Abstract:
A road paver with a movable screed and with a layer thickness measuring device, which includes at least two sensors. The sensors are formed to carry out a distance measurement to the paving plane at intervals to determine a distance between the sensor and a point on the surface of the plane, said point being registered by the sensor. One of the two sensors defines a coordinate system. The layer thickness measuring device is formed to add as vectors the distance to the plane measured by the sensor that defines the coordinate system and a movement, relative to the plane and resulting from the driving of the road paver, of the coordinate system that is defined by the one sensor. In this way, a first vector can be determined which, together with a second vector, is used by the measuring device for determining the layer thickness of the laying material.

Description:
The present invention relates to a road paver with a moveable screed and a layer thickness measuring device. 
     BACKGROUND OF THE INVENTION 
     The thickness of a layer produced during the construction of a road should be measured continuously and, if possible, in real time. Because the requirements with regard to the precision of the paving dimensions are relatively high in practice, it is not sufficient if the plane is recorded at a place in front of the screed and combined directly with a rear edge of the screed for the thickness of the produced road pavement, because this fails to take into consideration unevennesses of the plane. 
     In practice, systems are known that determine the thickness of the road pavement by means of direct measurement, for example, by means of ultrasound or radar. The disadvantage in this case, however, is that to some extent, reflective bodies must be employed for an exact measurement of the thickness of the road pavement in order to be able to locate the boundary layer precisely. 
     In another method for determining the layer thickness of a newly produced road pavement, measurement bodies are slid into a newly laid layer. This, however, can result in substantial damage to the newly laid pavement. 
     A further known approach provides for storage and combination of the values measured on the plane, whereby a reference is used in order to connect all values to one another. 
     In practice, the layer thickness is also calculated using an inclination sensor. This is described, for example, by DE 100 25 462 A1. This document discloses a device for determining the thickness of the paving layer on the basis of a predetermined distance between the position of a height sensor and the screed rear edge, the registered height above the subgrade and a determined tow arm-screed assemblage inclination, which is determined by means of an inclination sensor. 
     The use of inclination sensors, however, has the technical disadvantage that these are susceptible to all sorts of accelerations and therefore lead to an imprecise measurement result. In particular, the inclination sensors are disturbed during their measurement by shaking or by vibrations transferred to the screed assemblage. Small errors in the inclination sensors thereby cause large errors in the calculation of the layer thickness. 
     It is likewise problematic if a large number of distance sensors is needed to faithfully reproduce the surface of the plane, because all measured values of the sensors must be connected to one another in order to create a suitable reference. This method is complicated and additionally requires expensive equipment in order to determine the suitable reference. 
     In practice, external references are also used in order to determine the layer thickness of a newly laid road pavement. The external references are thereby positioned or mounted alongside the road paver. This is impractical, however, because either the complete section must first be provided with such references or the references must be moved at regular distances in order to allow a continuous determination of the layer thickness. 
     DE 100 25 474 A1 describes a device for determining the layer thickness of a laying material to be applied to a subgrade by a road paver, whereby the road paver has a tractor, at least one tow arm mounted rotatably on the tractor in a manner that allows the height to be adjusted, a floating screed rigidly attached to the at least one tow arm that, with the tow arm, forms a tow arm-screed assemblage and an undercarriage lying with its bottom side on the base. The device furthermore comprises a distance sensor attached in a stationary manner to the tractor, for measuring a relative distance between a reference position of the tractor and a reference position of the tow arm-screed assemblage, whereby the reference position of the tractor has a predetermined positional relationship to the bottom side of the undercarriage, and the reference position of the tow arm-screed assemblage has a predetermined positional relationship to a lower back edge of the screed. 
     DE 198 51 153 C1 relates to a method for measuring a driving surface, in which the driving surface profile is registered metrologically. Here, a driving surface is, in particular, a traffic area, e.g. a street or a highway, that is set up with a number of coats or layers in the framework of highway construction or repair work. The invention furthermore relates to a system for carrying out the method with a mobile testing device. 
