WO1990002099A1

WO1990002099A1 - Process and device for measuring a flow rate

Info

Publication number: WO1990002099A1
Application number: PCT/EP1989/000913
Authority: WO
Inventors: Kurt Jansen
Original assignee: Battelle-Institut E.V.
Priority date: 1988-08-17
Filing date: 1989-08-16
Publication date: 1990-03-08
Also published as: DE3827913C2; DE3827913A1

Abstract

In a process and device for measuring a flow rate, the beam preferably emitted by a semiconductor laser (4) is split in the visible or infra-red range by means of a collimating lens system (5) and a twin hole diaphragm (6) connected downstream of the latter into two partial beams (1, 2) which are guided through the flow containing scattering centres. The total intensity of the partial beams or the reflected parts thereof passing through the flow is measured preferably by a photoelectric cell (7) and then amplified (8, 9). The autocorrelation function is derived from the measurement signal and processed (10, 11) in order to determine the transit time of the scattering centres through the partial beams. The process and device also yield satisfactory results in the case of high scattering particle densities and unknown flow profiles. Hardware costs are minimal and only a fairly small amount of data need to be evaluated, because only one receiving unit is required and only one measurement signal has to be evaluated.

Description

Method and device for determining the speed of a flow

The invention relates to a method for determining the speed of a flow and a device for performing the method and is based on one

Optical method with the features in the preamble of claim 1 or a device with the features in the preamble of claim 7.

Optical measuring methods allow the determination of the propagation velocities and flow properties of flows, for example gas or liquid flows, in which particles or other scattering centers move at the same speed as the flow, through a contact-free access to the measuring medium.

A known optical method is LaserDoppler anemometry, in which an interference pattern is generated by the superposition of two laser beams at the measurement location. If a particle flies through this interference pattern, a frequency component can be detected in the scattered light spectrum of the particle, the position of which in the spectrum is proportional to the flow velocity. This method determines speeds using frequency measurement due to its small measurement volume with high spatial resolution. However, if knowledge of the mean flow velocity with an unknown flow profile is desired, this method cannot be used with only selective spatial resolution.

The light zone anemometer, which is also known, determines average velocities of a flow cross section through the parallel projection of a grating into the flow. Particles that fly through the stripe pattern generate scattered light with a preferred frequency component, the frequency of which in turn depends on the speed of propagation of the particles and the lattice constant. However, scattering particle concentrations that fluctuate slightly over time and large scattering particles can cause disturbances. Even with a large scattering particle density, the signal-to-noise ratio becomes ever lower, since many scattering particles simultaneously emit signals with a generally statistically distributed phase position.

It has also been proposed to use correlation methods for determining the speed as a statistical evaluation method. For example, about the

Forming the cross-correlation function of two signals from measuring sensors arranged spatially separated in the flow direction, the transit time of flow inhomogeneities can be determined. The speed to be determined results from this running time as the average speed over the known probe spacing.

The cross-correlation function can also be formed by signals from two measuring sensors onto which scattered radiation originating from two small measuring volumes is projected. To form the measurement volumes, e.g. two laser beams focused. In order to split a laser beam into two non-parallel beams and to focus them in the two measuring volumes and for the exact projection of the scattered light from the volumes onto the measuring sensors, however, complex arrangements and adjustments of optical components are required.

Regardless of the manner in which the measurement signals are obtained for the cross-correlation analysis, two detectors are always necessary. In addition, for the Signal evaluation of the two detector signals requires a relatively large amount of data, so that in some cases an off-line evaluation via intermediate storage is not possible due to the finite storage space.

The invention is based on the object, based on the features in the preamble of patent claims 1 and 7, of specifying a new method and a new device for determining the speed of a flow with the least possible technical outlay and requirements for the evaluation unit.

This object is solved by the subject matter of claims 1 and 7, respectively.

In the method according to the invention, which forms the runtime determination with the aid of the autocorrelation function of a single signal, the need for two detectors is avoided. A simple split of the

Radiation in two parallel beams and subsequent

Detection with a sensor in order to obtain the required information of the scattering center runtime by forming the autocorrelation function. For this purpose, only a collimator and a double-hole diaphragm are to be provided in the device according to the invention. The sum signal of an optoelectronic receiver unit originating from both partial beams serves as the evaluation signal.

In particular in the transmission measurement method, a single photo element operated in the short circuit, the surface of which detects both partial beams, supplies a measurement signal of sufficient amplitude without the need for additional amplification.

Since only one measurement signal is to be recorded and evaluated, the amount of data is reduced by half compared to the application of the cross-correlation method, which means that the conditions for off-line processing are improved, ie the memory requirement is reduced.

Both the small number of optical components in the

The beam path as well as the reduced receiving unit with only one detector, possibly with a downstream amplifier, and the need for only one analog / digital converter result in low hardware costs.

