Abstract:
An inductive sensing arrangement enables the distance of the cutting head of an automated sheep shearing apparatus from the skin of a sheep to be determined. Sensors on the cutting head include a coil, the inductance of which varies in accordance with the distance of the coil from a conductive surface such as the skin of a sheep. An output voltage represents the distance of the coil from the conductive surface.

Description:
This is a continuation of application Ser. No. 07/102,204, filed Sept. 29, 1987, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to remote sensing, and in particular relates to proximity sensing in automatically controlled operations, for example in sensing the postion of a shearing head, in relation to the skin of a sheep, in an automated sheep shearing system. 
     2. Description of the Prior Art 
     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. 
     BRIEF SUMMARY OF THE INVENTION 
     It is an object of this invention to provide an alternative form of proximity sensing. 
     The invention provides an inductive sensing arrangement including means for processing information relating to inductance changes resulting from the change in proximity relative to a surface, to produce data on the distance of said surface. 
     The invention also provides an arrangement characterized in that the distance of said surface is represented by a voltage across a tuned circuit. 
     The invention further provides a method of determining the proximity of a surface, including the steps of detecting the change in inductance which results from a change in proximity, and processing information representing said inductance change to provide a representation of the distance from the said surface. 
     Preferably, said arrangement includes a coil capable of generating a magnetic field, and said means operates to determine the inductance change of said coil resulting from the induction of an eddy current coil in the material of said element. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram relating to the theoretical basis of the invention; 
     FIG. 2 is a block diagram of one embodiment of a two-sensor inductance sensor arrangement according to the invention; 
     FIG. 3 is a more detailed diagram of one sensor channel of the arrangement of FIG. 3; and 
     FIG. 4 is a graph of V OUT  plotted against 1/h. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     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 γ and permeability μ part of this field penetrates into the material to the conventional depth of ##EQU1## The field induces eddy currents in the material, which have circular paths parallel to the material surface 10. If the source of the magnetic field and in turn the eddy currents is a coil 12, these currents will change the inductance of the coil 12. Assuming that the coil is of a single circular loop made from a wire of small section and the surface conductivity is high, the exciting coil 12 will induce an effective eddy current coil 14 of the shape shown in FIG. 1, mirroring the shape of exciting coil 12. The mutual inductance of the two coils is mathematically described as: ##EQU2## where: ##EQU3## J(k) and K(k)--elliptic integrals of 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&lt;&lt;r) ##EQU4## 
     Turning now to FIGS. 2 to 4, the sensor package 16 consists of two sensors placed side by side, designated functionally `left` (L) and `right` (R), which are part of the inductance sensing system shown on FIG. 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 and contols the frequency of a radio frequency plug-in unit 26, preferably at HP 86220A. Radio frequency amplifier 28 (preferably involving mini-circuits ZHL-2-12) feeds to power splitter 18 (preferably MCL ZFSC-4-1, which in turn feeds to the sensor package. 
     Detectors 20, 22 for right and left sensors R and L respectively have their signals amplified by amplifiers 30, 32 respectively, which produces V OUT  for each sensor, for supply to a data acquisition system 34. 
     FIG. 3 shows a single sensor channel where U i  is the powering signal, R is the characteristic impedance of the splitter 18, and Dl and C D  form a RF detector 20. In the sensor package there is a single turn coil 12, printed on double-sided copper-clad epoxy glass board (not shown) which is connected in series with a capacitance trimmer C to form a tuned circuit, Cp representing a parasitic capacitance. A parallel arrangement of the circuit is also possible, although in such an arrangement the influence of the parasitic capacitance is much bigger. 
     The other side of the board is grounded and there is a radial Faraday shield in front of each sensor. 
     The self inductance of the single turn coil 12 is affected by the ground plane according to equation (3) as used for proximity measurement, so the resonant frequency had to be found experimentally. It is desirable to use a high operating frequency to decrease the depth of penetration (δ) and increase the change of inductance; one frequency used in experiments was in the vicinity of 830 MHz. 
     When the sensor approaches a conductive material 10 the value of inductance L decreases, the circuit gets out of tune, which changes the voltage across the circuit. In experiments, the measured change of Vs was approximately 1.5 V over a distance of 60 mm. 
     FIG. 4 shows characteristics of the sensor measured on the outputs (V OUT ) 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&#39;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 only from a certain range of conductivity and permeability. The inductive sensor can also be employed directly for conductivity and permeability measurement.