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Evaluating the Different Protocols for Low Power Wireless Networks in Industrial Automation



Low power is becoming an increasingly important factor in the design and implementation of industrial automation networks, and there are new protocols available to help drive down power and cost.

Despite the perception that power consumption is not a key design issue in industrial automation, low power design has become important over the last few years for several reasons. Firstly is the obvious cost of the power. More systems are controlled remotely, so the power consumption of controllers and the wireless systems increases. This amounts to millions of dollars around the world, and cutting this bill is a key focus with the price of oil so high. Many industrial users also have tariffs that are dependent on the availability of power, and can have their supply cut back at times of greater demand, making the ability to operate with less power essential.

Wireless technologies can be used in a number of ways to cut power costs. Reducing the power consumption of the wireless transceivers, through low power silicon, appropriate protocols, and power-aware software can help reduce the direct cost. Additionally, wireless controllers can also be used around the factory floor to control levels of heating, lighting and high voltage air conditioning, depending on the activity and staffing levels at the time. This can dramatically reduce the overall direct power consumption of a facility.

There are other drivers for cutting the power consumption. Technology developments in the design and manufacture of the transceiver silicon are now bringing wireless devices and modules within the abilities of energy harvesting. This means that power scavenged from the environment – often from vibrations or the temperature of the factory floor – can be used to power a sensor and a wireless link without the need to replace a battery regularly. This allows sensors to be placed in a larger number of locations that were not previously accessible if they needed a cable or battery replacement. This allows equipment to be more closely monitored, and repaired or replaced before a failure brings the manufacturing line to a halt. This use of wireless sensors also brings direct cost savings to industrial customers.

However, there are issues to be addressed when reducing the power consumption of wireless systems, either by reducing the power of the transceiver or using an inherently lower power topology such as a mesh network. There are also several different protocols that can be used depending on the data rates, latency and distance between nodes of the network.

There is also a wide choice of frequencies to choose from, each with their own advantages and challenges.

An increasingly popular range is the unlicensed 2.4 GHz with many different approaches from Bluetooth to Wi-Fi to Zigbee. Standards bodies are all developing low power versions of these technologies, but the band itself is inherently more power hungry than other lower frequencies.

As this frequency is available around the world and used for a wide range of consumer and enterprise applications, the transceivers benefit from the economies of scale for lower prices, and systems integrators have a wide range of devices and modules to choose from. However, this is a congested band of spectrum and sensitive to interference from those same consumer and enterprise systems, as well as microwave and other emissions. Many of the protocols are not optimized for real time data, using a best case algorithm, and can also increase the power consumption if they have to keep re-sending packets as a result of interference.

Other wireless protocols have developed either from a proprietary approach or from existing industrial standards. Cyan Technology for example has developed its own CyNet mesh network for ultra-low power links in the 875 MHz band, while Hart has extended its HARTbus protocol for WirelessHART to link devices, again in a mesh network.

Mesh networks have a key advantage for low power, but have other issues with data throughput and latency. A mesh network uses each node to forward a message onto the next until the message arrives at its destination. This means the power and therefore the size requirement is much lower as a transceiver only has to reach its nearest neighbor. However, the data may have to hop several times to reach its destination, creating increased latency. Each node also has to handle two sets of data traffic, cutting the available bandwidth in half. This can be compensated for with small packets of data from sensors and monitoring equipment. This is not usually an issue, but the latency effect is. WirelessHART uses the 2.4 GHz band to get more throughput and the required latency for real time data still using a mesh network to keep the power consumption and node cost low.

Figure 1: The WirelessHART mesh network.

SP100.11a uses the same protocols as WirelessHART, but is an open standard that supports multiple protocols including Profibus on a single wireless infrastructure. It is designed to support performance levels for factory automation applications, not just process control, and is an ANSI standard.

2.4 GHz ISM

Atmel’s AT86RF230-ZU is a CMOS 2.4 GHz transceiver specifically designed for 802.15.4 and Zigbee, as well as WirelessHART and the SP100.11a protocol. As it operates in the 2.4 GHz space, it can also be used for systems using the 6LoWPAN protocol (see below).

Figure 2: Atmel’s AT86RF230 2.4 GHz transceiver supports Zigbee, WirelessHART, SP100.11a and 6LoWPAN.

The AT86RF230 is a true SPI-to-antenna design with all the RF-critical components except the antenna, crystal and de-coupling capacitors integrated on-chip and can be used with an external microcontroller like Atmel’s AVR family. It integrates the analog radio transceiver and the digital demodulation, including time and frequency synchronization and data buffering, requiring only an antenna, a crystal and four decoupling capacitors. The bidirectional differential antenna pins are used for transmission and reception, so no external antenna switch is needed.

The received RF signal is fed on a differential line through the low-noise amplifier (LNA) to the RF filter (PPF) to generate a complex signal. This signal is converted down by mixers to an intermediate frequency and fed to the integrated channel filter. The limiting amplifier provides sufficient gain to drive the analog-to-digital converter (ADC) and generate the digital signal that is then sampled by the digital baseband receiver.

For transmission, the device uses offset-QPSK (O-QPSK) modulation generated in the digital transmitter and then fed to the power amplifier, while the internal 128-byte RAM buffers the data to be transmitted or the received data. This combination provides a programmable output power from -17 dBm up to 3 dBm, and a link budget of 104 dB from a receiver sensitivity of -101 dBm.

Zigbee

TI’s CC2531 low power SoC for Zigbee is a USB-enabled, true system-on-chip (SoC) solution for IEEE 802.15.4 and Zigbee applications such as industrial control. With a built-in, crystal-less USB interface, it allows USB dongles or USB upgradeable network nodes to be built with low total bill-of-material costs. The CC2531 combines the performance of Chipcon’s RF transceiver with an industry-standard, enhanced 32 MHz 8051 microcontroller, in-system programmable flash memory, and 8 Kbytes of RAM. It also has various operating modes for systems where ultra-low power consumption is required and short transition times between operating modes further ensure low energy consumption.

Figure 3: The CC2531 Zigbee integrated transceiver USB SoC.

Bluetooth

Bluetooth is also now emerging as a viable technology for industrial automation. The latest version of the standard, Bluetooth v4.0, introduced low energy technology to the Core Specification, enabling new devices that can operate for months or even years on tiny, coin-cell batteries.

With this technology being adopted in security and consumer applications from multiple vendors, the cost becomes attractive for use in industrial applications, especially where there is a need for extended range links.

The BISM2 Bluetooth module from Laird Technologies adds a power amplifier and a low noise signal amplifier that can provide Bluetooth connections of up to 1,000 m. The 25 mm x 35 mm module interfaces via a serial data port to a high-performance, integrated Bluetooth antenna. As well as incorporating a fully approved, embedded Bluetooth protocol stack, the module includes a comprehensive AT style interface, which reduces the development time of applications from months to days.

The integrated power amplifier provides 25 mW output with power control, while the low noise amplifier improves the receive sensitivity to better than -84.5 dBm to get the range. This is particularly useful if devices and equipment are spread out across a site.

Figure 4: The BISM2 long distance Bluetooth module from Laird Technologies.

WirelessHART

As specified by IEEE802.15.4, the 2.4 GHz ISM frequency band is divided into sixteen non-overlapping frequency channels. WirelessHART uses a pseudo-random channel hopping sequence to reduce the chance of interference with other networks, particularly Wi-Fi. The channel hopping means the devices do not fix on a channel being used by an IEEE802.11b/g network for any lengthy period of time. Using a Time Division Multiple Access (TDMA) protocol ensures that only one device is talking on a channel at any given time, preventing message collisions and giving a defined latency by using 10 ms timeslots.

