WO2009153689A1 - Apparatus for determining a relative velocity - Google Patents

Apparatus for determining a relative velocity Download PDF

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Publication number
WO2009153689A1
WO2009153689A1 PCT/IB2009/052360 IB2009052360W WO2009153689A1 WO 2009153689 A1 WO2009153689 A1 WO 2009153689A1 IB 2009052360 W IB2009052360 W IB 2009052360W WO 2009153689 A1 WO2009153689 A1 WO 2009153689A1
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WO
WIPO (PCT)
Prior art keywords
determination unit
frequency
frequency shift
velocity
radiation
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Application number
PCT/IB2009/052360
Other languages
French (fr)
Inventor
Johannes Baier
Original Assignee
Philips Intellectual Property & Standards Gmbh
Koninklijke Philips Electronics N. V.
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Publication of WO2009153689A1 publication Critical patent/WO2009153689A1/en

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Classifications

    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
    • G01S17/00—Systems using the reflection or reradiation of electromagnetic waves other than radio waves, e.g. lidar systems
    • G01S17/02—Systems using the reflection of electromagnetic waves other than radio waves
    • G01S17/50—Systems of measurement based on relative movement of target
    • G01S17/58—Velocity or trajectory determination systems; Sense-of-movement determination systems
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
    • G01S17/00—Systems using the reflection or reradiation of electromagnetic waves other than radio waves, e.g. lidar systems
    • G01S17/88—Lidar systems specially adapted for specific applications
    • G01S17/93—Lidar systems specially adapted for specific applications for anti-collision purposes
    • G01S17/931—Lidar systems specially adapted for specific applications for anti-collision purposes of land vehicles

