Hydraulic lift apparatus for a battery driven lift truck

A hydraulic lift apparatus for a battery driven lift truck includes a hydraulic lift cylinder. A hydraulic pump operates as a pump in a load raising mode, so as to feed pressure fluid to the lift cylinder. The hydraulic pump operates as a motor in a load lowering mode. The hydraulic pump is driven by the pressure fluid displaced by the lift cylinder. A direct current machine is coupled to the hydraulic pump, so as to operate as an electric motor in the load raising mode and to operate as a generator in the load lowering mode. A useful brake circuit is energized by the direct current machine in the load lowering mode. A valve assembly is disposed in the pressure fluid path between the lift cylinder and the hydraulic pump. Control apparatus are provided for controlling the valve assembly and including speed regulating apparatus for varying the speed of the direct current machine. At least one secondary hydraulic consumer is adapted to be connected to a hydraulic pressure source via an associated control valve. A lowering branch is provided between the valve assembly and a connection is provided between a check valve and the inlet of the hydraulic pump. The secondary hydraulic consumer receives fluid from the hydraulic pump via the valve assembly. There is provided an externally excited direct current machine having its speed controlled by the speed regulating apparatus in response to operation of the valve assembly.

FIELD OF THE INVENTION 
The present invention relates to a hydraulic lift apparatus for a battery 
driven lift truck or similar vehicle. 
BACKGROUND OF THE INVENTION 
In such hydraulic lift apparatuses, the pressure fluid source generally is 
comprised of a constant pump which is driven by an electric motor. The 
speed of the motor is controlled in response to the positions of a valve 
lever. This allows to change the raising speed without any substantial 
throttling losses during the raising of a load. It has become known to 
make the lowering speed also dependent on the positions of the valve 
lever; this is achieved by a way valve in the lowering branch. The 
potential energy of the load is converted into heat energy and the 
throttle of the way valve and is fed into the tank along with the 
hydraulic fluid. Furthermore it has become known to use the motor pump 
unit for performing the load holding function during the lowering 
operation and to refeed a portion of the potential energy of the load into 
the battery via the electric motor which operates as a generator in this 
situation. 
From German 20 14 605, German 26 18 046, German 30 18 156, it has become 
known to set the lowering speed by means of hydraulic reservoirs or by 
means of throttles. As an alternative, the motor operates only in one 
operative mode. As a result, the electric motor has only a very limited 
operative range when it operates as a generator. From U.S. Pat. No. 
3,947,744, it has become known to control the electric motor by a simple 
field control during the lowering operation, and it has become known from 
German 36 02 551 to use a series-wound motor, which operates in a limited 
operative range. The operative ranges which are not covered by the 
electric motor driven as a generator have to be covered by further 
hydraulic throttles; it is inherent that the potential energy of the load 
cannot be used in this case. 
In lift trucks it is generally necessary to feed hydraulic pressure to 
additional consumers. It has become known to pressurize these hydraulic 
consumers also via the hydraulic pump by setting a constant speed value 
for performing the secondary functions, with a valve lever position 
responsive speed component being added to the constant speed value for 
performing the raising function. During the lowering operation, a 
hydraulic pump cannot operate as a generator and perform the secondary 
functions at the same time. Accordingly, it has become known from U.S. 
Pat. No. 3,947,744 to provide an additional motor pump unit for performing 
the secondary functions. 
It is an object of the present invention to provide a hydraulic lift 
apparatus for a battery driven lift truck wherein the drive machine for 
the hydraulic pump is driven over the complete operative range required by 
the hydraulic system for raising and lowering the load, while no 
additional motor pump unit is required for performing the secondary 
functions. 
The present invention and further developments of the invention are defined 
in the patent claims. 
According to one aspect of the invention as defined in patent claim 1, an 
externally excited direct current machine is provided which, according to 
one embodiment of the invention, provides for controlling of the 
excitation and armature voltages independently of each other. To this end, 
there is provided a separate field current regulating means including a 
desired value generator which determines a desired value of the field 
current from predetermined relationships of the speed and the armature 
current power switches actuatable by said regulating means are associated 
with the field coil and the armature, the arrangement and operation of 
said power switches determining the amount and the direction of the 
current through the armature and the field coil, and control means 
comprises a directional means for the raising and lowering operations. 
