Measurement device and method

Abstract

A protective element (126) for an optical measurement device, such as a break beam tool setting device (2) for a machine tool, is described. The protective element (126) includes a conduit (140) through which light and air can pass. The conduit (140) is configured such that, in use, a light beam is passed through the conduit along an optical axis (O) and air flow is guided out of the conduit along an air flow axis (A). The optical axis (O) is non-parallel to the air flow axis (A), and the conduit (140) has a cross-sectional profile that varies along the air flow axis (A). Improved measurement repeatability is provided.

Description

[Technical field]

[0001] The present invention relates to optical (non-contact) measurement apparatus, and in particular to such apparatus in which an exhaust air flow is used to protect the various optical components from contaminants of the type commonly found in machine tool environments. [Background technology]

[0002] It is known to protect non-contact measurement devices, such as non-contact tool measurement devices, from contaminants generated from machining operations performed by a machine tool, such as coolant, cutting debris, etc. One example of such a non-contact tool measurement device is a laser tool setter, which detects when a tool held by the machine tool breaks a thin laser beam, thereby allowing tool measurements, such as the tool length and diameter, to be obtained.

[0003] Nos. 5,233,633 and 5,233,633 describe examples of such laser tool setters in which the laser beam is passed from the transmitting portion to the receiving portion through a region of free space. The laser beam enters and exits the device through narrow channels or conduits in each of the transmitting and receiving portions. In such devices, each channel is formed (e.g., by drilling) at an oblique angle to the optical axis along which the laser beam passes between the transmitting and receiving portions. Thus, the exhausted air is directed (angled) away from the free space optical path traversed by the laser beam. These angled air passages are shown, for example, in FIG. 4c of US Pat. No. 5,233,633 and in FIG. 4 of US Pat. No. 5,233,633. The NC4 non-contact tool setting system sold by Renishaw Public Limited Company of Wotton-under-Edge, UK, is an example of a device that includes such an angled opening. An example of a break beam tool setter device that exhausts air along its optical axis is described in US Pat. No. 5,233,633. US Pat. No. 5,399,633 describes an alternative arrangement in which a tubular air shield is provided from an array of air nozzles surrounding a central opening through which the laser beam passes. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] European Patent No. 1050368 [Patent Document 2] European Patent No. 1502699 [Patent Document 3] US Patent Application Publication No. 2018 / 111240 [Patent Document 4] US Patent Application Publication No. 2010 / 0206384 Summary of the Invention [Problem to be solved by the invention]

[0005] Although known devices of the above type reduce air turbulence in the free space optical path traversed by the laser beam, the inventors have discovered that some air turbulence effects are still present, thereby degrading measurement reproducibility. [Means for solving the problem]

[0006] According to the invention there is provided a protective member for an optical measurement device, the protective member comprising a conduit through which light and air can pass, the conduit being configured such that, in use, a light beam is passed through the conduit along an optical axis and a flow of air is directed out of the conduit along an air flow axis, the optical axis being non-parallel to the air flow axis, and the conduit having a cross-sectional profile that varies along the air flow axis.

[0007] The present invention therefore relates to a protective member for an optical measurement device, such as a non-contact tool measurement device. The protective member comprises a conduit or passageway through which, in use, both a light beam and an air stream are passed. In use, light is passed through the conduit along an optical axis which may be perpendicular to an outer surface of the protective member. Air guided through the conduit is exhausted from the conduit along an air flow axis which is inclined or angled relative to the optical axis. The exhausted air is thus directed away from the light beam so as to prevent disturbance of the air flow along the free space path of the light beam.

[0008] The present invention is characterized by a conduit having a cross-sectional profile that varies (i.e., changes) along the length of the airflow axis. In particular, the cross-sectional area and / or cross-sectional shape of the conduit is varied along its length to reduce turbulence of the air exhausted therefrom. This differs from the conduits of U.S. Pat. Nos. 5,399,623 and 5,433,363, which are formed, for example, by drilling holes through a blank protective member along the airflow axis (thereby creating a conduit with a constant cross-sectional profile along the length of the airflow axis). It has been found that the varying shape of the conduit reduces the overall airflow turbulence of the exhausted air, thereby reducing the amount of turbulence present in the free-space beam path. The varying shape can also be used to reduce the amount of air that needs to be exhausted to provide a constant level of protection against external contaminants. Both of these improvements have been found to provide improvements in the repeatability of measurements.

