The present invention relates to a load sensing system, comprising a first assembly of actuators for controlling a first hydraulic function, a pump adapted to supply said actuators with pressurized hydraulic fluid, and an electrically controlled valve adapted to control the output pressure of the pump via a hydraulic signal. In particular, the invention relates to a working machine comprising the system. A working machine in the form of a wheel loader has several different work functions which are hydraulically controlled, such as lifting and tilting of an implement and steering of the machine. As a rule, the actuators are constituted of linear motors in the form of hydraulic cylinders. The invention further relates to a method for controlling a hydraulic function.
Below, the invention will be described in connection with the operation 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.
Thus, the hydraulic system is load sensing. This means that the pump senses the pressure (a LS signal) from the activated hydraulic cylinders. The pump then sets a pressure which is a specific number of bar higher than the pressure of the cylinders. This brings about an oil flow out to the control cylinders, the level of which depends on the extent to which the activated control valve is operated. As a rule, the so called control pressure is fixed. The control pressure is the difference between the pump pressure and the pressure of the load. As a rule, the pump is adapted to set a fixed pressure which is 20-30 bar higher than the detected load pressure. Thereby, a level of the control pressure which is suitably balanced for different working positions is selected. There are also systems where the control pressure can be varied.
It is desirable to achieve a load sensing system which creates prerequisites for a more efficient operation with respect to energy consumption and which is reliable in operation. The invention particularly, according to an aspect thereof, seeks to achieve a more optimal system, where the control pressure can be adapted to different working positions or operating conditions.
According to an aspect of the present invention, a load sensing system comprises                a first assembly of actuators for controlling a first hydraulic function;        a pump adapted to supply said actuators with pressurized hydraulic fluid;        an electrically controlled valve adapted to control the output pressure of the pump via a hydraulic signal, characterized in that the system comprises        a first pressure sensor for detecting a load pressure of the first actuator assembly, and        a control unit adapted to receive a signal with information about the load pressure detected by the first pressure sensor and to generate a control signal, corresponding to the detected load pressure, to the electrically controlled valve.        
The first hydraulic function is preferably constituted of a lift function, but could be constituted of another function, such as the tilt or steering function.
By means of achieving a variable control pressure in this way, the pump can for example be at a low basic level and set on a pressure which is only 5-10 bar higher, that is to say as small a pressure increase as possible (the limit is defined by requirements on lubricating and cooling ability). If a higher control pressure is required, for example 30 bar, the electrically controlled pump control valve (LS valve) has to compensate for this. According to an example, the actuator (cylinder) has the pressure level 100 bar. The LS valve then sets the pressure level 125 bar and the pump itself adds an additional 5 bar, which means that the pump pressure becomes 130 bar in total. Thereby, lower drag losses (idling losses) can be obtained, thanks to low standby pressure. When no flow is required from the pump, its idles at for example 5 bar instead of 30 bar.
Furthermore, prerequisites for smaller control losses are created with lower output flows to the functions. The smaller the flow demanded by a function is, the lower a control pressure can be used, since the slide in a control valve for the function is opened more. If the operator demands 50% more flow to a function, the slide can be fully opened and the control pressure can be decreased, for example, from 30 bar down to 8 bar via the electrically controlled LS valve. In practice, this means that the larger the lever deflection being used is, the higher a control pressure will be used.
Furthermore, prerequisites for a powerful shake-out function are created. In certain situations, it is desired to be able to shake a function, for example shaking the bucket in order to shake out the contents properly. In these situations, shaking can be activated by means of a button or by moving the lever back and forth in a certain pattern. If the computer registers that the operator desires to perform shaking, the pump can be set to a higher pressure level via the electrical LS valve and thereby generate larger flow via a higher control pressure.
According to a preferred example, the electrically controlled pump control valve is adapted to assume such a position that the hydraulic signal to the pump generates a substantially constant pump pressure when the input signal to the valve from the control unit drops out. The constant pump pressure preferably constitutes a maximum pressure. This means that the hydraulic system then acts as a constant pressure system. Accordingly, the pump provides flow as required, but operates at maximum pressure all the time. Thus, the operator can continue with his/her work also in case of an electronics malfunction.
According to another preferred embodiment, the system comprises a steering function. As a rule, a valve unit in the form of an orbitrol unit is used for the steering function. According to prior art, there are problems to get the orbitrol steering stable, since resonances with respect to pressure fluctuations arise. This is amplified since the hydraulic LS signal from the orbitrol unit is also oscillating. By instead reading the LS signal of the orbitrol unit via pressure sensors and setting a higher LS pressure via the electrical LS valve, a stable LS signal to the pump can be obtained, which means a filtered signal.
According to another preferred example, the system comprises a position sensor for the actuator. In this way prerequisites for end position damping are created. If the system has a separate feed pump for a function, for example for the steering function, the electrical LS signal can be used for end position damping. This means that the control unit registers that the actuator (cylinder) is approaching the end position via positions sensors. The electrical LS valve can then lower the control pressure to a suitable level, so that the maximum steering rate is reduced, which means that the operator cannot exceed a certain steering rate.
Positions sensors also create prerequisites for a power control. In certain situations, it is desired to reduce the maximum possible hydraulic power output, for example because the engine does not have the power at low rpms. The electrically controlled functions can easily be limited, but the problem is the steering orbitrol (which is not electrically controlled). By means of electrical LS control, the maximum flow can be reduced by decreasing the control pressure. The hydraulic power can be calculated if the pressure level of the pump (pumps) and its/their flow and the efficiency of the system are known. For the orbitrol unit, the flow can be calculated by means of the computer reading the position of the steering cylinders in time via position sensors. If the flow is too high in this working position (depending on, amongst other things, the pressure level and other factors), a reduction, of the control pressure, to a suitable, acceptable maximum flow can be made. If a common pump is used for steering and working hydraulic, this is not a problem either. The working hydraulic can be reduced via its electrically controlled valves, and since the steering has higher priority than other functions, the computer can check how much power the operator demands via the steering (flow via the position sensor of the steering cylinder and pressure via pressure sensors). If this power level falls below the allowed power level, the remaining power can be used for the working hydraulic and limitation only takes place there. In this power level exceeds the allowed power level, the working hydraulic gets no power at all, which means that the pressure level of the pump is only dependent on the LS pressure of the steering and thereby reduction of maximum flow can be accomplished with a suitable LS pressure via the electrical LS valve. A certain degree of steering should always be available, wherein the lowest LS level is obtained directly from the orbitrol unit if the electrical LS signal is set at zero, that is to say the control pressure then becomes equal to the control pressure of the pump, which is at the level 5-10 bar. This also provides better security, since the LS signal for the steering never can be completely set at zero via the electrical LS valve.
According to another preferred example, the system comprises a plurality of assemblies of actuators, which are adapted to control different functions, and at least one pressure sensor associated with each of the actuator assemblies for detecting a load pressure of the respective assembly.
In this way, prerequisites for a reduction of pressure fluctuations in the system are created. The control unit registers pressure at different positions in the system. If the control unit registers an abnormal pressure oscillation, a change of the control pressure can be induced, with the purpose of moving away from the point of resonance of the system. If several functions are used simultaneously, the slides in the control valves can be opened or closed more, depending on whether the control pressure is increased or decreased, with the purpose of obtaining the same flow level. If the control pressure is at a low level when the resonance occurs, an increase of the control pressure can be made. If the control pressure already is high, a decrease with a certain allowed increment can be made. The temporary change will remain until a certain change with respect to pressure and flow occurs in the system.
It is desirable to achieve a method which, with respect to energy consumption, provides an efficient control of a load sensing system. In particular, the invention, according to an aspect thereof, aims at a method where the control pressure can be adapted to different working positions, or operating conditions.
According to an aspect of the present invention, a method comprises detecting a load pressure of an actuator, adapted to control a hydraulic function via a pressure sensor, and controlling a pump, adapted to supply said actuator with pressurized hydraulic fluid corresponding to the detected load pressure, via a hydraulic signal.
According to a preferred example, the method comprises the step of actuating an electrically controlled valve via an electrical signal corresponding to the detected load pressure, said valve controlling the output pressure of the pump correspondingly via said hydraulic signal. This creates prerequisites for a system reliable in operation, since the electrically controlled valve can be arranged so that the pump receives a hydraulic signal even if the input signal to the electrically controlled valve should drop out.
According to another preferred embodiment, the method comprises the steps of determining a desired pump pressure in proportion to (and usually at a level above) the detected load pressure, and controlling the pump correspondingly. Accordingly, the control pressure can be varied based upon different operating conditions. This is preferably done by means of detecting also an output pressure from the pump, and generating the control signal based also upon the detected output pump pressure.
According to another preferred example, the method comprises the steps of detecting the position of an operator-controlled element associated with said hydraulic function, and actuating a control valve arranged between the pump and the actuator based upon the detected position of the operator-controlled element. The method preferably comprises the step of coordinating the control of the pump and the actuation of the control valve. This creates prerequisites for lower control losses at smaller output flows to the functions. The lower the flow requested for a function is, the lower a control pressure can be used, since the slide in the control valve is opened more.
Further preferred embodiments of the invention and advantages associated therewith are apparent from the following description.