EP3981918B1 - Road finisher and method for levelling the screed of a finisher - Google Patents

Description

The present invention relates to a road paver according to the preamble of claim 1. The invention further relates to a method according to independent method claim 16.

In DE 196 47 150 A1 , DE 296 19 831 U1 and DE 100 25 474 B4 Leveling systems for a paver screed are disclosed. These leveling systems have a tow point control loop that works taking into account a difference between a tow arm inclination detected using an inclination sensor and a target inclination value for the tow arm. The target inclination value is calculated based on height monitoring carried out in the area of the rear edge of the screed. During height monitoring, distance measurements are carried out to a reference in the area of the rear edge of the screed and compared with a target distance in order to determine the inclination setpoint. With this device, unevenness in the subsurface in front of the screed can only be taken into account imprecisely or not at all during the leveling process.

In the practice described above, the use of inclination sensors in particular has proven to be problematic, as they can react sensitively to unevenness in the surface and vibrations during installation, which can negatively influence a leveling control based on them. In addition, a high level of control and regulation effort is required for the tow point control loop described above due to the fact that the tow point control takes place at the same time as height monitoring.

DE 100 25 462 A1 discloses a road paver with a layer thickness measuring device for determining a layer thickness of the paving layer produced in an area of a rear edge of the screed. To determine the built-in layer thickness on the rear edge of the screed, a height signal from a sensor that is stationarily arranged on a screed-towing arm assembly and detects a distance to the ground, as well as an inclination signal from an inclination sensor arranged on the screed-towing arm assembly, is used.

The DE 11 2009 001 767 T5 discloses a paver that has a control for leveling the screed. The control has a first sensor on the front of the paver is arranged in front of the material bunker in order to record a height to the ground. The control also includes a second sensor that detects the height of the front pulling point on the plank beam in relation to the ground.

DE 691 26 017 T2 discloses a road paver with a leveling device, which takes into account a determined thickness of the surface produced. In particular, a leveling setpoint is calculated based on the recorded thickness of the covering, with the height of the screed being adjusted when the calculated leveling setpoint deviates from a measured value that is recorded by a height sensor in front of the installation bowl.

EP 1 672 122 A1 discloses a road paver with a leveling device for a screed, which is operated on the basis of a target/actual value comparison between a determined working height of the screed and a set target height.

EP 2 535 456 A1 discloses a road paver with a measuring device mounted on it, the vectorially detectable movement of which is recorded during paving operation and taken into account when calculating the layer thickness at the rear edge of the paving screed.

US 4,807,131A discloses a road construction machine with a working part, the working height of which can be detected by means of a measuring device, with the working height being able to be regulated based on this.

The invention is based on the object of equipping a road paver with a leveling system which reliably enables improved leveling of the paving screed of the road paver using simple, practical technical means and, above all, is suitable for producing a more precise evenness of the installed paving layer. Furthermore, it is the object of the invention to provide a leveling method for a paving screed of a road paver, by means of which a flat paving layer can be produced more easily.

This task is solved by a road paver according to claim 1. Furthermore, this task is solved using a method according to claim 16.

Advantageous developments of the invention are given by the respective subclaims.

The road paver according to the invention comprises a paving screed for producing a paving layer on a surface on which the road paver moves along in the paving direction moved along an installation route. The screed is mounted in a height-adjustable manner and has a pull arm which is attached to a front pull point formed thereon by means of a leveling cylinder on the paver. In addition, the paver according to the invention comprises a measuring device for carrying out a distance measurement, a storage device, a control device and a functionally connected controller device for adjusting a setting of the leveling cylinder.

According to the invention, the control unit is designed to calculate a correction value depending on at least one distance measurement of the measuring device carried out relative to the subsurface and/or to a reference, which can be carried out at a measuring point located in front of the front edge of the screed in the installation direction. The correction value preferably represents an unevenness detected at the measuring point as a difference between a planum and the actual subsurface with unevenness. Furthermore, the control unit is designed to at least temporarily store the correction value in the memory device and, when the installation operation continues, to calculate a leveling setpoint for the measuring point, taking into account the stored correction value, based on which the leveling cylinder of the screed is controlled when the front edge of the screed reaches the measuring point .

