Note: Descriptions are shown in the official language in which they were submitted.
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MAGNETOSTRICTIVE LINEAR DISPLACEMENT TRANSDUCER
UTILIZING AXIAL STRAIN PULSES
BACKGROUND OF THE INVENTION
This invention relates to devices for measuring linear displacement of an
object, such as a
piston in a hydraulic cylinder, and, in particular, to such devices using a
magnetostrictive
effect.
It is desirable or imperative in some instances to know the position of one
component which
moves relative to another. Examples of such applications include position
indicators for
industrial, mobile, construction and agricultural equipment, off road vehicles
and m~~rine
steering systems used on boats equipped with autopilots. What needs to be
ascertained in
some instances is the position of a piston within a hydraulic cylinder. Such
devices are also
useful for other applications including motion control devices for processes
and robotics,
closed-loop and fuzzy logic control systems, servo valves and fluid level
sensing in tanks.
There are two general classes of such position sensors, namely contact and non-
contact
sensors. Contact sensors include potentiometers, rheostats, and resistive film
sensors. Non-
contact sensors include proximity sensors, optical encoders, hall effect
sensors, radar and
microwave devices, acoustic devices, linear displacement transducers, and
linear variable
differential transformers. However, prior art devices are not ideal for all
purposes. For
example, in the case of non-contact sensors, prior art devices are often too
inaccurate,
complicated, expensive or bulky for many applications. In addition they may be
too fragile
for some applications such as use in mobile equipment. They may also be
affected b;y the
earth's magnetic field or extraneous sources of magnetism and therefore would
not provide
the required degree of accuracy for applications such as position sensing on
mobile
equipment.
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A magnetostrictive effect has been utilized previously for linear displacement
transducers.
Examples are found in United States Patent No. 3,898,555 to Tellerman and
5,017,867 to
Dumais et al. A torsional motion sensor is used to detect torsional motion
within the
magnetostrictive wave guide tube induced by passage of an electrical pulse
down a wire
which interacts with a magnetic field of an adjacent magnet. The position of
the magnet
along the tube can thereby be determined.
United States Patent No. 5,198, 761 to Hashimoto et al. discloses a stroke
detector including
a driving coil wound around a member with a large magnetostriction
coefficient. A pulse
input current to the coil causes magnetostriction phenomena on the
magnetostriction line
generating an ultrasonic wave. A detecting coil wound on the member induces a
detection
signal generated by reverse magnetostriction when the ultrasonic wave passes
by the position
of the magnet on the magnetostriction member.
The prior art magnetostrictive transducers sold under the trademark
TEMPOSONICS are
adapted t:o fit within the piston rod of an hydraulic or pneumatic cylinder.
This means that
a piston rod has to be bored out at considerable cost in order to insert the
probe. It employs
a relatively delicate tubular wave guide. There is considerable dead space at
both ends of the
probe which limits the measurable stroke of the piston. The device typically
measures the
- position of four magnets which are oriented with their poles being spaced-
apart radially with
respect to the center line of the tube. A current is pulsed down the tube, or
a wire within the
tube, which puts the tube into magnetostriction during the pulse. A twisting
strain is
produced at the position of the magnet which is sensed by a torsional strain
sensor at one end
of the tube. The connection of the torsional sensor to the wave guide tube
tends to be fragile.
Also the location of the connection is vital to transducer accuracy.
However some such magnetostrictive transducers are not well adapted for
applications where
power consumption is critical or where they must fit into space constrained
locations. In
addition., they may be too expensive, by virtue, for example, of their
sensitive torsional
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measuring means, mounting and dampening requirements and driver and detection
circuity
requirements, to be practical for some applications and may be prone to
inaccuracies or
damage caused by shock and vibration in applications such as mobile equipment.
Accordingly it is an object of the invention to provide an improved
magnetostrictive linear
displacement transducer which is well adapted for use in relatively
unfavourable
environrrrents such as mobile equipment.
It is also an object of the invention to provide an improved magnetostrictive
linear
displacement transducer with minimal dead zones which can fit into tight
locations, for
example in conj unction with hydraulic cylinders where the space availability
may be limited
very closely to the length and diameter of the cylinder.
It is a further object of the invention to provide an improved
magnetostrictive linear
displacement transducer which is economical to produce, highly accurate and
durable and
reliable in operation especially for such applications as industrial mobile
equipment.
It is still a further object of the invention to provide an improved
magnetostrictive linear
displacement transducer which has reduced power consumption compared to the
prior art.
