GB2046523A - Float operated switching assembly - Google Patents

Abstract

A float operated switching assembly has a float (4) which follows the liquid level and a primary magnet (7) which follows the movement of the float. The magnet (7) controls the angular position of a pivoted secondary bar magnet (16) on the other side of a non-magnetic wall (9) and the angular position of the magnet (16) controls by magnetic attraction the angular position of a tertiary pivoted magnet (17). The angular position of the magnet (17) controls via a coupling (20, 23) a pneumatic valve (24) or electric contacts (not shown). In each end position the magnets (16) and (17) adopt a mutually magnetically held position. The geometry is such that this retension of the magnet (16) is unaffected by movements of the magnet (17) resulting from pneumatic surges in the valve (24). The geometry is also such that the magnet (16) receives from the magnet (17) less torque than the magnet (17) receives from the magnet (16). <IMAGE>

Description

SPECIFICATION Float operated switch assembly The invention is concerned with a float operated switch assembly, for use with a boiler or other liquid container. The assembly has a flbat which follows the liquid level and is mounted with a primary magnet on the wet side of the assembly so that the primary magnet moves upon movement of the float. The primary magnet controls the movement of a secondary switch magnet by magnetic influence through a non-magnetic wall. The secondary magnet is mounted on the dry side of the assembly and its movement controls the operation of a switch. Such an assembly is hereinafter referred to as of the kind described.

In a typical example the primary magnet reciprocates in a vertical non-magnetic tube at the upper end of a stem projecting upwards from the float. As is accepted in the art, the float assembly, that is the float body and parts such as the stem and primary magnet carried by the float body, may be provided with spring assistance to the buoyancy of the float body. This is particularly useful when liquids of low specific gravity are involved as the weight of liquid displaced by the float body when fully immersed may be less than the weight of the float assembly.

The secondary magnet is a bar magnet pivoted about a vertical axis adjacent to the outside of the tube. The poles of the primary magnet are one above the other so that when the primary magnet rises or falls with the float, the secondary magnet pivots between two end positions with a snap action depending upon which pole of the primary magnet is closer to the secondary magnet. It is frequently desired to provide a number of switch functions each dependent upon a different level of liquid. For this purpose a number of the secondary magnets are provided at different heights along the tube for cooperation with a common primary magnet. However, the difficulty then arises of maintaining the function of one switch when the primary magnet has moved out of the sphere of influence of the respective secondary magnet.In order to hold the secondary magnet in the latched position in which it has last been pivoted by the primary magnet, it is known, as described for example in our British Specification No. 688402, to provide end to end with the secondary magnet a pivotally mounted tertiary magnet with the adjacent ends of the secondary and tertiary magnets acting in magnetic repulsion. This has the disadvantage that, upon switch over, when the secondary magnet begins to pivot to its other end position, the repulsive force between the adjacent poles of the secondary and tertiary magnet increases, giving rise to the possibility of hover of the secondary and tertiary magnets in an intermediate position in which the switching function is neither in one configuration or another.A similar problem arises with the arrangement disclosed in British Patent Specification No. 976743, in which an end of the secondary magnet is attracted in its end positions by one or other end of a pivoted magnetic armature.

A factor in any magnetically operated switch assembly is that the forces available for providing the switch function, e.g. contact faces between electrical switch contacts or operating forces for valve actuators, are limted by the magnetic influences.

In accordance with the present invention, in a float operated switch assembly of the kind described, the secondary magnet is a bar magnet pivotally mounted to swing between two limited end positions under the influence of the primary magnet, and a tertiary magnetic member is pivotally mounted to swing between two limited end positions about an axis which is substantially parallel to the pivotal axis of the secondary magnet such that when the secondary magnet swings to one end position, one pole of the secondary magnet attracts an adjacent pole of the tertiary magnetic member to cause the tertiary magnetic member to swing to a corresponding one end position thereby latching the secondary magnet and tertiary magnetic member in their one positions, and when the secondary magnet swings to its other end position, the other pole of the secondary magnet attracts the adjacent other pole of the tertiary magnetic member to cause the tertiary magnetic member to swing to its other end position thereby latching the secondary magnet and tertiary magnetic members in their other end positions, and the swinging of the tertiary magnetic member effecting a switching function; and the arrangement being such that at least in one or other end positions of the secondary magnet and tertiary magnetic member, the torque experienced by the secondary magnet from the tertiary magnetic member is less than the torque experienced by the tertiary magnetic member from the secondary magnet.

The tertiary magnetic member may be a member of magnetizable material, such as soft iron or mu metal in which poles are induced by the adjacent poles of the secondary magnet. However, the available forces will be greater if the tertiary magnetic member is also a permanent magnet. In any case the tertiary magnetic member may by of any appropriate shape provided that it presents, adjacent to the poles of the secondary magnet, poles of opposite polarity for attraction by the adjacent poles of the secondary magnet. It may thus be of horseshoe shape but most simply is of bar magnet shape.

The tertiary magnetic member serves the purpose of latching the secondary magnet in one or other of its end positions, thereby providing a switch memorgy when the primary magnet has moved to a position in which it no longer effectively influences the position of the secondary magnet. The arrangement also provides an increase in the available torque for operating the switch function. Thus, in at least one or other of its end positions, the tertiary magnetic member experiences a greater torque from the secondary magnet than the secondary magnet experiences from the tertiary magnetic member, and only the torque experienced by the secondary magnet has to be overcome by the primary magnet for switch over.

The mutual magnetic torques experienced by the secondary magnet and tertiary magnetic member will depend upon the geometry, in particular the separation of the two pivotal axes, the separation of the poles of the secondary magnet and of the tertiary magnetic member, the relative positions of the poles to the pivotal axes, and the angles to and through which the secondary magnet and tertiary magnetic member are able to swing. The geometry will be such that in a position in which the torque on the tertiary magnetic member is greater than that on the secondary magnet, the line of action between their poles which are attracting one another to determine the end position, passes closer to the pivotal axis of the secondary magnet than to that of the tertiary magnetic member.Furthermore, in order that the secondary magnet, in swinging between its end positions can capture the other end of the tertiary magnetic member, and ensures its swinging over as well, it is anticipated that the limited angular movement of the tertiary magnetic member will be less than that of the secondary magnet. A lock out features may be incorporated if the secondary magnet andlortertiary magnetic member are able to swing so far in one sense that an adjacent pair of their poles lie so close in an end position and provide such attraction that the magnetic influence of the primary magnet is insufficient to rotate the secondary magnet away from this end position. This may be useful for example for a low level steam boiler alarm and will require a manual reset.

A further advantage of the new assembly is that, as the secondary magnet and tertiary magnetic member are arranged side by side, rather than end to end, and that magnetic attractive rather than repulsive forces are involved, intial movement of the secondary magnet away from an end position reduces rather than initially increases the mutual force with the adjacent pole of the tertiary magnetic member. The previously discussed problem of hover is thus avoided.

The switch function may be an electrical function involving electrical switch contacts which are opened or closed by the swinging movement of the tertiary magnetic member. However, we envisage the application of the new assembly for the operation of a valve, such as a pneumatic valve. Such valve might be operated for example by means of a push rod, an end of which engages the tertiary magnetic member or a part which is carried and swings with the tertiary magnetic member.

When used in this way to operate a valve, such as a pneumatic valve, pressure surges in the pneumatic line might, transiently, be sufficient to swing the tertiary magnetic member to its opposite end position.

This could lead to the possibility of unlatching of the secondary magnet and the possibility of the secondary magnet being relatched in its opposite end position. The switch would then have been changed over by the transient feed back from the pneumatic circuit and could have serious consequences.

In order to avoid this possibility, the geometry of the secondary magnet and tertiary magnetic mem bers are preferably such that when the secondary magnet is latched in an end position with the tertiary magnetic member in a corresponding position, rotation of the tertiary magnetic member to its opposite end position does not affect the latching of the sec ondary magnet. In practice this will normally be achieved by limiting the angular swinging movement of the tertiary magnetic member to an angle of say up to 10 either side a central position, whilst allowing the secondary magnet to swing through a larger angle of say between 20 and 60 , preferably 40 , either side a central position.With this arrangement when the secondary magnet is in an end position, irrespective of the angular position of the tertiary magnetic member, one pole of the secondary magnet will always be nearer to the adjacent pole of the tertiary magnetic member than is the other pole of the secondary magnet to the other pole of the tertiary magnetic member. In contrast, when the secondary magnet swings over between its end positions, the other pole of the secondary magnet will be nearer to the adjacent pole of the tertiary magnetic member, irrespective of the angular position of the tertiary magnetic member. The tertiary magnetic member can then never be responsible for changing over the secondary magnet between its end positions and the stable switched position is always determined by the secondary magnet. This feature is also useful in reducing the effects of vibration on the switch.

An example of a switch assembly constructed in accordance with the invention is illustrated diagrammatically in the accompanying drawings, in which: Figure 1 is a diagrammatic elevation, with parts broken away in vertical section, showing two switch assemblies connected to a liquid container; Figure 2 is a diagrammatic plan view of one switch assembly; and Figure 3 is a perspective view of the one switch assembly.

As shown in Figure 1, a float 4 is buoyant in a liquid 5 within a container 6 and carries a vertically polarized bar magnet 7 at the top of a stem 8. The magnet 7 moves vertically, upon movement of the float 4, within a non-magnetic, e.g. stainless steel or glass, tube 9 to the outer wall of which are attached two switch units 10, all within a housing 11 having a lower wall 12 which seals the container 6. The wet side of the assembly is the part within the container 6 and within the tube 9 and the dry side the part within the housing 11 outside the tube 9.

As shown in Figures 2 and 3, each switch assembly is shown as having a mounting plate 13 on which are pivotally mounted about vertical axes, and between plates 14 and 15, a secondary switch bar magnet 16 and a tertiary bar magnet 17. The angular movement of the secondary magnet 16 is limited by a pair of abutments 18 mounted on the plate 14, and that of the tertiary magnet 17 by a pair of similar although smaller abutments 19. Angular movement of the tertiary magnet 17 is followed buy a rocking member 20 which is pivotally mounted about a vertical axis 21 and has at one end a roller 22 which bears against the magnet 17. At its other end the rocking member 20 is coupled to the end of an operating rod 23 of a pneumatic valve 24. In practice air lines to and from the pneumatic valves 24 will pass out of the housing 11 through conventional hoses.

As will be apparent from Figure 2, the tertiary magnet 17 is free to rotate through an angle a, of approximately 200 whereas the secondary magnet 16 is free to rock through a larger angle p, of approximately 80". The adjacent poles of the two magnets 16 and 17 are of opposite polarity, thus the poles P1 and P2 shown in Figure 2 will be one a south pole and the other a north pole. It follows that there are two stable positions, one shown in full lines and one shown in chain dotted lines. In each of these positions, it will be apparent that the line of force LF between the two closer poles represents the direction of an equal attraction on the poles P1 and P2 of the two magnets.However, owing to the geometry of the sizes of the two magnets and their angular freedom of movement, the line of force LF is inclined to the line joining the pivotal axes of the two magnets. Consequently the perpendicular distance Y between the pivotal axis of the magnet 16 and the line LF is less than the perpendicular distance X between the pivotal axis of the magnet 17 and the line LF. Consequently the torque experienced by the magnet 17 is greater than that experienced by the magnet 16. Although other magnetic interactions exist, owing to the inverse square law only the magnetic attarction between the poles p1 and p2 is significant in the full line position of the magnets.

As one or other of the north and south poles of the magnet7 moves up or down the tube 9 closely adjacent to the secondary magnet 16, the magnet 16 is urged to rotate in the same sense by the mutual attraction and repulsion between its respective poles and the adjacent pole of the magnet 7 so that it adopts one of its end positions. In either of the end positions the pole of the magnet 16 closer to the magnet 17 attracts the adjacent end of the magnet 17 and both magnets are held in their illustrated full or chain dotted line positions by their mutual attraction.

This is a stable latched configuration which is maintained until the other pole of the magnet 7 moves into proximity with the secondary magnet 16. This causes the magnet 16, and hence the magnet 17 to change over to their other mutually latched end positions. As has been explained, in each of the end positions of the two magnets, the torque experienced by the magnet 16 is less than that experienced by the magnet 17 so that the magnet 7 can change over the switch by providing a smaller torque on the magnet 16, to unlatch the magnet 16 from the magnet 17, than is available from the magnet 17 to operate the pneumatic switch 24.

It is assumed that the operating rod 23 is resiliently urged out of the pneumatic switch 24 so that the roller 22 is maintained in engagement with theterti- ary magnet 17. Rotation of the magnet 17 between its two end positions thus rocks the member 20 between its two end positions and causes the rod 23 to reciprocate between two end positions, in each of which a different pneumatic configuration is provided, for example by means of a spool directly connected to the rod 23, within the switch 24.

It is possible that transient fluctuations in the pneumatic circuit connected to the switch 24 may cause transient movements of the rod 23 outwardly of the switch 24 when the tertiary magnet 17 is in its full line position. This could momentarily overcome the magnetic attraction between the poles P1 and P2 and force the magnet 17 to rotate clockwise as shown in Figure 2 to its other end position. However the geometry of the magnets 16 and 17, and particumarly the greater angular movement of the magnet 16 than that of the magnet 17, prevents this movement of the magnet 17 from affecting the latching between the two magnets. Consequently the secondary magnet is maintained by the proximity of the pole P1 in its illustrated full line position and when the pressure in the pneumatic circuit has equalised again, the tertiary magnet 17 will readopt its full line position.

The particular geometric arrangement of the primary, secondary and tertiary magnets 7, 16 and 17, as illustrated, optimizes the transmission of energy from the float to the rocking member 20 and hence to the switch 24. The torque provided by the tertiary magnet 17 over its smaller angular movement compared to that of the secondary magnet 16 gives an output of work which is a large proportion of the energy absorbed from the movement of the primary magnet 7 working, prior to switch changeover, against the repulsion from the adjacent pole of the secondary magnet 16.

Claims (7)

1. Afloat operated switch assembly having a float which is mounted with a primary magnet on a wet side of the assembly so that the primary magnet moves upon movement of the float, the primary magnet controlling the movement of a secondary switch magnet by magnetic influence through a non-magnetic wall, and the secondary magnet being mounted on a dry side of the assembly and its movement controlling the operation of a switch, wherein the secondary magnet is a bar magnet pivotally mounted to swing between two limited end positions under the influence of the primary magnet, and a tertiary magnetic member is pivotally mounted to swing between two limited end positions about an axis which is substantially parallel to the pivotal axis of the secondary magnet such that when the secondary magnet swings to one end position, one pole of the secondary magnet attracts an adjacent pole of the tertiary magnetic member to cause the tertiary magnetic member to swing to a corresponding one end position thereby latching the secondary magnet and tertiary magnetic member in their one positions, and when the secondary magnet swings to its other end position, the other pole of the secondary magnet attracts the adjacent other pole of the tertiary magnetic member to cause the tertiary magnetic member to swing to its other end position thereby latching the secondary magnet and tertiary magnetic members in their other end positions, and the swinging of the tertiary magnetic member effecting a switching function; and the arrangement being such that at least in one or other end positions of the secondary magnet and tertiary magnetic member, the torque experienced by the secondary magnet from the tertiary magnetic member is less than the torque experienced by the tertiary magnetic member from the secondary magnet.

2. An assembly according to claim 1, in which the tertiary magnetic member is a permanent magnet.

3. An assembly according to claim 1 or claim 2, in which the tertiary magnetic member is of bar magnet shape.

4. An assembly according to any one of the preceding claims, in which the limited angular movement of the tertiary magnetic member is less than that of the secondary magnet.

5. An assembly according to any one of the preceding claims, in which the geometry of the secondary magnet and tertiary magnetic members are such that when the secondary magnet is latched in an end position with the tertiary magnetic member in a corresponding position, rotation of the tertiary magnetic member to its opposite end position does not affect the latching of the secondary magnet.

6. An assembly according to any one of the preceding claims, in which the switching function involves the operation of a pneumatic valve, an operating member of which is coupled to the tertiary magnetic member.

7. An assembly according to claim 1, substantially as described with reference to the accompanying drawings.