The control device thus reacts to an unevenness in the subsurface detected at the measuring point at a later point in time during the paving journey, namely when the front edge of the towed screed reaches the measuring point at which the unevenness in the subsurface was recorded directly based on the correction value. The determination of the correction value for detecting unevenness in the subsurface, which precedes the actual control process, is based on a simple height measurement technology that is ideal for use on pavers. Furthermore, the invention offers the advantage that inclination sensors can be dispensed with, which means that the leveling system according to the invention is designed to be more robust overall for use on construction sites. In addition, the measuring device arranged in front of the screed in the invention is less influenced by the vibrating operation of the screed, so that the distances measured by the measuring device can be taken into account more precisely when leveling the screed. Furthermore, the invention offers a cost-effective solution that can be easily installed and retrofitted to the paver. Because in the invention the control device only responds to the detected unevenness of the measuring point when the front edge of the screed reaches the measuring point, reaction times of the leveling cylinder can be better compensated, whereby an installation layer with high evenness can be produced.

The measuring device is preferably attached to the tension arm of the screed. Movements of the pull arm, in particular raising and lowering the pull arm, can thus be taken into account in the distance measurements. Above all, the measuring device can detect precise unevenness in the ground in front of the working area of the paving screed from the tension arm on the side of the paver, i.e. directly next to the chassis, and/or measure a distance to a reference provided along the ground on the side of the paving screed, for example as a There is a stretched guide wire next to the paver. As an alternative to the guide wire, a tensioned rope, a curb and/or an already prepared paving layer could be considered as a reference.

According to a variant, the measuring device can be attached to a tractor of the paver, the measured values of which can optionally be offset against measured values of a further measuring device, which is arranged on the traction arm or on the screed, in order to regulate a specific screed height.

A particularly advantageous variant provides that the measuring device is arranged in the area of the front pull point of the pull arm. This means that directly at the location of the leveling cylinder, i.e. without any noticeable influence of the tension arm inclination, a distance measurement to the subsurface and/or to the reference can be carried out, based on which precise leveling of the screed is possible.

Preferably, the measuring device is rotatably attached to the pull arm, in particular at the front pull point of the pull arm or at least in the immediate vicinity thereof. This ensures that it maintains an equilibrium position or at least moves back into it automatically, regardless of a change in the inclination of the tension arm controlled during the leveling process. In other words, this means that the measuring device does not follow the changes in inclination of the pull arm. This means that the height measurements of the measuring device are not influenced by changes in the inclination of the tension arm, but only record changes in distance to the ground and/or to the reference.

One variant provides that a linear guide is formed on the pull arm for the measuring device, along which the measuring device can be positioned in an adjustable manner in the installation direction. This allows the distance between the measuring device and the front edge of the screed to be adjusted. The measuring device can be rotatably mounted on the linear guide to ignore changes in inclination of the pull arm.

According to one embodiment of the invention, the measuring device has at least one first sensor for measuring a distance to the reference and at least one second sensor for measuring a distance to the ground. These two height measurements can be taken into account when calculating the correction value in order to record unevenness in the ground. One variant provides that the measuring device has a sensor that is designed to detect both a distance to the ground and a distance to the reference. For example, a radar sensor can be used for this.

Preferably, the first and second sensors are at the same distance from the front edge of the screed in the installation direction. This means that the two sensors can carry out height measurements at the same measuring point in the installation direction, based on which any unevenness that may be present at the measuring point can be precisely recorded as a deviation from the planum. In this variant, two distance measurements are carried out at the same point in front of the screed, one to the subsoil and the other to the reference, in order to determine the correction value for this measuring point based on this.

The first and/or the second sensor are preferably in the form of an optical or acoustic sensor, for example a laser or ultrasonic sensor. The height measurements can can be carried out using a transit time measurement, a phase position measurement and/or a laser triangulation.

It is conceivable that the determined correction value can be visualized as a measure of an unevenness recorded in the subsoil compared to an average subsoil course (grade) on the paver, for example by means of a display on the screed operator's station. On the display, the correction factor can show small and comparatively large unevennesses in differentiated colors.

