GB1581793A - Damping control device - Google Patents

Abstract

In the ram (10) which is actuable by the operating cylinder (11), there are arranged counter-pressure cylinders (13, 14), pressure spaces (17, 18) of which are connected to a pressure cell (33) and a proportional control valve (28). The pressure cell (33) which supplies an electrical actual value signal is connected via two resistors (34, 36) to a desired value transmitter (37). The signal indicating the difference between the desired and actual values is fed to an amplifier (40), the output signal of which triggers the exciting winding (29) of the proportional control valve (28). The desired value transmitter (37) is set to a pressure which corresponds to the force required to extend the piston rods (19, 20) of the counter-pressure cylinders (13, 14) during the return stroke of the ram (10). An integrating circuit composed of a resistor (42) and an adjustable capacitor (43) is designed in such a way that the time taken by a slow change in pressure is not affected by the control circuit (33, 34, 36, 40, 29, 28) while a rapid change is delayed. A dampening device of this type is not dependent on the viscosity of the pressure liquid, can be easily adjusted and is suitable for presses which are driven in different ways. <IMAGE>

Description

(54) IMPROVEMENTS IN OR RELATING TO A DAMPING CONTROL DEVICE (71) We, FRIESEKE & HOEPFNER GmbH, a body Corporate of the Federal Republic of Germany of Postfach 16 60, P 8520 Erlengen, Federal Republic of Germany, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The invention relates to a damping control device, and more particularly to a cutting impact damping control device for a punching press. As used herein, the term "cutting impact" means the relatively sudden movement of the ram that occurs when a tool breaks through a workpiece.

It has been proposed to provide a punching press with a device comprising a hydraulic piston and cylinder disposed between the ram and the press table and having means which restrict the discharge of pressure fluid from the cylinder and therefore cause delayed relaxation of mechanical strains in the press frame at the moment at which the tool breaks through the workpiece.

A press provided with such a damping device is disclosed in the Swiss Patent Specification 554 237. Delayed relaxation of the press frame as disclosed in this specification, is achieved by an adjustable flow restrictor through which the hydraulic fluid must discharge after the punching tool has broken through the workpiece. The restrictor is-brought into operation by actuation of a valve. The valve is controlled by a control cam, for example a cam lobe, which is mounted on the crankshaft of the press.

In practice, this kind of cutting impact damping device suffers from various die advantages. The action of the flow restrictor depends to a large extent on the viscosity of the working fluid.

Thus, immediately after starting the press the action of the cutting impact damping system will be completely different from that after a certain amount of time has elapsed during which the hydraulic fluid has become heated due to the flow resistance offered by the restrictor. Furthermore, restrictors of this kind do not operate without delay; a specific flow velocity must first be achieved before the restrictor can develop its full action. The sudden relaxation of the press frame and its drive resulting from the breakthrough of the tool is thus not immediately counteracted, but is only counteracted after a delay. In workpieces of different thickness, the breakthrough takes place at different points along the travel of the upper tool or punch. Accordingly, the time at which the flow restrictor is brought into action by actuation of the valve must be adjusted in dependence on the workpiece thickness. i.e. the control cam on the crankshaft of the press must be appropriately adjusted. Due to the operation of the valve by the control cam, it follows that all inaccuracies in the press drive for example those due to wear of the power-transmitting parts, influence the time at which the flow restrictor is set into operation. Since the drive mechanism of the press is utilised for controlling the valve, it follows that the known method of damping the cutting impact can be used only for presses which are driven by crankshafts. Moreover, the system can be subsequently fitted to existing presses only with great difficulty.

It is the object of the invention to provide a cutting impact damping control system generally of the kind described hereinbefore which operates completely independently of the viscosity of the working fluid, comes into operation substantially without delay by virtue of its operating principle, requires no adjustment, can be fitted subsequently to existing presses with virtually any kind of drive and any kind of relaxation properties, can be adjusted in the simplest possible manner with respect to the nature of the relaxation of the mechanical strains in the press, operates purely by hydraulic-electronic means and is moreover characterised by exceptionally low expenditure in terms of material and installation.

