US3556225A - Automatic level control system for earth moving machines - Google Patents

Automatic level control system for earth moving machines Download PDF

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Publication number
US3556225A
US3556225A US3556225DA US3556225A US 3556225 A US3556225 A US 3556225A US 3556225D A US3556225D A US 3556225DA US 3556225 A US3556225 A US 3556225A
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Prior art keywords
image receiving
blade
image
receiving device
light source
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Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Expired - Lifetime
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Atsushi Matsuzaki
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Hitachi Ltd
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Hitachi Ltd
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    • EFIXED CONSTRUCTIONS
    • E02HYDRAULIC ENGINEERING; FOUNDATIONS; SOIL SHIFTING
    • E02FDREDGING; SOIL-SHIFTING
    • E02F3/00Dredgers; Soil-shifting machines
    • E02F3/04Dredgers; Soil-shifting machines mechanically-driven
    • E02F3/76Graders, bulldozers, or the like with scraper plates or ploughshare-like elements; Levelling scarifying devices
    • E02F3/80Component parts
    • E02F3/84Drives or control devices therefor, e.g. hydraulic drive systems
    • E02F3/841Devices for controlling and guiding the whole machine, e.g. by feeler elements and reference lines placed exteriorly of the machine

Abstract

This level control system can effectively be utilized in the case where earth moving machines are employed to smooth and level various courses of soil, gravel etc. That is, the system is so designed as to control automatically the level of working tool of earth moving machine and to actuate the working tool to correct a deviation between the tool level and the desired level.

Description

United States Patent inventor Atsushi Matsuzaki Tokyo, Japan Appl. No. 748,768 Filed July 30, 1968 Patented Jan. 19, 1971 Assignee Hitachi, Ltd.

Tokyo, Japan a corporation of Japan Priority Aug. 2, 1967 Japan No. 42/49314 AUTOMATIC LEVEL CONTROL SYSTEM FOR EARTH MOVING MACHINES 1 Claim, 7 Drawing Figs.

US. Cl l72/4.5

Int. Cl E021 3/76 Field of Search 172/4.5; 3 7/AC, AL

References Cited UNITED STATES PATENTS 6/1957 12/1959 9/1961 11/1961 l/l963 10/1963 3/1966 l/1968 Bensinger Stewart et a1. Martin et a]. Kuehne et al. Pizzarotti et a1.

Hogan et a1 Layne Jones Primary Examiner-Robert E. Pulfrey Assistant Examiner--James W. Peterson Attorney Craig and Antonelli l72/4.5X 172/4.5 172/4.5 172/4.5 172/4.5X 172/4.5UX l72/4.5UX 172/4.5UX

ABSTRACT: This level control system can effectively be utilized in the case where earth moving machines are employed to smooth and level various courses of soil, gravel etc. That is, the system is so designed as to control automatically the level of working tool of earth moving machine and to actuate the working tool to correct a deviation between the tool level and the desired level.

PATENTED JAN 1 9m! SHEEI 2 BF 3 INVENTOR fiTSUSl-H lrmaz/p ATTORNEYS remarkable.

tion.

, circuitfor the image receivin device.

AUTQMATICL manner that the groundl surface, becomes ,uneven or; convex and concavel, then the bulldozer carries out the earth moving operation while; pitchingsi'nce the tractor of the bulldozer rides ona higherportion and falls intoa. lower portion. Thus, the unevenness of the ground surfacebecomes more and more In order toflatlye tcavate' uch uneven ground surface, it is necessary that the height ofthe bulldozer blade bealways adjusted during the-{movement of the bulldozer. U

ln the conventionall bulldozers, an operator actuates the blade control neans manually operated, upon noting a height of the blade above the desired level. With such arrangement, if the blade'isgcausedjto cutdownward, the frontportion of the tractor tends to sinlc when the tractor moved to the position cut d WnwardIaridthen, with movement ofthe bulldozer, the

level of the blade becomes lower. Thus, it'takes'a long time to become sltilled in the operation. Furthermore, for high speed bulldozer mate in, high skill in the operation'is requiredfor adjusting the he of the. blade and much difficulty is encountered inra cliieving such adjustme nt. I

, stJMMAiiY on THE INVVENTYIONQ j -,L ,V 1 .J

Itis a primaryob ect of the present invention to provide an m apparatusforautomatically controlling a heigh'tIof the bulldozer'blade, which enables the bladet o follow a desired excavation locus irrespective of the pitching motion of thebulldour dueto unevenness ofthe ground surface, wherein the spot image of a light source mounted on the blade is remotely received and detected to thereby automatically control the level of the-blade, and an image receiving-deviceautomatically follows thermovernent of the light source regardless the direction of the bulldozer movement.

