SE2250039A1 - Material handling vehicle comprising diagnostic coverage of pressure in lift system - Google Patents

Abstract

The invention relates to a material handling vehicle (1) comprising: a control unit (3) n hydraulic system (31), comprising a first and a second hydraulic cylinder (7, 9), and a lift device (11), wherein the hydraulic system (31) is connected to the lift device (11) for lifting a load. The vehicle (1) further comprises a mast (13) to which the lift device (11) is connected movable along the mast (11) by means of at least a first and second hydraulic cylinder (7, 9).The material handling vehicle (1) further comprises a first sensor (15), arranged to determine the pressure of the first hydraulic cylinder (7), and a second sensor (23), arranged to determine the pressure of the second hydraulic cylinder (9), wherein the control unit (3) is arranged to set each hydraulic cylinder (7, 9) in a resting state, when not in active use, a pressurized state, when pressurized but not lifting, and a working state, when pressurized during lifting with said each cylinder (7, 9). The material handling vehicle is arranged such that when one of the first and second hydraulic cylinder is in the working state, the control unit (3) shifts the state of the hydraulic cylinder set in resting state to a pressurized state, wherein the first and second sensors (15, 23) measures the pressures of the cylinder in the working state and the cylinder in the pressurized state, the control unit (3) is then arranged to receive values of pressure from the first and second sensor (15, 23), and compare the difference thereof to a maximum predetermined safety value. The invention further relates to a method to perform a safety control in such a material handling vehicle (1), a computer program (49), and a computer-readable medium (47), comprising the computer program.

Description

1 MATERIAL HANDLING VEHICLE COMPRISING DIAGNOSTIC COVERAGE OF PRESSURE IN LIFT SYSTEM TECHNICAL FIELD The present disclosure relates to a material handling vehicle and to a method for handling such a vehicle according to the appended claims. The present disclosure also relates to a computer program comprising instruction to perform said method, and a computer-readable medium, comprising such a computer program.

BACKGROUND ART Generally in material handling situations, in particular order picking in a warehouse, floor conveyors, forklifts, tiller trucks etc., are used. There are numerous advantages with this type of vehicles, they can be arranged to lift heavy goods, they can be easily operated by a trained operator, they are in general not demanding a considerable investment and they are often rugged and have a considerable life expectancy. lt is possible to adapt these vehicles by changing load carriers and also by applying a mast such that goods can be fetched from considerably heights. Lifting operations performed by such material handling vehicles need to be safe to use and the hydraulic systems operating the lifting devices need to be both reliable and secure. Providing safety measures to such vehicles may not always be trivial in ways of construction thereof, as space is valuable in a storage facility. Thus, safety measures taken must also take space and volume of devices in consideration SUMMARY OF THE INVENTION As space is such an important consideration in a storage facility, material handling devices must often be made as slim and volume efficient as possible. lt is therefore desirable to use as few and/or bulky component as possible. Furthermore, if new safety regulations are introduced into a safety standard, it may not always be simple to just swap out components to fulfill such a new standard if the newer components are larger than the old ones, and new components also introduces further costs, which of course is not desirable.

For material handling vehicles such as forklifts and reach trucks, certain types of operations require safety redundancy when used. For example, when a forklift/reach truck lifts up the forks, the hydraulic cylinder performing the work is pressurized during such a lifting operation. A pressure sensor arranged to measure the pressure of the hydraulic cylinder provide the pressure thereof, which assists in determining the state of said cylinder with regards to its current load etc.. However, according to a 2 safety standard EN 1175, any such material handling vehicle is required to live up to a certain performance level, wherein in this case the pressure sensor signal is included in a safety function with a performance level that require a safety check of signal input to fulfil said performance level requirements.

To install additional sensors to each hydraulic cylinder to perform such a safety check, or to replace existing devices with other devices in which the safety check is included, induces additional costs and may be cumbersome with regard to such an operation, which is not desirable.

Despite prior art there is a need to develop an improved material handling vehicle that provides an improved safety by means of redundancy in pressure measurements. There is also a need to develop such a material handling vehicle without the need to change the hydraulic cylinder setup thereof. Further, there is also a need to develop a method for measuring pressures in hydraulic cylinders of such material handling vehicles.

An object of the invention is thus to provide an improved material handling vehicle that provides an improved safety by means of redundancy in pressure measurements. Another object is respectively to provide such a material handling vehicle without the need to change the hydraulic cylinder setup thereof. An even further object is to provide a method for measuring pressures in hydraulic cylinders of such material handling vehicles.

According to a first aspect, a material handling vehicle is provided. The Material handling vehicle comprises: a control unit, an electric drive engine; a hydraulic system, comprising an hydraulic pump powered by an electric pump engine, for powering a comprised hydraulic pump; a first and a second hydraulic cylinder, and a lift device. The hydraulic system is connected to the lift device for lifting a load. The vehicle further comprises a mast to which the lifting device is connected, movable along the mast by means of at least a first and second hydraulic cylinder. The first cylinder is arranged to lift the lift device a first predetermined distance, wherein the second cylinder is arranged to lift the lift device a second predetermined distance. Wherein the first and second predetermined distances are arranged in consecutive manner, such that the start of lifting the second distance is arranged to be initiated when the first distance reaches its predetermined endpoint.

According to the first aspect a first sensor is arranged to determine the pressure of the first hydraulic cylinder, and a second sensor is arranged to determine the pressure of the second hydraulic cylinder. The control unit is arranged to set each hydraulic cylinder in a resting state, when not in active use; a pressurized state, when pressurized but not lifting; and a working state, when pressurized during lifting with said each cylinder. Wherein the material handling vehicle is arranged such that when one of the first and second hydraulic cylinder is in the working state, the control unit shifts the state of the 3 hydraulic cylinder in a resting state to a pressurized state, wherein the first and second sensors measures the pressures of the hydraulic cylinder in the working state and the hydraulic cylinder in the pressurized state. The control unit is then arranged to receive values of pressure from the first and second sensor, and compare the difference thereof to a maximum predetermined safety value.

This has the advantage that the control unit can measure the hydraulic cylinder doing a lifting operation in an additional measurement without the need to install an additional sensor. The sensor arranged to the cylinder doing the lifting can measure its cylinder in normal manner, wherein a first pressure measurement of said cylinder is received. Additionally, when setting another resting cylinder to a pressurized state, its sensor can measure said another cylinder simultaneously to the measurement of the lifting cylinder. As the hydraulic system during such a simultaneous measurement couples the two separate cylinders to be in fluid communication with the same pump at the same time, and that all cylinders are working to lift the same applied load, the measurement of the another cylinder can indirectly act as a redundancy measurement of the lifting cylinder. The control unit thus receives an additional input signal to utilize for determining the state of the lifting cylinder during operation.

As the value for the another cylinder in a pressurized state but not actively in extension can be measured and stored in a memory of the control unit beforehand, the control unit can subtract said value from the combined input from the two cylinders. Thus, control unit may provide a redundancy in measurements of a hydraulic cylinder, wherein addition of extra physical components are unnecessary. ln other words, a faulty measurement using only one sensor for the cylinder performing work can be dismissed, as the simultaneous measuring directly control the accuracy and correctness of said one sensor. This in turn hold the physical volume of the vehicle to a minimum, and lower manufacturing costs as well.

According to an aspect, each hydraulic cylinder comprises a control valve, wherein the control unit is arranged to control the opening of the control valve in a controlled manner so as to set said hydraulic cylinder in its pressurized state.

This has the advantage that the measurements wanted to be performed are all performed fully automatically in a fast and efficient manner by means of the control unit. A fast measurement is thus both safer and more user friendly as the operator of the vehicle does not need to manually perform the measurement operation.

According to an aspect, the hydraulic system may further comprise a safety sensor, arranged to measure the pressure of the hydraulic system. When all hydraulic cylinders are in their working state, 4 the control unit is arranged to measure the pressures of said cylinders, measure the pressure of the safety sensor, and compare all measured pressures combined to a second maximum predetermined safety value.

This has the advantage that the safety control may be performed even if all cylinders are busy performing work.