     EP 0 510 215 B1 describes a device for regulating a driving surface cover thickness. The device comprises height sensors and inclination sensors, all of which are arranged on a screed assemblage in such a way that they follow a movement of the screed. 
     U.S. Pat. No. 7,172,363 B2 refers to a paving machine. The paving machine can have a power source, a traction system, a hopper that is configured to hold paving material, and one or more conveyors that are configured to transfer the paving material from the hopper. The paving machine can also have a plate or a screed board that is configured to lay out a mat of the paving material. The paving machine furthermore comprises at least one front sensor that is mounted on a front part of the paving machine and configured in order to measure the height from a surface to the front sensor, and at least one rear sensor that is mounted on the paving machine and configured in order to measure the height from a surface of the mat to the rear sensor. The paving machine also comprises a controller that is configured to determine a thickness of the mat by determining a difference between one or more front height measurements that have been recorded by at least one front sensor, and one or more rear height measurements that have been recorded by the at least one rear sensor. The sensors are mounted on a support that is permanently connected to the screed. 
     Although the use of inclination sensors mentioned at the beginning is not problem free, it would likewise not be reasonable to disregard the inclination reference generated in this way on an uneven subgrade, because doing so would make it impossible to carry out an exact measurement of the layer thickness of the road pavement. 
     SUMMARY OF THE INVENTION 
     In light of the problems and disadvantages that result from the state of the art, the object of the invention is based on registering, as precisely and practicably as possible, a layer thickness of a newly laid road pavement using simple, constructive technical features. 
     This object is solved with the pavement thickness measuring arrangement and method of the present invention. 
     The invention relates to a road paver with a movable screed and with a layer thickness measuring device that comprises at least two sensors. The sensors are formed to carry out a distance measurement to the plane at intervals, in order to determine by vectors a distance between the sensor and a point on the surface of the plane, said point being registered by the sensor. One of the two sensors furthermore defines a coordinate system. According to the invention, the layer thickness measuring device is adapted to combine, for example to add, by vectors the distance to the plane measured by the sensor that defines the coordinate system, with a movement, relative to the plane and caused by the driving of the road paver, of the coordinate system defined by the one sensor to the plane. As a result, a first vector can be determined that can be used, together with a second vector, by the layer thickness measuring device for determining the layer thickness of the laying material. 
     The invention makes it possible to measure with precision the layer thickness of the laid road pavement while the road paver is driving. It is advantageous that the layer thickness measuring device according to the invention manages to determine the layer thickness without the use of inclination sensors. Furthermore, the sensors used for distance measurements are insensitive to accelerations of the road paver. 
     Using the invention, it is furthermore possible to determine an exact layer thickness of the road pavement without thereby providing the same with additional reflective bodies. In the case of the invention, there is likewise no need for external references that would have to be positioned along the paving section with what is usually a great effort or that possibly would have to be moved during the paving run of the road paver. 
     By means of registering the displacement of the coordinate system, the inclination reference can be produced in a simple way in order to conclude from it the layer thickness by means of vector calculation in a simple, unconventional way. In this way, it is likewise possible to take unevennesses of the paving into consideration in the calculation of the layer thickness. 
     In addition, the layer thickness measuring device according to the invention can be manufactured simply and economically and can be mounted to different places on the road paver without a great effort. 
     In a further embodiment of the invention, the layer thickness measuring device comprises an evaluation unit that registers the displacement of the coordinate system relative to the subgrade during the paving run. The evaluation unit is formed to register the measurement results of the height measurement carried out at intervals by the sensors in order then to determine the longitudinal and height displacement, but also the change in inclination of the coordinate system between two points in time. Using the evaluation unit, it is possible at a certain points in time during the paving run to register the layer thickness at one place at which a height measurement has previously been carried out and at which paving material has already been incorporated. 
