POSITIONING METHOD FOR AN ELECTRICALLY-DRIVEN INJECTION MOLDING MACHINE

Abstract: A positioning method uses widths of allowable positioning sections for the axes that are independently set depending on the operation directions of the operation units of the electrically-driven injection moulding machine and on the functions to be achieved. Positioning of the screw to the injection completion position is finished when the practical screw position (PA) is equal to or smaller than the sum of a desired injection completion position (PS) and a width (W1) which is set to "O" on the positive side region, and is further equal to or greater than a value which is obtd. by subtracting a width (W2) of the negative side region from the desired position (PS). The screw is positioned at the desired injection completion position or to a screw move position slightly in excess of the desired position by an allowable distance, and the amt. of shot is correctly controlled.

Description: The present invention relates to a positioning method according to the preamble of claim 1. An electrically-operated injection molding machine is provided with various operating units including an injection unit arid the like, each of which is driven by a corresponding one or more servomotors, to perform an injection molding cycle including an injection process, etc. In a respective process of the injection molding cycle, positioning control for one or more control axes, corresponding to the same process, is carried out. For instance, in a final stage of the injection process, positioning control to an injection end position on an injection axis is carried out. To this end, an injection molding machine of this kind is provided with servo circuits for individual axes so as to control the drive of the servomotors for these axes. Each of the servo circuits receives a pulse train, indicative of a target motor rotation position and supplied from a numerical control unit, and another pulse train, indicative of an actual motor rotation position and supplied from a position detector mounted on the servomotor concerned, to effect positioning to the target rotation position. Generally, in order to effect the positioning with a desired accuracy and within a desired setting time, the positioning is completed when the actual motor rotation position falls within an allowable positioning section having a predetermined width and consisting of positive-side and negative-side regions, which are adjacent to each other with respect to a boundary at which the target rotation position is located. Here, the positive-side region indicates part of the allowable positioning section at the side away from the origin of a coordinate system, which is set for an associated axis, with respect to the target position, and the negative-side region indicates part of the section at the side facing the coordinate origin. Conventionally, the width of the positive-side region of the allowable positioning section and the width of the negative-side region thereof are set to the same value of "W/2", as shown in Fig. 4, wherein reference symbol W indicates the width of the allowable positioning section, and PS indicates the target position. In certain cases, it is preferable to effect the positioning for each of control axes in a specific manner which is determined in dependence on an operating mode of the associated operating unit such as the direction to which the operating unit is operated and a function to be achieved by the operating unit. For instance, in order to effect the injection by which a predetermined amount of shot can be attained, preferably, the injection process should be completed and a shift to the hold process should be made at a screw-moving-position which slightly exceeds a target injection end position. On the other hand, in order to protect a mold of a type which is liable to be damaged when a mold-opening operation is continued even after a predetermined mold-opening stroke is reached, if such a type of mold is employed, the mold-opening process should preferably be completed at a movable-platen-moving-position slightly short of a target mold-opening end position. In other words, if the respective origins of coordinate systems set for the injection and the mold-clamping axes are located at the sides opposite the directions of injection and mold-clamping, the injection screw should preferably be positioned to a position falling within the negative-side region of the allowable positioning section associated with the target injection end position, whereas the movable platen should preferably be positioned to a position falling within the positive-side region of the allowable positioning section associated with the target mold-opening end position. However, the aforementioned conventional method cannot achieve the positioning to a position falling within an arbitrary one of the positive-side and negative-side regions of the allowable positioning section, although the positioning within this section can be surely made. In this respect, conventionally, the target position is shifted to either one of positive and negative sides by the half width "W/2" of the allowable positioning section, where required, in view of the operating manner of an associated operating unit of the injection molding machine. More specifically, if the positioning to a moving position exceeding the target position PS should be made, as with the aforesaid case for shifting from the injection process to the hold process, the target position is set to "PS + W/2", as shown in Fig. 5. On the other hand, if the positioning to a moving position short of the target position should be made as in the case of mold-opening end operation, the target position is set to "PS - W/2", as shown in Fig. 6. In this manner, the respective target position should be shifted, if necessary, in the light of the operating manner of the associated operating unit. Accordingly, labor is required for the preparation of a control program. Moreover, positioning accuracy deteriorates because an effective width of a corresponding one of the positive-side and negative-side regions of the allowable positioning section is increased due to the shift of the target position. A positioning method according to the preamble of the attached claim 1 is disclosed in JP-A-62-290513. The object of the present invention is to provide a positioning method which is capable of positioning an operating position of a respective operating unit of an electrically-operated injection molding machine, with ease, preferably in an optimum way which is determined in dependence on an operating manner of the operating unit. This object is achieved according to the invention by a positioning method as specified in claim 1. Particular embodiments of the invention are disclosed in the dependent claims 2 to 5. Preferably, said widths of said positive-side and negative-side regions are set in dependence on an operating direction to which said operating unit is operated and an operation to be achieved by said operating unit. As mentioned above, if the widths of the positive-side and negative-side regions of the allowable positioning section, associated with the operation of the operating unit of the electrically-operated injection molding machine, are set in a manner independently from each other, the positioning of the operating unit can be effected in an optimum manner suitable for the mode in which the operating unit is operated, with reduction of load to an operator or a programmer and with no deterioration in positioning accuracy. In other words, the positioning to an optimum target position can be carried out with an optimum positioning accuracy in the light of the direction to which the operating unit is operated and the function to be achieved by the operation section. Brief Description of the Drawings Fig. 1 is a flowchart showing a positioning-end discriminating processing in a positioning method according to an embodiment of the present invention; Fig. 2 is a schematic view showing part of an injection molding machine for embodying the method of Fig. 1; Fig. 3 is a view showing, by way of example, an allowable positioning section, which is set in accordance with the method of Fig. 1; Fig. 4 is a view showing, by way of example, an allowable positioning section set by a conventional positioning method; Fig. 5 is a view showing another example of an allowable positioning section set by the conventional method; and Fig. 6 is a view showing still another example of an allowable positioning section set in accordance with the conventional method. An electrically-operated injection molding machine to which a positioning method according to an embodiment of the present invention is applied, comprises various operating units (not shown) such as injection unit (mentioned later), screw rotation unit, mold-clamping unit, eject unit, and is so arranged as to control the drive of these operating units by means of a numerical control unit (hereinafter, referred to as NC unit), mentioned later, and a programmable machine controller (not shown). Referring to Fig. 2, the injection unit includes an injection servomotor 2 having a pulse coder 3 and operable to axially drive a screw 1, disposed within a heating cylinder 4, through a rotary motion/linear motion conversion mechanism (not shown). The NC unit 100 includes a central processing unit 101 for numerical control (hereinafter, referred to as NCCPU) to which are connected a ROM 101a storing therein a management program for universally controlling the injection molding machine, and a RAM 101b for temporal or temporary storage of data. Further, servo circuits (only the servo circuit corresponding to the injection servomotor 2 is shown by numeral 106) for controlling the drive of servomotors of the various operating units are connected to the NCCPU through a servo interface 107. The NC unit 100 further includes a central processing unit 102 for the programmable machine controller (hereinafter, referred to as PMCCPU) to which are connected a ROM 102a storing therein, e.g., a sequence program for controlling a sequence operation of the injection molding machine, and a RAM 102b for temporal storage of data. Reference numeral 103 denotes a non-volatile shared RAM, consisting of bubble memory, CMOS memory or the like, for storing therein an NC program and various molding condition parameters. In the NC program, widths of positive-side and negative-side regions, set in an independent manner, of allowable positioning sections for each of individual axes (various operating units) are stated together with an associated target value. Here, the positive-side region of the allowable positioning section indicates part of the same section at the side away from the origin of a coordinate system, set for an associated control axis, with respect to the target position on the same axis, and the negative-side region indicates part of the allowable positioning section at the side facing the coordinate origin. In an example of Fig. 3, the coordinate