U.S. patent application number 09/804817 was filed with the patent office on 2004-09-23 for microelectromechanical system (mems) analog electrical isolator.
Invention is credited to Dummermuth, Ernst H., Harris, Richard D., Knieser, Michael J., Kretschmann, Robert J..
Application Number | 20040183617 09/804817 |
Document ID | / |
Family ID | 46298757 |
Filed Date | 2004-09-23 |
United States Patent
Application |
20040183617 |
Kind Code |
A1 |
Harris, Richard D. ; et
al. |
September 23, 2004 |
MICROELECTROMECHANICAL SYSTEM (MEMS) ANALOG ELECTRICAL ISOLATOR
Abstract
A microelectromechanical system (MEMS) analog isolator may be
created in which an actuator such as an electrostatic motor drives
a beam against an opposing force set, for example, by another
electrostatic motor. Motion of the beam may be sensed by a sensor
also attached to the beam. The beam itself is electrically isolated
between the locations of the actuator and the sensor. The structure
may be incorporated into integrated circuits to provide on-chip
isolation.
Inventors: |
Harris, Richard D.; (Solon,
OH) ; Knieser, Michael J.; (Richmond Heights, OH)
; Dummermuth, Ernst H.; (Chesterland, OH) ;
Kretschmann, Robert J.; (Bay Village, OH) |
Correspondence
Address: |
Rockwell Technologies, LLC
Attention: John J. Horn
Patent Dept./704P Floor 8 T-29
1201 South Second Street
Milwaukee
WI
53204
US
|
Family ID: |
46298757 |
Appl. No.: |
09/804817 |
Filed: |
March 13, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09804817 |
Mar 13, 2001 |
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09406364 |
Sep 28, 1999 |
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6188322 |
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09804817 |
Mar 13, 2001 |
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09406654 |
Sep 27, 1999 |
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6463339 |
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09804817 |
Mar 13, 2001 |
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09400125 |
Sep 21, 1999 |
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6417743 |
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Current U.S.
Class: |
333/24.2 |
Current CPC
Class: |
B81B 3/0072 20130101;
G05B 2219/21109 20130101; G01R 33/0286 20130101; G05B 19/07
20130101; H03H 9/462 20130101; G05B 2219/24181 20130101; G01R
15/148 20130101 |
Class at
Publication: |
333/024.2 |
International
Class: |
H04B 003/04 |
Claims
We claim:
1. A microelectromechanical system (MEMS) analog isolator,
comprising: a substrate; an element supported from the substrate
for movement between a first and second position with respect to
the substrate, where at least a portion of the element between a
first and second location on the element is an electrical insulator
to electrically isolate the first and second locations from each
other; an actuator attached to the first portion of the element to
receive an input electrical signal and exert a force dependent on
the input electrical signal urging the element toward the second
position; a control element attached to the element to exert a
force dependent on the displacement of the element toward one of
the first position and the second position; and a sensor assembly
communicating with the second portion of the element to provide an
output electrical signal dependent on movement of the element
between the first position and the second position.
2. The MEMS analog isolator of claim 1 wherein the control element
is a spring and the sensor assembly includes a sensor providing the
analog output electrical signal.
3. The MEMS analog isolator of claim 1 wherein the control element
is a second actuator attached to the element to receive a feedback
electrical signal and exert a force dependent on the feedback
electrical signal urging the element toward the first position; and
including wherein the sensor assembly including a sensor indicating
a location of the element with respect to a null position and an
error detector receiving the output electrical signal to generate
the feedback electrical signal so as to tend to restore the element
to the null position and wherein the output electrical signal is
derived from the feedback signal.
4. The MEMS analog isolator of claim 1 wherein the control element
further includes a third actuator attached to the element to
receive a second feedback signal and exert a force dependent on the
second feedback electrical signal urging the element toward the
second position; whereby more complex feedback control of the
element may be accomplished.
5. The MEMS analog isolator of claim 3 wherein the error detector
produces a binary electrical feedback indicating a position of the
beam with respect to a null location between the first and second
positions and further including a pulse width demodulator circuit
evaluating the duty cycle of the feedback signal to produce the
output electrical signal.
6. The MEMS analog isolator of claim 1 wherein the actuator is
selected from the group consisting of: an electrostatic motor, a
Lorenz-force motor, a piezoelectric motor, a thermal-expansion
motor, and a mechanical-displacement motor.
7. The MEMS analog isolator of claim 1 wherein the control element
is selected from the group consisting of: an electrostatic motor, a
Lorenz-force motor, a piezoelectric motor, a thermal-expansion
motor, a mechanical-displacement motor, and a mechanical
spring.
8. The MEMS analog isolator of claim 1 wherein the sensor is
selected from the group consisting of a capacitive sensor, a
piezoelectric sensor, a photoelectric sensor, a resistive sensor,
or an optical switching sensor.
9. The MEMS analog isolator of claim 1 wherein the element is a
beam attached to the substrate for sliding motion between the first
and second positions.
10. The MEMS analog isolator of claim 8 wherein the beam moves with
respect to the substrate along a longitudinal axis and including
flexing transverse arm pairs attached at longitudinally opposed
ends of the beam to extend outward therefrom to support the beam
with respect to the substrate.
