Workpiece measurement methods

Abstract

Workpiece measurement methods for dimensional measurement of workpieces whose surface is at least partially covered by contamination in the form of a surface layer, for example a thin oil film, in which the workpiece including the surface layer is measured by means of an optical first measuring method to obtain at least one first measured value, wherein the first measuring method comprises a shadow image method, in which, by means of a second measuring method, the layer thickness of the surface layer is measured at at least one point on the workpiece in order to obtain at least one second measured value, wherein, to determine the dimension of the workpiece without surface layer, at least one first measured value is corrected using at least one second measured value.

Description

[0001] The invention relates to a workpiece measurement method for determining the dimensions of workpieces.

[0002] In dimensional metrology, appropriate workpiece measurement methods are used to determine the dimensions of a workpiece.

[0003] During the rough and fine machining of metallic components, it frequently occurs that the surface of the component becomes partially or almost completely covered with a thin surface layer and is thus contaminated. This surface layer can be, for example, and in particular, a thin film of oil.

[0004] When the workpiece is subjected to dimensional measurement, optical methods are used in particular.

[0005] In the shadow image principle, a telecentric light source illuminates an opposing camera. The workpiece is placed in the beam path and casts a shadow on the camera. The dimensions of the workpiece in the measuring plane are then calculated from the information of the light-dark transition.

[0006] Another well-known method for optical dimensioning follows the laser measurement principle. Here, a transmission unit emits a fine laser beam that scans the measurement field (camera) at high speed. The workpiece temporarily interrupts the beam path, and the dimension of the workpiece in that plane can be calculated by measuring the duration of the shadow.

[0007] The known methods allow for the quick and accurate determination of workpiece dimensions. However, a disadvantage is that the measurement results of both methods are influenced by the presence of a surface layer on the workpiece, resulting in dimensions that are determined to be larger than those of a clean workpiece, i.e., a workpiece without a surface layer.

[0008] To solve this problem, it is known to select the wavelength of the light source such that the measurement results are not affected by the surface layer. For example, it is known to use a light source with a wavelength of 0.1 nm, which largely avoids distortion of the measurement results by a surface layer, such as an oil film. However, the disadvantages of this approach are the additional costs for both the light source and the necessary optical elements and safety precautions.

[0009] DE 10 2011 077 567 A1 discloses a method for determining the surface topography of coated, reflective surfaces, in which the three-dimensional topography of the coating is determined by means of a chromatic white light measurement and subsequently the thickness of the coating is determined by means of UV interferometry and the surface topography of the coated surface is determined by comparison with the total dimensions of the coated surface.

[0010] US Patent 6 091 500 A discloses a method in which the inner diameter of a pipe section is determined by a combination of a shadow image method with an ultrasonic measuring method.

[0011] The invention is based on the objective of providing a workpiece measurement method that is relatively simple and inexpensive to implement and delivers highly accurate measured values.

[0012] The invention provides a workpiece measurement method for determining the dimensions of a workpiece whose surface is at least partially covered with a surface layer. In the method according to the invention, the workpiece, including the surface layer, is measured using an optical first measurement method to obtain at least one first measurement value. Furthermore, the thickness of the surface layer is measured at at least one location on the workpiece using a second measurement method to obtain at least one second measurement value. To determine the dimensions of the workpiece without the surface layer, at least one first measurement value is corrected using at least one second measurement value. The invention provides that the first measurement method is a shadow image method. In such a method operating on the shadow image principle, a telecentric light source illuminates an opposing camera.The workpiece is positioned in the beam path and casts a shadow on the camera. The dimensions of the object in the measurement plane are determined from the information about the light-dark transition in the camera image.

[0013] The invention thus applies the known method of layer thickness measurement to dimensional metrology. In the method according to the invention, initial measured values ​​are determined that represent the dimensions of the workpiece, including the surface layer. According to the invention, determining the dimensions of a workpiece means that at least one dimension of the workpiece, for example, the diameter of a shaft, is determined, or that several dimensions of the workpiece are determined.

[0014] In the method according to the invention, the thickness of the surface layer is further measured at at least one point on the surface of the workpiece. The measured layer thickness constitutes at least a second measurement point.

[0015] To determine the dimensions of the workpiece without the surface layer, for example, the dimensions of an uncontaminated workpiece, the initial measurements are corrected using at least one second measurement. Since the surface layer increases the dimensions of the workpiece, the second measurement(s) can be subtracted from the first measurement(s).

[0016] The invention thus provides a workpiece measurement method that is relatively simple and inexpensive to implement and delivers highly accurate measured values.

[0017] A further advantage of the workpiece measurement method according to the invention, which will also be referred to as the method below, is that fundamentally known and proven optical measurement methods, for example a method based on the shadow image principle or a laser measurement method, can be retained, so that the existing measuring equipment can continue to be used unchanged. This measuring equipment is merely supplemented by a suitable apparatus for measuring the thickness of the surface layer.

