Abstract:
A die cushion controlling apparatus for controlling an operation of a cushion pad, which comprises a pad drive mechanism for driving to raise or lower the cushion pad while applying an upward energizing force, a load measuring unit for measuring a load generated in the cushion pad, a time detecting unit for detecting a generating time and a vanishing time of the load, and a control unit for controlling the pad drive mechanism so that a load measured value measured by the load measuring unit follows a preset load pattern during a period from when the time detecting unit detects the generating time of the load until when the time detecting unit detects the vanishing time of the load.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a die cushion controlling apparatus and a die cushion controlling method which controls an operation of a cushion pad synchronously with an operation of a slide of a press machine. 
   2. Description of the Related Art 
   In a press machine, a die cushion apparatus (hereinafter merely referred to as a “die cushion”) is provided for controlling folds in a throttling work. A conventional die cushion generates a cushion pressure while driving to raise or lower a cushion pad by using a hydraulic pressure or an air pressure. In order to raising throttling workability of the press machine and prevent a work from being broken or strained, it is necessary to control the cushion pressure of the die cushion in high accuracy, and particularly, it is necessary to control the cushion pressure at the time of lowering operation of the cushion pad in high accuracy. 
   The die cushion using only the air pressure cannot control the cushion pressure in high accuracy at the time of operating the cushion pad. The die cushion using the hydraulic pressure can control the cushion pressure in high accuracy at the time of operating the cushion pad under the control of a hydraulic pressure. However, there is a drawback that the structure of a hydraulic apparatus is complicated, and severe maintenance and management is required. Therefore, recently, a die cushion having an electric servomotor which has a simple structure and which does not need severe maintenance and management is noted. 
   In Japanese Patent Application Laid-Open No. 10-202327, a control technology of a die cushion having a rotary electric servomotor is disclosed.  FIG. 18  is a view showing a conventional press machine and its control system. 
   In the press machine shown here, a slide  2  is coupled to an eccentric portion of a crankshaft in a slide drive mechanism  1 . The slide  2  is raised or lowered in response to a rotation of the crankshaft. An encoder is provided in the crankshaft, and a signal is outputted from the encoder to a controller  100  in response to the rotation of the crankshaft. The controller  100  obtains a position of the slide  2  by using this signal. 
   Also, in the die cushion shown here, the output shaft of the servomotor  16  is coupled to a screw portion  112   b  of a ball screw  112 , and this screw portion  112   b  is screwed into the cushion pad  11 . When the screw portion  112   b  of the ball screw  112  rotates in response to the rotation of the servomotor  16 , the cushion pad  11  is raised and lowered along the screw portion  112   b . The servomotor  16  is provided with an encoder and a signal is outputted from the encoder  19  to the controller  100  in accordance with the rotation of the servomotor  16 . The controller  100  obtains a position of the cushion pad by using this signal. 
   At an initial time of one stroke operation of the slide  2  from a top dead point, the controller  100  controls the position of the cushion pad  11  in accordance with the position of the slide  2 . By this control, the cushion pad  11  is lowered at a lower speed than a lowering speed of the slide  2  and operated so that an upper die  3   a  is contacted with a work  4  at a predetermined position. When the upper die  3   a  is contacted with the work  4 , the cushion pad  11  starts receiving a load of the slide  2 . At this time, a current value of the servomotor  16  is changed. When this current change is detected, the controller  100  obtains a cushion pressure based on the current value, and controls the servomotor  16  so that the obtained cushion pressure follows the pressure pattern of the preset cushion pressure. Then, the cushion pad  11  lowers while generating an upward energizing force, and reaches a bottom dead point. 
   An error of the control affects to throttling workability, and causes the work  4  to be broken or strained. Therefore, the controller  100  needs to control the operation of the cushion pad  11  so that the obtained cushion pressure follows to the set pressure pattern. 
   With respect to an accuracy of the operation of the cushion pad, the above-mentioned Japanese Patent Application Laid-Open No. 10-202327 has a problem. Generally, a feedback control must be a closed loop for measuring a physical amount in a control object and controlling the control object based on its measured value. If the feedback control of the cushion pressure of the cushion pad is performed, it is necessary to measure the load generated in the cushion pad. 
   However, in the above-mentioned Japanese Patent Application Laid-Open No. 10-202327, no physical amount is measured from the cushion pad side, and the current value of the servomotor for driving the cushion pad is merely measured. Though the load generated in the cushion pad and the current value of the servomotor have a certain relative relation, but it cannot be said that they always have a predetermined relationship. Therefore, it is severely said that the feedback control of the above-mentioned Japanese Patent Application Laid-Open No. 10-202327 does not become a closed loop. In the technology of the Japanese Patent Application Laid-Open No. 10-202327 from such a point, there is possibility of being not able to accurately control the operation of the cushion pad. In the worst case, the work generates a breakage or a strain. 
   The present invention is made in view of the above-mentioned circumstances, and aims to solve the problem by performing the feedback control of the cushion pressure in a closed loop and controlling a cushion pressure of a cushion pad in high accuracy. 
   SUMMARY OF THE INVENTION 
   A first aspect of the present invention is a die cushion controlling apparatus for controlling an operation of a cushion pad, comprising: 
   a pad drive mechanism for driving to raise or lower the cushion pad while applying an upward energizing force; 
   a load measuring unit for measuring a load generated in the cushion pad; 
   a time detecting unit for detecting a generating time and a vanishing time of the load; and 
   a control unit for controlling the pad drive mechanism so that a load measured value measured by the load measuring unit follows a preset load pattern during a period from when the time detecting means detects the generating time of the load until when the time detecting means detects the vanishing time of the load. 
