Abstract:
A voice coil actuator having a capacitive sensor. A magnetic housing contains at least one magnet, and has a wall that defines a first cavity. A magnetic core is coupled to the magnetic housing and extend from an interior surface of the magnetic housing in a direction of a center axis of the wall of the magnetic housing. A coil assembly has a wall defining a second cavity that at least partly envelops the magnetic core, disposed at least partly inside the first cavity, and adapted to move linearly with respect to the magnetic housing. The coil assembly forms a capacitive sensor with the magnetic core, the capacitive sensor adapted to measure at least one of position, velocity and acceleration of the coil assembly with respect to the magnetic housing.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention generally relates to sensors and more specifically to motion, position and/or acceleration sensors capable of operating in the presence of significant magnetic fields.  
       BACKGROUND  
       [0002]     A simple voice coil actuator is an ideal solution for many applications requiring precise movement, such as semiconductor equipment, defense systems and life-sustaining medical systems due to the simple, non-contacting structure of the design. The structure is typically the same as that found in a simple speaker.  
         [0003]     The voice coil actuator is a direct drive, limited motion device that utilizes a permanent magnetic field and a coil winding (conductor) to produce a force proportional to the current applied to the coil. The permanent magnetic field is provided by a permanent magnetic housing containing one or more permanent magnets, while the coil winding is a part of a coil assembly that moves in-and-out of the permanent magnetic housing along the axis thereof.  
         [0004]     The Lorentz principle governs the electromechanical conversion mechanism of a voice coil actuator. This law of physics states that if a current-carrying conductor is placed in a magnetic field, a force will act upon it. The magnetic flux density, “B”, the current, “I”, and the orientation of the field and current vectors determine the magnitude of this force. Further, if a total of “N” conductors (in series) of length “L” are placed in the magnetic field, the force acting upon the conductors is given by: F=KBLIN, where K is a constant. Hence, the force applied between the coil assembly and the permanent magnetic housing is proportional to the amount of current flowing through the coil.  
         [0005]     For voice coil actuator applications, it is desirable to measure the motion, position and/or acceleration of the coil assembly with respect to the permanent magnetic housing when a current of certain magnitude is applied. Due to the strong magnetic field in the voice coil actuator, linear variable displacement transducers (LVDTs) are not suitable for such measurements.  
         [0006]     Currently, potentiometers and optical sensors are used with the voice coil actuator, but they have their own shortcomings. By way of example, using potentiometers, variable resistors or other contact sensors will turn the voice coil actuator into a contact device, which is limited by the lifecycle due to wear and tear of the contacts. In addition, much noise is generated under vibration due to the use of contact fingers. Further, optical sensors must be mounted externally to the voice coil actuator, and is very costly.  
         [0007]     Therefore, it is desirable to provide a non-contact sensor that can be embedded within the voice coil actuator to measure a movement between the coil assembly and the permanent magnetic housing, which is substantially impervious to the strong magnetic field in the voice coil actuator.  
       SUMMARY  
       [0008]     In an exemplary embodiment according to the present invention, a voice coil actuator has a capacitive sensor. A magnetic housing contains at least one magnet, and has a wall that defines a first cavity. A magnetic core is coupled to the magnetic housing and extend from an interior surface of the magnetic housing in a direction of a center axis of the wall of the magnetic housing. A coil assembly has a wall defining a second cavity that at least partly envelops the magnetic core, disposed at least partly inside the first cavity, and adapted to move linearly with respect to the magnetic housing. The coil assembly forms a capacitive sensor with the magnetic core, the capacitive sensor adapted to measure at least one of position, velocity and acceleration of the coil assembly with respect to the magnetic housing.  
         [0009]     In another exemplary embodiment of the present invention, a position control system is provided. The position control system includes a voice coil actuator including a magnetic housing containing at least one magnet, a magnetic core coupled to the magnetic housing and extending from an interior surface of the magnetic housing, and a coil assembly adapted to move linearly with respect to the magnetic housing. The coil assembly forms a capacitive sensor with the magnetic core, the capacitive sensor adapted to measure at least one of position, velocity and acceleration of the coil assembly with respect to the magnetic housing and generates an output. The position control system also includes a signal conditioning circuit, a position/velocity control circuit and a driver. The signal conditioning circuit receives the output of the capacitive sensor, and processes the output to generate a voltage output. The position/velocity control circuit provides a feedback signal using the voltage output from the signal conditioning circuit. The driver drives the voice coil actuator using the feedback signal from the position/velocity control circuit.  
         [0010]     In yet another exemplary embodiment according to the present invention, a method of measuring at least one of position, velocity and acceleration of a coil assembly with respect to a magnetic housing in a voice coil actuator, is provided. A capacitance variance generated when the coil assembly moves with respect to the magnetic housing, is measured. A feedback signal to control a movement of the coil assembly with respect to the magnetic housing, is generated using the capacitance variance.  
         [0011]     These and other aspects of the invention will be more readily comprehended in view of the discussion herein and accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]      FIG. 1  is a perspective cut-away view of a voice coil actuator including a capacitive sensor in accordance with an exemplary embodiment of the present invention;  
         [0013]      FIG. 2   a  is a schematic cross-sectional view of the voice coil actuator of  FIG. 1 ;  
         [0014]      FIG. 2   b  is an equivalent circuit diagram of the capacitive sensor illustrated in  FIG. 2   a;    
         [0015]      FIG. 3   a  is a top view of two regions of electrically conductive material in accordance with an exemplary embodiment of the present invention;  
         [0016]      FIG. 3   b  is a top view of two regions of electrically conductive material in accordance with another exemplary embodiment of the present invention;  
         [0017]      FIG. 4   a  is a schematic cross-sectional view of a voice coil actuator having a capacitive sensor in accordance with another exemplary embodiment of the present invention that includes areas of electrically conductive material located on an insulated rod attached to a coil assembly housing;  
         [0018]      FIG. 4   b  is an equivalent circuit diagram of the capacitive sensor illustrated in  FIG. 4   a;    
         [0019]      FIG. 5  is a schematic cross-sectional view of an insulated rod in accordance with an exemplary embodiment of the present invention;  
         [0020]      FIG. 6  is a circuit diagram of a signal conditioning circuit for a capacitive sensor in accordance with an exemplary embodiment of the present invention;  
         [0021]      FIG. 7  is an equivalent circuit diagram of a signal conditioning circuit implemented using Application Specific Integrated Circuit (ASIC) for a capacitive sensor in accordance with an exemplary embodiment of the present invention;  
         [0022]      FIG. 8  is a circuit diagram for a timing circuit used in a frequency oscillator technique in accordance with an exemplary embodiment of the present invention; and  
         [0023]      FIG. 9  is a block diagram for a position control circuit in accordance with an exemplary embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0024]     In exemplary embodiments of the present invention, capacitive sensors are used to measure the relative movement, relative positions and/or relative acceleration between a permanent magnetic housing and a coil assembly of a voice coil actuator. Turning now to the drawings, voice coil actuators that include capacitive sensors are shown. The capacitive sensors typically include three plates that are equivalent to a pair of series capacitors, although in other embodiments a greater number of plates can be used. In several exemplary embodiments, the position of the coil assembly with respect to the permanent magnetic housing in the voice coil actuator can be determined by measuring the capacitance of the capacitive sensor. Once the position of the coil is determined, the output of the capacitive sensor can be processed by control circuitry to regulate the position, motion and/or acceleration of the coil.  
         [0025]     An exemplary embodiment of a linear voice coil actuator according to the present invention is shown in  FIG. 1 . A voice coil actuator  10  includes a coil assembly housing  12  around which is wrapped a tubular coil  14  of electrically conductive material such as copper wire. The coil assembly housing  12  and the tubular coil  14  together may be referred to as a coil assembly.  
         [0026]     The coil assembly housing  12  forms a generally cylindrical tube that is closed at one end. The coil assembly housing  12  is at least partly contained within (or enveloped by) a magnetic circuit housing  16 , which may also be referred to as a magnetic housing or a permanent magnet housing. The magnetic circuit housing  16  has a generally cylindrical shape and is open at one end. A cylindrical core  18  (or magnetic core) extends from the closed end of the magnetic circuit housing  16  and is set along the axial centerline of the magnetic circuit housing  16 . The core  18  as shown in  FIG. 1  has a solid body. In other embodiments, the core  18  may have one or more cavities formed therein.  
         [0027]     The magnetic circuit housing  16  includes one or more magnets  20  mounted on an interior surface of a shell  22  made of ferromagnetic material such as soft iron. In the exemplary embodiment shown in  FIG. 1 , the one or more magnets  20  have a generally cylindrical shape and conforms to the contour of the interior surface of the shell  22 , which also has a generally cylindrical shape. In one exemplary embodiment, a number of magnets are arranged so that they are facing radially inward and are all of the same polarity.  
         [0028]     The shell  22  contacts the core  18 , which is also constructed from a ferromagnetic material such as soft iron. The shell  22  may be fixedly attached to the core  18  or may be formed as a single integrated piece with the core  18 . The magnet(s)  20 , shell  22  and core  18  form a magnetic circuit that generates a magnetic field extending radially between the magnets  20  and the core  18 . The coil assembly housing  12  is inserted into the magnetic circuit housing  16  so that the open end of the coil assembly housing  12  at least partly contains (or envelops) the core  18  and the open end of the magnetic circuit housing  16  at least partly contains (or envelops) the coil assembly housing  12 .  
         [0029]     In the illustrated embodiment, an area of electrically conductive material  24  is located on the end of the core that faces the interior surface of the closed end of the coil assembly housing  12 . The area of the electrically conductive material  24  is electrically isolated from the core  18  by a layer of material  28 , which is a poor conductor of electricity, and forms a part of a capacitive sensor. In other embodiments, the area of electrically conductive material  24  may not be provided, and instead, the end surface of the core  18  may be used to for the capacitive sensor.  
         [0030]     The capacitive sensor also includes two or more areas of electrically conductive material  30  located inside the coil assembly housing  12  opposite the area of electrically conductive material  24 . Two of these areas can be connected to electrical contacts  25 . The areas of electrically conductive material  30  are separated by the coil assembly housing  12  by a layer of material  32 , which is similar to the layer of material  28  in that it is a poor conductor of electricity. Another area of electrically conductive material  33  is disposed between the coil assembly housing  12  and the layer of material  32  in the embodiment illustrated in  FIG. 1 . The areas of electrically conductive material  30 , the layer of material  32  and the area of electrically conductive material  33  may have a simple printed circuit board structure.  
         [0031]     The elements of a capacitive sensor in accordance with an exemplary embodiment of the present invention can be illustrated by taking a cross-section of the inventive voice coil actuator shown in  FIG. 1  along the line  26 . Such a cross-section is shown in  FIG. 2   a.  As all elements of  FIG. 2   a  that are essential for the complete understanding of the illustrated embodiment have been described in reference to  FIG. 1 , they will not be discussed again in reference to  FIG. 2   a.    
         [0032]     Each of the two areas of electrically conductive material  30  mounted within the coil assembly housing  12  forms a capacitor with the area of electrically conductive material  24  mounted on the end of the core  18 . The two capacitors are linked in series by the area of electrically conductive material  24 . Therefore, the areas of electrically conductive material form a circuit including two capacitors in series between the contacts  25 . Although the present invention is in no way intended to be limited by theory, the ideal capacitance of the two capacitors formed by the areas of electrically conductive material can be considered as follows:  
       C   =         C   1     ⁢     C   2           C   1     +     C   2             
 
         [0033]     where C 1  and C 2  represent capacitances of the capacitors C 1  and C 2 , respectively. In this and other embodiments/equations, the same symbol will be used for a capacitor and its capacitance for ease of description.  
         [0034]     As the coil assembly housing  12  (or coil assembly) moves within the magnetic circuit housing  16 , the distance between the two areas of electrically conductive material  30  mounted within the coil assembly housing  12  and the area of electrically conductive material  24  mounted on the end of the core  18  varies. This variance also varies the capacitance of C 1  and C 2 . Although not linear, the value C can change significantly with small variations in the position of the coil assembly housing  12  relative to the magnetic circuit housing  16 . The present invention is not limited by theory, however, theory predicts that changes in the capacitance C, which is the total capacitance of the capacitive sensor, will vary ideally as the reciprocal of the change in distance.  
         [0035]     It can be seen in  FIG. 2   b,  which is an equivalent circuit diagram of the capacitive sensor of  FIG. 2   a,  that there actually are additional capacitors C A  and C B  that are present. The capacitors C A  and C B  are respectively formed between the areas  30  and the area  33 . Hence, the total capacitance of the capacitive sensor is given as follows:  
       C   =           C   1     ⁢     C   2           C   1     +     C   2         +         C   A     ⁢     C   B           C   A     +     C   B               
 
         [0036]     However, since the capacitors C A  and C B  have fixed capacitances, they do not affect the distance measurements performed using variable capacitors C 1  and C 2 .  
         [0037]     In one embodiment, the areas of electrically conductive material are formed from plates of metal such as copper. In this case, the areas of electrically conductive material  24 ,  30  and  33  may be referred to as plates or metal plates. In other embodiments, any other suitable material may be used to form the areas  24 ,  30  and  33 . The layers of material that are poor conductors of electricity are constructed from any suitable dielectric material such as epoxy glass (e.g., G10), TEFLON® or any other suitable dielectric material. TEFLON® is a registered trademark of E.I. Du Pont De Nemours and Company, a Delaware corporation.  
         [0038]     An arrangement of the two areas of electrically conductive material  30  mounted to the interior of the closed end of a coil assembly housing  12  in accordance with an exemplary embodiment of the present invention is shown in  FIG. 3   a,  which shows the pattern used in the capacitive sensor of  FIG. 2   a.  The two areas of electrically conductive material  30  resemble half circles and are separated by a gap  36 . As discussed above, these areas of electrically conductive material  30  combine with the area of electrically conductive material  24  shown in  FIGS. 1 and 2   a  to form capacitors.  
         [0039]     In other embodiments, capacitors can be formed using a wide variety of patterns of electrically conductive material involving areas that are electrically isolated from each other. For example, an embodiment of the present invention where the two areas of electrically conductive material are a circle  40  and a concentric ring  42  is shown in  FIG. 3   b.  The circle and the concentric ring are separated by a gap  44 .  
         [0040]     Another exemplary embodiment of a voice coil actuator including a capacitive sensor in accordance with the present invention is shown in  FIG. 4   a.  The voice coil actuator of  FIG. 4   a  is similar to the voice coil actuator shown in  FIGS. 1 and 2   a  in that it includes a core assembly housing  12 ′ having a tubular coil  14 ′ mounted thereon, and a magnetic circuit housing  16 ′ having one or more permanent magnets  20 ′ mounted thereon. However, the configuration of the capacitive sensor is different.  
         [0041]     In the exemplary embodiment illustrated in  FIG. 4   a,  the core assembly housing  12 ′ includes an insulated rod  50 . The insulated rod  50  is connected to a closed end of the core assembly housing  12 ′ so that the two structures are co-axial. The insulated rod  50  includes two areas of electrically conductive material  52 . In order to accommodate the insulated rod  50  when the voice coil actuator is assembled, a core  18 ′ is hollow and has a cavity  54 . In the completed structure, the insulated rod  50  is inserted into the cavity  54  within the core  18 ′.  
         [0042]     The position of the areas of electrically conductive material are shown in the cross section taken along the line  56  in  FIG. 4   a,  which is shown in  FIG. 5 . The insulated rod  50  has two areas of electrically conductive material  52  lining the external surface of the insulated rod that are separated by two gaps  57 . The interior of the insulated rod can be filled with air or another material that is a poor conductor of electricity. The insulated rod  50  has a generally cylindrical shape, and the two areas of electrically conductive material  52  are formed to have a generally semi-circular cross-section and conform to the contour of the insulated rod  50 . Because of the gaps  57 , the cross-sections of the two areas of the electrically conductive material  52  are not complete semi-circles.  
         [0043]     The insulated rod  50  may be constructed from any suitable dielectric material such as epoxy glass (e.g., G10), TEFLON®, or the like. In one embodiment, the two areas of electrically conductive material  52  are constructed from copper plates, or any other suitable metal. The two areas of electrically conductive material  52 , when they are formed in a form of plates, may also be referred to as plates or metal plates.  
         [0044]     Each of the two areas of electrically conductive material  52  shown in  FIGS. 4   a  and  5  forms a capacitor with the ferromagnetic material used in the construction of the core  18 ′. The ferromagnetic material of the core  18 ′ also serves to connect the two capacitors in series. A third capacitor is formed by the two areas of electrically conductive material  52 . The third capacitor is in parallel with the two capacitors connected in series.  
         [0045]     As the coil assembly housing  12 ′ moves relative to the core  18 ′, the proportion of the areas of electrically conductive material  52  on the insulated rod  50  that are contained within (or enveloped by) the core  18 ′ can vary. This variation results in a variation in the capacitance of the two capacitors formed by the areas of electrically conductive material  52  and the core  18 ′. Theory predicts that a capacitor&#39;s capacitance will vary directly with respect to the area of the plates of the capacitor. In the case of the two capacitors formed by the areas of electrically conductive material  52  and the ferromagnetic material of the core  18 ′, the area of the plate of each capacitor that provides variable capacitance corresponds to the portion of the area of electrically conductive material  52  on the insulated rod  50  that is contained within (or enveloped by) the core  18 ′.  
         [0046]     The capacitance of the third capacitor does not vary with the position of the coil assembly housing  12 ′, because the two areas of electrically conductive material  52  on the insulated rod  50  are fixed relative to each other.  
         [0047]     It can be seen in  FIG. 4   b,  which is an equivalent circuit diagram of the capacitive sensor of  FIG. 4   a,  that a capacitor C 3  is arranged in parallel with the variable capacitors C 1  and C 2  that are arranged in series. As discussed above, the capacitors C 3  is formed between the areas of electrically conductive material  52 .  
         [0048]     As mentioned previously, the scope of the present invention is not intended to be limited by theory. That said, the capacitance of the sensor shown in  FIG. 4   a  will ideally have a capacitance given by the following equation:  
       C   =           C   1     ⁢     C   2           C   1     +     C   2         +     C   3           
 
         [0049]     where C 1  and C 2  are the capacitances of the two capacitors formed by the areas of electrically conductive material and the ferromagnetic material of the core; and C 3  is the capacitance of the capacitor formed by the two areas of electrically conductive material  52 .  
         [0050]     As discussed above, the capacitances C 1  and C 2  vary linearly with the position of the coil assembly housing  12 ′, and the capacitance C 3  is fixed. Therefore, theory predicts linear variation of the capacitance C with movement of the, coil.  
         [0051]     As discussed above, capacitive sensors in exemplary embodiments according to the present invention have capacitances that vary with the position of a coil of a voice coil actuator with respect to the magnetic circuit housing of the voice coil actuator. A variety of circuits can be used to monitor the output of sensors in accordance with the present invention. There are several techniques for monitoring and signal conditioning an output of a capacitive sensor. The most common methods are a differential amplifier technique and a frequency oscillator technique.  
         [0052]     A signal conditioning circuit for use with a capacitive sensor in accordance with an exemplary embodiment of the present invention is shown in  FIG. 6 , which is a simple bridge circuit. In  FIG. 6 , a variable capacitor C 11  represents the variable capacitance C of the capacitive sensor of  FIG. 2   a  or  FIG. 4   a.  The variable capacitor C 11  is connected between an inverting input of an amplifier  100  and a voltage source  102 . A capacitor C 13  is connected between a non-inverting input of the amplifier  100  and the voltage source  102 . Further, a capacitor C 14  is coupled between the non-inverting input of the amplifier  100  and ground. In addition, a capacitor C 12  is connected between the inverting input of the amplifier  100  and an output of the amplifier  100 . The output of the signal conditioning circuit of  FIG. 6  has a voltage of Vout with respect to ground.  
         [0053]     The output Vout is defined by the equation of Vout=½(1−C 12 /C 11 )Vin where C 11  is the sensor capacitance (i.e., the capacitance C of the capacitive sensor), C 12 =½C 11 max (i.e., one-half of the maximum capacitance of the variable capacitance C 11 ) and C 13 =C 14 . When C 11 =C 12 , Vout=0.  
         [0054]     Although the signal conditioning circuit of  FIG. 6  is adequate and provides an output that would be proportional to the changes of the variable capacitance C 11 , with the current advances in Application Specific Integrated Circuit (ASIC) technology, a typical off-the shelf capacitive sensor driver as shown in  FIG. 7  is readily available and provides a more ideal signal conditioning circuit. The circuit of  FIG. 7  is based on a charge compensation feedback loop, and converts the difference of two capacitances (i.e., C 21  and C 22 ), relative to their sum, into an analog voltage. Here, C 21  is the variable capacitance C of the capacitive sensor of  FIG. 2   a  or  4   a.  The output characteristic of the signal conditional circuit of  FIG. 7  is  
       Vout   =       (     1   +     G   ·         C   21     -     C   22           C   21     +     C   22             )     ·     Vcc   2           
 
 where G is the gain of the amplifier and Vcc is the supply voltage of the ASIC chip. Any other suitable circuitry known to those skilled in the art may be used to generate the analog voltage output. 
 
         [0055]     As can be seen in  FIG. 8 , in the frequency oscillator technique, when a variable capacitance C 31  is applied to an RC oscillator circuit using a timer  120  (e.g., 555 Timer), the output frequency of the timer  120  varies according to the changes of the capacitance. It can be seen in  FIG. 8  that a supply voltage +Vcc is divided by a voltage divider resistors Ra and Rb, and applied to the timer  120 . The variable capacitor C 31  is coupled between the timer  120  and ground. A capacitor C 32  is also coupled between the timer  120  and ground. Here, the variable capacitor C 31  represents the variable capacitance C of the capacitive sensor of  FIG. 2   a  or  FIG. 4   a.    
         [0056]     As discussed above, voice coil actuators can be used in a variety of applications. One typical application of the voice coil actuator in exemplary embodiments of the present invention is in position control operations. In position control operations, the position and velocity of the coil are sensed and a feedback signal is used to control the position of the coil. The capacitive sensors in exemplary embodiments of the present invention may, for example, be used to sense the position, velocity and/or acceleration and provide the feedback.  
         [0057]     As can be seen in  FIG. 9 , a voice coil actuator  200  includes a capacitive sensor  202 . The voice coil actuator  200  and the capacitive sensor  202 , for example, can be the voice coil actuator and the capacitive sensor, respectively, of  FIG. 2   a  or  FIG. 4   a.  The capacitive sensor output of the capacitive sensor  202  is provided to a charge conditioning circuit  204 , which provides a voltage output to a position/velocity control circuit  206 . The signal conditioning circuit  204  may, for example, be any of the signal conditioning circuits illustrated in  FIGS. 6-8 , or any other suitable signal conditioning circuit. The make and use of the position/velocity control circuit  206  for providing a feedback to a driver  208  to control the position, velocity and/or acceleration of the coil assembly movement in the voice coil actuator  200  is known to those skilled in the art. The driver  208  drives the voice coil actuator to adjust position, velocity and/or acceleration of the sensor assembly with respect to the magnetic housing of the voice coil actuator  200 . By way of example, the driver  208  may provide a current for driving the voice coil actuator  200 .  
         [0058]     Although the present invention has been described in reference to certain exemplary embodiments, those skilled in the art would understand that additional variations, substitutions and modifications can be made to the system, as disclosed, without departing form the spirit or scope of the invention. For example, although the above description depicts circular coils, coils of any shape such as square coils can be used. In addition, the other components of a voice coil actuator in accordance with the present invention can be of shapes compatible with the shape of the coil. Accordingly, the scope of the invention should be determined not by the embodiments illustrated, but by the appended claims and their equivalents.