Anti-magnetic tampering system for automobile ignition lock

A lock for the ignition circuit for an automobile including a Hall effect control sensor which is activated by a control magnet on the cylinder member of the lock so that when the cylinder is rotated by the key to its start position, the control sensor is activated. The activation of the control sensor provides one of several starting criteria to an ignition for starting an automobile. An anti-tampering sensor spaced from and adjacent to the control sensor has a trip level which is lower than the trip level of the control sensor so that magnetic forces induced from the exterior of the lock cannot trip the control sensor. A blocking plate is disposed between the control sensor and the anti-tampering sensor to prevent the anti-tampering sensor from being tripped by the control magnet when the cylinder member is rotated to its start position and to enable the anti-tampering sensor to be tripped first to produce a deactivation signal in response to a magnetic force induced from the exterior of the lock.

BACKGROUND OF THE INVENTION 
The present invention relates to locks, and more particularly to an 
anti-tampering mechanism for the ignition lock of an automobile. 
Various types of locks for use in connection with the ignition circuit of 
an automobile are known in the art. Many of such locks include anti-theft 
and/or anti-tampering mechanisms which are incorporated to deter 
unauthorized use of automobiles. One such lock is disclosed in U.S. Pat. 
No. 5,202,580. The lock includes a Hall effect sensor dement which is 
activated by a control magnet on the cylinder of the lock so that when the 
cylinder is rotated by a key to its start position, the Hall effect sensor 
element is activated. The activation of the sensor element provides one of 
several starting criteria to an ignition circuit for starting an 
automobile. An anti-tampering magnet on the cylinder lies directly under 
the sensor element in the off position and biases the Hall effect sensor 
element to its off position so hard that other magnetic forces induced 
from the exterior of the lock cannot overcome the bias and activate the 
sensor. 
Other anti-theft and/or anti-tampering mechanisms which utilize magnetic 
elements are shown in U.S. Pat. Nos. 4,862,139 and 4,546,266. Although 
many of such devices are effective for their intended purpose, there 
remains a need for improved devices of this type. 
SUMMARY OF THE INVENTION 
A lock for the ignition circuit of an automobile that utilizes two magnetic 
field sensors in combination with a magnetic field blocking means disposed 
between the two sensors. The first sensor is a control sensor which 
functions in the conventional manner with a permanent magnet on the 
rotating cylinder. In other words, the lock includes a control Hall effect 
sensor element which is activated by a control magnet on the cylinder 
member of the lock so that when the cylinder is rotated by the key to its 
start position, the control Hall effect sensor element is activated which 
in turn enables the ignition circuit to start an automobile. The second 
sensor is an anti-tampering sensor which is spaced from the control 
sensor. The control sensor trip level is biased high while the 
anti-tampering sensor trip level is biased low in order to prevent 
actuation or tapering with the control sensor through the use of an 
external magnet. The blocking means is preferably in the form of a ferrous 
plate and has two functions. First, the blocking means prevents the 
anti-tampering sensor from being tripped by the control magnet on the 
cylinder member when the cylinder is rotated to its start position. 
Secondly, the blocking means causes the anti-tampering sensor to be 
tripped first which provides a false or deactivation signal to the 
ignition circuit should an external magnet be employed from the exterior 
of the lock in an attempt to actuate the control sensor. 
Accordingly, the present invention provides a lock for an automobile 
ignition circuit comprising a hollow sleeve defining a cylindrical inner 
surface; a cylinder rotatably mounted within the sleeve and having a 
cylindrical outer surface defining a cylindrical interface with the inner 
face of the sleeve, said cylinder member having a keyway therein for 
receiving a key used to rotate the cylinder between off and start 
positions; a control Hall effect sensor on the sleeve for detecting when 
the cylinder member is rotated to its start position, said control sensor 
having a first trip level for producing an activation signal; a control 
magnet on the cylinder member for activating the control sensor when the 
cylinder member is rotated to its start position; an anti-tampering Hall 
effect sensor spaced from and adjacent to the control sensor for producing 
a deactivation signal, said anti-tampering sensor having a second trip 
level which is lower than the trip level of the control sensor; and 
blocking means disposed between the control sensor and the anti-tampering 
sensor to prevent the anti-tampering sensor from being tripped by the 
control magnet when the cylinder member is rotated to its start position 
and to enable the anti-tampering sensor to be tripped first to produce the 
deactivation signal in response to a magnet force induced from the 
exterior of the lock. 
The blocking means preferably comprises a plate composed of a ferrous 
material. The plate may be fiat so that the control sensor is located 
adjacent its inner surface and the anti-tampering sensor is located 
adjacent its outer surface, or the plate may be H-shaped defining inner 
and outer chambers with the control sensor located within the inner 
chamber and the anti-tampering sensor located within the outer chamber. In 
addition, the control sensor and anti-tampering sensor are spaced radially 
with respect to each other and preferably in radial alignment with each 
other. However, the sensors may also be spaced axially and/or 
circumferentially in a staggered relationship to each other.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to the drawings, FIGS. 1 and 2 illustrate an ignition lock 
generally designated by the numeral 1 constructed in accordance with the 
principles of the present invention. As illustrated, lock 1 includes a 
hollow cylindrical sleeve 2 which in turn is fixed within a housing 3. 
Housing 3 may, for example, comprise a steering column of an automobile, 
although it is within the scope of the present invention to mount ignition 
lock 1 in any desired location. 
Sleeve 2 includes a cylindrical inner surface 4 and receives an elongated 
rotatable cylinder member 5 therein. Cylinder number 5 includes a 
cylindrical outer surface 6 which forms a rotational interface with inner 
surface 4 of sleeve 2. Cylinder member 5 is rotatable about longitudinal 
axis 7, and includes a keyway 8 along the axis 7 for receiving a key (not 
shown) therein. Cylinder member 5 is rotatable between an off position, as 
shown in FIG. 2 and a start position wherein cylinder member 5 is rotated 
in a clockwise direction from the position shown in FIG. 2 until magnet 10 
is located beneath control sensor 9, as will hereinafter be described. 
When activated, sensor 9 produces an activation signal which is utilized 
by the automobile's, ignition circuit to start the automobile. After 
starting, cylinder 5 rotates in a counter-clockwise direction from the 
start position to a run position, as is conventional. Reference is made to 
the description contained in allowed U.S. Patent application Ser. No. 
07/946,017 filed Sep. 15, 1992, the disclosure of which is specifically 
incorporated herein by reference, for a complete detailed description of a 
typical electronic interlock control circuit for an automobile's ignition. 
A control sensor element 9, preferably a Hall effect sensor device, is 
mounted in a sensor module 12 on sleeve 2. Control sensor 9 functions to 
detect the rotational position of cylinder member 5 in order to insure 
that the ignition circuit for starting the engine of an automobile is 
operable only when a proper key is utilized to rotate cylinder member 5. 
In order to accomplish this, control sensor 9 is located along the 
rotational interface of surfaces 4 and 6 in order to sense the change in 
magnetic flux of a control magnet 10 located on cylinder member 5. Control 
magnet 10 is located along outer surface 6 and is positioned such that 
rotation of cylinder member 5 causes control magnet 10 to rotate past 
control sensor 9. Magnet 10 has a magnetic pole orientated radially 
outwardly at surface 6. Thus, when control magnet 10 passes control sensor 
9, sensor 9 is activated or enabled to produce an activation signal which 
in turn provides one of several starting criteria to the starting module 
(not shown) of an automobile's ignition circuit. 
As is readily apparent, control sensor 9 may easily be compromised when 
cylinder member 5 is in its off position by employing an external magnet 
i.e. one located outside of sleeve 2 and/or housing 3. This may be 
accomplished by simply moving a magnet of sufficient strength adjacent to 
housing 3 so that it produces a magnetic flux of sufficient strength to 
trip control sensor 9. In order to prevent this from occurring, lock 1 
includes an anti-tampering sensor element 11, preferably a Hall effect 
sensor device, mounted in sensor module 12 on sleeve 2. As shown best in 
FIGS. 1 and 2, sensor element 11 is mounted in radial alignment with 
sensor element 9. However, other orientations of sensor elements 9 and 11 
may also be employed, such as ones where sensors 9 and 11 are axially 
spaced with respect to each other, circumferentially spaced with respect 
to each other, and/or other staggered relationships. Anti-tampering sensor 
11 functions to provide a deactivation signal to the ignition circuit in 
response to a magnetic force induced from the exterior of the sleeve 2 or 
housing 3. This is accomplished by setting the trip level for 
anti-tampering sensor 11 lower than the trip level control sensor 9, i.e. 
sensor 11 is biased low whereas sensor 9 is biased high. Thus, magnetic 
forces applied from the exterior of sleeve 2 or housing 3 would cause 
anti-tampering sensor 11 to be tripped first, thus resulting in a 
deactivation signal. The magnetic forces would be insufficient to enable 
control sensor 9 without also enabling anti-tampering sensor 11. 
However, due to the fact that the trip level of anti-tampering sensor 11 is 
lower than the trip level of control sensor 9, it is possible that 
anti-tampering sensor 11 might become enabled or tripped by control magnet 
10 when magnet 10 is rotated with cylinder member 5 from its off position 
to its start position beneath control sensor 9. Therefore, in order to 
prevent anti-tapering sensor 11 from being tripped when an automobile is 
started, the lock 1 includes a magnetic flux blocking means disposed 
between control sensor 9 and anti-tapering sensor 11. As shown best in 
FIG. 2, this blocking means comprises a plate 13, preferably composed of a 
ferrous material. As shown in the embodiment of FIGS. 1 and 2, plate 13 is 
substantially fiat and defines a radially outer surface and a radially 
inner surface such that control sensor 9 is located adjacent its inner 
surface and the anti-tampering sensor 11 is located adjacent its outer 
surface. Thus, plate 13 functions to attenuate the magnetic field of 
control magnet 10 from reaching anti-tampering sensor 11 during a stating 
operation. Likewise, plate 13 attenuates the magnetic filed supplied by an 
external tampering magnet from reaching the control sensor 9. This is 
accomplished since the ferrous plate 13 acts as a pole piece increasing 
the magnetic field to the sensor on whichever side of the plate the magnet 
is located, i.e. the lines of magnetic flux are more concentrated by plate 
13, and are prevented from passing through plate 13 by its ferrous 
composition which provides a return path for the lines of flux. 
Referring now to FIG. 13, there is illustrated a second embodiment for the 
lock of the present invention. In this embodiment, the lock also includes 
a cylinder member 15 rotatably mounted within a sleeve 16 together with a 
control sensor 17 and anti-tampering sensor 18 mounted within a module 19. 
In this embodiment, however, the blocking means comprises an H-shaped 
plate 20, once again preferably made of a ferrous material. The H-shaped 
plate 20 defines a radially outer chamber and a radially inner chamber 
with the control sensor 17 located within the inner chamber and the 
anti-tampering sensor 18 located within the outer chamber. In all other 
respects, plate 20 functions identically as fiat plate 13 to prevent 
anti-tampering sensor 18 from being tripped by the control magnet on 
cylinder member 15, and to cause the anti-tampering sensor 18 to be 
tripped first should an external magnet be employed in an attempt to 
actuate control sensor 17. 
In operation, an operator of an automobile would first insert the 
appropriate key into cylinder member 5 and rotate cylinder member 5 in a 
clockwise direction until control magnet 10 passes control sensor 9 at 
which time sensor 9 is enabled or activated. When cylinder member 5 is 
returned to its off position, i.e. the position shown in FIG. 2, 
anti-tampering sensor 11 and plate 13 prevent compromising of the control 
sensor 9 by an external magnet. Thus, upon removal of a key by the 
operator, external magnetic forces cannot activate or enable sensor 9 
without also activating or enabling sensor 11. 
Various modes of carrying out the invention are contemplated as being 
within the scope of the following claims particularly pointing out and 
distinctly claiming the subject matter regarded as the invention.