Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
434
This invention relates to pieæo electric transducers
for generating an electrical signal related to the instan-
taneous velocity of the transducer.
Velocity responsive electromagnetic transducers of the
seismic variety are widely used in vibration analysis. See
U.S. patent 3,157,852. Velocity responsive eddy current
devices also are employed to measure instantaneous velocity
of vibrating bodies. See Canada patent 850,825. Such ~ -
velocity responsive devices have limited sensitivity at
vibration frequencies below ten cycles per second and above
one thousand cycles per second.
Prior art accelerometer devices employing compressed
piezo electric crystals have been employed to measure the ;
instantaneous acceleration of a vibrating body. The prior
art piezo electric crystal accelerometers develop an elec-
trical charge which is proportional to the instantaneous
acceleration. Such accelerometer i~lstallations are limited
to low-sensitivity piezo electric crystals, e.g., sensitivi-
ties below 1,000 pico coulombs per g. The instantaneous
acceleration responsive electric charge is delivered by
cable to a remote monitoring circuit for indicating the
instantaneous acceleration by converting the "charge" to a
corresponding voltage. The limitation of sensitivity
results from the fact that the more sensitive piezo electric
crystals have a low natural frequency which tends to be
excited in the crystal acceleration mod~ to yield a large
output signal which saturates the charge amplifier. Thus to
avoid amplifier saturation, the less sensitive piezo elec-
tric crystals have been employed, e.g., crystals having
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sensitivities of the order of 10 to 100 pico-coulombs
per g.
According to the present invention a high-sensitivity
piezo electric crystal stack with interposed electrodes is
compressed between a base and a seismic mass O Alternate
electrodes are connec-ted in parallel with each other and in
series with one or two electrical resistors to develop
an output electrical charge which is proportional to the
instantaneous velocity of the transducer. High sensitivity
piezo electric crystal devices are employed in this instal-
lation, i.e., crystals having a sensitivity greater than
1,000 pico-coulombs per g. Preferably the devices have a
sensitivity of 5,000 to 10,000 pico coulombs per g. The
high-sensitivity piezo electric crystals may be employed in
the present assembly because the instantaneous velocity of
the transducer is relatively low at the natural frequency of
the crystals. The output electrica:L charge from the present
transducer is preferably applied to a cable for delivery to
a remote monitoring circuit. Inasmuch as cable capacitance ;~
is not a factor in the system output, cable length is not a~
practical limiting factor. Cables up to 1,000 feet may be
employed. The cable delivers the electrical charge into a
charg~ convert~r whlch develops an electrical voltage corre-
sponding to the instantaneous velocity of -the transducer.
As an alternative embodiment, the resistor or resistors
may be applied to the system adjacent to the charge ampli- -
fier and remote from the transducer.
The resulting transducer is useful in measuring instan-
taneous velocities over a relatively wide range of velocities ~; ~
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from about one cycle per second through about 4,000 cycles
per second with an acceptable accuracy.
FIGURE 1 is a partly cross-sectional, partly schematic
illustration of the present transducer showing the actual
transducer in cross-section and the connectin~ cable and
circuitry schematically.
FIG~RE 2 is a perspective illustration of a typical
doughnut-shaped piezo electric crystal.
FIGURE 3 is a schematic illustration of a doughnut-
shaped electrode.
FIGURE 4 is a schematic illustration of typical cir-
cuitry useful in combination with the present transducer.
FIGURE 5 is a graphical illustration showing the range
of the present transducer and the range of prior art veloc-
ity responsive vibration transducers. ~,
FIGURES 6, 7, 8 and 9 are schernatic illustrations of
four alternative embodiments o the present invention.
Vibration transducers are employed in a variety of
installations. They may be employed in unbalance analysis
of rotating bodies; vibration measurements both repetitive
.
~and translent; contlnuous monitoring of instantaneous
vibrations. Such installations requira an electromechanical
transducer of some type which can generate an electrical
signal responsive to the instantaneous mechanical vibration
of the body under inspection. Seismic transducers are
securely mounted so as to move in unison with the body under
inspection. The transducer is customarily connected by an
electrical conductor to a conversion circuit which receives
an electrical signal and generates a useful result in the
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34
form of a meter reading, a visible manifestation on an
oscilloscope or similar read-out device; an alarm signal
such as a lamp illumination or an audible sound when the
observed vibration exceeds pre-established threshold levels;
circuit opening devices for terminating further activity
when observed vibrations exceed pre-established threshold
levels; and the like.
In its preferred embodiment the present transducer 10
comprises a cannister device as shown in FIGURE 1 including
a circular base 11, a seismic mass 12, a piezo electric
crystal stack 13, a connecting tension bolt 14, all secured
within a cylindrical casing 15 and cover cap 16. The piezo
electric crystal stack 13 includes alternating piezo elec-
tric crystals 17 (see FIGURE 2) having a central bore 18 and
electrodes 19 (see FIGURE 3) having a central bore 20 and a
radial connector tab 21. Shown in FIGURE 1 are seven piezo
electric crystal elements 17 and eight electrodes 19. A
mica doughnut-shaped disc 22 is applied to the top of the
uppermost electxode 19 to insulate that uppermost electrode
from the seismic mass 12. Another mica disc 23 is applied
beneath the bottom electrode 19 to insulate that bottom
electrode from the base 11.
The base 11 and the seismic mass 12 are fabricated from
non-magnetic steel, e.g., austenite stainless steel. The
seismic mass 12 has a central clearance bore 24 for re-
ceiving the tension bolt 14. The base 11 has a central,
internally threaded well 25 for receiving the threaded end
of the tension bolt 14. A suitable insulating plastic
sleeve 26 is applied over the body of the tension bolt 14
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where it passes through the central bores 18, 20 of the
crystals 17, 19, respectively. The plastic sleeve 26 pref-
erably is a shrink fit plastic, such as Teflon. The tension
bolt 14 is tightened until a compressive stress on the piezo
electric crystals of the order of 2000 to 4000 psi is
achieved. The crystals 17 should be maintained in a pre-
loaded state at the highest anticipated acceleration exposure.
The piezo electric crystal elements 17 are ceramic
materials having piezo electric properties, for example,
lead-zirconate-titanate crystals are suitable. The seismic
mass 12 in a typical unit weighs about one pound in order to
achieve a desirably high sensitivity in the unit. The base
member 11 should be sufficiently large to provide adequate
rigid mounting to the body under inspection.
It will be observed from FIGURE 1 that four of the
electrode tabs are connected by conductors to a resistor 27
and the other four alternating elect:rode tabs are connected
by conductors to another resistor 2~. The resistors 27, 28
are connected to output conductors 29, 30 which extend
through an aperture 31 in the cylindrical casing 15.
It will be observed that the cover cap 16 is fitted
into shoulders at the upper edges of the cylindrical casing
15 and the outer top portions of the cylindrical casing 15
are rolled over the perimeter of the cover plate 16. Simi-
larly the base member 11 has a circular shoulder which
engages an internal shoulder in the bottom of the casing 15 -
and the outer surface of the casing 15 is rolied into an
undercut portion of the base member 11 to provide a tamper-
resistant seal for the unit.
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While the resistors 27, 28 appear to be unsupported, it
is preferred that they be secured by an adhesive to the base
member 11 to prevent vibration of the resistors and con-
ductors. : :
The two conductors 29, 30 are connected to a cable 32
and ultimately to the input terminals 33, 34 of a conversion
circuit 35 which includes a dif~erential charge converter
36, a low pass filter 37 and a voltage amplifier 38 which
produces an output signal at a terminal 39.
The conversion circuitry is illustrated in more detail
in FIGURE 4 wherein the differential charge converter is
equipped with a high voltage input protection circuit
including a pair of resistors 40, 41 and a pair of back-to- : .
back Zener diodes 42, 43. The signal applied to the ter- ~: .
minals 33, 34 is a velocity responsive electrical charge .:
related to the instantaneous velocity of the transducer 10
(FIGURE 1). The velocit~ responsive electrical charge is :~
applied to the input terminals of an integrated circuit 44 ` -~-
operating as a charge converter to produce an output signal .-. .:
at the terminal 45 which is an electrical voltage related to
the instantaneous velocity of the transducer. The inte~
:
grated circuit 44 operates as a high gain amplifier with
negative capacitance feedback. The voltage of the terminal
45 is delivered through a low pa;s filter 37 and delivered -to the input terminals of an operational amplifier 46,
another operational amplifier 47 and a third operational ,';:~
amplifier 48 for delivery to the output terminal 39. A
potentiometer 49 permits adjustment of the level of output : -
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voltage at the output terminal 39.
