The current invention is related to a system and a method for measuring the distance to a target and/or the velocity of the target. The invention is further related to the use of such a system.
In US 2007/0058157 A1 a system for measuring velocity of a target is disclosed. The system includes a laser device, a detector and a signal processor. The laser device produces a frequency-modulated laser beam, wherein the frequency-modulated beam is split into at least two laser beams, wherein a reflected beam comprising one split beam is directed to the target and reflected from the target. The detector detects a combination of the reflected beam and a reference beam comprising another split beam, wherein the reflected beam and the reference beam have unequal delays. The signal processor obtains velocity information from the combined beam by use of Doppler-shifted harmonics of the modulation frequency. Dynamic range and flexibility of the disclosed sensor module is an issue.
It is an object of the current invention to provide an improved sensor module for measuring the velocity of a target and/or the distance of the target.
The object is achieved by means of a sensor module for measuring a distance to a target and/or a velocity of the target, the sensor module comprising at least one laser source for emitting laser light, at least one detector being adapted to detect modulated laser light and at least one control element, the control element being adapted to vary a focus point of the laser light and/or intensity of the laser light and/or direction of the laser light.
The detector may be for example a classical interferometer with a beam splitter, mirrors and a photodiode well known by those experienced in the art. The modulation of the laser light may be caused by a Doppler shift of reflected laser light due to the movement of the target interfering with the laser light emitted by the laser source. The photodiode may convert the optical signal of the modulated laser light to an electrical signal, the electrical signal may be analyzed by means of the detector or the control element and depending on the result the laser light emitted by the laser source for example a laser diode may be varied by means of the control element. The laser source may for example be slanted in a controlled way with respect to a reference axis and the slanting angle is monitored. Slanting of the laser source may cause a variation of the velocity component of the target being parallel to the laser light. The combination of the known slanting angle with respect to the reference axis and the velocity component parallel to the laser light may enable the determination of the velocity component of the target parallel to the reference axis. Due to the fact that the frequency of the Doppler shift and the resulting frequency modulation of the laser light that may be detected by the photodiode is proportional to the velocity component of the target being parallel to the beam of the laser light emitted by the laser source, controlled slanting of the laser source may enable to increase the range of velocities that may be detected by the sensor module. Depending on the slanting angle the same modulation frequency of the laser light may represent different velocities of the target parallel to the reference axis. It may be advantageous to combine the laser source with a lens in order to focus the emitted laser light on the target. The sensor module may comprise a driver circuitry or driver circuitries for the laser source, the detector and/or the control element. Further one or more power supplies as for example a battery may be added to the sensor module.
The variation of the intensity of the laser light is meant as variation between intensities different from zero intensity excluding pulsed driving of the laser source. Further the variation may be continuous or discontinuous.
As an alternative to the classical interferometer approach the detector may detect a modulation of resonating laser light in the laser source. The modulation of the laser light in the laser source may be induced by laser light reflected by the target reentering the laser source. The effect is known as Self-Mixing-Interference (SMI) by those skilled in the art. Depending on the electrical driving scheme of the laser source either the distance and/or the velocity of the target can be detected. The detector may be a photo-diode and the laser source may be a laserdiode as for example a side emitting laser-diode a Vertical Cavity Surface Emitting Laser (VCSEL) or Vertical Extended Cavity Surface Emitting Laser (VECSEL). Especially VCSEL or VECSEL allow integrating the photodiode and the laser-diode in one device by means of semiconductor processing. Details about detection of velocities by means of self-mixing interference can for example be found in WO 02/37410 A1
In another embodiment according to the current invention the control element comprises an optical element and the optical element is placed between the target and the laser source. The intensity of the laser light may for example be varied by means of a variable optical attenuator between the laser source and the target, which may improve signal to noises ratio of the detected modulation of the laser light by preventing non-linearities caused by high intensities of the laser light reflected by the target. Alternatively or in addition the focus point of the laser light may be varied by means of a variable focus lens, which may increase the detection range of the sensor module in comparison to sensor modules with a lens with a fixed focus. Further the laser light may be slanted with respect to a reference axis by means of a moveable mirror between the laser source and the target, which may increase the range of velocities those may be detected as discussed above in connection with the slanting of the laser source itself.
