DE19644791C2  Method and device for determining the light propagation time over a measuring section arranged between a measuring device and a reflecting object  Google Patents
Method and device for determining the light propagation time over a measuring section arranged between a measuring device and a reflecting objectInfo
 Publication number
 DE19644791C2 DE19644791C2 DE19644791A DE19644791A DE19644791C2 DE 19644791 C2 DE19644791 C2 DE 19644791C2 DE 19644791 A DE19644791 A DE 19644791A DE 19644791 A DE19644791 A DE 19644791A DE 19644791 C2 DE19644791 C2 DE 19644791C2
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 signal
 max
 characterized
 correlation function
 spectrum
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Classifications

 G—PHYSICS
 G04—HOROLOGY
 G04F—TIMEINTERVAL MEASURING
 G04F10/00—Apparatus for measuring unknown time intervals by electric means

 G—PHYSICS
 G01—MEASURING; TESTING
 G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
 G01J7/00—Measuring velocity of light

 G—PHYSICS
 G01—MEASURING; TESTING
 G01S—RADIO DIRECTIONFINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCEDETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
 G01S7/00—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00
 G01S7/48—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S17/00
 G01S7/483—Details of pulse systems
 G01S7/486—Receivers
 G01S7/4865—Details of time delay measurement, e.g. time of flight or time of arrival measurement, determining the exact position of a peak
 G01S7/4866—Details of time delay measurement, e.g. time of flight or time of arrival measurement, determining the exact position of a peak by fitting a model or function to the received signal

 G01S17/14—
Description
The present invention relates to a method for determining measurement of the signal transit time between a measuring device device and a reflecting object arranged measuring section, in which a transmitter contained in the measuring device a mo Dulated signal s (t) sends over the test section, the egg receive receiver contained in the measuring device, in a received signal e (t) converted and in the measuring device is evaluated. The invention is also based on a Vorrich directed to perform this procedure.
Such methods and devices are used at a distance measurements used to determine the distance to be determined between rule measuring device and reflecting object or the Län ge of the measuring section from the signal transit time over the measuring section to calculate.
It is often the case with such methods and devices problematic that a at relatively large distances very precise resolution must be achieved, besides Lich the existing radiant power Regulations regarding eye safety are observed have to.
In order to meet the abovementioned requirements, gene or methods according to the prior art a high to operate technical effort that negatively affects the Efficiency affects.
DE 34 35 949 C2 describes a CW radar system in which the transmission signal is modulated and for determining the Distance of a target object echo signals with un differently delayed repetitions of the transmission signal to be corrected for the delay due to the transit time To be able to determine echo signals. The distance searched results from the correlation result in which the best Adjustment between a certain delayed retry the transmission signal and the reception signal can be determined. Comparable systems are also in DE 39 37 787 C1 and described in DE 33 16 630 C2.
According to DE 32 39 403 C2 are used to determine the running time received signals into a complex frequency spectrum converts and the complex frequency spectra conjugate complex multiplied together. The cross spectrum obtained is then transformed back into the time domain. From the The amount of the backtransformed will then be the terms of signals received at different locations are determined.
An object of the present invention is to provide a Method or a device of the type mentioned above train that with the lowest possible economic up wall, ensuring eye safety even with large ones Intervals a high resolution can be achieved.
