KR20000076209A - Non-contact position sensor - Google Patents

Abstract

The apparatus for measuring the displacement of an object movable in a predetermined direction includes a fixed light source 10 for generating an incident light beam. A target feature 17 attached or incorporated to the object reflects the incident light beam to form a first image of the light source in the vicinity of it. An imaging lens 26 receives the reflected light beam to reconstruct the first image of the light source on the photodetector 20 as a second image. A photodetector spaced apart from the object receives the received light beam and generates an electrical signal having a property that is proportional to the reception position on the photodetector of the second image and indicates the position of the object. The target feature includes a curved surface that reflects the light beam such that a small first image, either point- or line-shaped, of the light source is formed and reshaped as a second image on the photodetector.

Description

Non-Contact Position Sensors {NON-CONTACT POSITION SENSOR}

Many applications are needed to monitor the position of a movable object relative to a given position and positions. For example, in a disk drive servo system, the head arm may be accurately detected so that the position of the rotatable read / write head arm relative to a known position can be read and written to such a position. It is desirable to be able to move in alignment with a given radial track position on the disc. The position sensing device is a device intended to provide such a position detection. The displacement sensor is a position sensing device that monitors the position of the movable object by repeatedly measuring the displacement of the object from a predetermined position.

Examples of conventional displacement sensors include optical-fiber proximity sensors and optical sensors, such as capacitive gauge devices, interferometer sensors commonly used in disk drive servo systems. Many prior art displacement sensors cannot achieve the resolution (minimum measurable displacement) required by any application (such as a disk drive servo system), or use expensive and / or complex circuits and hardware to achieve this resolution. in need. In order to achieve high resolution, some displacement sensors, for example, require a significant proximity between an accurate and powerful laser light source or sensing element and a movable object, making the device expensive and difficult to implement. Thus, the balance between the performance and simplicity of the device must be balanced.

U. S. Patent No. 5,315, 372 to Tsai discloses a conventional disc drive servo system using an optical displacement sensor. The Tsai device includes a light source and photodetector array attached to a separately spaced position on a rotatable master arm located outside of the disk drive. The reflector is attached to the rotatable read / write head arm at a position between the read / write head and the axis of rotation of the arm. In operation, the master arm is first accurately aligned with the desired radial track position on the disc using an interferometer device. Next, the displacement sensor is activated to position the head arm relative to the master arm so that the head arm can be moved in alignment with the master arm.

The light source of the displacement sensor produces an incident light beam reflected by the reflector onto the photodetector array. The position on the photodetector array where the light beam is reflected depends on the relative radial position of the head and master arm. Each photodetector element of the array produces an electrical signal having an amplitude proportional to the intensity of the received light. Thus, the signal generated by the array represents the relative radial position of the head and master arm. The processing circuit receives and decodes the signal output by the array to determine the relative head arm position and controls the motor to rotate the head arm until it is properly aligned.

The device disclosed in the Tsai patent suffers from a number of drawbacks. While the Tsai device is relatively structurally simple and inexpensive to implement, it is burdensome to operate. Tsai devices require prior knowledge of the location of each master arm and relative head arm in order to accurately analyze the electrical signal generated by the photodetector array. In addition, due to the significant separation between the reflector and the photodetector on the head arm, and also due to the dual axis of rotation of the head arm and the master arm, the performance accuracy is significantly lower. In addition, Tsai's displacement sensor is adapted to monitor the radial movement (caused by rotation about the axis) of the head arm relative to the master arm. Because the reflector reflects the incident beam directly onto the photodetector significantly spaced from the reflector, the sensor is sensitive to the radial movement of the head arm as well as the angular movement of the reflector. Thus, the exact angular orientation of the reflector on the head arm is crucial for correct operation. Any angular shift of the reflector relative to the head arm causes an error measurement.

It is therefore an object of the present invention to provide a simple but accurate displacement sensor.

