Tactile and / or optical distance sensor, system with such a distance sensor and method for calibrating such a distance sensor or such a system

Abstract

Tactile and / or optical distance sensor (100; 300) with a housing (102; 302) which has at least one elongated section (108; 340), a measuring arm (118; 344) which is arranged in the housing (102; 302), extends at least partially through the elongated section (108; 340) and has a tactile and / or optical scanning element (122; 322) at one end, a converter (128) configured to detect a position of the tactile scanning element (122) or a signal of the optical scanning element (322) and to generate associated scanning element measurement signals, and with a feed unit (52) with which the housing (102; 302) can be moved linearly along a feed direction (V), characterized in that a strain sensor (132; 332) is located in the area of ​​the measuring arm (118; 344) which runs through the elongated section (108; 340) or in an adjacent area directly bordering this area and wherein the strain sensor (132; 332) has a fiber Bragg grating (332).

Description

BACKGROUND OF THE INVENTION 1. Field of the invention

[0001] The invention relates to a tactile distance sensor, such as is typically attached to a coordinate measuring machine or a robot. Such distance sensors are used, for example, to measure the surface quality of high-quality workpieces such as engine blocks. The invention also relates to a system with such a distance sensor and a method for calibrating such a distance sensor or such a system with such a distance sensor. 2. Description of the state of the art

[0002] Tactile and / or optical distance sensors are used to measure the roughness of workpiece surfaces or to measure their geometry. These sensors are usually integrated into systems such as coordinate measuring machines (CMMs). Such CMMs typically include a table that supports the workpiece being measured and a positioning device that allows the distance sensor to be moved with high accuracy in three orthogonal directions (X, Y, and Z) relative to the table. CMMs with a movable table that can be moved relative to the stationary measuring device are also known.

[0003] Furthermore, coordinate measuring machines have an evaluation and control unit that controls the movements of the positioning device and evaluates the measured values ​​supplied by the distance sensor.

[0004] The distance sensor could, for example, be a so-called stylus measuring device. This typically has a movable measuring arm with a scanning element, such as a diamond stylus, attached to its end. During measurement, the scanning element is deflected by contact with the workpiece surface and moved perpendicular to the deflection direction along a feed direction by means of a linear drive. In this way, the workpiece surface to be measured can be scanned.

[0005] In recent years, the range of applications for such measuring devices has expanded considerably. In modern production processes, workpieces often have to be manufactured with such tight tolerances that continuous process monitoring is required.

[0006] A common problem is that the workpieces whose surfaces are to be measured automatically often have very complex shapes. Consequently, the surfaces to be measured are frequently located in hard-to-reach places. Modern measuring devices for coordinate measuring machines therefore often feature a movable and relatively slim arm that carries the distance sensor. This arm, with its multiple degrees of rotational freedom, can position the correspondingly slim and long distance sensor in virtually any desired pose relative to the workpiece.

[0007] This design, in turn, brings with it other problems. The longer and slimmer the arm on which a distance sensor is held, and / or the longer and slimmer the distance sensor itself, the more noticeable static or dynamic deformations of the arm and / or the distance sensor become during the measurement process. This leads to greater sensitivity to probing force fluctuations and floor vibrations. Such force fluctuations can cause parts within the measuring circuit to bend, both in skid-based and skid-less systems, which can then be recorded as measurement errors. The same problem arises when, within the same installation space, one wants to reduce sensitivity and thus create a measuring system suitable for systems with low damping, such as robot arms, or in environments with increased vibrations—such as manufacturing environments.

[0008] Document DE 10 2010 052 503 A1 describes a coordinate measuring machine in which deformations during the movement of the measuring machine are detected by means of an optical free-jet arrangement and the position of the probe head is thus corrected.

[0009] Document DE 10 2017 103 954 A1 describes a system with a tactile roughness sensor.

[0010] Document DE 30 11 003 A1 describes the use of an accelerometer which converts the forces occurring during a measurement process into signals which in turn serve to evaluate the measurement process. SUMMARY OF THE INVENTION

[0011] It is an object of the invention to provide an optical or tactile distance sensor which, on the one hand, enables a long and slim design and thus excellent accessibility of measuring points, and on the other hand offers a higher tolerance to natural vibrations or bending.

