Patent Publication Number: US-6660993-B2

Title: Safety apparatus and protection method for machines

Description:
This Application is a continuation of application Ser. No. 09/101,719, filed Apr. 16, 1999, now U.S. Pat. No. 6,316,763B1. 
    
    
     TECHNICAL AREA 
     The present invention relates to a safety apparatus and protection method for machines having a moving member. 
     BACKGROUND ART 
     The invention has particular, although not exclusive, utility with respect to press brakes and other types of machines having a working member that operates against a work-piece supported by a supporting member, whereby one of the members is controlled to selectively move convergingly towards the other member of the machine. With these types of machines an operator generally works in close proximity to, and can have a body member enter, the neighbourhood of the working member during traversing of the moving member. This moving member may comprise the working member, such as in a normal down-stroking press brake or it may comprise the work-piece supporting member such as occurs in an up-stroking press brake. 
     The invention is a development of the safety apparatus disclosed in Australian Patent Application No. 27084/92, which is incorporated herein by reference. It is an object of the present invention to protect objects entering the neighbourhood of a working member of a machine that operates against a workpiece supported by a supporting member, whereby one of the members is controlled to selectively move convergingly towards the other member of the machine. 
     Moreover, it is a preferred object of the invention to prevent the object from being impinged by that member of the machine which moves. 
     In accordance with one aspect of the present invention, there is provided a safety apparatus for a machine having a working member provided with a working end and a work-piece supporting member, whereby one of the members is controlled to selectively move convergingly towards the other member of the machine, the safety apparatus including: 
     corresponding light emitting means and light receiving means for mounting in fixed relationship with the working end of a working member, so as to define a protected region fixedly spaced therefrom; 
     said light emitting means being adapted for emitting a beam of light and the corresponding light receiving means being adapted for receiving the beam of light so that normally the beam may be emitted and received by said corresponding light emitting means and light receiving means along an uninterrupted path fixedly spaced from the working end of the working member; 
     control means to activate said light emitting means to emit the light beam and said light receiving means to sense receipt of the emitted light beam during a range of prescribed movement of the moving member, said range of prescribed movement being completed at a mute point and the moving member being capable of continuing to move through a further range of prescribed movement past said mute point; 
     halting means for halting advancing movement of the moving member in response to some contingency as detected or sensed by said control means. 
     Preferably, said control means includes moving member control means to control the direction or speed of movement of the moving member of the machine. 
     Preferably, said moving member control means includes: 
     input signals from one or more of: 
     said halting means, 
     said machine operator or operators, via hand control, switches, or other input device, or 
     machine sensors, such as pressure and limit switches; 
     moving member control processing means; and 
     output control signals, to control valves, relays, or other control devices. 
     Preferably, said halting means is disabled from halting advancing movement of the moving member for certain contingencies during said further range of prescribed movement. 
     Preferably, said control means includes position processing means to continuously track the movement of the moving member and check that said movement is in accordance with said prescribed movement; and wherein said halting means halts advancing movement of the moving member in response to said position processing means determining that said advancing movement is not in accordance with said prescribed movement. 
     Preferably, said position processing means includes tracking means to measure instantaneous movement of said moving member, said position processing means recording said mute point relative to the position of said moving member as measured by said tracking means and thereafter continuously checking for the occurrence of the measured location of the mute point as determined by said tracking means for effecting control of the movement of said moving member. 
     Preferably, said control means includes pulsing means to cause said light emitting means to generate the light beam so that it is pulsed in a prescribed manner, and light control and processing means to control the operation of said pulsing means and process signals received by said light receiving means to determine when the emitted light beam is not received or pulsed in said prescribed manner; and wherein said halting means halts advancing movement of the moving member in response to said light control and processing means determining that the emitted light beam is not received or pulsed in said prescribed manner during said range of prescribed movement. 
     Preferably, the safety apparatus includes a plurality of corresponding light emitting means and light receiving means to be disposed to define a barrier of light paths around the leading edge, and said pulsing means being adapted to cause each of said corresponding light emitting means and light receiving means to be pulsed in a different manner to differentiate between them. 
     Preferably, said control means includes vibration sensing means to analyse signals in response to said light receiving means sensing receipt of the light beam and distinguish between normal vibration of the light beam and abnormal interruption of the light beam; and wherein said halting means halts advancing movement of the moving member in response to said vibration sensing means sensing said abnormal interruption of the light beam during said range of prescribed movement. 
     Preferably, said light emitting means is mounted at one end of the active member and said light receiving means is mounted at the opposing end of the active member so that vibrational movement of the corresponding light emitting means and light receiving means which causes oscillatory movement of the emitted light beam is damped in one dimension transverse to the path of said light beam to essentially confine the resultant oscillatory movement of the light beam to a single transverse dimension substantially orthogonal to said one dimension to reduce the detection time taken in sensing a said abnormal interruption of the light beam. 
     Preferably, a plurality of said corresponding light emitting means and light receiving means are provided so that said light emitting means are integrally mounted in substantially parallel relationship adjacent to each other as a discrete unit relative to the active member and said light receiving means are integrally mounted in substantially similar parallel relationship adjacent to each other as a separate discrete unit relative to both the active member and said discrete unit, but in substantial alignment with said corresponding light emitting means to receive the respective emitted light beams therefrom, such that vibrational movement is imparted to each discrete unit as a whole causing synchronous and corresponding oscillation to said light beams and synchronous and corresponding sensing of uninterrupted passage of said light beams by said light receiving means, thereby facilitating the analysis and discrimination of the received light beams by said vibration sensing means. 
     Preferably, said moving member tracking means continuously checks the distance advanced by the moving member beyond said mute point with a prescribed maximum distance and if receipt of the emitted light beam is not interrupted within said prescribed maximum distance signals said halting means to halt further advancing movement of the moving member. 
     Preferably, said safety apparatus includes operator interface means to accept instructions from the operator, and to display information back to the operator; whereby said operator interface is constructed to identify certain specific conditions, and other information being coded to identify different faults and operation conditions. 
     Preferably, said control means includes step-up means to disable said halting means and selectively determine the limits of said prescribed movement and position of said mute point via said tracking means. 
     In accordance with another aspect of the present invention, there is provided a method for protecting an object entering into the path of a moving member of a machine, the moving member being either having a working member provided with a working end, or a work-piece supporting member, whereby one of the members is controlled to selectively move convergingly towards the other member of the machine, the method including: emitting a light beam at a fixedly spaced distance along a normally uninterrupted path in advance of the working end of the working member whilst the moving member moves through a range of prescribed movement, said range of prescribed movement being completed at a mute point and the moving member being capable of continuing to move through a further range of prescribed movement past said mute point; 
     continuously sensing for the receipt of said light beam after it has traversed in advance of the working end; and 
     halting the advancing movement of the moving member in response to any failure to receive and sense the emitted light beam at any time during said range of prescribed movement or in response to some contingency. 
     Preferably, the method includes preventing the moving member from being halted for certain contingencies during said further range of prescribed movement. 
     Preferably, the method also includes continuously tracking the movement of the moving member, checking that said movement is in accordance with said prescribed movement ,and halting advancing movement of the moving member whenever the movement is not in accordance with said prescribed movement. 
     Preferably, the method includes pulsing said light beam in a prescribed manner and halting the advancing movement of the moving member in response to failing to receive and sense the emitted light beam as pulsed in said prescribed member at the other end of the active member. 
     Preferably, the method includes emitting and sensing the receipt of a plurality of light beams disposed to define a barrier of light paths around the working end of the working member, each light beam being pulsed in a different manner to differentiate between them. 
     Preferably, the method includes analysing the received light beam to discriminate between normal vibration and abnormal interruption of the light beam and halting advancing movement of the moving member in response to sensing said abnormal interruption during said range of prescribed movement. 
     Preferably, the method includes damping vibrational movement of the light beam in one dimension transverse to the path of said light beam to essentially confine resultant oscillatory movement of said light beam to a single transverse dimension substantially orthogonal to said one dimension to reduce the detection time taken in sensing a said abnormal interruption of the light beam. 
     Preferably, the method includes emitting a plurality of light beams in substantially parallel relationship to each other such that vibrational movement from the machine is imparted equally to said light beams causing synchronous and corresponding oscillation of said light beams facilitating the analysis and discrimination of same. 
     Preferably, the method includes completing said range of prescribed movement at a mute point and continuing movement of the moving member through a further range of prescribed movement past said mute point during which advancing movement of the moving member is not halted by certain contingencies. 
     Preferably, the method includes continuously checking the distance advanced by the moving member beyond said mute point with a prescribed maximum distance and if interruption of the light beam is not sensed within said prescribed maximum distance, halting the advancing movement of the moving member. 
     Preferably, the method includes continuously measuring the instantaneous movement of the moving member, separately detecting the position of the mute point when the moving member is physically disposed thereat, checking that the detected mute point coincides with the measured mute point whenever wither mute point condition is sensed, and halting further advancing movement of the moving member immediately upon establishing a difference in the occurrence between the two. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be better understood in light of the following description of two specific embodiments thereof. The description is made with reference to the accompanying drawings, wherein: 
     FIG. 1 is a front view of a press brake to which the safety apparatus is fitted; 
     FIG. 2 is a fragmentary perspective view showing the safety apparatus of the first embodiment fitted to the press brake of FIG. 1; 
     FIGS.  3 ( a ),  3 ( b ) and  3 ( c ) are partial schematic side view cross sections of the press brake with the moving blade disposed in different positions to show the relative location of the light beams with respect to the leading edge of the blade during a pressing operation; 
     FIGS.  4 ( a ),  4 ( b ) and  4 ( c ) are partial schematic side view cross sections of the press brake in use during a pressing operation and in which the path of the moving blade is impeded by an object; 
     FIG. 5 is a schematic diagram showing the arrangement of the laser transmitting and receiving circuits; 
     FIG. 6 is a front view of the control box of the safety apparatus; 
     FIG. 7 is a schematic diagram of the controller of the first embodiment showing the various inputs and outputs connected thereto; 
     FIG. 8 is a block diagram showing the logical configuration of the controller of the first embodiment; 
     FIG. 9 a  is a schematic end view of the disc and shaft of the optical encoder; 
     FIG. 9 b  is a schematic side view of the blade tracking means incorporating the optical encoder; 
     FIG. 10 is a graphical representation showing the relative position of the blade with respect to various input and output signals of the controller during normal operation of the press in accordance with the first embodiment; 
     FIG. 11 is a block diagram showing the various states of operation of the controller during normal operation of the press in accordance with the first embodiment; 
     FIG. 12 is a graphical representation showing the relative position of the blade with respect to various input and output signals of the controller during the setup procedure of the press in accordance with the first embodiment; 
     FIG. 13 is a block diagram showing the various states of operation of the controller during the setup procedure of the press in accordance with the first embodiment; 
     FIG. 14 is a graphical representation showing the relative position of the blade with respect to various input and output signals of the controller during the mute forced mode of operation of the press in accordance with the first embodiment; 
     FIG. 15 is a block diagram showing the various states of operation of the controller during the mute forced mode of operation of the press in accordance with the first embodiment; 
     FIG. 16 is a graphical representation showing the relative position of the blade with respect to various input and output signals of the controller during normal operation of the press in accordance with the second embodiment; 
     FIG. 17 is a block diagram showing the various states of operation of the controller during normal operation of the press in accordance with the second embodiment; and 
     FIG. 18 is a block diagram showing the various states of operation of the controller during the mute forced mode of operation of the press in accordance with the second embodiment. 
