Posted by arlene

It may seem that using wireless systems to replace common links, like serial cables, is easy and straightforward. That is not often the case. Here are some common problems encountered when replacing wired systems with wireless:

1. RF links are not as dependable as wires. Anyone who has used a cell phone, portable radio, or CB knows firsthand about RF links. The signal is constantly changing as conditions change between the two points.
2. Expanding or moving an RF point is not always as easy as claimed, because a new position on the network may be out of range of the control point for the wireless network. This control point is commonly placed at the control panel in an industrial application.
3. Wireless installers sometimes offer the assistance of professional technicians who perform RF site surveys to determine control points for the wireless network based on planned coverage areas. Although useful, this adds highly skilled labor back into the installation cost and doesn’t address ease of reconfiguration or expansion.
4. Some installations require additional wireless control points (sometimes referred to as wireless access points) in addition to the control panel.

This is a very serious set of problems to face when reliability is of prime importance. Thus, despite the increasing popularity of IEEE 802.11 wireless local area network (LAN) systems and the promise of Blue-tooth systems, wireless communication has yet to be widely adopted in industrial applications. Although wireless systems seem like an obvious solution for industrial applications, in reality the cure can be worse than the disease—due to the fact that solutions based on these standards were not designed with the industrial environment in mind. Industrial users need a network architecture that takes the unique challenges of the industrial environment into account.

The ARBORNET research team at the Massachusetts Institute of Technology (MIT) Media Lab examined embedded wireless solutions for smart devices from 1997 to 2001. The study concluded that traditional cell phone-style wireless systems were simply inadequate for industrial applications and could never gain widespread acceptance in their current form, but that a fresh approach was needed and, ultimately, was identified.

Control and Sensing Networks versus Data Networks

To understand why standard wireless networks do not work well for industrial applications, it is important to distinguish between control and sensing networks and data networks.

Wireless data networks are primarily designed to link together computers, personal digital assistants (PDAs), printers, Internet access points, and other elements where large amounts of data are sent in both directions. In data networks, the emphasis is on speed: faster is better. The design and evolution of 802.11 networks is a good example. 802.11b, one of the first wireless LAN standards to be widely adopted, connects devices at up to 11 Mbps. One of the latest updates to this standard, 802.11a, will allow for data speeds up to 54Mbps, enabling more rapid downloads of music and video files by end-users.

But wireless networks for industrial control and sensing must be, above all, reliable, adaptable, and scalable. Because industrial sensors send only a few of bits of data per second or minute, providing information like temperature, pressure, and flow, data rates of 11 Mbps or even 54Mbps are rarely needed. Although speed is often the focus for data networks, the primary design objectives for industrial control and sensing networks are reliability, adaptability, and scalability.

Requirement #1: Reliability

For most industrial applications, reliability is crucial: wireless systems must be just as reliable as traditional copper wire. Depending on the application, garbled or dropped data can result in anything from a disruptive glitch to a devastating failure. Three factors determine the signal reliability between a radio transmitter and receiver:

Reliable Wireless Networks for Industrial Applications

• Path loss
• RF interference
• Transmit power

Consider a conversation between two people. Path loss corresponds to how muted a person’s voice becomes to another, either due to distance or the obstacles between them. The listener will have a hard time understanding the speaker who is too far away or talking through a closed door.

RF interference corresponds to ambient noise: it will be difficult for the listener to understand the speaker in a noisy environment. Many other factors—including receiver sensitivity and data encoding technique—affect the reliability of a link. However, between a given radio transmitter and receiver, the path loss, interference, and transmit power determine the bit- error rate.

The problems of path loss or interference can be overcome by moving closer to the listener or by shouting loud enough to be heard. In the wireless world, this corresponds to repositioning radios or by transmitting with a higher power. Unfortunately, neither of these are generally viable options. Increasing the transmit power creates a situation similar to people shouting over loud music at a party. A sophisticated antenna design that directs the RF signal towards the receiving radio might help. But this is much like using a megaphone to shout at the listener.

It does improve the path loss situation, but may fail if the listener or the megaphone moves.

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