This object is achieved according to the invention by a method for determining the signal transit time over a measuring path arranged between a measuring device and a reflecting object, in which a transmitter contained in the measuring device detects a modulated signal s (t) over the measuring path, which is detected by a sensor in the Receiving measuring device contained receiver received, converted into a reception signal e (t) and evaluated in the measuring device, between the transmission and reception signals s (t) and e (t) a correlation function k (t) is formed by the transmission and Receive signals s (t) and e (t) are transformed into the frequency range, in the frequency range the spectrum K (f) of the correlation function k (t) from the spectra S (f) and E (f) of the transmit and receive signals s ( t) and e (t) are calculated and the correlation function k (t) is then determined by backtransformation into the time domain, the maximum k _{max} = k (t _{max} ) of the correlation function k (t) is determined , t _{max is} evaluated to determine the signal transit time, to form the spectrum K (f) of the correlation function k (t) the product A (f) from the spectrum E (f) and the conjugate complex spectrum S * (f) or is formed from the spectrum S (f) and the conjugated complex spectrum E * (f) and the spectrum A (f) is split symmetrically with respect to the frequency f into the subspectra A _{1} (f) and A _{2} (f) and between the subspecs A _{1} (f) and A _{2} (f) q = (i × p)  p amplitude values w _{j} (1 ≦ j ≦ q) are inserted, where i represents the integer interpolation factor.
According to the invention, a correlation function between the transmit and receive signals formed, this formation is carried out in the frequency domain. This can price inexpensive components are used, since only with Fre sequences that are used in video technology are.
The signal s (t) to be emitted is preferred in the form of p (p <0) digital values stored in a transmit memory, the stored digital values sequentially read and one acted on with a control clock signal, control the transmitter are fed to the D / A converter, the received signal e (t) supplied with the control clock signal A / D converter, the values supplied by the A / D converter in one Received memory stored and the spectra S (f) and E (f) from the signals stored in the transmit and receive memories s (t) and e (t) are calculated.
According to the invention, the D / A converter and the A / D Transducer in sync with the identical control clock signal strikes, which has the consequence that always exactly when a Won digital value from the transmit memory into an analog value delt and is broadcast by the broadcaster in the same eye view of the analog value delivered by the receiver into a digital value converted and stored in the receive memory. To a send and receive cycle are consequently in the send and signals of substantially the same curves in the reception memory form, whereby, neglecting internal signal propagation times the received signal compared to the transmitted signal time Lich corresponds to that of a certain number of clock cycles signal runtime is shifted over the test section.
The number of clock cycles corresponding to the light travel time is determined according to the invention in that between the Sen de and the received signal formed a correlation function , the formation of this correlation function being invented is carried out appropriately in the frequency domain and after Reverse transformation of the spectrum of the correlation function in the time range determines the maximum of the correlation function is communicated. At the maximum of the correlation function, and received signals shifted so far relative to one another, that their curves are a minimal distance apart or largely overlap.
Since this shift exactly the signal transit time or the on number of clock cycles between sending and emp catches of the signal have passed, the will Shift characteristic time value in a preferred Embodiment used as the signal delay to be determined.
Thus, according to the invention, the signal transit time can be reduced computational effort using inexpensive components determine without, for example, as in the prior art a great optical effort operated or a GHz counter must be used.
Due to the interpolation according to the invention already in frequency area is achieved that no systematic error in the Calculating the correlation function arises, the back transformed time signal of the correlation function over the Interpolation factor i is expanded in time, so that ei ne results in an increased number of support points by a factor of i, which lead to higher accuracy.
According to a further advantageous embodiment of the invention, the maximum of the correlation function k (t) is determined by a curve fitting method, preferably a particular quadratic polynomial p (t) is determined, the distance from or the square of the distance to the correlation function k (t) is minimal in the range of k _{max} . After determining the maximum p _{max} = p (t _{max / pol} ), t _{max / pol is} evaluated instead of t _{max} to determine the signal run time and is used in particular directly as a representative for the signal run time. Three or more values of the correlation function k (t) can be used to determine the polynomial.
The problem with the described methods is the fact that signal propagation times within the measuring device lead to measuring errors. These can be eliminated by switching on a reference path in which reference signals pass through identical or corresponding components for the measurement signals. The signal propagation time cleaned of measurement errors can be obtained in this case by using the reference signal r (t) received over the reference path when determining the correlation function instead of the transmission signal s (t), since the correlation function between the reference signal is used to determine the correlation function r (t) and the received signal e (t) mutually compensate for the internal runtimes and after determining the maximum of the correlation function the time value t ' _{max} is cleared of corresponding measurement errors.