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     The first embodiment is directed towards a safety apparatus fitted to a conventional press brake where the blade is the working or active member and also the moving member of the press, and to a method of protecting an object entering into the path of movement of the blade. This is converse to an upstroking press brake where the blade is still the working or active member, but is stationary, and the bed of the press is both the work-piece supporting and moving member that converges towards the blade. 
     As shown in FIG. 1 of the drawings, a conventional press brake  10  comprises a housing  12 , a blade frame  14 , a base  16 , a blade  18  being both the active and moving member of the press, a pair of movement control switches disposed within a foot pedal  34 , and a hydraulic system and press controller therefor (not shown) for moving the blade. The blade frame  14  houses the blade  18  and is movable in conjunction with the blade in a vertical plane from within the housing  12  by operating the press controller and hydraulic system using the movement control switches via the foot pedal  34 . The blade  18  is mounted in a substantially upright manner and has a leading edge  36  as shown in FIGS. 2 to  4 . The base  16  has a co-operating bed member  38  having a recess  40  provided therein which functions as a die. 
     An item of material to be pressed into the recess  40 , for example a sheet  42 , may be provided on the bed  38 , as shown in FIGS. 3 and 4. The recess  40  may take any form required to effect the desired shape to which the material is pressed. In the present embodiment it is a right V-shape to generate an angle member as shown in FIG.  3 (C). 
     In FIG. 4, there is shown a user&#39;s hand  44  manipulating the sheet  42 . Accordingly, it is the function of the safety apparatus to protect incursions of the limbs of an operator into the path of movement of the blade  18  so that any limb is not impinged by the blade, which otherwise would generally result in the limb being amputated if the blade is not halted before meeting the bed  38 . 
     The safety apparatus generally comprises a pair of adjustable bracket arms  20 , a pair of adjustable vertical supports  21  mounted thereto, corresponding light emitting means  22  and light receiving means  24 , and control means, part of which is housed in a control box  25 . 
     The pair of bracket arms  20  are affixed to the blade frame  14  and extend outwardly and oppositely from the opposing ends of the blade frame to respectively locate the vertical supports  21  for the corresponding light emitting means  22  and light receiving means  24  to be mounted thereon. Accordingly, the light emitting means  22  and light receiving means  24  are disposed in housings  28  at either end of the blade  18 , and furthermore, are precisely aligned so that a light beam  26  emitted from the light emitting means  22  traverses a path disposed in substantially parallel and advanced relationship to the leading edge  36  of the blade  18  as shown in FIG. 2 of the drawings. The light receiving means  24  is correspondingly disposed to normally receive the light beam  26  when the path of the same is uninterrupted. 
     As shown in FIGS. 3 and 4, the light beams are emitted to be disposed in close and parallel proximity to the leading edge of the blade  18 . In the present embodiment, three light beams represented by paths  26   a ,  26   b  and  26   c  are provided which define a barrier in a plane typically 4 millimeters (mm) below the leading edge of the blade with the front and rear beams disposed typically 20 mm either side of the blade centre line. 
     The housings  28  for the light emitting means  22  and light receiving means  24  are hooded to minimise the effects of reflected light scatter from ambient light striking the light emitting means. 
     In the present embodiment, as shown in FIGS. 2 to  5 , the light emitting means  22  comprises three lasers, each having well defined and accurately focussed light beams to prevent the beams from scattering and reflecting upon the bed and blade surfaces. In the present embodiment, the lasers have an angle of diffusion of less than 0.04° and are laterally spaced to dispose the light beams in a horizontal plane relative to the vertical plane of the blade  18 . One laser is disposed in advance of, and marginally to the front side  18   a , of the blade  18  to direct an emitted light beam along the path represented by the line  26   a  of the drawings; a second laser is disposed in advance of, and in coplanar relationship with, the blade  18  to direct an emitted light beam along the path represented by the line  26   b ; and finally a third laser is disposed in advance of, and marginally to the rear side  18   b , of the blade to direct an emitted light beam along the path represented by the line  26   c  of the drawings. 
     The lasers include laser diodes  39   a ,  39   b  and  39   c  which are specially designed to be switched in a prescribed manner at a frequency in the kilohertz range so as to produce a pulsed light beam having a period in the millisecond range. At this frequency, the pulsing of the light beam is not apparent to the naked eye and so the light beam appears as a continuous beam. 
     The light receiving means  24  comprises a laser light sensor  41  including lens  43  and a mask  45  having three apertures  47   a ,  47   b  and  47   c  which are set up to be precisely aligned in corresponding relationship with the respective laser diodes  39   a ,  39   b  and  39   c  to receive the light beams emitted therefrom. The mask  45  is provided to allow only correctly aligned light beams through the apertures  47  to the laser light sensor  41 . 
     The laser diodes are mounted in adjustable sights (not shown) to focus and direct the light beams along their respective paths. The sights are fixedly and integrally mounted to form a separate discrete unit by press fitting them into a solid nylon mounting block. This mounting block spacially fixes their position relative to each other so that any vibrational effects upon the light emitting means  22  are transmitted equally and simultaneously to all of the lasers. 
     The laser sensor  41  is mounted in a cylinder with the lens  43  and the mask  45  is integrally mounted to the front of the cylinder, whereby all elements of the light receiving means  24  form a separate discrete unit by similarly being press fitted into another solid nylon mounting block for minimising and uniformly transmitting any vibrational effects applied to the light receiving means  24  to the laser sensor. 
     The lens  43  is disposed in relatively close proximity to the front of the cylinder and the mask to enable receipt of all three light beams and enable the use of a single sensor  41 . This is required when the light beams are disposed in particularly close relationship with each other where reliable sensing cannot be obtained using individual sensors. However, in cases where the light beams can be disposed further apart, individual sensors and lenses can be used for respective light beams. 
     The housings  28  of the corresponding light emitting means  22  and receiving means  24  are adjustably mounted in the vertical and horizontal planes with respect to the vertical supports  21 . This allows for variance in different blades which may be fitted to the press brake. 
     The bracket arms  20  and the vertical supports  21 , in conjunction with the housings  28  and the laser mounting arrangement, are strong enough to ensure that the laser diodes do not vibrate separately of the blade  18  and are light enough to ensure that vibration frequency at the end of the blade is not reduced too much, since an increased mass at the end of the blade results in an increase in the laser detection time. 
     It should be appreciated that laser beams are typically not directionally stable, whereby the emitted light beam generally oscillates in two transverse dimensions about a mean sensing point for the corresponding mask aperture  47 . A typical average oscillation time for this is 14 ms and the oscillation distance can be as wide as 3 cm. This would generally result in the laser detection time being too excessive to be workable in the present application. This is overcome in the present embodiment however, by fixedly mounting the light emitting means  22  and the light receiving means  24  directly to the blade frame  14  using the bracket arms  20  and the vertical supports  21  as previously described. In this manner, the mass of the blade frame  14  and the blade  18  in the vertical plane is so great relative to the transverse plane that very little vertical oscillation occurs. Consequently, most of the vibration is in the transverse plane, reducing the laser detection time to a satisfactory level. 
     With vertical oscillation being negligible, the lasers can be vertically disposed in close proximity to the leading edge  36  of the blade  18 . This is particularly advantageous with respect to setting the “mute point” of the apparatus to close tolerances relative to the material to be pressed. The mute point will be described in more detail later. 
     Notwithstanding this, the significant horizontal oscillation will still have an effect on the lasers and laser sensor, as they are disposed in relatively close proximity to each other in the horizontal plane. Accordingly, the horizontal oscillation can result in occasional cross aligning of a laser light beam with the mask aperture  47  for an adjacent laser. Consequently, the manner in which each laser diode  39  is switched is unique to that laser so that the laser sensor  41  can differentiate between the receipt of a particular light beam and the receipt of light beams emitted from any of the other adjacent lasers, thereby enabling a single sensor  41  and lens  43  to be used for multiple light beams. Hence each laser is electrically connected to a corresponding driver circuit  52   a ,  52   b  or  52   c , all of which form part of the control means for switching the laser diode  39  connected thereto in a particular way to differentiate the light beam emitted therefrom from the other light beams. Similarly, the laser sensor  41  is electrically connected to a corresponding laser receiver circuit  53   a ,  53   b  or  53   c , all of which also form part of the control means for sensing receipt of only that light beam emitted from the laser diode to which the laser receiver circuit corresponds. 
     Although the light emitting means  22  in the present embodiment has been described using three lasers and a mask  45  provided with corresponding apertures  47   a ,  47   b  and  47   c  to each of the light beams  26   a ,  26   b  and  26   c , by locating the lens  43  in relatively close proximity to the front of the cylinder in the mask, other embodiments can be provided which include additional numbers of differently modulated lasers having corresponding laser driver circuits  52  and laser receiver circuits  53 . In such embodiments, the mask is only necessary to mask the outside lasers since by correct installation of the light emitting means and the formation of same as a discrete integral unit, adjacent light beams will be correctly aligned relative to one another once the outer light beams are correctly aligned with respect to the light receiving means  24 . Accordingly, the receipt of different lasers can be differentiated between each other by virtue of their unique modulation or encoding as opposed to alignment with corresponding apertures. 
     As shown in FIG. 7, the control means comprises a main controller  51  is arranged to receive various input signals for controlling the operation of the press brake and sensor signals effecting predetermined safety parameters of the apparatus, process these signals and generate relevant output signals for operating the press brake, driving the lasers, halting advancing movement of the blade in response to the occurrence of certain prescribed contingencies and providing relevant information to an operator of the safety apparatus and press brake. 
