Patent Publication Number: US-9897197-B2

Title: Valve system for controlling a system pressure control system

Description:
FIELD OF THE INVENTION 
     This invention relates to a valve system for controlling a system pressure control system, in particular a hydraulic control unit of an automatic transmission. 
     BACKGROUND 
     Hydraulic systems, in particular in hydraulic control units of automatic transmissions, typically use a pilot-operated pressure relief valve, hereinafter referred to as system pressure valve, in order to adjust the system pressure in such a manner that the power consumption of the pump of the transmission is not unnecessarily high, and/or that the actuating elements are supplied with at least the amount of pressure that ensures safe operation of the system. 
     In order to adjust the system pressure, this is typically controlled by a pilot valve in such a manner that it may be continuously adjusted between a minimum pressure that is at least necessary for operation and a maximum pressure that is necessary for safe insertion and for full power transmission of the shift elements. 
     In hydraulic control units of automatic transmissions, the system pressure range may be varied in such a manner that the maximum and minimum system pressure are found in a very high relationship with each other, by which the valve reinforcement of the system pressure valve between the system pressure and the pilot pressure valve takes on a value of &gt;5. In the low pressure range, this leads in particular to position inaccuracies in a negative manner, which may result in an increased pump absorption torque and thus increased consumption. 
     SUMMARY OF THE INVENTION 
     This invention is subject to a task of specifying a valve system for controlling a system pressure control system, in particular a hydraulic control unit of an automatic transmission, through the use of which the position inaccuracies that arise in the lower pressure range under the state of the art are largely avoided. Additional objects and advantages of the invention will be set forth in part in the following description, or may be obvious from the description, or may be learned through practice of the invention. 
     In particular, a high resolution between system pressure and pilot pressure is to be achieved in the low pressure range. 
     These tasks are solved by the characteristics of the embodiments of a valve system claimed herein. Additional arrangements and advantages in accordance with the invention emerge from the following description. 
     Accordingly, what is proposed is a valve system for controlling a system pressure control system, in particular a hydraulic control unit of an automatic transmission, comprising a multi-stage system pressure valve, a pilot valve and a control valve with a control piston downstream from the pilot valve, which governs the pressure at a differential area of the system pressure valve, depending on the pilot pressure. 
     Thereby, the system pressure may be controlled in such a manner that, in a low pressure range with low valve reinforcement of the system pressure, there is a high resolution between system pressure and pilot pressure, whereas, in a second, higher pressure range of the system pressure, a low valve resolution is achieved, which is used to achieve the maximum system pressure. 
     In accordance with the invention, the switching of the system pressure ranges takes place without a break in the pilot pressure/system pressure characteristic curve, but on the basis of a sudden change to the dependency of the change to the system pressure as a function of the change to the pilot pressure in a desired pressure range. 
     This is achieved by the fact that the control valve corresponds to a pressure-reducing valve with inverted characteristics, which at low system pressure adjusts a control pressure, which is higher than the supply pressure at the control valve, by which the control valve is located in a fully open position, which produces a direct pressure connection between system pressure and the system pressure valve. 
     Upon an increase in the pilot pressure, the supply pressure, and at the same time the control pressure of the control valve, increase continuously. At the moment at which the control pressure of the control valve and the supply pressure are the same, the control valve proceeds into a modulation position, such that, upon a further increase in the pilot pressure, the control pressure is lower than the system pressure. 
     The concurrent increase in the pilot pressure and the reduction in the control pressure at the system pressure valve, each acting in an opposite direction, lead to a split in the system pressure characteristics. 
     Thus, in the lower pressure range of the pilot pressure (for example, between 0-2.5 bar), a good pressure position accuracy of the system pressure (for example, 3.5-10 bar) can be achieved, while, in the higher pressure range of the pilot pressure (for example, between 2.5-5 bar), the system pressure may vary in the range between 10-40 bar. In this high pressure range, high requirements for position accuracy, like those in the lower pressure range, are not required. 
     This means that the pilot pressure/system pressure characteristic curve features a continuous development, and that, from a predefined pilot pressure, on the basis of a sudden change to the dependency of the change to the system pressure as a function of the change to the pilot pressure, the gradient of the system pressure as a function of the pilot pressure is significantly steeper than that prior to the predefined pilot pressure. 
