GB2536167A - Surface shape measuring device and machine tool provided with same, and surface shape measuring method - Google Patents

Abstract

Provided is a surface shape measuring device (140) wherein a displacement meter (100) comprises: a light emitting unit (110) that projects a light beam (116) toward a measurement subject (130); an optical system (118) that focuses the scattered light of the light beam from the measurement subject (130); and a light receiving unit (120) that detects the focus position of the optical system (118). A movement mechanism (146) scans the light beam (116) by causing the displacement meter (100) and the measurement subject (130) to move relatively. A measurement control unit (156) is configured as follows: in order that the light receiving unit (120) is positioned, with respect to the light emitting unit (110), ahead of or following the scanning direction of the light beam (116), the light beam (116) is scanned by the movement mechanism (146), and during scanning of the light beam (116), the surface displacement of the measurement subject (130) is continuously measured, as surface shape data, by the displacement meter (100).

Description

DESCRIPTION

TITLE OF INVENTION

Surface Shape Measurement Apparatus and Machine Tool Including the Same, and Surface Shape Measurement Method

TECHNICAL FIELD

The present invention relates to a surface shape measurement apparatus that measures a surface shape by a non-contact displacement sensor using a light beam, a machine tool including the surface shape measurement apparatus, and a surface shape measurement method.

BACKGROUND ART

There is a growing demand for on-machine measurement techniques on machine tools. Conventionally, on-machine measurement has been limited to such applications as the positioning of, and the measurement of a geometric dimension of, a workpiece (also referred to as a "work"). In recent years, on-machine measurement has also been used for correction in order to improve the accuracy of finishing by comparison of on-machine measurement results with CAD data. Studies have also been conducted on automatic space error correction of a machine tool itself by using on-machine measurement results.

A touch probe is commonly used for on-machine measurement. A touch probe is attachable to a machine tool body by means of an ATC (Automatic Tool Changer). A touch probe is also capable of data transfer to and from a data processing computer through wireless communication, and has been enhanced as a measurement tool.

However, there are structural limitations of a touch probe. That is, due to its being of a contact type, the possibility that the probe will damage a finished workpiece cannot be excluded. Further, since there is only a small relief stroke at the time of contact, the shape of an object to be measured must be known in advance. When detecting the position of a workpiece that has not been machined, it is required for the workpiece to be positioned in advance with accuracy within a range of the small relief -1 -stroke With non-contact measurement, on the other hand, a workpiece is not damaged, and a relatively large distance of several tens of mm can be provided between a displacement sensor and the workpiece. This is thus suitable for such applications as the measurement to determine a machining offset before machining a casting, a forged part or the like, and the high-speed scanning of a shape of a finished workpiece.

Typical non-contact measurement methods include a laser triangulation method (for example, see Japanese Patent Laying-Open No. 10-332335 (PTD 1)). Conventionally, it has been difficult to measure a surface shape of a metallic material by means of a laser displacement meter due to an insufficient amount of diffusely reflected light on a metal glossy surface. This has resulted in the need for pretreatment such as the application of powders to the surface, thus hindering the progress toward commercialization of a laser displacement meter for on-machine measurement. It has recently become possible to measure a metal glossy surface owing to the improvement in sensitivity of an image sensor serving as a light-receiving element and the development of a semiconductor laser element. This has allowed the use of a diffuse reflection method for a triangulation laser displacement meter in on-machine measurement.

CITATION LIST

PATENT DOCUMENT

PTD I: Japanese Patent Laying-Open No. 10-332335

NON PATENT DOCUMENT

NPD 1: Yasuhiko Arai and three others, "High Resolution Electronic Speckle Pattern Interferometry by Using Only Two Speckle Patterns," publication "Optical Review," Vol. 41, No. 2, pp. 96 to 104, February, 2012

SUMMARY OF INVENTION

TECHNICAL PROBLEM

The precision of a triangulation laser displacement meter is usually indicated with repeatability, where submicron precision is usually ensured. However, because -2 -laser light is used and a laser spot has a certain size instead of an ideal dot shape, a particular measurement error is observed. This measurement error is characterized in that it includes significantly high spike-like noise as compared to the actual surface roughness, and cannot be removed by a time average process. In addition, the measurement error noted above is observed in a triangulation displacement meter not only using a coherent laser beam, but also using an incoherent light beam.

The present invention has been made in view of the problem noted above, and a main object of the present invention is to provide a surface shape measurement apparatus capable of reducing an error which is observed during measurement of a surface shape by triangulation using a light beam.

SOLUTION TO PROBLEM

In one aspect, the present invention is a surface shape measurement apparatus including a displacement meter, a movement mechanism, and a measurement control unit. The displacement meter includes a light-emitting unit that emits a light beam toward an object to be measured, an optical system that collects scattered light of the light beam from the object to be measured, and a light-receiving unit that detects a focusing position of the light collected by the optical system. The displacement meter measures a displacement of a surface of the object to be measured based on the focusing position of the light on the light-receiving unit The movement mechanism scans the light beam by moving the displacement meter and the object to be measured relative to each other. The measurement control unit controls the movement mechanism and the displacement meter. The measurement control unit is configured to cause the movement mechanism to scan the light beam such that the light-receiving unit is located in front of or behind the light-emitting unit in a scanning direction of the light beam, and during the scanning of the light beam, continuously measure a variation in the displacement of the surface of the object to be measured by means of the displacement meter as surface shape data.

According to the configuration of the measurement control unit described above, the aforementioned spike-like error can be extracted as an error pattern of a -3 -characteristic shape. By removing the extracted error pattern, therefore, the noise included in the surface shape data can be efficiently reduced.

Preferably, the surface shape measurement apparatus further includes a characteristic section extraction unit that extracts a characteristic section from a measurement range of the surface shape data. This characteristic section has a length equal to or smaller than a spot size of the light beam, and the surface shape data varies to reach a maximal value in a first half or a second half of the characteristic section, and varies to reach a minimal value in the other half of the characteristic section.

Preferably, the characteristic section has a length equal to or smaller than a spot size of the light beam and satisfies at least one predetermined condition. In this case, the at least one predetermined condition includes a condition that the surface shape data varies in a portion of the first half of the characteristic section in one direction beyond a range predetermined relative to an average value of the surface shape data, and the surface shape data varies in a portion of the second half of the characteristic section in a direction opposite to that of the first half beyond the range predetermined relative to the average value.

Alternatively, the characteristic section has a length equal to a spot size of the light beam and satisfies at least one predetermined condition. In this case, the at least one predetermined condition includes a condition that a coefficient of correlation between a waveform of the surface shape data in the first half of the characteristic section and a waveform obtained by rotating a waveform of the surface shape data in the second half of the characteristic section 180 degrees around a data point at a center of the characteristic section exceeds a predetermined reference value.

With the characteristic section extraction unit having any of the configurations described above, the error of the characteristic pattern shape included in the data measured by the laser displacement meter can be extracted. By removing the extracted error pattern, therefore, the noise included in the surface shape data can be efficiently reduced.

In one preferred embodiment, the surface shape measurement apparatus further -4 -includes a data correction unit that corrects the surface shape data such that a variation in the surface shape data relative to the average value of the surface shape data is reduced in the extracted characteristic section or each of a plurality of extracted characteristic sections.

Preferably, the data correction unit corrects the surface shape data by averaging a measured value at any first measurement point in each of the characteristic sections and a measured value at a second measurement point symmetric to the first measurement point about a midpoint of the section, and by replacing each of the measured values at the first and second measurement points with the obtained average value Alternatively, the data correction unit preferably corrects the surface shape data by replacing the data in each characteristic section with the average value of the surface shape data.

