Method and system for calibrating a distance measuring device

Abstract

Method for calibrating a distance measuring device (2), wherein the distance measuring device (2) is mounted on a test track (3), wherein a flat target object (4) is arranged displaceably on the test track (3) for reflecting a measurement signal emitted by the distance measuring device (2), wherein at least one distance measurement between the distance measuring device (2) and the target object (4) is carried out by means of the distance measuring device (2), and wherein a distance measurement value (d m ) with a reference value (d ref ), characterized in that an inclination of the target object (4) is detected.

Description

[0001] The present invention relates to a method and a system for calibrating a distance measuring device. The distance measuring device is mounted on a test track, a flat target object being arranged displaceably on the test track for reflecting a measurement signal emitted by the distance measuring device. At least one distance measurement between the distance measuring device and the target object is performed using the distance measuring device, and a distance measurement value determined by the distance measuring device is compared with a reference value. The distance measuring device is, in particular, a fill level measuring device for determining the fill level of a liquid or bulk material in a container using radar technology.

[0002] Radar level measurement devices are manufactured and sold by the applicant in various designs. For example, a free-radiating measuring device is offered under the name "Micropilot." Microwaves are emitted toward the medium via an antenna. The waves reflected from the surface of the medium are received by the measuring device, and an echo function is generated, which represents the echo amplitude as a function of distance. From this function, the desired echo is determined, and the corresponding travel time is determined. The distance between the antenna and the medium is determined based on the travel time.

[0003] Various measurement methods are known, with pulse radar and frequency modulation continuous wave (FMCW) radar being the most well-known. Pulse radar periodically emits short microwave pulses, and the pulses reflected from the surface of the medium are received. The received signal amplitude as a function of time represents the echo function, where each value corresponds to an echo reflected at a specific distance from the antenna.

[0004] In the FMCW method, a continuous microwave is transmitted, which is periodically frequency-modulated. At the time of reception, the frequency of the received signal differs from the frequency of the transmitted signal by a certain amount, which depends on the echo's propagation time. The distance between the antenna and the medium can thus be determined from the frequency difference. The echo function is represented by a Fourier spectrum of the mixed signal consisting of the transmitted and received signals.

[0005] To calibrate distance measuring devices, particularly radar measuring devices, a test track is often used whose length corresponds to the measuring range of the distance measuring device and on which a flat target object, also called a baffle plate, is arranged so that it can be moved. The distance measuring device to be calibrated is positioned, for example, at the starting point of the test track, the measurement signal is emitted towards the target object, and the reflected measurement signal is received. In the case of a radar measuring device, the distance between the radar measuring device and the target object is determined based on the signal propagation time. The measured distance is compared with a reference value determined using a calibrated standard. The calibrated standard is, for example, a laser interferometer with an accuracy that is higher by a defined factor than the accuracy of the distance measuring device to be calibrated. Typically, several measurement points are approached to determine linearity.

[0006] A measuring device for adjusting a distance measuring device in the automotive sector, which uses a tiltable reflector, is described in publication DE 198 40 307 A1. A radar-based level measuring device with a connected reflector for on-site recalibration of the level measuring device is described in publication EP 2199 763 A1.

[0007] Particularly for distance measuring devices that determine the distance to a surface, the measurement accuracy depends on the orientation of the target object's surface. If the target object's impact surface is not perpendicular to the radiation direction but is inclined, the distance is determined based on the portion reflected perpendicular to the impact surface. This uncertainty in the distance measurement can result in the measurement uncertainty of the distance measuring device determined during calibration being outside the required specifications.

[0008] The object of the invention is therefore to provide a method for calibrating a distance measuring device that enables a reduction in measurement uncertainty. Furthermore, a corresponding calibration system is to be provided.

[0009] With regard to the method, the problem is solved by detecting an inclination of the target object. Preferably, at least one inclination angle of the target object is determined. In particular, both an inclination in the horizontal direction and in the vertical direction are detected and quantitatively recorded. This allows the orientation of the impact surface of the measurement signal relative to the propagation direction of the measurement signal to be determined.