In a further embodiment according to the current invention the sensor module comprises at least two laser sources and at least two detectors and the laser sources are adapted to emit laser light in essentially the same direction. The adaption of the laser sources to emit laser light in essentially the same direction means that the laser sources are aligned in parallel in order to emit laser light in the same direction. After the emission the laser light emitted by the different laser sources may be manipulated e.g. by means of the control element causing a deviation from the direction of emission of the laser light, whereby the deviation from the direction of emission of the laser light may be different for different laser sources. Each laser source may have a dedicated detector only detecting the modulation of the laser light emitted by the belonging laser source. The laser sources and the detectors may for example be integrated SMI sensors manufactured by semiconductor processing. The laser light of at least two different laser sources may be varied differently by means of the control element with respect to the focus point of the laser light and/or the intensity of the laser light and/or the direction of the laser light. The latter may be done for example by means of variable optical attenuators, variable focus lenses or moveable mirrors provided for at least two or all laser sources integrated in the sensor module. Alternatively a passive optical element may be used in order to vary or manipulate the laser light emitted by the laser sources differently. This may for example be done by means of a curved mirror reflecting laser light emitted by one laser source depending on the point on the mirror where the emitted laser light hits the mirror. Using an array of laser sources and detectors may enable a variation of the slanting angle with respect to a reference axis without moving parts. Further lenses with different focal length may be used in order to vary the focus points of laser sources in an array of laser sources. Alternatively or in combination with lenses and/or mirrors the passive optical element may be an optical attenuator. Using for example a first optical attenuator with a first optical attenuation in combination with a first laser source and a first detector and a second optical attenuator with a second optical attenuation different from the first optical attenuation in combination with a second laser source and a second detector may simplify the sensor module. The different laser sources may be driven in parallel or the laser sources may be driven sequentially by using an appropriate electrical driving circuit. Driving the different laser sources sequentially (one after the other) may reduce the complexity of the electronic circuitry. Using passive optical elements in combination with an array of laser sources and detectors (three, four, five or more laser sources and detectors) may enable a flexible, cost effective and reliable sensor module.
In another embodiment according to the current invention the range of detection of a first detector overlaps with the range of detection of at least one second detector. The range of detection may be the range of distances to the target or velocities of the target that can be detected by one defined detector (in combination with the belonging laser source and optical element respectively belonging part of an optical element). That means the first detector may be able to detect velocities between 1 m/s and 10 m/s and the second detector may be able to detect velocities between 3 m/s and 30 m/s. The overlapping range of detection between 3 m/s and 10 m/s may be used to verify the first measurement signal resulting from the detection of modulated laser light detected by the first detector by comparing the first measurement signal with the second measurement signal resulting from the detection of modulated laser light detected by the second detector by means of an analyzer. A velocity measurement of for example 7.6 m/s detected by means of the first detector may be compared with and verified by the velocity measurement of the second detector. Having an array of detectors and belonging laser sources more than one detector may be used to verify the measurement result of one defined detector.
In a further embodiment of the current invention the sensor module further comprises a computing unit and an interface, the computing unit (for example a processor, an ASIC or the like) being arranged to compute distance and/or velocity data of the target based on measurement signals resulting from the detection of modulated laser light detected by at least one detector and the interface being arranged to communicate the distance and/or velocity data of the target for further processing. Further processing comprises displaying the distance and/or velocity data on a display or transmit the distance and/or velocity data to other system(s).
A system comprising a sensor module according to the current invention may be used in automotive applications as driver information systems and brake assistant systems. Further it may be used in one application selected from the group:
It is further an object of the current invention to provide an improved method for measuring the velocity of a target and/or the distance of the target.
The object is achieved by means of a method for measuring a distance to a target and/or a velocity of the target comprising the steps of:
Using two, three, four or even an array of laser sources in combination with one dedicated detector for each laser source and passive optical elements acting as control elements may result in a highly reliable method for measuring the velocity of a target and/or the distance of the target
Additional features will be described below which can be combined together and combined with any of the aspects. Other advantages will be apparent to those skilled in the art, especially over other prior art. Numerous variations and modifications can be made without departing from the claims of the present invention. Therefore, it should be clearly understood that the form of the present invention is illustrative only and is not intended to limit the scope of the present invention.
