01 January 2004

Wireless solves legacy systems

Each new communication protocol that comes along creates more legacy equipment.

By Timothy Cutler

The demand for low-cost flexible communications on the factory floor and in industrial environments has given rise to the use of wireless technologies.

In particular, the ability to eliminate or reduce costly wiring and conduit in industrial automation applications has created tremendous interest in wireless solutions to replace wire.

But despite some of the hype, it is not quite as simple as unplugging a wire and connecting a wireless device. Wireless transmissions have their own characteristics, which one must consider when applying wireless in industrial automation applications.

One must consider issues such as latency, transmission retries, immunity to noise, and jamming before deploying wireless in industrial automation applications.

Here we'll look at the specific communication needs of industrial automation applications and at the various wireless technologies that are available.

Another issue facing engineers is how to incorporate wireless solutions for various legacy communications without having to scrap existing equipment. We'll also look at this issue and provide some options for dealing with it.

Regardless of the solution chosen, proper range of frequency (RF) planning and installation techniques are key to a successful wireless deployment.

BUILD IN ERROR DETECTION

TERMINOLOGY Ethernet 10BaseT - an implementation of IEEE 802.3 standard that operates over unshielded twisted-pair cable at 10 Mbps OFDM - orthogonal frequency division multiplexing, a technology for the next generation of high-speed wireless data products and services ZigBee - low-speed, low-data rate, low-cost wireless technology

There are two environmental effects that impact wireless communication performance: multipath fading and interference. Multipath fading occurs when multiple copies of the transmitted signal, generated by the signal bouncing off reflective surfaces, arrive at the receiver at about the same time but out of phase, thereby reducing the strength of the received signal. The magnitude of the fade and the range of the frequencies affected depend on the geometry of the area where the system resides. An effective approach to mitigate the effects of multipath fading is to use as wide a range of frequencies as possible.

Interference occurs when unwanted RF energy exists in the frequency band of interest. Sources of interference include other wireless devices, lighting systems, arc welders, and electric motors. When the unwanted noise results in the signal-to-noise ratio falling below the minimum needed for a given bit error rate, higher bit error rates will be encountered up until communication is no longer possible and the RF system is considered jammed. As with multipath fading, an effective technique to reduce the susceptibility to jamming is to use as wide a range of frequencies as possible.

All communications systems have some latency. In wireless systems, the latency is often more pronounced because of the change in medium. That is, data from a wired system converts to RF energy and transmits to a receiver, which then converts the data back to a wired medium.

Additionally, wireless systems tend to be serial in nature, where wired systems can be serial or parallel with equal ease. Latencies for wireless systems can be relatively short, on the order of a few milliseconds. But the best-case latency is not the figure of interest.

Rather, the worst-case latency is the specification that is important. Due to their very nature, RF systems have more failed communication attempts than wired systems. Because of this characteristic, RF systems build in automatic error detection and retransmission schemes to handle instances when initial attempts fail.

COMPELLING CASE FOR WIRELESS

Communications between automation components take place at several levels. At the lowest level is intramachine communication. Above that are the intermachine communications and finally the intrafacility communications. Each class as described here has similar range, latency, and protocol implications for wireless communications.

Intramachine communications take place between components on a single piece of production equipment, be it a computer-numerical-control milling machine or a mixing machine. The distances between components are typically very short, on the order of a few meters. Tolerance to latency between these components is very low, as many are fast running state machines and the communications control precise, fast moving operations. These types of communications use low-level protocols such as DeviceNet. Given the short distances and low latencies required, intramachine communication is not a good candidate for wireless communications. In fact, replacing these cable runs with wireless devices will usually be more expensive than wired options.

Intermachine communications take place between different pieces of production equipment. The ranges involved can be a few meters to a few hundred meters. Because the product is transferring between production equipment, this communication can tolerate longer latencies than intramachine communications. Especially when mechanical operations are taking place, latencies of a few milliseconds to as much as several tens of milliseconds are tolerable. Even though this communication is controlling processes, longer latencies are okay.

The data transmitting in these instances is relatively low level and is relatively short in length. A few bytes to a few tens of bytes are typical packet lengths. This type of communication uses protocols such as Modbus, Profibus, fieldbus, and more and more, Ethernet. Because production lines can stretch for hundreds of meters, wireless communications can be a good fit to replace wires in these instances.