Definitions

  • the invention relates to an apparatus, a method and a computer program for determining a relative velocity between a first object and a second object.
  • the invention further relates to a vehicle comprising the apparatus.
  • SI self-mixing interference
  • a laser beam is emitted and reflected by an object, wherein the relative velocity between the SMI sensor and the object is determined based on the Doppler frequency shift of the reflected laser beam.
  • the frequency of the reflected beam / refl is shifted to higher values with respect to the emitted laser frequency / Las resulting in a Doppler frequency shift
  • an apparatus for determining a relative velocity between a first object, to which the apparatus is attachable, and a second object comprises: a radiation source for emitting radiation that is to be reflected by the second object, a frequency shift determination unit for determining a frequency shift between the emitted radiation and the reflected radiation reflected by the second object, a moving unit for moving the frequency shift determination unit relative to the first object such that a relative velocity between the frequency shift determination unit and the second object is reduced, a relative velocity determination unit for determining the relative velocity between the first object and the second object, based on the determined frequency shift and the velocity of the movement of the frequency shift determination unit relative to the first object.
  • the invention is based on the idea that by moving the frequency shift determination unit relative to the first object, in particular relative to an attaching element of the apparatus for attaching the apparatus to the first object, such that the determined relative velocity between the frequency determination unit and the second object and, thus, the determined frequency shift, is reduced, the requirements on the frequency shift determination unit and, thus, on the corresponding electronics can be reduced, because the frequency shift determination unit has to determine a smaller frequency shift only. Therefore, the frequency shift and, thus, the relative velocity between the first object and the second object, can be determined with a simpler frequency shift determination unit.
  • the frequency shift determination unit comprises preferentially several units for detecting the reflected radiation and for determining the frequency shift of the detected reflected radiation.
  • the whole frequency shift determination unit is moving, and in another embodiment, only one or some units of the frequency shift determination unit are moving, in particular at least the unit for detecting the reflected radiation.
  • the radiation source is preferentially a laser emitting a laser beam.
  • the frequency shift determination unit is preferentially adapted to determine the Doppler frequency shift as the frequency shift caused by the relative movement between the frequency shift determination unit and the second object.
  • the radiation source and the frequency shift determination unit are integrated into a radiation and determination unit, wherein the radiation and determination unit comprises:
  • a laser comprising a laser cavity for emitting a laser beam that is to be reflected by the second object, wherein the laser cavity is adapted such that a reflected laser beam reflected from the second object can be fed back into the laser cavity,
  • a detection unit for generating a detection signal, depending on a change of a laser intensity
  • a determination unit for determining a frequency shift of the reflected laser beam, based on the detection signal. This allows the relative velocity between the first object and the second object to be determined using self-mixing interference. By using self-mixing interference, the relative velocity can be determined very accurately with only one measurement at a time.
  • the frequency of the detection signal is the frequency shift which has to be determined. If the detection signal comprises several frequencies, the smallest frequency of the detection signal is preferentially the frequency shift. If the detection signal comprises a fundamental frequency and at least one higher harmonic, the fundamental frequency is preferentially the frequency shift which has to be determined.
  • the frequency shift is preferentially determined by using a Fourier transformation, in particular a Fast Fourier Transformation (FFT).
  • FFT Fast Fourier Transformation
  • the determination unit comprises a frequency filter with a filter frequency allowing a detection signal of the detection unit, which has the filter frequency, to pass the frequency filter, wherein the moving unit is adapted to move the radiation and determination unit with varying velocity during determining the frequency shift and/or the frequency filter is adapted to vary the filter frequency during determining the frequency shift, wherein, if the detection signal has passed the frequency filter, the passed filter frequency is the determined frequency shift and the relative velocity determination unit is adapted to determine the relative velocity between the first object and the second object, based on the passed filter frequency and the velocity of the radiation and determination unit relative to the first object at the time at which the detection signal has passed the frequency filter.
  • This allows the use of a simple analogue or digital filtering technique, without the need to use, for example, a FFT. This further simplifies the frequency shift determination unit.
  • the determination unit comprises several frequency filters, each with a filter frequency allowing a detection signal of the detection unit, which has the respective filter frequency, to pass the respective frequency filter, wherein filter frequencies of different frequency filters are different, wherein the moving unit is adapted to move the radiation and determination unit with varying velocity during determining the frequency shift and/or wherein the frequency filters are adapted to vary the filter frequencies during determining the frequency shift, and wherein, if the detection signal has passed a frequency filter, the passed filter frequency is the determined frequency shift, and the relative velocity determination unit is adapted to determine the relative velocity between the first object and the second object, based on the passed filter frequency and the velocity of the radiation and determination unit relative to the first object at the time at which the detection signal has passed the frequency filter.
  • the velocity range of the velocities of the movement of the radiation and determination unit can be decreased, without decreasing the velocity range of the relative velocity between the first object and the second object, which can be detected by the apparatus.
  • the requirements on the performance of the moving unit can be reduced.
  • the filter frequencies of the several frequency filters are non-equidistantly arranged in the frequency domain. This allows the resolution of the velocity determination of the apparatus to be increased in a frequency region of interest, which preferentially corresponds to a velocity region of interest.
  • the filter frequencies of the frequency filters are arranged equidistantly. For example, if the frequency region of interest is 40 MHz to 50 MHz, preferentially the filter frequencies are located at 40 MHz, 42 MHz, 44 MHz, 46 MHz, 48 MHz and 50 MHz.
  • the apparatus comprises a control unit for controlling the moving unit, depending on the determined relative velocity between the first object and the second object.
  • control unit for controlling the moving unit, depending on the determined relative velocity; in particular if the relative velocity is so high that it cannot be determined by the apparatus by using the present speed of the movement generated by the moving unit, the speed generated by the moving unit can be increased in order to reduce the relative velocity and, thus, the frequency shift such that the frequency shift and therefore the relative velocity can be determined by the apparatus.
  • the moving unit is adapted for moving the frequency shift determination unit relative to the first object such that the frequency shift determination unit performs a periodic movement, wherein the periodic movement is adapted for moving the frequency shift determination unit during a phase of the periodic movement such that the relative velocity between the frequency shift determination unit and the second object is reduced.
  • the apparatus is adapted to determine a relative velocity within a predefined velocity range between the first object and the second object, wherein the moving unit is adapted such that the absolute value of the velocity of movement of the frequency shift determination unit relative to the first object is smaller than or equal to an absolute value of a relative velocity within the velocity range.
  • the absolute value of the velocity of movement of the frequency shift determination unit relative to the first object is smaller than or equal to the absolute value of the relative velocity having the smallest absolute value within the velocity range.
  • the absolute value of the velocity of the frequency shift determination unit relative to the first object is preferentially 50 km/h or smaller. If the frequency shift determination unit is periodically moved with respect to the first object, preferentially the velocity of the periodic movement is modified by modifying the frequency, wherein the amplitude of the periodic movement is preferentially constant.
  • a vehicle which comprises an apparatus for determining a relative velocity between a first object being the vehicle and a second object as defined in claim 1.
  • a method of determining a relative velocity between a first object and a second object by using an apparatus as defined in claim 1 comprises the following steps: emitting radiation, that is to be reflected by the second object, by means of a radiation source, determining, by means of a frequency shift determination unit, a frequency shift between the emitted radiation and the reflected radiation reflected by the second object, moving the frequency shift determination unit relative to the first object by means of a moving unit in such a manner that the relative velocity between the frequency shift determination unit and the second object is reduced, determining, by means of a relative velocity determination unit, the relative velocity between the first object and the second object, based on the determined frequency shift and the velocity of the frequency shift determination unit relative to the first object.
  • a computer program for determining a relative velocity between a first object and a second object comprises program code means for causing an apparatus 1 as defined in claim 1 to carry out the steps of the method as defined in claim 10, provided the computer program is run on a computer controlling the apparatus 1.
  • the apparatus of claim 1 , the vehicle of claim 9, the method of claim 10 and the computer program of claim 11 have similar and/or identical preferred embodiments as defined in the dependent claims.
  • Fig. 1 shows schematically and exemplarily an embodiment of a vehicle comprising an apparatus for determining a relative velocity
  • Fig. 2 shows schematically and exemplarily an embodiment of an apparatus for determining a relative velocity
  • Fig. 3 shows schematically and exemplarily another embodiment of an apparatus for determining a relative velocity
  • Fig. 4 shows schematically and exemplarily in more detail an apparatus for determining a relative velocity
  • Fig. 5 shows schematically and exemplarily a detection unit, a determination unit and a relative velocity determination unit of an apparatus for determining a relative velocity
  • Fig. 6 shows schematically and exemplarily a detection unit, a determination unit and a relative velocity determination unit of an embodiment of an apparatus for determining a relative velocity
  • Fig. 7 shows exemplarily a flowchart illustrating an embodiment of a method for determining a relative velocity.
  • Fig. 1 shows schematically and exemplarily a vehicle 13, which can be regarded as a first object and which comprises an apparatus 1 for determining a relative velocity between the vehicle 13 and a second object 6, which can be, for example, a wall.
  • the second object can be any object which is able to reflect radiation from the apparatus such that a frequency shift of the reflected radiation can be determined by the apparatus.
  • the second object 6 can also be a ground or another vehicle.
  • the apparatus 1 for determining a relative velocity between the first object 13, to which the apparatus 1 is attached, and the second object 6 is adapted to emit radiation 14 that is to be reflected by the second object 6 and to determine a frequency shift between the emitted radiation 14 and reflected radiation 15, which is reflected by the second object 6, wherein the frequency shift is caused by a relative movement between the first object 13 and the second object 6, in particular by a relative movement between a frequency shift determination unit, which will be explained further below, and the second object 6.
  • the apparatus 1 for determining a relative velocity between the first object 13 and the second object 6 is schematically and exemplarily shown in Fig. 2.
  • the apparatus 1 comprises a radiation source 2 for emitting radiation 14 that is to be reflected by the second object 6.
  • the apparatus 1 further comprises a frequency shift determination unit 11 for determining a frequency shift between the emitted radiation 14 and the reflected radiation 15, which has been reflected by the second object 6, wherein the frequency shift is caused by a relative movement between the first object 13 and the second object 6, in particular by a relative movement between the frequency shift determination unit 11 and the second object 6.
  • the apparatus 1 further comprises a relative velocity determination unit 12 for determining the relative velocity between the first object 13 and the second object 6, based on the determined frequency shift, and a moving unit 8 for moving the frequency shift determination unit 11 relative to the first object 13 such that the relative velocity is reduced.
  • the frequency shift determination unit is moved towards the second object 6 relative to the first object 13, if the distance between the first object 6 and the second object 13 increases, and the frequency shift determination unit is moved away from the second object 6 relative to the first object 13, if the distance between the first object 6 and the second object 13 decreases.
  • the apparatus 1 further comprises a control unit 9 for controlling the moving unit, depending on the determined relative velocity.
  • the moving unit is controlled such that the speed of the movement of the frequency shift determination unit 11 is adapted such that a relative velocity can be measured. For example, the speed of the movement of the frequency shift determination unit towards the second object 6 is increased, if the relative velocity increases.
  • the moving unit 8 is preferentially adapted for moving the frequency shift determination unit 11 relative to the first object such that the frequency shift determination unit 11 performs a periodic movement, wherein the periodic movement is adapted for moving the frequency shift determination unit 11 during a phase of the periodic movement such that the relative velocity between the frequency shift determination unit and the second object is reduced.
  • the frequency shift determination unit 11 performs a forward-backward movement, wherein preferentially the forward or the backward movement of the frequency shift determination unit is a movement towards or away from the second object 6.
  • the apparatus 1 is adapted to determine a relative velocity within a predefined velocity range between the first object 13 and the second object 6, wherein the moving unit 8 is adapted such that an absolute value of the velocity of movement of the frequency shift determination unit 11 relative to the first object 13 is smaller than or equal to an absolute value of a relative velocity within the velocity range.
  • expected relative velocities are known which define a predefined velocity range, and, preferentially, the moving unit is adapted such that the absolute value of the velocity of the frequency shift determination unit relative to the first object 13 is equal to or smaller than the absolute value of the velocity having the smallest absolute value within the velocity range. If the frequency shift determination unit is periodically moved with respect to the first object, preferentially the velocity of the periodic movement is modified by modifying the frequency, wherein the amplitude of the periodic movement is preferentially constant.
  • FIG. 3 A further embodiment of an apparatus 25 for determining a relative velocity between a first object 13, to which the apparatus 25 is attachable, and a second object 6 is schematically and exemplarily shown in Fig. 3.
  • the apparatus 25 comprises a radiation and determination unit 16, in which the radiation source and the frequency shift determination unit are integrated.
  • the radiation and determination unit 16 will be described in more detail further below with reference to Fig. 4.
  • the radiation and determination unit 16 emits radiation 14 that is to be reflected by the second object 6 and determines a frequency shift between the emitted radiation 14 and the reflected radiation 15, which has been reflected by the second object 6, wherein also this frequency shift is caused by a relative movement between the first object 13 and the second object 6, in particular by a relative movement between the radiation and determination unit 16 and the second object 6.
  • the apparatus 25 comprises a relative velocity determination unit 18 for determining the relative velocity based on the determined frequency shift and a moving unit 17 for moving the radiation and determination unit 16 relative to the first object 13 such that the relative velocity between the radiation and determination unit 16 and the second object is reduced.
  • the apparatus 25 comprises a control unit 24 for controlling the moving unit 17 depending on the determined relative velocity, and preferentially also the moving unit 17 is adapted for moving the radiation and determination unit 16 relative to the first object 13 such that the radiation and determination unit 16 and, thus, the frequency shift determination unit, which is a part of the radiation and determination unit 16, performs a periodic movement, wherein the periodic movement is adapted for moving the radiation and determination unit 16 during a phase of the periodic movement such that the relative velocity between the radiation and determination unit 16 and the second object 6 is reduced.
  • the apparatus 25 is preferentially adapted to determine a relative velocity within a predefined velocity range between the first object 13 and the second object 6, wherein the moving unit 17 is adapted such that an absolute value of the velocity of movement of the frequency shift determination unit 20, 22, 23 relative to the first object 13 is smaller than or equal to an absolute value of a relative velocity within the velocity range.
  • Fig. 4 shows schematically and exemplarily an embodiment of the apparatus 25 in more detail.
  • a radiation source 20 and a frequency determination unit 20, 22, 23 are integrated into a radiation and determination unit 16.
  • the radiation source 20 is a laser 20 comprising a laser cavity 21 for emitting a laser beam 14 that is to be reflected by the second object 6, wherein the laser cavity 21 is adapted such that a reflected laser beam 15, which has been reflected from the second object 6, can be fed back into the laser cavity 21.
  • the laser cavity 21 comprises two reflectors 30, 31 and an active medium 40.
  • the reflected laser beam 15 interferes with the radiation within the laser cavity 21, as a result of which the laser intensity of the laser 20 is changed. This intensity change is detected by a detection unit 22 for generating a detection signal depending on the change in the laser intensity.
  • the generated detection signal of the detection unit 22 is used by a determination unit 23 for determining a frequency shift of the reflected laser beam, and the relative velocity determination unit 18 determines the relative velocity between the first object and the second object, based on the determined frequency shift and the velocity of the movement of the radiation and determination unit 16 caused by the moving unit 17, which is, in this embodiment, a Piezo element controlled by the control unit 24.
  • the radiation and determination unit 16 is a SMI sensor based on a surface emitting semiconductor laser (VCSEL), which consists of an electrically pumped gain medium 40 (in this embodiment, InGaAs quantum wells embedded in GaAs) embedded between two Distributed Bragg Reflectors (n-DBR 30 and p-DBR 31), which form the laser cavity 21.
  • the lower DBR 31 is highly reflective (reflectivity > 99.8%), while the reflectivity of the upper DBR 30 is smaller in order to increase feedback from the light 15 scattered back by the target object.
  • One of the DBRs is p-doped 31 and the other n-doped 30 to enable efficient current feeding into the gain region.
  • n- and p-type doping take place as described above (p-doping of the DBR with higher reflectivity), but also doping in the reversed order is possible as, for example, disclosed in WO 2005/117070 A2, which is herewith incorporated by reference.
  • the detection unit 22 which is preferentially a photodetector and which is attached to the VCSEL chip, measures the small amount of radiation leaking out of the highly reflective p-DBR mirror 31 and thus monitors the influence of the back-scattered light 15, i.e. of the reflected light, from the target object 6, being the second object, on the laser 20, from which information on the relative velocity between the first object and the second object and, if the velocity of the first object 13 is known, the velocity of the target object 6, i.e. the second object, can be extracted.
  • the detection unit 22 in particular the photodetector, can also be placed separate from the VCSEL chip, e.g. by separating a small portion of the emitted laser radiation from the main beam with the help of a suited beam splitter.
  • the complete laser sensor i.e. the radiation and determination unit 16 is mounted on the moving unit 17 being preferentially a Piezo element, which is driven at a certain frequency and amplitude, which are known and can be controlled very accurately by the control unit 24.
  • the preferred movement direction is denoted by reference number 32.
  • a motion frequency of -139 kHz would be necessary to compensate a target movement of 100 km/h with respect to the first object. This frequency is also well within reach for common Piezo elements.
  • the SMI laser sensor i.e. the radiation and determination unit 16 is based on a VCSEL, but also other semiconductor laser sources, such as edge emitting lasers or VECSELS (VCSEL with additional extended cavity), can be used, as well as arrays of VCSEL or VECSEL sensors.
  • VCSEL semiconductor laser sources
  • VECSELS VCSEL with additional extended cavity
  • the speed of the target object is usually obtained by performing a FFT of the photodiode signal in suited evaluation electronics, which yields a peak related to the Doppler frequency according to the above mentioned equation (2), allowing the determination of the relative velocity v rel Dop between sensor 16 and target object 6.
  • This is also possible here, with the additional benefit of a reduced detection frequency due to the reduced relative velocity between velocity sensor 16, i.e. the radiation and determination unit 16, and target object 6.
  • Equation (1) and (2) The sign of the relative velocity v rel Dop between sensor 16 and target object 6 is defined by equations (1) and (2), while all other velocities will be positive if they point in the direction of movement of the first object 13 and negative in the opposite case.
  • equations (1) and (2) the relative velocity v re i DoP i s larger than 0 if the sensor and the target object approach each other, and negative in the opposite case.
  • the target velocity v 2 can then easily be obtained via equation (4), wherein the velocity of the first object 13 V 1 is preferentially known from another velocity measurement.
  • the vibration frequency of the Piezo element is preferentially substantially lower than the expected Doppler frequency in order to guarantee a proper FFT result with sufficient accuracy.
  • the FFT sampling frequency should be high enough to obtain a sufficiently high frequency and thus velocity resolution.
  • the laser structure of the SMI sensor is preferentially epitaxially grown on a GaAs substrate 33.
  • the apparatus for determining a relative velocity between a first object and a second object is preferentially adapted to measure the relative velocity along the emission direction of the light emitted by the radiation source.
  • the apparatus can be adapted to measure the relative velocity also in other directions, for example, by using several radiation sources and frequency shift determination units, in particular, several SMI laser sensors, pointing in different directions, wherein, for example, the resulting relative velocities determined in different directions can be combined to a relative velocity in a desired direction.
  • the moving unit 17 is attached to an attaching element for attaching the apparatus to the first object.
  • the attaching element is preferentially a mounting plate 34, which is preferentially mounted on the first object 13.
  • the attaching element can be any other attaching element which allows the apparatus 25 to be attached to an object such that the frequency shift determination unit, in particular the radiation and determination unit 16, which is, in this embodiment, a laser SMI sensor, is moveable with respect to the object, in particular with respect to the mounting element.
  • the determination unit is adapted such that a FFT is not needed; such an embodiment of the determination unit 23 is schematically and exemplarily shown in Fig. 5.
  • the determination unit 23 comprises a frequency filter 26 with a filter frequency for allowing a detection signal of the detection unit 22, which has the filter frequency, to pass the frequency filter 26.
  • the moving unit 17 is preferentially adapted to move the radiation and determination unit 16, i.e., in this embodiment, the laser SMI sensor, with varying velocity, wherein, if the detection signal has passed the frequency filter 26, the passed filter frequency is the determined frequency shift and the relative velocity determination unit 18 is adapted to determine the relative velocity based on the passed filter frequency and the velocity of the radiation and determination unit 16 relative to the first object at the time at which the detection signal has passed the frequency filter 26.
  • the use of the frequency filter 26 allows a detection concept, which does not require FFT, but may allow simpler analogue or digital filtering.
  • the detection signal can be filtered with a narrow bandwidth at a suited fixed frequency, while the frequency of the movement of the moving unit 17, which is preferentially a Piezo element or a Piezo actuator, is varied, in particular swept.
  • the control unit 24 registers at which moving unit frequency, in particular at which moving unit velocity, an electronic signal passes the frequency filter 26.
  • the filter frequency then corresponds to the frequency shift, in particular to the Doppler frequency, of the relative movement between the radiation and determination unit 16, in particular the laser SMI sensor, and the target object 6, and the determination of the radiation and determination unit movement velocity via the motion frequency at filter resonance conditions allows determining the velocity of the target object.
  • the variation of the radiation and determination unit motion velocity may, for example, either be realized by continuously varying in a controlled way the carrier frequency and/or the amplitude of a sawtooth pattern governing the radiation and determination unit movement or by using another suited drive scheme such as, for example, a sinusoidal pattern. In the latter case, phase-resolved detection of the filter resonance with respect to the radiation and determination unit motion must be applied, as the speed of the moving unit 17 also varies within the sinusoidal pattern.
  • Fig. 6 shows schematically and exemplarily a further embodiment of the determination unit 23.
  • This embodiment of the determination unit comprises several frequency filters 27, 28, 29, each with a filter frequency allowing a detection signal of the detection unit 22, which has the respective filter frequency, to pass the respective frequency filter 27, 28, 29.
  • the filter frequencies of different frequency filters 27, 28, 29 are different.
  • the moving unit 17 is adapted to move the radiation and determination unit 16 with varying velocity and, if the detection signal has passed a frequency filter 27, 28, 29, the filter frequency of the passed frequency filter 27, 28, 29 is the determined frequency shift.
  • the relative velocity determination unit 18 is adapted to determine the relative velocity, based on the filter frequency of the passed frequency filter 27, 28, 29 and the velocity of the radiation and determination unit 16 relative to the first object at the time at which the detection signal has passed the respective frequency filter 27, 28, 29.
  • the filter frequencies of the several frequency filters 27, 28, 29 can be arranged equidistantly or non-equidistantly in the frequency domain.
  • a frequency range of interest has been predefined and the spacing of the frequency filters in the frequency domain is smaller in the frequency range of interest than outside this frequency range of interest.
  • a non-equidistant spacing of the frequency filters can increase the resolution and/or accuracy of the radiation and determination unit, i.e. of the laser sensor device, if the frequency spacing of the filters is reduced in a region of special interest.
  • a fixed moving unit velocity and frequency- tunable electronic filters can be applied, as well as a combined variation of both the sensor movement velocity and the filter frequencies.
  • the above described radiation and determination unit which is a laser sensor based on SMI, provides the possibility of measuring velocities, vibrations and distances and thus covers a broad range of applications as, for example, disclosed in the article "An overview of self-mixing sensing applications", T. Bosch, Proc. SPIE Conf. on Optoelectr. Microelectr. Mat. Dev., 385, 2004. SMI sensors make use of the effect that laser light which is scattered back, i.e.
  • the laser When the laser is operated not too far above the laser threshold, the response to the back-coupled light is linear, and the resulting variations in output power or frequency contain traceable information on the movement or the distance of the target object with respect to the sensor.
  • the laser output signal which contains the information, is preferentially collected via a photodiode. If the laser is operated with a defined current shape (e.g. a periodic sawtooth or triangular current), the output frequency almost instantaneously follows those current variations due to the simultaneously changed optical resonator length.
  • the resulting difference in frequency between the resonating light and the back-scattered light can be evaluated in a suitable electronic system and can be translated back into information about the position of the target object.
  • Information on the velocity of a target object can be obtained even simpler, as it is sufficient to operate the laser sensor with constant current; the velocity information then is contained in the Doppler frequency shift of the back-scattered laser light.
  • the radiation and determination unit comprises preferentially an infrared
  • the laser cavity preferentially consists of two stacks of Distributed Bragg Reflectors (DBRs), which are epitaxially grown on a suited substrate, and which enclose a gain region made up of several quantum wells.
  • DBRs Distributed Bragg Reflectors
  • the DBR layers also take over the task of feeding current into the gain region, which is the reason why one is usually n-doped and the other p-doped.
  • One DBR is designed to be highly reflective (typically the p-DBR with a reflectivity > 99.8%), while the other one allows a higher degree of outcoupling and thus also feedback to the laser cavity.
  • VCSELs Due to their surface emitting properties they can be produced and tested on wafer level in large quantities, which opens the possibility of a low-cost production process. Furthermore the output power can to a certain extent be scaled via the area of the emitting surface, and larger output powers can be achieved by using VCSEL arrays.
  • the working principle of the radiation and determination unit, in particular of the SMI laser sensor is, however, not restricted to surface emitting laser diodes. The SMI effect can also be harvested in edge emitting devices.
  • an external cavity is used for increasing both the coherence length and the power density of the laser sensor, thereby increasing the detection range, i.e. the possible distance between the first object and the second object, wherein a relative velocity is still measurable.
  • Such an external cavity is, for example, disclosed in WO 01/67563 A2, which is herewith incorporated by reference.
  • the apparatus for determining a relative velocity between a first object and a second object reduces the detection frequency, i.e. the frequency shift, which has to be determined, enables cheaper electronics and reduces the signal level necessary for a reliable measurement result.
  • the relative velocity between the laser sensor, i.e. the first object, and the target object, i.e. the second object, is reduced by moving the laser sensor with a known speed relative to the target object, e.g. with the help of a Piezo actuator or a magnetically driven vibrator operated at a suited and well-defined current shape, frequency and amplitude.
  • the reduction of the relative velocity immediately results in a reduced frequency shift, in particular a reduced Doppler frequency according to equation (2).
  • the radiation source emits radiation that is to be reflected by the second object 6.
  • the radiation is reflected, in particular backscattered, by the object 6 and the frequency shift determination unit determines a frequency shift between the emitted radiation and the reflected radiation in step 102.
  • the frequency shift determination unit is moved relative to the first object by the moving unit such that the relative velocity between the frequency shift determination unit and the second object is reduced, and in step 104, the relative velocity between the first object and the second object is determined based on the determined frequency shift and the velocity of the frequency shift determination unit relative to the first object is determined by a relative velocity determination unit.
  • a laser SMI sensor is used, in other embodiments other sensors can be used, which use the Doppler effect, for determining the frequency shift, which can be used for determining the relative velocity.
  • a radar can be used, which uses the radar Doppler shift.
  • the apparatus for determining a relative velocity between a first object and a second object is attached to a vehicle like a car, a bus or a truck, in other embodiments the apparatus can be attached to another moveable object or to an immobile object like, for example, a building.
  • Other variations to 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.
  • the emission of radiation, the determination of a frequency shift, the determination of a relative velocity and/or the movement of the frequency shift determination unit, in particular of the radiation and determination unit, performed by one or several units or devices can be performed by any other number of units or devices.
  • the determinations and/or the control of the apparatus in accordance with the above described method can be implemented as program code means of a computer program and/or as dedicated hardware.
  • a computer program may be stored/distributed on a suitable medium, such as an optical storage medium or a solid-state medium, supplied together with or as part of other hardware, but may also be distributed in other forms, such as via the Internet or other wired or wireless telecommunication systems. Any reference signs in the claims should not be construed as limiting the scope.

