Patent Description:
As a shape measurement apparatus that measures a shape of an object to be measured, for example, a three-dimensional coordinate measurement apparatus that obtains a shape of an object to be measured by detecting three-dimensional coordinate values of various measurement points of the object to be measured using an optical probe in a noncontact manner is known. The optical probe radiates measurement light from a light source toward the measurement points of the object to be measured and receives reflected light of the measurement light reflected at the measurement points. Then, the shape measurement apparatus measures a shape of the object to be measured based on a result of detection of distances from the optical probe to the measurement points by a known measurement method using an interferometer.

As such an optical probe, an optical rotation probe capable of performing rotational scanning around the longitudinal axis of the probe as disclosed in <CIT> is known. In a three-dimensional coordinate measurement apparatus equipped with the optical rotation probe can, for example, measure an inner shape of a cylindrical object to be measured and concurrently measure a surface shape by performing rotational scanning using the optical rotation probe. <CIT> relates to a probe for optical imaging and an optical measuring device. <CIT> relates to an interchangeable reflective assembly for a chromatic range sensor optical pen.

In recent years, there is a need for measurement of a shape of a local portion with a great curvature in shape measurement of an object to be measured. However, it is difficult with the three-dimensional measurement apparatus disclosed in <CIT> to accurately set an emission direction of measurement light of the optical rotation probe with respect to a reference direction in calibration, and there is a problem that a measurement error becomes large in a case where such a shape of a local portion with a great curvature is measured.

It is an object of the present invention to provide a calibration method for an optical rotation probe capable of calibrating an emission direction of measurement light emitted from an optical rotation probe with high accuracy. According to the present invention said object is solved by a calibration method having the features of the independent claim <NUM>. Preferred embodiments are laid down in the dependent claims.

According to the calibration method for the optical rotation probe according to the preferred embodiment, the adjustment error of the emission direction of the measurement light with respect to the reference direction can be calculated, so that even in the case where an adjustment error remains after calibration of the emission direction, the emission direction can be recalibrated based on the calculated adjustment error. This can improve calibration accuracy of the emission direction of the optical rotation probe and eventually reduce a measurement error in a three-dimensional coordinate measurement apparatus equipped with the optical rotation probe.

According to the presently disclosed subject matter, an adjustment error with respect to a reference direction can be calculated for an emission direction of measurement light emitted from an optical rotation probe, so that it is possible to perform calibration of the optical rotation probe with high accuracy.

Embodiments according to the presently disclosed subject matter will be described below in accordance with the accompanying drawings. Prior to description of a calibration method for an optical rotation probe, a configuration of a three-dimensional coordinate measurement apparatus will be described.

<FIG> is a schematic view of a three-dimensional coordinate measurement apparatus <NUM>. An X axis, a Y axis and a Z axis in <FIG> are a machine coordinate system defined based on a machine coordinate origin specific to the three-dimensional coordinate measurement apparatus <NUM>.

As illustrated in <FIG>, the three-dimensional coordinate measurement apparatus <NUM> performs shape measurement of a workpiece W, for example, shape measurement of an inner periphery of the cylindrical workpiece W using an optical rotation probe <NUM>. A shape of the workpiece W described here may include a three-dimensional shape, a two-dimensional shape, a surface shape, a profile shape, various kinds of dimensional shapes such as a length and a diameter, and the like of the workpiece W. Further, a shape and a type of the workpiece W to be measured is not particularly limited.

The three-dimensional coordinate measurement apparatus <NUM> includes a mount <NUM>, a table <NUM> (surface plate) provided on the mount <NUM>, a right Y carriage 16R and a left Y carriage <NUM> provided to stand on both end portions of the table <NUM>, and an X guide <NUM> that connects an upper portion of the right Y carriage 16R and an upper portion of the left Y carriage <NUM>. The right Y carriage 16R, the left Y carriage <NUM> and the X guide <NUM> constitute a portal frame <NUM>.

Sliding surfaces on which the right Y carriage 16R and the left Y carriage <NUM> slide along the Y axis direction are formed on upper surfaces and lateral surfaces of the both end portions of the X axis direction of the table <NUM>. Air bearings (not illustrated) may be provided at positions facing the sliding surfaces of the table <NUM>, in the right Y carriage 16R and the left Y carriage <NUM>. This allows the right Y carriage 16R and the left Y carriage <NUM> to move in the Y axis direction along with the X guide <NUM>.

An X carriage <NUM> is attached to the X guide <NUM>. A sliding surface on which the X carriage <NUM> slides is formed along the X axis direction on the X guide <NUM>. Further, an air bearing (not illustrated) is provided at a position facing the sliding surface of the X guide <NUM>, in the X carriage <NUM>. This allows the X carriage <NUM> to move in the X axis direction.

A Z carriage <NUM> (also referred to as a Z spindle) is attached to the X carriage <NUM>. Further, an air bearing (not illustrated) for guide in the Z axis direction for guiding the Z carriage <NUM> in the Z axis direction is provided at the X carriage <NUM>. By this means, the Z carriage <NUM> is held so as to be movable in the Z axis direction by the X carriage <NUM>. A measurement head <NUM> that selectively and detachably holds various kinds of known probes including the optical rotation probe <NUM> is provided at a lower end of the Z carriage <NUM>.

While not illustrated, a Y axis driving unit that moves the portal frame <NUM> in the Y axis direction, an X axis driving unit that moves the X carriage <NUM> in the X axis direction, and a Z axis driving unit that moves the Z carriage <NUM> in the Z axis direction may be provided in the three-dimensional coordinate measurement apparatus <NUM>. This enables the measurement head <NUM> (optical rotation probe <NUM>) to move in three-axis directions (XYZ axis directions) that are orthogonal to one another.

A Y axis linear scale (not illustrated) is provided at an end portion on the right Y carriage 16R side of the table <NUM>. Further, an X axis linear scale (not illustrated) is provided at the X guide <NUM>, and a Z axis linear scale (not illustrated) is provided at the Z carriage <NUM>.

On the other hand, a Y axis detecting unit (not illustrated) that reads the Y axis linear scale is provided at the right Y carriage 16R. Further, an X axis detecting unit (not illustrated) and a Z axis detecting unit (not illustrated) that respectively read the X axis linear scale and the Z axis linear scale are provided at the X carriage <NUM>. Detection results of the respective detecting units are output to a control apparatus <NUM> via a controller <NUM>.

A head driving unit (not illustrated) such as a motor that rotates the optical rotation probe <NUM> in each of a direction around a rotation axis parallel to the Z axis direction and a direction around a rotation axis perpendicular to the Z axis is provided at the measurement head <NUM>. By this means, the measurement head <NUM> can steplessly adjust a rotation angle of the optical rotation probe <NUM> in directions around two rotation axes. This results in enabling a posture of the optical rotation probe <NUM> to be arbitrarily displaced (rotated).

A probe rotation angle detecting unit (not illustrated) such as a rotary encoder that detects each rotation angle of the optical rotation probe <NUM> may be provided at the measurement head <NUM>. A detection result by the probe rotation angle detecting unit is output to the control apparatus <NUM> via the controller <NUM>.

