Patent ID: 8434301

Claim:
A hydraulic control system comprising: a hydraulic actuator having a cylinder and piston assembly; first and second fluid lines coupled to the hydraulic actuator; a high pressure fluid line; a low pressure fluid line; a central pressure source coupled to the high pressure fluid line to place high pressure on the first and second fluid lines; a servo valve positioned between the central pressure source and the hydraulic actuator and being controllable to selectively connect the first and second fluid lines to the high pressure fluid line and the low pressure fluid line to control movement of the hydraulic actuator; a variable displacement pump adjacent to the servo valve, the variable displacement pump being coupled to the high pressure fluid line upstream of the servo valve to place pressure on the high pressure fluid line; an induction motor connected to the variable displacement pump to drive the variable displacement pump; a pump mechanism controlling a pressure from the variable displacement pump by changing a fluid displacement of the variable displacement pump, the pump mechanism placing the pressure from the variable displacement pump on the high pressure fluid line; a first check valve in the high pressure fluid line adjacent to the servo valve, the first check valve preventing the high pressure fluid from entering from the variable displacement pump to the central pressure source; a second check valve adjacent to the variable displacement pump, the second check valve preventing the high pressure fluid from entering from the central pressure source to the variable displacement pump; and a controller which performs (i) a sequential-power mode in which, when the pressure from the central pressure source is lost, supply of the pressure to the high pressure fluid line is maintained by placing pressure from the variable displacement pump to the high pressure fluid line by activating the induction motor and (ii) a parallel-power mode in which, when it is required to switch the pressure source quicker than in the sequential-power mode, the induction motor is activated in advance for a failure of the central pressure source so that the pressure from the central pressure source and the pressure from the variable displacement pump are simultaneously supplied to the high pressure fluid line; the pump mechanism including a starting valve which returns the pressure from the variable displacement pump to the variable displacement pump until the pressure from the variable displacement pump reaches a predetermined pressure, and which applies the pressure from the variable displacement pump to the high pressure fluid line after the pressure from the variable displacement pump reaches the predetermined pressure.