Patent Application
20160320188
The invention concerns a fiber optic system as well as a method for reducing bias errors in such a fiber optic system.
Sensors for measuring angular velocities that are used for example in navigation systems or systems for analysis or control of movements or of the position of an object may comprise instead of mechanical gyroscopes also interferometric fiber gyroscopes (also called fiber optic gyroscopes or fiber gyroscope). These analyze the interference of two light waves running in opposite directions in a coiled optical wave guide, the interference resulting from different transit times of the oppositely running wave trains and the therefrom resulting phase difference, which is caused by a rotation of the optical wave guide around an axis perpendicular to the plane in which the wave guide is arranged. The amount of change in intensity of the detected light, which is described by an interferometer characteristic, is a measure for the rotation rate applied to the gyroscope.
The interferometer characteristic characterizes, hence, the dependency of the light intensity, which is to serve as observation parameter for determining the rotation, from the phase difference of the two light waves. Often, the working point of the fiber gyroscope on the cosine-formed interferometer characteristic is adjusted such that it is located in a point of a maximal gradient of the characteristic, i.e. approximately at a half of maximal light intensity. This should ensure that already for a small rotational movement a maximal sensitivity of the interferometer is guaranteed.
Such a fiber optic gyroscope comprises in principle a light source, which generates the circulating light waves or light beams, a coiled optical wave guide (e.g. a fiber coil), an optical device for irradiating the oppositely running light beams into both ends of the coiled optical wave guide and for receiving of the oppositely running light beams that have passed through the coiled optical waveguide (also called multifunctional integrated optical chip (MIOC)), as well as a detector that determines the interference of the oppositely running wave trains. Additionally, a polarizer that polarizes the light waves emitted from the light source and/or an optical coupler that guides the light beams either from the light source to the optical device (to the coiled optical wave guide) or from the optical device (from the coiled optical wave guide) to the detector are present. Further, a fiber gyroscope comprises an analog amplifier as well as an evaluation unit.
If a closed loop method for operating the gyroscope is used, the evaluation unit comprises an analog/digital converter, a control loop that generates a phase reset signal, and a digital/analog converter that provides a modulation signal obtained from the phase reset signal to a phase modulator in the optical device such that the phase of the supplied light waves can be modulated such that the intensity of the detected light remains constant, i.e. that the working point of the interferometer is maintained. Here, the value of the necessary phase reset signal is a measure for the rotation rate applied to the gyroscope.
A bias error may occur in such a fiberoptic gyroscope, which manifests itself as zero point error of the gyroscope. Due to this, also in a rest position, i.e. while no real mechanic intrinsic rotation rate is applied, a rotation rate larger than zero is output or determined. This may be caused by different effects. A known mechanism is the so-called amplitude modulation at the optical device, which is a parasitic effect of the phase modulators present on the optical device. These should ideally only modulate the phase of the light waves, but in fact also modify to a small degree the amplitude and hence the intensity of the light. In addition, the electrical modulation signal that is supplied to the optical device may leak into the detector path.
An object of the present invention is to provide a method for reduction of this bias error in a fiber optic system as well as a fiber optic system that is configured to carry out such a method.
This object is solved by the subject-matter of the independent claims. Preferred embodiments are defined in the dependent claims.
A fiber optic system according to the invention comprises a fiber optic gyroscope comprising a light source, a fiber coil, an optical device having a phase modulator, a photo detector, an amplifier and an evaluation unit as well as a control device. The optical device is configured to split a light beam coming from the light source by means of beam splitting and to modulate the phase of at least one of the generated light beams to adjust, hence, a current working point of the fiber optic gyroscope. The optical device is further configured to irradiate the generated light beams in directions opposite to each other into the fiber coil and to unify the light beams leaving the fiber coil. The photo detector is configured to generate from an interference signal that is generated due to the reunification of the light beams a signal corresponding to the light intensity of the interference signal, while the amplifier is configured to amplify the signal generated in the photo detector. The evaluation unit is configured to generate by means of the signal generated by the amplifier a modulation signal that controls the phase modulator such that a reset of the interferometer signal according to the current working point of the fiber optic gyroscope is achieved or such that a predetermined value is adjusted as current working point of the fiber optic gyroscope. Moreover, the evaluation unit is configured to determine on the basis of the modulation signal a rotation rate of the fiber optic gyroscope. The control device is configured to control at least one element of the fiber optic gyroscope such that at least two different values for the rotation rate are determined by the evaluation unit for almost constant rotation rate applied to the fiber optic gyroscope.
By a comparison of the at least two determined values for the rotation rate the bias error of the fiber optic gyroscope can be determined. Alternatively or additionally the bias error may be reduced by a linear combination of the at least two determined values for the rotation rate and a corrected value for the rotation rate applied to the fiber optic gyroscope can be obtained. For these purposes, the fiber optic system may further comprise a processing unit.