The optical rotation probe <NUM> is detachably attached to the measurement head <NUM>. The optical rotation probe <NUM> emits measurement light LA input from a wavelength swept light source <NUM> via an optical fiber cable <NUM> and a fiber circulator <NUM>, toward a measurement surface (here, an inner periphery) of the workpiece W. Further, the optical rotation probe <NUM> receives reflected light LB reflected on the measurement surface of the workpiece W and outputs the reflected light LB and reference light LC (see <FIG>) which will be described later to a photodetector <NUM> via the fiber circulator <NUM> and an optical fiber cable <NUM>.

Further, while described in detail later, the optical rotation probe <NUM> is configured such that a tip portion (rotating optical system <NUM>, see <FIG>) of the optical rotation probe <NUM> is rotatable in a direction around a longitudinal axis 62a (corresponding to a probe axis in the presently disclosed subject matter, see <FIG>) which will be described later. This enables the optical rotation probe <NUM> to perform rotational scanning with the measurement light LA along the measurement surface of the workpiece W by rotating its tip portion.

<FIG> is a cross-sectional diagram of the optical rotation probe <NUM>. As illustrated in <FIG>, the optical rotation probe <NUM> includes a fixed optical system <NUM> fixed at the measurement head <NUM>, and the rotating optical system <NUM> rotating in a direction around the longitudinal axis 62a of the optical rotation probe <NUM> by the fixed optical system <NUM>.

The fixed optical system <NUM> includes an optical fiber cable <NUM>, a head attaching portion <NUM>, a collimator lens <NUM>, and a hollow motor <NUM>.

As the optical fiber cable <NUM> (the same applies to other optical fiber cables <NUM> and <NUM>), known various kinds of optical fiber cables such as a single mode optical fiber cable and a polarization maintaining optical fiber cable are used.

One end side of the optical fiber cable <NUM> is inserted inside the measurement head <NUM> and inside of the Z carriage <NUM> and is connected to the fiber circulator <NUM>. Further, the other end side of the optical fiber cable <NUM> is connected to the head attaching portion <NUM>. An end surface on the other end side of the optical fiber cable <NUM> serves as an emission/incident end 44a from which the measurement light LA input from the wavelength swept light source <NUM> via the optical fiber cable <NUM>, and the like, is emitted and to which the reflected light LB, and the like, reflected on the measurement surface of the workpiece W are incident. A reference character P in the drawing designates an optical path of the measurement light LA and the reflected light LB.

Further, part of the measurement light LA input from the wavelength swept light source <NUM>, or the like, to the optical fiber cable <NUM> is reflected at the emission/incident end 44a as reference light LC (see <FIG>).

The head attaching portion <NUM> is a hollow cylindrical body extending in a direction parallel to the optical path P (longitudinal axis 62a). One end side of the head attaching portion <NUM> is detachably attached to the measurement head <NUM> described above. Further, the hollow motor <NUM> is fixed on the other end side of the head attaching portion <NUM>. Still further, a cable connecting portion 46a to which the other end side of the optical fiber cable <NUM> is connected is provided on the one end side of the head attaching portion <NUM>. The cable connecting portion 46a holds the emission/incident end 44a of the optical fiber cable <NUM> inside the head attaching portion <NUM> and at a position at which the emission/incident end 44a matches (including substantially matches, the same will apply below) a central axis of the head attaching portion <NUM>.

The collimator lens <NUM> is provided inside the head attaching portion <NUM> and at a position between the emission/incident end 44a and the hollow motor <NUM>. An optical axis of the collimator lens <NUM> matches a central line of the optical path P. The collimator lens <NUM> converts the measurement light LA emitted from the emission/incident end 44a into parallel light and then emits the measurement light LA toward an imaging lens <NUM> inside a shaft <NUM> which will be described later. This can prevent degradation of photosensitivity of the reflected light LB due to misalignment between the fixed optical system <NUM> and the rotating optical system <NUM>. Further, the collimator lens <NUM> emits the reflected light LB incident from the imaging lens <NUM> toward the emission/incident end 44a.

The hollow motor <NUM> rotates the shaft <NUM> which will be described later in a direction around the longitudinal axis 62a (hereinafter, simply referred to as a direction around the longitudinal axis). The hollow motor <NUM> includes a hollow stator <NUM> (also referred to as a stator) constituted by a coil (not illustrated) being wound, and a hollow rotor <NUM> (also referred to as a rotor) rotating in a direction around the longitudinal axis inside the stator <NUM>. A detailed structure of the hollow motor <NUM> is a known technique, and thus, detailed description will be omitted.

A hollow portion 54a through which the optical path P passes and extending in a direction parallel to the optical path P (the longitudinal axis 62a) is formed in the rotor <NUM>. By this means, the measurement light LA emitted from the collimator lens <NUM> passes through inside the hollow portion 54a and is incident on the imaging lens <NUM> which will be described later, and the reflected light LB emitted from the imaging lens <NUM> passes through inside the hollow portion 54a and is incident on the collimator lens <NUM>.

The rotor <NUM> rotates around the longitudinal axis 62a in accordance with application of a drive signal (voltage) to the stator <NUM> from the controller <NUM>. A rotor rotation angle detecting unit (not illustrated) such as a rotary encoder that detects each rotation angle of the rotor <NUM> may be provided in the hollow motor <NUM>. A detection result by the rotor rotation angle detecting unit is output to the control apparatus <NUM> via the controller <NUM>. The rotation angle of the rotor <NUM> may be made detectable by, for example, controlling the rotation angle, and the like, of the rotor <NUM> by known servo control instead of using the rotor rotation angle detecting unit.

Further, a shaft holding plate <NUM> which will be described later, included in the rotating optical system <NUM> is fixed at a circular tip surface 54b on a side facing the rotating optical system <NUM>, of the rotor <NUM>.

The hollow motor <NUM> may not be limited to the configuration (structure) illustrated in <FIG>, and known various kinds of hollow motors may be used instead.

The rotating optical system <NUM> rotates in a direction around the longitudinal axis in accordance with rotation of the rotor <NUM>. The rotating optical system <NUM> includes the shaft holding plate <NUM>, the shaft <NUM>, the imaging lens <NUM>, and a right angle prism mirror <NUM>.

The shaft holding plate <NUM> is formed in a shape (circular shape) substantially the same as a shape of the tip surface 54b of the rotor <NUM> and fixed on the tip surface 54b in a posture parallel to the tip surface 54b. A fitting hole through which the optical path P passes and which extends in a direction parallel to the optical path P (longitudinal axis 62a) is formed at the shaft holding plate <NUM>. One end portion of the shaft <NUM> is fitted into the fitting hole. By this means, the shaft holding plate <NUM> holds the shaft <NUM> in a state where the longitudinal axis 62a matches the central line of the optical path P.

