Note: Descriptions are shown in the official language in which they were submitted.
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Mold Actuator Stroke Control
TECHNICAL FIELD OF THE INVENTION
[0001] The present invention relates to injection molding machinery. More
specifically, the
present invention relates to the control systems used to open and close a
mold.
BACKGROUND OF THE INVENTION
[0002] Some examples of known molding systems are: (i) the HyPETTm Molding
System, (ii)
the QuadlocTM Molding System, (iii) the E-IylectricTM Molding System, and (iv)
the HyMetTm
Molding System, all manufactured by Husky Injection Molding Systems Ltd.
[0003] FIG. 1 is the perspective view of a molding system 20 (preferably an
injection molding
system hereafter referred to as the "system 20") according to the first
exemplary embodiment.
The system 20 is used to mold one more molded articles (not shown). The system
20 includes
components that are known to persons skilled in the art and these known
components will not be
described here; these known components are described, by way of example, in
the following
references: (i) Injection Molding Handbook by Osswald/Turng/Gramann ISBN: 3-
446-21669-2;
publisher: Hanser, and (ii) Injection Molding Handbook by Rosato and Rosato
ISBN: 0-412-
99381-3; publisher: Chapman & Hill.
[0004] The system 20 includes (amongst other things): (i) an injection-type
extruder 22
(hereafter referred to as the "extruder 22"), (ii) a hopper 24, (iii) a
control cabinet 26, (iv) a
human-machine interface, hereafter referred to as the "HMI 28", (v) a
stationary platen 30, (vi) a
moveable platen 32, and (vii) an ejector assembly 34 (described in greater
detail below). FIG. 1
depicts an approximate location of the ejector assembly 34 relative to the
system 20. The
extruder 22 has a barrel and a reciprocating screw disposed in the barrel.
Alternatively, the
extruder 22 could be a two stage shooting pot configuration. The hopper 24 is
coupled to a feed
throat of the extruder 22 so as to deliver pellets of moldable material to the
extruder 22. The
extruder 22 is configured to: (i) process the pellets into an injectable
molding material, and (ii)
inject the injectable material into a mold that is held closed by the platens
30, 32 after the platens
30, 32 have been stroked together. The control cabinet 26 houses control
equipment that is used
to control the system 20. The HMI 28 is coupled to the control equipment, and
the HMI 28 is
used to assist an operator in monitoring and controlling operations of the
system 20.
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[0005] The stationary platen 30 is configured to support a stationary mold
portion of a mold (not
shown). The moveable platen 32 is configured to: (i) support a moveable mold
portion of the
mold, and (ii) move relative to the stationary platen 30 so that the mold
portions of the mold
(neither shown) may be separated from each other or closed together. A mold
stroke actuator 36
(hereafter referred to as the "actuator 36") is coupled to the platens 30, 32.
Preferably, there are
two platen stroke actuators, each of which are mounted, respectively, at
opposite diagonal
corners of the platens 30, 32. The mold stroke actuator 36 is used to stroke
the moveable platen
32 relative to the stationary platen 30. Preferably, during mold closure, the
actuator 36
decelerates shortly before achieving contact between the two mold halves to
reduce the impact
and preserve the lifespan of the mold.
[0006] The stationary platen 30 supports four clamp actuators 38 that are each
positioned in
respective comers of the stationary platen 30. Four tie bars 40 each extend
from their respective
clamp actuators 38 toward respective corners of the moveable platen 32. The
tie bars 40 are
lockable relative to the moveable platen 32 by usage of respective tie-bar
locks 41 that are each
supported in respective comers of the moveable platen 32.
[0007] Fig. 2 provides a graph showing various defined and measured
performance parameters
for a typical mold stroke actuator during a mold close operation, such as the
one used on the
above systems. In this system, an operator calibrates the maximum daylight
between the
movable and stationary mold halves. Line 80 shows the position of movable
platen 32 as it
moves from the fully open position to the fully closed position. In this
example, the maximum
daylight is just over 400 mm. The operator further defines the period of the
mold close operation,
which in this example is just under 0.5 seconds.
