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BACKGROUND AND SUMMARY 
       [0001]    The present invention relates to a method for automatically limiting a pressure generated during operation in a hydraulic system when needed, said system being adapted to deliver a pressurized hydraulic fluid to at least one actuator adapted to perform a work function. 
         [0002]    Below, the invention will be described in connection with a working machine in the form of a wheel loader. This is a preferred, but by no means limiting application of the invention. The invention can for example also be used for other types of working machines (or work vehicles), such as a backhoe loader, an excavator, or an agricultural machine such as a tractor. 
         [0003]    A wheel loader can be utilised for a number of fields of activity, such as lifting and transportation of rock and gravel, loading pallets and logs. In each of these activities, different equipment is used, including implements in the form of a bucket, a fork implement and gripping arms. More particularly, the equipment comprises a load-arm unit, or boom, which is pivotally arranged relative to the wheel loader frame. Two actuators in the form of hydraulic cylinders are arranged between the frame and the load-arm unit in order to achieve a lifting and lowering movement of the load-arm unit. The implement is pivotally arranged on the load-arm unit. An additional actuator in the form of a hydraulic cylinder is arranged between the implement and the load-arm unit in order to achieve a tilting movement of the implement. 
         [0004]    The hydraulic system comprises a pump adapted to supply the hydraulic cylinders with pressurized hydraulic fluid via a hydraulic circuit comprising a plurality of control valves. 
         [0005]    As a rule, a wheel loader has more hydraulic functions than the above-mentioned lift and tilt function. Such additional hydraulic functions include steering, 3rd, 4th, and in some cases even more functions. Each function generally needs two shock valves, except lift which has one shock valve. For a machine with a 3rd and a 4th function, this implies nine shock valves. 
         [0006]    Different functions require different flow rates. Furthermore, the same function requires different flow rates for piston and piston rod side. Machines of different sizes also have different flow rate requirements. In practice, only a few shock valves are used, where the one having the highest flow requirement decides the flow rate. This implies that most functions have unnecessarily large shock valves. 
         [0007]    It is desirable to achieve a method which creates prerequisites for a more cost efficient system with maintained or improved service life. 
         [0008]    A method according to an aspect of the present invention includes
       detecting a pressure in at least one position in the system;—comparing the detected pressure value, or a value associated with the detected pressure value, with a first predetermined limit value; and   opening a flow communication between the actuator and a tank via a first conduit if the detected pressure value, or the value associated with the detected pressure value, exceeds the predetermined limit value.       
 
         [0011]    Thus, in this way, drainage to tank is actively controlled when needed. Preferably, at least one pressure sensor is adapted to detect the pressure to the respective function. 
         [0012]    In this way, the limit value (for example the opening pressure) can be set as low as possible in all situations, which results in a smaller load on the constituent components. 
         [0013]    According to a preferred embodiment, the flow communication is opened via a control valve being arranged on the first conduit and having the function to control the supply of the hydraulic fluid to and from, respectively, the actuator with the object of performing the work function. In case of an unexpected pressure increase, this control valve functions as a controlled shock valve. Preferably, separate inlet and outlet valves to the actuator are provided in order to control the function (for example a lifting and lowering movement). 
         [0014]    According to another preferred embodiment, the method further comprises the step of opening a flow communication between the actuator and the tank via a second conduit via a shock valve. The shock valve is also called pressure limiting valve. The shock valve is preferably arranged in a conventional way as a passive (directly controlled by the pressure), for example spring-loaded, shock valve. By means of combining the opening of the control valve and the shock valve, drainage to tank at a desired rate can be obtained in case of a pressure shock. 
         [0015]    Owing to the smaller size of the possibly included directly controlled shock valves and to fewer variants, a lower cost can be achieved. Furthermore, owing to the smaller directly controlled shock valves, the valve housing can be made smaller. 
         [0016]    As a rule, the control valve opens more slowly than the shock valve, which in many cases implies that said flow communication between the actuator and the tank via the first conduit is opened after the shock valve has opened the flow communication between the actuator and the tank via the second conduit. In other words, the control valve is opened with a certain delay, so that the shock valve is opened first. It is possible, however, to ensure that the control valve opens substantially simultaneously as, or before the shock valve. 
         [0017]    Preferably, a shock valve of a smaller size, i.e. with a lower nominal flow rate, than the electrically controlled outlet valve is used. The directly controlled shock valve, which is fast-acting, opens directly and flow drainage is initiated. Then, the electrically controlled control valve, which is capable of handling the larger flow requirement and draining it to tank, is opened. 
         [0018]    According to another preferred embodiment, the method comprises the step of determining the flow rate to the tank on the basis of the detected pressure. In this way, the characteristics of the shock control function can be determined. The opening degree of the control valve is controlled, for example, on the basis of the pressure change in the actuator. 
         [0019]    Further preferred embodiments of the invention and advantages associated therewith are apparent from the remaining claims and the following description. 
     
    
     
       BRIEF DESCRIPTION OF FIGURES 
         [0020]    The invention will be described more closely in the following, with reference to the embodiments shown in the attached drawings, wherein 
           [0021]      FIG. 1  shows a side view of a wheel loader, and 
           [0022]      FIG. 2  shows a system for performing the method during operation of the wheel loader. 
       
    
    
     DETAILED DESCRIPTION 
       [0023]      FIG. 1  shows a side view of a wheel loader  101 . The wheel loader  101  comprises a front vehicle section  102  and a rear vehicle section  103 , said sections each comprising a frame and a pair of drive shafts  112 ,  113 . The rear vehicle section  103  comprises a driver&#39;s cab  114 . The vehicle sections  102 ,  103  are connected to each other in such a way that they can be pivoted relative to each other about a vertical axis by means of two actuators in the form of hydraulic cylinders  104 ,  105  which are connected to the two sections. Accordingly, the hydraulic cylinders  104 ,  105  are disposed on different sides of a centre line in the longitudinal direction of the vehicle for steering, or turning the wheel loader  101 . 
         [0024]    The wheel loader  101  comprises an equipment  111  for handling objects or material. The equipment  111  comprises a load-arm unit  106  and an implement  107  in the form of a bucket which is fitted on the load-arm unit. Here, the bucket  107  is filled with material  116 . A first end of the load-arm unit  106  is pivotally connected to the front vehicle section  102  in order to achieve a lifting movement of the bucket. The bucket  107  is pivotally connected to a second end of the load-arm unit  106  in order to achieve a tilting movement of the bucket. 
         [0025]    The load-arm unit  106  can be raised and lowered relative to the front section  102  of the vehicle by means of two actuators in the form of hydraulic cylinders  108 ,  109 , each of which is 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 a third actuator (hydraulic cylinder)  110 , 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. 
         [0026]    A first embodiment of the system is shown in  FIG. 2 . The system  201  comprises a pump  205  adapted to supply the hydraulic cylinders with pressurized hydraulic fluid via a hydraulic circuit. The pump  205  is driven by the vehicles propulsion engine  206 , in the form of a diesel engine. The pump  205  has a variable displacement. The pump  205  is preferably adapted for infinitely variable control. The system  201  comprises a valve device  208  (se the dash-dotted line) which comprises a hydraulic circuit having a plurality of control valves for controlling the lift and tilt function. 
         [0027]    Two control valves, in the form of flow valves,  207 ,  209 , are arranged between the pump  205  and the lift cylinders  108 ,  109  in the circuit in order to control the lifting and lowering movement. While a first one of these valves  207  is arranged to connect the pump  205  to the piston side, a second one of these valves  209  is arranged to connect a tank  243  to the piston rod side. Furthermore, the first valve  207  is arranged to connect the tank  243  to the piston side and the second valve  208  is arranged, correspondingly, to connect the pump  205  to the piston rod side. This offers large possibilities for varying the control. In particular, it is not necessary to connect the pump and tank simultaneously to the function. 
         [0028]    The system  201  further comprises a control unit  213 , or computer, which contains software for controlling the functions. The control unit is also called a CPU (central processing unit) or ECM (electronic control module). The control unit  213  suitably comprises a microprocessor. 
         [0029]    An operator-controlled element  211 , in the form of a lifting lever, is operatively connected to the control unit  213 . The control unit  213  is adapted to receive control signals from the control lever and to actuate the control valves  207 ,  209  correspondingly (via a valve control unit  215 ). The control unit  213  preferably controls more general control strategies and the control unit  215  controls basic functions of the valve unit  208 . Naturally, the control units  213 ,  215  can also be integrated into a single unit. When controlling the pump  205 , there is an oil flow out to the cylinders  108 ,  109 , the level of which depends on the extent to which the actuated valves  207 ,  209  are opened. 
         [0030]    An operator-controlled element  219 , in the form of a steering-wheel, is hydraulically connected to the steering cylinders  104 ,  105 , via a valve unit in the form of an orbitrol unit  220 , for direct-control thereof. 
         [0031]    Similarly as for the lift function, two control valves  223 ,  225  are arranged between the pump  205  and the tilt cylinder  100  for controlling the forward and return movement of the implement relative to the load-arm unit. An operator-controlled element  227 , in the form of tilt lever, is operatively connected to the control unit  213 . The control unit  213  is adapted to receive control signals from the tilt lever and to actuate the control valves  223 ,  225  correspondingly. 
         [0032]    A prioritizing valve  220  is arranged at the outlet conduit  245  from the pump in order to automatically prioritize that the steering function receives the required pressure before the lift function (and the tilt function). 
         [0033]    The system  201  is load-sensing and comprises, for this purpose, a plurality of pressure sensors  229 ,  231 ,  233 ,  235 ,  237  for detecting load pressures of each of said functions. The lift function of the system comprises two pressure sensors  229 ,  231 , out which one is arranged on a conduit to the piston side of the lift cylinders and the other on a conduit to the piston rod side of the lift cylinders. In a corresponding way, the tilt function of the system comprises two pressure sensors  235 ,  237 , out of which one is arranged on a conduit to the piston rod side of the tilt cylinder and the other on a conduit to the piston side of the tilt cylinder. The steering function comprises a pressure sensor  233  on a conduit connected to the steering cylinders  104 ,  105 . More precisely, the pressure sensor  233  is situated on the LS-conduit which receives the same pressure as on one cylinder side when steering in one direction and as on the other cylinder side when steering in the other direction. In neutral, the LS-conduit is connected to tank. 
         [0034]    The system further comprises an electrically controlled valve  241  adapted to control the output pressure of the pump via a hydraulic signal. The system  201  comprises an additional pressure sensor  239  for detecting a pressure which is indicative of an output pressure from the pump. More precisely, the pressure sensor  239  is adapted to detect the pressure in a position downstream the electrically controlled valve  241 . Accordingly, the pressure sensor  239  senses the pump pressure directly when the valve  241  is fully open. In normal driving conditions, the pressure sensor  239  detects the output pressure from the valve  241 . Accordingly, the control unit  213  is adapted to receive a signal from the pump pressure sensor  239  with information about of the pressure level. 
         [0035]    Accordingly, the control unit  213  receives electrical signals from the pressure sensors  229 ,  231 ,  233 ,  235 ,  237 ,  239  and generates an electrical signal for controlling the electrical valve  241 . 
         [0036]    As previously stated, the control unit  213  is adapted to receive signals from the control levers  211 ,  227 . When the operator wants to lift the bucket, the lift lever  211  is operated. The control unit receives a corresponding signal from the lift lever  211  and actuates the control valves  207 ,  209  to such a position that the pump is connected to the piston side of the lift cylinders  108 ,  109  and the piston rod side of the lift cylinders is connected to the tank  243 . Furthermore, the control unit receives signals from the load pressure sensor  229  on the piston side of the lift cylinders and from the pressure sensor  239  downstream the pump. Based upon the received signals, a desired pump pressure at a level above the detected load pressure is determined, and the electrically controlled pump control valve  241  is actuated correspondingly. 
         [0037]    The control unit  213  is preferably adapted to coordinate the opening degree of the control valves  207 ,  209  and the output pressure of the pump  205  for optimum operation. 
         [0038]    The tilt function is controlled in a corresponding manner as the lift function. When steering the machine, the pressure sensor  233  of the steering function detects a load pressure of the steering and generates a corresponding load signal. The control unit  213  receives this load signal and a signal from the pressure sensor  239  on the outlet conduit of the electrically controlled valve  241 . Based upon the received signals, a desired pump pressure at a level above the detected load pressure is determined, and the electrically controlled pump control valve  241  is actuated correspondingly. 
         [0039]    When several functions are used simultaneously, the detected load pressures are compared and the pump  205  is controlled corresponding to the highest of the detected load pressures. 
         [0040]    Accordingly, the electrically controlled pump control valve  241  is adapted to be infinitely adjustable between two end positions, a first end position which corresponds to the pump producing a minimum pressure and a second end position which corresponds to the pump producing a maximum pressure. 
         [0041]    A hydraulic means  253 , in the form of a reversing valve, is arranged on a conduit  251  between the electrically controlled pump control valve  241  and the pump. The reversing valve  253  is adapted to receive the hydraulic signals from the steering function and the pump control valve  241 . 
         [0042]    Furthermore, the reversing valve is adapted to control the pump  205  corresponding to the received signal having the largest load pressure. Accordingly, the hydraulic means (reversing valve)  253  selects the higher pressure in an output signal made up of two input pressure signals. 
         [0043]    The system further comprises a sensor  255  for detecting lift cylinder position. The sensor  255  is operatively connected to the control unit  213 . In this way, the control unit  213  can decide whether a lifting or lowering movement of the load is performed. 
         [0044]    The system  201  further comprises a number of shock valves  261 ,  263 ,  367 , for the lift function and the tilt function, for draining hydraulic fluid to the tank  243  in case of a strong pressure increase. The lift function of the system comprises a shock valve  261  which is arranged on a conduit  273  to the piston side of the lift cylinders. The tilt function of the system comprises two shock valves  263 ,  267 , out of which one  263  is arranged on a conduit  277  to the piston rod side of the tilt cylinder and the other  267  on a conduit  279  to the piston side of the tilt cylinder. 
         [0045]    Below, a method for automatically limiting a pressure generated during operation in the system when needed is described in a few different examples. The method is described with respect to the lift function, but the corresponding also applies to, for example, the tilt function. 
         [0046]    An external force initiates a movement of the hydraulic cylinders  108 ,  109 . The control unit  213  detects that the pressure exceeds a certain first level (for example 350 bar) via the pressure sensor  229 . The control unit  213  then emits a signal to the outlet valve  207  to drain oil to the tank  243  via a first conduit  271 . Accordingly, the outlet valve  207  acts like a shock valve by means of software control. The directly controlled shock valve  261  opens when the pressure exceeds a certain second, predetermined level (for example 360 bar) and initiates draining of flow to the tank  243  via a second conduit  273 . The electrically controlled outlet valve  207  now has had time to open for a larger drainage flow to the tank  243 . The pressure, which is recorded continuously, drops and the electrically controlled outlet valve  207  and the directly controlled shock valve  261  close at specific pressure levels. 
         [0047]    The first level can be equal to the second level, but preferably the first level is smaller than the second level. This in order to obtain a substantially simultaneous, or earlier, opening of the control valve relative to the shock valve. 
         [0048]    As a supplement or an alternative to the foregoing, the electrically controlled outlet valve  207  is controlled on the basis of the pressure derivative (in order to obtain faster opening of the electrically controlled outlet valve  207 ). For example, the control valve is controlled to serve as a shock valve as soon as the pressure derivative in the cylinder  108 ,  109  exceeds a certain level, irrespective of whether the pressure level is low. If an external force initiates a movement of the cylinder, the control valve will initiate its opening procedure before the pressure level reaches the upper limit (for example 350 bar). If the upper limit is not reached, the control valve will still close when the pressure derivative falls short of a certain level. 
         [0049]    According to a further variant, the electrically controlled shock valve  207  has a variable opening pressure. Preferably, the pressure level is set depending upon the operating condition (such as load-arm position and/or bucket position). The directly controlled shock valve  261  is then set to open only at the maximum pressure level. In certain situations, a large shock resistance is needed, for example when the bucket is pushed into a material pile with maximum propulsion, and in other situations, the shock function can open at a lower pressure. This means that the machine/iron is subjected to less stress. 
         [0050]    The opening pressure of the electrically controlled valve  207  is, for example, dependent on the following operating parameters: 
         [0051]    Cylinder positions for different functions. For example, when the bucket is pushed with maximum propulsion into the material pile (when the unit is lowered and the bucket is in a level position) an exceptionally high resistance is needed on the piston side of the lift cylinder. 
         [0052]    Type of implement. Implements which are not influenced by the propulsion (for example a pallet fork assembly) do not need as high an opening pressure as a bucket. 
         [0053]    Type of handling. One handling example is loading timber onto a truck. Another example is bucket handling for loading gravel/rocks. Furthermore, it is conceivable to use the same implement, for example a bucket, for different handling operations. Accordingly, type of handling can be independent of type of implement. 
         [0054]    According to one example, the system is adaptive. The control unit can then record how the wheel loader is operated during a certain period of time through detecting operating parameters and concluding which handling operation is performed and/or which implement type is used. Alternatively, or as a supplement, the limit value is determined on the basis of a signal from an operator-controlled element, such as a lever, button, or another control means in the cab. 
         [0055]    Machine speed. At high machine speeds, it is safer if the opening pressures of the shock valves are at a higher level. 
         [0056]    According to a further variant, the electrically controlled valve  207  has different pressure drops for the same flow rate, wherein the pressure drop is dependent on the following:—the function concerned and/or—the cylinder position. When subjected to shock loading with the load-arm in a high position, it is not desirable that the unit falls to the ground, but is lowered at a controlled speed. With this system all functions and all machine sizes can have the same shock characteristics, that is to say, when the shock function opens, the same degree of resistance can be felt irrespective of the type of machine concerned. 
         [0057]    Furthermore, an adaptive shock control on the basis of a pressure level can be utilized. The basic idea is to have as low an opening pressure as possible, with the purpose of “sparing” the machine. The machines which are handled most aggressively are the ones which to a great extent decide the opening levels. Therefore, according to a further variant, an adaptive opening pressure is introduced. Thereby, most of the machines can be at lower levels and the machines which require higher levels will also get such levels. The idea is that the control unit  213  records the extent of shock loading which occurs. If this exceeds a certain level, the opening pressure for the electrically controlled shock valve  207  is temporarily increased within certain limits. The opening pressure can be a function of all or certain of the following: shock loading frequency, shock loading time, shock loading time expressed as a percentage of total machine time (with diesel engine running) and/or shock loading time expressed as a percentage of total active time for the function concerned. 
         [0058]    Similar adaptive action can also occur when the electrically controlled shock valve  207  opens at a certain pressure derivative. The pressure derivative limit can be adjusted depending upon how often/much the electrically controlled shock valve  207  opens as a result of the pressure derivative. The same function dependent parameters as described above can be used, but where, as mentioned before, only those cases where the shock loading control occurs as a result of the pressure derivative are taken into consideration. 
         [0059]    The invention should not be regarded as limited to the above-described exemplary embodiments, but a number of further variants and modifications are conceivable within the scope of the following claims. In particular, the preferred embodiments can be combined in a number of different ways.

Summary:
A method is provided for, when necessary, automatically limiting a pressure in a hydraulic system during operation. A system is arranged to deliver pressurized hydraulic fluid to at least one fluid actuated device arranged to perform a work function, where the procedure includes sensing a pressure in at least one position of the system, comparing the detected pressure value, or an associated value, with a first predefined pressure limit, and opening a communication of fluid between the fluid actuated device and a reservoir through a first conduit if the sensed pressure value, or an associated value, exceeds the predefined limit.