In a preferred embodiment of the invention, the reference signal r (t) received over the reference path is therefore used to determine the signal transit time instead of the signal s (t), in particular the spectrum R (f) is obtained from the reference signals r stored in the reference reception memory (t) calculated, the product A '(f) = E (f) × R * (f) or A' (f) = E * (f) × R (f) determines, the spectrum K '(f) according to the equation
formed and the correlation function k '(t) determined by backtransformation in the time domain. After determining the maximum k ' _{max} = k' (t ' _{max} ) of the correlation function k' (t), t ' _{max} can be used directly as the signal propagation time to be determined.
According to a further embodiment of the invention, it is also possible not to use the reference signal r (t) instead of the transmission signal s (t), but in addition to forming the correlation function k (t) between the transmission signal s (t) and the reception signal e (t) in an analogous manner to form a correlation function k "(t) between the reference signal r (t) received via the reference link and the transmission signal s (t). The two determined correlation functions k (t) and k" (t) then both contain the same measurement errors caused by the components. After determining the maximum of the correlation functions k (t) and k "(t), the difference between the two determined signal transit times t _{max} and t" _{max} , both of which are associated with the measurement error of the signal transit times within the measuring device, can be formed, so that this measurement error is eliminated by the difference.
Lightsi signals, ultrasonic signals or microwave signals are used become.
A possible embodiment of a measuring device for carrying out the method according to the invention has, for example, a transmit memory for storing the signal to be transmitted s (t) in the form of digital values, a D / A converter acted upon by the transmit memory and controlling the transmitter, one with the received signal e (t) acted upon A / D converter, a reception memory for storing the values supplied by the A / D converter and a clock generator which respectively applies the identical control clock signal to the D / A converter and the A / D converter, the Evaluation circuit with means for transforming the transmit and receive signals s (t) and e (t) into the frequency domain, for forming the spectrum K (f) of a correlation function k (t) between the signals s (stored in the transmit and receive memories) t) and e (t) for transforming the spectrum K (f) back into the time domain and for determining the maximum k _{max} = representing the transit time t _{max} to be determined k (t _{max} ) of the correlation function k (t) is provided, and the product A (f) from the spectrum E (f) and the conjugate complex spectrum S being used to form the spectrum K (f) of the correlation function k (t) * (f) or from the spectrum S (f) and the conjugate complex spectrum E * (f) is formed and the spectrum A (f) symmetrically with respect to the frequency f in the subspectra A _{1} (f) and A _{2} ( f) is split and inserted between the subspectra A _{1} (f) and A _{2} (f) q = (i × p)  p amplitude values w _{j} (1 ≦ j ≦ q), where i represents the integer interpolation factor.
At the end of an additionally provided reference path, a reference receiver which can be coupled to the evaluation circuit for generating a reference signal r (t) can be provided. In this case, the measuring device advantageously comprises an A / D converter acted upon by the reference signal r (t) and the control clock signal, a reception memory for storing the values supplied by the A / D converter and means for transforming the reference signals r ( t) in the frequency range, to form the spectrum K "(f) a correlation function k" (t) between the signals stored in the receive and in the reference receiver memory e (t) and r (t), for backtransforming the spectrum K "(f) in the time domain and to determine the maximum k" _{max} = k "(t" _{max} ) of the correlation function k "(t). The same A. can be used for the received signal e (t) and the reference signal r (t) / D converter and the same receive memory can be used by alternately feeding the signals to the A / D converter via a changeover switch and storing them in different address areas of the receive memory.
Such a device can compensate for the measuring errors caused by the measuring device, for example by providing means for forming the difference Δt _{max} =  t _{max}  t " _{max}  representing the running time to be determined Components for the received signal e (t) and the reference signal r (t) prevent additional systematic errors.
