Fuzzy inference thermocontrol method for an injection molding machine with a plurality of means for heating or cooling

To eliminate a temperature overshoot or an undershoot during thermocontrol of thermocontrolled components, e.g. an injection cylinder, with respect to an object temperature in each operating status of the injection molding machine, the Fuzzy Control theory is used for controlling the injection molding machine. By using the Fuzzy Control theory, the object temperature of the thermocontrolled components can be attained with practically eliminated overshoot and undershoot.

BACKGROUND OF THE INVENTION 
The present invention relates to a thermocontrol method for an injection 
molding machine, more precisely relates to a method for controlling 
temperature of thermocontrolled components, e.g. an injection cylinder, of 
the injection molding machine having means for heating and/or cooling, 
which are arranged at the prescribed intervals, to correspond to the 
instant operating status thereof. 
Conventionally, the PID (Proportional band-Integral time-Differential time) 
control method is usually adopted to control the temperature of 
thermocontrolled components, e.g. an injection cylinder, of an injection 
molding machine. 
The PID control method is based on a proportional action (P), which is in 
proportion to a control deviation; an integral action (I), which is based 
on an integrated value of the control deviation; and a differential action 
(D), which is based on differential coefficients of the thermocontrolled 
components. Using the PID control method, the object temperature can be 
maintained when the thermocontrolled components are under certain stable 
conditions. 
However, the injection molding machine has various operating states such as 
stop, temperature rise, mold, pause, etc. Furthermore, heating elements 
and cooling elements are different devices in each status. For example, in 
the temperature rise state, the heat of the heaters is the main heating 
element; radiant heat is the main cooling element. On the other hand, heat 
from the heater, the heat of friction caused by an injection screw acting 
upon the resin, etc. are the heating elements in the mold status, while 
the natural radiant heat, the endothermic reaction by resin supplied, etc. 
are the cooling elements therein. 
The interior temperature of injection cylinders is controlled to maintain a 
prescribed temperature distribution so as to inject resins smoothly. 
Generally, a plurality of heaters are arranged in the axial direction of 
the injection cylinder at prescribed intervals. Heat of each 
thermocontrolled component, which is conducted via separating parts 
between adjacent thermocontrolled components, affects the temperature of 
adjacent thermocontrolled components. 
In the conventional PID control of the injection cylinder, the heating 
elements and the cooling elements are different in each status. The 
temperature of each thermocontrolled component of the injection cylinder 
is shown in FIG. 7. 
FIG. 7 shows temperature change of each thermocontrolled component of the 
injection cylinder in the status of temperature rise, wherein the 
injection cylinder has three heaters which are serially provided between 
an injection nozzle and a hopper. 
In the injection cylinder, the relationship among the object temperature 
T01 of a first thermocontrolled component whose temperature is controlled 
by a first heater H1 provided on the injection nozzle side; object 
temperature T03 of a third thermocontrolled component whose temperature is 
controlled by a third heater H3 provided on the hopper side; and object 
temperature T02 of a second thermocontrolled component whose temperature 
is controlled by a second heater H2 provided between the first heater H1 
and the third heater H3 is: 
EQU T01&gt;T02&gt;T03 
The temperature of an injection cylinder, for example, may be higher 
(overshoot, P.sub.o) or may be lower (undershoot, P.sub.u) than the object 
temperature, as shown in FIG. 7: A Temperature Graph of the Injection 
Cylinder. 
Moreover, the second thermocontrolled component, which is controlled by the 
second heater H2, is affected by heat conducted from the first heater H1 
and the third heater H3, so that the overshoot P.sub.o and the undershoot 
P.sub.u in the graph T2 of the second thermocontrolled component is 
greater than that of the other graphs T1 and T3. 
The overshoot P.sub.o and undershoot P.sub.u of the injection cylinder have 
a large effect on the viscosity of molten resin, so that they may become 
factors in inferior production quality. 
Moreover, the overshoot P.sub.o causes resin deterioration when the object 
temperature is exceeded and the resin deterioration temperature is 
attained, resulting in inferior products. To avoid inferior production 
quality, manual control of the temperature of the injection cylinder, 
based on the experience of a skilled operator, is required. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a thermocontrol method for 
an injection molding machine, which is capable of eliminating as much as 
possible the overshoot and the undershoot with respect to the object 
temperature of thermocontrolled components arranged at prescribed 
intervals, e.g. the injection cylinder, and which is capable of 
automatically controlling the temperature of the thermocontrolled 
components. 
To achieve the object, the inventor first tried the method disclosed in the 
Japanese Patent Publication Gazette No. 63-48591. The method consists of 
designing an estimated heat conduction model of thermocontrolled 
components, and performing thermocontrol in accordance with the heat 
conduction model. 
When the actual heat conduction is similar to the model, this method is 
capable of reducing the overshoot and the undershoot. However, significant 
overshoot or undershoot may occur when the actual heat conduction varies 
from the model. 
The inventor then determined that the Fuzzy Control theory would prove 
effective for controlling the temperature of the thermocontrolled 
components, e.g. the injection cylinder, and thus created the present 
invention. 
Namely, the thermocontrol method for an injection molding machine having a 
plurality of means for heating and/or cooling, which are arranged at 
prescribed intervals, so as to control thermocontrolled components to 
maintain an object temperature corresponding to an operating status of the 
injection molding machine comprises the steps of: 
detecting the operating status of the injection molding machine and the 
temperature of the thermocontrolled components thereof; 
calculating a first deviation between the object temperature of the 
thermocontrolled components corresponding to the present status of the 
injection molding machine and the present temperature detected, and the 
rate of deviation change between the present first deviation and the 
previous first deviation; 
calculating a second deviation between an object temperature of separating 
parts, which are parts between the thermocontrolled components, and the 
present temperature thereof; 
performing fuzzy inference to define a control value for the means for 
heating or cooling by inferring the status of the injection molding 
machine, the first deviation calculated, the rate of deviation change and 
the second deviation wherein the status of the injection molding machine, 
the first deviation, the rate of deviation change, the second deviation 
and the control value of the means for heating or cooling are defined as 
fuzzy variables, and wherein the inference is based both on rules 
governing a mutual relationship among groups within membership functions, 
and membership functions which have groups to which are previously 
assigned optional probabilities which correspond to respective optional 
values; and calculating an actual control value of the means for heating 
and/or cooling based on the fuzzy inference. 
In the present invention, the Fuzzy Control theory is used for controlling 
the temperatures of the thermocontrolled components, whose temperatures 
are mutually effected by heat conduction via the separating parts, so that 
changing the control value of the means for heating and/or cooling, which 
are assembled in the injection cylinder, molds, etc. can be automatically 
executed as if by a skilled operator. 
By utilizing the Fuzzy Control theory, the actual temperature of the 
thermocontrolled components can quickly reach the object temperature, 
during which period temperature overshoot and undershoot can be eliminated 
as much as possible.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT 
Preferred embodiments of the present invention will now be described in 
detail with reference to the accompanying drawings. 
FIG. 1 is a block diagram showing the present embodiment. 
In FIG. 1, an injection molding machine M has an injection cylinder 1, 
which is divided into three thermocontrolled components: a first 
thermocontrolled component, a second thermocontrolled component and a 
third thermocontrolled component positioned at intervals between an 
injection nozzle side and a hopper side. Each thermocontrolled component 
has a thermosensor 5 for detecting the temperature of the thermocontrolled 
component. 
A first thermocontrolled component, located on the injection nozzle side, 
has a first electric heater H1; a third thermocontrolled component, 
located on the hopper side, has a third electric heater H3; and a second 
thermocontrolled component, located between the first thermocontrolled 
component and the third thermocontrolled component, has a second electric 
heater H2. 
The relationship of the object temperatures T01, T02 and T03 of the first, 
second and third thermocontrolled components is: 
EQU T01&gt;T02&gt;T03. 
Conventional thermosensors and conventional electric heaters may be used as 
the thermosensors 5 and the heaters H1-H3. 
Note that, the injection cylinder 1 excludes a nozzle portion, which comes 
into contact with molds. 
As an example, the thermocontrol of the second thermocontrolled component 
of the injection cylinder 1 will be explained. 
The temperature of the second thermocontrolled component is affected by the 
heat of the adjacent first and third thermocontrolled components, so that 
its overshoot and undershoot may be greater than that of the first and the 
third thermocontrolled components as shown in FIG. 7. 
The injection molding machine M is controlled by a programmable controller 
3, and the states of the injection molding machine M such as temperature 
rise, mold, pause, etc. are known from the controller 3. 
