Abstract:
A vehicle suspension strut is disclosed that includes a housing having a chamber and a piston slidably disposed in the chamber. A piston rod is connected to the piston and extends out of the housing. The piston rod includes a bore extending longitudinally therein. A magnetostrictive transducer provides an output signal indicative of the piston position with respect to the housing. The magnetostrictive transducer includes a magnetostrictive waveguide disposed in the bore and a magnet joined to the housing that is operably coupled to the magnetostrictive waveguide and the magnetostrictive waveguide abruptly terminates at one end.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation in part of U.S. application Ser. No. 08/828,193, filed Mar. 21, 1997, now U.S. Pat. No. 5,952,823, which claims the benefit of U.S. Provisional Application No. 60/013,985, entitled “Magnetostrictive Linear Displacement Transducer for a Shock Absorber,” filed Mar. 22, 1996. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a sensor to measure the position and velocity of movement of a piston in a cylinder. More particularly, the present invention relates to a magnetostrictive linear displacement transducer for use in a vehicle suspension strut. 
     2. Background Art 
     Various devices have been advanced to measure the distance traveled by a piston in a cylinder. One common application is determining the movement of the piston in a hydraulic, pneumatic, or hydro-pneumatic vehicle strut suspension, where the piston moves axially in a cylinder filled with at least one damping medium. See U.S. Pat. Nos. 4,502,006; 4,638,670; and 5,233,293, each of which discloses a displacement sensor to perform this function. None of the devices so far advanced has been widely accepted in the automotive industry. Although many systems can accurately measure motion of a piston in a cylinder or linear displacement or angular displacement, a simple transducer that can be easily incorporated into, for example, the strut suspension system to keep manufacturing costs down is still desired by many. 
     BRIEF SUMMARY OF THE INVENTION 
     A vehicle suspension strut is disclosed which includes a housing having a chamber and a piston slidably disposed in the chamber. A piston rod is connected to the piston and extends out of the housing. The piston rod includes a bore extending longitudinally therein. A magnetostrictive transducer provides an output signal indicative of the position and/or velocity of the piston with respect to the housing. The magnetostrictive transducer includes a magnetostrictive waveguide disposed in the bore and a magnet joined to the housing that is operably coupled to the magnetostrictive waveguide. 
     Another aspect of the present invention is a magnetostrictive transducer having a waveguide secured by a suspension sleeve fully surrounding the waveguide for use as automotive devices for applications requiring linear and/or angular measurement such as brake peddle position, steering wheel position, throttle position, mirror position and air valve position. The suspension sleeve and the waveguide are disposed in an inner cavity of an enclosure tube. In the preferred embodiment, the present invention further includes pins or connectors to electrically connect the waveguide assembly to an electric circuit that generates electric pulses and provides an output signal corresponding to, for example, the time required for torsional strain wave pulses to be received by a coil. A damping element is secured to the waveguide with heat shrinkable tubing or some other means, and dampens strain pulses not used by the electrical circuit. The suspension sleeve also serves to mechanically isolate the waveguide assembly from shock, vibration and contact with the enclosure tube. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a further understanding of the nature and objects of the present invention, reference should be made to the drawings in which like parts are given like reference numbers and wherein: 
     FIG. 1 is a schematic sectional view of a vehicle suspension strut having a magnetostrictive transducer of the present invention. 
     FIG. 2 is a cross-sectional view taken along section lines  2 — 2  of FIG.  1 . 
     FIG. 3 is a cross-sectional view taken along section lines  3 — 3  of FIG.  2 . 
     FIG. 4 is an enlarged view of the upper portion of FIG.  2 . 
     FIG. 5 is a cross-sectional, schematic view generally taken along section lines  5 — 5  of FIG. 1 but including a position magnet. 
     FIG. 6 is a cross-sectional, schematic view generally taken along section lines  5 — 5  of FIG. 1 but including a position magnet. 
     FIG. 7 is a side view partly in section of the upper portion of the magnetostrictive transducer. 
     FIG. 8 is a plan view of the apparatus of FIG.  7 . 
     FIG. 9 is a plan view of the bobbin cap of the present invention. 
