Abstract:
A protective device for securing to a component movable along a track and for the monitoring of a protected zone moving with the component includes a light transmitter for the radiation of a divergent transmitted light beam into a spatial zone containing the protected zone, a light receiver for the reception of at least some of the light of the transmitted light beam of the light transmitter radiated into the spatial zone and for the emitting of corresponding received signals, and a restriction device which is arranged in the direction of a transmitted light path after a section of the transmitted light path which contains the protected zone and by means of which the transmitted light beam can be restricted to a less divergent received light beam defining the width of the protected zone in at least one direction.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims priority to German Patent Application No. 102 47 136.3 filed Oct. 9, 2003, which application is herein expressly incorporated by reference. 
   FIELD OF THE INVENTION 
   The present invention relates to a protective apparatus for securing to a component movable along a track and for the monitoring of a protected zone to be moved with the component as well as to a method for the monitoring of a protected zone which has to be moved together with a component moved along a path. 
   BACKGROUND OF THE INVENTION 
   Machines are used in many areas which have components moved automatically by a drive device or which are moved themselves. If persons are standing in the movement zone of these components, the persons can be potentially at risk from the movement of the components. For the protection of people, protective apparatuses can therefore be arranged at the moved component at such machines which allow a monitoring of a protected zone moved along with the component. If the presence of a person is detected in the moved along protected zone, the movement of the component can be interrupted. 
   An example for such a machine is a bending press which is shown schematically in part in  FIG. 1  and which has a bending tool  10  driven by a drive device not shown in  FIG. 1  and having, for example, a lower side formed in a V shape and a stationary lower tool  12  whose upper side can, for example, have a V groove corresponding to the shape of the lower side of the bending tool  10 . For the shaping of a workpiece  14 , it is placed by an operator onto the lower tool  12 . To achieve a short cycle time, the bending tool  10  is moved from a starting position at high speed along a direction of movement B toward the workpiece  14 , but is then braked again such that it impacts the workpiece  14  at a lower speed suitable for the bending process and presses said workpiece into the lower tool  12 . The slowing down of the movement of the bending tool  10  can not take place instantaneously due, among other things, to its inertia. The path which is necessary to brake the bending tool  10  down from the fast speed to a standstill is also termed the trailing path N. 
   For the protection of a person who places workpieces  14  into the bending press, a light transmitter  20  and a light receiver  22  are attached to two holding arms  16  and  18  and together form a one-way light barrier. The light transmitter  20 , which has a light source  24  and a focusing optical transmitting system  26 , radiates a substantially parallel transmitted light beam  27  at a distance to the trailing path N parallel to the lower side of the bending tool  10  onto the light receiver  22  which has a photo-detection element with evaluation electronics  28  and an optical reception system  30  for the focusing of the received light ray onto the photo-detection element  28  with the evaluation electronics. The evaluation electronics are made such that, when the light path between the light transmitter  20  and the light receiver  22  is interrupted, a signal can be emitted to the drive device so that it brakes the movement of the bending tool  10  as fast as possible. The parallel transmitted light beam emitted by the light transmitter  20  therefore defines a protected zone which prevents injury to a person thanks to its distance from the lower side of the bending tool  10  corresponding to the size of the trailing path N. 
   In practice, however, vibrations of the holding arms  16  and  18  occur due to the movement of the bending tool  10 , whereby the light transmitter  20 , and thus also the parallel transmitted light beam  27  emitted by the light transmitter, is, for example, tilted relative to the light transmitter  20  (illustrated in  FIG. 1  by short-dotted lines). Due to the long light path, even small angles of tilt can result in large movements of the transmitted light beam  27  in the plane of the light receiver  22 , and in particular in a migration from a reception surface of the light receiver  22 . The transmitted light beam  27  can thereby no longer be received by the light transmitter  22  even without the presence of a body part of a person between the bending tool  10  and the workpiece  14 . The evaluation optics will therefore detect an interruption of the light path such that the bending press is stopped. This increases the downtimes of the bending press in an unnecessary manner. 
   To reduce the downtime, a reception surface of the light receiver  22  can alternatively be enlarged such that the transmitted light ray  27  of the light transmitter  20  does not migrate so easily from the light receiver  22  on vibrations. However, the case can then occur that the presence of a body part of a person between the bending tool  10  and the workpiece  14 —in  FIG. 1 , for example, close to the light receiver  22 —cannot be detected although it is at a distance beneath the bending tool  10  inside the trailing path N. This then means a reduction in labor safety. 
   Bending presses secured in this manner therefore have the disadvantage that they either conceal increased safety risks for the operators or have undesirably high downtimes. 
   SUMMARY OF THE INVENTION 
   It is therefore the underlying object of the present invention to provide a protective apparatus for securing to a component movable along a track and for the monitoring of a protected zone to be moved with the component, which has good functional reliability, as well as a method for the monitoring of a protected zone which has to be moved along with a component moved along a track. 
   The object is satisfied by a protective apparatus having the features of claim  1 . 
   The protective apparatus in accordance with the invention for securing to a component movable along a track and for the monitoring of a protected zone to be moved with the component includes a light transmitter for the radiation of a divergent transmitted light beam into a spatial zone containing the protected zone, a light receiver for the reception of at least some of the light of the transmitted light beam of the light transmitter radiated into the spatial zone and for the emitting of corresponding received signals and a restriction device which is arranged after a section of the transmitted light path, which contains the protected zone in the direction of a transmitted light path and by means of which the transmitted light beam can be restricted to a less divergent received light beam defining the width of the protected zone in at least one direction. 
   The object is furthermore satisfied by a method for the monitoring of a protected zone having the features of claim 21. 
   In the method in accordance with the invention for the monitoring of a protected zone which has to be moved along with a component moved along a track, a divergent transmitted light beam is transmitted and moved along with the component, the divergent transmitted light beam is restricted after covering a light path including the protected zone to a less divergent received light beam moved along with the component and defining the width of the protected zone in at least one direction and the received light beam is received and monitored. 
   The component along with which the protected zone is to be moved can principally be any desired component movable along a path, preferably along a fixedly pre-determined path, and in particular movable by an at least semi-automatic drive. 
   The protective apparatus in accordance with the invention is provided for securing to this component for the moving along of the protected zone. 
   In accordance with the invention, a divergent transmitted light beam is transmitted and moved along with the component. For this purpose, the light transmitter is provided for the radiation of the divergent transmitted light beam into a spatial zone containing the protected zone such that both this spatial zone and the transmitted light beam can be moved along with the component. The light transmitter can include a light source for example for light in the visible or in the infrared wavelength of light and, optionally, an optical transmitter system for the shaping of the divergent transmitted light beam from light emitted by the light source. The transmitted light beam is divergent in at least one direction of divergence, i.e. in particular in a sectional plane through the transmitted light beam along a direction of propagation of the light. 
   The restriction of the transmitted light beam in accordance with the invention takes place in the protective apparatus in accordance with the invention by the restriction device which is arranged such that the transmitted light beam is restricted at the earliest after covering a path containing the protected zone in the direction of divergence of the transmitted light beam. The restriction device can therefore also be moved along with the component and can in particular be held at it. 
   A part beam of the light of the divergent transmitted light beam determined by the restriction device is furthermore received and monitored after the restriction as a received light beam moved along with the component. In the apparatus in accordance with the invention, the light receiver serves for this purpose which is to be secured to the component, can then be moved along with the component and can, for this purpose, have a photo-detection element for light of the light transmitter and, optionally, an optical reception system for focusing the received light beam onto the photo-detection element. 
   The transmitted light beam, which is divergent contrary to the prior art, does not define the protected zone, but only contains it, in accordance with the invention. 
   The definition of the protected zone rather takes place in that the divergent transmitted light beam is restricted after covering the light path including the protected zone to a less divergent received light beam determining the width of the protected zone in at least one direction. The received light beam can in particular also be parallel or even convergent. The term of the less divergent received light beam therefore includes these two alternatives in the sense of the present invention. A narrower, that is less divergent, part beam is therefore so-to-say cut out of the divergent, i.e. spread out, transmitted light beam by the restriction, said part beam defining the protected zone and being received and monitored after the restriction or after the restriction device as a received light beam moved along with the component. Only the blocking of the part beam, and thus of the corresponding received light beam, is therefore monitored such that it, and in particular its width, in one direction or the part beam and its corresponding width define the protected zone. The restriction device therefore determines the part beam of the transmitted light beam between the light transmitter and the restriction device which can be received by the light receiver as the received light beam and thus defines the protected zone. 
   On a movement of the transmitted light beam by vibrations in a direction in which the transmitted light beam is divergent and in which a restriction takes place, only one other part beam of the transmitted light beam is used as the received light beam. The position of the protected zone, however, remains the same, since the received light beam or the restriction device are moved along with the component and a vibration-determined movement of the restriction device or of the light receiver only results in small displacements of the protected zone, but not in pivot movements with some large deflections of the part beam. 
   The protected zone can therefore also be held in a constant position relative to the component on a vibration of the light transmitter by a suitable arrangement of the restriction device. 
   A transmitted light beam is expediently used which is divergent at least in a direction tangential to the track of the component, for which purpose the light transmitter has to be aligned correspondingly. The restriction then takes place in the same direction, which can take place by a corresponding arrangement of the restriction device. The protected zone can be moved along in front of the component in this manner. 
   A migration of the transmitted light beam from the light receiver is thus largely avoided such that an interruption of the light path caused by a relative movement of the light transmitter and of the light receiver and an incorrect report of an infringement of the protected zone which thus takes place and which can, for example, result in a braking of the component is avoided. 
   A particularly large functional reliability of the protective apparatus in accordance with the invention thus also results on vibrations of the component to which the protection device is secured. 
   The use of a divergent transmitted light beam furthermore allows a particularly simple adjustment of the light transmitter relative to the restriction device or to the light receiver since, on a tilting of the light transmitter relative to the light receiver, only a different part beam of the transmitted light beam can be received as the received light beam. 
   Further developments and preferred embodiments of the invention are described in the description, in the claims and in the drawings. 
