DE3702691C2 - Non-contact distance sensor - Google Patents

Description

The invention relates to a non-contact distance sensor, the Scheimpflug condition being met for the arrangement of the projection axis, detectors and optical imaging elements. Such distance sensors are described in the following documents:
DE-A1-28 53 978, DE-A1-33 02 948, EP-A1-01 63 347, EP-A1-01 56 991, US-43 73 805 and US-44 53 083.

In these known distance sensors, a light beam is in usually projected perpendicular to the surface of the object, whose distance is to be determined. The projected light point is then from one with its optical axis against the Projection axis inclined lens on a linear, photo electrical detector such as B. a diode row or a so-called named CCD array shown. This creates one of the Position of the striking light spot dependent signal, which is a measure of object distance.

The usable measuring range of such a distance sensor is Part of the projection axis that is on the photosensitive surface of the detector is mapped, and its resolution is determined by the Number of individual elements of the detector determined. For highly accurate Measurements are therefore only available in a relatively small measuring range Available, which is also limited by the fact that Signals of the elements in the peripheral areas of the detector are often for other tasks such as B. needed for machine tracking and therefore not for the actual Measurement task can be used.

If you use a distance sensor of the type mentioned as an optical probe in a coordinate measuring machine, there is the demand for the highest possible measurement accuracy or on solution with the largest possible measuring range. Because if the measuring range is too small, the case can occur that at Scan the workpiece to be measured over an edge  and the measurement range is abruptly left without the machine control contains data about the tracking direction. In such cases, the probe would have to be controlled manually its measuring range can be placed back on the workpiece. Such faults interrupt the otherwise automatically ongoing measuring process.

From DE-A1-33 00 333 a distance sensor is known for which the problem of exceeding the measuring range is solved, that either the imaging lens or a deflecting mirror in Imaging beam path are shifted when the imaged Measuring point on the detector reaches its edge areas. By the mechanical movement of the optical component in question the measuring point is returned to the center of the detector, which equates to a shift in the measuring range. A disadvantage here is, however, the use of mechanically moving parts that the control loop with which the measuring range shift causes will make you lazy. The well-known probe is therefore for one fast scan operation not suitable.

EP-A1-013 45 97 describes a distance sensor based on the triangulation principle ( FIGS. 4, 5) which has two detectors and two imaging optics which are arranged symmetrically to one another. As a result, one and the same area of the projection axis is imaged on both detectors with the same imaging scale. This symmetrical arrangement serves to improve the signal-to-noise ratio compared to false light. The problem of exceeding the measuring range is neither addressed nor solved in this document.

It is the object of the present invention, one non-contact distance sensor of the type mentioned at the beginning create that as an optical probe in fast scan mode can be used.

This task is accomplished by training with those in claim 1 specified features solved.  

The result is two distance sensors different measuring ranges and different Measurement accuracy combined in one probe. This is without mechanically moving parts for fast scanning suitable optical probe created, which also over high Edges can be passed. Because when measuring in Scan operation leave the short but high-resolution measuring range the signals of the one behind it or him comprehensive measuring range with lower resolution than Control data for the return of the probe in the more precise Measuring range available.

Since the projection part of the distance sensor and also the Control electronics for the different measuring ranges assigned detectors can be used together the solution according to the invention is inexpensive and it can be a very compact probe can be realized, the universal can be used.

The simultaneous presence of two different ones Measuring ranges also allows an operation in which the Measurement object in a so-called orientation scan first quickly leaves the measuring line to get the track data for the to win the actual measuring process. With this Orientation scan becomes the larger measuring range with smaller Resolution used, which also has a larger working distance has, so that the risk of collision during the orientation scan is greatly reduced. After the orientation scan you can immediately be started with the actual precision measuring process without a probe change would have to be made.

Further advantages of the invention can be found in the subclaims and the following description of exemplary embodiments with reference to FIGS . 1 to 6 of the drawing.

Fig. 1 is a schematic diagram of a first embodiment of the invention;

Fig. 2 is a schematic diagram of a second embodiment of the invention;

Fig. 3 is a schematic diagram of a third embodiment of the invention;

Fig. 4 is a schematic diagram of a fourth embodiment of the invention;

Fig. 5 is the schematic diagram of a fifth embodiment of the invention;

Fig. 6 is the schematic diagram of a sixth embodiment of the invention.

The distance sensor shown in Fig. 1 consists in the projection part of a light emitting diode or laser diode (L₁) and a projection object, which projects a light beam with a narrow diameter along the projection axis (Z₁) approximately perpendicular to the workpiece to be measured (W₁). The resulting light spot on the workpiece surface (P₁) is imaged by a first lens (O₁) on a detector (D₁) in the form of a diode array or a CCD array and by a second lens (O₂) on a similar second detector (D₂). The arrangement of the projection axis (Z₁), the main planes (H₁) and (H₂) of the lenses (O₁) and (O₂) and the plane (E₁) in which the two detectors (D₁) and (D₂) are located chosen so that the Scheimpflug condition is met. For the case shown in FIG. 1, this condition means that the planes mentioned intersect the projection axis at one point. This makes it possible to always sharply image the point (P₁) projected onto the workpiece surface regardless of the distance in the direction of the projection axis (Z₁) on the detectors (D₁) and (D₂).

