Mark-sensing Photosensor

Abstract

A periodically oscillating electro-optical scanning apparatus for indicating the position of registration lines or edges relative to itself by providing an electrical signal output indicative thereof. The optical scanner is positioned relative to the material containing the indicia to eliminate output signal deficiencies.

Parent Case Text

OTHER APPLICATIONS

This application is a division of the application of SAMUEL P. WILLITS et al., Ser. No. 650,835, filed July 3, 1967, titled MARK SCANNING PHOTO SENSOR AND SERVO SYSTEM.

Description

BACKGROUND OF THE INVENTION

The field of the invention is generally related to improvements in apparatus for automatically positioning registration marks or indicia and, more particularly, to an electro-optical sensing apparatus having outputs either useful for positioning the apparatus or elements associated therewith relative to the registration indicia or for providing indications of the excursion thereof.

In an earlier application of Samuel P. Willits, now issued as U.S. Pat. No. 3,335,281, an apparatus is described which is responsive to registration indicia having linelike width characteristics relative to the effective width of the active electro-optical elements of the sensing head. In that apparatus the electrical signal output of a cyclically scanning sensing head was amplified and filtered to provide a control signal for a servo positioning device which mechanically repositioned the linelike registration indicia into a preselected alignment with the sensing head and, when this alignment was perfected, generated an auxiliary control signal indicative of satisfactory completion of the repositioning. The auxiliary control signal typically would be used for initiating a further operation of an associated machine such as tripping of a shear blade, actuation of a ram, etc.

A particular feature of the Willits patent is a special angular relationship of the material and indicia to the scan direction and light source. This special relationship has several advantages including elimination or reduction of errors caused by parallax and shadow effects or signal polarity reversals during a scan. In some embodiments, novel image offsetting means are used to advantage.

Various other devices are known for indicating the position of a linelike registration indicia and for servo positioning in response thereto. In some of these there is no movement of the scanning element and detection requires the movement of the registration indicia past the elements to evoke a signal. A device typical of this type is disclosed by J. C. Frommer in U.S. Pat. No. 2,840,371, issued June 24, 1958. Frommer's device provides repositioning of plural indicia upon a moving strip relative to his sensor and thus maintains registration during continuous feed color printing operations.

In other known devices there is movement of the scanning elements making movement of the indicia an unnessary prerequisite to detection. Devices typical of this type are described by M. A. McLennan in U.S. Pat. No. 2,489,305, issued Nov. 29, 1949, and W. P. Frantz in U.S. Pat. No. 2,892,948, issued June 30, 1959. The Frantz device however, unlike the others, requires the presence of two registration indicia to enable it to provide a suitable servo output signal.

Included in the prior art is the disclosure of H. R. Summerhays, Jr. in U.S. Pat. No. 2,674,917, issued Apr. 13, 1954. Summerhays discloses a noncontacting electro-optical width gauge incorporating fixed prisms and mirrors, wieldable independently of the prisms, to effect a scan of the edge of strip material moving beneath the prisms and mirrors. No apparatus is disclosed for any automatic repositioning, the device providing only meter indications of strip width variations.

From the foregoing brief discussion of prior art devices taken in conjunction with their complete disclosures, it can be seen that the prior art makes no provision for eliminating parallax errors due to positioning of the electro-optical sensor relative to the registration indicia nor is there provision for eliminating shadow effects such as occur when the indicia is either depressed below or elevated above the surface bearing it. The inability of the prior art to overcome parallax errors or shadow effects are overcome by the present invention.

SUMMARY OF INVENTION

A principal object of the invention is the elimination of parallax errors, shadow effects and polarity reversals in the output signals of a periodically oscillating electro-optical scanning apparatus. This object is realized by maintaining a scanning radiation source and oscillating scanning sensor in a special angular relationship to each other and a registration indicia and the material upon which the indicia is located.