11. The MEMS analog isolator of claim 9 wherein the flexing
transverse arms attached to the substrate at points proximate to
the beam and where the flexing transverse arms include: (i)
cantilevered first portions having first ends attached to the beam
and second ends attached to an elbow portion removed from the beam;
and (ii) cantilevered second portions substantially parallel to the
first portions and having first ends attached to the substrate and
second ends attached to the elbow portion; whereby expansion of the
first portion is offset by substantially equal expansion of the
second portion so that the amount of stress in the beam can be
controlled.
12. The MEMS analog isolator of claim 9 wherein the flexing
transverse arms attach to the substrate through a spring section
allowing angulation of the end of the transverse arm with respect
to the substrate.
13. The MEMS analog isolator of claim 9 wherein the beam and
transverse arms are symmetric across a longitudinal axis.
14. The MEMS analog isolator of claim 9 including further a
magnetic field crossing the beam and wherein at least one flexing
transverse arm pair is conductive to receive an electrical signal
and exert a force dependent on the electrical signal urging the
beam toward position.
15. The MEMS analog isolator of claim 9 including transverse
extending primary capacitor plates attached to the beam and
extending outward from the beam proximate to secondary capacitor
plates.
16. The MEMS analog isolator of claim 14 wherein an effective area
of the primary capacitor plates is equal across the longitudinal
axis of the beam.
17. The MEMS analog isolator of claim 14 wherein the capacitor
plates attach to the beam between the attachment points of at least
two of the flexing transverse arm pairs.
18. The MEMS analog isolator of claim 14 wherein the primary
capacitor plates are positioned with respect to the secondary
capacitor plates so as to draw the primary capacitor plates toward
the secondary capacitor plates on one side of the beam while to
separate the primary capacitor plates from the secondary capacitor
plates on the other side of the beam with a given motion.
19. The MEMS analog isolator of claim 14 wherein the primary
capacitor plates are positioned with respect to the secondary
capacitor plates so as to draw the primary capacitor plates toward
the secondary capacitor plates on both sides of the beam with a
given motion.
20. The MEMS analog isolator of claim 1 wherein the beam includes
first and second micro-machined layers, the first of which is
insulating to provide the portion of electrical insulator in a
region where the second layer is removed.
21. The MEMS analog isolator of claim 1 wherein the portion of
electrical insulator of the beam is between the actuator and the
controlling device.
22. The MEMS analog isolator of claim 1 wherein the portion of
electrical insulator of the beam is between the controlling device
and the sensor.
23. An isolated circuit module comprising: a substrate; a plurality
of interconnected solid-state electronic devices formed on the
substrate into an integrated circuit having analog input and output
points; a mechanical analog isolator also formed on the substrate
and electrically attached to at least one of the integrated circuit
input and output points, the mechanical analog isolator including:
a substrate; an element supported from the substrate for movement
between a first and second position with respect to the substrate,
where at least a portion of the element between a first and second
location on the element is an electrical insulator to electrically
isolate the first and second locations from each other; an actuator
attached to the first portion of the element to receive an input
electrical signal and exert a force dependent on the input
electrical signal urging the element toward the second position; a
control element attached to the element to exert a force dependent
on the displacement of the element toward the first position; a
sensor assembly communicating with the second portion of the
element to provide an output electrical signal dependent on
movement of the element between the first positions.
24. The isolated circuit module of claim 23 wherein the actuator of
the mechanical analog isolator is attached to at least one output
point of the integrated circuit whereby the output electrical
signal provides an isolated output for the at least one output
point.
25. The isolated circuit module of claim 23 wherein the sensor of
the mechanical analog isolator is attached to at least one input
point of the integrated circuit whereby the output electrical
signal provides an isolated input to at least one output point.
26. The MEMS analog isolator of claim 1 including further a second
sensor at the first portion of the element to provide a second
output electrical signal indicating movement of the element to the
second position, the second output electrical signal being
electrically isolated from the output electrical signal.
27. The MEMS analog isolator of claim 26 including further a second
actuator at the second portion of the element to receive a second
input electrical signal and exert a force dependent on the second
input electrical signal urging the element toward the second
position.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation in part of U.S. patent
application Ser. No. 09/406,364 filed Sep. 28, 1999; Ser. No.
09/406,654 filed Sep. 27, 1999 and Ser. No. 09/400,125 filed Sep.
21, 1999.
FIELD OF THE INVENTION
[0002] The present invention relates to electrical isolators and in
particular to a microelectromechanical system (MEMS) device
providing electrical isolation in the transmission of analog
electrical signals.
BACKGROUND OF THE INVENTION
[0003] Electrical isolators are used to provide electrical
isolation between circuit elements for the purposes of voltage
level shifting, electrical noise reduction, and high voltage and
current protection.
[0004] Circuit elements may be considered electrically isolated if
there is no path in which a direct current (DC) can flow between
them. Isolation of this kind can be obtained by capacitive or
inductive coupling. In capacitive coupling, an electrical input
signal is applied to one plate of a capacitor to transmit an
electrostatic signal across an insulating dielectric to a second
plate at which an output signal is developed. In inductive
coupling, an electrical input signal is applied to a first coil to
transmit an electromagnetic field across an insulating gap to a
second coil, which generates the isolated output signal. Both such
isolators essentially block steady state or DC electrical
signals.
