Abstract:
An improved system and method for detecting solenoid-related faults in a solenoid-controlled actuator assembly. The system includes a processing circuit which controls a solenoid driver circuit for energizing and de-energizing the solenoid, a current sense circuit for sensing the current in the solenoid, and a power supply switching circuit for selectively connecting and disconnecting a supply voltage from the solenoid driver circuit. The processing circuit implements fault detection logic wherein the processing circuit senses the current in the solenoid and, if the sensed current exceeds a predetermined current value for a predetermined time period, disconnects the supply voltage from the solenoid driver circuit and measures the rate of voltage decay in the solenoid driver circuit to determine if an open circuit fault or a short circuit fault exists.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a control system and/or method for sensing faulty solenoids. In particular, the present invention relates to a control system and/or method for sensing faults in the solenoids of solenoid-controlled actuators for vehicular automated or semi-automated change gear transmission systems. 
     2. Description of the Prior Art 
     Change-gear mechanical transmissions are well known in the prior art, as may be seen by reference to U.S. Pat. Nos. 3,105,395; 3,335,616; 4,428,469; 4,754,665; 4,920,815; 4,944,197; 5,086,897; 5,370,013; and 5,390,561, the disclosures of which are incorporated herein by reference. Two-position and three-position, fluid-actuated, actuator piston assemblies and actuator systems associated therewith also are well known in the prior art, as may be seen by reference to U.S. Pat. Nos. 4,899,607; 4,928,544; 4,936,156; 5,054,591; 5,193,410; 5,263,379; 5,272,441; 5,329,826; 5,651,292; and 5,661,998, the disclosures of which are incorporated herein by reference. 
     Transmission systems of particular reference are disclosed in the following patent applications, the disclosures of which are incorporated herein by reference: 
     Ser. No. 09/178,346 filed Oct. 22, 1998 entitled “ROBUST CONTROL FOR THREE-POSITION TRANSMISSION SHIFT ACTUATOR ASSEMBLY” 
     Ser. No. 08/053,089 filed Apr. 1, 1998 entitled “RANGE SHIFT CONTROL” 
     Ser. No. 08/053,090 filed Apr. 1, 1998 entitled “ADAPTIVE NEUTRAL SENSING” 
     Ser. No. 08/053,091 filed Apr. 1, 1998 entitled “JAW CLUTCH ENGAGEMENT CONTROL FOR ASSISTED, MANUALLY SHIFTED, SPLITTER-TYPE TRANSMISSION SYSTEM” 
     Ser. No. 08/053,092 filed Apr. 1, 1998 entitled “ENGINE FUEL CONTROL FOR COMPLETING SHIFTS IN CONTROLLER-ASSISTED, MANUALLY SHIFTED TRANSMISSIONS” 
     Ser. No. 08/053,093 filed Apr. 1, 1998 entitled “ADAPTIVE UPSHIFT JAW CLUTCH ENGAGEMENT CONTROL” 
     Ser. No. 08/053,095 filed Apr. 1, 1998 entitled “DYNAMIC RANGE SHIFT ACTUATION” 
     Ser. No. 08/053,181 filed Apr. 1, 1998 entitled “ADAPTIVE SPLITTER ACTUATOR ENGAGEMENT FORCE CONTROL” 
     Ser. No. 08/902,603 filed Aug. 7, 1997 entitled “PARTIALLY AUTOMATED, LEVER-SHIFTED MECHANICAL TRANSMISSION SYSTEM” 
     Ser. No. 08/990,678 filed Dec. 15, 1997 entitled “ASSISTED LEVER-SHIFTED TRANSMISSION” 
     Controls for automated and semi-automated transmission systems, including fault detection systems and/or methods, are known in the prior art as may be seen by reference to U.S. Pat. Nos. 4,595,986; 4,702,127; 4,922,425; 4,888,577; 4,849,899; and 5,272,441, the disclosures of which are hereby incorporated by reference. 
     It is known that solenoid-related faults sometimes occur in the solenoid-controlled actuators used in vehicular transmission systems. If such a fault occurs, it is highly desirable to indicate such a condition to the operator of the vehicle, and to initiate some safe form of recovery logic and/or temporary mode of operation. These actions will notify the operator that correction action is required, and will minimize the possibility of the transmission suffering mechanical damage or behaving in an unintended manner. 
     In order to properly respond to a solenoid-related fault, a reliable fault detection system must be implemented. Although examples of fault detection systems can be found in the prior art, such systems typically are complicated, cannot detect a wide variety of solenoid-related faults, or cannot reliably be implemented in all transmission control configurations. One particular problem with the prior art fault detection systems is their inability to be reliably implemented in low side solenoid driver circuit configurations, such configurations being desirable in that they enable a less complicated and lower cost electronic control unit to be used in the transmission control. 
     SUMMARY OF THE INVENTION 
     The present invention provides an improved fault detection system and method which minimizes or overcomes the problems of the prior art. 
     The fault detection system of the present invention includes a solenoid driver circuit for energizing and de-energizing the solenoid, a current sense circuit for sensing the current in the solenoid, and a power supply switching circuit for selectively connecting and disconnecting a supply voltage from the solenoid driver circuit. Also included is a processing circuit that is connected to and controls the solenoid driver circuit, the current sense circuit, and the power supply switching circuit. The processing circuit implements fault detection logic wherein the processing circuit senses the current in the solenoid and, if the sensed current exceeds a predetermined current value for a predetermined time period, disconnects the supply voltage from the solenoid driver circuit and measures the rate of voltage decay in the solenoid driver circuit to determine if an open circuit fault or a short circuit fault exists. 
     Other objects and advantages of the present invention will become apparent from a reading of the following description of the preferred embodiment taken in connection with the attached drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic illustration of a fluid-actuated, three-position shift actuator system used in connection with the present invention. 
     FIG. 2 is a schematic illustration of a compound transmission advantageously utilizing the actuator system of FIG.  1 . 
