Level And Slope Control For Surfacing Machines

Abstract

A level and slope control for road graders and similar surfacing machines is described in which one or more very narrow electromagnetic beams are projected along the path to be graded, and photoelectric sensors on the grading machine are utilized to receive and detect positional information from the light beams and to generate correction signals which both adjust the position of the surfacing blade and steer the machine in the desired direction.

Description

DESCRIPTION OF THE INVENTION

The present invention relates to surfacing machines and equipment, and more particularly concerns an automatic control apparatus for regulating the grade and cross slope of a road grader or similar earthmoving machine. In surfacing machines having ground engaging blades for earth moving and road grading it is often important to achieve very accurate control over the angle and height of the blade as the machine passes over the surface to be graded. This is especially true in the final cuts of the grading operation where the finished grade must be accurate to within a fraction of an inch in order to provide the proper crown of a road, for example, or to create a desired drainage pattern.

A number of attempts have been made in the past to provide automatic control apparatus for achieving the necessary accuracy in grading operations. The design of modern grading and surfacing machines lends itself to automatic operation in that the earthmoving blade element which engages the ground is generally carried by hydraulic cylinders which allow the ends of the blade to be raised or lowered either independently or simultaneously to control both the depth of the cut and the cross slope. In the language of the earthmoving art the angle to be cut along a fore-and-aft path with respect to the grading vehicle is known as the grade, while the angle along a lateral transverse axis to the machine is known as the cross slope, or simply as the slope. In addition, on most machines the angle of the blade with respect to the direction of travel can also be adjusted, and a like adjustment can be made to change the vertical angle of the blade with respect to the ground. The latter adjustments change the angle of attack of the cutting edge of the blade with respect to the ground to suit various operating conditions.

In the past, control over the operation of grading machines has been achieved by locating a large number of grade stakes along the path of machine travel by the use of surveying equipment. The stakes would be driven into place and the desired grade would be marked on them in a manner visible to the machine operator. The operator would then adjust the blade of his machine by reference to the line of stakes in order to achieve the desired cut as he moved along the grade. It is easily appreciated that this procedure requires considerable skill and close attention on the part of the operator, and even then some trial and error operation is often necessary due to soil conditions and other variables that might be encountered. The grade stakes used in such an operation must be placed very closely together so that the machine operator may easily observe them and thereby work to the desired level or grade between the stakes. Such procedures generally result in the grading operation proceeding at a very slow rate in comparison to the rate at which the machine itself was capable of operating, principally because of the operator's need to carefully and slowly adjust the operation of his machine to conform to the level and slope set by the grade stakes.

Various attempts at automatic operation have also been made. In one such system, a long wire is stretched taut along one or both sides of the path to be graded in order to establish a reference level for the equipment. Although this system furnishes a continuous reference line for the operator instead of the spaced reference points afforded by the stakes, the presence of a stretched wire across the working area caused considerable disadvantages. Such a wire must be supported at intermediate points if over a long grade because the weight of the wire will cause considerable sagging and thus create a deviation from the desired grade. The locating and installation of the guide wire also requires almost as much time and effort as the previous grade stakes. Finally, the presence of the wire is a considerable nuisance, considering that it is easily tripped over or run over by equipment and vehicles.

Other systems have used light beams in one fashion or another in order to establish grade reference lines, but none are known which ever achieved any practical utility. Ordinary light beam apparatus suffers from lack of adequate collimation of the beam, and the grade line thus established becomes inaccurate at long distances. An operator using a machine controlled by such a light beam is generally unable to determine whether his machine is on the beam or off, and he may easily lose the beam entirely without being aware of it. In addition, such beams are useful only for setting up the primary grade to be cut by the earthmoving machine, and do not have the capability of allowing control over cross slope as well.

