121  and damper  122  thereof. Secondly, the magnetic coupling  140  includes a magnetically cooperating element  142 , which may be of ferromagnetic material or an electromagnet, which is connected to the coupling interface  132 . More specifically, the coupling interface  132  has an opening, the perimeter of which is provided with the magnetically cooperating element  142 , which may be a metallic ring element, coating surrounding the opening. Alternatively, a metallic plate, two metallic parts above and below the magnet or a magnet may be used. 
         [0027]    As an alternative embodiment ( FIG. 4 ), the coupling interface can be also constructed so that the magnet  141  is in the middle of the magnetic element  142  or magnet  141  holds the magnetic element  142  from the side. Thus extra holding force is provided by the friction between the elements  141 ,  142  of the magnetic coupling  140 . A great number of varieties may be constructed without using an opening in the coupling interface as shown in the Figs. Alternatively, an L, C or U shaped interface (not shown) could be used for establishing a similar effect. 
         [0028]    It is clear that in all embodiments, the mutual positions of the elements  141 ,  142  of the magnetic coupling  140  may be reversed without affecting the function of the magnetic coupling  140 . 
         [0029]    The second suspension element  120  may have dynamic properties that are different from those of the first suspension element  110 . More specifically, the second suspension  120  element is considerably stiffer than the first suspension element  110 . While this is the case in the illustrated embodiments, it would also be possible to arrange similar suspension elements to act as the first and second suspension element  110 ,  120  or second suspension element  120  can be softer than first suspension element  110 . 
         [0030]    During normal operation, where the force between the frame  200  and first and second suspension element  110  and  120  is less than holding force of the magnetic coupling  140  between the permanent magnet  141  and the magnetically cooperating element  142 , the forces transmitted between the frame  200  and the object  130  are transmitted through both the first and second suspension element  110 ,  120 . When a shock loading occurs, the force between the frame  200  and object  130  exceeds the holding force of magnetic coupling  140 , the contact between the elements of the magnetic coupling  140 , namely the magnetically cooperating element  142  and the magnet  141 , is lost. Consequently, the force between the frame  200  and object  130  is transmitted only through the first suspension element  110 . In this context, the term shock is meant to refer to any impulse or transient loading or such impact transmitted from the frame  200  or from the object  130 . Examples of such shocks include explosions, wheel of a vehicle (car, motorcycle, bicycle etc.) hitting a bump or rough terrain, start up or stop of an engine, failure in machine etc. 
         [0031]    Next, the dynamics of the suspension are discussed in greater detail. In particular, the dynamic properties of the first and second suspension element  110 ,  120  are described. In this context the expression dynamic properties refers to typical suspension properties, which affect the performance of the suspension set-up. Such typical suspension properties include, among others, spring constant, damping constant or the combination thereof. The force transmitted from the motion of the frame  200  and transmitted to the object  130  via the suspension  110 ,  120  in normal operation (cf.  FIG. 1 ) may be equated as follows: 
         [0000]        F   e   =m{umlaut over (x)}   2   +c   1 ( {dot over (x)}   2   −{dot over (x)}   1 )+ k   1 ( x   2   −x   1 )+ c   2 ({dot over (x)} 2   −{dot over (x)}   1 )+ k   2 ( x   2   −x   1 ),   (1)
 
