Flexible capacitive strip for use in a non-contact obstacle detection system

[0002] This application claims the benefit of U.S. provisional application No. 60/330,171, Attorney Docket No. DP-304424, filed Oct. 17, 2001, the contents of which are incorporated by reference herein in their entirety.

[0003] This application further claims the benefit of U.S. provisional application No. 60/330,173, Attorney Docket No. DP-306074, filed Oct. 17, 2001, the contents of which are incorporated by reference herein in their entirety.

[0004] This application further claims the benefit of U.S. provisional application No. 60/361,803, Attorney Docket No. DP-304424, filed Mar. 5, 2002, the contents of which are incorporated by reference herein in their entirety.

[0005] The present disclosure relates generally to proximity detecting systems and, more particularly, to a flexible capacitive strip for use in a non-contact obstacle detection system, such as may be implemented in conjunction with a motor vehicle power lift-gate, power operated device, or perimeter system.

[0006] Various systems have been devised for detecting obstacles in the path of a moveable panel such as an automotive power window, power sliding door or power hinged door. When an obstacle is detected, forward movement (e.g., closing) of the panel is interrupted and, optionally, the movement of the panel may be thereafter reversed (e.g., opened). These detection systems may generally be characterized as either “contacting” or “non-contacting”. In a contacting system, an obstacle is detected only after some form of physical contact occurs between the panel and the obstacle, and may include devices such as pneumatic/pressure sensitive strips, or possibly sensors responsive to changes in mechanical or electrical loading in the apparatus that moves the panel.

[0007] On the other hand, in a non-contacting system, an obstacle is detected before actual contact occurs. One specific type of non-contacting obstacle detection system employs the use of a capacitive element(s) as a proximity sensor(s). Capacitive proximity sensors may include one or more electrical conductors formed along the leading edge of a moveable panel, as well as a capacitance sensitive circuit (e.g., a bridge circuit or an oscillator) coupled to the conductor(s). An obstacle (e.g., a human hand) in proximity to the conductor(s) changes the capacitance of the sensor, and the change is thereafter detected by the capacitive sensitive circuit.

[0008] Unfortunately, certain problems may arise in creating an appropriate capacitive proximity sensor having the requisite nominal, steady-state capacitance desired for high-sensitive applications, such as the non-contacting obstacle detecting system described above. First, the nominal capacitance value of a capacitive sensing device is inherently dependent upon (among other parameters) the physical dimensions and relative positions of the capacitor electrodes with respect to one another. More specifically, the degree to which the capacitor electrodes are dimensionally consistent with one another along their length will have an effect upon the characteristics of the device. Given the fact that a sensor could be located upon a contoured surface such as a vehicle pillar, panel or moveable lift-gate, the prospect of forming a reliable, sensitive capacitive proximity sensor can therefore be difficult.

[0009] Furthermore, it may also be desirable for the sensor to be located in a “tight” area that effectively limits the profile of the sensor, and/or provided in an outdoor setting (such as upon a motor vehicle) that is necessarily exposed to moisture. These variables also have an effect on capacitance and, as such, should be taken into account when designing a high-sensitivity, precision capacitive proximity sensor.

[0010] The above discussed and other drawbacks and deficiencies of the prior art are overcome or alleviated by a flexible, capacitive strip for use in a non-contact obstacle detection system. In an exemplary embodiment, the strip includes an elongated body for flexible mounting to a panel along a bottom surface of the elongated body. A first elongated planar conductor is contained within an upper section of the elongated body, and a longitudinal cavity is formed through a central portion of the elongated body, the longitudinal cavity being disposed between the planar conductor and the bottom surface. The first elongated planar conductor forms a first electrode of a sensing capacitor and the longitudinal cavity defines a portion of a dielectric material of the sensing capacitor.

[0011] In a preferred embodiment, the first elongated planar conductor is a first electrode of a sensing capacitor coupled to the capacitance detector circuit. The longitudinal cavity comprises a portion of a dielectric material of the sensing capacitor. In addition, the panel comprises a second electrode of the sensing capacitor. The planar conductor is preferably a flexible material, such that a substantially constant distance is maintained between the first elongated planar conductor and the panel. The elongated body is formed by extrusion of an insulating material such as santoprene rubber.

[0012] Referring to the exemplary drawings wherein like elements are numbered alike in the several Figures:

[0013] FIG. 1 is a partial perspective view of a flexible capacitive strip that may be used in a non-contact obstacle detection system, in accordance with an embodiment of the invention;

[0014] FIG. 2ais a lateral cross-sectional view of the capacitive strip of FIG. 1, shown mounted to a panel;

[0015] FIG. 2bis a lateral cross-sectional view of an alternative embodiment of FIG. 2a; and

[0016] FIG. 2cis an end view of still an alternative embodiment of the capacitive strip.

[0017] A flexible capacitive strip is disclosed herein. The strip is inexpensive to manufacture, and has a suitably low nominal capacitance (on the order of about 20 picofarads (pF), for example) while still maintaining three dimensional flexibility for mounting to a contoured surface such as on a motor vehicle. The strip has a relatively low profile so as to be able to fit between, for example, a vehicle pillar and a door that closes upon the pillar. In addition, the strip is preferably formed so as to provide moisture resistant protection for the electrode(s) contained therein. In a further aspect, the capacitive sensor strip is also configured for optionally housing capacitance detection circuitry therein.

[0018] Referring initially to FIGS. 1 and 2a, there is shown a perspective and a lateral cross-sectional view (respectively) of a flexible capacitive sensor strip 12. In accordance with an embodiment of the invention, the sensor strip 12 has a flexible, elongated body 14 with an adhesive strip 16 affixed to a bottom surface 18. The adhesive strip 16 provides a means for securing the strip 12 to a metal panel 20 (e.g., a motor vehicle pillar or lift-gate). The sensor body 14 is preferably formed by extrusion of an insulating, flexible material such as santoprene rubber. Having a dielectric constant or property of 2.3. Of course, other materials having other dielectric properties are contemplated for use in the present disclosure.

[0019] A flexible conductor 22 serves as a first electrode of the capacitive strip 12, and is insert molded into an upper portion 24 of the sensor body 14. The conductor 22 is further disposed substantially parallel to the bottom surface 18 and thus should be substantially parallel to the metal panel 20 when the strip 12 is attached thereto, regardless of the particular contours of the panel 20. Preferably, conductor 22 is a flat, braided conductor of tinned copper or the like so as to be flexible in any direction without being damaged, and without resulting in any change to its electrical characteristics along the length thereof. One example of a suitable choice for conductor 22 includes a flat tinned copper braid manufactured by the Alpha Wire Company. The santoprene rubber sensor body 14 covers the entire surface of the flat braid conductor 22 to prevent it from being exposed to moisture and, in addition, the extrusion of the rubber provides a cost effective method for producing several strips 12 (that may be cut to a desired length) from a single process.

[0020] In addition to containing the flexible conductor 22, the sensor body 14 further includes a central longitudinal cavity 26 between the upper portion 24 and the lower surface 18. In one aspect, the cavity 26 acts as an air core (i.e., dielectric) separating the conductor 22 from the metal panel 20. In this particular embodiment, the metal panel also serves as a second electrode or ground plane of the strip 12. However, it should also be appreciated that in applications where the strip 12 is to be affixed to a nonmetallic panel or a nonmetallic carrier, a second elongated conductor may be insert molded into the body 14 between the cavity 26 and the bottom surface 18, as is indicated in phantom by the conductor 28 in FIG. 2a.

[0021] FIG. 2billustrates an alternative embodiment of FIG. 2a. Instead of attaching the strip 12 to a metal panel by means of an adhesive strip, the sensor strip 12 may include grooves 27 formed within the outer sides of body 14. The strip 12 may then be slid (by way of grooves 27) into a plastic carrier 29 that holds the strip in place. In turn, the plastic carrier 29 may then be attached to a vehicle surface (not shown), for example, at an appropriate place. It will be noted, however, that in this embodiment, the conductor 28 is used as the second capacitor electrode as there is no direct contact between the strip 12 and a metal surface.

[0022] In general, the capacitance C of the sensor strip 12 may be determined from the following equation:

[0023] C=(ε_oε_rA)/d

[0024] where ε_ois the permittivity of free space (8.85 pF/m), ε_ris the relative permittivity (dielectric constant) of the material(s) used to form the strip body 14 (in this case, air and santoprene rubber), A is the surface area of the braided conductor 22, and d is the distance between the electrodes of the capacitor within the strip 12. In the embodiment wherein the panel 20 forms the ground plane of the strip 12, d is the distance from the braided conductor 22 to the panel 20. Alternatively, d is the distance from the braided conductor 22 to the second braided conductor 28.

[0025] Accordingly, it will be appreciated that the above described configuration for the capacitive strip 12 is advantageous in that its flexibility allows it to conform to the profile of a contoured surface such as a motor vehicle while still providing a uniformly shaped conductor therein that can remain substantially parallel to the contoured surface itself. Moreover, the santoprene rubber material, among other advantages, provides protection for the conductor from outside elements, such as moisture, which could otherwise adversely affect the sensitivity of the strip 12. It will further be noted that the size of the strip 12 may be tailored according to the specific application, taking into consideration the value of capacitance desired to be used in the associated sensing circuitry. For example, a capacitive strip 12 intended for use in a capacitive proximity sensor for a motor vehicle lift gate may have a width of about 10 millimeters (mm) and a thickness or height of about 8 mm. Again, however, it will be understood that the general configuration for the capacitive strip may have several other applications and, thus, other sizes.

[0026] When the capacitive strip 12 is used in conjunction with some type of capacitance detection circuitry, an external connection may be needed to connect the conductor 22 to the circuitry. Accordingly, any external wire, lead, or other connection to conductor should also be provided with a sealing material so as to maintain the protection of the conductor 22 from moisture and other conditions. This is also the case for any end of the strip 12 that has been cut to a desired length; a cut end of the strip 12 is preferably also provided with a protective end mold (e.g., rubber) to seal the tip of the conductor 22.

[0027] Finally, FIG. 2cillustrates an end view of an alternative embodiment of the capacitive strip 12. The capacitive strip 12 may have a cross section that is generally trapezoidal in shape, wherein the side surfaces 66 of the strip are upwardly tapered as determined by an acute angle, α, with respect to the bottom surface 18. In addition, the cavity 26 may be formed to include curved protrusions 68 within the material of the strip 12. Thus configured, strip 12 may be made more resistant to inward compression of the side surfaces 66. If pressure is applied to the upper portion 24 of the strip 12, the upper portion 24 will move straight downward, but will more readily return to its original position when the compressive force is removed.

[0028] Additionally, cavity 26 provides a means or area for receiving and holding an electronic circuit configured for use with the capacitive strip of the present disclosure.

[0029] While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.