Accordingly, it is a principal object of the present invention to provide an improved system for controlling the ground engaging element of an earthmoving or leveling machine which allows the machine to cut a predetermined grade and follow a predetermined linear path without the necessity of placing grade stakes, stringing wires, or setting up any permanent grade reference indications whatever. It is intended that the system make use of a linear beam of electromagnetic energy, preferably a light beam, which can be quickly and easily set up or shifted from place to place on the job site.

It is also intended that the system of the present invention be suitable for multiple use in which both ends of the ground engaging blade of the leveling apparatus may be independently controlled. Alternatively, it is intended that the system be useful in conjunction with other forms of control apparatus, including pendulum-operated cross slope detection means. In order to make the system wholly automatic, automatic, it is contemplated that the system include steering correction means which enable the machine to follow the electromagnetic beam automatically without intervention by the operator.

A more detailed object is to provide such a system in which the leveling and steering operations are carried out sequentially, so that the ground engaging element is first adjusted vertically to the desired position, and then the direction of the vehicle is adjusted by steering correction so that the more important grade and cross slope adjustments are always accomplished first. A proportioned response is contemplated in which the corrective control reaction is proportioned to the relative degree of deviation of the blade from the desired path as determined by the beam.

A further refinement in this respect is the provision of a compensating system which takes into account the widely varying positions of the ground engaging element in its mounting structure so that the control system will operate accurately without regard to changes in the mounting position of the blade in the machine.

Other objects and advantages of the present invention will become apparent upon reading the following detailed description in conjunction with the attached drawings, in which:

FIG. 1 is a perspective view of a self-propelled surfacing machine, specifically a grader, which embodies the present invention for automatic steering and grader blade attitude control;

FIG. 2 is an enlarged, fragmentary perspective view to better illustrate the support means for the grader blade shown in FIG. 1;

FIG. 3 is an enlarged, fragmentary view, partly in elevation and partly in vertical section, showing the grader blade with its associated lift cylinders and illustrating the mounting of sensing units provided in accordance with the present invention; and

FIG. 4 is a simplified schematic diagram of a representative form of the automatic control system.

While the invention is described in connection with certain preferred embodiments and procedures, it will be understood that it is not intended to so limit the invention. Instead, it is intended to cover all alternative and equivalent constructions as may be included within the spirit and scope of the invention as defined by the appended claims.

Turning to the drawings, there is shown in FIG. 1 a grading machine employing a blade level control system constructed according to the present invention. In this instance, the machine is a road grader having a chassis 10 which carries a scraper blade or moldboard 11. The grader is equipped with a cab 12 for the operator, a rear mounted engine 13, and a pair of powered rear driving wheels 15 which exert tractive effort to propel the machine along its intended path of travel. At the front, a pair of steerable wheels 16 is provided. The rear wheels 15 may also be made steerable in order to give the machine greater versatility of operation. The grader is generally of conventional design in most respects, save for the novel control system of the present invention.

The scraper blade 11 is suspended from the chassis 10 in a manner which allows it to be moved over a wide range of adjustment. Adjustment during operation is achieved by a pair of hydraulic cylinders consisting of a right cylinder 17 and a left cylinder 18, as seen from the operator's compartment, each of which is pivoted to the chassis 10 directly above the blade 11. The cylinders 17, 18 each terminate in a ball joint 20 on a blade mounting ring 21 carried beneath the arched central portion of the chassis 10.

The blade 11 is suspended from the chassis 10 in a manner which allows a wide degree of freedom in positioning the blade with respect to the ground. To start with, the operating plane of the blade 11 is determined by a mounting ring 21 to which the blade is attached. Although the blade 11 may be made pivotable or shiftable with respect to the mounting ring 21, the ring is always determinative of the plane in which the cutting surface will lie. For example, the cutting angle of the blade 11 with respect to the ground may be varied by a hydraulic blade circle reverse cylinder (not shown) linking the drawbar yoke 22 and the mounting ring 21. Another possible adjustment is accomplished by making the blade 11 shiftable longitudinally with respect to the mounting ring 21 so that the blade may be extended beyond the track of the vehicle for certain grading operation. This is accomplished by another hydraulic cylinder (not shown) located behind the blade 11.