         [0032]    where F e  is the excitation force transmitted to the mass  131 , 
         [0033]    m is the mass of the object  130 , 
         [0034]    {umlaut over (x)} 2  is the acceleration of the mass  131 , 
         [0035]    c 1  is the damping constant of the damper  112  of the first suspension element  110 , 
         [0036]    k 1  is the spring constant of the spring  111  of the first suspension element  110 , 
         [0037]    c 2  is the damping constant of the damper  122  of the second suspension element  120 , 
         [0038]    k 2  is the spring constant of the spring  121  of the second suspension element  120 , 
         [0039]    x 1  and x 2  are the positions of the frame  200  and object  130  in a reference coordinate, respectively, whereby x 2 −x 1  is the displacement of the frame  200  in respect to the mass  131 . Accordingly, x and x denote the first and second derivatives of the position with respect to time, i.e. velocity and acceleration. It is to be noted that the damper  112  and  122  can also be based on friction or pneumatic etc. This calculation example is based on viscose damper elements. Furthermore, the excitation movement can also be caused by the object  130  and then the suspension arrangement is isolating the frame  200  (in the calculation example above the excitation movement comes from the frame  200  and suspension arrangement isolates the object  130 ). 
         [0040]    While the excitation force F e  is greater than the holding force of the magnetic coupling  140 , the contact between the magnetically cooperating element  142  and the permanent magnet  141  is lost and the holding power is decreased significantly. When the permanent magnet  141  is not in contact with the magnetically cooperating element  142  (cf.  FIG. 2 ), the force transmitted to the object  130  may be equated roughly as follows: 
         [0000]        F   e   =m{umlaut over (x)}   2   +c   1 ( {dot over (x)}   2   −{dot over (x)}   1 )+ k   1 ( x   2   −x   1 )   (2)
 
         [0041]    where F e  is the excitation force transmitted to the mass  131 , 
         [0042]    m is the mass of the object  130 , 
         [0043]      {umlaut over (x)}   2  is the acceleration of the mass  131 , 
         [0044]    c 1  is the damping constant of the damper  112  of the first suspension element  110 , 
         [0045]    k 1  is the spring constant of the spring  111  of the first suspension element  110 , 
         [0046]    x 1  and x 2  are the positions of the frame  200  and mass  131  in a reference coordinate, respectively, whereby x 2 −x 1  is the displacement of the frame  200  in respect to the object  130 . Accordingly, x and x denote the first and second derivates of the position with respect to time, i.e. velocity and acceleration. 
         [0047]    In view of the equations (1) and (2) above, the holding force F h  of the magnetic coupling  140  may be designed with a simplified equation: 
         [0000]        F   h   =m·a,    (3)
 
         [0048]    where m is the mass of the object  130 , 
         [0049]    a is the acceleration of the frame  200 , and 
         [0050]    F h  is the holding force of the magnetic coupling  140 , when (x 2 −x 1 ) k 1 &lt;&lt;F h  and c 1 ({dot over (x)} 2 −{dot over (x)} 1 )&lt;&lt;F h . 
         [0051]    For example, when the mass of the object  130 —such as protected equipment or personnel—is 100 kg and holding force of the magnetic coupling  140  is 80 kg, the maximum acceleration that can be transmitted through the suspension arrangement  100  is: 
         [0000]      0.8·g   (4)
 
