Patent Publication Number: US-2022220695-A1

Title: Work machine

Description:
TECHNICAL FIELD 
     The present invention relates to a work machine provided with a controller that computes a position of a machine body on the basis of satellite signals received from a plurality of satellites, and that computes a position of a control point on the work machine on the basis of a result of the computation and a posture of a work implement. 
     BACKGROUND ART 
     In recent years, computerized construction has been increasingly introduced in construction sites. The computerized construction is a system that realizes higher efficiency in construction by utilizing electronic information and ICT (Information and Communication Technology) with focus placed on the construction among a series of construction processes including survey, design, construction, inspection, management, and so on. As machines that support the computerized construction, work machines, typified by hydraulic excavators, are known in which a guidance function of displaying, on a monitor, a position of a machine body and a position and posture of a front work implement together with position information regarding a construction target surface, and a machine control function of controlling the front work implement such that a bucket will not excessively excavate the construction target surface are implemented. Such a work machine that supports the computerized construction provides a function of providing assistance in a work and an operation by presenting information to an operator on the basis of computerized construction data having three-dimensional coordinate information. In machine guidance for a hydraulic excavator, for example, a position of a bucket tip is computed from information regarding a position and posture of a machine body and information regarding a posture of a front work implement, and a position of a bucket relative to a construction target surface is presented to an operator via a monitor. 
     In order to compute position information regarding an upper swing structure (a machine body) in a global coordinate system (a geographic coordinate system), a hydraulic excavator of this type is sometimes equipped with a satellite positioning system (e.g., a GNSS (Global Navigation Satellite System)) that receives positioning signals from positioning satellites via a positioning antenna attached to the upper swing structure, thereby computing a position of the upper swing structure. However, a front member of the hydraulic excavator, such as a boom, an arm, or a bucket, sometimes exists above the positioning antenna of the satellite positioning system, and accordingly, sometimes interferes with reception of positioning signals over a straight route. When this happens, it is probable that the positioning antenna will receive the positioning signals as diffracted waves or reflected waves called multipath. Using the received diffracted waves or reflected waves for positioning computation leads to an increased likelihood of an error in a positioning result. Thus, Patent Document 1, for example, describes a technique that aims to reduce an effect of multipath. 
     In a hydraulic excavator described in Patent Document 1, an outer surface of an upper structure, such as a front work implement, that can be located above a positioning antenna is covered with an electromagnetic wave absorber to prevent a positioning signal from being reflected by the upper structure and received by the positioning antenna to prevent a reduction in precision of position measurement. 
     PRIOR ART DOCUMENT 
     Patent Document 
     Patent Document 1: JP-2017-75820-A 
     SUMMARY OF THE INVENTION 
     Problem to be Solved by the Invention 
     However, the technique described in Patent Document 1, which is able to reduce an effect of a signal (a reflected wave) that is reflected by the upper structure (the front work implement), is not able to reduce an effect of a signal (a diffracted wave) that is diffracted at the upper structure. 
     In general, hydraulic excavators are work machines that perform a work by repeating a series of operations, such as excavation, swing, and soil dumping, with use of a front work implement, and the posture of the front work implement can frequently be changed during the work. A change in the posture of the front work implement can lead to a change in a positioning satellite that transmits positioning signals interrupted by the front work implement, which may change a combination of positioning satellites used for positioning computation. A change in the combination of the positioning satellites used for the positioning computation can cause great variation in positioning results, and can cause a reduction in reproducibility of positioning results even while the same work is being performed at the same position. This can cause a deterioration in precision in computing the position of the front work implement (a bucket claw tip), which can lead to a problem of the shape of an actual finished surface being different from that of a construction target surface, for example. 
     The present invention has been conceived to solve the above-described problem, and an object thereof is to provide a work machine capable of computing the position of the work machine with high precision on the basis of positioning signals from positioning satellites. 
     Means for Solving the Problem 
     The present application includes a plurality of means for solving the above-described problem, and one example thereof is a work machine including: a lower travel structure; an upper swing structure mounted to the lower travel structure so as to be capable of swinging; a work implement mounted to the upper swing structure so as to be capable of rotating; an antenna that receives positioning signals from a plurality of positioning satellites; a receiver configured to compute a position of the upper swing structure on the basis of the positioning signals received by the antenna; a posture sensor that detects a posture of each of the upper swing structure and the work implement; and a controller configured to compute a position of a control point set in the work implement on the basis of the position of the upper swing structure computed by the receiver, an azimuth angle of the upper swing structure, the posture of the work implement detected by the posture sensor, and dimensional data on the work implement, in which the controller is configured to set, as a mask range, a range in a field of vision in a sky over the antenna for which the work implement can become an obstacle when the antenna receives the positioning signals from the plurality of positioning satellites on the basis of a position at which the antenna is installed, a movable range of the work implement, the posture of the upper swing structure detected by the posture sensor, and the azimuth angle of the upper swing structure, in a case where it has been determined that the work implement is performing a work that demands precision, in which the receiver is configured to compute the position of the upper swing structure on a basis of positioning signals from, out of the plurality of positioning satellites, positioning satellites that remain after excluding a positioning satellite or satellites located in the mask range, in a first case in which the controller has determined, on the basis of satellite positioning condition data acquired from the receiver, that setting of the mask range leads to improved precision of the computation of the position of the upper swing structure by the receiver, and in which the receiver is configured to compute the position of the upper swing structure on the basis of the positioning signals transmitted from the plurality of positioning satellites without using the mask range, in a second case in which the controller has determined, on the basis of the satellite positioning condition data acquired from the receiver, that the setting of the mask range leads to reduced precision of the computation of the position of the upper swing structure by the receiver. 
     Advantages of the Invention 
     According to the present invention, a position of a work machine can be computed with high precision on the basis of positioning signals from positioning satellites. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of a structure of a hydraulic excavator according to an embodiment of the present invention. 
         FIG. 2  is a schematic diagram of a hardware configuration of a control system according to the embodiment of the present invention. 
         FIG. 3  illustrates a functional block diagram of a controller according to the embodiment of the present invention. 
         FIG. 4  illustrates an example of positional relations between the hydraulic excavator and positioning satellites. 
         FIG. 5  illustrates an example of arrangement of positioning satellites in the sky over GNSS antennas. 
         FIG. 6  illustrates a case in which a satellite mask range is set in  FIG. 5 . 
         FIG. 7  illustrates a processing flow of the controller according to the embodiment of the present invention. 
         FIG. 8  illustrates a processing flow of the controller (a precision demand determination section) according to the embodiment of the present invention. 
         FIG. 9  illustrates a processing flow of the controller (a satellite mask setting section) according to the embodiment of the present invention. 
     
    
    
     MODES FOR CARRYING OUT THE INVENTION 
     Hereinafter, an embodiment of the present invention will be described with reference to the accompanying drawings. In the present embodiment, the present invention is applied to a crawler-type hydraulic excavator as a work machine, and a control point (a point on the hydraulic excavator the position of which is computed by a controller  100  for control) is set at a bucket tip of the hydraulic excavator. Note that the control point may be set at any position on the hydraulic excavator, and that a work tool of a front work implement is not limited to a bucket as described below. Also note that the present invention is applicable not only to the hydraulic excavator described below but also to any other work machine provided with a receiver that receives positioning signals from positioning satellites via an antenna and computes the position of a main body (an upper swing structure) thereof in a global coordinate system. Also note that, in the accompanying drawings, like members are designated by like reference characters, and redundant description may be omitted. 
       FIG. 1  is a schematic diagram of a hydraulic excavator  1 . In  FIG. 1 , the hydraulic excavator  1  includes a main machine body  2  and a multi-joint front work implement  3 . The main machine body  2  includes a crawler-type lower travel structure  5 , and an upper swing structure  4  mounted on the lower travel structure  5  so as to be capable of swinging in a left-right direction. The front work implement  3  is mounted to the upper swing structure  4  so as to be capable of rotating. The front work implement  3  includes a plurality of front members including a boom  6 , an arm  7 , and a bucket  8 , which are driven by a boom cylinder  9 , an arm cylinder  10 , and a bucket cylinder  11 , respectively, which are hydraulic cylinders. 
     A plurality of posture sensors  20  ( 20   a,    20   b,  and  20   c ) for detecting the posture of the front work implement  3  are installed on the front work implement  3 . The posture sensors  20   a,    20   b,  and  20   c  are installed on the front members  6 ,  7 , and  8 , respectively, and output posture data for computing the postures of the respective front members  6 ,  7 , and  8 . The posture of each of the front members  6 ,  7 , and  8  can be computed from the outputted posture data, and the posture of the front work implement  3  can be computed by combining the postures thereof. As each of the posture sensors  20   a,    20   b,  and  20   c,  an inertial measurement unit (IMU) capable of detecting an angle of inclination with respect to a predetermined plane (e.g., a horizontal plane), for example, can be used. 
     A posture sensor  20   d  (see  FIG. 3 ) for detecting the posture (inclination angles) of the upper swing structure  4  is installed on the upper swing structure  4 . The posture of the upper swing structure  4  can be computed from posture data outputted by the posture sensor  20   d.  As the posture sensor  20   d,  an inertial measurement unit (IMU), for example, can be used as is the case with each of the posture sensors  20   a  to  20   c.    
     The upper swing structure  4  has installed therein a cab  12 , a hydraulic swing motor (swing drive device)  13 , an engine (not depicted), a hydraulic pump (not depicted) that is driven by the engine to supply a hydraulic operating fluid to hydraulic actuators in the hydraulic excavator, and other devices. 
     Moreover, the upper swing structure  4  has installed therein a satellite positioning system  21  (antennas  21   a  and  21   b  and a receiver  21   c ) for detecting the position of the upper swing structure  4  in the global coordinate system, a controller  100  capable of computing the position (e.g., the position in a site coordinate system) of the control point set in the hydraulic excavator  1  (including the front work implement  3 ), and a communication device  23  (see  FIG. 2 ) for the controller  100  to communicate with an external terminal, particularly, an external server  102 . 
     The cab  12  has provided therein a plurality of operation levers (operation devices)  18  for operating the hydraulic actuators in the hydraulic excavator. In the present embodiment, an operator is able to drive each of the boom cylinder  9 , the arm cylinder  10 , the bucket cylinder  11 , the hydraulic swing motor  13 , and hydraulic travel motors  15   a  and  15   b  by operating a corresponding one of the operation levers  18 . An operation amount inputted by the operator to each of the operation levers  18  is detected by a corresponding one of operation sensors  17  (see  FIG. 2 ), and is outputted to the controller  100 . When the operation lever  18  is of a hydraulic pilot type, for example, a pressure sensor can be used as the operation sensor  17 . 
     In addition, the cab  12  is provided with a construction target surface setting device  24  for registering three-dimensional data (3D design surface data) on a design surface for creating a construction target surface in a storage device (not depicted) in the controller  100 , a monitor (display device)  19  for displaying various types of information, including a positional relation between the front work implement  3  and the construction target surface, a control mode selection switch  26  for the operator to select any one of a rough excavation mode and a finishing mode as a control mode of limiting control (machine control), which will be described below, and a storage device (e.g., a semiconductor memory)  25  (see  FIG. 2 ), which is a device for storing various types of information inputted to and outputted from the controller  100 . The controller  100  and the storage device  25  are installed inside the cab  12  in the present embodiment, but may alternatively be installed outside of the cab  12 . In addition, the storage device in the controller  100  may be used as a substitute for the storage device  25 . 
     The lower travel structure  5  has crawler belts  14   a  and  14   b  on left and right sides, and the crawler belts  14   a  and  14   b  are driven by the hydraulic travel motors  15   a  and  15   b,  respectively, to cause the lower travel structure  5  to travel. It is to be noted that, in  FIG. 1 , only the crawler belt  14   a  and the hydraulic travel motor  15   a  on the left side are illustrated, and the crawler belt  14   b  and the hydraulic travel motor  15   b  on the right side are not illustrated. The upper swing structure  4  is rotatably connected to the lower travel structure  5  through a swing ring  16 , and is driven by the hydraulic swing motor  13 . 
       FIG. 2  illustrates an example system configuration of a position measurement system according to an embodiment of the present invention. 
     In  FIG. 2 , the satellite positioning system  21  is connected to wireless equipment  30  installed on the upper swing structure  4 . The wireless equipment  30  is used to receive correction data transmitted from a satellite positioning system installed in a reference station having known coordinates in the global coordinate system, and the received correction data is outputted to the receiver  21   c  in the satellite positioning system  21 . 
     As illustrated in  FIG. 1 , the satellite positioning system  21  is installed on an upper surface of the upper swing structure  4 , and includes two GNSS antennas  21  (a first GNSS antenna  21   a  and a second GNSS antenna  21   b ) for receiving positioning signals from a plurality of positioning satellites, and the receiver  21   c  capable of computing a position and an azimuth angle of the upper swing structure  4  in real time on the basis of the positioning signals received by the first GNSS antenna  21   a  and the second GNSS antenna  21   b  and the correction data received by the wireless equipment  30 . 
     Each of the first GNSS antenna  21   a  and the second GNSS antenna  21   b  is an antenna for RTK-GNSS (Real Time Kinematic-Global Navigation Satellite Systems). 
     The receiver  21   c  computes position data (position data on the upper swing structure  4 ) on the GNSS antennas  21   a  and  21   b  from the positioning signals received by the first GNSS antenna  21   a  and the second GNSS antenna  21   b,  computes a vector from one of the GNSS antennas to the other GNSS antenna, using the position data on the two GNSS antennas  21   a  and  21   b,  and calculates an orientation (azimuth angle) of the upper swing structure  4  on the basis of a direction of the vector. Data on the azimuth angle and the position data as to the upper swing structure  4  computed by the receiver  21   c  are outputted to the controller  100 . 
     Inclination angle data on the front members  6 ,  7 , and  8  and the upper swing structure  4  detected by the posture sensors  20  ( 20   a,    20   b,    20   c,  and  20   d ) are outputted to the controller  100 . 
     Data on the operation amounts on the operation levers  18  detected by the operation sensors  17  is outputted to the controller  100 . The operation sensors  17  are provided for the respective objects operated by the operation levers  18 , and detect pressures (pilot pressures) of the hydraulic operating fluid outputted as operation signals of the boom  6 , the arm  7 , the bucket  8 , the upper swing structure  4 , and the lower travel structure  5 . 
     The operation levers  18  are provided with the control mode selection switch  26 . The control mode selection switch  26  is a switch capable of selecting, as the control mode of the limiting control (machine control), which will be described below, one of the rough excavation mode, in which a higher priority is given to the operation velocity of the front work implement  3  (e.g., the operation velocity of the arm  7 ) than to precision of the construction target surface, and the finishing mode, in which a higher priority is given to the precision of the construction target surface than to the operation velocity of the front work implement  3  (e.g., the operation velocity of the arm  7 ). 
