DE2943237C2 - Thermoelectric calibration-free method and device for measuring the flow velocity of a medium - Google Patents

Description

The invention relates to a method and a device for performing the method according to the preamble of claims 1 and 5.

Hot wire anemometry (HDA) is a measurement method that is widely used in fluid engineering. she is based on the utilization of the cooling of a fine, electrically heated wire in the form of a so-called

Hot wire probe through a flowing fluid. Upper Electronic control circuits will either control the temperature of the hot wire or the electrical Tension on the wire kept constant. The electrical controlled variable is a non-linear function of the on Flow velocity acting on the wire For convenient and accurate measurement of the flow velocity is therefore generally the entire measuring device by means of known flow velocities calibrated and the relationship between the controlled variable and the flow velocity special electronic circuits linearized

The calibration of a hot wire anemometer for small flow velocities below about 5 m / s causes difficulties, at least in gaseous media. The comparison measurement necessary for the calibration is in many cases a measurement of the dynamic pressure of the flow, for example with the help of a Prandtl tube is carried out. Due to the quadratic relationship between the speed and the dynamic pressure, this is already very small at the speed mentioned. For example, it is 1 m / s in an air flow and approximately in the normal state 7 microbar. Such small pressures are only sufficient using complex, highly sensitive pressure gauges precisely detectable.

On the one hand, these properties make the applicability of hot wire anemometry very small Flow velocities limits. On the other hand, such speeds are essential Characteristic of the flow phenomena occurring in heating, ventilation and air conditioning technology. The on Measurement techniques customary in these areas generally do not allow very high measurement accuracies and do not offer the possibility, apart from the amount, also the direction and the sense of direction of the flow velocity to investigate.

The invention to be described here relates to a method and a device with which it is under Use of an uncalibrated hot wire is possible, small flow velocities according to amount, direction and to measure the sense of direction in one measuring process. As areas of application are opening up especially flow measurement problems of air conditioning and ventilation technology as well as meteorology, that is, those Problems in which largely stationary convection and creeping currents occur.

The invention is based on the prior art, which is given by the vibration anemometer, which is listed as a low-velocity anemometer 55 D 80/81 in the 1974/75 general catalog from DISA-Elektronik, Denmark, is described. With this device, an uncalibrated hot wire probe is as parallel as possible the flow velocity to be measured, if this is known at all, and transversely to the probe axis vibrated. Observing the periodic electrical signal supplied by the probe the frequency and / or the amplitude of the oscillation is changed until the speed amplitude the probe oscillation is equal to the amount of the flow velocity. In the electric In this case, the hot wire signal disappears if the alignment is not complete Intermediate maximum. This is based on the insensitivity of the hot wire sensor to the change in sign the relative speed c during the oscillation process. With knowledge of the speed amplitude of the probe oscillation, the amount of the flow velocity is obtained after the adjustment has been carried out Their direction is determined by taking a series of measurements in different Oscillation directions the maximum oscillation amplitude required for the adjustment is determined.

The state of the art also includes mechanical devices that use gear technology the hot wire probes carried by them on defined guided tracks with predetermined known ones Moving speeds (Lit. Soviet Invent Illustr., Sect II Elects Vol. W No. 32, Issued lfi. Sept 75, SU 433 403).

Such devices are used to calibrate hot wires or the behavior of calibrated hot wires under second unsteady flow conditions known in advance in the given flow field to study (Lit Journal Phys. E, Sei. Instr., Vol. 11,1978, Pp. 679-681).

A periodically moving, in particular rotating system for increasing the sensitivity of the measuring sensor, in this case a pitot tube, is also known (Lit Feinwerktechnik, 75, 1971, issue 3, pp. 131-133). By superimposing the speed of the pitot tube moving on a circular path with the absolute speed of the flow field, the level of dynamic pressure at which the measurement is carried out is increased. A comparison of the circumferential speed of the probe with the flow speed does not take place and is not necessary for the measurement.

The invention is therefore based on the object of a method and a device for implementation to create this method, with which it is possible to use an uncalibrated hot wire sensor except the amount of the speed to be measured also its direction and the sense of direction in one to measure continuous measuring process. The measuring process has the character of an extreme value process.

This object is achieved according to the characteristics of claims 1 and 5, respectively.

For this purpose, a hot wire probe at the end of a rotating probe shaft is set in a circular motion with the radius R and the speed η . The hot wire can in principle be oriented in various ways within the device, but an orientation parallel to the axis of rotation of the probe shaft is a particularly clear and optimal case in terms of its mode of operation. without restricting the general case.

The movement of the hot wire around the probe axis is a revolutionary movement. In adaptation to the However, in the present case, the general use of language that is incorrect in this regard is intended to be the Concept of rotation can be used.

The electrical supply of the hot wire or the signal transmission from the rotating part of the probe to the fixed part is carried out by means of a rotary transmitter for electrical signals.

The mode of operation of the measuring method will first be described using the simplified case in which the velocity vector to be measured v * already parallel to that of the circling hot wire described rotation planes, which follow-

to be referred to collectively as the »plane of rotation of the hot wire«.

The circumferential speed vector / 7 with the amount it of the hot wire on its circular path is added vectorially to the flow velocity vector v * to be measured with the amount ν. The hot wire signal proportional to the resulting relative speed vector Γ with the amount c is then (φ = angle of rotation of the probe rotation)

E = Kc

E = K- vV + v ² + 2 ■ 11 ■ ν cos ψ.

(D

E = K ■ (v + w)
E = K ■ (ν - μ)

at φ = 0, 2 / r, 4 η etc. at φ = 3 ff, 5 π etc.

u = 2 · π ■ η ■ R
the adjustment condition

(2)

V = W

10

K is a proportionality factor.

It can be seen that the signal is between the following extreme values fluctuates:

By varying the amount of the circumferential speed with the speed η of the probe and with the circular path radius R of the hot wire according to

20th

25th

30th

fulfill. In this case, the electrical signal £ becomes a minimum for a certain angle of rotation of the probe φ = φ ₀ + Δ φ.

In the case of the more general starting situation, in which the vector v * of the flow velocity is not already parallel to the plane of rotation of the hot wire, Zf only drops to a relative minimum with a fixed probe alignment, which must not be fallen below due to speed variation. With a constant speed of rotation of the probe, a sufficient change in the inclination of the probe in any plane of the axis of rotation by the angle A ψ causes the signal minimum to be additionally minimized to an absolute value. Then the plane of rotation of the hot wire is aligned parallel to the direction of flow and the vectorial adjustment ν * = u "is achieved in the angular position ψ = ψ ₀ + A φ.

With regard to an arbitrary but known initial direction φ _{0 of} the angle of rotation and tp _{0 of} the probe inclination, φ and φ then indicate the direction of the spatial velocity vector v ", the amount of which is determined according to equation (2).

The absolute minimum of the hot wire signal is only one of several possible calibration criteria.

Another criterion for achieving the adjustment is the modulation depth of the hot wire signal, here referred to as modulation for short. Its use assumes that the hot wire signal is not linearized while the absolute signal minimum retains its informative value even with a linearized hot wire signal. Modulation is the difference in signal size between the absolute maximum and the absolute minimum within a period. The basic signal behavior described with equation (1) applies to all cases u ^ v. In any case where the adjustment has not yet been achieved, the relative speed c> 0 acting on the hot wire is without the amount c passing through zero. In connection with the insensitivity of the sign of the hot wire signal to the direction of flow, this property means a particularly favorable uniqueness behavior of the hot wire signal and thus an advantage over the prior art.

The measurement of the angle of rotation position of the signal minimum can be done in various ways, expediently with electronic aids. For example, an electrically, optoelectrically or magnetically acting pulse generator between the rotating and the non-rotating part of the probe device can generate a pulse in the rotational angle position ψ = φο. In a two-beam oscillogram for signal and pulse train triggered with this pulse, the rotation angle difference Δφ between the pulse and the signal minimum can then be observed. This angular position corresponds to the direction of flow rotated back by π / 2 , projected onto the plane of rotation of the hot wire.

Other options for measuring the angle of rotation result from the use of co-rotating fine divided line or coding disks, scanned electrically, optically or magnetically

Q ^: A wide range of electronic options is available for evaluating the angle of rotation measurement, from a simple two-beam oscillogram, which is merely observed, to fully automatic electronic control circuits or computer analyzes.

Incidentally, the angle of rotation measurement does not set a comparison of the speed values that has already taken place ahead, but it can take place before this comparison. Will she also be in front of an already carried out adjustment of the direction of rotation of the plane of rotation to the direction of flow, it delivers the angle of rotation of the projection of the flow velocity vector onto the plane of rotation of the Hot wire.

The individual steps of the measurement process are as follows:

I. At any but constant speed, the inclination of the plane of rotation of the hot wire is inclined in any direction to maintain the spatial position of the measuring point around the center of the circular path of the hot wire until the selected calibration criterion is met with respect to this inclination. The plane of rotation of the hot wire is now parallel to the direction of the flow velocity vector v ".

II. While maintaining the probe inclination, the probe will now rotate? Ah! varies until the selected calibration criterion, now with regard to this speed, is fulfilled. As a result, the amounts of the circumferential speed of the hot wire and the flow speed u = ν are compared.

III. Measurement of the probe speed and calculation of the amount u = ν according to equation (2).

IV. Measurement of the rotational angle position A φ of the signal minimum with respect to the arbitrarily selected reference _{angular position Ji 0} and the inclination angle position φ with respect to an arbitrarily selected reference inclination ψ _{0 of} the probe or the plane of rotation of the hot wire.