         [0052]    where g is the gravity (˜9.82 m/s 2 ), when the natural frequency of the first suspension element  110  and object  130  is less than 1 Hz and the excitation displacement/force is in a reasonable area (e.g. a car driving to a bump or a mine explosion near a vessel or vehicle). This rough estimation is based to the fact that when k 1  and c 1  of the first suspension element  110  are chosen to be very loose, the excitation force does not reach the mass  131  because of the excellent vibration isolation properties of the loose first suspension element  110 . 
         [0053]    In normal operation ( FIG. 1 ), the loose first suspension element  110  would not be optimal as the sole suspension element between the frame  200  and object  130  because the object  130  would not be stable in most applications. As an example, let us consider a gyroscope on a sea vessel. Under normal circumstances the gyroscope should be rigidly attached to the frame of the sea vessel for accurate measurements. The more rigid second suspension element  120  is therefore provided with a magnetic coupling  140 . The second suspension element  120  may be rigid, even a steel bar for example, or at least much more rigid than first suspension element  110  to keep the protected or isolated object  130  stable. Because the magnetic coupling  140  keeps the object  130  suspended to the frame  200  in normal circumstances, the first and second suspension elements  110 ,  120  act in parallel. Therefore the second suspension element  120 , which is stiffer than the first  110 , is dominant, whereby the overall suspension characteristics of the suspension arrangement  100  is determined by the stiffer second suspension element  120 . 
         [0054]    When the sea vessel experiences a sudden shock in the excitation direction ED, such as a large upcoming wave or an underwater explosion for example, the delicate gyroscope should be gently suspended to the frame of the sea vessel. For switching from the stiff suspension provided by the second suspension element  120  to a more loose suspension, the second suspension element  120  is released from the object  130  by means of appropriately dimensioned magnetic coupling  140  between the second suspension element  120  and the object  130  (see principle above). During the shock (cf.  FIG. 2 ), the excitation force (F e ) originating from the frame  200  exceeds the magnetic holding force (F h ) between the magnetically cooperating element  142 , whereby the magnetic coupling  140  detaches thus decoupling the second suspension element  120  from the object  130 . It is to be noted that all the forces act in the excitation direction ED. With the second suspension element  120  detached from the object  130 , the object  130  (e.g. gyroscope) is suspended to the frame  200  (e.g. sea vessel) only through the first suspension element  110 , which is softer than the second  120 . The looser suspension isolates the object  130  from the frame  200  during the shock allowing the frame  200  to experience violent displacements without exerting excessive forces to the object  130 . 
         [0055]    After the frame  200  has returned to the rest position, the magnetic coupling  140  resumes its coupled configuration as the permanent magnet  141  attached to the second suspension element  120  and the magnetically cooperating element  142  attached to the object  130  return to the connected state ( FIG. 1 ). The object  130  is therefore again rigidly suspended to the frame  200 . 
         [0056]    The above-described embodiment represents a mere example of the inventive concept for arranging a suspension for a mass in respect to a frame. It is to be understood that a similar inventive suspension arrangement could be established in a great number of variants to the examples of  FIGS. 1 and 2 . For example, the suspension elements  110 ,  120  could alternatively contain only a spring or damper or the first suspension element  110  could only include a spring, whereas the second suspension element  120  could only include a damper. Alternatively, the suspension elements  110 ,  120  could be set up in a combination of the examples given above. The suspension elements  110 ,  120  may also be actively controlled by adjusting the damping properties of the dampers, for example, by means of an electromagnetic adjustment. In addition to mere stiffness, the suspension elements  110 ,  120  may be set up differently in suspension characteristics in that the spring  111  of the first suspension element  110  may be regressive, whereas the spring  121  of the second suspension element  120  may be progressive or degressive, for example. 
         [0057]    As concerns the magnetic coupling  140 , the reattachable coupling between the second suspension element  120  and the object  130  may be provided in a number of different ways to that disclosed above. For example, the magnetically cooperating element may alternatively be provided to the second suspension element to combine the spring and damper. Similarly, the magnet may be provided to the coupling interface. Instead of a permanent magnet, which is disclosed as the preferred option, the magnet may be provided as an active magnet, which is employed electronically, when sensors detect a shock from the frame. While this option is feasible, it is not as fast and robust as the virtually instantaneous permanent magnet arrangement disclosed above. 
