Patent Publication Number: US-7905126-B2

Title: Die cushion controller of press machine

Description:
This application is a U.S. National Phase Application under 35 USC 371 of International Application PCT/JP2007/053476 filed Feb. 26, 2007. 
     TECHNICAL FIELD 
     The present invention relates to a die-cushion controlling device for a press machine used for drawing or the like, the die-cushion controlling device controlling operations of a die-cushion pad in synchronization with operations of a slide. 
     BACKGROUND ART 
     As a die-cushion controlling device for controlling elevating operations of a die-cushion pad driven by a servo motor, a die-cushion controlling device proposed in, for instance, Patent Document 1 has been conventionally known. According to the die-cushion controlling device disclosed in Patent Document 1, a cushion stroke of a die cushion is controlled by a position control until an upper die of a slide contacts a die-cushion pad with a workpiece sandwiched therebetween. On detecting a change in a current of a servo motor when loading starts to be applied on the die-cushion pad, the control is switched from the position control to a pressure control by a detection signal of the current change, so that a preset cushion pressure is applied on the die-cushion pad. Since such a die-cushion controlling device as arranged above can switch the control from the position control to the pressure control, drawing can be favorably conducted. 
     In a die-cushion controlling device in which a position control and a pressure control are switchable, a control is switched from the pressure control to the position control again when a slide has reached a bottom dead center, so that a die-cushion pad is moved up within a predetermined time after an upward movement of the slide is started so as to return to a stand-by position for the next processing. In addition, when the slide has reached the bottom dead center position, the die-cushion pad is controlled to stop at its own bottom dead center position for a predetermined time (bottom dead center locking) in order to assure that a drawing is reliably conducted to the end. 
     [Patent Document 1] JP-A-10-202327 (page 3) 
     DISCLOSURE OF THE INVENTION 
     Problems to Be Solved by the Invention 
     While the die cushion is being applied with loading as in the middle of processing, a compressive force is being applied on the die-cushion pad and the like, thereby elastically deforming the die-cushion pad and the like. Thus, the die-cushion pad stops at the bottom dead center with a shrinkage being caused therein. Thus, when the bottom dead center locking is conducted in the above state, a workpiece bounces therefrom as soon as the loading applied on the die cushion is removed due to the upward movement of the slide, and the workpiece cannot be reliably held thereon. In particular, when a hydraulic chamber, a spring or the like for mitigating an impact caused at the time of a contact of the upper die with the workpiece is provided as the die cushion, an amount by which the die cushion is elastically deformed is so large that an amount by which the workpiece bounces therefrom is considerably increased. A solution for the problem has been demanded. 
     An object of the present invention is to provide a die-cushion controlling device capable of reliably preventing a workpiece from bouncing when a bottom dead center locking is conducted. 
     Means for Solving the Problem 
     A die-cushion controlling device for a press machine according to an aspect of the present invention includes: a position-command-signal outputting section that outputs a position command signal corresponding to a target position value of a die-cushion pad; a position detecting means that detects a position of the die-cushion pad; a position comparing section that outputs a position deviation signal corresponding to a deviation between the target position value based on the position command signal and a position detection value based on a position detection signal from the position detecting means; a position controlling section that outputs a position-speed command signal based on the position deviation signal; a speed controlling section that outputs a motor-current command signal based on the position-speed command signal from the position controlling section; a servo amplifier that feeds an electric servo motor for driving a die cushion with a current corresponding to the motor-current command signal; and an escape-position-command-signal outputting section that outputs a position command signal for commanding the die-cushion pad to downwardly escape from a bottom dead center position by a predetermined escape height based on a press signal outputted when a slide of the press machine has reached a bottom dead center position. 
     When the slide reaches the bottom dead center position, the die-cushion pad follows the slide and moves downward to also stop at its own bottom dead center position. At the bottom dead center position, shrinkages are present on the die-cushion pad and the like. As a solution, according to the aspect of the present invention, the cushion pad is moved downward from the its own bottom dead center position by the predetermined escape height by the position command signal from the escape-position-signal outputting section immediately after the slide has reaches the bottom dead center. With this arrangement, by the downward movement of the die-cushion pad to the escape position, the shrinkages generated due to the loading from the slide can be eliminated. Thus, when the loading is removed by the upward movement of the slide, bouncing of the workpiece at the time of bottom dead center locking, which is attributed to restoration to the original by the shrunk die-cushion pad and the like, can be reliably prevented. 
     Since the die-cushion pad is controlled to escape based on the press signal outputted when the slide reaches the bottom dead center, the die cushion does not move downward to the escape position before the slide reaches the bottom dead center or after the upward movement of the slide is started. The escaping of the die-cushion pad can be reliably conducted, thereby reliably preventing the processing or bouncing of the workpiece. 
     Preferably in the die-cushion controlling device for a press machine according to the present invention, the escape-position-command-signal outputting section generates the position command signal for escape based on the position detection signal outputted when the die-cushion pad is actually positioned at the bottom dead center. 
     According to the aspect of the present invention, the escape position of the die-cushion pad is determined not based on the bottom dead center position given as the target position value but based on the bottom dead center position at which the die-cushion pad actually stops. Thus, even if the actual bottom dead center position is fluctuated due to thickness errors or processing errors of the workpiece, the escape position can be uniformly determined based on the bottom dead center without being affected by such fluctuation, thereby contributing to more reliable elimination of the shrinkages. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  schematically shows an arrangement of a press machine according to an embodiment of the present invention. 
         FIG. 2  schematically shows an arrangement of a die-cushion according to the embodiment of the present invention. 
         FIG. 3  is a functional block diagram for explaining an arrangement of a die-cushion controlling device. 
         FIG. 4  is a control block diagram for explaining an arrangement of the die-cushion controlling device. 
         FIG. 5  shows a relationship between time and position-speed command signal. 
