Method of automatic zero adjustment of an injection-molding machine and an apparatus therefor

A method and an apparatus for automatic zero adjustment of an injection-molding machine, in which an axis driven by a servomotor can be automatically returned to its origin with accuracy and speed. When a one-revolution signal is produced after the axis reaches a deceleration position, a preset coordinate position of a reference point is written in a current-value register. Subsequently, when the axis driven toward an absolute position reaches the absolute position, the preset coordinate position of the reference point is corrected with use of a correction value calculated on the basis of a known coordinate position of the absolute position and the register value. When the axis driven toward the reference point reaches the corrected position, the preset reference point coordinate position is written in the current-value register.

TECHNICAL FIELD 
The present invention relates to a method of zero adjustment for drive 
axes, such as an injection axis, clamp axis, ejector axis, etc., of an 
injection-molding machine and an apparatus for effecting the same. 
BACKGROUND ART 
In injection-molding machines using oil pressure for drive sources for its 
injection unit, clamping (mold clamping) unit, etc., the strokes of a 
screw for injection and a clamp axis depend on the stroke of associated 
hydraulic cylinders, and the origin positions or zero points of the screw 
and the clamp axis are determined mechanically by themselves. 
In injection-molding machines driven by means of servomotors, however, the 
rotation of the servomotors is subject to no mechanical restrictions. 
Therefore, the origin positions of an injection axis, clamp axis, and 
ejector axis, which are driven by these servomotors, cannot be uniformly 
determined mechanically. Unless the origin points of the axes driven by 
the servomotors are positioned accurately with respect to the body of the 
injection-molding machine, however, the injection-molding machine body and 
equipment associated therewith may be possibly damaged. Unless the 
position of the injection axis, for example, or the screw position, 
relative to the injection-molding machine body, is detected accurately, 
and if the screw is not positioned accurately, the screw may possibly run 
against a heating cylinder, thereby damaging the screw or heating 
cylinder. Also in cushion amount adjustment or in injection-speed 
switching control, the position of the screw relative to the 
injection-molding machine body must be detected accurately for screw 
positioning. 
Likewise, unless the clamp axis is positioned accurately with respect to 
the injection-molding machine body, a mold may be possibly damaged. 
Generally, zero return operation along an axis driven by means of a 
servomotor is effected by locating the origin of the axis at a reference 
point which is preset on a predetermined coordinate position in a 
coordinate system of the machine body. The reference point is set to the 
position which is reached by the axis when the servomotor is rotated 
through a predetermined angle from a rotational position corresponding to 
an invariable coordinate position in the coordinate system of the 
injection-molding machine. More specifically, the injection-molding 
machine typically comprises a position detecting system which includes an 
absolute-value pulse coder adapted to deliver a signal indicative of one 
revolution of the servomotor with every arrival of a grid at a 
predetermined rotational position, a deceleration dog attached to the 
injection-molding machine body, and a sensor attached to the axis for 
detecting the decelerated dog. The position detecting system is adjusted 
so that the grid of the pulse coder takes a position opposite to the 
predetermined rotational position when a deceleration dog signal, 
delivered from the sensor, goes low while the axis is passing the 
deceleration dog during the return to the origin. Meanwhile, the reference 
point is set to the position reached by the axis when the servomotor makes 
a half turn after the deceleration dog signal diminishes. 
Accordingly, if the axis position after the zero return is deviated from 
the reference point during fine adjustment or the like of gears, driving 
belt, etc. of a drive system for the axis, the rotational position of the 
servomotor, after the zero return, is at a halfturn distance, in both 
positive and negative directions, from the position corresponding to the 
reference point, thus falling within a one-revolution range. Zero point 
adjustment can be performed within this range. 
In this zero adjustment, the axis is moved toward the reference point, 
whereupon the deceleration dog signal diminishes. When a one-revolution 
signal is then produced, the axis is stopped. If the stop position of the 
axis is deviated from the set reference point, the position of the 
deceleration dog or the sensor for deceleration dog detection, e.g., a 
limit switch, is manually corrected by shifting the location of the dog or 
the sensor so that the stop position coincides with the set reference 
point. 
In machine tools, robots, etc., whose axes are driven by means of 
servomotors, their accuracy is subject to no special problems, in general, 
even though the zero return is based on the conventional method as 
aforesaid. In injection-molding machines, however, the positional 
relationships between the machine body and the axes to be driven require a 
strict accuracy in microns. Therefore, the individual axes must be 
positioned accurately with respect to the coordinate system of the 
injection-molding machine body. Thus, accurate zero return is needed. 