     The rough excavation mode is, for example, a mode in which no limitation is placed on the operation velocity of the arm  7  (the arm cylinder  10 ) during the limiting control. In this mode, no limitation is placed on the operation velocity of the arm  7 , and the arm  7  operates at a velocity that matches the operation by the operator, allowing a speedy excavation work. However, because no limitation is placed on the velocity of the arm even when the bucket  8  is close to the construction target surface, the bucket  8  can break downwardly into the construction target surface depending on the velocity of the arm. 
     The finishing mode is, for example, a mode in which a limitation is placed on the operation velocity of the arm  7  (the arm cylinder  10 ) during the limiting control. In this mode, a maximum value of the operation velocity of the arm  7  is set to be smaller as the bucket  8  approaches the construction target surface, for example. In this case, the velocity of the arm when the bucket  8  is in the vicinity of the construction target surface tends to become lower than a velocity specified by the operation by the operator, but it is made easier to position the bucket  8  on or over the construction target surface, enabling a highly precise excavation work without doing damage to the construction target surface. 
     A switch of a push button type, for example, can be used as the control mode selection switch  26 . In the present embodiment, when the operator performs construction using the limiting control, the control mode can be switched to a desired control mode by pressing the control mode selection switch  26 . In particular, in a work for which the operator demands precision in construction, a switching between the finishing mode which allows highly precise construction, and the rough excavation mode which does not allow highly precise construction, can be made by pressing the switch near at hand. While the control mode selection switch  26  is provided in the operation levers  18  in the example of the present embodiment described above, the control mode selection switch  26  can be separately provided at another place in the cab  12 , or can be provided as an image of a switch on the monitor  19 . 
     The construction target surface setting device  24  is a device for registering the 3D design surface data for creating the construction target surface in the storage device (not depicted) in the controller  100 . The construction target surface setting device  24  is, for example, a controller (e.g., a tablet terminal provided with a touch panel display) that serves also as a display prepared for computerized construction, and can be used to make various types of settings, including a setting of contents of a work, and also to make settings concerning machine guidance. 
     The 3D design surface data can be inputted to the construction target surface setting device  24  through, for example, a nonvolatile semiconductor memory or the like. In addition, the 3D design surface data can also be loaded by being inputted from a server through a network. This device may be combined with the controller  100 , or the functions of the construction target surface setting device  24  may be implemented on the controller  100 . 
     The display device  19  is a device that displays various types of information. In the present embodiment, the display device  19  is a liquid crystal display monitor installed inside the cab  12 , and on the monitor  19 , information, such as an image of a side view of the hydraulic excavator  1  generated on the basis of the information acquired by the posture sensors  20 , and the shape of a section of the construction target surface, is presented to the operator. 
     The storage device  25  is a device for storing various types of information. The storage device is, for example, a nonvolatile storage medium, such as a semiconductor memory, and is detachably attachable via a dedicated insert port inside the cab  12 . 
     The communication device  23  is a device used for transmission and reception of data between the controller  100  and an external terminal (e.g., an external management server  102 ). The communication device  23  performs transmission and reception of data with the server  102 , which is at a location distant from the excavator  1 , via satellite communication, for example. Specifically, the communication device  23  transmits data recorded on the storage device  25  or secondary data generated on the basis of such data to the server  102  through a base station. In addition, the communication device  23  may realize exchange of data between the excavator  1  and the base station via a mobile phone network, a short-range wireless communication network, or the like. 
     Next, processes performed by the controller  100 , with the processes classified into functions, will be described below.  FIG. 3  illustrates a diagram (functional block diagram) depicting blocks representing functions realized by some of the processes performed by the controller  100  in the present embodiment. 
     The controller  100  is a computer having an arithmetic processing device (e.g., a CPU (not depicted)) and a storage device (e.g., a semiconductor memory, a hard disk drive, or the like (not depicted)), and by executing a program stored in the storage device on the arithmetic processing device, functions as various sections (an excavator posture computation section  42 , an abnormal state determination section  43 , a construction target surface computation section  44 , a hydraulic system control section  45 , a monitor display control section  46 , a precision demand determination section  47 , a satellite mask setting section  48 , and a data storage section  49 ) illustrated in  FIG. 3 . Hereinafter, processes performed by the respective sections will be described. 
     The excavator posture computation section  42  functions as a position data detection section  40  and a posture computation section  41 . 
     The position data detection section  40  performs computation of converting data on the positions (latitudes, longitudes, and altitudes) of the first GNSS antenna  21   a  and the second GNSS antenna  21   b  in the global coordinate system computed by the receiver  21   c  of the satellite positioning system  21  to position data (coordinate values) in a machine body coordinate system. For example, coordinate values of the first GNSS antenna  21   a  in the machine body coordinate system are known from design dimensions of the excavator  1  and a measurement using a surveying instrument, such as a total station, and therefore, conversion between the machine body coordinate system and the global coordinate system is possible by use of a coordinate conversion parameter obtained on the basis of the global coordinate system, the inclination angles (a pitch angle and a roll angle) of the upper swing structure  4 , and the coordinate values of the first GNSS antenna  21   a  in the machine body coordinate system. This makes it possible to calculate coordinate values of a center of a boom pin, which corresponds to an origin in the machine body coordinate system, in the global coordinate system. Computation of coordinate values in the global coordinate system of any point of the excavator  1  in the machine body coordinate system is possible. 
     The posture computation section  41  computes the posture of the hydraulic excavator  1  (the front work implement  3  and the upper swing structure  4 ) on the basis of the angle information regarding the front members  6 ,  7 , and  8  and the upper swing structure  4  computed via the posture sensors  20   a  to  20   d.    
     The excavator posture computation section  42  is capable of calculating position data (coordinate values) on the control point (e.g., a center of a bucket claw tip in a width direction of the bucket) in the bucket  8 , which the excavator  1  uses for construction, on the basis of the position coordinate data on the boom pin (the origin in the machine body coordinate system) calculated by the position data detection section  40 , the orientation (azimuth angle) of the upper swing structure  4 , the posture data calculated by the posture computation section  41 , and data on the dimensions of the front members  6 ,  7 , and  8  measured and stored in advance. In addition, the excavator posture computation section  42  is also capable of computing posture data for calculating a posture when the image of the side view of the excavator  1  is displayed on the monitor  19 . Note that, while the control point is set in the bucket claw tip for execution of the limiting control in the present embodiment, the control point may be set at another position as appropriate according to the content of processing of the machine control. 
     The construction target surface computation section  44  computes the construction target surface to be excavated by the excavator  1  on the basis of the 3D design surface data inputted from the construction target surface setting device  24  and current position data on the excavator  1  computed by the excavator posture computation section  42 , and outputs the computed construction target surface to the hydraulic system control section  45 . The construction target surface corresponds to a line of intersection of an operation plane of the front work implement  3  with the 3D design surface data, and the operation plane of the front work implement  3  is a plane perpendicular to each of the front members  6 ,  7 , and  8  and the boom pin and passing through an axial center of the boom pin. The calculated construction target surface can be used in the form of a construction target surface displayed on the monitor  19  as well, and is outputted to the monitor display control section  46 . In addition, the calculated construction target surface is outputted to the precision demand determination section  47  as well. 
     The monitor display control section  46  computes the positional relation between the construction target surface and the front work implement  3  on the basis of the construction target surface computed by the construction target surface computation section  44  and the posture of the front work implement  3  computed by the excavator posture computation section  42 , and displays the construction target surface and the front work implement  3  on the monitor  19 . This, for example, realizes a function as a machine guidance system that displays the posture of the front work implement  3 , including the boom  6 , the arm  7 , and the bucket  8 , and the tip position and angle of the bucket  8  on the monitor  19  to provide assistance in the operation by the operator. 
