Method of and apparatus for automatically controlling pressure in holding furnace incorporated in low pressure die-casting system

An apparatus for automatically controlling the pressure in a holding furnace incorporated in a low pressure die-casting system, comprising:

• a pressurized gas source;
• a pressure reducing valve;
• a proportional pressure control valve and a surge tank which are connected in that order between said pressurized gas source and said holding furnace so as to introduce the gas from said pressurized gas source to the interior of said holding furnace;
• a first pressure sensor for detecting the inside pressure (Pm) of said holding furnace;
• a second pressure sensor for detecting the inside pressure (Ps) of said surge tank;
• a microcomputer for controlling said proportional pressure control valve;
• a first A/D converter for A/D-converting the output of said first pressure sensor and inputting the result to said microcomputer;
• a second A/D converter for A/D converting the output of said second pressure sensor and inputting the result to said microcomputer;
• a D/A converter for D/A converting the output of said microcomputer and sending the same to said proportional pressure control valve, wherein said microcomputer stores a desired pressurization pattern of the time--pressure curve associated with said holding furnace, as well as steady values of the output pressure which are associated with the input command values of said proportional pressure control valve, reads out a desired pressure P.sub.i+1 at a time t.sub.i+1 which is a unit of time ahead of a time t.sub.1, reads out an input command value Vo of said proportional pressure control valve which corresponds to said desired pressure P.sub.i+1, calculates a pressure deviation P from said desired pressure P.sub.i+1 and said pressure Pm in said holding furnace which has been sent from said first pressure sensor, calculates a pressure deviation P, between the pressure Ps in said surge tank and the pressure Pm in said holding furnace, and sends to said proportional pressure control valve an input command value V which is obtained by adding to said input command value Vo a correction value Vd obtained by multiplying said pressure deviation P by a gain G set in accordance with said pressure deviation (P.sub.1).

An apparatus for automatically controlling the pressure in a holding furnace incorporated in a low pressure die-casting system, comprising:

• a pressurized gas source, a first supply including a pressure reducing valve and a proportional pressure control valve which are connected in that order between said pressurized gas source and said holding furnace so as to introduce the gas from said pressurized gas source to the interior of said holding furnace;
• a second supply including a second pressure reducing valve, a solenoid valve and a flow rate control valve which are connected in that order between said pressurized gas source and said holding furnace for supplying gas to the interior of said holding furnace separately from said first supply;
• a pressure sensor for detecting the inside pressure of said holding furnace;
• a microcomputer for controlling said proportional pressure control valve and said solenoid valve;
• an A/D converter for A/D-converting the output of said pressure sensor and inputting the same to said microcomputer;
• a D/A converter for D/A converting the output of said microcomputer and sending the same to said proportional pressure control valve, wherein said microcomputer stores a desired pressurization pattern of the time--pressure curve associated with said holding furnace, as well as steady values of the output pressure which are associated with the input command values of said proportional pressure control valve, reads out a desired pressure P.sub.i+1 at a time t.sub.i+1 which is a unit of time ahead of a time t.sub.i, reads out an input command value Vo of said proportional pressure control valve which corresponds to said desired pressure P.sub.i+1, calculates a pressure deviation P from said desired pressure P.sub.i+1 and said pressure Pm in said holding furnace which has been sent from said pressure sensor, and sends to said proportional pressure control valve an input command value V which is obtained by adding to said input command value Vo a correction value Vd obtained by multiplying said pressure deviation P by a present gain G;
• and sends to said solenoid valve a signal for opening said solenoid valve at a time in said desired pressurization pattern which corresponds to an inflection point J+1 where the rate at which the pressure is raised increases with respect to time, and a signal for closing said solenoid valve before the time corresponding to an inflection point J+2 where the rate at which the pressure is raised decreases.