Abstract:
The invention relates to a method and a device for determining proportions of solid matter in a test material. In order to reliably and easily determine proportions of solid matter in a test material, even in the case of transparent matter or matter of a similar color to the test material, the test material is exposed to an electric field and dielectric properties of the field are determined with the test material present. Two electrical quantities are determined from the dielectric properties and combined, resulting in a characteristic value which is independent of the mass of the test material. The characteristic value is compared with a previously determined characteristic value for the matter in question and the proportion of solid matter is determined therefrom.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a method and a device for determining proportions of solid matter in a test material. 
     One example of a situation in which it is desirable to determine proportions of solid matter in a test material is in the detection of foreign matter and foreign fibers in a textile formation. In this case the detection is usually carried out by optical means. For example, foreign matter and foreign fibers are detected by their reflection properties, which in the majority of known cases differ from the reflection properties of a pure textile formation. The textile formation to be tested is therefore illuminated with light. The light absorbed by the test material and/or the reflected light is then detected. Instantaneous or local deviations of the received quantity of light give an indication of the proportion of desirable and undesirable matter. 
     A disadvantage of known methods and devices of this kind lies in the failure to detect colorless, transparent or translucent matter such as, for example, polypropylene sheets, or matter of a color which is similar to that of the textile formation, such as cables or cords, which are used when packing raw textile materials such as cotton, etc. This results in parts of such polypropylene sheets, cables and cords being subsequently processed with the raw material, with the result that these are later enclosed in a yarn, for example. 
     It is possible to detect solid matter such as polypropylene parts when spun in a yarn. In this respect it is assumed that this matter or these polypropylene parts change the structure of the yarn and, for example, the hairiness in a section of the yarn in particular. This is detected when measuring the diameter, for example by a change in the mass of the yarn or in the hairiness due to polypropylene parts projecting from the yarn instead of hairs. Attempts have therefore been made in this case to detect foreign parts by measuring the diameter, the mass or the hairiness. 
     One disadvantage of this detection of proportions of solid matter or of foreign matter by measuring the diameter or the hairiness lies in the fact that much of the foreign matter is not detected. This is primarily due to the fact that it is not located at the surface of the test material. As a result, unfavorable proportions of solid matter and other foreign matter give rise to an end product which is weakened or contains defects. A product of this kind may also be the cause of difficulties during subsequent processing, so that a defect-free end product cannot be produced. 
     SUMMARY OF THE INVENTION 
     The present invention solves the object of providing a method and a device by means of which proportions of solid matter in a test material can be easily and reliably determined, even in the case of transparent matter or matter of a similar color as the test material. 
     This result is achieved by exposing the test material to an electric field and determining dielectric properties of the field with the test material. In order to determine the dielectric properties of the field, electrical measurable quantities are measured, from which at least two electrical quantities are determined and combined, resulting in a characteristic value which is independent of the mass of the test material. The characteristic value is compared with comparative values, and information on the proportion of solid matter or the change thereof in the test material is obtained from the comparison. The dielectric properties can be detected on the basis of a plurality of electrical quantities. 
     A first possibility lies, for example, in determining, as an electrical quantity, the change in capacitance caused by the test material or the relative permittivity ∈ r  in an electric alternating field of at least two frequencies from measurable quantities such as voltage, current, phase shift between voltage and current and any reference resistances, and forming therefrom a quotient as a characteristic value. 
     A second possibility lies, for example, in determining, as a characteristic value, electrical quantities such as the power factor cos φ of the change in capacitance caused by the test material from measurable quantities such as voltage, current, phase shift between voltage and current and any reference resistances. 
     The characteristic value determined from the electrical quantities by, for example, forming a quotient, remains constant as long as the proportion of solid matter in the test material remains constant. Should the proportion change, this fact is indicated by a corresponding change in the characteristic value. The absolute proportion of solid matter in the test material may also be determined by forming a relation from the constant characteristic value and from the changed characteristic value. 
     A device for performing this method consists of at least one precision capacitor, which is disposed in the region of the test material to create an electric field, a frequency generator connected to the capacitor to generate at least one frequency in the electric field, measuring elements connected to the frequency generator for electrical measurable quantities, and an evaluation circuit for forming the electrical quantities and the characteristic value and for comparing the characteristic value with predetermined values. 
     In order to be able to cancel out the basic capacitance of the capacitor without the test material, a reference capacitor, connected to the same or to an inverted signal source, may be provided. The frequency generators preferably form a bridge circuit with the precision capacitor and the reference capacitor. 
