Patent Description:
In general, a press brake includes a lower table for holding a lower die on an upper side of the lower table, an upper table for holding an upper die on a lower side of the upper table, and an elevating actuator for lifting and lowering the upper table relative to the lower table. In the press brake, in order to bend a workpiece, the upper die and the lower die cooperate by a lowering operation of the upper table with driving of the elevating actuator.

In recent years, it has become common to equip the press brake with an optical safety device to sufficiently secure the safety of the bending work (referring Patent Literature <NUM> and Patent Literature <NUM>). The optical safety device is provided with a projector being provided on one side in a length direction of the upper table for projecting a monitoring light directly under the upper die toward the other side in the length direction of the upper table, and a light receiver being provided on the other side in the length direction of the upper table for receiving the monitoring light. The light receiver has a light receiving element that outputs an electric signal (voltage signal) corresponding to the amount of light received as the monitoring light. A control device provided in the press brake detects (and monitor) the presence or absence of foreign body between the upper die and the lower die based on the light receiving state (result of light reception) of the light receiving element of the light receiver during the lowering operation of the upper table. When a presence of foreign body between the upper die and the lower die is detected, the control device of the press brake controls the elevating actuator so as to stop the lowering operation of the upper table.

When an environmental temperature in a factory drops in an evening or winter and a temperature difference occurs between the environmental temperature in the factory and a member temperature of an upper mold, air on a side far from the upper mold is cold, but air on a side close to the upper mold is heated by the upper mold. Therefore, as shown in <FIG>, a region HA of warm air and a region CA of cold air are formed around the upper mold P, and a layer of air with a temperature gradient is generated. Then, when a monitoring light B passes through the air layer having a temperature gradient, the monitoring light B is bent toward the cold air region CA due to the difference in a refractive index of the air. As a result, as shown in <FIG>, a shadow S of the upper mold P is enlarged by the monitoring light B projected on a side of a light receiver RX.

<FIG> is a schematic view showing how the monitoring light B is bent toward the cold air region CA. <FIG> is a schematic view showing that the shadow S of the upper mold P has the same size as the outline of the upper mold P before the monitoring light B is bent. <FIG> is a schematic view showing a state in which the shadow S of the upper mold P is enlarged after the monitoring light B is bent. In the drawing, "FF" indicates a forward direction, "FR" indicates a backward direction, "L" indicates a left direction, "R" indicates a right direction, "U" indicates an upward direction, and "D" indicates a downward direction.

The amount of expansion of the shadow S of the upper mold P due to the bending of the monitor light B is small when the optical path length of the monitor light B or a length of the upper mold P is short, but increases when the optical path length of the monitor light B and the length of the upper mold P are long. Therefore, when bending is performed in a state that a long upper die P is attached to a press brake having a long optical path length of the monitoring light B or a long overall length, if a layer of air having a temperature gradient is generated around the upper die P, the shadow S of the upper die P sometimes expands to a light receiving element RXa. In such a case, although there is no foreign body between the upper mold P and the lower mold, the control device of the press brake erroneously detects a presence of foreign body between the upper mold P and the lower mold by the shadow S of the upper mold P. As a result, the lowering operation of the upper table is stopped. That is, when the press brake is equipped with such optical safety device, there is a problem that the working efficiency (productivity) of the press brake is lowered.

Accordingly, it is an object of the present invention to provide an optical safety device for a press brake having a novel structure which can solve the problem of the aforementioned working efficiency (productivity) reduction.

A feature of the present invention is an optical safety device for a press brake comprising: a projector provided on one side in a length direction of an upper table in the press brake for projecting a monitoring light directly under an upper die and toward the other side in the length direction of the upper table; a photodetector provided on the other side in the length direction of the upper table and having a plurality of light receiving element for outputting electric signals (voltage signal) corresponding to the amount of the monitoring light and receiving the monitoring light; and an invalidation unit which invalidates the photodetectors when a shadow of the upper die projected on the photodetector by the monitoring light extends to the photodetectors positioned in a vicinity of the upper die.

Another feature of the present invention is the optical safety device, further comprising a determination unit for determining whether the shadow of the upper mold extends to the light receiving element located in the vicinity of the upper mold, and the invalidation unit invalidates the light receiving element when it is determined that the shadow of the upper mold extends to the light receiving element located in the vicinity of the upper mold.

