Likewise, regional water management association Aggerverband recently automated operations at its 200 x 41-meter Genkel Dam and its 8-million cubic-meter drinking water reservoir in Gummersbach and Meinerzhagen, Germany, with help from water management contractor HST Hydro-Systemtechnik in Meschede, Germany. To monitor water withdrawal, water levels, temperature, evaporation, wind velocity, and constantly check physical dam shifts in relation to its substrate, Genkel's new process control system needed to link 12 stations, including a "measuring raft" located in the middle of the reservoir and Aggerverband's central control room that monitors and controls several dams.
Consequently, 11 of the 12 stations implemented BC2000 bus terminal controllers with PLC functions from Beckhoff Automation, which were networked via a 1.5-km Lightbus ring and Beckhoff's ADS TwinCAT data transport protocol. However, reaching the raft required using Beckhoff's KN6551 wireless terminal, a radio link based on IEEE 802.15.2 and a solar power supply.
"These terminals, which can be integrated easily into the bus terminal system, use the 2.5 Ghz band," says Frank Huetger, HST's IT systems product manager. "A directional antenna ensures a stable radio link."
Revising When Needed
Because wireless is still a relatively new technology in many places, those reluctant to use it in the first place are quick to blame and dismiss it again when it has growing pains. So what wireless really needs most of all is a willingness to sometimes rethink, readjust and renovate wireless components, designs and layouts to achieve stable communications.
For instance, St1 Energy's petroleum refinery in the harbor area of Gothenburg, Sweden, uses Emerson's Rosemount tank gauging equipment and a mix of wired and wireless components for level and temperature measurements, but found it had no direct access from its control room to its Smart Wireless Gateway, which is based on the WirelessHART (IEC 62591) standard and collects data from the wireless field network. Ironically, to monitor their wireless network's status and configure devices, St1's technicians previously had to go into the field and investigate.
So St1 implemented a wireless link from its control room to the gateway via Emerson's Wi-Fi-based Pervasive Field Network (PFN) solution. PFN at St1 includes three of ProSoft Technology Inc.'s RadioLinx industrial hotspot units indoors connected to three remote, outdoor panel antennas. The first hotspot is connected to the gateway; the second serves as a repeater to achieve line-of-sight and relay data; and the third is in St1's control room, where it creates a Wi-Fi zone (Figure 3).
This layout enables operators to access the refinery's wireless field network from anywhere in the control room, via laptop PCs equipped with AMS Wireless Configurator, AMS Wireless Snap-On and/or TankMaster software. In fact, Snap-On software gives users a graphical overview of the tank farm, devices on the network and their status.
Likewise, Sunstate Cement Ltd. in Queensland, Australia, recently completed an $85-million expansion to increase capacity to 1.5 million tonnes, including installing a wireless local area network (WLAN) between three ship unloaders and four access points. However, the WLAN malfunctioned, and the unloaders couldn't communicate back to the main controller.
"At first, Sunstate Cement thought the problem was with Siemens' wireless equipment. They had very low signal strength, even from only a few meters away from the WLAN's access points, and there were communication dropouts between the main controller and the three RTU PLCs on the unloaders," says Falk Holman, Siemens Australia's networks product manager. "The first thing I found was that the initial installer hadn't conducted any WLAN channel planning, and there were so many collisions and interferences because the traffic wasn't coordinated between the four WLAN access points."
Holman explains that access collisions can be avoided by conducting space division multiple access (SDMA) or frequency division multiple access (FDMA) procedures. He adds that WLAN simulation software like Siemens' Sinema E can improve communication performance before installation.
"We also discovered that plant staff had installed 5-GHz antennas, but the network was configured for a 2.4-GHz frequency range," explains Holman. "Also, antennas on the access points were installed below structures and were too low and adjacent to solid concrete walls. So, after selecting the 5-GHz range for the network, we reallocated channels to each of the access points and the corresponding loaders." Holman adds that channels available for the network ranged between 149 and 165 (5,745 MHz and 5,825 MHz) for Sunstate's outdoor application. However, because there's a 2-MHz overlap between channels, Holman arranged the channels in an ad hoc order to prevent interference.
A second round of tests with a Cisco spectrum analyzer showed slightly weaker signal strength (-70 dBm or 48%), but much greater signal quality.
"Unfortunately, we still experienced communication dropouts," says Holman. "So, what else was wrong and why was it only 48%? Well, we found the installer used omni directional antennas, which can have high gain but a very small vertical lobe. So, when the loaders were parked, it was impossible for the antennas to have line-of-sight due to a big difference in the height of the mounting positions. Also, we noticed the antennas were mounted too close together. To use antenna diversity properly in a 5-GHz radio cell, we recommend using 20 times the wavelength as the clearance. And, locating two access-point antennas under a belt conveyor didn't help either."
Consequently, Sunstate raised its antennas from 2 meters to 5 meters and relocated redundant cable. "We found 5 meters of excess cable stuffed into a 1-meter channel, which can cause cable attenuation," adds Holman. "Luckily, we used this extra cable when the antennas were relocated away from the overhead structures and raised higher.
After making these corrections and using Siemens iPCF rapid roaming functionality, Sunstate's wireless system is operating as originally designed. In fact, Holman reports it's been running without interruption for more than 12 months without any replacement or added hardware.