GB2073892A - Position sensing - Google Patents

Abstract

The passage of a moving body such as a vehicle (1) past a predetermined position (x1) with a view to making speed measurements or initiating auxiliary processes in or outside the body can be sensed using inductive loop means for having induced therein or for inducing elsewhere a detectable signal. For accurate position sensing, the inductive loop means herein includes a first inductive loop (3) and at least one more loop (4) for sharpening-up the resultant field gradient at the position (x1). The sensing can be done on-board the moving body by a ferrite-cored coil/threshold detector combination (6, 7, 8), or externally to the moving body by detector means connected to the inductive loop means. <IMAGE>

Description

SPECIFICATION Position sensing This invention relates to the sensing of the passage of a moving object such as a vehicle past a predetermined position with a view, for example, to initiating some auxiliary process which is to take place at that time, on-board the object or otherwise, or to operate a position indicator or to provide a timing signal for measuring the velocity and/or acceleration of the object.

For sensing the passage of a moving body some part of which is made of metal, it is known to use an inductive technique whereby the body moves through or past a looped electrical conductor carrying a magnetic-field generating current. For example, in one known arrangement, two coaxial loops are used one of which carries a current so that, as the metal object passes through the loops, a signal is induced in the other, the signal being sensed by suitable apparatus connected to this loop.

With the known arrangements, the accuracy of position sensing may not be sufficient.

According to the present invention, there is provided apparatus for signalling the passage of a moving body through a predetermined position comprising inductive loop means for producing a magnetic field about said position, the loop means including a first inductive loop arranged for said body to pass therethrough, means for supplying electrical current to the first loop for the loop to produce a component of said magnetic field, and one or more further inductive loops arranged near the first and operable for substantially increasing the gradient of said field.

For a better understanding of the invention, reference will now be made, by way of example, to the accompanying drawing, in which: Figure 1 is a diagrammatic, perspective view of an arrangement for indicating to a sensor on board a moving body or vehicle the passage of the body or vehicle through a predetermined position, and Figure 2 comprises curves illustrating the form of magnetic field components associated.

with the Fig. 1 arrangement and the variation of a signal induced in a coil used in the arrangement.

In Fig. 1, a body 1 moving along axis AA carries apparatus 2 which is to sense transition of the body through position x1. Two fixed inductive loops 3 and 4 are arranged coaxial to axis AA so that the respective planes containing the loops intersect axis AA positions to and x2 respectively, to being downstream and x2 upstream of x. The loop 4 has half the number of turns of loop 3 and the two loops are connected in series with one another and to an alternating current source 5 such that the current flows around the two loops in respective opposite directions.Thus, the loops produce respective anti-phase magnetic field components of which the component produced by loop 4 has a maximum field strength equal to half the maximum field strength of the component produced by loop 3. The frequency of the current produced by source 5 may be chosen as desired---one factor which might be taken into account is the desirability of avoiding interference from or with local broadcast stations. By way of example, for an experiment in one particular geographical location, a frequency of 100 KHz was found suitable.

The apparatus 2 comprises a coil 6 having a ferrite core 7 and arranged to have a signal induced therein as the body 1 moves through the resultant field produced by the loops 3 and 4. Increase of the induced signal to above a predetermined threshold is sensed by a threshold detector 8 which thereafter operates a switch 9 recording the transition of the body through position x,. Although the Fig. shows the operation of switch 9 as being the endproduct of sensing the transition of body 1 through position x1, it will be appreciated that this is for illustration purposes only and generally the switch would be arranged to operate, in turn, some auxiliary apparatus such as an indicator, timer, apparatus for carrying out some further process and so on. Possibly the switch could be discarded and the detector 8 connected direct to the auxiliary apparatus.