Another preferred measuring device for performing the method according to the invention has instead of the means for processing the transmit and receive signals s (t) and e (t) means for the corresponding processing of the reference and received signals r (t) and e (t), so that a correlation function k '(t) between the reference signal r (t) and the received signal e (t) can be calculated by transformation into the frequency domain and subsequent inverse transformation, the maximum of which k' _{max is located} at a time value t ' _{max} which immediately represents the signal delay adjusted by the measuring errors caused by the measuring device.
The method according to the invention and the measurement according to the invention devices can preferably be in a reflection light use the barrier. With such reflection light barriers light transmitters and light receivers are arranged in one housing net, whereby the light emitted by the light transmitter by a reflecting object reflected in itself and from Light receiver is received. Such a reflection The light barrier always emits a warning or control signal if the received radiation power has a predetermined Pe gel falls below.
For certain applications, especially in the area of Security technology, it is necessary in addition to the informa tion on the level of the received signal power or the Presence of an object in the area to be monitored too yet to determine the location of the reflective object, so For example, to be able to recognize manipulations on the reflector NEN.
The method according to the invention can be used for the latter purpose or a corresponding device can be used, so information about the distance between the measuring device and gain the reflective object. The components In this case, the light transmitter and light receiver fulfill one Double function as it is for the actual light barrier function and on the other hand for distance measurement be used. This also helps to reduce costs at, since the corresponding components in the device only just have to be there.
Can also be used in retroreflective sensors instead of visible light signals, ultrasonic signals or Microwave signals are used.
Further preferred embodiments of the invention The method and the devices according to the invention are in specified in the subclaims.
The invention is illustrated below with the aid of an embodiment game described with reference to the drawings ben; in these show:
Fig. 1 is a block diagram of an apparatus for imple out the method according to the invention,
Fig. 2 shows the real part of the spectrum E (f) of a reception signal e (t),
Fig. 3 shows the real part of the spectrum C (f) of a erfindungsge Mäss interpolated correlation function k (t),
Fig. 4 shows the overall view of the present invention INTERPO profiled correlation function k (t),
Fig. 5 support points of a calculated in the time domain Cor relations functions,
Fig. 6, the approximated by a polynomial correlation function after Fig. 5,
Fig. 7 shows a detailed illustration of the interpolated correla tion function of Fig. 4,
Fig. 8 shows the correlation functions of FIGS. 5 and 7 as well as two quadratic polynomials for approximation of these correlation functions,
Fig. 9 is a detailed representation of Fig. 8 and
Fig. 10 is a block diagram of another device constructed according to the invention.
The block diagram shown in Fig. 1 shows a light transmitter 1 , which sends light 2 over a measuring path to a reflector 3 , which in turn reflects the light back over the measuring path to a light receiver 4 , light transmitter 1 and light receiver 4 preferably in a common housing se are housed.
The light transmitter 1 is acted upon by a D / A converter 5 , which in turn is stored in a transmit memory 6 stored digital values.
The output signal of the light receiver 4 is fed to an A / D converter 7 , which generates corresponding digital values and stores them in a reception memory 8 . The D / A converter 5 and the A / D converter 7 are operated synchronously and acted upon with a common clock signal which is generated by a clock generator 9 .
The digital signals stored in the transmit memory 6 and in the receive memory 8 are called up by a processor unit 10 , which calculates the transit time of the light over the measuring path from these signals and makes them available via an output 11 for further processing.
The principle of the method according to the invention and the device designed according to the invention is described in more detail below:
The pulse stored in the transmit memory 6 as a digital signal with pulses and pulse pauses of different widths is converted by the D / A converter 5 into an analog signal, which opens the light transmitter 1 , which is preferably designed as a laser diode. The sequence of Lichtim pulses generated in this way is emitted over the measuring path in the direction of the reflector 3 , which reflects the light pulse sequence in the opposite direction over the measuring path to the light receiver 4 .
The light receiver 4 generates a corresponding electrical signal, amplifies this if necessary by means of an additional amplifier component and feeds it to the A / D converter 7 , which converts the received signal into a sequence of digital values, which in turn is then stored in the receive memory 8 .
