Abstract:
Control method of the position of a solenoid valve using dithering; the control method provides the steps of: determining a target current across the solenoid valve which is expressed in digital form with a given minimum quantization interval; adding a dithering square wave to the target current; controlling the voltage applied to the solenoid valve to cause the current across the solenoid valve to track the target current added to the dithering square wave; jiggling the dithering square wave with a frequency which is a sub-multiple with respect to the maximum variation frequency of the current across the solenoid valve; and varying the amplitude of the dithering square wave for a fraction of the period of the dithering square wave itself and by an amount equal to the minimum quantization interval for temporarily determining a deviation of the mean value of the dithering square wave with respect to zero and therefore obtaining a corresponding variation of the mean value of the target current.

Description:
RELATED APPLICATION 
       [0001]    This application claims the benefit of priority under 35 U.S.C. Section 119 to Italian Patent Application Serial No. B02010A 000140, filed on Mar. 9, 2010, which is incorporated herein by reference in its entirety. 
       TECHNICAL FIELD 
       [0002]    The present document relates to a method to control the position of a solenoid valve using dithering. Some examples reference control of a solenoid valve of a driving hydraulic circuit for an automatic manual transmission which will be explicitly referred to in the following discussion without however losing in generality. 
       BACKGROUND 
       [0003]    Automatic manual transmissions (commonly referred to as “AMT”) are increasing in popularity. A typical AMT is structurally similar to a traditional type manual gearbox, except that the clutch pedal and the gear selector lever typically operated by the user are instead operated by corresponding hydraulic servo-controls controlled by solenoid valves. 
       SUMMARY 
       [0004]    In some examples, an automatic manual transmission is provided with a transmission control unit which drives hydraulic servo-controls associated with one or both of a clutch and gearbox to disengage the current gear and engage the next gear, during a gear change. According to some examples, the transmission control unit comprises a main microcontroller that communicates with sensors and the other components of the vehicle (essentially, an engine control unit), thus defining a target position for each hydraulic servo-control and therefore translating such target position into a target current for the corresponding solenoid valve. In order to relieve the main microcontroller from the intensive task of directly implementing the current control of the solenoid valves, the main microcontroller does not directly implement the current control of each solenoid valve, yet communicates the target current to a corresponding supporting microcontroller which autonomously achieves the current control of the solenoid valve for tracking the target current received from the main microcontroller. 
         [0005]    Typically, the main microcontroller uses the current control using dithering, i.e. the main microcontroller overlaps a dithering square wave, which has a zero mean value and has a high oscillation frequency, to the target current determined according to the target position; the period of the dithering square wave is too small to disturb the hydraulic system driven by the solenoid valve, but it allows to inhibit the occurrence of static friction phenomena within the solenoid valve. In other words, the solenoid valve is kept “in fibrillation” with minor fast and small scale oscillations around the target position for preventing the moving parts of the solenoid valve from “sticking” by increasing the breakout static friction. 
         [0006]    Each supporting microcontroller comprises a digital input which is connected to the main microcontroller for receiving the target current to be tracked from the main microcontroller itself. The target current to be tracked (i.e. the desired value of current moment by moment) is provided by the main microcontroller as a fraction of the maximum value and has a resolution defined by the number of bits of the digital input; for example, an 8-bit digital input allows a resolution of 1/256, a 9-bit digital input allows a resolution of 1/512, and a 10-bit digital input allows a resolution of 1/1024. 
         [0007]    Specifically, the control of the solenoid valve which drives the clutch requires a high accuracy, since the hydraulic servo-control of the clutch must on one hand be able to develop a very high thrust for transmitting a high torque through the clutch (especially in the case of a clutch in oil bath), and on the other hand it must be able to accurately make small movements by exerting a moderate thrust when the clutch plates start to interact with each other (i.e. in the first moments of closing of the clutch). 
         [0008]    It may happen that the digital input resolution of the supporting microcontrollers is insufficient as compared with the accuracy to be achieved in the current control of the solenoid valves, i.e. when it is desired to have a more accurate current control of the solenoid valves than it is allowed by the digital input resolution of the supporting microcontroller. Currently, such situation is remedied only by replacing the existing supporting microcontrollers with higher performance supporting microcontrollers; however, such replacement is relatively cost-effective when it is possible to take action during the designing step of the transmission control unit, but it is extremely expensive (especially in the presence of small volumes) when an already marketed transmission control unit is to be modified. 