It is a still further obj ect of the invention to provide an improved
magnetostrictive linear
displacement transducer which can be retrofitted in place of prior art devices
but which also
permits a broader range of mounting configurations such as on the exterior of
pneumatic or
hydraulic cylinders.
SUMMARY (JF THE INVENTION
In accordance with these objects, there is provided a magnetostrictive linear
displacement
transducer including an elongated member of a material with a large
magnetostrictive
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coefficient. There is an excitation coil about the member. There is means for
generating
electrical signals connected to the coil to produce axial magnetic fields
along the member.
There is a magnet adj acent the member. There is means for detecting axial
strain pulses
propagated along the member and caused by the magnetostrictive effect. There
is also means
for determining time delays between the signals and the axial strain pulses
propagated along
the member as induced by the signals and thereby a position of the magnet
along the
member.
The magnet may be oriented so its poles are spaced-apart in a direction
parallel to the
elongated member.
Orienting; the magnet parallel to the elongated member produces a stronger
magnetic field,
and hence permits a larger magnetic gap between the magnet and the sensor,
than in devices
where thf: poles are oriented radially with respect to the member. Accordingly
the device can
be mounted outside a cylinder instead of inside as in some prior art devices.
The means for detecting may include a piezoelectric detector connected to one
end of the
member.
The means for generating may include a pulse generator. The means for
determining may
include a time delay comparator electrically connected to the pulse generator
and to the
piezoelecaric detector.
There may be a part of the member folded back in a hook-like manner at an end
thereof
opposite the piezoelectric detector.
The invention offers significant advantages over the prior art. The device can
fit into tight
location; where physical length is equal to or close to the length of the
elongated member.
Devices according to the invention are much more producible and simpler than
prior art
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devices requiring a torsional strain sensor delicately connected to a wave
guide tube as part
of the transducer. They can be smaller in diameter and length to fit into
smaller locations.
They have inherently better resistance to vibration than some prior art
devices.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a diagrammatic view of a magnetostrictive linear displacement
transducer according to a first embodiment of the invention.
Fig. 2 is a fragmentary view of one end of a transducer according to a second
embodiment of the invention;
Fig. 3 is a fragmentary view of one end of a transducer according to a third
embodiment of the invention;
Fig. 4 is a view similar to 1~ ig. 1 of a fourth embodiment of the invention;
Fig. 5 is a longitudinal section of a fluid actuator with a magnetostrictive
linear displacement transducer installed externally thereon;
Fig. 6 is a view similar to Fig. 5 with the transducer installed internally
along
the piston rod thereof;
Fig. 7 is a graph showing a typical current pulse for the transducers of Fig.
1-
6;
Fig. 8 is a graph showing the output signal delivered to the amplifier of Fig.
1 or Fig. 4;
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Fig. 9 is a graph showing the magnetostriction level of the magnetostrictive
member of Fig. 1-6 plotted against the core axial magnetic field; and
Fig. 10 is a schematic diagram of the electronic components of the embodiment
of
Fig. 1.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring to Fig. 1, there is a magnetostrictive linear displacement
transducer 10 which can
be used for a number of different applications such as sensing the position of
a piston within
a hydraulic cylinder. There is an elongated member 12 which in this example is
a wire of
a material having a high magnetostriction coefficient. The range from +20 X 10-
bol/l to +30
X 10-6ohZ is preferred. The material used in this example is NIRON 52
available from
Carpenter Technology Corporation. The wire utilized has a diameter of 0.025".
C)ther
materials and other types of elongated members could be substituted. However,
the material
should have a high magnetostriction coefficient and a high stiffness.
Iron/cobalt, metglass
and ferrites are also suitable. A tubular member could also be substituted.
The member 12
has a first end 14, a second end 16 and a straight portion 17 between the
ends. Alternatively
the member could be curved or flexible.
An excitation coil 20 of an electrically conductive material is wound about
the member 12
along they straight portion between ends 14 and 16. Alternatively one or more
coils could be
positioned adjacent the member and along the member. They could be wound about
an inert
casing about the elongated member. The coil is of copper foil, 1 /16" wide and
.002" thick
in this p;~rticular example, but other conductive materials, such as wire or
film could be
substituted. The width of the foil strip, or the gage of the wire, can be
selected, along with
the turns per inch of the coil, to adjust the inductance of the coil. Through
this means a wide
variety of operative D.C. voltages and transducer lengths can be accommodated.
In this
example the winding is such as to use a standard +5 v DC and this minimizes
circuit costs.
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The coil is connected to a current pulse generating circuit 22 which, together
with the coil,
provides first means for magnetizing the member 12 for short, discrete periods
of time
corresponding to pulses generated by circuit 22. Typical pulse durations are 3
to 1 S,us long
and the pulses are repeated at a frequency of one pulse per millisecond. This
is suitable for
a magnet 24, described below, which is 1/8" to 1!4" wide.
Magnet 24 adjacent the member 12 is movable along a path represented by arrows
26
adjacent the member and extending between ends 14 and 16 of the member.
Alternatively
the magnet could be stationary and the transducer could move. The magnet in
this example
is oriented so that north pole 28 and south pole 30 are aligned parallel to
the member 12 and
to oppose the field produced by the excitation coil. The magnet could also be
oriented 90°
from the position shown as well. This may give a higher precision result, due
to a more
narrowly defined saturation zone, but gives a reduced magnetic gap capability.
The magnet 24 may comprise, for example, the piston of a hydraulic cylinder or
may be
mounted on such a piston. The elongated member 12 and the coil would be
typically
mounted coaxially with the piston rod or on the exterior of the cylinder which
would have
a wall of a non-ferromagnetic material. The transducer 10 would be used in
such an
application to ascertain the position of the piston within the cylinder.
Details of such a
combination are included below.
The pulse generating circuit 22 in this example provides pulses of D.C.
current 10
microseconds long and spaced-apart by 1 millisecond intervals. The pulses,
shown in Fig.
7, have a rise time of 3-5 microseconds in this example. The peak current in
this example
is approximately 5 amps at 5 volts D.C. power input.
Details of circuit 22 are shown in fig. 10. It includes five amplifiers A1 -
A5, three resistors
R12 - R14, a comparator C3 and a transistor T4.
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FET F1 acts as a driver to excite coil 20 in association resistors R15 and
R16, capacitor C4,
Zener diode Z1 and Schottky diode S1.
Trim center 23 acts as a retriggerable monostable as described below. It
includes transistor
T3 , comp arator C 5, resistors R 10 and R 11, amplifier A6 and potentiometer
V 1.
Other types of pulse drivers or means could be utilized in other examples of
the invention
to provide relatively short, but discrete pulses of current through the coil.
Alternatively other
means could be used for magnetizing the member 12 for such short discrete
periods of time.
The effect of the pulse generating circuit and coil is to produce axial
magnetic fields in the
member 12. As used herein the term "axial" refers to directions along the
longitudinal
direction of the member 12. The axial magnetic filed produces a uniform field
along the
member .and hence uniform magnetostriction. Also the axial magnetostrictive
pulse results
in less end bounce and distortion and is easier to dampen compared to
torsional pulses. In
this example the magnetic fields are sufficient to produce a magnetostrictive
effect along the
portion of the member co-extensive with the coil. The field produced is
counter to the field
of magnc;t 24 in this embodiment.
The magnet is movable along path 26. When the coil is de-energized only a
localized portion
of member 12 adjacent the magnet exhibits magnetostriction. In this example
this portion
is in mag;netostriction saturation. When the coil is energized, the rest of
the member apart
from this; localized portion exhibits magnetostriction, to a saturation level
in this example.
However the magnetic field created by the magnet counters the magnetic field
created by the
pulse acting on the coil in the localized region. In this embodiment this
means that this
region is taken out of the saturation caused by the magnet. This sudden change
in the
magnetostriction in the localized portion causes a strain pulse to propagate
axially along the
member fiom a point adjacent to the magnet in the form of sound waves, ultra
sonic waves
in this example. Fig. 9 shows the level of magnetostriction in member 12
plotted against the
axial magnetic field of member 12 (the core).
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There is also means for measuring time lags between initiation of each of the
separate
discrete periods of time when the pulse circuit provides pulses of current to
the coil and
detection of corresponding sound waves formed in the member 12 by the
magnetostrictive
effect adjacent the magnet as each pulse is provided by circuit 22. Each pulse
of current for
all practical purposes instantaneously magnetizes the entire member. Peak
magnetization
occurs at the peak of each pulse shown in Fig. ~ The effect is repeated as
each pulse is
conducted from the pulse generating circuit to the coil. The rapidly changing
magnetization
creates rnagnetostrictive strain pulses in this example, in the member 12
which start close to
the position of the magnet and are propagated along the member towards both
ends at about
15,000 ft./sec. In this example a piezoelectric element 34 is connected
directly to end 14 of
the member 12. It takes a finite time for ultrasonic waves to move along the
member l2
from the position of magnet 24 to the piezoelectric element 34 at end 14. This
time delay
is indicative of the position of the magnet 24 along path 26 and along member
12. It may be
appreciated that the time delay is greater when the magnet is near end 16 and
smaller as the
magnet approaches the piezoelectric element 34 at end 14.