V. The complete flow velocity vector v "results from the quantities ν = u, φ = φ ₀ + Αφ, φ = ψ ₀ + Αφ as well as from the direction of rotation of the probe.

IO

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Steps I. and II. Can basically be exchanged in the order, step IV. Can already be after step I.

Compared to the previous state of the art, there are the advantages of this new measuring method

a) in the simultaneous measurement of the amount, direction and sense of direction of the flow velocity through a continuously running measuring process,

b) in the increased measuring accuracy through more precise knowledge of the peripheral speed of the Hot wire versus the velocity amplitude of a mechanically oscillating system additionally lower mechanical stress on the hot wire,

c) in the lower disturbance of the flow at the measuring location, since the rotating hot wire is far away The area around the balanced state is not in its own fluid mechanical and thermodynamic (Heat cloud) wake moved,

d) in the uniqueness of the electrical signal in each operating state, since the relative The direction of flow for the hot wire only changes steadily and does not have a jump in direction, like a vibrating system.

The larger dimensions of the rotation area of the compared to the hot wire dimensions With the nature of the problems of large-volume spatial and climatic currents, hot wire as a "measuring point" fall does not matter.

For the mechanical centrifugal load on the hot wire, for a typical 5 ^ m hot wire, a turning circle radius of /? = 5.3mm and for the to measuring flow velocity of 2 m / s an acceleration acting on the hot wire in the case of adjustment of 77 times the acceleration of gravity. The resulting centrifugal load on the wire is, however less than the load caused by the flow resistance of the hot wire itself.

Since the hot wire does not have to be calibrated or linearized, the measurement can be carried out according to the described method with electronic means of different expansion stages to high resolution both with regard to the amount of the speed to be measured as well as with regard to their direction. the The lower limit of the measurable speed is due to the influence of thermal convection on the Given hot wire cooling, which increases with decreasing relative flow velocity, while the forced convection cooling decreases. The GR

with this

The measurable flow velocity is thus in the order of magnitude of about 5 cm / s.

The invention will now be explained with reference to FIGS. 1 to 5 explained in more detail

F i g. 1 shows a possible arrangement of the rotating hot wire probe in order to implement the measuring method. It consists of the rotating probe shaft 1 and the non-rotating probe shaft 2. Both are supported one inside the other by smooth-running bearings 3 and 3 '. These are shaped so that the hot wire 5 stretched between their ends is aligned parallel to the axis of rotation of the probe shaft 1 and is at a distance R from it through the electrical supply line 12 by means of a rotary transmitter, here for example in the form of sliding contacts 6 and 6 ', from the spatially fixed probe shaft 2 to the rotating probe shaft 1 and is forwarded there to the probe prongs 4 and 4', where it is fed to the hot wire 5. The probe shaft 1 is driven by means of a drive motor 7 which can be regulated in terms of its speed, which can be integrated into the probe shaft 2 and draws its electrical drive power from the supply line 10.

The probe shaft 1 also carries a pulse generator 8 which is as contact-free as possible and which, for example, generates one or more electrical or optical pulses per revolution of the probe shaft 1 by interacting with a pulse receiver 9 located in the probe shaft 2. This signal or a signal which is proportional to the speed of rotation taken from the motor 7 can be used for the precise determination of the speed η of the probe shaft 1. The angle of rotation position g> o of the pulse receiver 9 on the probe shaft 2 represents the reference direction for the direction of the velocity vector to be measured.

Fig. 2 shows for some rotational angular positions φ the vector diagram of the velocities v * and "", which are composed to the resulting flow velocity Γ of the hot wire. Since ti rotates φ for an unbalanced operating condition of the probe with the angle changes of the Hot-wire acting vector 7. It is easy to see from the diagram that at w ^ ν the amount cn never passes through zero and that it can only reach zero as an extreme value ^{in the calibration case JT = V -.}

F i g. 3 shows for a full period of φ the normalized course of c in cases 14 for v> u, 15 for v <u and 13 for v = u (balance state). It can be seen that for v / t / −1 the signal minimum becomes increasingly sharper

FIG. 4 shows the course 13 of the essentially signal-proportional speed c according to FIG. 3, as shown in a two-beam oscillogram, the second beam of which records the directional reference pulses 16, 16 'and 16 ". The minimum of the signal curve 13 has the phase difference Δφ compared to the assigned reference pulse 16, which is determined by the periodic repetition at the reference pulses 16' , 16 ", etc. reproduced from Δφ, the direction of the flow velocity to φ ^ ψο + Δφ + π / 2 is calculated with the rotation of the Hitzdrahtrotation be taken into account in the case of signal layer 13 'is Δφ = 0 and the direction of flow is equal to the rotational angular position of the Reference pulse plus π / 2.