WirelessHART uses 2.4 GHz radios with 10 dB amplifiers to allow communication of up to 200 m to the next node, which in turn can serve as a router to pass the message along the mesh. When the full distance is not needed, the node can transmit at a lower power to reduce the chance of interfering with other networks.

It uses Direct Sequence Spread Spectrum (DSSS) to provide about 8 dB of additional gain with the transmission spread over the entire frequency range of the selected channel. Devices with the correct decoding information can receive the data while others see the transmissions as white noise and disregard it. This allows multiple overlapping radio signals to be received and co-exist.

6LoWPAN

At the same time, the Internet Task Force has been working on a higher-level protocol to link sensors and equipment using IPv4 and IPv6. 6LoWPAN, or IPv6 over Low power Wireless Personal Area Networks, has defined encapsulation and header definitions that allow IPv6 packets to be sent over IEEE 802.15.4 networks such as Zigbee so those packets can be used across the wider Internet.

The target for 6LoWPAN is low-power radio communication at lower data rates with very limited form factor, with industrial automation as a key area. By specifying ways to send the same packets across different networks, there is less need for converting the packets, resulting in less complexity, power consumption and cost.

While this is a higher level protocol, it impacts directly on the processing requirements and packet size that are implemented in the wireless network and so impacts on the power consumption of each node.

Other ISM bands

The 2.4 GHz band is not the only frequency band used for industrial automation around the world. Other lower ISM bands have the advantage of more range for the same power budget, giving significant power advantages. These lower bands cannot necessarily carry large amounts of data, but this is not an issue for industrial automation. Using mesh network topologies, devices using these lower bands can achieve dramatically lower power consumption. However, these are not high volume applications and the end equipment cost can be higher as a result. Equipment in these bands also tends to use proprietary protocols, limiting interoperability.

The Semtech XE1205 is a low-power, Zero-IF integrated transceiver, operating in the 433/868/915 MHz ISM bands, that can handle both wide and narrow band links, giving it a key advantage in using protocols for industrial networking and process control.

All the major RF communication parameters in the device are programmable and most of them can be set dynamically. It can be used in both the European (ETSI EN 300-220-1), with 25 kHz channel spacing, and the North American (FCC part 15) regulatory standards.

Semtech’s TrueRF technology eliminates the need for a SAW filter, reducing the component count and system cost while still satisfying ETSI and FCC regulations.

Similarly the Silicon Labs SI1012-A-GM is a low power 240 to 960 MHz transceiver with an integrated 25 MHz 8051 microcontroller and 12-bit ADC.

The transceiver supports FSK, GFSK, and simple on-off OOK amplitude modulation with a receive sensitivity of –121 dBm and an output power of 13 dBm. It uses 18.5 mA in receive mode and just 18 mA for a short distance +1 dBm link.

Figure 5: The Silicon Labs Sl1012 240-960 MHz integrated transceiver

Conclusion

The choice of frequency band and protocol is vital for reducing the power of the wireless link and allowing sensor networks in industrial applications to be powered by energy harvesting. The use of such sensor networks allows for pre-emptive maintenance and closer monitoring of processes and equipment, reducing costs in multiple areas, not just the power bill. The lower ISM bands provide acceptable data rates and low power, especially with mesh networks, but tend to be proprietary, not scaling as well as other approaches.

While the mainstream 2.4 GHz band struggles with interference, new techniques coupled with approaches such as mesh networks and industrial protocols such as WirelessHART and the open SP100.11a protocol are bringing more credibility to this frequency band. The high volume Zigbee and Bluetooth technologies are both coming up with low power versions that, with the high volumes, will drive more usage in industrial applications simply from the lower cost and interoperability that allows system integrators to use a wider range of suppliers. With the device, module and standards groups all pushing lower power, industrial equipment makers have a dramatically wider range of options for reducing the power in their systems and reducing costs in many different ways.

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