Measuring and testing – Specimen stress or strain – or testing by stress or strain... – By loading of specimen
Reexamination Certificate
1999-08-12
2001-09-25
Noori, Max (Department: 2855)
Measuring and testing
Specimen stress or strain, or testing by stress or strain...
By loading of specimen
Reexamination Certificate
active
06293155
ABSTRACT:
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
Gebr, Schmidt Fabrik für Feinmechanik
Harness & Dickey & Pierce P.L.C.
Noori Max
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