Patent Application
20190360881
Disclosed embodiments relate to electro-mechanical force transducers.
Force transducers have widely been used in many different applications. Known force transducers include tension force sensors, strain gauge load cells which can be based on different principles such as a piezoelectric crystal, hydraulic, pneumatic, Linear Variable Differential Transformer (LVDT), capacitive, tuning fork, or a vibration wire. With such a large variety of force sensors, the measuring ranges covered are generally from 0.1 N to 100,000 N. However, the low end force measuring range from about 0.01 N to 0.5 N still remains a challenge to provide. A few highly sensitive force transducers can measure such small magnitude forces in this low force range, but are expensive and not sufficiently durable for industrial applications such as for servo gauging when measuring the stratified density distribution of a fluid using a submerged displacer placed in a container (e.g., an oil tank).
This Summary is provided to introduce a brief selection of disclosed concepts in a simplified form that are further described below in the Detailed Description including the drawings provided. This Summary is not intended to limit the claimed subject matter's scope.
This Disclosure recognizes there is an unmet need for a durable, relatively low cost, and high sensitivity electro-mechanical force transducer suitable for measuring the low end of the force measuring range being about 0.01 N to 0.5 N, which may be under harsh industrial conditions. This unmet need is met by disclosed dynamic inductance force transducers.
Disclosed aspects include a dynamic inductance force transducer comprising a variable inductor including an induction coil having a wire configured as a plurality of turns with a hollow center having an inner opening with an inner core within the inner opening. The inner core can move in and out of the opening responsive to an applied pulling force, which changes the inductance of the variable inductor depending on the magnitude of the pulling force. An elastic spring has an end for securing to a fixture and an opposite end secured to a first end of the inner core. The inner core has a second end opposite the first end that has a coupling feature coupled thereto for attaching a load which provides the pulling force.
Disclosed embodiments are described with reference to the attached figures, wherein like reference numerals are used throughout the figures to designate similar or equivalent elements. The figures are not drawn to scale and they are provided merely to illustrate certain disclosed aspects. Several disclosed aspects are described below with reference to example applications for illustration. It should be understood that numerous specific details, relationships, and methods are set forth to provide a full understanding of the disclosed embodiments.
The induction coil 120 comprises a wire configured as a plurality of turns. For example, 20 turns to generally more than 100 turns, for instance 50 to 100 turns, as long as the generated resonant frequency is detectable. The induction coil 120 can comprise materials such as copper, stainless steel, aluminum with a coating such as of a gold-nickel alloy, or other electrically conductive materials that can be formed into coils. The variable inductor 115/120 dynamically changes its inductance depending on a magnitude of the applied pulling force which changes the position of the inner core 115 based on the elasticity of the spring 110 that is coupled to the first end of the inner core 115 which is opposite to its second end which receives the pulling force.
The cords 117, 119 and 121, or pins for 117 and 119, are generally of high strength and can in one particular aspect be less than 0.25 mm (9.84252 mils) in diameter to be suited for a small drum for servo gauge applications that spools cord on its threading groove. For example, comprising stainless steel 316 which is known for robust chemical resistance in oil/gas and petrochemical applications. In some particular applications where even higher strength wire may be needed to minimize the elongation of very long thin wire, Molybdenum and/or Rhenium alloys may be used. The diameter of the cords can also be larger than 0.25 mm of stainless steel 316 in order to reduce the elongation, in this case a larger drum for servo gauge applications is usually needed to make a wider threading groove to wind a thick cord.
The wire of the induction coil 120 can be coated with a higher electrical conductivity layer, such as a metal coating on copper. For example, the conductive coating material can comprise gold-nickel alloy, silver, or nanoparticle based graphene which possesses inert properties to some harsh industrial application environments such as found in oil and gas, and petrochemical refining. Such a coating is generally advantageous because wire resistance causes a resistive loss of energy, where a highly electrically conductive coating will enable less resistive loss and thus more current to flow in the induction coil 120 which provides an increase strength of magnetic field flux, hence a higher efficiency and sensitivity.