     The controller  51  in the present embodiment is in the form of twin microprocessor systems, configured in a dual master-slave system, providing for constant cross-checking of each other to ensure that there is no logic malfunction between the two. The master microprocessor is designed to check the input/output devices connected to the controller and the slave processor, and the slave microprocessor is designed to check the master processor and PLD. 
     The microprocessors are programmed to provide specific processes as logically shown in FIG. 8, comprising: a laser control and processing means  54 , incorporating pulsing means  55  to switch the laser driver circuits  52  in a prescribed manner; blade movement control means  56  for controlling the general movement of the blade  18  of the press brake; halting means  57  to halt advancing movement of the blade; blade position processing means  58  including tracking means  59  to separately track and check correct movement of the blade; and setting means  60  to set the control parameters of the safety apparatus, including the mute point, and provide for testing of the same. 
     The mute point is defined to be a position reached by the leading edge  36  of the blade  18  pursuant to advancement towards the base  16  at a prescribed distance above the workpiece material  42  intended to be pressed upon the bed  38 . At this point, the halting means is disabled, ceasing protective sensing of the light beams and allowing the blade to progress beyond the initial prescribed range of protective movement, through a further prescribed range of movement, to engage the material  42  and press it against the bed  38  of the press brake. 
     The mute point is normally set at a distance from the outer surface of the work piece being pressed, which is smaller than the height of an object intended to be protected by the safety apparatus. In the present embodiment, such an object is a finger of an operator and so the mute point is normally set to a distance of 6 mm to 7 mm above the outer surface of the work piece being pressed. 
     The laser control and processing means  54  is designed to operate the pulsing means  55  so as to cause the laser driver circuits  52  to generate laser light beams in the prescribed manner. The pulsing means  55  modulates the respective laser driver circuits  52  with a unique code at the same or different frequencies in order to differentiate between the respective emitted light beams. This code is a prescribed repetitive digital code which can be accurately sensed by the corresponding laser receiving circuits  53  of the light receiver means and be processed by the laser control and processing means  54  to detect an interruption caused by the incursion of an object into any one of the light beams. Accordingly, the laser control and processing means  54  is programmed to continuously monitor the signals received from the laser receiving circuits  53  in conjunction with operating the pulsing means  55  and determine whether all three beams are received and sensed at the same time within a prescribed period of time. If this does not occur, then it is assumed that one or more light beams have been obstructed and a contingency procedure is invoked involving the laser control and processing means  54  immediately triggering the halting means  57  to halt advancement of the blade  18 . The blade movement control means  56  will then instigate a different mode of operation for the press brake, which will be described in more detail later. 
     The laser control and processing means  54  is programmed to include vibration sensing means to distinguish between normal vibration of the light beams caused by the operation of the press brake and abnormal interruption of any one of the beams caused by an obstruction. The vibration sensing means also provides some tolerance to the false detection of light beams and consequently reduces occurrences of false triggering of the halting means. 
     As previously described, due to the accurate focussing of the laser light beams, there is a significant horizontal oscillation, whereby the light beams will only periodically be received and sensed by the light receiving means. However, due to the integral mounting of the laser diodes and the laser sensor with the mask apertures, when the light beams are momentarily aligned with their corresponding mask aperture, and thus are received and sensed by the corresponding laser receiving circuits  53 , this will occur instantaneously and simultaneously amongst all three sensors, as will be the period of their nonalignment. Accordingly, the vibration sensing means analyses the signals output by the laser receiving circuits  53 , recognising the particular characteristics of these signals which indicate normal, uninterrupted transmission of the light beams. These characteristics include the unique coding or pulsing to differentiate between the three beams; the instantaneous and simultaneous receipt of the beams; and the periodicity of such. From this analysis, the vibration sensing means can detect departures from these characteristics as being representative of either false detection of a beam, actual obstruction of a beam, or some other abnormality requiring triggering of the halting means. 
     For example, in the event that one of the light beams, during its period of nonalignment, was actually sensed from a reflection, then as a result of the analysis of the vibration sensing means, the laser control and processing means  54  would recognise this as being a falsely detected light beam, if none of the other light beams were aligned at that moment, and it was not expected that any of the light beams would be so aligned at that moment. Similarly, if one of the light beams during its period of nonalignment actually impinged upon the laser sensor through another mask aperture other than its own, then it would also be recognised as being a falsely detected beam, due to it not being modulated correctly for that particular laser sensor. 
     It should be noted that the detection time taken by the laser control and processing means to discriminate between normal, uninterrupted transmission of the light beams and an actual obstruction of one or more of the beams is required to be substantially less than the time taken for the blade to advance from the mute point to impinge the material  42  on the bed  38 . This is necessary to cater for the situation where an obstruction of a beam by a limb occurs just as the blade is approaching the mute point. Obviously if the laser control and processing means  54  cannot detect that such an obstruction has occurred in time, then the blade will impinge the material  42  before it can be halted by the halting means  57 . 
     The laser control and processing means  54  is also programmed to sense for the receipt of any ambient light that could be sensed as a legitimate light beam during the period that the laser diodes are switched off and are not transmitting any light. This may arise from welding flashes or the like in the vicinity of the press, and if detected as legitimately received light beams, could cause erroneous sensing during a time that one or more of the light beams were actually obstructed by the incursion of an object into the path of an advancing blade, severely impugning the integrity of the safety apparatus. If such erroneous ambient light is sensed and processed by the laser control and processing means  54 , then an appropriate contingency procedure is invoked involving triggering of the halting means  57 . 
     The laser control and processing means  54  is additionally programmed to operate the pulsing means  55  so that the laser driver circuits  52  switch the lasers at a frequency which produces light beams at a particular luminance that is able to be just detected by the naked eye. The switching of the laser diodes improves their longevity as opposed to switching them on continuously and allows for the unique modulation of different lasers as previously described. 
     Additionally, the laser control and processing means  54  is programmed to increase the switching frequency of a particular laser diode to produce a light beam of higher luminance when that light beam is not being received by a corresponding laser sensor, when it is determined by the laser control and processing means that it should be so received. This would arise if the light beam was intercepted by an object or was misaligned for some reason. Consequently, this has the advantage that in the event of a light beam being obstructed by an object, the light beam instantly increases in luminance making it highly visible to the operator to alert them to the danger and the problem, quite apart from any contingent action to halt advancement of the blade. 
     A further advantage of this aspect of the invention is that in the situation of initially aligning the lasers with their corresponding mask apertures  47 , the higher luminance gives immediate feedback to the operator of any misalignment of the laser and facilitates their adjustment, whereby the light beam or beams would immediately decrease their luminance upon being correctly aligned. 
     In addition to interacting with the laser driver circuits  52  and the laser receiving circuits  53 , the laser control and processing means is programmed to indicate the status of the laser sensing facility via a series of indicator lights. Accordingly, the controller  51  has outputs connected to a series of display lamps  107  provided on the control box  25 . As shown in FIG. 6 of the drawings, on the left side of the control box  25 , these display lamps include two sensor lights, one green sensor light  46  to signify whether all of the light beams are clear, ie. being received by the laser sensors, and one red indicator light  48  which indicates whether any one of the light beams is obstructed. Accordingly, when the laser control and processing means  54  determines that the light beams are clear, the green indicator light  46  is caused to be illuminated, and when any light beam is determined by the laser control and processing means to be obstructed, the green indicator light is extinguished and the red indicator light  48  is illuminated to signify that the beams are not clear. 
     On the right side of the control box  25  are a set of laser sensor clear lights comprising three green lights  50   a ,  50   b  and  50   c  respectively corresponding to each of the light beams. Accordingly, the light  50   a  corresponds to the front light beam, the light  50   b  corresponds to the middle light beam and the light  50   c  corresponds to the rear light beam. The laser control and processing means  54  causes the relevant green light to be illuminated in response to sensing the receipt of a clear light beam by the corresponding laser receiving circuit  53 . The instant a light beam is obstructed, the laser control and processing means  54  causes the corresponding light to be extinguished. 
     The blade movement control means  56  is designed to effect direct control of the movement of the press brake in response to receiving input signals which effect movement of the blade  18 , subject to the overriding operation of the halting means  57 . These input signals are derived from: the pair of movement control switches operable via the foot pedal  34 , which comprise a blade advance switch  61  for normally causing advancing movement of the blade  18  and a blade retract switch  62  for normally causing retracting movement of the blade; a plurality of limit switches disposed within the housing  12 , which include a mute switch  63 , an upper limit switch  64  and a lower limit switch  65 ; and a pair of control switches provided on the control box  25 , which include a mode switch  31  and a blade speed switch  32 . Accordingly, the controller  51  has inputs connected to these switches and a plurality of outputs isolated by a bank of relay switches  66  connected to the electrical control lines of the hydraulic system of the press brake. 
     The mute switch  63  is adjustably mounted within the housing  12  at a predetermined location and is operated by a striker  30  fixedly mounted to the blade frame  14 . The mute switch  63  and the striker  30  are relatively and precisely positioned so that the mute switch  63  is momentarily closed by the striker when the leading edge  36  of the blade  18  reaches the mute point, following advancing movement of the blade frame  14  and blade  18  towards the bed  38  of the press. 
     The upper and lower limit switches  64  and  65  are similarly mounted within the housing  12  and are operated by the striker  30  and pressure sensors (not shown) connected into the hydraulic system to trigger the appropriate limit switch when the pressure of the hydraulic system exceeds a prescribed threshold representative of the blade reaching a maximal elevation or depression. The upper limit switch  64  is relatively disposed with respect to the blade frame  14  so that it is momentarily closed by the striker  30  when the blade is at its maximally elevated position within the housing  12  or by activation of its associated pressure sensor. The lower limit switch  65  is relatively disposed with respect to the blade frame  14  so that it is momentarily closed when the leading edge of the blade meets the bed  38  of the press at its maximally depressed position or by activation of its associated pressure sensor. 
     The mode switch  31  is key operated to move between two selectable positions, one denoting a normal protected mode of operation for the safety apparatus and press brake and the other denoting a mute forced mode of operation, where the press is not protected by the safety apparatus to the same extent as in the normal protected mode. The normal mode of operation is selected for normal operation of the press with full protection. The mute forced mode is only selected for special occasions such as pressing odd shaped materials, initialisation of the upper and lower limit switches  64  and  65 , described later with respect to the setting means  60 , changing of the blade, press maintenance and the like, which do not constitute part of normal use of the press. As previously mentioned, the mode switch  31  can only be operated by a key. Accordingly, it is intended that this key be retained by management and not the operator to limit switching of the mode switch  31  to the mute forced position on only those aforementioned special occasions. 
     In the mute forced mode, sensing provided by the laser control and processing means  54  is muted completely, however, sensing of all other aspects of the operation of the press is maintained so that advancement of the blade is locked in response to any of the press related contingencies, which will be described in more detail later. In order to accommodate this reduced protection, the advancement of the blade is different to that when the mode switch is in the normal position. Moreover, the blade advances under full speed to a prescribed distance above the mute point and thereafter proceeds in a crawling mode of operation. This aspect of operation will be described in more detail later. 
     The blade speed switch  32  is also key operated for switching between two selectable positions, one providing for normal continuous advancement of the blade  18 , when the blade advancement switch  61  is closed by operation of the foot pedal  34 , and the other providing for a crawling or pulsing advancement of the blade when operated by the foot pedal, as occurs in mute forced mode, but which adopts this mode of advancement throughout the entire range of movement of the blade. As is the case in the mute forced mode of operation, in the crawling position, the press is not protected by the safety apparatus to the same extent as in the normal protected mode. 
     When the mode switch  31  is disposed in the mute forced position or the blade speed switch  32  is disposed in the crawling advancement position, the halting means  57  is not responsive to contingencies arising from processing by the laser control and processing means  54 . The reason for this is that press brakes are sometimes required to press oddly shaped or bowed pieces of material having an uneven profile in the vertical plane. In these situations, certain portions of the surface of the material, when placed on the bed ready for pressing, are disposed considerably closer to the leading edge of the blade than other portions of the material. These closer portions will be engaged by the blade and be forced towards the bed before the blade fully engages the other portions of the material and presses the entire longitudinal extent of the material into the bed. Consequently, if the apparatus was operating with the halting means being responsive to the laser sensing system, a mute point would need to be set relative to the uppermost portion of the material, requiring the blade to travel a considerable distance before engaging the remainder of the surface of the material and pressing the same into the bed, during which the operator would not be protected at all. The problem is further exacerbated by the fact that in most of these situations, the operator is required to work in close proximity to the leading edge of the blade to maintain the correct position of the material until the blade fully engages the entire surface of the material before pressing it into the bed. 
     By allowing for the apparatus to be operated with crawling advancement of the blade, the press is permitted to be operated with total muting of the laser sensing facility provided by the laser control and processing means  54  and without regard to the mute point at all. With crawling or pulsing advancement of the blade, however, the blade can only be operated at a relatively slow speed. Thus, whilst there is no laser sensing protection for the operator at all, protection is indirectly provided by the slowed speed of the blade advancement, and is still directly provided in response to processing by the blade position processing means  58 . 
     Switching of the blade speed switch  32  between the two selectable positions denoting normal advancement of the blade and crawling or pulsing advancement of the blade, is not as critical insofar as safety of the operator is concerned. Accordingly, the key for operating the blade speed switch  32  is intended to be left with the operator to facilitate switching between the two speed positions as desired by the operator. 
     A comparatively large field muted light  35  is associated with switches  31  and  32  being disposed in the mute forced mode and the crawling advancement positions, respectively. Accordingly, the controller  51  has an output connected to the light  35  which is coloured amber to enable operation of the same. This light illuminates when the sensing of the light beams is disabled or muted to signify to the operator when the protective light beams around the leading edge of the blade are muted, which will be the case when either of the switches are disposed in the aforementioned positions. The output line is also connected to a field muted light fault sensor  35   a  which triggers an alarm whenever the circuit connected to the field muted light  35  is open when in fact it should be closed, as might be the case if the light were blown or otherwise faulty. 
     The relays  66  include: a pair of blade advancing relays  67  and  68 , which have a pair of duplicate blade advancing control lines  69  and  70  for connection to the blade advancing terminals of the hydraulic system; a secondary press locking relay  71 , which has a press locking control line  72  for connection to the press locking terminal of the hydraulic system; a blade crawling relay  73 , which has a blade crawl control line  74  for connection to the blade crawl control terminal of the hydraulic system; and a blade retracting relay  75 , which has a blade retraction control line  76  for connection to the blade retraction terminal of the hydraulic system. 
     Closing the blade advancing relays  67  and  68  operates the hydraulic system to advance the blade  18 , subject to closing of the secondary relay  71 , and opening them stops movement of the blade. 
     The secondary press locking relay  71  provides for locking out operation of the press as part of the fail safe operation of the safety apparatus and must be closed in order for any operation of the hydraulic motor via control line  63 . Accordingly, whenever the secondary relay  71  is open, movement of the blade  18  is halted absolutely, so that operation of any of the blade advancing relays  67  and  68 , the blade crawling relay  73  or the blade retracting relay  75  is of no effect. 
     Closing the blade crawling relay  73 , subject to closing of the secondary relay  71 , operates the hydraulic system to cause the blade to periodically advance in a staged manner at a prescribed rate, which will be described in more detail later. 
     Closing the blade retracting relay  75  operates the hydraulic system to retract the blade, again subject to closing of the secondary relay  71 . 
     The blade movement control means  56 , in conjunction with and subject to operation of the halting means  57 , simply effects movement of the blade frame  14  and blade  18  by closing and opening appropriate relays  67 ,  68 ,  73  and  75  in response to operation of the blade advance switch  61  and the blade retract switch  62 , closing of the mute point switch  63 , and the position of the mode switch  31  and blade speed switch  32 . 
     When the mode switch  31  is in the normal mode position, the blade movement control means  56  is operated in the normal manner, activating the blade advancing relays  67  and  68  in response to closure of the blade advance switch  61  and the blade retracting relay  75  in response to closure of the blade retract switch  62 . The laser control and processing means  54 , halting means  57  and blade position control means  58  are also operated normally, providing full protection of the press brake and overriding operation of the blade movement control means  56  in response to particular contingencies arising. 
     In the mute forced mode position, where the light beam sensing facility provided by the laser control and processing means  54  is disabled or muted, but the blade position processing means  58  and other press monitoring facilities of the protection apparatus are maintained, the blade advancing relays  67  and  68  and the blade retracting relay  75  are still operated by the blade movement control means  56  in the same manner as before, except that the halting means  57  does not effect overriding operation of the blade movement control means in response to particular contingencies arising from processing by the laser control and processing means  54 . 
     The particular manner of operation of these relays in response to these contingencies will be described in more detail later. 
     The halting means  57  is the kernel of the safety apparatus, being designed to operate in a fail safe manner to halt advancing movement of the blade  18  in response to being triggered when some predetermined contingency has been sensed. In this respect, the halting means can automatically override the operation of the blade movement control means  56  to effect operation of the various relays  66  to halt advancement of the blade, dependent upon the particular contingency. 
     In the normal protection mode, as selected by the mode switch  31 , the halting means  57  responds to various contingencies which can arise from processing of the received laser light beams by the laser control and processing means  54 , as previously described, or as a result of processing by the blade position processing means  58 , which will be described shortly, or as a result of sensing low power to the press and/or the safety apparatus by means of a low voltage sensor  101  connected as an input to the controller  51 . 
     In the mute forced mode, the halting means  57  responds to only those contingencies which arise as a result of processing by the blade position processing means  58  and the low voltage sensor  101 . 
     Accordingly, the halting means  57  is responsive to the state of the mode switch  31 , as represented logically by the control line  102 , as well as to outputs from the laser control and processing means  54 , as represented logically by control line  103 , the blade position processing means  58 , as represented logically by the control line  104 , and the low voltage sensor  101 , as represented logically by the control line  105 , all as shown in FIG.  7 . 
     The effective operation of the halting means  57  in response to contingencies arising from processing by the laser control and processing means  54  and from processing by the blade position processing means  58  is different, depending upon the particular nature of the contingency. In relation to the former, upon the laser control and processing means  56  sensing an obstruction of any of the light beams and triggering the halting means, the halting means  57  immediately overrides the operation of the blade movement control means  56 , causing opening of any of the blade advancing and blade crawling relays  67 ,  68  or  73  that may be closed, and closing of the blade retracting relay  75  for a short period of time to retract the blade so that it effectively bounces back a prescribed distance. It then causes the blade advancing relays  67  and  68  or the blade crawling relay  73  to be closed, after the blade advance switch  61  is opened by releasing the foot pedal and then closed by depressing the foot pedal once again. If the beam is obstructed when the blade reaches the same point again, the halting means  57  again causes opening of the advancing or crawling relays  67 ,  68  or  73 , halting movement of the blade. Instead of bouncing the blade back, the halting means permits the blade to advance by crawling or pulsing movement, closing the crawling relay  73  in response to the next closure of the blade advance switch  61  by operation of the foot pedal. The particular method of operation will be described with reference to FIG. 10 later. 
     With respect to other contingencies which may be representative of a malfunction of the press brake or the safety apparatus, the halting means  57  effects overriding operation of the blade movement control means  56  in a different manner causing locking up of the press. This will be described hereinafter with respect to the blade position processing means. 
     The blade position processing means  58  is designed to continuously track movement of the blade  18  of the press brake using the blade tracking means  59 . Further, it is designed to trigger the halting means  57  in response to detecting a discrepancy in the occurrence of signals corresponding to key positions of the blade derived from the blade tracking means  59 , compared with the occurrence of corresponding signals derived from the limit switches  63 ,  64  and  65 . Accordingly, the blade position control means  58  is required to be initialised by operation of the setting means  60  to store the position of the blade  18 , as derived by the blade tracking means  59 , at corresponding times to the occurrence of signals derived from the aforementioned limit switches as a result of movement of the blade through its entire range of movement. This initialisation process will be described in more detail later. 
     In the present embodiment, the blade tracking means  59  comprises an optical encoder  77  and a rectilinear-to-rotational motion transfer means  79  connected to the blade frame  14 . 
     As shown diagrammatically in FIGS. 8 a  and  8   b  of the drawings, the optical encoder  77  is of conventional design comprising a disc  81  fixedly mounted to a rotatable shaft  83  connected to the rectilinear-to-rotational motion transfer means  79  to rotate directly in response to rectilinear movement of the blade  18 , and an opto-electrical counter comprising a light emitting device  85 , a photoelectric sensor  87  and a digital up/down counter  89 . The disc  81  has a series of equidistantly spaced apertures  91  transversely disposed around the circumference of the disc  81  and is mounted relative to the opto-electrical counter so that the light emitting device  85  emits a beam of light aligned with the photo-electric sensor  87  axially through the apertures  91  from one side of the disc to the other side of the disc. Accordingly, the counter  89  sequentially counts pulses of light passing through successive apertures  91  as they rotate past the opto-electric counter in an incremental or decremental manner relative to the direction of rotation of the disc  81  and the extent of rotation thereof as caused by movement of the blade  18 . 
     The rectilinear-to-rotational motion transfer means  79  in the present embodiment is in the form of a chain and sprocket drive assembly comprising a sprocket  93  and a chain  95  engaged therewith. One end  95   a  of the chain is connected to the blade frame  14  and the other end  95   b  is connected to a return spring  97  fixedly mounted to the housing  12 . The sprocket  93  is rotatably mounted with respect to the housing  12 , intermediate the ends of the chain  95 , and is fixedly mounted to the shaft  83  so that it causes the shaft to rotate in unison therewith in response to longitudinal movement of the chain around the sprocket. The return spring  97  applies constant tension to the chain so that movement of the blade  18  in either the advancing or retracting direction directly causes longitudinal movement of the chain  95  in a direction away from the return spring or towards the return spring, respectively, which in turn causes rotation of the sprocket  93  in either a clockwise or ant-clockwise direction. 
     Rotation of the sprocket  93  and shaft  83  directly rotates the disc  81  in unison herewith resulting in the counter  85  providing an accurate measure of the position of the blade  18  at any particular point in time. Typically, the accuracy of measurement is to 0.4 mm. Further, by analysing the rate at which pulses are sensed, a measure of the speed of the blade can be determined. This is particularly useful for effecting control of the speed of movement of the blade in the crawl mode, as will become evident later. 
     The blade position processing means  58  continuously analyses information supplied by the optical encoder  77  and compares this with information separately derived from the mute switch  63  to trigger the halting means to halt blade movement in response to any discrepancy sensed in the information derived from the two sources. This will be discussed in more detail later. 
     Accordingly, if there is any discrepancy beyond a prescribed tolerance between the sensed occurrence of closure of the limit switches  64 ,  65  and  66  and the corresponding locations of the blade  18  as sensed by the blade tracking means  59  at these locations as originally set, by the setting means, then the blade position processing means  58  issues a signal to trigger the halting means  57  to halt movement of the blade and lock the press brake out from further operation until the fault is cleared. 
     The response of the halting means  57  to such triggering signals from the blade position processing means  58  is different than in other situations, as previously mentioned. Moreover, the halting means  57  immediately overrides the operation of the blade movement control means  56  by opening any of the blade advancing, blade crawling and blade retracting relays  67 ,  68 ,  73  and  75  which may be closed, and closing the secondary press locking relay  71 . 
     This same operation of the halting means  57  is performed in relation to processing of the received light beam by the laser control and processing means  54  which tends to indicate a malfunction of the laser sensing system or a contingency affecting the normal operation of same, as opposed to the sensing of an obstruction. For example, in the aforementioned contingency arising from sensing erroneous ambient light during the period that the laser diodes are switched off, which could cause false sensing by the laser sensors, the halting means  57  will be similarly triggered to cause closing of the secondary relay and locking of the press in the manner described above. 
     It should be appreciated that the cross checking operation of the blade position processing means  58  effectively provides dual protection covering either failures in the operation of the press brake, on the one hand, or failures in the blade tracking means  59  of the safety apparatus itself, on the other hand, whereby failures in either instance would cause a discrepancy which would result in triggering the halting means  57 . 
     It should be noted that this cross checking is also applied to the location of the mute point, whereby the halting means is triggered to override the operation of the blade position processing means  58 . As previously described, the mute point is initially set via the mute switch  63 , and is thereafter continuously confirmed and cross checked by the blade position processing means  58  against the set position of the blade as separately determined via the blade tracking means  59 . Accordingly, upon detecting any discrepancy beyond a prescribed tolerance between the occurrence of the mute switch signification of the mute point and the blade tracking means signification of the mute point, the blade position processing means triggers the halting means. However, on triggering, the halting means  57  responds slightly differently than in the case of sensing discrepancies with other limit switches. Moreover, the halting means  57  causes the press brake to stop normal operation by opening any of the blade advancing, blade crawling and blade retracting relays  67 ,  68 ,  73  and  75  which may be closed, as is normally the case, but instead of closing the secondary press locking relay  71 , it causes the control means to revert to the test mode to cause resetting of the mute point, which will be described hereinafter with respect to the setting means  60 . 
     The setting means  60  is designed to provide for initialising of the key positions of the blade for the operation of the blade position processing means  58  and setting of the mute point of the press brake and testing thereof, as well as attendance to faults detected by the controller. Accordingly, the controller  51  has inputs connected to the main power on/reset switch  98  of the press, which has an accompanying watchdog circuit  99  connected therewith, a setup link  100  for initialising the limit switches of the safety apparatus and press, and a test/lock switch  33 , and outputs connected to a test lamp  37  forming part of the series of display lamps  107  provided on the control box  25  to enable operation of the setting means  60 . 
     The setting means  60  effects initialisation of the limit switches following switching on of the press, by activating the setup link  100 . The setting means is programmed to then follow a routine whereby the blade is required to be retracted by the operator to its maximally elevated position. At this point the upper limit switch  64  will be activated, signifying to the blade position processing means  58  to set the particular position of the blade, as determined by the blade tracking means  59 , as the maximal elevated position of the blade. Thereafter, whenever this point is reached by the blade tracking means  59 , the blade position processing means  58  expects the upper limit switch  64  to be closed within an allowable tolerance. 
     Next the routine requires the blade to be fully depressed by the operator to strike the bed  38  at its maximally depressed position. At this point the lower limit switch  65  will be activated, signifying to the blade position processing means  58  to set this particular position of the blade, as determined by the blade tracking means  59 , as the maximal depressed position of the blade. Thereafter, whenever this point is reached by the blade tracking means  59 , the blade position processing means  58  expects the lower limit switch  64  to be closed within an allowable tolerance. 
     The setup link  100  is an electronic switch incorporated into the internal circuitry of the safety apparatus and cannot be operated by the operator without switching the mode switch  31  on the control box  25  to the mute forced position to disable or mute the laser control and processing means  54 . Once the setup procedure has been completed, the setup link  100  is deactivated, and the mode switch  31  is returned to the normal position. 
     With respect to setting the mute point, the test/lock switch  33  is used in association with the test lamp  37 . Accordingly, the setting means  60  is programmed so that when the press is first turned on, the test lamp  37  flashes to indicate that the mute point must be set before the press can be operated. Further still, the setting means causes the test lamp  37  to flash whenever the mute point requires setting or resetting during the normal operation of the press brake. This will occur upon first turning on the press brake and safety apparatus for operating during a work session or resetting of the press by operating the power on/reset switch  98 , or in response to the blade position processing means  58  triggering the setting means  60  whenever a discrepancy in the location of the mute point is detected between what is specified by the limit switches  30  and what is cross checked by the blade tracking means, as represented logically by the control line  106  in FIG.  7 . 
     In order to set the mute point, the setting means  60  is programmed to allow the mute point to be manually set via the mute switch  63  on the press brake whilst the test lamp  37  is flashing. As previously mentioned, this point is typically set 6 to 7 mm from the outer surface of the material  42  being pressed. After the mute point is set by the mute switch  63 , the test/lock switch  63  is required to be pressed in response to which the setting means will cause the test lamp  37  to change from its flashing condition to a continuously on condition. 
     Next, the blade advance switch  61  is required to be closed by the operator depressing the foot pedal  34  so as to advance the blade  18 . The setting means  60  is programmed to automatically stop the blade at the mute point as set by the mute switch  63 . 
     At this point, the test/lock switch  33  is required to be pressed again whereupon the test lamp  37  will be extinguished. This second pressing of the test/lock switch  33  provides a signal to the setting means  60 , causing it to confirm the position of the new point with the blade position processing means  58  as determined by the blade tracking means  59 , whereupon the press is ready to use. 
     The setting means  60  is also programmed to signify by means of different flashing sequences of the test lamp  37  and the laser sensor lights  46  to  50  as to different faults detected by the system and the cause of lock out of the press brake, if this arises. Faults are ranked as either major or minor, and as a consequence of the controller  51  comprising master and slave processors, are divided up into faults detected by one or the other processor. In the case of faults detected by the master processor, the setting means causes these to be flashed on the test lamp  37 . In the case of faults detected by the slave processor, the setting means causes these to be flashed on the laser sensor lights  46  to  50 . 
     With respect to the master processor, the minor faults are classified as follows: 
     1 flash: Power up detected 
     2 flashes: Mute forced mode change 
     3 flashes: Low power 
     4 flashes: Mute switch moved 
     5 flashes: Lasers detected ambient light 
     The major faults are classified as follows: 
     7 flashes: Stopping performance 
     9 flashes: Relay disparity 
     10 flashes: Opto-encoder too fast 
     11 flashes: Opto-encoder has travelled too far in one direction 
     12 flashes: The mute LED has failed 
     13 flashes: The setup link mode has changed during operation 
     14 flashes: The master processor has tried to execute an invalid mode 
     With respect to the slave processor, all faults are major and are classified as follows: 
     1 flash: Sensors not checked by master 
     2 flashes: PLD fail 
     3 flashes: Master doesn&#39;t stop at mute when laser beams have been broken in previous 500 ms 
     4 flashes: Relay disparity 
     5 flashes: Laser status check code from master processor incorrect 
     6 flashes: Serial data from master failed 
     In each of these instances, the setting means  60  is programmed to require the fault to be corrected before enabling the press to be operated again. For minor faults the operator is required to press the test/lock switch  33  to signify to the setting means that the fault has been cleared, whereupon the setting means proceeds with executing the appropriate part of the setup procedure to reset the parameters of the controller concerned with that fault. Subsequently, the sequence of flashes of the test lamp will be extinguished and the halting means  57  will allow the press brake to continue to operate. If the faulty condition prevails, the halting means will be triggered again, locking out the press once more. Hence it is necessary for the fault to be properly rectified in order for the press to become fully operational once more. 
     With respect to major faults, the operator is required to hold down the test/lock switch  33  for a period of 5 seconds , whereupon the setting means executes the entire setup procedure in order to rectify the fault. If this does not clear the fault then the setting means will cause the appropriate lights to flash once more and more serious maintenance is required to be undertaken before the press can be made operational again. 
