Patent Description:
Rolling weight deflectometers and their use are well established in surveying of pavements, such as road or airport runways covered with concrete or tarmac, for faults and defects. A rolling weight deflectometer comprises a heavy weight, e.g. <NUM>,<NUM> or more, supported by a wheel, which is rolled over the pavement at relatively high speed such as <NUM>/h. The weight causes a localised depression basin in the pavement surface around the wheel. Because of the elastic properties of the pavement, the deflection basin moves along the pavement surface together with the rolling weight, leaving no permanent depression in the surface. The depth of the depression basin has been used as an indicator for the elasticity module of the pavement, in turn, indicating defects and faults in the pavement, in particular the deeper layers, which need further investigation. Due to the high speed, the use of a rolling weight deflectometer is an efficient way of surveying the pavement.

However, as explained in e.g. <CIT>, <CIT> or the article <NPL>, the deflection caused in the pavement is rather minute, e.g. in the magnitude of <NUM> micrometers, compared to the general surface roughness of the pavement, and it is thus not an easy task to measure at <NUM>/h. As explained in <CIT> and <CIT>, the traditional rolling weight deflectometer uses a number of distance sensors, e.g. four, arranged with equidistant spacing along a horizontal beam and measuring the distance downwardly to the pavement. One of the distance sensors is arranged above the point where the loaded wheel engages and deflects the pavement surface, whereas the others are arranged with equidistant spacing along the beam in front of the loaded wheel, i.e. leading as seen in the direction of motion when, during measurement, the rolling weight deflectometer is moved along the pavement. When comparing measuring data from all sensors in two subsequent situations, namely when the loaded wheel has moved exactly one sensor-spacing distance, that is from a first position to a second position corresponding exactly to the position where the preceding range sensor was when the loaded wheel was in the first position, the actual deflection caused can be calculated quite precisely, using a suitable algorithm, e.g., the Harr algorithm as explained in <CIT> and <CIT>.

For calculating the deflection using the Harr algorithm, however, the assumption is made that the spacing between the range sensors is so long that all three leading sensors are outside the deflection basin, allowing the geometry of the undeflected pavement surface to be determined.

<CIT> discloses a rolling weight deflectometer with range sensors and an inclination sensor.

However, with equidistant spacing of the range sensors and a spacing between them sufficient for the assumption of the three leading sensors to be outside of the basin to hold true for practical purposes, the rolling weight deflectometer must have a substantial length. In practice, the spacing in prior art rolling weight deflectometers is about <NUM> meters, and the overall length of the rolling weight deflectometer including wheels and tow bar normally exceeds <NUM> meters. The prior art rolling weight deflectometers are therefore long and quite unhandy, considering that a towed rolling weight deflectometer, will have to be manoeuvred like any other trailer, including turning and reversing, when not running in a straight line during surveying.

Based on this prior art it is the object to provide a rolling weight deflectometer which does not suffer from the above drawbacks.

According to a first aspect of the present invention this object is achieved by an apparatus for rolling weight deflection measurement according to claim <NUM>.

By incorporating the inclination sensor it becomes possible to calculate a curvature parameter value of the depression basin, which is an improvement compared to the quantity that is calculated using the traditional Harr algorithm mentioned above. This, in turn, allows more knowledge to be gained about the layers below the surface. Furthermore, combined with the realization that absolute deflections are not a requirement for interpretation and modelling of the pavement condition, it allows that the rolling weight deflectometer can be made shorter than the prior art rolling weight deflectometers.

Thus, according to the second aspect of the invention, the object is achieved by a method according to claim <NUM> for surveying a pavement using the rolling weight deflectometer.

According to a first preferred embodiment of the first aspect of the invention, a first of said range sensors is arranged at a location corresponding to said rolling wheel, and the remainder of the range sensors being arranged in spaced apart manner preceding said first range sensor in the first direction.

This is the traditional way of arranging the sensors, and it would therefore make it easy to implement the present invention in existing rolling weight deflectometer constructions or even retrofit rolling weight deflectometers with the present invention.

According to a preferred embodiment of the first aspect of the invention, said inclination sensor comprises at least a pair of accelerometers. Due to their widespread use in e.g. smart phones accelerometers are readily available and incur only low costs as compared to e.g. a gyroscope.

According to a further preferred embodiment, the range sensors and the inclination sensor provide input to a data processing means adapted to calculate a curvature parameter value of a depression basin formed by the apparatus in the measuring surface when the apparatus is moved along the measuring surface. Calculating a curvature parameter yields the possibility of better estimating the condition of the pavement, in particular the hidden lower layers.

According to another preferred embodiment, the number of range sensors is three. Using an inclination sensor, no more than three range sensors is needed. This in turn allows the overall length of the rolling weight deflectometer to be reduced as compared to the prior art rolling weight deflectometers, using four range sensors.

According to the invention the curvature parameter is the value K calculated using the formula: κi = di-<NUM>di+<NUM> + di+<NUM> where di is the deflection measured with the i'th range sensor, di+<NUM> is the deflection measured with the (i+<NUM>)'th range sensor, and di+<NUM> is the deflection measured with the (i+<NUM>)'th range sensor. This has the advantage that it yields information based on only three range sensors. Accordingly, this reduces the necessary length of the rolling weight deflectometer, because only two range sensors need to be located outside of the deflection basin.

The invention will now be described in greater detail based on nonlimiting exemplary embodiments and with reference to the drawing, on which:.

Turning first to <FIG>, the upper part schematically shows a rolling weight deflectometer <NUM> according to the invention in a first position. The rolling weight deflectometer <NUM> is adapted to be moved, typically towed, in a first direction generally indicated with the arrow <NUM> along a test or measuring surface <NUM> formed by the pavement to be surveyed. The pavement could e.g. be a road or an airport runway covered with concrete or asphalt, which has to be surveyed for faults and defects. As the name suggests, the rolling weight deflectometer comprises a weight. The weight acts on a load wheel <NUM>. For that reason, the rolling weight deflectometer is sometimes referred to as rolling wheel deflectometer. In either case, the abbreviation RWD is widely used. The load wheel <NUM> support is generally arranged at the trailing end of the rolling weight deflectometer <NUM> as defined by the motion in the first direction <NUM>. The load wheel <NUM> is weighed down by a substantial mass so as to provide a down force on the pavement of e.g. <NUM> kN, <NUM> kN or <NUM> kN. This down force creates a deflection basin <NUM> around the load wheel <NUM> which propagates along the surface as the load wheel <NUM> moves. For illustration purposes, the depth of the deflection in the deflection basin <NUM> around the load wheel has been exaggerated, both in the upper and lower part of in <FIG>, but in particular in <FIG>. The actual deflection is in fact only in the micrometer range, typically in the range from <NUM> micrometers to <NUM> micrometers, whereas the typical overall length of the rolling weight deflectometer <NUM> in the first direction would be about a few meters as will be explained below.

Along the length of the rolling weight deflectometer <NUM> runs a carrier in the form of an essentially horizontal beam <NUM>. The beam <NUM> may be a part of the overall frame of the rolling weight deflectometer <NUM>. Preferably the beam <NUM> carries a number of range sensors <NUM>, <NUM>, <NUM>, <NUM> directed towards the test surface, but the range sensors <NUM>, <NUM>, <NUM>, <NUM> may also be supported otherwise. Since the intention is to perform measurements in the micrometer range a laser alignment system <NUM> is preferably used to keep track of variations in the position of individual range sensors <NUM>, <NUM>, <NUM>, <NUM> due to flexibility of the beam <NUM>, variations in thermal expansion along the length thereof etc., as e.g. described in <CIT>. The range sensors <NUM>, <NUM>, <NUM>, <NUM> are preferably equidistant, i.e. with the same spacing between any two neighbouring range sensors <NUM>, <NUM>, <NUM>, <NUM>. Different spacing may also be used. Important is that the spacing is known. As will be explained below the rolling weight deflectometer may be made shorter when, in accordance with the present invention, an inclination sensor <NUM> has been provided in a fixed relationship with the range sensors <NUM>, <NUM>, <NUM>, <NUM>, e.g. rigidly connected to the essentially horizontal beam <NUM> or the frame. In this respect it should be noted that the essentially horizontal beam <NUM> is to be understood as a beam which is essentially parallel to an arbitrary reference plane of the measuring surface. If the pavement to be surveyed goes up or downhill deviations from horizontal will be evident. Furthermore, the beam <NUM> is only essentially horizontal, as the very purpose of the inclination sensor <NUM> is to detect the very minute angular deviations of the beam <NUM> that inter alia stems from movements as the rolling weight deflectometer <NUM> moves over the measuring surface, i.e. the pavement being surveyed.

