Inertia activated electrical power source

Disclosed is an inertia activated power source in which the moving element is typically a mercury cell shearing a restraining wire at a predetermined G force. The moving cell is then caught between two spring contacts that extend exterior the case providing electrical terminals. Once activated, a spring latch locks the cell in place between the contacts. The action of the spring contacts materially reduces the shock on the battery when the device is subjected to forces many times greater than the desired activation source for the power source.

BACKGROUND OF THE INVENTION 
The field of the invention is in the art of accelerometers and more 
particularly that of inertia switching devices. 
It is frequently desirable to "turn on" ("enable" and "activate" also are 
synonymous, frequently uses terms) and electrical circuit when a 
relatively sudden and abrupt change of velocity occurs, i.e., an 
acceleration greater than a predetermined amount. The term "acceleration" 
is presumed to include both the positive and negative. Thus, 
de-acceleration is negative acceleration with the velocity vector still in 
the same direction. Inertia is that tendency of a body to resist 
acceleration. The inertia-activated electric power source of this 
invention provides a voltage on a pair of external terminals on the 
happening of a predetermined magnitude of acceleration. Generally, 
inertia-activated devices are well known. The use of an inertia device to 
activate restraining air bags that inflate on the collision of an 
automobile is a typical well-known example. The term "set back-actuated" 
is frequently applied to devices sensing forward thrust or positive 
acceleration, while "impact-actuated" is frequently applied to similar 
devices sensing sudden slowing or stopping, i.e., de-acceleration or 
negative acceleration. Typical examples of the former are devices for 
sensing the launching of airplanes, missiles, and projectiles, while 
typical examples of the latter are collision-sensing devices, excessive 
package handling load sensing devices, and safety beacon actuated devices. 
Generally, inertia devices are rated as to the number of G's required to 
activate them, G being the acceleration of gravity under standard 
conditions on the surface of the earth. Generally, inertia devices may be 
used for either positive G forces or negative G forces by merely turning 
them around. In some instances, it may be desirable to take into 
consideration a slight change in calibration due to a change in static 
load. 
An inertia-activated device must generally be very rugged and reliable. 
Quite frequently, they must withstand and continue to function after 
experiencing shock forces many times greater than that at which they 
initially operate. Many prior art devices are quite complex and their 
reliability to withstand shocks of greater magnitude than they were 
designed to sense is relatively low. Generally, prior art 
inertia-activated power sources are merely inertia switches connected by 
wiring to a battery with each requiring shock protection. It is thus an 
object of the present invention to provide an inertia-activated electrical 
power source in which a battery, in addition to supplying the electrical 
power is also the actuating element in sensing the acceleration. 
It is another object of the invention to provide an extremely rugged, 
relable inertia-activated power source that may readily be adjusted for 
various G load activating forces. 
It is another object of the invention to provide an inertia-activated 
electrical power source that upon activation latches in the "on" position. 
It is another object of the invention to provide a positive action device 
that is either "off" or "on" without any chattering, or partial or 
intermittent contacting of the internal electrical circuits of the device. 
It is another object of the invention to provide an inertia device in which 
the moving element may readily be cushioned to arrest its motion toward 
the end of its travel thus preventing any undue shock damage. 
It is yet another object of the invention to provide an inertia-activated 
electrical power source that has extremely few parts, is simple and 
economical to manufacture. 
SUMMARY OF THE INVENTION 
The invention provides a rugged, reliable, self-contained inertia-activated 
power source that is readily adjusted for predetermined actuating G 
forces. The device is low in electrical "noise", with no indefinite stage 
of operation, and once actuated, the device locks in the actuated ("on") 
position.

DESCRIPTION OF A PREFERRED EMBODIMENT 
FIG. 1 illustrates pictorially a typical embodiment of the invention. The 
case 11 of the device is conventionally attached to the structure which 
will undergo the acceleration. No voltage is present on terminals 13 and 
15 until the case 11 (and the structure to which it is attached) 
experiences an acceleration 17 of a predetermined magnitude. At the 
predetermined magnitude of acceleration, an output voltage providing 
electrical power becomes present between terminals 13 and 15. This voltage 
and electrical power remains after the acceleration causing its activation 
increases, decreases, or ceases. The internal structure of the device 
provides shock protection for acceleration or de-acceleration forces many 
times greater than that required to actuate the device. 
Referring further to FIGS. 2 and 3 of the drawing, the inertia-activated 
electrical power source 11 has a self-contained battery 19, such as a 
conventional silver, mercury, alkaline, or similar cell, that also 
functions as a moving element of an inertia sensor. The cell, or battery, 
as it is commonly called, is contained in the elongated cavity 21 of case 
11. The material from which the case is fabricated is not critical. It 
should be rugged and not short circuit the battery. High impact phenolic 
material such as used in printed circuit board material is generally 
suitable. A suitable embodiment of the invention may be fabricated from 
one side copper clad insulating board with a copper coating on the 
exterior of the case. A case may readily be fabricated by soldering and it 
may also be attached to other structures by soldering. The copper cladding 
must be removed in the areas adjacent the terminals to prevent a short 
circuit of the output voltage. 
Under sufficient downward (in the illustration) acceleration, the battery, 
due to its inertia, will slide from its original position 19 in the lower 
end of the cavity to a position 23 in the upper end of the cavity. In 
doing so, it wedges between the spring contacts 25 and 27 and the battery 
voltage is provided at terminals 13 and 15. However, before the battery 
can move appreciably, it must shear the shear member 29. A small copper 
wire is a suitable shear member. The shear wire is conventionally soldered 
at its ends 31 and 33 to the copper surface of the board. Knowing the 
shearing constant of the material of the wire, the mass of the battery, 
and the desired activating G force, the diameter of the wire may readily 
be calculated. However, as is common with many such similar devices, the 
calculated value is substantially a first approximation figure and exact 
values will later be determined depending on manufacturing tolerances 
employed and other variables. It is to be observed that the contact of the 
battery with the wire is not an ideal shear relationship, and that tension 
is also present due to necessary side clearances between the battery and 
the case. The magnitude of acceleration required to activate the device 
may readily be adjusted by merely changing the size of the shear wire to 
suit the desired application. 
Spring latch 35 readily compresses into recess 37 as the battery moves from 
the lower to the upper position allowing the battery to pass. After the 
battery has passed the latch, the latch moves back out and prohibits the 
battery from moving away from the contacts once contact has been 
established. 
Inertia-activated power sources such as being disclosed frequently are 
designed to function at for example a 10 G force but must continue to 
function even though the acceleration forces proceeds to several hundred 
G's. A novel feature of the invention is that the battery which provides 
electrical power in its function of providing a moving inertia element 
also is undergoing self-protection from excessive shock that might 
otherwise damage the battery or at least require additional components to 
physically protect the battery from damage. This shock protection is 
accomplished first through the restraining action experienced with the 
battery in shearing the wire 29 and second by the additional restraining 
action provided by the wedging effect created as the electrical contact 
areas 39 and 41 of the battery engage and slide into the spring contacts 
25 and 27. While the latching spring 25 does contribute to the arresting 
of the movement of the battery, it is generally comparatively negligible. 
If further cushioning of the battery is necessary, energy absorbing 
material such as rubber may be cemented to the surface 43 at the upper end 
of the cavity. 
While the principles of the invention in connection with specific apparatus 
have been described, it is to be understood that the foregoing description 
is by way of example only and not as a limitation to the scope of the 
invention as set forth in the accompanying claims.