EP4295061A1

EP4295061A1 - Condition monitoring system

Info

Publication number: EP4295061A1
Application number: EP22707737.7A
Authority: EP
Inventors: Andreas Schumacher
Original assignee: Danfoss Power Solutions GmbH and Co OHG
Current assignee: Danfoss Power Solutions GmbH and Co OHG
Priority date: 2021-04-21
Filing date: 2022-02-24
Publication date: 2023-12-27
Also published as: DE102021203992B3; WO2022223180A1; CN117769627A

Abstract

A condition monitoring system embedded to a hydraulic circuit comprising a master sensor device for continuously sensing a first physical operational parameter of the hydraulic propel unit. A storage unit is provided on the master sensor device for storing sensed and/or processed data. The condition monitoring system further comprises a computing unit for processing the sensed data. The condition information of the propel unit is obtained by means of a condition monitoring method and/or a predictive maintenance method applied on the propel unit. An output unit indicates the condition information of the propel unit and/or communicates the condition information of the propel unit to a telltale and/or a control unit and/or a vehicle controller.

Description

CONDITION MONITORING SYSTEM

The present invention is related to hydraulic circuits. More particular, the invention is related to a condition monitoring system for hydraulic circuits. The invention is further related to a method to monitor the condition of hydraulic circuits.

Condition monitoring systems are applied in work vehicles equipped with hydraulic circuits, e.g. with hydraulic propel units, and fulfil different tasks, e.g., predicting product failure by monitoring relevant system variables or providing a base for improvements by logging relevant system variables. Additionally, condition monitoring systems are capable of increasing the safety of the hydraulic circuits and work vehicles they are installed to, as the probability of dangerous hazards can be monitored. Condition monitoring systems provide the base for predictive maintenance systems, wherein the relevant system parameters are continuously monitored and maintenance actions are suggested if the monitored parameter(s) deviate(s) significantly from a design magnitude.

Typically, sensors are required for detecting the operating conditions of the monitored systems and a computation as well as a storage unit is required for data storage and sensor data processing. Software algorithms have to be designed for calculating the desired result from the sensor data. As condition monitoring systems become more and more relevant in the field of work vehicles, a wide range of applications has to be covered by manufacturers of hydrostatic circuits. Therefore, the hydrostatic component/unit suppliers for work vehicles are required to design a condition monitoring system for every distinct application. Synergy effects cannot be used, as the condition monitoring system components are distributed over the whole work vehicle.

DE 11 2014 000 132 T5 discloses a control device which adjusts a capacity ratio between a hydraulic motor and a hydraulic pump in a closed hydraulic circuit. The control device comprises a determining unit, which determines, whether an operator intends to decelerate the working vehicle or not, and causes an adaptation of the capacity ratio depending on the operator’s command. It is therefore an objective of the invention to provide a condition monitoring system for hydraulic circuits, e.g. fulfilling propel or work functions. The system should be capable of monitoring the hydraulic circuits and of communicating the states of the circuit without requiring an infrastructure specifically tailored to the circuit, wherein the system according to the invention should be cheap and versatile. Further, a hydrostatic circuit according to the invention shall be equipped with a cheap and flexibly applicable condition monitoring system. The invention shall further provide a method to monitor the condition of the regarded hydraulic circuits and to predict maintenance points in time.

The objective of the invention is solved by a condition monitoring system according to claim 1 , a hydrostatic unit according to claim 10, a hydraulic circuit according to claim 11 , and by a method to monitor the condition of hydraulic circuits and to predict maintenance points in time according to claim 14. Preferred embodiments of the invention are disclosed in the corresponding subclaims.

The condition monitoring system according to the invention is embedded in a hydraulic circuit. The term “embedded” means that the condition monitoring system as an electronic system is integrated into the mechanical and/or hydraulic periphery of the hydraulic circuit and fulfils a certain functionality as a part of the whole unit. The condition monitoring system comprises a master sensor device for continuously sensing a first physical operational parameter of the hydraulic circuit. The master sensor device senses at least a first physical parameter and converts the physical parameter into an electric signal. The master sensor device can be connected to a power supply if, e.g., electric energy is required to perform the conversion from the physical to the signal domain. However, as known by a person with skills in the relevant art, there are also sensors, which do not require a power connection for producing an output signal, e.g., photoelectric or hall sensors.

