DE2943237A1 - Fluid flow velocity vector measurement - uses nulling summation of peripheral rotation vector and flow from rotating heated wire - Google Patents

Abstract

The appts. includes a heated wire set in a circular motion with a peripheral speed vector to which the measurement flow velocity vector is added, resulting in a relative velocity. This results in a periodic electrical signal in the heated wire. The periodic electrical signal is nulled by varying the revolution rate and if necessary the inclination of the probe to the circular motion. The flow velocity vector is then fully represented by the measurable parameters of the wire peripheral velocity vector. The arrangement for calibration-free measurement of flow speed vector magnitude, direction, and sign is applicable in air conditioning and ventilation technology as well as meteorology where significant convection and stray currents occur. It differs from conventional arrangements in using an uncalibrated heated wire sensor.

Description

Rotating hot wire device for calibration-free measurement of speed

Small amount keitsvectors The invention relates to a rotating hot wire device in which the hot wire interacts periodically with the resulting speed vector c, which is derived from the vector u the circumferential speed of the hot wire and the vector to be measured v the flow speed composed. The uncalibrated periodic hot wire signal is supplied by a sharp and clearly defined minimum, which in the case of the comparison u = v to zero becomes, the complete information about the speed vector 4.

This practicable process opens up as areas of application especially flow measurement problems in air conditioning and ventilation technology as well as meteorology, So those problems in which largely stationary convection and creeping currents play a special role.

The hot wire anemometry (HDA) is one of the flow measurement technology widely used measurement method. It is based on the use of the cooling effect of a flowing medium on a very fine, electrically heated wire in the form of a so-called hot wire probe. via electronic control circuits are either the temperature or the tension on the wire is kept constant. The electrical rule magnitude is a generally non-linear function of those acting on the hot wire Flow velocity. For a convenient and accurate measurement of the flow velocity the relationship between the controlled variable and the speed is generally determined special electronic circuits linearized and the entire measuring device calibrated by means of known flow velocities.

The calibration of a hot wire anemometer for low flow velocities below about 5 m / s causes difficulties, at least in air and other gases. The comparison measurement necessary for the calibration is carried out in most cases Measurement of the dynamic pressure of the Flow, for example by means of a Prandtl pipe. The pressures to be measured at these low speeds are very small and only sufficient with complex, highly sensitive pressure gauges precisely detectable. In addition, increasing deviations often occur with falling speeds the measuring characteristics of the measuring sensor on which the comparison measurement is based on (Reynolds number dependence). Ultimately, the faulty influence also grows of convection currents around the heated wire.

The ¢ properties make the applicability of the HDA very small Flow velocities limits. On the other hand, such small flow velocities form an essential feature, for example, of the entire heating, ventilation and air conditioning technology. The measuring techniques customary in these fields are generally not applicable very high measuring accuracies and only rarely offer the possibility, besides the Amount also to determine the direction and the sense of direction of the flow velocity.

The invention to be described here relates to one having a hot wire working device, which enables small flow velocities To be measured in terms of amount, direction and sense of direction without calibration and in one measuring process.

It is based on the state of the art, which is provided by the vibration anemometer is given, which is currently manufactured by a relevant MeS device manufacturer and is sold (cf. Low-Velocity-Anemometer from DISA-Elektronik, Denmark). In this device, an uncalibrated hot wire probe, if known, is used if possible in oscillations parallel to the direction of flow to be measured, transversely to the probe axis offset. While observing the electrical signal supplied by the probe, becomes the frequency of the oscillation changed until the speed amplitude the probe oscillation is equal to the amount of the flow velocity. In the electric In this case, the signal disappears if the adjustment is incomplete Intermediate maximum. This is based on the insensitivity of the hot wire to the sign of the relative speed c and arises when it passes through zero speed acting on the hot wire.

If the velocity amplitude of the probe oscillation is known information about the amount of the Flow velocity, however, no clear information about the direction of flow, for which a number of measurements in different directions becomes necessary.

The invention relates to a prior art modified measuring device with which it is possible by means of an uncalibrated In addition to the amount of the speed to be measured, the hot wire sensor also shows its direction and to determine the sense of direction in one measuring process.

For this purpose, a hot wire probe is attached to the end of a rotating The probe shaft is set in a circular motion with the radius R and the speed n. The hot wire can in principle be oriented in different ways within the device However, its orientation is parallel to the axis of rotation of the probe shaft a particularly clear and optimal case in the mode of action The parallel alignment on which this description is based is therefore possible for others Superior to arrangements due to their properties.

