The Wireless I/O World

Just because you can't see large sections of an industrial network doesn't mean they aren't there. Wireless networks need much of the same TLC as their hardwired and twisted-pair counterparts, even though caring for a wireless system means different tasks, tools, and an awareness of the differing physics of wireless communications. This article includes additional Online material.

By Jim Montague February 1, 2004
  • Simpler, but slower

  • A pure wire replacement

  • Handling interference

  • Future congestion

Exclusive: Opto 22 kit makes GSM easier

Just because you can’t see large sections of an industrial network doesn’t mean they aren’t there. Wireless networks need much of the same TLC as their hardwired and twisted-pair counterparts, even though caring for a wireless system means different tasks, tools, and an awareness of the differing physics of wireless communications.

Still, visible or not, an industrial network’s mission remains the same: move signals and data about physical conditions accurately, quickly, and securely to where they can be analyzed and used to make decisions, and bring back instructions and further queries if needed. And, as with any industrial network, you must know what you need and want to accomplish, and then find out what your application and environment can handle, before you can decide what type of wireless technology to use.

Wireless I/O basics

One of the most promising wireless methods being implemented by some users is wireless input/output (I/O), which basically collects 4-20 mA analog or other raw signals from I/O points, and sends those data via radio to a central processing device, such as a PC or PLC. This method doesn’t use wireless local area networks (WLANs) or fieldbuses, which means it’s slower and proprietary, but it’s also simpler to implement than a WLAN-based network. This simplicity makes wireless I/O easier to apply and also means no cooperation is required with existing IT infrastructures. In addition, wireless I/O users save on the usual materials and labor by adopting wireless, plus they save even more time and money by not having to configure and program a WLAN or fieldbus.

As a result, wireless I/O is becoming increasingly popular in retrofit and expansion projects, often to help reduce expenditures, according to a recent report, "Wireless I/O: The Electricians’s Radio," by Harry Forbes of ARC Advisory Group. ARC says most wireless I/O systems are used for data acquisition, but some closed-loop control applications have been successfully deployed, though the effective bandwidth limits their control speed. Wireless I/O can carry analog and discrete sensor signals, generally at 1-20 Hz, which is enough for many control applications. Wireless I/O’s main limitation is the smaller number of signals that can be transmitted by each wireless I/O. As point counts increase, WLANs become more attractive, says ARC.

"Wireless I/O is still very new in the psyche of plant-floor engineers, even though it’s a simple idea. The concept tends to throw people, so we call it wireless I/O that doesn’t need a twisted pair," says Graham Moss, Elpro Technologies’ GM. "Even the word ‘wireless’ remains a bit frightening, most people no longer assume the worst and are beginning to realize they could use wireless in many of their applications."

To help potential users and ease implementation, several manufacturers have developed solutions that integrate wireless I/O with other sensing and networking capabilities. For example, Accutech’s Wireless Instrumentation System combines integrated, calibrated sensors; five-year-battery-powered field transceivers for temperature, pressure, acoustic, and multi-input (analog and discrete); a base radio transceiver with RS-232, RS-485 Modbus, and 4-20 mA output options; and software for data management and exception reporting. These units are approved for use in hazardous locations. In fact, New York-based Consolidated Edison reports that its technicians were able to install Accutech’s system at metering stations for the city’s steam distribution network in 45 minutes, compared to two hours previously.

In addition, Honeywell Process Solutions just launched its XYR5000 wireless transmitters, which monitor pressure, temperature and ultrasonic noise. They also include an analog input interface for adding wireless capabilities to 4-20mA devices, and wirelessly transmit their measurements to a base radio connected to a control system or data acquisition device, such as a recorder or PC. Each base radio accepts the signals of up to 50 transmitters. XYR5000 enables process and asset monitoring by eliminating wiring costs, and allows measuring on rotating equipment.

