Abstract:
An output shaft of a transmission is fixed to a planetary gearset carrier. Output torque is measured via a surface acoustic wave sensor affixed to a face of the carrier between two adjacent planet gears and radially inside a weld joining the carrier face to an opposite carrier face. In this location, the level of strain at typical transmission output torques produces a level of strain within the measuring range of a surface acoustic wave sensor. The sensor may be powered and signals communicated across an air gap defined by signal rings. Due the stable position and orientation of the carrier, a small, consistent air gap is possible.

Description:
TECHNICAL FIELD 
     This disclosure relates to the field of motor vehicle transmissions. More particularly, the disclosure pertains to transmission with a Surface Acoustic Wave (SAW) output torque sensor. 
     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. 
     The transmission speed ratio may be controlled by adjusting the pressure of hydraulic fluid supplied to various clutches. During a shift between speed ratios, a controller may reduce the hydraulic pressure to an off-going clutch and, in a coordinated manner, increase the hydraulic pressure to an on-coming clutch. If the pressures are not carefully coordinated, the shift feels rough to vehicle occupants. A number of noise factors such as environmental conditions, component wear, and part-to-part variation influence the transmission response to controller signals. In order to consistently achieve excellent shift quality, the controller may rely closed loop control using one or measurements of transmission response as feedback signals during a shift. Measurement of transmission output torque is particularly useful. 
     SUMMARY OF THE DISCLOSURE 
     A planetary gearset includes a carrier, a plurality of planet gears, a ring gear, a sun gear, and a surface acoustic wave sensor. The carrier includes a rear plate welded to a front plate. The rear plate and the front plate support a plurality of planet shafts. The planet gears are supported for rotation about the planet shafts. The ring gear and sun gear are each in continuous meshing engagement with each of the planet gears. The surface acoustic wave sensor is mounted to the rear plate between two adjacent planet shafts on a side opposite from the front plate. In this location, the level of strain is suitable for accurate detection with the surface acoustic wave sensor. This characteristic is further enhanced if the sensor is mounted radially inside the weld joining the front plate to the rear plate. The planetary gearset may further include a first signal ring mounted to the rear plate and in electrical communication with the sensor. The first signal ring may be configured to communicate across an air gap to a second signal ring that is fixed to a non-rotating member such as a transmission case. Due to the stable orientation and axial position of the carrier, the air gap has a constant thickness even as the carrier rotates and transmits torque, increasing the reliability of signal transmission. An output shaft may be fixed to the rear plate, in which case the sensor may be used to determine transmission output torque. 
     A transmission includes a planetary gearset, a surface acoustic wave sensor, and first and second signal rings. A carrier of the planetary gearset is supported for rotation with respect to a transmission housing. The surface acoustic wave sensor is affixed to the carrier. For example, the sensor may be mounted between adjacent planet gears. The first sensor ring is supported for rotation with the carrier. For example, it may be mounted directly to the carrier or mounted to the sensor. The second sensor ring is fixed to the housing. The two rings define an air gap having a constant thickness. This is possible because, during transmission operation, the axis of rotation of the carrier does not vary and the axial position of the carrier does not vary relative to the housing. The second signal ring receives signals from the sensor via the first ring and the air gap. An output shaft may be fixedly coupled to the carrier, in which case strain detected by the sensor may be used to accurately estimate transmission output torque. A controller may utilize these signals to adjust signals to a valve body to control the transmission. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is schematic diagram of a vehicle powertrain. 
         FIG. 2  is a cross section of a planetary gear set with a Surface Acoustic Wave (SAW) strain sensor configured to measure torque. 
         FIG. 3  is a pictorial view of the carrier of the planetary gear set of  FIG. 2 . 
     
    
    
     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. 
       FIG. 1  illustrates a vehicle powertrain system  10 . Mechanical power flow connections are indicated with solid bold lines while the flow of hydraulic fluid is indicated with dotted lines. Electrical control signals are indicated by bold dashed lines. An internal combustion engine  12  drives a crankshaft  14  which supplies input power to transmission  16 . The transmission  16  adjusts the speed and torque and delivers the power to differential  18 . Differential  18  divides the power between left and rights wheels  20  and  22  while allowing slight speed differences as the vehicle turns a corner. 
     Within transmission  16 , the speed and torque are adjusted by two components, torque converter  24  and gearbox  26 . Torque converter  24  includes an impeller and turbine that transmit power hydro-dynamically whenever the impeller rotates faster than the turbine. It may also include a stator that multiplies the torque. The torque converter may also include a bypass clutch that, when engaged, transmits power mechanically from the impeller to the turbine without the parasitic losses associated with hydro-dynamic power transfer. Gearbox  26  includes gearing and shift elements arranged such that engaging various subsets of the clutches establish various power flow paths. The different power flow paths have different speed ratios. 
     Valve body  28  supplies fluid to torque converter  24  and gearbox  26  at controlled pressures to control the torque capacity of shift elements. The valve body also supplies fluid to the hydro-dynamic chamber of torque converter  24  and supplies fluid for lubrication to gearbox  26 . Controller  30  sends electrical signals to solenoids within valve body  28  to adjust the various controlled pressures. Controller  30  determines what pressures to command based on a number of input signals. These input signals may include signals from engine  12 , torque converter  24 , and gearbox  26 . The input signals may also include signals from a driver operated accelerator pedal and a shift lever (not shown). 
     Among the most useful signals is an indication of transmission output torque. Several types of torque sensors are known. Most types of torque sensors operate best if placed on a section of shaft through which the torque of interest is transmitted. For example, output torque sensors are generally located on the output shaft. Often, it is necessary to mount any torque sensor on a section of the shaft that is sufficiently long and sufficiently separated from components such as gears that impact the torque. Consequently, it may be necessary to lengthen the shaft in order to add a torque sensor, resulting in an increase in the overall length of the transmission. While additional transmission length may pose few problems in some vehicle applications, it is critical in other vehicle applications. 
     One technology for measuring shaft torque is a surface acoustic wave (SAW) strain sensor. The strain in certain transmission components is proportional to torque transmitted by the component. For example, the strain on the surface of a long shaft is proportional to the torque transmitted by the shaft. SAW sensors are most accurate for sensing moderate levels of strain. The strain on the surface of an output shaft of a transmission is too high for accurately measuring typical transmission output shaft torques. Therefore, previous attempts to utilize SAW sensors as transmission output torque sensors have modified a region of the output shaft to locally reduce the strain near the sensor. For example, the sensor may be mounted to a post that sticks out radially from the shaft. Alternatively, a groove may be formed into the shaft around the sensor. These shaft modifications result in a larger overall shaft diameter than otherwise necessary. 
       FIG. 2  illustrates a cross section of a planetary gear set with a SAW sensor mounted to the planetary carrier. The top half of  FIG. 2  is a cross section through a planet gear while the bottom half of  FIG. 2  is a cross section between planet gears. A rear plate  40  of the carrier is fixed to output shaft  42 . They may be integrally formed, splined, welded, etc. The output shaft  42  and rear plate  40  are supported for rotation with respect to transmission case  44  by bearings. A front plate  46  of the carrier is welded to the rear plate at several circumferential locations between the planet gears as shown on the bottom of  FIG. 2 . A plurality of planet shafts  48  are fixed between the front plate  46  and the rear plate  40  at regularly spaced circumferential intervals. A plurality of planet gears  50  are supported for rotation about the planet shafts  48  by bearings. A sun gear  52  and a ring gear  54  are in continuous meshing engagement with each of the planet gears. Thrust bearings axially locate the sun and ring with respect to the carrier. 
     A SAW strain sensor  56  is mounted to the rear face of rear plate  40 . The sensor is mounted approximately midway between two of the planet shafts  48  and at approximately the same radial distance from the centerline as the planet shafts. Optionally, additional SAW sensors may be mounted between other pairs of adjacent planet shafts. In some cases, the rear plate may include features to counteract the mass of the sensor to ensure that the carrier is properly balanced. A rotating signal ring  58  is also fixed for rotation with the carrier. For example, rotating signal ring  58  may be fixed directly to SAW sensor  56  or mounted to the rear plate separately. A stationary signal ring  60  is mounted to transmission case  44  adjacent to rotating signal ring  58 . A small air gap separates rotating signal ring  58  from stationary signal ring  60 . The power to operate sensor  56  is transmitted across the air gap from ring  60  to ring  58  and then conducted to sensor  56  by circuitry on rotating signal plate  58 . Similarly, output signals from sensor  56  are transmitted from rotating signal ring  58  to stationary signal ring  60  across the air gap. 
       FIG. 3  is a pictorial view of the carrier and output shaft of  FIG. 2 . During assembly of the planetary gear set, a planet shaft  48  is inserted into each of the holes  62 . A portion of the torque exerted on the carrier by the ring gear and sun gear is transmitted through the planet gears and the planet shafts to the edges of these holes. The remainder of the torque is transmitted to the front plate and then to the rear plate  40  through the welded joints. SAW sensor  56  is mounted to a rear face of the rear plate  40  approximately midway between two of the holes  62  and at approximately the same radial distance from the centerline. This places the SAW sensor closer to the centerline than the welded joints between the rear plate and the front plate. At typical transmission output torque levels, the strain in this location is suitable for a SAW strain sensor. 
     Placing a SAW strain sensor in this location offers various advantages relative to other potential torque sensor locations in a transmission. Locating a torque sensor on a shaft generally leads to an increase in transmission length. Mounting the sensor to a face of carrier that is adjacent to fixed structure is often feasible with little or no increase in transmission length. As discussed previously, the strain on the surface of an output shaft is not suitable for a SAW sensor. Modifying the shaft to locally reduce the strain causes an increase in radial space required. Some components, such as flexplates, are designed to move axially relative to the adjacent fixed structure. Consequently, it is difficult to provide a small, consistent air gap between a rotating signal ring and a stationary signal ring to enable efficient communication with the sensor. Some transmission components have an inconsistent relationship between strain and torque, making them less suitable for torque sensors. For example, the strain at a particular location on a ring gear depends upon the relative position of the meshing planet gears at the moment. 
     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.