Intrafacility communications encompass functions such as downloading control programs, collecting historical data, and monitoring production processes in real time. This is the most latency tolerant of the industrial automation communications. However, the amount of data transmitted and the ranges to be covered are greater than the other communications. At this level of communications, bridging from an industrial automation communication protocol to a more general protocol is commonplace, as production information becomes available to nonproduction personnel and manufacturing information systems.

The protocols encountered here are similar to the intramachine protocols of Modbus, Profibus, and to an even greater extent, Ethernet. Because of these characteristics, it is this class of industrial communications that presents the most compelling case for wireless systems.

WIRELESS TECHNOLOGIES

There are several different wireless technologies that are usable for industrial automation communications. They include 802.11a,b, and g, Bluetooth, and proprietary solutions. ZigBee is a new, developing standard to address some of the industrial automation communication needs.

The 802.11 standard came about to allow interoperability between wireless networking devices. Subsequent versions of the standard, a, b, and g, have been put forth to reflect advances in technology. The 802.11 standards were developed for in-office wireless Ethernet networking as a wireless alternative to 10BaseT and then 100BaseT Ethernet networks. As such, the emphasis was on maximizing data throughput. Range was a much lower priority. In addition, because the intended environment was the office, concerns about RF noise and interference were secondary and rightly so. The typical application envisions employees with notebook computers being able to move around a corporate office and have network connectivity without finding an Ethernet jack. Distance from an access point is typically on the order of ten meters or so. The 802.11b standard has been wildly successful in both office and home wireless networks. Because of the large volume of units sold, the cost of 802.11b products is very low, drawing interest from many different application areas including industrial automation.

Standard 802.11b has not been as successful in industrial environments as it has in office and home environments for several reasons. First, industrial environments are noisy RF environments and have severe multipath fading. The 802.11b standard employs direct sequence spread spectrum technology. Although direct sequence offers high data rates, it is susceptible to jamming and fading. While 802.11b has an 11-megabits-per-second (Mbps) maximum data rate, the technology slows down the data rate to improve receive sensitivity in hopes of achieving better range. But simply slowing down the data rate does not improve immunity to jamming. The 802.11b standard occupies about 22 megahertz (MHz) of spectrum out of the 83.5 MHz available. Thus its ability to avoid frequency selective fading and jamming is somewhat limited.

Standard 802.11b has found application on the factory floor as a wireless monitoring system for automation systems in short-range applications. In a typical application, a manufacturing engineer with a personal digital assistant (PDA) or other handheld device approaches a piece of production equipment and from a range of a few feet, is able to query the status of and manually control the equipment. Even in these applications, the requirement of the production equipment to support 802.11b means sufficient intelligence to handle the 802.11b protocol stack. This is a different protocol stack than a TCP/IP-Ethernet-protocol stack, which has also limited the deployment of 802.11b. Add in the ability to have just three 802.11b networks in a location and the well-publicized security issues, and the limitations of 802.11b are even more apparent.

Standard 802.11a is the 802.11b protocol operating in the 5.2-gigahertz (GHz) band. As such, it really offers no benefits for industrial automation over 802.11b. In fact, 802.11a is encountering difficulty penetrating the office wireless networking market due to a lack of a reason to change from 802.11b.

Standard 802.11g adds higher data rates to 802.11b, boosting the maximum data rate to 54 Mbps by using orthogonal frequency division multiplexing (OFDM) technology. From the above discussion on RF theory, it is apparent that the higher data rate will limit the range of 802.11g. Even the use of OFDM technology cannot overcome the reduced receive sensitivity of the higher data rate.

Bluetooth technology is another low-cost radio technology. Bluetooth was created for the personal area network (PAN). Its original goal was to provide wireless connectivity between cell phones, PDAs, and notebook computers. Other applications include wireless headsets and keyboards. Unlike 802.11b, Bluetooth uses frequency-hopping spread spectrum technology. Bluetooth radios change frequencies, or hop, every 600 microseconds or 1600 times per second. Transmitting at 1 Mbps, Bluetooth's protocol is limited to networks of eight devices over ranges of ten to thirty meters. There are three classes of Bluetooth radios, each with a different transmit power. Class 1 devices transmit at 100 milliwatts; Class 2 devices transmit at 10 milliwatts, and Class 3 devices transmit at 1 milliwatt.

