Patent Publication Number: US-11047402-B2

Title: Electrically controllable hydraulic system for a vehicle transmission and method for controlling the same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is the U.S. National Stage of PCT/EP2018/072364, filed Aug. 17, 2018, which claims priority to Belgium Application No. 2017/5570, filed Aug. 18, 2017, the contents of each of which is incorporated herein by reference in its entirety. 
     BACKGROUND 
     The present invention pertains to an electrically controllable hydraulic system for a vehicle transmission. 
     The present invention further pertains to a method of electrically controlling a hydraulic system for a vehicle transmission. 
     A vehicle transmission for coupling and transmitting engine power to wheels of the vehicle may comprise a plurality of transmission elements, e.g., clutch elements and torque control elements that need to be controllably actuated within a continuous actuation range. Another example of a transmission element is a friction element that serves to clamp a flexible element like a chain or a belt in a variable transmission system. In an electrically controlled hydraulic system for a vehicle transmission the transmission elements are actuated by a hydraulic pressure which is controlled by electrically controlled hydraulic pressure controlling modules comprising a hydraulic valve element and an electromagnetically controllable operating element, such as a solenoid, for operating the hydraulic valve element. In practice a controlled continuous actuation may be complicated by friction occurring in these elements. In particular small changes in actuations may be difficult to achieve in that a change in hydraulic force required to achieve a small change may not be sufficient to outweigh a static friction within a pressure controlling module. An overshoot easily occurs once the electromagnetic force is strong enough to overcome the static friction. 
     A known way to address this complication is to add a low-frequency component, also denoted as dither, that causes the pressure controlling module to oscillate in accordance with the frequency of this dither component, therewith attenuating the effect of static friction within the pressure controlling module. 
     An example of an electronic controller having a hysteric drive mode providing a dither signal is the MC33816: SD6 Programmable Driver for Solenoid Control available from NXP. 
     The controller is arranged to independently control a respective one pair of switching elements arranged in a bridge circuit driving the load. In a first operational mode both switching elements are conductive to drive the load from the power supply terminals Vboost and Ground. In a second operational mode only a first one of the switching elements is conductive, so that a freewheel current occurs through the first conductive switching element and the freewheel diode attached to the same power supply terminal as the first conductive switching element. In a third operational mode only a second one of the switching elements is conductive, so that a freewheel current occurs through the second conductive switching element and the freewheel diode attached to the same power supply terminal as the second conductive switching element. 
     A sensing resistor is provided in series with the ground terminal. 
     SUMMARY 
     It is an object to provide an electrically controllable hydraulic system for a vehicle transmission allowing for a more accurate control of an actuation of a transmission element in a vehicle transmission. 
     It is a further object to provide a more accurate way of electrically controlling an actuation of a transmission element in a vehicle transmission. 
     In accordance with the above-mentioned object an improved electrically controllable hydraulic system for a vehicle transmission is provided as claimed in claim  1 . The improved electrically controllable hydraulic system comprises a pressure pump system and a subsystem comprising the transmission element and an electrically controlled hydraulic pressure controlling module including a hydraulic valve element for controlling a hydraulic pressure supplied to the at least one transmission element to actuate said transmission element within an actuation range and an electromagnetically controllable operating element for operating the hydraulic valve element: Therein the subsystem has a first cutoff frequency, and the electrically controlled hydraulic pressure controlling module has a second cut-off frequency (f 2 ) higher than said first cutoff frequency (f 1 ). 
     The improved electrically controllable hydraulic system further comprises a driver circuit to provide a drive signal for controllably driving the electromagnetically controllable valve. Therein the driver circuit comprises a bridge circuit having a first supply branch and a second supply branch provided between a first power supply terminal and a second power supply terminal. Therein the first power supply terminal and the second power supply terminal define a power supply polarity. The first supply branch comprises a first controllably conductive channel of a first switching element between the first power supply terminal and a first connection node and a first unidirectional conductive element arranged between the first connection node and the second power supply terminal. The second supply branch comprises a second controllably conductive channel of a second unidirectional conductive element between the first power supply terminal and a second connection node and a second switching element between the second connection node and the second power supply terminal. The first and the second unidirectional conductive element are each arranged with their non conductive direction with respect to the power supply polarity, and the electromagnetically controllable operating element is provided as a load between said first and said second connection node to receive the drive signal. 
     The driver circuit is controlled by a control circuit having an input for receiving an input signal indicative for a desired value of the current to be supplied to the electrically controlled hydraulic pressure controlling module and for accordingly providing a control signal, e.g. a pulse width modulated control signal, for simultaneously controlling the first and the second switching element in a first mode wherein the switching elements are both conductive and a second mode wherein the switching elements are both non-conductive. 
     In this way it is achieved that the current through the electromagnetically controllable operating element of the electrically controlled hydraulic pressure controlling module is forced to decline rapidly. Therewith a substantially broader tuning range of dither frequency and amplitude is available to mitigate static friction, resulting in a more stable and accurate pressure response of the solenoid valve. Therein the control signal has a frequency and a duty cycle that corresponds to a dithered input value, being an input value of said input signal modified by a dither value, wherein the dither value varies according to a periodic function with a dither frequency in a range determined by said first frequency (f 1 ), and said second frequency (f 2 ). Preferably the dither frequency is in a range determined by said first frequency (f 1 ), and said second frequency (f 2 ) in that the dither frequency is well below the second frequency. However, the present invention is also applicable if the dither frequency does not significantly exceed the second frequency, for example if the dither frequency has a value substantially equal to the second frequency, or if the dither frequency has a slightly higher value such that the response is not lower than 5 dB of the nominal response. 
     In accordance with the above-mentioned further object a corresponding improved method of controlling a hydraulic system for a vehicle transmission is provided as claimed in claim  7 . 
     In an embodiment the electrically controllable hydraulic system further comprises a sensing element for providing a sense signal indicative for an actual value of an actuation of the electrically controlled hydraulic pressure controlling module, wherein the control circuit is further configured to provide the control signal in accordance with a deviation between an actual value of said actuation as indicated by the sense signal and a desired value of said actuation as indicated by said dithered input value. In an embodiment of this embodiment the sensing element is a current sensing element arranged between the first and the second connection node in series with the electromagnetically controllable operating element of the electrically controlled hydraulic pressure controlling module. Therewith a highly accurate feedback signal is obtained to further improve the pressure response of the electrically controlled hydraulic pressure controlling module. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other aspects are described in more detail with reference to the drawing. Therein: 
         FIG. 1  illustrates an embodiment of an electrically controllable hydraulic system according to the present invention, 
         FIG. 2  illustrates a part of the electrically controllable hydraulic system in more detail, 
         FIG. 3  illustrates a driver as used in this embodiment in more detail, 
         FIG. 4A-4D  illustrate an operation of a electrically controllable hydraulic system according to the prior art, 
         FIG. 5A-5C  illustrate an operation of a electrically controllable hydraulic system of the embodiment of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Like reference symbols in the various drawings indicate like elements unless otherwise indicated. 
       FIG. 1  schematically illustrates an electrically controllable hydraulic system  1  for a vehicle transmission  60  for coupling and transmitting engine power to wheels of the vehicle by actuation of at least one transmission element  2  of the vehicle transmission via the hydraulic system. Generally, a transmission provides controlled application of engine power by conversion of speed and torque from a power source, such as for example an internal combustion engine or an electrical machine. The hydraulic system may provide for actuation of friction elements in the vehicle transmission for coupling the transmission input to the geartrain to transmit engine power to the wheels of the vehicle. Such a transmission, e.g. a CVT transmission may comprise friction elements. The friction elements can be embodied as pulleys between which a flexible element such as a chain or a belt can be clamped by means of friction force in a continuous variable transmission. Alternatively the vehicle transmission  60  may provide for a stepwise adjustable transmission ratio. As shown in  FIG. 1 , the vehicle transmission  60  comprises at least one transmission element  2  for example a friction element, and may comprise one or more further transmission elements, e.g. a clutch  3 . 
     In the embodiment shown the controllable hydraulic system  1  comprises a pressure pump system  4   a ,  4   b , an electrically controlled hydraulic pressure controlling module  1 B including a hydraulic actuation element  15 , and an electromagnetically controllable operating element  21  for operating the hydraulic actuation element  15 . The controllable hydraulic system  1  further comprises a driver circuit  32  and a control circuit  42 . 
     In this embodiment the hydraulic system  1  comprises a pressure pump system  4  having two outlet lines  5 ,  6 . The pressure pump system  4  can be embodied as a pump having two pump chambers, or as a two pumps each having a pump chamber, etc. Many variants are possible. Here, the pressure pump system  4  is schematically represented by two pumps  4   a ,  4   b  having outlet line  5  and  6  respectively. 
     The pump system  4  may be powered by the engine that also serves to provide power to the wheels of the vehicle, and pressurizes the hydraulic fluid of the hydraulic system  1 . The pressurized fluid is supplied to the hydraulic system  1 . The hydraulic system  1  is typically a dual system comprising a line pressure circuit  7  in which the fluid has a relatively high pressure (approximately 5-80 bar, preferably 7-70 bar) and a lubrication circuit  8  with a lower pressure (approximately 5-10 bar, preferably 6-9 bar). The lubrication circuit  8  is mainly for cooling and lubrication of components of the transmission and will not be elaborated further. It is noted that the high pressure range and the low pressure range are overlapping, but it is also noted that the high pressure is at any time higher than the low pressure, so there is no overlap of the pressure during use. 
     In the line pressure circuit  7 , operating elements or solenoids operate valves in the line pressure circuit for controlling the pressure on components of the transmission, such as the transmission elements  2 ,  3 , or a clutch, or the pressure in the line pressure circuit itself, etc. 
     In the embodiment shown, the pressure pump system  4  has two outlet lines  5 ,  6  which are coupled to the line pressure circuit  7 . The pressure pump system  4  is further provided with a bypass circuit  9  that is controlled by a bypass valve  10 . When the bypass circuit  9  is open, with bypass valve  10  open, there is flow through the bypass circuit and the output flow of the pressure system  4  is reduced. Therewith the pressure in one of the outlet lines, here outlet line  5 , becomes reduced. When the bypass circuit  9  is closed, the bypass valve  10  is closed, and the output flow of one of the outlet lines, here outlet line  5 , is supplied to the line pressure circuit  7 . This is also referred to as the “boost” function, as the output flow of the pump system  4  then rapidly, almost immediately, increases. The bypass valve  10  is controlled by a bypass operating element  11 . By providing the bypass circuit  9 , the pressure and output flow of the second pump or pump chamber  4   a  is always available, but is not always supplied to the line pressure circuit  7 . As such, when the additional pump flow is not required (no boosting), less energy is consumed from the engine. 
     In the line pressure circuit  7  various valves are provided for controlling pressure on components of the transmission and/or hydraulic system. There is a solenoid feed valve, not shown here, that controls the pressure on the operating elements or solenoids. A line pressure valve  13  is provided that controls the pressure in the line pressure circuit  7 . An output of the line pressure valve  13  is supplied to the lubrication circuit  8  with a lubrication valve. An output of the lubrication valve typically is fed back to the pump system  4 . 
     There is also provided a hydraulic valve element  15  for controlling the pressure on the transmission element  2 . For example, the hydraulic valve element  15  may provide for a further fluid flow to a selection valve, not shown here, that controls a forward clutch and a reverse clutch. Further, a hydraulic valve element  19  is provided in the line pressure circuit  7  that controls a further transmission element  3 . In an embodiment, not shown in  FIG. 1 , the line pressure circuit  7  also comprises a clutch valve and torque control valve, not shown here. 
     By controlling the pressure on the transmission element  2  the hydraulic valve element  15  actuates the transmission element  2  within an actuation range. 
     An electrically controlled hydraulic pressure controlling module  1 B is defined by the hydraulic valve element  15  and an electromagnetically controllable operating element  21  such as a solenoid that operates the hydraulic valve element  15 . Analogously, in the embodiment shown, further electromagnetically controllable operating elements  11  and  20  are provided to control further valve elements  10 ,  19  and  13 . 
     The electrically controlled hydraulic pressure controlling module  1 B and the transmission element  2  form a subsystem  1 A with a first cutoff frequency (f 1 ), and the electrically controlled hydraulic pressure controlling module  1 B has a second cut-off frequency (f 2 ) higher than the first cutoff frequency (f 1 ). A cutoff frequency, also denoted as corner frequency, is defined herein as a frequency where the response of the element or subsystem is reduced to −3 dB of the nominal response. 