<Summary of invention>

One embodiment of the invention is directed to an apparatus for measuring displacement of a movable object. The apparatus includes a fixed light source for generating an incident light beam. The target feature attached or incorporated to the object reflects the incident light beam and forms a first image of the light source in close proximity to the target feature. The first image moves according to the target feature. The imaging lens receives the reflected light beam and forms a second image of the light source on the fixed photodetector. The photodetector spaced from the object receives the second image and in response generates an electrical signal having a property (such as amplitude) that is proportional to the instantaneous position of the photodetector of the second image and indicates the position of the object. The target feature includes a curved surface that reflects the light beam such that a small, preferably point- or line-shaped, first image of the light source is formed in the vicinity of the target feature and reassembled on the photodetector as the second image. As a result, the device of the present invention measures only the horizontal movement of the object and is insensitive to other movements, such as the angular orientation of the object.

In one embodiment of the invention, the photodetector comprises a spatially aligned photodetector, such as a twin-cell photodetector. In another embodiment of the invention, the photodetector comprises a position sensor.

In an embodiment of the invention, the target feature has a radius of curvature in the range of 0.2-0.5 mm.

In an embodiment of the invention, the incident light beam and the reflected light beam form a plane substantially perpendicular to the moving direction to be sensed. In this embodiment, the axis formed through the central point of each cell of the pair-cell detector is substantially perpendicular to the plane and substantially parallel to the direction of movement of the object.

Another embodiment of the invention is directed to an optical displacement sensor for use in a disc drive servo system. The sensor monitors the relative radial position of the read / write head arm and the master arm. The apparatus includes a light source attached to the master arm and generating an incident light beam. The target feature attached or incorporated at the end of the head arm reflects the incident light beam and forms a first image of the light source in close proximity to the target feature. The first image moves according to the target feature. The imaging lens receives the reflected light beam and forms a second image of the light source on the photodetector. The photodetector attached to the master arm receives the second image and in response generates an electrical signal having an amplitude that is proportional to the instantaneous position on the photodetector of the second image and indicative of the position of the head arm. The target feature includes a curved surface that reflects the light beam such that a small, preferably point- or line-shaped, first image of the light source is formed near the target feature and reshaped on the photodetector as a second image. . As such, the displacement sensor is sensitive only to the horizontal movement of the target feature and insensitive to other movements. Thus, the angular orientation of the target feature on the head arm does not affect the measurement and need not be accurate.

Another embodiment of the invention is directed to a method of measuring displacement of a movable object. The method includes generating an incident light beam with a fixed light source, reflecting the incident light beam from a curved surface of a target feature to form a first image of the light source in close proximity to a target feature attached or incorporated into an object, Reforming the small first image of the light source onto its fixed photodetector as its second image, the electrical signal having an amplitude proportional to each instantaneous position on the photodetector of the second image and having an amplitude indicative of the position of the object; Generating.

The features and advantages of the invention will be more readily understood and apparent from the following description of the invention which should be read with reference to the accompanying drawings, and from the claims appended at the end of the description.

FIELD OF THE INVENTION The present invention generally relates to a method and apparatus for accurately measuring the fine displacement of a movable object, and more particularly to a simple but accurate optical method and apparatus for providing such displacement measurement.

1 is a block diagram of a general embodiment of the displacement sensor of the present invention.

2 is a detailed view of a particular embodiment of the displacement sensor of the present invention.

3 is a graph showing electrical signals generated from the output of a bi-cell photodetector in accordance with one embodiment of the present invention.

4 is a detailed block diagram of a disk drive servo system according to another embodiment of the present invention.

Fig. 5 is a block diagram of component functions and component functions illustrating the operation of the disc drive servo system of the present invention.

6A and 6B are electrical block diagrams of processing circuits of the disc drive servo system of the present invention.

7 is a detailed view of one embodiment of a target feature used in a disk drive servo system of the present invention.

FIG. 8 is a side view of the target feature and head arm shown in FIG. 7; FIG.

9 is a detailed view of a portion of an embodiment of the displacement sensor of the present invention.

1 is a general block diagram of one embodiment of a displacement sensor of the present invention. As shown, the sensor includes a fixed light source 10, a movable object 14, a static imaging lens 26 and a fixed photodetector 20. The target feature 16 is attached to or merges with the movable object 14. The object 14 is movable in the direction shown by arrow X, and the displacement sensor monitors the position of the object 14 by measuring the displacement of the object 14 from a reference position.

