GAP VOLTAGE DETECTOR FOR WIRE CUT ELECTRIC DISCHARGE MACHINES

Abstract: This invention provides a gap voltage detector for wire cut electric discharge machines, which is capable of accurately detecting a gap voltage between a wire electrode and a work while eliminating the influence by variations in the position of electric discharging. An operational amplifier (11) of the gap voltage detector is adapted to receive via one input terminal thereof the sum of a voltage between an upper electric member (4) and a work (3), which is detected by voltage dividing circuits (R11, R12), and a voltage between a lower electric member (5) and the work (3), which is detected by voltage dividing circuits (R21, R22), and via the other input terminal thereof and through variable resistors (RV1, RV2) an output corresponding to a differentiation value of a discharge current and sent out from a detecting coil (10), and an output corresponding to a discharge current from an integrating circuit (12), into which an output from the detecting coil (10) is inputted, as signals for compensating for a voltage drop ascribed to the wire resistance and inductance between the two electric members (4), (5), the sum of these voltages being compensated by the voltage drop, the resultant signal being outputted as a signal representative of a gap voltage.

Description: The present invention relates to a gap voltage detector in a wire-cut electric discharge machine, which is capable of detecting a gap voltage between a wire electrode and a workpiece. In a wire-cut electric discharge machine, it is known to effect appropriate control of electric discharge machining on the basis of a detected value of a gap voltage between a wire electrode (hereinafter referred to as wire) and a workpiece. Conventionally, a voltage between the workpiece and a conductor disposed above or below the workpiece for connecting a machining power source with the wire, is detected as the gap voltage. However, the thus detected voltage includes voltage drop components attributable to a wire resistance between the conductor and the workpiece and an inductance therebetween. As a position at which electric discharge occurs between the wire and the workpiece varies, a variation occurs also in these voltage drop components. In particular, a large variation in these components is found when the workpiece has a large thickness. As explained above, the detected voltage includes variable components attributable to a variation in the electric discharge position, and hence cannot indicate an accurate gap voltage. Further, it is also known to subtract a correction value representative of a voltage drop component attributable to an inductance between the conductor and the workpiece from the detected voltage, so as to correct the detected voltage, as disclosed in Japanese Provisional Patent Publication No. 60-180719. However, even in this case, the detected voltage thus corrected cannot indicate an accurate gap voltage because the just-mentioned correction value does not reflect a variation of the voltage drop component. Also, the gap voltage cannot be detected accurately, in the case of effecting correction for the voltage drop component associated with the wire resistance. According to the present invention there is provided a wire-cut electric discharge machine comprising wire guides operable to guide a machining wire along a path towards and away from a workpiece at entry and exit sides of the workpiece respectively, a first conductor disposed at one of said workpiece sides so as to contact the wire, and a first voltage detecting means for detecting a first voltage between the first conductor and the workpiece, characterised by: a second conductor disposed at the opposite side of the workpiece so as to contact the wire, a second voltage detecting means for detecting a second voltage between the second conductor and the workpiece, and means for detecting gap voltage between the wire and workpiece on the basis of the sum of the thus detected first and second voltages. An embodiment of the present invention may provide a gap voltage detector in a wire-cut electric discharge machine, which is capable of eliminating or reducing an adverse influence of a variation in an electric discharge position, thereby permitting an accurate detection of a gap voltage between a wire electrode and a workpiece. Preferably, the gap voltage detector includes means for detecting an electric discharge current, and means for detecting a differential value of the electric discharge current, and is operable to detect a voltage drop component, attributable to a wire resistance between the first and second conductors, and another voltage drop component, attributable to an inductance between these conductors, on the basis of the detected electric discharge current and its differential value, and is further operable to correct the sum of the first and second voltages by the use of the detected voltage drop components, to thereby detect the gap voltage. As explained above, according to the present invention, since the gap voltage is detected on the basis of the sum of the voltages between the first conductor and the workpiece and between the second conductor and the workpiece, and preferably, the sum is corrected by the voltage drop components attributable to the wire resistance and the inductance between these conductors, an accurate gap voltage can be detected free from an adverse affection of a variation in electric discharge position. Brief Description of the Drawings Fig. 1 is a schematic view showing part of a