Free space laser communication is a promising candidate for a high bit-rate, interference-free data link at lower power consumption compared to radio and microwave frequency systems. On the other hand, an optical link has its own challenges: pointing, stabilization, and acquisition. Considering the directional laser beam, with 1 miliradian (.017 deg) divergence, we can say that pointing accuracy must be a fraction of 1 miliradian, which is the beamwidth.
Vibrations on the hosting platform can easily be large enough to disturb the beam heading and interrupt the link. A small airplane, with a 1 meter wing span, is our main candidate as the host vehicle for the laser transceiver. Inertial sensors have demonstrated effectiveness in detecting and canceling the effects of undesired vehicle vibrations. Stabilization as described above, however, needs a steering capability of +/- 10 degrees optical.
The third challenge, acquisition, needs a laser beam to scan an area that the opposite end is likely to be in. An estimation of the target's position narrows the scan area, and its dimensions are limited by the errors in the estimation. In this particular application, we need another +/- 10 degrees of beam steering for acquisition. So, the total range of the mirror needs to be +/- 20 degrees optical.
Considering the requirements above, the beam steering element in the system must be precise, fast, and have a large dynamic range. A 2-DOF MEMS mirror built on SOI wafers will do the beam steering. Small dimensions give the mirror a reasonably large bandwidth to meet the speed requirement.
The goal of the research summarized here is to design and implement a feedback loop around the mirror in order to achieve the resolution, which is more than 12 bits. The 20 degree range requires high voltages in the actuator. Position sense for the mirror is another essential component in the feedback system. In most of the cases, separate sense fingers are needed for a better performance. Additional fingers bring up layout and stability challenges for the mirror. Various methods for drive and the sense have been investigated. The current research direction implements the high voltage actuation without a need for high voltage circuitry. The same method also has a chance of using the same set of fingers for both sense and drive.
Figure 1: A 2 DOF mirror built on SOI wafer
The Steered Agile Laser Transceivers (SALT) project focuses on developing a system capable of doing two-way >1 Mb/s laser communication between small airplanes over distances as far as 5 km. Precision beam steering systems are essential for using highly directional lasers in free-space communication. As part of the SALT project, the goal of this research is to design a precision pointing and stabilization system using MEMS devices.
Our project aims to develop and investigate digital post processing methods and circuit techniques that allow efficient, low power implementation of high performance analog-to-digital converters (ADC) in fine line technologies. The research focuses on a continuous background self-calibration technique applied to a high-speed pipeline ADC topology.
Digital signal processing in system-on-chip applications has created a need for high performance ADCs that are compatible with deep sub-micron technology. These applications typically demand high linearity, speed, and resolution while maintaining low power consumption. While digital circuits benefit from the aggressive downscaling of CMOS technology, the trend of decreasing supply voltage in deep sub-micron processes tends to increase the power dissipation of high resolution ADCs. This research focuses on a novel concept through which analog domain precision can be traded off for low power digital signal processing.
In most high-speed pipelined A/D converters, high-gain residue-amplifiers dominate overall power dissipation. Substantial power savings are possible when precision amplifiers are replaced by simple open-loop gain stages. To take advantage of this opportunity, we propose a new digital calibration technique capable of correcting errors arising from amplifier nonlinearity and temperature drift. Our approach uses a statistics-based signal processing technique to measure and cancel gain- and nonlinearity errors of the imprecise, low power residue amplifiers. Critical converter stages are switched randomly between two transfer characteristics without interrupting normal A/D operation. Comparison of the two distinct cumulative distributions in the converter back-end allows estimation of the required calibration parameters.
To evaluate the proposed scheme, we have designed and implemented a 12-bit, 75 Msample/s prototype ADC in 0.35 µm CMOS technology . For simplicity, the digital calibration is applied only to the first converter stage and implemented off chip. Compared to a state-of-the-art reference design , we achieve more than 60% power savings in this critical portion of the ADC. Measurement results show that the digital post-processing technique improves the signal to noise + distortion ratio (SNDR) of the converter from 48 dB to 67 dB. Future work will focus on expanding the concept to multi-stage calibration applied to a high performance design in a low-voltage, deep-submicron technology.
Figure 1: Chip micrograph
A typical MEMS gyroscope measures rotation rate by sensing the Coriolis acceleration of a vibrating proof-mass. The gyroscope design can be divided roughly into three parts: the proof-mass, the actuator for vibrating the proof-mass, and the sensor for detecting the Coriolis acceleration of the proof-mass. This research focuses on the design of 5V CMOS electronics and the mechanics of the actuator to increase sensitivity to rotation and reduce sensitivity to process variation and temperature.
