JP-5-116592A and JP-7-190732A propose an optical-fiber type collision detecting apparatus for a vehicle, that extends an optical fiber throughout, for example, the front surface of the vehicle and uses a light-leaking optical fiber sensor for detecting a decrease in the amount of transmission of light of the optical fiber caused by the collision load of the vehicle. Moreover, the patent document 1 discloses: that the optical-fiber type collision detecting apparatus for a vehicle includes a cylindrical collision load transmitting member having a protrusion, in which an optical fiber sensor is built, so as to concentrate collision load from the outside on a predetermined local portion of the optical fiber to increase the amount of local bending of the optical fiber; and that the optical fiber is covered with an elastic body such as rubber.
When the above-described conventional optical-fiber type collision detecting apparatus for a vehicle is interposed between the bumper reinforcement and the front bumper of the vehicle, there is advantageous to the high-speed detection of a collision of a pedestrian with the respective portions of the front bumper. The optical fiber sensor has at least an optical fiber part that has an optical fiber deformed by a collision load and changes the amount of transmission of light according to the collision load, and a light sending/receiving circuit for sending and receiving light to and from this optical fiber.
To enhance the detection sensitivity of the collision load of this optical fiber sensor, it is necessary for the optical fiber part of the optical fiber sensor to be effectively deformed within the range of its allowable rate of deformation by the collision load inputted to the optical fiber sensor. To realize this, it is preferable to prevent the entire optical fiber part from being moved backward by the collision load applied thereto from the front, by mechanically supporting the optical fiber part, at the front of a bumper reinforcement, using the bumper reinforcement.
Moreover, considering that the front bumper has a considerable width in the direction of height and is easily deformed, it is preferable that a load transmitting plate for concentrating the collision load applied to the front bumper on the optical fiber sensor is interposed between the front bumper and the optical fiber sensor, and using it to collect the collision load inputted to the front bumper and then to deform the optical fiber part while transmitting the collision load to the bumper reinforcement. With this, the collision load inputted to the front bumper can be applied to the optical fiber part.
Moreover, the optical fiber of the optical fiber sensor is usually covered with an elastic body so as to mechanically protect the optical fiber and to provide the optical fiber with a property of recovery in deformation to a change in the collision load. In addition, the optical fiber sensor is usually provided with a stress concentrating plate so as to locally concentrate the collision load inputted thereto from the load transmitting plate on a predetermined portion in the left and right directions of the optical fiber.
However, in the above-described conventional optical-fiber type collision detecting apparatus for a vehicle, the characteristics, in particular, optical characteristics or mechanical characteristics of the optical fiber part of the optical fiber sensor are changed by a change in temperature or aging degradation. As a result, it is found that the above-described conventional optical-fiber type collision detecting apparatus for a vehicle has a problem that it cannot detect the collision load with high accuracy because the output signal of the optical fiber sensor is fluctuated by a change in these characteristics.
An affect of a temperature change exerted on an optical fiber sensor of a conventional structure in which an optical fiber covered with a rubber tube and a stress concentrating plate adjacent to this are buried in soft resin will be considered below.
It is here assumed that an optical fiber is a resin-based optical fiber made of methacrylic resin. First, the shape, for example, radius and the density of the optical fiber is changed by a temperature change. The modulus of elasticity of the rubber tube and the soft resin for covering the optical fiber is also changed. In addition, although detailed description will be omitted, the light emission amount of the light emitting circuit of the optical fiber sensor and the light receiving sensitivity of the light receiving circuit thereof are also changed by the effect of the temperature change exerted on an electronic circuit. The total sum of these affects produces fluctuations to be considered before and after the temperature change in the output signal of the optical fiber sensor with respect to equal collision loads inputted to the optical fiber.
Next, similarly, the affect of aging degradation will be considered below. First, the affect of the aging degradation is significant, in particular, in the modulus of elasticity of the rubber tube and the soft resin for covering the optical fiber, and the sensitivity of the optical fiber (the amount of change in output signal per unit collision load) is fluctuated by its change. In addition, the light emission amount of the light emitting circuit of the optical fiber sensor is decreased by the aging degradation, whereby the sensitivity of the optical fiber sensor is changed. The total sum of these affects produces fluctuations to be considered before and after the aging degradation in the output signal of the optical fiber sensor with respect to equal collision loads inputted to the optical fiber.
Describing a change in the characteristics of the optical fiber sensor caused by the above-described temperature change and aging degradation, the value of output signal is changed in the respective values of collision loads inputted to the optical fiber sensor by the temperature change and the aging degradation of the optical fiber sensor, but this change can be simply classified into fluctuation in zero point that is the value of the output signal when the collision load is not inputted and fluctuation in sensitivity that is the amount of change in the output signal per unit collision load.
In a general optical fiber sensor, a signal voltage nearly proportional to the amount of light received by the light receiving circuit is compared with a predetermined threshold voltage to determine the kind of a colliding body and hence the above-described fluctuation zero point means fluctuation in a threshold voltage. As a result, it is found that the kind of a colliding body cannot be determined with high accuracy to thereby raise the possibility of presenting the problem of causing errors in determination.