Automotive security systems often include immobilizer devices, i.e., devices that prevent unauthorized use of a vehicle. Generally, an immobilizer protects a vehicle in one of three ways: (1) disabling the vehicle's starter circuit to prevent unauthorized starting of the vehicle; (2) disabling the vehicle's ignition system, engine computer, or another system necessary for the vehicle to run (“necessary system”); or (3) disabling both the starter circuit and the engine computer or another necessary system.
Because of safety concerns, care should be exercised when immobilizing a running vehicle. For example, it is considered preferable to disable the vehicle's engine after the vehicle reaches a full stop, or is near a full stop. But even an immobilizer properly designed not to disable an engine in a moving vehicle can malfunction. The malfunction can be a simple software “glitch” from which the immobilizer can recover, or it can be a complete failure, such as a power disconnect. Whatever the nature of the malfunction or failure, it can lead to disabling the vehicle while it is moving. It is desirable to provide an immobilizer device that reduces the likelihood of disabling a running vehicle in the event of immobilizer malfunction.
One immobilizer design relies on normally closed relay contacts in a circuit that enables a necessary system, for example, the ignition system. When the relay is de-energized, the normally closed contacts are held in contact with each other by a spring. Such structure, however, has a self-resonant frequency. Therefore, the contacts can come apart due to external vibration and shock. For example, the normally closed contacts can come apart when the relay vibrates at a frequency that is close to the self-resonant frequency of the contacts-spring structure, or when the relay is subjected to a sudden shock of short duration, which approximates a delta function and has one or more spectral components near the self-resonant frequency. In comparison, normally open contacts held together by electromagnetic force provide a more reliable connection. It follows that immobilizer design that relies on normally closed contacts renders the ignition circuit (or another necessary system connected to the immobilizer) susceptible to environmental effects, such as shock and vibration, and more prone to failure. It would be desirable to provide an immobilizer device that does not rely on normally closed relay contacts.
The immobilizer can also malfunction when the vehicle it not running, possibly preventing subsequent starting of the vehicle. Some prefer that the vehicle not be operable when a malfunction occurs, particularly when the malfunction is caused by a power disconnect; this is generally the case in the United Kingdom and other European countries. In contrast, others prefer that the vehicle be drivable until the security system is repaired; this is generally the case in the United States. It would be desirable to provide an immobilizer circuit that could be easily configured for each of these preferences, and that would reduce the likelihood of immobilizer malfunction causing a failure mode contrary to the configured preference.
A need thus exists for immobilizer-equipped security systems that reduce the likelihood of disabling a running vehicle because of immobilizer malfunction, but do not rely upon normally closed relay contacts to accomplish this. Still another need exists for immobilizer-equipped security systems that can be easily configured either to prevent or to allow a non-running vehicle to start in the event of immobilizer failure or a deliberate attempt to disable the immobilizer function, while at the same time protecting against unintentionally disabling a running vehicle due to immobilizer malfunction.