   A second aspect of the present invention is the die cushion controlling apparatus according to the first aspect of the invention, wherein the load measuring unit further comprises a strain gauge for measuring a strain of the cushion pad or a support for supporting the cushion pad, and wherein the load measuring unit obtains a value corresponding to the load by using a measured result of the strain gauge. 
   A third aspect of the present invention is the die cushion controlling apparatus according to the first aspect of the invention, wherein the load measuring unit further comprises a hydraulic chamber interposed between the cushion pad and the pad drive mechanism, and a pressure sensor for measuring a pressure in the hydraulic chamber, and wherein the load measuring unit obtains a value corresponding to the load by using a measured result of the pressure sensor. 
   The first to the third aspects of the present invention will be described. 
   The upper die is provided in the lower portion of the slide of the press machine, and the work is provided above the cushion pad of the die cushion. When the upper die is contacted with the work as the slide is operated to be lowered, the load caused by the weight of the slide is generated in the cushion pad. The cushion pad is lowered to a bottom dead point synchronously with the cushion pad while generating an upward energizing force by the drive force of the servomotor (pad drive mechanism). 
   The strain gauge is adhered to the side face of the cushion pad. The pressure generated in the cushion pad, that is, the cushion pressure is measured as the load by this strain gauge (load measuring unit). The measured value of the strain gauge is outputted to the pad controller. When the pressure is generated in the cushion pad, the measured value of the strain gauge becomes a predetermined value or more. In the pad controller, this time is detected and it is judged that the slide is operated to be lowered and the upper die is contacted with the work. Also, when the pressure of the cushion pad is vanished, the measured value of the strain gauge becomes a predetermined value or less. In the pad controller, this time is detected, and it is judged that the slide is changed to the raising operation from the bottom dead point (time detecting unit). In the pad controller, a pressure pattern of the cushion pressure is previously set. During a period from the load generating time to the vanishing time, the pad controller compares the measured pressure value with the set pressure pattern, and controls the servomotor so that the pressure value follows the pressure pattern (control unit). 
   Instead of the strain gauge, a pressure sensor may be used to measure the load generated in the cushion pad. In such a case, a hydraulic chamber is provided in a portion, where the weight of the cushion pad is received, of a power transmission route between the servomotor and the cushion pad. The pressure in the hydraulic chamber is measured by this pressure sensor. 
   According to the first to the third aspects of the present invention, a value showing the load from the cushion pad, which is an object to be controlled, is directly measured, and feedback control is performed. 
   A fourth aspect of the present invention is the die cushion controlling apparatus according to the first aspect of the invention, wherein a plurality of the cushion pads, the pad drive mechanisms, the load measuring units and the control units are provided in one working station of a press machine, and operations of the respective cushion pads are controlled independently. 
   The fourth aspect of the present invention will be described. 
   A plurality of the pads are provided in one working station of the press machine. 
   The strain gauge is adhered to the side face of each cushion pad, and the pressure generated in the corresponding cushion pad, that is, the cushion pressure is measured as the load by this strain gauge. The measured value of the strain gauge is outputted to the pad controller. In the pad controller, the pressure pattern of the cushion pressure corresponding to each cushion pad is previously set. The pad controller compares the measured pressure value with the set pressure pattern, and controls the corresponding servomotor so that the pressure value follows the pressure pattern. 
   According to the fourth aspect of the present invention, a value showing the load is directly measured from each cushion pad, which is an object to be controlled, and the individually independent feedback control is performed for each cushion pad. 
   A fifth aspect of the present invention is a die cushion controlling method for controlling an operation of a cushion pad, comprising: 
   a position control step of measuring a position of the cushion pad and controlling the position of the cushion pad so that a position measured value follows a preset position pattern; and 
   a load control step of measuring a load generated in the cushion pad and controlling the load generated in the cushion pad so that a load measured value follows a preset load pattern, wherein: 
   the position control step is switched to the load control step at a time when the load starts to be generated in the cushion pad. 
   The fifth aspect of the present invention will be described. 
   In the press machine, a preliminary acceleration is performed to alleviate an impact when the upper die is contacted with the work. The position of the cushion pad is measured during this preliminary acceleration, this position measured value is compared with the preset position pattern, and so called position feedback control is performed for controlling the servomotor so that the position measured value follows the position pattern. 
   When the upper die is contacted with the work, the load starts generating in the cushion. After the load generated in the cushion pad is detected or the cushion pad reaches the position where the upper die is contacted with the work, the load generated in the cushion pad is measured, this load measured value is compared with the preset load pattern, and so called the pressure feedback control is performed for controlling the servomotor so that the load measured value follows the preset load pattern. 
   As described above, the position feedback control is switched to the pressure feedback control at the time when the upper die contacts with the work. 
   According to the fifth aspect of the present invention, the value showing the position is measured directly from the cushion pad, which is the object to be controlled, during the preliminary acceleration, and the feedback control is performed. After the preliminary acceleration, the value showing the load is measured directly from the cushion pad, which is the object to be controlled, and the feedback control is performed. 
   According to the present invention, since the pressure feedback control of the closed loop for feeding back the cushion pressure measured from the cushion pad itself is performed at a timing at which the pressure feedback control of the cushion pad is required, the cushion pressure of the cushion pad can be controlled in high accuracy. Therefore, the workability of the press can be improved. 
   According to the fourth aspect of the present invention, since the cushion pressure in one working station can be partly changed, the accuracy of the press machine can be further improved. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Embodiments of the present invention will be described in detail based on the following figures, wherein: 
       FIG. 1  is a schematic view showing a structure of a press machine; 
       FIG. 2  is a schematic view of a die cushion according to a first embodiment; 
       FIG. 3  is a top view of the die cushion according to the first embodiment; 
       FIG. 4  is a control block diagram of feedback control performed in the first embodiment; 
       FIG. 5  is a view showing an operation of a slide and a die cushion pad; 
       FIG. 6  is a schematic view of the die cushion according to a second embodiment; 
       FIG. 7  is a schematic view of the die cushion according to a third embodiment; 
       FIG. 8  is a top view of the die cushion according to the third embodiment; 
       FIG. 9  is a control block diagram of feedback control performed in the third embodiment; 
       FIG. 10  is a schematic view of the die cushion according to a fourth embodiment; 
       FIG. 11  is a schematic view of the die cushion according to another form of the fourth embodiment; 
       FIG. 12  is an oil pressure circuit diagram according to a fifth embodiment; 
       FIG. 13  is a control block diagram of feedback control performed in the fifth embodiment; 
       FIG. 14  is an oil pressure circuit diagram according to another form of the fifth embodiment; 
       FIG. 15  is an oil pressure circuit diagram according to another form of the firth embodiment; 
       FIG. 16  is a view for explaining an arrangement of the cushion pad and its drive mechanism; 
       FIG. 17A  to  FIG. 17D  are top views of one working station; and 
       FIG. 18  is a view showing a conventional press machine and its control system. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Embodiments of the present invention will be described in detail with reference to the accompanying drawings. 
     FIG. 1  is a schematic view showing a structure of a press machine. 
   In the press machine, a slide  2  disposed in an upper portion and a bolster  8  disposed in a lower portion are provided oppositely to each other. The slide  2  is raised or lowered by receiving a power from a slide drive mechanism  1  located above the slide  2 . An upper die  3   a  is mounted on the lower portion of the slide  2 . On the other hand, the bolster  8  is fixed to an upper portion of a bed  9 , and a lower die  3   b  is mounted on an upper portion of the bolster  8 . A plurality of holes are provided vertically in the bolster  8  and the lower mold  3   b . Cushion pins  7  are respectively inserted into these holes. An upper end of the cushion pin  7  is contacted with the lower portion of a blank holder  5  provided in a recess portion of the lower die  3   b , and a lower end of the cushion pin  7  is contacted with a cushion pad  11  of a die cushion  10  provided in the bed  9 . A beam  6  is provided between inner wall surfaces of the bed  9 , and the die cushion  10  is supported by the beam  6 . 
   First Embodiment 
     FIG. 2  is a schematic view of the die cushion according to the first embodiment.  FIG. 3  is a top view of the die cushion according to the first embodiment. 
   In the die cushion  10 , the cushion pad  11  is coupled to the rotary shaft of a servomotor  16  via a ball screw  12 , a coupling member  25 , a large pulley  13 , a belt  14  and a small pulley  15 . Powers of the cushion pad  11  and the servomotor  16  are transmissible to each other. A nut portion  12   a  of the ball screw  12  is coupled to a lower portion of the cushion pad  11 . A threaded portion  12   b  of the ball screw  12  is engaged with the nut portion  12   a . A lower part of the threaded portion  12   b  is connected to the coupling member  25 . The coupling member  25  is supported to the beam  6  by a bearing, etc. and its lower part is coupled to the large pulley  13 . The small pulley  15  is connected to the rotary shaft of the servomotor  16 . The belt  14  is wound on the large pulley  13  and the small pulley  15 , and the powers of the large pulley  13  and the small pulley  15  are transmissible to each other. 
   The rotary type servomotor  16  has a rotary shaft that is rotated in normal and reverse directions by the supply of a current. When the current is supplied to the servomotor  16  and the rotary shaft is rotated, the small pulley  15 , the large pulley  13 , the coupling member  25 , and the threaded portion  12   b  are rotated. As the threaded portion  12   b  is rotated, the nut portion  12   a  is linearly operated in upward and downward directions, that is, in raising and lowering directions along the threaded portion  12   b . Then, the cushion pad  11  is raised and lowered together with the nut portion  12   a . The energizing force applied to the cushion pad  11 , that is, the cushion pressure generated in the cushion pad  11  is controlled under the current control to the servomotor  16 . 
   Various types of measuring devices are provided in the die cushion  10 . To measure a load generated in the cushion pad  11 , a strain gauge  17  is adhered to a side face of the cushion pad  11 . A pressure generated in the cushion pad  11  is measured by this strain gauge  17 . A linear scale  18  in which a raising or lowering direction is set as a measuring direction is provided between the cushion pad  11  and the bed  9 . A scale portion  18   a  of the linear scale  18  is provided on an inner wall surface of the bed  9 , and a head portion  18   b  is fixed to the cushion pad  11  side closely to the scale portion  18   a . As the cushion pad  11  is raised or lowered, the head portion  18   b  moves along the scale  18   a . The raised or lowered position of the cushion pad  11  is measured by this linear scale  18 . An encoder  19  is provided on a periphery of the rotary shaft of the servomotor  16 . The rotational speed of the servomotor  16  is measured by this encoder  19 . Each measured value is inputted to a pad controller  30 , and a supply current to the servomotor  16  is outputted. The pad controller  30  will be described later. 
   Further, one or more guides  21  are provided between each side face of the cushion pad  11  and the inner wall surface of the bed  9  opposed to each side face of the cushion pad  11 . The guides  21  include a pair of inner guides  21   a  and outer guides  21   b  engaged with each other. The inner guides  21   a  are provided on the side faces of the cushion pad  11 , and the outer guides  21   b  are provided on the inner wall surface of the bed  9 . The guides  21  guide the cushion pad  11  in the raising and lowering direction. 
