EP0003745A1 - Apparatus for suppressing cutting shocks in hydraulic presses - Google Patents

Abstract

In hydraulic presses, a reciprocating piston (2) carrying a cutting tool (4a, 4b) is driven by a press cylinder working on both sides. The press cylinder is equipped with a servo valve control to control the reciprocating piston movement. A damping device is provided to dampen the so-called cutting impact when the tool passes through the workpiece. It has a displacement sensor (7) which scans the stroke adjustment range. The output signal of the displacement sensor is given as an actual variable to an electrical control circuit (6); A signal corresponding to a predetermined continuous function is supplied to the control circuit (6) as the setpoint during the cutting movement of the tool. As a result, the tool follows a predetermined path-time function and the workpiece is passed through at a relatively low speed.

Description

The invention relates to a device for damping the cutting impact in hydraulic presses according to the preamble of claim 1.

Hydraulic presses are used, among other things, to punch workpieces, for example sheet metal. Step through the material as the tool moves relatively high forces between the punch and the die of the tool, which lead to very considerable deformation energies in the workpiece and the press, in particular in the hydraulic oil of the press cylinder of the press.

These deformation energies are released abruptly when the tool passes through the workpiece by accelerating the punch like a smell, which results in an unpleasant noise and a heavy load on the press. To dampen the so-called cutting impact, it is already known to provide shock-absorbing elements, for example damping cylinders, on the stationary part of the press, which absorb the olfactory movement of the hydraulic piston after the tool has passed through the workpiece. It is also known to provide such damping elements on the tool itself. However, the use of such damping elements is associated with considerable difficulties since the point of engagement is difficult to set, which is particularly the case when several such damping elements are used on the press table distributed around the tool. Damping cylinders attached to the tool also hinder the accessibility of the workpiece. If the damping elements are attached directly to the tool, they have to be very small and are therefore expensive. In addition, they have to be adapted to the tool each time the tool is changed.

The present invention has for its object to provide a device of the type mentioned, which is easy to use and leads to an improved shock absorption.

This object is achieved by the characterizing features of claim 1.

The present invention is based on the knowledge that most materials, in particular metal, change with increasing load and the beginning of deformation into a state in which the further material deformation takes place at a significantly lower load. When using the specified control loop, in which the adjustment path of the tool is controlled using a continuously increasing setpoint, this material characteristic is taken into account in that when the material gets into the easily deformable state, the force exerted on the tool is automatically reduced becomes. As a result, the energy stored in the press, the tool, the workpiece and the oil in the hydraulic system is already reduced while the tool is still in the workpiece. In known presses, the initially required force is maintained until the tool exits the workpiece. When the material then gets into its flowable state and requires less force for further movement, the excess force is converted into movement energy, which accelerates the stamp of the press, which contributes significantly to the cutting impact that occurs anyway due to the relaxation of the compressed parts. In contrast, the control used here guides the tool through the workpiece according to a predetermined travel-time function and leaves it at a relatively low speed. It is essential that the force exerted on the tool at the end of the punching movement is automatically significantly reduced compared to the force initially to be applied. If, in this control, the currently set target value for the tool's travel is exceeded for a short time because the force exerted on the tool is too great for the specified cutting movement of the tool, the control starts and reduces the hydraulic pressure and thus the force on the tool until actual and target value match again. After relatively small forces are sufficient towards the end of the cutting movement to bring the cutting movement to an end at an approximately constant movement speed of the tool, the pressure in the hydraulics is already reduced during the cutting movement by the control system, so that the elastic energies initially built up in the press and the hydraulics are almost completely degraded when the tool exits the workpiece. These energies are therefore used here for the punching process and do not need to be dampened after the tool has passed through the workpiece, as is the case with known presses with cut impact damping. The cut that occurs with conventional presses does not occur here at all.

In claim 2, an embodiment is specified which leads to a very simple control loop and in which the predetermined function for the setpoint is generated by superimposing a constant basic level and a signal which changes over time.

Depending on the application, it can be expedient to generate a linear or a logarithmically increasing output variable in the function generator. With a logarithmically increasing output variable, the tool is moved at a greater speed at the beginning of the cutting movement than at the end of the cutting movement, which leads to the tool exiting the workpiece extremely slowly, as a result of which extremely strong noise damping is achieved.

The use of a servo valve specified in claim 4 has led to very good results in connection with the present regulation. Although the structure of the regulation is relatively uncritical because Usually the movement of the tool through the workpiece does not take place extremely quickly, since it is expedient to use a servo valve with the highest possible sensitivity.

