HK1212533B - Systems reduce temperature induced drift effects on a liquid lens - Google Patents

Description

System for reducing temperature-induced drift effects on liquid lenses

CROSS-REFERENCE TO RELATED APPLICATIONS

not applicable

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

not applicable

Background Art

The present technology relates to tunable lenses for use in lens systems, and more particularly to systems and methods for reducing temperature-induced drift effects on microfluidic or liquid lenses used in vision systems.

Vision systems have been developed for a variety of different applications. For example, machine vision systems have been developed for reading bar codes and other types of symbols placed on packaging or products to obtain information therefrom. Other machine vision systems have been developed for inspecting the features/characteristics of manufactured parts.

Many vision systems include a camera for capturing an image of a symbol or object to be imaged. A processor receives the image and extracts information that can then be used to perform one or more vision processes. In many applications, the distance between the camera sensor and the symbol or object to be imaged can vary between applications. In these cases, an adjustable lens and/or autofocus system is often provided to capture a useful image, i.e., an image from which the data required to complete the machine vision process can be extracted. In these cases, when the system is activated to perform a vision process, the lens and autofocus system automatically focus the lens so that a clear image of the symbol or object to be imaged is produced on the camera sensor. After the focusing process is complete, a clear image of the symbol or object to be imaged is captured and processed to complete the vision process.

One type of tunable lens that can be used in machine vision processes is a liquid lens. A liquid lens is composed of one or more fluids of different refractive indices, and the liquid lens can be modified by controlling the meniscus or surface of the liquid. For example, in one type of liquid lens, two fluids are contained in a tube with transparent end caps. The first is a conductive aqueous solution and the second is a non-conductive oil. The interior of the tube is coated with a hydrophobic material, which causes the aqueous solution to form a hemispherical lens that can be adjusted by applying a DC voltage across the coating to reduce its water repellency in a process called electrowetting. Electrowetting adjusts the surface tension of the liquid, which changes the radius of curvature and adjusts the focal length of the lens. Several liquid lens configurations that utilize the electrowetting process are known.

Another type of tunable liquid lens utilizes an electromechanical actuator system to induce motion to adjust the lens's focus. For example, a voice coil-type tunable lens features an annular voice coil actuator pressed against a transparent membrane that serves as the transparent sidewall of a container. The container is filled with a transparent liquid. An electric current applied through the actuator induces the actuator to apply a force that deforms the membrane into a convex shape. This convex shape acts as a lens and can be adjusted by adjusting the current.

Liquid lenses are extremely versatile, offering highly variable focal lengths, some without moving parts. However, liquid lenses are inherently subject to unwanted changes in focal length (referred to herein as drift) caused by temperature variations and aging of the liquid in the lens. For example, temperature and aging can change the refractive index or dielectric constant of the liquid, thereby changing the focal length. For example, when imaging small symbols at a fixed, large distance, temperature drift of the lens will cause blur in the image and degrade reading performance. This unwanted drift can cause a liquid lens at a first temperature to have a first focal length, while the same liquid lens at a second temperature will have a second focal length that is different from the first.

For an adjustable lens that uses an electric current applied through an actuator to adjust the focus of the lens, the electric current applied through the actuator not only heats the actuator, but also heats the lens. Undesirably, this causes the temperature of the lens to vary with the applied control current. Since a higher electric current is required for a larger optical power, the lens will become hotter at a large optical power (close object distance) than at a small optical power (large object distance) when in use.

Various attempts have been made to compensate for liquid lens drift. These attempts measure the thermal properties of the liquid lens during a calibration process and then compensate the lens during normal operation by adjusting the liquid lens driver voltage or current based on the measured thermal properties. Not only does this require a time-consuming calibration process for each lens, but the measured thermal properties are based on typical drift behavior during calibration, which has limited accuracy.

Therefore, when using a variable lens in an application that causes lens temperature variations, the focusing of the variable lens will produce different results at different temperatures. For these applications, other systems and methods must be used in an attempt to maintain a more consistent focus and a sharper resulting image. The present technology provides a solution to these problems.

