US3029715A

US3029715A - Control system for paving machine

Info

Publication number: US3029715A
Application number: US717873A
Authority: US
Inventors: John T Bowen
Original assignee: Preco LLC
Current assignee: Preco LLC
Priority date: 1958-02-27
Filing date: 1958-02-27
Publication date: 1962-04-17
Anticipated expiration: 1979-04-17

Description

April 17, 1962 Filed Feb. 27, 1958 5 J. T. BOWEN 3,029,715

CONTROL SYSTEM FOR PAVING MACHINE 2 Sheets-Sheet 1 (Io/4N 17. flows/v,

JM Eura Q April 17, 1962 Filed Feb; 27, 1958 J T. BOWEN CONTROL SYSTEM FOR PAVING MACHINE 2 Sheets-Sheet 2 5 yi gh QOHN .72 Bows,

United States 3,023,715 CONTRQL SYSTEM FOR PAVMG MAC John T. Bowen, La Hahra, alil., assignor to Preco lncorporated, Los Angeles, Calif., a corporation or California Filed Feb. 27, 1958, Ser. No. 717,873 7 Claims. (Cl. 94-46) This invention has to do with improved means for controlling the operation of machines for depositing a layer or mat of paving material on a roadway or the like. 1

The invention relates more particularly to such machines which deposit the paving material adjacent the leading edge of a horizontal screed which extends transversely of the machine and is supported by the freshly laid mat. The level of the mat surface produced by such machines is typically controlled by variation of the inclination of the screed in the direction of travel. Manual adjustment means for varying the screed inclination are ordinarily provided at both ends of the screed, which is suificiently flexible torsionally to permit its independent adjustment at both sides of the machine. An operator, known as the screedman, ordinarily manipulates both of those adjustments in a manner to control the thickness of the mat at its lateral edges, thus determining the transverse slope of the mat surface.

A primary purpose of the present invention is to provide control means for machines of the type described whereby the transverse slope of the mat surface may be controlled, preferably automatically. An important advantage of such automatic control is that the screedman is released to devote additional attention to proper control of the mat thickness.

The invention may be employed to supplement existing devices which provide automatic or semi-automatic control of the mat thickness.

Another important object of the invention is to provide a control system which will directly control the paving machine, or facilitate its manual control, to produce a mat surface having a transverse slope of any desired value, which value may be varied conveniently and accurately during operation of the machine.

' A further object of the invention is to provide a system capable not only of detecting and indicating any error in the transverse slope of the mat surface that is being produced, but in large measure of anticipating the appearance of such errors and indicating or automatically producing appropriate adjustment of the machin to prevent their occurrence.

The present invention provides improved control means of the type described and. claimed in the copending patent application of Reeford P. Shea, Serial No. 717,874, filed on February 27, 1958 under the title Paving Machine Control System. A primary purpose of the present invention is to provide an improved control system that is potentially simpler and more economical and which is capable of improved accuracy and convenience in operation.

The invention further permits convenient and effective adjustment, during operation if desired, of the degree of correction produced by the control mechanism under 3,029,715 Patented Apr. 17, 1962 machine progresses along the roadway. That second signal typically corresponds to the angular twist-of the screed along its length. From those two distinct signals is developed a control signal which typically represents a selected algebraic combination of thescreed twist and the departure of the mat slope from the desired value. Means are then provided for drivingthe angle adjustment at one end of the screed in accordance with that'- control signal. With suitable selection of the sense of the various signals and actions of such a system, the screed twist is typically maintained substantially zero it the mat slope has the desired value. If the mat slope differs from that value, the screed twist automatically assumes a value tending to progressively increase the mat thickness at one end of the screed and decrease it, relatively, at the other end, thereby gradually restoring the desired slops.

The invention further provides means for adjustably varying the reference value from which the mat slope is measured in deriving the slope signal; and provides means for adjustably varying the relative weight that is accorded to the current slope signal and the screed twist signalin deriving the overall control signal. I p v A full understanding of the invention and ofits fur ther objects and advantages will be had from the fol lowingdescription of an illustrative manner in which.

it may be carried out. The particulars of that descrip-' tion, and of the accompanying drawings which form a part of it, are intended only as illustration of the inven tion and not as a limitation upon its scope, which is defined in the appended claims. In the drawings: FIG. 1 is a side elevation of an illustrative'paving machine to which the invention pertains; I

FIG. 2 is a rear elevation of that machine; FIG. 3 corresponds to a portion of FIG. 2 at enlarged scale and represents an illustrative embodiment of the invention in the machine of FIGS. 1 and 2; i .1 FIG. 4 is a detail of FIG. 3 at further enlarged scale and partly broken away;

