HK1171806B - Internal position and limit sensor for free piston machines - Google Patents

Abstract

A sensor for sensing the position of a reciprocating free piston in a free piston Stirling machine. The sensor has a disk mounted to an end face of the power piston coaxially with its cylinder and reciprocating with the piston The disk includes a rim around its outer perimeter formed of an electrically conductive material A coil is wound coaxially with the cylinder, spaced outwardly from the outer perimeter of the disk and mounted in fixed position relative to the pressure vessel, preferably on the exterior of the pressure vessel wall.

Description

Internal position and limit sensor for free piston machines

Background

The present invention relates to free-piston stirling engines (free-piston stirling engines), coolers and heat pumps, and more particularly to a position sensor for sensing the position of the reciprocating power piston of such free-piston machines. The sensor has the advantage that it only does not significantly increase the length of the free-piston machine. Furthermore, it has the advantage that the coil part of the sensor providing the electrical output signal can be mounted outside the pressure vessel head of the free piston machine, so that it does not require any electrical conductors for supplying power through the pressure vessel.

This application claims priority to provisional patent application serial No. 61/305986, filed on 19/2/2010, which is incorporated herein by reference. U.S. patents 4,667,158, 4866378, and 4,912,409 are also incorporated by reference into this application.

Free piston stirling machines are thermo-mechanical oscillators well known in the art. Free piston stirling machines offer a number of advantages including the ability to control their frequency, phase and amplitude, the ability to seal against their surroundings, and their ability to not require mechanical fluid seals between moving parts to prevent mixing of the working gas with the lubricating oil. Typically, free piston stirling machines include a power piston that reciprocates in a cylinder and is attached to a spring to form a resonant system. The power piston is also attached to a load when the stirling machine is operating as an engine and to a reciprocating prime mover for reciprocally driving the piston when the stirling machine is operating as a heat pump or cooler. In free piston stirling machines, the piston and displacer (if any) are not connected to each other or to a load or prime mover by mechanical linkages (e.g., connecting rods and crankshafts) that limit their reciprocating motion to a fixed stroke. Instead, the stroke of the oscillating piston is freely changed.

Free piston machines are typically designed with pistons having a nominal design stroke. However, when the machine encounters varying operating parameters (such as varying loads or varying operating temperatures), the piston stroke deviates from the nominal design stroke because its stroke is not limited by the mechanical linkage. If the working stroke is sufficiently increased, the piston may collide with other mechanical structures of the machine, such as a displacer or a member fixed at an axially opposite end of a cylinder in which the piston reciprocates.

Free-piston machines typically have an electronic control system, as the stroke varies depending on the operating parameters and the likelihood of collision. One of the most important parameters sensed and used by the control system is the linear position of the piston. For example, it is sometimes desirable to sense the instantaneous linear position or translation of the piston as it reciprocates through its cyclical motion and/or to sense the opposite end limits of the piston reciprocation.

The three U.S. patents cited above, which are robert-W-radly, show position sensors having an elongated coil and a tube (tube) that reciprocates inside and outside the coil. The inductance and therefore the impedance of the coil decreases as a function of the insertion length of the tube in the coil. While effective, the ledrick sensor occupies a length in the stirling machine that is at least an order of magnitude greater than twice the piston stroke. The reason is that in order to sense the position along the entire stroke, both the coil and the tube must have a length at least equal to the stroke. The tube must be capable of reciprocating movement between a maximum position of extraction from the coil and a maximum position of insertion into the coil, and the distance between maximum extraction and maximum insertion must be at least equal to the stroke. Thus, the length of the entire radley sensor must be at least twice the stroke length at the maximum extraction position. In addition to the length of the radley sensor, the design of the stirling machine must be provided in order to position both the coil and the reciprocating tube in the machine. Thus, the radley sensor adds volume and length requirements to the free piston stirling machine. Furthermore, because the coil of the reed-relay must be located within the sealed pressure vessel of the stirling machine, the electrical conductor leads from the coil must extend through the pressure vessel wall to connect to the control circuitry. This reduces the reliability of the machine, since such electrical feedthroughs must be sealed to withstand high voltages. The seal provides an additional risk of failure.