The spring 110 has one of its ends secured to a fixture 125, such as a secured by a mechanical coupler 122 that although shown being wire-like in
For an inner core comprising a fully non-magnetic material, the inductance variation of the variable inductor 115′/120 will not be notably affected by movement of the inner core 115′. In this embodiment, the turns of the inductor coil 120 are instead a variable being a function of the position of the non-magnetic inner core 115′ through movement of the contactor 115b resulting from movement of the non-magnetic inner core 115′. A length of the contactor 115b is generally least 2 turns of the induction coil 120, which is generally at least 20 turns, where spacing between two adjacent turns will generally be equal to or less than the diameter of the wire of the induction coil 120. It is recognized that the hysteresis for the non-magnetic inner core 115′ is generally much less being a non-magnetic material as compared to a magnetic material for the inner core.
The spring 110 is a coiled wire spring that functions as an elastic device that enables the force transducer 100 to be a high-precision frequency-force detector. The spring 110 can be a low cost spring that has high robustness, since wire springs with good compression, extension, and torsion can be commercially found as they are already used in a wide variety of other applications, such as portable weight scales and keyboards. Advances in materials and manufacturing technology have improved springs since they were introduced more than a century ago, but the basic principle is the same. In a coiled spring such as spring 110, the entire length of its wire contributes to elasticity because the forces and moments are distributed end-to-end.
Being able to be made from generally all low cost materials, force transducer 100 is generally manufactured at a low cost. Force transducer 100 is durable having a durability suitable for challenging industrial applications (e.g., having large temperature variations e.g., −40° C.˜+85° C.) due to its simplicity and durable components and sturdy interconnection between respective components. When the variable inductor (115/120 or 115′/120) of the force transducer 100 is electrically coupled to nodes within an oscillator circuit or an oscillator circuit portion as described below in
Elasticity is a property of a material which allows it to return to its original shape or length after being distorted (stretched or compressed). One example of a suitable material for the spring 110 having high elasticity and tensile strength is music spring wire ASTM A228 (ASTM is an international standards organization). According to Hooke's Law, there is a linear relationship between the force (F) needed to extend a spring and the resultant spring displacement (x), expressed as:
F
w
=k·x (1)
Where k is the so-called spring constant.
The inductance of an electronic inductor such as the induction coil 120 comprising a wire coil is determined by the following Equation 2:
L=N
2
μA/Z (2)
where the magnetic permeability μ=μrμ0, L is the inductance of the coil in Henrys, N is the number of turns in the wire coil, μ is the permeability of the inner core material, μr is the relative magnetic permeability, μ0 is the permeability of free space equal to 4π×10−7 henry/m, A is the area of the coil in square meters, for a circular cross section A=πr2, and z is the average length of the induction coil 120 in meters.
The disclosed movable inner core-based inductor is now described having an inner core comprising a magnetic material that is dragged by an externally applied pulling force applied by a cord 121 coupled to a coupling feature 130 coupled to a cord 119 that is coupled to the inner core 115. The inductance of the variable inductor (115/120) of the force transducer 100 is changed by pulling (or dragging) the inner core 115 out of induction coil 120, so that when the inner core 115 comprises a magnetic material the total inductance (LT) of the variable inductor (115/120) includes an air core inductance portion (Lair) and a magnetic core inductance portion (Lferrite) when a ferrite core.
L
T
=L
air
+L
ferrite (3)
From Equations 2 and 3, LT of the variable inductor (115/120) with its movable ferrite inner core can be expressed as:
L
T=μ0πr2n2((μferrite−1)d+z) (4)
f
T=1/{2π√{square root over ((LTC1))}} (5)
Where n is the number of turns of the induction coil 120 per unit length; Nair=n(z−d) is number of turns of the induction coil that is an air core inductor, while NFerrite=nd is number of turns of the induction coil that is a ferrite core inductor, LT is total inductance of the variable inductor (115/120) in henry; μFerrite is permeability of ferrite core; z is total length of the induction coil 120 and d is length of the coil occupied by ferrite core, fT is the resonance frequency, and C1 denotes a fixed capacitance in the oscillator circuit or oscillator circuit portion.
Equation 4 expresses a linear relationship between movable positions (d) of the inner core 115 versus LT. In this way, a variable inductance is created by dragging the inner core 115 out of induction coil 120, while the spring 110 connected to the other end of the inner core 115 which keeps the equilibrium of the magnetic core's position with the dragging force. If the external pulling force becomes zero, the spring 110 is able to precisely restore the inner core 115 to its original position where a zero external force (no pulling force) generally occurs during calibration. The same is true for maximum force that is dragging the inner core 115 out of the induction coil 120, where the maximum position of the inner core 115 is again determined by the spring constant of the spring 110 and the maximum pulling force to be experienced. The pulling force to be experienced is generally configured to not exceed about 95% of the total length of the induction coil 120.
