Abstract:
An electromechanical clamping device with hydraulic assistance. A thrust mechanism driven by an electric motor displaces two clamping jaws with respect to each other along a guide device. One clamping jaw is divided into two mutually displaceable part java. A chamber to hold pressure medium is formed between the part jaws. The first part jaw can be displaced along the guide device and locked with respect to the latter. One part jaw is provided with a first cylinder coupled to the output of the thrust mechanism. In order to close the clamping device, in a first step the thrust mechanism pulls the part jaws against the other clamping jaw, and in a second step the thrust mechanism delivers pressure medium from the first cylinder into the chamber between the two part jaws, with the first part jaw locked with respect to the guide device. To open the clamping device, in a first step a second cylinder arranged between the two part jaws pulls the part jaws against each other, with the first part jaw locked with respect to the guide device, and then in a second step the thrust mechanism forces the part jaw coupled to it in the opening direction, forcing the other part jaw with it, with the first part jaw unlocked from the guide device. The clamping device is provided for production machines, in particular for injection molding machines.

Description:
The invention pertains to an electromechanical clamping device with hydraulic assistance for a production machine, in particular for an injection molding machine. 
   FIELD AND BACKGROUND OF THE INVENTION 
   In this case, the invention is based on an electromagnetic clamping device with hydraulic assistance. Clamping devices of this type are used for production machines, for example injection molding machines. The clamping device has two clamping jaws on which in each case a part mold of a divided mold is held. A thrust mechanism driven by an electric motor displaces the clamping jaws with respect to each other along a guide device. The first clamping jaw is divided into two part jaws, which can be displaced with respect to each other in the direction of the guide device. The part jaws interengage in such a way that a chamber to hold pressure medium is formed between them. The first part jaw can be displaced along the guide device. The output of the thrust mechanism acts on one of the part jaws. In order to close the clamping device, in a first step the thrust mechanism pulls one part jaw against the second part jaw, pulling the other part jaw with it, and in a second step the thrust mechanism delivers pressure medium into the chamber between the two part jaws, with the first part jaw locked against the guide device. During the closing of the clamping device, the thrust mechanism is loaded in tension. By contrast, during the opening of the clamping device, the thrust mechanism is loaded in compression. In this case, in order to pull the part molds apart, a substantially higher force is required than for the following action of moving apart the clamping jaws bearing the part molds. The thrust mechanism therefore has to be dimensioned in accordance with the high force required to pull the mold apart. 
   SUMMARY OF THE. INVENTION 
   The invention is based on the object of providing a clamping device with hydraulic assistance in which the thrust mechanism needs to be dimensioned only in accordance with the force required to pull the clamping jaws apart, without there being any overloading of the thrust mechanism when pulling the mold apart. 
   This object is achieved by a device according to the invention, wherein by means of the hydraulic assistance during the pulling-apart procedure, the compressive force exerted by the electric motor on the thrust rod can be limited in such a way that no impermissibly high bending of the thrust rod takes place during the pulling-apart procedure. 
   Advantageous developments of the invention are respectively the subject matter of further features set forth herein. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be explained in more detail below, with its further details, using exemplary embodiments illustrated in the drawings, in which 
       FIG. 1  shows the structure of a first clamping device according to the invention in a simplified illustration, 
       FIG. 2  shows the structure of a second clamping device according to the invention in a simplified illustration, and 
