Electricity: circuit makers and breakers – Electrostrictive or electrostatic
Reexamination Certificate
2001-02-20
2003-06-24
Scott, J. R. (Department: 2832)
Electricity: circuit makers and breakers
Electrostrictive or electrostatic
C073S514190, C073S514160, C073S514260, C073S514320, C073S514330, C073S514340, C310S306000, C310S309000, C438S050000, C257S415000
Reexamination Certificate
active
06583374
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to electrical isolators and in particular to a microelectromechanical system (MEMS) device providing electrical isolation in the transmission of digital signals.
BACKGROUND OF THE INVENTION
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.
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.
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.
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.
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.
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.
Particularly in the area of industrial controls where many isolated control points are required, the use of optical isolators may be very costly or impractical.
BRIEF SUMMARY OF THE INVENTION
The present invention provides a mechanical isolator manufactured using MEMS techniques and suitable for transmitting 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 input signal to move the beam against a biasing force provided by a biased device. The other isolated end of the beam is attached to a sensor detecting movement of the beam only when the actuator force exceeds the countervailing force of the biased device. The small scale of the total device provides inexpensive, fast and reliable response.
Specifically, the present invention provides a microelectromechanical system digital isolator having a substrate and an element supported by the substrate for movement between the first and second position with respect to the substrate. 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 is 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 bias structure is attached to the element to exert a predetermined opposite force on the element urging the element toward the first position. Finally, a sensor is attached to the second portion of the element to provide an output electrical signal indicating movement of the element between the first position and the second position whereby an input signal above a predetermined magnitude overcomes the opposite force to cause the element to move rapidly from the first to the second position to produce the output electrical signal electrically isolated from the input electrical signal.
It is one object of the invention to produce a simple mechanical isolation system using MEMS techniques suitable for a wide variety of binary signals and yet which overcomes 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 voltage or current is applied directly to the device with no pre-processing.
The actuator may be an electrostatic motor or a Lorenz force motor or a piezoelectric motor or thermal-expansion motor or a mechanical displacement motor.
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.
Similarly, the bias structure may be an electrostatic motor, a Lorenz force motor, a piezoelectric motor, a thermal-expansion motor, a mechanical displacement motor, or a mechanical spring.
Thus the invention may provide both for an extremely simple force biasing that requires no electrical connection (e.g. a mechanical spring) or an adjustable bias structure that allow the threshold of activation of the device to be freely tailored to different circumstances. In this way, unlike with optical isolators, an input threshold voltage may be tailored to the particular application.
The sensor may be a capacitive sensor or a piezoelectric sensor or a photoelectric sensor or a resistive sensor or an optical switching sensor.
It is therefore another object of the invention to provide flexible variety of sensing techniques suitable for different purposes.
The travel of the element may be limited by stops to between the first and second position.
In this way, the invention may provide signal limiting comparable to that provided by an optical isolator for signals beyond the threshold needed to trigger the device.
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.
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 supportin
Dummermuth Ernst H.
Harris Richard D.
Herbert Patrick C.
Knieser Michael J.
Kretschmann Robert J.
Gerasimow Alexander M.
Quarles & Brady
Rockwell Automation Technologies Inc.
Scott J. R.
Walbrun William R.
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