The present invention relates to shock sensors for monitoring machines
, such as motor vehicles for stresses and crashes in general, and to shock sensors
utilizing the magnetostrictive effect in particular.
The modern motor vehicle is equipped with many active safety systems,
from seat belt tensioners, to air bags and fuel cutoff valves. To properly trigger
the activation of the various active safety systems, shock sensors are used to detect
the onset of a crash and to determine the severity of a crash. To optimize the use
of active safety systems it is important to know as soon as possible the likely
severity of the crash. Better results can be achieved by early detection of crash
severity and early deployment of active safety systems. At the same time, considerable
cost can be saved if safety systems are not deployed in less severe crashes where
passive restraints such a seat belts are likely to be sufficient to prevent serious
injury. As motor vehicles employ larger numbers of air bags and other deployable
safety systems, the cost of replacing deployed safety systems becomes a considerable
portion of the cost of repairs following a crash. Of course, in a severe crash,
when deployment of all safety systems is desired, the motor vehicle may have little
residual value due to the extensive damage caused by the crash. This tension between
the benefits of early deployment versus the cost of unnecessary deployment focuses
attention on sensors that can give an indication of crash severity early in a crash.
One type of known shock sensor that employs the inverse magnetostrictive effect
or the Villari effect can detect shock waves in ferrous structural elements. Shock
waves can be signal processed to give an indicator of crash severity early in the
crash sequence. However, the ability to detect shock waves in non-ferrous structural
elements, and a sensor having a larger output voltage are desirable to increase
the utility of sensors that detect shock waves in structural elements an motor vehicle
during a crash.
The shock sensor of this invention employs a Terfenol-D sensing element
positioned inside a sensing coil. A permanent biasing magnet is positioned in engagement
with the Terfenol-D sensing element, and a spacer engages the Terfenol-D sensing
element and extends from a housing that surrounds the biasing magnet, the Terfenol-D
sensing element, and the sensing coil. The housing has a beam with two spaced-apart
mounting holes through which fasteners extend to mount the shock sensor to a motor
vehicle's structural element. The mounting of the beam places the spacer in compression
against the motor vehicle's structural element. The spacer, the Terfenol-D sensing
element, and the biasing magnet are packaged in a sleeve that is positioned in a
cylindrical portion of the beam that extends perpendicular to the beam. The beam
is mounted by the fasteners to the motor vehicle's structural element. A bobbin
about which the sensing coil is wound is positioned over the Terfenol-D sensing
element and the biasing magnet, and the bobbin is over-molded to the beam. Compressive
waves introduced in the motor vehicle's structural element to which the shock sensor
is mounted travel through the spacer which is held in engagement with the structural
element, and from the spacer to the Terfenol-D sensing element. The strain in the
Terfenol-D sensing element under the influence of the magnetic field of the biasing
magnet produces a substantial change in magnetic field strength that results in
the generation of voltage in the output leads of the coil.
In the preferred embodiment the sensor is passive and has a large
voltage output that is easily detected and digitized. In an alternative embodiment,
a DC current can be supplied to the sensing coil to provide the biasing magnetic
field. The voltage produced by shock traveling through the Terfenol-D sensing element
can be detected by a high frequency filter that separates the shock sensing signal
from the applied DC biasing current. A simple series-connected capacitor in the
sensing output of the shock sensor can function as the high frequency filter.
It is a feature of the present invention to provide a shock sensor
that detects shock waves in the structural elements of a machine such as a motor
It is another feature of the present invention to provide a shock
sensor that detects shock waves in the nonferrous structural elements of a machine
such as a motor vehicle.
It is a further feature of the present invention to provide a shock
sensor for early detection of crash severity.
Further features and advantages of the invention will be apparent
from the following detailed description when taken in conjunction with the accompanying
- FIG. 1 is an exploded isometric view of the crash sensor of this invention.
- FIG. 2 is an isometric view of the crash sensor of FIG. 1 with the over-molding
of the coil shown in phantom view.
- FIG. 3 is an alternative embodiment of the crash sensor of this invention.
- FIG. 4 is a side elevation, somewhat schematic view, of a motor vehicle cut
away to show the mounting of the crash sensor of FIG. 1.
- FIG. 5 is a circuit diagram a further alternative embodiment of the crash sensor
of this invention.
Referring to FIGS. 1-4, wherein like numbers refer to similar parts,
a shock sensor 20 is shown in FIG. 2 that may be employed with machines, such as
used as a crash sensor in motor vehicles. The shock sensor 20, as shown in FIG.
