MicroServers and Motes
From Seamonster
Introduction
We use sensor network community terminology: Small low-power field devices (computer + sensors + radio) are called motes. Bigger more powerful devices (single board computer + medium range radio) are called microservers that may act as data gateways.
The first specific distinction between them is that mote radio range is typically one hundred meters whereas microserver radio range is 15--100 km. Also motes tend to run fairly small programs whereas microservers employ single-board computers to run a full-up operating system (Linux, embedded-XP, etcetera).
The Vexcel microservers were designed to operate in harsh environments, particularly in polar regions, and to over-winter by using very little power when sunlight gets scarce.
Microserver Functional Objectives
- Survivability in harsh environments for 2+ years of data acquisition
- Low power consumption (less than 1 watt average).
- Moderate telemetry range (8--15 km) + telemetry capability to overflying fixed wing aircraft.
- Moderate telemetry bandwidth (802.11G; 50Mbps nominal)
- Precision GPS to <10 cm relative accuracy
- Analog-to-digital signal recording to 3khz sampling frequency.
- Supports electronic interface standards (Ethernet, USB, RS232, ...)
- Single Board Computer capable of storing 16+ GB data
- Power management capabilities
- External battery recharge off photovoltaics
- Component duty cycles tied to annual solar illumination.
- Graceful shutdown under low-power conditions.
- Graceful recovery after battery recharge.
- Wireless network data sharing: Multiple redundant copies of all datasets.
- Open development environment (see below)
- Provide external (chassis) power connections and external data ports
- Support short-range wireless communication protocols such as 802.15.4
We want a device that can be installed anywhere in the world (on land) and connected to a rechargeable external battery and a photovoltaic panel. It must connect to any manner of electronic-signal-producing sensor. It must acquire, analyze, store, reduce and share data with other nodes connected in the wireless network. The WSN node must know where it is (via GPS) and be capable of time-stamping its data stream with appropriate precision (for example to < 10 microseconds for 3 khz seismic sample data). These nodes are intended to be available "at production cost" to geoscience investigators; depending on functionality this will run $2000-$5000 per unit.
Vexcel microserver specifications
Vexcel microservers were developed under NASA STTR support beginning in 2002 in collaboration with Penn State University. They run complex data acquisition programs and use reasonably sophisticated communication and power management strategies. In the Seamonster project their functions will include supporting local mote sub-networks, acquiring, storing, compressing and analyzing data, hauling data packets back to the Internet-connected Server, and routing network management (query) traffic.
The chief "harsh environment" attributes of Vecel microservers are tough cases and internal power management systems that operate independently of the main computer (note batteries in the image below). The (fairly obvious) 'trick' to meeting the power management low-power-consumption objective listed above is to attach a solid state switch to each power supply and control this switch with a logic output from a microcontroller. The microcontroller also talks to the main computer which can thereby turn other components on and off according to need.
This is the Generation 3 Version 1 (Gen-3.1) MicroServer
Gen 3.1 MicroServer Specifications
External Specifications Dimensions: (24 cm x 30 cm) x 11 cm depth (= 9.5 x 11.5 x 4.25 inches) Weight 4kg Enclosure: NEMA type 6 (Environmental protection / occasional submersion in water) Enclosure lid attachment: Screwdown (4 screws) Mounting: Two case-width drillable rails along bottom 24 cm edge (shown below). External Connectors * Ground lug * N-type wireless antenna connector (lightning surge protector) * N-type GPS antenna connector * 3-pin MILSPEC external power connector * 3-pin MILSPEC external photovoltaic connector
Internal Specifications Single Board Computer: ARM-9 Technologic Systems TS-7260 GPS: Novatel SuperStar-II L1 Carrier Phase Wireless Ethernet Bridge: 802.11G typically LinkSys WET54G Radio Amplification: HyperLink Technologies 1/4 watt 802.11 (2.4GHz) amplifier Power Supply * Internal typical: 4 LiSO3 D-cells 7+ Amp-hours * External typical: 12.5 Volts rechargeable lead-acid typical * Photovoltaic typical: 30 watt panel Power Consumption: Operational * Moderate duty cycle operation: * Sleep mode < 75 microwatts (microcontroller only) * Microcontroller normal operation: 75 milliwatts Power Consumption: Components * Single board computer, low power consumption mode * Single board computer, normal operational mode * Wireless Bridge plus Amplifier * GPS board (Novatel SuperStar-II L1-carrier phase) * "Full-bore" (all components on):
Gen 3.2 MicroServer Specifications
The principle changes from Gen 3.1 to Gen 3.2 are mote/subnet support and additional sensor support. The latter includes digitizers internal to the case as well as through-case connectors for things like web cams, met stations, and GPS devices.
This diagram indicates how power management is accomplished using a low-power microcontroller.
