~6~33
1 Background of the Invention
_
This invention is concerned with stroke control For a press and
in particular with methods of and apparatus for controlling the stroke
of a press which comprises a stationary platen, a movable platen~ and
5 a plurality of columns on which the movable platen is mounted and
which move to move the movable platen towards the stationary platen in
a stroke oF the press.
Jn the operation of many presses, a workpiece is placed on the
stationary platen of the press and a stroke of the press takes place
10 to bring the movable platen into pressing engagement with the
workpiece. In such presses, it is necessary to terrninate the stroke
of the press once the workpiece has been pressed sufficiently and
before damage occurs to the press. To cause the stroke to be
terminated at the correct time, presses are provided wi-th stroke
15 control apparatus which senses the position of a portion o-f the
movable platen and causes the stroke to be terminated when that
portion reaches a preselected point. The stroke control apparatus is
set for a particular type of workpiece and then in many cases operates
many times without further setting. Consistent results can be
.~
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1 achieved provided that the workpiece is always located in the same
position on the stationary platen but this may not be possible or
desirable. For example, where a sheet material is spreac! Gn the
s-tationary platen and is press-cut by having a shaped knife pressed
through it, the knife is generally moved along the stationary platen
between strokes of the press. Furtherrnore, positioning of workpieces
continually at the same position causes the platens to become worn
more rapidly than if the workpieces are positioned at different
positions.
Once the stroke control apparatus of a press has been set with
the workpiece in a particular position~ its accuracy will be reduced
if the workpiece is positioned elsewhere. Generally, i-f the worl<piece
is positioned near an edge of the stationary platen, once the movable
platen contacts the workpiece there is a tendency For the movable
platerl to tilt thereby affecting the accuracy of the stroke control
apparatus. Since stroke control apparatuses sense the position of an
edge portion of the movable platen and are generally set with a
workpiece in the center of the stationary platen, if the workpiece is
near the portion sensed, the stroke control apparatus may react later
20 than is desirable whereas, if the workpiece is remote from -the portion
sensed, the stroke control apparatus may react sooner than is
desirable. ~ltho~lgh many presses have balancing mechanisms that
alleviate this problem by endeavouring to maintain the two platens
parallel to one ano-ther, they do not react suFficiently rapidly to
25 overcome the problem. The problem could be reduced by sensing the
~6~33
1 movemernt of a central portion of the movable platen but to position
stroke control apparatus in such a position is highly inconvenient.
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~6~33i
1 Summary of the Invention
It is an object of the present invention to provide a method of
controlling the stroke or a press in which the effects of the problem
described above are reduced.
The invention provio'es a method of controlling the stroke of a
press which comprises a stationary platen, a movable platen, and a
plurality oF columns on which the movable platen is mounted and which
move to move the rnovable platen towards the stationar~ platen in a
stroke of the press, the method compris-ing continuously sensing the
movements of two portions of the movable platen during a stroke, which
portions are symmetrically located with respect to the columns,
generating a signal the magnitude of which at any time during a stroke
is dependent on the sum of the distances moved by the two portiorls
during the stroke, comparing the signal with a preselectecl value,
which value is selected in accordance with the length of stroke
clesired, and terminating the stroke when the magnitude of the signal
ec~uals the preselected value.
Preferably, to enable the stroke control apparatus to be
conveniently positioned, for example beneath the movable platen, the
two portions of the movable platen whose movements are sensecl are
portions thereof to which columns of the press are attached and their
movements are sensing the movement of the columns.
For increased accuracy in controlling the stroke of a -Four-column
press -in which the columns are arranged at the corners of a rectangle,
the movements of two diagonally-opposed columns are sensed.
3~
1 The invention also provides a stroke control apparatus suitable
for use in a method as described in the last preceding paragraph but
two comprisin~ a variable impedance device having two relatively
movable portions and the output of which is dependent on the relative
positions of the two relatively movable portions thereofg mechanical
connections connecting each of the two portions oF the movable platen
to one of the relatively movable portions of the variable impedance
device so that movement of each of the two portions of the movable
platen causes movement of one of the relatively movable portions of
the variable impedance device, comparing means operable to compare the
output oF the variable impedance device with a preselected value, and
terminating means operable to terminate a stroke of the press when the
magnitude of the output of the variable impedance device equals the
preselected value.
Where the press has a balancing valve operable to maintain the
two platens parallel to nne another, the mechanical connections can be
utilized in two ways if they are also connected to a spool of the
balancing valve in such a way that the spool is moved i-F one of the
portions of the movable platen moves more than the other and the valve
acts to balance the movements.
The invention also provides a stroke control apparatus suitable
for use in a method as described in the last preceding paragraph but
four comprising two variable impedance devices, one associated with
each oF the two portions of the movable platen and each havin~ a
movable portion whose position determines the output of the deviceg
3~3
1 the movable portion of each device being mechanically connectect to its
associated portior Qf the movable platen so that movemert of the
portion causes movement of the movable portion, averaging means
operable to average the outputs of the two variable impedance devices,
comparing means operable to compare the average of the OUtpllts of the
two variable impedance devices with a preselected value~ and
terminating means operable to terminate a stroke of the press when the
magnitude of the average of the outputs of the two devices equals the
preselected value.
There now follows a detailed description, to be read with
reference to the accompanying drawings, oi` two apparatwses and their
methods of operation which are illustrative of the invention. It is
to be understood that the illustrative apparatuses and methods have
been selected for description by way of example and not of limitation
o-F the invention.
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1 Brief Description f the Drawings
In the drawin~s:
Figure I is a perspective view, with parts broken away, of the
first illustrative press which is a four~column press;
Figure 2 is a diagrammatic view of electrical circuitry of the
first illustratlve press;
Figure 3 is a front elevational view of the second illustrative
press which is a two-column press; and
Figure 4 is a diagrammatic view of electrical circuitry of the
second illustrative press.
2Q
1 Description of the Preferred Embodiment
The first illustrative press shown in Figure 1 comprises a
framework 10 on which is mounted a stationary lower platen 12. The
lower platen 12 is substantially rectangular in plan view and, at
corner portions of this rectangle, four piston and cylinder assemblies
14, 16, 18 and 20 are mounted on the framework 10. The assemblies 14
and 16 and the assemblies 18 and 20 respectively are
diagonally-opposed to one another. The piston (not shown) of each of
the assemblies 14, 16, 18 and 20 is attached to a piston rod 22 which
projects upwardly and downwardly of the assembly so that operation of
the assembly causes vertical longitudinal movement o-F -the piston rod
22.
The four piston rods 22 are all attached at an upper end portion
thereof to a cnrner portion of a movable upper platen 28 of the press
so that operation of the Four assemblies 14~ 16, 18 and 20 to move the
piston rods 22 downwards moves the upper platen 28 downward towards
the lower platen 12 in an operative stroke of the press, and operation
of the four assemblies 14, 16, 18 and 20 to move the piston rods 22
upwards moves the upper platen 2~ upwards away fronl the lower platen
20 12 in a return stroke of the press. The four piston rods 22 thus form
columns of the press on which the movable platen 28 is mounted and
which move to move the movable platen 28 towards the stationary platen
12 in a stroke of the press.
The four piston and cylinder assemblies 14, 16, 18 and 20 are
2~ connected together in the same hydraulic circuit (not shown) which is
33
1 corntrolled by solenoid valves which are in turn controlled by
electrical switches S1, S2 and 53 (visible in Figure 2). The
arrarlyt-~ment is such that, when the switch S1 is operated a return
s~ruke of the press is terminated and the press comes -to rest in an
open condition. I~hen either the switch S2 or the switch S3 is
operated, an operative stroke of the press terminates and a return
stroke begins. Operator-operated switches (not shown) are also
provided which cause the press to commence an operative stroke. Which
of the switches S2 and S3 is effective to terminate a stroke is
selected by which operator-operated switches are used.
The hydraulic circuit of the first illustrative press also
comprises two balancing valves 30 and 32 (Figure 1). The valve 30 is
associatec! with the first and second assemblies 14 and 1& antl is
operable, in response to either of these assemblies becoming advanced
in its movement relative to the other oF those assemblies, to -throttle
the exhaust of hydraulic fluid from the advanced assembly so that the
press is brought back into balance. The valve 30 throttles an exhaust
when a spoo1 34 thereo-f is moved into or out of the valve 30. If the
spool 34 is moved into the valve 30~ the exhaust from the assembly 14
is throttled while, if the spool 34 is moved out of the valve 30, the
exhaust from the assembly 16 is throttled. Thus, the valve 30 acts to
maintain the upper platen 28 horizontal along a diagonal line between
the assemblies 14 and 1~. The valve 32 acts in the same way with
regard to the assemblies 18 and 20. The balancing valves 30 and 32
2~ are thus operable to maintain the two platens 12 and 28 parallel to
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33
1 one another.
