Near the end of 2005 the plant agreed to serve as a beta test site for Emerson's early wireless transmitters. It established in A Caustic a small Smart Wireless network, which ran for several months. At one time, that unit boasted 900-MHz networks from Emerson and a second supplier, plus WiFi and WiMax; coexistence never was an issue.
Early prototypes used existing instrument housings modified simply by having holes drilled, O-rings affixed and antennas attached to the wireless transmitters. All failures stemmed from either water intrusion or low battery voltage. With no sunlight for solar panels, power had to come from batteries. The original ones lasted only about three months, which was satisfactory because this was only a test.
Figure 1-- Always on track: The wireless mesh network
The basic concept of a self-organizing wireless mesh network proved itself in A Caustic, where the infrastructure is extremely dense with pipes, buildings and cranes. Users don’t need to configure the network. Instead, it actually organizes itself in response to plant changes that affect the way radio signals propagate — whether those changes are physical, such as equipment starting or stopping and railcars rolling by, other radio traffic, or due to an outside influence such as a thunderstorm (Figure 1). As a result, there’s no need for preliminary radio frequency (RF) site surveys or assumptions about what the RF characteristics are going to be like at any one time. It simply doesn't make any difference.
Because the RF space can't be controlled, Emerson's Smart Wireless systems are based on the assumption that changes will occur fast and often. The industrial environment is expected to be dynamic with no time for technicians to react. Therefore, the wireless network automatically adjusts. If an obstruction blocks line-of-sight communications, the network finds a new path for transmissions to reach the gateway (receiver).
This works because all field devices are transmitters and repeaters and are a part of the mesh. They are transceivers with an inherent system-level power-saving characteristic. The self-organizing mesh technology significantly reduces requirements for communications infrastructure (fewer gateways) because it allows transmissions to avoid obstacles and adapt to changing conditions in the facility. The network operates perfectly well in very dense plant environments.
A wireless network must take battery life into account. The more frequently a battery-powered instrument updates, the sooner the batteries run down. Setting the devices to a low-power mode can save energy, enabling them to operate for an extended time on one battery. Run times depend on how often the device is used but battery life currently can exceed 10 years by using a low update rate.
A mesh also inherently needs less power than direct connections. For example, to transmit a signal 1,000 feet, two devices, each with a range of 500 feet, in a mesh network don’t require nearly as much power as a single device transmitting over the entire 1,000-ft distance.
The devices also are secure and meet the requirements established by the PPG wireless team. The first of these is encryption with seemingly random symbols that surround each transmission. Even if a message is intercepted, it takes too long to decode to be of use. Encryption keys are changed frequently so anyone trying to read intercepted messages by comparing them won’t be able to break the code before it's changed.
In addition, a transmission is ignored unless it's authenticated — meaning that the sending and receiving devices must recognize one another. A third step is data verification by the receiving device. The authentication and verification rules are built into the devices, so no foreign device can intercept a transmission or send bogus information to the receiving station.