A METHOD AND APPARATUS FOR MEASURING LINEAR DISPLACEMENTS
FIELD OF THE INVENTION
The present invention relates to displacement transducers and more
particularly to capacitive linear displacement transducers.
BACKGROUND OF THE INVENTION
Several types of continuous linear displacement transducers are
known. A most common one is the linear potentiometer, wherein an
electrically contacting wiper slides along a resistive strip to generate a DC
voltage proportional to its position. The main weakness of the linear
potentiometer is the friction inherent to the sliding contact and the
reliability problems associated with it. The friction results in the wear of
the resistive strip and in a discontinuous electrical contact. This drawback
is especially severe in applications where in measured displacement has a
vibrational nature and is concentrated about a particular value. This limits
the application of the linear potentiometer to cases in which a finite
number of cycles is expected, typically ten millions.
Another transducer for measuring linear displacement is the Linear
Variable Differential Transformer (LVDT). The operation of this
transducer is based on a displacement dependent magnetic coupling
between a primary winding and two secondary windings. The differential
voltage induced in the secondary windings is proportional to the
differential magnetic coupling between the primary and secondary
windings, which is linearly dependent on the position of a moving
ferromagnetic member. The advantage of this transducer is that there is
no sliding electrical contact. However, the ratio of effective (linear)
measurement range of the transducer to its physical length is at most 1/2
but usually closer to 1/3 - compared to almost unity in the linear
potentiometer. Another disadvantage of the LVDT is its relatively high
cost, which results from the labor intensive winding and encapsulation
involved in its manufacturing.
Concepts for linear displacement transducers based on variable
capacitive coupling are described in US Patent 3.860.918 and US Patent
3.784.897. However, none of them proved viable for commercial use.
A displacement transducer is thus desirable that combines the advantageous
of the aforesaid two types of transducers.
It is a purpose of this invention to provide a variable capacitance
linear displacement transducer that essentially combines the advantages of
the linear potentiometer and the LVDT, i.e.:
1. Low cost.
2. Ratio of effective measurement range to physical length which is
close to unity.
3. Absence of sliding contact, resulting in high reliability.
It is another purpose of this invention to provide such a
displacement transducer, which is immune to extraneous magnetic and
electrical fields.
It is a further purpose of this invention to provide such a
displacement transducer which has low power consumption.
It is a still further purpose of this invention to provide such a
displacement transducer which permits optional hermetic sealing of the
moving member.
It is a still further purpose of this invention to provide such a
displacement transducer which can be constructed entirely from inorganic
materials to enable high-temperature operation
Other purposes and advantages of the invention will become
apparent as the description proceeds.
SUMMARY OF THE INVENTION
The device for measuring the displacement of a body or element
comprises a first and a second fixed or stationary member (or plate or
electrode), each of said stationary members comprising one or more than
one component parts; a movable member (or plate or electrode); means for
applying an input alternating tension to one of said first or second
stationary members; means for connecting said movable member to the
body or element the displacement of which is to be measured (hereinafter
"the monitored body"), whereby to cause the displacements of said
movable member to be a function of the displacements of said monitored
body; and means for monitoring the current induced in said second or first
stationary member. Said first stationary and movable members constitute
a first capacitance, said movable and second stationary members constitute
a second capacitance, and said second stationary and first stationary
members constitute a third capacitance. At least one, and preferably only
one, of said first and second capacitances, and consequently the current
induced in said second stationary member, vary as said movable member
is displaced and as functions of its displacement, and therefore as a
function of the displacement of said monitored body.
The means for monitoring the current induced in said second
stationary member are preferably means, such as a transimpedance
amplifier alternating tension. As a whole, therefore, the device produces
a current or preferably a voltage that is a function of a mechanical
displacement, and therefore may properly be called a transducer.
The input alternating tension is preferably applied to the first
stationary member. Therefore, the first stationary member may be called
hereinafter "excitation plate" or "electrode". The second stationary
member may be called "signal plate" or "electrode". The movable
member may be called "displacement plate" or "electrode"
In a preferred embodiment of the invention, the first stationary
member comprises two stationary excitation plates, to which two opposite
phase AC voltages are applied. In a particularly desirable configuration,
said two plates are triangular and arranged so that the base of each of them
faces the apex of the other. In a preferred constructional implementation,
the stationary plates may be arranged in cylindrical form, e.g., as
conductive films or layers applied to a non-conductive cylindrical base;
and the movable plate may be formed on a piston which slides within said
cylindrical base.
