Abstract:
This invention provides a removably mountable lens assembly for a vision system camera that includes an integral auto-focusing liquid lens unit, in which the lens unit compensates for focus variations by employing a feedback control circuit that is integrated into the body of the lens assembly. The feedback control circuit receives motion information related to the bobbin of the lens from a position sensor (e.g. a Hall sensor) and uses this information internally to correct for motion variations that deviate from the lens setting position at a desired lens focal distance setting. Illustratively, the feedback circuit can be interconnected with one or more temperature sensors that adjust the lens setting position for a particular temperature value. In addition, the feedback circuit can communicate with an accelerometer that reads a direction of gravity and thereby corrects for potential sag in the lens membrane based upon the spatial orientation of the lens.

Description:
FIELD OF THE INVENTION 
       [0001]    This application relates to cameras used in machine vision and more particularly to automatic focusing lens assemblies. 
       BACKGROUND OF THE INVENTION 
       [0002]    Vision systems that perform measurement, inspection, alignment of objects and/or decoding of symbology (e.g. bar codes, or more simply “IDs”) are used in a wide range of applications and industries. These systems are based around the use of an image sensor, which acquires images (typically grayscale or color, and in one, two or three dimensions) of the subject or object, and processes these acquired images using an on-board or interconnected vision system processor. The processor generally includes both processing hardware and non-transitory computer-readable program instructions that perform one or more vision system processes to generate a desired output based upon the image&#39;s processed information. This image information is typically provided within an array of image pixels each having various colors and/or intensities. In the example of an ID reader, the user or automated process acquires an image of an object that is believed to contain one or more IDs. The image is processed to identify ID features, which are then decoded by a decoding process and/or processor to obtain the inherent information (e.g. alphanumeric data) that is encoded in the pattern of the ID. 
         [0003]    Often, a vision system camera includes an internal processor and other components that allow it to act as a standalone unit, providing a desired output data (e.g. decoded symbol information) to a downstream process, such as an inventory tracking computer system or logistics application. It is often desirable that the camera assembly contain a lens mount, such as the commonly used C-mount, that is capable of receiving a variety of lens configurations. In this manner, the camera assembly can be adapted to the specific vision system task. The choice of lens configuration can be driven by a variety of factors, such as lighting/illumination, field of view, focal distance, relative angle of the camera axis and imaged surface, and the fineness of details on the imaged surface. In addition, the cost of the lens and/or the available space for mounting the vision system can also drive the choice of lens. 
         [0004]    An exemplary lens configuration that can be desirable in certain vision system applications is the automatic focusing (auto-focus) assembly. By way of example, an auto-focus lens can be facilitated by a so-called liquid lens assembly. One form of liquid lens uses two iso-density liquids—oil is an insulator while water is a conductor. The variation of voltage passed through the lens by surrounding circuitry leads to a change of curvature of the liquid-liquid interface, which in turn leads to a change of the focal length of the lens. Some significant advantages in the use of a liquid lens are the lens&#39; ruggedness (it is free of mechanical moving parts), its fast response times, its relatively good optical quality, and its low power consumption and size. The use of a liquid lens can desirably simplify installation, setup and maintenance of the vision system by eliminating the need to manually touch the lens. Relative to other auto-focus mechanisms, the liquid lens has extremely fast response times. It is also ideal for applications with reading distances that change from object-to-object (surface-to-surface) or during the changeover from the reading of one object to another object—for example in scanning a moving conveyor containing differing sized/height objects (such as shipping boxes). In general, the ability to quickly focus “on the fly” is desirable in many vision system applications. 
         [0005]    A recent development in liquid lens technology is available from Optotune AG of Switzerland. This lens utilizes a movable membrane covering a liquid reservoir to vary its focal distance. A bobbin exerts pressure to alter the shape of the membrane and thereby vary the lens focus. The bobbin is moved by varying the input current within a preset range. Differing current levels provide differing focal distances for the liquid lens. This lens advantageously provides a larger aperture (e.g. 6 to 10 millimeters) than competing designs (e.g. Varioptic of France) and operates faster. However, due to thermal drift and other factors, there may be variation in calibration and focus setting during runtime use, and over time in general. A variety of systems can be provided to compensate and/or correct for focus variation and other factors. However, these can require processing time (within the camera&#39;s internal processor) that slows the lens&#39; overall response time in coming to a new focus. It is recognized generally that a control frequency of at least approximately 1000 Hz may be required to adequately control the focus of the lens and maintain it within desired ranges. This poses a burden to the vision system&#39;s processor, which can be based on a DSP or similar architecture. That is vision system tasks would suffer if the DSP were continually preoccupied with lens-control tasks. 
       SUMMARY OF THE INVENTION 
       [0006]    This invention overcomes disadvantages of the prior art by providing a removably mountable lens assembly for a vision system camera that includes an integral auto-focusing, liquid lens unit, in which the lens unit compensates for focus variations by employing a feedback control circuit that is integrated into the body of the lens assembly. The feedback control circuit receives motion information related to and actuator, such as a bobbin (which variably biases the membrane under current control) of the lens from a position sensor (e.g., a Hall sensor) and uses this information internally to correct for motion variations that deviate from the lens setting position at a target lens focal distance setting. The defined “position sensor” can be a single (e.g. Hall sensor) unit or a combination of discrete sensor&#39;s located variously with respect to the actuator/bobbin to measure movement at various locations around the lens unit. Illustratively, the feedback circuit can be interconnected with one or more temperature sensors that adjust the lens setting position for a particular temperature value. In addition, the feedback circuit can communicate with an accelerometer that senses the acting direction of gravity, and thereby corrects for potential sag (or other orientation-induced deformation) in the lens membrane based upon the spatial orientation of the lens. 
         [0007]    In an illustrative embodiment, a lens assembly for a vision system camera having variable focus provides a lens body having a variable lens assembly and a fixed optics assembly. A controller (control circuit) is located within the body. The controller is constructed and arranged to receive a target focal distance from the vision system camera. The controller generates a target position of an actuator that controls curvature of the variable lens assembly. Based upon an actual measured position of the actuator, the controller corrects the measured position of the actuator to the target continuously, in a feedback loop. Illustratively, the variable lens assembly includes a membrane-based liquid lens element in which the membrane curvature is driven by a moving actuator. The liquid lens element can include a position sensor located to measure movement of the actuator associated with movement of a membrane of the membrane-based liquid lens assembly. This position sensor can comprise one or more linear Hall sensor(s) that measure(s) a magnet positioned to move on the actuator. The actuator can be a bobbin that is driven by current using a current controller operatively connected with the controller. The target position information illustratively defines a position that focuses an image acquired by the vision system camera. Additionally, the target position information can be further corrected by the controller for at least one of temperature of the liquid lens assembly, spatial orientation and/or other parameters (e.g. flange-to-sensor distance tolerance) of the liquid lens assembly. Thus, the controller converts this information into a corrected target position value for the Hall sensor. The corrected position information is determined by the controller based upon stored calibration parameters that reside in the memory (e.g. an EEPROM of the lens assembly). The calibration parameters can relate to temperature of the lens, provided by a temperature sensor, spatial orientation of the lens, provided by an accelerometer, and/or other parameters, such as flange-to-sensor distance tolerance. The controller can also allow for upgrade of its process instructions (firmware) via the communication network (e.g. an I2C communication interface), typically upon startup. This firmware upgrade is received from the vision system if newer information is available from it. 
         [0008]    Illustratively, the controller can reside on a circuit board that is positioned on a shelf surrounded by a cap assembly. The (e.g., cylindrical) cap assembly surrounds a filler having the shelf and a main barrel assembly that contains the fixed optics therein. The cap assembly is operatively connected to the filler containing the shelf. It is selectively rotatable about an optical axis with respect to the main barrel assembly. The main barrel assembly includes a mount base constructed and arranged to removably secure to a mount of the vision system camera so that the lens assembly is exchangeable. The controller illustratively indicates when the lens position has moved to a corrected position. 
         [0009]    In an illustrative embodiment, a method for controlling focus of a membrane-based liquid lens assembly of a vision system camera in the form of a “local” feedback loop (i.e. using a lens-assembly based controller/processor includes measuring of a present position of an actuator of the membrane-based liquid lens assembly with a position sensor. A target position of the actuator is received from an interconnected vision system processor of the vision system camera in the form of a focal distance. This distance is interpreted into the target position of the actuator by the controller. The controller (locally mounted in a body of the lens assembly) compares the measured, actual position of the actuator with the target position, and determines whether the two positions are currently substantially equal. If the values are substantially equal, then a correct position is indicated by the controller. If the values are sufficiently unequal, then the controller sends a correction to the actuator and repeats the above steps in a feedback loop that continuously maintains correct position based upon the current target. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    The invention description below refers to the accompanying drawings, of which: 
           [0011]      FIG. 1  is a perspective view of the external structure of an exchangeable auto-focus lens assembly with integrated feedback-loop-based focus control according to an illustrative embodiment; 
           [0012]      FIG. 2  is a side cross section of the lens assembly of  FIG. 1  showing the layout of internal mechanical, optical, electro-optical and electronic components; 
           [0013]      FIG. 3  is a perspective view of the lens assembly of  FIG. 1  with outer cap removed to reveal the arrangement of components; 
           [0014]      FIG. 4  is a perspective view of the lens assembly of  FIG. 1  with the outer cap and spacer assembly removed to reveal the interconnection between the liquid lens and the control circuit; 
           [0015]      FIG. 5  is a block diagram of the generalized electrical connection and data flow between the liquid lens, integrated controller and camera vision system processor for the lens assembly of  FIG. 1 ; 
           [0016]      FIG. 5A  is a flow diagram of a feedback loop-based bobbin position control process for the lens assembly of  FIG. 1 ; 
           [0017]      FIG. 6  is a block diagram of the stored data in the control circuit memory of  FIG. 5 ; and 
           [0018]      FIG. 7  is a temperature correction process that generates temperature-corrected bobbin position values for use with the control circuit of  FIG. 5  and process of  FIG. 5A . 
       
    
    
     DETAILED DESCRIPTION 
       [0019]      FIG. 1  details the external structure of an exchangeable, auto-focus lens assembly (also simply termed “lens assembly”)  100  according to an illustrative embodiment. The lens assembly includes an outer cap  110  defining a generally cylindrical shape. This outer cap  110  provides a protective and supporting shell for a variable focus lens element (comprising an Optotune membrane-based liquid lens model EL-6-18 or EL-10-30 in this exemplary embodiment)  120 . By way of useful background information the present data sheet with specifications for various models of this lens is available on the World Wide Web at www.optotune.com/images/products/Optotune%20EL-6-18.pdf. It is expressly contemplated that the teachings of the embodiments herein can be applied to a variety of electronically focused lens types including other forms of liquid lens technology and electro-mechanically adjusted solid lenses. For the purposes of this description, the variable focus lens element  120  (also simply termed the “liquid lens”) of the overall auto-focus lens assembly  100  is assumed to operate based upon predetermined inputs of current (or voltage in alternate arrangements), and provides various outputs that the user can employ to monitor and control the lens using conventional techniques. Such outputs can include the position of the bobbin using, for example, one or more Hall sensors (described further below) and/or the present temperature of the lens using one or more conventional temperature sensors. 
         [0020]    By way of further background, it has been observed that such liquid lenses exhibit excessive drift of its optical power over time and temperature. Although the lens can be focused relatively quickly to a new focal position (i.e. within 5 milliseconds), it tends to drift from this focus almost immediately. The initial drift (or “lag”) is caused by latency in the stretch of the membrane from one focus state to the next—i.e. the stretch takes a certain amount of time to occur. A second drift effect with a longer time constant is caused by the power dissipation of the lens&#39; actuator bobbin heating up the lens membrane and liquid. In addition the orientation of the lens with respect to the acting direction of gravity can cause membrane sag that has an effect on focus. The system and method of the embodiments described herein address disadvantages observed in the operation and performance such liquid lenses. 
         [0021]    The rear  130  of the lens assembly  100  includes a threaded base that can be adapted to seat in a standard camera mount, such as the popular cine or (C-mount). While not shown, it is expressly contemplated that the lens assembly  100  can be (removably) mounted a variety of camera types adapted to perform vision system tasks with an associated vision system processor. 
