GB2101360A - Light scanning apparatus - Google Patents

Abstract

A light scanning apparatus uses a modulated laser source (10) directed upon a multifaceted reflective polygon (28) so that in use the beam is reflected from successive facets (26) of the rotating polygon and sweeps along a scan path to provide successive raster lines extending across a photoreceptor (32). The speed of rotation of the polygon is controlled by means including a sweep velocity detector which comprises a pair of electro-optical sensing means (58, 62) positioned along the scan path at a fixed reference ahead of the scan format and along the scan path rearwardly of the photoreceptor. Each means comprises at least one photo-detector and provides a signal indicative of the beam falling on the photodetector. The signals of each of the sensing means are used to give a control signal for changing the speed of rotation of the polygon. <IMAGE>

Description

1 GB 2 101 360 A 1

SPECIFICATION Light scanning apparatus

This invention relates to a laser scanning apparatus. There is disclosed a laser spot scanning apparatus for communicating information to a scanned medium. The apparatus utilises reflected light from a multifaceted rotating polygon.

In particular, the invention is directed to a spot scanning system using acousto-optical and electro-optical means and methods to compensate for defects in facet to facet relationship and facet to axis error.

A recurring problem in known scanner systems is to reduce or eliminate error introduced as a result of inherent defects in the construction of rotating polygonal mirrors. Such defects occur usually in the angular relationship between adjacent facets facets (facet-to-facet) and between facet planes and the polygonal rotational axis (facet-to-axis). A typical proposal for solution of this problem is to employ non-spherical optics to partially correct the effects of facet angular error as is shown in the apparatus described in the U.S. Patent Specification No. 4 002 830. Another attempt to solve the problem is the use of optical reflecting or refracting elements pivotally mounted in the path of radiation and utilizing electromechanical devices that are energized by timed electrical signals in such a way that the refracting element is pivoted to correct the scanning errors caused by angular defects in the rotating mirror. The control of the electromechanical devices is pre-programmed to make proper adjustments and the fabrication of these optical systems is thus expensive, as is alignment 100 of the same.

This application is a divisional from Patent Application No. 79 23509, Publication Serial No.

2027950.

According to the present invention, there is 105 provided a light scanning apparatus comprising means for generating a laser beam, means modulating the amplitude of the beam in accordance with a writing program, a multifaceted reflective polygon positioned in the beam path, means for rotating the polygon, the beam being reflected from successive facets of the polygon and sweeping along a scan path to provide successive raster lines, a photoreceptor positioned to have the raster lines extending thereacross, and 115 control means for controlling the speed of rotation of the polygon, the control means including a sweep velocity measurement means which comprises a pair of electro-optical sensing means, one of the sensing means being positioned along the scan path at a fixed reference ahead of the scan format and the other sensing means being positioned along the scan path rearwardly of the photoreceptor, each sensing means comprising at least one photodetector and means providing a signal indicative of the beam failing on the respective photodetector, means receiving the signals of each of the sensing means and in response thereto providing a control signal for changing the speed of rotation of the polygon.

In a preferred version of the invention, each of the sensing means comprises a pair of photodetectors juxtaposed along an axis perpendicular to the scan path axis.

The invention will be better understood from the following nonlimiting description of an example thereof given with reference to the accompanying drawings in which:

Figure 1 is a diagrammatic representation of the major components of one example of an optical scanning device according to the present invention, the angle of reflection of the beam being distorted for purposes of illustration; Figure 2 is a schematic representation of the longitudinal travel of a modulated laser beam approaching and crossing a spot detector and photoreceptor; Figure 3 is a schematic diagram of the spot correction logic; and Figure 4 is a diagrammatic illustration of spot displacement during corrective modulation.

Referring now to the drawing, Figure 1 illustrates an overall view of the scanning system of this invention. The light source, such as a laser 10, which may be a 3 mw helium-neon laser, generates a collimuted beam 12 of monochromatic light which is directed through a neutral density filter 14 to control the light intensity. The beam 12 then passes through a modulator 16, such as an acousto-optical modulator. The beam 12 is next directed through a first lens 20 and intercepted by a knife edge 22 placed at the focal point of the first lens 20. The knife edge 22 is employed for stopping the zero order Bragg beam. The first order beam is thus separated and passes the knife edge 22 unattenuated. An example of a commercially available acousto-optic modulator is Model 1209 by Isomet Corp. Springfield, VA., which provides a built-in Bragg angle adjustment. The modulator 16 can typically be operated by a digital driver, such as Model No. 220 available from Isomet Corp.

wherein transistor-transistor logic compatible digital input controls an RF switch for on-off gating of the modulator 16. Another acoustooptical modulator is Model 304 manufactured by Coherent Associates, Danbury, Connecticut, U.S.A.