Still another object of the invention is to provide new and improved electro-optical sensing apparatus having wieldable optical means for physically offsetting the image of the scanned registration indicia a fixed distance from a plane containing the optical axes of the source of scanning radiation and the scanning sensor. This object is achieved by placing two reflecting surfaces, which desirably are supplied by a rhomboid prism, in the optical paths between the indicia and combination of sensor and light source.

The foregoing and other objects of the invention are achieved by a novel electro-optical sensing head which produces electrical signals having characteristics related to the relative position of the registration indicia or mark. The scanning radiation source and oscillating scanning sensor are maintained in very special angular relationships with respect to each other and the registration indicia and material bearing it while the scan is taking place.

The nature of the invention and its several features and objects will appear more fully from the following description made in connection with the accompanying drawings.

DESCRIPTION OF DRAWINGS

FIG. 1 is a combination mechanical schematic-electrical block diagram of the principal elements of the invention;

FIG. 2 is a waveform diagram illustrating output waveforms from the inventive sensing head for the various positions of the registration indicia relative thereto illustrated in FIG. 3 and of the waveforms appearing at the indicated point in the circuitry of FIG. 1;

FIG. 3B through 3F illustrate various positions of a registration mark sensed by the invention relative to the active scanning area of the invention; and

FIG. 4 is a partial mechanical-optical schematic of an embodiment of the sensing head of the invention showing the method of achieving an offset optical path.

DESCRIPTION OF INVENTION

The present invention utilizes elements of the aforementioned Willits application. Accordingly, FIG. 1 illustrates in simplified schematic form the principal elements of the Willits sensing apparatus. There, material 20, bearing a suitable registration mark 22 is positioned to be within the field of view of a photodetector 24 mounted on an oscillating arm 26. Arm 26 is oscillated about pivot axis 28 through an angle .alpha. between the two fixed positions indicated in phantom outline at 26' and 26" by a conventional moving coil type of electromechanical drive schematically indicated by arm drive coil 30. Coil 30 is excited by an amplified AC reference signal applied at terminal 58. The combination of oscillation and the physical size of the active area of detector 24, results in a scanned area indicated at 32.

A light source 34 is focused by condensing lens 36 on scanned area 32 to provide illumination for detector 24. Suitable aperture stops (not illustrated) may be used in some instances to restrict the light from source 34 to scan area 32 or nearly so, although such steps are not essential to the invention. The only requirement in this regard is the prevention of direct irradiation of detector 24 by source 34. Light source 34 is excited by a DC source 38 to insure no AC signal components will be generated by detector 24 by any rapid variations in intensity of the source. Use of such a DC excited light source has proven advantageous as will become apparent later in this description.

An image of the active scanning area 32 is formed by objective lens 40 in or substantially in the plane of movement of oscillating arm 26 and specifically in the plane of an aperture mask 42 positioned on that arm in front of detector 24. To provide an enhanced signal amplitude and an averaging effect to overcome any problem created by any variations in line width or smoothness, the shape of the aperture in mask 42 has been made long and narrow with the major axis of the aperture positioned parallel to the axis of scan arm 26. Detector 24 is positioned relative to the aperture in mask 42 so that only light passing through the aperture from lens 40 can fall on the detector's sensitive surface. In this manner the aperture in mask 42 defined the active area of detector 24. In the majority of embodiments constructed, the detector employed has been a silicon photovoltaic cell. This particular type of cell was chosen for its small size, logarithmic saturation characteristics and low impedance which matches the transistorized signal processing circuitry employed. Obviously, however, other types of cells may be employed depending on operating parameters.

In operation, oscillating arm 26 oscillates through a small angle .alpha. about axis 28 in a plane perpendicular to the optical axis 44 of lens 40. While the absolute scan amplitude 47 plus the width of the aperture in mask 42 is necessarily greater than the imaged width of registration mark 22, the showing of FIG. 1 has been exaggerated for illustration purposes. More nearly to scale is the illustration of FIG. 3 discussed below. The elements of the scanning head are so arranged that this oscillation of arm 26 periodically translates aperture mask 42 and detector 24 across the image of active sensing area 32 in a direction substantially perpendicular to the long axis of the image of registration mark 22. Thus, at any instant, the light falling on detector 24 through the aperture in mask 42 is a measure of the brightness of the particular part of the image of the active sensing area visible through the aperture in the mask. The resulting electrical output of detector 24 is thus proportional to the brightness of a particular part of the image of active sensing area 32 falling at any instant on the aperture in the mask 42.

