Abstract:
A transmission utilizes an output torque sensor that relies upon magnetization of a section of the output shaft. The sensor produces an electrical current that varies as the torque transmitted by the shaft varies. However, the relationship between output torque and electrical current is impacted by part-to-part variability of the shaft and of the sensor. Conventional methods of compensating for this variability are hampered because the sensors and shafts are not paired until they are assembled into the transmission. A portable test may be used to characterize each shaft and each sensor. This characterization data includes average zero torque current and variability of zero torque current with respect to shaft position. A mapping is selected based on the shaft characterization and the sensor characterization and programmed into the controller.

Description:
TECHNICAL FIELD 
       [0001]    This disclosure relates to the field of vehicle controls. More particularly, the disclosure pertains to a method of calibrating a torque sensor. 
       BACKGROUND 
       [0002]    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. Transmission speed ratio is the ratio of input shaft speed to output shaft speed. 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. 
         [0003]    A common type of automatic transmission includes a gearbox capable of alternately establishing a fixed number of power flow paths, each associated with a fixed speed ratio. The gearbox includes a number of shift elements such as clutches and brakes. A particular power flow path is established by engaging a particular subset of the shift elements. To shift from one power flow path to another power flow path with a different speed ratio, one or more shift elements must be released while one or more other shift elements must be engaged. Some shift elements may be passive devices such as one way clutches, while other shift elements engage or disengage in response to commands from a controller. For example, in many automatic transmissions, the shift devices are hydraulically controlled friction clutches or brakes. The controller regulates the torque capacity of the shift element by regulating an electrical current to a solenoid, which adjusts a force on a valve which, in turn, adjusts a pressure in a hydraulic circuit. 
         [0004]    Most transmissions are equipped with a launch device. When the vehicle is stationary or moving very slowly, the gearbox input speed is less than the minimum operating speed of the engine. A launch device transmits torque from the engine to the gearbox input while permitting the engine to rotate at an acceptable speed. A common launch device is a torque converter which includes an impeller driven by the engine and a turbine driving the gearbox input. Torque is transferred from the impeller to the turbine hydro-dynamically. Many torque converters also include a hydraulically controlled lock-up clutch that couples the impeller to the turbine, bypassing the hydro-dynamic power transfer path to improve efficiency at higher vehicle speeds. Other transmissions use an actively controlled launch clutch as a launch device. 
         [0005]    A modern automatic transmission is controlled by a microprocessor which adjusts the torque capacity of each shift element, including any lock-up clutch, at regular intervals. At each interval, the controller gathers information indicating the driver&#39;s intent, such as the positions of the shifter (PRNDL), the accelerator pedal, and the brake pedal. The controller also gathers information about the current operating state of the vehicle, engine, and transmission, such as the speed of various elements and the torque transmitted by various elements. Using this information, the controller determines whether to maintain the currently established power flow path or to shift to a different power flow path. If the controller decides to shift to a different power flow path, the controller then adjusts the torque capacities of the off-going shift elements and the on-coming shift elements in a coordinated manner in order to make the transition as smoothly as possible. 
         [0006]    The capability to shift smoothly depends on the ability to accurately determine operating state of the vehicle from sensor inputs. Various noise factors complicate this by influencing the relationship between the sensed quantity and the sensor output. These noise factors include environmental conditions such as temperature, component wear over time, and part-to-part variability in the manufacturing process. Methods which measure and compensate for these noise factors improve the ability to accurately determine operating state and therefore improve the ability to control the transmission. 
       SUMMARY OF THE DISCLOSURE 
       [0007]    A method of manufacturing transmission includes fabricating a plurality of shafts and sensors, testing each of the shafts and sensors, assembling the transmissions, and entering compensation data into controllers based on the test results. Each shaft has a magnetized section adapted for use with a magneto-elastic torque sensor. Each production shaft is tested with a master torque sensor, which may be either a pre-selected torque sensor with known sensitivity or may be an alternative magnetic flux sensing instrument. Similarly, each production sensor is tested with a master shaft, which may be either a pre-selected shaft with known magnetic signature or may be an alternative device that emits a known magnetic signature. The testing may produce characterizing data such as average zero torque electrical output and a metric of electrical output variability with respect to rotational position. The characterizing data may be utilized to identify a range for each shaft and a range for each sensor which are then used to select the compensation data to be entered into each controller. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  is a schematic representation of a vehicle powertrain. 
           [0009]      FIG. 2  is a schematic cross section of a transmission output shaft with an output torque sensor. 
           [0010]      FIG. 3  is a flow chart for a first method of adjusting data in a controller to account for part-to-part variability of the shaft and the sensor unit. 
           [0011]      FIG. 4  is a schematic cross section of a portable tester suitable for performing some of the test steps in the method of  FIG. 3 . 
           [0012]      FIG. 5  is a flow chart for a second method of adjusting data in a controller to account for part-to-part variability of the shaft and the sensor unit. 
           [0013]      FIG. 6  is a flow chart for a third method of adjusting data in a controller to account for part-to-part variability of the shaft and the sensor unit. 
       
    
    
     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. In a front wheel drive vehicle, the engine and transmission are typically oriented parallel to the front axle and the differential is typically integrated with the transmission in an assembly called a transaxle. 
         [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 on-coming and off-going friction clutches 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]    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 estimate the transmitted torque by processing the electrical signal. Such a torque sensor is described in U.S. Pat. No. 6,698,299. 
         [0018]      FIG. 2  illustrates one way of installing an output shaft torque sensor in a rear wheel drive transmission. Transmission output shaft  30  protrudes from the rear portion of transmission housing  32 . A seal  34  may preclude contaminants from getting inside the housing. The transmission output shaft may be supported with respect to the housing by ball bearings  36  which locate the shaft both axially and radially. A section of the output shaft surface  38  behind the transmission case is treated to produce a magnetic field that fluctuates with fluctuations in the output shaft torque. A sensor  40  is mounted to the housing  32  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  42 . Seals  44  prevent contamination from entering the gap between the sensor and the shaft surface. A wiring harness attached to sensor  40  includes a plug  46  that connects with plug  48  of the transmission wiring harness. The output shaft may be fixed to a planetary carrier  50 . Carrier  50  supports a number of planet gears  52  that mesh with a sun gear formed into shaft  54  and with a ring gear fixed to shell  56 . 
         [0019]    Several factors, in addition to shaft torque, influence the electrical signal generated by the sensor  40 . These other factors are collectively known as noise factors. Accurately determining shaft torque requires measures to minimize and/or compensate for these factors. The noise factors include temperature and part-to-part variability of the shaft and the sensor components. One way to compensate for temperature is to test the sensor and shaft at a variety of known shaft torques and known temperatures to produce a map indicating shaft torque as a function of voltage and temperature. Using this map, the controller measures the voltage and temperature and looks up a corresponding torque value, interpolating as necessary. 
         [0020]    Ideally, the testing that generates the map would be performed with the actual sensor and shaft used in the transmission. In that way, the map would also compensate for part-to-part variability of the shaft and sensor manufacturing processes. In practice, this may be impractical. The shafts and sensors may be manufactured by different suppliers and not paired to one another until the transmission is assembled. Once the transmission is assembled, it may be impractical to run the testing on every transmission at the necessary variety of known temperatures and known shaft torques. 
         [0021]      FIG. 3  illustrates a method of compensating for sensor and shaft part-to-part variability. At  60 , an assembly including a representative shaft called a master shaft and representative sensor called a master sensor is tested. A master shaft may be an actual sensor shaft which is pre-selected for a known magnetic signature as a reference. Alternatively, it may be a device that emits a known magnetic signature whose magnitude and circumferential pattern may differ from those of an actual sensor shaft, but serve as a standardized reference. A master sensor may be an actual sensor unit with a known sensitivity to magnetic flux magnitude and patterns. Alternatively, it may be a magnetic flux sensing instrumentation with sufficient sensitivity to magnitude and spatial profiles to classify the magnetized shafts. This testing may be done at a full range of controlled temperatures and controlled shaft torques. The testing may be done by the transmission manufacturer, the shaft manufacturer, the sensor manufacturer, or a designated testing laboratory. This testing produces a master mapping. After the testing, the master shaft is provided to the sensor manufacturer and the master sensor is provided to the shaft manufacturer. 
         [0022]    At  62 , the sensor manufacturer fabricates a production sensor. At  64 , the sensor manufacturer assembles the production sensor to the master shaft and runs tests. Since this testing is performed on every sensor, cost may dictate a less comprehensive set of controlled temperatures and controlled torques than the testing used to produce the master mapping. The result of this testing is a sensor mapping. Differences between the sensor mapping and the master mapping are attributable to part-to-part variability of the sensor manufacturing process. Similarly, at  66  and  68 , a production shaft is fabricated and tested with the master sensor to produce a shaft mapping. When the production sensor and shaft are shipped to the transmission manufacturer, the sensor and shaft mapping are also transmitted to the transmission manufacturer. Alternatively, data that characterizes the difference between the sensor or shaft mapping and the master mapping may be transmitted. In a mass production environment, the shafts and sensors are marked or otherwise individually identified so that each unit is associated with a particular corresponding mapping. 
         [0023]    Once a particular shaft and sensor are paired by the transmission manufacturer, the transmission manufacturer combines the master mapping, the shaft mapping, and the sensor mapping at  70  to produce a production mapping. One way to accomplish this is to compute differences between the master mapping and the sensor and shaft mappings respectively and add these differences to the master mapping. After the transmission is assembled at  72 , the production mapping is entered into the transmission controller at  74 . This production mapping at least partially compensates for the part-to-part variability of the sensor and the shaft. If there is interaction between the sensor variability and shaft variability, the production mapping does not capture that interaction. Also, if the sensor and shaft mappings are produced at the component manufacturing facility, the production mapping does not compensate for any changes that may occur while the components are in transit. 
         [0024]    The process may be modified to compensate for changes in shaft magnetization that occur in transit by doing the shaft mapping at the assembly facility using a portable tester like the tester illustrated in  FIG. 4 . Tester  80  includes a pedestal  82 , sensor housing  84 , and a test controller  86 . The shaft  30  is placed into the pedestal  82  such that the carrier rest on thrust bearings  88 . A ring gear  90  may be fixed to the pedestal. A shaft  92  with a sun gear may extend into the pedestal from below. The output shaft is positioned radially such that the planet gears mesh with the sun gear and the ring gear. Sensor housing  84  includes the master sensor  40 ′, bearings  42 ′, and seals  44 ′. The sensor housing  84  slides over the production shaft  30 . Sensor housing rests on pedestal  82  to locate the master sensor axial with respect to the magnetized region  38 . The wiring harness of the master sensor is plugged into a wiring harness of the test controller. A very similar portable tester may be used to perform sensor testing on production sensors using a master shaft. 
         [0025]    While no torque is applied to the shaft, the shaft may be rotated by rotating shaft  92 . Test controller  86  measures and records the electric current at a number of different shaft rotational positions. If the maximum and/or minimum current are outside of a specification, the shaft may be discarded or returned for rework. This data may be processed to generate scalar values characterizing the average zero torque electrical output and the variability of zero torque electrical output as a function of rotational position. This testing may be repeated with a known torque applied to the exposed end of the shaft to characterize the rate of change of voltage with respect to torque. The tester may also include a temperature sensor to record the temperature at which the test results are taken. The test results may be shifted to compensate for temperature differences using temperature sensitivity data derived from the master mapping. 
         [0026]      FIG. 5  illustrates an alternate method which compensates for interactions between shaft variability and sensor variability. At  100 , M sensors are selected which represent the anticipated spectrum of sensor variability. Each of the M sensors is tested with a master shaft at  102  to create a set of M reference sensor mappings. A master shaft may be an actual sensor shaft with a known magnetic signature as a reference. Alternatively, it may be a device that emits a known magnetic signature whose magnitude and circumferential pattern may differ from those of an actual sensor shaft, but serve as a reference. The sensor testing may be simplified for producing a reduced sensor map to characterize sensor behaviors only under selected conditions that are sufficient for the purpose of sensor classification. 
         [0027]    Similarly, at  104  and  106 , N shafts representing the anticipated spectrum of shaft variability are tested with a master sensor to generate a set of N reference shaft mappings. A master sensor may be an actual sensor unit which is pre-selected for a known sensitivity to magnetic flux magnitude and spatial profiles. Alternatively, it may be a magnetic flux sensing instrumentation with adequate sensitivity to magnitude and spatial profiles to classify the magnetized shafts. The shaft testing may be simplified for a reduced shaft map to characterize magnetic behaviors only under selected conditions that are sufficient for the purpose of shaft classification. At  108 , each selected sensor is tested with each selected shaft to generate M×N production mappings. As in  FIG. 3 , each production sensor is tested with the master shaft at  64  and each production shaft is tested with the master sensor at  68 . Both shaft and sensor testing may be simplified to characterize magnetic behaviors only under selected conditions for the purpose of shaft and sensor classification, corresponding to the tests at  102  and  106 . At  110 , the sensor mapping is compared to the M reference sensor mappings to determine which of the M sensors most closely resembles the production sensor. For example, each reference sensor may be associated with a range of average electrical outputs at zero torque and a range of electrical output variability metrics. Similarly, at  112 , the shaft mapping is compared to the N reference shaft mappings to determine which of the N shafts most closely resembles the production shaft. Like the reference sensors, each reference shaft may be associated with a range of average electrical outputs at zero torque and a range of electrical output variability metrics. 
         [0028]    Then, at  114 , the production mapping corresponding to the closest matches is entered into the controller. The testing performed at  108  to create the production mappings may be much more thorough than the testing performed at steps  102 ,  106 ,  110 , and  112 . To further increase the accuracy, the mapping entered into the controller may be modified based on the difference between the sensor mapping and the reference sensor mapping and/or the difference between the shaft mapping and the reference shaft mapping. 
         [0029]      FIG. 6  describes a hybrid method. As in the method of  FIG. 5 , a representative set of sensors are selected at  100  and tested with the master shaft at  102 . Unlike the method of  FIG. 5 , the testing performed at  102  should fully map the behavior over the entire range of torques and temperatures. The selected M sensors are then assembled into M portable testers. Each production sensor is tested at  64  and the closest reference sensor is selected at  110 . After a production sensor and a production shaft are paired for use together in a transmission, the shaft is tested at  116  using the portable tester having the closest reference sensor. At  118 , the reference sensor mapping corresponding to the closest sensor is modified based on the shaft mapping produced at  116  to produce a production mapping. For example, the reference sensor mapping may be shifted to have the average zero torque voltage as measured at  116 . To further increase the accuracy, the mapping may also be modified based on the difference between the sensor mapping produced at  64  and the reference sensor mapping. Finally, at  120 , the corresponding production mapping is entered into the controller. 
         [0030]    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.