     The result of the calculated layer thicknesses can preferably be retrievable visually and in real-time from the evaluation unit by an operator interface, which is available to the operating personnel of the paver. 
     The layer thickness measuring device preferably comprises three (or more) sensors. With the three sensors, the layer thickness measuring device can produce with particular precision an inclination reference for calculating the layer thickness. It would also be conceivable to provide to the layer thickness measuring device at least two sensor groups, each comprising at least two sensors, in order to further improve the measurement result. It is advantageous if the plurality of sensors of each sensor group are arranged in a row either in the direction of travel or perpendicular to the direction of travel. 
     It is also advantageous if all sensors are formed to carry out the distance measurement to the plane at intervals. This makes an exact layer thickness measurement retrievable at regular distances. It would furthermore be possible to form the evaluation unit in such a way that, by means of the measurement results of the respective sensor groups, it is capable of forming for each sensor group an averaged distance in order to use said averaged distance to measure the displacement of the coordinate system. The evaluation unit could thereby also be formed to filter extreme height differences out of the determination of the respective average value. As a result, construction tools lying around next to the paving section cannot negatively influence the measurement result. 
     In a further embodiment of the invention, the rearmost sensor or the rearmost sensor group in the direction of travel defines the coordinate system. As a result, the calculation of the first vector, but also the registration of the displacement of the coordinate system, can be simplified. 
     In an improved embodiment, all sensors or sensor groups are arranged equidistant from one another with regard to a distance in the direction of travel. By means of this distance, it is possible to stipulate a measure for the distance measurement to the plane at intervals in a simple way. 
     The layer thickness measuring device is preferably formed to measure the layer thickness at a place that is set back, at least by the distance or by a multiple of the distance, from the sensor that defines the coordinate system. In this way it is possible to measure the layer thickness by looking back at a place that is set back relative to the place of the momentary height measurement by the distance separating the sensors or sensor groups or by a multiple thereof. 
     In a further embodiment of the invention, the layer thickness measuring device comprises a support that is arranged rigidly relative to the road paver. The sensors of the layer thickness measuring device are preferably arranged equidistantly on the support. The mounting of the support directly on the road paver has the technical advantage that the support itself, and particularly the layer thickness measuring device mounted thereupon, is supported robustly and therefore absorbs no or only few vibrations that could act in a disruptive manner during the distance measurement. 
     As an alternative to this, the layer thickness measuring device comprises a holder that is arranged rigidly with respect to the screed. The sensors should also be arranged equidistantly on this holder. The rigid attachment of the layer thickness measuring device relative to the screed has the technical advantage that the position of the actuating cylinder for raising and lowering the screed does not have to be taken into consideration for calculating the layer thickness. This simplifies the calculation of the layer thickness and reduces the potential for error during the calculation. 
     In a further embodiment of the invention, the sensors are arranged directly on a tow arm that supports the screed. This arrangement allows a total weight of the road paver that is less than that when the sensors are arranged on a support or on a holder, whereby this weight reduction leads to reduced fuel consumption. 
     A further embodiment of the invention provides for the screed to comprise a rear edge whose distance from the layer thickness measuring device, particularly to the sensor that defines the coordinate system, defines the second vector. The rear edge of the screed forms a reliable and simple reference in order to define the second vector. 
     In a further advantageous embodiment of the invention, the layer thickness measuring device comprises an (e.g. fourth) sensor that is arranged, seen in the direction of travel of the road paver, at a certain distance behind the screed, meaning it is arranged behind the rear edge. This sensor is preferably formed to measure the distance to the freshly laid material behind the screed. The downstream sensor can lead to an especially precise layer thickness determination, because it is positioned at a sufficient distance away from the rear edge, where the paving material no longer deforms. 
     Like the previously mentioned sensors or sensor groups, the sensor arranged behind the screed is preferably mounted on the same support or on the same holder, whereby the support or holder projects backwards beyond the screed. As a result, it is possible to keep the distance between the sensor and the sensor that defines the coordinate system constant. As an alternative to the (fourth) sensor, it is possible to supplement the same with a further sensor that is arranged in a row with the fourth sensor, perpendicular to the direction of travel. In this way, again, as previously described with respect to the sensor groups, it is possible to calculate an averaged distance to the newly laid pavement by means of the evaluation unit. 
     The sensors for the distance measurement to the plane or to the surface of the newly laid road pavement are preferably acoustic and/or optic sensors, particularly ultrasound or laser sensors. In this way, contactless distance measurement is possible. 
     In addition to the road paver according to the invention, the invention relates to a method for determining a layer thickness of a laying material laid by a road paver on a plane. The road paver thereby supports a movable screed and a layer thickness measuring device, which comprises at least two sensors, whereby the sensors carry out a distance measurement at intervals in order to determine a distance between the respective sensor and a point on the surface of the plane, said point being registered by the sensor. One of the sensors furthermore defines a coordinate system. According to the invention, the layer thickness measuring device determines a first vector in that it adds as vectors a distance previously determined by the sensor that defines the coordinate system to a movement, relative to the plane and resulting from the driving of the road paver, of the coordinate system defined by the one sensor. It is furthermore provided that the layer thickness measuring device adds the first vector to at least a second vector in order to determine the layer thickness. 
     The same advantages result due to the method according to the invention that were already described above with reference to the road paver according to the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the invention are explained on the basis of the drawings. 
       Shown are: 
         FIG. 1  An embodiment of the layer thickness measuring device as it is used in the invention for a road paver for determining the layer thickness, 
         FIG. 2  A position of the advantageous layer thickness measuring device from  FIG. 1  relative to the plane at a point in time T- 1  and at a point in time T, 
         FIG. 3  A relocation at intervals of the advantageous layer thickness measuring device along the points in time T- 3 , T- 2 , T- 1  and T, 
         FIG. 4  A schematic depiction for determining the first vector, which is used for determining the layer thickness, 
         FIG. 5  An advantageous layer thickness measuring device with a support for mounting to the road paver, 
         FIG. 6  An advantageous layer thickness measuring device with a holder for mounting on the screed, 
         FIG. 7  An embodiment of the invention in which the sensors are mounted directly to a towing part supporting the screed, and 
         FIG. 8  An embodiment of the layer thickness measuring device with a fourth sensor that is positioned behind the screed. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows an advantageous embodiment of the layer thickness measuring device  1 . The layer thickness measuring device  1  comprises three sensors  2   a ,  2   b ,  2   c , that are arranged equidistant to one another, spaced apart by a distance L 1 . The sensors  2   a ,  2   b ,  2   c  are arranged on a holder  3  which is shown in  FIG. 1  essentially horizontal to a plane  4 . Seen in the direction of travel V, the rear-most sensor  2   c  defines a coordinate system  6 . The coordinate system  6  comprises an X-axis that is arranged essentially parallel to the holder  3 , as well as a Y-axis that is arranged essentially perpendicularly to the holder  3 . 
       FIG. 1  likewise shows a road pavement with a surface  5  that has been laid on the plane  4 . A first vector  9  is directed from the origin of the coordinate system  6  to a point  8  that lies on the plane  4 . A second vector  7  likewise points from the origin of the coordinate system  6  to a point  10  that lies on the surface  5  of the newly laid layer. The layer thickness  11  of the road pavement is shown between the points  8  and  10 . 
     It furthermore follows from  FIG. 1  that there is a distance L 2 , in a horizontal projection or perpendicular to the measuring direction of the sensor  2   c , lying between the sensor  2   c  that defines the coordinate system  6  and the place provided for the layer thickness determination, whereby the layer thickness of said place is reflected by the points  8  and  10  that are spaced a distance apart. The distance L 2  determines a distance between the origin of the coordinate system  6  and the place at which the layer thickness measurement should be carried out. The distance L 2  is preferably equal to the distance L 1 , or it corresponds, as shown in  FIG. 1 , to a multiple of the distance L 1 . 
