GB2169711A - Proximity switch - Google Patents

Abstract

A precision switch includes two LC oscillators the amplitude of oscillations of which are changed by a damping member (D), and has the coils (B1, B2) forming the oscillator inductances (L1, L2) positioned in a closely spaced parallel relationship so that the damping member D can be made to interact with both oscillators. The electromagnetic fields produced by the coils (B1, B2) are directed in either the same or opposite directions. <IMAGE>

Description

SPECIFICATION Precision proximity switch Field of invention The invention lies in the field of sensor technology and relates to a proximity switch having an oscillator section and sensing electronics on the one hand and a damping member on the other.

Background to the invention The principle of such non-contacting switches (usually referred to as proximity switches) is essentially based on the positive or negative damping of a free oscillator by the introduction of a paramagnetic material into an electromagnetic field generated by the oscillator coil. In general, use is made of coils through which an alternating current flows and which includes a ferromagnetic core which concentrates the magnetic flow. Eddy currents are induced by the cutting of the lines of flow by an approaching metallic damping member and these eddy currents increase as the distance between the damping part and the coil (measured along the coil axis) decreases. The energy loss in the coil, caused by the eddy currents, is perceived as damping of the oscillator.

The moving of the damping member towards the oscillator can take place along the coil axis at an angle thereto or even orthogonal thereto.

The speed of approach of the damping member (which may be attached to a heavy machine part) along the field axis ie towards the oscillator is generally limited for safety reasons. Generally preference is given to lateral movement ie along an inclined or orthogonal path of movement of a damping member relative to the oscillator coil with an unimpeded exit.

It has hitherto been a disadvantage of such proximity switches, particularly where for safety reasons lateral relative movement at high speed is employed, in that the reproducibility of the switching point has been inadequate for accurate positioning purposes and inter alia this has also been found to be highly temperature-dependent.

It is also not possible to avoid hysteresis between the increase and the decrease in damping as the damping member moves across from one dirction and the opposite effect as the damping member moves in the opposite direction. This is mainly due to electromagnetic and mechanical inertia of the system which cannot be ignored because it is inherent in such systems. Nevertheless it is desirable to use a proximity switch as a precision position delivering switch, which is able to reproducibly switch with a high degree geometrical resolution but whose construction involves virtually the same complexity and cost as a conventional proximity switch.

Summary of the invention According to one aspect of the invention this is achieved by incoporating at least- two oscillators in one switch, so that the inductive coils are aligned substantially parallel or antiparallel in close spatial association, so that the exit sides of the electromagnetic field are directed in the same or oppos ite direction and the same damping member can be made to interact with several oscillators.

Such an arrangement constitutes a novel proximity switch with two or more switching points which can be directly influenced by one damping member.

The close spatial association of two oscillator coils can be achieved by using two separate switches and would be covered by the inventive concept, but this would involve greater cost than that of producing switches with two or more oscillators.

The invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figures 1A and 1B show in the Y and Z planes one embodiment of the invention, which incorporates an oscillator section B1/B2 with two oscillators and a damping member A forming a precision proximity switch, Figure 2 illustrates oscillator signal patterns UB1 and UB2 as a function of the path of the damping part A, Figure 3 shows two superimposed, experimentally measured signal patterns, stressing one oscillator, Figure 4 shows experimentally measured signal patterns of superimposed signals from two oscillators, which can be used to control the switch, Figure 5 is a block diagram of a low cost precision proximity switch, Figure 6 is a detailed circuit of the switch of Figure 5, and Figure 7 illustrates the parallel arrangement of the field-producing components with approximate dimensional details.

Detailed description of the drawings As indicated hereinbefore, the inert interaction between damping member and oscillator section leads to a speeddependent hysteresis with respect to the path of movement of the damping member. No attempt is made to speed up this interaction, so that the switch reacts "faster" and in fact the invention is based on unchanged circumstances and the following principle: The switch must permit a reproducible, geometrically highly resolved detection of the position of a metal object (damping member), when the electromagnetic sensor field is formed from two or more field-producing elements or several oscillators instead of one.In the case of an orthogonal or inclined approach of a metal object in the heterodyne field built up from several oscillators, except in the special build up from several oscillators, except in the special case of a symmetry-invariable approach in the main axial direction of the radiated oscillator field, each oscillator is influenced in a non-synchronous manner with respect to the others. The time difference between the interactions of damping member and oscillators can be evaluated in accordance with the signal pattern of the combined oscillators.

A two oscillator system constructed as a special embodiment is shown in Figure 1 A.

Related to an orthogonal spatial system two oscillators B1, B2 eg at the same distance from the Z axis and on the same side of the X plane, produce an electromagnetic sensor field which propogates in the same direction Z. The individual fields are preferably associated in a linear superimposed manner. No details will be given here of the physical characteristics of the total field formed by the superimposition of the individual fields.

In the case of a lateral ie orthogonal aproach of the damping member A to the oscillator field in the X direction and in this case substantially at right angles to the Z axis, oscillator B1 is influenced first. nowever, oscillator B2 remains stationary in its behaviour.

Successively and with a time delay Xp, both oscillators are influenced by the damping member and at the symmetry point Xo both are influenced substantially uniformly. Finally one oscillator B2 interacts with damping member A.

It can be seen that movements of the damping member in the X, Y-plane, other than in the Y direction (figure 1B) leads to unequal behaviour of the oscillators in relation to time and distance moved, whereas a movement in the Z or Y direction, whilst maintaining the clearly visible symmetry condition causes the oscillators to approach with the angle a (alpha) to the Z axis, the phase relationship Xp between the two oscillators is a Xp=trig (a). Generally therefore the damping member is moved at right angles to the propogation direction of the magnetic field.

In the case of the complete passage of the damping member through a heterodyne field, the individual signal patterns simulated in Figure 2 are obtained. The diagram shows individual oscillator voltages UB1 and UB2, as a function of the position of the damping member (not shown) with respect to the two oscillators B1, B2 and will be used as a basis for the discussion of the specific embodiment with two oscillators.

When the damping member approaches the two oscillators B1, B2 in the direction of the path-voltage diagram, the ferromagnetic object initially damps oscillator B1 by removing energy which is required for forming eddy currents in the damping member. The voltage at oscillator B1 successively collapses, whilst the spatially remote oscillator B2 is still not affected. Between the axes of the two oscillator fields, the damping part starts to move away from oscillator B1, and to approach oscillator B2, after which it moves away from both oscillators B1 and B2.This leads to superimposed damping and damping-removed curves with a distinct intersection point precisely between the oscillators, but only if both oscillators within a minimum divergence have the same steepness of damping and damping removal curves with the same oscilator/damping member spacing. The diagram also shows intersection points as a function of the minimum distance between oscillator and the damping member, which is expressed by a change in the steepness. For representation purposes, this is based on a fixed damping member approached by the two oscillators B1, B2 with the minimum, ie vertical spacings, A=a, B=b and C=c.

The indicated positions of the intersections of curved pairs A,a or B,b or C,c reveals the relative insensitivity of intersection points with respect to the spacing between the damping part and the oscillator in the Z direction.

In the case of two oscillators unequally spaced from the X,Y plane, curves with different steepnesses are obtained. This can be utilised for setting or displacement of the intersection point with respect to the direction of movement of the damping member. As a result of the skew symmetry obtained, there is an increasing sensitivity to displacements in the Z direction corresponding to the skew.

As has been stated, the steepness of the damping and damping removed curve is also a function of the distance between the oscillator (field exit point) and the damping member.

The substantially sigmoid pattern between maximum damping and complete damping removed placed natural limits on a group of precisely detectable intersection points SAa, SBb, and SCc. This is represented by curves Cc, which also shows the influence of the reciprocal spacing D of the oscillators.

Figure 3 shows an experimentally measured group of curves zl to z4 in the voltage-path diagram. Each oscillator B1 and B2 is individually measured and its voltage supply plotted.

Thus, these are not the voltage patterns of circuitry-combined oscillators. Parameter z stands for the distance on the major axis between the oscillator coil and the damping member in the Z direction, ie as stated above, the main propagation direction of the electromagnetic oscillator field. The intersection points of the paired signal patterns are, as expected, linearly distributed in the Z direction. It is possible to see this in the case reverse path of the damping removal, -dam ping-damping removal of oscillator B2 on passing relative to the damping member. In order not to complicate the drawing, the curves for oscillator B1 are only shown in part.

Figure 4 shows an experimentally determined heterodyne field, ie the superimposed signal of oscillators B1, B2 as a function of a passage past a damping member. The signal minimas corresponding to the intersection points are also linearly distributed in the Z direction, so that the same conditions apply as described hereinbefore.

It is possible to evaluate the curve of Figure 4 in several different ways, but a preferred procedure will be described later.

As stated, the inherent inertia of the system remains unchanged. If the switch is now to react more rapidly or is to be operated at a higher speed, then (eg in the case of an approach from the left), the signal pattern over B1 is used for reducing the approach speed or for introducing a preliminary stop. Thus the switch is conditioned. The target point xo can now be approached with an optimum slow speed, the target point being determind by a minimum detection. This type of detection is known per se. As a result of its symmetrical construction, this is obviously also possible in the opposite direction. It is to be observed that the convex curvature is adequate for a precise detection of the minima, eg by threshold detection.The definition of the curvature, but not the position on the X or Y-axis is a function of the spacing of the oscillator coil and damping member on the Z-axis.

It must be borne in mind that an oscillator can be "positively" damped as well as ''nega- tively" damped, the latter having been assumed hitherto. With positive damping there is a voltage amplification or increase in amplitude of the oscillator signal when using a ferrite sintered member in which the formation of eddy currents is prevented.

The inventive idea is the use of more than one oscillator in a proximity switch and in producing and evaluating a heterodyne field of at least two oscillators, which interact with the same damping member. Thus, such a switch has several switching points directly influenced by the common damping member.

Figure 5 shows in very general form a block diagram of an embodiment of a precision switch with two oscillators B1 and B2, their common evaluation circuit, a comparator C and a series-connected amplifier V. In accordance with the characteristic of the comparator, the output signal shows which oscillator is more strongly influenced by the damping member. The comparator and series-connected amplifier are constructed in accordance with known technology.

Figure 6 shows the circuitry details of an embodiment of the two oscillators B1 and B2, which are in this case shown with mirror-symmetry. It is possible to see the resonant circuits L1/C1 and L2/C2, as well as the ouput terminals Al and A2. Any type of oscillators is suitable, such as eg a Hartley or Colpitts and modifications thereof, and are preferably used as selected pairs.

The inventive arrangement of the oscillators or at least their tuning coils L1 and L2 in close spatial association in the discussed embodiment, is shown in Figure 7 and is roughly dimensioned as follows. The coils of oscilllators B1 and B2 are wound in the pot cores with a diameter of 5.8 mm. Two such pot cores are arranged with a B1--B2 spacing of approximately 3 mm, so that the open core sides of both cores approximately form a plane. The electromagnetic field of the oscillator propogates in the same direction for both coils. The overall diameter D of the oscillator part B1/B2 of the precision switch is approximately 18 to 20 mm. As a rule, the complete oscillator circuit, together with the comparator and optionally with the output amplifier can be housed in a common housing S, so that only a single binary switching signal has to be led away from the sensor point. According to Figure 7, housing S is approximately 30 mm long.

Claims (7)

1. Non-contacting proximity switch with an oscillator section and evaluation electronics on the one hand and a damping part on the other, characterised by the arrangement of at least two free running oscillators such that the fields emitted by the field-producing components (B1, B2) are spatial close to one another and the exit points of the components which determine the electromagnetic fields are aligned substantially parallel or antiparallel to one another and that a single damping part (A) can be made to interact with several oscillators (B1, B2, etc).

2. A proximity switch as claimed in claim 1, characterised by the arrangement of at least two LC-oscillators (B1, B2), such that the inductance-forming coils ( L1, L2) are in close spatial association and substantially parallel, the exit points of the electromagnetic field being directed in the same direction and the same damping part (A) can be made to interact with several oscillators (B 1, B2 etc).

3. A proximity switch as claimed in claim 1, characterised by the arrangement of two oscillators (B1, B2), so that the inductance-forming coils ( L1, L2) are in spatial association and are aligned in a substantially antiparallel manner, the exit points of the electromagnetic field being directed in the opposite direction and the same damping part (A) being made to interact with both oscillators (B1, B2).

4. A proximity switch as claimed in claims 2 or 3, characterised in that the oscillator coils are arranged in pot core ferrite bodies.

5. A proximity switch as claimed in any one of claims 2, 3 or 4, characterised in that at least one of the oscillators (B1 or B2) is displaceable and/or pivotable in the main axial direction of its electromagnetic field.

6. A proximity switch as claimed in any one of the claims 1 to 4, characterised in that there is a common evaluation circuit for the oscillators comprising a comparator (C).

7. Proximity switches constructed, arranged and adapted to operate substantially as herein described with reference to and as illustrated in the accompanying drawings.