Summary of the Invention

The present technology provides systems and methods for reducing temperature-induced drift effects on liquid lenses used in vision systems. A processor can receive a temperature value from a temperature sensor and, based on the received temperature value, energize or de-energize a heating element on at least one circuit board to maintain the temperature value within a predetermined control temperature range to reduce drift effects. The processor can also control a bias signal applied to the lens or lens actuator to control temperature changes and the associated induced drift effects. Image sharpness can also be determined from a series of images alone or in combination with controlling the temperature of the liquid lens to adjust the lens's focal length.

In one aspect, the present technology provides a vision system and method for maintaining the temperature of a liquid lens at a controlled temperature, thereby reducing drift effects on the liquid lens. The vision system includes a focusable liquid lens having a field of view. At least one circuit board is in thermal contact with at least a portion of the liquid lens. A heating element is disposed on the at least one circuit board, the heating element being controllable to heat the at least one circuit board. A temperature sensor is configured to measure a temperature value of the liquid lens. A feedback loop controls electrical power to the heating element based on a difference between the measured temperature of the liquid lens and a predetermined control temperature.

In other aspects, the present technology provides a vision system and method for controlling a bias signal applied to a liquid lens to control the temperature of the liquid lens. The vision system includes a focusable liquid lens having a field of view, wherein the focus of the liquid lens can be adjusted for image capture using a control signal applied to the liquid lens. When the liquid lens is not adjusted for image capture using the control signal, a bias signal is applied to the liquid lens. The bias signal is applied to the liquid lens to control the temperature of the liquid lens.

In some embodiments, the bias signal may be controlled in relation to the average heat dissipation from the liquid lens. In other embodiments, the bias signal may be dependent on a sensed temperature value of the liquid lens or the ambient temperature.

Other embodiments include systems and methods for optimizing the focus of an adjustable lens in a vision system having a field of view. The method includes several steps, including: adjusting the focus of the adjustable lens by a predetermined adjustment step; acquiring a first image of the field of view including a region of interest; calculating a first sharpness score for the region of interest within the first image of the field of view; adjusting the focus of the adjustable lens by the predetermined adjustment step; acquiring another image of the field of view including the region of interest; calculating another sharpness score for the region of interest within the another image of the field of view; comparing the first sharpness score to the another sharpness score; and defining a direction for a next adjustment step in focus based on the comparison.

Still further embodiments include systems and methods for optimizing the focus of an adjustable lens in a vision system having a field of view. The methods include: adjusting the focus of the adjustable lens by a predetermined adjustment step; acquiring a first image of the field of view; measuring a first ambient temperature near the adjustable lens; adjusting the focus of the adjustable lens by the predetermined adjustment step; acquiring another image of the field of view; measuring another ambient temperature near the adjustable lens; comparing the first ambient temperature to the other ambient temperature; and defining a direction for a next adjustment step in the focus based on the comparison.

To achieve the foregoing and related objectives, the present technology includes the features described more fully below. The following description and accompanying drawings set forth in detail various aspects of the present technology. However, these aspects represent only a few of the many ways in which the principles of the present technology can be employed. Other aspects, advantages, and novel features of the present technology will become apparent by reference to the following detailed description of the technology in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

1 is a perspective view of a fixed-mount reader device acquiring an image of a symbol on an item of interest in accordance with an embodiment of the present technology;

FIG2 is a perspective view of a fixedly mounted reader device showing the front end of the reader device;

FIG3 is a schematic diagram illustrating components that may include the reader device of FIGS. 1 and 2 ;

FIG4 is an exploded view illustrating an embodiment of a liquid lens and components of a reader device disposed in thermal communication with the liquid lens;

FIG5 is a schematic diagram illustrating values and data that may be stored in a memory;

FIG6 is a side schematic diagram showing a liquid lens and a circuit board in contact with the liquid lens;

FIG7 is a flow chart of a method associated with controlling the temperature of a liquid lens;

FIG8 is a side schematic diagram illustrating an additional embodiment of a liquid lens including an actuator and a circuit board in contact with the liquid lens;

9 is a diagram illustrating the relative positions to which a liquid lens is driven and the default positions to which the associated lens is returned.

FIG10 is a diagram similar to FIG9 and showing the same relative position to which the liquid lens is driven and, instead, showing a calculated return position to which the lens is returned for controlling the temperature of the liquid lens; and

11 , 12 , and 13 are flow charts of methods associated with controlling the temperature of a liquid lens according to embodiments of the present technology;

While the present technology is susceptible to various modifications and alternative forms, specific embodiments thereof will be shown by way of example in the drawings and will be described in detail herein. However, it should be understood that the description of specific embodiments herein is not intended to limit the present technology to the particular forms disclosed, but on the contrary, it is intended to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the present technology as defined by the appended claims.