The light source 10 generates an incident light beam 12 that is reflected by the target feature 16 as a reflected light beam 18. The first point-like or line-like image (not shown) of the light source 10 is very close to the target feature 18 due to the small curved reflective surface on the target feature. Is formed. The first image moves in accordance with the movement of the target feature. The light beam 18 is focused by the lens 26 such that a second image 19, similar to the first image of the light source 10, is formed on the surface of the photodetector 20. In response to receiving the reflected light beam 18 (and the second image), the photodetector 20 is proportional to the position on the surface of the photodetector 20 in which the reflected light beam is received (ie, the second image is formed) and In addition, it generates an electrical signal having a characteristic (such as amplitude or frequency) corresponding to the current position of the object 14. Processing circuitry (the specific embodiment described below with reference to FIG. 6B) may be coupled to the photodetector to receive an electrical signal generated for the photodetector and determine the position of the object and / or the displacement of the object from a known reference position. In order to process such a signal. The circuit can include both analog and digital processing circuits.

As described in more detail with reference to FIG. 9, the target feature 16 has a first small, preferably point- or line-shaped image of the light source 10 formed in close proximity to the target feature and the lens A curved surface 17 reflecting the incident light beam 12 to be reshaped as a second image on the surface of the photodetector 20 by 26. Photodetector 20 is more sensitive to small displacements than when the size of the image is reduced. Therefore, it is desirable to refocus a small point- or line-shaped image of the light source onto the photodetector surface.

As described in more detail below, the curved surface 17 of the target feature 16 can be concave or convex. Target feature 16 may be formed integrally with object 14 using a suitable stamping tool. For example, target feature 16 may include a curved surface that is stamped as part of object 14. Optionally, target feature 16 may be a separate device attached to object 14. For example, target feature 16 may be a cylindrical element such as a pin that is welded, soldered, screwed or otherwise attached to an object. The curved surface 17 of the target feature 16 should be sufficiently flexible and preferably polished so that the incident light beam is effectively reflected.

Referring to FIG. 1, when the object 14 is moved in the direction of arrow X, the position on the surface of the photodetector 20 where the reflected beam 18 is received moves in the opposite direction, shown by arrow X '. . In response, photodetector 20 generates an electrical signal, where each signal has a characteristic (i.e. amplitude or amplitude) that is proportional to the position on the photodetector surface of reflected beam 18 and also to the relative position of object 14 Frequency).

The displacement resolution of the sensor increases as the image size of the light source reflected onto the photodetector surface decreases. In addition, the displacement resolution of the sensor increases as the intensity of the incident light beam increases. Since the use of target features with curved surfaces significantly reduces the size of the light source image (ie, point- or line-shaped image) reflected onto the photodetector surface, high efficiency and expensive The need for a laser light source is significantly reduced.

2 is a detailed view showing a specific embodiment of the displacement sensor of the present invention. Similar components in FIG. 2 are referred to by the same reference symbols as in FIG. 1. As shown, the sensor includes a laser diode light source 22, an object 14 to which the target feature 16 is attached, and a bi-cell photodetector 28. It also includes a gradient index (GRIN) incident laser diode beam collimating lens 24 and a reflective beam focusing lens 26.

The laser diode 22 generates an incident laser beam 12 (shown in FIG. 2 as two separate laser beams) provided to the target feature 16 through the GRIN collimating lens 24. Each laser diode 22, GRIN collimating lens 24 and pair-cell photodetector 28 may be conventional devices. The incident laser beam 12 reflects from the curved surface 17 of the target feature 16 to form a first image (not shown) in close proximity to the target feature to reflect the incident beam 12. 18) to reflect. The small first image, either point- or line-shaped, is reshaped by the lens 26 as a second image 30 on the upper surface 31 of the pair-cell photodetector 28. Lens 26 receives beam 18 to focus the second image on photodetector 28. The curved surface 17 of the target feature 16 allows a point- or line-shaped small image 30 of the light source 22 to be refocused on the surface 31 of the photodetector 28.

Since the first point-shaped image is formed very close to the target feature, the system of the present invention cannot be considered to be measured insensitive to the movement of objects and target features (such as radial movement). For example, if the object 14 is weakly rotated around an axis formed in the Y direction (parallel to the longitudinal axis of the target feature), the measurement is not affected. This is because the portion of the beam 18 (and the first image) reflected on the surface of the lens 16 changes but the position of the second image 30 on the photodetector 31 does not change. Thus, the system of the present invention is insensitive to small radial movement of an object.