The valve 30 is mounted on the frarne work 10 at a front central
portion thereof while the valve 32 is mounted on a rear central
portion of the framework 10. The spool 34 o-f the valve 30 is
pivotally connected to a central portion of a lever 36. A lower end
portion of the lever 36 is pivotally connected to a link 38 which is
in turn pivotally connected to one arm of a bell-crank lever 40 which
is pivotally mounted on the framework 10 adjacent the assembly 16.
The other arm of the bell-crank lever 40 carries a cam (nct shown)
which is received in a groove 42 in the piston rod 22. Movement of
the piston rod 22 causes the bell-crank lever 40 to pivot moving the
link 38 and thus the lower end portion of the lever 36. An upper end
portion of the lever 36 is pivotally connected to a link 44 which is
in turn pivotally connected to a lever 46 which is fixed on a shaft
15 48. The shaft 48 is pivotally mounted on the frame work 10 and
carries a lever 50 which is fixed on the shaft 48 and carries a cam
(not shown) which is received in a groove 52 in the piston rod 22 of
the assernbly 14. Movement of the piston rod 22 of th` dSSembly 14
causes the shaft 48 to pivot moving the link 44 and thus the upper end
portion of the lever 36. Thus, during an operative stroke of the
press, the links 38 ancl 44 are moved in opposite directions and, so
long as the links 38 and 44 move an equal amount, the center of the
lever 36 does not move and nor does the spool 34. ~lowever, if one of
the links 38 and 44 moves more than the other the spool 34 will be
2~ moved to correct the imbalance. As shown in Figure 1, a similar
3~
1 arran(Jement of levers and links is associated with the valve 32 and
the assemblies 18 and 20. Thus each diagonally-opposed pair of
assemblies has its own balancing means.
A variable impedance device of the first illustrative press is
associated with the balancing means of the assemblies 14 and 16. The
device is in the form of a linear potentiometer having a
horizontally-extending coil 60 and a moving contact 62 (Figure 2)
attached to a rod 64 which extends into the coil 60. The coil 60 and
the contact 62 form two relatively movable portions o-f the
potentiometér and the output of the potentiometer is dependent on the
relative positions of these two relatively movable portions. The coil
60 is attached to a vertically-extencling bar 66 which is clamped to
the link 38 so that movement of the link 38 caused by movement of the
p;ston rod 22 of the assembly 16 causes horizontal movement of the
coil 60. The rod 64 is attached to a vertically-extending bar 68
which is clarnped to the link 4~ so that movement of the link 44 caused
by movement of the piston rod 22 of the assembly 14 causes horizontal
movement of the moving contact 62. The links 38 and 44~ the bell
crank lever 40 the levers 46 and 50 and the shaft 48 thus form
mechanical connections which through the piston rods 22 connect each
of two portions of the movable platen 28 (those portions to which the
piston rods 22 of the assemblies 14 and 16 are attachcd those
portions being symmetrically located with respect to the columns of
the press) to one of the relatively movable portions 60 and 62 o-f the
potentiometer. The arrangement is such that movement of each of the
~12~ 6~333
1 two portions of the movable platen 28 causes movement of one of the
relative movable portions 60 and 62. Thus the voltage on the moving
contact 62 is dependent on the position of both the coil 60 and the
contact 62 and is hence dependent on the sum of the movements of the
piston rods 22 of the assemblies 14 and 16. Since the piston rods 22
are directly attached to the upper platen 28 the voltage on the
movi!-~g contact 62 is dependent on the average separatiorl o-F the
platens 12 and 28 along the line between the assemblies 1~ and 16 and
this average separation is equal to the separation half way between
the assemblies 14 and 16. The voltage on the moving contact 62 thus
indicates the separation of the platens 12 ancl 2~ at the center of the
operative area of the press.
The potentiometer 60 62 forrns part of the electrical circuit
shown in Figure 2. This circuit receives a d.c. power input on a
positive line P and a negative line N. The coil 60 of the
potentiometer 60 62 is connected between the lines P and N. The
circuit also comprises three operator-settable potentiometers POT ];
POT 2 and POT 3~ each of which has its moving contact connected to a
voltage stabilizer in the form of an amplifier with a feedback loop
(A1 A2 and A3 respectively) which acts to prevent undesired
Fluctuations in the voltage received from the movirlg contact 62. The
potentiometer POT 1 has its coil connected in series with a setting
potentiometer SP 1 between the lines P and N. The potentiometers POT
2 and POT 3 are connected in parallel in series with two setting
potentiometers SP2 and SP3 between the lines P and N. The purpose oF
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~6~
l the setting potentiometers SP1, SP2, and SP3 is to allow tuning so
that the full range oF the potentiometers POT 1, POT 2 and POT 3 can
be utilized. The moving contact 62 is connected to a voltage
stabilizer in the form o-F an amplifier A~ with a feedback loop.
The outputs of the amplifiers Al and A4 are connected to a
comparator C1 which produces an output in the form of a positive
voltage so long as the input it receives from the amplifier A4 is
greater than that which it receives from the amplifier A1 but, when
this is not the case, the comparator C1 produces no output. The
output of the comparator C1 is connected to the base of che transistor
T1 which has its collector connected to the line P and its e~itter
connected to the base of a transistor T4. The transistor T4 has its
emitter earthed and 2 relay RL1 is connected between the line P and
the collector o-f the transistor T~. The relay RL1 operates the switch
Sl. The arrangement is such that, as the upper platen 2~ of the first
illustrative press is raised in a return stroke of the press, the
voltage on the moving contact 62 becomes increasingly negative until
it is no longer greater than the voltage from the potentiometer POT 1.
When this occurs~ the comparator C1 switches off its output thereby
switching off the transistor T1 which in turn switches off the
transistor T~ thereby de-energizing the relay RL1 so tha'c the switch
S1 is operated to terminate the return stroke.
The potentiometers POT 2 and POT 3 are both arranged to terminate
operative strokes of the press and the se-ttings thereof a'llow two
alternative stroke terminating positions. Which of the potentiometers
-14- ~6~3~
1 POT 2 and POT 3 is effective to terminate a particular opera~ive
stroke is determined by electrical connections (not shown) to the
switches S2 anrl S3. Since the functions of and connections to the
potentiometers POT 2 and POT 3 are identical only those relating to
the potentiometer POT 2 will be described, with reference numbers
relating to the potentiometer POT 3 being given in brackets after
those relating to the potentiometer POT 2.
As described above the potentiometer POT 2 (POT 3) has its moving
contact connected to the amplifier A2 (A3). The outputs of rhe
amplifiers A2 (A3) and A4 are connected to a comparator C2 (C3) which
produces an output in the form of a positive voltage so long as the
input it receives from the amplifier A2 (A3) is greater than that
which it receives from the amplifier A~ but, when this is not the
case, the comparator C2 (C3) produces no output. The output of the
lS comparator C2 (C3) is connected to the base of a trarlsistor T2 (T3)
which has its collector connected to the line P and its emitter
connected to the base o-f a transistor T5 (T6). The transistor T5 (T6)
has its emitter earthed and a relay RL2 (RL3) is connected between the
line P and the collector of the transistor T5 (T6). The relay RL2
(RL3) operates the switch S2 (S3). The arrangement is such that, as
-the upper platen 28 is lowered in an operative stroke, the voltage on
the moving contact 62 increases until it equals that on the moving
contact of the potentiometer POT 2 (POT 3). When this occurs, the
comparator C2 (C3) switches off its output thereby switching off the
transistor T2 (T3) which in turn switches off the transistor T5 (T6)
-15~ 33~
1 thereby de-energizing the relay RL2 (RL3) so that the switch S2 (~3)
is operated to terrllinate the operative stroke.
The elec-trical circuit shown in Figure 2 comprises comparing
means ln the Form of the comparator C2 operable to compare the output
nf the potentiometer 60, 62 with a preselected value selected by the
setting of the potentiometer 60, 62 and terminating means T in the
form of the relay RL2 operable to terminate a stroke of the press when
-the magnitude of the output equals the preselected value. The circuit
also comprises alternative comparing means in the form of the
comparator C3 operable to compare the output of the potentiometer 60,
62 with an alternative preselected value selected by the setting of
the potentiometer POT 3, and alternative terminating means in the form
of the relay RL3 operable to terminate a stroke oF the press when the
magnitude of the output is equal to the alternative preselected value.
The circuit also comprises further comparing means in the form of the
comparator C1 operable to compare the magnitude of the output of the
potentiometer 60, 62 with a further preselected value selected by the
setting o~ the potentionleter POT 1 in accordance with the length of
return stroke of the movable platen 28 away from the stationary platen
12. The circuit also comprises further terminating means in the form
of the relay RL1 which is operable to terminate a return stroke when
the magnitude of the output is equal to the further preselected value.
In the operation of the first illustrative press, the nlovements
of the two portions of the movable platen 28 which are attached to the
piston rods 22 of the assemblies 1~ and 16 are continuously sensed
~16- ~18~33
l during a stroke by means of the mecharllcal cGnnections 38, 40, 44, 46,
48 and 50 by sensing the movement of the piston rods 22 Form1rlg the
columns. The potentiometer 60, 62 generates a signal in the Form of a
voltage the ma~nitude of which at any time duriny a stroke is
dependent on the sum of the distances moved by the two portions during
the stroke. The comparator C2 or alternatively C3 compares the signal
with a preselected value from the potentiometer POT 2 or POT 3 and the
stroke is terminated by the relay RL2 or RL3 when the ma~nitude of the
signal e~uals the preselected value.
The second illustrative press shown in Figure 3 comprises a
framework lOQ on which is mounted a stationary lower platen 112 which
has a piston and cylinder assembly 114 and 116 mounted at each end
thereof. The pistons (not shown) of the assemblies 114 and 116 are
each attached to a piston rod 122 of the assembly which projects
upwardly and downwardly of the assembly so that operation of the
assembly causes vertical longitudinal movement of the piston rod 122.
The two piston rods 122 are both attached at an upper end thereof
to an upper platen 128 of the press which is supported between the two
piston rods 122. The two piston rods 122 form columns on which the
movable platen 128 is mounted and which move to move the movable
platen toward the stationary platen 112 in a stroke of the press.
~peration of the two assemblies 114 and 116 to move the piston rod 122
downwards moves the upper platen 128 downwards towards the lower
platen 112 in an operative stroke of the press, and operation of the
assemblies 114 and 116 in the opposite direction moves the upper
-17~ 6~33
1 platen 128 upwards in a return stroke of the press.
The assemblies 114 and 116 are connected together in the same
hydraulic circuit (not shown) which is controlled (in like manner to
that of the first illustrative press) by solenoid valves controlled by
eleccrical switches Sl, S2 and S3. The electrical circuil of the
second illustrative press is shown in Figure 4 and is identical in
construction and operation to that of the first illustrative press
except for differences described below.
The hydraulic circuit of the second illustrative press also
comprises a balancing valve 130 which operates in identical manner to
the valves 30 and 32 of the first illustrative press. Associated with
the valve 130 is a spool 134~ a lever 136, two links 138 and two
bell-crank levers 140 which are pivoted by the movement of the piston
rods 122 and act to move the links 138 so that unequal movenlent of the
piston rGds 122 causes movement of the spool 134 to throttle the
exhaust from whichever assembly 114 or 116 has become advanced
relative to the other.
Associated with each of the assemblies 114 and 116 is a variable
impedance device in the form of a potentiometer having a
vertically-extending coil 160 and a moving contact 162 (Figure 4)
attached to a rod 164 which extends into the coil 160. ~ach coil 160
is mounted so that it depends from the cylinder of one of the
assemblies 114 and 116 and the rod 164 of each potentiometcr is
attached to a lower end portion of the piston rod 122 Gf the assembly
so that movement of the piston rod 122 causes the rod 164 to move
6~3~
1 verticall~ and hence the moving contact 162 to move withir Ihe coil
160. The second illustrative press thus comprises two variable
impedance clevices, one associated with each of the two portions of the
rnovable platen 128 to which the piston rods ].22 are attached. Each
device has a movable portion in the form of the contact 162 whose
position determines the output of the device. The contacts 162 are
mechanically connected through the piston rods 122 to the assnciated
portion of the movable platen 128 so that movement of the portion of
.he movable platen causes movement of the contact 162.
The potentiometers 160, 162 form part of the electrical circuit
shown in Figure 4. This circuit is identical to that shown in Figure
2 except that, instead of one potentiometer with an associated
ampli-fier A~, the circuit contains the two potentiometers 160, 162
connected in parallel and amplifiers A4 and A5 are associated one with
each potentiolneter. The two amplifiers A4 and A5 have their outputs
connected to an amplifier A6 which acts as averaging means operable to
average the outputs of the two potentiometers 160, 162 and produce an
output equal to the average of the inputs it receives froln the
ampl-ifiers A4 and A5 and supplies this output to the comparator C1.
The operation of the second illustrative press is identical to
that o-f the first illustrative press described above except that where
the first illustrative press uses the output of the potentiometer 60,
62, the second illustrative press uses the average of the outputs of
the two potentiometers 160, 162.
It will be apparent that, in both the first and the second
-19- ~6~3~
1 illustrative press, the accuracy of the stroke control apparatus is
enhanced because the control depends on the sum o-f the movements of
two portions of the movable platen. Furthermore, because -the movement
of columns is sensed, the stroke control apparatus can be conveniently
positioned beneath the stationary platen. Furthermore, the mechanical
connections of the presses are utilized in two ways for both the
balancing valve and the stroke con-trol apparatus.
It will also be apparent that variable impedance devices other
than linear potentiometers may be utilized in variations of the first
and the second illustrative presses, for example inductance devices.