[0005] Such isolators, although simple, block the communication of
signals that have significant low frequency components. Further,
these isolators can introduce significant frequency dependent
attenuation and phase distortion in the transmitted signal. These
features make such isolators unsuitable for many types of signals
including many types of high-speed digital communications.
[0006] In addition, it is sometimes desirable to provide high
voltage (>2 kV) isolation between two different portions of a
system, while maintaining a communication path between these two
portions. This is often true in industrial control applications
where it is desirable to isolate the sensor/actuator portions from
the control portions of the overall system. It is also applicable
to medical instrumentation systems, where it is desirable to
isolate the patient from the voltages and currents within the
instrumentation.
[0007] The isolation of digital signals is frequently provided by
optical isolators. In an optical isolator, an input signal drives a
light source, typically a light emitting diode (LED) positioned to
transmit its light to a photodiode or phototransistor through an
insulating but transparent separator. Such a system will readily
transmit a binary signal of arbitrary frequency without the
distortion and attenuation introduced by capacitors and inductors.
The optical isolator further provides an inherent signal limiting
in the output through saturation of the light receiver, and signal
thresholding in the input, by virtue of the intrinsic LED forward
bias voltage.
[0008] Nevertheless, optical isolators have some disadvantages.
They require a relatively expensive gallium arsenide (GaAs)
substrate that is incompatible with other types of integrated
circuitry and thus optical isolators often require separate
packaging and assembly from the circuits they are protecting. The
characteristics of the LED and photodetector can be difficult to
control during fabrication, increasing the costs if unit-to-unit
variation cannot be tolerated. The power requirements of the LED
may require signal conditioning of the input signal before an
optical isolator can be used, imposing yet an additional cost.
While the forward bias voltage of the LED provides an inherent
noise thresholding, the threshold generally cannot be adjusted but
is fixed by chemical properties of the LED materials. Accordingly,
if different thresholds are required, additional signal
conditioning may be needed. Finally, the LED is a diode and thus
limits the input signal to a single polarity unless multiple LEDs
are used.
[0009] It is common to process analog electrical signals using
digital circuitry such as microprocessors. In such situations, the
analog signal may be periodically sampled and the samples converted
into digital words input by an analog to digital converter (A/D) to
and processed by the digital circuitry. Conversely, digital words
produced by the digital circuitry may be converted into an analog
signal through the use of a digital-to-analog converter (D/A) to
provide a series of analog electrical values that may be filtered
into a continuous analog signal. Isolation of such signals at the
interface to the digital circuitry is often desired and may be
performed by placing an optical isolator in series with the
electrical signal representing each bit of the relevant digital
word after the A/D converter and before the D/A converter.
Particularly in the area of industrial controls where many isolated
analog signals must be processed and output, a large number of
optical isolators are required rendering the isolation very costly
or impractical.
BRIEF SUMMARY OF THE INVENTION
[0010] The present invention provides a mechanical isolator
manufactured using MEMS techniques and suitable for transmitting
analog signals without prior conversion to digital signals. A
special fabrication process forms a microscopic beam whose ends are
insulated from each other. One end of the beam is connected to a
microscopic actuator, which receives an analog input signal to move
the beam in proportion to a generated actuator force. The other
isolated end of the beam is attached to a sensor detecting movement
of the beam to provide a corresponding analog value. The small
scale of the total device provides inexpensive, fast and reliable
response.
[0011] Specifically, the present invention provides a
microelectromechanical system (MEMS) analog isolator having a
substrate and an element supported from the substrate for
continuous movement between a first and second position with
respect to the substrate, where at least a portion of the element
between a first and second location on the element is an electrical
insulator to electrically isolate the first and second locations
from each other. An actuator attached to the first portion of the
element receives an input electrical signal and exerts a force
dependent on the input electrical signal urging the element toward
the second position. A control device attached to the element to
exert a force dependent on the displacement of the element toward
the first position and a sensor assembly communicating with the
second portion of the element provide an analog output electrical
signal dependent on movement of the element between the first
position and the second position.
[0012] It is one object of the invention to produce a simple
mechanical isolation system using MEMS techniques suitable for
direct isolation of an analog signal overcoming the need for many
optical isolators and further avoiding many of the disadvantages of
current optical isolators in costs, interdevice consistency, and
incompatibility with other integrated circuit components. In
addition, the present invention requires no preconditioning of the
input signal. The input voltage, current, or mechanical
displacement can be applied directly to the device with no
pre-processing.
[0013] The control element may be a spring or its equivalent and
the sensor assembly may include a sensor providing the analog
output electrical signal based on the amount of movement of the
element.
[0014] Thus another object of the invention is to provide the
possibility of a simple open-loop analog isolator where the analog
signal is transmitted over an insulated beam by motion of the
beam.
[0015] Alternatively, the control element may be a second actuator
attached to the element to receive a feedback electrical signal and
exert a force dependent on the feedback electrical signal urging
the element toward the first position. In this case, the sensor
assembly may include a sensor indicating a location of the element
with respect to a null position and an error detector receiving the
output electrical signal to generate the feedback electrical signal
so as to tend to restore the element to the null position. The
output electrical signal is derived from the feedback signal.
