SCANNING DEVICE AND APPARATUS INCORPORATING SAID SCANNING DEVICE

Abstract: An optical scanning device is described, which comprises a scanning element having a reflecting measuring reference face (31), and a position detection system (80) which supplies, inter alia, a measuring signal (jz) for the rotational position of the scanning element. The scanning element is supported by an electromagnetical bearing which causes the scanning element to assume certain positions of equilibrium. A reliable start-up is achieved by selectively amplifying said signal for frequencies near the natural frequency (fb) of the scanning element in said positions of equilibrium.

Description: Scanning device and apparatus incorporating said scanning device. The invention relates to a scanning device comprising a frame carrying coil means, a rotor comprising permanent magnetic means and deflecting means for deflecting a radiation beam, magnetic bearing means for supporting said rotor relative to the frame while allowing the rotor to rotate around an axis of rotation in a rotational direction, said bearing means causing a position of equilibrium of the rotor to be established in the rotational direction, and - commutation means for energizing said coil means at an operational frequency so as to drive the rotor. The invention also relates to an apparatus for writing information on an object by means of a radiation beam, comprising a radiation source and/or a radiation detector and a scanning device for moving said radiation beam relative to said object and/or detector. The invention also relates to an apparatus for reading information from an object by means of a radiation beam, comprising a radiation detector and a scanning device for moving said radiation beam relative to said object. Such a scanning device is known from EP-A-459 585. The known scanning device comprises a rotor consisting of a polygon mirror secured to a permanent magnetic body, a set of coils for driving said mirror around an axis of rotation and for positioning the rotor, and a measurement system for detecting five degrees of freedom of the rotor. The measurement system generates signals which are used by a processing unit to control the electric currents through the coils. In this way the rotor can be positioned relative to the coils without any mechanical contact with the rotor. The rotor can be driven by generating an alternating magnetic field with some of the coils in that the permanent magnetic body generates a permanent magnetic field which is not rotationally symmetric relative to said axis of rotation. The electromagnetic forces exerted on the magnetic body by means of the coils result in positions of equilibrium of the rotor in the direction of rotation. It has been found that by generating an alternating magnetic field with said coils it is possible to drive the rotor once it rotates but said alternating magnetic field does not reliably cause the rotor to start rotating once it has come to a standstill. It is an object of the invention to provide a scanning device as defined in the first paragraph, in which the rotor reliably starts rotating upon energization of the scanning device. To achieve this object the scanning device according to the invention is characterized in that the scanning device further comprises detection means which generate a signal dependent on the rotational position of the rotor, said commutation means being conceived to energize said coil means in dependence on said signal, and - filter means generating a filtered signal, said filter means having an amplitude transfer function with a relatively high value for frequencies near the natural frequency of said rotor for rotational movements about said position of equilibrium and a relatively low value for frequencies near said operational frequency. The invention is based on the following. When the scanning device is energized, the magnetic bearings will position the rotor in a position of equilibrium. This position is sensed by the detection means and results in a commutation state of the commutation means depending on the detected position. Due to mechanical disturbances from the environment of the scanning device or due to fluctuations in the magnetic field of the magnetic bearing, the rotor will oscillate about said position of equilibrium at a natural frequency. This natural frequency depends on the mass moment of inertia of the rotor and on the forces resulting from the magnetic bearing. These oscillations are detected by the detection means and result in a different energization of the coil means. Because the magnetic bearing is frictionless and the energization of the coil means is stronger for frequencies near the natural frequency of the rotor, the rotor starts to oscillate about said position of equilibrium at said natural frequency. The amplitude of this oscillation will rapidly increase until the amplitude of said oscillation becomes so large thai the rotor reaches a position from which it starts rotating. In fact, the position of equilibrium has been made unstable by amplifying movements of the rotor at its natural frequency. For frequencies near the operational frequency, this amplification is undesired because it would cause clipping in said commutation means, resulting in an irregular drive of the rotor and corresponding vibrations of the rotor. A selective amplification can be realized with very simple means. Moreover, it has been found that with the measures according to the invention it is even possible to start the rotor when the same coils are used for the magnetic bearing and for the generation of an alternating magnetic field to rotate the rotor. When the same coils are used, the magnetic field of the bearing and the alternating magnetic field are parallel so that no torque can be exerted on the rotor when it is in said position of equilibrium. In spite of this, the measures according tot the invention ensure that the rotor is reliably set into rotation because small excursions of the rotor, which are always present, cause the rotor to oscillate and hence move to positions in which it is possible to exert a torque on the rotor. It is to be noted that EP-A-105 851 discloses a reluctance motor which is started by causing it to oscillate at increasing amplitude. However this oscillation is caused by a special design of the rotor poles and by energizing the coils of said reluctance motor with an alternating current of a fixed frequency. An embodiment of the scanning device according to the invention is characterized in that said high value is at least five times said low value and at least a part of the coil means energized by the commutation means is coreless. Due to these measures it is possible to generate large electromagnetic forces on the rotor during start-up of the rotor while at the same time the heating of the coils is limited. With coreless coils magnetic saturation effects are not a limiting factor and said forces are only limited by heating of the coil due to ohmic losses generated by the electrical current through the coil. Due to said measures said current is only large for a short period during start-up of the rotor so that said heating is limited. It has been found that the selective amplification can even be increased to 10 or 20 times, resulting in a shorter start-up time of the rotor. An embodiment of the scanning device according to the invention is characterized in that said signal is AC-coupled to the commutation means by means of a high-pass filter with a cross-over frequency below said natural frequency. As said signal represents the rotational position of the rotor, this seems not sensible at first sight because for slow movements of the rotor the position information is lost which seems to make it more difficult to start the rotor. However, it has been found that this AC-coupling does not preclude starting of the rotor to start. AC-coupling is favourable because adjustments for correcting a DC component in said signal can be dispensed with. An embodiment of the scanning device according to the invention is characterized in that said detection means are conceived to generate a sine-shaped signal as a function of the rotor position. Due to this measure said signal only comprises one frequency component once the rotor is rotating at the desired rotation speed. This is advantageous because the absence of other frequency components reduces the likelihood of any resonances being excited in the scanning device. Such resonances are preferably avoided because they influence the path of the radiation beam and may affect the information and/or image quality in an apparatus according to the invention. An embodiment of the scanning device according to the invention is characterized in that the commutation means are conceived to energize the coil means with a current which is proportional to said filtered signal. This is a very simple way to realize the commutation means as it only requires a current source having an output proportional to an input signal. An embodiment of the scanning device according to the invention is characterized in that the permanent magnetic means have a direction of magnetization which is parallel to said axis of rotation, and that the permanent magnetic means are conceived to generate a magnetic field whose magnitude varies along the circumferential direction of the rotor. Due to these measures this permanent magnetic means can also serve as the movable part of the magnetic bearing means as disclosed in EP-A-459 585. An embodiment of the scanning device according to the invention is characterized in that said filter means comprise a filter with a cross-over frequency which is more than twice said natural frequency. Due to these measures the rotor will even start when the natural frequency of the rotor changes. This change may be caused by a change of the magnetic forces applied to the rotor by the magnetic bearing means, for example, due to a different orientation of the scanning device relative to the field of gravity. The invention also relates to an apparatus for reading and/or writing information on an information carrier, comprising a radiation source which generates a radiation beam, and a scanning device according to the invention for moving said radiation beam relative to said information carrier. The use of a scanning device according to the invention for such an apparatus is very advantageous because the rotor can be rotated at very high frequencies, enabling a high data-rate to be obtained. In addition, a quick and reliable start-up of the rotor is obtained, enabling a quick and reliable start-up of the