Control system for hydraulic circuit apparatus

A control system for a hydraulic circuit apparatus including at least first and second hydraulic pumps of a variable displacement type, a first hydraulic actuator arranged for hydraulic connection with said first pump through a first valve means to be driven thereby, a second hydraulic actuator arranged for respective selective hydraulic connection with said first and second pumps through second and third valve means, an order of priority for the hydrualic connection being set beforehand in such a manner that when an operation signal for the second hydraulic actuator is received while the first pump is inoperative, the first pump takes priority over the second pump for hydraulic connection with the second actuator and, when an operation signal for the first actuator is received whle the first pump is in hydraulic connection with the second actuator, the first actuator takes priority over the second actuator for hydraulic connection with the first pump and the second actuator is brought into hydraulic connection with the second pump, the displacement vcolume of the first pump and switching of the second valve means are controlled in such a manner that, when the first pump which is in hydraulic connection with the second actuator is to be brought into hydraulic connection with the first actuator, the displacement volume of the first pump is returned to zero before changing the hydraulic connection, the control system comprising:

• first means for judging whether the first pump is in hyraulic connection with the second actuator when an operation signal for the first actuator is received, and for generating a command for backing up a reduction in a supply of hydraulic fluid in the second actuator simultaneously when the displacement volume of the first pump begins to return to zero, when it is judged that the first pump is in hydraulic connection with the second actuator;
• second means for generating a command for switching the third valve means to an open position in accordance with the backup command from the first means;
• third means for generating a command for initiating a displacement of the second pump in accordance with the backup command from the first means;
• and
• fourth means for generating a command in accordance with the backup command from the first means for rendering an abosolute valve of a rate of change in a displacement volume of the first pump upon returning to zero and the absolute value of a rate of change of the displacement volume of the second pump after a starting of its displacement substantially equal to each other and larger than a predetermined maximum value of rates of change in the displacement volume of said first and second pumps during normal operation thereof, said fourth means includes first and second means for generating preset maximum rates of change in the displacement volume of said first and second pumps during normal operation thereof, respectively, third means for generating preset rates of change in the displacement volume of the first and second pumps during a back-up operation thereof larger than the preset maximum rates of change during normal operation, means for selecting the preset rates of change generated by the third means as maximum rates of change in the displacement volume for the first and second pumps in the presence of the backup command from the first means, and means for inverting one of the selected preset rates to take a negative value.