It is advantageous if the control device is designed to calculate the correction value for the measuring point based on the distance to the ground measured at the measuring point using the second sensor, minus the distance to the reference measured using the first sensor and further minus a preset altitude of the reference to the subgrade determine. A correction value calculated using this equation for the measuring point using the control device precisely depicts the unevenness that deviates from the subsurface, i.e. an elevation or a depression in the subsurface.

Preferably, the control device is configured to derive the leveling setpoint for the measuring point, i.e. to form the setpoint for a distance of the sensor from the reference, in an intermediate step to form a difference between a preset base leveling setpoint and the stored correction value. The basic leveling setpoint provides a guideline value for the control and regulation function, on the basis of which the screed should be towed, assuming a flat, averaged surface, i.e. a fictitious surface without unevenness. The correction value is used to adjust the basic leveling setpoint in the practical case that the measuring device detects an unevenness in the subsurface, whereby a more precise leveling setpoint adapted to the unevenness can be calculated for the measuring point. This means that the detected unevenness can be optimally compensated for.

An advantageous development provides that the control device is configured to calculate the leveling setpoint from the difference between the preset basic leveling setpoint and the stored correction value minus a distance to the reference currently measured by the measuring device. This leveling setpoint is then available to the control device as an input variable, based on which the leveling cylinder can be controlled to level the screed.

According to one embodiment, the measuring device has a plurality of sensors for measuring a distance to the ground and/or to the reference, the control device for this purpose is designed to form a respective average value as a basis for determining the correction value based on several distance measurements carried out at the same time to the background and / or to the reference. The fact that several distance measurements to the subsurface and/or to the reference are averaged to determine the correction value creates a filter function so that smoother transitions when leveling the screed are possible because the control device thus responds to unevenness during the installation operation in a somewhat dampened manner.

A further development of the invention provides that the control device is configured to multiply the calculated correction value by a compensation factor that is dependent on a geometry of the screed. It is conceivable that in addition to or instead of the geometry of the screed, the compensation factor includes, for example, the weight of the screed and/or at least one operating parameter set and/or recorded thereon during operation of the screed, for example a tamper speed and/or a heating output of the screed, is taken into account. It is also conceivable that the compensation factor takes into account the density of the ground on which the paver moves during paving. This would allow the flexibility of the subsurface to be taken into account when leveling the screed, meaning that any unevenness can be compensated for by operating the screed. One embodiment provides that the correction factor takes into account an installation temperature of the installed installation layer that is currently measured behind the screed.

Preferably, the paver has at least one distance measuring device for detecting a distance traveled by the front edge of the screed, the calculation of the leveling setpoint being triggerable on the control device when the distance traveled by the screed recorded by means of the distance measuring device corresponds to a distance between the measuring device and the front edge of the screed . This makes it possible for the control device to carry out a precise leveling of the screed at the right time and at the right place, i.e. at the measuring point, based on the correction value calculated there, so that any unevenness measured at the measuring point can be reliably compensated for.

It is particularly advantageous if the control device is designed to continuously calculate correction values during an paving journey of the paver along the paving route, to store these for the respective measuring points and to use the respective stored correction values to determine adapted leveling setpoints. This will ensures that the control device responds reliably to all unevenness in the subsurface along the installation route, so that a flat installation layer can be produced along the entire installation route.

The control device is preferably designed to use a GPS data-based underground data model to determine the correction value. One variant provides that the GPS data-based subsurface data model can be made available to the control device using a web-based application, in particular using a cloud-based application, in order to supply the paver, in particular the control device trained on it, with updated geosubsoil data along the paving route.

According to one embodiment of the invention, the control device is designed to calculate the correction value taking into account a piston position of the leveling cylinder currently set at the measuring point. The piston position can be represented, for example, by means of an extension path of the piston that can be detected, in particular using the measuring device. This would make it possible to detect unevenness in the subsurface even if the measuring device only carries out the distance measurement to the reference, for example to a taut guide wire, with no distance measurement to the subsurface otherwise taking place. Detecting the piston position of the leveling cylinder can replace measuring the distance to the ground. This can be advantageous for certain types of substrate, especially open-pored substrate surfaces.