Signals from the controller 3 are sent to a microprocessor (MPU)) 9 to 
indicate the present operating status of the injection molding machine M: 
temperature rise, mold, pause, etc. 
The object temperature T01 and T03 of the first and the third 
thermocontrolled components corresponding to the present operating status 
of the injection molding machine M, is defined on the basis of temperature 
data stored in areas (1) and (2) of a computer memory 13. 
The present temperature T2 of the second thermocontrolled component, 
detected by the sensor 5, and the object temperature T02, defined on the 
basis of signal of the controller 3, are sent to the MPU 9. A first 
deviation .DELTA.T2 (.DELTA.T2=T2-T02) is calculated by an arithmetic 
logical unit (ALU) 11 of the MPU 9, written in an area (3) of the memory 
13. 
Furthermore, the ALU 11 calculates the rate of deviation change 
.DELTA.(.DELTA.T)2 of the present first deviation .DELTA.T2 and the 
previous first deviation .DELTA.T2' stored in the area (3) of the memory 
13 (.DELTA.(.DELTA.T)2=.DELTA.T2-.DELTA.T2'). 
In the present embodiment, the ALU 11 calculates the second temperature 
deviation .DELTA.T12, which is the deviation between the object 
temperature and the present temperature of the separating part located 
between the first and the second thermocontrolled components. The second 
temperature deviation .DELTA.T12 is calculated as follows: 
EQU .DELTA.T12=[(T1+T2)-(T01+T02)]/2 
wherein 
T1: detected temperature of the first thermocontrolled component, 
T2: detected temperature of the second thermocontrolled component, 
T01: object temperature of the first thermocontrolled component, 
T02: object temperature of the second thermocontrolled component. 
The separating part between the first and the second thermocontrolled parts 
is usually narrow, and the temperature deviation therebetween is about 
10.degree. C., so that the temperature deviation between the calculated 
temperature of the separating part, which is calculated on the basis of 
the detected temperature T1 and T2, and the detected temperature thereof 
is very small. 
Fuzzy inference is performed on the basis of the membership functions and 
the rules which are stored in the areas (4) and (5) of the memory 13, 
using the status of the injection cylinder 1, the first deviation 
.DELTA.T2, the rate of the deviation change .DELTA.(.DELTA.T)2 and the 
second deviation .DELTA.T12, which are each detected or calculated. 
Then, the control value for the second heater H2 is calculated on the basis 
of fuzzy inference by the ALU 11. The calculated control value is sent to 
the second heater H2 as a control signal by the MPU 9. The above stated 
sequence, from reading data to sending control signals to the second 
heater H2, will be continuously repeated, so that the actual temperature 
of the second thermocontrolled component can quickly reach the object 
temperature T02, which corresponds to the present status of the injection 
molding machine M. Note that, the contents of the memory 13 can be shown 
on an output unit 15, e.g. a display, and can be changed with an input 
unit 14, e.g. a keyboard. 
The membership functions for each fuzzy variable have been stored in an 
area (4) of the memory 13 (see FIG. 2). The fuzzy variables are: the value 
(A) indicating the status of the injection molding machine; the first 
deviation .DELTA.T2 (B) of the second thermocontrolled component; the rate 
of deviation change .DELTA.(.DELTA.T)2 (C) of the second thermocontrolled 
component; the second temperature deviation .DELTA.T12 between the first 
and the second thermocontrolled components; and the degree of output for 
operation (E), i.e. control voltage of the second heater H2. 
Each membership function is divided into a plurality of groups including or 
excluding mutual overlapped sectors. Each group has been respectively 
assigned a grade or a probability (0-1). 
The membership function for the fuzzy variable (A) is divided into five 
distinct groups. The grades of the groups are "0" or "1". The membership 
function for the fuzzy variable (B), which is the first deviation 
.DELTA.T2, is divided into seven groups with overlapped sectors. Five 
groups of the seven are graphically depicted as triangles. In the graph, 
temperature change is indicated along the horizontal axis--the base edge 
of the overlapping triangular groups--and is defined in 10.degree. C. 
increments which correspond to the length of each triangular group's base 
edge. 