According to the aforementioned system in which the axes are positioned 
manually at the reference position, however, high-accuracy zero adjustment 
cannot be achieved. Moreover, the adjustment requires much time, and 
cannot be performed automatically. 
SUMMARY OF THE INVENTION 
Accordingly, the object of the present invention is to provide a method and 
an apparatus for automatic zero return, capable of automatic zero return 
with accuracy and speed, in an injection-molding machine whose injection 
axis, clamp axis, ejector axis, etc., are driven by means of servomotors. 
In order to achieve the above object, a method of automatic zero adjustment 
according to the present invention comprises steps of: (a) driving a 
servomotor to move an axis of an injection-molding machine from a zero 
return start position toward a reference point; (b) storing a preset 
coordinate position of the reference point as a current coordinate 
position of the axis when the servomotor further rotates to take a 
predetermined rotational-angle position after the axis reaches a 
deceleration start position situated short of the reference point; (c) 
updating the memory value of the current coordinated position while moving 
the axis toward an absolute position whose coordinate position is known; 
(d) correcting the preset reference point coordinate position with use of 
a correction value calculated on the basis of the memory value of the 
current coordinate position and the known coordinate position of the 
absolute position when the axis reaches the absolute position; (e) 
updating the memory value of the current coordinate position while moving 
the axis toward the reference point; and (f) replacing the memory value 
with the preset reference point coordinate position when the memory value 
reaches the corrected reference point coordinate position. 
An automatic zero point adjusting apparatus according to the present 
invention comprises: axis position detecting means for severally detecting 
the arrival of an axis, driven by means of a servomotor of an 
injection-molding machine, at a zero origin return start position, a 
deceleration start position, and an absolute position of the axis; motor 
rotational-position detecting means adapted to produce a one-revolution 
signal with every revolution of the servomotor; current-value memory means 
whose memory value is updated as the axis moves; memory control means for 
renewing the memory value of the current-value memory means; and 
correcting means for correcting the preset reference point coordinate 
position with use of a correction value calculated on the basis of a 
preset coordinate position of the absolute position and the memory value 
of the current-value memory means at the time of the arrival of the axis 
at the absolute position, the current-value memory means being adapted to 
store the preset reference point coordinate position when the 
one-revolution signal is first produced after the axis reaches the 
deceleration start position, and the memory value of the current-value 
memory means being replaced with the preset reference point coordinate 
position when the axis reaches the corrected reference point coordinate 
position. 
According to the present invention, as described above, if the position of 
the axis relative to the body of the injection-molding machine is 
dislocated due to adjustment of gears, belt or the like of a drive system 
for the axis, which is driven by means of the servomotor, the amount of 
dislocation is detected accurately and automatically to the minimum unit 
of movement of the servomotor. Also, the dislocation is automatically 
corrected for the adjustment of the origin position of the axis. Thus, the 
axis is accurately positioned with respect to the injection-molding 
machine, so that accurate zero return can be effected with an accuracy in 
microns which is required by the injection-molding machine.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Best Mode of Carrying Out the Invention 
FIG.1 shows the positional relationships between dogs and limit switches, 
with respect to an injection axis, as an example, in an apparatus to which 
is applied a method of automatic zero adjustment according to an 
embodiment of the present invention. In FIG. 1, numeral 2 denotes a 
stationary platen fixed to a base (not shown) of the housing of an 
injection-molding machine. A dog 1, which is fixed to the stationary 
platen 2, includes a zero return start dog 1a, used to set a zero return 
start position mentioned later, and a deceleration start dog 1b for 
setting a deceleration start position and an absolute position mentioned 
later. Symbols A and B designate limit switches which serve as sensors for 
detecting the zero return start dog 1a and the deceleration start dog 1b, 
respectively. Both the limit switches A and B are fixed to the injection 
axis or screw axis 100. The dogs 1a and 1b are arranged sot that the limit 
switch B is situated on the same side of the deceleration start dog 1b as 
the stationary platen 2 when the zero return start dog 1a is detected by 
the limit switch A. Thus, there is a positional relationship (indicated by 
symbols A' and B' in FIG. 1) such that the limit switch B cannot step on 
the deceleration start dog 1b. A reference point R is situated at a 
position on the right of the deceleration start dog 1b, as in FIG. 1, and 
at a distance L1 from the stationary platen 2. In other words, the 
coordinate position of the reference point R, with respect to the 
screw-axis moving direction, is set to L1. When the injection axis 100, 
positioned accurately, reaches the right-hand end or trailing end of the 
deceleration start dog 1b, a grid G (FIG. 2) of an absolute-value pulse 
coder attached to a servomotor takes a rotational position diametrically 
opposite to a predetermined rotational position (uppermost rotational 
position) where a one-revolution signal mentioned later is produced. In 
connection with this, the coordinate position L1 of the reference point R 
is set so as to be coincident with the moved position of the injection 
axis reached when the grid G reaches the uppermost position, as the 
servomotor makes a half turn after the injection axis 100 reaches the 
trailing end of the deceleration start dog, and when the one-revolution 
signal is delivered from the pulse coder. 