       FIG. 3  shows the block diagram of a control system for the clamping devices illustrated in FIGS.  1  and  2 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1  shows the structure of a clamping device according to the invention in a simplified illustration. The clamping device has a fixed clamping jaw  11  and a clamping jaw  13  which can be displaced along a guide device  12 . The guide device  12  is formed by two crossbeams  12   a  and  12   b , which are provided with a latching division. In order to lock the displaceable clamping jaw  13  to the guide device  12 , a latching device  15 , which is illustrated only schematically and is controlled by a signal y v3 , latches into a detent in the crossbeam  12   a . In a corresponding way, a further controlled latching device which, for reasons of clarity, is not illustrated in  FIG. 1 , latches into a detent in the crossbeam  12   b . The clamping jaw  13  is formed from two interengaging part jaws  16  and  17 , which can be displaced with respect to each other in the direction of the guide device  12 . The latching device  15  is held on the part jaw  16 . The part jaws  16  and  17  enclose between them a chamber  20  to hold hydraulic pressure medium. Between the clamping jaws  11  and  13 , a mold  23  comprising two part molds  21  and  22  is held. The first part mold  21  is held on the clamping jaw  11 . The other part mold  22  is held on the part jaw  17 . An electric motor  25 , which is held on the fixed clamping jaw  11 , drives a thrust mechanism  26 . The electric motor  25  is supplied, by a machine control system illustrated in  FIG. 3 , with a desired rotational speed value n s  as input variable. The current i Mi  drawn by the electric motor  25  is a measure of the force acting on the thrust rod  27 . The current i Mi  is supplied to the control device, illustrated in  FIG. 3 , as input signal. The thrust mechanism  26  translates the rotational movement of the electric motor  25  into a longitudinal movement of the thrust rod  27 , which forms the output of the thrust mechanism  26 . A coupling  28  connects the thrust rod  27  via a piston rod  31  to the piston  32  of a double-ended cylinder  33 . In accordance with the publication “Grundlagen und Komponenten der Fluidtechnik—Der Hydraulik Trainer, Band 1” [Principles and Components of Fluid Technology—the Hydraulic Trainer, Vol. 1] from Mannesmann Rexroth GmbH, RD 00290/10.91 (second edition, 1991), pp. 124 to 125, double-ended cylinder here designates a double-acting cylinder with piston rods on both sides, in which the diameters of the two piston rods can also be of different sizes. The double-ended cylinder  33  is held on the displaceable part jaw  17 . On the side facing away from the thrust rod  27 , the piston  32  is connected to a second piston rod  34 . The piston rod  34  penetrates into a further chamber  35 . The chamber  35  is connected via a first nonreturn valve  36  to the chamber  40  on the rod side of a differential cylinder  41 . In addition, the chamber  35  is connected to a tank  43  via a replenishing valve  42 . The differential cylinder  41  is held on the part jaw  16 . It is connected via a piston rod  45  to the part jaw  17 . The piston of the differential cylinder  41  is provided with the designation  46 , and the chamber on the crown side with the designation  47 . A displacement transducer  50  converts the output signal from a displacement sensor  51 , which is connected to the piston  46 , into an electric signal x i , which is a measure of the position of the piston  46  and therefore also a measure of the distance between the part jaws  16  and  17 . A pressure transducer  52  converts the pressure in the chamber  40  into an electric signal p i  corresponding to said pressure. A hydraulic control device in the form of a proportional valve  53  controls the flow of pressure medium from a pump  54  to the differential cylinder  41  and from the latter to the tank  43  as a function of an actuating signal y v1 . A nonreturn valve  55  is arranged between the pump  43  and the proportional valve  41 . A pressure limiting valve  56  limits the output pressure of the pump  54  in the usual way. Arranged between the pump  54  and the double-ended cylinder  33  is a hydraulic control device in the form of a control valve  60  with three control positions. The control valve  60  controls the flow of pressure medium from the pump  54  to the double-ended cylinder  33  and from the latter to the tank  43  as a function of an actuating signal y v2 . Seals are designated  62 ,  63  and  64 . 
   In order to act uniformly with force on the part jaw  17 , in addition to the electric motor  25 , a further electric motor  25 * is provided, which is connected via a thrust mechanism  26 *, a thrust rod  27 * and a coupling  28 * to the piston rod  31 * of a further double-ended cylinder corresponding to the double-ended cylinder  33 . The piston rod of a further differential cylinder corresponding to the differential cylinder  41  is designated  45 *. The construction and pressure medium supply of the further double-ended cylinder and of the further differential cylinder correspond to the cylinders already described above. 
