GB2054867A - Eddy-current distance measuring apparatus - Google Patents
Eddy-current distance measuring apparatus Download PDFInfo
- Publication number
- GB2054867A GB2054867A GB8022329A GB8022329A GB2054867A GB 2054867 A GB2054867 A GB 2054867A GB 8022329 A GB8022329 A GB 8022329A GB 8022329 A GB8022329 A GB 8022329A GB 2054867 A GB2054867 A GB 2054867A
- Authority
- GB
- United Kingdom
- Prior art keywords
- amplifier
- detection coil
- oscillator
- output
- resonant circuit
- 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.)
- Granted
Links
Classifications
-
- 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
- G01B7/02—Measuring arrangements characterised by the use of electric or magnetic techniques for measuring length, width or thickness
- G01B7/023—Measuring arrangements characterised by the use of electric or magnetic techniques for measuring length, width or thickness for measuring distance between sensor and object
Landscapes
- Physics & Mathematics (AREA)
- General Physics & Mathematics (AREA)
- Measurement Of Length, Angles, Or The Like Using Electric Or Magnetic Means (AREA)
- Measurement Of Levels Of Liquids Or Fluent Solid Materials (AREA)
- Inductance-Capacitance Distribution Constants And Capacitance-Resistance Oscillators (AREA)
Abstract
A resonant circuit comprising a detection coil (3) in parallel with a capacitor (C0) is connected to the feed-back circuit of a positive feedback amplifier (4). A voltage controlled oscillator (5) applies an AC signal to the amplifier, and a phase comparator (6) controls the oscillation frequency in accordance with any phase difference between the outputs of the amplifier and the oscillator so that the oscillator oscillates at the resonant frequency of the resonant circuit. The positive feedback impedance of the amplifier thus comprises only a real component and the feedback factor is maintained constant irrespective of any variation in the detection coil inherent impedance due to temperature change, etc., thereby automatically compensating for the effect of temperature changes on the distance measuring characteristic. <IMAGE>
Description
SPECIFICATION
Eddy-current distance measuring apparatus
The present invention relates to apparatus for measuring the distance from a metallic body to a detection coil by utilizing the effect of eddy currents induced in the metallic body.
An apparatus has been disclosed in British
Patent No. 1,512,799 in which a reference signal having a fixed frequency and fixed amplitude is applied to the inverting input terminal of a differential amplifier whose output is applied to its non-inverting input terminal through a positive feedback circuit and a detection coil is excited by the feedback signal, whereby the distance from a metallic body disposed opposite to the coil is measured in accordance with a change in the coil impedance.This type of distance measuring apparatus is designed so that the post-feedback amplification factor of a positive feedback amplifier is controlled in accordance with the impedance value of a detection coil and the output voltage of the amplifier is measured, thereby detecting as a distance measurement value a detection coil impedance change corresponding to the distance between the metallic body and the detection coil.
With the apparatus disclosed in the abovementioned prior patent specification, no means is included to correct for the effect of changes in the impedance of the detection coil'due to temperature changes and there has existed a need for an improved apparatus designed so that the effect of such temperature changes on the detection coil to metallic object distance output characteristic is compensated for with a high degree of accuracy.
The rate of change of the detection coil impedance with temperature changes differs in dependence on the material for the detection coil bobbin, coil shape, coil winding method and so on.
Moreover, in the case of a detection coil whose bobbin core is made of a ferromagnetic material such as ferrite, variation in the permeability of the ferrite due to temperature changes results in a hysteresis characteristic and this makes it more difficult to provide the previously-mentioned temperature compensation for variations in the output characteristic of a detection coil including such a ferrite bobbin core.
According to the present invention there is provided a distance measuring apparatus comprising a positive feedback amplifier, a resonant circuit connected to a feedback circuit of said amplifier so as to control the post-feedback amplification factor of said amplifier, said resonant circuit comprising a capacitor connected in parallel with a detection coil, a variable frequency voltage controlled oscillator connected to apply an
AC signal to said amplifier to excite said detection coil, and a phase comparator connected to generate a voltage output representative of a phase difference between an output of said oscillator and an output of said amplifier and to apply said voltage output to control said oscillator
to oscillate at an oscillation frequency
substantially equal to a resonant frequency of said
resonant circuit, whereby the amplitude of the
output signal of said amplifier embodies a
measure of the distance between said coil and a
metallic object whose distance from the coil it is
desired to measure.