     FIG. 3 is a sectional view of a preferred embodiment of the transmission of FIG.  2 . 
     FIG. 4 illustrates a typical shift pattern and typical gear ratios for the transmission of FIG.  2 . 
     FIG. 5 is a graphic representation of variable force applied by the actuator assembly of the present invention in response to variable pulse width modulation of the single controlled supply valve or variable fluid pressure provided thereby to the first chamber. 
     FIG. 6 is a schematic illustration of a fluid-actuated shift actuator system for the range clutch of the transmission. 
     FIG. 7 is a schematic circuit diagram of a solenoid controller of the present invention. 
     FIGS. 8 and 9 are flowcharts of the programming of the electronic control unit implementing the fault detection logic of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Certain terminology is used in the following description for convenience only and is not limiting. The words “upwardly,” “downwardly,” “rightwardly” and “leftwardly” will designate directions in the drawings to which reference is made. The words “forward” and “rearward” will refer respectively to the front and rear ends of the transmission as conventionally mounted in the vehicle, being respectively to the left and right sides of the transmission as illustrated in FIG.  2 . The words “inwardly” and “outwardly” will refer respectively to directions toward and away from the geometric center of the device and designated parts thereof. Said terminology includes the words above specifically mentioned, derivatives thereof and words of similar import. 
     The term “compound transmission” is used to designate a change-speed or change-gear transmission having a main transmission section and an auxiliary drive train unit, such as an auxiliary transmission section, connected in series whereby the selected gear reduction in the main transmission section may be compounded by further selected gear reduction in the auxiliary transmission section. The term “upshift” as used herein shall mean the shifting from a lower speed gear ratio to a higher speed gear ratio, and the term “downshift” as used herein shall mean the shifting from a higher speed gear ratio to a lower speed gear ratio. The terms “low speed gear” or “low gear” as used herein shall designate a gear utilized for relatively low forward speed operation in a transmission (i.e., a set of gears having a higher ratio of reduction of output shaft speed relative to the speed of the input shaft). “Synchronized clutch assembly” and words of similar import shall designate a clutch assembly utilized to non-rotatably couple a selected gear to a shaft by means of a positive clutch in which attempted engagement of said clutch is prevented until the members of the clutch are at substantially synchronous rotation and relatively large capacity friction means are associated with the clutch members and are sufficient, upon initiation of a clutch engagement, to cause the clutch members and all members rotating therewith to rotate at a substantially synchronous speed. 
     The fluid-actuated, three-position shift actuator system  10  of the present invention, and the three-position actuator piston assembly  12  utilized therein, may be especially advantageously utilized as a splitter clutch actuator in a compound transmission  110 , as illustrated in FIGS.  24 . 
     Transmission  110  includes a mainsection  112  and an auxiliary section  114 , both contained within housing  116 . Housing  116  includes a forward end wall  116 A and a rearward end wall  116 B, but not an intermediate wall. 
     Input shaft  118  carries input gear  120  fixed for rotation therewith and defines a rearwardly opening pocket  118 A wherein a reduced diameter extension  158 A of output shaft  158  is piloted. A non-friction bushing  118 B or the like may be provided in pocket or blind bore  118 A. The forward end of input shaft  118  is supported by bearing  118 C in front end wall  116 A while the rearward end  158 C of output shaft  158  is supported by bearing assembly  158 D in rear and wall  116 B. Bearing assembly  158 D may be a pair of opposed taper bearings or a single roller or ball bearing as is illustrated in FIG.  3 . 
     The mainshaft  146 , which carries mainshaft clutches  148  and  150 , and the mainshaft splitter clutch  180  is in the form of a generally tubular body  146 A having an externally splined outer surface  146 B and an axially extending through bore  146 C for passage of output shaft  158 . Shift forks  152  and  154  are provided for shifting clutches  148  and  150 , respectively. Mainshaft  146  is independently rotatable relative to input shaft  118  and output shaft  158  and, preferably, is free for limited radial movements relative thereto. 
     The mainsection  112  includes two substantially identical mainsection countershaft assemblies  122 , each comprising a mainsection counter-shaft  124  carrying countershaft gears  130 ,  132 ,  134 ,  136  and  138  fixed thereto. Gear pairs  130 ,  134 ,  136  and  138  are constantly meshed with input gear  118 , mainshaft gears  140  and  142  and idler  157 , which is meshed with reverse mainshaft gear  144 , respectively. 
     Mainsection countershaft  124  extends rearwardly into the auxiliary section, where its rearward end  124 A is supported directly or indirectly in rear housing end wall  116 B. 
     The auxiliary section  114  includes two substantially identical auxiliary countershaft assemblies  160 , each including an auxiliary countershaft  162  carrying auxiliary countershaft gears  168 ,  170  and  172  for rotation therewith. Auxiliary countershaft gear pairs  168 ,  170  and  172  are constantly meshed with splitter gear  174 , splitter/range gear  176  and range gear  178 , respectively. Splitter clutch  180  is fixed to mainshaft  146  for selectively clutching either gear  174  or  176  thereto, while synchronized range clutch  182  is fixed to output shaft  158  for selectively clutching either gear  176  or gear  178  thereto. Preferably, the splitter clutch is axially positioned by a shift fork  180 A controlled by the actuator system  10  of the present invention. 
     Auxiliary countershafts  162  are generally tubular in shape defining a through bore  162 A for receipt of the rearward extensions of the mainsection countershafts  124 . Bearings or bushings  162 B and  162 C are provided to rotatably support auxiliary countershaft  162  on mainsection countershaft  124 . Bearing  162 D directly or indirectly supports the rear ends of countershafts  124  and  162  in the rear end wall  116 B. 