         FIG. 6  shows a relationship between time and pressure-speed command signal. 
         FIG. 7  is an illustration for explaining a switch operation for switching a control between a position control and a pressure control. 
         FIG. 8  shows a position pattern. 
         FIG. 9  shows a pressure pattern. 
         FIG. 10  is an illustration for explaining movements of a slide and a die-cushion pad. 
         FIG. 11  is an illustration for explaining operations of an output switching section and an escape-position-command-signal outputting section. 
     
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     Next, embodiment(s) of a die-cushion controlling device according to the present invention will be described with reference to the attached drawings. 
       FIG. 1  schematically shows an arrangement of a press machine according to an embodiment of the present invention.  FIG. 2  schematically shows an arrangement of a die cushion  13  according to a first embodiment of the present invention. 
     A press machine  1  shown in  FIG. 1  includes: a slide  4  supported by a body frame  2  to be movable up and down, and driven to move up and down by a slide driving mechanism  3 ; and a bolster  6  disposed to face the slide  4 , and mounted on a bed  5 . A lower surface of the slide  4  is attached with an upper die  7  while an upper surface of the bolster  6  is attached with a lower die  8 . With this arrangement, a workpiece  9  placed between the upper die  7  and the lower die  8  experiences press work (drawing) with the slide  4  moved up and down. 
     Among the above arrangement, the bed  5  houses a die cushion(s)  13  therein. The die cushion  13  includes: a required die-cushion pin  14 ; a die-cushion pad  15  supported by the bed  5  to be movable up and down in the bed  5 ; and a die-cushion-pad driving mechanism  16  for driving the die-cushion pad  15  to move up and down. 
     The die-cushion pin  14  is inserted into holes formed respectively in the bolster  6  and the lower die  8  in such a manner as to vertically penetrate the bolster  6  and the lower die  8 . An upper end of the die-cushion pin  14  abuts on a blank holder  17  disposed in a recessed portion of the lower die  8  while a lower end of the die-cushion pin  14  abuts on the die-cushion pad  15 . 
     One or more guide member(s) (not shown) for vertically guiding the die-cushion pad  15  is provided between lateral surfaces of the die-cushion pad  15  and an inner wall surface of the bed  5  facing the lateral surfaces of the die-cushion pad  15 . The one or more guide member(s), which includes a mutually-engageable pair of an inner guide and an outer guide, is attached thereto such that the inner guide is attached to the lateral surfaces of the die-cushion pad  15  while the outer guide is attached to the inner wall surface of the bed  5 . In this manner, the die-cushion pad  15  is supported by the bed  5  to be movable up and down within the bed  5 . 
     As shown in  FIG. 2 , the die-cushion-pad driving mechanism  16  includes: an electric servo motor  21  serving as a driving source; a ball-screw mechanism  22  serving as an elevating means for elevating the die-cushion pad  15 ; and a wound transmission mechanism  23  and a connecting member  24  provided in a power transmission path lying between the electric servo motor  21  and the ball-screw mechanism  22 . According to the arrangement of the die-cushion-pad driving mechanism  16 , the die-cushion pad  15  and the electric servo motor  21  can transmit mutual power to each other. 
     The electric servo motor  21  is a rotary AC servo motor having a rotary shaft, rotary speed and torque of which are controlled by controlling of a motor current i (current) fed to the electric servo motor  21 . A body of the electric servo motor  21  is fixed on a beam  25  that bridges the inner wall surface of the bed  5 . The electric servo motor  21  is additionally provided with an encoder  36  (position detecting means). The encoder  36  detects an angle and angular speed of the rotary shaft of the electric servo motor  21 , and outputs the detection values respectively as a motor-rotary-angle detection signal θ and as a motor-rotation-angular speed detection signal ω. The motor-rotary-angle detection signal θ and the motor-rotation-angular speed detection signal ω outputted from the encoder  36  are inputted into a later-described controller  41 . 
     The ball-screw mechanism  22  includes a screw portion  26  and a nut portion  27  screwed to the screw portion  26 , and converts rotative power inputted from the nut portion  27  into linear power by the screw portion  26  to output the converted power. A lower end of the screw portion  26  is adapted to be advanced into and retracted from a space formed at the center of the connecting member  24  while a lower end of the nut portion  27  is linked to an upper end of the connecting member  24 . The connecting member  24  is supported by the beam  25  via a bearing device  28  that includes a required bearing and a bearing housing for housing the bearing. 
     The wound transmission mechanism  23  is provided by winding a timing belt  31  between a small pulley  29  fixed to the rotary shaft of the electric servo motor  21  and a large pulley  30  fixed to a lower end of the connecting member  24 . 
     With the above arrangement, the rotary power of the electric servo motor  21  is transmitted to the nut portion  27  of the ball-screw mechanism  22  via the small pulley  29 , the timing belt  31 , the large pulley  30  and the connecting member  24 . The rotary power transmitted to the nut portion  27  moves the screw portion  26  of the ball-screw mechanism  22  up and down, thereby driving the die-cushion pad  15  to move up and down. By controlling the motor current i fed to the electric servo motor  21 , a biasing force fed to the die-cushion pad  15  is also controlled. 
     In the die cushion  13 , a lower end of the die-cushion pad  15  is connected with a plunger rod  80 . A lateral surface of the plunger rod  80  is slidably supported by a tubular plunger guide  82 . The plunger guide  82  guides the plunger rod  80  and the die-cushion pad  15  connected to the plunger rod  80  in the elevating direction. A lower portion of the plunger rod  80  is provided with a cylinder  80 A having a downward opening, in which a piston  81  is slidably housed. 