DETAILED DESCRIPTION

Various aspects of the present technology will now be described with reference to the accompanying drawings, wherein like reference numerals correspond to similar elements throughout the several views. It should be understood, however, that the drawings and detailed description referred to below are not intended to limit the claimed subject matter to the particular forms disclosed. On the contrary, the present invention is intended to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the claimed subject matter.

As used herein, the terms "component," "system," "method," and the like are intended to refer to hardware, a combination of hardware and software, software, or software in execution. The word "exemplary" as used herein means serving as an example, instance, or illustration. Any aspect or design described herein as "exemplary" is not necessarily to be construed as preferred or advantageous over other aspects or designs.

Furthermore, the disclosed subject matter may be implemented as a system, method, apparatus, or article of manufacture by using standard programming and/or engineering techniques and/or programming to produce software, firmware, hardware, or any combination thereof to implement the aspects detailed herein.

Unless otherwise specified or limited, the terms "connected" and "coupled" and variations thereof are used broadly and encompass direct and indirect mountings, connections, supports, and couplings. Furthermore, "connected" and "coupled" are not limited to physical or mechanical connections or couplings. As used herein, unless expressly stated otherwise, "connected" means that one element/feature is directly or indirectly connected to another element/feature, and not necessarily electrically or mechanically. Similarly, unless expressly stated otherwise, "coupled" means that one component/feature is directly or indirectly coupled to another component/feature, and not necessarily electrically or mechanically.

As used herein, the term "processor" may include one or more processors and multiple memories and/or one or more programmable hardware components. As used herein, the term "processor" is intended to include any type of processor, CPU, microcontroller, digital signal processor, or other device capable of executing software instructions.

As used herein, the term "memory" includes non-volatile media, such as magnetic media or hard disks, optical storage, or flash memory; volatile media, such as system memory, such as random access memory (RAM), such as DRAM, SRAM, EDO RAM, RAMBUS RAM, DR DRAM; or installation media, such as software media, such as CD-ROMs, on which configuration data and programs can be stored and/or data communications can be cached. The term "memory" may also include other types of memory, whether known or developed in the future, or combinations thereof.

The following describes embodiments of the present technology by using diagrams to illustrate structures or processes for implementing embodiments of the present technology. Presenting embodiments of the present technology using diagrams in this manner should not be construed as limiting their scope. The present technology contemplates systems and methods for reducing and/or controlling temperature-induced drift effects on tunable lenses and improving image quality.

Various embodiments will be described in conjunction with a liquid lens as part of a fixed-mount symbol reader adapted to acquire images of an object or indicia thereon. This is because the features and advantages of the present technology are well suited for this purpose. Nevertheless, it should be understood that various aspects of the present technology are applicable to other forms of electronic devices and are not limited to the use of a liquid lens as part of a reader, as it will be appreciated that numerous types of electronic devices incorporating heat-sensitive lenses can benefit from reduced temperature-induced drift according to the features described herein.

Referring now to the drawings, in which like reference numerals correspond to like elements throughout the several views, and particularly to FIG1 , the present technology will be described in the context of an exemplary fixed-mount symbol reader 20 that can be used to capture an image of a symbol (e.g., a two-dimensional symbol 22 disposed on an article 24) and to decode the symbol in the captured image. While the technology herein is described in the context of a fixed-mount symbol reader 20, it should be understood that the technology is also applicable to handheld symbol readers and fixed cameras, as non-limiting examples, such as where a conveyor moves articles or packages of various sizes through the reader 20's field of view so that the distance between the reader lens/sensor and the package or article having the symbol applied thereon can vary from article to article.

Referring now to Figures 1 and 2, the reader 20 may include a metal or rigid plastic housing 26. An adjustable focus lens 36 may be disposed behind a lens housing 40 located near the distal end of the reader housing 26 and having a field of view 42. The lens 36 may be a known, commercially available multifocal liquid lens. In these types of lenses, the focus is adjusted by varying a control signal applied to the liquid lens.

