Seat belt warning system
A safety belt warning system for vehicles comprises a sensor mat for the detection of a force acting on its surface, which includes, for the formation of two electrical capacitors, two dielectric layers which are located one above the other in sandwich fashion and arranged in each case between electrically conductive coats, and which have different compressibility at least in the direction of loading due to force, so that the capacitances of the two capacitors vary differently with a load on the sensor mat.
1. A seat belt warning system comprising
a sensor mat having a surface and adapted for detecting a force acting upon the surface in a direction, said sensor mat comprising
a first conductive layer, a second conductive layer and a third conductive layer disposed in parallel, spaced relationship;
a first dielectric layer interposed between the first conductive layer and the second conductive layer and having a first compressibility in response to the force, said first conductive layer, said first dielectric layer and said second conductive layer forming a first electrical capacitor acted upon by the force and characterized by a first capacitance that varies dependent upon the force; and
a second dielectric layer interposed between the second conductive layer and the third conductive layer and having a second compressibility in response to the force, said second compressibility being different from the first compressibility, said second conductive layer, said second dielectric layer and said third conductive layer forming a second electrical capacitor acted upon by the force and characterized by a second capacitance that varies dependent upon the force different from the first capacitance.
2. The seat belt warning system according to claim 1 , characterised in that the second dielectric layer is substantially incompressible.
3. The seat belt warning system according to claim 1 , characterised in that the first capacitance and the second capacitance are substantially equal when no force is applied to the sensor mat.
4. The seat belt warning system according to claim 1 , further comprising an evaluating device adapted to provide a belt warning signal based upon the first capacitance and the second capacitance.
5. The seat belt warning system according to claim 4 , wherein the evaluating device is configured to provide the belt warning signal based upon a variation in a capacitance of the first capacitor relative to a variation in a capacitance of the second capacitor.
6. The seat belt warning system according to claim 4 , wherein the evaluating device comprises a bridge circuit having a first branch and a second branch, said first capacitor being associated the first branch and the second capacitor being associated with the second branch.
7. The seat belt warning system according to claim 6 , wherein the bridge circuit is balanced when no load is applied to the sensor mat.
8. The seat belt warning system according to claim 1 , wherein the first dielectric layer has a high compressibility relative to the second dielectric layer and is arranged above the second dielectric layer.
9. The seat belt warning system according to claim 1 , wherein the sensor mat comprises a flexible film, said first conductive layer being formed by a metal coating applied to a first portion of the flexible film, said second conductive layer being formed by a metal coating applied to a second portion of the flexible film, said third conductive layer being formed by a metal coating applied to a third portion of the flexible film, said flexible film being folded to form a first intermediate space adapted to receive the first dielectric layer and a second intermediate space adapted to receive the second dielectric layer.
10. The seat belt warning system according to claim 9 , wherein the flexible film is formed of a PET or PEN composition.
11. The seat belt warning system according to claim 9 , wherein the metal coatings are composed of copper.
12. The seat belt warning system according to claim 1 , wherein at least one of the first dielectric layer and the second dielectric layer is air-permeable.
13. The seat belt warning system according to claim 1 , wherein one of the first dielectric layer and the second dielectric layer has a higher compressibility and is composed of silicone rubber.
14. The seat belt warning system according to claim 1 , wherein one of the first dielectric layer and the second dielectric layer has a lower compressibility and is formed of a three-dimensional mesh structure.
15. The seat belt warning system according to claim 1 , wherein the first dielectric layer has a first layer temperature expansion coefficient or a first layer moisture expansion coefficient, and the second dielectric layer has a second layer temperature expansion coefficient substantially equal to the first layer temperature expansion coefficient or a second layer moisture expansion coefficient substantially equal to the first layer moisture expansion coefficient.