apparatus. Such an apparatus may be, for example, a laser printer or an optical tape apparatus for recording and/or reproducing information. The invention also relates to an apparatus for displaying information on a display, comprising a radiation source which generates a radiation beam, means for modulating said radiation beam, and a scanning device according to the invention for scanning said radiation beam over said display. Such an apparatus may be, for example, a display apparatus as described in EP-A-517 517 or EP-A-374 857. The use of the scanning device according to the invention enables a fast and reliable start-up of the display apparatus. The invention also relates to an apparatus for converting an image into an electrical signal, comprising a radiation sensitive sensor and a scanning device according to the invention for imaging said image on said sensor. Such an apparatus may be an infrared camera as described in US-A-3, 706,484. The measures according to the invention improve the reliability of such an apparatus. The invention will now be elucidated with reference to the drawings in which Fig. 1 shows the basic elements of an apparatus for scanning an optical tape, Fig. 2 shows an exploded view of a part of the scanning device, Fig. 3 shows the principle of the optical detection system for checking the position of the mirror polygon in said apparatus, Fig. 4 shows an embodiment of a strip pattern provided on the mirror polygon, Fig. 5 shows the signal which is dependent on the rotational position of the mirror polygon and which is obtained by means of this pattern, Fig. 6 shows the principle of the scanning device according to the invention, Fig. 7 shows a detailed representation of the commutation means, Fig. 8 shows the circuit diagram of a laser printer, Fig. 9 shows the circuit diagram of a picture display apparatus with a picture display panel which can be scanned by means of radiation beam, and Fig. 10 shows the circuit diagram of an infrared camera. Fig.1 shows the basic elements of an apparatus for scanning a record carrier in the form of a tape. This record carrier 1 is directly transported from a supply reel 3 to a take-up reel 2 over a stationary guiding element 4. Both reels are driven by separate motors (not shown). The tape travel direction is indicated by means of the arrow 5. The scanning device of the apparatus comprises a radiation source detection unit 10, which supplies a scanning beam b, a rotating mirror polygon 20, which reflects the, (for example parallel) beam to an objective lens 30 which focuses the beam to a radiation spot V on the tape. The mirror polygon comprises, for example, ten mirror facets f j -fio which extend, for example, parallel to the axis of rotation of the mirror polygon. During operation, the polygon 20 rotates in the direction indicated by the arrow 22. Each facet which rotates in the radiation path of the beam b, facet f 2 in the drawing, will move the beam b in the direction of the arrow 25, perpendicularly to the tape travel direction 5, across the entrance pupil of the objective lens 30. The radiation spot V formed by this lens then scans a track extending in the direction perpendicular to the direction 5. A second, a third, etc. track are consecutively scanned by means of the facets f l s f 10 , etc. The beam b is deflected, for example, through an angle of 48°. The objective lens has, for example, an effective focal length of 1.25 mm and a numerical aperture of 0.45. The scanning spot V can then be moved, for example, through a distance of 1 mm in the vertical direction. In this way, it is possible to write and read tracks having a length of 1 mm in the direction perpendicular to the tape travel direction 5. Reading a recorded tape is effected in a manner similar to that for writing because the beam reflected by the tape 1 traverses the same optical path in the reverse direction to the radiation source detection unit 10. The information signal, the focus error signal and the tracking error signal are obtained in a way similar to that in an optical audio disc (CD) player. The radiation source detection unit comprises a high-power diode laser having a wavelength of, for example, 780 nm. If the objective lens has an NA of 0.45, a resolving power which is comparable to that of the Compact Disc system is obtained. Then an information density of 1 bit/ m can be achieved, and a tape having a width of 12.7 mm and a length of 42 m can store 50 Gbytes of information. The information density in the track direction is, for example, 0.6 μ /bit so that a track may comprise approximately 1600 bits. The nominal rotation frequency of the mirror polygon is, for example, 2000 revolutions per second. The scanning frequency of a mirror polygon with ten facets is then 20 kHz. At 1600 bits per track a bitrate of 32 Mbits per second is achieved. The track period is, for example, of the order of 1.6 μm. At a scanning frequency of 20 kHz the tape speed is then 3.2 cm/sec during reading and writing. This is a relatively low speed so that no complicated tape transport mechanism is required. Fig. 2 shows an exploded view of a part of the scanning device. The scanning device comprises a frame 7 carrying a set of coils 8. A permanent magnet 9 is situated in the magnetic field of the coils 8 and is secured to the mirror polygon 