Electro-mechanical liquid level sensor

An electro-mechanical liquid level sensor is disclosed. The sensor comprises a magnetically operable switch in response to liquid level to complete a circuit for actuating a warning light or the like. The switch comprises a movable electrical contact operative to be moved by magnetic force which is variable in accordance with liquid level whereby the circuit is completed upon a predetermined change in liquid level. The movable parts of the sensor and the structure associated therewith are self-aligning.

FIELD OF THE INVENTION 
The present invention relates to liquid level sensors and especially those 
for automotive applications such as in master brake cylinders. 
DESCRIPTION OF THE PRIOR ART 
Liquid level sensors are well known in the art for sensing the height of 
liquid in a container. One of the problems in sensing levels in a liquid 
container which is subject to motion such as in automobile master brake 
cylinders is that the motion imparted to the liquid can give erroneous 
false warnings of actual liquid level. This is caused especially when the 
container is for example, only half full whereby substantial movement of 
the liquid in the container can result even from such minor actions as 
acceleration and braking of a motor vehicle. Additionally, many known 
sensors required careful alignment between parts in order for the sensor 
to give an accurate indication of a predetermined change in liquid level. 
This adds time and expense to manufacturing in that assembly of the 
container requires manual adjustments between parts. It is also 
disadvantageous where the sensors are installed as aftermarket equipment 
or where the sensor is inadvertently disturbed during refilling of the 
liquid container. 
In accordance with the present invention, erroneous readings of liquid 
level are materially reduced and careful adjustments of sensor parts of 
obviated by employing a self-aligning sensor. 
SUMMARY OF THE INVENTION 
The present invention is embodied in and carried out by a liquid level 
sensor comprising a container for liquid; first magnetic means positioned 
in the container operative to move in relation to the liquid level in the 
container; second magnetic means positioned in the container in a 
predetermined relationship with the first magnetic means operative to be 
moved from a first position to a second position upon reaching a threshold 
magnetic force between it and the first magnetic means and an electrical 
switch positioned in the sensor such that movement of the second magnetic 
means to its second position will close the electrical switch thereby 
energizing a signal means associated therewith. Further in accordance with 
the invention, the first magnetic means preferably comprises ferromagnetic 
or magnetically attractable material secured to a float which rises and 
falls with liquid level in the container and the second magnetic means 
comprises permanent magnet material positioned below the float means. To 
protect against erroneous liquid level readings, means may be provided in 
the float to maintain the float in an essentially predetermined attitude 
within the container regardless of container motion and vibration. 
These and other aspects of the present invention will be more apparent from 
the following detailed description of the invention when considered in 
conjunction with the accompanying drawing.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
In FIG. 1 is shown a fluid reservoir 10 with a liquid 12 therein. While 
liquid 12 may be any liquid whose level it is desired to monitor, for 
purposes of illustrating the preferred embodiment, liquid 12 is suitably 
vehicle brake fluid. A switch 14 comprising conductive member 28 is 
engageable with contacts 16 and 18 and has a permanent magnet 20 affixed 
to the top thereof. Floating on the top of the liquid is a float assembly 
22, suitably a hollow plastic assembly having an annular or other 
appropriately shaped hole essentially in the middle thereof to assist in 
preventing pivotable movement of the float as described more fully 
hereinafter. A magnetically attractable material 24 such as metal is 
disposed in the bottom of the float assembly. The magnetically attractable 
material is conveniently of toroidal shape to reduce weight. A spacing rim 
26 is preferably employed to maintain the member 24 in the proper position 
in the fluid reservoir 10 and permit it to move in linear fashion so that 
as the float assembly drops the metal 24 will be directly above the magnet 
20. When the float assembly drops to a position proximate the magnet, the 
magnet will be attracted upwardly thereby urging conductive member 28 to 
move linearly upward into engagement with contacts 16 and 18 to complete 
the circuit and light warning lamp 30. 
Alternatively and, in fact, preferably the float is made of a magnetically 
attractable material, suitably ferromagnetic stainless steel. In this 
manner there need not be a two piece construction as is true with a 
plastic float having a magnetically attractable material therewithin. 
Furthermore, making the float of metal is highly desirable since it can 
withstand much greater temperatures than can most plastics. 
As shown in FIG. 2, switch 14 comprises movable contact 28 which is secured 
to magnet 20. Contact 28 is U-shaped and is disposed within pendulum 
assembly 29 to be translationally movable therein. Pendulum assembly 29 is 
mounted on pins 16 and 18 and is rotatable thereabout. Pendulum inner 
portion 33, made of non-conductive material, has openings 35 through which 
the pendulum is mounted on the pins. Openings 35 are sized to accept pins 
16 and 18 in a pivotable manner. About inner portion 33 is positioned the 
U-shaped electrically conductive contact 28 which has openings that are 
larger than openings 35 as well as the diameter of pins 16 and 18. A 
counterweight 32 is positioned in the bottom of pendulum 29 to weigh the 
bottom of the pendulum thereby tending to maintain the magnet 20 in the 
position shown in FIG. 2. As shown in FIG. 2, the distance between magnet 
20 and magnetic material 24 is sufficient so that the magnetic force 
therebetween is insufficient to lift contact 28 in slot 37. Contact 28, 
therefore, rests on pendulum inner portion 33 in a non-contacting position 
with respect to pins 16 and 18. The openings in contact 28 are preferably 
at least 20% larger than the diameter of pins 16 and 18 so that there is 
not contact therebetween with contact 28 in its rest position shown in 
FIG. 2. A shroud 34 of non-magnetically attractable material is disposed 
about the switch assembly 14 to maintain it free of contact with the 
liquid whereby short circuiting is prevented. The shroud may be made of a 
high temperature resistant plastic but it is preferably formed of a 
non-magnetically attractable metal such as brass or non-magnetic stainless 
steel so that it can withstand the high temperature of today's automobile 
engines and can also serve as an electrical conduit as more fully 
discussed hereinafter. The shroud 34 is spaced from the magnet 20 a 
distance A which is more than the distance B between contact 28 and pin 
contacts 16 and 18. Thus, when contact 28 is urged upward by the proximity 
of the magnetic material 24 in float 22, contact 28 will make positive 
contact with pins 16 and the linear upward movement of the contact 28 will 
be restrained by pins 16 and 18 rather than the shroud 34. With the fluid 
level shown in FIG. 2, the distance between the magnetic material 24 and 
magnet 20 is sufficient so that the magnetic force therebetween is 
insufficient to lift the magnet and contact 28. It is important to note 
that the float 22 is constrained within the container for movement while 
remaining essentially normal to the container side wall by means of 
extension or rim 26. Pendulum 29 also assists in preventing erroneous 
readings which would otherwise be caused by movement of the floating 
contact 28 in the vertical direction due to movement of the container. By 
providing pendulum 29 including counterweight 32, the assembly carrying 
the floating contact will tend to pivot under motion and vibration of the 
container rather than "bounce" or "jump" in the vertical direction. As the 
liquid level in container 10 falls, float 22 follows and after the liquid 
level in container 10 has fallen a predetermined level, magnetically 
attractable material 24 will be sufficiently close to magnet 20 so that 
the magnetic force therebetween will be at the threshold value which will 
lift the magnet 20 and the floating contact 28. Upon being moved 
vertically upwards, the opening in the floating contact 28 will move 
upwards until the contact 28 strikes and is stopped by pins 16 and 18. 
This closes the circuit shown in FIG. 1 to indicate a low fluid level. 
In the embodiment shown in FIG. 2, the magnet 20, and the contact 28 are an 
integral but separate structure from the pendulum portion 31, 33 and the 
weight 32, the latter all being joined to each other. A hole (not shown) 
is provided in the top of the outer portion 31 of the pendulum through 
which the magnet 20 extends and is free to move thus permitting the 
contact 28 to engage the pins 16 and 18. 
It will be appreciated that there can be various changes and modification 
in the pendulum structure; for example, the structure can be an integral 
unit. The disadvantage to this construction is that the magnetic force 
must be sufficient to lift the entire unit including the weight 32 whereas 
such is not required with the preferred embodiment as hereinbefore 
described. 
The hole in the center of the float assembly 22 is to present disengagement 
of the magnet 20 and the magnetic material 24 if there is a total loss of 
fluid in the reservoir. Were the hole not present, the float 22 would 
pivot sharply about the end of the shroud 34, which could cause 
disengagement of the magnet 20 and the magnetic material 24 thereby giving 
an erroneous reading. The hole in the center of the float, resulting in an 
essentially toroidally shaped float, substantially eliminates this 
potential problem. 
While for purposes of illustration the permanent magnet has been shown in 
the sensor assembly and the magnetic material such as metal has been shown 
in the float, and while this is the preferred form of construction, it 
will be appreciated that the materials could be reversed. The potential 
problem with such a reversal is that the reservoir 10 is usually formed of 
metal which could interfere with proper operation of the float. In a still 
further alternate embodiment, item 20 and 24 could both be of magnetic 
material but with their polarities oppositely aligned. 
Because of the construction of the present invention in which the switch is 
operated by positive magnetic attraction rather than by repulsion, a 
positive reading is obtained once the threshold magnetic force has been 
reached. In the preferred form of the present invention, the magnetic 
attraction between elements 20 and 24 in the closed position is 
sufficiently great so that even subsequent raising of the liquid level in 
the reservoir will not separate the two elements but rather they must be 
separated by manual force. 
It will be appreciated that the sensor described hereinbefore is 
self-aligning due to the float structure, fixed shroud 34 and pendulum 
assembly 29. As the housing 36 is screwed into the container 10, the 
pendulum rotates so that any point at which screwing is stopped the sensor 
is properly aligned. Similarly, minor rotation of the housing 36 by 
inadvertence, vibration or the like will not have a deleterious effect on 
alignment. 
In the embodiment shown in FIG. 2, the contact 16 is not directly connected 
to ground. Rather, the brake fluid container is itself grounded and the 
contact 16 is grounded through the shroud 34 which is of brass or 
non-magnetic stainless steel. Because of the conductive effect of the 
housing 36, contact 18 is electrically isolated from the housing 36 of the 
switch assembly 14 by a suitable insulating material 38. 
While the preferred embodiment describes the invention with respect to 
monitoring brake fluid level, it is to be understood that the invention 
has application in monitoring other fluids. However, the invention has 
particular application for use in vehicles and especially for indicating a 
low level of hydraulic brake fluid. The sensor described hereinbefore is 
essentially unaffected by normal vehicles movement and vibration because 
of the self-aligning feature, pivotable mounting of the floating contact 
and the positive force associated with closing the contacts. The sensor is 
simple in design, rugged and essentially unaffected by fluid temperature. 
The advantages of the present invention, as well as certain changes and 
modifications of the disclosed embodiment thereof, will be readily 
apparent to those skilled in the art. It is the applicant's intention to 
cover by his claims all those changes and modifications which could be 
made to the embodiment of the invention herein chosen for the purpose of 
the disclosure without departing from the spirit and scope of the 
invention.