     The hydraulic system control section  45  computes and outputs command values for controlling a hydraulic system of the hydraulic excavator  1 , the hydraulic system including the hydraulic pump, a control valve, the hydraulic actuators, and the like. In the present embodiment, as machine control, limiting control of controlling the front work implement  3  on the basis of a distance (target surface distance) between the front work implement  3  and the construction target surface such that the range of operation of the front work implement  3  is limited to the construction target surface and a space over the construction target surface can be performed. 
     More specifically, in the limiting control according to the present embodiment, the controller  100  automatically adds an operation of raising the boom  6  to an operation (e.g., an arm crowding operation) of the arm  7  controlled by the operator such that the velocity component of the bucket claw tip in a direction perpendicular to the construction target surface and toward the construction target surface will approach zero as the target surface distance approaches zero. As a result, on the construction target surface, the velocity component of the bucket claw tip in the direction perpendicular to the construction target surface is maintained at zero. At this time, the velocity components of the bucket claw tip in directions parallel to the construction target surface are not zero, and therefore, the operator is able to cause the claw tip of the bucket  8  to move parallel to and along the construction target surface to carry out an excavation along the construction target surface by inputting an arm crowding manipulation to the corresponding operation lever  18 , for example. When the operation of raising the boom  6  is automatically added by the controller  100 , a control signal is outputted from the controller  100  to a solenoid proportional valve (not depicted) that outputs a pilot pressure according to the inputted control signal to control a control valve (not depicted) that controls the operation of the boom cylinder  9  through the pilot pressure. Note that, while the limiting control is mentioned as the machine control in the present embodiment, examples of machine control that are applicable include other types of control such as entry prohibition control of automatically controlling various types of hydraulic actuators so as to prevent the control point from entering an entry prohibited area set around the hydraulic excavator  1 . 
     The precision demand determination section  47  is a section that determines whether or not the front work implement  3  is performing a work that demands precision. While a detailed description will be provided with reference to a flowchart of  FIG. 8 , which will be described below, the precision demand determination section  47  grasps the content of the work of the hydraulic excavator  1  and a demand of the operator on the basis of the position data on the front work implement  3  calculated via the operation levers  18  and the excavator posture computation section  42 , data on the construction target surface computed by the construction target surface computation section  44 , and the like, and determines whether or not the precision demanded for control of the position of the control point set in the hydraulic excavator  1  is high. 
     The abnormal state determination section  43  is a section that identifies an abnormality caused in the excavator  1  on the basis of information concerning equipment installed on the excavator  1 , such as the posture sensors  20 , the satellite positioning system  21 , the construction target surface setting device  24 , and the communication device  23 . For example, in a computerized construction machine on which a machine guidance function is implemented, the operator operates the front work implement  3  in such a manner that the claw tip of the actual bucket  8  will move along the construction target surface while referring to the positional relation between the bucket  8  and the construction target surface presented on the monitor  19 . If the construction target surface has nevertheless been excessively excavated by the bucket  8 , the operator will recognize that some abnormality has occurred in the hydraulic excavator  1 . When such an abnormality has occurred, the abnormal state determination section  43  determines whether a cause thereof is a failure in the equipment or poor conditions for reception of the positioning signals from the positioning satellites or reception of the correction data via the wireless equipment  30 . The abnormal state determination section  43  outputs a “positioning condition abnormality flag” to each of the data storage section  49  and the monitor display control section  46  when it has been determined that the cause of the abnormality is a deterioration in satellite positioning conditions, and outputs an “equipment abnormality flag” to each of the data storage section  49  and the monitor display control section  46  when it has been determined that the cause of the abnormality is a failure in the equipment. 
     (Output of Positioning Condition Abnormality Flag) 
     Regarding the positioning condition abnormality flag, the receiver  21   c  regularly outputs the degree of reliability of the positioning via the receiver  21   c  itself to the controller  100 , and when the degree of reliability is equal to or smaller than a predetermined value Vt 1 , the abnormal state determination section  43  (the controller  100 ) determines that the cause of the abnormality is a deterioration in the satellite positioning conditions, and outputs the positioning condition abnormality flag to each of the monitor display control section  46  and the data storage section  49 . 
     Accepting input of the positioning condition abnormality flag, the display control section  46  displays information (a message or an image) indicating a low reliability of the positioning via the receiver  21   c  on the monitor  19 . The operator is thus able to recognize a need to suspend the work until precision of positioning is recovered when, for example, the limiting control (machine control) is being used for a finishing work, because of a deterioration in the precision of the positioning via the satellite positioning system  21 . 
     Accepting input of the positioning condition abnormality flag, the data storage section  49  collects and stores satellite positioning condition data (which will be described below) while the degree of reliability of the positioning is equal to or smaller than the predetermined value Vt 1 , and outputs the stored satellite positioning condition data, together with times of acquisition of the data, to the external management server  102  via the communication device  23 . At this time, the satellite positioning condition data may be outputted to the storage device  25  to be stored on the hydraulic excavator  1  as well. 
     The satellite positioning condition data is a generic term referring to data indicating the satellite positioning conditions outputted from the receiver  21   c,  and includes at least one item of data among, for example, the number of positioning satellites used for the positioning, variance of positions (altitudes) in a heightwise direction among positions (latitudes, longitudes, and altitudes) computed by satellite positioning, VDOP (Vertical Dilution of Precision) computed from the geometrical arrangement of the positioning satellites used for the positioning, and the degree of reliability of the positioning via the receiver  21   c.  The recording of the satellite positioning condition data on the external management server  102  and the storage device  25  makes it possible to make it clear that the cause of the abnormality is not a failure in the equipment but the satellite positioning conditions, and increases the likelihood that a detailed cause of the deterioration in the satellite positioning conditions can be identified after the event. 
     (Output of Equipment Abnormality Flag) 
     Examples of equipment related to the equipment abnormality flag include, in addition to standard equipment installed on the hydraulic excavator  1 , such as the engine and the hydraulic pump, equipment installed thereon for computerized construction. Examples of the equipment for computerized construction include the posture sensors  20  installed on the front members  6 ,  7 , and  8  and the upper swing structure  4 , the receiver  21   c  in the satellite positioning system  21 , the operation sensors (pressure sensors)  17 , the communication device  23 , and the wireless equipment  30 . When an occurrence of an abnormality has been detected, the abnormal state determination section  43  determines whether or not an abnormality has occurred in each of such pieces of equipment on the basis of output values from various types of sensors that detect operation conditions of such pieces of equipment, data outputted from such pieces of equipment, and the like, and, when it is determined that an abnormality has occurred therein, outputs the equipment abnormality flag to each of the monitor display control section  46  and the data storage section  49 . 
     Accepting input of the equipment abnormality flag, the display control section  46  displays information (a message or an image) indicating an occurrence of an abnormality (failure) in the equipment on the monitor  19 . The operator is thus able to recognize a need for a quick and detailed abnormality diagnosis of the equipment, including a determination as to whether or not a replacement of equipment is necessary, for example, because of the occurrence of the abnormality in the equipment. 
     Accepting input of the equipment abnormality flag, the data storage section  49  collects and stores equipment operation condition data (snapshot data) for a predetermined time around the time of reception of the flag, and outputs the stored equipment operation condition data, together with times of acquisition of the data, to the external management server  102  via the communication device  23 . At this time, the equipment operation condition data may be outputted to the storage device  25  to be stored on the hydraulic excavator  1  as well. The equipment operation condition data is a generic term referring to various types of data used for the determination as to whether or not an abnormality has occurred in the equipment, and includes, for example, the position data and the posture data on the hydraulic excavator  1  including the front work implement  3 , the operation amounts of the operator, pressure values of the hydraulic operating fluid detected by pressure sensors installed on the hydraulic actuators and hydraulic lines connected thereto, and the like. The recording of the equipment operation condition data on the external management server  102  and the storage device  25  makes it possible to make it clear that the cause of the abnormality is not the satellite positioning conditions but a failure in the equipment, and increases the likelihood that a detailed cause of the failure in the equipment can be identified after the event. 
