Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
1~17785
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 float which
follows the liquid level and is mounted with a primary
5. 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
10. 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
15. upper end of a stem proJecting upwards from the
float. As is accepted in the art, the noat 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
20. float body. This is particularly useful when llqu1ds
o~ 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
25. pivoted about a vertical axis adjacent to the
outside of the tube. The poles of the pr~mary 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
30. depending upon which pole of the primary magnet is
c~oser to the secondary magnet. It is frequently
desired to provide a number of switch functions each
dependent upon a different level of li~uid. For this
purpose a number of the secondarymagnets are pro-
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. .
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vided at different heights along the tube for cooperationwith 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 e~rample in our British Specifi-
cation No. 688402, to provide end to end with the second-
ary magnet a pivotally mounted tertiary magnet with theadaacent 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 inter-
- mediate 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 o~ the
secondary magnet is attracted in its end positions by
one or other end of a pivoted magnetic armatu.e.
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 limited by the magnetic influences.
In accordance with the present invention, in a
3 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 in~lu-
ence 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
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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 exper-
ienced by the tertiary magnetic member from the secondarymagnet.
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 ~t
presents,adjacent to the poles o~ the secondary magnet,
3 poles of opposite polarity for attraction by the
ad3acent 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 o~
latching the secondary magnet in one or other of its end
positions, thereby providing a switch memory when the
J 1117~85
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primary magnet has moved to a position in which it no
longer effectively influences the position of the second-
ary magnet. The arrangement also provides an increase in
the available torque for operating the switch function.
mus, 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.
me 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 ensure its
swinging o~er 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 and/
or tertiary magnetic member are able to swing so far in
one sense that an adiacent pair of their poles lie so
close in an end position and pro~ide such attraction
that the magnetic influence of the primary magnet is
78
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insufficient to rotate the secondary magnet away from
this end position. This may be use~ul 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, initial 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 pro-
blem of hover is thus avoided.
The switch ~unction may be an electrical fw ^tion
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 ~alve, pressure surges in the pneumatic line
mi~ht, transiently, be sufficient to swing the tertiary
magnetic member to its opposite end position. This could
lead to the possibility of unlatching of the seconda~y
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 con-
sequences.
In order to avoid this possibility, the geometry
of ~he secondary magnet and tertiary magnetic members are
3~ preferably such that when the secondary magnet is latched
( 111778
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in an end position with the tertiary magnetic member in
a corresponding position, rotation of the tertiary
magnetic member to its opp~site end position does not
aIfect the latching ol the secondary magnet. In pr~ct-
5. ice 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
10. 60, preferably 40 , either side a central position
With this arrangement when the secondary magnet is in
an end position, irrespecl~ive of the angular position
of the tertiary ma~netic memb~r, one pole of the
secondary magnet will always be nearer to the adjacent
15. 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 position~,
the other pole of the secon~ary magnet will be nearer
20. to the adjacent pole of the tertiary ma~netic member,
irrespective of the angular position of the tertiary
magnetic ~e~ber. The terti ry magnetic member can then
never be responsible for ch~nging over the secondary
m~gne~ between its end positions and the stable
25. switched position is always determined by the secondary
m~gnet. This feature is also useful in reducing the
effects of vibration on ~he switch.
An example o~ a switch assembly constructed in
accordance with the in-~enti~n is illustrated diagramm-
30. atically i~. the accompanyi~g drawings, in which:-
Figure 1 is a diagr2mmatic eleva-tion7 with parts
bro~en away in vertical s~ction, showing ~wo switch
assemblies co~nected to a liquid container;
Fi~ure 2 is a diagr~at~c plan ViW of one
~5. switch assembly; and,
111~785
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
5. 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
10. 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
15. 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. me angular movement
of the secondary magnet 16 is limited by a pair of
20. abutments 18 mounted on the plate 14, and that of the
tertiary m~gnet 17 by a pair of similar although
smaller abutments 19. Angular movement of the tertiary
magnet 17 is followed by a rocking member 20 which is
pivotally mounted about a vertical axis 21 and has at
25. 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
30. hoses.
As will ~e apparent from Figure 2, the tertiary
magnet 17 is free to rotate through an angle a, of
approxi~ately 20 whereas the secondary magnet 16 is
free to rock through a larger angle ~ , of approx-
35- imately 80. The adJacent poles of the two magnets 16
1~1778
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and 17 are of opposite polarity, thus the poles Pl 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
5. 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 e~ual
attraction on the poles Pl and P2 of the two magnets.
However, owing to the geometry of the siz~sof the two
10. 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 magr.et 1~ and the line LF is less than the perpendi-
15. cular distance X between the pivotal axis of the magnet17 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
20. magnetic attraction between the poles pl and p2 is
significant in the full line position of the magnets.
As one or other of the north and south poles of
the magnet 7 moves up or down the tube 9 closely adJacent
to the secondary magnet 16, the magnet 16 is urged to
25. rotate in the same sense by the mutual attraction and
repulsion between its repsective 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
30. 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
con~iguration which is maintained until the other pole
of the magnet 7 moves into proximity with the secondary
35. magnet 16. This causes the magnet 16, and hence the
g
magnet 17 to change over to their other mutually
latched end p~sitions. 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
5. 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.
10. 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 the
tertiary magnet 1?. Rotation of the magnet 17 between
its two end positions thus rGcks the member 20 between
15. its two end positions and causes the rod 23 to recipro-
cate 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.
20. 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. m is could momentarily o~ercome
25. the magnetic attraction be~ween the poles P1 and P2 andforce the magnet 17 to rotate clockwise as shown in
Fi~ure 2 to its other end position. However the geo-
metry of the magnets 16 and 17, and particularly the
greater angular movement of the magnet 16 than that of
30. the magnet 17, pre~ents this movement of the magnet
17 ~rom affecting the latching between the two magnets.
Consequently the secondary magnet is maintained by the
proxim}ty of the pole Pl in its illustrated full line
position and when the pressure in the pneumatic circuit
35. has equalised again, the tertiary magnet 17 will readopt
its full line position.
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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.