[0010] According to a first embodiment of the method, the distance measurement value and / or the reference value are corrected according to the detected inclination of the target object. For example, the distance measurement value is corrected using software in the electronics unit of the distance measuring device, taking the inclination of the target object into account. When correcting the reference value, the actual distance between the distance measuring device to be calibrated and the target object is determined based on the inclination, and the distance measurement value is assigned to the corrected reference value. This embodiment offers the advantage that the correction can be performed outside the distance measuring device to be calibrated.

[0011] According to another embodiment, if an inclination is present, the alignment of the target object or the test track is corrected to compensate for the inclination of the target object. As an alternative to adjusting the distance measurement, if an inclination is present, the position or alignment of the target object is corrected directly or indirectly by adjusting the test track. Adjusting the test track could, for example, be the leveling of a bump.

[0012] In one embodiment, the reference value and / or the inclination of the target object are determined non-contact. A non-contact determination of the inclination, for example, by determining one or more inclination angles, prevents the measuring device from changing the orientation of the target object to detect the inclination.

[0013] In one embodiment, the reference value and / or the inclination of the target object are measured using a laser measuring device. The laser measuring device can be arranged on the same side of the target object as the distance measuring device, or preferably on the opposite side, and can measure from behind, so to speak. Advantageously, the same laser measuring device is used to perform a distance measurement to determine the reference value and an environmental scan to determine the inclination of the target object.

[0014] According to one embodiment of the method, at least one environmental parameter is measured, and the influence of the environmental parameter on the measured distance value and / or the reference value is compensated. In a related embodiment, at least one of the following variables is measured as an environmental parameter. Environmental parameters influence the propagation speed of measurement signals and thus the measured value of a distance measuring device that evaluates the signal propagation time. By determining the environmental parameters, compensation for their influences is possible, so that the measured distance value or the reference value can be determined with improved accuracy.

[0015] An advantageous embodiment of the invention includes continuously recording the reference value and / or the inclination of the target object. The reference value and the inclination are recorded at least during the distance measurement with the distance measuring device to be calibrated. Continuous determination of the reference value and / or the inclination is particularly advantageous when recording a measured value with the distance measuring device for different distances to the target object. With continuous determination of the reference value and angle of inclination, the actual distance to the target object, as well as its orientation, are known throughout the entire calibration process. By continuously determining the reference value for the distance measurement, changes in the distance, for example due to a temperature-related change in the length of the test section, are known or can be compensated for at any time, so that the reference value is always available with a high level of accuracy.By continuously recording the inclination, the orientation of the target object is known at all times, so that measurement uncertainties due to an impact surface that is not perpendicular to the propagation direction of the measurement signal can be compensated.

[0016] In one embodiment of the invention, a radar level gauge is calibrated. For this purpose, the target object is designed such that it has optimal reflection properties for the radar measurement signal.

[0017] The invention further relates to a system for calibrating a distance measuring device, comprising at least one test track on which the distance measuring device can be mounted, and a flat target object displaceably arranged on the test track for reflecting a measurement signal emitted by the distance measuring device back into the distance measuring device, so that a distance measurement for determining the distance between the distance measuring device and the target object can be carried out by means of the distance measuring device. With regard to the system, the object underlying the invention is achieved in that the system has at least one device mountable on the test track for detecting an inclination of the target object.

[0018] According to the invention, the device for detecting the inclination is designed to determine at least one angle of inclination without contact.

[0019] According to a further embodiment of the system, the device for detecting the inclination is a laser tracker. A laser tracker is a combination of a distance measuring device, in particular an absolute distance measuring device, for example in the form of a laser interferometer, and an angle measuring device. A laser tracker is therefore suitable for detecting the three-dimensional point coordinates of an object.

[0020] In a further embodiment, the system comprises means for determining a reference value for the distance measurement using the distance measuring device. The means for determining the reference value is, for example, a laser interferometer. In an advantageous embodiment of the system, the same device serves both to detect the inclination and to determine the reference value. A laser tracker is advantageously mounted on the test track for this purpose.

[0021] The proposed calibration system enables the detection of a tilt of the target object in any direction, which can be caused, for example, by an unevenness of the ground or the test track on which the target object is mounted. By knowing the inclination of the target object, for example in the form of at least one angle of inclination, the distance variation associated with the inclination can be determined and compensated for or taken into account when determining the measured distance value or the reference value. This reduces the measurement error in distance measurements. The system according to the invention also offers the advantage that calibration can be easily performed by production personnel using the system, without the presence of a specialist.