With undisturbed reflection and disregard for any interference signals that may occur, signals of the same form are present after a transmission / reception cycle of a sequence of light pulses in the transmission memory 6 and in the reception memory 8 , which signals are shifted in time due to the light propagation time over the measuring path and due to internal signal propagation times.
Both the digital signals stored in the transmitter memory 6 and in the receiver memory 8 have a defined length of p pulses of n bits each.
The processor unit 10 calculates the Fourier transform S (f) from s (t) and E (f) from e (t). For computing time minimization, this can be done, for example, using known FFT routines.
Fig. 2 shows the real part of the spectrum E (f) generated in this way of a received signal e (t) with 256 samples, each encoded with 8 bits.
To obtain an interpolation of the correlation function k (t) ten, which has no systematic error, is in one next step of S (f) that conjugates complex Spectrum S * (f) formed and in a further process step the spectrum E (f) with the conjugate complex spec multiplied by S * (f). This way the spectrum A (f) = E (f) × S * (f) obtained.
The spectrum A (f), like the two output spectra S (f) and E (f), is symmetrical with respect to p / 2 and is divided into the two subspectra A _{1} (f) and A _{2} (f) in a next process step , These result from this
Then, by inserting q "complex" zeros (0 + j0) in the range between p / 2 and p / 2 + 1, the modified spectrum K (f) is formed according to the following equation, the real part of which is shown in FIG. 3:
The number q of zeros is calculated according to the equation
q = (i × p)  p,
where i represents the integer interpolation factor. According to the equation
where t _{res is} the runtime increment, f _{s is} the sampling frequency and n is the resolution of the entire system in bits, the interpolation factor can be sensibly determined up to i = 2 ^{n} . When the signals are encoded with 8 bits and a clock frequency of 30 MHz, this results in a possible resolution of 130.21 ps.
The spectrum K (f) is then, for example, by a inverse fast Fourier transform in the time domain backtransformed, creating the desired correlation radio tion k (t) with interpolated nodes is obtained.
The correlation function k (t) is shown in FIG. 4 for an interpolation factor i = 10 and a signal length of 256 samples. The time signal obtained is temporally stretched over the interpolation factor i, so that 256 × 10 = 2560 support points result.
Fig. 5 shows the support points of a correlation function kt (t) generated in the time domain by calculating a respective correlation value for each sample. It can be seen that the maximum of this function can only be determined at intervals of 1 / sampling frequency, ie in the example corresponding to 33.33 ns. If one determines the maximum of this correlation function calculated in the time domain, for example by interpolation with a polynomial of the second degree, then the polynomial 12 shown in FIG. 6 is held to determine the maximum kt _{max} (t _{max} ). As can be seen from FIG. 6, the structures of the correlation function kt (t) and the determined polynomial 12 differ significantly, so that a not inconsiderable, systematic error occurs in the determination of the maximum and thus in the determination of the signal transit time ,
FIG. 7 shows a detailed representation of the interpolated correlation function ascertained with the method according to the invention, as shown in FIG. 4, which includes the searched maximum of the correlation function k (t). In order to clarify the improvement compared to the determination of the correlation function in the time range, the same standards were chosen as in FIG. 5. Since the conversion from the time scale shown in FIG. 4 (support points) into ns takes place in accordance with the selected clock frequency of the A / D and D / A converters used. With the clock frequency of 30 MHz selected in the example (corresponding to 33.33 ns), the distance between the nodes is 3.333 ns. The maximum of the correlation function k (t) according to FIG. 4 is thus approximately 1301 × 3.333 ns = 4.336 μs based on the first sample value (= 0 ns). Since in practice only a range of approximately 1 µs is of interest for distances up to approx. 150 m, there are ± 150 interpolated sampling points for this range around the maximum for the selected example with i = 10, as shown in FIG. 7 is.
