Abstract:
A printed circuit board (PCB) coil linear actuator is disclosed. The actuator includes a coil assembly and a magnet assembly. The coil assembly includes a plurality of PCB coils electrically connected in series. The PCB coils arranged in a row and adjacent PCB coils are separated by a gap. Each PCB coil includes a low aspect ratio, multi-layer coil member disposed on a board member. The actuator assembly includes a plurality of magnet units arranged in a row, wherein adjacent magnet units are separated by a gap. When the actuator is assembled, the PCB coils arranged in alternating sequence with the magnet units. The PCB coil linear actuator is intended to replace traditional slotted bobbin voice coil actuators (VCAs) and is particularly useful in fast steering mirror (FSM) applications. The PCB coil linear actuator provides many advantages over a VCA of an equivalent motor constant, including improved performance, lower weight and a lower profile.

Description:
[0001]    This invention was made with Government support under contract FA9453-05-M-0070 awarded by the AFRL/PK8VV Det 8 Air Force Research Laboratory. The Government has certain rights in this invention. 
     
    
     BACKGROUND 
       [0002]    A voice coil actuator (VCA) is typically used as the actuator in fast steering mirrors (FSM). A VCA typically includes a coil wrapped around a slotted bobbin. An example of a conventional VCA  10  is shown in  FIGS. 1 and 2 . As shown in  FIGS. 1 and 2 , the VCA  10  includes a housing  20 , a magnet assembly  40  disposed within the housing  20  and a coil assembly  60  disposed within the housing  20 . The coil assembly  60  includes a coil member  62  wound about a slotted bobbin  64 . 
         [0003]    It is desirable to provide an actuator having improved performance, lower weight and a lower profile in comparison to conventional voice coil actuators. 
       SUMMARY 
       [0004]    A PCB coil linear actuator is disclosed. The actuator includes a coil assembly for attaching to a base side of a device, such as a fast steering mirror (FSM), and a magnet assembly for attaching to a moveable side of the device, such as a mirror side of a FSM. In some cases the reverse is also possible where the coil assembly is attached to the mirror and the magnet assembly is attached to the base. 
         [0005]    The coil assembly contains a plurality of PCB coils connected in series. Each PCB coil includes a low-profile, oval-shaped coil member having multiple coil layers, wherein the coil member is disposed on a board member that is secured to base of the coil assembly. 
         [0006]    The actuator further includes a magnet assembly having a plurality of magnet units secured to a base of the magnet assembly and positioned in line with the PCB coils such that the PCB coils and magnet units are arranged in alternating fashion along the length of the actuator. The magnet units each include a lower magnet and an upper magnet having opposite polarizations, with their polar axes being perpendicular to the planes of the coil members. Pole pieces, or shunts, are positioned outside of the magnets in the outermost magnet units in order to close the magnetic circuit in the actuator. 
         [0007]    The inventive PCB coil actuator provides several advantages over known voice coil actuators (VCAs). Such advantages include:
       Highly reliable manufacturing without coil winding   Modular design enabling quick application matching   Reduced assembly time   Simpler electromechanical interface   Parallel/serial winding combinations can be jumpered directly on the PCB coil   Excellent thermal conductivity to the mounting base of the coil assembly   Very high shock and vibration tolerance   High rigidity resulting in exceptional high frequency performance       
 
