Abstract:
A vibration device comprises a motor having a stator and a rotor each having one or more magnetic elements. A magnetic element on at least one of the rotor and the stator comprises an electromagnet which can be supplied with an electric current so as to interact with a magnetic element on the other of the rotor and the stator to cause rotation of the rotor with respect to the stator; and the one or more magnetic elements of the rotor, considered together, have a center of mass which is offset from the axis of rotation of the rotor. The rotor and the stator are arranged in, or together form, a cavity containing a liquid.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention relates to vibration devices. 
   2. Description of the Prior Art 
   Vibration devices are used in applications such as pagers or mobile telephones (for “silent” call alerts and the like) and in computer game hand-held controllers (to give tactile feedback to the game player). 
   One type of previously proposed vibration device comprises a conventional rotary motor driving an eccentric mass or cam or the like, having its centre of mass displaced away from the axis of rotation. As the motor drives the offset mass around, a vibration is transmitted to the casing or other member to which the motor is attached. 
   Another previously proposed vibration device comprises a solenoid with a moveable mass (e.g. connected to a metal core within the solenoid) influenced by the magnetic field of the solenoid. The solenoid is fed with an appropriately alternating supply current so that the moveable mass is caused to oscillate and thereby to generate a vibration. 
   There is a constant need for vibration devices to be smaller and lighter, particularly in respect of mobile telephone applications. 
   SUMMARY OF THE INVENTION 
   This invention provides a vibration device comprising a motor having a stator and a rotor each having one or more magnetic elements, in which: 
   a magnetic element on at least one of the rotor and the stator comprises an electromagnet which can be supplied with an electric current so as to interact with a magnetic element on the other of the rotor and the stator to cause rotation of the rotor with respect to the stator; 
   the one or more magnetic elements of the rotor, considered together, have a centre of mass which is offset from the axis of rotation of the rotor; and 
   the rotor and the stator are arranged in, or together form, a cavity containing a liquid. 
   Embodiments of the invention can address the need for a smaller and lighter vibration device by using the magnetic elements (permanent magnets, electrically-energised magnets or a combination of both) of the motor itself as the offset or eccentric mass which causes vibration. Thus, the need for additional masses is avoided. 
   The invention is applicable to devices in which the rotor is arranged to rotate inside the stator or to devices in which the rotor is arranged to rotate at least partially outside of the stator. 
   Preferably the device comprises two or more rotationally displaced electromagnets; and a drive circuit arranged to supply respective drive currents to the electromagnets so that the rotational displacement of the electromagnets and the relative timing of the respective drive currents tends to promote rotation of the rotor with respect to the stator. In this arrangement, the drive circuit can cause the motor to move with a stepped or jerky movement, providing an enhanced vibration effect. 
   The rotor and the stator are arranged in, or together form, a cavity containing a liquid such as an oil. This has various potential advantages. For example, the liquid can help to transfer vibrational motion from the rotor to the casing; the liquid can provide lubrication; and/or the liquid can cause the motor to stop more quickly when a drive current is removed. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects, features and advantages of the invention will be apparent from the following detailed description of illustrative embodiments which is to be read in connection with the accompanying drawings, in which: 
       FIG. 1  schematically illustrates an electric motor having a rotor disposed inside a stator; 
       FIG. 2  schematically illustrates at electric motor having a rotor disposed around a stator; 
       FIGS. 3 and 4  schematically illustrate electric motors having asymmetric rotors; 
       FIGS. 5A to 5D  schematically illustrate the operation of a brushless motor; 
       FIG. 6  schematically illustrates electrical connections to a brushless motor; 
       FIGS. 7A to 7C  schematically illustrate rotors for a brushless motor; 
       FIG. 8  schematically illustrates a game controller; 
       FIG. 9  schematically illustrates a pager; and 
       FIG. 10  schematically illustrates a mobile telephone. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring now to the drawings,  FIG. 1  schematically illustrates an electric motor having a rotor  10  disposed inside a stator  20 ,  30 . 