   An aperture angle of the transmitted light beam which determines the divergence of the transmitted light beam in the direction of divergence can preferably be selected, among other things, in dependence on the spacing between the light transmitter and the restriction device, since this spacing, together with the aperture angle, forms the width of the transmitted light beam at the restriction device. It is preferred for an aperture angle of the transmitted light beam to be selected such that a width of the transmitted light beam in a pre-determined direction orthogonal to the transmitted light path directly in front of the restriction device is larger by a factor of 10 than a corresponding width of the received light beam directly at the restriction device. (See  FIG. 2A  where a&gt; 10   b .)Such a large divergence of the transmitted light beam relative to that of the received light beam allows a particularly simple adjustment and a high functional reliability, in particular if the pre-determined direction is arranged tangentially to the track of the component after the fitting to the component. 
   When the aperture angle of the transmitted light beam is pre-determined independently of the special features of the component, and in particular of its movement, a comparatively large amount of light of the light transmitter can remain unused during the total operation due to the divergence of the transmitted light beam. It is therefore preferred for an aperture angle of the transmitted light beam to be selected such that a width of the transmitted light beam directly at the restriction device is larger in a pre-determined direction orthogonal to the transmitted light path than a mean movement amplitude of the transmitted light beam to be expected caused by a relative movement of the light transmitter and the restriction direction directly at the restriction device. (See  FIG. 2B  where a&gt;c.) In this manner, with a given functional reliability, as much light as possible of the light transmitter is also actually used for the monitoring of the protected zone on the operation of the protective apparatus. 
   The restriction device can generally include any desired means by means of which the divergent transmitted light beam can be restricted in its width to the received light beam in the pre-determined direction. It can, for example, be a restriction of the light-sensitive surface of the photo-detection element. It is, however, preferred for the restriction device to include a diaphragm. Such a restriction device is in particular suitable for use in one-way light barriers, i.e. for the case that the light transmitter and the light receiver are arranged on opposing sides of the protected zone. The protected zone can be defined very easily by the shape and size of the diaphragm restricting the transmitted light beam. The diaphragm can in particular also be made as a part of the light receiver. 
   It is furthermore preferred for a reflecting element to be used for the restriction of the transmitted light beam. In particular, it is preferred in the protective device in accordance with the invention for the restriction device to include a reflecting element at which a part beam of the divergent transmitted light beam to be supplied to the light receiver can be reflected. The reflecting element acting in a reflecting manner for light of the light transmitter thus acts in a similar manner to a diaphragm in that only light of the transmitted light beam incident onto the surface of the reflecting element is reflected as the received light beam. The size, shape and alignment of the reflecting surface, which is preferably surrounded by a region reflecting the light of the light transmitter not at all or only a little, determine the width or divergence of the received light beam and the direction in which the transmitted light beam is restricted. Furthermore, it simultaneously allows a deflection of the light path. 
   In another embodiment of the invention, it is preferred for a retro-reflective element to be used for the restriction of the transmitted light beam. For this purpose, it is preferred in the protective apparatus in accordance with the invention for the restriction device to include a retro-reflective element at which a part beam of the divergent transmitted light beam to be supplied to the light receiver can be reflected. A retro-reflective element is here understood as an element which reflects back the incident light again in the direction of the incident light. The retro-reflective element can in particular be a 90° prism, a triple reflector or appropriate reflection foils. Even when the retro-reflective element is tilted with respect to the light transmitter, light can thus again be reflected back in the direction of the light transmitter. Analogously to the case of the reflecting element of a restriction device, the size, shape and alignment of the retro-reflective element or of its optically effective surface also determine the effect on the transmitted light beam here. The resulting received light beam can in particular be convergent. 
   In the two aforesaid alternatives, it is particularly preferred for the light transmitter and the light receiver to be arranged on the same side of the protected zone. The protective apparatus therefore then includes a special reflection light barrier in which the light transmitter and the light receiver can preferably be arranged in one housing. The arrangement of the light transmitter and the light receiver on the same side of the protected zone in particular allows a particularly simple adjustment of the light transmitter and of the light receiver even before the mounting to the component. 
   In this case, it is furthermore particularly preferred for the light transmitter and the light receiver to have a common beam splitter and a common transmission/reception lens arranged after the beam splitter at least in the transmission direction. The beam splitter, which can in particular include a semi-transmitting mirror, allows a spatially separated arrangement of a light source of the light transmitter and of a photo-detection element of the light receiver, although the transmitted light beam and the received light beam overlap at least in the region of the transmission/reception lens. In this manner, an optical transmission system and an optical reception system of the protective apparatus in accordance with the invention are at least partly integrated, which results, in particular in conjunction with a retro-reflective element as the restriction device, in a simple design with a simultaneously high accuracy of the beam guiding. 
