GB2076159A - Displacement Transducer - Google Patents
Displacement Transducer Download PDFInfo
- Publication number
- GB2076159A GB2076159A GB8016064A GB8016064A GB2076159A GB 2076159 A GB2076159 A GB 2076159A GB 8016064 A GB8016064 A GB 8016064A GB 8016064 A GB8016064 A GB 8016064A GB 2076159 A GB2076159 A GB 2076159A
- Authority
- GB
- United Kingdom
- Prior art keywords
- magnetic fluid
- cavity
- transducer
- force
- transducer according
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Withdrawn
Links
Classifications
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01L—MEASURING FORCE, STRESS, TORQUE, WORK, MECHANICAL POWER, MECHANICAL EFFICIENCY, OR FLUID PRESSURE
- G01L9/00—Measuring steady of quasi-steady pressure of fluid or fluent solid material by electric or magnetic pressure-sensitive elements; Transmitting or indicating the displacement of mechanical pressure-sensitive elements, used to measure the steady or quasi-steady pressure of a fluid or fluent solid material, by electric or magnetic means
- G01L9/0041—Transmitting or indicating the displacement of flexible diaphragms
- G01L9/007—Transmitting or indicating the displacement of flexible diaphragms using variations in inductance
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
- G01B7/00—Measuring arrangements characterised by the use of electric or magnetic techniques
- G01B7/16—Measuring arrangements characterised by the use of electric or magnetic techniques for measuring the deformation in a solid, e.g. by resistance strain gauge
- G01B7/24—Measuring arrangements characterised by the use of electric or magnetic techniques for measuring the deformation in a solid, e.g. by resistance strain gauge using change in magnetic properties
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01L—MEASURING FORCE, STRESS, TORQUE, WORK, MECHANICAL POWER, MECHANICAL EFFICIENCY, OR FLUID PRESSURE
- G01L13/00—Devices or apparatus for measuring differences of two or more fluid pressure values
- G01L13/02—Devices or apparatus for measuring differences of two or more fluid pressure values using elastically-deformable members or pistons as sensing elements
- G01L13/025—Devices or apparatus for measuring differences of two or more fluid pressure values using elastically-deformable members or pistons as sensing elements using diaphragms
Landscapes
- Physics & Mathematics (AREA)
- General Physics & Mathematics (AREA)
- Measuring Fluid Pressure (AREA)
Abstract
In a displacement transducer, displacement of a magnetic fluid 107 moveable in a cavity 108 is used to produce an output signal. The displacement can be the result of movement of diaphragms 103, 104, in a differential pressure cell. In a force-balance transducer, the magnetic fluid has a balancing force applied to it magnetically so as to keep it substantially stationary within the cavity. Means are provided for producing an output signal dependent on either the magnetically detected position of the magnetic fluid or on the magnitude of the balancing force, as appropriate. <IMAGE>
Description
SPECIFICATION
Transducer
This invention relates to transducers, and
particularly to transducers for measuring a displacement or a force.
The transducer of the invention comprises a
magnetic fluid contained within a cavity and
moveable within the cavity in response to either a
measureand displacement or a measurand force,
magnetic means either for detecting the position of the magnetic fluid within the cavity or for applying a balancing force to the magnetic fluid so as to keep the magnetic fluid substantially stationary within the cavity, and means for producing an output signal dependent either on the magnetically detected position of the
magnetic fluid or on the magnitude of the balancing force, as appropriate.
It will be apparent that the transducer can be a displacement transducer or a force-balance transducer. Examples of both kinds are described below, but use of the invention in displacement transducers, preferably linear displacement transducers, provides a wide variety of applications since so many physical properties are measured by means of converting them into a displacement.
Examples of the latter are pressure, weight, force, torque or temperature (the last by expansion or differential expansion of a metal or fluid element). Instances where displacement itself is desired to be measured are strain or shear strain in a constructional member, and positional control in machine tool operations.
Use of the invention in a force-balance transducer can for example be in connection with the measurement of a force or of a pressure, weight or torque.