The signal outputs of detector 24 for various positions of registration mark 22 relative thereto are discussed in conjunction with FIGS. 2 and 3. However, before continuing with a description of these signals and of the signal processing circuitry, a description and understanding or other characteristics of the scanning head of the invention will aid in understanding the detector signal outputs and processing circuitry.

In certain applications, the material 20 bearing the registration marks 22 has a highly polished surface. In such an instance, the specular reflections from the polished surface contain a very high percentage of the total light reflected from that surface. Additionally, depending on the angle of the optical axis 46 of lens 36 relative to the polished surface and the angle of optical axis 44 relative thereto, the image of the registration mark will appear either light or dark regardless of its absolute contrast relative thereto. Obviously this can present problems of signal polarity as the material 20 is positioned. To provide consistent polarity of the detector signal in the face of such a condition and to enhance signal levels and reduce spurious signals, it has been found desirable to have the optical system, including the surface of material 20 in a particular optical alignment that also maximizes specular reflections. I have found that this particular optical alignment can be achieved by arranging optical axes 44 and 46 in such a manner as to cause them to intersect each other at or very near the surface of material 20, by having axes 44 and 46 and a line 48 positioned between the axes in the same plane, and by further arranging the axes 44 and 46 relative to line 48 so that the angles .PHI. formed between each axis and that line are equal.

Another type of material variation that can affect the accuracy of scanner type sensing apparatus of the type employed in my invention, occurs when the distance from the sensing head to the registration mark varies. This distance variation most frequently is caused by material thickness changes, but also may be caused by raised or depressed registration indicia 22 causing shadow effects. Either material thickness variations or shadow effects would ordinarily cause errors in measurement of indicia location which errors can result in positional errors of the work piece 20 relative to the desired reference position. It is an advantageous feature of my invention that these positional errors caused by parallax or shadow effects are eliminated by a particular geometrical arrangement of the entire illuminating, imaging and scanning system. The particular geometrical arrangement used operatively places these elements in a plane that contains the axis of the registration indicia and that is mutually perpendicular to both the scanning direction and the surface of the material 20 being registered. Such an arrangement is shown in FIG. 1.

In many applications of my automatic sensing apparatus, mechanical interference with machine elements physically prevents achieving the above set forth requirement for operating the active elements of the sensing head in a single plane which includes the registration mark. To overcome this difficulty and retain the advantages of operating the elements of the sensing head in a single plane, we provide an optical offset achieved by the insertion of a rhomboid prism into the optical system near or adjacent to the surface bearing the registration mark. This arrangement is illustrated in FIG. 4 where rhomboid prism 50 is shown positioned adjacent active scanning area 32 and material 20 to provide the desired offset in optical axes 44 and 46 relative to fixed shear blade 52 and movable shear blade 54. As will be apparent to those versed in the optical arts, reflectors and other optical elements can be used to obtain optical offsets in place of the illustrated rhomboid prisms and such elements are employed when the optical path is modified to reduce its height. However, a special feature of a rhomboid prism is that optical systems employing such a prism are relatively insensitive to small positional changes of the prism relative to the sensing head.

In certain applications of my automatic sensing apparatus, some mechanical part or machine element, such as a shear blade, must be moved through the space occupied by the optical system of the invention. It is a special feature of the invention that such movements of machine elements can be effected without resultant damage to the optical system. This advantageous result is obtained by utilizing a wieldable rhomboid prism in the inventive optical system. Because of the properties of the rhomboid prism, the prism can be reciprocally displaced in the directions of double ended arrow 56 during the shear cycle and any small positional changes in prism location to the optical axes 44 and 46 that occur when the prism is returned to normal operating position after displacement do not result in any error in location of the reference position on subsequent cycles. As will become apparent from description which follows, prism 50, when necessary, may be moved to clear a moving machine element when an output signal which actuates the shear is developed in the circuitry of the invention. The apparatus to move prism 50 has not been illustrated since it is conventional in nature and forms no part of my invention. However, one mechanism successfully employed for this purpose consisted solely of a solenoid-actuated bellcrank to which prism 50 was cemented. Other mechanisms may be employed as well depending largely on the type of machine tool with which my inventive apparatus is associated.