   It is furthermore preferred for the light receiver to have at least one reception element whose reception surface has a greater extent in a divergence direction of the received light beam which corresponds to the direction in which the divergent transmitted light beam has been restricted by the restriction device than in a direction orthogonally thereto. This reception element can in particular be a photo-detection element. As previously mentioned in connection with the term of the less divergent reception light beam, the received light beam does not necessarily have to be divergent in the direction of divergence, but can also be parallel or convergent. In this manner, a migration of the received light beam from a reception surface of the reception element, which is caused by a tilting of the light transmitter and/or of the light receiver or of the restriction device with a reflecting or retro-reflective element, can be avoided at least with small tilting angles. The extent of the reception element in the direction of divergence of the received light beam is particularly preferably larger by a factor of at least 4 than its extent in a direction orthogonal thereto. 
   To achieve a particularly high light intensity on a photo-detection element of the light receiver, the light transmitter preferably has an optical transmission system and/or the light receiver preferably has an optical reception system. To obtain an increased depth of field, it is then preferred for the light receiver to have a slit diaphragm whose slit is arranged parallel to a direction of divergence of the received light beam which corresponds to the direction in which the divergent transmitted light beam can be restricted by the restriction device. The slit is aligned parallel to the direction of divergence of the received light beam at the location of the slit; this means that the direction of divergence is defined with respect to the direction of the received light beam. An increased depth of field is thus achieved in the direction of divergence of the received light beam, but not in a direction orthogonal thereto, which is in particular not necessary when asymmetric reception elements are used. The accuracy requirements for the adjustment of the diaphragm or of the light receiver in the direction of the slit as well as the accuracy requirement on production are hereby reduced. 
   In accordance with the invention, the transmitted light beam only needs to be divergent in a direction which is preferably aligned tangentially to the track along which the component is moved. A very narrow, i.e. almost linear, protected zone, can then result. It is, however, desirable for many applications for an a real protected zone to be monitored. It is therefore preferred for the light transmitter to be designed such that a transmitted light beam can be emitted by it which diverges to a different extent in two directions orthogonal to one another. Whereas, for example, an aperture angle selected in accordance with the aforesaid criteria can be provided in a direction in which the restriction device restricts the divergent transmitted light beam, the transmitted light beam can, for example, be selected to substantially diverge less in a direction orthogonal thereto such that an areal protected zone results whose surface normal is preferably tangential to the track of the component. 
   It is therefore particularly preferred for the light transmitter and the restriction device to be made for the monitoring of a substantially areal protected zone in a direction orthogonal to a direction of movement of the component. The restriction device can in particular have a greater extent transversely to the direction in which it restricts the transmitted light beam than in the direction orthogonal thereto. 
   It is then furthermore preferred for the light receiver to have a diaphragm with a butterfly-shaped diaphragm aperture which includes two wing regions which are arranged transversely to the direction in which the divergent transmitted light beam can be restricted by the restriction device. A minimum width of the diaphragm aperture is particularly preferably approximately as large as the extent of a reflecting or retro-reflective element of the restriction device in the direction in which the restriction device restricts the divergent transmitted light beam. In this manner, a maladjustment between the restriction device and the sensor in the direction of restriction, in which the restriction device restricts the divergent transmitted light beam, can be recognized easily, on the one hand, since the received light beam can no longer pass through the diaphragm aperture on a sufficiently large maladjustment. The position of the protected zone can therefore be kept constant relative to the component in the direction of restriction. On the other hand, however, there is an adjustment tolerance with respect to an inclination of the restriction device transversely to the direction of restriction, since the diaphragm aperture widens toward the outside such that a tilting of the protected zone around the beam direction by correspondingly small angles does not result in an interruption of the received light beam. Furthermore, rotational vibrations of the restriction device which bring about a corresponding inclination do not have a strong effect. Since the marginal region of the protected zone has the function of recognizing an intervention approaching from the side, the difference in the spacing of the protective field from the component which can occur under certain circumstances in the direction of restriction is of less importance. 
   It is then furthermore particularly preferred for the restriction device to include at least two reflecting or retro-reflective elements which represent an encoding. In this manner, a lateral resolution of the protected zone into zones is also possible. 
   It is furthermore preferred for the reflecting or retro-reflective elements to be inclined with respect to one another such that a drop in intensity of the transmission and/or reception light beam in the direction of a margin of the light beam can substantially be compensated. A uniformly illuminated protected zone thus results for the light receiver. Typically, zones of retro-reflective elements are tilted in the image central region by an angle different from beams incident on them perpendicularly. The magnitude of the tilt of the retro-reflective elements decreases down to zero toward the image margin. Due to the efficiency of the retro-reflection decreasing as the angle of tilt increases, the imaged energy in the image central region can be reduced and the intensity drop toward the image margin typical for images can be compensated. When retro-reflective elements are used, in particular the retro-reflective elements lying in the center of the transmitted light beam can therefore be tilted by a small angle with respect to the retro-reflective elements lying at the margin of the transmitted light beam such that the corresponding received light beam—in contrast to a received light beam of a retro-reflective element at the margin of the transmitted light beam—is only partially incident on the light receiver or on a corresponding light-sensitive element. 
   It is in particular preferred for the monitoring of an areal protected zone for the light receiver to have a linear arrangement of photo-detection elements. In this manner, in particular in conjunction with a restriction device having an encoded reflector unit, a resolution is also made possible in a direction perpendicular to the track of the component. 
