US2536802A - Accelerometer - Google Patents

Description

Patented Jan. 2, 1951 ACCELEROMETER Robert 0. Fehr, Augustus n. Fiske. in, and Richard J. Wells, Schenectady, N. Y., asaignors to G neral Electric Company, a corporation of New York Application November 28, 1948, Serial No. 62,122

1 Claim. (Cl. 171-327) This invention relates to an improved accelerometer, and in particular to an accelerometer adapted to measure shock waves traveling through the ground as a result of large exploslons.

An object of the invention is to provide an accelerometer of the type described which has high sensitivity, and at the same time is simple and rugged in construction. Other objects and advantages will appear as the description proceeds.

The features of the invention which are believed to be novel and patentable are pointed out in the claim forming a part of this specification. For a better understanding of the invention, reference is made in the following description to the accompanying drawing, in which Fig. 1 shows a vertical section of a preferred embodiment of the invention; Fig. 2 shows a section along the line 2-2, Fig. 1; Fig. 3 is a view showing in greater detail the construction of the crystal assembly and its attachment to the accelerometer post; and Figs. 4 and 5 are circuit diagrams showing electrical connections to the crystals. Where the same part appears in more than one figure of the drawing, it is represented in each case by the same reference numeral.

Referring now to Figs. 1 and 2, the accelerometer includes a base I, and a massive unitary post 2 which is rigidly attached at one of its ends to the base. Preferably, base I and post 2 are machined from a single piece of metal, and post 2 has a uniform square cross section throughout its length. A cylindrical bore extends along the axis of the post, and a co-axial cable 3 is fitted within this bore. A gasket 4, washer 5, and nut 6 provide a moisture-tight seal between the cable and base I. A cover I and gasket 8 complete the moisturetight assembly so that all working parts are protected from moisture and other disturbing influences.

Thin piezoelectric crystals 8 and III are attached to opposite sides of post 2 near the base, as shown. An insulating sheath I I, which may be 7 of paper but preferably is an impermeable thermosetting plastic, is provided about the post to electrically insulate it from the crystals. Electrical connections from the crystals to the coaxial cable are made through a terminal board It. The inner conductor of the co-axial cable is represented at I3 and the outer conductor at ll.

Refer now to Fig. 3, which shows in greater detail the construction of a crystal assembly and its attachment to the accelerometer post. Crystal 8 may be an ADP crystal approximately ten mils thick, flve-eighths inch long. and the same width as post 2. Such a crystal has been form! to give good results with a steel post having a one inch square cross section. In'contact with and substantially covering the two large facu of crystal 9 are two electrodes II and it. These electrodes may be formed by depositing a thin coating of metal on the large faces of the crystal. The entire assembly is held, together, and one large face of the crystal is rigidly attached to post 2, by a strong glue, cement, or other bonding agent. An alkyd resin cement has been found satisfactory for this purpose. Leads I1 and I! are respectively connected to the two electrodes, as shown. 4

Refer now to Figs. 4 and 5, which are circuit diagrams of two alternative electrical connections. The two crystals may be connected in series, as in Fig. 4, or in parallel, as in Fig. 5, with their polarities opposed. For the series connection, as shown in Fig. 4, if the two crystals are deformedin the sarne direction, as by tension or compression of the accelerometer post, the outer face of each crystal becomes positive with respect to the face adjacent post 2, so that the voltages across the two crystals are of opposite polarity inthe circuit and balance out. For the parallel connection shown in Fig. 5, one of the crystals is reversed so that when they are deformed in the same direction the outer face of one has the same polarity as the inner face of the other. The final result is the same, but with the parallel circuit it is current produced by the opposing voltages rather than the voltages themselves which balance out.

The accelerometer is responsive to accelerating forces normal to the axis of post 2 and the plane of crystals 9 and I0. For example, when used as a ground wave detector. the accelerometer may be installed so that any shock waves traveling through the ground will be transmitted to base I. Assume that these shock waves are from left to right in the plane of the section shown in Fig. 1. As base I is acceleratedby the shock waves, the inertia of post 2 produces stresses which slightly bend the post. In other words. the post acts as a cantilever beam which is deflected by the inertia of its own mass in response to acceleration normal to its axis. It should be noted that in this accelerometer, the inertia mass is distributed throughout the length of the post. In this respect, the accelerometer differs from the lumped-mass. type in which the inertia mass acts as a unit upon a spring or other resilient member.

when post 2 bends, one side of the post is placed in tension and the other side in compression. This mechanically deforms crystals I and I0 respectively in opposite directions, that is, one of the crystals is shortened in a direction parallel to its large faces, while the other crystal is lengthened in the same direction. Voltages of opposite polarities are thus produced between the large faces of the respective crystals, but since the crystals are connected in the electrical circuit with their polarities opposed, these voltages add together and produce an output voltage of double value between conductors i3 and ll of the co-axial cable. The other end of cable 3 is connected to suitable. voltage-responsive apparatus, such as an amplifier and an-indicating or recording instrument, which may produce an indication proportional to the voltage between conductors i 3 and It, and hence to the magnitude of the acceleration measured.

Acceleration in other than the desired direction has negligible effect upon the output voltage. For example, acceleration in the direction of the axis of post 2 produces tension or compression simultaneously on both sides of the post. In this case, the voltages produced across crystals 9 and ill have polarities such that the two voltages baance out. as hereinbefore explained, and consequently no net voltage is produced between conductors l3 and II responsive to such acceleration.

Having described the principle of this invention and the best mode in which we have contemplated app'ying that principle, we wish it to be understood that the apparatus described is illustrative only, and that other means can be employed without departing from the true spirit and'scope. oi the invention defined by the following claim.

What we claim as new and desire to secure by Letters Patent oi the United States is:

' assaeos rigidly attached at one of its ends to the base so that the post deflects as a cantilever beam responsive to acceleration normal to its axis, said post having a. substantially uniform square cross section throughout its length, a coaxial cable, the post and base having a cylindrical bore extending along the axis of the post into which the coaxial cable fits, thinpiezoelectric crystals respectively attached to opposite sides of said post near the base, each such crystal having two relatively large opposite faces one of which is rigidly attached to a side of the post, electrodes in contact with and substantially covering the large faces of the crystals, an insulating sheath about the post to electrically insulate it from the crystals and their electrodes, and electrical connections between electrodes and the co-axial cable such that crystals on opposite sides of the post are connected with opposed polarities.

ROBERT O. FEHR. AUGUSTUS .H. FIBKE, Jn. RICHARD J. WELLS.

REFERENCES CITED The following references are 01 record in the idle 01' this patent:

UNITED STATES PATENTS Number Name Date 1,693,806 Cady Dem- 4, 1928 1,912,213 Nieolson May 30,; 1933 2,266,449. Ullrich et al Dec. 16, 1941 2,359,245 Ritzmann Sept. 26, 1944 2,371,626 Keckemetl Mar. 20, 1945

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