METHOD OF CONTROLLING WIRE TENSION OF A WIRE-CUT DISCHARGE MACHINING APPARATUS

Abstract: A method of controlling wire tension of a wire-cut discharge machining apparatus avoids break in the wire by preventing the wire tension from changing excessively because of abnormal operation of an electromagnetic powder brake when the tension to the wire is applied, removed or is changed. When it is judged that he practical wire tension may change excessively based on the of the presently instructed wire tension and a newly instructed wire tension, the processing is repeated to add or subtract the upper-limit change quantity which has been set in advance to a value that will not excessively change the wire tension, to or from a present desired wire tension (tO) at a predetermined period (U) until the desired wire tension reaches the instructed wire tension (tn). The electromagnetic powder brake undergoes the braking operation with an operation voltage corresponding to a desired wire tension that gradually changes, and the practical wire tension gradually changes until the instructed wire tension is reached.

Description: The present invention relates to a method of controlling the wire tension in a wire cut electric discharge machine adapted to control the wire tension by means of an electric brake. In general, a wire cut electric discharge machine includes an apparatus for controlling the tension applied to a wire electrode (hereinafter referred to as a wire). Typically, the wire tension controlling apparatus includes an electromagnetic powder brake for applying a braking force to a braking roller arranged in contact with the wire. An operation voltage of the powder brake is changed to adjust the braking force applied to the braking roller, to thereby control the wire tension. However, according to the wire tension controlling apparatus of this type, there is a possibility of the wire being broken or disconnected upon start of the application of the wire tension and upon increase of the wire tension. Further, the wire may be slacked when the application of the wire tension is released and the wire tension is reduced, so that occurrence of the wire breakage may be erroneously detected by a wire breakage detection apparatus disposed in the vicinity of the wire path. In addition, there is a possibility that the wire may vibrate upon start and release of application of the wire tension, and upon increase and decrease of the wire tension, thus giving bad influences on various operating sections of the electric discharge machine. According to the present invention there is provided a wire tension controlling method in a wire cut electric discharge machine adapted to control a wire tension applied to a wire electrode by means of an electric brake, said method being characterised by steps of: (a) predictively determining, when a wire tension setting/updating instruction is delivered, presence/absence of a possibility of an actual wire tension being excessively changed if said electric brake is operated in accordance with said instruction; (b) changing a target wire tension to a predetermined amount above or below the present target wire tension so as to prevent an excessive change of said actual wire tension when the presence of the possibility of said actual wire tension being excessively changed is determined, then controlling the actual wire tension accordingly, and (c) periodically effecting said step (b) until said target wire tension, and then the actual wire tension, reaches an instructed wire tension specified by said instruction; wherein in step (a) the presence of the possibility of said actual wire tension being excessively changed is determined when it is determined on the basis of a present instructed wire tension and a new instructed wire tension that a variation rate of said instructed wire tension is larger than a predetermined variation rate corresponding to said predetermined amount used in said step (b); and wherein said instructed wire tension is immediately set as a new target wire tension when it is determined in said step (a) that there is no possibility that said wire tension may be excessively changed. As described above, according to the present invention, when there is a possibility of the actual wire tension being excessively changed if the electric brake is operated in accordance with the wire tension setting/updating instruction, the target wire tension is gradually changed until it reaches the instructed wire tension. Therefore, the wire tension can be controlled to the original target value, with an abnormal operation of the electric brake at the time of setting/updating the wire tension kept prevented, i.e., without the operation of the electric discharge machine being interrupted, which abnormality might otherwise cause an excessive change of the wire tension which might incur wire breakage, erroneous determination in wire breakage detection, and vibration of the wire. Fig. 1 is a schematic view showing a wire delivery apparatus of a wire cut electric discharge machine to which a wire tension controlling method according to an embodiment of the present invention is applied, and peripheral equipment of the wire delivery apparatus; Fig. 2 is a flowchart showing wire tension setting/updating process executed by the electric discharge machine of Fig. 1; Fig. 3 is a graph showing changes of an operation voltage of an electromagnetic powder brake and of an actual wire tension with lapse of time in conventional wire tension setting/updating operation; Fig. 4 is a graph similar to Fig. 3 and associated with the wire tension setting/updating process of Fig. 2; and Fig. 5 is a graph showing a change of the target wire tension with lapse of time during the process of Fig. 2. First, the cause of occurrence of the aforesaid problems such as wire breakage in the conventional wire tension control with the use of the electromagnetic powder brake will be