     The structure and function of double-acting jaw clutch collar  180  is substantially identical to the structure and function of the sliding clutch collars  148  and  150  utilized in the main transmission section  112  and the function of double-acting synchronized clutch assembly  182  is substantially identical to the structure and function of prior art double-acting synchronized clutch assemblies, examples of which may be seen by reference to U.S. Pat. Nos. 4,462,489; 4,125,179 and 2,667,955, the disclosures of which are incorporated herein by reference. The synchronized clutch assembly  182  illustrated is of the pin type described in aforementioned U.S. Pat. No. 4,462,489. 
     When used in connection with the actuator system  10  of the present invention, the splitter jaw clutch  180  is a three-position clutch assembly which may be selectively positioned in the rightwardmost (direct) or leftwardmost (overdrive) positions for engaging either gear  176  or gear  174 , respectively, to the mainshaft  146  or in an intermediate non-engaged (neutral) position. The neutral position refers to the range of intermediate positions of clutch  180  wherein neither gear  174  nor  176  is engaged to the mainshaft  146 . 
     As may be seen by reference to FIGS. 2-4, by selectively axially positioning both the splitter clutch  180  and the range clutch  182  in the forward and rearward axial positions thereof, four distinct ratios of mainshaft rotation to output shaft rotation may be provided. Accordingly, auxiliary transmission section  114  is a three-layer auxiliary section of the combined range and splitter type providing four selectable speeds or drive ratios between the input (mainshaft  146 ) and output (output shaft  158 ) thereof. The mainsection  112  provides a reverse and three potentially selectable forward speeds. However, one of the selectable mainsection forward gear ratios, the low speed gear ratios associated with mainshaft gear  142 , is not utilized in the high range. Thus, transmission  110  is properly designated as a “(2+1)×(2×2)” type transmission providing nine or ten selectable forward speeds, depending upon the desirability and practicality of splitting the low gear ratio. 
     The shift pattern for shifting transmission  110  is schematically illustrated in FIG.  4 . Divisions in the vertical direction at each gear lever position signify splitter shifts, while movement in the horizontal direction from the ¾ and ⅚ leg of the H pattern to the ⅞ and {fraction (9/10)} leg of the H pattern signifies a shift from the low range to the high range of the transmission. As discussed above, splitter shifting is accomplished in the usual manner by means of a vehicle operator-actuated splitter button or the like, usually a button located at the shift lever knob, while operation of the range clutch shifting assembly is an automatic response to movement of the gear shift lever between the central and rightwardmost legs of the shift pattern, as illustrated in FIG.  6 . Range shift devices of this general type are known in the prior art and may be seen by reference to U.S. Pat. Nos. 3,429,202; 4,455,883; 4,561,325; 4,663,725 and 4,974,468, the disclosures of which are incorporated herein by reference. 
     Referring again to FIG. 4, and assuming that it is desirable that a transmission have generally equal ratio steps, the mainsection ratio steps should be generally equal, the splitter step should be generally equal to the square root of the mainsection ratio steps, and the range step should equal about the mainsection ratio step raised to the N th  power, where N equals the number of mainsection ratio steps occurring in both ranges (i.e., N=2 in the (2+1)×(2×2) transmission  110 ). Given the desired ideal ratios, gearing to approximate these ratios is selected. In the above example, the splitter steps are about 33.3%, while the range step is about 316%, which generally is suitable for a “2+1” main transmission section having about 78% steps, as the square root of 1.78 equals about 1.33, and 1.78 raised to the second power (i.e., N=2) equals about 3.16. 
     For certain at least partially automated mechanical transmission systems utilizing mechanical transmissions similar to transmission  110  illustrated in FIGS. 2-4, it may be desirable under certain operating conditions to cause the splitter jaw clutch  180  to move to and remain in the neutral position thereof and/or to vary the force applied to the shift fork  180 A and clutch  180 . The shift actuator piston assembly  12  and actuator system  10  of the present invention provides a relatively simple, inexpensive and reliable means to provide these desirable splitter clutch control features. 
     Referring to FIG. 1, the fluid-actuated, three-position actuator piston assembly  12  includes a two-piece cylinder body  14 , including a main body piece  14 A and an end cap  14 B. The cylinder body defines a blind bore  16  from which extends a piston shaft  18  carrying a shift actuator such as the shift fork  180 A for axial movement therewith. The blind bore  16  includes an enlarged interior diameter portion  16 A, a reduced interior diameter portion  16 B, and an intermediate interior diameter portion  16 C interposed between the large and small interior diameter portions. Shoulders  16 D and  16 E, respectively, are defined at the intersections of bore portions  16 A and  16 C and of bore portions  16 C and  16 B, respectively. By way of example, for a heavy-duty transmission, the interior diameters  16 F,  16 G and  16 H of bore portions  16 A,  16 B and  16 C, respectively, may be about 2.203 inches, 1.197 inches and 1.850 inches, respectively. 
     An enlarged differential area piston member  20  is slidably and sealingly received in enlarged bore portion  16 A and is fixed to shaft  18  for axial movement therewith. Piston  20  defines a leftward-facing large face  20 A and a rightward-facing smaller face  20 B. 
     Shaft  18  is slidably received in smaller diameter bore portion  16 B and carries an annular, tubular piston member  22  on the outer diameter surface  18 A thereof. Annular, tubular piston member  22  defines an interior diameter surface  22 A slidably and sealingly carried by the outer diameter surface  18 A of shaft  18  and an outer diameter surface  22 B slidably and sealingly received in intermediate diameter portion  16 C. The tubular piston also defines a leftward-facing piston face  22 C. 
     Although shaft  18  is illustrated in connection with shift yoke  180 A, it also may be utilized to operate other devices such as shift mechanisms of the types illustrated in U.S. Pat. No. 4,920,815, the disclosure of which is incorporated herein by reference. 
