Claims:1. An eddy current proximity sensor, the sensor comprising:
a LC oscillator;
a pickup coil, wherein the pickup coil is a planar multilayered coil incorporating at least three inductors connected in series, and wherein the pickup coil is part of tank circuit of the LC oscillator; and
a target positioned at a distance from the pickup coil,
wherein oscillating frequency of the LC oscillator is a function of the distance between the pickup coil and the target.

2. The sensor as claimed in claim 1, wherein the LC oscillator is integral to PCB incorporating the pickup coil.

3. The sensor as claimed in claim 1, wherein circuit of the LC oscillator is a cold electronic based circuit.

4. The sensor as claimed in claim 1, wherein the LC oscillator is a digital Inverter based LC oscillator.

5. The sensor as claimed in claim 1, wherein the target is metallic.

6. The sensor as claimed in claim 1, wherein the sensor has operating temperature range of 4.2K to 300K..

7. The sensor as claimed in claim 1, wherein the sensor has sensing range of 0-5mm.

8. The sensor as claimed in claim 1, wherein the sensor has an accuracy of ±10µm.
, Description:FIELD OF THE INVENTION
[0001] The present disclosure relates to technical field of cryogenics proximity sensing. In particular, the present disclosure pertains to a multilayer planar inductor based proximity sensor and associated electronics operating at cryogenic temperatures.

BACKGROUND OF THE INVENTION
[0002] Background description includes information that may be useful in understanding the present invention. It is not an admission that any of the information provided herein is prior art or relevant to the presently claimed invention, or that any publication specifically or implicitly referenced is prior art.
[0003] Eddy-Current sensors or inductive proximity sensors are well known. These sensors typically include a length of conductive wire wound in coil fashion around a non-ferrous core of either air, plastic or ceramic or a ferrite core to form a probe. The wire coil is electrically energized by a low level radio frequency current to produce an electromagnetic field surrounding the probe. When a metallic material forming a target is placed within this magnetic field, eddy currents are generated on surface of the target. The generated eddy currents in turn create their own magnetic field which interacts with the coil. Interaction of the magnetic fields is dependent on distance between the probe and the target. As the distance changes, associated electronics sense change in the field interaction and produce a signal which is proportional to the change in distance between the probe and target.
[0004] In many applications involving cryogenics, viz. low temperature physics, space applications, cryo-engines and superconducting magnetic suspensions, proximity / displacement measurement is one of the critical requirements. Potentiometer or LVDT based sensors are preferred for cryogenic applications. These proximity sensors require moving part to be physically coupled to the sensor part. Though Eddy current sensors are noncontact, low cost devices capable of high-resolution measurement and are currently being used in variety of applications requiring high precision even in harsh industrial environments, their application in cryogenic environment is limited by two factors: deformation of coil’s frame and the resistance variation of cooper coil at the low temperature. Temperature drift that occurs when the ambient temperature changes makes the output signal unstable and inaccurate as reported by Q. Li, F. Ding, Novel displacement eddy current sensor with temperature compensation for electrohydraulic valves, Sensors and Actuators A: Physical 122 (1) (2005) 83–87.Non-Patent Literature “A frameless eddy current sensor for cryogenic displacement measurement, Sensors and Actuators” by Wang et al, 1998 and “An inductive position sensor for the measurement of large displacements at low temperature” by Backes et al, 2010, disclose eddy current sensors and matched oscillator circuit designed using flat coils of Manganese copper, instead of general copper to measure displacement in cryogenic environment. However, these references illustrate usage of a flat sensing coil (single layer) at cryogenic temperatures up to 20 K only and the associated electronics at a stable room temperature. This greatly affects the signal to noise ratio due to long leads connecting the sensor and the electronics.
[0005] Thus, conventionally proximity sensors working at sub-10 K temperature ranges are mainly based on either LVDT or potentiometer. They suffer from drawback that moving parts have to be physically coupled to the sensor part. Furthermore, conventional eddy current sensors working at cryogenic temperatures have their electronics kept at room temperature. This introduces unnecessary noise due to long leads connecting sensor to electronics.
[0006] There is thus a need in the art for a sensitive non-contact eddy current proximity/displacement sensor capable of operating at sub-10 K temperature ranges.
[0007] The present invention satisfies the existing needs, as well as others, and generally overcomes the deficiencies found in the prior art.