The magnetic fluid is a fluid (normally a liquid) having ferromagnetic properties. Such magnetic fluids are obtainable commercially, for example from Ferrox Limited, United Kingdom, or Ferrofl uidics Corporation, U.S.A. These commercial magnetic fluids are permanent colloidal suspensions of ferromagnetic particles such as for example magnetite (Fe304) in a liquid, such as water or an organic iquid, and containing stabilisers to prevent agglomeration. Suitable organic liquids include organic solvents, hydrocarbons, halohydrocarbons or esters such as bis(2-ethylhexyl)azelate.
The magnetic fluid is contained within a cavity.
Most conveniently the cavity is a bore or tube, preferable of circular cross-section, although it can be of annular cross-section or of square, rectangular, elliptical or other regular or irregular cross-section. Nevertheless, circular or annular cross-sections are preferred. The cross-section is preferably constant along the length of the cavity.
The diameter of the cross-section, or its smallest dimension where it is not circular, is preferably from 10-6 metre to 5 x 10-3 metre (1 micron to 5 millimetres) for example from 10-5 metre to 10-3 metre. Thus the cavity is often conveniently a capillary tube, for example of glass, a synthetic resin or a non-ferromagnetic metal.
The magnetic fluid is moveable within the cavity in response to the measurand displacement or force. It is particularly convenient to achieve this by means of hydraulic pressure acting through an appropriate liquid that is not miscible with the magnetic fluid, for example, an oil. In the examples that follow, such hydraulic transmission is employed in pressure transducers with appropriate diaphragms. Alternatively, a bellows or piston can be used. By using a diaphragm bellows or piston that is large in relation to the cross-sectional area of the cavity, a considerable amplification of linear movement can be achieved. Not only does this improve sensitivity and linearity of output, it enables the use of a relatively small balancing force when the transducer is a force-balance transducer.Of course, the reverse arrangement can be used where comparatively large displacements are to be measured and it is appropriate to displace the magnetic fluid by a smaller distance. Suitable hydraulic liquids include water (where the magnetic fluid is non-aqueous or immiscible with water), or a hydrocarbon oil or synthetic oil.
The use of a small cross-section cavity also assists, through surface tension, in discouraging mixing of the magnetic fluid with any hydraulic liquid with which it is in contact. Afurther method is to apply a strong magnetic fiel across the cavity cross section. This can provide a perfect seal between the magnetic fluid and the inner walls of the cavity.
Detection of the position of the magnetic fluid is by magnetic means. Preferably, an electrical coil surrounds the cavity so that movement of the magnetic fluid within the cavity alters the inductance of the coil. The coil can form part of, for example, a differential transformer or a differential oscillator and appropriate electronic circuitry is used to produce the output signal.
The application of a balancing force in the case of a force-balance transducer is also by magnetic means. A convenient way of bringing this about is to employ a pair of electromagnets positioned so that lines of force cross the cavity; in this case the electromagnet coils each carry a direct current and these currents are controlled so as to produce the required balancing force to keep the magnetic fluid substantially stationary. If an alternating current ripple is superimposed on each direct current and small "search coils" are positioned at either end of the coil, emergence of the magnetic fluid from either end of the solenoid coil alters the degree to which a ripple voltage is induced in the corresponding search coil, and the resulting signal can be used to control the two direct currents so as to counteract the movement of the magnetic fluid that caused that signal to be produced.Force balance is thereby achieved and the relative magnitudes of the electromagnet currents can then be used as a signal indicative of the measurand force.
It can happen, particularly of course in a force balance device according to the invention, that the pressure on one side of the magnetic fluid may be higher than that on the other side. This pressure difference should not be so great as to cause a breakdown in the seal between the magnetic fluid and the walls of the cavity. Such breakdown can occur if the pressure difference is greaterthan about 4x104 Pascals (6 pounds per square inch). However, if there are several portions of magnetic fluid spaced along the length of cavity, each portion is subjected to only part of the total pressure difference, which total can thus be higher than that withstandable by a single portion of magnetic fluid.
Examples of transducers according to the invention are shown in the accompanying drawings, in which:
Figure 1 is a cross-section of a differential pressure transducer comprising a displacement transducer according to the invention interposed between two diaphragms and acting as secondary transducer;
Figure 2 shows a force-balance transducer according to the invention;
Figure 3 is a cross-section of a differential pressure transducer similar to that of Figure 1 but including an overload protection device;
Figure 4 shows another example of a differential pressure transducer, comprising a stiffened diaphragm interposed between two displacement transducers according to the invention; and Figure 5 shows another Example of a forcebalance transducer according to the invention, employing a permanent magnet and an external balancing force.
Referring to Figure 1, a differential pressure transducer (D.P. Cell) is shown suitable for use in measuring the difference in pressure betwen two process fluids (gas or liquid) 101 and 102.
Process fluid 101 is in contact with a diaphragm 103 and process fluid 102 is in contact with a diaphragm 104. At least one of these diaphragms is a stiff, rated diaphragm that deforms by definite amounts when subjected to pressure differences within a certain range, the spring action of this diaphragm exerting a restoring force when it is deformed by pressure. The other diaphragm can similarly be a stiff rated diaphragm so that the two diaphragms act together in providing a restoring force. Alternatively, one of the diaphragms can be a floppy diaphragm exerting little if any restoring force and employed to isolate the process fluid from the interior of the pressure transducer.
The side of each diaphragm remote from the relevant process fluid contacts a portion of hydraulic fluid 105 or 106 for example an oil, and the two portions 105 and 106 are separated by a magnetic fluid 107 within a straight capillary tube
108. It is normally convenient to arrange that when the differential pressure between the two process fluids 101 and 102 has the required value for the particular application in view, the
magnetic fluid 107 is positioned about midway
between the two ends of the capillary tube 108. It
will be seen that movement of the diaphragms
103 and 104 results in an amplified displacement
of the magnetic fluid along the tube 108. An
electrical coil 109 surrounds the capillary tube
108 at its mid-point, and the change in
inductance of this coil as the magnetic fluid
moves within it can be converted to an electrical
signal.The coil 109 preferably forms the central
winding of a linear variable differential
transformer whereby the direction as well as
magnitude of a change can be detected by known
methods.
In order to compensate for liquid volume changes as a result of changes in temperature, the hydraulic fluids 105 and 106 can be in communication with chambers (not shown) having walls of a material of relatively high coefficient of thermal expansion, for example a nickei alloy, according to known techniques.
Referring to Figure 2, the two diaphragms 203 and 204 are both floppy. A horse-shoe electromagnet 209 has a coil 210 and pole pieces 211 and 212 positioned either side of the capillary tube 208 so that when the electromagnet is energised the resulting magnetic lines of force cross the capillary tube. A similar electromagnet 213 has a coil 214 and pole pieces 21 5 and 21 6 and is placed adjacent to the electromagnet 209. The coils 210 and 214 each carry a direct current with a superimposed a.c.
ripple. There are small search coils 21 7 and 218 surrounding the capillary tube on either side of the pair of electromagnets and movement of the magnetic fluid 217 into either search coil increases an induced ripple voltage in that coil: the induced ripple signal can then be used to vary the relative magnitudes of the respective direct components of the currents in the coils 210 and 214, thereby producing a restoring force on the magnetic fluid that substantially balances the externally applied force due to the pressure difference between process fluids 201 and 202.
The relative magnitudes of these direct current components then provide a measure of this pressure difference.
In Figure 3, the diameter of the capillary tube 308 is exaggerated so as to permit the illustration of two steel balls 310 and 311 within the magnetic fluid. There are two resilient end stops 312 and 313, each shaped so that it will form a seal when one of the balls 310 and 311 presses against it. These act as an overload protection device; when an overload in either direction occurs, one of the steel balls engages one of the end stops and prevents further movement of the diaphragms 303 and 304 and escape of the magnetic fluid from the capillary tube 308. There can if desired be only one ball, or more than two can be employed. The balls can be magnetised.
This assists in ensuring that the balls stay within the magnetic fluid. One can use a cylindrical body instead of such balls.
An alternative or additional form of overload protection could be provided by making the capillary tube very long so that the magnetic fluid stays within the capillary tube even during overload conditions. The extra length can be accommodated by one or more loops of capillary potted in a suitable resin, for example an epoxy or polyester resin. Where such a loop of capillary tube is employed in this way, an electrical sensing coil for detecting the position of the magnetic fluid can be wound around the loop in toroidal form.
Referring now to Figure 4, a stiff rated diaphragm 410 is situated between two floppy isolating diaphragms 403 and 404, and there are two capillary tubes 411 and 412 each containing two portions of hydraulic fluid and a magnetic fluid 413,414.
Capillary tube 411 is shorter than tube 412 and thus of smaller volume; it has resilient sealing end stops 415,416 as shown in Figure 3 and there is a steel ball (not shown) within magnetic fluid 413. Magnetic fluid 414 is surrounded by electrical coil 409 employed for detection or force balance purposes as described above. It will be seen that the use of a capillary tube 411 of smaller volume than capillary tube 412 means that the overload protection device operates before magnetic fluid 414 can escape from capillary tube 412.
Another example of a force-balancing transducer is shown in Figure 5. There are floppy diaphragms 503 and 504, hydraulic fluids 505 and 506 and magnetic fluid 507 within a capillary 508, all as described in connection with Figure 2.
A permanent horse-shoe magnet 509 has shaped pole pieces 510 and 511 on either side of capillary 508 at the normal position of the magnetic fluid. Interaction between the magnet and the magnetic fluid keeps the latter between the pole pieces 510 and 511, so that a force on the magnetic fluid parallel to the capillary 508 is transmitted to the magnet. An external opposing measurable force can be applied to the magnet as indicated by arrows (and thence magnetically to the magnetic fluid), and utilised for force-balance purposes.
As a modification to the force-balance transducer of Figure 5, a cylindrical slug of ferromagnetic material can be positioned inside the tube 412 which can then be of larger than capillary cross-seCtion, the magnetic fluid then being contained in two portions, one near either end of the cavity of annular cross-section between the slug and the interior surface of the tube. An annular magnet surrounds the tube and the slug, and the portions of magnetic fluid thus complete a magnetic circuit where the lines of force lie transversely across the annular cavity, and also provide a seal between the two hydraulic fluids. An external balancing force can then be applied to the magnet as before.
While the invention has been particularly described with reference to the employment of a magnetic fluid, it is nevertheless possible to use other fluids having an electrical effect. The electrical effect can for example be conductive, resistive, capacitive, inductive or magnetic.
Molten metals, for example mercury or a molten tin/lead or other alloy where the temperature is above its melting point, and aqueous solutions of inorganic salts are examples of conductive fluids.
Mercury is often preferred. Aqueous salt solutions can also be employed for their resistive effect or for their relatively high permittivity where the electrical effect utilised is capacitive. Preferably, however, the fluid is one having a magnetic effect (which term includes an inductive effect) and the invention is particularly described and exemplified above with reference to transducers comprising a magnetic fluid. However, it is to be understood that other fluids having an electrical effect can be employed. For example one can introduce an electrically conductive fluid (such as mercury) either as a separate "slug" in a capillary cavity or as one half of a hydraulic liquid filling the transducer, or merely sense the interface between two hydraulic liquids that differ in electrical effect, one liquid being in each half of the transducer.
Where an electrically conductive fluid such as mercury is employed, sensing can be by causing fluid movement to change the capacity of a capacitor. Alternatively, movement of a conductive liquid "slug" along an electrical resistance track can provide a signal. A possible construction comprises one or more electrically resistive tracks, for example of carbon or a metal film, formed on a mandrell by known resistance manufacturing techniques so that when the mandrell is inserted in a hole there is a clearance between the mandrell and the hole wall. The clearance then constitutes the cavity, within which movement of a conductive fluid alters the electrical resistance. Appropriate shaping of either the resistive track or the mandrell would allow the transducer to have a predetermined response characteristic.
An interface between two fluids of different permittivities can be detected by positioning it between the plates of a capacity so that the capacity of the latter varies with the position of the interface. Optical detection of a fluid interface is another possibility.
Claims (9)
1. A transducer comprising a magnetic fluid contained within a cavity and moveable within the cavity in response to either a measurand displacement or a measurand force, magnetic means either for detecting the position of the magnetic fluid within the cavity or for applying a balancing force to the magnetic fluid so as to keep the magnetic fluid substantially stationary within the cavity, and means for producing an output signal dependent either on the magnetically detected position of the magnetic fluid or on the magnitude of the balancing force, as appropriate.
2. A transducer according to Claim 1, that is a differential pressure transducer wherein movement of the magnetic fluid within the cavity is by means of hydraulic pressure acting through a liquid that is not miscible with the magnetic fluid.
3. A transducer according to Claim 2, that is a differential pressure transducer.
4. A differential pressure transducer according to Claim 3, in which pressure is transmitted to the hydraulic fluid by means of a diaphragm, piston or bellows.
5. A differential pressure transducer according to Claim 4, in which the cross-sectional area of the diaphragm, piston or bellows is large in relation to the cross-sectional area of the cavity so as to achieve amplification of the movement of the diaphragm, piston or bellows.
6. A transducer according to any of the preceding claims, in which production of a signal output is achieved by means of an electrical coil surrounding the cavity so that movement of the magnetic fluid within the cavity alters the inductance of the coil and this inductance alternation is employed in producing the signal output.
7. A transducer according to any of Claims 2 to 5, having an overload protection device comprising at least one ferromagnetic ball within the magnetic fluid, and at least one end stop situated at or adjacent to an end of the cavity and shaped to form a seal when the ball presses against it.
8. A transducer according to Claim 7, in which the ball is magnetised.
9. A transducer substantially as hereinbefore described with reference to, and as illustrated in, any of Figures 1 to 5 of the accompanying drawings.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB8016064A GB2076159A (en) | 1980-05-15 | 1980-05-15 | Displacement Transducer |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB8016064A GB2076159A (en) | 1980-05-15 | 1980-05-15 | Displacement Transducer |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| GB2076159A true GB2076159A (en) | 1981-11-25 |
Family
ID=10513444
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| GB8016064A Withdrawn GB2076159A (en) | 1980-05-15 | 1980-05-15 | Displacement Transducer |
Country Status (1)
| Country | Link |
|---|---|
| GB (1) | GB2076159A (en) |
Cited By (5)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN104019932A (en) * | 2014-05-29 | 2014-09-03 | 北京交通大学 | Section-type self-inductance magnetic liquid micro differential pressure sensor |
| CN112577661A (en) * | 2020-11-05 | 2021-03-30 | 清华大学 | Magnetic liquid micro-pressure difference sensor with elastic membrane |
| CN112577660A (en) * | 2020-11-05 | 2021-03-30 | 清华大学 | Horizontal thin tube type magnetic liquid micro differential pressure sensor |
| CN112577659A (en) * | 2020-11-05 | 2021-03-30 | 清华大学 | High-sensitivity capillary type magnetic liquid micro-differential pressure sensor |
| EP4137782A4 (en) * | 2020-04-17 | 2024-03-27 | MultiDimension Technology Co., Ltd. | Capillary channel environmental sensor and preparation method therefor |
-
1980
- 1980-05-15 GB GB8016064A patent/GB2076159A/en not_active Withdrawn
Cited By (6)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN104019932A (en) * | 2014-05-29 | 2014-09-03 | 北京交通大学 | Section-type self-inductance magnetic liquid micro differential pressure sensor |
| CN104019932B (en) * | 2014-05-29 | 2017-10-31 | 北京交通大学 | A kind of section type self-induction type magnetic-liquid micro differential pressure sensor |
| EP4137782A4 (en) * | 2020-04-17 | 2024-03-27 | MultiDimension Technology Co., Ltd. | Capillary channel environmental sensor and preparation method therefor |
| CN112577661A (en) * | 2020-11-05 | 2021-03-30 | 清华大学 | Magnetic liquid micro-pressure difference sensor with elastic membrane |
| CN112577660A (en) * | 2020-11-05 | 2021-03-30 | 清华大学 | Horizontal thin tube type magnetic liquid micro differential pressure sensor |
| CN112577659A (en) * | 2020-11-05 | 2021-03-30 | 清华大学 | High-sensitivity capillary type magnetic liquid micro-differential pressure sensor |
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| WAP | Application withdrawn, taken to be withdrawn or refused ** after publication under section 16(1) |