     An inner wall surface of the cylinder  80 A and an upper surface of the piston  81  define a hydraulic chamber  83 , an inside of which is filled with pressure oil. The hydraulic chamber  83  is disposed coaxially with the plunger rod  80  and the ball-screw mechanism  22 . A pressure-oil port of the hydraulic chamber  83  is connected to a hydraulic circuit, such that the pressure oil is transferred between the hydraulic chamber  83  and the hydraulic circuit. The pressure oil of the hydraulic chamber  83  mitigates an impact resulting from a contact of the upper die  7  with the workpiece  9 . When the hydraulic pressure has reached a predetermined level or more, the pressure oil is ejected to a tank. The pressure oil of the hydraulic chamber  83  provides protection from an excessive load. 
     A lower end of the piston  81  abuts on an upper end of the screw portion  26  of the ball-screw mechanism  22 . The lower end of the piston  81  is provided with a spherical recessed surface  81 A while the upper end of the screw portion  26 , which faces the recessed surface  81 A, is provided with a spherical projecting surface. Conversely, the lower end of the piston  81  may be provided with a projecting surface while the upper end of the screw portion  26  may be provided with a recessed surface. A rod-like member such as the screw portion  26  is vulnerable to bending moment while being invulnerable to axial forces applied on ends thereof. In an arrangement where the upper end of the screw portion  26  is spherical, even when bending moment is applied on the upper end of the screw portion  26  due to inclination of the die-cushion pad  15 , only an axial force is applied on the entirety of the screw portion  26 . With such a structure, damages on the screw portion  26  due to eccentric loading can be prevented. 
     In the die cushion  13 , a pressure of the hydraulic chamber  83  is detected within the above-described hydraulic circuit. The port of the hydraulic chamber  83  is communicated with a conduit line  85  for forming the hydraulic circuit. In the middle of the conduit line  85 , a pressure gauge  93  (pressure detecting means) is provided. The pressure gauge  93  detects the pressure of the hydraulic chamber  83 , i.e., loading applied on the die-cushion pad  15 . The pressure gauge  93  outputs a pressure detection signal Pr to the controller  41 . 
     Next, an arrangement of a die-cushion controlling device  40  for controlling the die cushion  13  will be described with reference to the functional block diagram of  FIG. 3  and the control block diagram of  FIG. 4 . 
     The die-cushion controlling device  40  shown in  FIGS. 3 and 4  includes a controller  41  and a servo amplifier  42  for feeding the electric servo motor  21  with a motor current i corresponding to a motor-current command signal ic outputted from the controller  41 . 
     The controller  41 , a description of which by detailed illustration will be omitted herein, includes: an input interface for converting and shaping various input signals; computer device mainly including a micro computer, a high-speed math coprocessor or the like for performing arithmetic or logic computation on input data in accordance with a predetermined procedure; and an output interface for converting the computed result into a control signal to output. The controller  41  is provided with various functional sections such as a die-cushion-pad-position calculating section  43 , a die-cushion-pad-speed calculating section  44 , a position-command-signal outputting section  45 , a position comparing section  46 , a position controlling section  47 , a pressure-command-signal outputting section  48 , a pressure comparing section  49 , a pressure controlling section  50 , a position/pressure-control switching section  51 , a speed comparing section  52 , a speed controlling section  53 , an output switching section  57  and an escape-position-command-signal outputting section  58 . The above functional sections are provided by software or the like processed by the computer. 
     The die-cushion-pad-position calculating section  43 : inputs the motor-rotary-angle detection signal θ from the encoder  36  provided to the electric servo motor  21 ; obtains a position of the die-cushion pad  15 , which is on a predetermined relationship with the motor rotary angle, based on the input signal; and outputs the result as a die-cushion-pad-position detection signal hr (position detection signal). 
     The die-cushion-pad-speed calculating section  44 : inputs the motor-rotation-angular speed detection signal ω from the encoder  36 ; obtains speed of the die-cushion pad  15  (elevating speed), which is on a predetermined relationship with the motor rotary speed, based on the input signal; and outputs the result as a die-cushion-pad speed detection signal υr. 
     The position-command-signal outputting section  45  obtains a target position value of the die-cushion pad  15  by reference to a predetermined position pattern  54 , and generates and outputs a position command signal hc based on the obtained target position value. The position pattern  54  shows a desirable correspondence relationship between time (or press angles or slide positions) and die-cushion pad positions. 
     The position comparing section  46  compares the position command signal hc outputted from the position-command-signal outputting section  45  via an output switching section  57  with the die-cushion-pad-position detection signal hr from the die-cushion-pad-position calculating section  43  to output a position deviation signal eh. 
     The position controlling section  47 , which includes a coefficient multiplier  55  for: inputting the position deviation signal eh from the position comparing section  46 ; multiplying the input signal by a predetermined position gain K 1 ; and outputting the multiplied signal, generates and outputs a position-speed command signal υhc that has a value in correspondence with the value of the position deviation signal eh. 
     The pressure-command-signal outputting section  48  obtains a target value of a pressure (cushion pressure) applied on the die-cushion pad  15  by reference to a predetermined pressure pattern  56 , and generates and outputs a pressure command signal Pc based on the obtained target pressure value. The pressure pattern  56  shows a desirable correspondence relationship between time (or press angles or slide positions) and pressures applied on the die-cushion pad  15 . 
     The pressure comparing section  49  compares the pressure command signal Pc from the pressure-command-signal outputting section  48  with a pressure detection signal Pr from a pressure gauge  93  to output a pressure deviation signal ep. 
     The pressure controlling section  50 , which includes: a coefficient multiplier  71  for inputting the pressure deviation signal ep from the pressure comparing section  49 , multiplying the input signal by a predetermined proportional gain K 2  and outputting the multiplied signal; an integrator  72  for inputting the pressure deviation signal ep from the pressure comparing section  49 , integrating the input signal and outputting the integrated signal (a code “s” in the block denotes Laplace operator); and a coefficient multiplier  73  for inputting the signal outputted from the integrator  72 , multiplying the input signal by a predetermined integral gain K 3  and outputting the multiplied signal, generates and outputs a pressure-speed command signal υpc by adding the output signal from the coefficient multiplier  71  with the output signal from the coefficient multiplier  73 . 