The oscillator and the phase comparator may
be formed by an integrated phase locked loop
(PLL) device.
It will be appreciated that even if the
inductance of the detection coil changes with
temperature, the resonant frequency of the
resonant circuit is varied in response to the
variation of the detection coil inductance and also
the excitation frequency of the coil always follows
or changes in response to the resonant frequency.
As a result, the feedback factor of the positive
feedback amplifier is maintained at a substantially
constant value determined by the composite
impedance of the resonant circuit at resonance
and any variation in the output voltage due to
temperature variation is substantially eliminated, thus making it possible to provide automatic temperature compensation. Further, by virtue of the fact that its resonant frequency is always
applied to the resonant circuit so that the complex
impedance of the resonant circuit is maintained at
a value representing only the real component without any imaginary component, substantially no phase variation takes place in the positive feedback circuit with the resulting improvement in operating stability and simpler and easier adjustment.
An embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings, in which:
Fig. 1 is a block diagram showing a prior art apparatus.
Fig. 2 is a block diagram showing an embodiment of the present invention.
Fig. 3 is a graph showing by way of example the measurements of the rate of change AL/L of the inductance (ordinate) of a detection coil with respect to its temperature T (abscissa).
Fig. 4 is a graph showing by way of example the measurements of the rate of change AVO/VO in the output voltage of amplifier (ordinate) with respect to the temperature T of the detection coil (abscissa).
Fig. 5 is a graph showing by way of example the measurements of the output voltage V0 (ordinate) of the amplifier in the distance measuring apparatus of the present invention with respect to the distance D (abscissa) to be measured.
Referring to Fig. 1, there is illustrated by way of example a known distance measuring apparatus of the type shown in the previously mentioned prior patent specification.
In the Figure, numeral 1 designates a metallic body, 2 a reference oscillator, 3 a detection coil, 4 an amplifier, and Zs a feedback impedance. The oscillator 2 supplies an AC signal of a fixed frequency and fixed amplitude to the amplifier 4.
The output of the amplifier 4 is applied to the detection coil 3 through the feedback impedance
Zs, so that when the AC magnetic flux generated from the detection coil 3 passes through the
metallic body 1, eddy currents are induced in the
metallic body 1 and the resulting reaction causes
a change in the impedance Z of the detection coil
3, thus changing the feedback factor /3p = ZZ(Z + Zs). As a result, the output voltage V0 of the amplifier 4 changes with the distance
between the detection coil 3 and the metallic body
1. Thus, by measuring the output voltage VO, it is possible to measure the distance between the
detection coil 3 and the metallic body 1 in a non
contact manner.
However, the feedback amplifier type eddy
current distance measuring apparatus shown in
Fig. 1 has the following disadvantages. More specifically, generally the impedance of a coil
changes more or less with a change in the temperature of the coil and it has been impossible to compensate with a high accuracy for the effect
of such temperature changes on the output
characteristic related to distance between the
detection coil 3 and the metallic body 1. The rate
of change with temperature of the impedance of the detection coil 3 differs in dependence on the
material and size of the coil bobbin, the coil
winding method, etc., of the detection coil 3.Thus,
in the case of a known coil bobbin using a ferrite
core which is a ferromagnetic material, the permeability of the ferrite changes with temperature and its rate of change shows a
hysteresis characteristic with respect to the temperature variation. The use of such ferrite core
makes the temperature compensation of the
output characteristic more difficult.
The foregoing deficiencies are overcome by the
apparatus of this invention in which the oscillation frequency of a reference oscillator is varied in
accordance with the inductance of a detection coil
and also a resistor is used in place of a feedback
impedance, that is, the imaginary component is
eliminated, thereby maintaining the feedback
factor (pp) at a constant value irrespective of
variations in the temperature of the detection coil.
A preferred embodiment of the present
invention will now be described in greater detail
with reference to Figs. 2 to 5.
Referring first to Fig. 2, numeral 1 designates a
metallic body, 3 a detection coil and 4 an
amplifier, and these elements are similar to their
counterparts which were described in connection
with Fig. 1. Symbol Rp designates a feedback
resistor, Co a parallel resonant capacitor, 5 a
voltage-controlled oscillator (VCO) whose
frequency of oscillation is varied in accordance
with a DC control voltage, and 6 a phase
comparator.
The detection coil 3 and the capacitor Co of Fig.
2 form a parallel resonant circuit and its
composite impedance Zo at a resonant frequency
fo is given by the following equation (1) (27lfo.L)2
zo = y +
=X+Q.27lfo.L where Q = wolly (1)
Where y is the resistance of the detection coil, L is the inductance of the detection coil, and a)o (2rrfo) is the angular frequency.
It will be seen that the composite impedance
Zo of the parallel resonant circuit at the resonant frequency fo comprises only the real component.
The parallel resonant circuit and the feedback resistor Rp form the positive feedback circuit of the amplifier 4, and applied to the amplifier 4 is the oscillation voltage of the oscillator 5 which oscillates at an oscillation frequency fm that is substantially equal to the resonant frequency fo of the parallel resonant circuit. This oscillation voltage is first amplified by the amplifier 4 and it is then applied to the phase comparator 6. The output voltage of the oscillator 5 is also applied to the phase comparator 6 so that the phase difference between the applied voltages is detected and the phase comparator 6 generates a
DC voltage corresponding to the phase difference.
The DC voltage is then applied to the oscillator 5 to control the same such that the oscillation frequency fm of the oscillator 5 becomes equal to the parallel resonant frequency fo and the phase of the output voltage of the oscillator 5 is also locked.
Assuming that the inductance of the detection coil 3 is varied with a change of its temperature, the resonant frequency fo also changes and its rate of change is given by the following equation (2) AL/L= (fo/fo')Z -- 1 (2) Where L = detection coil inductance
AL = detection coil inductance varied with
detection coil temperature
fo = resonant frequency of parallel resonant
circuit before temperature change
fo' = resonant frequency of parallel resonant
circuit after temperature change
Thus, with respect to the composite impedance
Zo of the parallel resonant circuit, it is automatically controlled so that even if the value of the inductance L of the detection coil 3 is increased, the resonant frequency fo is decreased and the composite impedance Zo is maintained constant. If the composite impedance Zo is maintained constant, the feedback factor pp = (Zo/(Rp + Zo)) is maintained constant and the output voltage V0 of the amplifier 4 is maintained constant.
Fig. 3 shows the measurements of variation in the inductance L of the detection coil 3 with temperature. Fig. 4 shows the measurements of variation in the output voltage of the amplifier 4 with variation in the temperature of the detection coil 3. The dotted line shows the output voltage variations according to the circuit construction of
Fig. 1, and the solid line shows the output voltage variations according to the circuit construction of
Fig. 2. Fig. 5 shows the measurements obtained with the distance measuring apparatus of Fig. 2.
With the present invention described hereinabove, there are the following advantages.
(a) Due to the fact that the capacitor Co is connected to the detection coil 3 to form a parallel resonant circuit whose resonant frequency is varied with variation of the coil inductance, even if the inductance of the detection coil 3 is varied with variation of its temperature, the feedback factor is maintained constant, thus making it possible to automatically compensate for the effects of temperature changes.
(b) Due to the fact that the oscillation frequency of the oscillator 5 is equal to the resonant frequency of the resonant circuit and the composite impedance of the resonant circuit comprises only the real part, the stability of the distance measuring apparatus is improved and its adjustment is also simplified.
(c) The oscillator used needs not be comprised of a highly stable oscillator employing a crystal element or the like.
(d) The circuits enclosed by a dotted line A in
Fig. 2 may be replaced with an integrated phase locked loop device with the resulting simplification of the circuit construction.
(e) By measuring the output voltage of the phase comparator 6, it is possible to continuously detect the amount of change in the inductance of the detection coil 3.