     Importantly, the controller  51  is arranged so that it constitutes the entire interface between the blade advance and blade retract switches  61  and  62  directing operation of the press brake by the operator and the motor relays  66  actually effecting operation of the press brake normally in direct response thereto, thereby enabling absolute fail safe operation of the press to be achieved. 
     Now the method of operating the press will be described with regard to FIGS. 9 to  18 . 
     Firstly having regard to the operation of the apparatus in normal use, reference is made to FIGS. 9 and 10. In normal use, the state of the controller  51  initially has the mode switch  31  and the blade speed switch  32  respectively disposed in the normal positions, the blade  18  initially disposed in the maximal elevated position  111 , the blade advancing relays  67  and  68  open, disabling the blade advancing control lines and  70 , and the field muted lamp  35  turned off, as represented by state block  112 . 
     To change the state of the controller  51  and advance the blade, the operator is required to depress the foot pedal  34  closing the blade advance switch  61 , then release the foot pedal to open the switch  61  for a period of at least 300 milliseconds, and then depress the foot pedal  34  once again as represented at point  113  to close the switch  61  once more. The blade position control means  56  will then close the advancing blade relays, asserting blade advancing control lines  69  and  70 , as represented by state block  114 , advancing the blade at full speed, whilst the foot pedal  34  remains depressed and the switch  61  is closed. 
     If there is no obstruction, and the secondary relay  71  is closed asserting the press locking control line  72 , the blade  18  will continue to advance straight through the mute point  115  without stopping, as represented by state line  116 . Subsequently, the pressing operation will be completed on the bed  38 , as represented at  117 . 
     At the mute point, however, there is a change of state of the controller  51 , whereby both blade advancing control lines  69  and  70  will be forcedly asserted, whilst the blade crawl control line  74  will be forcedly disabled together with the retract control line  76  and the field muted lamp will be turned on to indicate that the laser sensing system is muted at that point, as represented by state block  118 . Thus after the blade reaches the mute point  115  and proceeds through to the bed, the response of the laser control and processing means  54  to the receipt of the light beams by the laser receiving circuits  53  is disabled so that the light beams do not react with the material  42  as it is pressed. 
     Activation of the lower limit switch  65  and corresponding pressure sensor of the press, automatically cause the blade to fully retract under the normal operation of the press, after completing the pressing operation, as represented by state line  120 , returning to state  112  where the field muted lamp  35  is turned off again. In addition, the blade can be retracted whilst it is above the mute point at any time from the state  114  by depressing the foot pedal  34  to close the blade retract switch  62  and assert the blade retract control line  76  as represented by state line  122 . 
     Thus, in normal use, the safety apparatus does not impede the pressing operation in any way. Further, as the beams can be positioned to within 20 mm of the blade, the operator can work very close to the press and blade whilst full protection is maintained. 
     After the blade is advanced from an elevated position as represented at  119 , and an object interrupts any one of the beams at  121  before the blade reaches the mute point  115  whilst in state  114 , the halting means  57  will be triggered to cause the blade to retract by disabling the blade advancing control lines  69  and  70  and assert the blade retract line  76 , as represented by the state block  124 . Thus the blade  18  is instantly reversed away from the obstruction. Average response times of 15 to 35 mm per second with maximum blade travel of 0.77 mm during the process of the blade coming to a stop and going into a reverse direction are designed into the system. Under these conditions, it is not possible for the blade to touch the object interrupting the light beam. 
     The halting means  57  maintains this state  124  for a prescribed time to enable the blade to reverse its travel for a for a distance of 10 mm, for example, before disabling the blade retract control line  76 , pausing movement of the blade at  123 . 
     The blade advance switch then has to be disabled for a period of at least 300 milliseconds by releasing the foot pedal and reasserted by depressing the foot pedal again at  125 , before the halting means will allow the blade to be advanced again. The blade advance relays  67  and  68  will then be closed and the blade advancing control lines  69  and  70  will be asserted, whilst the blade retract control line is disabled, as represented by state block  126 . 
     If there is no further obstruction, this state  126  will be maintained until the blade reaches the mute point  115 , as represented by state line  128 . If there is another obstruction, such as at  127 , which may be at the same point  121  as the previous obstruction, the halting means causes a change of state of the controller  51  as represented by state line  130 , whereby the blade advance relays  67  and  68  will be opened, disabling blade advancing control lines  69  and  70 , and instead of asserting the blade retract control line  76 , maintaining it in its disabled state, causing a halting of the movement of the blade at  129 , as represented by state block  130 . 
     As in the case of state  114 , the blade can be retracted from its advancing state  126  at any time before it reaches the mute point  115  by asserting the blade retract control line  76 , as represented by state line  134 . Similarly, from state  132 , the blade can be retracted to return to its state  112 , by asserting the blade retract control line as represented by state line  136 . 
     In order to advance the blade from its halted state  132 , the blade advance switch needs to be disabled for at least 300 ms by releasing the foot pedal and reasserting the same by depressing the foot pedal, as shown at  131 . Thereafter the state of the controller is changed as represented by state line  138 , by asserting the blade advancing control lines  69  and  70  and the blade crawl control line  74 , as represented by state block  140 . 
     Thus the blade is only permitted to advance beyond the point  127  in a crawling or pulsing manner, as shown at  133 . In this condition, the blade travels at a reduced speed, for example 10 mm or less per second, compared with the normal speed for advancement of the blade which is typically 20 to 50 mm per second. This reduced speed is achieved by the halting means selectively asserting and disabling the blade advance control lines, thereby repeatedly starting and stopping movement of the blade, to reduce its average speed to an acceptable level, which is less than 10 mm per second in the present embodiment, as determined by the blade position processing means  58 . 
     This crawling state of advancement  140  is maintained, as represented by state line  142  until the blade reaches the mute point  115 . During this state  140 , however, the blade can be retracted to return to its fully elevated state  112 , by asserting the blade retract control line  76 , as represented by state line  144 . 
     Upon reaching the mute point  115 , either via state line  128  or  142 , the halting means effects another change in state of the controller by opening the blade advance relays  67  and  68  to disable the blade advancing control lines  69  and  70 , thereby halting movement of the blade as shown at  135  and turning the field muted lamp  35  on, as represented by state block  146 . At this point, movement of the blade is paused as shown at  137 , whereupon it can be either fully retracted by asserting the blade retract control line  76 , returning to state  112 , as represented by state line  148 , or be advanced again at  139  to complete the pressing operation at  141  by proceeding to state  118 , as represented by state line  150 . 
     With the latter, the halting means maintains the blade at the mute point  115  and does not permit the blade to be advanced until the blade advance switch  61  has been disabled once more for at least 300 ms by releasing the foot pedal and reasserted again by depressing the foot pedal, as shown at  139 . 
     As previously described, the controller is programmed to operate the press in this manner in order to accommodate material of unusual profiles such as those formed of wavy material which have a profile projecting past the outer surface of the material reposed upon the bed and beyond the mute point distance. Hence, if the safety apparatus did not have such a pulse mode facility, the safety apparatus would halt the blade and prevent completion of the pressing operation. By the same token, it should be appreciated that safety is not greatly compromised by causing the press to adopt the crawling or pulsing mode of advancement, whereby ample time is available to the operator to withdraw a hand or finger in the event that this may have caused the obstruction, or take their foot off the foot pedal before the pressing operation is completed. 
     Having described the normal operation of the apparatus, the various states and procedure involved with setting up and testing the apparatus following a power on or reset will now be described with reference to FIGS. 11 and 12. 
     In response to a power on or reset signal provided by the power on/reset switch  98 , the controller  51  adopts an initial state where the blade  18  is initially disposed at its maximally elevated position  151 , the blade movement control means  56  causes disabling of the blade advancing control lines  69  and  70 , the setting means  60  causes the test lamp  37  to be flashed, and the halting means  57  causes the secondary relay  71  to be closed, asserting the press locking control line  72 , as represented by state block  152 . 
     The operator is then required to press the test/lock switch  33 , with the blade speed switch  32  disposed in the normal position and not in the crawl position to change the state of the controller  51 . The setting means  60  then allows the blade movement control means  56  to assert the blade advancing control lines  69  and  70  and disable the blade crawl control line  74  in response to the operator depress the foot pedal and operate the blade advance switch  61  as shown at  153 , this change being represented by state block  154 . Accordingly, the blade advances towards the bed  38  as shown at  155 . 
     After the blade has travelled 10 mm, the state of the controller changes again at  157  by the halting means  57  opening the secondary relay  71  and disabling the press lock control line  72  to lock further movement of the blade, as represented by state block  156 . If the blade does not stop quickly enough as determined by the blade position processing means  58  and the blade tracking means  59 , the controller is programmed to cause a stopping performance error to be generated. 
     The setting means  60  is programmed to pause the blade at the position  159  in the locked state  156  for a period of at least 3 seconds during which the blade advance switch  61  is disabled by the operator releasing the foot pedal. 
     The locked state  156  is changed at  161 , only after the operator restarts the press, causing the halting means to reassert the press lock control line  72 , and closing the blade advance switch  61  again by depressing the foot pedal. As shown in FIG. 10, the setting means causes the blade crawl control line  74  to be asserted, advancing the blade in a crawling advancement mode, as represented by state block  158 . It should be noted that the blade advancing control lines  69  and  70  are maintained in an asserted state during this locked state to ensure fail safe operation of the secondary relay  71 . 
     With this change of state at  161 , the blade advances as shown at  163  and is required to be deliberately obstructed by the operator before reaching the mute point  115 . This obstruction is shown to occur at  165 , whereupon the halting means would normally be triggered by the laser control and processing means  54  to cause a change in state, whereby the blade advancing control lines  69  and  70  are disabled, halting further advancing movement of the blade, as represented by state block  160 . 
     This state  160  is a standard lock state for the apparatus that is entered into and maintained by the controller, as shown at  167 , every time that the mode switch  31  is changed to the mute forced mode position or that a minor fault occurs, such as detection of power up, low power, moving of the mute switch  63 , or detection of ambient light by the laser sensors. In order to change the state, the test lamp switch  33  is required to be pressed and the blade advance switch  61  opened and then closed by operating the foot pedal  34 , as shown at  169 . 
     If the standard lock state is entered above the mute point  115 , the controller reverts to the previous advance state  158 , advancing the blade towards the bed crawling mode again, as shown at  171 . 