Because frequency-hopping technology is inherently better against fading and jamming, Bluetooth has the potential to provide reliable, short-range wireless communications in industrial environments. Although attractive due to its low cost, devices with the higher transmit power trade the extra power for cost. Bluetooth should be a solution for the short-range monitoring application that 802.11b serves, as well as the intermachine communications. A potential limiting factor is the need to handle the Bluetooth protocol stack. While the Bluetooth stack is smaller than the 802.11b stack, it still requires some 250 kilobytes of code space. Another limitation is the inability of Bluetooth to handle industrial automation protocols such as Modbus or Profibus. Nevertheless, designers are currently integrating Bluetooth into some programmable logic controllers and sensors.

There are a number of proprietary wireless solutions, some of which are well suited for industrial automation applications. The advantage proprietary solutions have is that they can be optimized to serve relatively small-volume applications. The wireless industrial automation market is very large, but relative to cell phones and office and home networks, the volumes are lower. Standards usually satisfy a number of vendors and applications to create large volumes to interest vendors such as chip makers. Proprietary solutions do not have the same requirement, and one can optimize them to a higher degree. The trade-off, of course, is somewhat higher cost and sole sources.

Proprietary solutions are available with data rates ranging from 9600 bits per second to 1+ Mbps. Frequency hopping and direct sequence solutions are available. An important consideration in industrial applications is a wide operating temperature range. Products with industrial temperature ranges are readily available.

With the range of data rates, transmit powers, and receive sensitivities, solutions that fit the needs of both intermachine and interfacility communications are available from multiple vendors. Typically these solutions have simpler protocols, even to the point of accepting unformatted serial data requiring no protocol at all. In addition, products are on the market that direct accept and transmit Modbus, Profibus, and Modbus TCP protocols, allowing installed protocols to be transmitted seamlessly.

ZigBee is a new wireless networking standard built around the 802.15.4 standard. It is a low-cost, low-power, and low-data rate wireless PAN solution for applications ranging from home automation and building automation to remote controls and industrial automation. Battery operation is a key goal of the ZigBee standard.

ZigBee is based on direct sequence technology and will offer a 250-kilobits-per-second (Kbps) data rate in the 2.4 GHz version, 40 Kbps in the 900 MHz version, and 20 Kbps in the 868 MHz version.

ZigBee is very new, so its ultimate performance and potential is still unproven. It may turn out to be an option for intermachine communications. It does not seem that it will be able to handle the range and duty cycle for interfacility communications. The issue of existing automation protocols is also present with ZigBee.

CONVERT WIRELESS TO PACKETS

There has been a big move in industrial automation to connect automation devices via Ethernet connections. The benefits are numerous: low-cost products, multivendor interoperability, protocol transparency, and the ability to tie production-floor systems seamlessly into corporate networks.

The problem that arises is what to do with the existing systems. Should we replace legacy products even though they are still functional? Should Ethernet converters be used to convert Modbus or Profibus outputs to Ethernet? Either option is expensive.

Fortunately, there is a wireless option that can solve this problem in a cost-effective manner. There are industrial access points on the market that can accept unformatted data, or Modbus or Profibus packets from a wireless remote, and convert the data into Ethernet packets to transmit over an Ethernet network.

To an application running on the network, the remote devices appear as nodes on the network. In one solution, the remote devices get an IP address or a port number under the IP address of the access point. These access points can be used with other wireless Ethernet access points to provide Ethernet access to all points on a manufacturing floor.

Wireless communications offer many benefits in industrial automation applications. However, there are some parts of industrial automation communications are best served by wired communications. While wireless makes sense for intermachine and intrafacility connections, intramachine communications is not a good candidate.

There are many products on the market, but not all are well suited for the demands of the industrial environment. With a good understanding of the communication needs and with careful planning, the benefits of wireless communications are eminently possible. IT

Behind the byline

Timothy Cutler has degrees in electrical engineering and business. He is a vice president at Cirronet Inc. and holds two patents for microprocessor-based design. Cutler has written about industrial wireless networking technology before for InTech. Read his article "Speed vs. Distance" at www.isa.org/intech/WirelessCutler. Write him at tcutler@cirronet.com.