Abstract:
The disclosed device and method relate to an actuator. The actuator includes a field structure assembly having an arrangement of permanent magnets and magnetically soft components, and a moving coil assembly. The arrangement of permanent magnets includes a conical magnet and a plurality of segmented ring magnets.

Description:
FIELD 
     The described embodiments relate to an actuator. In particular, the exemplary embodiments to an actuator having improved acceleration for payloads at an optimum volume and mass of actuator. 
     BACKGROUND 
     Typical moving coil assembly  100  actuators utilise radial magnets in the field structure, or axial central magnets. A typical “loudspeaker” design uses an annular axial magnet. Production of a large payload acceleration with little electrical power requires a large radial magnetic flux. To increase the magnetic flux of such designs requires that the external dimensions of the actuator be increased. This may not be an option as the space required for an increased-size actuator may not be available, so generally a compromise or work-around has to be found. 
     The described embodiments seek to mitigate the problems associated with the known designs described above. 
     SUMMARY 
     The exemplary embodiments provide an actuator comprising a field structure assembly comprising an arrangement of permanent magnets and magnetically soft components, and a moving coil assembly, wherein the arrangement of permanent magnets comprises a conical magnet and a plurality of segmented ring magnets. 
     The actuator according to the exemplary embodiments includes a magnetic assembly which allows a larger air gap to be formed in a field structure of such an actuator, allowing the coil assembly greater movement within the field structure. Such an actuator can therefore have an more optimal overall mass and volume, allowing it to fit into restricted spaces, and the moving coil assembly (as part of an angular motion mechanism) can travel through a relatively large angle respective to the fixed part. Further, the higher magnetic flux provided by the magnetic assembly is increased relative to that of conventional known designs. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       Specific exemplary embodiments will now be described, by way of example only and with reference to the accompanying drawings that have like reference numerals, wherein:— 
         FIG. 1  is a diagram illustrating an actuator according to the present invention; 
         FIG. 2  is a diagram showing a cross-section of the actuator according to the present invention as shown in  FIG. 1 ; and 
         FIG. 2A  is a diagram showing a plan view of the actuator according to the present invention as shown in  FIGS. 1 and 2 . 
     
    
    
     DETAILED DESCRIPTION 
     A specific embodiment of the invention is shown in  FIGS. 1 to 3 . The actuator  10  consists of two portions: a field structure assembly  200  and a coil assembly  100 . 
     The field structure assembly  200  is a hollow cylindrical structure formed with a closed end, the closed end having a centrally-located hole  280 . Along the central axis of the field structure assembly  200 , there is positioned a cylindrical pole piece  260  which defines a radial space  270  between an outer surface of the pole piece  260  and the inner surface of the field outer pole  290 . A retaining screw  250  is fixed through both the centrally-located hole  280  in the closed end of the field outer pole  290 , and the cylindrical pole piece  260 . 
     In the radial space  270  located towards the closed end of the field outer pole  290  there is located an arrangement of permanent magnets that form an inwardly-facing single pole face. The magnet assembly is formed from a conical magnet  210  and several segments of a ring magnet  220 . The conical magnet  210  has an inclined circumferential face. The upper face of the conical magnet  210  abuts the lower surface of the pole piece  260  while the lower face of the conical magnet  210  abuts the inward-facing surface of the closed end of the field outer pole  290 . The ring magnet segments  220  are provided having inner radial surfaces abutting the outer surface of the pole piece  260  and outer radial surfaces abutting the inner cylindrical walls of the field outer pole  290 . The lower surfaces of the ring magnet segments  220  are inclined to co-operate with the inclined circumferential face of the conical magnet  210  such that these faces abut. The conical magnet  210  and ring magnet segments  220  are fixed in place with adhesive. 
     Towards the open end of the radial space  270  between the inner surface of the field outer pole  290  and the outer surface of the pole piece  260 , an air gap is formed. 
     The coil assembly  100  is a hollow cylindrical structure with one end closed, arranged to fit within the air gap defined at the open end of the radial space  270  between the inner surface of the field outer pole  290  and the outer surface of the pole piece  260 . Around the outer surface of the hollow cylindrical structure a coil  110  is provided. The cylindrical structure is selected from a material that has good thermal conductivity but is electrically non-conductive. A ceramic is a class of material that would fit this requirement. This material characteristic eliminates the production of eddy currents which are detrimental to the response time of the actuator assembly. 
     The field structure  200  is assembled by the following steps: First, the conical magnet  210  is placed against the inward facing surface of the field outer pole  290  and fixed in place with adhesive, the adhesive being applied between the inward facing surface of the closed end of the field outer pole  290  and the conical magnet  210 . Next, the segments of the ring magnet  220  are inserted to abut the inner surface of the field outer pole  290  and the inclined circumferential surface of the conical magnet  210  using a specially designed tool that forces the magnets to remain in place. While the magnets are retained in place, they are fixed in place with adhesive injected through adhesive holes  240  provided in the field outer pole  290 . Then the pole piece  260  is inserted into the gap defined by the conical magnet  200  and assembled ring magnet segments  220 . The pole piece  260  is retained in place with a retaining screw  250  inserted through a centrally located hole  280  in the closed end of the field outer pole  290 . An end stop  230  is then inserted into the still open end of the shaft in the pole piece  260  to act as a shock absorber for when, in use, the coil assembly  100  strikes the top of the end stop  230 . 
     Due to the novel magnetic topology created by the above described arrangement of magnets, the actuator  10  can move a mirror connected to the mating point  140  of the coil assembly  100  through a relatively large angle as the large air gap allows a large range of movement and the significant radial magnetic flux allows large payload acceleration at an optimum volume and mass of the actuator  10 . 
     It is to be understood that any feature described in relation to any one embodiment may be used alone, or in combination with other features described, and may also be used in combination with one or more features of any other of the embodiments, or any combination of any other of the embodiments. Furthermore, equivalents and modifications not described above may also be employed without departing from the scope of the invention, which is defined in the accompanying claims.