The shaft <NUM>, which is a hollow cylinder extending in a direction parallel to the optical path P, has the longitudinal axis 62a parallel to the optical path P. Further, in a state where one end portion of the shaft <NUM> is fixed at the shaft holding plate <NUM>, the longitudinal axis 62a matches (substantially matches) the central line of the optical path P, and an inner surface 62b of the shaft <NUM> encloses the optical path P.

Further, the imaging lens <NUM> is provided inside the shaft <NUM> and on the other end portion that is an opposite side of the one end portion described above of the shaft <NUM>. Further, the right angle prism mirror <NUM> is provided at the other end portion of the shaft <NUM> so as to cover an opening portion on the other end side of the shaft <NUM>.

The imaging lens <NUM> is disposed at a position at which the optical axis of the imaging lens <NUM> matches the central line of the optical path P. The imaging lens <NUM> causes an image of the measurement light LA incident from the collimator lens <NUM> to be formed on the measurement surface of the workpiece W through the right angle prism mirror <NUM>. Further, the imaging lens <NUM> emits the reflected light LB incident through the right angle prism mirror <NUM> toward the collimator lens <NUM>.

The right angle prism mirror <NUM> reflects the measurement light LA incident through inside of the shaft <NUM> and the imaging lens <NUM> toward the measurement surface of the workpiece W. Specifically, the right angle prism mirror <NUM> refracts the measurement light LA incident from the imaging lens <NUM> at <NUM>° (including substantially <NUM>°) to obtain a light flux parallel to a rotation plane (plane perpendicular to the longitudinal axis 62a (rotation axis)) of the right angle prism mirror <NUM>, or the like, and then, emits the light flux toward the measurement surface of the workpiece W.

Further, the right angle prism mirror <NUM> reflects the reflected light LB reflected on the measurement surface of the workpiece W toward the imaging lens <NUM>. By this means, the reflected light LB is incident on the emission/incident end 44a of the optical fiber cable <NUM> from the right angle prism mirror <NUM> through the collimator lens <NUM>.

The shaft holding plate <NUM>, the shaft <NUM>, the imaging lens <NUM> and the right angle prism mirror <NUM> included in the rotating optical system <NUM> integrally rotate in a direction around the longitudinal axis in accordance with rotation of the rotor <NUM>. Then, as a result of the right angle prism mirror <NUM> rotating in the direction around the longitudinal axis, rotational scanning is performed with the measurement light LA along the measurement surface of the workpiece W.

Returning to <FIG>, in a case where the three-dimensional coordinate measurement apparatus <NUM> is in a manual measurement mode, the controller <NUM> drives the respective driving units (the XYZ driving units and the head driving unit) (not illustrated) in response to operation input to an operating unit (not illustrated) to displace a position and a posture of the optical rotation probe <NUM> and drives the hollow motor <NUM> to rotate the right angle prism mirror <NUM>, and the like, in the direction around the longitudinal axis. Further, in a case where the three-dimensional coordinate measurement apparatus <NUM> is in an automatic measurement mode, the controller <NUM> drives the respective driving units and the hollow motor <NUM> under control by the control apparatus <NUM> to displace the position and the posture of the optical rotation probe <NUM> and rotate the right angle prism mirror <NUM>, and the like, in the direction around the longitudinal axis.

Further, the respective detecting units (not illustrated) (the XYZ axis detecting units, the probe rotation angle detecting unit and the rotor rotation angle detecting unit) described above are connected to the controller <NUM>, and signals, and the like, output from the respective detecting units are output to the control apparatus <NUM>.

<FIG> is an explanatory diagram for explaining shape measurement of the measurement surface of the workpiece W using the optical rotation probe <NUM>. As illustrated in <FIG>, and <FIG> and <FIG> described above, the wavelength swept light source <NUM> emits the measurement light LA to the fiber circulator <NUM> via the optical fiber cable <NUM>. The measurement light LA is wavelength swept light having a wavelength varying in a fixed wavelength band in a sine wave in a fixed wavelength sweep cycle (with a fixed wavelength sweep frequency).

The fiber circulator <NUM> is connected to the wavelength swept light source <NUM> via the optical fiber cable <NUM>, is connected to the photodetector <NUM> via the optical fiber cable <NUM> and is connected to the optical fiber cable <NUM> of the optical rotation probe <NUM>.

The fiber circulator <NUM>, which is, for example, a non-reciprocating type and one-direction type device having three ports, outputs the measurement light LA input from the wavelength swept light source <NUM> via the optical fiber cable <NUM> to the optical fiber cable <NUM>. By this means, the measurement light LA is input to the optical rotation probe <NUM> from the wavelength swept light source <NUM>. As a result, the reflected light LB reflected on the measurement surface of the workpiece W and the reference light LC reflected at the emission/incident end 44a are input to the fiber circulator <NUM> via the optical fiber cable <NUM>.

Further, the fiber circulator <NUM> outputs an interference signal SG of the reflected light LB and the reference light LC input from the optical rotation probe <NUM> to the photodetector <NUM> via the optical fiber cable <NUM>.

As the photodetector <NUM>, for example, a silicon photo diode, an indium gallium arsenic (InGaAs) photodiode, a photoelectric tube, a photo multiplier tube, or the like, is used. The photodetector <NUM> converts the interference signal SG input from the fiber circulator <NUM> via the optical fiber cable <NUM> to obtain an electrical signal and amplifies and outputs the electrical signal to the control apparatus <NUM> under control by the control apparatus <NUM>.

The control apparatus <NUM> includes, for example, an arithmetic apparatus such as a personal computer and includes an arithmetic circuit constituted with various kinds of processors and a memory, and the like. Various kinds of processors include a central processing unit (CPU), a graphics processing unit (GPU), an application specific integrated circuit (ASIC), a programmable logic device (for example, simple programmable logic devices (SPLD), a complex programmable logic device (CPLD) and a field programmable gate arrays (FPGA)), and the like. Here, various kinds of functions of the control apparatus <NUM> may be implemented by one processor or may be implemented by processors of the same type or different types.

For example, during the automatic measurement mode, the control apparatus <NUM> drives the respective driving units (not illustrated) described above and the hollow motor <NUM> in accordance with a measurement program determined in advance to execute displacement of the position and the posture of the optical rotation probe <NUM> and rotation of the right angle prism mirror <NUM>, and the like, in the direction around the longitudinal axis. By this means, the optical rotation probe <NUM> executes rotational scanning of the measurement surface of the workpiece W with the measurement light LA, and the photodetector <NUM> detects the interference signal SG for each of measurement points on the measurement surface. As a result, a detection result of the interference signal SG for each of the measurement points is input to the control apparatus <NUM> from the photodetector <NUM>. The control apparatus <NUM> may cause rotational scanning with the measurement light LA described above to be executed in response to operation input with respect to the operating unit (not illustrated) during the manual measurement mode.

Further, the control apparatus <NUM> calculates a distance L2 from the right angle prism mirror <NUM> to the measurement point on the measurement surface of the workpiece W for each of the measurement points based on the detection result of the interference signal SG for each of the measurement points detected by the photodetector <NUM> in both the automatic measurement mode and the manual measurement mode.