[0012] This problem is solved by a tactile and / or optical distance sensor according to independent claim 1, comprising a housing having at least one elongated section, a measuring arm arranged in the housing extending at least partially through the elongated section and having a tactile and / or optical scanning element at one end, a transducer configured to detect a position of the tactile scanning element or a signal of the optical scanning element and to generate associated scanning element measurement signals, and a feed unit with which the housing can be moved linearly along a feed direction.

[0013] According to the invention, a strain sensor comprising a fiber Bragg grating is located in the area of ​​the measuring arm that extends through the elongated section, or in an adjacent area directly bordering this area. Such a strain sensor can detect any deformation or bending of the elongated section and, if necessary, correct the measurement result of the distance sensor or the measuring arm by taking the measured value of the strain sensor into account.

[0014] This may include, for example, making a geometric correction to the length of the elongated section or the areas connected to the elongated section.

[0015] This allows, on the one hand, the distance sensor or measuring arm to be manufactured with a smaller diameter and / or thinner wall for better accessibility to the surface being measured, without negatively affecting the measurement accuracy. Conversely, the measurement accuracy of an existing distance sensor or measuring arm design can be increased.

[0016] The use of a strain gauge also offers the advantage that it can be positioned where a component bends that is relevant to the measurement result. Other deformations that have no effect on the measurement result can thus be disregarded.

[0017] The strain sensor is designed as a fiber Bragg grating, so that components already present for the optical scanning element, such as light sources or spectral analyzers, can be advantageously used.

[0018] Advantageously, the strain sensor is mounted at a point of high local strain. As already mentioned, this enables the detection of particularly relevant bending events and a corresponding correction of the scanning element's deflection values.

[0019] In one embodiment of the invention, the strain sensor can be integrated into the housing. This allows for particularly simple detection of any bending moments that may occur within the housing.

[0020] In a distance sensor that has at least one support element, the support element is preferably located on the elongated section. This embodiment is particularly advantageous when touch detection is to be performed using the strain sensor.

[0021] The task is also solved by a system for measuring the roughness of a workpiece surface, which includes a coordinate measuring machine with a positioning device, a distance sensor as described above, a measuring head which is mounted between the positioning unit and the distance sensor and is configured to exert actuating forces along all directions and to measure forces and / or deflections acting on the measuring head along all directions, and a control device for controlling the system.

[0022] In a preferred embodiment of the system, the control unit is configured to receive strain sensor values ​​from the strain sensor, receive sensor deflection values ​​from the scanning element, and correlate the strain sensor values ​​and the deflection values. "Correlate" here means relating the strain sensor values ​​and the deflection values ​​to each other in such a way that the scanning element deflection values ​​can be corrected using the strain sensor values. For example, a strain sensor value can be assigned to a specific scanning element deflection value by recording both at the same time – or by recording at the same point during a measurement process.Furthermore, in the case of a tactile scanning element, a specific contact force of the scanning element on the surface to be measured can be assigned to a specific strain sensor value, and a specific geometric change of the scanning element can be determined. Similarly, in the case of an optical scanning element, a specific geometric change of the scanning element can be inferred from a specific strain sensor value, which in turn can be used to correct the scanning element deflection value.

[0023] In a further development of the invention, the control unit is configured to detect probing based on a strain sensor reading. This improves the reliability of the system and helps to prevent damage to the system or the surface under investigation.

[0024] In a particular embodiment of the invention, the system comprises an optical distance sensor with an associated light source – for example, a white light source and / or a laser – and a spectrometer. According to the invention, the light source and / or the spectrometer can also be used as a light source and / or spectrometer for the strain sensor. This significantly reduces the complexity of the additional strain sensor in the system and enables a particularly cost-effective design.

[0025] In a further development of the invention, the system includes a beam splitter and / or an optical multiplexer. A beam splitter or an optical multiplexer can be used to couple the light from the light source into the strain sensor and / or back out again.

[0026] The problem is also solved by a method for calibrating a distance sensor as described above or for calibrating a system as described above, wherein the method comprises the following steps: Performing a probing with the distance sensor on a surface; acquiring a strain sensor value, a scanning element deflection value, and / or a contact force value; performing the probing and acquiring steps for multiple contact force values.

[0027] The surface can be the surface to be examined. However, it can also be any other surface suitable for mechanical or optical contact with the scanning element.