         [0009]    To address at least some of these design aspects, some examples of the present subject matter provide a method to control the position of a solenoid valve using dithering. Certain control method examples are free of one or more of the above-described drawbacks and are easy and/or cost-effective to implement. 
         [0010]    Examples provide a method to control the position of a solenoid valve using dithering as claimed in the attached claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    The present invention will now be described with reference to the attached drawings, which show a non-limitative embodiment thereof, in which: 
           [0012]      FIG. 1  is a schematic and plan view of a rear-wheel drive vehicle provided with an automatic manual transmission; 
           [0013]      FIG. 2  is a schematic view of the automatic manual transmission of the vehicle in  FIG. 1 ; 
           [0014]      FIG. 3  is a schematic view of a driving hydraulic circuit of a clutch of the automatic manual transmission in  FIG. 2  provided with a solenoid valve which is controlled according to the control method of some examples; and 
           [0015]      FIGS. 4-13  are six graphs showing corresponding time trends of an electric current flowing across the solenoid valve in  FIG. 3 . 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    In  FIG. 1 , numeral  1  indicates as a whole a vehicle (specifically a car) provided with two front wheels  2  and two rear drive wheels  3 ; in front position there is arranged an internal combustion engine  4  which is provided with a crankshaft  5  and produces a driving torque which is transmitted to the rear drive wheels  3  by means of an automatic manual transmission  6 . The transmission  6  comprises a dual-clutch gearbox  7  arranged at the rear and a drive shaft  8  which connects the crankshaft  5  to an input of the gear  7 . A self-locking differential  9 , from which originates a pair of axle shafts  10 , each of which is integral with a rear drive wheel  3 , is connected in cascade to the gear  7 . 
         [0017]    Vehicle  1  comprises a control unit  11  of the engine which supervises the control of the engine  4 , a transmission control unit  12  which supervises the control of the transmission  6 , and a BUS line  13  made according to the CAN (Car Area Network) protocol which is extended to the entire vehicle  1  and allows the control units  11  and  12  to communicate with each other. In other words, the control unit  11  of the engine  4  and the transmission control unit  12  are connected to the BUS line  13  and therefore can communicate with each other by means of messages sent over the BUS line  13  itself. Furthermore, the control unit  11  of the engine  4  and the transmission control unit  12  can be directly connected to each other by means of a dedicated synchronization cable  14  which is able to directly transmit a signal from the transmission control unit  12  to the control unit  11  of the engine  4  without the delays introduced by the BUS line  13 . 
         [0018]    As shown in  FIG. 2 , the dual-clutch gearbox  7  comprises a pair of primary shafts  15  which are coaxial with each other, independent and fitted within each other. Furthermore, the dual-clutch gearbox  7  comprises two coaxial clutches  16  arranged in series, each of which is adapted to connect a corresponding primary shaft  15  to the crankshaft  5  of the internal combustion engine  4  by means of the interposition of the transmission shaft  8 . The dual-clutch gear  7  comprises a single secondary shaft  17  connected to the differential  9  which transmits the motion to the rear drive wheels  3 ; according to an alternative and equivalent embodiment, the dual-clutch gearbox  7  comprises two secondary shafts  17  which are both connected to the differential  9 . 
         [0019]    The dual-clutch gearbox  7  has seven forward gears indicated by Roman numerals (first gear I, second gear II, third gear III, fourth gear IV, fifth gear V, sixth gear VI and seventh gear VII) and a reverse gear (indicated by the letter R). The primary shaft  15  and the secondary shaft  17  are mechanically coupled with each other by means of a plurality of pairs of gears, each of which defines a corresponding gear and comprises a primary gear  18  fitted on the primary shaft  15  and a secondary gear  19  fitted on the secondary shaft  17 . In order to allow a proper operation of the dual-clutch gearbox  7 , all the odd gears (first gear I, third gear III, fifth gear V, seventh gear VII) are coupled to the same primary shaft  15 , while all the even gears (second gear II, fourth gear IV and sixth gear VI) are coupled to the other primary shaft  15 . 