     The dielectric properties of a field are represented by at least one quantity from a group of electrical quantities comprising capacitance, relative permittivity, loss angle and power factor. 
     The advantages achieved by the invention lie in particular in the fact that it enables the most varied foreign matter, compositions or proportions of solid matter in a test material to be detected, irrespective of whether certain matter is visible, invisible or of a similar color to the test material or whether it occurs inside or at the surface. The means provided for carrying out the method are of a simple structure and allow it to be combined problem-free with other measuring devices which measure other parameters in pursuit of other objects. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is illustrated in detail in the following on the basis of an example and with reference to the accompanying figures, in which: 
     FIG. 1 is a diagrammatic and simplified representation of a first device in accordance with the present invention; 
     FIGS. 2 and 3 are graphical representations of physical relations; and 
     FIGS. 4 to  6  are further embodiments of the device according to the invention. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 shows a first embodiment of a device according to the invention with a precision capacitor  1  for an elongate test material  2  that is moved longitudinally such as, for example, a yarn, roving, tape, filament, etc. The precision capacitor  1  is connected on one side, via a resistor  3 , to a frequency generator  4  and on the other side to a ground reference potential. One output  5  of the precision capacitor  1  is connected via an amplifier  6  to one input  7  of an evaluation circuit  8 . The same output  5  is also connected to one input  9  and, via the resistor  3 , to the other input  10  of an operational amplifier  11 , which is connected in parallel with the resistor  3 . The output of the operational amplifier  11  forms a further input  12  for the evaluation circuit  8 . The inputs  7  and  12  are respectively connected in the evaluation circuit  8  to measuring elements  13 ,  14 , which are known per se, for electrical quantities. The evaluation circuit  8  has an output  15 . 
     FIG. 2 is a graphical representation of characteristic signals along two axes  16  and  17 . Values for frequencies of an electric field are plotted along the axis  16  and values for the relative permittivity ∈ r  of the same field along the axis  17 . Curves  18 ,  19  and  20  represent the relative permittivity ∈ r  as a function of the frequency for fields with a test material of cotton with 68% moisture, of cotton with 47% moisture and of polypropylene. It can be seen here that the relative permittivity for polypropylene according to curve  20  is largely independent of frequency, while the relative permittivity for cotton is highly frequency-dependent, as shown by the curves  18  and  19 . The frequency dependency increases with the moisture content. As the change in capacitance of the precision capacitor changes proportionally with the relative permittivity, the same relative dependency on frequency applies to the change in capacitance. However the change in capacitance is at the same time also influenced by the mass of the test material. 
     FIG. 3 is a graphical representation of other characteristic signals with two axes  21  and  22 . Values for frequencies of an electric field are plotted along the axis  21  and values for the power factor cos φ of the test material, for different materials, along the axis  22 . Curves  23 ,  24  and  25  represent the power factor cos φ as function of the frequency for cotton with 68% moisture, for cotton with 47% moisture and for polypropylene as the test material. Whereas the power factor of polypropylene is largely independent of frequency, the power factor of cotton is highly frequency dependent. Here too the dependency increases with the moisture content of the cotton. However the power factor of the change in capacitance of the precision capacitor is independent of the mass of the test material. 
     FIG. 4 shows a second embodiment of a device according to the invention with two precision capacitors  35  and  36  for an elongate test material  37  that is moved longitudinally such as, for example, a yarn, roving, tape, filament, etc. The precision capacitors  35 ,  36  are connected on one side, via resistors  38 ,  39 , to a ground potential and on the other side to a respective frequency generator  40 ,  41 . The precision capacitors  35 ,  36  are connected via lines  42 ,  44  and  43 ,  45  to an evaluation unit  46  which, for example, is formed as a processor and has an output  47 . 
     FIG. 5 is a simplified representation of a third embodiment with a precision capacitor  50 , a reference capacitor  51  and a frequency generator  52 . A tap  53  lies between the capacitors  50  and  51 , and an inverting or non-inverting amplifier  54 ,  55 , is respectively associated with each. Also provided is an evaluation circuit  57 , which is connected via the tap  53  and a line  56 . 
