RU2526200C1 - Method to tune string accelerometer - Google Patents

Abstract

FIELD: measurement equipment.

SUBSTANCE: method to tune a string accelerometer, comprising a string of rectangular cross section and a plate bracket fixed in a cantilever manner with a weight, including a fixture of string ends between two planes, previously mechanically treated in two mutually perpendicularly directions across and along the string, and differs by the fact that the string is set along the axis of symmetry of the bracket perpendicularly to its plane, string ends are fixed in series on the weight and the body with matching of the fixation surfaces into one plane, they compare frequency of self-excited oscillations of the string with the specified one and if required, they correct the length of the string, based on the expression: $$\Delta l=\frac{\left(f-f_0\right)}{f}\frac{l}{\frac{l}{2y}+1},$$

where Δl - variation of string length; f and f₀ - actual and specified frequency of string oscillations; l and y - string length and bracket sagging with string arrangement in the same plane, at the same time they again mechanically process the fixation surfaces to their arrangement in one plane, besides, the length of the string is reduced, if the frequency is below the specified one, and increased, if it is more, then force is applied to the weight in the area of string fixation, which smoothly varies tension of the string in the working range of frequencies, and assess variation of the signal amplitude from the string, trying through accurate setting of the string to bring the frequency and amplitude of the signal into the specified allowance, afterwards they perform thermomechanical ageing of the accelerometer.

EFFECT: invention makes it possible to reduce duration of parameters stabilisation, time of assembly and to increase yield of good string accelerometers during manufacturing.

5 dwg

Description

The invention relates to measuring technique, and more specifically to string accelerometers for the autonomous determination of the motion parameters of aircraft, and can be used in the manufacture of string accelerometers.

Known string sensors containing a housing, a string stretched in it and a device for exciting self-oscillations. The design and method of attaching the strings are of paramount importance to ensure a given initial frequency and its stability during tuning of the main parameters of string sensors. Known designs of string sensors of physical quantities with different ways of attaching strings (see the book Kartsev EA, Korotkov VP Unified string measuring transducers, M., Engineering, 1982, pp. 34-38). The greatest frequency stability is provided by the method of fastening strings of rectangular cross section between two planes with their preliminary grinding and grinding in two planes — perpendicular to the string at its exit and in the plane along the string. However, setting a predetermined frequency of string oscillations in the transducers is difficult and leads to a deterioration in the stability of the oscillation frequency.

Known string accelerometer (see ed. St. SU No. 1840379, G01P 15/10), which uses a ribbon string with thickened ends, simplifying the fastening of the string, with an adjusting device that allows you to change the tension of the string, and therefore, control the frequency of oscillations. Despite the movement of the adjusting slide along the radius, it is not possible to exclude the relative displacement of the string terminations, which leads to a bend in the terminations of the thinned part of the string and, therefore, to additional stresses, which affects the stability of the oscillation frequency.

The known differential string accelerometer and method of its manufacture (see RF patent No. 2258230, G01P 15/10, publ. 10.08.2005, adopted by the authors as a prototype), which consists in obtaining the string by flattening from wire. The attachment points before installing the strings are ground in two mutually perpendicular planes - in the plane of the string and perpendicular to it at the points of exit of the string from the terminations with fixation of the fastening elements by pins. By tensioning the strings with an elastic suspension, the required initial frequency of the string vibration is selected, while the surfaces of the string attachment points may not lie in one previously polished plane. This leads to deformation of the string at the exit points. Bending stresses in terminations impair the stability of the frequency of self-oscillations and the accuracy of the measurement. In addition, the straightness of the string is violated, as a result of which side resonances are possible in the working measurement range, which reduce the quality factor and lead to a breakdown of vibrations; the stabilization time of the parameters increases and the yield of suitable accelerometers decreases.

The task of the invention is to reduce the stabilization time of the parameters, the assembly time and increase the yield of string accelerometers in the manufacture.

The specified technical result during the implementation of the invention is achieved by the fact that in the known method of tuning a string accelerometer containing a rectangular cross-section string and a cantilever fixed plate suspension with a load, including fixing the ends of the string between two planes, previously machined in two mutually perpendicular directions across and along the string, the peculiarity lies in the fact that the string is set along the axis of symmetry of the suspension perpendicular to its plane, the string is fixed tionary loads on the ends of the string and casing when aligned mounting surfaces in a single plane, compared with self-oscillation frequency of the string set and, if necessary, adjusting the length of the string, from the expression:

, $$\Delta l=\frac{\left(f-f_0\right)}{f}\frac{l}{\frac{l}{2y}+\mathrm{one}}$$

,

where Δl is the change in string length;

f and f ₀ - the actual and given frequency of the string;

1 and y are the string length and suspension deflection when the string is in the same plane, the fastening surfaces are again machined to be located in the same plane, and the string length is reduced if the frequency is less than the specified one and increased if more, then applied to the load at the point of attachment of the string, the force smoothly changing the tension of the string in the working frequency range, and evaluate the change in the amplitude of the signal from the string, achieving accurate installation of the string hit the frequency and amplitude of the signal in a given tolerance, after four to carry out thermomechanical aging accelerometer.

The essence of the invention is illustrated by drawings, in which Fig. 1 shows a structural diagram of the string fastening in an accelerometer (side view), the dotted line shows the initial position of the undeformed suspension, Fig. 2 - suspension with a load (bottom view), Fig. 3 shows the installation of the string with a deviation from perpendicularity to the plane of the suspension; figure 4 presents the installation of the string, offset from the axis of symmetry of the suspension (view from the side of the suspension), figure 5 is the same, but in cross section of the string.

In the string accelerometer (Fig. 1), a rectangular cross-section string 1 is fixed at one end to the casing 2, and the other - to the load 3, which is rigidly fastened to the elastic plate suspension 4, isolated from the casing 2, to which it is attached with glue and screws (on figure 1 is not shown). The self-oscillations of the string 1 are excited by a magnetoelectric drive including a permanent magnet and an electronic excitation circuit. The string 1 is located in the field of a permanent magnet, and its ends are pressed against the body 2 and the load 3 with flat plates 5 and 6 using screws (not shown in Fig. 1). To eliminate the uncertainty of mounting the strings 1 in the seals, the machining of the exit points of the strings 1 is perpendicular to the plane of oscillation, excluding the so-called "step". Also, grinding of the surfaces of the load 3 and the housing 2 in the plane of the string 1 is carried out to eliminate twisting and bending of the string 1. Mechanical processing perpendicular to the string 1 is carried out in a device fixing the suspension 4 in a strained straightened position, and the distance between the terminations becomes equal to the length l of the string 1 during installation . Due to the deviation of the geometric and elastic parameters of the suspension 4 and string 1, it is not possible to provide the required initial frequency f ₀ with a tolerance of the order of 1%. The adjusting devices cause additional stresses in the tension circuit of the string 1 and are therefore undesirable. In the prototype, the tension of the string 1 is changed by changing the deflection of the suspension 4, but the mounting surfaces of the ends of the string 1 on the load 3 and the housing 2 come out of the same plane. Moreover, the absolute values of the change in the deflection Δy of the suspension 4 and the length Δl of the string 1 are equal. This can be seen from figure 1: how much the deflection of the suspension 4 increases, so the length of the string 1 decreases. The relative deviation of the frequency f from the given initial value f ₀ during tuning is determined by the expression:

, $$\frac{f-f_0}{f_0}=\frac{\Delta y}{2y}+\frac{\Delta l}{l}=\frac{\Delta l}{l}\left(\frac{l}{2y}+\mathrm{one}\right)$$

,

where l and y are the length of the string 1 and the deflection of the suspension 4.

.The length of the string 1 is ensured by the installation of the gauge in the device during machining with high accuracy ~ (0.1-0.2)%, and the deflection of the unwanted suspension 4 in this case ranges from (1-10)%. When tuning the frequency, part of the accelerometers falls into the 1% tolerance, ensuring the length of the string 1 during machining. The other part requires adjusting the length of the string 1 depending on the deviation of the initial frequency $$\Delta l=\frac{\left(f-f_0\right)}{f}\frac{l}{\frac{l}{2y}+\mathrm{one}}$$

.

