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01 June 2004

Top of the heap

Going wireless helps clean up a landfill.

By Garrett Schmidt

Operators of the Roanoke, Va., regional landfill had a problem. The solid waste disposal facility needed to monitor the leachate storage tank level at a remote point of the landfill. The problem was operators had to constantly monitor the tank because the runoff of leachate is a primary concern due to Environmental Protection Agency monitoring. In addition, the runoff could cause considerable damage to the nearby property.

Several times a day an operator had to drive a half mile to the tank, take a manual reading, and log the data manually, because an old buried line had failed.

Because of the distance and constantly changing landscape of the landfill, laying conduit and running wire was expensive and impractical.

The landfill operators thought wireless, but they had to educate themselves on the various forms of wireless they needed for their operation.

The word "wireless" evokes several different emotions from people in an industrial context. Some get excited about the opportunity to employ a new technology, while others believe a copper wire is the only way to get information from point A to point B in a reliable manner. Trust it or not, wireless is here to stay. The questions are, "is wireless technology right for my application?" and if so, "which wireless technology is right?" The first step toward answering these questions is to determine exactly what the application is. The radio frequency (RF) environment, distance you need to transmit, type of information, amount of information, and alternatives all affect the final decision about the appropriate technology.

The narrowband interference is de-spread at the receiver, leaving the original user data intact.

Options

Several mediums exist for the transmission of data, including copper, fiber optics, and wireless. Users often look first to traditional cabling or fiber for their solutions, finding that it may be entirely too expensive, time consuming, labor intensive, or even impossible to implement. Once users make the decision to use wireless, they face a whole new set of choices:

As the number of wireless device manufacturers grows, so do the choices for wireless technology. The first big decision is whether to use a licensed or an unlicensed system. A licensed system gives the user a dedicated bandwidth to transmit on, with no interference from other radios, as well as a high-powered radio. Unfortunately, these features come with a steep price tag. The user must renew the license with the Federal Communications Commission (FCC) yearly to maintain rights to the frequency. An unlicensed system is much less expensive, but the user must accept limitations on transmit power and interference.

New license-free radio transmission techniques and standards are emerging almost daily, giving the user a host of choices for the radio output power (up to the FCC limitation of 1 watt), RF baud rate, transmission distance, transmission type, operating frequency, and proprietary or standard protocol. Most industrial wireless applications fall in the license-free frequency ranges of 902–928 megahertz or 2.4 gigahertz, which are allocated by the FCC for Industrial, Scientific, Medical (ISM) use. In the ISM bands, two transmission methods are typically used.

The first is frequency hopping spread spectrum (FHSS). Frequency hopping radios function by constantly changing their transmit frequency, moving in a pseudorandom pattern around the frequency spectrum at a high rate of speed. This transmission method has a high interference tolerance, guaranteeing that information will pass between the radios until 100% of the frequency band clogs with RF noise. Frequency hopping is also inherently secure; the moving target is extremely difficult to track and lock onto. Typically, users who employ this technique require a relatively low baud rate, which allows for a great amount of energy per data bit, improving the probability of successful radio transmissions. Many proprietary radio systems implement FHSS to move serial data or I/O updates over the air. Bluetooth devices (802.15.1) also use frequency hopping.

Direct sequence spread spectrum (DSSS) is another transmission method employed in the ISM bands. DSSS is able to move large packets of information, but is more prone to interference. Direct sequencing starts with the transmitter replacing every bit of user data with a random code word. The code word then spreads out across the RF spectrum to occupy a much wider bandwidth for transmission. At the receiver the code word is de-spread, leaving the original bit intact. Interference that enters the packet suppresses at the receiver when the code word is de-spread, but a certain jamming margin exists. The DSSS radio will function perfectly until the jamming margin is exceeded, at which point the radio will shut down completely. For this reason, a user is better off if the DSSS is in low-to-medium interference environments; 100% of the data will pass, and larger packet sizes can go out at higher baud rates. Wireless Ethernet (802.11) devices, as well as ZigBee devices (802.15.4), employ direct sequencing.

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." W

Behind the byline

Garrett Schmidt is with Phoenix Contact Inc. in Harrisburg, Pa.