     Rightward axial movement of the annular, tubular piston  22  relative to shaft  18  is limited by stop member  24 , while leftward axial movement of the piston  22  relative to shaft  18  is limited by piston face  20 B. Rightward axial movement of tubular piston  22  relative to bore  16  and body  14  is limited by shoulder  16 E. Piston face  20 A and bore portion  16 A define a first chamber  26  connected by passage  28  to a selectably pressurized and exhausted fluid conduit A, while piston face  20 B, bore portion  16 A and the leftward face  22 C of tubular secondary piston  22  define a second chamber  30  connected by passage  32  to a constantly pressurized conduit B. 
     A position sensor  34  may be provided to provide an input signal indicative of the axial position of shaft  18  and/or shift yoke  180 A. As is illustrated, shift yoke  180 A may be fully leftwardly displaced to engage the overdrive splitter ratio (i.e., gear  174  engaged to mainshaft  146 ), fully rightwardly displaced to engage the direct drive splitter ratio (i.e., gear  176  engaged to mainshaft  146 ), or centered in a neutral position area (mainshaft  146  not engaged to either gears  174  or  176 ). 
     A microprocessor-based controller (or electronic control unit (ECU))  36  may be provided to receive various input signals  38  which are processed according to predetermined logic rules to issue command output signals  40  to various system actuators, such as the pulse-width-modulated, solenoid-controlled valve assembly  42  used to control the pressurization and exhaust of conduit A and piston chamber  26 . Controllers of this type are known in the prior art, as may be seen by reference to U.S. Pat. Nos. 4,360,060; 4,595,986; 5,281,902 and 5,445,126, the disclosures of which are incorporated herein by reference. 
     A source of onboard filtered and regulated air  44 , usually from the vehicle compressor, is constantly connected directly to chamber  30  through conduit B and passage  32  in body  14 . The chamber  26  is selectably connected by conduit A and passage  28  to the source  44  or to atmosphere (ATMO), depending upon the position of the three-way, two-position pulse-width-modulated, solenoid-controlled valve assembly  42 . In a typical heavy-duty vehicle, the source of pressurized air  44  will be regulated at about 80 psi. 
     The microprocessor-based controller  36  may receive input signals from an electronic data link, such as those conforming to industry-established protocols such as SAE J1922, SAE J1939 and/or ISO 11898 and/or from various sensors such as sensors indicative of throttle pedal position, vehicle speed, transmission shaft speed, engine speed, engine torque, shift lever and/or splitter selector manipulation, master clutch condition and the like. The controller  36  also may issue command output signals  40  to display devices, transmission main section and/or range section actuators, engine controllers, master clutch operators, driveline retarder operators and the like. The controller  36  also may issue command output signals to pulse-width-modulated, solenoid-controlled valve assembly  42 . 
     A sensor  158 E (FIG. 3) may be provided to provide a signal indicative of the rotational speed of output shaft  158  (also indicative of vehicle speed), and a sensor  60  (FIG. 2) may be provided to provide a signal indicative of operation of the splitter selector switch  62  located on the shift lever  64 . 
     It is important to note that when piston shaft  18 /shift yoke  180 A (FIG. 1) is in the neutral position and tubular piston  22  is on stop  24  (which is the natural consequence of constantly pressurizing chamber  30 ), then tubular piston  22  will be in contact with shoulder  16 E. Accordingly, if the shaft  18 /yoke  180 A is displaced in the overdrive (leftward) direction, the face  22 C of piston  22  will apply a rightward force on the shaft and yoke (about 130 pounds in the current example), which force will abruptly cease as the shaft  18 /yoke  180 A moves to neutral or to the direct drive (rightward) direction from neutral. This behavior is utilized to control positioning of the three-position actuator  12 , as will be described in greater detail below. 
     FIG. 5 is a graphic representation of the forces applied to the shift yoke  180 A at various degrees of pulse width modulation, depending on the positioning of the yoke, and assuming the dimensions set forth above, and an 80-psi source of pressurized fluid (i.e.,  20 A=2.203 inches;  20 B=1.197 inches;  20 C=1.850 inches; and source  44 =80 psi). In FIG. 5, the percentage of pulse width modulation (% PWM) varies from 0% modulation (fully energized) to 100% modulation (energized none of the time), and positive force is in the direct (rightward) direction, while negative force is in the overdrive (leftward) direction. Line  50  represents the forces applied to the yoke if the yoke is displaced to the overdrive (rightward) direction of neutral, while line  52  represents the forces applied to the yoke if the yoke is displaced to the direct (leftward) direction of neutral. At any given level of pulse width modulation of solenoid-controlled valve  42 , or in the corresponding resulting pressurization of conduit A, the difference between lines  50  and  52  is the approximately 130 pounds of force to the right which the tubular piston will provide if the yoke  180 A is located to the left (overdrive) side of neutral. 
     By way of example, if engaged in overdrive, if a 0% modulation (i.e., full energization) of solenoid-controlled valve  42  is commanded, the yoke will be biased from the overdrive position toward neutral with about 220 pounds of force until reaching neutral (line  50 ) and then will be biased with about 90 pounds of force from neutral to the direct position (line  52 ). Similarly, at 20% modulation (i.e., valve energized 80% of time), the yoke will be urged toward neutral with about 170 pounds of force and then from neutral into direct with about 40 pounds of force. 
     As represented by line  54 , at about 38% modulation (i.e., valve solenoid energized 62% of time), regardless of the position of the yoke, the yoke will be biased toward neutral with about 65 pounds of force and then abruptly stopped in the neutral position. Theoretically, at about 28% modulation (line  56 ) to about 52% modulation (line  58 ), the yoke will be biased to neutral with various amounts of force and will remain in or near the neutral position. 
     Accordingly, using the three-position actuator system  10  of the present invention, requiring only a single pulse-width-modulation-controlled, solenoid-controlled valve assembly  42 , an actuator having three selectable and maintainable positions and selectably variable actuation forces is provided. 
