EP1733193A1

EP1733193A1 - Zero crossing detection of an ultrasound signal with a variable threshold

Info

Publication number: EP1733193A1
Application number: EP05701608A
Authority: EP
Inventors: Tobias Lang
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 2004-03-25
Filing date: 2005-01-26
Publication date: 2006-12-20
Also published as: KR20060134114A; WO2005093379A1; DE102004014674A1; US20080190213A1; JP2007530933A; US7543508B2

Abstract

The invention relates to an ultrasound flow sensor provided with at least one ultrasound converter (A,B) for transmitting and receiving ultrasound signals (A0,B0) and a receiver (4) which is connected to the ultrasound converter (A,B) and which determines a zero crossing (N) of the ultrasound signal (A0,B0) as a reception moment (to) after the ultrasound signal (A0,B0) exceeds a given threshold (SW). Measuring accuracy can be improved substantially if the receiver (E) determines information on the amplitude (Amp) of the ultrasound signal (A0,B0) and varies the threshold (SW) according to the information thus determined.

Description

description

Zero crossing detection of an ultrasound signal with a variable threshold

The invention relates to an ultrasonic flow sensor according to the preamble of patent claim 1 and a method for detecting the time of reception of an ultrasonic signal according to the preamble of patent claim 7 measure that flows through a pipeline. A known type of ultrasonic flow sensors comprises two offset in the flow direction. arranged ultrasound transducers, which each generate ultrasound signals and send them out to the other ultrasound transducer. The ultrasonic signals are received by the other transducers and evaluated using electronics. The transit time difference between the signal in the flow direction and the signal in the opposite direction is a measure of the flow velocity of the fluid. The desired measured variable, such as a volume or mass flow, can be calculated from this. Fig. 1 shows a typical arrangement of an ultrasonic flow sensor with two ultrasonic transducers A, B, which are arranged within a pipe 3 and face each other at a distance L. A fluid 1 flows in the pipeline 3 at a speed v in the direction of the arrow 2. The measuring section L is inclined by an angle α with respect to the flow direction 2. During one Measurement, the ultrasonic transducers A, B send each other ultrasonic signals that are either slowed or accelerated by the flow, depending on the direction. The transit times of the sound signals are a measure of the flow velocity to be determined.

Fig. 2 shows a highly simplified schematic representation of a transducer arrangement with an associated control and evaluation electronics 4. The flow sensor can e.g. work according to the so-called "sing-around" process. The reception of an ultrasonic signal AO or B0 at one of the transducers A, B triggers an ultrasonic signal in the opposite direction.

For the transit time measurement of an ultrasonic signal AO or B0, it is essential that the time of reception of an ultrasonic signal A0, B0 can be clearly and precisely determined. A method known from the prior art for determining a reception time is explained below with reference to FIG. 3.

3 shows the signal curve of an individual ultrasonic signal A0, B0. The "reception time" of the signal A0, B0 is defined here as the first zero crossing N _{0 of} the signal after the signal amplitude Amp has exceeded a predetermined threshold value SW (the so-called pretrigger level). In the example shown, the time t _{o would be} the reception time of the signal. (Alternatively, the reception time of the signal could also be determined by evaluating the phase of the signal.)

Contamination, drifting or aging of the ultrasonic transducers, or turbulence in the flowing fluid can lead to the amplitude of the ultrasonic signals A0, B0 varying greatly. As long as the signal amplitude does not change too much, the zero crossing detection is hardly impaired, since the same zero crossing always as Reception time is detected and the frequency of the

Signal remains essentially the same. However, as soon as the maximum signal amplitude of the half-wave comes into the range of the threshold value SW before the time to, incorrect measurements of the reception time can occur if the ultrasound signal exceeds the threshold value e.g. to a later one

Time exceeds and thus an incorrect zero crossing is detected as the reception time.

It is therefore the object of the present invention to improve the measuring accuracy of an ultrasonic flow sensor which determines the time of reception of an ultrasonic signal by means of zero-crossing detection.

This object is achieved according to the invention by the features specified in claim 1 and in claim 7. Further embodiments of the invention are the subject of dependent claims.

An essential idea of the invention is to determine information about the amplitude of the ultrasound signal and to adapt the threshold value (pretrigger level) to the amplitude of the ultrasound signal. This means that when the signal amplitude changes, the correct, i.e. the same zero crossing or the correct event is detected as the reception time.

Information about the signal amplitude can be determined in different ways: A first possibility is to measure a signal maximum, preferably the maximum amplitude of the ultrasound signal by means of an appropriate device. Another possibility is to rectify the ultrasound signal and to determine an average. This mean value is also a measure of the signal amplitude and can therefore be used as a reference variable for adapting the threshold value. In addition, many other signal evaluation methods are conceivable from which information about the signal amplitude can be obtained.

According to a preferred embodiment of the invention, the receiving unit of the ultrasonic flow sensor comprises a device for measuring the maximum amplitude of the

Ultrasonic signal. The threshold value can thus be adapted to the current maximum signal amplitude. This greatly reduces incorrect measurements.

A preferred embodiment of the amplitude measuring device comprises a first S / H stage (sample and hold element), at the input of which the ultrasound signal or a corresponding transducer output signal is present and which stores the maximum value of the signal amplitude, and a downstream second S / H level, which accepts and stores the maximum value of the first S / H level. A desired threshold value (pretrigger level) can finally be generated from the maximum amplitude value determined in this way.

For this purpose, the output signal of the second S / H stage is divided by a voltage divider and the partial voltage (= threshold value) is fed to a comparator. The comparator preferably switches its output when the converter output signal exceeds the threshold. The zero crossing detection can then be carried out.

In order to prevent the threshold value from fluctuating too much, a low-pass filter is preferably provided with which the amplitude information or the threshold value information (i.e. the corresponding signal) is filtered.

Another embodiment of the receiving unit comprises a rectifier, with which the converter output signal is rectified. The rectified signal can e.g. integrated by means of an integrator or by means of a

Low pass filter. The integrator output signal and the filter output signal in turn provide information about the signal amplitude of the ultrasound signal and thus allow the threshold value to be adjusted.

A further embodiment of the receiving unit comprises a differentiator, with which the transducer output signal is differentiated, and a downstream unit for zero-crossing detection, with which the times of the maxima of the ultrasound signal are recorded. The maxima can e.g. stored in an S / H level and from this the maximum with the highest value can be determined.

Another embodiment of the receiving unit comprises two lock-in amplifiers in which the converter output signal is amplified on the basis of two reference clock signals, the reference clock signals having the frequency of the ultrasonic signals and being mutually phase-shifted, for example by pi / 2. If the two amplifier output signals generated in this way are integrated or filtered with a low-pass filter, a value can be determined from the resulting signals un and u _P i ₂ by quadratic averaging + u ² , _{/ 2} , which represents a measure of the signal amplitude and one Threshold adjustment allowed.

The invention is explained in more detail below by way of example with reference to the accompanying drawings. Show it:

1 shows an ultrasonic flow sensor known from the prior art with two ultrasonic transducers;

2 shows an ultrasonic flow sensor with associated control and reception circuit;

3 shows the signal curve of an individual ultrasound signal;

4 shows a receiving unit for adapting the threshold value according to a first embodiment of the invention; FIG. 5 shows a signal flow diagram of the signals from FIG. 4; and

6a-6d different embodiments of receiving units for adapting the threshold value.

With regard to the explanation of FIGS. 1-3, reference is made to the introduction to the description.

FIG. 4 shows part of a receiving unit 4 with which the threshold value SW can be adapted to the signal amplitude of the ultrasonic signals AO or BO. In this embodiment, the receiving unit 4 measures the maximum amplitude Ampm _a x of the ultrasound signal A0, B0.

The receiving unit 4 comprises a first comparator 10, at the input US of which the converter output signal 5 is present, and the other input of which is connected to the output 19 of a first S / H stage 12. The comparator 10 changes its initial state when the converter output signal 5 becomes greater than the amplitude value previously stored in the S / H stage 1.2. This activates the first S / H stage 12, which then takes over and stores the current amplitude value. 3 shows the course of the output signal 19 of the first S / H stage 12.

When the ultrasonic signal A0 or B0 has decayed, a second S / H stage 13 takes on the maximum amplitude value Amp _{max of} the first S / H stage 12. For this purpose, the second S / H stage 13 is controlled by a control signal “stop” The output signal 20 of the second S / H stage 13 is supplied to a voltage divider 14 with an adjustable divider factor 14. The voltage divider 14 is implemented here as a trimming potentiometer, the partial voltage U _t forming the new threshold value SW for the detection of the reception time. The partial voltage U _t is fed to a second comparator 16 as a reference voltage. The other input of the comparator 16 is connected to the input US of the receiving unit 4. The output signal of the second comparator 16 thus changes the switching state when the ultrasonic signal AO, BO exceeds or falls below the threshold value SW. The switching state is stored in a monoflop 18.

FIG. 5 shows the signal curve of some signals of the circuit of FIG. 4. The signal curve is shown hatched where the corresponding signal either depends on previous events or the signal state is undefined.

The measuring method begins with the generation of a start signal "start", which is fed to the first S / H stage 12 via an OR gate 11 in order to activate it. Upon reception of an ultrasonic signal A0, B0 at the input US stores the first S / H stage 12 (signal s / hl) the maximum signal amplitude Amp _m ax, as was described above. With the arrival of a stop signal "stop" assumes the second S / H stage _13: the value of the first S / H stage 12 (signal s / h2).

With the next start signal, the first S / H stage 12 is reset and a new measurement can begin.

6a shows another embodiment of a receiving unit 4, in which the converter output signal is first fed to a rectifier 21. The output signal of the rectifier 21 is finally integrated by means of an integrator 22, with the signal being averaged. This value is in turn a measure of the maximum amplitude of the ultrasonic signal A0, B0. The integrator output signal is then sampled and stored by the second S / H stage 13. The rest of the circuit for generating the partial voltage U _t can be implemented identically as in FIG. 4. 6b shows a further embodiment of a receiving unit 4 with a rectifier 21 and a downstream low-pass filter 15. The rectified and filtered converter output signal 5 can in turn be evaluated, for example, by means of the circuit of FIG. 4.

6c shows a further different embodiment of a receiving unit 4 with a differentiator 23 and a unit 24 for zero-crossing detection. With the help of the differentiator 23 and the unit 24, the times of the signal maxima of the ultrasonic signal A0, B0 are determined and a downstream S / H stage 25 is activated, which takes over the maximum signal values. In order that lower amplitude values are not accepted again when the ultrasonic signal A0, B0 decays, the number of

S / H stage 25 scans e.g. be limited by a counter or a monoflop.

6d shows yet another embodiment of a receiving unit 4 with two lock-in amplifiers 40, 42 or

41.43. The lock-in amplifiers can be constructed, for example, as multipliers 40, 41 with subsequent integrators 42, 43. In this case, the ultrasonic signal A0, B0 is inverted in a phase-controlled manner by means of the reference clocks Ref ₀ and Ref _{pi / 2} , which have exactly the ultrasonic frequency. Another possibility is, for example, that an inverting and a non-inverting amplifier is used instead of the two multipliers, the amplification factors of which are essentially the same except for the sign. In this case a

Signal multiplexer used to switch between inverting and non-inverting operation according to the reference clocks. The reference clocks are phase-shifted by pi / 2. The resulting signals are then integrated and thus the amplifier output signals un and

Upi _{/ 2} generated. It is an alternative to the integration of the signals For example, low-pass filtering is also conceivable. The signals u _o and u _P i _{/ 2} are added squarely via a digital or analog arithmetic circuit 44 and sampled with an S / H stage 13. The output signal out of the circuit of FIG. 6d is in turn a measure of the amplitude of the ultrasound signal and allows the threshold value SW (U _t ) to be adjusted. The rest

Circuit for generating the partial voltage U _t can be implemented identically as in FIG. 4.

