Research

Robert White, Mech. Eng.







Liquid Metal Interconnects

MEMS Surface Shear Sensors

MEMS Ultrasound Navigation

Aerosol Jet Printed Interconnect

Structural Vibrations for Robotic Communication

MEMS Microphone Arrays for Aeroacoustics

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Research Videos:

MEMS microphone array-on-a-chip for measuring small scale turbulent pressure fluctuations in wind tunnel testing. January 2013.

MEMS floating element shear stress sensor for direct dynamic skin friction measurement at high spatial resolution in wind tunnel testing. January 2013.

MEMS ultrasonic velocity sensing technology for mobile navigation systems using high frequency in-air ultrasonics. January 2013.


Updated July 26, 2016

Liquid Metal Interconnects for Conformable Sensor Packaging Enabling Inertial Measurements of Animals and Soft Robots

 

Nikolas Kastor1 and Robert D. White2

 

1 Tufts Mechanical Engineering Graduate Student     2 Tufts Mechanical Engineering Faculty

 

 

In biomechanics, inertial measurements units (IMUs) are used to map the dynamic modes and gates of locomotion of animals.  Typically, thin wires are soldered to the IMU and the package is bonded to the location of interest, on the animal, using cyanoacrylate or epoxy.  These types of adhesives and the solder of the interconnects are brittle and typically fail from cyclic loading of the animal flexing its body.  The same situation can be found in soft robotics, where a compliant and durable way of connecting electrical components within the body of the robot is required to maintain its “soft” characteristics.  To solve this problem, we propose a self-contained package, which encapsulates an IMU, made from a flexible elastomer with room temperature eutectic metal interconnect “wiring.”  Because of the compliant nature of the materials used, the electronics package can then be bonded to a flexible surface with van der Waals forces.  Using eutectic metal allows for compliant interconnects that will not break or change their resistivity under large strains. The electrical connections between the solder pads of a 3x3x1mm IMU are bridged to the required capacitors in 100x50µm microfluidic channels.  88µm diameter wires that exit the package to measurement electronics are attached by submersion of their stripped conductor in 300µm diameter wells.  A positive pattern for molding the microfluidic system was manufactured by standard SU-8 photolithography on a Si chip where the IMU, capacitors  and wires were placed on specific features of the micro-channels and encapsulated and filled with liquid metal.

 

Fig: A soft PDMS encapsulated sensor system with embedded inertial measurement system IC, discrete components, and microfluidic channels for liquid-metal interconnects.  This packaging method enables all-soft-material packaging of sensors and electronics for soft robotic systems, internet-of-things applications, and biometric measurements on small, soft animals.

Relevant Publications:

Nikolas Kastor and Robert D. White, "Liquid Metal Interconnects for Conformable Sensor Packaging Enabling Inertial Measurements of Animals and Soft Robots" in the 43rd iMAPS New England Symposium and Expo, Boxborough, MA, May 3, 2016.

 


MEMS Surface Shear Sensors for Aerodynamic Applications

 

Zhengxin Zhao2, Nikolas Kastor2 , Daniela Torres1, and Robert D. White3

 

1 Tufts Mechanical Engineering Undergraduate      2 Tufts Mechanical Engineering Graduate Student     3 Tufts Mechanical Engineering Faculty

 

This project aims to develop "direct" surface shear stress measurement sensors that can provide real time measurement of the time resolved local shear stress at the surface of a wind tunnel model or vehicle during ground or flight testing. The goal is to provide spatial resolution of 1 mm, bandwidth of 100 kHz, and to cover the shear stress range of 0.1 Pa to 1000 Pa. Our approach is to micromachine “floating element” style MEMS shear stress sensors in arrays, and to integrate these with high performance capacitance to digital converters in a small form factor package. There are challenges associated with low topology packaging of the sensors, and sensitivity to other variables such as fluctuating pressures, pressure gradients, and environmental variables such as temperature and humidity. Collaborators have included industry and government partners.

The current design employs 256 “floating elements” on a 1 cm x 1 cm chip. The sensor has been able to measure shear stresses from approximately 0.5 Pa to approximately 15 Pa in laminar flow cell and turbulent boundary layer environments. Current bandwidth is limited to about 10 Hz. Sensitivity to pressure gradient has been characterized. Additional characterization of environmental sensitivities to temperature, humidity, vibration, and fluctuating pressures are needed. Additional developments to improve resolution and bandwidth, as well as reduce surface topology, are under way.

 

We have applied the array to the measurement of boundary layer shear in a flat plate subsonic wind tunnel study. We are interested in extending our applications areas to additional wind tunnel studies with structured models, flight testing, and turbomachinery applications.

 

Fig: The most recent design of the Tufts shear sensor includes 4 separate MEMS sensor groups for measuring surface shear stress in both directions, as well as pressure gradient in both directions.  Overall dimension of the system in 0.75”.  The chip is 1 cm2.


Representative publications:

Zhao, Z., Shin, M., Gallman, J. M., and White, R. D. "A Microfabricated Shear Sensor Array on a Chip with Pressure Gradient Calibration", Sensors and Actuators A: Physical, Volume 205, 2014, Pages 133-142, ISSN 0924-4247. LINK PDF of the paper.

