DEVICE FOR CONTROLLING WIRE TENSION AND DETECTING BREAKAGE OF WIRE

Abstract: A device for correctly controlling the wire tension and detecting breakage of the wire, which is produced at a reduced cost and which is adapted to a wire-cut electric spark machine. A signal that represents the wire tension is applied to differential amplifiers (13, 14) from a load cell (9) that receives the load which corresponds to the wire tension via a cantilever (7) and a guide roller (8) supported by the cantilever. Output voltages corresponding to the deviation between a practical value of wire tension and the setpoint value are applied from the two amplifiers to a transistor (15) and to a comparator (16), respectively. The braking force of an electromagnetic braking device (6) is variably controlled by the transistor, and the wire tension is fed back to the setpoint value. In case the wire tension diminishes as a result of breakage of the wire, the output voltage of the amplifier applied to the comparator becomes greater than a reference voltage for discriminating the breakage of wire, and the comparator produces a signal that indicates the breakage of wire.

Description: The present invention relates to an apparatus for wire tension control for a wire-cut electric discharge machine. In a conventional wire-cut electric discharge machine, a tension is applied to a running wire electrode (hereinafter referred to as the wire) by subjecting the wire to a braking force by means of a brake gear, the braking force is open-loop-controlled to adjust the wire tension to a predetermined value, and flexure of the wire caused by disconnection is detected by means of a limit switch opposed to the wire, thereby effecting disconnection detection. According to the electric discharge machine with the construction described above, the braking force is controlled in an open-loop manner, so that the wire tension control sometimes cannot be accurately performed. Since a wire tension control system and a disconnection detection system are arranged independent of each other, moreover, high cost is entailed. JP-A 57-144633 also discloses a wire tension control apparatus and the precharacterising part of attached claim 1 is based on this document. However, the apparatus of this document is complex and intrusive. A separate wire disconnection detection device is disclosed in JP-B2 58-2011. The object of the present invention is to provide an apparatus for wire tension control and disconnection detection for a wire-cut electric discharge machine, the apparatus being capable of accurate wire tension control and disconnection detection and entailing only low cost. According to the present invention there is provided an apparatus for wire tension control in a wire-cut electric discharge machine comprising: a wire tension detecting device including a roller upon which the wire extends when the electric discharge machine is in use, a pivotally mounted lever supporting the roller for rotation of the latter, and a detector for detecting pivotal force exerted on the lever by the wire via the roller when the apparatus is in use; and a wire tension controller for producing and controlling the wire tension, when the apparatus is in use, in dependence upon a signal representing a set value of wire tension and another signal, from the detector, representing an actual value of wire tension, the apparatus being characterised in that: the roller is a wire guide roller which with other wire guides defines the operating path of the wire through the electric discharge machine and has such a position that the wire runs on it and thereby applies a pivotal force to the lever; the detector is a load cell having a pressure detecting portion upon which the lever is supported against said pivotal force and which produces said another signal representing the actual value of wire tension; the wire tension controller comprises a braking device for applying a braking force to the wire; and the apparatus further comprises a wire disconnection detector operable to detect disconnection of the wire on the basis of said another signal representing the actual value of wire tension. According to the present invention, as described above, feedback control of the wire tension and the disconnection detection are effected in accordance with the output signal of the tension detecting means, which is indicative of the wire tension, so that the wire tension control and the disconnection detection can be accurately achieved, and a common sensor can be used for both the control and the detection, thus ensuring reduction in cost. Brief Description of the Drawing The single drawing is a schematic view showing the principal part of a wire-cut electric discharge machine provided with an apparatus for wire tension control and disconnection detection according to one embodiment of the present invention. In the accompanying drawing, a wire-cut electric