     The three sensors  2   a ,  2   b ,  2   c  have a geometrically defined position with respect to the reference coordinate system  6 . Using the three sensors  2   a ,  2   b ,  2   c  it is possible to carry out a distance measurement to the plane  4  at intervals, in order to register, in addition to the covered path and the height displacement, likewise the change in the inclination of the reference coordinate system  6  at different points in time T- 3 , T- 2 , T- 1 , T (see  FIG. 3 ). 
       FIG. 2  shows a displacement of the reference coordinate system  6  of the layer thickness measuring device in the direction of travel V at different points in time T 1  and T.  FIG. 2  shows a displacement of the reference coordinate system  6  by means of a height displacement vy, a longitudinal displacement vx, along the direction of travel V, as well as a change in inclination a 1 . At the point in time T- 1 , the sensors  1   a ,  1   b ,  1   c  each register a distance to the plane  4  that extends up to the points P 2 , P 3 , P 4 . Offset in time, due to a movement of the road paver in the direction of travel V, the reference coordinate system  6  defined by the rear-most sensor  2   c  is relocated along the height displacement vy, as well as along the longitudinal displacement vx, and by the inclination angle α 1 , which results in the position of the layer thickness measuring device at the point in time T. At the point in time T, the sensors  2   a ,  2   b ,  2   c  determine the distance to the plane  4 , which in each case extends up to point P 1 , P 2 , P 3 . The distance measurement has thereby taken place in an interval that corresponds to the distance between the sensors  2   a ,  2   b ,  2   c , meaning L 1 . 
       FIG. 3  shows a relocation of the layer thickness measuring device  1  along the direction of travel V at the points in time T- 3 , T- 2 , T- 1  and T, whereby the sensors  2   a ,  2   b ,  2   c  carry out a distance measurement to the plane  4  at an interval of the distance L 1 . It can be seen that the layer thickness measuring device  1 , particularly the reference coordinate system  6 , moves differently from interval to interval, meaning during the coverage of the distance L 1 , which can be explained by unevennesses in the plane  4 , i.e., to a different height setting of the screed to offset the unevennesses. The distances to the plane  4  are registered by the sensors  2   a ,  2   b ,  2   c  per interval. 
       FIG. 3  shows that, for example, at the point in time T- 1 , the sensor  2   a  is directed at the same point P 2  as the sensor  2   b  is at the point in time T. This calculation assumes that only small changes in the inclination take place during the paving run. 
     The displacement of the reference coordinate system  6  across the points in time T- 3  to T is shown in  FIG. 4  by vectors. The vector  9  leads from the reference coordinate system  6  at the point in time T to the point P 6 , which indicates the place  8  shown in  FIG. 1 . The vector  9  results from the transformation of the distance  12  registered by the sensor  2   c  at the past point in time T- 3  to be covered to the positioned reference coordinate system  6  at the point in time T. This corresponds to the sum of the measured value  12  at the point in time T- 3  and the vectorial path through vy 3 , vx 3 , vy 2 , vx 2 , vy 1  and vx 1 . As a result, the changes in inclination α 1 , α 2 , α 3  of the reference coordinate system  6  that occur during the drive are taken into account for calculating the layer thickness  11 . The vector  9  determines the place  8 , i.e., the point P 6  at which the layer thickness is measured. 