With the data correction unit having any of the configurations described above, the aforementioned characteristic error pattern can be removed.

In the embodiment described above, more preferably, the surface shape measurement apparatus further includes a filter processing unit that performs a low-pass filter process on the surface shape data corrected by the data correction unit, the low-pass filter process leaving only a variation of a cycle longer than the spot size of the light beam. Consequently, the noise included in the surface shape data can be further reduced.

In another preferred embodiment, the surface shape measurement apparatus further includes a moving average processing unit that performs a moving average on the surface shape data using a moving average window of a variable width. Here, the width of the moving average window is greater than the spot size of the light beam.

The width of the moving average window when the moving average is performed in a section including the characteristic section is greater than the width of the moving average window when the moving average is performed in a section including no characteristic section. -5 -

In still another preferred embodiment, the surface shape measurement apparatus further includes a moving average processing unit that performs a weighted moving average on the surface shape data. Here, a weighted moving average window has a width greater than the spot size of the light beam. A weight at a measurement point in the characteristic section is smaller than a weight at a measurement point outside the characteristic section.

With the moving average processing unit having any of the configurations described above, the error of the characteristic pattern shape included in the data measured by the laser displacement meter can be suppressed.

In another aspect, the present invention is a machine tool including the surface shape measurement apparatus described above.

In still another aspect, the present invention is a surface shape measurement method using a non-contact displacement meter. The displacement meter includes a light-emitting unit that emits a light beam toward an object to be measured, an optical system that collects scattered light of the light beam from the object to be measured, and a light-receiving unit that detects a focusing position of the light collected by the optical system. The surface shape measurement method includes the steps of moving the displacement meter relative to the object to be measured along a scanning direction, while maintaining positional relation between the light-receiving unit and the light- emitting unit such that the light-receiving unit is located in front of or behind the light-emitting unit in the scanning direction, and determining a surface shape of the object to be measured based on a variation in measured value from the displacement meter due to the relative movement of the displacement meter.

Preferably, the step of determining a surface shape includes the step of extracting a characteristic section from a measurement range of the surface shape of the object to be measured. The characteristic section has a length equal to or smaller than a spot size of the light beam, and measured data on the surface shape of the object to be measured varies to reach a maximal value in a first half or a second half of the characteristic section, and varies to reach a minimal value in the other half of the -6 -characteristic section.

Preferably, the step of determining a surface shape further includes the step of correcting the measured data on the surface shape of the object to be measured such that a deviation of each measured value relative to an average value of the measured data is reduced in the extracted characteristic section or each of a plurality of extracted characteristic sections.

ADVANTAGEOUS EFFECTS OF INVENTION

According to the present invention, therefore, an error which is observed during measurement of a surface shape by triangulation using a light beam can be reduced.

BRIEF DESCRIPTION OF DRAWINGS

Fig. 1 is a diagram schematically showing the configuration of a laser displacement meter.

Fig. 2 is a perspective view schematically showing the configuration of a linear image sensor of Fig. 1.

Fig. 3 is a diagram showing an example of data detected by the linear image sensor of Fig. I Fig. 4 is a block diagram schematically showing an example of the configuration of a surface shape measurement apparatus according to a first embodiment.

Fig. 5 is a diagram showing an example of measurement results of a metal surface by means of the laser displacement meter.

Fig. 6 is a diagram illustrating a variation in image-forming spot on the linear image sensor, when an object to be measured has non-uniform reflectivity.

Fig. 7 is a plan view schematically showing a surface of the object to be measured of Fig. 6.

Fig. 8 is a diagram showing examples of brightness distribution detected by the linear image sensor, in the cases of Figs. 6 and 7.

Fig. 9 is a diagram illustrating the magnitude of a measurement error caused by the non-uniformity of the reflectivity of the object to be measured. -7 -

Fig. 10 is a diagram showing results of repeated measurements of a displacement in a height direction in a microscopic section on a gauge block by means of the laser displacement meter.

Fig. 11 is a diagram showing measurement results of the displacement in the height direction over a range of 0.4 mm on the gauge block by means of the laser displacement meter.

Fig. 12 is a diagram showing a maximum value of an amount of light received by the image sensor at each measurement point of Fig. 11.

Fig. 13 is a diagram showing relation between the displacement in the height direction and the maximum amount of received light in a region RC of Figs. 11 and 12.

Fig. 14 is a diagram showing a measurement result of a surface shape of a square region 0.5 mm on a side, by means of a laser beam having a spot size of 50 htm in diameter.

Fig. 15 is a diagram showing a measurement result of a surface shape of the same region as that of Fig. 14, by varying the laser spot size to 400 him in diameter.

Fig. 16 is a flowchart showing surface shape measurement and a procedure of processing measured data.

Fig. 17 is a diagram illustrating a method of extracting a characteristic section in step SIO5 of Fig. 16.

Fig. 18 is a diagram illustrating another method of extracting the characteristic section in step S105 of Fig. 16.

Fig. 19 is a diagram illustrating a method of correcting surface shape data in step S110 of Fig. 1_6.

Fig. 20 is a diagram showing a result of performing the data correction shown in step S110 of Fig. 16 on the measured data of Fig. 11.

Fig. 21 is a diagram showing a result of performing a low-pass filter process shown in step S115 of Fig. 16 on the data of Fig. 20.

Fig. 22 is a diagram showing a result of performing the low-pass filter process shown in step S115 of Fig. 16 on the measured data of Fig. 11, without performing the -8 -data correction shown in step S110.

Fig. 23 is a block diagram schematically showing the configuration of a surface shape measurement apparatus according to a second embodiment.

Fig. 24 is a flowchart showing an example of surface shape measurement and a procedure of processing measured data, in the surface shape measurement apparatus according to the second embodiment.

Fig. 25 is a flowchart showing another example of surface shape measurement and a procedure of processing measured data, in the surface shape measurement apparatus according to the second embodiment.

Fig. 26 is a perspective view schematically showing the configuration of a machine tool according to a third embodiment.

Fig. 27 is a block diagram showing a functional configuration of a portion, which relates to a surface shape measurement apparatus, of the machine tool of Fig 26. DESCRIPTION OF EMBODIMENTS Embodiments will now be described in detail with reference to the drawings.

Although each of the following embodiments describes a surface shape measurement apparatus using a laser displacement meter by way of example, the present invention is also applicable to a non-contact displacement meter using an incoherent light beam instead of laser light. In the following description, the same or corresponding parts are designated by the same reference characters and the descriptions thereof may not be repeated.

<First Embodiment> [Overview of Laser Displacement Meter] Fig. 1 is a diagram schematically showing the configuration of a laser displacement meter. Referring to Fig. 1, a laser displacement meter 100 includes a light-emitting unit 110, a condenser lens 118 as an optical system, and a linear image sensor 120 as a light-receiving unit. Light-emitting unit 110 includes a laser diode 112 and a lens 114.

A laser beam 116 emitted from laser diode 112 is collimated into substantially -9 -parallel light by lens 114, and applied to an object to be measured 130. A spot size w (also referred to as a spot diameter) of laser beam 116 on the object to be measured is, for example, 50 jun in diameter. The light diffusely reflected on object to be measured 130 is collected by condenser lens 118 onto linear image sensor 120 which is disposed in an angular direction of y relative to laser beam 116.