[0016] Another object of the invention is thus to permit a more
complex analog isolator using feedback techniques where the analog
signal is transmitted as forces permitting minimal movement of the
beam thus avoiding mechanical non-linearities.
[0017] The control element may further include a third actuator
attached to the element to receive a second feedback signal and
exert a force dependent on the second feedback electrical signal
urging the element toward the second position.
[0018] It is thus a further object of the invention to permit a
feedback control of the beam allowing feedback signals that may
exert either a force urging the element toward the first position
or a force urging the element toward the second position.
[0019] The above described error detector may produce a binary
electrical feedback signal indicating a position of the beam with
respect to the null location between the first and second positions
and further including a pulse width demodulator circuit evaluating
the duty cycle of the feedback signal to produce the output
electrical signal.
[0020] It is thus another object of the invention to provide a
simple method of extracting a multi-bit digital signal from the
isolator of the present invention. The duty cycle demodulator may
be a simple counting circuit.
[0021] The actuator may be an electrostatic motor or a Lorenz-force
motor or a piezoelectric motor or a thermal-expansion motor or a
mechanical-displacement motor.
[0022] It is therefore another object of the invention to provide
an isolator that may receive a variety of different electrical
signals that may not be compatible with an optical isolator LED,
for example, those having a voltage of less than 0.7 volts.
[0023] Similarly, the control element may be an electrostatic
motor, a Lorenz-force motor, a piezoelectric motor, a
thermal-expansion motor, a mechanical-displacement motor, or a
mechanical spring.
[0024] Thus the invention may provide both for an extremely simple
control element that requires no electrical connection (e.g. a
mechanical spring) or an adjustable control element that allows the
null point of the beam to be freely adjusted.
[0025] The sensor may be a capacitive sensor or a piezoelectric
sensor or a photoelectric sensor or a resistive sensor or an
optical switching sensor.
[0026] It is therefore another object of the invention to provide
flexible variety of sensing techniques suitable for different
purposes.
[0027] In one embodiment of the invention, the element may be a
beam attached to the substrate for sliding motion between the first
and second positions. The beam may be supported by flexing
transverse arm pairs attached at longitudinally opposed ends of the
beam to extend outward therefrom.
[0028] Thus it is another object of the invention to provide a
simple mechanism that may be implemented on a microscopic scale
using MEMS technologies for supporting the element for motion.
[0029] The flexing transverse arms may include a cantilevered first
portion having first ends attached to the beam and second ends
attached to an elbow portion removed from the beam and a
cantilevered second portion substantially parallel to the first
portion and having a first end attached to the substrate proximate
to the beam and a second end attached to the elbow portion. Further
the beam and the transverse arms may be symmetric across a
longitudinal beam access.
[0030] Thus it is another object of the invention to provide a
microscopic structure that is resistant to thermal expansion due to
processing temperatures or to changes in the operating temperature.
The symmetry ensures that the beam remains centered with thermal
expansion while the doubling back of the flexible transverse arms
provides for a degree of stress relief.
[0031] The flexing transverse arms may attach to the substrate
through a spring section allowing angulations of the ends of the
transverse arms with respect to the substrate.
[0032] It is thus another object of the invention to allow an
effective pivoting of the flexible transverse arms so as to
decrease the stiffness of the beam structure.
[0033] One embodiment of the invention may include a magnetic
field, which may be produced by a magnet, crossing the beam and at
least one flexing transverse arm may be conductive to an electrical
signal and exert a force dependent on the electrical signal urging
the beam toward a position.
[0034] It is thus another object of the invention to provide that
the same structure used to support the beam may provide for its
actuation or control.
[0035] The beam may include transverse extending primary capacitor
plates attached to the beam and extending out from the beam
proximate to secondary capacitor plates. The effective area of the
primary capacitor plates may be equal across the longitudinal axis
of the beam and the capacitor plates may be attached to the beam
between attachment points of at least two of the flexing transverse
arm pairs. In one embodiment, the capacitors may include
interdigitated fingers. Parallel plate capacitors will also work
(although they have less linearity).
[0036] Another object of the invention is to provide a method for
the integration of an electrostatic motor to the isolator in a way
that balanced and well-supported forces may be obtained.
[0037] The primary capacitor plates may be positioned with respect
to the secondary capacitor plates so as to draw the primary
capacitor plates toward the secondary capacitor plates on one side
of the beam while to separate the primary capacitor plates from the
secondary capacitor plates on the other side of the beam.
Conversely, the capacitor plates may be positioned so that all draw
together with a given motion.
[0038] Thus it is another object of the invention to allow the
capacitor plates to be used as a sensor in which a comparison of
capacitance values reveals a position of the beam or as an
electrostatic motor.
[0039] The beam may include a first and second micro-machined
layer, the first of which is insulating to provide the portion of
the electrical insulator in a region where the second layer is
removed.
[0040] Thus it is another object of the invention to provide a
simple method for forming insulating and conductive elements
required by the present invention.
[0041] The electrical insulator of the beam may be between the
actuator and the control element or between the control element and
the sensor or both.
[0042] It is further an object of the invention to provide that the
controlling circuit may be placed on either side of the isolation
or to provide redundant isolation for greater total isolation.
[0043] The analog isolator may include a second sensor at a first
portion of the element to provide a second output electrical signal
indicating movement of the element to the second position, the
output second electrical signal being electrically isolated from
the first output electrical signal.
[0044] Thus it is another object of the invention to provide for an
isolator that produces a signal indicating movement of the beam and
thus operation of the isolator from the isolated side.
[0045] The isolator may further include a second actuator as a
second portion of the element to receive a second input signal and
exert a force dependent on the second input electrical signal
urging the element toward the second position.
[0046] Thus it is another object of the invention to provide for a
bi-directional electrical isolator suitable for use in multi-level
control loops or for the purpose of resetting a scaling factor.
[0047] The foregoing objects and advantages may not apply to all
embodiments of the inventions and are not intended to define the
scope of the invention, for which purpose claims are provided. In
the following description, reference is made to the accompanying
drawings, which form a part hereof, and in which there is shown by
way of illustration, a preferred embodiment of the invention. Such
embodiment also does not define the scope of the invention and
reference must be made therefore to the claims for this
purpose.
BRIEF DESCRIPTION OF THE DRAWINGS
[0048] FIG. 1 is a simplified block diagram of the present analog
isolator showing its elements of an actuator, a control element and
a sensor communicating along a single mechanical element that may
move to transmit data between the actuator and sensor and showing
insulating portions of the moving element;
[0049] FIG. 2 is a top plan view of one embodiment of the isolator
of FIG. 1 using three electrostatic motors and a capacitive sensor
showing support of a moving beam connecting these components by
means of flexible transverse arms and showing implementation of the
insulating sections of the beam;
[0050] FIG. 3 is a simplified perspective view of an insulating
section of the beam of FIG. 2 showing the use of laminated
conductive and nonconductive layers and the removal of the
conductive layer to create the insulating section;
[0051] FIG. 4 is a fragmentary view of one transverse arm of FIG. 2
showing an optional doubling back of the arm at an elbow so as to
provide stress relief;
[0052] FIGS. 5a and 5b are fragmentary detailed views of the elbow
of FIG. 4 showing the incorporation of a spring allowing angulation
of the portion of the transverse arm attached to the beam for
improved force characteristics;
[0053] FIG. 6 is a view of one pair of transverse arms of FIG. 2
showing electrical separation of the arms of the pair to allow a
current to be imposed on the arm to create a Lorenz-force motor
such as may be substituted for the electrostatic motors of FIG.
2;
[0054] FIG. 7 is a figure similar to that of FIG. 1 showing the
addition of a second sensor and second actuator on opposite ends of
the beam to allow for a bi-directional isolator or with the
additional sensor alone, a high reliability isolator; and
[0055] FIG. 8 is a detailed view of the sensor of FIG. 1 and its
associated processing electronics for extracting a digital word
from the isolator of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0056] Referring now to FIG. 1, a MEMS analog isolator 10 per the
present invention includes an actuator 12, control element 14, and
a sensor 18 mechanically interconnected by a movable beam 20.
[0057] The actuator 12 includes terminals 22a and 22b and 22c+22d
through which an analog electrical input signal 21 may be received
and converted into a mechanical force tending to move the beam 20
in an actuation direction 24 indicated by an arrow. In the
microscopic scale of the MEMS analog isolator 10, the actuator may
be a piezoelectric actuator, a thermal-expansion motor, a
mechanical-displacement motor, an electrostatic motor, or a
Lorenz-force motor generally known in the art, the latter two to be
described in more detail below. For a Lorenz-force motor or
thermal-expansion motor, the analog electrical input signal 21 will
be a current, for the piezoelectric or electrostatic motor, the
input electrical signal will be a voltage.
[0058] The actuator 12 communicates with a first end of the beam
20. An opposite end of the beam 20 is received by the sensor 18
which detects movement of the beam 20 and through its terminals 26a
and 26b and 26c+26d produces an electrical signal that may be
measured directly or further processed by processing electronics 28
to produce the output signal 30 indicating movement of the beam 20.
The sensor 18 may be a piezoelectric-type sensor, a photoelectric
sensor, a resistive sensor, an optical switching sensor, or a
capacitive sensor according to techniques known in the art of MEMS
design. In the preferred embodiment, the sensor 18 uses
counterpoised movable plate capacitors as will be described in more
detail below.
[0059] Attached to the beam 20 between the actuator 12 and the
sensor 18 is the control element 14 which provides both a force on
the beam 20 opposite the actuation direction 24 and tending to
resist the operation of the actuator 12 or with the actuation
direction 24 augmenting the operation of the actuator 12, as
indicated by double headed arrows 35.
[0060] Absent an analog electrical input signal 21, the control
element 14 may hold the beam 20 in a position toward the sensor 18.
Ideally, the control element 14 provides a force that increases
with motion of the beam 20 in the actuation direction 24. In this
way, a simple relationship between actuation force and movement of
the beam 20 is generated (e.g., with a simple spring-type system).
The MEMS analog isolator 10 provides extremely low friction and
inertia so this movement or force is consistent and rapid.
Alternatively, the control element 14 may provide a rapidly
increasing force (in a feedback system) arresting the movement of
the beam 20 for any actuation force. Here the magnitude of the
arresting force provides the output signal.
[0061] As described, the force provided by the control element 14
may be adjustable by varying a current or voltage to the structure
and used in a feedback mode to essentially eliminate all but a
small movement of the beam 20. Some movement of the beam 20 is
necessary for the sensor 18 to provide the necessary countervailing
feedback, but the movement may be reduced to an extent that
non-linearities in the actuators and mechanical elements of the
MEMS analog isolator 10, that might occur with more pronounced
movement, are eliminated. Specifically, in this mode, the movement
of the beam 20 is detected by processing electronics 28 to produce
a position signal. The position signal is compared against a
reference signal 29 to produce an error signal 31 which is directed
to the control element to produce a restoring force returning the
beam 20 to the null point. The connection between the error signal
to the control element 14 may be direct or may be further modified
by a feedback network 33 providing compensation for the system
according to well-known feedback techniques. The feedback network
33 may steer voltage to either terminals 38c and 38d with a return
at terminal 50 for actuation toward the sensor 18 or to terminals
38a and 38b with a return at terminal 50 for actuation toward the
actuator 12 reflecting the fact that the electrostatic motors
provide only a single direction of force.
[0062] The beam 20 includes conductive portions 32a and 32b,
located at the actuator 12 and sensor 18, respectively, and such as
may form part of the actuator 12 or sensor 18. Insulating portions
34a and 34b separate conductive portions 32a and 32b from a
centermost conductive portion 32c that may be part of the control
element 14; the insulating portions 34a and 34b thus defining three
regions of isolation 36a-c. The first region 36a includes the
actuator 12 and conductive portion 32a, the second region 36b
includes the center conductive portion 32c and the control element
14, and the third region 36c includes the conductive portion 32b
and sensor 18.
[0063] The insulated beam 20 provides a mechanism by which the
analog electrical input signal 21 acting through the actuator 12
may produce a corresponding output signal 30 at the sensor 18
electrically isolated from the analog electrical input signal 21.
The control element 14 may be electrically isolated from either the
input signal and/or the output signal 30.
[0064] The control element 14 is preferably a Lorenz-force motor or
an electrostatic motor of a type that will be described below. For
the former of these two control elements, terminals 38a and 38b and
return 50 are provided to provide a bi-directional current
dictating the countervailing force provided by the control element
14. The direction of the current dictates the direction of the
force. For the latter electrostatic structure, terminals 38a, 38b,
38c, and 38d are provided. Voltage is applied either to terminal
pair 38a and 38b (with reference to return 50) or to terminal pair
38c and 38d (with respect to return 50) to determine the direction
of the force.
[0065] Referring now to FIG. 2, the beam 20 may extend above a
substrate 42 along a longitudinal axis 40 passing along a midline
between transversely opposed pylons 44 attached to a substrate 42.
The pylons form the terminals 22a and 22b, 38a-38d, 26a, and 26b
described above. Ideally, the substrate 42 is an insulating
substrate and thus pylons 44 are all mutually isolated and
particular conductive layers are placed or wire bonding used to
make the necessary connections.
[0066] The beam 20 is supported away from the substrate 42 and held
for movement along the longitudinal axis 40 by means of flexing arm
pairs 46 extending transversely on opposite sides of both ends of
the beam 20 and its middle. The flexing arms 46 extend away from
the beam 20 to elbows 48 transversely removed from the beam 20 on
each side of the beam 20. The elbows 48 in turn connect to
expansion compensators 50, which return to be attached to the
substrate 42 at a point near the beam 20. As mentioned above, these
expansion compensators are not absolutely required. They serve as
stress relief if that is needed. The flexing transverse arms 46 are
generally parallel to the expansion compensators 50 to which they
are connected. The flexing transverse arms 46, elbows 48 and
expansion compensators are conductive to provide electrical
connections between the conductive portions 32a, 32b, and 32c and
stationary electrical terminals (not shown).
[0067] Referring now to FIG. 4, the length L.sub.1 of each
expansion compensator 50 between its point of attachment 52 to the
substrate 42 and its connection to a corresponding flexing
transverse arm 46 at elbow 48 and the length L.sub.2 of the flexing
transverse arm 46 defined as the distance between its connection to
beam 20 and the elbow 48 are set to be nearly equal so that
expansion caused by thermal effects in the flexing transverse arm
46 is nearly or completely canceled by expansion in the expansion
compensator 50. In this way, little tension or compression develops
in the flexing transverse arm 46. Both the flexing transverse arm
46 and the expansion compensator 50 in this embodiment are
fabricated of the same material, however it will be understood that
different materials may also be used and lengths L.sub.1 and
L.sub.2 adjusted to reflect the differences in thermal expansion
coefficients. Note that a doubling back of the arm is not required.
A straight connection will also work. The doubling back of the arm
is a stress-relieving feature. Stress in the beam will affect the
spring constant. Depending on the spring constant desired, and
other geometric and process (e.g. substrate choice) considerations,
stress relief may or may not be needed or desirable.
[0068] Referring to FIG. 5a, the elbow 48 may include a serpentine
portion 54 extending longitudinally from the expansion compensator
50 to its flexing transverse arm 46. As shown in FIG. 5b, the
serpentine portion 54 allow angulation .alpha. between the flexing
transverse arm 46 and expansion compensator 50 such as provides
essentially a radius adjusting pivot, both decreasing the force
exerted by the flexing transverse arm pairs 46 on the beam 20 with
movement of the beam 20 and decreasing the stiffness of the
structure.
[0069] Referring again to FIGS. 2 and 3, in between the flexing
transverse arm pairs 46 the beam 20 expands to create T-bars 56
flanking insulating portion 34a and 34b. Insulating material 58
attached to these T-bars 56 create the insulating portions 34.
Generally the beam 20 may be fabricated using well-known MEMS
processing techniques to produce a structure suspended above the
substrate 42 and composed of a laminated upper conductive layer 60
(for example polycrystalline silicon or crystalline silicon
optionally with an upper aluminum layer) and a lower insulating
layer 62 such as silicon dioxide or silicon nitride. The insulating
portions 34 may be obtained simply by etching away the upper layer
in the region 34a or 34b according to techniques well known in the
art using selective etching techniques. An improved method of
fabricating these structures is described in U.S. Pat. No.
6,159,385 issued Dec. 12, 2000 hereby incorporated by reference.
The edges and comers of the T-bars 56 may be rounded to increase
the breakdown voltage between them.
[0070] Each of the upper conductive layer 60 and lower insulating
layer 62 are perforated by vertically extending channels 64 such as
assists in conducting etchant beneath the layers 60 and 62 to
remove a sacrificial layer that normally attaches layers 60 and 62
to the substrate 42 below according to techniques well known in the
art.
[0071] Referring now to FIG. 2 again, portion 32a of the beam 20,
such as provides a portion of the actuator 12 may have transversely
outwardly extending, moving capacitor plates 66 overlapping with
corresponding transversely inwardly extending stationary capacitor
plates 68 attached to the pylons 44 representing terminals 22a and
22b. Each of the moving capacitor plates 66 and their corresponding
stationary capacitor plates 68 may have mutually engaging fingers
(as opposed to being simple parallel plate capacitors) so as to
provide for a more uniform electrostatic force over a greater range
of longitudinal travel of the beam 20. The thus formed
electrostatic motor operates using the attraction between the
capacitor plates 66 and 68 with the terminals 22b and 22a connected
to a more positive voltage than that of beam 20 (connected to
terminals 22c+22d), to urge the beam 20 in the actuation direction
24. For this reason, stationary capacitor plates 68 are after the
moving capacitor plates 66 on both sides of the beam 20 as one
travels along the actuation direction. Capacitor plates 66 and 68
are cantilevered over the substrate 42 by the same under etching
used to free the beam 20 from the substrate 42.
[0072] The pylons 44 flanking portion 32c of the beam such as form
pads 38a-38d likewise include moving and stationary capacitor
plates 66 and 68 in two distinct pairs. As noted, this section
provides the control element 14 and as such, two electrostatic
motors; one (using terminals 38c and 38d) created to produce a
force in the opposite direction of actuator 12 with the moving
capacitor plates 66 following the stationary capacitor plates 68 as
one moves in the actuation direction 24 and the other (using
terminals 38a and 38b) created to produce a force in the same
direction to the actuator 12 with the moving capacitor plates 66
preceding the stationary capacitor plates 68 as one moves in the
actuation direction 24. These two actuators are used in combination
to give the best possible control of the closed loop system.
[0073] Referring still to FIG. 2, portion 32b of the beam also
supports moving capacitor plates 66 and stationary capacitor plates
68. However in this case, the capacitor plates do not serve the
purpose of making an electrostatic motor but instead serve as a
sensing means in which variation in the capacitance between the
moving capacitor plates 66 and stationary capacitor plates 68
serves to indicate the position of the beam 20. In this regard, the
order of the stationary and moving capacitor plates 66 and 68 is
reversed on opposite sides of the beam 20. Thus, the moving
capacitor plates 66 precede the stationary capacitor plates 68 on a
first side of the beam (the upper side as depicted in FIG. 2) as
one moves in the actuation direction 24 (as measured between
terminal 26a and terminals 26c+26d) whereas the reverse order
occurs on the lower side of the beam 20 (as measured between
terminal 26b and terminals 26c+26d). Accordingly as the beam 20
moves in the actuation direction 24, the capacitance formed by the
upper moving capacitor plates 66 and stationary capacitor plates 68
increases while the capacitance formed by the lower plates
decreases. The point where the value of the upper capacitance
crosses the value of the lower capacitance precisely defines a null
point and is preferably set midway in the travel of the beam
20.
[0074] Techniques for comparing capacitance well known in the art
may be used to evaluate the position of the beam 20. One circuit
for providing extremely accurate measurements of these capacitances
is described in co-pending application Ser. No. 09/677,037 filed
Sep. 29, 2000, hereby incorporated by reference.
[0075] Generally, the operating structure of the MEMS analog
isolator 10 is constructed to be symmetric about an axis through
the middle of the beam 20 along the longitudinal axis 40 such as to
better compensate the thermal expansions. In addition, the
operating area of the plates of the capacitors, plates 66 and 68 on
both sides of the beam 20 for the actuator 12 and the control
element 14, are made equal so as to be balanced. For similar
reasons, the capacitors of the electrostatic motors and the control
element 14 are placed between flexing transverse arm pairs 46 so as
to better control slight amounts of torsion caused by uneven forces
between the capacitor plates 66 and 68.