         [0020]    Each primary gear  18  is keyed to a corresponding primary shaft  15  for always rotating integrally with the primary shaft  15  itself and permanently meshes with the corresponding secondary gear  19 ; on the contrary, each secondary gear  19  is idly fitted on the secondary shaft  17 . Furthermore, the dual-clutch gearbox  7  comprises four double synchronizers  20 , each of which is coaxially fitted to the secondary shaft  17 , it is arranged between two secondary gears  19 , and is adapted to be actuated for alternatively engaging the two corresponding secondary gears  19  with the secondary shaft  17  (i.e. for alternatively making the two corresponding secondary gears  19  angularly integral with the secondary shaft  17 ). In other words, each synchronizer  20  may be moved in a direction for engaging a secondary gear  19  with the secondary shaft  17 , or it may be moved in the opposite direction for engaging the other secondary gear  19  with the secondary shaft  17 . 
         [0021]    As shown in  FIG. 3 , the transmission  6  comprises a driving hydraulic circuit  21  (only partially shown in  FIG. 3 ) which actuates the clutches  16  and the synchronizers  20  by means of respective hydraulic servo-controls  22  (only one of which is shown in  FIG. 3 ). Specifically, for the sake of simplicity,  FIG. 3  shows a single hydraulic servo-control  22  which is coupled to a clutch  16  and is provided with a thrust chamber  23  which may be filled with oil under pressure; when the thrust chamber  23  is filled with oil under pressure, an axial thrust on the plates of the clutch  16  is generated with an intensity essentially proportional to the pressure P of the oil within the thrust chamber  23 . 
         [0022]    The hydraulic circuit  21  comprises a reservoir  24  for oil at atmospheric pressure, from which originates a conduit  25  provided with a pump  26  and a check valve  27  for feeding oil under pressure to a hydraulic accumulator  28 ; the hydraulic accumulator  28  communicates by means of a conduit  29  with an inlet of a proportional solenoid valve  30 , from which originate a conduit  31  flowing to the thrust chamber  23  and a conduit  32  flowing to the reservoir  24 . In use, the solenoid valve  30  is able to keep the thrust chamber  23  isolated from the reservoir  24  for keeping the pressure P of the oil in the thrust chamber  23  constant, it is able to connect the thrust chamber  23  to the reservoir  24  for reducing the pressure P of the oil in the thrust chamber  23 , and is adapted to connect the thrust chamber  23  to the hydraulic accumulator  28  for increasing the pressure P of the oil in the thrust chamber  23 . 
         [0023]    The solenoid valve  30  is provided with a control coil  34  which is crossed by an electric current I generated by the transmission control unit  12  by applying a voltage V variable over time to the ends of the control coil  34 . The transmission control unit  12  comprises a main microcontroller  35  which communicates with the sensors of the transmission  6  (such as for example a pressure sensor  36  which measures the pressure P of the oil within the thrust chamber  23 ) and with the other components of vehicle  1  (essentially with the engine control unit  11 ), thus defining a target position for each hydraulic servo-control  22  and therefore translating such target position to a corresponding target current I OBJ  (shown in  FIGS. 4-13 ) for the corresponding solenoid valve  30 . In order to relieve the main microcontroller  35  from the intensive task of directly implementing the current control of the solenoid valves  30 , the main microcontroller  35  does not directly implement the current control of each solenoid valve  30 , yet communicates the target current I OBJ  to a corresponding supporting microcontroller  37  which autonomously achieves the current control of the solenoid valve  30  for tracking the target current I OBJ  received from the main microcontroller  35 . In particular, each supporting microcontroller  37  tracks the target current I OBJ  received from the main microcontroller  35  by means of a feedback control and is therefore provided with a current sensor  38  which measures the intensity of the electric current I which crosses the control coil  34  of the solenoid valve  30 . 
