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Wireless Systems for Environmental MonitoringThis application note describes how to deploy Crossbow’s MICA family of wireless mesh networking hardware, software, and sensors for environmental monitoring. The sensor nodes are easy to deploy, self-configure, and report into a database and graphical software package. Software for these applications is readily downloaded from Crossbow’s website and TinyOS distribution. All of the software components described in this application note are freely available for use with our hardware/sensors.This application note also covers the technical and performance aspects of Crossbow’s mesh networking protocol and stack known as XMesh (a.k.a. Surge Time Sync). The XMesh stack is self-healing, ad-hoc mesh network which combines two breakthrough technologies – Distributed Time Synchronization and Low-Power Listening. Networks of tens to hundreds of motes can be deployed with one year or more of battery life.This combination of software and hardware delivers a complete solution for environmental monitoring applications. Example deployments include:•Light, temperature, and soil conditions within a green house•Soil moisture and temperature in a vineyard or other high value crop•Wind speed and wind direction measurement in mountainous regions•Frost detection and warning•Measurement of localized ET (Evapo-Transpiration) for Irrigation Control•Indoor comfort monitoring, including HVAC tune-upSystem OverviewMost environmental monitoring deployments have a number of common characteristics. The Crossbow solution addresses the following characteristics:•Monitoring temperature, humidity, barometric pressure and other environmental parameters•Low sampling rates, typically slower than 2 minutes per sensor measurement.•Outdoor environments•Deployment of sensors over several acres or more•Battery operation for at least one year•Remote logging of data and remote data access.Crossbow utilizes its MICA2 and MICA2DOT programmable processor radio modules as the heart of its environmental monitoring system. The MICA2 and/or MICA2DOT are configured with the appropriate sensor and data acquisition boards and packaging. The MICA2 andMICA2DOTs are loaded with TinyOS-based firmware that performs sensor monitoring and power control on top of the XMesh protocol. These packaged sensor modules communicate with the Crossbow Stargate access point which receives sensor readings and logs them into a local database. The sensor data is then accessed, monitored, plotted, and optionally exported with the MOTE-VIEW graphical user interface from any network connected PC. Figure 1 shows the environmental monitoring system architecture and the way these various components fit together.Figure 1. Crossbow’s Wireless Environmental Monitoring System OverviewThe software components that come together to form this system are listed below and shown in Figure 2. All of these software components are available from Crossbow at no charge for use with our hardware.MICA2 / MICA2DOT -based Enviromental Sensors:•TinyOS operating system*•XMesh (Surge Time Sync)* component which handles all networking and radio communications.•XSensor sensor application* which is customized for sampling environmental sensors available from Crossbow (see Sensor section of Appnote)Stargate Base Station and Sensor Data Server / Database :•Basestation MICA2 mote running XMesh (Surge Time Sync) * for interfacing to Sensor Network•XListen* software which parses mote data packets and stores into Postgres database.•Postgres* database for local storage of data•Linux* operating system for StargatePC:•MOTE-VIEW** data visualization program for Windows based machines Notes:*Open source code in ** Free runtime executable from CrossbowFigure 2. Software Components and Architecture of Mesh NetworkSensorsCrossbow’s MICA2/MICA2DOT platforms and wireless mesh network software are easily integrated with a wide variety of sensors. Presently, Crossbow offers the following low-power sensor combinations for environmental monitoring:Packaging: •MEP410 is packaged in a water resistant plastic box with externalantenna and external battery box. The battery box canaccommodate 2 C size batteries.•MEP510 is packaged in a water resistant plastic box with externalantenna and an internal lithium 2/3 AA battery.•The MTS400/410 and MDA300 are not presently environmentallypackaged, but Customer can easily add external housingHumidity Sensor:Type:Accuracy:Long Term Stability:Min OperatingVoltage:•Sensirion SHT11•3% from 20% to 80% RH•5% from 0% to 20% RH•5% from 80% to 100% RH•<1% RH per year•2.4VTemperature Sensor:Type:Accuracy:•Internal to Humidity and Barometric Sensor•Humidity Temp: +/- 1.5°C from -100 C to+600 C•Barometric Temp: +/- 0.8° C from -100C to+60° CBarometricPressureSensor: Type: Accuracy: Long Term Stability: Min. Operating Volts: • Intersema MS5534A • +/- 1.5 mbar from 750 to 1100 mbar• -1 mbar/yr• 2.2VLight Sensor: MEP410Type: Location: Min. Operating Voltage:• Hamamatsu S1337, UV range: 190-1100 nm• Hamamatsu S1087,Visible range: 320-730 nm• One each on top and bottomof package (both lightsensors)• 2.2VAccelerometer Sensor:Type:Range: • Analog Devices ADXL202 • +/-2 g GPS:Type: • SIRF, 12-Channel Receiver The MEP410 can optionally be loaded with an internal humidity sensor along with the external humidity sensorMDA300: The MDA300 is a general purpose data acquisition board that can interface to a variety of external sensors. It also includes an internal temperature and humidity sensor. See MDA300 datasheet for complete list of data acquisition features.Commonly Used Sensors for MDA300 include: • Echo EC-20 and EC-10 Dielectric Aquameter for measuring soil moisture content. Accuracy: .03m/m (% water content) • Spectrum Soil Temperature ProbeWireless CommunicationCrossbow’s MICA2 and MICA2DOT motes have integrated radios that are designed to operate in the ISM band. Crossbow manufactures motes in several different frequency ranges to support the various ISM bands throughout the world. Users should purchase the appropriate frequency range for ISM band rules of their country. Users should also consider radio range requirements when selecting the appropriate MICA2 or MICA2DOT mote. The following table summarizes currently available options recommended for Environmental Monitoring.MPR400, MPR500 MICA2, MICA2DOT 868/900-928MHz (US, Europe) MPR410, MPR510 MICA2, MICA2DOT 432-434MHz (US, Europe)MPR420, MPR520 MICA2, MICA2DOT 315MHz (Japan only)Antenna: Crossbow recommends an external, ¼ wave whip antennae. These antennae are inexpensive and offer improved coverage. The MICA2 has an MMCX connector for easy connection to many types of external antennas. All sensors should be placed such that theantennas are oriented vertically. Horizontal placement of the antennae will result in a substantial loss of distance.Radio Range: Lower radio frequencies for example, 433 MHz, will have longer ranges in an outdoor deployment. Depending on the foliage and environmental conditions, expect ranges of 200-500 feet at 433 MHz and 100-300 feet with 916 MHz. Remember, the XMesh stack willautomatically configure the network and allow for radio range extension via message hopping across multiple deployed sensors.Placement: Units are usually placed at least 1-3 feet above the ground. Placing units at ground level will decrease communication range. Grass and other foliage will also decrease distance. Crossbow recommends the following average mesh grid density:916 MHz: 1 mote/2500 sq feet (50’ by 50’).433 MHz: 1 mote/10000 sq feet (100’ by 100’).Foliage and other RF obstacles will decrease distance. However, if the units are deployed such that motes can find other motes (parents) then the network will automatically reroute radio traffic.Figure 3. Aerial photo of Sensor Network DeploymentXMesh Networking StackCrossbow’s deployments are based on the XMesh (Surge Time Sync) networking protocol which has been optimized for low-duty cycle, environmental monitoring, applications. The XMesh network has the following features:•Low power (typically less than 350 µA average current).•Network time synchronization to +/- 1 msec.•Low power listening with an 8 time per second wake-up interval, allowing for rapid message transfer across the network.The default sampling period is 3 minutes, although many other sampling intervals are allowable. The following average currents were measured at different sampling intervals:Data Interval (minutes) Average MICA2 or MICA2DOT Current(µA)1 6773 3156 196The XMesh network has been extensively tested both at indoor and outdoor locations. In a typical indoor test, nodes were placed at every 300 sq ft. to cover a 10,000 sq ft facility. To simulate larger distance between nodes, the radio transmit power was turned down to -6dBm. In outdoor tests nodes were spread across several acres of rugged terrain with an average density of 1 mote per 10,000 sq ft and at full radio power. Statistical analysis across many deployments shows on average greater than 90% of all traffic generated at any node will be collected at the base station without the use of end-end acknowledgements.Stargate Base StationFigure 4. Stargate gateway as a base stationThe Crossbow Stargate gateway and microserver is an embedded Linux computer designed to be the primary access point for wireless sensor networks. The Stargate’s small form factor, reliability, and optional communication interfaces makes it ideal for remote, environmental monitoring. A base station mote (MICA2) is connected to the 51-pin connector on the Stargate.A resident program, XListen, takes an input stream of wireless sensor data from the base station mote and stores it in a local Postgres database.The Crossbow Stagate can be connected to a wide-area or back-haul network in one of the following ways:•Wired Ethernet connection•WiFi, using a Compact Flash card (Netgear MA701, AmbicomWL1100C-CF, etc.)•Long Range WiFi using PCMCIA card (ZComax or SMC Wireless SMC2532W-B).