It is an object and feature of the present invention to provide a position sensor which only minimally increases the length and volume of a free piston stirling machine.

It is another object and feature of the present invention to provide an embodiment of the present invention that does not require a conductor lead (lead) extending through the pressure vessel.

Disclosure of Invention

The present invention is a sensor for sensing the translation or position of a reciprocating free piston in a free piston stirling machine. As is well known in the art, a stirling machine has an external pressure vessel for containing a working gas and at least one power piston freely reciprocable in a cylinder within the pressure vessel at a nominal design maximum stroke along an axis of reciprocation. The present invention has a disc (disk) mounted to an end face of a piston coaxially with a cylinder and reciprocating together with the piston. The disk includes a rim around its outer periphery formed of an electrically conductive material and is preferably formed entirely of an electrically conductive material. The coil is wound coaxially with the cylinder, externally spaced from the outer periphery of the disc and mounted in a fixed position with respect to the pressure vessel, preferably on the outside of the pressure vessel. The disc is much shorter than the coil in its axial direction. Preferably, the coil has a length at least approximately equal to the nominal design maximum stroke of the piston and is approximately centered at the center of the nominal design maximum stroke. Two preferred embodiments with coils, distributed coils and end concentrated coils.

Drawings

Fig. 1 is a diagrammatic axial cross-section of a portion of a free piston stirling machine having one embodiment of the present invention mounted thereon.

Fig. 2 is a block diagram of a simplified coil and sensor circuit embodying the present invention and illustrating its operating principles.

Fig. 3 is an axial cross-sectional view of a free piston stirling machine having another embodiment of the present invention mounted thereon.

Fig. 4 is a cross-sectional view of the embodiment of fig. 3, taken generally along line 4-4 of fig. 3.

FIG. 5 is a graph showing apparent coil impedance as a function of piston position for a distributed coil embodiment of the present invention.

Fig. 6 is a graph showing apparent coil impedance as a function of piston position for an end concentrated coil embodiment of the present invention.

FIG. 7 is a block diagram of the coil and detection circuitry for detecting a signal from the coil as a function of piston position.

Fig. 8 is a graph showing the output signal from the detection circuit of fig. 7 as a function of time for the distributed coil embodiment of the present invention.

Fig. 9 is a graph showing the output signal from the detection circuit of fig. 7 as a function of time for the end concentrated coil embodiment of the present invention.

Fig. 10 is a diagram showing two different coil arrangements.

In describing the preferred embodiments of the present invention illustrated in the drawings, specific terminology will be resorted to for the sake of clarity. However, it is not intended that the invention be limited to the terms so selected and it is to be understood that each specific term includes all technical equivalents which operate in a similar manner to accomplish a similar purpose.

Detailed Description

Fig. 1 shows diagrammatically one embodiment of the invention mounted to a free piston stirling machine known in the art. The prior art component of a stirling machine includes a power piston 10 reciprocating in a cylinder 12 along an axis 14. As is also known in the art, the magnet or series of magnets 16 are arranged in an annular configuration about the axis 14. The magnet 16 is attached to the piston 10 such that the magnet 16 reciprocates with the piston 10 within an armature comprising an armature coil 18 and a low reluctance ferromagnetic core 20. These components are contained within an outer pressure vessel 22 which is sealed and contains the working gas of the stirling machine. The structure of fig. 1 may operate as a linear alternator driven by a stirling engine or as a stirling cooler or heat pump driven by a linear electric motor, as is known in the art. As is also well known in the art, the power piston may alternatively be connected to other loads or prime movers. The present invention is a sensor for sensing the position of a reciprocating free piston 10 along its axis of reciprocation 14. The sensor has a disc 24 mounted to an end face 26 of the piston 10 coaxially with the cylinder 12 and reciprocating with the piston 10. The disk 24 has a rim 28 around its outer periphery formed of an electrically conductive material (such as aluminum, which is preferred because of its high conductivity). Although it is only necessary for the outer edge 28 to be an electrical conductor, it is generally desirable for the entire disk to be made of metal.

