Switching device for motor vehicle anti-theft system

An anti-theft system for a motor vehicle comprises an electrically-powered alarm, a switching device for actuating the alarm, and an electrical circuit connecting the switching device and the alarm. The switching device is attached to the motor vehicle and is provided with electrical contacts which automatically close the electrical circuit to actuate the alarm when the vehicle is in an inclined position.

This invention relates to an anti-theft system for a motor vehicle, and in 
particular to an anti-theft system having an electrically-powered alarm 
which is actuated by a switching device connected to a motor vehicle. 
BACKGROUND 
Anti-theft systems for motor vehicles are known that are equipped with 
electrical contacts which actuate the vehicle horn when an attempt is made 
to open the door. These known anti-theft systems are, however, only 
effective when unauthorized persons attempt to enter the vehicle. They do 
not respond upon removal of vehicle parts, particularly road wheels, which 
are accessible from the outside. 
Anti-theft systems are also known which become effective when the vehicle 
is shaken or rocked. Although these known anti-theft systems set off the 
alarm when an unauthorized person interferes with the outside of the 
vehicle, they are very sensitive and also react to external influences 
that have nothing to do with attempts at theft, for example, when a heavy 
fast-moving vehicle passes close by, or when strong gusts of wind shake or 
rock the parked vehicle. 
THE INVENTION 
The aim of the present invention is to provide a simple, economical and 
reliable anti-theft system that responds when an attempt is made to remove 
the road wheels from the vehicle, or to load the entire vehicle onto 
another and to carry it away, the system not reacting, however, to 
innocent external influences. 
The present invention provides an anti-theft system for a motor vehicle, 
the system comprising an electrically-powered alarm, a switching device 
for actuating the alarm, and an electrical circuit connecting the 
switching device and the alarm, the switching device being attached, in 
use, to the motor vehicle, wherein the switching device is provided with a 
housing defining a chamber which is partially filled with an 
electrically-conductive fluid, and with electrical contacts in the form of 
elongated immersible sensors which extend downwards into said chamber to 
differing extents, and which automatically close the electrical circuit 
when electrically connected by the fluid, when the vehicle is shifted into 
an inclined or differently inclined position. 
This system offers the advantage that the alarm is not actuated by 
short-term vibrations of any kind. However, the alarm is reliably actuated 
when the vehicle is moved along an inclined surface or ramp onto another 
vehicle, or when the vehicle is tilted to one side, for example, for the 
purpose of removing a road wheel. 
A switching device of this kind can be produced in a simple and economical 
manner and operates reliably. It can be fitted in a very small space and 
at practically any position on the vehicle, and the chamber can be filled 
to a level that exactly suits that inclined position of the particular 
vehicle that is necessary for raising a wheel of the vehicle from the 
gound. The same switching device can, therefore, be used for all types of 
vehicles. 
The use of elongated, downwardly extending immersible sensors as contacts 
offers the advantage, among others, that point-contact is established at 
the bottom ends of the sensors, and surface leakage currents cannot occur 
along the inner wall of the housing when said fluid washes back from the 
contacts. In this arrangement, the lower end of each immersible sensor 
defines a certain level, up to which said fluid can rise when relatively 
pronounced tilting or inclination of the vehicle occurs. 
Advantageously, the housing is a cup-shaped vessel whose top is closed off 
by a cover, and the sensors depend from the cover into said chamber and 
are connected to an electronic analysis device. The analysis device is so 
designed that it determines the initial orientation of the housing when 
the alarm is primed, and only actuates the alarm when the orientation or 
inclination of the housing is changed. 
Preferably, there are a plurality of groups of sensors, the sensors of each 
group having the same length and the sensors of the different groups 
having different lengths, in which case the sensors of each group are 
connected by a common conductor to an electronic analysis device. 
Conveniently, one group of sensors extends into said chamber to terminate 
closely adjacent to the floor thereof. This ensures a reliable switching 
action, irrespective of the side to which the housing is tilted along with 
the vehicle. 