     By carrying out PI action (a combination of proportional action (P action) and integral action (I action)), the pressure controlling section  50  outputs a pressure-speed command signal υpc having such a value that is in correspondence with the value of the pressure deviation signal ep and that is increased as long as the pressure deviation signal ep is inputted thereinto. With this arrangement, the detected pressure is rapidly and accurately equalized with the target pressure. 
     The position/pressure-control switching section  51 , which switches a control between a position control for controlling the position of the die-cushion pad  15  and a pressure control for controlling the pressure applied on the die-cushion pad  15 , includes: a switch  60  for switching a connection between a connection of contacts (a) and (b) and a connection of contacts (c) and (b); and a position/pressure comparing section  61  for selecting the switching operations of the switch  60 . 
     When the switch  60  connects the contact (b) with the contact (a) (this connecting operation is hereinafter called “contacts-(b)-to-(a) connecting operation”), the position-speed command signal υhc from the position controlling section  47  flows into the speed comparing section  52 . When the switch  60  connects the contact (b) with the contact (c) (this connecting operation is hereinafter called “contacts-(b)-to-(c) connecting operation), the pressure-speed command signal υpc from the pressure controlling section  50  flows into the speed comparing section  52 . 
     The position/pressure comparing section  61  is set to compare the pressure-speed command signal υpc from the pressure controlling section  50  with the position-speed command signal υhc from the position controlling section  47  and to select the one having the smaller value of the two signals. 
     Switching logic of the position/pressure comparing section  61  will be described with reference to  FIGS. 5 to 7 .  FIG. 5  shows the position-speed command signal υhc. In  FIG. 5 , when the position pattern (target position value) of the die-cushion pad  15  is set constantly at 0 (stand-by position), the position of the die-cushion pad  15  before the upper die  7  contacts the workpiece  9  is identical with the stand-by position. Accordingly, the position deviation signal eh has a value of 0, so that the position-speed command signal υhc has a value of 0. Subsequently, after moving downward at a predetermined acceleration, the die-cushion pad  15  moves downward at a constant speed. Thus, the value of the position-speed command signal υhc is decreased from the value in the stand-by state at a predetermined time constant, and is subsequently maintained at a constant value. While the die-cushion pad  15  moves downward together with the slide  4  in actuality after the upper die  7  has reached a touch position during a preliminary acceleration, the position pattern  54  is set at a higher position than the actual position of the die-cushion pad  15 . Therefore, the value of the position deviation signal eh is gradually increased upward, so that the value of the position-speed command signal υhc is also increased in accordance therewith. 
       FIG. 6  shows the pressure-speed command signal υpc. In  FIG. 6 , when the pressure pattern (target pressure value) of the die-cushion pad  15  is set constantly at a constant value, no pressure is applied on the die-cushion pad  15  before the upper die  7  contacts the workpiece  9 . Accordingly, the value of the pressure deviation signal ep is identical with the constant value of the pressure pattern, so that the value of the pressure-speed command signal υpc corresponds to the constant value of the pressure pattern. When the upper die  7  has subsequently reached a position to contact the workpiece  9  (touch position), the upper die  7  presses the die-cushion pad  15 , thereby applying the pressure thereto. The pressure is increased in accordance with the downward movement of the die-cushion pad  15  so as to approximate to the initially-set target pressure value. Therefore, the value of the pressure deviation signal ep is gradually decreased, so that the value of the pressure-speed command signal υpc is also decreased in accordance therewith. 
     As shown in  FIG. 7 , the position/pressure comparing section  61  is set to compare the position-speed command signal υhc with the pressure-speed command signal υpc and to select the one having the smaller value of the two signals. With this arrangement, during the downward movement of the die-cushion pad  15  before the upper die  7  contacts the workpiece  9 , the position-speed command signal υhc is selected because the value of the position-speed command signal υhc is smaller than the value of the pressure-speed command signal υpc. By the above selection, the switch  60  connects the contact (b) with the contact (a), so that the position-speed command signal υhc flows into the speed comparing section  52 , thereby conducting a position control. 
     When the upper die  7  has subsequently reached the touch position to contact the workpiece  9 , the value of the position-speed command signal υhc is increased while the value of the pressure-speed command signal υpc is decreased. When the magnitude relation of the values of the speed command signals υhc, υpc is reversed, the position/pressure comparing section  61  selects the pressure-speed command signal υpc having the smaller value than the position-speed command signal υhc, so that the contacts (b) and (c) of the switch  60  are connected. By this operation of switching the connection, the pressure-speed command signal υpc flows into the speed comparing section  52 , thereby conducting a pressure control. 
     Since the position/pressure comparing section  61  is set to constantly compare the position-speed command signal υhc with the pressure-speed command signal υpc so as to select the one having the smaller value of the two signals, the switch between the position control and the pressure control can be automatically performed at a suitable timing. Accordingly, influence (e.g., influence brought about by an impact, oscillation etc.) caused by the contact of the upper die  7  with the die-cushion pad  15  via the workpiece  9  can be minimized, and the switch between the position control and the pressure control can be stably and reliably performed at a suitable timing. In addition, since both of the position-speed command signal υhc and the pressure-speed command signal υpc are constantly being observed, the touch position when the upper die  7  contacts the workpiece  9  can be reliably obtained. Thus, a rapid and reliable switch can be performed. 