Abstract

The invention relates to an apparatus for determining a relative velocity between a first object (13), to which the apparatus (1) is attachable, and a second object (6). The apparatus (1) comprises a radiation source for emitting radiation that is to be reflected by the second object and a frequency shift determination unit for determining a frequency shift between the emitted radiation (14) and the reflected radiation (15). The apparatus further comprises a moving unit for moving the frequency shift determination unit relative to the first object (13) such that a relative velocity between the frequency shift determination unit and the second object (6) is reduced and a relative velocity determination unit for determining the relative velocity between the first object (13) and the second object (6), based on the determined frequency shift and the velocity of the movement of the frequency shift determination unit relative to the first object (13).

Description

APPARATUS FOR DETERMINING A RELATIVE VELOCITY
FIELD OF THE INVENTION
The invention relates to an apparatus, a method and a computer program for determining a relative velocity between a first object and a second object. The invention further relates to a vehicle comprising the apparatus.
BACKGROUND OF THE INVENTION
The article "A Double-Laser Diode Onboard Sensor for Velocity Measurements", X. Raoul, T. Bosch, G. Plantier and N. Servagent, IEEE Transactions on Instrumentation and Measurement, Vol. 53, February 2007, discloses a self-mixing interference (SMI) sensor on a car for a real-time velocity measurement.
When velocities are measured in e.g. automotive applications, using the SMI sensor, a laser beam is emitted and reflected by an object, wherein the relative velocity between the SMI sensor and the object is determined based on the Doppler frequency shift of the reflected laser beam. In the case of the object approaching the sensor, the frequency of the reflected beam /refl is shifted to higher values with respect to the emitted laser frequency /Las resulting in a Doppler frequency shift
A/ rel,Dop ~ /refl ~ J Las V-U larger than 0. In the opposite case of the sensor and the object drifting apart from each other, the Doppler frequency shift is negative. Since in automotive applications typical velocity values up to 100 km/h or more have to be dealt with, the resulting Doppler frequency shift
Δ/rel,Dop = ^|^ , (2)
which is to be detected and which depends on the relative velocity vrel Dop between the sensor and the object and on the wavelength λ of the laser beam, is then in the order of ~55 MHz or more for an emission wavelength of ~1 μm. Such a high frequency shift can be determined with expensive and sophisticated frequency shift determination units only.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an apparatus, a method and a computer program for determining a relative velocity between a first object and a second object, which allow the relative velocity to be determined with a simpler frequency shift determination unit, in particular simpler electronics for determining the frequency shift. It is a further object of the present invention to provide a corresponding vehicle comprising the apparatus.
In an aspect of the present invention an apparatus for determining a relative velocity between a first object, to which the apparatus is attachable, and a second object is presented, wherein the apparatus comprises: a radiation source for emitting radiation that is to be reflected by the second object, a frequency shift determination unit for determining a frequency shift between the emitted radiation and the reflected radiation reflected by the second object, a moving unit for moving the frequency shift determination unit relative to the first object such that a relative velocity between the frequency shift determination unit and the second object is reduced, a relative velocity determination unit for determining the relative velocity between the first object and the second object, based on the determined frequency shift and the velocity of the movement of the frequency shift determination unit relative to the first object.
The invention is based on the idea that by moving the frequency shift determination unit relative to the first object, in particular relative to an attaching element of the apparatus for attaching the apparatus to the first object, such that the determined relative velocity between the frequency determination unit and the second object and, thus, the determined frequency shift, is reduced, the requirements on the frequency shift determination unit and, thus, on the corresponding electronics can be reduced, because the frequency shift determination unit has to determine a smaller frequency shift only. Therefore, the frequency shift and, thus, the relative velocity between the first object and the second object, can be determined with a simpler frequency shift determination unit.
The frequency shift determination unit comprises preferentially several units for detecting the reflected radiation and for determining the frequency shift of the detected reflected radiation. In an embodiment, the whole frequency shift determination unit is moving, and in another embodiment, only one or some units of the frequency shift determination unit are moving, in particular at least the unit for detecting the reflected radiation.
The radiation source is preferentially a laser emitting a laser beam. Furthermore, the frequency shift determination unit is preferentially adapted to determine the Doppler frequency shift as the frequency shift caused by the relative movement between the frequency shift determination unit and the second object.
It is preferred that the radiation source and the frequency shift determination unit are integrated into a radiation and determination unit, wherein the radiation and determination unit comprises:
- a laser comprising a laser cavity for emitting a laser beam that is to be reflected by the second object, wherein the laser cavity is adapted such that a reflected laser beam reflected from the second object can be fed back into the laser cavity,
- a detection unit for generating a detection signal, depending on a change of a laser intensity, and
- a determination unit for determining a frequency shift of the reflected laser beam, based on the detection signal. This allows the relative velocity between the first object and the second object to be determined using self-mixing interference. By using self-mixing interference, the relative velocity can be determined very accurately with only one measurement at a time.
In a preferred embodiment, the frequency of the detection signal is the frequency shift which has to be determined. If the detection signal comprises several frequencies, the smallest frequency of the detection signal is preferentially the frequency shift. If the detection signal comprises a fundamental frequency and at least one higher harmonic, the fundamental frequency is preferentially the frequency shift which has to be determined. The frequency shift is preferentially determined by using a Fourier transformation, in particular a Fast Fourier Transformation (FFT). It is further preferred that the determination unit comprises a frequency filter with a filter frequency allowing a detection signal of the detection unit, which has the filter frequency, to pass the frequency filter, wherein the moving unit is adapted to move the radiation and determination unit with varying velocity during determining the frequency shift and/or the frequency filter is adapted to vary the filter frequency during determining the frequency shift, wherein, if the detection signal has passed the frequency filter, the passed filter frequency is the determined frequency shift and the relative velocity determination unit is adapted to determine the relative velocity between the first object and the second object, based on the passed filter frequency and the velocity of the radiation and determination unit relative to the first object at the time at which the detection signal has passed the frequency filter. This allows the use of a simple analogue or digital filtering technique, without the need to use, for example, a FFT. This further simplifies the frequency shift determination unit.
In a preferred embodiment, the determination unit comprises several frequency filters, each with a filter frequency allowing a detection signal of the detection unit, which has the respective filter frequency, to pass the respective frequency filter, wherein filter frequencies of different frequency filters are different, wherein the moving unit is adapted to move the radiation and determination unit with varying velocity during determining the frequency shift and/or wherein the frequency filters are adapted to vary the filter frequencies during determining the frequency shift, and wherein, if the detection signal has passed a frequency filter, the passed filter frequency is the determined frequency shift, and the relative velocity determination unit is adapted to determine the relative velocity between the first object and the second object, based on the passed filter frequency and the velocity of the radiation and determination unit relative to the first object at the time at which the detection signal has passed the frequency filter. Since several frequency filters having different filter frequencies are present, the velocity range of the velocities of the movement of the radiation and determination unit can be decreased, without decreasing the velocity range of the relative velocity between the first object and the second object, which can be detected by the apparatus. Thus, the requirements on the performance of the moving unit can be reduced.
It is further preferred that the filter frequencies of the several frequency filters are non-equidistantly arranged in the frequency domain. This allows the resolution of the velocity determination of the apparatus to be increased in a frequency region of interest, which preferentially corresponds to a velocity region of interest. Preferentially, within the frequency region of interest, the filter frequencies of the frequency filters are arranged equidistantly. For example, if the frequency region of interest is 40 MHz to 50 MHz, preferentially the filter frequencies are located at 40 MHz, 42 MHz, 44 MHz, 46 MHz, 48 MHz and 50 MHz.
In a preferred embodiment, the apparatus comprises a control unit for controlling the moving unit, depending on the determined relative velocity between the first object and the second object. This allows control of the moving unit, depending on the determined relative velocity; in particular if the relative velocity is so high that it cannot be determined by the apparatus by using the present speed of the movement generated by the moving unit, the speed generated by the moving unit can be increased in order to reduce the relative velocity and, thus, the frequency shift such that the frequency shift and therefore the relative velocity can be determined by the apparatus. It is further preferred that the moving unit is adapted for moving the frequency shift determination unit relative to the first object such that the frequency shift determination unit performs a periodic movement, wherein the periodic movement is adapted for moving the frequency shift determination unit during a phase of the periodic movement such that the relative velocity between the frequency shift determination unit and the second object is reduced.
It is further preferred that the apparatus is adapted to determine a relative velocity within a predefined velocity range between the first object and the second object, wherein the moving unit is adapted such that the absolute value of the velocity of movement of the frequency shift determination unit relative to the first object is smaller than or equal to an absolute value of a relative velocity within the velocity range. In particular, the absolute value of the velocity of movement of the frequency shift determination unit relative to the first object is smaller than or equal to the absolute value of the relative velocity having the smallest absolute value within the velocity range.
If, for example, the predefined velocity range is 50 km/h to 200 km/h, the absolute value of the velocity of the frequency shift determination unit relative to the first object is preferentially 50 km/h or smaller. If the frequency shift determination unit is periodically moved with respect to the first object, preferentially the velocity of the periodic movement is modified by modifying the frequency, wherein the amplitude of the periodic movement is preferentially constant.
In a further aspect of the present invention a vehicle is presented, which comprises an apparatus for determining a relative velocity between a first object being the vehicle and a second object as defined in claim 1.
In a further aspect of the present invention a method of determining a relative velocity between a first object and a second object by using an apparatus as defined in claim 1 is presented, wherein the method comprises the following steps: emitting radiation, that is to be reflected by the second object, by means of a radiation source, determining, by means of a frequency shift determination unit, a frequency shift between the emitted radiation and the reflected radiation reflected by the second object, moving the frequency shift determination unit relative to the first object by means of a moving unit in such a manner that the relative velocity between the frequency shift determination unit and the second object is reduced, determining, by means of a relative velocity determination unit, the relative velocity between the first object and the second object, based on the determined frequency shift and the velocity of the frequency shift determination unit relative to the first object.
In a further aspect of the present invention a computer program for determining a relative velocity between a first object and a second object is presented, wherein the computer program comprises program code means for causing an apparatus 1 as defined in claim 1 to carry out the steps of the method as defined in claim 10, provided the computer program is run on a computer controlling the apparatus 1. It will be understood that the apparatus of claim 1 , the vehicle of claim 9, the method of claim 10 and the computer program of claim 11 have similar and/or identical preferred embodiments as defined in the dependent claims.
