Position control system for a construction implement such as a road grader

Claim 2

A sensor grade-establishing construction tool elevation control for maintaining the vertical position of the construction tool relative to a reference, said construction tool carried by the frame of a construction implement having a frame supported for horizontal motion in a longitudinal direction and said construction tool positioned with respect to said frame by a lateral-positioning device that responds to a hydraulic signal to adjust the lateral position of said construction tool and a vertical-positioning device that responds to a hydraulic signal to adjust the vertical position of the construction tool, comprising: a first transducer configured to be carried by a grade-establishing construction tool for transmitting an acoustic wave and receiving a first reflection of said wave from a reference; a second transducer configured to be carried by said construction tool for receiving a second reflection of said acoustic wave from said reference; a sensor control coupled to said first and second transducers for monitoring the time at which said first and second reflections of the acoustic wave are received respectively by said first and second transducers in order to produce at least one electrical signal indicative of the vertical and lateral distances of said construction tool from the reference; and a hydraulic control responsive to said at least one electrical signal to provide a hydraulic signal to a vertical-positioning device as a function of the vertical distance of said construction tool from said reference in order to establish the grade with said construction tool at a predetermined vertical relationship to said reference, said hydraulic control further providing a hydraulic signal to a lateral-positioning device as a function of the lateral distance of said construction tool from said reference in order to maintain a lateral position of said construction tool with respect to said reference and thereby reduce grading errors resulting from lateral variation of said transducers with respect to said reference.

Claim 3

The sensor as defined in claim 1, wherein said sensor control converts said reflections of said acoustic waves into pulses and includes means responsive to said pulses for measuring the time it takes a wave to travel from said first transducer to said first and second transducers.

Claim 4

The sensor as defined in claim 2, wherein said sensor control measures the duration of said reflections, and said sensor control further includes adjustment means for adjusting said time measured when the reflections have a short duration.

Claim 5

The sensor as defined in claim 3, wherein said duration of said reflections is measured by counting the number of pulses received by said means responsive to said pulses.

Claim 6

A grade-establishing construction tool elevation control for maintaining the vertical position of the construction tool relative to a reference, said construction tool carried by the frame of a construction implement having a frame supported for horizontal motion in a longitudinal direction and positioned with respect to said frame by a tool-positioning device that responds to a hydraulic signal to adjust the position of the construction tool, comprising: a first ultrasonic transducer configured to be carried by a construction tool for transmitting an acoustic wave and receiving a first reflection of said wave from a reference; a second ultrasonic transducer configured to be carried by said construction tool for receiving a second reflection of said acoustic wave from said reference; a sensor control coupled to said first and second transducers for monitoring the times at which the first and second reflections of the acoustic wave are received by said first and second transducers in order to produce a signal indicative of the position of a construction tool with respect to said reference as a function of said times and the velocity of said acoustic wave; a third transducer positioned a predetermined distance from said first transducer for receiving a side lobe of the acoustic wave transmitted from said first transducer in order to determine said velocity of said acoustic wave; and a hydraulic control responsive to said signal indicative of the position of the construction tool to provide a hydraulic signal to a tool-positioning device in order to maintain a given position of said construction tool with respect to said reference.

Claim 7

A grade-establishing construction tool elevation control for maintaining the vertical position of the construction tool relative to a reference, said construction tool carried by the frame of a construction implement having a frame supported for horizontal motion in a longitudinal direction and positioned with respect to said frame by a tool-positioning device that responds to a hydraulic signal to adjust the position of the construction tool, comprising: a first ultrasonic transducer configured to be carried by a construction tool for transmitting an acoustic wave and receiving a first reflection of said wave from a reference; a second ultrasonic transducer configured to be carried by said construction tool for receiving a second reflection of said acoustic wave from said reference; a sensor control coupled to said first and second transducers for monitoring the times at which the first and second reflections of the acoustic wave are received by said first and second transducers in order to produce a signal indicative of the position of a construction tool with respect to said reference as a function of said times and the velocity of said acoustic wave; a third transducer positioned a predetermined distance from said first transducer for receiving a side lobe of the acoustic wave transmitted from said first transducer in order to determine said velocity of said acoustic wave; and a hydraulic control responsive to said signal indicative of the position of the construction tool to provide a hydraulic signal to a tool-positioning device in order to maintain a given position of said construction tool with respect to said reference.

Claim 8

The grade-establishing construction tool elevation control as defined in claim 5, wherein said third transducer is positioned generally orthogonally to said first transducer.

