Abstract:
A method is provided for measuring the milling depth of a road milling machine, the machine being operative to mill a ground surface with a milling roller lowered to a milling depth to create a milling track, the machine including at least one side plate located to at least one side of the milling roller to engage an untreated ground surface, and the machine including a stripping plate operative to be lowered onto the milling track generated by the milling roller. The method includes measuring the milling depth of the milling track, the measuring including detecting a measurement value of a ground engaging sensor engaging the milling track.

Description:
RELATED APPLICATIONS 
     The present application claims the priority of the German Patent Application No. 10 2006 062 129.8 of Dec. 22, 2006, the disclosure of which is herewith incorporated herein by reference. This application is also a divisional of U.S. patent application Ser. No. 12/003,094 filed Dec. 20, 2007, the disclosure of which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention refers to a self-propelled road milling machine, especially a cold milling machine, as well as a methods for measuring the milling depth. 
     2. Description of Related Art 
     With such road milling machines, the machine frame is supported by a track assembly comprising wheels or caterpillar tracks connected to the machine frame through lifting columns, the lifting columns allowing to maintain the machine frame in a horizontal plane or in parallel to the ground or under a predetermined longitudinal and/or transversal inclination. 
     A milling roll for working a ground or traffic surface is supported at the machine frame. 
     Near the front end sides of the milling roll height-adjustable side plates are provided as edge protectors at an outer wall of the road milling machine, which side plates, in operation, rest on the ground or traffic surface at the lateral non-milled edges of the milling track. Behind the milling roll, seen in the travelling direction, a height-adjustable stripping means is provided which, in operation, may be lowered into the milling track formed by the milling roll to strip off milling material remaining in the milling track. Further, the road milling machine has a control means for controlling the milling depth of the milling roll. 
     It is a problem with known road milling machines that the milling depth can not be controlled accurately enough and that, for this reason, the milling depth has to be measured repeatedly by hand during the milling operation. Especially in cases where a hard traffic surface, e.g. concrete, is milled, the tools are worn heavily so that the milling depth set is corrupted by the decreasing diameter of the cutting circle. For example, the wear of the tools, when milling concrete, can cause a difference in the milling radius of 15 mm after only a few 100 m, so that the measuring of an adjustment of side plates, for example, with respect to the machine frame is not sufficiently accurate. If the milling depth is insufficient, a time-consuming reworking of the milling track has to be carried out. Should the milling track be too deep, more building material has to be applied afterwards in order to achieve the desired ground or traffic surface level. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to improve the accuracy of measuring the milling depth during the operation of a road milling machine and to thereby minimize deviations from a predetermined milling depth. 
     The invention advantageously provides that at least one measuring means detects the lifting of a first sensor means resting on the ground or traffic surface and/or the lowering of a second sensor means to the bottom of the milling track, the lifting or lowering being effected in correspondence with the present milling depth. From the measured values supplied by the at least one measuring means, the control means can determine the milling depth at the level of the measuring means of the milling roll or the second sensor means. 
     Here, the measurement is effected preferably at the level of the stripping means arranged closely behind the milling roll, or immediately behind the stripping means, if a separate sensor means is provided. 
     Using the stripping means as a sensor means is advantageous in that no measuring errors are caused by some unevenness in the milling track. It is another advantage that the stripping means is protected against wear at its bottom edge. 
     As an alternative, the control means can use the measurement values of the at least one measuring means to determine the current milling depth of the milling roll at the level of the milling roll axis. Preferably, this is done by a calculation that may also take into account an inclined position of the machine frame. 
     The measuring means are preferably formed by position sensing means. In one embodiment it is provided that the first sensor means is formed by at least one of the side plates arranged on either side at the front sides of the milling roll so as to be height-adjustable and pivotable with respect to the machine frame. The side plates rest on the ground or traffic surface or are pressed against these, so that a change of their position relative to the machine frame during operation allows for an exact detection of the milling depth, if a measurement of the change of the position of a second sensor means is performed additionally in the milling track relative to the machine frame. 
     Also for side plates, there is an advantage that their bottom edges are protected against wear. 