explained on the basis of the knowledge of present inventors, It is considered that the above problems may be caused by an excessive variation in the actual wire tension as the result of an abnormal operation in which a braking force generated by the powder brake will excessively overshoot or undershoot the target value when the wire tension is set/updated. That is, in the conventional typical wire tension control using a numerical control unit for effecting a series of various control operations at predetermined intervals of cycle, an instructed wire tension is read from the numerical control program by the numerical control unit upon start of application of the wire tension, for instance. Then, a control signal for generating the instructed wire tension is supplied from the numerical control unit to the electromagnetic powder brake. As a result, in response to the instructed wire tension, the operation voltage of the powder brake rapidly increases within a short time period, which belongs to a corresponding one control cycle, 8 msec or 16 msec, for instance, of the numerical control unit, and varies in dependence on the response characteristic of the powder brake. Specifically, upon start of the application of the wire tension, the electromagnetic powder brake is temporarily locked, if the operation voltage of the electromagnetic powder brake rapidly increases from 0 V, corresponding to the target wire tension of 0.0 N set for a state where no wire tension is applied, to a voltage of V1, corresponding to the target wire tension of 29,43 N [3.0 kgf] set for applying the wire tension, within one control cycle U (=16 msec) of the numerical control unit, as shown on the left half portion in Fig. 3, for example. As a result, the actual wire tension rapidly increases to temporarily overshoot up to a value of 39,24 N [4.0 kgf] which is larger than the target value 29,43 N [3.0 kgf], so that the wire of some kind may be broken. On the other hand, when the operation voltage of the electromagnetic powder brake is rapidly reduced from a voltage of V1 to 0 V, with a change of the target wire tension from 29,43 N [3.0 kgf] to 0.0 N, as shown on the right half portion of Fig. 3, the powder brake is rapidly set into the release operation state, thereby causing the actual wire tension to be rapidly reduced and significantly undershoot the target value As a result, the wire is temporarily slacked, so that occurrence of the wire breakage may be erroneously detected by the breakage detection apparatus despite that the wire is not actually broken. Such an abnormal condition tends to more frequently occur as the target variation rate (the target variation amount per unit time) of the wire tension becomes larger. In the case of Fig. 3, the above abnormal condition seldom occurs when the target variation amount of the wire tension per one control cycle of 16 msec is less than about 19,62 N [2.0 kgf], but may frequently occur when it becomes about 29,43 N [3.0 kgf]. Referring to Fig. 1, a wire cut electric discharge machine to which a wire tension controlling method according to an embodiment of the present invention is applied comprises a wire delivery mechanism 1 and a numerical control unit 2 which accommodates therein a computer connected to a keyboard 2′. The numerical control unit 2 is connected in control relation to the wire delivery mechanism 1, a table driving mechanism (not shown) for moving, relative to a wire 3, a working table (not shown) on which a work 4 is disposed, and various operating sections of the electric discharge machine including a breakage detection apparatus 5. The breakage detection apparatus 5 is arranged in a facing relation to a wire delivery path, and has a detection lever 5′ thereof which is operable when the wire 3 is slacked upon wire breakage, to thereby detect occurrence of the breakage of the wire. In the wire delivery mechanism 1, the wire 3 delivered from a wire supply reel 11 is fed to a feed roller 18 for applying a tractive force to the wire into drive the same wire, via a brake roller 13 cooperating with an electric brake 12 to apply a brake force to the wire 3, an upper guide roller 14, an upper guide 15, a lower guide 16 and a lower guide roller 17. The roller 18 is operatively coupled to a motor 19 which is connected to the numerical control unit 2. Reference numerals 13' and 18' respectively denote pinch rollers, which are arranged to be in urged contact with the brake roller 13 and feed roller 18, respectively. For instance, the electric brake 12 consists of an electromagnetic powder brake, which has an electromagnetic coil connected to an external driving circuit (not shown) which is operable under control of the numerical control unit 2, and a rotary shaft disposed in a housing which is filled with ferromagnetic particles. This powder brake is so arranged that coupling forces between the ferromagnetic particles, i.e., a resultant friction force, varies in dependence on a voltage applied from the driving circuit to the electromagnetic coil, i.e., a brake operating voltage ( or operating power or operating current). so as to change a braking force applied to a brake roller 13 having a rotary shaft connected to the rotary shaft of the electromagnetic powder brake 12, i.e., the tension applied to the wire 3. Now, the wire tension controlling operation of the electric discharge machine with the above construction, in particular, the wire tension setting/updating process will be explained. Before starting the electric discharge machining operation by the electric discharge machine, in order to prevent occurrences of the above problems such as the wire breakage caused by an excessivey increased or decreased variation in the actual wire tension, an operator sets an upper limit value Δt (>0) of the variation rate of the target wire tension (the variation amount per one control operation cycle of the numerical control unit 2) by the use of the