Patent Publication Number: US-3879149-A

Title: Method and apparatus for providing grade control for the screed of a paver or the like

Description:
El 1: 18C  
 States atent Smith et al.  
 PROVIDING GRADE CONTROL FOR THE SCREED OF A PAVER OR THE LIKE [75] Inventors: Donald W. Smith; Ronald W. Beaty;  
 Harry A. Barber, all of Aurora, 111.  
 [73] Assignee: Barber-Greene Company, Aurora.  
 [22] Filed: Sept. 18, 1972 [52] US. Cl. 404/72; 404/84 [51] int. Cl. E0lc 19/00 [58] Field of Search 404/84, 83, 72; 37/D1G. 20  
 [56] References Cited UNITED STATES PATENTS 2.962.979 12/1960 McCormick....., 404/84 X 3.181.441 5/1965 Flom 404/84 3.264.958 8/1966 Babb..... 404/84 3.264.959 8/1966 Shea 404/84 3.272.099 9/1966 Drake 404/84 3.285.148 11/1966 Munyon 404/84 3.323.427 6/1967 Schrimper.... 404/84 3.334.560 8/1967 Long 404/84 3.604.512 9/1971 Carter... 404/84 X 3.618.484 11/1971 Long 404/84 3.637.026 1/1972 Snow 404/84 Apr. 22, 1975 3.674.094 7/1972 Kuntz 404/84 X Primary Examiner-Nile C. Byers, Jr. Attorney. Agent, or FirmMcDougal1, Hersh &amp; Scott [57] ABSTRACT A mobile grade reference device includes first means for sensing the vertical distance between successive points along a predetermined path on the subgrade and a grade reference point on such grade reference device. Second means are provided for continuously generating and storing first intelligence representative of the successive vertical distances sensed by such first means as the grade reference device travels over the subgrade. Third means on the paver senses the vertical distance between successive points along said predetermined path on the subgrade and a point on the screed. Fourth means continuously generate second intelligence representative of the successive vertical distances sensed by the third means as the paver travels along the aforesaid predetermined path. Fifth means compare the first and second intelligence as the latter is generated and produce grade control signals representative of the differences between the vertical distances sensed by the first and third means at the same points on the subgrade. A control mechanism for the screed is actuated by these grade control signals.  
 22 Claims, 6 Drawing Figures METHOD AND APPARATUS FOR PROYIDING GRADE CONTROL FOR THE SCREED OF A PAVER OR THE LIKE I BACKGROUND OF THE INVENTION 1. Field of the Invention The field of the present invention relates to control systems for roadway making machinery, and more particularly to systems for providing grade control for a mobile apparatus of the type which forms a roadway by applying a layer of roadway material to the subgrade. Specifically, the invention concerns a method and apparatus for providing grade control to the screed of a paver.  
 2. The Prior Art In the use of an asphalt paving machine, for example, an external grade reference device, from which grade sensings are taken, is preferably provided. As is known to those skilled in the art, an asphalt paving machine for use with an external grade reference device includes an automatic screed control system. Such a system includes a feeler or sensor which engages the external reference wire or stringline; this external reference establishes a reference plane independently of the paving machine. The feeler or sensor activates a control mechanism, usually hydraulically or electrically operated, to change the screed level; this may be achieved by raising or lowering the pivot points of the draft arms which mount the screed or by pivoting the screed with respect to the draft arms.  
  The external reference may be provided by erecting a wire stringline supported in parallel relation to the desired gradeline. This is disadvantageous in that erection of such stringline is time consuming. Further, the stringline interferes with the free movement of men and equipment along the subgrade.  
  In some instances, an existing curb or adjacent pavement surface parallel to the desired grade may be used as the external reference device. Quite often, such a curb or pavement surface is not available; in such a situation, a mobile grade reference device is frequently employed. Such a mobile grade reference device is connected to the paver for movement therewith. This mobile grade reference device, which usually mounts a stringline defining the external reference plane, includes plural shoes or skids which ride over the subgrade during movement of the paving machine. As used herein, the term subgrade&#34; has reference to the surface which is to receive the material being laid by the roadway making machine, whether bituminous material, graded earth, or a layer of aggregate placed on the earth.  