For determining the at least two different values for the rotation rate for almost constant rotation rate applied to the fiber optic gyroscope at least two different working points of the fiber optic gyroscope may be adjusted at the very same interferometer characteristic of the fiber optic gyroscope. Alternatively, at least two different interferometer characteristics of the fiber optic gyroscope may be generated by a change of the amplification of the control path outside of the evaluation unit.
By means of the method according to the invention and the fiber optic system according to the invention the bias error can be reduced independent of the age of the fiber optic gyroscopes and other external influences, as the error is compensated continuously based on the at least two different determined values for the rotation rate. Then, for example aging of components of the fiber optic gyroscope or changes in the ambient temperature have a substantially smaller influence on the rotation rates determined by the fiber optic system if compared to systems in which the bias error is only determined in one calibration process and afterwards used as a constant value in generating the corrected rotation rate. Embodiments of the present invention are explained in what follows based on the figures, wherein same elements are indicated by the same reference signs.
The fiber optic system 1 illustrated in
Phase modulation set by the phase modulator 130 determines the current working point of the fiber optic gyroscope 10 on an interferometer characteristic of the fiber optic gyroscope 10, the interferometer characteristic being defined by parameters of the components of the fiber optic gyroscope 10. Such parameters that determine the interferometer characteristic are for example the light power of the light source 11, an amplitude modulation of the used light, which is caused by the optical device 13 or other components of the fiber optic gyroscope 10, the light sensitivity of the photo detector 14 or the analog amplification of the amplifier 15. By means of the interferometer characteristic a rotation rate applied to the fiber optic gyroscope 10 can be determined based on the signal generated by the amplifier in the evaluation unit 16. In closed loop operation of the fiber optic gyroscope 10 the evaluation unit 16 comprises a control loop that generates a modulation signal that is suitable for controlling the phase modulation 130 such that a reset of the interference signal according to the current working point of the fiber optic gyroscope 10 can be achieved. Based on the amount of deviation of the signal generated by the amplifier and of the signal predetermined by the working point of the fiber optic gyroscope 10 or, respectively, based on the value of the necessary reset signal the evaluation unit 16 determines the rotation rate that is applied to the fiber optic gyroscope 10. In addition, by means of a modified modulation signal also a new current working point of the fiber optic gyroscope 10 may be set.
The fiber optic gyroscope 10 has a bias error BF, whose origin has already been described above. The size of the bias error BF depends on the position of the working points of the fiber optic gyroscope on the interferometer characteristic of the fiber optic gyroscope as well as on the characteristic values of the interferometer characteristic of the fiber optic gyroscope as such, wherein in particular the gradient of the characteristic at the working point, i.e. the size or amplification of a signal deviating from the working point of the characteristic, matters.
These features of the bias error BF are used in the method according to the invention, which is illustrated in
For this first working point or, respectively, for this first interferometer characteristic at step S21 a first rotation rate Ω1 is determined by the evaluation unit 16 for the rotation rate Ω that is actually applied to the fiber optic gyroscope 10.
Then, the control device 20 controls the components of the fiber optic gyroscope 10 such that at step S22 a second working point is set at the given interferometer characteristic of the fiber optic gyroscope 10 or that a second interferometer characteristic of the fiber optic gyroscope 10 is obtained. It is also possible to newly adjust the working point, i.e. the phase modulation generated by the phase modulation 130, as well as the interferometer characteristic.
At step S23 for this second working point or, respectively, for the second interferometer characteristic a second rotation rate Ω2 is determined by the evaluation unit 16 for the rotation rate Ω that is actually applied to the fiber optic gyroscope 10.
As the change of working point or, respectively, of the interferometer characteristic as well as the determination of the rotation rate by the evaluation unit are carried out in a very short time period, while the rotation rate Ω applied to the fiber optic gyroscope 10 changes only comparatively slow, the first determined rotation rate Ω1 as well as the second determined rotation rate Ω2 correspond to almost identical rotation rates Ω that are actually applied to the fiber optic gyroscope 10. This means that the difference between the two determined rotation rates Ω1 and Ω2 is caused by different sizes of the bias error for the underlying working points or, respectively, interferometer characteristics.