In the present embodiment, the position of the trailing end of the 
deceleration start dog 1b is used as an absolute position L2, and the 
limit switch B doubles as a sensor for detecting the absolute position L2. 
FIG. 2 is a diagram for illustrating the principle of operation of the 
present embodiment. In FIG. 2, symbol G designates the grid of the 
absolute-value pulse encoder P which is attached to the servomotor M used 
to drive a injection axis. The pulse encoder produces a one-revolution 
pulse when the grid G reaches the uppermost rotational position as 
illustrated. In FIG. 2, (a) shows a state such that the injection axis is 
positioned normally, while (b) and (c) show cases in which the injection 
axis is dislocated. 
If the injection axis is positioned accurately, as indicated by (a) in FIG. 
2, and it moves from left to right of FIG. 2, when the limit switch B 
reaches the trailing position of the deceleration start dog 1b, the grid G 
is situated in its lowermost rotational position. Thereafter, when the 
rotating shaft of the pulse coder makes an additional half turn so that 
the grid G reaches the uppermost position where the one-revolution pulse 
is delivered from the pulse coder, the injection axis reaches the 
reference point coordinate position L1. 
However, if the injection axis is dislocated after gears or belt of a drive 
system for the injection axis of the injection-molding machine is 
adjusted, the grid G does not take the uppermost rotational position or 
one-revolution pulse generating position, although the reference point 
coordinate position L1 is reached by the injection axis, as indicated by 
(b) and (c) in FIG. 2. If the injection axis position at the point of time 
of generation of the one-revolution pulse is set as the reference point in 
a current-value register, a difference .+-..alpha. between the reference 
point coordinate position L1 and the one-revolution pulse generating 
position is produced as an error, as indicated by (b) and (c) of FIG. 2. 
Thereupon, according to the present embodiment, the preset reference point 
coordinate position L1 is written in the current-value register R when the 
one-revolution signal is produced after the limit switch B slides down the 
deceleration start dog 1b, that is, when a tentative reference point is 
reached. Thus, a tentative coordinate system for zero adjustment is 
established. Subsequently, the injection axis is moved from right to left 
of FIG. 2 until the limit switch B steps on the deceleration start dog 1b. 
Then, the absolute position L2 of the trailing end of the deceleration 
start dog 1b is compared with the value in the current-value register R, 
which is indicative of the injection axis position reached when the limit 
switch B reaches the dog 1b. Thus, the error .+-..alpha. is obtained. The 
coordinate position of the reference point in the tentative coordinate 
system is detected with use of the obtained error .alpha.. When the 
injection axis reaches the reference point coordinate position in the 
tentative coordinate system, moreover, the reference point coordinate 
position L1 is set in the current-value register R, that is, the ordinary 
coordinate system is restored. Thus, return to the origin is finished. 
The zero return operation of the present invention will now be described on 
the assumption that there is a relationship shown in FIG. 2(c) between the 
dog and the rotational position of the grid, for example. Let it be 
supposed that the distance between the normal reference point and the 
trailing end of the deceleration start dog 1b is L3. When the first 
one-revolution pulse is produced after the limit switch B slides down the 
deceleration start dog 1b, the reference point coordinate position L1 is 
set in the current-value register R, although the actual absolute 
coordinate position of the axis at that time is L1+.alpha. (FIG. 2(c). 
Accordingly, the axis is driven in the opposite direction (from right to 
left of FIG. 2) thereafter, and the value in the current-value register R 
obtained when the limit switch B is moved so as to step on the 
deceleration start dog 1b is L1--(L3+.alpha.). Meanwhile, this position is 
the absolute coordinate position L2. In order to examine the error at this 
point of time, the absolute coordinate position L2 (=L1-L3) is subtracted 
from the value in the current-value register R, whereupon we obtain 
EQU {L1-(L3+.alpha.)}-L2=L1-L3-.alpha.-L2=-.alpha. 