     Once the mute point  115  is reached, signified by closure of the mute switch  63  as shown at  173 , the controller enters the standard lock state  160  again. It remains in this state as shown at  175  until the test/lock switch  33  is pressed again and the blade advance switch is opened and then closed by operating the foot pedal, as shown at  177 . 
     Whilst at the mute point  115 , the blade position processing means  158  records the mute point position as indicated by the blade tracking means  59 , which is then checked every time the mute switch  63  is activated for any discrepancy therewith to ensure that the mute switch  63  has not moved. 
     Thereafter the controller adopts the state as represented by state block  162  and disables the blade crawl control line  74  to advance the blade at normal speed once more to complete pressing against the bed as shown at  179 . Upon completing pressing, the blade remains in contact with the bed until the blade retract control line  76  is asserted by the operator by appropriately depressing the foot pedal, whereupon the blade retracts normally back to the maximal elevated position  151 . The blade advancing control lines  69  and  70  are disabled when the blade travels above the mute point and are asserted again when the blade advance switch  61  is closed. 
     When any major fault occurs the controller enters a state whereby the press is locked by disabling the press lock control line  72 , together with all blade advancing, crawl and retracting control lines  69 ,  70 ,  74  and  76 . In order to change the controller from this state, the test/lock switch  33  is required to be closed for a period of 5 seconds by the operator pressing it. This will simply result in the setting means stopping strobing of the watchdog circuit  99 , which will automatically cause both processors to be reset, invoking the full setup procedure. 
     The operation of the safety apparatus in the mute forced mode will now be described with reference to FIGS. 13 and 14. 
     In this mode, the mode switch  31  is disposed in the mute forced position and the test/lock switch  33  is momentarily closed by pressing it to unlock the press. Initially, the controller  51  adopts a state represented by state block  180  where the blade advancing control lines  69  and  70  are disabled as shown at  181 . 
     In order to change from this state and move the blade  18  towards the bed  38 , the blade advance switch  61  needs to be opened for at least 300 ms and then be closed by operating the foot pedal  34 , as shown at  183 , and represented by state line  182 . The controller then adopts a state as represented by state block  184 , where the blade advancing control lines  69  and  70  are asserted, causing the blade to advance normally, as shown at  185  to a limit height  187 , which in the present embodiment is preset to within 15 mm of the mute point  115 . 
     At the limit height  187 , the controller enforces a crawl advancement of the blade to be adopted, as shown at  189 , by asserting the blade crawl control line  74 . The crawl advancement is then regulated by the halting means  57  and blade position processing means  58  to limit the average speed of advancement to below 10 mm/s by disabling and asserting the blade advancing control lines  69  and  70  periodically, as shown at  189   a . The blade advancing control lines are continuously asserted, as shown at  189   b , when the blade normally advances at a speed of less than 10 mm/s. 
     The controller  51  maintains this crawling advancement state until the blade reaches the mute point  115 , as shown at  191 . At the mute point, the controller reverts to its original state  180 , as represented by state line  186 , where the blade advancing control lines are disabled by the halting means  57 , halting further advancement of the blade, as shown at  193 . 
     In order to change from this state  180 , the blade advance switch  61  is required to be opened for at least 300 ms and closed again by the operator operating the foot pedal, as shown at  195  and represented by state line  188 . Consequently, the controller adopts a state represented by state block  190 , where the blade advancing control lines  69  and  70  are asserted and the blade crawl control line  74  is disabled and pressing of the material into the bed is completed, as shown at  197 . 
     Thereafter, the blade retracts, whereupon the blade advancing control lines  69  and  70  are automatically disabled after the blade retracts beyond the mute point  115 , as shown at  199  and represented by state block  192 . The controller can change its state to advance the blade once more towards the mute point by adopting the state  184 , upon the operator opening the blade advance switch  61  for at least 300 ms and then closing it again by operating the foot pedal, as represented by state line  194 , without having to return to its initial state  180 . 
     The second embodiment is substantially identical to the first embodiment, except that instead of relying upon relatively fixed limit switches disposed within the housing  12  of the press brake to determine the mute point, the safety apparatus uses a different technique of establishing the mute point and checking same during actual operation, to ensure safety of the operator and prevent bypassing of the protection system during normal operation of the press. 
     The physical components of the safety apparatus of the present embodiment are the same as those of the previous embodiment except that the mute switch and associated striker bar are omitted, and the control means includes a controller made up of the same microprocessor arrangement, but programmed slightly differently with respect to the functions performed by the laser control and processing means, the blade position processing means and the setting means, in order to provide for and accommodate the different determination and monitoring method for the mute point. Accordingly, the same reference numerals will be in describing the various components of the safety apparatus of the present embodiment as were used in the preceding embodiment, where appropriate. 
     The safety apparatus of the present embodiment finds particular and essential utility in those types of press brakes which have relatively complex electronic control systems for operating the hydraulic system of the press brake and which do not rely upon or cater for the provision of mechanical limit switches. 
     The laser control and processing means  54  of the present embodiment is substantially identical to that of the preceding embodiment, working in conjunction with the pulsing means  55  and laser driver and receiving circuits  52  and  53  respectively, in the same manner to determine faults and obstructions of the light beams as before. The laser control and processing means  54  differs in the manner of its operation, however, by continuing to process laser receiving circuit information in an unmuted manner, after the blade  18  reaches the predetermined mute point of the press. Moreover, the laser control and processing means  54 , with the aid of the blade position processing means  58 , checks that the central light beam  26   b  is interrupted within a prescribed distance of advancement of the blade beyond the mute point, and if not interrupted within this distance, maintains unmuted sensing of the light beams, triggering the halting means  57 , immediately upon sensing an obstruction to any one of the light beams. In the event that the central light beam is interrupted within the prescribed distance, further sensing of the light beams is muted, allowing the blade to continue advancing and presumably complete its pressing operation. 
     The blade position processing means  58  again is substantially identical to that of the preceding embodiment, working in conjunction with the blade tracking means  59  to monitor the movement of the blade and provide relevant position and speed information to the other processing elements of the controller  51 . However, the blade position processing means does not provide for cross checking of the mute point with a mute switch and instead simply relies upon the initial setting of this by the setting means  60  with respect to the positional data obtained from the blade tracking means  59 . 
     Consequently, the blade position processing means  58  interacts with the halting means  57  in a slightly different manner than in the case of the preceding embodiment, whereby mute point information is used to determine unmuted sensing of the light beams, after the mute point has been reached by the leading edge  36  of the blade, and muting of further sensing of the light beams is contingent upon the central light beam being interrupted within the prescribed distance, as previously described. 
     The setting means  60  is different from that of the preceding embodiment insofar as the establishment of the mute point is concerned. In all other respects, the setting means is substantially the same as that of the preceding embodiment. Hence, the setting means  60  derives upper and lower limits of the blade from either limit switches or the controller system of the press brake itself, as in the case of the preceding embodiment, and includes appropriate software to establish upper and lower limits depending upon the actual type of press in relation to which the safety apparatus is used. 
     In the present embodiment, the mute point is established as part of the normal operation of the press and does not involve a separate setting up procedure as was the case in the previous embodiment. Accordingly the setting means operates continuously to indicate when a particular point attained by the leading edge of the blade is to be recognised and recorded as the mute point. This is simply achieved in the normal operation of the press by advancing the blade until receipt of the central light beam  26   b  is interrupted. In this situation, as is normally the case, the blade will bounce back a prescribed distance as a result of overriding operation of the halting means  57 . The blade then be advanced again and if the central beam  26   b  is broken at the same point, within a prescribed tolerance, the halting means will halt the blade at that point and enable the setting means  60  to operate. The setting means causes the field muted lamp  37  to flash, signifying to the operator that the mute point can be set at this point by pressing the test/lock switch  33 . If the test/lock switch  33  is closed at this time, then the blade position processing means  58  will record that position of the blade as the mute point. Thereafter the blade position processing means  58  will assign a prescribed tolerance and zone to the mute point, within which interruption of the central light beam is expected to be consistent with the recorded mute point being the actual mute point for the pressing operation and not a falsely recorded mute point. 
     Operation of the press with respect to its various states and establishment of the mute point in accordance with the present embodiment will now be described with reference to FIGS. 15 to  18 . 
     Firstly having regard to the normal operation of the press, in which the mode switch  31  and the blade speed switch  32  are both disposed in the normal position, the controller  51  adopts an initial state, represented by state block  200 , at which the blade  18  is disposed at its maximally elevated position  201  above the bed  38 . In this state, the blade advancing control lines  69  and  70  are disabled and the field muted lamp  37  is off. 
     To change from this state  200 , the blade advance switch needs to be opened for at least 300 ms and then closed by the operator operating the foot pedal  34 , as represented by state line  202  and as shown at  203 . Consequently the blade advancing control lines  69  and  70  are asserted and the blade advances towards the bed  38 , as represented by state block  204  and as shown at  205 . From this state  204 , the blade can be fully retracted to return to its initial state  200  at its maximally elevated position  201 , by the blade movement control means  56  asserting the blade retracting control line  76  and disabling the blade advancing control lines  69  and  70 , in response to closure of the blade retract switch  62  by the operator, at any time before reaching the mute point, as represented by state line  206 . 
     The blade will continue to advance towards the bed in the advancing state  204  until the central light beam  26   b  becomes obstructed, as shown at  207  and represented by state line  208 . It should be noted that the mute point has not yet been set at this stage and so there is no permitted zone within which the interruption is required to be sensed. 
     At this point the controller will adopt a state, represented by state block  210 , where the halting means  57  exercises overriding control of the blade movement control means  56  by disabling the blade advancing control lines  69  and  70  and asserting the blade retracting control line  76  for a prescribed period to retract the blade a prescribed distance of 4.5 mm in the present embodiment, or until the blade advance switch  61  has been opened by releasing the foot pedal for a period of at least 300 ms, whichever occurs first. This changing of state is represented by state line  212 , whereupon the blade will be halted by the halting means  57 , as shown at  209 , until the blade advance switch  61  has been opened for the requisite 300 ms period and closed again, as shown at  211 , causing the blade advancing control lines  69  and  70  to be asserted once more, advancing the blade towards the bed, as represented by state block  214 . 
     The blade will remain in this state  212  until the light beams are interrupted or obstructed once more, as shown at  213 , and represented by state line  216 . During this advancing movement, the blade can be retracted to its initial position and the controller adopt its initial state  200 , by the operator closing the blade retract switch  62  as previously described, as represented by state line  218 . 