FIGS. 2B through 2F illustrate the output waveforms of detector 24 for the various positions of the registration mark as illustrated at the correspondingly lettered subfigure of FIG. 3. In FIG. 2, time increases from left to right. FIG. 2A illustrates the time-varying position of oscillating arm 26. Because of the very low inertia of arm 26, wave train 2A may also be considered to be identical or nearly identical with the AC reference voltage supplied to the system circuitry at terminal 58.

When registration mark 22 is displaced from the center 60 of the scanning area 32 to a position to the left and near one edge thereof as shown in FIG. 3B, the output signal of detector 24 is shown in FIG. 2B. The flattop or straight line portion of the wave train occurs when the detector scans the image of the uniform background 62 of material 20 adjacent mark 22. The negative-going loop occurs once each cycle as the detector cyclically moves into and out of the image of the registration mark. Note that in this and the following examples, the reference position with respect to which registration mark 22 is positioned, has been assumed to be the center 60 of the active scanning area 32. Other reference positions, offset as desired from the position of this example, could be employed. However, normally the reference position is located at the center of the active scan area.

If the registration mark is then positioned nearer to the center 60 of active scanning area 32 as shown in FIG. 3C, detector output is as shown in FIG. 2C. There again the straight line portion of the wave train occurs when the detector does not "see" the registration mark and only "sees" the uniform background area 62 of material 20. The double negative-going loop is generated as the detector cyclically scans back and forth through the image of the registration mark 22. Note that when compared with FIG. 2B the duration of the scan through the image of the mark 22 has increased relative to the duration of the scan of the uniform background area 62 and that the two scan durations have almost become equal.

When the registration mark 22 is positioned at the center 60 of the active scanning area as illustrated in FIG. 3D, the output wave train of detector 24 is as illustrated in FIG. 2D. Two complete cycles each consisting of a negative-going loop and a straight line portion appear in the output wave train for each scan cycle with identical waveforms generated on each side of the scan cycle. The output wave train is, for this position of the mark, composed only of even harmonics of the frequency of displacement of oscillating arm 26 with no fundamental component. Thus, the output wave train comprises a signal whose component frequencies, other than the constant term, are all even harmonics of the frequency of FIG. 2A.

If the registration mark 22 is moved to the right of center 60 of scanning area 32 as shown in FIG. 3E, the output wave train of detector 24 is as illustrated in FIG. 2E. A move of the work further to the right as shown in FIG. 3F results in output wave train 2F. By inspection of FIG. 2B through 2F, it can be seen that as the registration mark 22 is displaced one way or the other from the center 60 of scanning area 32, the fundamental component in the output wave train appears. Further, as the displacement of the mark from center increases, the fundamental component increases in amplitude with its phase determined by the direction of displacement from center 60. Displacement of the mark one way produces a fundamental component in phase with the wave train of FIG. 2A and displacement in the other direction produces a fundamental component 180.degree. out of phase with that wave train. Thus, the characteristics of the fundamental contained in the output wave train are exactly those required to provide a servocontrol signal indicative of the position of registration mark 22. The means by which I convert these characteristics of the detector signal are illustrated schematically in FIG. 1.

The output signal from detector 24 is amplified in signal amplifier 64. Signal amplifier 64 is a cascaded grounded emitter transistor amplifier whose final stages are operated saturated to maintain a comparatively uniform servo loop gain over large ranges of signal level from detector 24. The output signals of amplifier 64 corresponding to its input signals 2B, 2C, 2D, 2E and 2F, are also illustrated in FIG. 2 as signals B, C, D, E and F respectively. The manner of achieving these characteristics of signal amplifier 64 and the reason therefore are explained in detail in the aforementioned Willits patent. The amplified output of signal amplifier 64 is processed by low-Q tuned filter 66 which passes the fundamental frequency contained therein and reduces the amplitude of any higher harmonics present.

The filtered signal is further amplified in push-pull cascaded emitter follower transistor power amplifier 68 before being applied to the control winding 72 of two-phase servomotor 70. Phase shift network 76 provides a signal on winding 74 phased 90.degree. from that on winding 72. The servo loop back to indicia 22 is closed by providing a mechanical link between the output shaft of servomotor 70 and material 20. This link is schematically shown as 78 in FIG. 1.

The Willits apparatus of this application also includes position-analyzing circuits capable of generating an auxiliary control signal whenever positioning of a registration mark has been satisfactorily accomplished. These circuits are described in detail in the Willits patent and comprise a logic circuit 80 and a signal presence detector 290. These two circuits together analyze the output signal of the power amplifier 68 in terms of voltage references supplied at terminals 87 and 96 and initiate the auxiliary control signal formed by control relay 100 after the passage of time occasioned by time delay 98. Time delay 98 insures that the auxiliary control signal will not be initiated prematurely as it otherwise might under condition of servomotor over- or undershoot.

While certain specific examples of the invention have been disclosed, it will be understood that further variations and modifications could be made in the system that are within the spirit and scope of the invention as described herein.

Claims

I claim:

1. An electro-optical sensing head for providing an output signal indicative of the location of a single registration mark on a surface praticularly positioned relative thereto comprising:

scanning means including a radiation detector cyclically oscillating over a substantially linear path in a direction predetermined relative to said registration mark,

a source of radiation,

a condensing lens focusing said radiation on said registration mark,

an objective lens for imaging said registration mark on said radiation detector, and

the optical axes of said condensing lens and said objective lens each having an identical angular relationship to a line drawn perpendicular to the surface bearing said registration mark and each axis intersecting said line at said surface, said optical axes being further positioned to lie in a common plane which is perpendicular to both the scanning direction and said surface.

2. An electro-optical sensing head for providing an output signal indicative of the location of a single registration mark on a surface particularly positioned relative thereto comprising:

scanning means including a radiation detector cyclically oscillating over a substantially linear path in a direction predetermined relative to said registration mark,

a source of radiation,

a condensing lens focusing said radiation on said registration mark,

an objective lens for imaging said registration mark on said radiation detector, and

the optical axes of said condensing lens and said objective lens each intersecting at said surface, said axes being further positioned to lie in a common plane which is perpendicular to both the scanning direction and said surface.

3. An electro-optical sensing head in accord with claim 2 further comprising optical means wieldable independently of said objective lens and said condensing lens for physically offsetting the image of said registration mark a fixed distance relative to said source of radiation and said scanning means while maintaining the planes containing said axes parallel on either side of said offset.

4. An electro-optical sensing head in accord with claim 2 wherein said predetermined direction of cyclical oscillation is about an axis substantially perpendicular to the optical axis of said objective lens and lying in said common plane.

5. An electro-optical sensing head for providing an output signal indicative of the location of a single registration mark on a surface particularly positioned relative thereto comprising:

scanning means including a radiation detector cyclically oscillating over a substantially linear path in a direction predetermined relative to said registration mark,

a source of radiation,

a condensing lens focusing said radiation on said registration mark,

an objective lens for imaging said registration mark on said radiation detector, the optical axes of said condensing lens and said objective lens each being positioned to intersect at said surface and to lie in a common plane which is perpendicular to both the scanning direction and said surface, and

optical means wieldable independently of said objective lens and said condensing lens for physically offsetting the image of said registration mark a fixed distance relative to said source of radiation and said scanning means while maintaining the planes containing said axes parallel on either side of said offset, said wieldable optical means comprising a rhomboid prism.