Method for measuring the lifting height of an industrial truck

Abstract

Method for detecting the lifting height (h) in an industrial truck (2) having a hydraulic lifting device that can lift a load-handling device (9) by means of a pressurized hydraulic fluid and is supplied with hydraulic fluid by a hydraulic pump (4), and having a first lifting height measuring system (11) that can detect an absolute lifting height (h) and / or a relative lifting movement, as well as having a second lifting height measuring system for checking the function of the first lifting height measuring system (11), wherein a delivery rate of the hydraulic pump (4) is detected for the second lifting height measuring system and, during a lifting of the load-handling device (9), a correlation is determined between the change in the lifting height (h), which is detected by the first lifting height measuring system (11), and a change in the lifting height determined by the delivery rate, characterized in that the correlation is only checked for time periods,in which no other hydraulic consumers are operated, in particular not a hydraulic steering system.

Description

[0001] The invention relates to a method for detecting the lifting height. In particular, the invention relates to a method for detecting the lifting height in an industrial truck having a hydraulic lifting device that can lift a load-handling device by means of a pressurized hydraulic fluid and is supplied with hydraulic fluid by a hydraulic pump, and having a first lifting height measuring system that can detect an absolute lifting height and / or a relative lifting movement, as well as a second lifting height measuring system for checking the function of the first lifting height measuring system, wherein a delivery rate of the hydraulic pump is detected for the second lifting height measuring system and, during a lifting of the load-handling device, a correlation is determined between the change in lifting height detected by the first lifting height measuring system and a change in lifting height determined by the delivery rate.

[0002] For industrial trucks with a lifting mast on which a load-handling device, most commonly a fork, is mounted in a height-adjustable manner to transport a load, various lifting height measuring systems are known. These can be used to determine the height at which the load-handling device is located. Sensors and measuring systems are used for this purpose that can continuously measure the lifting height. Common sensors for this purpose are those driven by toothed belts, racks, cables, or retracting cable drums when the lifting mast is extended and retracted. Other common sensors are optical or wireless sensor systems that measure, for example, the path of a laser beam between the sensor and the target on the raised part of the lifting mast.

[0003] The mechanical systems described above use both incremental and absolute measurement methods, in which an absolute stroke height or a relative change in the stroke height is recorded. Non-mechanical systems ultimately perform time-of-flight measurements, thus recording absolute values ​​of the distance traveled by the light beam, or alternatively, by sound waves or similar.

[0004] Since the drive system, braking devices, and possibly safety devices of the industrial truck are often controlled depending on the lift height, there is a need for reliable measurement of the lift height and monitoring of the existing lift height measurement systems for errors or failures. This is achieved using state-of-the-art methods through the use of redundant systems.

[0005] To create such a redundant system, for example, two identical sensors are installed to measure the lifting height. These sensors read their signals separately, and their signal values ​​can be compared to verify their validity. It is also known to perform verification measurements at specific lifting heights using simpler, cost-effective, additional reference sensors, which can, for example, be simple switches that trigger a signal when the corresponding lifting height is passed.

[0006] The disadvantage of this state of the art technology is that it requires additional installation space and incurs additional costs. Furthermore, there is the problem that identical sensors installed very close to one another can cause common-cause errors. These are particularly difficult to detect, and where high safety requirements exist, different measuring methods and installation positions are often deliberately requested. The latter, in turn, requires even greater effort. If monitoring by an additional sensor at a specific lifting height is only carried out point-wise at one or more locations, gaps in monitoring arise, and under unfavorable operating conditions, longer periods can arise during which the lifting height sensor or lifting height measuring system cannot be monitored.

[0007] It is also known to perform monitoring using a plausibility check instead of redundancy. For example, in analog sensor systems, a zero value of the sensor's current or voltage output can be compared with an offset. This can be used to detect, for example, a break in a mechanical measuring device such as a cable pull or cable drum. Potential short circuits can also be detected. It is also possible to compare the temporal change of the signal, possibly even higher derivatives, with maximum possible values ​​and to check signal values ​​for exceeding possible maximum or minimum values ​​or, more generally, system limits.

[0008] A disadvantage of this state of the art, however, is that monitoring by means of a plausibility check has only limited significance as long as the sensor signal values ​​and any time derivatives remain within the system specifications.

[0009] From US 5 526 673 A a method with the features of the preamble of patent claim 1 is known.

[0010] US 2015 / 0 100 210 A1 discloses a reach truck in which the lowering speed of the load forks can be controlled depending on the lifting height, which is measured with a lifting height sensor.

[0011] US 4 942 529 A discloses an industrial truck control system

[0012] The present invention is therefore based on the object of providing a method for measuring the lifting height of an industrial truck which avoids the aforementioned disadvantages and with which the function of a lifting height measuring system can be monitored.

[0013] This object is achieved by a method having the features of independent patent claim 1. Advantageous developments of the invention are specified in the subclaims.

