Apparatus for focusing an image

Abstract: An apparatus for focusing an image onto an image receptor (20) is disclosed. The apparatus inculdes a stationary lens (10), a rotatable frame (40) that is rigidly connected to the image receptor (20), and a base. The base is pivotally connected to the rotatable frame (40) about a rotational axis. The image is focused by the stationary lens (10) onto the image receptor (20) by pivoting the rotatable frame (40) about the rotational axis. In an alternate embodiment, an apparatus for focusing an image onto an image receptor (20) includes a stationary lens (10), a voice coil actuator (100), a servo loop for driving the voice coil actuator, and an electromechanical transducer (80) for measuring the position of the image receptor (20). The electromechanical transducer (80) is electrically connected to the servo loop. Connecting means for rigidly connecting the image receptor (20) to the voice coil actuator (100) are also provided. The image is focused by the stationary lens (10) onto the image receptor (20) by driving the voice coil actuator (100). A method for focusing an image onto an image receptor pivotally connected to a base about a rotational axis is disclosed. The method includes detecting information representative of the distance between s stationary lens and an object being imaged, and pivoting the image receptor about the rotational axis in accordance with the detected information.

Description: The present invention relates to an apparatus and a method for focusing an image onto an image receptor according to the preambles of the independent claims. Such an apparatus and such a method are known from EP-A-0 267 549. In particular, the invention relates to the automatic focusing of images onto an image receptor and to automatic focusing systems capable of reliably performing successive focusing cycles at high speed over prolonged periods of time. As bar codes and other symbologies used to encode packaging labels grow more dense, the optical path of an electronic scanner often must be focused in order to successfully read the code or symbol. High speed package handling operations which use bar codes or other symbologies to identify packages typically require rapid automatic focusing mechanisms which must operate at high speed for prolonged periods of time. The high package quantities processed by such systems require the automatic focusing mechanism employed to have a very long life. For example, in certain applications, the focusing mechanism may be required to perform as many as one million focusing cycles per year without breakdown. In conventional automatic focusing systems, image focusing is achieved by holding the focal plane of an image receptor in a stationary position and moving a focusing lens. In such systems, the lens is typically moved by mounting the lens in a screw mount and turning the mount with a small servo motor. Such conventional systems are not suitable for cameras which must be focused and refocused quickly and repetitively over prolonged periods of time. For example, a conventional automatic focusing system could not be successfully employed to perform overhead scanning of a continuous stream of packaging labels of varying height positioned on a rapidly moving belt. Due to the high mass of their moving parts, conventional automatic focusing systems are unable to achieve the very fast focusing slew times required by such applications. In addition, due to the friction and wear between moving parts in conventional automatic focusing systems, such systems are unable to perform continuously in high volume operations for prolonged periods of time. EP-A-0 267 549 discloses a method and means for self-referencing and self-focusing a bar code reader. In this system a sensor may be mounted in the optical axis of an optical system. The sensor may be mounted on a moving beam to move cyclically back and forth along the optical axis. It is therefore an object of the present invention to provide an automatic focusing system capable of reliably performing in a high speed automated environment for prolonged periods of time. It is a further object of the present invention to provide an automatic focusing system which does not require the movement of high mass parts to achieve focusing. It is a still further object of the present invention to provide an automatic focusing system which operates with a minimum of friction and wear between its moving parts. These objects are solved in accordance with the features of the independent claims. Dependent claims are directed on preferred embodiments of the invention. An apparatus for focusing an image onto an image receptor is disclosed. In a preferred embodiment, the apparatus includes a stationary lens, a rotatable frame that is rigidly connected to the image receptor, and a base. The base is pivotally connected to the rotatable frame about a rotational axis. The image is focused by the stationary lens onto the image receptor by pivoting the rotatable frame about the rotational axis. In an alternate embodiment an apparatus for focusing an image onto an image receptor includes a stationary lens, a voice coil actuator, a servo loop for driving the voice coil actuator and an electromechanical transducer for measuring the position of the image receptor. The electromechanical transducer is electrically connected to the servo loop. Connecting means for rigidly connecting the image receptor to the voice coil actuator are also provided. The image is focused by the stationary lens onto the image receptor by driving