   The protective apparatus in accordance with the invention preferably has an evaluation circuit for the reception of received signals of the light receiver and for the emitting of corresponding monitoring signals. In particular a drive for the moved component can then in particular be controlled using these monitoring signals. 
   A further subject of the invention is a component for an apparatus which can be moved on a pre-determined track with a drive device of the apparatus and which is characterized in that a protective apparatus in accordance with the invention is held at the component, with the protected zone including at least some of the track. The protective apparatus is particularly preferably held at the component such that the transmitted light beam diverges in a direction tangential to the track of the component and such that the restriction device likewise restricts the divergent transmitted light beam in a direction substantially tangential to the track. Vibration influences in the direction of the track can thereby be largely suppressed, in particular without any limitation in safety, such that only a few or no infringements of the protected zone caused only by vibrations are recognized with high reliability. Lower downtimes thus result. 
   It is preferred for the light transmitter and the restriction device to be held at two holding arms spaced apart spatially transversely to the track of the component. In this manner, a protected zone can be obtained very easily over the total width of the component. 
   A further subject of the invention is a shaping apparatus, in particular a bending press, having a component in accordance with the invention made as a shaping tool, in particular as a bending tool, with a protective apparatus in accordance with the invention, with the protected zone including at least some of the track. Large advantages in particular result for such apparatuses, since high safety demands are made, on the one hand, due to the high forces and high tool speeds which occur as a rule in shaping, and since only low downtimes are desired at the same time. The use of the protective apparatus in accordance with the invention therefore allows a particularly safe and efficient manner of operation of the shaping apparatus in accordance with the invention. 
   Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein: 
       FIG. 1  is a schematic, partly side view of a bending press with a conventional protective apparatus; 
       FIG. 2  is a schematic, partly side view of a bending prees with a protective apparatus in accordance with a first preferred embodiment of the invention, and  FIGS. 2A and 2B  contain labeled dimensions of features shown in  FIG. 2 ; 
       FIG. 3  is a schematic, partly side view of a bending press with a protective apparatus in accordance with a second preferred embodiment of the invention; 
       FIG. 4  is a schematic, partly side view of two holding arms with a protective apparatus of a bending press in accordance with a third preferred embodiment of the invention; 
       FIG. 5  is a schematic, partly plan view of the protective apparatus in  FIG. 4 ; 
       FIG. 6  is a schematic representation of a restriction device of the protective apparatus in  FIG. 4 ; 
       FIG. 7  is a schematic representation of photo-detection elements in a light receiver of the protective apparatus in  FIG. 4 ; 
       FIG. 8  is a schematic representation of a diaphragm of a protective apparatus of a bending press in accordance with a fourth preferred embodiment of the invention; and 
       FIG. 9  is a schematic representation of a protected zone position in the protective apparatus in  FIG. 8 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The following description of the preferred embodiment(s) is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses. 
   In  FIG. 2 , a bending press includes a bending tool  32 , which can be moved in a direction of movement B along a pre-determined linear path by a drive device not shown in  FIG. 2 , and a stationary lower tool  34 , on which a workpiece  36  can be stored for working. 
   The bending tool  32  has a V-shaped lower side. The upper side of the lower tool  34  is made complementary to this lower side of the bending tool  32  and has a corresponding V-shaped bending groove which is not shown in  FIG. 2 . 
   A protective apparatus of the invention in accordance with a first embodiment of the invention is held at holding arms  38  and  40  secured to the bending tool. It includes a light transmitter  42  held at the holding arm  38  and a light receiver  44  held at the holding arm  40  and connected to the drive device of the bending press by a lead (not shown). 
   The light transmitter  42  includes—in a housing—a light source  46  in the form of a red light LED or an infrared light LED and an optical transmission system  48  arranged in front of the light source  46  in the radiation direction. 
   The optical transmission system  48  includes at least one lens and is designed such that light of the LED  46  can be radiated as a transmitted light beam  50  which is divergent in a direction parallel to the direction of movement B of the bending tool  32 . 
   Furthermore, the light transmitter  42  is held at the holding arm  38  such that the divergent transmitted light beam  50  can be radiated in a direction substantially along the lower side of the bending tool  32  and parallel thereto. A light discharge aperture of the light transmitter  42  has a spacing from a lower edge of the bending tool  32  of the magnitude of a trailing path N, which is defined as above, such that the transmitted light beam  50  also has this spacing from the lower edge of the bending tool  32  on average. 
   The light receiver  44  includes—in a housing—a diaphragm  52  as a restriction device, an optical reception system  54 , a photo-detection element  56  for the reception of red light or of infrared light of the light transmitter  42  and for the emitting of corresponding received signals and an evaluation circuit  57  connected to the photo-detection element  56  for the monitoring of the received signals and for the emitting of monitoring signals to the drive device. 
