Jeffrey N. Schoess, David K. Arch, Wei Yang, Cleopatra Cabuz, Ben. Hocker, Burgess R. Johnson, Mark L. Wilson, "MEMS sensing and control: an aerospace ...
Invited Paper
MEMS sensing and control: An aerospace perspective Jeffrey N. Schoess, David Arch, Wei Yang, Cleopatra Cabuz, Ben Hocker, Burgess Johnson, and Mark Wilson Honeywell Technology Center, Minneapolis, MN ABSTRACT Future advanced fixed- and rotary-wing aircraft, launch vehicles, and spacecraft will incorporate smart microsensors to monitor flight integrity and provide flight control inputs. This paper provides an overview ofHoneywell's MEMS technologies for aerospace applications of sensing and control. A unique second-generation polysilicon rsonant microbeam sensor design is described. It incorporates a micron-level vacuum-encapsulated microbeam to optically sense aerodynamic parameters and to optically excite the sensor pickoff Optically excited self-resonant microbeams form the basis for a new class of versatile, high-performance, low-cost MEMS sensors that uniquely combine silicon microfabrication technology with optoelectronic technology that can sense dynamic pressure, acceleration forces, acoustic emission (AE), and many other aerospace parameters of interest. Honeywell's recent work in MEMS tuning fork gyros for inertial sensing and a MEMS freepiston engine are also described.
Keywords: MEMS, optical sensors, acoustic emission, condition-based maintenance (CBM), inertial gyro
1. WHAT IS MEMS? MEMS (microelectromechanical systems) is a class of physically small systems that have both electrical and mechanical components. Originally, modified integrated circuit (computer chip) fabrication techniques and materials were used to create these very small mechanical devices. Today many more fabrication techniques and materials are available. Sensors and actuators are the two main categories of MEMS. Sensors are typically noninvasive, whereas actuators tend to modify their environment. Microsensors are useful because their small physical size (1 00 jim) allows them to be less invasive and work in smaller areas. Key examples ofmicrosensors include devices that measure pressure, acceleration, strain, temperature, vibration, rotation, proximity, acoustic emission, and many others. Microactuators are useful because the amount of work they perform on the environment is small and precise. Microsensors measure the environmental effects.
2. AEROSPACE SENSING APPLICATIONS Honeywell has been developing a family ofMEMS based on a polysilicon resonant transducer design that convers environmental changes to changes in a resonating micromechanical beam ofpolysilicon."2'3 The resonant frequency change can be sensed electronically by resistors fabricated into the resonating beam. The polysilicon resonant design approach is called "resonant integrated micromachined sensor (RIMS)." Figure 1 illustrates the RIMS design. The RIMS microbeam design typically has elements 100 to 400 im long, 46 im wide, and 2 jim thick with characteristic resonance frequencies of 100 kHz to more than 1 MHz. As the sensor flexes, the induced strain is read out as a change in frequency of the microbeam. Figure 2 illustrates the effect of stretching the microbeam by applied stress (i.e., external forces of vibration or AE, which causes a measured shift in resonant frequency. Figure 3 highlights an optical micrograph ofa resonant microbeam structure. The typical mechanical Q factor ofthe RIMS exceeds 20,000, and values of 100,000 have been measured. Honeywell has successfully developed prototype versions of the RIMS that are capable of measuring pressure, vibration, and temperature for environmental control, engine condition monitoring, pump diagnostics, and process control applications. Figure 4 illustrates two packaged RIMS sensors.4 Honeywell has also demonstrated the feasibility of using RIMS to detect wide-bandwidth (>500 kHz) acoustic phenomena, which is useful for structural integrity monitoring applications.