The curve 10 in Fig. 2 shows the relationship between the signal induced in coil 6 as the body moves along axis AA through the loops 3 and 4. This curve corresponds to the strength along axis AA of the resultant magnetic field due to loops 3 and 44 and the strengths along axis AA of the respective component fields are shown by dashed-line curves 11 and 12. As can be seen, the distance between the two loops, i.e. between x0 and x2 is such that, at x2, the strength of the field component due to loop 3 has fallen to half its maximum, i.e. to a level equal to the strength of the component due to loop 4 at this point.Thus, since the two components are in antiphase, the strength of the resultant field at position x2 is substantially zero and this resultant field and hence the signal induced in coil 6 increase quite rapidly from x2 to xO. The loops 3 and 4 are positioned relative to position x such that the threshold value 1 3 of the signal induced in coil 6 to which the detector 8 responds is reached at xl.

It will be appreciated that practical limitations of any electrical apparatus result in a tolerance in the threshold value sensed by detector 8, i.e. it can normally only be guaranteed that the detector will respond when the induced signal is within a tolerance range around the predetermined value. Thus, there will be a corresponding tolerance xt in the position sensing. However, as can be seen, this tolerance xt is much less than the tolerance Xf that would be obtained if the loop 4 were not present and the signal induced in coil 6 was due to the relatively slowly varying component field of the loop 3 alone.Also, with the arrangement shown, the accuracy of position sensing is far less sensitive to deviations of the body 1 from the axes of the loops as it passes therethrough than would be the case were loop 4 not present. Thus, the loops do not have to be coaxial to axis AA although this is normal.

It will be apreciated of course that although a preferred arrangement is shown, the increased field gradient in the area of position x, relative to that obtained with loop 3 alone can be achieved by various other arrangements within the scope of the invention. For example, the loops 3 and 4 may be able to be moved to different relative positions and/or arranged to produce respective field components having relative strengths and phase other than those described. Instead of two loops, there could be three or even more than three. For example, a further loop could be arranged downstream of loop 3 in Fig. 1 and fed so as to reinforce the field at xO and hence steepen the gradient near x1, the field from loop 4 possibly needing to be adapted to suit.

Apart from the above, it will be understood that even with the arrangement shown the loops do not have to be positioned or phased accurately to obtain a very substantial reduction in position sensing tolerance relative to the single loop case.

It is not essential that the transition through x1 be sensed on-board the body 1. Instead, in order to sense this transition from a fixed position, suitable sensing means could be connected to the loop 3 and/or 4 and made operable to sense signals fed back therethrough due to the passage of any metal part(s) of body 1.

To measure the velocity of a vehicle, it can be timed in its passage between two positions and, for this, an arrangement such as that shown can be set up at each such position.

Claims (8)

1. Apparatus for signalling the passage of a moving body through a predetermined position comprising inductive loop means for producing a magnetic field about said position, the loop means including a first inductive loop arranged for said body to pass therethrough, means for supplying electrical current to the first loop for the loop to produce a component of said magnet field, and one or more further inductive loops arranged near the first and operable for substantially increasing the gradient of said field.

2. Apparatus according to claim 1, wherien the or one of the said further inductive loop(s) is operable for producing a magnetic field component -of which the maximum strength is about half that of the field component due to the first loop and which opposes the field component due to the first loop, this further loop being at a position where the strength of the field component to the first loop is about half its maximum.

3. Apparatus according to claim 2, wherein the said further loop has half the number of turns of the first loop and these two loops are interconnected to pass the same value of current but in opposite directions.

4. Apparatus according to claim 1, 2 or 3, including induced signal detector means connected to the inductive loop means.

5. Apparatus for signalling the passage of a moving body through a predetermined substantially as hereinbefore described with reference to the accompanying drawing.

6. A combination of apparatus according to any preceding claim and detecting apparatus adapted for installation on-board said moving body and comprising inductive means and detector means connected to the inductive means and operable for sensing a signal induced in the inductive means by its passage through said magnetic field.

7. A combination according to claim 6, wherein said inductive means comprises a ferrite-cored coil.

8. A combination of signalling and detecting apparatus substantially as hereinbefore described with reference to the accompanying drawing.