     Within the framework of a further variant of the invention, the control valve has an increasing pilot pressure/control pressure characteristic curve, and, from a predefined pilot pressure, for example from a pressure of 2.5 bar, acts on a differential area of the system pressure valve, which additionally affects the pilot pressure. Below the predefined pilot pressure, for example below 2.5 bar, the control pressure is constant and does not additionally affect the pilot pressure. Above the predefined pilot pressure, the control pressure increases in a manner clearly steeper than that in the low pilot pressure range. 
     In addition, within the framework of an additional form of the invention, it can be provided that the even system pressure range is controlled in the high pilot pressure range. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the following, the invention is more specifically illustrated as an example on the basis of the attached figures. The following are represented in these: 
         FIG. 1 : a schematic view of an embodiment of the valve system according to the invention for controlling a system pressure control system; and 
         FIG. 2 : a diagram for illustrating the pilot pressure/system pressure characteristic curve and the pilot pressure/control pressure characteristic curve in accordance with one embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made to embodiments of the invention, one or more examples of which are shown in the drawings. Each embodiment is provided by way of explanation of the invention, and not as a limitation of the invention. For example features illustrated or described as part of one embodiment can be combined with another embodiment to yield still another embodiment. It is intended that the present invention include these and other modifications and variations to the embodiments described herein. 
     In accordance with the invention, and with reference to  FIG. 1 , the valve system in accordance with the invention for controlling a system pressure control system comprises, in particular, a hydraulic control unit of an automatic transmission of a multi-stage system pressure valve  1 , a pilot valve (labeled in  FIG. 1 ) and a control valve  2  with a control piston  3  downstream from the pilot valve, which governs the pressure at a differential area “Adiffpregel” of the system pressure valve  1 , depending on the pilot pressure (pEDS). 
     The system pressure valve features a channel for the system pressure “psys”, a channel for the secondary circuit pressure “psek”, through which excess oil is led to the secondary circuit, and a channel through which, if the excess oil exceeds a predefined pressure, the excess oil is led with the pressure “psaug” to the suction side of the pump. A channel for the control pressure “pregel” is also provided. 
     The control valve  2  is structured as a ventilated 3/2-way valve and, in accordance with the invention, corresponds to a pressure-reducing valve with an inverted characteristic, which at low system pressure adjusts a control pressure, which is higher than the supply pressure at the control valve  2 , by which the control valve  2  is located in a fully open position, which produces a direct pressure connection between system pressure and the system pressure valve, whereas, upon an increase in the pilot pressure, the supply pressure, and at the same time the control pressure of the control valve  2 , increase continuously. In this lower pressure range of the pilot pressure, a high resolution between the system pressure and the pilot pressure, thus a good pressure position accuracy of the system pressure, can be achieved, as can be seen in  FIG. 2 . Thereby, this curve “A” represents the development of the control pressure as a function of the pilot pressure, whereas curve “B” represents the development of the system pressure as a function of the pilot pressure. 
     At the moment at which the control pressure of the control valve  2  and the supply pressure are the same, the control valve  2  proceeds into a modulation position, such that, upon a further increase in the pilot pressure, the control pressure is lower than the system pressure. The concurrent increase in the pilot pressure and the reduction in the control pressure at the system pressure valve, each acting in an opposite direction, lead to a split in the system pressure characteristics as a function of the pilot pressure. 
     The moment at which the control pressure of the control valve  2  and the supply pressure are equal is designated in  FIG. 2  as “t_1”. As is evident in  FIG. 2 , a continuous pilot pressure/system pressure characteristic curve arises, which features a bend at point in time t_1. At this point in time, the gradient of the system pressure as a function of the pilot pressure is considerably steeper than that prior to this point in time. 
     Through the concept according to the invention, in the higher pressure range of the pilot pressure, i.e. from the point in time t_1, a low valve resolution is achieved, whereas the system pressure in the range of 10-40 bar is varied and can achieve the maximum value. 
     In an advantageous embodiment, the switching of the system pressure ranges takes place without a break in the pilot pressure/system pressure characteristic curve, on the basis of a sudden change to the dependency of the change to the system pressure as a function of the change to the pilot pressure in a desired pressure range. 
     Modifications and variations can be made to the embodiments illustrated or described herein without departing from the scope and spirit of the invention as set forth in the appended claims.