EP0630853B1 - Hydraulic lifting equipment for battery-propelled handling trucks or the like - Google Patents

Description

The invention relates to a hydraulic lifting device for battery-powered industrial trucks or the like according to the preamble of patent claim 1.

For electrically operated industrial trucks it is known to use hydraulic lifting devices and to generate the delivery pressure from a constant pump driven by an electric motor. The speed of the engine is controlled depending on a valve lever. This means that when lifting the load, the lifting speed is changed without significant throttling losses. It is in this Context also known to set the lowering speed accordingly via a valve lever and to realize it via a directional valve in the lowering branch. The potential energy of the load of a throttle of the directional control valve is converted into heat and discharged into the tank with the fluid. However, it is also known to feed the potential energy of the load back into the battery via the electric motor operating as a generator.

From DE-A-20 14 605 it has become known to use a direct current shunt motor as a direct current machine and to use a rotary lobe pump with adjustable delivery rate as a pump. By adjusting the control means of the pump, the delivery rate is brought to a maximum value from zero regardless of the direction of movement of the control means according to the deflection from a central position, the pump, while maintaining its direction of rotation in one direction of movement of the control means as a pump and in the other direction of movement as Engine works.

From DE-A-26 18 046 it has become known to provide separate hydraulic branches for lifting and lowering, to which a direct current motor and a pump as well as a hydraulic motor and a generator are assigned. When lowering a constant flow valve generates a fixed lowering speed. A manual control valve switches over between lifting and lowering.

From DE-A-30 18 156 it has become known to provide controllable solenoid valves for lifting and lowering in order to implement starting and braking ramps. A volume flow measurement is used to control the motor or generator. A cage induction motor is used as the drive machine. From SE-A-84 05 088 it has become known to use a double-closing machine as the drive machine or generator, a part of the series winding being removed when it is operating as a generator in the lowering mode of the lifting device. The lifting cylinder is controlled via a lever-operated valve in the pressure medium line.

From US-A-3 947 744 it is known to use a separate three-phase machine for energy recovery when lowering a hoist. The braking force when lowering can be adjusted by controlling the field of the generator.

Finally, it has become known from EP-A-230 884 to couple a series locking machine to a hydraulic unit. A control valve arrangement is provided in the pressure medium path, which has a proportional valve, the hoist control in load-lowering mode opening the proportional valve in accordance with a ramp function and, depending on the output current of the DC machine working as a generator, effectively switching the useful brake circuit if the generator output current exceeds a predetermined value. Over a restricted area, the device described therefore also operates via a hydraulic throttle point, so that the potential energy of the load cannot be recovered. Furthermore, when changing from the hydraulic control of the lowering speed via the throttle point to an electrical via the electric motor with the pump, transitions occur which are noticeable in a jerky change in the lowering speed.

The invention has for its object to provide a hydraulic lifting device for battery-powered industrial trucks, which also enables sensitive lowering without significant hydraulic losses and optimal energy recovery in lowering mode.

This object is achieved according to the invention by the features of patent claim 1 and patent claim 6.

In the invention, only one load holding valve is provided in the pressure medium path, which is either open or closed and does not produce any throttling losses in the open position.

It is also essential to the invention that an externally excited DC machine is provided which enables the excitation and armature voltages to be set independently of one another both in motor and in generator operation. For this purpose, the lifting device according to the invention provides a separate field current control device with a setpoint generator, which determines the setpoint for the field current from predetermined relationships between the speed and the armature current. From "Microprocessor-Based High-Efficiency Drive of a DC Motor" from IEEE Transactions on Industrial Electronics Vol. IE 34 No. November 4, 1987, pages 433 to 440, has become known to control armature and field by determining the target value for the field current from predetermined relationships of speed and armature current, here the actual armature current value. A corresponding algorithm or table must be provided for this.

With the circuit described, it is possible to operate the DC machine over the entire working range hydraulic system when lifting and lowering the load requires to operate. Additional hydraulic components are not required. A maximum recoverable potential energy is obtained in the lowering phase. In addition, there is no longer a transition from hydraulic to electrical load maintenance during the lowering process, which makes the lowering function easier for the operator to control.