20. Fig.2 further shows several parts forming an air tight housing, allowing a low air pressure inside so that the polygon can rotate almost without friction. The magnet 9 is magnetized in the direction 9a, which is parallel to the axis of rotation 20a of the mirror polygon. The permanent magnet 9a is uniformly magnetized but, due the two flat sides 9b and 9c, it generates a magnetic field which varies along the circumferential direction of the polygon 20. Because of this variation it is possible to drive the polygon 20 in the rotational direction by energizing the coils 8a-8d as is shown in Fig.7. The mirror polygon is supported electromagnetically and can move in six degrees of freedom. These movements must be detected so that they can be corrected, if necessary. To this end, a position detection system is provided, with which movements of the mirror polygon along three axes and tilts about two of these axes can be measured. This system also provides the possibility of measuring the rotation of the mirror polygon about its rotation axis. This system has a simple set-up and the available measuring radiation is used efficiently so that measuring signals having the maximum strength are obtained. Fig. 3 illustrates the principle of the position detection system 80. The system is arranged at the side of the mirror polygon where the spherical element 23, see Fig. 1, is disposed. In Fig. 3, the reference numeral 33 denotes a diode laser which emits a radiation beam 40. This beam 40 is first converted into a parallel beam by a collimator lens 35. Subsequently, the beam 40 is incident on a splitting cube 36 with a separating face 37, which reflects a part of the measuring beam 40 as a measuring sub-beam 50 towards the polygon. This polygon is represented by the face 31, which is visible in Fig. 1, and has been referred to hereinbefore as the reference face. This face 31 is reflective and in its center it carries a spherical element 23, which is also shown in Fig. 1 and is also reflective. The part of the measuring beam 40 which is not reflected by the splitting cube is passed towards a reflector 38, which reflects this part as a measuring sub-beam 45 towards the spherical element 23 on the reference face 31. This first measuring sub-beam is focused in the center of curvature of the spherical element 23 by an objective lens 39. The measuring sub-beam reflected by the element 23 traverses the objective lens 39 again and is reflected by the reflector 38 towards the beam splitter 36, which reflects a part 45 of the beam towards a radiation-sensitive detection system 60 comprising a plurality of detection elements 71-78. A lens 41, which converts the measuring sub-beam into a converging beam 55, is arranged between the beam splitter and the detection system 60. When the mirror polygon is moved in the X direction, in the plane of the drawing, and in the Y direction, perpendicular to the plane of the drawing, the radiation spot formed by the first measuring sub-beam 45 in the detection plane moves in the X direction and the Y direction, respectively, with respect to the elements of the detection system 60. This movement can be measured by combining the output signals of the detection elements in a known manner. A cylindrical iens 34 is arranged in the radiation path for measuring the movement of the polygon in the Z direction. This lens 35 converts the diode laser beam into an astigmatic beam. After reflection by the reference face, such a beam forms a radiation spot in the detection plane, which spot has a shape which is dependent on the degree of focusing of the beam in the center of curvature of the spherical element 23. If the beam is sharply focused at this point, i.e. if the reference face has the correct position with respect to the position detection system, said radiation spot is circular. When the position of the reference face deviates from the desired position, i.e. when the beam is no longer sharply focused in said center of curvature, said radiation spot has an elliptical shape. The shape of the radiation spot, and hence the Z position of the reference face of the mirror polygon, can be detected in known manner by means of a four- quadrant detector accommodated in the detection system 60. The second measuring sub-beam 50 reflected by the beam splitter is incident on a flat portion of the reference face 31. This beam is reflected by the reference face and a part thereof is passed to the detection system 60 by the beam splitter, which beam is also converged by the lens 41. When the reference face 31 is tilted aboui: the X and/or Y axis, the radiation spot formed by the second measuring sub-beam 50 in the detection plane is moved across the detection elements 71-74 of the system 60 in the X and/or Y direction, so that these tilts can be measured. By giving an area of the flat portion of the reference face a varying reflection coefficient in the circumferential direction, as is indicated by means of the area 24 in Fig. 1 , the intensity of the second measuring beam will decrease or increase when the polygon rotates. Hence, the position of rotation or the rotation frequency of the mirror polygon can be determined. Said area 24 may consist of a dark or dull area, or of a diffusing area obtained by roughening or the provision of a grating. The reflection