Phoenix, Omnex partnership

For the past couple of years, Phoenix Contact and Omnex Control Systems have jointly focused their wireless I/O efforts on plant-floor users, who are usually more concerned with securing a signal than building a network. Phoenix initially investigated frequency-hopping spread spectrum (FHSS) to wirelessly perform the I/O-based gathering and transmitting of its wired 4-20 mA signal conditioners.

"Process applications are always measuring and transmitting temperature, pressure, level and flow, usually with 4-20 mA analog signals that indicate status, or with digital alarm signals," says Davis Mathews, Phoenix’s instrumentation and wireless marketing manager. "So, we and Omnex came up with a device that would wirelessly accept one 4-20 mA signal or two digital inputs, but one that required no set up or programming. We took a simplistic approach on purpose, though at our level there isn’t much fear because we’re only making one device wireless, rather than a whole fieldbus network. This allows users to try one or two wireless I/O devices, and add more when they’re ready."

Using Omnex’s Trusted wireless I/O and data radios, Phoenix developed its Measurement Control Regulation-Radio Analog Digital (MCR-RAD) technology, which transmits small packets, usually 16-18 bits at 96 kbps and at relatively high power, up to the 1-watt maximum allowed by federal regulations. This allows MCR-RAD’s signals to penetrate objects better and transmit over longer distances. The partners’ first product was RAD-UD (uni-directional), which accepts one signal. In response to user requests, they next produced RAD-BD (bi-directional), which accepts multiple I/O signals, up to 33 analog or 66 digital or a mix of the two.

To eliminate cables between serial nodes and point-to-multipoint networks, Phoenix and Omnex also released RAD-Data Series (DS) in January 2004. "Users will be able to put a RAD-Data Bus at each station, and connect I/O points directly to them, which will eliminate the need for a PLC at each station in applications where the PLC is usually surrounded by multiple I/O points," says Mathews. "This will allow users to make their controller the master in a Modbus network. Meanwhile, the stations will automatically become slaves in this network because they will look like PLCs to the master controller. Once this is done, Modbus will register analog and discrete signals like it would in a normal, wired PLC network. This is basically a wireless way to do some traditional networking, but it’s possible without having to change what’s already been done."

Managing tanks, water treatment

To reduce alarms and communication shutdowns, which interfered with vital tank level data during product transfer, Marathon Ashland Petroleum recently installed Phoenix and Omnex’s MCR-RAD radios to help manage five tanks at its remote tank farm and 19 tanks at it main farm in Louisville, KY. The two farms are located a mile apart, and are used to store and transfer gasoline, oil, and jet fuel. The tanks are hardwired to two remote terminal units (RTUs) at each farm, and communicate continuously via wireless I/O in a 30-second loop cycle.

"We’d been using radios for about four years, but we were getting five or six alarms per day, each of which shut down communications for 30-40 seconds. This was driving the terminal manager nuts because it’s so important to know the tanks’ levels when transferring product," says Donnie Shultz, Marathon’s electronic and instrumentation technician. "We first field tested Phoenix’s radios for three months, and they never missed a lick, even though we used the old antennas. This system took me about 10 minutes to get up and running because the software for addressing was all point and click, and now we have better packet data and 19.2 kbps communication speed. We estimate that we’ve saved between $70,000 and $80,000 by using the new radios, and we’re going to start using them in other applications.

"I was very reluctant about using wireless I/O because I generally think hardwiring is better. We were frustrated and depressed with our first radios, but the new ones took off running and changed our whole outlook. The best thing you can do if you’re looking at using radios is to get a field trial, and really try them out."

Similarly, Elpro reports that CDR Flint Ink recently encountered a problem with analog signals between its aeration and wastewater treatment plants (WWTPs). Oxygen reduction potentional (ORP), pH, temperature and flow signals from the aeration plant were connected to a paperless chart recorder in the WWTP via multi-pair cabling, which was strung close to high-voltage power wires. This placement caused surge and ground loop problems, and frequent failures of the chart recorder and aeration plant transmitters. However, it was difficult to justify replacing the signal cabling, even though it led to high maintenance costs and long outages of the remote monitoring.

