Keywords: ultrasonic shadow method, measuring the width of the packaging tape, primary measuring transducers of width.

Transcription

1 UDC UTRASONIC SHADOW METHOD OF MEASURING THE WIDTH OF PACKING TAPE IN THE AIR AND THE RESEARCH OF OPTIONS OF MEASURING TRANSDUCERS FOR ITS IMPEMENTATION Rishan A.І., PhD in Technical Sciences 1 Novachevskyi Ya.V., Master s degree student 1 Zaiko V.М., Master s degree student 1 1 National University of Food Technologies, 68, Volodymyrska Str., Kyiv, Ukraine, The article presents the results of research and development of ultrasonic shadow method of measuring the width of the packaging tape. The essence of this method is to control the deviation from the initial position of edge of the packaging tape using two identical primary measuring transducers of width (PMTW), each of which contains rigidly fixed and directed towards each other planes of emission and receiving of ultrasonic vibrations, they are oscillator and measuring receiver, which create acoustic control zone of edge position of the tape which is measured by the width in the air. Each emitter is a linear group of emitters that are excited by common-mode voltage from the transformer with plenty of windings, that leads to creation of equally distributed acoustic beam by the intensity and length, which overlaps by the edge of the acoustically nontransparent tape. The latter gives an opportunity to get a proportional correlation between the acoustic pressure on the receiver and the position of the edge of the tape. There are derived basic mathematical dependencies that describe the considered shadow measurement method in its implementation in the air, and dependencies by choosing of parameters of acoustic measurement zone PMTW in the implementation of the method and the results of experimental researches are presented. Keywords: ultrasonic shadow method, measuring the width of the packaging tape, primary measuring transducers of width. Formulation of the problem. Ultrasonic shadow method of measurement is commonly used in the flaw detection search for cracks or presence of impurities in the metal products. The same method can be used to measure the width of the tape semi products in the air, which in their turn can be either optically transparent or easily susceptible to deformation [4], such as sheets for packaging products in the food industry, etc. The main condition for the implementing of the shadow method is that there must be a big difference between acoustic air resistance and semiproducts [1], that will allow to get a clear shadow from the edge of the packing list on the receiver. Usually this condition is satisfied because the acoustic resistance of the air and the sheet are different almost by three ranks [2].

2 One more condition for the implementation of this shadow method is to create in the air equally divided by intensity ultrasound beam at some length, that overlap the edge of packing sheet. To solve it, there is used well-known hydroacoustics dependency for pressure on the distance H, which is formed by a linear group of ultrasonic oscillator located in the hard screen with radiation in halfspace along the normal to their surface [3]: Р 2 * m* Q j( t ) С е, (1) 2 where С - acoustic impedance environment; λ - length of ultrasonic waves; m - number of 2 radiators, which form linear group; - Setting emitted wave; t - time factor; - cyclic frequency radiation; Q - volume velocity of ultrasonic vibrations source, which describes the ability of the emitter to the formation of acoustic field. From the dependence (1) we can see that pressure, which linear group of emitter at a distance H develops, depends on the number of emitters in the group which create common acoustic beam and if that common beam overlaps the edge of the acoustically nontransparent tape, then it is possible to receive a proportional correlation between the pressure and the position of the edge of the tape to implement the shadow method of measuring the position of the edge of the tape. Purpose of the article is the development of a functional scheme of the device of measuring the width of the tape in the air and the research of conditions of building acoustic zone of measurement by the implementation of the shadow amplitude method. Presenting of the main material. Dependence (1) for pressure of the ultrasonic wave may be submitted in the following form: Р С * m* Q SС РО, (2) 2 2 where Q S * V - volume velocity of ultrasonic radiation source; S plane of one emitter in a РО group; S С m* S V - total area which form linear group; С - oscillatory speed on the surface of one emitter in a group; Р О - pressure which one emitter group develops in its surface; Dependence (2) unlike the dependence (1) describes the resulting amplitude value of pressure of ultrasonic vibrations, which is proportional to the general plane of the linear group of emitters, and point to a basic condition that must be executed to achieve a proportional dependence between the change of pressure and size of the overlap ultrasound beam by the edge of the tape. Such a condition is a proportional change in the total area of linear group of emitters which can easily be made in case when the group of emitters is rectangular. The latter points to feasibility of a set of emitter of single rectangular emitters and their location in the package along the length of its overlapping by the edge of the tape on the minimum allowable distances causing acoustic and electrical solution (pic.1).

3 In this case, the excitation of all single emitters package by the common-mode voltage, the package can be viewed as monolithic rectangular emitter, and the dependence of the amplitude of pressure which develops such a linear group of emitters at the distance H along the normal to the surface of radiation takes the form: Р m* О * П П РО РО, (3) 2 2H where О - size of single emitter in the direction of dislocation of the edge of the tape; package size of emitters in the direction of dislocation of the edge of the tape; П - size of single emitter in the direction of the tape movement, which is controlled by the width; and О * m* П * П SС ; Dependency (3) allows the determining of the amplitude of the voltage at the output of the ultrasonic receiver that measures this pressure on the distance from the surface of emitter considering the size of overlapping ultrasound beam: - U К П Р О SС (1 Х 2 ), (4) where К П - receiver sensitivity; X - displacement of the tape edge that causes overlapping of ultrasonic beam of the package of emitters. The gain (sensitivity conversion) in the shadow method of the width measurement is defined by the dependency: К П дu ш SС РО. (5) дх 2 The signal of the receiver reaches the maximum value while Х = 0, and minimal by the full overlapping a parameter of the package of emitters, i.e. when Х. Thus, the parameter determines the range of measurement of the position of the edge of the tape and must be determined from the conditions: X 2 В, (6) 2 where Х - is a possible deviation from the edge of the tape from the starting position caused by a change of the width of the tape; В - displacement of the axis of the tape relatively to the starting position at a constant width of it. Dependency (5) makes it possible to determine the number of single emitters m that are necessary to ensure the specified range of measurement of the condition: 2 x 2 B m. (7) О О

4 Dependence (4) in the shadow method of measuring the width of the tape of the packing list is implemented with the structural scheme that is shown in pic.2. The scheme implements the differential measurement method only when the width deviation of a tape of its nominal value is measured which is algebraically added to the nominal value after each measurement. The method is performed by using two 1 and 2 of the primary measuring transducers of width (PMTW), each of which accommodates rigidly fixed and directed towards each other areas of emission and receiving of ultrasonic vibrations emitter 3 and measurement receiver 4 (pic.2). Both PMTW can move through bracket-line 5. PMTW are fixed in relation to the 6 strip width of which is measured in such a way that the left and right edge of the tape were between transmitter and receiver of both PMTW. Nominal (required) value of the tape is exposed corresponding to displacement in both bracket-line level 5 on of overlap of both ultrasound beams that correspond half of the maximum length and in reference 7. Adder 10 determines the sum of the average values of signals from both measuring receivers. After their detection by the detectors 8, 9, and zooming into amplifiers 11, it is considered as the beginning of the measurement of deviation of the tape width from its nominal value. At excitation of emitters of both PMTW ultrasonic beams of left and right PMTW are overlapped by the edges of the tape 6. A part of beams of each PMTW is reflected from the surface of the strip and does not get on receivers. With the change of the tape width in the direction of its increasing, the degree of overlap of acoustic beams of both PMTW increases. The total output signal decreases relative to its value which corresponded to the nominal value of the initial configuration. Negative difference between these signals is determined by algebraic adder-indicator 12, which determines the sign and the inclination of the tape width from its nominal value and reflects the actual value of the tape width 6. Decreasing the tape width causes opposite effect. By using a sum of signals of both PMTW, as informational, the independence of the output signal is ensured in the shadow method from a possible displacement of constant across the width of the tape toward the left or right PMTW. Dependency (5) characterizes the transfer factor in the shadow method for specific values of parameters of measurement zone between the emitter and receiver (S,,, λ), but in an explicit form does not assess the impact of diffraction on permitting property of the method and on his error. Diffraction is caused by the presence of the angular deviation of the ultrasound beam (angle of diffraction extension) and is manifested in the fact that violates the straightness of the ultrasound beam. This in turn leads to the fact that the acoustic shadow from the edge of the tape is blurred and not clear. Accordingly, in order to decrease an additional error from the impact of diffraction, the width of diffraction tape on the receiver should be as small as possible. In this case, diffraction takes a place in parallel beams from a half-plane that is formed by a tape that covers the acoustic beam. The width of the diffraction of the tape in this case depends on the length of the ultrasonic wave, direction of emission (angle difference of emission). In addition, it depends on the distance between the surface that forms a shadow, and the measuring receiver. To reduce the diffraction of the tape, which puts an additional error into measurement result, it is necessary to use emitting systems with high value of coefficient of direction, i.e. to

5 reduce the angle of diffraction extension, which is given for a rectangular emitter with length by: sin /. (8) During the synthesis of the PMTW parameters, which is shown in the pic.1, and in which pressure measurement is carried out by distributing along the length of overlapping rectangular measuring receiver of length, there is taken into account the condition of ensuring specified absolute total error of measurement of edge position of the sheet that can be written in the following form: Ф, where - nonlinearity error; and Ф - fluctuation error (the impact of air environment on the measurement results). The acceptable error value from the impact of the phenomenon of diffraction at a selected frequency of emitting and the length of emitter is achieved by choosing the distance between the surface of the sheet that made public by ultrasonic vibrations and measuring receiver with conditions: tg sin * * F С, (9) where F - resonant frequency of used ultrasound emitters ; C - speed of ultrasound in the area of measurement of position of the edge of sheet. Since the choice of the emission frequency and size of linear group of emitters are achieved by the value of the angle, divergence of radiation within the degrees units, in dependence (9) tg for small angles is replaced by sin The acceptable error value oft the nonlinearity characteristics of PMTW transformation, which is due to nonlinearity of the diagram of the linear group direction of ultrasonic transducers constituting the emitter, is reached if a linear group of a single emitters generates and ultrasonic beam with the same intensity across its cross-section. For this purpose the addition of acoustic pressure, produced in an area of measuring receiver, should be carried out taking into account the characteristics of direction of each single emitter

6 within the angle of acuity of the main maximum. The angle of acuity of the main maximum is one of the key factor of the assessment of direction of emitter and is characterized by an angle И divergence (pic.1) from the axis of the direction of the main maximum of radiation, which corresponds to a relatively small change in pressure of the ultrasonic wave relative to the pressure at of maximum. For the rectangular single piston oscillator with the length О, the angle И is determined by the formula: sin 6 И. (10) О If we consider that the value in the implementation of the shadow method is a relative error of non-linearity within overlapping of ultrasound beam of a single emitter, i.e. / О,(11) then the distance between emitter and emitter receiver in which within the limits of the length of a single emitter nonlinearity in distribution of the intensity of ultrasonic field and thus the non-linearity of the output signal by measuring receiver in functions of overlapping his edge of the sheet will not extend beyond dependency (11), is determined by the formula: 2 О О О * F 0,64 2tg. (12) И 6 * С 2 О Dependency (12) determine the conditions of optimal adding of acoustic pressure from individual emitters in the plane of the measuring of receiver to obtain a given measurement range with the required nonlinearity, and points to the possible impact on the overall nonlinearity of output analog signal of measurement receiver by changing the excitation of each separate single emitter. In order to provide the linearization, the output transformer with many windings of ultrasonic generator (pic.1) is used with the possibility of individual voltage control single separate emitter by a change of the additional resistance in each of the windings of excitation.

7 In order to eliminate the occurrence of the standing wave in the shadow method of width control and its impact on the beam distributed by intensity ultrasound, plane of emission of acoustic system of emitters, it must be installed not across to the plane of measuring receiver but at an angle to it which is determined by the condition when the reflected beam from surface of the sheet does not shoot in the plane of the oscillator. This condition is determined by dependence (pic. 1): 2 П cos 2H О tg( ), (13) where - the angle of acoustic system of emitters in the direction of a sheet from the normal position between the plane of emitters and the surface of a sheet; - divergence angle of emission of acoustic system of emitters in the direction of a sheet whose width is measured and determined by dependence (8) for length П, and H О - the distance between the plane of emitters and the surface of a sheet that is under consideration. So, arctg 4 П. (14) О Considering that for the small angles of divergence the displacement of measuring receiver is calculated, which corresponds to the maximum pressure on him of the ultrasound beam after the turn of the emitters system: В П * tg H ( ). (15) 4 П О The actual pressure value of the amplitude of pressure of ultrasonic vibrations on the measuring receiver will be slightly smaller than at parallel location of planes of oscillator and receiver and it is determined by dependence (2) which is multiplied by the value cos. Conclusions. The device designed according to the structural scheme on the pic.2 to control the tape width, which implements the shadow method and in which piezoceramic transducers of the prismatic type ТБК-3 on resonant frequency of khz (by the size in the lower base of the

8 prism П = 10 mm and = 50 mm for receiver and = 25 mm for oscillator when the number m = 6 of oscillators in the package) are used, there should be noted that oscillators and receivers were used as piezoceramic transducers. Such device provides absolute error of ± 2 mm in deviation measuring range ± 50 mm. REFERENCES 1. Analysis and development of a method of measuring the level of substances in the air in spatial ultrasonic beatings / Rishan A.// Automation of production processes (22). P Research of basic parameters of ultrasonic interference level gauges on the standing wave/ Rishan A., Borodkina U. // Scientific and technical information P Basics of hydroacoustics/ Tyurin A.and others//.: Shipbuilding p. 4. An ultrasonic device for measuring the width of the tape/ Rishan A., Gumanyuk M..// С.С , BI 8, 1983.