Gyroscope sensitivity depends on the velocity of the proof-mass, which is affected by the size of the actuator, the mechanical spring, and the mechanical damping. Increasing the sensitivity requires more actuation, vibration at the mechanical resonance, and reduced damping. Reducing the sensitivity to process variation and temperature requires position sensing and feedback control to electronically adjust the spring constant and maintain a constant sinusoidal velocity.
The actuator design will use capacitive position sensing and electrostatic forces, which are easy to integrate but tend to be nonlinear for large motion of the proof-mass. The design will use parallel-plate actuators, which can generate significantly larger forces than the more common lateral comb drive and can generate an electrostatic negative spring that adjusts the mechanical spring constant. CMOS circuits will be designed to measure the position, stabilize the parallel-plate actuator over large motions, and reduce the nonlinearity of the negative spring effect. Additionally, the mechanical design of the actuator will minimize damping and maximize stiffness of undesired mechanical modes.
The actuator design will be demonstrated first in a z-axis gyroscope and later in a six-degree-of-freedom inertial measurement unit (6 DOF IMU), which includes three gyroscopes and three accelerometers. The actuator design will be applicable to other MEMS such as scanning mirror displays and micropositioners.
The goal of this project is to develop the actuator for a novel integrated digital output z-axis gyroscope with frequency-locked resonance.
The objective of this research is to develop electronic interface circuits to measure strain with a silicon micromachined resonant sensor. These sensors are analogous to a guitar string. When the sensor is stretched (tensile strain) or compressed, its resonant frequency increases or decreases accordingly. Resonant sensors have several attributes that make them attractive. First, the information we want is contained in their output frequency and therefore sensor output is immune to AM noise. Second, their sensitivity to applied strains has been shown to be quite high .
While resonate sensors have the promise of high sensitivity, challenges remain in the development of the sensors. In fact, there are two components of the design that need to be improved from the current state of the art. The first component is the actual resonant sensor/oscillator, which is composed of a micromachined resonator and oscillator circuitry. The micromachined resonators have a high Q, but they suffer from high motional resistance . This makes it difficult to make an oscillator, as we must match the impedance of the resonator with an equivalent negative resistance for oscillation to occur. The impedance match is performed by the oscillator circuitry. Improvements in design of the resonator and oscillator circuitry can significantly improve the linearity and phase noise of the oscillator, resulting in better sensor resolution. The second component is the method used to measure the sensor's output. In many applications, the change in resonant frequency is measured by frequency counting . With this method, high accuracy measurements can be obtained, but at the expense of bandwidth. Another method of measurement uses FM demodulation to measure the change in frequency at the sensor output and is usually done with a PLL. With this method, better bandwidth is achieved; however, the phase noise of the VCO and the resonant oscillator make the DC measurement resolution quite poor.
This research will focus mainly on methods to improve oscillator circuit design and frequency measurement techniques to yield good sensor resolution over a large bandwidth.
The goal of this project is to develop a system for the measurement of strain in automobile roller bearings achieving a resolution of .1 microstrain (me) over a range of ± 1000 me and a bandwidth of 10 kHz.
The goal of this project is to develop and verify diagnostic assays for infectious diseases currently presenting significant threats to public health, including Dengue, Malaria, and HIV. The reporting elements in these assays are paramagnetic beads, which are detected using a CMOS based platform. Our goal is to demonstrate improved protocol simplicity compared to ELISA, the current immunoassay standard, with special emphasis on the applicability of the assay in a point of care or at home setting where the advents of a research laboratory are not available.
The goal of this project is to develop a high-order sigma-delta sense interface for micromachined gyroscope sensors. The main priorities are providing sufficient stability margin and low power consumption while maintaining the intrinsically high resolution of the sensor element.
The goal of this project is to develop a high-order sigma-delta sense interface for micromachined gyroscope sensors. The main priorities are providing sufficient stability margin and low power consumption while maintaining the intrinsically high resolution of the sensor element. Research so far has shown that implementing the gyroscope sense interface as a closed loop system using sigma-delta modulation has the advantage of low sensitivity to parameter variations and high linearity as well as an intrinsic digital output. In this work a fourth order sigma-delta loop is being developed. The advantage of the fourth-order topology is that it can provide high attenuation of the quantization noise in the signal band and allow for sufficient compensation of the loop as well as operation at low sampling frequency without introducing an additional noise penalty. The current status of the project is summarized below: