Patent Publication Number: US-7710460-B2

Title: Method of compensating for an effect of temperature on a control system

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application is related to the following applications, all of which are filed on the same date as this application, and all of which are assigned to the assignee of this application:
         Ferrofluid suspension for image stabilization (U.S. application Ser. No. 10/896,566), which was published on Jan. 26, 2006 as pending U.S. Patent Application Pub. No. 2006/0018645;   Apparatus and method for heat sinking a sensor (U.S. application Ser. No. 10/896,565), which was published on Jan. 26, 2006 as pending U.S. Patent Application Pub. No. US 2006/0018644;   Flexible suspension for image stabilization (U.S. application Ser. No. 10/896,470), which was published on Jan. 26, 2006 as pending U.S. Patent Application Pub. No. 2006/0017815; and   Magnet configuration for image stabilization (U.S. application Ser. No. 10/896,526), which was published on Jan. 26, 2006 as pending U.S. Patent Application Pub. No. 2006/0018643.       

   FIELD OF THE INVENTION 
   The present invention relates generally to photography, and more specifically to image stabilization. 
   BACKGROUND OF THE INVENTION 
   Image blur caused by camera shake is a common problem in photography. The problem is especially acute when a lens of relatively long focal length is used, because the effects of camera motion are magnified in proportion to the lens focal length. Many cameras, including models designed for casual “point and shoot” photographers, are available with zoom lenses that provide quite long focal lengths. Especially at the longer focal length settings, camera shake may become a limiting factor in a photographer&#39;s ability to take an unblurred photograph, unless corrective measures are taken. 
   Some simple approaches to reducing blur resulting from camera shake include placing the camera on a tripod, and using a faster shutter speed. However, a tripod may not be readily available or convenient in a particular photographic situation. Using a faster shutter speed is not always feasible, especially in situations with dim lighting. Shutter speed may be increased if a larger lens aperture is used, but larger-aperture lenses are bulky and expensive and not always available. In addition, the photographer may wish to use a smaller lens aperture to achieve other photographic effects such as large depth of field. 
   Various devices and techniques have been proposed to help address the problem of image blur due to camera shake. For example, Murakoshi (U.S. Pat. No. 4,448,510) uses an accelerometer to detect camera shake, and provides an indication to the user of the camera if the acceleration exceeds a threshold level. The photographer can then make appropriate adjustments. 
   Satoh (U.S. Pat. No. 6,101,332) also senses camera shake, and combines the shake information with other camera parameters to estimate how much image blur might result. A set of light emitting diodes communicates the estimate to the photographer, who can then make adjustments. 
   Another approach has been to automate the camera operation, and let the camera choose settings that will minimize blur. For example, Bolle et al. (U.S. Pat. No. 6,301,440) applies a variety of image analysis techniques in an attempt to improve several aspects of photographs. 
   Some cameras or lenses are equipped with image stabilization mechanisms that sense the motion of the camera and move optical elements in such a way as to compensate for the camera shake. See for example Otani et al. (U.S. Pat. No. 5,774,266) and Hamada et al. (U.S. Pat. No. 5,943,512). 
   In a digital camera, the photosensitive element is an electronic array light sensor onto which a scene image is projected by the camera&#39;s lens. Some recent digital cameras compensate for camera shake by moving the sensor during the exposure in response to camera motions so that the sensor approximately follows the scene image projected onto it, thus reducing blur. 
   SUMMARY OF THE INVENTION 
   A method of compensating for an effect of temperature comprises characterizing the performance of a control system that performs image stabilization by moving an electronic array light sensor in relation to a lens, and adjusting a control system parameter when the performance departs significantly from a nominal performance due to an effect of temperature. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a simplified block diagram of a digital camera. 
       FIG. 2  shows a perspective view of a digital camera, and illustrates a coordinate system convenient for describing motions of the camera. 
       FIG. 3  shows a schematic top view of the camera of  FIG. 2 , and illustrates how camera rotation can cause image blur. 
       FIG. 4  depicts a cutaway and simplified perspective view of a camera comprising a sensor mounting system in accordance with an example embodiment of the invention. 
       FIG. 5  shows, in perspective, an exploded partial view of a suspension assembly in accordance with an example embodiment of the invention. 
       FIG. 6  shows a side view of the suspension assembly of  FIG. 5  in its assembled state. 
       FIG. 7  depicts a circuit carrier in accordance with an example embodiment of the invention, shown in an unfolded configuration. 
       FIG. 8  shows the circuit carrier of  FIG. 7  in a folded configuration. 
       FIGS. 9A ,  9 B, and  9 C depict the flexing of a service loop of the circuit carrier of  FIG. 7 , as a portion of the circuit carrier is displaced from its nominal position. 
       FIG. 10  shows, in perspective, an exploded partial view of a sensor mounting system in accordance with a second example embodiment of the invention. 
       FIG. 11  shows a side view of the mechanism for image stabilization of  FIG. 10 . 
       FIG. 12  depicts the example sensor mounting system of  FIG. 10 , with additional components shown. 
       FIG. 13  shows a simplified block diagram of a control system for performing image stabilization in one axis of motion. 
       FIG. 14  depicts the control system of  FIG. 13  configured for self-characterization. 
       FIG. 15  depicts example responses of the control system of  FIG. 13  to a step input, at several different example temperatures. 
       FIG. 16  illustrates example frequency responses of the control system of  FIG. 13  at different temperatures, presented in a Bode plot. 
       FIG. 17  illustrates the effect of increased gain on the frequency response of the control system of  FIG. 13  at a cold temperature. 
       FIG. 18  shows, in perspective, an exploded partial view of a sensor mounting system in accordance with another example embodiment of the invention. 
       FIG. 19  shows the sensor mounting system of  FIG. 18  in its assembled state. 
       FIGS. 20 and 21  illustrate a technique for heat sinking the sensor in the sensor mounting system of  FIGS. 18 and 19 . 
   

   DETAILED DESCRIPTION 
     FIG. 1  shows a simplified block diagram of a digital camera. A lens  101  gathers light emanating from a scene, and redirects the light  102  such that an image of the scene is projected onto an electronic array light sensor  103 . Electronic array light sensor  103  may be an array of charge coupled devices, commonly called a “CCD array”, a “CCD sensor”, or simply a “CCD”. Alternatively, electronic array light sensor  103  may be an array of active pixels constructed using complementary metal oxide semiconductor technology. Such a sensor may be called an “active pixel array sensor”, a “CMOS sensor”, or another similar name. Other sensor technologies are possible. The light-sensitive elements on electronic array light sensor  103  are generally arranged in an ordered rectangular array, so that each element, or “pixel”, corresponds to a scene location. 
   Image data signals  104  are passed to logic  110 . Logic  110  interprets the image data signals  104 , converting them to a numerical representation, called a “digital image.” Logic  110  may perform other functions as well, such as analyzing digital images taken by the camera for proper exposure, adjusting camera settings, performing digital manipulations on digital images, managing the storage, retrieval, and display of digital images, accepting inputs from a user of the camera, and other functions. Logic  110  also controls electronic array light sensor  103  through control signals  105 . Logic  110  may comprise a microprocessor, a digital signal processor, dedicated logic, or a combination of these. 
   Storage  111  comprises memory for storing digital images taken by the camera, as well as camera setting information, program instructions for logic  110 , and other items. User controls  112  enable a user of the camera to configure and operate the camera, and may comprise buttons, dials, switches, or other control devices. A display  109  may be provided for displaying digital images taken by the camera, as well as for use in conjunction with user controls  112  in the camera&#39;s user interface. A flash or strobe light  106  may provide supplemental light  107  to the scene, under control of strobe electronics  108 , which are in turn controlled by logic  110 . Logic  110  may also provide control signals  113  to control lens  101 . For example, logic  110  may adjust the focus of the lens  101 , and, if lens  101  is a zoom lens, may control the zoom position of lens  101 . 
     FIG. 2  shows a perspective view of a digital camera  200 , and illustrates a coordinate system convenient for describing motions of the camera  200 . Rotations about the X and Y axes, indicated by rotation directions θ X  and θ Y  (often called pitch and yaw respectively), are the primary causes of image blur due to camera shake. Rotation about the Z axis and translations in any of the axis directions are typically small, and their effects are attenuated by the operation of the camera lens because photographs are typically taken at large inverse magnifications. 
     FIG. 3  shows a schematic top view of camera  200 , and illustrates how camera rotation can cause image blur. In  FIG. 3 , camera  200  is shown in an initial position depicted by solid lines, and in a position, depicted by broken lines, in which camera  200  has been rotated about the Y axis. The reference numbers for the camera and other parts in the rotated position are shown as “primed” values, to indicate that the referenced items are the same items, shifted in position. In  FIG. 3 , a light ray  300  emanating from a particular scene location, passes through lens  201  and impinges on sensor  203  at a particular location  204 . If the camera is rotated, the light ray is not affected in its travel from the scene location to the camera. (Its travel within the camera, after it encounters lens  201 ′ may be slightly affected, depending on the point of rotation of the camera. It is shown as unaffected in  FIG. 3 , as if the camera has been rotated around the lens nodal point, but even if the camera is rotated about a different point so that there is a deviation of ray  300 , the deviation is generally small enough to be neglected by an image stabilization system.) However, sensor  203  moves to a new position, indicated by sensor  203 ′. The light ray, emanating from the same scene location, now impinges on sensor  203 ′ at a different sensor location than where it impinged on sensor  203 , because position  204  has moved to position  204 ′. If the rotation occurs during the taking of a photograph, then each of the sensor locations where the light ray impinged will have collected light from the same scene location. A photograph taken during the rotation will thus be blurred. 
   If sensor  203  can be made to move within the camera by an amount just sufficient to keep the sensor position  204  in the path of light ray  300 , then the mapping of scene locations to sensor locations can be held fixed, and a sharp photograph can be taken even though the camera may be rotating. The rotation shown in  FIG. 3  has been exaggerated for clarity of explanation. In an actual application, the fact that the sensor has rotated slightly can be ignored, and translations of the sensor in the camera&#39;s X-direction are sufficient to substantially counter rotations of the camera about the Y axis. Similarly, translations of the sensor in the Y-direction are sufficient to substantially counter rotations of the camera about the X axis. 
     FIG. 4  depicts a cutaway and simplified perspective view of a camera  400  comprising a sensor mounting system in accordance with an example embodiment of the invention. The lens elements and much of the internal support structure and electronics of example camera  400  are omitted from  FIG. 4  for clearer viewing. Camera  400  comprises a suspension assembly  402 , which further comprises an electronic array light sensor  401 , mounted in suspension assembly  402 . Electronic array light sensor  401  is generally rectangular, having a top that faces the camera lens, a bottom opposite the top, and four sides. Suspension assembly  402  enables sensor  401  to move in the camera&#39;s X and Y axes. An appropriate control system (not shown) drives sensor  401  in response to rotations of the camera about the Y and X axes in order to compensate for camera shake. Sensor  401  may be, for example, a Sony ICX282AK CCD sensor, or another similar kind of sensor. 
     FIG. 5  shows, in perspective, an exploded partial view of suspension assembly  402 . First plate  501  is substantially rigid, and may be made of steel, aluminum or another suitable material. Affixed to plate  501  are magnets  502 . Magnets  502  are arranged in pairs, with each pair comprising magnets placed with their polarities opposite. For example, each pair of magnets  502  has one magnet with its north pole facing away from plate  501  and one magnet with its south pole facing away from plate  501 . The pairs of magnets may be fabricated from separate pieces of magnetic material, or may be oppositely-magnetized regions on a single piece of magnetic material. 
   A second plate  503  also comprises pairs of magnets on the side facing plate  501 . (The magnets on plate  503  are not readily visible in  FIG. 5 .) The magnets on plate  503  are of complementary polarity to the corresponding magnets on plate  501 . That is, opposite each of magnets  502  having its south pole facing plate  503  is a magnet on plate  503  with its north pole facing plate  501 . In this way, magnets  502  and their corresponding magnets on plate  503  set up magnetic fields between the two plates. A magnet on plate  501  and its complementary magnet on plate  503  make up a set of complementary magnets. 
   When assembly  402  is assembled, plates  501  and  503  are in fixed relationship to each other, and in fixed relationship to the body of camera  400 . They may be held in relative position with spacer studs  504  or by other suitable mechanical means. The attachment of the pair of plates to the body of camera  400  may be by any suitable mechanical means, many of which are known in the art. 
   Between plates  501  and  503  is a generally planar circuit carrier sensor mounting portion  505 . Circuit carrier sensor mounting portion  505  is semirigid or substantially rigid, and may be a common printed circuit board. Alternatively, circuit carrier sensor mounting portion  505  may be a “flex circuit”. A flex circuit is similar to a printed circuit board, but has as its substrate a flexible material such as polyimide, polyester, or another suitable material. A flex circuit may be used to electrically interconnect electronic components while enabling their physical relationship to be configured to an available space. Sensor mounting portion  505  may also be a flex circuit with multiple conducting layers, and may have a stiffening member attached. 
   Mounted on sensor mounting portion  505  are electronic array light sensor  401 , and coils  506 - 509 . Sensor mounting portion  505  may also hold circuitry such as bypass capacitors, a buffer amplifier for conditioning the analog image signal produced by electronic array light sensor  401 , or other circuitry. Coils  506 - 509  may be wound from traditional magnet wire and affixed to sensor mounting portion  505 , or may be formed by circuit traces integrated into sensor mounting portion  505 , or may be formed by other means. If circuit carrier sensor mounting portion  505  has multiple layers, each coil may be made up of circuit traces on more than one of the layers. Each coil is positioned so that when assembly  402  is assembled, each coil is substantially centered between complementary pairs of permanent magnets on plates  501  and  503 . When an electric current is passed through any of coils  506 - 509 , a force is generated, acting on the coil. The magnitude of the force is generally proportional to the strength of the magnetic field in which the coil is positioned, the magnitude of the current, and the number of conductors in the coil. The direction of the force is perpendicular to both the direction of current flow and the magnetic field. Thus, current flowing in coils  506  and  508  produces force acting on the coils, and therefore also on current carrier sensor mounting portion  505  and sensor  401 , parallel to the Y axis. The force may be in the positive Y direction or the negative Y direction, depending on the direction of current flow in the coil. Similarly, current flowing in coils  507  and  509  produces force parallel to the X axis. The pairs of coils may be wired in series or parallel, or controlled individually. 
   Thus, each coil  506 - 509  and its associated set of complementary magnets forms a moving coil linear motor, wherein the magnets are the stator of the linear motor, and the coil is part of the moving member of the linear motor. The linear motors comprising coils  506  and  508  work in concert to move sensor mounting portion  505  in directions parallel to the Y axis, and the motors comprising coils  507  and  509  work in concert to move circuit carrier sensor mounting portion  505  in directions parallel to the X axis. When all four linear motors are operated in concert, generalized X-Y motion of sensor mounting portion  505  can be accomplished. Because the moving coil linear motors are positioned symmetrically about the center of circuit carrier sensor mounting portion  505 , the forces generated do not produce any significant torque on sensor mounting portion  505  that would tend to rotate sensor  401  about an axis parallel to the Z axis. That is, the line of action of each motor, or pair of motors working in the same axis, passes as nearly as practicable through the center of mass of the moving assembly. 
   In an alternative arrangement, coils may be placed on plates  501  and  503 , and permanent magnets placed on sensor mounting portion  505 , so that each set of coils and magnets forms a moving magnet linear motor. For the purposes of this disclosure, the term linear motor encompasses the motors as depicted in  FIG. 5  and moving magnet linear motors, as well as a linear voice coil actuator. 
     FIG. 6  depicts a side view of assembly  402  in its assembled state. Plates  501  and  503  are spaced sufficiently apart that circuit carrier sensor mounting portion  505  and coils  506 - 509  can move freely between the magnets attached to plates  501  and  503 . Sufficient travel is provided to enable circuit carrier to move sufficiently in the X and Y directions that most common camera shake signals can be compensated. A preferred amount of sensor travel is +/−1 to 2 millimeters in each axis. Gaps are provided on each side of the moving parts, between circuit carrier sensor mounting portion  505  and magnets  502 , and between coils  506 - 509  and the magnets on plate  503 . Preferably, the gaps are 0.1 to 0.5 millimeters thick. 
   These gaps are substantially filled with a ferrofluid  601 . A ferrofluid is a suspension of magnetic particles in a fluid, and reacts to magnetic fields acting on it. Ferrofluids are available from FerroTec, USA corporation, of Nashua, N.H. Ferrofluid  601  is strongly attracted to the region of greatest magnetic flux between the magnets. This attraction, together with capillary action, causes ferrofluid  601  to remain in the gaps, and to hold circuit carrier sensor mounting portion  505  and coils  506 - 509  relatively stiffly at an equilibrium position between the magnets. That is, coils  506 - 509  and sensor mounting portion  505  are held away from the magnets, and little movement will occur of circuit carrier sensor mounting portion  505  and coils  506 - 509  in a direction parallel to the Z axis. However, motion in the X and Y axes (that is, in directions parallel to the X and Y axes) is essentially free of static friction, and is only moderately impeded by dynamic friction, due to the moderate viscosity of ferrofluid  601 . Ferrofluid  601  thus forms a fluid bearing, enabling free movement of sensor  401  in the directions desirable for compensating for camera shake, and constraining the movement of sensor  401  in other directions. 
   While the example embodiment shown in  FIGS. 5 and 6  uses four linear motors positioned one on each side of electronic array light sensor  401 , other embodiments may comprise fewer than four motors. For example, generalized X-Y motion may be accomplished using only two linear motors, positioned proximate two adjacent sides of an electronic array light sensor. In an application in which vibration is expected in only one axis, a single linear motor may be used to provide motion compensation in that axis. 
     FIG. 7  depicts a more complete representation of circuit carrier  702 , shown in an unfolded configuration. Service loops  701  connect main sensor mounting portion  505  of circuit carrier  702  with connecting portion  703  and other logic mounting portion  704 . Preferably, service loops  701  are flex circuit regions each with a single circuit layer, for maximum flexibility. Other logic mounting portion  704  may preferably comprise multiple circuit layers, and may hold circuitry that interacts with sensor  401  and coils  506 - 509 . Such circuitry may comprise a timing generator for sensor  401 , power amplifiers for controlling the current flowing in coils  506 - 509 , buffer memory, motion sensors, or other devices. Connector  705  further connects the circuitry on circuit carrier  702  with other camera subsystems, such as a microprocessor system, non-volatile storage, or other components. Connector  706  is configured to receive connecting pads on connecting portion  703 . Alternatively, the connecting pads may be soldered to other logic mounting portion  704 , or another kind of connection may be provided. 
   Many other variations of which circuitry components to put on which circuit carrier portion are possible. For example, service loops  701  may connect sensor mounting portion  505  to more than one other logic mounting portion, as when connecting portion  703  has logic mounted on it. The other logic mounting portions may be connected together, or independently connected to another circuit board. 
   Preferably, critical control and data signals relating to sensor  401  will be routed through the loops  701  most directly connected to other logic mounting portion  704 , where the signals may be digitized, strengthened, or otherwise processed. This routing minimizes the trace length between sensor  401  and the interface circuitry, thus minimizing the opportunity for noise contamination of critical signals. Other, less critical signals may be routed through the other loops, through connecting portion  703  to other logic mounting portion  704 . 
   Service loops  701  are placed, as nearly as is practicable, symmetrically about the center of mass of the moving assembly of the system. Full rotational symmetry is not required; service loops  701  may be substantially mirror-symmetric about orthogonal axes passing through the center of mass of the moving assembly. (Mirror symmetry is shown in  FIG. 7 .) Once circuit carrier  702  is folded into the configuration of  FIG. 8 , any forces exerted on sensor mounting portion  505  by service loops  701  are symmetrical, and therefore do not induce significant rotation of sensor mounting portion  505  about the Z axis. Other logic mounting portion  704  remains stationary during operation, while service loops  701  enable translation of sensor mounting portion  505 , and therefore also translation of sensor  401 , in the X and Y axes.  FIG. 9A  depicts a detail view of one of service loops  701  in its nominal position.  FIG. 9B  shows the same loop flexing as sensor-mounting portion  505  moves in the in the negative X direction, and  FIG. 9C  shows the same loop flexing as sensor-mounting portion  505  moves in the negative Y direction. 
   Other service loop configurations may be envisioned as well. For example, a system using only two service loops may be used. The two service loops may emanate from opposite edges of sensor mounting portion  505 . 
     FIG. 10  shows, in perspective, an exploded partial view of a sensor mounting system in accordance with a second example embodiment of the invention. In this example embodiment, a generally planar heat sink  1001  is interposed between circuit carrier sensor mounting portion  1002  and sensor  1003 . Heat sink  1001 , circuit carrier sensor mounting portion  1002 , and sensor  1003  are attached together, so that they move as a unit during image stabilization. In particular, heat sink  1001  is preferably in close contact with the bottom surface of sensor  1003 , so that heat transfer is facilitated from sensor  1003  into heat sink  1001 . 
     FIG. 11  shows a side view of the mechanism for image stabilization of  FIG. 10 . Plates  1004  and  1005  are spaced apart such that the unit comprising heat sink  1001 , circuit carrier sensor mounting portion  1002 , and sensor  1003  can move freely between the magnets  1006  mounted on plate  1004  and complementary magnets on plate  1005 . Preferably, a gap of 0.1 to 0.5 millimeters may be provided between heat sink  1001  and magnets  1006 , and a similar gap may be provided between circuit carrier sensor mounting portion  1002  and the magnets mounted on plate  1004 . Coils  1007 - 1010  may be wire coils affixed to circuit carrier sensor mounting portion  1002  or may be circuit traces that are part of sensor mounting portion  1002 . Circuit carrier sensor mounting portion  1002  may comprise multiple circuit layers. 
   A quantity of ferrofluid is inserted into each gap. The quantity is sufficient to substantially fill the gap between a pair of magnets and the nearby surface of heat sink  1001  or circuit carrier sensor mounting portion  1002 . The ferrofluid is naturally drawn to the region of highest magnetic flux between the magnets, and, in moving to that region, pushes the unit comprising heat sink  1001  and circuit carrier sensor mounting portion  1002  to an equilibrium Z position between the magnets. Thus, a fluid bearing is formed that holds heat sink  1001  and sensor mounting portion  1002  relatively stiffly in the Z axis, while enabling motion in the X and Y axes substantially unimpeded by static friction. 
   The ferrofluid also provides an enhanced heat conduction path for removing heat from sensor  1003 . The performance of sensor  1003  may be dependent on its operating temperature. For example, if sensor  1003  is a CCD sensor, it generates heat during much of the time the camera is operating, and its dark noise level is strongly correlated to its operating temperature. It is desirable to draw excess heat away from sensor  1003  and dissipate it. Heat sink  1001  is preferably made of a lightweight, rigid or semi-rigid material that is a good conductor of heat. The thickness of heat sink  1001  is chosen by balancing its effect on the performance of the control system performing the image stabilization, the mechanical stiffness of heat sink  1001 , and the thermal effectiveness of heat sink  1001 . Preferably, heat sink  1001  is about 0.5 to 1.0 millimeters thick, and made of aluminum. 
   Heat is transferred into heat sink  1001  from the bottom of sensor  1003 , and is carried by heat sink  1001  toward lower-temperature areas. Ferrofluid  1101  provides a heat conduction path to the magnets mounted on plates  1004  and  1005 , which typically operate at a lower temperature than does sensor  1003 . Plates  1004  and  1005  may provide further thermal mass, in addition to the thermal mass supplied by components already encountered, into which heat may flow, to be ultimately dissipated through the body of the camera comprising the stabilization mechanism and into the surrounding environment. The term thermal mass refers to material capable of absorbing a relatively large amount of thermal energy without changing its temperature substantially. 
   In an alternative example embodiment, the heat sinking function is provided by a layer of conductive material comprised in circuit carrier sensor mounting portion  1002 . For example, if sensor mounting portion  1002  is a flex circuit comprising multiple circuit layers, one of the layers may be devoted to providing a substantially contiguous copper sheet that facilitates the conduction of heat away from sensor  1003 . Alternatively, a thermally conductive cladding layer may be provided on sensor mounting portion  1002 . In yet another embodiment, interstitial areas between circuit traces in any and all layers of sensor mounting portion  1002  may be substantially filled with circuit trace material, generally copper, in order to enhance the thermal conductivity of sensor mounting portion  1002 . The infilling material may be electrically isolated from active circuit traces, or may be formed by enlarging the active circuit traces. 
     FIG. 12  depicts the example sensor mounting system of  FIG. 10 , with additional components shown. Hall effect sensors  1201  and  1202  are mounted on the back side of circuit carrier sensor mounting portion  1002 , opposite electronic array light sensor  1003 . Preferably, Hall effect sensors  1201  and  1202  are “analog”, or “linear” type sensors. An analog or linear Hall effect sensor, when connected to appropriate driving circuitry, produces a voltage proportional to the strength of a magnetic field acting on it. Hall effect sensors are widely available. 
   A sense magnet plate  1203  holds sense magnet pairs  1204  and  1205 . Magnet plate  1203  is preferably made of steel, or another suitable magnetic material. Magnet pairs  1204  and  1205  are affixed on plate  1203  and positioned such that when plate  1203  is in its assembled position and circuit carrier sensor mounting portion  1002  is in the nominal center of its available travel, the sensing element of Hall effect sensor  1201  is positioned over the center of magnet pair  1204 , and the sensing element of Hall effect sensor  1202  is positioned over the center of magnet pair  1205 . The sensing element of each Hall effect sensor is much smaller than the device package. Each magnet pair comprises a permanent magnet with its north pole facing away from magnet plate  1203 , and a magnet with its south pole facing away from magnet plate  1203 . 