Another feature of the present invention is the optical safety device, wherein the determination unit determines, based on a light receiving state (result of light reception) of the light receiving element located in the vicinity of the upper mold, whether or not the shadow of the upper mold extends to the light receiving element located in the vicinity of the upper mold.

Another feature of the present invention is the optical safety device, wherein the optical safety device for press brake is provided with a calculation unit for calculating an amount of enlargement of the shadow of the upper mold, and the determination unit determines whether or not the shadow of the upper mold is enlarged to the light receiving element located in the vicinity of the upper mold based on the calculated amount of enlargement of the shadow of the upper mold.

Another feature of the present invention is the optical safety device, wherein the optical safety device for press brake is provided with a changeover switch for switching the light receiving element positioned in the vicinity of the upper die to an invalidated state, and the invalidating unit invalidates the light receiving element positioned in the vicinity of the upper die when the changeover switch is operated.

Another feature of the present invention is a press brake provided with one of the above mentioned optical safety device for the press brake.

A feature of the present invention is an optical monitoring method, by using a light projector provided on one side in a length direction of an upper table in a press brake for projecting monitoring light directly under an upper die toward other side in the length direction of the upper table and a light receiving element provided on the other side in the length direction of the upper table and receiving the monitoring light, for monitoring a presence or an absence of a foreign substance between the upper die and a lower die based on a light receiving states (result of light reception) of the plurality of light receiving elements in a photodetector, comprising; a step of determining whether or not a shadow of the upper die due to the monitoring light projected on the light receiving element side is expanded to the light receiving element located in the nearest vicinity of the upper die, before the press brake starts a lowering operation of the upper table relative to the lower table in the press brake, and a step of invalidating the light receiving element when it is determined that the shadow of the upper mold extends to the light receiving element positioned nearest to the upper mold.

Hereunder, embodiments and modification according to the present invention will be explained with reference to the drawings.

Embodiments and modifications of the present invention will be described sequentially with reference to <FIG>.

In the description and claims of the present application, the term "be provided with" means to include indirect provision via a separate member in addition to direct provision. The term "foreign body" refers to an object other than a work, such as a worker's finger and a tool. The term "Length direction" refers to the longitudinal direction of the press brake, and in the embodiment of the present invention, refers to the lateral direction. The term "one side along the length" refers to one side in a longitudinal direction of the press brake, and in the embodiment of the present invention, refers to a right side. The term "other side in the length direction" refers to the other side in the longitudinal direction of the press brake, and in the embodiment of the present invention, refers to the left side. The term "depth direction" refers to a depth direction of the press brake, and in the embodiment of the present invention, refers to the front-rear direction. In the drawings, "FF" indicates a forward direction, "FR" indicates a backward direction, "L" indicates a left direction, "R" indicates a right direction, "U" indicates an upward direction, and "D" indicates a downward direction.

As shown in <FIG> and <FIG>, a press brake <NUM> according to an embodiment of the present invention is a working machine which performs bending work on a plate-like workpiece (sheet metal) W as a part to be worked by the cooperation (engagement) of an upper die <NUM> and a lower die <NUM>. The press brake <NUM> includes a body frame <NUM>. And, the body frame <NUM> includes a pair of side plates <NUM> spaced apart and opposed to each other in the longitudinal direction (lateral direction), and a plurality of connecting members <NUM> for connecting the pair of side plates <NUM>.

A lower table <NUM> extending in the length direction is provided at a lower part of the body frame <NUM>, and a lower mold <NUM> is detachably held on an upper side of the lower table <NUM> through a lower mold holder <NUM>. An upper table <NUM> extending in the longitudinal direction is provided on an upper portion of the body frame <NUM> so as to be movable up and down (move up and down), and the upper table <NUM> vertically opposes the lower table <NUM>. The upper table <NUM> detachably holds the upper mold <NUM> through the upper mold holder <NUM> on its lower side. Further, above each of the side plates <NUM>, a hydraulic lifting cylinder <NUM> is provided as a lifting actuator for lifting and lowering the upper table <NUM>.

