Doppler effect based deflectometry system and method using multiple reference sensors

Abstract

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Description

[0001] DOPPLER EFFECT BASED DEFLECTOMETRY SYSTEM AND METHOD USING

[0002] MULTIPLE REFERENCE SENSORS

[0003] TECHNICAL FIELD

[0004] The present invention relates to the field of deflectometry and deflection of surface placement, in particular non-contact deflectometry based on Doppler shift measurements.

[0005] BACKGROUND OF THE INVENTION

[0006] The present invention relates to the field of non-contact deflectometry, a technique pivotal for the measurement of surface deflection. Specifically, it addresses the challenges in deflectometry systems that employ the Doppler effect for detecting surface deviations.

[0007] Deflectometry, as a practice, is instrumental in gauging the characteristics of various surfaces, such as roads, conveyor belts, and similar structures. Key attributes like carrying capacity, durability, wear, and material integrity are often determined by measuring the deflection of these surfaces under predetermined loads. Traditional methods have utilized various techniques for this purpose, with one prevalent approach being the detection of Doppler shifts in coherent light reflected off a surface moving relative to the detection system.

[0008] Doppler-based deflectometry offers distinct advantages, primarily its non-contact nature. This aspect is particularly beneficial as it eliminates the need for direct physical interaction with the surface being measured. Consequently, it allows for continuous operation of the system involved, such as uninterrupted traffic on a road or ongoing activity on a conveyor belt, while measurements are conducted.

[0009] However, these methods have inherent limitations. The high sensitivity to minute distance changes between the sensor and the surface leads to accuracy issues due to speckle decorrelation on optically rough surfaces and interference from external contaminants like dirt. Speckle decorrelation, in particular, causes fluctuations in the Doppler signal, resulting in misleading readings. Although solutions like receiver diversification have been suggested to maintain a constant distance to the measured surface, these solutions are technically complex and expensive. Measurement noise causes uncertainty for the determined deflection and when applying multiple sensors for measurements along a deflection basin, any small error at each point accumulates for the final signal. Typically, this is handled by a reference sensor, but if the reference sensor does not measure correctly or is not sufficiently outside the deflection basin, the results will be unreliable. The calculated slope for each of the measuring sensor heads contains additive white gaussian noise from both the slope-measuring sensor and the reference sensor. By averaging readings from multiple sensors aimed at the same target for every measured point, noise can be reduced, however this is expensive. Often it is desirable to integrate the deflection velocity over time or distance to obtain the deflection depth. Since all calculated deflection velocities contain correlated noise from the reference source, the noise of a reference sensor scales up with the length of the deflection basin independently of the number of measuring positions along the deflection basin.

[0010] In light of these challenges, there is a need for a more efficient and cost-effective solution for non-contact deflectometry providing improved signals less affected by small variations in surface roughness.

[0011] SUMMARY OF THE INVENTION

[0012] Numerous objects and advantages, which will be evident from the description of the present invention, are, according to a first aspect of the present invention, obtained by:

[0013] An optical system for measuring the deflection of a surface subjected to a load, said optical system comprising: a plurality of sensor heads, for the measurement of a Doppler shift, each sensor head of said plurality of sensor heads being configured to emit laser light and comprising a detector arranged such that laser light may be emitted towards a surface and said detector may collect light reflected from said surface, a mounting beam for fixing said plurality of sensor heads relative to each other, said sensor heads being arranged along said mounting beam, two or more of said sensor heads being reference sensors, said reference sensors being arranged as a reference cluster, at least one of said sensor heads being a deflection sensor, said deflection sensor being arranged away from said reference cluster. The sensor heads are arranged to measure the deflection of the surface in an axial direction, by which is understood a direction that will intersect with the surface the deflection of which is to be measured. By the deflection is understood the dynamical change in distance between the detector and the surface, e.g. due to a deflection under an applied force or due to vibrations. In other words: the deflection of the surface is the distance the surface is moved when a force is applied relative to when no force is applied to the surface. The coherent light emitted from the sensor head will be emitted in the axial direction.

[0014] Emission of laser light may be effectuated in various way. Each sensor head may comprise a laser source, or laser light may be generated centrally for one or more of the sensor heads and directed to the sensor head, e.g. by optical fibre. In the following, the term laser source will be used to refer to the point of origin of laser light from the sensor head, regardless of where the light is generated, e.g. laser source may in the context of a sensor head also refer to emission point from a waveguide, such as a fibre comprised in the sensor head.

[0015] Preferably, all deflection sensors are arranged such that the laser sources emit coherent laser light along the same axial direction, i.e. such that the emitted laser beams are substantially parallel. In some variants, the optical system may be arranged such that the axial direction is substantially perpendicular to the surface, such as 0-10 degrees from perpendicular, typically 0 to 2 degrees from perpendicular. In some variants, the optical system may be arranged such that the axial direction is at an angle ranging from 30 to 150 degrees with respect to the surface. In some variants, the angle between the optical system and the surface may vary during the time in which measurements are performed. For example, if variation of the detected velocity is measured due to the deflection of the surface, a load which is causing the deflection of the surface may also cause the angle to change. The angle may also change due to the relative movement of the optical system and the surface, as unevenness of the surface may cause variation in the angle, depending on the relative position at the time of measurement.

[0016] Preferably at least one of said reference sensors is arranged to emit coherent laser light in an axial direction being parallel to the axial direction of emission of the one or more deflection sensors of the optical system. The sensor heads are arranged on said mounting beam such that all sensor heads may simultaneously emit light towards said surface. Preferably, all sensor heads are arranged on the same face of said mounting beam.

[0017] Based on the measured velocity and the known route or distance, the optical system has travelled in the transverse direction. It is possible to correlate the measured velocity with a position of the surface, e.g. to determine the location of weakened or damaged regions in the support under the measured surface.

[0018] In a variant, the reference sensors and the deflection sensors are of the same type, i.e. having the same performance characteristics and configuration aside from placement along the mounting beam. During use the one or more deflection sensors are arranged to measure the deflection of the surface, preferably by being placed closer to the load causing deflection of the surface than the reference sensors which are arranged to measure the surface with no or minimum deflection. Further difference in relation to the deflection sensors and the reference sensors is how the data collected from those sensors are used in the processing of the determined deflection.

[0019] A benefit of having all sensor heads of the optical system arranged on a mounting beam is that the sensor heads will move collectively following the same movement. In particular, arranging the two or more reference sensors together as a reference cluster minimises relative variance between those reference sensors, e.g. due to pitch or yaw.