According to an aspect, the control unit may limit or prevent operation of the material handling vehicle if the maximal predetermined safety value is exceeded, wherein the limitation comprises connecting the control unit to the electric drive engine for restricting the operable speed of the material handling vehicle.

This has the advantage that a safety feature is introduced for control of the vehicle, which safety feature hinders the vehicle to be in operation if a faulty cylinder is detected. Thus increasing the safety and reliability of the material handling vehicle.

According to a second aspect, a method for safety control in a material handling vehicle, comprising at least two hydraulic cylinders having one pressure sensor each, is provided. The method utilizes a material handling vehicle according to the present disclosure, and comprise the steps of: a) measuring the pressure in a first hydraulic cylinder in operation, b) opening of a second hydraulic cylinder, actualizing the opening so that the second cylinder does not extend, c) comparing the first pressure with the second pressure, d) determining if a predetermined maximal deviation value between the first hydraulic cylinder and the second hydraulic cylinder is surpassed; and, if the predetermined maximal deviation value is surpassed; e) taking safety measures for restricting or stopping the operation of the material handling vehicle.

This has the advantage that a method is provided, which method exhibits the same advantages as for the material handling vehicles according to the present disclosure.

According to a third aspect, a computer program is provided. The computer program comprises computer-readable instructions. The instructions are executable by at least one processor (of a control system) of a control unit of a material handling vehicle according to the disclosure. When the instructions are executed, the control unit causes the material handling vehicle to perform a safety control of at least one pressure sensor of said material handling vehicle, by means of performing the steps of the method as defined by the disclosure herein.

This has the advantage that the method according to the disclosure may be performed with any material handling vehicle comprising the needed components. Thus the method is performed by the software without the need to modify the components of the vehicle.

According to a fourth aspect, a computer-readable medium, comprising the computer program according to the disclosure is provided.

This has the advantage that the software needed to performed the method may be transferred to any suitable material handling vehicle in which the method is desired to be used.

BRIEF DESCRIPTION OF THE DRAWINGS Below is a description of, as examples, embodiments with reference to the enclosed drawings, in which: Fig. 1 shows a material handling vehicle in a side view according to an embodiment, Fig. 2 shows a front view of a mast of a material handling vehicle according to an embodiment, Fig. 3 shows a top down view of a mast of a material handling vehicle according to an embodiment, Fig. 4 shows a schematic view of a hydraulic system of a material handling vehicle according to an embodiment, Fig. 5 shows a schematic representation of a method safety control of at least two pressure sensors in a material handling vehicle, and Fig. 6 shown a schematic representation of a computer-readable medium, comprising the computer program for a material handling vehicle.

DETAILED DESCRIPTION The detailed description with reference to the embodiments depicted are to be viewed as exemplary embodiments comprising a combination of certain features, which features have been described in detail above. lt is thus to be understood that additional embodiments may be achieved by combining other features into embodiments not depicted herein. The figures are to be viewed as examples and not mutually exclusive combinations. lt should also be noted that all figures shown and described are schematically represented, wherein generic parts of machinery is not depicted for the sake of simplicity.

Fig. 1 shows a material handling vehicle 1 in a side view according to an embodiment. The material handling vehicle may be for example a forklift, a reach truck, or a similar material handling vehicle, wherein the inventive concept of the disclosure may be utilized for any such vehicle comprising and using at least two hydraulic cylinder in a manner conforming to the concept as will be described herein. 6 The material handling vehicle 1 comprises a control unit 3, an electric drive engine 5; a hydraulic system (shown in Fig. 4), comprising a hydraulic pump 6 powered by an electric pump engine, for powering a comprised hydraulic pump. The vehicle 1 further comprises at least a first and a second hydraulic cylinder 7, 9, and a lift device 11, wherein the hydraulic system is connected to the lift device for lifting a load. The vehicle further comprise a mast 13, shown in a front view in Fig.2 and a top down view in Fig. 3, to which the lift device 11 is connected movable along the mast 13 by means of at least the first and second hydraulic cylinder 7, 9.