The pair-cell detector 28 comprises two photodetector cells A and B separated into narrow strips 32. When the object 14 moves in the X direction, the point image 30 moves across the surface 31 of the pair-cell detector 28 in the opposite direction X 'from cell A to cell B. The center of the point image 30 is shown in FIG. 2 as located in the strip 32 between cells A and B. FIG. The sensor may be calibrated to have a zero or reference position (where the object 14 is at a predetermined position) when the center of the point image 30 is focused on the strip 32 between cells A and B.

Each cell A or B of the pair-cell photodetector 28 produces an analog electrical signal having an amplitude that depends on the location of the point image 30 on the cell surface. Each cell produces the strongest electrical signal when the point image 30 is located entirely on the surface of that cell. By monitoring the ratio of (AB) / (A + B), where A and B represent the amplitude of the electrical signal generated by cells A and B of detector 28, respectively, on the surface of photodetector 28 The position of the point image 30 can be monitored, whereby the position of the object 14 (relative to the fixed light source and photodetector) can also be monitored. This is because the position of the point image 30 on the surface of the photodetector 28 is linearly related to the relative position of the object 14. The ratio of (A-B) / (A + B) can be monitored by the use of a processing circuit (see FIG. 6B) electrically connected to the output of the pair-cell detector 28. Such circuitry may include digital signal processing circuitry coupled to a computer having a display device such that a user can easily and visually monitor the position of the object 14.

The sensor is insensitive to movement in the Y, Y ', Z, or angle (near Y axis) directions. In this embodiment, the plane formed by the incident laser beam 12 and the reflected laser beam 18 to reduce the sensitivity of the sensor to the vertical movement of the object (direction away or towards the laser diode and photodetector). It is substantially orthogonal to the axis formed between the center points of these cells A and B and is substantially parallel to the strip 32. Thus, for example, if the object 14 moves vertically from the first position to the second position (away from the laser diode and photodetector) in the direction of arrow Z as shown in FIG. 2, the point image 30 is Perpendicular to the direction X, it moves in the direction of the arrow Y 'on the surface of the photodetector 28 and does not affect the electrical output signal of the photodetector.

Additionally, as shown in FIG. 2, in one embodiment of the present invention, the target feature 16 includes an incident laser beam 12 and reflections such that the sensor is sensitive to fine horizontal movement (in the Y direction or the Y ′ direction). Parallel to the plane formed by the laser beam 18 (shown in dashed lines), parallel to the strip 32 of the photodetector 28, and perpendicular to the axis formed through the center point of each cell of the pair-cell detector. It has a longitudinal axis.

The present invention provides insensitivity to the movement of the object in the Z (upward), Y or Y '(horizontal to the plane formed by the incident and reflected light beam), and radial (near axes formed in the Y direction) directions. Only the movement of the object in the X direction (perpendicular to the plane formed by the incident and reflected beams) is measured.

FIG. 3 is a graph showing the time versus the difference (A-B) in the amplitudes of signals A and B as the object 14 moves in the direction of arrow X (shown in FIG. 2). In this embodiment, the object 14 is in position at time 0 such that the point image 30 is not focused on the surface 31 of the pair-cell photodetector 28 (before reaching the surface of cell A). Assume that When the object 14 moves in the X direction, the point image 30 first reaches the surface 31 of cell A and then crosses the strip 32 to cell B.

When the leading edge of the point image 30 (after size 1 and significantly larger than the width of the gap between A and B) reaches the end of cell A (after time 1), signal AB is at 0 It starts to increase, reaching the maximum when the image is entirely in cell A (at time 2). When the leading end of the image reaches the gap between cells A and B (at time 3), signal AB begins to decrease so that the point image is in the center of the gap between cells A and B and cells A and B Pass zero when placed together As the point image continues to move to cell B, the signals A-B continue to decrease, reaching a maximum when the image is entirely on cell B (at time 4). When the point image completely leaves cell B (time 5), the signal (A-B) increases back to zero. The slope of the curve (AB) representing the image position is maximum when the image is in both A and B, and increases in value when the size of the point image decreases, while still being greater than the width of the gap between cells A and B. Is kept larger.