It is conceivable that the control device is designed to set the correction value for the measuring point based on the distance to the reference measured at the measuring point using the first sensor plus the height of the reference to the subgrade plus a distance of the measuring device to the pull point height plus one set based on the piston position To determine the extension path of the leveling cylinder and minus a constructive height between the underside of the paver's chassis and the pull point of the leveling cylinder when retracted.

The present invention also relates to a method for leveling a paving screed of a paver, wherein a control device of the paver depends on at least one distance measurement carried out to the subsoil and / or to a reference by means of a measuring device provided on the paver, the distance measurement being carried out on a front edge in the paving direction The measuring point lying on the screed is carried out, a correction value is calculated and this is stored at least temporarily in a storage device is stored and, when the paving operation continues, a leveling setpoint is calculated for the measuring point, taking into account the stored correction value, based on which at least one leveling cylinder of the screed is controlled when the front edge of the paving screed reaches the measuring point.

To determine the correction value, the measuring device preferably carries out at least two distance measurements at the measuring point in front of the screed, one to the reference and one to the subsurface. This means that any unevenness in the subsurface at the measuring point can be determined precisely as a deviation from the subgrade and used precisely to level the screed.

The leveling system according to the invention and the leveling method according to the invention can be carried out on both sides of the paver. The embodiments presented previously in connection with the invention can therefore be used on both sides of the paver.

Figur 1

einen Straßenfertiger zur Herstellung einer Einbauschicht auf einem Untergrund,

Figur 2

eine schematische, isolierte Darstellung der Einbaubohle des Straßenfertigers mit einer Messeinrichtung gemäß einer Variante der Erfindung,

Figur 3

eine schematische, isolierte Darstellung der Einbaubohle mit einer daran befestigten Messeinrichtung gemäß einer anderen Variante der Erfindung,

Figur 4

eine schematische Darstellung eines erfindungsgemäßen Regelkreises zum Durchführen der Nivellierung der Einbaubohle aus den Figuren 2 und 3,

Figur 5

eine schematische, isolierte Darstellung der Einbaubohle mit einer daran befestigten Messeinrichtung gemäß einer weiteren Variante der Erfindung, und

Figur 6

eine schematische Darstellung eines Regelkreises zum Nivellieren der Einbaubohle gemäß der Variante aus Figur 5.

Embodiments of the invention are explained in more detail using the following figures. Show it:

Figure 1

a paver for producing a paving layer on a surface,

Figure 2

a schematic, isolated representation of the paver's paving screed with a measuring device according to a variant of the invention,

Figure 3

a schematic, isolated representation of the screed with a measuring device attached to it according to another variant of the invention,

Figure 4

a schematic representation of a control circuit according to the invention for carrying out the leveling of the screed from the Figures 2 and 3 ,

Figure 5

a schematic, isolated representation of the screed with a measuring device attached to it according to a further variant of the invention, and

Figure 6

a schematic representation of a control circuit for leveling the screed according to the variant Figure 5 .

The same components are given the same reference numerals throughout the figures.

Figure 1 shows a paver 1, which produces a paving layer 2 on a surface 3, on which the paver 1 moves along a paving direction R during a paving journey. The road paver 1 has a height-adjustable paving screed 4 for (pre-)compacting the paving layer 2. The paving screed 4 is attached to a pull arm 5, which is connected at a front pull point 6 to a leveling cylinder 7 on a tractor 22 of the road paver 1. The pull arm 5 serves as a lever to convert a varying leveling cylinder position into a corresponding change in the angle of attack of the screed 4, in particular to compensate for unevenness 8 in the subsurface 3.

Figure 2 shows an isolated, schematic representation of the screed 4, the pull arm 5 and the leveling cylinder 7. A measuring device 10 is arranged on the pull arm 5 between a front edge 9 of the screed and the front pull point 6. The measuring device 10 is designed to carry out at least one distance measurement to the substrate 3 and/or to a reference 11. According to Figure 2 the reference 11 is constructed as a guide wire, with the reference 11 occupying an average height h ₁₁ above the substrate 3. The reference 11 is stretched to the side of the paver 1 and, as will be explained in more detail below, serves a leveling function of the screed 4.

