Kris Pister

OpenWSN: a standards‐based low‐power wireless development environment

The OpenWSN project is an open-source implementation of a fully standards-based protocol stack for capillary networks, rooted in the new IEEE802.15.4e Time Synchronized Channel Hopping standard. IEEE802.15.4e, coupled with Internet of Things standards, such as 6LoWPAN, RPL and CoAP, enables ultra-low-power and highly reliable mesh networks, which are fully integrated into the Internet. The resulting protocol stack will be cornerstone to the upcoming machine-to-machine revolution. This article gives an overview of the protocol stack, as well as key integration details and the platforms and tools developed around it. The pure-C OpenWSN stack was ported to four off-the-shelf platforms representative of hardware currently used, from older 16-bit microcontroller to state-of-the-art 32-bit Cortex-M architectures. The tools developed around the low-power mesh networks include visualisation and debugging software, a simulator to mimic OpenWSN networks on a PC, and the environment needed to connect those networks to the Internet. Experimental results presented in this article include a network where motes operate at an average radio duty cycle well below 0.1% and an average current draw of 68  μA on off-the-shelf hardware. These ultra-low-power requirements enable a range of applications, with motes perpetually powered by micro-scavenging devices. OpenWSN is, to the best of our knowledge, the first open-source implementation of the IEEE802.15.4e standard. Copyright

Reliability through frequency diversity: why channel hopping makes sense

Wireless sensor networks (WSNs) face the challenge of ensuring end-to-end communication while operating over individually unreliable wireless links. This paper addresses channel hopping, a class of frequency diverse communication protocols in which subsequent packets are sent over different frequency channels. Channel hopping combats external interference and persistent multipath fading, two of the main causes of failure along a communication link. This paper is, to our knowledge, the first to address the impact of channel hopping on routing. We simulate the performance of channel hopping and single channel solutions on connectivity traces gathered from a real-world office WSN deployment. Results indicate that the most basic channel hopping protocol increases connectivity along communication links, improving network efficiency (measured by the expected transmission count ETX) by 56% and network stability (measured by the average churn) by 38%. Further improvement can be achieved through the use of whitelisting - selective channel hopping over a subset of the available frequencies.

A planar air levitated electrostatic actuator system

The design and testing of a micromotor capable of moving multiple objects in three dimensions is discussed. The fixed surface of the motor, fabricated on a silicon wafer, contains air nozzles that levitate rigid platforms and conductive plates that generate an electric field to apply forces to platforms. Initial testing shows that the air nozzles form a stable very low friction bearing, and the motor is capable of several hundred microns of motion in two dimensions with fields generated by a 2 volt potential. Applications of this motor include a microrobotic work cell. Testing was limited to verification of the fundamental principles of operation, but experimental results indicate that a two-dimensional linear electrostatic stepper motor should be realizable with this process. In addition, it should be possible to control the vertical motion of the platforms by varying either the supply voltage or bearing pressure.<<ETX>>

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.

Smart dust mote forerunners

We have demonstrated a 138 mm/sup 3/ autonomous uni-directional sensing/communication mote that optically transmits a measure of the ambient light level. We have also developed a 63 mm/sup 3/ autonomous bi-directional communication mote that receives an optical signal, generates a pseudorandom sequence based on this signal to emulate sensor data, then optically transmits the result, although it has only been demonstrated in a bench configuration at this time. The latter system contains a micromachined corner cube reflector, a 0.078 mm/sup 3/ CMOS chip that consumes 75 /spl mu/W, and a Mn-Ti-Li cell, but we have also demonstrated operation from an /spl sim/2 mm/sup 2/ solar cell. These motes allow us to demonstrate necessary concepts of Smart Dust such as optical data transmission, data processing, energy management, miniaturization, and system integration.

The Swarm at the Edge of the Cloud

Mobile devices such as laptops, netbooks, tablets, smart phones and game consoles have become our de facto interface to the vast amount of information delivery and processing capabilities of the cloud. The move to mobility has been enabled by the dual forces of ubiquitous wireless connectivity combined with the increasing energy efficiency offered by Moores law.

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.

Implementation of Gradient Routing in Wireless Sensor Networks

IETF ROLL has recently proposed gradient routing as a fundamental building block for data collection in Wireless Sensor Networks. This paper seconds this choice by presenting an implementation of gradient routing on current hardware, and by showing experimentally that gradient routing is robust against topological changes. To stress its self-healing quality, we design and implement a complete communication stack in which neighbor tables are built in a purely reactive fashion. We quantify the resulting topological changes, and show how gradient routing elegantly handles these dynamics. This paper presents, to the best of our knowledge, the first experimental study on gradient routing as advocated by IETF ROLL.

3D structures with piezoresistive sensors in standard CMOS

Aluminum hinges and polysilicon piezoresistors have been fabricated in a standard commercial CMOS process with one maskless post-processing step. The hinges and piezoresistors are formed using the metal interconnect and transistor gate layers in the CMOS process. Xenon difluoride is shown to be a simple and effective alternative to standard bulk etchants for this process, because of its extreme selectivity and gentle gas phase etch. Preliminary results from a piezoresistive accelerometer are given.

Low power RF design for sensor networks

The design of RF circuits for short-range, low-power wireless communication is discussed. A derivation of optimum link range and transceiver power budget is presented based on simple models for indoor path loss and power vs. performance tradeoffs in a generic transceiver. Design techniques aimed at efficiently reaching these parameters are discussed for individual circuit blocks. Finally, some published transceivers are discussed with respect to the optimization and design techniques presented.