The electrical supply of the hot wire or the signal pick-up from the rotating part of the device is carried out through rotary transmitters for electrical Signals or appropriately acting means.

The operation of this device should be based on the simplified case can be described at which the velocity vector to be measured v is parallel to the planes described by the rotating hot wire.

The circumferential speed vector u with the amount u of the hot wire on its circular path is added vectorially to the flow velocity vector v to be measured with the amount v. The hot wire signal proportional to the resulting relative speed vector c with the amount c is then E = Kc K is the proportionality factor between the electrical measured variable E of the hot wire signal and the amount c.

It can be seen that the signal fluctuates between the following extreme values: E. = K. (v + u) at f = G, 9w, 4 # etc.

E = K. (v - u) at> = 7. 3Tt, 5 # etc.

By varying the amount u of the circumferential speed over the speed n of the probe and with the circular orbit radius R of the hot wire according to #. n 30 the condition # = # can be set so that for a certain angle of rotation = Yo + ## = 7-Z (z = odd number) the electrical signal E becomes zero.

In the case where the vector v is the flow velocity does not lay parallel to the circular plane of the wire movement, E is initially only noticeable a positive minimum 6, which should not be undercut despite the speed variation is. Only through an additional change in inclination to the angle tp = b + ## der Measuring device in any plane in which the axis of rotation of the measuring device must be, the complete adjustment to v = u with E = 0 can then be achieved.

The minimum of the electrical signal is assigned to the angular positions fund 4> to. With regard to any but known initial direction # ° and Give 0 danr; ## and ## the direction of the spatial velocity vector v, the amount of which is determined according to equation (2).

This basic signal behavior applies to all cases v9 and In in each case of the not yet achieved adjustment is the one acting on the hot wire Relative speed c> 0 without the speed vector crossing zero is carried out. In connection with the insensitivity of the sign of the hot wire compared to the direction of flow, this property means a particularly favorable one Uniqueness behavior of the signal es E.

The measurement of the angular position of the signal minimum can be carried out on various Because of this, it is best done with simple electronic tools.

For example, an electrically, optically or magnetically acting Pulse generator between the rotating and the non-rotating part of the device cause a pulse at the angular position y = p +. In one with this impulse triggered two-beam oscillogram for signal and pulse train can then determine the angle difference can be read between the signal minimum and the pulse. The for this purpose fixed reference direction) C, can also be changed so that Af = 0. In this case the single pulse is mapped to the signal minimum and gives the Direction of flow turned back by s / 2.

Other possibilities for angle measurement are given, for example through the use of finely divided co-rotating graduated or coding disks, scanned electrically, optically or magnetically.

There is a wide range of options for the evaluation from a simple two-beam oscillogram to fully automatic electronic ones Circuits with digital output of the complete characteristic data of the speed vector.

The essence of the invention will now be described with reference to FIGS Fig. 1 shows a possible arrangement of the measuring device as a conceptual design. It consists of the rotating probe shaft 1 and the non-rotating probe shaft 2. Both are supported in one another by a play-free, smooth bearing 3 and 3 '.

The probe shaft carries on its aerodynamically designed free one End the probe prongs 4 and 4 '. These are shaped so that the one between their ends stretched hot wire 5 is aligned parallel to the axis of rotation of the probe shaft and has the distance R from this. The supply of the hot wire 5 with the electrical Heating power, which is generally also the carrier for the measurement signal, takes place via the electrical supply line 12 by means of rotary transmitters, here for example in the form of sliding contacts 6 and 6 'from the non-rotating probe shaft 2 into the probe shaft 1 and is forwarded there to the probe tines 4 and 4 ', where it is fed to the hot wire 5.

The drive of the probe shaft 1 takes place by means of a speed controllable drive motor 7, which can be integrated into the probe shaft and draws its electrical drive power via the supply line 10.

The probe shaft 1 also carries one (or more) with as little contact as possible acting pulse generator 8, which for example one (or more) electrical or optical pulse per revolution of the probe shaft by interacting with a (or several) n the probe shaft located pulse receiver 9 generated. This or a signal taken from the motor 7 can also be used in addition to the precise speed determination serve the probe shaft.

The angular position of the pulse receiver 9 on the probe shaft represents the Reference direction f for the velocity vector to be measured. The electrical The impulses for the angle of rotation measurement are carried away via the line system 11.

Fig. 2 shows the vector diagram of the speeds for some angular positions v and u, which are combined to form the resulting relative speed c. Since u revolves with the prong f, the vector acting on the hot wire changes c accordingly. It can easily be seen from the diagram that with u v the vector c does not experience a smooth zero crossing, but only in the calibration case at u = v reaches zero as the extreme value.

Figure 3 shows for a full period of; the normalized course for c in cases 14 for v> u, 15 for v <u and 13 for v = u (adjustment).

It can be seen that the signal minimum increases for v / u + 1 trains sharper.

4 shows the course 13 of the signal-proportional speed according to FIG. 3, as it is represented in a two-beam oscillogram, its second Beam records the angle pulses 16, 16 'and 16 ".

The general position of the signal 13 relative to the rotation angle pulses 16, 16 ', 16 "etc. can be adjusted by turning the probe shaft 2 with the Impuisempüanger 9 (Fig. 1) move so that with the signal curve 13 'the signal minima exactly above pulses 16, 16 'and 16 ".

In this case, the angle indicating the direction of flow shrinks af to zero and the position yO of the pulse receiver is equal to that rotated back by / 2 Direction of flow.

5 shows the angular situation with regard to the position of the pulse generator 8 and 9 at the moment of the impulses and the speed adjustment. From this it can be seen that the actual direction of speed is in the calibration case determined direction f = 90 ## + hy leads by # / 2.

Compared to the previous state of the art, this has the advantages Invention a) in the simultaneous measurement of magnitude, direction and sense of direction the flow velocity within the circular plane of the probe rotation through a single measuring process, with additional inclination adjustability of the probe even des spatial velocity vector v; b) in greater accuracy through more accurate Knowledge of the circumferential speed of the hot wire versus the speed amplitude a mechanically oscillating system with additionally lower mechanical stress of the hot wire; c) in the lower disturbance of the flow at the measuring point, there the rotating hot wire does not move in a wide area around the balance state moving in its own wake area; d) in the uniqueness of the electrical signal in every unbalanced state as well as the increasing sharpness of the signal minimum when approaching the matched state.

The slightly larger dimensions of the rotation area of the probe head do not fall with the nature of the problems of large-volume spatial and climatic flows weight.

As an example, for a probe speed of 3600 rpm and a flow rate of 2 m / s to be measured, a turning circle radius for the hot wire of R = 30-v = S, 31 mm # .n A constant acceleration acts on the probe wire from = n 2 = 77 (9 = acceleration due to gravity) g 30 which is mechanically tolerable for common hot wire designs.

Since the hot wire does not have to be calibrated or linearized, it can the measurement with this device with simple electronic means to high resolution both in terms of the amount of the speed to be measured as well as in terms of their direction.

The lower limit should also be due to the influence of the thermal Convection currents are given around the heated wire, which by as little as possible The upper temperature of the wire compared to the flow can be minimized.

Compared to the one considered here parallel to the axis of rotation of the probe Wire, other wire directions (e.g. radial alignment) are conceivable, but with Disadvantages.

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Claims (5)

Claims 1. Device for calibration-free measurement of flow rates. according to amount, direction and sense of direction, characterized. that one in the optimal case parallel to the axis of rotation of the device Aligned hot wire around it is set in a circular motion deart, that is related to the vector v of the flow velocity to be measured in the area of the hot wire the vector u of its peripheral speed to the vector c of the resulting relative speed added, which, acting on the hot wire, the emergence of a periodic electrical signal of the hot wire causes the minimum by changing the Speed and possibly also the inclination of the probe perpendicular to the circular movement to zero made, the adjustment shows v = u, which in consequence of the speed, phase position and circle radius of the device determinable variables u and angular position of the signal minimum the flow vector v is completely determined. 2. Apparatus according to claim 1, characterized in that the electrical Supply of the hot wire and the generally associated signal removal from the rotating part of the device into its resting part by means of suitable Rotary transmitter takes place, for example in the form of sliding contacts or suitable constructed miniature mercury rotary joint. 3. Apparatus according to claim 1 and 2, characterized in that the relative angular position of the signal minimum by reference to one or more electrical, optically or magnetically generated pulse signals is determined, which by a or several correspondingly acting pulse generators coupled with the rotary movement of the probe be generated. 4. Apparatus according to claim 1 to 3, characterized in that the measuring device is stored so that it is around the rotating probe head in a any longitudinal plane of the probe body can be inclined so that the adjustment can take place with velocity vectors v, which are not parallel from the outset lie to the circular plane of the hot wire movement. 5. Apparatus according to claim 1 to 4, characterized in that the speed and inclination adjustment of the probe is completely controlled and by means of suitable control electronics and servo drives is regulated, as well as from the according to the existing parameters of the probe speed and the angular position of the Signal minimum the information about the amount, direction and sense of direction of the speed vector v is calculated automatically by the electronics and output in a suitable manner.