A condition monitoring system according to the invention comprises a storage unit on the master sensor for storing sensed and/or processed data. Electric energy can be supplied to the storage unit via the power supply to which the master sensor device is connected to.

The condition monitoring system further comprises a computing unit for processing the sensed data in order to determine condition information of the hydraulic circuit, preferably according to a condition monitoring method and/or a predictive maintenance method applied on the hydraulic circuit. The computing unit is at least capable of receiving sensed data, or reading sensed data from the storage unit, and writing processed data to the storage unit. The computing unit can also be able to read processed data from the storage unit or is even able to overwrite sensed data, if, e.g., an obvious error in the sensed data is to be corrected. The condition monitoring system further comprises an output unit for indicating the determined condition information of the hydraulic circuit and/or for communicating the determined condition information of the hydraulic circuit to a tell-tale and/or a control unit and/or a vehicle controller.

The condition information, which is determined by the computing unit and indicated and/or communicated by the output unit, can comprise performance characteristics of main components of the hydraulic circuit or of the hydraulic circuit itself, and/or values of efficiency of the hydraulic circuit and/or other parameters which are suitable for indicating the condition of a hydraulic circuit. The condition information determined according to the invention may either be indicated directly via the output unit, e.g., by using a display, or may be sent to an information processing device, e.g., a tell-tale and/or a control unit and/or a vehicle controller in order to be processed further and/or to be displayed.

The condition monitoring system according to the invention can further comprise at least one slave sensor for sensing a second physical operational parameter and a communication path between the at least one slave sensor and the master sensor for communicating the sensed values of the at least one slave sensor to the master sensor. The second physical operational parameter can either be of the same physical domain as the first physical operational parameter which is sensed by the master sensor device, or can be from a different physical domain. If more than one slave sensor is provided, every sensor can sense a different physical operational parameter or some of the slave sensors can sense the same physical operational parameters at different locations of the hydraulic circuit. It is also covered by the concept of the invention that the same physical operational parameter is sensed by more than one slave sensor, or by a slave sensor and the master sensor, if, e.g., measurement errors have to be detected in a reliable way. The communication path between the at least one slave sensor and the master sensor can be a one-directional communication path via which, for example, sensed data can only be sent from the slave sensor to the master sensor, or can be a two-directional communication path by means of which sensed data from the slave sensor can be sent to the master sensor, and information and commands from the master sensor can be sent to the slave sensor.

According to the inventive concept, the master sensor device, the storage unit, the computing unit, the output unit, the at least one slave sensor and/or the communication path(s) can be designed as an integral part or as an assembly group - in other words as a package - which can be arranged or fastened to the inside/outside of a hydrostatic unit of the hydraulic circuit or to the inside/outside of the hydraulic circuit itself, e.g., at an external surface. In one embodiment of the invention, the storage unit, the computing unit and the output unit are integrated in the master sensor device which is arranged at the inside of a hydrostatic unit of a hydraulic circuit. This facilitates the handling of the components during and after assembling the condition monitoring system according to the invention and also the installation of the condition monitoring system to a hydraulic circuit or a hydrostatic unit. It is also covered by the inventive idea to arrange one or more slave sensors and/or communication paths to and from the slave sensors separately from the other aforementioned components which can form an integral part or assembly integrated to the hydraulic circuit.

In one embodiment of the invention the master sensor - which comprises at least a storage unit, and can be designed as an assembly group comprising computing and output capabilities - can be seen as the “intelligent” head of the system collecting and processing data sensed by the slave sensors. Normally the slave sensors are cheaper than the master sensor device, as they are not equipped with a storage unit, a computing unit or an output unit, such that the overall costs for a condition monitoring system can be reduced. Providing such an intelligent master sensor device together with at least one slave sensor for sensing physical signals at locations, which are distant from the master sensor device, leads to a cost efficient condition monitoring system. However, there is no loss of information, as all data is stored at the central master sensor device. Moreover, this facilitates the data processing as information may be only exchanged between the master sensor and the slave sensor(s) during the measuring step and not during the subsequent data processing which is performed solely at the master sensor device.

The first and/or second physical operational parameter can be selected from the group comprising: hydraulic input pressure and/or output pressure, hydraulic fluid temperature, hydraulic circuit temperature, hydraulic fluid flow rate, displacement angle, rotational or linear/longitudinal speed, acceleration/deceleration, hydraulic fluid viscosity, vibration, noise level, ambient temperature, roughness, deformation, and/or comparable parameters. If more than two physical operational parameters are monitored, those parameters can be selected from the aforementioned list as well, however other, not aforementioned parameters can be sensed also.

