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METROLOGY SUPPORT OF ANEMOMETRY IN MINING INDUSTRY

SEMYON SHKUNDIN

Moscow State Mining University,

Leninsky Ave., 6, Moscow, 117935, Russia

Abstract. In Moscow State Mining University an acoustic method of gas-air flow rate measurement has been developed. The method differs from others known in that the process of radiating of vibrations into the gas flow travelling through the air duct and the manner in which they are received. It does not destroy the aerodynamic structure of the flow, does not add any airdynamic resistance, is practically inertialess, reacts to the average velocity in the cross-section. The cylindrical anemometric channel through which the stream flows and the acoustic waves propagate is simultaneously an air duct and a wave guide. The report performs the new means of metrology support for anemometry in mine industry. The background of this metrology support - the acoustic anemometer named above installed in the metrology installation. The main advantages of the offered anemometer are wide dynamic range, sensitivity, inertialessness. It can be used in different tasks of safety and environmental monitoring.

Key Words. Mine ventilation, environment monitoring, acoustics anemometer, metrology, aerometric installation.


I NTRODUCTION

The control of mine workings ventilation conditions demands not only the development of modern devices for anemometer control but also means of their metrology support. Just these facilities are necessary for the problems decision in surface environment and meteorology monitoring.

At present time in Russia control and checking of anemometer devices are being produced, basically, by bassin departments of State Standart Cometee and S c ientific Production Asso c iation "Metrology S c ientific Research Institute by Mendeleev". The remotion of the named above organizations from mining enterprises rezults in fact that anemometers are not checked even ones a year. At the same time coal seam mining intensification, gasa i rdynamic s parameters control in the mining districts and in the whole mine, the tendency to increase permitted methane concentration level strenthen the demands to anemometers accuracy and their metrological support.

Objectiv e ly there are two possibilities of anemometer control improv ement in mines: development of new anemometers, satisfying modern demands, including metrological, and the delivery of metrological characteristics of the existing devices on the appropriate level, reg ula ted by the device passport.

The situation is complicated by the fact that the predominating type of anemometers are ta ch ometric ones, which increase their error due to construction properties being kept even without expluatation. The experience shows that anemometers must be checked when regularly expluatated, at least, every month. It is not necessary to prove safe expedien cy and even necessity of che c king, it is enough to say that there are no means to measure the lowest limit of control le d speeds (0.15 m/s), dictated by Safety R egulations in Russia, and there are no means of check ing hypothetical mine device, which would be able to measure this speed.

All the mentioned circumstances rezulted to necessity of such anemometer checking means creation, which might be used directly at places of their expluatation, in the mine buildings and units or hydrometeostations all over the state territory.

Description of the suggested method of anemometry

Aerometric installation, which presents the combination of air flow source and calibrating chamber with reference anemometer, measuring speed of this flow, is the base of mertrology support for industrial anemometry. The ventilator of air flow source-incentiver in anemometry channel creates flow speeds in the range of (0.1-20) m/s, measured by the named acoustic anemometer. The principle of the aerometric installation operation is based on the reproduction of the controlled unit of airflow speed , comparison of indications of the reference anemometer and she one being check ed and recount of tarirastic characteristic s of the checked anemometer. The suggested reference acoustic anemometer uses the phenomenon of acceleration or slowdown of acoustic vibrations coming to the rece i ver s by the changing of the movement speed of medium. The difference consists in the method of radiatin g and receiv ing of vibrations. The following paragraph describes operation of acoustic anemometer more in detail . The careful analysis of publications and preliminary laboratory estimation have showed that on the one hand acoustic s methods in flow measurement s ha ve not realized their potential possibilities, and on the other hand non of the existing acoustic s means of measurement of the flow allows to create a nemometer, which should be able:

- to measure speeds of flow in the range of ( 0.05 ... 30 ) m/s;

- not to add aerodynamical resistance in the control l ed flow,

- not to disturb aerodynamical curve;

- to measure average flow speed in the cross section;

- to have stab i le characteristics, allow ing to decrease erro r .

The suggested measurement method satisf ie s all mentioned here demands [1] . The method consists in radiatin g and rece iving of acoustic waves in air conduit, characteristics comparison of the radiated and received waves. It differs from others known in that w aves are being radiated and received by excitement of the air conduit elements, acoustically isolated from each other . This method gives accuracy and prov ides exception of air conduit effect on the aerodynamical field of gasair flow .