Claim 9

The grade-establishing construction tool elevation control as defined in claim 5, wherein said first and third transducers are spaced by a predetermined distance, and said sensor control measures the time it takes for said side lobe of said acoustic wave to travel said predetermined distance to determine the velocity of said pulse.

Claim 10

A grade-establishing construction tool elevation control for maintaining the vertical position of the construction tool relative to a reference, said construction tool carried by the frame of a construction implement having a frame supported for horizontal motion in a longitudinal direction and said construction tool positioned with respect to said frame by a lateral-positioning device that responds to a hydraulic signal to adjust the lateral position of said construction tool and a vertical-positioning device that responds to a hydraulic signal to adjust the vertical position of the construction tool, comprising: a first ultrasonic transducer configured to be carried by the construction tool for transmitting an acoustic wave and receiving a reflection of said wave from a reference; a second ultrasonic transducer configured to be carried by said construction tool for transmitting a second acoustic wave and receiving a reflection of said second acoustic wave from said reference; a control means connected to said first and second ultrasonic transducers for calculating the vertical and lateral distances of said construction tool from said reference using outputs of said first and second ultrasonic transducers in order to provide at least one signal indicative of the position of said construction tool laterally from said reference and vertically from said reference; and a hydraulic control responsive to said at least one electrical signal to provide a hydraulic signal to a vertical-positioning device as a function of the vertical distance of said construction tool from said reference in order to establish the grade with said construction tool at a predetermined vertical relationship to said reference, said hydraulic control further providing a hydraulic signal to a lateral-positioning device as a function of the lateral distance of said construction tool from said reference in order to maintain a lateral position of said construction tool with respect to said reference and thereby reduce grading errors resulting from lateral variation of said transducers with respect to said reference.

Claim 11

A grade-establishing construction tool elevation control for maintaining the vertical position of the construction tool relative to a reference, said construction tool carried by the frame of a construction implement having a frame supported for horizontal motion in a longitudinal direction and said construction tool positioned with respect to said frame by a lateral-positioning device that responds to a hydraulic signal to adjust the lateral position of said construction tool and a vertical-positioning device that responds to a hydraulic signal to adjust the vertical position of the construction tool, comprising: a first ultrasonic transducer configured to be carried by the construction tool for transmitting an acoustic wave and receiving a reflection of said wave from a reference; a second ultrasonic transducer configured to be carried by said construction tool for transmitting a second acoustic wave and receiving a reflection of said second acoustic wave from said reference; a control means connected to said first and second ultrasonic transducers for calculating the vertical and lateral distances of said construction tool from said reference using outputs of said first and second ultrasonic transducers in order to provide at least one signal indicative of the position of said construction tool laterally from said reference and vertically from said reference; and a hydraulic control responsive to said at least one electrical signal to provide a hydraulic signal to a vertical-positioning device as a function of the vertical distance of said construction tool from said reference in order to establish the grade with said construction tool at a predetermined vertical relationship to said reference, said hydraulic control further providing a hydraulic signal to a lateral-positioning device as a function of the lateral distance of said construction tool from said reference in order to maintain a lateral position of said construction tool with respect to said reference and thereby reduce grading errors resulting from lateral variation of said transducers with respect to said reference.

Claim 12

The grade-establishing construction tool elevation control as defined in claim 8, wherein said reference has a first surface and a second surface, and wherein said first and second surfaces are generally orthogonal to one another, and wherein said first and second ultrasonic transducers are positioned generally orthogonally to one another.

Claim 13

The grade-establishing construction tool elevation control as defined in claim 8, wherein said reference comprises first and second surfaces generally orthogonal to one another and wherein said first ultrasonic transducer is positioned to reflect from one of said surfaces, and wherein said second ultrasonic transducer is spaced from said first ultrasonic transducer and positioned to sense the intersection of said first and second surfaces of said reference.