Each membership function for the fuzzy variable (C), which is the rate of 
the first deviation change .DELTA.(.DELTA.T)2 and the second deviation 
.DELTA.T12, is divided into five groups with overlapped sectors. Three 
groups of the five are also graphically depicted as triangles. In the 
triangular groups, the rate of deviation change or the second deviation at 
the base edge is defined in 5.degree. C. increments which correspond to 
the length of each triangular group's base edge. 
The second heater H2, whose rated voltage is 200 V, is controlled by 
inputting 100 V plus or minus the control voltage. Thus, the membership 
function whose fuzzy variable is the degree of output for operation (E), 
i.e. the control voltage of the second heater H2, is graphically divided 
into five overlapping sectors whose points of intersection delineate 50 V 
increments. Three groups of the five are graphically shown as triangles. 
The relationship among the groups of each membership function is defined by 
a rule previously stored in an area (5) of the memory 13. A rule for 
temperature rise status, as an example, is shown in the following TABLE: 
TABLE 
______________________________________ 
if then 
INPUT OUTPUT 
No. A B C D E 
______________________________________ 
1 TEMP. RISE NB NB -- PB 
2 TEMP. RISE NB NS -- PB 
3 TEMP. RISE NB ZERO -- PB 
4 TEMP. RISE NB PS -- PB 
5 TEMP. RISE NB PB -- PB 
6 TEMP. RISE NM NB -- PB 
7 TEMP. RISE NM NS -- PB 
8 TEMP. RISE NM ZERO -- PB 
9 TEMP. RISE NM PS -- PS 
10 TEMP. RISE NM PB -- PS 
11 TEMP. RISE NS NB -- PB 
12 TEMP. RISE NS NS -- PS 
13 TEMP. RISE NS ZERO -- PS 
14 TEMP. RISE NS ZERO PB NS 
15 TEMP. RISE NS ZERO ZERO PS 
16 TEMP. RISE NS ZERO NB PB 
17 TEMP. RISE NS PS -- ZERO 
18 TEMP. RISE NS PB -- ZERO 
19 TEMP. RISE ZERO PS -- PS 
20 TEMP. RISE ZERO PB -- PS 
21 TEMP. RISE ZERO NB PS ZERO 
22 TEMP. RISE ZERO NS -- ZERO 
23 TEMP. RISE ZERO PS -- NS 
24 TEMP. RISE ZERO PS PS NS 
25 TEMP. RISE ZERO PB -- NS 
26 TEMP. RISE PS NB -- PS 
27 TEMP. RISE PS NS -- ZERO 
28 TEMP. RISE PS NS ZERO PS 
29 TEMP. RISE PS NS PS ZERO 
30 TEMP. RISE PS ZERO -- ZERO 
31 TEMP. RISE PS ZERO PS NS 
32 TEMP. RISE PS PS -- NS 
33 TEMP. RISE PS PB -- NB 
34 TEMP. RISE PM NB -- NS 
35 TEMP. RISE PM NS -- NS 
36 TEMP. RISE PM ZERO -- NB 
37 TEMP. RISE PM PS -- NB 
38 TEMP. RISE PM PB -- NB 
39 TEMP. RISE PB NB -- NB 
40 TEMP. RISE PB NS -- NB 
41 TEMP. RISE PB ZERO -- NB 
42 TEMP. RISE PB PS -- NB 
43 TEMP. RISE PB PB -- NB 
______________________________________ 
In the TABLE, the INPUTs A, B, C and D in the "if" column respectively 
indicate the fuzzy variables: the status of the injection molding machine 
M (A); the first deviation .DELTA.T2 (B); the rate of deviation change 
.DELTA.(.DELTA.T)2 (C); and the second deviation .DELTA.T12. 
On the other hand, the OUTPUT E in the "then" column indicates the control 
value of the second heater H2 (E). 
In the horizontal direction of the TABLE, for example in row No. 1, the 
relationship among the INPUTs A, B, C and D is logical "AND"; in the 
vertical direction thereof, for example, the relationship between row No. 
1 and row No. 2 is logical "OR". Note that, there are shown in the TABLE 
all combinations of all the groups in the membership function for the 
INPUTs A, B, C and D, but combinations impossible or very rare may be 
omitted from the TABLE. 
Next, the fuzzy inference for defining the input value or the input voltage 
to the heaters will be explained. Note that, this case will be explained 
under the following conditions: the variable (A), Operating Status, is X 
(temperature rise); the variable (B), Latest First Temperature Deviation 
.DELTA.T2, is Y; the variable (C), Latest Rate of Deviation Change 
.DELTA.(.DELTA.T)2 is Z; and the second deviation .DELTA.T12 is R. 