The determined condition information can be continuously processed by a predictive maintenance method to determine remaining time until next maintenance service or end of life cycle. Preferably, the processing of the condition information is based on the sensed data, which is stored in the storage unit and is executed by the computing unit. The term “continuously” means that the processing is executed in regular intervals, whose length depends, e.g., on the available computing power, the available storage capacities and the system which is monitored. Various predictive maintenance methods are known by a person skilled in the art, wherein the sensed physical signals, the corresponding sensors, the amount of storage required at the storage unit and the computing power of the computing unit will be selected based on the predictive maintenance method to be applied and the intervals at which the predictive maintenance method shall be executed.

The master sensor device according to the invention can be a multi-physics sensor, capable of continuously measuring more than one physical operational parameter. The physical operational parameters measured by the multi-physics sensor may be from the same physical domain, e.g., the electrical domain or the hydraulic domain, or from different physical domains, e.g., a combination of electric and hydraulic domain.

The communication paths between the one or more slave sensors and the master sensor can be a wired line, a LAN or WLAN system, a bus system, a network or internet (cloud) interface or the like. As each type of communication path has its advantages and disadvantages compared to the others, a particular type has to be selected in accordance with system requirements. For example, a wire line can be the most cost effective communication path type, whereas a WLAN system is not limited to space restrictions imposed by the design of the hydraulic circuit.

In one embodiment of the invention, the computing unit is adapted for receiving continuously sensed values of physical operational parameters of the hydraulic circuit. A calculated speed (rotational or longitudinal/linear) of a hydrostatic unit of the hydraulic circuit can be determined from these parameters. The computing unit is further adapted to determine deviation values between the calculated speed and a sensed speed of the hydrostatic unit of the hydraulic circuit. The computing unit is also adapted to store the deviation values in a timely order, starting at the point in time when the hydraulic circuit is put into service or at start of operation.

The calculated rotational or longitudinal/linear speed of the hydrostatic unit can, e.g., be determined based on the output pressure of a hydraulic pump arranged in the hydraulic circuit. The correlation between the pressure and the speed can be derived from a physical model of the system or from measurement results recorded during the operation of the hydraulic circuit under experimental conditions. The calculated speed represents nominal system behaviour, wherein the measured speed can be seen as performance indicator of the current system behaviour. By determining deviation values between the calculated speed and the sensed speed of the hydrostatic unit, a measure is obtained of how much the performance of the hydraulic circuit deviates from the nominal system behaviour. These values can be stored in a timely order, e.g., in the storage unit of the condition monitoring system. Depending on the usage, the values may alternatively be stored sorted by magnitude or in a database format. Preferably, the timely sorted deviation values are used to predict the system behaviour by comparing the curve of the deviation values to a reference curve in order to predict points in time at which maintenance is required or system failure is likely to occur.

The output unit can emit an alarm signal if, during a time period longer than a predefined limit, the deviation values are bigger than a predefined threshold value. This means that the current system behaviour deviates significantly from the nominal system behaviour for a relevant period of time. By selecting the limit for the magnitude of the predefined time period and the predefined deviation threshold value, a system designer can influence the behaviour of the condition monitoring system. The lower the deviation threshold value and the shorter the relevant time period are set, the faster the system will react to deviations. If, in contrast to that, high magnitudes of relevant time periods and deviation threshold values are predefined, alarm signals will only be sent if the calculated rotational speed deviates significantly from the sensed rotational speed for a long time period. Experiments may be conducted with the monitored system to evaluate which magnitudes for the deviation threshold values and what length of the relevant time period provide a suitable combination of robust system behaviour and reliable maintenance predictions.

A hydrostatic unit equipped with a condition monitoring system according to the invention can be part of a hydraulic circuit of the open hydraulic circuit type or closed hydraulic circuit type, wherein, preferably, at least the hydrostatic pump is equipped with a condition monitoring system according to the invention.

Further preferably, at least one hydrostatic actor, e.g. a hydrostatic motor, of the hydraulic circuit is equipped with a slave sensor according to the invention, wherein the condition monitoring system monitors the condition of both the hydrostatic pump and the at least one hydrostatic motor. This arrangement makes optimal usage of the concept of locally divided master and slave sensors and, therefore, provides an efficient option to monitor the condition of a hydraulic circuit comprising more than one hydrostatic unit (hydrostatic displacement device).

The inventive condition monitoring system can also be applied to hydraulic circuits comprising hydrostatic pump-cylinder systems. A hydrostatic pump-cylinder system according to the invention comprises at least one hydrostatic cylinder driven by a hydrostatic pump, wherein, according to the invention, the hydrostatic pump is equipped with a condition monitoring system, and the at least one hydrostatic cylinder is equipped with a slave sensor. The condition monitoring system monitors the condition of both the hydrostatic pump and the at least one hydrostatic cylinder. Similar to the condition monitoring of a hydraulic circuit comprising a hydrostatic motor and a hydrostatic pump, the hydraulic circuit with hydrostatic pump and cylinder comprises at least two hydrostatic units (displacement devices), which are arranged at different locations of the circuit. Therefore, multiple sensors are necessary to measure and monitor the condition of the circuit but only one of these sensors - the master sensor device - is equipped with a computational means, such as a storage or a computation unit. The sensor data is processed centrally at the master sensor device and provided to the output unit, which displays the processed data and/or communicates the processed condition information of the circuit to a superordinate unit, e.g., to a vehicle. At the superordinate level, the condition information can be displayed using a tell-tale and/or other visual interfaces and/or can be further processed by a control unit and/or a vehicle controller.

According to the invention, a method for monitoring the condition of hydraulic circuits and for predicting maintenance points in time of hydraulic circuits is provided as follows:

In a first step a) a speed (rotational or longitudinal) of a hydrostatic unit of the hydraulic circuit is calculated based on sensed values of operational parameters of the hydraulic circuit at predetermined points in time during its operation. The selection of the predetermined point in time may be based on, e.g., a time pattern with constant time steps, but may also be based on a different rule to obtain predetermined time steps, for example, the calculation could be triggered when one of the sensed values exceeds a predefined threshold value.

The calculation of the (theoretical) speed of the hydrostatic unit can be performed in various ways based on the sensed values of operational parameters, for example, based on the system pressure or a vibration frequency, wherein the parameters providing the base for the calculation can be directly or indirectly related to the speed. The calculation basis for determining a speed can also be derived from empirical values gathered from measurements or experiments. It is preferred to perform the calculation based on the empirical relation between the calculated speed and the sensed operating parameters of the hydraulic circuit, as this requires low computational power. The empirical relation can for example be obtained from tuning the elements of a matrix serving as a proportionality factor between the measured parameter and the speed during experiments, such that a reliable estimation/calculation of the speed can be obtained with low computational effort from multiplying a vector containing the measured system parameters with the matrix comprising the tuned values. The relation between the calculated speed and the sensed values of system parameters can also be obtained by means of drawing up a look-up table or approximating the physical equations representing the hydraulic circuit with a low degree Taylor series. However, a person with skills in the relevant art will find different solutions requiring low computational effort for calculating a speed from measured system parameters.

In step b), the real speed of the hydrostatic unit is sensed at that predetermined point in time, at which the parameters providing the base for the calculation have been measured. Depending on the specific application, the speed of the hydrostatic unit can be sensed throughout the operation of the hydraulic circuit or can be sensed only when a trigger signal is provided at the predetermined point in time.

In step c) an absolute deviation value representing the deviation of the calculated speed from the sensed (real) speed is determined at the predetermined point in time. An indication of how significantly the system behaviour deviates from the designed system behaviour is obtained by comparing the calculated rotational speed to the sensed rotational speed. This deviation is expressed in an absolute deviation value. Therefore, conclusions about the current condition of the hydraulic circuit can be drawn from the absolute deviation value.

The absolute deviation value may be filtered depending on its temporal resolution and scattering. If absolute deviation values are calculated frequently and the values are significantly distributed, for example a moving average filter can be applied to facilitate further processing of the absolute deviation values. However, the absolute deviation value may also be processed further in an unfiltered way, or different filter techniques may be applied.

The steps a) to c) are repeated periodically during operation of the hydraulic circuit. The repetition is labelled as step d).