[0028] The steps of probing and recording the aforementioned values ​​can be carried out for at least two, but preferably for a larger number of values.

[0029] During data acquisition, at least the strain sensor value and the scanning element deflection value can be recorded. Correlating these two values ​​alone improves the accuracy of the recorded deflection values. If the contact force exerted on the scanning element and / or the total deflection of the distance sensor are also determined, the value calculated as scanning element deflection can be further refined.

[0030] A further development of the procedure includes the step of creating a lookup table and / or a functional relationship for scanning element deflection values, strain sensor values, total deflection, and / or contact force values. This enables improved correction of the deflection value using the strain sensor values, the total deflection, and / or the contact force values. In particular, statistical tools, physical, and / or other mathematical models can be used for the correction.

[0031] A preferred embodiment of the method comprises the step of determining a correction value for the scanning element deflection value based on the strain sensor value using the lookup table and / or the functional relationship.

[0032] A particularly preferred embodiment of the invention provides that a temporal correlation between the probing and the acquisition of the strain sensor value, the scanning element deflection value, the total deflection, and / or the contact force value is recorded. This enables a particularly precise assignment of the acquired values ​​and thus allows for a particularly high accuracy of the correction. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Exemplary embodiments of the invention are explained in more detail below with reference to the drawings. These show: Fig. 1 a perspective view of a coordinate measuring machine with a distance sensor according to the invention attached to it; Fig. 2 in a schematic sectional view a first embodiment of a distance sensor according to the invention; Fig.3 in a schematic sectional view a second embodiment of a distance sensor according to the invention; Fig. 4 in a schematic sectional view a third embodiment of a distance sensor according to the invention and Fig. 5 a method for calibrating a distance sensor or a system. DESCRIPTION OF PREFERRED EXAMPLES

[0034] Fig. Figure 1 shows a schematic perspective view of a coordinate measuring machine 10. The coordinate measuring machine 10 comprises a table 12, which includes a base 14 and a plate 16 made of hard stone. The plate 16 serves to hold a workpiece 18, the surface of which is to be measured. In the illustrated embodiment, the measurement is a spatially resolved roughness measurement.

[0035] The table 12 carries a positioning device 20, with which a distance sensor 100 according to the invention can be positioned relative to the table 12 with high accuracy. In the illustrated embodiment, the positioning device 20 is designed as a portal and comprises a portal 24, which is supported by two feet 26, 28 at the edges of the table 12 and can be moved by motor in the horizontal X-direction along the table 12. A boom 32 is mounted on a portal crossbeam 30, which connects the two feet 26, 28, such that it can be moved by motor along the longitudinal direction of the portal crossbeam 30, i.e., in the also horizontal Y-direction. A measuring carrier 36 is received in a vertically oriented receptacle 34 of the boom 32 and can be moved by motor along the vertical Z-direction. The movement is indicated by a double arrow in each case.

[0036] The distance sensor 100 according to the invention is attached to the measuring carrier 36 by means of an automatic coupling, so that it can be automatically exchanged for a differently constructed distance sensor. Typically, the positioning device 20 places the distance sensor 100 in a holder (not shown) at the edge of the table 12 and then moves it to another distance sensor held in the holder, activating the automatic coupling. In addition to a purely mechanical connection, the automatic coupling can also establish a communication link, enabling the exchange of control and measurement data between the distance sensor 101 and a control and evaluation unit 38. Alternatively, such communication can also take place via a radio interface.

[0037] The space that can be reached by the measuring carrier 36 by traversing movements along the X, Y and Z axes is, in the illustrated embodiment, on the order of about 2 m. 3 , so that significantly larger workpieces can be measured than in Fig. 1 is shown.

[0038] The positioning device 20 has at least one transducer for each of the three travel directions X, Y, Z, which sends information about the traveled paths back to the evaluation and control unit 38. This ensures that the position of the automatic coupling, to which the distance sensor 100 is attached and which essentially corresponds to the tool center point (TCP) of robots, is known with high accuracy in all travel positions.

[0039] The evaluation and control unit 38 controls the movements of the positioning unit 20 and evaluates the measured values ​​transmitted by the distance sensor 100. The evaluation also includes the computational correction of the measured values ​​supplied by the distance sensor 100. This allows static and dynamic influences of the positioning unit 20, thermal deformations of the table 12, and also the bending of tactile probes caused by probing forces to be taken into account.

[0040] The following refers to the Fig. 2 the construction of the roughness probe 100 according to the invention is explained in more detail.

[0041] Fig. Figure 2 shows the distance sensor 100 in a schematic, partially cut-away side view.

[0042] The distance sensor 100 has a housing 102 that can be divided into a connecting section 104, an intermediate section 106, an angled section 108, and a vertical section 110. The connecting section 104 extends with its longitudinal axis along the feed direction V, along which the distance sensor 100 is moved during operation. The connecting section 104 attaches the distance sensor 100 to a movable mounting element 56 of a feed unit 52. In addition to the mounting element 56, the feed unit 52 also includes a drive unit 54 and is part of an arm 40. The arm 40 serves to position the distance sensor 100 in a specific pose at a specific location on the workpiece 18 to be measured.

[0043] The intermediate section 106 of the housing 102, which adjoins the connecting section 104, also extends with its longitudinal axis along the feed direction V, but has a smaller diameter than the connecting section 104. The intermediate section 106 transitions into the developed section 108 at a bend 112. The longitudinal axis of the angled section 108 extends at the Fig.In the embodiment shown in Figure 2, the intermediate section 106 extends along a longitudinal axis that forms an angle of 45° with the vertical longitudinal axis. The angled section 108 transitions into the vertical section 110 at a second bend 114. Consequently, the vertical section 110 is oriented perpendicular to the feed direction V. A support element 116 is attached to the end of the vertical section 110 that faces the object 18 under investigation or its surface. The support element 116 is rigidly connected to the housing 102 and rests against the workpiece 18 during measurements, preventing the actual scanning element from vibrating during the measurements.

[0044] The housing 102 contains an internal measuring arm 118, which is rotatably mounted about a rotary axis 120. At its end facing the workpiece 18, the measuring arm 118 carries a scanning element 122, which can, for example, be a diamond stylus. When the scanning element 122 is deflected by the surface 124 of the workpiece 18, the opposite end 126 of the measuring arm 118 pivots. This deflection is detected by a transducer in the form of a measuring system 128. Depending on the rotational position of the measuring arm 118, the measuring system 128 generates measurement signals that represent the rotational positions and thus the deflection of the scanning element 122. The measuring system 128 can detect the rotational position of the measuring arm 118 inductively, optically, magnetically, or pneumatically.In the present embodiment, the measurement signals generated by the measuring system 128 are transmitted as scanning element deflection values ​​via a line 130 to the connection section 104 and finally to the control and evaluation unit 38 for further processing.

[0045] During a measurement, the distance sensor 100 is moved along the feed direction V using the feed unit 52. Based on a time- and / or location-dependent detection of the deflections of the scanning element 116, the roughness of the surface 124 can be determined.

[0046] Despite the fact that in Fig. Due to the double angulation of the housing 102 shown in Figure 2, it can be subject to a bending moment during a measurement and / or probing and thus deform elastically. In order to detect such deformation and take it into account when evaluating the scanning element deflection value, the distance sensor 100 features Fig.2 a strain sensor 132. This is shown by way of example in the embodiment of the Fig. 2 attached to the angled section 108. However, this mounting location is merely an example. The strain sensor 132 can be mounted wherever critical bending moments are expected to occur. In a skidless system – as in Fig. As shown in Figure 2, the transition between the support element and the needle to the suspension of the measuring system, as well as between the suspension of the measuring system and the exchange interface or the feed mechanism, is critical. One or more strain sensors 132 can be attached at all these points. In the present embodiment of the Fig. 2, this would therefore in principle be along the entire housing 102 including the connection section 104, possibly also in the area of ​​the fastening element 56.

[0047] The strain sensor 132 is as described in Fig.Figure 2 shows the strain sensor attached to the outside of housing 102. However, this is also only an example. The strain sensor 132 could also be integrated inside housing 102 or even into the housing wall of housing 102 itself.

[0048] The strain sensor 132 is connected to the connection section 104 via a line 134 - comparable to the line 130.

[0049] If the distance sensor 100 is moved along the feed direction V, and the support element 116 slides over the surface 124 of the workpiece 18, the forces transmitted to the housing 102 are detected by the feed unit 52, and the distance sensor 100 is adjusted accordingly. Nevertheless, bending moments can occur on parts of the housing 102, on the entire housing 102 relative to the connection section 104, and / or at the transition to the fastening element 56 due to the forces that occur. These bending moments can be detected by the strain sensor 132, provided it is installed at the appropriate location, and transmitted via line 134 through an electrical interface to the control and evaluation unit 38 for further processing. The control and evaluation unit 38 can then correct the scanning element deflection values ​​based on the strain sensor values, if necessary with knowledge of the contact force values ​​and the total deflection.

[0050] Depending on the design and construction of the distance sensor 100, the strain sensor 132 itself can be implemented using a suitable technology. For example, the strain sensor can be designed as an electrically based strain gauge or based on an optical technology, e.g., a fiber Bragg grating.

[0051] Fig. Figure 3 illustrates an alternative embodiment of a distance sensor 200 in a schematically represented sectional view. Identical or comparable features are designated with the same reference numerals – plus 100 – as used in Fig. 2 have been used.

[0052] While the design of Fig. 2 a distance sensor 100 which operates without a skid, but only with a support element 116, indicates the Fig.3. A distance sensor 200, which operates on a skid-based design. Accordingly, the housing 202 of the distance sensor 200 also has a connection section 204, which transitions into an intermediate section 206. In contrast to the distance sensor 100, the distance sensor 200 of the Fig. 3 subsequently a straight section 240, which has a skid 242 at its end facing the object 18 to be examined or its surface 224 to be examined.

[0053] As the skid 242 slides across the surface 224 of the workpiece 18 along the feed direction during movement of the distance sensor 200, the measuring arm 218 moves due to the contact of the stylus tip 222 with the surface 224. During this movement, the measuring arm 218 rotates about its axis of rotation 220. This movement is detected in the measuring system 228 and transmitted via an electrical line 230 to the control and evaluation unit 38 for further processing. The distance sensor 200 also has a strain sensor 232 in the straight section 240 of the housing 202, which can transmit the generated strain sensor values ​​to the control and evaluation unit 38 via a line 234. Here, too, the strain sensor 232 is positioned at a location subject to particularly high bending moments. The installation can be carried out externally, integrated into the housing wall, or even inside the housing 202.

[0054] In the Fig. 2 and Fig. 3. Strain sensors 132 and 232 were shown as examples. These can both be electrical and optical in nature. The scanning techniques shown are described in the Fig. 2 and Fig. 3 mechanically, i.e. tactilely. Alternatively, the scanning elements 118, 218 can also operate on an optical basis.

[0055] Fig. Figure 4 shows a schematic cross-sectional view of a third embodiment of a distance sensor 300. Identical or comparable features are designated with the same reference numerals, to which 100 is added.

[0056] The distance sensor 300 also has a housing 302, which has a support element 316 at its end facing the object to be examined. The support element 316 can also be designed as a skid. The housing 302 can be moved along a feed direction V by means of a feed unit 352.

[0057] Inside the housing 302, a fiber Bragg grating is arranged in the elongated section 340 as a strain sensor 332. The fiber Bragg grating is connected to an optical splitter 341 via a fiber optic cable 334. The optical splitter 341, or optical multiplexer, has a feed line 342 in the form of an optical waveguide. The fiber Bragg grating 332 can be supplied with the appropriate light source via this feed optical waveguide 342.

[0058] In this embodiment, the scanning element 322 is also an optical distance sensor. This sensor also has an optical line 344 to the optical multiplexer 340. In this way, the scanning element 322 can also be supplied with the required light source, and the optical signals can be transmitted from both the optical distance sensor 322 and the fiber Bragg grating 332, which functions as a strain sensor, via the optical multiplexer 340 and the waveguide 342 to the control and evaluation unit 38. There, the light source and a spectrometer (not shown) can introduce the corresponding signals into the optical waveguide 342 and read them out. This embodiment has the significant advantage that the optical distance sensor can be used without requiring additional components such as a light source and spectrometer, and the fiber Bragg grating 332 can also be used for strain measurement.

[0059] Fig. Figure 5 illustrates a procedure for calibrating a distance sensor as described above or a system as described above.

[0060] In a first step (S1), a distance sensor is pressed onto a flat surface with an initial force. This flat surface could be, for example, a glass or a ceramic plate. This step is also referred to as probing.

[0061] In a second step (S2), the deflection of the scanning element, such as a needle or an optical distance sensor, the strain sensor value of a strain sensor, and the force applied to the scanning element are measured.

[0062] Steps S1 and S2 are repeated with different force values ​​until a sufficient number of measurements have been obtained (S3).

[0063] A look-up table is created from the measured values ​​obtained in this way (S4).

[0064] To correct a needle deflection measurement recorded during a measurement, the corresponding value can now be looked up in the lookup table. Missing needle deflection values ​​can be interpolated using the available data points.

[0065] Such a table might look like this, for example: Power Total displacement elongation value Deflection 100 mN 3 µm 150 nm 800 nm 200 mN 5 at 230 nm 1300 nm 300 mN 10 µm 270 nm 1700 nm

[0066] For example, if a later measured value for the strain value is 230 nm, the total deflection of the needle can be corrected by 5 µm to an actual needle deflection of 1.3 µm.

Claims

[1] Tactile and / or optical distance sensor (100; 300) with a housing (102; 302) which has at least one elongated section (108; 340), a measuring arm (118; 344) which is arranged in the housing (102; 302), extends at least partially through the elongated section (108; 340) and has a tactile and / or optical scanning element (122; 322) at one end, a converter (128) configured to detect a position of the tactile scanning element (122) or a signal of the optical scanning element (322) and to generate associated scanning element measurement signals, and with a feed unit (52) with which the housing (102; 302) can be moved linearly along a feed direction (V), characterized by , that a strain sensor (132; 332) is located in the area of ​​the measuring arm (118; 344) which runs through the elongated section (108; 340) or in an adjacent area directly bordering this area and wherein the strain sensor (132; 332) has a fiber Bragg grating (332). [2] Distance sensor (100; 300) according to one of the preceding claims, wherein the strain sensor (132; 332) is mounted at a location with a high local strain. [3] Distance sensor (100; 300) according to one of the preceding claims, wherein the strain sensor is integrated in the housing (102; 302). [4] Distance sensor (100; 300) according to one of the preceding claims with at least one support element, wherein the support element (116; 316) is located on the elongated section (108; 340). [5] System for measuring the roughness of a workpiece surface (18), with a) a coordinate measuring machine (10) which has a positioning device (20), b) a distance sensor (100) according to one of the preceding claims, c) a measuring head (36) which is mounted between the positioning device (20) and the distance sensor (100) and is configured to exert actuating forces in all directions and to measure forces and / or deflections acting on the measuring head (36) in all directions and d) a control device (38) for controlling the system. [6] System according to claim 5, wherein the control device (38) is configured to receive strain sensor values ​​from the strain sensor (132), to receive scanning element deflection values ​​from the scanning element (118) and to correlate the strain sensor values ​​and the deflection values. [7] System according to one of claims 5 or 6, wherein the control device (38) is configured to detect probing based on a strain sensor value of the strain sensor (132). [8] System according to one of claims 5 to 7 comprising an optical distance sensor (300) as well as an associated light source and a spectrometer, wherein the light source and / or the spectrometer can also be used for the strain sensor (332). [9] System according to claim 8, wherein the system comprises a beam splitter (341) or optical multiplexer. [10] Method for calibrating a distance sensor (100; 300) or a system according to any of the preceding claims, comprising the steps: Performing a probing operation with the distance sensor (100; 300) on a surface; Acquiring a strain sensor value of the strain sensor (132; 332), a scanning element deflection value of the scanning element (122; 322), a total deflection and / or a contact force value; Performing the probing and sensing steps for multiple contact force values. [11] The method of claim 10, comprising the step of: Creating a lookup table and / or a functional relationship for contact force values, scanning element deflection values, total deflection and / or strain sensor values. [12] Method according to claim 11, comprising the step Determining a correction value for the scanning element deflection value based on the strain sensor value using the lookup table and / or the functional relationship. [13] Method according to one of claims 10 to 12, wherein a temporal correlation of probing and acquiring the strain sensor value of the strain sensor (132; 332), the scanning element deflection value of the scanning element (122; 322), the total deflection and / or the contact force value is acquired.

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