[0020] By a cluster of reference sensors is understood a grouping of reference sensors arranged together, thereby minimising the effects of spatially different locations of each reference sensor of the reference cluster. Preferably, the reference sensors of a reference cluster are more closely spaced than any of the deflection sensors of the optical system. It is preferable that the distance between each neighbouring reference sensor of a reference cluster is smaller than the distance between each neighbouring deflection sensor as well as smaller than the distance between the reference cluster and the closest deflection sensor. By such arrangement of the reference sensors of the reference cluster it is ensured that the reference sensors will collect data from the same region being away from the deflection region where the deflection sensors are preferably measuring the deflection. Due to their close spacing, all reference sensors of a reference cluster may during operation of the optical system be arranged outside of the deflection region such that reference sensors will for all configurations of the optical system solely be used as reference sensors.

[0021] In a preferred variant, the distance between each neighbouring reference sensor of a reference cluster is the same.

[0022] In a variant, a first reference cluster may be arranged at a first end of the mounting beam while a second reference cluster may be arranged at a second end of said mounting beam such that correlation of measurements collected by the first reference cluster and the second reference cluster may reveal rotation of the mounting beam.

[0023] Measuring the reflected beam in the axial direction, i.e. on axis with the emitted laser beam, causes interference between the emitted coherent laser beam and the Doppler- shifted reflected beam. This interference pattern may be measured by the detector and from such measurements the relative velocity between the optical system and the surface may be determined. It is to be understood that while it is the reflection from the surface that is being measured, the measurements relate to the properties of the underlying structure, e.g. material integrity and how much the material can give way under the applied deflection force rather than to surface structure.

[0024] Coaxial measurement of the reflection, i.e. the coherent laser beam and the reflected beam being coaxial, is preferable, as measurements are simplified. However, it is to be understood that in other variants the system may be configured to measure a reflected beam which is not coaxial with the emitted coherent laser beam, e.g. by having the reflected beam collected at an angle or guided to the detector via a separate path.

[0025] The plurality of sensor heads comprises two or more references sensors and at least one deflection sensor. Preferably, the plurality of sensor heads comprises a plurality of deflection sensors. The deflection sensors are arranged to detect the deflection of the surface by determination of a Doppler shift as described above. The reference sensors are used to collect reference data for the processing of the data collected by the deflection sensors. Various types of reference data may be collected by the two or more reference sensors, depending on their relative arrangement. At least one reference sensor of the two or more reference sensors arranged as a reference cluster may be arranged to detect along an axis parallel to the axis of detection of a deflection sensor of the optical system. For such an arrangement, the sensor data of the reference sensor may be used to normalize the data detected by the deflection sensors. Having such a reference sensor is vital to compensate for the movement of the optical system itself relative to the surface. The reference sensors are arranged to be outside the deflection basin or at least so far from the deflection basin that the effect of the deflection is minimised. Hence the reference data collected by the reference sensor may be used to determine the deflection of the surface affected by a load relative to the unloaded state. By relating the sensor data of both the reference sensor and the deflection sensor to the position of the surface based on knowledge of the movement speed of the optical system relative to the surface, it is possible to process deflection sensor data and reference sensor data for the same position.

[0026] As the optical systems of the invention preferably comprises multiple deflection sensors and typically more deflection sensors than reference sensors, data from the same reference sensor is used for the processing of the data collected by each of the deflection sensors. Hence, noise and uncertainty of the reference signal data will be multiplied as it affects every reference sensor point. Therefore, having multiple reference sensors located together as a reference cluster is an efficient and cost- effective way of minimising adverse effects of noise and / or errors of the reference sensor data and thereby of the resulting analysed deflection data. Reference sensor data may be improved by having multiple reference sensors arranged to collect reference data parallelly at closely spaced locations of a reference region, i.e. away from the deflection basin, and averaging the reference sensor data.

[0027] The optical system may further comprise a processing unit for on board processing of the collected sensor data, e.g. averaging reference sensor data, determining translation velocity of the optical system relative to the surface and / or relating the deflection sensor data to the reference sensor data. The optical system may further comprise a transmission unit for transmitting raw data for processing and / or transmitting processed data to external processing units for further analysis and use.

[0028] In some variants of the invention the optical system comprises more than one reference cluster, such as a first reference cluster and a second reference cluster.

[0029] According to a further embodiment of the first aspect of the invention, each of the reference sensors of the reference cluster is arranged closer to a neighbouring reference sensor than to the closest located deflection sensor.

[0030] By having the reference sensors more closely spaced to each other than the deflection sensors it is made easier to achieve similar reference data from the reference sensors while locating all reference sensors at a reference region outside the deflection region. By having all reference sensors outside the deflection region, the benefits of multiple reference datasets are maximised as the decrease of the noise is increased by the enhanced knowledge of and decreased noise impact of the reference level.

[0031] According to a further embodiment of the first aspect of the invention, each of said reference sensors is arranged to be within 100 mm of an adjacent reference sensor, such as within 75 mm of an adjacent reference sensor, more preferably within 50 mm of an adjacent reference sensor.

[0032] The close spacing of the reference sensors of a reference cluster has multiple benefits. When using multiple reference sensors to average the reference data, ideally each reference sensor detects the same point of the surface or as closely spaced points as possible such that variation of the surface is minimised. Minor variations in the position may however be beneficial as they enable determining whether the reference sensors are outside the deflection basin or if a consistent velocity is detected, e.g. sensors closer to the reference basin detecting a larger velocity than those further from the contact point, indicating that they are not all fully outside the deflection region. Close spacing of reference sensors of a reference cluster further minimises effects of rotation of the mounting beam, e.g. pitch when the mounting beam moves uphill or yaw when the mounting beam follows the turn of a road, as the effects of such rotation increase with distance between the measurement points. It is to be understood that the close spacing relates to the two or more reference sensors arranged as a single reference cluster. Some variants of the optical system may comprise multiple reference clusters, such as a first reference cluster and a second reference cluster. The multiple sensors of the first reference cluster will be closely spaced, as will the reference sensors of the second reference cluster, while the first reference cluster and the second reference cluster may be located with a larger spacing between them, e.g. on opposite sides of one or more deflection sensors.

[0033] Having a first reference cluster and a second reference cluster arranged spaced apart along the mounting beam may enable comparison of reference data which may be used to determine various features such as the aforementioned pitch and yaw as well as providing information of the extent of the deflection basin in opposite directions along the longitudinal direction of the mounting beam.

[0034] According to a further embodiment of the first aspect of the invention, two or more reference sensors are arranged abutting a neighbouring reference sensor.

[0035] By a reference sensor being arranged to abut a neighbouring reference sensor is understood that it is arranged such that the neighbouring sensors are as closely spaced as possible given the physical limitations of the mounting due to the size of the sensor heads. In other words, the spacing of the reference sensors of the reference cluster may be limited by their physical dimensions only. It is understood that a first reference sensor may be arranged to abut multiple neighbouring reference sensors arranged on either side of said first reference sensor along the longitudinal direction of the mounting beam.

