Abstract:
An assembly that includes a target component mounted for rotation about an axis, a sensor mounted adjacent the inner member and directed toward the inner member to measure the rotational speed of the target component and an outer component interposed between the sensor and the target component. Low magnetic permeability of the outer component is assured by appropriate selection of the material, maintaining the concentration of martensite in the outer component below a reference concentration as indicated by certain reference indices such as the Instability Function, and/or by maintaining the temperature at which a stamping operation is performed on the inner member above a pre-determined temperature.

Description:
BACKGROUND OF THE INVENTION 
   This invention relates to the field of sensing an operating condition when the target is concealed from a sensor by another component. 
   Currently electronic transmission controls rely on accurate information regarding the rotational speed of transmission components located within a case. The speed signals are used as input information to sophisticated powertrain control algorithms. The speed of most components in the case can be accessed directly by magnetic sensing devices, but occasionally such access can only be obtained upon relocating the target component adjacent a sensor. Frequently these relocations compromise the power flow arrangement in the transmission and add cost and complexity to the mechanical design, hydraulic actuation and electronic controls. Indirect access using surrogate speeds in combination with algorithmic corrective calculations, in place of the true target speed, sacrifices response time and accuracy. 
   When the target component has interposed between it and the speed sensor another component formed of ferrous metal, magnetic flux exchange between the sensing device and the surface profile of the target element will be attenuated. To avoid this difficulty it is preferable that the interposed component have low magnetic permeability while providing high structural strength. 
   U.S. Pat. No. 5,825,176 describes an apparatus in which the speed of a rotating inner member is represented by a signal produced by a speed sensor located adjacent the outer surface of an outer member, which covers at least partially the inner member. The outer member is formed with a pattern of angularly and axially spaced windows through its thickness. These windows provide intermittent direct access of a magnetic flux path from the sensor to the target component and interrupted direct access as each window rotates past the flow path. 
   In an alternate approach using non-magnetic material for the interposed element, a high cost magnetic ring is pressed onto the target component in order to provide sufficient magnetic signal penetration through the interposed outer component. 
   SUMMARY OF THE INVENTION 
   The present invention produces a time varying electrical signal that represents rotational speed, or another suitable operating variable of the target component, the signal being used for electronic transmission control. Interposed between the signal-producing sensor and the target component is a second component having low magnetic permeability, which permits uninterrupted passage of magnetic flux between the sensor and target. 
   One embodiment of the present invention for producing a signal indicating rotational speed, includes a target component mounted for rotation, a second component having a portion thereof at least partially overlapping the target component; and a sensor including a coil and a magnet generating a flux path extending through said portion of the second component to said target component, the flux path having a magnetic reluctance that varies with rotation of the target component, the coil carrying a signal generated in response to changes in said reluctance, the signal having a predetermined pear-to-peak amplitude and a frequency indicative of the rotational speed of the target component. 
   A system for determining a rotational speed of a target component according to the present invention includes another component having at least a portion surrounding the target component and being formed of material having a relative magnetic permeability equal to or less than 25.0, a magnetic source generating a magnetic flux path within which the target component and second component are located, rotation of the target component causing changes in a characteristic of the magnetic flux path, a detector generating a position signal that varies in response to changes in said characteristic, and a controller for determining a rotational speed of the target component based on values of said position signal over time. 
   Because the interposed element must also carry relatively large drivetrain torque loads, the material of that component has high structural strength and is readily welded without loss of strength and without adversely affecting the function of the sensor. 
   Another advantage of the present invention is avoiding need to relocate components in order that a sensor has direct access to a target. Instead, the target component may be covered or otherwise concealed from the sensor, thereby avoiding the complexity and increase variable costs and manufacturing cost that such component relocation causes. 
   The interposed, concealing or covering component according to the present invention may be formed of stainless steel that is resistant to martensite formation, which is a crystalline phase transformation that frequently occurs when a component of stainless steel is formed by stamping. The mechanical strength of the interposed element according to the present invention is high and provides the opportunity to minimize the thickness and weight of that element. The possibility of deforming the part is eliminated because no post-stamping heat treatment is required. 
   No separate magnetic ring mounted on the target element is required to enhance the magnetic flux transfer through the speed sensor. A conventional splined surface profile on the target element, or another tooth profile, provides sufficient signal excitation. Importantly, there is no need to compromise the optimal power flow through the transmission by relocating components. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic diagram representing a cross section through of a portion of an automatic transmission showing a speed sensor, target component and outer component. 
       FIG. 2  is an end view of the target component of  FIG. 1 . 
