Abstract:
A method for determining whether an accumulator is filled to a predetermined level with a hydraulic fluid includes commanding a current to a solenoid, providing pressurized hydraulic fluid to the accumulator, setting a timer to an initial value, incrementing the timer, determining if the timer is greater than a timer threshold value, measuring a current to the solenoid if the timer is greater than the timer threshold value, calculating a modified current as a function of the measured current, the timer value, and the commanded current, comparing the modified current to a threshold, and determining that the accumulator is filled to the predetermined level if the modified current is greater than the threshold.

Description:
FIELD 
     The present disclosure relates to a system and method for determining an accumulator fill state, and more particularly to a system and method for determining an accumulator fill state using measured current in a solenoid. 
     BACKGROUND 
     The statements in this section merely provide background information related to the present disclosure and may or may not constitute prior art. 
     A typical automatic transmission includes a hydraulic control system that, among other functions, is employed to actuate a plurality of torque transmitting devices. These torque transmitting devices may be, for example, friction clutches and brakes. The conventional hydraulic control system typically includes a main pump that provides a pressurized fluid, such as oil, to a plurality of valves and solenoids within a valve body. The main pump is driven by the engine of the motor vehicle. The valves and solenoids are operable to direct the pressurized hydraulic fluid through a hydraulic fluid circuit to the plurality of torque transmitting devices within the transmission. The pressurized hydraulic fluid delivered to the torque transmitting devices is used to engage or disengage the devices in order to obtain different gear ratios. 
     In certain hydraulic control systems an accumulator is used to augment or in some cases replace the pump as the source of pressurized hydraulic fluid. Accordingly, the charge state of the accumulator is critical to properly controlling the transmission. While pressure sensors may be employed to determine the pressure of the hydraulic fluid within the accumulator, and therefore its charge state, there is room in the art for a method of determining the charge state of an accumulator that minimizes the use of additional components and sensors. 
     SUMMARY 
     A method for determining whether an accumulator is filled to a predetermined level with a hydraulic fluid is provided. The accumulator is in fluid communication with at least one solenoid. The method includes the steps of commanding a current to the solenoid, providing pressurized hydraulic fluid to the accumulator, setting a timer to an initial value, incrementing the timer, determining if the timer is greater than a timer threshold value, measuring a current to the solenoid if the timer is greater than the timer threshold value, calculating a modified current as a function of the measured current, the timer value, and the commanded current, comparing the modified current to a threshold, and determining that the accumulator is filled to the predetermined level if the modified current is greater than the threshold. 
     In one aspect, the method further includes the step of comprising filtering the measured current. 
     In another aspect, calculating the modified current includes calculating a rate of change of the measured current over time. 
     In yet another aspect, calculating the modified current includes calculating a magnitude of change in the measured current over time. 
     In yet another aspect, calculating the modified current includes calculating a running average of the measured current over time. 
     Further areas of applicability will become apparent from the description provided herein. It should be understood that the description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure. 
    
    
     
