Sachin Nadig

Planar laser-micro machined bulk PZT bimorph For in-plane actuation

We demonstrate a PZT bimorph technology for integrated chip-scale in-plane nano-motion actuators. Bulk PZT actuators are micro-machined to define beams by laser-cutting through a PZT-4 plate while also defining arbitrary 2D electrode patterns on top and bottom surface of the beams by using a commercial laser cutting tool with backside alignment capability. Lateral actuators that are 5, 10 and 20 mm long, 0.45 mm wide, and 0.5 mm thick were characterized with single and dual side laser patterned electrodes. The in-plane bimorph displacements are in the range of 0.02 to 0.64 microns/volt. The out of plane displacement sensitivity of these actuators were simulated to be ~ 100s ppm/V compared to the desired in plane actuation. We also report doubling of the actuator tip displacement for the same applied voltages in the dual side electrode actuators, when compared to the single sided actuators.

DOME-DISC: Diffractive optics metrology enabled dithering inertial sensor calibration

We demonstrate 107-ppm accurate scale-factor and bias calibration of a commercial Coriolis force gyroscope, in which the typical un-calibrated scale factor variations are ~100,000-ppm. In this paper, we present a proof-of-concept result on calibration architecture - Diffractive Optics Metrology Enabled Dithering Inertial Sensor Calibration (DOME-DISC). DOME-DISC consists of a piezoelectric dither stage to provide built-in mechanical stimulus to the gyroscope attached to it. The motion of the dithering stage is measured by imaging an optical diffraction pattern created by an incident laser off diffraction gratings on the dither stage. In order to calibrate the gyroscope, the stage motion needs to be measured accurately and precisely. The motion of the stage is measured using the Nano Optical Ruler Imaging System (NORIS), with absolute accuracy of ~30nm over several millimeters, stable over several hours. NORIS provides parts-per-million stage motion measurement accuracy. In this paper, an angular dither motion of 0.1 to 0.5 degrees was optically measured with ~ 0.1 millidegree resolution. By measuring the scale factor and bias for a gyroscope on the dither stage mounted directly on a commercial rate table, and matching the gyroscope input-output curve to 100ppm, we demonstrate the capability to measure in-package gyroscope characteristics within the error limits of the commercial rate table.

Monolithic piezoelectric in-plane motion stage with low cross-axis-coupling

We present a rotary dither stage that can provide rotation stimulus to objects mounted on it. The stage is in planar form-factor allowing compatibility with planar packages that can house both the stage and the inertial sensor. We used laser micromachining of bulk PZT-4 plates to form PZT beams to achieve monolithic integration of lateral actuators and flexures. This process enables high-aspect ratio PZT beams (500μm thick, 150μm wide) resulting in high out-of-plane stiffness, helping in reducing the out-of-plane motion to parts-per-thousand of the inplane motion. The micro stage technology opens up a new design space of 100-micron scale lateral bimorphs for mm-scale stages with large motion. A dither stage was designed and fabricated that can achieve in plane dither of ~ 1.2 millidegree/V and dither rates up to 1800 degree/s. Low dither rates of 20-60 millidegree/s are demonstrated and measured.

Multi-modal mechanical stimuli stage for in-situ calibration of MEMS gyroscopes

We report a multi-axis piezoelectric multi-modal mechanical stimuli stage capable of in-situ calibration of MEMS gyroscopes attached to the stage. The system is fully self-contained with electronics for portable operation. In this work, we demonstrate active calibration of a commercial Z-axis gyroscope ADXRS646 for high bandwidth applications. The dithered stage is capable of pure sinusoidal angular rates at 0-300 deg/s even after being loaded with a gyroscope die, to extract the scale factor and the bias. The stage has the added capability of X-Y in-plane acceleration stimulus 0-90 m/s2 to extract the cross-axis sensitivities which are essential parameters. The measured maximum in-plane cross-axis sensitivity for unpackaged gyro ~ 13.64% (Sx/Sy x 100) operated at ~800Hz bandwidth, providing high bandwidth calibration data useful for personal navigation. The maximum power consumption of the unloaded calibration stage is ~396mWatts during calibration.

In-run scale factor and drift calibration of MEMS gyroscopes with rejection of acceleration sensitivities

We report a ~50PPM precision in-run calibration platform for MEMS gyroscopes. In this work we demonstrate an integrated system capable of extracting instantaneous scale factor, drifts and cross axis sensitivities of the commercial ADXRS646 Z-axis gyroscope, with complete in-run rejection of acceleration sensitivities. The demonstrated method is applicable to any MEMS Coriolis force gyroscopes. The 50ppm calibration result is limited by the inherent limit of the gyroscope, with the calibration system being potentially able to extract even higher levels of accuracy.

Monolithic 2-axis in-plane PZT lateral bimorph energy harvester with differential output

We report a 2-axis (X-Y) piezoelectric energy harvester, whose sensitive axis in-plane is rotationally invariant, a result achieved by spiral in-place bimorph, which can be modeled as a cascade of lateral bimorphs. This is different than conventional piezoelectric energy harvesters that are sensitive only along one axis or can realize multi-axis sensitivity through package level assembly of multiple devices at different orientations. Rotational invariance of the sensitive axis is useful when deployed in applications where the device orientation with respect to the vibration is stochastic in nature. At 1-g applied acceleration, our device generates maximum differential voltages of ~ ±6.8Vpp across a 1MΩ load, at a resonance frequency of ~163.5Hz. The harvester has a Q-factor of 129.5 and electromechanical coupling coefficient of kt2 ~3% for the fundamental lateral mode and a peak normalized output power density of ~0.89μWatt/mm3g-2.

3-axis MEMS gyroscope calibration stage: Magnetic actuation enabled out-of-plane dither for piezoelectric in-plane calibration

This paper reports a mechanical stage with 3-axis angular dither capability used to calibrate 3-axis gyroscope chip. A combination of piezoelectric and magnetic actuation provides in-plane and out-of-plane motion. The 3-axis dither capability is crucial for fully calibrating the scale factor and bias of a commercial 3-axis MEMS gyroscope. The measured magnetically actuated dither stage scale factor for out-of-plane mode is 0.250 (°/s)/mA as a function of current, or 5.57 × 10−2 (°/s)/Hz as a function of frequency. The power consumption of the magnetic stage is 33.5 mW/mdeg. The maximum dither angle obtained is 8.95 mdeg at 300 mW. The impact of the high power can be minimal on periodic low duty-cycle calibration.

Piezoelectric micro dither stage calibration of 6-axis IMU

We demonstrate simultaneous extraction of the scale factor and cross axis sensitivity of a commercial 6-DOF Inertial Measurement Unit (IMU), which has three gyroscopes and three accelerometers, using a multi-axis, mm-scale piezoelectric dither stage. The stage is 25.4×25.4×0.5 mm, with a platform disk of diameter 7.5 mm, onto which the IMU chip is adhesively attached. The stage has small inertia allowing for high bandwidth on-the-fly calibration and tracking of scale factor drift using rotation dither rates and accelerations as high as 100 deg/sec and 90 m/s2, respectively. We measure the gyroscope cross-axis sensitivities of 2.4%, which is within the IMU specifications. Using the cross-axis sensitivity and the scale factors, we demonstrate a pathway to improving commercial IMUs, reaching performance needed for personal navigation.

Vibration Powered RF-Transponder for Sensing Low Frequency Motion Events

Vibration energy harvesting offers a pathway to developing battery-less sensing solutions to be deployed in wireless sensor network nodes. The integration of the energy harvesters require regulation by power conditioning and control circuitry that consume some of the energy generated, reducing the effective energy available for node function. By designing a unique 3D-printed plastic structure for low frequency sensitivity and mechanical switching, and a lateral PZT bimorph for capturing energy from environmental vibrations, we report a zero-power consumption RF-transponder capable of detecting and reporting motion events without a battery. We have successfully picked up wireless transmissions on an external receiver placed ~25cm away from the transponder, shaken at 0.75 g and 20 Hz. We have additionally demonstrated the ability to harvest energy from 5 Hz vibrations up to just under 150 Hz. When placed on an oil-based electric generator, which vibrates when operating, the RF-transponder has successfully picked up the differing harmonics to identify the mode of operation as the economy or regular power setting.

Self-calibration compatible Z-axis bulk PZT vibratory gyroscope

We report a laser micromachined piezoelectric bulk-PZT (Lead Zirconate Titanate) Z-axis Coriolis force gyroscope, which utilizes spring-mass resonances of the in-plane PZT bimorphs and a PZT proof-mass. The gyroscope is compatible to be monolithically integrated with a PZT dither stage for in-situ self-calibration. The design exploits the large piezoelectric coefficients and high mass density of bulk-PZT to obtain high sensitivity, even at low resonance quality factors, compared to other piezoelectric gyroscopes. The gyroscope provides high dynamic range owing to elimination of micro-gaps needed in capacitive transduction. The gyroscope has unamplified sensitivity of ~1.3μV/°/sec under mode-mismatched operation, at drive amplitude of 10Vp-p, drive resonance of 108.9 kHz, with electromechanical coupling factor kt of ~0.15 and Q of ~100.