Thus, by observing signals A-B, it can be seen that the position of the point image on the surface 31 of the pair-cell photodetector 28 and the linearly related relative position of the object 14 can be monitored. Similarly, the displacement between the first position and the second position of the object 14 can be measured.

The dynamic range of the sensor (position range of the object for which accurate measurements can be made) is within the linear range of the A-B signal curve and is shown between the dashed labels DR (about 2.5 and 3.5 hours) on the graph of FIG. 3. In addition, the system has a minimum acquisition range that is exactly the minimum detectable level of signals A-B. This capture range is shown between the dashed labels CR. The slope (milliamps / micron) of the A-B signal curve in the linear dynamic range corresponds to the reactivity (or sensitivity) of the pair-cell detector. As the slope of the curve in this area increases, the sensor's achievable resolution (the minimum detectable displacement of the object) increases but the sensor's dynamic range decreases. Thus, with the sensor of the present invention, it should be noted that equilibrium between dynamic range and displacement resolution must be maintained.

The magnitude of signal A + B (when point image 30 is focused on surface 31 of pair-cell detector 28) indicates that monitoring of the ratio of (AB) / (A + B) monitors signal AB. It can be kept at a constant constant so that it can be more simply achieved. The maintenance of the signal A + B constant is used to determine the intensity of the input laser beam 12 using an automatic gain control (AGC) circuit electrically connected to the laser diode 22 (an embodiment is described below with reference to FIG. 6A). Can be achieved by controlling. As described below, the AGC circuit monitors the signal A + B while controlling the power provided to the laser diode. Thus, to simplify the processing circuitry necessary to monitor the output electrical signal generated by the pair-cell detector 28, an AGC circuit can be used.

The light source 10 can be any type of light source that produces an incident light beam having a sufficient intensity to be detected by a photodetector to meet the sensitivity requirements of a particular application. Light sources include, for example, laser diodes, light emitting diodes (LEDs), and LEDs, tungsten light sources, or optical fiber light sources. If an optical fiber light source is used, a pair of optical fibers, or split fibers, may be used to direct the reflective laser beam to the photodetector. In one embodiment of the invention, the power of the light source preferably falls within 0.1 milliwatts to 20 milliwatts. In addition, the intensity of the incident laser beam is preferably approximately 1 mwatt / 100μ.

Various forms of focusing optical elements can be used to direct incident and reflected light or laser beams. For example, conventional lenses, spherical or cylindrical mirrors, GRIN lenses, or molded aspherical lenses can be used.

The photodetector may be any form of photodetector that produces an electrical signal having a property that is proportional to (eg, magnitude or frequency) relative to the position on the detector surface of the received light beam. For example, the photodetector may be a position sensor in which the magnitude of the generated electrical signal is directly proportional to the position on the sensor where the reflected beam is received. The photodetector preferably comprises a spatially arranged photodetector capable of generating a signal or signals in which a ratio (such as (A-B) / (A + B)) can be generated. For example, the photodetector may be a pair-cell detector, quad-cell detector, CCD array, or the like.

An advantage of the sensor of the present invention is that by monitoring the ratio of (A-B) / (A + B), the sensor is somewhat insensitive to the intensity of the light source, the reflectance of the target feature and the sensitivity of the photodetector.

In one embodiment of the invention, each cell of the pair-cell detector has a dimension of 0.6 mm to 1.2 mm and the gap between the cells (width of the strip 32) is approximately 10 μm. The width of the image 30 may fall within the range of 50 μm-100 μm. The standard sensitivity of such a photodetector is approximately 0.4 amps / watt.

In typical applications, such as servo system applications (to be described below), lens 26 may be located at approximately the same distance between target feature 16 and photodetector 28, approximately 20 mm from each. The reflective beam lens 26 conventionally has a 10 mm focal length. The radius of curvature of the curved surface 17 of the target feature 16 preferably falls in the range of 0.5-5.0 mm in one embodiment of the invention.

4 is a diagram illustrating another embodiment of the present invention in which an optical displacement sensor is used in a disk drive servo system. As shown, a conventional servo system includes a rotatable master arm 40 (not shown) and a read / write head arm 42 (located within the disk drive) (located outside of the disk drive). Both master arm 40 and head arm 42 rotate about a common axis 44. A read / write head (not shown) is attached to the top surface end 54 of the head arm 42 via a bend (not shown). A disk (not shown) in which data is written and read is located on the head arm.