     FIG. 6 is a simplified representation of a fourth embodiment with a precision capacitor  60 , a reference capacitor  61  and a frequency generator  62 . A tap  63  lies between the capacitors  60  and  61 , and each is associated with an inverting or non-inverting amplifier  64 ,  65 , respectively. Another frequency generator  66  is also connected in series with the frequency generator  62 . The tap  63  leads into an element  67  for frequency separation with two outputs  68 ,  69 . 
     The invention operates as follows: 
     In order to determine proportions of solid matter such as, for example, the proportion of polypropylene in a cotton yarn or cotton tape, the test material, e.g. the cotton yarn or cotton tape, is placed in an electric alternating field which is generated with alternating current at a certain frequency f 1 . This produces an electric alternating field whose dielectric properties are determined not just by measurable values such as current, voltage and phase angle, but also by the preset frequency. The dielectric properties may be expressed in particular by the relative permittivity ∈ r  and/or the power factor cos φ. It is also possible to repeat this operation in further electric alternating fields with further frequencies f 2 , f 3 , etc. and then also obtain second, third, etc. differing values for the relative permittivity ∈ r  and the power factor cos φ. 
     If the values which express the dielectric properties for the matter present in the test material are known separately, as represented, for example, in FIGS. 2 and 3, the proportion of the matter in the test material can be determined from these. There are various possibilities for this. 
     A first possibility lies in determining the change in capacitance AC of the precision capacitor, as a consequence of a test material introduced therein, for at least two frequencies f 1 , f 2 , . . . of the measuring field in the precision capacitor from the dielectric properties and the quantity of matter. This is effected on the one hand for pure test material such as polypropylene or cotton by calculation with values from FIG.  2  and on the other hand by measuring corresponding values of the dielectric properties at the precision capacitor with actual test material. Assuming that the test material consists of two types of matter, the change in capacitance of the precision capacitor with a test material consisting of just one of these types of matter in each case can be calculated using values from FIG.  2 . From this it is possible to determine the change in capacitance of the precision capacitor with pure and with actual test material at two or more frequencies. A quotient can be determined as the characteristic number from the determined changes in capacitance at different frequencies for the test material, this quotient ideally being 1 for polypropylene and greater than or less than 1 for cotton, for example. This characteristic number is independent of the mass of the test material and can be continuously monitored for actual test material by comparison with a reference value. The occurrence of a deviation means that the proportion of foreign matter in the test material has changed. Alternatively, the proportion of pure matter in the actual test material can be quantified from a formula, with the characteristic number for a pure test material and the characteristic number for the actual test material. 
     A second possibility lies in determining in a known manner, from the electrical measurable quantities which characterize an electric field, the power factor cos φ of the change in capacitance caused by the test material introduced into the field. This power factor is independent of the mass of the test material, which is why it is sufficient to determine the power factor for pure test material and the power factor for actual test material. A characteristic number which is proportional to the proportion of one of the types of matter in the test material can be determined from the two power factors according to a formula and can also be monitored. 
     In the device according to FIG. 1, the test material  2  in the precision capacitor  1  is exposed to an electric field with a frequency f 1 . The precision capacitor  1  is fed with appropriate alternating currents from the frequency generator  4  via the resistor  3 , resulting in an alternating field with a frequency f 1  in the gap in the precision capacitor  1 . The voltage at the output  5  of the precision capacitor  1  is amplified in the amplifier  6  and applied to the input  7 , so that this voltage is quantified in the measuring element  14  and then fed to the evaluation circuit  8 . The voltage across the resistor  3  is amplified in the operational amplifier  11  and fed via the input  12  to the measuring element  13 , where it is likewise quantified. Since the value of the resistor  3  is known, the evaluation circuit  8  can also calculate the current in the resistor  3  from this. The change in capacitance of the precision capacitor  1  caused by the test material can be determined according to laws which are known per se from this and from the fixed quantities known for the precision capacitor  1 . The power factor cos φ is also determined by measuring the phase difference between the current and the voltage. The values for the power factor cos φ of the change in capacitance are independent of the mass of the test material  2  in the precision capacitor  1 . This change in capacitance is determined in the evaluation circuit  8  using values from the measuring elements  13 ,  14  and other fixed inputs. The evaluation circuit  8  may be formed as an electric circuit or as a computer which digitally processes measured values and inputs. The evaluation circuit delivers via the output  15  a signal which indicates the presence of other solid matter in the test material or a changed proportion of one of the types of matter. A signal of this kind may be continuously delivered by the evaluation unit  8 , processed to form mean values therein and compared with mean values of this kind or other reference values. Distinct deviations then point to changes in the composition of the test material  2 . 