The mounting ring 21 to which the blade 11 is secured is in turn carried by the chassis 10 by means of a yoke or drawbar 22. The latter is a triangular structure having a ball joint 23 at one end which connects to the forward portion of the chassis 10. The ball joint 23 allows complete freedom of articulation of the yoke 22 with respect to the chassis, allowing the mounting ring 21 to swing from side to side, to move up and down, and to rotate about a longitudinal axis extending through the ball joint 23. The entire tractive effort of the machine is thus exerted through the ball joint 23 and in turn through the yoke 22 and blade mounting ring 21 to the blade 11.

In order to locate the mounting ring 21 and its attached blade 11 for performing a particular grading operation, a locating link 25 is provided at the rear of the yoke 22. The link 25 has another ball joint 26 at one end which may be attached to one or the other of two mounting points 27 located on each side of the yoke 22. The locating link 25 is selectively positioned by a locating hydraulic cylinder 28 which operates in conjunction with the two hydraulic cylinders 17, 18 previously mentioned to locate the blade 11 in a desired position. The locating link is articulated about a pivot 29 in order to allow a range of adjustment in the new position. For instance, if it is desired to swing the blade over to one side in order to perform a certain operation, the locating link 25 is attached to the particular locating ball joint 26 opposite the side on which the grading operation is to be performed. The locating hydraulic cylinder 28 is then actuated to shift the locating link 25 toward the desired side, thereby bringing the yoke 22 with it. This establishes the final position of the mounting ring 21 with respect to the chassis 10, with the mounting ring then being rotatable about an axis determined by a line extending between the forward ball joint 23 and the locating link pivot 29. The hydraulic cylinders 17, 18 are actuated in unison if it is desired to raise or lower the mounting ring 21 through a path of travel determined by the yoke 22 and locating link ball joint 26, or to rotate the mounting ring 21 about an axis determined by the ball joint 23 and the locating link pivot 29 as previously mentioned.

As a principal feature of the invention, a means is provided whereby the position of the blade 11 with respect to the chassis 10 is adjusted by an automatic control system which uses as its reference a collimated beam of light projected along the path of desired blade travel. Such a beam is preferably generated by a laser unit, this term being an acronym for Light Amplification by Stimulated Emission of Radiation. Such a beam is characterized by its very great intensity and almost total lack of dispersion or diffusion from a straight line path. Within the distances contemplated in the present application the dispersion or widening of the light beam from such a source is negligible and the beam approximates a straight line over its entire useful path of travel. Because the beam does not diverge, its energy per unit area is substantially undiminished except for losses which may result from haze, smoke, dust or the like on the job site. In the present application, a laser beam generating unit 30 is set up on a tripod mount 30a or similar support, and located with respect to the grade line by ordinary surveying means, such as a plumb line 30b. The laser unit 30 is supplied by a source of electrical energy (not shown) which may comprise a field generating unit or the like.

While the present embodiment is described with a beam generation unit 30 which consists of a laser it will be appreciated that any similarly intense, highly collimated electromagnetic beam would also be satisfactory in carrying out the invention. However, the properties of the laser unit described make it preferable for use in the present application.

In the illustrated embodiment, the beam from the laser unit 30 is received by a sensing unit 31 carried by one end of the blade 11. The sensing unit 31 consists generally of a broad plate having a plurality of sensitive areas which respond to the impingement of the laser beam by generating an electrical output voltage. This voltage is then used to control the various hydraulic cylinders which effect positional changes in the blade 11. In addition, directional changes in the vehicle itself are accomplished by means of a control circuit which effects steering changes in the vehicle through a steering hydraulic control cylinder 32 which is operatively connected to the front wheels 16 of the grader.

Referring to FIG. 4 it may be seen that the sensing unit 31 is provided with photovoltaic surface areas, each of which is responsive to a particular direction of deviation of the blade 11 from the desired path of travel as determined by a shift in the impingement of the laser beam. Deviation in a vertical direction is indicated by an upper sensing surface 35 and a lower sensing surface 36. Lateral deviation is detected by a left sensing surface 37 and a right sensing surface 38, while the correct path of travel is indicated by the impingement of the laser beam on a central or null surface 40. In keeping with the invention, the cross-sectional area of the laser beam is relatively small as compared with the surface area of the various sensitive photovoltaic elements 35, 36, 37, 38. This allows provision for a proportional control system in which the various photovoltaic sensing elements are made progressively move responsive as the distance away from the null surface 40 increases. This may be achieved either by graded deposition of the photovoltaic material on the photosensitive surfaces 35, 36, 37, 38, or may alternatively be accomplished by the application of a graded shading material over the photosensitive surfaces, such as smoked glass. The shading is arranged so that the areas nearest the null area 40 generate a proportionately lower error signal than do the areas further away.

The error signal generated by the sensing unit 31 is translated into movement of the blade 11 by a system of electrically actuated control valves which govern the movement of the various hydraulic cylinders. The control cylinders 17, 18 may be controlled simultaneously in order to effect an overall raising or lowering of the blade 11, but in the illustrated embodiment an additional feature is included by which the cross slope of the blade is kept constant relative to the true vertical. More specifically, in one form of the present invention the position of the blade 11 relative to true horizontal transversely of the machine is detected by a pendulum sensor 41 which in the illustrative embodiment is mounted on the yoke 22. This sensor 41, illustrated schematically in FIG. 4, is of a known pendulum type in which a pendulum element is operatively connected to a potentiometer 42 so that the resistance value of the potentiometer is a function of the pendulum position. With the blade 11 at a given transverse angle relative to true horizontal, the resistance value of the potentiometer 42 will be at a predetermined value and any variation from this angle will cause a corresponding deviation in the resistance value.

It will be observed that changes in the rotational position of the blade 11 with respect to the yoke 22 will cause a change in the true cross slope which is cut as the vehicle moves along. This is especially true when the blade 11 is cocked over at a steep angle by suitable positioning of the locating link 25. In such instances, the yoke 22 will be swung about its axis between the ball joint 23 and the locating link pivot 29, thereby introducing further variation into the system. The net effect of such shifts of the blade 11 with respect to the chassis 10 is to cause an error to appear between the position indicated by the pendulum sensor 41 and the actual cross slope grading position of the blade 11.

To eliminate this difficulty, and according to a further aspect of the invention, the error resulting from shifting the position of the blade 11 with respect to the chassis 10 is compensated by the provision of a blade circle potentiometer 43 and a yoke position potentiometer 45. As shown in FIG. 4, these two potentiometers are connected in series with the output of the pendulum potentiometer 42 so that the net output of the three potentiometers 42, 43, 45 is used for positional information of the blade 11 rather than the output of the pendulum sensor potentiometer 42 alone. In the case of the yoke position potentiometer 45, this instrument is arranged with a suitable actuating means to indicate vertical angularity change of the yoke 22 and its attached blade mounting ring 21 with respect to the chassis 10. In other words, the potentiometer 45 supplies a signal that is representative of the position of the blade 11 relative to true horizontal longitudinally of the machine. This signal is simply added to or reduced from the output of the pendulum sensor potentiometer 42 in order to generate a corrective signal corresponding to the angular position of the blade 11 with respect to the true horizontal transversely of the machine. The blade circle potentiometer 43 is used to generate a further correction signal which is dependent on the rotation position of the blade 11 and mounting ring 21 in respect to the yoke 22. This is done because rotation of the blade 11 will effect a change in the true cross slope cut by the grader, making compensation necessary in the control system.

