EP0284903B1 - Piston press - Google Patents

Description

The invention relates to a piston press with a plunger, which is connected to a piston running in a cylinder, the cylinder can be pressurized with compressed air via electrically actuated valves, the plunger interacts with a displacement measuring system, an output signal of the displacement measuring system is compared in a controller with predefined setpoints is and the controller controls the valves for pressurizing the cylinder according to a predetermined path / time curve.

Such a piston press is known from European patent application EP-A-0 025 987.

To deflect the plunger, the compressed air is supplied via switching valves to the piston in generally known piston presses, which moves the plunger until it hits the workpiece. The plunger is either placed on the workpiece with full force, which corresponds to the preset pressure of the compressed air, or limit switches are provided which close the valves again when the plunger reaches a predetermined end position or cause the plunger to be retrieved.

The known piston presses with pneumatic drive thus have the disadvantage that sensitive control, in particular displacement / time control, is not possible because of the ram Known pneumatic piston presses with full force on the workpiece and at most decreases when the limit switch is reached in the operating pressure.

In the piston press known from EP-A-0 025 987 mentioned at the outset, a displacement measuring system is assigned to the plunger, which transmits information about the state of movement of the plunger to a computer. This computer acts as a controller and controls the electrically actuated switching valves. During the delivery stroke, a first switching valve is opened and the piston is moved downward by means of a first pressurization. As soon as the tool hits the workpiece to be machined, the piston no longer moves due to the high back pressure, which is registered by the position measuring system. Now a second switching valve is opened so that the working stroke can now be carried out because of the higher pressure.

At the end of the working stroke, which is again registered by the position measuring system, the computer switches the switching valves in such a way that the piston carries out its return stroke to the starting position.

In the known piston press, the switching times of the individual switching valves are set by the computer in such a way that the dwell time of the tool on the workpiece (between the delivery stroke and the working stroke) and after the working stroke is as short as possible. In other words, the combination of computer and position measuring system serves here to replace the limit switches from the prior art described above. This piston press is, so to speak, "self-learning" because it recognizes because of the position measuring system the computer automatically, when the work stroke has to start and when it has ended. It is therefore no longer necessary to adjust the limit switches.

However, sensitive regulation is not possible here either, because the plunger must first touch the workpiece with full force before the computer receives the information from the position measuring system that the working stroke can begin.

The invention is based on the object of developing a piston press of the type mentioned in such a way that the ram can be deflected precisely in a positionally controlled manner.

This object is achieved in that the electrically actuated valves are electropneumatic servo valves and that the path / time curve is divided into discrete areas of different gradients, the plunger being moved from its rest position to an end position with gradually decreasing speed.

The object on which the invention is based is completely achieved in this way because the pressing process can be adapted to a wide variety of pressing tasks by means of the controllable path / time profile. In particular, particularly sensitive pressing tasks can be solved, and the paths to be measured and the speeds at which the paths are measured can be adapted to the respective pressing task.

There is the further advantage that initially with a relatively high travel speed, i.e. in rapid motion, the workpiece is approached, then the majority of the pressing task is carried out in one working stroke and finally the fine adjustment for reaching the predetermined end position can be carried out in one positioning stroke.

In a preferred development of this variant, the controller is a state controller with adjustable controller parameters, which forms control signals as a function of the travel signal, a speed signal and an acceleration signal, and the controller parameters can be switched over automatically when changing from a first area to a second area.

This measure has the advantage that the control can be optimally controlled in the different areas of different gradients of the path / time curve, because the parameters of the controller that determine the control process can be adapted to the respective gradients and thus the respective workpiece.

In a further embodiment of the invention, a pressure sensor detects the pressure acting on the cylinder or the pressing force exerted by the ram, compares these values with a predetermined target value range and actuates a display element if the range is not complied with.

These measures have the particular advantage that it can also be checked whether a certain pressure or a certain pressing force has been used to measure the predetermined path, in particular to achieve the predetermined end position of the ram, or whether this pressure or this pressing force has a certain tolerance range -or was below. In this way, conclusions can be drawn about the gelled workpieces. If, for example, the specified stroke of the ram was measured with too little pressure or too little pressing force, this can indicate that a workpiece to be deformed consisted of a material that was too soft or that the raw dimensions of the workpiece were too loose exhibited, while when the predetermined pressure or. Press force range was either too hard a workpiece or the fit of the workpieces to be pressed together was too narrow. In both cases, since there may be a workpiece that is not suitable for the specific application, a distinction can be made between a quality workpiece and a reject workpiece.

Finally, an embodiment of the invention is particularly preferred in which the servo valves are 3/2-servo valves with pilot valve. This combination of a directional servo valve with a pressure servo valve enables both sensitive pressure control and exact positioning of the press ram.

Further advantages result from the description and the attached drawing.