     Here, the measuring means may comprise cable lines coupled with the side plates and/or the stripping means, and associated cable-line sensors as the position sensors which measure the changes of the position of the side plates and the stripping means relative to the machine frame or the relative displacement of at least one of the side plates in relation to the stripping means or the second sensor means. 
     Preferably, the cable lines coupled with the side plates and the stripping means are arranged transversely to the milling track in a substantially vertical plane extending approximately at the level of the stripping means. 
     Hereby, it can be avoided that a measurement error is caused by using different reference planes for the measurement at the side plates with respect to the measurement at the stripping plate. 
     To achieve this, it may be provided that a cable line is coupled on the one hand with the stripping means and, on the other hand, with at least one of the side plates via a guide roller, such that a cable-line sensor immediately measures the milling depth, e.g. at the guide roller. 
     In another alternative it may be provided that the side plate has a respective measuring means at the side edges facing the side plates, which measures the relative displacement of the stripping means with respect to the at least one adjacent side plate or the relative displacement of at least one side plate with respect to the stripping means. 
     According to another alternative embodiment, the stripping means may include at least one height-adjustable beam as the first sensing means, which is guided vertically and linearly in the stripping means and extends transversely to the travelling direction, said beam resting on the ground or traffic surface beside the milling track, the position of the beam relative to the stripping means, preferably with respect to height and/or inclination, being measurable by the measuring means. 
     Due to gravity, the side plates may rest on the edges of the ground or traffic surface beside the milling track milled by the milling machine, or they may alternatively be pressed on the edges by hydraulic means. 
     The stripping means may also be pressed on the surface of the milling track using hydraulic means. 
     The hydraulic means for pressing the side plates on the ground or traffic surface or for pressing the stripping means on the bottom of the milling track may comprise integrated position sensing systems. 
     For lifting or lowering the side plates and/or the stripping means, a plurality of, preferably two respective piston/cylinder units with integrated position sensing systems may be provided, whose position sensing signals are used by the control means to calculate the current milling depth from the relative difference between the positions of the stripping means and the at least one first sensor means. 
     The control means that receives the position sensing signals from the measuring means is adapted to automatically control the lifted condition of the rear lifting columns, seen in the travelling direction, to establish parallelism between the machine frame and the ground or traffic surface at a desired milling depth. 
     The side plates resting on the traffic surface so as to be pivotable with respect to the machine frame may comprise measuring means spaced apart in the travelling direction, the control means being capable to measure the longitudinal and/or the transversal inclination of the machine frame with respect to the ground or traffic surface from the difference between the measurement signals from the side plates and the stripping means. 
     The front and/or rear lifting columns may include a position sensing system to detect the lifted condition. The control means that receives the position sensing signals from the measuring means can control the condition of all lifting columns such that the machine frame has a predetermined inclination or a predetermined travel-distance-dependent transverse inclination across the travelling direction. 
     Preferably, the current set value for the milling depth of the milling roll is adjusted using the front lifting columns. 
     The following is a detailed description of a preferred embodiment of the invention with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a cold milling machine. 
         FIG. 2  illustrates a first sensor means attached to the stripping plate. 
         FIG. 3  shows two piston/cylinder units for lifting or lowering the stripping plate of a stripping means. 
         FIG. 4  illustrates an optical device for measuring the positional difference between the side plates and the stripping means. 
         FIG. 5  shows a cable line measuring means provided between the side plates and the stripping means. 
         FIG. 6  illustrates a preferred embodiment. 
         FIGS. 7   a, b, c  are schematic illustrations of the measurement error occurring at the stripping plate of the stripping means in the absence of parallelism between the machine frame and the ground or traffic surface. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The road milling machine illustrated in  FIG. 1  comprises a machine frame  4  supported by a track assembly having two front chain tracks  2  and at least one rear chain track  3 . The chain tracks  2 ,  3  are connected with the machine frame  4  via lifting columns  12 ,  13 . It is understood that wheels may be used instead of the chain tracks  2 ,  3 . 