Furthermore, it is of inventive importance that the lowering branch is 
provided between the valve assembly and a connection between a check valve 
and the inlet of the hydraulic pump, with the hydraulic pump pressurizing 
the secondary hydraulic consumer. As a result, the direct current machine 
is driven at all times in the same direction of rotation, no matter 
whether there is a raising or lowering operation. Accordingly, the 
hydraulic consumer can be operated directly by the hydraulic energy from 
the lowering operation, so that efficiency losses due to additional energy 
conversions are avoided. 
The present invention allows for a load independent control of the lowering 
speed both when the direct current machine operates as a generator and 
when there is a controlling operation solely via the valve assembly. The 
load holding function can be realized by the manually actuatable control 
valve which is infinitely adjustable and accordingly allows for a very 
sensitive regulation of the raising and lowering speeds. 
During the lowering operation, the normal case is that the secondary 
hydraulic consumer is operated by the hydraulic energy resulting from the 
lowering operation. The excess volume flow is refed into the tank. 
Furthermore, it is possible that, during the lowering operation, the 
pressure is greater than necessary for operating the secondary consumer, 
while only a too small volume flow is available. The speed control causes 
the electric motor to drive the pump. 
In order to prevent that the lowering speed will become excessive, one 
embodiment of the invention provides a hydraulic volume flow limiter in 
the lowering branch, which limiter limits the volume flow from the 
hydraulic cylinder at the inlet to a value predetermined by the valve 
lever position. It may be comprised of a pressure balance which is 
controlled by the volume flow and the inlet pressure of the control valve 
assembly. This constellation ensures that the lowering speed is maintained 
to be substantially constant. As soon as the flow rate in the pressure 
balance exceeds a predetermined value relative to the valve lever position 
at the control valve, the pressure balance starts its controlling 
function. It operates in connection with the throttle of the control valve 
as a two-way flow regulator and accordingly maintains the said lowering 
speed at a constant value. In this manner, it is ensured that the 
hydraulic pump can draw fluid via the check valve. Preferably, in this 
mode the control is such as to obtain a constant speed of a value such 
that the volume stream fed by the pump is sufficient to accommodate the 
maximum volume flow demand of the additional consumer. A valve lever 
position responsive velocity or speed of the secondary consumer is 
adjusted via an additional control valve, with the excess volume flow 
being refed into the tank. 
Finally, it is also possible that both the pressure and the volume flow 
during the lowering operation are smaller than required for the secondary 
function. In this case, an embodiment of the invention provides that a 
switch valve is connected between the pressure limiter and the hydraulic 
pump. The switch valve directs the hydraulic fluid directly to the tank 
when the pressure and volume flow in the lowering branch are smaller than 
necessary for operating the hydraulic consumer. The combination of the 
pressure balance and switch valve then works as a two-way flow rate 
regulator. The pressurizing of the secondary hydraulic consumer occurs in 
the same manner as already described. 
The lift mast of a hydraulic lift truck includes among others a free lift 
cylinder and at least one mast lift cylinder. The oil volume of the free 
lift cylinder is discharged during the lowering operation only after the 
mast lift cylinder has been completely retracted and accordingly does not 
contain any oil. As a result thereof, there will be a transition between 
the lowering operation in the mast lift mode and in the free lift mode. 
Due to the different cylinder faces in the mast lift mode and free lift 
mode there will be different lowering speeds at the same generator speed. 
In order to compensate for this, a further embodiment of the invention 
provides for detecting whether the lift mast is in the mast lift range or 
in the free lift range, and in response thereto proportionality factors 
between the desired lowering speed value and the motor speed in the 
control are changed so as to obtain the same lowering speed both in the 
mast and free lift modes. 
From "Microprocessor-based High-Efficiency Drive of a DC Motor" in IEEE 
Transactions on Industrial Electronics, vol. IE 34, No. 4 November 1987, 
pages 433 to 440, it has become known to control or regulate the armature 
and the field of a direct current machine so that the desired value for 
the field current is determined by predetermined relationships between the 
speed and the armature current, i.e. the actual armature current value. In 
this connection, a corresponding algorithm or a corresponding table is to 
be provided. 
The control or the desired value generation for the lift apparatus is 
obtained by an electric signal, for example via a manually actuated 
potentiometer, with directional means providing signals indicating the 
operations raising or lowering, respectively. At the moment when the 
hydraulic fluid starts to flow in the lowering branch via the control 
valve assembly, the hydraulic pump drives the direct current machine which 
operates as a generator. Since, however, the desired value for the speed 
still equals zero, the control tries to obtain this value, whereby the 
associated power switch for the armature is completely fired. The power 
switches for the field coil are operated such that the current is at a 
maximum. From this results a maximum brake torque which is sufficient to 
lower the load at a minimum speed if this is desirable. By providing 
corresponding desired values for the speed on the other hand, the raising 
and lowering speeds can be adjusted to the desired value. 
SUMMARY OF THE INVENTION 
A preferred embodiment of the invention provides that the desired field 
current value generator determines the desired value for the field current 
from the desired armature current value and the actual speed. This 
provides for the advantage that the speed control may operate also in the 
operative range wherein a higher armature voltage than the battery voltage 
is necessary to lower the load at an optimum efficiency. 
Instead of an externally excited direct current machine, it is possible to 
use a three-phase induction machine which is operated accordingly via 
converters. Speed control means determines by means of a speed sensor the 
actual rotor frequency of the machine in order to determine a control 
deviation from a desired speed value or a desired frequency value in order 
to obtain the desired speed both for the raising and lowering operations. 
Depending on whether the difference between the actual and desired 
frequency indicates a positive or negative slip, the three-phase induction 
machine operates as a motor or as a generator. Refeeding of electrical 
energy into the battery is obtained automatically without special 
provisions being required.

DETAILED DESCRIPTION OF THE INVENTION 
An externally excited direct current machine 10 drives a hydraulic pump 12 
which selectively operates as a motor. The pump 12 draws hydraulic fluid 
via a check valve 16 from a tank 14 and feeds it, via a control valve 
assembly 18, to a lift cylinder 20 or a group of secondary functions 22. 
The valve assembly 18 is manually actuated. For actuating the lift 
cylinder 20, there is provided for example a manual lever 24. For 
actuating the secondary functions 22, there are provided additional (not 
shown) manual levers. A speed regulating means to be described later is 
set so that initially a constant speed of the direct current motor 10 is 
used to operate the secondary functions 22. Excess volume flow is refed 
into the tank 14 via a line 26 and a filter 29. Raising and lowering of 
the lift cylinder 20 is then performed in a speed responsive manner. The 
lowering operation is obtained by actuation of the manual lever 24; a 
lowering branch 28 of the control valve assembly 18 is connected to a 
location between the pump 12 and the check valve 16. The volume flow 
during the lowering operation drives the hydraulic pump 12 in the same 
direction as it is driven when it operates as a pump so that it operates 
as a hydraulic motor driving the direct current machine 10 which now 
operates as a generator in order to convert the potential energy from the 
lift cylinder into electrical energy used for loading the battery. The 
lowering branch 28 includes a hydraulic pressure balance 30 and a switch 
valve 32 which is in an open position when it is not actuated by its 
solenoid 34. When it is, however, actuated, it communicates the upper 
portion of the lowering branch 28 via a line 36 with the line 26 and the 
tank 14. The hydraulic balance 30 receives, via a control inlet, the 
pressure in the lowering branch between the lift cylinder 20 and the 
control valve 18 as indicated by the dotted line 36a. The hydraulic 
balance 30 receives the differential pressure which is present across the 
valve 18 and which is valve lever position responsive. When the 
differential pressure will exceed a predetermined value, the pressure 
balance performs its controlling function, and the load pressure is 
decreased across the valve 30 so that ambient pressure prevails at the 
pump 12 and fluid can be pumped from the tank. If the volume flow in the 
pressure balance 30 exceeds a value as determined by the manual valve 18, 
a limiting function will be obtained so that the volume flow is not 
abruptly increased when the valve 32 is switched; at this time the pump 12 
can draw hydraulic fluid from the tank 14 if it is driven by the direct 
current machine 10. 