The control or the setpoint specification for the lifting device takes place by means of an electrical signal, for example via a manually operated potentiometer, a direction indicator also having to be provided, the signals of which indicate the operating process of raising or lowering and which also control the load holding valve. As soon as the load holding valve is opened when the load is raised, a hydraulic current begins to flow and drives the direct current machine acting as a generator via the hydraulic unit. However, since the setpoint is still zero, the control tries to reach this value, whereby the associated circuit breaker for the armature is completely switched through. The circuit breakers for the field winding are operated so that the current is maximum. This creates a maximum braking torque that is sufficient to lower the load at minimum speed if desired. By setting the appropriate setpoint for on the other hand, the speed can be set to the desired value, the lifting and lowering speed.

In a preferred embodiment of the invention it is provided that the field current setpoint generator determines the setpoint for the field current from the armature current setpoint and the actual speed. This has the advantage that the speed control device can also operate in the operating range in which a higher armature voltage than the battery voltage is necessary in order to reduce the generator with optimal efficiency.

Instead of an externally excited DC machine, a three-phase asynchronous machine can also be used, which is fed accordingly via converters. A speed control device uses a tachometer to determine the actual rotor frequency of the machine and forms a control deviation with a speed setpoint or frequency setpoint in order to achieve the desired speed for both lifting and lowering. Depending on whether the difference between the actual and target frequency indicates a positive or a negative slip, the asynchronous machine works as a motor or generator. Electrical energy is fed back into the battery automatically without any special precautions being taken.

In the case of a lifting mast for conveyors with at least one extendable mast part on which the load suspension device is arranged such that it can be moved vertically, a distinction is made between the mast lift and the so-called free lift. Free lift is understood to mean the adjustment of the load handler on the movable mast part and mast lift is the adjustment of the movable mast part. It goes without saying that the lifting cylinders for the parts mentioned can have different cross-sections and therefore also displace different volumes. Therefore, if no special precautions are taken for this case, different lowering speeds occur in lowering mode. Therefore, an embodiment of the invention provides that a sensor is provided on the lifting mast, which determines whether a lowering operation of the movable mast part or the load-carrying means takes place and whose signals are sent to the speed setpoint generator in order to modify the speed setpoint.

In known conveyors, auxiliary functions are also performed by the hydraulic circuit of the lifting device. In the device according to the invention, however, a separate unit consisting of a pump and a DC motor is provided for supplying the secondary functions. Otherwise there would be no energy recovery during the lowering process if the hydraulic unit was also used as a pump Supplying the secondary functions should serve. The additional effort for the additional pump unit is justified in view of the optimal energy recovery during lowering operation.

In some cases, the secondary functions require a relatively large volume flow or a high pressure. Since this occurs very rarely, an appropriate design of the additional pump unit would be useless for most applications. It can therefore be considered to use the hydraulic unit as a pump in this case and to temporarily forego recovery during the lowering process when this secondary function is called up.

Fig. 1

zeigt eine hydraulische Hubvorrichtung nach der Erfindung.

Fig. 2

zeigt ein Blockschaltbild zur Regelung der Hubvorrichtung nach Fig. 1.

Fig. 3

zeigt ein Blockschaltbild des Leistungsteils der Gleichstrommaschine nach Fig. 1 bzw. Fig. 2.

Fig. 4 bis 6

zeigen Diagramme von verschiedenen Steuersignalen der erfindungsgemäßen Hubvorrichtung.

Fig. 7

zeigt eine Handbetätigung für die Hubvorrichtung nach der Erfindung.

Fig. 8

zeigt ein Blockschaltbild zur Regelung einer Hubvorrichtung ähnlich der nach Fig. 1 mit einer Drehstromasynchronmaschine.

Fig. 9

zeigt ein Blockschaltbild des Leistungsteils der Asynchronmaschine nach Fig. 8.