     As shown in  FIG. 1  the electromagnetically controllable operating element  21  of the electrically controlled hydraulic pressure controlling module  1 B is driven by a driver circuit  32 , controlled by a PWM signal from control circuit  42 . Likewise a respective control circuit  41 ,  43  and driver circuit  31 ,  33  are provided for electromagnetically controllable operating elements  11  and  20  of other electrically controlled hydraulic pressure controlling modules. The control circuits  41 ,  42 ,  43  are controlled by respective input signals I 1 , I 2 , I 3  from a main control unit  50  indicative for a desired actuation of the corresponding transmission element of the vehicle transmission  60  and/or an element of the hydraulic system. In the sequel, the signal I 2 , that indicates the desired actuation of the transmission element  2 , is also indicated as I set . 
     In  FIG. 2 , part of the electrically controllable hydraulic system of  FIG. 1  is shown, with the control circuit  42  in more detail. Similar circuitry as shown in  FIG. 2  may be provided for other electromagnetically controllable operating elements, e.g. electromagnetically controllable operating elements  11  and  20 . 
     As shown in  FIG. 2 , control circuit  42  has an input for receiving an input signal Let, indicative for a desired value of the actuation of the transmission element  2 . The control circuit  42  includes a modulation element  424 , e.g. an adder, that modifies an input value of the input signal I set  with a dither value (indicated by I dith ) to render a dithered input signal I dithset . The dither value varies according to a periodic function with a dither frequency (f dith ) in a range determined by the first cutoff frequency f 1  and the second cutoff frequency f 2 . Preferably the range for the dither frequency (f dith ) is determined in that the dither frequency (f dith ) is higher than the first cutoff frequency f 1  and lower than the second cutoff frequency f 2 . However, embodiments may be contemplated wherein the dither frequency (f dith ) is equal to the second cutoff frequency f 2  or even slightly higher than to the second cutoff frequency f 2 , provided that the contribution of the dither component in the driver signal is still capable to induce an oscillation of the electrically controlled hydraulic pressure controlling module  1 B. 
     The control circuit  42  is configured for providing a control signal PWM 2 , e.g. a pulse width modulated control signal, for controlling the driver circuit  32  in accordance with the dithered input signal I dithset , so as to achieve that the driver circuit  32  causes the electromagnetically controllable operating element  21  to operate the hydraulic valve element  15  with the actuation as specified by the dithered input signal. I.e. the control circuit outputs the control signal PWM 2  as a periodic signal with a frequency f PWM  and having a duty cycle that corresponds to the instantaneous value of the dithered input signal. 
     The dither signal I dith  added to the input signal I set  introduces a periodic variation relative to the desired actuation value of the transmission element  2  as indicated by the input signal I set . Typically the PWM frequency f PWM  is higher than both the first and the second cutoff frequency. The dither frequency, introduces a periodic variation in the actuation of the electromagnetically controllable operating element  21  and the associated hydraulic valve element  15 , therewith mitigating effects of static friction. As the dither frequency (as well as the PWM frequency) is higher than the first cutoff frequency, this periodic variations do not result in disturbing vibrations of the transmission element By way of example, the PWM-frequency (f PWM ) may be 10 to 1000 times higher than the dither frequency. For example, the PWM-frequency may be in the range of 500 to 10000 Hz, and the dither frequency may be in the range of 10 to 500 Hz. Nevertheless, also a higher dither frequency e.g. up to 10 kHz may be applicable, provided that it is not significantly above the cutoff frequency of the electrically controlled hydraulic pressure controlling module, comprising a hydraulic valve element and the electromagnetically controllable operating element. 
     In the embodiment shown in  FIG. 2 , a sensing element is arranged to provide a sense signal I sense  indicative for an actual value of the actuation of the transmission element. The control circuit  42  includes a PWM generation circuit module  423  to provide the control signal PWM 2  in accordance with a deviation between an actual value of the actuation as indicated by the sense signal I sense  and a desired value of the actuation of the electrically controlled hydraulic pressure controlling module  1 B as indicated by the dithered input signal I dithset . The sensing element may directly sense an actuation of the hydraulic valve element  15  of the electrically controlled hydraulic pressure controlling module  1 B, for example by a position sensor. In an embodiment, for example as shown in  FIG. 3 , the sensing element  25  is a current sensing element that is arranged between the first and the second connection node  3231 ,  3241  in series with the electromagnetically controllable operating element of the electromagnetically controllable operating element  1 B. 
     The control circuit  42  may be further configured to provide the control signal PWM 2  in accordance with the deviation between the indicated actual value and the desired value as indicated by the dithered input signal I dithset , for example in that it not only tends to achieve the desired value, but also tends to reduce the difference in a manner depending on a magnitude of the difference. In the embodiment shown, the control circuit  42  comprises a subtraction element  421  to determine a difference signal indicative for the deviation and an amplification circuit  422  to control the PWM generation circuit module  423 . The amplification circuit  422  is for example a PID control circuit. The latter is for example implemented as software in a general purpose processor or in a dedicated signal processor or as a dedicated hardware module. 
     An embodiment of the driver circuit  32  of  FIG. 2  is now described in more detail with reference to  FIG. 3 . As shown therein, the driver circuit comprises a bridge circuit having a first supply branch  323  and a second supply branch  324  provided between a first power supply terminal  321  and a second power supply terminal  322 . The first power supply terminal  321  and the second power supply terminal  322  define a power supply polarity. In this case a voltage V+ having a positive polarity with respect to a Ground voltage at power supply terminal  322  is provided. 
     The first supply branch  323  comprises a first controllably conductive channel of a first switching element  3232  between the first power supply terminal  321  and a first connection node  3231 . The first supply branch  323  further comprises a first unidirectional conductive element  3233  arranged between the first connection node  3231  and the second power supply terminal  322 . The second supply branch  324  comprises a second controllably conductive channel of a second switching element  3242  between the second power supply terminal  322  and a second connection node  3241 . The second supply branch  324  further comprises a second unidirectional conductive element  3243  between the second connection node  3241  and the first power supply terminal  321 . The first and the second unidirectional conductive element  3233 ,  3243  are each arranged with their non conductive direction with respect to the power supply polarity V+/Ground and the electromagnetically controllable operating element  21  is provided as a load between said first and said second connection node  3231 ,  3241  to receive the drive signal. 
     As shown in  FIG. 3 , the first and the second switching element  3232 ,  3242  are simultaneously controlled by the control signal PWM 2 . 
     In the embodiment as shown in  FIG. 3  the sensing element  25  is a current sensing element arranged in series with the electromagnetically controllable operating element  21  between the first and the second connection node  3231 ,  3241 . Alternatively, the current sensing element may be provided that inductively senses a current through the electromagnetically controllable operating element  21 . Alternatively, or in addition sensing elements may be provided that measure an actuation of the hydraulic valve element  15  of the electrically controlled hydraulic pressure controlling module  1 B and/or of the transmission element  2 . 
       FIG. 4A-D  illustrates operation of a prior art driver stage wherein only one of the switching elements is switched off at a time. In the example shown dither is applied with an amplitude of 150 mA and a frequency of 75 Hz. The targeted average current is 200 mA. 
     Therein  FIG. 4A  shows a first stage of a driving cycle, wherein both switching elements are conducting,  FIG. 4B  shows a second stage of a driving cycle, wherein the upper one of the switching elements is conducting and the lower one is non-conducting.  FIG. 4C  shows a third stage of a driving cycle, wherein the lower one of the switching elements is conducting and the upper one is conducting. 
     Corresponding signals are illustrated in  FIG. 4D . Therein I dithref  indicates the desired value for the current, having the targeted average value of 200 mA and including a dither to mitigate static friction. Io indicates the actual current through the electromagnetically controllable operating element  21 . This signal includes a high frequency component introduced by the PWM modulation at a frequency of about 2 kHz. The average current, without the high frequency component, but including the lower frequency dither component, is denoted as Ioa. As becomes apparent from  FIG. 4D , a strong deviation occurs between the desired current indicated by I dithref , and the average current Ioa. This implies that in this known arrangement, the dithering introduces a systematic deviation. This systematic deviation resides in a relatively slow decrease of the current through the electromagnetically controllable operating element during the second and third stage of the driving cycle as is demonstrated below. Typically, the electromagnetically controllable operating element of the electrically controlled hydraulic pressure controlling module is a solenoid having inductive characteristics. 
     For simplicity assume that the switching elements and the diodes have ideal switching characteristics. Let the solenoid valve time constant be τ and current flowing through the solenoid just when PWM switches the mosfet ON be I on . Then the current through solenoid during ON time is given by: 
     