  As is known to those skilled in the art, a mobile grade reference device of the type under consideration provides optimum results when the device is of substantial length, say at least thirty feet, and is preferably provided with a considerable number of longitudinally spaced subgrade engaging shoes or skids. In this regard, reference is made to the copending application of Munyon and Beaty, Ser. No. 202,127, filed Nov. 26, 1971, now Pat. No. 3,771,882, and assigned to the assignee of the present invention. A linkage system connects the stringline adjacent the longitudinal midpoint of the mobile grade reference device. As explained in Munyon Pat. 3,285,148, assigned to the assignee of the present application, optimum results are obtained when the sensor is mounted forwardly of the forward portion of the screed, preferably adjacent such forward portion. With these last two mentioned requirements in mind, it necessarily follows that the mobile grade reference device must be mounted adjacent one side of the paver with the longitudinal midpoint of the grade reference device located just forwardly of the forward portion of the screed.  
 In many instances, the mobile grade reference device cannot be mounted adjacent one side of the paver for movement therewith because the surface adjacent the subgrade is unsuitable for movement of the mobile grade reference devices. For example, the area adjacent and to one side of the subgrade may have a grade profile which does not correspond to the grade profile of the subgrade; or, obstructions may be located on such area adjacent the subgrade. When such conditions have been encountered, one practical alternative is to mount the mobile grade reference device forwardly of the screed and within the lateral extremities thereof using the linkage mechanism shown in the copending application of Beaty and Martenson, Ser. NO. 278,299, filed Aug. 7, 1972, now Pat. No. 3,81 1,787, and assigned to the assignee of the present application. The mechanism described and claimed in the aforementioned Beaty and Martenson application provides quite satisfactory results; however, such mechanism does require special means for attaching the grade reference device to the screed and also requires that the grade reference device be limited to an effective length of twenty feet or so.  
 SUMMARY AND OBJECTS OF THE INVENTION The present invention has to do with methods and apparatus whereby a mobile grade reference device, which may be of any desired length, is moved over the subgrade in advance of the roadway making machinery. According to one embodiment of the apparatus of the invention, the mobile grade reference device is attached to the paver forwardly thereof and is moved or pushed ahead of the paver. The mobile grade reference device includes means for sensing the vertical distance between successive points along a predetermined path on the subgrade and a grade reference point on the longitudinal midpoint of the grade reference device. Means are provided for continuously generating and storing first intelligence representative of the successive vertical distances sensed by the mobile grade reference device as it travels along the subgrade. The paver includes sensing means adjacent the forward portion of the screed for sensing the vertical distance between a reference point on the screed and successive points on the subgrade along said predetermined path. Means are provided for continuously generating second intelligence representative of the successive vertical distances sensed at the screed as the paver moves along the same predetermined path. The mobile grade reference device carries a subgrade engaging wheel. Rotation of this wheel provides information representative of the horizontal distance between the grade reference sensing points on the grade reference device and on the screed. This information is used for comparing the first intelligence with the second intelligence as the latter is generated thereby to produce grade control signals representative of the differences between vertical distances sensed at the grade reference device and at the screed for each of the same points on the subgrade. These grade control signals operate the automatic screed level control mechanism.  
  In another embodiment of the invention, the mobile grade reference device is not connected with the paver and is propelled independently over the subgrade by powered means separate from the power or motive means of the paver. In this embodiment, the first intelligence generated and stored by the mobile grade reference device is correlated with horizontal position identification data representative of the progressive horizontal distances between the grade reference point on the mobile grade reference device and a fixed point adjacent the subgrade. This first intelligence is retrieved as the second intelligence is generated at the screed thereby to provide the aforementioned grade control signals representative of the differences between the corresponding vertical distances sensed at the mobile grade reference device and at the screed.  
  A primary object of the present invention is the provision ofa new and improved method and apparatus for providing grade control for a roadway making apparatus wherein a mobile grade reference device is moved over the subgrade in advance of the roadway making machinery.  
  Another object of the present invention is the provision of a method for controlling the screed of a paver or the like which includes the steps of moving the mobile grade reference device over the subgrade in advance of the paver, storing the grade profile reference data generated by sensing means on the mobile grade reference device, and then retrieving this data when the paver is moved over the subgrade.  
  Another object of the present invention is the provision of a method for controlling the screed of a paver or the like which includes the steps of advancing a mo bile grade reference device over the subgrade along a predetermined path in advance of the paver, generating and storing first intelligence representative of the successive vertical distances between the subgrade and a reference point on the mobile grade reference device, moving the paver along said path over the subgrade and generating second intelligence representative of the successive vertical distances between the subgrade and a grade reference point on the screed and then comparing the first intelligence with the second intelligence as the latter is generated thereby to produce grade control signals representative of the differences between corresponding vertical distances sensed by the grade reference device and the paver, respectively.  
  Still another object of the present invention is the provision of a new and improved apparatus for carrying out the aforesaid method, which apparatus includes, a mobile grade reference device which may be connected to the paver forwardly thereof or which may be independent of the paver and moved over the subgrade by separate powered means, first means on the grade reference device for sensing the vertical distance between a point on the subgrade and a grade reference point on the mobile grade reference device, second means for continuously generating and storing first intelligence representative of the successive vertical distances sensed by such first means as the mobile grade reference device travels along a predetermined path over the subgrade, third means on the paver for sensing the vertical distances between a point on the subgrade and a reference point on the screed, fourth means for continuously generating second intelligence representative of the successive vertical distances sensed by said third means as the paver travels along said predetermined path over the subgrade, and fifth means for comparing the first intelligence with the second intelligence as the latter is generated thereby to produce grade control signals representative of the differences between the corresponding vertical distances sensed by said first and third means, respectively.  
  These and other objects and advantages of the present invention will become apparent from the following specification disclosing a preferred embodiment shown in the accompanying drawings.  
 DESCRIPTION OF THE DRAWINGS FIG. 1 is a side elevational view, largely schematic in form, of one embodiment of the apparatus of the present invention;  
  FIG. 2 is a reduced top plan view of the apparatus of FIG. 1 showing the same positioned on a subgrade;  
  FIG. 3 is an enlarged side view of the vertical distance sensing means attached to the screed;  
  FIG. 4 is an enlarged side view of the vertical distance sensing means attached to the mobile grade reference device;  
  FIG. 5 is a block diagram of a system for generating and storing the profile grade reference data sensed at the mobile grade reference device and for comparing such data with the profile grade reference data sensed at the screed of the paver; and  
  FIG. 6 is a side view, largely schematic in form, of another embodiment of the apparatus of the present invention.  
 DESCRIPTION OF THE METHOD AND PREFERRED EMBODIMENTS OF THE INVENTION The method and apparatus of the present invention is herein described and illustrated within the context of an asphalt paving machine. However, the invention is not to be limited for use with such a machine as the invention has broader applicability. For example, the present invention may be used with other roadway making equipment of the type adapted to apply roadway material in the form of a layer to an existing subgrade, concrete finishing machines and certain types of graders being representative of this type of equipment.  
  Referring now to FIGS. 1 and 2, an asphalt paver, generally designated 10, of the type well known to those skilled in the art includes a screed 11 towed behind the paver by a pair of draft arms 12 and 14. These draft arms are pivotally mounted at their forward ends to the paver. Powered means, hydraulically or electrically operated, are provided for raising or lowering the forward ends of the draft arms to vary the angle of attack of the screed for decreasing or increasing the thickness of the asphalt layer so as to provide grade control. The angle of attack of the screed may also be varied by pivoting the screed with respect to the draft arms in which case the forward ends of the latter are secured to the paver by fixed pivots.  
  The paver 10 is preferably provided with an automatic screed control system of the type shown in the aformentioned Munyon Pat. No. 3,285,148. According to the automatic system disclosed in this Munyon pa-&#39; tent, a sensor mounted adjacent one side of the paver engages the external reference device and supplies signals of the appropriate direction and magnitude for actuating the powered means for one of the draft arms. Transverse slope control means are provided to raise and lower the pivot axis of the other draft arm thereby to maintain the predetermined slope. Reference should be had to the Munyon patent for a detailed explanation of the automatic screed control system.  
  A sensing device, generally designated 15, is mounted to the screed for movement in unison therewith, as by means of a bracket 16. The sensing device includes a rotary transducer 18 (FIG. 3) including a shaft 19 mounting an arm 20. This arm is pivotally engaged with a shoe or skid 21, the latter being adapted to engage the subgrade just forwardly of the forward portion of the screed 1 1. As will be explained hereinbelow, the transducer 16 produces an electrical signal which varies in proportion to the amount of rotation of the shaft 19 thereby producing a signal representative of the vertical distance sensed between the subgrade and a given reference point on the screed. Of course, other forms of transducers, such as a linear transducer, could be provided to produce the appropriate vertical distance signal. v i  
  Referring againto FIGS. 1 and 2, a mobile grade reference device is generally designated 24. The mobile grade reference device includes plural pairs of subgrade engaging shoes or skids each pivotally engaged with arms or links 26. The arms 26 are pivotally engaged with other linkage 27 which supports a stringline or wire 28. A preferred form of mobile grade reference device is disclosed and claimed in the aforementioned application of Munyon and Beaty, Ser&#39;. No. 202,127, now Pat. No. 3,771,892, reference should be had to that application for a detailed description of the mobile grade reference device.  
  A sensing device, generally designated 30, is connected with the mobile grade reference device 24 adjacent the longitudinal midpoint of the latter. This sensing device includes a rotary transducer 31 including a shaft 32 mounting an arm 33. A follower element 34 is mounted on the distal end of the arm 33, which follower engages the stringline 28. The transducer 31 is mounted on a subgrade engaging skid or shoe 36, which shoe is pivotally connected with a link 37, the latter being pivotally connected to &#34;the linkage 27 forming part of the mobile grade reference device.  
  It will be apparent that the sensing device will continuously sense or measure the vertical distance between the stringline 28 and the subgrade at the shoe 36. The rotary transducer 31, as will be explained hereinbelow, produces an electrical signal which varies in proportion to the vertical distance between the subgrade and the stringline.  
  In the embodiment of the present invention now being described, the grade reference device 24 is moved along thepath over the subgrade by a powered wheel apparatus 40 of any suitable form, this apparatus being connected with the mobile grade reference device by a bracket or arm 41. Thus, it is seen that the mobile grade reference device 24 is independent of the paver and is propelled over the subgrade by powered means separate from the motive or power means of the paver 10. At this time it should be mentioned that the mobile grade reference device, since it isseparate from the paver, may be of any desired length, say forty or fifty feet or even longer, to provide the optimum trace approximating the means profile grade of an undulating or otherwise uneven surface to be paved.  
  A subgrade engaging wheel 44 is connected to the linkage 27 of the mobile grade reference device by appropriate pivotally mounted arms or links 45, 46. Such wheel is rotated by reason of its engagement with the subgrade as the mobile grade reference device is moved over the subgrade thereby to provide the necessary horizontal control data which is correlated with the vertical sensings taken by the sensing device 30. The wheel 44 may be fitted with mechanical or magnetic switches, for example, to provide electrical pulse signals for each unit of length of travel along the subgrade, which unit may be a single revolution of the wheel 44 as well as a fraction or multiple thereof. Since an unacceptable accumulation of errors would result if the wheel 44 were to measure horizontal distances of substantial length along the roadway subgrade, preferably horizontal distance control points in the form of stakes or the like are longitudinally spaced apart along the path of travel of the mobile grade reference device at predetermined locations, say 50 to foot intervals, such that the component (to be referred to hereinbelow) which accepts the horizontal distance data accumulated by the wheel 44 may be reset to zero as the mobile grade reference device passes each such stake.  
  Reference should now be had to FIG. 5 which illustrates, in block form, a suitable solid-state memory system for use with the embodiment of the apparatus just described. The rotary transducer 31 of the sensing device 30 on the mobile grade reference device may be in the form of an optical encoder, whereby two trains of pulses are transmitted representing clockwise or counterclockwise rotation of the encoder shaft 32. The time relationship of these pulses, as determined by a pulse circuit 48, will cause an up or down pulse to be generated and supplied to a pulse dividing circuit 49. This&#39;last mentioned circuit, depending upon information supplied by a selector switch (not shown), will divide these pulses by 1, 2, or 3, thereby establishing three ranges of elevation sensing, e.g., :L 1 inch, 1 2 inches, or i 3 inches, respectively. Pulses from the dividing circuit 49 are then fed to an encoder-follower counter 50 which maintains a running tally of the present state of the optical encoder 31 relative to an initial null or zero position which may be preset. The counter 50 not only maintains this information on the position of the encoder shaft, but also converts this position information into parallel format for further processing.  
  A data latch 51, upon command, accepts new data from the encoder-follower 50 and retains this data regardless of subsequent changes in the counter data. An over-range latch 52 will be activated whenever a specific (hardwired) count is reached and will inhibit new data from entering the data latch until the count is reduced below this present maximum.  
  A comparator 53 receives data from the data latch and a compare counter 54 detects any difference between such data and enables a main clock 55 to drive the compare counter 54 either up or down until a match is obtained. The number of counts required to match the compare counter are fed to a difference counter 56 and represent the difference between the last vertical distance sensed or measured by the device 30 and the current vertical distance sensed; this func tion represents the change in vertical distance between successive points measured or sensed as the mobile grade reference device is moved over the subgrade.  