Specifically, the control apparatus <NUM> calculates for each of the measurement points, a total value of distances (L1 + L2) of a distance L1 from the emission/incident end 44a to the right angle prism mirror <NUM> and the distance L2 for each of the measurement points described above based on the detection result of the interference signal SG for each of the measurement points. Here, a calculation method of the total value of the distances is a known technique (for example, <CIT> and <CIT>), and thus, specific description will be omitted here.

Then, the control apparatus <NUM> calculates the distance L2 for each of the measurement points based on the calculation result of the total value of the distances for each of the measurement points and the known distance L1. Then, the control apparatus <NUM> calculates a three-dimensional coordinate for each of the measurement points based on the position and the posture of the optical rotation probe <NUM> for each of the measurement points, the rotation angle in the direction around the longitudinal axis of the right angle prism mirror <NUM>, and the like, for each of the measurement points, and the calculation result of the distance L2 for each of the measurement points. As a result of this, the control apparatus <NUM> can calculate a shape of the measurement surface of the workpiece W scanned with the measurement light LA.

Here, the optical rotation probe illustrated in <FIG> and <FIG> is merely an example, and of course, does not intend to limit the configuration of the optical rotation probe.

A calibration method for the optical rotation probe <NUM> according to the first embodiment will be described next using <FIG> which are related examples which are not claimed but are useful to understand the invention. <FIG> is a flowchart indicating outline of a calibration method of the emission direction of the measurement light LA. While description will be provided below using a case where the X axis direction is set as a reference direction that is a reference of emission, the description does not intend to limit the reference direction.

First, as illustrated in <FIG> which is a related example which is not claimed but is useful to understand the invention, the reference direction of emission of the measurement light LA is set. Here, the reference direction of emission of the measurement light LA with respect to the X axis direction on the XY plane, in other words, a direction in which the rotation angle s of the optical rotation probe <NUM> with respect to the X axis direction becomes <NUM>° is set (step S10).

The reference direction of emission of the measurement light LA is preferably set using, for example, a beam profiler <NUM>. This enables setting of the emission direction of the measurement light LA with high accuracy. A method for setting the reference direction of emission on the XY plane using the beam profiler <NUM> will be described below with reference to <FIG> which is a related example which is not claimed but is useful to understand the invention. <FIG>, which is a related example which is not claimed but is useful to understand the invention, illustrates an aspect where a position of a peak of intensity of the measurement light LA is detected using the beam profiler <NUM>.

First, the control apparatus <NUM> drives the respective driving units (the XYZ driving units and the head driving unit) (not illustrated) to set the posture of the optical rotation probe <NUM> such that the shaft <NUM> of the optical rotation probe <NUM> becomes parallel to the Z axis direction (step <NUM>). Subsequently, as illustrated in <FIG>, which is a related example which is not claimed but is useful to understand the invention, the control apparatus <NUM> causes the optical rotation probe <NUM> to emit the measurement light LA toward the beam profiler <NUM> in the X axis direction (step S1 <NUM>). The control apparatus <NUM> measures intensity distribution of the measurement light LA using the beam profiler <NUM> while relatively moving the optical rotation probe <NUM> with respect to the beam profiler <NUM> by a distance L in the X axis direction (in a direction of an arrow in <FIG>) and detects a displacement amount of the position (coordinate) of the peak of the intensity of the measurement light LA (step S <NUM>). An angle (displacement angle) at which the measurement light LA is displaced from the X axis direction on the XY plane is calculated based on the detection result in step S <NUM> using the following expression (<NUM>) (step S <NUM>).

Here, respective symbols mean as follows.

The control apparatus <NUM> updates numerical values of control software within the memory based on the displacement angle from the X axis direction calculated in step S114 and sets the reference direction in which the rotation angle s of the optical rotation probe <NUM> becomes <NUM>° (step S118). By this means, setting of the reference direction for the XY plane is completed. For example, a user adjusts the angle (displacement angle) of the measurement light LA displaced from the X axis direction on the XZ plane using a mechanical adjustment mechanism (such as a gauge). Description regarding this setting will be omitted.

While in the above description, a method for setting the reference direction of emission of the measurement light LA using the beam profiler <NUM> has been described, the reference direction of emission of the measurement light LA may be visually set.

Returning to <FIG>, which is a related example which is not claimed but is useful to understand the invention, after the reference direction of emission of the measurement light LA is set in this manner (step S10), an optical path length of the measurement light LA of the optical rotation probe <NUM> is calibrated (step S20). The optical path length of the measurement light LA is calibrated by, for example, setting a length LS of the shaft <NUM> of the optical rotation probe <NUM> at a predetermined length. Here, if an intersection of a reflection surface of the prism mirror <NUM> and the optical path P is set at PX in <FIG>, the length LS of the shaft <NUM> corresponds to a distance from the emission/incident end 44a of the optical fiber cable <NUM> to the intersection PX.

Calibration of the optical path length of the measurement light LA using a ring gauge <NUM> will be described below as an example with reference to <FIG> which is a related example which is not claimed but is useful to understand the invention,. As illustrated in <FIG>, the control apparatus <NUM> rotates the optical rotation probe <NUM> around the longitudinal axis 62a to measure an internal diameter of the ring gauge <NUM> having a known internal diameter D. Here, a diameter of the ring gauge <NUM> to be used in calibration may preferably be about double a distance from the longitudinal axis 62a of the optical rotation probe <NUM> to a focal point of the measurement light LA.

Subsequently, the length LS of the shaft <NUM> of the optical rotation probe <NUM> is set based on the following expression (<NUM>) such that a measurement value of the internal diameter of the ring gauge <NUM> becomes equal to the known internal diameter D of the ring gauge <NUM>. By this means, the optical path length of the measurement light LA is calibrated.

Subsequently, returning to <FIG>, which is a related example which is not claimed but is useful to understand the invention, after the optical path length of the measurement light LA is calibrated (step S20), a relative angle from a reference angle is calibrated (step S30). Here, a rotation angle s of the optical rotation probe <NUM> refers to a rotation angle in a rotation direction around the longitudinal axis 62a (hereinafter, also referred to as an S axis) of the optical rotation probe <NUM> (that is, a rotation angle around the longitudinal axis 62a in the emission direction of the measurement light LA). Further, a state where the emission direction of the measurement light LA of the optical rotation probe <NUM> is made to match the reference direction (X direction) in step S10 is set as a reference angle (s = <NUM>°), and a rotation angle when the optical rotation probe <NUM> rotates around the longitudinal axis 62a from the reference angle (that is, a relative angle from the reference angle) is represented as a "rotation angle s of the optical rotation probe". A calibration method of the relative angle using a calibration sphere <NUM> will be described below as an example with reference to <FIG> and <FIG>. <FIG> is a flowchart indicating calibration procedure of the relative angle, and <FIG> is a view illustrating a positional relationship between the optical rotation probe <NUM> and the calibration sphere <NUM> in a case where the rotation angle s = <NUM>° and s = <NUM>° in calibration of the relative angle using the calibration sphere <NUM>.