[0008] An open loop control system 78 regulates the velocity of the movable
platen 32 based
upon elapsed time in order to achieve this cycle time. An idealized velocity
profile 82 is
provided by a lookup table. For each moment of the actuator stroke cycle, a
closing velocity set
point is provided for movable platen 32. Thus, at T=0 seconds the velocity
setpoint is 0 mm/s. At
T=0.1 seconds, the velocity set point peaks at just over 1600 mm/s and then
begins decelerating
to avoid the mold halves from crashing together. The acceleration is linear,
causing a sharp peak
for the velocity profile.
[0009] The actual velocity 84 of movable platen 32 deviates from the velocity
profile 82, given
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the large inertial mass of the movable platen 32 as well as measurement
latency in the system.
For example, from T=0 seconds to T=0.2 seconds, the movable platen is slower
than its setpoint
and from T=0.2 seconds to T=0.475, the movable platen 32 is faster than its
target set point.
[00010] A mold stroke command 86 indicates the control voltage used to open
or close a
valve in system 20's hydraulic circuit, thereby increasing or decreasing
hydraulic pressure within
actuator 36. Changes to the hydraulic pressure within actuator 36 accelerates
or decelerates the
movable platen 32.
SUMMARY OF THE INVENTION
[00011] According to a first aspect of the present invention, there is
provided a controller
for a mold stroke actuator connected to a movable platen, the controller
operable to receive
operational measurements and change the output of the mold stroke actuator
using a closed loop
control system.
The present invention can provide closed loop control with feed forward
compensation
for a movable platen of an injection molding machine. The closed loop control
is on velocity
and acceleration. The acceleration is computed from net force (pressure
multiplied by cylinder
area) and moving mass. Feed forward compensation is obtained from the velocity
profile
The invention provides a predetermined velocity profile that is optimized for
mold stroke
to achieve its fastest mold close and mold open times with a minimum of jerky
motion. The
invention further provides a closed loop control system and method that
controls the mold stroke
to track the predetermined velocity profile. The invention also provides auto-
calibration of a
mold stroke valve, which can have considerable tolerance variations.
BRIEF DESCRIPTION OF THE DRAWINGS
[00012] Exemplary embodiments of the present invention will now be
described with
reference to the accompanying drawings in which:
Fig. 1 is a perspective view of a prior art molding machine;
Fig. 2 is a graph showing a prior art open loop control system for the molding
machine of
Fig. 1;
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Fig. 3A and 3B are hydraulic schematics for a mold stroke actuator for the
molding
machine of Fig. 1 in accordance with a non-limiting embodiment of the
invention; and
Fig. 4 is a graph showing a closed loop control system for a molding machine
in
accordance with a non-limiting embodiment of the invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[00013] The inventors have determined that known prior-art closed loop
control systems
such as PIDs do not work well for mold stroke actuators. The large mass of the
movable platen
and mold half coupled with a relatively small actuator caused significant
latency in the system.
The natural operational frequency of the system is low and error correction
generates instability
in the system. Even with modern processing architecture, control systems do
not update quickly
enough to achieve smooth operation. When the machine is running quickly, the
mold stroke
motion can become rough and jerky. Reliability and repeatability are not
easily achieved with an
open-loop control system, and machines can experience considerable variation
which requires
manual calibration of the valves and the machine.
[00014] In contrast, the present control loop system obviates these
problems, and can
result in reduced cycle time, improved mold open position repeatability, and
reduced machine
shaking and vibrations.
[00015] Referring now to Fig. 3A and 3B, the operation of actuator 36 is
described in
greater detail. Fig. 3A provides a schema for a mold close operation, and Fig.
3B provides a
schema for a mold open operation. For simplicity, pilot lines have been
omitted. Actuator 36 is
operable to linearly translate movable platen 32 between an open and a closed
position and is
motivated by a hydraulic circuit 50. As used herein, the movable platen 32
includes the attached
mold, unless otherwise stated.
[00016] Hydraulic circuit 50 can include a pump 52, an optional accumulator
54, the
actuator 36, a circuit switch (not shown), proportional shutoff valves 58, 60,
and 68, pressure
sensors or gauges 62, and a tank 64. In the circuit shown, pump 52 is a
unidirectional, fixed
displacement pump, but other types of pumps can be used, depending on the
configuration of the
machine 20. Actuator 36 is a double-acting piston. Preferably, the circuit
switch (not shown) is a
ports / 3 position directional valve as is known to those of skill in the art
that provides
regenerative fluid capacities to actuator 36. One port of the circuit switch
leads to the cylinder
side of actuator 36, another port leads the rod side 60A of actuator 36,
another port leads to
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pump 52, another port leads to tank 64, and yet another port leads back to rod
side 60A. The
pressure gauges 62 measure the difference in pressure between the cylinder and
rod sides of
actuator 36. As is known to those of skill in the art, proportional valves 58
and 60 can have wide
operational tolerances, approaching as much as 10% between valves.
[00017] As shown in Fig. 3A, in order to move the movable platen 32 to the
closed
position, the switch is in the first position so that flow from the pump 52
and/or accumulator 54
is directed to cylinder side of actuator 36 in order to motivate the movable
platen 32 to move
towards the stationary platen 30. Hydraulic fluid from rod side of actuator 36
is directed back to
the cylinder side as part of a regenerative circuit to improve performance and
minimize the
amount of hydraulic fluid required by machine 20.
[00018] As shown in Fig. 3B, in order to move the movable platen 32 to the
open position,
the switch is in a second position so that the flow from pump 52 (and less
likely, accumulator 54)
is directed to the rod side of actuator 36 in order to motivate the movable
platen 32 away from
the stationary platen 30. Hydraulic fluid from cylinder side of actuator 36
drains to tank 64.
Greater or lesser flow control can be provided by the restriction/expansion of
shut-off valves 60,
greater or lesser outputs from pump 52, or greater or lesser discharges from
accumulator 54.
[00019] Referring now to Fig. 4, a control system 100 is described to
regulate mold stroke
closure for the injection machine 20 and the like. Control system 100 defines
a predetermined
velocity profile 102 that is generally sinusoidal and divided between an
acceleration phase 110
and a deceleration phase 112 of the movable platen 32 as it moves from the
open to closed
position. That is to say, movable platen 32 accelerates and decelerates in a
generally symmetrical
fashion that can be graphed over time as a sine function. Velocity profile 102
graphs the ideal,
predetermined velocity (in mm/s) of movable platen 32 over time (in s). As is
described in
greater detail below, acceleration of movable platen 32 is a function of time,
and deceleration is
a function of position. A similar velocity profile is provided to move the
movable platen 32 from
the closed position to the open position.
[00020] A mold stroke command 104 indicates the control voltage (V) used to
open or
close the valve 58 (during mold close), thereby increasing or decreasing
hydraulic pressure
within actuator 36. Changes to the hydraulic pressure within actuator 36
accelerates or
decelerates the movable platen 32. During mold open, the control voltage
regulates valve 60.
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[00021] The actual velocity 106 of movable platen 32 is measured in real-
time (mm/s). As
is discussed in greater detail below, control loop 100 with feed forward error
correction
continually adjusts the actual velocity 106 of movable platen 32 to better map
to the desired
velocity profile 102.
[00022] Position 108 shows the measured position of movable platen 32
relative to the
closed position (in mm) over time. Fig. 4 shows movable platen 32 moving from
the fully open
position to the fully closed position (approx. 400 mm in the example provided
by Fig. 4).
[00023] As mentioned previously, in control system 100 the acceleration of
movable
platen 32 is based on time during the acceleration phase 110, but the
deceleration of movable
platen 32 during the deceleration phase 112 is based upon the position of the
platen. Control
system 100 corrects for velocity, acceleration and position, and also provides
feed forward
correction.
[00024] The target velocity of movable platen 32 for any point of the
velocity profile 102
during the acceleration phase is determined by time, and is defined by the
following equations.
[00025] Acceleration for any point of time during the acceleration phase
110 of velocity
profile 102 is defined by the function:
a(t)= am sin(a) where
.0 9Fa.
a.= __________
m
co 1 =amax
and where
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xmaõ
a(t). acceleration of movable platen 32 at time t
= Maximum acceleration of movable platen 32
co = Circular frequency of the sinuisodal profile
F.õõ = Maximum force applicable by mold stroke actuator 30
m = Moving mass of movable platen 32 (including the attached mold)
x.õ = Distance travelled by movable platen 32 between the open and closed
positions
[00026] Given the above determination of acceleration, velocity profile 102
during the
acceleration phase 110 can be defined as follows:
vaõ = Max(vaõ(t ¨1),v aõ (t ¨1) + a(t) At) where
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V acc = Velocity profile 102 during the acceleration phase 110
v. (t ¨1) = Velocity during acceleration at time t ¨1 or previous scan
[00027] The maximum velocity of movable platen 32 (which may exceed the
peak
velocity of velocity profile 102) is determined as follows:
2 x
v = max wh
mm,ere
27r
= ¨
co
v. = Maximum velocity of movable platen 32
T = Period of the sinuisodal profile, i.e., the duration of the mold stroke
operation
[00028] The target velocity of movable platen 32 for any point of the
velocity profile 102
during the deceleration phase 112 is determined by the measured position of
movable platen 32
(to prevent mold crash), and is defined by the following equations.
[00029] The velocity of movable platen 32 during deceleration can be
determined as
follows:
r
vmax ¨1/2 a deoei(x(t) ¨ xo)
v dem = vavg ¨ vspan cos Max ,271- _______
vmax
= Vmax Vcontact
V avg
2 where
, _ Vmax Vcontact
span ¨
2
a = v' = amax
decel
2T 27r
V decel = Velocity of movable platen 32 during deceleration
vaõg = Average velocity of movable platen 32
Van = Velocity span, i.e., a period where the movable platen 32 moves at its
peak velocity (and can be 0)
v. = Maximum velocity of movable platen 32
V contact = Contact velocity or final velocity of movable platen 32 when it
reaches the fully closed position
a decel = Position based deceleration, which provides an estimate of the
required deceleration based upon time
(i.e., assuming a perfect sinosoidal profile)
x(t) = Position at time t
x0 = Safety distance (typically set to 0 mm)
amax = Maximum acceleration
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1000301 Given the above definitions, the velocity profile 102 can be
defined for any point
along the mold stroke by the following equation:
vsp (t) = Min(v aõ (t), v . (t), v .4(0) where
vp (t) = Velocity setpoint for movable platen 32 at time t
s
V acc(t) = Velocity during acceleration at time t
v .(t) = Velocity during acceleration at time t
v1 (1) and Vdecel(t ) will always meet in the middle of the mold stroke,
thus providing
symmetry for the velocity profile 102.
[00031] Control system 100 compensates when the actual velocity 106
deviates from the
velocity profile 102. A PID controller or the like is used to perform the
compensation functions.
Control system 100 does not simply correct for errors in velocity of movable
platen 32, but can
correct for other factors as well. Errors in the position of movable platen 32
(i.e., e P(t) ) are
preferably zeroed out. Also, as the acceleration of the movable platen 32 is
very difficult (and/or
expensive requiring accelerometers) to measure directly, these errors (i.e.,
ea (t)) are preferably
zeroed out. Instead, acceleration based upon the force provided by actuator 36
is used (a (t)
)based on the pressure differential between cylinder and rod sides of actuator
36 (and measured
by pressure gauges 62).
1000321 The corrected velocity can be defined as follows:
V corrected (t) ev(t)kp+ep(t)k,+ea(t)kd+eaf(t)kdf +1,v where
eõ(t) = v sp (t) ¨ v ,(t) e p (t) = x sp(t)¨ x (t)
ea (t) = a (t) ¨ a põ(t) e (t) = as,, (t) ¨ a (t) and where
dv PA (t)A A ¨ PB(t)AB F f (V)
a sp = a f (t) = ______________
dt
V corrected (t) = Velocity output at time t, after being corrected by the
closed
loop feedback
e,, (t) = Error in velocity at time t
e (t) = Error in position at time t
e (t) = Error in acceleration at time t
eaf (t) = Error in acceleration force at time t
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k p = Proportional gain
k = Integral gain (often zeroed out)
k = Derivative gain (often zeroed out)
kdf = Derivative gain for acceleration force
vsp (t) = Velocity setpoint at time t
Vace (t) = Velocity during acceleration at time t
Vm (t) = Maximum velocity at time t entered by the user
V decel (t) = Velocity during deceleration at time t
v(t) = Velocity at time t
x(t) = Position at time t Subscripts sp and põ denote setpoint and actual
value
a(t) = Acceleration at time t
a (t) = Calculated acceleration force at time t
PA (t) = Pressure in the cylinder side of the actuator 30 at time t
PB (t) = Pressure in the rod side of the actuator 30 at time t
AA = Cylinder area
AB = Rod area
F (v) = Friction force as a function of velocity
m = Moving mass
For an actuator 36 having a regenerative hydraulic circuit, the following
equation defines the
voltage required to enlarge or reduce the opening in the proportional valve:
V corrected (t) APT:ef
V
[00033] ref VI (t) ¨ P A(t) Z(V)11PB(t)¨ PA(t)
where
z(V)= kB(V)
kA(V)
v ref (t) = Reference velocity at time t, which is used to determine the
command voltage
to the valve from a reference table (look - up table)
V corrected (t) = Velocity output at time t, after being corrected by
closed loop feedback
APref = Pressure drop used in the reference (look - up) table
(t) = Supply pressure at time t
P, (t)= Pressure in the cylinder side of the actuator 30 at time t
PB (t) = Pressure in the rod side of the actuator 30 at time t
z(V) = Ratio of flow resistance as a function command voltage to the valve
kB (V) = Flow resistance of valve from rod to cylinder side
k ,(V) = Flow resistance of valve from supply to cylinder side
[00034] Given the above equations, an operator can set a target speed, and
control system
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100 will calculate the required period and velocity profile. Alternatively, an
operator could set a
target period, and control system 100 will calculate the required velocity.