External Specifications Dimensions: Slightly larger/deeper Weight 5 kg Enclosure/Mounting: As above for Gen-3.1 External Connectors * Ground lug * N-type wireless antenna connector * N-type GPS antenna connector * 3-pin MILSPEC external power connector * 3-pin MILSPEC external photovoltaic connector * 2 USB ports * 1 Ethernet port * 1 3-pin power supply connector (12/5/Gd)
Internal Specifications Single Board Computer: ARM-9 GPS: Novatel SuperStar-II L1 Carrier Phase Wireless Ethernet Bridge: 802.11G typically LinkSys WET54G Radio Amplification: HyperLink Technologies up to 1 watt amplifier (802.11B/G)
Power Supply As above (Gen-3.1) with additional power draw for optional components as selected
Motes
(Placeholder remarks.) Motes (short for 'remote devices') are low-power CPU chips combined with radios, often connected to one or more sensors. Motes are intended to operate with low power consumption and have commensurately low data handling and transmission range characteristics. (MicroServers can act as local network anchor points for Motes.)
MicroServer Back Story
"The purpose of the Seamonster program is to facilitate geophysical data acquisition in harsh environments by means of a general-purpose field computer; in other words is to get researchers out of the electronics lab and into the field doing good science, and to get those researchers more and better data per research budget dollar."
From 2002--2006 I worked under NASA sponsorship to evolve backbone WSN nodes through 3 "generations": A prototype Gen-1, a working field prototype Gen-2, and a professionally packaged commercial device, Gen-3.1. (WSN incidentally stands for Wireless Sensor Network.) These nodes are sometimes informally referred to as backbone 'vertebrae' or even as 'bricks', but their proper designation is microserver because they have modest but complete server functionality.
Power Consumption Elaborations
Electronic devices have varying power consumption based on what they are doing, and obviously zero power consumption when they are off. Like this remark our approach to power managemnet is quite self-evident: We use a very low-power microcontroller with its own batteries that can operate for up to three years to monitor the primary rechargeable power supply and only permit the rest of the system to operate if sufficient power is available from the external battery. We assume that in addition to this external supply there is also an external photovoltaic panel connected to the unit.
We have a passive charge controller internal to the device that receives the photovoltaic input and uses it to recharge the main/external battery, typically a lead-acid battery of some type. This baseline approach means that the device can eventually recover from low-power conditions after an adequate recharge period. When this happens the autonomous low-power microcontroller re-enables the main computer.
The second part of the power management strategy centers on this main computer. The sensor network is deployed with a built-in duty-cycle strategy that anticipates both the system power consumption tendencies and the likely available power as a function of time of year. This strategy is implemented on the main computer by means of a task manager program that turns other components on and off as they are needed. Since the microcontroller already "has its hand" on the power switches of the other components we simply interface the main computer to the microcontroller by means of General Purpose I/O (GPIO) ports.
Meeting Gen 3.1 / Gen 3.2 Objectives
Generation 2 microservers were built for high-frequency seismic data acquisition via external geophones. More detailed description is available here. Generation 3.1 devices do not incorporate all of the Generation 2 functionality, in partiucular a digitizer. For Gen 3.1 simplifications were made to concentrate on packaging and power management. Generation 3.2 devices will re-incorporate the 4-channel digitizer in order to cover the entire functional objective list given above.
Have We Field Tested These Devices?
Yes, but not enough. The devices have been cold-soaked for short periods and have operated properly. They have operated in ambient temperature near Denver Colorado over winter with no ill effects; temperature range -10 to +21C (10--70F). Primary field testing has taken place in summer on Alaskan temperate glaciers only, Generation 2 only. We continue with cold-environment exposure during winter 2006/2007 in the Colorado Rockies. The first severe cold-soak test is anticipated for winter 2007-2008 in the Canadian Arctic. The devices are engineered to operate at -40C for extended periods of time but we can not claim that they operate reliably in these conditions until we test them.
What Is Meant By "Raw Cost"?
Commercialization projects proceed, broadly speaking, in two stages: An R&D phase and a commercial product phase. While we can now offer commercialized versions of these microservers, they are still in their R&D phase until they have proven themselves in the field. In short, we have built commercial devices that have not yet been certified for commercial release. For that reason I describe cost per unit as a "base cost", that necessary to cover production expenses. The devices are built by a manufacturing company in Boise Idaho, KimCo manufacturing. The bottom line is that these devices are cheap and experimental, not expensive and field-proven.
What Is Meant By "Open Development Environment"?
An Open Development Environment (ODE) means that WSN nodes are fully functional as delivered but can be modified and enhanced by a developer for further purposes. For example if the developer wishes to implement a different power management strategy, the means for doing so is transparently presented in the system documentation (at least) if not built into the system architecture as an API or by other means (future concepts). Facilitating the premise stated above is the motivation for maintaining an ODE approach; as I make further progress in integrating this work with other programs at Microsoft I will be able to give a more complete picture of Open Development.