для конкретной конструкции акселерометра величина постоянная и составляет, например, при l=6 мм и y=1 мм величину 1,5. Если при закреплении второго конца получили частоту f автоколебаний больше заданной начальной частоты f₀, например, на 4%, то длину струны 1 надо увеличить на 0,06 мм, а если меньше f, чем f₀ на 4%, то укоротить струну 1 на 0,06 мм. Чтобы струна 1 с измененной длиной осталась прямолинейной, необходимо вновь провести механическую обработку мест крепления струны 1, обеспечив при новом калибре в приспособлении (в нашем примере калибр изменен на 0,06 мм) расположение струны 1 в одной плоскости. Далее закрепляют струну 1 с измененной длиной и проверяют начальную частоту автоколебаний, при непопадании в допуск повторяют операции с изменением длины и механической обработкой плоскости крепления струны 1. Для точных акселерометров нестабильность частоты автоколебаний струны 1 должна составлять величину не хуже 1·10^-6-1·10^-7. Наибольшее влияние на стабильность частоты оказывают величина, вид и неравномерность напряжений в сечении и по длине струны 1, а также отклонение от прямолинейной формы струны 1. Обычно напряжения в струне 1 составляют 0,1-0,2 предела прочности порядка 400 Н/мм². Максимальная стабильность частоты будет, если струна 1 установлена таким образом, что испытывает только равномерные растягивающие напряжения в сечении по всей длине колеблющейся части струны 1. Струна 1, растягиваемая подвесом 4 прямоугольного сечения (фиг.2) с осью симметрии 7, устанавливается перпендикулярно плоскости подвеса 4 по его оси симметрии 7 и в таком положении закрепляется на грузе 3. Если струна 1 имеет отклонение от перпендикуляра на угол α (фиг.3), т.е. смещение верхнего конца от нижнего - центра приложения растягивающей силы подвеса 4 приводит к появлению поперечных изгибающих напряжений в плоскости струны 1, причем максимальные напряжения у верхней заделки. В частности, при отклонении на угол α=30' (смещение ~0,05 мм) и растягивающим струну 1 усилием 4 Н максимально изгибающие напряжения составят ~500 Н/мм². При смещении центра сечения струны 1 от оси симметрии подвеса 4 (фиг.4 и 5) на величину Δ возникает поперечный изгибающий момент по ширине струны 1, приводящий к изгибающим напряжениям по всей длине струны 1 в ее плоскости. Струна 1 испытывает сложное напряженное состояние -внецентренное растяжение. Изгибающие напряжения при Δ=0,1 мм составят ~900 Н/мм² и в сочетании с растягивающими напряжениями делают напряжения неравномерными по сечению и длине струны 1, что ухудшает стабильность частоты колебаний. Изгибающие напряжения в заделках по толщине струны 1 возникают, если поверхности мест крепления струны 1 не лежат в одной плоскости. При неплоскостности ~0,03 мм эти напряжения составят около 700 Н/мм², причем струна 1 у выхода из заделок имеет перегибы по радиусу, равному 1-2 толщины струны 1. О недостаточной точности установки струны 1 относительно подвеса 4 и повышенной неплоскостности заделок свидетельствует нестабильность частоты хуже 2·10^-6. Окончательно о качестве закрепления струны 1 судят по величине изменения сигнала со струны 1 при приложении к грузу 3 в месте крепления струны 1 плавно изменяющего усилия, вызывающего увеличение и уменьшение частоты колебаний. Усилие прикладывается по оси струны 1 (на фиг.1 показано стрелками) и составляет величину не менее ±mng, где m - масса груза 3, n - перегрузка, g - ускорение свободного падения. Это соответствует изменению начальной частоты f₀ на величину ±kng (где k - коэффициент преобразования). Сигнал со струны 1 контролируется, например, по осциллографу и не должен изменяться более чем на 10%. Это говорит о возможности обеспечения заданных точностных характеристик акселерометра и его работоспособности во всем диапазоне измеряемых ускорений. Если изменение сигнала превысит указанный допуск, следует более точно выставлять струну 1. После обеспечения заданной начальной частоты колебаний струны 1 и допустимого изменения сигнала со струны 1 проводят термомеханическое старение акселерометра, стабилизирующее напряжения в цепи натяжения струны 1. При этом повышается стабильность частоты до 1·10^-7, а это в свою очередь повышает точность выставки измерительной оси и уменьшает погрешность определения коэффициента преобразования.Attitude $$\frac{l}{\frac{l}{2y}+\mathrm{one}}$$

