运用声学的方法和装置测量棉纤维中的麦克隆指数

运用声学的方法和装置测量棉纤维中的麦克隆指数

In 1993 the new state standard RSt Uz 604-93 “Сotton Fibre. Technical conditions” was implemented in Republic of Uzbekistan. In accordance with this standard the micronaire index was added to the number of the important characteristics of cotton fiber, which widely used on the world trade and characterizing the linear density and maturity of the fibre.

The base range of a micronaire index in world trade of Upland cotton is 3,5 - 4,9. If the value is below 3.5 and more than 4,9 the discount is made from the price of cotton fibre. In this regard, it is important to control instrumentally a micronaire index at preparation and processing of seed cotton, and also at developing new varieties of cotton.

For definition of a micronaire index in the world practice a method of air permeability is used, which realized in such devices as "МIcronaire", "Fibrofine", "Portar" etc. These devices are also entered into HVI measuring systems.

Recently in various industries the acoustic devices are used for nondestructive testing of quality of fibrous materials.

Devices that use acoustic methods are simple in design and reliable in operation. For accuracy, they are not inferior to similar devices using, for example, the method of air permeability of samples of cotton fiber in the air stream.

In [1] it is shown, that by using the acoustic method it is possible to determine the grade of a seed cotton and cotton fibre by breaking load of fiber. At the same time the dependence of acoustic attenuation in a sample of fibers from thEir breaking load was used.

In [2], the results of studies of the phase difference depending on sound waves passing through a sample of cotton fiber from the micronaire index, the coefficient of linear density and maturity of cotton fibers are shown.

This paper discusses the use of acoustic method for measuring of micronaire index from the damping of acoustic oscillations.

The basis of the acoustic measurement method is the process of distribution of acoustic vibrations in a sample of cotton, placed in the chamber and compacted in the form of a cylindrical tube with a flat base. The direction of passage of sound fluctuations is perpendicular to the bases. A sample of fibers is possible to simulate a two-component isotropic porous medium (fiber, air). With the passage of a plane sound wave through such an environment there will be loss of energy due to friction of the fiber, which will lead to a change in the pressure amplitude of sound vibrations.

Figure 1.The appearance of acoustic micronaire instrument

Figure 2. The block diagram of acoustic micronaire instrument

The device operates as follows. The output signal measured by unit 6 after passing through the sample connected with the parameters of the fiber by the ratio [1]:

; (1)

; (2)

Where U0 voltage at the microphone in the absence of the sample, mV;

a - damping coefficient of acoustic waves, 1/m;

L - thickness of the fibrous mass, m;

Т - linear density, mtex;

B - a constant factor;

e – porosity of the sample;

f - frequency of sound vibrations, Hz.

Linear density is connected with the micronaire index through a ratio[3]:

; (3) ; (4)

where

(5)

By logarithming the equation (4), we found

(6)

or

(7)

where b0=ln U0.

From the formula (7) we can see that linear regression relationship exists between a signal on an exit of a microphone at testing sample and micronaire index in logarithmic scale.

This equation was used in the regression analysis in the construction of the instrument calibration graph.