Control system for mine roof supports

Claim 1

IN A MINE ROOF SUPPORT UNIT HAVING HYDRAULIC PROPPING AND SHIFTING DEVICES, A CONTROL SYSTEM COMPRISING: VALVE MEANS INTERPOSED BETWEEN A HYDRAULIC POWER SOURCE AND THE PROPPING AND SHIFTING DEVICES, SAID VALVE MEANS INCLIUDING TWO ELECTROMAGNETICALLY ACTUATED TWO-POSITION VALVES TOGETHER PROVIDING A MAXIMUM OF FOUR OPERATIVE COMBINATIONS OF VALVE POSITIONS, IN EACH OF WHICH OPERATIVE COMBINATIONS SAID VALVE MEANS IS ARRANGED TO EFFECT ONE OF A CORRESPONDING NUMBER OF DIFFERENT COMBINED OPERATIONS OF THE PROPPING DEVICES AND SHIFTING DEVICES; FIRST AND SECOND SOLENOIDS COUPLED WITH SAID VALVES AND ADAPTED FOR BOTH TO BE DEENERGIZED, ONE OR THE OTHER ENERGIZED SINGLY, OR BOTH ENERGIZED, TO PROVIDE THE SAID DIFFERENT OPERATIVE COMBINATIONS OF VALVE POSITIONS; A CONTROL CIRCUIT INCLUDING TWO CONDUCTORS ACROSS WHICH SAID SOLENOIDS ARE CONNECTED, CURRENT RECTIFYING MEANS ASSOCIATED WITH SAID SOLENOIDS TO CAUSE ENERGIZATION OF ONE OR OTHER OF SAID SOLENOIDS FOR OPPOSITE DIRECTIONS OF CURRENT FLOW RESPECTIVELY THROUGH THE TWO CONDUCTORS, AN ALTERNATING CURRENT SOURCE, SWITCH MEANS ARRANGED TO CONNECT THE ALTERNATING CURRENT SOURCE ACROSS SAID TWO CONDUCTORS, AND ADDITIONAL RECTIFYING MEANS ASSOCIATED WITH SAID SWITCH MEANS WHEREBY SAID SWITCH MEANS IS OPERABLE TO ISOLATE SAID ALTERNATING CURRENT SOURCE FROM SAID TWO CONDUCTORS, OR TO DIRECT ELECTRIC CURRENT THROUGH THE TWO CONDUCTORS IN ONE DIRECTION, ANOTHER DIRECTION, OR IN BOTH DIRECTIONS.