Method for operating an electric press

A method is described for operating an electric press which comprises an electrically actuated press ram, at least one displacement sensor for sensing positions of the displacement of the press ram during its working stroke, at least one force sensor for sensing compressive force applied by the press ram during the working stroke onto workpieces to be processed, and a control system which controls the working stroke in terms of displacement and compressive force. Parameters which indicate a successful course or completion of the pressing operation are dynamically adapted as a function of the profile of the compressive force versus the displacement of the press ram.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates to a method for operating an electric press
 which comprises an electrically actuated press ram, at least one
 displacement sensor for sensing positions of the displacement of the press
 ram during its working stroke, at least one force sensor for sensing
 compressive force applied by the press ram during the working stroke onto
 workpieces to be processed, and a control system which controls the
 working stroke in terms of displacement and compressive force, having the
 steps:
 a) moving the press ram into its initial position;
 b) lowering the press ram onto the workpieces to be processed, and
 measuring the compressive force;
 c) detecting the onset of pressing based on a rise in the compressive
 force;
 d) further lowering the press ram to perform the pressing operation, and
 monitoring the compressive force being applied; and
 e) halting the press ram when the latter has reached a preset joining or
 end position.
 2. Related Prior Art
 A method of this kind is known from U.S. Pat. No. 5,483,874.
 The known method is carried out on an electric press which comprises a
 spindle drive driven by an electric motor. The threaded spindle of the
 spindle drive is mounted rotatably but axially nondisplaceably, while the
 spindle nut is mounted nonrotatably but axially displaceably, and is
 joined to the press ram.
 The electric press has displacement sensors and force sensors in order to
 sense the profile of the compressive force versus the displacement of the
 press ram during its working stroke and report it to a control system
 which, on the basis of these data, controls the working stroke.
 At the beginning of a pressing operation, the press ram is moved into its
 initial position above the workpiece, and is then lowered onto the
 workpiece or workpieces to be processed.
 The compressive force is measured during this lowering, and the fact that
 the pressing operation is beginning is detected on the basis of a rise in
 this compressive force, whereupon the lowering rate of the press ram is
 decreased.
 The press ram is lowered further during the pressing operation which then
 follows, the applied compressive force then being monitored to determine
 whether it remains constant during the pressing operation. The
 displacement of the press ram also continues to be monitored in order to
 detect when it has reached a joining position (therein called the end
 position) in which the press ram essentially does not move any farther
 down. When this end position has been reached, the press ram is retracted.
 If the end position is not reached, or if a constant pressure is not
 applied during the pressing operation, the pressing operation is
 terminated.
 It has now been found that it is possible, in the context of this kind of
 method, for a compressive force that it is sometimes too high and also
 sometimes too low to be applied, depending on tolerances of the workpieces
 to be processed, so that some of the workpieces are not correctly joined
 and some are damaged by excessive compressive force.
 In order to process the workpieces gently and reproducibly, they therefore
 must have very narrow tolerances; if these tolerances are exceeded upward
 or downward, the known method terminates the pressing operation because
 the end position is not reached and/or the compressive force is not
 constant; this can result in unnecessary wastage.
 SUMMARY OF THE INVENTION
 In view of the above, it is an object of the present invention to improve
 the method mentioned at the outset so as to make possible gentle
 processing even of parts with coarser tolerances, so as thereby to prevent
 unnecessary wastage or reduce the wastage.
 In the case of the method mentioned at the outset, this object is achieved
 according to the present invention in that parameters which indicate a
 successful course and/or completion of the pressing operation are
 dynamically adapted as a function of the profile of the compressive force
 versus the displacement of the press ram.
 The object underlying the invention is completely achieved in this fashion.
 Specifically, the inventors of the present application have recognized that
 the high wastage with the known method is attributable in particular to
 the fact that the beginning of the pressing operation is dynamically
 sensed, but not the completion of the pressing operation. In the prior
 art, a fixed end position is defined here; whether it is reached or not
 reached determines the success of the pressing operation. In addition, a
 constant compressive force is required during the pressing operation, any
 deviation from that constant compressive force also being considered as
 wastage.
 What is critical to the successful completion of a pressing operation,
 however, is not so much the beginning of the pressing operation but rather
 the profile of the pressing operation versus the displacement of the press
 ram, as well as the location of the joining point and the compressive
 force applied in the joining point.
