Seth Hollar

System architecture directions for networked sensors

Technological progress in integrated, low-power, CMOS communication devices and sensors makes a rich design space of networked sensors viable. They can be deeply embedded in the physical world and spread throughout our environment like smart dust. The missing elements are an overall system architecture and a methodology for systematic advance. To this end, we identify key requirements, develop a small device that is representative of the class, design a tiny event-driven operating system, and show that it provides support for efficient modularity and concurrency-intensive operation. Our operating system fits in 178 bytes of memory, propagates events in the time it takes to copy 1.25 bytes of memory, context switches in the time it takes to copy 6 bytes of memory and supports two level scheduling. The analysis lays a groundwork for future architectural advances.

Single mask, large force, and large displacement electrostatic linear inchworm motors

We have demonstrated a family of large force and large displacement electrostatic linear inchworm motors that operate with moderate to high voltages. The inchworm motor design decouples actuator force from total travel and allows the use of electrostatic gap-closing actuators to achieve large force and large displacement while consuming low power. A typical inchworm motor measures 3 mm /spl times/ 1 mm /spl times/ 50 /spl mu/m and can lift over 130 times its own weight. One motor has achieved a travel of 80 /spl mu/m and a calculated force of 260 /spl mu/N at 33 V. The force density of that motor was 87 /spl mu/N/mm/sup 2/ at 33 V and the energy efficiency was estimated at 8%. Another motor displaced the shuttle at an average velocity of almost 4 mm/s and achieved an estimated power density of 190 W/m/sup 3/. Motors were cycled 23.6 million times for over 13.5 h without stiction. This family of motors is fabricated in silicon-on-insulator (SOI) wafers using a single mask.

Solar powered 10 mg silicon robot

We have demonstrated an autonomous two-legged microrobot which has taken its first steps. The body of the robot is fabricated in a planarized silicon-on-insulator (SOI), two-layer polysilicon process and is 8.5 mm /spl times/ 4 mm /spl times/ 0.5 mm in size. We previously reported initial leg motion from an off-board controller but have now incorporated control and power supplies onto the robot, resulting in autonomous operation for the first time. This solar-powered microrobot has two, one degree-of-freedom (DOF) legs and drags its tail end. Leg motion is generated via electrostatic inchworm motors on the robot body. The robot is a three chip hybrid assembled from one chip which contains the robots motors and legs, a second chip which integrates both solar cells and high voltage buffers, and a third chip which incorporates CMOS circuitry for sequencing the legs. The robot has demonstrated 3 mm of motion shuffling sideways and has lifted its front end more than 300 /spl mu/m above the surface. The total weight of the three-chip robot is only 10.2 mg.

Acceleration sensing glove (ASG)

A glove with 2-axis accelerometers on the finger tips and back of the hand has been built using commercial-off-the-shelf components. Taking advantage of gravity induced acceleration offsets, we have been able to identify pseudo static gestures. We have also developed software that allows the glove to be used as a mouse pointing device for a Windows 95 or NT machine.

An SOI process for fabrication of solar cells, transistors and electrostatic actuators

We have developed a new process for fabricating integrated, solar-powered microelectromechanical systems (MEMS) on a silicon-on-insulator (SOI) wafer. The intended applications for this process are autonomous microsystems, such as microrobots and distributed sensor networks. Two versions of the process have been created. The first combines solar cells and MEMS devices with NMOS transistors utilizing metal gates. This version has yielded solar cell efficiencies greater that 11%, a 200 cell array with an output of 88.5 V and transistor breakdown voltages above 25 V. Using this process, a fully integrated device has been demonstrated consisting of an electrostatic, gap-closing actuator being powered by an on-board buffer consisting of an NMOS inverter. The only external connections were ground and a 5 V control signal. A second version has also been developed which provides better solar cell performance and CMOS circuits utilizing polysilicon gates. This version has yielded solar cell efficiencies greater than 14%, a 90 cell array with an output over 50 V, and NMOS and PMOS devices with breakdown voltages greater than 50 V.