In Fig. 1, a direction of laser beam 116 is a Z-axis direction. A plane including a central axis of laser beam 116 and an optical axis of condenser lens 118 is referred to as an optical path plane. A direction parallel to this optical path plane and perpendicular to the Z-axis direction is an X-axis direction. A direction perpendicular to both the X-axis direction and the Z-axis direction is a Y-axis direction. In the case of Fig. 1, the Y-axis direction is a direction perpendicular to the sheet of the drawing, and an X-Z plane is parallel to the sheet of the drawing (optical path plane).

A beam size of laser light (a spot size on an object to be measured) is now described. There are various definitions of a beam size of laser light. For example, a beam size of laser light having a symmetric beam profile such as in the TEMOO mode is defined, on a surface orthogonal to the optical axis, with a width of intensity distribution corresponding to the reciprocal of the square of e (note that e is the base of natural logarithm) (13.5%) relative to a peak value. If the beam profile has been distorted, a circle encompassing 86.5% of the entire power of the beam with reference to peak power is calculated, for example, and the diameter of this circle is defined as a beam size. In this specification, in order to include various definitions, a range equal to or greater than the diameter of a circle encompassing 50% of the entire power and equal to or smaller than the diameter of a circle encompassing 95% of the entire power is substantially equal to the beam size (the spot size on the object to be measured).

Fig. 2 is a perspective view schematically showing the configuration of the linear image sensor of Fig. I. Referring to Fig. 2, linear image sensor 120 includes linearly arranged 1024 pixels 122. Each pixel 122 outputs a signal of a brightness level ranging from 0 to a maximum of 255 depending on the amount of received light.

Fig. 3 is a diagram showing an example of data detected by the linear image -10-sensor of Fig. 1. A horizontal axis of Fig. 3 represents a pixel position, and a vertical axis represents the brightness level. Referring to Figs. 2 and 3, the light diffusely reflected on object to be measured 130 is collected by condenser lens 118 to a spot 124 on linear image sensor 120, resulting in data having the shape of a Gaussian distribution as shown in Fig. 3. A distance to the object is calculated by triangulation based on a centroid position of the data of Fig. 3.

Referring back to Fig. 1, linear image sensor 120 is disposed at an angle based on the Scheimpflug condition. That is, a detection plane of linear image sensor 120 and a principal plane of condenser lens 118 intersect each other by a straight line, where these planes form an angle [3. A plane including laser beam 116 serves as a subject plane. In this case, a movement magnification M of image-forming spot 124 on linear image sensor 120 relative to a variation in distance between object to be measured 130 and laser displacement meter 100 is given by the following equation (1). It is noted that fo represents a focal length of condenser lens 118, and I represents a distance from an irradiation position of laser beam 116 (laser spot 132) on object to be measured 130 to condenser lens 118.

M = (fo * siny) / (1 cosfl) ...(1) In the case of this embodiment, fo = 55 mm, 1= 80 mm, = fr16, and f3 = 511/18 in the above equation (1). Movement magnification M is thus calculated as follows: M = [55 x sin(t/6))-/ (80 x cos(57x18)} = 0.53...(1A) [Configuration of Surface Shape Measurement Apparatus] Fig. 4 is a block diagram schematically showing an example of the configuration of a surface shape measurement apparatus according to the first embodiment. Referring to Fig. 4, a surface shape measurement apparatus 140 includes a table 144 on which object to be measured 130 is placed, a saddle 142, laser displacement meter 100, an X-axis drive mechanism 146X, a Y-axis drive mechanism 146Y, a Z-axis drive mechanism 146Z, and a computer 150.

Table 144 is disposed on saddle 142, and is movable in the X-axis direction. Saddle 142 is movable in the Y-axis direction. X-axis drive mechanism 146X moves table 144 in the X-axis direction. Y-axis drive mechanism 146Y moves saddle 142 in the Y-axis direction. Z-axis drive mechanism 146Z moves laser displacement meter 100 in the Z-axis direction. X-axis drive mechanism 146X, Y-axis drive mechanism 146Y and Z-axis drive mechanism 146Z function as a movement mechanism 146 for moving laser displacement meter 100 and object to be measured 130 relative to each other. Thus, laser beam 116 scans a surface of object to be measured 130 by movement mechanism 146.

It is noted that the configuration of movement mechanism 146 is not limited to the example of Fig. 4. For example, the configuration may be such that object to be measured 130 is fixed and laser displacement meter 100 is movable in three X, Y and Z directions.

Computer 150 includes a processor 152, a memory 154, as well as a display device, an input/output device and the like which are not shown. Processor 152 functions as a measurement control unit 156 and a data processing unit 158 by executing a program stored in memory 154.

Measurement control unit 156 scans laser beam 116 by controlling laser displacement meter 100 and movement mechanism 146. During this scanning of laser beam 116, measurement control unit 156 continuously measures surface shape data 166 on object to be measured 130 by means of laser displacement meter 100. Measured surface shape data 166 is stored in memory 154. Surface shape data 166 is a data series in which a scanning position on object to be measured 130 (the position irradiated with the laser beam) is associated with a displacement in the Z direction of the surface of object to be measured 130 in that scanning position.

Data processing unit 158 performs data processing on the data measured by laser displacement meter 100 (surface shape data 166) in order to remove characteristic noise included in the measured data. The details of the contents of the data processing will be described later with reference to Figs. 16 to 22.

Surface shape measurement apparatus 140 according to this embodiment is characterized by a relation between a scanning direction of laser beam 116 and an -12-orientation of laser displacement meter 100. Specifically, as shown in Fig. 1, the scanning of laser beam 116 takes place such that the light-receiving unit (linear image sensor 120) is located in front of or behind light-emitting unit 110 in the scanning direction of laser beam 116 (+X direction or -X direction in the case of Fig 1). In other words, the scanning direction of laser beam 116 is matched with the optical path plane (parallel to the X-Z plane in the case of Fig. 1) including the central axis of laser beam 116 and the optical axis of condenser lens 118.

By matching the scanning direction of laser beam 116 with the orientation of laser displacement meter 100 as described above, a characteristic error pattern such as shown in regions RA, RB and RC of Fig. 11 appears in the measured data. By extracting and removing this characteristic error pattern from surface shape data 166, therefore, the relatively high noise included in surface shape data 166 can be efficiently reduced.

It is noted that the scanning of laser beam 116 may take place such that the laser spot follows a curved trajectory on the surface of object to be measured 130. In this case, in order for the light-receiving unit to be located in front of or behind light-emitting unit 110 in the scanning direction, a drive mechanism would be needed to rotate object to be measured 130 or laser displacement meter 100 around the Z-axis (C-axis direction).

[Regarding Cause for Measurement Error of Laser Displacement Meter] Fig. 5 is a diagram showing an example of measurement results of a metal surface by means of the laser displacement meter. Specifically, Fig. 5 shows results of measurements of a displacement of a surface of a metal gauge block having a smooth surface, at intervals of 0.1 mm by means of laser displacement meter 100 A scanning direction of the laser beam is parallel to the surface of the gauge block. As shown in Fig. 5, noise as high as 36 p.m was observed in 3(3 value (a indicates standard deviation) in the measured data, whereas the gauge block has a surface roughness of about 0.06 pm. As will be described later in detail, this noise is characterized in that it cannot be removed by a time average process (a process of performing averaging by -13 -repeatedly making the same measurement).

Possible causes for an error of the laser displacement meter include electrical noise, effect of temperature variation, a motion error of movement mechanism 146 (X-axis drive mechanism 146X, Y-axis drive mechanism 146Y and Z-axis drive mechanism 146Z), an error based on non-uniformity of reflectivity of the surface of object to be measured 130, laser speckle, and the like.

The electrical noise can be improved by a time average process on measured values. The repeatability of a commercially available laser displacement meter is often indicated with a numeral after the time average process has been performed, which is submicron precision Thus, this is not a cause for the relatively high noise shown in Fig. 5.

The error due to temperature variation includes an error caused by ambient temperature, but is mainly an error caused by a thermal displacement of a measurement optical system, which occurs when an electric circuit within the laser displacement meter serves as a heat source. In the presently used laser displacement meter, a shift of 10Rm in measured value was observed after turn-on. However, a shift in measured value due to temperature variation is usually stabilized in about 30 to 60 minutes after turn-on, and thus cannot be a cause for the noise shown in Fig. 5.

The motion error of movement mechanism 146 includes an error due to a thermal displacement and a mechanical error. The error due to a thermal displacement is observed as a temporally slow variation, as with the effect of temperature within the laser displacement meter described above. The mechanical error is caused by an error in positional accuracy of movement mechanism 146 during the scanning of the laser beam for surface shape measurement. However, the mechanical error appears as a large wave as compared to the aforementioned spike-like noise, and is thus unlikely a cause for the noise shown in Fig. 5.

The motion error of movement mechanism 146 may also include an error due to vibration of a servo shaft. However, noise due to vibration of a servo shaft can be removed by a time average process as with the electrical noise, and thus is not a cause -14-for the noise shown in Fig. 5.

In view of the consideration above, it is considered that one of the causes for the noise shown in Fig. 5 is the non-uniformity of microscopic reflectivity of the surface of object to be measured 130. The non-uniformity of the reflectivity results from non-uniformity of a material, and a flaw, asperities and the like of the metal surface. Since the laser beam for measurement has the spot size, a brightness variation occurs due to the non-uniformity of the microscopic reflectivity within the laser spot, which may result in a measurement error.

Fig. 6 is a diagram illustrating a variation in image-forming spot on the linear image sensor, when the object to be measured has non-uniform reflectivity. Fig. 7 is a plan view schematically showing the surface of the object to be measured of Fig. 6. Fig. 8 is a diagram showing examples of brightness distribution detected by the linear image sensor in the cases of Figs. 6 and 7.

Referring to Figs. 6 to 8, in order to accurately determine a displacement in a height direction by a triangulation method, it is important that the brightness distribution within image-forming spot 124 on linear image sensor 120 present a Gaussian distribution so that that the center of image-forming spot 124 can be detected. When a brightness variation occurs due to the non-uniformity of the microscopic reflectivity of the surface of object to be measured 130, the brightness distribution deviates from the Gaussian distribution due to this brightness variation, which may result in a measurement error.

As shown in Figs. 6 and 7, a surface 130A of object to be measured 130 includes a region smaller than the spot size of laser spot 132 and higher in reflectivity than a surrounding region. The position of this high-reflectivity region relative to laser beam 116 moves successively from P1 to P2 to P3, as the scanning of laser beam 116 takes place in the +X direction (scanning direction) (namely, as the object to be measured moves in the -X direction).

When the high-reflectivity region is located in P1 relative to laser beam 116, as shown in Fig. 8(A), a centroid 136 of the data deviates from a center 134 of the image- -15-forming spot of image sensor UO. As a result, object to be measured 130 is measured as if it is in a position farther from light-emitting unit 110 (lower position) than the actual position.

When the high-reflectivity region is located in P2 relative to laser beam 116, as shown in Fig. 8(B), center 134 of the image-forming spot of image sensor 120 matches centroid 136 of the data. As a result, object to be measured 130 is measured as if it is in the actual position.

When the high-reflectivity region is located in P3 relative to laser beam 116, as shown in Fig. 8(C), centroid 136 of the data deviates from center 134 of the image-forming spot of image sensor 120 in a direction opposite to that of Fig. 8(A). As a result, object to be measured 130 is measured as if it is in a position closer to light-emitting unit 110 (higher position) than the actual position.

Fig. 9 is a diagram illustrating the magnitude of a measurement error caused by the non-uniformity of the reflectivity of the object to be measured. To facilitate the illustration, laser spot 132 is shown enlarged in size in Figs. 9(A) and (B). Fig. 9(A) shows a case where, as in the case of Fig. 8(A), the surface of the object to be measured is measured as if it is located farther than the actual position by a-. Fig. 9(B) shows a case where, as in the case of Fig. 8(C), the surface of the object to be measured is measured as if it is located closer than the actual position by 6+.

The magnitude of a measurement error caused by the non-uniformity of the reflectivity is determined by spot size w of laser spot 132 and a light-receiving angle y. Specifically, a maximum value of the measurement error (i.e., an error when a peripheral portion of laser spot 132 is erroneously recognized as a center) st. is given by the following equation (2): emax = W / (2 * 1a-11(7)) ...(2) The laser displacement meter used for the measurements in Fig. 5 has a spot size w = 50 nm and a light-receiving angle y = z/6. In this case, maximum value emax of the measurement error is given by the following equation (2A), which is approximately the same as the measurement error in Fig. 5. -16-

max = 25 / tan(it/6) = 43 [um]...(2A) If a laser diode is used as a light source of the triangulation method, it is considered that speckle resulting from interference of reflected light also contributes, in addition to the non-uniformity of the reflectivity, to the noise shown in Fig. 5.

Speckle is an interference phenomenon of laser light, and therefore, measured noise caused by speckle is closely related to surface roughness. Specifically, when an optical path length deviates by 2/2 relative to a laser light wavelength X, the light is darkened on image sensor 120 due to interference, whereas the light is brightened on image sensor 120 when the optical path length deviates by X. In terms of the surface asperities of the object to be measured, a brightness variation occurs on image sensor at asperities of an integral multiple of 2c/4.

The gauge block, which is the object to be measured in the case of Fig. 5, has a surface asperity of several tens of nm or less, which is approximately one-tenth the wavelength X of the laser light. In that case, the speckle does not present a high contrast state but causes a low contrast variation.

Various studies have been conducted to date on laser speckle. It is known that an average speckle diameter on an image-forming surface is given by the following equation (3) (for example, see Yasuhiko Arai and three others, "Optical Review," Vol. 41, No. 2, pp% to 104, February, 2012 (NPD 1)): a = 1.22 x (1+M) * X, * f/d...(3) In the equation (3) above, G represents an average speckle diameter on an image-forming surface, M represents a magnification of an image-forming optical system, X represents a wavelength of laser light, f represents a focal length of a lens, and d represents an aperture of the lens. The laser displacement meter used for the measurements in Fig. 5 has M = 0.53, X = 655 nm, d = 10 mm and f= 33 mm, and therefore, cy = 4.0 p.m holds.

Since each pixel of linear image sensor 120 has a width of 12 um, it is considered that the effect of average-sized speckle as calculated above is averaged over -17-the pixels of linear image sensor 120. However, speckle having a diameter larger than an average value that appears due to the non-uniformity of a particular location is not averaged in the pixels of linear image sensor 120 Accordingly, speckle noise can be a cause for the noise shown in Fig. 5 by the same mechanism as that when the reflectivity is non-uniform as was described with reference to Figs. 6 to 9. Usually, speckle noise is treated as a statistic because it is unpredictable. In a microscopic range, however, speckle noise appears deterministically with good reproducibility, and thus cannot be removed by an average process with a time filter.