       
         
           
             
               
                 
                   
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     Let current flowing through the solenoid just when PWM switches the mosfet OFF be I off . Then the current through solenoid during OFF time is given by: 
     
       
         
           
             
               
                 
                   
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     The current controller controls the duty cycle of the PWM signal to follow desired current I dithref . However, the electrical time constant and inertia of the spool is higher for direct acting solenoid valves since the required output force is high. Using the state of the art circuit input PWM controls the voltage across solenoid between the power supply voltage and forward drop of diode (≈0). The energy stored in the solenoid winding need to discharge in the winding resistance and in diode. This gives a slower discharge path for stored energy as given in equation (2), as per equation (1) and (2) the negative slope of solenoid current is limited. If the applied dither slope exceeds this limit, the solenoid current becomes uncontrollable resulting in an average current Ioa higher than the specified current I dithref . This enforces significant limits to the applicable dither amplitude and frequency for direct acting solenoid valves. 
       FIG. 5A-C  illustrate operation of a driver in the inventive electrically controllable hydraulic system. In this example, dither is applied with an amplitude of 200 mA and a frequency of 200 Hz. The average current is 300 mA. 
     In the inventive system, the switching elements are simultaneously switched in a conductive state as shown in  FIG. 5A  and a non-conductive state as shown in  FIG. 5B . 
     Corresponding signals are illustrated in  FIG. 5C . Therein I dithref  indicates the desired value for the current, having the targeted average value of 300 mA and including a dither to mitigate static friction. Io indicates the actual current through the electromagnetically controllable operating element  21  of the electrically controlled hydraulic pressure controlling module  1 B. This signal includes the high frequency component introduced by the PWM modulation at a frequency of about 2 kHz. The average current, without the high frequency component, is denoted as Ioa. As becomes apparent from  FIG. 5C , the average current Ioa closely follows the desired value I dithref . This is achieved in that in the second stage of the driving cycle, as shown in  FIG. 5B  both switching elements are simultaneously in their non-conductive state. As a result, the only available current path through which the solenoid can discharge is formed by the chain comprising the unidirectional conductive element  3233 , the solenoid  21 , an optional current sensing element  25  and the unidirectional conductive element  3243 . Therewith the supply voltage present on the terminals  321 ,  322  causes an active reduction of the current according to the following equation (3). 
     
       
         
           
             
               
                 
                   
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     As compared to the case specified by equation (2), therewith a substantially faster reduction of the current is achieved in a dither cycle. Hence, the new electrically controllable hydraulic system provides a substantially higher tuning range of dither frequency and amplitude in order to compensate static friction. Therewith more stable and accurate pressure responses from the electrically controlled hydraulic pressure controlling module  1 B can be achieved. 
     In particular a highly accurate pressure response is achieved in that the sensing element  25  in the electrically controllable hydraulic system is a current sensing element arranged in series with the electromagnetically controllable operating element  21  of the electrically controlled hydraulic pressure controlling module  1 B between the first and the second connection node  3231 ,  3241 . The sensing element  25  is capable to sense the current Io through the electromagnetically controllable operating element during the full driver cycle, i.e. not only in the conductive state of the switching elements  3232 ,  3242 , but also in their non-conductive state. Therewith the average current Ioa can be determined more accurately, allowing for an even more accurate control of the transmission element by the electrically controlled hydraulic pressure controlling module  1 B.