The first cylinder 7 is arranged to lift the lift device 11 a first predetermined distance, which distance relates to the lift device being lifted up its maximum lifting height, performed by the first cylinder 7 alone, along the mast 13 that is not extended during said lifting operation. The pressure provided to the hydraulic cylinder is normally measured along its path from the hydraulic pump 6 to said cylinder 7, 9. Thus, a first sensor 15 is arranged to the first cylinder 7 to measure the pressure of said cylinder 7. Said first sensor 15 is seen and described in more detail with reference to Fig. 4. lf the first cylinder 7 would experience a mechanical failure, such as not extending; or leaking, such a first sensor 15 would thus show a non-expected pressure, wherein a safety system comprised in the vehicle would signal that something is wrong with the first cylinder 7, when operated during said mechanical failure, wherein an operator can act accordingly. For safety reasons such an important feature of the vehicle, lifting a potentially very heavy load that could fall down and cause damage and injuries, may demand a secondary reading of said pressure when the cylinder is in use. However providing a secondary sensor to give another reading of the pressure, or providing another sensor being able to perform a plurality of measurements simultaneously is not desired, as it would be both costly and/or making the hydraulic system bulkier. As the disclosure herein will show, there are however alternatives to re-designing the hydraulic system, while still providing multiple safety measurements of the first hydraulic cylinder.

The second cylinder 9 is arranged to lift the lift device 11 a second predetermined distance, which distance is related to the mast being extended such that a first mast section 17 lifts at least one remaining mast section upwards. The first and second predetermined distances may thus be arranged in consecutive manner, such that the start of lifting the second distance is arranged to be initiated when the first distance reaches its predetermined endpoint. lt is however possible to move and extend the mast the second predetermined distance without first having moved the lifting device the first predetermined distance if desirable.

The simplest form of such a material handling vehicle 1 thus comprise a mast having a first mast section 17 and a second mast section 19, wherein the second hydraulic cylinder 9 is arranged to move the second mast section vertically, along an extending direction of said two sections 17, 19. However, 7 the second hydraulic cylinder may also be utilized to extend a plurality of sections of the mast. As seen in Fig. 2 and Fig. 3, the mast therein comprises an additional third mast section 21 compared to the above described simplest form. Therein it should thus be perceived that the second predetermined distance is achieved when all three mast sections 17, 19, 21 are extended from each other to an maximal extension of the mast 13. The mast sections 17, 19, 21 may extend sequentially or simultaneously from each other. lt should however be mentioned that a mast 13 may comprise an ever larger amount of mast sections, and that the second predetermined distance may be divided between a plurality of hydraulic cylinders, performing work simultaneously or sequentially. lf the vehicle comprises a larger amount of hydraulic cylinders, the inventive concept as described herein may of course be used by means of the same principal but utilizing other combinations of cylinders.

Presuming the concept in a simple form, the material handling vehicle 1 comprises a first sensor 15, arranged to determine the pressure of the first hydraulic cylinder 7; and a second sensor 23, arranged to determine the pressure of the second hydraulic cylinder 9. The control unit 3 is arranged to control each hydraulic cylinder 7, 9 and to set them in a resting state, when not in active use; a pressurized state, when pressurized but not lifting; and a working state, when pressurized during lifting with said each cylinder. When a lift of a load then is initiated with either of the two cylinders, and said cylinder is put in a working state; the control unit is arranged to set another (in this example the other) hydraulic cylinder to a pressurized state. The first and second sensors then measure the pressures of the cylinder in the working state and the cylinder in the pressurized state. The control unit is then arranged to receive values of pressures from the first and second sensor 15, 23, and compare the difference thereof to a maximum predetermined safety value.

Each hydraulic cylinder 7, 9 may comprise a control valve 25, 26, wherein the control unit 3 is arranged to control the opening of the control valve 25, 26 in a controlled manner so as to set said hydraulic cylinder 7, 9 in its pressurized state. As the pressurized state requires only a pressurization of the cylinder but no elongation of the same, the amount of hydraulic fluid entering the cylinder is what needs to be provided thereto in a similar controlled manner. To control the amount of hydraulic fluid flowing into the hydraulic cylinder, the opening off the control valve may be performed by means of controlling the time the valve is opened, the size of the opening of the control valve, or a combination of the two. Pressurized state thus is defined that the pressure in the hydraulic cylinder is maximal, highest possible, without inducing elongation of the hydraulic cylinder.