         [0022]    With further reference also to  FIGS. 2-4 , the construction of the lens assembly  100  is described in further detail. It is expressly contemplated that the depicted construction is illustrative of a range of possible arrangements of components that should be clear to those of skill in the art. The cap  110  defines a metal shell (for example aluminum alloy) that includes a side skirt  140  and unitary front face  150 . The cap overlies a spacer/filler  210  (see also  FIG. 3 ). This filler  210  includes a pair of threaded holes  310  ( FIG. 3 ) that receive threaded fasteners  160  to removably secure the cap over the filler  210 . A pair of opposing threaded fasteners  170  are recessed in corresponding holes  172  of the cap and pass through holes  320  in the filler  210  and into threaded holes  410  ( FIG. 4 ) on two keys  440  that rotatably engage the main lens barrel assembly  220  ( FIGS. 2 and 4 ). This relationship is described further below. These fasteners  170  maintain the main lens barrel assembly  220  in axial alignment with the filler  210 . 
         [0023]    As shown in  FIG. 2 , the lens barrel assembly  220  contains a series of fixed lenses  230 ,  232 ,  234 ,  236  and  238  arranged according to ordinary optical skill behind the liquid lens  210 . These lenses allow the image projected along the optical axis OA to the vision system sensor to be sized appropriately to the sensor&#39;s area over a range of varying focal distances specified for the lens assembly. By way of example, the range of optical power can be −2 to +10 diopter. The lenses  230 - 238  are arranged in a compressed stack within the main barrel assembly  220  with appropriate steps and/or spacers therebetween. The overall stack is held in place by a threaded retaining ring  240  at the rear end ( 130 ) of the lens assembly  110 . At the front of the main barrel is located an aperture stop disc  250  that reduces the system aperture to an appropriate, smaller diameter. This allows customization of brightness/exposure control and/or depth of field for a given vision system application. 
         [0024]    The main barrel assembly  220  includes a rear externally threaded base  260  having a diameter and thread smaller than that of a C-mount—for example a conventional M-12 mount size for interchangeability with camera&#39;s employing this standard, or another arbitrary thread size. A threaded mount ring  262  with, for example, a C-mount external thread  264  is threaded over the base thread  260 . This ring  262  allows the back focus of the lens with respect to the camera sensor to be accurately set. In general, the shoulder  266  of the ring is set to abut the face of the camera mount when the lens is secured against the camera body. A pair of set screws  360  ( FIGS. 3 and 4 ) pass through the ring  262 , and removably engage the base thread  260  to maintain the mount ring  262  at an appropriate back focus setting. 
         [0025]    An O-ring  267  is provided on the front face of the liquid lens  120  to cancel out tolerances. In addition, and with reference also to  FIG. 4 , filler  210  is adapted to rotate with respect to the main barrel assembly  220 . A pair of semi-circular keys  440 , held together by an O-ring  450  engage a groove in the filler  210  and allow the filler  210  and cap  110  to rotate with respect to the barrel assembly  220  about the axis OA, while fixing these components along the axial direction. In this manner, after the lens assembly threaded base is properly seated in the camera housing with desired back focus, the cap is rotated to align the cable  270  with the camera&#39;s connecting socket. This rotation is secured via the knob  180  ( FIG. 1 ) that threads through a hole  380  in the filler  210  and can be tightened to bear against the barrel assembly  220 , thereby rotationally locking these components together at the desired rotational orientation therebetween. 
         [0026]    As shown in  FIG. 3 , the front end of the filler  210  includes a somewhat rectangular recess  330  to support the shape of the liquid lens  120  in a position at the front of the assembly and in front of the main barrel assembly  220 . The filler  210  also includes a flattened top end (shelf)  340  with appropriate raised retaining tabs  342  to support a lens control circuit board  350  according to an illustrative embodiment. The arrangement of the shelf  340 , circuit board  350  and cap  110  define a sufficient gap G ( FIG. 2 ) between the inner surface of the cap and the circuit board to provide clearance for the board. In an embodiment, the approximate diameter of the cap is approximately 32 millimeters. 
         [0027]    Notably, the barrel assembly  220  is an interchangeable component so that different fixed lens arrangements can be provided in the overall lens assembly (i.e. with the same liquid lens, cap and control circuitry). Thus, this design provides substantial versatility in providing a range of possible focal distances for different vision system applications. 
