Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01P—MEASURING LINEAR OR ANGULAR SPEED, ACCELERATION, DECELERATION, OR SHOCK; INDICATING PRESENCE, ABSENCE, OR DIRECTION, OF MOVEMENT
  • G01P5/00—Measuring speed of fluids, e.g. of air stream; Measuring speed of bodies relative to fluids, e.g. of ship, of aircraft
  • G01P5/18—Measuring speed of fluids, e.g. of air stream; Measuring speed of bodies relative to fluids, e.g. of ship, of aircraft by measuring the time taken to traverse a fixed distance
  • G01P5/20—Measuring speed of fluids, e.g. of air stream; Measuring speed of bodies relative to fluids, e.g. of ship, of aircraft by measuring the time taken to traverse a fixed distance using particles entrained by a fluid stream

Definitions

• 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 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.
• this method cannot be used with only selective spatial resolution.
• the light zone anemometer 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.
• 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.
• 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.
• the measurement volumes e.g. two laser beams focused.
• complex arrangements and adjustments of optical components are required.
• 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.
• Detection with a sensor in order to obtain the required information of the scattering center runtime by forming the autocorrelation function.
• 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.
• 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.
• 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.
• the dimensions of the device according to the invention are also small as a result.
• 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.
• 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.
• the autocorrelation function already enables the determination of the transit time from the position of a relative one
• AGC automatic gain control
• 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.
• Fig. 1 shows an embodiment of a device according to the invention for performing the invention
• 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,
• FIG. 2B shows the normalized autocorrelation function of the signal from FIG. 2A plotted against the time shift
• FIG. 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
• FIG. 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
• FIG. 4B shows the normalized autocorrelation function of the signal from FIG. 4A
• FIGS. 4A and 4C shows the processed normalized autocorrelation function of the signal from FIGS. 4A and
• FIG. 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
• A serves as the radiation source
• 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
• the receiving unit of the measuring device has
• 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.
• 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
• the notch filter is tuned to the frequency of 50 Hz, so that
• 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.
• 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
• the storage capacity is still not sufficient, it can be buffered accordingly or an online evaluation can be carried out.
• Measurement volumes are detected, can be evaluated with the photo element already without post-amplification measures
• 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)
• I (t) I 1 (t) + I 2 (t) (2)
• 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 ⁇
• 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 ⁇ 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.
• Equations (2) and (3) give:
• the flow rate can be specified.
• Equations (3) and (4) change analogously.
• Correlation functions are no longer just from Time shift ⁇ , but also dependent on the observation time t c and the observation period 2T.
• 2A shows the measured time profile of the photo element signal at a point in time at which a
• the scattering particle traverses the laser beams.
• the scattering particle has one in the direction of flow
• Propagation speed can be met sufficiently.
• the data of the standardized AKF are prepared by appropriate software.
• the function of the scattering particle is with
• Runtime is almost identical to that of the previously explained case is random.
• 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.
• Radiographic procedures have been included.
• the reflection component can of the scattered light can be used at a suitable angle, as already mentioned above.
• the method according to the invention and the device are also suitable for monitoring and controlling transport processes (production lines, assembly lines) in industry.

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• Engineering & Computer Science (AREA)
• Aviation & Aerospace Engineering (AREA)
• Physics & Mathematics (AREA)
• General Physics & Mathematics (AREA)
• Investigating Or Analysing Materials By Optical Means (AREA)

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• 1988
  • 1988-08-17 DE DE19883827913 patent/DE3827913A1/de active Granted
• 1989
  • 1989-08-16 WO PCT/EP1989/000913 patent/WO1990002099A1/de unknown

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Non-Patent Citations (3)

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