     When the position control is selected as a result of the switching operation by the position/pressure-control switching section  51 , the speed comparing section  52  compares the position-speed command signal υhc from the position controlling section  47  with the die-cushion-pad-speed detection signal υr from the die-cushion-pad-speed calculating section  44 , and outputs a speed deviation signal ev. When the pressure control is selected as a result of the switching operation by the position/pressure-control switching section  51  on the other hand, the speed comparing section  52  compares the pressure-speed command signal υpc from the pressure controlling section  50  with the die-cushion-pad-speed detection signal or from the die-cushion-pad-speed calculating section  44 , and outputs the speed deviation signal ev. 
     According to the present embodiment, when the pressure control is performed, the pressure controlling section  50  outputs the pressure-speed command signal υpc having such a value that is in correspondence with the value of the pressure deviation signal ep and that is increased as long as the pressure deviation signal ep is inputted thereinto. With this arrangement, a pressure deviation can be rapidly and reliably reduced. Accordingly, accuracy of the pressure control can be enhanced. 
     The speed controlling section  53 , which includes: a coefficient multiplier  62  for inputting the speed deviation signal ev from the speed comparing section  52 , multiplying the input signal by a predetermined proportional gain K 4  and outputting the multiplied signal; an integrator  63  for inputting the speed deviation signal ev from the speed comparing section  52 , integrating the input signal and outputting the integrated signal (the code “s” in the block denotes Laplace operator); and a coefficient multiplier  64  for inputting the signal outputted from the integrator  63 , multiplying the input signal by a predetermined integral gain K 5  and outputting the multiplied signal, generates and outputs a motor-current command signal ic (torque command signal) by adding the output signal from the coefficient multiplier  62  with the output signal from the coefficient multiplier  64 . 
     By also carrying out PI action (a combination of proportional action (P action) and integral action (I action)), the speed controlling section  53  outputs a motor-current command signal ic that has such a value that is in correspondence with the value of the speed deviation signal ev and that is increased as long as the speed deviation signal ev is inputted thereinto. With this arrangement, the detected speed is rapidly and accurately equalized with the target speed. In this manner, a stable position/pressure control can be performed. 
     The output switching section  57 , which serves as a switch switched by a press signal S from a press-signal generating section  10  provided to the press machine  1 , includes contacts (d), (e) and (f) in terms of the control block. At the time of a contacts-(e)-to-(d) connecting operation by which the contacts (e) and (d) are connected in the output switching section  57 , the position command signal hc from the position-command-signal outputting section  45  is output to the position comparing section  46  as described above. On the other hand, at the time of a contacts-(e)-to-(f) connecting operation whereby the contacts (e) and (f) are connected in the output switching section  57 , a position command signal hc from the escape-position-command-signal outputting section  58  is output to the position comparing section  46 . 
     The escape-position-command-signal outputting section  58  includes a bottom-dead-center-position holding means  581  and an escape-position-command generating means  582 . 
     The bottom-dead-center-position holding means  581  singly reads the die-cushion-pad-position detection signal hr from the die-cushion-pad-position calculating section  43  when determining that the slide  4  has reached the bottom dead center by reference to the press signal S, and holds the signal hr as a bottom-dead-center-position detection signal hr′ of the die-cushion pad  15 . 
     The escape-position-command generating means  582  generates a position command signal hc for downwardly escaping the die-cushion pad  15  by predetermined height from the bottom dead center position. Specifically, the escape-position-command generating means  582  generates the position command signal hc by subtracting an escaping height Δh from the value of the bottom-dead-center-position detection signal hr′ (hc=hr′−Δh), and outputs the generated signal to the output switching section  57 . The escape height Δh may be suitably set in view of loading applied to the die cushion  13  and a structure thereof. An operator sets the escape height at a suitable value through an operation on a setting screen of an operation panel (not shown). In the present embodiment, the escape height Δh is exemplarily set approximately at 2 mm. 
     The press signal S generated by the press-signal generating section  10  is related with positions of the slide, and generated based on a motor-rotary-angle signal θp from the encoder  12  provided on the servo motor  11  for driving the slide  4 . During a period until the slide  4  is moved to the bottom dead center from the top dead center, an ON signal is outputted as the press signal S. On the other hand, during a period until the slide  4  is subsequently elevated up to a predetermined height from the bottom dead center, an OFF signal is outputted as the press signal S. During a period until the slide  4  is moved to the top dead center from the predetermined height, an ON signal is outputted again (see  FIG. 8 : bottom dead center Hldc, predetermined height H). During a period when the press signal S is OFF, a bottom-dead-center-position locking is performed by the die cushion  13 . 
     The servo amplifier  42  includes a current comparing section  65 , a current controlling section  66  and a current detecting section  67 . In the servo amplifier  42 , the current detecting section  67  detects the motor current i fed to the electric servo motor  21  and outputs the detected value as a motor-current detection signal ir. The current comparing section  65  compares the motor-current command signal ic from the speed controlling section  53  with the motor-current detection signal ir from the current detecting section  67  to output a motor-current deviation signal ei. The current controlling section  66  controls the motor current i fed to the electric servo motor  21  based on the motor-current deviation signal ei from the current comparing section  65 . 
     The position pattern  54  of the position-command-signal outputting section  45 , the pressure pattern  56  of the pressure-command-signal outputting section  48  and the press signal S according to the present embodiment will be described in detail below.  FIG. 8  shows the position pattern  54  and the press signal S according to the present embodiment in a manner that is associated with operations of the slide.  FIG. 9  shows the pressure pattern  56  according to the present embodiment. 