Instead of the upper table <NUM> configured to be movable up and down, the lower table <NUM> may be configured to be movable up and down. Instead of using a hydraulic lift cylinder <NUM> as the lift actuator, a lift servomotor (not shown) may be used.

The press brake <NUM> includes a linear encoder <NUM> for detecting the height position of the upper table <NUM>, that is, the height position of the tip (bottom) of the upper mold <NUM>. The linear encoder <NUM> has a linear scale <NUM> provided on one side plate <NUM> and extending in the vertical direction, and a read head (not shown) provided at an appropriate position on the upper table <NUM> for reading the scale of a linear scale <NUM>.

On a front side (Forward) of the lower table <NUM>, a foot switch <NUM> as an activation switch for outputting a predetermined activation signal by a stepping operation of an operator M is installed. The predetermined start signal is a signal for starting the lowering operation of the upper table <NUM> by driving the pair of elevating cylinders <NUM>. A back gauge <NUM> for positioning the workpiece W in the depth direction (anteroposterior direction) with respect to the lower mold <NUM> is provided on the back side (rearward) of the lower table <NUM>. The back gauge <NUM> has a butting member <NUM> capable of butting a end face of the work W, and the butting member <NUM> is position-adjustable in the depth direction.

The press brake <NUM> is provided with an optical safety device <NUM> for monitoring intrusion of foreign bodies other than the work W between the upper mold <NUM> and the lower mold <NUM>. The specific configuration of the optical safety device (Optical safety device for press brake) <NUM> is as follows.

As shown in <FIG> and <FIG>, on one side (right) in the length direction of the upper table <NUM>, a projector <NUM> which projects six laser beams B as monitoring light toward the other side (left) in the length direction of the upper table <NUM>, passing directly under the upper mold <NUM> and in the vicinity of its front side, is provided via a first support mechanism <NUM>. The projector <NUM> has one laser diode <NUM> as a light emitting element for emitting a laser beam B, and a mask <NUM> provided on the emission side of the laser diode <NUM> and having six openings (Not shown) formed therein. The first support mechanism <NUM> has, for example, a known structure as disclosed in the aforementioned Patent Literature <NUM>:<CIT>, and the relative height position of the light projector <NUM> with respect to the upper table <NUM> can be adjusted by the height of the upper mold <NUM>.

As shown in <FIG>, <FIG>, and <FIG>, a photodetector <NUM> for receiving six laser beams B is provided on the other lengthwise side (Left) of the upper table <NUM> via a second support mechanism <NUM>. The light receiver <NUM> has six photodiodes <NUM> (<NUM> ~ <NUM>) serving as six light receiving elements for outputting electric signals (voltage signal) corresponding to the amount of received light of the laser beam B, and each photodiode <NUM> (<NUM> ~ <NUM>) faces a corresponding opening of the mask <NUM>. The three photodiodes <NUM> ~ <NUM> positioned on the front side (FF side) receive three laser beams B projected in the vicinity of the front side immediately below the upper mold <NUM> toward the other side in the longitudinal direction of the upper table <NUM>. The three photodiodes <NUM> ~ <NUM> located on the rear side (FR side) receive the three laser beams B projected in the longitudinal direction of the upper table <NUM> directly under the upper mold <NUM>. The photodiode <NUM> is located nearest to the upper mold <NUM>. The second support mechanism <NUM> has, for example, a known structure as described in the aforementioned Patent Literature <NUM>, and the relative height position of the light receiver <NUM> with respect to the upper table <NUM> can be adjusted by the height of the upper mold <NUM>.

The number of laser beams B generated by one laser diode <NUM> and the mask <NUM> may be appropriately changed. Instead of six laser beams B being generated by one laser diode <NUM> and a mask <NUM>, six laser beams B may be generated by six (same number of laser beams B) laser diodes <NUM>. The mask <NUM> may be omitted, and one laser beam B may be received by six photodiodes <NUM>. The monitoring light may be a parallel light, and an LED light may be used instead of the laser light B. In this case, a light emitting diode (Not shown) is used as the light emitting element instead of the laser diode <NUM>.

