DE102022113728A1 - Method for measuring changes in distance to a measurement object - Google Patents

Abstract

In a method for measuring changes in distance to a measurement object (2), a frequency-modulated continuous wave radar signal is generated and sent as a transmission signal to the measurement object (2) via a radar antenna. Radar radiation reflected by the measurement object (2) is received by the radar antenna and the received signal is mixed with the transmitted signal to form an intermediate frequency signal. The intermediate frequency signal is sampled in discrete time to generate a digital data stream. In the method, the change in distance is determined by evaluating the phase spectrum of the digital data stream. During the duration of a disturbance state in which an obstacle is arranged in the radar transmission path between the radar antenna and the measurement object (2) or there is a risk that this will be the case, the frequency-modulated continuous wave radar signal continues to be generated, but the measurement of the change in distance is stopped. After the disruption has ended, the change in distance is determined from the difference between a phase value determined before the disruption occurred and a phase value determined after the disruption occurred.

Description

The invention relates to a method for measuring changes in distance to a measurement object, wherein a frequency-modulated continuous wave radar signal is generated and sent as a transmission signal via a radar antenna to the measurement object, with radar radiation reflected by the measurement object being received by the radar antenna and the received signal thus obtained to form an intermediate frequency signal with the transmission signal is mixed, with the intermediate frequency signal being sampled in a time-discrete manner to generate a digital data stream.

Distance measurement using radar is a widely used technology in industrial environments when it comes to measuring distances without contact and under difficult environmental conditions such as dust, steam, smoke, lighting fluctuations and noise. With the invention, distance changes in the µm range can be measured under difficult environmental conditions. This enables new applications, for example in the areas of CNC machines, steel and rolling mills and sheet metal thickness measurement.

The state of the art is radar distance measurement with µm accuracy, for example described in the publication by Steffen Scherr, et. Al. “Miniaturized 122 GHz ISM Band FMCW Radar with Micrometer Accuracy,” Proceedings of the 12th European Radar Conference, 9-11 Sept. 2015, Paris. Here, a distance measurement accuracy of +/- 400 µm is demonstrated over distance ranges from 1000 to 2900 mm. In sections in the distance range between 2000 and 2005 mm, the publication shows a distance error of +/- 2 µm.

With an FMCW radar, the high-frequency transmit signal and the high-frequency received signal, which are in the mm-wave range, are mixed down into the audio or video frequency range. This signal is digitized and converted into a discrete frequency and phase spectrum using the complex discrete Fourier transformation (DFT). The maxima in the frequency spectrum can be assigned to objects that are within the measuring range of the radar lobe. As a first approximation, the frequency of a frequency maximum is proportional to the distance of the object. In practice, the accuracy of the distance measurement is limited to values of the order of half a wavelength of the average transmission frequency.

The accuracy of the distance measurement can be estimated using the Cramer-Rao limit. The Cramer-Rao limit is a theoretical limit that cannot be achieved in practice and depends on many factors of a real radar system, such as the signal-to-noise ratio of the received signal, the phase noise of the transmission frequency, the phase error caused by the distance from Transmitting and receiving radar antenna, the bandwidth of the transmitted signal, the linearity of the frequency chirp, but also the selected window function and the digitization of the radar signal. A theoretical estimate of the achievable accuracy is described in Steffen Scherr's dissertation, "FMCW radar signal processing for distance measurement with high accuracy", 2016, ISBN 978-3-7315-0607-2. In order to increase the distance measurement accuracy of a radar, the FMCW radar signals are evaluated using a combined frequency and phase analysis at the output of the high-frequency mixer and is state of the art. The algorithm described in Scherr's dissertation is the basis for distance measurements with high accuracy. Within a uniqueness range in which each frequency in the Fourier spectrum can be assigned to a unique phase, phase evaluation can be used to achieve distance changes even in the sub-µm range if the appropriate technical effort is applied. Above all, a larger bandwidth of the transmission frequency increases the resolution. In order to achieve bandwidths of more than 10% of the average transmission frequency, sophisticated and correspondingly expensive radar components are required. In addition, the use of large frequency bandwidths requires approval by the authorities and is not always guaranteed.

The transmission frequency of the transmission signal s _s (t) of an FMCW radar changes linearly with time and is described by Equation 1. $$s_s\left(t\right)=A_s\cdot \text{exp}\left(-\text{j}\left({\omega }_s+{\phi }_s\right)\right)=A_s\cdot \text{exp}\left(-j{\mathrm{\Phi }}_s\right)$$

In Equation 1, A _s is the amplitude, ω _s is the angular frequency, φ _{s is} the phase and Φ _{s is} the frequency description of the transmitted signal. The angular frequency is defined by ω _s = 2πf _s , where f _s the transmission frequency in an FMCW radar changes linearly over time and can be described in a time period T by equation 2. This change in frequency is also known as chirp. $$f_s\left(t\right)=f_a\pm \frac{b}{T}t$$

In Equation 2, f _a is the initial frequency, B is the bandwidth of the transmission frequency and thus the difference between the initial and final frequencies, and T is the chirp time. i.e. the time in which the transmission frequency changes linearly from the initial frequency f _a to the final frequency. ± means that in the case of Addi tion, the transmission frequency increases linearly from small to large frequencies and, in the case of subtraction, decreases from large to small frequencies. In further consideration, only the rising case is considered, as the falling case must be viewed in the same way.

Analogous to the transmitted signal, the received signal that returns from the reflecting object to the radar can be described by equation 3. $$s_e\left(t\right)=A_e\cdot \text{exp}\left(-\text{j}\left({\omega }_e+{\phi }_e\right)\right)=A_e\cdot \text{exp}\left(-j{\Phi }_e\right)$$

The signal at the output of a high-frequency mixer of an FMCW radar is also referred to as the intermediate frequency signal s _IF (t) and can be written by equation 4 by multiplying the transmitted and received signals. $$\begin{array}{l}{s}_{IF}\left(t\right)=A_{IF}\cdot \text{exp}\left(-j\left({\omega }_{IF}+{\phi }_{IF}\right)\right)\\ =A_s\cdot \text{exp}\left(-j\left({\omega }_s+{\phi }_s\right)\right)\times A_e\cdot \text{exp}\left(-\left(j{\omega }_e+{\phi }_e\right)\right)\end{array}$$

In Equation 4, A _IF is the amplitude, ω _IF is the angular frequency, and φ _IF is the phase of the intermediate frequency signal.

In a simplified frequency analysis of equation 4, which makes use of the fact that the exponents add or subtract in a multiplication, equation 5 results. $${\omega }_{IF}={\omega }_s\pm {\omega }_e,\text{or}{f}_{IF}=f_s\pm f_e$$

The case of addition in Equation 5 is of no practical importance since the subsequent electronics act as a low pass.

A radar signal that is reflected from an object that is at a distance R from the radar is received by the radar receiver with a time delay according to equation 6. $$\tau =\frac{2\mathrm{<figure-callout\; id="0"\; label="R\; c"\; filenames="DE102022113728A1\_0014.png,DE102022113728A1\_0015.png"\; state="\{\{state\}\}">R</figure-callout>}}{c_{}}$$

In equation 6, τ is the transit time that the transmission signal needs to be received by the radar receiver. R is the distance to the object and c ₀ is the speed of propagation of electromagnetic waves in a vacuum, which hardly differs from the speed of propagation in air. This results in equation 7 using equations 2, 5 and 6. $$f_{IF}=\frac{b}{T}\tau =\frac{2b}{T\cdot c_0}{R}_f$$

Equation 7 means that the intermediate frequency f _IF is proportional to the distance R if the bandwidth B, the chirp time T and the propagation speed of the electromagnetic wave c ₀ are known. The difference in the propagation speed of the electromagnetic wave in vacuum and in air is small and is neglected in practical radar applications. The person skilled in the art naturally sets B and T so that the intermediate frequency f _IF is in the range of the audio or video frequency and can therefore be determined technically well and cost-effectively. By determining f _IF, the distance R within the errors considered below can be determined.

A closer look at the mixed product of Equation 4 without considering phase leads to Equation 8. $${\Phi }_{IF}\left(t\right)=2\pi \left(\frac{b}{T}\tau t-\frac{{\mathrm{<figure-callout\; id="2"\; label="b"\; filenames="DE102022113728A1\_0013.png,DE102022113728A1\_0014.png"\; state="\{\{state\}\}">b</figure-callout>}}^2}{2\mathrm{<figure-callout\; id="2"\; label="T\; \tau "\; filenames="DE102022113728A1\_0013.png,DE102022113728A1\_0014.png"\; state="\{\{state\}\}">T</figure-callout>}}{\tau }^{}{}^2+f_a\tau \right)$$

wird vernachlässigt, da die Signallaufzeit τ für Abstände im Bereich weniger Meter im Bereich von Nanosekunden liegt und damit erheblich größer ist als die Chirpzeit T, die üblicherweise im Millisekunden-Bereich liegt.In Equation 8, Φ _IF (t) is the frequency description of the intermediate frequency signal at the output of the high frequency mixer. The term $\frac{b^{}}{}$$\frac{{}^2}{2\mathrm{<figure-callout\; id="2"\; label="T\; \tau "\; filenames="DE102022113728A1\_0013.png,DE102022113728A1\_0014.png"\; state="\{\{state\}\}">T</figure-callout>}}{\tau }^{}{}^2$

is neglected because the signal transit time τ for distances of a few meters is in the range of nanoseconds and is therefore considerably longer than the chirp time T, which is usually in the millisecond range.