As illustrated in <FIG>, which is a related example which is not claimed but is useful to understand the invention, the control apparatus <NUM> rotates the optical rotation probe <NUM> around the longitudinal axis 62a of the optical rotation probe by a predetermined rotation angle s and aligns the focal point of the measurement light LA on a surface of the calibration sphere <NUM> as far as possible at the position (step S210). For example, in a case where the calibration sphere <NUM> is measured at a position where the rotation angle s is <NUM>°, the control apparatus <NUM> rotates (autorotation which will be described later) the optical rotation probe <NUM> around the longitudinal axis 62a by <NUM>° and further rotates (revolution which will be described later) the optical rotation probe <NUM> around the calibration sphere <NUM> by <NUM>°. As a result of this, the optical rotation probe <NUM> and the calibration sphere <NUM> in a case where the rotation angle s = <NUM>° have a positional relationship as illustrated in <FIG>. Here, the surface of the calibration sphere <NUM> is preferably located at a position of approximately ± focal depth/<NUM> from the focal position of the measurement light LA.

Subsequently, the control apparatus <NUM> drives the X axis driving unit, the Y axis driving unit and the Z axis driving unit of the three-dimensional coordinate measurement apparatus <NUM> without altering the emission direction of the measurement light LA (that is, in a state where the optical rotation probe <NUM> is fixed without being rotated around the longitudinal axis 62a) to scan the surface of the calibration sphere <NUM> with the measurement light LA and measures three-dimensional coordinates of measurement points on the surface of the calibration sphere <NUM> at the rotation angle s (step S212). <FIG>, which is a related example which is not claimed but is useful to understand the invention, illustrates the measurement points M on the calibration sphere <NUM> in cases where the rotation angle s = <NUM>° and s = <NUM>° as an example.

Subsequently, the control apparatus <NUM> calculates a three-dimensional coordinate of the center of the calibration sphere <NUM> from the three-dimensional coordinate values of the measurement points on the surface of the calibration sphere <NUM> for the rotation angle s (step S214). In this calculation, for example, a least-squares method using a known spherical diameter of the calibration sphere <NUM> may be used.

The control apparatus <NUM> repeats the processing from step S210 to step S214 for rotation angles s. For example, the processing from step S210 to step S214 is repeated for each rotation angle s while the rotation angle s of the optical rotation probe <NUM> is altered at intervals of <NUM>° so that s = <NUM>°, <NUM>°, <NUM>°,. , <NUM>° (step S216: No). Here, intervals of the rotation angle s of the optical rotation probe <NUM> during measurement are not limited to the intervals of <NUM>°.

After the processing from step S210 to step S214 is performed for all the rotation angles s (step S216: Yes), further, the control apparatus <NUM> corrects numerical values of the control program within the memory such that the three-dimensional coordinate of the center of the calibration sphere <NUM> calculated for each rotation angle s matches the three-dimensional coordinate of the center of the calibration sphere <NUM> calculated for the rotation angle s = <NUM>° (step S218). Two correction methods will be described below as an example.

The first correction method is a method in which correction is performed using the measurement value obtained in step S210. For example, in an ideal case, the three-dimensional coordinate of the center of the calibration sphere <NUM> calculated for the rotation angle s = <NUM>° matches the three-dimensional coordinate of the center of the calibration sphere <NUM> calculated for the rotation angle s = <NUM>°, and thus, a vector component of the measurement light LA is (x, y, z) = (<NUM>, <NUM>, <NUM>).

In a case where the rotation angle s is not correctly <NUM>° due to an error, for example, the control apparatus <NUM> corrects the vector component (x, y, z) of the measurement light LA to be (<NUM>, <NUM>, <NUM>) so that the three-dimensional coordinate of the center of the calibration sphere <NUM> calculated for the rotation angle s = <NUM>° matches the three-dimensional coordinate of the center of the calibration sphere <NUM> calculated for the rotation angle s = <NUM>°. For the rotation angle s with no measurement value, linear interpolation may be performed for rotation angles s with measurement values to calculate a correction value of the vector component of the measurement light LA.

The second correction method is a method in which correction is performed using a trigonometric function. In this method, an error Δs of the rotation angle of the emission direction of the measurement light LA with respect to the X axis direction within the XY plane (hereinafter, simply referred to as an S angle error) and an error angle Δcp of the emission direction of the measurement light LA with respect to the XY plane (hereinafter, simply referred to as an elevation angle error) are calculated from the following expression (<NUM>) and expression (<NUM>).

A reference numeral 10A in <FIG>, which is a related example which is not claimed but is useful to understand the invention, is a graph of the S angle error Δs calculated for each rotation angle s based on the above expression (<NUM>), and a reference numeral 10B is a graph of the elevation angle error Δcp calculated for each rotation angle s based on the above expression (<NUM>). The control apparatus <NUM> updates numerical value of the control software stored in the memory based on the calculated S angle error Δs and the elevation angle error Δcp.

Returning to <FIG> which is a related example which is not claimed but is useful to understand the invention, after the relative angle is calibrated in step S30, the optical path length of the measurement light LA is recalibrated (step S40). Procedure of recalibrating the optical path length is the same as that in step S20, and thus, description will be omitted.

Further, the reference direction of emission set in step S10 is recalibrated (step S50). While the reference direction is set in step S10, as will be described later using <FIG>, in this setting, displacement occurs from the intended reference direction of emission (in this example, the X axis direction) due to its accuracy, which provides an error to the reference angle of the S angle. Recalibration of the emission direction is one of the features of the presently disclosed subject matter. Here, the S angle means the rotation angle of the emission direction of the measurement light LA with respect to the X axis direction (reference direction) within the XY plane.

Principle of recalibration of the emission direction in step S50 will be described below. In the present embodiment, as an example, the adjustment error of the emission direction is measured using a cylindrical pin gauge <NUM> (corresponding to the reference object in the presently disclosed subject matter) having a known radius R, and the emission direction is recalibrated based on the measured adjustment error. <FIG> and <FIG> are respectively a perspective view and an XY plan view illustrating a positional relationship between the optical rotation probe <NUM> and the pin gauge <NUM> upon measurement of the adjustment error of the emission direction (which will be described in detail later). As illustrated in <FIG>, the longitudinal axis of the pin gauge <NUM> is disposed in parallel to the longitudinal axis 62a of the optical rotation probe <NUM>. Then, as illustrated in <FIG>, if the radius R of the pin gauge <NUM> is measured in a state where the emission direction of the measurement light LA has a reference angle error Δs0 of the S angle with respect to the reference direction (in this example, the X axis direction) on the XY plane, an error ΔR occurs in the measurement value. This error ΔR can be expressed using the following expression (<NUM>) as a function of the reference angle error Δs0 of the S angle. [Expression <NUM>] <MAT>.

Here, F is a focal distance from the longitudinal axis 62a of the optical rotation probe <NUM>, and symbols a and b can be expressed using the following expressions (<NUM>) and (<NUM>). [Expression <NUM>] <MAT>
[Expression <NUM>] <MAT>.