[0014] The object is achieved according to the invention in that, in a method for detecting the lifting height in an industrial truck having a hydraulic lifting device which can lift a load-handling device by means of a pressurised hydraulic fluid and is supplied with hydraulic fluid by a hydraulic pump, and having a first lifting height measuring system which can detect an absolute lifting height and / or a relative lifting movement, as well as having a second lifting height measuring system for checking the function of the first lifting height measuring system, a delivery rate of the hydraulic pump is detected for the second lifting height measuring system and, during a lifting of the load-handling device, a correlation is determined between the change in the lifting height which is detected by the first lifting height measuring system and a change in the lifting height determined by the delivery rate, wherein the correlation is only checked for periods in which no other hydraulic consumers are actuated,especially not a hydraulic steering system.

[0015] The signal from the first lift height measuring system, a non-redundant system that can, for example, consist of the previously described optical measuring systems, an ultrasound system, or systems that mechanically record the lift height, such as a cable pulley with a potentiometer, is compared with a change in lift height determined by the delivery rate. The delivery rate of the hydraulic pump is integrated, thus resulting in a relative change in lift height. The hydraulic pump can, for example, be an adjustable pump, where the pump setting in conjunction with the speed determines the delivery rate, or a non-adjustable pump, where the delivery rate is directly dependent on the speed, so that only the speed of the hydraulic pump needs to be recorded. This time-integrated flow rate signal is correlated with the signal from the first lift height measuring system over a measuring period.The measurement period can be selected based on boundary conditions such that, firstly, only the load-handling device is lifted and the lifting movement has stabilized, i.e., the vibrations typically occurring at the start of the lifting movement have subsided. Further boundary conditions can be selected such that no other hydraulic consumers are activated, in particular, no hydraulic steering is activated, and finally, the lifting movement has not been moved against an upper stop. A temporal correlation is recorded during the lifting movement. This correlation is independent of a constant displacement and also of an extension.This means that as long as a relative lift can be detected in parallel with the first lift height measurement system, which can be assigned with a proportionality factor, namely the extension, the check of the first lift height measurement system, based on the correlation, shows that the first lift height measurement system is operating correctly. If the boundary conditions are not met or cannot be fulfilled, the process can be reset and started again. Advantageously, a redundant check of the signal from a lift height measurement system can be implemented very cost-effectively using existing sensor data. For example, a single speed of the hydraulic pump may already be sufficient.

[0016] The flow rate of the hydraulic pump can be measured via the speed of the hydraulic pump.

[0017] Determining the flow rate of the hydraulic pump is particularly easy via the speed in the case of, for example, a gear pump or other hydraulic pumps with a flow rate proportional to the speed.

[0018] In one embodiment of the method, a lifting movement is detected by the actuation of a control element, in particular an opening of an electromagnetically actuated hydraulic valve.

[0019] This allows the method to start recording the correlation. Another example is recording the deflection of a lifting lever, such as a joystick. It is also possible to detect movement from the time derivative of the lifting height of the signal from the first lifting height measurement system.

[0020] In the invention, the correlation is only checked for periods in which no other hydraulic consumers are operated, in particular not a hydraulic steering system.

[0021] The volume flow of hydraulic fluid required for additional consumers is often very low. In particular, in a hydraulic steering system, the hydraulic flow required by the hydraulic pump during typical operation is significantly lower than the hydraulic flow required for the lifting device, e.g. a lifting cylinder. It is therefore possible to design a correlation threshold so that the hydraulic flow diverted by such additional consumers or a hydraulic steering system only occurs as an error within the tolerance threshold. This results in a greater temporal coverage of the monitoring of the first lifting height measuring system. If greater accuracy is desired, however, time periods in which no other hydraulic consumers are operated can be selected for the correlation check. This can be determined, for example, by monitoring electromagnetically operated hydraulic valves.

[0022] For example, in the case of a hydraulic steering system, a steering angle signal and, above all, a temporal change in a steering angle signal can also be used for this purpose.

[0023] A tolerance value may be provided for the correlation, in particular a correlation threshold, preferably one of 95%.

[0024] Advantageously, in the event of a missing correlation, an error is detected and an error is counted in a memory, whereby the error in the memory is deleted again after an expiration time and only when a specified number of stored errors have been reached is the first lifting height measuring system assessed as faulty.

[0025] This provides greater stability against incorrect evaluations, as an error is forgotten after a certain period of time. Only when a specified number of errors are recorded in the memory of such an error counter is the first lift height measurement system reported as defective.

[0026] The industrial truck may have hydraulic steering and at low lifting speeds and simultaneous steering, the correlation cannot be recorded.

[0027] It is advantageous to specifically avoid monitoring at low lifting speeds, resulting in low hydraulic flow into the hydraulic lifting device, while simultaneously operating a hydraulic steering system. In such a case, the diverted hydraulic flow into the hydraulic steering system would be relatively significant. The same applies to other hydraulic consumers.

[0028] The correlation is advantageously recorded when the lifting movement is constant, in particular when it does not exhibit any oscillation.

[0029] When starting a lifting movement, it is advisable to wait until it has settled down.

[0030] In one embodiment of the method, the correlation is not evaluated if the lifting device has moved against an upper stop.

[0031] The first stroke height measuring system can be an optical measuring system, an ultrasonic measuring system and / or a mechanical length measuring system.

[0032] Tests have shown that the use or differentiation of a target speed from an actual speed of the hydraulic pump has little influence on the method according to the invention.

[0033] Further advantages and details of the invention are explained in more detail with reference to the embodiment shown in the schematic figures. Fig. 1 schematically shows a forklift truck as an example of an industrial truck in which the method according to the invention is used, and Fig. 2 schematically shows the method according to the invention in a diagram.

[0034] The Fig. 1 schematically shows a forklift truck 1 as an example of an industrial truck 2 in which the method according to the invention is used. The forklift truck 1 has a hydraulic lifting device 3 comprising a hydraulic pump 4, the hydraulic fluid of which is fed to a lifting cylinder via hydraulic valves 5, controlled in the present example by a joystick 6. The hydraulic pump 4 draws the hydraulic fluid from a hydraulic tank 7. The hydraulic lifting device 3 raises a load-handling device 9 on a lifting mast 10. The lifting mast 10 has several lifting stages in the form of successively extending mast sections, and a lifting stage sensor 8 detects the extension of the next stage. A first lifting height measuring system 11 in the form of an optical distance sensor 12, which is directed towards a target reflector 13, detects a lifting height h of the load-handling device 9.

[0035] The Fig.2 shows a schematic diagram of the method according to the invention. In a check 14 to determine whether only a lifting movement is active, input data from a hydraulic steering actuator 15, a signal 16 from a hydraulic valve for lifting or lowering, a signal 17 for a hydraulic valve of a tilting device, and a signal 18 from a hydraulic valve for additional hydraulic units are combined. The signal 19 from the first lifting height measuring system 11 is used for a check 20 to determine whether the lifting height has reached the upper stop. The results of check 14 and check 20 are combined in the AND operator 21 and fed to an integrator 22 for a hydraulic pump speed 23 and to a correlator 24 with a trigger. The signal 19 from the first lifting height measuring system 11 is also fed to a differentiator 25, which triggers the correlator 24 when the lifting height changes.The signal from correlator 24 undergoes a comparison 26 with a correlation threshold, resulting in a warning condition 27 for a possible sensor error. This is recorded in an error counter 28, with the counted errors being deleted after a certain expiration time. When a minimum number of counted errors is recorded in the error counter 28, a warning 29 is issued indicating a sensor error.

Claims

[1] Method for detecting the lifting height (h) in an industrial truck (2) having a hydraulic lifting device which can lift a load-handling device (9) by means of a pressurised hydraulic fluid and is supplied with hydraulic fluid by a hydraulic pump (4), and having a first lifting height measuring system (11) which can detect an absolute lifting height (h) and / or a relative lifting movement, and having a second lifting height measuring system for checking the function of the first lifting height measuring system (11), wherein a delivery rate of the hydraulic pump (4) is detected for the second lifting height measuring system and, during a lifting of the load-handling device (9), a correlation is determined between the change in the lifting height (h) detected by the first lifting height measuring system (11) and a change in the lifting height determined by the delivery rate, characterized bythat the correlation is only checked for periods in which no other hydraulic consumers are operated, in particular not a hydraulic steering system. [2] Method according to claim 1, characterized by that the delivery rate of the hydraulic pump (4) is detected via the speed (23) of the hydraulic pump (4). [3] Method according to claim 1 or 2, characterized by that a lifting movement is detected by the actuation of a control element, in particular an opening of an electromagnetically actuated hydraulic valve (5). [4] Method according to one of claims 1 to 3, characterized by that a tolerance value is provided for the correlation, in particular a correlation threshold, preferably one of 95%. [5] Method according to one of claims 1 to 4, characterized bythat in the event of a missing correlation, an error is detected and an error is counted in a memory, wherein the error in the memory is deleted again after an expiration time and only from a predetermined number of stored errors is the first lifting height measuring system (11) assessed as faulty. [6] Method according to one of claims 1 to 5, characterized by that the industrial truck (2) has hydraulic steering and that at low lifting speeds and simultaneous steering, the correlation is not recorded. [7] Method according to one of claims 1 to 6, characterized by that the correlation is detected when the lifting movement is constant, in particular when it does not exhibit any oscillation. [8] Method according to one of claims 1 to 7, characterized by that an evaluation of the correlation does not take place if the lifting device (3) has moved against an upper stop. [9] Method according to one of claims 1 to 8, characterized by that the first lifting height measuring system (11) is an optical measuring system, ultrasonic measuring system and / or mechanical length recording measuring system.

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