Abstract:
A safety device for an automatically operating installation determines at least one movement variable of a moving installation part in a safety-related manner. The safety device includes an acceleration sensor and an evaluation unit. The acceleration sensor is adapted to be coupled to the moving installation part in order to detect any acceleration of the moving installation part. The evaluation unit determines a movement velocity and/or a movement travel of the installation part on the basis of the acceleration.

Description:
CROSS REFERENCES TO RELATED APPLICATIONS  
       [0001]     This application is a continuation of international patent application PCT/EP2005/011768, filed on Nov. 3, 2005 designating the U.S., which international patent application has been published in German language as WO 2006/056300 and claims priority from German patent application DE 10 2004 058 471.0, filed on Nov. 24, 2004. The entire contents of these priority applications are incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     The present invention relates to the safety-relevant or fail-safe determination of a movement variable on an automatically operating installation (or machine), such as a manufacturing installation and/or conveyor installation. More particularly, the invention relates to the fail-safe determination of a movement velocity and/or the movement travel of the beam of a press brake or a similar installation having a tool which moves like a stamp. The invention also relates to a safety device and to a method for fail-safely stopping of a moving part of such an installation in response to the movement variable.  
         [0003]     DE 100 27 156 A1 discloses a press brake or a similar machine, in which a first machine part carries out a stamp-like working movement with respect to a second machine part. This working movement can be used to shape a workpiece under pressure, or else for cutting or stamping. As will readily be appreciated, such a stamp-like working movement is highly dangerous to the operator who, for example, has to insert and align the workpiece under the moving part. In fact, accidents occur again and again, in which an operator is subjected to severe crushing, even with body parts being cut off, as a result of carelessness and/or incorrect actions. In order to prevent such accidents, the installation from DE 100 27 156 A1 has a non-contact protective device. The protective device comprises a light barrier arrangement, whose light beams run parallel to the front edge of the moving tool, with the light barrier arrangement being moved with the working movement of the tool. If one or more of the light beams of the light barrier arrangement is interrupted, the working movement is typically stopped immediately. However, the light barrier arrangement has to be deactivated (what is called muting) shortly before the press closes, because the interruption of the light beams by the workpiece would otherwise prevent the press from closing.  
         [0004]     In order to allow older presses to be retrofitted with such a light barrier arrangement, DE 100 27 156 A1 proposes that the light barrier arrangement be deactivated (muted) when the movement velocity of the stamp-like tool is below a predetermined velocity. This is because, in the case of a press, the stamp-like tool is normally driven from its initial rest position (what is called upper dead point) at high speed towards the workpiece. However, the workpiece is shaped at a low velocity (creeping speed). According to DE 100 27 156 A1, deactivation of the light barrier arrangement solely as a function of the movement velocity of the tool has the advantage that even old presses can be easily retrofitted with the described light barrier arrangement. On the other hand, the proposed procedure is dependent on the capability to fail-safely determine the respective velocity of the tool, i.e. the movement velocity of the tool must be determined in a safety-related manner such that a dangerous state for the operator is prevented to occur even in the event of a fault or functional failure in the safety device.  
         [0005]     In the case of the press from DE 100 27 156 A1, the velocity of the tool is therefore determined by two measurement modules, with one measurement module being in the form of an incremental shaft encoder, in which a cable which is fixed to the tool is wound up and unwound by the working movement. The rotary movement which is produced by the winding up and unwinding is detected by an incremental shaft encoder or a rotation sensor. The second measurement module has an inductive sensor, through which a magnetic measuring tape is moved, with the magnetic measuring tape likewise following the movement of the tool. Alternatively, measurement devices are disclosed which include a toothed rod, a linear potentiometer, a translucent scale with a light barrier, or an inductive sensor with a perforated strip of sheet metal.  
         [0006]     These sensors all have in common that their physical dimensions depend on the size of the installation and the movement travel of the moving installation part. If, for example, the intention is to monitor a movement travel of 1000 mm, a correspondingly long cable, measuring tape or scale is required. The proposed measurement means are therefore physically large, and they involve complex installation with correspondingly high costs. This applies even more to the safety-relevant determination of movement variables of a moving installation part, since redundant measurement means generally have to be used, for safety reasons.  
         [0007]     A similar installation, again in the form of a press, is known from WO 97/25568. An optical encoder disk is used here in order to monitor the working movement of the tool. The encoder disk is rotated by a chain, a cable or the like, which is attached to one end to the moving tool. This has the same disadvantages in terms of material costs, dimensions and installation effort as the installation from DE 100 27 156 A1.  
       SUMMARY OF THE INVENTION  
       [0008]     In view of the above, it is an object of the invention to provide a cost-effective and physically small alternative for fail-safely determining a movement variable, such as a movement velocity and/or movement travel, of a moving part of a generally stationary installation.  
         [0009]     According to one aspect, there is provided an automatically operating installation comprising at least one installation part which is moved in an automated manner, comprising a control unit designed to control the movements of the installation part, and comprising a safety device which is designed to determine at least one movement variable of the installation part in a safety-related manner, said safety device comprising at least one acceleration sensor and an evaluation unit, wherein the acceleration sensor is adapted to be coupled to the moving installation part for detecting any acceleration of the moving installation part, and wherein the evaluation unit is designed to determine at least one of a movement velocity and a movement travel of the installation part on the basis of the acceleration detected by the acceleration sensor.  
         [0010]     According to another aspect, there is provided a safety device for fail-safely determining at least one movement variable of a moving part of an automatically operating installation, the safety device comprising at least one acceleration sensor and an evaluation unit, wherein the acceleration sensor is adapted to be coupled to the moving installation part for fail-safely detecting an acceleration of the moving installation part, and wherein the evaluation unit is designed to fail-safely determine at least one of a movement velocity and a movement travel of the installation part on the basis of the acceleration.  
         [0011]     According to yet another object, there is provided a method for safety-related stopping of a moving part of an automatically operating installation, comprising the steps of: fail-safely determining a movement variable of the moving part, fail-safely comparing the determined movement variable with a predefined reference value, and fail-safely stopping the moving part as a function of the determined movement variable and the defined reference value, wherein, in order to determine the movement variable, an acceleration of the moving part is first detected by means of an acceleration sensor, and wherein at least one of a movement velocity and a movement travel of the moving part is determined as the movement variable on the basis of the detected acceleration.  
         [0012]     The new apparatus and method depart from the principle which has always been used until now for the safety-relevant determination of movement and/or velocity on stationary installations, namely the principle of using measurement means whose measuring range and physical dimensions depend on the size of the installation, and the magnitude of the measurement variable to be detected. This is because an acceleration sensor allows measurement detection of any acceleration irrespective of the size of the accelerated installation part, and irrespective of the distance which is traveled during the acceleration. The physical dimensions of an acceleration sensor are independent of the installation size and are independent of the distance which is traveled by the moving installation part.  
         [0013]     Furthermore, acceleration sensors, such as capacitively or piezo-resistively operating acceleration sensors, are commercially available as integrated components, whose dimensions are only a few millimetres or centimetres. Suitable acceleration sensors are used, for example, in motor vehicles in order to trigger an airbag on the basis of the detected accelerations which occur, for example, in the event of an accident. In such situations, the detected acceleration itself is used as the measurement variable while, in contrast, the present invention determines a movement velocity and/or movement travels on the basis of the detected acceleration.  
         [0014]     The physical relationship between acceleration, velocity and distance traveled has been known for a long time. For example, the velocity of a moving installation part can be calculated from the acceleration detected by measurement, by integrating the detected acceleration values over time. The distance traveled can equally be determined by integration over velocity. In order to obtain the actual values of the velocity or travel, however, the velocity and the position of the moving installation part at the start of the integration time must be known. However, this is not a problem in the case of stationary automatically operating installation since, in general, the installation is in a defined rest state at least after it has been switched on, or has started an initialization process. In the case of a press, a defined rest point (velocity zero, defined position) always occurs at the start of a working cycle, since every working cycle starts at what is called upper dead point of the press tool.  
         [0015]     If the acceleration of the tool (or of an installation part which is connected to it) is now detected continuously, or at least at regular time intervals, from the start of the working movement, the actual velocity and the actual position can be determined mathematically.  
         [0016]     The value of the velocity and of the distance traveled depend not only on the known start point, but also on the accuracy with which the acceleration sensor detects the acceleration of the moving installation part, and on the integration accuracy of the evaluation unit. However, there is no need for an exactly measured value for safety-relevant determination of velocity, travel or position. In fact, a threshold value analysis is sufficient in this case, which allows a statement that is reliable from the safety point of view as to whether the velocity, travel or position do not exceed or undershoot a threshold value which is defined for safety reasons. Since a suitable safety margin is planned in, it is easily possible to compensate for any measurement inaccuracies in the novel approach.  
         [0017]     The new apparatus and method allow to use small and cost-effective acceleration sensors for the determination of velocity, travel and/or position of the moving installation part, even when the “measurement tolerances” that can be achieved in this way are inadequate for other control applications.  
         [0018]     In addition to low component costs and small dimensions, the novel safety device further has the advantage that the assembly effort is considerably less than that of previous devices. The acceleration sensor does not require any reference point on the fixed part of the installation, in contrast to all previous measurement means. It can therefore be positioned virtually anywhere on the moving installation part. In the case of a press of the type described initially, it is particularly advantageous to integrate the at least one acceleration sensor in or on the receiver for the light barrier arrangement, thus considerably reducing the wiring complexity.  
         [0019]     Furthermore, the novel safety device has the advantage that it can be used largely independently of the respective installation type, and thus for a plurality of different installations.  
         [0020]     Finally, the new approach allows non-contact and therefore wear-free determination of the movement velocity, travel and/or position.  
         [0021]     In a refinement, the safety device has at least two acceleration sensors, and the evaluation unit is designed to determine the movement velocity and/or movement travel of the installation part in a redundant form, by means of the at least two acceleration sensors.  
         [0022]     This refinement is a particularly simple way to achieve better fail-safety by means of a plausibility check. Because of the small physical size of commercially available acceleration sensors, this advantageous refinement profits from the advantages mentioned above without any restrictions.  
         [0023]     In a further refinement, the at least two acceleration sensors are designed such that they detect accelerations of the moving installation part on in each case one of at least two different sensor axes.  
         [0024]     This refinement reduces the probability of the occurrence of what is called common-cause errors. This further enhances the safety of the novel safety device. On the other hand, a vectorial breakdown of the measured values along the at least two different sensor axes also allows an advantageous plausibility check.  
         [0025]     In a further refinement, the at least two acceleration sensors are integrated into a common sensor housing.  
         [0026]     This refinement makes it possible to further reduce the physical space required, and to further reduce the component and/or installation costs. Furthermore, it is particularly advantageous for the at least two integrated acceleration sensors to detect different sensor axes. This is because numerous versions of suitable acceleration sensors are commercially available, with the original purpose of these commercial acceleration sensors being multi-dimensional detection of accelerations in a plane or in three dimensions. For the purposes of the present refinement, however, the at least two sensor axes are used for redundant detection of accelerations of the moving installation part in one movement direction. This can be achieved highly cost-effectively in accordance with the preferred refinement.  
         [0027]     In a further refinement, the at least two acceleration sensors are coupled to the moving installation part in at least two different mounting positions, such that different gravitational biases occur at the at least two acceleration sensors.  
         [0028]     In this case, it is particularly preferable if the mounting positions of the at least two acceleration sensors are arranged offset through 180° with respect to one another, because they then produce output signals in opposite senses, which allow advantageous evaluation.  
         [0029]     Since acceleration sensors typically comprise a moving mass whose displacement caused by inertia forces is detected by measurement, different mounting positions will affect the measurement. The reason is the displacement of the moving mass caused by the earth&#39;s gravity. This leads to a stationary bias value in the output signal from the acceleration sensors, i.e. a gravitational bias. The different gravitational biases can be used well for a plausibility check, thus further increasing the functional safety of the novel safety device. Furthermore, noise-voltage components can easily be eliminated by means of a particularly preferred subtraction process. This results in the safety-relevant determination of velocity and/or travel and/or position being possible even more accurately and reliably.  
         [0030]     In a further refinement, the evaluation unit is designed to read the at least two acceleration sensors with a time offset between them.  
         [0031]     This refinement makes it possible to achieve higher measurement resolution, and thus a faster reaction, from the safety device in a highly cost-effective manner.  
         [0032]     In a further refinement, the at least one acceleration sensor has a test input for feeding in a test signal. The test signal is preferably connected to the evaluation unit, which is thus able to feed a test signal into the acceleration sensor.  
         [0033]     Correct operation of the acceleration sensor during continuous operation of the safety device can be easily monitored by means of an expectation embodied in the test signal. For example, the test signal can be used to produce a defined displacement of the moving mass in the acceleration sensor, which must be reflected in the output signal from the acceleration sensor. This refinement therefore allows safe operation even when only a single acceleration sensor is used for determining velocity and/or travel. This allows the component costs to be further reduced.  
         [0034]     In a further refinement, the evaluation unit is designed to determine a movement direction of the moving installation part on the basis of the acceleration, in a safety-related manner.  
         [0035]     The movement direction of the moving installation part can easily be derived from the “mathematical sign” of the acceleration values detected by measurement. Furthermore, the movement direction can also be determined during the movement process from the velocity or position values obtained. The present refinement therefore offers information, without any significant additional costs, which can advantageously be included in the safety-critical assessment of an operating situation. For example, different movement velocities can often be allowed in an installation, depending on the respective movement direction, thus making it possible to increase the productivity of the installation without any adverse effect on safety.  
         [0036]     In a further refinement, a detector is provided, which is designed to detect when the moving installation part has reached a defined rest position, with the detector being connected to the evaluation unit. In this case, the detector is preferably designed to be fail-safe in terms of the pertinent standards, such as within the meaning of Category 4 of European Standard EN 954-1 or similar safety regulations).  
         [0037]     Such a detector advantageously can be used to reliably determine the start point of the movement of the moving installation part. This makes it possible to increase the accuracy and reliability of the measured values for the velocity, travel and/or position. In consequence, safety margins can be reduced, thus increasing the productivity of the monitored installation. It is particularly preferable, if the detector is a detector which is physically installed on the installation since, in this case, it produces a “real” measured value. As an alternative to this, however, the detector may also be “virtual”, for example by the start point being derived from the data from the control system.  
         [0038]     In a further refinement, a memory is coupled to the evaluation unit, which memory is designed to store a time reference profile of the at least one movement variable of the moving installation part. It is preferable for the evaluation unit to be designed to compare an instantaneous movement profile of the moving installation part with the reference profile, and to stop the moving installation part as a function of this.  
         [0039]     The recording and monitoring of a reference profile, such as by way of example the time profile of the acceleration or velocity of the moving installation part during a working cycle, enhances reliable monitoring of the installation. In particular, it is very simple to quickly and reliably identify changes in the movement process of the installation, such as a prolonged slow-down movement resulting from ageing phenomena, wear or external influences, and to react quickly and reliably. Furthermore, the state of the installation can also be deduced from the recording of a movement profile, so that, for example, it is possible to match a safety margin to a specific installation. This makes it possible to match a safety device to an installation being monitored in such a way as to minimize the reduction in productivity.  
         [0040]     In a further refinement, the moving installation part is a tool which moves like a stamp. It is furthermore preferable if at least one of the following variables is determined as the movement variable: movement velocity of the tool, slowing-down travel of the tool (this is, so to speak, the braking movement including the reaction time after initiation of an emergency stop), muting point of a non-contact protective device, switching time between high-speed and creeping speed of the tool, and movement direction of the tool.  
         [0041]     As has already been mentioned above, the present invention is not restricted to presses and similar machines with installation parts moving in opposite directions with respect to one another. On the other hand, however, these examples are a particularly preferred application, since the movements which occur in such installations can be monitored easily and advantageously by means of the novel safety device. In particular, the movement velocities and movement paths of a press or of a similar tool are well suited for allowing determination of velocity, travel and/or position on the basis of the acceleration. Furthermore, in such installations, there is a particularly high risk, and therefore a very urgent requirement for a safety device of the present type.  
         [0042]     In a further refinement, the safety device comprises a transmitter and a receiver forming a non-contact protective device which moves with the tool, with the at least one acceleration sensor being arranged in the transmitter and/or the receiver.  
         [0043]     The protective device is not restricted to a conventional light barrier arrangement, but can also contain a camera as a receiver. The arrangement of the at least one acceleration sensor in the area of the receiver allows to check the vertical adjustment of the transmitter/receiver very easily, and even during monitoring operation, on the basis of the gravitational bias and the accelerations. In addition, the installation effort for the novel safety device is further reduced, since the transmitter and receiver have to be fitted to the installation in any case.  
         [0044]     It goes without saying that the features mentioned above and those which are still to be explained in the following text can be used not only in the respectively stated combination but also in other combinations or on their own, without departing from the scope of the present invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0045]     Embodiments of the invention will be explained in more detail in the following description, and are illustrated in the drawing, in which:  
         [0046]      FIG. 1  shows a simplified illustration of a preferred embodiment of the invention,  
         [0047]      FIG. 2  shows a simplified illustration of a capacitive acceleration sensor, which can advantageously be used in embodiments of the invention,  
         [0048]      FIG. 3  shows a schematic illustration of a velocity profile of the moving installation part of the installation shown in  FIG. 1 , and  
         [0049]      FIG. 4  shows a simplified flowchart in order to explain an embodiment of the method according to the invention. 
     
    
     DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0050]     In  FIG. 1 , a press which is a preferred embodiment of the novel installation, is designated by reference number  10 .  
         [0051]     The press  10  has an upper tool  12  (moving installation part) and a lower tool  14 . The reference numbers  16  represent two drives, in a simplified form, by means of which the upper tool  12  can be moved towards the lower tool  14  in the direction of an arrow  18 . A bending tool  20  is arranged on the upper tool  12 . The reference number  22  represents a foot-operated button, in a simplified form, which must be operated in order to start and to carry out a working cycle of the press  10 . As an alternative to this, the press  10  can also be operated via other control elements.  
         [0052]     A die  24  is arranged on the lower tool  14 , and a workpiece  26 , such as a sheet-metal part, rests on it. The lower end of the bending tool  20  is designed to be complementary to the die  24  and allows the workpiece  26  to be shaped as it is driven into the die  24 .  
         [0053]     The reference numbers  27  and  28  denote two holders, which are arranged on the upper tool  12  and on the left and right of the bending tool  20 . A transmitter  29  is located at the end of the holder  27 , and a receiver  30  is located at the end of the holder  28 . The transmitter  29  and receiver  30  form a light barrier arrangement, which produces one or more light beams  32  which run parallel to the lower edge of the bending tool  20 , a short distance away from it. The light beam or beams  32  move downwards together with the bending tool  20  in the direction of the arrow  18 ; in other words, the transmitter  29  and receiver  30  form a non-contact protective device which moves together with the upper tool, as is generally known from the documents cited initially. As an alternative to this, the non-contact protective device can also be provided by a camera unit or by other optical means.  
         [0054]     The reference numbers  34  and  36  schematically represent two limit switches, which are closed only when the upper tool  12  is at its upper dead point. The closed position of the limit switches  34 ,  36  thus signals that the press  10  is in its initial state for carrying out a working cycle.  
         [0055]     Reference number  40  represents a control unit, in a simplified form, which controls at least some of the functions of the press  10 . In a preferred embodiment, the control unit is a fail-safe PLC as is marketed by the present applicant under the brand name PSS®. The control unit  40  contains (illustrated in simplified form) an interface part  42  and two or more redundant signal processing channels. In this case, the signal processing channels are illustrated only with a respective processor  44   a ,  44   b , a first memory  46   a ,  46   b  and a second memory  48   a ,  48   b . The processors  44   a ,  44   b  can communicate with one another via an appropriate interface (such as a bus link or a dual-ported RAM), and they carry out a plausibility check on the respective processing results. The interface part  42  has a plurality of inputs and outputs, to which the sensors and actuators for the press  10  are connected. In particular, the foot-operated button  22 , the limit switches  34 ,  36  and at least the receiver  30  of the light barrier arrangement are connected. Furthermore, the drives  60  for the press can be switched off (via suitable actuators, such as contactors, not illustrated here).  
         [0056]     Reference number  50  schematically represents an acceleration sensor which in this case, by way of example, is arranged on the upper tool  12  of the press  10 . In this embodiment, the acceleration sensor  50  has two sensor axes  52 ,  54 , i.e. it provides acceleration values along the two axes  52 ,  54 , which run at right angles to one another. In the illustrated embodiment, the acceleration sensor  50  is arranged such that each sensor axis  52 ,  54  runs at an angle of 45° to the movement direction  18  of the upper tool  12 . Redundant information about the acceleration of the upper tool  12  along the movement direction  18  can be derived by vectorial evaluation of the acceleration measured values along the two sensor axes  52 ,  54 .  
         [0057]     As an alternative to or in addition to the acceleration sensor  50 , the press  10  in another embodiment has two individual acceleration sensors  50   a ,  50   b , which are arranged adjacent to the receiver  30  (or adjacent to the transmitter  29 ), or at least in their area. The acceleration sensors  50   a ,  50   b  are integrated in the receiver  30  in one embodiment. In the preferred embodiment, the acceleration sensors  50   a ,  50   b  are arranged in two different mounting positions, namely rotated through 180° with respect to one another. In consequence, the acceleration sensors  50   a ,  50   b  provide different gravitational biases and the voltage signals at the output of the sensors are in opposite senses, thus allowing advantageous subtraction.  
         [0058]     It is self-evident that the acceleration sensor  50  can likewise be integrated in the transmitter  29  or the receiver  30 , as an alternative to the simplified illustration. Furthermore, the two individual acceleration sensors  50   a ,  50   b  could, in contrast to the illustrated arrangement, be arranged in or adjacent to the receiver, or else at some other point on the upper tool  12 , or at some other point at which the accelerations of the upper tool, or of the bending tool  20 , can be measured. If required, further acceleration sensors can also be used in order to increase the redundancy, and/or to determine further movement variables.  
         [0059]     The output signals from the acceleration sensor or sensors  50 ,  50   a ,  50   b  are likewise fed to the control unit  40 , as is indicated by the reference numbers  52 ,  54  for the control unit  40 .  
         [0060]     In the present embodiment, a movement velocity of the upper tool  12  and the slowing-down travel (“braking travel”), the switching point between high speed and creeping speed, as well as the muting point for the light barrier arrangement are determined in the control unit  40 . An appropriate program module for this purpose is stored in the memories  46   a ,  46   b . In other words, the program modules in the memories  46   a ,  46   b  each form an evaluation unit in terms of the present invention. However, as an alternative to this, the evaluation unit could also be provided separately from the control unit  40 . In an embodiment, the evaluation unit is, for the purposes of the present invention, completely integrated in the receiver  30  of the light barrier arrangement. All of the other safety-relevant tasks might also reside there, so that the control unit  40  may be a conventional, non-safe control unit.  
         [0061]     In order to start a working cycle of the press  10 , the upper tool  12  must be located at its upper dead point (as illustrated in  FIG. 1 ). This initial position can be detected in a fail-safe form by the limit switches  34 ,  36 .  
         [0062]     On operation of the foot-operated button  22 , the upper tool  12  is moved, together with the bending tool  20 , downwards at a high movement velocity (high speed). The high movement velocity is maintained until the bending tool  20  reaches a predefined switching point  56 . The distance traveled at high speed is designated d 1  in  FIG. 1 . After the switching point, the upper tool  12  together with the bending tool  20  moves only at creeping speed in order to complete the shaping process. The distance traveled is indicated by d 2  in  FIG. 1 .  
         [0063]     The upper tool  12  then returns to its initial position (upper dead point) again. This generally once again takes place at high speed, but in the opposite movement direction.  FIG. 3  shows a simplified form of a corresponding velocity profile, with the profile of the velocity being plotted against time. During a first phase  62 , the upper tool  12  assumes its maximum velocity (high speed), and is then braked again on reaching the switching point  56  (flank  64 ). The movement is then continued at a lower speed (creeping speed, phase  66 ). In some presses, the movement at the creeping speed in phase  66  must be specifically initiated by renewed operation of a control switch. Once the workpiece has been shaped, the upper tool returns at high speed, but in the opposite movement direction (phase  68 ).  
         [0064]      FIG. 2  schematically shows the design of a capacitive acceleration sensor  50   a , as may be used in the embodiment shown in  FIG. 1 . The acceleration sensor  50   a  has a measurement element which can be considered, in a simplified form, to be a “duplicated-plate capacitor” with three mutually parallel plates  70 ,  72 ,  74 . The central plate  72  is mounted such that it can move. In the rest state, the distance between the plates  70 ,  72  is approximately the same as the distance between the plates  72 ,  74 . When the acceleration sensor  50   a  is accelerated, the distances d 3  and d 4  change as a result of the inertia of the central plate  72 . The changed distances result in a change of the capacitance values C 1  and C 2  of the two capacitors, which can be detected by measurement.  