[0036] According to a further embodiment of the first aspect of the invention, two of said reference sensors are arranged to emit substantially parallel laser beams.

[0037] As previously described, the reference sensor data collected by separate reference sensors may be averaged to decrease the noise of the reference sensor data. Having the reference sensors arranged to emit parallel beams ensures that the collected reference sensor data is of similar character, thereby minimising the necessary processing of the averaging of the signals. In particular, two or more reference sensors being arranged to be closely spaced and emit parallel laser beams will in the ideal case collect similar data as they measure the surface under similar conditions and for a similar location, thereby allowing for the direct averaging of the collected reference sensor data for use in the processing of the deflection data when determining the magnitude of the deflection of the surface.

[0038] By a sensor axis is understood the axis along which a sensor head emits a laser beam.

[0039] According to a further variant of the first aspect of the invention, the reference cluster comprises at least three reference sensors.

[0040] In a preferred variant, the mounting beam has a length of at least three meters.

[0041] In a preferred variant, the first reference cluster is arranged at least one meter from the centre of the mounting beam, such as at least 1,25 m from the centre of the mounting beam.

[0042] In a preferred variant, the first reference cluster is arranged within a meter of a first end of the mounting beam, such as within thirty centimetres from the first end of the mounting beam.

[0043] It is to be understood that it is preferable that the entirety of the reference cluster may be arranged within a meter of a first end of the mounting beam, such as within thirty centimetres from the first end of the mounting beam.

[0044] According to a further variant of the first aspect of the invention, the reference sensors of the reference cluster are arranged as a reference unit.

[0045] By a reference unit may be understood a mechanically connected component comprising multiple reference sensors. A reference unit may comprise a reference unit housing to which the reference sensor of the reference cluster may be mounted. A reference unit may comprise one or more mounting bars for connecting the reference sensor of the reference cluster to each other such that their relative distance may be determined by the mounting bars and such that the reference unit may be mounted collectively to the mounting beam of the optical system. Another object of the present invention is to provide a detection apparatus for measuring the deflection of a surface subjected to a load, the detection apparatus comprising an optical system according to the first aspect of the invention, a host vehicle for carrying said optical system and applying a load to said surface. The optical system is mounted to the host vehicle via the mounting beam. The mounting beam is arranged to extend longitudinally along the direction of travel of the host vehicle. The mounting beam is arranged such that at least one sensor head will be located at a deflection region.

[0046] A host vehicle may be any means for carrying the optical system along a surface to be detected and applying a load to the surface. The host vehicle may be a self-propelling vehicle, such as a truck, or it may be a unit which can be attached to another vehicle and dragged, such as a trailer to be towed by a car, a truck, or a train.

[0047] The mounting beam may be mounted to the host vehicle by any means known in the art, e.g. it may be screwed on, welded to the host vehicle, or may be mounted releasably via specific mounting brackets.

[0048] By having the mounting beam arranged to extend longitudinally along the directing of travel of the host vehicle is understood that it is arranged longitudinally along the host vehicle. In other words, the mounting beam being arranged to extend longitudinally along the direction of travel of the host vehicle may mean that the mounting beam will be parallel to the diameter of a wheel of said host vehicle. Preferably, the mounting beam is parallel to the diameter of a wheel of said host vehicle being a contact point applying a predetermined deflection load to the surface to be measured. The rigidity and length of the mounting beam may lead to one or more of the plurality of sensors not following the trajectory of movement of the host vehicle at all times, however the mounting beam will follow the direction of movement of the host vehicle.

[0049] Arrangement of the mounting beam carrying the plurality of sensor heads along the longitudinal direction has several benefits. Preferably, the reference cluster is arranged such that the two or more reference sensors detect the surface at a reference region, i.e. outside the reference basin, i.e. where the surface is not affected by the load. As the effect of the load on the surface decreases with distance from the load, it is preferable to have the reference cluster spaced away from the load, e.g. from the wheel of the host vehicle contacting the surface. The longitudinal direction of the vehicle provides opportunity for the necessary spacing, e.g. by arrangement of the reference cluster between the axels of wheel pairs and / or extending behind the host vehicle where the reference cluster will not enter the deflection basin of another load point, e.g. of another wheel.

[0050] Furthermore, arranging the mounting beam along the direction of travel of the host vehicle enables the monitoring of discontinuities of the surface in the direction of travel where many surfaces will experience significant wear due to the continued load from vehicles travelling along the surface. Measurements collected along the direction of travel enables the integration of various measurement points to find the area of maximum deflection depth in the direction of travel. This is the direction most relevant to monitor for roads and / or rails. Furthermore, if a more complete picture is necessary, e.g. of all lanes of a multi-lane highway, it is possible to obtain the full picture by providing multiple passes, having the host vehicle travel along each lane to determine if a defect extends in the transverse direction as well.

[0051] By a deflection region is understood a section of the surface being deflected due to the load applied to the surface by the host vehicle. The deflection region may also be called the deflection basin. In a preferred variant, a deflection sensor may be arranged adjacent the load of the host vehicle such that the sensor may be configured to detect the deflection at the expected centre of the deflection basin. As mentioned, it is preferable that the optical system has multiple deflection sensors such that they may be arranged along the deflection region allowing a mapping of the deflection region.

[0052] The deflection apparatus may comprise a single optical system. The deflection system may comprise two or more optical systems. It may be preferable for the deflection apparatus to comprise two optical systems arranged on either side of the host vehicle, such that it is possible to measure deflection of the surface caused by contact from both the right-hand side wheels and the left-hand side wheels.

[0053] According to a further variant of the second aspect of the invention, the first reference cluster is located at a first reference region.

[0054] According to a further embodiment of the second aspect of the invention, the first reference cluster is arranged to continuously measure at a reference region being outside the deflection region. According to a further embodiment of the second aspect of the invention, the first reference cluster is located at a first reference region, the centre of the reference region being at the midway point between two points of maximum deflection caused by the host vehicle.

[0055] By the reference region is understood a region away from a deflection region, i.e. a region which is nominally unaffected by the load applied to the surface by the host vehicle. It is to be understood that while it is the intent that the reference cluster is located at a reference region, this may not be the case during every instance of operation. For example, a surface may have sections which are particularly damaged, which will cause the deflection and the deflection region to grow, whereby the deflection region may extend to a region expected to be a reference region. For systems known in the art, such deflection of the intended reference region might cause the measurements of the deflection to be skewed as the reference measurements are affected. A benefit of the present invention is that the multiple reference sensor of the reference cluster may be arranged to reveal when an unintended change of the reference region occurs, e.g. if the deviation between two reference datasets of two reference sensors increases.