     The 3D design surface data, information regarding the soil of a construction site, information regarding a terrain including an area surrounding the construction site, an area in the construction site in which communication is possible, and the like can be stored in the external management server  102 , and the external management server  102  can be configured to be capable of grasping communication conditions as well. In addition, data on another work machine that is performing a work in an area surrounding the work machine in which an abnormality has occurred is also simultaneously uploaded to the external management server  102  to make it possible to grasp abnormality data concerning an environment, such as satellites and communication. Furthermore, because a matter concerning the environment, such as a communication system, the satellite positioning, or the like, may cause a deterioration in construction conditions, the state of the arrangement (e.g., values of DOP, including VDOP) of the satellites at the time of an occurrence of an abnormality, and signal levels, and the like, at that time are surveyed and recorded for reporting in the present embodiment. 
     In particular, when computerized construction is carried out, especially during a work involving machine control that automates a part of an operation of the work machine, a phenomenon such as an excessive excavation of the construction target surface, or a failure for the bucket  8  to approach the construction target surface, may occur. When such a phenomenon happened in the past, a person in charge of a service has needed to visit the site to see the behavior of the actual machine and check the states of various types of sensors and so on, thereby determining whether an abnormality of the machine or a deterioration in positioning conditions due to an influence of surrounding conditions had occurred, for example. In the present embodiment, various types of data at the time of the occurrence of the abnormality are transmitted to the external management server  102 , and the content of the work and the states of the installed equipment can be checked, and therefore, an efficient support can be provided after the occurrence of the abnormality. 
     Next, an example of a scene in which a deterioration in the satellite positioning conditions has occurred during an excavation work will be described below with reference to  FIGS. 4 to 6 . 
       FIG. 4  illustrates posture changes (posture A and posture B) of the hydraulic excavator in a scene in which a positioning signal from a positioning satellite is interrupted by the front work implement  3  performing an excavation operation, while  FIGS. 5 and 6  each illustrate a field of vision in the sky over the first GNSS antenna  21   a  at that time. 
     In  FIG. 4 , the hydraulic excavator  1  is performing a finishing work on a face of slope. While referring to the positional relation between the bucket  8  and the construction target surface displayed on the monitor  19 , the operator performs an excavation manipulation of the hydraulic excavator such that the claw tip of the bucket  8  will move along the construction target surface. An area  60  enclosed by a broken line in  FIG. 6  represents an area (which may be referred to as a “covered area”) for which the front work implement  3  (the boom  6 , the arm  7 , and the bucket  8 ) can become an obstacle that interrupts a positioning signal before reaching the GNSS antenna  21   a  or  21   b,  and in this covered area  60 , satellites (e.g., a satellite G 1 ) that transmit positioning signals that can be interrupted exist. In the situation illustrated in  FIG. 4 , the satellite G 1  is visible as illustrated in  FIG. 5  when the boom  6  is lowered to excavate an area lower in level than the excavator  1  as indicated by a posture A, but when the boom  6  is raised along with arm crowding as indicated by a posture B, a positioning signal from the satellite G 1  is interrupted by the boom  6  as illustrated in  FIG. 6 . Therefore, during a series of movements as illustrated in  FIG. 4 , regular changes occur between a case where direct reception of positioning signals from the satellite G 1  is possible (posture A) and a case where the positioning signals from the satellite G 1  are affected by reflection, diffraction, and the like (posture B). In this case, the combination of positioning satellites that are used for position calculation can change, leading to a greater variation in position measurement results. 
     When performing a position measurement, the receiver  21   c  selects the satellites (positioning signals) used for position calculation by determining various conditions, including the qualities of positioning signals from satellites received by the two GNSS antennas  21   a  and  21   b,  and the arrangement of GNSS satellites from which the positioning signals are received. The arrangement of the satellites is assessed using numerical values called DOP (Dilution Of Precision), and when, for example, too many positioning satellites are distributed on one side in the field of vision in the sky, the DOP is poor (has a large numerical value), resulting in reduced precision of the position calculation. Meanwhile, when the satellites are evenly distributed in the field of vision in the sky, the DOP is good (has a small numerical value), resulting in improved precision of the position calculation. This is because the position measurement by the satellite positioning system  21  employs a measurement system that applies triangulation. The position measurement by the satellite positioning system  21  has error factors, other than the DOP, that may slightly vary between the satellites, such as an error in trajectory information, a timepiece error of the GNSS satellites, and even when the hydraulic excavator  1  stays at the same position, a difference in the combination of the satellites used for the position calculation may cause a difference in the positions calculated. 
     In addition, in the construction as described above, a soil dumping work may be performed after the excavation operation, and in this case, after the excavation operation is completed, the boom  6  is raised, a swing operation is performed in a posture with the arm  7  and the bucket  8  folded, and a movement to a soil dumping location is made. At the soil dumping location, the arm  7  and the bucket  8  are caused to make a dump to complete the soil dumping work, and thereafter, a swing is made again, and a return to the excavation location is made to perform a work. In such a scene also, a change in the satellites used for the positioning may occur, because a swing may cause a satellite that has been picked up so far to be invisible or cause a satellite that has not been picked up so far to be visible. Therefore, even when the front work implement  3  has returned to the original position, reproducibility of position measurement may have become deteriorated. Such a deterioration in the reproducibility of position calculation results may cause the positional relation between the construction target surface and the bucket tip displayed on the monitor  19  to be different in each excavation work, which may cause a problem, such as a discontinuity in a finished work. 
     To overcome such a problem, in the present embodiment, the controller  100  sets a satellite mask range  65  (see FIG.  6 ) on the machine body coordinate system to limit the positioning satellites that can be employed by the receiver  21   c  for satellite positioning. In the satellite positioning, the receiver  21   c  performs a process of computing the position of the first GNSS antenna  21   a  on the basis of positioning signals transmitted from positioning satellites that remain after excluding the positioning satellite or the positioning satellites located in the satellite mask range  65  set by the controller  100 . 
     Next, details of a process of setting the satellite mask range  65  performed by the controller  100  according to the present embodiment will be described below with reference to a processing flow of  FIG. 7 . In  FIG. 7 , the processing flow is repeatedly computed at intervals of a fixed time (e.g., 100 ms). 
     (Process (1) for Setting Satellite Mask Range) 
     At step S 101 , the controller  100  acquires the position data on the first GNSS antenna  21   a  computed by the satellite positioning system  21  (i.e., the receiver  21   c ), and data on the vector from the first GNSS antenna  21   a  to the second GNSS antenna  21   b.    
     At step S 102 , the controller  100  computes, in the excavator posture computation section  42 , the posture data on each of the members  5  to  8  on the basis of data (posture sensor data) outputted from the posture sensors (IMUs)  20  attached to the front members  6 ,  7 , and  8  and the upper swing structure  4 . At the next step S 103 , the excavator posture computation section  42  performs a common vector computation and a common coordinate transformation on the basis of the data (the three-dimensional positions of the two GNSS antennas  21   a  and  21   b,  and the data on the vector from the first GNSS antenna  21   a  to the second GNSS antenna  21   b ) computed by the satellite positioning system  21  and acquired at step S 101 , and the posture data on each of the members  5  to  8  computed at step S 102 , thus computing the three-dimensional position of the bucket tip (the control point) and the position and posture of the hydraulic excavator  1  in each of the global coordinate system and the site coordinate system. The position and posture of the hydraulic excavator  1  and the position of the bucket tip (the control point) computed at step S 103  are outputted to the abnormal state determination section  43 , the construction target surface computation section  44 , the hydraulic system control section  45 , the monitor display control section  46 , the precision demand determination section  47 , the satellite mask setting section  48 , and so on within the controller  100  as necessary (step S 110 ). 
     At step S 104 , the controller  100  determines, in the precision demand determination section  47 , whether or not the front work implement  3  is performing a work that demands precision (i.e., whether or not a highly precise position computation is required for the position computation on the control point (i.e., the bucket tip)). Here, the description of  FIG. 7  is suspended for a time, and details of a process (a computation process for a precision demand flag) performed by the precision demand determination section  47  to determine whether or not precision is demanded will be described below with reference to  FIG. 8 .  FIG. 8  illustrates a processing flow of a method of computing the precision demand flag. In the flow of  FIG. 8 , the precision demand determination section  47  computes the precision demand flag (i.e., determines ON/OFF of the precision demand flag) on the basis of control mode data outputted from the control mode selection switch  26 , construction target surface data outputted from the construction target surface computation section  44 , and excavator posture data outputted from the excavator posture computation section  42 . 
     (Computation Process for Precision Demand Flag) 
     At step S 201 , the precision demand determination section  47  acquires the excavator posture data (data computed at step S 103  in  FIG. 7 ) computed by the excavator posture computation section  42 . 
     Next, the precision demand determination section  47  accepts input of the control mode (the control mode data) selected by the control mode selection switch  26  at step S 202 , and determines whether or not the selected control mode is the finishing mode at step S 203 . 
     When it is determined at step S 203  that the finishing mode is selected, it means that the precision of the satellite positioning also needs to be high because the finishing mode naturally requires the position of the bucket tip to be calculated with high precision. Accordingly, the precision demand determination section  47  sets the precision demand flag to ON at step S 204 , and outputs this result to the satellite mask setting section  48 . 
     Meanwhile, when it is determined at step S 203  that the finishing mode is not selected (i.e., when the rough excavation mode is selected in the present embodiment), the precision demand determination section  47  acquires, at step S 205 , the construction target surface data computed by the construction target surface computation section  44 . At the next step S 206 , the precision demand determination section  47  computes a distance between the construction target surface and the bucket tip (the front work implement  3 ) on the basis of the excavator posture data acquired at step S 201  and the construction target surface data acquired at step S 205 , and proceeds to step S 207 . At step S 207 , the precision demand determination section  47  determines whether or not the distance between the construction target surface and the bucket tip (the front work implement  3 ) computed at step S 206  is equal to or smaller than a predetermined value D 1  (e.g., equal to or smaller than 30 cm). 
     When it is determined at step S 207  that the distance between the construction target surface and the bucket tip is greater than the predetermined value D 1 , the precision demand determination section  47  proceeds to step S 210 , sets the precision demand flag to OFF, and outputs this result to the satellite mask setting section  48 . 
     Meanwhile, when it is determined at step S 207  that the distance between the construction target surface and the bucket tip is equal to or smaller than the predetermined value D 1 , the precision demand determination section  47  proceeds to step S 208 , and computes a change in the posture of the front work implement  3 . At step S 208 , the precision demand determination section  47  computes the change in the posture of the front work implement  3  on the basis of a difference between an excavator posture acquired in an immediately previous iteration of the control and an excavator posture acquired in the current iteration, and proceeds to step S 209 . At step S 209 , the precision demand determination section  47  determines whether or not there is a change in the excavator posture calculated at step S 208 . 
     When it is determined at step S 209  that there is a change in the excavator posture, it means that the position of the bucket tip needs to be computed with high precision, considering a possibility that the bucket tip will do damage to the construction target surface, since the distance between the construction target surface and the bucket tip is equal to or smaller than the predetermined value D 1 , meaning a very short distance therebetween, and, further, the excavator posture is being changed. Accordingly, the precision demand determination section  47  proceeds to step S 204 , sets the precision demand flag to ON, and outputs this result to the satellite mask setting section  48 . 
     Meanwhile, when it is determined at step S 209  that there is not a change in the excavator posture, the precision demand determination section  47  proceeds to step S 210 , sets the precision demand flag to OFF, and outputs this result to the satellite mask setting section  48 . That is, in this case, it is determined that the front work implement  3  is not performing a work that demands precision, because the hydraulic excavator  1  is stationary while the distance between the construction target surface and the bucket tip is short, being equal to or smaller than the predetermined value D 1 . Although, in the present embodiment, it is determined through computation whether or not the front work implement  3  is in operation on the basis of the change in the posture of each of the front members  6 ,  7 , and (step S 208 ), it may alternatively be determined whether or not the front work implement  3  is in operation on the basis of information inputted from the operation levers  18 , i.e., information inputted from the operation sensors  17 . Also note that it may be determined whether or not the excavator  1  is excavating on the basis of the position data on the hydraulic excavator  1 , the posture sensor data, pressures (the operation amounts) of the operation levers  18 , and the construction target surface data. In addition, because the aforementioned items of information are not sufficient to clearly determine whether or not the excavator  1  is actually performing an excavation work, an output from a pressure sensor attached to each of the hydraulic cylinders  9 ,  10 , and  11  may be used for the determination. 
     Note that the flow of  FIG. 8  may be omitted, and the precision demand flag may be set to ON at all times. Also note that the posture data as to the excavator  1  may be acquired not at step S 201  but at, for example, step S 205 , after it is determined that the control mode is not the finishing mode. Also note that, while the ON/OFF of the precision demand flag is determined on the basis of (1) whether or not the selected control mode is the finishing mode, and (2) a combination of the distance between the bucket tip and the construction target surface and whether or not there is a change in the excavator posture, in the example of  FIG. 8 , the ON/OFF of the precision demand flag may be determined on the basis of at least one of the above (1) and (2). Also note that whether or not there is a change in the excavator posture may be omitted from the condition (2). The change in the excavator posture can include a posture change other than the change in the posture of the front work implement  3  described above. 
     (Process (2) for Setting Satellite Mask Range) 
     The description of  FIG. 7  is resumed. At step S 105 , the controller  100  determines whether or not the precision demand flag is ON in the satellite mask setting section  48 . When it is determined in this determination that the precision demand flag is ON, the satellite mask setting section  48  proceeds to step S 106 . 
     At step S 106 , the satellite mask setting section  48  computes setting parameters for the satellite mask range  65  to set, on the machine body coordinate system, for example, the satellite mask range  65 , which is a range in the field of vision in the sky over the GNSS antennas  21   a  and  21   b  for which the front work implement  3  can become an obstacle when the GNSS antennas  21   a  and  21   b  receive positioning signals from a plurality of positioning satellites in the sky over the excavator. Examples of the setting parameters for the satellite mask range  65  include a minimum azimuth angle and a maximum azimuth angle that define an azimuth range of the mask, and a minimum elevation angle and a maximum elevation angle that define an elevation angle range of the mask (see  FIG. 6 ) with reference to the position of each of the GNSS antennas  21   a  and  21   b.  As illustrated in  FIG. 6 , the satellite mask range  65  is a range enclosed by both the azimuth range from the minimum azimuth angle to the maximum azimuth angle and the elevation angle range from the minimum elevation angle to the maximum elevation angle. Note that, as illustrated in  FIG. 6 , the azimuth angle (machine body azimuth angle) of the upper swing structure  4  may be set as the minimum azimuth angle. 