Preferably, a gyroscope is used as the inclination sensor <NUM>. Gyroscopes come in many variations, such as classical spinning wheel gyroscopes, ring laser gyroscopes, fibre optic gyroscopes, and micro machined vibrating gyroscopes. The latter have become widely used in consumer electronics and now comes at reasonable prices. Alternatively, a pair of accelerometers could be used. Unlike the gyroscope which may be mounted at any suitable place in rigid connection with the range sensors <NUM>, <NUM> ,<NUM>, <NUM> on the rolling weight deflectometer <NUM>, e.g. on the beam <NUM>, the accelerometers should be placed at spaced apart locations e.g. at either end of the rolling wheel deflectometer <NUM>, e.g. at either end of the beam <NUM>.

The range sensors <NUM>, <NUM>, <NUM>, <NUM> used in the present invention are preferably line scanners as disclosed in <CIT>. Currently preferred is a Gocator <NUM> line scanner, available from LMI Technologies, Inc. Line scanners of this type project a line onto the surface at an angle using a laser fanning out from a point source.

The individual range sensors <NUM>, <NUM>, <NUM>, <NUM> are preferably arranged to scan lines along the surface in a direction across the direction of movement. Repeating this scanning at suitable small intervals triggered e.g. by a tacho linked to the load wheel <NUM>. Thus consecutive line scans will be made forming a virtual image of the surface with a given resolution depending inter alia on the resolution of the camera recording the undulations on the line and on the frequency with which the scan is triggered. The image is referred to as virtual because the resulting pixel values do not represent actual visual image data but distances. The distances need not be the vertical elevation of the sensor over the measuring area for the specific point but can, as seen from <FIG>, be measured at an angle depending on the inclination of the beam <NUM>, i.e. the minute angular deviation from essentially horizontal of the beam <NUM>, as measured using the inclination sensor <NUM>.

It should be noted that the above method of matching subsequent distance measurements with one and the same location as disclosed in <CIT> is only one way of doing so, and that the use of an inclination sensor <NUM> according to the present invention is per se independent of the way the matching is done.

The rolling weight deflectometer <NUM> preferably further comprises a data processing means <NUM> such as a computer, which could, however, possibly also be located in a towing vehicle or at a remote location.

Turning now to <FIG>, a schematic diagram of the measuring system is shown in conjunction with a deflected surveyed pavement with a measuring surface <NUM>. Upper and lower parts of the diagram in <FIG> correspond to upper and lower parts of <FIG>, only for illustration purposes are the deflection in and the unevenness of the measuring surface even more exaggerated.

Thus, in <FIG> the beam <NUM> is illustrated by the gray line. On the beam the four range sensors <NUM>, <NUM>, <NUM>, <NUM> (not shown) are located at positions <NUM>, <NUM>, <NUM>, and <NUM>. As mentioned above, the range sensors <NUM>, <NUM>, <NUM>, <NUM> are preferably laser range sensors, and for ease of explanation this will be assumed in the following, Accordingly, from the laser range sensors <NUM>, <NUM>, <NUM>, <NUM> on the beam <NUM> four laser beams <NUM>, <NUM>, <NUM>, <NUM> emanate in a direction towards the measuring surface <NUM>, shown with a thick full line. For the purpose of explaining the algorithms involved in the measurement an arbitrary reference plane <NUM> is defined, and shown with a thin full line. Furthermore, using a thick dashed line the un-deflected measurement surface <NUM> is shown, i.e. the surface as it would have been, was the weight of the rolling weight deflectometer not deflecting it.

As can be seen, the laser beams <NUM>, <NUM>, <NUM> of the three of the four range sensors <NUM>, <NUM>, <NUM> at time t<NUM> (top) aim at the same location on the measurement surface as the laser beams <NUM>, <NUM>, <NUM> of range sensors <NUM>, <NUM>, <NUM> at time t<NUM> (bottom), allowing the comparison between deflected and un-deflected measurement surface.