Thus, the result of subtraction, which is to be zero if the axis is 
returned normally to the reference point coordinate position L1, is found 
to be -.alpha., that is, a value smaller than the true value by .alpha. is 
set in the current-value register R. In other words, the reference point 
is situated at a position deviated toward the deceleration start dog 1b by 
a distance .alpha. from the axis position corresponding to the aforesaid 
one-revolution generating position. As shown in FIG. 2 (c)-; after all, if 
the axis is moved so that the value in the current-value register becomes 
equal to L1-.alpha., which is obtained by adding a correction value 
-.alpha. to the reference point coordinate position L1, after switch B 
defects position L2--and if the reference point coordinate position L1 is 
written in the current-value register R when the destined position is 
reached (i.e. when axis moved to right by the amount L1-9) then a correct 
reference point coordinate position is stored in the current-value 
register R. 
FIG. 3 is a block diagram showing the principle part of the 
injection-molding machine which, exemplifying the present invention, 
performs the aforementioned operation. In FIG. 3, numeral 10 denotes a 
numerical control unit (hereinafter referred to as NC unit) for 
controlling the injection-molding machine. The NC unit 10 includes a 
microprocessor (hereinafter referred to as CPU) 11 for NC, and a CPU 12 
for a programmable machine controller (hereinafter referred to as PMC). 
The PMCCPU 12 is connected with a ROM 15 which stores a zero return 
program, mentioned later, a sequence program for various operations of the 
injection-molding machine, etc. The NCCPU 11 is connected with a ROM 14 
which stores a control program for generally controlling the 
injection-molding machine. The NCCPU 11 is also connected, through a 
servo-interface 16, with servo-circuits for controlling the drive of 
servomotors for various axes for injection, clamping, screw rotation, 
ejector operation, etc. Among the servomotors, only the motor for the 
injection axis is designated by symbol M. Among the servo-circuits, only 
the circuit associated with the motor M is denoted by numeral 17. Symbol 
PC designates an absolute-value pulse coder attached to the servomotor M. 
Those servomotors, absolute-value pulse coders, and servo-circuits 
associated with the other axes than the injection axis 100 are omitted in 
FIG. 3. 
Numeral 18 denotes a nonvolatile common RAM which, including a backup power 
source, stores programs for controlling the various operations of the 
injection-molding machine, various set values, parameters, etc. Numeral 13 
denotes a bus-arbiter controller (hereinafter referred to as BAC), which 
is connected with the respective buses of the NCCPU 11, the PMCCPU 12, the 
common RAM 18, an input circuit 20, and an output circuit 21. The bus to 
be used is controlled by means of the BAC 13. The BAC 13 is also connected 
with a manual-data input device 30 with a display (hereinafter referred to 
as CRT/MDI) through an operator panel controller 19. Numeral 22 denotes a 
RAM used for tentative storage of data during various processes of 
operation by the NCCPU 11. Also, the PMCCPU 12 can be shown connected 
selectively with a RAM. In the present embodiment, no RAM is connected to 
the PMCCPU 12. 
Symbols A1 and B1 designate limit switches attached to the injection axis 
100; A2 and B2, limit switches attached to a clamp axis (not shown); and 
A3 and B3, limit switches attached to an ejector axis (not shown). The 
limit switches A1, A2 and A3, which correspond to the limit switch A of 
FIG. 1, are used to detect the zero return start dog, out of the dogs 
provided individually for the axes. On the other hand, the limit switches 
B1, B2 and B3, which correspond to the limit switch B of FIG. 1, are used 
to detect the deceleration start dog, out of the dogs provided 
individually for the axes. These limit switches A1 to A3 and B1 to B3 are 
connected to the input circuit 20. 
Referring now to the operational flow chart of FIG. 4, the operation of the 
present embodiment will be described. 
If a command for zero return is inputted through the CRT/MDI 30, the PMCCPU 
12 reads out the zero return program from the ROM 15, thereby successively 
returning the individual axes to their origins. When starting the process 
with the injection axis 100, the PMCCPU 12 drives the motor M for the 
injection axis with the aid of the BAC 13, NCCPU 11, servo-interface 16, 
and servocircuit 17, thereby moving the injection axis toward the zero 
return start position (Step S1). Then, the PMCCPU 12 determines, through 
the medium of the BAC 13 and the input circuit 20, whether or not a 
detection signal is inputted which is delivered when the zero return start 
dog 1a is detected by the limit switch A1 (Step S2). If the limit switch 
A1 is turned on so that the detection signal is inputted, the servomotor M 
is reversed to move the injection axis toward the reference point (Step 
S3). 