In Figure 2 the measuring device 10 has a first sensor 12 for measuring a distance y ₁ to the reference and a second sensor 13 for measuring a distance y ₂ to the substrate 3. Preferably, the first and second sensors 12, 13 are positioned in the installation direction R at an equal distance x ₉ from the front edge 9 of the installation screed 4. Thus, at a measuring point 14 according to Figure 2 two distance measurements were carried out, one to measure the distance y ₁ and one to measure the distance y ₂ .

Furthermore shows Figure 2 that the measuring device 10 can detect an unevenness 8 in the subsurface 3 at the measuring point 14 below the measuring device 10 by means of the two sensors 12, 13. The unevenness 8 represents a difference to a planum P. To compensate for the unevenness 8 Figure 2 A corresponding leveling of the screed 4 takes place when, during continued installation operation in the installation direction R, the front edge 9 of the screed 4 arrives above the unevenness 8, ie at the measuring point 14. In other words, the leveling system used according to the invention reacts according to the in Figure 2 shown variant on the unevenness 8 detected by the measuring device 10 at the measuring point 14 when the front edge 9 of the screed 4 is in Figure 2 distance x ₉ shown.

Figure 3 shows a variant for attaching the measuring device 10 Figure 2 . The arrangement Figure 3 differs from Figure 2 in that the measuring device 10 is positioned directly at the front pull point 6. At this position, so to speak at the front end of the pull arm 5, the distances y ₁ , y ₂ detected by the two sensors 12, 13 can be used particularly advantageously to compensate for unevenness 8 when leveling the paving screed 4 to produce a flat paving layer 2, because at this point the height of the pull point 6 is recorded exactly and is not superimposed by the changes in inclination of the screed 4.

Figure 4 shows a schematic representation of a leveling system 15. The leveling system 15 can according to Figure 2 and Figure 3 Use the recorded height measurements to level the screed 4 in order to compensate for unevenness 8 in the subsurface 3.

The leveling system 15 has a memory device 16, a control device 17 and a functionally connected controller device 18 for adjusting a setting of the leveling cylinder 7. According to Figure 4 the measured distances y ₁ , y ₂ of the sensors 12, 13 are supplied to the control device 17. Based on the measured distances y ₁ , y ₂ and taking into account the established height h ₁₁ of the reference 11 above the subgrade P, the control device 17 can determine a correction value K.

The control device 17 off Figure 4 is designed to calculate the correction value K for the measuring point 14 based on the distance y ₂ to the ground 3 measured at the measuring point 14 using the second sensor 13 minus the distance y ₁ to the reference 11 measured using the first sensor 12 and further minus the preset height h ₁₁ of reference 11 to determine. Furthermore, the control device 17 can be configured to continuously store the correction values K determined during the installation operation along the installation route in the installation direction R in the memory device 16 for the respective measuring points 14, so that the correction values K reach the corresponding ones when the front edge 9 of the installation screed 4 is reached Measuring points 14 can be used along the installation route for leveling the installation screed 4.

Furthermore shows Figure 4 that the control device 17 can display a current paving speed v _E of the paver 1 by means of a speed sensor 19. The installation speed v _E transmitted to the control device 17 can be used to determine the distance x ₉ . Coupled with this or as a functionally independent unit can be according to Figure 4 a distance measuring device 20 for the leveling system 15 is present in order to Distance x ₉ or a distance traveled by the front edge 9 of the paving screed 4 when the paver 1 moves in the paving direction R during the paving journey.

Furthermore shows Figure 4 that the control device 17 is supplied with a preset basic leveling setpoint y _1-basis . Furthermore, a compensation factor c can be stored in the control device 17, which may be dependent on a geometry of the screed 4.

The control device 17 off Figure 4 is configured to determine for each stored correction value K the path traveled, ie the distance traveled, which the screed 4, in particular the front edge 9 formed thereon, has covered from the time of storage. As soon as the distance traveled corresponds to the distance x ₉ , the correction value K is subtracted from the basic leveling setpoint y _{1 basis} using the control device 17. Optionally, the correction value K can be multiplied beforehand by the compensation factor c.