To solve the problem, Flint installed Elpro’s 905U wireless I/O system, reportedly at a fraction of the cost of cable replacement. Analog signals at the aeration plant are connected to one 905U unit, which transmits signals to another 905U unit at the treatment plant. And, because the 905Us are two-way transceivers, Flint also connected and established a return signal from the WWTP to the aeration plant, which allows remote control of a 10-in. throttling valve. After proving its wireless system, Flint decided to use 905U’s peer-to-peer features, and installed a third 905U unit at the firm’s boiler control center. As a result, analog signals from the aeration plant are transmitted to the WWTP and the boiler, which increases the plant operating flexibility.

"The problem with modems is that they’re not very radio-friendly because they’re too fast. Fieldbuses expect an immediate response when they send data, but a radio’s 5 ms average response time can cause some high-speed protocols to time out. This is why wireless gateways, or wireless protocol converters, evolved over the past few years. Gateways make data across a radio the same as across a wired network, so all you have to do is modulate the data onto the right frequency," says Moss. "On the databus side, the gateway responds to the I/O device, so the PLC is happy because it got its response. Meanwhile, the gateway converts the data into another protocol, and transmits it via wireless." Moss adds that wireless gateways operate at 900 MHz and 115 kbps, or at 2.5 GHz and 250 kbps.

Handling interference

Wireless I/O’s primary disadvantage is that it can be more susceptible than hardwiring to electrical noise and interference, typically caused by obstacles or machinery. However, users and standards organizations already have alleviated many of these problems.

For now, spread spectrum solves many interference and security difficulties by dividing its signal among many channels in a preset pattern, and then reassembling or correlating them from identical portions of apparent noise. Likewise, digital packet switching organizes signals more efficiently for faster, high-volume transmissions. The two primary types of spread-spectrum are FHSS, which uses predetermined timing on each frequency by transceivers on each end to prevent interference, and direct sequence spread spectrum (DSSS), which uses numeric codes that identify units intended to communicate with each other, preventing unauthorized communication on the network.

However, some users report that spread spectrum isn’t particularly secure or reliable because the 900 MHz-to-5.4 GHz Industrial, Scientific and Medical (ISM) bands it uses and many wireless devices remain unlicensed and legally unprotected from interference. So, while spread spectrum is easy to intercept, its receivers are more subject to intermodulation distortion, blocking, de-sensing and other narrow-band phenomena.

While spread spectrum uses a technique called process gain to reduce narrow-band interference, some users add that manufacturers apparently don’t care about this, and spread signals at minimally required rates to boost power up to regulatory limits. These spreading rates are sometimes applied below the data transmission rate, which means they have no process gain, and are susceptible to all interference on the band. Process gain is similar to the quieting effect of a wide-band FM signal, and users need to ask about it before implementing a wireless system.

To avoid these and other potential problems, wireless’ supporters say users must test and evaluate wireless solutions in their own applications and facilities because each has its own characteristics. Most wireless manufacturers will gladly bring solutions to users for tryouts.

"Wireless also means that users don’t have to rip and replace existing networks. They can implement wireless slowly, leave their former network as is, and simply add a layer of mobility they didn’t have before," says Renee Robinson, Wonderware’s visualization products manager.

Better SCADA; less wiring

To help Normal, IL’s water department replace its outdated RTU-based SCADA system, system integrator Rick Caldwell of SCADAware recently implemented an innovative, PC-driven, FHSS solution. As a result, Normal now uses a primary and secondary server in its water treatment plant for HMI and PC-based control. These computers collect and monitor data from all of Normal’s wells, tanks, and lift stations via a Locus wireless serial network, with data rates of 57.6 kbps, compared to the previous system’s 0.3 kbps.

Meanwhile, Wago I/O programmable field couplers allow plant staffers to make adjustments and activate controls in their system. Also, a Sixnet Ethernet-to-serial converter is used to convert incoming serial data to Ethernet, so the data can be accessed on the plant’s LAN. SCADAware’s software features a simple-to-use graphical user interface, allowing easy and immediate access to data as well as visual indicators of possible problems. These improvements reduce the department’s drive and response times and aid diagnosis, maintenance, and expansion of the SCADA and water distribution system.

Similar savings were achieved by Akzo Nobel Surface Chemistry LLC, which recently needed to add six temperature transmitters to monitor and track raw material in a storage tank. However, the tank’s location made it impossible to route wiring to the transmitters without adding substantial costs and delays. In fact, the company estimated that the labor costs of preparing the path for the traditional 4-20 mA wires would have been about 10 times the cost of a wireless transmitter/receiver set, according to Marc Ayala, Akzo’s control system specialist.

Consequently, Akzo opted to use wireless to reach the tank, and integrate it with Emerson Process Management’s DeltaV automation system in just a few days, rather than the two weeks that would have been required to install conduits for the wiring. Akzo also selected a wireless monitoring solution from Omnex. So far, Akzo has installed about 10 of these receiver/transmitter sets, and has used them in another application to perform closed-loop control on a slow acting temperature control loop.

Future congestion, standardization?

Reliability, security, and expandability are three present concerns of wireless I/O in process plants, but radio spectrum congestion may be the big headache in the future. This hasn’t been a problem so far, simply because there aren’t enough users to cause congestion. However, Moss says wireless devices are multiplying so quickly that congestion will definitely become a problem, and that the same will happen with industrial wireless products.

For instance, plants with 10 or fewer wireless applications now, such as wireless I/O or radio modems connecting PLCs, are expected to have 100 to 200 by the end of 2010, according to Moss. The problem is further exacerbated by "spill-over," in which wireless transmissions don’t stop at the plant’s boundary. Relatively soon, each wireless transmitter will have to compete with others in the same plant, not to mention others in nearby facilities.

Fortunately, developers are seeking solutions based on the fact that wireless communications often use the same type of polled or timed update messaging as PLCs. Moss adds that wireless developers can lessen congestion issues by using smarter messaging protocols. However, the difficulty with polling or timed updates is that messages need to be sent continuously to get good time response. By changing to exception reporting, which is sending a message only when there is a change, the total traffic density of most wireless systems can be reduced 50 to 100 times. Of course, this change introduces new problems with network control, because each node must be able to act as a network master.

In addition, open standards for wireless are expected within the next 5-10 years, which should lessen the potential incompatibility of proprietary radio equipment. Similarly, increased standardization may allow RF to be replaced where appropriate by wireless WANs (WWANs) that use cellular telephone networks and Internet protocol (IP) to move data, according to Benson Hougland, Opto22’s technical marketing director. "It helps to remember that wireless is just another network or an extension of a wired network," he says.

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To improve analog signals between its aeration and wastewater treatment plants, as well as its boiler control center, Flint Ink used a 905U wireless I/O system with two-way transceiver units from Elpro Technologies.

Normal, IL’s municipal water department uses a 56.7-kbps Locus wireless serial network to collect data from and monitor all of the town’s wells, tanks, and lift stations. Wago I/O programmable field couplers allow treatment plant staffers to adjust and activate controls in the system, and a Sixnet serial-to-Ethernet converter allows data access on the plant’s local area network.

Using mesh networking to make RF wireless reliable

How can you convince plant-floor veterans that radio frequency (RF) communications can be reliable when cell phones regularly drop connections? Robert Poor, Ember Corp.’s co-founder, chairman and CTO, says the answer is mesh networking.
“For example, assume there’s temperature sensor on the plant-floor that’s sends out a reading once per minute, and that we’ve decided to use an Ember-enabled wireless link to get the data from sensor to controller,” says Poor. “Make the extremely conservative assumption that the wireless link is only 90% reliable—that one in 10 readings are lost. In practice, the reliability would be much higher. With an ordinary star topology, which is used by IEEE 802.11, this means one reading will be lost every 10 minutes.