         [0028]    Also notably, the provision of a lens control circuit within the overall structure of the lens assembly allows certain control functions to be localized within the lens itself. This is described in further detail below. The circuit board  350  is connected via a connector  422  and standard ribbon cable  420  to the liquid lens  120  as shown in  FIG. 4 . The filler  210  provides a gap to run the cable  420  between these components. Additionally, the control circuit board  350  is connected to a cable  270  and multi-pin end connector  272 . These are arranged to electrically connect to a receptacle on the camera housing (typically along its front face adjacent to the lens mount). This cable provides power to the lens assembly (the circuit board and liquid lens) from the camera body, and also provides a data interconnect between the lens and the camera&#39;s vision system processor, as described in further detail below. A cutout  274  at the rear edge of the cap  110  provides a chase for the cable  270  to pass from the interior to the exterior of the lens assembly  110 . Appropriate seals and/or close-tolerance fits prevent incursion of moisture or contaminants from the environment. 
         [0029]    The control functions of the circuit board  350  are now described in further detail with reference to  FIG. 5 . As described above, it has been observed that the drift or lag can be controlled by measuring the position of the actuator and the temperature of the lens and using this data to control the current through the lens actuator bobbin (a magnetic coil that compresses the lens variable under different current settings). In an illustrative embodiment, such drift/lag is compensated by a control circuit  510  (also termed simply “controller”) on the circuit board that integrates a (digital) feedback loop completely into the lens barrel of the lens assembly avoiding the use of the camera&#39;s vision system processor to control these adjustments. The control circuit includes an associated memory (e.g. an EEPROM)  512  that, as shown in  FIG. 6  can be divided into data memory  610  and program memory  620 . As described further below, the data memory  610  can include correction parameters for temperature  612 , orientation with respect to gravity  614 , and other appropriate parameters  616 . Such other parameters  616  can include tolerance control parameters, such as the flange tolerance correction (described below). The program memory can include the feedback-loop control software and correction application  622 . 
         [0030]    At startup, the vision system  520  communicates to the lens assembly circuit  350  the tolerance value of its flange-to-sensor distance. This value is the deviation from the ideal C-mount distance (typically 17.526 millimeters), which has been measured after assembly of the vision system and has been stored in the memory  526  (e.g. a non-volatile flash memory) of the vision system. The control circuit  510  is arranged to correct for the flange tolerance as described further below. 
         [0031]    Upon startup, the control circuit  510  can request the vision system processor  522  of the vision system camera  520  to provide the latest firmware upgrade  528  so that the function lens assembly is synchronized with the software and firmware of the vision system. If the firmware is up-to-date, then the processor indicates this state to the lens control circuit and no upgrade is performed. If the firmware is out-of-date, then the new firmware is loaded in the appropriate location of the lens assembly&#39;s program memory  620  ( FIG. 6 ). This communication typically occurs over the lens assembly&#39;s I2C communication interface ( 531 ) transmitted over the cable  270  ( FIG. 2 ). 
         [0032]    Note, as used herein the terms “process” and/or “processor” should be taken broadly to include a variety of electronic hardware and/or software based functions and components. Moreover, a depicted process or processor can be combined with other processes and/or processors or divided into various sub-processes or processors. Such sub-processes and/or sub-processors can be variously combined according to embodiments herein. Likewise, it is expressly contemplated that any function, process and/or processor herein can be implemented using electronic hardware, software consisting of a non-transitory computer-readable medium of program instructions, or a combination of hardware and software. 
         [0033]    The control circuit  510  can be implemented using a variety of electronic hardware. Illustratively a microcontroller is employed. The control circuit  510  receives focus information  530  (e.g. focal distance, which is translated by the controller into target bobbin position) from the vision system camera  520  (i.e. via cable  270  and interface link  531 ). This focus information can be derived from a focus process  532  that operates in the camera processor  522 . The focus process can use conventional or custom auto-focus techniques to determine proper focus. These can include range-finding or stepping through a series of focus values in an effort to generate crisp edges in the image  534  of an object acquired by the sensor  536 . While highly variable a 2K×1K-pixel sensor is used in the exemplary embodiment. 