wire-cut electric discharge machine equipped with a gap voltage detector according to an embodiment of the present invention; and Fig. 2 is a view for explanation of operational principle of the arrangement shown in Fig. 1. Referring to Fig. 1, a wire-cut electric discharge machine equipped with a gap voltage detector according to an embodiment of the present invention, comprises an upper guide roller 6 and an upper wire guide 8 disposed above a workpiece 3, respectively, and a lower wire guide 9 and a lower guide roller 7 which are respectively disposed in a facing relation with these elements 6, 8 with respect to the workpiece 3, and is so designed as to guide a wire 2 by means of these elements 6 - 9. An upper conductor 4 and a lower conductor 5 are respectively disposed in the vicinity of the wire guides 8, 9 for contact with the wire 2, and each of these conductors is connected to one output terminal of a machining power source 1 having another output terminal connected to the workpiece 3 which is grounded. The upper conductor 4 is connected to the workpiece 3 through a first voltage divider which consists of resistors R11 and R12 for detecting a voltage between the upper conductor 4 and the workpiece 3, and a joint of these resistors is connected, through a resistor R13, to a first input terminal of an operational amplifier (adder) 11 for detecting a gap voltage between the wire 2 and the workpiece 3. On the other hand, the lower conductor 5 is connected to the workpiece 3 through a second voltage divider consisting of resistors R21 and R22 for detecting a voltage between the lower conductor 5 and the workpiece 3, and a joint of the resistors is connected to the first input terminal of the operational amplifier 11 through a resistor R23. Reference numeral 10 designates a detection coil for detecting a differential value of an electric discharge current. A lead wire, connecting the machining power source 1 with the workpiece 3, extends through the detection coil 10. The coil 10 has one end which is grounded, and another end which is connected to a second input terminal of the operational amplifier 11 through a first variable resistor RV1 and to an input terminal of an integration circuit 12 for detecting the electric discharge current. Further, an output terminal of the integration circuit 12 is connected to the second input terminal of the operational amplifier 11 through a second variable resistor RV2. Meanwhile, the direction of winding of the detection coil 10 is so determined that voltages applied to the respective input terminals of the operational amplifier 11 are opposite in polarity from each other. In the following, operation of the electric discharge machine constructed as above will be explained. Upon start of operation of the electric discharge machine, an electric discharge power is supplied from the machining power source 1 to the wire 2, which is being delivered along the elements 6 - 9 shown in Fig. 1 and passing an initial hole or a machining groove formed in the workpiece 3 to move downwardly as viewed in Fig. 1. As a result, electric discharge occurs between the wire 2 and the workpiece 3 for electric discharge machining. Here, let it be assumed !hat electric discharge occurs at an electric discharge position indicated by reference symbol P in Fig. 2. Voltages VU and VL between the upper conductor 4 and the workpiece 3 and between the lower conductor 5 and the workpiece 3, and the sum VG (=VU+VL) of these voltages are represented by the following equations (1) - (3), respectively. Herein, the resistors R11, R12, R21, R23 and the like are not taken into consideration. VU = Vg + r1 · i1 + L1 · (di1/dt) VL = Vg + r2 · i2 + L2 · (di2/dt) VG = 2Vg + r1 · i1 + r2 · i2 + L1 · (di1/dt) + L2 · (di2/dt) where Vg denotes the gap voltage; r1, the wire resistance between the upper conductor 4 and the electric discharge position P ; L1, the inductance between the upper conductor 4 and the discharge position P ; r2, the wire resistance between the lower conductor 5 and the discharge position P ; L2, the inductance between the lower conductor 5 and the discharge position P ; il, the electric current flowing through the upper conductor 4; and i2, the electric current flowing through the lower conductor 5 (See, Fig. 2). The inductances L1, L2 are considerably smaller than the inductance L1′ between the machining power source 1 and the upper, lower conductors 4, 5 and the inductance L2′ between the workpiece 3 and the machining power source 1. Accordingly, the electric currents i1 and i2 are approximately equal to each other (il ≒ i2 ≒ i), and the following approximate equation (4) can be obtained by modifying equation (3). VG ≒ 2Vg + i(r1+r2) + (L1+L2)di/dt In equation (4), the value r1+r2 represents the wire resistance between the conductors 4, 5 and the value L1+L2 represents the inductance therebetween, respectively. These values are definitely determined in dependence on the construction of the electric discharge machine, and do not change even when the electric discharge position P varies. Accordingly, the value of the right hand side of equation (4) accurately corresponds to the gap voltage Vg. During the electric discharge machining, a voltage indicative of the voltage VU appears at a junction of the resistors R11, R12 of the first voltage divider which connects the upper conductor 4 and the workpiece 3, whereas a voltage indicative of the voltage VL appears at an junction of the resistors R21, R22 of the second voltage divider which connects the lower conductor 5 and the workpiece 3. The sum of