MEMS sensing and control An aerospace perspective

Invited PaperMEMSsensing 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

ABSTRACTFuture advanced fixed- and rotary-wing aircraft, launch vehicles, and spacecraft will incorporate smart microsensors tomonitor flight integrity and provide flight control inputs. This paper provides an overview ofHoneywell's MEMStechnologies for aerospace applications of sensing and control. A unique second-generation polysilicon rsonant microbeamsensor design is described. It incorporates a micron-level vacuum-encapsulated microbeam to optically sense aerodynamicparameters and to optically excite the sensor pickoff Optically excited self-resonant microbeams form the basis for a newclass of versatile, high-performance, low-cost MEMS sensors that uniquely combine silicon microfabrication technologywith optoelectronic technology that can sense dynamic pressure, acceleration forces, acoustic emission (AE), and many otheraerospace parameters of interest. Honeywell's recent work in MEMS tuning fork gyros for inertial sensing and a MEMS free-piston engine are also described.

Keywords: MEMS, optical sensors, acoustic emission, condition-based maintenance (CBM), inertial gyro1. WHAT IS MEMS?MEMS (microelectromechanical systems) is a class of physically small systems that have both electrical and mechanicalcomponents. Originally, modified integrated circuit (computer chip) fabrication techniques and materials were used to createthese 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 tomodify their environment. Microsensors are useful because their small physical size (1 00 jim) allows them to be less invasiveand 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 theamount of work they perform on the environment is small and precise. Microsensors measure the environmental effects.

2. AEROSPACE SENSING APPLICATIONSHoneywell has been developing a family ofMEMS based on a polysilicon resonant transducer design that conversenvironmental changes to changes in a resonating micromechanical beam ofpolysilicon."2'3 The resonant frequency changecan be sensed electronically by resistors fabricated into the resonating beam. The polysilicon resonant design approach iscalled "resonant integrated micromachined sensor (RIMS)." Figure 1 illustrates the RIMS design. The RIMS microbeamdesign typically has elements 100 to 400 im long, 46 im wide, and 2 jim thick with characteristic resonance frequencies of100kHzto 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, whichcauses 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, andtemperature 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 detectwide-bandwidth (>500 kHz) acoustic phenomena, which is useful for structural integrity monitoring applications.

InSmart Structures and Materials 2000: Smart Electronics and MEMS,

22Vijay K. Varadan, Editor, Proceedings of SPIE Vol. 3990 (2000) • 0277-786X/00/$1 5.00ResonatingMicrobeam

Figure 1. RIMS DesignAppliedDeflectionForceStresses

Micro beam

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Figure 3. High-Q Polysilicon Microbeam Oscillator for Precision Digital Sensors23

C00073G IResultingAxial Force

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MicrobeamSenseResistor

CIC73-O2Figure 2. Effect of Stretching the Microbeam by Applied Stress