[0018] Assuming that the thickness of the surface layer is the same or substantially the same along the surface of the workpiece, as is often the case with a thin oil film, for example, it may be sufficient according to the invention to determine the thickness of the surface layer at one point on the workpiece and, based on this measurement, to assume that the thickness of the surface layer is substantially constant along the surface of the workpiece. In such an embodiment of the method according to the invention, the same second measurement is always used to correct the first measurement or measurements. To improve the accuracy of the method according to the invention, a further development of the invention provides that the thickness of the surface layer is determined at least section by section with spatial resolution using the second measurement method.In this embodiment, the layer thickness is determined at several or a multitude of locations along the workpiece surface, so that the spatial distribution of the surface layer thickness along the workpiece surface is determined and used to correct the initial measurement(s). This embodiment also enables accurate measurement results of the workpiece dimensions without a surface layer, even if the surface layer thickness varies along the workpiece surface.

[0019] Another advantageous embodiment of the invention provides that the second measuring method is a non-contact measuring method. This embodiment has the advantage that a measuring apparatus for carrying out the second measuring method does not come into contact with the workpiece, so that contamination of the measuring apparatus, for example by an oil film adhering to the workpiece, is reliably avoided. A further advantage of this embodiment is that the first measuring method and the second measuring method can be carried out simultaneously or overlapping in time.

[0020] According to the invention, the first and second measurement methods can be carried out sequentially. However, an advantageous embodiment of the invention provides that the first and second measurement methods are carried out simultaneously or overlapping in time. In this way, the measurement times are reduced, which has a positive effect on cycle times, for example, when a number of identical workpieces are to be measured sequentially in series production.

[0021] Another advantageous embodiment of the invention provides that the first measuring method is a laser measuring method. In this embodiment, a transmission unit emits a fine laser beam that scans the measuring field (camera) at high speed. The workpiece temporarily interrupts the beam path. By measuring the shadow duration, the dimensions of the workpiece in this plane can be calculated.

[0022] As a second measurement method, any suitable method for measuring layer thickness can be used.

[0023] An advantageous further development of the invention provides that the second measuring method is an ultrasonic measuring method. Such ultrasonic measuring methods can be carried out with relatively little equipment and enable the determination of the surface layer thickness with high accuracy.

[0024] According to the invention, the second measurement method can, for example, be a method for determining electrical parameters. In this respect, an advantageous embodiment of the invention provides that the second measurement method is an eddy current method and / or a capacitive method. In an eddy current method, an alternating magnetic field is generated by a coil, which induces eddy currents in the material under investigation. During the measurement, the eddy current density is detected by a sensor through the magnetic field generated by the eddy current, whereby the amplitude and / or the phase shift relative to the excitation signal can be measured.

[0025] Another advantageous embodiment of the invention provides that the second measuring method is an optical measuring method.

[0026] A further development of the aforementioned embodiment provides that the second measurement method comprises an optical interferometry method. Such interferometry methods are generally known to those skilled in the art and are therefore not explained in detail.

[0027] Another further development of the embodiment, in which the second measuring method is an optical measuring method, provides that the second measuring method comprises a microscopic measuring method, in particular one operating according to the principle of confocal technology.

[0028] Another training method stipulates that the second measurement method is a measurement method that works according to the principle of ellipsometry.

[0029] Another advantageous embodiment of the invention provides that the second measuring method is based on the use of electromagnetic waves to determine the thickness of the surface layer. In this embodiment, electromagnetic waves of any suitable wavelength can be used, for example, microwaves or radar waves.

[0030] Another embodiment of the invention provides that, within the framework of the second measuring method, the optical layer thickness is measured and, using known or determined, in particular measured, properties of the surface layer, especially the refractive index of the material of the surface layer, the mechanical layer thickness is determined from the optical layer thickness.

[0031] In this respect, the invention provides for the use of a layer thickness measurement method in dimensional metrology for correcting measured values ​​that represent dimensions of a workpiece including a surface layer covering at least part of the workpiece surface.

[0032] In the context of the invention, the terms "first measurement method" and "second measurement method" serve to indicate that these are measurement methods based on different measurement principles. They do not imply a chronological sequence in which the measurement methods are performed. If the first and second measurement methods are performed sequentially, either the first measurement method can be performed first, followed by the second measurement method, or the second measurement method can be performed first, followed by the first measurement method. The first and second measurement methods can also be performed simultaneously or with any suitable temporal overlap.

[0033] The use according to the invention can be further developed analogously as specified in dependent claims 2 to method claim 1.

[0034] According to the invention, the thickness of the surface layer is measured using a layer thickness measurement method. The determined layer thickness measurements are used to correct measurements, or at least one measurement, that represent the dimension, or at least one measure, of the workpiece, including the surface layer.

[0035] The invention is explained in more detail below with reference to the accompanying drawing, in which an arrangement for carrying out an embodiment of the method according to the invention is shown in a highly schematic and block-diagram-like manner.

[0036] The single figure in the drawing depicts a workpiece in the form of a shaft 2, whose axial direction is perpendicular to the plane of the drawing. Reference numeral 4 in the drawing indicates that the surface of the shaft 2 is covered, at least partially, and in the example shown, circumferentially, with a surface layer.

[0037] In the method according to the invention, the workpiece 2, including its surface layer 4, is measured using an optical first measuring method to obtain at least one first measured value. In the illustrated embodiment, the first measuring method comprises a shadow image method in which light rays 6 from a telecentric light source illuminate an opposite camera 8. The workpiece 2 is arranged in the beam path of the light source and produces a shadow on the camera 8, whereby the dimension of the workpiece 2 in the measuring plane, i.e., the diameter of the shaft 2 in the illustrated embodiment, is determined from the information of the light-dark transition. It is evident that the surface layer 4 influences the measured value in the shadow image method such that the diameter of the shaft 2 is determined to be too large compared to a clean shaft, i.e., without the surface layer 4.If the diameter of the shaft is to be determined at only a single axial point on the shaft, it is sufficient to perform a single measurement, so that the result of the measurement is a single initial measurement value, which is transmitted from the camera 8 to a memory location 10 and stored there. If the diameter of the shaft 2 is to be determined at different axial points, the measurement is carried out using the shadow image method at different axial points on the shaft 2, whereby the measurement values ​​are then stored in the memory location 10 and assigned to the respective axial point.

[0038] The method according to the invention provides that the thickness of the surface layer 4 is measured at at least one point on the workpiece using a second measuring method to obtain at least one second measured value. In the illustrated embodiment, it is shown, purely by way of example, that the second measuring method is an ultrasonic measuring method using an ultrasonic transmitter / receiver 12. The layer thickness determined by the ultrasonic transmitter / receiver is supplied to the memory 10. Assuming that the thickness of the surface layer 4 is constant in the circumferential direction of the shaft 2, it is sufficient according to the invention to measure the layer thickness at only one circumferential point.

[0039] According to the invention, to determine the diameter of the shaft 2 without the surface layer 4, the first measured value, i.e., the diameter determined using the shadow image method, is corrected using at least one second measured value, i.e., the thickness of the surface layer determined by the ultrasonic transmitter / receiver. In the illustrated embodiment, the determined layer thickness is accordingly multiplied by two and subtracted from the diameter determined using the shadow image method.

[0040] If the layer thickness cannot be assumed to be constant in the circumferential direction of shaft 2, the method according to the invention is carried out for different circumferential locations of shaft 2 by, for example, rotating it about its axis of rotation. This rotation can be carried out continuously or stepwise, whereby corresponding measured values ​​can be recorded continuously or stepwise using the first measuring method and the second measuring method.

[0041] If, in addition, the diameter of the shaft 2 is to be determined at different axial locations, the shaft 2 can be moved perpendicular to the plane of the drawing and measurements can be carried out at the respective axial location, whereby the movement of the shaft can again be continuous or stepwise.

[0042] The workpiece measurement method according to the invention enables the measurement of workpieces with high accuracy in a relatively simple and cost-effective manner.

Claims

[1] Workpiece measurement methods for dimensional measurement of workpieces whose surface is at least partially covered by contamination in the form of a surface layer, for example a thin oil film, in which the workpiece including the surface layer is measured by means of an optical first measuring method to obtain at least one first measured value, wherein the first measuring method comprises a shadow image method, in which, by means of a second measuring method, the layer thickness of the surface layer is measured at at least one point on the workpiece in order to obtain at least one second measured value, wherein, to determine the dimension of the workpiece without surface layer, at least one first measured value is corrected using at least one second measured value. [2] Method according to claim 1, characterized by, that the layer thickness of the surface layer is determined at least section by section with spatial resolution along the surface of the workpiece using the second measuring method. [3] Method according to claim 1 or 2, characterized by that the second measuring method is a non-contact measuring method. [4] Method according to any one of the preceding claims, characterized by that the first measurement procedure and the second measurement procedure are carried out simultaneously or overlapping in time. [5] Method according to any one of the preceding claims, characterized by that the first measurement method includes a laser measurement method. [6] Method according to any one of the preceding claims, characterized by that the second measurement method includes an ultrasound measurement method. [7] Method according to any one of claims 1 to 5, characterized by that the second measurement method includes an eddy current method and / or a capacitive method. [8] Method according to any one of claims 1 to 5, characterized by that the second measurement method includes an optical measurement method. [9] Method according to claim 8, characterized by that the second measurement method includes an optical interferometry method. [10] Method according to claim 8 or 9, characterized by that the second measurement method includes a microscopic measurement method, in particular one that works according to the principle of confocal technology. [11] Method according to any one of claims 8 to 10, characterized by that the second measurement method includes a measurement method operating on the principle of ellipsometry. [12] Method according to any one of claims 1 to 5 or 7 to 11, characterized by , that the second measurement method is based on the use of electromagnetic waves to determine the layer density of the surface layer. [13] Method according to any one of claims 1 to 5 or 8 to 12, characterized by, that the optical thickness of the surface layer is measured and, using known or determined, in particular measured, properties of the surface layer, especially the refractive index of the material of the surface layer, the mechanical thickness of the surface layer is determined from the optical thickness.

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