     At this stage the mute point still has not been set and there is no permitted zone within which an obstruction is required to be sensed. Consequently, the controller adopts the state represented by state block  220 , where the halting means halts further advancement of the blade by disabling the blade advancing control lines  69  and  70 , pausing the blade at this position as shown at  215 . As in the previous state block  214 , the blade can be fully retracted to the initial position  201  and state  200 , in the manner previously described, and as represented by state line  222 . 
     In order to change the state of the controller at this point, the blade advance switch  61  must be opened for a period of at least 300 ms and closed again by the operator operating the foot pedal, as shown at  217  and represented by state line  224 . 
     The controller then changes to a state represented by the state block  226  from which the mute point can be set. Thus the controller causes the blade advancing relays  69  and  70  to be asserted, and forces the blade to adopt a crawling movement by asserting the blade crawl control line  74 . The setting means  60  simultaneously causes the field muted lamp  37  to flash, whilst the laser control and processing means continues to assert that the central light beam  26   b  remains obstructed. This signifies to the operator whether the point  215 , at which the blade was previously halted, is to become the mute point for the pressing operation, thereafter. 
     To accept the point  215  as the mute point, the operator is required to press the test/lock switch  33 , momentarily closing it, whereupon the blade position processing means records the particular position of the blade at which it was halted as the mute point, as represented by state block  228 . 
     If the point  215  is not accepted as the mute point, the controller will simply maintain its state  226 , where the blade will simply continue to advance in crawl mode until retracted by the operator, in the manner previously described, or by the automatic operation of the press, on completing its pressing operation. This change in state involving retraction of the blade is represented by state line  230 . It should be noted that during this crawling advancement of the blade, the field muted lamp  37  will be continuously flashing, signifying that the blade is advancing in a muted mode. 
     If the mute point is set at state  228 , the controller changes state once more as represented by state block  232 . Consequently, the blade advancing control lines  69  and  70  will continue to be asserted, the blade crawl control line  74  will be disabled and the field muted lamp  37  will be turned on to be illuminated continuously. Thus the blade will be allowed to advance at its normal speed to complete its pressing action, as shown at  219 . Thereafter the blade will be retracted to its initial position  201 , as shown at  221 , and the controller will return to its initial state  200 , as represented by state line  234 . 
     With the mute point set, the permitted zone within which the central light beam is required to be interrupted or obstructed is defined a prescribed distance above and below the actual point. In FIG. 15, this is represented by the shaded area  223 . Three different scenarios are actually depicted in FIG. 15, the first showing the mute point set too high with a permitted zone shown as  223   a , the second showing the mute point set too low with a permitted zone  223   b , and the third showing the mute point set correctly with a permitted zone  223   c . The manner in which the safety apparatus of the present embodiment accommodates these incorrect settings of the mute point and still maintains full protection for the operator will become evident in describing how the apparatus operates normally with the mute point set. 
     Thus, on the return of the blade  18  to the initial position  201  and the controller  51  being in state  200 , the operator operates the press as before to proceed to state  204 , as shown at  225 . The blade then advances towards the permitted zone  223 , as shown at  227 , in each of the instances shown in FIG.  15 . 
     If the laser control and processing means  54  senses an obstruction before reaching the mute point set within the permitted zone  223 , and outside of the zone (not shown), the controller reverts to state  210  and causes the blade to bounce back a prescribed distance, in the manner previously described. The operator then operates the press as before to proceed to state  214 , whereupon the blade will advance normally once more. 
     If there is no further obstruction, the blade will continue to advance until it reaches the permitted zone  223 . 
     In the case of a correctly set mute point, such as would occur within the permitted zone  223   c , the central light beam  26   b  will become obstructed within the permitted zone  223   c  and cause the controller to change state, as represented by state line  236 , and adopt the state represented by state block  238 . 
     In this state  238 , the halting means  57  will cause the blade advancing control lines  69  and  70  to be disabled, halting advancement of the blade, and the setting means  60  will cause the field muted lamp  37  to be turned on. This is a mandatory state for the controller to enter following sensing an obstruction of the light beams before the mute point. 
     To change this state, the blade can either be retracted in the usual manner by the operator closing the blade retract control line  76 , as represented by the state line  240 , or be advanced after the operator opens the blade advance switch  61  for a period of at least 300 ms and then close it using the foot pedal, as represented by state line  242 . Performing the latter results in the controller proceeding to state  232  to complete the pressing operation. 
     In the case of an incorrectly set mute point, such as the one which occurs in permitted zone  223   a , where the mute point is too high, the blade will advance through the permitted zone  223   a , without obstruction. Immediately the blade passes the permitted zone  223   a , the laser control and processing means  54  will remain active and will not be muted, maintaining full protection to the operator as represented by the further shaded region  229 . 
     If any of the light beams  26  were previously obstructed before the blade reached the permitted zone  223   a , the controller will be in state  214  as it passes through the permitted zone. If no light beams were previously obstructed, the controller will be in state  204  as it passes through the permitted zone  223   a . 
     Thus the next obstruction of the central light beam  36   b  in the case of the former will result in the controller proceeding through to state  220  where the blade will be halted, and then with appropriate operation of the press by the operator, the controller will proceed to state  226 , enabling a new mute point to be set by proceeding through states  228  and  232 , or requiring the blade to advance in crawl mode to complete the pressing operation before retracting, where the controller follows state line  230 . 
     In the case of the latter where the controller is in the state  204 , which is the actual scenario depicted in FIG. 15, the next obstruction of the central light beam  26   b , will be that shown at  231 , which will result in the controller proceeding to state  210  causing the blade to bounce back from the obstruction and then to proceed to state  214  to pause the blade at position  233 . Upon appropriate operation of the press by the operator, the controller will proceed through to state  220 , where the advancement of the blade will be halted again at point  235  by the controller adopting state  226 , in response to sensing the obstruction again. The operator is then provided with an opportunity to reset a new mute point at this position by progressing the controller through to state  228  and subsequently  232  to complete the pressing operation as shown at  237  and retracting the blade at  221 , or to complete the pressing operation in crawl mode before retracting where the controller follows the state line  230 . 
     In the case of an incorrectly set mute point, such as the one which occurs in permitted zone  223   b , where the mute point is set too low, the controller will sense an obstruction before actually reaching the permitted zone. In this case the controller will proceed from state  204  to state  210 , bouncing back as shown at  239  to ultimately adopt state  214  as shown at  241 . Upon appropriate operation of the press by the operator, the controller will proceed through to state  220 , where the advancement of the blade will be halted again at point  243  by the controller adopting state  226 , in response to sensing the obstruction again. The operator is then provided with an opportunity to reset a new mute point at this position by progressing the controller through to state  228  and subsequently  232  to complete the pressing operation as shown at  245  and retracting the blade at  221 , or to complete the pressing operation in crawl mode before retracting where the controller follows the state line  230 . 
     In the case of a correctly set mute point such as shown at  223   c , with the controller advancing the blade in state  204 , as shown at  227 , without any previous obstruction encountered, the blade will reach the permitted zone  223   c , expecting the central light beam  26   b  to be interrupted or obstructed. This will occur if the mute point is correctly set with required tolerance for the permitted zone, resulting in the controller proceeding directly to state  232  without the blade being halted by the halting means. Consequently, the field muted lamp  37  will be turned on at  247  to indicate that the laser sensing facility is muted at that point, and the blade will be allowed to continue advancing to complete the pressing operation as shown at  249 , retracting as shown at  221 , with the controller following state line  234  to return to its initial state  200 . 
     Obviously the most expedient pressing procedure to adopt is the latter, where the mute point is correctly set and there are no obstructions, whilst full protection is provided to the operator by the safety apparatus. This encourages the operator to use the safety apparatus in the intended manner, whilst deterring misuse or abuse of the apparatus. 
     The setup operation for the controller of the present embodiment is substantially identical to that of the preceding embodiment with the exception that there is no setting of the mute point as previously described. Accordingly, the states followed by the controller are the same as described in the preceding embodiment with respect to FIG. 12, with the exception that the setup procedure is completed with state  158 . 
     The mute forced procedure where the mode switch  31  is in the mute forced position, is substantially identical to that of the preceding embodiment, with the additional facility of flashing the field muted lamp  37  at different stages to provide an additional indication of the status of the controller, and the exception that a new mute point is allowed to be set at different states following sensing of an obstruction of the central light beam  26   b . Accordingly, FIG. 17 shows the same states followed by the controller as described in the preceding embodiment with respect to FIG. 14, with the following exceptions: 
     state block  244  is the same as state block  180 , with the addition that the field muted lamp  37  is also turned on by the setting means 
     state line  246  is the same as state line  182   
     state block  248  is the same as state block  184 , with the addition that the field muted lamp  37  is also flashed by the setting means when the blade advance switch  61  is opened, otherwise it is on 
     state line  250  is the same as state line  186  and state line  252  is the same as state line  188   
     state block  254  is the same as state block  190 , with the addition that the field muted lamp  37  is also flashed when the blade advance switch  61  is opened, otherwise it is on 
     state block  256  is the same as state block  192  and state line  258  is the same as state line  194   
     from states  248  and  254 , the controller permits a new mute point to be set by pressing the test/lock switch  33  whereupon the blade position processing means  58  records the position that the blade was in at the time that the central light beam  26   b  was obstructed as measured by the blade tracking means  59  as the new mute point, as represented by state block  258   
     the controller reverts to state  244  following setting of the new mute point. 
     It should be appreciated that the scope of the present invention is not limited to the particular embodiments hereinbefore described. In particular, the safety apparatus and method is not limited to use in conjunction with a down-stroking press brake, but may find equal utility with up-stroking press brakes as previously mentioned, and with other sorts of mechanical machines such as lathes, drills, other types of presses, milling machines and the like, where an operator may need to work in close proximity to a moving tool which could cause injury if a limb or body part were to enter into the path of movement of the tool member. Accordingly, appropriate modifications and changes to the safety apparatus and method that would be obvious to a skilled person in the field of manufacturing and installing such apparatus to suit different machines are envisaged to fall within the scope of the present invention. 
     In addition, the invention is not limited to light beams that traverse a pencil line. As is common knowledge with the shaping of collimated light such as lasers, various cross-sectional shaped beams of light may be generated and projected, such as planar beams, arcuate beams, cylindrical beams etc, that may be chosen to provide enhanced enveloping of the working end of the working member. 
     Further still, the invention is not limited to increasing luminance of the light beams by increasing the frequency of the pulsing alone. As is common knowledge in the art, increased luminance can be provided by increasing the duty cycle of the pulsing of the light beams.