There is known a hydraulic excavator as a typical one of such construction machines. In a hydraulic excavator, front members such as a boom and an arm, which constitute a front device, are operated by respective manual control levers. However, because the front members are coupled to each other through articulations for pivoting motion, it is very difficult to carry out excavation work over a predetermined area, particularly an area set by linear lines, by operating the front members. For this reason, there is a demand for enabling such work to be performed in an automatic manner. Various proposals for automating such work have been made.
According to International Laid-open Publication WO95/30059, for example, an excavation enable area is set on the basis of a body as a reference, and excavation is controlled such that when part of a front device, e.g., a bucket, comes close to the boundary of the excavation enable area, only movement of the bucket toward the boundary is slowed down, and when the bucket reaches the boundary of the excavation enable area, the bucket is allowed to move along the boundary of the excavation enable area while it is kept from moving out of the excavation enable area.
When a hydraulic excavator is designed to perform the above-mentioned work in an automatic manner, the posture and height of the hydraulic excavator itself are varied due to change in topography of the work site if a body of the excavator is moved. This means that the area set with respect to the body must be set again whenever the body is moved. In view of the above, JP, A, 3-295933 proposes an automatic excavation method for overcoming that drawback. The proposed automatic excavation method comprises the steps of detecting a height of an excavator body by a sensor, which is mounted on the body, using a laser beam from a laser oscillator installed on the ground to be excavated, determining an excavation depth (corresponding to the limited area in the above related art) based on the detected height of the body, excavating the ground linearly over a predetermined length while the body is kept stopped, then traveling the body by a predetermined distance, detecting change in height of the body by using the laser beam before excavating the ground linearly again while the body is kept stopped, and modifying the excavation depth in accordance with the detected change in the body height.
Also, U.S. Pat. No. 4,829,418 proposes another automatic excavation method in which the excavation depth is modified by using a laser beam. This proposed automatic excavation method comprises the steps of setting a desired excavation depth (HTTRGT) with a laser beam as a basis, mounting a laser sensor on an arm, calculating a distance (HTACT) from the laser beam to a bucket prong of a front device at the moment the laser sensor detects the laser beam during excavation, and controlling associated actuators in accordance with a result of comparison between HTTRGT and HTACT so that the bucket prong is moved near the desired excavation depth.