[0056] In preferred operation of the detection apparatus the reference sensors of the reference cluster will only measure at a reference region. In other words, in the preferred operation of the detection apparatus the reference sensor will not measure at the deflection region. Preferably, the reference sensors will never be moved to or measure at the deflection region. In preferable variants, all reference sensors are solely used for reference measurements.

[0057] For a host vehicle having multiple sets of wheels, e.g. a truck, deflection basins will extend around the wheels where they contact the surface and provide a load to the surface in a manner such that the maximum deflection occurs behind the contact point relative to the direction of travel. By locating the reference region at a midway point between two maximum deflection points, e.g. between two different sets of wheels shifted somewhat towards the rearmost set of wheels, the reference region is spaced as far from the load points as possible, thereby decreasing the risk of the reference region being deflected. By the points of maximum deflection is understood the locations of theoretical maximum deflection based on the load points of the host vehicle, i.e. the points which through mathematical analysis are the theoretical maximum of deflection for an ideal surface and support subjected to the load of the host vehicle. Hence, in a preferred variant, the reference region is arranged between two neighbouring wheel axels, closer to the rearmost of the neighbouring axels relative to the intended direction of travel of the host vehicle.

[0058] In a variant, the width of the deflection region in the longitudinal direction of the mounting beam is a third of the distance between the two neighbouring axels between which the reference region is located or less, such as a quarter of said width, such as a tenth of said width.

[0059] According to a variant of the second aspect of the invention, the centre of the reference region is at least two meters from each load point of the host vehicle, such as at least three meters from each load point of the host vehicle. Preferably, this ensures that the centre of the reference region is at least two meters from the centre of the deflection region, such as at least three meters from the centre of the deflection region. Such distance increases the chance of the reference region experiencing a minimum of effect from the deflection caused by the load of the host vehicle.

[0060] A third object of the present invention is to provide a method for optically measuring deflection of a surface subjected to a load comprising: providing an optical system, said optical systems comprising a plurality of sensor heads, each sensor head being configured to emit laser light and comprising a detector, the plurality of sensor heads comprising one or more deflection sensors and two or more reference sensors arranged as a reference cluster; emitting a laser beam of coherent light from a sensor head of said optical system in an axial direction towards said surface while said surface is moving in a transversal direction relative to said emitted laser beam; measuring an optical signal of a reflected beam reflected from said surface; determining the Doppler frequency of said reflected beam; determining the deflection of said surface in said axial direction based on measurement data of said deflection sensors and measurement data of said two or more reference sensors of said reference cluster. The force causing deflection of the surface and underlying material, e.g. an applied force, must move relative to the surface in a transverse direction for any velocity in the axial direction to occur, as the axial velocity is due to how much the material under the surface is affected by the deflection force. Alternatively, a standing wave or a vibration through the material may produce the axial velocity. Velocity in the axial direction is necessary for the determination of the Doppler frequency, which is in turn used to determine the deflection of the surface. For example, a deflection means, i.e. a means for applying a deflection force, may be mounted along with the optical system, such that the optical system and the deflection force move together relative to the surface, such that the axial velocity varies in response to the constant deflection force as the material is deflected to a varying degree depending on the structural strength of the material.

[0061] For the present system, reference data is collected by the two or more reference sensors of the reference cluster. This reference data is subsequently used for the processing of the deflection sensor data to determine the deflection of the surface. For example, reference sensor data may be used to normalize the deflection sensor data to determine the deflection relative to the surface when the surface is not subjected to a load. As another example, the reference sensor data may be used to determine the transverse velocity of the movement of the optical system relative to the surface. As previously described, multiple types of reference data may be collected by the reference cluster simultaneously and used for the determination of the deflection of the surface.

[0062] For cases where the optical system is mounted on a vehicle moving across the surface to be measured, the transversal direction of movement of the surface relative to the emitted laser beam may also be considered the direction of travel.

[0063] In a preferred variant, all sensor heads of the optical system emit laser beams toward the surface simultaneously.

[0064] In a preferred variant, the optical system comprises a plurality of deflection sensors. For such a configuration, each deflection dataset collected by a deflection sensor is postprocessed using the reference sensor data collected by the two or more reference sensors of the reference cluster. Multiple deflection sensors thus enable simultaneous determination of surface deflection at various points along the surface. The processing of the collected sensor data may be carried out during multiple steps. For example, the reference sensor data may be analysed separately, e.g. to determine a reference velocity relative to the surface and / or velocity in the transversal direction of travel. Subsequently this analysed reference sensor data may be used during the analysis of the data collected by each of the deflection sensors of the optical system.

[0065] According to a further embodiment of the third aspect of the invention, the data collected by two or more reference sensors of the reference cluster is used for the normalization of data collected by one or more deflection sensors when determining the deflection of said surface in said axial direction.

[0066] It is known in the art to use a single reference sensor of the normalization of the data collected by deflection sensors. Using the data from multiple reference sensors provide several benefits as previously discussed. In particular, the noise is decreased as the reference data may be averaged between the multiple reference sensors, thereby decreasing the effect of local variations and noise from each sensor head.

[0067] Normalization of the deflection sensor data may be carried out in any order or manner known in the art. The Doppler frequency shift may be determined for the reference sensors and the deflection sensors, whereupon the deflection may be determined. The reference velocity may be determined for the reference sensors and for the deflection velocity for the deflection sensors separately, and normalization may be carried out subsequently.

[0068] According to an embodiment of the third aspect of the invention, collection of reference sensor data and collection of deflection sensor data takes place simultaneously.

[0069] By collecting reference sensor data and collecting deflection sensor data at the same time it is made possible to continuously measure such that it is not necessary to pause deflection sensor measurements to provide a reference level, making the process more efficient. Furthermore, the simultaneous collection of reference sensor data while operating the system to collect deflection data allows for a moving reference level, i.e. accounting for an underlying change in the supporting surface, such as due to inclination of the surface or change in the material unrelated to the integrity of the supporting material being investigated. According to a variant of the third aspect of the invention, the reference sensor data collected by at least a first reference sensor and a second reference sensor of the reference cluster is compared to verify correct functioning of the optical system.

[0070] The two or more reference sensors of a reference cluster are arranged closely spaced such that they will detect areas of the surface which are expected to have similar characteristics, e.g. similar distance to the reference sensors, leading to similar Doppler frequency shifts detected by the reference sensors. Therefore, each reference sensor of a sensor cluster being arranged in a similar manner, e.g. having parallel sensor axes, is expected to collect similar data sets. Therefore, increased difference between a first reference sensor dataset and a second reference sensor dataset is indicative of a change of state of the system. Such a change may be indicative of various different issues, e.g. an error may have occurred for one of the reference sensors, such that its output is faulty, e.g. the noise has increased, there may be dirt or debris on the surface causing variations unrelated to the surface itself, or damage of the surface may be so severe that the deflection basin is increased, affecting some of the reference sensors such that they are not in a reference region.