Evaluating equation 4 with respect to phase leads to equation 9. $${\phi }_{IF}=2\pi f_a\tau =\frac{4\pi f_a}{c_0}{R}_{\phi }$$

By measuring the phase φ _IF , the distance R can also be determined. The uniqueness range of the phase is naturally limited to +/- half a wavelength, but can be determined much more precisely than the frequency. A combination of phase and frequency evaluation of the intermediate frequency signal can therefore significantly improve the accuracy of a distance measurement compared to pure frequency measurement if the frequency measurement achieves an accuracy that is better than half a wavelength. Unfortunately, this cannot be guaranteed in real radar systems with reasonable effort.

In Scherr's dissertation, the measurement accuracy of radars is measured during a continuous change in distance to the object. This means that the phase relationship to the distance is guaranteed at all times, since only monotonically increasing and decreasing distance measurements were examined. An FMCW radar that provides absolute distance to an object using the method described in Scherr's dissertation can therefore have an error of half a wavelength if the determination of the distance by pure frequency evaluation is not more accurate than half a wavelength. If the radio link between the object and the radar is interrupted or disturbed during the distance measurement, for example by a second object that temporarily comes into the radar beam, this can lead to a measured jump in distance of half a wavelength or a multiple of half a wavelength.

An “absolute distance” is the distance between the radar and the object. A “relative distance” is the change in distance between the radar and the object.

According to the state of the art, the absolute distance can only be reliably determined with an accuracy of at most half a wavelength using inexpensive radar systems, which are limited by their resolution of the discrete frequency spectrum and bandwidth. The relative distance can be determined significantly more accurately than half a wavelength through phase evaluation if the radio link is not disturbed during the change in distance. A disturbance can occur due to an interruption in the radio link, or due to a sudden jump in the distance between two radar measurements, in which the object has moved faster than half a wavelength between two measurements. Further interference can occur due to electromagnetic interference, temperature changes, changes in air composition, electronic noise and other influences.

The disadvantage of the prior art is that in practice the absolute distance to an object can only be measured with limited accuracy by an FMCW radar. Limited accuracy means that either the range in which an accuracy of better than half a wavelength can be achieved is limited to limited distance ranges, and/or the inaccuracy increases at the full distance range. In the cited prior art, the limited distance range is given as 5 mm and the full distance range as 1800 mm, with continuous measurement, without disturbing the radio link.

1. 1. Zu schnelle Bewegung des Messobjekts
2. 2. Herausbewegen des Messobjekts aus dem Messbereich
3. 3. Störobjekte zwischen Messobjekt und Sensor

The prior art describes continuous changes in distance with a change of less than half a wavelength between two measurements. If the change in distance between two measurements is greater than half a wavelength, the phase evaluation of the measurement signal leads to ambiguous distance measurements. Uniqueness cannot be guaranteed in industrial applications. Changes in distance of more than half a wavelength between two measurements can be caused by the following reasons, among others:

1. 1. The measurement object moves too quickly
2. 2. Moving the measurement object out of the measurement area
3. 3. Interfering objects between the measurement object and sensor

These causes can have a “foreseeable” and “unforeseeable” character. If there are ambiguities in the phase evaluation of the measurement signal, measurement errors arise. Measuring errors can mean that the industrial process in which the measuring method is used has to be interrupted in order to restart the measuring process. Manual intervention in the industrial process is usually unacceptable as this leads to increased costs and creates additional sources of error. The cited state of the art leads to previously unsatisfactory results, which have so far severely limited or even made impossible the use of radar technology in industrial environments.

The object of the invention is therefore to create a method of the type mentioned at the outset, which enables safe, robust, cost-efficient and precise distance change measurement.

1. a) für einen ersten Zeitpunkt Erzeugen eines ersten Phasenspektrums aus dem digitalen Datenstrom, Ermitteln eines ersten Phasenwerts, den dieses Phasenspektrum bei einer Bezugsfrequenz aufweist, und Speichern des ersten Phasenwerts in einem Datenspeicher,
2. b) für einen weiteren Zeitpunkt Prüfen, ob ein Störzustand vorliegt, bei dem ein Hindernis in der Radarübertragungsstrecke zwischen der Radarantenne und dem Messobjekt angeordnet ist,
3. c) falls für den in Schritt b) genannten Zeitpunkt das Vorliegen des Störzustands detektiert wird, Wiederholen der Schritte b) und c),
4. d) für den in Schritt b) genannten weiteren Zeitpunkt Erzeugen eines weiteren Phasenspektrums aus dem digitalen Datenstrom, Ermitteln eines weiteren Phasenwerts, den dieses Phasenspektrum bei der Bezugsfrequenz aufweist,
5. e) Ermitteln eines zur Differenz aus dem gespeicherten Phasenwert und dem in Schritt f) genannten weiteren Phasenwert proportionalen Messwerts (ΔP_φN) für die Abstandsänderung,
6. f) optional Ablegen des in Schritt g) genannten weiteren Phasenwerts im Datenspeicher und Wiederholen der Schritte b) bis f), wobei in Schritt e) dieser Phasenwert als gespeicherter Phasenwert verwendet wird.

According to the invention, this object is achieved with the features of patent claim 1. In a method for measuring changes in distance to a measurement object, these provide that a frequency-modulated continuous wave radar signal is generated and sent as a transmission signal via a radar antenna to the measurement object, that radar radiation reflected by the measurement object is received by the radar antenna and the received signal thus obtained is used to form an intermediate frequency signal The transmission signal is mixed, that the intermediate frequency signal is sampled in a time-discrete manner in order to generate a digital data stream, and that the method has the following steps:

1. a) for a first time, generating a first phase spectrum from the digital data stream, determining a first phase value that this phase spectrum has at a reference frequency, and storing the first phase value in a data memory,
2. b) checking for a further point in time whether a fault condition exists in which an obstacle is arranged in the radar transmission path between the radar antenna and the measurement object,
3. c) if the presence of the fault condition is detected at the time mentioned in step b), repeating steps b) and c),
4. d) for the further time mentioned in step b), generating a further phase spectrum from the digital data stream, determining a further phase value that this phase spectrum has at the reference frequency,
5. e) determining a measured value (ΔP _φN ) for the distance change that is proportional to the difference between the stored phase value and the further phase value mentioned in step f),
6. f) optionally storing the further phase value mentioned in step g) in the data memory and repeating steps b) to f), whereby in step e) this phase value is used as the stored phase value.

The distance change measurement is therefore determined by a phase evaluation of the phase spectrum. This advantageously enables measurement accuracies in the µm range. However, the phase repeats every 2π. Pure phase evaluation therefore cannot decide whether the distance between two measurements has changed by more than one wavelength or not. The invention assumes that the distance of the measurement object from the radar antenna only changes so slowly that the difference between the stored phase value and the further phase value mentioned in step g) is always smaller than π for an interference-free distance measurement.

In practice, however, it happens that disturbance conditions occur in which an obstacle is temporarily arranged in the radar transmission path between the radar antenna and the measurement object. Such an obstacle can be, for example, a robot arm that is temporarily moved through the measuring section. So that such disturbances in step d) cannot lead to an incorrect distance measurement, the invention provides that at the point in time mentioned in step b) it is checked whether a disturbance condition is present in which an obstacle is in the radar transmission path between the radar antenna and the measurement object is arranged or there is a risk that this will be the case. This test can be carried out on the robot arm, for example, by the robot's machine control issuing a corresponding signal, which is evaluated when measuring the change in distance. If no fault condition is detected, the measured value for the change in distance is determined from the difference between the stored phase value and the further phase value mentioned in step g). However, if the test shows that a fault condition exists, the test is repeated until the fault condition has ended. Then, in step e), a phase spectrum is generated from the digital data stream and the measured value for the change in distance is determined from the difference between the phase value determined from this phase spectrum and the last phase value that was measured before the disturbance occurred.

• - Es kann eine kostengünstige Radarelektronik verwendet werden mit reduzierter Bandbreite.
• - Durch die Möglichkeit begrenzte Bandbreiten zu nutzen kann das hochauflösende Radar in den lizenzfreien ISM-Bänder betrieben werden.
• - Das Radar kann trotz Bandbreitenlimitierung Abstandsänderungen im µm-Bereich messen.
• - Gegenüber optischen Messverfahren, die ähnliche Genauigkeiten erzielen, ist das erfindungsgemäße Radarverfahren deutlich robuster und kostengünstiger.
• - Wegen der erheblich größeren Wellenlänge der elektromagnetischen Strahlung eines Radars gegenüber optischen Systemen, ist die Erfindung fehlertoleranter gegenüber Staub, Rauch, Dampf, Temperaturstrahlung und Partikeln.
• - Die Oberflächenrauigkeit des Messobjekts ist für die Messgenauigkeit unbedeutend.
• - Gegenüber magnetisch arbeitenden Abstandsmessverfahren hat die Erfindung den Vorteil, dass die Abstandsänderung aus einem größerem Abstand gemessen werden kann.
• - Unterschiede in der Oberflächenleitfähigkeit des Messobjekts haben kaum bis keinen Einfluss auf die Messgenauigkeit.
• - Gegenüber Ultraschall Messmethoden hat die Erfindung erhebliche Genauigkeitsvorteile und wird durch akustische Störungen nicht beeinflusst.
• - Die Erfindung ermöglicht neue Anwendungen.