Referring now to FIG. 3 , in addition to the components described above with reference to FIG. 1 and FIG. 2 , the reader 20 may include a processor 50, a camera sensor 52, a power source 54, a memory 56, and one or more interface devices 58, such as an audible sound generator, an LED to indicate a successful symbol decode, wireless and/or limited communication, and the like. As is known, the power source 54 may be replaced with a battery for power. The processor 50 may be coupled to the memory 56, which may store programs executed by the processor 50. Furthermore, the memory 50 may direct the storage of images obtained via the camera sensor 52 in the memory 56. The processor 50 may also be coupled to the camera sensor 52 to receive image data therefrom. A known triggering/actuation device or method 34 may be coupled to or executed by the processor 50 to initiate the symbol reading process. The processor 50 may also be coupled to the variable-focus liquid lens 36 to modify the focal position or focal length of the liquid lens 36.

In typical operation, reader 20 is positioned so that the field of view 42 of the camera or lens is directed toward the surface of article 24 to which symbol 22 has been applied, thereby placing symbol 22 within reader's field of view 42. Once so positioned, trigger 34 may be activated, causing reader 20 to acquire one or more images of symbol 22 within field of view 42. Once a suitable focused image of symbol 22 has been obtained, processor 50 within reader 20, or a processor remote from reader 20 using communication interface 58, may attempt to decode symbol 22 and may then provide the decoded information to other software applications for use. Furthermore, following successful decoding of symbol 22, reader 20 may provide an indication to the user that the decode was successful. Here, although not shown in FIG. 1 or 2 , the indication of a successful decode may be provided via an audible beep or noise, or via illumination of an LED or the like, or both.

Liquid lenses, such as liquid lens 36, are typically composed of one or more fluids of different refractive indices and are variable by controlling the meniscus or surface of the liquid. The liquid lens can be adjusted by applying a control signal 64 to the liquid lens or to a liquid lens actuator. For example, the control signal 64 can include a control voltage or a control current. For example, in some types of known liquid lenses, two fluids are contained in a tube with transparent end caps. The first is a conductive aqueous solution and the second is a non-conductive oil. The interior of the tube is coated with a hydrophobic material, which causes the aqueous solution to form a hemispherical lens that can be adjusted by applying a DC voltage across the coating to reduce its water repellency in a process called electrowetting. Electrowetting adjusts the surface tension of the liquid, which changes the radius of curvature and adjusts the focal length of the liquid lens.

As discussed above, the optical properties of a liquid lens differ from those of a typical glass or plastic lens. For example, the optical power of a liquid lens decreases as the temperature of the lens increases and as the lens ages. Furthermore, when focusing the liquid lens, there is a lag between the control signal 64 and the optical power. That is, as the control signal 64 increases and decreases, the incremental change in optical power varies, which can adversely affect the feedback loop.

Embodiments of the present technology control the temperature of the adjustable lens 36 in order to reduce drift effects caused by changes in lens temperature. To minimize drift effects, the application of heat can be controlled alone or in combination with various aspects of controlling the bias signal 66 to the lens 36 or the lens actuator 96. As described below, the control signal 64 can be removed between the acquisition of consecutive images. The bias signal 66 can be applied in place of the control signal 64. For example, the bias signal 66 can include a bias voltage or a bias current. The level of the bias signal 66 and the length of time the bias signal is applied can be adjusted. When adjusted in this manner, the effects of temperature (both ambient temperature and lens temperature) can be offset.

Generally speaking, higher temperatures cause a decrease in the optical power of the liquid lens 36. In this example, the current approach increases the focal length of the reader 20 to adjust for the decrease in optical power. This change in focal length can be used to compensate for the effects of temperature on the liquid lens, but any time the focus of the liquid lens changes, there is a risk associated with reduced clarity of the acquired image due to uncertainty about the exact focus to which the liquid lens should be adjusted.

Referring now to FIG. 4 , an embodiment is shown that can be used to significantly reduce or eliminate focus drift of liquid lens 36 by stabilizing the temperature of liquid lens 36. In this embodiment, a portion of housing 26 has been removed to provide an exploded view of liquid lens 36 and components disposed in contact with and/or near liquid lens 36. In this embodiment, liquid lens 36 can be maintained at a predetermined control temperature 60 despite variations in ambient temperature 62 that may occur around reader 20. Data, such as predetermined control temperature value 61 and ambient temperature value 63, can be stored in memory 56 (see FIG. 5 ). Ambient temperature 62 can be measured at or near liquid lens 36 within housing 26, or outside reader 30, or both. Control temperature 60 can be maintained at a constant temperature and/or a near-constant temperature, for example, within a few degrees. Furthermore, control temperature 60 can be maintained within the operating range of liquid lens 36, for example, between -50 degrees Celsius and 70 degrees Celsius.