As shown in <FIG>, the press brake <NUM> includes a controller <NUM> for controlling a pair of elevating cylinders <NUM>, a back gauge <NUM>, an optical safety device <NUM> and the like based on a machining program, a monitoring program and the like. The controller <NUM> is constituted by one or a plurality of computers, and a linear encoder <NUM>, a foot switch <NUM>, and the like are connected to the controller <NUM>.

The controller <NUM> has a memory (not shown) for storing a processing program, a monitoring program, and the like, and a CPU (not shown) for interpreting and executing the processing program, the monitoring program, and the like. The control device <NUM> is provided with an input part <NUM> for inputting die information, work information, product information, etc., and a display part <NUM> for always displaying the die information, work information, product information, etc. The die information includes information representing the shape and dimensions of the upper die <NUM>, the distance from the light receiver <NUM> to the end face of the upper die <NUM>, and the shape and dimensions of the lower die <NUM>. The work information includes information representing the material, shape, dimensions, etc. of the work W, and the product information includes information representing the shape, dimensions, etc. of the product (not shown).

As shown in <FIG> and <FIG>, the input unit <NUM> has a start button <NUM> for starting the programmed operation of the press brake <NUM>. The input unit <NUM> has a first changeover switch <NUM> for switching the three photodiodes <NUM> ~ <NUM> positioned on the front side to the invalidated state, and a second changeover switch <NUM> for switching the photodiodes <NUM> positioned closest to the upper mold <NUM> to the invalidated state.

The controller <NUM> has a function as the determination unit <NUM>, a function as an invalidation unit <NUM>, a function as a detection unit <NUM>, and a function as a control unit <NUM>. Specific contents of the determination unit <NUM>, the invalidation unit <NUM>, the detection unit <NUM>, and the control unit <NUM> are as follows.

As shown in <FIG>, and <FIG>, before starting the lowering operation of the upper table <NUM> (previous), the determination unit <NUM> determines whether or not the shadow of the upper mold <NUM> by the laser beam B projected on the side of the light receiver <NUM> is enlarged to the photodiode <NUM>, based on the light receiving state (result of light reception) of the photodiode <NUM> located in the nearest vicinity of the upper mold <NUM>. Specifically, before starting the descending operation of the upper table <NUM>, the determination unit <NUM> determines whether or not the amount of light received by the photodiode <NUM> (amount of light received per unit time) is equal to or less than a predetermined first threshold value. If the amount of light received by the photodiode <NUM> is equal to or less than the predetermined first threshold value, the determination unit <NUM> determines that the shadow of the upper mold <NUM> is expanded to the photodiode <NUM>. If the amount of light received by the photodiode <NUM> exceeds the predetermined first threshold value, the determination unit <NUM> determines that the shadow of the upper mold <NUM> has not expanded to the photodiode <NUM>. The predetermined first threshold value is a value set in advance for determining whether or not the shadow of the upper mold <NUM> extends to the photodiode <NUM>.

As shown in <FIG> and <FIG>, the invalidating section <NUM> invalidates the photodiode <NUM> when it is determined that the shadow of the upper mold <NUM> is enlarged to the photodiode <NUM> located nearest to the upper mold <NUM>. The invalidating unit <NUM> invalidates the three photodiodes <NUM> ~ <NUM> positioned on the front side when the first changeover switch <NUM> is operated by the worker M to perform box bending of the work W. The invalidating unit <NUM> invalidates the photodiode <NUM> located in the closest vicinity of the upper mold <NUM> when the second changeover switch <NUM> is changed operation by the worker M. Note that, in <FIG>, the hatching attached to the photodiode <NUM> located in the closest vicinity of the upper mold <NUM> indicates that the photodiode <NUM> is in the invalidated state.

As shown in <FIG>, <FIG>, and <FIG>, during the lowering operation of the upper table <NUM>, the detector <NUM> detects the presence or absence of foreign body between the upper mold <NUM> and the lower mold <NUM> based on the light receiving states (result of light reception) of the effective plurality of photodiodes <NUM>. Specifically, the detector <NUM> detects whether or not the amount of light received by any effective photodiode <NUM> (amount of light received per unit time) is equal to or less than a predetermined second threshold value during the lowering operation of the upper table <NUM>. When the amount of light received by any effective photodiode <NUM> is equal to or less than a predetermined second threshold value, the detection unit <NUM> detects the presence of foreign body between the upper mold <NUM> and the lower mold <NUM>. When the amount of light received by all the effective photodiodes <NUM> exceeds a predetermined second threshold, a detection part <NUM> detects no foreign body between the upper mold <NUM> and the lower mold <NUM>. The predetermined second threshold value is a value set in advance for detecting the presence or absence of a foreign substance between the upper mold <NUM> and the lower mold <NUM>, and may be the same value as the first threshold value.

The control unit <NUM> controls the pair of elevating cylinders <NUM> (hydraulic unit of the elevating cylinder <NUM>) so as to start the lowering operation of the upper table <NUM> when a predetermined start signal is output from the foot switch <NUM>. The control unit <NUM> controls the pair of elevating cylinders <NUM> so as to reduce the lowering speed of the upper table <NUM> when the height position of the tip of the upper mold <NUM> is positioned at a predetermined height position close to the work W while monitoring the detection result from the linear encoder <NUM>. The predetermined height position is either a first height position or a second height position higher than the first height position. In normal bending processing, the first height position is selected as the predetermined height position. When the three photodiodes <NUM> ~ <NUM> positioned on the front side and the photodiode <NUM> located in the closest vicinity of the upper mold <NUM> is in the invalidated state, the second height position is selected as the predetermined height position.

When it is detected that there is a foreign substance between the upper mold <NUM> and the lower mold <NUM>, the control unit <NUM> controls the pair of elevating cylinders <NUM> (hydraulic unit of the elevating cylinder <NUM>) so as to stop the lowering operation of the upper table <NUM>. When the height position of the tip of the upper mold <NUM> is at a predetermined height position, the control unit <NUM> need not control the pair of elevating cylinders <NUM> so as to stop the lowering operation of the upper table <NUM>.

Here, the first changeover switch <NUM>, the second changeover switch <NUM>, the determination unit <NUM>, the invalidation unit <NUM>, and the detection unit <NUM> are regarded as a part of the optical safety device <NUM>.

Next, the operation of the embodiment of the present invention including the optical monitoring method according to the embodiment of the present invention will be described with reference to <FIG> and the like. An optical monitoring method according to an embodiment of the present invention is a method of monitoring the presence or absence of foreign body between an upper mold <NUM> and a lower mold <NUM> using a light projector <NUM> and a light receiver <NUM>, based on the light receiving state of a plurality of photodiodes <NUM> in the light receiver <NUM>. An optical monitoring method according to an embodiment of the present invention includes a determination step, an invalidation step, a detection step, and a stop step.

The operator M turns on the power of the press brake <NUM> (Step <NUM> in <FIG>) and depresses the start button <NUM> (Step <NUM> in <FIG>). Then, six laser beams B are projected from the projector <NUM> toward a position directly below the upper mold <NUM> and the vicinity of its front side. Then, the determination unit <NUM> determines whether or not the shadow of the upper mold <NUM> by the laser beam B projected on the side of the light receiver <NUM> is enlarged to the photodiode <NUM> based on the light receiving state of the photodiode <NUM> located in the closest vicinity of the upper mold <NUM> (Step <NUM> in <FIG>, a determination step).

When it is determined that the shadow of the upper mold <NUM> extends to the photodiode <NUM> (Yes in step <NUM> in <FIG>), the invalidating section <NUM> invalidates the photodiode <NUM> (Step <NUM> in <FIG>, the invalidating step). When box bending is performed on the work W and the first changeover switch <NUM> is operated by the worker M, the invalidating section <NUM> invalidates the three photodiodes <NUM> ~ <NUM> positioned on the front side.

After that, when the operator M performs a foot operation of the foot switch <NUM>, a predetermined start signal is outputted from the foot switch <NUM> (Step <NUM> of <FIG>). Then, the control unit <NUM> controls the pair of elevating cylinders <NUM> to start the lowering operation of the upper table <NUM> (Step <NUM> in <FIG>). Then, the detection unit <NUM> detects the presence or absence of the foreign body between the upper mold <NUM> and the lower mold <NUM> based on the light-receiving states (result of light reception) of the plurality of photodiodes <NUM> (Step <NUM> and detection step in <FIG>).

When it is detected that there is no foreign body between the upper mold <NUM> and the lower mold <NUM> (No in step <NUM> in <FIG>), the lowering operation of the upper table <NUM> is continued. When the height position of the tip of the upper mold <NUM> is positioned at a predetermined height position close to the workpiece W, the control unit <NUM> controls the pair of elevating cylinders <NUM> to reduce the lowering speed of the upper table <NUM>. Thus, the work W can be bent by the cooperation (engagement) of the upper mold <NUM> and the lower mold <NUM> (Step <NUM> of <FIG>). On the other hand, when the presence of foreign body between the upper mold <NUM> and the lower mold <NUM> is detected (Yes in step <NUM> in <FIG>), the control unit <NUM> controls the pair of elevating cylinders <NUM> to stop the lowering operation of the upper table <NUM> (Step <NUM> and the stop step in <FIG>).

When the CPU of the control device <NUM> determines that no portion to be bent remains in the workpiece W after the bending of the workpiece W (No in step <NUM> in <FIG>), the CPU finishes the series of processes. On the other hand, when the CPU of the control device <NUM> determines that a portion to be bent on the workpiece W remains after the bending of the workpiece W (Yes in step <NUM> in <FIG>), the CPU returns the process to the step <NUM> in <FIG>.

It should be noted that the process of step <NUM> in <FIG> (decision step) may be omitted. In this case, in the invalidating process of step <NUM> in <FIG>, the invalidating unit <NUM> invalidates the photodiode <NUM> located nearest to the upper mold <NUM> when the second changeover switch <NUM> is operated by the operator M.

As described above, the invalidating unit <NUM> invalidates the photodiode <NUM> when the shadow S of the upper mold <NUM> extends to the photodiode <NUM> located in the closest vicinity of the upper mold <NUM>. Thus, it is prevented that although there is no foreign body between the upper mold <NUM> and the lower mold <NUM>, the presence of foreign body between the upper mold <NUM> and the lower mold <NUM> is erroneously detected by the detecting part <NUM> by the shadow S of the upper mold <NUM>.

Therefore, according to the embodiment of the present invention, it is possible to sufficiently prevent the lowering operation of the upper table from being stopped by the shadow S of the upper die <NUM> even when the bending is performed with the long upper die <NUM> attached to the press brake <NUM> having the long overall length. Therefore, according to the embodiment of the present invention, the press brake <NUM> is equipped with the optical safety device <NUM>, and the working efficiency (productivity) of the press brake <NUM> is improved while sufficiently securing the safety of the bending work.

(Modifications of the embodiment of the present invention) A modification of the embodiment of the present invention will be described only with respect to a portion different from the above-described embodiment of the present invention.

As shown in <FIG> and <FIG>, a first temperature sensor <NUM> for measuring the temperature of the member at the tip of the upper mold <NUM> and a second temperature sensor <NUM> for measuring the environmental temperature in the factory are connected to the control device <NUM>. The control device <NUM> has a function as the calculation unit <NUM> and a function as the determination unit <NUM> in place of the function as the determination unit <NUM>. The specific contents of the calculation unit <NUM> and the determination unit <NUM> are as follows.

As shown in <FIG> and <FIG>, the calculation unit <NUM> calculates the enlargement amount of the shadow S of the upper mold <NUM> based on the member temperature of the tip of the upper mold <NUM>, which is a measurement result from the first temperature sensor <NUM>, the environmental temperature in the factory, which is a measurement result from the second temperature sensor <NUM>, the length of the upper mold <NUM>, and the distance from the light receiver <NUM> to the end surface of the upper mold <NUM>. The enlargement amount of the shadow S of the upper mold <NUM> refers to the maximum amount by which the shadow S of the upper mold <NUM> protrudes from the outline of the upper mold <NUM>. It is preferable that the calculation unit <NUM> calculates the enlargement amount of the shadow S of the upper mold <NUM> in consideration of the distance from the light receiver <NUM> to the end face of the upper mold <NUM>. The calculation unit <NUM> may calculate the enlargement amount of the shadow S of the upper mold <NUM> based on the captured image captured from the side of the projector <NUM> by the camera (not shown).