It is desirable to use the first order beam to produce a spot because the position of the spot can be displaced in accordance with frequency modulation applied to the modulator which will selectively defiect the beam 12 in a desired direction such as indicated by the arrows a, b. The first order beam 12 is then directed toward a second lens 24 which directs a converging beam onto a reflecting face or facet 26 of a rotating polygonal mirror, herein referred to as a polygon 28. The polygon 28 is continuously driven by a motor drive 30 and preferably is maintained at a constant velocity. In the preferred embodiment as shown, the polygon 28 has thirty facets 26 and is designed for generating approximately 240 scan lines per second. A moderate spot velocity is 2 GB 2 101 360 A 2 preferred for implementing the optical spot sensing and closed loop feedback correction circuitry.

The beam 12 is thus reflected successively from each of the facets 26 of the rotating polygon 28 and onto a photoreceptor 32. The reflection of the beam 12 from the polygon 28 is distorted for purposes of illustration as it will be appreciated that the incident beam and reflecting beam will be in the same plane rather than at an angle to one another as indicated by Figure 1. The modulated beam 12 may appear as a succession of dots 34 which will generate a scan line forming a raster across the moving photoreceptor 32. The photoreceptor 32 may be any image plane and can be mounted on a rotating drum such as for use with an electrophotographic copier.

It should thus be apparent that the light scanning system of the present invention can be readily interfaced with an electrophotographic copier having panchromatic photoreceptors and can thus function as a high quality nonimpact printer.

It is well known that various types of errors are inherent in the geometric fidelity of a commercially available rotatable polygon. In particular, deviation in parallelism of each facet relative to the axis of rotation introduces a facetto-axis error and the resulting scan lines will correspondingly contain these inaccuracies which manifest themselves as alignment deviations from a desired scan line travel axis, i.e., line to line spacing variation. A spot correction assembly 36 is used for optically detecting and correcting for these facet-to-axis errors. The spot correction assembly 36 in the preferred embodiment, is provided with an optical detector in the form of a split detector 38 optically positioned in the scan format plane and divided in half to form cells A, B (Figure 3) with a common electrode. A division C (Figure 2) formed between the two cells A, B is registered with the desired scan path axis and has a dimension substantially less than the diameter of the spot 34. A signal will thus be generated from either or both cells A, B when the spot 34 sweeps the split detector 38. Since the alignment of division C is parallel to the scan direction, the division C provides a reference for indicating deviations of spot 34 from the desired travel axis on the photoreceptor 32.

Referring now to Figure 2, maximum allowable uncorrected facet-to-axis angular error may cause the spot 34 to fall anywhere within a transverse zone, within the light sensitive area of split detector 38. Successive scans, as determined by the rotating polygon 28, travel a distance D that extends from the outside edge 33 of the photoreceptor 32 to the outside edge 39 of the split detector 38. The distance D is greater than the width D' by a distance D2 which is at least equal to the distance a spot 34 will travel as a result of the greatest facet-to-facet deviation it should thus be evident that the correction of each successive spot 34 is achieved during a---dead- time, i.e., the period of travel prior to transversing 130 the photoreceptor 32.

A typical logic circuit implementing the present invention for control of an acousto-optical deflector to provide compensating deflection of the laser beam such that the spot 34 will exit the split detector 38 in registration with the division C between the individual cells A and B is shown in Figure 3. When an uncorrected spot 34 of the laser beam enters the detector 38, a comparator 40 compares the signal generated at either photocell A or B with a reference voltage V, and provides a low output signal to an inverter 42 to generate a high enabling signal at an AND gate 44. A system clock 45 provides correction count pulses as a second input to the AND gate 44. The presence of the spot 34 at either cell segment A or B thus provides a high signal at the AND gate 44 enabling clock pulses to pass through the AND gate 44 and register at a counter 46. The instantaneous count of the counter 46 drives a digital to analog converter 48 which in turn provides an analog correction signal via a voltage controlled oscillator 50, an amplitude control 51, and an amplifier 52 to an acousto-optical deflector 54 which is incorporated into the modulator 16. The beam 12 will thus be displaced in the appropriate direction a or b. The amplitude control 51 is connected to a summer 53 that measures the amount of light failing on A and B of the split detector 38 to maintain the light output constant to the acousto-optical deflector 54.

It should be appreciated that the direction of count of the counter 46 determines the direction of corrective deflection applied by the deflector 54. In order to control the direction of count, a second comparator 56 compared the output of cell A with respect to the output of cell B. If the terminal spot 34 of the laser beam enters cell A, the output of cell A will be greater than that of cell B, and the comparator 56 will provide a low output. The low output of comparator 56 determines the count direction of the counter 46. As the deflecting correction is applied, the spot 34 progresses towards cell B while translating across the detector 38. As soon as the spot 34 crosses into cell B, the signal of cell B will be greater than the signal of cell A which causes the comparator 56 to switch to a high output. The high output of the comparator 56 reverses the direction of the counter 46 and thus provides an opposite direction of corrective deflection of the laser beam such that the laser beam will progress towards cell A. Thus, the spot 34 will track the division C until it exits from the detector 38 at which time the clock 45 is disabled and the correction value for the particular facet 26 is digitally stored in counter 46 until the next uncorrected spot enters the detector.

The control process is further detailed in Figure 4. The spot 34 is shown entering the split detector 38 at cell A. The displacement of the spot 34 resulting from an incremental change in the counter is reflected through the closed loop control circuitry through a deflection in either direction a or b. The uncertainty of the exit point of q 3 GB 2 101 360 A 3 spot 34 is D, due to the digitization of the signal. 50 The amount D. is less than the tolerable error.

With a given voltage controlled oscillator and acousto-optical modulator, D. can be varied by changing the scaling factor of the digital to analog converter. Since the most extreme error would be 55 equivalent to D1 2 the number of correction steps to bring the spot 10 34to the division C is a maximum of D1 n =-. 2D.

Assuming it takes a given time (Q to perform a corrective step, the total time is Dits (n)(t2) =_ ' 21)s If the spot velocity is V.. then the minimum length of photocell required is Dit. D2 =- (vr). 2D.

A typical value for V, is 2200 inches per second.

Other alternate closed loop means of control include a successive approximation technique which converges more rapidly than the above described counter system.

A further method capable of converging more rapidly in the use of a precision sensing detector such as United Detector Technology PIN-SC/1 OD, which senses the centroid of a light spot and gives analog output proportional to spot position. These outputs can be digitized directly and coupled to a voltage controlled oscillator without intervening conversions.

Scan line spot detection will not be discussed with reference to Figure 1. Since the modulator 16 is controllable by internal logic which determines exposure on the scan format 32, an edge detector 58 is positioned adjacent the leading edge of an exposure slot 60 formed in an opaque shield 61 for indicating when the spot 34 is at the precise location. The edge detector 58 and a logic circuit can thus be used as an implement to synchronize the internal logic with the location of the scan line. It should therefore be clear that each sweep of the scan line is independently referenced and will thus negate any facet-to-facet polygon error which will manifest itself as jitter in the time that successive scans cross a geometric reference point.

The edge detector 58 of the preferred 110 embodiment utilizes a split photocell similar in construction to detector 38 except for orientation of the division perpendicular to the scan path.

Each half of the split photocell forming detector 58 is essentially identical in area, location, material and temperature. Thus, the use of a split photocell has advantages over a single cell edge detector in that it will be relatively insensitive to laser light intensity change, temperature changes and ambient light.

A second edge detector 62 of similar construction to the edge detector 58 is located at a trailing edge of the exposure slot 60. The edge detector 62 will indicate when spot 34 has passed a fixed terminal point beyond the scan path. The time differential as detected between the first edge detector 58 and the second detector 62 can be interpreted through logic circuitry to indicate the flight time for spot 34 to cover a fixed length scan path. Thus, the speed of the spot can be computed. Variations in speed for different scan lines can be detected, and a feedback loop can then be utilized for speed control of the motor drive 30.

With regard to the aforementioned, it has been found that as a beam 12 is deflected or detuned from the Bragg angle, the efficiency will change. The efficiency of the scanning apparatus disclosed herein, however, can be improved by introducing an intensity modulator 64 for applying an amplitude modulated correction signal for maintaining laser illumination at a constant level. The intensity modulator 64 could also be used for control of spot size by varying the intensity. The use of different spot sizes can be effectively employed as letters or numbers are created so as to avoid roughened edges and improve character formation. The system of this invention can also employ two power sources using parallel laser beams with each of the beams being of a different diameter and corresponding spot size. This will provide a matrix of dots having different sizes for forming a single generated character. The different size dot will intermesh to create letters and numerals having a smoother appearance.

It will be seen that the laser scanning apparatus illustrated herein is well suited to meet conditions of practical use.

As various changes may be made in the apparatus as above particularly described, it is to be understood that the matter shown in the accompanying drawings is to be interpreted as illustrative and not in a limiting sense.

Claims (2)

1. A light scanning apparatus comprising means for generating a laser beam, means modulating the amplitude of the beam in accordance with a writing program, a multifaceted reflective polygon positioned in the beam path, means for rotating the polygon, the beam being reflected from successive facets of the polygon and sweeping along a scan path to provide successive raster lines, a photoreceptor positioned to have the raster lines extending thereacross, and control means for controlling the speed of rotation of the polygon, the control means including a sweep velocity measurement means which 4 GB 2 101 360 A 4 comprises a pair of electro-optical sensing means, one of the sensing means being positioned along the scan path at a fixed reference ahead of the scan format and the other sensing means being positioned along the scan path rearwardly of the photoreceptor, each sensing means comprising at least one photodetector and means providing a signal indicative of the beam failing on the respective photodetector, means receiving the signals of each of the sensing means and in response thereto providing a control signal for changing the speed of rotation of the polygon.

2. Apparatus according to claim 1 in which each of the sensing means comprises a pair of photodetectors juxtaposed along an axis perpendicular to the scan path axis.

Printed for Her Majesty's Stationery Office by the Courier Press. Leamington Spa, 1983. Published by the Patent Office 25 Southampton Buildings, London, WC2A lAY, from which copies may be obtained.

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