<FIG> indicates a graph of an error of a radius calculated based on the expression (<NUM>) in a case where the radius R = <NUM>, and the distance F = <NUM>. <FIG> indicates the reference angle error Δs0 (°) of the S angle on the horizontal axis and indicates the error ΔR (µm) of the radius on the vertical axis.

In the presently disclosed subject matter, the angle of the reference direction is temporarily changed by updating values of the control software within the memory with an angle obtained by intentionally displacing the calibrated emission direction of the measurement light LA of the optical rotation probe <NUM> by a minute angle Δs1 with respect to the reference direction set in step S10. The radius R of the pin gauge <NUM> is measured in a state where the reference angle is displaced by the minute angle Δs1 from the setting in step S10, and the error ΔR of the radius R is calculated. If the emission direction of the measurement light LA of the optical rotation probe <NUM> does not have the reference angle error Δs0 of the S angle, the calculated error ΔR should match the theoretical ΔR. Thus, in the presently disclosed subject matter, the reference angle error Δs0 of the S angle of the calibrated emission direction of the optical rotation probe <NUM> with respect to the reference direction is calculated as the adjustment error based on a difference between the calculated error ΔR and the theoretical ΔR. By recalibrating the emission direction based on the calculated adjustment error, calibration accuracy of the optical rotation probe <NUM> is improved, and eventually, the measurement error by the optical rotation probe <NUM> is reduced.

A recalibration method of the emission direction will be described below using <FIG>. <FIG> is a flowchart indicating procedure of recalibrating the emission direction. As indicated in <FIG>, first, the pin gauge <NUM> is disposed so as to achieve a predetermined positional relationship with the optical rotation probe <NUM> (step S310). The radius R of the pin gauge <NUM> is preferably approximately from ten times to tens of times of a beam diameter of the measurement light LA. <FIG> and <FIG> are views respectively illustrating a positional relationship between the optical rotation probe <NUM> and the pin gauge <NUM> upon measurement of the adjustment error of the emission direction. As illustrated in <FIG>, the pin gauge <NUM> is disposed in parallel to the longitudinal axis 62a of the optical rotation probe <NUM>. Here, preferably, a distance between the optical rotation probe <NUM> and the pin gauge <NUM> is set such that a distance between the optical rotation probe <NUM> and the pin gauge <NUM> is constant, and a surface of the pin gauge <NUM> is preferably located in a focal distance of the measurement light LA.

More specifically, the distance between the optical rotation probe <NUM> and the pin gauge <NUM> is preferably set such that the surface of the pin gauge <NUM> is located at a position of approximately ± focal depth/<NUM> from the focal position of the measurement light LA. In the examples illustrated in <FIG> and <FIG>, a distance between the longitudinal axis 62a of the optical rotation probe <NUM> and the measurement surface of the pin gauge <NUM> is equal to the focal distance F.

Further, in the example in <FIG>, the longitudinal axis 62a of the optical rotation probe <NUM> is disposed at the origin on the XY plane, and the center C of a circle of the cross-section of the pin gauge <NUM> is disposed on the X axis.

Through the calibration procedure from step S10 to step S40 described above, ideally, the optical rotation probe <NUM> is adjusted such that the emission direction of the measurement light LA matches the X axis direction that is the reference direction at the rotation angle s = <NUM>°. Thus, ideally, the measurement light LA is expected to be emitted toward an apex of the pin gauge <NUM> perpendicularly to the axial direction of the pin gauge <NUM> at the rotation angle of the optical rotation probe <NUM> = <NUM>°.

After the optical rotation probe <NUM> and the pin gauge <NUM> are disposed in such a positional relationship, the control apparatus <NUM> sets an angle obtained by changing the optical rotation probe <NUM> around the longitudinal axis 62a (S axis) by a predetermined minute angle Δs1 from the reference angle set in step S10 by the head driving unit (not illustrated) of the measurement head <NUM> as a temporal reference angle (step S312). The change of the optical rotation probe <NUM> in this event corresponds to movement indicated with a dashed arrow A in <FIG>.

Subsequently, the control apparatus <NUM> emits the measurement light LA from the optical rotation probe <NUM> toward the pin gauge <NUM> while moving the optical rotation probe <NUM> along a rotation trajectory (in a direction of a solid arrow B in <FIG>) centering the pin gauge <NUM> by controlling the XYZ driving units of the three-dimensional coordinate measurement apparatus <NUM> and continuously rotating the optical rotation probe <NUM> around the longitudinal axis (rotation axis) 62a (that is, continuously varying the rotation angle s of the optical rotation probe <NUM>) and measures the radius R of the pin gauge <NUM> (step S314).

In other words, the control apparatus <NUM> brings rotational movement (revolution movement) of the optical rotation probe <NUM> centering around the pin gauge <NUM> in synchronization with rotational movement (autorotation movement) around the longitudinal axis (rotation axis) 62a of the optical rotation probe <NUM> and emits the measurement light LA from the optical rotation probe <NUM> toward the pin gauge <NUM> perpendicularly with respect to the longitudinal axis direction of the pin gauge <NUM> while keeping a constant distance between the optical rotation probe <NUM> and the pin gauge <NUM>. In this case, measurement is preferably performed after the optical rotation probe <NUM> is rotated around the pin gauge <NUM> by one or more revolutions (equal to or greater than <NUM>°). Further, the apex of the pin gauge <NUM> is preferably irradiated with the measurement light LA emitted from the optical rotation probe <NUM>.

Subsequently, the control apparatus <NUM> calculates the error ΔR of the radius R of the pin gauge <NUM> at the predetermined minute angle Δs1 based on the measurement result obtained in step S314 (step S316). Here, the radius R of the pin gauge <NUM> is known, and calculation of the error ΔR is simple calculation of a difference, and thus, description will be omitted.

The control apparatus <NUM> repeats measurement of the radius R of the pin gauge <NUM> and calculation of the error ΔR while altering the reference angle of rotation around the longitudinal axis 62a of the optical rotation probe <NUM> by a minute angle Δs1 (step S318: Yes). After the processing from step S312 to S316 is repeated the number of times sufficient to obtain an approximate expression (step S318: No), the control apparatus <NUM> further obtains an approximate expression indicating a relationship between the minute angle Δs1 and the error ΔR (step S320). Preferably, the control apparatus <NUM> obtains a quadratic polynomial using the least-squares method.

<FIG> indicates an example of a graph in which the error ΔR calculated from the measurement value in step S314 is plotted in a superimposed manner on the graph in <FIG>, the graph indicating the minute angle Δs1 on the horizontal axis and indicating the error ΔR on the vertical axis. In <FIG>, a rhombic indicates the error ΔR calculated from the measurement value, and a thin solid line indicates the approximate expression obtained using the least-squares method. The approximate expression indicated in <FIG> is as follows.