     In order to quantify the proportion of a solid matter in the test material  2 , it is assumed that the matter forming the test material  2  is known. For example, the test material  2  consists of cotton with 47% moisture and possibly polypropylene. Power factors for this matter at arbitrary frequencies f 1 , f 2  can be read from FIG.  3 . Assuming that the proportions of both types of matter together amount to 100% of the mass of the test material  2 , the proportion of the first matter can be determined according to the following formulae (1) and (2). 
     In the device according to FIG. 4 the test material is moved through two precision capacitors  35 ,  36  in succession, these producing fields of different frequencies f 1 , f 2 , as produced by a.c. voltages, which are supplied by the frequency generators  40 ,  41 . The evaluation unit  46  therefore receives values for currents and voltages at two different frequencies in parallel via the lines  42  to  45 . Changes in capacitance can be calculated from these values and, in turn, a characteristic number from these changes. All the values which are represented in FIG. 2 are stored as tables in the evaluation unit  46  for this purpose. The proportion of a first matter K 1  in the test material is therefore obtained from 
     
       
           K 1=( A−F 2)/( F 1− F 2).  formula (1) 
       
     
     The proportion of the second matter is obtained from 
     
       
           K 2=( A−F 1)/( F 2− F 1).  formula (2) 
       
     
     Here the values F 1  and F 2  are the ratios of the relative permittivities of cotton and polypropylene at the first frequency f 1  and at the second frequency f 2 . The factors F 1  and F 2  are therefore obtained by forming a quotient, for example, from the values 26/28 and 27/29 in FIG.  2 . The parameter A corresponds to the quotient of the changes in capacitance in the precision capacitor for the actual test material or the characteristic number already known. 
     In the device according to FIG. 5 an a.c. voltage with a frequency is produced in the frequency generator  52 , fed to the two amplifiers  54 ,  55  and from these applied to both capacitors  50 ,  51 . A phase shift of 180° is produced in the amplifier  55 , for example, by delaying the signal, so that the capacitors  50 ,  51  are each supplied with a signal  70 ,  71  which cancel each other out. A zero voltage is thus present at the tap  53  at least when there is no test material in the precision capacitor  50 . All influences of the empty capacitors are in this way neutralized. The signal at the tap  53  changes as soon as a test material is introduced into the precision capacitor  50 . The cosine of the phase angle of the signal is the inverse of the power factor cos φ of the test material in the precision capacitor  50  and independent of the quantity of test material. The mixture ratio of the two types of matter in the test material can be determined from the power factor by a simple rule of allegation with the values from FIG. 3 for the frequency of the applied a.c. voltage. This takes place in the evaluation circuit  57 , which also receives via the line  56  information on the phase angle of the unamplified signal, which is not influenced by the capacitors  50  and  51 , of the frequency generator  52 . 
     The operations in the device according to FIG. 6 are identical to those in the device according to FIG. 5, although with the difference that a signal with two superimposed frequencies is applied to the capacitors  60 ,  61  by the two frequency generators  62 ,  66 , which signal is resolved into its two frequencies in the element  67 . The ratio of the two components in the test material can be determined as is known for the device according to FIG.  4 . This construction, as well as that according to FIG. 5, has the advantage with respect to the construction according to FIG. 4 of the influence of the empty capacitor being compensated by the bridge circuit principle. The signal voltage is zero when the precision capacitor is empty and this voltage changes proportionally with the dielectric properties of the test material and the quantity thereof. 
     As this method enables the proportions of solid matter in a test material to be determined, a prerequisite is for proportions of non-solid matter, such as water, in one of the types of matter for measurement to be constant. For example, the moisture in the cotton must be known and constant before measuring takes place. However this is the case in the textile industry anyway, for most operations relating to the processing of raw materials take place in air-conditioned spaces. 
     The capacitive detection of foreign matter and the proportions thereof in a test material may be combined with known measurements of other parameters such as, e.g. uniformity, mass, etc., as a signal can be used both for measuring such parameters and for detecting the proportions of foreign matter. As mentioned above, the proportions of foreign matter can be determined by calculating the capacitance at a plurality of frequencies or by calculating the power factor at just one frequency. However it is also possible to do this by calculating the power factor for a plurality of frequencies. Suitable frequencies are, for example, 10 kHz to 100 kHz and 10 MHZ.