Just before starting the lowering operation of the upper table <NUM> (Previous), a determination part <NUM> determines whether or not the shadow S of the upper mold <NUM> is enlarged up to a photodiode <NUM> positioned nearest to the upper mold <NUM> based on the calculated enlargement amount of the shadow S of the upper mold <NUM>. Specifically, before starting the lowering operation of the upper table <NUM>, the determination unit <NUM> determines whether the enlargement amount of the shadow S of the upper mold <NUM> is equal to or larger than a predetermined enlargement amount. If the enlargement amount of the shadow S of the upper mold <NUM> is equal to or larger than a predetermined enlargement amount, the determination unit <NUM> determines that the shadow of the upper mold <NUM> is enlarged up to the photodiode <NUM>. When the enlargement amount of the shadow S of the upper mold <NUM> is less than a predetermined enlargement amount, the determination part <NUM> determines that the shadow of the upper mold <NUM> is not enlarged to the photodiode <NUM>. The predetermined enlargement amount is a preset enlargement amount for determining whether or not the shadow of the upper mold <NUM> is enlarged to the photodiode <NUM>.

In the modification of the embodiment of the present invention, immediately before the processing of step <NUM> in <FIG>, the calculation unit <NUM> calculates the enlargement amount of the shadow S of the upper mold <NUM> (calculation step). In other words, the optical monitoring method according to a modification of the embodiment of the present invention includes a calculation step immediately before the processing of step <NUM>. In the process of step S <NUM> in <FIG>, the determination unit <NUM> determines, based on the calculated enlargement amount of the shadow S of the upper mold <NUM>, whether or not the shadow S of the upper mold <NUM> has been enlarged up to the photodiode <NUM> located in the vicinity of the upper mold <NUM> (decision step).

The modification of the embodiment of the present invention also produces the same effect as the embodiment of the present invention described above.

It should be noted that the present invention is not limited to the description of the above-described embodiments, and may be implemented in various forms, for example, as follows.

The determination unit <NUM> may determine whether or not the shadow of the mold <NUM> is enlarged to any of the four photodiodes <NUM>, <NUM>, <NUM> based on not only the light receiving state of the photodiode <NUM> located in the closest vicinity of the upper mold <NUM> but also the light receiving states of the three photodiodes <NUM>,<NUM>,<NUM>,<NUM>. Similarly, the determination unit <NUM> may determine, based on the calculated enlargement amount of the shadow S of the upper mold <NUM>, whether or not the shadow S of the upper mold <NUM> has been enlarged to any one of the photodiodes <NUM>,<NUM>,<NUM> in addition to the photodiode <NUM> located in the vicinity of the upper mold <NUM>.

The scope of rights included in the present invention is not limited to the above-describedembodiments but by the scope of the appended claims.

Claim 1:
An optical safety device (<NUM>) for a press brake (<NUM>) comprising:
a projector (<NUM>) provided in use on one side in a length direction of an upper table (<NUM>) in the press brake (<NUM>) for projecting a monitoring light directly under an upper die (<NUM>) and toward the other side in the length direction of the upper table (<NUM>);
a photodetector (<NUM>) provided in use on the other side in the length direction of the upper table (<NUM>) and having a plurality of light receiving elements (<NUM>, <NUM>-<NUM>) for receiving the monitoring light and outputting electric signals corresponding to an amount of the monitoring light; and
a detection unit (<NUM>) for detecting a presence or absence of a foreign body between the upper die (<NUM>) and a lower die (<NUM>) based on light receiving states of effective light receiving elements (<NUM>, <NUM>-<NUM>) during the lowering operation of the upper table (<NUM>) relative to the lower table (<NUM>);
characterized by
a determination unit (<NUM>) for determining whether or not a shadow of the upper die (<NUM>) generated by refracting the monitoring light projected on the side of the light receiving elements (<NUM>, <NUM>-<NUM>) extends to the light receiving element (<NUM>) located closest to the upper die (<NUM>) before starting a lowering operation of the upper table (<NUM>) relative to a lower table (<NUM>); and
an invalidation unit (<NUM>) for invalidating the light receiving element (<NUM>) located closest to the upper die (<NUM>) when it is determined that the shadow of the upper die (<NUM>) extends to the light receiving element (<NUM>) located closest to the upper die (<NUM>).