Further, a thick solid line indicates a theoretical error ΔR. As indicated in <FIG>, compared to the graph of the theoretical error ΔR, the graph of the error ΔR calculated from the measurement value is displaced in the vertical axis direction and extends in a width in the horizontal axis direction. Here, displacement in the vertical axis direction may be caused by a uniform length measurement error occurring at the wavelength swept light source <NUM> of the measurement light LA. Extension in the width in the horizontal axis direction may be caused by extension of the beam diameter of the measurement light LA as a result of the measurement position being displaced from the focal position of the measurement light LA.

Subsequently, the control apparatus <NUM> calculates the adjustment error of the emission direction of the measurement light LA from the approximate expression obtained in step S320 (step S322). For example, the following quadratic polynomial (<NUM>) using the least-squares method can be transformed to expression (<NUM>). <MAT>
[Expression <NUM>] <MAT>.

From the above, the control apparatus <NUM> can perform calculation assuming that the reference angle error Δs0 (adjustment error) of the optical rotation probe <NUM> is the minute angle Δs1 when ΔR in the expression (<NUM>) has an extreme value, and is "-B/2A" (°). This adjustment error "-B/2A" (°) corresponds to an error of the emission direction of the measurement light LA remaining after calibration from step S10 to step S40.

Specifically, in a case of the graph indicated in <FIG>, the control apparatus <NUM> can obtain <NUM>/(<NUM> × <NUM>) = <NUM>° as the reference angle error Δs0 (adjustment error) from the approximate expression "ΔR = -<NUM>. 044Δs1<NUM> + <NUM>. 86Δs1 + <NUM>". The control apparatus <NUM> recalibrates the emission direction by updating the numerical values of the control software stored in the memory again based on the calculated adjustment error (step S324).

According to the calibration method for the optical rotation probe <NUM> in the present embodiment as described above, the emission direction of the measurement light LA emitted from the optical rotation probe <NUM> can be calibrated with high accuracy. It is therefore possible to reduce the measurement error of the optical rotation probe <NUM>, which eventually makes it possible to reduce the measurement error of the three-dimensional coordinate measurement apparatus <NUM> equipped with the optical rotation probe <NUM>. By this means, it is possible to measure a shape with a great curvature of a local portion of the object to be measured with high accuracy.

While the calibration method of the presently disclosed subject matter can improve calibration accuracy of the optical rotation probe <NUM> as described above, the calibration method of the present embodiment further has the following advantages.

First, to measure a fine shape, it is necessary to make the beam diameter of the measurement light LA smaller. While at the focal position of the measurement light LA, a beam diameter of several micrometers to several tens of micrometers can be obtained, if the measurement position is deviated from the focal position of the measurement light, the beam diameter of the measurement light LA becomes larger. Further, there is a case where the beam diameter of the measurement light LA becomes larger due to influence of white noise, or the like.

A reference numeral 16A in <FIG> designates intensity distribution of the measurement light LA obtained in a case where the beam diameter is small, and a reference numeral 16B designates intensity distribution of the measurement light LA obtained in a case where the beam diameter is large. While the intensity distribution of the measurement light LA is measured upon calibration of the measurement light LA, in a case where the beam diameter of the measurement light LA is large as designated with the reference numeral 16B in <FIG>, a gradient of intensity becomes small in the vicinity of the peak, which makes it difficult to detect a position of the peak of the intensity of the measurement light LA with high accuracy. Thus, in related art, there is a problem that the reference direction (reference of the rotation angle of the optical rotation probe) of emission of the measurement light LA of the optical rotation probe <NUM> cannot be set with high accuracy.

For example, in a case where the focal distance of the imaging lens <NUM> of the optical rotation probe <NUM> is <NUM>, and the beam diameter of the measurement light LA is <NUM>, at a measurement position displaced from the focal distance by <NUM>, the beam diameter becomes <NUM>. In this event, setting accuracy of the position of the peak of the intensity of the measurement light LA is approximately <NUM>, and calibration accuracy of the emission direction is approximately <NUM>°. In a case where a distance from the longitudinal axis 62a (optical axis of the measurement light LA) of the optical rotation probe <NUM> to the focal distance is <NUM>, an error of the measurement position becomes <NUM>.

In related art, there is a problem that a shape with a great curvature of a local portion of the object to be measured cannot be measured with high accuracy due to such a large error. On the other hand, in the present embodiment, the radius R of the pin gauge <NUM> is measured in a state where the calibrated emission direction of the measurement light LA of the optical rotation probe <NUM> is intentionally displaced by a minute angle Δs1 with respect to the reference angle set in step S10, and the error ΔR of the radius R is calculated.

As illustrated in <FIG>, in a case where the beam diameter of the measurement light LA extends as a result of the measurement position being displaced from the focal position of the measurement light LA, a graph of the calculated error ΔR extends in the horizontal axis direction, but the graph maintains symmetry in the horizontal axis direction. Thus, extension in the width in the horizontal axis direction of the graph does not affect the calculation result of the adjustment error based on the approximate expression of the calculated error ΔR. Thus, in the present embodiment, even in a case where the beam diameter of the measurement light LA extends, the emission direction of the measurement light LA can be calibrated with high accuracy based on the calculation result of the adjustment error.

In a similar manner, even in a case where a uniform length measurement error remains in the wavelength swept light source <NUM> of the measurement light LA after calibration, as indicated in <FIG>, the graph of the error ΔR calculated from the measurement value is only displaced in the vertical axis direction compared to the graph of the theoretical error ΔR, and does not affect the calculation result of the adjustment error based on the approximate expression of the calculated error ΔR. Thus, in the present embodiment, even in a case where a uniform length measurement error occurs at the wavelength swept light source <NUM> of the measurement light LA, it is possible to calibrate the emission direction of the measurement light LA with high accuracy.

In the above description, the reference angle error Δs0 of the S angle is calculated by measuring the radius R of the pin gauge <NUM> having a known radius. However, a surface shape of the pin gauge <NUM> may be measured instead of measuring the radius R of the pin gauge <NUM>. <FIG> indicates a recalibration method of the emission direction of the measurement light LA according to the second embodiment. As illustrated in <FIG>, while the recalibration method of the emission direction in the second embodiment is substantially the same as the flowchart indicated in <FIG>, the recalibration method is different from the flowchart in <FIG> in that processing in step S410 and S412 is performed in place of steps S314 and S316. The difference will be described below.

In the second embodiment, in step S410, instead of the radius R of the pin gauge <NUM>, the surface shape of the pin gauge <NUM> is measured in a state where the reference angle of the S angle the optical rotation probe <NUM> around the longitudinal axis 62a (S axis) is changed by a minute angle Δs1 from the setting in step S10. A reference numeral 18A in <FIG> designates a graph in which the measurement result of the surface shape at a certain minute angle Δs1 is plotted on an XY coordinate.

Subsequently, in step S412, the control apparatus <NUM> calculates standard deviation σ (ΔR) of the error ΔR of the radius R of the pin gauge <NUM> based on the measurement value at each minute angle Δs1. An example of a method for calculating the standard deviation σ (ΔR) for a certain minute angle Δs1 will be specifically described below with reference to the graph indicated in <FIG>. First, the control apparatus <NUM> calculates an approximate value of the radius R of the pin gauge <NUM> by approximating the measurement result of the surface shape using the least-squares method. In a graph designated with the reference numeral 18A, an approximate value of the radius R is designated with a thick solid circle. The reference numeral 18B designates a graph obtained by converting the graph of the XY coordinate of each measurement point designated with the reference numeral 18A into the measurement value (vertical axis) of the radius R with respect to the rotation angle (horizontal axis) centering around the center C of the pin gauge <NUM>. Subsequently, the control apparatus <NUM> calculates the error ΔR that is a difference between the approximate value of the radius R and the measurement value of the radius R. A reference numeral 18C designates a graph of the error ΔR (vertical axis) with respect to the rotation angle s (horizontal axis) centering around the center C of the pin gauge <NUM>. Further, the control apparatus <NUM> calculates the standard deviation σ (ΔR) of the error ΔR from variation of the obtained error ΔR and further calculates ±2σ that is a range including about <NUM>% of all the measurement points in normal distribution of the error ΔR as a shape error. In a case of the graphs designated with the reference numerals 18A to 18C in <FIG>, the shape error (±2σ) is <NUM>. The control apparatus <NUM> performs such calculation of the shape error for different minute angles Δs1.

The subsequent procedure is similar to that in the first embodiment. <FIG> indicates a graph plotted with the minute angle Δs1 of rotation around the longitudinal axis 62a of the optical rotation probe <NUM> on the horizontal axis and the shape error (±2σ) calculated in step S314 on the vertical axis. For example, if the graph in <FIG> is approximated to a quadratic polynomial using the least-squares method, "±2σ = -<NUM>. 508Δs1<NUM> + <NUM>. 141Δs1 + <NUM>" (see the solid line graph in <FIG>).

In this manner, also in the second embodiment in which the surface shape is measured, results substantially the same as those obtained in the first embodiment in which the radius R is measured, can be obtained. By changing the reference angle of the optical rotation probe <NUM> by the minute angle Δs1 from the setting in step S10, in a case where the measurement light LA does not perpendicularly hit the measurement point on the pin gauge <NUM>, a width of the interference signal of the measurement light LA and the reflected light LB becomes wide. In a case of the second embodiment in which the shape measurement is performed, this causes an error, and thus, a difference occurs between the approximate expression in the first embodiment and the approximate expression in the second embodiment.

Also in the second embodiment, in a similar manner to the first embodiment, the calibration accuracy can be improved, and the measurement error of the optical rotation probe <NUM> can be reduced, which eventually makes it possible to reduce the measurement error of the three-dimensional coordinate measurement apparatus <NUM> equipped with the optical rotation probe <NUM>.

In the above first and second embodiments, the emission direction is recalibrated using the radius R of the cylindrical pin gauge <NUM>. Here, the radius R of the pin gauge <NUM> is, for example, approximately from ten times to tens of times of the beam diameter of the measurement light LA. For example, in a case where the beam diameter is <NUM>, the radius R of the pin gauge <NUM> is approximately from <NUM> to several millimeters. It is not easy to manually position the pin gauge <NUM> having such a small radius R for measurement.

Thus, in place of the pin gauge <NUM>, a jig <NUM> in which a pin gauge <NUM> is provided to perpendicularly (in the Z axis direction) stand on a cuboid block <NUM> as illustrated in <FIG> may be used. Here, perpendicularity of the pin gauge <NUM> may not be strictly required. For example, in a case of the pin gauge <NUM> having the radius R of <NUM>, if an inclination angle with respect to the XY plane is less than <NUM>°, influence provided to the measurement error of the radius R by the inclination angle is <NUM>, which is small enough to be used in calibration.

A recalibration method in a case where this jig <NUM> is used will be described below. First, before recalibration indicated in <FIG> is performed, a position of the pin gauge <NUM> on the block <NUM> in the jig <NUM> is measured. Normally, it is enough if this measurement is performed once for the jig <NUM>, and thereafter, a numerical value obtained through this measurement is repeatedly used in recalibration.

First, a design coordinate (x0, y0, z0) of the center (longitudinal axis) of the pin gauge <NUM> is assumed. Subsequently, three surfaces B1, B2 and B3 of the block <NUM> of the jig <NUM> are measured, a workpiece coordinate system (XYZ orthogonal coordinate system) of the block <NUM> of the jig <NUM> is created, and further, a diameter of the pin gauge <NUM> is measured, and thereby a position of the center of the pin gauge <NUM> is measured. There is a case where displacement occurs in a position measurement result due to tolerance of the position of the pin gauge, and thus, the measurement result is different from a design position in many cases. Further, there is also a case where a measurement error occurs as a result of the measurement light LA not perpendicularly hitting the apex of the pin gauge <NUM>. Here, the measurement result is assumed as (x1, y1, z1 ≈ z0). In a case where the measurement result is different from the design position, thereafter, the jig <NUM> is used in recalibration indicated in <FIG> in the first and the second embodiments assuming that the center of the pin gauge <NUM> is located at (x1, y1, z0) of the measurement result in the workpiece coordinate system.

According to the third embodiment, before the recalibration method indicated in <FIG> is performed, the coordinate of the center of the pin gauge <NUM> has already been specified on the workpiece coordinate system, and thus, the pin gauge <NUM> can be positioned only by placing the jig <NUM> on the table <NUM> of the three-dimensional coordinate measurement apparatus <NUM>. This can provide an effect that the pin gauge <NUM> can be easily positioned in addition to the effects in the above-described embodiments.

As described above, according to the first to the third embodiments, the emission direction of the measurement light LA emitted from the optical rotation probe <NUM> can be calibrated with high accuracy. It is therefore possible to reduce the measurement error of the optical rotation probe <NUM>, which eventually makes it possible to reduce the measurement error of the three-dimensional coordinate measurement apparatus <NUM> equipped with the optical rotation probe <NUM>. By this means, for example, it is possible to measure a shape with a great curvature of a local portion of the object to be measured with high accuracy.

Claim 1:
A calibration method for an optical rotation probe (<NUM>) capable of emitting measurement light in a direction perpendicular to a probe axis (62a) and capable of rotating an emission direction of the measurement light around the probe axis (62a), the calibration method comprising:
a change step of changing the emission direction of the measurement light by a minute angle from a reference direction set in advance;
an acquisition step of acquiring a shape error for a reference object (<NUM>) after the change step by emitting the measurement light from the optical rotation probe (<NUM>) toward the reference object (<NUM>) while rotating the emission direction of the measurement light around the probe axis (62a) and varying a relative position between the optical rotation probe (<NUM>) and the reference object (<NUM>); and
an adjustment error calculation step of calculating an adjustment error of the emission direction of the measurement light with respect to the reference direction based on a theoretical value of the shape error for the reference object (<NUM>) to be obtained in a case where the emission direction of the measurement light matches the reference direction and a measurement value of the shape error for the reference object (<NUM>) obtained in the acquisition step.