 

Zhao, Z., Long, K., Gallman, J., and White, R.D. "Flow Testing of a MEMS Floating Element Shear Stress Sensor", AIAA2014-1235, in the Proceedings of the 52nd AIAA Aerospace Sciences Meeting, National Harbor, MD, January 13-17, 2014. PDF of the paper.

 

Zhao, Z. "MEMS Floating Element Sensor Array for Wall Shear Stress Measurement under a Turbulent Boundary Layer", Ph.D. Thesis, Tufts University, January, 2014. PDF of the thesis (6.5 MB).

 

Liu, S. "Cochlear Properties and Micromachined Hair-like Shear Sensors”, Ph.D. Thesis, Tufts University, February 2012. PDF of the thesis (12 MB).



MEMS Ultrasound Arrays for Navigation Applications

 

Minchul Shin2, Kevin Ligonde1, and Robert D. White3

 

1 Tufts Mechanical Engineering Undergraduate      2 Tufts Mechanical Engineering Graduate Student     3 Tufts Mechanical Engineering Faculty

 

This project aims to develop and characterize MEMS ultrasound transmit and receive array-on-a-chip devices for navigation applications. The goal is to demonstrate the ability to measure 3D velocity and distance to an acoustic reflector using frequency modulated continuous wave Doppler ultrasound. The project includes design, microfabrication, and testing.

 

The current system has been successful at measuring 1D velocity of a moving reflector with 60 velocity updates per second, at a resolution of approximately 0.5 cm/s and a maximum measurable range to the reflector of 1.5 meters. Work is ongoing in increase resolution, increase range, extend the technology to measur 3D velocities and distance veotors, and reduce package size and power for portable applications. We are interested in collaborating with potential end users who would find a low power, low weight distance and velocity measurement system useful. We can imagine applications in mobile robotics and personal navigation systems and are interested in other application areas such as acoustic anemometry.

 

http://emerald.tufts.edu/~rwhite07/images/nickel_cmut.png

Fig: Images of the packaged array chip (left), and a microscope image of a few sensor elements (right).


Representative publications:

Minchul Shin, Zhengxin Zhao, Paul DeBitetto, Robert D. White, "Micromachined ultrasonic Doppler velocity sensor using nickel on glass transducers", Sensors and Actuators A: Physical, Volume 208, 1 February 2014, Pages 37-49, ISSN 0924-4247. LINK. PDF of the paper.

 

Shin, M., Krause, J. S., DeBitetto, P., and White, R. D. "Acoustic Doppler Velocity Measurement System using Capacitive Micromachined Ultrasound Transducer Array Technology", Journal of the Acoustical Society of America, 134 (2) pp. 1011-1020, 2013. PDF of the paper. (c) 2013 Acoustical Society of America. LINK

 

Shin, M. "MEMS Based Doppler Velocity Measurement System", Ph.D. Thesis, Tufts University, August 2012. PDF of the thesis (6.5 MB).

 


 


Printed Transceiver Circuit for System-on-chip Sensor and Processor

 

Peter Lewis1, Brian Smith2 and Robert D. White3

 

1 Tufts Mechanical Engineering Graduate Student     2 Technical Staff Member, Draper Labs     3 Tufts Mechanical Engineering Faculty

 

 

This research employs aerosol jet printing to rapidly manufacture a multilayer PCB with COTs component integration. Specifically, this research provides a method to making a system-on-a-chip (SOC) circuit on a variety of substrates with novel integration methods. This fits well into the recent research phenomenon deemed “the internet of things.” By advancing the manufacturing capabilities for a system that can measure and transmit data, one can integrate such circuits with minimal spatial and geometric interference to the broader device. A unique manufacturing process was developed for non-embedded components which involves the building up of the circuit around the system’s microprocessor. The transceiver circuit and its microprocessor, based off a commercially available circuit, has been successfully programmed and has shown to be working. The matching network is still being worked on, however, the goal is to have it working by the conference. To the best of the researchers’ knowledge, this research is novel in that it is the most complicated multilayer circuit that has been built using aerosol jet printing technology. Rapid ageing tests for a silver and a CNT ink were done and analyzed to determine the reliability of an aerosol jet printed circuit board. Results show adhesion is the primary mechanism of device failure as opposed to conductor degradation.

 

Fig: A microcontroller and RF transceiver circuit created entirely using aerosol jet printing of both dielectric and interconnect layers.  This methodology of integrated system manufacturing allows rapid iteration on design and printing over topology, on curved surfaces, and on soft substrates for compliant soft-material electronics applications and the internet-of-things.

Relevant Publications:

Peter Lewis, Brian Smith and Robert D. White, "Printed Transceiver Circuit for System-on-chip Sensor and Processor" in the 43rd iMAPS New England Symposium and Expo, Boxborough, MA, May 3, 2016.

Peter Lewis, Parshant Kumar, Robert D. White, and Brian R. Smith, "Deposition Characteristics and Electrical Properties of Silver and CNT Inks Deposited by Aerosol Jet", at the International Microelectronics and Packaging Society (iMAPS) New England Chapter 42nd Symposium and Expo, Boxborough, Massachusetts, May 5, 2015.PDF of the poster.