coefficient may have several maxima and minima along the circumferential direction. The detector signal generated by the second measuring sub-beam 50 then has a corresponding number of minima and maxima per revolution. If the area 24 has a monotonic linear variation of the reflection through 360°, the detector signal is a sawtooth-shaped signal with a period corresponding to one rotation. The linearly increasing or decreasing reflection can be obtained by linearly varying the extent of dullness or by linearly varying the strip density if the reference face is provided with strips through 360°. The detection system 60 is shown in an underneath view in the upper part of Fig. 3. This system comprises two quadrant detectors 70 and 75 with detection elements 71, 72, 73 and 74 and 76, 77, 78 and 79, respectively. If the signals of the detection elements 71, 72, 73 and 74 are represented by a, b, c and d, and those of the detection elements 76, 77, 78 and 79 by p, q, r and s, then the translations Mx, My and Mz in the X, Y and Z directions, respectively, are given by: Mx = (p+s)-(q+r) My = (p+q)-(r+s) Mz = (p+r)-(q+s) and the rotations j x , j y , j z about the X, Y and Z axes are given by: j x = (a-r-b)-(c-r-d) j y = (a+d)-(b+c) j z = a+b+c+d, in which the signal j z represents the reflection of the sub-beam 50 by the reference face. Fig. 4 shows another pattern to be provided on the reference face. As is shown in Fig. 4, the pattern comprises two groups of strips 85 and 86 each extending through 180°. In each group, the density of the strips 87 initially increases and then decreases. Dependent on the rotational position phi, the intensity of this beam will vary and hence the detector signal generated by this beam, as is shown in Fig. 5. In Fig.5, the rotational position phi is plotted horizontally and the value of the detector signal j z representing the reflection of the sub-beam 50 is plotted vertically in arbitrary units. The interval between the vertical lines 83 and 84 corresponds to one revolution. During one revolution, the sinusoidal signal j z changes sign four times. This signal j z is very suitable for directly driving the polygon in the rotational direction. Fig. 6 shows the principle of the scanning device according to the invention. The signal j z supplied by the quadrant detector 70 and the associated processing electronic circuitry is converted into a filtered signal k z by a filter 66 and an AC-coupling 67, which is shown diagrammatically as a resistor R and a capacitor C. The filter 66 has a cross-over frequency f a , which is about half the natural frequency f b of the polygon 20 in the magnetic bearing. Signals with a frequency in the range below this cross-over frequency f a are transmitted with an attenuation about 10 times as low as that of signals near the operational frequency f c at which the coils 8 are energized at normal operation of the scanning device. The filtered signal k^. is applied to the commutation means 68. Fig.7 shows a detailed representation of the commutation means. The commutation means comprises eight power amplifiers 68a-68g which each supply an electrical current to one of the eight coils 8a-8f shown in Fig.2. Each current is proportional to two of the signals Mx, My, Mz, j x> j y , l^. By energizing the coils 8a-8h according to this circuit diagram, the polygon mirror is supported electromagnetically and driven in die rotational direction. Driving forces for driving the polygon mirror in the rotational direction are obtained by energizing the coils 8a-8d with a current linearly proportional to the filtered signal k... The currents through the coils 8a and 8c increase or decrease simultaneously in dependence on the filtered signal k.. The same holds for the currents through the coils 8b and 8d. Hence, the position in the X and Y-direction (see Fig.3) of the polygon is not influenced by the filtered signal k z . The pattern shown in Fig.4 is so positioned relative to the magnet 9 that the positions of equilibrium of the electromagnetic bearing correspond to, for example, the indications 82, 83 or 84 shown in Fig.5. When the scanning device is energized, the coils 8a-8d are energized to position the polygon 20. This energization will drive the polygon 20 to a position of equilibrium due to the rotational asymmetry of the magnet 9 and the rotational asymmetry of the magnetic field generated by the coils 8a-8d. However, the polygon 20 will always make small excursions out of said position of equilibrium due to mechanical vibrations generated in its environment or because of its mass inertia and the speed it has been given by the initial energization of the coils. These excursions result in a variation in the signal j z , which results in a corresponding variation of the magnetic field generated by the coils 8a- 8d. Since the magnetic bearing supports the polygon almost without friction and the signal j z is selectively amplified (k^) at the natural frequency f b of the polygon, the polygon will oscillate in the rotational direction until it reaches a position 90 degrees beyond the position of equilibrium, after which it will start rotating. Fig. 8 shows the principle of a laser printer 90. In such a printer a photosensitive layer is first inscribed with a scanning laser beam. Subsequently, this layer is passed through an