According to this invention there is provided a damping control device suitable for use with a punching press which press comprises a piston and cylinder disposed operatively between a ram and a press table, the damping control device comprising a pressure regulating arrangement adapted to be connected to a cylinder chamber of said piston and cylinder, said pressure regulating arrangement comprising a pressuresensitive load cell which is adapted to be connected in fluid communication with the cylinder chamber and is adapted to provide an electric signal representative of the pressure of working fluid within the cylinder chamber; a means for generating an electric signal representative of a set point, a set point-measured fluid pressure value signal comparator which is connected to receive the signals generated in use by the pressure load cell and by the set point signal generating means, an amplifier connected to receive in use signals from the said set point-measured value comparator, and a proportional regulating valve which is arranged to be controlled by signals from the amplifier in use and which is adapted to be hydraulically connected between the cylinder chamber on the one hand and a pressure fluid supply source and an oil reservoir on the other hand, the set point signal generating means being adjustable to provide a set point signal representative of a fluid pressure which corresponds to the pressure required to extend the piston of the cylinder for a return stroke thereof so that a pre-determined pressure is substantially maintained within said cylinder chamber, the pressure regulating arrangement either being provided additionally with an electrical integrating element or being arranged so that parts thereof constitute a mechanical integrating element, which integrating element is so disposed and which has such a time constant that slowly changing signals delivered by the pressure-sensitive load cell cause virtually immediate operation of said regulating valve but rapidly changing signals of the pressure load cell resulting from the break-through of a tool cause operation of said regulating valve with a delav defined by the integrating element therefore causing delayed relaxation of mechanical strain in a press frame forming part of said press at the moment at which the tool breaks through the workpiece.

Preferably the integrating element is adjustable to regulate said delay and preferably comprises adjustable electronic components.

In a preferred embodiment a part of the proportional regulating valve, which has been selected in terms of its operating rate from a range of commercially available valves constitutes the integrating element.

Thus the selected valve will have sufficient inertia to provide the integrating effect that is to say the inertia will be such that the valve will only react relatively slowly in response to input signals.

ln order that the invention may be more readily understood and so that further preferred features may be appreciated, a preferred embodiment of the invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 is a diagrammatic view together with a block circuit diagram illustrating a hydraulically driven press with a pressure regulating circuit, Figure 2 is a graphical figure which shows the change of pressure in the cylinder chamber with respect to time. As used herein, the term "cylinder chamber" means the portion of the space within the cylinder which is in fluid communication with the damping control device, Figure 3 is a graphical figure which shows the amount of opening of the proportional regulating valve with respect to time, and Figure 4 is a graphical figure which shows the distortion of the press frame with respect to time during a working stroke.

Referring to Figure 1, a hydraulically driven press is provided with a press frame comprising a press base 1, two columns 2 and 3 and a top cross-member 4. A press table 5 is situated between the columns 2 and 3. The said press table supports a bottom die 6 and two conventional guide rods 7 and 8. A workpiece 9 which is to be blanked out is located on the bottom die 6.

A ram 10 is provided which is movable in the upward and downward direction within the space defined by the columns 2 and 3 on guide rods in a manner not shown. The motion of the ram is produced by a hydraulic working jack 11 having a piston rod 12, one end of which is mounted in the ram 10.

Two pistons 15 and 16, lying in cylinders 13 and 14 and thus forming cylinder chambers 17 and 18, are installed in or integrally formed in the ram 10. The piston rods 19 and 2() of the above-mentioned pistons terminate in openings 21, 22. These openings are in alignment with the guide rods 7 and 8 in the press table 5. The cylinder chambers 17 and 18 are connected parallel with each other and are connected to a hydraulic fluid supply duct 23. The lowermost surface of the ram 10 is provided with an upper tool or punch 24.

The damping device includes a pump 25 adapted to draw hydraulic fluid from a tank 26. The pump 25 is connected via a nonreturn valve 27 and a proportional regulating valve 28 to the duct 23 which leads to the cylinder chambers 17 and 18. A central hydraulic pipeline system can of course also be provided in place of the pump 25 and the tank 26. The proportional regulating valve 28 is actuated in response to excitation of a solenoid 29 and has the limiting position in which the valve elements 30 and 31 are respectively in the operative position. Depending on the polarity and intensity of the current applied to the solenoid 29, the valve can assume any intermediate position or it can also be completely closed. A conventional pressure-limiting valve 32 is also connected to the output of the pump 25, and valve 32 is set to a relatively low value, for example 15 bar. The purpose of the valve 32 is to conduct the oil delivered by the pump 25 into the tank 26 if the proportional regulating valve 28 is closed causing the output pressure of the pump to rise.

A pressure load cell 33 is connected to the duct 23 to sense the pressure of hydraulic fluid within the duct 23 and to provide an output electrical signal representative of the detected pressure. It will be appreciated that since duct 23 is in fluid communication with the chambers 17, 18. the pressure within that duct will be equal to the pressure within the chambers 17. 18. The output of the load cell is connected via a resistor 34 to a terminal 35. The terminal 35 is also connected to a resistor 36, which in turn is connected to the wiper of a potentiometer 37. One end of the resistive winding of the said potentiometer is connected via a terminal 38 to a negative potential, and the other end of the resistive winding is connected to ground.

As will be appreciated the operational amplifier 40 functions as a simple amplifier.

The output of the amplifier is connected to the solenoid 29 which controls the proportional regulating valve 28. A feedback element. comprising a resistor 42 and an adjustable capacitor 43, is connected between the output of the amplifier 40 and the input 39.

The pressure load cell 33. the resistors 34 and 36. the amplifier 40 and the proportional regulating valve 28 form a control circuit for the pressure in the line 23. or in other words for the pressure in the chambers 17 and 18.

A "set point" signal may be entered into the said control circuit by adjusting the potentiometer 37 thus causing a predetermined potential. representative of the "set point" signal to be supplied to terminal 35. A signal representative of the measured value of pressure in the line 23 is supplied by the pressure load cell 33 to the terminal 35. The terminal 35 therefore represents a set pointmeasured, fluid pressure value signal comparator. The capacitor 43 together with the resistor 42 is the integrating element in the above-mentioned control circuit.

The numeral 44 in Figures 2, 3 and 4 refers to the point at which the tool breaks through the workpiece. These Figures will be explained in the course of the description of the function of the exemplified embodiment illustrated in Figure 1.

For the purposes of explanation it is assumed that the ram 10 is initially in its top position and the pistons 15 and 16 are also in their top position, and thus the apparatus is in the condition illustrated in Figure 1. The pump 25 is started by a control device, not shown, simultaneously with actuation of the press cylinder. The cylinder chambers 17 and 18 are filled with hydraulic fluid via the non-return valve 27 and the proportional regulating valve 28. Accordingly, the piston rods 19 and 20 of the said pistons travel in the downward direction.

The pressure which is then set in the duct 23 is maintained by the regulating circuit or damping device at a value which is just sufficient to overcome the frictional resistances between the cylinders 13 and 14 and the pistons 15 and 16, and in the remaining pipeline system. To this end. the pressure in the duct 23 is measured by the pressure load cell 33 which provides a corresponding voltage which is supplied via the resistor 34 to the set point-measured, fluid pressure value signal comparator. namely the point 35, to which a voltage representative of the set point, adjusted by the potentiometer 37, is fed via the resistor 36. The pressure regulated in the duct 23- and in the cylinder chambers 17 and 18 corresponds to the set point voltage which is adjusted by adjusting the potentiometer. This pressure can be. for example. 10 bar. This is indicated by the broken line 45 in Figure 2. In this drawing, the voltage Up generated by the pressure load cell 33 and proportional to the pressure in the duct 23 is plotted against the time.

As the downward motion of the ram 10 continues the guide rods 7 and 8 strike against the piston rods 19 and 20. Hydraulic fluid is therefore discharged from the cylinder chambers 17 and 18. The said oil is discharged through the valve 28, which has the element 30 in the appropriate position, into the tank 26. The control circuit described above ensures that the pressure in the chambers 17 and 18 maintains the low value specified by the potentiometer 37, for example the above-mentioned 10 bar, as illustrated bv the line 45 in Figure 2. As soon as the top tool 24 has broken through the workpiece 9. the entire force of thepress and more particularly the mechanical stress produced by the distortion of the press frame resulting from the working stroke, acts suddenly on the cylinders 13 and 14.

Accordingly, an enormous pressure is suddenly produced in chambers 17 and 18, as indicated in Figure 2 by the steeply rising line 46. This pressure can have a maximum value of, for example, 350 bar.

The voltage delivered by the pressure load cell 33 at this pressure would normally suddenly open the valve 28 to its full extent over the entire control surface. However, this is not the case in this embodiment, because of the presence of the integrating element 42, 43 in the regulating circuit.

Owing to the time constant which is thus provided by the control circuit, the valve 28 is first opened relatively slowly in accordance with an exponential function. The pressure in the cylinder chambers 17, 18 therefore diminishes slowly as illustrated by the line 47 in Figure 2. The amount of opening M of the valve 28 with respect to time t is plotted in Figure 3. The line 48 shows the percentage amount of opening of the said valve until the tool 24 breaks through the workpiece 9. As can be seen, opening of the valve is exceptionally small at the point 44 at which the breakthrough occurs and the valve opening increases at first gradually in accordance with the above mentioned exponential function. This illustrated by the curve 49.

The distortion w of the press frame relative to the non-stressed state over the working stroke of the press is shown in Figure 4. The gradually rising line 50 represents the press frame flexure until breakthrough occurs at the point 44. An exceptionally short and hard relaxation in accordance with the curve 51 would occur immediately after breakthrough in the absence of a damping device such as that which forms the subject of the present invention, but the amplitude of such curve in actual fact would be far greater than shown. The time during which this occurs is very short and would amount to approximately 1 to 3 milliseconds. In the interests of clarity, the decaying oscillation of the relaxation is shown greatly expanded in Figure 4. The use of a damping system in accordance with the invention ensures that such relaxation decays slowly in accordance with an exponential function in the manner described above.

This is indicated by the line 52.

As is evident from the description hereinabove of the exemplified embodiment, the cutting impact is damped in accordance with the invention by purely hydraulic-electronic means. No connection is required between the device and any part of the press drive or the press frame.

Accordingly, a damping control system in accordance with the invention can readily be incorporated subsequently on existing presses. To this end, it is merely necessary to provide the ram 10 with the cylinders 13, 14 and pistons 15, 16 or to replace the original ram with one which is so equipped; all other parts are independent of the blanking press in use. For the above-mentioned reasons, the invention can also be employed for presses of all kinds, for example in the types of press driven by means of a crankshaft.

As can be seen more particularly by reference to Figure 3, damping of the cutting impact commences basically directly at the time of breakthrough, because up to that time the proportional regulating valve 28 provides a small aperture intended for the slow discharge of oil from the chambers 17, 18 and because due to the presence of the integrating element in the regulating circuit the said aperture is opened only very slowly in relation to a normal cutting impact. The regulating circuit also automatically compensates for any change of viscosity of the hydraulic fluid; the system according to the invention is therefore intrinsically completely independent of viscosity.

The invention also does not call for adapting the system to presses with working strokes of different velocity. Such adaptation is obtained automatically by the regulate ing circuit. For a relatively slow working stroke, the aperture in the valve 28 is relatively small; the automatic regulating system sets the aperture to a somewhat larger value for more rapid operating strokes, so that the low pressure value preset by the potentiometer 37 is maintained in the chambers 17 and 18 or in the duct 23.

The adjustable capacitor 43 in the integrating element enables the time of relaxation of the mechanical stresses to be altered in the simplest possible manner to suit presses of the most diverse kind. It should be noted that this does not refer to any form of difficult adjustment, but in the simplest case relates to a simple setting by ear to obtain the quietest cutting impact. For example, in a first experimental mode, the noise which occurred on breakthrough of the tool was reduced by 14 dBA at the first attempt.

The cylinders 13, 14 and pistons 15, 16 exert a counterforce during the working stroke which is only minimal and has practically no significance since the pressure in the duct 23 is preferably regulated by the regulating circuit only to the lowest value required for overcoming the frictional resistances in the hydraulic system.

The integrating action in the control circuit can be obtained in the simplest possible manner for cutting impact damping control systems of the kind according to the invention, intended only for a specific press by selecting a proportional regulating valve 28 with adequate inertia from the range of commercial types.

However, the integrating action will be produced in accordance with Figure 1 or in like manner by electronic means for universal cutting impact damping control systems which can thus be used on many different types of press, with appropriate setting or adjustments being made at the time of installation.

As will be appreciated from the above description, the preferred embodiment of the invention comprises only very few commercially available and inexpensive components and calls for only a slight additional expenditure for installation compared with embodiments disclosed by the prior art.

WHAT WE CLAIM IS: 1. A damping control device suitable for use with a punching press which press comprises a piston and cylinder disposed operatively between a ram and a press table, the damping control device comprising a pressure regulating arrangement adapted to be connected to a cylinder chamber of said piston and cylinder, said pressure regulating arrangement comprising a pressuresensitive load cell which is adapted to be connected in fluid communication with the cylinder chamber and is adapted to provide an electric signal representative of the pressure of working fluid within the cylinder chamber; a means for generating an eiectrical signal representative of a set point, a set point-measured fluid pressure valve signal comparator which is connected to receive the signals generated in use by the pressure load cell and by the set point signal generating means, an amplifier connected to receive in use signals from the said set point-measured value comparator, and a proportional regulating valve which is arranged to be controlled by signals from the amplifier in use and which is adapted to be hydraulically connected between the cylinder chamber on the one hand and a pressure fluid supply source and an oil reservoir on the other hand, the set point signal generating means being adjustable to provide a set point signal representative of a fluid pressure which corresponds to the pressure required to extend the piston of the cylinder for a return stroke thereof so that a pre-determined pressure is substantially maintained within said cylinder chamber, the pressure regulating arrangement either being provided additionally with an electrical integrating element or being arranged so that parts thereof constitute a mechanical integrating element, which integrating element is so disposed and which has such a time constant that slowly changing signals delivered by the pressure-sensitive load cell cause virtually immediate operation of said regulating valve but rapidly changing signals of the pressure load cell resulting from the breakthrough of a tool cause operation of said regulating valve with a delay defined by the integrating element therefore causing delayed relaxation of mechanical strain in a press frame forming part of said press at the moment at which the tool breaks through the workpiece.

2. A device according to claim 1. wherein said integrating element is adjustable to regulate said delay.

3. A device according to claim 2 wherein the integrating element comprises adjustable electronic components.

4. A device according to claim 1 wherein the integrating element forms part of the pressure regulating arrangement and comprises part of the proportional regulating valve having sufficient inertia to provide the integrating effect.

5. A damping device substantially as herein described by way of example with reference to and as shown in the accompanying drawings.

6. A press provided with a damping device according to any one of the preceding

Claims (1)

1. claims.