In some embodiments, the control temperature 60 can be maintained at or near the high end of the operating range, for example, 70 degrees Celsius. At higher temperatures, some liquid lenses change to a new focal length more quickly. Thus, maintaining the control temperature 60 at or near the high end of the operating range will not only provide the largest possible operating range for the reader 20, but will also serve to reduce or eliminate drift and increase the focusing speed of the liquid lens 36 due to the improved reaction time of the liquid in the liquid lens. For example, it is contemplated that the control temperature 60 can be maintained at a low, or mid-range temperature, or any temperature within or above the operating range of the ambient temperature.

Referring now to Figures 4, 5, and 6, and by way of non-limiting example, the liquid lens 36 can be positioned in thermal and/or physical contact with a first circuit board 70, or, for example, in thermal and/or physical contact between the first circuit board 70 and a second circuit board 72. One or both of the first circuit board 70 and the second circuit board 72 can include a temperature sensor 74 as part of a control circuit 76 for the liquid lens 36 and/or the reader 20. By way of example, the first circuit board 70 can include contacts 78 to electrically couple the control circuit 76 to the liquid lens 36, and the control circuit 76 on the second circuit board 72 can include liquid lens drive circuitry. A control cable 80 can extend from the second circuit board 72 to electrically connect the control circuit 76 to the processor 50. A rubber ring 88 can be included to maintain constant pressure on one or both of the first circuit board 70 and the second circuit board 72 with the liquid lens 36 therebetween. It should be appreciated that other configurations and arrangements of components are contemplated.

In some embodiments, one or both of the first circuit board 70 and the second circuit board 72 can be made of a thermally conductive material. An exemplary thermally conductive material is "Thermal Clad Insulated Metal Substrate" developed by Bergquist Corporation. Furthermore, one or both of the first circuit board 70 and the second circuit board 72 can include a controllable heating element 82. The heating element 82 is controlled to heat the circuit board on which it is located, e.g., the second circuit board 72, and to heat the ambient air in or near the liquid lens 36.

In some embodiments, one or both of the first circuit board 70 and the second circuit board 72 may be in electrical, thermal, and/or physical contact with the liquid lens 36. When in thermal or physical contact, the heating element 82 is controlled to generate heat that thermally affects the liquid lens 36. Referring to FIG7 , a method 83 for controlling the temperature of the liquid lens is shown. At process block 84, the temperature sensor 74 may sense a temperature value 132 associated with the liquid lens 36. At decision block 85, a feedback loop may compare the temperature value 132 to the control temperature 60. If the temperature value 132 is not at the control temperature 60 or within the control temperature range, at process block 86, the heating element 82 may be energized to increase the temperature of one or both of the first circuit board 70 and the second circuit board 72, and thereby increase the temperature of the liquid lens 36. At process block 87, when the temperature value 131 is at the control temperature 60 or within the control temperature range, the heating element 82 may be de-energized, and the liquid lens properties may be maintained.

Additional reader 20 components, when assembled, can enclose the liquid lens 36 and the first and second circuit boards 70, 72. For example, the guide 90 and lens housing 40 can physically and thermally enclose all or a portion of the liquid lens 36. The lens barrel 94 and lens housing 40 can physically and thermally enclose the liquid lens 36 and all or a portion of the first and second circuit boards 70, 72. The guide 90 can be used to center the liquid lens 36 within the lens barrel 94. Any additional components, such as the rubber ring 88, the guide 90, the lens housing 40, and the lens barrel 94, can be further optimized for thermal insulation, for example, by adjusting the shape and material properties, so that only minimal power is required to maintain the liquid lens 36 at a controlled temperature 60.

In an additional embodiment, focus drift of the liquid lens 36 is reduced or eliminated by stabilizing the temperature of the liquid lens 36. This embodiment may be used alone or in combination with the embodiments described above and shown in Figures 4 to 7.

For example, other known tunable lens configurations utilize electro-mechanical actuator systems (such as piezo actuators, small electric motors, and electromagnetic actuators (e.g., voice coils)) to induce motion to control one or more lenses, such as the meniscus of a liquid lens. In some embodiments, other variable lens elements may also be used, for example by changing the refractive index of a transparent material. FIG8 illustrates an exemplary variable lens 95. Variable lens 95 may include an annular voice coil actuator 96 induced to press against a transparent membrane 98, which serves as the transparent sidewall of a container 108. The container is filled with liquid 36. A control signal 64 applied via voice coil 99 induces actuator 96 to apply a force to deform membrane 98 into a convex shape. This convex shape serves as liquid lens 36 and can be adjusted by adjusting control signal 64. In these liquid lens configurations, as control signal 64 is applied to the actuator to change the focus of the liquid lens, actuator 96 itself may induce a temperature change in liquid lens 36. The power consumption in actuator 96 is generally proportional to the square of the power of control signal 64. For example, when the liquid lens 36 is driven to provide a high optical power, such as when focusing on a close symbol, more control current is required to the actuator 96, and heat generation and associated dissipation from the liquid lens 36 is high. Conversely, when the liquid lens 36 is driven at a lower optical power, such as when focusing on a farther symbol, less control current is required to the actuator, and heat generation and associated dissipation from the liquid lens 36 is lower. In some applications, accurately detecting temperature changes induced in the liquid lens 36 using the temperature sensor 74 is a challenge because the thermal coupling between the actuator 96 and the liquid lens 36 is better (e.g., faster) than the thermal coupling between the liquid lens 36 and the temperature sensor 74. This is at least partially due to the physical contact between the liquid lens 36 and the actuator 96.

Accordingly, undesirable actuator-induced temperature variations in the liquid lens 36 can be controlled by controlling the bias signal 66 to the actuator 96. When the control signal 64 is not applied to the actuator, the bias signal 66 is applied to adjust the focus of the lens to facilitate image acquisition, thereby controlling the induced temperature variations and the associated induced drift effects. The bias signal 66 through the actuator can be controlled to reduce temperature variations caused by internal heating and/or ambient temperature.

9 , after each focusing operation 106, the liquid lens generally operates in which the liquid lens is driven back to a default position 100, which is typically located in the middle 102 of the focus range 104. The default position 100 does not take into account any past operation of the liquid lens, such as whether the liquid lens 36 was recently driven at a high or low optical power. As shown in FIG9 , the liquid lens 36 is driven at a higher optical power more often than it is at a lower optical power. This operation typically increases the temperature of the liquid lens, thereby causing a drift effect and reducing the clarity of the acquired image.

Referring to FIG. 10 , in some embodiments, the bias signal 66 to the actuator 96 can alternatively be controlled such that the average heat dissipation of the liquid lens 36 and the actuator 96 remains substantially constant. Constant heat dissipation can be equivalent to a constant temperature, and a constant temperature can be equivalent to reduced or no drift effects. For example, a history 68 of liquid lens operation can be maintained in the memory 56 , and the processor 50 can indicate a return position based on analyzing past history. For example, if the liquid lens 36 is driven to the same optical power as shown in FIG. 9 , the processor can determine that the temperature of the liquid lens 36 will increase. Instead of returning the liquid lens 36 to the middle 102 of its focal range, the bias signal 66 can be used to return the liquid lens 36 to the desired optical power position 110 , where the bias signal can be sufficiently reduced to balance the higher control signal 64 for the higher optical power. The processor 50 can manage the application of the bias signal 66 to the actuator 96 to average the current applied to the actuator to reduce induced temperature changes and the associated induced drift effects.

Similarly, the bias signal 66 to the actuator 96 can be controlled in such a manner that the bias signal is dependent on the measured temperature of the liquid lens 36 to reduce induced temperature variations and associated induced drift effects. For example, the liquid lens 36 can be driven with a bias signal 66 that is temporarily reduced after the liquid lens 36 has been set to a high optical power for image acquisition and that is temporarily increased after the liquid lens has been set to a low optical power.

11 , method 114 illustrates a method in which a temperature factor 116 is maintained and tracked for easy query by processor 50. Temperature factor 116 may be a value associated with the amount of time a particular control signal 64 is applied to liquid lens 36. In this example, temperature factor 116 does not include measured temperature value 132, although in some embodiments, measured temperature value 132 may be included. When liquid lens 36 is not actively driven by control signal 64 for image acquisition, processor 50 may adjust bias signal 66 to compensate for past control signal applications. At process block 120, processor 50 drives liquid lens 36 with the particular control signal 64 for a particular amount of time to acquire an image. At process block 122, both time value 112 for the particular amount of time the particular control signal is applied and control value 118 for the particular control current may be stored in memory 56 as elements of temperature factor 116 (see FIG. 5 ). After the image has been acquired and the temperature factor 116 has been stored, the processor 50 may query the temperature factor 116 from the memory in process block 124 to calculate the return position of the liquid lens based on the temperature factor 116.

As a non-limiting example, if a 100 mA control signal 64 is applied to the actuator 96 for 10 milliseconds, the processor 50 may then determine that the liquid lens 36 should be driven with a 10 mA bias signal 66 current for 100 milliseconds to reduce the temperature of the liquid lens 36 to the control temperature 60. At process block 126, the processor 50 may then drive the liquid lens to the return position based on the analysis temperature factor 116. This method may be repeated at process block 120.

Depending on when the liquid lens 36 is driven to optical power during use of the reader 20, a counter 128 operable in the memory 56 and controllable by the processor 50 may be included to count up or down to track the temperature factor. For example, the liquid lens 36 may be driven to a new position before the application of 10 mA for 100 milliseconds is complete. The counter 128 may record how many of the 10 mA for 100 milliseconds have been applied and continue to apply the bias signal 66 after the liquid lens 36 has completed image acquisition. It should be recognized that these are merely examples and that many factors will affect the specific bias signal and application time, as will be understood by those skilled in the art.

Referring to method 130 in FIG. 12 , in some embodiments, temperature sensor 74 may be read to provide temperature value 132, and bias signal 66 may be controlled (i.e., by decreasing or increasing the bias signal) to attempt to maintain a consistent and/or predetermined control temperature 60, either alone or in combination with temperature factor 116. The use of temperature sensor 74 has the advantage of including ambient or external temperatures that affect reader 20 and, in particular, liquid lens 36. At process block 134, temperature value 132 is obtained from temperature sensor 74. Optionally, at process block 136, temperature value 132 may be stored in memory 56 (see FIG. 5 ). After the image has been acquired and temperature value 132 has been stored, processor 50 may query temperature value 132 from memory 56, based on temperature value 132, at process block 138, to calculate a return position of liquid lens 36. At process block 140, processor 50 may then drive liquid lens 36 to the return position using bias signal 66 based on temperature value 132 and/or temperature factor 116. Furthermore, in some embodiments, tracking temperature factor 116 may be omitted. This method may be repeated at process block 134.

In some applications, the induced drift may not be completely eliminated, such as, for example, when the reader device is subjected to large ambient temperature shocks, or when the liquid lens 36 is operated in a manner that does not allow sufficient time to control the bias signal 66 to control the temperature of the liquid lens. In these applications, the sharpness of the image may be determined from a series of images alone, or in combination with controlling the temperature of the liquid lens 36 to adjust the focus of the lens.

In most reader applications, a series of images is typically acquired. This series of images can be acquired within a single trigger (such as in a known continuous or manual mode) or over several triggers (such as in a known single-touch mode). Image acquisition parameters (e.g., focus) can be changed by predetermined small adjustment steps between each of the series of images. For one or more images in the series, reader 20 can use a sharpness calculation 146, operable in memory 56, to determine a sharpness score 148 for each image. The sharpness score 148 from one image can be compared with the sharpness score from another image to determine the effect of the predetermined small adjustment step between each image. The predetermined small adjustment step can improve the sharpness score, reduce the sharpness score, or maintain the sharpness score unchanged. Based on the comparison of the sharpness scores, processor 50 can determine the direction of the next predetermined small adjustment step, e.g., a larger or smaller focus. In some embodiments, either alone or in combination with the sharpness score 148, processor 50 can also use ambient temperature changes (e.g., an increase or decrease in ambient temperature) to determine the direction of the predetermined small adjustment step.

Referring to FIG. 13 , in some embodiments, the sharpness calculation 146 may analyze a small region of interest (ROI) 152 within the field of view of one or more images. At process block 154 of method 156 , the ROI 152 may be automatically defined by a symbol (e.g., barcode 22 as shown in FIG. 1 ) or user-defined (e.g., label symbol 160 as shown in FIG. 1 ). For example, the sharpness calculation 146 process may be enabled by placing a known ROI 152 (e.g., barcode 22 or symbol 160) within the field of view 42 of each image for which a sharpness score 148 is to be calculated. The focal length of the adjustable lens 36 may be adjusted by predetermined small adjustment steps at process block 158. At process block 162 , an image containing the ROI 152 may be acquired. Optionally, at process block 163 , the processor 50 may confirm that the ROI 152 is located within the acquired image. At process block 164, processor 50 may then run sharpness calculation 146 on known ROI 152 determined to be located in the image to generate a sharpness score 148 for ROI 152 in the acquired image. Next, at process block 166, the focus of adjustable lens 36 may be adjusted again by predetermined small adjustment steps. At process block 168, additional images of the field of view containing ROI 152 may be acquired. Optionally, processor 50 may again confirm that ROI 152 is located in the acquired image. At process block 170, processor 50 may then run sharpness calculation 146 on known ROI 152 determined to be located in the additional image to generate a subsequent sharpness score 148. At process block 172, first sharpness score 148 may be compared to subsequent sharpness score 148. Based on the comparison of the sharpness scores, at process block 174, processor 50 may define a direction for the next predetermined adjustment step, and the focus of adjustable lens 36 may be adjusted in the defined direction by the predetermined small adjustment steps. This method may then be repeated at process block 168 by acquiring another image containing the ROI 152 and comparing the sharpness score to the previously calculated sharpness score.

To ensure that the reader 20 does not slowly drift from focusing away from the potentially small ROI 152 to the background, the focus can be limited to predetermined small adjustments. This can include limiting the adjustment to one image acquisition parameter at a time, and/or limiting the amount of adjustment to one or more image acquisition parameters.

While the present technology has been described with reference to preferred embodiments, those skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the present technology. For example, the present technology is not limited to reducing temperature-induced drift effects on liquid lenses used in machine vision systems and may be implemented in other systems that include liquid lenses. For example, while a fixed-mount system is shown and described above, the machine vision system may be a handheld system. In a handheld system, the distance from the vision system to the symbol or character being read may be known or determined, and in these cases, adjustment of focus may be simplified in some applications.

The particular embodiments disclosed above are illustrative only, as the present technology may be modified and practiced in different but equivalent manners, as will be apparent to those skilled in the art having the benefit of the teachings herein. Furthermore, the invention is not limited to the specific constructions or designs shown herein, unless specifically stated in the appended claims. It is therefore apparent that the particular embodiments disclosed above may be altered or modified, and all such variations are considered within the scope and spirit of the present technology. The protection sought herein is therefore set forth in the appended claims.

Claims (9)

1. A vision system, comprising: Adjustable focusing liquid lens with field of view; At least one circuit board is in thermal contact with at least a portion of the liquid lens; A heating element is placed on the at least one circuit board, and the heating element is controllable to heat the at least one circuit board. A temperature sensor is configured to measure the temperature of the liquid lens; and A feedback loop controls the electrical energy supplied to the heating element based on the difference between the measured temperature of the liquid lens and a predetermined control temperature. 2. The vision system of claim 1, further comprising a processor that receives a temperature value from a temperature sensor and, based on the received temperature value, supplies or de-energizes the heating element on the at least one circuit board to maintain the temperature value within a predetermined control temperature range. 3. The vision system as claimed in claim 1, wherein the predetermined control temperature is near the high end of the operating temperature range of the liquid lens. 4. The vision system as claimed in claim 1, wherein the temperature sensor is placed on the at least one circuit board. 5. The vision system as claimed in claim 1, wherein the at least one circuit board includes a liquid lens driving circuit. 6. The vision system as claimed in claim 1, wherein the at least one circuit board includes a vision system control circuit. 7. The vision system of claim 1, wherein the at least one circuit board includes a first circuit board and further includes a second circuit board, the first circuit board including at least one contact to electrically couple control circuitry on the second circuit board to the liquid lens, and the liquid lens is positioned between the first circuit board and the second circuit board. 8. The vision system as claimed in claim 1, wherein the focal point of the adjustable liquid lens is adjustable using a control signal applied to the liquid lens. 9. The vision system as claimed in claim 1, wherein the vision system is at least one of a fixed-mount vision system and a handheld vision system.