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Die cushion control deviceRelated Patent Categories: Metal Deforming, By Use Of Closed-die And Coacting Work-forcer (e.g., Push-drawing), Cup Or Shell Drawing (i.e., Deep Drawing), With Application Of Frictional Restraining Force To Work During Deformation, With Yieldable Actuation Of Work-gripper (e.g., Spring-biased Blank-holder)Die cushion control device description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20060169018, Die cushion control device. Brief Patent Description - Full Patent Description - Patent Application Claims
BACKGROUND OF THE INVENTION [0001] 1. Field of the Invention [0002] The present invention relates to a die cushion control device for performing a die cushion control in a press machine. [0003] 2. Description of Related Art [0004] In a press machine, when sheet metal is pressed between a set of dies by relative motion of the set of dies along a press axis, a die cushion control is performed in order to prevent the sheet metal from being applied with abrupt pressure and to moderate a change of pressure applied to the sheet metal. [0005] A die cushion control device of the press machine conventionally performs hydraulic or pneumatic pressure control. An upper die movably provided by a press axis driving device moves downward to hit the sheet metal, and subsequently, the die cushion control device implements pressure control. In the case of die cushion control using hydraulic or pneumatic pressure, however, there is time lag between provision of a control signal and a response. For this reason, it is difficult to carry out the control in such a way that the sheet metal does not receive surge pressure that is created by impact at the beginning of the press. [0006] In order to resolve this problem, a die cushion device that has a servomotor for elevating and lowering a die cushion member and is used while switching between the position control of stroke of the cushion member and the pressure control performed by current torque control by means of the servomotor is proposed (see JP 10-202327A). [0007] In the case of a die cushion device that carries out the drive of the die cushion member by the servomotor and implements the control while switching between the position control of stroke of the cushion member and the pressure control as in the invention described in JP 10-202327A, the pressure control is performed as the sheet metal is pinched between the die cushion member and upper die driven by the press axis driving device, and in the other periods, the position control is carried out. The position control is performed to drive-control the servomotor so as to follow a commanded position. When the die cushion member is applied with great force, the servomotor produces great torque for the purpose of maintaining the commanded position and acts to maintain the position. [0008] On this account, during die change or during position control as in adjustment of the press machine and the like, if the upper die driven by the press axis driving device hits the die cushion member, or if the press axis is driven with a foreign matter pinched between the die cushion member and the upper die fixed to the press axis to apply great force to the die cushion member and the like, great impact force is created, which does a lot of damage to the die cushion member and the die in some cases. [0009] As a die cushion control device, there is another known device in which the motion of the die cushion member is started at a slower velocity than the motion velocity of the upper die driven by the press axis driving device in the direction drawing away from the upper die before the upper die is brought into contact with the die cushion member, and the die cushion member is so controlled as to hit the upper die during the motion by difference of relative motion velocities, to thereby achieve the reduction of hitting noise made in the event of a collision or the like. [0010] In case that such control is implemented by the die cushion control device, if the press hits the die cushion for another reason, such as erroneous setting and wrong operation, and position control is carried out while the control is not switched to pressure control, the commanded position of the die cushion and the actual position thereof deviate from each other. The servomotor then generates great torque in order to maintain the commanded position, which causes the problem that there occurs a mechanical damage such as damage to the die cushion member. SUMMARY OF THE INVENTION [0011] A die cushion control device of the present invention is provided for a press machine that presses metal between a set of dies relatively movable along a press axis while controlling a pressure exerted on the sheet metal. The die cushion control device comprises: a die cushion member for cushioning the pressure exerted on the sheet metal by the relative motion of the set of dies; a servomotor for moving the die cushion member in a direction of the press axis; and a controller for controlling the servomotor by selectively performing one of a position control and a pressure control of the die cushion member, and having torque limit means for effecting a torque limit on the servomotor during the position control of the die cushion member. [0012] The torque limit means may apply a torque limit to a torque command for the servomotor to move the die cushion member in a direction that the set of dies press the sheet metal, and does not apply the torque limit to a torque command for the servomotor to move the die cushion member in a direction opposite to the direction that the set of dies press the sheet metal. [0013] Further, the toque limit means may apply a first torque limit to a torque command for the servomotor to move the die cushion member in a direction that the set of dies press the sheet metal, and apply a second torque limit of a value different from a value of the first torque limit to a torque command for the servomotor to move the die cushion member in a direction opposite to the direction that the set of dies press the sheet metal. [0014] When the die cushion members are under position control, the servomotor for driving the die cushion members is provided with torque limits, and the servomotor does not output torque of a predetermined value or more. Therefore, even if the upper die hits the die cushion members due to malfunctions or wrong operation, the die cushion members do not generate drag equal to or more than a torque limit value. This prevents damage to the die fixed to the press axis, the die cushion members, etc. BRIEF DESCRIPTION OF THE DRAWINGS [0015] FIG. 1 is a schematic view of a press machine in which a die cushion control device of a first embodiment of the present invention is applied; [0016] FIG. 2 is a block diagram of the die cushion control device in the first embodiment; [0017] FIG. 3 is a view for explaining a collision between die cushion members and an upper die in a position control state of the first embodiment; [0018] FIG. 4 is a schematic view of a press machine in which a die cushion control device of a second embodiment of the present invention is applied; and [0019] FIG. 5 is a view for explaining a switch from position control to pressure control in the second embodiment. DETAILED DESCRIPTION [0020] FIG. 1 is a schematic view of a press machine in which a die cushion control device of a first embodiment of the present invention is applied. A lower die 2 of a set of dies is fixed to a base of the press machine, and an upper die 1 is driven by a press axis driving device (not shown), so as to face the lower die 2. The press axis driving device may be constituted by a crank mechanism, by a hydraulic or pneumatic pressure mechanism, or by a position control mechanism using a servomotor, thereby driving the upper die 1 according to a predetermined pattern and moving the same upward and downward in the figure. Continue reading about Die cushion control device... Full patent description for Die cushion control device Brief Patent Description - Full Patent Description - Patent Application Claims Click on the above for other options relating to this Die cushion control device patent application. ###  How KEYWORD MONITOR works... a FREE service from FreshPatents1. Sign up (takes 30 seconds). 2. Fill in the keywords to be monitored. 3. Each week you receive an email with patent applications related to your keywords. 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