The piezoelectric element 34 produces electrical pulses at the same frequency
as the pulses
of circuit 22, but with the time delay caused by the propagation of ultrasonic
waves from the
position on the member 12 adjacent magnet 24 to end 14 thereof. The
piezoelectric element
in this example is approximately 0.1" square although other configurations
such as circular
elements could be substituted. Fit;. 8 shows the pulses produced by element
34.
The piezoelectric element is connected to a high gain, narrow band amplifier
36 which is
tuned to the frequency of the pulses described above. As seen in Fig. 10 the
amplifier is
connected to a pulse shaper 38 v~hich serves to sharpen the pulses originating
with the
piezoelectric element. As seen in Fig. 10 there are two transistors T1 and T2,
nine resistors
R 1 - R9, two caparators C 1 and C2 and two diodes D 1 and D2 in the
amplifler/pulse shaper
portion of the circuit.
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Time delay monostable circuit 23 is connected to the current pulse generator
22 as seen in
Fig. 10. The time delay monostable circuit is set to define the end point of
the waveguide
(member 12). The current pulse l:riggers the monostable circuit and defines a
start point.
Between the start and end points m electronic time window is defined. This
corresponds to
the mono~stable output and to the maximum length of travel of the magnet along
the member
12.
Compara.tor 40 produces a signal proportional to the time delay between
initiation of the
pulse from circuit 22 to receipt of the signal from pulse shaper 38. An
integrator 41 ramps
first in one direction from the start of the time window until the digitizer
output indicates that
the ultrasonic pulse has been detecaed. The integrator ramps in the opposite
direction until
the end of the time window. The cycle is then repeated.
Fig. 2 and 3 show alternative embodiments of the invention. Member 12.2 of
Fig. 2 has an
extension 42 beyond coil 20.2. In Fig. 3, member 12.3 has a folded-back or
hook-like
portion 44 extending beyond coil 20.3 . The purpose of the extension and of
the folded-back
or hook-like portion of the members is to void interference and a null point
near end 16 of
the member 12 which would otherwise result from reflections of pulses off the
end of the
wire. This extension or hook-like portion does not serve as means for damping
as in the
prior art, but instead repositions th.e reflection/null point such that it
does not influence the
signal received by piezoelectric elE;ment 2.4. The member could have an
extension 42 at one
or both ends. Alternatively the end of tile member could be crimped or cut at
an oblique
angle. As a further alternative software signal processing could be used to
identify and
remove the effects of end reflection.
An alternative embodiment is shown in Fig. 4 where like parts have like
numbers as in Fig.
1 with the additional designation ".4". In this embodiment there is an
additional
piezoelectric element 46.4 at the opposite end 16.4 of piezoelectric element
34.4. There is
also a second amplifier 48.4 and a second pulse shaper 50.4. A more accurate
calculation
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of the position of the magnet can lbe obtained with this embodiment because
two separate
time delays are measured from the pulses received by piezoelectric elements
34.4 and 46.4.
The ratio of these two time delay, indicates the position of the magnet along
the member
12.4. Noise signals can be filtered out because the sum of the time delays for
true signals
must be constant for a transducer of a given length.
Fig. 5 shows a typical installation of the transducer 10 of Fig. 1 on an
hydraulic actuator 51
which includes a cylinder 56, a pi;>ton 52 and a piston rod 54 connected to
the piston. The
transducer 10 is mounted in an exterior tubular casing 60 connected to the
outside of the
cylinder such that the member 12 extends parallel to the piston rod. The
member 12 is
supported by a series of annular inserts 5 8 in the tube. The casing has plugs
62 and 64 at
opposite ends thereof.
Fig. 6 shows an alternative installation of the transducer 10 which is more
conventional from
the point of view of the prior art. Here the components of the cylinder and
mounting system
equivalent to those in Fig. 5 are given the same numbers but with the
additional designation
".6". Here the transducer 10 is mounted coaxially with piston rod 54.6 within
central bore
55 thereof The casing 60.6 is fixedly connected to a plug 68 located in
central aperture 67
at the end of the cylinder. A seal 68 is compressed between the plug and the
end of the
cylinder.
It will be understood by someone skilled in the art that many of the details
provided above
are by way of example only and axe not intended to limit the scope of the
invention which
is to be interpreted with reference to the following claims.
