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Title:
A SUSPENSION SYSTEM FOR A WORKING MACHINE AND A METHOD FOR CONTROLLING THE SUSPENSION SYSTEM
Document Type and Number:
WIPO Patent Application WO/2019/086133
Kind Code:
A1
Abstract:
The invention relates to a suspension system (200) for a working machine (100), comprising: a first swing arm (202) pivotally connected to a body (103) of the working machine and to a first wheel hub (208), a first pressure controlled actuator (210) connects an end portion of the first swing arm and the first wheel hub to the body of the working machine; a second swing arm (212) pivotally connected to the body of the working machine and to a second wheel hub (218), the second wheel hub being on the opposing side of the working machine as the first wheel hub, a second pressure controlled actuator (220) connects an end portion of the second swing arm and the second wheel hub to the body of the working machine, a pressure means (222) configured to provide pressure to the actuators; a valve arrangement (224) arranged between the pressure means and the actuators and configured to control a pressure in the actuators. The system further comprises: a vibration sensor (226) configured to detect vibrations of the working machine; and a suspension control unit (228) connected to the vibration sensor and configured to determine modal vibration properties of the working machine based on an input from the vibration sensor, wherein the suspension control unit control the valve arrangement to control the pressure of the actuators so that the modal vibrations are counteracted by the actuators.

Inventors:
SELVAM, Kausihan (Västra Ringvägen 21B, Västerås, 724 61, SE)
STENER, Patrik (Mats Ålännings Gata 14, Torshälla, 644 30, SE)
Application Number:
EP2017/078355
Publication Date:
May 09, 2019
Filing Date:
November 06, 2017
Export Citation:
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Assignee:
VOLVO CONSTRUCTION EQUIPMENT AB (631 85 Eskilstuna, 631 85, SE)
International Classes:
E02F9/22; B60G1/00; E02F9/02; E02F3/65
Domestic Patent References:
WO2017061911A12017-04-13
Foreign References:
JPS6322718A1988-01-30
JPH0640232A1994-02-15
JP2006298048A2006-11-02
Attorney, Agent or Firm:
VOLVO TECHNOLOGY CORPORATION (Volvo Group Intellectual Property, BF14100 M1.7, Göteborg, 405 08, SE)
Download PDF:
Claims:
CLAIMS

1 . A suspension system (200) for a working machine (100), the suspension system comprising:

a first swing arm (202) having a first end portion (204) pivotally connected to a body (103) of the working machine and a second end portion (206) connected to a first wheel hub (208), wherein a first pressure controlled actuator (210) connects the second end portion of the first swing arm and the first wheel hub to the body of the working machine;

a second swing arm (212) having a first end portion (214) pivotally connected to the body of the working machine and a second end portion (216) connected to a second wheel hub (218), the second wheel hub being on the opposing side of the working machine as the first wheel hub, wherein a second pressure controlled actuator (220) connects the second end portion of the second swing arm and the second wheel hub to the body of the working machine;

a pressure means (222) configured to provide pressure to the first and second pressure controlled actuator;

a valve arrangement (224) arranged between the pressure means and the first and second pressure controlled actuator and configured to control a pressure in the first and second pressure controlled actuator; characterized in that the system further comprises:

a vibration sensor (226) configured to detect vibrations of the working machine; a suspension control unit (228) connected to the vibration sensor and configured to determine modal vibration properties of the working machine based on an input from the vibration sensor, wherein the suspension control unit is further configured to control the valve arrangement to control the pressure of the first and second pressure controlled actuator so that the determined modal vibrations are counteracted by the pressure controlled actuators.

2. The system according to claim 1 , wherein the valve arrangement is configurable to form a coupling between a piston (302) of the first actuator and a piston (304) of the second actuator such that a movement of the piston in one direction of one of the first and second actuators results in a movement in the opposing direction of the piston of the other one of the first and second actuators.

3. The system according to claim 1 or 2, wherein the valve arrangement is configured to individually control the pressure in each of the first and second actuator.

4. The system according to claim 1 or 2, wherein the valve arrangement is configured to 5 provide a predetermined combined average pressure of the first and second actuators.

5. The system according to any one of the preceding claims, wherein the pressure means comprises an accumulator (306) coupled to the valve arrangement and configured to provide a pressure to the first and/or the second actuator via the valve arrangement.

10

6. The system according to any one of the preceding claims, wherein the first actuator and the second actuator are hydraulic cylinders.

7. The system according to claim 6, further comprising a hydraulic machine (226) 15 arranged and configured to provide a flow of hydraulic fluid to the first and second hydraulic cylinder via the valve arrangement.

8. The system according to any one of claims 1 to 5, wherein the first actuator and the second actuator are pneumatic cylinders.

20

9. The system according to any one of the preceding claims, wherein the control unit comprises a bandpass filter having a passband corresponding to a predetermined modal frequency of the working machine, and wherein the control unit is configured to filter the input from the vibration sensor using the bandpass filter.

25

10. The system according to claim 9, wherein the bandpass filter has a lower cutoff frequency in the range of 1 to 2 Hz and an upper cutoff frequency in the range of 2 to 3 Hz.

30 1 1 . The system according to any one of the preceding claims, wherein the first and second swing arms are connected to rear wheels (1 12) of the working machine.

12. The system according to any one of the preceding claims, wherein the vibration sensor is an inertial measurement unit, IMU.

35

13. The system according to any one of the preceding claims, wherein the vibration sensor is located in a cab (1 14) of the working machine.

14. The system according to any one of the preceding claims, wherein the vibration 5 sensor is located at an implement (107) of the working machine.

15. The system according to any one of the preceding claims, wherein the working machine is a wheel loader.

10 16. A working machine comprising a system according to any one of the preceding claims.

17. A method for controlling a suspension system (200) in a working machine (100), the suspension system comprising:

15 a first swing arm (202) having a first end portion (204) pivotally connected to a body (103) of the working machine and a second end portion (206) connected to a first wheel hub (208), wherein a first pressure controlled actuator (210) connects the second end portion of the swing arm and the first wheel hub to the body of the working machine; a second swing arm (212) having a first end portion (214) pivotally connected to

20 the body of the working machine and a second end portion (216) connected to a second wheel hub (218), the second wheel hub being on the opposing side of the working machine as the first wheel hub, wherein a second pressure controlled actuator (220) connects the second end portion of the swing arm and the second wheel hub to the body of the working machine;

25 a pressure means (222) configured to provide pressure to the first and second actuator;

a valve arrangement (224) arranged between the pressure means and the first and second actuator and configured to control a pressure in the first and second actuator; a vibration sensor (226) configured to detect vibrations of the working machine;

30 and

a suspension control unit (228) connected to the vibration sensor, wherein the method comprises:

detecting (S1 ) vibrations of the working machine;

determining (S2) modal vibration properties of the working machine; and controlling (S3) the valve arrangement to control the pressure of the first and second pressure controlled actuator so that the determined modal vibrations are counteracted by the pressure controlled actuators. 18. The method according to claim 17, further comprising controlling the valve arrangement to form a coupling between a piston of the first actuator and a piston of the second actuator such that a movement of the piston in one direction of one of the first and second actuators results in a movement in the opposing direction of the piston of the other one of the first and second actuators.

19. The method according to claim 17 or 18, further comprising individually controlling the pressure in each of the first and second actuator.

20. The method according to any one of claims 17 to 19, further comprising controlling the valve arrangement to provide a predetermined combined average pressure of the first and second actuators.

21 . The method according to any one of claims 17 to 20, further comprising bandpass filtering the input from the vibration sensor using a bandpass filter having a passband corresponding to a predetermined modal frequency of the working machine.

22. The method according to claim 21 , wherein the bandpass filter has a lower cutoff frequency in the range of 1 to 2 Hz and an upper cutoff frequency in the range of 2 to 3 Hz.

23. A computer program comprising program code means for performing the steps of any of claims 17 to 22 when said program is run on a computer.

24. A computer readable medium carrying a computer program comprising program code means for performing the steps of any of claims 17 to 22 when said program product is run on a computer.

Description:
A suspension system for a working machine and a method for controlling the suspension system

TECHNICAL FIELD

The invention relates to an active suspension system for a working machine comprising swing arms.

The invention is applicable on working machines within the fields of industrial construction machines or construction equipment, in particular wheel loaders. Although the invention will be described with respect to a wheel loader, the invention is not restricted to this particular machine, but may also be used in other working machines such as articulated haulers, excavators and backhoe loaders.

BACKGROUND

A working machine, such as a wheel loader, is usually provided with a bucket, container, gripper or other type of implement for digging, carrying and/or transporting a load. For example, a wheel loader has a lift arm unit for raising and lowering the implement. Usually one or a pair of hydraulic cylinders is arranged for raising the load arm and a further hydraulic cylinder is arranged for tilting the implement relative to the load arm. In addition, the working machine is often articulated frame-steered and has a pair of hydraulic cylinders for turning/steering the working machine by pivoting a front section and a rear section of the working machine relative to each other. The hydraulic system generally further comprises at least one hydraulic pump, which is arranged to supply hydraulic power, i.e. hydraulic flow and/or hydraulic pressure, to the hydraulic cylinders.

A working machine, such as a wheel loader, may experience undesired movements, in particular when the wheel loader is moving forward to transport a load in the implement from one place to another. The surface, e.g. at a construction site, on which the working machine is moving may be rough and uneven. Undesired movements may cause a jerky ride which is uncomfortable for the driver, and which may also cause safety risks, e.g. due to the risk that a portion of the load in the implement will fall out due to the undesired movements. It is known to provide a wheel loader with a so called boom suspension system (BSS) to smoothen the ride of the machine. The system adds flexibility between the lifting arm and a body of the wheel loader. The system includes an accumulator connected to the hydraulic cylinders for the lift arm. A well working boom suspension system requires that the accumulator is well tuned. Since the load of the implement may change, such tuning may be difficult. Also, there are drawbacks with the described boom suspension system since such systems add cost and energy consumption to the wheel loader. A further drawback is that such systems have a low bandwidth and the changes in the pressure in the hydraulic system act as a low pass filter, causing a delay of the dampening function. Also, the damping force produced by the BSS depends on the load on the bucket where the lesser the load, the lesser the damping. Moreover, the boom suspension system attempts to dampen the reaction caused by machine suspension and hence is an indirect suspension system. By providing more than one accumulator in the boom suspension system, the useable frequency of the accumulators is increased and hence the load range. However, adding more accumulators to the system increases system cost and complexity.

WO2017/06191 1 describes a method and system for dampening movements of a working machine by controlling the propulsion torque and braking torque of individual wheels of the working machine. The described solution comprises generating a torque to a selected wheel to counteract a detected undesirable movement of the working machine.

However, there is still a need for further improved suspension systems for a wheel loader.

SUMMARY

An object of the invention is to provide an improved active suspension system for a working machine and a method for controlling such a suspension system. According to a first aspect of the invention, the object is achieved by a suspension system for a working machine according to claim 1 .The suspension system comprises a first swing arm having a first end portion pivotally connected to a body of the working machine and a second end portion connected to a first wheel hub, wherein a first pressure controlled actuator connects the second end portion of the swing arm and the first wheel hub to the body of the working machine and a second swing arm having a first end portion pivotally connected to the body of the working machine and a second end portion connected to a second wheel hub, the second wheel hub being on the opposing side of the working machine as the first wheel hub, wherein a second pressure controlled actuator connects the second end portion of the swing arm and the second wheel hub to the body of the working machine. The suspension system further comprises a pressure means configured to provide pressure to the first and second actuator and a valve arrangement arranged between the pressure means and the first and second actuator and configured to control a pressure in the first and second actuator. The system further comprises: a vibration sensor configured to detect vibrations of the working machine; and a suspension control unit connected to the vibration sensor and configured to determine modal vibration properties of the working machine based on an input from the vibration sensor, wherein the suspension control unit is further configured to control the valve arrangement to control the pressure of the first and second actuator to so that the determined modal vibrations are counteracted by the actuators.

Modal vibration properties include both frequency and amplitude of the detected modal vibration. The modal vibrations can be seen as the detected vibrations or oscillations within a predetermined frequency range approximately corresponding to a resonance frequency of the working machine. It is here assumed that properties of vibrations occurring at the resonance frequency for a specific working machine are known beforehand. The resonance properties of the working machine can for example be estimated analytically or acquired empirically. Since the vibrations occurring at the working machine resonance frequency are enhanced, they are particularly important to dampen. A working machine has a relatively large pitching motion tendency on an uneven surface due to a relatively large inertia in the pitching direction combined with a relatively short wheel base and large springy tires. This tendency is aggravated in a wheel loader where a load in the implement can be quite heavy, in particular where the implement is outside of the wheel base of the wheel loader. The described system detects and measures the modal vibrations of the working machine during operating an in particular when being driven on an uneven surface, and based on the measured vibrations the system can actively manipulate the pressure of the suspension system based on the measured vibrations in order to counteract the vibrations. Thus, the vibrations are actively damped and the ride is made more comfortable and secure. According to one embodiment of the invention, the valve arrangement may be configurable to form a coupling between a piston of the first actuator and a piston of the second actuator such that a movement of the piston in one direction of one of the first and second actuators results in a movement in the opposing direction of the piston of the other one of the first and second actuators. The described system brings the advantage that both wheels will automatically be in contact with the ground on an uneven surface since a movement in one direction (i.e. up or down) on one wheel is counteracted by a movement in the opposing direction on the other wheel, similarly to in a conventional type of suspension system used in e.g. cars. The present system is however stiff since there is no spring connected to the wheels. Accordingly, the described suspension system provides dampening also in a stiff system. The described coupling can be seen as simulating a pivot axle, but here with a hydraulic or pneumatic coupling between the two wheels. A pivot axle functionality is desirable during normal operation to provide suspension for the working machine when travelling on uneven terrain.

According to one embodiment of the invention, the valve arrangement may advantageously be configured to individually control the pressure in each of the first and second actuator. Thereby, the actuators can be controlled to handle vibrations originating form objects such as bumps and potholes influencing only one side of the working machine and thereby only one of the rear wheels, which provides a better overall comfort for the vehicle operator.

According to one embodiment of the invention, the valve arrangement may be configured to provide a predetermined combined average pressure of the first and second actuators. Thereby, if the pressure is increased in one actuator it is correspondingly decreased in the other such that is one wheel is raised the other wheel is lowered, similarly to a pivot axle Thereby, the working machine can be operated comfortably on uneven terrain. According to one embodiment of the invention, the pressure means may comprise an accumulator coupled to the valve arrangement and configured to provide a pressure to the first and/or the second actuator via the valve arrangement. The accumulator is thus the means by which the desired pressure is provided to each of the pressure controlled accumulators. It is further assumed that the accumulator can be charged when required, for example by a pump associated with the suspension system. According to one embodiment of the invention the first pressure controlled actuator and the second pressure controlled actuator may advantageously be hydraulic cylinders. Thereby, already existing hydraulic components of the working machine may be used together with the hydraulic cylinders of the suspension system. The suspension system may further comprise a hydraulic machine arranged and configured to provide a flow of hydraulic fluid to the first and second hydraulic cylinder via the valve arrangement. The hydraulic machine may further be connected to an accumulator which can be charged by the hydraulic machine. Thereby, the pressure means can be seen as comprising the accumulator and hydraulic machine, alone or in combination.

According to one embodiment of the invention, the first pressure controlled actuator and the second pressure controlled actuator may pneumatic cylinders. A pneumatic system may be used in applications where cost savings are important since pneumatic systems generally can be made cheaper than hydraulic systems. However, a pneumatic system may not be practical for large machines, in part due to the difference in compressibility of air compared to hydraulic fluid. Moreover, the components of a pneumatic system are typically larger than corresponding components in a hydraulic system, which may make a pneumatic system inconvenient in some applications.

According to one embodiment of the invention, the control unit may advantageously comprise a bandpass filter having a passband corresponding to a predetermined modal frequency of the working machine, and wherein the control unit is configured to filter the input from the vibration sensor using the bandpass filter. Thereby, an oscillating motion occurring at the resonance frequency can be more easily detected.

According to one embodiment of the invention, the bandpass filter may advantageously have a lower cutoff frequency in the range of 1 to 2 Hz and an upper cutoff frequency in the range of 2 to 3 Hz, where the passband of the bandpass filter is preferably selected to include the resonance frequency of the vehicle. The resonance frequency is a property of the vehicle geometry and wheel stiffness.

According to one embodiment of the invention, the first and second swing arms are advantageously connected to rear wheels of the working machine. In a wheel loader, the swing arms are located only at rear portion to maintain the stability of the wheel loader. The front wheel axle of the wheel loader is typically fixed.

According to one embodiment of the invention, the vibration sensor may advantageously be an inertial measurement unit, IMU. An IMU is advantageously used since it is provided as one unit which can provide all the information required for determining the vibration properties of the working machine. The IMU is connected to the control unit so that the valve arrangement can be controlled to counteract the vibrations detected by the IMU. However, it should be noted that the vibrations also can be measured by other types of sensors or sensor systems such as accelerometers, angle sensors etc.

According to one embodiment of the invention, the vibration sensor may advantageously be located in a cab of the working machine. Thereby, the measured vibrations correspond to the vibrations or oscillations experienced by the operator of the vehicle, and the suspension system can be controlled to optimize the comfort of the driver. It also possible to use several vibration sensors arranged at different locations of the working machine. For example, if it is of particular importance that the vibrations of an implement of the working machine are minimized, a vibration sensor may be located near the implement. There is also provided working machine, such as a wheel loader, comprising a system according to any one of the above described embodiments.

According to a second aspect of the invention, the object is achieved by a method for controlling a suspension system in a working machine. The suspension system comprises: a first swing arm having a first end portion pivotally connected to a body of the working machine and a second end portion connected to a first wheel hub, wherein a first pressure controlled actuator connects the second end portion of the swing arm and the first wheel hub to the body of the working machine; a second swing arm having a first end portion pivotally connected to the body of the working machine and a second end portion connected to a second wheel hub, the second wheel hub being on the opposing side of the working machine as the first wheel hub, wherein a second pressure controlled actuator connects the second end portion of the swing arm and the second wheel hub to the body of the working machine; a pressure means configured to provide pressure to the first and second actuator; a valve arrangement arranged between the pressure means and the first and second actuator and configured to control a pressure in the first and second actuator; a vibration sensor configured to detect vibrations of the working machine; and a suspension control unit connected to the vibration sensor. The method comprises: detecting vibrations of the working machine; determining modal vibration properties of the working machine; and controlling the valve arrangement to control the pressure of the first and second actuator to so that the determined modal vibrations are counteracted by the actuators.

According to one embodiment of the invention the method may further comprise controlling the valve arrangement to form a coupling between a piston of the first actuator and a piston of the second actuator such that a movement of the piston in one direction of one of the first and second actuators results in a movement in the opposing direction of the piston of the other one of the first and second actuators. This can be achieved by controlling the valve arrangement to cross-couple the two actuators. Effects and features of this second aspect of the present invention are largely analogous to those described above in connection with the first aspect of the invention.

There is further provided a computer program comprising program code means for performing the steps of the above method when the program is run on a compute and a computer readable medium carrying a computer program comprising program code means for performing the steps of the above method when the program product is run on a computer.

Further advantages and advantageous features of the invention are disclosed in the following description and in the dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

With reference to the appended drawings, below follows a more detailed description of embodiments of the invention cited as examples.

In the drawings:

Fig. 1 is a schematic illustration of a working machine comprising a suspension system according to an embodiment of the invention, Fig. 2 is a schematic illustration of a suspension system for working machine according to an embodiment of the invention, Fig. 3 is a schematic illustration of a suspension system for a working machine according to an embodiment of the invention, and

Fig. 4 is a flow chart outlining the general steps of controlling a suspension system for a working machine according to an embodiment of the invention.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS OF THE INVENTION

In the present detailed description, various embodiments of a suspension system and a method for controlling a suspension system according to the present invention are mainly discussed with reference to a hydraulic suspension system in a wheel loader. It should be noted that this by no means limits the scope of the present invention which is equally applicable to pneumatic suspension systems, and that the invention also may be applicable for other types of working machines.

FIG. 1 shows a frame-steered working machine in the form of a wheel loader 100. The body of the wheel loader 100 comprises a front body section 102 and a rear body section 103, which sections each comprises a pair of wheels 1 12,1 13. The rear body-section 103 comprises a cab 1 14. The body sections 102,103 are connected to each other in such a way that they can pivot in relation to each other around a vertical axis by means of two first actuators in the form of hydraulic cylinders 104,105 arranged between the two sections 102, 103. The hydraulic cylinders 104,105 are thus arranged one on each side of a horizontal centerline of the vehicle in a vehicle traveling direction in order to turn the wheel loader 101 .

The wheel loader 100 comprises equipment 1 1 1 for handling a load 1 16 such as objects or material. The equipment 1 1 1 comprises a load-arm unit 106, also referred to as a linkage, and an implement 107 in the form of a bucket fitted on the load-arm unit 106. A first end of the load-arm unit 106 is pivotally connected to the front vehicle section 102. The implement 107 is pivotally connected to a second end of the load-arm unit 106. The load-arm unit 106 can be raised and lowered relative to the front section 102 of the vehicle by means of actuators in the form of one or more hydraulic cylinders 108, connected at one end to the front vehicle section 102 and at the other end to the load-arm unit 106. The bucket 107 can be tilted relative to the load-arm unit 106 by means of an actuator in the form of a hydraulic cylinder 1 10, which is connected at one end to the front vehicle section 102 and at the other end to the bucket 107 via a link-arm system 1 15.

Fig. 2 schematically illustrates a suspension system 200 for the working machine 100 according to an embodiment of the invention. The suspension system 200 comprises a first swing arm 202 having a first end portion 204 pivotally connected to a body 103 of the working machine and a second end portion 206 connected to a first wheel hub 208, wherein a first pressure controlled actuator 210, here in the form of a hydraulic cylinder 210, connects the second end portion 206 of the first swing arm 202 and the first wheel hub to the body 103 of the working machine. The system 200 further comprises a corresponding second swing arm 212 having a first end portion 214 pivotally connected to the body 103 of the working machine 210 and a second end portion 216 connected to a second wheel hub 218, the second wheel hub 218 being on the opposing side of the working machine 100 as the first wheel hub 208, wherein a second pressure controlled actuator 220, also embodied by a hydraulic cylinder 220, connects the second end portion 218 of the swing arm 212 and the second wheel hub 218 to the body 103 of the working machine. Accordingly, the rear wheels have individual hubs 206, 218, without any direct connection between the wheels, and each wheel is actuated individually.

Even though the present description is related to a hydraulic suspension system where the pressure controlled actuators 210, 220 are illustrated as hydraulic cylinders 210, 220. Embodiments of the invention are equally applicable for a pneumatic system where the pressure controlled actuators 210, 220 are embodied by pneumatic actuators.

In the wheel loader 100 illustrated herein, the swing arms 202, 212 are connected to the rear body section 103 so that the rear wheels 1 12 are suspended by the hydraulic cylinders 210, 220 while the front wheels 1 13 are connected to the front body section 102 by a fixed front wheel axle. Thereby, the stability of the wheel loader 200 is maintained. The swing arms 202, 212 are thus rigid elements arranged to pivot about a fixed point connected to the body 103 of the working machine 100 so that the rear wheels 1 12 are allowed to move independently in a substantially vertical direction, thereby incurring a pivoting motion of the swing arm 202, 212 about the point where it is connected to the body 103.

The suspension system further comprises pressure means 222 configured to provide pressure to the first and second hydraulic cylinders 210, 220. The pressure means 222 is in Fig. 2 embodied by a hydraulic machine 222 arranged and configured to provide a flow of hydraulic fluid to the first and second hydraulic cylinder 210 via a valve arrangement 224 arranged between the hydraulic machine and the first and second hydraulic cylinders 210, 220. The valve arrangement 224 is configured to control a pressure in the first and second hydraulic cylinders 210, 220. The valve arrangement 224 comprises a plurality of valves 308a-d controlling the flow of hydraulic fluid between the accumulator 306 and the hydraulic cylinders 210,220.

The suspension system further comprises a vibration sensor 226 configured to detect vibrations of the working machine 100. The vibration sensor 226 may for example be a IMU 226, an internal measurement unit, configured to detect vibration properties of the working machine. In the present description, the vibration sensor is located in the cab 1 14 of the working machine in order to detect the vibrations experienced by the driver of the working machine.

A suspension control unit 228 connected to the vibration sensor 226 is configured to determine modal vibration properties of the working machine 100 based on an input from the vibration sensor 226, wherein the suspension control unit 228 is further configured to control the valve arrangement 224 to control the pressure of the first and second hydraulic cylinder 210, 220 so that the determined modal vibrations are counteracted by the hydraulic cylinders 210, 220.

The suspension control unit 228 may include a microprocessor, microcontroller, programmable digital signal processor or another programmable device. The control unit 228 may also, or instead, include an application specific integrated circuit, a programmable gate array or programmable array logic, a programmable logic device, or a digital signal processor. Where the control unit 228 includes a programmable device such as the microprocessor, microcontroller or programmable digital signal processor mentioned above, the processor may further include computer executable code that controls operation of the programmable device. The suspension control unit 228 is connected to the various described features of the suspension system 200 and is configured to control the suspension system 200. Moreover, the control unit 228 may be embodied by one or more control units, where each control unit may be either a general purpose control unit or a dedicated control unit for performing a specific function.

Fig. 3 is a schematic illustration of parts of the described suspension system 200. In particular, Fig. 3 illustrates the hydraulic cylinders, 210, 220, the valve arrangement 224 and the pressure means 222. The pressure means 222 is here embodied by a hydraulic machine 226, which may be a motor but which also may be configured to act as a generator, and a hydraulic accumulator 306. In an example embodiment, hydraulic pressure is built up in the hydraulic accumulator 308 by the hydraulic machine 226. When a pressure is needed in either of the hydraulic cylinders 210, 220, the valve arrangement 224 is controlled so that the pressure can be supplied by the hydraulic accumulator 308. However, the pressure can also be provided directly from a hydraulic machine acting as a pump or via one or more accumulators. It is also possible that the required pressure is provided from other hydraulic systems present in the working machine. Accordingly, the pressure means should be understood to include any arrangement or device capable of providing sufficient pressure, in the present case hydraulic pressure, to the hydraulic cylinders 210, 220.

Fig. 3 further illustrates an example valve arrangement 224 comprising four hydraulic valves 308a-d. In the following, selected use cases will be described. It should however be noted that the described valve arrangement 224 and use cases are merely exemplifying and that many variations of the valve arrangement 224 are possible.

If all of the valves 308a-d are closed, the piston 302 of the first actuator 210 is connected to the piston 304 of the second actuator 220, and the rod side 303 of the first actuator 210 is connected to the rod side 305 of the second actuator 220. Accordingly, the piston sides 302, 304 of the two hydraulic cylinders 210, 220 are connected and the rod sides 303, 305 of the two hydraulic cylinders 210, 220 are connected.

If valves 308b and 308d are opened, hydraulic fluid is evacuated from the respective rod sides 303, 305 and hydraulic fluid is sent to the corresponding piston sides 302, 304 of the hydraulic cylinders 210, 220, meaning that the suspension system 200 will behave as a virtual rear axle acting to push the rear portion 103 of the working machine 100 upwards. If instead valves 308a and 308c are opened and valves 308b and 308d are closed, hydraulic fluid is evacuated from the respective piston sides 302, 304 and hydraulic fluid is sent to the corresponding rod sides 303, 305 of the hydraulic cylinders 210, 220, meaning that the suspension system 200 will behave as a virtual rear axle acting to lower the rear portion 103 of the working machine 100.

The suspension control unit 228 is configured to control the suspension system 200 to perform the steps of a control method as illustrated by the Flow chart of Fig. 4. The method for controlling the suspension system 200 comprises detecting S1 vibrations of the working machine 200, determining S2 modal vibration properties of the working machine 200 and controlling S3 the valve arrangement 224 to control the pressure of the first and second hydraulic cylinder 210, 220 so that the determined modal vibrations are counteracted by the hydraulic cylinders 210, 220.

Accordingly, the suspension system 200 is configured to actively control and change the pressure of the hydraulic cylinders 210, 220 in real time. It is possible to change the pressure for the two hydraulic cylinders 210, 220 in a manner simulating the functionality of a wheel axle, or it is possible to control the hydraulic cylinders 210, 220 individually to control the suspension of each wheel.

The method thus comprises detecting a movement, more particularly a pitching movement, of the working machine 100. A pitching movement is a rotational movement around a horizontal, lateral axis. I.e. the pitching movement is a rotational movement in a vertical plane oriented in parallel with the travel direction of the working machine 100. The pitching movement detection comprises the suspension control unit 228 receiving signals from the vibration sensor 226. Even more specifically, the detected movement is an oscillating pitching movement of the working machine 100. Such a movement may be triggered by a bump on the surface supporting the working machine 100, as it is moving ahead. Another trigger could be actuation, e.g. upon a driver command, of the lifting hydraulic cylinder 108 to move the equipment 1 1 1 as the working machine 100 is moving ahead. The oscillating pitching movement may thus cause a jerky ride which is uncomfortable for the driver, and which may also cause safety risks, e.g. due to a portion of a load 1 16 in the implement 107 falling out due to the movement. The oscillating pitching movement may be aggravated if the implement 107 carries a load 1 16.

The working machine 100 may have a relatively large pitching motion tendency on an uneven surface due to a relatively large inertia in the pitching direction combined with a relatively short wheel base, a lack of wheel suspension dampeners and large "springy" tires. If the implement 1 16 is loaded, the pitching inertia is increased, which further increases the pitching motion tendency. In addition, if the loaded implement is positioned so as to be outside of the wheel base of the working machine, the pitching oscillation may be further aggravated.

For identifying the oscillating pitching movement, the detection comprises processing the signals from the vibration sensor 226, to filter out frequencies higher than 0.5 Hz, preferably higher than 1 Hz, and lower than 4 Hz, preferably lower than 3 Hz, in this example around 2 Hz.

The bandpass filter for filtering out the vibrations at the modal frequency is advantageously provided as a digital filter where an analog sensing signal is first AD- converted before being filtered. However, it is in principle also possible to use analog filters to perform the described bandpass filtering. Next, a compensating PI (proportional integral) controller is used to control the valve arrangement 224 and hydraulic cylinders 210, 220 to maintain this filtered value to zero. If the modal vibrations are maintained to zero then there is no modal vibration in the working machine, and hence efficient damping of the working machine is achieved.

Accordingly, the purpose of the suspension system and associated control functionality is to control the pressure and oil flow in the hydraulic cylinders 210, 220 such that the filtered vibration value is zero, hence the frequency component of the pitching moment that is related to machine frequency is damped or completely nullified.

It is to be understood that the present invention is not limited to the embodiments described above and illustrated in the drawings; rather, the skilled person will recognize that many changes and modifications may be made within the scope of the appended claims.