     In the illustrated system, a modulation of 0% to about 28% will result in the actuator shifting to the direct position, a modulation of about 28% to 52% will result in the actuator shifting to neutral, and a modulation of about 52% to 100% will result in the actuator shifting to the overdrive position. Alternatively, the same results may be obtained by simply providing a variable source of pressure to conduit A by a source selectably variable between 0 psi and 80 psi. The operating characteristics of system  10  may be varied, as required, by varying the relative effective areas of the piston faces  20 A,  20 B and  22 C. 
     The valve assembly  42  includes a solenoid  42 A for controlling the positioning of two-position valve member  42 B. A solenoid controller  42 C, operated by command signals  40  from ECU  36 , is provided to selectively energize and deenergize the coils of the solenoid  42 A from an onboard source (not shown) of electrical power, such as a battery or alternator. All or part of controller  42 C may be integral with ECU  36 . The valve may be of the structure illustrated in aforementioned U.S. Pat. No. 5,661,998. 
     The voltage V applied to the solenoid valve  42  directly affects the valve response time and, as such, the PWM values that result in a neutral state. This valve response time can vary by more than a factor of 2 over the range of 9 to 18 VDC that a typical onboard system must operate under. System voltage V is sensed by controller  42 C and provided to the ECU  36  to adjust the valve PWM value so that the splitter will achieve a neutral state. The control of solenoid  42 A, thus, is done as a variable function of sensed voltage V applied to the solenoid. In particular, response times (and, thus, required lead times) are considered to vary inversely with sensed voltage. 
     The response time of the valve turning off is directly affected by the maximum current in the coil. A circuit  46  is employed in the ECU and/or controller  42 C that rolls back the current in the coil of the solenoid valve to a lower and constant value, regardless of the voltage at the coil. By starting from the same point every time, valve off times are very constant and, as such, the variable effects of coil current levels are greatly reduced. 
     A spring-loaded plunger  48  that fits into a notch  50  in the splitter rod  18  or piston is used to hold the splitter piston  20  in neutral. This spring-loaded plunger or detent increases the range of PWM values that keep the splitter in the neutral state by requiring additional force to move out of this state. The detent is designed such that it gives added force to hold the piston in neutral during PWM conditions but not so much force that it slows the response time for the cylinder as it moves from neutral into gear. 
     The larger the flow orifice in the valve and/or conduits, the smaller the PWM range that will result in the neutral state for the splitter piston. This is because larger orifices flow so much air that the valve can only be open for a very short time before the pressure in the cylinder rises to a point that the splitter moves through neutral. 
     Since the same valve  42 B used for the splitter piston also may be used for the range piston (one that requires high flow, if required), an orifice or restriction  52  was added between the splitter solenoid valve  42 B and the splitter piston chamber  26  to improve this situation. This significantly increases the PWM range to achieve neutral and allows common valves to be used for the splitter and the range pistons. 
     Solenoid-controlled valve assembly  42  is described above in connection with fluid-actuated, three-position actuator piston assembly  12  for controlling shift fork  180 A of splitter clutch  180 . A similar valve assembly may be used in connection with a range piston assembly for controlling the shift fork of range clutch  182 . A preferred exemplary embodiment of such a valve assembly and range piston assembly is shown in FIG.  6 . 
     FIG. 6 shows a fluid-actuated shift actuator system  10 ′, and an actuator piston assembly  12 ′ utilized therein. The structure of system  10 ′ and assembly  12 ′ are substantially similar to system  10  and assembly  12  described above in connection with FIG.  1 . However, piston shaft  18 ′ carries a shift actuator such as shift fork  182 A (for axial movement therewith) which axially positions range clutch  182 . Also, system  10 ′ does not include a notch or a spring-loaded plunger, such as notch  50  and plunger  48  of FIG.  1 . Furthermore, (for high flow purposes) system  10 ′ preferably does not include a restriction such as restriction  52  of FIG.  1 . 
     FIG. 6 also shows a solenoid-controlled valve assembly  42 ′ that is controlled by an ECU  36 ′ which preferably is the same as ECU  36 . Valve assembly  42 ′ includes two (2) two-position valve members  42 B′, with the position of each controlled by a solenoid  42 A′. Each solenoid  42 A′ is selectively energized and de-energized by a solenoid controller  42 C′, each of which is operated by command signals  40 ′ from ECU  36 ′. All or part of each solenoid controller  42 C′ may be integral with ECU  36 ′. Valve members  42 B′ are preferably of the same structure as valve member  42 B described above in connection with the splitter piston assembly of FIG.  1 . 
     ECU  36 ′ receives various input signals  38 ′ which are processed according to predetermined logic rules to issue command output signals  40 ′ to solenoid-controlled valve assembly  42 ′ for controlling the pressurization and exhaust of both conduit A′ (and piston chamber  26 ′) and conduit B′ (and piston chamber  30 ′). In particular, chamber  26 ′ is selectively connected by conduit A′ and passage  28 ′ to a source of pressurized air  44 ′ (preferably the same as source  44  of FIG. 1) or to atmosphere (ATMO), depending upon the position of one of the valve members  42 B′. Chamber  30 ′ is selectively connected by conduit B′ and passage  32 ′ to source  44 ′ or to atmosphere (ATMO), depending upon the position of the other valve member  42 B′. In a manner known to one of ordinary skill in the art, such pressurization and exhaust of chamber  26 ′ and chamber  30 ′ controls the movement of piston shaft  18 ′ and shift fork  182 A, and thus the position of range clutch  182 . 