[Regarding Spike-Like Noise] Results of closer observation of the spike-like noise shown in Fig. 5 will now be described.

Fig. 10 is a diagram showing results of repeated measurements of a displacement in a height direction in a microscopic section on a gauge block by means of the laser displacement meter. Fig. 10 shows measured data at each measurement point (a range of ±3a of an average value, CY represents standard deviation) after measurements were made 20 times at a sampling interval of 1 tim over a measurement range of 0.1 mm on the same gauge block as that of Fig. 5. Since the laser beam has a spot size of 50 um, the measurement interval (1 um) is sufficiently smaller than the spot size.

As shown in Fig. 10, what appeared to be random spike-like noise in Fig. 5 had excellent measurement reproducibility, and measurement results were obtained as if the surface has minute asperities. The gauge block had a surface roughness of several tens of nm, and asperities approximately 100 times the actual roughness were detected. A data variation at each measurement point is 2.3 um in 3cs value. An error at each measurement point is electrical noise, or noise caused by fluctuations in air, vibration of a servo motor or the like, and can be canceled by a time average process.

Fig. 11 is a diagram showing measurement results of the displacement in the height direction over a range of 0.4 mm on the gauge block by means of the laser displacement meter. Fig. 11 shows measured data (surface shape data) after a -18-measurement was made only once at a sampling interval of 1!Am over a measurement range of 0.4 mm on the same gauge block as that of Fig. 5.

Here, the measurement in Fig. 11 is characterized in that the scanning of laser beam 116 takes place such that the light-receiving unit (linear image sensor 120) is located in front of light-emitting unit 110 of Fig. 1 in the scanning direction. That is, the scanning direction of the laser beam is matched with the optical path plane (the X-Z plane in the case of Fig. 1) including the central axis of the laser beam and the optical axis of condenser lens 118.

As shown in Fig. 11, along with the fine noise with good reproducibility, a relatively large error occurs as shown in regions RA, RB and RC. The error seen in these regions RA, RB and RC has a characteristic shape. Specifically, as the measurement point (the scanning position of the laser beam) moves from left to right, the displacement in the height direction measured by the laser displacement meter presents a variation of initially becoming lower than an average level of the data, then rising above the average level, and finally returning to the original average level.

The variation in measured data seen in regions RA, RB and RC of Fig. 11 is the same as that described with reference to Figs. 6 to 8. That is, as the high-reflectivity region moves from right to left within the laser spot (note that this is a direction opposite to the scanning direction of the laser beam), the displacement in the height direction measured by the laser displacement meter becomes lower than the actual displacement in the first half and becomes higher than the actual displacement in the second half It should be noted that the direction of the error variation described above is when the light-receiving unit (linear image sensor 120) is located in front of light-emitting unit 110 in the scanning direction in Fig. 1. The error variation will occur in an opposite direction when the light-receiving unit (linear image sensor 120) is located behind light-emitting unit 110 in the scanning direction.

Fig. 12 is a diagram showing a maximum value of an amount of light received by the image sensor at each measurement point of Fig. 11. A vertical axis of Fig. 12 -19-represents the brightness level in a pixel with a maximum amount of received light. It can be read that the amount of received light fluctuates sharply in regions RA, RB and RC where the relatively large measurement error having the characteristic shape is observed in Fig. 11.

Fig. 13 is a diagram showing relation between the displacement in the height direction and the maximum amount of received light in region RC of Figs. 11 and 12.

Referring to Fig. 13, as the measurement point (the scanning position of the laser beam) moves, the maximum amount of received light detected by image sensor 120 gradually increases, and the centroid position of the brightness distribution on image sensor 120 deviates. As a result, the detected value from the laser displacement meter (the displacement in the height direction) varies significantly to a negative side (a section indicated with an arrow Al).

When the amount of light received by image sensor 120 reaches a maximum level, the centroid position of the brightness distribution on image sensor 120 moves as the measurement point (the scanning position of the laser beam) moves. As a result, the detected value from the laser displacement meter (the displacement in the height direction) changes from a negative value to a positive value (a section indicated with an arrow A2).

Finally, the amount of light received by image sensor 120 decreases, causing the detected value from the laser displacement meter (the displacement in the height direction) to return to the right value (average value) (a section indicated with an arrow A3).

As has been described, the characteristic phenomenon noted above can be explained by the reflection variation in the minute region (the region smaller than the laser spot size) on the surface of the object to be measured. Alternatively, this characteristic phenomenon can be explained as a case where the speckle diameter is too large to ignore relative to the spot size of the laser light.

The average speckle diameter is calculated to be 4.0 p.m as was described with the equation (3), which is smaller than the pixel width of 12 um of linear image sensor -20 -and is approximately 1/12 the spot size of 50 p.m. Thus, the effect of the speckle is averaged in each pixel, so that a variation in measured value due to the effect of the average speckle diameter is small and can be removed by an average process with a space filter.

However, errors that occur due to speckle having a large diameter that appears locally are very large and cannot be readily removed by an average process in a small area, thus greatly affecting the measured value from the laser displacement meter. Nevertheless, a range where such large-diameter speckle affects the measured data is limited to be within the range of the laser spot size, as can be seen from Fig. 10 as well.

Fig. 14 is a diagram showing a measurement result of a surface shape of a square region 0.5 mm on a side, by means of a laser beam having a spot size of 50 p.m in diameter. Fig. 15 is a diagram showing a measurement result of a surface shape of the same region as that of Fig. 14, by changing the laser spot size to 400 pm in diameter. In Figs. 14 and 15, $ indicates the diameter.

As shown in Figs. 14 and 15, when the spot size of the laser light was changed from 50 p.m in diameter to 400 pm in diameter, a maximum value of the asperities of the surface measured by the laser displacement meter increased from 67 p.m to 80 p.m.

As the laser spot size increases, errors due to speckle of an average size are averaged, thus reducing the effect of the errors. However, a range affected by large speckle that appears locally increases as the laser spot size increases, as indicated in the equation (2) above. It is thus considered that the measurement error of the laser displacement meter increases as the spot size increases.

[Regarding Method of Removing Noise] As described above, it is considered that a measurement error of the laser displacement meter is due to the local non-uniformity of the reflectivity of the object to be measured, and the speckle of a large diameter that appears occasionally. An error caused by these factors has the following characteristics.

(a) A range affected by the non-uniformity of the reflectivity and the speckle of a large diameter is determined by the laser spot size. -21 -

(b) By orientating the laser displacement meter such that the light-receiving unit (image sensor) is located in front of or behind the laser beam in the scanning direction, a characteristic error pattern appears as seen in regions RA, RB and RC of Fig. 11. Specifically, the error varies in a positive or negative direction in the first half of a section where this characteristic error pattern appears (which is referred to as a characteristic section), and the error varies in a direction opposite to that of the first half in the second half Further, absolute values of the error at points equidistant from a median value of the characteristic section are substantially equal with each other (that is, the error pattern is substantially point-symmetric with respect to the median value).

The section where such a characteristic error pattern is seen (characteristic section) is equal to or smaller than the laser spot size. According to the sampling theorem, therefore, in order to detect this error pattern (upward and downward variation in measured value), a sampling interval of the laser displacement meter needs to be V2 or less the laser spot size. In order to accurately detect a shape of the upward and downward variation in measured value, the sampling interval of the laser displacement meter is desirably 1/10 or less the spot size More desirably, the sampling interval of the laser displacement meter is 1/20 or less the spot size.