The sensor coil 30 is wound coaxially with the cylinder 12 and is spaced externally from the outer periphery 28 of the disc 24 and mounted in a fixed position relative to the pressure vessel. In the embodiment of fig. 1, the coil is mounted around the exterior of the pressure vessel 22, and the pressure vessel is constructed of a non-ferromagnetic material (such as stainless steel or inconel) so that it does not have any significant effect on the magnetic coupling between the disk 24 and the coil 30. The coil 30 has conductor leads 32 for connection to the detector circuit. Alternatively, the coil may be wound around the interior of the pressure vessel, but this has the disadvantage of requiring the conductor wire to extend through the wall of the pressure vessel 22 and be sealed with respect to the wall of the pressure vessel. However, this has the advantage that the magnetic coupling between the disk and the coil is greater. It is also preferred to have an annular ferromagnetic shield positioned outside the coil to avoid interference from stray electromagnetic fields and to maximize magnetic coupling between the disk and the coil.

The basic operating principle is that the reactance of the coil, and therefore the impedance, varies as a function of the position of the disc in or near the end of the coil. The reactance of the coil is a decreasing function of the magnetic coupling between the disc and the coil; i.e. the greater the magnetic flux (from the coil current) at the location of the disc, the lower the reactance of the coil. The length of the coil 30 is at least approximately equal to the nominal design maximum stroke of the piston 10, and the coil is centered at the center of the nominal design maximum stroke. An example of a typical piston stroke is 10mm, although the design stroke varies significantly depending on the size and purpose of the free piston stirling machine. Free piston stirling machines may have a diameter in the order of 10cm to 30cm, again depending on the size and purpose of the machine. Thus, the coil 30 has an aspect ratio of diameter to length such that it is a short coil. The coil diameter is so much larger than the coil length that there is a coil end effect over the entire length of the coil. Thus, the magnetic flux in the coil and the magnetic coupling between the coil and the disk change along the axis of the entire coil. There is substantially no increase in disc translation that does not result in a change in the impedance of the coil.

Referring to fig. 2, an alternating current power supply 40 having a carrier frequency on the order of 50kHz to 200kHz is applied to the coil. The alternating current induces eddy currents around the periphery of the disk, causing the disk to act like a shorted secondary coil of a transformer. The magnetic field for eddy currents is magnetically coupled to the coil 30, decreasing the coil impedance in response to increased magnetic coupling. The magnetic coupling is at its maximum when the disk is centered in a coil having turns distributed along its entire length. The impedance of the coil therefore varies as a function of the magnetic coupling between the disk and the coil, and is at its minimum when the disk is centered in a coil having turns distributed along its entire length. As the disk moves within the coil, the amplitude of the voltage across the coil at the carrier frequency changes as a function of the disk position because the AC power source 40 is a constant current source and the impedance of the coil changes as a function of the disk position. This amplitude variation is detected by the amplitude modem 42 to provide an envelope signal as a function of the disc position.

As the magnetic coupling between the disk and the coil increases, the resistive losses generated by eddy currents in the disk cause an increase in the apparent resistance (apparent resistance) seen at the coil terminals. However, the decrease in the reactive part of the coil impedance is dominant and significantly greater than the increase in the resistive part of the apparent coil impedance.

Disc-shaped element

Preferably, the disc 24 is circular and has a thickness in the axial direction of no more than thirty percent and most preferably ten percent of the nominal design maximum stroke. Furthermore, the radial dimension perpendicular to the axis, or radius for an ideal circular disc, is greater than the thickness of the disc in the axial direction. More preferably, the radial dimension is at least 10 times greater than the axial thickness of the disc. This aspect ratio of the disks and the relatively small thickness of the disks allow the sensor of the present invention to increase the length of the stirling machine very little. The thickness of the disc is subject to engineering compromises. A thinner disc will provide greater resolution. However, it is also desirable that the disc is sufficiently stiff that it does not significantly bend due to the alternating acceleration and deceleration of the reciprocating motion of the piston. For example, we have used a disc with a thickness of about 2 mm.

Whether the coil is positioned inside or outside the pressure vessel, it is desirable that the disk extend radially outwardly to place its outer periphery as close as possible to the coil to maximize magnetic coupling therebetween. For the outer coil this means as close as possible to the pressure vessel wall. As close as possible means as close as is allowed by engineering judgment, with no risk of physical contact taking into account the designed radial motion tolerances.

Although the disks may be constructed from a single unperforated plate of electrically conductive material, forming the disks with perforations, and particularly in a wheel configuration, has several advantages. The provision of perforations in the disc allows the working gas to move more freely within the stirling machine and thereby reduces pumping losses. Fig. 3 and 4 show a free piston stirling machine with a disc 49 advantageously formed with a central hub 50 mounted to the piston 52 and an outer peripheral rim 54, spokes 56 connecting the hub 50 to the rim 54.

Another advantage of the disc of the invention, particularly in spoke-type configuration, is that it can also be used as a damper to damp the piston 52 at the opposite end of its reciprocating movement if the stroke of the piston exceeds the maximum allowable stroke. To cushion such over-stroke (over stroke), at least one pair of annular stops 60 and 62 are mounted within the pressure vessel 64 in a fixed position relative to the pressure vessel 64 on opposite sides of the disc 49. Stops 60 and 62 are spaced apart a distance equal to the maximum nominal design allowed stroke. Stops 60 and 62 are fixed equidistantly on axially opposite sides of the center of the nominal design maximum stroke and are aligned to be contacted by the edge 54 of the disc 49 in the event that the stroke exceeds the maximum nominal design allowed stroke. The stops 60 and 62 may alternatively be discrete stops disposed at intervals and distributed in a ring for similar contact with the edge 54 of the disk 49.

The radial spokes act like leaf springs. Any substantial over-stroke deflects the spokes and gradually absorbs the kinetic energy of the piston as it stops. The spoke spring then releases the stored kinetic energy, pushing the piston back in the opposite direction. To further enhance this advantageous feature, the disk 49 may be constructed of two different materials. The rim 54 may be formed of a conductive material having a greater electrical conductivity than the spokes 56, and the spokes may be formed of a spring material. For this two-part construction of the disc 49, the rim is typically manufactured to have a radial width of 5% to 15% of the radius of the disc 49.

To maximize the advantage of the spring and damper action from above, a spacer 64 is inserted coaxially between the hub 50 of the disc 49 and the end face 66 of the piston 52. The spacer 64 has a diameter that is smaller than the diameter of the piston 52 so that the spokes 56 can bend between the rim 50 and the spacer 64. The disc 48 is attached to the piston 52 by a machine screw or nut through a central bore coaxially through the hub 50.

Coil

The coils used in the sensor of the present invention can be manufactured in a number of different embodiments. Two of the most important are distributed coils (distributed coils) and end concentrated coils (end concentrated coils). The difference is the distribution of the windings of the coil along the length of the coil.

Fig. 10 shows these two coil embodiments. In the distributed coil embodiment 30A, the turns of the coil are distributed, preferably evenly distributed, along the entire length of the coil. In concentrated coil embodiment 30B, the windings are concentrated at the ends of the coil. Preferably, the concentrated coil may be considered as two short coil portions, each preferably concentrated at opposite ends of the designed maximum nominal working stroke but connected in series. Preferably, each of the two short coil portions is approximately 25% of the piston stroke in length, the series connection extending between the two coil portions.

Although the number of turns in any coil embodiment is not critical, the greater the number of turns, the greater the change in impedance within the coil as a function of the disc position. Thus, the selection of the number of turns is an engineering tradeoff made between enough turns to obtain a useful signal and not as many turns to provide a reduced benefit for additional turns.

