Abstract:
A method for detecting backlash and adjusting driveline variables, includes measuring torque transmitted between components across a torsional discontinuity, measuring driveline twist across the discontinuity, using a ratio of driveline twist divided by torque and an inverse of driveline stiffness to determine whether the driveline is entering, exiting or in a backlash zone, and using measured driveline twist and torque at the backlash zone to adjust reference values of driveline twist and torque.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to detecting a lash crossing in a motor vehicle driveline particularly with the use of a device that produces signals indicating the magnitude of torque being transmitted and the directional sense of the transmitted torque. 
     2. Description of the Prior Art 
     Automotive drivetrains include meshed gear teeth that exhibit deadband when the direction of torque applied to them changes, as shown in the diagram of  FIG. 1  and in  FIG. 2 . 
     Such deadband causes discontinuity of the transmitted torque and excitation of the driveline, as the shafts feature certain compliance. Due to that, the deadband is typically called “backlash” or being in the deadband zone during the change of direction of torque: “lash crossing.” If there were no backlash associated with the meshed gears, the shaft twist to torque relationship could have been expressed using the torsional spring equation (Hooke&#39;s law): t=Ks q, where Ks is the shaft stiffness. 
     Lash crossing has been one of the key factors contributing to drivability issues. There has been a continuing effort in the automotive control engineering community to address the problem of backlash via predicting, detecting, avoiding, and mitigating the effects of lash crossing. 
     Using the measurements available on production vehicles today, reliable detection of the crossing the deadband zone is a formidable challenge. Various publications explore the Hooke&#39;s law to derive the driveline torque based on shaft position sensors, which are typically used for angular speed and known as “speed sensors.” Others use the speed difference across the unlocked torque converter as an indication of torque direction at the input of the gearbox. 
     The numerical and resolution problems associated with estimation methods solely based on speed sensors position (tooth count) readings, remain limited to higher speed operation and are sensitive to various noise factors: synchronization, resolution at low speeds, engine and road disturbances, measurement and numerical errors, to name a few. 
     SUMMARY OF THE INVENTION 
     A method for detecting backlash and adjusting driveline variables, includes measuring torque transmitted between components across a torsional discontinuity, measuring driveline twist across the discontinuity, using a ratio of driveline twist divided by torque and an inverse of driveline stiffness to determine whether the driveline is entering, exiting or in a backlash zone, and using measured driveline twist and torque at the backlash zone to adjust reference values of driveline twist and torque. 
     The method avoids the numerical and resolution problems associated with estimation methods solely based on speed sensors position readings, high speed operation and sensitivity to various noise factors including synchronization, resolution at low speed, engine and road disturbances and measurement error. 
     The scope of applicability of the preferred embodiment will become apparent from the following detailed description, claims and drawings. It should be understood, that the description and specific examples, although indicating preferred embodiments of the invention, are given by way of illustration only. Various changes and modifications to the described embodiments and examples will become apparent to those skilled in the art. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       The invention will be more readily understood by reference to the following description, taken with the accompanying drawings, in which: 
         FIG. 1  is a schematic diagram illustrating two shafts of a driveline with deadband; 
         FIG. 2  is an end view of meshing gear teeth showing backlash between the gears 
         FIG. 3  is a graph showing the variation of driveline torque and twist from negative to positive twist including discontinuities where entry into and exit from backlash occur; 
         FIG. 4  is a schematic diagram of an electronic controller for controlling the torque converter clutch; 
         FIG. 5  is an algorithm for lash crossing and size detection, using shaft torque and twist measurements; and 
         FIG. 6  shows time traces of lash crossing detection using the ratio between crude estimate of twist and measured shaft torque. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       FIG. 1  shows a driveline  10  having two shafts  12 ,  14  connected by a torsional discontinuity  16  in which backlash can occur. The discontinuity  16  is be represented by the meshing gears  18 ,  20  of  FIG. 2 , each gear being secured to one of the shafts  12 ,  14 . A production-suitable magneto-elastic shaft torque sensor  24  having ability to measure torque directly at a transmission input shaft or output shaft is enabling many features in vehicle control. The dynamic response of the shaft torque sensor  24  is within a fraction of a millisecond and its accuracy is far superior to any currently available on-board torque estimates. Hence, this torque measurement can enable robust backlash detection and useful feedback for drivability control of motor vehicles. 
       FIG. 3  is a graph showing the variation of driveline torque τ and twist θ from a negative twist angle to positive twist angle including discontinuities θn, θp where entry into or exit from a backlash zone  26 , i.e., a lash crossing occur. 
     Theoretically, a zero crossing torque measurement at a shaft close to the main source of deadband, which occurs typically at the differential, should directly indicate the lash crossing. In practice, however, there are several sources of uncertainty that can be eliminated by combining information from the torque sensor and speed sensor signals. Offset in the torque sensor reading, due to either measurement noise or location on the driveline, can cause the lash crossing to be at a torque signal different from zero. Utilizing the speed sensors teeth count information in the torque region around zero, can help pinpoint the lash crossing and, in turn, zero out the torque measurement offset. 
     The discontinuity of the slope Ks, as seen in  FIG. 1 , can be used to determine when the driveline is in the non-contact zone. More precisely, the torque-twist relationship is as follows:
 
 K   s (θ−θ p ),θ&gt;θ p &gt;0
 
τ=0,θ n &lt;θ&lt;θ p ,|θ n |=|θ p |
 
 K   s (θ−θ n ),θ&lt;θ n &lt;0
 
where:
 
θ is measured angle of one side of the torsional discontinuity with respect to the other side
 
θ p  is the reference positive direction lash angle
 
θ n  is the reference negative direction lash angle and
 
τ is torque on the shaft element
 
     When the driveline is in the deadband zone  26 , slope Ks is zero. 
       FIG. 4  illustrates a controller  50  comprising an electronic microprocessor  54  accessible to electronic memory  56 , which contains control algorithms; a data communication bus  58  interconnecting components of the controller; inlet ports  60  communicating the controller with various powertrain sensors, and outlet ports  62  communicating signals from the controller in response to the results produced by execution of the algorithms. The RAM component of memory  56  contains reference data associated with torque sensor  24  including ε and reference data related to driveline  10  including Ks, θp, θn, a signal  55  representing torque τ measured at gearbox input or output, and the transmission angular displacement or twist θ1, θ2. The input signals to controller  50  include wheel angular displacement or twist θ3, being transmitted typically by the ABS module. 
     There are many ways this can be used to detect the backlash zone using the measurement of shaft torque and the shaft twist based on measured position count of the differential output shaft speed and the wheel speed sensors. 
     The most direct way to determine that the driveline is in the non-contacting (backlash) region is by observing that the torque measurement (τ) is constant, while the twist (θ) measurement (or estimation) is changing. Note that using the torque signal only to detect when lash crosses zero may not be sufficient, as the signal can have offset or measurement noise. 
     One way would be to calculate the ratio (τ/θ) and, when close to 0, to conclude that the driveline is in the backlash region. 
     Another method, which is robust to numerical errors and other noise factors, is to use the inverse ratio (θ/τ). That ratio will increase to large magnitudes as soon as the driveline enters the non-contacting region, because the measured torque will be close to zero. An algorithm  30  of this method is described with reference to  FIG. 5 . At step  32  a test is made to determine if the measured torque |τ|&lt;ε, where ε is the measurement error range specified for the torque sensor  24  in the vicinity where the torque sensor indicates zero torque is being transmitted in the driveline  10 . ε is the maximum allowed measurement torque error of the torque sensor  24  at a zero torque reading, where a lash crossing is occurring. For example, a torque sensor specification might have a tolerance ε of +/−5 Nm for torque values up to 100 Nm and 5% error for larger torques. 
     If the result of test  32  is false, control passes to step  44 , which is explained below. 
     If the result of test  32  is true, at step  36  a test is made to determine whether |θ/τ|&gt;δ, where δ is the slope detection threshold, to be calibrated accordingly with Ks. Since τ=Ks θ, then θ/τ=1/Ks. Therefore, the angular displacement threshold δ is related to 1/Ks. |θ/τ| should be calibrated as a scalar greater than the estimated 1/Ks, to ensure robustness. The key is that, in the region where a lash crossing occurs, the torque τ is approximately zero; therefore, the division of angular displacement θ by torque τ results in a very large number for θ/τ. 
     Therefore, if the result of test  36  is true, driveline  10  is operating in the backlash zone  26 . Step  36  can be made more robust, e.g., by checking also |θ/(τ−ε)|&gt;δ, |θ/(τ+ε)|&gt;δ for a more robust detection in the presence of noise. 
     A test is made at step  38  to determine whether the lash flag is zero. If the lash flag is zero, no change in the lash state has occurred since the last execution of algorithm  30 . Lash flag is set to 1 at step  40  indicating entry into the backlash zone  26 , and lash flag is reset to 0 at step  46  indicating exit from the backlash zone. Therefore, because the driveline  10  is in the lash zone according to test  36 , and because the driveline was exiting the lash zone at the last execution of the algorithm according result of test  38 , step  40  related to the driveline currently entering the backlash zone  26 . 
     It is helpful to determine whether the driveline  10  is transitioning in and out of lash crossing backlash zone  26 . If step  36  indicates that driveline  10  is currently operating in the backlash zone  26 , thereafter different logic paths are taken, depending on whether transitioning is occurring or continuing in the previous lash state is occurring. 
     When the results of steps  36  and  38  are both true, indicating that the driveline is entering the backlash region zone between θ p  and θ n , at step  40  the offset (if any) of the driveline angular displacement can be eliminated by either (i) immediate resetting it to 0; (ii) filtering to 0; or (iii) gradual adaptation over time. For example, if θ p  and θ n  are expected to occur at 3 degrees, but successive executions of algorithm  30  indicate that θ p  or θ n  occur at 5, 6, 5, 4 degrees, the currently measured shaft twist θ can be compared to the model stored values, θp and θn, and θp and θn can be adjusted in memory  56  into conformance with the measured twist θ. 
     At step  40  the lash flag is set to 1 indicating that the lash state has changed since the last execution of algorithm  30 . After executing step  40 , control passes to step  34  where execution of algorithm  30  is terminated. 
     When the result of step  36  is true and the result of step  38  is false, indicating that the driveline  10  is operating is the backlash zone  26 , at step  42  the torque τ offset is adapted, i.e., memory  56  is updated to account for the τ offset. After step  36  indicates that the driveline is operating in the backlash zone  26 , the torque measurement produced by torque sensor  24  should be 0. If that measurement is other than zero, the measured torque is the torque offset or torque error, which can be used gradually or instantaneously corrected in memory  56  for the torque error. 
     After executing step  42 , control passes to step  34  where execution of the algorithm is terminated. 
     If the result of test  36  is false, indicating that torque and twist are low and driveline  10  is not operating in the backlash region  26 , a test is made at step  44  to determine whether the lash flag is equal to 1. If the result of test  44  is false, control passes to step  34 , where execution of the algorithm is terminated. 
     If the result of test  44  is true, the lash flag being zero indicates that the lash state has not changed since the flag was set upon entry into the backlash zone  26  at the last execution of algorithm  30 . Therefore, the driveline is exiting the backlash zone  26 . At step  46  the offset (if any) of the driveline twist θ can be eliminated, i.e., adjusted by either (i) immediate resetting twist to 0; (ii) filtering to 0; or (iii) gradual adaptation over time. For example, if θ p  or θ n  are expected to occur at 3 degrees, but successive executions of algorithm  30  indicate that θ p  or θ n  occur at 5, 6, 5, 4 degrees, the currently measured shaft twist θ can be compared to the model stored values, θp and θn, and θp and θn can be adjusted in memory  56  into conformance with the measured twist θ. At step  46  the lash flag is reset to 0. 
     At step  48 , the gap in terms of driveline twist |θp−θn| can be determined for the current gear in which the transmission is operating. More precisely, θp and θn can be determined and drift from the driveline twist estimate or twist measurement is eliminated upon recognizing that θp and θn are symmetric around 0. Upon exiting the backlash zone  26 , the driveline crosses θp or θn, as illustrated in  FIG. 3 . If the measured angle at the θp or θn transitions does not coincide with the expected values of θp and θn stored in memory  56 , those values can be adjusted. For example, if θp is 4.0 degrees and θn is 2.0 degrees, |θp−θn| is adjusted to 3.0 degrees, i.e. they converge to the corrected center value 3.0 degrees. 
     The time traces in  FIG. 5  show the shaft torque measurement  70 , the lash detection flag (top)  72 , a crude estimate of driveline twist  74 , the signal  76  resulting from the division of the estimated driveline twist  74  by the torque measurement  70 , and the threshold δ  78 . 
     Since vehicle data indicating the twist angle signal produced by the driveline shaft speed sensors is unavailable, the estimated shaft twist  74  is obtained via crude integration of a wheel speed sensor and the transmission output shaft speed sensor signals. 
     In accordance with the provisions of the patent statutes, the preferred embodiment has been described. However, it should be noted that the alternate embodiments can be practiced otherwise than as specifically illustrated and described.