The basic leveling setpoint y _1-basis can be set manually by an operator on a control panel of the paver, so that a desired height of the screed 4 can be set accordingly for the paving operation. The height of the screed 4 can be determined manually by the operator or measured by a layer thickness sensor, not shown.

Furthermore shows Figure 4 that the leveling setpoint y _{1 -} setpoint determined taking into account the correction value K using the control device 17 for the measuring point 14 _is supplied to the control device 18. Furthermore, the control device 18 is supplied with the measured distance y ₁ . The controller device 18 is designed to calculate a controller variable u, which is fed to an actuator 21, based on a difference between the leveling target values y _{1 -target} calculated on the basis of unevenness 8 and the distance y ₁ currently measured at the measuring point 14. The actuator 21, for example a hydraulic drive component, then sets an extension path s ₇ of the leveling cylinder 7, so that a pull point height h ₆ can be adjusted in order to position the screed 4, in particular its rear edge, at a desired height h _bo .

Figure 5 essentially shows the arrangement Figure 3 , wherein the measuring device 10 according to Figure 5 only has the first sensor 12 for measuring the distance y ₁ to the reference 11. By means of the arrangement Figure 5 The correction value K can be calculated primarily based on the measured distance y ₁ and based on the extension path s ₇ of the leveling cylinder 7 become. For an unevenness 8 detected using the measuring device 10, the correction value K can be a sum of the distance y ₁ , the height h ₁₁ to the reference 11, a distance h _s of the first sensor 12 to the front pull point 6 and the extension path s ₇ of the leveling cylinder 7 minus a height h _zp , which indicates a constructive height of the underside of the chassis F to the front pull point 6 when the leveling cylinder 7 is retracted.

Figure 6 shows a schematic representation of a leveling system 15 'for the in Figure 5 arrangement shown. Here, the measured distances y ₁ and the detected extension paths s ₇ of the leveling cylinder 7 are continuously transmitted to the control device 17, based on which the correction value K is calculated and stored in the storage device 16 for each measuring point 14 along the installation route. The correction value K can be calculated using the sum described above minus the height h _zp , which is present when the leveling cylinder 7 is retracted. The basic leveling setpoint y _1-setpoint held by the control device 17 is calculated minus the correction value K to the leveling setpoint y _1-setpoint , which is fed to the control device 18 as an input variable at the latest when the front edge 9 of the screed 4 is at the measuring point 14 for the measured distance y ₁ has arrived, whereby the control device 18 determines the control variable u for the actuator 21 from a difference between the calculated leveling setpoint y _1-setpoint and the measured distance y ₁ , which adjusts the leveling cylinder 7 accordingly in order to level the screed 4.

Claims (16)