system of the axis is set in such a manner that the coordinate position (the operating position of an associated operating unit of the injection molding machine) decreases along the direction (shown by arrow a ) to which the operating unit is operated. The widths of the positive-side and negative-side regions of the allowable positioning section are set to "W1" and "W2", respectively. The values W1, W2 are positive or zero, respectively, and the width W of the whole section is equal to the sum of W1 and W2. In the present embodiment, the width W1 of the positive-side region of the allowable positioning section associated with the injection axis is set to "0", so that a shift from the injection process to the hold process is made when the screw 1 reaches the target injection end position or a screw-moving position which slightly exceeds the target injection end position toward the mold-clamping unit by a permissible distance. Referring to Fig. 2 again, in the NC unit 100, respective busses of the CPUs 101, 102, the shared RAM 103, an input circuit 104, and an output circuit 105 are connected to a bus arbiter controller (hereinafter, referred to as BAC) 108 for control of selection of a bus to be enabled at a certain information processing cycle of the NC unit 100. Further, a manual data input device with a CRT display (hereinafter, referred to as CRT/MDI) 110 is connected to the BAC 108 through an operator panel controller 109, so that an operator is permitted to operate various operative keys of the CRT/MDI 110 including software keys and ten-key pad, so as to input various control parameters including the molding condition parameters. In the following, operation of the injection molding machine constructed as above will be explained. During operation of the injection molding machine, the NCCPU 101 performs the pulse distribution to servo circuits of associated operating units of the molding machine through the servo interface 107, and the PMCCPU 102 performs sequence control for associated operating units, respectively, in accordance with the NC program and various control parameters stored in the shared RAM 103 and with the sequence program stored in the ROM 102a. As a result, the injection molding cycle which consists of a series of processes of mold-opening, mold-closing, mold-clamping, metering, injection, hold, ejection of a molded product, etc. is carried out in basically the same manner as in the conventional molding machine. During the time period in which the pulse distribution operation is performed, each of registers (not shown) accommodated in the servo interface 107 is supplied with a command pulse train from the NCCPU 101 and a feedback pulse train from the pulse coder 3 corresponding to the same register, so that the register is stored with a value, indicative of the actual rotation position of the servomotor corresponding to the same register (the actual operating position of the associated operating unit). The NCCPU 101 periodically reads out the stored values of the respective registers in the servo interface 107, and writes these values into present-value registers (not shown) in the shared RAM 103, respectively, through the BAC 108. The PMCCPU 102 executes a positioning-end discriminating processing for the individual axes at predetermined intervals of cycle. With reference to Fig. 1, the processing for the injection axis, by way of example, will be explained hereinbelow. After start of the injection process, the PMCCPU 102 reads out the actual moving position PA, at the present time, for the injection axis through the BAC 108 from the present-value register for the injection axis in the shared RAM 103 at predetermined intervals of cycle (step S1), and reads out, from the shared RAM 103, the target injection-end position PS and the widths W1, W2 of the positive-side and negative-side regions of the allowable positioning section associated therewith, which are stated in the NC program. Whereupon, the PMCCPU determines whether or not the screw 1 has moved to a position falling within the allowable positioning section, associated with the injection-end position. More specifically, the PMCCPU determines, at the step S2, whether or not the value PA is equal to or less than the sum PS + W1 of the target injection-end position PS and the width W1 (=0) of the positive-side region of the allowable positioning section, and further determines, at the step S3, whether or not the value PA is equal to or larger than the value PS - W2 obtained by subtracting the width W2 of the negative-side region of the allowable positioning section from the value PS. In case that the coordinate system is set for the injection axis as shown in Fig. 3, the actual position PA gradually decreases, with advancement of the injection process. At an initial stage of the injection process, the actual value PA is larger than the value PS + W1, and hence the result of determination at the step S2 is negative (NO). In this case, the PMCCPU 202 determines that the positioning of the screw 1 to the target injection-end position has not been completed as yet, and completes the determination processing of the present cycle, and then performs the same determination in the next determination processing cycle. Thereafter, if the result of the determination at the step S2 becomes affirmative (YES) in a certain discrimination processing cycle, then the PMCCPU determines, at the step S3, whether or not the value PA is equal to or larger than the value PS - W2 in the aforementioned