Claim 14

A grade-establishing construction tool position control for maintaining the position of the construction tool relative to a reference, said construction tool carried by the frame of a construction implement having a frame supported for horizontal motion in a longitudinal direction and positioned with respect to said frame by a tool-positioning device that responds to a hydraulic signal to adjust the position of the construction tool, comprising: a first ultrasonic transducer configured to be carried by the construction tool for transmitting an acoustic wave toward a reference and receiving a reflection of the acoustic wave from said reference; a second ultrasonic transducer configured to be carried by said construction tool and positioned for receiving a side lobe of said acoustic wave; and a sensor control coupled to said first and second transducers for monitoring the time at which said side lobe of said acoustic wave is received at said second transducer in order to determine the actual velocity of said acoustic wave and for monitoring the time at which said reflection of the acoustic wave is received at said first ultrasonic transducer in order to produce a signal indicative of the position of said construction tool with respect to said reference as a function of said velocity of said acoustic wave and said time at which said reflection is received at said first ultrasonic transducer; and a hydraulic control responsive to said signal indicative of the position of said construction tool to provide a hydraulic signal to a tool-positioning device in order to maintain a given position of said construction tool with respect to said reference.

Claim 15

A grade-establishing construction tool position control for maintaining the position of the construction tool relative to a reference, said construction tool carried by the frame of a construction implement having a frame supported for horizontal motion in a longitudinal direction and positioned with respect to said frame by a tool-positioning device that responds to a hydraulic signal to adjust the position of the construction tool, comprising: a first ultrasonic transducer configured to be carried by the construction tool for transmitting an acoustic wave toward a reference and receiving a reflection of the acoustic wave from said reference; a second ultrasonic transducer configured to be carried by said construction tool and positioned for receiving a side lobe of said acoustic wave; and a sensor control coupled to said first and second transducers for monitoring the time at which said side lobe of said acoustic wave is received at said second transducer in order to determine the actual velocity of said acoustic wave and for monitoring the time at which said reflection of the acoustic wave is received at said first ultrasonic transducer in order to produce a signal indicative of the position of said construction tool with respect to said reference as a function of said velocity of said acoustic wave and said time at which said reflection is received at said first ultrasonic transducer; and a hydraulic control responsive to said signal indicative of the position of said construction tool to provide a hydraulic signal to a tool-positioning device in order to maintain a given position of said construction tool with respect to said reference.

Claim 16

The grade-establishing construction tool position control as defined in claim 11, wherein said first and second transducers are spaced by a predetermined distance and wherein said control means measures the time it takes for said side lobe to travel from said first acoustic transducer to said second acoustic transducer; and wherein said predetermined distance is divided by said measured time to calculate the velocity of said acoustic wave.

Claim 17

A grade-establishing construction implement comprising: a frame supported for horizontal motion in a longitudinal direction and means for carrying a grade-establishing construction tool on said frame, said means including a lateral-positioning device that responds to a hydraulic signal to adjust the lateral position of the construction tool and a vertical-positioning device that responds to a hydraulic signal to adjust the vertical position of said construction tool; and a sensor being carried by said construction tool, said sensor including a first ultrasonic transducer for generating an acoustic wave toward a reference and for receiving a first reflection of said acoustic wave from said reference and a second transducer for receiving a second reflection of said acoustic wave from said reference; a sensor control coupled to said first and second transducers to measure time periods between transmission of said acoustic wave by said first transducer and reception of said first and second reflections, respectively, by said first and second transducers in order to produce indications of vertical and lateral positions of said construction tool relative to said reference; and a hydraulic control responsive to said indications to provide a hydraulic signal to said vertical-adjusting device as a function of the vertical position of said construction tool relative to said reference in order to establish a grade with said construction tool at a predetermined vertical relationship to said reference, said hydraulic control further providing a hydraulic signal to said lateral-positioning device as a function of the lateral position of said construction tool relative to said reference in order to maintain a lateral position of said construction tool with respect to said reference and thereby reduce grading errors resulting from lateral variation of said transducers with respect to said reference.

Claim 18

A grade-establishing construction implement comprising: a frame supported for horizontal motion in a longitudinal direction and means for carrying a grade-establishing construction tool on said frame, said means including a lateral-positioning device that responds to a hydraulic signal to adjust the lateral position of the construction tool and a vertical-positioning device that responds to a hydraulic signal to adjust the vertical position of said construction tool; and a sensor being carried by said construction tool, said sensor including a first ultrasonic transducer for generating an acoustic wave toward a reference and for receiving a first reflection of said acoustic wave from said reference and a second transducer for receiving a second reflection of said acoustic wave from said reference; a sensor control coupled to said first and second transducers to measure time periods between transmission of said acoustic wave by said first transducer and reception of said first and second reflections, respectively, by said first and second transducers in order to produce indications of vertical and lateral positions of said construction tool relative to said reference; and a hydraulic control responsive to said indications to provide a hydraulic signal to said vertical-adjusting device as a function of the vertical position of said construction tool relative to said reference in order to establish a grade with said construction tool at a predetermined vertical relationship to said reference, said hydraulic control further providing a hydraulic signal to said lateral-positioning device as a function of the lateral position of said construction tool relative to said reference in order to maintain a lateral position of said construction tool with respect to said reference and thereby reduce grading errors resulting from lateral variation of said transducers with respect to said reference.