. Other objectsffeatures and advantages of the present'invention willrbecome apparent from the following description and the accompanying drawi j BR EFDECRIi Q Q HE PR WNQ FIG. 1wis anelevationalside view showing a bulldozer 50 equipped with the apparatus according to the present inven- FIG. 2 is a perspective view, partly cutaway, showing an example of the image receiving device incorporated .in the present apparatus.

FIGS. 34 and 3b are schematic diagrams showing the blade controlling circuit incorporated in the present apparatus.

l T A hydraulic cylinder 5 for driving theibl'ade S upward and -tected by a potentiometer 26.

downwardis rotatably supported on the tractor 6 by means of fitting'7, andthe free end of a pistonrod therefor is rotatably attached to the framel by means ofa pin 8. l a

Referring now to FIG. 2, an image receiving device 9 adaptedfor receiving the light from the light source 4 on the blade includes an image receiving (surface 10 for detecting a deviation of the blade height from the desired height, an image receiving surface ll for causing the image receiving device 9 r to follow the movement of thebulldozer, and a half mirror 13 for focusing thelight rays emanating-from the light source 4 at theimage receiving surfaces 10 and 11 through lenses 12. On the image receiving surface 10 are provided luterally extend- 1 ,ing' photoelectric converter elements [5 and 16 such as photocells-with'a lateral bandlike space ofa suitable width.

On the image'receiving surface ll-arc provided vertically extending photoelectric converterelezments 1 8 and 19 such as photocells' with a ve 'rticalibandlike spacel7 of a suitable width. The image receivingrdeviceflfis' mounted on the sup port table 20 for elevation or pivotal movement about a shaft 22 supported by flanges 21 provided onithe support table 20 having gear teeth formed in its outer periphery. The image receivingdevice "9 is turned by means of a servomotor 23 through a horizontal gear 24 interme shing with the gear teeth formed in the outer periphery of the support table 20 l v The image receiving-.device9 is elevated by-means of the servomotor 25 fixedt'o the. support table 20fthrough a gearing mechanism, for example, and the extent of elevation-is de- As shown in'FlGS. 3a and 3b, a relay is connected with the photoelectricconverter-element lsprovided on the image receiving surface 10 ofthe device 9' throughan'amplifier 27, and a relay 30 is coupled to the photoelectric converter elernent 16 through an amplifier 29. l-

'Thus, when the image of the light source 4 is formed at the space IA-between the photoelectric converter elements 15 and 16 by means of the lenses 12 through the half mirror 13,

neither the relay 28 her 30 ise'nergized.

' .An oscillator circuit 31 produces an electric signal having a frequency .off when a normally open relay 28 is'closed, and

anoscillator circuit'32 produces an electric signal having a frequency off, when a normallyoperi relay 30 is closed. These signals are suppliedto a synthesizer 33. The electric-signal having the frequency off or f supplied to the synthesizer 33 l "is same a transmitter 35 through acontrol circuit 34 and FIG. 4 is a 'blockdiagram showing the circuit for controlling l the rotation ofthe image receiving device.

FIG. 5 is a vlaloclt diagram showing the elevation controlling R FIG. 6 is a perspective view, partly cutaway, showing an example of the? light source means incorporated in the present apparatus. I M

FIG. 7 is: a perspective view, partly cutaway, showing in the present apparatus. 9 r v i DESCRIPTIONOF THE PREFERRED EMBODlMENTS Referring to FIG; 1 of the drawings, framel is rotatably atanother example of'the image receiving device incorporated tached to the tractor 6 of the bulldozer, and a blade 3 is pro*-.

vided at the free ends ofthe framel.