 The new method now makes available intelligent assembly even of parts with
 coarser tolerances, since the parameters critical to the result of the
 pressing operation are dynamically derived and adapted from the profile of
 the compressive force during the working stroke of the press ram. Based on
 those parameters, after completion of a pressing operation a conclusion
 can be drawn as to whether the pressing operation was successful and
 corresponds to predefined test values.
 It is especially preferred in this context if the end position is adapted
 dynamically as a function of the position of the press ram at the onset of
 pressing.
 The advantage here is that in the simplest case, the end position is
 shifted by the same magnitude by which the onset of pressing shifts. This
 is done, for example, by storing a sample curve in the control system, a
 constant distance between onset of pressing and end position always being
 assumed and defined.
 On the other hand, it is preferred if the end position is dynamically
 adapted or detected as a function of a sharp rise in the compressive force
 in the region of the end position.
 The advantage here is that in addition to or instead of the coarse
 adaptation of the end position as a function of the onset of pressing, the
 direct joining point--at which, for example when joining two workpieces,
 the latter were pressed into a unit--is detected. The joining or end
 position can differ for different workpieces as a function of workpiece
 tolerances, so that a determination of the end position solely from the
 onset of pressing is not as reliable as deriving the joining position from
 the sharp rise in the compressive force. This can be done, for example, by
 continuously monitoring the change in compressive force with displacement
 or with time, so that the joining point is detected in real time by
 analyzing that rise.
 It is further preferred if the pressing operation is terminated in
 dynamically adapted fashion as a function of the sharp rise in compressive
 force.
 The advantage here is that not only the joining position itself, but also
 the compressive force yet to be applied in the joining position, are
 adapted dynamically as a function of the workpiece tolerances. This is
 because depending on the workpiece tolerances, it is possible that a
 relatively low compressive force was applied in one case at the onset of
 the actual joining operation, while for workpieces having different
 tolerances, a very high compressive force was already necessary simply to
 press the workpieces into a unit in the joining position. What is done
 now, in order to compensate for these tolerances, is not to predefine a
 high compressive force that must be reached, which is sufficient for all
 expected tolerances but in some cases is much too high. Instead the
 pressing operation is dynamically completed, as a function of the change
 in slope of the compressive force profile, as soon as the workpieces have
 arrived in the joining position.
 In the case of the method described so far, the parameter "joining or end
 position" is thus dynamically adapted based on the position of the press
 ram at the onset of pressing, the sharp rise in compressive force upon
 reaching the joining position, and optionally the instantaneous value of
 the compressive force upon reaching the joining position; the result is
 greatly to reduce the effect of workpiece tolerances, so that altogether
 the wastage declines sharply as compared with the method known from the
 prior art.
 It is further preferred, however, if the profile of the compressive force
 versus the displacement of the press ram is monitored to determine if
 certain parameter sets are being observed, the parameter sets being
 dynamically adapted as a function of the position of the press ram at the
 onset of pressing.
 The further advantage here as compared with the method described so far is
 that depending on how the press is used, not only the joining position but
 also further intermediate positions, at which the compressive force must
 lie within specific ranges, are monitored. What may be observed, for
 example, is the fact that when parts are being joined, the compressive
 force exhibits a certain superelevation when the press ram has travelled a
 certain distance since the onset of pressing and stiction transitions into
 sliding friction. Characteristic curve profiles of this kind can be
 described by parameter sets which define a "window" through which the
 curve for the compressive force versus displacement must pass in order for
 the result of the pressing operation to be satisfactory. If the curve does
 not pass appropriately through one of these windows, a decision can then
 be made relatively promptly that this pressing operation can no longer be
 satisfactorily completed, and it is thus terminated immediately. This
 early discontinuation can prevent damage to the electric press itself as
 well as destruction of the workpieces, which may simply have been
 assembled incorrectly, so that realignment of the workpieces will still
 allow successful joining; the result is thus once again to decrease the
 wastage.
 It must also be mentioned that the monitoring windows need only to be
 shifted as a function of the onset of pressing in order to allow parts
 with different tolerances to be sensed. In this context it is possible on
 the one hand to shift the windows along the displacement axis, but a shift
 along the compressive force axis is also possible.
 It is further preferred if the parameter sets are ascertained by processing
 and measuring sample workpieces.
 The advantage here is that by processing and taking averages for several
 workpieces, it is possible to define reliable parameter sets or windows
 which are important for quality control of the pressing operation. For
 example, a permissible deviation in terms of compressive force or
 compressive displacement can be defined in the parameter sets; workpieces
 for which the window is missed are then picked out as waste.
 It is preferred in this context if, at least during the processing of
 sample workpieces, the compressive force profile is displayed on a screen
 in real time.
 The advantage here is that the profile of the compressive force can easily
 be observed even during the "teach-in" process, so that even at that early
 point corresponding windows can be defined which can then be checked or
 further modified when additional sample workpieces are processed. This
 determination of the parameter sets or windows with the greatest possible
 accuracy allows good control over the actual pressing operation on
 workpieces intended for further processing, so that their wastage can be
 greatly decreased.
 Lastly, it is also preferred if, at least during the processing of sample
 workpieces, the applied compressive force is modified via an electronic
 handwheel.
 The advantage here is that the necessary compressive force can be
 ascertained and defined in simple and very accurate fashion, since not
 only the working stroke (i.e. the displacement of the press ram) but also
 the compressive force applied, in particular, in the joining position, can
 be adjusted manually. This makes it possible to prevent the definition of
 an excessive compressive force which might allow the destruction of
 workpieces with appropriate tolerances despite the features according to
 the present invention described above.
 In other words, it is possible by the use of an electronic handwheel to
 adjust the compressive force, and by way of the compressive
 force/displacement curves to be monitored in real time, to ascertain
 optimum parameter sets with the aid of sample workpieces; those parameter
 sets make it possible to achieve gentle and reliable joining or processing
 even with workpieces having coarser tolerances. The dynamic modification
 of these parameter sets as a function of the instantaneous compressive
 force profile while processing workpieces makes possible on the one hand a
 decrease in the wastage thanks to optimal adjustment of the compressive
 force and joining position, and on the other hand timely detection of
 failed pressing operations, as already described above.
 Further advantages are evident from the specification and from the appended
 drawings.
 It is understood that the features mentioned above and those yet to be
 explained below can be used not only in the respective combinations
 indicated, but also in other combinations or in isolation, without leaving
 the context of the present invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT
 In FIG. 1, 10 generally designates an electric press which is operated via
 a control system indicated at 11. A screen 12, a keyboard 14, and an
 electronic handwheel 15 are connected to control system 11.
 Also connected to control system 11 is an interface 16 of electric press 10
 which makes possible control and monitoring of the pressing operations, as
 will be described later.
 Electric press 10 has an electric motor 17 which is mounted on a housing
 18. A schematically indicated press ram 19, which is actuated via electric
 motor 17, projects downward out of housing 18.
 Schematically indicated below electric press 10 are two workpieces 21 and
 22 which are to be joined to one another by electric press 10, for which
 purpose press ram 19 performs a working stroke indicated at 23.
 The profile for compressive force over displacement which thereby results
 is displayed in real time on screen 12 as curve 24, for which purpose a
 displacement sensor 25 and a force sensor 26 are provided in electric
 press 10.
 In the position shown in FIG. 1, press ram 19 is in its initial position
 27. When press. ram 19 is moved farther downward in FIG. 1, at the onset
 of pressing 28 it comes into contact with workpieces 21 and 22 which it
 then, during the further course of its working stroke 23, presses into a
 unit, this occurring in its joining position 29. Press ram 19 is then
 retracted back into its initial position 27.
 FIG. 2 depicts, by way of example, two curves 24, 24' representing the
 profile of compressive force versus displacement; curve 24 is intended to
 be a comparison curve ascertained on the basis of sample workpieces. In
 its position S.sub.B, press ram 19 has reached the position in which the
 pressing operation begins, i.e. it must exert force in order to join
 workpieces 21, 22 to one another. What is first observed is a force
 increase, up to a superelevation 31 at which stiction transitions into
 sliding friction; as the working stroke continues further, the compressive
 force initially declines again, and then rises again until upon reaching
 joining position S.sub.F it has attained a force F.sub.F. The compressive
 force F then rises steeply until the pressing operation is terminated and
 ram 19 is retracted.
 Control system 11 continuously monitors the rise in the compressive force,
 and at point F.sub.F now detects a sudden change in the slope dF/ds or
 dF/dt, thus detecting that the joining point has been reached. Since the
 rise cannot be sensed in an arbitrarily short time, a certain force
 superelevation .DELTA.F occurs (to F.sub.m), but this does not cause any
 damage to the workpieces.
 The profile of curve 24 is also critical to the result of the pressing
 operation, and control system 11 therefore monitors a whole series of
 parameters to ensure they are observed. These parameters include the
 joining position S.sub.F and compressive force F. If, for example, the
 joining position S.sub.F is not reached, the pressing operation was not
 successful. Since, however, a deviation of this kind does not necessarily
 indicate a failed pressing operation, but rather might be attributable to
 the fact that the parts being joined have different tolerances, the
 parameters being monitored are now dynamically adapted based on the
 profile of curve 24.
 In a first step, the onset of the pressing operation, i.e. position 28, is
 monitored. With curve 24' the compressive force begins much later, only at
 position S.sub.B'. Thus in the simplest case, the joining point S.sub.F'
 is also modified, by a value exactly equal to that offset, in the set of
 monitoring parameter, as indicated in FIG. 2.
 It is also evident that when the joining point S.sub.F' is reached, curve
 24' has a lower compressive force, namely a compressive force F.sub.F'. If
 control system 11 now waited to terminate the pressing operation until
 force F.sub.m had been reached, an unnecessarily large force would be
 expended, which is undesirable for the reasons already mentioned. Control
 system 11 once again, however, detects the steep rise in the force and
 immediately terminates the pressing operation, so that the force can rise
 only to a value F.sub.m'.
 In other words, this means that curve 24' in FIG. 2 is merely shifted to
 the right and downward; this was detected, by way of points S.sub.B' and
 dF/ds, by the control system which thereupon dynamically adapts the
 parameter S.sub.F' and terminates the pressing operation.
 It is not only the beginning and completion of the pressing operation which
 are responsible for a successful result, however; the profile of the
 compressive force during the working stroke also plays a major role. For
 example, the superelevation 31 is an indication that workpiece 21 has been
 completely pressed into workpiece 22, because continuation of the pressing
 operation initially requires a lesser force once this joining has
 occurred. This superelevation 31 is monitored using a further parameter
 set W which is indicated in FIG. 2 by a window 32. A further prerequisite
 for a successful pressing operation is thus the fact that curve 24 passes
 through window 32, and exhibits a superelevation 31. The curve can have
 any desired position in window 32.
 It is evident, however, that curve 24' does not pass through window 32 at
 all, so that pressing operation 24' would not of itself be considered
 successful. The fact that curve 24' does not intersect window 32 is due,
 however, to the fact that because of the workpiece tolerances, the onset
 of pressing 28' and thus the entire curve 24' in FIG. 2 was shifted to the
 right. Since the control system detects this based on the shift of point
 S.sub.B to S.sub.B', a dynamically adapted parameter set W' is generated,
 indicated by a window 34 which has been shifted to the right by a
 magnitude 35 as compared to window 32. As long as curve 24' now passes
 through window 34, the pressing operation will be considered successful.
 Of course various such windows 32, 34 can be laid over curve 24 in order to
 monitor different portions of the pressing operation. The number and
 location of the windows depend on the workpieces 21, 22 being joined.
 This means, however, that a new sample curve 24, and new windows 32, 34,
 must be ascertained for different types of workpieces 21, 22.
 This is done by joining sample workpieces, which involves using electronic
 handwheel 15 to adjust not only working stroke 28 of electric press 10 but
 also, in particular, the compressive force F of press ram 19. The
 instantaneous profile of the compressive force is displayed in real time
 on screen 12, so that the operator recording a new curve 24 can make fine
 adjustments to the compressive force using electronic handwheel 15, and
 can immediately check the result on screen 12. Windows 32, 34 can then
 also be adjusted on screen 12. New sample workpieces are then pressed; the
 instantaneous compressive force profile can once again be displayed on
 screen 12, and at the same time the position of windows 32, 34 can be
 checked. Once a large number of sample workpieces has been pressed in this
 fashion, appropriate parameters sets are available, comprising windows 32,
 34 and the positions S.sub.B at onset of pressing 28 and S.sub.P at the
 end of the pressing operation.