     FIG. 10 is a side view of the bobbin cap of the present invention. 
     FIG. 11 is a front view of the bobbin cap of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 illustrates a magnetostrictive transducer  10  of the present invention incorporated in a vehicle suspension strut illustrated schematically at  12 . As is well known, a vehicle suspension strut  12  includes a cylinder  14  and a piston  16 . A piston rod  18  is secured to the piston  16  and extends through a sealed aperture  20  provided on an end of the cylinder  14 . The piston rod  18  connects to a frame member, not shown, of a vehicle, while the cylinder  14  is connected to a frame portion supporting a tire and wheel assembly, also not shown. The vehicle suspension strut  12  minimizes acceleration between the frame portions of the vehicle. As will be described below, the magnetostrictive transducer  10  provides a convenient sensor to monitor the position and/or velocity of the piston  16  with respect to the cylinder  14 . 
     Referring to FIG. 2, the magnetostrictive transducer  10  includes waveguide  26 , having ends  26 A and  26 B, that is protected by a plastic or other non-conducting material enclosure tube  28 . Enclosure tube  28  is bonded to and mounted on a bracket  24  on near end  28 B and preferably spin welded or otherwise sealed with an end cap  50  on remote end  28 A. A suspension sleeve  22  surrounds waveguide  26  to securely fix the position of waveguide  26  within enclosure tube  28  without applying excessive physical pressure on waveguide  26 . Excessive pressure applied to waveguide  26  by any means of positioning the waveguide  26  within the enclosure tube  28  will restrict the microscopic movement (magnetostrictive response) resulting from the application of an electrical interrogation pulse, decreasing the amplitude of the torsional strain wave that is detected and used for determination of the longitudinal displacement of a position magnet  34 , such as a doughnut magnet, joined in the cylinder  14 . If the amplitude of the torsional strain wave diminishes below a predetermined threshold, the magnetostrictive transducer  10  will not function. Intermediate amplitudes of the torsional strain wave cause less accurate position measurements. 
     As shown in FIG.  1  and FIG. 2, enclosure tube  28  is disposed in a longitudinal bore  31  provided in the piston rod  18 . The position magnet  34  (FIG.  5 ), joined to the cylinder  14  is oriented such that the magnetic field generated by the position magnet  34  passes through the piston rod  18 , and enclosure tube  28  to waveguide  26 . Enclosure tube  28  is preferably constructed from non-conducting material which has a low co-efficient of linear thermal expansion, such as less than 20 ppm per degree centigrade. The low thermal co-efficient of expansion is useful by minimizing the length of bore  31  and thus the overall strut length (FIG. 1) as well as minimizing the distance between end  26 A and cap  50  (FIG.  2 ). 
     A return conductor  38  is electrically connected, such as by a laser weld or other mechanism, to the waveguide  26  at end  26 A of waveguide  26 . Return conductor  38  completes the electrical circuit as is necessary to provide an electrical path for the electric current pulse applied to the waveguide  26  to interrogate the magnetostrictive transducer  10  to make a time or distance measurement. 
     The bracket  24  incorporates a bobbin  42  onto which numerous turns of small diameter insulated copper wire are wound, forming a coil  44 . Return conductor  38  electrically interconnects with external circuitry (not shown) through an interconnecting pin  52 A. Waveguide  26  electrically interconnects with external circuitry (not shown) through an interconnecting pin  52 B by welding, soldering, braising, crimping, glueing or other suitable means, but preferably welding. Bracket  24  interrelates coil  44 , waveguide  26 , enclosure tube  28 , return conductor  38 , and electrically interconnecting pins  52 A and  52 B. Return conductor  38  is preferably connected to pin  52 A by inductive heating for soldering to minimize heating the surrounding area. 
     A pulse generator (not shown) provides a stream of electric pulses, each of which is also provided to a signal processing circuit (not shown) for timing purposes. Referring to FIG. 5, when an electric pulse is applied to the waveguide  26  with a current in a direction indicated by arrow  40 , a magnetic field is formed surrounding waveguide  26 . Interaction of this field with the magnetic field from a position magnet  34  causes a torsional strain wave pulse to be launched in the waveguide  26  in both directions away from the position magnet  34 . A sensing tape  46  is joined to the waveguide  26  proximate the end  26 B and extends into the coil bobbin  44  forming a mode converter  44 ′. The coil bobbin  44  has an opening  54  at its end away from the waveguide  26 . This opening  54  is sealed for protection against the introduction of foreign matter by a tape cap  48 . The strain wave causes a dynamic effect in the permeability of the sensing tape  46  which is biased with a permanent magnetic field by bias magnet  46 A, as shown in FIG.  3 . 
     The dynamic effect in the magnetic field of the coil  44  due to the strain wave pulse results in an output signal from the coil  44  that is provided to the signal processing circuit (not shown) through a first connecting pin  60 A and a second connecting pin  60 B illustrated in FIG.  8 . Referring again to FIG. 5, by comparing the time of application of a particular electric pulse and a time of return of a sonic torsional strain wave pulse back along the waveguide  26 , the signal processing circuit (not shown) can calculate a distance of the position magnet  34  from the sensing tape  46 . By processing the rate of change of the distance between the position magnet  34  and the sensing tape  46 , the signal processing circuit (not shown) can calculate the relative velocity of the movement of the position magnet  34  along the waveguide  26 . The signal processing circuit (not shown) provides an output signal, either digital or analog, which is proportional to the calculated distance or velocity. 
     The waveguide  26  is a solid magnetostrictive alloy that can conduct electric current. A suitable magnetostrictive alloy is a nickel-iron alloy of approximately 30% to 50% nickel. Damping is provided on the waveguide  26  to control propagation of the strain wave pulse and mechanically isolate the waveguide  26  from shock and vibration. A damping element  59  is illustrated in FIG.  2 . The damping element  59  intentionally limits the propagation of one of the strain wave pulses. When the electric pulse forms a magnetic field surrounding the waveguide  26 , interaction of this field with the magnetic field from the position magnet  34  causes a torsional strain wave pulse to be launched in the waveguide  26  in both directions away from the position magnet  34 . A first strain wave pulse propagates down the waveguide  26  toward the sensing tape  46 . A second strain wave pulse propagates up the waveguide  26  away from the sensing tape  46 . The damping element  59  attenuates or dampens the second strain wave pulse so that it is not reflected back down the waveguide  26 , potentially causing erroneous position readings in the output. 
     The damping element  59  can be made of a suitable braided material that is attached to the waveguide  26 . In the preferred embodiment, the damping element  59  is secured to the waveguide  26  with a portion of heat shrink tubing  62 , as illustrated in FIG.  2 . It should be understood that the stroke or active sensor region of the magnetostrictive transducer  10  is defined by a portion of the waveguide  26  extending between the damping element  59  and the sensing tape  46 . To maximize the electrical stroke of the magnetostrictive transducer  10  and minimize the overall length of the magnetostrictive transducer  10 , the damping element  59  is disposed on a remote end  26 A of waveguide  26 . A braided damping element as used in the preferred embodiment is the subject matter of U.S. Pat. No. 5,545,984 to Gloden et al., which is assigned to the same assignee as this application. 
     Waveguide  26  is bendable but also substantially rigid such that it retains its shape. A unique and novel configuration of the waveguide  26  in the present invention is a Z-bend  26 C at near end  26 B to rachet  24  with a first abrupt bend  26 F and a second abrupt  26 G. The first abrupt bend  26 F (FIG. 4) provides a means for reflecting the first strain wave pulse, as described below. The desirability of reinforcing the signal amplitude of the first strain wave pulse by reflection is the subject matter of U.S. Pat. No. 4,952,873 to Tellerman, which is assigned to the same assignee as this application. If the distance between the sensing tape  46  and the first abrupt bend  26 F is substantially the same distance as one-half of the wavelength of the first strain wave pulse, the signal detected by the sensing tape  46  will be reinforced by in-phase addition of the reflected portion of the signal to the non-reflected portion of the signal. The reinforced pulse significantly improves the signal-to-noise ratio of the transducer, resulting in higher immunity to errors induced by external noise sources. The abrupt bend is to have an upset, which upset is an abrupt impedance change resulting from a change in macro geometry, such as a change of moment of inertia. The increase in the angle of the abrupt bend lessens the sharp tooling requirement to insure the appropriate change in moment of inertia. Therefore, it is preferable to have an abrupt bend of 85° to 95°. Further, the abrupt bend enhances manufacturing by enhancing contact between the waveguide  26  and plastic surface  205 . Thus, it is preferable to use an angle of approximately 89°. Alternatively, an anchor may be used for better performance. 
     The use of the Z-bend  26 C constitutes a significant improvement in manufacturability over previous methods of creating controlled reflections of the first strain wave pulse. A brass or similar anchor is used in the device that is the subject matter of U.S. Pat. No. 5,590,091 to Gloden et al., which is assigned to the same assignee as this application. The brass reflection collar used at the remote end of a transducer and detailed in U.S. Pat. No. 5,017,867 to Dumais et al. must be installed by hand and is secured with a set screw. In the magnetostrictive transducer industry, transducers have typically been built in quantities of a few thousand per month. Because of the high quantity demands of the automotive industry, a primary market for the present invention, the installation of a brass or similar reflecting anchor using conventional techniques is time and cost prohibitive. The Z-bend  26 C of the present invention for the first time permits high-volume manufacturing of magnetostrictive transducers using automated machinery. 
     The Z-bend  26 C in waveguide  26  permits the waveguide  26  to be “snap fitted” into the bracket  24  for durability, ease of manufacturing, and low cots. A center portion  26 D of the Z-bend in the waveguide  26  securely anchors and rigidly fixes the waveguide  26  into the bracket  24 . A shorter outward portion  26 E after the Z-bend  26 C is welded to interconnecting pin  52 B that provides the electrical interconnection of the magnetostrictive transducer  10  to electronic circuitry (not shown). Referring to FIG. 5, electrical interconnection of magnetostrictive transducer  10  with external circuitry is accomplished with a printed circuit board  36 . Because of the minimal conductor length necessary to interconnect the magnetostrictive transducer  10  with printed circuit board  36 , the amount of ringing (continuing oscillations occurring for a period of time following an electrical or magnetic stimulation of the reactance of the coil  44 ) is minimized. The reduced ringing permits more accurate position measurements with the position magnet  34  being positioned closer to the near end, thereby allowing the overall mechanical length of the magnetostrictive transducer  10  to be reduced without requiring the electrical length to be reduced, a major objective of the present invention. 
     As previously stated, return conductor  38  provides a current return path for electric interrogation pulses applied to the waveguide  26  by an electronic circuit (not shown). A feature of the present invention is that the return conductor  38  can be either insulated or uninsulated. Therefore, in the preferred embodiment, the enclosure tube  28  is an electrical insulator, as is the suspension sleeve  22 . For manufacturing efficiency, the return conductor  38  is readily sandwiched between the enclosure tube  28  and the suspension sleeve  22  without the possibility that the return conductor  38  will short against electrical components causing a failure of the magnetostrictive transducer  10 . 
     Another feature of the present invention is an integral strain relief  64  for the return conductor  38 . In previous designs, transducers have failed from return conductors that have broken at their points of electrical interconnection with other transducer circuitry. This problem is typically caused by small movement of the return conductor  38  caused by shock and vibration induced into the magnetostrictive transducer  10  resulting in a fatigued segment of the return conductor  38  that eventually severs. Without compromising the compactness of the magnetostrictive transducer  10 , the present invention uses a bracket base cap  72  with an integral strain relief  64 . After return conductor  38  is positioned within a channel  94  in the bracket base cap  72 , a slotted stub  96  is melted such that molten plastic flows around and engages the return conductor  38 . When the plastic cools and hardens, it surrounds and securely anchors return conductor  38  to relieve the stress caused by the return conductor  38  pulling against the circuit board pin  52 A and eventually breaking. 
     All of the features of a particular preferred embodiment of the assembly are not shown in the above disclosure in order to emphasize the generality of the disclosure. 
     Because many varying and different embodiments may be made within the scope of the invention concept taught herein which may involve many modifications in the embodiments herein detailed in accordance with the descriptive requirements of the law, it is to be understood that the details herein are to be interpreted as illustrative and not in a limiting sense.