AU604059B2

AU604059B2 - Inductive sensing

Info

Publication number: AU604059B2
Application number: AU79229/87A
Authority: AU
Inventors: Jan Baranski
Original assignee: University of Western Australia
Current assignee: WESTERN AUSTRALIA THE, University of
Priority date: 1986-09-29
Filing date: 1987-09-29
Publication date: 1990-12-06
Anticipated expiration: 2007-09-29
Also published as: AU7922987A

Description

604059 COMMONWEALTH OF AUSTRALIA PATENTS ACT 1952 COMPLETE SPECIFICATION

(ORIGINAL)

Form FOR 13FFICE USE Class Int. Class Application Number: Lodged: Complete Specification-Lodged: Accepted: Published: Priority: Related Art: 4 arr:~:trn.is ,-,jjdc unri.r Sectii)1, 4L) and is cnrcmct tfr 7BE COMPLETED BY APPLICANT Name of Applicant: ,Address of Applicant: Actual Inventor: Address for Service: THE UNIVERSITY OF WESTERN AUSTRALIA NEDLANDS, WESTERN AUSTRALIA, 6009 JAN BARANSKI SANDERCOCK, SMITHE BEADLE 207 RiVersdais koad, Box 410) Hawthorn, Victoria, 3122 Complete Specification for the Invention entitled: INDUCTIVE SENSING The f'ollowing statement is a f#411 description of this Invention, Including the best method of performing it known to me:- 2 This invention relates to remote sensing, and in particular relates to proximity sensing in automatically controlled operations, for example in sensing the position of a shearing head, in relation to the skin of a sheep, in an automated sheep shearing system.

In AU-A-32064/84, capacitance proximity sensing in automated sheep shearing systems is described, and AU-A- 35303/84 relates to the use of resistance proximity sensing in such systems.

It is an object of this invention to provide an S alternative form of proximity sensing.

The invention provides apparatus for measuring the S proximity of an object to a surface of low conductivity material, comprising at least one sensor located on said object, said sensor including a single turn coil connected in a tuned circuit, the inductance of said coil varying in accordance with changes in said proximity, and RF means coupled to said tuned circuit for applying a UHF RF signal to said tuned circuit, and for detecting at the output of said tuned circuit a voltage amplitude representing said proximity.

The invention also provides an automated sheep shearing apparatus including: a cutter head; an inductive sensing arrangement including a sensor package located on said cutter head, said package including a pair of sensors, each of said sensors including a single turn coil connected in series with a capacitance trimmer to j form a tuned circuit; psspe.016/uniwa 90 7 3 S2a means coupled to said sensors for applying a UHF RF signal to each said coil; and means coupled to said sensors for detecting voltage changes across each said tuned circuit and for determining the distance between the cutter head and the skin of the sheep in response to said detected voltage changes.

SThe invention still further provides a method for measuring the proximity of an object to a surface comprising: providing at least one single turn coil in a tuned circuit having a predetermined shape for inducing an eddy o# o. o, current flow path in said surface, said eddy current flow o, p o path mirroring said predetermined coil shape; to detecting changes in the inductance of said coil as .9 'I :said proximity varies, including the steps of applying a UHF RF frequency signal across said tuned circuit and sensing voltage across said tuned circuit; and representing proximity of said coil with said surface in response to said sensed voltage.

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3 1 2 3 4 6 7 8 9 11 S 12 13 14 16 17 18 I I 1 L I r Oi-j.d el c- f An embodiment of the invention will be described in detail hereinafter, with reference to the accompanying dr-wings, in which:- Figure 1 is a diagram relating to the theoretical basis of the invention; Figure 2 is a block diagram of one embodiment of a twosensor inductance sensor arrangement according to the invention; Figure 3 is a more detailed diagram of one sensor channel cf the arrangement of Figure 3; and Figure 4 is a graph of VOUT plotted against 1/h.

The inductive sensor of the present invention utilizes the eddy currents phenomenon. When a magnetic field is produced in the vicinity of a material characterized by conductivity y and permeability p part of this field penetrates into the material to the conventional depth of wPy 22 The field induces eddy currents in the material, which have 23 circular paths parallel to the material surface 10. If the 24 source of the magnetic field and in turn the eddy currents is a coil 12, these currents will change the inductance of 26 the coil 12. Assuming that the coil is of a single 27 circular loop made from a wire of small section and the 28 surface conductivity is high, the exciting coil 12 will 29 induce an effective eddy current coil 14 of the shape shown 870929, jcspe.006,uwaaust.spe, hil.

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4 in Figure 1, mirroring the shape of exciting coil 12. The mutual inductance of the two coils is mathematically described as k 2 M J(k) K(k) 4r 2 where K J(k) and K(k) elliptic integrals of 4r +d first and second order;r,d,h as in Fig. 1.

A complete description of the inductance change is only possible for small distances (h tr t A 1 aM &L L M IY6 Dh 18 19 21 22 23 24 26 27 28 Turning now to Figures 2 to 4, the sensor package 16 consists of two sensors placed side by side, designated functionally 'left' and 'right' which are part of the inductance sensing system shown on Figure 2. The package was designed to fit the cutter head of a sheep shearing robot. It is suggested that a third, 'back', sensor could be added (reference the three capacitance sensors of AU-A-32064/84) and that the system could be integrated by including the power splitter 18 and detectors 20, 22 in the sensor package 16.

A frequency control unit 24 feeds into a radio frequency plug-in unit 26, preferabl P 86220A. Radio frequency plug-in unit 26, preferablyga* HP 86220A. Radio 870929,tIcspe.006,uwaaust.spe 7

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5 1 frequency amplifier 28 (preferably involving mini-circuits 2 ZHL-2-12) feeds to power splitter 18 (preferably MCL ZFSC-4- 3 which in turn feeds to the sensor package.

4 Detectors 20, 22 for right and left sensors R and L respectively have their signals amplified by amplifiers 6 32 respectively, which produces VOUT for each sensor, for 7 supply to a data acquisition system 34 8 Figure 3 shows a single sensor channel where U i is the 9 powering signal, R is the characteristic impedance of the splitter 18, and DI and CD form a RF detector 20. In the 11 sensor package there is a single turn coil 12, printed on 12 double-sided copper-clad epoxy glass board (not shown) which 13 is connected in series with a capacitance trimmer C to form 14 a tuned circuit, Cp representing a parasitic capacitance. A parallel arrangement of the circuit is also possible, 16 although in such an arrangement the influence of the 17 parasitic caps itance is much bigger.

18 The other side of the board is grounded and there is a 19 radial Faraday shield in front of each sensor.

The sIlf inductance of the single turn coil 12 is 21 affected by the ground plane according to equation as 22 used for proximity measurement, so the resonant frequency 23 had to be found experimentally. It is desirable to use a 24 high operating frequency to decrease the depth of penetration 6) and increase the change of inductance; one 26 frequency used in experiments was in the vicinity of 830 27 MIz.

28 When the sensor approaches a conductive material 10 the 4 9 value of inductance L decreases, the circuit gets out of 0 870929,ljcspe.006,uwaaust..spe,

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1 2 3 4 6 7 8 9 11 12 13 14 16 17 18 19 21 22 23 24 26 27 7 6 tune, which changes the voltage across the circuit. In experiments, the measured change of V was approximately 1.5V over a distance of Figure 4 shows characteristics of the sensor measured on the outputs (VOUT) of the amplifiers 30, 32.

The nature of fleece, especially near the skin 10, of a sheep, suggests an electrical anisotropy. When the surface conductivity is considered, the weakest properties are found on any plane locally parallel with the skin. Such a position naturally corresponds with a normal cutter position during sheep shearing. The inductive sensor discriminates between the surface conductivity of the sheep's skin and the wool surface conductivity. This significantly decreases sensitivity to the wool conditions.

The sensor of this invention can be tuned to different working points on the resonant characteristic and to different resonant frequencies. A 'band sensitivity' can be obtained to detect materials cnly from a certain range of conductivity and permeability. The inductive sensor can also be employed directly for conductivity and permeability measurement.

The entire contents of the provisional speifications lodged with Australian Patent Applications of which this is the complete specification are hereby imported into this specification and form part of the disclosure of this specification. The claims form part of the disclosure of this specication.

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Claims

1. Apparatus for measuring the proximity of an object to a surface of low conductivity material, comprising at least one sensor located on said object, said sensor including a single turn coil connected in a tuned circuit, the inductance of said coil varying in accordance with changes in said proximity, and RF means coupled to said tuned circuit for applying a UHF RF signal to said tuned circuit, and for detecting at the output of said tuned circuit a voltage amplitude representing said proximity.

2. Apparatus according to claim 1 wherein said UHF S signal is in the vicinity of 830 MHz.

3. Apparatus according to claim 2, wherein said coil is connected in series with a capacitor trimmer.

4. Apparatus according to claim 1, 2 or 3, further including a pair of sensors oriented in side-by-side relationship. Apparatus according to claim 4 wherein said sensors are incorporated into a cutter head of a sheep shearing device, and wherein said surface comprises thG skin of the sheep.

6. A method for measuring the proximity of an object to a surface comprising: providing at least one single turn coil in a tuned circuit having a predetermined shape for inducing an eddy current flow path in said surface, said eddy current flow path mirroring said predetermined coil shape; detecting changes in the inductance of said coil as said proximity varies, including the steps Of applying a UHF B 4/ psspe.016/uniwa 90 7 3 8 RF frequency signal across said tuned circuit and sensing voltage across said tuned circuit; and representing proximity of said coil with said surface in response to said sensed voltage.

7. A method according to claim 6 wherein said UHF signal has a frequency of about 830 MHz. 1j-

8. A method according to claim 7 wherein the change in inductaics e L is determined by the following equation: PAL '6 where U is the permeability of the surface; is the i 0l conductivity of the surface; is the depth of penetration of the eddy current in the surface; M is the mutual Sinductance; and h is the distance between the surface and a centre of the eddy current coil.

9. An automated sheep shearing apparatus including: a cutter head; an inductive sensing arrangement including a sensor package located on said cutter head, said package including a pair of sensors, each of said sensors including a single turn coil connected in series with a capacitance trimmer to form a tuned circuit; means coupled to said sensors for applying a UHF RF signal to each said coil; and means coupled to said sensors for detecting voltage changes across each said tuned circuit and for determining the distance between the cutter head and the skin of the sheep in response to said detected voltage changes. S 10 The automated sheep shearing apparatus as defined in sspe 016/uniwa 90 7 3 7 i 9 cq~t i claim 9 wherein said means for detecting voltage changes includes means for amplifying each of said sensor generated signals and data acquisition means for receiving and acquiring said amplified signals.

11. Apparatus as in claim 1 wherein: said coil comprises a planar coil; said tuned circuit further comprises a trimmer capacitance connected in series with said coil to form a resonant circuit; said RF means includes radio frequency generating means coupled to said tuned circuit for generating said UHF RF signal in the form of an alternating current signal and for applying said alternating current signal to said tuned circuit; and said coil discriminates between wool and sheep skin in proximity to said coil.

12. A method as in claim 6 wherein: said single turn coil comprises providing a planar coil connected in series with a trimmer capacitance to form a series resonant circuit; and said method further includes applying said UHF signal to said series resonant circuit and discriminating between wool and sheep skin in proximity to said coil.

13. Apparatus as in claim 9 wherein: each said coil comprises a planar coil; and said means coupled to said sensors for applying UHF RF signal to each coil includes radio frequency generating means coupled to said sensors for generating said UHF signal psspe.016/uniwa 90 7 3 10 and for applying said alternating current signal to said sensors; and said coil discriminates between wool and sheep skin in proximity to said coil.

14. Apparatus according to claim 13 wherein each said coil is printed on a printed circuit board each said coil and trimmer capacitor together form a series resonant tuned circuit resonating at the frequency of said UHF signal and said generated UHF signal corresponds to the resonant frequency of said tuned circuit, proximity between said coils and said sheep skin changing the inductance of said coils and thereby detuning said tuned circuit from said resonant frequency, Apparatus according to claim 14 wherein said coils induce eddy current to flow through said sheep skin in a path mirroring the shape of the coils and said eddy current flow changes the inductance of said coils,

16. Apparatus according to claim 15 whgsin said inductance change is dependent upon the au il i tance between said eddy current paths and said coi.s,

17. Apparatus according to claim 16 further including a ground plane *conductive surface disposed on another surface of said printed circuit board.

18. Apparatus according to claim 17 further including a radial Farady shield operatively coupled to said col~,

19. Apparatus according to any one 'f r 1 9, 10 and 13-18 wherein said UHP RrF igqal of about 830 MHz. S 20. Apparatus for measuring th w of an object psspe,016/uniwa 90 7 3 r i 11 to a surface substantially as hereinbefore described with reference to the accompanying drawings.

21. A method of determining the proximity of an object to a surface substantially as hereinbefore described with reference to the accompanying drawings. DATED this 3 July 1990 SMITH SHELSTON BEADLE Fellows Institute of Patent Attorneys of Australia Patent Attorneys for the Applicant: THE UNIVERSITY OF WESTERN AUSTRALIA ago psspe.016/uniwa 90 7 3 L