Since the characteristic error pattern described above is dominant over the measurement error of the laser displacement meter, if this error pattern can be extracted and removed by software processing, the measurement error of the laser displacement meter can be efficiently reduced. A procedure for noise removal will be described more specifically below.

Fig. 16 is a flowchart showing surface shape measurement and a procedure of processing measured data. Referring mainly now to Figs. 4 and 16, the operation of measurement control unit 156 and data processing unit 158 of Fig. 4 will be described.

First, measurement control unit 156 orients laser displacement meter 100 such that the light-receiving unit (linear image sensor 120) is located in front of or behind light-emitting unit 110 of Fig. 1 in the scanning direction of laser beam 116 (+X direction or -X direction), and causes movement mechanism 146 to scan laser beam -22 - 116. In addition, during the scanning of laser beam 116, measurement control unit 156 continuously measures a displacement of the surface of object to be measured 130 by means of laser displacement meter 100 (step S100). The measured data is stored in memory 154 as surface shape data 166. A sampling interval of surface shape data 166 needs to be 1/2 or less the spot size of the laser beam, and desirably 1/10 or less, more desirably 1/20 or less the spot size.

After the measurement by measurement control unit 156, data processing unit 158 performs data processing on surface shape data 166. As shown in Fig. 4, data processing unit 158 includes a characteristic section extraction unit 160, a data correction unit 162, and a filter processing unit 164.

First, characteristic section extraction unit 160 extracts a characteristic section where the aforementioned characteristic error pattern is observed from a measurement range of the surface shape data (step S105). As has been described, the characteristic section has a length equal to or smaller than the spot size of the light beam, and the surface shape data varies to reach a maximal value in the first half or the second half of the characteristic section and varies to reach a minimal value in the other half of the characteristic section. Referring to Figs. 17 and 18, the method of extracting the characteristic section will be described.

Fig. 17 is a diagram illustrating the method of extracting the characteristic section in step S105 of Fig. 16. Referring to Fig. 17, characteristic section extraction unit 160 of Fig. 4 successively cuts measured data MD of a section 11 equal to spot size w of the laser beam from the measurement range of the surface shape data. Then, characteristic section extraction unit 160 determines a coefficient of correlation between a waveform of cut measured data MD in a first half 12 of section II and a waveform obtained by rotating a waveform of measured data MD in a second half 13 of section II 180 degrees around a data point MP at a center of section II. If the determined coefficient of correlation exceeds a predetermined reference value, characteristic section extraction unit 160 identifies cut section Il as the characteristic section.

-23 -Fig. 18 is a diagram illustrating another method of extracting the characteristic section in step 5105 of Fig. 16. Referring to Fig. 18, characteristic section extraction unit 160 of Fig. 4 extracts, within a section 14 equal to or smaller than spot size w of the laser beam, a portion where measured data MD varies in both positive and negative directions beyond a range TB predetermined relative to an average value AV. Then, if measured data MD varies in a portion of a first half IS of section 14 in one direction (positive direction or negative direction) beyond range TH predetermined relative to average value AV of measured data MD, and measured data MD varies in a portion of a second half 16 of section 14 in a direction opposite to that of first half 15 beyond range III predetermined relative to average value AV, then characteristic section extraction unit 160 identifies this section 14 as the characteristic section.

Referring back to Figs. 4 and 16, data correction unit 162 corrects surface shape data 166 such that a variation relative to the average value of surface shape data 166 (namely, deviation of each measured value from the average value) is reduced in one or each of the plurality of extracted characteristic sections (step S110). Possible methods include, for example, correcting surface shape data 166 by utilizing the symmetry of the error pattern in each characteristic section.

Fig. 19 is a diagram illustrating the method of correcting surface shape data 166 in step S110 of Fig. 16. Referring to Fig. 19, a solid line graph represents data MD (surface shape data 166) measured in a characteristic section 17. A chain-dotted line graph represents folded data RD obtained by folding, at a border BR between a first half 18 and a second half 19 of characteristic section 17, measured data MD in the second half over the first half and measured data MD in the first half over the second half Since measured data MD and folded data RD are substantially symmetric to each other about border BR, by averaging measured data MD and folded data RD at each measurement point, corrected data AD from which the characteristic noise has been removed (a broken line in Fig. 19) can be obtained.

Specifically, data correction unit 162 corrects the surface shape data by averaging a measured value at any first measurement point in each characteristic -24 -section and a measured value at a second measurement point symmetric to the first measurement point about a midpoint of the section, and by replacing the measured values at the first and second measurement points with the obtained average value.

Alternatively, data correction unit 162 may correct surface shape data 166 by replacing the data in each characteristic section with the average value of surface shape data 166.

Referring back to Figs. 4 and 16, filter processing unit 164 performs a spatial low-pass filter process on the surface shape data corrected by data correction unit 162 (step S115). in this case, since a range where the characteristic error pattern appears is equal to or smaller than the laser spot size, a cutoff wavelength of a spatial low pass filter for noise removal may be matched with the laser spot size during the measurement. As a result, only a variation of a cycle longer than the spot size of the laser beam remains. As the spatial low-pass filter process, for example, a moving average process can be employed where the width of a moving average window is set to be equal to the spot size.

A result of performing the data processing of steps S105, S110 and S115 in Fig. 16 on the measured data of Fig. 11 will now be described.

Fig. 20 is a diagram showing a result of performing the data correction shown in step 5110 of Fig. 16 on the measured data of Fig. 11. By performing the data correction shown in step 5110, a fluctuation range (noise component) of the data was reduced from 3cy = 0.026 mm to 3cy = 0.012 mm.

Fig. 21 is a diagram showing a result of performing the low-pass filter process shown in step S115 of Fig. 16 on the data of Fig. 20. Specifically, a moving average process was performed by setting the width of the moving average window to 50 p.m which is equal to the spot size. By performing the moving average process, the fluctuation range of the data was reduced from 3cs = 0.012 mm to 3c = 0.004 mm. Fig. 22 is a diagram showing a result of performing the low-pass filter process shown in step S115 of Fig. 16 on the measured data of Fig. 11, without performing the data correction shown in step S110. Specifically, a moving average process was -25 -performed by setting the width of the moving average window to 50 pm which is equal to the spot size. In this case, the fluctuation range of the data was only reduced from 30 = 0.026 mm to 3o = 0.009 mm.

In the case of Fig. 22, if the width of the moving average window is made larger than the spot size of the laser beam, the measurement noise can be further reduced, however, the noise resulting from the characteristic error pattern cannot be completely removed. Further, increasing the width of the moving average window too much would disadvantageously result in inability to detect asperities of a level which can normally be detected.

[Conclusion of First Embodiment]

As described above, when the surface shape of the object to be measured is measured by means of the laser displacement meter with the scanning of the laser beam, a large spike-like error appears. The error has the following characteristics: (a) The error of the laser displacement meter has very good repeat reproducibility. Thus, the noise cannot be removed by averaging a plurality of measurements (time average process).

(b) By orientating the laser displacement meter such that the light-receiving unit (image sensor) is located in front of or behind the laser beam in the scanning direction, the characteristic error pattern having a point-symmetric shape appears.

(c) The range where the aforementioned characteristic error pattern appears is determined by the laser spot size.