   When circuit carrier sensor mounting portion  1002  is in the center of its available travel range, the effects of the north and south magnets of each pair on its corresponding Hall effect sensor tend to cancel, and the voltage produced by the Hall effect sensor is a reference value. Using magnet pair  1204  and Hall effect sensor  1201  as an example, as circuit carrier sensor mounting portion  1002  (and thus Hall effect sensor  1201 , which is mounted on sensor mounting portion  1002 ) move in the X direction, the sensing element of Hall effect sensor  1201  is increasingly affected by the magnetic field from the “south” magnet of magnet pair  1204 , while the effect of the “north” magnet diminishes. The voltage produced by Hall effect sensor  1201  changes from its reference value approximately in proportion to the distance moved by circuit carrier sensor mounting portion  1002 . When circuit carrier sensor mounting portion  1002  moves in the negative X direction, the “north” magnet increasingly dominates, and the voltage produced by Hall effect sensor  1201  changes in the opposite sense, in rough proportion to the position of circuit carrier sensor mounting portion  1002 . For example, motion in the X direction may produce an increasing voltage, while motion in the negative X direction may produce a decreasing voltage. 
   Similarly, Hall effect sensor  1202  and magnet pair  1205  provide a voltage that is related to the position of circuit carrier sensor mounting portion  1002  in the Y axis. Hall effect sensors  1201  and  1202  thus provide feedback signals indicating the position of sensor mounting portion  1002 . These position feedback signals may be used by an appropriate control system that measures rotations of the camera, and drives circuit carrier sensor mounting portion  1002  (and thus sensor  1003 ) in such a way as to counter the camera rotation, providing an image stabilization function. 
   As has been previously described, circuit carrier sensor mounting portion  1002  is suspended between plates  1004  and  1005  by a ferrofluid bearing. The performance of the control system performing image stabilization depends on several factors, including the mass of the assembly moved by the control system, the characteristics of the linear motors, and the viscosity of ferrofluid  1101 , as well as other factors. The viscosity of ferrofluid  1101 , in turn, is dependent on its temperature. Ferrofluid  1101  is more viscous at relatively colder temperatures and less viscous at relatively higher temperatures. Thus, it resists motion of circuit carrier sensor mounting portion  1002  more strongly at colder temperatures, and provides more damping to the control system. 
   It is desirable for the camera comprising the stabilization system to operate over a wide temperature range, and for its performance to be generally consistent at all temperatures in the range. A camera in accordance with an example embodiment of the invention may compensate for the effects of varying temperature in one of several ways. For example, the camera may characterize the dynamic performance of the control system and, when the performance departs significantly from a nominal performance, adjust at least one control system parameter in response to the characterization in order to maintain consistency of operation. Alternatively, the camera may measure its internal temperature and modify at least one control system parameter based on a previous characterization of the effect of temperature on the camera&#39;s designed performance. For example, a temperature sensing element such as a thermistor may be designed into the camera&#39;s circuitry, or the camera may use a control processor that has a built-in temperature measuring capability. And finally, a camera may compensate for the effect of temperature by warming the ferrofluid, thereby bringing its viscosity, and therefore also the camera&#39;s dynamic performance, closer to its nominal condition. 
     FIG. 13  shows a simplified block diagram of a control system for performing image stabilization in one axis of motion. For example, the control system of  FIG. 13  may move circuit carrier sensor mounting portion  1002  in the X axis to compensate for camera rotation about the Y axis. A corresponding control system (not shown) compensates for camera rotation about the X axis by moving circuit carrier sensor mounting portion  1002  in the Y axis. In block  1301 , camera rotation is sensed. The sensing may be accomplished using an accelerometer, a rate gyroscope, or another suitable device. In conversion block  1302 , the output of the sensing device is converted to the proper units and magnitude for the subsequent control loop. For example, if rotation is sensed using a rate gyroscope, then the sensing device produces a signal indicating the rate of camera rotation. The conversion at block  1302  would then comprise integrating the signal to obtain a signal indicating the rotational position of the camera. The conversion at block  1302  may further comprise scaling the position signal based on the focal length of the camera lens, and scaling the signal to match the transfer gain characteristics and dynamic range of the subsequent control loop. The output of conversion block  1302  is a position command, indicating the position of circuit carrier sensor mounting portion  1002  required to compensate for the measured camera rotation. 
   At differencer  1303 , the commanded position is compared with the actual position of circuit carrier sensor mounting portion  1002 , as indicated by position measurement block  1304 . Position measurement block  1304  may comprise, for example Hall effect sensor  1201  and magnet pair  1204 . Differencer  1303  produces a difference signal  1308 , indicating the magnitude and direction of the present error in the position of sensor mounting portion  1002 . This difference signal is amplified at amplifier  1305 , and is fed to the image stabilization plant  1306 . Image stabilization plant  1306  represents the dynamics of the image stabilization mechanism, comprising the linear motors driving circuit carrier sensor mounting portion  1002 , the mass of sensor mounting portion  1002  and its associated circuitry, the viscous friction induced by ferrofluid  1101 , and other items. The output of the image stabilization plant is the sensor position  1307 . 
   Differencer  1303  is preferably performed digitally. That is, preferably, conversion block  1302  and position measurement block  1304  comprise analog-to-digital (A/D) converters so that the commanded position output from conversion block  1302  and the measured position output from position measurement block  1304  are numerical values. The function of differencer  1303  is then preferably performed in a microprocessor, digital signal processor, or similar digital logic. Amplifier  1305  may be implemented digitally as well, and the resulting signal converted, using a digital-to-analog (D/A) converter, to a signal for driving image stabilization plant  1306 . 