The lens (O₁) with its main plane (H₁) by one Angle α₁ equal to 30 ° inclined to the detector plane (E₁), while the angle of inclination α₂ of the lens (O₂) is 45 °. This results in different for both lenses Object and image distances, so that the middle figure scales under which the projection axis (Z₁) on the two Detectors (D₁) and (D₂) is shown, even with the same Focal length of the lenses (O₁) and (O₂) are different, so two differently sized, superimposed measuring probes are created rich (₁) and (₂) for the detectors (D₁) and (D₂). In the The table below shows the size of the measuring area range depending on the focal length of the lenses (O₁) or (O₂) based on a usable line length of 12 mm for the detectors (D₁) or (D₂) can have:

Table 1

The values relate to the arrangement sketched in Fig. 1 and assume that the detector plane (E₁) and the projection axis (Z₁) form a right angle with each other.

In the described embodiment, the length of the two Diode rows (D₁ and D₂) and also the number of light sensitive individual elements on both lines the same (e.g. 512 pixel). However, it can prove to be advantageous if, for. B. for the diode array assigned to the more precise measuring range (Z₂)  (D₂) a copy with a larger number of individual elements (e.g. 1024 pixels) is selected to enable the resolution of the Measuring range (₂) to improve.

About the signals of the diode row (D₂), the distance (Z) to the workpiece surface (W₁) in the measuring range (₂) with high Accuracy can be measured. At the same time, the signals the diode row (D₁) assigned to the larger measuring range (₁) serve as control data for a measuring machine, on the in the distance sensor in several spatial directions is attached and from which he on the contour of the workpiece (W₁) is moved along so that the workpiece surface is always in the Measuring range (₂) remains. If the measuring range (₂) is left, by the distance sensor for example the edge (K) of the work pieces (W₁) runs over, the Dio denzeile (D₂) the machine drive stopped and the edge (K) then traveled at a lower speed at which the machine drives can follow the workpiece contour.

In the embodiment shown in Fig. 1, the scanning direction perpendicular to the plane spanned by the axes of the projection system and the imaging systems must be selected to carry out the above-mentioned mode of operation in order to avoid switching off one of the two measuring ranges by the edge K, ie the arrow A and the workpiece contour W 1 extend perpendicular to the plane of the paper. This restriction with regard to the scanning direction is no longer necessary if the structure described below according to FIG. 2 is selected:
The distance sensor shown in the embodiment of FIG. 2 differs from that of FIG. 1 only in that the planes in which the two imaging systems consisting of the lenses (O₁₁) and the detector (D₁₁) or the lens (O₁₂) and and the detector (D₁₂) lie around the projection axis are rotated and enclose an angle β 90 °. In this spatial arrangement of the two imaging systems in different planes, a very compact mechanical structure for the distance sensor can also be realized.

It should also be noted that the two diode rows (D₁₁) and (D₁₂) in this embodiment alternating from one common electronics unit (L) can be controlled and also an operation is possible in which only a single one of the two diode rows is activated.

In the exemplary embodiment according to FIG. 3, the distance sensor has, in addition to the projection lens (P₂₁,) which is assigned to the light source (L₂₁), only a single imaging lens (O₂₁) with a large image angle for the two diode lines (D₂₁) and (D₂₂) arranged one behind the other. This lens is inclined with its main plane (H₂₁) by an angle α = 45 ° to the detector plane (E₂), so that the average imaging scale, under which the projection axis is imaged, is 1: 1. But since for imaging on the detectors (D₂₁) and (D₂₂) under different image angle ranges of the lens ( 021 ) are used, the measuring ranges (₂₁) and (₂₂) shown on the diode rows are different in size and the spatial resolution in the two measuring ranges differently. The larger and in its lower edge area clearly non-linear measuring range (Z₂₁) is assigned a longer working distance. With this measuring range, the object to be measured can be quickly scanned, for example in the orientation scan operating mode mentioned at the outset, the path data for subsequent measurement in the higher-resolution measuring range (₂₂) being obtained.

Since the mechanical dimensions of the diode rows (D₂₁) and (D₂₂) exceed their photosensitive surface, there is a dead zone between the two measuring ranges (₂₁) and (₂₂). This can be avoided if, as in Fig. 4 Darge provides behind the imaging lens (O₃₁) a geometric beam splitter in the form of a reflective cutting edge (S₃₁). This beam splitter (S₃₁) deflects the imaging beam path for the angular range under which the measuring range (₃₁) is mapped onto the diode array (D₃₂). Thus, the two detectors (D₃₁) and (D₃₂) can be mounted in different planes and the measuring ranges adjoin one another. The Scheimpflug condition is also kept in this arrangement, because the intersection of the main plane of the lens (O₃₁) with the projection axis lies in the plane of the virtual len image (V₃₂).

