Magnetostrictive Pressure Transducer

This invention relates to a magnetostrictive pressure transducer and, more specifically, to a device for measuring relatively low pressures, wherein the force from the pressure being sensed is directed along one axis of an element having the property of changing magnetic induction proportionally to the force applied thereto and means for measuring the changes in the magnetic induction as a quantitative indication of the force and hence the applied pressure.

It is well known that the magnetic properties of many ferromagnetic materials undergo changes with stress. For example, the magnetic induction of nickel-iron alloys and iron-cobalt alloys increases and that of nickel decreases with tension stress. Conversely, if these metals are subjected to magnetic fields, their volume will increase in the case of nickel-iron and iron-cobalt alloys or contract in the case of nickel. This expansion may be as large as 70 .times. 10.sup.-.sup.6 in/in for 60 cobalt-iron (H-450 amp-turn/M) alloy. Some attempts have been made to utilize the above described magnetostrictive principle for pressure sensing purposes. U.S. Pat. No. 2,370,845 is an example of prior art which uses magnetostrictive sensors to measure stresses in the 1000 psi + region.

The prior art as well as initial logical attempts to provide a transducer for measuring low pressure is not satisfactory for relatively low pressures in that at hydrostatic pressure levels below 200 psi the output voltage obtained is low and zero levels are erratic. Such systems have a low output/noise ratio. In accordance with the present invention, there is provided a pressure transducer which overcomes the above noted problems. Briefly, the above is accomplished by providing a diaphragm for applying pressure to be measured along an axis of a magnetostrictive element and providing circuitry for sensing the changes in magnetic induction of the magnetostrictive element.

It is therefore an object of this invention to provide a magnetostrictive pressure transducer capable of measuring relatively low pressure levels accurately.

It is a further object of this invention to provide a magnetostrictive pressure transducer which is a small size relative to prior art transducers.

It is a yet further object of this invention to provide a magnetostrictive pressure transducer which is substantially insensitive to ambient temperature.

It is a yet further object of this invention to provide a magnetostrictive pressure sensing device wherein pressure is directed along one axis of the magnetostrictive device.

The above objects and still further objects of the invention will become apparent from the following description of preferred embodiments of the invention which are provided by way of example and not by way of limitation, wherein:

FIG. 1 is a cross section of a pressure sensing member or cartridge in accordance with the present invention;

FIG. 2 is a cross section of a transducer assembly mounting two pressure sensing members of FIG. 1 in accordance with the present invention;

FIG. 3 is an excitation circuit for use with the assembly of FIG. 2; and

FIGS. 4-6 are circuit embodiments employing pressure sensing members of FIG. 1.

Referring first to FIG. 1, there is shown a pressure sensing cartridge for sensing of applied pressure in accordance with the present invention. The cartridge includes a magnetostrictive stud 10 made of material having magnetic properties. The stud is preferably composed of iron-cobalt alloys, nickel-iron alloys or ferrites. In particular alloys of iron-cobalt have been found to be effective since such alloys have a level of permeability which optimizes the output to noise ratio of the system and also since such alloys have magnetic properties which are less temperature dependent than other known magnetic materials. To minimize the volume required for the electronics (required for energizing the cartridge and detecting the output signal) and to optimize the frequency response of the pressure transducer, relatively high excitation frequencies are required. At higher frequencies the reactive components of the electronics decrease in volume. However the depth of penetration (skin depth) of the magnetic field decreases with frequency so that smaller cross-sectional areas of the magnetostrictive stud are active in influencing the electrical circuit. The skin depth depends on the electrical resistivity and the magnetic permeability of the material. For instance, for a frequency of 1000 Hz and average magnetic material, the skin depth is less than 10 mils. Thus in the preferred embodiment the sensor element is made in a tubular configuration as by rolling one end of a sheet toward a second end but spaced therefrom. The spacing may be maintained by placing a non-magnetic core of plaster if desired. The sheet may also be rolled upon itself in the form of a scroll to increase the active surface area without a significant increase in volume. The tube or scroll is configured by taking a thin sheet, for instance between 0.0005 and 0.01 inches in thickness of iron-cobalt alloy, coating opposite sides with a thin layer of insulating material such as for example a polyimide synthetic resin, such as sold by E.I. DuPont De Nemours and Company under the trademark "KAPTON". This sheet is then rolled into a tube or on itself into a relatively tight scroll.

A sensor made in accordance with this invention may be constructed by taking a sheet of iron-cobalt alloy (PERMENDUR a trademark of Allegheny Ludlum Steel Corp. for cobalt-iron alloys of very high magnetic saturation) .002 inches thick annealing it in an inert atmosphere such as argon at a temperature range from 840.degree. to 860.degree. C, preferably at 850.degree. C, for 1 to 2 hours and then quenching it in brine. The sheet is cut and coated with 0.2 .times. 10.sup.-.sup.3 inches thick layer of polyimide on opposite faces of the sheet. The polyimide is composed of pyromelletic dianhydride, polyamide acid and M-pyrol (vehicle) and is precured at 150.degree. C for twenty minutes to remove the vehicle, and rolled into a tube or scroll if desired with an outer diameter of approximately 0.125 inches. Retaining rings may be placed about the scrolls to keep them tightly wound while the tubes or scrolls are heat treated first to react the pyromelletic dianhydride and the polyamide acid at 220.degree. to 260.degree. C for 1 hour to obtain the polyimide and then to develop the magnetic properties of the alloy material. This heat treatment consists of baking the tubes or scrolls from 1 to 60 hours at 250.degree. to 350.degree. C, preferably for 50 hours at 300.degree. C. This configuration not only gives good mechanical stability but it also results in a sensor which has excellent sensitivity in the desired pressure range of 0-100 psi through use of a diaphragm exerting 2-5 Kg/mm.sup.2 on the sensor with essentially a straight line output versus force relationship. The ends of the scrolls may have a conical shape which can conveniently be produced by cutting the sheet into a trapezoidal shape prior to rolling. The conical shape produces a uniform stress per area of material and prevents the possibility of excessive eddy currents through short circuits. Pressure to be sensed is applied along the longitudinal axis of the magnetostrictive stud 10 and is coupled to it through end pieces 4 and diaphragm 26. The diaphragm may be dome shaped formed of material 0.004 inches thick and having a spring rate less than 0.5 lb/.001 in. This low spring rate can be achieved by forming two concentric corrugations (not shown) in the diaphragm which also compensates for differences in temperature expansion between the sensing stud and the mount for the sensing stud. It should be noted that other axially directed force transmitting means could be used in place of the diaphragm if desired, such as other membranes, a piston or the like. The end pieces are of bronze or brass and are attached by means of an hydrostatic pressure transfer medium such as potting compounds having high strength, good chemical resistance and good performance at high temperatures such as an epoxy novolac resin. A coil 13 is centrally located about stud 10. The free portions of stud 10 between the end pieces and coil 13 are inserted in respective slots 16 of "U" shaped yoke 12 formed of ferrite material. Slots 16 may be formed so that stud 10 fits closely therein with little or no air gap. For convenience of manufacture a slight space between the stud and the yoke can be provided and then filled with ferrite powder 18 suspended in a binder such as paste or vacuum grease or some thermoplastic or elastic epoxy such as "FLEXANT" 95 a trademark of Devcon Corporation, Danvers, Massachusetts for a non sagging urethane putty. Alternately yoke 12 of magnetic material could be molded so that the legs of the yoke are molded about spaced portions of the stud.

A pair of sensors of the type in FIG. 1 may be utilized in an electronic transducer circuit (to be explained later). The pair of transducers, as shown in FIG. 2, is positioned in a case 24; the transducers comprising a sensing transducer 20 and a reference transducer 22. The sensing transducer 20 is mounted so that an end cap 14 contacts a membrane 26. A calibration device 28 is provided in the form of a mount screw so that adjustment of the screw will determine the initial force of the stud 10 in cartridge 20. Electronic circuitry may be positioned within or without case 24.

An excitation voltage is applied to the sensor by an excitation oscillator circuit 30 such as that shown in FIG. 3 which can for example provide up to three volts peak to peak at one kilohertz. The excitation circuit includes a voltage regulated power supply comprising NPN transistor Q1, zener diode D1, capacitor C1 and resistor R1 and an oscillator comprising twin T filter T limited by back to back zener diodes D5, D6 connected between the negative input and the output of an operational amplifier. Positive feedback to the amplifier is provided through a voltage divider comprising resistors R4, R5 and R6. Resistor R5 is a variable resistor to permit adjustment of the amount of feedback. An auxiliary circuit for providing the proper voltage level for the operational amplifier comprises a voltage divider including resistors R2 and R3. A buffer and amplifier portion comprises a temperature compensated voltage divider to provide pre bias for output transistors including resistor R7, diode D2 for NPN transistor Q2 and resistor R8, diode D3 for PNP transistor Q3 with capacitor C2 to provide DC isolation. The above circuit is well known in the art and hence operation of it will not be described in detail.

Referring now to FIG. 4 the excitation output (line L1) is applied across a voltage divider comprising resistor R9 and coil L5 providing an output voltage at junction A. Coil L5 corresponds to transducer 20 of FIG. 2 with the axial force to be measured indicated by arrow F. Resistor R9 can be temperature sensitive to provide ambient compensation to offset any temperature sensitivity of transducer 20. For instance temperature sensitive material such as nickel or nickel-silicon alloy wire or negative temperature coefficient (NTC) thermistors can be used for the resistor. Alternatively, temperature compensation can be obtained by constructing the magnetic coil itself with temperature sensitive wire. Thus a change in force F causes a change in the inductance of coil L5 and a corresponding change in the output voltage at junction A. This output voltage is fed to a signal recovery circuit or detector, well known in the art. It is desirable that the detector provide a high impedance load on the sensor such that changes in voltage be detected with high impedance amplifier to minimize error. As shown in FIG. 4 an asymmetrical detector 40 is employed. Any suitable indicating means (not shown) can then be connected to detector 40.

In FIG. 5 an extra coil L6' is provided on stud 10 to recover the output. This embodiment is particularly useful if electrical isolation between the excitation circuit and sensor and the detector is desired. This embodiment is also useful for obtaining a symmetrical output and hence useful with differential amplifier 50 shown in the Figure. It will be noted the FIG. 4 embodiment changes in inductance only are detected while in the FIG. 5 embodiment changes in magnetic coupling due to stress between the primary and secondary coils are detected along with changes in inductance.

FIG. 6 shows yet another embodiment similar to FIGS. 4 and 5. In the embodiment a sensing bridge is employed comprising coil L4 (corresponding to transducer 22 of FIG. 2) and resistor R10 in one leg of the bridge and coil L5 (corresponding to transducer 20 of FIG. 2) and resistor R11 in the other leg of the bridge. Resistors R10 and R11 may be temperature dependent for the same reason as mentioned above relative to resistor R9 in FIGS. 4 and 5. Variable resistor R12 can be used to provide correction for zero output. The detector circuit comprises differential amplifier 60 with symmetrical or asymmetrical output as desired. Thus any output from the sensing bridge due to bridge imbalance is amplified to provide an output voltage which is proportional to the force applied to the transducer 20 (coil L5).

Although the invention has been described with respect to specific preferred embodiments any variations and modifications will immediately become apparent to those skilled in the art. It is therefore the intention that the appended claims be interpreted as broadly as possible in view of the prior art to include all such variations and modifications.