     FIG. 7 shows a preferred exemplary embodiment of a solenoid controller circuit  300  which may be characterized as a “low side solenoid driver” circuit. As described below, this circuit is preferably used in solenoid controller  42 C of FIG.  1  and solenoid controllers  42 C′ of FIG.  6 . Circuit  300  includes a power supply MOSFET Q 10  having its drain connected to a power supply  302  providing, in the preferred embodiment, vehicle battery voltage of typically 12-14 volts. An amplifier A 1  receives a power supply control signal from an electronic control unit (preferably serving as both ECU  36  and ECU  36 ′) via connection  304 , and passes this signal to the gate of MOSFET Q 10 . When MOSFET Q 10  is driven (ON) by an appropriate voltage signal at its gate, the power supply voltage is supplied to point P 1 . Point  1  is electrically connected to a voltage divider consisting of resistors R 17  and R 18 , with a test point T 1  connected to a digital input of the ECU. The voltage divider functions to convert the supply voltage at point P 1  to 0-5 volts at test point T 1  so that it may be read as a digital input voltage signal by the ECU. Also electrically connected to point P 1  is a solenoid which may be solenoid  42 A of FIG. 1 or one of solenoids  42 A′ of FIG.  6 . Connected in parallel with the solenoid is a diode D 32  in series with a resistor R 132 . 
     Circuit  300  also includes a solenoid driver MOSFET Q 13  which, when driven (ON) by an appropriate voltage signal at its gate, energizes the solenoid (assuming supply voltage from power supply  302  is provided to the solenoid) and passes the solenoid current through a resistor R 9  to ground. Between MOSFET Q 13  and resistor R 9  is a test point T 2  that is electrically connected to an amplifier A 2 . When MOSFET Q 13  is ON and the solenoid is energized, the voltage that is input to amplifier A 2  (the voltage at test point T 2  which is the voltage drop across resistor R 9 ) is proportional to the current through the solenoid. After being electronically scaled and filtered by means known to one of ordinary skill in the art, this voltage signal is supplied via connection  306  to the analog input of the ECU, for reasons discussed below. Connection  306  also feeds back this voltage signal (indicative of solenoid current) to a solenoid current control amplifier A 3  which combines it with a solenoid current level request signal received from the ECU via connection  308  (that is appropriately scaled to an analog voltage signal). The output voltage signal of amplifier A 3  is supplied to the input of a control circuit  310  which also receives a solenoid driver ON/OFF request signal from the ECU via connection  312 . Control circuit  310  determines whether or not MOSFET Q 13  is driven, and thus whether or not the solenoid is energized (assuming supply voltage from power supply  302  is provided to the solenoid). The voltage signal from amplifier A 3  functions as a PWM (pulse width modulation) control which may be used by circuit  310  to control the level of solenoid current. 
     In the preferred embodiment, control circuit  310  is a NOR gate. Also, in the preferred embodiment, resistor R 17  is a 3920 ohm resistor, resistor R 18  is a 5111 ohm resistor, resistor R 132  is a 3 ohm resistor, and resistor R 9  is a 0.1 ohm resistor. 
     In the preferred embodiment described below, solenoid controller circuit  300  is implemented within solenoid controller  42 C of FIG. 1 (used in connection with the control of solenoid  42 A, valve  42 B, and ultimately splitter clutch  180 ) and within the two solenoid controllers  42 C′ of FIG. 6 (used in connection with the control of solenoids  42 A′, valves  42 B′, and ultimately range clutch  182 ). A common power supply  302 , MOSFET Q 10 , and amplifier A 1  are preferably used in all three solenoid controller circuits  300  so implemented, such that one transistor controls the supply of power to all three solenoids (solenoid  42 A and the two solenoids  42 A′). However, in other embodiments, each solenoid controller circuit  300  may be completely separate, including a separate power supply  302  for each. Such alternative embodiments are less preferred due to higher cost considerations associated with more than one such power supply  302 . In addition to the common components mentioned above, a common voltage divider (consisting of resistors R 17  and R 18  with test point T 1  therebetween) is preferably used in all three circuits  300 . 
     The ECU is programmed with fault detection logic of the present invention which is implemented in connection with solenoid controller circuits  300  for detecting faults in solenoid  42 A or in solenoids  42 A′. Shown in FIGS. 8 and 9 are preferred exemplary flowcharts of the programming of the ECU wherein the fault detection logic is implemented. For the sake of simplicity, the flowcharts are directed to the detection of faults within only one of the three solenoids. Of course, in the preferred embodiment, the fault detection logic is likewise applied to the other two solenoids. 
     Referring first to FIG. 8, the fault detection logic begins with block  398  which is executed during normal operation of the ECU programming. In block  398 , the program causes a counter, “counter 1 ”, to be initialized to zero in preparation for its use in subsequent programming. The program then proceeds to block  400  wherein the current in the solenoid is periodically sampled by the ECU via connection  306  of its solenoid controller circuit  300  (as described above in connection with FIG.  7 ). If the solenoid that is being checked for faults is not initially energized (when the fault detection logic is executed), block  400  causes the ECU to briefly energize the solenoid (provide an appropriate drive signal to the gate of MOSFET Q 13 ) so that the current flows and a voltage signal indicative of current is supplied to the ECU via connection  306 . As known to one of ordinary skill in the art, the duration of such energization is kept to a minimum so that the valve associated with the solenoid is not inadvertently actuated. 
     The program then proceeds to block  402  which determines whether or not the sampled solenoid current is abnormally “low” (below a predetermined current value) for more than a predetermined period of time. In the preferred embodiment, the predetermined current value (against which solenoid current is compared) is a value representing approximately half of the current that is normally expected in the solenoid when in its then-current operating condition. For example, in one embodiment, normal solenoid current (for an initially active solenoid) is in the range of 1-1.3 amps, and the predetermined current value (against which actual current value is compared) is appropriately 500 millliamps. The predetermined period of time will depend on whether the solenoid was initially energized or if it had to be briefly energized in order to sample the current (as described above). If the solenoid was initially energized, the predetermined time period preferably is approximately 100 milliseconds. If the solenoid was not initially energized, the predetermined time period preferably is approximately 300 milliseconds. 
     If block  402  determines that the sampled solenoid current is not below the predetermined current value for more than the predetermined period of time (“NO”), then the possibility of a solenoid fault is not indicated (or likely), and the program proceeds to other portions of the ECU programming. 
     If, however, block  402  determines that the sampled solenoid current is below the predetermined current value for more than the predetermined time period (“YES”), then a possibility exists that the solenoid has a fault. Faults possibly indicated include (1) an opened solenoid coil winding, (2) a shorted solenoid coil winding, (3) the low side power leads of the solenoid shorted to ground, and (4) opened solenoid power leads. The program then proceeds to block  406  which causes the DIAGNOSTICS PROCEDURE shown in FIG. 9 to be executed. The DIAGNOSTICS PROCEDURE, described in greater detail below, returns with an “inconclusive” result, an “open fault” result (corresponding to either fault (1) or (4) listed above), or a “shorted fault” result (corresponding to either fault (2) or (3) listed above). The program then proceeds to block  408  which determines whether or not a fault result (“open” or “shorted”) has been returned. If so (“YES”), the program proceeds to block  410  which causes the ECU to operate the transmission in an appropriate “fallback” mode of operation which is designed to prevent damage to the transmission, or undesirable transmission operation, despite the existence of the solenoid fault. The manner of selection of an appropriate fallback mode depends, at least in part, upon whether the fault result is an “open fault” or a “shorted fault” result, as well as the state of the transmission when the fault is detected. Block  410  preferably also causes the ECU to indicate the detected fault to the operator of the vehicle. Preferably, a warning lamp on the operator&#39;s shift knob is employed for this purpose, and preferably identifies the faulty solenoid by implementing a specific blink rate for that particular solenoid (blink codes). 
     If, however, block  408  determines that a fault result (“open” or “shorted”) has not been returned (“NO”), then an “inconclusive” result must have been returned and the program proceeds to block  412 . An “inconclusive” result occurs when solenoid current abnormally dropped but the DIAGNOSTICS PROCEDURE is unable to detect a solenoid fault. Such an “inconclusive” result can be caused by a variety of things, most notably when an EMI (electromagnetic interference) event is experienced by the vehicle. An EMI event can drive LOW the analog input lines to the ECU, resulting in an erroneously “low” reading of solenoid current that triggers the solenoid fault DIAGNOSTICS PROCEDURE. Because an EMI event is normally of short duration, a vehicle may be able to “ride through” such an event until the ECU&#39;s analog input lines return to normal and sampled current readings are once again accurate. Block  412  facilitates such a “ride through” attempt. Block  412  determines whether or not counter “counter 1 ” is equal to 6. If not (“NO”), then the program proceeds to block  414  which adds one to the counter, and the program loops back to block  400  to sample the solenoid current again, thereby beginning another fault detection logic cycle. By means of blocks  412  and  414 , the program repeats the fault detection logic cycle seven times (in the preferred embodiment), thereby trying to wait out an EMI event (or other cause of an “inconclusive” result). If, during one of these seven cycles, the cause of the “inconclusive” result terminates and the sampled current returns to normal levels, then block  402  will cause the program to exit the fault detection logic and proceed to other portions of the ECU&#39;s programming. If, however, throughout the seven cycle time period, block  402  continually determines than an abnormally low current is present and “inconclusive” results are continually returned from the DIAGNOSTICS PROCEDURE, then block  412  will eventually determine that counter 1  counted to 6 (“YES”), and the program proceeds to block  416 . Block  416  then causes the ECU to operate the transmission in an appropriate fallback mode, and to indicate the detected “inconclusive fault” to the operator of the vehicle (preferably in the manner described above in connection with block  410 ). In this situation, a problem exists with the solenoid, but it is a problem which the DIAGNOSTICS PROCEDURE was unable to identify. 
     Referring now to FIG. 9, shown is the DIAGNOSTICS PROCEDURE that is executed by block  406  of FIG.  8 . The procedure begins with block  450  that causes two counters, “counter 2 ” and “counter 3 ”, to be initialized to zero in preparation for their use in subsequent programming. Next, block  452  begins an “open circuit solenoid test”, the purpose of which is to detect the possible existence of an “open fault” such as an opened solenoid coil winding or opened solenoid power leads. In the preferred embodiment, block  452  causes the other two solenoids (those which are not the present subject of the fault detection logic) to be (or remain) de-energized. The purpose of this de-energization is to isolate the potentially faulty solenoid (all three solenoids share a common power supply  302  in the preferred embodiment) so that the “decay test” described below is only affected by the potentially faulty solenoid. De-energization is achieved via appropriate control signals from the ECU to control circuit  310  of each solenoid&#39;s solenoid controller circuit  300 , whereby each solenoid&#39;s solenoid driver MOSFET Q 13  is turned OFF. Block  452  also causes the potentially faulty solenoid to be (or remain) energized by an appropriate drive signal to the gate of its driver MOSFET Q 13 . 
     The program then proceeds to block  454  which causes the ECU to turn OFF power supply MOSFET Q 10  so that power supply voltage is no longer supplied to point P 1  of circuit  300  (FIG.  7 ). At this time, a “decay test” begins whereby the voltage at point P 1  begins to decay at a certain rate. As the voltage decays, the voltage divider of circuit  300  converts the decayed voltage to 0-5 volts at test point T 1 . After a predetermined time period (approximately 40 milliseconds in the preferred embodiment), block  454  causes the ECU to read the voltage at test point T 1  as a digital input voltage. If the voltage at point P 1  has decayed (during the 40 millisecond time period) to such an extent that the converted voltage at test point T 1  drops below approximately 0.8 volts (in the preferred embodiment), then the ECU will digitally read the T 1  voltage value as a LOW logic state. On the other hand, the ECU will digitally read the T 1  voltage value as a HIGH logic state if the T 1  voltage is 0.8 volts or higher. (In the preferred embodiment using the circuit  300  component values set forth above, a voltage of 0.8 volts at test point T 1  corresponds to a voltage of 1.4 volts at point P 1 .) After the logic state of test point T 1  has been read by the ECU, block  454  then causes the ECU to turn ON power supply MOSFET Q 10  so that power supply voltage is again supplied to point P 1  of circuit  300 , and causes all three solenoids to return to their original energized or de-energized states. This reconnection of the power supply voltage and return of the solenoids to their original states must be performed within a particular time period in order to prevent undesired valve changes that could affect transmission operation. A time period of approximately 90 milliseconds was experimentally determined to be an appropriate time period within which to have block  454  perform the described restorations. 