OBJECTIVES OF THE INVENTION
[0008] A general objective of the present disclosure is to provide a non-contact proximity sensor that is capable of operating at sub-10 K temperature ranges
[0009] An objective of the present disclosure is to provide a pickup coil that is small in size.
[0010] Another objective of the present disclosure to provide an eddy current proximity sensor having a pickup coil in form of a multilayer planar inductor that also integrates electronic circuit.
[0011] Yet another objective of the present disclosure to provide a cold electronics based LC oscillator operating in cryogenic environment down to 4.2 K.
[0012] Still another objective of the present disclosure to provide highly sensitive eddy current position/displacement sensor for cryogenic displacement measurement in the range of 0-5 mm.

SUMMARY OF THE INVENTION
[0013] Aspects of the present disclosure relate to anon-contact type proximity sensor for operating in cryogenic environment without being affected by noise. In an aspect, the disclosed sensor is an eddy current proximity sensor that is capable of operating down to liquid helium temperature.
[0014] In an aspect, the disclosed eddy current proximity sensor incorporates its electronics as an integral part of the pickup coil, and the electronics is positioned within cryogenic environment. Thus the sensor is free from noise due to long connections between the pickup coil and the electronics.
[0015] In an aspect, the disclosed eddy current proximity sensor incorporates a multilayer planar inductor (interchangeably referred to as pickup coil) having three planer coils. The three planer coils can be connected in series to form an inductor that forms part of the tank/resonant circuit of a LC oscillator. The LC oscillator can be oscillating at the circuit's resonant frequency, the resonant frequency being a function of inductance of the inductor in the resonant circuit of the oscillator.
[0016] In an aspect, arrangement of three planer coils arranged in three layers and connected in series provides an enhanced inductance due to magnetic coupling. Therefore, effective impedance value of these three layer planar pickup coils when connected in series is not a simple addition of individual values but can be equivalent to more than six layers. Thus the multilayer planer configuration of the coils helps to keep the dimensions of pickup coil as small as possible which is essential due to various considerations.
[0017] As is known, eddy current generated in a metallic target positioned near multilayer planar inductor/pickup coil can affect inductance of the pickup coil, and therefore, distance of the pickup coil from a metallic target determines inductance of the multilayer planar inductor. Thus when the distance between the multilayer planar inductor and the metallic target changes, the inductance of the multilayer planar inductor also changes resulting in change in frequency of oscillation of the LC oscillator. This makes frequency of the LC oscillator a function of height, i.e. distance between the multilayer planar inductor/pickup coil and the metallic target. When the height is a time varying function, output of the LC oscillator would be a frequency modulated (FM) wave wherein the modulated wave corresponds to the position of the metal target with respect to the multilayer planar inductor/pickup coil.
[0018] In an aspect, LC oscillator in the disclosed proximity sensor can be a cold electronic LC oscillator so that it can be positioned within cryogenic environment duly integrated with pickup coil. In an aspect, integrating the LC oscillator with the pickup coil eliminates the need of long connections that result in noise. In an aspect, placing the LC oscillator within cryogenic environment makes its working independent of variations in ambient temperature.
[0019] In an aspect, the disclosed eddy current proximity sensor can function satisfactorily in temperature range from room temperature (300 K) to temperature of liquid Helium (4.2 K). Further it has a sensing range of 0-5 mm with an accuracy of ±10 µm.
[0020] In an aspect, the proposed eddy current proximity sensor has been validated for its functioning for different distances of the target at different cryogenic temperatures down to 4.2 K using an operating frequency of 150 KHz for the three layer inductor (inductance of the coil being a function of the operating frequency). The sensor response was observed and the frequency values for different temperatures and at different distances with respect to the sensor and the target were plotted and studied.
[0021] Various objects, features, aspects and advantages of the inventive subject matter will become more apparent from the following detailed description of preferred embodiments, along with the accompanying drawing figures in which like numerals represent like components.

