DE29824820U1 - Truck - Google Patents

Description

Weickmann & Weickmann

Patent Attorneys European Patent Attorneys · European Trademark Attorneys

DIPL.-ING. H. WEICKMANN (until 31.1.01) Dipu-iNG. FA WEICKMANN

DIPL.-CHEM. B. HUBER

DR.-ING. H. LISKA

DIPL.-PHYS. DR. J. PRECHTEL

DIPL.-CHEM. DR. B. BÖHM

DIPL.-CHEM. DR. W. WEISS

DIPL.-PHYS. DR. J. TIESMEYER

DIPL.-PHYS. DR. M. HERZOG

DiPL-PHYS. B. RUTTENSPERGER

DIPL.-PHYS. DR.-ING. V.JORDAN

DIPL.-CHEM. DR. M. DEY DIPL.-FORSTW. DR. J. LACHNIT

Our sign:

16433G DE/Tlct

Applicant:

STEINBOCK GmbH Steinbockstr. 38

85368 Moosburg

Industrial truck

P.O. Box RRO R?O 81635 Munich. Germany. Tel. (089) 45563 0. Fax (0891 45563 999, email<P>weickmann.de

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Industrial truck Description

The invention relates to an industrial truck according to the preamble of claim 1 and an industrial truck according to the preamble of claim 2.

Such industrial trucks with a position measuring device for determining the position of the load-carrying device relative to a reference point of the industrial truck in question are known from EP 0 335 196 A1.

In the industrial truck according to EP 0 335 196 A1, the position measuring device has a gearwheel that is rotatably mounted in a stationary lifting mast part and that meshes with the teeth of a row of teeth that is formed on the movable lifting mast part that is telescopically height-adjustable relative to the stationary lifting mast part. In the subject matter of EP 0 335 196 A1, the concept of the quasi-positive drive of the "roller body" is taken up and a gearwheel-rack arrangement is used for this purpose.

The disadvantages of a gear-tooth row pairing, as known from EP 0 335 196 A1, are in particular that the comparatively complex tooth row on the telescopic lifting mast part must be prepared with the required precision.

For example, DE 195 08 346 C1 discloses an industrial truck with a position measuring device for determining the lifting height of a height-adjustable load-carrying device. The load-carrying device is driven by a hydraulic cylinder fed by a hydraulic pump, whereby the hydraulic pump is driven by an electric motor. Starting from a zero position of the load-carrying device, the revolutions of the hydraulic pump are measured incrementally in one direction of rotation and

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recorded decrementally in the opposite direction of rotation and evaluated taking into account the overall efficiency of the lifting system to determine the respective lifting height.

In the case of industrial trucks, it has already been proposed to provide proximity switches at predetermined intervals on the mast for a load-handling device, which respond to a marking that can be moved with the load-handling device in order to determine the respective lifting height of the load-handling device.

From DE 32 11 486 A1 a forklift truck with the features mentioned above is known, in which the position measuring device comprises a rotary disk provided with radial edge slots, which is seated on the shaft of a gear arranged in the upper area of the movable part of the lifting mast, which is intended to deflect a lifting chain for the load-carrying fork. To detect the rotary movement of the disk, an optical sensor arrangement with a light-emitting diode and a phototransistor is provided near the edge of the disk. The light path formed by the light-emitting diode and the phototransistor is alternately released by the edge slots or interrupted by the teeth located between the edge slots when the disk rotates, so that the phototransistor delivers a pulse-shaped, electrical signal, the respective number of pulses of which corresponds to the angle of rotation of the disk and the chain gear connected to it in a rotationally fixed manner, whereby the respective change in the lifting height of the load-carrying fork is determined from the angle of rotation of the chain gear engaged with the lifting chain. Since the respective rotation of the chain gear when raising and lowering the load-carrying fork depends directly on the length of the chain section deflected at the chain gear, changes in the chain length, which often occur when operating under load, lead to errors in the lifting height determination. A further disadvantage of this known solution is that the sensor components (light-emitting diode, phototransistor,

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Turntable) must be arranged on the moving part of the lifting mast, since the chain gear connected to the turntable must be arranged on the moving part of the lifting mast for functional reasons. This not only means a structural determination, but in any case brings with it the problems that arise when deriving electrical signals from moving signal transmitters.

Likewise, US-A-5 056 437 discloses a position measuring device for an automatic storage system in which a coding disk as a distance detection means is driven to rotate by a chain. The disadvantages mentioned in connection with DE 32 11 486 A1 with regard to chain elongation apply accordingly to US-A-5 056 437.

The invention is based on the object of specifying an industrial truck of the type mentioned at the beginning, in which the position measuring device can be implemented cost-effectively using simple means and yet reliably delivers position measurement results of high accuracy and resolution.

This object is achieved according to the invention on the basis of an industrial truck having the features of the preamble of claim 1 by the features of the characterizing part of claim 1. The stated object is also achieved by an industrial truck having the features of claim 2.

In the case of an industrial truck in the form of a forklift truck, the load-carrying device is usually a load-carrying fork which is arranged on a fork carrier and can be moved vertically together with the fork carrier on a lifting frame or mast. To determine the lifting height of the load-carrying fork, the roller body according to claim 1 is arranged on the fork carrier or an element connected to it for movement along the guide in such a way that it is, for example, on a line parallel to the lifting direction.

The track rolls off the lifting frame. The rotary movement of the roller body is detected by the signal generator, so that the evaluation circuit connected to the signal generator can evaluate the electrical signal supplied by the signal generator to determine the lifting height.

On the other hand, according to claim 2, the roller body can be arranged rotatably on an element that is stationary with respect to the guide, with its circumference resting on the element that can be moved with the load-carrying means, such that it is forced to rotate when the element that can be moved with the load-carrying means moves. The embodiment according to claim 2 has the advantage that the signal lines do not have to move and can be laid in a fixed manner.

According to the invention, the roller body is part of a rolling bearing, for example the outer ring of a rolling bearing. The use of a rolling bearing with an integrated angle sensor offers the advantage that extremely low friction moments have to be overcome and the roller body can therefore roll on its roller track without any significant counter-torque. In test measurements, the roller body did not show any slip error that would noticeably impair the reproducibility of the measurement results, even after a large number of translatory movements of the load-bearing device. Even under the conditions of a roller track contaminated with lubricant, very reproducible measurement results were obtained.

The advantages of the invention can also be obtained if the roller body is rotatably arranged on the element movable with the load-carrying means by means of a rolling bearing.

The signal generator is preferably a digital angle sensor designed as an incremental encoder, whereby the evaluation circuit contains a counting circuit which counts the values emitted by the angle sensor in accordance with the change in the angle of rotation of the roller body.

counts pulses. The incremental angle sensor is preferably designed with at least two channels, so that when the roller body rotates it emits two counting pulse signals, preferably 90° out of phase. The evaluation circuit evaluates the counting pulse signals in order to determine the direction of rotation of the roller body and, depending on the direction of rotation, to count up or down the counting pulses of at least one of the counting pulse signals. When the load-carrying device is raised, for example, it counts up, whereas when the load-carrying device is lowered it counts down, so that the current count can be used to determine the lifting height. The evaluation circuit can be designed in such a way that it evaluates the two phase-shifted counting pulse signals redundantly for safety reasons in order to be able to detect any measurement errors.

The invention also relates to industrial trucks in which the position of the load-carrying means can be influenced by superimposing movements of several elements that can be moved relative to one another. Such a case is, for example, in a forklift truck with a telescopically length-adjustable mast, which has a lower mast part and an upper mast part that can be extended telescopically relative to it, with the load-carrying means being movable on the upper mast part. For such a forklift truck, it is proposed that the movement of the upper mast part relative to the lower mast part be detected with a first roller body that is rotatably mounted on the upper mast part and can roll on the lower mast part. To detect the movement of the load-carrying means relative to the upper mast part, it is proposed that a second roller body is rotatably mounted on an element connected to the load-carrying means for joint movement relative to the upper mast part and can roll on the upper mast part. The evaluation circuit evaluates the rotational movement signals emitted by the signal transmitters of the roller bodies in order to determine the positions

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of the load-handling device and the upper mast section relative to the lower mast section. The measuring principle can of course be extended to masts with additional telescopic mast sections.

Additional elements that can be moved relative to one another can be provided between the load-handling device and a lifting frame of a forklift truck, as is the case, for example, with a so-called three-way order picking forklift with a height-adjustable driver's cabin. In such a forklift truck, a driver's cabin is arranged on a main lifting frame so that it can be adjusted in height, with a swivel-push device being arranged on the driver's cabin, which has an additional lifting frame that can be moved transversely to the lifting direction of the driver's cabin, on which the load-handling device is arranged parallel to the lifting direction of the driver's cabin and so that it can be adjusted in height relative to the driver's cabin, with the load-handling device also being pivotable relative to the driver's cabin about an axis that is parallel to the lifting direction of the driver's cabin.

To detect the translational movements, it is proposed that a roller body with the relevant signal transmitter is arranged rotatably on the driver's cab and can roll along a track on the main lifting frame, that another roller body is arranged rotatably on an element connected to the load-carrying device for joint movement relative to the additional lifting frame and can roll along a track on the additional lifting frame. The evaluation device can then use the signals from the signal transmitters to determine the height positions of the load-carrying device and the driver's cab relative to the main lifting frame or the height position of the load-carrying device relative to the additional lifting frame. Furthermore, a roller body with a signal transmitter can be provided in a corresponding manner to determine the side thrust of the load-carrying device on the driver's cab. Of course, in the case of a forklift truck with a height-adjustable driver's cab and a load-carrying device additionally arranged to be movable thereon, it is also possible in the sense of the present invention that

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For example, only the main lift, namely the height position of the driver's cab, is monitored with a roller body with a corresponding signal transmitter and some other measuring principle is used to record the position of the load-handling device relative to the driver's cab.

An embodiment of the invention is explained in more detail below with reference to the drawings.

Fig. 1 is a highly simplified side view of an industrial truck according to the invention,

Fig. 2 Pulse signals with phase shift, as emitted by angle sensors of the position measuring device, and

Fig. 3 is a simplified partial representation of a telescopic lifting frame to explain a preferred referencing process.

The forklift truck 1 according to Fig. 1 is a so-called three-way order picking forklift truck. The forklift truck 1 has a telescopically length-adjustable lifting frame 3 with a lower lifting frame part 5 that is stationary in relation to the chassis of the forklift truck 1 and an upper lifting frame part 7 that can be extended or retracted in the vertical direction relative to the lower lifting frame part 5 and on which a driver's cabin 9 is arranged so that its height is adjustable. At the front of the driver's cabin 9 there is a swivel-push device 11 that is arranged so that it can be moved laterally relative to the driver's cabin 9, i.e. perpendicular to the plane of the drawing in Fig. 1, and has an additional lifting frame (additional mast) 13 on which a load-carrying device (fork) 15 with its holder 16 is guided so that its height is adjustable relative to the driver's cabin 9. The additional mast 13 can be pivoted together with the load-carrying device 15 about an axis 17 by approximately 180°.

As a sensor of a position measuring device, a roller bearing 18 designed as an incremental angle sensor is arranged on the upper mast part 7, the rotating outer ring 19 of which serves as a roller body with a roller axis perpendicular to the lifting direction of the upper mast part 7, the roller body 19 resting with its circumference on a surface 21 of the lower mast part 5, which forms a track running in the lifting direction of the upper mast part 7, on which the roller body 19 rolls when the upper mast part 7 is telescopically displaced relative to the lower mast part 5. The roller bearing 18 is attached to the mast part 7 in such a way that the roller body 19 is spring-loaded towards its track 21 and thus always has contact with the track.

In Fig. 1, the upper mast part 7 is shown partially extended, while the cabin 9 is shown in its uppermost position relative to the upper mast part 7. The load-carrying device 15 is in its lowest position relative to the additional mast 13 and is pivoted sideways towards the viewer's side according to Fig. 1. The hydraulic drive means for the elements 7, 9, 11 and 15 are not shown.

When the roller body 19 rotates, the angle sensor 18 emits two pulse trains, phase-shifted by 90°, as electrical signals, as shown in Fig. 2. Each pulse interval corresponds to a specific change in the angle of rotation of the roller body 19. The phase-shifted electrical signals are fed to an evaluation circuit (not shown) which has an up/down counting circuit for counting the respective measurement signal pulses and which determines the direction of rotation by comparing the two measurement signals. When the upper mast part 7 is raised, the counting circuit counts the pulses of the respective measurement signal upwards, whereas when the upper mast part 7 is lowered and the direction of rotation of the roller body 19 is reversed, the pulses are counted downwards. The evaluation circuit determines the position of the upper mast part 7 from the respective counter reading.

part 7 relative to the lower mast part 5. The evaluation circuit can also determine the respective lifting speed from the pulses counted per unit of time, wherein the lifting speed values can be used as actual values for a lifting speed control, for example depending on the respective position of the upper mast part 7 relative to the lower mast part 5, for example in the sense that the lifting speed is reduced in a controlled manner when the upper mast part 7 approaches its maximum permissible lifting height position or another predetermined position.

In the embodiment according to Fig. 1, reference sensors are also provided for the position measuring device. In the example, these are proximity sensors 23 and 25, which are arranged on the lower mast part 5 and, when they are opposite a reference sensor element (marking) 27 attached to the upper mast part 7 at a predetermined location, emit a respective reference signal to the evaluation circuit. Using the reference signal, the evaluation circuit can check the position value derived from the angle sensor 18 and correct it if necessary. The reference value sensors can also be used to calibrate the measuring range of the position measuring device, with the upper mast part 7 being extended from its lowest basic position so that the reference value sensor element 27 is guided past the proximity sensors 23 and 25 one after the other. The evaluation circuit determines the number of pulses emitted by the angle sensor 18 per channel between the occurrence of the first reference signal from the proximity sensor 23 and the occurrence of the second reference signal from the proximity sensor 25 in order to normalize the respective number of pulses to the predetermined distance between the proximity sensors 23 and 25, so that a very precise correlation can be made between position changes of the upper lifting frame part 7 and angle of rotation changes of the roller body 19. The sensors 23 and 25 can be designed as inductive proximity sensors, light barrier switches or the like.

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and, if necessary, take on additional functions, for example as part of an end position detection circuit.

For referencing, within the scope of the invention, it would also be possible to use a single reference sensor, for example the reference sensor 23, which is arranged, for example, at a predetermined distance above the lowest possible position of the reference element 27, which the reference element 27 assumes when the upper lifting frame part 7 is completely retracted into its lowest basic position. Another possibility is to use a single reference transmitter, in which the relevant reference sensor and the reference transmitter element interact over a predetermined lifting section.

To explain a further method of referencing, Fig. 3 shows a lower mast part 5a and an upper mast part 7a, which can be moved telescopically relative thereto, of a length-adjustable mast of an industrial truck according to the invention.

The upper mast part 7a is shown in Fig. 3 in a position in which it is raised by a predetermined reference distance r compared to its lowest possible rest position. The sensor 23a at the level of the reference distance r changes its output signal when the mast part 7a is moved upwards beyond the reference distance r or when it returns to the reference distance range when moving downwards. In Fig. 3, the upper mast part 7a is shown in a snapshot in which it causes a signal change at the sensor 23a. From the signal state of the sensor 23a, it can be clearly deduced whether the mast part 7a is outside the reference distance range r and that it must be lowered in order to bring its lower end into the reference distance range r for referencing.

For example, the following referencing process can take place:

1. Starting from the completely lowered basic position of the mast part 7a, the mast part 7a is raised until a change in the signal state is detected at the sensor 23a. The change in the signal state indicates that the sensor 23a is functioning.

2. Starting from the position shown in Fig. 3, the mast part 7a is lowered over the entire reference distance r until it has reached its lowest basic position. While the mast part 7a is being lowered, the evaluation circuit checks the two phase-shifted electrical signals of the angle sensor 18a for the correct phase relationship in the event of lowering. The angle sensor signal is also evaluated in order to measure the reference distance r.

3. From the lowest basic position, the mast part 7a is raised again until the reference sensor 23a changes its output signal state.

The evaluation circuit checks the phase-shifted electrical signals of the angle sensor 18a for correct phase response in the event of lifting. The reference distance r is also measured.

If the mast part 7a is initially located outside the reference range r, referencing can be carried out in a corresponding manner, whereby the step mentioned above under point 1 can be omitted.

The following errors can be detected using the referencing process described above:

Defect of the reference sensor 23a,

Defect or faulty signal of the angle sensor 18a,
any elongation or stretching of the lifting chain normally used to extend the mast part 7a,
Error in the evaluation circuit or counting circuit.

Fig. 3 further shows the possibility that the angle sensor 18a is rotatably arranged on the fixed mast part and is set in rotation when the movable mast part 7a is moved upwards or downwards.

In the embodiment shown in Fig. 1, an angle sensor 18' corresponding to the angle sensor 18 is arranged on the driver's cab 9, the associated roller body 19' rolling on a track 21' running in the longitudinal direction of the upper mast part 7 when the driver's cab 9 is raised or lowered relative to the upper mast part 7. To determine the position of the driver's cab 9 relative to the upper mast part 7 or to the lower mast part 5, the evaluation circuit evaluates the corresponding pulse signals of the angle sensor 18' arranged on the driver's cab 9. Reference sensors of the type described above can also be provided with regard to determining the position of the driver's cab 9.

A further angle sensor 18" corresponding to the angle sensor 18 is arranged on an element 16 that is firmly connected to the load-handling device 15, wherein the associated roller body 19" rolls along a vertically extending path of the additional mast 13 when the load-handling device 15 is raised or lowered relative to the additional mast 13. The evaluation circuit also evaluates the pulse signals of the latter angle sensor 18" and can determine the lifting height of the load-handling device 15 relative to the driver's cabin 9 and relative to the mast parts 7 and 5 from the respective angle sensor information.

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Of course, an angle sensor corresponding to the angle sensor 18 can also be provided on the driver's cab 9 to detect the sideshift of the load-handling device 15.

The invention enables precise and simple position monitoring of the load-handling device or the elements that can be moved with the load-handling device (in the exemplary embodiment, elements 7, 9, 11 and 16) relative to one another and relative to a fixed reference point of the industrial truck. The position values and position change speed values provided by the position measuring device can be used, for example, as the respective actual comparison values for a drive control that controls the movement sequences of these elements.

Claims (12)

1. Industrial truck with a load-carrying means ( 15 ), a device ( 3 , 9 , 11 ) for moving the load-carrying means ( 15 ) on the industrial truck ( 1 ), which has at least one element (7 or 9 or 16 ) that can be moved along a substantially straight guide (on 5 or 7 or 9 or 13) together with the load-carrying means ( 15 ), and with a position measuring device for monitoring the position of the element ( 7 or 9 or 16) that can be moved with the load-carrying means ( 15 ) or of the load-carrying means ( 15 ) relative to the guide (on 5 or 7 or 9 or 13), wherein the position measuring device comprises at least one roller body ( 19 , 19 ', 19") that, when the element ( 7 or 9 or 16 ) that is connected to the load-carrying means (15) moves, is moved in a direction parallel to the guide (on 5 or 7 or 9 or 13 ). 15 ) movable element ( 7 or 9 or 16 ) and which cooperates with a signal generator which emits an electrical signal in accordance with the rotary movement of the roller body to an evaluation circuit which evaluates the signal to determine the position of the element ( 7 or 9 or 16 ) movable with the load-carrying means ( 15 ) or of the load carrier ( 15 ) relative to the guide (on 5 or 7 or 9 or 13), characterized in that the roller body ( 19 , 19 ', 19 ") is part of a roller bearing and is rotatably arranged on the element ( 7 or 9 or 16 ) movable with the load-carrying means ( 15 ) and is supported with its circumference on a track ( 21 , 21 ', 21 ") in such a way that when the element ( 7 or 9 or 16 ) movable with the load-carrying means ( 15 ) moves along the guide (on 5 or 7 or 9 or 13), it is forced to roll along the track ( 21 , 21 ', 21 "). 2. Industrial truck with a load-carrying means, a device for moving the load-carrying means on the industrial truck, which has at least one element ( 7a ) that can be moved along a substantially straight guide (on 5a) together with the load-carrying means, and with a position measuring device ( 18a ) for monitoring the position of the element ( 7a ) that can be moved with the load-carrying means or of the load-carrying means relative to the guide (on 5a), wherein the position measuring device comprises at least one roller body that is rotatably arranged on an element that is stationary with respect to the guide and rests with its circumference on the element that can be moved with the load-carrying means in such a way that it is forcibly rotated when the element ( 7a ) that can be moved with the load-carrying means moves, and wherein the roller body interacts with a signal transmitter that emits an electrical signal in accordance with the rotary movement of the roller body to an evaluation circuit that uses the signal to determine the position of the element that can be moved with the load-carrying means. Element ( 7a ) or the load carrier relative to the guide (5a), characterized in that the roller body is part of a rolling bearing. 3. Industrial truck according to claim 1, characterized in that the rolling bearing has an integrated angle sensor. 4. Industrial truck according to claim 1, 2 or 3, characterized in that the signal generator is a digital angle sensor ( 18 , 18 ', 18 "). 5. Industrial truck according to claim 4, characterized in that the digital angle sensor ( 18 , 18 ', 18 ") is designed as an incremental angle sensor and that the evaluation circuit contains a counting circuit for counting the pulses emitted by the angle sensor in accordance with the rotation of the roller body ( 19 , 19 ', 19 "). 6. Industrial truck according to claim 5, characterized in that the incremental angle sensor ( 18 , 18 ', 18 ") emits two phase-shifted pulse signals (A, B) when the roller body ( 19 , 19 ', 19 ") rotates and that the evaluation circuit is set up to process the pulse signals to determine the direction of rotation of the roller body ( 19 , 19 ', 19 ") and to count up or down the pulses of at least one of the pulse signals depending on the direction of rotation. 7. Industrial truck according to one of the preceding claims, characterized in that the element ( 7 or 9 or 16 ) which can be moved with the load-carrying means ( 15 ) is guided on a lifting frame ( 3 ) in a height-adjustable manner and that the position measuring device is designed to determine the lifting height of the load-carrying means ( 15 ). 8. Industrial truck according to one of claims 1-6, characterized in that it has a lifting frame ( 3 ) with a driver's cabin ( 9 ) guided thereon in a height-adjustable manner and carrying the load-carrying means ( 15 ), and that the roller body ( 19 ') is rotatably arranged on the driver's cabin ( 9 ) and can roll on the lifting frame ( 3 ). 9. Industrial truck according to one of the preceding claims, characterized in that it has a length-adjustable lifting frame ( 3 ) for the load-carrying means ( 15 ) with a lower lifting frame part ( 5 ) and an upper lifting frame part ( 7 ) which can be extended telescopically relative thereto, and that the roller body ( 19 ) is rotatably arranged on the upper lifting frame part ( 7 ) and can roll on the lower lifting frame part ( 5 ). 10. Industrial truck according to one of the preceding claims, characterized in that it has a length-adjustable lifting frame ( 3 ) for the load-carrying means ( 15 ) with a lower lifting frame part ( 5 ) and an upper lifting frame part ( 7 ) which can be extended telescopically relative thereto, and that the roller body ( 19 ) is rotatably arranged on the lower lifting frame part ( 7 ) and can roll on the upper lifting frame part ( 5 ). 11. Industrial truck according to one of the preceding claims, characterized in that at least one reference transmitter ( 23 , 25 ) is provided which emits a reference signal to the evaluation circuit in a predetermined position of the element ( 7 ) movable with the load-carrying means ( 15 ), and in that the evaluation circuit compares the position measurement value of the position measuring device present upon receipt of the reference signal with a position target value and, depending on this comparison, calibrates the position measuring device if necessary. 12. Industrial truck with a load-carrying means ( 15 ), a device ( 3 , 9 , 11 ) for moving the load-carrying means ( 15 ) on the industrial truck ( 1 ), which has at least one element (7 or 9 or 16) that can be moved along a substantially straight guide (on 5 or 7 or 9 or 13) together with the load-carrying means ( 15 ), and with a position measuring device for monitoring the position of the element ( 7 or 9 or 16 ) that can be moved with the load-carrying means ( 15 ) or of the load-carrying means ( 15 ) relative to the guide (on 5 or 7 or 9 or 13), wherein the position measuring device comprises at least one roller body ( 19 , 19 ', 19 "), which - either a) is arranged rotatably on the element ( 7 or 9 or 16 ) which is movable with the load-carrying means ( 15 ) and rests with its circumference on a track ( 21 , 21 ', 21") running along the guide (on 5 or 7 or 9 or 13) in such a way that it is forced to roll along the track (21, 21', 21 " ) when the element ( 7 or 9 or 16 ) which is movable with the load-carrying means ( 15 ) moves along the guide (on 5 or 7 or 9 or 13), - or b) is arranged so as to be rotatable on an element which is stationary with respect to the guide and rests with its circumference on the element which is movable with the load-carrying means in such a way that it is forced to rotate when the element which is movable with the load-carrying means moves, and that the roller body ( 19 , 19 ', 19 ") cooperates with a signal transmitter which emits an electrical signal in accordance with the rotational movement of the roller body to an evaluation circuit which evaluates the signal to determine the position of the element ( 7 or 9 or 16 ) movable with the load-carrying means ( 15 ) or of the load carrier ( 15 ) relative to the guide (on 5 or on 7 or on 9 or on 13).