     After block  454 , then program proceeds to block  456  which determines whether or not the logic state read by the ECU at test point T 1  is HIGH. If so (“YES”), then the solenoid is assumed to have an “open fault”, and the program proceeds to block  458 . The reason for this assumption is that an energized solenoid, without an “open fault”, would cause a fast discharge of energy whereby the logic state at test point T 1  would be read as LOW. The fact that such a fast discharge did not take place is strongly indicative of the existence of an “open fault”. However, before the assumption of an “open fault” is accepted as a fact, it is desirable, in the preferred embodiment, to repeat the “open circuit solenoid test” in order to confirm the results. Block  458  facilitates such a confirmation. Block  458  determines whether or not counter “counter 2 ” is equal to 2. If not (“NO”), then the program proceeds to block  460  which adds one to the counter, and the program loops back to block  452  to begin another “open circuit solenoid test”. By means of blocks  458  and  460 , the program repeats the test three times (in the preferred embodiment). If, during one of these three test cycles, the logic state at test point T 1  is read as LOW, then confirmation is not achieved, and block  456  will cause the test to be exited. If, however, block  456  determines that the logic state is HIGH for three consecutive test cycles, then confirmation is achieved, and block  458  will eventually determine that counter 2  counted to 2 (“YES”). The program then proceeds to block  462  which causes the DIAGNOSTICS PROCEDURE to return an “open fault” result to block  406  of FIG.  8 . 
     In the preferred embodiment, the assumption of an “open fault” can be confirmed in another way when the possibly faulty solenoid is solenoid  42 A of FIG. 1 (which is used in the actuation of splitter piston assembly  12 ), and the possibly faulty solenoid was originally energized (when fault detection logic was initiated). In these circumstances, if splitter shift fork  180 A slips into neutral, then a high probability exists that an “open fault” is present. Therefore, detection by the ECU of such slippage (when the proper circumstances exist) can serve as confirmation of an “open fault”, and is included in the logic of block  458  (or elsewhere) in the preferred embodiment. Because the aforementioned slippage is highly undesirable, inclusion of such logic is especially preferable since it speeds up the identification of the fault in those circumstances. 
     Returning now to block  456 , if it determines that the logic state read by the ECU at test point T 1  is not HIGH (“NO”), then the program proceeds to block  464 . Block  464  begins a “short solenoid test”, the purpose of which is to detect the possible existence of a “shorted fault” such as a shorted solenoid coil winding or when the low side power leads of the solenoid are shorted to ground. Similar to block  452 , block  464 , in the preferred embodiment, causes the other two solenoids (those which are not the present subject of the fault detection logic) to be (or remain) de-energized, for the purpose described above. Block  464 , however, also causes the potentially faulty solenoid to be (or remain) de-energized by turning OFF its driver MOSFET Q 13 . 
     The program then proceeds to block  466  which performs the same operations as block  454 , described above. After block  466 , the program proceeds to block  468  which determines whether or not the logic state read by the ECU at test point T 1  is LOW. If so (“YES”), then the solenoid is assumed to have a “shorted fault”, and the program proceeds to block  470 . The reason for this assumption is that a de-energized solenoid, without a “shorted fault”, would result in a slow discharge of energy (during the “decay test”) whereby the logic state at test point T 1  would be read as HIGH. The fact that a fast discharge instead took place is strongly indicative of the existence of a “shorted fault”. However, before the assumption of a “shorted fault” is accepted as a fact, it is desirable, in the preferred embodiment, to repeat the “short solenoid test” to confirm the results. In the manner similar to that described above in connection with blocks  458  and  460 , blocks  470  and  472  facilitate such a confirmation by repeating the test three times (in the preferred embodiment). If, during one of these three test cycles, the logic state at test point T 1  is read as HIGH, then confirmation is not achieved, and block  468  will cause the test to be exited. If, however, block  468  determines that the logic state is LOW for three consecutive test cycles, then confirmation is achieved, and block  470  will eventually determine that counter  3  counted to 2 (“YES”). The program then proceeds to block  474  which causes the DIAGNOSTICS PROCEDURE to return a “short fault” result to block  406  of FIG.  8 . 
     In the preferred embodiment, the assumption of a “short fault” can be confirmed in another way when the possibly faulty solenoid is solenoid  42 A (used in the actuation of splitter piston assembly  12 ), and the possibly faulty solenoid was originally de-energized (when fault detection logic was initiated). In these circumstances, if splitter shift fork  180 A slips into neutral, a high probability exists that a “short fault” is present. Therefore, detection by the ECU of such slippage (when the proper circumstances exist) can serve as confirmation of a “short fault”, and is preferably included in the logic of block  470  (or elsewhere) whereby it advantageously speeds up the identification of the fault in those circumstances. 
     Returning now to block  468 , if it determines that the logic state read by the ECU at test point T 1  is not LOW (“NO”), then the program proceeds to block  476 . Block  476  causes the DIAGNOSTICS PROCEDURE to return an “inconclusive” result to block  406  of FIG.  8 . 
     Although the preferred embodiment of the present invention is described above in connection with a low side solenoid driver circuit, the fault detection logic of the present invention may, of course, be implemented with other appropriate circuit configurations. Furthermore, the present invention may be implemented for the detection of faults in solenoids used in any portion of a transmission as well as in other non-transmission related systems. 
     Although the preferred embodiment of the present invention has been described with a certain degree of particularity, various changes to form and detail may be made without departing from the spirit and scope of the invention as hereinafter claimed.