The reference numeral 4 represents a light source which is adapted to produce light rays of suitable brightness and wavelength. v

frequency modulated there so as to be transmitted from a transmitter antenna 36. r r

An amplifier 37 in FlG. 4 is connected with the photoelec tric converter element 18provided on the image receiving surface 11 of the aforementioned device 9,, andan amplifier 38 is connected with the photoelectric converter element 19,

Signals from the amplifiers 37 and 38 are supplied to the servomotor 23 through a synthesizer circuit 39 andamplifier 40 to controlthe rotation of the support: table 20.

if the image of the light source 4 is shifted from the space 17 so as to be formed on the photoelectric converter element 19 or 18, then the servqmotor 23 drives the support table 20 in such direction that the shifted image comes to'the space 17 again. Thus, the image receiving device is able to follow the movement of the bulldozer. I

The reference numeral 41 representsa blade operating circuit mounted on the tractor 6, and 42 a solenoid valve inserted.

in an operating hydraulic circuit for the cylinder 5. The valve 42 is so designed that when a coil 42a thereof is energized, it is .switched to A to cause the cylinder 5 to raise the blade 3, and that when a coil 42b thereof is energized, it is switched to "B" to cause the cylinder 5 to move the blade plate 3 downward. r

When neither the coil 42a nor 42b is energized, the solenoid valve assumes a neutral position C so that the blade 3 does not move. I

The reference numerals 43 and 44 represent a receiving antenna and areceiver for receiving the signal having the frequency f or f transmitted from the transmitting antenna 36, respectively, and 45 a band-pass filter which is so designed as to pass the signal having the frequency of f to the circuit of an amplifier 46 and coil 42a and the signal having the frequency off to the circuit of an amplifier 47 and coil 4217.

Referring to FIG. 5, the reference numeral 48 denotes a commanding device which is adapted to externally command the angle of elevation of the image receiving device 9. If there exists a difference between a command signal provided by the commanding device and a signal from the potentiometer 26 detecting the extent of elevation of the image receiving device, then the difference signal is supplied to the servomotor 25 through an amplifier 49 to drive the servomotor 25. When a desired angle of elevation of the image receiving device is reached so that the signal difference becomes zero, the servomotor 25 is stopped from being driven.

FIG. 6 shows an example of the light source device wherein a rotary mask 53 formed with a plurality of windows 52 of a suitable size which are arranged in uniformly spaced relationship with each other isrotatably provided in a transparent frame 51 mounted on a mount 50, a lamp 54 is fixedly provided in the rotary mask 53, and a motor 56 is connected with a support shaft for the rotary mask 53 through a coupling means 55. The rotary mask 53 is rotated by means of the motor 56 to interrupt light rays emanating from the lamp 54 to provide a light source of a suitable frequency. in front of the lenses 12, there are provided a hood and filter to interrupt the sunbeams as much as possible and pass only the light rays emanating from the light source 4 to thereby prevent error operation due to the sunbeams as surely as possible.

In order to give a better understanding of the present system to control the bulldozer blade automatically, description will now be made of the operation of an embodiment of the present invention with reference to the drawings. in the ap paratus as shown in the drawings, the width of the space 14 between the upper element 15 and the lower element 16 provided on the image receiving surface is selected in the range of allowable error for an excavating locus of the blade edge, and the space is positioned substantially at the center of the image receiving surface 10. Further, the size of the space 17 between the left-hand side element 18 and the right-hand side element 19 provided on the image receiving surface 11 is selected to a suitable value, and the space 17 is positioned substantially at the center of the image receiving surface 11.

When the image of the light source 4 is formed at the space 14 of the image receiving surface 10, no signal is supplied to the amplifiers 27 and 29, so that both relays 28 and 30 are open. Hence, no signal is transmitted from the transmitting antenna 36, and the solenoid valve 42 assumes the neutral position C since the coils 42a and 42b are not energized, so that the blade does not move.

When the image of the light source 4 is formed at the space 17 of the receiving surface 11, no signal is supplied to the amplifiers 37 and 38 so that the servomotor 23 is not driven. Thus, the rotary motion of the device 9 does not occur.

With sinking movement of the bulldozer to cause the blade 3 to descend below a desired excavating locus, the image of the light source 4 is formed at the photoelectric converter element above the space 14 on the image receiving surface 10, so that the resulting electric signal energizes the relay 28 through the amplifier 27 to close the relay 28.

