Ernesto Lasalandra

Control of a Z-axis MEMS vibrational gyroscope

The paper describes the design of the control loops in a Z-axis, MEMS vibrational gyroscope. In this device, a silicon mass is driven through electrostatic actuator so that it has a sinusoidal linear motion, with a controlled speed. The design of a suitable controller, capable of maintaining the required speed and with prescribed restoring capabilities after shocks has been derived and described in the paper. Attached to the driving mass, a second mass, free to move in the direction orthogonal to the motion of the first mass, is subjected to a Coriolis force, proportional to the product of the first mass speed by Z-axis rotational speed. The sensing of the Coriolis force and, in turn, of the Z-axis rotational speed, is performed in closed loop fashion, with a 1-bit quantized actuation. The restoring force that brings the motion of the second mass to zero is equivalent to the output bit stream of a sigma-delta converter and contains the information of the Coriolis force. The design of this second control loop and a detailed analysis on the signal-to-noise ratio achievable with the proposed design is reported.

Interface for MEMS-based rotational accelerometer for HDD applications with 2.5 rad/s/sup 2/ resolution and digital output

A 0.6 /spl mu/m BiCMOS interface for microelectromechanical systems (MEMS) based rotational accelerometers is presented. It is housed in an inexpensive standard SO-24 plastic package with a capacitive rotational accelerometer sensor produced using MEMS technology. This sensitive interface chip includes the analog-to-digital conversion, filtering, and interface functions. The analog-to-digital conversion is realized through a single-bit electromechanical /spl Sigma/-/spl Delta/ loop able to detect capacitive unbalancing as low as 50 aF (50/spl times/10/sup -18/ F). The produced bitstream is then processed by a digital chain and made available through a standard 3.3 V (5 V tolerant) three-wire serial bus. The signal bandwidth is about 800 Hz, the sensitivity is 2.5 rad/s/sup 2/, with a full-scale sinewave of 200 rad/s/sup 2/ and a signal-to-noise ratio peak of 38 dB over 30-800 Hz. Through the serial bus, it is also possible to program device characteristics including gain, offset, filter performance, and phase delay. The complete sensor is used in a feed-forward compensation scheme to cancel external disturbances acting on computer hard-disk drives so as to steadily keep the read-write heads on track: this allows greater track densities and better speed performance.

MEMS Motion Sensors Based on the Variations of the Fringe Capacitances

In this paper, a MEMS motion sensor is described which uses fringe field capacitances as sensing elements. Each capacitance basic cell is formed using thin strips of standard electrical paths, running side by side, and close one another in a plane parallel to the substrate: in this way, the capacitance between the paths is mainly formed by the fringe field streamlines. A grounded seismic mass, suspended in the air volume where the electric fringe field streamlines are concentrated, determines changes in the capacitance value when subject to an external force that makes it move. With respect to parallel plates capacitive sensors, this implementation has the advantage to be more immune from the pull-in instability. A test device is built using an industrial surface micromachining process. The experimental results show a sensitivity close to 1 fF of capacitance variation per unit of earth acceleration for a device taking up an area of (430 μm)2. A detailed analysis performed through FEM simulations predicts possible consistent sensitivity improvements.

MEMS-based Accelerometers and their Application to Vibration Suppression in Hard Disk Drives

Hard disk drives (HDDs) are currently the most inexpensive devices for storing large amounts of data. Since their introduction in the late 1950’s [1], they have been improved in their reliability, data capacity and throughput. Moreover, their cost per byte has consistently decreased such that they have been able to maintain their position as the most economical solution for storing large amount of data. For those readers not familiar with HDDs, a brief description of their operation is provided. Figure 1 shows the interior of a commercial HDD, with a stack of magnetic disks (platters) containing the user’s data and a rotating arm supporting the magnetic heads. The rotating arm is moved by the torque generated by a voice coil motor (VCM) mounted at one end of the arm.

An Efficient Earth Magnetic Field MEMS Sensor: Modeling, Experimental Results, and Optimization

A new z-axis Lorentz force microelectromechanical systems magnetometer was designed, fabricated, and tested. The proposed device is characterized by simple design, reduced dimensions, and high efficiency. Furthermore, possible parasitic acceleration sensitivity is mechanically canceled in the proposed device. The initial design was subsequently studied through an ad hoc formulated multiphysics model used to compute the sensor dynamics; optimality of the design configuration was then obtained by means of a structural optimization approach. A wide scenario of design configurations, obtained with the proposed optimization approach, is finally discussed.

Geometry optimization of a Lorentz force, resonating MEMS magnetometer

Abstract In this paper, we propose a multiphysics-based optimization of the geometry of a microbeam, which represents the sensing part of a Lorentz force, resonating, MEMS magnetometer. The optimization problem is framed by accounting for a (weak) electro-thermo-mechanical coupling, which induces a state of residual stresses in the microbeam, clamped at both its ends. Nonlinearities caused by the aforementioned residual stresses and by the electrostatic sensing are taken into account up to second order. In the optimization procedure, two objective functions are handled, as linked to the sensitivity (to be maximized) and to the power consumption (to be minimized) of the system; a constraint is also accounted for to ensure the sensor to work within a required frequency regime. By considering the microbeam width and length as design parameters, we show that a set of possible optimal designs can be obtained by tuning the weighting factors mixing the two aforementioned objective functions.

A Pierce oscillator for MEMS resonant accelerometer with a novel low-power amplitude limiting technique

A low-power very compact Pierce oscillator for resonant accelerometers is presented in this paper. The resonator of a surface micro-machined uniaxial resonant accelerometer is embedded in the oscillating loop as a frequency selective filter. External accelerations cause a shift of the resonator resonance frequency resulting in the frequency modulation of the oscillator output signal. The proposed circuit exploits two sub-threshold operated FETs as the amplitude limiting control (ALC) network, capable of driving the resonator in its linear region of operation. The low power dissipation and the very compact 5 transistor (5T) architecture are the clear advantages of this oscillator. Experimental results demonstrate that the oscillation amplitude can be effectively limited to a desired value and that consumer grade performance can be achieved: an acceleration noise density of 360μg/VHz for a power dissipation of 21.6μW is demonstrated.

2.5 rad/sec/sup 2/ resolution digital output MEMS-based rotational accelerometer for HDD applications

Mechanical vibration of a HDD can cause head mis-positioning. This effect can be compensated by using the device presented in the following. It is a capacitive rotational accelerometer sensor which uses MEMS. The vibrations of the disk drive cause tiny rotational displacements of the moving part of the MEMS structure, which are measured by sensing the small variations in capacitance that they cause. Capacitance changes as small as 0.05 fF can be measured by the device. The device operates as follows: the minute rotational vibrations are detected and measured in order to generate a feed-forward correction signal for the voice coil drive circuit to keep the head in the correct position. This allows the track spacing to be reduced significantly, increasing the storage density per unit area. The circuit has been realized in a 0.61 /spl mu/m BiCMOS technology with an area of 9.5 mm/sup 2/. The power consumption is 150 mW from a single 5V supply.

Design of a Delta-Sigma Bandpass Demodulator for a Z-Axis MEMS Vibrational Gyroscope

This paper describes the design of a ∆Σ bandpass demodulator for a Z-axis MEMS vibrational gyroscope. In this MEMS device, a silicon mass is driven through electrostatic actuator so that it has a sinusoidal linear motion, with a controlled speed. Attached to such driving mass, a second mass, free to move in the direction orthogonal to the motion of the first mass, is subjected to a Coriolis force, proportional to the product of the first mass speed by Z-axis rotational speed. The sensing of the Coriolis force and, in turn, of the Z-axis rotational speed, is performed in closed loop fashion, by measuring the restoring force needed for keeping the sensing mass at the equilibrium position. The restoring force is applied to the sensing mass through a quantized actuation signal, which is obtained from the output bit stream of a band- pass Delta-Sigma converter, containing the information of the Coriolis force. The design of the sensing control loop and simulation results regarding the signal-to-noise ratio achievable with the proposed design is reported. Finally, the issue of the realization of the proposed solution with switched capacitor (SC) technology is also addressed. I. INTRODUCTION

Compact MEMS magnetometers for inertial measurement units

This work discusses the performance, in particular in terms of minimum obtainable resolution, for Lorentz force based MEMS magnetometers, both accounting for technological constraints (minimum air gap, package pressure, standard industrial process, ...) and considering the final application in consumer electronics (bandwidth, noise, full scale range and linearity). Following an in-depth analysis of the operating principle, and carefully accounting for the damping behavior in the molecule-flow regime (typical for the used package), the expressions of the magnetometer sensitivity and intrinsic resolution (i.e. limited by thermo-mechanical noise) are derived. The predictions are compared to previous literature works and to recent experimental results obtained on a magnetometer designed and built using the ST Microelectronics ThELMA process. The given considerations are relevant for the co-design of a suitable low-power driving and readout electronics, an example of which is described in details.