Due to this, at step S24 it is possible to determine by a comparison of the two determined rotation rates Ω1 and Ω2 or by further processing the two determined rotation rates Ω1 and Ω2 the bias error BF of the fiber optic gyroscope 10 for a specific working point on a specific interferometer characteristic. Moreover, at step S25 a corrected value SIT (for the rotation rate can be generated by a linear combination of the two determined rotation rates Ω1 and Ω2 that corresponds to the rotation rate Ω that is actually applied to the fiber optic gyroscope 10 and that has a reduced bias error. Ideally, the bias error BF may even be compensated or eliminated totally. Steps S24 and S25 are carried out by the processing unit 30 illustrated in
In addition, also the steps S20 and S21 as well as S22 and S23 may be carried out several times after each other such that several values for the first rotation rate Ω1 and several values for the second rotation rate Ω2 are determined, from which then the mean is taken to obtain the rotation rate values Ω1 and Ω2 that are used in steps S24 and S25. In this process, the steps for setting the different working points or for obtaining the different interferometer characteristics, and for determining the corresponding associated rotation rate values Ωi should be carried out alternating or—for more than two working points or interferometer characteristics—in a predetermined sequence that allows relating of a specific determined rotation rate value Ωi to a specific working point or a specific interferometer characteristic.
Step S24 is optional. It may for example be carried out only during a calibration or test phase of the fiber optic gyroscope to judge the quality of the fiber optic gyroscope based on predetermined limits for the bias error. The determined bias error BF may then be stored and/or used for further processing steps.
As examples for the linear combination carried out at step S25 of the determined rotation rate values Ωi only forming of (weighted) differences or averages has been indicated here. However, also other different linear combinations are possible.
The control device 20 and the processing device 30 that are illustrated in
The control loop 162 comprises different components as for example adders, multipliers, and dead time elements that serve for a corresponding processing of the applied signals and to some of which signals from supporting control loops are supplied, which are not shown in the control loop 162 illustrated in
The unit 61 for generating the modulation deviation may for example generate a modulation deviation of π/2, to which one of the generally used working points on the interferometer characteristic corresponds.
The unit 63 for generating the dither amplitude can be controlled by the control device 20 by means of a control signal such that it generates at least two different values for the dither amplitude. For example, a first amplitude having the value (π/2)/256, which corresponds to the standard amplitude, and a second, strongly increased amplitude having for example the value of 70·(π/2)/256 may be set. As the dither signal with the adjusted dither amplitude serves for generating the second digital signal S2, which corresponds after digital/analog conversion to the modulation signal, thus, different working points of the fiber optic gyroscope 10 on the interferometer characteristic of the fiber optic gyroscope 10 can be set.
To be able to evaluate the rotation rate information of the different working points of the fiber optic gyroscope 10 separately and in relation to the corresponding working points, the modulation-random-bit and the dither-random-bit are interrelated in the toggle control unit 65 by an XOR logic. This ensures that a series of working points can be set that have either a sequence that is strictly alternating from cycle to cycle or that is predetermined. Further, it is ensured that rotation rate values Ωi can be generated and then related to the respectively corresponding working point. To this end, by using the XOR logic of the modulation-random-bit and the dither-random-bit it is determined on which of the predetermined levels the working point will be located in the next cycle. If in a comparison of this level with the working point level of the previous modulation cycle identity is determined, an alternating sequence of working points can be achieved by inverting the dither-random-bit. If more than two different working points are to be set, the toggle control unit 65 is formed accordingly.
Using the signal of the toggle control unit 65 a toggle demultiplexer 67 is operated after the signal passed a corresponding dead time element 66 (in
The dead time elements may be adapted to the number of working points and the working speed of the components comprised in the fiber optic gyroscope 10. In this manner, also retardations other than the illustrated retardation of two cycles can be realized by the corresponding dead time elements.
To adjust the different light source current values and to relate the determined third digital signal S3 or, respectively, the determined rotation rate values to a specific interferometer characteristic, the control device 20 comprises besides a control unit 21, which provides a signal for adjusting the respective light source current in the light source 11, a toggle control unit 22 and a dead time element 23. The toggle control unit 22 serves for adjusting a predetermined sequence of interferometer characteristics of the fiber optic gyroscope 10 to be obtained, similarly as already described with respect to the first embodiment of
Using the signal from the toggle control unit 22 a toggle demultiplexer 67 in the evaluation unit 16 is operated after the signal has passed a dead time element 23, which is adapted to the implementation. Here, the toggle demultiplexer 67 relates the determined rotation rate values Ωi to a specific interferometer characteristic. The rotation rate values Ωi that are related to the same interferometer characteristic or the corresponding third digital signals S3 are averaged in a corresponding integrator 68 and output as rotation rate values.
In contrast to the control loop 162 of the first embodiment illustrated in
Also an alternative or additional control of other components of the fiber optic gyroscope 10 is possible.
The combination of the first and the second embodiment of the fiber optic system with the method for operating the fiber optic system is also conceivable. Stated differently, components of the control loop 162 for setting different working points of the fiber optic gyroscope 10 as well as other components of the fiber optic gyroscope 10 for obtaining different interferometer characteristics of the fiber optic gyroscope 10 may be controlled simultaneously or in a time shifted manner.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2015 106 550.9 | Apr 2015 | DE | national |