It will be understood that a preferred embodiment of the invention can also be any combination of the dependent claims with a respective independent claim.
BRIEF DESCRIPTION OF THE DRAWINGS
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. In the following drawings:
Fig. 1 shows schematically and exemplarily an embodiment of a vehicle comprising an apparatus for determining a relative velocity, Fig. 2 shows schematically and exemplarily an embodiment of an apparatus for determining a relative velocity, Fig. 3 shows schematically and exemplarily another embodiment of an apparatus for determining a relative velocity, Fig. 4 shows schematically and exemplarily in more detail an apparatus for determining a relative velocity,
Fig. 5 shows schematically and exemplarily a detection unit, a determination unit and a relative velocity determination unit of an apparatus for determining a relative velocity, Fig. 6 shows schematically and exemplarily a detection unit, a determination unit and a relative velocity determination unit of an embodiment of an apparatus for determining a relative velocity, and Fig. 7 shows exemplarily a flowchart illustrating an embodiment of a method for determining a relative velocity.
DETAILED DESCRIPTION OF EMBODIMENTS
Fig. 1 shows schematically and exemplarily a vehicle 13, which can be regarded as a first object and which comprises an apparatus 1 for determining a relative velocity between the vehicle 13 and a second object 6, which can be, for example, a wall. The second object can be any object which is able to reflect radiation from the apparatus such that a frequency shift of the reflected radiation can be determined by the apparatus. For example, the second object 6 can also be a ground or another vehicle.
The apparatus 1 for determining a relative velocity between the first object 13, to which the apparatus 1 is attached, and the second object 6 is adapted to emit radiation 14 that is to be reflected by the second object 6 and to determine a frequency shift between the emitted radiation 14 and reflected radiation 15, which is reflected by the second object 6, wherein the frequency shift is caused by a relative movement between the first object 13 and the second object 6, in particular by a relative movement between a frequency shift determination unit, which will be explained further below, and the second object 6.
An embodiment of the apparatus 1 for determining a relative velocity between the first object 13 and the second object 6 is schematically and exemplarily shown in Fig. 2. The apparatus 1 comprises a radiation source 2 for emitting radiation 14 that is to be reflected by the second object 6. The apparatus 1 further comprises a frequency shift determination unit 11 for determining a frequency shift between the emitted radiation 14 and the reflected radiation 15, which has been reflected by the second object 6, wherein the frequency shift is caused by a relative movement between the first object 13 and the second object 6, in particular by a relative movement between the frequency shift determination unit 11 and the second object 6.
The apparatus 1 further comprises a relative velocity determination unit 12 for determining the relative velocity between the first object 13 and the second object 6, based on the determined frequency shift, and a moving unit 8 for moving the frequency shift determination unit 11 relative to the first object 13 such that the relative velocity is reduced. Preferentially, the frequency shift determination unit is moved towards the second object 6 relative to the first object 13, if the distance between the first object 6 and the second object 13 increases, and the frequency shift determination unit is moved away from the second object 6 relative to the first object 13, if the distance between the first object 6 and the second object 13 decreases.
The apparatus 1 further comprises a control unit 9 for controlling the moving unit, depending on the determined relative velocity. In particular, the moving unit is controlled such that the speed of the movement of the frequency shift determination unit 11 is adapted such that a relative velocity can be measured. For example, the speed of the movement of the frequency shift determination unit towards the second object 6 is increased, if the relative velocity increases.
The moving unit 8 is preferentially adapted for moving the frequency shift determination unit 11 relative to the first object such that the frequency shift determination unit 11 performs a periodic movement, wherein the periodic movement is adapted for moving the frequency shift determination unit 11 during a phase of the periodic movement such that the relative velocity between the frequency shift determination unit and the second object is reduced. In particular, the frequency shift determination unit 11 performs a forward-backward movement, wherein preferentially the forward or the backward movement of the frequency shift determination unit is a movement towards or away from the second object 6. The apparatus 1 is adapted to determine a relative velocity within a predefined velocity range between the first object 13 and the second object 6, wherein the moving unit 8 is adapted such that an absolute value of the velocity of movement of the frequency shift determination unit 11 relative to the first object 13 is smaller than or equal to an absolute value of a relative velocity within the velocity range. Thus, depending on the application of the apparatus, expected relative velocities are known which define a predefined velocity range, and, preferentially, the moving unit is adapted such that the absolute value of the velocity of the frequency shift determination unit relative to the first object 13 is equal to or smaller than the absolute value of the velocity having the smallest absolute value within the velocity range. If the frequency shift determination unit is periodically moved with respect to the first object, preferentially the velocity of the periodic movement is modified by modifying the frequency, wherein the amplitude of the periodic movement is preferentially constant.
A further embodiment of an apparatus 25 for determining a relative velocity between a first object 13, to which the apparatus 25 is attachable, and a second object 6 is schematically and exemplarily shown in Fig. 3.
The apparatus 25 comprises a radiation and determination unit 16, in which the radiation source and the frequency shift determination unit are integrated. The radiation and determination unit 16 will be described in more detail further below with reference to Fig. 4.
Also the radiation and determination unit 16 emits radiation 14 that is to be reflected by the second object 6 and determines a frequency shift between the emitted radiation 14 and the reflected radiation 15, which has been reflected by the second object 6, wherein also this frequency shift is caused by a relative movement between the first object 13 and the second object 6, in particular by a relative movement between the radiation and determination unit 16 and the second object 6. Similar to the apparatus 1, also the apparatus 25 comprises a relative velocity determination unit 18 for determining the relative velocity based on the determined frequency shift and a moving unit 17 for moving the radiation and determination unit 16 relative to the first object 13 such that the relative velocity between the radiation and determination unit 16 and the second object is reduced. Furthermore, also the apparatus 25 comprises a control unit 24 for controlling the moving unit 17 depending on the determined relative velocity, and preferentially also the moving unit 17 is adapted for moving the radiation and determination unit 16 relative to the first object 13 such that the radiation and determination unit 16 and, thus, the frequency shift determination unit, which is a part of the radiation and determination unit 16, performs a periodic movement, wherein the periodic movement is adapted for moving the radiation and determination unit 16 during a phase of the periodic movement such that the relative velocity between the radiation and determination unit 16 and the second object 6 is reduced. Also the apparatus 25 is preferentially adapted to determine a relative velocity within a predefined velocity range between the first object 13 and the second object 6, wherein the moving unit 17 is adapted such that an absolute value of the velocity of movement of the frequency shift determination unit 20, 22, 23 relative to the first object 13 is smaller than or equal to an absolute value of a relative velocity within the velocity range.
Fig. 4 shows schematically and exemplarily an embodiment of the apparatus 25 in more detail.
As already mentioned above, a radiation source 20 and a frequency determination unit 20, 22, 23 are integrated into a radiation and determination unit 16.
The radiation source 20 is a laser 20 comprising a laser cavity 21 for emitting a laser beam 14 that is to be reflected by the second object 6, wherein the laser cavity 21 is adapted such that a reflected laser beam 15, which has been reflected from the second object 6, can be fed back into the laser cavity 21. The laser cavity 21 comprises two reflectors 30, 31 and an active medium 40. The reflected laser beam 15 interferes with the radiation within the laser cavity 21, as a result of which the laser intensity of the laser 20 is changed. This intensity change is detected by a detection unit 22 for generating a detection signal depending on the change in the laser intensity. The generated detection signal of the detection unit 22 is used by a determination unit 23 for determining a frequency shift of the reflected laser beam, and the relative velocity determination unit 18 determines the relative velocity between the first object and the second object, based on the determined frequency shift and the velocity of the movement of the radiation and determination unit 16 caused by the moving unit 17, which is, in this embodiment, a Piezo element controlled by the control unit 24.
The radiation and determination unit 16 is a SMI sensor based on a surface emitting semiconductor laser (VCSEL), which consists of an electrically pumped gain medium 40 (in this embodiment, InGaAs quantum wells embedded in GaAs) embedded between two Distributed Bragg Reflectors (n-DBR 30 and p-DBR 31), which form the laser cavity 21. The lower DBR 31 is highly reflective (reflectivity > 99.8%), while the reflectivity of the upper DBR 30 is smaller in order to increase feedback from the light 15 scattered back by the target object. One of the DBRs is p-doped 31 and the other n-doped 30 to enable efficient current feeding into the gain region. In most of the known VCSEL devices, n- and p-type doping take place as described above (p-doping of the DBR with higher reflectivity), but also doping in the reversed order is possible as, for example, disclosed in WO 2005/117070 A2, which is herewith incorporated by reference.
Via n- and p-DBR electrical contacts (not shown in Fig. 4) a current is fed into the gain region. The detection unit 22, which is preferentially a photodetector and which is attached to the VCSEL chip, measures the small amount of radiation leaking out of the highly reflective p-DBR mirror 31 and thus monitors the influence of the back-scattered light 15, i.e. of the reflected light, from the target object 6, being the second object, on the laser 20, from which information on the relative velocity between the first object and the second object and, if the velocity of the first object 13 is known, the velocity of the target object 6, i.e. the second object, can be extracted. In an alternative embodiment the detection unit 22, in particular the photodetector, can also be placed separate from the VCSEL chip, e.g. by separating a small portion of the emitted laser radiation from the main beam with the help of a suited beam splitter.
The complete laser sensor, i.e. the radiation and determination unit 16, is mounted on the moving unit 17 being preferentially a Piezo element, which is driven at a certain frequency and amplitude, which are known and can be controlled very accurately by the control unit 24. The preferred movement direction is denoted by reference number 32. A preferred current pattern has a triangular or sawtooth shape. Assuming a linear response of the Piezo element to the applied current, the velocity of the sensor movement vsens can be easily calculated. For a sawtooth current with a frequency/ and a maximum amplitude A(u) of the movement, which is directly related to the voltage U applied to the Piezo element, the following relation holds: ym = 2- A(U)- f . (3)
For a maximum amplitude of 100 μm, which can be typically achieved with commercially available Piezo elements without difficulties, a motion frequency of -139 kHz would be necessary to compensate a target movement of 100 km/h with respect to the first object. This frequency is also well within reach for common Piezo elements.
In this embodiment the SMI laser sensor, i.e. the radiation and determination unit 16, is based on a VCSEL, but also other semiconductor laser sources, such as edge emitting lasers or VECSELS (VCSEL with additional extended cavity), can be used, as well as arrays of VCSEL or VECSEL sensors.
In conventional SMI -based velocity sensors the speed of the target object is usually obtained by performing a FFT of the photodiode signal in suited evaluation electronics, which yields a peak related to the Doppler frequency according to the above mentioned equation (2), allowing the determination of the relative velocity vrel Dop between sensor 16 and target object 6. This is also possible here, with the additional benefit of a reduced detection frequency due to the reduced relative velocity between velocity sensor 16, i.e. the radiation and determination unit 16, and target object 6. The relative velocity V12 = V1 - V2 (4) between the first object 13 and the second object 6, i.e. the target object, can be obtained by the following equation: v12 = vrel;Dop - vseils . (5)
The sign of the relative velocity vrel Dop between sensor 16 and target object 6 is defined by equations (1) and (2), while all other velocities will be positive if they point in the direction of movement of the first object 13 and negative in the opposite case. For example, according to equations (1) and (2) the relative velocity v reiDoP is larger than 0 if the sensor and the target object approach each other, and negative in the opposite case.
The target velocity v2 can then easily be obtained via equation (4), wherein the velocity of the first object 13 V1 is preferentially known from another velocity measurement.
In this case it should of course be ensured that for the FFT only data are evaluated from sample intervals during which the sensor moves with a well-defined constant velocity vsens , and during which the sensor movement heads in the right direction, which reduces the relative velocity between sensor and target object. Furthermore, the vibration frequency of the Piezo element is preferentially substantially lower than the expected Doppler frequency in order to guarantee a proper FFT result with sufficient accuracy. On the other hand the FFT sampling frequency should be high enough to obtain a sufficiently high frequency and thus velocity resolution.
The laser structure of the SMI sensor is preferentially epitaxially grown on a GaAs substrate 33. The apparatus for determining a relative velocity between a first object and a second object is preferentially adapted to measure the relative velocity along the emission direction of the light emitted by the radiation source. In other embodiments, the apparatus can be adapted to measure the relative velocity also in other directions, for example, by using several radiation sources and frequency shift determination units, in particular, several SMI laser sensors, pointing in different directions, wherein, for example, the resulting relative velocities determined in different directions can be combined to a relative velocity in a desired direction.
The moving unit 17 is attached to an attaching element for attaching the apparatus to the first object. The attaching element is preferentially a mounting plate 34, which is preferentially mounted on the first object 13. In other embodiments, the attaching element can be any other attaching element which allows the apparatus 25 to be attached to an object such that the frequency shift determination unit, in particular the radiation and determination unit 16, which is, in this embodiment, a laser SMI sensor, is moveable with respect to the object, in particular with respect to the mounting element.
Preferentially, the determination unit is adapted such that a FFT is not needed; such an embodiment of the determination unit 23 is schematically and exemplarily shown in Fig. 5.
The determination unit 23 comprises a frequency filter 26 with a filter frequency for allowing a detection signal of the detection unit 22, which has the filter frequency, to pass the frequency filter 26. The moving unit 17 is preferentially adapted to move the radiation and determination unit 16, i.e., in this embodiment, the laser SMI sensor, with varying velocity, wherein, if the detection signal has passed the frequency filter 26, the passed filter frequency is the determined frequency shift and the relative velocity determination unit 18 is adapted to determine the relative velocity based on the passed filter frequency and the velocity of the radiation and determination unit 16 relative to the first object at the time at which the detection signal has passed the frequency filter 26.
The use of the frequency filter 26 allows a detection concept, which does not require FFT, but may allow simpler analogue or digital filtering. In this case the detection signal can be filtered with a narrow bandwidth at a suited fixed frequency, while the frequency of the movement of the moving unit 17, which is preferentially a Piezo element or a Piezo actuator, is varied, in particular swept. The control unit 24 registers at which moving unit frequency, in particular at which moving unit velocity, an electronic signal passes the frequency filter 26. The filter frequency then corresponds to the frequency shift, in particular to the Doppler frequency, of the relative movement between the radiation and determination unit 16, in particular the laser SMI sensor, and the target object 6, and the determination of the radiation and determination unit movement velocity via the motion frequency at filter resonance conditions allows determining the velocity of the target object. The variation of the radiation and determination unit motion velocity may, for example, either be realized by continuously varying in a controlled way the carrier frequency and/or the amplitude of a sawtooth pattern governing the radiation and determination unit movement or by using another suited drive scheme such as, for example, a sinusoidal pattern. In the latter case, phase-resolved detection of the filter resonance with respect to the radiation and determination unit motion must be applied, as the speed of the moving unit 17 also varies within the sinusoidal pattern.
Fig. 6 shows schematically and exemplarily a further embodiment of the determination unit 23.
This embodiment of the determination unit comprises several frequency filters 27, 28, 29, each with a filter frequency allowing a detection signal of the detection unit 22, which has the respective filter frequency, to pass the respective frequency filter 27, 28, 29. The filter frequencies of different frequency filters 27, 28, 29 are different. In this embodiment, the moving unit 17 is adapted to move the radiation and determination unit 16 with varying velocity and, if the detection signal has passed a frequency filter 27, 28, 29, the filter frequency of the passed frequency filter 27, 28, 29 is the determined frequency shift. The relative velocity determination unit 18 is adapted to determine the relative velocity, based on the filter frequency of the passed frequency filter 27, 28, 29 and the velocity of the radiation and determination unit 16 relative to the first object at the time at which the detection signal has passed the respective frequency filter 27, 28, 29. The filter frequencies of the several frequency filters 27, 28, 29 can be arranged equidistantly or non-equidistantly in the frequency domain. In an embodiment, a frequency range of interest has been predefined and the spacing of the frequency filters in the frequency domain is smaller in the frequency range of interest than outside this frequency range of interest.
The use of several frequency filters in parallel, which are preferentially electronic filters, reduces the demands on the performance of the moving unit, in particular on the Piezo actuator. In the above mentioned example of an operation range up to -100 km/h, which results in a Doppler frequency of ~55 MHz, a set of for instance 10 electronic filters equally spaced between 5 MHz, 10 MHz, ... 50 MHz would either reduce the required frequency of the moving unit, in particular the Piezo frequency, or the amplitude range by an order of magnitude, allowing the use of less complicated and less expensive moving unit devices, in particular Piezo devices.
As already mentioned above, a non-equidistant spacing of the frequency filters can increase the resolution and/or accuracy of the radiation and determination unit, i.e. of the laser sensor device, if the frequency spacing of the filters is reduced in a region of special interest.
Instead of using fixed filter frequencies and varying the velocity of the moving unit, in particular the vibration velocity of the moving unit, a fixed moving unit velocity and frequency- tunable electronic filters can be applied, as well as a combined variation of both the sensor movement velocity and the filter frequencies. The above described radiation and determination unit, which is a laser sensor based on SMI, provides the possibility of measuring velocities, vibrations and distances and thus covers a broad range of applications as, for example, disclosed in the article "An overview of self-mixing sensing applications", T. Bosch, Proc. SPIE Conf. on Optoelectr. Microelectr. Mat. Dev., 385, 2004. SMI sensors make use of the effect that laser light which is scattered back, i.e. reflected, from a target object and re-enters the laser cavity, interferes with the resonating radiation and thus influences the output properties of the device. When the laser is operated not too far above the laser threshold, the response to the back-coupled light is linear, and the resulting variations in output power or frequency contain traceable information on the movement or the distance of the target object with respect to the sensor. The laser output signal, which contains the information, is preferentially collected via a photodiode. If the laser is operated with a defined current shape (e.g. a periodic sawtooth or triangular current), the output frequency almost instantaneously follows those current variations due to the simultaneously changed optical resonator length. The resulting difference in frequency between the resonating light and the back-scattered light can be evaluated in a suitable electronic system and can be translated back into information about the position of the target object. Information on the velocity of a target object can be obtained even simpler, as it is sufficient to operate the laser sensor with constant current; the velocity information then is contained in the Doppler frequency shift of the back-scattered laser light. The radiation and determination unit comprises preferentially an infrared
VCSEL laser. The laser cavity preferentially consists of two stacks of Distributed Bragg Reflectors (DBRs), which are epitaxially grown on a suited substrate, and which enclose a gain region made up of several quantum wells. The DBR layers also take over the task of feeding current into the gain region, which is the reason why one is usually n-doped and the other p-doped. One DBR is designed to be highly reflective (typically the p-DBR with a reflectivity > 99.8%), while the other one allows a higher degree of outcoupling and thus also feedback to the laser cavity.
The big advantage of VCSELs is that due to their surface emitting properties they can be produced and tested on wafer level in large quantities, which opens the possibility of a low-cost production process. Furthermore the output power can to a certain extent be scaled via the area of the emitting surface, and larger output powers can be achieved by using VCSEL arrays. The working principle of the radiation and determination unit, in particular of the SMI laser sensor is, however, not restricted to surface emitting laser diodes. The SMI effect can also be harvested in edge emitting devices.
In an embodiment, an external cavity is used for increasing both the coherence length and the power density of the laser sensor, thereby increasing the detection range, i.e. the possible distance between the first object and the second object, wherein a relative velocity is still measurable. Such an external cavity is, for example, disclosed in WO 01/67563 A2, which is herewith incorporated by reference.
As already mentioned above, when measuring velocities in automotive applications, typical speed values up to 100 km/h or more have to be dealt with. The resulting Doppler frequency shift is then in the order of ~55 MHz or more for an emission wavelength of ~1 μm. Such a high value imposes high demands on the frequency resolution of the detection system and requires expensive electronics and measures for noise reduction. The apparatus for determining a relative velocity between a first object and a second object reduces the detection frequency, i.e. the frequency shift, which has to be determined, enables cheaper electronics and reduces the signal level necessary for a reliable measurement result.
The relative velocity between the laser sensor, i.e. the first object, and the target object, i.e. the second object, is reduced by moving the laser sensor with a known speed relative to the target object, e.g. with the help of a Piezo actuator or a magnetically driven vibrator operated at a suited and well-defined current shape, frequency and amplitude. The reduction of the relative velocity immediately results in a reduced frequency shift, in particular a reduced Doppler frequency according to equation (2). In the following a method of determining a relative velocity between a first object and a second object will be illustrated with reference to a flowchart shown in Fig. 7.
In step 101, the radiation source emits radiation that is to be reflected by the second object 6. The radiation is reflected, in particular backscattered, by the object 6 and the frequency shift determination unit determines a frequency shift between the emitted radiation and the reflected radiation in step 102. In step 103, the frequency shift determination unit is moved relative to the first object by the moving unit such that the relative velocity between the frequency shift determination unit and the second object is reduced, and in step 104, the relative velocity between the first object and the second object is determined based on the determined frequency shift and the velocity of the frequency shift determination unit relative to the first object is determined by a relative velocity determination unit.
Although in the above described embodiment, a laser SMI sensor is used, in other embodiments other sensors can be used, which use the Doppler effect, for determining the frequency shift, which can be used for determining the relative velocity. For example, a radar can be used, which uses the radar Doppler shift. Although in the above described embodiment the apparatus for determining a relative velocity between a first object and a second object is attached to a vehicle like a car, a bus or a truck, in other embodiments the apparatus can be attached to another moveable object or to an immobile object like, for example, a building. Other variations to 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.
In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. A single unit or device may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.
The emission of radiation, the determination of a frequency shift, the determination of a relative velocity and/or the movement of the frequency shift determination unit, in particular of the radiation and determination unit, performed by one or several units or devices can be performed by any other number of units or devices. The determinations and/or the control of the apparatus in accordance with the above described method can be implemented as program code means of a computer program and/or as dedicated hardware.
A computer program may be stored/distributed on a suitable medium, such as an optical storage medium or a solid-state medium, supplied together with or as part of other hardware, but may also be distributed in other forms, such as via the Internet or other wired or wireless telecommunication systems. Any reference signs in the claims should not be construed as limiting the scope.

Claims

CLAIMS:
1. An apparatus for determining a relative velocity between a first object
(13), to which the apparatus (1; 25) is attachable, and a second object (6), wherein the apparatus (1; 25) comprises:
- a radiation source (2; 20) for emitting radiation (14) that is to be reflected by the second object (6), - a frequency shift determination unit (11; 20, 22, 23) for determining a frequency shift between the emitted radiation (14) and the reflected radiation (15) reflected by the second object (6),
- a moving unit (8; 17) for moving the frequency shift determination unit (11; 20, 22, 23) relative to the first object (13) such that a relative velocity between the frequency shift determination unit (11 ; 20, 22, 23) and the second object (6) is reduced,
- a relative velocity determination unit (12; 18) for determining the relative velocity between the first object (13) and the second object (6), based on the determined frequency shift and the velocity of the movement of the frequency shift determination unit (11; 20, 22, 23) relative to the first object (13).
2. The apparatus as defined in claim 1, wherein the radiation source (20) and the frequency shift determination unit (20, 22, 23) are integrated into a radiation and determination unit (16), wherein the radiation and determination unit (16) comprises:
- a laser (20) comprising a laser cavity (21) for emitting a laser beam (14) that is to be reflected by the second object (6), wherein the laser cavity (21) is adapted such that a reflected laser beam (15) reflected from the second object (6) can be fed back into the laser cavity (21),
- a detection unit (22) for generating a detection signal, depending on a change in laser intensity, and - a determination unit (23) for determining a frequency shift of the reflected laser beam (15), based on the detection signal.
3. The apparatus as defined in claim 2, wherein the determination unit (23) comprises a frequency filter (26) with a filter frequency allowing a detection signal of the detection unit (22), which has the filter frequency, to pass the frequency filter (26), wherein the moving unit (17) is adapted to move the radiation and determination unit (16) with varying velocity during determining the frequency shift and/or the frequency filter (26) is adapted to vary the filter frequency during determining the frequency shift, wherein, if the detection signal has passed the frequency filter (26), the passed filter frequency is the determined frequency shift and the relative velocity determination unit (18) is adapted to determine the relative velocity between the first object (13) and the second object (6), based on the passed filter frequency and the velocity of the radiation and determination unit (16) relative to the first object (13) at the time at which the detection signal has passed the frequency filter.
4. The apparatus as defined in claim 3, wherein the determination unit (23) comprises several frequency filters (27, 28, 29), each with a filter frequency allowing a detection signal of the detection unit (22), which has the respective filter frequency, to pass the respective frequency filter, wherein filter frequencies of different frequency filters (27, 28, 29) are different, wherein the moving unit (17) is adapted to move the radiation and determination unit (16) with varying velocity during determining the frequency shift and/or wherein the frequency filters (27, 28, 29) are adapted to vary the filter frequencies during determining the frequency shift, and wherein, if the detection signal has passed a frequency filter (27, 28, 29), the passed filter frequency is the determined frequency shift and the relative velocity determination unit (18) is adapted to determine the relative velocity between the first object (13) and the second object (6), based on the passed filter frequency and the velocity of the radiation and determination unit (16) relative to the first object (13) at the time at which the detection signal has passed the frequency filter (27, 28, 29).
5. The apparatus as defined in claim 4, wherein the filter frequencies of the several frequency filters (27, 28, 29) are non-equidistantly arranged in a frequency domain.
6. The apparatus as defined in claim 1, wherein the apparatus (1; 25) comprises a control unit (9; 24) for controlling the moving unit (8; 17), depending on the determined relative velocity between the first object (13) and the second object (6).
7. The apparatus as defined in claim 1, wherein the moving unit (8; 17) is adapted for moving the frequency shift determination unit (11; 20, 22, 23) relative to the first object (13) such that the frequency shift determination unit (11; 20, 22, 23) performs a periodic movement, wherein the periodic movement is adapted for moving the frequency shift determination unit (11; 20, 22, 23) during a phase of the periodic movement such that the relative velocity between the frequency shift determination unit (11; 20, 22, 23) and the second object (6) is reduced.
8. The apparatus as defined in claim 1, wherein the apparatus (1; 25) is adapted to determine a relative velocity within a predefined velocity range between the first object (13) and the second object (6), wherein the moving unit (8; 17) is adapted such that an absolute value of the velocity of movement of the frequency shift determination unit (11; 20, 22, 23) relative to the first object (13) is smaller than or equal to an absolute value of a relative velocity within the velocity range.
9. A vehicle comprising an apparatus (1; 25) for determining a relative velocity between a first object (13) being the vehicle and a second object (6), as defined in claim 1.
10. A method of determining a relative velocity between a first object (13) and a second object (6) by using an apparatus as defined in claim 1, wherein the method comprises the following steps:
- emitting radiation, that is to be reflected by the second object (6), by means of a radiation source (2), - determining, by means of a frequency shift determination unit (11; 20,
22, 23), a frequency shift between the emitted radiation and the reflected radiation reflected by the second object,
- moving the frequency shift determination unit (11; 20, 22, 23) relative to the first object, by means of a moving unit (8), in such a manner that the relative velocity between the frequency shift determination unit (11; 20, 22, 23) and the second object (6) is reduced,
- determining, by means of a relative velocity determination unit (12), the relative velocity between the first object (13) and the second object (6), based on the determined frequency shift and the velocity of the frequency shift determination unit (11; 20, 22, 23) relative to the first object (13).
11. A computer program for determining a relative velocity between a first object and a second object, the computer program comprising program code means for causing an apparatus (1) as defined in claim 1 to carry out the steps of the method as defined in claim 10, when the computer program is run on a computer controlling the apparatus (1).
PCT/IB2009/052360 2008-06-19 2009-06-04 Apparatus for determining a relative velocity WO2009153689A1 (en)

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EP08104466 2008-06-19

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Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
GB887646A (en) * 1958-01-31 1962-01-24 Gen Precision Inc Improvements in or relating to the measurement of velocities
DE4127528A1 (en) * 1991-08-20 1993-02-25 Alexander W Dr Ing Koch Laser velocimetry of moving object surface layers in plasma processing - using vibrating mirror whose reflection is correlated with that from object mirror, for evaluation of speed-dependent Doppler shift
WO2001067563A2 (en) * 2000-03-06 2001-09-13 Novalux, Inc. Coupled cavity high power semiconductor laser
US20030031221A1 (en) * 2000-04-05 2003-02-13 Coretek, Inc. Single mode operation of microelectromechanically tunable, half-symmetric, vertical cavity surface emitting lasers

Patent Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
GB887646A (en) * 1958-01-31 1962-01-24 Gen Precision Inc Improvements in or relating to the measurement of velocities
DE4127528A1 (en) * 1991-08-20 1993-02-25 Alexander W Dr Ing Koch Laser velocimetry of moving object surface layers in plasma processing - using vibrating mirror whose reflection is correlated with that from object mirror, for evaluation of speed-dependent Doppler shift
WO2001067563A2 (en) * 2000-03-06 2001-09-13 Novalux, Inc. Coupled cavity high power semiconductor laser
US20030031221A1 (en) * 2000-04-05 2003-02-13 Coretek, Inc. Single mode operation of microelectromechanically tunable, half-symmetric, vertical cavity surface emitting lasers

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