The present invention relates to a low pressure sensor for sensing
a change in the mass of a gas within a pressure vessel, and more particularly,
to a temperature compensating low gas pressure sensor for use in a hybrid inflator
of a vehicle safety restraint assembly.
Numerous types of inflators have been disclosed in the prior art
for expanding an inflatable air bag of a vehicle safety restraint system. One type
of inflator utilizes a quantity of high pressure gas stored in a storage cylinder
or body, which is selectively released to inflate the air bag. Another type of
inflator derives the gas source from a combustible gas generating material, which,
upon ignition, generates a quantity of hot gas for inflating the air bag. In still
another type, the inflator includes both stored compressed gas and gas generating
material for inflating the air bag. Such an inflator is referred to as a hybrid
inflator, an example of which is disclosed in U.S. Patent No. 5,360,232, assigned
to the assignee of the present invention.
In a vehicle safety restraint system which partly or solely utilizes
stored compressed gas it is very important to monitor the pressurized bottle containing
the stored gas to detect any leakage in the container. If the gas pressure of the
bottle falls below a predetermined level due to an undetected gas leak, the airbag
effectiveness would degrade and the system will not operate properly.
Typically hybrid inflators are filled with compressed argon gas to
approximately 3000 psi at 21°C. However, this same volume of gas will diminish
to a pressure of 2200 psi at -30°C and increase to a pressure of 4000 psi at 80°C.
Given that normal operating temperatures span such a wide range, compensation for
temperature of the gas is a necessary function of a low pressure gas sensor. If
such temperature compensation is not included in the low pressure gas sensor, the
sensor would signal "low gas pressure" at cold temperatures and would not sense
an actual low gas pressure at high temperatures.
It is known to merely detect loss of pressure in a pressurized vessel,
with such loss being attributed to a leak in the vessel or other damage to the
vessel. See U.S. Patent Nos. 3,771,121, 4,049,935 and 5,296,659. U.S. Patent No.
5,225,643, assigned to the assignee of the present invention, discloses a differential
pressure switch disposed within a pressurized vessel.
A major disadvantage of the above differential pressure switches
is the ineffectiveness for differentiating between a drop in pressure attributed
to a change in temperature of the gas or a drop in pressure due to a leak.
U.S. Patent Nos. 3,818,764 and 3,944,769 disclose pressure sensors
which are temperature compensated by charging the sensor reference chambers with
the same gas as the inflator. Thus, the switch must be pressurized and this pressurized
gas may also leak. Moreover, continuous adjustment of the pressure is required.
U.S. Patent No. 5,356,176 discloses a complex leakage detecting assembly
which generates a signal in response to a change in temperature of the vessel through
the use of a plurality of strain gauges and a layered bimetallic disk.
It is an object of the present invention to overcome the deficiencies
of the prior art by providing a low pressure sensor which both senses a change
in a mass of a fluid, such as compressed gas, of a pressurized vessel due to a
leak in the vessel and compensates for temperature changes of the compressed gas.
Another object of the invention is to provide a low gas pressure
sensor which compensates for temperature changes of the compressed gas without
utilizing a compressed reference gas chamber.
In accomplishing these and other objectives of the present invention,
there is provided a low pressure sensing device for sensing a change in a mass
of a fluid of a pressurized vessel including a housing disposed within the pressurized
vessel. A pressure diaphragm is attached to the housing in communication with
the fluid in the pressurized vessel. A retainer is in contact with the pressure
diaphragm and is movably disposed within the housing. A temperature sensitive thermostat
is also movably disposed within the housing. The thermostat moves in relation to
a change in temperature of the fluid. Contact means provide contact between the
diaphragm and the thermostat. Spring means disposed in the housing in communication
with the retainer, counteract the movement of the thermostat due to a change in
temperature of the fluid, wherein when the diaphragm is acted upon by a shift in
pressure due to a temperature change of the fluid, the spring means moves the
pressure diaphragm at the same rate as the thermostat to maintain contact between
the thermostat and the contact means.
The pressure of the gas is measured by the movement of the pressure
diaphragm, however, the bimetallic thermostat compensates for pressure changes
due to temperature changes, thus the sensor can indicate a loss in the actual mass
of the gas present in the inflator.
Other features and advantages of the present invention will become
apparent from the following description of the invention which refers to the accompanying
Fig. 1 is a cross-section of the low pressure sensor according to
the present invention.
Fig. 2 is a cross-section of the housing of the low pressure sensor
of the present invention.
Fig. 3 is a cross-section of the pressure diaphragm according to
the present invention.
Fig. 4 is a cross-sectional view of the spring backup disc of the
low pressure sensor.
Fig. 5 is a cross-sectional view of the thermostat holder of the
Fig. 6 is a cross-sectional view of the low pressure sensor illustrating
a break in contact due to a low pressure condition in a pressurized vessel.
Referring to Fig. 1, in order to verify an undetected gas leak failure,
a low pressure sensor (LPS) 10 is installed inside a pressure vessel 40, such as
an inflator bottle of a hybrid gas inflator. The low pressure sensor 10 senses
a change in the mass of the gas within the vessel 40.
The temperature sensitive low pressure switch of the present invention
includes a housing 12, which can, for example, be positioned within the fill port
of the bottle. As shown in Figs. 1 and 2, housing 12 includes an upper chamber
11 for receiving a diaphragm 16, a spring retainer 18, and a stack of spring washers
20, all of which will be described further herein. A lower chamber 13 of housing
12 receives a thermostat holder and spring support 28 and a bimetal thermostat
30, which will also be described further herein.
Referring to Figs. 1 and 3, diaphragm 16 is made of thin, circular,
flexible metal and may include a spherical convolution 17 to maximize the deflection
capability of the diaphragm. Diaphragm 16 is also hermetic to allow pressure to
build up on one side 15. Pressure diaphragm 16 is attached to the housing 12 at
a shoulder 14 by welding or some other hermetic attachment means, and is responsive
to the pressure of the fluid in the vessel.
Floating backup retainer 18 is positioned between spring washers
20 and diaphragm 16. If the springs 20 and backup retainer 18 were not present,
the high pressure would burst the thin pressure diaphragm. Backup retainer 18 floats
within the upper portion of chamber 11. As shown in Fig. 4, the upper surface of
retainer 18 matches the shape of the pressure diaphragm upon engagement of the
retainer and diaphragm. As retainer 18 floats within chamber 11, a lip 21 of the
retainer abuts against shoulder 14 of the housing to limit the upward movement
of the retainer within the housing.
One of the criteria for choosing a pressure sensor is that it must
compensate for temperature variations. The low pressure sensor of the present invention
is a temperature sensitive device of a simple, low cost design.
The temperature is compensated for by providing bimetallic thermostat
30 which moves at the same rate as the pressure diaphragm 16, when acted upon by
a shift in gas pressure due to a temperature change of the gas. Thermostatic bimetals
react to temperature by distorting in relation to the temperature change. Bimetals
are available in a variety of materials, thickness' and shapes. The bimetal is
designed to allow movement at the same rate the spring compresses and expands with
relation to temperature and pressure.
The thermostat 30 is in the shape of a disc which may be snap fit
within the non-conductive thermostat holder 28. As shown in Fig. 5, holder 28 includes
a slot 29 into which the edge of thermostat 30 is received. Holder 28 also includes
a central opening 27 through which the contact arm extends.
The pressure of the gas acting upon the diaphragm 16 is counteracted
by the stack of spring washers 20. It should be appreciated that a standard coil
spring can also be used, however, it would have to be large enough to counteract
the given pressures. As force pressure increases or decreases against the diaphragm
16, the spring washers 20 compress and expand respectively with the force.
A contact arm 24 is attached electrically, by welding or other known
means, to retainer 18. Contact arm extends towards bimetal thermostat 30 such that
electrical contact is made between contact arm 24 and bimetal thermostat 30 when
there is adequate gas in the inflator.
As shown in Fig. 6, when pressure drops due to a gas leak in the
vessel, pressure diaphragm 16 will move away, (to the right in Fig. 6), under its
own bias, disengaging from retainer 18 which also moves rightwardly and away and
from thermostat 30. Contact arm 24 attached to retainer 18, will move away, rightwardly
as shown in Fig. 6, breaking contact with thermostat 30. Thus, the circuit opens
when pressure is lost to gas leakage. Contact arm 24 is designed such that it's
tip 25 will compress and spring back to shape when pushed against the thermostat.
This is necessary because there is a requirement to proof test the gas bottle to
its maximum expected operating pressure (MEOP). The MEOP occurs at high temperature
conditions, usually around 6000 psi at 80°C, and the proof test is performed at
ambient temperature (21°C). Therefore, the diaphragm and contact arm would move
during proof testing, while the thermostat would not move. For this reason, the
contact arm must be compressible.
Electric leadwires 32 depicted in Fig. 1, demonstrate how the continuity
between the contact arm and thermostat could be verified. There are many other
possible means of connecting the contact arm and the thermostat. If the pressure
drops below a certain threshold, the sensor circuit is broken which triggers an
audible alarm or light illumination 36 (Fig. 1), alerting the vehicle operator
that the gas pressure has dropped below a proper operating level.
The total movement of the diaphragm with respect to pressure change
is governed by the spring washer size, shape, number, material and stacking configuration.
The total flexing of the bimetal thermostat is governed by size,
thickness, geometry and material. Given the wide range of possible design parameters,
several designs utilizing the present invention are possible. An example of the
calculations required to determine an operational diaphragm/spring washer and bimetal
thermostat combination is as follows:
In order for the low pressure sensor of the present invention to
function properly the thermostat movement (M) must be equal to the spring movement
(Y) and can be expressed as:
M = Y
The thermostat movement is defined as:
M = (Kbimetal x ΔT x Dthermostat )/ tbimetal
where constant Kbimetal = 2.13x10-6 in./°F (for ASTM T2 bimetal),
ΔT = temperature change, Dthermostat = diameter of the thermostat,
= thickness of the bimetal thermostat.
The spring movement is defined as:
Y = Kspring x ΔF x nsprings
where constant Kspring = 5.9 x 10-6 in/lb.
(for a spring washer having an outer diameter = 0.75 in., an inner diameter =
0.38 in., a thickness of 0.035 in. and a 0.057 in. taper), nsprings =
number of springs in washer stack, and ΔF = change in force.
The change in force is defined as:
ΔF = ΔP x Adiaphragm
where ΔP = change in pressure and Adiaphragm
= area of the pressure diaphragm.
With a pressure diaphragm having a diameter of 0.6 in., a thermostatic
disc having a diameter of 0.75 in., P-30°C = 2200 psi, P+80°C
= 4000 psi, and using 5 springs, the spring and bimetal parameters can be calculated
ΔF = ΔP x Adiaphragm
ΔF = (4000-2200)lb./ in2 x 0.283 in2 = 510 lb.
Y = Kspring x ΔF x nsprings
Y = 5.9x 10-6 in/lb. x 510 lb. x 5 springs
Y = 0.015 in.
M = 0.015 in.
tbimetal = Kbimetal x ΔT x Dthermostat
= (2.13x10-6 in/°F x 198°C x 0.75 in.) / (0.015 in.)
tbimetal = 0.021 in.
Therefore, given the above, the bimetal thermostat disc should have
a thickness of 0.021 in. in order to move at the same rate as the 5 springs.