Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
  • B21D—WORKING OR PROCESSING OF SHEET METAL OR METAL TUBES, RODS OR PROFILES WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
  • B21D24/00—Special deep-drawing arrangements in, or in connection with, presses
  • B21D24/10—Devices controlling or operating blank holders independently, or in conjunction with dies
  • B21D24/14—Devices controlling or operating blank holders independently, or in conjunction with dies pneumatically or hydraulically

Definitions

• the invention relates to a device for controlling and regulating the pressure in hydraulic die cushions of presses according to the preamble of patent claim 1.
• the displacement principle In hydraulic die cushions in presses, the displacement principle often occurs, in which the press ram drives the pistons of the die cushion cylinders and displaces the oil from the cylinder chambers.
• a proportional valve arranged in the discharge line influences the discharge cross-section in such a way that the desired pressure profiles in the cylinder are established during the drawing process.
• the current pressure in the die cushion cylinder is recorded by means of a pressure sensor and fed to a controller as an actual signal.
• the controller forms a corresponding correction signal for controlling the proportional valve from the comparison of the nominal and actual signals.
• the impact of the tappet on the extended pistons of the die cushion cylinders is regarded as a critical moment during the regulation, since the discharge volume flow suddenly changes from zero to its maximum value.
• the main control piston of the proportional valve has to be moved into the open position at high speed in order to avoid undesirable pressure peaks.
• the disturbing influences on the regulation of the die cushion pressure are comparatively low, so that generally simple control concepts (PI controllers) already ensure good results.
• the ram speed is gradually reduced to zero.
• the die cushion pressure should correspond to the specified courses.
• a circuit arrangement for regulating the holding force is known from EP 0 330 718 (closest prior art), which a suitable way Proportional valve actuated.
• a number of coordinated control measures result in a reduction in the pressure peaks when the tappet hits the piston rods of the die cushion cylinders.
• This circuit arrangement requires proportional valves with high control speeds.
• Another assessment criterion is the effort required to commission the control loop and the options for influencing its behavior by the maintenance staff. In this case, special knowledge of maintenance personnel is required.
• the pressure reducing valve is located in the inlet line and the pressure relief valve is located in the outlet line. Both valves are controlled by a controller using a specific procedure.
• the object of the invention is therefore to design a device for controlling and regulating the pressure in such a way that the overshoot of the pressure when the ram strikes is reduced with robust methods that can be understood by the operating personnel, and the pressure build-up path of the die cushion is also minimized.
• the object is achieved by the features of claim 1 and claim 3.
• the advantages of the invention are that the proportional valve for regulating the die cushion pressure is acted upon by a defined back pressure at its tank connection. This back pressure is chosen so large that the mass forces acting on the cylinder pistons are compensated, even if the proportional valve completely releases the drain.
• Another feature of the invention consists in the now possible way of controlling the proportional valve. In principle, it is assumed that the proportional valve is preferably controlled in the first phase of the drawing process (pressure build-up in the cylinder) and the control of the proportional valve starts in the second phase (after pressure build-up has taken place). The first, controlled mode of operation is the "pre-opening" of the proportional valve.
• the main control piston Before the plunger hits the displacement piston, the main control piston is positioned on a predefined, defined opening gap.
• the size of the pre-opening results from the speed of impact of the tappet on the die cushion and the specified pressure setpoint for the start of control.
• This pre-opening value is determined once empirically or simulatively and stored, for example, in a table within a controller. With the help of this valve pre-opening it is achieved that the main piston of the proportional valve has already covered a certain distance in the opening direction and when the plunger hits, only the difference to the maximum value has to be overcome.
• the pre-opening value is generally smaller than the maximum required valve opening, which is necessary for setting the desired pressure in the volume flow caused by the impact speed, in order to enable the pressure in the cylinder to build up in a short time.
• the rate of pressure rise is therefore dependent on the selected pre-opening value and the pressure setpoint.
• a compromise between the shortest possible pressure build-up path and the maximum permissible overshoot can be regarded as the optimum.
• the transition from the controlled mode of operation to the regulated mode takes place towards the end of the build-up of pressure.
• the criterion for the switchover is derived from the process itself if no path measuring system is available for the detection of the die cushion path.
• the switchover takes place when the difference between the pressure setpoint and actual pressure value falls below a predetermined amount. This ensures that the excitation of the control loop is reduced when switching.