The present invention will be explained in greater detail with reference to the figures, in which the same reference signs indicate similar parts, and in which:
A schematic view of a second embodiment of a sensor module according to the current invention is depicted in
In
In addition the anticipated velocity and therefore frequency may be used to electronically generate a frequency, which is mixed with the measured signal. By means of this method it may be possible to reduce the bandwidth of the signal, which has to be analyzed. Only the velocity differences are then measured. It should be noted that no accuracy is lost, because the difference between the anticipated velocity and the real velocity is measured, which gives a correct result. This method is still superior to any method trying to integrate over measured acceleration values. The method also offers some redundancy. If one of the sensors is not working (e.g. obstructed) the second best signal is still better than nothing.
This set-up adapts itself to large variations in speed. It is ensured that always a good signal is available. The multiplication of laser sources 100 and detectors 200 is in practice very simple and the measurement becomes very robust. The multiplication of laser sources 100 and detectors 200 is much more cost efficient than a very broadband amplification and signal analysis electronics. An array of VCSELs with an integrated micro-optics is also very small (about 1 mm), therefore yielding an extremely compact sensor module.
An analyzer 600 compares the different results of the detectors 200. The contribution of higher harmonics (the figure illustrates less higher harmonics in the attenuate case . . . ) as well as the height of the first peak over the noise level are analyzed and the best measurement is selected. This result is communicated via the interface 610 for example to a car management system or to a display, which informs the driver. The method also gives an indication about the feedback and the quality of the signal. This information can be used for a reliability indication or as an information about the road surface (an extreme example is the strongly changing feedback along a broken white line).
The method also offers some redundancy. If one of laser sources 100 or detectors 200 is not working (e.g. obstructed) the second best signal to noise may be used.
This set-up adapts itself to changing feedback conditions (like e.g. changing road surface). It is ensured that always a good signal is available. The multiplication of laser sources 100 and detectors 200 is in practice very simple and the measurement becomes very robust.
Disturbing objects in between the road and the sensor, which may move at different speed may be detected by means of the sensor module according to this embodiment as well. A practical example is water from rain or from the street. An unfocussed measurement set-up would yield several different velocities. An auto-focus system would most likely fail. The proposed invention would yield different velocities for laser sources 100 with integrated detectors 200 due to the different focus of the control element 400 with respect to the single laser source. This makes it simple for the analyzer 600 to decide, which velocity belongs to the road surface and not to the rain. There is also additional information about the speed and amount of disturbing objects available.
The embodiment according to the current invention may also be used to derive information about car load and driving conditions, as the distance to the road can be determined, too.
The method also offers some redundancy. If one of the sensors is not working (e.g. obstructed) the second best signal may be used. Alternatively an overlapping range of detection 800 may be used to control the measurement signals generated by different detectors 200.
This set-up adapts itself to changing distance to the object. It is ensured that always a good signal is available. The multiplication of laser sources 100 and detectors 200 is in practice very simple and the measurement becomes very robust.
The present invention will be described with respect to particular embodiments and with reference to certain drawings, but this is not to be construed in a limiting sense, as the invention is limited only by the appended claims. Any reference signs in the claims shall not be construed as limiting the scope thereof. The drawings described are only schematic and are non-limiting. In the drawings, the size of some of the elements may be exaggerated and not drawn to scale for illustrative purposes. Where the term “comprising” is used in the present description and claims, it does not exclude other elements or steps. Where an indefinite or definite article is used when referring to a singular noun, e.g. “a” or “an”, “the”, this includes a plural of that noun unless specifically stated otherwise.
Furthermore, the terms first, second, third and the like in the description and in the claims are used for distinguishing between similar elements and not necessarily for describing a sequential or chronological order. It is to be understood that the terms so used are interchangeable under appropriate circumstances, and that the embodiments of the invention described herein are capable of operation in other sequences than described or illustrated herein.
Moreover, the terms top, bottom, first, second and the like in the description and the claims are used for descriptive purposes and not necessarily for describing relative positions. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other orientations than described or illustrated herein.
Other variations of the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims.
Number | Date | Country | Kind |
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08100525.8 | Jan 2008 | EP | regional |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/IB09/50109 | 1/12/2009 | WO | 00 | 9/29/2010 |