In a further preferred embodiment, each stationary plate is
constituted by a plurality, e.g., a pair, of plate elements connected in
parallel.
In a still further preferred embodiment, the excitation plate further
comprises two additional sinusoidal electrode pairs shifted by 90
mechanical degrees, each electrode pair being excited by a complementary
voltage having a different frequency.
The means for connecting the movable plate to the monitored body
may be of any kind, and may be e.g., kinematic means of any convenient
structure or may include an electromagnetic coupling.
The method for measuring the linear displacements of a monitored
body according to the invention, comprises providing at least two
capacitances including at least one movable plate, displacing said movable
plate as a function of the displacements of the monitored body, applying
to at least one plate of said capacitances an alternating input tension, and
measuring the changes in said capacitances due to the displacements of
said movable body by measuring an output tension induced by said input
tension. Preferably, three capacitances including one movable plate and
two stationary plates are provided.
According to an aspect of the invention, the changes in the
capacities are measured by measuring the changes in the current induced
in a stationary plate of one of said capacitances, preferably by transducing
said current to an output voltage and measuring said voltage.
BRIEF DESCRIPTION OF THE DRAWINGS
Figs. 1-a and 1-b are a diagram of a device according to a first
embodiment of the invention, illustrating its basic concept, and the
equivalent circuit thereof, respectively;
Figs. 2-a and 2-b are a diagram of a device according to a second
embodiment of the invention and its equivalent circuit, respectively;
Figs. 3-a and 3-b are a diagram of a device according to a third
embodiment of the invention and its equivalent circuit, respectively;
Fig. 4-a and 4-b show two constructional implementations,
schematically illustrated as developed on a plane, of the device of Figs.
3-a and 3-b;
Fig 5 illustrates a cylindrical constructional implementation of Figs.
3-a and 3-b;
Fig. 6 shows a constructional implementation, schematically
illustrated as developed on a plane, of a multiple plate embodiment of a
device according to the invention;
Fig. 7 shows a constructional implementation, schematically
illustrated as developed on a plane, of a device according to an
embodiment of the invention including coarse and fine channels;
Fig. 8 shows an actual mask use for screen printing coarse and fine
stationary plates.
Fig. 9 illustrates an hermetically sealed device according to an
embodiment of the invention, with a magnetic coupling of the displacement
electrode to the monitored body;
Fig. 10 illustrates a device according to a further embodiment of the
invention, with a rod-less mechanical coupling of the displacement
electrode to the monitored body; and
Fig. 11 illustrates a device according to a still further embodiment
of the invention, with a cable mechamcal coupling of the displacement
electrode to the monitored body.
DETAILED DESCRIPTION OF THE INVENTION
The concept of the invention is illustrated in Fig. 1. It is based on
capacitive coupling between two conductive stationary members through
a movable intermediate, electrically conductive, member that is isolated
from said stationary members and is displaceable with the body or element
the displacement of which is to be measured.
Fig. 1(a) diagrammatically illustrates a transducer device according
to a first embodiment of the invention. A first conductive, stationary
member 1 - the excitation plate or electrode - is excited by an input AC
voltage source 4. A second conductive, stationary member 3 - the signal
plate or electrode - is connected to the virtual ground of an operational
charge or transimpedance amplifier 5, that converts the current that is
capacitively coupled into said second member to an output voltage. A
third, movable, conductive member 6 - the displacement electrode - is
operatively connected to the momtored body in such a way that its
displacement is equal to or a known function of the displacement of said
body. Electrode 6 is capacitively coupled to said electrodes 1 and 3.
The equivalent circuit of the transducer device of Fig 1(a) is shown
in Fig. 1(b) which illustrates the fact that the total capacitance between
members 1 and 3 consists of a fixed capacitance C 3 and two
displacement-dependent, variable capacitances C,.6 and C6.3. Capacities
Cι.6 and C^ vary as a function of the displacement of element 6.
Therefore the charge in said two capacities, and consequently the current
from Co.3 to .3 also varies as a function of the said displacement. The
voltage V0 , which is the output of the transimpedance amplifier 5, also
varies as a function of the displacement of movable plate 6, and therefore
of the momtored body, so that it provides a measure of the displacement
of the momtored body. If Vex is the excitation voltage applied to
stationary plate 1, V
0 = V
ex.
and if C, is small relatively to the
Cj-6, the said output voltage is approximately proportional to the said
excitation voltage.
A modified configuration of the transducer device is shown in Fig.