The aforementioned error can be explained by speckle noise having a diameter too large to ignore relative to the spot size of the laser light, or the local non-uniformity of the reflectivity of the surface of the object to be measured. By extracting and removing the characteristic error pattern of the error described above, the measurement error of the laser displacement meter can be efficiently reduced. Further, since the amount of light received by the image sensor varies sharply in a portion where the error increases, the variation in the amount of light received by the image sensor can be used as an indicator of measurement reliability, and can also be used to distinguish the -26 -characteristic error pattern from the original asperities on the object to be measured. <Second Embodiment> Fig. 23 is a block diagram schematically showing the configuration of a surface shape measurement apparatus according to a second embodiment. A data processing unit 158A of Fig. 23 is different from data processing unit 158 of Fig. 4 in that includes a moving average processing unit 163 instead of data correction unit 162 and filter processing unit 164. Except for data processing unit 158, a surface shape measurement apparatus 140A of Fig. 23 is identical to surface shape measurement apparatus 140 of Fig. 4, and thus the same or corresponding parts are designated by the same reference signs and the descriptions thereof will not be repeated.

Fig. 24 is a flowchart showing an example of surface shape measurement and a procedure of processing measured data, in the surface shape measurement apparatus according to the second embodiment. In the data processing procedure of Fig. 24, step S120 is executed instead of steps 5110 and S115 of Fig. 16. Steps S100 and S105 are the same as in Fig. 16 and thus the descriptions thereof will not be repeated.

In the example of Fig. 24, moving average processing unit 163 of Fig. 23 performs a moving average on surface shape data 166 using a variable window width (step S120). The width of the moving average window is greater than the spot size of the light beam. When performing the moving average in a section where the characteristic error pattern as seen in regions RA, RB and RC of Fig. 11 is observed (referred to as a characteristic section), moving average processing unit 163 sets the width of the moving average window to be greater than that when performing the moving average in a section including no characteristic section. For example, the width of the moving average window when the moving average is performed in a section including the characteristic section is set to be five times or more the spot size.

Fig. 25 is a flowchart showing another example of surface shape measurement and a procedure of processing measured data, in the surface shape measurement apparatus according to the second embodiment. In the data processing procedure of Fig. 24, step 5125 is executed instead of steps S110 and S115 of Fig. 16. Steps S100 -27 -and S105 are the same as in Fig. 16 and thus the descriptions thereof will not be repeated.

In the example of Fig. 25, moving average processing unit 163 of Fig. 23 performs a weighted moving average on surface shape data 166 (step S125). The window width of the weighted moving average is greater than the spot size of the light beam, and is set to be five times or more the spot size, for example. Moving average processing unit 163 sets a weight at a measurement point in the characteristic section where the characteristic error pattern as seen in regions RA, RB and RC of Fig. 11 is observed to be smaller than a weight at a measurement point outside the characteristic section.

As described above, by suppressing the data variation in the characteristic section more than the data variation outside the characteristic section, the error included in the laser displacement meter can be efficiently removed.

<Third Embodiment> A third embodiment discloses a machine tool including the surface shape measurement apparatus in the first or second embodiment. Although the machine tool will be described as a vertical machining center below, the machine tool may be of another type such as a horizontal machining center or a lathe.

Fig. 26 is a perspective view schematically showing the configuration of the machine tool according to the third embodiment. Referring to Fig. 26, a machine tool includes a machining device 10, an NC (Numerical Control) device 24, an ATC (Automatic Tool Changer) 28, and computer 150 Machining device 10 includes a bed 12, a column 14 placed on bed 12, a spindle head 20 having a spindle 22, and a saddle 16 having a table 18.

Spindle head 20 is supported on a front surface of column 14, and is movable in a vertical direction (Z-axis direction). A tool (not shown) or a measurement head 42 is detachably attached to a tip of spindle 22. Spindle 22 has a central axis line (CL in Fig. 2) parallel to the Z axis, and is supported on spindle head 20 rotatably around the central axis line. Measurement head 42 includes therein laser displacement meter 100 -28 -shown in Figs. 4 and 23, a control circuit and a driving battery for this laser displacement meter, and a communication device for conducting wireless communication.

Saddle 16 is disposed on bed 12 and movable forward or backward in a horizontal direction (Y-axis direction). Table 18 is disposed on saddle 16. Table 18 is movable to the right or left in a horizontal direction (X-axis direction). A work 2 is placed on table 18. Saddle 16 corresponds to saddle 142 of Figs. 4 and 23, and table 18 corresponds to table 144 of Figs 4 and 23 Work 2 corresponds to object to be measured 130 of Figs. 4 and 23.

Machining device 10 is a machining center that performs three-axis control of linearly moving measurement head 42 and work 2 relative to each other in directions of three orthogonal X, Y and Z axes. Unlike the configuration of Fig. 1, machining device 10 may be configured to move spindle head 20 supporting measurement head 42 in the X-axis and Y-axis directions relative to work 2.

NC device 24 controls the entire operation of machining device 10 including the three-axis control noted above. ATC (Automatic Tool Changer) 28 automatically changes the tool and measurement head 42 for spindle 22. ATC 28 is controlled by NC device 24.

Fig. 27 is a block diagram showing a functional configuration of a portion, which relates to a surface shape measurement apparatus, of the machine tool of Fig. 26.

Fig. 27 shows a Z-axis feed mechanism 34, a Y-axis feed mechanism 32 and an X-axis feed mechanism 30 provided on machining device 10.

Referring to Figs. 26 and 27, Z-axis feed mechanism 34 drives and moves spindle head 20 supported on column 14 in the Z-axis direction. Y-axis feed mechanism 32 drives and moves saddle 16 disposed on head 12 in the Y-axis direction.

X-axis feed mechanism 30 drives and moves table 18 placed on saddle 16 and supporting work 2 in the X-axis direction. NC device 24 controls each of Z-axis feed mechanism 34, Y-axis feed mechanism 32 and X-axis feed mechanism 30. X-axis feed mechanism 30, Y-axis feed mechanism 32 and Z-axis feed mechanism 34 -29 -correspond to X-axis drive mechanism 146X, Y-axis drive mechanism 146Y and Z-axis drive mechanism 146Z of Figs. 4 and 23, respectively.

Computer 150 includes processor 152, memory 154, a communication device 168 for conducting wireless communication with measurement head 42, and the like. Processor 152 functions as measurement control unit 156 and data processing units 158, 158A described in Figs. 4 and 23 by executing a program stored in memory 154.

Measurement control unit 156 cooperates with NC device 24 to continuously change the positional relation between measurement head 42 and work 2, whereby laser beam 116 scans along a surface of work 2. During the scanning of laser beam 116, measurement control unit 156 obtains displacement data in a height direction (Z-axis direction) at a plurality of measurement points in the scanning direction of laser beam 116 from measurement head 42 as surface shape data on work 2. A specific procedure is as follows.

First, based on the control by measurement control unit 156, NC device 24 drives one of X-axis feed mechanism 30 and Y-axis feed mechanism 32, or at least two of X-axis feed mechanism 30, Y-axis feed mechanism 32 and Z-axis feed mechanism 34, to continuously change the positional relation between measurement head 42 and work 2. Here, the laser displacement meter is oriented such that the light-receiving unit of the laser displacement meter is located in front of or behind the light-emitting unit of the laser displacement meter in the scanning direction of laser beam 116.