Of course it is also possible instead of the geometric one Beam splitter (S₃₁) a physical beam splitter set if the associated loss of intensity does not disturbs.

Corresponding examples are shown in FIGS . 5 and 6 Darge. In the exemplary embodiment according to FIG. 5, a physical beam splitter (S₄₁) is arranged behind the imaging lens (O₄₁) inclined at an angle of 60 ° relative to the projection axis, which, like the geometric beam splitter ( 31 ) from FIG. 4, shows the partial beam path for imaging the Meßbe Reichs (₄₁) deflected. The first diode row (D₄₁) is arranged in the image plane in the deflected part of the beam path.

In the other partial beam path of the light passing through the beam splitter (S₄₁) there is an optical element (O₄₂) in the form of a negative lens. This lens forms together with the lens (O₄₁) a telephoto lens, the main planes (H₄₁) and (H₄₂) are in front of the lens and which has a longer focal length than the single lens (O₄₁). The image plane (E₄₂) of this (O₄₁) and (O₄₂) telephoto lens is inclined at an angle α₄₂ of 60 ° to the main planes (H₄₁) and (H₄₂). This arrangement, which is symmetrical with respect to the projection axis and image plane, can, for. B. achieve in which the focal length (f₁) of (O₄₁) to 16 mm, the focal length (f₂) of (O₄₂) to - 13 mm and a mutual distance between (O₄₁) and (O₄₂) of d = 13 mm selected becomes. With it, the measuring area (Z₄₂) is mapped onto the diode array (D₄₂) under an average magnification of 1: 1. In contrast, the average imaging scale for the beam reflected on (D₄₁) is 3 : 1.

Also in the embodiment of Fig. 6 additional behind arranged in the imaging beam path of the objective (O₅₁) physical beam splitter (S₅₁) optical elements (O₅₂) and (O₅₃) are arranged. This is a field lens (O₅₂) in the image plane (E₅₁) of the lens (O₅₁) and a relay lens (O₅₃), the central area of the measuring range (₅₁) shown in the plane (E₅₁) again enlarged on the diode row (D₅₂) maps. For the repeated image through the relay lens (O₅₃) the Scheimpflug condition is also met again, because the plane (E₅₁) in which the intermediate image is created, the main plane of the relay lens (O₅₃) and the image plane (E₅₂) intersect at one point. In effect, the smaller measuring range (₄₂) is assigned by the two-stage mapping of the diode row (D zugeordnet₂), which can be resolved accordingly higher.

In the exemplary embodiments in FIGS . 1 to 6, diode rows have been mentioned as position-sensitive photoelectric detectors. Of course, it is also possible to use so-called CCD arrays or other position-sensitive transducer elements instead of diode rows. Finally, the number of measuring ranges placed one above the other or lying one behind the other is not limited to two, but may be greater.

Claims (9)

1. Non-contact distance sensor based on the triangulation principle

• a projection axis along which a light beam is emitted,
• - At least two position-sensitive photoelectric detectors, and
• - Optical imaging elements which, taking the Scheimpflug condition into account, map at least two sections of the projection axis lying one on top of the other or one behind the other, each with a different imaging scale and / or at a different image angle, as measuring areas of different sizes and / or spatial resolution, assigned to the at least two position-sensitive photoelectric detectors.

2. Distance sensor according to claim 1, characterized in that the detectors (D₁, D₂) from a plurality of Individual elements exist, the number of which varies is. 3. Distance sensor according to claim 1 or 2, characterized characterized in that each detector (D₁, D₂) has one Objective (O₁, O₂) is assigned and the Image scale of the lenses (O₁, O₂) different is. 4. Distance sensor according to claim 3, characterized in that the each from the projection axis and the optical Axis of one of the objectives (O₁₁, O₁₂) spanned planes include an angle (β) that is less than 90 °. 5. Distance sensor according to claim 1 or 2, characterized characterized in that the detectors (D₂₁, D₂₂)  common lens (O₂₁) is assigned and the Figure on the detectors under different Image angles are done. 6. Distance sensor according to claim 5, characterized by at least one beam splitter (S₃₁) in the image side Beam path of the lens (O₃₁). 7. Distance sensor according to claim 6, characterized in that behind the beam splitter (S₄₁, S₅₁) more optical Elements (O₄₂, O₅₂, O₅₃) are arranged. 8. Distance sensor according to claim 7, characterized in that the lens (O₄₁) in conjunction with another optical element (O₄₂) represents a telephoto lens. 9. Distance sensor according to claim 7, characterized in that the other optical elements (O₅₂, O₅₃) one repeated illustration of the image plane (E₅₁) of the lens with generate a reproduction scale different from 1.