Abstract:
A transmission utilizes an electro-magnetically actuated selectable one-way-clutch. The one-way-clutch prevents rotation of a transmission member in both directions when a current exceeds a threshold and permits rotation in only one direction otherwise. To prevent un-intended engagement, a switch interrupts the current unless a second current exceeds a threshold. In order to engage the one-way-clutch, both currents are set above their respective thresholds by a controller. In the event of a single fault such as a short circuit, the system continues to function normally. The controller may periodically test for a fault by intentionally setting one current above its threshold and the other below its threshold and determining the state of the one-way-clutch by measuring speeds of transmission elements.

Description:
TECHNICAL FIELD 
     This disclosure relates to the field of transmission systems. More particularly, the disclosure pertains to a transmission control system including a selectable one-way-clutch, an electric switch, and methods of controlling these devices via a controller. 
     BACKGROUND 
     Many vehicles are used over a wide range of vehicle speeds, including both forward and reverse movement. Some types of engines, however, are capable of operating efficiently only within a narrow range of speeds. Consequently, transmissions capable of efficiently transmitting power at a variety of speed ratios are frequently employed. When the vehicle is at low speed, the transmission is usually operated at a high speed ratio such that it multiplies the engine torque for improved acceleration. At high vehicle speed, operating the transmission at a low speed ratio permits an engine speed associated with quiet, fuel efficient cruising. Typically, a transmission has a housing mounted to the vehicle structure, an input shaft driven by an engine crankshaft, and an output shaft driving the vehicle wheels, often via a differential assembly which permits the left and right wheel to rotate at slightly different speeds as the vehicle turns. 
     A common type of automatic transmission utilizes a collection of shift elements. Various subsets of the shift elements are engaged to establish the various speed ratios. A common type of shift element utilizes a clutch pack having separator plates splined to a housing and interleaved with friction plates splined to a rotating shell. When the separator plates and the friction plates are forced together, torque may be transmitted between the housing and the shell. A controller controls the torque capacity on such a friction clutch by adjusting the force squeezing the clutch pack. Friction clutches are capable of transmitting torque in the presence of relative speed between the separator plates and the friction plates. Some drag torque is transmitted even when a friction clutch is disengaged reducing efficiency. 
     Another type of shift element is a one-way-clutch. A one-way-clutch permits relative rotation between two races in one direction, but prevents relative rotation in the opposite direction. One-way-clutches have advantages and disadvantages relative to friction clutches. Shift quality is improved when the off-going element is a one-way-clutch because the clutch passively releases at the correct time. Generally, the parasitic drag of one-way-clutches is lower than that of friction clutches. However, one-way-clutches cannot transmit torque in the presence of relative rotation between the races. Some one-way-clutches are selectable. A selectable one-way-clutch (SOWC) can be actively controlled to be in one of multiple states which vary in terms of which directions of relative rotation are permitted. For example, a selectable one-way-clutch may permit relative rotation in one direction but not the other in one state and preclude relative rotation in both directions in another state. Like non-selectable one-way-clutches, selectable one-way-clutches are not capable of transmitting torque in the presence of relative rotation. 
     SUMMARY OF THE DISCLOSURE 
     A transmission includes a selectable one-way-clutch, a switch, and a controller. The selectable one-way-clutch is configured to prevent relative rotation between two transmission elements in response to a first current exceeding a first threshold and to permit relative rotation in only one direction in response to the first current being less than the first threshold. One of the elements may be a non-rotating transmission case. The switch is configured to interrupt the first current in response to a second current not exceeding a second threshold. The controller sets the state of the selectable one-way-clutch in response to manipulation of a shift lever by manipulating the first and second currents. Specifically, the controller may be programmed to engage the selectable one-way-clutch in response to selection of a reverse range by setting the first and second currents to levels exceeding the first and second thresholds respectively, and to dis-engage the selectable one-way-clutch in response to selection of a drive range by setting the first and second electrical currents to levels less than the first and second thresholds respectively. A first wire may electrically connect the controller to the selectable one-way-clutch while a second wire electrically connects the switch to the controller such that the first current flows through the first and second wires. In some embodiments, a third wire electrically connects the switch to the controller such that the second current flowing through the second and third wires. In other embodiments, a third wire electrically connects the switch to the controller while a fourth wire electrically connects the switch to the controller such that the second current flows through the third and fourth wires. 