In step e) a signal is output if the absolute deviation value exceeds a predetermined admissible threshold value for the deviation for a time period longer than a predetermined time period. The predetermined admissible deviation threshold value and the predetermined time period are set by a system designer depending on the application. The higher the predetermined deviation threshold value is set, the more deviation between calculated (theoretical) and sensed (real) speed, i.e. deviation between nominal and current system behaviour, is allowed before an output signal is generated. By setting a minimum length for a predetermined time period, measurement errors, singularities or short-term influences influencing the absolute deviation value, eventually evoking an output signal can be eliminated from being considered as faulty condition by the condition monitoring method according to the invention. The periodical repetition of the steps a) to c) does not inevitably lead to an execution of the steps a) to c) in a strict time pattern. The steps a) to c) can alternatively be performed when an operation condition that is periodically monitored becomes true, e.g., when a system pressure rises above a certain value.

According to the invention, a predictive maintenance signal can be output, for example, when a first admissible threshold value for absolute deviation values is exceeded during a time interval being larger than the predefined time period. A failure signal can be output if a second admissible threshold value for absolute deviation values is exceeded. Preferably the first admissible threshold value representing the point in time at which predictive maintenance is required, is lower than the second admissible threshold value which represents the point in time at which failure of the hydrostatic circuit has a high probability to occur. For example, the first threshold value for absolute deviation values is bigger than 50 rpm. For example, the second threshold value for absolute deviation values is bigger than 100 rpm.

Preferably the first and/or second admissible threshold values are defined by a system designer or by an operator, for example, before the method to monitor the condition of hydraulic circuit and to predict maintenance points in time of a hydraulic circuit is activated on the hydraulic circuit. According to the invention the predetermined threshold values can be stored, for example, in the storing unit of a master sensor device of a condition monitoring system.

In one embodiment of the invention, the magnitude of the first and/or second admissible threshold value can be changed during the operation of the hydraulic circuit by service or maintenance personal, for example. The values which probably have been defined by a system designer can be adapted to the specific characteristics of the particular vehicle/system to which the condition monitoring method according to the invention is applied to, preferably by means of updates which are implemented by a system designer or an operator or as well by means of self-learning algorithms or other computational or artificial intelligence means, for example.

The invention described above in general is now detailed further with the help of annexed Figures in which preferred embodiments and preferred design possibilities are shown. However, these preferred embodiments do not limit the scope of the inventive idea. The shown, preferred embodiments can be combined with one another without leaving the spirit of the invention. Furthermore, modifications within the possibilities of the knowledge of a person with skills in the relevant art can be implemented without leaving the spirit of the invention. In the Figures, it is shown:

Figure 1 shows schematically a condition monitoring system according to the invention; Figure 2 shows schematically a second embodiment of a condition monitoring system according to the invention;

Figure 3 shows a flow diagram of the method according to the invention;

Figure 4 shows exemplarily a diagram of an absolute deviation value over operational time;

In Figure 1, a hydraulic circuit 1 comprising a condition monitoring system 10 according to the invention is shown schematically. The hydraulic circuit 1 which in this case serves as hydraulic propel unit 1 comprises two hydrostatic units, i.e. a hydraulic pump 2 providing hydraulic pressure to a hydraulic motor 6, wherein the pump 2 and the motor 6 are arranged in a closed loop hydraulic circuit. The closed loop circuit comprises a high pressure line 3 which connects the pump outlet with the motor inlet, and a low pressure line 5 connecting the motor outlet with the pump inlet. The pump 2 is driven by an internal combustion engine 4 connected to the pump 2 via a shaft or gear means.

The hydraulic pressure which is generated by the pump 2 is transferred via the high pressure line 3 to the hydraulic motor 6, where it is converted to mechanical energy and provided to a mechanical consumer 8 via a shaft or gear means. A condition monitoring system 10 is arranged at the outlet of the hydraulic pump 2 in the high pressure line 3. The condition monitoring system 10 comprises a master sensor device 12 sensing data 14 from the high pressure line 3. In this case, the master sensor device 12 can sense, for example, the outlet pressure of the hydraulic pump 2, the volumetric flow rate through the pump and/or through the high pressure line 3 and/or other system parameters, like temperature or fluidity. A storage unit 16 is arranged on the master sensor device 12. The sensed data 14 from the master sensor device 12 is stored in the storage unit 16. A computing unit 20 is connected to the storage unit 16 by means of data connections, and exchanges sensed data 14 and/or processed data 18 with the storage unit 16. The processed data 18 can be stored on the storage unit 16 additionally or alternatively to the sensed data 14. The computing unit 20 processes the sensed data 14 and calculates condition information about the hydraulic circuit/propel unit 1 based on the sensed data 14 applying a condition monitoring method and/or a predictive maintenance method; for example a predictive maintenance method according to the embodiment shown with Figure 3. The condition monitoring system 10 further comprises an output unit 24 capable of receiving data, especially condition information 22, from the computing unit 20. The condition information 22 of the propel unit 1 can be indicated directly by the output unit 24, e.g., via a visualization device. However, the condition information can also be communicated by an output unit 24 towards an information processing device 26, e.g., a tell-tale, a control unit and/or a vehicle controller.