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In Smart Structures and Materials 2000: Smart Electronics and MEMS, Vijay K. Varadan, Editor, Proceedings of SPIE Vol. 3990 (2000) • 0277-786X/00/$1 5.00
Applied Force
Resonating Microbeam
Deflection Stresses Micro beam
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Resulting Axial Force
t Figure 1. RIMS Design
Vacuum Cavity Enclosure
Drive Electrode Microbeam
Sense Resistor
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Figure 2. Effect of Stretching the Microbeam by Applied Stress
Figure 3. High-Q Polysilicon Microbeam Oscillator for Precision Digital Sensors
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Temperature
Pressure
Figure 4. Honeywell RIMS Sensors
2.1 MEMS tuning fork gyro Honeywell is currently developing a tuning fork version inertial gyro to place a functioning gyroscope on a chip. Today's platforms, which require sophisticated guidance and navigation applications, are extremely sensitive to size and weight. A gyro on a chip could greatly expand the tactical uses of the technology. The tuning fork gyro technology could he used in autos for braking and steering, as well as for next-generation airhags. Figure 5 illustrates a block diagram of the gyro design. A performance goal of 1 deg/hr is expected in the near future. The MEMS tuning fork (TFG) mechanism is shown in the optical micrograph of Figure 6. A detailed view of the I'FG is shown in Figure 6.
2.2 Free-piston MEMS engine Another exciting MEMS technology being developed at Honeywell is the free-piston MEMS cngine.' ftc engine is being developed to provide autonomous power generation capability for remote sensing and control applications. The goal of this development effort is a high-energy density of 10 W (2000 Wh!kg) from a 1 cm' engine package. The approach fiatures a free-piston design for knock combustion using opposing dual pistons and a compression ratio of 30-60. Figure 7 illustrates the design approach and engine cycling sequence. Input Rate
Output
Motion
Figure 5. Honeywell MEMS Tuning Fork Cyro
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Figure 6. MEMS TFG Mechanism (left); Detailed View of TFG (right)
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Figure 7. Free-Piston MEMS Engine
2.3 Optical polychromator Figure 8 illustrates a schematic view of an optical polychromator, an array of mirrors supported by flexures. This 1024element array is being developed to provide a tunable reference grating for use in a dark field cross-correlation spectroscopy measurement. The successful development of this MEMS tunable grating will provide the flexibility of being able to identify multiple chemicals based on their JR absorption spectra without carrying these reference chemicals or multiple reference gratings with the instrument. A schematic of operation is shown in Figure 9.
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Figure 8. Optical Polychromator Broadband Light In
Polychromatic Spectrum Out
Deflectable Micromirror Grating Elements
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=3 = =3 =3 = T
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=3 =3 =3 =3 =3 = =3 =3 =3 L=3 =3 =3 =3 =3 Si Wafer CC()3 03
Individually Addressable Array of Driver Electrodes
Figure 9. Principle of Operation for Polchromator
2.4 Electrostatically actuated microvalve array The focus of this effort is to develop a low-power (5 rns) actuator for controlling airflow to mainstage control valves or actuators. The design uses electrostatic valve actuation for a micromachined cantilevered valve plate. The rolling action of this valve plate allows a very large displacement to be achieved. Operation of valve prototypes with greater than 100 million cycles has been achieved. A fabricated device is shown in Figure 10.
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Figure 10. Silicon Microvalve for Smart Valve Actuators
3. SUMMARY Several MEMS-based devices currently being developed by Honeywell have been presented tbr aerospace applications. The MEMS devices are being developed for airflow, power generation, inertial sensing, engine condition monitoring, structural integrity assessment, and environmental control.
4. REFERENCES I.
J. Zook, J. N. Schoess. et al.. "Fiber-optic vibration sensor based on modulation of light-excited oscillators," IEEE Transducers'99, Senadi. Japan, 1999.
2.
J. Zook and J. N. Schoess, "Optically resonant microbeams," SPIE Photonics West, San [)iego. February 6, 1995.
3.
J.
N. Schoess and J. Zook, "Smart MEMS for smart structures," 1995 SPIE Smart Structures and Materials Symposium. Paper no. 2448-12, 1995.
4. M. Wilson et a!., "An optical network of silicon rmcromachine sensors, SPIE, 1998. 5.
David Arch, Honeywell personal correspondence. February 16, 2000.
6. Wei Yang. Honeywell personal correspondence, February 15, 2000.
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