     A method of operating a transmission includes responding to selection of a reverse range by engaging a selectable one-way-clutch and preparing for an upshift by disengaging the selectable one-way-clutch in one direction. The selectable one-way-clutch is engaged by regulating a first and a second current to levels exceeding first and second thresholds respectively. The selectable one-way-clutch is dis-engaged in one direction by regulating the first and second currents to levels less than the first and second thresholds respectively. The controller may test for a fault by regulating the first current to a first level exceeding the first threshold, regulating the second current to a second level less than the second threshold, and increasing a torque capacity of a friction clutch. If a transmission speed ratio reacts in a manner inconsistent with the selectable one-way-clutch being disengaged, an error code is set. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of a transmission gearing arrangement. 
         FIG. 2  is a schematic diagram of a first electrical circuit for controlling a selectable one-way-clutch. 
         FIG. 3  is a schematic diagram of a second electrical circuit for controlling a selectable one-way-clutch. 
         FIG. 4  is a schematic diagram of a third electrical circuit for controlling a selectable one-way-clutch. 
         FIG. 5  is a schematic diagram of a fourth electrical circuit for controlling a selectable one-way-clutch. 
         FIG. 6  is a flow chart for a method of controlling a selectable one-way-clutch. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present disclosure are described herein. It is to be understood, however, that the disclosed embodiments are merely examples and other embodiments can take various and alternative forms. The figures are not necessarily to scale; some features could be exaggerated or minimized to show details of particular components. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for teaching one skilled in the art to variously employ the present invention. As those of ordinary skill in the art will understand, various features illustrated and described with reference to any one of the figures can be combined with features illustrated in one or more other figures to produce embodiments that are not explicitly illustrated or described. The combinations of features illustrated provide representative embodiments for typical applications. Various combinations and modifications of the features consistent with the teachings of this disclosure, however, could be desired for particular applications or implementations. 
     A transmission gearing arrangement is illustrated schematically in  FIG. 1 . Mechanical connections are illustrated with solid lines, whereas control signals are indicated with dashed lines. Input shaft  10  is driven by an internal combustion engine, potentially via a launch device such as a torque converter. Output  12  drives the vehicle wheels, potentially via gearing and a differential. The various components of the gearing arrangement are supported within a transmission case  14  that is fixed to vehicle structure. The transmission utilizes four simple planetary gear sets  20 ,  30 ,  40 , and  50 . A planet carrier  22  rotates about a central axis and supports a set of planet gears  24  such that the planet gears rotate with respect to the planet carrier. External gear teeth on the planet gears  24  mesh with external gear teeth on a sun gear  26  and with internal gear teeth on a ring gear  28 . Sun gear  26  and ring gear  28  are supported to rotate about the same axis as the carrier. Gear sets  30 ,  40 , and  50  are similarly structured. 
     Sun gear  46  is fixedly coupled to input shaft  10 . Ring gear  38  and carrier  52  are fixedly coupled to output  12 . Carrier  22  is fixedly coupled to sun gear  36 . Ring gear  28 , carrier  42 , and ring gear  58  are mutually fixedly coupled. Carrier  32  is fixedly coupled to ring gear  48 . Clutch  62  selectively couples ring gear  28  to input shaft  10 . Sun gear  26  is selectively coupled to input shaft  10  by clutch  60  and selectively held against rotation by brake  64 . Brake  66  selectively holds sun gear  56  against rotation. Brake  68  selectively holds carrier  22  and sun gear  36  against rotation. Selectable one-way-clutch  70  has two states. In a disengaged state, selectable one-way-clutch  70  passively holds carrier  32  and ring gear  48  against rotation in one direction while permitting rotation in the other direction. In an engaged state, selectable one-way-clutch  70  holds carrier  32  and ring gear  48  against rotation in both directions. 
     As shown in Table 1, engaging shift elements  60 - 70  in combinations of two establishes eight forward speed ratios and one reverse speed ratio between input shaft  10  and output  12 . An X indicates that the shift element must be engaged to establish the speed ratio. The (X) for selectable one-way-clutch  70  in 1st gear indicates that selectable one-way-clutch  70  may be in either the engaged state or the disengaged state in order to transfer power from the input shaft to the output shaft in 1st gear. To transfer power from the output shaft to the input shaft in 1st gear, selectable one-way-clutch  70  must be in the engaged state. Selectable one-way-clutch  70  must be in the disengaged state prior to shifting into 2nd gear. 
     
       
         
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                 60 
                 62 
                 64 
                 66 
                 68 
                 70 
                 Ratio 
                 Step 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Rev 
                 X 
                   
                   
                   
                   
                 X 
                 −3.79 
                 89% 
               
               
                 1st 
                   
                   
                   
                 X 
                   
                 (X) 
                 4.26 
                   
               
               
                 2nd 
                   
                   
                   
                 X 
                 X 
                   
                 2.73 
                 1.56 
               
               
                 3rd 
                   
                   
                 X 
                 X 
                   
                   
                 2.19 
                 1.25 
               
               
                 4th 
                 X 
                   
                   
                 X 
                   
                   
                 1.71 
                 1.28 
               
               
                 5th 
                   
                 X 
                   
                 X 
                   
                   
                 1.33 
                 1.29 
               
               
                 6th 
                 X 
                 X 
                   
                   
                   
                   
                 1.00 
                 1.33 
               
               
                 7th 
                   
                 X 
                 X 
                   
                   
                   
                 0.85 
                 1.18 
               
               
                 8th 
                   
                 X 
                   
                   
                 X 
                   
                 0.69 
                 1.23 
               
               
                   
               
             
          
         
       
     
     Controller  80  adjusts the states of each shift element by sending control signals. The torque capacities of shift elements  60 ,  62 ,  64 ,  66 , and  68  are adjusted by sending electrical control signals  82  to valve body  84 . In response, valve body  84  adjust the pressures in hydraulic circuits  86  routed to the respective shift elements. Controller  80  sets the state of selectable one-way-clutch  70  using electrical signal  88 . Specifically, controller  80  sets current in an electrical circuit  88  above a threshold to engage selectable one-way-clutch  70  such that relative rotation is prevented in both directions. When the electrical current in circuit  88  is less than the threshold, one-way-clutch  70  is in a disengaged state in which carrier  32  and ring gear  48  are permitted to rotate in a forward direction (same direction as normal engine operation) but precluded from rotating in a negative direction. 
       FIG. 2  schematically illustrates an electrical circuit for controlling selectable one-way-clutch  70 . Electrical power is provided by a battery  100  having a positive terminal  102  and a negative terminal  104 . Alternatively, power may be provided by other types of electric power sources such as an engine driven alternator. The negative terminal  104  may be connected to a ground plane  106 . Controller  80  receives electrical power by electrically connecting a power terminal  108  to the battery positive terminal  102  and connecting a ground terminal  110  to the battery negative terminal  104  or the ground plane  106 . Selectabel one-way-clutch  70  enters the engaged state when current in excess of a threshold flows from terminals  112  to  114 . Terminal  112  is connected to terminal  116  by wire  118  and signal wire  88  connects terminal  120  to terminal  114 . Internal switches within controller  80  selectively connect terminal  108  to terminal  116  and selectively connect terminal  110  to terminal  120 . These circuits may have specified resistances such that a desired level of current flows when the internal switches are closed. Wire  118  is used to provide electrical current to various other transmission components, such as solenoids in valve body  84 , so the internal switches that selectively connect terminal  108  to terminal  116  are typically closed whenever the transmission is operating. In order to engage selectable one-way-clutch  70 , controller  80  closes the internal switches that connect terminal  110  to terminal  120 , providing a complete circuit through selectable one-way-clutch  70 . To disengage one-way-clutch  70 , controller  80  opens the internal switches that connect terminal  110  to terminal  120 , preventing current from flowing through selectable one-way-clutch  70 . 
     In addition to provided intended function when all components are connected and operating as designed, it is important to anticipate the types of error conditions that might occur and ensure that the consequences of such error conditions are not too severe. One type of error condition that may occasionally occur in an electrical control system is electrical connection of wires or components that are supposed to be electrically insulated from one another, called short circuiting. If a short circuit occurs between wire  88  and ground plain  106 , then electrical current will flow through selectable one-way-clutch  70  whenever terminal  116  is powered. As mentioned above, since other components need power during many operating conditions, terminal  116  is powered whenever the transmission is operating. Consequently, selectable one-way-clutch  70  may be engaged even when the internal switches within controller  80  that are intended to engage it are open. 
     The consequences of unintended engagement of selectable one-way-clutch  70  depend upon the operating state of the vehicle when the error condition first occurs. If the unintended engagement occurs while the transmission is in reverse or in 1st gear with positive torque, then no change will be noticed until an upshift from 1st gear is attempted. When an upshift is attempted, selectable one-way-clutch  70  will not release properly, so the transmission will enter a tie-up state with loss of output torque. However, if the unintended engagement occurs while selectable one-way-clutch  70  is in an over-running condition, the sudden engagement will result in very abrupt speed changes of transmission components and very high torques imposed on transmission components. In some cases, the components may fail as a consequence of the stresses imposed. 
       FIG. 3  schematically illustrates an electrical circuit for controlling selectable one-way-clutch  70  that prevents the failure mode described above from occurring in the presence of any single short circuit. Switch  130  is interposed between terminal  116  of controller  80  and terminal  112  of selectable one-way-clutch  70 . When a current above a threshold flows from terminal  132  to terminal  134 , switch  130  electrically connects terminal  132  to terminal  136 . When the current from terminal  132  to terminal  134  is below the threshold, on the other hand, switch  130  electrically disconnects terminal  132  from terminal  136 . Terminal  132  is connected to controller terminal  116 , terminal  136  is connected to controllable one-way-clutch terminal  112 , and terminal  134  is connected to controller terminal  138 . Like the circuit of  FIG. 2 , internal switches in controller  80  connect terminal  116  to terminal  108  during most transmission operating conditions. In order to engage selectable one-way-clutch  70 , controller  80  closes internal switches to connect both terminals  120  and  138  to terminal  110 . If terminal  138  is not connected to terminal  110 , then switch  130  opens such that terminal  136  is not powered. Therefore, no current would flow through selectable one-way-clutch  70  even if a short circuit condition exists between wire  88  and ground plane  106 . Similarly, if a short circuit occurs between terminal  134  and ground plane  106 , then the circuit behaves the same as the circuit of  FIG. 2 . 
     Various types of switches may be utilized depending upon the magnitude of the current required to engage the selectable one-way-clutch. For relatively low current applications, transistors may be used. For higher current applications, relays may be used. The switch may be physically located near the selectable one-way-clutch, near the controller, or at an intermediate location. If the switch is integrated with the selectable one-way-clutch, then terminal  136 , terminal  112 , and the electrical connection between them may not be discrete identifiable components. 
     In  FIG. 3 , the powered terminal is shared among components while each component has a separately controlled ground terminal. Similar functionality could be achieved with a shared ground terminal and separately controlled powered terminals. In such an arrangement, it may be preferable to locate the switch on the ground (shared) side of the selectable one-way-clutch. 
       FIG. 4  schematically illustrates another circuit for controlling selectable one-way-clutch  70 . Although controller  80  may not have a sufficient number of separately controlled power terminals to provide separate power supplies for each transmission component, it may have more than one power terminal. In the circuit of  FIG. 4 , some transmission components receive electrical power via terminal  116  while other components receive electrical power via terminal  140 . The components may be grouped such that some transmission functionality is available even if a failure disables one of the two terminals. Switch  142  is interposed between terminal  116  of controller  80  and terminal  112  of selectable one-way-clutch  70 . When a current above a threshold flows from terminal  144  to terminal  146 , switch  142  electrically connects terminal  148  to terminal  150 . When the current from terminal  144  to terminal  146  is below the threshold, on the other hand, switch  142  electrically disconnects terminal  148  from terminal  150 . Terminal  144  is connected to controller terminal  140 , terminal  146  is connected to controller terminal  138 , terminal  148  is connected to controller terminal  116 , and terminal  150  is connected to selectable one-way-clutch terminal  112 . Operation of the circuit of  FIG. 4  is identical to operation of the circuit of  FIG. 3 . 
       FIG. 5  illustrates an alternative circuit that may be suitable when the electrical current required to engage the selectable one-way-clutch is high relative to the current draw of the controller. Terminal  148  of switch  142  is electrically connected to the positive terminal  102  of the battery  100 . As with the circuit of  FIG. 4 , terminal  112  of the selectable one-way-clutch is powered whenever controller  80  internally connects terminal  138  to terminal  110  causing current to flow between terminals  144  and  146  of switch  142  to close the switch. A second switch  160  is utilized to selectively connect terminal  114  to ground plain  106 . When current above a threshold flows between terminals  162  and  164 , switch  160  closes to electrically connect terminal  166  to terminal  168 . The current that engages selectable one-way-clutch  70  does not flow through controller  80 . Controller  80  may be powered by the same power source as the selectable one-way-clutch as shown or may be powered from a different source of electrical energy. 
       FIG. 6  is a flow chart illustrating logic that may be used by controller  80  to switch and selectable one way clutch (or, in the case of  FIG. 5 , the two switches). Starting at  170 , the controller first determines at  172  whether reverse is commanded. This is determined primarily based on a position of a shift selector which is manipulated by a driver. However, there may be some conditions in which reverse is not commanded immediately such as if the driver moves the shift lever to the reverse position while the vehicle is moving forward rapidly. If reverse is commanded, then, at  174 , the controller sets both the switch current and the SOWC current above the required threshold to engage selectable one-way-clutch  70 . If reverse is not commanded, the controller determines at  176  whether low range is commanded. In low range, the transmission should be configured to transmit negative torque, so SOWC  70  should be engaged when the transmission is in 1st gear and the torque is negative. If the transmission is not in 1st gear, as determines at  178 , then the current in both circuits is set below the corresponding threshold at  180  to ensure that SOWC  70  is disengaged. Similarly, if the transmission is in drive, as determined at  182 , then the SOWC is disengaged at  178  regardless of which forward gear is currently selected. If the transmission is not in park, as determined at  184 , then the controller determines that it must be in neutral and disengages SOWC at  180 . 
     Since the behavior of the system does not change in the presence of a single failure, such as a short circuit, it is likely that such as failure would go unnoticed. One a single failure has occurred, however, the system is no longer safe if an additional failure occurs. Therefore, it may be desirable to periodically check for the presence of a single failure.  FIG. 6  illustrates several potential ways of accomplishing that. These tests may be carried out during operating conditions in which unintended engagement can be detected but would not cause damage. If the transmission is in low range and in 1st gear, the controller determines at  180  whether such a test should be conducted. Although illustrated in  FIG. 6  in low range, the test could also be conducted in drive. The decision may be based on how much time has elapsed since the previous test. Also, if the torque is currently negative, then no test should be conducted. If no such test is desired, then controller immediately engages SOWC at  174  in the normal way. If a test is be conducted, then controller  80  sets one current above the threshold and the other below at  182 . To test for a short between wire  88  and ground, the switch is turned on and the SOWC is turned off. To test for a short between terminal  146  and ground, the switch is turned off and the SOWC is turned on. If a short-circuit is present, the SOWC will be in an engaged state. Otherwise, it will be in a dis-engaged state. To determine whether the SOWC is engaged, the torque capacity of  68  is gradually increased at  184 . If the SOWC is disengaged, the transmission ratio will begin to decrease as it would during a 1-2 shift which can be detected at  186 . This process may be timed to coincide with a scheduled 1-2 shift such that the vehicle occupants will not notice as long as no error is present. If no ratio change occurs, then an error code is set at  188  to provide information to a service technician. The controller may take additional actions such as turning off power to terminals  116  or  140  or restricting the transmission to reverse and 1st gear. 
     Another opportunity to test for a single failure occurs at  190  while the vehicle is in park. Normally, while the vehicle is in park, no shift elements are engaged, including SOWC  70 . If a test is desired, one of the currents is set above the threshold and one below at  192 . If the switch is off and the SOWC is on, a short circuit would result in current at terminal  120 . If the controller detects this at an error code is set at  196  and further error mitigation actions may be taken. If no current is detected, or if the controller is not capable of measuring this current, then the controller test whether SOWC is engaged by engaging clutch  60  at  198 . While the transmission is in neutral, the turbine shaft will rotate at near the speed of the engine. If the transmission is shifted into a gear with the output shaft stationary, the turbine shaft will slow to a stop. If SOWC is disengaged, then the transmission will still be in a neutral state after engaging clutch  60 . However, if the SOWC is engaged, engaging clutch  60  will place the transmission in reverse, causing the turbine shaft to stop which can be detected at  200 . 
     While exemplary embodiments are described above, it is not intended that these embodiments describe all possible forms encompassed by the claims. The words used in the specification are words of description rather than limitation, and it is understood that various changes can be made without departing from the spirit and scope of the disclosure. As previously described, the features of various embodiments can be combined to form further embodiments of the invention that may not be explicitly described or illustrated. While various embodiments could have been described as providing advantages or being preferred over other embodiments or prior art implementations with respect to one or more desired characteristics, those of ordinary skill in the art recognize that one or more features or characteristics can be compromised to achieve desired overall system attributes, which depend on the specific application and implementation. As such, embodiments described as less desirable than other embodiments or prior art implementations with respect to one or more characteristics are not outside the scope of the disclosure and can be desirable for particular applications.