BRIEF DESCRIPTION OF THE DRAWINGS
[0022] The accompanying drawings are included to provide a further understanding of the present disclosure, and are incorporated in and constitute a part of this specification. The drawings illustrate exemplary embodiments of the present disclosure and, together with the description, serve to explain the principles of the present disclosure.
[0023] FIG. 1 illustrates working principle of a conventional non-contact multilayer planar inductor based eddy current proximity sensor device.
[0024] FIGs. 2A and 2B illustrate exemplary images of small sized and high inductance multi-layer pickup coil integrating cold electronics based LC oscillator in accordance with embodiments of the present disclosure.
[0025] FIG. 3 illustrates an exemplary circuit diagram of a digital inverter based LC oscillator incorporating multi-layer pickup coils in accordance with embodiments of the present disclosure.
[0026] FIG.4 illustrates an exemplary plot of variation of frequency of oscillator for different temperatures in accordance with embodiments of the present disclosure.
[0027] FIGs. 5A and 5B illustrate exemplary schematic of the cryostat and image showing sensor mounting on sample rod respectively in accordance with embodiments of the present disclosure.
[0028] FIG. 6 illustrates an exemplary block diagram indicating measurement system used during validation of the disclosed sensor in accordance with embodiments of the present disclosure.
[0029] FIG. 7A illustrates an exemplary plot of sensor response at various temperatures with sensor to target displacement varied in accordance with embodiments of the present disclosure.
[0030] FIG.7B illustrates an exemplary plot of sensor response at various displacements at different temperatures in accordance with embodiments of the present disclosure.
[0031] FIG. 7C illustrates an exemplary plot of sensor output at 4.2 K and asymptotic curve fit in accordance with embodiments of the present disclosure