       FIG. 3  is a cross section taken at plane  3 — 3  of  FIG. 2 . 
       FIG. 4  is a cross section taken at plane  4 — 4  of  FIG. 3 . 
       FIG. 5  is an end view of the outer component. 
       FIG. 6  is a cross section taken at plane  6 — 6  of  FIG. 5 . 
       FIG. 7  is an isometric view showing the location of the speed sensor and target component with an outer component moved axially to uncover the target. 
       FIG. 8  is a schematic cross section of the sensor about a central plane. 
       FIGS. 9 and 10  schematically represent the flux paths generated by the sensor. 
       FIG. 11  is a schematic diagram of a system for determining the rotational speed of the target. 
       FIG. 12  is a graph illustrating the temperature dependence of martensite formation at various magnitudes of plastic strain. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Turning now to  FIG. 1 , the components of a transmission  10  are enclosed in a transmission case  12 , which may be formed with a locally increased wall thickness at a boss  14 . A speed sensor  16  is mounted on the transmission case at the boss by a bolt  18  passing through a flange  20  extending from the sensor. The bolt engages threads  21  tapped into the thickness of boss  14 . Interposed between sensor  16  and the outer surface of a forward clutch cylinder  22 , the target component whose speed of rotation is to be determined, and sensor  16  is a shell  24  having a radial disc  26 . The shell is welded or riveted at  28  to a reverse sun gear wheel  30 , and includes an axially directed arm  32  extending between sensor  16  and the outer surface of cylinder  22 . 
   Sensor  16  provides a surface  52  adapted to receive an electrical connector that latches to the sensor at  54  and completes an electrical connection with terminals (not shown) connected to a coil of the sensor. 
   Shell  24  rotates at a different speed than that of cylinder  22  under most operating conditions, and it may be stationary or synchronous with cylinder  22  at other conditions. The location of shell  24  between sensor  16  and target cylinder  22  presents a problem using conventional technology for producing an electric signal produced by the sensor representing the speed of the target. 
   Referring now to  FIGS. 2–4 , the forward clutch cylinder  22  is formed with a radial web  40  that extends radially outward in a series of steps to an axially directed flange  44 . Located on the outer surface of flange  44  are angularly spaced spline teeth  46 , each tooth extending radially outward between successive lands  48 , located between each of the splines. The crest of each spline is also formed with a radially directed rib  50 . In this way the air gap located between the inside surface of the sensor  16  and the outer surface of cylinder  22  varies in length as the spline teeth  46  and lands  48  pass under the sensor while cylinder  22  rotates about its axis. The material of the target component  22  may be any material including a broad range of highly magnetic ferrous materials, preferably SAE J403 1010 low carbon steel. 
     FIGS. 5 and 6  show a detailed configuration of the second component or shell  24 , which includes a radial disc  26  and an axially directed flange  32  extending from the disc. The material of the second component  26  is selected as described below. 
   In order to show clearly the target component  22 ,  FIG. 7  shows the second, outer component  24  moved axially rightward along axis  62  from the as-assembled position, which shown best in  FIG. 1 . Clutch cylinder  22  and shell  24  rotate about axis  22 . As assembled, flange  32  covers the target component and blocks the direct path between the sensor  16  and target  22 . 
   The sensor is directed toward the interior of case  12  and is located directly, radially above target cylinder  22 .  FIG. 8  shows schematically the sensor in cross section. The sensor is preferably a variable reluctance speed sensor that includes a magnet  68  having magnetically opposite poles, an iron pole piece  70 , and an inductive coil  74  that is wound around a plastic bobbin  76 . The coil and bobbin surround the pole piece  70  so that magnetic flux change generated by the magnet and teeth  46  generates a corresponding electrical signal in the coil. The coil includes a pair of lead wires  84  connected to a controller  82  (shown in  FIG. 11 ) having a signal conditioning circuit or programmed logic for processing the electrical signal generated by the sensor  16 , and determining from that signal  80  the rotational speed of target  22 . 
   As the target component rotates, the spline teeth  46  rotate past the sensor  16  causing a sinusoidal variation in the flux due to changes in reluctance. This variation in reluctance, and therefore flux, generates a frequency and amplitude variation in the electrical signal generated on the coil of the sensor. The frequency of that signal is directly related to the rotational speed of the target component  22 . 
   By selecting appropriate material for the second component such that the material has a relatively low concentration of martensite, its magnetic permeability is low. Therefore, the signal induced in the coil of the sensor is substantially unaffected by the presence of the second component, which is essentially magnetically transparent to the sensor. 
     FIGS. 9 and 10  schematically represent the flux path generated by the magnet  68  in the vicinity of the target component  22  and second component  24 .  FIG. 9  shows one of the teeth  46  of the target component angularity aligned with the sensor&#39;s pole piece  70 . Flux generated by the magnet flows from the pole piece  70  through the material of the second component portion  32 , which overlaps the target  22 , along the axially directed teeth  46  on the outer surface of the target component  22 , and back to the opposite pole end of the sensor  16 . 
     FIG. 10  illustrates schematically the flux path generated by the sensor and passing in the land  48  between alternate teeth  46  on the target  22 . The flux path is the same as the flux path shown in  FIG. 9 , except that the reluctance of the target component to flux is changed because, rather than being aligned with a tooth  46  on the target component, the sensor is now aligned with a land  48  between successive teeth on the target component  22 . This change in the reluctance, and therefore flux, causes a corresponding change in the output signal generated on the coil  74  of sensor  16 . 
   The rotational speed of the target  22  is determined from the signal generated in the coil, which is connected to an appropriate signal conditioning device, which may include a microprocessor for analyzing the signal. As  FIG. 11  shows, the voltage signal  80  generated by sensor  16  varies sinusoidally with time and has a predetermined amplitude  86  (preferably about 240 mV). The amplitude is recognizable by, and compatible with the signal conditioning circuit of a controller  82  that converts the signal induced in coil  74  to the rotational speed of target  22 . The signal  80  is monitored continually by the controller. The state of the sampler goes high or to 1 each time the sampled voltage reaches +240 mV, and that time is recorded. Similarly the state goes low or to zero each time the sampled voltage reaches −240 mV. The controller  82  maintains a running count of the number of high states and low states. Electronic memory accessible to the controller stores the number of teeth  46  on the target component, which in a preferred example is  34  teeth per revolution. The controller  82  uses the length of the period between the occurrence of  34  high states to determine the speed of rotation of the target  24  and to produce a digital signal representing that speed, which is carried on line  88  to a powertrain control module  90 . This speed information is used to control various functions of the transmission including electronically controlled gear ratio changes. 
   A magnetic field is produced due to the presence of the permanent magnetic located in the sensor  16 . As cylinder  22  rotates, the magnetic field expands and collapses continuously as the splines  46  and lands  48  rotate pass the sensor. This expansion and collapse of the magnetic field induces in the coil an electrical signal having an acceptable voltage amplitude and a frequency that is an accurate measure of the rotational speed of cylinder  22 . The controlling factors affecting the magnitude of the induced voltage are the magnet strength, coil turns, rotational speed of the target  22 , air gap, diameter of the target wheel, and the material of the component  24  interposed between sensor  16  and target cylinder  22 . Preferably the sensor produces a signal whose peak-to-peak magnitude is greater than ±240 mV (480 mV peak-to-peak) when the speed of the target  22  is 450 rpm. Depending on the requirements of the electronic system that receives and processes the signal produced by sensor  16 , such as a powertrain control module for a motor vehicle, other acceptable peak-to-peak magnitudes of the sensor signal include ±72 mV (144 mV peak-to-peak) and ±160 mV (320 mV peak-to-peak). 
   A sensor capable of producing an acceptable output signal magnitude is a variable reluctance sensor available from HI-STAT Manufacturing, a division of StoneRidge, Inc. of Novi, Mich., the sensor having StoneRidge part number PN 8624-201. The sensor may be an electromagnetic sensor, a Hall-type sensor (such as Allegro ATS640-two-wire), or a magneto-resistive MR-type sensor. 
   Acceptable materials for the outer component  32  include, by way of example but not limitation, aluminum, titanium, stainless steel, and other materials having a relative magnetic permeability, relative to that of air, in the range of 1.0–25.0. Austenitic stainless steel is generally acceptable for the outer component provided suitable steps are taken to maintain relative magnetic permeability equal to or less than 25.0. Martensite concentration of stainless steel and other materials provides another indication of the acceptability of a material for the portion  32  of the second component  24  that is located between the sensor  16  and target component  22 . Ferritic stainless steel, which is magnetic, is preferably avoided because its relative magnetic permeability exceeds 25.0. Martensitic stainless steel, which is also magnetic, is preferably avoided unless its martensite concentration is low, or its relative magnetic permeability is less than 25. These factors affect the ability of the sensor  16  to generate a signal having an acceptable peak-to-peak amplitude without excessive electrical noise. 
