Accelerometer Damped With Self Contained Viscous Material

The invention disclosed herein relates generally to accelerometers and more particularly to a damping arrangement for cantilever beam type accelerometers used for relatively low acceleration levels at relatively low frequencies. Applicant's invention has been found to be particularly applicable to acceleration levels below 2 g's and frequencies below 50 Hz, where 1 "g" is equal to the acceleration of the earth's gravity at its surface.

In the past accelerometers have used a fluid such as oil to achieve damping by locating the accelerometer within a cavity and filling the cavity with oil. The oil then slows or damps the movement of the inertial element of the accelerometer and provides the desired damping effect. The manufacturing processes associated with providing and maintaining oil filled cavity devices and the effects of ambient temperature variation on the oil and upon the accelerometer performance make oil filled cavities undesirable for many accelerometer applications.

Thus a need exists for an accelerometer damping arrangement that does not require the use of an oil filled cavity and provides substantially uniform performance under varying ambient temperatures.

The present invention solves these and other needs by providing a damping arrangement for an accelerometer having a stationary member and an inertial member spaced from and movable with respect to the stationary member. The damping is provided by a drop of self contained viscous material bridging the gap between the stationary and the inertial members. In one embodiment the inertial member is a cantilever beam with the deflectable end carrying a magnet and with a Hall chip located on the stationary member for providing a signal representative of the movement of the magnet. The deflectable end of the inertial member may be configured with two legs extending toward the stationary member, the two legs being partially immersed in two portions of silicone gel deposited on the stationary member.

FIG. 1 is a side elevation view of an accelerometer in accordance with the applicant's invention shown partially broken away to better reveal the damping feature embodied therein.

FIG. 2 is a plan view of the accelerometer of FIG. 1.

FIG. 3 is an end elevation view of the accelerometer of FIG. 1.

In FIGS. 1-3, reference numeral 10 identifies an accelerometer having a damping arrangement in accordance with applicant's invention. As shown in these features, accelerometer 10 includes a stationary member or substrate 12 and a deflectable cantilever beam 14. Beam 14 has a fixed end 16 spaced from substrate 12 by support block 20, and a deflectable end 18. Beam 14 is secured to support block 20 by screw 22 through clamping piece 24. Beam 14 can be of spring type stainless steel or other suitable material.

Magnet 26 is secured to deflectable end 18 of beam 14. Preferably a rare earth type magnet will be used to provide a very high flux density, but other types of magnets could be used. An opening cut in the beam with extending tabs may be used in combination with epoxy to secure magnet 26 to beam 14, or other means can be used.

Well known thick film processing techniques are used to screen resistors and interconnects onto lower surface 42 of substrate 12 as needed for the electronics to support a magnetic field sensor. A magnetic field sensor or Hall chip 28 is mounted to lower surface 42 so that the Hall chip is in alignment with magnet 26.

A damping arrangement in accordance with the present invention preferably utilizes a silicone gel as a damping material, although other self contained viscous materials having suitable characteristics may also be used. For example certain silicone oils may be suitable in some applications. For purposes of the present application, self contained means having the property of retaining a predetermined shape under all expected environmental conditions without the requirement for a confining container.

Silicone gel known as SYLGARLD.TM. 527 primerless silicone dielectric gel commercially available from Dow Corning Corporation has been found to be a very satisfactory damping material. This is a two component silicone encapsulant that cures to form a cushioning, self-healing, resilient, gel like mass. The cured gel retains much of the stress relief and self healing qualities of a liquid, while developing much of the dimensional stability and non-flow characteristics of a solid elastomer. After curing, the physical properties of resistance to penetration and volumetric expansion are insensitive to the normal changes in ambient temperature and other environmental parameters.

Deflectable end 18 of beam 14 includes a first depending leg 30 extending toward substrate 12 and terminating in end 32 and a second depending leg 34 extending toward substrate 12 and terminating in end 36. Legs 30 and 34 are preferably spaced apart. End 32 and end 36 are spaced from substrate 12 by a predetermined distance.

A first portion or drop 44 of silicone gel is deposited between end 32 and upper surface 40 of substrate 12 and a second portion or drop 46 is deposited between end 36 and upper surface 40. The silicone gel will adhere to substrate 12 and legs 30 and 34. A hypodermic needle or the like may be used for depositing the silicone gel. The silicone gel may be allowed to cure at room temperature or the curing may be accelerated through the use of an oven.

In one satisfactory model of the applicant's invention the displacement of the deflectable end 18 of cantilever beam 14 at acceleration levels below 2 g's and frequencies below 50 hertz was estimated to be of the order of 0.001 inch in either direction relative to the 0 g acceleration position of the beam. With this amount of deflection satisfactory results were obtained and the accelerometer output was relatively flat over the desired operating range of frequencies when depending legs 30 and 34 were immersed to a depth of approximately 0.004 to 0.005 inches in first portion 44 and second portion 46 respectively of silicone gel. The model had a natural resonant frequency of approximately 250 hertz.

The damping effect apparently results from the deforming of the gel by the movement of legs 30 and 34 and the rubbery or elastic property of the gel which causes it to resiliently return to its original shape or form. The amount of damping can therefore be varied somewhat by varying the width of depending legs 30 and 34 and the quantity of gel used. For example, increasing the width of the depending legs increases the leg surface area immersed in the gel and quantity of gel that is deformed by movement of the legs, thereby increasing the damping effect of the gel. It will also be appreciated that other variations in the shape or form of the legs will vary the damping action. Similarly the depth of immersion of the legs in the gel may be varied to vary the damping effect.

In accordance with the foregoing description, applicant has developed a simple damping arrangement that is easily incorporated into the design of conventional deflectable beam accelerometers. Although a specific embodiment of the applicant's damping arrangement is shown and described for illustrative purposes, a number of variations and modifications will be apparent to those of ordinary skill in the relevant arts. It is not intended that coverage be limited to the disclosed embodiment, but only by the terms of the following claims.