The invention has several advantages over the prior art:

• - Cost-effective radar electronics can be used with reduced bandwidth.
• - Due to the possibility of using limited bandwidths, the high-resolution radar can be operated in the license-free ISM bands.
• - The radar can measure distance changes in the µm range despite bandwidth limitations.
• - Compared to optical measurement methods that achieve similar accuracies, the radar method according to the invention is significantly more robust and cost-effective.
• - Because of the significantly longer wavelength of the electromagnetic radiation of a radar compared to optical systems, the invention is more fault-tolerant towards dust, smoke, steam, temperature radiation and particles.
• - The surface roughness of the measurement object is insignificant for the measurement accuracy.
• - Compared to magnetically operating distance measuring methods, the invention has the advantage that the change in distance can be measured from a greater distance.
• - Differences in the surface conductivity of the measurement object have little to no influence on the measurement accuracy.
• - Compared to ultrasound measurement methods, the invention has significant advantages in terms of accuracy and is not influenced by acoustic interference.
• - The invention enables new applications.

Further advantageous embodiments of the invention are described in the subclaims.

In a preferred embodiment of the invention it is provided that at least three first support points are provided for the first phase spectrum, that the first phase spectrum is formed from the first support points by linear interpolation and that the first phase value is determined with the aid of this interpolation, and/or that for In step d) of claim 1, at least three further support points are provided, so that the further phase spectrum is determined by linear interpolation from the width ren support points are formed and the further phase value is determined with the help of this interpolation. This linear interpolation allows the change in distance between the measurement object and the radar to be determined with even greater precision.

The first and/or the second linear interpolation is preferably carried out over a number of at least 2, optionally at least 3 and preferably at least 10 support points. This allows the influence of scatter in the support points on the measured value to be compensated for.

In an advantageous embodiment of the invention, a first amplitude spectrum is generated from the digital data stream for the first point in time and a sampling point is determined in the first amplitude spectrum, at which the amplitude spectrum has a maximum assigned to the measurement object, the reference frequency in step a) of claim 1 being the Frequency of a sampling point of the first amplitude spectrum is selected, which is a maximum of n sampling points away from the sampling point having the maximum, where n is an integer between 0 and 10, in particular between 0 and 5, optionally between 0 and 3 and preferably 1 or 0. Optionally, a further amplitude spectrum can be generated for the further point in time mentioned in step b) of claim 1 and a sampling point can be determined in the further amplitude spectrum, at which the further amplitude spectrum has a maximum assigned to the measurement object, with steps b) to f) being repeated. of claim 1, the frequency of a sampling point of this further amplitude spectrum is selected as the reference frequency, which is a maximum of n sampling points away from the sampling point having the maximum, where n is an integer between 0 and 10, in particular between 0 and 5, possibly between 0 and 3 and is preferably 1 or 0. Since relatively large signal levels occur in the area of the maximum of the amplitude spectrum, the linear interpolation can be carried out particularly precisely in this area.

1. i) dass für den in Schritt a) von Patentanspruch 1 genannten ersten Zeitpunkt aus dem digitalen Datenstrom ein erstes Amplitudenspektrum erzeugt und ein erster Frequenzwert (Δf_od) bestimmt wird, bei dem das erste Amplitudenspektrum ein Maximum aufweist, und der erste Frequenzwert (Δf_od) im Datenspeicher gespeichert wird,
2. ii) dass für den in Schritt b) von Patentanspruch 1 genannten weiteren Zeitpunkt aus dem digitalen Datenstrom ein weiteres Amplitudenspektrum ermittelt und ein weiterer Frequenzwert (Δf_nd) bestimmt wird, bei dem das weitere Amplitudenspektrum ein Maximum aufweist,
3. iii) dass zwischen aufeinanderfolgenden Zeitpunkten, für die jeweils ein Amplitudenspektrum ermittelt wird, der Abstand zwischen der Radarantenne und dem Messobjekt nicht um einen Wert geändert wird, dessen Betrag größer ist als die halbe Wellenlänge der mittleren Radarfrequenz des Sendesignals,
4. iv) dass ein zur Differenz aus dem gespeicherten Frequenzwert (Δf_od) und dem weiteren Frequenzwert (Δf_nd) aus Schritt ii) proportionaler Abstandsänderungswert (ΔR_fN) gebildet und betragsmäßig mit einem Grenzwert verglichen wird, der kleiner als ein Abstandsänderungswert ist, der bei der Messung einer Abstandsänderung auftritt, die der halben Wellenlänge der mittleren Radarfrequenz des Sendesignals entspricht,
5. v) dass in Schritt b) von Patentanspruch 1 der Störzustand detektiert wird, wenn der Vergleich in Schritt iv) ergibt, dass der Abstandsänderungswert (ΔR_fN) größer ist als der Grenzwert,
6. vi) optional Speichern des weiteren Frequenzwerts (Δf_nd) im Datenspeicher und Widerholen der Schritte ii) bis vi), wobei in Schritt iv) dieser Frequenzwert (Δf_nd) als gespeicherter Frequenzwert (Δf_od) verwendet wird.

In a further development of the invention it is provided

1. i) that for the first point in time mentioned in step a) of claim 1, a first amplitude spectrum is generated from the digital data stream and a first frequency value (Δf _od ) is determined, at which the first amplitude spectrum has a maximum, and the first frequency value (Δf _od ) is stored in the data storage,
2. ii) that for the further point in time mentioned in step b) of claim 1, a further amplitude spectrum is determined from the digital data stream and a further frequency value (Δf _nd ) is determined, at which the further amplitude spectrum has a maximum,
3. iii) that between successive times for which an amplitude spectrum is determined, the distance between the radar antenna and the measurement object is not changed by a value whose amount is greater than half the wavelength of the average radar frequency of the transmission signal,
4. iv) that a distance change value (ΔR _fN ) proportional to the difference between the stored frequency value (Δf _od ) and the further frequency value (Δf _nd ) from step ii) is formed and compared in terms of amount with a limit value which is smaller than a distance change value which is at the measurement of a change in distance occurs which corresponds to half the wavelength of the mean radar frequency of the transmission signal,
5. v) that in step b) of claim 1, the fault condition is detected if the comparison in step iv) shows that the distance change value (ΔR _fN ) is greater than the limit value,
6. vi) optionally storing the further frequency value (Δf _nd ) in the data memory and repeating steps ii) to vi), whereby in step iv) this frequency value (Δf _nd ) is used as the stored frequency value (Δf _od ).

In this development of the invention, the amplitude spectrum is evaluated in order to automatically check whether a fault condition exists. The process can therefore be carried out in a simple manner. The limit value can be smaller than a distance change value that occurs when measuring a distance change that corresponds to 100%, possibly 25% and in particular 50% of the wavelength of the average radar frequency of the transmission signal.

In an advantageous embodiment of the invention, steps b) to f) of claim 1 are run through at least twice, the measured values (ΔP _φN ) determined in these runs being added up in order to obtain a total value by which the distance increases to the measurement object changed during these runs. This means that distance changes that are greater than half the wavelength of the average transmission frequency can also be measured.

• - wobei die Werkzeugmaschine einen Positionierantrieb aufweist, mittels dem der Werkstückhalter oder Werkzeughalter relativ zu der Stelle, an welcher der Positionierantrieb an dem Basisteil angeordnet ist, positioniert wird,
• - wobei die Wegstrecke, um welche die Positionierung in Richtung der Radarübertragungsstrecke relativ zu dieser Stelle des Basisteils erfolgt, erfasst wird,
• - wobei die Radarantenne an dem Werkstückhalter und das Messobjekt an dem Werkzeughalter oder die Radarantenne an dem Werkzeughalter und das Messobjekt an dem Werkstückhalter angeordnet ist, und
• - wobei zum Ermitteln einer Drift und/oder eines Verzugs der Werkzeugmaschine die Differenz aus dem Messwert (ΔP_φN) für die Abstandsänderung zwischen der Radarantenne und dem Messobjekt und der erfassten Wegstrecke gebildet wird.

In one application of the invention, a machine tool is provided which has a base part on which a workpiece holder and a tool holder are arranged, the workpiece holder being a workpiece and the tool holder being a Carrying a machining tool for removing material from the workpiece, carrying material from the workpiece,

• - wherein the machine tool has a positioning drive by means of which the workpiece holder or tool holder is positioned relative to the point at which the positioning drive is arranged on the base part,
• - the distance over which the positioning takes place in the direction of the radar transmission path relative to this point of the base part is recorded,
• - wherein the radar antenna is arranged on the workpiece holder and the measurement object on the tool holder or the radar antenna is arranged on the tool holder and the measurement object is arranged on the workpiece holder, and
• - whereby to determine a drift and/or a delay of the machine tool, the difference is formed from the measured value (ΔP _φN ) for the change in distance between the radar antenna and the measurement object and the recorded distance.

Due to its design, the drift of a CNC machine tool is usually less than half a wavelength of the average radar transmission frequency between the disturbances. Compared to other measuring methods, the invention has the advantage of working without contact and reacting less sensitively to contamination from coolants and lubricants than, for example, optical measuring methods. Compared to mechanical methods, which often use microswitches, the method according to the invention has the advantage of faster measurement. Microswitches must be approached comparatively slowly until mechanical contact is made. The wear at the contact point leads to a measurement error drift, which is eliminated by the non-contact invention. The method can be expanded to three radar measuring sections arranged orthogonally to one another in order to capture a complete drift measurement in all dimensions. If necessary, the influence of the drift or distortion on the distance between the workpiece holder and the tool holder can be automatically compensated for using the positioning drive.

