CLAMP FORCE CONTROL FOR LOAD-HANDLING DEVICE

COMMANDE DE FORCE DE SERRAGE D'UN ORGANE DE PREHENSION

English Abstract

In a control for a load-handling device that includes an hydraulic gripper (14) which is intended to grip objects, such as one or more rolls or bales of paper material or other goods, through the medium of pressure in a clamping pressure line (32) leading to one or more hydraulic cylinders (30) that actuate gripping arms on the gripper (14), and on the other hand to lift said objects by actuation from a lifting device (22, 42), there is proposed the possibility of adapting the clamping force to the lifting force during a clamping and lifting operation, among other things by monitoring the lifting force of the lifting device (42) with the aid of sensor means (60), wherein a pressure reduction valve (36) is provided in the clamping pressure line (32), and a control means (50) is provided for controlling the pressure reduction valve (36) to a pressure in the clamping pressure line (36) that is proportional to the lifting force detected by the sensor means (60).

French Abstract

Dans un contrôle d'un dispositif de manutention de charge qui comprend une pince hydraulique (14) qui est destinée à saisir des objets, tels qu'un ou plusieurs rouleaux ou balles de papier ou d'autres marchandises, par le moyen de pression dans une ligne de pression de serrage (32) menant à un ou plusieurs vérins hydrauliques (30) qui actionnent les bras de préhension sur la pince (14), et d'autre part pour soulever lesdits objets par l'actionnement d'un dispositif de levage (22, 42), la possibilité est proposée d'adapter la force de serrage à la force de levage lors d'une opération de serrage et de levage, entre autres choses, en contrôlant la force de levage de l'appareil de levage (42) à l'aide d'un capteur (60), où un robinet de réduction de pression (36) est prévu dans la ligne de pression de serrage (32), et un moyen de contrôle (50) est prévu pour régler le robinet de réduction de pression (36) à une pression dans la ligne de pression de serrage (36) qui est proportionnelle à la force de levage détectée par le capteur (60).

Claims

-8-
The embodiments of the invention in which
the exclusive property or privilege is claimed are
defined as follows:
1. A control for a load-handling device includ-
ing a hydraulic gripper adapted on the one hand to
firmly grip an object by means of pressure in a
clamping pressure line leading to one or more hydrau-
lic cylinders that apply clamping force to gripping
arms on the gripper and, on the other hand, adapted to
lift said object by actuation of a hydraulic lifting
device, comprising:
a pressure-regulating valve to limit said
pressure in said clamping pressure line;
said pressure-regulating valve being adapted
to limit said pressure in said clamping pressure
line in a preset relationship to a magnitude of
lifting force exerted by said lifting device on
said object so that said pressure increases in
response to increases in said lifting force and
without regard to any downward sliding of said
object relative to said load-handing device, said
clamping pressure line and a hydraulic supply
line for said lifting device being interconnected
in parallel with each other.
2. The control according to claim 1, wherein
said pressure-regulating valve is adapted to control
increases in said pressure in said clamping pressure
line in predetermined proportion to said increases in
said lifting force.
3. The control according to claim 1, including
a sensor for measuring said magnitude of lifting
force.

Description

CA 02555839 1997-04-22
1
ChAMP FORCE CONTROL FOR LOAD-HANDLING DEVICE
The present invention relates to a force control for controlling
a load-handling device that includes an hydraulic gripper adapted
on the one hand to firmly grip objects, such as one or more rolls
or bales of paper material or other goods, through the medium of
pressure in a clap pressure line leading to one or more hydrau-
lic cylinders that actuate gripper-mounted gripping arms, and on
the other hand to lift said objects by actuation from a lifting
device.
Great demands are placed on paper-rolls, particularly for
printing presses, that. the rolls will. not be squeezed to an oval
shape during handling of the rolls from a paper mill to a
printing department. For instance, the tolerance placed on a roll
125 cm in diameter may sometimes be as narrow as 2 mm measured on
the-radius of the roll. Different solutions have been proposed in
endeavours to optimally control the clamping force of the gripper
against the roll periphery. EP-A-0664272 and FI-B-84715, amo~~.~.
others, teach devices for controlling the clamping or gripping
forces in. response to detected sliding at the gripping surfaces.
One object of the present invention is to provide a control of
the kind defined in._ahe introduction that is capable of continu-
ously controlling the clamping force towards an optimal value
during a combined clamping and lifting movement sequence.
According to one aspect of the invention, there is sensed the
gravitational force acting on the lifting device during the
combined clamping and lifting operation, wherein a pressure
reduction valve in the clamping pressure line controls the

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2
pressure in said line proportionally to the sensed lifting force.
This ensures that the clamping force will never exceed a prede-
termined percentage of the weight of the load.
Embodiments of the invention will now be described with reference
to the accompanying drawings, in which FIG. 1 is a simplified
perspective view of a load-handling device with which the
invention is intended to be applied; FIG. 2 shows an hydraulic
circuit that includes the principles of the invention; and FIG.
3 is a mare detailed hydraulic circuit for an inventive load-
handling device.
In, FIG. 1, the reference numeral 10 identifies a carrier or a
vehicle, such as a fork-lift truck, an articulated wheel-mounted
loader or like vehicle-, on which there is mounted a known load-
handling device in the form of a roll-clamping unit. The roll-
clamping unit of the illustrated embodiment is intended to grip
and lift two rolls 12 simultaneously and includes a gripper 14
having mutually opposed gripping arms 16 and 18, 20 respectively.
The gripping arms are hinged on a frame 22 so as to be raisable
and lowerable thereon. The frame 22 is mounted on a generally
vertical stand 24 which, in turn, can be swung or tilted relative
to the truck 10 about an axis 26. The frame 22 may also be
rotatable about a horizontal axis with respect to the stand 24,
so as to enable the rolls 12 to be brought to a horizontally
lying position (not shown):
The gripper 14 may be one of a number of different variants
having different numbers of fixed and movable gripping arms. In

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3
the examples illustrated in FIGS. 2 and 3, the movable gripping
arms are operated by double-acting hydraulic cylinders 30,
wherein the six hydraulic cylinders 30 of the FIG. 3 embodiment
are intended for operating the outer gripping arms of a multi-
roll unit, for simultaneous gripping of a pair of juxtaposed roll
stacks (not shown) towards a centre tongue of the unit.
Each double-acting hydraulic cylinder 30 receives clamping or
closing pressure from an hydraulic line (clamping pressure line)
32 and an opening pressure from an hydraulic line 34. The frame
22 is lifted through the medium of one or more hydraulic~cylin-
ders 42 (only one is shown) which receive lifting pressure from
an hydraulic line 44. The stand 12 is tilted through the medium
of double-acting hydraulic cylinders 66 which are driven in
opposite directions by pressure arriving from respective hydrau-
lic lines 68 and 70. The pressure in the lines 68, 70 is control-
led by an electromagnetically controlled directional valve 72, to
which pressure is delivered via an inlet line 74 and an outlet
line 76.
The output of an electromagnetically proportion-controlled
pressure reduction valve 36 is~ connected to the line 32 that
delivers clamping pressure to the gripper 14, and the input of
said valve 36 is connected to a supply line 40. A check valve 38
connected in parallel with the pressure reduction valve 36
ensures that hydraulic fluid in the line 36 can flow back past
the pressure reduction valve 36 when opening the gripper 14.
Extending between the supply line 40 and the supply line 44 to
the single-acting cylinder 42 of the lifting means 22 is an
hydraulic line 46 in which there is connected a sequence.valve 48
which is adapted to connect. the supply line 40 with the supply
line 44 and to drive the lifting means 22 when the pressure in
the supply line 40 reaches a pressure level set on the sequence

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valve 4 8 .
As will be seen from the principle presentation of the circuitry
in FIG. 2, there is also included a pair of directional valves 86
'and 88 and a drive source that includes an hydraulic pump 90. The
directional valve 86 is adapted for operator actuation of the
hydraulic cylinder 42. Correspondingly, the directional valve 88
is also adapted for operator-actuation of the grippes 14, i.e.
closing and opening of the grippes. The valve 72 may also be
operator-actuable for controlling the tilt function independently
of the control system described in the following.
The various functions, the clamping force of the grippes 14, the
lifting force of the lifting means 22, and the angle to which the
tilt means 24 is inclined relative to the vehicle 10, are
monitored by respective sensors 54, 60 and 78. More specifically,
the sensors 54 and 60 of the illustrated embodiment are pressure
sensors which sense the pressure prevailing _in respective
cylinders 30 and 42 via lines 56 and 62 respectively. Thus, the
pressure sensed by the sensor 54 is a measurement of the clamping
force exerted by the grippes, and the pressure sensed by the
sensor 60 is a measurement of the lifting force exerted by the
lifting means 22. The sensor 78 is a position sensor and senses
the position of a point on the stand 24 in relation to the truck,
wherewith this position can be readily converted to a measurement
of the angle to which the stand 24 is tilted about the axis 26 in
relation to the truck 10. By way of an alternative, the afore
described pressure sensors 54 and 60 may be replaced with force,
sensors, e.g., in the form of load cells for detecting.clamping
force and pressure force (not shown).
The outputs of the sensors 54, 60, 70 are connected to a central
electronic control unit 50 by means of respective signal conduc-
tors 52, 58 and 80. The control unit 50 is connected to the

CA 02555839 1997-04-22
control input of the proportional control reduction valve 36 by
means of a signal conductor 64. A pair of further outputs of the
control unit 50 are connected to the control inputs of the
directional valve 72 by means of a respective signal conductor 82
5 and 84.
The control unit 50 includes a programmable microcomputer of
known kind capable of registering continuously the incoming
sensor signals and continuously varying the outgoing signals in
response to changes in the incoming signals with the aid of a
computing program.
The inventive control functions in the following manner:
When one or more rolls 12 standing on a support are to be gripped
and lifted by the load-handling device, the first line 40 is
pressurized, e.g.. by setting the directional valve 88 to its
lower position in FIG. 2. The directional valves 72 and 86
initially remain in their illustrated closed states. The pressure
2o reduction valve 36 is initially open until there is reached in
the line 2 a pressure that provides a desired initial roll-
clamping force in the cylinder 3,0. This force is detected by the
sensor 54, which then sends to the control unit 50 a signal to
the effect that the pressure reduction valve 36 shall be closed.
The pressure in the line 40 connected to the input ~of the
sequence valve 48 then increases, such that when the inlet
pressure overcomes the spring force of the sequence valve 48
hydraulic fluid is forced into the lines 46 and 44 and the
cylinder 42 then commences to lift the gripper that clamps the
rolls.
Installed in the computing programme of the control unit 50 is an
adjustable, predetermined proportionality constant which. gives
the optimal quotient Q between the clamping pressure p~ in the

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line 32 and the lifting pressure py in the line 44, so that the
clamping force exerted by the gripper during the entire clamping
and lifting sequence will have a magnitude such that the gripper
14 will not deform the rolls and such that the rolls are unable
to slide down between the gripping arms of said gripper 14.
Accordingly, it is ascertained continuously in the control unit
50 during the entire lifting sequence whether or not the real or
prevailing value of p~ signalled by the sensor 54 coincides with
the set-point value or control value Q x pL. If such is not the
case, the control unit 50 delivers a signal on the line 64 which
adjusts the setting of the pressure reduction valve 36 in a
direction towards the set-point value on p~ in the line 32.
During the aforedescribed sequence, the clamping force exerted by
the gripper 14 will thus be adapted (increased) continuously in
proportion to the increasing lifting force during the moment at
which the gripper l4 releases the paper roll or rolls 12 from the
support.
During the described sequence in which the paper roll or rolls 12
leave the supportive surface, the truck 10 will also unavoidably
"curtsy" as a result of the load. In order to prevent the bottom
front edge of the bottom roll 12 from being subjected to exces-
sively large local pressure forces against the supportive
surface, such as to deform the roll and render it unusable, the
tilt function of the load-handling device is controlled in the
following way:
The aforesaid "curtsying" of the truck is generally proportional
to the force that acts on the lifting device 22 and the cylinder
42,land therewith generally proportional to the pressure pL in the
line 44. It is therefore necessary for the gripper 14 and the

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stand 24 to be tilted back about the axis 26 during the gripping
and lifting sequence proportionally to the increasing pressure pt,
with the 'aid of the cylinder 66. To this end, the computing
program of the control unit 50 includes a function which controls
the directional valve 72, via the signal lines 82, 84, to adjust
the hydraulic cylinder 66 proportionally to the pressure pz
detected by the sensor 60. The real value of the adjustment or
tilt is monitored by the, position sensor 78 and is corrected
towards the set-point value by signals sent from the control unit
50 to the directional valve 72.
This will thus ensure that the load-handling device, with or
without load, will constantly maintain a preset inclination
relative to the supportive surface, particularly so that the roll
12 standing on the supportive surface will be lifted~essentially
parallel from said surface. Control of the tilt function is also
reversible, so as to enable a roll to be deposited flatly on the
supportive surface without danger of the roll edges being
damaged.

Representative Drawing