these voltages is applied to the first input terminal of the operational amplifier 11. The sum applied to that terminal corresponds to the value of the right hand side of equation (4), and hence accurately corresponds to the gap voltage Vg. During the electric discharge machining, a voltage is induced in the detection coil 10, the magnitude of the induced voltage varying in dependence on a change (differential value) di/dt in the electric discharge current, flowing between the machining power source 1 and the workpiece 3, with respect to time. The detection coil 10 supplies, through the first variable resistor RV1, the second input terminal of the operational amplifier 11 with an output signal indicative of the differential value of the electric discharge current. The integration circuit 12 receives the output signal from the detection coil 10 and integrates the same signal, and then supplies, through the second variable resistor RV2, the second input terminal of the amplifier 11 with an output signal indicative of the electric discharge current i . Meanwhile, the variable resistors RV1, RV2 are adjusted beforehand so that the voltages applied to the second input terminal of the amplifier 11 through these resistors represent the third term ((L1+L2)di/dt) and the second term (i(r1+r2)) of the right hand side of equation (4), respectively. The operational amplifier 11 is supplied at its first input terminal with a voltage corresponding to the sum of the first, second and third terms of the right hand side of equation (4), and is also supplied at its second input terminal with a voltage corresponding to the sum of the second and third terms of equation (4) and having an opposite polarity from that of the voltage applied to the first input terminal, to thereby generate a voltage corresponding to the first term (2Vg) of equation (4). Accordingly, the output voltage of the operational amplifier 11, free from an adverse influence of a variation in the electric discharge position P , corresponds to a value obtained by subtracting voltage drop components attributable to the wire resistance and inductance between the conductors 4 and 5 from the sum of the same voltage drop components and the voltage which accurately corresponds to the gap voltage Vg. As a consequence, the output voltage of the amplifier represents the gap voltage accurately. The present invention is not limited to the aforementioned embodiment, but may be modified in various ways. For instance, although, in the embodiment, the sum of the voltage between the upper conductor 4 and the workpiece 3 and the voltage between the lower conductor 5 and the workpiece 3 is corrected by voltage drop components so as to detect the gap voltage, the sum itself may be detected as the gap voltage.

Claims: 1. A wire-cut electric discharge machine comprising wire guides (6,7,8,9) operable to guide a machining wire (2) along a path towards and away from a workpiece (3) at entry and exit sides of the workpiece (3) respectively, a first conductor (4) disposed at one of said workpiece sides so as to contact the wire (2), and a first voltage detecting means (R11,R12,R13) for detecting a first voltage (VU) between the first conductor (4) and the workpiece (3), characterised by: a second conductor (5) disposed at the opposite side of the workpiece so as to contact the wire (2), a second voltage detecting means (R21,R22,R23) for detecting a second voltage (VL) between the second conductor (5) and the workpiece (3), and means (11) for detecting gap voltage between the wire (2) and workpiece (3) on the basis of the sum of the thus detected first and second voltages. 2. A wire-cut electric discharge machine according to claim 1, comprising a first current detector (10) for detecting a differential value (di/dt) of an electric discharge current when the machine is in use, a first signal generator (RV1) coupled to the first current detector (10) and operable to generate therefrom a signal indicating the value [(L1+L2).di/dt] of a first voltage drop component attributable to the wire inductance (L1+L2) between the first and second conductors (4,5), a second current detector (12) for detecting the value (i) of said electric discharge current when the machine is in use, and a second signal generator (RV2) coupled to the second current detector (12) and operable to generate therefrom a signal indicating the value [i(r1+r2)] of a second voltage drop component attributable to the wire resistance (r1+r2) between said first and second conductors (4,5), the means (11) for detecting the gap voltage being coupled to the first and second signal generators (RV1,RV2) so that said sum of the first and second voltages is corrected by the outputs of the first and second signal generators (RV1,RV2), thereby to provide a corrected detected gap voltage. 3. A wire-cut electric discharge machine according to claim 2, wherein the first current detector (10) comprises a detection coil around a wire for conducting the electric discharge current, and the second current detector (12) is an integration circuit coupled to receive the output of said detection coil. 4. A wire-cut electric discharge machine according to claim 2 or 3, wherein the first signal generator (RV1) is a first variable resistor adjustable to represent said wire inductance (L1+L2) between the first and second conductors (4,5), and the second signal generator (RV2) is a second variable resistor adjustable to represent said wire resistance (r1+r2) between the first and second conductors (4,5).

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