Design of low-power silicon articulated microrobots

We are creating a class of autonomous low-power silicon articulated microrobots fabricated on a 1 cm2 silicon die and mounted with actuators, a controller, and a solar array. By taking advantage of the high force-density of electrostatic actuators in the micro scale, low-power actuators can be made for microrobots. A micromotor with an energy efficiency of 4%, that uses CMOS-compatible supply voltage, and has a motion resolution of 2 μm has been demonstrated in a volume of 0.015 mm3. Articulated two degrees-of-freedom legs with built-in mechanical couplings have been fabricated in a commercial micromachining foundry (MUMPs) and successfully assembled. Lowpower CMOS electronics will be used to control the robot locomotion and a solar array chip will be used to power the microrobot.

Robot leg motion in a planarized-SOI, two-layer poly-Si process

With the ultimate goal of creating autonomous microrobots, we developed a five-mask process that combines two polysilicon structural layers with 50-/spl mu/m-thick SOI structures and a backside substrate etch. The polysilicon layers provide three-dimensional (3-D) hinged structures, high compliance structures, and electrical wiring. The SOI structural layer yields much stronger structures and large-force actuators. This process was developed as a part of a three-chip solution for a solar-powered 10-mg silicon robot. Here, we describe the fabrication of this planarized-SOI, two-layer poly-Si process (henceforth called the SOI/poly process), basic modules in the design of robot legs in this process, and lastly, the results of fabricated robot legs. In designing the leg structures, we developed guidelines and test structures to provide a better understanding of the robot leg performance. These guidelines include understanding the relationship between the lateral etch depth to the actuator spacing and performing static friction tests of polysilicon flaps to more accurately model the frictional forces of the linkages. Last, we report on the performance of the robot legs and inchworm motors. On an 8 mm /spl times/ 3 mm robot, we have demonstrated a 1 degree-of-freedom (DOF) robot leg, 1 mm in length, which demonstrates up to 60 /spl mu/N of vertical leg force with an angular deflection of almost 30/spl deg/. A two-DOF robot leg, also 1 mm in length, operated with at least 90/spl deg/ of angular deflection, and each inchworm motor demonstrated a shuttle displacement of 400 /spl mu/m with speeds up to 6.8 mm/s. In addition to robot legs, a bidirectional inchworm motor that produces equivalent forces in both directions was also fabricated in this SOI/poly process. This motor uses an additional set of gap-closing-actuator (GCA) arrays to prebias the drive frame.

Bidirectional inchworm motors and two-DOF robot leg operation

We have demonstrated a bidirectional inchworm motor that produces equivalent forces in both directions. This motor uses an additional set of gap-closing-actuator arrays to pre-bias the drive frame. To obtain the highest force densities possible, the motors are designed close to the limiting resolution of the process. We describe the inequalities relating the lateral etch depth to the actuator gap spacing and tooth width of the inchworm motors. In addition, we have demonstrated a two degree-of-freedom (DOF) robot leg operated with an external controller. The leg, 1 mm in length, was fabricated in a planarized SOI/2-poly process and was operated by two electrostatic inchworm motors. Each joint of the leg has demonstrated at least 90/spl deg/ of static angular deflection, and each inchworm motor has demonstrated a shuttle displacement of 400 /spl mu/m with speeds up to 6.8 mm/s. This corresponds to a robot foot speed of over 0.75 m/s and over 4 full steps per second. Endurance tests have shown that the shuttle and leg are visually undamaged after 60,000 full leg sweeps for 16.5 hours of operation (/spl sim/10 million inchworm cycles).

Video semaphore decoding for free-space optical communication

Using teal-time image processing we have demonstrated a low bit-rate free-space optical communication system at a range of more than 20km with an average optical transmission power of less than 2mW. The transmitter is an autonomous one cubic inch microprocessor-controlled sensor node with a laser diode output. The receiver is a standard CCD camera with a 1-inch aperture lens, and both hardware and software implementations of the video semaphore decoding algorithm. With this system sensor data can be reliably transmitted 21 km form San Francisco to Berkeley.