Note: Descriptions are shown in the official language in which they were submitted.
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ARRANGEMENT FOR DETERMINING THE TRAVEL OF A PISTON
_ _ _
BACKGROUND OF TXE INVENTION
FIELD OF TEE INVENTION
The invention relates generally to an arrangement for
determining ~he position or travel of a piston in hydraulic,
pneumatic or hydro-pneumatic assemblies, such as vibration
dampers, gas springs and hydro-pneumatic suspensions, and more
particularly, to vi.bration dampers having the piston arranged to
slide axially by means of a piStQn rod in a cylinder filled with
at least one damping medium.
DESCRIPTION OF PRIOR ART
Suspensions are, for example, known from British Laid-Open
Patent Application No. 8110972 and its corresponding German Laid-
Open Patent Application DE-OS 32 12 433 in which each suspension
unit of a vehicle has a sensor for adjusting its platform height
producing an output magnitude which changes progressively with
the platform height. The drawback here is that for adjusting the
height of the platform, i.e. the spacing between the sprung and
unsprung parts of the vehicle, a platform height sensor is
provided at each respective suspension unit. Such a device is
particularly expensive as the corresponding measured values
between the vehicle suspension and the actual suspension unit or
strut must be ascertained. Further, this determination of the
measured value is expensive as corresponding measurement trans-
ducers must be provided on the widely different parts of the
vehicle.
In arrangements for the contactless measurement of the
travel of a piston relating to some aspects of the invention, it
has been found that because of different relatively moving
$
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eLectLode components, precise evaluation of the electric signals
produced by the electrodes becomes difficult owing to disturbing
influences arising through unfavorable lengths of lead.
OBJECTS OF THE INVENTION
Taking the above as a starting point, it is the object of
the invention to provide a method of measuring the piston travel,
the apparatus of which operates without physical contact between
the parts of the transducer thereby providing a high degree of
reliability.
It is a further object of the invention to integrate the
measuring aLrangement into the suspension of a vehicle, which
arrangement has relatively small dimensions with respect to its
incorporating unit and thereby requires no major modifications to
existing hydraulic, pneumatic or hydro-pneumatic assemblies such
as vibration dampers, gas springs and hydro-pneumatic
suspensions.
It is yet a further object of this invention to
substantially eliminate movable connecting leads so that
disturbing influences arising from external variations of the
measured value are avoided as far as possible so that a reliable
evaluation of the variable capacitance is achieved.
SUMMARY OF THE INVENTION
One aspect of the invention resides broadly in a cylinder
having an arrangement for determining a position of piston means
therewithin, said cylinder having a space therewithin with said
piston means disposed in said space for axial slidable movement
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within said cylinder; at least one damping medium being disposed
within said cylinder for damping sald axlal movement of said
piston means with respect to said cylinder; said arrangement for
determining the position of said piston means comprising: means
for sensing a position of said piston means within said cylinder;
said position sensing means having a first element and a second
element; said first element of said position sensing means being
disposed to be movable with said piston means; said second
element of said position sensing means being disposed to be
substantially stationary with respect to said cylinder; said
first element and said second element being electrically
insulated one from the other; said two elements forming a
variable impedance component which is subs~antially electrically
reactive; said variable reactive electrical impedance varying, in
operation, with said position of said piston means in said
cylinder; an electrical signal being generated by at least one of
said elements and being indicative of said position of said
piston means; electrical connecting means, connected to at least
one of said elements, for sensing said electrical signal,
indicative of said position of said piston means; said first
element comprising a first electrode and said second element
comprising a second electrode of a capacitor; dielectric means
being disposed at least in part between said first and second
electrodes of said capacitor; said electrical connecting means
being connected to said first electrode and to said second
electrode; said piston means including a piston rod; said first
electrode comprising at least a portion of said piston rod; said
piston rod having a hollow portion; said cylinder having a base
at one end thereof; said second electrode comprising a first,
cylindrical tube which is disposed on and extends from said base
of said cylinder means into said space within said cylinder; said
hollow portion of said piston rod having a longitudinal axis
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being substantially axially aligned with a longitudinal axis of
said first tube; and said first tube at least during operation
extending, at least partially, into said hollow portion of said
piston rod.
Arother aspect of the invention resides in a cylinder having
an arrangement for determining a position of piston means there-
within, said cylinder having a space therewithin with said piston
means disposed in said space for axial slidable movement within
said cylinder; at least one damping medium being disposed within
said cylinder for damping said axial movement of said piston
means with respect to said cylinder; said arrangement for
determining the position of said piston means comprising: means
for sensing a position of said piston means within said cylinder;
said position sensing means having a first element and a second
element; said first element of said position sensing means being
disposed to be movable with said piston means; said second
element of said position sensing means being disposed to be
substantially stationary with respect to said cylinder; said
first element and said second element being electrically
insulated one from the other; said two elements forming a
variable impedance component which is substantially electrically
reactive; said variable reactive electrical impedance varying, in
operation, with said position of said piston means in said
cylinder; an electrical signal being generated by at least one of
said elements and being indicative of said position of said
piston means; electrical connecting means, connected to at least
one of said elements, for sensing said electrical signal,
indicative of said position of said piston means; said first
element comprising a first electrode and said second element
comprising a second electrode of a capacitor; dielectric means
being disposed at least in part between said first and second
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electrodes o said capacitor; said electrical connecting means
being connected to said first electrode and to said second
electrode; said cylinder having a base; said piston means having
a face disposed towards said base, said base of said damper
cylinder being insulated from the cylinder to form said second
electrode of said capacitor; and said face of said piston means
forming said first electrode of said capacitor.
Yet another aspect of the invention resides broadly in a
cylinder having an arrangement for determining a position of
piston means therewithin, said cylinder having a space there-
within with said piston means disposed in said space for axial
slidable movement within said cylinder; at least one damping
medium being disposed within said cylinder for damping said axial
movement of said piston means with respect to said cylinder; said
arrangement for de~ermining the position of said piston means
comprising: means for sensing a position of said piston means
within said cylinder; said position sensing means having a first
element and a second element; said first element of said position
sensing means being disposed to be movable with said piston
means; said second element of said position sensing means being
disposed to be substan~ially stationary with respect to said
cylinder; said first element and said second element being
electrically insulated one from the other; said tWG elements
forming a variable impedance component which is substantially
electrically reactive; said variable reactive electrical
impedance varying, in operation, with said position of said
piston means in said cylinder; an electrical signal being
generated by at least one of said elements and being indicative
of said position of said piston means; electrical connecting
means, connected to at least one of said elements, for sensing
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said electrical signal, indicative of said position of said
piston means; and an impedance bridge, connected to said
connecting means, for generating an electrical signal indicative
of a position of said piston means.
A further aspect of the invention resides in a cylinder
having an arrangement for determining a position of piston means
therewithin, said cylinder having a space therewithin with said
piston means disposed in said space for axial slidable movement
within said cylinder; at least one damping medium being disposed
within said cylinder for damping said axial movement of said
piston means with respect to said cylinder; said arrangement for
determining the position of said piston means comprising: means
for sensing a position of said piston means within said cylinder;
said position sensing means having a first element and a second
element; said first element of said position sensing means being
disposed to be movable with said piston means; said second
element of said position sensing means being disposed to be
substantially stationary with respect to said cylinder; said
first element and said second element being electrically
insulated one from the other; said two elements forming a
variable impedance component which is substantially electrically
reactive; said variable reactive electrical impedance varying, in
operation, with said position of said piston means in said
cylinder; an electrical signal being generated by at least one of
said elements and being indicative of said position of said
piston means; electrical connecting means, connected to at least
one of said elements, for sensing said electrical signal,
indicative of said position o~ said piston means; said first
element comprising a first electrode and said second element
comprising a second electrode of a capacitor; dielectric means
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being disposed at least in part between said first and second
electrodes of said capacitor; said electricaI connecting means
being connected to said first electrode and to said second
electrode; said piston means including a piston rod;
said first electrode comprising at least a portion of said piston
rod; capacitance measuring circuitry for connection to said
capacitor; said capacitance measuring circuitry comprising a
capacitor bridge; said variable capacitor forming one of the arms
of one half of sai.d capacitor bridge; and an amplifier being
connected to said capacitor bridge for generating an electrical
signal corresponding to the position of said piston in said
damper cylinder.
A yet further aspect of the invention resides broadly in a
cylinder having an arrangement for determining a position of
piston means therewithin, said cylinder having a space
therewithin with said piston means disposed in said space for
axial slidable movement within said cylinder; at least one
damping medium being disposed within said cylinder for damping
said axial movement of said piston means with respect to said
cylinder; one of said at least one damping medium being hydraulic
damping oil; said arrangement for determining the position of
said piston means comprising: said piston means including a
circular, hollow piston rod; said cylinder having a base at one
end thereof from which a hollow circular cylindrical, first tube
extends into said space and also into said hollow piston in a
telescopic relationship therewith, thereby forming, with a
dielectrical material, comprising said hydraulic damping oil,
disposed between said first tube and said piston rod, a variable,
first capacitor; a longitudinal axis of said piston rod being
substantially aligned with a longitudinal axis of said first
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tube; said first tube being insulated from said cylinder and said
hollow piston rod; a first lead being connected to said first
tube; and a second lead being connected to said hollow piston
rod.
Yet another further aspect of the invention resides broadly
in an arrangement for measuring relative travel of a piston in a
cylinder, comprising: a supporting member housing a first
electrode which is mounted substantially fixed but substantially
electrically insulated with respect to the supporting member; a
second electrode spaced from the first electrode to define a
space therebetween, said second electrode being substantially
fixed with respect to the first electrode and being substantially
electrically insulated from the supporting member, said first and
second electrodes in use forming a capacitive reactance; a
reactance varying member which is mounted to move in accordance
with said relative travel between the piston and the cylinder,
said reactance varying member being so disposed that its movement
changes the capacitance between said first and second electrodes
in accordance with said relative movement between the piston and
the cylinder; and electrical connecting means being connected to
said fixed first and second electrodes for measuring the capaci-
tance formed therebetween and for producing a signal indicative
of relative movement between the piston and the cylinder.
An additional aspect of the invention resides broadly in an
arrangement for measuring relative travel of a piston in a
cylinder which is part of a hydro-pneumatic assembly, comprising:
a suppor~ing member in the form of a hollow piston rod and
housing having fixed first and second spaced substantially
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coaxial tubular electrodes which are bo~h substantially electri-
call~ insulated from the supporting member and mounted in a
substantially fixed relationship with respect to each other to
form a substantially annular gap; electrical leads connected to
said first and second electrodes, said first and second elec-
trodes in use forming a variable capacitance; a sleeve-like
member which is mounted for movement reflecting said relative
travel of the piston in said cylinder, said sleeve-like member
being disposed to enter said substantially annular gap to vary in
use said capacitance between said first and second electrodes;
and said variable capacitance producing in use a signal indica-
tive of relative travel of said piston in said cylinder.
A yet additional aspect of the invention resides broadly in
an arrangement for measuring relative travel of a piston in a
cylinder which is part of a mechanical suspension assembly,
comprising: a hollow piston rod housing first and second,
substantially coaxial tubular electrodes which are spaced from
one another and which are both substantially electrically
insulated from the piston rod and mounted in a substantially
fixed relationship with respect to each other to form a
substantially annular gap, said first and second electrodes
forming a variable capacitance; a sleeve-like member which is
mounted for movement reflecting said relative travel of the
piston in said cylinder, said sleeve-like member being disposed
to slidingly enter said substantially annular gap to vary said
variable capacitance; a third electrode substantially electri-
cally insulated from both the first and second electrodes and
spaced from the second electrode to form a substantially fixed
reference capacitance in use; and electrical connecting means
connected to said fixed first, second and third electrodes.
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A further additional aspect of the invention resides broadly
in an arrangement for measuring relative travel of a piston in a
cylinder, comprising: a supporting member housing a first
electrode which is mounted fixed but electrically insulated with
respect to the supporting member; a second electrode spaced from
the first electrode to define a space therebetween, said second
electrode being immovable with respect to the first electrode and
being electrically insulated from the supporting member, said
first and second electrodes in use forming a capacitive reac-
tance; a reactance varying member which is mounted to move in
accordance with said relative travel between the piston and the
cylinder, said reactance varying member being so disposed that
its movement changes the capacitance between said first and
second electrodes in accordance with said relative movement
between the piston and the cylinder; and electrical measuring
means being connected to said fixed first and second electrodes
to measure the capacitance formed therebetween and produce a
signal indicative of relative movement between the piston and the
cylinder.
A yet further additional aspect of the invention resides
broadly in an arrangement for measuring relative travel of a
piston in a cylinder which is part of a hydro-pneumatic assembly,
comprising: a supporting member in the form of a hollow piston
rod and housing fixed first and second spaced substantially
coaxial tubular electrodes which are both electrically insulated
from the supporting member and mounted in fixed relationship with
respect to each other to form a substantially annular gap; elec-
trical leads connected to said first and second electrodes, said
first and second electrodes in use forming a variable capaci-
tance; a sleeve-like member which is mounted for movement
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reflecting said relative travel of the piston in said cylinder,
said sleeve-like member being disposed to enter said substan-
tially annular gap to vary said capaci~ance between said first
and second electrodes; and electrical measuring means being
connected to said electrical leads to produce a signal indicative
of relative travel of said piston in said cylinder.
Another further additional aspect of the invention resides
broadly in an arrangement for measuring relative travel of a
piston in a cylinder which is part of a mechanical suspension
assembly, comprising: a hollow piston rod housing first and
spaced substantially coaxial tubular electrodes which are both
electrically insulated from the piston rod and mounted in fixed
relationship with respect to each other to form a substantially
annular gap, said first and second electrodes forming a variable
capacitance; a sleeve-like member which is mounted for movement
reflecting said relative travel o~ the piston in said cylinder,
said sLeeve-like member being disposed to slidingly enter said
substantially annular gap to vary said variable capaci~ance; a
third electrode electrically insulated from both the second
electrode and spaced from the second electrode to form a fixed
capacitance in use; electrical leads connected to said fixed
first, second and third electrodes; and an electrical measuring
bridge connected to at least said first, second and third
electrodes to measure said variable capacitance to produce a
signal, by using said fixed capacitance as a reference, said
signal indicating the extent of movement of said piston in said
cylinder.
A yet another additional aspect of the invention resides
broadly in a damping cylinder of a vibration damper, said damping
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cylinder having an arrangement for determining a position of
piston means therewithin, said cylinder having a space there-
within with said piston means disposed in said space for axial
slidable movement within said cylinder; at least one damping
medium being disposed within said cylinder for damping said a~ial
movement of said piston means with respect to said cylinder; said
arrangement for deter~ining the position of said piston means
comprising: means for sensing a position of said piston means
within said cylinder; said position sensing means having a first
element and a second element; said first element of said position
sensing means being disposed to be movable with said piston
means; said second element of said position sensing means being
disposed to be substantially stationary with respect to said
cylinder; said first element and said second element being
electrically insulated one from the other; said two elements
forming a variable impedance component which is substantially
electrically reactive; said variable reactive electrical
impedance varying, in operation, with said position of said
piston means in said cylinder; an electrical signal being
generated by at least one of said elements and being indicative
of said position of said piston means; electrical connecting
means, being connected to at least one of said elements, for
sensing said electrical signal, indicative of said position of
said piston means; said first element comprising a first
electrode and said second element comprising a second electrode
of a capacitor; dielectric means being disposed at least in part
between said first and second electrodes of said capacitor; said
electrical connecting means being connected to said first elec-
trode and to said second electrode; said piston means including a
piston rod; said first electrode comprising at least a portion of
said piston rod; said piston rod having a hollow portion; said
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cylinder having a base at one end thereof; said second electrode
comprising a first, cylindrical tube which is disposed on and
extends from said base of said cylinder means into said space
within said cylinder; said hollow portion of said piston rod
having a longitudinal axis being substantially axially aligned
with a longitudinal axis of said first tube; and said first tube
at least during operation extending, at least partially, into
said hollow portion of said piston rod.
Another yet further aspect of the invention resides broadly
in a damping cylinder of a vibration damper, said damping
cylinder having an arrangement for determining a position of
piston means therewithin, said cylinder having a space there-
within with said piston means disposed in said space for axial
slidable movement within said cylinder; at least one damping
medium being disposed within said cylinder for damping said axial
movement of said piston means with respect to said cylinder; said
arrangement for determining the position of said piston means
comprising: means for sensing a position of said piston means
within said cylinder; said position sensing means having a first
element and a second element; said first element of said position
sensing means being disposed to be movable with said piston
means; said second element of said position sensing means being
disposed to be substantially stationary with respect to said
cylinder; said first element and said second element being
electrically insulated one from the other; said two elements
forming a variable impedance component which is substantially
electrically reactive; said variable reactive electrical
impedance varying, in operation, with said position of said
piston means in said cylinder; an electrical signal being
generated by at least one of said elements and being indicative
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of said position of said piston means; electrical connecting
means, being connected to at least one of said elements, for
sensing said electrical signal, indicative of said position of
said piston means; said first element comprising a first
electrode and said second element comprising a second electrode
of a capacitor; dielectric means disposed at least in part
between said first and second electrodes of said capacitor; and
said electrical connecting means being connected to said first
electrode and to said second electrode; said cylinder having a
base; said piston means having a face disposed towards said base,
said base of said damper cylinder being insulated from the
cylinder to form said second electrode of said capacitor; and
said face of said piston means forming said first electrode of
said capacitor.
A still further aspect of the invention resides broadly in a
damping cylinder of a vibration damper, said damping cylinder
having an arrangement for determining a position of piston means
therewithin, said cylinder having a space therewithin with said
piston means disposed in said space for axial slidable movement
within said cylinder; at least one damping medium being disposed
within said cylinder for damping said axial movement of said
piston means with respect to said cylinder; said arrangement for
determining the position of said piston means comprising:
means for sensing a position OI said piston means within
said cylinder; said position sensing means having a first element
and a second element; said first element of said position sensing
means being disposed ~o be movable with said piston means; said
second element of said position sensing means being disposed to
be substantially stationary with respect to said cylinder; said
first element and said second element being electrically
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insulated one from the other; said ~wo elements forming a
variable impedance component which is subst&ntially electrically
reactive; said variable reactive electrical impedance varying, in
operation, with said position of said piston means in said
cylinder; an electrical signal being generated by at least one of
said elements and bein8 indicative of said position of said
piston means; and electrical connecting means, being connected to
at least one of said elements, for sensing said electrical
signal, indicative of said position of said piston means; and an
impedance bridge, being connected to said connecting means, for
generating an electrical signal indicative of a position of said
piston means.
A still further additional aspect of the invention resides
broadly in a damping cylinder of a vibration damper, said damping
cylinder having an arrangement for determining a position of
piston means therewithin, said cylinder having a space there-
within with said piston means disposed in said space for axial
slidable movement within said cylinder; at least one damping
medium being disposed within said cylinder for damping said axial
movement of said piston means with respect to said cylinder; said
arrangement for determining the position of said piston means
comprising: means for sensing a position of said piston means
within said cylinder; said position sensing means having a first
element and a second element; said first element of said position
sensing means being disposed to be movable with said piston
means; said second element of said position sensing means being
disposed to be substantially stationary with respect to said
cylinder; said first element and said second element being
electrically insulated one from the other; said two elements
forming a variable impedance component which is substantially
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electrically reactive; said variable reactive electrical
impedance varying, ln operation, with said position of said
piston means in said cylinder; an electrical signal being
generated by at least one of said elements and being indicative
of said position of said piston means; electrical connecting
means, being connected to at least one of said elements, for
sensing said electrical signal, indicative of said position of
said piston means; said first element comprising a first
electrode and said second element comprising a second electrode
of a capacitor; dielectric means being disposed at least in part
between said first and second electrodes of said capacitor; and
said electrical connecting means being connected to said first
electrode and to said second electrode; said piston means
including a piston rod; said first electrode comprising at least
a portion of said piston rod; capacitance measuring circuitry for
connection to said capacitor; said capacitance measuring
circuitry comprising a capacitor bridge; said variable capacitor
forming one of the arms of one half of said capacitor bridge; and
an amplifier being connected to said capacitor bridge for
generating an electrical signal corresponding to the position of
said piston in said damper cylinder.
Another still further additional aspect of the invention
resides broadly in a damping cylinder of a vibration damper, said
cylinder having an arrangement for determining a position of
piston means therewithin, said cylinder having a space there-
within with said piston means disposed in said space for axial
slidable movement within said cylinder; at least one damping
medium being disposed within said cylinder for damping said axial
movement of said piston means with respect to said cylinder; one
of said at least one damping medium being hydraulic damping oil;
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~LZ49354
said arrangement for determining the position of said piston
means comprising: said piston means including a circular, hollow
piston rod; said cylinder having a base at one end thereof from
which a hollow circular cylindrical, first tube extends into said
space and also into said hollow piston in a telescopic
relationship therewith> thereby forming, with a dielectric
material, comprising said hydraulic damping oil, disposed between
said first tube and said pi.ston rod, a variable, first capacitor;
a longitudinal axis of said piston rod being substantially
aligned with a longitudinal axis of said first tube; said first
tube being insulated from said cylinder and said hollow piston
rod; a first ].ead being connected to said first tube; and a
second lead being connected to said hollow piston rod.
Yet another still further additional aspect of the invention
resides broadly in a damping cylinder in a vibration damper, said
cylinder having an arrangement for measuring relative travel of a
piston in said cylinder, comprising: a supporting member housing
a first electrode which is mounted substantially fixed but sub-
stantially electrically insulated with respect to the supporting
member; a second electrode being spaced from the first electrode
to define a space therebetween, said second electrode being
substantially fixed with respect to the first electrode, said
first and second electrodes in use forming a capacitive
reactance; a reactance varying member which is mounted to move in
accordance with said relative travel between the piston and the
cylinder, said reactance varying member being so disposed that
its movement changes the capacitance between said first and
second electrodes in accordance with said relative movement
between the piston and the cylinder; and electrical connecting
means connected to said fixed first and second electrodes for
measuring the capacitance formed therebetween and for producing a
signal indicative of relative movement between the piston and the
cylinder.
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Still another yet further additional aspect of the invention
resides broadly in a damping cylinder ln a vibration damper, said
cylinder having an arrangement for measuring relative travel of a
piston in said cylinder, comprising: a supporting member in the
form of a hollow piston rod and housing having fixed first and
second spaced substantially coaxial tubular electrodes which are
both substantially electrically insulated from the supporting
member and mounted in a substantially fixed relationship with
respect to each other to form a substantially annular gap;
electrical leads connected to said first and second electrodes,
said first and second electrodes in use forming a variable
capacitance; a sleeve-like member which is mounted for movement
reflecting said relative travel of the piston in said cylinder,
said sleeve-like member being disposed to enter said substan-
tially annular gap to vary in use said capacitance between said
first and second electrodes; and said variable capacitance
producing in use a signal indicative of relative travel of said
piston in said cylinder.
Another still further yet additional aspect of the invention
resides broadly in an arrangement for measuring relative travel
of a piston in a damping cylinder of a vibration damper for a
mechanical suspension assembly for a vehicle, comprising: a
hollow piston rod housing first and second, substantially coaxial
tubular electrodes which are spaced from one another and which
are both substantially electrically insulated from the piston rod
and mounted in a substantially fixed relationship with respect to
each other to form a substantially annular gap, said first and
second electrodes forming a variable capacitance; a sleeve-like
member which is mounted for movement reflecting said relative
travel of the piston in said cylinder, said sleeve-like member
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being disposed to slidingly enter said substantially annular gap
to vary said variable capacitance; a third electrode
substantially electrically insulated from both the first and
second electrodes and spaced from the second electrode to form a
substantially fixed reference capacitance in use; and electrical
connecting means connected to said fixed first, second and third
electrodes.
The problems related to the prior art are solved, according
to one embodiment of the invention, by providing a capacitor
which is formed in the interior of the cylinder in such a way
that a first axially displaceable electrode is formed directly or
indirectly by the piston and/or the piston rod and a second
electrode, insulated from the first, is held fixed within the
cylinder. These first and second electrodes are separated from
one another by a dielectric.
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3~4
In this first embodiment, it is preferable that a compact
measuring system assem~ly is provided by a physical contactlessly
workable method of measurement, so that the measuring system can
be integrated into, for example, a vibration damper, a gas spring
or a hydro-pneumatic suspPnsion without major modifications.
Further advantages of this embodiment of the invention are the
low-cost o~ manufacture and the providing of vehicle suspension
which can readily f~nction together with the arrangement for
determining the travel or position of a piston therein, as a
self-contained unitary system.
According to a further important feature of the invention,
it is provided that the piston rod is made hollow, thereby
forming the first electrode. A second electrode comprising a
tube, is provided and disposed within the hollow space in the
piston rod and spaced therefrom. This tube is secured in the
base of the cylinder and is insulated from the first electrode
and also preferably from the cylinder. The second electrode is
disposed substantially coaxially with the first electrode and at
least partially therewithin. The first electrode is axially
displaceable in a telescopical fashion relative to the second
electrode.
It has been found particularly desirable to design and make
a hydro-pneumatic vibration damper so that a cylindrical
capacitor can be provided therein with minimal structural
modifications. The typically oil-filled vibrational damper
cylinder and its often hollow piston rod have associated there-
with an additional hollow cylinder which is held by an insulating
body in the base o~ the vibration damper cylinder. The hollow
piston rod and the additional cylinder form a cylindrical
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capacitor which, as a result of the varying overlapping surfaces9
form a first and second electrode, which arrangement produces a
variation in capacitance so that the desired value of a physical
magnitude to be measured is provided by a change in the
capacitance between the electrodes.
The travel of the piston results in the change of capaci-
tance as a function of the piston displacement. The position of
the piston is uniquely related to the capacitance. The capaci-
tance of the cylindrical capacitor as a function of piston
position (L) or travel from the position just before any portion
of the two circular cylinders telescope together or become
radially adjacent to one another is:
Cylinder 2~ x Eo x Er x L
ln 2
r
where
ACCylinder = change in capacitance
Eo = the dielectric constant of free space
Er = the relative dielectric constant of the oil in
the damper
L = length of piston travel
r2 = radius of the bore of the piston rod
rl = outer radius of the hollow cylinder secured to
the cylinder base
~ = 3.14159
This equation follows from the fact that the movable
circular piston rod is displaced from and concentrically disposed
over the inner hollow circular cylinder which inner cylinder is
secured in an insulating manner to the base o~ the outer,
~ 3~ NHL-BGG-02 Canada
vibration damper cylinder. This results in a variation of the
cylinder surfaces being radially adjacent to one another and
results in the variation of the capacitance in accordance with
~he travel or position of the piston. The electrodes are formed
by the structure, such that, one electrode on the one hand is
formed by the hollow piston rod ancl the other electrode on the
other hand is formed by the tube fixed to the base of ~he
cylinder.
According to an alternative capacitive embodiment, it is
provided that the base of the cylinder is arranged to be
insulated from the cylinder to form the second electrode and the
first electrode is formed by that face of the piston which faces
towards the base of the cylinder.
Also, in this alternative embodiment, the capacitor is
formed by the components of the assembly itself provided to form
the suspension which includes the vibrational damper of which the
capacitor is a part. Here the change in capacitance is produced
by the varying spacing or distance between the two electrodes
which capacitance is expressed by the equation:
C = Eo x Er x A
L
where,
Eo = dielectric constant of free space
Er = relative dielectric constant of oil dielectric
or in the vibrational damper
A = effective surface area of the electrodes ~faces
of piston and base of cylinder)
C - capacitance
L = distance between the electrodes
~24~354 NHL-BGG-02 Canada
By calculating the capacitance for two different positions
of the piston, this equation shows that a change in the spacing,
between the piston and the cylinder base, produces a change in
the capacitance by which the corresponding position change or
travel can be derived.
To produce a corresponding electric signal, which can be
used, for example, for controlling the height of a vehicle, it is
provided according to a yet further embodiment that the elec-
trodes of the capacitor are connected in a capacitance measuring
bridge and produce an electric signal corresponding to position.
This signal is fed through an amplifier for use in making adjust-
MentS to the pressure of the damping medium, thereby changing the
position of the piston to a desired position.
It is a further aim to arrange such a device so that, in
addition to the evaluation of the variable capacitance, a
simultaneous compensation for pressure and temperature is made
possible such that the pressure is regulated by the position
and/or change and/or rate of change of position of the piston
in operation.
To solve this problem, it is provided in a yet further and
third, capacitive embodiment of the invention that, within the
cylindrical interior of the inner, first tube, forming an
electrode, is secured in the base of the vibrational damper
cylinder, there is arranged and spaced from it, a further second
inner fixed tube which forms an additional electrode defining,
with the first tube, a second capacitor having a fixed
capacitance value under conditions of a fixed temperature and a
fixed pressure.
~g354 NHL-BG~-02 Canada
An advantage of this third embodiment is that by the
insulated incorporation of an additional, second, metallic,
circular cylinder, as a further third electrode within the
electrode already present on the base of the vibrational damper
cylinder, a second capacitor having a fixed capacitance under
specific condition is produced. This gives the advantage of a
capacitive half-bridge which is made up of the variable
capacitance (varied by displacement of the piston rod~ and the
fixed capacitance secured in the base of the cylinder which
fixed, second capacitor varies in capacitance in the same way
that the variable first capacitor varies from influences other
than displacement of the piston and/or piston rod.
This arrangement in the th~rd capacitive embodiment not only
allows improved evaluation of the variable capaci~ance but also
achieves a simultaneous compensation for at least pressure a~d
temperature in that, by means of a suitable electronic circuit,
e.g. a Wheatstone Bridge, the ratio of the two capacitances is
taken into account.
In one embodiment of the invention, it is provided that the
first tube and the further, second tube are arranged to be
insulated from one another. The further, second tube can like-
wise be secured in the base of the cylinder.
In an alternative, magnetic or inductive, embodiment of the
invention, it is provided for solving the problem posed according
to the invention that the outer surface of the cylinder has, over
at least a portion of the region of the piston travel, at least
one winding producing a magnetic field.
It is advantageous that an inductive or magnetic field
should be produced. The piston and piston rod of the vibration
~ ~ NHL-BGG-02 Canada
damper, being disposed in this magnetic field, produces an
increase in the inductance as the piston and piston rod move
further into the damper cylinder so that after measuring and
converting the inducta~ce into an electrical magnitude the
determination of the piston position and thus travel can likewise
be obtained. Also this embodiment is suitable for the output
signal to be able to be used as a basis for, for example, vehicle
height control.
A further, inductive embodiment of the invention envisages
that two windings are provided, having longitudinal axes which
are substantially coextensive with the longitudinal axis of the
cylinder of the vibration damper. Each of the windings covers
preferably the distance of half the piston travel. In these
versions it is advantageous that the piston rod can be made solid
and not necessarily hollow. Ferromagnetic materials with rela-
tively high permeance are advantageously used for making at least
portions of the piston and/or the piston rod.
By such an arrangement of the windings, two inductances are
formed which can be used directly as the series components of one
side of an inductance measuring bridge .
To solve the problem of disturbing influences arising from
external variations of the measured value because of the movement
of movable connecting leads, according to one embodiment of the
invention, there is provided in a vibration damper a first
electrode insulated with respect to a supporting component that
receives it, and a second electrode arranged non-displaceably
within the interior of the first electrode and spaced from it; a
dielectric member having a known dielectric constant is disposed
so as to be able to enter between ~he first and second electrodes,
during the functioning of the vibration damper.
~ 35~ NHL-BGG-02 Canada
In such an arrangement, it is significant that no unfavor-
able lengths of lead are present. The electric connections for
the two electrodes are taken out of the supporting components
that receive ~hem and accordingly are close to one another. By
the avoidance of unnecessary lengths of lead, any disturbing in-
fluences Erom outside are largely eliminated, so that a reliable
evaluation of the measured values can be achieved. Further, it
is of advantage that the mounting of a measuring arrangement can
be selected according to the type of vehicle so that measuring
means are mounted either on the piston rod or on the cylinder,
and this allows one to avoid mounting on the unsprung mass of the
vehicle and allows mounting on the bodywork to be achieved
without difficulty.
According to an important feature of the invention, the
first and second electrodes are arranged advantageously in the
interior of the piston rod, and the dielectric medium used is
secured in the cylinder.
Expediently, in such an arrangement, a contactlessly
operating method of making the physical measurement can be
received in the interior of the piston rod as a compact assembly
so that simple integration is possible. Since all the components
employed are tubular, they are matched without any problems, to
the geometry of the piston rod. Such a system can be employed
without major modifications, for example, in the piston rod of a
vibration damper, a gas spring or a hydro-pneumatic suspension.
In another preferred embodiment of the invention, the first
and second electrodes are mounted in the cylinder whereas the
dielectric is provided inside the piston rod.
~4~5~ NHL-BGG-02 Canada
Preferably, the dielectric comprises a ceramic tube; by
the use of such a tube because of the characteristics of the
ceramic material, compensation for pressure and temperature in
the measuring system will not be necessary.
In an improvement of the foregoing embodiment, the cylindri-
cal inner surface of the ceramic tube is advantageously connected
electrically to the cylindrical outer surface of the second
electrode. By such a conducting inner layer, the oil gap which
would otherwise be made necessary by the tolerances can be
eliminated. This oil gap acts in principle as a further dielec-
tric because of the tolerances and the consequently varying
spacing, whereby variations arise in the measured values; such
undesirable variations can be eliminated by the electrically
conducting connection. In order to further ~acilitate formation
of the electrically conducting connection, the cylindrical inner
surface of the ceramic tube may be provided with a metallic
layer. In order to additionally amplify effectiveness of the
electric conducting layer, preferably, an electrically conducting
ring is mounted on the cylindrical outer surface of the second
electrode, this ring having an outside diameter corresponding to
the inside diameter of the dielectric.
To evaluate the variable capacitance with simultaneous com-
pensation for pressure and temperature, it is proposed according
to a further embodiment that all the electrodes should be fixed
relative to one another and should be insulated from the sup-
porting component that receives them, the firs~ and second
electrodes being separated from each other by a dielectric.
Also, a third electrode is spaced from the second electrode in
such a way that a tubular body can enter the space between them.
12
NHL-BGG-02 Canada
3 Z~35~
In such an arrangement, it is of significance that no ~mfavorable
lengths of conduc~ing lead are present. The electric connections
for the individual electrodes are taken directly out of ~he
component and accordingly lie close to one another. By elimi
nating unnecessary lengths of lead, disturbing influences from
outside are largely eliminated, so that a reliable evaluation of
the measured signals can be achieved. Furthermore, it is an
advantage that the mounting of the measuring system can be chosen
in accordance with the type of vehicle so that the apparatus is
mounted ei~her on the piston rod or on the cylinder, which allows
one to avoid mounting the arrangement on the ~msprung mass of the
vehicle and allows it to be mounted on the bodywork.
According to a further important feature of the invention,
all ~he electrodes are mounted in the interior of the piston rod,
an insulating body being provided between the outer surface of
the first electrode and the cylindrical inner surface of the
piston rod, the tubular body being connected rigidly to the
cylinder.
In this embodiment, it is advantageous that the contactless
arrangement of making the physical measurement can be received in
the interior of the piston rod as a compact assembly unit so that
simple integration is possible with the components which are all
of tubular shape.
It is furthermore advantageous that all the connections and
the actual capacitor are received within the piston rod. This
allows the possibility of mounting the evaluating electronic
circuits without ups~tting acceleration influences, since the
piston rod is usually mounted on the bodywork, so that during
operation of the vehicle no noticeable acceleration acts on the
piston rod.
NHL-BGG-02 Canada
In order ~o take advan~age of the geometry of the hollow
piston rod and simultaneously realize all cost-favorable and
manufacturing advantages, a tube of synthetic resin, such as an
epoxy resin, is used as the insulating body in another embodiment.
In a further important modifieation, an earthed electrically
conducting tube is provided as the tubular body. This tubular
body is secured in the base of the cylinder and3 on inward
movement of the piston rod it enters between the second and third
electrodes. The first and second electrodes form a fixed capaci-
tance which is introduced for compensation of varying temperature
and pressure influences on the measurement.
The second and third electrodes, between which the tube
enters, produce a resultant variable capacitance for determining
the travel of the piston. In this connection it is advantageous
that the penetration of the earthed tube between the second and
third electrodes reduces the capacitance since the effective
capacitive surface area is reduced.
In an important modification, it is envisaged that the
tubular body comprises an electrically non-conducting tube. In
this way, full use is made of the fact that the non-conducting
tube acts as a dielectric and accordingly on entry between the
second and third electrodes, a parallel circuit of two capaci-
tances is obtained. This parallel circuit is a consequence of
the two different dielectrics, since the insulating tube and the
medium (e.g. mineral oil, such as commercially available shock
absorber oil) present in the remaining part have different
dielectric constants. During the measurement of the variable
capacitance, this characteristic is represented by a parallel
connection of two capacitances, the overall capacitance being
14
~2~93~4 NHL-BGG-02 Canada
derived by addition of the two individual capacitances.
A particularly preferred material for the tubular body
comprises synthetic resin.
BRIEF DESCRIPTION OF THE DRAWINGS
Preferred embodiments of the invention by way of example are
illustrated diagrammatically in the accompanying drawings
wherein:
Figure 1 shows a sectional view of a vibration damper, in
which the hollow piston rod forms a variable capacitor in
conjunction with a tube in a cavity of the damper.
Figure 2 shows a sectional view of a diagrammatic represen-
tation of a cylindrical capacitor.
Figure 3 shows a diagram of capacitance of the cylindrical
capacitor of Figure 2 with respect to piston position or piston
travel.
Figure 4 shows a sectional view of a further embodiment of a
vibration damper, in which the face of the piston and the base of
the cylinder form plates of a capacitor.
Figure 5 shows a sectional view of a cylindrical vibration
damper wherein an inductive coil is disposed on the surface of
the cylinder.
Figure 6 shows a sectional view of a vibration damper wi~h
another coil arrangement.
Figure 7 shows a sectional view of a vibration damper in
which the tube forms, with a further cylindrical tube, a second
fi~ed capacitor.
Figure 8 shows schematically the embodiments of Figure 1 and
Figure 7 arranged in an impedance measuring bridge.
NHL-BGG-02 Canada
~24~3S~
Figure 9 shows schematically the inductive coil arrangement
oE Figure 5 connected into an impedance measuring bridge.
Figure 10 shows schematically the coil arrangement of Figure
6 connected into an impedance measuring bridge.
Figure 11 shows a sectional view of another embodiment of
which a tubular capacitor is arranged in the interior or the
piston rod.
Figure 12 shows a sectional view of a vibration damper in
which a compensator is mounted in the in~erior of the piston rod.
Figure 13 depicts a piston travel/capacitance diagram for a
cylindrical capacitor with compensation for $emperature.
DESCRIPTION OF THE, PREFERRED EMBODIMENTS
The vibration damper illustrated in Figure 1 comprises
substantially a cylinder 1, a piston 2 and a piston rod 3, well-
known in the prior art. In the interior cavity 4 of the cylinder
1 there is at least one damping medium serving for damping
vibrations of the piston 2. Attachment means 5 and 6, well-known
in the prior art, are provided for mounting the vibration damper
in a vehicle. The piston rod 3, which is made hollow, slides
telescopically over a tube 7, the tube 7 being located in the
cylinder 1, and at~ached thereto by an insulating body 8 which
insulates the tube 7 from the remainder of the components of the
damper. The hollow piston rod 3 and the tube 7 form together a
variable circular cylindrical capacitor. The damping medium
present in the interior cavity 4 for~s a dielectric for the
capacitor. The hollow piston rod 3 forms the first electrode and
is connected through a lead 10 connected through an insulating
terminal (not shown) to appropriate ter~inals of an appropriate
measurement device. The piston rod 3 forms a second electrode
~2~93~5~ NHL BGG~02 Canada
which is connected ~hrough a lead 9 to appropriate terminals of
the measurement device.
The steepness of the change in capacitance is dependent upon
the spacing between the outer surface of the tube 7 and the inner
surface of the hollow piston rod 3, and on the medium present
between them. The hollow piston rod 3 is arranged coaxially with
and is concentrically spaced from the tube 7; a space between
them receives the dielectric which may be a damping medium such
as oil, or even a dielectric solid between the facing surfaces
of the tube 7 and the piston rod 3. In Figure 2, there is
diagrammatically illustrated a cylindrical capacitor, ~he firs~
elec~rode thereof comprising the hollow piston rod 3 and the
second electrode comprising tube 7. The spacing 11 indicates a
practical minimum overlap. The spacing 12 indicates the maximum
overlap between the two surfaces when the piston 2 is as far to
the right in Figure 1 as it can move. These overlaps produce a
minimum and maximNm capacitance when appropriately converted, and
represent the minimum and maximum travel positions of the piston.
By interpolating between these extremes, the position of the
piston in the cylinder is indicated by a capacitance therein-
between. The leads 9 and 10 serve, for example, for connection
to a capacitance measuring bridge as shown in Figure 8 infra.
Figure 3 shows a diagram in which the capacitance of such a
cylindrical capacitor is plotted against piston position or
travel. It is seen from Figure 3 that the change in capacitance
is proportional to the travel of the piston, and the steepness of
the curve is dependent on the ratio of the inside radius of the
bore in the piston rod to the outside radius of the tube 7 as
well as the relative dielectric constant Er The relative
~93~4 NHL-BGG-02 Canada
dielectr;c constant Er is predeterm:ined in any known embodiment
by the characteristics of the damping medium used. The two
solid straight lines show two different cylindrical capacitor
arrangements having, for example, d:ifferent maximum overlap-
spacings between the piston rod 3 and the cylinder, or relative
dielectric constants Er of the dielectric.
Figure 4 shows diagrammatically an alternative, second
capacitive embodiment of the invention illustrating a vibration
damper again having a cylinder 1, a piston 2 and a piston rod 3.
The piston 2 and the base 13 of the cylinder respectively form
the first and second electrodes of a capacitor. The base 13
of the cylinder 1 is mounted in this cylinder 1 but insulated
therefrom so that a desired change in capacitance ca~ be obtained
by varying the distance between the piston 2 and the base 13 of
the cylinder 1. By appropriate conversion of the measured value,
likewise in this embodiment, the position of the piston can be
obtained.
As an alternative inductive embodiment, there is shown in
section in Figure 5, a vibration damper wherein again the princi-
pal components are the cylinder 1, the piston 2 and the piston
rod 3.
The outer surface 14 of the vibration damper cylinder 1 is
provided with windings 15, comprising a first coil 16 and a
second coil 17, for respective generation of a magnetic field.
The coils 16 and 17 when connected appropriately as the arms of
a half-bridge generate a positional signal indicative of the
position of the piston 2.
The resultant inductive half-bridge generates a bridge
voltage determined by the degree or position of the insertion of
18
~2~3~ NHL-BGG-02 Canada
the piston 2 in the damper cylinder, so that, a suitable signal
corresponding to the piston travel is produced.
Figure 6 shows a modified inductive embodiment. The prin-
cipal components of a vibration damper are here again the
cylinder 1, the piston 2 and the piston rod 3 which in the
embodiments using inductance; this piston rod 3 does not have to
be made hollow. A winding is disposed on the outer surface of
the vibration damper cylinder 1 extending preferably over at
least the entire range of travel of the piston. On insertion of
the piston within this region, there is a resulting change in
inductance which can be correspondingly evaluated. The piston
rod 3, at least, in Figure 5 and Figure 6, is preferably made of
a ferromagnetic material in order to vary the inductance as much
as possible from one relative position of the piston 2 and piston
rod 3 to another position thereof.
The vibration damper illustrated in Figure 7 comprises a
fourth capacitance embodiment of the invention which is made up
of basically the cylinder l, the piston 2 and the piston rod 3 as
in Figure 1. Within the interior cavity 4 of the cylinder 1,
there is the damping medium serving for damping vibrations.
As in the embodiment of Figure 1, mounting means 5 and 6 are
provided for mounting the damper in the vehicle in a manner
well-known in the prior art. The piston rod 3, w~ich is made
hollow, can slide telescopically over the tube 7, the tube 7
being secured in the cylinder in an insulating body 8 and being
insulated with respect to the remainder of the components of the
damper. The hollow piston rod 3 and the tube 1 form together
the cylindrical capacitor. The damping medium present in the
19
~2~35~ NHL-B&G-02 Canada
interior cavity ~ preferably forms the dielectric for the capaci-
tor as in Figure l. The hollow piston rod 3 forms the first
electrode and the tube 7 forms the second fixed electrode and is
connected through the lead 10, passing through an insulated
terminal arrangement (not shown), to the input of an appropriate
measurement device such as shown in Figure 8 infra.
The steepness of the changP in capacitance is dependent ~pon
the spacing between the outer surface of the tube 7 and the inner
surface of the hollow piston rod 3 and on the dielectric medium
present between them. The hollow piston rod 3 is arranged to be
coaxial with and spaced from the tube 7; the space between them
receives a suitable dielectric~
The spacing ll represents the minimum overlap and the
spacing 12 the maximum overlap between the two surfaces, produc-
ing accordingly a minimum and a maximum capacitance, which,
appropriately converted, indicate the momentary piston travel,
i.e., the position of the piston in the cylinder.
Secured within the cylindrical interior 19 of the tube 7 is
a further tube 18 which is likewise arranged spaced from the tube
7. Between the two tubes, the damping fluid here again acts
preferably as a dielectric as in Figure l; the dielectric may be
solid. By the fixed arrangement of the two tubes 7 and 18 with
respect to one another, there is produced a capacitor having a
fixed capacitance at a particular temperature and pressure. The
cylindrical tube 18 which acts as the further electrode is
mounted in the insulating body 8 of the cylinder again in an
insulating manner and is connected to the measurement receiver
through a lead 20. The tube 7 and the tube 18 again form a
tubular circular capacitor which serves for compensating for at
~g35~ NHL-BGG-02 Canada
least the pressure and temperature of the dielectric medium
between the tube 7 and the piston rod 3. The manner of operation
of the overall system is well-known in the prior art of Wheatstone
Bridge as a capacitive half-bridge.
In Figure 8, a schematic diagram is shown representing a
bridge circuit 108, e.g., an impedance Wheatstone Bridge, for
sensing the changes in capacitance of a variable capacitor 110
formed by the tube 7 and the hollow piston rod 3. The leads 9
and 10 are connected in the bridge 108 so that the capacitor 110
forms half on one arm of the bridge 108. Either a fixed capaci-
tor external to the vibration damper or a temperature variable
capacitor, as shown in Figure 7, and formed by the tube 7 and the
tube 18, comprises a capacitor 112 forming one-half of the bridge
108 connected to the capacitor 110. Two other impedance
elements, preferably capacitors 114 and 116 form the other half
of the bridge 108. Series capacitors 110 and 112 form one arm
and the series capacitors 114 and 116 form the other arm of the
bridge 108. A generator 118, preferably supplying alternating
current, is connected across the bridge 108 at the connectors
between the capacitors 110 and 114 and the capacitors 112 and
116. The operation of a Wheatstone Bridge is well-known in the
electrical prior art.
It is within the purview of the invention that this gener-
ator 118 may also be a generator of direct voltage if the imped-
ance levels of the bridge 108 are appropriate therefor. Alterna-
tively, within the purview of the invention, a pulse generator
may also be used under special circumstances. The terminal 10
forms one input of an amplifier 120; a junction of the capacitors
114 and 116 forms ~he other input of the amplifier 120 which
N~L-BGG-02 Canada
935~
generates a signal proportional to the position of the piston 2.
In the case of the inductive coil embodiment of the inven-
tion as shown in Figure 5 being adapted to the circuit in Figure
8, the first coil 16 replaces the capacitor 110 and the second
coil 17 replaces the capacitor 112 as in shown in Figure 9. The
other capacitors 114 and 116 are preferably replaced by impedances.
However, other impedance elements may be used for capacitors 114
and 116 if their impedance is chosen appropriately in a manner
which is well known in the impedance bridge art.
For example, if the capacitive impedances llO and 112 in
Figure 8 both increase by the same percentage amount due to
change in the mutual dielectric because of temperature, pressure,
etc., the voltage at the terminal 10 will remain constant, as is
well-known in the art of electrical bridge measurements such as
the Wheatstone Bridge.
Fig1lre 10 shows the coil arrangement 22 of Figure 6 con-
nected into the bridge 108 replacing the capacitor 110.
The vibration damper illustrated in Figure 11 generally
comprises the cylinder 201, the piston 202 and the piston rod
203. In the interior cavity 204 of the piston 201, there is
provided a damping medium serving for damping the vibrations.
Attachment means 205 and 206 are provided for mounting the
vibration damper on the vehicle.
The piston rod 203 is made hollow, and there~ithin are
disposed a first elec~rode 208 and the second electrode 211. The
first electrode 208 is insulated from the piston rod 203 by an
insulating layer 207. Between the first electrode 208 and the
second electrode 211 is a gap 212.
354 MHL-BGG-02 Canada
Since the first electrode 208 i.s substantially concentric to
and spaced away from the second electrode 211, the tubular body
2.13, which is secured to the base 214 of the cylinder 201, can
enter the gap 212 axially during operation of the damper in the
vehicle. By the entry of the tubular body 213, a variation
occurs in the capacitance between the first electrode 208 and the
second electrode 211.
In the specific embodiment of Figure 11, a ceramic tube is
provided as the tubular body 213. The variation in capacitance
arises because of different dielectric characteristic~. of the
ceramic tube and the rest of the medium which may be oil.
The ceramic tube and the mineral oil which is present as the
damping fluid each have a different dielectric constant, and on
insertion of the ceramic tube 213 in the gap 212, the immersed
surface of the ceramic tube acts as a different dielectric from
oil.
In order to eliminate as far as possible the effect of the
oil in the gap, between the electrode 211 and the ceramic tube
213, the ceramic tube 213 is provided with an electrically
conducting surface on its inner cylindrical surface 209. This
conducting surface is electrically connected to the second
electrode 211 by an electrically conducting ring 210 provided on
the outer surface of the second electrode 211. Since the inner
cylindrical surface 209 electrically becomes a part of the second
electrode 211, the oil in the gap between the second electrode
211 and the ceramic tube 213 does not have any electric filed
generated therein. Therefore, this gap does not produce any
capacitive effect and thereby variations in this gap due to
NHL-BGG-02 Canada
~;~4935~:
tolerances, wear of the parts, etc., do not deleteriously affect
the performance accuracy.
By virtue of the arrangement of the electrodes 208 and 211
either in the hollow piston rod 203 or in the cylinder 201,
structurally preferred possibilities are provided for making the
connections for the leads 215. The leads could be made to
terminate as connecting terminals on the outer surface either of
the cylinder 201 or the piston rod 203. The electrodes 208 and
211 are held fixed relative to one another. The tubular body 213
is fixed also; however, the tubular body is mounted in the
cylinder 201 so that during operation of the vibration damper, a
telescopic displacement of the components within each other take~
place, and the tubular capacitor can perform as desired.
The vibration damper illustrated in Figure 12 comprises
substantially the cylinder 201, the piston 202 and the piston rod
203. In the interior cavity 204 of the cylinder 201, there is
the damping medium serving for damping the oscillations. Securing
devices 205 and 206 are provided as attachment means for mounting
the damper in the vehicle.
Provided in the piston rod 203, which is made hollow, are
the first electrode 217 and the second electrode 218. The first
electrode 217 is insulated with respect to the piston rod 203 by
a tube 219 of, for example, synthetic resin. A dielectric 220 is
disposed between the first electrode 217 and ~he second electrode
218.
Spaced away ~rom the second electrode 218 is a third elec-
trode 221. The tubular body 223 is disposed to be able to enter
the intermediate space 222 between the second electrode 218 and
the third electrode 221 during active functioning of the damper
24
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~2~93~4
in the vehicle. By the insertion of the tubular body 223 into
the space 222, there occurs a change of capacitance between the
second electrode 218 and the third electrode 221. The first
electrode 217 forms, together with the second electrode 218, a
fixed capacitance by contrast. The space 222 may be filled with
a dielectric fluid, e.g., mineral oil.
Preferably, as illustrated, the tubular body is metallic;
with the use of an electrically conducting tubular body 223, the
latter must be earthed through the insulating body 224 of the
cylinder. This results in the following manner of operation:
with the piston rod 203 extended, the three electrodes form two
fixed capacitances of known magnitude. 0~ inward movement of the
grounded electrically conducting tubular body 223, the capacitance
between the second electrode 218 and the third electrode 221 is
reduced because the effective capacitive surface area is
diminished.
As an alternative in regard to the material of the tubular
body 223, it is also possible for the tubular body 223 to be made
in the form of an electrically non-conducting tube. For example,
a tube of synthetic resin is suitable for this purpose. If a
synthetic resin tube is provided to form the tubular body 223
then on entry of the synthetic resin tube between the second
electrode 218 and the third electrode 221, there is produced a
parallel circuit of two capacitors. The two capacitances of
different values arise by the different dielectrics so that the
overall capacitance of the second electrode 218 and the third
electrode 221 results from the addition of the two individual
capacitances. These different values of capacitance arise partly
since the synthetic resin tube and the mineral oil which may be
~æ~35~ NHL-~GG-02 Canada
present as the damping fluid, each have different dielectric
constants. If the tubular body 223 is made of synthetic resin,
on insertion of the synthetic resin tube, the inserted surface
area of the tubular body 223 acts as a dielectric.
The variable capacitance follows the formula:
CgeS (1) - Ca + Cb
where:
Cges ~ overall capaci~ance
Ca = the individual capacitance of the first
medium (e.g. Mineral oil)
Cb = the individual capacitance of the second
medium (e.g. Synthetic resin)
In Figure 13, a diagram is shown in which the capacitance
for such a cylindrical capacitor is drawn against piston travel
or displacement. The individual capacitance Ca falls steadily on
insertion of the piston 202, whereas the individual capacitance
Cb, in which the tubular body 223 is used as the dielectric,
steadily increases. The two individual capacitances add up and
form the resultant characteristic line Cges.
By the disposition of the electrodes 217, 218 and 221,
either within the hollow piston rod 203 or in the cylinder 201,
there are advantageous possibilities for making connections to
the leads 225 since these only need to be mounted as connecting
pins on the outside surace of either the cylinder 201 or the
piston rod 203. The electrodes are advantageously kept fixed
relative to one another, and the tubular body 223 is likewise
fixed, but in the other component. Accordingly during the action
of a vibration damper, a telescopic displacement of the parts
26
~z~93$4 N~L-B&G-02 Canada
within one another takes place, so that the desired measured
variable capacitance is formed.
The invention, as described hereinabove in the context of a
preferred Pmbodiment, is not be taken as limited to all of the
provided details thereof, since modifications and variations
thereof may be made without departing from the spirit and scope
of the invention.
27