• Another method for the combination of control and regulation is described in the embodiment. After the tappet has hit the die cushion, the expected disturbances are relatively small, so that a generally known PID control algorithm is sufficient for pressure control. It is also recommended to make the D component of the controller switchable. An additional minimization of the pressure build-up path can be achieved while maintaining the valve pre-opening in that the die cushion cylinder is acted upon by a further pressure oil source.
• the press ram 1 strikes with the upper part 2 of the die 3, which rests on the sheet metal holder 4 of the lower part 5 of the tool.
• the sheet metal holder 4 holds the circuit board 3 at a distance " ⁇ s", which corresponds to the pressure build-up path, above the lower part 5 of the die.
• the die cushion force required for the forming process is applied to the die plate by pressure-actuated die cushion cylinder 11 via its piston rod on the cushion plate 8 and via pressure bolts 7 4 applied and counteracts this in the downward movement of the plunger 1.
• the die cushion cylinders 11 provided for the generation of force and a central cylinder 9 for controlling the position of the die cushion for its upward motion are fastened in the press table 6.
• the hydraulic control 10 for generating pressure is each assigned to a die cushion cylinder 11 in the form of a control block.
• a die cushion cylinder 11 For external prestressing of the die cushion cylinder 11, its piston rod must be fixed in its upper working position by a stop 29, which is fixed or adjustable.
• one or more drawing cushion cylinders 11 are respectively connected on the discharge side, in special cases not shown together, via a line 28 to a proportional valve 12.
• the respective actual value of the die cushion pressure is recorded via pressure transducer 17 and fed to the cushion force regulator 23.
• the proportional valve 12 is connected on the discharge side via the continued line 28 to the prestressed tank 13 which is pressurized with compressed air from the pressure source 14.
• the hydraulic oil flows only then from the die cushion cylinder 11 via the opened proportional valve 12 into the prestressed tank 13 when the press ram 1 moves the pistons of the die cushion cylinder 11 downward.
• the hydraulic oil flows back into the die cushion cylinder 11 via the check valve 15.
• Another check valve 18 separates the die cushion cylinder 11 from a switchable pressure source 20 in such a way that when the switching valve 19 is activated, hydraulic oil flows from the pressure source 20 into the die cushion cylinder 11 as long as there is a pressure difference.
• the pressure in the pressure source 20 is adjusted by means of a pressure-controlled pump 22 to the target pressure of the die cushion in such a way that the storage pressure is slightly lower.
• a higher-level controller 24 transfers the current actual values with respect to the cushion travel "s K ", the number of press strokes “n” and the crank angle “ ⁇ ” to the cushion force controller 23.
• the cushion force controller 23 of the set pressure “p to” transmits a threshold pressure “p SW” the drawing depth "s Z” and.
• the cushion force controller 23 contains a generally known PID controller with switchable I and D components and a resettable I component. The PID controller itself can also be switched. All switching functions of the cushion force controller 23 are triggered by an internal control unit 25 as a function of the cushion travel "s K ".
• the cushion force controller 23 receives the actual pressure value "p ist " directly from a pressure sensor 17.
• the pressure setpoint "p soll” is fed to the PID controller via a reference variable generator 26 to form the control deviation "x d ".
• the command variable generator 26 changes the pressure value "p set” depending on the Kissenweg “s K” such that it is increased in the “passive” pressure build-up phase of a reduced proportion of more than a defined distance up to its original value.
• the controller output is the Vorö réelleswert summed “to p” which in a module 27 in response to the Kissenweg “s K", the pad speed “ds K (t) / dt", the draw depth “s Z”, the pressure command value, the number of press “n” and the crank angle " ⁇ ” calculated the pre-opening value and activated during the pressure build-up.
• a pressure threshold value "p SW” is transmitted to the module 27, which is used when the die cushion does not have its own measuring system.
• the pre-opening value is then deactivated as a function of the pressure threshold "p SW ".
• the command variable "w” and the pre-opening value are determined as a function of the number of press strokes.
• the cushion force controller 23 delivers the switching signal to a valve 19 by comparing a reference angle " ⁇ ref " transferred by the control 24 with the current value of the crank angle " ⁇ ".

Landscapes

• Engineering & Computer Science (AREA)
• Mechanical Engineering (AREA)
• Presses And Accessory Devices Thereof (AREA)
• Shaping Metal By Deep-Drawing, Or The Like (AREA)
• Braking Arrangements (AREA)
• Paper (AREA)

Applications Claiming Priority (2)

Publications (2)

Family

ID=7776963

Family Applications (1)

Country Status (3)

Families Citing this family (4)

Family Cites Families (4)

• 1995
  • 1995-11-09 DE DE19541693A patent/DE19541693A1/de not_active Withdrawn
• 1996
  • 1996-11-05 EP EP96117676A patent/EP0773076B1/de not_active Expired - Lifetime
  • 1996-11-05 ES ES96117676T patent/ES2145362T3/es not_active Expired - Lifetime
  • 1996-11-05 DE DE59604877T patent/DE59604877D1/de not_active Expired - Lifetime

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