         [0024]    Each supporting microcontroller  37  comprises a digital input  39  which is connected to the main microcontroller  35  for receiving the target current I OBJ  to be tracked from the main microcontroller  35  itself. The target current I OBJ  to be tracked (i.e. the desired value moment by moment of the current I which crosses the control coil  34  of the solenoid valve  30 ) is provided by the main microcontroller  35  as a fraction of the maximum value and has a resolution defined by the number of bits of the digital input  39 ; for example, an 8-bit digital input  39  allows a resolution of 1/256, a 9-bit digital input  39  allows a resolution of 1/512, and a 10-bit digital input  39  allows a resolution of   1/1024. Consequently, the resolution of the digital input 39 (i.e. the number of bits of the digital input 39) defines the minimum quantization interval Δ   min  of the target current I OBJ  (shown in  FIGS. 6-13 ); in other words, the higher the resolution of the digital input  39  (i.e. the greater the number of bits of the digital input  39 ), the smaller the minimum quantization interval Δ min  of the target current I OBJ . For example, an 8-bit digital input  39  allows a resolution of 1/256 and therefore the minimum variation of the target current I OBJ  is 1/256 (i.e. it is not possible to increase or decrease the target current I OBJ  by a quantity smaller than 1/256). 
         [0025]    As shown in  FIG. 4 , the main microcontroller  35  uses the control current using dithering, i.e. the main microcontroller  35  overlaps a dithering square wave I DITH , which normally (i.e. under normal conditions) has a zero mean value and has a high oscillation frequency, to each target current I OBJ  determined according to the target position; the period T DITH  of the dithering square wave is too small to disturb the hydraulic circuit  21  driven by the solenoid valves  30 , but allows to inhibit the occurrence of static friction phenomena within the solenoid valves  30 . In other words, each solenoid valve  30  is kept “in fibrillation” with minor fast and small scale oscillations around the target position for preventing the moving parts of the solenoid valve  30  from “sticking” by increasing the breakout static friction. 
         [0026]    As shown in  FIG. 4 , the main microcontroller  35  determines the target current I OBJ , it determines the dithering square wave I DITH  which normally has a zero mean value, and adds the dithering square wave I DITH  to the target current I OBJ , for each solenoid valve  30 ; the main microcontroller  35  communicates the addition of the target current I OBJ  and the dithering wave square I DITH  to the digital input  39  of the corresponding supporting microcontroller  37 , such that the supporting microcontroller  37  drives the coil  34  of the solenoid valve  30  to track such addition. As apparent from  FIG. 5 , the supporting microcontroller  37  drives the coil  34  of the solenoid valve  30  by means of the known control technique called “chopper” which provides the application, to the terminals of the coil  34  of the solenoid valve  30 , of a positive voltage which determines an increase of the current I which crosses the coil  34  and alternatively of a zero (or negative) voltage which determines a decrease of the current I which crosses the coil  34 . 
         [0027]    The oscillation frequency of the dithering square wave I DITH  is chosen such that this oscillation frequency is an integer sub-multiple of (i.e. is smaller than) the maximum variation frequency of the current I across the solenoid valve  30  (i.e. the maximum “speed” by which it is possible to modify the current I across the solenoid valve  30 ); in other words, the period T DITH  of the dithering square wave I DITH  is an integer multiple of (i.e. is higher than) the minimum period T min  by which it is possible to modify the current I across the solenoid valve  30 , as clearly shown in  FIGS. 6-13 . In this manner, during a single period T DITH  of the dithering square wave I DITH , it is possible to vary the intensity of the current I across the solenoid valve  30  for multiple times; in the example shown in  FIGS. 6-13 , the period T DITH  of the dithering square wave I DITH  is equal to eight times the minimum period T min  by which it is possible to modify the current I across the solenoid valve  30  (i.e. the oscillation frequency of the dithering square wave I DITH  is equal to ⅛ of the maximum variation frequency of the current I across the solenoid valve  30 ) and therefore it is possible to vary the current I across the solenoid valve  30  for eight times at every period T DITH  of the dithering square wave I DITH . 
         [0028]    Due to the fact that during a single period T DITH  of the dithering square wave I DITH  it is possible to vary the intensity of the current I across the solenoid valve  30  for multiple times, it is possible to vary the amplitude of the dithering square wave I DITH  for a fraction of the period T DITH  of the dithering square wave I DITH  itself and by an amount equal to the minimum quantization interval Δ min  of the target current I OBJ  for temporarily determining a deviation of the mean value of the dithering square wave I DITH  with respect to zero and therefore obtaining a corresponding variation of the mean value of the target current I OBJ  by an amount equal to a fraction of the minimum quantization interval Δ min . 
         [0029]    In  FIG. 6 , the dithering square wave I DITH  of the first period is identical to the dithering square wave I DITH  of the second period, and therefore the mean value of the target current I OBJ  remains constant between the first period and the second period. 
         [0030]    In  FIG. 7 , the dithering square wave I DITH  of the second period differs from the dithering square wave I DITH  of the first period in that, during ¼ of the period T DITH , the dithering square wave I DITH  of the second period is increased by an amount equal to the minimum quantization interval Δ min  for ¼ of the period T DITH ; in this manner, the mean value of the target current I OBJ  increases by ¼ of the minimum quantization interval Δ min  between the first period and the second period. As shown in  FIG. 7 , the increase of the dithering square wave I DITH  of the second period is distributed half in a first positive half period and half in a second negative period; as shown in  FIGS. 8 and 9 , it is also possible to set all the increase of the dithering square wave I DITH  of the second period in a single half period (in the first half period as shown in  FIG. 8  and in the second half period as shown in  FIG. 9 ). 
         [0031]    In  FIG. 10 , the dithering square wave I DITH  of the second period differs from the dithering square wave I DITH  of the first period in that, during 2/4 of the period T DITH , the dithering square wave I DITH  of the second period is increased by an amount equal to the minimum quantization interval Δ min ; in this manner, the mean value of the target current I OBJ  increases by 2/4 of the minimum quantization interval Δ min  between the first period and the second period. 
         [0032]    In  FIG. 11 , the dithering square wave I DITH  of the second period differs from the dithering square wave I DITH  of the first period in that, during ¾ of the period T DITH , the dithering square wave I DITH  of the second period is increased by an amount equal to the minimum quantization interval Δ min ; in this manner, the mean value of the target current I OBJ  increases by ¾ of the minimum quantization interval Δ min  between the first period and the second period. 
         [0033]    In  FIG. 12 , the dithering square wave I DITH  of the second period differs from the dithering square wave I DITH  of the first period in that, during ⅝ of the period T DITH , the dithering square wave I DITH  of the second period is increased by an amount equal to the minimum quantization interval Δ min ; in this manner, the mean value of the target current I OBJ  increases by ⅝ of the minimum quantization interval Δ min  between the first period and the second period. 
         [0034]    In  FIG. 13 , the dithering square wave I DITH  of the second period differs from the dithering square wave I DITH  of the first period in that, during ⅞ of the period T DITH , the dithering square wave I DITH  of the second period is increased by an amount equal to the minimum quantization interval Δ min ; in this manner, the mean value of the target current I OBJ  increases by ⅞ of the minimum quantization interval Δ min  between the first period and the second period. 
         [0035]    As set forth above, it is apparent that it is possible to obtain a variation of the mean value of the target current I OBJ  by an amount equal to a fraction of the minimum quantization interval Δ min , and it is therefore possible to effectively increase the effective resolution on the control of the target current I OBJ . 
         [0036]    The above-described control method of the position of a solenoid valve  30  using dithering has many advantages. 
         [0037]    Firstly, the above-described control method of the position of a solenoid valve  30  using dithering allows to increase the effective resolution of the control of the target current I OBJ  which is higher than the “hardware” resolution (i.e. defined by the number of bits of the digital input  39  of the corresponding supporting microcontroller  37 ). In other words, by means of a moderate complication of the software control, it is possible to increase the effective resolution of the control of the target current I OBJ  as compared with the limits defined by the “hardware”. 
         [0038]    Furthermore, the above-described control method of the position of a solenoid valve  30  using dithering is simple and cost-effective to be implemented, since it does not require the installation of additional physical components and does not involve an expansion of the transmission control unit  12  as it does not require a significant additional processing power.