the voice coil actuator. A method for focusing an image onto an image receptor pivotally connected to a base about a rotational axis is also disclosed. The method includes detecting information representative of the distance between a stationary lens and an object being imaged, and pivoting the image receptor about the rotational axis in accordance with the detected information. Figure 1 shows a first cross-sectional view of a preferred embodiment of the present invention. Figure 2 shows a second cross-sectional view of a preferred embodiment of the present invention. Figure 3 shows a third cross-sectional view of a preferred embodiment of the present invention. Figure 4 is a block diagram illustrating the operation of a preferred servo loop according to the present invention. Referring now to Figures 1-3, there is shown a voice coil focusing system according to a preferred embodiment of the present invention. The system includes stationary lens 10 for focusing an image traveling along optical path 10a onto image receptor 20. Image receptor 20 is preferably a charge coupled device (CCD) having focal plane 20a. Image receptor 20 is rigidly mounted on circuit board 30 which in turn is rigidly mounted on rotatable frame 40. Base 50 and rotatable frame 40 are pivotally connected by hinges 60. Rotatable frame 40 and first connecting rod 70 are rigidly connected at actuating point 42. Second connecting rod 75 rigidly connects the front face of voice coil actuator 100 to electromechanical transducer core 85. Mounting chassis 90 is provided for rigidly securing the body of electromechanical transducer 80 above the front face of voice coil actuator 100. Electromechanical transducer core 85 rigidly connects first connecting rod 70 and second connecting rod 75 without touching the outer body of electromechanical transducer 80. In the preferred embodiment, the position of image receptor 20 is controlled by three points which form an Isosceles triangle with base 50. The two lower vertices of the triangle are connected to base 50 at hinges 60. The apex of the triangle is connected to voice coil actuator 100 via in-line electromechanical transducer core 85 and connecting rods 70, 75. The position of image receptor 20 (and focal plane 20a) is measured by electromechanical transducer 80, which sends a position feedback signal to the servo drive loop. In the preferred embodiment, electromechanical transducer 80 is an absolute position feedback device such as a linear differential variable transformer (LDVT) having an inner core 85. A suitable LDVT for use in connection with the present invention is the Schaevitz model 050 HR AC LDVT. The use of an absolute position feedback device ensures that focal plane 20a is precisely positioned and also serves to quickly stabilize focal plane 20a at a desired position. In the preferred embodiment, base 50 is fastened to outer base 116 with adjustment thumbwheels 114 and lockscrews 112. According to the preferred embodiment, voice coil actuator 100 provides the actuating force for positioning rotatable frame 40. Voice coil actuator 100 is preferably formed from a conventional acoustic loudspeaker. The voice coil of the acoustic loudspeaker preferably serves as the rear bearing of and provides return resilience for the actuator link formed by connecting rods 70, 75 and transducer 80. The voice coil diaphragm also aids the servo damping. A suitable loudspeaker for use in connection with the present invention is a Realistic™ model 40-1325 8,9 cm (3.5") speaker rated at 20 watts and 8 ohms. Such a loudspeaker provides for movement of focal plane 20a over a range of approximately 0,76-1,2 mm (.030" - .050"). Since conventional acoustic loudspeakers are typically non-linear, an LDVT is preferably employed in a servo loop with voice coil actuator 100 to remove any non-linearity from the system. Each hinge 60 is preferably formed from a rectangular piece of flat brass shim stock about 0,5 mm (.020") in thickness, although other materials (including beryllium copper) and thicknesses may be used. Screws 62 secure one end of each hinge 60 to rotatable frame 40, and the other end of each hinge 60 to stationary base 50. When hinges 60 are formed of rectangular pieces of brass shim and secured as shown, each hinge 60 is able to flex (or pivot) about rotational axis 64 when an actuating force is applied to rotatable frame 40 at actuating point 42. In the preferred embodiment, image receptor 20 is oriented so that focal plane 20a intersects axis 64. According to the preferred embodiment, connecting rod 70 is affixed to rotatable frame 40 at actuating point 42 such that a right angle is formed between connecting rod 70 and a reference line 66. Reference line 66 is perpendicular to and intersects axis 64. Connecting rod 70 is preferably threaded at one end. The threaded end of connecting rod 70 passes through an opening affixed to rotatable frame 40 at actuating point 42. Connecting rod 70 is then affixed to rotatable frame 40 at actuating point 42 by hex nuts 44. In a preferred embodiment, hex nuts 44 may be used to calibrate a nominal position of focal plane 20a such that an object in the middle of the system's focusing range is focused on image receptor 20 when no force is applied by voice coil actuator 100. Also as part of the initial calibration, focal plane 20a is made perpendicular to optical path 10a by turning adjustment thumbwheels 114 to rotate the angle of base 50 relative to optical path 10a. When the proper angle is found, base 50 is secured to outer base 116 with lock screws 114. In a preferred embodiment, mirror 110 is provided for reflecting an image traveling along optical path 10a from a vertical to a horizontal direction. Mirror 110 allows the focusing system of the present invention to be positioned horizontally (as shown in Figure 1) while performing overhead scanning of a continuous stream of packaging labels of varying height positioned on a rapidly moving belt. In an alternate preferred embodiment, the focusing system of the present invention may be oriented in a vertical direction to perform such overhead scanning. In this alternate embodiment, a spring (not shown) is preferably mounted between rotatable frame 40 and chassis 90 to counteract the force of gravity and establish the nominal position of focal plane 20a. Referring now to Figure 4, there is shown a block diagram illustrating the operation of a preferred servo loop for driving voice coil actuator 100 according to the present invention. LDVT signal processor 120 is provided for converting the AC output of LDVT 80 to a DC voltage proportional to deflection distance. Distance information from a sensor (not shown) is provided to D/A converter 130. The distance information provided is representative of the distance between stationary lens 10 and an object being imaged. An error signal representing the difference between the output of LDVT signal processor 120 and the output D/A converter 130 is determined by differential amplifier 135 and provided to FID servo 140. The output of PID servo 140 is amplified by power amplifier 150 to drive voice coil actuator 100. Error detector 160 is provided to detect any failures in the servo loop. Error detector 160 sends an error bit to a computer if a failure in the loop is detected. A preferred embodiment of the present invention has been tested for focusing speed and has achieved a focusing settling time of 20ms. In addition, life tests of the present invention show more than 58 million successful focusing cycles without failure.

Claims: An apparatus for focusing an image onto an image receptor (20) comprising: (A) a stationary lens (10); (B) a support (40), said image receptor (20) being rigidly connected to said support (40); and (C) a base (50) to which the support (40) is movably connected, characterized in that the support is a rotatable frame (40) pivotally connected to said base (50) about a rotational axis, wherein said image is focused by said stationary lens (10) onto said image receptor (20) by pivoting said rotatable frame (40) about said rotational axis. The apparatus of claim 1, characterized in that said image receptor (20) has a focal plane which intersects said rotational axis. The apparatus of claim 1, characterized in that said image receptor (20) comprises a charge coupled device. The apparatus of claim 1, characterized in further comprising a hinge (60) for pivotally connecting said base (50) and said rotatable frame (40). The apparatus of claim 4, characterized in that said hinge (60) comprises a flat piece of flexible metal having first and second ends, said first end being rigidly connected to said rotatable frame, said second end being rigidly connected to said base (50). The apparatus of claim 1, characterized in further comprising (D) a voice coil actuator (100); (E) a servo loop for driving said voice coil actuator (100); (F) an electromechanical transducer (80) for measuring the position of said image receptor (20), said electro-mechanical transducer (80) being electrically connected to said servo loop; and (G) connecting means for rigidly connecting said rotatable frame (40) to said voice coil actuator (100) wherein said image is focused by said stationary lens (10) onto said image receptor (20) by driving said voice coil actuator (100) to pivot said rotatable frame (40) about said rotational axis. The apparatus of claim 6, characterized in that said electro-mechanical transducer (80) is a linear differential variable transformer. The apparatus of claim 6, characterized in that said voice coil actuator (100) comprises an acoustic loudspeaker. A method for focusing an image onto an image receptor, said image receptor being movably connected to a base (50), characterized in pivotably connecting said image receptor to said base (50) about a rotational axis, the method comprising the steps of: (A) detecting information representative of the distance between a stationary lens (10) and an object being imaged; and (B) pivoting said image receptor (20) about said rotational axis in accordance with said detected information. The method of claim 9, characterized in that step (B) comprises the step of driving a voice coil actuator (100) in accordance with said detected information to pivot said image receptor (20) about said rotational axis. The method of claim 10, characterized in that said image receptor (20) is mounted on a rotatable frame (40) which is pivotally connected to said base (50), and said voice coil actuator (100) is rigidly connected to said rotatable frame (40). The method of claim 9, characterized in that said image receptor (20) has a focal plane (20a) which intersects said rotational axis. The method of claim 12, characterized in that said image receptor has an optical path (10a) which is perpendicular to said focal plane (20a).

Inventors: ESSLINGER ROBERT, GARLAND LANCE, MULLIGAN JOSEPH, ROTH PAUL, ROTH STEPHEN, ESSLINGER, ROBERT, GARLAND, LANCE, MULLIGAN, JOSEPH, ROTH, PAUL, ROTH, STEPHEN
Applicants: UNITED PARCEL SERVICE INC, UNITED PARCEL SERVICE OF AMERICA, INC.
Application year: 1993
Main classification: G06K7
IPC codes: G02B7