         [0016]    The invention can best be understood in the following detailed description with reference to the attached drawing figures. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]      FIG. 1  is a perspective view of a conventional voice coil actuator. 
           [0018]      FIG. 2  is a perspective view of a bobbin and voice coil of the voice coil actuator of  FIG. 1 . 
           [0019]      FIG. 3  is a side sectional view of a PCB coil linear actuator according to an embodiment of the invention. 
           [0020]      FIG. 4  is a perspective view of a coil assembly of the PCB coil linear actuator of  FIG. 3 . 
           [0021]      FIG. 5  is a top view of a PCB coil of the coil assembly of  FIG. 4 . 
           [0022]      FIG. 6  is a cut-away side view showing the stacked layers of the coil members of the PCB coil of  FIG. 5 . 
           [0023]      FIG. 7  is a schematic drawing showing the electrical interconnection of the stacked layers of the coil members of  FIG. 6 , wherein the stacked layers are connected to each other in series. 
           [0024]      FIGS. 8A-8C  are top, side and end views, respectively, of a coil base of the coil assembly of  FIG. 4 . 
           [0025]      FIGS. 9A and 9B  are top and side views, respectively, of a coil clamp of the coil assembly of  FIG. 4 . 
           [0026]      FIGS. 10A and 10B  are top and side views, respectively, of a coil standoff of the coil assembly of  FIG. 4 . 
           [0027]      FIG. 11  is a perspective view of a magnet assembly of the PCB coil linear actuator of  FIG. 3 . 
           [0028]      FIGS. 12A and 12B  are perspective views showing two faces of a magnet of the magnet assembly of  FIG. 11 . 
           [0029]      FIGS. 13A-13C  are top, side and end views, respectively, of a magnet base of the magnet assembly of  FIG. 11 . 
           [0030]      FIGS. 14A and 14B  are top and side views, respectively, of an inner lower magnet clamp of the magnet assembly of  FIG. 11 . 
           [0031]      FIGS. 15A and 15B  are top and side views, respectively, of an inner magnet holder of the magnet assembly of  FIG. 11 . 
           [0032]      FIGS. 16A and 16B  are top and side views, respectively, of an inner upper magnet clamp of the magnet assembly of  FIG. 11 . 
           [0033]      FIGS. 17A and 17B  are top and side views, respectively, of an outer lower magnet clamp of the magnet assembly of  FIG. 11 . 
           [0034]      FIGS. 18A and 18B  are top and side views, respectively, of an outer magnet holder of the magnet assembly of  FIG. 11 . 
           [0035]      FIGS. 19A and 19B  are top and side views, respectively, of an outer upper magnet clamp of the magnet assembly of  FIG. 11 . 
           [0036]      FIGS. 20A and 20B  are top and side views of magnet shunts forming the pole pieces of the magnet assembly of  FIG. 11 . 
           [0037]      FIG. 21  is a graph showing the magnitude response of a PCB coil linear actuator. 
       
    
    
     DETAILED DESCRIPTION 
       [0038]    A PCB coil linear actuator according to one embodiment of the invention is described in the following paragraphs with reference to  FIGS. 1-21 . Such an actuator is particularly useful in fast steering mirror (FSM) applications, however other uses are envisioned. 
         [0039]      FIG. 3  shows a PCB coil linear actuator  100 . The actuator  100  includes a coil assembly  110  and a magnet assembly  150 , which will now be described in detail. 
         [0040]    Referring to  FIGS. 3 and 4 , the coil assembly  110  includes a plurality of PCB coils  112  arranged in a row on a coil assembly base  120 , such that a gap  114  is formed between adjacent coils  112 . The PCB coils  112  are electrically connected in series. As shown in  FIGS. 3 ,  4  and  8 A- 8 C, the coil assembly base  120  may be mounted to a base  302  of an FSM device  300  using mounting holes  122 . As shown in  FIGS. 3 ,  8 A and  8 C, the base member  120  includes retaining slots  124  for retaining a bottom end of the PCB coils  112 . The coil assembly  110  further includes coil standoffs  126  (shown in  FIGS. 4 ,  10 A and  10 B), which are arranged in two rows along opposing sides of the base member  120  and attached to the base member  120  via fastening holes  130 ,  132 , and include retaining slots  134  for retaining longitudinal ends of the PCB coils  112 . A pair of coil clamps  136 , shown in  FIGS. 9A and 9B , are arranged in two rows along the two opposing sides of the base member  120 , and are fastened to the upper end ends of the coil standoffs  126   a ,  126   b  via fastening holes  130 ,  138  and matching screws (not shown). Screws, not shown, are used to fasten the coli standoffs  126   a ,  126   b  to the base member  120 . The coil clamps are positioned to secure top edges of the PCB coils  112  within retaining slots  140 . 
         [0041]    As illustrated in  FIGS. 5-7 , each PCB coil  112  includes a low-profile, substantially flat, coil member  116  mounted on a board member or PC board  118 . The coil member  116  is wound in a substantially oval shape on the surface of the board member  118  and includes multiple coil layers  116   a - 116   h  stacked one on top of another and connected in series. The layers  116   a - 116   h  are insulated from each other by lamination (not shown).  FIG. 7  shows a detailed schematic of the electrical connection of the coil layers  116   a - 116   h . As shown in  FIG. 7 , individual coil layers are connected in series and pairs of coils are bridged together. In the example given, each coil member  116  includes eight coil layers configured to yield eight turns per board. The interconnect from the top layer  116   a  to the fifth middle layer  116   f , the second middle layer  116   b  to the sixth middle layer  116   g , etc. is with conductive vias through the board member  118 . There are two turns per layer for the first four layers  116   a - 116   d  that are then connected in parallel to the next four layers  116   e - 116   h . A PCB coil with the described configuration may have a thickness of about 0.04 in. Although one particular embodiment of a PCB coil is described above, it should be understood that other embodiments are possible, having varying numbers of layers and turns, varying connection schemes between layers and varying shapes and sizes. 
         [0042]    Turning to  FIGS. 3 and 11 , the magnet assembly  150  includes inner magnet units  160  at an interior area of the magnet assembly  150  and outer magnet units  180  placed at the ends of magnet assembly  150 . The magnet units  160 ,  180  are arranged in a single row and are secured to a magnet assembly base  200  such that adjacent magnet units  160 ,  180  are separated by a gap  152  for accommodating a PCB coil  112 . As shown in FIGS.  11  and  13 A- 13 C, the base member  200  may be mounted to a mirror  304  of the FSM device  300  using mounting holes  192 . It is also possible, and in some cases desirable, to mount the base member  120  of the coil assembly  110  to the FSM mirror side  304  and to mount the base member  200  of the magnet assembly to the FSM base  302 . 
         [0043]    As shown in  FIGS. 3 , and  12 , the inner magnet units  160  each include a first or lower magnet  162  and a second or upper magnet  164 . The lower magnet  162  and upper magnet  164  are arranged such that they have opposite polar orientation (indicated by “N” and “S”), with their polar axes being perpendicular to the planes of the coil members  116 . With reference to FIGS.  3  and  14 A- 16 B, the inner magnet units  160  further include an inner lower magnet clamp  166  ( FIGS. 14A and 14B ), an inner central magnet holder  168  ( FIGS. 15A and 15B ) and an inner upper magnet clamp  170  ( FIGS. 16A and 16B ) The inner lower magnet clamps  166  and inner upper magnet clamps  170  are substantially rod-shaped. The inner central magnet holder  168  is essentially H-shaped, having a horizontal portion  168   a  and vertical portions  168   b  at opposite ends of the horizontal portion  168   a . The inner lower magnet clamps  166  and inner upper magnet clamps  170  attach to lower and upper ends, respectively, of the vertical portions  168   b  of the inner central magnet holder  168  via fastening holes  166   a ,  168   c  and  170   a  and matching fasteners (not shown). Thus, the lower magnet  162  is secured between the inner lower magnet clamp  166  and the inner central magnet holder  168 , while the upper magnet  164  is secured between the inner upper magnet clamp  170  and the inner central magnet holder  168 . The inner lower magnet clamps  166  are attached to the magnet assembly base  200  by fasteners (not shown) inserted in fastening holes  166   b  and matching holes in the base  200  (not shown). 