The housing may be cylindrical, and the elongated sensors may be 
distributed on radial lines extending from the central axis of the 
housing. These lines may extend from the central axis of the housing in a 
star-like formation, and form with each other angles of 180.degree., 
90.degree. or, preferably, 120.degree.. In this case, it is advantageous 
if the negative terminal of the switching device is connected to one 
immersible sensor which extends almost to the floor of the housing. This 
immersible sensor can be arranged in the central region or central axis of 
the housing. 
Advantageously, the sensors of each group may be arranged at the corners of 
a substantially equilateral triangle centered on the housing axis. This 
results in great reliability in the switching action, irrespective of the 
direction in which the housing, along with the vehicle, is tilted. 
Preferably, the sensors are arranged adjacent to the inner surface of the 
outer wall of the housing. This offers the advantage that the highest 
possible switching position is achieved, since, even when the vehicle is 
slightly inclined, the greatest fluctuation in fluid-level in the housing 
occurs at the outer wall and thus can be used for the switching operation. 
This is because each sensor in this case is electrically connected, via 
said fluid, with a more or less diametrically-opposed sensor which leads 
to the negative terminal and is more or less disposed diametrically 
opposite thereto. 
Advantageously, the housing chamber is of annular form, the chamber being 
adjacent to the outer wall of the housing and surrounding a cylindrical 
inner or central chamber which accommodates the electronic analysis 
device. This arrangement offers the advantage that relatively little 
electrically-conductive fluid is needed within the fluid-containing 
chamber. It also has the advantage of accommodating the analysis device 
within the protection of the housing interior. A further advantage is that 
the leads from the immersible sensors to the analysis device are very 
short. 
When use is made of an annular fluid-containing chamber, the immersible 
sensors are expediently distributed around this chamber in a circle. Such 
an arrangement is easy to produce, and guarantees fluid-tight, 
mutually-insulated seats for the individual sensors in the cover. 
Moreover, the leads running to the sensors can extend or be passed, in a 
fluid-tight manner, through the cover and into the interior of the 
housing. 
Advantageously, the fluid-containing chamber is divided into a plurality of 
chamber parts or compartments by means of upright divider walls, the 
divider walls being so formed that the chamber compartments are in fluid 
communication with each other. Preferably, the divider walls are arranged 
to be immersible in the electrically-conductive fluid, and to terminate 
adjacent to i.e., spaced a short distance from the floor of the housing. 
The immersible divider walls extending into the electrical-conductive fluid 
allow said fluid to rise and fall at only a moderate rate when the 
vehicle, and thus the housing, is swung or rocked back and forth. This is 
because the fluid can only flow between the chamber compartments through 
the passageways below the immersible divider walls. Splashing of the fluid 
is thus suppressed, since the surface area of the fluid is relatively 
small in each of the chamber compartments. The immersible divider walls 
may extend upwardly as far as the cover of the housing. 
Preferably, there are four walls arranged at right-angles to each other. 
This is particularly advantageous where the chamber is annular, as the 
immersible divider walls divide the chamber into a plurality of segmental 
or arcuate chamber compartments, each of which contains immersible 
sensors. Advantageously, the housing is attached, in use, to the motor 
vehicle in such a manner that each of the chamber divider walls is 
inclined, i.e. horizontally angled to the longitudinal axis of the 
vehicle. Thus, maximum damping down of fluctuations in the fluid level 
during shaking or rocking movements of the vehicle is achieved both in the 
longitudinal and the transverse directions. 
Various electrically-conductive fluids can be used in the switching device, 
provided they are able to retain their electrically-conductive properties 
in the temperature range prescribed. Triphenylchloromethane or 
triphenylfluoromethane may be used as the electrically-conductive fluid. 
It is preferable, however, for the fluid to contain a metal salt, and a 
mixture of glycerine and copper sulphate is particularly suitable. A fluid 
of this kind possesses a certain viscosity and flows in a relatively inert 
or slow manner, so that movements in the fluid level are also relatively 
slow.

PREFERRED EMBODIMENTS 
Referring to the drawings, FIGS. 1 and 2 show a first form of switching 
device 10 according to the invention having a hollow, cup-shaped 
cylindrical housing 14, the cylindrical wall 15 and the floor 16 of which 
are made of an electrically non-conductive material, for example a 
polyethylene or polypropylene plastics material. 
As shown in FIGS. 1 to 3, the housing 14 has an inner cylindrical wall 50 
concentric with the outer cylindrical wall 15. The two cylindrical walls 
15 and 50 define an annular chamber 56 which accommodates a partial 
filling or body of an electrically-conductive fluid 23. This fluid may be, 
for example, a mixture of glycerine and copper sulphate, 
triphenylchloromethane or triphenylfluoromethane. The bottom of the 
chamber 56 is closed by a surface 51 which forms part of the housing floor 
16. At the top, the chamber 56 is closed off by an annular cover 19, which 
fits between the outer wall 15 and the inner wall 50 in a fluid-tight 
manner. Preferably, the cover 19 is bonded or welded to the walls 15 and 
50, so that a fluid-tight seal is obtained. The walls 15 and 50, the floor 
16 and the cover 19 are made of a plastics material, for example 
polyethylene, which cannot be wetted by the fluid 23. Immersible sensors 
57, 58, 59, 60 and 61 of generally rod-like form are arranged in the 
annular chamber 56, the sensors depending from the cover 19 and being 
sealed against the cover by sealing means 21. Three each of the immersible 
sensors 57 to 61 are provided, and the sensors are so distributed, in a 
circle, around and concentric with the annular chamber 56 that the three 
sensors of each group 57, 58, 59, 60 and 61 form the corners 57a, 57b, 
57c; 58a, 58b, 58c; 59a, 59b, 59c; 60a, 60b, 60c; and 61a, 61b, 61c of 
respective of substantially equilateral triangles, which are indicated in 
FIG. 2 by different kinds of broken lines and are centered on the central 
axis of the chamber and angularly displaced therearound relative to one 
another. The immersible sensors 57 are all connected to a negative 
terminal 13, whereas the sensors of the groups 58, 59, 60 and 61 are 
connected, via conductors 46, 47, 48 and 62, respectively, to a 
conventional form of electronic analysis device 49 for the digital 
analysis of the orientation tilt of the housing 14. The electronic 
analysis device 49 is arranged in the interior or inner chamber 52 of the 
housing 14. 
The conductors 13, 46, 47, 48 and 62, which are shown only diagrammatically 
in FIG. 2, are in fact in the form of a printed circuit provided on a 
conductor plate 63 which is disposed on the cover 19, so that the printed 
conductors make electrical contact with their corresponding immersible 
sensors 57 to 61 held in the cover 19. 
As will be seen from FIG. 3, the immersible sensors 57 are long enough to 
extend into the fluid 23 even when the axis of the housing 14 is vertical. 
The immersible sensors of the groups 58 to 61, on the other hand, are 
shorter, but all of the sensors of each group 58, 59, 60 and 61, 
respectively, are of the same length, and their bottom ends, in each case, 
define a switching surface parallel to the floor 16. As can be seen from 
FIG. 3, the switching surfaces formed by the sensors 58, 59, 60 and 61 are 
at different levels or distances from the floor 16 all greater than the 
distance of the sensors 57 therefrom, which distances are, however, 
somewhat exaggerated in this Figure. 
As shown in FIG. 1, the housing 14 is provided with a cap 66 having a 
peripheral dependent flange 67, which engages round a bead 65 formed 
around the upper outside edge 65 of the housing. A pressure ring 69 is 
provided between the top 68 of the cap 66 and the conductor plate 63, the 
pressure ring being of U-shaped cross-section and being made of resilient 
material, for example glass-fibre reinforced polypropylene. Thus, the 
conductor plate 63 is pressed firmly against the cover 19, by the 
compression of the pressure ring 69 between the conductor plate and the 
cap 66, so that an electrically-conductive connection is established 
between the conductive upper ends of the sensors 57 to 61 and respective 
printed circuit conductors on the conductor plate 63. 
The electronic analysis device 49, which is secured to the lower face of 
the conductor plate 63, is connected through conductors (not shown) to an 
alarm (not shown), for example the horn of the vehicle. These conductors 
pass out of the cap 66 through a central opening 72 therein. 
The switching device 10 is secured to a motor vehicle, at a suitable 
position thereon that is not accessible to unauthorized persons. 
Preferably, the switching device 10 is arranged so that the axis of its 
cylindrical housing 14 is vertical. Then, when the vehicle is parked and 
the alarm unit is primed, the analysis device 49 ascertains the particular 
set position or orientation, i.e. inclination, of the vehicle, this being 
defined by the sensors which are immersed in the fluid 23. If the vehicle, 
and therefore the switching device 10, is then tilted or shifted to a 
different oriented position, for example as a result of unauthorized 
persons raising one side of the vehicle so as to remove a road wheel, the 
position of the fluid level 24 changes in relation to the immersible 
sensors of the groups 58 to 61. For example, the fluid level 24 may change 
in such a way that, not only the sensors of group 58, but also those of 
group 59 are then contacted by the fluid 23. The analysis device 49 then 
closes the circuit of the alarm unit so that the alarm is set off. 
FIGS. 4 and 5 show a second form of switching device 10 whose housing 14 
has a cylindrical wall 15 and a dished floor 16. An immersible sensor 57, 
connected to a negative terminal 13, is located in the central region or 
central axis of the housing 14, and extends into the body of 
electrically-conductive fluid 23 which covers part of the floor 16. Other 
sensors 58, 59 and 60 are arranged along radial lines 75, 76, 77 and 78 
extending from the central axis of the housing 14. The lines 75 to 78 form 
angles of 90.degree. with each other about the central axis of the housing 
and, in plan view, form a right-angled cross. Here again, the sensors of 
each group 58, 59 and 60 are connected to respective conductors 46, 47 and 
48. In other respects, the switching device of FIGS. 4 and 5 is similar to 
that of FIGS. 1 to 3 and operates in a similar manner. 
FIG. 6 shows a third form of switching device 10. This device is similar to 
that of FIGS. 4 and 5 in that its housing 14 is of the same shape as that 
of FIGS. 4 and 5, and in that a sensor 57, connected to a negative 
terminal 13, is located in the central region or central axis of the 
housing. The remaining sensors, however, are arranged along radial lines 
79, 80 and 81, which are at angles of 120.degree. to each other about the 
central axis of the housing. Here again, this switching device operates in 
a similar manner to that of the embodiment of FIGS. 1 to 3. 
FIGS. 7 and 8 show a fourth form of switching device 10 according to the 
invention. In this embodiment, an annular chamber 56 is divided off in the 
housing 14 by a cylindrical inner wall 50, the chamber 56 accommodating a 
body of electrically-conductive fluid 23. Sensors 57 to 61 extend 
downwardly into the chamber 56 from the cover 19. The chamber 56 is 
divided by four immersible upright divider walls 102, 103, 104 and 105 
into four segmental or arcuate chambers or compartments 56a, 56b, 56c and 
56d. These immersible divider walls 102, 103, 104, 105 terminate at a 
short distance, i.e. are spaced from the floor 16 of the housing 14 so as 
to leave passageways between adjacent ones of the compartments 56a-56d, 
and they are immersed in the electrically-conductive fluid 23 the level of 
which is indicated by the numeral 24. The immersible divider walls 102 to 
105 may extend up to the cover 19; however, in the arrangement shown, they 
terminate at, i.e. are spaced a short distance below the cover. As can be 
seen from FIG. 8, the four immersible divider walls 102 to 105 are 
arranged at angles of 90.degree. to each other about the central axis of 
the housing 14, and preferably in such a way that they all extend at a 
horizontal angle of about 45.degree. to the longitudinal axis, and thus to 
the normal straight-ahead direction of travel of the motor vehicle on 
which the switching device 10 is mounted, which direction is indicated by 
the arrow F. 
The immersible divider walls 102 to 105 may be formed integrally with the 
cylindrical outer and inner side walls 15 and 50 of the housing 14. 
Alternatively, they may be bonded to the walls 15 and 50 or secured 
thereto in some other way. 
Each segmental chamber 56a, 56b, 56c and 56d contains five immersible 
sensors 57, 58, 59, 60 and 61 of differing lengths and angularly spaced 
apart around the chamber axis, with the sensors of each group thereof 
angularly spaced approximately 90.degree. apart around the chamber axis, 
as shown. All the sensors 57 to 61 are press-fitted into the cover 19, and 
are sealed thereagainst by means of a seal 21. The immersible sensors 57 
have the greatest length so that their bottom ends lie closest to the 
housing floor 16, and they extend into the fluid 23 when the housing axis 
is vertical. The sensors 57 are all connected to the negative terminal 13 
of the alarm circuit. The immersible sensors 58, 59, 60 and 61 in each 
pair of diametrically opposed segmental chambers 56a, 56, and 56b, 56d, 
are connected by conductors 46, 47, 48 and 62, respectively, to an 
electronic analysis device 49 for the digital analysis of the orientation 
or inclination of the housing 14. The analysis device 49 is disposed in 
the interior 52 of the housing 14. As with the embodiment of FIGS. 1 to 3, 
the conductors 46, 47, 48 and 62 are in the form of a printed circuit 
provided an the surface of the cover 63. 
When a motor vehicle equipped with the switching device 10 of FIGS. 7 and 8 
is parked and the alarm unit is primed, the level 24 of the fluid 23 in 
the housing 14 is detected by the sensors 57 to 61. If the vehicle, and 
therefore the housing 14, is then tilted, the fluid 23 touches other 
sensors, and, after a certain time-lag, the analysis device 49 sets off 
the alarm. If, on the other hand, the vehicle, and therefore the housing 
14, is swung or rocked backwards and forwards, then, although the fluid 
level 24 alters, it assumes its original position again after a certain 
time, so that the alarm is not set off even though in the meantime other 
sensors have come into contact with the fluid. During the above-mentioned 
swinging moment, the movement of the fluid 23 in the housing 14 is 
retarded and damped down by the immersible divider walls 102, 103, 104 and 
105 so that a surface wave of the fluid 23, which moves right around the 
annular chamber 56 and would cause a false alarm, cannot occur. 
FIG. 9 shows a fifth form of switching device 10 according to the invention 
having immersible sensors 56, 58, 59 and 60 arranged in two rows crossing 
each other at right-angles in a hollow, cylindrical housing 14. The 
housing 14 is divided into four sectors 106, 107, 108 and 109 by 
immersible radial divider walls 102, 103, 104 and 105 which cross each 
other at right-angles at the center of the housing. The floor 16 of the 
housing 14 may be dished. The immersible sensors 57, which are connected 
to the negative terminal 13 of the alarm circuit, are located in the 
central region of the housing 14, and extend almost to the floor 16. Thus, 
in any position of the housing 14 they reach into the body of 
electrically-conductive fluid 23 which, as in FIG. 5, covers part of the 
housing floor 16. The sensors 58, 59 and 60 are shorter than the sensors 
57 and of differing lengths, and they are interconnected by respective 
conductors which lead to the electronic analysis device 49. Thus, the 
sensors 58, 59 and 60 define switching surfaces at different levels in the 
housing 14, which levels are reached by the fluid in the housing when the 
vehicle is in an inclined position. The immersible divider walls 102 to 
105 retard and damp down the movements of the fluid 23 when the vehicle is 
swung or rocked backwards and forwards.