DIGITAL ACCELEROMETER reached, the two beams come into contact with each
„D„CC DECrDEwrnc Tri DC, ATCrv other- The contacting of one beam with the other beam
CROSS REFERENCES TO RELATED defines the closing of the switch.
APPLICATIONS Irrespective of which switch is used, an electronically
This application is related to copending application 5 controllable switching means, in the form of a
Ser.-No. 121,040 filed Mar. 4, 1971 entitled "Elec- transistor, is connected to each of the switch contacts,
tronic Fuse System" by M. H. White, R. G. Hamel, and A source of voltage is also connected to the transistor.
D. D. O'Sullivan and assigned to the same assignee as Before the object is subjected to a predetermined ac
the present invention. celeration, the switch contacts are open and the
10 transistor is conducting. When the threshold accelera
BACKGROUND OF THE INVENTION tion is reached, the switch contacts close causing the
1. Field of the Invention transistor to become non-conductive and resulting in a In general, the present invention relates to ac- high voltage being sensed at the output of the
celerometers. More specifically, it relates to digital ac- j 5 transistor.
celerometers — that is, accelerometers which indicate The acceleration profile of an object can also be whether one predetermined threshold level of accelera- measured by providing a plurality of the above switches tion has been exceeded. and transistors each of which is designed to indicate
2. Description of the Prior Art when a different threshold acceleration level is One prior art device uses piezoelectric crystals. Two 20 reacned- In each case, the conductivity of each
such crystals are placed between a mass and a support transistor is controlled by a separate switch of either base. The entire assembly is then mounted on an ob- type.
ject. As the object is subjected to increasing accelera- BRIEF DESCRIPTION OF THE DRAWINGS
tions, the mass will cause increased pressure to be exerted on the crystals. The crystals will generate a 25 For a better understanding of the invention, changing output voltage which is proportional to the in- reference may be had to the preferred embodiment, excrease or decrease in acceleration. emplary of the invention, shown in the accompanying
Another device for sensing changes in acceleration is drawings, in which: the piezojunction transducers in the form of pressure FIG. 1 shows a perspective view of a preferred emsensitive transistors. In this case, as the acceleration in- 30 bodiment of the switch;
creases, pressure is applied to the emitter-base junction FIG. 2 shows a schematic representation of a of the transistor thereby changing its current gain. preferred circuit using the switch with a transistor;
However, many problems result from these devices. FIG. 3 shows another embodiment of the switch; First, they require a seismic mass and spring assembly FIG. 4 shows a schematic representation of how a to transduce the accelerational force to the sensor and 35 plurality of switches and transistors are connected the entire assembly must be critically mounted on the together to measure the acceleration profile of an obsensor. Second, because the sensors are analog sensors, ject; and
they continuously sample the acceleration, thereby FIG. 5 shows a typical acceleration profile and requiring power dissipation. Third, they are extremely ^ graphically shows the sequential switching necessary to large and cause weight problems. obtain it.
BRIEF SUMMARY OF THE INVENTION DETAILED DESCRIPTION OF THE INVENTION
The invention includes a switch which is mounted on The present invention is a very small, digital solidan object which may be subjected to a particular ac- 45 state accelerometer which senses threshold acceleraceleration. The switch is mounted on a support and is tion levels and which is compatible with integrated cirmade up of a stationary conductive member and a cuitry. "Threshold" means a single, predetermined movable conductive member both of which are level of acceleration. Therefore, the accelerometer to disposed on the support means. When the object on be described can only sense a single, predetermined acwhich the switch is located is subjected to a predeter- 50 celeration level.
mined acceleration, the switch contacts close. It is often desirable to know when an object is ac
Two types of switches are shown. The first switch is a celerating at a certain rate or what the acceleration cantilever-type switch. That is, one extremity of the forces are on a particular object in a particular movable conductive member rests on the support base direction. For example, it might be desirable to know while the other extremity is able to swing free. When 55 the centrifugal acceleration of a rotating machine such the object is stationary, the free end of the cantilever as a turbine or a generator. As another example, it beam is fixed to be a predetermined distance from the might be desirable to know when a projectile, fired by a stationary conductive member. rifle reaches a particular acceleration. This is an exam
In the second embodiment, the movable conductive pie of setback acceleration. For still another example, member takes the form of a beam which is clamped at ^ it might also be desirable to measure the acceleration both ends. Again, a predetermined fixed relation is set forces on an automobile which is involved in an acup between the movable conductive member and the cident. This is an example of impact acceleration. In Stationary conductive member. Here, the center of the addition to the above three examples, there are innudouble-clamped beam is flexible. When the object is g5 merable situations where it might be desirable to measubjected to a predetermined acceleration, the center sure acceleration forces.
of the beam is flexed toward the stationary conductive FIG. 1 shows a preferred embodiment of the invenmember until, when the threshold acceleration is tion. It shows a switch assembly 10 which can easily be