         [0044]    As shown in  FIGS. 3 ,  12 A and  12 B, each of the outer magnet units  180  also includes a first or lower magnet  162  and a second or upper magnet  164  arranged such that they have opposite polar orientations (indicated by “N” and “S”), with their polar axes being perpendicular to the planes of the coil members  116 . The outer magnet units  180  further include a pole piece or shunt  182  ( FIGS. 20A and 20B ). With reference to FIGS.  3  and  17 A- 19 B, the outer magnet units  160  further include an outer lower magnet clamp  186  ( FIGS. 17A and 17B ), an outer central magnet holder  188  ( FIGS. 18A and 18B ) and an upper magnet clamp  190  ( FIGS. 19A and 19B ) The outer lower and outer upper magnet clamps  186  and  190  are substantially bar-shaped. The outer central magnet holder  188  is essentially H-shaped, having a horizontal portion  188   a  and vertical portions  188   b  at opposite ends of the horizontal portion  188   a . The outer lower magnet clamps  186  and outer upper magnet clamps  190  attach to lower and upper ends, respectively, of the vertical portions  188   b  of the outer central magnet holder  188  via fastening holes  186   a ,  188   c  and  190   a . Thus, the lower magnet  162  is secured between the outer lower magnet clamp  186  and the outer central magnet holder  188 , while the upper magnet  164  is secured between the outer upper magnet clamp  190  and the outer central magnet holder  188 . The pole piece  182  is positioned outside of the upper and lower magnets  162 ,  164  in the outer magnet unit  180  and is secured between the outer lower magnet clamp  186  and the outer upper magnet clamp  190 . The outer lower magnet clamps  186  attach to the magnet assembly base  200  by mounting holes  187  and  194 . 
         [0045]    The magnets  162 ,  164  may be, by way of example, 35 MGOe Neodymium Boron Iron (NdBFe) energy product magnets. For higher performance, the magnets  162 ,  164  may be 50 MGOe NdBFe permanent magnets. For high temperature operation, high energy product samarium cobalt magnets  162 ,  164  may be used. Other types of magnets may be used, as well. The pole pieces may be made of readily available carbon steel, for example, C1008, which has a saturation flux density of about 18,000 Gauss. Higher performance can e achieved, however, by constructing the pole pieces  182  from Hiperco 50A (also known as Vanadium Permendur), which has a saturation flux density up to 23,000 Gauss. Because of the higher flux density, less Hiperco 50A would be required than C1008 by the ratio of the flux densities, which would reduce the weight of the actuator. 
         [0046]    The other, non-magnetic members of the magnet assembly (i.e., magnet assembly base  200 , lower magnet clamps  166 ,  186 , central magnet holders  168 ,  188  and upper magnet clamps  170 ,  190 , as well as the coil base member  120 , coil standoffs  128   a ,  128   b  and coil clamps  136  may be made from aluminum or another suitable material. One other possible material is Torlon (polyimide-amide, PIA), which is strong has high temperature resistance and exhibits a thermal coefficient of expansion which is very close to that of C1008 and Neodymium magnets. Aluminum is stronger than Torlon, but is twice as heavy. These and other materials may be used based on design considerations such as weight, structural rigidity and cost. 
         [0047]    According the embodiment shown, seven magnet units are provided, including five inner magnet units  160  and two outer magnet units  180 , thereby providing a total of fourteen magnets. The magnet assembly  150  is designed with the coil assembly  110  in mind. When the actuator  100  is assembled ( FIG. 3 ), the coil assembly  110  and the magnet assembly  150  interface such that the coil assembly base  120  and the magnet assembly base  200  are positioned at opposing sides, the PCB coils are inserted into the gaps  152  between adjacent magnet units  160  and  180 , the inner magnet units  160  are inserted into the gaps  114  between adjacent inner PCB coils  112  and the outer magnet units  180  are positioned outside of the outer PCB coils  112 . Thus, the PCB coils  112  and magnet units  160 ,  180  are arranged in alternating fashion along the length of the actuator  100 . It should be understood that the actuator  100  can be scaled up or down in similar configurations having various numbers of PCB coils and magnet units  160 ,  180 . 
         [0048]    In operation, when a current is applied to the PCB coils  112 , the coils  112  produce a magnetic field which interacts with the magnetic fields produced by the magnet assemblies  160 ,  180 , thereby providing movement of the mirror side  304  of the FSM  300  proportional to the applied current. The force applied by the actuator is determined by the equation: 
         [0000]        F (newtons)= NILB    
         [0000]    where: 
         [0049]    N=the effective number of turns; 
         [0050]    I=the current flowing into the PCB coil (amps); 
         [0051]    L=the effective length of  1  turn (meters) normal to the magnetic field; 
         [0052]    and 
         [0053]    B=the average magnetic field flux density (tesla) applied normally through the PCB coils 
         [0054]    The performance characteristics and benefits of the inventive actuator can be appreciated from the following example. 
       EXAMPLE 
       [0055]    A prototype PCB coil linear actuator was evaluated for a FSM application. Some key parameters for the actuator were a required stroke of ±0.032 in. (0.8 mm), a gap between the coil assembly and magnet assembly of 0.020 in. (0.5 mm) and a peak force greater than 50 Newtons. Further parameters for the prototype were as follows. 
         [0056]    The prototype PCB coil linear actuator employed six, eight-layer PCB coils having a thickness of 0.040 in. and an equivalent of eight turns per board. The PCB coils were connected in series, resulting in 48 effective turns. The footprint of each PCB coil was 3.00 in.×0.675 in. The prototype further employed fourteen 35 MG NdBFe magnets (2 in. long and about ⅜ in. wide) and C1008 pole pieces. The non-magnetic, structural members of the actuator were constructed of aluminum. The predicted and actual test results test results for the first prototype, in comparison with the test results for a conventional voice coil actuator (VCA), appear in the following Table 1. 
         [0000]    
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Actual vs. Predicted Performance of PCB Coil Linear Actuator Prototype 
               