     The satellite mask range  65  is set on the basis of the positional relation between the position (e.g., a coordinate position in the machine body coordinate system set in the upper swing structure  4 ) at which each of the GNSS antennas  21   a  and  21   b  is installed on the upper swing structure  4  (the hydraulic excavator  1 ) and the movable range of the front work implement  3  (the boom  6 , the arm  7 , and the bucket  8 ), the inclination angles of the upper swing structure  4  detected by the posture sensor  20   d,  and the azimuth angle of the upper swing structure  4  computed from the data on the vector from the first GNSS antenna  21   a  to the second GNSS antenna  21   b.  In addition, when there is an existing building or a wall or the like that can become an obstacle, a range for which such an obstacle can cause an adverse effect on the positioning may be added to the satellite mask range  65 . Note that the satellite mask range  65  can be set individually for each of the two GNSS antennas  21   a  and  21   b.  Also note that the satellite mask range  65  defined by an angle of traverse and an angle of elevation as illustrated in  FIG. 6  is merely an example, and that the satellite mask range may be defined by other parameters as long as the satellite mask range is an area that is set with reference to each of the GNSS antennas  21   a  and  21   b.    
     At step S 107 , the satellite mask setting section  48  receives data (the satellite positioning condition data) representing the satellite positioning conditions outputted from the receiver  21   c  in the satellite positioning system  21 . The satellite positioning condition data includes, for example, the number of positioning satellites used for the positioning, the variance of the positions (altitudes) in the heightwise direction among the positions (latitudes, longitudes, and altitudes) computed by satellite positioning, the VDOP (vertical dilution of precision), and the degree of reliability of the positioning via the receiver  21   c.    
     At step S 108 , the satellite mask setting section  48  computes a satellite mask prohibition flag on the basis of the satellite positioning condition data acquired at step S 107 , and determines whether or not the satellite mask range  65  is to be set. Here, the description of  FIG. 7  is suspended again, and details of a process performed by the satellite mask setting section  48  at step S 108  will be described below with reference to  FIG. 9 . 
     (Computation Process for Satellite Mask Prohibition Flag) 
       FIG. 9  illustrates a processing flow concerning a method of calculating a prohibition flag (the satellite mask prohibition flag) for a satellite mask process performed by the satellite mask setting section  48 . Here, the satellite positioning condition data acquired from the receiver  21   c  at step S 107  is used to determine whether or not the satellite mask range  65  is to be set, and when it is determined that the setting of the satellite mask range  65  leads to reduced precision of positioning computation (e.g., the position data on the upper swing structure  4 ) performed by the receiver  21   c,  the setting of the satellite mask range  65  is canceled. The satellite positioning condition data is various types of information concerning a plurality of positioning satellites acquired by the receiver  21   c,  and examples thereof include NMEA messages obtained from the receiver  21   c  for the satellite positioning, for example. Data regarding a result of the positioning computation by the receiver  21   c  can also be included in the satellite positioning condition data. 
     First, at step S 301 , the satellite mask setting section  48  acquires the satellite positioning condition data, which is various types of information concerning the satellite positioning system  21 . The satellite positioning condition data acquired in the present embodiment includes the signal level of each of the positioning satellites used for the positioning computation, the number of positioning satellites from which positioning signals are receivable, the number of positioning satellites the positioning signals of which have been used for the positioning, positioning results and variance thereof, the VDOP (Vertical Dilution of Precision), which is an indicator of the degree of poorness of the arrangement of the positioning satellites, and the degree of reliability of the result of the positioning computation (the degree of reliability of the positioning). 
     At step S 302 , the satellite mask setting section  48  determines whether or not, out of the satellite positioning condition data acquired at step S 301 , the number of positioning satellites used for the positioning is equal to or greater than a predetermined threshold value Ns (for example, Ns=4). The satellite mask process can improve positioning performance by setting the satellite mask range  65  for a range in the field of vision in the sky over the GNSS antennas  21   a  and  21   b  for which the front members  6 ,  7 , and  8  and so on are likely to affect the positioning signals, and excluding any satellites located within the satellite mask range  65  for the positioning. However, masking a satellite when the number of satellites available is small can result in an insufficient number of satellites being used for the positioning computation and in unfavorable conditions for the positioning computation. Accordingly, in the present embodiment, a performance of the satellite mask process is prohibited in such a case. When it is determined in the determination at step S 302  that the number of satellites used for the positioning is smaller than the threshold value Ns, control proceeds to step S 306 , and the satellite mask prohibition flag is set to ON. 
     Meanwhile, when it is determined at step S 302  that the number of satellites used for the positioning is equal to or greater than Ns, control proceeds to step S 303 . 
     Note that a reason that the example threshold value Ns of 4 is assumed here is that, while the minimum number of satellites for computing a position in a three-dimensional space is three, positioning signals from four positioning satellites are required to correct a time difference between timepieces. 
     At step S 303 , using the satellite positioning condition data acquired at step S 301 , the satellite mask setting section  48  determines whether or not the satellite mask range  65  is to be set on the basis of whether or not variation in the positions (altitudes) in the heightwise direction among the positions of the excavator  1  (the upper swing structure  4 ) computed by the receiver  21   c  falls within a predetermined range. To explain a specific method employed in the present embodiment, the satellite mask setting section  48  determines whether or not deviation (deviation of altitude) dh from an average value of altitudes obtained via the satellite positioning over a predetermined period falls within a predetermined range. Here, the satellite mask setting section  48  determines whether or not the following inequality holds: −σ&lt;deviation dh&lt;σ. For example, σ is a standard deviation of the altitudes obtained via the satellite positioning. The standard deviation of the altitudes is the square root of the average (variance of the altitudes) of squared values of differences between the altitudes obtained via the satellite positioning and the average value of the altitudes obtained over the predetermined period. The deviation dh and the standard deviation σ can be computed by the satellite mask setting section  48  on the basis of a time series of positioning results outputted from the receiver  21   c.    
     When it is determined at step S 303  that the variation in the altitudes falls outside of the predetermined range, control proceeds to step S 306 , and the satellite mask prohibition flag is set to ON. 
     Meanwhile, when it is determined at step S 303  that the variation in the altitudes falls within the predetermined range, control proceeds to step S 304 . 
     At step S 304 , using the satellite positioning condition data acquired at step S 301 , the satellite mask setting section  48  determines whether the satellite mask range  65  is to be set on the basis of whether or not the VDOP, computed from the geometrical arrangement of the positioning satellites used for the satellite positioning, is smaller than a predetermined dilution-of-precision threshold value α (for example, α=1). Examples of DOP include PDOP (Position Dilution of Precision) and HDOP (Horizontal Dilution of Precision) in addition to the VDOP, and the above determination can be made on the basis of the PDOP or the HDOP, but it is preferable that the determination is made on the basis of the value of the VDOP. 
     When it is determined at step S 304  that the VDOP is equal to or greater than the dilution-of-precision threshold value α, control proceeds to step S 306 , and the satellite mask prohibition flag is set to ON, because the arrangement of the satellites is poor. Meanwhile, when it is determined at step S 304  that the VDOP is smaller than the dilution-of-precision threshold value α, control proceeds to step S 305 , and the satellite mask prohibition flag is set to OFF. 
     (Process (3) for Setting Satellite Mask Range) 
     The description of  FIG. 7  is resumed. At step S 109 , the controller  100  determines whether or not the satellite mask prohibition flag is OFF in the satellite mask setting section  48 . When it is determined in this determination that the satellite mask prohibition flag is ON (a second case), control proceeds to step S 111 , and the satellite mask setting section  48  resets the setting parameters (e.g., the minimum azimuth angle, the maximum azimuth angle, the minimum elevation angle, and the maximum elevation angle) for the satellite mask range  65  computed at step S 106 , determining that the setting of the satellite mask range  65  leads to reduced precision of the positioning via the receiver  21   c.  The setting of the satellite mask range  65  is thus canceled, allowing the receiver  21   c  to use positioning signals from any satellite for which the front work implement  3  can become an obstacle as well for the positioning. 
     Meanwhile, when it is determined at step S 109  that the satellite mask prohibition flag is OFF (a first case), control proceeds to step S 112 . At step S 112 , the satellite mask setting section  48  outputs the setting parameters for the satellite mask range  65  computed at step S 106  to the receiver  21   c  (the satellite positioning system  21 ), determining that the setting of the satellite mask range  65  leads to an improvement (i.e., not a reduction) in the precision of the positioning via the receiver  21   c.  The satellite mask range  65  based on the setting parameters is thus set, suspending use of positioning signals transmitted from any positioning satellite located within the satellite mask range  65  for the positioning via the receiver  21   c.    
     (Advantages) 
     In the hydraulic excavator according to the present embodiment having the above-described configuration, the satellite mask range  65  can be set to prevent any positioning satellite located in a range for which the front work implement  3  can become an obstacle to positioning signals for each of the GNSS antennas  21   a  and  21   b  from being used for the positioning, and this leads to improved precision in computing the position of a predetermined control point (e.g., the bucket claw tip) set in the hydraulic excavator  1 , which in turn leads to improved precision in controlling the control point and improved precision in construction (the first case). 
     In addition, regarding the hydraulic excavator according to the present embodiment, even when the precision demand flag is ON and a work that demands high precision of the control of the front work implement  3  is being performed, the setting of the satellite mask range  65  will lead to reduced precision of the positioning with high probability in a case where the number of satellites available for use is insufficient, a case where there is great variation in positioning results, or a case where the satellites available for use are unevenly distributed in arrangement (the second case). Therefore, in such a case, the satellite mask prohibition flag is set to ON to cancel the setting of the satellite mask range  65 . This increases the number of satellites that can be used for the positioning, making it possible to make the precision of the positioning higher than in a case where the satellite mask range continues to be set. 
     In particular, in the present embodiment, the satellite mask prohibition flag is set with focus placed on changes in parameters concerning the heightwise direction (vertical direction), such as the VDOP and the variation in the altitudes out of the positioning results. When the hydraulic excavator  1  is performing a work that demands high precision, the hydraulic excavator  1  does not normally move in the heightwise direction, and the changes in the parameters concerning the heightwise direction are limited. Therefore, determining the ON/OFF of the satellite mask prohibition flag with focus placed on the parameters concerning the heightwise direction can achieve higher reliability than in a case where a parameter concerning a horizontal direction is used for the same determination. 
     (Others) 
     Note that the present invention is not limited to the above-described embodiment, and that a variety of modifications are included in the scope of the invention without departing from the gist thereof. For example, the present invention is not limited to embodiments having all of the above-described features of the above-described embodiment, but encompasses embodiments that do not have some of the features. Also note that some features of an embodiment may be added to or replace features of another embodiment. 
     In the flow of  FIG. 9 , the satellite mask prohibition flag is set to ON or OFF on the basis of three of the number of satellites (step S 302 ), the variation in the altitudes (step S 303 ), and the VDOP (step S 304 ), but it is to be understood that the satellite mask prohibition flag may be set to ON or OFF on the basis of at least one of the above three. For example, in the flow of  FIG. 9 , the satellite mask prohibition flag is set to OFF when the number of satellites is equal to or greater than the predetermined threshold value, the variation in the altitudes falls within the predetermined range, and the VDOP is smaller than the predetermined threshold value, but it is to be understood that the satellite mask prohibition flag may be set to OFF when at least one of the above three conditions is satisfied. Also note that the order in which the determinations as to three of the number of satellites (step S 302 ), the variation in the altitudes (step S 303 ), and the VDOP (step S 304 ) are made is not limited to the order illustrated in the flow of  FIG. 9 , but may be any order. 
     The determination made at step S 303  may not be only as to whether or not the variation in the altitudes falls within the predetermined range as illustrated in  FIG. 9 , but the latitudes and the longitudes, which are computed by the receiver  21   c  as are the altitudes, may also be used for the determination. In this case, the determination may be made on the basis of whether or not the variation in at least one of the altitudes, the latitudes, and the longitudes falls within a predetermined range. Although, in the foregoing description, it is determined whether the deviation from the average value of the positions (altitudes) obtained via the satellite positioning over the predetermined period falls within the range of ±σ (σ=standard deviation), it may be determined whether or not the deviation falls within a range defined by actual numerical values, e.g., ±β [mm], instead of the standard deviation. A specific example of ±β is ±20 [mm]. This is because it is preferable that the variation falls within this numerical range when a machine error and errors of other posture sensors are taken into account, while, in machine guidance or machine control, precision is demanded at the bucket located at a distal end of the front work implement. 
     At step S 304 , it may be determined whether or not not only the VDOP illustrated in  FIG. 9  but also each of the HDOP and the DOP is smaller than a predetermined dilution-of-precision threshold value. 
     Also note that the components of the above-described controller  100 , the functions and processes performed by the components, and so on may be implemented partially or entirely in hardware (for example, logic implementing each function may be designed on an integrated circuit). Also note that the components of the controller  100  may be implemented by a program (software) that is loaded and executed by the arithmetic processing device (e.g., the CPU) to realize the functions of the components of the controller  100 . Information related to the program can be stored in, for example, a semiconductor memory (a flash memory, an SSD, or the like), a magnetic storage device (a hard disk drive or the like), a recording medium (a magnetic disk, an optical disk, or the like), or the like. 
     In addition, in the description of each of the embodiments described above, control lines and information lines that are considered to be necessary for explaining the embodiment have been presented, but it is to be understood that all control lines and information lines related to the product may not have been presented. In practice, almost all the components may be considered to be interconnected. 
     DESCRIPTION OF REFERENCE CHARACTERS 
     
         
           1 : Hydraulic excavator 
           3 : Front work implement 
           4 : Upper swing structure 
           5 : Lower travel structure 
           6 : Boom 
           7 : Arm 
           8 : Bucket 
           9 : Boom cylinder 
           10 : Arm cylinder 
           11 : Bucket cylinder 
           12 : Cab 
           13 : Hydraulic swing motor 
           14   a:  Crawler belt 
           14   b:  Crawler belt 
           15   a:  Hydraulic travel motor 
           15   b:  Hydraulic travel motor 
           16 : Swing ring 
           17 : Operation sensor 
           18 : Operation lever 
           19 : Monitor (display device) 
           20   a  to  20   d:  Posture sensor (IMU) 
           21 : Satellite positioning system 
           21   a:  First GNSS antenna 
           21   b:  Second GNSS antenna 
           21   c:  Receiver 
           23 : Communication device 
           24 : Construction target surface setting device 
           25 : Storage device 
           26 : Control mode selection switch 
           30 : Wireless equipment 
           40 : Position data detection section 
           41 : Posture computation section 
           42 : Excavator posture computation section 
           43 : Abnormal state determination section 
           44 : Construction target surface computation section 
           45 : Hydraulic system control section 
           46 : Monitor display control section 
           47 : Precision demand determination section 
           48 : Satellite mask setting section 
           49 : Data storage section 
           60 : Covered area 
           65 : Satellite mask range 
           100 : Controller 
           102 : External management server