A hydraulic excavator has a plurality of working units such as a boom, an arm, a bucket, a swing mechanism and a travel mechanism. These units are operated by an operator who manipulates two operation levers in combination. The type of work to be done is different depending on the situation of a work site, and the order of priority of using these working units varies depending on the type of work. However, when a heavy load is applied to a working unit which is desired to be operated with priority over other working units, the operator has to manipulate the two control levers in a complicated way to decrease the load as well as to move that working unit with priority over the other working units. This is to lay an excessive burden upon the operator and requires him to be skillful. In view of the above circumstances, the applicant of this invention has filed a patent application (Japanese Patent Application No. 19769/1987) for a control device which adds a weight to a command value for a working unit which is desired to be moved with priority over other working units so that, even by naturally manipulating the levers, it can be moved faster than when being moved by a normal manipulation. The invention of this application has been put into practice.
However, the above-mentioned control device is intended to control a swash plate inclination angle of a hydraulic pump in addition to a valve opening degree of a flow rate control valve for controlling a flow rate of pressure oil supplied to hydraulic actuators such as hydraulic cylinders to drive the working units, and to comprehensively control the above-mentioned angle and opening degree depending on the priority order of the working units. In the case of this type of control device, it is necessary to decide at a stage of designing a hydraulic circuit that the control device is built in it and to design an exclusive hydraulic circuit. Therefore, the conventional control device cannot be installed in the existing hydraulic circuits at a later stage by way of so-called "after-installation".
Accordingly, there have been demanded for a cost reduction purpose a simply structured device which can be added to the existing hydraulic circuits so as to be able to control working units according to the order of priority.