   In a first technique useful in compensating for the effects of temperature changes on the viscosity of ferrofluid  1101  and the resulting changes in the performance of the control system, the logic that implements the control system characterizes the system by subjecting the position control loop to a standardized signal, and monitoring the resulting sensor position. 
     FIG. 14  depicts the control system of  FIG. 13  configured for self-characterization. In  FIG. 14 , logic  1401  produces a calibration command signal  1402 . Calibration command signal may be a step command, or a cyclic signal such as a sine wave or square wave. Preferably, the signal is in digital form. Logic  1401  also receives actual position signal  1403 , which is the output of position measurement block  1304 . Preferably, position signal  1403  is also in digital form. By monitoring position signal  1403 , logic  1401  can measure the response of the position control loop to calibration command signal  1402 . 
   For example,  FIG. 15  depicts example responses of the system to a step input, at several different example temperatures. Trace  1501  represents the step input, normalized so that the step commands a movement of one displacement unit. Curves  1502 ,  1503 , and  1504  represent example responses of the system at normal, cold, and hot temperatures respectively. Because the ferrofluid is more viscous at cold temperatures, the system responds more slowly at the cold temperature. By measuring the fraction of the step input command the system has moved at a fixed time, such as 0.2 seconds after the command in the example of  FIG. 15 , the responsiveness of the system can be determined. Alternatively, logic  1401  may sample the step response at several times so that the response of the system can be more completely characterized. 
   As an alternative to a step input position command, logic  1401  may subject the system to a periodic calibration command signal  1402 , and characterize the performance of the system by measuring its frequency response. For example, a sinusoidal calibration command signal  1402  will result in a generally sinusoidal position signal  1403 , but position signal  1403  will be shifted in phase in relation to calibration command signal  1402 , and will have an amplitude that is a function of the dynamics of the control system and the frequency of the sinusoidal calibration command signal  1402 .  FIG. 16  illustrates example frequency responses of the system at different temperatures, presented in a Bode plot. Curves  1601 ,  1602 , and  1603  represent the frequency responses of the system at normal, cold, and hot temperatures respectively. The increased viscosity of the ferrofluid at cold temperatures tends to attenuate the amplitude of position signal  1403 . By subjecting the system to a sinusoidal input at a known frequency, for example five Hertz, and noting the corresponding amplitude of position signal  1403 , the responsiveness of the system can be characterized. Alternatively, logic  1401  may sample the amplitude of position signal  1403  at several frequencies in order to more completely characterize the system, and may use a periodic calibration command signal that is other than sinusoidal. For example, calibration command signal  1402  may be a square wave. 
   In a second technique useful in compensating for the effects of temperature changes on the viscosity of ferrofluid  1101 , the control system may be adjusted based on the results of a system characterization in order to make the system performance relatively more consistent over a range of temperatures. For example, when the characterization indicates that a cold temperature has caused the system to be sluggish, the logic implementing the control system may increase the gain of amplifier  1305 . At elevated temperatures, the viscosity of ferrofluid  1101  is reduced, and the control system may become so responsive that undesirable oscillations, sometimes called “ringing” are introduced. In that case, the logic implementing the control system may decrease the gain of amplifier  1304 . 
   If amplifier  1305  is implemented digitally, the increase or decrease may be accomplished with a simple numerical multiplication.  FIG. 17  illustrates the effect of the increased gain on the system frequency response of a cold system. Curve  1701  is an example frequency response of the system at a normal temperature. Curve  1702  is an example frequency response at a cold temperature. Curve  1703  illustrates that elevating the system gain can adjust the system frequency response of the cold system to approximate the normal temperature behavior. In an example method for compensating for temperature variations, logic  1401  measures the amplitude of position signal  1403  in response to a preselected periodic calibration command signal  1402 . If the amplitude of position signal  1403  differs from the amplitude expected at a normal operating temperature, logic  1401  may increase or decrease the gain of amplifier  1305  and remeasure the amplitude of position signal  1403 , repeating the procedure as necessary until the amplitude of position signal  1403  approaches that of a system operating at a normal temperature operation or is otherwise satisfactory, or the operating limits of the system have been reached. Alternatively, the system may apply a preselected gain adjustment, determined from prior experiment, that is selected to compensate for a particular frequency response measurement. 
   In a third technique useful in compensating for the effects of temperature changes on the viscosity of ferrofluid  1101 , the control system may adjust the actual temperature of ferrofluid  1101  in order to improve the system performance. For example, in the sensor mounting system of  FIGS. 10 and 11 , coils  1007 - 1010  are positioned between layers of ferrofluid  1101 , separated from it only by heat sink  1001  or by circuit carrier sensor mounting portion  1002 . The control system performing image stabilization is configured to supply electrical current through coils  1007 - 1010  as part of the process of image stabilization. Coils  1007 - 1010  are preferably made of copper or a suitable copper alloy, which resists the flow of current. As a result, current flowing through any of the coils causes the coil to dissipate energy in the form of heat. This heat-generating effect may be used to warm ferrofluid  1101  in order to lower its viscosity and improve the performance of the image stabilization system. 
   For example, when it is detected that the system performance is sluggish, logic  1401  may pass a current through coils  1007 - 1010  for a period of time estimated, based on the characterization of system performance, to warm the ferrofluid sufficiently to bring the system performance to a level similar to a system operating at a normal temperature. Alternatively, the system may pass a current through coils  1007 - 1010  for a preselected time and the recharacterize the system performance, repeating the process until the system performance is satisfactory, or until a budget of energy allocated to ferrofluid heating is depleted. 