discharge machine causes wire transportation means (not shown) to drive a wire 1 delivered from a wire supply bobbin 3, thereby running the wire along a pinch roller 4, a brake roller 5, a guide roller 8, an upper guide 10, a machining groove formed on a workpiece 2, a lower guide 11, and a guide roller 12, and applies a machining voltage between the wire 1 and the workpiece 2, thereby causing electric discharge between them to effect electric discharge machining. Numeral 6 denotes an electromagnetic brake gear, which is arranged so as to brake the brake roller 5 to apply a braking force to the wire 1, thereby producing a wire tension. Numeral 17 denotes a backup plate for preventing the wire 1 from slipping off the guide roller 8 in case of, for example, disconnection. Further, the electric discharge machine comprises an apparatus for wire tension control and disconnection detection, which is provided with a cantilever 7. One end of the lever 7 is swingably supported by a shaft 7a, while the other end supports the guide roller 8 for rotation. Also, the cantilever 7 is supported, at the middle portion thereof, by a pressure detecting portion 9a of a load cell 9 as tension detecting means. The output terminal of the load cell 9 is connected to one input terminal of a differential amplifier 13, which constitutes brake control means in conjunction with a transistor 15, and a reference voltage VL1 corresponding to a set value of wire tension is applied to the other input terminal of the amplifier 13. The output terminal of the amplifier 13 is connected to the base of the transistor 15, whose collector is connected to a solenoid (not shown) of the brake gear 6, and whose emitter is grounded. Further, the output terminal of the load cell 9 is connected to one input terminal of a differential amplifier 14, which constitutes disconnection detecting means in conjunction with a comparator 16, and receives the reference voltage VL1 at the other input terminal thereof. The output terminal of the amplifier 14 is connected to one input terminal of the comparator 16, which receives a reference voltage VL2 for disconnection discrimination at the other input terminal thereof. The gain of the amplifier 13 is set to a large value, while the gain of the amplifier 14 is set to a small value. The output terminal of the comparator 16 is connected to a numerical control unit (not shown) for controlling various operations of the electric discharge machine and delivering the aforesaid reference voltages VL1 and VL2. The following is a description of the operation of the electric discharge machine with the aforementioned construction. First, an operator operates a keyboard of the numerical control unit to input the set value of wire tension, and then starts the electric discharge machine. At the start of the operation, the electric discharge machine performs conventional wire threading as required. As the wire 1 travels along the illustrated wire transportation path, an operative force, resulting from the wire tension, to rotate the cantilever 7 in the counterclockwise direction, as viewed in the drawing, around the shaft 7a acts on the lever 7 through the medium of the guide roller 8. As a result, a load corresponding to the wire tension acts on the pressure detecting portion 9a of the load cell 9 through the medium of the lever 7, and an electrical output signal corresponding in voltage to the load is delivered from the load cell 9 to the one input terminal of each of the differential amplifiers 13 and 14. Since the weights of the cantilever 7 and the guide roller 8 bear on the load cell 9, moreover, they are compensated for by calibrating the load cell 9 beforehand in accordance with the aforesaid weights, or by offsetting the reference voltage VL1 in dependence on these weights. The amplifier 13 applies an output voltage, which corresponds to the difference between the output voltage of the load cell 9 and the reference voltage VL1 corresponding to the set wire tension, to the base of the transistor 15. As a result, an electric current corresponding to the aforesaid difference flows through the solenoid of the brake gear 6, so that the braking force of the brake roller 5 is subjected to variable control. Thus, if the wire tension is smaller than the set value, the output voltage of the load cell 9 is low. Accordingly, the output voltage of the differential amplifier 13, the current flowing through the brake gear 6, and the braking force of the brake roller 5 increase individually, so that the wire tension increases. If the wire tension is greater than the set value, on the other hand, the elements 9, 13, 6 and 5 operate contrary the manner for the small wire tension, so that the wire tension is reduced. Thus, the wire tension is controlled to the set value in a feedback manner. In case of no disconnection, the aforementioned wire tension control is executed, so that the difference between the load cell output voltage and the reference voltage VL1, applied individually to both input terminals of the differential amplifier 14 of the disconnection detecting means, is small, and the output voltage of the amplifier 14 corresponding to this difference is lower than the reference voltage VL2 for disconnection discrimination. Thus, an output signal from the comparator 16 is at an L-level indicative of a no-disconnection state. If the wire 1 snaps so that the wire tension is gone, the output voltage of the load cell 9 lowers, so that the difference between the load cell output voltage and the reference voltage VL1 increases. As a result, the output voltage of the amplifier 14 exceeds the reference voltage VL2, and an output voltage of an H-level indicative of a disconnection state is delivered from the comparator 16 to the numerical control unit. The present invention is not limited to the embodiment described above, and various modifications may be effected therein. In the above embodiment, for example, the differential amplifier 14 is used to input the reference voltage VL1 corresponding to the set value of wire tension and the output voltage of the load cell 9. Instead of using this, however, an amplifier may be used simply to amplify the load cell output so that the output of the amplifier is applied to the comparator 16. In the above embodiment, moreover, the cantilever 7 is supported by means of the load cell. Alternatively, however, a spring may be additionally used to support the lever. Furthermore, the apparatus for wire tension control and disconnection detection according to the present invention may be mounted on an electric discharge machine of a type delivering a command for the start of application of wire tension in response to an operation of an exclusive-use switch therefor. When no wire tension is produced, in the electric discharge machine of this type, a disconnection detection signal is delivered from the comparator 16 even though the wire is not subject to disconnection. It is therefore necessary to provide a circuit for invalidating this signal.

Claims: 1. An apparatus for wire tension control in a wire-cut electric discharge machine comprising: a wire tension detecting device including a roller (8) upon which the wire (1) extends when the electric discharge machine is in use, a pivotally mounted lever (7) supporting the roller (8) for rotation of the latter, and a detector (9) for detecting pivotal force exerted on the lever (7) by the wire (1) via the roller (8) when the apparatus is in use; and a wire tension controller (5,6) for producing and controlling the wire tension, when the apparatus is in use, in dependence upon a signal representing a set value of wire tension and another signal, from the detector (9), representing an actual value of wire tension, the apparatus being characterised in that: the roller (8) is a wire guide roller which with other wire guides (10,11,12) defines the operating path of the wire (1) through the electric discharge machine and has such a position that the wire (1) runs on it and thereby applies a pivotal force to the lever (7); the detector (9) is a load cell having a pressure detecting portion (9a) upon which the lever (7) is supported against said pivotal force and which produces said another signal representing the actual value of wire tension; the wire tension controller (5,6) comprises a braking device for applying a braking force to the wire (1); and the apparatus further comprises a wire disconnection detector (14,16) operable to detect disconnection of the wire (1) on the basis of said another signal representing the actual value of wire tension. 2. An apparatus for wire tension control according to claim 1, and comprising a differential amplifier (13) coupled to receive the output signal of the detector (9) and a signal (VL1) indicative of said set value of wire tension, and to produce an output signal corresponding to the difference between these two signals and supply it to the wire tension controller (5,6). 3. An apparatus for wire tension control according to claim 2, wherein said wire tension controller (5,6) includes an electromagnetic brake gear (6) and a brake roller (5), the electromagnetic brake gear (6) being controlled by a transistor (15) for adjusting the driving current of the electromagnetic brake gear (6) in accordance with said output signal of said differential amplifier (13). 4. An apparatus for wire tension control according to any preceding claim, wherein said disconnection detector (14,16) includes an amplifier (14) adapted to receive said output signal of said detector (9), and a comparator (16) for comparing an output signal from this amplifier (14) and a reference signal (VL2) for disconnection discrimination.

Inventors: IZUMIYA, SYUNZO, OBARA, HARUKI
Applicants: FANUC LTD.
Application year: 1989
Main classification: B23H7
IPC codes: B23H7