The dynamic behavior of the receiving unit 4 can also be influenced by suitable filters. A low-pass filter, for example, prevents the threshold value SW from being adjusted too quickly. Such a low pass filter could e.g. be realized as an RC element, which is connected between the second S / H stage 13 and the trimming potentiometer 14. Alternatively, the low-pass filter 15 could also be between the

Trimming potentiometer and the second comparator 16 can be switched.

LIST OF REFERENCE NUMBERS

1 fluid

2 flow direction

3 pipeline

4 control and receiving unit

5 converter output signal

10 first comparator

11 OR gate

12 First S / H level

13 Second S / H level

14 voltage divider

15 low pass filters

16 Second comparator

17 AND gate

18 monoflop

19 Output of the first S / H stage 12

20 Output of the second S / H stage 13

21 rectifiers

22 integrator

23 differentiators

24 unit for zero crossing detection

25 S / H level

40.41 multiplier

42.43 integrator

44 arithmetic unit

Refo, Ref _{pi /} 2 reference clock signals to receive time

SW threshold

A0, B0 ultrasonic signals

A, B ultrasonic transducers

L measuring distance u _t partial voltage

Claims

claims

1. Ultrasonic flow sensor with at least one ultrasonic transducer (A, B) for transmitting and receiving ultrasonic signals (A0, B0) and a receiving unit (4) connected to the ultrasonic transducer (A, B), which monitors when the ultrasonic signal (A0, B0 ) exceeds a predetermined threshold value (SW) and determines a reception time (to) of the ultrasound signal (A0, B0) as a function of this event, characterized in that the reception unit (4) has a

Information about the amplitude (Amp) of the ultrasound signal

(A0, B0) is determined and the threshold value (SW) is set as a function of the determined information.

2. Ultrasonic flow sensor according to claim 1, characterized in that the receiving unit (4) has a first S / H stage (12), at whose input (US) a transducer output signal (5) is present, and a downstream second S / H level (13), which takes over and stores the maximum value (Amp _max ) of the first S / H level (12).

3. Ultrasonic flow sensor according to claim 2, characterized in that a voltage divider (14), which divides the output signal (20) of the second S / H stage (13), and a comparator (16) are provided, to which the partial voltage of the Voltage divider (14) is supplied.

4. Ultrasonic flow sensor according to one of the preceding claims, characterized in that a low-pass filter (15) is provided with which the information on the Signal amplitude (Amp _max ) or information derived therefrom (U) is filtered.

5. Ultrasonic flow sensor according to one of the preceding claims, characterized in that the receiving unit (4) has a rectifier (21) with which the converter output signal (5) is rectified.

6. Ultrasonic flow sensor according to one of the preceding claims, characterized in that the receiving unit (4) has a differentiator (23) to which the transducer output signal (5) is fed, and a downstream unit (24) for zero-crossing detection.

7. Method for detecting the time of reception (to) of a received on an ultrasonic transducer (A, B)

Ultrasonic signal (A0, B0) by means of a receiving unit (4) which monitors when the ultrasonic signal (A0, B0) exceeds a predetermined threshold value (SW) and determines a reception time (to) of the ultrasonic signal (A0, B0) depending on this event ,. characterized in that the receiving unit (4) determines information about an amplitude (Amp) of the ultrasonic signal (A0, B0) and the threshold value (SW) is set as a function of the determined information (Amp).

8. The method according to claim 7, characterized in that the maximum amplitude value (Amp _max) of the ultrasonic signal (A0, B0) H-stage (12) is in a first S / stored, and the maximum value (Amp _max) of the first S / H level (12) is sampled and stored by a second S / H level (13).

9. The method according to claim 7, characterized in that the amplitude information (Amp, out) is obtained from the output signal (u ₀ , U _p nv?) Of two lock-in amplifiers (41, 42; 41, 43).