FIG. 5 is a section on line 5--5 of FIG. 3; and FIG. 6 is a schematic diagram representing an illus trative electrical system in accordance with the invention. In the present illustrative embodiment, the inventionis utilized for controlling a typical paving finishing ma chine of known type. In that machine, as illustratively and somewhatschematically represented, the main body 20 of the machine is supported for forward movement over a roadway base 29, typically on two endless trac tor treads 28. Those treads are driven by an engine 3t via suitable reduction gear transmission and clutch mechanism, not specifically shown, under manual control of an operator in seat 32. At the forward end of machine 20 is a transverse roller 34 adapted to push ahead of the machine a truck loaded with paving material, indicated at 33. A receiving hopper 26 is provided in the forward portion of the machine in position to receive paving material 27'from the truck and to supply it to suitable conveying and dis-' tributing mechanism within the body of the machine. That mechanism is indicated generally by the numeral 24, and includes the power-driven transverse distributing screws 25. Those screws spread a loose pile 35 of pav ing material across roadway base 29, just to the rear of treads 28 and typically extending transversely the entire width of the machine;

Paving material is spread and tamped'to an accurately controllable level to form a compacted mat36. That is accomplished by cooperative action of the-vertically reciprocating tamping bars 42 and the screed 44. The latter is a generally horizontal plate extending transversely mediately behind the' tamping bars. Each stroke of tamping bars 42 strikes off a limited amount of paving material and compacts it to uniform density directly adjacent the leading edge 45 of the screed. As the machine moves forward, screed 44 is drawn over the freshly formed mat, further compacting it and smoothing its upper surface.

The term paving material is employed inthe present specification and claims to refer to any material that can be handled in the described manner. Such material may comprise broken rock or gravel, for example, or bltllIIli-r nous adapted for producing the first course or the finishing course of a pavement. The base on which the machine operates may comprise an actual roadway, or a surface of any other type on which a course of material is to be laid. The term roadway will be employed ordinarily in the generic sense of including any surface on which material is to be spread.

An important feature of the type of machine with which the present invention is primarily concerned is the manner in which the thickness of the mat, or, more precisely, the level of its top surface, is determined. The entire screed assembly, designated generally by numeral 40 and comprising the screed itself, tamping bars 42 and the eccentric mechanism for reciprocating the latter, is supported directly on the top surface of the freshly laid mat. The screed assembly is coupled to the main body 20 of the machine by coupling means which permit it to vary in vertical level and thus to float on the finished mat while following at a fixed distance behind the machine tractor. Tamping bars 42 are driven from engine 30 by transmission means indicated at 43 adapted to accommodate vertical movement of the screed assembly. The mat is produced by depositing and compacting the paving material immediately in front of the leading edge of the screed, and the freshly laid mat is further compacted and smoothed as the screed moves over it. The level of each new portion of the mat surface may be considered to be determined jointly by the level of the mat portion just previously laid, upon which the main body of the screed is supported, and upon the inclination angle of the screed surface in the direction of forward travel. The latter angle, indicated at 48 in FIG. 5, will be referred to as the angle of attack of the screed, by analogy with the angle of attack at which the airfoil of an aircraft engages the supporting air. Variation of the angle of attack of the screed does not produce an abrupt change in the level of the mat surface, but causes a gradual transition from one level to another. For any particular conditions of operation, including a particular paving material, there is typically an equilibrium value of the angle of attack for which the mat surface extends horizontally in the direction of travel. That value of the angle of attack will be referred to for convenience as the critical value.

I It is usual in paving machines of the present type to make the screed torsionally flexible about its longitudinal axis, and to provide independently operable means for adjusting the angle of attack of its respective end portions. The mat surface level is then separately adjustable along the two side edges of the mat. Manipulation of those two adjustments in suitable mutual relationship permits the operator to control the transverse slope of the resulting mat. For example, by increasing the angle of attack at both ends of the screed, the mat thickness may be caused to increase substantially uniformly across the mat; and by increasing the angle of attack at one end of the screed and decreasing it at the other end the transverse slope of the surface can be varied without substantially affecting the average thickness of the mat.