Claims (3)
1. Distance measuring apparatus comprising a positive feedback amplifier, a resonant circuit connected to a feedback circuit of said amplifier so as to control the post-feedback amplification factor of said amplifier, said resonant circuit comprising a capacitor connected in parallel with a detection coil, a variable frequency voltage controlled oscillator connected to apply an AC signal to said amplifier to excite said detection coil, and a phase comparator connected to generate a voltage output representative of a phase difference between an output of said oscillator and an output of said amplifier and to apply said voltage output to control said oscillator to oscillate at an oscillation frequency substantially equal to a resonant frequency of said resonant circuit, whereby the amplitude of the output signal of said amplifier embodies a measure of the distance between said coil and a metallic object whose distance from the coil it is desired to measure.
2. Apparatus as claimed in claim 1 wherein said positive feedback amplifier includes an inverting input terminal, a non-inverting input terminal and an output terminal, wherein said oscillator has an output connected to said inverting input terminal, and said resonant circuit is connected to said noninverting input terminal, and wherein a feedback resistor is connected between said non-inverting input terminal and said output terminal.
3. Distance measuring apparatus substantially as described herein with reference to Figure 2 of the accompanying drawings.
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP8747279A JPS5612502A (en) | 1979-07-12 | 1979-07-12 | Feedback amplification type vortex flow range finder |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| GB2054867A true GB2054867A (en) | 1981-02-18 |
| GB2054867B GB2054867B (en) | 1983-06-29 |
Family
ID=13915846
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| GB8022329A Expired GB2054867B (en) | 1979-07-12 | 1980-07-08 | Eddy-current distance measuring apparatus |
Country Status (4)
| Country | Link |
|---|---|
| JP (1) | JPS5612502A (en) |
| DE (1) | DE3026389C2 (en) |
| FR (1) | FR2461233A1 (en) |
| GB (1) | GB2054867B (en) |
Cited By (7)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| GB2136580A (en) * | 1983-03-16 | 1984-09-19 | Thyssen Industrie | Method and apparatus for determining the distance from a conductor |
| EP0168696A1 (en) * | 1984-06-30 | 1986-01-22 | Nippon Kokan Kabushiki Kaisha | Eddy current distance signal formation apparatus |
| FR2575820A1 (en) * | 1985-01-10 | 1986-07-11 | Equip Construction Electriq | METHOD AND DEVICE FOR MEASURING THE DISTANCE BETWEEN A TARGET AND A SENSOR |
| FR2638837A1 (en) * | 1988-11-10 | 1990-05-11 | Phytrans | Method for detecting the temperature drift of the response signal of an inductive sensor and device for its implementation |
| WO1997021070A1 (en) * | 1995-12-05 | 1997-06-12 | Skf Condition Monitoring | Driver for eddy current proximity probe |
| US5854553A (en) * | 1996-06-19 | 1998-12-29 | Skf Condition Monitoring | Digitally linearizing eddy current probe |
| EP3141502A1 (en) * | 2015-09-10 | 2017-03-15 | Kabushiki Kaisha Toshiba | Sheet processing apparatus and method including an apparatus and a method for detecting sheet thickness |
Families Citing this family (6)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| DE3409448A1 (en) * | 1983-03-16 | 1984-09-20 | Thyssen Industrie Ag, 4300 Essen | Method and device for determining the distance of a magnetic probe from a conductive reaction rail |
| JPS6093316A (en) * | 1983-10-27 | 1985-05-25 | Nippon Kokan Kk <Nkk> | Eddy current type hot water level measuring method |
