Where Wireless
Sensor Communications and
the Internet Meet
The CrossNet architecture extends the Internet to the sensor using wireless connections based on the Bluetooth protocol.
Mike Dunbar,
Crossbow
Technology, Inc.
Communicating with sensors has long been limited either to wired connections between the sensing element and the data acquisition (DA)/control electronics or to expensive, proprietary wireless communications protocols. Significant progress has been made in providing greater access to networked measurements and in enabling faster response to changing proc ess or environmental conditions. Using a ubiquitous and inexpensive wireless communications technology to extend the network to the sensor or control site can accelerate this wave of network implementation.
Figure 1. The CrossNet architecture extends the Internet to the analog sensor world over a wireless communications link.
Crossbow Technology’s CrossNet architecture (see Figure 1) extends the Internet using wireless connections to sensor nodes via the Bluetooth protocol. Bluetooth is an emerging, worldwide standard for inexpensive, local wireless communications. The standard is being implemented in laptops, handheld computers, mobile phones, desktop PCs, and a wide range of Internet appliances. The CrossNet architecture will reduce installation costs, improve users’ ability to easily and quickly reconfigure the DA and control system, and provide an opportunity to connect measurement and control data directly to the Internet.
CrossNet Architecture
The CrossNet architecture is flexible, modular, and adaptable to a broad range of test, measurement, monitoring, and security applications. The three basic building blocks are sensors, nodes, and hubs, all selected and configured to meet application requirements. The architecture can support data monitoring applications in which a number of sensors transmit data via a node to a single computer or handheld display. The computer can be connected to the Internet as shown in Figure 1, enabling the data to be accessed remotely by multiple users. More extensive sensor networks can be created in which tens or hundreds of sensors are connected via multiple hubs to the central network. In CrossNet, the hub (which can be a PDA or a PC) is typically the Bluetooth communications master, and the nodes are the slaves.
Figure 2. CrossNet smart I/O cables contain a transducer electronic data sheet (TEDS) for each sensor, as well as programmable sensor signal conditioning.
The Sensor. CrossNet supports a broad range of sensor types. The sensor connects to the node and consists of the sensing element, signal conditioning circuitry, and the transducer electronic data sheet (TEDS).
The TEDS is defined in the IEEE 1451.2 standard and allows sensors to identify themselves and to be interchangeable in networked systems. With this information, a host computer can determine the sensor type (e.g., pressure or temperature), meas urement range, and scaling.
These features can either be built di rectly into the sensor or formed by connecting a sensor to a CrossNet smart I/O (SI/O) cable, which contains the required circuitry (see Figure 2). The SI/O cables are available for sensors with voltage, current, or resistive bridge outputs. Spe cialized cables can easily be developed for specific sensor configurations, such as thermocouples, strain gauges, and other sensor technologies. The TEDS for each sensor is programmed into static memory attached to the sensor during initial system configuration.
The Node. In a typical CrossNet application, the user mounts sensors on a machine, test stand, or in a facility at any number of points to be monitored. The individual sensors are then connected to CrossNet nodes. Each node incorporates a Bluetooth radio for wireless communication with a computer or handheld device. The nominal transmission range is 10 m, with an optional capability of 100 m in a higher power version.
Photo 1. The CrossNet node transmits sensor data to a PC or network hub over a Bluetooth radio link.
The node (see Photo 1) collects data from multiple sensors and transmits the data via Bluetooth wireless communications to a network hub or other Internet appliance, such as a computer. The node can supply 5 V excitation to each sensor, or an external source can supply sensor excitation. As many as four channels are available on each node for analog inputs and digital outputs (see Figure 3). The sensor signal is digitized (16 bit A/D resolution) for transmission along with the TEDS for each sensor, allowing each channel to identify itself to the host system. The node can operate from either an external power supply or an internal battery.
The node can activiate operation in a number of ways:
• The host computer can request that data be acquired and returned.
• An event trigger can initiate data acqui sition at a user-defined sampling rate based on one of the sensor channels entering or exiting a user-defined region or threshold.
• An external TTL signal can start data sampling.
The data can be a single measurement from each channel, an average of a user-defined number of measurements, or data that have been logged in the node over a predetermined amount of time. In the data log mode, a time stamp is included with the data. The initial CrossNet node will support a sampling rate of up to 100 Hz for each channel. Future versions will support sampling speeds in excess of 5 kHz.
Because the nodes are compact (~4 by 3 by 1 in.), lightweight, rugged units, you can mount them close to the sensors they are monitoring. Any number of nodes can be installed in a building or on a machine.
Figure 3. The wireless node allows multiple sensor channels to be transmitted to a PC, PDA, or other Internet appliance for data acquisition, building control, or other applications.
Hub/Internet Appliance. A CrossNet hub can be any Bluetooth-enabled device for DA or analysis. The hub downloads data—including the TEDS information—from multiple nodes. Application software running on the host system (hub) displays the data in a user-defined format, or it links the data to a number of Windows-based DA and control software packages (e.g., LabVIEW or Excel). Application software for PC environments and PDAs is available.
Hubs can range in complexity from single computers or handheld devices logging/ displaying data to Web servers using the TCP/IP Internet communications protocol for wide area networking. Hubs can also use the TCP/IP protocol to connect with other wired or wireless network architectures, such as Ethernet and LonWorks. Crossbow is developing a family of network gateways, which allow multiple nodes to be connected to the Internet or other network architectures.
Applications
Applications for CrossNet include DA and control, environmental and building monitoring, HVAC diagnostics, automotive diagnostics, home networking and automation, and a host of applications in which the wireless extension of the Internet to real world data is of value. In some applications, the wireless access to the data is used to supplement the installed, wired control system for engineers and technicians to monitor ma chine status on demand.
Wireless access to sensor data reduces the time required to install a measurement system and significantly cuts the time needed to reconfigure a test stand or DA setup. The elimination of sensor wiring can be a major cost savings, and it provides access to remote sensing sites. CrossNet also lets you perform remote or mobile DA by untethering the sensors from the DA system.
Mike Dunbar is Director, Strategic Partner ships, Crossbow Technology, Inc., 41 E. Daggett Dr., San Jose, CA 95134; 408-965-3354, fax 408-324-4840, mdunbar@xbow .com.