A control valve may be designed to be open for only 0,010-0,500 seconds, preferably only 0,020-0,200 seconds, for the hydraulic cylinder to reach its pressurized state. 8 The pressures received are then processed by the control unit 3 by means of algorithms in its software, which calculates the expected difference in pressure between the two cylinders 7, 9 based on the configuration and variables of the material handling vehicle 1 and its specific cylinders. A potential load held by the lifting device is also taken into consideration as a variable, as such a load may lead to different pressures within different types of cylinders with different designs and chamber sizes etc.. lf the control unit 3 detects a deviation from the expected measured value, allowing a predetermined margin of error, when comparing it to the calculated value, the control unit is 3 arranged to provide an error signal to the vehicle 1.

The warning signal may be utilized in various manners. To add a safety feature to the vehicle, the control unit 3 may limit or prevent operation of the material handling vehicle 1 if a maximal predetermined safety value is exceeded, wherein the limitation comprises connecting the control unit 3 to the electric drive engine for restricting the operable speed of the material handling vehicle 1. Utilizing such a safety feature in a material handling vehicle 1 will thus stop or restrict an operator of the vehicle 1 to continue using the vehicle when/if the pressure within a working cylinder is shown to not be right, which could mean that said cylinder is dangerous to use.

The mast 13 of the vehicle 1 may further be movable in horizontal direction by means of at least a third hydraulic cylinder 27. Such a horizontal displacement is shown in Fig. 1, wherein the mast 3 drawn in solid lines shows the mast 3 in its default position and the mast 3 shown in dotted lines shows the mast fully extended horizontally. This provides a so called reach function/feature to the material handling vehicle 1, making it possible for the vehicle to perform material handling operations with a larger horizontal range, making the vehicle having a greater reach when it cannot drive further forward towards a load to be handled.

Fig. 4 shows a schematic view of a hydraulic system 31 of a material handling vehicle 1 according to an embodiment. The schematics therein shows connections of the components comprised in the hydraulic system 31. The control unit 3 is to be perceived as controlling the system in its entirety and thus being electrically connected to controllable components, but as it is not using hydraulic fluid, which are the couplings primarily shown herein, it is not line connected to the components. The electric drive engine 5 of the vehicle 1 is shown to be connected to the control unit but not being a part of the hydraulic system 31. The hydraulic system 31 comprises a hydraulic pump 6, powered by an electric pump engine (not shown), a first and a second hydraulic cylinder 7, 9. The first cylinder 7 is arranged to lift the lift device a first predetermined distance, wherein the second cylinder 9 is arranged to lift the lift device a second predetermined distance. 9 Furthermore, it is herein shown that the hydraulic pump 6 feeds the first and second cylinders 7, 9 via a feed line 33, which is divided to a first and a second feed line 34, 35, and controlled by means of a first and second control valves 25, 26, coupled to the first and second cylinder 7, 9 respectively. The first and second control valves 25, 26 are operated electrically by the control unit 3, so as to control the first and second hydraulic cylinders 7, 9 to perform operations thereof. lt may also be seen that the first sensor 15 is coupled to the first feed line 34 and that the second sensor 23 is coupled to the second feed line 35, wherein the pressures of said feed lines 34, 35 can be measured when their respective control valves 25, 26 are operated to open said feed lines to pressurize the first and second hydraulic cylinders respectively 7, 9.

Each control valve 25, 26 is arranged to be opened via the control unit 3 for only for 0,010-0,500 seconds, so as to set the separate hydraulic cylinders 7, 9 in their pressurized states.

Preferably, each control valve 25, 26 is arranged to be opened via the control unit 3 for only for 0,010- 0,500 seconds, so as to set the separate hydraulic cylinders 7, 9 in their pressurized states.

After being pressurized, both for being set into its pressurized state or its working state, each cylinder may be set into its respective resting state by a first return valve 37 and a second return valve 39 comprised in the hydraulic system. Said return valves 37, 39 lowers the pressures on their respective cylinder 7, 9, so that the hydraulic fluid is returned to a hydraulic tank 41 comprised in the system 31, via a return line 43 also comprised therein.

The hydraulic system 31 of the material handling vehicle 1 may further comprise an additional safety sensor 45, arranged to measure the pressure of the hydraulic system 31. Said safety sensor 45 may be arranged after the hydraulic pump 6 feeding hydraulic fluid to the feed line 33, as is shown in dotted lines in Fig. 4. Thus, when all hydraulic cylinders (all being the first and second cylinders in Fig. 4) are in their working state, the control unit 3 may measure the pressures of said lifting cylinders in their working state by means of their respective first and second sensors 15, 23, and measure the pressure of the safety sensor 45 simultaneously. The control unit 3 may then compare all measured pressures combined to a second maximum predetermined safety value. The principle is used in the same manner as described with reference making a safety measure of one of the first and second cylinders 7, 9 by means of using the other one to receive an additional signal. However herein the safety sensor 45 provides the additional signal and is subtracted from the combined measurement of all three simultaneously used sensors 15, 23, 45. lf an error signal is detected, the control unit 3 may first conclude that one of the first and second cylinders 15, 23 are faulty, wherein subsequent measurements may be performed, isolated to the two cylinders 7, 9 consequently, so as to identify which of the two cylinders 7, 9 that induces the error.

The safety sensor 45 may also be used with either of the two cylinders 7, 9 when used individually, wherein the safety feature provided to the hydraulic system 31 is diversified even further.

When an error signal is detected for any of the various usages of the hydraulic system 31, the control unit 3 may limit or prevent operation of the material handling vehicle 1 if the maximal a predetermined safety value is exceeded. Such a limitation comprise connecting the control unit 3 to the electric drive engine 5, wherein the electric drive engine 5 may either restrict or fully stop the operable speed of the material handling vehicle 1, and/or specific features thereof.

Fig. 5 shows a schematic representation of a method for safety control in a material handling vehicle 1 comprising at least two pressure sensors 15, 23. The method is used with a material handling vehicle 1 having the configuration and features as described in the disclosure herein. The method comprise the steps of: a) measuring the pressure in a first hydraulic cylinder 7 in operation, thus the first hydraulic cylinder 7 is in its working state, b) opening of a second hydraulic cylinder 9, actualizing the opening so that the second cylinder 9 does not extend; thus the second cylinder 9 is in its pressurized state, c) comparing the first pressure with the second pressure, and d) determining if a predetermined maximal deviation value between the first hydraulic cylinder 7 and the second hydraulic cylinder 9 is surpassed. lf the predetermined maximal deviation value is surpassed, step e) is performed, wherein e) taking safety measures for restricting or stopping the operation of the material handling vehicle 1.

Step e) may further be diversified to be split into the sub-steps el) or el), wherein el) comprises taking safety measures for restricting operation of the vehicle, and el) comprises fully stopping operation of the vehicle 1, wherein el) or el) is performed, based on if the predetermined maximal deviation value is surpassed by a predetermined difference value. lf the predetermined maximal deviation value is surpassed, but the difference thereof is lesser than the predetermined difference value, step el) is performed, wherein safety measures for restricting operation of the vehicle is taken. lf the predetermined maximal deviation value is surpassed, and the difference thereof is equal to, or greater than the predetermined difference value, step el) is performed, wherein safety measures for fully stopping operation of the vehicle is taken.

Fig. 6 shows a schematic representation of a computer-readable medium 47, said computer-readable medium 47 comprising a computer program 49. The computer program 49 comprising computer- readable instructions which, when executed by at least one processor of a control unit 3 of a material handling vehicle 1, causes the at least one processor to perform a safety control of at least one pressure sensor 15, 23 of said material handling vehicle 1, by means of performing the steps of the method as described in the disclosure herein. 11 The foregoing description of the embodiments has been furnished for illustrative and descriptive purposes. lt is not intended to be exhaustive, or to limit the embodiments to the variants described. Many modifications and variations will obviously be apparent to one ski||ed in the art. The embodiments have been chosen and described in order to best explicate principles and practical applications, and to thereby enable one ski||ed in the art to understand the invention in terms of its various embodiments and with the various modifications that are applicable to its intended use. The components and features specified above may, within the framework of the embodiments, be combined between different embodiments specified.