• Fig. 1 eine schematische Darstellung einer üblichen C-Presse mit einem beidseitig arbeitenden Preßzylinder und einem daran angeschlossenen Regelkreis;
• Fig. 2 ein Blockschaltbild des verwendeten Regelkreises;
• Fig. 3 ein Schaltbeispiel des Regelkreises nach Fig. 2 und
• Fig. 4 ein Weg-Zeit-Diagramm eines typischen Bewegungsablaufs des Werkzeugs der Presse.

Based on an embodiment of the invention shown in the drawing, this is explained in more detail below. Show it:

• Figure 1 is a schematic representation of a conventional C-press with a bilaterally operating press cylinder and a control loop connected to it.
• Fig. 2 is a block diagram of the control loop used;
• Fig. 3 is a circuit example of the control circuit according to Fig. 2 and
• Fig. 4 is a path-time diagram of a typical movement of the tool of the press.

Fig. 1 shows a typical press as it is used for punching materials, for example sheet metal. It has a press frame 1, on the C-shaped arm of which a reciprocating piston 2 is movably mounted in a cylinder acting on both sides in the vertical direction. On the lower piston rod 3 of the piston, a punch 4a of a tool is attached, the associated die 4b of which rests on the lower part of the frame 1. A quantity servo valve 5, which is controlled by control electronics 6, is connected to the two chambers of the reciprocating piston 2. A displacement sensor 7 is arranged on the lower part of the frame 1 of the press, the output signal of which is fed to the control electronics 6. Reference number 8 is a setpoint generator referred to, the output signal is also supplied to the control electronics 6.

In Fig. 2, the entire control loop, which also includes the press itself, is shown in more detail in a block diagram. The controlled system of this control circuit contains a position control amplifier 6a, the quantity servo valve 5 and the press cylinder 2. In the feedback branch there is the displacement sensor 7, the output signal of which is fed to a summation point 9 with a negative sign. The output signal of the setpoint generator 8 and the output signal of the setpoint generator 8 and the output signal of a function generator 10 are also fed to this summation point. The input of this function generator 10 is connected to the displacement sensor 7. The reference numeral 11 designates all those parts of the control circuit which supply disturbance variables, for example the workpiece which bends during the stamping process and the parts of the press which deform. These variables are effective in the control circuit in front of the reciprocating piston 2. The disturbance variable block 11 is therefore coupled to the input of the reciprocating piston 2. This disturbance variable block is irrelevant for understanding the control loop.

The function of the control loop shown in FIG. 2 is explained in more detail below. It is assumed that there is a die for punching the workpiece 12 on the die 4b of the press 1. The punch 4a of the tool is in an initial position, as shown in FIG. 1. From this starting position, the plunger 4a is moved down relatively quickly in that hydraulic oil is pumped into the upper chamber of the reciprocating piston 2 via the pump connected to the hydraulics 5. This is done in the usual way. If the punch 4a just touches the workpiece 12, or a short distance in front of it, the displacement sensor 7 is replaced by a suitable one actuated part connected to the piston. The further movement of the piston now takes place via the control loop, as shown in FIG. 2. The now following movement of the plunger 4a caused by the control loop lies in the time diagram according to FIG. 4 between the times t _a and t _b . The setpoint generator 8 emits a constant basic output signal. When the displacement transmitter 7 is actuated by the reciprocating piston 2, the function generator 10 is triggered, which now emits a signal which rises in time and which is superimposed on the signal from the setpoint transmitter 8 at the summation point 9. This control loop can be viewed in such a way that it is controlled by a setpoint, which is composed of the sum signal from the signals from the setpoint generator 8 and the function generator 10. The actual actual size is determined on the displacement sensor 7 and fed to the summation point 9 with a negative sign. Since the signal from the function generator 10 rises in time, the actual size, that is to say the adjustment of the stamp 4a, is continuously adjusted to the desired size. This is done in that the servo valve 5 receives an electrical output signal via the position control amplifier 6a, which causes the pump connected to the servo valve 5 to pump hydraulic oil into the upper chamber of the reciprocating piston 2. Electrically controlled servo valves which are suitable for this purpose are known and need not be explained in more detail here. If the plunger 4a touches the workpiece 12, the control circuit has the effect that the hydraulic pressure in the upper chamber of the reciprocating piston 2 is increased considerably, so that the actual displacement path of the piston is tracked to the predetermined desired value. During the movement of the punch 4a through the workpiece 12, a condition occurs in which the further movement of the punch 4a through the workpiece can take place with less pressure than was previously necessary because the workpiece has reached its "yield point". The actual value will briefly become somewhat larger than the previous one Given setpoint, which is immediately corrected again by the control circuit by actuating the servo valve 5 so that the pressure acting on the reciprocating piston 2 is reduced. Depending on the control characteristic of this control loop, the adjustment path of the plunger 4a, that is to say the actual size of the control loop, will more or less rigidly follow the predetermined, constantly increasing target value. At the end of the path of the punch 4a through the workpiece 12, a state will be reached in which there is only a relatively low pressure in the upper chamber of the reciprocating piston 2, so that the forces acting on the press arm, the workpiece 12 and the hydraulic oil accordingly are low. The stamp 4a will therefore emerge from the workpiece 12 practically without a cut. It is expedient to start regulating the movement of the punch 4a before the punch 4a has reached the workpiece 12 and to maintain it somewhat after the punch 4a has left the workpiece 12 in order to compensate for any movement tolerances. In Fig. 4, the thickness of the connecting piece 12 is indicated with the horizontally running broken lines. The end of the control, like the onset of the control, can be triggered again by the solenoid 7, which overrides the control in a predetermined end position and reconnects the servo valve to the usual servo circuit via a circuit, not shown here ensures a rapid return movement of the plunger 4a by pressurizing the lower chamber of the reciprocating piston 2. This ead movement is shown in FIG. 4 after the time t _b .

In principle, the function generator 10 only has to deliver a steadily increasing output signal which is superimposed on the constant signal of the setpoint generator 8. For example, a linearly increasing output signal can lead to very useful results. The exit area Speed of the stamp 4a from the workpiece 12 can be further reduced by generating a logarithmic signal instead of a linearly increasing signal in the function generator 10, which increases significantly more at the time t _a at the beginning of the control than at the time t _b at the end of the control . The time period between the times t _a and t _b is advantageously chosen to be 1 to 2 seconds.

The cheapest setting of the setpoint generator 8 can easily be found in practice. First, a value is set at which the punch 4a breaks through the workpiece 12, which naturally results in a cut blow. In the subsequent punching operations, the setpoint is gradually reduced until the press works without a cut.

FIG. 3 shows a practical exemplary embodiment of the control loop, as is shown schematically in FIG. 2. A potentiometer is used as setpoint generator 8, which is connected to a fixed voltage with its end connections. The constant basic setpoint, which is supplied to the summation point 9 in FIG. 2, is taken off via the center tap of this potentiometer. The summation point 9 shown in FIG. 2 is formed by the negative connection of an operational amplifier 13, which is contained in the position control amplifier 6a. The actual signal from the actual value transmitter 7 is also fed to this negative connection. The output of this operational amplifier 13 is connected to the electrically controllable servo valve 5, of which only the control coil is shown in FIG. 3. The function generator 10 contains a capacitor 14, which acts as a charging capacitor and supplies a linearly increasing voltage at the output of the function generator 10. A constant voltage of a correction potentiometer 15 is superimposed on the signal from the actual value transmitter 7, as a result of which the starting point t _a the regulation can be set. It is within the scope of the skilled person to modify the function generator so that, for example, it delivers a logarithmic function such as that shown in broken lines as a second alternative between the times t _a and t _b in FIG. 4.

A displacement potentiometer, the end connections of which are at a constant voltage, can be used, for example, as the displacement sensor 7. Other conventional sensing elements can also be used which are suitable for reproducing the position of a mechanical element, here the die 4b or the piston rod 3, in an analog form. For example, photoelectric sensing elements also appear to be very suitable here.

The specified regulation has the advantage that it can also be used to easily retrofit presses that have not yet had a cut impact damping. The control only needs to be connected to the already existing hydraulic system in such presses with double-acting pulling winches, without any structural modifications being necessary.

Claims (4)

1.Device for damping the cutting impact in hydraulic presses, with a reciprocating piston driven by a press cylinder working on both sides and carrying the cutting tool and a servo valve control connected to it, characterized by a displacement sensor (7) scanning the reciprocating piston displacement path and an electrical control circuit (6) for the movement of Reciprocating piston (2), in which the reciprocating piston adjustment path is used as the actual variable and the target value is increased according to a predetermined continuous function during the cutting assignment of the tool (4a, 4b). 2. Device according to claim 1, characterized in that a function generator (10) is provided for increasing the setpoint according to the predetermined function, the output signal of which is added to a constant setpoint signal. 3. Device according to claim 1 or 2, characterized in that the function generator (10) generates a linear or a logarithmically increasing output signal. 4. Device according to one of claims 1 to 3, characterized by an electrically controllable servo valve 5 with a time constant for the output flow rate 0% to 90% at 100%. Step input of the control current is less than 20 msec., Preferably less than 10 msec.

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