Miscellaneous active electrical nonlinear devices – circuits – and – Specific identifiable device – circuit – or system – With specific source of supply or bias voltage
Reexamination Certificate
1999-12-02
2001-05-15
Cunningham, Terry D. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Specific identifiable device, circuit, or system
With specific source of supply or bias voltage
C327S538000, C323S315000
Reexamination Certificate
active
06232831
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to electrical power supplies used to provide electrical power to amplified measurement transducers, and more particularly to an electrical power supply including a floating current source facilitating the use of line-powered circuits operating from a utility a.c. power source.
2. Description of the Related Art
Piezoelectric materials develop surface electrical charges when subjected to a mechanical force. When an applied force distorts or deforms a piezoelectric crystalline structure, electrical charges within the crystalline structure are displaced, and a net electrical charge is developed across opposed surfaces of the crystalline structure. In many cases, the developed electrical charge is directly proportional to the applied force. Piezoelectric materials include quartz, tourmaline, and man-made piezoelectric ceramic materials (e.g., lead zirconate titanate or PZT).
A piezoelectric transducer includes a piezoelectric material as a sensing element to measure, for example, force, pressure, or acceleration. The electrical signals produced by piezoelectric transducers are often conveyed to remote readout devices by multi-conductor cables. Modem piezoelectric transducers include electronic components which form an electrical interface between the piezoelectric transducers and the cables in order to reduce distortion of the electrical signals (e.g., amplitude reduction, noise pickup, etc.) during transmission along the cables.
FIG. 1
is a diagram of an exemplary prior art measurement system
10
including an acceleration transducer (i.e., accelerometer)
12
, a power supply
14
, and a readout unit
16
. Accelerometer
12
produces an electrical signal proportional to an acceleration experienced by accelerometer
12
. Accelerometer
12
may be mechanically coupled to a physical structure or unit under test (UUT) undergoing shock or vibration testing. The electrical signal produced by accelerometer
12
is transmitted to readout unit
16
. Readout unit
16
may be, for example, an oscilloscope, a data recorder, or a chart recorder.
Accelerometer
12
includes a seismic mass
18
, a piezoelectric sensing element
20
, and a signal amplifier
22
enclosed within a housing
24
. Piezoelectric sensing element
20
may be a piezoelectric crystalline material (e.g., quartz). Seismic mass
18
is mechanically coupled to piezoelectric sensing element
20
such that when accelerometer
12
experiences an acceleration, seismic mass
18
imposes a mechanical force upon sensing element
20
which distorts a crystalline structure of sensing element
20
. For example, when accelerometer
12
experiences acceleration a along a defined axis extending through accelerometer
12
, seismic mass
18
may impose a compression force F upon sensing element
20
where F=m·a. Alternately, seismic mass
18
may impose a tension force F within sensing element
20
, or a shear stress within sensing element
20
.
When accelerometer
12
experiences an acceleration, and seismic mass
18
imposes a mechanical force upon sensing element
20
, sensing element
20
produces an electrical signal (e.g., a charge signal or a voltage signal) between an input terminal of signal amplifier
22
and a reference node
26
. When sensing element
20
produces a charge signal, signal amplifier
22
may be a charge amplifier which converts the charge signal to a voltage signal. Signal amplifier
22
produces a voltage signal V
S
at an output terminal, where a known relationship exists between voltage signal V
S
and the electrical signal produced by sensing element
20
.
In the embodiment of
FIG. 1
, voltage signal V
S
is transmitted along a first two-conductor cable
28
to power supply
14
, and along a second two-conductor cable
30
to an input of readout unit
16
. A signal loop is thus formed between accelerometer
12
and readout unit
16
. Signal amplifier
22
preferably has a relatively low output impedance in order that other impedances around the signal loop may be kept relatively low. For example, when the output impedance of signal amplifier
22
is reduced, an input impedance of a differential amplifier
32
within readout unit
16
which receives voltage signal V
S
may also be reduced. As a result, the amount of noise introduced into voltage signal V
S
during transmission from accelerometer
12
to readout unit
16
is reduced. It is noted that power supply
14
may be incorporated into readout unit
16
, thus eliminating the second two-conductor cable
30
.
Power supply
14
produces a direct current (d.c.) bias voltage V
B
and bias current I
B
required by signal amplifier
22
. Power supply
14
includes a battery
34
and a constant current diode
36
connected in series between the two conductors of the first two-conductor cable
28
. Such power supplies for amplified transducers are well known in the art. Battery
34
produces a d.c. voltage, and constant current diode
36
passes constant d.c. current I
b
.As shown in
FIG. 1
, positive bias voltage V
B
is developed between the output terminal of signal amplifier
22
and reference node
26
, and bias current I
B
flows into the output terminal of signal amplifier
22
. Signal voltage V
S
produced by signal amplifier
22
is superimposed upon bias voltage V
B
such that an electrical voltage of (V
B
+V
S
) exists between the output terminal of signal amplifier
22
and reference node
26
. A d.c. blocking capacitor may be inserted in the signal loop between power supply
14
and readout unit
16
, or readout unit
16
may include a d.c. level shifter to remove bias voltage V
B
.
FIG. 2
is a diagram of one embodiment of signal amplifier
22
according to the prior art. In the embodiment of
FIG. 2
, signal amplifier
22
includes an n-channel, depletion-mode metal oxide semiconductor (MOS) transistor
40
having a gate terminal G coupled to a first terminal of sensing element
20
, a source terminal S coupled to a second terminal of sensing element
20
and reference node
26
, and a drain terminal D coupled to the output terminal of signal amplifier
22
. MOS transistor
40
is biased into a linear operating region by bias voltage V
B
and bias current I
B
provided by power supply
14
. Connected in a common source configuration as shown in
FIG. 2
, MOS transistor
40
has a relatively high input impedance, a relatively low output impedance, and amplifies the voltage produced by sensing element
20
. Signal voltage V
S
produced by MOS transistor
40
reproduces the electrical signal produced by sensing element
20
, and is superimposed upon bias voltage V
B
such that an electrical voltage of (V
B
+V
S
) exists between the output terminal of signal amplifier
22
and reference node
26
.
FIG. 3
is a graph of the voltage between the output terminal of signal amplifier
22
and reference node
26
versus time. The linear operating region of MOS transistor
40
exists between a maximum voltage V
MAX
and a minimum voltage V
MIN
. Bias voltage V
B
may be about midway between V
MAX
and a minimum voltage V
MIN
as shown in
FIG. 3
, allowing for equally-sized positive and negative voltage swings of signal voltage V
S
. An electrical voltage of (V
B
+V
S
) exists between the output terminal of signal amplifier
22
and reference node
26
as described above and shown in FIG.
3
.
One reason batteries are often used to generate bias voltage V
B
and bias current I
b
is to ensure electrical isolation between the two conductors of two-conductor cable
28
and a ground electrical potential G
2
existing at power supply
14
. Referring back to
FIG. 1
, housing
24
of accelerometer
12
may be made of metal (e.g., stainless steel) for strength, durability, and long-term transducer reliability as is typical. In this case, metal housing
24
is also electrically conductive. When accelerometer
12
is mechanically coupled to an electrically conductive structure under test, and the electrically conductive structure is connected to a ground electri
Becker Alvin G.
Lennous Paul A.
Conley Rose & Tayon
Cunningham Terry D.
Hood Jeffrey C.
National Instruments Corporation
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