keyboard 2′. This upper limit variation rate Δt is set to a value enough to prevent an occurrence of an excessive variation in the actual wire tension even when the target wire tension increases or decreases at the upper limit variation rate, which excessive variation may cause breakage of the wire 3, an erroneous determination by the breakage detection apparatus 5, and vibration of the wire giving bad influences on various operating sections of the electric discharge machine. Further, since the upper limit variation rate Δt may vary in dependence on the type of the electric discharge machine, the type of the wire 3, etc., this variation rate is experimentally decided beforehand, for instance. In addition, the upper limit value Δt is set again where required before electric discharge machining is restarted after completion of automatic wire extension, etc. When the upper limit variation rate Δt of the target wire tension is set, a central processing unit (hereinafter referred to as CPU) of the computer (not shown) provided in the numerical control unit 2 causes a first register accommodated therein to store the upper limit variation rate Δt. Before starting the electric discharge machining operation or during the same operation, the CPU executes the wire tension setting/updating process of Fig. 2 at predetermined intervals of cycle. Meanwhile, after the start of the electric discharge machining, the CPU repeatedly effects, at the aforesaid intervals, a series of control operations, including a control operation associated with the wire delivery operation by the motor 19 via the feed roller 18, a working table positioning control operation, etc., so as to control the various operating sections of the electric discharge machine in a conventional manner. In the wire tension setting/updating process, the CPU determines whether or not the instructed wire tension is initialized or updated through a manual operation of the operator by the use of the keyboard 2' or through the NC program (hereinafter, the operation of initializing or updating the instructed wire tension is referred to as the operation of updating the instructed wire tension). If no updating operation of the instructed wire tension is effected, the CPU supplies the driving circuit with a control signal, corresponding to a value indicative of the present instructed (target) wire tension t0 (t0=0 when no wire tension is applied) and stored in a second register T, so as to cause the driving circuit to apply a desired voltage to the electromagnetic coil of the electromagnetic powder brake 12, thereby controlling the actual wire tension to the target value t0. When it is determined that the instructed wire tension is updated, the CPU sets a new instructed wire tension tn into a third register Tn (step S1), and compares the new instructed wire tension tn with the present target wire tension t0, to determine whether or not the absolute value of the difference therebetween (variation amount in the instructed wire tension per one control operation cycle, i.e., the instructed variation rate) is larger than the upper limit variation rate Δ t of the target wire tension (step S2). If the instructed variation rate is larger than the upper limit value Δ t, then it is determined whether or not the new instructed value tn is larger than the present target value t0 (step S3). If the value tn is larger than the value t0, that is, if it is predictively determined that there is a possibility that the actual wire tension may excessively increase when the new instructed value is immediately used as the new target value, the total sum of the present target value t0 and the upper limit variation rate Δ t is stored into a second register T as a new target value (step S4), and then a control signal corresponding to the total sum (t0+Δ t) stored in the second register T is supplied to the electromagnetic powder brake 12 (step S5). Next, the step S2 is entered, and then the steps S2 to S5 are repeatedly executed. As a result, as shown in Fig. 5, the target wire tension t0 increases from the value t0, obtained before the time point tA at which the updating operation starts, towards the instructed value tn by the upper limit value Δt each time the wire tension updating process of Fig. 2 is effected. Then, in the step S2 of the processing cycle corresponding to the time point tB at which the updating process is completed, the CPU determines that the absolute value of the difference between the instructed value tn and the present target wire tension t0 is not larger than the value Δt, and sets the instructed value tn as a new target value into the second register T (step S6). Whereupon, the CPU supplies a control signal corresponding to the value tn stored in the second register to the brake 12, thus completing the updating process (step S7). As a result, as shown in the left half portion of Fig. 4, the operation voltage of the electromagnetic powder brake 12 gradually increases from a value 0 obtained at the start of the updating process to the value V1 obtained at the completion of the same process. At this time, the actual wire tension gradually increases from the values 0.0 N obtained before the updating operation to the instructed wire tension 29,43 N [3.0 kgf] during the updating processing period ΣU (for example, 1 to 3 sec.). Therefore, the wire will not vibrate and the brake 12 will not be locked so that the wire will not be broken. On the other hand, when it is determined in the step S2 of the processing cycle, corresponding to the time point at which the updating process starts, that the absolute value of the difference between the instructed wire tension tn and the present target wire tension t0 is larger than the upper limit value Δ t, and furtherit is determined in the step S3 that the value tn is not larger than the value t0, that is, when it is determined that there is a possibility that the actual wire tension may be