CN112697124A - Square wave quadrature demodulation implementation method and device of closed-loop resonant optical gyroscope - Google Patents
Square wave quadrature demodulation implementation method and device of closed-loop resonant optical gyroscope Download PDFInfo
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Abstract
The invention discloses a square wave quadrature demodulation implementation method and device of a closed-loop resonant optical gyroscope, and belongs to the technical field of optical sensing and signal detection. Detecting light output by the optical resonant cavity by using a photoelectric detector, and then carrying out orthogonal demodulation on a signal output by the photoelectric detector through a square wave orthogonal demodulation module to obtain an orthogonal demodulation signal; the square wave orthogonal demodulation module comprises two channels, and in the first channel, a signal output by the photoelectric detector obtains a first demodulation signal according to a first channel reference signal; in the second channel, the signal output by the photoelectric detector obtains a second demodulation signal according to a second channel reference signal; and obtaining a final output square wave quadrature demodulation signal according to the difference value of the first demodulation signal and the second demodulation signal. The invention can effectively inhibit the problem of performance reduction of the closed-loop resonant optical gyro system caused by phase fluctuation of the square wave signal, and improves the precision of the optical gyro.
Description
Technical Field
The invention relates to the technical field of optical sensing and signal detection, in particular to a square wave quadrature demodulation implementation method and device of a closed-loop resonant optical gyroscope.
Technical Field
The resonant optical gyroscope is a rotation angular velocity measuring sensor based on the Sagnac effect and taking an optical ring-shaped resonant cavity as a core sensitive element.
The phase modulation and demodulation technology can be used for detecting the signal of the resonant optical gyro, and the technology is widely applied to the resonant optical gyro system. In order to improve the accuracy of the resonant optical gyro system, methods of phase modulation and demodulation using various modulation waveforms have been proposed and applied to the resonant optical gyro system, including sine wave modulation, triangular wave modulation, sawtooth wave modulation, and the like. In an actual resonant optical gyro system, the phase of a signal to be demodulated fluctuates due to the influence of factors such as ambient temperature, and the like, so that the output performance of the gyro is influenced. The application of the sine wave quadrature demodulation technology in the open-loop resonant optical gyroscope effectively inhibits the influence of the phase fluctuation of the signal to be demodulated on a gyroscope system.
Compared with an open-loop detection system, the closed-loop detection system of the resonant optical gyroscope has higher linearity and larger dynamic range, so that the realization of closed-loop detection has important significance for improving the performance of the resonant optical gyroscope. The bipolar sawtooth wave phase modulation and demodulation technology is one of important methods for realizing system closed loop detection.
In the prior art, a closed-loop resonant optical gyro system based on digital sine-bipolar sawtooth phase modulation is reported, and the closed-loop system adopts two LiNbO3A phase modulator and employs bipolar sawtooth phase modulation in only one loop. When the bipolar sawtooth phase modulation is adopted, the square wave signal output after modulation needs to be synchronously demodulated. In an actual closed-loop resonant optical gyro system, the phase of a square wave signal to be demodulated can fluctuate under the influence of factors such as ambient temperature and the like. The phase fluctuation of the square wave signal can affect the slope of a demodulation curve, so that disturbance is introduced to closed loop locking, and the performance of the gyroscope is affected finally. However, at present, the deep research on the effect of reducing the phase fluctuation of the square wave signal by adopting the quadrature demodulation technology in the closed-loop resonant optical gyro system has not been carried out yetSee reports.
Disclosure of Invention
The invention provides a square wave quadrature demodulation implementation method and device of a closed-loop resonant optical gyroscope, aiming at the problem of phase fluctuation of a square wave signal to be demodulated caused by changes of environmental factors such as temperature and the like in a closed-loop resonant optical gyroscope system. The invention analyzes the influence of the phase fluctuation of the square wave signal on the demodulation curve, provides a method for realizing square wave orthogonal demodulation in the closed-loop resonant optical gyroscope, optimizes the square wave orthogonal demodulation method by analyzing the characteristics of the square wave orthogonal demodulation adopted in the closed-loop resonant optical gyroscope system, can effectively inhibit the problem of performance reduction of the closed-loop resonant optical gyroscope system caused by the phase fluctuation of the square wave signal, and improves the precision of the optical gyroscope.