     Using the lifting columns  12 ,  13 , the machine frame  4  can be lifted or lowered or moved to take a predetermined inclined position with respect to the ground or traffic surface  8 . The milling roll  6  supported in the machine frame  4  is enclosed by a roll case  9  which is open at the front, seen in the travelling direction, towards a conveyor belt  11  that conveys the milled material in a front part of the machine frame  4  to a second conveyor means  13 . The second conveyor means  13  with which the milled material may be delivered onto a truck, for example, is not fully illustrated in  FIG. 1  because of its length. Behind the milling roll  6 , a height-adjustable stripping means  14  is arranged which, in operation, has a stripping plate  15  engage into the milling track  17  formed by the milling roll  6  and strip the bottom of the milling track  17  so that no milled material is left in the milling track  17  behind the stripping plate. 
     Above the milling roll  6 , a driver&#39;s stand  5  with a control panel for the vehicle operator is provided for all control functions of the driving and milling operations. It also includes a control means  23  for controlling the milling depth of the milling roll  6 . 
     The side plates  10 , arranged on either side near the front end of the milling roll  6 , and the stripping means  14  are provided with measuring means  16  that allow the determination of the current milling depth at the level of the stripping means  14  or the calculation of the milling depth at the level of the rotational axis of the milling roll. Here, the milling depth is determined in a plane orthogonal to the ground or traffic surface, which plane is parallel to the rotational axis of the milling roll and includes the rotational axis. 
     The position of a first sensor means, e.g. the side plates  10 , on the ground or traffic surface  8  and/or the lowering of a second sensor means, e.g. the stripping means, can thus be detected. Measuring means  16 , preferably formed by position sensing means, measure the displacements of the sensor means, e.g. the side plates  10  or a beam  20  or the stripping plate  15 , with respect to the machine frame  4  or relative to each other. 
     The embodiment illustrated in  FIG. 2  shows a beam  20  as the sensor means, resting on the ground or traffic surface  8  and guided at the stripping plate  15  of the stripping means in a slot  24  extending linearly and orthogonally to the bottom edge  19  of the stripping plate  15 . It is understood that two mutually parallel slots  24  can be provided in the stripping plate  15  or that the beam  20 , serving as the sensing means, can be guided in a different manner so as to be height-adjustable at the stripping means  14 . The measuring means  16 , provided in the form of a position sensing means, detects the displacement of the beam  20  with respect to the stripping means  14 . Should two horizontally spaced slots  24  be used, it is possible to separately detect the milling depth on the left side of the milling track  17  and on the right side of the milling track  17 . Moreover, this offers the possibility to determine an inclination of the machine frame  4  with respect to the ground or traffic surface  8 . 
       FIG. 3  illustrates another embodiment wherein the stripping plate  15  of the stripping means  14  can be lifted or lowered by means of hydraulic means. The hydraulic means are formed by piston/cylinder units  26 ,  28  with an integrated position sensing system. This means that the piston/cylinder units  26 ,  28  not only allow for the stroke movement of the stripping means, but moreover generate a position signal. 
     As is evident from  FIG. 3 , the piston/cylinder units  26 ,  28  have one end connected to the machine frame  4  and the other end connected to the stripping plate  15 . 
       FIG. 4  illustrates an embodiment, wherein the relative movement between the side plates  10  and the stripping plate  15  is measured directly in order to detect the milling depth of the milling track  17 . To achieve this, elements  38 ,  40  of the measuring means  16  are provided, e.g., at the side plates  10  and opposite thereto at the stripping plate  15 , which elements allow for the detection of the relative displacement of the stripping plate  15  with respect to the side plates  10 . This displacement corresponds to the milling depth s in  FIG. 4 . For example, such a measuring means, which measures relative displacements, may be formed by an optical system, e.g. by reading a scale with an optical sensor, or by an electromagnetic or inductive system. 
     As an alternative and as illustrated in  FIG. 5 , the relative position sensing system between the side plates  10  and the stripping plate  15  may also be formed by a cable line  22  in combination with a cable-line sensor  21 . the cable line  22  is coupled with the stripping plate  15  of the stripping means  14  on the one hand and, on the other hand, with at least one of the side plates  10  via a guide roller  35 , so that the signal from the cable-line sensor  21  can immediately indicate the value of the current milling depth. 