ink bath and then a print on paper is made. The photosensitive layer 92 can be provded on a roller 91 which is rotated about a shaft 93 to inscribe consecutive lines. The line scanning is realized by means of a mirror polygon 20 having, for example, six reflecting facets f. The reference numeral 30 denotes an objective lens which focuses the radiation from a radiation source 11, for example, a high-power diode laser, which radiation is reflected by the mirror facets f to form a radiation spot V on the medium 92. The intensity of the laser beam is modulated in accordance with the information to be written by modulating the current through the diode laser or by means of a separate, for example acoustic-optical or electro-optical, modulator 96. To obtain a reliable start-up of the laser printer, the apparatus is provided with a position detection system 80 and a filter 66 as described with reference to Fig.6. Fig. 9 shows the circuit diagram of a picture display apparatus 100 in which the picture is generated by a reflecting, radiation-sensitive, i.e. radiation- inscribable, picture display panel 110. The use of such a panel in an image projection apparatus is described in European Patent Application 0,517,517. The advantage of a radiation-inscribable panel as compared with a conventional active matrix panel is that a high light efficiency can be achieved therewith because it is not necessary to provide a matrix of electronic switches and conducting electrodes on the panel surface and because this panel absorbs hardly any radiation. This panel is line-sequentially scanned by a write beam 130 coming from a unit 125 accommodating a radiation source, preferably a laser, as well as a beamshaping optical system to which the information to be displayed, for example a video signal, is applied so that the laser beam is intensity-modulated in conformity with this information by a modulator 127. The laser beam 130 is incident on a rapidly rotating mirror polygon 20 and subsequently on a second scanning element 131, which moves more slowly and is formed by, for example, a vibrating plane mirror or by a second mirror polygon. The scanning element 131 reflects the beam towards the panel 110. The mirror polygon 20 reflects the converging beam 130 in such a way that the radiation spot formed on the photosensitive layer of the panel describes a line. The second scanning element 131 ensures a relatively slow movement of this radiation spot in a second direction perpendicular to the line direction. Thus, the photosensitive layer 113 of the panel 110 is scanned in two dimensions and a two- dimensional matrix of pixels is written. The use of a mirror polygon for scanning a picture display panel by means of a write beam is known from the English language abstract of Japanese Patent Application 62-56931. As is shown in Fig. 9, the panel 110 with the write system can be used in an image projection apparatus. This apparatus is provided with an illumination unit comprising a radiation source 140 and a beam-shaping optical system 141, which unit supplies an illumination beam 145. This beam illuminates the panel 110 via a polarization- sensitive beam splitter 142. The image formed in this panel is projected on a projection screen 144 by the beam 146 reflected by the panel and by a projection lens 143. The inscribable panel with the write system may alternatively be used in a direct-vision apparatus in which a viewer directly watches the panel. To determine the movements of the mirror polygon 20 in six degrees of freedom, the position detection system 80 and the filter 66 described above with reference to Fig.6 may be used. The picture display system, which is known as laser TV and with which an image is directly written on a projection screen or on a wall functioning as such by means of a scanning laser beam, or three laser beams in the case of a colour image, can be further used as the scanning device with a mirror polygon. A laser TV apparatus is described in, for example, European Patent Application 0, 374,857. In such an apparatus, scanning must take place at a high velocity so that also in this case preferably a mirror polygon is used which rotates at a high speed and which is free floating in a vacuum, as described above. Another use of the invention is in the field of scanning cameras, notably infrared cameras, in which an image of a scene or object formed by an objective lens is moved across a detector or a row of detectors. Such a camera 200 is shown m Fig.10 and is also described in, for example, US Patent 3,706,484. An image 205 is made from a remote object by means of optical elements 201 and 202. A rapidly rotating mirror polygon 220 is used for line-sequentially moving the image 205 across a detector 208. The invention can be used for realizing a high scanning velocity and a reliable start-up. Mirror polygons are also used in apparatuses for inspecting objects or workpieces during or after their manufacture, or for reading codes, for example bar codes, on objects, so that the invention may also be used in these apparatuses. The invention has been elucidated on the basis of embodiments in which the deflecting means are a mirror polygon. However, the deflecting means can also be implemented in another way, for example as a transmission or reflection grating or a hologram.

Claims: CLAIMS: 1. A scanning device comprising a frame (7) carrying coil means (8), a rotor (9, 20) comprising permanent magnetic means (9) and deflecting means (20) for deflecting a radiation beam (b), - magnetic bearing means (7, 8) for supporting said rotor relative to the frame while allowing the rotor to rotate around an axis of rotation (20a) in a rotational direction, said bearing means causing a position of equilibrium of the rotor to be established in the rotational direction, and commutation means (68) for energizing said coil means (8) at an operational frequency (f c ) so as to drive the rotor, characterized in that the scanning device further comprises detection means (80) which generates a signal (]_) dependent on the rotational position (phi) of the rotor (9, 20), said commutation means (68) being conceived to energize said coil means (8) in dependence on said signal, and - filter means (66) generating a filtered signal (k^, said filter means having an amplitude transfer function with a relatively high value for frequencies near the natural frequency (f b ) of said rotor for rotational movements about said position of equilibrium and a relatively low value for frequencies near said operational frequency (f c ). 2. A scanning device as claimed in Claim 1, characterized in that said high value is at least five times said low value and at least a part of the coil means (8) energized by the commutation means (66) is coreless. 3. A scanning device as claimed in Claim 1 or 2, characterized in that said signal (j_) is AC-coupled to the commutation means (68) by means of a high-pass filter (67) with a cross-over frequency below said natural frequency (f b ). 4. A scanning device as claimed in Claim 1, 2 or 3, characterized in that said detection means (80) are conceived to generate a sine-shaped signal (]_ as a function of the rotor position (phi). 5. A scanning device as claimed in Claim 1, 2 3 or 4, characterized in that the commutation means (68) are conceived to energize the coil means (8) with a current which is proportional to the filtered signal ( ^. 6. A scanning device as claimed in Claim 1, 2, 3, 4 or 5, characterized in that the permanent magnetic means (9) have a direction of magnetization (9a) which is parallel to said axis of rotation (20a), and the permanent magnetic means are conceived to generate a magnetic field whose magnitude varies along the circumferential direction of the rotor. 7. A scanning device as claimed in any one of the preceding Claims, characterized in that said filter means (66) comprise a filter (66) with a cross-over frequency (f which is more than twice said natural frequency (f b ). 8. An apparatus (90; 100) for writing information on an object (1; 92; 110) by means of a radiation beam (b), comprising a radiation source and a scanning device as claimed in any one of the preceding Claims, for moving said radiation beam relative to said object. 9. An apparatus (200) for reading information from an object (1) by means of a radiation beam (b), comprising a radiation detector and a scanning device as claimed in any one of the preceding Claims, for moving said radiation beam relative to said object. 10. An apparatus (90) as claimed in Claim 8 and/or Claim 9, in which the object is an information carrier (1; 92). 11. An apparatus (100) as claimed in Claim 8, for displaying information in which the object is a display (144), the apparatus comprising means (127) for modulating said radiation beam. 12. An apparatus as claimed in Claim 9, in which the object, is an image of a scene.

Inventors: KAHLMAN JOSEPHUS ARNOLDUS HENR, KAHLMAN, JOSEPHUS, ARNOLDUS, HENRICUS, MARIA
Applicants: PHILIPS ELECTRONICS NV, PHILIPS NORDEN AB, PHILIPS ELECTRONICS N.V.
Application year: 1997
Main classification: G11B7
IPC codes: G02B26, G11B7, H02P6