In order to achieve the purpose, the invention adopts the following technical method:
one of the purposes of the invention is to provide a square wave orthogonal demodulation implementation method of a closed-loop resonant optical gyroscope, which comprises the steps of detecting light output by an optical resonant cavity by using a photoelectric detector, and then carrying out orthogonal demodulation on a signal output by the photoelectric detector through a square wave orthogonal demodulation module to obtain an orthogonal demodulation signal;
the square wave orthogonal demodulation module comprises two channels, and in the first channel, a signal output by the photoelectric detector obtains a first demodulation signal according to a first channel reference signal; in the second channel, the signal output by the photoelectric detector obtains a second demodulation signal according to a second channel reference signal; and obtaining a square wave quadrature demodulation signal which is finally output according to the first demodulation signal and the second demodulation signal.
The second channel reference signal is orthogonal to the first channel reference signal.
Another objective of the present invention is to provide a square wave quadrature demodulation implementation apparatus for implementing a closed-loop resonant optical gyroscope of the above method, where the apparatus includes at least one square wave quadrature demodulation module, and the square wave quadrature demodulation module is used to implement the square wave quadrature demodulation implementation method.
The invention has the following beneficial effects:
1) the invention analyzes the influence of the phase fluctuation of the square wave signal on the demodulation curve, namely in the actual closed-loop resonant optical gyro system, the fluctuation of the slope of the demodulation curve can be caused by the phase fluctuation of the square wave signal, and the performance of the closed-loop resonant optical gyro system is finally influenced.
Based on this, the present invention adopts the method of quadrature demodulation in the closed-loop RFOG for suppressing the demodulation curve fluctuation caused by the phase fluctuation.
2) The invention analyzes the phase working point theta in the quadrature demodulation process by adopting square waves0The influence on the demodulation curve is found that the phase error delta theta is in the fluctuation range of-pi/4 rad to pi/4 rad, and the theta is measured0After adjusting from 0 to pi/4 rad, the quadrature demodulation signal can be represented by a unique expression VI-VQIt is obtained that the determination of the sign of Δ θ is no longer necessary, and thus the fluctuation of the demodulated signal in the vicinity of the resonance point due to the symbol misdetermination can be avoided.
Based on this, the invention proposes to set the phase working point to pi/4 rad in the square wave quadrature demodulation method adopted by the closed-loop RFOG. The optimized square wave orthogonal demodulation method ensures that the result of square wave demodulation is not influenced by phase fluctuation, and effectively inhibits the performance reduction of the closed-loop resonant optical gyroscope caused by the phase fluctuation; meanwhile, the fluctuation of the demodulation signal caused by symbol misjudgment in the square wave orthogonal demodulation process is also inhibited. The square wave quadrature demodulation method effectively improves the precision of the optical gyroscope.
3) According to the invention, through the demodulation curve test, the zero-offset stability test and the long-time effectiveness test before and after square wave quadrature demodulation, the slope fluctuation of the demodulation curve is inhibited after the quadrature demodulation method is adopted, and the slope fluctuation is almost kept near the maximum value, the zero-offset stability is effectively improved, and the effectiveness is better achieved for a long time.
Drawings
FIG. 1 is a device structure of a closed-loop resonant optical gyroscope based on sine-bipolar sawtooth phase modulation;
FIG. 2 is a graph of simulation results of demodulation curves at different phase differences;
FIG. 3 shows the simulation results of the slope of the demodulation curve under different phase differences;
FIG. 4 is a square wave quadrature demodulation schematic;
FIG. 5 is a diagram of an orthogonal demodulation algorithm;
FIG. 6 is V at different Δ θF_NA simulation result graph related to delta f';
FIG. 7 is a diagram of a simulation result of a demodulation curve under correct judgment and incorrect judgment;
FIG. 8 is a diagram of an optimized square wave quadrature demodulation algorithm;
fig. 9 shows the test results of the demodulation curves before and after the square wave quadrature demodulation in example 1, (a) square wave-free quadrature demodulation, and (b) square wave quadrature demodulation;
FIG. 10 shows θ in example 20A demodulation curve test result at 0 and pi/4 rad;
fig. 11 is an Allan variance analysis result of the system 1h output signal test data before and after the square wave quadrature demodulation method is optimized in embodiment 3;
FIG. 12 is the results of Allan analysis of variance of the output signal test data of system 1h in example 3 without the square wave quadrature demodulation method;
FIG. 13 shows the results of Allan ANOVA of the output signal test data of system 6h under different conditions in example 4.