1, has three functional elements: a Terfenol-D (Tb0.3 Dy0.7 Fe1.92) sensing element
22, a biasing magnet 24, and a sensing coil 26 mounted on a housing 27. The housing
27 has a cylindrical portion 32 that projects from a beam 34. The shock sensor 20
incorporates a bobbin 28 on which the sensing coil 26 is wound. The bobbin 28 has
a central aperture 30 that fits over the cylindrical housing portion 32. The sensing
coil 26 can have for example, around 1,000 turns of thirty-six gauge wire. The beam
34 extends on either side of the cylindrical housing portion 32 and has two apertures
36, one formed in each end 38 of the beam. As shown in FIG. 1, a sleeve 40 is loaded
with a cylindrical biasing magnet 24, a cylindrical Terfenol-D sensing element 22
and a cylindrical spacing element 42 that protrudes from the sleeve 40. The biasing
magnet 24, the Terfenol-D sensing element 22 and the cylindrical spacing element
42 can be press fit or bonded to the sleeve 40. The sleeve 40 is positioned within
the cylindrical housing 32 so that the spacing element 42 protrudes beyond a land
44, on the bottom surface 46 of the beam 34. The sleeve 40 can be press fit or bonded
within the cylindrical housing 32. The bobbin 28 about which the sensing coil 26
is wound is surrounded by an over molded enclosure 48, as shown in FIG. 2.
End bosses or lands 50 surround the apertures 36 and define a mounting
plane. Fasteners 52 extend through the end apertures 36 of the beam 34 and mount
the shock sensor 20 to a structural element 54 of a machine, such as for use as
a crash sensor on a motor vehicle 56, as shown in FIG. 4. The cylindrical spacing
element 42 extends beyond the mounting plane so that when the end bosses 50 are
brought into contact with a portion of a structural element 54, the spacing element
42 is resiliently compressed by the flexure of portions 57 of the housing 27 between
the fasteners 52 and the Terfenol-D sensing element. This resilient flexure of housing
portions 57 compresses the Terfenol-D sensing element against the structural element
54 of the motor vehicle 56. This compressive loading assures good transmission of
shock waves from the structural element 54 to the Terfenol-D sensing element 22.
To improve the transmission of shock from the spacing element 42 to the Terfenol-D,
the spacing element 42 is preferably bonded with an adhesive to the sensing element.
The output from the coil may be from about 0.2 to 2.0 volts or greater,
depending on the number of turns in the sensing coil 26, the biasing field of the
biasing magnet 24, the composition of the structural element to which the shock
sensor 20 is mounted, the force of the pre-load on the spacing element 42 and other
factors affecting coupling between the Terfenol-D sensing element 22 and the structural
material to which the shock sensor 20 is mounted.
An alternative embodiment shock sensor 58 is shown in FIG. 3. The
shock sensor 58 is similar to the shock sensor 20, but has only one mounting aperture
60 in an end 62 of a cantilever beam 64, rather than the two apertures 36 on the
opposite ends of the beam 34 of the shock sensor 20. The shock sensor 58 has a housing
59 with a cantilever beam 64 that forms a biasing member that resiliently compresses
the sensing element by way of a spacing element 66 against the structural element
54 of the motor vehicle 56. The spacing element 66, as in the device 20, engages
a biasing magnet that extends within the sensing coil 26 formed on a bobbin.
The shock sensor 58 housing has a boss 68 that surrounds the opening
into which the spacing element 66 is fitted. A mounting boss 70 projects from the
housing around a mounting aperture 60, and a mounting plane is defined by the mounting
boss 70. The spacing element 66 extends from the housing beyond the mounting plane.
Flexure of the cantilever beam 64 caused by the spacing element 66 extending beyond
the mounting plane causes the compressive loading between the spacing element 66
and the structural element 54 of the motor vehicle 56.
The spacing element 42 could be omitted and the Terfenol-D sensing
element extended to engage the motor vehicle structural element 54. A second biasing
magnet could also be used between the spacing element 42 and the Terfenol-D sensing
element 22, or instead of the spacing element 42 to increase the strength of the
biasing magnetic field. The magnet will preferably be of a high-strength type such
as those fabricated with a rare earth metal, for example neodymium-iron-boron magnets.
The biasing magnet 24 can be replaced with a DC voltage 72 as shown
in FIG. 5 which is applied to the sensing coil 74 that contains a Terfenol-D sensing
element 76. The output of the coil 74 may be applied to a high frequency filter
such as formed by capacitor 78. The high frequency filter separates the voltage
produced by the shock wave passing through the Terfenol-D sensing element from the
supplied DC biasing voltage 72. The output of the high frequency filter can be supplied
to a safety system 80 or other processing circuit for characterizing an motor vehicle
Terfenol-D is an alloy of Tb0.3 Dy0.7 Fe1.92 but the term giant magnetostrictive
material is defined to include Terfenol-D and various alloys of highly magnetostrictive
rare earths such as Tb and Dy, as disclosed in US 4 308 474. A sensing assembly
is defined which include the sensing element 22 alone or the sensing element plus
the spacing element 42.