The basic principle, also known as the Harr algorithm, as explained in <CIT> and <CIT>, assumes that at least four laser range sensors <NUM>, <NUM>, <NUM>, <NUM> are attached equidistantly to the rigid beam. The laser range sensors record the distance zi from laser i to the ground simultaneously at times controlled by the user. The distance reading zi from laser i can be divided into four contributions:.

The objective is to measure each deflection di independently, or at least some combination of deflections: as illustrated below, such combinations could be the so-called curvature or curvature-difference. In other words, the goal is to remove quantities from the observed data that do not relate to the deflections, such as the beam height h, the beam rotation θ and the surface texture ri.

The equations relating the four laser range sensor measurements and their contributions at time t<NUM> are <MAT>.

Using image correlation, a time t<NUM> is identified such that the measurements by lasers range sensors <NUM>, <NUM>, <NUM> at positions <NUM>, <NUM> and <NUM> on the beam <NUM> hit the same place on the ground as the measurements by lasers <NUM>, <NUM>, <NUM> at positions <NUM>, <NUM> and <NUM> on the beam <NUM> at time t<NUM>, see <FIG>. At time t<NUM>, we denote the time-dependent variables [zi,h,θ] with a prime and the equations are <MAT>.

Note how the recording by laser i at time t<NUM> and t<NUM> involves the texture at location i and i+<NUM>, respectively. We have assumed that the shape of the deflection basin does not change between t<NUM> and t<NUM> such that <MAT>.

The texture ri can be eliminated by subtracting the recording from laser i+<NUM> at time t<NUM> from that of laser i at time t<NUM> <MAT>.

If we define a new quantity called the deflection-difference <MAT> as well as the beam height-difference Δh=h'-h, then equation (<NUM>) can be written as <MAT>.

Equations (<NUM>) still contain both the beam height-difference Δh and the beam angles θ and θ'. Δh can be eliminated by subtracting (<NUM>)b from (<NUM>)a and (<NUM>)c from (<NUM>)b <MAT>.

Inspired in part by numerical finite difference calculus, we introduce a new quantity called the curvature <MAT> Then equation (<NUM>) simplifies to <MAT>.

We assume that the angles are small, θ≪<NUM> and θ'≪<NUM>, and remind that a Taylor Series expansion of the Tangent function around zero reveals that a linearization is a good approximation, tanx=x+O(x<NUM>), and thus <MAT> where <MAT> is the beam angle-difference between the two measurement times. The beam angle-difference can be measured very accurately using a gyroscope and the curvature can thereby be determined according to equation (<NUM>).

This is the new and inventive algorithm relating a curvature of a depression basin to the measurements from only three laser range sensors and a gyroscope. The new and inventive algorithm constitutes an improvement over the classical Harr algorithm, in which the so-called curvature-difference is determined, as demonstrated below. This furthermore allows the rolling weight deflectometer <NUM> to be made shorter than prior art rolling weight deflectometers.

To highlight the difference to the classical Harr algorithm, we now give a short description of it. In our terminology, the classical Harr algorithm measures a quantity called the curvature difference <MAT>.

We then recover the classical Harr algorithm by subtracting (<NUM>)b from (<NUM>)a to eliminate the beam angle-difference <MAT> or slightly rewritten <MAT>.

This is the classical Harr algorithm relating the curvature-difference of a deflection basin to the measurements from four lasers.

Table <NUM> below shows a summary of the differences between the new inventive curvature K and the classical curvature-difference Δκ from the Harr algorithm.

We note that despite the fact that the measurement system can determine the curvature using only three laser range sensors, and the curvature-difference using only four laser range sensors it can be expanded with more equidistantly-spaced lasers than the three/four considered in the example above. For each new laser added, both a new curvature and curvature-difference can be formed. For instance, with a fifth laser, we can form three curvatures: κ<NUM>=d<NUM>-<NUM>d<NUM>+d<NUM>, κ<NUM>=d<NUM>-<NUM>d<NUM>+d<NUM> and κ<NUM>=d<NUM>-<NUM>d<NUM>+d<NUM>, and two curvature-differences: Δκ<NUM>=d<NUM>-<NUM>d<NUM>+<NUM>d<NUM>-d<NUM> and Δκ<NUM>=d<NUM>-<NUM>d<NUM>+<NUM>d<NUM>-d<NUM>.

So in pursuit of the object of the present invention it has been realized that by deviating from the classic Harr algorithm and not relying solely on distance measurements, it becomes possible to not only make a substantially shorter rolling weight deflectometer than those of the prior art, but also to gain additional knowledge about the pavement, in particular the sub-surface layers.

In particular, as can be understood from table <NUM>, the inventors have realized that using an inclination sensor as in the above, it will be possible to reduce the length of the rolling wheel deflectometer <NUM> as one laser range sensor <NUM> can be omitted.

Thus, referring now to <FIG> with the new curvature algorithm only three range sensors, e.g. laser range sensors <NUM>, <NUM>, <NUM>, and an inclination sensor <NUM>, e.g. a gyroscope, are needed. Using only laser range sensors <NUM>, <NUM>, <NUM>, the laser measurements and their contributions at time t<NUM> are <MAT> and at time t<NUM> <MAT>.

Again, the texture ri can be eliminated by subtracting the recording from laser i+<NUM> at time t<NUM> from that of laser i at time t<NUM> <MAT>.

Equations (<NUM>) can be rearranged as <MAT>.

Equations (<NUM>) still contain both the beam heights and the beam angles. The beam heights can be eliminated by subtracting (<NUM>)b from (<NUM>)a <MAT>.

When, as indicated above, the curvature is defined as <MAT> then equation (<NUM>) is written as <MAT>.

Assuming still that the angles are small, θ ≪ <NUM> and θ' ≪ <NUM>, and reminding that a Taylor Series expansion of the Tangent function around zero reveals that a linearization is a good approximation, tanx = x + <NUM>(x<NUM>), thus <MAT> where <MAT> is the beam angle-difference between the two measurement times. The beam angle-difference can be measured very accurately using a gyroscope and the curvature can thereby be determined according to equation (<NUM>).

This is the novel algorithm relating a curvature to the distance measurements from three range sensors and one inclination sensor. As is readily derivable form <FIG> this allows shortening the length of the rolling weight deflectometer <NUM> with an amount corresponding to the distance between the two laser range sensors, viz. the distance from the laser range sensor <NUM> in <FIG> to the now omitted laser range sensor <NUM>.

Claim 1:
An apparatus (<NUM>) for rolling weight deflection measurement comprising a rolling wheel (<NUM>) to be moved along a measuring surface (<NUM>) in a first direction (<NUM>)
a frame extending essentially along said measuring surface in said first direction (<NUM>),
at least one carrier (<NUM>),
a number of spaced apart range sensors (<NUM>, <NUM>, <NUM>, <NUM>) arranged on said at least one carrier (<NUM>) and configured to measure a distance to said measuring surface at pavement locations passed by the apparatus (<NUM>), a first of said range sensors (<NUM>) being arranged at a predetermined location with respect to said rolling wheel (<NUM>), and the remainder of the range sensors (<NUM>, <NUM>, <NUM>) being arranged in spaced apart manner in line with said first range sensor (<NUM>) in the first direction,
at least one inclination sensor (<NUM>) configured to measure at least a change in inclination of said at least one carrier (<NUM>),
and a data processing means (<NUM>),
characterized in that the data processing means (<NUM>) is
adapted to calculate the curvature of a depression basin (<NUM>) formed by the apparatus (<NUM>) in the measuring surface (<NUM>) when the apparatus (<NUM>) is moved along the measuring surface (<NUM>), where the calculating is based on said measured distances and corresponding inclination measurements from said at least one inclination sensor to yield a curvature parameter of the depression basin (<NUM>), the curvature parameter being a value κ calculated using the formula: <MAT>
where
di is the deflection measured with the i'th range sensor and the at least one inclination sensor (<NUM>),
di+<NUM> is the deflection measured with the (i+<NUM>)'th range sensor and the at least one inclination sensor (<NUM>),
di+<NUM> is the deflection measured with the (i+<NUM>)'th range sensor and the at least one inclination sensor (<NUM>).