In the figure: the tunable semiconductor laser comprises a tunable semiconductor laser 1, an isolator 2, a light splitting coupler 3, a first loop phase modulator 4, a second loop phase modulator 5, a first loop sine wave modulation signal generation module 6, a second loop bipolar sawtooth wave modulation signal generation module 7, an input end coupler 8, an optical resonant cavity 9, an output end coupler 10, a first loop photoelectric detector 11, a second loop photoelectric detector 12, a first loop phase-locked amplifier 13, a second loop phase-locked amplifier 14, a first loop servo control module 15, a second loop servo control module 16 and a low-pass filter module 17.
Detailed Description
For a better understanding of the present invention, the technical means of the present invention will be described in detail below with reference to the accompanying drawings and specific embodiments.
The invention provides a square wave quadrature demodulation implementation method of a closed-loop resonant optical gyroscope, aiming at the problem of phase fluctuation of a square wave signal to be demodulated caused by changes of environmental factors such as temperature and the like in a closed-loop resonant optical gyroscope system. Detecting light output by the optical resonant cavity by using a photoelectric detector, and then carrying out orthogonal demodulation on a signal output by the photoelectric detector through a square wave orthogonal demodulation module to obtain an orthogonal demodulation signal;
the square wave orthogonal demodulation module comprises two channels, and in the first channel, a signal output by the photoelectric detector obtains a first demodulation signal according to a first channel reference signal; in the second channel, the signal output by the photoelectric detector obtains a second demodulation signal according to a second channel reference signal; and obtaining a square wave quadrature demodulation signal which is finally output according to the first demodulation signal and the second demodulation signal.
The second channel reference signal is orthogonal to the first channel reference signal.
The invention also provides a square wave quadrature demodulation implementation device of the closed-loop resonant optical gyroscope, which comprises at least one square wave quadrature demodulation module, wherein the square wave quadrature demodulation module is used for implementing the square wave quadrature demodulation implementation method. That is to say, the square wave quadrature demodulation module comprises two channels, each channel comprises a square wave demodulation module, and in the first channel, the signal output by the photodetector obtains a first demodulation signal according to a first channel reference signal; in the second channel, the signal output by the photoelectric detector obtains a second demodulation signal according to a second channel reference signal; and obtaining a final output square wave quadrature demodulation signal according to the difference value of the first demodulation signal and the second demodulation signal.
The method of the present embodiment is described based on the closed-loop resonant optical gyro shown in fig. 1, but the present invention is not limited to the closed-loop resonant optical gyro having this configuration, and is also applicable to closed-loop resonant optical gyros having other configurations.
In an embodiment of the present invention, a device structure of a closed-loop resonant optical gyroscope using sine-bipolar sawtooth phase modulation, as shown in fig. 1, includes a tunable semiconductor laser 1, an isolator 2, a light splitting coupler 3, a first loop phase modulator 4, a second loop phase modulator 5, a first loop sine wave modulation signal generation module 6, a second loop bipolar sawtooth modulation signal generation module 7, an input end coupler 8, an optical resonant cavity 9, an output end coupler 10, a first loop photodetector 11, a second loop photodetector 12, a first loop lock-in amplifier 13, a second loop lock-in amplifier 14, a first loop servo control module 15, a second loop servo control module 16, and a low-pass filter module 17.
The tunable semiconductor laser 1 is connected with an isolator 2, the isolator 2 is connected with a light splitting coupler 3, two paths of output of the light splitting coupler 3 are respectively connected with a first loop phase modulator 4 and a second loop phase modulator 5, output light of the first loop phase modulator 4 and output light of the second loop phase modulator 5 enter an optical resonant cavity 9 through an input end coupler 8, two paths of light in the optical resonant cavity 9 are respectively connected with a first loop photoelectric detector 11 and a second loop photoelectric detector 12 after being output through an output end coupler 10, and the output of the first loop photoelectric detector 11 is sequentially connected with a first loop phase-locked amplifier 13, a first loop servo control module 15 and a tuning end of the tunable semiconductor laser 1; the output of the second loop photoelectric detector 12 is connected with a second loop phase-locked amplifier 14 and a second loop servo control module 16 in sequence, the output of the second loop servo control module 16 is simultaneously connected with a second loop bipolar sawtooth wave modulation signal generation module 7 and a low-pass filtering module 17, the output of the first loop sine wave modulation signal generation module 6 is connected with a first loop phase modulator 4, and the output of the second loop low-pass filtering module 17 is used as the output of a gyroscope.
The method for implementing square wave quadrature demodulation of the closed-loop resonant optical gyro device comprises the following steps:
in the first loop of the sine wave modulation, the difference between the resonant frequency of the first loop and the center frequency of the tunable semiconductor laser 1 is obtained by detecting the amplitude of a primary frequency signal of the sine wave modulation frequency in the output signal of the first loop photodetector 11, a frequency difference signal is input to the first loop servo control module 15, a feedback signal is generated and input to the tuning end of the tunable semiconductor laser 1, the center frequency of the output light of the tunable semiconductor laser 1 is controlled, and thus the center frequency of the tunable semiconductor laser 1 is locked on the resonant frequency of the first loop.
In the second loop of the bipolar sawtooth wave modulation, after the bipolar sawtooth wave modulation, the second loop lock-in amplifier 14 obtains a demodulation value proportional to the resonance frequency difference between the first and second loops and also proportional to the rotation angular velocity of the object. And an output signal obtained after a second loop demodulation value is input into a second loop servo control module 16 is input into a second loop bipolar sawtooth wave modulation signal generation module 7, the waveform of the bipolar sawtooth wave is adjusted in real time, and the equivalent frequency shift quantity of the bipolar sawtooth wave is fed back and controlled, so that the resonant frequency locking of the second loop is realized. And under the condition that the first loop and the second loop are locked, the feedback signal of the second loop is used as a gyro output signal.
In square wave quadrature demodulation scheme, the second loop photodetector PD _ CW outputs a signal VD(t) is transmitted through two square wave demodulation channels. One of the channel reference signals is r1(t) the output demodulation signal is set to VIThe other channel reference signal is r2(t) with a reference signal r1(t) quadrature, output demodulation signal is set to VQ. Will VIAnd VQInputting the signal into a quadrature demodulation algorithm module to obtain a final output signal Vout. The square wave quadrature demodulation is implemented by the second loop lock-in amplifier 14, wherein the square wave signal to be demodulated output by the second loop photodetector 12 can be represented as:
in the formula, VDIs the square wave signal to be demodulated, V, output by the second loop photodetector 121And V2The voltage of the first half period and the second half period of the square wave signal to be demodulated is respectively, and F is the frequency of the square wave signal and is the same as the repetition frequency of the bipolar sawtooth wave; n is an integer, Z represents a set of integers, t is time; k is a radical ofC0、kC1And kC2The coupling coefficients of the strength of the optical coupler 3 (abbreviated as C0), the input end coupler 8 (abbreviated as C1) and the output end coupler (abbreviated as C2) in fig. 1 are respectively; alpha is alphaC0、αC1And alphaC2Insertion losses of couplers C0, C1, and C2, respectively; alpha is alphaPM_CWIs the insertion intensity loss factor of the second loop phase modulator 5 (briefly described as PM _ CW); p is a photoelectric conversion coefficient of the photodetector; i is0Is the output optical power of the laser; τ ═ nrL)/c is the transit time of the optical cavity 9 (abbreviated as FRR), L is the fiber ring length of FRR, nrIs the refractive index of the fiber, c is the propagation speed of light in vacuum; Δ f ═ fCW-f0' is the equivalent resonant frequency difference, fCWIs the resonant frequency of the second loop, f0' is the laser equivalent center frequency; f. ofaIs the equivalent square wave frequency modulation signal amplitude; alpha is alphaL/2Is the intensity loss factor of light transmitting half a turn in the FRR.
Square wave reference signal r of one of the square wave demodulation channels1(t) can be expressed as:
where θ is the phase difference between the reference signal and the signal to be demodulated. The demodulated output signal is:
wherein, G is the gain of the square wave demodulation module. According to formula (3), VIAfter normalization, it can be expressed as:
the normalized slope of the demodulation curve at the resonance point can be expressed as:
however, in an actual resonant optical gyro system, the phase difference θ fluctuates, which may cause fluctuation in the slope of the demodulation curve. Based on equation (4), fig. 2 shows the results of simulation of the demodulation curves when the phase difference θ is 0, pi/20 rad, and pi/10 rad. The simulation parameters are as follows: coefficient of intensity coupling kC1And kC2All are 0.05, insertion loss coefficient kC1And kC2Are all 0.0228, the refractive index n of the optical fiberr1.455, the ring length L of FRR is 14m, and the propagation velocity c of light in vacuum is 3X 108m/s, intensity loss factor alpha of half turn of light transmission in FRRL/2Is 0.0883, equivalent square wave frequency modulation signal amplitude faIs 263.1 kHz.
Fig. 2 shows that the slope at the resonance point changes with the change in the phase difference θ. In this embodiment, the minimum amplitude of the signal detectable by the digital system is set as VI_N_minAs shown by the solid line in fig. 2. From the vertical dashed lines of different gray levels in fig. 2, it can be seen that the resonant frequency locking accuracy decreases as the slope of the demodulation curve deviates from its maximum value.
FIG. 3 shows the simulation results for θ ranging from- π/2rad to π/2rad with k, according to equation (5). It can be seen that k is decreasing as θ deviates from 0. For example, when θ ═ π/10rad, k is 80% of its maximum value. Therefore, when the non-orthogonal square wave demodulation method is used, the fluctuation of the phase difference theta can cause the change of the slope k of the demodulation curve, and finally the performance of the closed-loop resonant optical gyro system is influenced.
The other square wave demodulation channel reference signal is r2(t) with a reference signal r1(t) orthogonal, which can be expressed as:
outputting a demodulated signal VQCan be expressed as:
will VIAnd VQInputting the signal into a quadrature demodulation algorithm module to obtain a final output signal VoutAs shown in fig. 4. According to VIAnd VQThe expression for two demodulated signals, the square wave quadrature demodulated output signal, can be expressed as:
from equation (8), it can be found that the calculation result of the final square wave quadrature demodulation is G (V)1-V2) And/2, not influenced by theta fluctuation. However, in order to obtain the above result, different calculations need to be performed in different θ ranges. Therefore, in a practical algorithm, it is necessary to determine the range of θ. Here, the present embodiment expresses θ as:
θ＝θ0+Δθ (9)
in the formula, theta0And delta theta is a phase error at a preset phase operating point. At theta0For example, 0, assume that Δ θ fluctuates between- π/4rad to π/4 rad. In thatIn this case, in combination with the formula (3) and the formula (7), Δ θ may be in the range of VIAnd VQReflects the product of (a), then the quadrature demodulation algorithm can be expressed as:
the schematic diagram of the quadrature demodulation algorithm obtained according to equation (10) is shown in fig. 5.
In order to simplify the analysis, the invention uses V in the algorithmoutAnd VFPerforming normalization processing to normalize VoutAnd VFRespectively expressed as:
according to the formula (11), this example simulates V when Δ θ is + - π/4rad, + - π/10rad, and + - π/30rad, respectivelyF_NThe relationship with Δ f' is shown in FIG. 6. It can be found that under ideal conditions, V fluctuates between- π/4rad to π/4radF_NThe sign of Δ θ is exactly opposite. This helps to accurately predict the algorithmic expression of equation (8). However, various noises such as quantization noise exist in an actual digital resonance type optical gyro system, which causes VFUncertain fluctuations may eventually lead to false positives for the delta theta sign.
To explain VFHow the fluctuation of (2) affects the mechanism of the performance of the resonant optical gyro system, the present embodiment assumes that the introduced fluctuation noise is in the range of-0.03 to 0.03, as shown by the range of the broken line in fig. 6, in which case misjudgment may occur. It can be found that when Δ θ is equal toWhen Δ f' is close to 0, erroneous determination is likely to occur. False positives may occur, for example, when Δ θ ± π/30rad, | Δ f' | is less than about 31 kHz.
In order to analyze the influence of the above-described erroneous judgment phenomenon on the resonant optical gyro system, the present embodiment simulates a demodulation curve in the case of correct judgment and erroneous judgment in the case where Δ θ is pi/30 rad, according to equation (11). As shown in fig. 7, when | Δ f' | is less than about 31kHz, a misjudgment phenomenon occurs, in which the slope of the demodulation curve is less than that in the case of a correct judgment. This causes the demodulated signal to fluctuate near the resonance point, which results in a reduction in detection accuracy, and ultimately affects the performance of the resonant optical gyroscope.
According to the theoretical analysis result, the working point theta is used as the working point in the orthogonal demodulation algorithm0When the value is 0, fluctuation of the demodulation signal in the vicinity of the resonance point is caused. This problem can be solved by setting the operating point θ0Solved for π/4 rad. In this case, the reference signal r1(t) can be expressed as:
thus, the demodulated signal VICan be expressed as:
correspondingly, another reference signal r2(t) can be expressed as:
thus, the demodulated signal VQCan be expressed as:
combining equations (13) and (15), the square-wave quadrature-demodulated output signal can be expressed as:
where G is the gain of the square wave demodulation module in the first channel and the second channel (as shown in fig. 4). According to the formula (16), when Δ θ fluctuates between- π/4rad to π/4rad, the demodulated signal can be represented by a unique expression VI-VQThus obtaining the product. Therefore, when the operating point θ0After the adjustment from 0 to pi/4 rad, the symbol of delta theta is not needed to be judged any more, thereby avoiding the fluctuation of the demodulation signal caused by misjudgment. The block diagram of the algorithm optimized according to equation (16) is shown in fig. 8.
The following embodiments verify that the square wave quadrature demodulation method can effectively improve the zero-offset stability of the closed-loop resonant optical gyro system through experimental test analysis results.
Example 1
Fig. 9 shows the test results of the demodulation curve before and after quadrature demodulation with a square wave, which is obtained by sweeping the center frequency of the laser. FIG. 9(a) is a demodulation curve under quadrature demodulation without square waves; FIG. 9(b) is a demodulation curve using square-wave quadrature demodulation, the operating point θ of which0Set to pi/4 rad. In the experiment, the values of Delta theta are artificially set to be 0, pi/20 rad and pi/10 rad. It can be found that when square wave quadrature demodulation is not employed, as shown in fig. 9(a), the slope of the demodulation curve fluctuates with Δ θ, and thus the slope of the demodulation curve cannot be always maintained at the maximum value; when the square wave quadrature demodulation method is employed, as shown in fig. 9(b), the demodulation curve slope fluctuation is suppressed and is kept almost in the vicinity of the maximum value.
Example 2
To illustrate the necessity of optimizing the phase operating point when using square wave quadrature demodulation, FIG. 10 shows the comparison results of the demodulation curve test with the operating point set to 0 and π/4 rad. It was found that when the operating point was set to 0, there was a significant fluctuation in the vicinity of the resonance point, and when the operating point was set to π/4rad, the fluctuation was suppressed.
Example 3
FIG. 11 shows Allan analysis of variance of gyro 1h output signal test data with operating points of 0 and π/4rad, respectively, when a square wave quadrature demodulation method is employed. The integration time was 0.1s and the sampling rate was 10 Hz. From the Allan analysis of variance results of FIG. 11, a zero bias stability of approximately 11.9deg/h was obtained when the quadrature demodulation operating point was set to 0, which is even worse than our reported test result of about 7.1deg/h without the square wave quadrature demodulation scheme shown in FIG. 12, which may be due to the fact that the demodulation curve fluctuates around the resonance point at non-ideal phase operating points. However, when the operating point is set to π/4rad, zero bias stability of about 6.0deg/h can be obtained. The zero bias stability is about 84.5% of the test results obtained for the non-square wave quadrature demodulation scheme shown in fig. 12.
Example 4
In order to further verify the effectiveness of the square wave quadrature demodulation scheme under a longer time, the gyro output signal is tested for 6h under different conditions in the embodiment, and fig. 13 is an Allan variance analysis result of the test data. According to the Allan analysis of variance results, the null-offset stability is about 33.2deg/h when the quadrature demodulation operating point is set to 0, and still worse than about 27.8deg/h when square-wave quadrature demodulation is not used. However, when the operating point is set to π/4rad, the zero bias stability with quadrature demodulation is about 11.7deg/h, which is about 42.1% of that without square-wave quadrature demodulation. Compared with the gyro output result of 1h, the performance improvement effect of square wave quadrature demodulation on the system is more remarkable in the long-term test result of 6 h.
The foregoing lists merely illustrate specific embodiments of the invention. It is obvious that the invention is not limited to the above embodiments, but that many variations are possible. All modifications which can be derived or suggested by a person skilled in the art from the disclosure of the present invention are to be considered within the scope of the invention.
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