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InTech, Jun 2004 by Schmidt, Garrett

Frequency hopping and direct sequencing have their place in the industrial world. Choosing a wireless device is an application-dependent decision. What works in one application may not work in another, and furthermore, what works in one location may not work in another. The low baud rates used with frequency hopping equate to more energy per bit and a greater transmission distance through obstacles. However, there is a compromise with the loss of speed. Direct sequencing supports higher baud rates and thus faster updates and data transfer.

Landfill's answer

Back at the Roanoke landfill the operators knew as the landscape changed, they would have to dig up conduit and place it in a new location, leaving no communications link across the site during construction. They then began to look into the possibility of using a wireless device to bring back the tank level information. They first thought about a licensed radio, but the high cost of a license coupled with the expense of an unnecessarily high-powered radio was a deterrent. A contractor servicing the landfill had already been using some unlicensed frequency hopping radios for several years at other locations, so operators thought that may be the way to go.

The landfill bought several frequency hopping serial data radios for the new system. One radio connected to a Modbus addressable I/O block that the level transmitter fed into, and another connected to the port of a display panel in the main control room. One problem did develop-the operators had to place the tank level transmitter in a pit about 200 feet below the level of the control room. So they had to find a different way to maintain the radio link. Because a repeater radio was necessary as an intermediate point between the tank and the control room, the landfill operators purchased a small solar cabinet in which to mount the repeater radio. They placed the cabinet at the top of a hill next to the pit, which allows the repeater to move around as the landscape of the facility changes. "Labor savings are pretty large since it no longer requires manpower to manually check the levels and create logs," said Steve Charles of Sterling Engineered Sales Co. Inc., who helped work on the project. "Compared to the cost of underground wiring, this installation almost paid for itself in the first couple of weeks it was in operation. Total installed cost was less than 10% of the cost of using conventional underground cable and conduit in an installation of this type."

With the use of data radios, operators can now monitor the tank level information via Modbus polling at the control room. The system has software to automatically log the data transmitted from the remote site, and a panel meter provides an operator with an immediate visual indication of the level status, giving the facility an early warning of any problems with the potentially harmful leachate. "This system enabled the [user] to constantly monitor the potential hazard and do something about problems before any spill occurs. This eliminates the costs involved in cleanup of toxic chemical spills," Charles said. "If a spill were to occur, it would most certainly have the effect of shutting down the operation until the spill could be cleaned up. Maintenance crews no longer have to worry about spills. The system has proven so reliable that very little has been needed to keep it up and running, and using serial data radios to create a Modbus wireless network leaves the system open to adding another site or more as the need arises."