   Then, the feedback control of the die cushion will be described. 
     FIG. 4  is a control block diagram performed in the first embodiment. 
   The pad controller  30  has a controller  31  and an amplifier  32 . In the controller  31 , a pressure pattern showing a desired corresponding relation between a time (or a press angle or a slide position) and a pressure generated in the cushion pad  11 , that is, a cushion pressure, and a position pattern showing a desired corresponding relation between and the time (or the press angle or the slide position) and a position of the cushion pad  11 , are set. In the controller  31 , the cushion pressure corresponding to the time (or the press angle or the slide position) is obtained by using the pressure pattern, and outputted as a pressure control signal Sp. The cushion position corresponding to the time (or the press angle or the slide position) is obtained by using the position pattern, and outputted as a position control signal Sh. The pressure control signal Sp, the position control signal Sh and the other measured values are inputted to the amplifier  32 . A supply current I from the amplifier  32  is outputted to the servomotor  16 . In the amplifier  32 , any of the pressure feedback control or the position feedback control is performed, and both are switched at a predetermined timing. 
   It should be noted that the “pressure” of the pressure pattern includes a load applied to the cushion pad  11  and a strain occurred in a member of the cushion pad  11 . Because the load and the strain are correlated with each other. In the case where the oil pressure chamber is provided, as described in the embodiments 4 and 5, the oil pressure in the oil pressure chamber may be used as the “pressure”. 
   Here, concerning the feedback control performed in the pad controller  30 , the pressure feedback control will be described first. 
   The pressure generated in the cushion pad  11 , that is, the cushion pressure is measured by the strain gauge  17 , and its value is outputted as a pressure feedback signal Spf to a pressure comparator  33 . In the pressure comparator  33 , a value of the pressure feedback signal Spf is compared with a value of the pressure control signal Sp, and a pressure correction signal Spc is generated. The pressure correction signal Spc is outputted to a pressure controller  34 . In the pressure controller  34 , a suitable speed of the servomotor  16  is obtained based on the pressure correction signal Spc, and a motor speed control signal Sr 1  is generated. The motor speed control signal Sr 1  is outputted to a speed comparator  35 . 
   A rotary speed of the servomotor  16  is measured by the encoder  19 , and its value is outputted as a speed feedback signal Srf to the speed comparator  35 . In the speed comparator  35 , a value of the motor speed control signal Sr 1  (Sr 2  in the case of the position feedback control) is compared with a value of the speed feedback signal Srf, and a motor speed correction signal Src is generated. The motor speed correction signal Src is outputted to the speed controller  36 . In the speed controller  36 , a suitable current value to the servomotor  16  is obtained based on the motor speed correction signal Src, and a current control signal Sc is generated. The current control signal Sc is outputted to a current comparator  37 . 
   The supply current to the servomotor  16  is measured by a current detector  39 , and its value is outputted as a current feedback signal Scf to the current comparator  37 . In the current comparator  37 , a value of the current control signal Sc is compared with a value of the current feedback signal Scf, and a current correction signal Scc is generated. The current correction signal Scc is outputted to a current controller  38 . In the current controller  38 , a suitable supply current I to the servomotor  16  is generated based on the current correction signal Scc. The supply current I is outputted to a current detector  39 , and supplied to the servomotor  16 . Then, the servomotor  16  drives the cushion pad  11 . In this case, the cushion pad  11  is lowered while generating the upward energizing force. Thus, the set cushion pressure is obtained. 
   Then, the position feedback control will be described. 
   A height position of the cushion pad  11  is measured by the head portion  18   b  of the linear scale  18 , and its value is outputted as a position feedback signal Shf to a position comparator  43 . In the position comparator  43 , a value of the position feedback signal Shf is compared with a value of a position control signal Sh, and a position correction signal Shc is generated. The position correction signal Sch is outputted to the position controller  44 . In the position controller  44 , a suitable speed of the servomotor  16  is obtained based on the position correction signal Shc, and the motor speed control signal Sr 2  is generated. The motor speed control signal Sr 2  is outputted to the speed comparator  35 . A flow of the signal after the motor speed comparator  35  is the same as the pressure feedback control. 
   Incidentally, in the pad controller  30 , functions up to the speed controller  36  may be incorporated in the controller  31  side, and functions after the current comparator  37  may be incorporated in the amplifier  32  side. 
   The pressure feedback control and the position feedback control are switched by a switch operation of a switching unit  45 . In this embodiment, when a first switching time in which the upper die is contacted with the work, is detected, the position feedback control is switched to the pressure feedback control. When a second switching time in which the cushion pad  11  reaches the bottom dead point, is detected, the pressure feedback control is switched to the position feedback control. 
   The first switching time is a time when the measured value of the strain gauge  17  reaches a first threshold value at the time the cushion pad  11  is lowered (when the upper die is contacted with the work and the pressure of the cushion pad  11  starts generating), or a time when the measured value of the head portion  18   b  of the linear scale  18  reaches a first predetermined position (when the cushion pad  11  reaches the position where the upper die is contacted with the work). The second switching time is a time when the measured value of the strain gauge  17  reaches a second threshold value at the time the cushion pad  11  is lowered (when the upper die is separated from the work and the pressure of the cushion pad  11  is vanished), or the measured value of the head portion  18   b  of the linear scale  18  reaches a second predetermined position (when the cushion pad  11  reaches the bottom dead point). 
   Then, the relationship between an operation of the cushion pad  11  and the pressure and position feedback controls will be described by using  FIG. 4  and  FIG. 5 . 
     FIG. 5  is a view showing an operation of the slide and the die cushion pad, which shows positional changes of the slide and the cushion pad along with the passage of time. 