“However, with two wireless neighbors listening, as in a mesh network, their reliability goes up to 99% or one reading lost every 100 minutes. With four wireless neighbors listening, their network’s reliability goes up to 99.99% or one reading lost per week, and six neighbors have 99.9999% reliability, or one data packet lost every 2.6 years!”
To calculate this reliability, Poor adds, let “p” represent the probability of sending a packet, in which 0 = nothing gets through; 1.0 = perfect transmission every time; and 0.5 = half the packets dropped. Next, let N be the number of neighbors around to relay the packets. Consequently, the total probability of success can be calculated using the equation: 1-(1-p)^N. “So, you can see that increasing N quickly increases the network’s reliability,” says Poor. “In the case where p= 0.9 or 90% reliability, you‘add a nine’ for every neighboring node added. With a more realistic initial value of 95% link reliability, the odds get better even faster:

Securing wireless networks

Users can make wireless networks as secure as they desire, just like with wired networks. They can use the basic built-in security features of a wireless access point (AP) to protect connections, and then increase security with more sophisticated technologies, such as virtual private networks (VPNs). Invensys Wonderware’s recent white paper, “Tablet PCs in Industrial Automation,” includes recommendations for securing wireless networks:

  • Plan the coverage of your AP. When possible, plan coverage of the area of interest to extend from the AP to the external walls and windows. Locating an AP near an external wall extends coverage outside facilities, where strangers could have access to it.

  • Change the service set identifier (SSID) from the default.

  • Disable SSID broadcasting. Broadcasting advertises the availability of your network, identifying it by name. An obvious name can make your network a target over other available networks.

  • Change administrator password from default. This prevents intruders from tampering with your settings.

  • Enable wired equivalent privacy (WEP) security. Although not perfect, this deters all but the most dedicated hackers.

  • Use the highest level of WEP encryption available in your AP, preferably 128-bit encryption over 64-bit encryption.

  • Change the encryption key periodically.

  • Limit the number of connections. For example, if you only need to support five machines on an AP, set the maximum to five. If you attempt to connect and are denied the connection, this may be an indication that an unauthorized computer has gained access to your Wi-Fi network.

  • Disable DHCP and use fixed IP addresses. This increases the difficulty for a potential intruder to gain access to your network by not providing an IP address automatically. However, this is only practical in small networks.

  • Use media access control (MAC) address filtering. It’s possible to configure an AP to accept connections only from devices whose MAC address belongs to an access list. However, this is only practical in networks with few wireless devices.

Four tasks before going wireless

Besides getting vendors to test and evaluate their wireless solutions in your specific application, there are four major tasks that users should perform before trying to implement a wireless I/O or other wireless network, according to Davis Mathews, Phoenix Contact’s instrumentation and wireless marketing manager.

  • Make sure to start any new, first-time wireless network in a non-critical application that can’t cause a significant failure.

  • Identify any signals that you’ve always wanted to get from Point A to Point B, but have always been too costly for a traditional solution.

  • Do an overall site survey, and check the actual site where you want to install your wireless equipment.

  • Look for a relatively slow, probably process, application for your first wireless network.

Exclusive: Opto 22 kit makes GSM easier

Opto 22 introduced the OptoGSM I/O Portal Starter Kit, said to contain everything required to quickly and cost-effectively deploy wireless machine-to-machine (M2M) solutions. The kit includes 24-hour access to a secure Internet portal, so customers can monitor and manage machines, equipment, and systems; establish alarms; and manage data. It also includes sensor interfaces to the connected equipment, wireless communications, and competitive rate plans for data transfer. Opto 22 calls the system "ideal for those seeking to perform simple monitoring and control of physical business assets and environments." It also reduces "risk of implementing an M2M solution since no large capital outlay, complicated integration, or long-term contracts are required."