         [0058]    Turning now to  FIGS. 3 a  to 3 c   , which present graphs showing measurements of two tests of an arrangement of  FIG. 1 . In the first study ( FIG. 3 a   ), a bicycle was tested effectively with three different rear suspension setups; first with the original stiff coil spring (stiffness: 132 N/mm) of the bicycle alone and then together with a loose coil spring (stiffness: 23 N/mm) coupled to a suspension arrangement  100  as shown in  FIG. 1 . The suspension arrangement was installed to the rear suspension of the bicycle with a loose coil spring (stiffness: 23 N/mm) in parallel with a magnet (holding force: 215 N) that was in series with stiff rubber spring (stiffness: 250 N/mm). First, the bicycle was driven uphill with a plain loose coil spring ( FIG. 3 a    thick line) and then with the novel suspension arrangement ( FIG. 3 a    thin line). The measurement results are presented in  FIG. 3 a    in time domain (X-axis: time and Y-axis: relative displacement of rear suspension). The bicycle with a loose spring was bouncing up and down during the ride to uphill. With the novel suspension arrangement the stability was greatly improved. The difference between the loose spring and the novel suspension arrangement can be seen in  FIG. 3   a:  with the loose spring the relative displacement is oscillating at natural frequency of spring-mass system with for example amplitude of +−8 mm. The oscillating amplitude is clearly lower with novel suspension arrangement and it is not steady state. The suspension is mainly taken care with stiff rubber spring during uphill ride with the novel suspension arrangement. If transient loads with higher than  215  N force occurs then the suspension changes to loose coil spring. 
         [0059]    In the second study ( FIGS. 3 b  and 3 c   ), after the uphill study, the same bicycle was driven to a 6 cm high bump with the novel suspension arrangement ( FIG. 3 c   ) and with the original stiff coil spring ( FIG. 3 b   ). The measurement results are presented in the Figs. in time domain (X-axis: time and Y-axis: acceleration to vertical direction measured from middle of the bicycle frame). While the stability of the bicycle with the original stiff coil spring was good, it is apparent from  FIG. 3 b    that the response during ride to bump was poor. 
         [0060]    With the novel suspension arrangement the stability was similar to original stiff spring, but and the response during ride to bump was excellent as can be seen from  FIG. 3 c   . The difference between the stiff spring and the novel suspension arrangement is clear. With the stiff spring, the acceleration of the bicycle frame in vertical direction was approximately 6 g and with the novel suspension arrangement approximately 1 g (where g is gravity: 9.82 m/s2). With the novel suspension arrangement the suspension is mainly handled by the stiff rubber spring during normal ride. When the transient load from the bump occurs, the suspension changes to loose coil spring giving smooth response. After the transient load the suspension changes back to stiff spring because the magnet holds, the loads during normal ride when the forces are less than 215 N. 
         [0061]    Next, exemplary dimensioning values for the components are presented by way of an example concerning isolating equipment in a vessel, which is anticipated to experience sudden shocks. A suspension arrangement was constructed similarly as illustrated in  FIGS. 5 a  to 5 c    with the following specifications: 
         [0000]      m=300 kg, 
         [0000]        c   1 =1400 Ns/m, 
         [0000]        k   1 =70 N/mm, 
         [0000]        c   2 =10000 Ns/m, 
         [0000]        k   2 =0 N/m, 
         [0000]        x   2   −x   1 =30 mm, 
         [0000]        {dot over (x)}   1 =2.2 m/s, 
         [0000]        {umlaut over (x)}   2 =500 m/s 2 , and 
         [0000]      F h =500 N. 
         [0062]    The results of the test are shown in  FIG. 7 , which shows the measurements of the example of the novel suspension arrangement used in a shock test table to isolate a 300 kg mass (the values are given above). The measurement result is in time domain (horizontal axis is time and vertical axis is a velocity). The thick line is a excitation velocity and thin line is a response velocity of isolated 300 kg mass. As may be concluded, a considerable dampening effect was achieved. 
         [0000]    
       
         
               
             
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 LIST OF REFERENCE NUMBERS. 
               
             
          
           
               
                 Number 
                 Part 
               
               
                   
               
               
                 100 
                 suspension arrangement 
               
               
                 110 
                 1 st  suspension element 
               
               
                 111 
                 spring 
               
               
                 112 
                 damper 
               
               
                 120 
                 2 nd  suspension element 
               
               
                 121 
                 spring 
               
               
                 122 
                 damper 
               
               
                 130 
                 object 
               
               
                 131 
                 mass 
               
               
                 132 
                 coupling interface 
               
               
                 140 
                 magnetic coupling 
               
               
                 141 
                 magnet 
               
               
                 142 
                 magnetically cooperating element 
               
               
                 200 
                 frame 
               
               
                 x 1   
                 position of the object in a reference coordinate 
               
               
                 x 2   
                 position of the object in a reference coordinate 
               
               
                 ED 
                 excitation direction