•Cell phone modem card (Sierra Wireless Aircard 555D) for remote cellular access.Remote administration and database replication for the sensor data logged onto the Stargate is done with a secure shell connection (ssh).Xlisten Logging Software for StargateThe Xlisten software which runs on Stargate acts as the intermediary between the sensor readings from the wireless mesh network of sensors and the Postgres database installed on Stargate. Xlisten is able to recognize and interpret packets in a standardized format. The standard packet generally includes at a minimum: node ID, parent, sensorboard ID, and voltage. Data transmitted by the motes is a raw analog or digital reading. Final conversion to engineering units (e.g., degree C) is done by Xlisten or MOTE-VIEW. The full C source code for conversion is available and provides a good reference for how to convert sensor readings for the entire line of Crossbow wireless products. Other sensor conversions are easily added. Most common conversions are in the shared xconvert.c library.Xlisten is written to be extensible so that support for new sensors and packets is straightforward to add. Each sensorboard is handled with a single source module in the boards directory. The particular board to use is decided by looking up the unique board ID field. The board file provides specific handling for printing parsed and converted packets, as well as data logging. Data Visualization Software: MOTE-VIEWMOTE-VIEW is a Windows .NET-based data visualization tool for monitoring and managing sensor networks. MOTE-VIEW connects to the remote Postgres database on the Stargate basestation through a TCP/IP link.MOTE-VIEW has a number of useful features for monitoring and understanding wireless sensor data. These features include:•Historical and Real-Time Charting•Topology Map•Data Export•PrintingExportMOTE-VIEW allows the users to export data as CSV (comma delimited text) or XML. All data is exported as raw data or converted to engineering units.VisualizationThere are three ways to visualize sensor data in MOTE-VIEW:•Data Grid – Last sensor result set received from each node.•Charts – Three charts of a sensor against time for multiple nodes.•Topology Map – Spatial view of last result from each node.Figure 5. MOTE-VIEW Charts each data point with linear extrapolation between them.Zoom and pan functionality is supported as well.Network Health MonitoringMOTE-VIEW provides diagnostics for evaluating network health that is quite effective in the field. Each node is color coded by the amount of time since the last data was received. Green signifies that data was received in the last 20 minutes and that the node is “healthy”. The node color then further erodes as the last result grows increasingly stale: light green color after 20 minutes, yellow after 40 minutes, orange after an hour, and red after a full day of no results.Figure 6. MOTE-VIEW topology map: Nodes are color coded by temperature. The possible colors are: blue (temp < 18C), green (18C < temp < 22C), yellow (22C < temp < 26C), orange(26C< temp < 30C), and red (temp > 30C)Battery LifeBattery life is a critical parameter for low power networks. The power consumption of individual motes within the network varies due to the number of radio messages that a mote hears and transmits. Most of the battery power is expended for these operations. Motes close to the base station see higher radio traffic and therefore have lower battery life. Due to this variability it has been difficult to predict and measure the actual power usage of individual motes as well as the entire network. Measuring battery life in the field is useful, but most customers want to have a good idea of battery life prior to deployment. For this reason, Crossbow has expended significant development to accurately measure and characterize the battery life and current consumption of its hardware in real-world deployments.In order to characterize this power usage, Crossbow has developed power monitoring software that runs in the motes during network testing. These diagnostic modes monitor various power states of the mote, such as:•Microprocessor on-time and sleep time power•Radio receive power for each packet•Radio transmit power time for each packetEach time the mote cycles through these states, the power consumed is computed and accumulated with the previous used power. Data is periodically transmitted to the base station for logging. Figure 7 below shows the analysis of a networking session. The left diagram shows the average current consumed for each mote in the network.Figure 7. The left diagram shows the average current consumed for each mote in the network. The right diagram shows the expected lifetime of each mote in the network using2000 mA-hr batteries.Battery Life and Average CurrentBy integrating the total current consumed, the ACM100 can accurately compute the average operational current even though the motes cycles through current consumption states that differs by up-to three orders of magnitude. Utilizing this real average, we accurately predict the battery life time. Below is a graph of a 2000 mA-hr battery based on different average currents.Figure 8. Battery Life vs. Average CurrentAchieving average operational currents below 500 µA is critical for multi-month operation on AA batteries. An average current below 250 µA is needed for one year operation. Larger batteries (C or D) can be used for extended operation. The table below estimates the battery life for different Alkaline battery sizes at different current drains:Battery Size Capacity(mA-hr)Battery Life@ 250µA (yrs)Battery Life@ 500µA (yrs)Battery Life@ 1mA (yrs)AA 2000 1 0.5 0.25C 6000 3 1.5 0.75D 12000 7 3.5 1.5。