The parts of the condition monitoring system 10 shown with Figure 1 , namely, the master sensor device 12, the storage unit 16, the computing unit 20 and the output unit 24 can be provided as, e.g., an integral part or an assembly group or as one micro-controller in combination with a sensor, for example. However, the sensor device 12 and the units 16, 20, 24 for storage, information and output signals can be provided as separate parts as well. Further, the condition monitoring system 10 according to the described preferred embodiment is integral part of the propel unit 1 and can be provided at the inside of a housing of the hydraulic propel unit 1 or at the outside of the housing.

A person with skills in the relevant art will detect that the high pressure line 3 and the low pressure line 5 can be interchanged. Similarly, the pressure in the low pressure line 5 could be measured by the master sensor device without parting from the spirit of the invention. A condition monitoring system according to the invention can also comprise multiple sensors in the high pressure line 3 and/or in the low pressure line 5 in order to determine a pressure difference.

Figure 2 shows schematically a second embodiment of a condition monitoring system 10 according to the invention. In the present example, the condition monitoring system 10 is integrated to a hydraulic circuit/propel unit 1. The second embodiment of the condition monitoring system 10 comprises all elements of the condition monitoring system 10 according to the first embodiment which is shown in Figure 1. The condition monitoring system 10 according to the second embodiment further comprises several slave sensors 30 sensing additional physical operational parameters of the hydraulic propel unit 1. For example, the rotational speed of the input shaft connected to the hydraulic pump 2 or the rotational speed of the output shaft connected to the motor 6 are monitored. Additionally, pressure sensors are provided at the inlet and the outlet of the hydraulic pump 2 and of the hydraulic motor 6. The slave sensors 30 are connected via communication paths 32 with the master sensor device 12. Information between the master sensor device 12 and the slave sensors 30 is exchanged via the communication paths 32. For example, the master sensor device 12 is capable of sending commands towards the slave sensors 30, whereas the slave sensors 30 provide sensed data 14 to the master sensor device 12. Preferably, only the master sensor device 12 is equipped with a means capable of performing computing actions, e.g., a storage unit 16 and/or a computing unit 20. As the slave sensors 30 of this embodiment are not equipped with a computational means the master sensor 12 coordinates the measurements executed by the slave sensors 30 via the communication paths 32. In systems comprising a high complexity, because of a high number of system components have to be monitored, for example, more than one intelligent master sensor device 12 can be provided if the storage and/or computing capacity of one master sensor is not enough to process and store all the data which is received from the slave sensors 30. Depending on the system environment to which the condition monitoring system 10 is installed to, the communication paths 32 can be provided as wire lines, LAN or WLAN systems, bus systems, networks or internet interfaces (also cloud interfaces), or the like. Therefore, the communication paths 32 are not limited to visible connections but can also be wireless technologies, e.g., Bluetooth or infrared connection may be applied.

Figure 3 shows schematically a method for monitoring the condition of hydraulic circuit 1 and to predict maintenance points of time of hydraulic circuits 1. In step a) a rotational speed 40 of the hydrostatic unit 6 is calculated based on sensed data 14, i.e. sensed values of operational parameters which are adequate for performing the calculation. Preferably, the calculation in step a) is executed repeatedly at predetermined points in time, for example periodically. The intervals for executing the calculation can be selected, for example, as a result of balancing of requirements with regard to a high resolution of the calculated data and a minimized storage demand.

In step b) a rotational speed 42 of the hydrostatic unit 6 is sensed by a sensor device 30, preferably at the same predetermined point in time as the calculation of the rotational speed in step a). The calculation of the operational parameters in step a) can be exemplarily fulfilled by a master sensor device 12 equipped with a means for computation. The sensing of the rotational speed 42 of the hydrostatic unit 6 in step b), exemplarily measured at the shaft connecting the hydrostatic unit/motor 6 with the mechanical consumer 8, can be executed by a slave sensor 30 without computational capabilities. However, the usage of different sensor types and the use of more than one sensor to measure the operational parameters are also covered by the inventive idea.

In step c), an absolute deviation value 44 is determined by comparison of the calculated rotational speed 40 and the sensed rotational speed 42, i.e. by comparing the two values which have been measured respectively sensed preferably at the same predetermined point in time. Therefore, the absolute deviation value 44 represents the deviation of the calculated speed 40 from the sensed rotational speed 42.

The steps a) to c) are repeated periodically or at least repeatedly during operation of the hydraulic circuit 1. This is indicated with step d).

In step e) a signal 46 is output if the absolute deviation value 44 exceeds a predetermined admissible threshold value during a time period longer than a predetermined time period.

In Figure 4, a diagram is shown which visualizes the development of the absolute deviation value 44 over operational time of a hydraulic circuit 1. The absolute deviation value 44 is calculated from the sensed rotational speed 42 and the calculated rotational speed 40 over time. A first admissible threshold value 50 and a second admissible threshold value 52 are visualized, wherein the first admissible threshold value 50 comprises a lower absolute value than the second admissible threshold value 52. As an example, the first threshold value can be assigned with a magnitude of 50 rpm and the second threshold value can comprise a magnitude of 100 rpm. If the absolute deviation value 44 exceeds the first threshold value, according to step e) of the method according to Figure 3 a predictive maintenance signal 56 can be output. If the absolute deviation value 44 exceeds the second admissible threshold 52 for a time span that is longer than the predetermined time period 54, a failure signal is output at the end 58 of the predetermined time period 54 indicating that a failure of the hydraulic propel unit is likely to occur.

The predetermined time period 54 can be for example set to one or several hours. However a person with skills in the relevant art derives that the predetermined time period(s) 54 can be defined, for example in the range of seconds, minutes, hours or days, without leaving the scope of the invention. If more than one time period is defined, the time periods can comprise the same length or a different length.

From the above disclosure and accompanying Figures and claims, it will be appreciated that the condition monitoring system 10 and the method to monitor the condition of hydraulic circuits 1 and to predict maintenance points in time offer many possibilities and advantages over the prior art. It will be appreciated further by a person skilled in the relevant art that further modifications and changes to the condition monitoring system 10 and the method for monitoring the condition of hydraulic circuits 1 known from the art can be made to the condition monitoring system and method according to the invention without parting from the spirit and scope of this invention. Therefore, all these modifications and changes are within the scope of the claims and are covered by them. It should be further understood that the examples and embodiments described above are for illustrative purposes only and that various modifications, changes or combinations of embodiments in the light thereof which will be suggested to a person skilled in the relevant art are included in the spirit and purview of this invention.

Reference Number List

1 Hydraulic circuit / propel unit

2 Hydraulic pump / Hydrostatic pump / hydrostatic unit

3 High pressure line

4 Internal combustion engine

5 Low pressure line

6 Hydraulic motor / Hydrostatic motor / hydrostatic unit

8 Mechanical consumer

9 Vehicle controller

10 Condition monitoring system

12 Master sensor device

14 Sensed data / Sensed values

16 Storage unit

18 Processed data

20 Computing unit

22 Condition information

24 Output unit

26 Information processing device

30 Slave sensor

32 Communication path

40 Calculated rotational speed

42 Sensed rotational speed

44 Absolute deviation value

46 Signal

50 First admissible threshold

52 Second admissible threshold

54 Predetermined time period

56 Predictive maintenance signal

58 End of predetermined time period

Claims

1. Condition monitoring system (10) for being arranged or fastened to a hydrostatic unit which is embedded to a hydraulic circuit (1) comprising: a master sensor device (12) for continuously sensing a first physical operational parameter of the hydraulic circuit (1 ); a storage unit (16) on the master sensor (12) for storing sensed and/or processed data (14, 18); a computing unit (20) for processing the sensed data (14) in order to determine condition information (22) of the hydrostatic unit of the hydraulic circuit (1); an output unit (24) for indicating the condition information (22) of the hydrostatic unit of the hydraulic circuit (1) and/or for communicating the condition information (22) of the hydrostatic unit of the hydraulic circuit (1 ) to a telltale and/or a control unit and/or a vehicle controller (9).

