in fig1 , 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 . 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 . as shown in fig2 , 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 . 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 . 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 . as shown in fig3 , the transmission 6 comprises a driving hydraulic circuit 21 ( only partially shown in fig3 ) which actuates the clutches 16 and the synchronizers 20 by means of respective hydraulic servo - controls 22 ( only one of which is shown in fig3 ). specifically , for the sake of simplicity , fig3 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 . 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 . 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 fig4 - 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 . 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 fig6 - 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 ). as shown in fig4 , 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 . as shown in fig4 , 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 fig5 , 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 . 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 fig6 - 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 fig6 - 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 . 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 . in fig6 , 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 . in fig7 , 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 fig7 , 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 fig8 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 fig8 and in the second half period as shown in fig9 ). in fig1 , 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 . in fig1 , 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 . in fig1 , 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 . in fig1 , 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 . 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 . the above - described control method of the position of a solenoid valve 30 using dithering has many advantages . 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 ”. 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 .