The invention is explained in more detail below with reference to drawings.

Fig. 1

shows a hydraulic lifting device according to the invention.

Fig. 2

shows a block diagram for controlling the lifting device of FIG. 1st

Fig. 3

2 shows a block diagram of the power section of the direct current machine according to FIG. 1 or FIG. 2.

4 to 6

show diagrams of various control signals of the lifting device according to the invention.

Fig. 7

shows a manual operation for the lifting device according to the invention.

Fig. 8

shows a block diagram for controlling a lifting device similar to that of FIG. 1 with a three-phase asynchronous machine.

Fig. 9

shows a block diagram of the power section of the asynchronous machine according to FIG. 8.

A DC machine 10 drives a hydraulic unit 12, which optionally works as a motor or pump. In pump operation, the pump 12 conveys to a lifting cylinder 18 via a load holding valve 14 and a valve block 16. The lifting cylinder 18 can be a single cylinder or represent a plurality of lifting cylinders with which the movable mast sections and / or the load suspension means can be raised and lowered ( the latter is not shown). The load holding valve 14 has a check valve 20. A bypass to the hydraulic unit 12 and a filter 22 contains a check valve 24. Hydraulics Medium is in the tank 26. A bypass to the tank 26 is formed via a pressure relief valve 28, which contains a further filter 30. It is also connected to a valve block 32, which is fed by a hydraulic pump 34, which in turn is driven by a DC motor 36. A number of secondary functions 38 are supplied via the valve block 32. The performance of the unit 34, 36 is relatively small in relation to that of the hydraulic unit 12 or DC machine 10. A pressure relief valve 38 is connected to the bypass described.

The valve block 16 is designed such that it optionally connects the load holding valve 14 to the lifting cylinder 18 or to an auxiliary consumer 40.

In the lifting operation of the lifting cylinder 18, the externally excited direct current motor 10 drives the pump 12 and conveys fluid from the tank 26 via the filter 22 and the ball valve 20 and the valve block 16 into the lifting cylinder 18. After the lifting process has ended, the load is supported on the ball valve 20 of the load holding valve 14, so that sagging of the load is prevented.

The series-circuit motor 36 connected via a contactor to the battery voltage drives the pump 34, which conveys the fluid from the tank 26 to the secondary functions 38 via the manually controlled valve block 32. The return flow takes place via the filter 30 into the tank 26. The lifting function and the secondary functions do not influence one another.

In lowering mode, the load holding valve 14 is switched electrically and directs the load pressure stored in the lifting cylinder to the hydraulic unit 12, which is operated as a motor in the opposite direction of rotation to the lifting function. The externally excited DC machine 10 operates in generator mode, the speed being directly proportional to the lowering speed, which neglects the leakage losses in the hydraulic unit 12 once. Apart from the load holding valve 14, the changeover valve 16 and the supply and discharge hoses, there are no further hydraulic components in the lowering branch which can lead to additional pressure losses and thus to a reduction in efficiency. If the mast has a free lift cylinder and two mast lift cylinders, the oil volume of the mast lift cylinder is emptied first during the lowering process. A sensor 42 is assigned to the lifting cylinder or the mast, by means of which it is indicated when a switch is made from the mast lifting to the free lifting during the lowering process.

In the secondary mode, an attachment, for example, can be supplied with the pressure medium flow of the pump 12 in parallel with the lifting cylinder 18. The volume flow is divided via valve 16, which can be designed as a load-sensing valve.

If the additional function 40 is requested during the lowering operation, the lowering function must be interrupted. The valve 16 blocks the volume flow from the lifting cylinder 18. The externally excited DC machine 10, which was operated as a generator during the lowering function, now reverses and drives the hydraulic unit 12, for example at a constant speed. The hydraulic unit 12 supplies the volume flow required to supply the additional function 40.

The speed control of the externally excited DC machine 10 of the device according to FIG. 1 is explained in more detail with reference to FIGS. 2 to 7.

Fig. 7 shows a hand lever 44 which can be pivoted to the left and right, the extent of the pivoting being indicated by -X or + X. He actuates a potentiometer, indicated at 46, which generates a signal P as a function of the deflection. The signal P is shown in FIG. 4. The deflection-dependent signals in FIG. 4 do not differ by the sign, therefore a pair of microswitches (not shown) are assigned to the lever 44, which specify the sign of the signal P. This is indicated by the signals S1 and S2 in FIGS. 5 and 6, respectively. A speed _setpoint generator 44 calculates a speed _setpoint n _set from the signals P, S1 and S2, the absolute value of P determining the absolute value of n _set and the signals S1 and S2 the corresponding sign. If a signal is received from the encoder 42, the speed setpoint is modified accordingly in order to maintain a constant lowering speed. (This will be discussed in more detail below). A speed sensor 46 a connected to the direct current machine 10 supplies an _actual speed value n Act to a target actual value comparison 48, and the control deviation is given to a speed controller 50. It forms a setpoint for the armature current _IAset , which is compared in a setpoint actual value comparison 52 with the armature current _{actual value} I _AIst . The control deviation arrives at an armature current controller 56 and from there to a control value transmitter indicated by 58.

Relationships between speed and armature current are stored in a value table 60. In a corresponding arithmetic stage 62, the setpoint is converted from the data in table 60 for field winding I _FSoll calculated. It is essential that the armature current _setpoint I _{Aset is} used for the calculation. The setpoint I _FSoll is compared in an actual setpoint comparison 64 with the field current actual value, the control deviation being given to a field current controller 66 which generates a corresponding actuating signal in the actuating value transmitter 68. The regulators 56, 66 are designed as digital regulators and generate pulse-width-modulated voltages via downstream power _units 58, 68, by means of which the predefined current _values I _{A target} and I _{F target are regulated} . Because the input current for calculating the field current setpoint I _{FSoll in} addition to the _actual speed n _is the armature current _setpoint I _ASoll , it is also possible to work in an operating range in which a higher armature voltage than the battery voltage U _Batt would be necessary in order to regenerate with optimal efficiency lower, as will be described.

As can be seen from FIG. 3, the armature of the externally excited DC machine 10 is connected to a battery 52 via a half bridge 50 consisting of the Mosfets T1 and T2. Diodes 54, 56 are antiparallel to the Mosfets T1 and T2. The field winding 58 is located in the diagonal branch of the bridge circuit 60 at the terminals of the battery 52, the bridge circuit consisting of the Mosfets T3 to T6 to which diodes 62 to 68 are antiparallel.

The Mosfets T1 and T2 are controlled cyclically, i.e. Mosfet T1 is off when T2 is on and vice versa. The size of the current flow thus results from the pulse duty factor for the Mosfets T1 and T2. The same applies to the Mosfets T3 to T6, which are opened or closed crosswise. Mosfet T1 works as a step-down converter in motorized lifting operation and Mosfet T2 works as a step-up converter in generator-based lowering operation.

If the lever 44 is deflected from the rest position in the direction of lowering to such an extent that the request for the lowering function is generated via the signal S2 and, on the other hand, the signal P signals a speed _setpoint of n _target = 0, the signal S2 causes the load holding valve 14 to open, whereby hydraulic volume flows through the hydraulic unit 12 and drives the DC machine 10. Due to the constant control deviation that occurs in this way, an I _{A target is given} to the target actual value comparison 52, and the armature current controller 56 ensures that the armature is short-circuited via Mosfet T2. In addition, the field winding 58 is supplied with maximum field current. The speed value that is now set is so small that the lowest possible lowering speed that is set is sufficient to ensure sensitive travel of the lifting cylinder 18.

At this operating point of the DC machine 10, however, no energy is fed back into the battery 52.

However, if a speed _setpoint n _target > 0 is set by a further deflection of the valve lever, the controller 56 withdraws the pulse width of the MOSFET T2 from 100% control until the desired speed n _{target is} set. The Mosfet T2 now operates in step-up mode at every pulse width <100%, and energy is fed back into the battery 52.

In FIG. 8, a speed _setpoint generator 44a generates a rotor frequency _setpoint f _2soll from the signals P, S1 and S2 for a three-phase asynchronous _machine 10a, which can be used in the circuit shown there instead of the separately excited DC machine according to FIG. The signal P fed into the setpoint generator 44a corresponds to the extent of the deflection, for example of the hand lever according to FIG. 7. The sign of the signal is indicated by microswitches (not shown) which are assigned to the hand lever 44. The sign is therefore determined by the signals S1 and S2. A speed sensor 46a connected to the machine 10a supplies an actual _speed value n _ist , which is passed to an arithmetic stage 84 which, according to the number of pole pairs p of the machine 10a, is the actual value f _{2ist of} the rotor frequency calculated. The actual frequency value is given to the target actual value comparison 48a, and the control difference reaches a speed controller 70.

The speed controller 70 generates a setpoint for the active component i _{qsoll of} the complex current _{space vector} i. The active component i _qsoll is proportional to the torque of the asynchronous machine 10a. The value i _dsoll is the target value of the _{reactive component} of the current _{space vector} i, which is proportional to the magnetizing current of the asynchronous machine. The setpoint for the slip frequency f _ssoll is determined at 86 from the setpoint of the active component i _{qsoll of} the current space vector i. A table can be stored in 86 which establishes the relationship between the active current and the slip frequency. It is also conceivable to store an equivalent circuit diagram of the asynchronous machine in 86 and to use it to determine the respective slip frequency relatively precisely.

The determined slip frequency f _{ssoll is} added to the _actual rotor frequency _value f _2act at 85. This results in the stator frequency _setpoint f _1set which is fed to a rotary transformation 74. The _{current space vector} i resulting from i _qsoll , i _dsoll and f _1soll is transformed to the _{string sizes} , and the setpoints result for the _string currents i _usoll and i _vsoll . The respective control differences, which result from subtracting the respective _actual current _values i _uist and i _vist at the addition points 75 and 77, are given to the current controllers 76 and 78, which output the manipulated variables for the phase _voltages U _usoll and U _vsoll . The setpoint of the third phase voltage U _wsoll can be calculated from the condition that the sum of all three voltages must be zero at the addition point 79.

The three voltage setting values are now converted into pulse width modulation signals in block 82, which control a power section 81 in such a way that the desired current values result in the asynchronous machine 10a.

Details of the power section 81 can be seen from the block diagram in FIG. 9.

It can be seen in FIG. 9 that one strand of the asynchronous machine 10a is located at a connection point of a pair of Mosfets which are connected in series and are connected to the battery voltage U _{Batt and are} denoted by T1 to T6. The transistors T1 to T6 are operated with a sine-weighted pulse width and are counter-cyclically controlled in pairs. The control of the three pairs of transistors is designed so that the sine-weighted pulse widths with which the transistor pairs are controlled are given a phase shift of 120 ° to the transistor pairs in the frequency of the sine weighting. With this control, a rotating rotating field is generated in the asynchronous machine 10a, which is variable in frequency and voltage.

From the comparison of the frequencies f _ssoll and f _{2ist it} follows from the sign of the target frequency f _ssoll whether the asynchronous machine 10a works in the motor or generator _mode . Thus, when the asynchronous machine 10a is operated as a generator, it is automatically fed back into the battery according to FIG. 9 without further precautions.

If the lever 44 in FIG. 7 is deflected so far from the rest position that the request for the lowering function is generated via the signal S ₂ , on the other hand the signal P signals a rotor frequency setpoint f ₂ = zero, the signal S ₂ causes Opening the load holding valve 14 (FIG. 1), whereby hydraulic medium flows through the hydraulic unit and this drives the asynchronous machine 10a. The controller now adjusts to the lower control limit, the lowest possible stator field frequency, which is approximately 0.2 Hz. Due to the slip in the asynchronous machine 10a there is a constant control deviation. The speed value that is set is so small that the lowest possible lowering speed that is set is sufficient to ensure smooth travel of the lifting cylinder 18 (FIG. 1).

Claims (7)

1. A hydraulic lift device for battery-propelled ground conveyor vehicles, comprising at least a hydraulic ram (18), a hydraulic machine (12) operating as a pump in a load raising cycle by delivering pressurized fluid to said ram (18) and operating as a motor in the load lowering cycle in driving the hydraulic machine (12) by pressurized fluid displaced from said ram (18), a DC machine (10) coupled to the hydraulic machine operating as an electric-motor in the load raising cycle and as a generator in the load lowering cycle, an energy recovering circuitry which is fed by the DC machine (10) in the load lowering cycle, a valve means comprising a load holding valve and disposed in the fluid pass between said hydraulic ram (18) and said hydraulic machine (12), a control means actuating said valve means and comprising directional setting means and a speed control means acting on said DC machine (10) for setting the speed, characterized in that the valve means comprises exclusively a single load holding valve (14) working without choke losses in the open position, that the DC machine (10) is of the separately excited type comprising a separate field current control means (66, 68) having a desired value setting means (62) determining a desired field current (I_Fsoll) from the predetermined relations between the speed (n_Ist) and the armature current (I_Asoll) depending on said control means, that power switches (Mosfets T1 through T6) are associated to the field winding (58a) and the armature which power switches are actuated by said control means, said power switches being arranged and controlled such that the intensity and the direction of the current through the armature and field winding are defined and that the speed control means controls the load holding for the lowering and raising cycle during the whole work range.
2. The device of claim 1, characterized in that the desired speed value setting means for the speed control means is a potentiometer (46) comprising an adjusting member (44) and that micro switches generating directional signals (S1, S2) are associated to said adjusting member (49).
3. The device of claim 1 or 2, characterized in that the desired field current value setting means (62) calculates the desired value for the field current (I_Fsoll) from the desired value of the armature current (I_Asoll) and the actual speed (n_Ist).
4. The device of one of the claims 1 to 3, characterized in that the armature is connected to the battery (52) through a half-bridge (50) including Mosfets (T1, T2) to which diodes (54, 56) are connected antiparallel which Mosfets (T1, T2) are cyclically controlled.
5. The device of one of the claims 1 to 4, characterized in that the field winding (58) is connected in the crossing branch of a bridge circuit comprising four Mosfets (T3 through T6) to which diodes (62 through 68) are connected antiparallel and which are cyclically controlled.
6. A hydraulic lift device for battery-propelled ground conveyor vehicles comprising at least a hydraulic ram, a hydraulic machine operating as a pump in a load raising cycle by delivering pressurized fluid to said ram and operating as a motor in the load lowering cycle in driving the hydraulic machine by pressurized fluid displaced from said ram, an electric-machine coupled to the hydraulic machine operating as a motor in the load raising cycle and as a generator in the load lowering cycle, an enery recovering circuitry which is fed by the machine in the load lowering cycle, a valve means disposed in the fluid pass between said hydraulic ram and said hydraulic machine, a control means actuating said valve means and having a directional setting means (44) for lifting and lowering and a speed control means acting on said machine for setting the speed, characterized in that the valve means is defined exclusively by a single load holding valve working without choke losses in the open position, that the machine (10) comprises a three-phase induction machine (10a) to be operated by a frequency converter and including a speed control means for controlling the stator frequency in response to an error signal determined by the actual speed value and a predetermined desired speed value and that the speed control means controls the load holding for raising and lowering during the whole work range.
7. The device of one of the claims 1 to 6 for a lifting mast comprising at least a movable mast portion and load carrying means which are adjustable in height with respect to the movable mast portion, characterized in that a sensor (42) is provided at the lift mast to determine whether a lowering cycle of the movable mast portion (mast lift) or the load carrying means (free lift) is performed and which signals are fed to the desired speed value setting means (44) for modifying the desired speed value signal (n_Soll).

Images