Claims (1)

1.Claims Material handling vehicle (1) comprising: a control unit (3), an electric drive engine (5), an hydraulic system (31), comprising an hydraulic pump (6) powered by an electric pump engine, for powering the hydraulic pump (6), a first and a second hydraulic cylinder (7, 9), a lift device (11), wherein the hydraulic system (31) is connected to the lift device (11) for lifting a load, a mast (13) to which the lift device (11) is connected movable along the mast (11) by means of at least a first and second hydraulic cylinder (7, 9), wherein the first cylinder (7) is arranged to lift the lift device (11) a first predetermined distance, wherein the second cylinder (9) is arranged to lift the lift device (11) a second predetermined distance, and wherein the first and second predetermined distances are arranged in consecutive manner, such that the start of lifting the second distance is arranged to be initiated when the first distance reaches its predetermined endpoint, characterized in that a first sensor (15) is arranged to determine the pressure of the first hydraulic cylinder (7), a second sensor (23) is arranged to determine the pressure of the second hydraulic cylinder (9), wherein the control unit (3) is arranged to set each hydraulic cylinder (7, 9) in a resting state, when not in active use, a pressurized state, when pressurized but not lifting, and a working state, when pressurized during lifting with said each cylinder (7, 9), wherein the material handling vehicle (1) is arranged such that when one of the first and second hydraulic cylinder (7,9) is in the working state and the other hydraulic cylinder is in resting state, the control unit (3) shifts the state of the hydraulic cylinder in resting state to a pressurized state, wherein the first and second sensors (15, 23) measures the pressures of the hydraulic cylinder (7) in the working state and the hydraulic cylinder (9) in the pressurized state, the control unit (3) is arranged to receive values of pressure from the first and second sensor (15, 23), and compare the difference thereof to a maximum predetermined safety value. Material handling vehicle (1) according to claim 1, wherein each hydraulic cylinder (7, 9) comprises a control valve (25, 26), wherein the control unit (3) is arranged to control the opening of the control valve (25, 26) in a controlled manner so as to set said hydraulic cylinder (7, 9) in its pressurized state. Material handling vehicle (1) according to any of the preceding claims, wherein the hydraulic system (31) further comprises a safety sensor (45), arranged to measure the 13 pressure of the hydraulic system (31), wherein when all hydraulic cylinders are in their working state, the control unit (3) is arranged to measure the pressures of said cylinders, measure the pressure of the safety sensor (45), and compare all measured pressures combined to a second maximum predetermined safety value. Material handling vehicle (1) according to any of the preceding claims, wherein the control unit (3) may limit or prevent operation of the material handling vehicle (1) if the maximal predetermined safety value is exceeded, wherein the limitation comprises connecting the control unit (3) to the electric drive engine (5) for restricting the operable speed of the material handling vehicle (1). A method for safety control in a material handling vehicle (1), comprising at least two hydraulic cylinders (7, 9) having one pressure sensor (15, 23) each, according to any of the claims above, comprising the steps of, comprising the steps of: a) measuring the pressure in a first hydraulic cylinder (7) in operation, b) opening of a second hydraulic cylinder (9) actualizing the opening so that the second cylinder does not extend, c) comparing the first pressure with the second pressure, d) determining if a predetermined maximal deviation value between the first hydraulic cylinder (7) and the second hydraulic cylinder (9) is surpassed, if the predetermined maximal deviation value is surpassed: e) taking safety measures for restricting or stopping the operation of the material handling vehicle (1). A computer program (49) comprising computer-readable instructions which, when executed by at least one processor of a control unit (3) of a material handling vehicle (1), causes the at least one processor to perform a safety control of at least one pressure sensor (15, 23) of said material handling vehicle (1), by means of performing the steps of the method according to claim A computer-readable medium (47), comprising the computer program (49) according to claim 6.