         [0034]    The focus information  530  is used by the control circuit  510  to generate a target bobbin position and to provide a digital signal with movement information  540  to the current controller  544 . The current controller applies the appropriate current to an annular bobbin assembly  550  (or “bobbin”), which thereby deforms the liquid lens membrane  552  to provide an appropriate convex shape to the bulged lensmatic region  554  within the central opening of the bobbin  550 . The bobbin  550  includes a magnet  558  that passes over a conventional linear Hall sensor  560 . This Hall sensor  560  generates a digital position signal  562  that is directed back to the control circuit  510  where it is analyzed for actual bobbin position (for example, calling up values in the memory  512 ) versus the target position represented by a corresponding Hall sensor target position. If, in a comparison of the actual Hall sensor value and target Hall sensor value, these values do not match, then the control circuit  510  applies a correction, and that is delivered to the current controller  544 , where it is used to move the bobbin  550  to a correct position that conforms with the target Hall sensor position. Once the bobbin  550  is at the correct position, the controller can signal that correction is complete. 
         [0035]    Note that additional Hall sensors (or other position-sensing devices)  566  (shown in phantom) can generate additional (optional) position signals  568  that are used by the control circuit to verify and/or supplement the signal of sensor  560 . In an embodiment, data is transmitted between components using an I2C protocol, but other protocols are expressly contemplated. In general, the commercially available Hall sensor operates in the digital realm (i.e. using the I2C interface protocol), thereby effectively avoiding signal interference due to magnetic effects. By way of non-limiting example, a model AS5510 Hall linear sensor (or sensors) available from AustriaMicrosystems (AMS) of Austria can be used. 
         [0036]    With reference to  FIG. 5A , a bobbin position-sensing/correcting feedback loop process  570  is shown in a series of flow-diagram process steps. A target focus distance is received from the vision system processor in step  572 . The control feedback loop  570  then initiates as this focus distance is used by the lens assembly control circuit (controller)  510  to determine a target value for bobbin position represented by a target Hall sensor value provided by one or more sensors on the bobbin. The target Hall sensor value(s) can be corrected based upon stored parameters in memory  512  (step  574 ). Such parameters include, but are not limited to temperature, spatial orientation and flange-to-sensor-distance tolerance, and this (optional) process is described further below. In step  576 , the control circuit  510  measures the actual position of the bobbin based upon the position of the Hall sensor(s) and associated signal value(s) ( 562 ). In step  578 , the control circuit  510  then compares the actual, returned Hall sensor value(s) with the target value. If the values are not substantially equal then decision step  580  branches to step  582  and the control circuit directs the current controller  544  to input a current that will move the bobbin to the corrected position. This can be based on the difference in current needed to move the bobbin between the actual and correct position. If the comparison in step  578  determines that the actual and target Hall sensor value(s) are substantially equal, then the decision step  580  branches to step  582  and the system indicates that correction is complete. The control circuit repeats correction steps  574 ,  576 ,  578 ,  580  and  582  until the actual and target Hall sensor values are substantially equal (within an acceptable tolerance), and the new correct bobbin position is indicated. This complete status can be reported to the camera&#39;s processor  522  for use in performing image acquisition. 
         [0037]    Note that this local feedback loop  570  can run continuously to maintain focus at a set position once established, and until a new bobbin position/focus is directed by the camera. Thus, the feedback loop  570  ensures a steady and continuing focus throughout the image acquisition of an object, and does so in a manner that avoids increased burdens on the camera&#39;s vision system processor. 
         [0038]    The determination of the target value for the Hall sensor(s) in step  574  can include optional temperature, spatial orientation and/or other parameter (e.g. flange distance) correction based upon parameters  612 ,  614 ,  616  ( FIG. 6 ) stored in memory  512 . Temperature of the lens unit is sensed (optionally) by an on-board or adjacent temperature sensor  588  ( FIG. 5 ). The temperature sensor  588 , like other components of the circuit  350 , can employ a standard interface protocol (e.g. I2C). 
         [0039]    As shown in  FIG. 7 , an optional temperature compensation process  700  operating within the control circuit  510  receives a temperature reading  710  from the sensor  536  and target focus or bobbin position information  720  and applies temperature calibration parameters  730 . These can be stored locally on the lens assembly circuit memory  512  as shown in  FIG. 6 . The correction parameters can define a curve or a series of table values associated with given temperature readings that are measured during calibration. The process  700  modifies the target Hall sensor value (and associated bobbin position) from a base value, based upon the focus distance provided by the vision system camera to a value that accounts for the variation of lens focus with respect to lens temperature. Thus, the base Hall sensor value can be added-to or subtracted from by the control circuit  510  based upon the prevailing temperature reading at the lens to generate a temperature corrected target value  740 . 
         [0040]    Likewise, correction for orientation with respect to gravity that can result in sag or other geometric deformation of the lens membrane in differing ways is compensated by an (optional) accelerometer  594  that transmits the spatial orientation  596  of the lens/camera with respect to the acting directing of gravity to the control circuit via, for example, an I2C protocol. In an embodiment, an orientation correction factor is determined (by reading the accelerometer  594 ), and applied to the target Hall sensor value by the control circuit in a manner similar to temperature correction ( FIG. 7 ) substituting orientation for temperature in block  710 . Since orientation typically remains constant (except in the case of a moving camera), the determination of orientation can be a one-time event (i.e. at camera setup/calibration), or can occur upon start up or at a timed interval based upon the control circuit&#39;s clock. Like temperature correction, orientation correction parameters can comprise a curve or lookup table mapped to differing orientations, which can be determined during calibration. The appropriate orientation parameter value is applied to the step of determining ( 574 ) the target Hall sensor value, and the target value is adjusted to include this further correction for spatial orientation. Note that in the case of a moving camera, the orientation parameter can be continuously updated in the same manner that temperature is updated to account for changes over time. 
         [0041]    Other parameters ( 616  in  FIG. 6 ), such as flange-to-sensor distance tolerance, can also be stored in the circuit memory  512 . These parameters can be updated from the data store of the vision system camera upon startup or at another interval of time. The value of each parameter is used by the control circuit&#39;s process to further adjust and correct the target Hall sensor value. This overall corrected value is used in the comparison step  578  against the actual measured value to thereby move the bobbin to the correct position. 
         [0042]    It should be clear that superior position correction, on the order of 1 millisecond, can be achieved using the local feedback loop instantiated in a control circuit packaged in the lens assembly. The entire lens assembly package fits within a standard C-mount lens affording a high degree of interoperability with a wide range of vision system camera models and types. The system and method for controlling and correcting the focus of a liquid (or other similar auto-focusing) lens described herein can be employed rapidly, and at any time during camera runtime operation and generally free of burden to the camera&#39;s vision system processor. This system and method also desirably accounts for variations in focus due to thermal conditions and spatial orientation (i.e. lens sag due to gravity). This system and method more generally allow for a lens assembly that mounts in a conventional camera base. 
         [0043]    The foregoing has been a detailed description of illustrative embodiments of the invention. Various modifications and additions can be made without departing from the spirit and scope of this invention. Features of each of the various embodiments described above can be combined with features of other described embodiments as appropriate in order to provide a multiplicity of feature combinations in associated new embodiments. Furthermore, while the foregoing describes a number of separate embodiments of the apparatus and method of the present invention, what has been described herein is merely illustrative of the application of the principles of the present invention. For example, while a Hall sensor is used to measure position, a variety of alternate position-sensing devices can be used in association with the feedback loop herein. For example an optical/interference-based position sensor can be employed in alternate embodiments. Also, it is contemplated that the principles herein can be applied to a variety of lenses (liquid and otherwise), in which the curvature of the lens is varied via electronic control. Thus the term “variable lens assembly” should be taken broadly to expressly include at least such lens types. In addition while various bobbin position corrections are performed within the lens control circuit and feedback loop, it is contemplated that some corrections can be performed within the vision system camera processor, and the corrected focal distance is then sent to the lens assembly for use in further feedback loop operations. As used herein, various directional and orientation terms such as “vertical”, “horizontal”, “up”, “down”, “bottom”, “top”, “side”, “front”, “rear”, “left”, “right”, and the like, are used only as relative conventions and not as absolute orientations with respect to a fixed coordinate system, such as gravity. Accordingly, this description is meant to be taken only by way of example, and not to otherwise limit the scope of this invention.