It goes without saying that the features described above and those yet to be explained below can be used not only in the combination specified in each case, but also in other combinations or on their own without departing from the scope of the present invention.

Fig. 1

eine schematisierte Gesamtansicht einer erfindungsgemäßen Kolbenpresse mit zugehöriger elektronischer und pneumatischer Steuerung;

Fig. 2

ein Weg/Zeit-Diagramm zur Erläuterung des geregelten Verfahrens des Pressenstößels bei der Presse gemäß Fig. 1;

Fig. 3

eine Kennlinie eines erfindungsgemäß verwendeten Komparators zum Erkennen von Ausschuß.

An embodiment of the invention is shown in the drawing and is explained in more detail in the following description. Show it:

Fig. 1

a schematic overall view of a piston press according to the invention with associated electronic and pneumatic control;

Fig. 2

a path / time diagram for explaining the controlled process of the press ram in the press of FIG. 1;

Fig. 3

a characteristic curve of a comparator used in accordance with the invention for detecting rejects.

In Fig. 1, 10 designates an embodiment of a piston press according to the invention. A machine table 11 stands on a solid surface. A workpiece 12 can be seen on the machine table 11, which is to be pressed, for example, with a bolt 13. A stand 14, which holds a cylinder 15 laterally, extends upward from the machine table 11. A piston 16 runs in the cylinder 15 and is firmly connected to a tappet 17.

The plunger 17 is connected to a position measuring system 18 at a suitable point. In the illustrated embodiment a rod 19 extends from the upper end face of the plunger 17 and carries an inductive element 20. The element 20 interacts with a coil 21. It goes without saying, however, that instead of an inductive displacement measuring system 18, any other suitable displacement measuring system, for example a capacitive or an optical or some other displacement measuring system, can be used.

am Steuergerät 34 an.From the measuring system 18, an electrical line 30 leads once directly, once via a differentiating stage 31 and again via a double differentiating stage 33 to a control unit 34. If the path signal detected by the measuring system 18 is denoted by s, then the signals s, ṡ and

on the control unit 34.

Setpoints s₀, s₁, s₂, s₃ ... of the travel path 17 of the plunger 17 can be specified to the control unit 34 via a first keyboard 35.

Additional setpoints can be entered into the control unit 34 via a second keyboard 36, for example an upper and a lower limit value p₀, p _{u of} an operating pressure p of the piston press 10.

The control device 34 comprises a controller 37, which is preferably a state controller of a type known per se. The controller characteristic can be defined in a manner known per se via controller parameters. In the exemplary embodiment shown in FIG. 1, the controller 37 is provided with adjustable controller parameters, as will be explained further below.

Furthermore, the control unit 34 comprises, in addition to comparators (not shown) for comparing the actual path s with the target values s₀, s₁ ... a comparator 38 for comparing pressure values p, as will be explained further below.

Above the piston 16, an upper space 40 extends in the cylinder 15. This is connected with a pneumatic line 41 to an electropneumatic 3/2-servo valve which is internally provided with a pilot valve, as described in detail for example in DIN ISO 1219 under 7.2 .

A lower space 43 of the cylinder 15 below the piston 16 is connected to a further electropneumatic 3/2 servo valve 45 by a pneumatic line 44. The servo valves 42, 45 are controlled separately by the controller 37.

A pressure sensor 50 detects, for example, the differential pressure between the spaces 40 and 43 and supplies a corresponding signal p to the comparator 38 in the control unit 34, the further input of which is connected to the limit values p₀, p _u specified by the second keyboard 36. Alternatively or additionally, a force sensor 70, for example a piezoelectric sensor, can be arranged on the plunger 17 in order to detect the pressing force F which the plunger 17 exerts on the workpiece 12/13. In another operating mode, limit values F _o , F _u can then be specified using the second keyboard 36.

The output of the comparator 38 controls a display element 51.

The operation of the piston press 10 according to FIG. 1 is as follows:
The user of the piston press 10 first enters 35 paths using the first keyboard, depending on the workpiece 12, 13 to be machined, s₀ being the rest position of the ram 17 when the piston 16 has assumed its upper end position in FIG. 1. Between the value s₀ and a further value s₁, the plunger 17 measures the empty space until it is placed on the workpiece 12, 13. From the value s₁ to a value s₂, there is now a working stroke in which the essential pressing work, for example the pressing in of the bolt 13 into the workpiece 12. Finally, there is a small way from the value s₂ to a further value s₃, in which, for example, the bolt 13 is positioned exactly on the final value. The user also specifies travel speeds for the plunger 17 in the various stroke ranges.

The user also uses the second keyboard 36 to enter a pressure range between a lower value p _u and an upper value p _o or a pressing force range F _o - F _u , it being checked whether the pressure p or the pressure required to deflect the plunger 17 the applied pressing force F lies within this range of values p _o - p _u or F _o - F _u .

zugeführt, und der Regler 37 erzeugt in Abhängigkeit von vorgegebenen Reglerparametern, die aus den vorgegebenen Werten s_o, s₁, s₂ und s₃ und den Verfahrgeschwindigkeiten bestimmt wurden, das analoge Steuersignal für die Servoventile 42 und 45.The piston press 10 is now switched on by means of a switch (not shown in FIG. 1). The first servo valve 42 is switched from the position shown in FIG. 1 so that compressed air flows from a reservoir via the pneumatic line 41 into the upper space 40. The piston 16 now moves downward, the instantaneous position of the piston 16 or the plunger 17 being detected by means of the displacement measuring system 18 and readjusted if necessary by actuating the valves 42, 45 in accordance with the specified displacement / time dependence. The As a result, controller 37 becomes a displacement signal s, a speed signal ṡ and an acceleration signal

fed, and the controller 37 generates depending on predetermined controller parameters, which were determined from the predetermined values s _o , s₁, s₂ and s₃ and the travel speeds, the analog control signal for the servo valves 42 and 45.

The associated path / time diagram is plotted in FIG. 2. The piston press 10 was turned on at time t₀ and up to time t₁ the idle stroke between the upper starting position s₀ and touchdown s₁ measured on the bolt 13. As can be seen from the relatively steep course in the interval t₀ to t₁, this idle stroke happens with a relatively high travel speed. At time t 1, the measuring system 18 recognizes that the predetermined value s 1 has been reached. The controller parameters are now switched in the controller 37, as can be seen from the short overshoot in FIG. 2. By appropriate actuation of the servo valves 42 and 45, the piston 16 with the plunger 17 continues its path somewhat more slowly until the working stroke has ended t 2 and the bolt 13 is almost in its end position. At the time t₂ it is recognized that the next predetermined limit value s₂ has been reached, and the controller 37 is switched again in its controller parameters, so that the end position of the bolt 13 can now be approached relatively slowly in a positioning stroke, until finally the final value s₃ at the time t₃ was reached.

A short holding phase then preferably follows until the piston 16 returns to its upper starting position by corresponding actuation of the servo valves 42, 45.

During the working stroke and the positioning stroke, ie between the times t 1 and t 3, the control unit 34 has compared the pressure applied in each case with the pressure interval p _u - p _o by means of the comparator 38. The corresponding characteristic of the comparator 38 is shown in FIG. 3. It can be seen that a quality signal Q is derived which assumes the value 1 in the interval p _o - p _u and the value 0 outside this interval. A display element 51 is therefore only (or with appropriate logic only then not) driven if a pressure had to be applied to measure the path up to the value s 3, which was in the interval p _o - p _u . The same applies in the case of the monitoring of the pressing force if it is determined whether the pressing force F is in the interval F _u - F _o .

In a practical exemplary embodiment of the piston press 10 according to the invention, 37 gain factors between 100 and 1000 were set in the controller. This resulted in a press-in accuracy of the path s of ± 0.01 mm. With a time constant of the drive system between 10 and 50 ms, a movement speed was in the positioning phase to achieve this press-in accuracy, i.e. between t₂ and t₃ about 1 mm / s required.

Claims (5)

1. Piston press (10) having a plunger (17) which is connected to a piston (16) running in a cylinder (15), the cylinder (15) being loadable with compressed air via electrically actuable valves (42, 45), the plunger (17) cooperating with a path-measuring system (18), an output signal (s) of the path-measuring system (18) being compared with predetermined desired values (S₀, S₁, S₂, S₃) in a controller (37), and the controller activating the valves (42, 45) for the pressure loading of the cylinder (15) according to a predetermined path/time curve (S_(t)), characterised in that the electrically actuable valves (42, 45) are electropneumatic servovalves (42, 45), and in that the path/time curve (S_(t)) is divided into discrete regions of differing gradient, the plunger (17) being moved out of a position of rest (S₀) at stepwise decreasing speed into an end position (S₃).
2. Piston press according to Claim 1, characterised in that the controller (37) is a state controller with adjustable controller parameters which forms regulating signals in dependence on the path signal (s), a speed signal (s) and an acceleration signal (s), and in that the controller parameters can be changed over automatically at the transition from a first region to a second region.
3. Piston press according to Claim 1 or 2, characterised in that a pressure sensor (50) detects the pressure (p) acting on the cylinder (15), compares it with a predetermined desired-value range (p₀ - p_u) and, if the range (p₀ - p_u) is not adhered to, actuates an indicator element (51).
4. Piston press according to Claim 1 or 2, characterised in that a force sensor (70) detects the pressing force (F) exerted by the plunger (17), compares it with a predetermined desired-value range (F₀ - F_u) and, if the range (F₀ - F_u) is not adhered to, actuates an indicator element (51).
5. Piston press according to one of Claims 1 to 4, characterised in that the servovalves are 3/2-way servovalves (42, 45) with a pilot valve.

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