Voice coil focusing system having an image receptor mounted on a pivotally-rotatable frame

An apparatus for focusing an image onto an image receptor is disclosed. 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 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.

FIELD OF THE INVENTION 
This invention relates to the automatic focusing of images onto an image 
receptor and, in particular, to automatic focusing systems capable of 
reliably performing successive focusing cycles at high speed over 
prolonged periods of time. 
BACKGROUND OF THE INVENTION 
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. 
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. 
Further objects and advantages of the invention will become apparent from 
the description of the invention which follows. 
SUMMARY 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 of the present invention, 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.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to FIGS. 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.TM. model 40-1325 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 0.030"-0.050". 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 0.020" 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 is perpendicular to and 
intersects axis 64. 
According to the preferred embodiment, connecting rod 75 is affixed to 
rotatable frame 40 at actuating point 42 such that a right angle is formed 
between connecting rod 75 and a reference line 66. Reference line 66 is 
perpendicular to and intersects axis 64. Connecting rod 75 is preferably 
threaded at one end. The threaded end of connecting rod 75 passes through 
an opening affixed to rotatable frame 40 at actuating point 42. Connecting 
rod 75 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 FIG. 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 FIG. 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 PID servo 140. 
The output of PID servo 140 is amplified by power amplifier 10 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 20 ms. In 
addition, life tests of the present invention show more than 58 million 
successful focusing cycles without failure. 
The present invention may be embodied in other specific forms without 
departing from the spirit or essential attributes of the invention. 
Accordingly, reference should be made to the appended claims, rather than 
the foregoing specification, as indicating the scope of the invention.