Of course, it will be appreciated that the specific mechanical mounting (not shown) of the potentiometers 43 and 45 may be affected in a variety of ways. For example, since the potentiometer 43 is to provide a signal representative of the rotational position of the blade 11 and mounting ring 21 with respect to the yoke 22, it may suitably be driven by the rotational motion relative to the yoke 22 of the mounting ring 21. On the other hand, since the potentiometer 45 is to provide a signal representative of the vertical angularity of the blade 11, it may suitably be driven by the vertical movement of the neck of the ball joint 23 as indicated in FIG. 2.

Up to this point, the blade position compensation system has been described as comprising a pair of additional potentiometers 43, 45 connected in series with the pendulum potentiometer 42. It will be appreciated that the pendulum sensor 41 could be mounted on either the blade mounting ring 21 or the yoke 22 without departing from the invention. The electrical output from the cross slope sensing system is directed to an electrically actuated blade slope hydraulic control valve 46. As indicated schematically in FIG. 4, the valve 46 has an electric actuator which is responsive to the output of the slope position sensor potentiometers, and also to the output of a slope selector potentiometer 47. In a typical installation, the slope selector potentiometer 47 would be located in the cab of the grader so that the operator could simply dial in the desired cross slope to be graded. This would cause a signal to actuate the blade slope control valve 46. As long as an electrical bias exists between the slope selector potentiometer 47 and the output of the pendulum sensor potentiometer 42 as corrected by potentiometers 43, 45, the unbalance will cause actuation of the control valve 46 and admit pressure fluid to one of the blade control hydraulic cylinders 17, 18. One end or the other of the blade 11 is thereby moved to a new position in which the cross slope actually cut by the grader (as indicated through the corrected output signal of the pendulum sensor potentiometer 42) is corrected, whereupon the signals fed to the electrically actuated blade slope control valve 46 are equalized and the system is brought back to neutral.

The foregoing description has been based on the blade slope control valve 46 being connected to one or the other of the blade control cylinders 17, 18. Thus it has been assumed that the opposite cylinder remained stationary during the correction process which brings the actual cross slope as cut by the machine into agreement with the desired cross slope as indicated by the position of the slope selector potentiometer 47. If one of the blade control cylinders, say cylinder 17, is thus connected to the blade slope control valve 46, the other cylinder 18 would then be used in connection with the blade height control system. As the blade 11 is shifted vertically by one hydraulic cylinder 18, it will be seen that the proper cross slope position is maintained by the cross slope control system previously described. As one edge of the blade is shifted vertically by the cylinder 18, the balance of the blade slope hydraulic control valve 46 is shifted and the other blade cylinder 17 is operated until the balance is again restored and the blade 11 is returned to the proper cross slope. Stated another way, the cylinder 18 moves one end of the blade to the proper vertical position, and the cylinder 17 then follows automatically to maintain the proper cross slope.

The operation of the blade height control system is governed by another electrically actuated control valve 48 which supplies pressure fluid to the blade control cylinder 18 as previously described. The blade height control valve 48 operates in a manner similar to that of the slope control valve 46, except that the electric actuators are connected to the electrical output of a corresponding portion of the sensing unit 31. For instance, the upper sensing surface 35 of the sensing unit 31 would be energized by the laser beam when the blade 11 is too low, and therefore this surface is connected through an amplifier 50 to that portion of the blade height control valve 48 which will admit fluid into the cylinder 18 which will raise the blade 11. Similarly, the lower surface 36 is connected through an amplifier 51 to the opposite side of the blade height control valve 48 to effect a lowering of the blade 11 when the laser beam falls too low on the sensing unit 31.

According to yet another aspect of the invention, the correction of the blade height and cross slope is accomplished prior to any steering correction, whereby the more important blade positioning function is always carried out prior to the steering correction. This result is achieved by positioning the steering correction control surfaces 37, 38 in the relatively narrow horizontal band containing the null surface 40. The horizontal band including the steering correction surfaces 37, 38 and null surface 40 functions as a null band for vertical corrections of the blade 11, and steering corrections are not initiated until the laser beam is in this region.

Once in this region, the steering control sections 37, 38 of the sensing unit 31 act through their associated amplifiers 52, 53 which in turn supply actuating signals to an electrically actuated steering control valve 44 similar to the control valves 46, 48 previously described. When the beam falls on the left steering control surface 37, an electrical signal is supplied through the amplifier 52 to that portion of the steering control valve 44 which turns the wheel 16 of the vehicle to the left, thereby bringing the path of travel back to the desired line. When the beam again falls on the null surface 40, the correction is removed and the vehicle continues to travel in a straight path. Preferably, the steering control surfaces 37, 38 are also provided with a graded surface which increases the relative output signal as the beam falls further from the null surface 40. In this way a proportional control is achieved which provides large corrections for large deviations from the desired path and smaller corrections for deviations requiring a smaller corrective action.

It will be appreciated that the blade positioning means of the present invention need not be restricted to the embodiment disclosed, but may be used in other ways. For instance, to replace the slope control potentiometers 42, 43, 45 and 47, an additional laser unit 60 can be used to project a beam generally parallel to the beam of the first laser unit 30. The blade 11 is then equipped with an additional sensing unit 61 to correctively control the cylinder 18 in a manner similar to the previously described manner in which the sensing unit 31 is used in conjunction with the first laser beam to correctively control the cylinder 17. In this manner both ends of the blade 11 are independently controlled to achieve the desired blade position along the entire path of vehicle travel. Only one laser sensing unit 31 is used to control steering when the second beam is used. If desired, a varying cross slope may be achieved with this apparatus by shifting one or the other of the laser beams so that they no longer lie in the same plane, but are still substantially equally spaced from each other at all points along their effective range. This distance will be dictated by the distance between the respective null points 40 of the two sensing units 31, 61.

An important advantage of the laser beam in the present invention is the fact that its impingement on the sensing unit 31 is visible to the machine operator. In this way he can maintain a constant check on the operation of his machine, and can even "find" the beam again in the event that the sensing unit 31 should shift out of the beam entirely. If desired, the operator can even use the beam as a visual guide for manual control of the machine.

Claims

We claim:

1. In a self-propelled surfacing machine having a chassis, ground engaging support means for said chassis, steering means for said support means, a ground engaging leveling element carried by said chassis, and power means connected between said chassis and said leveling element for adjusting the vertical position of said leveling element relative to said chassis; an automatic control system for steering said machine along a predetermined path while maintaining said leveling element along a predetermined grade by following a collimated electromagnetic beam projected along said path and parallel to said grade by a remotely stationed transmitter; said control system comprising the combination of:

a beam target mounted on said leveling element and having a target area that is large relative to the cross-sectional area of said beam, said target area including a central null region positioned to intercept said beam under nominal operating conditions, vertically spaced sensing regions responsive to impingement of said beam thereon for supplying a first control signal, and horizontally spaced sensing regions responsive to impingement of said beam thereon for generating a second control signal; said vertically and horizontally spaced sensing regions having sensitivities which vary outwardly from said central null region so that said first and second control signals have magnitudes that are respectively proportional to the vertical and horizontal deviation of said beam from said central null region;

first control means coupled between said target and said power means and proportionally responsive to the magnitude of said first control signal for correctively actuating said power means to maintain said leveling element along said grade; and

second control means coupled between said target and said steering means and proportionally responsive to the magnitude of said second control signal for correctively actuating said steering means to steer said machine along said path.

2. In a self-propelled surfacing machine having a chassis, ground engaging support means for said chassis, steering means for said support means, a support structure pivotably mounted on said chassis for rotational movement about its own longitudinal axis and vertical movement relative to said chassis, a ground engaging leveling element pivotably mounted on said support structure for rotation about an axis perpendicular to the longitudinal axis of said support structure, first power means connected between said chassis and said leveling element for varying the vertical position of said leveling element relative to said chassis, and second power means connected between said chassis and said leveling element for varying the angle of said leveling element transversely of said chassis; an automatic control system for maintaining said leveling element along a predetermined grade with a predetermined slope by following a collimated electromagnetic beam projected along a predetermined path of travel for said machine and parallel to said grade by a remotely stationed transmitter; said control system comprising the combination of:

a beam target mounted on said leveling element and having a target area which is large relative to the cross-sectional area of said beam, said target area including a null region positioned to intercept said beam under nominal operating conditions and vertically spaced sensing regions which are responsive to impingement of said beam thereon for supplying a first control signal;

first control means coupled between said target and said first power means for correctively actuating said first power means in response to said first control signal to thereby return said leveling element to said grade;

reference means mounted on said machine for providing a reference signal indicative of the predetermined slope for said leveling element;

detector means for supplying a second control signal indicative of the actual slope of said leveling element, said detector means including the series connected combination of a first gravity-operated potentiometer mounted on said support structure for providing a signal representative of the angle between said grade and true horizontal transversely of the machine, a second potentiometer coupled between said support structure and said chassis for operation in response to vertical movement of said support structure relative to said chassis for providing a signal representative of the angle between said leveling element and true horizontal longitudinally of said machine, and a third potentiometer coupled between said leveling element and said support structure for operation in response to rotation of said leveling element about said perpendicular axis for providing a signal representative of the rotational position of said leveling element; and

second control means coupled to said reference means, said detector means and said second power means for correctively actuating said second power means in response to an imbalance between said reference signal and said second control signal to thereby return said leveling element to said predetermined slope.

3. In a self-propelled surfacing machine having a chassis, ground engaging support means for said chassis, steering means for said support means, a ground engaging leveling element pivotably mounted on said chassis, first power means connected between said chassis and said leveling element for varying the vertical position of said leveling element relative to said chassis, and second power means connected between said chassis and said leveling element for varying the angle of said leveling element transversely of said chassis; an automatic control system for maintaining said leveling element along a predetermined grade with a predetermined slope by following a collimated electromagnetic beam projected along a predetermined path of travel for said machine and parallel to said grade by a remotely stationed transmitter; said control system comprising the combination of:

a beam target mounted on said leveling element and having a target area which is large relative to the cross-sectional area of said beam, said target area including a null region positioned to intercept said beam under nominal operating conditions, vertically spaced sensing regions which are responsive to impingement of said beam thereon for supplying a first control signal and a pair of horizontally spaced sensing regions one on either side of said null region for supplying a second control signal in response to the impingement of said beam on either of them;

first control means coupled between said target and said first power means for correctively actuating said first power means in response to said first control signal to thereby return said leveling element to said grade;

a second control means coupled between said target means and said steering means for correctively actuating said steering means in response to said second control signal to maintain said machine along said predetermined path of travel;

reference means mounted on said machine for providing a reference signal indicative of the predetermined slope for said leveling element;

detector means coupled to said leveling element for supplying a third control signal indicative of the actual slope of said leveling element; and

third control means coupled to said reference means, said detector means and said second power means for correctively actuating said second power means in response to an imbalance between said reference signal and said third control signal to thereby return said leveling element to said predetermined slope.

4. The combination of claim 3 wherein said leveling element is mounted on a support structure for rotation about an axis perpendicular to the longitudinal axis of said support structure, said support structure is pivotably mounted on said chassis for vertical movement relative to said chassis and for rotational movement about the longitudinal axis of said support structure; and said detector means includes the series-connected combination of a first gravity-operated potentiometer mounted on said support structure for providing a signal representative of the angle between said leveling element and true horizontal transversely of the machine, a second potentiometer coupled between said support structure and said chassis for operation in response to vertical movement of said support structure relative to said chassis for providing a signal representative of the angle between said leveling element and true horizontal longitudinally of the machine, and a third potentiometer coupled between said leveling element and said support structure for operation in response to rotation of said leveling element about said perpendicular axis for providing a signal representative of the rotational position of said leveling element.