In the position Y of the variable (B) or the first deviation .DELTA.T2, the 
groups "ZERO" and "NS" are overlapped; in the position Z of the variable 
(C) or the rate of the deviation change .DELTA.(.DELTA.T)2, the groups 
"NS" and "NB" are overlapped; and in the position R of the second 
deviation .DELTA.T12, the groups "ZERO" and "PS" are overlapped. 
Therefore, combinations of the INPUTs A, B, C and D result in five rules, 
which are shown in FIG. 3 as rules Nos. 11, 12, 19, 20 and 21. In the 
rules No. 11, 12, 19 and 20, the OUTPUT E is defined by the INPUTs A, B, 
and C; in the rule No. 21, the OUTPUT E is defined by the INPUTs A, B, C 
and D. The relationship among the INPUTs A, B, C and D in each rule is 
logical "AND", so that OUTPUT E for each combination will infer a range 
including the INPUTs A, B, C and D. Namely, the range of the OUTPUT E is 
shown as an area which is partitioned by minimum input values of the 
INPUTs A, B, C and D, marked by the shaded area in FIG. 3. 
The actual control voltage to the second heater H2 is calculated by the ALU 
11 on the basis of the inferred OUTPUTs E of the rules. 
The steps of the calculation will be explained. 
The mutual relationship among OUTPUTs E is logical "OR". First, the 
inferred range (marked by shading in FIG. 3) of each OUTPUT E, is composed 
as shown in FIG. 4. 
Next, the centroid of the inferred range in FIG. 4 is determined and the 
control value, corresponding to the input voltage for the second heater 
H2, is calculated. The control value calculated by the ALU 11 is sent to 
the second heater H2 and the control voltage thereto is controlled by the 
MPU 9. 
Furthermore, the control value for the first heater H1 and the third heater 
H3 will be determined in the same manner and controlled by the MPU 9. 
Utilizing the above described Fuzzy Control theory, temperature overshoot 
P.sub.o and undershoot P.sub.u (see the graph F in FIG. 5) in the 
injection cylinder 1 can be virtually eliminated, and the temperature of 
the injection cylinder 1 is automatically adjusted to correspond to the 
object temperature of the instant operating state of the injection molding 
machine M. Furthermore, inferior production can be prevented. Even in 
cases of using a resin with low thermostability, no manual control by 
experienced operators is required. 
Note that, in the present embodiment, new membership functions, e.g. a 
deviation between the current rate of change and previous rate thereof, 
may be added to the membership functions shown in FIG. 2. 
Additionally, each thermocontrolled component may be controlled 
respectively after all the thermocontrolled components are heated together 
to prescribed temperatures as shown in FIG. 6. 
In the present embodiment, an injection nozzle, which may be provided to 
the front end of the injection cylinder, may also be controlled utilizing 
the Fuzzy Control theory as applied to the injection cylinder. 
Furthermore, the temperatures of the molds also need precise control, so 
they too may be controlled on the basis of the fuzzy inference. 
There are provided means for heating, e.g. an electric heater, and means 
for cooling, e.g. a water circulation pipe, in some molds. In this case, 
the means for heating and cooling may be controlled on the basis of the 
fuzzy theory, so that the overshoot and the undershoot can be eliminated 
as much as possible. Thus, an unskilled operator using a preprogrammed 
Fuzzy Control system is capable of controlling the temperature of the 
molds as if he were a skilled operator applying manual control. 
In the present invention, the temperature of the thermocontrolled 
components, which mutually affect one another, is capable of automatically 
adjusting to the object temperature corresponding to the operating status 
of the injection molding machine. During the afore described automated 
thermocontrol, overshoot and undershoot are eliminated as much as 
possible. Therefore, the present invention contributes to the effective 
operation of injection molding machines and to the reduction of inferior 
molded products. 
The invention may be embodied in other specific forms without departing 
from the spirit or essential characteristics thereof. The present 
embodiment is therefore to be considered in all respects as illustrative 
and not restrictive, the scope of the invention being indicated by the 
appended claims rather than by the foregoing description and all changes 
which come within the meaning and range of equivalency of the claims are 
therefore intended to be embraced therein.