   For the monitoring of a protected zone beneath the bending tool  32 , infrared light is emitted by the light emitting diode  46  during a movement of the bending tool  32  and shaped by the optical transmission system  48  into the transmitted light beam  50  which is divergent in a direction parallel to the direction of movement B of the bending tool  32 . The transmitted light beam  50  passes through a corresponding spatial zone moved beneath the bending tool  32  along with the bending tool and is restricted by the diaphragm  52  acting as a restriction device to a received light beam  58  which is substantially less divergent than the transmitted light beam  50 . The received light beam  58  is then focused onto the photo-detection element  56  by the optical reception system  54 . The photo-detection element  56  receives the infrared light and converts it into an electrical signal which is monitored by the evaluation circuit  57 . The evaluation circuit  57  emits an alarm signal to the drive device for the bending tool  32  on the absence of a received signal such that the bending tool&#39;s movement is stopped. 
   The diaphragm  52  therefore only allows a part beam  60  of the transmitted light beam  58  to pass which is supplied to the photo-detection element  56  as a received light beam  58  via the optical reception system  54 . Only if an object blocks the part beam  60  of the divergent transmitted light beam  50  can no light be received by the light receiver  44  such that the part beam  60  defined by a diaphragm aperture of the diaphragm  52  forms a protected zone  62  of the protective apparatus formed by the light transmitter  42  and by the light receiver  44  shown as hatched. 
   As can easily be recognized in  FIG. 2 , a tilting of the light transmitter  42  admittedly results in a correspondingly pivoted divergent transmitted light beam  50 ′ but another part beam  60 ′ of the divergent transmitted light beam  50 ′ is now transmitted through the diaphragm  52  as the restriction device and has the same position and the same width relative to the bending tool  32  as the part beam  60 . The position of the protected zone  62  relative to the bending tool  32  therefore remains substantially unchanged even on a movement of the light transmitter  42 . A movement of the light receiver  44  also does not substantially change the position of the protected zone  62  relative to the bending tool  32  since, in contrast to movements of the light transmitter  42 , small movements of the light receiver  42 , and thus of the restriction device in the form of the diaphragm  52 , do not result in a pivoting of the part beam  60 , but at most in small displacements. 
   To achieve a sufficient insensitivity to the influence of vibrations, an aperture angle α of the divergent transmitted light beam  50  amounts, for example, to approximately 1° with a 2 m long bending tool, with the diaphragm  52  then being able to have a diaphragm aperture with a width of, for example, approximately 5 mm in the direction of restriction parallel to the direction of movement B such that the width of the transmitted light beam  50  at the diaphragm  52  amounts to approximately 3.5 cm. The angles and lengths used in the Figures are, if not otherwise stated, only selected for a better illustration and are therefore not necessarily realistic. 
   To allow a stopping of the bending tool  32  in good time, the light transmitter  42  and the light receiver  44  are held at the holding arms  38  or  40  such that the protected zone  62  extends substantially parallel to the lower edge of the bending tool  32  at a spacing from the trailing path N which designates the path which is required to brake the bending tool  32  down to a standstill as quickly as maximally possible. 
   In  FIG. 3 , a bending press is shown with a protective apparatus in accordance with a second preferred embodiment of the invention which only differs from the first embodiment by the design of the protective apparatus. The same statements therefore apply with respect to the same components, in particular to the bending press, as in the preceding embodiment and the same reference numerals are used. 
   The protective apparatus now includes a light transmitter and a light receiver which are combined to from one transmission/reception unit  64  and are held at the holding arm  38 . A restriction device in the form of a reflecting element  68  is arranged at a holder  66  at the holding arm  40 . 
   The transmission/reception unit  64  includes as a light source for the light transmitter a laser diode  69  for red light, a beam splitter  70  and an optical transmission/reception system  72  and for the light receiver, in addition to the optical transmission/reception system  72  and the beam splitter  70 , a further optical reception system  74 , a photo-detection element  56  and an evaluation circuit  57 . The photo-detection element  56  and the evaluation circuit  57  are made as in the preceding embodiment. 
   The beam splitter  70  is formed by a semi-transmitting mirror. 
   The reflecting element  68  is formed by a mirror reflective for light of the laser diode  69 , whereas the holder  66  does not reflect the light at all or only weakly. 
   For the monitoring light of the laser diode  69  is shaped by the beam splitter  70  and by the optical transmission/reception system  72  to a divergent transmitted light beam  76  which is radiated between the bending tool  32  and the lower tool  34  and which is divergent in a direction parallel to the direction of movement B of the bending tool  32 . 
   The transmitted light beam  76  is then incident on the reflecting element  68  and on the holder  66 , with it substantially only being reflected at the reflecting element  68 . A part beam  78  of the transmitted light beam  76  is therefore reflected back to the transmission/reception unit  64  as a received light beam  80 . The width of the part beam  78  and that of the received light beam  80  are defined by the extent of the reflecting element  68  parallel to the track of the bending tool  32 , i.e. to the direction of movement B in the example. The reflecting element  68  therefore represents a restriction device within the sense of the invention. 
   The received light beam  80  is focused or deflected onto the photo-detection element  56  by the optical transmission/reception system  72 , by the beam splitter  70  and by the further optical reception system  74 . 
   Analog to the part beam  60  of the transmitted light beam  50  in the preceding embodiment, a protected zone  82  is now substantially provided by the part beam  78  since only its blocking results in a change of the received light beam  80  received by the transmission/reception unit  64 . 