  This vertical distance change information is supplied to a shift register memory 57. This shift register memory retains the vertical distance change data as a group of six binary bits and sequentially moves this information one position further back with each command of the distance clock 58, which clock is actuated by the distance wheel 44. The ouput of the shift register memory 57 is used to preset a drive pulse counter 59; presetting this drive pulse counter causes the same to start counting down to zero, thereby converting the parallel vertical distance change data into serial data once again.  
  This serial vertical distance change data from the drive pulse counter 59 is supplied to a motor pulse sequencer 60 which converts the single-line serial data into four-line sequential pulses required to drive a stepper motor which constitutes the rotary transducer 18 of the sensing device attached to the screed of the paver. The motor drive pulses are fed to the stepper motor through a current amplifier 61 where they are converted from low power logic pulses to high power pulses capable of driving the stepper motor 18.  
  The motor pulse sequencer 60 also receives up and down commands directly from the shift register memory 57 and uses this data to determine the count se quence, i.e., l, 2. 3, 4 or 4, 3, 2, 1. This sequence determines the direction of rotation of the stepper motor shaft 19.  
  A sub-routine generator 62 and the main clock 55 serve as part of the over-all memory system and perform a variety of the necessary tasks. For example, these components are used to drive the counters and sequence the comparator 53 through a fixed series of operations.  
  Summarizing the operation of the embodiment of the invention heretofore described, the mobile grade reference device 24 is advanced longitudinally over the subgrade along a predetermined path. This path may be determined by a steering stringline or by an existing reference device, such as a curb. The sensing device 30 continuously senses the successive vertical distances between the stringline 28 and points on the subgrade over which the shoe 36 travels. This varying vertical distance information is programmed, i.e., this information is generated and stored along with correlated horizontal distance information resulting from rotation of the distance measuring wheel 44.  
  This vertical distance information or intelligence along with the horizontal information may be stored on tape or wire in a reel or cassette. Since this grade profile information is recorded and stored, it is apparent that such information may be generated well in advance of the paving operation. For example, this vertical distance information may be generated and stored a day or even weeks in advance of the paving operation, the only requirement being that there be no appreciable change in the profile of the subgrade between the time it is traversed by the mobile grade reference device and the paver.  
  When the paver is advanced over the subgrade along the same path followed by the mobile grade reference device, the sensing device 15 on the screed senses the successive vertical distances between a given point on the screed and the successive points on the subgrade over which the shoe 21 rides. As this vertical distance information is continuously generated, it is compared with the retrieved vertical distance information generated by the mobile grade reference device for corresponding, i.e., the same, points on the subgrade as determined by the horizontal position information. In essence, these corresponding sensed vertical distances are compared and the differences, if any, can be considered as error signals which are supplied as inputs to the automatic screed control system. lt will be appreciated that by the system just described, any given reference point on the screed will follow the path of travel of the grade reference point on the mobile grade reference device, which last mentioned point may be assumed, for purposes of this discussion and by way of illustration, as the point of engagement between the follower 34 and the stringline 28; in other words, the grade reference point on the screed will follow the grade profile defined by the path of movement of the grade reference point on the mobile grade reference device.  
  It will be appreciated that systems other than the one shown in block diagram form in FIG. 5 may be used in practicing the present invention. By way of example, the grade profile data generated by the sensing device 30 may be recorded and stored, along with the horizontal position control data, using magnetic tape or wire, punched paper tape or multiple shift registers. If such data are recorded on tape or wire, the latter might be deposited directly on the subgrade along the path of travel of the mobile grade reference device. The paver would then subsequently be moved along the same path described by the mobile grade reference device, and the sensing means on the paver would pick up the recorded vertical data on the tape or wire. Correlation of grade control data and horizontal control data would be automatically accomplished using this system.  
  In lieu of appling such tape or wire directly to the subgrade, a magnetic type of paint could also be applied to the subgrade, the amount of paint being proportional to the vertical distance between the subgrade and the grade reference point of the mobile grade reference device. When applying either the tape or wire or magnetic paint directly to the subgrade, the latter would act as a memory and would obviate storage of the vertical grade data and horizontal control data in the manner described in connection with the explanation of the system shown in FIG. 5.  
  As an alternative to the use of the horizontal distance measuring wheel 44, it is contemplated that the horizontal control data could also be developed by means of a wire antenna deposited along one side of the subgrade and through which radio frequency signals would be transmitted. The horizontal distance along such antenna could be determined by measuring the time for a signal to travel from a transmitter to a transponder or other target.  
  Another embodiment of the apparatus of the present invention is illustrated in FIG. 6. Here, a screed 65 of an asphalt paving machine is disposed behind a transverse auger 66 and supported from the rearward ends of the usual draft arms, one such arm being illustrated and designated 67. The forward end of the draft arm 67 is pivotally mounted to the ram 68 of a hydraulic cylinder 69, which cylinder is pivotally engaged with the frame 70 of the paving machine. The other draft arm is of course mounted to the paver in the same manner, the fluid cylinder 69 and the fluid cylinder for the other draft arm being controlled by an automatic screed control system of the type referred to in the aforementioned Munyon patent.  
  A mobile grade reference device, generally designated 72 is disposed forwardly of the screed and within the lateral extremities thereof. This mobile grade reference device includes a beam 73 having downwardly inclined legs 74, 75 pivotally engaged with subgrade engaging shoes or skids 76, 77 respectively. A bracket 78 is connected to the beam 73, which bracket is pivotally engaged with a link 79. The link 79 is pivotally connected to a bracket 80, the latter being connected to the frame 70 of the paver. In other words, the mobile grade reference device 72 is connected to the paver and is propelled over the subgrade thereby.  
  A distance measuring wheel 82 is connected to an arm 83, which arm is pivotally engaged with another arm 84 mounted from the beam 73. The wheel 82 drives a belt 85 which in turn rotates the input shaft of a suitable rotary transducer 86 for accumulating the horizontal position information.  
  A subbeam 88 has downwardly and outwardly inclined legs 89, 90. The leg 89 is pivotally connected with both the beam 73 and the shoe 77. The leg 90 is pivotally connected with a shoe 92.  
  A bracket 94 depending from the draft arm 67 mounts a rotary transducer 95, the rotary shaft of which mounts a sensing arm 96. This sensing arm is engaged by an upright member 97, the latter being connected to the shoe 77. It is apparent that the transducer 95 will continuously sense vertical distances between the subgrade and a given reference point on the screed or draft arm.  
  Another rotary transducer 99 is mounted on the beam 73. This transducer includes a sensing arm 100 engaged by an upstanding member 101, the latter being connected to the shoe 92. It is apparent that the transducer 99 will sense the vertical distance between the subgrade at the shoe 92 and a grade reference point on the mobile grade reference device 72, which point may be considered for purposes of illustration as defined by the engagement between the arm 100 and the upstanding member 101.  
  Since the mobile grade reference device 72, according to the embodiment of the invention shown in FIG. 6, is attached to the paver, there is a constant horizontal distance between the point where vertical measurements are sensed by the transducer 95 and the point wherein the vertical distances are sensed by the transducer 99. When using this embodiment of the invention, a multiple shift register may be conveniently employed. The vertical grade control data generated by the transducer 99 would be entered as incremental bits&#34;in a shifting digital register. The entry of this data to the register would be controlled by signals from the horizontal distance transducer 86. Each bit of such data is shifted down the register one step as a new bit is entered and the last bit drops out of the register as an input to the automatic screed control system. In essence, such a multiple shift register provides that the vertical distance measured at the shoe 77 is compared with the vertical distance which was sensed by the shoe 92 when the latter occupied the position along the subgrade occupied by the shoe 77.  
  It will be apparent that the mobile grade reference device 24 in the embodiment of the invention illustrated in FIGS. 1 through 4 could be directly connected to the paver for movement in unison with the latter in which case the separate powered means 40 would not be required. With such an arrangement, the distance measuring wheel 44 could supply the input information to a multiple shift register of the type just mentioned to correlate the vertical distances sensed by the sensing devices 15 and 30. It should also be understood that the mobile grade reference device 72 of the FIG. 6 embodiment might be considerably greater in length than illustrated and provided with additional subgrade engaging shoes along with appropriate linkage to provide an optimum trace approximating the mean profile grade.  
 We claim:  
  I. The method of controlling a traveling roadway surface preparation machine for making a continuous roadway surface, which machine has a grade control element controllable in response to a grade control signal, said method comprising the steps of:  
 a. moving a mobile grade reference device over the subgrade along a predetermined path and continuously generating grade control intelligence having a horizontal control data factor and a factor representative of the vertical distances between successive points on the subgrade and an averaged grade reference point on said device;  
 b. storing said grade control intelligence;  
 c. moving the roadway surface preparation machine over the subgrade along said predetermined path and continuously generating grade profile intelligence representative of the vertical distances between a point on said grade control element and successive points on the subgrade corresponding to said first mentioned successive points;  
 d. integrating said grade control intelligence with said grade profile intelligence and generating grade control signals representative of the differences between the respective vertical distances of the for mer and the latter at corresponding ones of said points on the subgrade; and  
 e. controlling said grade control element in response to said grade control signals.  
  2. The method according to claim 1 wherein a fixed horizontal distance is maintained between said grade reference device and said machine and wherein the step of storing said grade control intelligence includes moving a member along an endless path at least a portion of which is representative of the horizontal distance between said grade reference point and said point on the machine.  
  3. The method according to claim 1 wherein the step of storing said grade control intelligence includes continuously recording the same on a medium representative of the progressive horizontal distances from a fixed point to said first mentioned points on the subgrade.  
  4. The method according to claim 1 wherein the step of storing said grade control intelligence includes continuously recording the latter by the use of a tangible medium and depositing the same on the subgrade according to a pattern corresponding to said predetermined path.  
  5. The method according to claim 1 wherein the step of storing said grade control intelligence includes continuously recording the latter by the use of a tangible medium in the form of a strip having vertical distance information thereon and depositing the strip on the subgrade along a line corresponding to said predetermined path.  
  6. The method of providing grade control for the screed of a paver comprising the steps of:  
 a. moving a mobile grade reference device over the subgrade along a predetermined path and generating first grade profile information having a horizontal control data factor and a factor representative of the vertical distances between successive points along the subgrade and an average grade reference point on the grade reference device;  
 b. storing said first grade profile information;  
 c. moving the paver over the subgrade along said predetermined path and generating second grade profile information representative of the vertical distances between an element on the screed and successive points along the subgrade corresponding to said first mentioned successive points;  
 d. comparing said second grade profile information with said first grade profile information at the time the former is generated and thereby generating error signals representative of the difference between the corresponding vertical distances of respective first and second grade profile information; and  
 e. operating said screed in response to said error signals.  
  7. The method according to claim 6 wherein a fixed horizontal distance is maintained between said grade reference device and said paver and wherein the step of storing said first grade profile information includes moving a member along an endless path at least a portion of which is representative of the horizontal distance between said grade reference element and said elment n the machine.  
  8. The method according to claim 6 wherein the step of storing said first grade profile information includes continuously recording the same on a medium representative of the progressive horizontal distances from a fixed point to said first mentioned points on the subgrade.  
  9. The method according to claim 6 wherein the step of storing said first grade profile information includes continuously recording the latter by the use of a tangible medium and depositing the same on the subgrade according to a pattern corresponding to said predetermined path.  
  10. The method according to claim 6 wherein the step of storing said first grade profile information includes continuously recording the latter by the use of a tangible medium in the form of a strip having vertical distance information thereon and depositing the strip on the subgrade along a line corresponding to said predetermined path.  
  11. The method of providing grade control for the screed of a paver comprising the steps of:  
 a. moving a mobile grade reference device over the subgrade along a predetermined path and generating first information having a horizontal control data factor and a factor representative of the vertical distances between successive first points on the subgrade and corresponding second points along the path described by movement of a grade reference element on the grade reference device as the latter moves laong said predetermined path;  
 b. storing said first information;  
 c. moving the paver over the subgrade and along said predetermined path and generating second information representative of the vertical distances between successive points on the subgrade corresponding to said first points and third points along the path described by movement of an element on the screed as the latter moves along said predetermined path;  
 d. comparing said first information with said second information as the latter is generated and thereby generating error signals representative of the differences between the corresponding vertical distances of said first and second information, respectively; and  
 e. operating screed grade control mechanism in response to said error signals such that said element on the screed describes a path as it travels over the subgrade corresponding to the path described by said grade reference element as the latter moves over the subgrade.  
  12. The method according to claim 11 wherein the step of storing said information includes recording the latter on a medium representative of the progressive horizontal distances said first points bear with respect to a fixed point on the subgrade.  
  13. The method according to claim 11 wherein the step of storing said first information includes continuously recording the latter by the use of a tangible medium and depositing the same on the subgrade longitudinally thereof.  
  14. The method acording to claim 11 wherein the step of storing said first information includes continuously recording the latter by the use of a tangible medium in the form of a strip having vertical distance information thereon and depositing the strip on the subgrade longitudinally thereof.  
  15. The method of making a roadway comprising the steps of:  
 a. moving a mobile grade reference device over the subgrade and along a predetermined path;  
 b. continouously sensing, during movement of said grade reference device, the vertical distances between the subgrade and a grade reference point on said grade reference device thereby to generate successive first bits of vertical distance information:  
 0. continously sensing, during movement of said grade reference device, the progressive horizontal distances between said grade reference point and a fixed reference point on the subgrade;  
 d. correlating and storing said first bits of information with said horizontal distance information;  
 e. moving roadway making machinery over the subgrade and along a predetermined path, which machinery includes a propulsion unit and grade control equipment movable vertically relative thereto;  
 f, continuously sensing, during such movement of the machinery, the vertical distances between the subgrade and a point on the grade control equipment thereby to generate successive second bits of vertical distance information;  
 g. comparing said second bits of information with the corresponding first bits of information as a determined by said horizontal distance information to generate successive error signals representative of the differences between corresponding first and second bits of information; and  
  h. operating said grade control equipment on said machinery in response to the generation of said error signals thereby to make a roadway having a surface defining a grade profile corresponding to the path of movement described by said grade reference point. 16. In a mobile roadway making apparatus of the type which deposits a layer of roadway material on the subgrade and which includes vertically movable mechanism for controlling the grade profile defined by the surface of said layer of material in response to grade control signals, the improvement comprising:  
 a. a mobile grade reference device and means for moving the same over the subgrade along a predetermined path; b. first means on said grade reference device for sensing the vertical distance between a point on the subgrade and a grade reference point on said grade reference device; c. second means for l. continuously generating first intelligence having a horizontal control data factor and a factor representative of the successive vertical distances sensed by said first means as it travels along said predetermined path; and  
 2. storing said first intelligence;-  
 d. third means on said mobile apparatus for sensing the vertical distance between a point on the subgrade and a point on said vertically movable mechanism;  
 e. fourth means for continuously generating second intelligence representative of the successive vertical distances sensed by said third means as it travels along said predetermined path; and  
 f. fifth means for comparing said first intelligence with said second intelligence as the latter is generated thereby to produce grade control signals representative of the differences between the corresponding vertical distances sensed by said first and third means, respectively.  
 17. The improvement according to claim 16 further defined by:  
 a. said mobile grade reference device including a plurality of subgrade engaging members which are spaced apart longitudinally of said predetermined path;  
 b. linkage means connecting said members;  
 c. said linkage means including a generally horizontally disposed linear element which is moved vertically in response to vertical movements of said subgrade engaging members, which linear element has a point therealong defining said grade reference point; and  
 d. said first means including a subgrade engaging element and transducing means engaged with the latter and said linear element at said grade reference point for supplying information to said second means.  
  18. The improvement according to claim 16 further defined by, other means connecting said mobile grade reference device with said mobile apparatus forwardly of the latter and at a fixed distance with respect thereto such that movement of the mobile apparatus imparts corresponding movement to the mobile grade reference device.  
  19. The improvement according to claim 18 further defined by, said second means including a subgrade engaging member and means mounting the same for movement with said mobile grade reference device and along an endless path at least a portion of which is representative of the horizontal distance between said grade reference point and said point on saidvertically movable mechanism.  
  20. In a mobile roadway making apparatus of the type which deposits a layer of roadway material on the subgrade and which includes vertically movable mechanism for controlling the grade profile defined by the surface of said layer of material in response to grade control signals, the improvement comprising:  
 a. a mobile grade reference device and means for moving the same over the subgrade along a predetermined path; b. first means on said grade reference device for sensing the vertical distance between a point on the subgrade and a grade reference point on said grade reference device; c. second means for l. continuously generating first intelligence representative of the successive vertical distances sensed by said first means as it travels along said predetermined path; and  
 2. storing said first intelligence;  
 (1. third means on said mobile apparatus for sensing the vertical distance between a point on the subgrade and a point on said vertically movable mechanism;  
 e. fourth means for continuously generating second intelligence representative of the successive vertical distances sensed by said third means as it travels along said predetermined path;  
 f. fith means for comparing said first intelligence with said second intelligence as the latter is generated thereby to produce grade control signals representative of the differences between the corresponding vertical distances sensed by said first and third means, respectively;  
 g. said mobile grade reference device being separate from said mobile apparatus; and  
 h. powered mobile means engaged with said mobile grade reference device for moving the same along said predetermined path independently of said mobile apparatus.  
  21. The improvement according to claim 20 further defined by, said second means including a subgrade engaging member and means mounting the same for movement with said mobile grade reference device and along an endless path at least a portion of which is representative of the progressive horizontal distances be tween a fixed point on the subgrade and said grade reference point.  
  22. The improvement according to claim 16 wherein said mobile apparatus is a paver and wherein said vertically movable mechanism is a screed.