For all speeds from
0 to vmaõ where the target speed is v
Tõ 1v y
= T max p = power index = 0,...,0.25,...,0.5,...1
V,
2x
0 < n <1
Tn(max) max Tn Tn(max)
vn
[00035] The presently preferred value for n = 0.25
27r
co ¨
Tõ
v õcon
an = ____________
2
a(t) = a n sin(cont)
[00036] In an idealized system, where the actual velocity follows tightly
with the velocity
setpoint, then deceleration of movable platen 32 would start at the midpoint
(assuming that there
was no interval where movable platen 32 moved at a constant velocity) and
could be simply
determined as follows:
= vmax = a max
"decel
2T 2r
[00037] However, in practice, using the above equation would cause movable
platen 32 to
lag and decelerate too late. Preferably, when the actual velocity 106 of
movable platen 32 lags
behind velocity profile 102, or if the actual velocity 106 is greater than the
setpoint defined by
velocity profile 102, the deceleration reference is determined as follows. The
starting time of the
deceleration phase 112 (i.e., when deceleration begins) can be calculated as
t decel = t accel t const_sp
Tõ
t accel = ¨2 subscript n indicates for a given speed
tconst sp = = velocity setpoint (minimum to maximum)
- Ax
Ax = xmax ¨ xo ¨ xaccei ¨ X dem where
Xmax = mold stroke setpoint or actual mold open position
xo = safety distance
I?, To
Xaccel = X decel
4
[00038] The starting point for the deceleration of movable platen 32 is
defined as follows:
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tz- max(v,
Xstart _SP =
vp, = actual velocity at time t
V nlOn
= _________
2
[00039] Deceleration of movable platen 32 is dynamically calculated to
correct for errors:
if t > t decelor x(t) < xsta,t_sp (i.e., the mold is closed) or x(t) >
Jcari_sp (i.e., the mold is open)
X start _decel = x(t) latched!
Vn 2
adecel = min adecel max' Qf
ukX start _decel 0) j
a max
= ¨
a decelmax
_
2ir
[00040] The present invention provides a control system for a mold system
that achieves
faster mold close and open times than known prior-art open loop control can
attain. The present
invention can provide a smooth mold stroke motion and improved mold stroke
repeatability. The
present invention can provide ease of set-up, auto valve calibration and no
need for machine
calibration. The present invention can further provide optimized velocity
profile to close and
open mold with the fastest times and smooth motion. The present invention can
provide closed
loop control with feed forward compensation. The closed loop control is on
velocity and
acceleration. The acceleration is computed from net force (pressure multiplied
by cylinder area)
and moving mass. Feed forward compensation is obtained through compensation of
the pressure
drop across the valve.
[00041] While the present invention has been described with respect to what
is presently
considered to be the preferred embodiments, it is to be understood that the
invention is not
limited to the disclosed embodiments. To the contrary, the invention is
intended to cover various
modifications and equivalent arrangements included within the spirit and scope
of the appended
claims. The scope of the following claims is to be accorded the broadest
interpretation so as to
encompass all such modifications and equivalent structures and functions.
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