   The rotor in this example is formed of 3 electrical windings which are arranged to rotate about an axle, the view of  FIG. 1  being of course a cross-section in a plane perpendicular to the axle. 
   The stator is formed of magnetic elements  20 ,  30  which generate a magnetic field which interacts with the fields generated by the coil windings  10 . The motor is encased in a housing  50 . 
   The operation of a simple electric motor of this type is well known and is thoroughly described elsewhere, but in brief summary an electric current is supplied selectively to the coil windings  10  via, for example, a commutator and associated brushes, so that the polarity of the supply current to each winding changes as the motor rotates. Magnetic fields generated by the coil windings  10  interact with magnetic fields provided by the stator  20 ,  30  to generate a rotational motion about the axle  40 . 
   The stator can be formed of permanent magnets, electromagnets or a combination of these. 
     FIG. 2  schematically illustrates another simple electric motor having a rotor  60  disposed about a stator  70 . In this case the stator  70  may be formed of permanent or electrically-energised magnets which are stationary with respect to the casing  50 . Similarly, the rotor  60  may be formed of permanent or electromagnets which are energised with appropriate polarities to cause a rotational motion around the axe  40 . 
   Very many different configurations of electric motors are known. While it is possible to use permanent magnets in an electric motor, it is noted that at least some of the magnetic elements need to be electromagnets, i.e. as part of the rotor, the stator or both. 
   The motors described with reference to  FIGS. 1 and 2  are previously proposed designs. They have in common the feature that the centre of mass of the rotor is coincident or substantially coincident with the axle  40 . This is considered to be a desirable feature of such a motor to provide reduced-vibration operation. 
   In contrast, motors will now be described in accordance with embodiments of the present invention, where the aim is to provide enhanced-vibration operation. Indeed, in many instances, the sole use or purpose of these motors is to generate vibration. 
     FIG. 3  illustrates a motor similar in operation to the motor of  FIG. 1 , but with an asymmetric rotor. In particular, the rotor is formed of one large section  80  and two smaller sections  90 , which co-operate to give a centre of mass which is displaced away from the axe  40 , being for example at a notional position  100 . 
   In  FIG. 3 , the angular displacement of the centres of the magnets forming the sections  80 ,  90  remain at 120° to provide a reliable operation of the motor. However, as the motor rotates, the displaced centre of mass causes the whole arrangement to vibrate. 
   Similarly,  FIG. 4  schematically illustrates a motor similar to  FIG. 2  except that the rotor is formed of two lighter segments  120  and one heavier segment  110 , again giving a displaced centre of mass (e.g. at a notional position  130  in the motor&#39;s current orientation). Again, as the motor rotates, the displaced centre of mass will rotate, causing a general vibration of the whole arrangement. 
   In other embodiments, the size of each of the rotor sections is substantially the same, but the sections have different masses, for example by being fabricated at least in part from different materials. 
   In the motors of  FIGS. 3 and 4  (and indeed in the motors to be described subsequently) the casing  50  is wholly or partially filled with a liquid such as an oil. Preferably the liquid is not electrically conducting so as not to inhibit the operation of any electrical connections such as brushes. The liquid can assist in lubrication of the motor, in damping the motor when power is removed and in transferring the vibration or motion from the rotor to the casing  50 . 
   The casing could be implemented, at least in part, by some of the magnetic elements themselves. For example, in  FIG. 3 , at least a part of the casing could be formed by the stator elements  20 ,  30 , for example with a non-magnetic but liquid-tight material occupying the circumferential gap between the stator elements. 
   The examples described so far are motors with brushes. A brushless motor arrangement will now be described by way of a further example. The drawings shown in  FIGS. 5A to 5D  schematically illustrate the operation of such a brushless motor. Again, they are schematic cross-sections along a plane perpendicular to the motor axle. For clarify a casing containing the liquid referred to above is not shown, although to provide an embodiment of the invention one could be provided or could be formed (at least in part) by the outside of the stator. 
   Turning to  FIG. 5A , the stator is shown as an outer structure  200  having 6 poles or “teeth”. These are arranged in pairs and are labelled as A, B, and C. Inside the stator is a soft steel rotor  210  which rotates about an axle  220 . The rotor has four poles. 
   As mentioned above, the windings around the stator poles are arranged in pairs A, B and C. Each pair has two coils connected in series. Such a set of windings is called a “phase”, so the example shown in  FIG. 5A  is a three-phase motor. Electric current is provided to the phases via respective switches, which are labelled A, B and C to correspond to the labelling of the phases. 
   In  FIGS. 5A to 5D , for clarity the rotor is shown as having a symmetric structure. In accordance with embodiments of the invention, an asymmetric structure is used, and examples of such structures will be shown below with reference to  FIGS. 7A to 7C . 
   Starting with  FIG. 5A , then, phase A is energised and two of the four rotor teeth are aligned with stator teeth for phase A. Next, in  FIG. 5B , the switch for phase B is closed and attraction between the teeth corresponding to phase B causes a counter-clockwise torque to act on the rotor. Looking at  FIG. 5C , this leads to a rotation of the rotor in a counter-clockwise direction. Referring to  FIG. 5D , the current supply to the teeth for phase A is removed and the rotor aligns with the teeth for phase B. The process then repeats with phase C being energised and phase B being released and so on. 
     FIG. 6  schematically illustrates electrical connections to the motor of  FIGS. 5A to 5D . The pairs of coils for each phase are labelled as A 1 , A 2  etc. and current is supplied from a driver circuit  230 , often implemented as a single integrated circuit device. 
   In a “normal” operation, the driver circuit would drive the respective coils in an appropriate order and frequency to provide substantially constant speed operation. If, however, the driver circuit applies a different time interval between the energising of the different phases of the motor, a rougher or “jerky” mode of operation can be achieved. In conjunction with the asymmetric rotors to be described below, this can give an enhanced vibrational effect. 
     FIGS. 7A to 7C  schematically illustrate motors for a brushless motor of the type described above. The rotors are shown in cross section about the axle  220  and may be formed, for example, of soft steel. It can be seen that the centre of mass of each of the rotors is displaced away from the axle, thus leading to a degree of vibration when the rotor is rotated. In other embodiments the general cross section of the rotor is uniform, but at least some of the teeth on the rotor extend along the axle for different distances to give the offset centre of mass. 
   Finally, some examples of the use of a motor of the type described above will now be given.  FIG. 8 , schematically illustrates a game controller  300  associated with a family computer entertainment system (such as a Sony®™ PlayStation 2™ family computer entertainment system)  310 . Various controls  320  on the game controller generate control signals which are passed via control lines  330  to the games processor  310  and control the operation of the particular game in use. At appropriate times in the games operation, signals are sent via control lines  240  to a vibration device  350  within the handheld controller. This causes the vibration device  350  to vibrate, giving an enhanced sensation for the user. 
     FIG. 9 , schematically illustrates a pager  400  having a message screen  410 , an audible warning device  420  and a vibration device  430 . When the pager is set in a so-called “silent” mode, the audible warning device  420  is switched off and only the vibration device  430  is used to notify the user that a message has arrived. Similarly,  FIG. 10  schematically illustrates a mobile telephone having a display screen  500 , an earpiece and audible warning device  510 , a keyboard  520  and a microphone  530 . For use in a “silent” mode a vibration device  540  which may be of the type described above is provided so that the user is notified only by the vibration (and perhaps a flashing light) when a call or message is received. 
   Although illustrative embodiments of the invention have been described in detail herein with reference to the accompanying drawings, it is to be understood that the invention is not limited to those precise embodiments, and that various changes and modifications can be effected therein by one skilled in the art without departing from the spirit and scope of the invention as defined by the appended claims.