manner. Ordinally, the result of determination at the step S3 is affirmative. In this case, the PMCCPU 102 determines that the screw 1 has reached the target injection-end position or a moving position which slightly exceeds the target position, and then writes a positioning-end signal into the shared RAM 103 (step S4), to complete the discrimination processing for the injection-end position. In response to the positioning-end signal being written, the injection process is completed. This results in a suitable shot whose amount is accurately controlled. Whereupon the hold process is entered. On the other hand, the PMCCPU determines that the positioning of the screw 1 to the injection-end position has not been completed as yet due to the presence of overshoot in controlling the drive of the screw 1, for instance, so long as the result of the determination at the step S3 is negative even when the determination result of the step S2 is affirmative, completes the present determination processing cycle, and repeats the processing of Fig. 1 in the next determination processing cycle. Thereafter, the determination result at the step S3 becomes affirmative, and then the processing advances to the aforesaid step S4. Further, positioning for other control axes of the injection molding machine is effected in the aforementioned manner. For instance, in case that the injection molding machine is mounted with a mold of a type which is liable to be damaged when the mold-opening operation is continued over a stroke larger than a predetermined mold-opening stroke, the width of a corresponding one of the positive-side and negative-side regions of the allowable positioning section, associated with the mold-opening-end position of the mold-clamping unit, is set to "0", so that the mold-opening operation is finished at the target mold-opening-end position or a movable-platen-moving position slightly short of the target position, to thereby protect the mold. The present invention is not limited to the foregoing embodiment, and various modifications thereof may be made. For instance, although the positioning widths W1, W2 are indicated in the NC program in the foregoing embodiment, the positioning widths W1, W2 may be stored in a predetermined storage region of the shared RAM 103 in response to a manual operation of a keyboard of the CRT/MDI 110 for setting the positioning widths, so that these stored values W1, W2 are referred to during the positioning-end discrimination processing. Further, the discrimination of completion of positioning may be made on the basis of an actual position error, although, in the embodiment, such discrimination is effected on the basis of target position PS, actual position PA and positioning widths W1, W2. In this case, the absolute value of the actual position error, which error is stored in a position error register provided in a servo circuit or a servo interface and then transferred to and stored in the shared RAM, is compared with each of the positioning widths W1, W2, and completion of the positioning is determined when the absolute value of the actual position error is less than both of the positioning widths.

Claims: A positioning method for positioning an operating position of an operating unit (1), driven by a servomotor (2), of an electrically-operated injection molding machine within a predetermined allowable positioning section containing a target position (PS), characterised by steps of: (a) setting a width (W1) of a positive-side region of said allowable positioning section at one side of said target position (PS); and (b) setting a width (W2) of a negative-side region of said allowable positioning section at the other side of said target position (PS) in a manner independently from said width (W1) of said positive-side region, said negative-side region being adjacent to said positive-side region. A positioning method according to claim 1, wherein said widths (W1 and W2) of said positive-side and negative-side regions are set in dependence on an operating direction to which said operating unit (1) is operated, and an operation to be achieved by said operating unit (1). A positioning method according to claim 1 or 2, further including a step of: stating said widths (W1 and W2) of said positive-side and negative-side regions of said allowable positioning section in a control program together with the target position (PS) associated with said allowable positioning section. A positioning method according to claim 1 or 2, further including a step of: storing set values of said widths (W1 and W2) of said positive-side and negative-side regions of said allowable positioning section into a memory (103) of the injection molding machine in a manner readable therefrom, in response to a manual setting of said set values. A positioning method according to any preceding claim, further comprising a step of: completing positioning for the operating position of said operating unit (1) when it is determined that an actual operating position of said operating unit (1) falls within said allowable positioning section on the basis of said actual operating position, a target operating position (PS), and said widths (W1 and W2) of said positive-side and negative-side regions of said allowable positioning section.

Inventors: KAMIGUCHI MASAO ROOM - FANUC, NEKO NORIAKI FANUC DAI- VIRA-K, KAMIGUCHI, MASAO ROOM 6-207, FANUC, NEKO, NORIAKI FANUC DAI-3 VIRA-KARAMATSU
Applicants: FANUC LTD, FANUC LTD.
Application year: 1989
Main classification: B29C45
IPC codes: B29C45, G05B19