DETAILED DESCRIPTION OF THE INVENTION
[0032] The following is a detailed description of embodiments of the disclosure depicted in the accompanying drawings. The embodiments are in such detail as to clearly communicate the disclosure. However, the amount of detail offered is not intended to limit the anticipated variations of embodiments; on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the present disclosure as defined by the appended claims.
[0033] Each of the appended claims defines a separate invention, which for infringement purposes is recognized as including equivalents to the various elements or limitations specified in the claims. Depending on the context, all references below to the "invention" may in some cases refer to certain specific embodiments only. In other cases it will be recognized that references to the "invention" will refer to subject matter recited in one or more, but not necessarily all, of the claims.
[0034] Unless the context requires otherwise, throughout the specification which follow, the word “comprise” and variations thereof, such as, “comprises” and “comprising” are to be construed in an open, inclusive sense that is as “including, but not limited to.”
[0035] Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment. Thus, the appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments.
[0036] As used in this specification and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the content clearly dictates otherwise. It should also be noted that the term “or” is generally employed in its sense including “and/or” unless the content clearly dictates otherwise.
[0037] In some embodiments, the numbers expressing quantities of ingredients, properties such as concentration, reaction conditions, and so forth, used to describe and claim certain embodiments of the invention are to be understood as being modified in some instances by the term “about.” Accordingly, in some embodiments, the numerical parameters set forth in the written description are approximations that can vary depending upon the desired properties sought to be obtained by a particular embodiment. In some embodiments, the numerical parameters should be construed in light of the number of reported significant digits and by applying ordinary rounding techniques.
[0038] Notwithstanding that the numerical ranges and parameters setting forth the broad scope of some embodiments of the invention are approximations, the numerical values set forth in the specific examples are reported as precisely as practicable. The recitation of ranges of values herein is merely intended to serve as a shorthand method of referring individually to each separate value falling within the range. Unless otherwise indicated herein, each individual value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g. “such as”) provided with respect to certain embodiments herein is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention otherwise claimed. No language in the specification should be construed as indicating any non-claimed element essential to the practice of the invention.
[0039] The headings and abstract of the invention provided herein are for convenience only and do not interpret the scope or meaning of the embodiments.
[0040] The following discussion provides many example embodiments of the inventive subject matter. Although each embodiment represents a single combination of inventive elements, the inventive subject matter is considered to include all possible combinations of the disclosed elements. Thus if one embodiment comprises elements A, B, and C, and a second embodiment comprises elements B and D, then the inventive subject matter is also considered to include other remaining combinations of A, B, C, or D, even if not explicitly disclosed.
[0041] Various terms as used herein are shown below. To the extent a term used in a claim is not defined below, it should be given the broadest definition persons in the pertinent art have given that term as reflected in printed publications and issued patents at the time of filing.
[0042] Embodiments of the present disclosure relate to a non-contact multilayer planar inductor based eddy current proximity sensor and associated electronics operating down to liquid helium temperature. In an aspect, the highly sensitive proximity sensor can be used for displacement measurement in the range of 0-5 mm under cryogenic conditions.
[0043] In an embodiment, the proposed sensor incorporates a multilayer planer inductor having three coils. In an aspect, arrangement of three planer coils in three layers and connected in series provides an enhanced inductance due to magnetic coupling. Therefore, effective impedance value of a three layer planar pickup coils connected in series is not a simple addition of individual values but can be equivalent to more than six layers. Thus the multilayer planer configuration of the coils helps to keep the dimensions of pickup coil as small as possible which is essential due to various considerations.
[0044] It is to be understood that though embodiments of the present disclosure have been explained with reference to three coils arranged in coplanar manner, there can be more than three coils to reap enhanced advantage of magnetic coupling and resultant increase in inductance, and all such applications are well within the scope of the present disclosure without any limitations.
[0045] In an embodiment, the proposed multilayer planer inductor/pickup coil is part of a tank/resonant circuit of a LC oscillator. Thus resonant frequency of the LC oscillator is a function of inductance of the pickup coil. In an embodiment, the LC oscillator can be a cold electronic LC oscillator so that it can be positioned within cryogenic environment duly integrated with pickup coil. In an aspect, integrating the LC oscillator with the pickup coil eliminates need of long connections that result in noise.
[0046] FIG.1 illustrates working principle of a non-contact multilayer planar inductor based eddy current proximity sensor device as known in the art. Shown there are constituent parts of the sensor such as planer coil 102 and a metallic target 104. The planer coil 102 and the metallic target 104 are spaced apart from each other by a distance say ‘h’. When an alternating current is passed through the coil 102, magnetic field of the coil 102 generates eddy current 106 in the metallic target 104. The generated eddy current 106 affects inductance of the coil 102, wherein distance ‘h’ of the coil 102 from the metallic target 104 determines inductance of the multilayer planar inductor.
[0047] Mathematical model of inductors in the presence of a metal have been extensively studied by C. Dodd et.al and Y. Kraftmakher et al. For a coil such as coil 104 having ‘n’ turns concentrated into a radius ‘r0’ at a height ‘h’ above the conductor of whose thickness ‘t’ is greater than the skin depth at the operating frequency; the change in impedance of the coil is given by, Dziczkowski et al "Eddy current measurements of surface roughness", PAMM 10.1 (2010): 605-606.

(1)
where,

(2)

(3)

(4)

are the generalized parameters, ? is the radian frequency and s is the conductivity of the target metal.

(5)

where, J1 is the first order Bessel function. The impedance term f(a, ß, ?) is a function of h as can be seen from the equations 1,2 and 5, f(a,ß,?) and ?Z are functions of ‘h’.
[0048] Thus when the distance ‘h’ between the coil 102 and the metallic target 104 changes, the inductance of the coil 104 also changes. Therefore, if the coil 102 were to be part of a LC oscillator, change in distance between the coil 102 and the metallic target 104 shall result in change in frequency of oscillation of the LC oscillator. This makes frequency of the LC oscillator a function of distance between the multilayer planar inductor/pickup coil and the metallic target. When ‘h’ is a time varying function, output of the LC oscillator would be a frequency modulated (FM) wave wherein the modulated wave corresponds to the position of the metal target 104 with respect to the coil 102.This is the operating principle of any eddy current proximity sensor.
[0049] In an embodiment, Table. 1 below presents exemplary values of a single layer planar inductor which is needed to keep the dimensions of the coil as small as possible due to various considerations such as localized measurement, restriction on space to accommodate the sensor, frequency of operation and Q factor of the coil.

Parameter Value
Din 162 mils
Dout 1062 mils
Conductor Thickness (w) 10 mils
Conductor Spacing 10 mils
Turns 23
Fill factor 0.7353

Table 1: Inductor Parameters

[0050] While the operating frequency is a function of the inductance of the coil, the Q factor determines the sensitivity. For a value of 10 and above for Q factor, the inductance needs to be = 10 µH. The inductance value can be computed using the values given in table with the help of “Greenhouse formula” below:

(6)

Here, T is called the fill factor given by,
(7)

The computed value of L1= 8018.9 nH agrees fairly with the measured value of 7800 nH using an LCR meter at room temperature.
[0051] In an aspect, the present disclosure provides a coil that possesses increased inductance within given constrains. In an embodiment, the disclosed coil employs three layers of inductor in such a way that the flow of current always follows the same direction. It quantitatively establishes its efficacy over single layer planar inductor. The impedance values of a three layer planar inductor are presented in Table 2 below along with modified equations.

Parameter Equation Experimental values
L total L1 [3+4K1 +2K2 ] 59.8 µH
Rs 3R1 8.4 O

Table 2: Modified Equations for 3 layer inductor along with experimental inductance.

[0052] In an aspect, as shown in the table 2 above, total impedance of three layers of coils connected in series is not a simple addition of the individual values. Due to magnetic coupling coefficients K1 and K2, it can increase the total inductance making it effectively equivalent to more than six layers.
[0053] FIG. 2A illustrates an exemplary image of small sized PCB 202 incorporating high inductance multi-layer planer inductor 204 in accordance with embodiments of the present disclosure. The inductor 204 incorporating three coils in three layers can be fabricated as per standard FRP PCB process. Alignment and current direction in the coils can be designed to give maximum additive mutual inductance value.
[0054] In an embodiment, LC oscillator can be integrated along with multi-layer planer inductor so as to avoid connecting them by long connection and thereby avoid noise on account of such connection. In an exemplary implementation the LC oscillator was integrated on same PCB 202 having multi-layer planer inductor 204. FIG. 2B illustrates an exemplary image of the LC oscillator 206 on the PCB 202.
[0055] In an embodiment, since LC oscillator 206 on account of its being integrated with inductor 204, has to work within cryogenic environment, cold electronic LC oscillator can be used for the LC oscillator 206. In an implementation, the cold electronic LC oscillator was realized making use of commercially available inverter ICs and the other components. In an embodiment, required performance and temperature stability of frequency can be achieved by screening individual components and ICs and further subjecting them to thermal cycling down to 77K.
[0056] FIG. 3 illustrates an exemplary circuit diagram of digital Inverter based LC oscillator 206 in accordance with embodiments of the present disclosure. Also shown therein is metallic target 302 forming complete eddy current based proximity sensor. As shown, the circuit can comprise of three inductors L1, L2 and L3 connected in series whose effective inductance is governed by Table 2.
[0057] In an embodiment, operating frequency of LC oscillator 206 can be derived from following equation:

(8)

Where, SRF is Self Resonant Frequency.
For three layer inductor 204, SRF was found out to be 3.71 MHz using an impedance analyzer. Hence the operating frequency has to be less than 371 KHz. Based on these details, an operating frequency of 150 KHz was chosen with corresponding skin depth of 0.1 mm for copper. Therefore copper target having a thickness higher than 0.1 mm can be used. In the exemplary embodiment target 302 of Copper having thickness of 6 mm was used.
[0058] Since frequency of the LC oscillator is given by:

(9)

a suitable ‘C’ value can be selected to have the frequency around 150 KHz.
[0059] FIG.4 illustrates an exemplary plot of variation of frequency of oscillator for different temperatures in accordance with embodiments of the present disclosure. From the plot it can be observed that temperature effect on the frequency of the oscillator is linear and is approximately 70 Hz/K.
[0060] FIGs. 5A and 5B illustrate exemplary schematic of the cryostat and image showing sensor mounting on sample rod of experimental set up to establish efficacy of the disclosed device at sub-10 K temperature respectively in accordance with embodiments of the present disclosure. As shown in FIG. 5A cryostat has a liquid nitrogen jacket 502 surrounding a liquid helium (LHe) reservoir 504 with inner and outer vacuum spaces 506 and 508 respectively. A capillary tube 510 couples test chamber 512 with the liquid helium reservoir 504. The flow of LHe to the test chamber 512 is controlled with a needle valve 514. A heater at bottom of the test chamber 512 maintains the DUT at constant temperature. Sensor under validation is mounted at sample holder 516 at the end of the position rod 518.
[0061] FIG. 5Billustrates an exemplary image of the sensor mounted on a sample position rod 518 with the nylon washers 552 used to maintain gap between the inductor/pickup coil 204 and copper target 302. An electrolytic grade copper disc of 35 mm × 30 mm × 6 mm was used for the experiment in accordance with embodiments of the present disclosure.
[0062] FIG. 6 illustrates an exemplary block diagram indicating measurement system used during validation of the disclosed sensor in accordance with embodiments of the present disclosure. The DAQ system used LabVIEW software 606 to interface the digital storage oscilloscope (DSO) 608, Keithley 2000 Digital Multimeter 604 and Lakeshore 332 temperature controller 610, and for data acquisition from the sensor and cold electronics kept in the cryogenic environment in the cryostat 602.
[0063] In an experimental aspect, experimental set up of FIG. 5A and 5B, and DAQ of FIG. 6 were used for observing sensor response. Frequency values for different temperatures and at different distances with respect to the sensor and the target were plotted and studied.
[0064] FIG. 7A illustrates exemplary temperature-frequency plots showing response of the disclosed sensor when it is cooled from room temperature to Liquid Helium temperature at different frequencies. The sensor to target displacement is varied from 0.85 mm to 4.5 mm.
[0065] FIG. 7B illustrates exemplary plots of variations in the frequencies for five fixed distances and at various cryogenic temperatures between 50 K and 4.2 K in accordance with embodiments of the present disclosure.
[0066] FIG. 7 C illustrate exemplary plots of the frequency outputs for different distances of the target metal at 4.2 K and the asymptotic fit which follows the equation of the type y=a+b.cx in accordance with embodiments of the present disclosure.
[0067] Table 3 shows sensitivity data in respect of experimental sensor at different distances between 0.85 mm and 4.5 mm:

Sl. No Linearity Range Sensitivity
1 0.85 mm-1.5 mm 67.70 KHz/mm
2 1.5 mm-2.5 mm 20.45 KHz/mm
3 2.5 mm-3.5 mm 11.83 KHz/mm
4 3.5 mm-4.5 mm 6.49 KHz/mm

Table 3: Piecewise linearized sensitivity of the sensor

[0068] In an embodiment, by calibrating the disclosed sensor for different distances between pickup coil and target for different operating temperatures down to 4.2 K, one can measure displacement in the range 0-5 mm at any cryogenic temperature with an accuracy of ±10 µm.
[0069] While the foregoing describes various embodiments of the invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof. The scope of the invention is determined by the claims that follow. The invention is not limited to the described embodiments, versions or examples, which are included to enable a person having ordinary skill in the art to make and use the invention when combined with information and knowledge available to the person having ordinary skill in the art.

ADVANTAGES OF THE PRESENT INVENTION
[0070] The present disclosure provides a non-contact proximity sensor that is capable of operating at sub-10 K temperature ranges
[0071] The present disclosure provides a pickup coil that is small in size.
[0072] The present disclosure provides an eddy current proximity sensor having a pickup coil in the form of a multilayer planar inductor that also integrates electronic circuit.
[0073] The present disclosure provides a cold electronics based LC oscillator operating in cryogenic environment at temperatures down to 4.2 K.
[0074] The present disclosure provides highly sensitive eddy current position/displacement sensor for cryogenic displacement measurement in the range of 0-5 mm.