The dimensions of the device according to the invention are also small as a result.

Compared to the light zone anemometer, large or temporally fluctuating scattering particle concentrations and the occurrence of large scattering particles are irrelevant in the statistical method according to the invention. One of many scattering particles or other types of scattering centers, e.g.

Fluctuations in density, resulting signal is just desired when forming the autocorrelation function. Random temporal correlations of signal components can then be found particularly well through the integral formation of the autocorrelation function.

In contrast to laser Doppler anemometry, the method according to the invention allows the determination of the mean flow velocity in a larger cross-section for flows even with an unknown flow profile.

In addition to the use of, for example, halogen lamps or gas lasers, a semiconductor laser as radiation source, which is small and compact and whose radiation does not have to be expanded in contrast to the gas laser, is recommended for the device or the method according to the invention. In the case of small scattering centers moving sufficiently fast, the autocorrelation function already enables the determination of the transit time from the position of a relative one

Maximum. Otherwise, the runtime results after two differentiations and after inverting the autocorrelation function, after the main maximum has been eliminated at zero time shift, based on the position of the new main maximum that then occurs, even with larger scattering particles, with good accuracy.

In the event of a strongly fluctuating particle density, the use of an automatic gain control (AGC) following the receiver unit is recommended, which means that an optimal measuring range resolution is always automatically obtained even with measurements that take longer.

If multiple scattering occurs and especially when

Measurements with optically transparent particles, e.g. Drops of water, it may be necessary to use another screen with two opening channels immediately in front of the receiving unit. This aperture causes an angle selection of the incident radiation. Only the radiation from the diaphragm that comes from the desired angular range selected for the examination is transmitted. Disturbances of stray light from another angular range are thus prevented.

The invention is explained in more detail below with reference to the drawings. Show it:

Fig. 1 shows an embodiment of a device according to the invention for performing the invention

Process,

2A is a measured with the device of FIG. 1 Signal as a function of time for a single, small particle crossing the laser beams,

2B shows the normalized autocorrelation function of the signal from FIG. 2A plotted against the time shift,

2C shows the processed normalized autocorrelation function of the signal from FIG. 2A,

3A shows a signal measured with the device according to FIG. 1 as a function of time for a single larger scattering particle,

3B shows the normalized autocorrelation function of the signal from FIG. 3A,

3C shows the processed normalized autocorrelation function of the signal from FIG. 3A,

4A shows a signal measured with the device according to FIG. 1 as a function of time for sand grains falling freely through the measurement volume,

4B shows the normalized autocorrelation function of the signal from FIG. 4A,

4C shows the processed normalized autocorrelation function of the signal from FIGS. 4A and

5 shows the time-dependent course of determined speed values for the measuring time range from FIG. 4A. An exemplary embodiment of the device according to the invention is shown in FIG. 1 as a schematic block diagram

shown. A serves as the radiation source

Semiconductor laser (4) with collimator optics (5) and an optical power of 3 mW, the wavelength of the laser radiation being in the optical range or in the near infrared. A wavelength of 870 nm was selected for the measurement results shown below. Not one

shown in the compact semiconductor laser device integrated monitor diode allows using a

electronic control loop (3) the control and

Stabilization of the laser power, so that a

Temperature stabilization of the laser diode is not necessary. A fluctuation in the laser power is otherwise irrelevant for the measuring method according to the invention, so that the control serves only to protect the laser diode. The laser beam leaving the collimator optics has a circular cross section with a diameter of 5 mm and is broken down into two parallel partial beams by the diaphragm (6) shown in FIG. 1. The panel has two square openings, each with an edge length of 1 mm. The centers of the openings are 4 mm apart. This distance is as large as possible

has been chosen. It is characterized by the dimensions of the

Laser beam diameter limited. When using a gas laser, the partial beams are generated

additional optical elements needed. These are unnecessary, for example, with halogen light sources.

The receiving unit of the measuring device has

large-area Si photo element (7) with the dimensions 20 mm x 9 mm. The short circuit current of a photo element is a linear function of the lighting intensity and is also proportional to the irradiated area. From

The total intensity of the two becomes the photo element

Laser partial beams in a proportional to this Short-circuit current implemented, which is converted into a measuring voltage by a preamplifier stage (8).

If scattering centers occur e.g. Scattering particles through the two partial beams (1,2), their running time is longer than

Partial beam spacing etc. contained as information in the measurement signal. The speed of the particles and their direction of flow perpendicular to the direction of propagation of the parallel beams are also indicated in FIG. 1.

A second amplifier stage (9) contains a high pass, a low pass and a notch filter of quality 17 and

reduces the measurement signal spectrum to the frequency range required for the evaluation. The notch filter is tuned to the frequency of 50 Hz, so that

Measurement signal is largely free of interference. The post-amplifier can optionally have a constant

Amplification or operated with an AGC circuit (automatic gain control). The advantage of the AGC circuit lies in the optimal measurement range resolution that automatically arises even with longer measurements under changing measurement conditions. The amplifier output voltages are digitized by an analog / digital converter (10) and transferred to a computer (11) or a microprocessor with display unit via fast data transmission.

The requirements for the computer or microprocessor with regard to the amount of data to be processed and the sampling rate for this are determined by the respectively to be measured

Flow velocities specified. For example, at a flow speed of 2 m / s

Minimum sampling frequency, which is proportional to the maximum occurring speed, must not be less than 2 kHz with the dimensions selected above. However, since only the measurement signal of a single photo element is to be evaluated, the measurement time range is also through in the case of off-line evaluations the amount of data to be processed is not too strong

limited. If the storage capacity is still not sufficient, it can be buffered accordingly or an online evaluation can be carried out. For

Laboratory measurements can usually be done on post-amplification

can be dispensed with, e.g. especially with long transmission lines between preamplifier and computer, as they often occur in production plants,

Any interference that may occur should be eliminated with the help of the post-amplifier.

The one used in the device shown here

Transmission measurement method in which the rays modified by scattering in the measurement volume following the

Measurement volumes are detected, can be evaluated with the photo element already without post-amplification measures

Sum signals. In the event of multiple scattering and in the presence of optically transparent particles in the flow (e.g. water droplets), a further orifice with two passages in front of the

Arranged photo element, which causes an angle selection.

The information contained in the measuring signal of the propagation time of scattering centers by the laser partial beams (1) and (2) is determined by calculating the autocorrelation function with subsequent processing. The total light intensity I detected by the photo element (7) results for the short circuit operation of the photo element to (1)

with i: intensity density

r: location coordinate

t: time

A: Photo element area. For the arrangement shown in FIG. 1, this results in:

I (t) = I ₁ (t) + I ₂ (t) (2) where I _1,2 corresponds to the intensity of the laser partial beam (1) or (2). The autocorrelation function of the intensity I ₁ or the cross-correlation function of the intensities I ₁ and I ₂ are said to be

α

defined, with I ₁ and I ₂ now being understood to mean the signal component free of direct component and T indicating the observation period, a the partial beam spacing and Ʈ a time shift. For the sake of simplicity, the same symbols are retained.

The autocorrelation function C of the total intensity measured by the photoelement can be calculated as follows for any, but frozen, scattered particle distribution which moves through the partial beams. The runtime is denoted by Ʈ = a / v; v denotes the particle velocity. The following applies to this special case:

Equations (2) and (3) give:

1 /

t

Are the correlation functions C ₁₁ (Ʈ) and C ₁₂ (Ʈ ', Ʈ) regarding Ʈ so narrow-banded that their functional courses in one

C- Ʈ diagram does not overlap, so follows for the

C (Ʈ ', Ʈ = 0) normalized autocorrelation function

The basic course of K (Ʈ ', Ʈ) can be read directly from equation (8): K (Ʈ', Ʈ) has an absolute maximum of height 1 at Ʈ = 0, is symmetrical with respect to Ʈ = 0 and has two secondary maxima of height 0.5 at c = Ʈ 'and Ʈ = - Ʈ'. The position of one of these secondary maxima is to be determined so that the relationship £

the flow rate can be specified.

Since in reality there are no infinite measuring times for

Are available and the observer has a temporally resolved flow

Want to determine the speed curve, is a

Short-term correlation function defined. Instead of equation (6) one obtains

Equations (3) and (4) change analogously. The

Correlation functions are no longer just from Time shift Ʈ, but also dependent on the observation time t _c and the observation period 2T.

The results explained below result for the measurements carried out and evaluated on the basis of the above considerations with the aid of the device outlined in FIG.

2A shows the measured time profile of the photo element signal at a point in time at which a

single scattering particle traverses the laser beams. The scattering particle has one in the direction of flow

Dimension <3 mm, since it can be clearly seen in FIG. 2A that the particle entry into the partial beam 2 only takes place after the scattering particle has left the beam 1. The normalized autocorrelation function (AKF) K (Ʈ ', Ʈ) determined by the computer is shown in FIG. 2B. The curve

corresponds approximately to that previously for a special case

estimated AKF course, since the requirements made for the special case discussed are small

Scattering particles or scattering centers with large enough

Propagation speed can be met sufficiently. In order to emphasize the position of the secondary maximum which can already be seen in FIG. 2B, the data of the standardized AKF are prepared by appropriate software. The

standardized AKF is differentiated and inverted twice after the time shift Ʈ. Then the negative parts of interest and those of the

Major maximums removed at zero time shift.

The further determination of the transit time Ʈ 'is thus reduced to a simple maximum determination. 2C shows the data that has been prepared in a special way. The

The function of the scattering particle is with

about 3 ms. According to equation (9), this corresponds to the present a value of a speed of approximately 1.3 m / s. The sequence of FIGS. 3A to 3C shows measurements with a scattering particle whose dimensions are larger than 3 mm. The particle successively shades both partial beams from the photo element so that the total intensity measured drops to zero. As a result, the calculated correlation functions of the partial beam intensities, see equations (3) and (4), overlap in FIG. 3B. Pronounced secondary maxima can no longer be found in the autocorrelation function shown. The same as the data preparation method explained with reference to FIGS. 2A to 2C, however, also clearly provides the runtime of the particle in this case, see FIG. 3C. The fact that the

Runtime is almost identical to that of the previously explained case is random.

In the third measurement shown, conditions such as e.g. occur in the compressed air conveyance of particles, simulated by grains of sand falling freely from a height of about 22 cm traverse the measuring volume over a width of 20 cm. The measurement signal (Fig. 4A) shows large fluctuations due to the large number of particles that the laser beams pass through. For a small time window from the measurement data shown, the standardized AKF and the prepared standardized AKF also show them

Many-particle measurement clearly the characteristic

Time shift (transit time), which is contained in the measurement signal (Fig. 4B + 4C). If you let this time window slide over the entire measuring time, a time-dependent one

Speed curve, as shown in Fig. 5, can be specified. It can be seen from the figure that the speed of the grains of sand at the measuring location fluctuates around the average value of 2.12 m / s with a satisfactory resolution.

All measurements are, as shown in Fig. 1, in each case

Radiographic procedures have been included. Depending on the measurement object and measurement conditions, however, the reflection component can of the scattered light can be used at a suitable angle, as already mentioned above.

In addition to the use of speed measurements of gases and liquids, in flow channels of all kinds and multi-phase and particle flows, the method according to the invention and the device are also suitable for monitoring and controlling transport processes (production lines, assembly lines) in industry.

Claims

1. A method for determining the speed of a flow, in which radiation in the visible or infrared range after passage or reflection through the flow containing scattering centers is measured with an optoelectronic receiving unit and the measurement signals supplied by the receiving unit using a correlation method to obtain the running time of the scattering centers evaluated by the radiation

become,

characterized,

that the radiation is converted into a parallel bundle of rays, which is then broken down into two parallel partial beams before passage or reflection through the flow, that the sum intensity of both partial beams is measured with the optoelectronic receiving unit, and that the autocorrelation function is formed from the measured sum signal.

2. The method according to claim 1,

characterized,

that the autocorrelation function of the measurement signal is differentiated twice according to the time and that after inverting the function and after eliminating the main maximum at zero time shift, the position of the new main maximum which is then present is determined.

3. The method according to any one of the preceding claims, characterized in

that the radiation from a semiconductor laser is used.

4. The method according to any one of the preceding claims, characterized in

that the measured sum signal is preamplified and optionally with simultaneous filtering with fixed gain or automatically controlled gain before digitization.

5. The method according to any one of the preceding claims, characterized in

that the non-parallel portions of the radiation are masked out immediately in front of the receiving unit.

6. The method according to any one of the preceding claims, characterized in that

that the sum intensity is measured with a photo element operated in the short circuit.

7. Device for carrying out the method according to claim 1 with a radiation source emitting radiation in the visible or infrared range, an optoelectronic receiving unit and an evaluation unit forming a correlation function of the measurement signals supplied by the receiving unit,

characterized,

that the device following the radiation source (4) has a collimator lens (5), which is followed by a double-hole diaphragm (6), which splits the radiation passing through into two parallel partial beams that the optoelectronic receiving unit (7, 8, 9) is designed for is to measure the sum intensity of the partial beams after passage or reflection through the flow, and that the evaluation unit (9, 10, 11) forms the autocorrelation function of the measurement signal and determines the running time of the scattering centers in the flow over the partial beam distance.

8. The device according to claim 7,

characterized,

that the light source (4) is a semiconductor laser.

9. The device according to claim 7 or claim 8,

characterized,

that the receiving unit has a photo element (7) which detects both partial beams and whose short circuit current is measured.

10. The device according to one of claims 7 to 9,

characterized,

that the receiving unit has a preamplifier (8) and an optional downstream filter unit (9) with main amplifier and / or automatic gain control (AGC).

11. The device according to one of claims 7 to 10,

characterized,

that the evaluation unit has an analog / digital converter (10) and a microprocessor with display unit or computer (11).

12. The device according to claim 8,

characterized,

that a device (3) for intensity control of

Semiconductor laser radiation is provided.

13. The device according to one of claims 7 to 12,

characterized,

that a further, thicker screen with two through-channels is provided for masking out non-parallel radiation components, upstream of the receiving unit (7, 8, 9).