[0009] It should again be noted that the conduit is also configured to allow the light beam to pass along an optical axis (i.e., an axis angled relative to the airflow axis). The light beam passing through the conduit may be restricted (e.g., attenuated or shaped) by the conduit, or may pass through the conduit unimpeded. Thus, the above-mentioned improvements to airflow characteristics may be achieved without affecting the optical performance of the protective member.

[0010] Advantageously, the conduit has an inlet opening for receiving air and an outlet opening for discharging air. The inlet opening advantageously has a different cross-sectional area than the outlet opening. Preferably, the cross-sectional area of ​​the inlet opening is larger than the cross-sectional area of ​​the outlet opening. In other words, the inlet of the conduit may be wider than the outlet. This change (e.g., decrease) in cross-sectional area along the conduit reduces the turbulence of the air in the conduit before it is discharged, thereby reducing the turbulence of the discharged air. The inlet opening may also have an at least partially grooved profile. For example, any "sharp" edges may be flattened to provide a smoother, less turbulent air flow. In a preferred embodiment, a hole may be drilled and then widened and appropriately shaped using a laser cutting process to form the conduit. This two-step process minimizes burrs that would otherwise cause some air turbulence.

[0011] Advantageously, at least a portion of the conduit has an asymmetric cross-sectional profile. For example, the circular or elliptical cross-section of the prior art conduit may be replaced with an asymmetric cross-section. Thus, the conduit may be non-cylindrical. Advantageously, the conduit has an outlet opening that is substantially D-shaped when viewed along the optical axis.

[0012] Advantageously, the conduit presents a substantially circular cross-section to light passing therethrough along the optical axis. In other words, the conduit is shaped such that a substantially circular light beam may pass therethrough along the optical axis. The conduit may restrict or shape the light beam. Alternatively, substantially all of the incoming light beam may pass straight through the conduit along the optical axis.

[0013] The protective member may include further components to help direct the flow of air through the conduit and / or to interact with the light beam. For example, the protective member may include an optical aperture (i.e., an aperture separate from the conduit) to constrict the light beam passed along the optical axis. The optical aperture may shape and / or attenuate the light beam. The cross section of the conduit for the light, as viewed along the optical axis, may be slightly larger than the associated optical aperture. For example, the effective radius of the conduit along the optical axis may be 0.1 mm larger than the radius of such optical aperture. Also, all of the air that is exhausted through the protective member may pass through the optical aperture. Alternatively, there may be other holes or passages through which some of the air may pass (i.e., without passing through the optical aperture).

[0014] All air exiting the conduit may be directed in substantially the same direction (i.e. along the air flow axis). For example, all air flows exiting the conduit may be directed either above, below or to the side of the optical axis. The air flow axis may be non-parallel (e.g. tilted or oblique) to the optical axis, so that the air flows have the least possible effect on the light passing along the optical axis. In particular, the protective air flow preferably crosses the optical path in only one direction, in a very localized area. This provides a stable air flow arrangement that improves the repeatability of the measurements. Advantageously, there may not be a substantial portion of the air flow exiting the conduit that moves in a direction along or parallel to the optical axis. In other words, the air is preferably exited in an off-axis direction.

[0015] Advantageously, the air flow axis may be angled at 5° or more relative to the optical axis. The air flow axis may be angled at 10° or more relative to the optical axis. The air flow axis may be angled at 15° or more relative to the optical axis. The air flow axis may be angled at 20° or more relative to the optical axis. The air flow axis may be angled at less than 45° relative to the optical axis. Preferably, the air flow axis may be angled at about 30° relative to the optical axis. A larger angle will direct the air further away from the path of the free space light beam, but it will require the conduit to be larger (i.e. to ensure that the beam can still pass along the optical axis) and therefore there will be reduced ingress protection and / or more air will have to be evacuated.

[0016] The protective element preferably consists of only a single conduit. The single conduit may have only a single outlet opening through which the air is discharged. The physical shape of the single conduit may define the direction of the air flow. Preferably, all air discharged through the protective element passes through a single conduit. Thus, a complex arrangement of multiple air nozzles of the type described in US Pat. No. 5,399,633 can be avoided. Also, the consumption of air required to provide a certain level of protection against the ingress of pollutants is reduced.

[0017] The protective member may be integrally formed with the optical measurement device. For example, the protective member may form part of a casing or shell of such a device. Conveniently, the protective member may include one or more structures for releasable attachment to a transmitter or receiver of the optical measurement device. For example, the protective member may be formed as a cap that is attached to and removed from the optical measurement device. And various such caps may be provided on different structures of the optical measurement device. Such caps may be configured to fit prior art measurement devices (i.e. they may be retrofittable). The optical components of the optical measurement device (e.g. lenses / detectors) are preferably recessed within the device (i.e. behind the conduit of the protective member).

[0018] The invention also extends to an optical measurement device comprising at least one protective member as described above. The optical measurement device may comprise a transmitter with a protective member and / or a receiver with a protective member. In such examples, the protective members may be nominally the same or they may be different. For example, protective members with different sized conduits and / or different sized optical apertures may be provided for the transmitter and the receiver. The air turbulence generated at the transmitter may be smaller than the air turbulence generated at the receiver. Also, a kit of multiple protective members with different sized conduits and / or optical apertures may be provided to enable the measurement device to be configured as required.

[0019] The optical measurement device may include other optical components (lenses, light sources, receivers, etc.). The optical measurement device may include other air flow control devices (control valves, flow restrictors, one-way valves, etc.). Advantageously, the optical measurement device may include an internal air chamber adjacent to each protective member conduit. Such an internal air chamber may help reduce turbulence of the air entering the protective member conduit, thereby reducing turbulence of the exhausted air. Also, the air flow through the internal air chamber may be positioned to avoid the optical path through the air chamber. Also, the air flow through the chamber may converge at the conduit. This may reduce the effect of turbulence on the optical beam as it passes through the air chamber.

[0020] The conduit size may be selected to give the best balance of repeatability and accuracy versus air consumption. For example, the optical aperture size may be maintained and repeatability improved. Alternatively, the optical aperture size may be increased to improve metrology while maintaining repeatability. Thus, an appropriately sized protective member could be used for each required application.

[0021] In a preferred embodiment, the optical measuring device is a tool measuring device, such as a break beam tool measuring device. The tool measuring device may be mountable to the bed of the machine tool. Air, or another gas, may be supplied to the device (e.g. from a machine shop compressed air supply). The air supply may be at a substantially constant flow rate. Alternatively, the air flow rate may be variable during use (e.g. based on the level of protection required). For example, low and high flow rates may be used. The air supply may be stopped entirely, for example when air protection is not required. The protective member may also form part of a shutter assembly, as described in the applicant's patent application PCT / GB2020 / 050581.

[0022] Also described herein is a protective member for an optical measurement device. The protective member may include a conduit through which light and gas (e.g., air) may pass. The conduit may be configured such that, in use, a light beam passes through the conduit along an optical axis. The conduit may be configured such that, in use, a flow of air (or other gas) is directed out of the conduit along an air flow axis. The optical axis may be non-parallel to the air flow axis. The conduit may have a cross-sectional profile that varies along the air flow axis. The conduit may be a substantially smooth aerodynamic conduit. The substantially smooth aerodynamic conduit may be Venturi-shaped. The conduit may be substantially smooth (e.g., by chamfering or blending arcs during manufacture). The member may include any of the other features described herein, alone or in combination.

[0023] The present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: [Brief description of the drawings]

[0024] [Figure 1] FIG. 1 illustrates a non-contact tool setting device. [Diagram 2] FIG. 1 illustrates a prior art cap for a non-contact tool setting device. [Diagram 3]1 shows a longitudinal section of a cap according to the present invention for use with a non-contact tool setting device; [Figure 4] FIG. 4 is a front view of the cap of FIG. 3. [Diagram 5] FIG. 13 shows a longitudinal section of a cap according to the present invention with an additional air flow insert. [Figure 6] 4 is a photograph of a cap made according to the structure shown in FIG. 3. [Figure 7] FIG. 2 illustrates air turbulence in the xy plane associated with a prior art cap. [Figure 8] FIG. 1 illustrates air turbulence in the xy plane associated with the cap of the present invention. [Figure 9] FIG. 2 illustrates air turbulence in the yz plane associated with a prior art cap. [Figure 10] FIG. 13 illustrates air turbulence in the yz plane associated with the cap of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0025] With reference to FIG. 1, a partial exploded view of a prior art tool setter device 2 is shown. The device comprises a transmitting unit 10, including a laser diode and suitable optics (not shown) for generating a light beam 12. A receiving unit 14 is also provided, including a photodiode (not shown) for detecting the intensity of the received light beam 12. Both transmitting unit 10 and receiving unit 14 are fixed to a common base 20, thereby maintaining a constant distance and orientation relative to each other. And the base 20 may be mounted directly to the bed of a machine tool, or indeed to any suitable part. It should also be noted that various alternative structures for mounting the transmitter and receiver could be used. For example, a common housing for the transmitter and receiver could be provided, or the transmitter and receiver units could be separately mounted to the machine tool. An electrical socket 22 is provided on the base 20 for connection via a cable to an associated interface (not shown) that provides power and receives the beam intensity signal from the detector of the receiving unit 14.

[0026] The tool setter device 2 is designed to operate in the harsh environment of a machine tool, where pressurized flows of coolant, coolant mist, cutting debris, etc. are often present. Long-term and reliable operation of the device under such harsh conditions is possible by using a so-called air protection system. The transmitting unit 10 and the receiving unit 14 therefore include an air cap 26 with a passage or conduit 28 through which the light beam 12 is passed. In FIG. 1 the air cap 26 is shown removed from the respective transmitting and receiving units for illustrative purposes (but would be attached to such units during use). In use, compressed air is supplied to the body of the device through an air inlet 30. The device is arranged such that at least a portion of such air is forced to flow out of the device via the same conduit 40 in the cap 26 through which the light passes. This continuous flow of air prevents contaminants from entering the device, while still allowing light to enter and exit the device as required, so that measurements can be performed.

[0027] Referring now also to FIG. 2, the configuration of the prior art air caps 26 is shown in more detail. Each air cap 26 includes an oblique passageway or conduit 40 through which air is forced to escape. The conduits 40 are formed by drilling a blank cap at an oblique angle to the surface normal of the outermost surface of the member. The size and oblique angle of the conduit 40 are selected so that a light beam passing through the optical aperture 42 along the normal to the outermost surface of the member (i.e., along the optical axis of the light beam) can also pass through the conduit. The conduit 40 also exhausts air, which is forced under pressure into the chamber 44 through a series of air holes 46, optionally via an air flow restrictor 48. The air is exhausted in a direction approximately parallel to the angle at which the conduits are formed. In other words, the air is exhausted along an air flow axis (A) that is at an oblique angle (θ) to the optical axis (O) of the light beam. As explained in U.S. Patent No. 5,393,633 and U.S. Patent No. 5,393,633, this arrangement serves to direct air flows away from the free space optical path of the beam (i.e., the free space path between the transmitting unit and the receiving unit traversed by the optical beam), thereby reducing the effect that such air flows would have on the measurement accuracy of the device.

[0028] It has been found by the inventors that, although the prior art arrangements provide reliable protection against contaminants, turbulence in the exhausted air stream still disturbs the air flow along the free space path of the beam, thereby detrimentally affecting the measurement performance of the device. The present invention has now found to improve the measurement accuracy of the device by reducing the air flow disturbances associated with the exhausted air, as described below. In particular, it has been found that modifying the cross-sectional profile along the conduit (i.e., in the direction along the air flow axis A) can significantly reduce the turbulence of the air exhausted from such a conduit, as compared to prior art conduits having a constant cross-section along their length. For example, it has been found that removing any sharp edges in the air flow path through the conduit and / or reducing the cross-sectional area of ​​the conduit along its length (i.e., in the direction of the air flow axis) reduces the turbulence of the exhausted air.

[0029] 3 and 4, an air cap 126 according to the present invention is illustrated. The air cap includes a conduit 140 having an area of ​​constant cross section 142, but including a region 144 having an increasing cross-sectional area. In particular, the sharp edge 90, visible in the air cap 26 of FIG. 2, is replaced with a flattened portion 190, shown in FIG. 3. This change in the shape of the opening of the conduit 140 thus provides a "D-shaped" conduit 140 when viewed from the direction of the surface normal (i.e., along the optical axis), as shown in FIG. 4. When installed in a tool setter apparatus (e.g., as shown in FIG. 1), air is exhausted along the air flow axis A while a light beam passes along the optical axis O. The air flow axis A is inclined, or angled, relative to the optical axis O.

[0030] Figure 5 shows a specification in which the air cap 126 described with reference to Figures 3 and 4 may further include an optical aperture 150, which may, for example, restrict the light beam passing therethrough. An optional air flow restrictor 152 is also provided, and it has been found that providing such an air flow restrictor with an opening that widens to a uniform cross section acts to further reduce turbulence in the air flow passing to the chamber 154. In this example, the diameter of the opening of the air flow restrictor 152 is about 1.7 mm, and the optical aperture 150 has a diameter of about 0.5 mm. A flow rate of about 30 liters per minute is allowed to pass through the air cap 126. The shape of the air flow restrictor 152 is preferably without steps or abrupt changes in cross section, which helps to reduce the effects of air turbulence.

[0031] Figure 6 is a photograph of an air cap manufactured according to the description of Figures 3 and 4. The D-shaped conduits are formed by drilling holes from the surface normal to the blank and laser cutting the remaining profile. Of course, other manufacturing techniques would be possible.

[0032] Referring now to Figures 7 to 10, air flow modeling results are shown to illustrate the reduced air turbulence generated when using an air cap as described with reference to Figures 3 to 6 compared to a prior art air cap as shown in Figure 2. In particular, Figure 7 shows the air turbulence in the XY plane for the prior art air cap as shown in Figure 2. It can be seen that the air turbulence extends to the optical axis O through which the light beam passes. Figure 8 shows the air turbulence in the XY plane for the air cap of the present invention as described with reference to Figures 3 to 6. It can be seen that the modifications to the geometry of the conduit reduce the overall turbulence associated with the exhausting air, and the amount of air turbulence near the optical axis O is significantly reduced. This can also be seen from the air turbulence in the YZ plane plots as shown in Figure 9 for the prior art air cap and Figure 10 for the air cap of the present invention. Of course, the air cap could be positioned in any desired orientation relative to the axes of the associated measuring device or machine tool.

[0033] It should be remembered that the above is only one example of the invention and one skilled in the art would understand variations that would be possible. For example, the air cap could be attached to only one of the receiver or transmitter units of the reflector and also to the combined transmitter / receiver unit. It would also be possible to use the air cap with other measurement devices, not limited to tool setters. Although a removable air cap is described, it would also be possible for the conduits etc. to be formed as an integral part of the measurement device. For example, the conduits could be provided as part of an access panel or housing portion.

Claims

1. 1. A protective element for an optical measurement device, the protective element comprising a conduit through which light and air can pass, the conduit being configured such that, in use, a light beam is passed through the conduit along an optical axis and an air flow is directed out of the conduit along an air flow axis, the optical axis being non-parallel to the air flow axis, and the conduit having a cross-sectional profile that varies along the air flow axis.

2. 2. The protective element of claim 1, wherein the conduit has an inlet opening for receiving air and an outlet opening for discharging air, the inlet opening having a different cross-sectional area than the outlet opening.

3. 3. The protective element of claim 2, wherein the cross-sectional area of ​​the inlet opening is greater than the cross-sectional area of ​​the outlet opening.

4. A protective element according to any one of claims 1 to 3, characterized in that at least a portion of the conduit has an asymmetric cross-sectional profile.

5. A protective element according to any one of claims 1 to 4, wherein the conduit has an entrance opening which is substantially D-shaped when viewed along the optical axis.

6. A protective element according to any one of the preceding claims, wherein the conduit presents a substantially circular cross-section to light passing therethrough along the optical axis.

7. A protective element according to any one of claims 1 to 6, further comprising an optical aperture for focusing a light beam passed along the optical axis.

8. A protective element according to any one of the preceding claims, characterized in that the air flow axis is angled at an angle of 10° or more with respect to the optical axis.

9. A protective element according to any one of claims 1 to 8, characterized in that the protective element comprises one or more structures for releasable attachment to a transmitter or receiver of an optical measurement device.

10. Optical measuring device, characterized in that it comprises at least one protective member according to any one of claims 1 to 9.

11. 11. The optical measurement device of claim 10 including an internal air flow control member for minimizing turbulence of airflow directed into the conduit of the protective member.

12. 12. An optical measurement device according to claim 10 or 11, characterized in that each protective member includes an internal air chamber adjacent the conduit.

13. An optical measuring device according to any one of claims 10 to 12, characterized in that the measuring device is a break beam tool measuring device.

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