The raising of the lift cylinder 20 as well as the operation of the 
secondary functions have already been described. Also, the lowering 
operation has been substantially explained. The valve lever position 
responsive control of the speed of the motor 10 is directly proportional 
to the lowering speed apart from negligible leakage losses of the pump 12. 
When during the lowering phase, there is a demand for a secondary function 
22, the fluid is throttled in the control valve 18 to the pressure 
necessary for the secondary function. The secondary function is provided 
directly with the energy resulting from the lowering operation when the 
pressure and the volume flow are sufficient. This is normally the case 
when the load is lowered. A reversal of the pump 12 is not required. If 
the pressure and the volume flow resulting from the lowering operation are 
sufficient to operate the secondary function, there will be a direct 
hydraulic pressurization by the volume flow resulting from the lowering 
operation, with the excess volume flow being fed into the tank 14 via the 
control valve assembly 18. 
As soon as there is demand for a secondary function, the control means 
compares the actual motor speed resulting from the lowering speed with the 
desired speed corresponding to the volume flow requirement of the 
secondary function. When the volume flow requirement of the secondary 
function exceeds the "offer" resulting from the lowering operation, the 
motor speed will be increased accordingly. For this constellation, it must 
be ensured that the lowering speed remains approximately constant. This is 
achieved, as already mentioned, by the pressure balance if 30 in 
cooperation with the variable throttle in the control valve assembly 18. 
As soon as the flow rate at the pressure balance 30 exceeds a 
predetermined value relative to the valve lever position at the valve 18, 
the pressure balance 30 starts to perform its controlling operation. It 
now operates in connection with the throttle in the valve 18 as a two-way 
flow rate regulator so as to maintain the set lowering speed at a constant 
value. As a result it is ensured that the pressure resulting from the 
lowering operation is decreased at the pressure balance 30 and that 
additional fluid is fed via the valve 18 to ensure the operation of the 
secondary function 22. The motor 10 is operated at a constant speed which 
is selected so that the volume flow fed by the pump 12 is sufficient to 
satisfy the maximum volume flow requirement of the secondary functions. A 
valve lever position responsive speed of the secondary functions is 
enabled by a lever actuated throttle in the control valve assembly 18. 
Excess volume flow is refed into the tank 14 via the filter 29. 
If the pressure resulting from the lowering operation is smaller and the 
volume flow is smaller or greater than necessary for the secondary 
function, the switch valve 32 is switched off and the fluid is fed 
directly into the tank 14 via the filter 29. The lowering speed is 
maintained at a constant value via the pressure balance in connection with 
the throttle in the control valve assembly 18 which is adjustable by the 
valve lever. This combination operates similar to a two-way flow rate 
regulator. Operating the secondary function 22 is obtained via the direct 
current motor 10 and the hydraulic pump 12 as described above. 
The speed control of the externally excited direct current machine 10 of 
the apparatus in FIG. 1 will now be explained in more detail with 
reference to FIGS. 2 to 7. 
FIG. 7 shows a manual lever 44 which is pivotal to the left and to the 
right, with the extent of pivotal movement being indicated by -X and +X, 
respectively. It actuates a potentiometer indicated at 46 and generating a 
signal P in response to the pivotal movement. The signal P is represented 
in FIG. 4. The pivotal movement responsive signals in FIG. 4 do not differ 
from each other as to their polarity; this is why a pair of microswitches 
(not shown) is associated with the lever 44, which microswitches determine 
the polarity of the signal P. This is indicated by the signals S1 and S2 
in FIG. 5 and FIG. 6, respectively. A desired speed value generator 42 
computes a desired speed value n.sub.Soll from the signals P, S1 and S2, 
with the absolute value of P determining the absolute value of n.sub.Soll 
and the signals S1 and S2 determining the corresponding polarities. If a 
signal is received from the generator 42, the desired speed value is 
modified correspondingly so as to maintain a constant lowering speed (this 
will be explained in more detail below). A speed sensor 47 connected to 
the direct current machine provides an actual speed value n.sub.1st to a 
desired/actual values comparator 48, and the control deviation is fed to a 
speed regulator 50. It provides a desired value for the armature current 
I.sub.ASoll which is compared with the actual armature current value 
I.sub.AIst in a desired/actual values comparator 52. The control deviation 
is fed to an armature current regulator 56 and from there to an actuator 
indicated at 58. 
A table 60 stores relationships between the speed and the armature current. 
In a respective computing stage 62, the desired value for the field coil 
current I.sub.FSoll is computed from the data of the table 60. In this 
connection, it is important that the desired armature current value 
I.sub.ASoll is used for the computation. The desired value I.sub.FSoll is 
compared to the actual field current value in an actual/desired values 
comparator 64, and the control deviation is fed to a field current 
regulator 66 which provides a corresponding positioning signal in the 
position signal generator 68. The controllers 56, 66 are digital 
controllers and generate, via following power components 58, 68, pulse 
width modulated voltages which are used to adjust the predetermined 
current values I.sub.ASoll and I.sub.FSoll. Due to the fact that the 
desired armature current value I.sub.ASoll in addition to the actual speed 
value n.sub.Ist is used as an input for computing the desired field 
current value I.sub.FSoll, it is possible to operate in an operative range 
wherein an armature voltage exceeding the battery voltage U.sub.Batt would 
be necessary to perform a lowering operation in a generator-type manner at 
optimal efficiency as will be described later on. 
As may be seen from FIG. 3 the armature of the externally excited direct 
current machine 10 is connected to a battery 53 via a semibridge 51 
consisting of the MOSFETs T1 and T2. Diodes 54a, 55a are connected in an 
antiparallel relationship to the MOSFETs T1 and T2. The field coil 57a is 
connected to the poles of the battery 53 in series connection to the 
MOSFET T3 and in parallel connection to the semibridge 51, a diode being 
connected in an antiparallel relationship to the field coil 57a and to the 
MOSFET T3, respectively. 
The MOSFETs T1 and T2 are operated in a cyclic manner, i.e. the MOSFET T1 
is switched off when MOSFET T2 is switched on, and vice versa. The amount 
of the current flow, accordingly, results from the duty cycle of the 
pulses for the MOSFETs T1 and T2. The same is true for the MOSFET T3. The 
MOSFET T1 operates during the motor-type lift operation as a so-called low 
setting means, and the MOSFET T2 operates during the generator-type 
lowering operation as a high setting means. 
When the lever 44 is pivoted from its rest position in a direction for a 
lowering operation so far that the demand for the lowering function is 
provided via the signal S2 while the signal P indicates a desired speed 
value n.sub.Soll =0, the signal S2 causes the valve 18 to open; as a 
result hydraulic fluid flows through the pump 12 and drives the direct 
current machine 10. Due to the control deviation resulting therefrom, an 
I.sub.ASoll is fed to the desired/actual values comparator 52, and the 
armature current regulator 56 causes the armature to be short circuited 
via the MOSFET T2. Furthermore the field coil 57a receives a maximum field 
current. The resulting speed value is so small that the resulting minimal 
lowering speed is sufficient to ensure a sensitive lowering of the lift 
cylinder 20. At this point of operation of the direct current machine 10 
no energy is refed into the battery 53. 
If, however, a desired speed value n.sub.Soll &gt;0 is set by a further 
pivotal movement of the valve lever, the regulator 56 reduces the pulse 
width of the MOSFET T2 relative to the 100% operation until the desired 
speed n.sub.Soll will result. The MOSFET T2 now operates at each pulse 
width&lt;100% in the high setting mode, and energy will be refed into the 
battery 53. 
In FIG. 8 a desired speed value generator 42a generates from the signals P, 
S1 and S2 a desired rotor frequency value f.sub.2Soll for a threephase 
induction machine 10a which can be used instead of the externally excited 
direct current machine of FIG. 1 in the circuit shown therein. The signal 
P fed into the desired value generator 42a corresponds to the extent of 
the pivotal movement of for example the manual lever in FIG. 7. The 
polarity of the signal is determined by microswitches (not shown) which 
are associated to the manual lever 44. Accordingly, the polarity is 
determined by the signals S1 and S2. A speed sensor 47a connected to the 
machine 10a provides an actual speed value n.sub.ist which is fed to a 
computing stage 84 which computes the actual value f.sub.2ist of the rotor 
frequency in accordance with the pole pair p of the machine 10a. The 
actual frequency value is fed to the desired/actual values comparator 48a, 
and the deviation is fed to a speed regulator 70. 
The speed regulator 70 generates a desired value for the active component 
i.sub.qsoll of the complex current space pointer i. The active component 
i.sub.qsoll is proportional to the torque of the induction machine 10a. 
The value i.sub.dsoll is the desired value of the reactive component of 
the current space pointer i which is proportional to the magnetizing 
current of the induction machine. The desired value for the slip frequency 
f.sub.ssoll at 86 is determined from the desired value of the active 
component i.sub.qsoll of the current space pointer i. 86 may include a 
table which interconnects the active current and the slip frequency. As an 
alternative, a replacement circuit diagram of the induction machine can be 
included in 86 and may be used to determine the respective slip frequency 
relatively precisely. 
The obtained slip frequency f.sub.ssoll will be added to the actual rotor 
frequency value f.sub.2ist at 85. The result is the desired stator 
frequency value f.sub.Isoll which is fed to a rotary transformation means 
74. The current space pointer i resulting from i.sub.qsoll, i.sub.dsoll 
and f.sub.Isoll is transformed into the phase parameters from which the 
desired values for the phase currents i.sub.usoll and i.sub.vsoll follow. 
The respective deviations which are obtained by subtracting the respective 
actual current values i.sub.uist and i.sub.vist at the adding stages 75 
and 77 are fed to the current regulators 76 and 78 which provide the 
values for the phase voltages U.sub.usoll and U.sub.vsoll. The desired 
value of the third phase voltage U.sub.wsoll can be computed at the adding 
stage 79 from the condition that the sum of all three voltages must equal 
zero. 
The three voltage values are converted, in block 82, into pulse width 
modulated signals which energize a power output stage 81 such that the 
desired current values in the induction machine 10a result. 
Details of the power output stage 81 are shown in the block diagram of FIG. 
9. 
As shown in FIG. 9, each phase of the induction machine 10a is connected to 
a connection point of a pair of MOSFETs T1 to T6, respectively, which are 
connected in series and subjected to the battery voltage U.sub.Batt. The 
transistors T1 to T6 are operated at a sinusoidal pulse width and are 
energized pairwise in an anticyclic manner. Energization of the three 
transistor pairs is designed so that the sinusoidal pulse width modulated 
drive signals are fed to the transistor pairs out of phase each for 
120.degree. at the frequency of the sinusoidal evaluation. Under these 
circumstances a rotating field which is variable as to frequency and 
voltage is generated in the induction machine 10a. 
A comparison of the frequency f.sub.ssoll and f.sub.2ist provides the 
polarity of the desired frequency f.sub.ssoll from which follows whether 
the induction machine 10a is to be operated as a motor or as a generator. 
As a result energy is automatically--without any further measures--refed 
into the battery in FIG. 9 when the induction machine 10a is operated as a 
generator. 
When the lever 44 in FIG. 7 is pivoted from its rest position in the 
direction for a lowering operation so far that the demand for the lowering 
function is generated via the signal S.sub.2 and on the other side the 
signal P indicates a desired rotor frequency value f.sub.2 =zero, 
hydraulic fluid flows to the hydraulic system and drives the induction 
machine 10a. The control means now performs a control operation such as to 
arrive at the lower control limit, i.e. the minimum stator field frequency 
which is about 0.2 Hz. The slip in the induction machine 10a provides for 
results a continuous control deviation. The resulting speed value is so 
small that the resulting minimum lowering speed is sufficient to ensure a 
sensitive lowering of the lift cylinder 20 (FIG. 1). 
The lift apparatus in accordance with the present invention can comprise a 
lift mast having at least one displaceable mast portion and load receiving 
means mounted on the displaceable mast portion so as to be adjustable in 
height. A sensor can also be provided on the lift mast for detecting 
whether there is a lowering operation of the displaceable mast portion or 
of the load receiving means. Such a sensor can provide signals H (see 
FIGS. 2 and 8) to the desired speed value generator 42 for modifying a 
desired speed value signal (n.sub.Soll).