As seen from FIG. 1, the limit switch B1 is off when the limit switch A1 is 
on. The PMCCPU 12 monitors, through the BAC 13 and the input circuit 20, 
to determine whether or not the limit switch B1 is turned on by stepping 
on the deceleration start dog 1b (Step S4). If the limit switch B1 is 
turned on, the PMCCPU 12 decelerates the servomotor M, and causes it to 
advance the axis movement toward the reference point. Then, the PMCCPU 12 
determines whether or not the limit switch B1 is turned off by sliding 
down the deceleration start dog (Step S6). Thereafter, when the grid 
reaches the uppermost rotational position as the rotating shaft of the 
absolute-value pulse coder P rotates, the one-revolution pulse is 
delivered from the pulse coder P. When the CPU 12 detects this through the 
BAC 13, NCCPU 11, servo-interface 16, and servocircuit 17 (Step S7). it 
stops the drive of the injection axis (Step S8), and stores the 
current-value register R in the common RAM 18 with the reference point 
coordinate position L1 which is set and stored in the common RAM 18 (Step 
S9). This injection axis stop position is a tentative reference position. 
Then, the servomotor M is driven to move the axis in the direction for 
absolute position detection. In the present embodiment, the deceleration 
start dog 1b doubles as a dog for absolute position detection, so that the 
injection axis moves to the left of FIG. 1 (Step S10). The CPU 12 monitors 
to determine whether or not an ON signal from the limit switch B1 is 
inputted (step S11). If the limit switch B1 steps on the deceleration 
start dog 1b to be turned on, the movement of the injection axis is 
stopped (Step S12). Then, the absolute position L2 set and stored in the 
common RAM 18 is subtracted from the value of the current-value register R 
to obtain the grid correction value (indicated by +.alpha. or -.alpha. in 
FIG. 2) (Step S13). Subsequently, the axis is moved again toward the zero 
return start position (Step S14). When the limit switch A is turned on 
(Step S15), the servomotor M is reversed to move the axis toward the 
reference point (Step S16). If the limit switch B1 steps on the 
deceleration start dog 1b in the same manner as aforesaid (Step S17), the 
speed of the axis movement is reduced. Thereafter, the axis is moved until 
the value of the current-value register R, which is updated as the axis 
moves, attains a value corresponding to a corrected reference point 
coordinate position, that is, a new reference point coordinate position 
which is obtained by adding the grid correction value .alpha., calculated 
in Step S13, to the reference point coordinate position L1 stored in the 
common RAM 18 (Step S18). When the aforesaid position is reached, the 
reference point coordinate position L1 is written in the current-value 
register (Step S19). Thus, when the injection axis is in the correct 
reference point coordinate position L1, the reference point coordinate 
position L1 is written in the current-value register R, and the injection 
axis is located accurately in a correct position relative to the 
injection-molding machine. 
When the zero return operation for the injection axis is completed in this 
manner, the same process is then executed for each of the clamp axis and 
the ejector axis. Thus, the injection axis, clamp axis, and ejector axis 
are returned automatically to their respective origins in succession. 
In the embodiment described above, the value stored as the absolute 
position in the common RAM 18 is used as the position of the trailing end 
of the deceleration start dog 1b on the reference-point side. 
Alternatively, however, the position of the trailing end of the zero 
return start dog 1b may be used as the absolute position, and the limit 
switch A may be given a function as a sensor for absolute position 
detection, as well as the function for zero return position detection. In 
this case, the limit switch A must only be checked in place of the limit 
switch B in Step S11 of FIG. 4, and Steps S14 and S15 may be omitted. More 
specifically, if the limit switch A is on, then the limit switch B is 
already situated on the same side of the deceleration start dog 1b as the 
front plate 2, after having stepped over the dog 1b. Thus, Steps S14 and 
S15 can be omitted. 
The dog for the absolute position and the sensor, such as a limit switch, 
for detecting the dog may be provided separately from those for the zero 
return start or deceleration start. In the aforementioned embodiment, 
moreover, the dog 1 is fixed to the injection-molding machine body, and 
the sensors or limit switches are fixed to the axes. In contrast with 
this, however, the limit switches or sensors may be fixed to the 
injection-molding machine body. In this case, the dog 1 is fixed to the 
axes. Instead of using the limit switches as the sensors, furthermore, 
photoelectric tubes of proximity switches may be used as the sensors. In 
this case, the dog must be constructed corresponding to these sensors.