[0071] In a preferred variant, reference sensor data of all similarly configured reference sensors is compared, thereby making it possible to determine if only a single sensor is affected, e.g. due to malfunction of that sensor. For such variants, the method may include determining if the signal of one or more reference sensors should be excluded from the reference sensor data which is used for the processing of the collected deflection sensor data. For example, if a single reference sensor collected a reference dataset exceeding the variation threshold, while other reference sensors of the reference cluster provide reference datasets within the acceptable range of each other, the data processing may be based on only the reference dataset falling within the acceptable range, while the diverging reference data set falling outside the acceptable reference range may be excluded from the processing.

[0072] For such a variant, the system may indicate an error state if the comparison indicates that the difference between the reference sensor data collected by a first reference sensor and a second reference sensor of the reference cluster exceeds a predetermined threshold. According to an embodiment of the third aspect of the invention, each reference sensor is solely used for reference measurements.

[0073] In other words, during the intended operation each reference sensor will collect reference sensor data only from a reference region and the collected reference sensor data will only be used for postprocessing, determining a reference level and / or normalizing the data collected from the deflection sensors. The reference sensors are not intermittently used as deflection sensors and are never intended to measure reflections from the deflection region.

[0074] SHORT LIST OF THE DRAWINGS

[0075] In the following, examples of embodiments are described according to the invention, where:

[0076] Fig. 1 illustrates an optical system arranged to detect the deflection of a surface, the optical system having multiple reference sensors arranged with parallel sensor axes.

[0077] Fig. 2 illustrates an optical system arranged to detect the deflection of a surface, the optical system having multiple reference sensors.

[0078] Fig. 3 illustrates an embodiment of a detection apparatus where an optical system is mounted to a truck.

[0079] Fig. 4 illustrates a top view at the tire level of part of a detection apparatus where the optical system is arranged relative to a wheel pair being a contact point for applying load to a surface.

[0080] Fig. 5 shows a flowchart overview of a method optically measuring the deflection of a surface subjected to a load.

[0081] DETAILED DESCRIPTION OF THE DRAWINGS

[0082] The invention will now be explained in more detail below by means of examples with reference to the accompanying drawings. The invention may, however, be embodied in different forms than depicted below, and should not be construed as limited to any examples set forth herein. Rather, any examples are provided so that the disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like reference numerals refer to like elements throughout. Like elements will, thus, not be described in detail with respect to the description of each figure. A detailed description of embodiments of the optical system for measuring the deflection of a surface, the detection apparatus with the optical system integrated, and the method of determining the surface deflection using the optical system is provided.

[0083] Fig. 1 illustrates an embodiment of an optical system 10 according to the invention. The optical system 10 comprises a mounting beam 40 along which sensor heads 12 are arranged. Each sensor head comprises a laser source emitting a coherent laser beam 22 and a detector for detecting a reflected beam 32 reflected from the surface 1 the deflection of which is to be determined.

[0084] The plurality of sensor heads 12 comprises a plurality of deflection sensors 20. The plurality of deflection sensors 20 is arranged spaced along the mounting beam 40 with parallel sensor axes, such that the coherent laser beams 22 emitted from the reference sensors 50 towards the surface 1 are emitted parallel to each other. The deflection sensors 20 are arranged such that at least one of the deflection sensors 20 will emit coherent laser beams 22 onto a deflection region 2 of the surface 1 where the surface is deflected due to the applied load. The applied load is illustrated by the arrow F causing deflection of the surface 1. The width of the deflection region 2, also called the deflection basin, will depend on the magnitude of the applied load F as well as the type of and integrity of the support under the surface 1. It is part of the underlying principle of the method that the magnitude and the deflection as well as the shape of the deflection basin may be used to determine the structural integrity of the material supporting the surface 1 , thereby having the potential to reveal defects of the foundation and / or weaker regions. Therefore, it cannot be ensured that every deflection sensor 20 will be arranged to detect deflection of a deflection region 2 for all use cases. In a preferred embodiment, a plurality of deflection sensors 20 is distributed along a section of the mounting beam 40 with a high density around the theoretical point of maximum deflection and with a lower density, i.e. further spacing, further away from said point of maximum deflection. Preferably, the section of the mounting beam 40 along which the deflection sensors 20 are arranged, regardless of whether their distribution is even or not, covers at least half of the length of the mounting beam 40.

[0085] The plurality of sensor heads 12 illustrated in Fig. 1 further comprises a subgroup of two reference sensors 50 arranged as a reference cluster 55. The two reference sensors 50 of the reference cluster 55 are arranged with parallel sensor axes, such that the coherent laser beams 22 emitted from the reference sensors 50 towards the surface 1 are emitted parallel to each other. The reference cluster 55 is arranged to emit coherent laser beams 22 onto a reference region 5 of the surface 1 where the surface is not deflected due to the applied load. By such an arrangement, the Doppler shift detected by the reference sensors 50 of the reference cluster 55 provides a reference velocity corresponding to no deflection of the surface. This reference velocity may be used to normalize the data collected by the deflection sensors 20, whereby the deflection caused by the load may be normalised. Hence, the reference velocity ensures that changes of the detected velocity are due to the deflection caused by the load, rather than e.g. a change in the pitch of the surface 1 or other changes unrelated to the load.

[0086] It is understood that while the reference cluster 55 is intended to be arranged to detect a reference region 2 where there is no deflection of the surface due to the applied load, the size of the deflection basin in the longitudinal direction of the mounting beam 40 depends on the support under the surface 1 and may vary along the surface 1 during the process of measuring the deflection, as the optical system 10 translates along the surface 1 in the longitudinal direction of the mounting beam 40. Hence, the reference cluster 55 is intended to measure an undeflected reference region 5 of the surface; however, at times the deflection basin may extend into the reference region 5. Therefore, the reference region may be defined alternatively as a specific distance from the point of contact where the load is applied, and / or the placement of the reference cluster 55 may be determined relative to the mounting beam 40 or a host vehicle (see Fig. 3) rather than from the reference region 5. In preferred embodiments of the invention, the reference cluster 55 is arranged closer to a first end of the mounting beam 41 than any of the deflection sensors 12 of the optical system 10, the first end of the mounting beam 41 being frontmost relative to the direction of travel v. Placement of the reference cluster 55 near the first end 41 is beneficial as the deflection basin will typically be shifted towards the opposite end of the mounting beam 40, whereby the first end of the beam 41 being the frontmost end is least likely to be near a deflected segment of the surface.