In another embodiment of the invention, the measurement object is an edge or a surface of a material web, in particular a sheet, which material web is moved relative to the radar antenna along the edge and / or the surface in a transport direction, the radar antenna being aligned in this way relative to the material web is that the change in distance to the measurement object is measured transversely and preferably orthogonally to the transport direction, and wherein the measurement of the change in distance is carried out while the material web is moved relative to the radar antenna in the transport direction.

In sheet metal rolling mills, the rolls must be regulated so that the sheet is kept as precisely as possible in the rolling track so that the thickness and width of the sheet remains within specified tolerances. Before the actual rolling process, it is advantageous to measure the distortion of the slab, as this must be compensated for during the rolling process. During the rolling process, the slab edge becomes the sheet edge. For elongated slabs, which are primarily used in rolling mills for coil metals, it is advantageous to determine the so-called “saber shape” of the slab before rolling. This refers to the curvature of the elongated slab or sheet. During the rolling process, it is advantageous to continuously record and monitor the saber shape, width, thickness and position of the sheet in the rolling train. While the slab or sheet moves past the radar measuring point, the distance between the edges and the measuring system only changes suddenly in rare cases. Sudden changes in distance occur, for example, when sheet or slab ends are welded together, or in the event of defects. With the help of the invention, such jumps in distance changes can be recognized automatically. In addition, the continuous high-precision distance change can be used to measure the saber shape and precisely control the rolling train.

During the rolling process, the deformation work causes the sheet metal to become hot to red-hot in sections. The sheet metal is sometimes cooled with water and steam is formed, which is why optical sensors are unsuitable. With the help of the invention, the change in distance between the slab or sheet edges can be measured continuously from a sufficient distance without contact and with accuracies in the µm range.

The invention enables extremely precise control of the rolling process for the first time. This means that the usual oversize of the rolled sheet width can be significantly reduced. At the end of the rolling train, the sheet metal is usually rolled onto coils. To prevent unwanted sheet metal protrusions from occurring on the sides of the coil, sheets are rolled to an excess size and cut to size on the sides before rolling up. This creates unwanted scrap, which consists of rolled sheet metal and is usually melted down again. Reducing the oversize therefore leads to energy savings and a reduction in CO2 emissions from the rolling mill.

1. I) dass ein erstes frequenzmoduliertes Dauerstrichradarsignal erzeugt und als Sendesignal über die erste Radarantenne zur ersten Kante der Materialbahn gesendet und von dort reflektierte Radarstrahlung von der ersten Radarantenne empfangen wird, während die Materialbahn in Transportrichtung bewegt wird, dass das so erhaltene erste Empfangssignal zur Bildung eines ersten Zwischenfrequenzsignals mit dem Sendesignal der ersten Radarantenne gemischt wird, dass das erste Zwischenfrequenzsignal zeitdiskret abgetastet wird, um einen ersten digitalen Datenstrom zu erzeugen,
   1. a1) dass für einen ersten Zeitpunkt ein erstes Phasenspektrum aus dem ersten digitalen Datenstrom gebildet und ein erster Phasenwert ermittelt wird, den das erste Phasenspektrum bei einer ersten Bezugsfrequenz aufweist, dass dieser Phasenwert in einem Datenspeicher gespeichert wird,
   2. b1) dass für einen weiteren ersten Zeitpunkt geprüft wird, ob ein Störzustand vorliegt, bei dem ein Hindernis in der Radarübertragungsstrecke zwischen der ersten Radarantenne und der ersten Kante angeordnet ist,
   3. c1) dass für den Fall, dass für den in Schritt b1) genannten Zeitpunkt das Vorliegen des Störzustands detektiert wird, die Schritte b1) und c1) wiederholt werden,
   4. d1) dass für den in Schritt b1) genannten weiteren Zeitpunkt ein weiteres erstes Phasenspektrum aus dem ersten digitalen Datenstrom gebildet und ein weiterer erster Phasenwert ermittelt wird, den dieses Phasenspektrum bei der ersten Bezugsfrequenz aufweist,
   5. e1) dass ein zur Differenz aus dem gespeicherten ersten Phasenwert und dem in Schritt d1) genannten weiteren ersten Phasenwert proportionaler erster Messwert für die Abstandsänderung erzeugt wird,
2. II) dass ein zweites frequenzmoduliertes Dauerstrichradarsignal erzeugt und als Sendesignal über die zweite Radarantenne zur zweiten Kante der Materialbahn gesendet und von dort reflektierte Radarstrahlung von der zweiten Radarantenne empfangen wird, während die Materialbahn in die Transportrichtung bewegt wird, dass das so erhaltene zweite Empfangssignal zur Bildung eines zweiten Zwischenfrequenzsignals mit dem Sendesignal der zweiten Radarantenne gemischt wird, dass das zweite Zwischenfrequenzsignal zeitdiskret abgetastet wird, um einen zweiten digitalen Datenstrom zu erzeugen,
   1. a2) dass für einen zweiten Zeitpunkt ein zweites Phasenspektrum aus dem zweiten digitalen Datenstrom gebildet und ein zweiter Phasenwert ermittelt wird, den dieses Phasenspektrum bei einer zweiten Bezugsfrequenz aufweist, dass der zweite Phasenwert in dem Datenspeicher gespeichert wird,
   2. b2) dass für einen weiteren zweiten Zeitpunkt geprüft wird, ob ein Störzustand vorliegt, bei dem ein Hindernis in der Radarübertragungsstrecke zwischen der zweiten Radarantenne und der ersten Kante angeordnet ist,
   3. c2) dass für den Fall, dass für den in Schritt b2) genannten Zeitpunkt das Vorliegen des Störzustands detektiert wird, die Schritte b2) und c2) wiederholt werden,
   4. d2) dass für den in Schritt b2) genannten weiteren zweiten Zeitpunkt ein weiteres zweites Phasenspektrum aus dem zweiten digitalen Datenstrom gebildet und ein weiterer zweiter Phasenwert ermittelt wird, den dieses Phasenspektrum bei der zweiten Bezugsfrequenz aufweist,
   5. e2) dass ein zur Differenz aus dem gespeicherten zweiten Phasenwert und dem in Schritt d2) genannten weiteren zweiten Phasenwert proportionaler zweiter Messwert für die Abstandsänderung erzeugt wird,
3. III) dass beim Auftreten von Abstandsänderungen, bei denen sich die linke und rechte Kante der Materialbahn in der von der Materialbahn aufgespannten Ebene quer zur Transportrichtung in dieselbe Richtung verschieben, die Lage der Achse wenigstens einer Walze des Walzenkalanders im Sinne einer Reduzierung dieser Lageverschiebung verändert wird,
4. VI) optional Speichern des in Schritt d1) genannten weiteren ersten Phasenwerts und des in Schritt d2) genannten weiteren zweiten Phasenwerts im Datenspeicher und Wiederholen der Schritte I) bis IV), wobei in Schritt e1) dieser erste Phasenwert und in Schritt e2) dieser zweite Phasenwert als gespeicherter Phasenwert verwendet wird.

In an advantageous embodiment of the invention it is provided that the material web is moved in the transport direction through a roll or roller gap between the rollers of at least one roll calender or is moved over a pair of rollers, that the material web has at least one first edge running in the transport direction and a first edge facing away from this, second edge running in the transport direction has that a first radar antenna is arranged in a stationary manner relative to the roll calender or the pair of rolls and a second radar antenna is arranged in a stationary manner in relation to the roll calender or pair of rolls,

1. I) that a first frequency-modulated continuous wave radar signal is generated and sent as a transmission signal via the first radar antenna to the first edge of the material web and radar radiation reflected from there is received by the first radar antenna while the material web is moved in the transport direction, so that the first received signal thus obtained forms a the first intermediate frequency signal is mixed with the transmission signal of the first radar antenna, so that the first intermediate frequency signal is sampled in a time-discrete manner in order to generate a first digital data stream,
   1. a1) that a first phase spectrum is formed from the first digital data stream for a first point in time and a first phase value is determined, which the first phase spectrum has at a first reference frequency, that this phase value is stored in a data memory,
   2. b1) that it is checked for a further first point in time whether there is a fault condition in which an obstacle is arranged in the radar transmission path between the first radar antenna and the first edge,
   3. c1) that in the event that the presence of the fault condition is detected at the time specified in step b1), steps b1) and c1) are repeated,
   4. d1) that for the further time mentioned in step b1), a further first phase spectrum is formed from the first digital data stream and a further first phase value is determined, which this phase spectrum has at the first reference frequency,
   5. e1) that a first measured value for the change in distance is generated which is proportional to the difference between the stored first phase value and the further first phase value mentioned in step d1),
2. II) that a second frequency-modulated continuous wave radar signal is generated and sent as a transmission signal via the second radar antenna to the second edge of the material web and radar radiation reflected from there is received by the second radar antenna while the material web is moved in the transport direction, so that the second reception signal thus obtained is formed a second intermediate frequency signal is mixed with the transmission signal of the second radar antenna, that the second intermediate frequency signal is sampled in a time-discrete manner in order to generate a second digital data stream,
   1. a2) that a second phase spectrum is formed from the second digital data stream for a second point in time and a second phase value is determined, which this phase spectrum has at a second reference frequency, that the second phase value is stored in the data memory,
   2. b2) that a check is made for a further second time as to whether there is a fault condition in which an obstacle is arranged in the radar transmission path between the second radar antenna and the first edge,
   3. c2) that in the event that the presence of the fault condition is detected at the time specified in step b2), steps b2) and c2) are repeated,
   4. d2) that for the further second point in time mentioned in step b2), a further second phase spectrum is formed from the second digital data stream and a further second phase value is determined, which this phase spectrum has at the second reference frequency,
   5. e2) that a second measured value for the change in distance is generated which is proportional to the difference between the stored second phase value and the further second phase value mentioned in step d2),
3. III) that when distance changes occur in which the left and right edges of the material web shift in the same direction in the plane spanned by the material web transverse to the transport direction, the position of the axis of at least one roll of the roll calender is changed in the sense of reducing this positional shift ,
4. VI) optionally storing the further first phase value mentioned in step d1) and the further second phase value mentioned in step d2) in the data memory and repeating steps I) to IV), whereby in step e1) this first phase value and in step e2) this second Phase value is used as the stored phase value.