   The reflecting element  68  and the transmission/reception unit  64  are in turn arranged such that the protected zone  82  is arranged substantially parallel to the lower side of the bending tool  32  at a spacing from the trailing path N. 
   To obtain high function reliability, the optical transmission/reception system  72  is made such that an aperture angle α of the transmitted light beam  76  is so large that its width in a plane parallel to the reflecting element  68  is approximately ten times larger directly at the reflecting element  68  than the extent of the reflecting element  68  in this direction. The widths of the part beam  78  and of the received light beam  80  directly at the reflecting element are therefore—defined by the corresponding extent of the reflecting element—smaller by a factor of approximately 10 than the width of the transmitted light beam  76  at this point. 
   The function of the protective apparatus of the second embodiment, and in particular also the insensitivity to vibrations, result analogously to that of the first embodiment. 
   A bending press with a protective apparatus in accordance with a third preferred embodiment of the invention is partly shown in  FIGS. 4 to 7 . It differs from the bending press in the second embodiment by the design of the restriction device and by the design of the transmission/reception unit. The same reference numerals are therefore used for the same components and the statements on these components also apply accordingly in this embodiment. 
   A transmission/reception unit  84  which includes a light transmitter and a light receiver and is held at the arm  38  has a laser diode  69  for red light, a cylindrical lens  86 , a beam splitter  70 , an aspheric lens  88 , a slit diaphragm  90 , a color filter  92  and a line-scan camera  94  which is connected to an evaluation circuit  96 . 
   The cylindrical lens  86  and the aspheric lens  88  are made and arranged such that light of the laser diode  69  is shaped to form a transmitted beam  98  which is divergent in two directions orthogonal to one another and has an aperture angle α parallel to the direction of movement B of the bending tool  32  which is smaller than a further aperture angle α′ in a direction orthognal of movement B. 
   The aspheric lens  88  is furthermore designed such that received light is focused onto the line-scan camera  94  through the slit diaphragm  90  and the color filter  92  after reflection at the beam splitter  70 . 
   The slit diaphragm  90  has a slit  99  which is arranged in a plane through the holding arms  38  and  40  perpendicular to the direction of movement B. 
   The color filter  92 , which includes an interference filter, serves for the elimination of outside light or interfering light and substantially only transmits light of the laser diode  69 . 
   The line-scan camera  94  includes a plurality of elongate photo-detection elements  100  in the CMOS or CCD technology (cf.  FIG. 7 ) which are arranged parallel to one another transversely to their longitudinal direction and which have a length along their longitudinal axis of approximately 250 μm and a length in a direction transversely thereto of approximately 12.5 μm (cf.  FIG. 7 ). 
   The line-scan camera  94  is connected to the evaluation circuit  96  which receives and monitors electrical signals emitted by the photo-detection elements  100  on the reception of light. When an interruption of a light path is detected, an alarm signal is emitted to the drive device of the bending tool  32 . 
   A restriction device  102  held at the holder  66  at the holding arm  40  has parallel, flatly rectangular, retro-reflective elements  104  (cf.  FIG. 6 ) which are arranged transversely to the linear track of the bending tool and thus of the direction of movement B. The short sides of the retro-reflective elements are aligned parallel to the direction of movement B. An encoding is formed by this arrangement. The retro-reflective elements  104  are formed from a conventional retro-reflective foil. 
   For the monitoring, in particular during the movement of the bending tool  32 , light of the laser diode  69  is beamed by the cylindrical lens  86  and by the aspheric lens  88  into the divergent transmitted light bean  98  which is less divergent in a direction parallel to the direction of movement B than in a direction perpendicular thereto (cf.  FIGS. 4 and 5 ). The aperture angle α can amount to, for example, 1° and the aperture angle α′ to, for example, approximately 0° at a spacing of the restriction device from the transmission/reception unit  84  of, for example, 2 m. 
   In accordance with the encoding, the retro-reflective elements  104  reflect a fan of part beams  106  into a fan of received light beams  108  which are now somewhat convergent due to the retro-reflection. The fanned part beams  106  or the received light beams  108  are only shown partly as separate beams in  FIG. 5  for reasons of clarity. 
   Furthermore, the transmitted light beam  98  is restricted in a direction of divergence parallel to the direction of movement B by the retro-reflective elements  104  whose short sides are aligned parallel to said direction of divergence. Their extent in the direction of movement B is selected in accordance with a mean amplitude of the transmission/reception unit  84  to be expected by a factor of 10 smaller than the width of the transmitted light beam  98  at the direction of restriction  102 . The part beams  106  reflected into the received light beams  108  are therefore less divergent than the transmitted light beam  98  in a direction parallel to the direction of movement. The width of the transmitted light beam  98  in the direction of divergence parallel to the direction of movement B at the restriction device  102  is thus also larger than a width of the received light beams  108  produced by the reflection directly at the restriction device  102 . 
   The part beams  106  and, due to the retro-reflective properties of the retro-reflective elements  104  also the received light beams  108 , thus define a protected zone  110  which is shown as hatched and whose position is fixed by the transmission/reception unit  84  and by the restriction device  102  and which is arranged in fan-like shape substantially areally perpendicular to the direction of movement B of the bending tool  32  parallel to its lower side at a spacing corresponding to the trailing path N. 
   The received light beams  108  are guided through the aspheric lens  88  via the beam splitter  70  through the slit diaphragm  90 . The slit  99  of the slit diaphragm  90  is arranged parallel to a direction of divergence of the received light beams  108  at the location of the slit diaphragm which corresponds to the direction in which the transmitted light beam  98  was restricted to the received light beam  108  by the restriction device  102  in the direction of movement B. The slit  99  is thus also arranged perpendicular to the direction of movement B. The slit diaphragm  90  therefore brings about an increase in the depth of field perpendicular to the direction of movement B of the bending tool  32 , but not in a direction orthogonal thereto which corresponds to the direction in which the divergent transmitted light beam  98  was restricted parallel to the direction of movement B. 
   After passing through the color filter  92 , where outside light and interfering light is filtered, the imaging of the retro-reflective elements  104  shown only schematically in  FIG. 7  appears on the line-scan camera  94  such that a spatial resolution is present in the viewfinder region perpendicular to the direction of movement B. In  FIG. 7 , the photo-detection elements  100  are shown too wide in comparison with the images of the retro-reflective elements  104  for reasons of clarity. 
   The photo-detection elements  104  are arranged with their elongate directions parallel to the slot  99  and thus to a direction of divergence of the received light bundles  108  at the location of the photo-detection elements  104  which corresponds to the direction in which the transmitted light beam  98  was restricted to the received light beam  108  by the restriction unit  102  in the direction of movement B. The effects of vibration-induced movements of the restriction device  102 , and thus the received light beam  108  parallel to the direction of movement B, can thereby be easily intercepted. 
   Analog to the first two embodiments, by the use of the restriction device  102  with retro-reflective elements  104 , a protected zone  110  which is arranged in a constant manner relative to the bending tool  32 , is narrow in the direction of movement B and allows a safe monitoring of the zone beneath the bending tool  32 , and thus low downtimes of the bending press, also results with vibrations of the transmission/reception unit  84  and of the restriction device  102  toward one another in the direction of movement B or on tilting movements of the transmission/reception unit  84  with respect to the restriction device  102  in a plane which contains the direction of movement B. 
   A bending press with a protective apparatus in accordance with a fourth preferred embodiment of the invention is shown partly in  FIGS. 8 and 9 . It is substantially shown like the bending press in the third embodiment such that the same reference numerals are used for the same components and the statements on these components also apply correspondingly in this embodiment. 
   In contrast to the bending press in the third embodiment, a diaphragm  112  is provided between the aspheric lens  88  and the beam splitter  70  and has a butterfly-like diaphragm aperture  114  (cf.  FIG. 8 ). Two wing regions  116  of the diaphragm aperture  114  are arranged substantially parallel to the areal protected zone  110  and thus transversely to the direction of movement B or to the direction in which the divergent transmitted light beam  98  can be restricted by the restriction unit  102  (cf.  FIG. 9 ). 
   A width D of a restriction of the diaphragm aperture  114  corresponds to the height of the retro-reflective elements  104  in the direction of the direction of movement B. 
   The diaphragm  112  is arranged at such a spacing from a lower edge  118  of the bending tool  32  that the protected zone  110  lies at the height of the restriction of the diaphragm aperture  114  and the received light beams  108  can pass through it (cf.  FIG. 9 ). 
   In the event of a maladjustment between the restriction device  102  and the transmission/reception unit  84  in the direction of movement B, the light path is interrupted at the middle of the protected zone  110  since the received light beams  108  can no longer pass through the only narrow diaphragm aperture  114  in this region. Such a maladjustment in the direction of high safety relevance directly beneath the lower edge  118  of the bending tool  32  is therefore easy to recognize. 
   A maladjustment in the angular position of the diaphragm  112  or of the transmission/reception unit  84  with respect to the restriction unit  102 , and thus to the protected zone  110 , for example by assembly imprecision or by vibrations, does not, in contrast, necessarily result in an interruption of the light path. This is illustrated in  FIG. 9  in which the diaphragm  112  is only shown schematically while omitting the aspheric lens  88 . The wing regions  114  are made such that the received light beams  108  can still pass through the diaphragm  112  in the event of maladjustments by angles of up to approximately 3° and no interruption of the light path takes place. Since the marginal region of the protected zone  110  has the function of recognizing an intervention approaching from the side, the different spacings of the protected zone  110  from the bending tool  32  in this region are of lesser importance. 
   Tiltings between the transmission/reception unit  84  and the restriction device along the lower edge  118  of the bending tool  32  caused by vibrations do not have a negative effect since the retro-reflective elements  104  substantially reflect the transmitted light beam  98  or the part beams  106  onto themselves. Furthermore, for example, retro-reflective foils have a certain scattered portion on reflection by which such maladjustments are likewise partly compensated. 
   The description of the invention is merely exemplary in nature and, thus, variations that do not depart from the gist of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention.