The so-called smart entry system is already put into practical use in vehicle doors. According to the smart entry system, once a capacitive sensor incorporated in the door handle has detected the user touching the door handle in such a manner that suggests the intention of the user to open the door, an ID code matching is made between a card key carried by the user and an onboard control unit, and the door may be unlocked only when an ID code matching is detected.
In a known door handle device for such a smart entry system, a circuit board for detecting a change in an electrostatic capacitance is incorporated in the door grip that is held by the user in his/her effort to open the door. Such a door handle device is disclosed, for instance, in U.S. Pat. No. 6,740,834 assigned to the same assignee. The contents of this patent are hereby incorporated by reference.
In conventional door handle devices such as the one disclosed in the aforementioned US patent, the circuit board for detecting changes in the capacitance is individually designed for each vehicle model. Therefore, when an attempt is made to manufacture door handle devices for different vehicle models in the same manufacturing line, the management of the required component parts becomes unacceptably complex.
To eliminate such an inconvenience, it is conceivable to design a universal capacitive sensor unit and apply the capacitive sensor unit of the same design to different vehicle models. However, it has not been possible because it necessitates the door handle to be adapted to the shape of the sensor unit.
A door handle is required to have a relatively large and uniform thickness to avoid any sink marks or other flaws to be produced on the outer surface and to give the door handle a required mechanical strength. Therefore, it is not practical to fit a common sensor unit to door handles of various different designs. In particular, if any gap is formed between the sensor unit and the inner wall of the cavity in the door handle, it means that an air layer having a low dielectric coefficient is interposed between the surface which the user touches and a detection electrode, and this reduces the detection sensitivity of the capacitive sensor. As can be readily appreciated, a vehicle door handle is designed as a part of the overall design of the vehicle body, and may significantly vary in size and shape for each particular design of the vehicle body.
It is sometimes desired that a metallic component to be placed on the outer surface of the door handle or apply a metallic plating to a part of the outer surface of the door handle for aesthetic reasons, In such a case, when a part of the metallic part is brought close to the capacitive sensor, the metallic part may form a part of the electric circuit for the capacitive sensor so that the capacitive sensor may be inadvertently activated when a part well outside the grip portion is touched. It means that the door may be unlocked simply by slightly touching the outer face of the door handle even without the user intending to unlock the door, and this obviously is not desirable.