       DRAWINGS 
       The drawings described herein are for illustration purposes only and are not intended to limit the scope of the present disclosure in any way. 
         FIG. 1  is a schematic diagram of a portion of an exemplary hydraulic control system; 
         FIG. 2  is a schematic diagram of a portion of another exemplary hydraulic control system; and 
         FIG. 3  is a flow chart illustrating a method of operating the hydraulic control systems of  FIGS. 1 and 2  according to the principles of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     The following description is merely exemplary in nature and is not intended to limit the present disclosure, application, or uses. 
     With reference to  FIG. 1 , a portion of a hydraulic control system is generally indicated by reference number  10 . At the outset it should be appreciated that the portion of the hydraulic control system  10  shown in  FIG. 1  is exemplary and that other configurations may be employed. The hydraulic control system  10  is operable to selectively engage torque transmitting devices (not shown) and to provide cooling and lubrication to a transmission (not shown) by selectively communicating a hydraulic fluid  12  from a sump  14  to a hydraulic circuit  16 . The hydraulic fluid  12  is communicated to the hydraulic circuit  16  under pressure from either an engine driven pump  18  or an accumulator  20 . 
     The sump  14  is a tank or reservoir to which the hydraulic fluid  12  returns and collects from various components and regions of the transmission. The hydraulic fluid  12  is forced from the sump  14  and communicated throughout the hydraulic control system  10  via the pump  18 . The pump  18  may be, for example, a gear pump, a vane pump, a gerotor pump, or any other positive displacement pump. The pump  18  includes an inlet port  22  and an outlet port  24 . The inlet port  22  communicates with the sump  14  via a suction line  26 . The outlet port  24  communicates pressurized hydraulic fluid  12  to a supply line  30 . The supply line  30  is in communication with a spring biased blow-off safety valve  32 , an optional pressure side filter  34 , and an optional spring biased check valve  36 . The spring biased blow-off safety valve  32  communicates with the sump  14 . The spring biased blow-off safety valve  32  is set at a relatively high predetermined pressure and if the pressure of the hydraulic fluid  12  in the supply line  30  exceeds this pressure, the safety valve  32  opens momentarily to relieve and reduce the pressure of the hydraulic fluid  12 . The pressure side filter  34  is disposed in parallel with the spring biased check valve  36 . If the pressure side filter  34  becomes blocked or partially blocked, pressure within supply line  30  increases and opens the spring biased check valve  36  in order to allow the hydraulic fluid  12  to bypass the pressure side filter  34 . 
     The pressure side filter  34  and the spring biased check valve  36  each communicate with an outlet line  38 . The outlet line  38  is in communication with a second check valve  40 . The second check valve  40  is in communication with a main supply line  42  and is configured to maintain hydraulic pressure within the main supply line  42 . The main supply line  42  supplies pressurized hydraulic fluid to the hydraulic circuit  16  and a control device  46 . The control device  46  is operable to “actively” control whether the accumulator  20  is charged or discharged. For example, when the control device  46  is open, the accumulator  20  may charge or discharge based on the level of pressure supplied by the pump  18 . When the control device  46  is closed, the accumulator  20  remains in either a charged or discharged state. The control device  46  may be an on/off solenoid or a pressure or flow control solenoid. 
     The control device  46  is electrically controlled by a control module  48 . The control module  48  may be a transmission control module, an engine control module, or both, or any other type of controller or computer. The control module  48  is preferably an electronic control device having a preprogrammed digital computer or processor, control logic, memory used to store data, and at least one I/O peripheral for supplying a signal current to the control device  46 . 
     The control device  46  is in fluid communication with the accumulator  20 . The accumulator  20  is an energy storage device in which the non-compressible hydraulic fluid  12  is held under pressure by an external source. In the example provided, the accumulator  20  is a spring type or gas filled type accumulator having a spring or compressible gas that provides a compressive force on the hydraulic fluid  12  within the accumulator  20 . However, it should be appreciated that the accumulator  20  may be of other types, such as a gas-charged type, without departing from the scope of the present invention. Accordingly, the accumulator  20  is operable to supply pressurized hydraulic fluid  12  back to the main supply line  42 . However, upon discharge of the accumulator  20 , the second check valve  40  prevents the pressurized hydraulic fluid  12  from returning to the pump  18 . The accumulator  20 , when charged, effectively replaces the pump  18  as the source of pressurized hydraulic fluid  12 , thereby eliminating the need for the pump  18  to run continuously. 
     Turning briefly to  FIG. 2 , a portion of an alternate hydraulic control system is indicated generally by reference number  10 ′. The hydraulic control system  10 ′ is similar to the hydraulic control system  10  shown in  FIG. 1  and like components are indicated by like reference numbers. However, the hydraulic control system  10 ′ is configured to “passively” charge the accumulator  20  rather than “actively” charge the accumulator  20 . For example, a supply line  50  communicates from the main supply line  42  to a third check valve  52  disposed in parallel with the control device  46 . The third check valve  52  is in communication with the accumulator  20  and is configured to maintain hydraulic pressure between the main supply line  42  and the accumulator  20 . The accumulator  20  is charged when pressure from the pump  18  exceeds the bias of the third check valve  52 . Discharge of the accumulator  20  occurs when the control device  46  is opened. 
     With reference to  FIG. 3 , and with continued reference to  FIGS. 1 and 2 , a method  100  for determining the charge or fill state of the accumulator  20  will now be described. The method  100  begins at step  102  where a current is supplied to the solenoid  46 . In the configuration of “active” accumulator fill, the current supplied to the solenoid  46  is sufficient to open the solenoid  46 . In the configuration of “passive” accumulator fill, the current supplied to the solenoid  46  is not sufficient to open the solenoid  46 . At step  104  pressurized hydraulic fluid  12  is supplied to the accumulator  20  by the pump  18 . The hydraulic fluid  12  is sufficiently pressurized to begin charging the accumulator  20 . 
     Once hydraulic fluid  12  has been provided to the accumulator  20 , a timer value is initialized at a reference value, such as zero, indicated at step  106 . At step  108  the timer value is incremented. At step  110  the timer value is compared to a timer threshold value. The timer threshold value is a predefined value representative of a minimum amount of time that should pass while charging of the accumulator  20  occurs. The timer threshold value may be a function of a temperature of the hydraulic fluid or proportional to the size of the accumulator  20 . If the timer value is less than the timer threshold value, the method  100  returns to step  108  where the timer value is incremented. If the timer value is greater than the timer threshold value, then the method  100  proceeds to step  112 . 
     At step  112  the current supplied to the solenoid  46  is measured by the controller  48 . Since the current commanded by the controller  48  has not changed, any change in the measured current is representative of a change in the forces acting on the solenoid  46 . For example, as fluid pressure between the solenoid  46  and the accumulator  20  increases, the change in pressure  20  induces a change in the current in the solenoid  46  due to the pressure forces acting on the armature or valve of the solenoid  46 . 
     At step  114  the controller  48  may optionally apply a filter to the measured current. This filter may be a low-pass, band-pass, high-pass, or other filter applied to the measured current in order to help detection by eliminating any unwanted or interfering frequency signals in the measured current. 
     At step  116  the characteristics in measured current over time and with respect to the commanded current is modified using one or more of several methods. In one example, the controller  48  calculates a derivative value of the measured current (i.e. the rate of change of the current). In another example, the controller  48  calculates a magnitude of the change of the measured current over time. In yet another example, the controller  48  calculates a running average of the measured current over time. 
     Once the measured current has been modified, the method  100  proceeds to step  118  where the modified current is compared to a threshold value. The value of the threshold value is dependent on the selected method of quantifying the measured current. Generally, however, the threshold value is determined based on learned behavior of the pressure of the hydraulic fluid acting on the solenoid  46  and the change in measured current induced by the hydraulic fluid. If the modified current is less than the threshold value, the method returns to step  112  and repeats. If, however, the modified current is greater than the threshold value, then the method  100  proceeds to step  120  where the controller  48  determines that the accumulator  20  is charged and the method  100  ends. 
     The description of the invention is merely exemplary in nature and variations that do not depart from the gist of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention.