TITLE: PROCESS AND DEVICE FOR MEASURING AND CONTROLLING STRUCTURAL DEFLECTIONS OF A PRESSING-BENDING MACHINE

DESCRIPTION

The present invention relates to a process and a device for measuring the deflections caused by bend stresses in a plate pressing-bending machine, using strain-gauge sensors (3 and 4 in Figure 1) suitably mounted to the upper cross member (1) and to the lower cross member (2) , to control the force of one or more structure deflecting jacks (10) , by means of a control system that processes the signal provided by said sensors, so that the distance between matrix-dies and punches, not shown in Figure 1, can be held constant and under control across the machine. In plate pressing-bending machines, any deflection of the structure (i.e. the upper cross member and the lower cross member) caused by bend stresses considerably affects accuracy and uniformity of bend angles obtained by the machine, as the bend angles formed in the internal, more deflected areas, are different from the angles expected and obtained in the external areas, where the displacements of the cross members of the machine are controlled with the utmost precision, generally by suitably applied optical lines. These deflections cannot be eliminated by further stiffening the structure, due to practical size, weight and cost problems.

Nevertheless, several different technical solutions have been proposed to attenuate this drawback, which generally do not alter the structure of the upper cross member, normally having as stiff a construction as possible, but affect the structure of the lower cross member, to obtain a controlled

deflection, known as "crowning", corresponding to that of the upper cross member, so that a constant pointwise distance between the tools (punches and matrix-dies) is obtained all along the machine . In other words , the shorter the penetration length of the punch in the central deflected area of the upper cross member, the higher is the matrix die in the corresponding central deflected area of the lower cross member. The final resulting bend so obtained with a deflectable pressing machine may be wholly comparable to what would be obtained using a virtual theoretically undeflectable press, provided that the deflected cross member is the same.

In common practice the lower cross member, i.e. the one that acts as a base for the matrix-dies and has to be subjected to controlled deflection is interposed between two external tables, which are joined to the rest of the machine structure, and support it in a well-designed manner. The various technical solutions are aimed at causing the lower cross member to be deflected as accurately as possible, and can be substantially divided into two classes:

1) Passive compensation, i.e. special design of the geometric structure of the lower cross member supports and the resulting elasticity, to obtain a deflection that is proportional to the force exerted thereon, but of different direction, which can compensate for the deflection of the upper cross member.

2) Active compensation, using one or more jacks

(typically hydraulic jacks that use oil under pressure) , which are controlled to obtain a well-

determined force, and thence a well-determined deflection, to compensate for the expected deflection of the upper cross member.

An example of type 1) passive compensation solutions of common use consists in placing the lower cross member supports in a central position with respect to the hydraulic cylinders of the machine, to obtain a convex deflection, which compensates for the concave deflection of the higher cross member. Another example of type 1) passive compensation solutions is disclosed in European Patent Application

EP 1 449 598 Al, in which the position of two longitudinal support bars for the lower tool-holding cross member may be varied to adjust the compliance of the structure, to consequently optimize the resulting elastic deflection line.

Common passive compensation methods require no deflection measurement, and no use of hydraulic crowning jacks for deflection compensation purposes. Nevertheless, this obvious advantage is limited by the unsatisfactory accuracy of the compensation deflection obtained through crowing, through the various operating positions. Furthermore, these compensation methods are of difficult implementation in large-width machines . Among type 2) active compensation solutions, an example is disclosed in European Patent Application EP 1 452 302 Al, in which pressure values for the compensation cylinders are calculated from a pre- registered gauging apparatus, empirically obtained from tests on particular plate specimens, and from the measurement of the force exerted during the previous bending operation. In bending force measurement, this method does not use upper and/or lower cross member-

mounted sensors , but shoulder-mounted sensors . Nevertheless, the determination of active compensation through mathematical tables and empirical tests on reference plate tests, is rather difficult in practice, due to the many initial test bends to be considered, and does not always ensure the highest accuracy. Particularly, no real-time accurate control of the actual deflection of the upper and lower cross members is provided at each bending operation, although this is the phenomenon that has to be accurately considered.

Other type 2) active compensation solutions actually measure the deflection of the upper or lower cross member of the pressing machine using direct or indirect geometric means, i.e. position sensors. Generally, these sensors include optical precision lines and are applied to the tables of the pressing machines, by means of more or less complex additional mechanical structures, which are used to obtain, at the ends of the position sensors, a rather precise displacement which is equal or proportional to the deflection of the corresponding table . In certain cases , levers or particular mechanisms may be used to amplify the detectable displacements and make them appreciable with a sufficient resolution. An example of a solution of this type is disclosed in patent EP 1027178 Bl, in which at least one upper position transducer is mounted between the upper cross member and an upper fixed cross member, for measuring the deflection of the cross member by geometrical calculation.

Solutions of this type are affected by the drawbacks of additional costs for position sensors, possible mechanical problems caused by the presence of

delicate parts in relative motion and possible complexity of the additional mechanical structures for mounting the position sensors to the machine. These structures must be particularly rigid, to provide a precise and undeflectable reference support.

A first object of the present invention is to obtain a direct, precise and simple measure of the deflection of the two cross members, which requires neither complex modeling of the pressing machine using a theoretical structural calculation, nor bending tests for empirical determination, on a case-by-case basis, of the deflection and the compensation action to be taken .

A second object of the present invention is to obtain a measure of the deflection of the two cross members without requiring any fixed external cross member, connected to other parts of the machine structure, as a position reference.

A third object of the present invention is to easily obtain a mechanical amplification of the stresses caused by the deflection for easier detection thereof, without using any mechanism, but simply through a concentration of stresses by reducing the sections of the counteracting elastic elements, at the stress sensors, i.e. the strain-gauges .

Another object of the present invention is to provide a device for controlling structural deflections using sensors that ensure ultra-high resolution while being simple, reliable, insensitive to the inevitable dirt present in the operating area of the machine, and not containing moving parts .

A further object of the present invention, which can be simply fulfilled by multiplying the number of

stress sensors on the same structural element, is to accurately measure and control even complex structural deflections, such as in the case of multi-position bending on large-width pressing machines. A particular advantage of this process is that it can be very easily adapted, by simply varying the position and number of the deflection sensors , to the particular conditions of any machine, such as an existing, pre-built machine, provided that it is equipped with hydraulic crowning cylinders, and that it implies no variation in existing numerical control functions and in the modes of operation by the machine operators .

These objects are fulfilled by the process and device of the present invention, which is characterized as set out in the annexed claims .

These and other features will be more apparent from the following description of a few embodiments , which are shown by way of example and without limitation in the accompanying drawings, in which:

Figure 1 shows a simple application of the present device, when installed on a machine having a single hydraulic compensation cylinder, as shown before the start of the bending step; - Figure 2 shows the operation of the device on the machine of Figure 1 , as shown during the bending step;

Figure 3 shows a more complex application of the present device, on a machine having three hydraulic compensation cylinders , as shown before the start of the bending step;

Figure 4 shows the detail of a preferred

embodiment of a measuring bar, which is an example of a deflection sensor comprising a strain-gauge, and an example of possible clearance-free supports ; - Figure 5 shows the principle schematic of an exemplary control system for use with the control process .

Generally, normal and complete compensation for shoulder deflection is already provided in a pressing- bending machine thanks to the particular structure for supporting the optical lines that control the main cylinders of the machine .

The process for controlling deflections in other structural parts of the machine, which also poses a difficult problem, is carried out, according to the present invention, by the addition of suitable devices for measuring the deflection of the upper and lower cross members of the pressing machine. In order to achieve high sensitivity and measuring resolution, as well as a precise correspondence of the deflection measures of the upper and lower cross members, a novel measuring device is provided, which is based on the use of strain gauges, whose particular features essentially depend on how such strain gauges are mounted and in which positions .

In the preferred embodiment, and with reference to Figure 1, in order not to be bound by the shape and structure of the machine as shown in Figures 1 and 2, which can change from model to mode, and to amplify the deflections that can be sensed, the strain gauges 3 and 4 are mounted to two identical measuring bars 5 and 6, which may consist of segments of a rigid elastic

material (such as steel, aluminum alloy, fiberglass reinforced resin, etc.) or multiple segments of different lengths, shapes or materials, rigidly coupled together. The measuring bars so formed are constructed in such a manner as to be relatively rigid at the portions far from the strain gauges and relatively compliant near the strain gauges (for further information see the description of Figure 4) . In the preferred embodiment, they have the same length, corresponding to the distance between the shoulders 7 and 9 of the machine . These bars are mounted in the same manner on the upper cross member and the lower cross member by means of supports 9 without clearance, which turn the deflection in the cross members into flexural deflection of the bars, near the strain gauges .

In the preferred embodiment as shown in Figure 1 , which relates to smaller machines or machines having a lower bending force, one pair of strain gauges 3 and 4 and one hydraulic cylinder 10 can be simply used. The bars have been intentionally mounted to the front part of the cross members of the machine, on the operator side, for clarity. It is evident that a mounting arrangement on the opposite back side may provide several different advantages .

In Figure 2 , the same machine as that of Figure 1 is shown during the bending step, and deflections are intentionally exaggerated for better clarity, in the simplified, illustrative, though not preferred condition in which the bar has the same bending strength (i.e. the same moment of inertia) throughout its length The upper cross member 1 is subjected to concave deflection and transmits such deflection to the

measuring bar 5 mounted thereto, thereby causing it to bend. Due to its mounting position, the strain gauge in the middle of the upper bar, senses a tensile stress

(in a different mounting position it could sense a bending, compressive or shear stress) .

The control system detects this signal and compares it with the signal transmitted from the corresponding strain gage 4 mounted in the same position to the bar 6 on the lower cross member 2, which would tend to bend in the opposite direction. The difference between the two signals causes a pressure increase in the hydraulic cylinder 10, which extends to impart a convex deflection to the lower cross bar 2, and hence to the lower measuring bar, which deflects in the same manner as the upper bar, until a steady equilibrium state is reached, when both the strain gauges 3 and 4 sense the same deflection, i.e. when the distance between the punch and the matrix-die is constant throughout the plate width. It shall be noted that the control system starts to operate as soon as deflection occurs , subsequently follows the force changes that occur during the different bending steps, and finally maintain the equilibrium state in the lower dead point until the cross member is finally lifted at the end of the bending process. This ensures optimized bending of the plate.

Particularly referring to the simple case as shown in Figures 1 and 2 , further simplifications may be envisaged, which are described hereinbelow, through not shown, and may be used at lower costs and with limited performances .

A single measuring bar may be used with a single strain gauge, and for the lower cross member only, in

the particular case in which a considerably oversized upper cross member has a negligible deflection with respect to that of the lower cross member.

However, one single measuring bar with one strain gauge, for the upper cross member only could be used if deflections of the cross member is expected to be anyway proportional to that of the lower cross member, and if this single available measurement is expected to allow direct compensation for both deflections , with a suitably calibrated open-loop control system.

According to the invention, in the most general case the number of strain gauges on the cross members (or the bars) and the number of cylindrical cylinders depend on the structure of the machine, on its size and the maximum available bending force: typically, each strain gauge on each cross member or each bar is mounted at each hydraulic cylinder, for optimal control of its action.

In the example of Figure 3 , the machine has three hydraulic compensation cylinders 11, 12 and 13, and three pairs of strain gauge pairs on the bars 14, 15, 16, 17, 18 and 19. (It is evident that, cost reduction issues might induce to use fewer strain gauges than those required in this example, which obviously affects precision.). The machine is controlled in substantially the same manner as in the case of Figures 1 and 2 , with each pair of strain gauges acting on its underlying cylinder. Decoupled operations in real cases can ensure a stable and precise overall deflection, especially by the use of feed forward control techniques , which are described in greater detail below, with reference to Figure 5.

Figure 4 is an axonometric detail view of a

preferred embodiment of one of the two bars 5 and 6 of Figure 1 as viewed from the bottom and not from the top like in the previous figures. For simplicity, the middle portion of the bar is only shown, which has a strain gauge 21 and the corresponding middle support 22, and with the two end portions 23 and 24 which include the end supports 25 and 26. In this type of preferred embodiment, the strain gauges are resistive strain gauges, that are particularly precise, cost- effective, reliable and widely used. Nevertheless, other types of sensors may be used, which are based on several different operation principles , such as , without limitation, semi-conductor based or piezoelectric sensors. In the simplest example as proposed herein, a single strain gauge is used, to detect tensile or compressive stresses consequent to bar bending.

To obtain signals of greater amplitude in the same area of the bar, other strain gauges may be mounted, in adjoining and/or opposed positions. Each strain gauge may either be of simple type, i.e. including one component, or have several different combined components ; it can be of half-bridge or full bridge type; if the deflection sensors are used in pairs, like in the above examples , care should be taken that each strain gauge is symmetrical with its corresponding sensor on the other bar . The deflection of bars , which proportionally reproduces the deflection of the two cross bars, may be locally amplified by suitably identically processing the bars, such as by providing a section restriction or a hole 27, with the techniques that are normally used for forming load cells : by reducing the section of the bar at a strain gauge, the

latter is stressed by a greater local deflection and provides a signal of higher lever. For the same reason, reinforced sections are used for the parts of the bar that are far from the strain gauges, i.e. the longer parts , not shown in the detail drawing of Figure 4 , which sections are formed of stiffer materials and/or with larger moments of inertia than those used in the middle portion 20 as shown, which comprises the middle support 22. This allows further concentration of stresses due to the elastic deflection imposed to the bar as a whole at the strain gauge 21.

In the preferred embodiment, the supports 22, 25 and 26 have the same construction at the upper and lower cross members, and are disposed symmetrically with respect to the centerline of the machine. They are designed to form simple constraints, with no mechanical clearance, which perfectly transmit vertical displacements in both directions . In the example as shown in Figure 4 , the supports are formed by connecting cylindrical plugs 28, 29 and 30, to be force fitted in the cross member, to housings 31, 32 and 33 formed in the bar, having a greater diameter of the plugs , with the clearance being taken up by tightening vertical screws 34, 35 and 36 into the bar. Each strain gauge 21 is mounted at the smaller section of the bar, within a hole 27 which also acts as a protective housing.

A control system processes the signals from each strain gauge, compares them and produces a pressure reference for each hydraulic cylinder or for several different hydraulic cylinders operating in parallel .

The control system of the preferred embodiment, as shown in the block diagram of Figure 5, is of the

electronic type and comprises a microcontroller μC, with analog-to-digital A/D converters for input signals . The microcontroller is interfaced at each output with an inductive current to pressure I/P transducer, and adjusts the pulse width modulated switched current, using a feedback provided by current measurement in the load using a transducer TA which can also detect direct current, with an output converted into voltage (I/V) , and with the corresponding input having an analog-to-digital A/D converter. Current measurement (by TA and I/V) can also be obtained using a shunt and a voltage amplifier.

The control system can also have a completely analog operation, or be integrated in the functions of the numerical control system of the machine, using inputs and outputs that are generally available therein .

For example, for greater simplicity and clarity, Figure 5 shows the operation of a control system having a single control loop, with two strain gauges SA and SB located on the upper cross member (SA) and the lower cross member (SB) respectively, and with a single output for crowning control through the cylinder C. By multiplying the number of strain gauges and compensation cylinders with an independent control, the process is substantially the same, repeated several times, and with simultaneous operation. In the example of the figures, the two strain gauges SA and SB, of the full bridge type, provide the signal to their respective differential amplifiers GA and GB allowing zero and gain calibration, to obtain the same zero and the same end-of-scale as the corresponding sensor on

the other bar. Zero calibration of each sensor is performed at the start, with undeflected bars, and may be automatically repeated on the machine from one bending process to another, when the machine is idle, for continuously compensating even for the slightest errors due to secondary thermal , electrical or mechanical deviations, affording a greatest long-time precision of the machine.

Balancing of gains GA and GB is not difficult, even due to the identical structure of the bars, and may be simply carried out before mounting to the machine, by coupling the bars together and causing identical deflection thereof. Such step can be also carried out after mounting, with a convenient semi- automatic calibration procedure, through which the machine is suitably deflected with a coining bend on a plate specimen laid on the center of the machine .

Control is based on an error signal obtained as the difference between the deflection value of the upper cross member (set-point) in one point and the corresponding deflection value of the lower cross member in the corresponding point (feed-back) , with a PID, i.e. proportional-plus-integral-plus-derivative control . The gain of the different proportional (P) , integral (I) and derivative (D) components is optimized for each particular application.

For a faster response and a greater stability of the control system, in the preferred embodiment the deflection value of the upper cross member is directly added, with the appropriate calibratable multiplicative coefficient K, to the output signal from the PID control system, with a feed-forward function, to

provide, through the power amplifier SW, the actual reference for the pressure limiting valve of the hydraulic cylinder that operates in that point. In the illustrated embodiment, a valve is used for linear control from a current level I , in the range from zero to 1 ampere or from zero to 2 ampere .

By calibrating the multiplicative coefficient K of the feed-forward action so that most of the required output signal corresponds thereto, the gain required by the PID control system can be minimized, thereby increasing its stability, especially when multiple control loops are actuated simultaneously for several different independent jacks.

Control systems other than PID exist in the art (e.g. with fuzzy logic or controlling state variables in more complex systems) and can be also used.

This deflection control, which can be theoretically always on, may be disabled by a logical signal UDP, provided by the Numerical Control system NC of the machine when the upper cross member is at the upper dead point or is not under stress. Thus, the integral component of the PID is reduced to zero and the PWM output is automatically disabled before each bend, for the correction of zero errors detected by the various deflection sensors, as described above.