     As shown in  FIG. 8 , the position pattern  54  is set such that: a position h 1  equivalent to the stand-by position of the die-cushion pad  15  is initially maintained during a period until a time t 1 ; and the position is subsequently lowered down to a position h 2  by a predetermined time constant during a period from the time t 1  to time t 2  due to a preliminary acceleration. Then, at a position h 12  on the way to the position h 2 , the upper die  7  contacts the workpiece  9  (time t 12 ). Since it is preferable to perform a pressure control when drawing is conducted with the upper die  7  contacting the workpiece  9 , the target position value according to the position pattern  54  until a time t 4  at which the slide  4  reaches the bottom dead center is set at a value higher than a value of the actual position of the die-cushion pad  15 . With this arrangement, when the die-cushion pad  15  moves downward, the value of the position deviation signal eh is increased, so that the pressure deviation signal ep, which has the smaller value than the position deviation signal eh, is selected for the pressure control to be performed. According specifically to the position pattern  54  at this time, a position command signal hc for commanding the position to be lowered to a position h 3  by another time constant during a period from the time t 2  to a time t 3  after the preliminary acceleration until the position h 2  is output. 
     Then, during a period from the time t 3  until the slide  4  reaches the bottom dead center Hldc, i.e., until an OFF signal as the press signal S is outputted from the press-signal generating section  10  to the output switching section  57  and the escape-position-command-signal outputting section  58 , a position command signal hc for commanding the die-cushion pad  15  to stop at the position h 3  is output. Since the die-cushion pad  15  follows the downward movement of the slide  4 , when the slide  4  has reached the bottom dead center, the die-cushion pad  15  also stops at the position h 4  (i.e., its own bottom dead center position of the die-cushion pad  15 ). At this time, since the position h 4 , at which the die-cushion pad  15  stops, is lower than the position h 3  (i.e., a stop position set by the target position value according to the position pattern  54 ), the die-cushion pad  15  remains stopped at the position h 4  while the position deviation signal eh of a certain value is being output. However, since the target pressure value in the later-described pressure pattern  56  is set at a sufficiently high value at the position h 4 , the value of the pressure deviation signal ep becomes larger than the value of the position deviation signal eh, so that the control is switched to the position control. 
     When an OFF signal as the press signal S is outputted at this time, switching to the contacts-(e)-to-(f) connecting operation is conducted by the output switching section  57 , so that the position command signal hc from the escape-position-command-signal outputting section  58  is outputted to the position comparing section  46 . Specifically, when the bottom-dead-center-position holding means  581  of the escape-position-command-signal outputting section  58  inputs the OFF signal as the press signal S, the bottom-dead-center-position holding means  581  inputs the die-cushion-pad-position detection signal hr (actually the position h 4 ) one time to hold this die-cushion-pad-position detection signal hr as the bottom-dead-center-position detection signal hr′. In addition, while the press signal S is OFF, the escape-position-command-signal generating means  582  outputs to the output switching section  57  a position command signal hc for commanding a position h 5 , which is lowered from the position h 4  (i.e., the bottom dead center position) by the escape height Ah, to be the target position value, and this position command signal hc is outputted to the position comparing section  46 . 
     Consequently, the position command signal hc for commanding the position h 5  to be the target position value is generated so that the position lowered from the position h 4  by the escape height Δh is targeted based on the position h 4  (i.e., the actual bottom dead center of the die-cushion pad  15 ). Thus, even when the actual bottom dead center of the die-cushion pad  15  is fluctuated for each workpiece  9  due to thickness errors or processing errors of the workpiece  9 , the die-cushion pad  15  can accurately escape to the position h 5  invariably lowered from the actual bottom dead center by the escape height Δh, thereby reliably eliminating shrinkages caused in the die-cushion pad  15  and the hydraulic chamber  83 . 
     When the die-cushion pad  15  moves further downward to the position h 5  (escape position) than the position h 4  (the bottom dead center), the value of the position deviation signal eh generated from the position command signal hc from the output switching section  57  and the die-cushion-pad-position detection signal hr from the die-cushion-pad-position calculating section  43 , which have been inputted in the position comparing section  46 , becomes 0. Accordingly, the die-cushion pad  15  stops at the position h 5  (escape position), such that the bottom-dead-center-position locking is conducted. In addition, since the shrinkages having been caused in the die-cushion pad  15  and the hydraulic chamber  83  during the drawing can be eliminated due to the downward movement of the die-cushion pad  15  to the escape position, the workpiece  9  can be prevented from bouncing even when a loading is lifted from the die cushion  13  by an upward movement of the slide  4 . 
     When the slide  4  is subsequently moved upward to a predetermined height H (when the time is elapsed from the time t 4  to the time t 5 ), the press-signal generating section  10  generates an ON signal as the press signal S. Thus, the output switching section  57  resumes the contacts-(e)-to-(d) connecting operation, so that the position command signal hc from the position-command-signal outputting section  45  in accordance with the predetermined position pattern  54  is outputted to the position comparing section  46  via the output switching section  57 . During a period from the time t 5  to a time t 6 , in which a supplementary lifting operation for upwardly moving the die-cushion pad  15  by a predetermined height is performed, the die-cushion pad  15  is set to be positioned at the position h 6  at the time t 6 . During a period subsequent to the time t 6 , the die-cushion pad  15  is set to return to the position h 1  equivalent to the stand-by position. 
     As shown in  FIG. 9 , according to the pressure pattern  56 , a predetermined value P 1  is set during a period until a time t 12 , which precedes the contacting of the upper die  7  with the workpiece  9 . The predetermined value P 1  is set at a value higher by a predetermined ratio than a value of precompression of the die-cushion pad  15 . With this arrangement, a predetermined pressure deviation signal ep is generated during a period before the upper die  7  contacts the workpiece  9 . During a period from the time t 12  to the time t 4  in which the upper die  7  contacts the workpiece  9  to conduct drawing, the pressure pattern  56  is set such that an optimum pressure is applied during each time period. 
     Specifically, when drawing is started, the target pressure value is diagonally increased from the predetermined value P 1  to a predetermined P 2  by a predetermined time constant, and maintains the predetermined value P 2  during a period until the time t 21 . Then, during a period from the time t 21  to a time t 22 , the target pressure value is diagonally decreased from the predetermined value P 2  to a predetermined value P 3  by a predetermined time constant, and maintains the predetermined value P 3  during a period from the time t 22  to the time t 4  (i.e., a period until the slide  4  reaches the bottom dead center). Since it is preferable to perform the position control after the slide  4  has reached the bottom dead center (subsequent to the time t 4 ), the target pressure value is set to jump to a high value of a predetermined value P 4  so that the value of the pressure deviation signal ep is increased. 
     Next, a relationship between the movement of the die-cushion pad  15  and the pressure/position control will be described below.  FIG. 10  shows an illustration for explaining movements of the slide  4  and the die-cushion pad  15 , in which positional variations of the slide  4  and the die-cushion pad  15  in accordance with lapse of time are illustrated by lines. 
     It should be noted that, in the description below, the die-cushion-pad-position detection signal hr from the die-cushion-pad-position calculating section  43  will be referred to as “position feedback signal hr”, the die-cushion-pad-speed detection signal υr from the die-cushion-pad-speed calculating section  44  will be referred to as “speed feedback signal υr” and the pressure detection signal Pr from the pressure gauge  93  will be referred to as “pressure feedback signal Pr”. In addition, the position control will be referred to as “position feedback control” while the pressure control will be referred to as “pressure feedback control”. 
     Initially, the die-cushion pad  15  is positioned at the position h 1  (i.e., stand-by position) during a period until the time t 1  since the press work is started. Thus, the value of the position-speed command signal υhc is 0 while the value of the pressure-speed command value υpc corresponds to the predetermined value P 1 . Accordingly, during the period until the time t 1  since the drawing operation is started, the position/pressure comparing section  61  selects the position-speed command signal υhc, and the switch  60  connects the contact (b) with the contact (a), so that the position feedback control is performed. In addition, the value of the pressure-speed command signal υpc also corresponds to the predetermined value P 1  during the period from the time t 1  to the time t 12 , so that the position feedback control is continued to be performed. 
     In the position feedback control, the position comparing section  46  subtracts the value of the position feedback signal hr from the value of the position command signal hc and outputs the position deviation signal eh, the position controlling section  47  outputs the position-speed command signal υhc for decreasing the value of the position deviation signal eh, the speed comparing section  52  subtracts the value of the speed feedback signal Or from the value of the position-speed command signal υhc and outputs the speed deviation signal ev, the speed controlling section  53  outputs the motor-current command signal ic (torque command signal) for decreasing the value of the speed deviation signal ev, and the servo amplifier  42  feeds the electric servo motor  21  with the motor current i corresponding to the motor-current command signal ic. With this operation, the position of the die-cushion pad  15  is controlled so that the position detection value detected by the encoder  36  follows the predetermined position pattern  54 . 
     With this operation, the die-cushion pad  15  is on standby at the stand-by position during the period until the time t 1 . Then, in order to mitigate an impact caused when the upper die  7  contacts the workpiece  9 , the preliminary acceleration of the die-cushion pad  15  is conducted during the period from the time t 1  to the time t 2 . 
     The upper die  7  subsequently contacts the workpiece  9  at the position h 12  at the time t 12  during the preliminary acceleration. The target position value according to the position pattern  54  keeps varying in the same manner toward the position h 2 , and subsequently varies toward the position h 3 . However, since the die-cushion pad  15  in actuality follows the slide  4  and moves further downward than the target position, the position deviation signal eh with a certain value or more is output. On the other hand, the pressure is increased due to the contact of the upper die  7  with the workpiece  9 , so that the value of the generated pressure approximates to the predetermined value P 1  (i.e., the target pressure value according to the pressure pattern  56 ). Accordingly, the value of the pressure deviation signal ep is gradually decreased. When the value of the pressure-speed command signal υpc based on the pressure deviation signal ep has become smaller than the value of the position-speed command signal υhc based on the position deviation signal eh, the position/pressure comparing section  61  selects the pressure-speed command signal υpc. 
     With this operation, the switch  60  connects the contact (b) and the contact (c) by the contacts-(b)-to-(c) connecting operation in the position/pressure-control switching section  51 , so that the control is automatically switched to the pressure feedback control from the position feedback control. Accordingly, owing to the automatic switching operation of the position/pressure-control switching section  51 , the control can be reliably switched between the position control and the pressure control immediately after the upper die  7  contacts the workpiece  9 . With the above operations, the slide  4  and the die-cushion pad  15  move downward in an integrated manner during the period from the time t 12  to the time t 4 , so that the workpiece  9  experiences the drawing. During this period from the time t 12  to the time t 4 , the pressure feedback control is performed. 
     In the pressure feedback control, the pressure comparing section  49  subtracts the value of the pressure feedback signal Pr from the value of the pressure command signal Pc and outputs the pressure deviation signal ep, the pressure controlling section  50  outputs the pressure-speed command signal υpc for decreasing the value of the pressure deviation signal ep, the speed comparing section  52  subtracts the value of the speed feedback signal ir from the value of the pressure-speed command signal υpc and outputs the speed deviation signal ev, the speed controlling section  53  outputs the motor-current command signal ic (torque command signal) for decreasing the value of the speed deviation signal ev, and the servo amplifier  42  feeds the electric servo motor  21  with the motor current i corresponding to the motor-current command signal ic. With this operation, the cushion pressure of the die-cushion pad  15  is controlled so that the pressure detection value detected by the pressure gauge  93  follows the predetermined pressure pattern  56 . 
     The slide  4  and the die-cushion pad  15  subsequently reach the bottom dead centers at the time t 4  respectively. The target pressure value according to the pressure pattern  56  jumps to the predetermined value P 4 , thereby increasing the value of the pressure deviation signal ep. Thus, the value of the position-speed command signal υhc based on the position deviation signal eh becomes smaller than the value of the pressure-speed command signal υpc based on the pressure deviation signal ep, so that the position/pressure comparing section  61  selects the position-speed command signal υhc. With this operation, the switch  60  connects the contact (b) and the contact (a) by the contacts-(b)-to-(a) connecting operation in the position/pressure-control switching section  51 , so that the control is automatically switched to the position feedback control from the pressure feedback control. In addition, the die-cushion pad  15  moves downward to the position h 5  (escape position) further than the position h 4  (actual bottom dead center position of the die-cushion pad  15 ) when the slide  4  reaches the bottom dead center. Accordingly, the shrinkages caused in the die cushion pad  15  and the hydraulic chamber  83  can be reliably eliminated, and the workpiece  9  can be prevented from bouncing even when the loading is lifted from the die-cushion pad due to the upward movement of the slide  4  at the time of initiation of a rotation locking. 
     Now, with reference to a flowchart shown in  FIG. 11 , operations of the output switching section  57  and the escape-position-command-signal outputting section  58  will be described in more detail. In  FIG. 11 , since the press-signal generating section  10  outputs the ON signal as the press signal during a period in which the slide  4  is in between the top dead center and the bottom dead center, the output switching section  57  during a period in which the ON signal is inputted therein performs the contacts-(e)-to-(d) connecting operation, obtains the position command signal hc based on the target position according to the position pattern  54  from the position-command-signal outputting section  45 , and outputs the obtained signal to the position comparing section  46  (ST 1 ). The position command signal hc is outputted for commanding the die-cushion pad  15  to remain positioned at the position h 3  when the die-cushion pad  15  has reached the position h 3 , so that the output switching section  57  waits for the input of the OFF signal as the press signal S (ST 2 ). 
     When the slide  4  reaches the bottom dead center position at the time t 4 , the press signal S is altered from the ON signal to the OFF signal, so that the OFF signal is inputted into the output switching section  57  and the escape-position-command-signal outputting section  58 . Then, the escape-position-command-signal outputting section  58  obtains the die-cushion-pad-position detection signal hr from the die-cushion-pad-position calculating section  43  at one time, and holds the signal hr as the bottom-dead-center-position detection signal hr′. In addition, the escape-position-command-signal outputting section  58  generates a position command signal hc for commanding the position calculated by subtracting the escape height Δh from the value of the signal hr′ to be the target position value, and outputs the generated signal to the output switching section  57 . On the other hand, the output switching section  57  switches the connecting operation to the contacts-(e)-to-(f) connecting operation, and outputs the position command signal hc from the escape-position-command-signal outputting section  58  to the position comparing section  46  (ST 3 ). The contacts-(e)-to-(f) connecting operation is maintained until the slide  4  reaches the predetermined height H and until the ON signal as the press signal S is inputted into the output switching section  57  (ST 4 ). When the ON signal is input, the output switching section  57  returns to ST 1  again to switch the connecting operation to the contacts-(e)-to-(f) connecting operation, and outputs the position command signal hc based on the target position according to the position pattern  54 . 
     Referring back to  FIG. 10 , as described above, during the period from the time t 4  to the time t 5 , the die-cushion pad  15  accurately performs locking at the position h 5 , i.e., the position lowered from the position h 4  (the actual bottom dead center position of the die-cushion pad  15 ) by the escape height Δh, and temporarily stops its upward movement. During the subsequent period of the time t 5  to time t 6 , the die-cushion pad  15  moves upward by an amount of the supplementary lifting. At the time t 6 , the die-cushion pad  15  resumes its upward movement, and stops upwardly moving after returning to the stand-by position h 1 . During the period subsequent to the time t 4 , the position feedback control is performed. By flows of various signals as described above, the position of the die-cushion pad  15  is controlled so that the position detection value detected by the encoder  36  follows the predetermined position pattern  54 . 
     It should be noted that, although the best structure, method and the like for implementing the present invention have been described in the above description, the present invention is not limited to the above description. Specifically, while the present invention has been described above with a specific embodiment(s) being particularly illustrated and mainly described, those skilled in the art may make various modifications to the above-described embodiments in terms of a shape, quantity or any other detailed configuration without departing from a scope of a technical idea and an object of the present invention. 
     Thus, a shape, quantity and the like described above merely serve as exemplifying the present invention for facilitating an understanding of the present invention, and do not serve as any limitations on the present invention, so that what is described by a name of a component for which the description of the shape, quantity and the like are partially or totally omitted is also included in the present invention. 
     For instance, while the pressure gauge provided in the hydraulic circuit is used as the pressure detecting means according to the present invention in the above embodiment, the pressure detecting means may be a strain gauge or the like provided on a lateral surface of the die-cushion pad. In addition, the position detecting means is also not limited to the encoder provided on the electric servo motor for driving the die cushion but may be a linear scale provided between the die-cushion pad and the bed. Further, the electric servo motor is not limited to the rotary servo motor but may be a linearly-moving servo motor such as a linear servo motor. 
     While the die-cushion controlling device switches the control between the position control and the pressure control in the above embodiment, it is only required that at least the position control be performed at the bottom dead center position. An arrangement in which the position control is performed through a stroke is also included in the present invention. 
     While the escape position is obtained based on the actual bottom dead center position of the die-cushion pad  15  in the above embodiment, the die cushion pad  15  may be moved to the escape position by the position command signal hc outputted from the position-command-signal outputting section  45  when the fluctuation of the bottom dead center position is so small as not to affect the elimination of the shrinkages. In other words, a target position value for the escape position may be set in advance as a value in the position pattern  54 . However, it is preferable to determine the escape position based on the actual bottom dead center position because, even when the bottom dead center position is fluctuated due to thickness dimension errors or processing errors of the workpiece, the die-cushion pad can be accurately moved downward from the bottom dead center position by the predetermined escape height Δh to reliably eliminate the shrinkages without being affected by such a fluctuation.