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It will be observed in connection with the difEerential
charge converter integrated circuit 44 that two parallel
capacitors 50, 51 are balanced against three parallel
capacitors 52, 53, 54 to permit accurate balancing of the
charge converter circuit 36. One of the capacitors 54 is a
variable capacitor to permit trim adjustments to accommodate
manufacturing tolerances of the circuit components.
Operation of the Present Transducer
FIGURE 5 illustrates graphically the relationship
between the frequency of repetitive vibrations (in cycles
per second) and the amplitude of vibration (in inches per
second~. FIGURE 5 shows a range of frequencies from one
cycle per second through 10,000 cycles per second and a -
corresponding range of vibration amplitude from 100 inches
per second through 0.0001 inch per second. Plotted on
FIGURE 5 at 45 angle in a positive direction are the
corresponding peak-to-peak vibration displacements ranging
from 1.0 inch ~hrough 10-5 inch. Also plot~ed in FIGURE 5
are the corresponding peak acceleration values ranging from
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100 g throu~h 0.0001 g.
` Within the graphical framework of FIGURE 5 there is a
bold-line bordered area A which establishes the approximate
limitations o~ existing velocity responsive transducers.
They are limited to frequency ranges between 10 cycles per
second and about 1,000 cycles per second. They are further
limited by the peak-to-peak displacement of the vibrating
body from a range of about 0.1 inch to about 0.00005 inch.
, They are further limited to accelerations below about 20
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The accelerometer device of the present invention
appears to be useful over the shaded area B of FIGURE 5.
Acceptable frequency ranges from one through 4,000 cycles
per second and over velocities from about 0.0005 through
about 5 inches per second. The device is particularly
useful in the low frequency ranges from about one through
about 10 cycles per second.
Actual tests of the device have indicated the following
characteristics. The device has a sensitivity of approxi-
mately 30 pico-coulombs per inch per second when measuring
velocity. This corresponds to an acceleration sensitivity
of approximately 5,000 pico-coulombs per g. The device has
a natural frequency much higher than 10,000 cycles per
second which indicates that the device at all times is
functioning well below its resonance.
The device has been tested with a spectrum of fre- ~
quencies and shows 92 percent response at one cycle per ~ ~-
second, 100 percent response at four cycles per second and
95 percent response at 4,000 cycles per second. The device
can be operated at temperatures up to about 500F without
damage.
The device exhibits only a 10 phase shift at 5 cycles
per second; 3 at 10 c.p.s.; and negligible above 20 c.p.s. ~ -
This negligible phase shift permits the device to be used as
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a balancing transducer if re~uired.
; Alternative Embodiments ~-
As illustrated in FIGURE 6 the preferred device employs ~ ~ -
two resistors 27, 28 and a differential charge converter 36.
It is feasible, as in FIGURE 7, to employ a single resistor
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28' and a single ended charge converter in the conversion
circuit 35' to generate an output signal at terminal 39'.
The equation for operation of ~he piezo electric
devices of FIGURES 6 and 7 is:
o i(j2~fCR ~ J
wherein QO is the transducer output charge;
Qi is the charge generated in the piezo electric crystals;
R is the series resistance;
C is the capacitance of the crystal;
f is the frequency (in cycles/second).
The resistance value R in the foregoing equation for the -
preferred embodiment of FIGURE 6 is the sum of the resis- -
tance of resistors 27, 28. In the alternative embodiment of
FIGURE 7, the resistance value R is the resistance of the
single resistor 28'.
By adjusting the value of R in the above equation, the
low frequency response and the sens:itivity of the transducer
can be varied. For example, if R is doubled, the lower
llmit of useful frequency is halved, but the sensitivity
also is halved. The present system permits the use of
transducers havlng a sensitivity of 1000 pico-coulombs/g and
higher.
As shown in FIGURE 8, the piezo electric crystal 13 has
its terminals connected directly to the cable 32 which is
joined to the input terminal of the charge amplifier 35
through two resistors 27a, 28a. In FIGURE 9~ only a single
resistor 28a' is provided between the cable 32 and the
charge amplifier 35'. The resulting output signal for both
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FIGURES 8 and 9 is determined by the equation
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Qo = Qi Lj~(cl+c2)R + lJ
wherein C2 is the capacitance of the cable 32;
R is the resistance (of 27a and 28a in FIGIJRE 8;
of 28a' in FIGURE 9);
and the other factors have the same meaning as hereinbefore
assigned.
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