FIELD OF THE INVENTION
The invention relates to a method and device for determining the presence of contaminants in an elongated textile material, in particular by determining electrical properties of the textile material, i.e. for detecting foreign substances in a strand-like textile material, as described in the preamble of the corresponding independent claims.
BACKGROUND OF THE INVENTION
US 6,346,819 discloses a method for determining the proportion of foreign materials in a test yarn. The test yarn is exposed to an electrical field and dielectric properties of the field are determined, from which a characteristic value independent of the mass of the test material is computed. The characteristic value is compared to a characteristic value obtained with a different measurement frequency, and the proportion of foreign material is determined from this comparison. The method therefore must operate at two different frequencies, which complicates the signal generation and analysis circuitry.
DESCRIPTION OF THE INVENTION
It is therefore an object of the invention to create a method and device for determining the presence of contaminants in an elongated textile material of the type mentioned initially, which overcomes the disadvantages mentioned above.
These objects are achieved by a method and device for determining the presence of contaminants in an elongated textile material, according to the corresponding independent claims.
The inventive method device for determining the presence of contaminants in an elongated textile material comprises the steps of
• moving the elongated textile material through a measurement capacitor;
• supplying an excitation signal to a first terminal of the measurement capacitor;

• determining a first component of the voltage at a second terminal of the
measurement capacitor, which first component is in phase with the excitation
signal and which represents the electrical equivalent of the mass on the
I
elongated textile material;
• determining a second component of the voltage at the second terminal of the measurement capacitor, which second component is shifted in phase with regard to the excitation signal and which represents the Loss Tangent Output of the Yarn Capacitor; and
• determining, from said first and second components, a material content of the elongated textile material; and
• determining, from the material content, the presence of a contaminant in the elongated textile material.
For a capacitor formed from a lossy dielectric material, the loss tangent is the ratio at any particular frequency between the real and imaginary parts of the impedance of the capacitor. A large loss tangent means that there is a high dielectric absorption. Instead of using the dielectric loss for detecting mass variations, it is used here to detect the presence of e.g. polypropylene contaminants in cotton yarn.
The capacitor formed by the yarn in a test gap can be modelled as a lossless capacitor C with a shunt resistor R representing the loss term, ignoring the series parasitic inductance and resistance. The Capacitor Dissipation Factor, also called Loss tangent, or tan 9, is the ratio of the capacitive reactance Xc to resistance R, that is
Loss tangent = tan 9 = Xc / R = 1 / co RC
Polypropylene is a nearly-perfect dielectric, or nearly lossless. The loss tangent of polypropylene corresponds to an angle of less than 0.01 degrees. This is relatively distinct to that of cotton and this difference in properties is measured: Cotton yarn is not a particularly good capacitor, so a degree or so of loss tangent is expected.

Naturally, the same principle will work for other contaminants such as polyethylene etc. that exhibit a dielectric loss that significantly differs from that of the base yarn.
The loss tangent information appears as a high value resistance across the capacitor being measured. In a preferred embodiment of the invention, the capacitor is excited with a periodic excitation signal, and an output signal of the capacitor is analysed component-wise. Any nonzero loss tangent changes the phase shift between the output signal component and the excitation signal. The loss tangent component is determined by synchronous demodulation of the output signal with a switching signal having a 90 degrees phase shift with respect to the excitation signal. The amplitude of this demodulated signal is proportional to the conductance, the reciprocal of resistance, corresponding to the loss tangent. An ideal capacitor will have zero conductance.
Thus, in more detail, the signal components representing the capacitor output signal are determined by
• determining the first component of the voltage at the second terminal of the measurement capacitor by demodulating a signal corresponding to said voltage synchronously with the excitation signal; i.e. by synchronous demodulation with respect to the excitation signal
• determining the second component of said voltage by demodulating the signal corresponding to said voltage with a constant phase shift with respect to the excitation signal, i.e. by synchronous demodulation with a phase shifted signal with respect to the excitation signal. Pref^rably, the absolute value of this phase shift is at least approximately 90 degrees.
The synchronously-demodulated 0 and 90 degree signal are preferably further filtered with low-pass filters.
The analog and/or digital processing circuits are also synchronised with the Loss Tangent measurement circuits so that they are in a known phase relative to the

excitation clock. In particular, an Analog-Digital Converter ADC used e.g. to digitise the sannpled signals for filtering, is preferably synchronised with the excitation frequency. As a result, filtering requirements are relaxed and out of band signals that might appear in the evaluated signal are eliminated.
A further preferred variant of the invention comprises the step of computing the abovementioned material factor MF as being proportional to
MF =(TM-LT)/TM where TM is the first component value and LT is the second component value.
In a further preferred embodiment of the invention, the loss tangent information is obtained by using a bridge circuit and determining the imbalance created thereupon. The capacitance signal is determined by exciting the bridge with a periodic signal having a reference phase of zero degrees. The bridge imbalance is determined by synchronously demodulating the bridge voltage with a phase shift of zero degrees with respect to the exciting signal. This demodulated signal is fed back to balance the bridge. The loss tangent signal is determined by synchronously demodulating the bridge voltage with a phase shift of preferably 90 degrees.
In more detail, the bridge is excited by supplying a further excitation signal to a first terminal of a reference capacitor whose second terminal is connected to the second terminal of the measurement capacitor. This connection point is the central terminal of the measurement bridge. The further excitation signal has the same shape as the excitation signal but with inverted polarity. The bridge is balanced by controlling the excitation signal and the further excitation signal to drive the first component of the voltage at the second terminal of the measurement capacitor to zero. In particular, the relative amplitude of the two excitation signals is adjusted to balance the bridge.
The device for determining the presence of contaminants in an elongated textile material comprised a measurement capacitor through which the elongated textile material can be moved; a first voltage source for supplying an excitation signal

connected to a first terminal of the measurement capacitor; a Tirst demodulator, for determining a first component of the voltage at a second terminal of the measurement capacitor, which first component is in phase with the excitation signal; a second demodulator, for determining a second component of the voltage at the second terminal of the measurement capacitor, which second component is shifted in phase with regard to the excitation signal; an evaluation circuit for computing, from said first component representing an electrical equivalent of the Mass and the second component representing the Loss tangent measured of the elongated material, combines both the components for determining the material content.
In a preferred embodiment of the invention, the first demodulator is configured to demodulate the voltage at the second terminal of the measurement capacitor synchronously with the excitation signal, and the second demodulator is configured to demodulate said voltage with a constant phase shift with respect to the excitation signal.
In a further preferred embodiment of the invention, the device comprises a second voltage source for supplying a further excitation signal to a first terminal of a reference capacitor whose second terminal is connected to the second terminal of the measurement capacitor, where the further excitation signal has the same shape as the excitation signal but with inverted polarity.
In a further preferred embodiment of the invention, the evaluation circuit comprises a comparison element for comparing the material factor to a predetermined reference value and for indicating the presence of contaminant when the difference between the material factor and the reference value exceeds a predetermined error limit. The predetermined reference value is determined e.g. by taking measurements with an uncontaminated reference yarn, determining the material factor each time and averaging. Furthermore, a long term average of the material factor may be determined during operation of the device and be used as a reference. This allows to compensate for slowly changing environmental conditions and e.g. yarn humidity.

The signal processing, from the measured voltage at the common terminal to the comparison with the reference value may be implemented by analog or digital signal processing means or a combination thereof.
In summary, the method for determining the presence of contaminants in an elongated textile material (20) comprises the steps of
• moving the elongated textile material (20) through a measurement capacitor
(1);
• supplying an excitation signal to a first terminal of the measurement capacitor (1);
• determining a first component of the voltage at a second terminal (6) of the measurement capacitor (1), which first component is in phase with the excitation signal and which represents the electrical equivalent of the mass;
• determining a second component of the voltage at the second terminal (6) of the measurement capacitor (1), which second component is shifted in phase with regard to the excitation signal and which represents the Loss Tangent of the Yarn Capacitor;
• determining, from said first and second components, a material content of the elongated textile material (20); and
• determining, from the material content, the presence of a contaminant in the elongated textile material.
In a preferred embodiment of the invention, the method further comprises the steps of
• determining the first component of the voltage at the second terminal (6) of the measurement capacitor (1) by demodulating a signal corresponding to said voltage synchronously with the excitation signal; and
• determining the second component of said voltage by demodulating the signal corresponding to said voltage with a constant phase shift with respect to the excitation signal.

In a preferred embodiment of the invention, the method further comprises the step of computing the material factor as being proportional to
MF =(TM-LT)/TM where TM is the first component value and LT is the second component value.
In a preferred embodiment of the invention, the absolute value of the phase shift for determining the second component is at least approximately 90 degrees.
In a preferred embodiment of the invention, the method further comprises the step of
• supplying a further excitation signal to a first terminal of a reference capacitor
(2) whose second terminal is connected to the second terminal (6) of the
measurement capacitor (1), where the further excitation signal has the same
shape as the excitation signal but with inverted polarity.
In a preferred embodiment of the invention, the method further comprises the step of
• controlling the excitation signal and the further excitation signal to drive the
first component of the voltage at the second terminal (6) of the measurement
capacitor (1) to zero.
The device for determining the presence of contaminants in an elongated textile material (20), comprises
• a measurement capacitor (1) through which the elongated textile material (20) can be moved;
• a first voltage source (3) for supplying an excitation signal connected to a first terminal of the measurement capacitor (1);
• a first demodulator (12, 14) for determining a first component of the voltage at a second terminal (6) of the measurement capacitor (1), which first component is in phase with the excitation signal;

• a second demodulator (11, 13) for determining a second component of the voltage at the second terminal (6) of the measurement capacitor (1), which second component is shifted in phase with regard to the excitation signal;
• an evaluation circuit (19) for computing, from said first and second components, a material content of the elongated textile material (20); and
• a comparison element for determining the presence of a contaminant in the elongated textile material (20) from the material content.
In a preferred embodiment of the invention, the first demodulator is configured to demodulate the voltage at the second terminal (6) of the measurement capacitor (1) synchronously with the excitation signal, and the second demodulator is configured to demodulate said voltage with a constant phase shift with respect to the excitation signal.
In a preferred embodiment of the invention, the evaluation circuit (19) is configured to compute the material factor as being proportional to
MF =(TM-LT)/TM where TM is the first component value and LT is the second component value.
In a preferred embodiment of the invention, the device further comprises a second voltage source (4) for supplying a further excitation signal to a first terminal of a reference capacitor (2) whose second terminal is connected to the second terminal (6) of the measurement capacitor (1), where the further excitation signal has the same shape as the excitation signal but with inverted polarity.
BRIEF DESCRIPTION OF THE DRAWINGS
The subject matter of the invention will be explained in more detail in the following text with reference to preferred exemplary embodiments which are illustrated in the attached drawings, in which:

Figure 1 schematically shows a block diagram of a device according to the invention.
The reference symbols used in the drawings, and their meanings, are listed in summary form in the list of reference symbols. In principle, identical parts are provided with the same reference symbols in the figures.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
Figure 1 schematically shows a block diagram of a device for determining the presence of contaminants in an elongated textile material, e.g. a yarn 20. A measurement capacitor 1 and a reference capacitor 2 are arranged in a series configuration, forming a measurement bridge. Ideally, the measurement capacitor 1 and reference capacitor 2 are of identical build and have the same parameters, compensating for variations in environmental conditions. A first voltage source 3 is arranged to provide, with respect to a reference or ground 5 an excitation signal or voltage to a first terminal of the measurement capacitor 1. A second voltage source 4 is arranged to provide, with respect to a reference or ground 5 an inverted excitation signal or voltage to a first terminal of the reference capacitor 2. The excitation signal and the inverted excitation signal are identical and only differ in polarity. Typically, they are square waves with frequencies e.g. at 100 kHz. As a result, the voltage in the middle of the bridge, i.e. at a common terminal 6 joining a second terminal of the measurement capacitor 1 and a second terminal of the reference capacitor 2, is zero for a perfectly balanced bridge.
The voltage of the common terminal 6 is amplified by an AC amplifier 7. The AC amplifier 7 comprises a positive amplifier output 8 and in negative amplifier output 9 of the amplified signal. For demodulating the amplified signal, electronic or solid state switches are provided which alternately connect to the positive amplifier output 8 or the negative amplifier output 9. A first switch 11 and a second switch 12 are controlled to do so synchronously and with a fixed phase shift with respect to the excitation signal. The phase shift of the first switch 11 and the phase shift of the

second switch 12 differ by 90 degrees. Thus, the first switch 11 is controlled by the same clock 10 as the first voltage source 3 and second voltage source 4, and the second switch 12 via a 90 degrees phase shift 17 element shifting the clock signal.
The output signals of the switches are filtered by respective low pass filters 13, 14, eliminating residual switching pulse spikes. The first switch 11 and first low pass filter 13 together constitute a first synchronous demodulator with a capacitance output 15 signal, and the second switch 12 and second low pass filter 14 together constitute a second synchronous demodulator with a loss tangent output 16 signal. The output of a synchronous demodulator is proportional to the amplitude of the signal component having the same period as the switching clock, and also proportional to the phase difference between said signal component and the switching clock.
The output of the first low pass filter 13 is coupled back to the excitation sources 3, 4 for a closed loop control to maintain the charge balance in the bridge output. The order of the feedback is fixed in the feedback attenuation network so that the overall frequency response of the whole system is well under limits to detect the smallest capacitance change resulting in the capacitive sensor. The capacitance value measured for a given test material - which is dependent on the mass of the test material - should not saturate the signal to the rails. The Minimum and the Maximum capacitance possible with different test material expected to be tested in the system with their inherent mass variations are calculated to determine the feedback attenuation level.
The output of the first low pass filter 13 is at zero degrees and is zero for an ideal perfectly matched measuring and reference capacitive sensors. Othenwise, the output will be proportional to
TM = Vs * ((Cm - Cr) / (Cm + Cr)) * (1 / Feedback Attenuation) with

TM - Capacitance Output in Voltage, mVolts
Cm - Measuring Capacitance in Farads
Cr - Reference Capacitance in Farads
Vs - peak to peak voltage of a rectangular source signal
Feedback Attenuation - as determined by the circuit components
The function of capacitance measurement output TM has the unit volts as the charge sensitive amplifier 7 converts the capacitive charge changes due the presence of the material in the capacitive sensor 1 electrostatic field to a voltage. Thus this signal value TM is proportional to the mass of the yarn, present in the measurement capacitor 1. This can be used to determine mass variations or imperfections in the yarn 20.
The loss tangent output LT for referencing composition of materials is measured after 90 degrees through the second synchronous demodulator 12, 14. The measured value LT is the output of the second low pass filter 14 and is again a capacitor referenced value or dependent on mass as the first value but represents the rush of charges at exactly 90 degree phase. The capacitance outputs, including Total Mass TM, and the Loss Tangent output are combined to generate contaminant content, e.g. polypropylene (PP) content PC as
PC = TM - LT
The percentage of PP Contamination can then be calculated as
%PP = (PC/TM)*100
While the invention has been described in present preferred embodiments of the invention, it is distinctly understood that the invention is not limited thereto, but may be othenA/ise variously embodied and practised within the scope of the description.

LIST OF DESIGNATIONS
1 measurement capacitor
2 reference capacitor
3 first voltage source
4 second voltage source
5 ground
6 common terminal
7 AC amplifier
8 positive amplifier output
9 negative amplifier output
10 clock, SyClk
11 first switch
12 second switch
13 first low pass filter, LP1
14 second low pass filter, LP2
15 capacitance output
16 loss tangent output
17 90 degrees phase shift, D90°
18 feedback block, FB
19 evaluation circuit
20 yarn

WE CLAIM :
1. A method for determining the presence of contaminants in an elongated textile
material (20), comprising the steps of
• moving the elongated textile material (20) through a measurement capacitor
(1);
• supplying an excitation signal to a first terminal of the measurement capacitor (1);
• determining a first component of the voltage at a second terminal (6) of the measurement capacitor (1), which first component is in phase with the excitation signal and which represents the electrical equivalent of the mass on the elongated textile material (20);
• determining a second component of the voltage at the second terminal (6) of the measurement capacitor (1), which second component is shifted in phase with regard to the excitation signal and which represents the Loss Tangent Output of the Yarn Capacitor;
• determining, from said first and second components, a material content of the elongated textile material (20); and
• determining, from the material content, the presence of a contaminant in the elongated textile material (20).
2. The method of claim 1, further comprising the steps of
• determining the first component of the voltage at the second terminal (6) of the measurement capacitor (1) by demodulating a signal corresponding to said voltage synchronously with the excitation signal; and
• determining the second component of said voltage by demodulating the signal corresponding to said voltage with a constant phase shift with respect to the excitation signal.
3. The method of claim 1, further comprising the step of computing the material
factor as being proportional to

MF =(TM-LT)/TM where TM is the first component value and LT is the second component value.
4. The method of claim 1, wherein the absolute value of the phase shift for determining the second component is at least approximately 90 degrees.
5. The method of claim 1, further comprising the step of
• supplying a further excitation signal to a first terminal of a reference capacitor
(2) whose second terminal is connected to the second terminal (6) of the
measurement capacitor (1), where the further excitation signal has the same
shape as the excitation signal but with inverted polarity.
6. The method of claim 5, further comprising the step of
• controlling the excitation signal and the further excitation signal to drive the
first component of the voltage at the second terminal (6) of the measurement
capacitor (1) to zero.
7. A device for determining the presence of contaminants in an elongated textile
material (20), comprising
• a measurement capacitor (1) through which the elongated textile material (20) can be moved;
• a first voltage source (3) for supplying an excitation signal connected to a first terminal of the measurement capacitor (1);
• a first demodulator (12, 14) for determining a first component of the voltage at a second terminal (6) of the measurement capacitor (1), which first component is in phase with the excitation signal;
• a second demodulator (11, 13) for determining a second component of the voltage at the second terminal (6) of the measurement capacitor (1), which second component is shifted in phase with regard to the excitation signal;
• an evaluation circuit (19) for computing, from said first and second components, a material factor of the elongated textile material (20); and

• a comparison element for determining the presence of a contaminant in the elongated textile material (20) from the material factor.
8. The device of claim 7, wherein the first demodulator is configured to demodulate
the voltage at the second terminal (6) of the measurement capacitor (1)
synchronously with the excitation signal, and the second demodulator is
configured to demodulate said voltage with a constant phase shift with respect
to the excitation signal.
9. The device of claim 7, wherein the evaluation circuit (19) is configured to
compute the material factor as being proportional to
MF =(TM-LT)/TM where TM is the first component value and LT is the second component value.
10. The device of claim 7, further comprising a second voltage source (4) for
supplying a further excitation signal to a first terminal of a reference capacitor,(2)
whose second terminal is connected to the second terminal (6) of the
measurement capacitor (1), where the further excitation signal has the same
shape as the excitation signal but with inverted polarity.