While the small points represent the support points for the correlation function k (t) according to the invention Representing procedures characterize those shown in bold Points the points of reference when calculating the correlation function in the time domain. For this ver it is immediately apparent that due to the higher resolution solution, the interpolated radio determined according to the invention values free of systematic errors regarding the location of the maximum of the correlation function k (t).
Thus, by determining the maximum of the invention determined correlation function k (t) with great accuracy the signal transit time can be determined.
The diagrams shown in FIGS . 5 to 7 show a correct scaling of the time intervals, the indicated absolute values being chosen arbitrarily and for better illustration starting with 0 ns corresponding to a maximum of approx. 155 ns. The zero point is calibrated in the real device for absolute measurements or calculated with known reference distance and negligible influence of the electronic signal transit times. In relatively measuring devices, only the difference between the current measured value and the previous measured value or that saved during commissioning is of interest. In this case, calibration is not necessary because the corresponding zero offset is automatically compensated.
In order to obtain the maximum resolution of 130.21 ps, the interpolation factor i must be selected to the maximum value 2 ^{n} , ie i = 256 in the present example. In this case, when the spectrum K (f) is transformed back into the time domain, an inverse FFT over 256 × 256 = 65536 points is required, which requires a great deal of computing time or fast and expensive special processors.
To avoid the use of such expensive special processors, the maximum determination of the correlation function k (t) can be carried out using the following approximation method:
First, according to the described method according to the invention, but with a reduced interpolation factor of, for example, i = 10, the interpolated correlation function k (t) or. their bases determined. Due to the reduced interpolation factor, only an inverse FFT over 256 × 10 = 2560 points is required, which can be carried out in a relatively short computing time.
Then the maximum function value of the determined Support points of the interpolated correlation function with tt and by this and by at least one neighboring barten function value a quadratic polynomial after the Me least squares method. The maximum of this qua dratic polynomial is determined and as the maximum of the corrections lation function used.
In FIG. 8, both the quadratic polynomial 12 of Fig. 6 is approximated as a polynomial 13, te to the present invention calculate interpolated correlation function, shown by way up. It can be seen at first glance that the maximum correlation function k (t) is approximated better by the polynomial 13 than by the polynomial 12 , so that the signal transit time obtainable via the maximum determination contains a smaller systematic error. The temporally different position of the maxima of the two polynomials 12 , 13 can be seen better in the data representation according to FIG. 9. The maxima are shifted from each other by 1.43 ns, which corresponds to a systematic error of approx. 21.45 cm as a change in distance.
FIG. 10 shows a measuring device similar to the measuring device according to FIG. 1, in which the light 2 emitted by the light transmitter is emitted not only via the measuring section to the light receiver 4 , but also via a reference section to a reference receiver 14 . The reference receiver 14 is located, for example, within the housing of the measuring device according to the invention, so that the signal transit time over the reference path is either negligibly small or sufficiently well known.
The outputs of the receiver 4 and the reference receiver 14 are connected to the A / D converter 7 via a changeover switch 15 which can be switched between a "Ref" and a "Measuring" position.
The reference receiver 14 generates an electrical signal corresponding to the received signal r (t), which is fed via the order switch 15 to the A / D converter 7 when the changeover switch 15 is in the "Ref" position. In the "measuring" position, however, the output signal generated by the receiver 4 is fed to the A / D converter 7 .
The switch is set to the "measurement" position for the duration of the measurement acquisition. The signal 2 emitted by the transmitter 1 and reflected by the reflector 3 is received by the receiver 4 and supplied to the A / D converter 7 via the switch 15 . The digital output signal generated by the A / D converter 7 is then stored in an address space of the reception memory 8 assigned to the reception signal.
Then the switch 15 is brought into the "Ref" position so that the signal emitted by the transmitter 1 , the reference path passing signal from the reference receiver 14 via the switch 15 to the A / D converter 7 is passed. The digital reference signal generated by the A / D converter 7 is then stored in an address space of the reception memory 8 assigned to the reference signal and different from the address space of the reception signal.
This process can be repeated one or more times. Successive measuring and refing are preferred stored in the reception memory until its memory capacity is exhausted. By an alternating measurement and reference signal acquisition can briefly occurring, for example caused by thermal drift processes Disturbances are compensated. By averaging over the respectively measured and reference signal values and on final calculation of the correlation function using the averaged values can compare the required computing time to calculate the correlation function for each te pair of values can be significantly reduced.
The following two methods are possible according to the invention to compensate for the delay components caused by the electronic components:
In a first embodiment, the correlation function k (t) between the transmit and receive signals s (t) and e (t) is determined, the maximum of k (t) is calculated and the associated time value t _{max} determined. Then the correlation function k "(t) between the transmission signal s (t) and the reference signal r (t) is determined analogously to the determination of the correlation function k (t) according to the method according to the invention. The maximum of k" (t) is also determined determined by one of the methods described and the associated running time t " _{max} , which represents the sum of the internal running times, calculated.
This reference transit time t " _{max} is then subtracted from the transit time t _{max} determined during the actual measurement process, as a result of which the signal transit time obtained with respect to the measuring section is freed from the internal signal transit times of the measuring device.
The determination of the reference term can either be done in one separate process, for example when switching on the device tes, or at periodic intervals, for example each parallel to the actual measuring process via the measuring distance.
If the signal transit time over the reference path does not have zero, but a certain known value, even without internal signal transit times, this reference path signal transit time can be taken into account by subtracting it after determining the transit time t " _{max} . The result of this subtraction represents thus exclusively the signal time due to the electronic components of the measuring device.
Since in the described method both for determining the correlation function k (t) between the transmitted signal s (t) and the received signal e (t) and for determining the correlation function k "(t) between the transmitted signal s (t) and the Re If the reference signal r (t) requires an inverse Fourier transformation, which requires considerable computing time, especially when using a large interpolation factor, the following embodiment of the inventive method can save computing time:
Instead of the correlation functions k (t) between the transmission signal s (t) and the reception signal e (t) and the correlation function k "(t) between the transmission signal s (t) and the reference signal r (t), only the correlation function k '(t) between the received signal e (t) and the reference signal r (t) ge forms.
To do this, the Fourier transformed E (f) and R (f) of the received signal e (t) and of the reference signal r (t) is formed.
In a next step, the conjugated kom plex spectrum R * (f) calculated and multiply this with E (f) adorned, so that the spectrum A '(f) = E (f) × R * (f) results. Alternatively, the conjugate complex spectrum E * (f) calculated and multiplied by R (f) so that the spectrum A '(f) results in A' (f) = E * (f) × R (f).
A '(f) is treated analogously to A (f), ie at p / 2 it is split into the subspectrums A _{1} (f) and A _{2} (f) and then filled symmetrically with q zeros, so that K' (f) according to following equation is generated:
K '(f) is transformed back into the time domain, for example, via an inverse FFT, so that the correlation function k' (t) is generated. Then the maximum of the correlation function k '(t) is determined by one of the above methods, ie determined either by direct calculation or by curve fitting and the time value t' _{max} belonging to this maximum is calculated.
In the event that the reference signal r (t) is shifted relative to the Sen designal s (t) solely on the basis of internal signal propagation times, that is to say that the signal transit time over the reference path is zero, the time value t ' _{max found} immediately provides the desired value Signal transit time, since the portions of the signal transit time contained in the received signal e (t), which are caused by the electronic components, are automatically eliminated by the formation of the correlation function.
If the signal transit time due to the finite length of the reference path has a known value that differs from zero, this must be added to the signal transit time t ' _{max} calculated via the correlation function k' (t).
In principle, it is also possible not to use the reference branch from additional electronic components, but from egg nem switched optical path with optical delay elements elements, for example light fibers. Furthermore, it is possible, the measurement signal optically, for. B. by means of optical fiber as far as to delay within the device that a separation of Measurement and reference signal at the time level. this has the advantage that the electronic components used are identical for the measuring and the reference branch, so that the systematic error caused by the different run times in different electronic components is caused, can be completely eliminated.
By determining the correlation function immediately between the received signal and the reference signal Time required to determine the signal transit time, in particular in that only a single inverse transformation in time area is required to be significantly reduced. Which he Interpolation of the correlation function according to the invention allowed the use of the correlation method in absolute mes send devices with high accuracy requirements. Farther is the method according to the invention in ultrasonic measuring technology Use nik for distance or speed measuring devices bar.
Since only frequencies that are used in the Vi Deotechnik are common, the Inter according to the invention polation of the correlation function is also particularly inexpensive realizable. Another advantage is that the Zeitli neal can be stacked regardless of amplitude and a very high S / N ratio is achievable.
The correlation function k '(t) between the received signal e (t) and the reference signal r (t) can also be determined directly in the time range between the signals stored in the receive and reference address space of the memory 8 the products of opposite reference point values of the two signals are calculated and stored, whereupon one of the stored signals, for example the signal stored in the reference address space of the memory 8, is shifted by a sampling interval or a function value and the said sum is in turn calculated and stored. This process is repeated until all values of the correlation function calculated in this way are available. The number of shifting, product and sum formation steps depends on the frequency of the generator 9 Taktge and the length of the measuring section.
For the correlation function calculated in this way determined the maximum in the manner described and from it the signal delay to be determined is calculated. Here are all Process steps described that are not obvious Lich the determination of the correlation function in the frequency domain assume rich in determining the correlation radio tion applicable in the time domain.
Claims (35)
a transmitter ( 1 ) contained in the measuring device sends a modulated signal s (t) over the measuring section, which is received by a receiver ( 4 ) contained in the measuring device, converted into a received signal e (t) and evaluated in the measuring device,
a correlation function k (t) is formed between the transmit and receive signals s (t) and e (t) by transforming the transmit and receive signals s (t) and e (t) into the frequency range, and the spectrum in the frequency range K (f) of the correlation function k (t) is calculated from the spectra S (f) and E (f) of the transmit and receive signals s (t) and e (t) and then the correlation function k ( t) is determined
the maximum k _{max} = k (t _{max} ) of the correlation function k (t) is determined,
t _{max is} evaluated to determine the signal transit time,
to form the spectrum K (f) of the correlation function k (t) the product A (f) from the spectrum E (f) and the conjugate complex spectrum S * (f) or from the spectrum S (f) and the conjugate complex spectrum E * (f) is formed and
the spectrum A (f) is split symmetrically with respect to the frequency f into the subspectra A _{1} (f) and A _{2} (f) and between the subspectra A _{1} (f) and A _{2} (f) q = (i × p)  p amplitude values w _{j} (1 ≦ j ≦ q) are inserted, where i represents the integer interpolation factor.
is formed.
that w _{j} = 0 and the spectrum K (f) of the correlation function k (t) according to the equation
is formed.
that the reference signal r (t) received over the reference path is used to determine the signal transit time instead of the signal s (t), in particular that the spectrum R (f) is calculated from the reference signals r (t) stored in the reference reception memory ( 16 ), the product A '(f) = E (f) × R * (f) or A' (f) = E * (f) × R (f) determines the spectrum K '(f) according to the equation
formed and the correlation function k '(t) is determined by backtransformation in the time domain.
that in order to determine the signal transit time in addition to processing the reception signal e (t), the reference signal r (t) received via the reference path is processed accordingly, in particular that the spectrum R (f) from the reference signals stored in the reference reception memory ( 16 ) r (t ) calculated the product
A "(f) = S (f) × R * (f) or A" (f) = S * (f) × R (f) determines the spectrum K "(f) according to the equation
is formed and the correlation function k "(t) is determined by backtransformation into the time domain.
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