[0087] The reference sensors 50 of the reference cluster 55 are arranged with sensor axes being parallel to each other as well as being parallel to the sensor axes of the deflection sensors 20 of the optical system.

[0088] In preferred embodiment of the invention, all sensor heads 12 are identical such that both the deflection sensors 20 and the reference sensors 50 are of the same make, such that their output may be directly relatable.

[0089] During use the optical system 10 must translate relative to the surface 1 in a direction transverse to the direction in which the coherent laser beams of the sensor heads 12 are emitted. This transverse translation is illustrated by the arrow v.

[0090] The two reference sensors 50 of the reference cluster 55 are arranged to abut each other such that they are contacting each other and the distance between the emitted coherent laser beams 22 is limited by the physical size of the sensor heads 12 such that the coherent laser beams 22 are emitted in a parallel manner, as closely spaced as possible. In other embodiments, the reference sensors 50 may be spaced further apart due to other constraints than the physical dimensions of the sensor heads 12, e.g. due to heating caused by the sensor heads 12.

[0091] While the embodiment of Fig. 1 illustrates the reference sensor cluster 55 as having two reference sensors 50, other embodiments within the scope of the invention may comprise additional reference sensors 50. Similarly, the number of deflection sensors 20 may vary between embodiments of the invention.

[0092] The sensor heads 12 are illustrated to be attached to the side of the mounting beam facing the surface 1 of which the deflection is to be detected. In other embodiments, the sensor heads 12 may be arranged on another side of the mounting beam 40, as long as they may be arranged to emit coherent laser beams 22 onto the surface 1. All sensor heads 12 may be arranged on the same side of the mounting beam, as illustrated in Fig. 1. In other embodiments, sensor heads 12 may be arranged on different sides of the mounting beam 40. Fig. 2 illustrates an embodiment of an optical system 10 according to the invention comprising a reference cluster 55 having three reference sensors 51 , 52, 53. The reference sensors of the reference cluster 55 are arranged adjacently along the longitudinal direction of the mounting beam 40.

[0093] The reference cluster 55 having three reference sensors 51, 52, 53 is arranged at a first end 41 of the mounting beam 40. Another reference cluster comprising two reference sensors 50 is arranged at the second end 42 of the mounting beam 40. In other embodiments, the reference clusters 55 at the first 41 and second ends 42 of the mounting beam 40 may be identical.

[0094] Both the first reference sensor 51 and the second refence sensor 52 emit coherent laser beams 22 onto the surface 1 , and a reflected beam 32 of light is then reflected back from the surface 1 to the detector of the sensor heads of the first 51 and second refence sensors 52.

[0095] Reference sensor data collected from the first reference sensor 51 may further be used for noise reduction in a direction perpendicular to the direction of translation v.

[0096] The first reference cluster 55 further includes a third reference sensor 53. The second reference sensor 52 and the third reference sensor 53 are arranged such that their sensor axes are parallel, i.e. such that the coherent laser beams 22 emitted from the second 52 and the third reference sensor 53 are parallel. The second 52 and third reference sensor 53 may preferably be arranged such that their sensor axes are perpendicular to the translation direction v. The benefits of having multiple reference sensors thusly arranged with parallel sensor axes, e.g. noise reduction and verification of intended functionality of the sensors, are as previously discussed.

[0097] The three reference sensors 51 , 52, 53 of the reference cluster 55 at the first end 41 of the mounting beam are arranged to parallelly emit coherent laser beams 22 onto the surface.

[0098] Having a reference cluster 55 comprising more than two reference sensors 50 provides further improvement of the reference signal as there is increased certainty, decreased noise and increased robustness to errors or faulty behaviour of any of the reference sensors 50 of a cluster 55. Other embodiments within the scope of the invention may comprise reference clusters having more reference sensors, e.g. three or more reference sensors arranged with parallel sensor axes, whereby the noise may be further reduced.

[0099] The exemplary embodiment illustrated in Fig. 2 comprises a first reference cluster 55 arranged at a first end of a mounting beam 41 and a second reference cluster 56 arranged at a second end of a mounting beam 42. In the illustrated embodiment the second reference cluster 56 comprises two reference sensors 50. The two reference sensors of the second reference cluster 56 are arranged parallel with respect to each other, i.e. they are arranged to emit parallel coherent laser beams 22. The two reference sensors of the second reference cluster 56 are spaced apart, such that they are not abutting, but are still both arranged to be located at a reference region away from the deflection of the surface.

[0100] By a reference cluster being arranged at an end of the mounting beam may be understood that the reference cluster is closer to that end of the mounting beam than any other sensor head of the plurality of sensor heads of the optical system. A reference cluster may alternatively be considered arranged at an end of a mounting beam if being arranged within a quarter of the length of the mounting beam in the longitudinal direction from that end of the mounting beam, more preferably within a fifth of the length of the mounting beam from that end, such as within a tenth of the length of the mounting beam of the end of the mounting beam.

[0101] It is understood that many other variants and constellations of reference sensor within one or more reference clusters of an optical system are within the scope of the invention. An optical system may comprise a single reference cluster, two optical reference clusters or more reference clusters. Each reference cluster of the invention may have any number of reference sensors which may be arranged in various ways. A reference cluster may comprise two or more reference sensors arranged abutting and one or more reference sensors spaced apart from the other reference sensors. Optical systems having two or more reference clusters may have different constellations of the reference sensors in the first and the second reference cluster as in the example of Fig. 2, or the first and second reference cluster may be identical placed at different positions along the optical system. As shown in Fig. 2, the deflection sensors 20 of the optical system 10 may be distributed along the beam 40 in an uneven manner, e.g. having closer spacing where the largest deflection is expected to occur, while being spaced further apart at the expected edges of the deflection basin. In other embodiments the deflection sensors 20 may be equidistant.

[0102] Fig. 3 is an illustration of an embodiment of a detection apparatus 100 having a host vehicle 110 taking the form of a truck travelling along a surface 1 in the direction indicated by the arrow v. The truck has a weight causing it to apply a load to the surface 1 at the contact points of the wheels that define load points 112. The load from the host vehicle 110 causes deflection basins of the surface 1. The shape and extent of the deflection basins, also called deflection regions, depend on the strength and structure of the area where the load vehicle is, i.e. on the material underlying the surface 1 which is what the method of the present invention investigates.

[0103] The detection apparatus 100 comprises an optical system 10 connected to the host vehicle 110 via the mounting beam 40. Various embodiments of the optical system may be used for the detection apparatus, and it is to be understood that the illustration of Fig. 3 is a simplified illustration of a single embodiment.

[0104] The optical system may comprise a plurality of deflection sensors 20. Preferably, the optical system may be arranged such that a deflection sensor 20 is arranged at the load point 112, e.g. at the axel of the wheel of a truck in the direction of the length of the truck, such that said deflection sensor 20 may detect the expected maximum deflection of the surface 1. Preferably, multiple deflection sensors 20 are arranged along the mounting beam 40 on either side of the load point 112, whereby it is possible to simultaneously map various points along the slopes of the deflection region.

[0105] A first reference cluster 55 of the optical system 10 may preferably be arranged equidistantly from two mathematically determined points of maximum deflection. Preferably, the first reference cluster 55 is arranged between neighbouring wheel pairs, such that it is closer to the rearmost wheel pair than the frontmost wheel pair, such that it is most likely that the reference cluster 55 is arranged away from the deflection basins created by the host vehicle. Preferably, the distance from the load point 112 to the first sensor cluster 55 in the direction of translation v is at least two meters, more preferably approximately three meters. Depending on the dimensions of the host vehicle 110 and the strength of the ground under the surface 1 along which the detection apparatus 100 is moving, the deflection region may extend to the areas from which light from one or more of the reference sensors of the optical system is reflected. In such suboptimal cases, it is particularly beneficial to have multiple reference sensors arranged adjacently as part of the first reference cluster 55, as the comparison of reference sensor data from adjacent reference sensors of the same reference cluster 55 enables confirmation of whether the detected region is a flat reference region or whether the region detected by the reference sensors slopes due to the deflection load.

[0106] For some embodiments of the detection apparatus, as illustrated in Fig. 3, the optical system 10 may comprise a second reference cluster 56 arranged at the end of the mounting beam opposite the arrangement of the first reference cluster 55. In some embodiments, as illustrated, the second end of the mounting beam 40 may extend beyond the length of the host vehicle, whereby the second reference cluster 56 may be arranged further from the deflection point 112, thereby increasing the chance of the second reference cluster 56 emitting coherent laser beams unto a reference region of the surface 1 where the load of the host vehicle 110 does not cause deflection of the surface 1. Other embodiments may have the optical system arranged such that the mounting beam does not extend beyond the length of the host vehicle in the direction of travel, such that the host vehicle causes a minimum of obstruction of the area where the surface 1 is being investigated, e.g. a highway.

[0107] Some embodiments of the deflection apparatus may comprise two or more reference clusters. Some embodiments of the deflection apparatus may comprise a single reference cluster of multiple reference sensors.

[0108] The reference clusters 55, 56 of Fig. 3 are illustrated as single units. In some embodiments a reference cluster 55, 56 may be a closely spaced collection of two or more reference sensors arranged in a manner similar to how each of the deflection sensors 20 of the optical system 10 is arranged. In other embodiments, a reference cluster 55, 56 may be a single reference unit being fixed to the mounting beam 10, the reference unit comprising two or more reference sensors 50 being fixedly arranged with respect to each other, e.g. connected by connection rods or mounted within a reference shell. In some embodiments, the reference sensors of a reference cluster are arranged adjacently in the longitudinal direction of the mounting beam 40. In other embodiments, a reference cluster may be arranged as a reference unit having two or more reference sensors arranged at the same longitudinal position along the mounting beam such that the reference sensors of the reference cluster are adjacently arranged in the transverse direction with respect to the mounting beam and the direction of travel, e.g. in a plane substantially parallel to the surface of the reference region.

[0109] Fig. 4 illustrates a possible arrangement of an optical system 10 relative to the wheels of a host vehicle with respect to the longitudinal direction. Note that the figure is not to scale, rather it provides an illustration of preferable relative arrangement locations. Similarly, the placement of the optical system 10 in the direction transverse the direction of travel v may differ. On the right-hand side of Fig. 4, an optical system 10 is illustrated as being arranged between the twin pair of tires of a twin tire arrangement, whereby the deflection sensors of the optical system may be best aligned with the point of maximum deflection. For host vehicles having twin tires such arrangement is preferable. On the left-hand side of Fig. 4, an optical system 10 is illustrated as being arranged between the tires connected by an axel 115. Such arrangement may be considered inside placement, having the optical system arranged such that wheels of the same axel 114 are arranged on opposite sides of the optical system 10. For host vehicles not having twin tires, such inside placement of the optical system 10 is preferable. Such arrangement may also be used for vehicles having twin tires, e.g. due to other limitations of the mounting of the optical system to the host vehicle. In other embodiments, the optical system 10 may be arranged on the outside of the wheels, i.e. such that all wheels of an axel 114 are on the same side of the optical system 10.

[0110] The host vehicle of the illustration has at least two sets of wheels, each of which includes two wheels and an axel 115 connecting said wheels. In a preferred embodiment, the optical system 10 is arranged adjacently with respect to one of said wheels, as the wheel acts as a load point, and such arrangement of the optical system ensures that one or more of the deflection sensors 20 may detect a deflection region of the surface 1.

[0111] In a preferred variant, the optical system is arranged such that a reference cluster 55 comprising multiple reference sensors is arranged closer to the rearmost of the axels 115 of a neighbouring set of wheel pairs of the host vehicle. Preferably, the reference cluster 55 is arranged where analysis has shown that it will, in the direction of travel, be furthest from both of the deflection basins created by the contact points of the neighbouring sets of wheels, whereby the reference cluster 55 is arranged at the point along the longitudinal axis of the mounting beam where the surface 1 will experience least deflection due to the load of the host vehicle applied at each load point of the wheels.

[0112] Such arrangement further allows the optical system 10 to be arranged with the longitudinal direction of the mounting beam 40 to be along the direction of travel indicated by the arrow v. Such arrangement is beneficial as for many host vehicles it allows the reference cluster 55 to be spaced as far as possible from two neighbouring load points without the mounting beam of the optical system extending beyond the host vehicle along the longitudinal axis of the mounting beam 40. Furthermore, such arrangement of the optical system 10 relative to the host vehicle allows the reference cluster 55 to be mounted rigidly relative to the deflection sensors 20, i.e. the optical system 10 may be an integrated system mounted to the host vehicle as a single unit. When the host vehicle is a vehicle travelling along the surface 1 similarly to other vehicles needing to use that surface, e.g. a truck travelling along a road or a train travelling along a set of tracks, it is further beneficial to arrange a plurality of deflection sensors 20 along the direction of translation v, as this enables the investigation of the structural integrity of the surface 1 along the direction where it will experience most wear, thereby giving a more realistic indication than a transverse image, which may more prone to determine local defects.

[0113] In a preferred variant where the host vehicle is equipped with twin tires, the optical system 10 is mounted to the host vehicle in a manner such that it is arranged between the two tires of a twin pair, such that the maximum deflection of the surface 1 from the load applied at the wheel is detected while the sensor heads of the optical system 10 may still emit coherent laser light onto the surface 1 without the beam being interrupted by any part of the host vehicle, such as the wheel. In other embodiments, the optical system 10 may be mounted as close as possible to the face of the wheel facing the axel 115, such that the optical system 10 is mounted on the inside, as shown in the left-hand side of Fig. 4.

[0114] A detection apparatus may have multiple optical systems as is illustrated in Fig. 4, e.g one related to each tire track. In a preferred embodiment, the detection apparatus comprises two optical systems, which may preferably be mounted in the same manner related to the wheels connected at opposite ends of an axel. In other embodiments, different arrangements may be used for two or more optical systems of a detection apparatus.

[0115] Fig. 5 shows a flowchart illustrating the steps of an embodiment of the method for optically measuring deflection of a surface subjected to a load according to the invention.

[0116] Providing an optical system 210 is necessary for the collection of data in accordance with the invention. The optical system may be an integral part of a piece of machinery measuring a surface of a moving surface, such as a conveyor belt. The optical system may be part of a detection apparatus having the optical system carried by a host vehicle as previously described. A load 211 is provided, the load being stationary relative to the optical system while both the load and the optical system translate relative to the surface 1.

[0117] The plurality of sensor heads provides emission of coherent laser beams 220 onto the surface the deflection of which is to be determined. Both deflection sensor emission 221 and reference sensor emission 222 occur during this step of the method.

[0118] The detectors of the plurality of sensor heads then detect reflected light 230 reflected from the surface the deflection of which is to be determined. Both deflection sensor detection 231 and reference sensor detection 232 occur during this step of the method.

[0119] While the step of emission of coherent laser beams must be initiated before the detection of reflected light originating from the laser beams is possible, it is to be understood that emission and detection may, once begun, take place continuously.

[0120] Once data has been collected by the detectors of the sensor heads, a data processing step 250 may take place. The data processing may fully or partially take place using an integrated processing unit or may fully or partially take place externally from the optical system after transmission to an external processing unit. Data processing 250 comprises determination of the Doppler frequency shift 251 , which is in turn used to determine the deflection output 260 being the output of the analysis and providing data relating to the deflection of the surface. The data processing 250 may comprise multiple sub-analyses, including but not limited to averaging reference sensor output 252, normalization of sensor data 253 and / or determination of translation speed 254.

[0121] LIST OF REFERENCES

[0122] 1 Surface the deflection of which is measured

[0123] 2 Deflection region

[0124] 5 Reference region

[0125] 10 Optical system

[0126] 12 Sensor head

[0127] 20 Deflection sensor

[0128] 22 Coherent laser beam

[0129] 30 Detector

[0130] 32 Reflected beam

[0131] 40 Mounting beam

[0132] 41 First end of mounting beam

[0133] 50 Reference sensors

[0134] 51 First reference sensor

[0135] 52 Second reference sensor

[0136] 53 Third reference sensor

[0137] 55 Reference cluster

[0138] 100 Detection apparatus

[0139] 110 Host vehicle

[0140] 112 Load point

[0141] 115 Axel

[0142] 210 Providing an optical system

[0143] 211 Provision of load

[0144] 220 Emission of laser beams

[0145] 221 Deflection sensor emission

[0146] 222 Reference sensor emission

[0147] 230 Detection of reflected light

[0148] 231 Deflection sensor detection

[0149] 232 Reference sensor detection

Claims

CLAIMS1. An optical system for measuring the deflection of a surface subjected to a load, said optical system comprising: a plurality of sensor heads, for the measurement of a doppler shift, each sensor head of said plurality of sensor heads being configured to emit laser light and comprising a detector arranged such that laser light may be emitted towards a surface and said detector may collect light reflected from said surface, a mounting beam for fixing said plurality of sensor heads relative to each other, said sensor heads being arranged along said mounting beam, two or more of said sensor heads being reference sensors, said reference sensors being arranged as a reference cluster, at least one of said sensor heads being a deflection sensor, said deflection sensor being arranged away from said reference cluster.

2. The optical system according to claim 1 , each of said reference sensors of said reference cluster being arranged closer to a neighbouring reference sensor than to the closest located deflection sensor.

3. The optical system according to any of the preceding claims, each of said reference sensors being arranged to be within 50 mm of an adjacent reference sensor.

4. The optical system according to any of the preceding claims, two or more reference sensors being arranged abutting a neighbouring reference sensor.

5. The optical system according to any one of the preceding claims, two of said reference sensors being arranged to emit substantially parallel laser beams.

6. The optical system according to any of the preceding claims, said reference sensors of said reference cluster being arranged as a reference unit.

7. The optical system according to any of the preceding claims, said reference cluster being arranged within a meter of a first end of said mounting beam, such as within thirty centimetres from said first end of the mounting beam.

8. The optical system according to any of the preceding claims, said optical system comprising a first reference cluster and a second reference cluster.

9. A detection apparatus for measuring the deflection of a surface subjected to a load, said apparatus comprising: an optical system according to any of the claims 1-6, a host vehicle for carrying said optical system and applying a load to said surface, said optical system being mounted to said host vehicle via said mounting beam, said mounting beam being arranged to extend longitudinally along the direction of travel of said host vehicle, said mounting beam being arranged such that at least one deflection sensor will be located at a deflection region.

10. The detection apparatus according to claim 9, the first reference cluster being located at a first reference region and the first reference cluster being arranged to continuously measure at a reference region outside the deflection region.

11. A method for optically measuring deflection of a surface subjected to a load, said method comprising: providing an optical system, said optical system comprising a plurality of sensor heads, each sensor head being configured to emit laser light and comprising a detector, the plurality of sensor heads comprising one or more deflection sensors and two or more reference sensors arranged as a reference cluster; emitting a laser beam of coherent light from a sensor head of said optical system in an axial direction towards said surface while said surface is moving in a transversal direction relative to said emitted laser beam; measuring an optical signal of a reflected beam reflected from said surface; determining the Doppler frequency of said reflected beam; determining the deflection of said surface in said axial direction based on measurement data of said deflection sensors and measurement data of said two or more reference sensors of said reference cluster.

12. The method according to claim 11, wherein the collection of reference sensor data and collection of deflection sensor data takes place simultaneously.

13. The method according to claim 11-12, using the data collected by two or more reference sensors of the reference cluster for the normalization of data collected by one or more deflection sensors when determining the deflection of said surface in said axial direction.

14. The method according to any of the claims 11-13, each reference sensor being solely used for reference measurements.

15. The method according to any of the claims 11-14, the reference sensor data collected by at least a first reference sensor and a second reference sensor of the reference cluster being compared to verify correct functioning of the optical system.