     FIG. 3  shows the block diagram of a control system for the upper half of the clamping device illustrated in FIG.  1 . The control of the components provided with an * of the lower half of the clamping device illustrated in  FIG. 1  is carried out in a corresponding way. The parameters and desired values required for the control of the operating sequence of the production machine are stored in a higher-order machine control system  70 . The machine control system  70  supplies the electric motor  25  with the desired rotational speed value n s . The machine control system  70  determines the position of the control piston of the control valve  60  via the actuating signal y v2  and controls the latching device  15  via the actuating signal y v3 . The machine control system  70  supplies an electric control device  71  with a desired position value x s  for the position of the piston  46 , a desired current value i Ma  for the electric motor  25 , a threshold value i Mu  for changing over from position control to force control, an actuating signal y v10  and a changeover signal y u . The desired current value i Ma  determines the maximum value of the force with which the electric motor  25  forces the thrust rod  27  against the part jaw  17 . As already described above, the control device  71  of the clamping device illustrated in  FIG. 1  is supplied with the actual current value i Mi  of the electric motor  25 , the actual position value x i  of the piston  46  and the actual pressure value p i  in the chamber  40 , as electric voltage signals. The actual current value i Mi  is here a measure of the force with which the electric motor  25  forces the push rod  27  against the part jaw  17 . The actual position value x i  of the piston  46  is a measure of the distance between the part jaws  16  and  17  of the clamping jaw  13 . The control device  71  processes these signals to form the actuating signal y v1 , which determines the position of the control piston of the proportional valve  53 . A difference former  75  forms a control difference x di  from the desired current value i Ma  and the actual current value i Mi  of the electric motor  25 . A differentiator  76  forms the time derivative of the actual pressure value from the actual pressure value p i . In order to improve the control procedure, this signal is combined with the control difference x di  in a computing device  77  and supplied to a force controller  78 , which forms an actuating signal y K  for a force control loop. A differentiator  80  forms a control difference x dx  from the desired position value x s  and the actual position value x i  of the piston  46 , said difference being supplied to a position controller  81 . The position controller  81  forms an actuating signal y x  for a distance control loop. A changeover device  83  with two controlled changeover switches  84 ,  85  supplies the proportional valve  53  either with the actuating signal y K , the actuating signal y x  or a constant actuating signal y v10  as actuating signal y v1 . A comparator  88  switches the changeover switch  84  into the lower position if the actual current value i Mi  of the electric motor  25  is greater than the threshold value i Mu . The threshold value i Mu  is chosen such that it lies between the largest value occurring during the movement of the clamping jaw  13  and the value i Ma . If, at the same time, the changeover switch  85  is in the upper position, the actuating signal supplied to the proportional valve  53  is equal to the actuating signal y K , and the force control loop is active. If the actual current value i Mi  of the electric motor  25  is less than the threshold value i Mu , the comparator  88  switches the changeover switch  84  into the upper position. If the changeover switch  85  is simultaneously in the upper position, the actuating signal supplied to the proportional valve  53  is equal to the actuating signal y x  and the distance control loop is active. If the changeover switch  85  is in the lower position, because of an actuating signal y u  which is output by the machine control system  70  and is supplied to a magnet  89 , both the force control loop and the distance control group are interrupted. The actuating signal supplied to the proportional valve  53  is equal to the constant actuating signal y v10 , which is chosen such that the control piston of the proportional valve  53  is in a neutral position, in this case in the central position. In this position, the chambers  40  and  47  of the differential cylinder  41  are connected to each other in a restricted manner, to the pump  54  and to the tank  43 . 
   In order to close the clamping device, in a first step the thrust mechanism  26  pulls the part jaw  17  coupled to it against the fixed clamping jaw  11 . In the process, the part jaw  17  pulls the part jaw  16  with it. Since, as the part jaws  16  and  17  are moved, the current i Mi  drawn by the electric motor  25  is less than the threshold value i Mu , the actuating signal y x  is supplied to the proportional valve  53 , and the distance control loop, which keeps the distance between the part jaws  16  and  17  constant, is active. In this case, the piston  32  of the double-ended cylinder  33  rests on the right-hand stop, but can also be clamped in an intermediate position. Once the part jaw  17  has reached the clamping jaw  11 , the machine control system  70  locks the latching device  15 . The machine control device  70  switches the control piston of the control valve  60  into the position in which the right-hand chamber, provided with the designation  92 , of the double-ended cylinder  33  is connected to the chamber  20 . If, then, in a second step the electric motor  25  pulls the piston  32  onward in the closing direction, pressure medium is displaced from the chamber  92  into the chamber  20  and the part jaw  17  is pressed against the clamping jaw  11  in the process. The relationships between the areas to which pressure is applied are chosen such that the force exerted on the piston  32  by the electric motor  25  is increased. In this step, the thrust rod  27  is loaded in tension. Since, when loaded in tension, the thrust rod  27  can be acted on with a greater force than is permissible in the case of compressive loading, limiting the tensile force exerted on the thrust rod  27  by the electric motor  25  is not required. 
