Measuring and testing – Speed – velocity – or acceleration – Acceleration determination utilizing inertial element
Patent
1997-02-13
1999-07-13
Williams, Hezron
Measuring and testing
Speed, velocity, or acceleration
Acceleration determination utilizing inertial element
G01P 1500
Patent
active
059229551
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a gravity-compensation type accelerometer. Compensating for the effect of gravity on the seismic mass of an accelerometer gives increased sensitivity to variations in acceleration.
The invention applies particularly to small apparatuses. The nature of the accelerometer makes it suitable for construction using mechanical, micromechanical or microelectronics techniques (for example micromachining).
The main field of application for an accelerometer using the present invention is for studying the movement or behavior of milieus subject to gravity (for example seismology).
2. Discussion of the Background
The invention thus makes it possible to design gravity-compensation accelerometers where known types of one-piece accelerometer do not allow for this compensation. Accelerometers using known techniques are described in, for example, M. Ueda, H. Inada, Y. Mine and K. Sunago: "Development of micromachined silicon accelerometer" in the Sumitomo Electric Technical Review, No. 38 of June 1994, pages 72-77 and in Michael E. Hoenk: "Small inertial measurements units-sources of error and limitations on accuracy" in the review SPIE, vol. 2220, pages 15-26.
The most common method for measuring acceleration consists in not directly measuring the acceleration itself but rather the force F applied to a mass M due to the effect of the acceleration .gamma. in question. According to the basic law of motion F=M..gamma., if the value of M is known, F can be measured and a value for the acceleration obtained.
The most common type of acceleration sensor thus consists of an inert, or seismic, mass, generally supported by one or more springs. When the mass is subjected to variations in acceleration, it moves and the springs are distorted. The system returns to its initial position as soon as the force due to the acceleration is canceled.
A horizontal acceleration sensor in the rest state is not sensitive to any disruptive effect. On the other hand, a vertically-sensitive accelerometer is subject to a minimum force equivalent to that of gravity, F=M.g, where g is the gravitational constant.
This minimum force due to gravity is inconvenient when attempting to measure very slight vertical accelerations (less than 10.sup.-6 G). It is therefore important in this situation to compensate for the effort due to gravity with a force tending in the opposite direction to that exerted by gravity. At the present time there are two classes of processes for compensating for the force of gravity: electrostatic field maintains the seismic mass in suspension. Such processes require complex servo systems, state of equilibrium, suspended by a pre-distorted spring.
There are also hybrid systems that use a combination of electrostatic or electromagnetic forces together with the return force of a spring. An example of this type of system is described in Shi Jung Chen and Kuan Chen: "The effects of spring and magnetic distortions on electromagnetic geophones" in J. Phys. E. Sci. Instrum. 21 (1988), pages 943-947.
These techniques have other disadvantages.
In electrostatic or electromagnetic apparatuses, the presence of an electronic servo system can generate interference noise that is incompatible with the desired sensitivity. Moreover, purely electrostatic compensation methods produce unstable systems that are difficult to servo-control.
Vertically-sensitive sensors where the effect of gravity on the seismic mass is compensated for by a spring are currently produced by assembling a variety of mechanical parts. This type of sensor is described in, for example, E. Wielandt and G. Streckeisen: "The leaf-spring seismometer: design and performance" in Bulletin of Seismological Society of America, Vol. 72 No. 6; pages 2349-2367, December 1982. By virtue of their construction, this type of apparatus does not have a very high Q quality factor. This structural parameter is related to the density of Brownian noise S of the apparatus using the following relation: ##EQU1## where:
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Brun Jean
Caplet Stephane
Ruzie Gerard
Touret Patricia
Commissariat a l''Energie Atomique
Moller Richard A.
Williams Hezron
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