Abstract:
A method of improving linear position sensing of clutch actuators with magnetic field sensors utilizes a dedicated magnetic field sensor solely to detect the ambient magnetic field. Typical three position hydraulic clutch actuators include a pair of active magnetic field sensors, one of such active sensors associated with each of a pair of pistons in such actuator and an adjacent pair of permanent magnets, one of such magnets associated with each of such pistons. The invention provides an additional magnetic field sensor disposed proximate the active magnetic field sensors which senses the surrounding (stray, background or parasitic) magnetic field proximate the active magnetic field sensors and provides a signal to an electronic circuit or software which actively and in real time corrects the signals from the active magnetic field sensors by cancelling out the magnitude of the stray magnetic field as detected by the additional sensor.

Description:
FIELD 
     The present disclosure relates to a method of providing robust linear position measurement in automotive applications and more particularly to a method of providing robust linear position measurement in automotive transmission applications by utilizing a magnetic field sensor for detecting background magnetic field strength. 
     BACKGROUND 
     The statements in this section merely provide background information related to the present disclosure and may or may not constitute prior art. 
     In many modern automotive transmissions, especially dual clutch transmissions (DCT&#39;s), a plurality of hydraulic actuators (operators) carry out commands that provide both a desired shift and shift sequence. In order to control such operators and confirm the attainment of a desired position, it is common practice to employ plural state or proportional linear position sensors that, in the former case, provide, for example, a signal that changes from a first state to a second state when a particular operator position has been achieved and, in the latter case, provide a signal that varies linearly (proportionally) between a first actuator position and a second actuator position. 
     Clearly the data from proportional sensors provides far more useful information as they not only indicate when multiple distinct actuator positions have been achieved but also provide the real time position of the actuator during translation and, if differentiated, the speed of the actuator. Because of these benefits, proportional sensors are by far the most commonly utilized linear position sensors and magnetic field sensors such as Hall effect sensors are the most common type. 
     In a typical magnetic field linear sensor assembly, a permanent magnet is mounted to a translating component such as the actuator piston, the output shaft or an associated shift rail and the magnetic sensor, which is stationary, is secured in proximate, sensing relationship with the permanent magnet to a housing, flange, web or other stationary transmission component. Translation of the permanent magnet thus varies the magnetic field strength sensed by the sensor and, with proper conditioning, scaling and software, the position of the actuator and associated shift components can be determined. 
     While accurate and dependable, the magnetic field strength sensor is not without drawbacks. Arguably the most problematic is its susceptibility to stray magnetic fields. A stray field can mimic the field produced by the associated permanent magnet such that the magnetic field sensor may provide a signal indicating that an actuator is in a certain position when it is not. Contrariwise, a stray magnetic field may interfere with the field from the permanent magnet and cause a sensor to indicate that an actuator and associated shift components have failed to achieve a desired position when, in fact, they have. 
     One approach to this problem is to provide shielding proximate the magnetic field sensor of materials such as mu metal which are designed to minimize stray magnetic fields and thus reduce inaccurate signals from the magnetic field sensor. Unfortunately, this solution adds weight and, in the case of an automatic motor vehicle transmission, occupies space in an already crowded environment. Furthermore, it is often difficult to sufficiently shield the sensor as certain regions must be left unshielded to allow the sensor to function properly. 
     From the foregoing, it is apparent that improvements in the art of magnetic field sensor isolation would be both desirable and beneficial. The present invention is so directed. 
     SUMMARY 
     The present invention provides a method of improving linear position sensing of clutch and shift actuators (operators) by magnetic field sensors by providing an additional dedicated magnetic field sensor intended solely to detect the ambient magnetic field and changes therein. Specifically, in a typical dual clutch transmission, a three position hydraulic clutch actuator will include a pair of active magnetic field sensors, one of such active sensors associated with each of a pair of pistons in such actuator and an adjacent pair of permanent magnets, one of such magnets associated with each of such pistons. The invention comprehends providing an additional magnetic field sensor disposed proximate the active magnetic field sensors which senses the surrounding (stray, ambient, parasitic or background) magnetic field proximate the active magnetic field sensors (but which is insensitive to the magnets associated with the pistons) and provides a signal to a compensating circuit which actively and in real time corrects the signals from the active magnetic field sensors by cancelling out the stray magnetic field as detected by the additional sensor. Thus, the stray (parasitic) magnetic field is effectively eliminated and the signals from the active sensors have a greatly improved integrity, accuracy, robustness and reliability. Depending upon the locations of the active sensors and the desired degree of accuracy required by the system, that is, how accurately and completely the stray or parasitic signal(s) must be cancelled or suppressed, more than one additional (compensating) magnetic field sensor may be utilized. 
     Thus it is an aspect of the present invention to provide a method of improving the accuracy and robustness of signals provided by magnetic field piston position sensors in a motor vehicle transmission. 
     It is a further aspect of the present invention to provide a method of sensing and cancelling the ambient or parasitic magnetic field sensed by magnetic field actuator position sensors in a motor vehicle transmission. 
     It is a still further aspect of the present invention to provide a method of utilizing active and compensating magnetic field sensors to eliminate ambient magnetic field signals from the outputs of the active actuator piston position sensors. 
     It is a further aspect of the present invention to provide apparatus including active and compensating magnetic field sensors to eliminate ambient magnetic field interference from the signals from the active actuator piston position sensors. 
     Further aspects, advantages and areas of applicability will become apparent from the description provided herein. It should be understood that the description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure. 
    
    
     
       DRAWINGS 
       The drawings described herein are for illustration purposes only and are not intended to limit the scope of the present disclosure in any way. 
         FIG. 1  is a schematic, side elevational view of a portion of a dual clutch transmission incorporating the present invention; 
         FIG. 2  is an enlarged, sectional view of a hydraulic actuator incorporating the magnetic field sensors of the present invention; 
         FIG. 3  is an enlarged, side elevational view of a linear magnetic field sensor assembly according to the present invention; 
         FIG. 4  is a schematic diagram of the mechanical and electrical components of an exemplary linear position sensing circuit incorporating the present invention; and 
         FIG. 5  is a graph presenting plots of actuator position sensor output, ambient magnetic field strength and compensated or corrected actuator position sensor output. 
     
    
    
     DETAILED DESCRIPTION 
     The following description is merely exemplary in nature and is not intended to limit the present disclosure, application, or uses. 
     With reference to  FIGS. 1 and 2 , a portion of a dual clutch transmission is illustrated and generally designated by the reference number  10 . The transmission  10  includes a cast metal housing  12  which receives, locates, supports and protects various mechanical components such as gears, shafts, bearings, actuators, hydraulic fluid passageways and the like. The transmission includes an input shaft  14  which drives a clutch input member or housing  16 . Disposed within the clutch input member or housing  16  is a first friction clutch plate or disc  18  coupled to a first clutch output member or shaft  20  and a second friction clutch plate or disc  22  coupled to a second clutch output member or quill  24 . 
     One of the transmission actuators  26  actuates or operates the friction clutches  18  and  22  and is illustrated in  FIG. 1 . The actuator  26  includes a cylindrical housing or cylinder  28  that may be integrally formed with or installed within the metal housing  12  and that receives a bi-directionally translating piston assembly  30 . Typically and preferably, the housing  12  and the cylinder  28  will be cast of aluminum, magnesium or other non-magnetic metal or alloy. The piston assembly  30  is coupled to the friction clutches  18  and  22  by a clutch linkage  32  that is connected to and bi-directionally and independently translates the friction clutch plates or discs  18  and  22  into contact with the clutch input member or housing  16  to independently provide drive torque to the first clutch output member or shaft  20  or the second clutch output member or quill  24  and thence to gear pairs (not illustrated) within the transmission  10 . 
     As noted, the piston assembly  30  independently engages one of the two friction clutches  18  and  22 . It thus translates between and provides three distinct positions: a center or neutral position, a first, terminal position at a first, for example, left, end which engages the first friction clutch  18  and a second terminal position at a second, for example, right end which engages the second friction clutch  22 . As such, the piston assembly  30  will typically include a first or inner piston  34  and a second or outer piston  36 . A controlled supply of pressurized hydraulic fluid to the faces of the first or inner piston  34  and the second or outer piston  36  determines the position of the clutch linkage  32  and friction clutches  18  and  22  as described above. 
     Referring now to  FIG. 3 , a typical and exemplary magnetic field sensor assembly  40  for sensing the position of the pistons  34  and  36  is illustrated. The magnetic field sensor assembly  40  includes a non-ferrous frame or mounting bracket  42  that supports and positions a magnetic sensor  44 . The magnetic sensor  44  is an elongate bar or member that encapsulates a plurality of magnetic coils  46 . The number of magnetic coils  46  is dependent upon the extent of motion of the pistons  34  and  36 , the size (length) of the magnetic sensor  44  and the desired degree of accuracy, that is, linear definition or resolution, to be provided by the sensor  44 , with more magnetic coils  46  providing higher accuracy and resolution. From four to ten magnetic coils  46  have been found to be satisfactory for most applications but more or fewer may be utilized as necessary and desired. 
     Referring again to  FIGS. 1 and 2 , secured to and disposed proximate the magnetic field sensor assembly  40  such that it is in sensing relationship therewith is a permanent magnet  50  that is attached to and translates with the first or inner piston  34 . Similarly, the second or outer piston  36  includes a second permanent magnet  52  that is attached to it and translates therewith and a second magnetic field sensor assembly  48  disposed proximate the second permanent magnet  52  thereby sensing translation the second or outer piston  36 . As noted above, the housing  12  and the cylinder  28  are typically and preferably cast of a non-magnetic metal or alloy such as aluminum or magnesium such that the (moving) magnetic fields of the magnets  50  and  52  may be readily sensed by the magnetic field sensor assemblies  40  and  48 , respectively. 
     Also disposed in the housing  12  preferably opposite, that is, diametrically opposed to, the magnetic field sensor assembly  40  and the second magnetic field sensor  48  but at a distance from the permanent magnet  50  and the second permanent magnet  52  is a compensating magnetic field sensor  60 . The compensating magnetic field sensor  60  is located so that it is essentially insensitive to and unaffected by the magnetic fields of the permanent magnets  50  and  52  but will sense and provide data regarding the overall magnetic field, that is, the ambient or surrounding magnetic field, that the transmission  10  is exposed or subjected to in real time. Accordingly, the compensating magnetic field sensor  60  preferably requires a single coil  62  which generally senses the ambient (parasitic) magnetic field or fields to which the transmission  10  is exposed at any given time. The term “parasitic” is utilized herein to denote the unwanted and interfering background or stray magnetic fields imposed on the transmission  10  by any source, for example, magnetized steel components in bridges and roads, tools and equipment or high current carrying wires. 
     Referring now to  FIG. 4 , a schematic and exemplary diagram of a compensated linear position sensing circuit for a vehicle transmission is designated by the reference number  70 . At the outset, it should be understood that the circuit  70  is provided primarily to illustrate the signal or data flow and processing and that integrated electronic compensating circuits and digital signal analysis and conditioning utilizing software and specific algorithms can be utilized to undertake the signal processing described herein. The circuit  70  includes the first magnetic field sensor assembly  40 , the first permanent magnet  50  on the first or inner piston  34 , the second magnetic field sensor assembly  48 , the second permanent magnet  52  on the second or outer piston  36  and the compensating magnetic field sensor  60 . Multiple conductor cables  72  carry the outputs of the first sensor assembly  40  and the second sensor assembly  48  to inputs of comparators  74  and  76 . Provided to the inverting inputs of the comparators  74  and  76  is the signal from the compensating magnetic field sensor  60  in a cable  78 . The comparators  74  and  76  sum the signals provided to their inputs thereby effectively cancelling out the stray, background or parasitic magnetic field signal. Stated somewhat differently, the comparators  74  and  76  essentially function as out-of-phase summing devices wherein the signals from, for example, the first and second magnetic field sensor assemblies  40  and  48  are of one phase and are applied to the positive inputs and thus may be utilized to cancel out the signal from the compensating magnetic field sensor  60  which is of the same phase but which is applied to the negative inputs. It should be understood that other electronic circuits as well as software and algorithms utilized by microprocessors wherein the signal from the compensating magnetic field sensor  60  representing the background or parasitic magnetic field is utilized to correct and suppress the interfering signal in the outputs of the first and second magnetic field sensor assemblies  40  and  48  are deemed to be within the purview of the present invention. 
     Because both the first and second magnetic field sensor assemblies  40  and  48  and the compensating magnetic field sensor  60  are exposed or subjected to an external or parasitic magnetic field whereas only the first and second magnetic field sensor assemblies  40  and  48  are exposed or subjected to the fields from their respective permanent magnets  50  and  52 , when the signal from the compensating magnetic field sensor  60  is inverted and summed (added) to the signals from the first and second magnetic field sensor assemblies  40  and  48 , the external or parasitic field sensed by the first and second magnetic field sensor assemblies  40  and  48  is cancelled out, leaving only the signal (data) from the first and second magnetic field sensor assemblies  40  and  48  generated by the permanent magnets  50  and  52 , respectively, which represents the positions of the pistons  34  and  36 . These signals (data) are provided to a transmission control module  80  where they are utilized to provide highly accurate information regarding the current, real time position of the pistons  34  and  36 . The transmission control module  80  may also contain a microprocessor which includes sub-routines, algorithms and look up tables which accept the signals from the comparators  74  and  76  or other electronic devices and provide data utilized in subsequent components and operations which represent the real time positions of the pistons  34  and  36  and thus of the shift linkage  32  and the friction clutches  18  and  22  (illustrated in  FIG. 1 ). 
       FIG. 5  provides a time based graph of various plots associated with operation of the present invention. The horizontal (X) axis represents time and the vertical (Y) axis displays multiple signals and transient operating states. At the outset, it should be noted that the vertical alignment of the plots is significant and permits ready comparison of signals (data) without and with the present invention and its effect. The first plot  82  illustrates a signal from one of the magnetic field sensor assemblies  40  or  48 , the transient or impulse to the right indicating achievement of a certain position of, for example, the piston  34  or  36 . The second plot  84  represents a transient interference event (on the left) wherein the transmission  10  is exposed or subjected to a momentary parasitic magnetic field from a magnetized bridge, road component or other source. 
     The third plot  86  is again the signal from one of the magnetic field sensor assemblies  40  or  48  which is the sum of the first plot  82  and the second plot  84 . That is, it is the signal from one of the magnetic field sensors assemblies  40  or  48  that has been affected by the transient interference event of plot  84  which is also providing data from the translation of the one of the pistons  34  or  36  as presented in plot  82 . Note that the spurious and unwanted data from the interference event may mimic the desired data from translation of the piston  34  or  36 , rendering accurate computation of the actual position of the piston  34  or  36  difficult or impossible. The fourth plot  88  represents the output of the comparators or operational amplifiers  74  and  76  provided to the transmission control module  80 . Here, the spurious, parasitic signal sensed by the compensating magnetic field sensor  60  of plots  84  and  86  has been removed, leaving the signal (data) that represents the true, real time position of the piston  34  or  36 . It should be understood that the rectangles of the plots  82 ,  84 ,  86  and  88  representing magnetic field strengths and signals have been employed to facilitate explanation and comparison and that actual time-based plots of the magnetic field strengths and signals may take various shapes such as sinusoids, trapezoids and shapes other than the rectangles shown. 
     As described above, it is assumed that a single compensating magnetic field sensor  60  provides a signal sufficiently representative of the parasitic magnetic field to which the transmission  10  and its various actuators and position sensors are exposed so that only one compensating magnetic field sensor  60  need be utilized. However, depending upon the required accuracy of the signals generated and provided to, for example, the transmission control module  80 , and the specific locations of the various magnetic field sensor assemblies  40  and  48  as well as magnetic field sensors associated with other hydraulic actuators, it may be preferable and desirable to utilize additional compensating magnetic field sensors, for example, in multiple locations proximate other actuators and in locations remote from that of the single compensating magnetic field sensor  60 . Such a transmission, with multiple compensating magnetic field sensors is fully within the purview of the present invention. 
     The description of the invention is merely exemplary in nature and variations that do not depart from the gist of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention.