Abstract:
A powertrain incorporates a link shaft between the transmission output shaft and downstream components in order to facilitate use of a torque sensor. The link shaft includes an inner section splined to the output shaft and an outer section treated to produce a magnetic field that varies based on transmitted torque. The inner section and outer section are separated by a circumferential gap that directs the torque past the treated surface. Use of the link shaft eliminates the need to directly treat the transmission output shaft. Separating the spline and the treated surface radially reduces the axial length required for torque sensing and allows commonality among variants with and without torque sensors and between two wheel drive and four wheel drive variants.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. provision application Ser. No. 62/105,969 filed Jan. 21, 2015, the disclosure of which is hereby incorporated in its entirety by reference herein. 
     
    
     TECHNICAL FIELD 
       [0002]    This disclosure relates to the field of automatic transmission and transfer cases. More particularly, the disclosure pertains to installation of a transmission output torque sensor. 
       BACKGROUND 
       [0003]    An automatic transmission establishes various power flow paths having different speed ratios by selectively engaging and disengaging a number of shift elements. The shift elements include friction clutches. A controller sets the torque capacity of each friction clutch by sending control signals. For example, a controller may send a pulse width modulated signal to a solenoid such that the electromagnetic force exerted by the solenoid is proportional to the pulse width. The solenoid may be connected to a spool valve in a valve body such that the hydraulic pressure in a particular passageway of the valve body is proportional to the electromagnetic force. Fluid from the passageway may be routed to a piston apply chamber of a clutch causing the piston to exert a force related to the fluid pressure. The piston force may squeeze friction plates between separator plates to establish a torque capacity of a clutch. Alternatively, the signal may influence the torque capacity by other mechanism such as causing a motor to rotate, thus causing a piston to apply force to a clutch pack. Due to the indirect causal chain between the signal and the clutch torque capacity, a number of unpredictable noise factors may influence the relationship. 
         [0004]    When the controller determines that a speed ratio change is needed, the controller may execute a shift by releasing one shift element called an off-going element and engaging another shift element called an on-coming element. For the shift to feel smooth to vehicle occupants, it is important that the torque capacity of the on-coming element and the torque capacity of the off-going element be coordinated with respect to one another and with respect to the engine torque. For example, if the off-going clutch is released prematurely, then the driveshaft torque will drop excessively and the engine speed will rise. This phenomenon is called an engine flare. On the other hand, if the off-going element is released too late, then the two shift elements will resist one another and the output torque will drop excessively. This phenomenon is called a tie-up. 
         [0005]    In order to adjust the control signals such that the torque capacities are proper in the presence of unknown noise factors, a controller may utilize a feedback signal. For example, a controller may utilize signals from an input speed sensor and an output speed sensor to compute the current speed ratio of the transmission. An unexpected or excessively large increase in the ratio of input speed to output speed may indicate an engine flare. In response to this information, the controller may increase the torque capacity of the on-coming shift element. However, some errors, such as a tie-up, are not necessarily reflected in the speed ratio. Furthermore, it takes time for an error in shift element torque capacity to show up as a change in the speed ratio. Therefore, it is desirable to supplement this information with a measurement of the transmission output torque. 
       SUMMARY 
       [0006]    A powertrain includes a link shaft adapted to couple a transmission output shaft to a downstream powertrain component. The link shaft includes an inner section and an outer section. The inner section is adapted for fixation to the transmission output shaft by, for example, a spline. The outer section is concentric with and axially overlapping the inner section. An outer surface of the outer section is treated to produce a magnetic field that varies as a transmitted torque varies. The inner and outer sections are separated by a circumferential gap to direct torque from the transmission shaft past the outer surface. 
         [0007]    In some embodiments, the link shaft may couple the transmission output shaft directly to a driveshaft. In such embodiments, the link shaft may include a flange fixed to the outer section and having holes such that the driveshaft may be attached to the flange by bolts. In such embodiments, a sensor may be mounted to the transmission housing in close proximity to the outer section of the link shaft and configured to vary an electrical signal in response to changes in the magnetic field. A controller may determine transmission output torque based on the electrical signal. 
         [0008]    In some embodiments, the links shaft may couple the transmission output shaft to an input shaft of a transfer case. In such embodiments, a magnetic isolation section may be included between the outer section of the link shaft and the transfer case input shaft. In such embodiments, a sensor may be mounted to the transfer case housing in close proximity to the outer section of the link shaft and configured to vary an electrical signal in response to changes in the magnetic field. A controller may determine transmission output torque based on the electrical signal. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is a schematic diagram of a rear wheel drive powertrain. 
           [0010]      FIG. 2  is a schematic diagram of a four wheel drive powertrain. 
           [0011]      FIG. 3  is a schematic cross section of a rear portion of a transmission and front portion of a driveshaft showing installation of a magneto-elastic output torque sensor. 
           [0012]      FIG. 4  is a schematic cross section of a rear portion of a transmission and front portion of a driveshaft showing an alternative installation of a magneto-elastic output torque sensor. 
           [0013]      FIG. 5  is a schematic cross section of a rear portion of a transmission and front portion of a transfer case showing installation of a magneto-elastic transmission output torque sensor. 
       
    
    
     DETAILED DESCRIPTION 
       [0014]    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. 
         [0015]      FIG. 1  schematically illustrates a powertrain of a rear wheel drive vehicle. Mechanical connections are illustrated with solid lines while dotted lines represent signals that convey information. Power to propel the vehicle is generated by internal combustion engine  10 . This power is conditioned to satisfy vehicle needs by transmission  12  and delivered to rear driveshaft  14 . In particular, when the vehicle is at low speed, transmission  12  reduces the speed and multiplies the torque relative to the power provided by the engine. When the vehicle is at high speed, transmission  12  causes driveshaft  14  to rotate faster than the engine crankshaft. Rear differential  16  divides the power from driveshaft  14  between left and right rear axles  18  and  20  which drive left and right rear wheels  22  and  24  respectively. Differential  16  permits the two axles to rotate at slightly different speeds relative to one another when the vehicle turns a corner. Differential  16  also multiplies the driveshaft torque by a fixed ratio called the final drive ratio and changes the axis of rotation by 90 degrees. Left and right front wheels,  26  and  28  respectively, are not powered. 
         [0016]    Transmission  12  establishes various power flow paths having different speed ratios by selectively engaging and disengaging a number of shift elements. Controller  30  adjusts the torque capacity of each friction clutch during shift events. For some types of transmissions, such as dual clutch transmissions, controller  30  also continuously adjusts the torque capacity of one of more clutches to launch the vehicle from a stationary position. Controller  30  may utilize signals from transmission  12 , such as input and output speed and torque sensors. Controller  30  may also send control signals to engine  10  to adjust the torque output of the engine. 
         [0017]      FIG. 2  schematically illustrates a four wheel drive vehicle powertrain. Transfer case  32  is interposed between transmission  12  and driveshaft  14 . Transfer case  32  may divert a portion of the power from rear driveshaft  14  to front driveshaft  34 . Front differential  36  divides the power from front driveshaft  34  between left and right front axles  38  and  40  which drive left and right front wheels  26  and  28  respectively. Like rear differential  16 , front differential  36  also multiplies the torque by a fixed final drive ratio and changes the axis of rotation by 90 degrees. 
         [0018]    Several types of transfer case are known. In a torque-on-demand (TOD) transfer case, the transmission output shaft is continuously coupled to the rear driveshaft  14 . An actively controlled clutch selectively drivably connects the transmission output to the front driveshaft  34  via axis transfer gears or a chain and sprockets. Controller  30  may engage the torque-on-demand clutch is response to sensing a loss of traction at the rear wheels or in anticipation of loss of traction. In a center differential type transfer case, a planetary gear set divides the transmission output torque between the front and rear driveshafts while allowing some speed difference. Many transfer cases of both types also provide a driver controlled coupler that, when engaged, forces the front and rear driveshafts to rotate at the same speed. 
         [0019]    Control of transmission clutches during a shift event, control of launch clutches during a launch event, and control of a transfer case torque on demand clutch may all be improved by use of a measurement of transmission output torque. In a four wheel drive powertrain such as the powertrain illustrated in  FIG. 2 , a measurement of rear driveshaft torque may be used instead of or in addition to a measurement of transmission output torque. One known type of torque sensor is based on materials that have magnetic properties which change in response to shear strain. When a shaft transmits torque, the surface of the shaft deflects in shear. For a given shaft geometry, the surface shear strain is proportional to the torque transmitted by the shaft. A magneto-elastic torque sensor produces an electrical signal that varies in response to the change in the magnetic field. A controller can accurately determine the transmitted torque by processing the electrical signal. Such a torque sensor is described in U.S. Pat. No. 6,698,299. 
         [0020]      FIG. 3  illustrates one way of installing an output shaft torque sensor in a rear wheel drive powertrain layout. Transmission output shaft  50  protrudes from the rear portion of transmission housing  52 . A seal  54  may preclude contaminants from getting inside the case. The transmission output shaft may be supported with respect to the housing by bearings  56 . A section of the output shaft surface  58  behind the transmission case is treated to produce a magnetic field that fluctuates with fluctuations in the output shaft torque. A sensor  60  is mounted to the housing  52  in close proximity to the treated section of the output shaft. The clearance between the sensor and the shaft surface is closely controlled using bearings  62 . Seals  64  prevent contamination from entering the gap between the sensor and the shaft surface. A flange  66  is splined to the output shaft  50  behind the sensor  60 . A universal joint  68  is bolted to the flange by bolts  70  to couple the driveshaft  14  to the output shaft  50 . 
         [0021]    The scheme of  FIG. 3  has several drawbacks. The output shaft must be lengthened to incorporate the treated section for torque sensing. For an existing transmission design, this requires re-design of the output shaft, which is an expensive component. It may also be necessary to revise the design of the transmission case to add provisions for mounting the sensor  60 . For some transmission gearing arrangements, the output shaft is a long component, making the process of treating it to produce the magnetic field cumbersome. 
         [0022]      FIG. 4  illustrates an improved scheme for installing an output shaft torque sensor in a rear wheel drive powertrain. The output flange of  FIG. 3  is replaced by a link shaft  72  that incorporates an output flange. Link shaft  72  is splined to output shaft  50  behind transmission housing  52 . Instead of treating a section of the output shaft  50 , an outer surface  74  of link shaft  72  is treated to produce a magnetic field that fluctuates based on transmitted torque. A circumferential gap  76  between the inner spline and the treated outer section directs the transmitted torque past the treated section. Gap  76  may be machined into the link shaft. Alternatively, link shaft  72  may be formed by welding an inner section having the spline to an outer treated section. Sensor  60  is mounted outside the treated section on a bracket  78  mounted to transmission housing  52 . For example, bracket  78  may attach to a boss that is designed for mounting of a transfer case. The same output shaft  50  and housing  52  may be used for transmission variants having an output torque sensor and for other variants not having a transmission torque sensor. Any axial length increase associated with output torque sensing is minimized. This scheme may also be used to install a torque sensor on the rear driveshaft of a transfer case. 
         [0023]      FIG. 5  illustrates a scheme for installing a transmission output shaft torque sensor in a four wheel drive powertrain. A transfer case housing  80  is fastened to the transmission housing  52  by bolts  82 . Transfer case input shaft  84  is supported within transfer case housing  80  by bearings  86 . Link shaft  88  is splined to transmission output shaft  50 . An outer surface of link shaft  88  is treated to produce a magnetic field that fluctuates based on transmitted torque. A circumferential gap  90  between the inner spline and the treated outer section directs the transmitted torque past the treated section. Link shaft  88  may be formed by machining gap  90  or may be formed by welding an inner section having the spline to an outer treated section. Sensor  60  is mounted to transfer case housing  80  outside the treated section. Magnetic isolator  92  is splined to both the transfer case input shaft  84  and to link shaft  88 . The torque on demand clutch within the transfer case may be electro-magnetically actuated, which may produce a magnetic field within transfer case input shaft  84 . Isolator  92  is made from a non-magnetic material, such as stainless steel, to prevent any magnetic field in transfer case input shaft  84  from influencing the magnetic field produced by the treated section of link shaft  88 . The scheme of  FIG. 5  permits using the same transmission output shaft for four wheel drive variants as for rear wheel drive variants and does not increase the axial length relative to variants without a torque sensor. 
         [0024]    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.