excessively reduced if the new instructed value is immediately used as the new target value, the CPU sets a value, derived by subtracting the upper limit value Δt from the present target wire tension t0, into the second register as a new target value (step S8). Whereupon, the step S2 is entered after execution of the step S5. Thereafter, a series of operations including the steps S2, S3, S8 and S5 are repeatedly effected until the target wire tension reaches the instructed value. As a result, the operation voltage of the powder brake 12 and the actual wire tension gradually decrease as shown in the right half portion of Fig. 4. Therefore, no wire vibration occurs, and a rapid release operation of the brake 12 and resulatant slackness of the wire 3 do not occur. Thus, an erroneous determination in the breakage detection operation by the breakage detection apparatus 5 will not occur. When it is determined in the step S2 of the processing cycle, corresponding to the time point at which the updating process starts, that the absolute value of the difference between the instructed wire tension tn and the present target wire tension t0 is not larger than the upper limit variation rate Δ t, an excessive variation in the actual wire tension and problems caused thereby do not occur even if the instructed wire tension tn is immediately used as a new target wire tension. Thus, the aforesaid steps S6, S7 are executed, thereby completing the updating process. The present invention is not limited to the above embodiment and various modifications may be made. For example, it is not inevitably necessary to linearly update the target wire tension in the wire tension setting/updating process of Fig. 2, and it is possible to non-linearly update the same. Further, it is possible to provide, between the steps S5 and S2 in the setting/updating process, a step of delaying the updating operation of the target wire tension by an appropriate time period. In this case, for example, a timer in which the delay time is set is started at the end of the step S5, and then the CPU is set in a stand-by mode until the timer is up.

Claims: A wire tension controlling method in a wire cut electric discharge machine adapted to control a wire tension applied to a wire electrode (3) by means of an electric brake (12), said method being characterised by steps of: (a) predictively determining, when a wire tension setting/updating instruction is delivered, presence/absence of a possibility of an actual wire tension being excessively changed if said electric brake (12) is operated in accordance with said instruction; (b) changing a target wire tension to a predetermined amount (Δt) above or below the present target wire tension so as to prevent an excessive change of said actual wire tension when the presence of the possibility of said actual wire tension being excessively changed is determined, then controlling the actual wire tension accordingly, and (c) periodically effecting said step (b) until said target wire tension, and then the actual wire tension, reaches an instructed wire tension specified by said instruction; wherein in step (a) the presence of the possibility of said actual wire tension being excessively changed is determined when it is determined on the basis of a present instructed wire tension and a new instructed wire tension that a variation rate of said instructed wire tension is larger than a predetermined variation rate (Δt) corresponding to said predetermined amount (Δt) used in said step (b); and wherein said instructed wire tension is immediately set as a new target wire tension when it is determined in said step (a) that there is no possibility that said wire tension may be excessively changed. A wire tension controlling method in a wire cut electric discharge machine according to claim 1, wherein said electric discharge machine includes a brake roller (13) arranged in contact with said wire electrode (3) and connected to said electric brake (12), and a control unit (2) connected to said electric brake (12); and wherein said control unit (2) has a step of controlling a braking force applied from said electric brake (12) to said brake roller (13) in accordance with said target wire tension obtained in said step (b), thereby controlling said actual wire tension to said target wire tension. A wire tension controlling method in a wire cut electric discharge machine according to claim 2, wherein said electric brake (12) is comprised of an electromagnetic powder brake operable to generate a braking force which varies in dependence on any one of operating power, operating voltage and operating current. A wire tension controlling method in a wire cut electric discharge machine according to any preceding claim, wherein said predetermined amount (Δt) used in said step (b) is set to such a value as to prevent occurrence of an excessive variation in said actual wire tension which may cause wire breakage. A wire tension controlling method in a wire cut electric discharge machine according to any preceding claim, wherein said electric discharge machine includes a breakage detection apparatus (5), and wherein said predetermined amount (Δt) used in said step (b) is set to such a value as to prevent occurrence of an excessive variation in said actual wire tension which may cause an erroneous determination in a breakage detection operation of said breakage detection apparatus (5). A wire tension controlling method in a wire, cut electric discharge machine according to any preceding claim, and wherein said predetermined amount (Δt) used in said step (b) is set to such a value as to prevent occurrence of an excessive variation in said actual wire tension which may cause harmful wire vibration.

Inventors: ARAKAWA, YASUO FANUC DAI-3 VIRA-KARAMATSU, ASO, TOSHIYUKI ROOM 7-208, FANUC MANSHONHARIMOMI, KATO, JUNICHI
Applicants: FANUC LTD.
Application year: 1989
Main classification: B23H7
IPC codes: B23H7