2(a), wherein the area of member 6 is such that the mutual capacitance
C^ between members 1 and 6 is constant regardless of displacement of
member 6 in the direction of the arrow over a certain range. The
equivalent circuit is shown in Fig. 2(b), which is self-explanatory. The
capacitance C,.6 is the only displacement-dependent one in this case.
Fig. 3(a) illustrates an embodiment wherein the first stationary
member is constituted by two excitation plates 1 and 2, which are excited
with two opposite phase AC voltages 4 and 4'. As shown in the
equivalent circuit of Fig. 3(b), said two plates induce two currents of
opposite polarities in the signal plate 3, through the displacement plate 6,
the net sum of said currents being converted to an output voltage signal by
amplifier 5 as before. In addition, two mutually canceling parasitic
currents are coupled through capacitances C,_3 and C2.3. As displacement
electrode 6 is translated along the direction of the arrow, the magnitude
and phase of the output voltage varies and is indicative of the sense and
magnitude of its displacement and therefore of the momtored body
displacement.
It will be obvious to those skilled in the art that the roles of said
excitation and signal members can be reversed by connecting each of
members 1 and 2 to the virtual ground input of a separate charge amplifier
and exciting member 3 with a single AC voltage source. The output
voltages of said charge amplifiers would then be subtracted to provide a
differential signal indicative of the displacement of member 6, whereas the
sum signal can be used to normalize the differential signal against common
mode effects, such as amplitude variations in the excitation voltage and
geometrical variations that symmetrically affect the stationary members.
A like arrangement is applicable to any embodiment of the invention: in
other words, the input AC voltage can be applied to the signal plate
instead of to the first stationary plate, no matter how this is structured.
The actual structural implementation of the conductive members in
Figs. 1, 2, and 3, is dependent upon the particular embodiment of the
invention. One such structure is illustrated in Fig. 4(a), which shows a
top view of a plane differential configuration of an embodiment in
accordance with Fig. 3. The plates can actually be flat bodies, or the
drawing can be understood to represent the development on a plane of
plates that are actually not flat bodies. Excitation plates 1 and 2 are
triangular, disposed in such a way that the tip of each of them faces the
base of the other, so that their cumulative cross-section is the same along
all their length, and are electrically coupled to signal plate 3, also
illustrated as a plane body, through movable, signal plate 6, which is held
at a fixed distance from the common plane of plates 1, 2, and 3.
Capacitance .3 is therefore constant and capacitances C,.6 and C2.6 are
linearly and oppositely dependent on the displacement of plate 6 along the
axis represented by the arrows. The net induced current in the signal plate
is zero, when the movable plate 6 is in the center position - viz. the
position in which it faces portions of excitation plates 1 and 2 of the same
width - and attains its maximum value with the appropriate polarity at the
extreme positions - viz. the positions corresponding to the maximum width
of one of the excitation plates and the minimum width of the other. The
effective measurement range of the configuration in Fig. 4(a) substantially
equals the length of the fixed plates 1 and 2 less the length of the movable
plate 6 along the travel direction, i.e., almost the full length of the fixed
plates. The smaller the dimension of the moving plate in the travel
direction compared to the length of the fixed plates, the closer to 1 is the
ratio of the effective measurement range to the physical length of the
transducer device.
Fig. 4(b) illustrates a modification to the configuration of Fig. 4(a)
wherein the parasitic capacitance between signal plate 3 and excitation
plates is decreased by enclosing it with guard plates 7 which are held at
the same electrical potential, i.e., ground potential in the specific case
shown.
A different embodiment of the invention, wherein the electrode
plates have a different configuration, is schematically illustrated in Fig. 5.
The stationary plates are cylindrical, in particular, they are disposed on a
hollow, non-conductive cylinder 8, and the movable member 9 is in the
shape of a piston. For example, the stationary plates can be made by
depositing a conductive film or coating on the outer surface of a glass
cylinder. Figs. 4(a) or 4(b) can be taken as representing the surface of the
device, viz. the stationary plates, as it would look if the device were cut
along a generatrix of the cylinder and developed on a plane, while the
movable piston can be made from a low friction conductive material such
as graphite. Alternatively, the piston can be made from a Teflon-coated
metal cylinder, the non-conductive cylinder from a plastic tube, and the
conductive, stationary plates by selectively metallizing the said tube. The
embodiment of Fig. 5 includes a rod 10 that serves mechanically to couple
the momtored body to the movable piston 9 of the transducer device.
The piston-and-cy Under construction of the embodiment of Fig. 5
enables the application of the invention to pneumatic actuators of the
piston-and-cylinder type in which the cylinder is non-metallic, so that the
stationary, conductive plates can be applied to it. The resulting integrated
actuator /displacement transducer is useful in various pneumatic systems,
by enabling a cost-effective closed-loop servo-control.
The stationary plates of the transducer of the present invention are
sensitive to parasitic capacitive coupling from neighboring conductive and
dielectric objects. A preferred embodiment of the configuration of Fig.
5 includes a grounded metallic concentric sleeve 11 , that serves both as a
screen for protecting the electrodes and as a housing for mechanical
protection of cylinder 8.
In the embodiment shown in Fig. 5 the capacitive coupling between
the stationary plates and piston 9 is sensitive to radial misalignment of the
piston in the cylinder, which misalignment would result in erroneous
output signal. This sensitivity can be decreased by employing, in place of
single stationary plates, sets of stationary plates, each of which comprises
a number of plates - in the embodiment illustrated, a pair, such as 1-1,
2-2, and 3-3 - that are connected in parallel, as shown in Fig. 6. This
construction decreases the said sensitivity, due to a compensating effect in
diametrically opposing set of plates. In Fig. 6, each set of stationary
plates is constituted by a pair, but it could be constituted by any number
of plates, such as four.
Fig. 7 illustrates another embodiment of the invention, wherein an
additional, fine channel is added to provide an improved accuracy. The
additional channel comprises two periodic patterns, shown as triangular in
the figure, that are shifted by 90 mechamcal degrees. The first pattern
includes the electrodes 12 and 13 whereas the second pattern includes
electrodes 14 and 15 . Each electrodes pair is excited with two opposite
polarity voltages of a frequency different from those of the other pair, and
is capacitively coupled to the common signal electrode 3 via piston 9. In
the preferred embodiment of this version the length of the piston element
is substantially one half the period length of the periodic pattern (not to
scale in the figure) in order to maximize the output signal. By
demodulating the output signal that is derived from the signal electrode 3,
three separate signals are obtained:
1. A signal that is linearly proportional to the displacement of
movable member 6.
2. A fine-channel signal that is sinusoidally proportional to the
displacement.
3. A fine-channel signal that is cosinusoidally proportional to the
displacement.
The fine channels enable an interpolation within an individual cycle
but are unable to identify the cycle. By combining the linear (coarse) and
fine channels information the individual cycle can be identified and the
interpolation enables an improved accuracy.
Fig. 8 illustrates an actual mask used for screen printing plates on
a cylindrical glass transducer.
The mechanical coupling to the moving piston 9 to the monitored
body, in the embodiment illustrated in Fig. 5, is provided by means of a
rod 10. Although this is a convenient coupling means, it does not permit
totally to seal the transducer device against external contaminating
materials when used in an uncontrolled environment. Fig. 9 illustrates
another embodiment of the invention, wherein the coupling of piston 9 to
the momtored body is obtained by the interaction between two concentric
magnets. Numeral 8 designates a non-conductive cylinder as in Fig. 5.
A first cylindrical magnet 16 is mounted within piston 9 and a second
annular magnet 17 is concentric to the first one. The two magnets are
axially magnetized, but in opposite senses. As a result, they exert an
attractive force on one another that maintains the internal magnet within
the external one. By attaching the external magnet to the monitored body,
piston 9 follows its displacement with an accuracy that depends on the
friction between it and the cylinder and on the attractive force between the
two magnets. An error of 0.5 mm is typical in a transducer that
comprises a glass cylinder and a graphite piston and employs
Samarium-Cobalt coupling magnets. A further advantage of the transducer
illustrated in Fig. 9 is that the total space needed is substantially one half
that of the embodiment in Fig. 5, since there is no need for the space
occupied by the rod while in full excursion.
A further embodiment of the invention is illustrated in Fig. 10.
This embodiment incorporates a cylinder that has an elongated slot 18
along its length to enable a direct mechanical coupling to the moving
element by means of an electrically non-conductive connection member 19.
Although this embodiment cannot be hermetically sealed, it occupies the
same reduced space as in Fig. 9.
A further embodiment of the invention is illustrated in Fig. 11. In
this embodiment the mechanical coupling of piston 9 to the monitored
body is obtained by means of two flexible cables 20 and 21 which enable
mechanical flexibility in certain situations.
Although a number of embodiments have been described by way
of illustration, it will be understood that the invention may be carried out
by persons skilled in the art with many modifications, variations and
adaptations, without departing from its spirit or exceeding the scope of the
claims.