F i g. 5 shows the angular situation with regard to the position of the pulse generator 8 or pulse receiver 9 at the moment of the pulse generation. It can be seen from this that the actual speed direction of the direction φ = ψο + Δφ is πΙ2 ahead of the direction determined from the angle of rotation measurement. In addition, it can be seen that the display and measurement of the phase difference does not require that the circumferential speed u of the hot wire is adjusted to the flow speed ν. This measurement is basically also possible in every non-adjusted operating state of the probe.

For this purpose 3 sheets of drawings

Claims (7)

Patent claims: 1. Thermoelectric calibration-free method for measuring the flow velocity of a medium, in which a hot wire probe exposed to the flow is in a periodic motion known speed is offset in such a way that the flow rate to be measured the own speed of the hot wire is added to a resulting relative speed, which by cooling on the hot wire generates a periodically modulated electrical signal that is transmitted by Variation of the periodic proper movement of the hot wire probe to the extreme value of a characteristic Signal characteristic is set so that the flow rate from their in this matching state occurring identity with the speed amplitude of the hot wire inherent. movement results, characterized in that the hot wire on a circular path around a Axis moves while defining a plane of rotation that the known flow direction The plane of rotation is set to the direction of flow or if the direction of flow is unknown first the inclination of the plane of rotation at constant probe speed from any Starting position in any plane perpendicular to the plane of rotation is varied so that the Modulation of the non-linearized hot wire signal with respect to this slope is maximal, whereby the The plane of rotation of the hot wire is aligned parallel to the direction of flow, and that the Probe speed is varied so that the now constant orientation of the plane of rotation that the Signal modulation with respect to the speed is maximum, whereby the amount of the peripheral speed of the hot wire is equal to the magnitude of the flow velocity, so that the vector the flow speed from the turning radius of the circular path of the hot wire and the probe speed, from the angle of rotation of a characteristic signal minimum within the plane of rotation and can be completely determined from the direction of rotation of the hot wire probe. 2. The method according to claim 1, characterized in that instead of the modulation of the not linearized hot wire signal a characteristic signal minimum that occurs periodically in the hot wire signal is used as an adjustment criterion, which in the adjustment case both with regard to the Inclination adjustment of the plane of rotation of the hot wire parallel to the direction of the flow velocity as well as with regard to the speed setting of the probe to adjust the amount of the Circumferential speed of the hot wire to the absolute value of the flow speed assumes the smallest value, which is both a non-linearized and a linearized Hot wire characteristic can be realized. 3. The method according to claim 1 and 2, characterized in that the speed and / or the Inclination adjustment of the probe by means of a suitable electronic signal evaluation and control system and is fully controlled and automatically adjusted with the help of servo drives, as well as from the information about the probe speed and angle of rotation that is available after the comparison has been carried out of the signal minimum the magnitude, direction and sense of direction of the flow velocity vector calculated by the electronic system and output in a suitable manner. 4. The method according to claim 1 and 2, characterized characterized in that after the adjustment and orientation of the plane of rotation of the hot wire Probe speed is varied periodically so that within each variation cycle at least once the adjustment state is set with respect to the speed, which is determined by a suitable electronic signal acquisition and evaluation system is determined, and that by further suitable electronics from the current calibration information about the speed of the probe and angle of rotation of the signal minimum according to the instantaneous flow velocity vector Amount, direction and sense of direction are calculated and output independently. 5. Apparatus for performing the method according to claim 1, wherein a hot wire, the electrical signal is generated with known means of hot wire measuring technology, relative to the the flowing medium, the speed of which is to be measured, is movable and its electrical signal, which is proportional to the relative speed of the medium to the hot wire, as an indicator for Extreme states of the relative speed are used and thus a defined balance state of the Indicates hot wire movement on the movement of the flowing medium, characterized in that a rotating probe shaft 1 by means of two prongs 4 and 4 'the hot wire 5 circularly around its own Axis moves that the probe shaft 1 is mounted in a probe shaft 2 by the bearings 3 and 3 'and by a Motor 7 with the electrical supply line 10 can be set in rotation that the probe speed by means of a pulse generator device, consisting of a transmitter 8 and a Receiver 9 and the electrical supply line II can be measured and that the electrical supply of the hot wire 5 or the extraction of the hot wire signal Via electrical rotary transmitters, for example in the form of sliding contacts 6 and 6 ', with the electrical supply line 12 takes place. 6. Apparatus according to claim 5, characterized in that the hot wire 5 is parallel to the Axis of rotation of the probe shaft 1 is aligned. 7. Apparatus according to claim 5, characterized in that the pulse generator device is a electrically, optically or magnetically acting principle is based and that, based on a Revolution of the probe shaft 1, single or multiple pulse generators can be used.