Claim 19

A grade-establishing construction implement comprising: a frame supported for horizontal motion in a longitudinal direction and means for carrying a grade-establishing construction tool on said frame; a sensor being carried by said construction tool, said sensor including ultrasonic transducer means for generating at least one acoustic wave and for receiving reflections of said at least one acoustic wave from a reference at at least two locations and sensor control means coupled to said transducer means to measure time periods between transmission of said acoustic wave and reception of said reflections of said acoustic wave at said two locations; and wherein said means for carrying said construction tool includes position control means for varying the vertical and lateral positions of said construction tool, said sensor control means is responsive to the measured time periods for calculating lateral and vertical distances of said sensor from said reference as a function of said measured time periods, said position control means is responsive to said lateral and vertical distances for positioning said construction tool laterally and vertically in order to maintain a predetermined height and a predetermined lateral offset of said construction tool with respect to said reference to thereby reduce height errors resulting from lateral variations of said sensor from said reference.

Claim 20

A grade-establishing construction implement comprising: a frame supported for horizontal motion in a longitudinal direction and means for carrying a grade-establishing construction tool on said frame; a sensor being carried by said construction tool, said sensor including ultrasonic transducer means for generating at least one acoustic wave and for receiving reflections of said at least one acoustic wave from a reference at at least two locations and sensor control means coupled to said transducer means to measure time periods between transmission of said acoustic wave and reception of said reflections of said acoustic wave at said two locations; and wherein said means for carrying said construction tool includes position control means for varying the vertical and lateral positions of said construction tool, said sensor control means is responsive to the measured time periods for calculating lateral and vertical distances of said sensor from said reference as a function of said measured time periods, said position control means is responsive to said lateral and vertical distances for positioning said construction tool laterally and vertically in order to maintain a predetermined height and a predetermined lateral offset of said construction tool with respect to said reference to thereby reduce height errors resulting from lateral variations of said sensor from said reference.

Claim 21

The grade-establishing construction implement as defined in claim 14, wherein said ultrasonic transducer means include first and second transducers.

Claim 22

The grade-establishing construction implement as defined in claim 15, wherein said first and second transducers each transmit and receive respective acoustic waves.

Claim 23

The grade-establishing construction implement as defined in claim 15, wherein said first transducer transmits acoustic waves and said first and second transducers receive reflections of said acoustic waves emitted by said first transducer.

Claim 24

A grade-establishing construction implement comprising: a frame supported for horizontal motion in a longitudinal direction and means for carrying a construction tool on said frame; a sensor being carried by said construction tool, said sensor including a first ultrasonic transducer for generating an acoustic wave and for receiving a first reflection of said acoustic wave from a reference and a second transducer for receiving a second reflection of said acoustic wave from said reference and a sensor control coupled to said first and second transducers to measure time periods between transmission of said acoustic wave and reception of said reflections of said acoustic wave, respectively, by said first and second transducers wherein said sensor further includes a third transducer positioned a predetermined distance from said first transducer for receiving a side lobe of the acoustic wave transmitted by the first transducer in order to determine the velocity of said acoustic wave; and wherein said means for carrying said construction tool includes position control means for varying the vertical and lateral positions of said construction tool, said sensor control is responsive to the measured time periods and to the determined acoustic wave velocity for calculating lateral and vertical distances of said sensor from said reference as a function of said time periods and said velocity, said position control means is responsive to said lateral and vertical distances for positioning said construction tool laterally and vertically in order to maintain a predetermined height and a predetermined lateral offset of said construction tool with respect to said reference to thereby reduce height errors resulting from lateral variations of said sensor from said reference.