A high-precision frequency oscillator circuit is thus formed by using the variable inductor 115/120 designed and described in
Regarding the sensitivity of the force transducer 100, sensing resolution is recognized to be important to enable distinguishing a small change of a physical parameter of an object under investigation. The higher the resolution, the better the sensing accuracy and sensitivity. As shown in
Where C1 denotes the fixed capacitance in the oscillator circuit or oscillator circuit portion. Substituting d=z−x into Equation 1, applying differential to Equation 6 with respect to Fw, generates Equation 7:
Where x denotes displacement of inner core with respect to the original position. Hence Equation 7 shows the sensitivity of force detection can be expressed by a relationship between the change in the resonant frequency ΔfT and the change in the applied force ΔFw in Equation 7. ΔFw results in change in the resonant frequency of the sinusoidal signal (ΔfT) output by the oscillator circuit which can be detected by a suitable frequency detection circuit.
The determining of the magnitude of the pulling force or force change can thus comprise using a force-frequency relation, such as shown above. Alternatively, a more practical method is generally to store the characterization data/table as a look-up table in a non-volatile memory, where a processor does the sensing calculation using a look-up table relating the oscillating frequency to a magnitude of the pulling force or a change in the oscillation frequency to a change in the pulling force.
In a typical application, the induction coil 120 having its inner core being magnetic is physically placed within an oscillator circuit or oscillator circuit portion and is electrically coupled by connecting its respective ends 120a, 120b to nodes in the oscillator circuit or oscillator circuit portion. An LC tank is one example oscillator circuit. More generally, the oscillator circuit or oscillator circuit portion for disclosed embodiments can be any circuit that can take an inductance L into account in its resonant frequency generation, such as typical timer module where oscillator circuit is built inside and connected to outside inputs from L, resistor(s) R, or a capacitor C.
To measure the applied pulling force, one can first measure the present resonant frequency from the signal at the output of an amplifier coupled to the oscillator circuit that has the variable inductor 115/120 or 115′/120 coupled thereto without a pulling force applied to the force transducer 100. Subsequently, any other force applied under measurement circumstance can be determined by sensing the force change or coupled thereto with the maximum force at maximum inner core displacement, subsequently any other force applied under measurement circumstance can be determined by sensing the force change. In another way, instead of sensing force changes, the absolute force can also be determined with reference to absolute resonant frequency, by directly measuring the absolute frequency output. The absolute force can be determined by Equation 6 and Equation 1 and its equivalent lookup table. In the above methods of force determination, the force detection sensitivity is expressed by Equation 7.
Being in the oscillator circuit, when a pulling force is applied, the force changes the inductance of the variable inductor 115/120 because as described above the pulling force drags the inner core 115 comprising out of a length of the induction coil 120. For a magnetic core material, the total inductance of the variable inductor 115/120 is thus based on a resulting first length portion with an air core (where the inner core 115 is not present) and a second length portion with the inner core 115 present, which changes total inductance, hence a resonant frequency of the oscillator circuit. An equation can then be used, such as Equation 7 shown above, that relates the change in the frequency of the oscillator circuit and a magnitude of the change in the applied pulling force that enables determining a present magnitude of the pulling force from the resonant frequency that can be measured.
One particular example of applications for disclosed force transducers is for servo gauges (see the servo gauge 310 in
A small error in density measurement can result in large error in mass, given the huge volume of a bulky tank. For example, bulk storage tanks in tank farms can have diameters up to 80 meters and height of 40 meters, which can store crude oil of 1.2 million barrels=50 million gallons=190,000 cubic meters (m3). For Weights and Measures (W&M) applications using level gauges, even if volume of contents is provided accurately by high precision servo level gauge, mass has to generally be determined by density which can vary from 790 kg/m3 to 1000 kg/m3, an error of 0.001% (e.g., 1 kg/m3) can cause large error in mass transactions in about 2 tons of oil, corresponding to revenue loss of US $72,000 (at an oil price of US$ 60/barrel).
Since the density measurement accuracy of available/state-of-the-art servo-gauge-based force transducers is about ±3 kg/m3 (±0.19 lb/ft3) and the measuring range is usually confined within apparent weight of 20 g to 265 g, servo-gauges are not often adopted for W&M density measurements in large volumetric tanks. One possible reason is the limits of current force transducers whose sensitivity and dynamic range directly determine the high accuracy of density measurements.
As noted above, the force measurement ranging from 0.01N to 0.5 N still remains quite challenging. A fewer high-sensitive force transducers can measure small force but very expensive and less durable for industrial applications, such as measurement of stratified density distribution of a fluid using a submerged displacer. For a level gauge application using force transducers based on so-called Archimedes principle, the upward force (buoyancy) of a displacer is determined by:
F
b
=ρ·g·V (8)
Where V is full volume of displacer submerged in liquid of density ρ, g is the gravitational acceleration constant on the geological spot, its nominal value is 9.78033 m/s2. According to Equation 8, to measure the density (ρ) of a fluid accurately, the buoyancy needs to be determined more accurately by measuring the overall force Fw exerted on the wire that is suspending the displacer with a weight W (Equation 9).
F
b
=W−F
w (9)
Given the displacer is being used at a fixed geological location, the only variable that changes with density of fluid is the force, Fw, i.e.,
Let Wm and Fwm denote W/g and Fw/g mass term in kilograms, then Equation 10 becomes:
In order to understand the sensitivity requirements of a force transducer, applying derivative to Equation 11 with respect to full immersion depth,
Thus, ΔFwm=−VΔρ[kg] (13)
For a crude oil tank with 1.2 million barrels, the measuring density accuracy should be at least 100 times better than provided by current state of the art force sensors, so that the revenue loss can be reduced by 100 times, e.g., to about US $1,200 per full tank. Therefore, measured density accuracy to meet this requirement should be:
Δρ≤0.01 kg/m3=0.00001 [g/cm3] (14)
According to Equation 13, then sensitivity of the force transducer should be:
ΔFwm≤0.00001V [g] (15)
Where V denotes immersed volume of density displacer usually not larger than 300 cm3, typical about 200 cm3. To meet the requirement of density accuracy of 0.01 g/m3, the sensitivity of force measurements should be:
ΔFwm≤0.002 [g] (16)
This is a challenge for all known industrial force transducers to accurately measure small change of force that is less than 2 mg that as described below based on simulation data disclosed force transducers can provide.
The implementation of a disclosed force transducer can be a low cost yet robust implementation to meet high sensitivity force measurement needs of a variety of applications.
For a discrete oscillator circuit, the oscillator circuit has at least one fixed capacitor. The oscillator circuit can also comprise an oscillator integrated circuit (IC), where the variable inductor 120 can be coupled to an input of an oscillator IC (an input pin) that can detect the resulting resonant frequency change when the variable inductance is changed by a pulling force. The feedback loop includes an amplifier 210 and a feedback network 215 that provides the needed feedback to sustain the oscillations at the induced resonant frequency that is based on the inductance of the variable inductor.
The dark dot shown in
Regarding where along the length of the induction coil 120 to connect the system ground to, a typical value is 25% to 30% of the total induction coil length, but it can also be another ratio number depending on the design requirements and the transistor performance. In this configuration as noted above, the phase of the feedback signal received by the induction coil 120 is reversed by 180 degrees so that this output signal is positively maintained.
The arrangement in
Sensing resolution is an important capability of a sensor that can distinguish a small change of physical parameter of an object under investigation. The higher the resolution, the better the sensing accuracy and sensitivity. As shown in
The methods and devices disclosed herein can be implemented by using ordinary commercially available components. The coiled wire for spring 110 has a high durability and robustness and is generally able to cope with millions of times of force changes and movements. Although the force transducer for servo gauge application is just an example application, it can improve the state-of-the-art servo gauge measuring accuracy density which has been around ±3 kg/m3 (±0.19 lb/ft3) that is challenge for fairly accurate fiscal mass-based transactions in W&M. Furthermore, disclosed force transducers can measure a small force which makes it possible to use larger volume of displacer to provide more reliable volumetric data under fluctuation of fluid, even though larger volume creates greater buoyancy that will reduce the overall force exerted on the cord 119. The restriction on the selection of a displacer 312 may also be alleviated to large degree, since they can cope with small and large variation of densities throughout entire contents of bulky storage tanks.
Disclosed methods and devices can also be used for other industrial and/or commercial applications where high force sensitivity and accuracy and large dynamic range are all needed. Disclosed force transducers are expected to fill in the gap where high sensitivity is required to distinguish a subtle change of the force under harsh industrial conditions.
Disclosed force transducers are flexible to implement for various sensitivity requirements at low cost, because the inner core displacement is determined by the spring constant of the spring 110 enabling the external pulling force to be measured. The wire springs for spring 110 can be chosen so that the displacement at corresponding frequency change can provide easy detection of small force changes.
Also the range of oscillating frequencies can be selected by selection of a variety of frequency generation system components which can be used for tuning the resonant frequency, such as having different capacitance component value(s) in the oscillator circuit to avoid possible electromagnetic interferences (EMI) to occur in the same frequency range. Other resonant frequency tuning parameters include the spring constant of the spring 110, the pulling force (e.g., by selecting the weight of the displacer 312), the total length of induction coil 120, the diameter of induction coil 120, the number of turns of the induction coil 120, and the relative permeability of inner core 115. Any of these parameters can provide flexibility and design freedom to set a resonant frequency to meet the frequency needs for a variety of applications.
Disclosed aspects have several significant advantages. In combination of linear or nonlinear movement of the inner core 115 or non-magnetic inner core 115′, essentially the exact detected frequency (from the overall inductance) should be accurately repeated under the same overall force, meaning from increase to decrease, and vice versa. The requirements of high sensitivity of the force transducer demands solution to mechanical hysteresis in the cord 121, which is described above may comprise stainless steel 316 for robust chemical resistance. In some particular applications as noted above higher strength wire may be required to minimize the elongation of long thin wire such as by using Molybdenum and/or Rhenium alloys, and the spring 110 can be made of a high elastic material such as ASTM A228.
Hysteresis of the inner core 115 material can impact the inductance when a magnetic material affects permeability of the combined induction coils, consequently the accuracy of measurements. To address this problem, the total flux density will generally not reach the saturation levels of core magnetic material, or as an alternative a non-magnetic material for the inner core can be used (see
To increase detection sensitivity, disclosed force transducers benefit from an extra spring coil that can confine the magnetic field distribution from disturbances so that the stability of the magnetic induction is retained, hence the sensitivity. Since the two ends of the spring 110 are not electrically connected to induction coil 120 or terminal 115a, the influence of its self-inductance can be negligible, so that mutual inductance in the overall inductance is minimally contributed by the spring 110, which is primarily used to provide coupling to wiring and the load with a high repeatability. The shape of the induction coil 120 also strengthens the magnetic field that is created. To figure out the exact effective mutual inductance values would be different in theory, but can be easily measured in practice, the length of spring 110 change provides an additional attribute to change of inductance due to displacement of the inner core, since the spring 110 is aligned with induction coil 120 on the axial direction, although it is not significant. More importantly the change of the inductance will be mainly determined by displacement of the core material.
Known variable inductors use strong magnetic core materials and large coil cross sections, and more turns to increase the sensitivity. In disclosed force transducers, in contrast, the induction coil 120 generally has a relatively small cross section dimension, such as a radius of 2 or 4 mms.
Disclosed force transducers and related sensing methods thus address the challenges of requirements of high dynamic range force transducers, by accurately detecting small and large forces with subtle force changes. Applications that require high sensitivity on the order of parts per million (ppm), while the absolute force can range from a few grams to hundreds or thousands of grams depending on the strength and elasticity of the spring 110, with servo gauging with a displacer 312 being is just one of the possible applications for disclosed force transducers.
Disclosed embodiments are further illustrated by the following specific Examples, which should not be construed as limiting the scope or content of this Disclosure in any way.
As described above, given the pulling force, displacement x of the inner core will depend on elastic spring constant of the spring 110, while output frequency range can be predetermined by the capacitance in the oscillator circuit and the range of dynamic inductance of the variable inductor. A design simulation was performed on a force sensing system using an induction coil 120 comprising wire-wound copper coils 4 mm in diameter with 25 turns for the data in both
While various disclosed embodiments have been described above, it should be understood that they have been presented by way of example only, and not limitation. Numerous changes to the subject matter disclosed herein can be made in accordance with this Disclosure without departing from the spirit or scope of this Disclosure. In addition, while a particular feature may have been disclosed with respect to only one of several implementations, such feature may be combined with one or more other features of the other implementations as may be desired and advantageous for any given or particular application.