The description of wave propagation process in the tubes without flow was suggested by E.Scuchic [2] . From this discription we ma d e the conclusion which waves could spread in the round channel of the given diameter. The spread rate of wave fronts of these vibrations is equal to one of sound velocity in open space with the same medium. In the air gas channel the sound speed in the direction of the waveguide axle is expressed :

C = C = 2 p / k x ( 1 )

where : k x - wave number for propagation to the waveguide axle direction;

- frequency of the radiated vibrations .

R eview of w ork s , concerning the description of the acoustic field in waveguide, being in this case also airduct, showed that non of the classical descriptions covers the process of vibration propagation in finite length channel with moving gas air flow. The models of wave propagating in the channels, considered in the literature, are not faithful to engineer task, borning our reference to them. In particular the known models describe either infinite airduct with moving gasair flow or channel without flow. The waves, reflec ted from ends of waveguide can be ignored only in infinite channel. At the same time, it is obvi ou s, that anemometer c an not has infinite length and its dimensions should be as less as possible. Engineering task of anemometer creation with cylindrical electroacoustic converter, which don't bring off the curve of aerodynamic velocities , demands solution of mathematical physics task with border conditions, representing the real anemometer channel. The main of these conditions is the one, taking into account the finite length of the anemometer. The solution of this task may be g o t using G. G o h nson's and K. Ogimot o 's work [3] .

The Anemometer Operating

The measuring channel of the reference anemometer presents the cylindrical air conduit, containing two semichannels. Fig. 1.

There is a radiating element i n the centre of air conduit, on each side of which rece iving elements are found at a distance L . The difference of phases in the right semichannel ( down the flow ) is expressed:

, ( 2 )

where : j 1 - initial phase difference.

Analog ous for the second , left semichannel (against the flow):

, ( 3 )

where j 2 - initial phase difference in the second semichannel.

The aerometric installation has to operate in accordence with technical task at temperatures from

15 to 25 0 C that corresponds to the reg im e of metrological tests. In this range of temperature change s , as it is easy to count, the sound speed does not exceed 345 m/s.

In the right semichannel minimal value of phase difference corresponds to the maximum of ( C + U ), i.e. C = 345 m/s and U = 20 m/s; that is , in assumption j 1 = 0 :

. ( 4 )

In the left semichannel D j 2 min correspond s to the case when C = 345 m/s and U = 0 . That is , in assumption j 2 =0 we get:

. ( 5 )

Now formules ( 2 ) and ( 3 ) will be the follow ing :

, ( 6 )

.

Before measurement the phase zero in the right semichannel is being adjusted to C + U = 365 m/s ; in the left semichannel the phase zero corresponds to C - U = 345 m/s. The phase zero schematically may be fixe d precisly, therefore anemometer phasing does not generate measurement error. We can describe the basic operations of measurement algorithm of the different phases from the above formules in the next way:


Fig. 1.


1. Automatic amplification adjustment of the received sinosoidal vibrations.

2. Tr ansformation of sinosoidal vibrations into meander.

3 . Correcting phase change.

4. Forming of binary codes o f the analog signals in each semichannel.

5. Time averaging of binary code s .

6. Calculation of binary code s corresponding to the flow speed.

Automat ic amplification adjustment is necessary for modification of radiator and receiver signals to standard level. The inertia of this adjustment does not limit the measuring possibilities because the main task isn't to measure puls speed but averaged one during some interval ( for example 1s). Just b ecause of this automatic amplification adjustment does not bring the errors into the process of measurements. The transformation of sinosoidal vibrations into meander, in fact , can cause the bias of pulse front relativ e ly to zeroes of harmon ic . However the correction of phases allows make it equal to zero. The transformation of informative im pulses into the binary code is made by filling out their duration by the im pulses of higher tact frequency. The choice of this frequency is made from the accuracy and sensitivity requirements to anemometer. We 'll detemine the value of this frequency in case when it is nesesary to measure U = 0.05 m/s. For this purpose w e differentiate the last expression s by the speed:

, ( 7 )

. ( 8 )

The expression (7) takes smallest value at C =345 m/s and U = 20 m/s. We'll find the value of informative parameter (phase difference), corresponding to U = 0.05 m/s at L = 0.5 m and f =10.3 k H z ( the resonance of piezoceramics transformers).

This value at the given frequency corresponds to time interval 150 10 -9 s. This means that frequency of filling (tact pulses) has to be not less than 6.7 k H z . The expression ( 8 ) takes minim um value at C = 345 m/s and U = 0. For increment = 0.05 m/s phase difference will be equal:

( 9 )

This value corresponds to time interval 228 10 -9 s, i.e. filling frequency has to be not less than 4.4 k H z . Thus when filling frequency is equal to 6.7 k H z and more, it is possible to follow the changing of flow speed, not exceeding 0.05 m/s in both cha nn e ls. During the filling of informative pulses by tact ones it can happen discrepancy of leading and (or) back fronts of informative and tact pulses. Error is resulted (plus or minus one tact pulse), maximal estimation of which corresponds to mistake 0.05 m/s. Mistake is not increased in the following step of logical division because the division is done in number view. As a result of this operation we have got sum ( C + U ) and difference ( C - U ) with accuracy o f constant multiplier.

Finally, as a result of logical subtract ion we get value, proportional to flow speed.

K [(C + U) - (C - U)] = K S U ;

where K S - coefficient, rezulting from equalization of semichannel characteristics slope .

Af t er the fulfilment of this operation maximum error value can be d = 0.05 + 0.05 = 0.1 m/s. Thus the resulting absolute error of calculation of measur ed speed does not exceed 0.1 m/s. The described algorithm has been realized in the installation for mine anemometer checking (aerometric installation). Multiple increasing of filling frequency allows to reduce application error into corresponding number of times.

The installation description

The hardware part of installation functionally includes two fundamental blocks - measuring (measure r ) and calculating (calculater). In the measur ing block informative parameter (phase shift) is extracted, and analog -digital transformat ion is proceeded. The measur ing block includes : radiator, high - stabile 20 k H z generator, controll ed divi der , transform er meander in to sin us , giving level schem e , power amplifier, two receiving transform ers (one in each semichannel), two schemes of automat ic amplifier adjustment ; two transfor mer s sin us into meander, two controll ed phase shifters , schem e of selections frequency control.

The calculator includes: two binary code form er s, two averaging schem es , converter, substractin g binary numbers, g o t in the two semich a nn e ls and transformin g the binary code into binary -decimal one, indicator.

The signals are synchrotreated in both semichann e ls. The temperature instability coefficient for quartz generator is 10 -6 , that suppl i es sufficient accuracy of filling the informative pulse by the tact one s , and also the stabil ity of the radiat ed vibrations frequency. In the control led diviser the assumed frequ e ncy is divided to the value , corresponding to the radiat o r resonance - 10.3 k H z with possible maxim um deviation - 1 H z . Such instability allows to have stab i le amplif ication in the receiv ing transducer . But really the frequency 20 k H z suppli es sensitivity of the anemometer to the flow speed 0.01 m/s, corresponding to the time interval o f 50 nanoseconds. The changing of divisor coefficient by the unit causes the changing of frequency at the exit for 10 H z in the range of 10-12 k H z .

Thus, generator and programm ed divisor has the foll o wing functions:

- creat e the possibility of radiator frequency adjusting ;

- supply stabile high-frequency filling of informative time interval with necess ar y step;

- define support frequencies for control signals forming.

On the radiating ceramic s element periodical voltage (4- 10 ) V must be applied . To form such signal, sequence of rectangular pulses of resonance or near to it frequ e ncy has to be transform ed into sinosoidal and strengthen. The level giving sc hem e match es the low frequencies filter exit with the entrance of the power amplifier and semu l tan e ously smooth tuning o f signal level. The choice of vibration s amplitude of the radiator is being produced with the reference of two conflicting to each other considerations. On the one hand, increasing o f the transformer signal causes warming of it and, consequently , changing of it s characteristic s. B esides, appearing warm asy m m etric al field in the anemometer channel results to the nonidenti cal sound speed value in semichannels. On the other hand, decreasing of vibration signal amplitude on the radiating element can cause reduction of measurement accuracy. In the aerometric installation anemometer the radiated through base L = 0.5 m and for it signal level on the receiver may be (1-10) mV when amplitude on the radiator - 2V. Power amplifier implemented with the use of microscheme, designed to work with the load of about unit s of O m . The received signal is reinforced to the value , necessary for normal work of sc hem e of the automatic amplification adjustment (5V). B esides the characteristics of the controll ed medium - temperature, moistour, pressure, which influence on the sound speed is compensated by the existence of two semichannals , the informative parameter - phase shift is subjected to the influence of the electron schemes characteristics. So receiving amplifier has not to bring phase shift more than 40 nanoseconds. To broaden the dynamic range of the input pulses in the receiving tract non linear amplifier providing the ratio of maximum to minimum signals equal to 40 is used .

The schem e of automatic amplification adjustment with phas e tuning allow s to chang e the input voltage from 0.5 to 20 mV and exclude s one of the basic errors of measurement - dependence of phase shift upon the amplitude in the receiver. The harmonic signal of several volts amplitute is removed from the automatic amplification adjustment and transformed into meander by the comparator, which follows to indicate the buckhead voltage - 0.15 V. Controlled phase shifter permit s to make zero phas e difference between the transmit ed and receiv ed signal s during the flow absence. The informative meander in the same block is being converted into the seque nce of short pulses, which open the count of each given measurement of the flow speed. This count is being produced with the frequ e ncy 10 k H z . The shift of the short pulses relativ e ly support signal may be changed by phase turning and, in particular, made to zero before measurement. Time dynamic s range consists of 100 ns, that permits work ing at frequ e ncy (10-12) k H z, to overlap 2 p dynamic s range. Decimal count er s are used as count ing elements.

Any measurement of flow speed is the average one during certain period of time. Acoustic method of measurement, being inertialess, permits to produce thousand s measurements of speed per second. Because of the turbulent structure of the stream these measurements, b eing momentary, are confirmed to the probability distribution, i.e. have the value spread.

To get the information about average flow speed i t is necessary to carry out the averaging of great number of measurements .In the installation anemometer 4096 measurements are being averaged and control s c hem e of samplings frequency permits to vary the average interval from 1 to 20 seconds . F or the follow ing calculations concerned with algorith of flow speed display , it is necessary to perform the informative parameter as the digital code. This is carried out in each semichann e l in the formers of the digital code. The transformation of pulse duration into the digital number is being made with the help of s y nchro nized counters , which work with frequ e ncy 20 k H z . During each interval of filling it is possible to get mistake which value is equal to 1 tact of frequ e ncy 20 k H z , i.e. in 50 nanosecond. But this mistake, if appeared, has equal probability in both chann e ls, that i s on the stage of sub s traction of code s corresponding to the phase climbs in the right and left semichannels, it is destroyed. Thus, at the exit of digital code form er , built on the synchronized digital count er s and controlling trigger, we get numbers in the digital code, which must be averaged.

As has been said, averag ing operation is necessary for exception of dependence of casual components measurement errors, and done in two stages: first, sum up 4096 measurements, second, division of result by 4096. Received digit number, equal to the average number for one measurement, rerecorded into register, where kept during one cycle of the averaging. Similar, a number - the rezult of measurement of the average speed in the second semichannel is being recorded. The process is repeated in the new cycle. So in the end of each cycle w e have two numbers, corresponding to the speed of vibration propagation ( i. e. sound speed together with flow speed) in the left and right semichann e ls. The fin al rezult, i.e. the number, corresponding to the speed value of air flow, is g o t as the function of two numbers. The indication represents the numbers from 00,00 to 20,00, that corresponds to the flow speed, expressed in m/s. The i ndication step 0.01 m/s . F or receiving of the necessary metrolog y characteristics it is necessary to compensate the basic anemometer errors. C ompensation of e rro r , causing by electron tract and phase characteristics of converter, is produced in each of channels with the help of controll ed phase shifters . C ompensation of e rro r , caused by asym m etry of anemometer semichann e ls as not changing in time, is being produced by device calibration, i. e. adjustment of phase shifts. C ompensation of e rror, concerned with reflecting waves, is made on the stage of airduct constraction .

Erro r compensation, concerned with value difference of sound speed in both semichannels, is made, as has been described, by averaging of the enough amount of measurements. This averaging is effective because the air volume, passing through the left and the right semichannels are the same, so sound speed in the left semichannel at the next moment will be sound speed in the right one . Thus, the same changes of sound speed will be in both semichannels, but with time shift. The averaging permits to exclude momentary assimetry of chann e ls.

CONCLUSION

Aerometric installation is designed for checking anemometers of different types. The aerometric installation comprises an aerodynamic channel and a control panel. The operation of the installation is based on the reproduction of the air stream velocity unit by generating and measuring of it. Air flow rate control is two-stage, using a thyristor circuit of fan drive control and a remote-controlled butterfly valve. The air flow rate is measured by using an ultrasonic method which has been developed at Moscow State Mining University and Which is based on sound vibrations propagation velocity depending upon air flow rate.

REFERENCES

Puckov L. Shkundin S. oth. : The method of measurement of airgas flow velocity. Author's certificate 16822590, 1991.

E. Scuchic. Basic acoustics. Moscow 1976, vol.

2, pages 112-116.

G. Gohnoson and K. Ogimoto. Sound radiation from a finite length unfledged circular duct with uniform axial flow. Acoustic Society of America papers. 68, 1980.

 
 
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