Electronic Design
Cut The Links To Your Sensor/Actuator Networks

Cut The Links To Your Sensor/Actuator Networks

Network everything. That seems to be the trend in wireless as in all other communications technologies. It’s difficult to identify any segment of electronics today that isn’t networked.

Local-area networks (LANs), personal-area networks (PANs), metro-area networks (MANs), wide-area networks (WANs), the Internet, and the forthcoming Smart Grid all envelop us. And now a newer form of network is finally being widely deployed: the wireless sensor network (WSN) or, more precisely, wireless sensor and actuator networks (WSANs).

Both have been discussed extensively over the years and have been the subject of intensive research and development in university, military, and other research labs around the globe. It’s only now that we’re beginning to see the many useful possibilities, especially for the home-area network (HAN) that is going to be the core of the coming Smart Grid rollout.

WSANs DEFINED

A WSAN is a network infrastructure that can sense its environment and react to specific conditions of interest. It can monitor and control its environment within its design capability. In many cases, it also can be set up to do some amount of relevant computing.

Many, if not most, WSANs are sense-only networks. As a result, they’re called WSNs since they don’t involve controlling functions within the environment. Some organizations refer to WSANs as wireless data acquisition or wireless telemetry. In these traditional functions, a major consideration is the recording and storage of the collected data along with some analysis and display.

The network is made up of miniaturized nodes that consist of a sensor and its related signal conditioning circuitry, a radio transceiver, some memory, and an embedded controller. The battery-powered unit is designed for very low power consumption. These nodes can communicate with a central master control point or with one another.

A central controller or master node with more extensive computing capability collects the information gathered by the sensors and passes it along to some data center, usually through the connection to some other network like a company LAN or the Internet. The nodes are usually stationary but could be mobile. They also could be location-aware.

The nodes can monitor any physical characteristic for which an electronic sensor has been developed. The most common sensors are for temperature, pressure, light, sound, motion, humidity, and pollutants. Some WSNs can accommodate video input. As for control, the actuators may be lights, motors, fans, valves, relays, solenoids, pumps, appliances, or any other electromechanical device.

A primary consideration of any WSAN is network topology. The two most widely used topologies are the star and mesh. The star network (Fig. 1a), also called multipoint-to-point (MPP), has a central master control node with computing power with multiple nodes. The nodes only talk to the controller rather than to one another.

In the mesh network (Fig. 1b), the nodes communicate with one another and offer a multi-hop capability back to a central collection point. In the mesh topology, the nodes report the status of their own sensors and act as relay points that simply retransmit the data from nearby nodes.

The method allows sensors to be spread over a wider range than the single-node range. It also provides a form of network reliability. If a node’s battery dies or its signals are blocked, the network automatically and dynamically reroutes the data through other adjacent nodes. WSANs can use other hybrid forms of network topologies as required as well. These may be a mix of tree, star, or mesh.

THE HARDWARE AND SOFTWARE

The main hardware element is the node. Nodes also are known as “motes,” a mote being a tiny particle, such as dust. The sometimes stated goal of WSNs is to make the nodes that small. Nodes as small as a dime or quarter are fairly common, but that’s about as small as they get today.

The node’s basic architecture (Fig. 2) has an embedded controller and memory at its core. The controller hosts a small operating system that runs the networking software and manages the I/O (see “Interfacing The Sensor”). The sensor, its signal conditioning, and the analog-to-digital converter (ADC) comprise another major section, while the radio transceiver with its antenna form yet another. In some cases, there may be multiple sensors and related circuitry.

An essential part of the node is the power-management portion. The power source is a battery, of course, but power management is critical to long battery life. Some of this control may be handled by the MCU.

The software consists of a small specialized operating system (OS) and all the related drivers and applications programs. More than a dozen OSs are associated with WSANs. A popular one is TinyOS and its related programming language called network embedded system C (nesC), an extension to C. TinyOS is an event-driven OS that calls event drivers for specific tasks as opposed to a threading OS. Other software is related to the sensor such as the communications media access controller (MAC), the protocol and networking functions, and any application software that performs related data manipulation.

RADIO TECHNOLOGY AND STANDARDS

Many existing wireless networking technologies are suitable for use in WSANs (see “Important Wireless Facts To Keep In Mind”). The most widely used are IEEE 802.15.4, ZigBee, Bluetooth, Z-Wave, and 802.11 Wi-Fi. There are also other proprietary technologies including RFID.

If any one technology dominates the WSAN arena, it’s IEEE 802.15.4 and the enhanced version known as ZigBee. The IEEE standard defines the physical layer (PHY) and MAC layer of the system while ZigBee adds the upper network and applications layers. This wireless technology is based on direct-sequence spread-spectrum (DSSS) and uses the carrier sense multiple access with collision avoidance (CSMA/CA) channel access method.

The standard defines several different modulation methods based on phase-shift keying (PSK). It also defines three primary operating bands using unlicensed spectrum. First is the 868.3-MHz frequency in which a maximum data rate of 20 kbits/s can be achieved with raised-cosine binary phase-shift keying (BPSK) modulation. The maximum range is about 1 km. This version is used primarily in Europe.

In the U.S., the 902- to 928-MHz band is often used. The standard defines 10 channels, each 600 kHz in width and spaced 2 MHz apart. Again, the raised-cosine BPSK modulation is used. A maximum data rate of 40 kbits/s can be achieved. Range is about 1 km.

For Wi-Fi, Bluetooth, and other technologies, the most widely-used band is the 2.4- to 2.4835-GHz range. The standard defines 16 channels, each 3 MHz wide and spaced 5 MHz apart. The modulation is offset quadradure PSK (O-QPSK), which permits a data rate to 250 kbits/s. The maximum range is about 220 m.

The protocol is relatively complex but has an addressing scheme with a 64-bit address so many nodes can be accommodated. The maximum packet size is 127 bytes. Data is transmitted in short packets in a burst mode so transmit time is minimal, saving considerable power. Most radios using this standard consume very little power thanks to the very short transmit duty cycle.

ZigBee adds more layers to the basic protocol stack. This allows a wide range of topologies and applications to be supported, including mesh, which may be the most widely used form in WSANs.

An interesting variation of the 802.15.4 standard is called “6lopan,” which means IPv6 over low-power wireless PANs. With 6lopan, extreme mesh networking over the Internet for the Smart Grid movement is a possibility. The Internet Engineering Task Force (IETF), an organization that develops and maintains Internet standards, is developing 61opan. The standard is designated as IETF RFC 4944 and 4919.

More and more devices are connected to the Internet, and each needs an Internet Protocol (IP) address. That’s where IPv6 comes in. The IP networking standard has a 128-bit address, unlike the 32-bit address of the older IPv4 standard. It permits IP packets to be carried over low-speed WSANs.

The maximum packet size of the 802.15.4 standard is 127 bytes. The RFC 4944 standard allows the WSAN to carry up to 1280 bytes as required by IPv6. It does this by using a form of encapsulation and header compression. The standard is still a work in progress, but a final version is expected this year.

Bluetooth (BT) is another potential radio technology for WSANs. BT is an ad-hoc PAN that also operates in the 2.4- to 2.4835-GHz band. It uses frequency-hopping spread-spectrum (FHSS) technology. The hop rate is 1600 hops per second over 79 frequencies spaced 1 MHz apart. Maximum data rate is 1 Mbit/s with a throughput of 723 kbits/s. Modulation is Gaussian frequency-shift keying (GFSK).

Yet another faster option afforded by Bluetooth V. 2.1, enhanced data rate (EDR), uses different modulation methods to achieve a 2- or 3-Mbit/s data rate. The most common range is about 10 m with a typical 4-dBm power amplifier (PA). An external PA with 20-dBm power output is defined to extend the range to almost 100 m.

An important feature for WSANs is the ability of BT nodes to form piconets, which comprise links to seven other BT devices. Piconets can then be interconnected to form scatter nets for a greater number of nodes as the application requires. An ultra-low-power version of BT is also available to extend battery life.

ZigBee/802.15.4 and Bluetooth radios are most common when distances between nodes are less than about 10 m. If the nodes are more widely dispersed, an alternative is the popular IEEE WLAN 802.11 (Wi-Fi) standard. Maximum range is about 100 m if the nodes are in the clear.

Another advantage of Wi-Fi is its higher data rate potential of 11 Mbits/s for .11b, 54 Mbits/s for .11a/g, and over 300 Mbits/s for .11n. However, it’s rare to find an application requiring the .11n data rate as sensor sampling is extremely infrequent. Thus, the low data rates defined by ZigBee and Bluetooth are more than adequate.

Furthermore, Wi-Fi consumes much more power than either ZigBee or Bluetooth, making it unfriendly to long battery life requirements. Another disadvantage is the lack of a defined mesh networking protocol, but that’s about to end. The IEEE Task Group recently approved a mesh networking standard (802.11s) that should be ratified later this year with products coming shortly. In general, Wi-Fi isn’t a widespread choice for WSANs. However, it’s most likely used as the link between the WSAN collection point and either a company LAN or the Internet.

There also is a mix of proprietary standards in the industrial, scientific and medical (ISM) bands. One of the most widespread, known as Z-Wave, was designed for low-power, short-range sensor and actuator applications. It uses the unlicensed frequency of 908.42 MHz in the U.S. and can deliver a data rate of 9.6 kbits/s or 40 kbits/s using FSK. The protocol is optimized for mesh networking in WSANs.

Another standard from EnOcean uses the 868-MHz or 315-MHz unlicensed band with a data rate to 125 kbits/s. Its maximum range is about 300 m, and it’s designed for ultra-low-power consumption and mesh networking. Crossbow Technology (now MEMSIC) has WSN modules that use 802.15.4/ZigBee but also a proprietary module using the 868/916-MHz frequencies with a data rate of 38.4 kbits/s.

Ultra-Wideband (UWB) has been used as the wireless link in WSANs. In its WiMedia orthogonal frequency-division multiplexing (OFDM) format, it consumes little power and has a very high data rate. For some applications it may be an alternative to consider.

Many other wireless technologies can be deployed in some applications. Two additional examples are cellular networks and RFID. Embedded cell-phone modules are widely available for what are called machine-to-machine (M2M) applications, in which sensors or actuators are interfaced to the radio module. The module then reports back to a monitor and control point via the cellular network. These modules can comprise a multipoint system but not a mesh network. The range is greater than 2 or 3 km, and the reliability is excellent.

Some systems may need to include RFID. The system would consist of multiple RFID readers near the objects that are wearing RFID tags. The readers can read many tags, but the range is only a few feet for a passive tag. Active tags that use a battery can have a range of up to a hundred feet depending upon the frequency of operation. The readers would be networked back to a central data collection place where the ID is made. Mesh or multipoint arrangements can be used.

WSAN APPLICATIONS

The number of potential applications for WSANs is astronomical. But as it turns out, there are a few widely implemented systems.

• Building automation: WSANs are used to monitor lights, temperature, humidity, and other conditions for HVAC control. They are also used to monitor motion, smoke, and environmental factors.

• Home automation and control: The primary use is in monitoring temperature and humidity to control HVAC systems. WSANs can also monitor and control the energy usage of lights and appliances as part of a Smart Grid system.

• Weather monitoring: Sensors monitor all common weather conditions, collecting, storing, and transmitting data over a large area.

• Environmental monitoring: Sensors are used to make desired measurements of pollutants and other factors. Applications include detection of forest fires, floods, and earthquakes, as well as crop monitoring and watering.

• Industrial automation: Sensors monitor machines to determine usage, wear, maintenance, and serviceability. They also provide environmental monitoring for pollutants and abnormal conditions.

• Civil engineering: Sensors monitor the structural integrity of buildings, bridges, and other structures. They also can monitor highways for traffic and road conditions.

• Medical and health care: Uses include patient monitoring, patient records, information sharing, and emergency communications.

• Logistics: WSANs are used to track items in warehouse storage, inventory control, and shipping and handling.

• Military: Uses include equipment location and tracking, battlefield monitoring and management, surveillance, and troop and weapon activity sensing.

• Security: WSANs have uses in presence, motion, and break-in detection as well as in video surveillance.

• Robotics and remote vehicles: WSANs can monitor all functions, surroundings, and controlling operations.

REPRESENTATIVE RECENT PRODUCTS

With dozens of both component and end-equipment sources, engineers have a rich environment to choose from when designing a WSAN. For example, sources of 802.15.4/ZigBee equipment abound. Chips  are available from Freescale, Texas Instruments, and Microchip Technologies.

Ember is another long-time participant in the field. Its latest ZigBee systems-on-a-chip (SoCs), the EM351 and EM357, include a full 802.15.4 2.4-GHz radio with ZigBee protocol stack. They also include a 32-bit ARM Cortex M3 processor to run the application. The EM351 has 128 kbytes of flash, while the EM357 offers 192 kbytes.

With a power output in the +3- to +8-dBm range and a receiver sensitivity of –102 dBm, the link budget is exceptional. Power consumption is low. With good power management, battery life can last many years. Users can obtain a development kit radio module using the EM35x (Fig. 3). One of the most common applications for the Ember modules is in HANs (Fig. 4).

An interesting proprietary technology comes from EnOcean, whose Dolphin platform was designed for building automation, home networks, and other systems requiring very low power consumption and long life. The radio technology uses the 868-MHz band or the 315-MHz band. Even at low power, practical ranges are possible because of the low-frequency design.

Typical range within buildings is 30 m, but up to 300 m can be achieved over a free-space path. The data rate is 125 kbits/s, transmission may be one-way or two-way, and a unique 32-bit ID is used. The basic radio modules (Fig. 5) can operate without batteries using three types of energy harvesting:

• Mechanical: A magnet and coil inside a light switch generates power each time the switch is actuated. A self-powered light switch generates power and converts it to a radio signal every time the light switch is pressed.

• Solar: Most of the sensors (occupancy/motion, door/window, photo/light) are powered by collecting and storing energy from light. When combined with smart and ultra-low-power radios, sensors can operate with just 40 lux of ambient light. (In a typical indoor setting, more than 400 lux is usually available.) The energy is stored in capacitors, which allows the sensors to do their job even when they’re in complete darkness for days.

• Thermal: When energy is needed to control sensors residing in permanent darkness, temperature differentials can generate energy for wireless communications. This is the newest form of micro energy harvesting, and it’s enabling self-powered controls such as valve actuators.

The Z-Wave products from Sigma Designs (formerly Zensys) are also unique. Using a mesh architecture, the nodes can be used with switches, lights, thermostats, and appliance controllers. They are also compatible with some of the Advanced Metering Infrastructure (AMI) electric meters being installed as part of the Smart Grid initiative to manage and control energy usage in the home.

The Z-Wave modules operate on 908.42 MHz in the U.S. using FSK modulation and can deliver a data rate of 9.6 or 40 kbits/s as needed. The Z-Wave ZM3102N’s 8051 controller runs the protocol and mesh network. With low power, battery life can be very long. Dozens of companies use the Z-Wave modules for home monitoring and control, such as the Z-Wave-enabled Trane thermostat (Fig. 6). These and other end products are widely available in Lowe’s and Radio Shack stores.

Microchip Technology has a line of 802.15.4/ZigBee products as well as some low-power ISM-band radio chips. But the company’s recent acquisition of ZeroG Wireless included a WSAN product called Wi-Fi I/O. The primary product is the ZG2100, an 802.11b Wi-Fi module designed for very low power consumption.

The ZG2100 runs the standard .11b protocol but speed is limited to 1 or 2 Mbits/s. It is Wi-Fi certified and runs the available WEP, WPA, or WPA2 security. Also, it uses a serial peripheral interface (SPI) and is only 21 by 31 by 3.7 mm. If you need the speed as well as low power consumption, this is an attractive option. And, it’s very easy to incorporate into existing LANs.

One of the more interesting new products to address the WSAN market is Silicon Laboratories’ Si10xx wireless MCU family. This series of devices packages an ISM-band radio along with an 8051 controller, giving designers multiple ways to design their product. A unique power system with an efficient low-dropout (LDO) regulator and dc-dc converter adds a new dimension to the need for low power consumption and super-long battery life.

The top-of-the-line device is the Si1000, which features a 25-MIPS 8051 with 64 kbytes of flash and the usual mix of I/Os and interfaces as well as timers. A 10- or 12-bit ADC is also on chip as well as a temperature sensor and voltage comparators.

The radio is a real gem. It can be programmed to operate over the 240- to 960-MHz range, which covers the standard ISM frequencies of 315, 433, or 868 MHz. Modulation is FSK or GFSK with a data rate to 250 kbits/s. The receive sensitivity is an amazing –121 dBm while the programmable transmit power can be up to 20 dBm for a net link budget of 141 dB. This can extend range up to 3 km over a clear line-of-sight path.

The big news is the internal LDO and dc-dc converter with their programmable power-management unit, which keeps total power consumption low under all possible operating conditions. The EZMac software lets designers create a protocol for point-to-point, multipoint-to-point, and simple mesh networks. The Wireless M-Bus software, also available, is widely used for metering in Europe. Availability is scheduled for the second quarter of this year.

DESIGN ISSUES

The main design issues for WSANs vary depending on the applications, but three stand out: power consumption, ease of network modifications, and security.

Because the nodes are battery operated, long life is essential to minimize the time, cost, and inconvenience of changing batteries. Some of the newer modules offer a battery life of years, though most are considerably less. Look for products that transmit data in packets at high speed to minimize transmitter on time. A short transmit duty cycle is essential to long battery life.

Next, how easy is it to remove modules or add modules? The most desirable situation is one in which the system is ad hoc and modules may come and go without any reprogramming or intermediation.

Finally, security may be an issue in your application. Most standards provide some level of security, but you have to verify that it is sufficient for your application.

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