   Although austenitic stainless steel is nonmagnetic, it is susceptible to changes in crystalline structure during forming operations, particularly due to stamping. These changes in crystalline structure increase its martensite concentration. Therefore, care should be taken, as disclosed and described below, in selecting a stainless steel material for the second component  32 , during its forming operations, and after forming to assure that the martensite concentration of the second component  24  will not prevent the sensor from generating an acceptable signal, one that is compatible with the requirements of the control system to which it is input. 
   The presence of martensite in the second component  24  blocks the flow of flux from the sensor magnet  68  to the target component  22  and lowers sensor voltage output. In addition, the degree of magnetism present in the second component  24  has only a slight influence on the magnitude of voltage output by the sensor. The shell  24  is a stamped part, and the stamping operation itself affects the concentration of martensite near the stamped metal. Furthermore, the temperature of the metal of the second component  24  when it is stamped also influences the concentration of martensite in the shell  24 . As  FIG. 12  shows, the concentration of martensite declines gradually with increasing temperature of the metal being stamped, and that concentration rapidly declines when the temperature of the material when stamped is in the approximate range −50° C. to +25° C. 
   When the material of the second component  24  is stainless steel, its martensite concentration can be predicted with reference to an instability factor, I (f), determined from the following equation, (1): I(f)=(37.19)−51.25(% C)−2.59(% Ni)−1.02(% Mn)−0.47(% Cr)−34.4(% N), wherein the symbols represent the concentrations by weight of carbon, nickel, manganese, chromium and nitrogen, respectively, in the material. This equation was published in U.S. Pat. No. 3,599,320. 
   Alternatively, the martensite concentration in the metal of the second component can be predicted from the martensite deformation, MD (30), which is determined from the following equation (2): MD (30)=(413)−462(% C+% N)−9.2(% Si)−8.1(% Mn)−13.7(% Cr)−9.5(% Ni)−18.5(% Mo), and Martensite Formation M s , which is determined from the following equation (3): M s =75(14.6−% Cr)+110 (8.9−Ni)+60(1.33−Mn)+50(0.47−Si)+3000[0.068−(C+N)]. Equations (2) and (3) appeared in the ASTM Specialty Handbook For Stainless Steel, 3d Edition, August 1999, published by ASTM International. 
   It has been discovered that if I (f) is less than 2.9, and the temperature at which component  24  is stamped from stainless steel is greater than approximately 32° F., then the martensite concentration in the second component adjacent the sensor (as measured by a ferrite scope) is less than 30 percent, and the peak-to-peak voltage magnitude of the signal produced by the sensor is within an acceptable range. If the stamping temperature is increased, then the instability factor and its corresponding martensite concentration can be increased, and the peak-to-peak voltage magnitude of the signal produced by the sensor is within an acceptable range. In another example of the application of the present invention, wherein I (f) is less than 1.0, and the minimum temperature of the material when stamped is greater than 50° F., the concentration of martensite in the stainless steel is less than 15 percent, and the peak-to-peak voltage magnitude of the signal produced by the sensor is within an acceptable range. Preferably, the material of shell  24  is AISI 304 low carbon stainless steel. 
   A method for producing the second component, the shell  24 , includes obtaining a certification of the concentrations of the various alloy elements in the sheet stock from which component  24  is to be stamped. Next, the instability factor is calculated using equation (1) and the martensite concentration is predicted from the magnitude of I(f). Then shell  24  is stamped from flat sheet stock provided its stamping temperature is greater than a temperature that would produce a martensite concentration in component  22  at the sensor exceeding a martensite concentration that would result in an output signal from the sensor outside an acceptable range. Next, a ferrite scope or magnetic permeability meter can be used to measure the concentration of martensite in the second component at the location of the sensor and target component. The shell can be secured at  28  by riveting or welding to the gear wheel  30  without loss of structural strength and without adversely affecting the condition of the shell in the vicinity of the sensor. The shell is installed in the assembly, provided the ferrite scope indicates the concentration of martensite will result in an acceptable sensor signal. If the ferrite scope check is not used, then the stamped shell can be installed in the assembly. 
   This process reduces the magnetic permeability of the shell so that it is magnetically transparent to the sensor. In this way the sensor produces an electric signal whose frequency is a correct measure of the rotational speed of the forward cylinder clutch  22 . 
   Although the form of the invention shown and described here constitutes preferred embodiments of the invention, it is not intended to illustrate all possible forms of the invention. Words used here are words of description rather than of limitation. Various changes in the form of the invention may be made without departing from the spirit and scope of the invention.