         [0065]     In the present case, the acceleration sensor  50   a  has a test input  76  to which a test signal can be applied. The test signal can be used to deliberately deflect the central plate  72 , which must show up at the output of the acceleration sensor as a corresponding “acceleration signal”. This allows the acceleration sensor to be checked for correct operation.  
         [0066]     However, it should be noted that the present invention is not restricted to capacitive acceleration sensors. For example, piezo-resistive acceleration sensors can also be used, in which case the displacement of a mass element is determined using piezo-elements. Other measurement principles can also be used for (preferably direct) detection of accelerations by measurement.  
         [0067]     The velocity of the upper tool can be determined from the detected accelerations by integrating the acceleration values over time. Mathematically, the relationship is:  
             v     A   /   B       ⁡     (   t   )       =       v   0     +       ∫     t   1       t   2       ⁢       a     A   /   B       ⁢     ⅆ   t             ,       
 
         [0068]     where  
         [0069]     v A/B (t) is the velocity profile over time, which is determined in the two evaluation channels A and B, respectively,  
         [0070]     v 0  is the velocity at the start of the integration process,  
         [0071]     a A/B  are the acceleration values detected by measurement in the respective channels A and B, and  
         [0072]     t 1 , t 2  are the start and end times, respectively, of the time interval over which the acceleration values a A/B  are integrated.  
         [0073]     The distance traveled can be determined in the same way from the resultant velocity using the following relationship:  
             s     A   /   B       ⁡     (   t   )       =       s   0     +       ∫     t   1       t   2       ⁢       v     A   /   B       ⁢     ⅆ   t             ,       
 
         [0074]     where  
         [0075]     s A/B (t) is the distance traveled at the time t,  
         [0076]     s 0  is the location at the start of the integration process,  
         [0077]     v A/B  is the velocity at which the distance s was traveled, to be precise separately for the channels A and B, and  
         [0078]     t 1 , t 2  are the start and end times of the time interval under consideration.  
         [0079]     Since the upper tool  12  is located at its upper dead point at the start of each working cycle, the velocity at the start of each working cycle is zero. The distance traveled with respect to the upper dead point at the start of each working cycle is likewise zero. Continuous or quasi-continuous detection of the acceleration values therefore allows continuous determination of the velocity and of the distance traveled. The respective instantaneous position of the upper tool  12  can also be determined from the distance traveled. It is also possible to determine whether the upper tool has reached the switching point for switching between high speed and creeping speed. Furthermore, a slowing-down measurement is possible if the initiation of the emergency stop is used as the start time t 1 , and the time at which the upper tool  12  becomes completely stationary is used as the end time t 2 .  
         [0080]     A preferred embodiment of the method according to the invention is illustrated in simplified form in  FIG. 4 . First, a check is carried out in step  80  to determine whether the upper tool  12  is at its upper dead point (designated s 0 ). The process does not move to step  82 , in which the acceleration values a A  and a B  are read in a redundant form, until this condition is satisfied. The movement velocity and the distance traveled are then determined by integration in steps  84  and  86 . Furthermore, the movement direction is determined in step  88 , in which case this step is optional and may be omitted if required. The information obtained is then produced in step  90 , in order to allow evaluation in step  92 .  
         [0081]     The velocity values and/or position values and direction values obtained are evaluated in a manner known per se. In particular, a check is carried out to determine whether the respective velocity of the upper tool is less than the maximum velocities defined for safe operation. Furthermore, another check is carried out to determine whether the upper tool is in a position (has traveled a movement distance) which corresponds to a correct working cycle. If this is not the case, an emergency stop is carried out in step  94 , since the press  10  is in an unsafe state then. In this situation, it might be envisaged not to initiate the emergency stop immediately on the basis of the velocity and/or position values obtained, but, for example, just to suppress the deactivation (muting) of the light barrier arrangement, so that the emergency stop is initiated when the light beam  32  is interrupted by the workpiece  26 .  
         [0082]     In the preferred embodiment, the determined velocity and/or position values are used to create a movement profile, as is illustrated in a simplified form in  FIG. 3 . The recorded movement profile is compared with a reference profile in step  98 . For example, it is possible to determine when the upper tool has been moving at its high speed for longer than intended, as is indicated in a simplified form by the reference number  102  in  FIG. 4 . In a situation such as this, the press is again switched off in step  100  in order to avoid a situation which would be dangerous for the operator.