Electronic Design

Wireless Sensor Architecture Uses Bluetooth Standard

Flexible network enables plug-and-play connection of real-time sensors to the Internet for low-cost data acquisition, measurement, and control.

Traditional data-acquisition systems require reams and bundles of cabling to interconnect multiple sensor types to multiple signal-conditioning circuits. This is both an expensive and time-consuming method, plus it isn't flexible when more sensors must be added to a network. This not only limits sensor portability, but also requires the storage of calibration data and engineering units with no standardized output format possible.

In the vast majority of applications, each of these sensors must be individually wired to some kind of data-monitoring/acquisition/control device. Moreover, wiring is frequently temporary and must be torn down or reconfigured as soon as a particular test is completed.

In response to this problem, a number of companies have developed proprietary wireless solutions. These solutions have proprietary protocols that usually operate in the Industrial, Scientific, and Medical (ISM) band with the popular frequencies of 40 MHz, 900 MHz, and 2.4 GHz. Such systems, though, are expensive, costing anywhere from $10,000 per transmitter for a high-end system to $100 per transmitter for a low-end system.

Well aware of this bottleneck, Crossbow Technology of San Jose, Calif., has decided to do something about it. To alleviate the problem, the company has introduced the industry's first wireless sensor architecture that makes use of the Bluetooth communications standard. This standard operates in the 2.4-GHz band and is being designed into next-generation notebook computers, personal digital assistants (PDAs), mobile phones, and a wide range of other Internet appliances.

Dubbed CrossNet, the architecture allows low-cost wireless data connections to the Internet, thereby saving time and money, reducing test errors, and increasing productivity in many applications (Fig. 1). These include remote data-acquisition, monitoring, security, compliance, actuation, data-broadcasting and logging, environmental facility mapping, machine-performance monitoring, and other applications. The architecture not only does away with costly wiring, but eliminates the need to manage different sensor types as well.

Different sensors can self-identify themselves in the CrossNet architecture. They do so by transmitting their IEEE 1451-defined Transducer Electronic Data Sheet (TEDS) to a node. This makes data management much easier in applications with large arrays of sensors.

"Communicating with sensors is no longer limited by the need for wired connections or for expensive, proprietary wireless protocols," explains Mike Horton, Crossbow Technology's president and CEO. "The emerging Bluetooth technology enables fast, efficient sensor-to-Internet connections using the CrossNet architecture, which eliminates wired connections and the associated manual tasks of initiating, checking, and changing sensor connections," Horton states. "We believe it will be immediately useful for a broad range of data-acquisition, measurement, control, monitoring, and similar applications," he adds.

Key Elements
Key CrossNet architecture elements include the CrossNet nodes, a "smart" I/O (SI/O), and software for PDAs, PCs, and the Internet. Each node unit, measuring 3.5 by 2.5 by 0.75 in., controls and monitors up to four sensors (Fig. 2). Any number of nodes can be mounted in a building or on a machine. Each node contains a wireless Bluetooth radio that can transmit and receive data or commands over a distance of 10 m, with a PDA or PC. (An optional version works up to 100 m.)

The CrossNet architecture supports a broad range of sensor vendors and sensor types with voltage, current, and resistive-bridge outputs. Just a few of the types of sensors that can work with the architecture include temperature, pressure, motion, flow, gas, chemical, GPS, humidity, magnetic, light, and strain sensors. Specialized CrossNet cables can be developed for specific sensor configurations, such as thermocouples and strain gauges.

The SI/O connects individual sensors of practically any type to a node employing the IEEE 1451 TEDS standard. After a sensor is configured with the SI/O, the node automatically detects the type of sensor that's in use, determines its usable range, and reports the data from the sensor in appropriate engineering units. Additionally, with the SI/O, sensors can be calibrated and reconfigured over the Internet.

A node can initiate operation through several ways. Data can be acquired and returned upon request from the host device. Or instead, an event trigger can be established to start acquiring data based on one of the sensor channels (enter or exit an alarm region or threshold). Yet another method is to have an external TTL signal be activated to initiate data sampling.

The initial CrossNet node product will support a sampling rate of up to 10 Hz for each channel. Future versions will support higher-speed sampling rates in excess of 5 kHz.

Crossbow Technology will supply software for Windows-based PCs to gather data from as well as transmit commands to and from the CrossNet nodes. Bluetooth-enabled PDA users can download a compact program that configures the PDA to receive data from any number of CrossNet nodes.

Most software may be obtained free of cost by downloading it from Crossbow Technology's web site, which is found at www.xbow.com. The data is presented in a format that can easily be integrated into any application program that's based on Labview, Visual Basic, C++, or Excel. In addition, Crossbow Technology provides ServeWare software. This extends the real-time access of CrossNet data to remote users that are connected via the Internet.

To help accelerate the adoption of the CrossNet architecture, Crossbow Technology is introducing its CrossLink partners program. This is open to sensor companies, systems integrators, software companies, and Internet-appliance makers.

Price & Availability
Crossbow Technology can provide a complete demonstration of its CrossNet solution now. Due to low availability of Bluetooth radios, however, CrossNet nodes won't be available until late this year. A node that's able to control and monitor up to four sensors will be priced at $1895 each in single quantities. Smart I/Os will be available at the same time for a price of $95 each, depending on configuration. Beta software will be available late next month for downloading at www.CrossNetWireless.com. Full-release software will be available when CrossNet nodes begin shipping.

Crossbow Technology Inc., 41 E. Daggett Dr., San Jose, Calif. 95134; (408) 965-3300; fax (408) 324-4840; e-mail: [email protected]; Internet: www.xbow.com.

TAGS: Medical
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