[0076] Referring now to FIG. 6, it will be understood that one or
both of the electrostatic motors forming the actuator 12 and the
control element 14 described above, may be replaced with
Lorenz-force motors 75 in which forces are generated not by
electrostatic attraction between capacitor plates but by the
interaction of a current with a magnetic field. In the Lorenz-force
motor 75, a magnetic field (e.g. with a permanent magnet, not
shown) may be generated adjacent to the MEMS analog isolator 10 to
produce a substrate-normal magnetic flux 70. The expansion
compensators 50 supporting the flexing transverse arm 46 on
opposite sides of the beam 20 are electrically isolated from each
other so that a voltage may be developed across expansion
compensators 50 to cause a current 72 to flow through the flexing
transverse arm 46. This current flow in the magnetic field
generated by the magnet will produce a longitudinal force on the
beam 20 that may act in lieu of the electrostatic motors. The
amount of deflection is generally determined by the flux density of
the magnetic field 70, the amount of current and the flexibility of
the flexing transverse arm pairs 46 in accordance with the right
hand rule.
[0077] The Lorenz-force motors 75 are two quadrant, meaning they
will accept currents in either direction to produce a force with or
opposed to the actuation direction 24. Hence with Lorenz-force
motors 75 (or the bi-directional electrostatic motor of the control
element 14 described above), the MEMS analog isolator 10 may
operate with two polarities unlike an optical isolator.
[0078] Referring now to FIG. 7, the actuator 12 positioned on beam
portion 32a, may be teamed with a second sensor 74 for sensing
motion of the beam 20 and that sensor 74 may be used to provide
isolated feedback to a device producing the analog electrical input
signal 21 as to motion of the beam 20 such as may be used to ensure
greater reliability in the transmission of signals.
[0079] Alternatively or in addition, the sensor 18 may be teamed
with an actuator 76 having the same orientation of actuator 12 but
positioned in isolation portion 32b. When actuator 76 is teamed
with sensor 74, they together provide a bi-directional analog
isolator in which isolated signals may be sent from either end of
the beam 20 to the other end. It will be understood that another
variation of this embodiment may eliminate the control element and
instead the actuators 76 and 12 may be used during transmission by
the other actuator as the control element. Such a device may be
useful in some multi-loop analog system or for scaling
adjustment.
[0080] It will be understood with greater circuit complexity that
certain of the elements of the actuator 12, control element 14 and
sensor 18 may be combined into individual structures and hence,
these terms should be considered to cover the functional
equivalents of the functions of actuator control element 14 and
sensor 18 whether or not they are realized as individual structures
or not. Further the relative location of the control element 14,
the actuator 12 and the sensor 18 may be swapped and still provide
isolated signal transmission.
[0081] Referring now to FIG. 8, a digital word output 100 can be
obtained from the sensor 18 by making use of an error signal 31
resulting directly from a comparison of the capacitors of the
sensor 18 by capacitive comparison circuit 102 of a type well known
in the art. One such circuit for providing extremely accurate
measurements of these capacitances in described in co-pending
application Ser. No. 09/677,037 filed Sep. 29, 2000, hereby
incorporated by reference. As so configured, the error signal 31
(when connected to the control element 14) will tend to restore the
beam 20 to a null position dependent on the location where the
values of the capacitors of the sensor 18 change their relationship
of which is greater than the other. The output of the capacitive
comparison circuit 102 will generally be a duty cycle modulated
square wave 104 produced as the beam 20 wanders back and forth
across the null point under the influences of the actuation force
and the restoring force. The beam 20 provides an inertial averaging
of the error signal 31 so that its average force is proportional to
the actuation force. Counter 106 measures the percentage of time
that the error signal 31 is in the high state. In one embodiment,
the output of the capacitive comparison circuit 102 may be
logically ANDed with a high rate clock signal to cause the counter
106 to count up during the time the error signal 31 is high and not
otherwise. The counter may be reset periodically by a second time
interval signal 110. The value on the counter 106 just prior to the
resetting will be proportional to the duty cycle of the error
signal 31 and therefore to the actuation signal. The frequency of
the clock signal 108 and the period of the time interval signal 110
may be selected according to the desired resolution in the digital
word output 100 according to methods well known in the art.
[0082] Referring again to FIG. 2, MEMS fabrication allows that a
portion of the substrate 42 may also include integrated circuits 73
having a number of solid-state devices such as may implement, for
example, the capacitor sense circuitry described above. A number of
the MEMS analog isolators 10 may be placed on a single integrated
circuit with appropriate interconnects made for providing them with
the currents required. Generally, using the MEMS analog isolator 10
of the present invention, a single integrated circuit of arbitrary
complexity, such as an industrial controller, may include isolators
on the same substrate 42 manufactured concurrently with each other.
These MEMS analog isolators 10 may provide for either inputs to the
remaining integrated circuitry in the form of a digital word or,
through the use of an on-board digital to analog converter,
isolated analog outputs from the integrated circuit 73.
[0083] It is specifically intended that the present invention not
be limited to the embodiments and illustrations contained herein,
but that modified forms of those embodiments including portions of
the embodiments and combinations of elements of different
embodiments also be included as come within the scope of the
following claims.
* * * * *