   In the press machine, to alleviate the impact when the upper die is contacted with the work, the cushion pad  11  is preliminarily accelerated. Between a time t 1  and a time t 2 , the preliminary acceleration is performed. During this period, the position feedback control is performed in the pad controller  30 , and the position of the cushion pad  11  is controlled so that the position measured value follows the preset position pattern. The cushion pad  11  is lowered in response to its result. 
   At the time t 2  (first switching time), the upper die is contacted with the work. At this time, the switch is switched in the switching unit  45  of the pad controller  30 , and the position feedback control is switched to the pressure feedback control. Between the time t 2  and a time t 3 , the slide  2  is lowered together with the cushion pad  1 , and the work is drawn. During this period, the pressure feedback control is performed in the pad controller  30 , and the energizing force applied to the cushion pad  11  is controlled so that the pressure measured value follows the preset pressure pattern. The cushion pad  11  is lowered in response to the result thereof. At the time t 3  (second switching time), the slide  2  and the cushion pad  11  reach the bottom dead point. At this time, the switch is switched in the switching unit  45  of the pad controller  30  and the pressure feedback control is switched to the position feedback control. Between the time t 3  and a time t 4 , the slide  2  and the cushion pad  11  are raised together for an amount of an auxiliary lift. Between the time t 4  and a time t 5 , the cushion pad is locked, and a raising operation is temporarily stopped. At the time t 5 , the cushion pad  11  again starts raising. As described above, after the time t 3 , the position feedback control is performed in the pad controller  30 , and the position of the cushion pad  11  is controlled so that the position measured value follows the preset position pattern. The cushion pad  11  is raised in response to the result thereof. 
   In this embodiment, a pressure generated in the cushion pad  11 , that is, the cushion pressure is measured, and the pressure feedback control is performed, but the feedback control based on the energizing force applied to the cushion pad  11  is considered to be one type of the pressure feedback control. 
   According to the first embodiment, since the pressure feedback control of the closed loop for feeding back the cushion pressure measured from the cushion pad itself is performed at a timing necessary for the pressure feedback of the cushion pad, the cushion pressure of the cushion pad can be controlled in high accuracy Therefore, the workability of the press can be improved. 
   Incidentally, the present invention can be applied to various types of die cushion. Part of them will be described in a second embodiment to a sixth embodiment. 
   Second Embodiment 
     FIG. 6  is a schematic view of a die cushion according to the second embodiment. Concerning the die cushion  50  shown in  FIG. 6 , only different portion from the die cushion  10  shown in  FIG. 2  will be described. 
   In the die cushion  50 , the cushion pad  11  is coupled to a rotary shaft of the servomotor  16  via a ball screw  52 , a coupling member  55 , a large pulley  13 , a belt  14  and a small pulley  15 . Between the cushion pad  11  and the servomotor  16 , powers are transmissible to each other. The threaded portion  52   b  of the ball screw  52  is coupled to the lower portion of the cushion pad  11 . The threaded portion  52   b  of the ball screw  52  is engaged with a nut portion  52   a . A lower part of the nut portion  52   a  is connected to the coupling member  55 . The coupling member  55  is supported by a bearing, etc. to the beam  6 , and its lower portion is coupled to the large pulley  13 . The small pulley  15  is connected to the rotary shaft of the servomotor  16 . A belt  14  is wound around the large pulley  13  and the small pulley  15  and their powers are transmissible to each other. 
   When a current is supplied to the servomotor  16  and the rotary shaft is rotated, the small pulley  15 , the large pulley  13 , the coupling member  55 , and the nut portion  52   a  are rotated. As the nut portion  52   a  is rotated, the threaded portion  52   b  is linearly moved in a vertical direction, that is, in a raised or lowered direction along the nut portion  52   a . Then, the cushion pad  11  is raised or lowered together with the threaded portion  52   b . The energizing force applied to the cushion pad  11  under the current control of the servomotor  16 , that is, the cushion pressure generated in the cushion pad  11  is controlled. 
   In the die cushion  50 , the strain gauge  17 , the linear scale  18 , the encoder  19 , and the pad controller  30  are similar to those in the die cushion  10  of the first embodiment. In the pad controller  30 , the feedback control similar to the feedback control of the first embodiment is performed. 
   According to the second embodiment, the similar effects to those of the first embodiment can be obtained. 
   Third Embodiment 
     FIG. 7  is a schematic view of a die cushion according to a third embodiment.  FIG. 8  is a top view of the die cushion according to the third embodiment. Concerning the die cushion  60  shown in  FIG. 7  and  FIG. 8 , only a portion different from the die cushion  10  shown in  FIG. 2  will be described. 
   A linear servomotor  61  is provided between each side face of the cushion pad  11  and each inner wall surface of the bed  9  opposed to the side face of the cushion pad  11 . The linear servomotor  61  includes a pair of a coil portion  61   a  and a magnet portion  61   b . The coil portion  61   a  is provided on each side face of the cushion pad  11 , and the magnet portion  61   b  is provided on the inner wall surface of the bed  9 . Contrarily, the magnet portion  61   b  may be provided on each side face of the cushion pad  11 , and the coil portion  61   a  may be provided on the inner wall surface of the bed  9 . Incidentally, in  FIG. 7 , the linear servomotor  61  is shown only on the right side face of the cushion pad  11  and the facing inner wall surface of the bed  9 . However, actually, the linear servomotor  61  is provided on each side face of the cushion pad  11  and the facing inner wall surface of the opposed bed  9 , as shown in  FIG. 8 . 
   In the case that the coil portion  61   a  is provided in the cushion pad  11 , when the coil portion  61   a  is excited, an attraction force and a repelling force act between the coil portion  61   a  and the magnet portion  61   b , thereby the coil portion  61   a  and the cushion pad  11  receive an energizing force of a raising and lowering direction. In the case that the magnet portion  61   b  is provided in the cushion pad  11 , when the coil portion  61   a  is excited, the attraction force and the repelling force act between the coil portion  61   a  and the magnet portion  61   b , thereby the magnet portion  61   b  and the cushion pad  11  receive an energizing force of the raising and lowering direction. When the supply current to the coil portion  61   a  is controlled, the energizing force applied to the cushion pad  11 , that is, the cushion pressure generated in the cushion pad  11  is controlled. 
   An air pressure type balancer  62  having a piston and a cylinder is provided in the lower portion of the cushion pad  11 . Though not shown, the piston of the balancer  62  is supported at a lower portion by the beam  6 . Thus, since the cushion pad  11  is supported by the beam  6  via the balancer  62 , even if a magnetic force between the coil portion  61  and the magnet portion  61   b  is eliminated as a power source of the linear servomotor  61  is cut off, the cushion pad  11  does not drop down. 
   In the die cushion  60 , the strain gauge  17 , the linear scale  18 , and the pad controller  30  are similar to those in the die cushion  10  of the first embodiment. 
   Concerning the feedback control, it is basically the same as the die cushion  10  of the first embodiment. However, since the rotary type servomotor and the linear drive type servomotor are different in structures, a feedback control system of the motor speed is slightly different. Here, only that difference will be described. 
     FIG. 9  is a control block diagram of the feedback control performed in the third embodiment. 
   The speed of the linear servomotor  61  is a relative speed of the coil portion  61   a  to the magnet portion  61   b . That is, a raising or lowering speed of the cushion pad  11 . The raising or lowering speed of the cushion pad  11  is obtained by differentiating a displacing amount with respect to time. The raising or lowering speed is differentiated based on a position signal measured by the head portion  18   b , and its value is outputted as a speed feedback signal Svf to the speed comparator  35 . In the speed comparator  35 , a value of the motor speed control signal Sv 1  (Sv 2  in the case of the position feedback control) is compared with a value of the speed feedback signal Svf, and a motor speed correction signal Svc is generated. The motor seed correction signal Svc is outputted to the speed controller  36 . In the speed controller  36 , a suitable current value to the servomotor  16  is obtained based on the motor speed correction signal Svc, and a current control signal Sc is generated. The current control signal Sc is outputted to the current comparator  37 . 
   Incidentally, the pressure feedback control system and the current feedback control system are similar to those in the first embodiment. 
   According to the third embodiment, the similar effects to the first embodiment can be obtained. 
   According to the third embodiment, a power transmission between the servomotor and the cushion pad is not performed by a mechanical contact using an engaging member, such as a gear, a belt, a ball screw, etc., but is performed by non-contact using a magnetic force. Therefore, a mechanical sound in the power transmission is eliminated and an operating sound of the press machine is reduced. 
   According to the third embodiment, the number of components is reduced as compared with the case of using the rotary servomotor. Therefore, maintenance of the die cushion is facilitated. 
   Fourth Embodiment 
     FIG. 10  is a schematic view of the die cushion according to a fourth embodiment. Concerning the die cushion  10  shown in  FIG. 10 , only a portion different from the cushion  10  shown in  FIG. 2  will be described. 
   In the die cushion  70 , the cushion pad  11  is coupled to the rotary shaft of the servomotor  16  via a plunger rod  73 , a piston  74 , a ball screw  72 , a coupling member  75 , a large pulley  13 , a belt  14  and a small pulley  15 . Between the cushion pad  11  and the servomotor  16 , powers are transmissible to each other. 
   The columnar plunger rod  73  is connected to the lower portion of the cushion pad  11 . The plunger rod  73  is slidably supported at its side face by a cylindrical plunger guide  76 . The plunger guide  76  is mountable on the beam  6 . When the plunger guide  76  is fixed to the beam  6 , the plunger rod  73  is raised or lowered while being supported by the plunger guide  76 . The plunger guide  76  guides the plunger rod  73  and the cushion pad  11  coupled to the plunger rod  73  in a raising or lowering direction. 
   A cylinder  73   a  having an opening in a downward direction is formed in a lower portion of the plunger rod  73 , and the piston  74  is slidably contained in the cylinder  73   a . An oil pressure chamber  77  is formed by the inner wall surface of the cylinder  73   a  and the upper face of the piston  74 , and pressure oil is filled in this oil pressure chamber  77 . The axial center of the oil pressure chamber  77  is the same as those of the plunger rod  73  and the ball screw  72 . The pressure oil for alleviating an impact is filled in the oil pressure chamber  77 . The pressure oil in the oil pressure chamber  77  alleviates the impact generated when the upper die contacts with the work. 
   As shown in  FIG. 11 , it may be arranged that a conduit  85  is communicated with the oil pressure chamber  77  to supply the pressure oil to the oil pressure chamber  77  and discharge the pressure oil from the oil pressure chamber  77 . An oil pressure circuit shown in  FIG. 12 ,  FIG. 14  and  FIG. 15  is connected to the oil pressure chamber  77  via the conduit  85 . Details of these oil pressure circuits will be described with reference to the fifth embodiment. 
   A lower end of the piston  74  is contacted with an upper end of the threaded portion  72   b  of the ball screw  72 . A spherical recess surface  74   a  is formed on the lower end of the piston  74 , and a spherical protruding surface  72   c  is formed on the upper end of the threaded portion  72   b  opposed to this recess surface  74   a . Contrarily, a protruding surface is formed on the lower end of the piston  78 , and a recess surface may be formed on the upper end of the threaded portion  72   b . A bar-like member like the threaded portion  72   b  is strong against the axial force acting on the end portion, but is weak to a bending moment. When the upper end of the threaded portion  72   b  is formed in a spherical shape, even if the cushion pad  11  is inclined so that the bending moment is generated at the upper end of the threaded portion  72   b , only the axial force acts on the entire threaded portion  72   b . A damage of the threaded portion  72   b  due to an eccentric load can be prevented by such a structure. 
   A coupling member  75  is interposed between the nut portion  72   a  of the ball screw  72  and the large pulley  13  and the coupling member  75  is supported to the beam  6  by a bearing, etc. The small pulley  15  is connected to the rotary shaft of the servomotor  16 . A belt  14  is wound on the large pulley  13  and the small pulley  15 , and their powers are transmissible to each other. 
   When a current is supplied to the servomotor  16  and the rotary shaft is rotated, the small pulley  15  and the large pulley  13  are rotated. Since the large pulley  13 , the coupling member  75  and the nut portion  72   a  are integral, the nut portion  72   a  is rotated along with the rotation of the large pulley  13 . As the nut portion  72   a  is rotated, the threaded portion  72   b  linearly moves along the nut portion  72   a  in a vertical direction, that is, in a raising or lowering direction. The cushion pad  11  is raised or lowered together with the threaded portion  72   b , the piston  74  and the plunger rod  73 . The energizing force applied to the cushion pad  11 , that is, the cushion pressure generated in the cushion pad  11  is controlled under the current control to the servomotor  16 . 
   In the die cushion  70 , concerning the strain gauge  17 , the linear scale  18 , the encoder  19  and the pad controller  30  are similar to those of the die cushion  10  of the first embodiment. In the pad controller  30 , a feedback control similar to the feedback control of the first embodiment is performed. 
   Incidentally, the strain gauge  17  may be provided on a side face of the plunger rod  73 , not on a side face of the cushion pad  11 . 
   According to the fourth embodiment, the similar effects to those in the first embodiment can be obtained. 
   Fifth Embodiment 
   Concerning the die cushion  70  shown in  FIG. 11 , it may also be considered to measure a pressure in the oil pressure chamber  77 , not measuring a pressure generated in the cushion pad  11  by the strain gauge  17 . 
     FIG. 12  is an oil pressure circuit diagram according to a fifth embodiment.  FIG. 13  is a control block diagram of the feedback control performed in the firth embodiment. 
   The pressure oil discharge port of an oil pressure pump  83  communicates with a pressure oil port of the oil pressure chamber  77  via a check valve  81  and a conduit  85 . A branch conduit is connected to a conduit between the oil pressure pump  83  and the check valve  81 , and this branch conduit communicates with a relief valve  82 . Further, the relief valve  82  communicates with a tank  84 . The pressure oil discharged from the oil pressure pump  83  is set to a predetermined pressure by the relief valve  82 , and the residual pressure oil is returned to the tank  84 . Incidentally, by the check valve  81 , the pressure change in the oil pressure chamber  77  does not affect influence directly to the oil pressure pump  83 . 
   A branch conduit is connected to the conduit  85 , and this branch conduit communicates with the relief valve  93 . Furthermore, the relief valve  93  communicates with the tank  84 . In the relief vale  93 , the maximum oil pressure for preventing overloading is set as a relief pressure. When the oil pressure in the oil pressure chamber  77  reaches the maximum oil pressure, the relief valve  93  is opened, and the pressure oil in the conduit  85  is returned to the tank  84  via the relief valve  93 . Then, the oil pressure in the conduit  85  lowers. When a measured value of a pressure sensor  86  becomes a predetermined pressure or lower, a controller, not shown, emergency stops the press machine. Therefore, the pressure oil in the conduit  85  is discharged to the tank  84  to thereby prevent overloading. 
   The pressure sensor  86  is provided in the conduit  85 . The pressure in the oil pressure chamber  77 , that is, a load generated in the cushion pad  11  is measured by the pressure sensor  86 . The measured value of the pressure sensor  86  is outputted to the pad controller  30 . The feedback control shown in the control block diagram of  FIG. 13  is fundamentally the same as the feedback control shown in the control block diagram of  FIG. 4 . 
     FIG. 14  is an oil pressure circuit diagram according to another form of the firth embodiment. 
   As shown in  FIG. 14 , a directional control valve  88  may be provided instead of the relief valve  93  of  FIG. 12 . Normally, the directional control valve  88  presses a spool, a poppet, etc., provided in itself by a spring force, and shuts off the conduit  85  and the tank  84 . When the measured value of the pressure sensor  86  exceeds a predetermined pressure, there might be overloading. The measured value of the pressure sensor  86  is outputted to a pressure controller  87 , and when the measured value exceeds a predetermined pressure, the pressure controller  87  outputs a relief signal to the directional control valve  88 . The directional control valve  88  which has received the relief signal, excites a coil provided in itself. When a propulsion force by the magnetic fore exceeds the pressing force by the spring force, the spool, the poppet, etc. move. Thus, the directional control valve  88  is switched, and the conduit  85  communicates with the tank  84 . Then, the oil pressure in the conduit  85  is returned to the tank  84  via the directional control valve  88 . The pressure controller  87  outputs an emergency stop signal to the controller of the press machine, not shown, together with the relief signal. The controller emergency stops the press machine in response to the input of the emergency stop signal. Thus, the overloading is prevented. 
     FIG. 15  is also an oil pressure circuit diagram according to another form of the fifth embodiment. 
   As shown in  FIG. 15 , a protector valve  95  may be provided instead of the relief valve  93  of  FIG. 12 . The protector valve  95  has a small diameter oil chamber  95   a  and a large diameter air chamber  95   b , and further has a piston  95   c  having a small diameter piston slidable in the oil chamber  95   a  and a large diameter piston slidable in the air chamber  95   b . The conduit  85  communicates with the oil chamber  95   a . The air chamber  95   b  communicates with an air pressure source  99  via a directional control valve  96 , a check valve  97  and a pressure regulator  98 . An oil pressure port is provided at a side face of the oil chamber  95   a . The oil pressure port communicates with the tank  84 . 
   The air pressure in the air chamber  95   b  is set by the pressure regulator  98 , so that the piston  95   c  is balanced when the oil pressure in the conduit  85  is the maximum oil pressure for preventing the overloading. That is, when the oil pressure in the conduit  85  becomes the maximum oil pressure or higher, the piston  95   c  moves to the air chamber  95   b  side. The conduit  95  communicates with the tank  94  by the movement of the piston  95   c . Then, the pressure oil in the conduit  85  is returned to the tank  84  via the protector valve  95 . When the piston  95   c  moves to the air chamber  95   b  side, a proximity switch detects the movement of the piston  95   c , and outputs an emergency stop signal to the controller of the press machine, not shown. The controller emergency stops the press machine in response to the input of the emergency stop signal. Thus, the overloading is prevented. 
   Normally, the directional control valve  96  presses the spool, the poppet, etc. provided in itself by a spring force to bring the conduit  85  into communication with the tank  84 . When the solenoid in the directional control valve  96  is energized, a propulsion force is generated by a magnetic force at the spool, the poppet, etc. When the propulsion force by the magnetic force exceeds the pressing force by the spring force, the spool, the poppet, etc. move. Thus, the directional control valve  96  is switched, and the air in the air chamber  95   b  is discharged to the atmosphere via a silencer  90 . Then, the oil in the oil chamber  77  is returned to the tank  84 . Such an operation of the directional control valve  96  is mainly performed at a maintenance time. 
   According to the fifth embodiment, the similar effects to the first embodiment can be obtained. 
   Sixth Embodiment 
   In the respective embodiments, the die cushion of a single piece has been described. However, a plurality of die cushions may be provided in one working station of the press machine. In this case, it is preferable to set the positional relationship between the cushion pad and its drive mechanism as follows. The positional relationship will be described with the die cushion  70 ′ shown in  FIG. 16  as an example. 
     FIG. 16  is a view for explaining the positional relationship between the cushion pad and its drive mechanism. 
   First, there is assumed a first projected image  91  when projected from perpendicularly above of the cushion pad  11  downward to a horizontal surface. Similarly, there is also assumed a second projected image  92  when projected from perpendicularly above of the drive mechanism, such as the plunger rod  73 , the plunger guide  76 , the ball screw  72  and the servomotor  16 , etc. disposed under the cushion pad  11 . The cushion pad  11  and its drive mechanism are disposed to include all the second projected image  92  in the first projected image  91 . According to this disposition, the space of the die cushion  70 ′ in the horizontal direction does not become larger than the area of the upper surface of the cushion pad  11 . That is, even if the cushion pads  11  are provided adjacently to each other, the drive mechanism of the lower portions of the respective cushion pads  11  may not interfere with each other, and a plurality of die cushions  70 ′ can be provided adjacent to one working station. 
   In  FIG. 16 , if the projected image to downward of the servomotor  16 , the belt  14  and the small pulley  15  is out of the first projected image  91 , it may be possible to dispose the adjacent die cushions  70 ′ close to each other by changing the height of the belt  14  or reversing the disposition of the servomotor  16  with each other. Thus, the area of the cushion pad  11  of the respective die cushions  70 ′ can be further reduced, the disposition of the die cushion  70 ′ is facilitated, and the degree of freedom of the disposition is increased. 
     FIGS. 17A to 17D  are top views of one working station. In  FIG. 17A , one die cushion  70 ′ is provided in one working station of the press machine. In  FIG. 17B , two die cushions  70 ′ are provided in one working station of the press machine. In  FIG. 17C , four die cushions  70 ′ are provided in one working station of the press machine. In  FIG. 17D , eight die cushions  70 ′ are provided in one working station of the press machine. 
   The respective die cushions  70 ′ are controlled independently from each other. Therefore, the cushion pressure in one working station becomes variable. Also, die cushions  70 ′ may be interlocked. 
   When comparing a case where one cushion pad having a plurality of drive mechanisms is provided in one working station and the operation of this cushion pad is controlled, with a case where a plurality of cushion pads each having one drive mechanism are provided in one working station and the operation of each cushion pad is controlled, it is said that the latter case has higher independent controllability since the cushion pads are divided. 
   In this embodiment, as the die cushion provided in plural in one working station, the die cushion  70 ′ equivalent to the die cushion  70  shown in  FIG. 10  has been described as an example. However, it may be the die cushion  10  shown in  FIG. 2 , the die cushion  50  shown in  FIG. 6 , or the die cushion equivalent to the die cushion  60  shown in  FIG. 7  may be adopted. However, in such a case, it is necessary to provide a guide member for guiding the die cushion on opposed side faces of the cushion pads  11  adjacent to each other. Since the cushion pad  70  ( 70 ′) has itself the guide member, that is, the plunger guide  76  to the die cushion  10 ,  50 , or  60 , it is not necessary to provide the guide member for guiding the cushion pad  11  to each other. 
   According to the sixth embodiment, the effects similar to the first embodiment can be obtained. Further, according to the sixth embodiment, since the cushion pressure in one work station can be changed partially, the accuracy of the press machine can be further improved.