for a specific design of the accelerometer, the value is constant and is, for example, at l = 6 mm and y = 1 mm, a value of 1.5. If, when the second end was fixed, the self-oscillation frequency f was greater than the specified initial frequency f ₀ , for example, by 4%, then string 1 must be increased by 0.06 mm, and if f is less than f ₀ by 4%, then shorten string 1 by 0.06 mm. In order for the string 1 with the changed length to remain straightforward, it is necessary to machine again the attachment points of the string 1, ensuring that with a new gauge in the device (in our example, the caliber is changed to 0.06 mm), the string 1 is in the same plane. Next, string 1 with a modified length is fixed and the initial frequency of self-oscillations is checked, if it does not fall into the tolerance, operations with changing the length and machining of the plane of attachment of string 1 are repeated. For accurate accelerometers, the instability of the frequency of self-oscillations of string 1 should be no worse than 1 · 10 ^-6 -1 · 10 ^-7 . The greatest influence on frequency stability is exerted by the magnitude, type and unevenness of the stresses in the cross section and along the length of the string 1, as well as the deviation from the rectilinear shape of the string 1. Typically, the stresses in the string 1 are 0.1-0.2 tensile strengths of the order of 400 N / mm ² . The maximum frequency stability will be if the string 1 is installed in such a way that it experiences only uniform tensile stresses in the cross section along the entire length of the vibrating part of the string 1. String 1, stretched by a suspension 4 of rectangular cross section (figure 2) with an axis of symmetry 7, is installed perpendicular to the plane of the suspension 4 along its axis of symmetry 7 and in this position is fixed on the load 3. If the string 1 has a deviation from the perpendicular to the angle α (figure 3), i.e. the displacement of the upper end from the lower - the center of application of the tensile force of the suspension 4 leads to the appearance of transverse bending stresses in the plane of the string 1, and the maximum stresses at the upper seal. In particular, when deflected by an angle α = 30 '(displacement of ~ 0.05 mm) and a tensile string 1 of 4 N, the maximum bending stresses will be ~ 500 N / mm ² . When the center of the cross section of the string 1 is shifted from the axis of symmetry of the suspension 4 (FIGS. 4 and 5) by Δ, a transverse bending moment occurs along the width of the string 1, leading to bending stresses along the entire length of the string 1 in its plane. String 1 experiences a complex stress state - eccentric tension. Bending stresses at Δ = 0.1 mm will be ~ 900 N / mm ² and in combination with tensile stresses make the stresses uneven in cross section and length of the string 1, which affects the stability of the oscillation frequency. Bending stresses in the seals along the thickness of the string 1 occur if the surfaces of the attachment points of the string 1 do not lie in the same plane. With a non-flatness of ~ 0.03 mm, these stresses will be about 700 N / mm ² , and the string 1 at the exit from the terminations has inflections along the radius equal to 1-2 thicknesses of the string 1. About the insufficient accuracy of the installation of the string 1 relative to the suspension 4 and increased non-flatness of the terminations indicates frequency instability worse than 2 · 10 ^-6 . Finally, the quality of fixing the string 1 is judged by the magnitude of the change in the signal from the string 1 when applied to the load 3 in the place of attachment of the string 1 smoothly changing forces, causing an increase and decrease in the frequency of oscillations. The force is applied along the axis of the string 1 (shown in Fig. 1 by arrows) and is not less than ± mng, where m is the mass of the load 3, n is the overload, g is the acceleration of gravity. This corresponds to a change in the initial frequency f ₀ by ± kng (where k is the conversion coefficient). The signal from string 1 is controlled, for example, by an oscilloscope and should not change by more than 10%. This suggests the possibility of providing the specified accuracy characteristics of the accelerometer and its performance over the entire range of measured accelerations. If the signal change exceeds the specified tolerance, string 1 should be set more accurately. After ensuring the specified initial oscillation frequency of string 1 and the permissible signal change from string 1, thermomechanical aging of the accelerometer is carried out, which stabilizes the voltage in the string tension chain 1. This increases the frequency stability to 1 · 10 ^-7 , and this in turn increases the accuracy of the exhibition of the measuring axis and reduces the error in determining the conversion coefficient.

Thanks to the proposed method for adjusting the accelerometer, the stabilization time of the parameters, the assembly time are reduced, and the yield of suitable string accelerometers during manufacture is increased.

Claims (1)

A method for tuning a string accelerometer containing a rectangular cross-section string and a cantilever fixed plate suspension with a load, including fixing the ends of the string between two planes previously machined in two mutually perpendicular directions across and along the string, characterized in that the string is set along the suspension symmetry axis perpendicular to it planes, sequentially fix the ends of the strings on the load and the body when combining the mounting surfaces in one plane, compare the frequency the oscillations of the string to a predetermined length and adjusting if necessary the string from the expression:

,
where Δl is the change in string length;
f and f ₀ - the actual and given frequency of the string;
l and y are the length of the string and the deflection of the suspension when the string is located in the same plane, while the fastening surfaces are again machined to be located in the same plane, and the string length is reduced if the frequency is less than the specified one and increased if more, then applied to the load in the place of string attachment, the force smoothly changing the tension of the string in the working frequency range, and the change in the amplitude of the signal from the string is evaluated, ensuring that the string accurately sets the frequency and amplitude of the signal to the specified tolerance, after which o conduct thermomechanical aging of the accelerometer.

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