As with a conventional disc drive servo system, the master arm is first rotated at the reference position in alignment with the radial track position to be written and read out. Next, through the use of the optical displacement sensor system of the present invention, the head arm is rotated to align with the master arm so that the read / write head can read or write to a predetermined radial track position of the disc.

The master arm is first aligned using an interfering light system. The interference system includes a laser source 46, a fold mirror 48, a laser interferometer 50, a retroreflective device 52 attached below the master arm 40, and an interference output signal detector 61 (in FIG. 5). Shown). The master arm VCM drive circuit 63 is connected between the interference output signal detector 61 and the voice coil motor (VCM) 65, which controls the rotational movement of the master arm 40.

As noted, the laser interferometer uses a first laser beam that reflects off of the moving target (ie, retroreflector 52) and a reference laser beam that reciprocates a fixed distance to determine the position (or displacement) of the moving target. . The laser source 46 generates a laser beam that reflects from the fold mirror 48 to the retroreflector 52 through the laser interferometer 50. The beam is reflected by the retroreflector 52 and mixed with the reference beam by the interferometer 50. Interferometer 50 provides a mixed beam to detector 61 that determines the displacement (or change in position) of master arm 40 in response. The detector 61 then sends a signal indicating a change in position to the drive circuit 63 which alternately provides a signal to the VCM 65 to rotate the master arm until it is aligned with the selected radial track position.

Once the mast arm 40 is properly aligned, the next head arm 42 is aligned with the master arm 40 using the displacement sensor of the present invention. The sensor includes a source module 56 (preferably comprising a laser diode), a reflector 58 and a detector module 60 (preferably including a pair-cell photodetector), all of which are master arms 40 It is attached to the upper surface of). As with the general embodiment of the displacement sensor of the present invention described above with reference to FIGS. 1 and 2, during operation, the laser diode in the source module 56 is integrated with the lower side of the end 54 of the head arm 42, or Create an incident laser beam that is reflected by reflector 58 to a target feature (not shown) to which it is attached. As described above, the target feature has a curved surface where the incident laser beam is reflected by forming an image of the source in close proximity to the target feature. The reflected laser beam is then reflected from the reflector 58 to the detector module 60. The image is refocused on the photodetector in module 60.

When the head arm 42 is properly aligned with the master arm 40, a small image 30 (see FIG. 2), either point- or line-shaped, shows cells A and a pair of cell detectors in the detector module 60. The system is calibrated to refocus on the surface of the strip 32, which is divided between B. Displacement sensor processing control circuit 67 (shown in FIG. 5) is electrically connected to source module 56 and detector module (FIGS. 4, 60), as described in detail below, and signals A + B and AB. Circuitry for determining and monitoring the value of the signal indicating the relative position of the head and master arm. The head arm VCM drive circuit 69 is connected between the displacement sensor processing control circuit 67 and the head arm VCM 71. The displacement sensor processing control circuit 67 drives the head arm VCM in response to provide a control signal to the VCM 71 to rotate the head arm 42 until the head arm 42 aligns with the master arm 40. Provide a digital output signal indicative of the head arm and master arm positions relative to circuit 69. As described in more detail below with reference to FIG. 6A, the sensor also includes an AGC circuit connected between the photodetector and the laser diode to maintain a constant signal A + B.

6A is a block diagram of a sensor AGC circuit of the present invention. The circuit is shown to include a pair-cell detector 62 (included in detector module 60) and a laser diode 72 (included in source module 56) for convenience of description. The circuit includes conventional analog signal amplifiers 64 and 66 connected to receive output signals A and B from pair-cell detector 62, respectively. Analog amplifiers 64 and 66 amplify analog signals A and B, respectively. The amplified analog signals A and B are provided as outputs and are also provided to the AGC circuit 73. The AGC circuit 73 is composed of an analog summing circuit 68 and a laser control power circuit 70. Summing circuit 68 generates summing signal A + B. Signal A + to the laser control power circuit 70 which monitors the magnitude of the sum signal A + B and alternately provides an output signal to control the power of the laser diode 72 in which the signal A + B actually remains constant. B is provided. It should be appreciated, for example, that the intensity of the incident laser beam generated by the laser diode may vary when the reflectance of the target varies so that signal A + B remains constant.

FIG. 6B is a block diagram of the head arm processing control circuit 69 for controlling the head arm VCM 71. As shown, signals A and B are received by block 74 including amplifiers and adder and subtractor circuits for generating signals A-B and A + B. Signal A-B is provided to an analog-to-digital converter (ADC) 76 that converts analog signal A-B into a digital signal. The digital signal is provided to a conventional buffer 78 which provides a digital output representing the signals A-B.

Signal A + B is provided to comparator 80 which compares signal A + B with a reference signal (minimum threshold level, i.e. 0) and provides a status output bit indicating whether signal A + B is greater than the minimum threshold level. . At the point when the signal AB goes to zero and the image point is focused on the strip between the cells of the pair-cell detector (indicating that the head arm 42 and the master arm 44 are aligned). Circuit 69 controls VCM 71 to rotate head arm 42 until such a zero crossing is determined to correspond. Signal A + B can also have a value of zero when the point image is not located on the surface of either of the cells of the pair-cell detector (indicating that the head arm 42 and master arm 40 are not significantly aligned). It should be appreciated (as discussed above with reference to FIG. 3). The circuit determines if an appropriate zero crossing is reached by monitoring signal A + B with comparator 80. Referring to FIG. 3, it should be noted that the proper zero crossing of signals A-B is reached when signal A-B goes to zero and signal A + B is greater than zero. Thus, when the comparator 80 determines that the signal A + B is greater than zero, a status bit is output. Thus, the digital output signal provided by the buffer 78 and the status bits provided by the comparator 80 indicate when the head arm 42 and the master arm 40 are properly aligned.

FIG. 7 shows the end 54 of the head arm 42 in more detail. The bottom surface of the head arm 42 is shown to illustrate the target feature 86. A slider flexture 84 is attached to the top surface of the head arm 42 by welding. Spot welding 82 is shown. A slider and read / write head (not shown) are connected to the tip of the flexure 84. This structure of the flexure, the slider and the read / write head is conventional.

In one embodiment of the invention, the target feature 86 is integrally formed with the end of the head arm 42. The target feature may be stamped at the end 54 of the head arm 42 using a stamping tool that preferably produces a curved surface having a longitudinal axis parallel to the longitudinal axis of the head arm. The target feature may be, for example, a cylindrical indent 88 in the head arm 42 as shown. The cylindrical indent 88 provides a curved surface of the target feature 86 that reflects the incident laser beam. The radius of curvature of the curved surface falls in the range of 0.5-5.0 mm in this embodiment.

8 is a side view of the end of the head arm 42 showing the slider 92 attached to the flexure 84. As shown, the slider 92 is located below the target feature 86 at approximately the same longitudinal point along the head arm 42. Also shown is a shoulder 90 which slightly raises the target feature 86 on the remainder of the head arm 42.

9 is a block diagram of the component function and component structure of the system portion of the present invention (such as FIGS. 1 and 2) illustrating the formation of a first image and a second image of a light source. Only the system portion of the present invention is shown in FIG. The illustrated portion includes the target feature 16, the imaging lens 26 and the detector 20.

Two positions of the target feature 16, a first position (shown in solid line) and a second position (shown in virtual form) are shown in FIG. 9. The second position shows the target feature moved in the horizontal direction X from the first position, the displacement being measured by the system of the present invention.

As shown, the incident beam 12 reflects from the curved surface 17 of the target feature 16 as a reflective beam 18. A small first image 100A in the form of a point of light source is formed very close to the target feature 16. When target feature 16 moves from a first position (referred to as position A) to a second position (referred to as position B), the first point-shaped image is also referred to as target feature 16 (referred to 100A). To the second position (referred to as 100B) from the first position.

At each position, a portion of the reflective beam 18 is received by the image lens 26 and focused on the surface of the photodetector 20. For position A, second image 100A 'is formed on the surface of photodetector 20. As can be seen, when the target feature 16 moves in the horizontal direction X from position A to position B, a second point-shaped image (on the surface 31 of the photodetector 20 in FIG. 2) Move from position 100A 'to position 100B', which corresponds to the movement of image 30 in the X 'direction. It should be noted that the second image 100A '(and 100B') corresponds to the second image 30 in FIG.