In synchronization with the driving of the feed mechanisms noted above, a PLC (Programmable Logic Controller) 26 included in NC device 24 outputs a trigger signal to communication device 168 at regular intervals. In response to the trigger signal, communication device 168 transmits a measurement command f to measurement head 42, and measurement head 42 measures a distance D from measurement head 42 to work 2 (namely, a displacement of the surface of work 2) in accordance with measurement command f Data F on measured distance D is transmitted from measurement head 42 to measurement control unit 156 via communication device 168 PLC 26 also obtains position information about X-axis feed mechanism 30, Y- -30-axis feed mechanism 32 and Z-axis feed mechanism 34 at the timing of the distance measurement by measurement head 42 described above, to detect data on the position of measurement head 42. PLC 26 transmits the detected position data on measurement head 42 to measurement control unit 156.

Based on the position data on measurement head 42 obtained from PLC 26 and data F on distance D obtained from measurement head 42, measurement control unit 156 stores the displacement data in the height direction (Z-axis direction) at each measurement point along the scanning direction of laser beam 116 in memory 154 as surface shape data 166.

Processor 152 also functions as data processing units 158, 158A that perform data processing for removing noise included in surface shape data 166 noted above. The operation of data processing units 158, 158A is as described in the first and second embodiments. An error included in surface shape data 166 can be efficiently reduced by data processing units 158, 158A It should be understood that the embodiments disclosed herein are illustrative and non-restrictive in every respect. The scope of the present invention is defined by the terms of the claims, rather than the description above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims. REFERENCE SIGNS LIST 2 work; 10 machining device; 16, 142 saddle; 18, 144 table; 24 NC device; 30 X-axis feed mechanism; 32 Y-axis feed mechanism; 34 Z-axis feed mechanism; 42 measurement head; 100 laser displacement meter; 110 light-emitting unit; 112 laser diode; 114 lens; 116 laser beam; 118 condenser lens (optical system); 120 linear image sensor (light-receiving unit); 130 object to be measured; 132 laser spot; 140, 140A surface shape measurement apparatus; 146 movement mechanism; 146X X-axis drive mechanism; 146Y Y-axis drive mechanism; 146Z Z-axis drive mechanism; 150 computer, 152 processor, 154 memory, 156 measurement control unit; 158, 158A data processing unit; 160 characteristic section extraction unit; 162 data correction unit, 163 moving average processing unit, 164 filter processing unit; 166 surface shape data, 168 -31 -communication device, 200 machine tool -32 -

Claims (10)

1. CLAIMS1. A surface shape measurement apparatus, comprising: a displacement meter, the displacement meter including a light-emitting unit that emits a light beam toward an object to be measured, an optical system that collects scattered light of the light beam from the object to be measured, and a light-receiving unit that detects a focusing position of the light collected by the optical system, the displacement meter measuring a displacement of a surface of the object to be measured based on the focusing position of the light on the light-receiving unit; a movement mechanism that scans the light beam by moving the displacement meter and the object to be measured relative to each other; and a measurement control unit that controls the movement mechanism and the displacement meter, the measurement control unit being configured to: cause the movement mechanism to scan the light beam such that the light-receiving unit is located in front of or behind the light-emitting unit in a scanning direction of the light beam; and during the scanning of the light beam, continuously measure a variation in the displacement of the surface of the object to be measured by means of the displacement meter as surface shape data.
2. 2. The surface shape measurement apparatus according to claim 1, further comprising a characteristic section extraction unit that extracts a characteristic section from a measurement range of the surface shape data, wherein the characteristic section has a length equal to or smaller than a spot size of the light beam, and the surface shape data varies to reach a maximal value in a first half or a second half of the characteristic section, and varies to reach a minimal value in the other half of the characteristic section.

   -33 -
3. 3. The surface shape measurement apparatus according to claim 2, wherein the characteristic section has a length equal to or smaller than a spot size of the light beam and satisfies at least one predetermined condition, the at least one predetermined condition including a condition that the surface shape data varies in a portion of the first half of the characteristic section in one direction beyond a range predetermined relative to an average value of the surface shape data, and the surface shape data varies in a portion of the second half of the characteristic section in a direction opposite to that of the first half beyond the range predetermined relative to the average value.
4. 4. The surface shape measurement apparatus according to claim 2, wherein the characteristic section has a length equal to a spot size of the light beam and satisfies at least one predetermined condition, the at least one predetermined condition including a condition that a coefficient of correlation between a waveform of the surface shape data in the first half of the characteristic section and a waveform obtained by rotating a waveform of the surface shape data in the second half of the characteristic section 180 degrees around a data point at a center of the characteristic section exceeds a predetermined reference value.
5. 5. The surface shape measurement apparatus according to any one of claims 2 to 4, further comprising a data correction unit that corrects the surface shape data such that a variation in the surface shape data relative to the average value of the surface shape data is reduced in the extracted characteristic section or each of a plurality of extracted characteristic sections.
6. 6. The surface shape measurement apparatus according to claim 5, wherein the data correction unit corrects the surface shape data by averaging a measured value at any first measurement point in each of the characteristic sections and a -34 -measured value at a second measurement point symmetric to the first measurement point about a midpoint of the section, and by replacing each of the measured values at the first and second measurement points with the obtained average value.
7. 7. The surface shape measurement apparatus according to claim 5, wherein the data correction unit corrects the surface shape data by replacing the data in each the characteristic section with the average value of the surface shape data
8. 8. The surface shape measurement apparatus according to any one of claims 5 to 7, further comprising a filter processing unit that performs a low-pass filter process on the surface shape data corrected by the data correction unit, the low-pass filter process leaving only a variation of a cycle longer than the spot size of the light beam.
9. 9. The surface shape measurement apparatus according to any one of claims 2 to 4, further comprising a moving average processing unit that performs a moving average on the surface shape data using a moving average window of a variable width, wherein the width of the moving average window is greater than the spot size of the light beam, and the width of the moving average window when the moving average is performed in a section including the characteristic section is greater than the width of the moving average window when the moving average is performed in a section including no characteristic section.10 The surface shape measurement apparatus according to any one of claims 2 to 4, further comprising a moving average processing unit that performs a weighted moving average on the surface shape data, wherein a weighted moving average window has a width greater than the spot size of the light beam, and -35 -a weight at a measurement point in the characteristic section is smaller than a weight at a measurement point outside the characteristic section.11. A machine tool, comprising the surface shape measurement apparatus according to any one of claims 1 to
10. 10.12. A surface shape measurement method using a non-contact displacement meter, the displacement meter including a light-emitting unit that emits a light beam toward an object to be measured, an optical system that collects scattered light of the light beam from the object to be measured, and a light-receiving unit that detects a focusing position of the light collected by the optical system, the surface shape measurement method comprising the steps of: moving the displacement meter relative to the object to be measured along a scanning direction, while maintaining positional relation between the light-receiving unit and the light-emitting unit such that the light-receiving unit is located in front of or behind the light-emitting unit in the scanning direction; and determining a surface shape of the object to be measured based on a variation in measured value from the displacement meter resulting from the relative movement of the displacement meter.13. The surface shape measurement method according to claim 12, wherein the step of determining a surface shape includes the step of extracting a characteristic section from a measurement range of the surface shape of the object to be measured, the characteristic section has a length equal to or smaller than a spot size of the light beam, and measured data on the surface shape of the object to be measured varies to reach a maximal value in a first half or a second half of the characteristic section, and varies to reach a minimal value in the other half of the characteristic section. -36-14. The surface shape measurement method according to claim 13, wherein the step of determining a surface shape further includes the step of correcting the measured data on the surface shape of the object to be measured such that a deviation of each measured value relative to an average value of the measured data is reduced in the extracted characteristic section or each of a plurality of extracted characteristic sections. -37-