1. Road finishing machine (1), comprising a screed (4) for producing a paving layer (2) on a subsoil (3) on which the road finishing machine (1) is moving in the laying direction (R) along a laying section, wherein the screed (4) is height-adjustable and has a pulling arm (5) which is fixed to the road finishing machine (1) at a front pulling point (6) formed thereon by means of a levelling cylinder (7), at least one measuring means (10) for performing at least one distance measurement, a storage means (16), a controlling system (17) and a closed-loop controller means (18) operatively linked thereto for adapting a setting of the levelling cylinder (7), wherein the controlling system (17) is embodied to calculate a correction value (K) in response to at least one distance measurement performed with respect to the subsoil (3) and/or to a reference (11), which is performable at a measuring point (14) located in front of a front edge (9) of the screed (4) in the laying direction (R) and to at least temporarily store the correction value (K) in the storage means (17), characterized in that the controlling system (17) is configured to calculate, with a continued laying operation, a desired levelling value (y_1-soll) for the measuring point (14), taking into consideration the stored correction value (K), on the basis of which the levelling cylinder (7) of the screed (4) is controlled when the front edge (9) of the screed (4) reaches the measuring point (14).
2. Road finishing machine according to claim 1, characterized in that the measuring means (10) is fixed at the pulling arm (5) of the screed (4).
3. Road finishing machine according to claim 1 or 2, characterized in that the measuring means (10) is arranged in the region of the front pulling point (6) of the pulling arm (5).
4. Road finishing machine according to one of the preceding claims, characterized in that the measuring means (10) has at least one first sensor (12) for measuring a distance (y₁) to the reference (11) and at least one second sensor (13) for measuring a distance (y₂) to the subsoil (3).
5. Road finishing machine according to claim 4, characterized in that the first and the second sensors (12, 13) have the same distance (x₉) to the front edge (9) of the screed (4) in the laying direction (R).
6. Road finishing machine according to claim 4 or 5, characterized in that the controlling system (17) is embodied to determine the correction value (K) for the measuring point (14) by means of the distance (y₂) to the subsoil (3) measured at the measuring point (14) by means of the second sensor (13), minus the distance (y₁) to the reference (11) measured by means of the first sensor (12), and furthermore minus a pre-set altitude (h₁₁) of the reference (11) to the subsoil (3).
7. Road finishing machine according to one of the preceding claims 4 to 6, characterized in that the controlling system (17) is configured to form, in an intermediate step, a difference of a pre-set desired basic levelling value (y_1-Basis) and the stored correction value (K) to derive the desired levelling value (y_1-soll) for the measuring point (14).
8. Road finishing machine according to claim 7, characterized in that the controlling system (17) is configured to calculate, from the difference between the pre-set desired basic levelling value (y_1-Basis) and the stored correction value (K), minus a distance (y₁) to the reference (11) currently measured by means of the measuring means (10), the desired levelling value (y_1-Soll).
9. Road finishing machine according to one of the preceding claims, characterized in that the measuring means (10) has a plurality of sensors (12, 13) for measuring a distance (y₁, y₂) to the subsoil (3) and/or to the reference (11), wherein the controlling system (17) is embodied to form, based on a plurality of distance measurements (y₁, y₂) to the subsoil (3) and/or to the reference (11) performed simultaneously, a respective average value as a basis for the determination of the correction value (K).
10. Road finishing machine according to one of the preceding claims, characterized in that the controlling system (17) is configured to multiply the calculated correction value (K) with a compensation factor (c) depending on a geometry of the screed (4).
11. Road finishing machine according to one of the preceding claims, characterized in that the road finishing machine (1) includes, for detecting a covered section of the front edge (9) of the screed (4), at least one path measuring means (20), wherein the calculation of the desired levelling value (y_1-soll) can be triggered by means of the controlling system (17), if the covered section of the front edge (9) of the screed (4) detected by the path measuring means (20) corresponds to a distance (x₉) between the measuring means (10) and the front edge (9) of the screed (4).
12. Road finishing machine according to one of the preceding claims, characterized in that the controlling system (17) is embodied to continuously calculate correction values (K) during a laying drive of the road finishing machine (1) along the laying section, to store them and to employ the respective stored correction values (K) to determine adapted desired levelling values (y_1-Soll).
13. Road finishing machine according to one of the preceding claims, characterized in that the controlling system (17) is embodied to employ a GPS data-based subsoil data model to determine the correction value (K).
14. Road finishing machine according to one of the preceding claims, characterized in that the controlling system (17) is embodied to calculate the correction value (K) taking into consideration a piston position of the levelling cylinder (7) currently set at the measuring point (14).
15. Road finishing machine according to one of the preceding claims, characterized in that the measuring means (10) is fixed to a tractor (22) of the road finishing machine (1), wherein its measured values can be calculated with measured values of a further measuring means which is arranged at the pulling arm or at the screed to control a certain screed height.
16. Method for levelling a screed (4) of a road finishing machine (1), wherein a controlling system (17) of the road finishing machine (1) calculates a correction value (K) in response to at least one distance measurement of a measuring means (10) performed with respect to the subsoil (3) and/or to a reference (11), wherein the distance measurement is performed at a measuring point (14) located in front of a front edge (9) of the screed (4) in a laying direction (R), and at least temporarily stores the correction value (K) in a storage means (16), characterized in that the controlling system (17) calculates a desired levelling value (y_1-soll) for the measuring point (14) with a continued laying operation, taking into consideration the stored correction value (K), by means of which at least one levelling cylinder (7) of the screed (4) is controlled when the front edge (9) of the screed (4) reaches the measuring point (14).

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