| DE3420330C1 (en) * | 1984-05-30 | 1985-12-05 | Deutsche Forschungs- und Versuchsanstalt für Luft- und Raumfahrt e.V., 5000 Köln | Inductive sensor and method for non-contact, three-dimensional position detection of holes, bores, bolts, rivets etc. in or on metal parts by means of such a sensor |
| IT1209599B (en) * | 1984-11-15 | 1989-08-30 | Siette Spa | LEVEL DETECTION SYSTEM FOR VEHICLES. |
| SE456606B (en) * | 1987-02-18 | 1988-10-17 | Tornbloms Kvalitetskontroll Ab | DEVICE AND / OR TEST DIMENSION AND / OR DISTANCE SAFETY DEVICE |
| DE3815010A1 (en) * | 1988-04-30 | 1989-11-09 | Leybold Ag | CIRCUIT ARRANGEMENT FOR THE COMBINED USE OF AN INDUCTIVE AND A CAPACITIVE DEVICE FOR THE DESTRUCTION-FREE MEASUREMENT OF THE RESISTANT THIN LAYERS |
Family Cites Families (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US3213694A (en) * | 1963-03-11 | 1965-10-26 | Palomar Scient Corp | Stabilized transducer system for measuring displacement and acceleration |
| US3851242A (en) * | 1972-06-27 | 1974-11-26 | J Ellis | Frequency-modulated eddy-current proximity gage |
| GB1512799A (en) * | 1974-11-06 | 1978-06-01 | Nippon Kokan Kk | Apparatus for non-contact measurement of distance between a metallic body and a detection coil |
| US4160204A (en) * | 1974-11-11 | 1979-07-03 | Kaman Sciences Corporation | Non-contact distance measurement system |
-
1979
- 1979-07-12 JP JP8747279A patent/JPS5612502A/en active Granted
-
1980
- 1980-07-08 GB GB8022329A patent/GB2054867B/en not_active Expired
- 1980-07-09 FR FR8015249A patent/FR2461233A1/en active Granted
- 1980-07-11 DE DE19803026389 patent/DE3026389C2/en not_active Expired
Cited By (11)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| GB2136580A (en) * | 1983-03-16 | 1984-09-19 | Thyssen Industrie | Method and apparatus for determining the distance from a conductor |
| FR2542865A1 (en) * | 1983-03-16 | 1984-09-21 | Thyssen Industrie | METHOD AND DEVICE FOR DETERMINING THE DISTANCE OF A MAGNETIC PROBE TO A CONDUCTIVE REACTION RAIL |
| US4866380A (en) * | 1983-03-16 | 1989-09-12 | Thyssen Industrie Ag | Method and apparatus for determining the distance between an electromagnetic sensor and a conductive rail |
| EP0168696A1 (en) * | 1984-06-30 | 1986-01-22 | Nippon Kokan Kabushiki Kaisha | Eddy current distance signal formation apparatus |
| US4716366A (en) * | 1984-06-30 | 1987-12-29 | Nippon Kokan K.K. | Eddy current distance signal apparatus with temperature change compensation means |
| FR2575820A1 (en) * | 1985-01-10 | 1986-07-11 | Equip Construction Electriq | METHOD AND DEVICE FOR MEASURING THE DISTANCE BETWEEN A TARGET AND A SENSOR |
| FR2638837A1 (en) * | 1988-11-10 | 1990-05-11 | Phytrans | Method for detecting the temperature drift of the response signal of an inductive sensor and device for its implementation |
| WO1997021070A1 (en) * | 1995-12-05 | 1997-06-12 | Skf Condition Monitoring | Driver for eddy current proximity probe |
| US5854553A (en) * | 1996-06-19 | 1998-12-29 | Skf Condition Monitoring | Digitally linearizing eddy current probe |
| EP3141502A1 (en) * | 2015-09-10 | 2017-03-15 | Kabushiki Kaisha Toshiba | Sheet processing apparatus and method including an apparatus and a method for detecting sheet thickness |
| US9714147B2 (en) | 2015-09-10 | 2017-07-25 | Kabushiki Kaisha Toshiba | Sheet processing apparatus and method of detecting thickness of sheet |
Also Published As
| Publication number | Publication date |
|---|---|
| GB2054867B (en) | 1983-06-29 |
| JPS5612502A (en) | 1981-02-06 |
| JPS623881B2 (en) | 1987-01-27 |
| DE3026389C2 (en) | 1984-04-19 |
| FR2461233B1 (en) | 1983-05-13 |
| FR2461233A1 (en) | 1981-01-30 |
| DE3026389A1 (en) | 1981-01-15 |
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| PCNP | Patent ceased through non-payment of renewal fee |
Effective date: 19960708 |