       FIG. 5  shows an advantageous layer thickness measuring device  1  that is mounted on a road paver  20 , especially on the chassis of the road paver  20 , by means of a support  13 . The support  13  is mounted perpendicularly on the holder  3 , which bears the sensors  2   a ,  2   b ,  2   c . The support  13  is aligned essentially perpendicularly to the plane  4 .  FIG. 5  furthermore shows a movable tow arm  17  of the road paver  20 , whereby a screed  16  is mounted on the tow arm  17 . The screed  16  comprises a rear edge  18 , which runs along with the place  10 , at which the layer thickness  11  is preferably determined. The place  10  is depicted by  FIG. 1 . The tow arm  17  is supported by actuating cylinders  14 ,  15  in such a way that the height can be adjusted. The actuating cylinders  14 ,  15  can be activated in such a way that, by means of a corresponding height adjustment, they offset unevennesses on the plane  4  in such a way that even then, a flat road pavement can be laid by the screed  16 . 
       FIG. 4  furthermore shows an evaluation unit  30  that, as shown schematically by means of a dashed line, is electrically connected to the sensors  2   a ,  2   b ,  2   c  in order to register their measurement results. The evaluation unit  30  is furthermore connected to sensors  31 ,  32  which register a position of the actuating cylinders  14 ,  15  and pass this on to the evaluation unit  30  for determining the layer thickness  11 . By means of the geometric shape of the road paver  20  and of the tow arm  17 , as well as by means of the positions of the actuating cylinder  14 ,  15  registered by the evaluation unit  30 , it is possible to determine the position of the rear edge  18  in relation to the reference coordinate system  6 . The evaluation unit  30  is consequently able to determine the second vector  7  between the coordinate system  6  and the determined position of the rear edge  18  in order to combine this with the first vector  9  into the layer thickness  11 . 
     Unlike in  FIG. 5 ,  FIG. 6  does not show a direct mounting of the layer thickness measuring device  1  on the road paver, and instead shows a mounting on the screed  16 . The layer thickness measuring device  1  is mounted on the screed  16  by means of a holder  19 , that results from an extension of the holder  3 . Due to the direct rigid connection of the three sensors  2   a ,  2   b ,  2   c  on the screed  16  by means of the holder  19 , a calculation of the position of the actuating cylinders  14 ,  15  is not necessary in order to determine the layer thickness  11  at the place  10  or underneath the rear edge  18  of the screed  16 . The distance of the rear edge  18  of the screed  16  in relation to the reference coordinate system  6  is thereby constant. Analogously, it is possible to manage without a holder  19  if the sensors  2   a ,  2   b ,  2   c  are mounted directly on the tow arm  17 . This is shown by  FIG. 7 . The sensors  2   a ,  2   b ,  2   c  are thereby arranged on the tow arm  17  in an ascending manner in the direction opposite the direction of travel V and are separated from one another by the distance L 1 . The horizontal distance between the rear-most sensor  2   c  and the rear edge  18  amounts to L 2 , which corresponds to a multiple of the distance L 1 . 
     In a further embodiment according to  FIG. 8 , it is shown that the layer thickness measuring device  1  additionally comprises a fourth sensor  2   d  which is arranged behind the screed  16 , seen in the direction of travel V. The layer thickness measuring device  1  comprises the sensor  2   d , in order to be independent of the geometric shape of the screed  16  and of the road paver  20  when calculating the layer thickness. In accordance with  FIG. 8 , the layer thickness  11  is not determined under the rear edge  18  of the screed  16 , and instead is determined underneath the sensor  2   d , which is positioned in a known position relative to the coordinate system  6 . The sensor  2   d  measures a distance  21  to the surface  5  of the constructed layer. By means of the distance  21  and the relative position of the sensors  2   c ,  2   d  it is possible for the evaluation unit  30  to determine the second vector  7 . The layer thickness  11  can then be calculated with the first vector  9 . 
     The present invention makes it possible to register an inclination reference without inclination sensors and with a reduced sensor expenditure, provided that the changes in inclination between two points to be measured are small. Due to the sensitivity of all inclination sensors to accelerations and vibrations and the imprecision as a consequence of this, there consequently additionally results an advantage in the achievable precision of the thickness measurement.