     The side plates  10  themselves can be used as first sensor means by monitoring their position with respect to the machine frame  4  or the second sensor means by means of a cable line and a cable-line sensor or by means of piston/cylinder units  30 ,  32  with integrated position sensing means. 
     For example, the measuring means can also measure the displacement of the side plates  10  with respect to the machine frame  4 . Should two measuring means be used, one in front of the side plates  10  and one behind the same, seen in the travelling direction, it is also possible to determine the longitudinal inclination of the machine frame  4  with respect to the ground or traffic surface  8  or to also determine the transverse inclination of the machine frame  4  by a comparison of the measured values for both side plates  10  on both sides of the milling roll  6 . 
       FIG. 6  illustrates a preferred embodiment, wherein cable lines  22  comprising cable-line sensors  21  mounted to the machine frame  4  are arranged on both sides of the stripping means  15 . On either side of the machine, the side plates  10  are also provided with cable lines  22  and cable-line sensors  21  fastened at the machine frame  4 . The milling depth s is determined from the difference between the measured values of the cable-line sensors  21  for the side plates  10  and the cable-line sensors  21  of the stripping means  15 . Here, the measurement should preferably be made in the same substantially vertical plane in order to avoid measurement errors. 
       FIGS. 7   a  to  7   c  illustrate the cable-line sensors  21  for the side plates  10  and the stripping plates  14 , the drawings only indicating one cable-line sensor  21 , since the cable-line sensors are arranged one behind the other in substantially the same plane. 
       FIGS. 7   a, b, c  are to illustrate the case where the ground or traffic surface  8  is not parallel to the machine frame  4 , the measured milling depth value indicated by the measuring means having to be corrected because of an angle error, because a longitudinal inclination of the machine frame  4  corrupts the measurement signal at the level of the stripping plate  15  or a second sensor means near the stripping means  14 . Due to the fixed geometrical relations, i.e. the distance of the stripping plate  15  from the rotational axis of the milling roll  6 , the measured milling depth value can be corrected, knowing the angular deviation from the horizontal in the travelling direction, and the current milling depth at the level of the milling roll axis can be calculated. The angular deviation in the travelling direction may be determined, for example, from the position of the lifting columns  12 ,  13  of the caterpillar track assemblies  2 ,  3  or the piston/cylinder units  30 ,  32 . 
     It is further evident from  FIGS. 7   a  to  c , to which extent the side plates  10  are pivotable with respect to the machine frame  4 . Since the piston/cylinder units  30 ,  32  are also provided with position sensing systems, these measuring signals may be used as an alternative to cable-line sensors  21  to determine the distance of the side plates  10  from the machine frame  4 . 
       FIG. 7   c  illustrates the position of the at least one side plate  10  for a ground-parallel position of the machine frame  4 . The stripping plate  15  illustrated in  FIGS. 7   a  to  7   c  is located at the roll case  9 , so that the distance of the stripping plate  14  from the rotational axis to the milling roll  6  can be determined unambiguously in order to allow for a calculation of the milling depth correction should the machine frame  4  not be parallel to the ground. 
     The control means  23  can calculate the current milling depth at the level of the milling roll axis from the position sensing signals received, and it can possibly also generate a control signal for a vertical adjustment of the milling roll  6 . 
     Preferably, the control means  23  can automatically control the lifted condition of the at least one rear lifting column  13 , seen in the travelling direction, to establish parallelism between the machine frame  4  and the ground or traffic surface  8  or to the horizontal plane or to a predetermined desired milling plane. 
     Although the invention has been described and illustrated with reference to specific embodiments thereof, it is not intended that the invention be limited to those illustrative embodiments. Those skilled in that art will recognize that variations and modifications can be made without departing from the true scope of the invention as defined by the claims that follow. It is therefore intended to include within the invention all such variations and modifications as fall within the scope of the appended claims and equivalents thereof.