2. Condition monitoring system (10) according to claim 1 further comprising: at least one slave sensor (30) for sensing a second physical operational parameter; a communication path (32) between the at least one slave sensor (30) and the master sensor (12) for communicating the sensed values (14) sensed by the at least one slave sensor (30) to the master sensor (12);

3. Condition monitoring system (10) according to claims 1 or 2, wherein the master sensor device (12), the storage unit (16), the computing unit (20), the output unit (24), the at least one slave sensor (30) and/or the communication path (32) are designed as an integral part or assembly group which is arranged at the inside or at the outside of a hydrostatic unit in the hydraulic circuit (1).

4. Condition monitoring system (10) according to one of claims 1 to 3, wherein the first and/or second physical operational parameter is selected from the group comprising: hydraulic input pressure and/or output pressure, hydraulic fluid temperature, hydraulic circuit (1) temperature, hydraulic fluid flow rate, displacement angle, speed, acceleration / deceleration, hydraulic fluid viscosity, vibration frequency, noise level, ambient temperature, roughness, deformation;

5. Condition monitoring system (10) according to any of the preceding claims, wherein the determined condition information (22) is continuously processed by a predictive maintenance method to determine remaining time for next maintenance service or end of lifecycle.

6. Condition monitoring system (10) according to any of the preceding claims, wherein the master sensor device (12) is a multi-physics sensor capable of continuously measuring more than one physical operational parameter.

7. Condition monitoring system (10) according to one of claims 2 to 6, wherein the communication path (32) is a wire line, a LAN or WLAN system, a BUS-system, a network or internet (cloud) interface, or the like.

8. Condition monitoring system (10) according to one of claims 1 to 7, wherein the computing unit (20) is adapted for receiving continuously sensed values (14) of physical operational parameters of the hydraulic circuit (1), from which a calculated speed (40) of a hydrostatic unit in the hydraulic circuit (1) can be determined, and is adapted for determining deviation values of the calculated speed (40) to a sensed speed (42) of the hydrostatic unit, and for storing the deviation values in an timely order starting at the point in time when the hydraulic circuit (1) is put into operation or service.

9. Condition monitoring system (10) according to claim 8, wherein the output unit (24) emits an alarm signal if, during a time period longer than a predefined time period (54), the deviation values are bigger than a predefined threshold value (50, 52).

10. Hydrostatic unit equipped with a condition monitoring system (10) according to any one of claims 1 to 9.

11. Hydraulic circuit (1 ) of the open hydraulic circuit or closed hydraulic circuit type, wherein at least a hydrostatic pump (2) is equipped with a condition monitoring system (10) according to one of claims 1 to 9.

12. Hydraulic circuit (1 ) according to claim 11 , wherein at least one hydrostatic motor (6) is equipped with a slave sensor (30) according to claim 2, and wherein the condition monitoring system (10) monitors the condition of both the hydrostatic pump (2) and the at least one hydrostatic motor (6).

13. Hydraulic circuit (1) comprising at least one hydrostatic cylinder driven by a hydrostatic pump (2), wherein the hydrostatic pump (2) is equipped with a condition monitoring system (10) according to one of claims 1 to 9 and the at least one hydrostatic cylinder is equipped with a slave sensor (30) according to claim 2, and wherein the condition monitoring system (10) monitors the condition of both the hydrostatic pump (2) and the at least one hydrostatic cylinder.

14. Method to monitor the condition of hydraulic circuits (1) and to predict maintenance points in time of a hydraulic circuit (1) comprising the steps of: a) Determining at a predetermined point in time during operation of the hydraulic circuit (1) a calculated speed (40) of a hydrostatic unit based on sensed values (14) of operational parameters; b) Sensing the speed of the hydrostatic unit at the predetermined point in time according to step a); c) Determining an absolute deviation value (44) representing the deviation of the calculated speed (40) from the sensed speed (42) at this predetermined point in time (54); d) Repeating steps a) to c) periodically during operation of the hydraulic circuit (1 ); e) Outputting a signal (46) if, during a time period longer than a predetermined time period (54), the absolute deviation value (44) exceeds a predetermined admissible threshold value (50, 52) for the deviation;

15. Method according to claim 14, wherein a predictive maintenance signal (56) is outputted when a first admissible threshold (50) value for absolute deviation values (44) is exceeded, and wherein a failure signal is outputted if a second admissible threshold (52) value for absolute deviation values (44) is exceeded.