The coupling means between the tractor and the screed assembly is ordinarily of a type which produces a certain degree of regulation of the angle of attack of the screed. In the present illustrative machine, that coupling means is a well known type which comprises two rearwardly extending

draft arms

50 and 51 which are pivotally mounted on opposite sides of tractor 20. With such coupling,

vertical movements of the tractor itself, whether in response to irregular or intended changes in level of the roadway, cause corresponding but relatively small change in the angle of attack of the screed. Mechanism is also provided for manually altering the angle of attack at each end of the screed. That is done on some paving machines by pivoting the forward ends of

draft arms

50 and 51 at points which are vertically adjustable with respect to the tractor body. In the present illustrative type of machine the draft armsare pivoted on a fixed trunnion axis 52, and adjustment of the angle of attack is accomplished by structure which permits adjustable variation of the angle between the screed surface and the length of the arm. The screed assembly is typically mounted at a fixed average distance below the draft arms by vertical supports, indicated at 54; and is rotatably adjustable with respect to support 54 about a transverse axis 55, as by the generally vertical screws 56 and 57. The lower end of each screw is rotatably mounted on a bracket member 58 which is universally related to the screed plate near its rearward outer corner; and the upper part of the screw is threaded in a bracket member 59 which is universally related to theend of the draft arm. Screw rotation thus varies the angle between the length of the draft arm and the Plane of the screed surface adjacent its end.

The screed is longitudinally stiffened by a frame structure indicated generally at 60. That frame structure typically comprises two distinct frame sections 61 and 62, on the right and left sides, respectively, of the central plane 64 of the machine, which are adjustably connected together to facilitate production of a crowned pavement. As shown, those frames are mutually swingable through a small angle about a pivot axis 63 parallel to the direction of travel of the machine and close to the plane of the screed. The crown angle is adjustable by the screw connection 66 between the upper portions of the two frames. The screed itself comprises two portions effectively hinged together parallel to axis 63, or is otherwise made sufliciently flexible to accommodate the described crown adjustment. The tamping mechanism at the leading edge of the screed is correspondingly constructed in two separately actuated portions to accommodate the crown adjustment.

With the described type of coupling between screed assembly and machine proper, the angle of attack of each end of screed 44 is determined substantially independently by two primary factors: the inclination of the associated draft arm, and the condition of adjustment of screw 56 or 57. For a given screw adjustment and type of paving material, and with the tractor moving over a flat horizontal roadway, there is a definite equilibrium mat thickness at which the angle of attack of the screed has the above defined critical value. If the mat initially has a different thickness at one or both ends, the surface level of the mat laid down tends to increase or decrease progressively, causing one or both of the draftv arms to swing about trunnion axis 52, until the angle of attack at both ends of the screed reaches the critical value at which the machine produces a horizontal mat surface. If the settings of screws 56 and57 are different, the resulting equilibrium mat level is correspondingly different at the two ends of the screed, that is, at the two transverse edges of the mat, so that the mat surface has a transverse slope. That equilibrium condition typically continues until disturbed either by variation in level of the roadway base, which causes one or both trunnions to rise or fall, or by a change in adjustment of one or both of the control screws 56 and 57. Either type of change shifts the angle of attack away from its critical or equilibrium value at one or both ends of the screed; and the mat level again changes progressively until equilibrium conditions are reestablished. r For example, if one track of machine 20 encounters a substantial depression, the trunnion on that side is lowered, reducing the angle of attack of that end of the screed. If no compensating adjustment is made, the mat level at that side gradually decreases toward a level at which the angle of attack equals the critical value. By suitable adjustment of the appropriate control screw, the angle of attack can, at least in theory, be maintained continuously at the critical value, regardless of vertical movement of the trunnions. It is thus possible to produce a mat surface that does not reflect irregularities in the roadway base. In performing such adjustment, it is necessary to distinguish between temporary deviations from level of the roadway base and intentional variations,

Such adjustment is rendered difficult by the same characteristic of the machine that tends inherently to smooth out errors in the roadbed. By the time an error in mat level has become noticeable, the error in slope of the mat surface in the direction of travel is typically so great that it can only be corrected over an appreciable distance; and throughout that distance the error in level continues to increase. It is therefore essential, so far as possible, to detect and correct errors in angle of attack of the screed even before they produce a measurable error in level of the mat surface.

The present invention applies that principle of anticipation to the particularly important. problem of controlling the transverse slope of the mat surface. In accordance with the invention, the transverse slope is controlled, preferably automatically, in accordance with two distinct types of error signal, one of which relates to the actual value" of the transverse slope and theother of which relates to a condition of the screed which is primarily determinative of variations in that slope. The first signal, representing the actual transverse slope of the mat that is currently being laid down, may be derived from sensing means of any suitable type for detecting the transverse inclination of the mat, as by comparing the height of the freshly laid mat at two transversely spaced points. It is convenient to utilize the inclination of the screed itself as a measure of the current slope of the mat. Whereas the trailin edge of the screed provides theoretically the most direct measure of the slope of the just completed mat, the slope signal may be obtained satisfactorily in actual practice from the leading edge of the screed, or, in fact, by comparing the height of any two points of the screed that are spaced along its length. Any dependence of the resulting slope signal upon the detailed selection of those spaced points, due, for example, to twist of the screed, is'typically small and can be compensated, for all practical purposes, by a slight change in the value of the correction ratio, to be defined. The slope signal initially developed may represent directly the difference between the actual and the desired values of the transverse slope. Alternatively, the initial signal may represent the absolute value of the transverse slope angle, in which case, a final slope signal may be derived from it by comparison with the value of the desired transverse slope.

Thev second signal, representing, in effect, the rate at which the transverse mat slope is being changed, may typically be derived from a comparison of the angles of attack of the screed at two longitudinally spaced points, typically adjacent its two ends. A preferred manner of obtaining such comparison is by sensing means directly responsive to the twist or torsion angle of the screed.

The two described signals may be employed in any suitable manner for control of the paving machine. In accordance with a further aspect of the invention, the sig-. nals are added, taking account of their relative signs, to develop a combined signal, and the latter is employed as error signal either for display to the operator or for directly controlling a power drive for the adjusting mechanism of one end of the screed.

In the present embodiment, the described first sensing means is denoted generally by the numeral 70. and is mounted in a cabinet 72. Cabinet 72 is mounted directly on one of the screed frames, shown as left frame 62.

A pendulum 74 mounted within the cabinet on a pendulum axis 75 parallel to the direction of travel of the case, indicated at 73, which is filled with a suitable damping fluid such as heavy oil.

Pendulum 74 is connected, for example via an amplify ing gear train 76, to an output shaft 77. Output shaft 77 is connected to an output device of any suitabletype for producing an electrical signal representing the shaft position. That output device or transducer may, for example, comprise a rotary potentiometer 80 with a winding 81 mounted on the wall of cabinet 72 and with a brush 82 fixedly mounted on shaft 77 (FIG. 6). electrical voltage, either alternating or direct current, is appliedacross potentiometer winding 81, the voltage tapped by brush 82 comprises an electricalsignal representing the actual transverse slope of the mat surface being laid down by the left portion of the screed. That signal is typically proportional to the slope angle measured from the horizontal.

Visual indicationof that slope angle may be provided as by a pointer 84 visible from outside cabinet 72 and driven from shaft 77. A scale 85 for pointer84 may be calibrated directly in terms of the transverse slope angle corresponding to the pointer position. amplification produced by gear train 76, that slope angle can be observed conveniently and accurately Whereas pointer 84 and potentiometer 80 are represented illustra tively as being driven from the same shaft 77, itwill be understood that separate drive mechanisms with different drive ratios, may .be provided if desired. V

Selection means are also preferably provided for designating the desired transverse slope angle. That selection control typically comprises a second potentiometenindicated at 120, with winding 121 and brush 122. Potentiometer 120 may be mounted within cabinet 72 and controlled by a manual knob 123 which is provided with a scale 124. That scale may be calibrated directly in terms of the desired transverse slope angle, expressed in degrees, percentage or any preferred units. An illustrative manner of combining the slope selection signal from potentiometer 120 with the signals from the two sensing means will be described. 1

An alternative manner of providing manual selectio of the desired slope angle is to mount the body of potentiometer 80 in a manner to permit rotational adjustment of the potentiometer winding about shaft 77.. A control knob, such as 123, may then be coupled to the potentiometer body to adjust its position in accordance with the desired mat slope. The output signal from the potentiometer then directly represents the slope error, and potentiometer 120 is not required. When, as in the present embodiment, the level-sensing device is mounted on one ofthe screed frame sections, the desired slope to be set in at knob 123 is, of course, the desired slope for the corresponding portion of the mat. Changes in the crown adjustment then do not disturb the control action.

In the present embodiment, a signal rep-resenting the screed twist is developed by second sensing means indicated generally at 90. A rotary transducer in the illustrative form of the potentiometer 92 has its winding 93 anchored with respect to the left edgeof screed 44, as shown in FIG. 3 and as indicated schematically in FIG. 6 by the broken line 151. jElectrical connections'between potentiometer 92 and cabinet 72 are represented as the cable 95 in FIG. 3. The potentiometer. brush 94 (FIG. 6) is driven by an input shaft 96 via gearing which amplifies the shaft rotation and is indicated schematically at 97. Shaft 96 extends parallel to the screed substan- Because of the the differential angle of attack at the respective ends of the screed. The voltage at brush 94 is thus a signal representing the screed twist. That voltage is typically proportional to the angle of twist measured from some se: lected position which will be taken as the position of zero twist.

Power means are provided for one or both of the screed adjustments, represented illustratively as the screws 56 and 57. As illustrated, a reversible hydraulic motor of conventional type is indicated at 100 with a reduction gear 102 and chain drive 103 for driving left screw 57. Pressurized fluid for actuating motor 100 is supplied by a pump indicated schematically at 104, driven by engine 30, Pump 104, which may be present as part of the paving machine, is connected via a pressure line 106 and a return line 107 to electrically actuated control valve 110 in cabinet 72 (FIG. 4). Valve 110 is connected to motor 100 by two

motor lines

111 and 112. Valve 110 typically has a normal or neutral position, in which themotor lines are cut off and motor 100 is idle. The valve is shiftable in response to electrical signals of two distinct types to connect pressure line 106 to one or other of the

motor lines

111 and 112, return line 107 being connected to the other motor line, to drive motor 100 in one direction or the other. For'definiteness, pressure supply to motor 100 via line 111 will be taken to drive screw 57 clockwise, lowering the left rear corner of screed 44 and increasing the angle of attack of the left end of the screed. Valve 110 is actuated, in accordance with the present invention, in response to signals derived from sensing means 70 and 90; and that actuation is preferably further modified by the slope selection signal which is manually adjustable in accordance with the desired value of the transverse slope of the mat surface.

An illustrative electrical system in accordance with the invention is represented in FIG. 6.

Potentiometer windlugs

81, 93 and 121 are connected in parallel with each other and with a resistance 126 between two

power lines

128 and 129. Resistance 126 has a fixed or manually With that typical arrangement, whenever the screedslope departs from the dial setting in one direction a-voltage signal of one phase is developed on lines 137; and for opposite slope error,. that voltage signal has the opposite phase.

Potentiometer 90 is typically so arranged and adjusted that when the screed twist is zero the voltage at brush 94 is the same as that at tap 127 on resistance 126, so that the signal onlines 138 is correspondingly zero. Tap 127 may be made variable on resistance 126 to facilitate such adjustment. The voltage signal on line 138- then has one phase for one direction of twist of the screed and opposite phase for the opposite direction of twist.

The two signals on lines 137 and 138 may be additively combined, with regard for their relative phase, by

any suitable means. As illustrated, that is accomplished by the transformer 140, which has two

primary windings

141 and 142 coupled to a common secondary winding 143. Primary winding 141 is connected between slope error signal lines 137, and primary winding 142 is connected between screed twist signal lines #138. The voltage produced in transformer secondary 143 is supplied via lines 144 as input signal to a phase sensitive amplifier of servo type, indicated schematically at 146. Amplifier 146 receives

vialthe lines

128a and 129a a phase reference voltage having definite phase relation to the voltage on

lines

128 and 129.

Amplifier 146 provides on lines 148 an output signal which corresponds in magnitude and polarity to the input signal from transformer secondary 143. That output signal is displayed by a suitable meter of center-zero type, indicated at 150 in FIGS. 4 and, 6, for example. The sneter reading is useful for adjusting the control system, and may be employed by the operator as a visual indication with reference to which to adjust the screed manually.

plifi r 146 p e a y a o s pp i s s on line @1 an output signal of suitable type to drive control valve 1110. The signals on

lines

147 and 148 may be the same,

adjustable center tap 127. Alternating current power is supplied to

lines

128 and 129, for example from a con-' verting device 130 of conventional type which receives direct current power via the main control switch 132 from a power source shown as the battery 134. The two lines 137 are connected respectively to the potentiometer brushes 82 and 122, and carry a signal that represents the transverse slope error. The two lines138 are connected respectively to potentiometer brush 94 and terminal 127, and carry a signal that represents the screed twist, A variable resistance 145 is connected in series with the screed twist signal circuit, and may be employed to vary the constant of proportionality between the screed twist angle and the resulting signal amplitude.

The connections for driving potentiometer 80 from pendulum 74 are such that the alternating current signal at potentiometer brush 82 represents the screed slope in a plane transverse of the pavement, the phase of the signal corresponding to the sign of the screed slope and the magnitude of the signal being substantially proportional to the magnitude of the screed slope with a definite constant of proportionality. Also, the calibrations of scale 124 are so placed that the phase of the alternating current signal at brush 122 corresponds to the sign of the desired transverse slope, and the magnitude of the signal is proportional to the magnitude of the desired slope, the constant of proportionality being the same as that between the signal at brush 82 and the actual screed slope.

or may be derived from suitable distinct portions of the amplifier circuit. The control signal for valve 110 may be of any suitable form. which drives valve 110 to one of its actuated positions when the input signal voltage on line 144 differs from zero by a critical threshold value and is in phase with the reference voltage; and drives valve 110 to its other actuated position when the input signal has a similar value and is in opposite phase to the reference voltage. Valve 110 may be of a proportional type, which supplies an increasing volume of pressurized fluid to motor as the amplitude of the electrical control signal increases. However, it is satisfactory for many purposes to provide positive motor response of onoff type, suitable means of known type being provided to prevent hunting. Whereas hydraulic drive rneans have been described, it will be understood that an electric motor of servo type may be used in place of valve and motor 100-if preferred.

Drive means such as motor 100 may be provided, if desired, for the right hand end of the screed, rather than .for the left hand end as illustratively shown, the

lines

111 and 112 from valve 110 being interchanged, for example,

so that'the direction of screw adjustment is reversed. Also, drive means and control valves may be provided for'both screed adjustments, with switching means, such as a conventional manually controlled double throw switch, in lines 147 for supplying the outpu signals'from When the direction of twist tends to reduce the magnitude of the slope error, as is true, for example, when the screed end that is too low has the greater angle of attack, the signals in

windings

141 and 142 are in opposite phase and tend to cancel. In the latter instance, the screed twist and the slope error will be referred to as oppositely directed. There is then a particular value of the ratio of the magnitude of the screed twist to the magnitude of the slope error for which the output from the transformer on lines 144 is zero. That ratio will be referred to for convenience as the correction ratio, since it determines in practice'the rapidity with which the slope is restored to its desired value as the machine progresses .along the roadway. 1

When the correction ratio is equal to unity,- for exam ple, the screed is driven 'by the servo control in a manner to maintain the screed'twist in degrees equal to the current slope error in degrees; whereas a correction ratio of five means that the angular screed twist under equilibrium conditions of the servo drive is five times the current slope error. The actual magnitude of the correction ratio depends upon many engineering factors of the described system, including, for example, the turns ratio of the two primary windings of transformer 140, the resistance per unit angle of

potentiometers

82 and 94, and the detailed linkage systems by which they are driven.

The system preferably includes means for conveniently adjusting the value of the correction ratio during operation of the machine. Such means are representedillustratively as the variable resistance 145, already described, adjustment of which changes the factor of proportionality between the screed twist angle and the resulting voltage signal supplied to transformer winding 142. Alternatively, that factor of proportionality may be adjusted mechanically, as by providing a variable gear ratio at 97 between shaft 96 and potentiometer 9t (FIG. 3). An advantage of the present system is that such adjustment can be made electrically. However, the scope of the invention is not limited to any particular mechanism for varying the value of the correction ratio.

In the described system, whenever the screed twist and the screed slope error are both zero, or are oppositely directed and in such ratio that their respective error signals cancel, the input signal to amplifier 146 is zero. Valve 110 then remains in neutral position, idling motor 100. The control system then produces no change in the screed position. On the other hand, if the ratio of the screed twist to the slope error is greater or less than the correction ratio, a signal of corresponding phase is supplied to amplifier 146, the amplitude of the signal increasing with the departure of the actual ratio from the correction ratio. Valve 110 is then shifted to one or other of its actuated positions, causing motor 100 to drive the angle of attack adjustment'of the controlled end of the screed in one direction or the other. The drive connection is so arranged that in each instance the angle of attack of the controlled end of the screed is altered in a direction to bring the ratio of screed twist to slope error into equality with the correction ratio.

Typical operation of the paver control system will now be described for several illustrative practical situations. It is supposed first that the paving machine is proceeding alonga levelroadway laying a mat of predetermined transverse slope, to which dial 123 has been set. The right hand screed adjustment, represented by screw 56, is under manual control of the screedman, while the left hand adjustment, represented by screw 57, is driven automatically by the control system. Under equilibrium conditions, the angle of attack at both ends of the screed idle, and the control system is quiescent.

If the screedman now anticipates the need for an increase of mat thickness at the same transverse slope, for example to conform to level stakes set along the right hand edge of the roadway, he operates screw 56 to increasethe angle of attack of the right end of the screed. That change by itself would cause a gradual increase in mat thickness at the right end of the screed, swinging draft arm 50 upward about its trunnion until the angle of attack is restored to its original critical value. That would gradually produce an error in transverse slope, eventually developing a slope error signal on lines 137 which would alter the position of left screw 57 and correct the slope error. With the present system however, correction is made immediately, without waiting for a measurable error in the transverse slope to appear. As soon as the screedman turns right screw 56, sensing means detects the resulting change in the screed twist. The resulting signal on lines 133 causes motor to drive left screw 57 to restore the previous condition of twist, which is zero in the present instance. Hence the angle of attack of the left end of the screed is automatically increased immediately to accord with the increase produced manually at the right end of the screed. As the machine proceeds forward the mat level therefore rises uniformly across the roadway, maintaining the correct transverse slope.

After equilibrium is again attained, assume that the left track 'drops into a lengthy, but shallow depression in the roadway. Due to the geometry of the pacing machine, the trunnion of the left draft arm drops correspondingly, decreasing the-angle of attack of the left end of the screed. With the present control system, that produces no error in the mat surface; for the resulting-screed twist immediately develops a control signal. Motor 1% drives left screed adjustment 57 down, increasing the angle of attack, until the screed twist is eliminated. The angle of attack at both-ends of the screed therefore maintains its critical value. Hence on the left side a thicker mat is laid to bridge the depression, and the mat surface remains uniform.

If, in spite of control actions of the type described, an error in the transverse slope should develop for any reason, such error is detected at an early stage by slope sensing means 7 0. In practice an error in transverse slope, as the term is here employed, occurs whenever the setting of dial 123 is altered, as may be done, progressively, for example, to produce desired superelevation as a roadway enters a curve. A slope error may also result from a de ,parture of the machine from theoretical performance in a manner causing a cumulative error. For example, the

critical value of the screed angle of attack may vary not agree with at set in at dial 123, the resulting slope error signal on lines 137 produces adjustment of the left end of the screed in the proper direction to'bring the slope gradually back toward its desired value. The degree of corrective adjustment for that purpose is automatically limited to an appropriate value. That limitation is due to the action of screed twist sensing means 90. The corrective change in angle of attack produced automatically at the left end of the screed causes the screed to twist; and the resulting twist signal tends to oppose the initial slope error signal in transformer 140. The corrective action resulting from a slope error of given magnitude thus produces only sufficient screed twist to can- 7 cel the slope error signal. The magnitude of screed twist correspondingly reduced. Correct mat slope is therefore approached smoothly, without any sharp transitions either in mat level or even in curvature of mat surface.

If opera'ng conditions are such as would normally tend to produce a slope error by cumulative action, such cumulative action is checked at an early stage. For, as soon as a slope error appears, the screed adjustment automatically produces a compensating screed twist proportional to that error. That twist rapidly becomes sufficient to prevent furthor cumulative increase of the slope error. The resulting equilibrium condition is typically reached when the slope error is still so small as to be negligible in practice. That is particularly true when the correction ratio, as already defined, has a relatively large value. That fact illustrates the utility of the described means for adjusting the value of the correction ratio in accordance with conditions of operation.

I claim:

1. In combination with a paving machine that is movable forwardly over the ground and that comprises two draft arms pivotally mounted at opposite sides of the machine on transverse pivot axes and extending rearwardly therefrom, an elongated, torsionally flexible screed having its end portions mounted on the respective draft arms adjacent their'rearward ends, means for depositing paving material adjacent the forward edge of the screed to form a mat on which the screed rides, and adjusting means operable to vary independently the angles of at? tack at which the respective screed end portions engage the deposited material; first sensing means responsive to the transverse inclination of the mat surface, second sens ing means acting independently of the first sensing means and responsive to the diiference between the angles of attack for respective longitudinally spaced portions of the sc eed mea s ont oll ntl b s d fir t and c nd sensing means and producing a signal that represents a linear combination of said transverse slope and said difference between the angles of attack for facilitating manual operation of said adjusting means to maintain a desired transverse slope of the mat surface.

2. In combination with a paving machine that is movable forwardly over the ground and that comprises two draft arms pivotally mounted at opposite sides of the machine on transverse pivot axes and extending rearwardly therefrom, an elongated, torsionally flexible screed having its end portions mounted on the respective draft arms adjacent their rearward ends, means for depositing pay- :ing material adjacent the forward edge of the screed to form a mat on which the screed rides, and adjusting means operable to vary independently the angles of attacl; at which the respective screed end portions engage the deposited material; first sensnig means responsive to the transverse inclination of the mat surface, second sens-- ing means acting independently of the first sensing means said signal to maintain a desired transverse slope of the mat surface. 7

3. In combination with a paving machine that is movable forwardly over the ground and that comprises two draft arms pivotally mounted at opposite sides of the machine on transverse pivot axes and extending rearwardly "therefrom, an elongated, torsionally flexible screed haying its end portions mounted on the respective draft arms adja ent thei rea ward nd e ns f de o i ing r ing material adjacent the forward edge of the screed to form a mat on which the screed rides, and adjusting means operable to vary independently the angles of attack at which the respective screed end portions engage the deposited material; first sensing means .acting to develop a first electrical voltage substantial- 1y proportional to the transverse inclination of the mat surface, second sensing means acting to develop a sec; ond electrical voltage'substantially proportional to the difference between the angles of attack for two longituw dinally spaced portions of the screed, andelectrical means acting tosum one of said voltages and an adjustably variable fraction of the other voltage to produce a combined voltage, means'f orvisually indicating the value of said combined voltage for facilitating manual operation of said adjusting means to maintain a desired transverse slope of the mat surface.

4. In combination with a paving machine that is movable forwardly over the ground andthat comprises two 7 draft arms pivotally mounted at opposite sides of the machine on transverse pivot axes and extending rearwardly therefrom, an elongated, torsionally flexible screed having its end portions mounted on the respective draft arms ad jacent their rearward ends, means for depositing paving material adjacent the forward edge of the screed to form a mat on which the screed rides, and adjusting means operable to vary independently the angles of attack at which the respective screed end portions engage the deposited material; first sensing means acting to develop a first electrical voltage substantially proportional to the transverse inclination of the mat surface, second sensing means acting to develop a second electrical voltage substantially proportional to the difference between the angles of attack for two longitudinally spaced portions of the screed, and electrical means acting to sum one of said voltages and an adjustably variable fraction of the other voltage to produce a combined voltage, and power means for driving one of said adjusting means in response to said combined voltage to maintain a desired transverse slope of the mat surface.

5. In combination with a paving machine that is movable forwardly over the groundand that comprises two draft arms pivotally mounted at opposite sides of the machine on transverse pivot axes and extending rearwardly therefrom, an elongated, torsionally flexible screed havingits end portions mounted on the respective draft arms adjacent their rearward-ends, means for depositing paving material adjacent the forward edge of the screed to form a mat on which the screed rides, and adjusting means operable to vary independently the angles of attack at which the respective screed end portions engage the deposited material; an electrical transducer comprising two relatively rotatable members, structure rotationally coupling themembers to respective longitudinally spaced portions of the screed, circuit means for producing an electrical signal in response to relative rotation of the members, and power means for ,driving one of said adjusting means in response to yariations of said signal to maintain a desired transverse slope of the mat surface.

wardly therefrom, an elongated, torsionally flexible screed having its end portions mounted on the respective draft arms adjacent their rearward ends, means for depositing paving material adjacent the forward edge of'the screed to form a mat on which the screed rides, and adjusting means operable to vary independently the angles of attack at which the respective screed end portions engage the deposited material; pendulum means mounted on the screed on a pendulum axis parallel to the direction of travel of the machine, first transducer means driven by the pendulum means and producing a first electrical i l, co t anss uss mea s m ri ins t re at ely 13 rotatable members, structure rotationally coupling the members to respective longitudinally spaced portions of the screed, means acting to produce a second electrical signal in response to relative rotation of said members, and circuit means acting to add said electrical signals to produce a combined electrical signal for facilitating manual operation of said adjusting means to maintain a desired transverse slope of the mat surface.

7. In combination With a paving machine that is movable forwardly over the ground and that comprises two draft arms pivotally mounted at opposite sides of the machine on transverse pivot axes and extending rearwardly therefrom, an elongated, torsionally flexible screed I having its end portions mounted on the respective draft arms adjacent their rearward ends, means for depositing paving material adjacent the forward edge of the screed to form a mat on Which the screed rides, and adjusting means operable to vary independently the angles of attack at which the respective screed end portions engage the deposited material; pendulum means mounted on the screed on a pendulum axis parallel to the direction of travel of the machine, first transducer means driven by the pendulum means and producing a first electrical signal, second transducer means comprising two relatively rotatable members, structure rotationally coupling the members to respective longitudinally spaced portions of the screed, means acting to produce a second electrical signal in response to relative rotation of said members, and circuit means acting to add said electrical signals to produce a combined electrical signal, and power means for driving one of said adjusting means in response to said signal to maintain a desired transverse slope of the mat surface.

References Cited in the fileof this patent UNITED STATES PATENTS