             
          
           
               
                   
                   
                 Typical Voice 
                 PCB Coil 
                   
               
               
                 Actuator Parameter 
                 Symbol 
                 Coil Actuator 
                 Actuator 
                 Units 
               
               
                   
               
             
          
           
               
                 ELECTRICAL 
                   
                   
                 Actual 
                   
               
               
                 Force Constant 
                 K F   
                 5.8 
                 2.54 
                 Newtons/Amp 
               
               
                 Motor Constant 
                 K M   
                 2.8 
                 4.33 
                 Newtons/√Watt 
               
               
                 Back EMF Constant 
                 K B   
                 5.8 
                 2.54 
                 Volts/(meter/sec) 
               
               
                 Nominal Force 
                 F nom   
                 6.75 
                 10.61 
                 Newtons 
               
               
                 Voltage @Nominal Force 
                 V nom   
                 5.12 
                 1.43 
                 Volts 
               
               
                 Current @ Nominal Force 
                 I nom   
                 1.164 
                 4.18 
                 Amps 
               
               
                 Power @ Nominal Force 
                 P nom   
                 6.0 
                 6.0 
                 Watts 
               
               
                 Peak Force (20% Duty Cycle) 
                 F pk   
                 33.8 
                 53.4 
                 Newtons 
               
               
                 Voltage @ Peak Force 
                 V pk   
                 26.2 
                 7.2 
                 Volts 
               
               
                 Current @ Peak Force 
                 I pk   
                 5.8 
                 21.05 
                 Amps 
               
               
                 Power @ Peak Force 
                 P pk   
                 152 
                 152 
                 Watts 
               
               
                 Voltage for 21 N Req&#39;d Force per 
                   
                 15.93 
                 2.84 
                 Volts 
               
               
                 Actuator 6 for 12″ Dia. BSM 
               
               
                 Current for 21 N Req&#39;d Force per 
                   
                 3.62 
                 8.28 
                 Amps 
               
               
                 Actuator for 12″ Dia. BSM 
               
               
                 Power for 21 N Req&#39;d Force per 
                   
                 57.68 
                 23.50 
                 Watts 
               
               
                 Actuator for 12″ Dia. BSM 
               
               
                 DC Winding Resistance 
                 R c   
                 4.4 
                 0.343 
                 Ω 
               
               
                 Winding Inductance 
                 L c   
                 1.4 
                 0.044 
                 milli-Henries 
               
               
                 MECHANICAL 
               
               
                 Nominal Actuator Length 
                 L nom   
                 1.56 
                 0.887 
                 inch (millimeters) 
               
               
                   
                   
                 (39.62) 
                 (22.4) 
               
               
                 Coil Weight 
                 m coil   
                 25 
                 67.0 
                 grams 
               
               
                 Magnet Weight 
                 m mag   
                 19.6 
                 142.0 
                 grams 
               
               
                 Actuator Total Weight 
                 m act   
                 22.1 
                 209.0 
                 grams 
               
               
                 Clearance each Side of Coil 
                   
                 ±0.020 
                 ±0.020 
                 inch (millimeters) 
               
               
                   
                   
                 (±0.51) 
                 (±0.51) 
               
               
                 Stroke 
                   
                 ±0.20 
                 ±0.032 
                 inch (millimeters) 
               
               
                   
                   
                 (±5.08) 
                 (±0.81) 
               
               
                 Coil Thermal Resistance to Base 
                 φ TH   
                 5.0 
                 &lt;5.0 
                 ° C./Watt 
               
               
                   
               
             
          
         
       
     
         [0057]    With reference to Table 1, the actuator, as tested, exhibited the following properties:
       force constant—2.54 Newtons/Amp   motor constant—4.33 Newtons/Watt/ 1/2      winding resistance—0.343 ohms   winding inductance—&lt;0.044 milli-Henries   nominal actuator length—0.887 in. (22.4 mm)   coil weight (base side)—67.0 g   magnet weight (mirror side)—142.0 g   total actuator weight—209 g   electrical time constant (L/R)—128 μsec       
 
       FIG. 20 is a graph of actuator magnitude response, showing the results of the prototype testing. 
       [0067]    By comparison, a typical example VCA has the following parameters:
       force constant—5.8 newtons/amp   motor constant—2.8 newtons/watt 1/2      winding resistance—4.4 ohms   winding inductance—&lt;1.4 milli-Henries   nominal actuator length—1.56 in. (39.62 mm)   coil weight—25 g   magnet weight—196 g   total actuator weight—221 g   electrical time constant (L/R)—318 μsec       
 
         [0077]    From the above, it can be seen that the prototype proved that superior performance is provided by the inventive actuator design, and exhibited actual performance above the predicted performance in the critical area of efficiency. For example, the actual motor constant was calculated to be 4.33 newtons/watt 1/2 , which was 54% higher than the typical VCA with approximately the same total actuator weight. The motor constant indicates the overall efficiency of the actuator. The VCA in this example has more effective turns resulting in a high force constant but with much less efficiency. The PCB coil actuator also can be configured for a higher force constant by increasing the effective number of turns, but in this case the design was for an ultra-low resistance actuator (0.343 ohms vs. 4.4 ohms for the example VCA. The motor constant is a function of the force constant and the resistance as follows: K M =K F /R 1/2 . 
         [0078]    The PCB coil actuator prototype was designed to generate 21 newtons of force. For example, the tested prototype required only 23.5 watts of power to generate 21 newtons of force, as opposed to the 57.68 watts required by the VCA. Thus, the prototype required only 41% of the power required by the VCA to produce the same force. Additionally, the tested prototype had a time constant of 128 μsec versus a time constant of 318 μsec for the VCA. The prototype&#39;s tested time constant is 40% of that currently being used in 1 kHz loops, so there is no reason to believe that the inventive actuator cannot be used to close 2 kHz loops. 
         [0079]    The performance of the PCB coil actuator can be improved by using higher performance materials such as vanadium permendur and 50 MGOe magnets. 
         [0080]    The embodiments described hereinabove are further intended to explain best modes know of practicing the invention and to enable others skilled in the art to utilize the invention in such, or other, embodiments and with various modifications required by the particular applications or uses of the invention. Accordingly, the description is not intended to limit the invention to the form disclosed herein. Also, it is intended that the attached claims be construed to include alternative embodiments not explicitly defined in the detailed description.