Then, the signal having a frequency of f is transmitted from the transmitting antenna 36. This signal is supplied to an amplifier 46 through a receiving antenna 43, receiver 44 and a band-pass filter 45 and then to a circuit related to the coil 42a.

The coil 42a is thereby energized so that the solenoid valve 42 is switched to Consequently, pressure oil from a pressure oil source (not shown) acts onto a piston rod side chamber 5a of the cylinder 5 to raise the blade 3. At this point, the light source 4 is also raised together with the blade 3. Thus, when the image of the light source 4 shifts back into the space 14 on the receiving surface 10, no signals enter the amplifier 27 or 29, so that no signal is transmitted from the transmitting antenna 36 since the relay 28 or 30 is opened.

And then the coil 42a or 42b of the solenoid valve 42 is deenergized, so that the valve assumes the neutral position C and the blade stops at the excavating height.

in case the blade 3 is raised above a desired excavation locus, the image of the light source 4 is formed at the photoelectric converter element 16 below the space 14 on the image receiving surface 10. Thus the resulting electric signal energizes the relay 30 through the amplifier 29 to close the relay 30. The signal having a frequency of f is transmitted from the transmitting antenna 36. This signal energizes the coil 42b of the solenoid valve 42 through the receiving antenna 43, receiver 44 and filter 45 to switch the solenoid valve to in this way, the pressure oil from the pressure oil source (not shown) acts onto a chamber 5b of cylinder 5 opposite to the piston rod to expand the cylinder, thus pushing down the blade. When the image of the light source 4 shifts back into the space 14 of the receiving surface 10 as a result of the downward movement of the blade 3, no signal enters the amplifier 27 nor 29 as in the above case, so that no signal is transmitted from the transmitting antenna since either relay 28 or 30 is opened.

Hence, the coil 42a or 42b of the solenoid 42 is deenergized so that the solenoid valve assumes the neutral position C and the blade stops.

As mentioned above, the height of the bulldozer blade is kept within the allowable deviation from the desired height. Similarly, with the forward movement of the bulldozer, the image of the light source 4 is shifted from the space 17 of the image receiving surface 11 so as to be formed at the righthand side photoelectric converter element 19. At this point, the servomotor 23 is driven through the amplifier 38, synthesizer circuit 39 and amplifier 40, thereby rotating the support table 20 in a direction to reduce the deviation of the shifted image from the space 17. In case the image of light source 4 is formed on the space 17 with the rotation of the image receiving device 9, the input to the servomotor 23 ceases so that the servomotor 23 stops. In this way, the image receiving device 9 is rotated following the movement of the light source 4.

FIG. 7 shows another example of the image receiving device incorporated in the present apparatus. The device 9 receiving the image of the light source 4 includes an image receiving surface 10 for detecting changes in the upward or downward movement of the blade 3, an image receiving surface 11 for enabling the device 9 to follow the movement of the bulldozer, and a half mirror 13 for causing the image of the light source 4 to be formed on the image receiving surfaces 10 and 11 through lenses. On the image receiving surface 10 are provided laterally extending photoelectric converter elements 15 and 16 such as photocell with a lateral bandlike space 14 of a suitable width. On the image receiving surfaces 11 are provided longitudinally extending photoelectric converter elements with a longitudinal bandlike space 17 of a suitable width.

The image receiving device 9 is mounted on a support 59 rotatably supported on a fixed base 57 by a shaft 58 for elevation about a shaft 60. The support 59 is rotated in a horizontal plane by means of a servomotor 23 fixed to the fixed base 57 through a worm gear mechanism 61. Thus, the image receiving device 9 is elevated by means of the servomotor 25 through a worm gear 62.

As described above, in the apparatus according to the present invention, the image of the light source mounted on the bulldozer blade is formed on the two image receiving surfaces of the image receiving device, and the difference between the present height of the blade and the height of the desired excavation locus is detected on the image receiving surfaces having laterally extending photoelectric converter elements provided thereon to control the height of the blade. Further, the image receiving device is able to follow the movement of the bulldozer by means of the image receiving surface having longitudinally extending photoelectric converter elements provided. Thus, in accordance with the present invention, the adjustment of the blade which has heretofore been effected by an operator can be automatically carried out with high accuracy, and yet high-speed operation can berealized with great advantage.

lclaim:

I. An apparatus in combination with a bulldozer blade for the automatic control thereof comprising a horizontally rotatable support means, an image receiving device rotatably mounted on said support means for varying the angle of elevation of said image receiving device wherein a light source is mounted on said blade and the image of, said light source,

mounted on the blade is formed on an imagereceiving surface of said image receiving device having photoelectric converter elements provided thereon within a longitudinal bandlike space therebetween and a second image receiving surface is provided at said image receiving device having photoelectric converter elements provided thereon with-a lateral bandlike space therebetween, means for detecting deviation of the actual height of the blade on a moving bulldozer from a desired excavationlocus, said means further converting the deviation into an electric signal in accordance with the relationship between the lateral bandlike space of'said second image receiving surface and the position where the image of said light source is formed and transmitting said electric signal, a blade operating cylinder being operatively connected with said blade, a hydraulic circuit being connected with said blade operating cylinder, a solenoid valve being arranged in said hydraulic circuit for control of the desired raising and lowering of said blade, means receiving said electric signal to control said solenoid valve in said hydraulic circuit means for detecting the difierence' between the actual visual direction of the moving bulldozer and the optical axis of the image receiving device in accordance with the relationship between the longitudinal bandlike space of the first image receiving surface and the position where therimage of said light source on said blade is formed and converting it into an electric signal to thereby control a servomotor for rotating the support means, a potentiometer operatively associated with said image receiving device for detecting the angle of elevation thereof, an external commanding device operatively associated with said image receiving device for externally commanding the angle of elevation thereof, and means for changing the angle of elevation of said image receiving device in accordance with signals from said potentiometer and saidexternal commanding device.

Claims (1)

1. An apparatus in combination with a bulldozer blade for the automatic control thereof comprising a horizontally rotatable support means, an image receiving device rotatably mounted on said support means for varying the angle of elevation of said image receiving device wherein a light source is mounted on said blade and the image of said light source mounted on the blade is formed on an image receiving surface of said image receiving device having photoelectric converter elements provided thereon within a longitudinal bandlike space therebetween and a second image receiving surface is provided at said image receiving device having photoelectric converter elements provided thereon with a lateral bandlike space therebetween, means for detecting deviation of the actual height of the blade on a moving bulldozer from a desired excavation locus, said means further converting the deviation into an electric signal in accordance with the relationship between the lateral bandlike space of said second image receiving surface and the position where the image of said light source is formed and transmitting said electric signal, a blade operating cylinder being operatively connected with said blade, a hydraulic circuit being connected with said blade operating cylinder, a solenoid valve being arranged in said hydraulic circuit for control of the desired raising and lowering of said blade, means receiving said electric signal to control said solenoid valve in said hydraulic circuit means for detecting the difference between the actual visual direction of the moving bulldozer and the optical axis of the image receiving device in accordance with the relationship between the longitudinal bandlike space of the first image receiving surface and the position where the image of said light source on said blade is formed and converting it into an electric signal to thereby control a servomotor for rotating the support means, a potentiometer operatively associated with said image receiving device for detecting the angle of elevation thereof, an external commanding device operatively associated with said image receiving device for externally commanding The angle of elevation thereof, and means for changing the angle of elevation of said image receiving device in accordance with signals from said potentiometer and said external commanding device.
US3556225A 1967-08-02 1968-07-30 Automatic level control system for earth moving machines Expired - Lifetime US3556225A (en)

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Cited By (10)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3678885A (en) * 1971-05-17 1972-07-25 William K Ferguson Slope and grade meter
US3831683A (en) * 1971-09-06 1974-08-27 Hitachi Construction Machinery System for controlling the level of an earth-removing blade of a bulldozer
US3876012A (en) * 1972-01-18 1975-04-08 Excel Ind Anti-overturning implement vehicle
US3957121A (en) * 1973-05-28 1976-05-18 Kabushiki Kaisha Komatsu Seisakusho Automatic control system for earth-moving equipment
US4053018A (en) * 1973-05-23 1977-10-11 Kabushiki Kaisha Komatsu Seisakusho Automatic control system for earth-moving equipment
US5462122A (en) * 1993-07-08 1995-10-31 Kabushiki Kaisha Komatsu Seisakusho Automatic drive control system for a bulldozer
US20110046857A1 (en) * 2009-08-18 2011-02-24 Caterpillar Inc. Implement Control System For A Machine
US9222770B1 (en) * 2012-06-06 2015-12-29 John C. Miller Gradient measuring apparatus and system
US20160316619A1 (en) * 2015-05-01 2016-11-03 Deere & Company Height of cut control system
US20170034996A1 (en) * 2015-05-01 2017-02-09 Deere & Company Height of cut control system

Citations (8)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2796685A (en) * 1951-12-15 1957-06-25 Bensinger Isabell Mck Method and apparatus for earth grading and allied arts
US2916836A (en) * 1955-06-24 1959-12-15 Karl J Stewart Photoelectric automatic blade level control
US3000121A (en) * 1956-05-21 1961-09-19 Collins Radio Co Automatic vertical control of earth moving machines
US3009271A (en) * 1959-11-18 1961-11-21 Collins Radio Co Automatic control of earth-moving machines
US3071049A (en) * 1959-12-21 1963-01-01 Impresa Pizzarotti & C S R L Method and apparatus for laying a bituminous road mat
US3107168A (en) * 1959-03-19 1963-10-15 Donald J Hogan Track maintenance apparatus
US3242340A (en) * 1963-02-12 1966-03-22 Melvin E Layne Photosensitive means for controlling leveling and grading apparatus and establishing reference levels
US3364356A (en) * 1964-05-01 1968-01-16 R B Pullin & Company Ltd Optical guidance apparatus for guiding a movable object along a straight path

Patent Citations (8)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2796685A (en) * 1951-12-15 1957-06-25 Bensinger Isabell Mck Method and apparatus for earth grading and allied arts
US2916836A (en) * 1955-06-24 1959-12-15 Karl J Stewart Photoelectric automatic blade level control
US3000121A (en) * 1956-05-21 1961-09-19 Collins Radio Co Automatic vertical control of earth moving machines
US3107168A (en) * 1959-03-19 1963-10-15 Donald J Hogan Track maintenance apparatus
US3009271A (en) * 1959-11-18 1961-11-21 Collins Radio Co Automatic control of earth-moving machines
US3071049A (en) * 1959-12-21 1963-01-01 Impresa Pizzarotti & C S R L Method and apparatus for laying a bituminous road mat
US3242340A (en) * 1963-02-12 1966-03-22 Melvin E Layne Photosensitive means for controlling leveling and grading apparatus and establishing reference levels
US3364356A (en) * 1964-05-01 1968-01-16 R B Pullin & Company Ltd Optical guidance apparatus for guiding a movable object along a straight path

Cited By (14)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3678885A (en) * 1971-05-17 1972-07-25 William K Ferguson Slope and grade meter
US3831683A (en) * 1971-09-06 1974-08-27 Hitachi Construction Machinery System for controlling the level of an earth-removing blade of a bulldozer
US3876012A (en) * 1972-01-18 1975-04-08 Excel Ind Anti-overturning implement vehicle
US4053018A (en) * 1973-05-23 1977-10-11 Kabushiki Kaisha Komatsu Seisakusho Automatic control system for earth-moving equipment
US3957121A (en) * 1973-05-28 1976-05-18 Kabushiki Kaisha Komatsu Seisakusho Automatic control system for earth-moving equipment
US5462122A (en) * 1993-07-08 1995-10-31 Kabushiki Kaisha Komatsu Seisakusho Automatic drive control system for a bulldozer
US8406963B2 (en) 2009-08-18 2013-03-26 Caterpillar Inc. Implement control system for a machine
US20110046857A1 (en) * 2009-08-18 2011-02-24 Caterpillar Inc. Implement Control System For A Machine
US8700273B2 (en) 2009-08-18 2014-04-15 Caterpillar Inc. Implement control system for a machine
US8762010B2 (en) 2009-08-18 2014-06-24 Caterpillar Inc. Implement control system for a machine
US9222770B1 (en) * 2012-06-06 2015-12-29 John C. Miller Gradient measuring apparatus and system
US20160316619A1 (en) * 2015-05-01 2016-11-03 Deere & Company Height of cut control system
US20170034996A1 (en) * 2015-05-01 2017-02-09 Deere & Company Height of cut control system
US9861035B2 (en) * 2015-05-01 2018-01-09 Deere & Company Height of cut control system

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