   The opening of the clamping device is likewise carried out in two steps. In the first step, the latching device  15  remains locked. The control piston of the control valve  60  is switched into a position in which the chamber  20  is relieved to the tank  43 . The force required to open the mold  23  is greater than the force which the thrust rod  27  can transmit under compressive loading without bending impermissibly. The current i Mi  drawn by the electric motor  25  rises to a value which is greater than the threshold value i Mu . The comparator  88  therefore switches the changeover switch  84  into the lower position, in which the force control loop is active. The pump  54  delivers pressure medium to the chamber  40  and, in this way, assists the force acting in the opening direction. The force controller  78  controls the proportional valve  53  in such a way that the current i Mi  drawn by the electric motor  25  is equal to the threshold value i Ma . In addition, pressure medium which is displaced from the chamber  35  and supplied to the chamber  40  via the nonreturn valve  36  assists the action of pulling the mold  23  open. Once the mold  23  has been pulled open, the machine control system  70  unlocks the latching device  15  for the second step of the opening procedure. The current i Mi  drawn by the electric motor  25  when moving the clamping jaw  13  in the opening direction is less than the threshold value i Mu . The comparator  88  thus switches the changeover switch  84  into the upper position again, that is to say from force control to distance control. As an alternative to the distance control, during the second step of the opening procedure the proportional valve  53  can be supplied with the actuating signal y v10 , which switches the control piston of the proportional valve  53  into the central position. 
     FIG. 2  shows the structure of a second clamping device according to the invention in a schematic illustration. In terms of its substantial parts, this clamping device corresponds to the clamping device illustrated in FIG.  1 . Identical components are therefore provided with the same designations as in FIG.  1 . The clamping device illustrated in  FIG. 2  likewise has a fixed clamping jaw  11  and a clamping jaw  13  which is formed from two part jaws  16  and  17  and can be displaced along a guide device  12 . The guide device  12  is constructed in the same way as in  FIG. 1. A  chamber  20  to hold hydraulic pressure medium is formed between the part jaws  16  and  17 . Two part molds  21  and  22  of a mold  23  are held on the part jaws  16  and  17 . A differential cylinder  95  is held on the part jaw  17 . A piston  97  connected to a piston rod  96  divides the differential cylinder  95  into a chamber  98  on the rod side and a chamber  99  on the crown side. An electric motor  25  is held on the fixed clamping jaw  11 . A thrust mechanism  26  translates the rotational movement of the electric motor  25  into a longitudinal movement of the thrust rod  27 , which a coupling  28  transmits to the piston rod  96  of the differential cylinder  95 . A control valve  60 , whose position is determined by an actuating signal y v2 , connects the chamber  98  to a pump  54  in a first position and to the chamber  20  in a second position. In the central position of the control valve  60 , both the chamber  98  and the chamber  20  are shut off. Instead of the control valve  60  with three control positions, a 2-way control valve can be used if the central position of the control valve  60  is not needed. The chamber  99  is continuously connected to the tank  43 , irrespective of the position of the control valve  60 . As in  FIG. 1 , a differential cylinder  41  is held on the part jaw  16 , its piston rod  45  being connected to the part jaw  17 . The proportional valve  53  controls the flow of pressure medium from the pump  54  to the chambers  40  and  47  of the differential cylinder  41  in accordance with the actuating signal y v1 . The components provided with an * are—as described in connection with FIG.  1 —provided in order to act uniformly with force on the part jaw  17 . 
   The control of the clamping device illustrated in  FIG. 2  is carried out in the same way as the control of the clamping device illustrated in  FIG. 1 , by means of the electric control device  71  illustrated in  FIG. 3 ; in conjunction with the higher-order machine control system  70 . The control device  71  receives from the clamping device illustrated in  FIG. 2  the actual position value x i  of the piston  46 , the actual pressure value p i  in the chamber  40  and the actual current value i Mi  of the electric motor  25 . The clamping device illustrated in  FIG. 2  receives from the control device  71  the actuating signal y v1  for the proportional  53  and, from the machine control system  70 , the actuating signal y v2  for the control valve  60 , the actuating signal y v3  for the latching device  15  and the desired rotational speed value n s  for the electric motor  25 . The combining of the individual signals is carried out in the same way as described above in connection with the control of the clamping device illustrated in FIG.  1 . 
   The force acting on the thrust rod  27  can—as described above—be determined from the current i Mi  drawn by the electric motor  25 . However, it is also possible to register the force acting on the thrust rod  27  directly, using a force sensor arranged in the force flow, or to determine it via a pressure measurement in the chamber on the rod side of the double-ended cylinder  33  or in the chamber  20 .