The length of the coil is approximately the length of the maximum nominal working stroke. These are considered. Free piston stirling machines can be designed to operate with a single stroke. The nominal stroke is the stroke that the design works to follow. The preferred length of the coil is at least the length of the stroke, the end of the coil being located radially outwardly from the disc at the end position of the stroke. However, with so many parameters in engineering and science, a small amount of deviation in operation brings only a small difference, so the device is still feasible and useful over a range extending on either side of the preferred length and positioning. This is true for the present invention. While it is preferred that the coil length be equal to or exceed the nominal design stroke, it is not necessary to be exactly equal to the nominal stroke. The coil length may be more or less than the nominal stroke, but as the coil length deviates even further from the stroke length, the desired features and characteristics of the present invention become progressively less. The invention is believed to be feasible and effective for coil lengths in the range of 90% to 110% of the stroke length, and it is preferred that the coil be slightly longer than the design stroke. It is believed that a significant deviation from this range may result in a deterioration in the working efficiency or unnecessarily taking up additional space within the pressure vessel. However, when it is desired to sense piston position over a wide range of piston positions, the coil may be extended further. Some free piston stirling machines are designed to operate with variable strokes that vary within a nominal design range. However, in this case, the length of the single coil embodiment of the present invention may preferably be the maximum stroke within the nominal design stroke range, and may deviate from the values described above.

Operation and detection circuit

Fig. 5 is a graph showing the variation of coil impedance as a function of disk position in a distributed coil embodiment (referred to on the graph as a single coil). The graph shows the impedance change as the piston moves from one end of its stroke to the opposite end of its stroke in a generally sinusoidal motion that is characteristic of piston motion. As can be seen in fig. 5, because the distribution of the magnetic flux in the coil has a sinusoidal distribution from one end to the other, the impedance of the coil varies substantially sinusoidally as a function of the position of the disk. Because the impedance is reduced by the presence of the disk, the minimum impedance occurs when the disk is in the center of the coil (where the magnetic flux is greatest). The diagram is horizontally symmetrical because of the parasitic effect caused by the presence of the piston moving alternately closer and further away from the coil and itself being a large conductive block located close to the disc. Further parasitic effects arise due to the fact that the internal structure of the free piston machine is symmetrical in the axial direction.

Fig. 6 is a graph showing the change in coil impedance as a function of disc position in an end concentrated embodiment of the invention as the piston moves in a generally sinusoidal motion from one end of its stroke to the opposite end of its stroke. The graph of fig. 6 shows that the apparent coil impedance decreases as the disk approaches the center of one of the two concentrated coil portions and increases as the disk approaches the center of the concentrated coil between the two coil portions. After the disk passes through the center of the coil between the two concentrated coil portions and comes closer to the second coil portion, the impedance decreases until the disk reaches the center of the second coil portion, and then the impedance increases as the disk moves toward the end of the coil.

Figure 7 shows a preferred circuit for detecting a signal as a function of piston position. The 125kHz sine wave generator drives a power supply to apply a 125kHz supply current to the coil 30. Because the coil is driven by an AC power source, the amplitude of the sine, 125kHz AC voltage across the coil, will be proportional to the coil impedance. In other words, the voltage on the coil, with a carrier frequency of 125KHz, is amplitude modulated by the instantaneous value of the coil impedance. Thus, the voltage across the coil can be amplified, filtered and demodulated by an AM demodulator to provide a signal at its output 70 that is proportional to the coil impedance. Because the coil impedance is a function of piston position, the signal at output 70 is a function of piston position. The amplifier detection circuit of fig. 7 includes an amplifier and inverts the output signal, it will be apparent to those skilled in the art that the output signal may be inverted again if desired, although it is generally not necessary.

The circuit of fig. 7 can be analyzed in more detail. The sine wave generator 72 has a counter to produce a 125kHz square wave that is heavily filtered to produce a 125kHz sine wave. The sine wave must be very stable and contain few other frequency components. The output from the sine wave generator 72 is applied to a constant current source 74 which generates a 125kHz sine current through the ILS coil 30. The constant current generator 74 uses the voltage wave generated by the sine wave generator and the sense resistor to generate a signal having a constant current characteristic. The current sink from the coil is the ground plane.

As the disc core (core) moves through the coil, the impedance of the coil 30 changes. Four wire measurements were used. Two wires supply current to the coil and two other wires measure the voltage across the coil. The phase adjuster 76 changes the phase of the sine wave by approximately 90 degrees. The phase change is formed by a low-pass filter. A sine to square circuit (sine to square circuit) 78 forms a square wave from the phase-shifted sine wave for use by the synchronous demodulator 80. Instrumentation amplifier 82 measures the voltage on the ILS coil. This voltage is generated by a 125kHz sinusoidal current flowing through the coil and depends on the impedance of the ILS coil. The pre-filter 84 is the front end to the demodulator, which prevents unwanted low frequency signals from passing through the demodulator.

The square wave generator does not generate a signal with a duty cycle of exactly 50%, which varies its average value. This would allow some undesired low frequency signals to pass through the demodulator without a high pass pre-filter. This pre-filter may not be necessary if the duty cycle of the modified square wave is exactly 50%. Demodulator 80 is a synchronous demodulator that recovers the modulated signal from the instrumentation amplifier. The 125kHz sinusoidal signal is modulated by changes in the impedance of the ILS coil. Demodulator 80 multiplies the signal from the instrumentation amplifier with a modified square wave. A low pass filter is then used to remove the higher frequency components, leaving only the adjustment signal representing the impedance of the ILS coil. Since the shape and level of the square wave is not ideal, it is used to control a switch. When closed, the switch multiplies the signal by 0, and when open, the switch effectively multiplies the signal by 1.

The output signal from demodulator 80 over one cycle of piston motion is shown as a function of time in fig. 8 for a distributed coil embodiment. It shows the output signal as the plunger moves from one end of its stroke near one end of the distributed coil to the opposite end of its stroke near the opposite end of the coil and then back again. As is apparent in the graph, the graph of fig. 8 has a first portion that is simply reversed from fig. 5, and the first portion is followed by a second portion that is a mirror image of the first portion when the plunger is returned. Fig. 9 shows the same case for the end concentrated coil and has the same relationship as fig. 6.

Where it is desired to linearize the signal from output 70 to obtain a signal having a magnitude that varies linearly with piston position, existing techniques for doing so may be used. For example, the output of the detector circuit of fig. 7 may be converted from an analog format to a digital format. During laboratory testing, the piston moves along an axis formed with a series of stopped reciprocating motions separated by small discrete translational intervals. The magnitude of the output signal is stored in a digital memory associated with the measured and correlated position of the piston at each stop. The collection of these associated data pairs provides a look-up table. During operation of the sensor of the present invention having a detector circuit, the output of the demodulator of the detector circuit is periodically sampled, converted to digital format and used to add to a look-up table and find the piston position corresponding to the magnitude of the detected output from the demodulator.

It is apparent from fig. 5 and 6 that although the sensor of the present invention provides a signal as a function of the absolute value of the position of the piston, it does not provide information about the polarity of the piston position, as described above; that is, the magnitude of the detected output signal does not indicate on which side of the central piston position the piston is located. However, if desired, this information can be obtained using existing techniques. For example, the polarity of the voltage on the armature coil 30 (fig. 1) may be monitored and indicate on which side of the center the piston is located. Alternatively, the asymmetry of the graph shown in the figures may be used, for example by including the asymmetric portion of the graph in the look-up table described above.

This detailed description, taken in conjunction with the drawings, is intended primarily as a description of the presently preferred embodiments of the invention, and is not intended to represent the only forms in which the present invention may be constructed or utilized. This specification sets forth the designs, functions, means, and methods of practicing the invention in connection with the illustrated embodiments. However, it is to be understood that the same or equivalent functions and features may be accomplished by different embodiments that are also intended to be encompassed within the spirit and scope of the invention, and that various modifications may be resorted to without departing from the scope of the invention or the claims that follow.

Claims (19)

1. A sensor for sensing translation or position of a reciprocating free piston in a free piston stirling machine having an external pressure vessel for containing a working gas and at least one power piston freely reciprocable in a cylinder within the pressure vessel at a nominal design maximum stroke along an axis of reciprocation, the sensor comprising:

(a) a disc mounted to an end face of the piston coaxially with the cylinder and reciprocating with the piston, the disc including an edge surrounding an outer periphery of the disc formed of an electrically conductive material; and

(b) a sensor coil wound coaxially with the cylinder, externally spaced from the outer periphery of the disc and mounted in a fixed position relative to the pressure vessel, the sensor coil having conductor leads for connection to a detector circuit; wherein the disc has a thickness in an axial direction of no more than thirty percent of the nominal design maximum stroke and a radial dimension perpendicular to the axis that is greater than the thickness of the disc in the axial direction.

2. The sensor of claim 1, wherein the sensor coil has a length at least approximately equal to the nominal design maximum stroke and is approximately centered at a center of the nominal design maximum stroke.

3. The sensor of claim 2, wherein the sensor coil is wound around an exterior of the pressure vessel, and the pressure vessel is a non-ferromagnetic material.

4. A sensor according to claim 3, wherein the disc comprises a central hub mounted to the piston and comprises an outer peripheral rim and spokes connecting the hub to the rim.

5. The sensor of claim 4, wherein the rim is a conductive material having a greater electrical conductivity than the spokes, and the spokes are formed of a spring material.

6. The sensor of claim 4, wherein the reciprocating free piston has a maximum nominal design allowed stroke, and the sensor further comprises at least one pair of stops mounted within the pressure vessel in a fixed position relative to the pressure vessel, equidistantly located on axially opposite sides of a center of the nominal design maximum stroke, spaced apart by a distance equal to the maximum nominal design allowed stroke, and aligned to be contacted by the edge of the disc in the event of a stroke exceeding the maximum nominal design allowed stroke.

7. Sensor according to claim 6, wherein a spacer is inserted coaxially between the disc and the end face of the piston, said spacer having a diameter smaller than the diameter of the piston to allow the spoke to bend between the rim and the spacer.

8. The sensor of claim 7, wherein the disk has a circular outer periphery.

9. The sensor of claim 2, wherein the sensor coil is wound such that turns are more concentrated at opposite ends of the sensor coil.

10. The sensor of claim 9, wherein the disc includes a central hub mounted to the piston and includes an outer peripheral rim and spokes connecting the hub to the rim.

11. The sensor of claim 10, wherein the rim is a conductive material having a greater electrical conductivity than the spokes, and the spokes are formed of a spring material.

12. The sensor of claim 9, wherein the reciprocating free piston has a maximum nominal design allowed stroke, and the sensor further comprises at least one pair of stops mounted within the pressure vessel in a fixed position relative to the pressure vessel, equidistantly located on axially opposite sides of a center of the nominal design maximum stroke, spaced apart a distance equal to the maximum nominal design allowed stroke, and aligned to be contacted by the edge of the disc in the event of a stroke exceeding the maximum nominal design allowed stroke.

13. The sensor of claim 12, wherein a spacer is inserted coaxially between the disc and the end face of the piston, the spacer having a diameter smaller than the diameter of the piston to allow the spoke to flex between the rim and the spacer.

14. The sensor of claim 13, wherein the disk has a circular outer periphery.

15. The sensor of claim 2, wherein the disc comprises a central hub mounted to the piston and comprises an outer peripheral rim and spokes connecting the hub to the rim.

16. The sensor of claim 15, wherein the rim is a conductive material having a greater electrical conductivity than the spokes, and the spokes are formed of a spring material.

17. The sensor of claim 15 wherein said reciprocating free piston has a maximum nominal design allowed stroke and said sensor further comprises at least one pair of stops mounted within said pressure vessel in a fixed position relative to said pressure vessel, equidistantly located on axially opposite sides of the center of said nominal design maximum stroke, spaced apart a distance equal to said maximum nominal design allowed stroke, and aligned for contact by said edge of said disc in the event of a stroke exceeding said maximum nominal design allowed stroke.

18. The sensor of claim 17, wherein a spacer is inserted coaxially between the disc and the end face of the piston, the spacer having a diameter smaller than the diameter of the piston to allow the spoke to flex between the rim and the spacer.

19. The sensor of claim 18, wherein the disk has a circular outer periphery.