1. Technical Field
The present invention relates generally to signal transmission and, more specifically, to keyless entry devices for vehicles.
2. Background Art
Remote electronic transmitters are commonly used for many applications to transmit codes or other control signals to receiving units. One common application has been in keyless entry devices, small hand-held transmitters used to lock and unlock vehicles and/or control vehicle security systems. These transmitters allow a person to easily lock the doors, turn on an alarm, and/or disable the vehicle by transmitting an appropriate signal to a receiving control unit. Likewise, these transmitters allow a person to unlock the doors, turn off the alarm, and/or enable the vehicle by transmitting an appropriate signal to the receiving control unit. These transmitters thus have the advantage of allowing easy access to the vehicle and improved control over any alarm and disabling system.
In the past these transmitters have used infrared, radio frequency, or other wireless transmission techniques to transmit the control signal from the keyless entry device to the receiving control unit. Unfortunately, by broadcasting these control signals they become available to other receivers. Thieves can thus use their own receivers to discover the transmitted control signal, and then use their own transmitter to transmit the control signal and unlock the doors, turn off the alarm, and/or enable the car.
This technique, often referred to as "grabbing" or "snooping", has become so widespread that a great deal of effort has been put into "anti-grabbing" protection devices. Such devices have included the use of algorithmic code encryption as well as code rejection techniques. Unfortunately, these techniques have not proven to totally effective in preventing grabbing. Thus, these arrangements continue to suffer from the disadvantage that the control signal is transmitted in a way that is detectable by other remote receivers.
Another technique for preventing grabbing was described in Great Brittan Patent No. 2 290 435, entitled "Signal Transmission System." This patent describes a method of transmitting a signal through an electrically conductive probe to a receiver circuit on the automobile. The conductive probe is selected to be substantially non-radiative, meaning that the probe will only transmit to the receiver through when the probe is proximate a conductive body (e.g. the automobile body) connected to the receiver. Because the signal is not transmitted as a radiative radio frequency signal it is not available to be grabbed by other receivers. Thus, the system described in this patent provides a transmission system that overcomes the disadvantages of the previous systems.
One disadvantage with this system is the difficulty in using the signal transmitting probe. The transmitting signal probe described in this patent is a metal tipped device. To effectively use this device, a person must hold the metal tipped probe against the car. Unfortunately, it is difficult to make an effective electrical contact with the metal tipped probe. The metal tip on the probe has a tendency to "bounce" off the metal car body and thus it is difficult to maintain a consistent conductive path to the receiver on the car. This is primarily because the metal probe is restricted in shape and cannot conform to uneven surfaces. This results in a relatively small contact surface area between the metal tipped probe and the car, reducing the conductivity of the contact between the metal tipped probe and the car. Additionally, the small size of the contact area means that metal tipped probe must be placed directly against a metal portion of the car, something that can be difficult with modern cars that make significant use of plastics and other non-conductive materials. The results is that a user may have to make multiple attempts before the control signal is received. This can annoy the user and waste the transmitter's power. Thus, while the system disclosed in this patent reduces the problems of code grabbing, there remains implementation difficulties.
Thus, there remains a need for improved vehicle entry system that reduces the problems of code grabbing while providing consistent performance and ease of use.