Through these measures, the material web can be automatically kept in a rolling track so that the thickness and width of the material web remains within specified tolerances. The process can be used, for example, in sheet metal rolling mills.

In an expedient embodiment of the invention, the material web is wound onto a coil or a winding element which is driven in rotation about an axis of rotation, the measurement object being the lateral surface of the coil and the change in distance to the lateral surface being measured while the material web is wound onto the coil, the change in the Angle of rotation of the coil is recorded and a measurement signal for the thickness of the material web is formed from the change in distance and the change in the angle of rotation. The invention can therefore also be used very advantageously when rolling up or unrolling a sheet or similar material web from a coil. By detecting the roll-up angle, for example with an angle incremental encoder on the roll-up mechanism of the coil and the radar unit according to the invention, which continuously detects the increase or decrease in thickness of the sheet metal roll, the sheet thickness is continuously measured, for example for quality assurance. With each revolution of the coil, the radius of the coil changes by the thickness of the sheet. By continuously measuring the change in distance to the coil surface with the high-precision radar according to the invention, the sheet thickness can be measured with µm accuracy.

Sheet metal surfaces can be optically reflective. This can make the use of optical measurement technology unreliable and unsuitable. The radar described in this invention always measures the change in distance to the surface normal of the sheet on the coil and is therefore inherently reliable. Compared to optical systems, there are further advantages due to the better robustness against measurement errors caused by surface roughness, steam, dust and particles. Condensation of vapor and adhesion of particles to optical surfaces lead to potential measurement errors in optical systems. The high-precision radar according to the invention is significantly more robust here.

In another advantageous embodiment of the invention, the measurement object is the surface of an object, in particular a plate or a disk, the radar antenna being moved in a plane relative to the surface and a position signal for the position of the radar antenna relative to the surface being generated, the change in distance between the radar antenna and the surface of the object is measured while the radar antenna is moved relative to the surface, and wherein a measurement signal for the flatness and / or warpage of the surface is generated depending on the measured values for the change in distance and the position signal. The flatness of plates can therefore be advantageously measured using the invention. To do this, the plates are moved on a conveyor belt under a radar beam. The radar according to the invention is located on the carriage of a linear motor, which moves back and forth transversely to the transport direction of the plates. As a result, the change in distance to the surface of the plate is measured in a meandering manner by the high-precision radar. There are spacing jumps at the edges of the plate. These mark the beginning of the plate, the sides of the plate and the end of the plate. These jumps in distance change can be automatically detected by the invention and thus the geometry of the plate. When a plate passes through the conveyor belt into the radar beam, the linear drive is started and the distance change measurement begins. From the distance change measurements, the distortion of the plate can be measured in the µm range. Instead of flatness, the deviation from a given surface curvature can also be measured.

Compared to optical measuring methods, the invention has the advantage of being able to measure independently of the surface roughness of the plate. Thanks to the invention, the surface of the plate can be mirror-smooth, milled or sawn-rough, without this having an influence on the measurement accuracy of the distortion. Radar measurement is also more reliable because it is less sensitive to dust. The plate material is irrelevant for the flatness measurement with the radar according to the invention as long as there is an evaluable partial reflection at the plate surface/measuring section interface. Optically transparent panels can therefore also be measured.

In another advantageous application of the invention, a press is provided which has two receptacles that can be moved towards and away from one another, on each of which at least one press molding tool is arranged, with the radar antenna being arranged on one receptacle and a radar reflector as a measurement object on the other receptacle , whereby a pressing process is carried out with the press and the change in distance between the radar antenna and the measurement object is measured. With the help of the invention, radar sensors can be advantageously used in sheet metal presses to control movements and detect changes over time due to temperature drift and other influences. During the pressing process, predictable jumps in distance changes can occur due to the rapidly moving reflection surface, for example when opening and closing the press. Before opening or firing the press, the Distance change measurement can be stopped according to the invention and continued again after the opening or closing process. This allows the distortion of the press due to material deformation, for example caused by temperature drift or other influences, to be measured in the µm range. In addition, unforeseeable jumps in distance changes can occur due to foreign bodies in the measuring section, which must be detected in order to detect incorrect pressing in good time and avoid rejects. Compared to other measuring methods, the radar method according to the invention has the advantage of better robustness and accuracy over a wide measuring range of 70 mm to 1000 mm.

Further details, features and advantages of the present invention result from the following description of exemplary embodiments with reference to the drawing.

• 1 ein schematisches Blockschaltbild der Radareinheit zur kontinuierlichen Messung der Abstandsänderung zu einem Objekt,
• 2 das Radar-Messsignal im Frequenzbereich mit dem Amplituden- und Phasenspektrum, wobei für das Amplituden- und Phasenspektrum jeweils zwei analoge Spektren dargestellt sind, von denen das eine durch eine durchgezogene Linie und das andere strichliniert markiert sind, und wobei äquidistante Abtastwerte durch Punkte dargestellt sind,
• 3 eine Darstellung ähnlich 2, wobei jedoch die Amplituden- und Phasenspektren durch Geradenstücke angenähert sind, welche die Abtastwerte miteinander verbinden,
• 4 ein Ablaufdiagramm für die Abstandsänderungsmessung,
• 5 eine Radareinheit zur Messung einer Drift oder eines Verzugs in einer Werkzeugmaschine,
• 6 eine Abstandsänderungsmessung von Blechkanten mit zwei Radareinheiten in einer Walzstraße,
• 7 eine kontinuierliche Blechdickenmessung beim Aufrollen eines Blechs am Ende einer Walzstraße,
• 8 ein Transportband auf dem Platten befördert werden, deren Ebenheit mit Hilfe eines Radar-Verfahrens zur Messung von Abstandsänderungen ermittelt wird,
• 9 eine Presse, deren Weg mit einem Radar mittels Abstandsänderung hochpräzise gemessen wird,
• 10 ein Weg - Zeit - Diagramm der Abstandsänderung einer in 9 beschriebenen Presse.

It shows:

• 1 a schematic block diagram of the radar unit for continuously measuring the change in distance to an object,
• 2 the radar measurement signal in the frequency range with the amplitude and phase spectrum, two analog spectra being shown for the amplitude and phase spectrum, one of which is marked by a solid line and the other by a dashed line, and where equidistant sample values are represented by dots ,
• 3 a similar representation 2 , however, the amplitude and phase spectra are approximated by straight lines that connect the samples with each other,
• 4 a flowchart for the distance change measurement,
• 5 a radar unit for measuring drift or distortion in a machine tool,
• 6 a change in distance measurement of sheet metal edges with two radar units in a rolling mill,
• 7 a continuous sheet thickness measurement when rolling up a sheet at the end of a rolling train,
• 8th a conveyor belt on which plates are conveyed, the flatness of which is determined using a radar method to measure changes in distance,
• 9 a press whose path is measured with high precision using a radar by changing the distance,
• 10 a path-time diagram of the change in distance of an in 9 described press.

One in 1 Radar unit 1, designated overall by 1, for continuously measuring the change in distance to a measurement object 2 has an FMCW radar head 3, which has a radar signal generator 4, a transmitting radar antenna 5, a receiving radar antenna 6 and a high-frequency mixer 7. The radar signal generator 4 generates an FMCW transmission signal and its output is connected to the transmission radar antenna 5, which emits a transmission signal Tx onto the measurement object 2 arranged at a distance 8 from the transmission radar antenna 5. Part of the transmission signal reflected by the measurement object 2 is received by the receiving radar antenna 6.

The receiving radar antenna 6 is connected to a first input Rx of the high-frequency mixer 7. A second input LO of the high-frequency mixer 7 is connected to the output of the radar signal generator 4. The high-frequency mixer 7 generates an intermediate frequency signal which is present at the output IF of the high-frequency mixer 7.

The output IF of the high-frequency mixer 7 is connected to the analog input of an analog-digital converter 9. The analog-digital converter 9 converts the analog intermediate frequency signal present at the output IF into a digital data stream. The digital data stream at the output of the analog-digital converter 9 is connected to the input of a signal processing unit 11 via a data line 10.

The signal processing unit 11 is connected via a synchronization line 12 to an input of the radar signal generator 4 and a trigger input of the analog-to-digital converter 9 and synchronizes the transmission signal with the digitized intermediate frequency data stream at the output of the analog-to-digital converter 9. The signal processing unit 11 is connected to a computing unit 15 via two data lines 13, 14.

The signal processing unit 11 converts the digital digitized intermediate frequency data stream from the time domain into the frequency domain using complex numerical Fourier transformation and synchronizes the radar signal generator 4 with the analog-digital converter 9.