Claim 25

A grade-establishing construction implement comprising: a frame supported for horizontal motion in a longitudinal direction and means for carrying a construction tool on said frame; a sensor being carried by said construction tool, said sensor including a first ultrasonic transducer for generating an acoustic wave and for receiving a first reflection of said acoustic wave from a reference and a second transducer for receiving a second reflection of said acoustic wave from said reference and a sensor control coupled to said first and second transducers to measure time periods between transmission of said acoustic wave and reception of said reflections of said acoustic wave, respectively, by said first and second transducers wherein said sensor further includes a third transducer positioned a predetermined distance from said first transducer for receiving a side lobe of the acoustic wave transmitted by the first transducer in order to determine the velocity of said acoustic wave; and wherein said means for carrying said construction tool includes position control means for varying the vertical and lateral positions of said construction tool, said sensor control is responsive to the measured time periods and to the determined acoustic wave velocity for calculating lateral and vertical distances of said sensor from said reference as a function of said time periods and said velocity, said position control means is responsive to said lateral and vertical distances for positioning said construction tool laterally and vertically in order to maintain a predetermined height and a predetermined lateral offset of said construction tool with respect to said reference to thereby reduce height errors resulting from lateral variations of said sensor from said reference.

Claim 26

A grade-establishing construction tool elevation control for maintaining the vertical position of the construction tool relative to a reference, said construction tool carried by the frame of a construction implement having a frame supported for horizontal motion in a longitudinal direction and positioned with respect to said frame by a tool-positioning device that responds to a hydraulic signal to adjust the position of the construction tool, comprising: a transmitter means for transmitting a signal toward a reference, said transmitter means configured to be carried by a construction tool; a first reflector in spaced relation to said transmitter means and providing a first reflection of a portion of said signal; a second reflector in spaced relation to said first reflector, said second reflector providing a second reflection of said first reflection; a receiver means for receiving said second reflection and a reflection of said signal from said reference, said receiver means in spaced relation to said second reflector; control means coupled to said transmitter means and said receiver means for producing an indication of the position of said construction tool with respect to said reference as a function of signal velocity and the time said reflection of said signal from said reference is received by said receiver and wherein the distance from said transmitting means to said receiving means via said first and second reflectors is a fixed predetermined distance whereby the time that it takes said signal portion to travel on a path from said transmitter means to said first reflector, from said first reflector to said second reflector, and from said second reflector to said receiver means is used to determine the signal velocity to thereby calibrate the sensor; and a hydraulic control responsive to said indication to provide a hydraulic signal to a tool-positioning device in order to maintain a given position of said construction tool with respect to said reference.

Claim 27

A grade-establishing construction tool elevation control for maintaining the vertical position of the construction tool relative to a reference, said construction tool carried by the frame of a construction implement having a frame supported for horizontal motion in a longitudinal direction and positioned with respect to said frame by a tool-positioning device that responds to a hydraulic signal to adjust the position of the construction tool, comprising: a transmitter means for transmitting a signal toward a reference, said transmitter means configured to be carried by a construction tool; a first reflector in spaced relation to said transmitter means and providing a first reflection of a portion of said signal; a second reflector in spaced relation to said first reflector, said second reflector providing a second reflection of said first reflection; a receiver means for receiving said second reflection and a reflection of said signal from said reference, said receiver means in spaced relation to said second reflector; control means coupled to said transmitter means and said receiver means for producing an indication of the position of said construction tool with respect to said reference as a function of signal velocity and the time said reflection of said signal from said reference is received by said receiver and wherein the distance from said transmitting means to said receiving means via said first and second reflectors is a fixed predetermined distance whereby the time that it takes said signal portion to travel on a path from said transmitter means to said first reflector, from said first reflector to said second reflector, and from said second reflector to said receiver means is used to determine the signal velocity to thereby calibrate the sensor; and a hydraulic control responsive to said indication to provide a hydraulic signal to a tool-positioning device in order to maintain a given position of said construction tool with respect to said reference.

Claim 28

The grade-establishing construction tool elevation control as defined in claim 19, wherein said means for transmitting is a first transducer and said means for receiving is a second transducer, and said signal is an acoustic wave.

Claim 29

The grade-establishing construction tool elevation control as defined in claim 19, wherein said signal reflects off a reference surface forming first and second echoes of said signal which are received by said means for transmitting and said means for receiving.

Claim 30

The grade-establishing construction tool elevation control as defined in claim 19, wherein said control means is responsive to said echoes received by said means for transmitting and said means for receiving for determining a vertical distance and a lateral distance relative to said reference.

Claim 31

A road grader comprising: a frame supported for horizontal motion in a longitudinal direction and means for carrying a blade on said frame; a sensor carried by said blade and including a transmitter for transmitting at least one acoustic signal and at least two laterally spaced receivers for receiving echoes of said at least one acoustic signal from a reference, said sensor including control means responsive to measured time periods between the transmission of said at least one acoustic signal and the reception of said echoes thereof, respectively, by said at least two receivers for producing indications of a vertical distance and a lateral distance of the sensor relative to the reference; and position control means carried by said road grader, said position control means for adjusting the vertical and lateral positions of said blade in response to said indications to maintain said vertical distance and said lateral distance constant to thereby reduce height errors resulting from lateral variations of said sensor from said reference.

Claim 32

A road grader comprising: a frame supported for horizontal motion in a longitudinal direction and means for carrying a blade on said frame; a sensor carried by said blade and including a transmitter for transmitting at least one acoustic signal and at least two laterally spaced receivers for receiving echoes of said at least one acoustic signal from a reference, said sensor including control means responsive to measured time periods between the transmission of said at least one acoustic signal and the reception of said echoes thereof, respectively, by said at least two receivers for producing indications of a vertical distance and a lateral distance of the sensor relative to the reference; and position control means carried by said road grader, said position control means for adjusting the vertical and lateral positions of said blade in response to said indications to maintain said vertical distance and said lateral distance constant to thereby reduce height errors resulting from lateral variations of said sensor from said reference.

Claim 33

The road grader as defined in claim 23, wherein said transmitter is defined by a first transducer and said receivers are defined by said first transducer and a second transducer, said transducers for transmitting and receiving an acoustic wave.

Claim 34

The road grader as defined in claim 24, wherein said sensor includes a third transducer for sensing a side lobe of an acoustic wave transmitted by said first transducer.

Claim 35

The road grader as defined in claim 24, wherein said first and second transducers are positioned side-by-side and wherein a reflection of an acoustic wave transmitted by said first transducer is received by said first and second transducers.

Claim 36

The road grader as defined in claim 24, wherein said first and second transducers are mounted generally orthogonal to one another whereby the echo of an acoustic pulse transmitted from said first transducer to a reference is received by said first transducer and an echo of an acoustic wave transmitted by said second transducer and reflected off said reference is received by said second transducer.

Claim 37

The road grader as defined in claim 24, wherein said sensor includes first and second reflectors whereby an acoustic wave transmitted from said first transducer will reflect off said first and second reflectors and be received by said second transducer and the velocity of said acoustic wave may be measured.

Claim 38

The road grader as defined in claim 23, wherein said position control means includes a first position control mechanism for adjusting the vertical position of said construction tool relative to said construction implement and a second position control mechanism for adjusting the lateral position of said construction tool relative to said construction implement.

Claim 39

A road grader comprising: a frame supported for horizontal motion in a longitudinal direction and positioning means for positioning a blade on said frame, said positioning means including a lateral-positioning device that responds to a hydraulic signal to adjust the lateral position of the blade and a vertical-positioning device that responds to a hydraulic signal to adjust the vertical position of said blade; a sensor being carried by said blade and coupled to said positioning means, said sensor including a first ultrasonic transducer for generating an acoustic wave and for receiving a reflection of said acoustic wave from a reference, a second transducer for transmitting a second acoustic wave and receiving a reflection of said second acoustic wave from said reference, and a sensor control coupled to said first and second transducers to measure time periods between transmission and reception of each of said acoustic waves and therefrom determine the vertical and lateral distances of said construction tool from said reference, wherein said positioning means is responsive to the determined vertical distance to position said blade in said vertical direction and to the determined lateral distance to position the blade in said lateral direction to thereby reduce blade height errors resulting from lateral variations of said sensor from said reference.

Claim 40

A road grader comprising: a frame supported for horizontal motion in a longitudinal direction and positioning means for positioning a blade on said frame, said positioning means including a lateral-positioning device that responds to a hydraulic signal to adjust the lateral position of the blade and a vertical-positioning device that responds to a hydraulic signal to adjust the vertical position of said blade; a sensor being carried by said blade and coupled to said positioning means, said sensor including a first ultrasonic transducer for generating an acoustic wave and for receiving a reflection of said acoustic wave from a reference, a second transducer for transmitting a second acoustic wave and receiving a reflection of said second acoustic wave from said reference, and a sensor control coupled to said first and second transducers to measure time periods between transmission and reception of each of said acoustic waves and therefrom determine the vertical and lateral distances of said construction tool from said reference, wherein said positioning means is responsive to the determined vertical distance to position said blade in said vertical direction and to the determined lateral distance to position the blade in said lateral direction to thereby reduce blade height errors resulting from lateral variations of said sensor from said reference.