The present invention generally relates to an acceleration sensor for measuring accelerations in one or a plurality of directions by means of a plate-like seismic mass that responds to the accelerations and is movably held by one or a plurality of webs, wherein a light beam from a light source is directed onto or over the seismic mass and is deflected as a function of the movement of the seismic mass, and wherein the deflected light is directed to at least one optical detector which measures the deflections as a function of the acceleration. An acceleration sensor having these features is disclosed, for example, in U.S. Pat. No. 3,789,674. More particularly, the present invention is directed toward such acceleration sensors which are produced, for example, as silicon micromechanisms and are composed of a plate-like seismic mass which is held by one or a plurality of webs so that the mass is able to react to the accelerations to be measured with a corresponding movement.
A plurality of such acceleration sensors have been developed in which seismic masses are connected, either on one side or edge or at opposite sides (edges) by one or a plurality (usually two) of webs with a fixed, i.e. stationary or rigid silicon frame. Examples can be found in the following publications: (a) "A Batch Fabricated Silicon Accelerometer", IEEE Trans. Electron. Devices, ED-26 (1979), 1911; and (b) "Micromechanic Capacitive Accelerometer", U.E. Gerlach-Meyer, Sensors and Actuators A, 25-27 (1991), page 558 et seq. In order to be able to record the movement of the seismic mass, measurements are made in (a) of changes in resistance in a piezo-resistor integrated in the web, or in (b) of changes in the voltage of the seismic mass configured as a capacitor plate relative to one or a plurality of fixed electrodes. In case (a) the great temperature dependence of the integrated resistors poses problems. In case (b), the temperature dependence is somewhat less, in principle, but the capacitance read-out is connected with greater expenditures for electronic equipment. The design of such capacitive acceleration sensors further requires special expenditures with respect to the attenuation and frequency behavior of the sensor due to the displacement of the quantity of air between the capacitor plates. The adjustment of the capacitor plates relative to one another and the design and mounting of the seismic plate are correspondingly critical. Far-reaching miniaturization encounters limits since they require capacitor surfaces in the mm.sup.2 range in order not to obtain excessively low capacitance values in the pF range and less.
For both of the above arrangements it is difficult to realize sensors that respond to accelerations in more than one direction. This also applies for the following, optically designed acceleration sensors in which the optical coupling, for example, between two optical fibers changes as a function of the acceleration to be monitored.
The above-mentioned U.S. Pat. No. 3,789,674 discloses an acceleration sensor which is provided with a cantilever arm that is held within a housing by means of an adjustment screw and supports a seismic mass whose top surface is made reflective and is disposed below a light source that is arranged in the ceiling of the block-shaped housing. On both sides of the light source, photocells are attached to the housing ceiling. These photocells receive different light reflection percentages depending on the acceleration acting on the seismic mass.
Federal Republic of Germany Laid-Open patent application No. DE 3,007,462.A1 discloses a shock indicator in which a single-fiber optical conductor is brought through each one of two oppositely disposed housing walls, with the one conductor projecting relatively far into the housing and there facing the other, shorter conductor that has been introduced less far into the housing. The indicator also includes a pushed-over sleeve, that serves as the seismic mass, for the longer conductor. If there are shocks, the longer optical conductor, which is configured as a spring, is excited to vibrate in some plane in its longitudinal axis so that the optical coupling between the photoconductors is periodically interrupted.
In Sov. Inv. Illustrated, Volume W, No. 4, published Mar. 4, 1975, page 8, SU 420935, a seismic mass is pressed by means of a spring against a holding plate in a housing. The end surface pressed against the holding plate is made reflecting where it faces the end faces of two optical fibers that are brought through the mounting plate. If accelerations perpendicular to the holding plate occur, the seismic mass leaves the plane of the holder as soon as the inertial force acting on it exceeds the force of the spring, so that a gap is formed which is sufficient to close the light path or light circuit via the reflecting surface of the seismic mass so that the light is conducted through the optical conductors and analyzed.
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