 

 

 


Structural Vibration for Robotic Communication and Sensing on One-Dimensional Structures

 

Maxwell Hill1, Jerry Mekdara2, Barry Trimmer3 and Robert White4

 

1 Tufts Mechanical Engineering Graduate Student  2 Tufts Biology Graduate Student   3 Tufts Biology Faculty   4 Tufts Mechanical Engineering Faculty

 

 

Presented at the 2015 IEEE/RSJ International Conference on Intelligent Robots and Systems (IROS 2015), Hamburg, Germany, September 28 - October 3, 2015.

 

Structure-borne vibrations in a one dimensional structure were examined as a means of communication and sensing for networks of robots. The concept was inspired by the observation that insects use structural vibrations to communicate and to detect features of their environment. A 12 x 5 x 6 cm mobile robot capable of traversing an acoustically favorable structure was developed. A technique for measuring distance between robots and communicating commands to robots using vibrations generated on a common one dimensional substrate were demonstrated.

 

 

Fig: The autonomous robot pictured was developed to navigate a rail system (one-dimensional environment).  The robot can communicate with an external computer and sense distance by sending structural vibrations along a “third rail”. (a) Robot navigating the track (b) a close-up of the three-rail system (c) a cross-section of the rail system with labels.

Relevant Publications:

Maxwell Hill, Prasong Mekdara, Barry Trimmer, Robert David White, "Structural Vibration for Robotic Communication and Sensing on One-Dimensional Structures" in the 2015 IEEE/RSJ International Conference on Intelligent Robots and Systems (IROS 2015), Hamburg, Germany, September 28 - October 3, 2015.PDF of the paper.

Hill, L., Mekdara, P., Trimmer, B., and White, R. D. "Structural Vibration for Robotic Communication and Sensing on One-Dimensional Structures" in the 7th International Symposium on Adaptive Motion of Animals and Machines (AMAM 2015), Cambridge, MA, June21-25, 2015. PDF of the paper.

 


 


Wind Tunnel Testing of a MEMS Microphone Array-on-a-Chip

 

Robert D White1, Joshua Krause2

1 Tufts Mechanical Engineering Faculty    2 Tufts Mechanical Engineering Graduate Student  

 

A microelectromechanical systems (MEMS) based microphone array on a chip has been developed and applied to aeroacoustic measurements.  The array is designed to measure the fluctuating pressures present under a turbulent boundary layer (TBL).  Each chip measures 1 cm2 and contains 64 individually addressable capacitively sensed microphones, with a center to center pitch of approximately 1.25 mm.  Surface topology, including the packaging, is kept to less than 0.13 mm.  Element to element sensitivity variation in the array is less than ±2.5 dB from least to most sensitive, and phase variation is less than ±6.5 degrees (at 1 kHz).  The microphone 3dB bandwidth is 300 Hz to 100 kHz, and the microphones are linear to better than 0.3% at sound pressure levels up to 150 dB SPL.  A unique switched architecture system electronics and packaging method are employed to reduce data acquisition channel count requirements, and to maintain a low surface roughness. 

 

The array was recently applied to the measurement of multi point turbulence spectra under a flat plate TBL in a low speed, low turbulence intensity wind tunnel at the University of Toronto Institute for Aerospace Studies (UTIAS).  Testing was at a maximum speed of 30 m/s, and maximum Reynolds numbers based on plate length of approximately 106. Evidence of turbulent structures was gathered by examining coherence data between rows of array elements spaced apart in the flow direction.  Coherence decreased with distance in the 300 Hz to 2 kHz band, indicating an ability to separate turbulent structures from acoustic pressure fluctuations in this band, even at the low speeds achievable in the UTIAS tunnel.

 

            http://emerald.tufts.edu/~rwhite07/images/Snap11_Scale.JPG

Fig: (Left) Prof. White in the UTIAS tunnel.  The MEMS microphone array is visible in the floor of the tunnel.  The array includes 64 microphones in a square grid over a 1 cm x 1 cm aperture.  Testing was carried out with partners from Bombardier Aerospace and the University of Toronto. (Right) Microscope picture of one of the 0.6 mm diameter elements in the MEMS array.

 

Representative publications:

Krause, J., Gallman, J., Moeller, M., and White, R. D., "A Microphone Array on a Chip for High Spatial Resolution Measurements of Turbulence", Journal of Microelectromechanical Systems, vol 23, n 5, pp 1164-1181, 2014.  LINK. PDF of the paper.

 

White, R.D., Krause, J., De Jong, R., Holup, G., Gallman, J., and Moeller, M. "MEMS Microphone Array on a Chip for Turbulent Boundary Layer Measurements" at the AIAA Aerospace Sciences Meeting, ASM 2012, Nashville, TN, Jan 8-12, 2012. PDF of the paper.

 

Krause, J. S. "Micromachined Microphone Array on a Chip for Turbulent Boundary Layer Measurements", Ph.D. Thesis, Tufts University, August 2011. PDF of the thesis (39 MB).

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