A first data line 13 transmits the amplitude values of the frequency spectrum calculated by the signal processing unit 11. A second data line 14 transmits the phase values of the frequency spectrum calculated by the signal processing unit 11 to the computing unit 15. The computing unit 15 calculates from the phase and Frequency values a distance value. When the distance changes, the measured phase changes. From the change in phase, which is measured in the frequency range at which the frequency spectrum has a maximum, the computing unit 15 calculates the change in distance with an accuracy in the µm range.

The computing unit 15 detects sudden changes in distance from the calculated distance data. The computing unit 15 transmits the calculated distance change data, but also, upon request from a higher-level controller, raw data from the analog-digital converter, the frequency and phase spectrum, as well as alarm, error and status information of the radar unit, such as temperature and air pressure, via a bidirectional data line 16 to a higher-level controller and/or to a display unit 17, which displays the measured values, the time, the distance and the change in distance, as well as errors due to sudden changes in distance. The higher-level controller can select the desired data stream to be displayed via the bidirectional data line 16. The computing unit can be informed when the measurement should be interrupted and resumed via the bidirectional data line 16 or a separate input unit 18, which can be designed, for example, as a simple switch.

The 2 and 3 describe the measurement signal s _IF (t) at the output ZF of the mixer 5 through the Fourier transformation in the frequency range with the associated phase. The measurement object 2, which is located at a distance of 8 from the antenna 5, 6, generates a signal by reflecting the transmission signal Tx at the IF output ZF of the high-frequency mixer 7. This signal contains the intermediate frequency Δf, which is equal to f _IF from equation 5 and is, in a first approximation, proportional to the distance 8 to the measurement object 2. The frequency Δf is determined by sampling and digitizing the signal at the IF output of the high-frequency mixer 7 in a time-discrete manner using the analog-digital converter 9 and converting it into the frequency range with the signal processing unit 11 and with the aid of the complex numerical Fourier transformation. The complex numerical Fourier transform provides a digitized amplitude and phase spectrum of the signal at the IF output of the high-frequency mixer 7. In 2 and 3 In the upper diagram the amplitude A is shown over the frequency f _IF and in the lower diagram the phase φ is shown over the frequency f _IF . In 2 are the spectra as analog signals with support points and in 3 the spectra are approximated by straight line sections that connect the support points.

The solid curve is the reference measurement to which the change in distance relates. The change in distance shifts the maximum of the curve. The nth measurement after the reference measurement is shown as an example as a dash-dotted line. Because of the discrete sampling with support points in the time domain, the numerical Fourier transformation obtains the spectrum as an interpolated function with support points in the frequency domain. As an example, support points from 1 to 9 are shown for the reference measurement (solid line) for both the amplitude spectrum (top) and the phase spectrum (bottom). The support points for the nth spectrum are not shown.

Since the course of the curve between the support points in the amplitude spectrum is not known exactly, the frequency at which the maximum of the amplitude lies cannot be determined with sufficient accuracy.

The course between the reference points in the phase spectrum can, however, be adjusted to a very good approximation using a straight line (see 3 ). The shift of the straight line from the phase spectra as a result of a change in distance can therefore be determined much more precisely than the shift of the maximum from the amplitude spectrum. The phase of a periodic function can only take values between -π and +π. Therefore, the absolute distance value cannot be determined from the evaluation of the phase spectrum. As long as it can be ensured that the change in distance between two successive spectra is not greater than half a wavelength, the change in distance can be calculated with µm accuracy. However, changes in distance that are greater than half a wavelength can be detected by evaluating the amplitude measurements.

A reference measurement is useful for calculating the change in distance. The maximum of the amplitude A _O of the reference measurement of the spectrum is at the frequency Δf ₀ . The frequency Δf is proportional to the distance 7 and, as described at the beginning, cannot be determined with any precision. The digitized amplitude values of the frequency spectrum are only available in frequency-equidistant support values. As those skilled in the art know, the number of support points can be improved by zero padding of the discrete (DFT) or fast (FFT) Fourier transform or the chirped Z transform (CZP), but this does not change the fundamental inaccuracy. The accuracy with which the frequency of the real amplitude maximum and thus the distance 8 can be determined in FMCW radars is theoretically limited by the Cramer-Rao limit. In real radar systems, the accuracy with which the frequency Δf is determined can be significantly lower than described by the Cramer-Rao limit and in inexpensive radar systems it is limited to the range of half a wavelength of the mean radar frequency. The phase values of the spectrum are between -π and +π. In the area of the amplitude maximum, the phase progression is linear and can be interpolated by a straight line. The distance measurement is carried out continuously one after the other. In 3B the reference measurement is labeled A _0d ,Δf ₀ and ^φ _0d and the nth measurement is labeled A _nd , Δf _nd and ^φ _nd .

The linear phase curves of the reference measurement and the nth measurement have, to a good approximation, the same slope and are shifted relative to one another on the frequency axis. This frequency shift can be determined much more precisely by averaging over many numerical support values of the phase spectrum than the shift in the amplitude maxima due to the change in distance. The phase shift of the linear phase curve of the nth measurement compared to the reference measurement can thus be determined with very high precision. In practice, changes in the distance 8 can therefore be determined with µm accuracy, but not the absolute distance 8, since it cannot be decided with sufficient accuracy which phase range belongs to which distance determined by frequency measurement.

1. a) Für einen ersten Zeitpunkt wird ein erstes Amplitudenspektrum erzeugt und es wird das dem Messobjekt 2 zugeordnete Maximum dieses Amplitudenspektrums ermittelt. Für den ersten Zeitpunkt wird außerdem ein erstes Phasenspektrum erzeugt und es wird ein erster Phasenwert ermittelt, den dieses Phasenspektrum bei einer Bezugsfrequenz aufweist, die mit der Frequenz übereinstimmt, an der das erste Amplitudenspektrum das vorstehend genannte Maximum hat. Der erste Phasenwert wird in einem Datenspeicher abgelegt.
2. b) Für einen weiteren Zeitpunkt wird geprüft, ob ein Störzustand vorliegt, bei dem ein Hindernis in der Radarübertragungsstrecke zwischen der Radarantenne und dem Messobjekt 2 angeordnet ist oder die Gefahr besteht, dass das der Fall ist.
3. c) Falls für den in Schritt b) genannten Zeitpunkt das Vorliegen des Störzustands detektiert wird, wird die Messung unterbrochen und die Schritte b) und c) werden wiederholt. Während der Unterbrechung werden keine neuen Messwerte für die Abstandsänderung erfasst. Während der Unterbrechung erzeugt der Radarsignalgenerator 4 aber weiterhin das FMCW-Sendesignal.
4. d) Für den in Schritt b) genannten weiteren Zeitpunkt wird ein weiteres Amplitudenspektrum aus dem digitalen Datenstrom erzeugt und es wird das dem Messobjekt 2 zugeordnete Maximum dieses Amplitudenspektrums ermittelt. Für den weiteren Zeitpunkt wird außerdem ein weiteres Phasenspektrum erzeugt und es wird ein weiterer Phasenwert ermittelt, den dieses Phasenspektrum bei der Bezugsfrequenz aufweist. Der weitere Phasenwert wird im Datenspeicher abgelegt.
5. e) Es wird ein zur Differenz aus dem gespeicherten Phasenwert und dem in Schritt d) genannten weiteren Phasenwert proportionaler Messwert (ΔP_φN) für die Abstandsänderung ermittelt.
6. f) optional wird für den weiteren Zeitpunkt ein weiteres Amplitudenspektrum aus dem digitalen Datenstrom erzeugt und es wird das dem Messobjekt 2 zugeordnete Maximum dieses Amplitudenspektrums ermittelt. Außerdem wird für den weiteren Zeitpunkt ein weiterer Phasenwert ermittelt, den das in Schritt d) genannte Phasenspektrum bei einer Bezugsfrequenz aufweist, die mit der Frequenz übereinstimmt, an der das in Schritt f) genannte weitere Amplitudenspektrum ein dem Messobjekt 2 zugeordnete Maximum hat. Dieser weitere Phasenwert wird im Datenspeicher abgelegt und die Schritte b) bis f) werden wiederholt, wobei in Schritt e) dieser Phasenwert als gespeicherter Phasenwert und in Schritt d) als Bezugsfrequenz die Bezugsfrequenz aus Schritt f) verwendet wird.

When measuring the change in distance using the radar unit 1, the following process steps are carried out:

1. a) A first amplitude spectrum is generated for a first point in time and the maximum of this amplitude spectrum assigned to the measurement object 2 is determined. A first phase spectrum is also generated for the first point in time and a first phase value is determined which this phase spectrum has at a reference frequency which corresponds to the frequency at which the first amplitude spectrum has the aforementioned maximum. The first phase value is stored in a data memory.
2. b) At a further point in time it is checked whether there is a fault condition in which an obstacle is arranged in the radar transmission path between the radar antenna and the measurement object 2 or there is a risk that this is the case.
3. c) If the presence of the fault condition is detected at the time specified in step b), the measurement is interrupted and steps b) and c) are repeated. No new distance change measurements are recorded during the interruption. During the interruption, the radar signal generator 4 continues to generate the FMCW transmission signal.
4. d) For the further time mentioned in step b), a further amplitude spectrum is generated from the digital data stream and the maximum of this amplitude spectrum assigned to the measurement object 2 is determined. A further phase spectrum is also generated for the further point in time and a further phase value is determined which this phase spectrum has at the reference frequency. The further phase value is stored in the data memory.
5. e) A measured value (ΔP _φN ) for the change in distance is determined which is proportional to the difference between the stored phase value and the further phase value mentioned in step d).
6. f) optionally, a further amplitude spectrum is generated from the digital data stream for the further point in time and the maximum of this amplitude spectrum assigned to the measurement object 2 is determined. In addition, a further phase value is determined for the further point in time, which the phase spectrum mentioned in step d) has at a reference frequency which corresponds to the frequency at which the further amplitude spectrum mentioned in step f) has a maximum assigned to the measurement object 2. This further phase value is stored in the data memory and steps b) to f) are repeated, with this phase value being used as the stored phase value in step e) and the reference frequency from step f) being used as the reference frequency in step d).

Ist die Abstandsänderung größer/gleich, so wird eine Fehlermeldung ausgegeben. Ist die Abstandsänderung kleiner so wird die errechnete Abstandsänderung zusammen mit dem Zähler, dem Zeitstempel der Messung und dem absoluten Messwert ausgegeben. Wenn die Messung weder pausieren soll noch gestoppt werden soll fängt die Schleife wieder beim Inkrementieren des Zählers an. Sollte die Messung Pausieren, weil beispielsweise eine Störung der Funkstrecke zu erwarten ist, so wird so lange gewartet bis die Messung durch ein „Weiter“ Kommando fortgesetzt werden soll, wenn beispielsweise die Störung der Funkstrecke behoben ist und sichergestellt ist, dass das Objekt wieder innerhalb einer Abstandsmessung zur letzten gültigen Messung von weniger als einer halben Wellenlänge ist. Die Kommandos „Pause“ und „Weiter“ können beispielsweise durch den Schalter 17 erfolgen. In einer erweiterten Ausführung kann die Pausierung auch automatisch beim Feststellen einer Störung erfolgen. 4 describes the flowchart for measuring the change in distance. After starting and initializing the radar unit 1, the counter N is set to zero. Then the absolute distance measurement is carried out according to equation 7 (R _f0 ~ f _IF ) and the distance measurement according to equation 9 (R _φ0 - φ _IF ), where R _f0 is the reference measurement of the distance change measurement. The counter N is then incremented in a programming loop and the measurement according to equations 7 and 9 is repeated. The change in distance ΔR _φN is calculated from the difference between the reference measurement R _φ0 and the Nth distance measurement R _φN . The time of the measurement is recorded in t _N. A decision is then made as to whether the measurement is valid, i.e. no phase jump has occurred between the measurements. To do this, the distance measurement according to equation 7 is compared with the previous distance measurement as equation 7 and it is determined whether the change in the distances is less than half a wavelength $\left(R_{fN}-R_{fN-1}\ge \frac{\lambda }{2}\right).$

If the change in distance is greater than/equal, an error message is output. If the change in distance is smaller, the calculated change in distance is used together with the counter, the time stamp of the measurement and the absolute measurement value issued. If the measurement should neither be paused nor stopped, the loop starts again when the counter is incremented. If the measurement is paused because, for example, a disruption in the radio link is to be expected, the measurement will be waited until the measurement is to be continued with a “Continue” command, for example if the disruption in the radio link has been eliminated and it is ensured that the object is within again a distance measurement from the last valid measurement is less than half a wavelength. The commands “Pause” and “Continue” can be given using switch 17, for example. In an extended version, the pause can also take place automatically when a fault is detected.

5 describes how the method according to the invention can be used for continuously measuring the drift in a machine tool 19. In the process, the machine tool and the in 1 radar unit 1 shown is provided.

The machine tool 19 has a workpiece holder 20, which positions a workpiece 21 relative to a base part 22 designed as a receiving block. For this purpose, the workpiece holder 20 can be moved relative to the base part 22 towards and away from a machining tool 24 by means of a positioning drive 23 which is fastened to the base part 22 and is designed as a linear motor. The processing tool 24, for example a grinding wheel, is arranged on a tool holder 25 which is attached to the base part 22. The workpiece holder 20 moves the workpiece 21, for example, against the processing tool 24 with the aid of the positioning drive 23.

The FMCW radar head 3 of the radar unit 1 is firmly connected to the workpiece holder 20. A radar reflector, which serves as a measurement object 2, is firmly connected to the tool holder 20. Changes in distance 8 between the tool holder 25 and the workpiece holder 20 are measured with the radar unit 1 and record the feed of the workpiece 21 together with the drift of the machine tool 19. The feed of the workpiece 21 is carried out in the usual way via a control of the positioning drive 23, not shown in the drawing µm accuracy known. The drift of the machine tool 19 is calculated from the comparison of the feed with the change in distance measured by the radar unit 1.

6 describes the application of the method according to the invention for the continuous measurement of the saber shape of a material web 26, namely a sheet metal strip in a rolling train. The material web 26, guided by rollers 27, moves tangentially to the lateral surface of the rollers 27 in a transport direction 28 past a measuring point at which two opposite FMCW radar heads 3, 3 'are arranged, each corresponding to a radar unit 1 1 assigned. The radar heads 3, 3' each send out radar lobes 29, 29'. The opening solid angle of the radar lobes 29, 29 'is of minor importance, since the respective changes in distance 8, 8' of the sheet metal edges on both sides are measured at the points where the radar lobes 29, 29' orthogonally meet the edges of the material web 26.

7 describes the application of the method according to the invention or in 1 Radar unit 1 shown for precise measurement of the thickness 32 of a material web 26, which is rolled up onto a coil 30 at the end of a rolling train. When the material web 26 is rolled up onto the coil 30, the distance to the FMCW radar head 3 of the radar unit 1 facing the peripheral surface of the coil 30 is continuously reduced. The angle of rotation of the coil 30 is continuously measured via an incremental encoder 31 as the material web 26 is rolled up. The thickness 32 of the material web 26 is calculated from the measured angle of rotation and the change in distance 8 measured by the radar unit 1. The thickness 32 of the material web 26 is calculated from the difference of the measured change in distance 8 between a first angle of rotation and a second angle of rotation, which occurs one full revolution or 360 ° later than the first.

8th shows a conveyor belt 33 on which plates 34 are conveyed in a direction 35, the flatness of which corresponds to the radar unit 1, from which in 8th For the sake of simplicity, only the FMCW radar head 3 is shown, is determined by means of distance change measurement 8. For this purpose, the radar beam 36 is aligned perpendicularly to the surface of the plate 34. The conveyor belt 33 moves the plate 34 under the radar beam 36. Changes in distance down to the µm range are recorded. And compared with a reference plate whose flatness is known.

9 shows a press 37 which has a stamp carrier 38 with a press stamp 39 and a lower frame part 40. The press 37 has an upper press molding tool 41 and a lower press molding tool 42 which can be moved towards and away from it. The distance 43 between the upper press molding tool 41 and the lower press molding tool 42 changes cyclically by moving the upper press molding tool 41 up and down. The upper press molding tool 41 is connected to the press stamp 39 via an upper tool holder 44 and the lower press molding tool 42 is attached to a lower tool holder 40 arranges. In the 1 Radar unit 1 shown measures the change in distance 45 between the upper tool holder 44 and the lower tool holder 40 on the measuring section. The temporal change in distance 45 is in the right sketch of 9 shown.

10 describes a path-time diagram of the change in distance of one of the in 9 Press 37 shown with cyclically repeating opening and closing movements. This is where the comes in 1 sketched radar unit 1 is used to monitor the press movement. On the ordinate 46 of 10 the distance measured by the radar unit 1 is plotted. The time during the measurement is plotted on the abscissa 47. The cycle duration 48 is given by passing through the same distance value in the same direction of movement. The maximum movement deflection is designated 49. The maximum distance between the measurement object and the radar sensor is given by 50. The maximum movement speed is reached at point 51. A possible fixed distance value, which can be defined as the measurement interruption and resumption point of the distance change measurement, is outlined by the straight line 52.

Claims (14)

Method for measuring changes in distance to a measurement object (2), wherein a frequency-modulated continuous wave radar signal is generated and sent as a transmission signal via a radar antenna to the measurement object (2), radar radiation reflected by the measurement object (2) being received by the radar antenna and the reception signal thus obtained being formed an intermediate frequency signal is mixed with the transmission signal, the intermediate frequency signal being sampled in a time-discrete manner in order to generate a digital data stream, and the method having the following steps: a) generating a first phase spectrum from the digital data stream for a first point in time, determining a first phase value, which this phase spectrum has at a reference frequency, and storing the first phase value in a data memory, b) for a further point in time checking whether there is a disturbance condition in which an obstacle is arranged in the radar transmission path between the radar antenna and the measurement object (2) or the There is a risk that this is the case, c) if the presence of the fault state is detected for the time mentioned in step b), repeating steps b) and c), d) for the further time mentioned in step b), generating a further one phase spectrum from the digital data stream, determining a further phase value that this phase spectrum has at the reference frequency, e) determining a measured value (ΔP _φN ) for the change in distance which is proportional to the difference between the stored phase value and the further phase value mentioned in step d). optionally storing the further phase value mentioned in step d) in the data memory and repeating steps b) to f), whereby in step e) this phase value is used as the stored phase value. Procedure according to Claim 1 , characterized in that at least three first support points are provided for the first phase spectrum, that the first phase spectrum is formed by linear interpolation from the first support points and the first phase value is determined with the aid of this interpolation, and / or that for that in step d). Patent claim 1 mentioned further phase spectrum at least three further support points are provided, that the further phase spectrum is formed by linear interpolation from the further support points and the further phase value is determined with the help of this interpolation. Procedure according to Claim 2 , characterized in that the first and/or the second linear interpolation is carried out over a number of at least 2, optionally at least 3 and preferably at least 10 support points. Procedure according to one of the Claims 1 until 3 , characterized in that - a first amplitude spectrum is generated from the digital data stream for the first point in time and a sampling point is determined in the first amplitude spectrum, at which the amplitude spectrum has a maximum assigned to the measurement object, which is used as the reference frequency in step a). Patent claim 1 the frequency of a sampling point of the first amplitude spectrum is selected, which is a maximum of n sampling points away from the sampling point having the maximum, where n is an integer between 0 and 10, in particular between 0 and 5, optionally between 0 and 3 and preferably 1 or 0 , and optionally - that for the one in step b) of Patent claim 1 at the further point in time mentioned, a further amplitude spectrum is generated and in the further amplitude spectrum a sampling point is determined, at which the further amplitude spectrum has a maximum assigned to the measurement object, and that when repeating steps b) to f). Patent claim 1 the frequency of a sampling point of this further amplitude spectrum is selected as the reference frequency, which is a maximum of n sampling points away from the sampling point having the maximum, where n is an integer between 0 and 10, in particular between 0 and 5, optionally between 0 and 3 and preferably 1 or 0. Procedure according to one of the Claims 1 until 4 , characterized in i) that for the step a) of Patent claim 1 at the first point in time mentioned, a first amplitude spectrum is generated from the digital data stream and a first frequency value (Δf _od ) is determined, at which the first amplitude spectrum has a maximum, and the first frequency value (Δf _od ) is stored in the data memory, ii) that for the in Step b) of Patent claim 1 mentioned further point in time, a further amplitude spectrum is determined from the digital data stream and a further frequency value (Δf _nd ) is determined, at which the further amplitude spectrum has a maximum, iii) that between successive points in time, for which an amplitude spectrum is determined, the distance between the radar antenna and the measurement object (2) is not changed by a value whose amount is greater than half the wavelength of the average radar frequency of the transmission signal, iv) that a difference between the stored frequency value (Δf _od ) and the further frequency value (Δf _nd ) from step ii) proportional distance change value (ΔR _fN ) is formed and compared in terms of amount with a limit value which is smaller than a distance change value that occurs when measuring a distance change which corresponds to half the wavelength of the mean radar frequency of the transmission signal, v) that in step b) from Patent claim 1 the disturbance condition is detected if the comparison in step iv) shows that the distance change value (ΔR _fN ) is greater than the limit value, vi) optionally storing the further frequency value (Δf _nd ) in the data memory and repeating steps ii) to vi), wherein in step iv) this frequency value (Δf _nd ) is used as a stored frequency value (Δf _od ). Procedure according to Claim 5 , characterized in that the limit value is smaller than a distance change value that occurs when measuring a distance change which corresponds to 100%, possibly 25% and in particular 50% of the wavelength of the mean radar frequency of the transmission signal. Procedure according to one of the Claims 1 until 6 , characterized in that the position of the obstacle is detected and compared with the position of the transmission link, and that the fault condition is detected if the comparison shows that the obstacle is arranged in the transmission link or is less than a predetermined distance from the transmission link. Procedure according to one of the Claims 1 until 7 , characterized in that steps b) to f) of Patent claim 1 be run through at least twice, and that the measured values (ΔP _φN ) determined for the change in distance during these runs are added up in order to obtain a total value by which the distance to the measurement object (2) has changed during these runs. Procedure according to one of the Claims 1 until 8th , characterized in that a machine tool (19) is provided which has a base part (22) on which a workpiece holder (20) and a tool holder (25) are arranged, that the workpiece holder (20) holds a workpiece (21) and the Tool holder (25) carries a processing tool (24) for removing material from the workpiece (21), - that the machine tool (19) has a positioning drive (23) by means of which the workpiece holder (20) or the tool holder (25) is relative to the point at which the positioning drive (23) is arranged on the base part (22), - that the distance over which the positioning takes place in the direction of the radar transmission path relative to this point on the base part is recorded, - that the radar antenna on the workpiece holder (20) and the measurement object (2) on the tool holder (25) or the radar antenna on the tool holder (25) and the measurement object (2) on the workpiece holder (20), and - that for determining a drift and / or a delay of the machine tool (19), the difference is formed from the measured value (ΔP _φN ) for the change in distance between the radar antenna and the measurement object (2) and the recorded distance. Procedure according to one of the Claims 1 until 8th , characterized in that the measurement object (2) is an edge or a surface of a material web (26), in particular a sheet, which material web (26) moves relative to the radar antenna along the edge and / or the surface in a transport direction (28). is that the radar antenna is aligned relative to the material web (26) in such a way that the change in distance to the measurement object (2) is measured transversely and preferably orthogonally to the transport direction (28), and that the measurement of the change in distance is carried out while the material web (26 ) is moved relative to the radar antenna in the transport direction (26). Procedure according to Claim 10 , characterized in that the material web (26) in the transport direction (28) through a roller or roller gap between the rollers at least a roll calender is moved through or moved over a pair of rollers, so that the material web (26) has at least one first edge running in the transport direction (28) and a second edge facing away from this and running in the transport direction (28), which is stationary to the roll calender or the A first radar antenna arranged in a pair of rollers and a second radar antenna arranged in a stationary manner relative to the roller calender or pair of rollers are provided, I) that a first frequency-modulated continuous wave radar signal is generated and sent as a transmission signal via the first radar antenna to the first edge of the material web (26) and radar radiation reflected from there by the first radar antenna is received while the material web (26) is moved in the transport direction (28), that the first received signal thus obtained is mixed with the transmission signal of the first radar antenna to form a first intermediate frequency signal, that the first intermediate frequency signal is sampled in a time-discrete manner to form one of the first digital to generate data stream, a1) that a first phase spectrum is formed from the first digital data stream for a first point in time and a first phase value is determined, which the first phase spectrum has at a first reference frequency, that this phase value is stored in a data memory, b1) that for a further first point in time is checked to determine whether there is a disturbance condition in which an obstacle is arranged in the radar transmission path between the first radar antenna and the first edge, c1) that in the event that the disturbance condition is present for the point in time mentioned in step b1). is detected, steps b1) and c1) are repeated, d1) that for the further time mentioned in step b1), a further first phase spectrum is formed from the first digital data stream and a further first phase value is determined, which this phase spectrum is at the first reference frequency has, e1) that a first measured value (ΔR _φN1 ) for the change in distance which is proportional to the difference between the stored first phase value and the further first phase value mentioned in step d1) is generated, II) that a second frequency-modulated continuous wave radar signal is generated and as a transmission signal via the second Radar antenna is sent to the second edge of the material web (26) and radar radiation reflected from there is received by the second radar antenna while the material web (26) is moved in the transport direction (28) so that the second received signal thus obtained forms a second intermediate frequency signal with the Transmission signal of the second radar antenna is mixed, that the second intermediate frequency signal is sampled in a time-discrete manner in order to generate a second digital data stream, a2) that a second phase spectrum is formed from the second digital data stream for a second point in time and a second phase value is determined, which this phase spectrum contributes a second reference frequency, that the second phase value is stored in the data memory, b2) that a check is made for a further second time as to whether there is a disturbance condition in which an obstacle is arranged in the radar transmission path between the second radar antenna and the first edge, c2 ) that in the event that the presence of the disturbance state is detected for the time mentioned in step b2), steps b2) and c2) are repeated, d2) that a further second phase spectrum is produced for the further second time mentioned in step b2). the second digital data stream is formed and a further second phase value is determined, which this phase spectrum has at the second reference frequency, e2) that a second measured value (ΔR _φN2 ) is proportional to the difference between the stored second phase value and the further second phase value mentioned in step d2). is generated for the distance change, III) that when distance changes occur in which the left and right edges of the material web (26) shift in the same direction in the plane spanned by the material web (26) transversely to the transport direction (28), the position the axis of at least one roll of the roll calender is changed in the sense of reducing this positional shift, VI) optionally storing the further first phase value mentioned in step d1) and the further second phase value mentioned in step d2) in the data memory and repeating steps I) to IV) , whereby in step e1) this first phase value and in step e2) this second phase value is used as the stored phase value. Procedure according to Claim 10 , characterized in that the material web (26) is wound onto a coil (30) or a winding element which is driven in rotation about an axis of rotation, that the measurement object (2) is the lateral surface of the coil (30) and the change in distance to the lateral surface is measured while the Material web (26) is wound onto the coil (30) so that the change in the angle of rotation of the coil (30) is detected and a measurement signal for the thickness of the material web (26) is formed from the change in distance and the change in the angle of rotation. Procedure according to one of the Claims 1 until 8th , characterized in that the measurement object (2) is the surface of an object, in particular which is a plate (34) or a disk, that the radar antenna is moved in a plane relative to the surface and a position signal for the position of the radar antenna relative to the surface is generated, that the change in distance between the radar antenna and the surface of the object is measured while the Radar antenna is moved relative to the surface, and that a measurement signal for the flatness and / or warpage of the surface is generated depending on the measured values for the change in distance and the position signal. Procedure according to one of the Claims 1 until 8th , characterized in that a press (37) is provided which has two tool holders (40, 44) that can be moved towards and away from each other, on each of which at least one press molding tool (41, 42) is arranged, and that the radar antenna is on one Tool holder and a radar reflector is arranged as the measurement object (2) on the other tool holder, that a pressing process is carried out with the press (37) and the change in distance (45) between the radar antenna and the measurement object (2) is measured.

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