[0022] The invention is explained in more detail with reference to the following figures with reference to a radar measuring device for level measurement. However, the invention is not limited to the illustrated embodiment. Fig. 1 shows a system for calibrating a distance measuring device; Fig. 2 illustrates the effects of a tilt of the target object on the distance measurement.

[0023] In Fig. 1 shows a system for calibrating a distance measuring device. The system has a test track 3, for example in the form of a rail, on which at least the distance measuring device to be calibrated, a target object 4, a calibrated distance measuring device that serves as a standard for the calibration, and a device for determining at least one angle of inclination of the target object 4 can be mounted. In the example shown, the device for determining at least one angle of inclination is designed as a laser tracker 5 and simultaneously serves as a calibrated distance measuring device for recording a reference value. In principle, any distance measuring device that can determine a distance with a higher accuracy than the distance measuring device to be calibrated is suitable as a calibrated distance measuring device. In the example shown, the distance measuring device to be calibrated is a freely radiating radar measuring device 2 for level measurement.The system is equally suitable for other level measuring devices operating according to the time-of-flight (ToF) principle, such as guided radar or ultrasonic measuring devices. Furthermore, any distance measuring device that emits a measurement signal and evaluates its reflection from a target object 4 to determine the distance d to the target object 4 can be calibrated using the system.

[0024] In a so-called free-radar radar, the measurement signal is emitted via an antenna, for example a planar antenna or, as shown, a horn antenna 21, in the direction of the object or medium to be detected. The measurement signal is a microwave signal, preferably with a frequency between 1 and 100 GHz. The measurement signal is generated and the received echo signal is evaluated in a measuring transducer 23. The measuring transducer 23 contains at least one electronic unit, for example in the form of a microcontroller. A radar measuring device 2 can also be designed as a guided radar. In the guided radar, a rod or cable probe serves as a waveguide for the measurement signal. Radar measuring devices 2 for level measurement are manufactured and marketed by the applicant under the names "Levelflex" - guided radar - and "Micropilot" - free-radar.

[0025] For attachment to the installation site, usually a container at least partially filled with the material to be detected, the radar measuring device 2 has suitable connection means. In the illustrated case, this is a flange 22. The radar measuring device 2 is mounted on the test section 3 via the flange 22. For example, at the beginning of the test section 3, there is a wall with a suitable recess into which the radar measuring device 2 can be inserted.

[0026] The target object 4 is arranged so that it can be moved at a distance d from the radar measuring device 2. The movable arrangement allows different distances d between the radar measuring device 2 and the target object 4 to be set. The positioning of the target object 4 can be done manually or automatically via an electronic control system, by means of which a predefined position on the test track 3 is approached. Preferably, at least five different positions are approached for calibration. The greatest distance d that can be measured with the radar measuring device 2 is also set. For example, this distance d = 30 m, so that the test track 3 should be at least 30 meters long in this case. The target object 4 has a flat design, i.e. it has a flat surface facing the radar measuring device 2, which serves as the impact surface 41 for the measurement signal, and a flat surface facing the laser tracker 5, which serves as the impact surface 41 for the laser beam.Target objects 4 suitable for calibration are well known to those skilled in the art, so their shape will not be discussed further. The target object 4 is oriented such that the impact surface 41 is oriented substantially perpendicular to the propagation direction of the measurement signal. Deviations from this alignment are detected and preferably also quantified according to the invention.

[0027] For this purpose, a laser tracker 5 is arranged on the test track 3. In the example, the laser tracker 5 is arranged behind the target object 4 as seen from the radar measuring device 2, so that the laser beam emitted by the laser tracker 5 hits the rear of the target object 4. The laser tracker 5 fulfills two functions: it is designed to perform an angle measurement and, on the other hand, to measure a distance using laser interferometry. Laser trackers 5 are offered, for example, by Leica Geosystems. In an alternative embodiment of the calibration system according to the invention, two separate measuring devices are provided for the angle measurement and the distance measurement.

[0028] The distance to the target object 4 determined with the laser tracker 5 is used to calculate a reference value d reffor the distance d between the target object 4 and the radar measuring device 2. The reference value essentially results from the difference between the known distance, which was fixed during calibration, between the radar measuring device 2 and the laser tracker 5 and the distance determined with the laser tracker 5. Other factors to be taken into account include dead spaces and the diameter of the target object 4.

[0029] During the calibration of the radar measuring device 2, for each of the set distances d between the radar measuring device 2 and the target object 4, a distance measurement value d m This distance measurement d m is compared with the reference value d ref compared and the deviation determined. A Laser Tracker 5 can measure distances within the measuring range of radar level gauges with an accuracy of 10 micrometers and is therefore particularly suitable as a calibration standard.

[0030] Due to the lobe-shaped radiation pattern of the radar measuring device 2, the emitted measurement signal does not impinge on the target object 4 as a point-like beam. Rather, the measurement signal covers a specific area. If the impact surface 41 is inclined relative to an imaginary plane oriented perpendicular to the propagation direction of the measurement signal, this results in a broadened echo pulse.

[0031] The inclination of the target object 4 in any direction leads to an increased measurement uncertainty in the distance measurement. This is referred to in connection with Fig. 2. To ensure the accuracy of the distance measurement d mTo improve, the laser tracker 5 performs a 3D scan and thus detects an inclination of the target object 4. The laser tracker 5 determines at least one inclination angle α of the target object 4. For example, two inclination angles are determined such that the inclination of the target object 4 in the vertical direction and in the horizontal direction is quantified.

[0032] In one embodiment, the information about the inclination of the target object 4 is provided to the radar measuring device 2 to be calibrated. The electronic unit in the measuring transducer 23 of the radar measuring device 2 takes into account the inclination of the target object 4 when determining the distance measurement value d m . Since the scattering in the distance d has been eliminated, the measurement uncertainty of the determined distance measurement value d m only depends on the measurement uncertainty which is attributable to the radar measuring device 2 itself.

[0033] In another variant, the actual distance to the reflection measuring point is determined using the information about the inclination, ie a trigonometric correction of the reference value d is carried out. ref .

[0034] Laser trackers 5 with integrated compensation for environmental influences are available on the market. For example, environmental conditions such as temperature, pressure, and humidity are measured and taken into account when measuring angles and / or distances, so that the reference value and at least one inclination angle are independent of environmental conditions and thus highly precise. If the laser tracker 5 does not have automatic compensation, it is advantageous to record the environmental conditions with separate sensors and to calculate the reference value. ref to be corrected accordingly. Preferably, the distance measurement value d measured with the radar measuring device 2 is also corrected m .

[0035] Thanks to the tilt detection feature, which compensates for unevenness in the test track 3 that could negatively impact calibration, the calibration system 1 can also be installed on a less than perfectly level surface. During installation, the calibration system 1 is aligned so that, in particular, the target object 4 is optimally aligned with the distance measuring device 2 to be calibrated. The alignment is then continuously monitored using the laser tracker 5.

[0036] Fig. 2 illustrates the effects of an inclination of the target object 4 on the determination of the distance measurement value d mThe target object 4 is positioned at a distance d from the radar measuring device 2. The test track 3 is uneven, which results in an inclination of the target object 4 mounted on the test track 3. The plane in which the impact surface 41 lies is therefore not perpendicular to the propagation direction of the measurement signal, but is inclined backward by an inclination angle a from the vertical. This inclination angle is determined by the laser tracker 5, for example, via a three-point measurement.

[0037] The distance d is checked by means of the laser tracker 5, which determines the reference value d ref as a current and calibrated value for the actual distance between radar measuring device 2 and target object 4. The reference value d ref at least during the recording of the distance measurement d m determined with radar measuring device 2.

[0038] If there was no inclination of the target object 4, the distance measurement value d mand the reference value d ref within the measurement accuracy of the radar measuring device 2, since the measurement only takes place on the optical axis between the distance measuring device and the reference measuring device. Due to the inclination of the target object 4, however, an actual reflection takes place outside the optical axis. The distance measurement value d determined by the radar measuring device 2 from the echo signal m deviates from the distance measurement value d determined on a vertical surface m Since the angle of inclination a is known, the determined distance measurement value d m or the reference value d ref be corrected accordingly.

[0039] The correction of the measured value d mThe radar measuring device 2 is, for example, supplied with information about the angle and the change in distance along the optical axis caused by the inclination of the impact surface, and from these values, together with the determined measured value, a corrected measured value for the actual distance d is calculated. This corrected measured value is then compared with the reference value d ref compared.

[0040] The correction of the reference value d ref For example, the angle, the set distance d and the distance change along the optical axis are used to determine the reference value d ref for the actual distance between radar measuring device 2 and target object 4. This corrected reference value d ref is measured with the value d m of radar measuring device 2.

[0041] As an alternative to the correction of the measured value d mor the reference value d ref the test section 3 or the orientation of the target object 4 can be corrected. All of the above measures lead to a minimization of the measurement uncertainty attributable to the test section, so that the remaining measurement uncertainty is attributable to the radar measuring device 2 itself to be calibrated.

[0042] For example, during the calibration of radar measuring device 2, the expanded measurement uncertainty is determined according to DIN EN ISO 14253-1, which is calculated by multiplying the standard deviation of the distance measurement value d by a factor k, usually k=2. m The reference value d ref lies with a probability of 95% within the limits given by the expanded measurement uncertainty around the distance measurement value d m .

[0043] By detecting the inclination of the target object 4 and taking appropriate measures to correct the inclination or the effects caused by the inclination during the measurement, the measurement uncertainty during calibration can be reduced to such an extent that the calibrated radar measuring device 2 meets the usual specifications for tank gauging, the main application area of ​​radar level measuring devices, such as OIML, NMI, PTB, and PAC. For example, calibration of the radar measuring device 2 with a maximum measurement uncertainty of + / - 0.4 mm at a distance of up to 30 m is possible. List of reference symbols 1 calibration system 2 radar measuring device 21 Antenna 22 flange 23 transmitters 3 test track 4 Target object 41 Impact surface 5 laser trackers a angle of inclination d distance

Claims

[1] Method for calibrating a distance measuring device (2), wherein the distance measuring device (2) is mounted on a test track (3), wherein a flat target object (4) is arranged displaceably on the test track (3) for reflecting a measurement signal emitted by the distance measuring device (2), wherein at least one distance measurement between the distance measuring device (2) and the target object (4) is carried out by means of the distance measuring device (2), and wherein a distance measurement value (d m ) with a reference value (d ref ) is compared, characterized by that an inclination of the target object (4) is detected. [2] Method according to claim 1, characterized by that the distance measurement value (d m ) and / or the reference value (d ref ) is corrected according to the detected inclination. [3] Method according to claim 1, characterized bythat in the event of an inclination, an alignment of the target object (4) or the test section (3) is corrected in such a way that the inclination of the target object (4) is compensated. [4] Method according to one of the preceding claims, characterized by that the reference value (d ref ) and / or the inclination is determined without contact. [5] Method according to claim 4, characterized by that the reference value (d ref ) and / or the inclination is determined by means of a laser measuring device (5). [6] Method according to one or more of the preceding claims, characterized by that at least one environmental parameter is measured and an influence of the environmental parameter on the distance measurement value (d m ) and / or the reference value (d ref ) is compensated. [7] Method according to the preceding claim, characterized by that at least one of the following environmental parameters is measured: temperature, humidity and pressure. [8] Method according to one or more of the preceding claims, characterized by that the reference value (d ref ) and / or the inclination is determined continuously. [9] Method according to one or more of the preceding claims, characterized by that a radar level gauge (2) is calibrated. [10] System (1) for calibrating a distance measuring device (2), at least comprising a test track (3) on which the distance measuring device (2) can be mounted, and a flat target object (4) displaceably arranged on the test track (3) for reflecting a measurement signal emitted by the distance measuring device (2) back into the distance measuring device (2), so that a distance measurement for determining the distance (d) between the distance measuring device (2) and the target object (4) can be carried out by means of the distance measuring device (2), characterized bythat the system (1) has at least one device (5) mountable on the test track (3) for detecting an inclination of the target object (4), and that the device (5) for detecting the inclination of the target object (4) is designed to determine at least one angle of inclination (α) in a contactless manner. [11] System according to claim 10, characterized by that the device (5) for detecting the inclination is a laser tracker. [12] System according to at least one of claims 10 to 11, characterized by that the system (1) comprises means (5) for determining a reference value (d ref ) for distance measurement by means of the distance measuring device (2).

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