TECHNICAL FIELD
[0001] The present invention relates to an industrial lift truck according to the preamble
of claim 1.
BACKGROUND ART
[0002] Load lifting devices of industrial lift trucks are often driven by means of hydraulic
cylinders supplied by hydraulic pumps. The operator can choose between different operation
modes of the load lifting device, in dependence of the actual material handling situation,
which correspond to different set point speeds of the load lifting device. The maximum
lifting and lowering speeds of the load lifting device are restrained due to safety
regulations so as to avoid accidents, e.g. due to unexpected load movements during
lifting/lowering of the load lifting device. Accordingly, the maximum speeds allowed
must fall within stipulated speed ranges.
[0003] However, the actual lifting and lowering speeds of the load lifting device are affected
by e.g. the weight of the load, component wear and variations in hydraulic fluid temperature.
Accordingly, the actual speed value of the load lifting device will deviate from the
set point speed value. These deviations are difficult to predict so the load lifting
devices and associated equipment are calibrated to fall well within the stipulated
speed ranges. Thus, the potentially allowed maximum speeds are not fully utilized.
[0004] One way to avoid load movements during lifting/lowering is to avoid speed variations
and to keep the load lifting device at a constant speed. However, industrial lift
trucks of today often comprise two or more lifting stages, each stage being driven
by a separate hydraulic cylinder. Thus, during transition from one stage to another,
e.g. when a free lift cylinder approaches its end stroke and the main lift cylinder
starting its stroke, such constant speed control may be difficult to accomplish.
[0005] One way of solving the problem of constant speed control is disclosed in
US 2005/0263354 A1. The transition between stages is accomplished by decelerating the initial stage
at a rate which equals the acceleration rate of the next stage to maintain the load
lifting device at a constant vertical speed. The fact that speed variations may occur,
e.g. due to variations of the weight of the load, component wear and variations in
hydraulic fluid temperature, as described above, is however not taken into consideration,
since the hydraulic fluid flow is only redirected from one hydraulic cylinder to another.
Accordingly, a constant speed during the transition stage can not be guaranteed.
SUMMARY OF THE INVENTION
[0006] An object of the present invention is thus to provide an industrial lift truck having
a load lifting device which vertical speed can be maintained at desired set point
speeds, e.g. constant, even though the weight of the load, component wear and the
hydraulic fluid temperature is varying.
[0007] Yet an object of the present invention is to maintain a constant speed of the load
lifting device during a transition stage, when the lifting/lowering operation shifts
from a first hydraulic cylinder to a second hydraulic cylinder.
[0008] Another object of the present invention is to increase the maximum speeds allowed
for the load lifting device, still falling within stipulated speed ranges.
[0009] These objects are accomplished by means of an industrial truck having the features
of the characterising portion of claim 1.
[0010] According to claim 1, the controller comprises means for determining any differences
between the calculated actual speed and the set point speed, and means for sending
signals to the hydraulic pump to change its speed, and/or signals to the hydraulic
valve assembly to change its valve opening degree, if a difference exist between the
calculated actual speed and the set point speed of the load lifting device, so as
to compensate for the difference. Hereby, the desired speed of the load lifting device
will always be maintained. Thus, during the transition period between the free lift
stage and the main lift stage, the load lifting device can be maintained at a constant
speed, so that load movements can be avoided. Moreover, it also becomes possible to
adapt set point speeds which are closer to stipulated maximum speeds of the load lifting
device, since the uncertainty due to load weights, variations in hydraulic fluid temperatures
and component wear automatically is compensated for.
[0011] Suitably, the vertical lifting frame comprises a stationary lifting frame section
and at least one telescopic lifting frame section, wherein the load lifting device
is movably arranged with respect to the telescopic lifting frame section, and the
telescopic lifting frame section is movably arranged with respect to the stationary
lifting frame section, and wherein the hydraulic cylinder assembly comprises a first
hydraulic cylinder for driving the load lifting device with respect to the telescopic
lifting frame section, and a second hydraulic cylinder for driving the telescopic
lifting frame section with respect to the stationary lifting frame section. Hereby,
a suitable way of handling the load and of driving the load lifting device is achieved.
[0012] Advantageously, the position measuring assembly comprises at least a first position
measuring device for measuring the vertical position of the load lifting device with
respect to the telescopic lifting frame section, and at least a second position measuring
device for measuring the vertical position of the telescopic lifting frame section
with respect to the stationary lifting frame section. Hereby, a suitable way of measuring
the height of the load lifting device is achieved.
[0013] Preferably, said means for calculating the actual speed of the load lifting device
is adapted to calculate a relative speed of the load lifting device with respect to
the telescopic lifting frame section based on vertical position measurements of the
first position measuring device, and to calculate a relative speed of the telescopic
lifting frame section with respect to the stationary lifting frame section based on
vertical position measurements of the second position measuring device, wherein the
actual speed of the load lifting device is the sum of the relative speeds. Hereby,
the control of the speed of the load lifting device can be divided into sub controls
of the relative speed of the load lifting device with respect to the telescopic lifting
frame section, and the relative speed of the telescopic lifting frame section with
respect to the stationary lifting frame section.
[0014] Suitably, said means for generating a set point speed of the load lifting device
based on the chosen operation mode is adapted to generate a relative set point speed
of the load lifting device with respect to the telescopic lifting frame section, and
to generate a relative set point speed of the telescopic lifting frame section with
respect to the stationary lifting frame section, wherein the set point speed of the
load lifting device is the sum of the relative set point speeds. Hereby, the set point
speed of the load lifting device can be dived into a sub set point speed of the load
lifting device with respect to the telescopic lifting frame section, and to a sub
set point speed of the telescopic lifting frame section with respect to the stationary
lifting frame section.
[0015] Advantageously, said means for comparing the calculated actual speed with the set
point speed is adapted to compare the relative speed with the relative set point speed
of the lifting device with respect to the telescopic lifting frame section, and to
compare the relative speed with the relative set point speed of the telescopic lifting
frame section with respect to the stationary lifting frame section. Hereby, separate
comparison steps are performed which makes the speed control more accurate.
[0016] Preferably, the variable hydraulic valve assembly comprises at least one variable
supply valve and at least one variable drainage valve.
[0017] Suitably, said means for sending signals to the hydraulic pump to change its speed,
and/or signals to the hydraulic valve assembly to change its valve opening degree
is adapted to send signals to the hydraulic pump and/or the at least one supply valve
only during lifting motions of the load lifting device. Hereby, an effective way of
compensating for the difference between the actual speeds and the set point speeds
is achieved.
[0018] Advantageously, said means for sending signals to the hydraulic pump to change its
speed, and/or signals to the hydraulic valve assembly to change its valve opening
degree is adapted to send signals to the at least one drainage valve only during lowering
motions of the load lifting device. Hereby, an effective way of lowering the load
lifting device is achieved.
[0019] Preferably, said means for sending signals to the hydraulic pump to change its speed,
and/or signals to the hydraulic valve assembly to change its valve opening degree
is adapted to send signals to the hydraulic pump and/or the at least one supply valve,
or to the at least one drainage valve, so as to maintain the load lifting device at
a constant speed when both the load lifting device and the telescopic lifting frame
section are in motion. Hereby, an effective way of maintaining the load lifting device
at a constant speed during the transition stage between the free lift stage and the
main lift stage is achieved.
[0020] Suitably, said means for sending signals to the hydraulic pump to change its speed,
and/or signals to the hydraulic valve assembly to change its valve opening degree
is adapted to send signals to the hydraulic pump to increase its speed, and/or signals
to the at least one supply valve to increase its valve opening degree, if the relative
speed is lower than the relative set point speed of the lifting device with respect
to the telescopic lifting frame section, or if the relative speed is lower than the
relative set point speed of the telescopic lifting frame section with respect to the
stationary lifting frame section. Hereby, an effective way of compensating for the
difference between the actual speed and the set point speed, when the actual speed
is lower than the set point speed, is achieved.
[0021] Advantageously, said means for sending signals to the hydraulic pump to change its
speed, and/or signals to the hydraulic valve assembly to change its valve opening
degree is adapted to send signals to the hydraulic pump to decrease its speed, and/or
signals to the at least one supply valve to decrease its valve opening degree, if
the relative speed is higher than the relative set point speed of the lifting device
with respect to the telescopic lifting frame section, or if the relative speed is
higher than the relative set point speed of the telescopic lifting frame section with
respect to the stationary lifting frame section. Hereby, an effective way of compensating
for the difference between the actual speed and the set point speed, when the actual
speed is higher than the set point speed, is achieved.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] The present invention will now be described with reference to accompanying drawings,
on which:
Fig. 1 shows a perspective view of an industrial truck according to the present invention,
Fig. 2 shows a schematic diagram of a controller for the industrial truck,
Fig. 3 shows a speed-position diagram for the free lift cylinder and the main lift
cylinder, i.e. a speed-position diagram for the load lifting device.
DETAILED DESCRIPTION
[0023] Reference is first made to fig. 1 which shows a perspective view of an industrial
lift truck 1 according to the present invention, and to fig. 2 which schematically
shows a controller as well as components of the hydraulic system and the position
measurement system of the industrial lift truck. The industrial lift truck 1 comprises
a vertical lifting frame 3 having a stationary lifting frame section 5 and at least
one telescopic lifting frame section 7. The telescopic lifting frame section 7 is
vertically movable within the stationary lifting frame section 5 between a retracted
and an extended position. A load lifting device 9 in the form of a fork is vertically
movable within the telescopic lifting frame section 7 between a retracted and an extended
position. This will be more fully described below.
[0024] The load lifting device 9 is driven by a hydraulic cylinder assembly 11 which comprises
a first hydraulic cylinder 13 (henceforth referred to as free lift cylinder 13) and
a second hydraulic cylinder 15 (henceforth referred to as main lift cylinder 15).
The free lift cylinder 13 connects the telescopic lifting frame section 7 with the
load lifting device 9, while the main lift cylinder 15 connects the stationary lifting
frame section 5 with the telescopic lifting frame section 7. Thus, the load lifting
device 9 will be movable with respect to the telescopic lifting frame section 7, while
the telescopic lifting frame section 7 will be movable with respect to the stationary
lifting frame section 5 during operation of the free lift cylinder and the main lift
cylinder.
[0025] The hydraulic cylinder assembly 11, i.e. the free lift cylinder 13 and the main lift
cylinder 15 are supplied with hydraulic fluid from a variable hydraulic pump 17, i.e.
having a variable speed. A variable hydraulic valve assembly 19 in the form of one
or more variable hydraulic valves (one or more supply valves 19a for the lifting operation
and one or more discharge valves 19b for the lowering operation) is arranged in the
hydraulic system. The one or more variable hydraulic valves 19a for the supply of
hydraulic fluid to the hydraulic cylinder assembly 11 are arranged between the variable
hydraulic pump 17 and the hydraulic cylinder assembly 11, while the one or more discharge
valves 19b for the lowering operation are arranged between the hydraulic pump 17 and
a not shown hydraulic fluid tank. The term "variable" means that the variable hydraulic
valves 19 have a variable valve opening degree, so that the amount of hydraulic fluid
that passes through the valves can be controlled.
[0026] The industrial lift truck 1 is provided with a position measuring assembly 25a-b
in the form of height sensors for measuring the vertical position of the load lifting
device 9. The position measuring assembly 25a-b comprises at least a first position
measuring device 25a for measuring the vertical position of the load lifting device
9 with respect to the telescopic lifting frame section 7, but also at least a second
position measuring device 25b for measuring the vertical position of the telescopic
lifting frame section 7 with respect to the stationary lifting frame section 5. The
first and second position measuring devices 25a-b may be composed of sensor bearings
or any other sensors/detectors, such as laser sensors, ultrasound sensors, which are
suitable to detect the position of the load lifting device 9 with respect to a fixed
point on the telescopic lifting frame section 7, as well as the position of the telescopic
lifting frame section 7 with respect to a fixed point on the stationary lifting frame
section 5.
[0027] Moreover, the industrial lift truck 1 comprises an operator interface 31 in the form
of an operator input device 31 for inputting an operation mode of the load lifting
device 9. The operation mode could e.g. be a preselected height setting or a setting
for manually control the load lifting device 9 upwards or downwards.
[0028] The industrial lift truck 1 comprises a controller 33 for controlling the speed of
the load lifting device 9 during the lifting/lowering operations of the load lifting
device 9.
[0029] The controller 33 comprises a set point generator 35 for generating set point speeds
and set point heights values in dependence on the operation mode chosen by the operator
and with which the lifting/lowering motion has to comply with, e.g. a maximum speed
for the load lifting device 9 or remaining distance to complete stroke of free lift
cylinder 13. In this respect, the set point speed of the load lifting device 9 generated
by the set point generator 35 can be divided into a relative set point speed of the
load lifting device 9 with respect to the telescopic lifting frame section 7 and into
a relative set point speed of the telescopic lifting frame section 7 with respect
to the stationary lifting frame section 5. The sum of the relative set point speeds
corresponds to the set point speed of the load lifting device 9.
[0030] The controller 33 also comprises means 37 for calculation of the actual position/height
and the actual speed of the load lifting device 9 based on measurements made by the
first and second position measuring devices 25a-b. The height of the load lifting
device 9 will correspond to the sum of the height of the load lifting device 9 with
respect to a fixed point on the telescopic lifting frame section 7 and the height
of the telescopic lifting frame section 7 with respect to a fixed point on the stationary
lifting frame section 5. In a corresponding way, the actual speed of the load lifting
device 9 will correspond to the sum of a relative speed of the load lifting device
9 with respect to the telescopic lifting frame section 7 and a relative speed of the
telescopic lifting frame section 7 with respect of the stationary lifting frame section
5. The relative speed of the load lifting device 9 with respect to the telescopic
lifting frame section 7 is based on the vertical position measurements of the first
position measuring device 25a, while the relative speed of the telescopic lifting
frame section 7 with respect to the stationary lifting frame section 5 is based on
the vertical position measurements of the second position measuring device 25b. The
actual speed of the load lifting device 9 can also be defined as the sum of the stroke
rates of the free lift cylinder 13 and the main lift cylinder 15.
[0031] The controller 33 further comprises means 39 for comparing the calculated actual
position and speed values with the set point values generated by the set point generator
35, so as to determine any differences between them. Accordingly, it is adapted to
compare the relative actual speed with the relative set point speed of the lifting
device with respect to the telescopic lifting frame section 7, and to compare the
relative actual speed with the relative set point speed of the telescopic lifting
frame section 7 with respect to the stationary lifting frame section 5.
[0032] Finally, the controller 33 comprises means 39 for sending compensation signals to
the variable hydraulic pump 17 to change its speed, and/or signals to the hydraulic
valve assembly 19 to change its valve opening degree in dependence of the difference
between the actual speed and the set point speed of the load lifting device 9 so as
to compensate for the difference. During a lifting motion, the compensation signals
are only sent to the variable hydraulic pump 17 to change its speed and/or to the
one or more supply valves 19a to change its valve opening degree, while during a lowering
motion of the load lifting device 9 the compensation signals are only sent to the
one or more discharge valves 19b to change its valve opening degree.
OPERATION
[0033] The operation of the load lifting device 9 will now be described with reference to
fig. 2-3. As described above the set point speeds of the load lifting device 9 depends
on the operation mode of the load lifting device 9 chosen by the operator of the industrial
lift truck 1. The operation modes correspond to speed values which secure a safe and
effective material handling, and which have been calibrated and programmed beforehand.
The invention serves to compensate for any deviations from these set point speed values
which can occur during various material handling situations, and which will be exemplified
below.
[0034] With reference to fig. 3 the operation of the load lifting device 9 will now be described.
The operation begins with the operator inputting an operation mode to be performed,
in this case a manual lifting mode. Starting with the load lifting device 9 being
positioned at its lowermost position carrying a pallet or any other suitable load,
a lifting operation can commence. To lift to the uppermost position, i.e. the position
where the telescopic lifting frame section 7 is fully extended and the load lifting
device 9 is at the uppermost position relative the telescopic lifting frame section
7, the lifting operation can be divided into three stages: a free lift stage, a transition
stage and a main lift stage.
[0035] During the free lift stage only the free lift cylinder 13 operates (the main lift
cylinder is in idle mode) and moves the load lifting device 9 with respect to the
telescopic lifting frame section 7. Thus, the speed of the load lifting device 9 during
this free lift stage equals the stroke rate of the free lift cylinder 13. The speed
of the load lifting device 9 starts gently to avoid load movements from zero speed
(not shown in the diagram) and reaches a maximum speed, which is to be held constant
during the remaining part of the free lift stage. At a predetermined height of the
load lifting device 9 with respect to a fixed point on the telescopic lifting frame
section 7, but before the free lift cylinder 13 has reached its full stroke, the free
lift stage ends and the transition lift stage begins. During this transition lift
stage, the main lift cylinder 15 begins its stroke with a linearly increasing stroke
rate. Meanwhile, the free lift cylinder 13 linearly decreases its stroke rate in a
corresponding way. Thus, the speed of the load lifting device 9 equals the sum of
the stroke rates of the free lift cylinder 13 and the main lift cylinder 15, which
thus is maintained at a constant speed level due to the balancing behaviour of the
stroke rates of the free lift cylinder 13 and the main lift cylinder 15. When the
free lift cylinder 13 reaches its full stroke, the transition lift stage ends, and
the main lift stage begins. The stroke rate of the main lift cylinder 15 has now reached
its maximum, while the stroke rate of the free lift cylinder 13 has become zero. The
speed of the load lifting device 9 now solely corresponds to the stroke rate of the
main lift cylinder 15. The shift from the free lift stage to the main lift stage is
thus performed so that the load lifting device 9 is held at a constant level, whereby
the risk of load movements is minimized.
[0036] During the free lift stage, the means for calculating the actual speed of load lifting
device 9 receives vertical position signals from the first position measuring device
25a, e.g. every 20 ms. These signals are processed so that the actual speed values
can be calculated for the free lift cylinder 13, i.e. the load lifting device 9 with
respect to the telescopic lifting frame section 7. Since the main lift cylinder 15
is in idle mode, the actual speed of the load lifting device 9 corresponds solely
to the relative speed of the load lifting device 9 with respect to the telescopic
lifting frame section 7, i.e. the stroke rate of the free lift cylinder 13. If the
actual speed value, at a given point of time during the free lift stage deviates from
the set point speed, which set point speed has been established by the set point generator
35 in dependence of the operation mode chosen by the operator, a compensation signal
is sent either to the variable hydraulic pump 17 and/or to the variable hydraulic
valve assembly 19. If for instance the actual speed of the load lifting device 9 is
higher than the set point speed, a compensation signal is sent to the variable hydraulic
pump 17 to decrease its speed. However, instead of sending a compensation signal to
the hydraulic pump to increase its speed, a compensation signal could alternatively
or as a complement be sent to the associated supply valve 19a to increase its valve
opening degree. On the other hand, if the actual speed is lower than the set point
speed, a compensation signal is sent to the variable hydraulic pump 17 to increase
its speed. However, instead of sending a compensation signal to the hydraulic pump
to decrease its speed, a compensation signal could alternatively or as a complement
be sent to the associated supply valve 19a to decrease its valve opening degree.
[0037] During the transition stage, if the actual speed of the load lifting device 9, i.e.
the sum of the relative speed of the load lifting device 9 with respect to the telescopic
lifting frame section 7 and the relative speed of the telescopic lifting frame section
7 with respect to the stationary lifting frame section 5, i.e. the sum of the stroke
rates of the free lift cylinder 13 and the main lift cylinder 15, is higher than the
set point speed several options exist: a compensation signal could be sent to the
variable supply valve 19a associated with the free lift cylinder 13 to decrease its
valve opening degree, if only the relative speed of the load lifting device 9 with
respect to the telescopic lifting frame section 7, i.e. the stroke rate of the free
lift cylinder 13 is deviating from the relative set point speed, while the relative
speed of the telescopic lifting frame section 7 with respect to the stationary lifting
frame section 5, i.e. the actual stroke rate of the main lift cylinder 15 equals the
relative set point speed, i.e. the set point stroke rate.
[0038] The opposite applies if the actual speed of the load lifting device 9, i.e. the sum
of the relative speed of the load lifting device 9 with respect to the telescopic
lifting frame section 7 and the relative speed of the telescopic lifting frame section
7 with respect to the stationary lifting frame section 5, i.e. the sum of the stroke
rates of the free lift cylinder 13 and the main lift cylinder 15, is lower than the
set point speed. That means that a compensation signal could be sent to the variable
supply valve 19a associated with the free lift cylinder 13 to increase its valve opening
degree, if only the relative speed of the load lifting device 9 with respect to the
telescopic lifting frame section 7, i.e. the stroke rate of the free lift cylinder
13 is deviating from the set point speed of the load lifting device 9 with respect
to the telescopic lifting frame section 7, while the relative speed of the telescopic
lifting frame section 7 with respect to the stationary lifting frame section 5, i.e.
the stroke rate of the main lift cylinder 15 equals the relative set point speed of
the telescopic lifting frame section 7 with respect to the stationary lifting frame
section 5. An analogous approach applies to the main lift cylinder 15 if its actual
stroke rate deviates from the set point stroke rate, while the stroke rate of the
free lift cylinder 13 equals the set point stroke rate.
[0039] On the other hand, if the actual speed of the load lifting device 9 is higher than
the set point speed and this is because both the relative speed of the load lifting
device 9 with respect to the telescopic lifting frame section 7 and the relative speed
of the telescopic lifting frame section 7 with respect to the stationary lifting frame
section 5 are higher than corresponding relative set point speeds, the speed of the
variable hydraulic pump 17 should be decreased, instead of decreasing the valve opening
degree for both the variable supply valves 19a associated with the free lift cylinder
13 and the main lift cylinder 15. The opposite applies if the actual speed of the
load lifting device 9 is lower than the set point speed and this is because both the
relative speed of the load lifting device 9 with respect to the telescopic lifting
frame section 7 and the relative speed of the telescopic lifting frame section 7 with
respect to the stationary lifting frame section 5 are lower than corresponding set
point speeds. That is, the speed of the variable hydraulic pump 17 should be increased,
instead of increasing the valve opening degree for both the variable supply valves
19a corresponding to the free lift cylinder 13 and the main lift cylinder 15.
[0040] Concerning the main lift stage, the same rules apply as for the free lift stage.
That is, if the actual speed value at a given point of time deviates from the set
point speed established by the set point generator 35, i.e. the relative actual speed
of the telescopic lifting frame section 7 with respect to the stationary lifting frame
section 5, a compensation signal is sent either to the variable hydraulic pump 17
and/or to the one or more supply valves 19a of the variable hydraulic valve assembly
19. If for instance the actual speed of the load lifting device 9, i.e. the actual
speed of the telescopic lifting frame section 7 with respect to the stationary lifting
frame section 5 (since the free lift cylinder 13 is in idle mode ) is higher than
the set point speed, a compensation signal is sent to the variable hydraulic pump
17 to decrease its speed. On the other hand, if the actual speed, i.e. the stroke
rate of the main lift cylinder 15, is lower than the set point speed, a compensation
signal is sent to the variable hydraulic pump 17 to increase its speed.
[0041] Concerning lowering of the load lifting device 9, the variable hydraulic pump 17
is not controlled at all so as to compensate for any speed deviations. Instead the
variable discharge valves 19b of the variable hydraulic valve assembly 19 are employed.
These discharge valves 19b are controlled in an analogous manner as for the supply
valves 19a of the variable hydraulic valve assembly 19 during the lifting operation,
and reference is therefore made to these passages.
[0042] It is of course conceivable that the stroke rates of the free lift cylinder and the
main lift cylinder during the transition stage of the lifting and lowering operation
of the load lifting device adopts a speed changing behaviour other than linear as
depicted in fig. 3., the speed changing behaviour could e.g. be S-shaped when plotted
in the fig. 2 diagram.
[0043] The invention has been described with reference to an industrial lift truck having
a lifting frame with a free lift stage and a main lift stage. It is conceivable that
the industrial lift truck has a lifting frame with only a main lift stage, i.e. a
transition between different stages will not occur.
1. An industrial lift truck (1), comprising:
- a lifting frame having a vertically movable load lifting device (9),
- a hydraulic cylinder assembly (11) for driving the load lifting device (9),
- a variable hydraulic pump (17) for supplying hydraulic fluid to the hydraulic cylinder
assembly (11),
- a variable hydraulic assembly (19a-b) arranged for controlling the hydraulic fluid
supplied to/drained from the hydraulic cylinder assembly (11),
- a position measuring assembly (25a-b) for measuring vertical positions of the load
lifting device (9) with respect to the lifting frame,
- an operator input device (31) for choosing an operation mode of the load lifting
device (9),
- a controller (33) comprising: means (37) for calculating the actual speed of the
load lifting device (9) based on the vertical position measurements, means (35) for
generating a set point speed of the load lifting device (9) based on the chosen operation
mode, and means (39) for comparing the calculated actual speed with the set point
speed,
characterised in that the controller (33) comprises:
means (39) for determining any differences between the calculated actual speed and
the set point speed, and means (39) for sending signals to the hydraulic pump to change
its speed, and/or signals to the hydraulic valve assembly (19a-b) to change its valve
opening degree, if a difference exist between the calculated actual speed and the
set point speed of the load lifting device (9), so as to compensate for the difference.
2. Industrial lift truck (1) according to claim 1, wherein the vertical lifting frame
(3) comprises a stationary lifting frame section (5) and at least one telescopic lifting
frame section (7), wherein the load lifting device (9) is movably arranged with respect
to the telescopic lifting frame section (7), and the telescopic lifting frame section
(7) is movably arranged with respect to the stationary lifting frame section (5),
and wherein the hydraulic cylinder assembly (11) comprises a first hydraulic cylinder
(13) for driving the load lifting device (9) with respect to the telescopic lifting
frame section (7), and a second hydraulic cylinder (15) for driving the telescopic
lifting frame section (7) with respect to the stationary lifting frame section (5).
3. Industrial lift truck (1) according to claim 2, wherein the position measuring assembly
(25a-b) comprises at least a first position measuring device (25a) for measuring the
vertical position of the load lifting device (9) with respect to the telescopic lifting
frame section (7), and at least a second position measuring device (25b) for measuring
the vertical position of the telescopic lifting frame section (7) with respect to
the stationary lifting frame section (5).
4. Industrial lift truck (1) according to claim 3, wherein said means for calculating
the actual speed of the load lifting device (9) is adapted to calculate a relative
speed of the load lifting device (9) with respect to the telescopic lifting frame
section (7) based on vertical position measurements of the first position measuring
device (25a), and to calculate a relative speed of the telescopic lifting frame section
(7) with respect to the stationary lifting frame section (5) based on vertical position
measurements of the second position measuring device (25b), wherein the actual speed
of the load lifting device (9) is the sum of the relative speeds.
5. Industrial lift truck (1) according to any of claims 2-4, wherein said means (33)
for generating a set point speed of the load lifting device (9) based on the chosen
operation mode is adapted to generate a relative set point speed of the load lifting
device (9) with respect to the telescopic lifting frame section (7), and to generate
a relative set point speed of the telescopic lifting frame section (7) with respect
to the stationary lifting frame section (5), wherein the set point speed of the load
lifting device (9) is the sum of the relative set point speeds.
6. Industrial lift truck (1) according to any of claims 2-5, wherein said means (37)
for comparing the calculated actual speed with the set point speed is adapted to compare
the relative speed with the relative set point speed of the lifting device with respect
to the telescopic lifting frame section (7), and to compare the relative speed with
the relative set point speed of the telescopic lifting frame section (7) with respect
to the stationary lifting frame section (5).
7. Industrial lift truck (1) according to any of claims 2-6, wherein the variable hydraulic
valve assembly (19a-b) comprises at least one variable supply valve (19a) and at least
one variable drainage valve (19b).
8. Industrial lift truck (1) according to claim 7, wherein said means (39) for sending
signals to the hydraulic pump to change its speed, and/or signals to the hydraulic
valve assembly (19a-b) to change its valve opening degree is adapted to send signals
to the hydraulic pump and/or the at least one supply valve (19a) only during lifting
motions of the load lifting device (9).
9. Industrial lift truck (1) according to claim 7, wherein said means (39) for sending
signals to the hydraulic pump to change its speed, and/or signals to the hydraulic
valve assembly (19a-b) to change its valve opening degree is adapted to send signals
to the at least one drainage valve (19b) only during lowering motions of the load
lifting device (9).
10. Industrial lift truck (1) according to any of claims 7-9, wherein said means (39)
for sending signals to the hydraulic pump to change its speed, and/or signals to the
hydraulic valve assembly (19a-b) to change its valve opening degree is adapted to
send signals to the hydraulic pump and/or the at least one supply valve (19a), or
to the at least one drainage valve (19b), so as to maintain the load lifting device
(9) at a constant speed when both the load lifting device (9) and the telescopic lifting
frame section (7) is in motion.
11. Industrial lift truck (1) according to any of claims 7-8, wherein said means (39)
for sending signals to the hydraulic pump to change its speed, and/or signals to the
hydraulic valve assembly (19a-b) to change its valve opening degree is adapted to
send signals to the hydraulic pump to increase its speed, and/or signals to the at
least one supply valve (19a) to increase its valve opening degree, if the relative
speed is lower than the relative set point speed of the lifting device with respect
to the telescopic lifting frame section (7), or if the relative speed is lower than
the relative set point speed of the telescopic lifting frame section (7) with respect
to the stationary lifting frame section (5).
12. Industrial lift truck (1) according to any of claims 7-8, wherein said means (39)
for sending signals to the hydraulic pump to change its speed, and/or signals to the
hydraulic valve assembly (19a-b) to change its valve opening degree is adapted to
send signals to the hydraulic pump to decrease its speed, and/or signals to the at
least one supply valve (19a) to decrease its valve opening degree, if the relative
speed is higher than the relative set point speed of the lifting device with respect
to the telescopic lifting frame section (7), or if the relative speed is higher than
the relative set point speed of the telescopic lifting frame section (7) with respect
to the stationary lifting frame section (5).