   The current passed through the coils may be direct or alternating current. A direct current will drive circuit carrier sensor mounting portion  1002  against its travel stops. An alternating current of a frequency below or similar to a resonant frequency of the control system will cause oscillating motion of sensor mounting portion  1002 . For example, a frequency between one-half of the resonant frequency and double the resonant frequency may be considered similar to the resonant frequency. The oscillating motion may have advantages in that it may induce additional frictional heating of ferrofluid  1101 , and may serve to distribute the heat from coils  1007 - 1010  more evenly through ferrofluid  1101 . An alternating current of higher frequency may induce little or no detectable motion of circuit carrier sensor mounting  1002 . 
     FIG. 18  shows, in perspective, an exploded partial view of a sensor mounting system in accordance with another example embodiment of the invention. The example mounting system of  FIG. 18  is especially compact. A magnet plate  1801  holds pairs of drive magnets of  1802 , each pair comprising a magnet with its north pole facing away from plate  1801  and a magnet with its south pole facing away from plate  1801 . The pairs of drive magnets  1802  surround a generally rectangular area  1803  of plate  1801 . Mounted inside area  1803  are sense magnet pairs  1804  and  1805 . A circuit carrier sensor mounting portion  1806  comprises coils  1807 - 1810 , which are formed of circuit traces comprised in circuit carrier sensor mounting portion  1806 , and thus do not add significant thickness to sensor mounting portion  1806 . 
   Also mounted on circuit carrier sensor mounting portion  1806  are Hall effect sensors  1811  and  1812 . Hall effect sensors  1811  and  1812  are positioned such that, when sensor mounting portion  1806  is in its nominal position, the sensing elements of sensors  1811  and  1812  are centered on sense magnet pairs  1804  and  1805 , respectively. Electronic array light sensor  1813  mounts on circuit carrier sensor mounting portion  1806 , straddling Hall effect sensors  1811  and  1812 . 
   Circuit carrier sensor mounting portion  1806  is suspended between magnet plate  1801  and second plate  1814 . The two plates are held apart by spacers  1815  such that a gap can be maintained between circuit carrier  1806  and magnets  1802 , and also between circuit carrier  1806  and second plate  1814 . Plate  1814  does not have magnets mounted on it, but is made of a magnetically permeable material, such as steel, so that it serves to complete a magnetic circuit between members of magnet pairs  1802 . Thus, coils  1807 - 1810  are positioned in areas of magnetic flux. Magnets  1802 , plate  1814 , and coils  1807 - 1810  are thus comprised in linear motors that move circuit carrier sensor mounting portion  1806 , and consequently sensor  1813 , in the X and Y axes. 
   The gaps between circuit carrier sensor mounting portion  1806  and magnets  1802 , and between sensor mounting portion  1806  and plate  1814  are substantially filled with a ferrofluid, which is strongly attracted to the areas of magnetic flux, and serves to hold circuit carrier sensor mounting portion  1806  in an equilibrium Z position between magnets  1802  and plate  1814 .  FIG. 19  shows the sensor mounting system of  FIG. 18  in its assembled state. The ferrofluid in the motor gaps is denoted as element  1901  in  FIG. 19 . Ferrofluid  1901  forms a fluid bearing, constraining circuit carrier sensor mounting portion  1806  in the Z axis, but enabling motion of sensor mounting portion  1806  in the X and Y axes substantially free of static friction. 
   In an alternative arrangement, the positions of the Hall effect sensors and the sense magnets may be interchanged, so that the sense magnets are comprised in the moving assembly and the Hall effect sensors are stationary with respect to the rest of the camera. In either arrangement, applications may be envisioned that do not require a full complement of two Hall effect sensors and two pairs of sense magnets. At a minimum, at least one Hall effect sensor and at least one sense magnet may suffice in some applications. 
     FIGS. 20 and 21  illustrate a technique for heat sinking sensor  1813  in the sensor mounting system of  FIGS. 18 and 19 . Heat conductor  2001  is placed between sensor  1813  and sensor mounting portion  1806 . Heat conductor  2001  extends away from sensor mounting portion  1806  sufficiently far that it is in close contact with sensor  1813 . Heat conductor  2001  is made of a thermally conductive material, preferably aluminum. 
   As shown in  FIG. 21 , a quantity of ferrofluid  2101  is placed between sensor mounting portion  1806  and sense magnet pairs  1804  and  1805 . (Ferrofluid  1901  in the motor gaps is not shown in  FIG. 21  so that ferrofluid  2101  is more readily visible.) A heat conduction path is thus provided for heat generated by the operation of sensor  1813 . The heat can flow through heat conductor  2001 , through sensor mounting portion  1806 , through ferrofluid  2101 , through sense magnet pairs  1804  and  1805 , and into plate  1801 , which serves as a thermal reservoir and facilitates the dissipation of the heat. A gap is provided between the magnets in sense magnet pairs  1804  and  1805  and sensor mounting portion  1806 . Ferrofluid  2101  is attracted to the gap by the magnetic flux generated by sense magnet pairs  1804  and  1805 , and does not substantially impede the motion of sensor mounting portion  1806  during image stabilization.