In the servo system embodiment of the present invention (shown in FIG. 4), only the rotational movement of the head arm will be measured near the axis 44 (corresponding to the movement of the object 14 in the X direction in FIG. 2). . Radial movement of the target feature relative to the head arm (corresponding to rotation of the object 14 near axis Y in FIG. 2), head arm vertically away from or facing the master arm (corresponding to movement in direction Z in FIG. 2). None of the movement of the target feature in either direction formed by the longitudinal axis of the head arm (corresponding to the movement in the direction Y in FIG. 2) will be detected by the system.

While described in at least one exemplary embodiment of the invention, various changes, modifications and improvements will be readily apparent to those skilled in the art. For example, while the sensor of the present invention is described as monitoring the position in one dimension (or direction) of an object by using a pair-cell, it should be understood that the present invention is not limited thereto. By using a quad-cell detector, the sensor can each monitor the position of the object in two dimensions. Such changes, modifications, and improvements are intended to be within the spirit and scope of the invention. Accordingly, the foregoing descriptions are mentioned by way of example only, and are not intended to be limiting. The invention is limited only by what is defined in the following claims and their equivalents.

Claims (18)

An apparatus for measuring a displacement of an object movable in a predetermined direction, A fixed light source for generating an incident light beam; A target feature attached to the object and reflecting the incident light beam to form a first image of the light source in the vicinity thereof; An imaging lens that receives the first image and reconstructs the first image on a photodetector as a second image of a light source; And The photodetector spaced apart from the object and detecting a linear displacement in a first direction of the second image And The movement of the object in the predetermined direction corresponds to the movement of the second image in the first direction so that the apparatus is relatively insensitive to movement of the object in a direction different from the predetermined direction. The method of claim 1, And the photodetector comprises a pair-cell photodetector. The method of claim 1, And the photodetector comprises a position sensor. The method of claim 1, And the target feature is incorporated with the object. The method of claim 1, Wherein the target feature has a radius of curvature in the range of 0.2-5.0 mm. The method of claim 1, And the incident light beam and the reflected light beam form a plane perpendicular to the predetermined direction. The method of claim 2, And an axis formed through the center point of each cell of the paired-cell detector is substantially parallel to the predetermined direction and substantially perpendicular to a plane formed by the incident light beam and the reflected light beam. An optical position sensing device for sensing a rotational position of a read / write head arm relative to a rotational position of a master arm of a disk drive, A light source attached to the master arm, the light source generating an incident light beam; A target feature attached to a head arm and reflecting the incident light beam to form a first image of the light source in the vicinity thereof; An imaging lens positioned in the optical path between the target feature and the photodetector and receiving the reflected light beam to refocus the first image of the light source on the photodetector as a second image; And The photodetector attached to the master arm and detecting a linear displacement in a first direction of the second image And The relative rotational movement of the head arm and / or master arm is directed in the first direction such that the device is relatively insensitive to the movement of the target feature other than the movement caused by the relative rotational movement of the head arm and / or master arm. And a position corresponding to the movement of the second image. The method of claim 8, And the photodetector comprises a pair-cell photodetector. The method of claim 8, And the photodetector comprises a position-sensing sensor. The method of claim 8, And the target feature is integrated with the head arm. The method of claim 8, And said target feature has a radius of curvature in the range of 0.2-5.0 mm. The method of claim 8, And the incident light beam and the reflected light beam form a plane perpendicular to the predetermined direction. The method of claim 9, An axis formed through the center point of each cell of the pair-cell detector is substantially parallel to the direction of movement of the head arm and substantially perpendicular to a plane formed by the incident light beam and the reflected light beam Optical position sensing device. The method of claim 8, And said target feature and read / write head are located proximal to a tip of said head arm. The method of claim 15, And said target feature and read / write head are located on opposite sides of said head arm. The method of claim 8, And the head arm and the master arm share a common axis of rotation. A method for measuring displacement in a predetermined direction of a movable object, Generating an incident light beam by a fixed light source; Reflecting the incident light beam from a curved surface of a target feature attached to the object to form a first image of a light source near the target feature; Refocusing the first image of the light source onto a fixed photodetector as a second image; And Detecting a linear displacement of the second image in a first direction corresponding to the displacement of the object in the predetermined direction with the photodetector Method comprising a.