Twenty years ago, coin-operated public pay phones were located on nearly every street corner in most American towns and cities. Today, finding one is about as easy as locating a needle in a haystack. In addition to altering the urban landscape, the wireless revolution has increased mobility and created a new world of on-demand communication and entertainment.

While most manufacturing engineers use cell phones, pagers, iPods, Palm Pilots and other wireless devices on a daily basis, the industrial world is still tethered to an intricate network of cable and wire. Traditionally, the motors and controls that drive and monitor conveyors, parts feeders, robots, fastening tools and other automated production equipment have been tethered to miles of conduit and pipe. Twenty years from now, that wiring maze will probably be a nostalgic memory-just like the public pay phone.

Recent advances in industrial wireless technology are allowing factories to finally cut the cord. "Success in warehouse and distribution applications has carried over into shop floor control environments," says Jake Millette, an analyst at Venture Development Corp. (VDC, Natick, MA). "The evolution of standards, enhanced security and increased bandwidth has propelled the use of wireless into harsh environments.

"Wireless operational real-time control holds many potential advantages over wired solutions," adds Millette. "As both users and vendors of wireless products gain experience in industrial monitoring and control applications, the capabilities will become more apparent."

Millette claims that the North American market for industrial wireless systems is growing 40 percent annually and is expected to reach $419 million by next year. However, before there is widespread acceptance on assembly lines, manufacturing engineers must still address a wide variety of wireless automation challenges, such as cost effectiveness, reliability, standards and interference issues.

Why Wireless?

Wireless technology offers numerous benefits for assembly applications, including:

• Cost. Wireless networks negate the cost of the cable previously needed to connect devices and controllers. Eliminating the need for wires inherently decreases installation time, providing a significant savings on materials and labor costs.
• Flexibility. Wireless systems allow easier add-ons, removals and changes to the network. This is especially valuable in applications where change-outs or expansions are frequent and expensive.
• Mobility. Portable operator interfaces having wireless monitoring and control capabilities allow operators, engineers and maintenance personnel to interface with equipment remotely, enhancing operator safety and providing the flexible network architectures desired in plant installations.

The biggest benefit to going wireless on the plant floor is the ability to dramatically reduce cabling, and the numerous headaches associated with it. Cable management costs include installing, maintaining, troubleshooting and upgrading bundles of wire. Wiring installation costs alone can run anywhere from $10 to $1,000 per foot, depending on the type of application. On a typical assembly line, that easily translates into thousands of dollars.

"It costs a typical manufacturing company $1,500 to connect one port in a factory to the Ethernet infrastructure," claims Kevin Prouty, senior director of manufacturing solutions at Symbol Technologies Inc. (Holtsville, NY). "That is $1,500 just to move it a few feet. Having wireless connections can save manufacturers millions of dollars in installation and relocation costs."

As cable management costs escalate, the cost for wireless technology will drop. "The allure to improving the bottom line comes from the promise of greater monitoring, tighter control and improved communication capabilities throughout a plant automation system," says Pat McCurdy, product marketing manager at Phoenix Contact Inc. (Harrisburg, PA). "Wireless technology can enable all these areas by providing fast, easy, cost-effective implementation. With running cable, you reach the point of diminishing returns faster than wireless implementations."

Cable management is a critical issue in factories that rely heavily on robots, such as automotive assembly plants. Cables are often the main source of sensor faults and robot downtime. And, because cable-related failures are often of an intermittent nature, they can be difficult to troubleshoot.

"Whenever there are applications with lots of movement, such as end-of-arm tooling in SCARA robots, cable fatigue causes numerous problems," notes Jesse Hayes, product manager for automation components at Schunk Inc. (Morrisville, NC). "Frequent stretching and bending can cause cable breaks and intermittent signals in articulated robots. When a cable has to be replaced, that creates downtime and lost productivity."

Assembly lines typically contain thousands of sensors and actuators that must be connected to power and control systems. During normal operating conditions, they are constantly subjected to mechanical stress and strain. Wired systems have a high failure rate, especially due to the high failure rate of connectors. Wireless sensors can often be redeployed in under 15 minutes, which is less time than it can take to write a request for the maintenance department to do the work.

Wireless systems also are more flexible and mobile than their wired counterparts. For instance, reconfiguration of wireless manufacturing equipment does not require a time-consuming design and installation task.

"Moving or rotating equipment poses a particular problem today, as they require expensive mechanical solutions, such as cable tracks and slip rings, to achieve an acceptable reliability," explains Jan-Erik Frey, head of wireless technology and business development at ABB Automation Technologies (Vasteras, Sweden). "Apart from high investment and maintenance costs, these mechanical solutions also greatly limit the design and movement of the equipment."

Mobility also enables entirely new ways of working, resulting in more effective operation of the plant. "That leads to improved productivity and quality of the final product," Frey points out. "A mobile workforce, although enabled by wireless technologies, is more of a management question than a technological challenge, though."

For example, if an operator can report a problem via a mobile terminal, and even directly order spare parts while on the plant floor, is he then part of the maintenance crew? "Traditional roles within an industrial plant need to be redefined in order to fully benefit from this new-found mobility," notes Frey.

Industrial-Strength Applications

Although most people still associate wireless communication with consumer products, the technology is slowly trickling down to the manufacturing world. According to a recent study by VDC, the majority of companies using wireless technology in industrial applications are applying it to control applications, or both control and monitoring. "The control applications are largely in setup and maintenance," says Millette. "Reliability concerns are inhibiting adoption for operational real-time use."

Wireless technology has been eagerly adapted by process industries, such as oil refineries, steel mills, electric power plants and water treatment facilities, which typically have equipment, such as pipes, valves and tanks, spread out over much wider areas than assembly lines. Wireless technology offers greater cost advantages vs. traditional wiring due to those longer communication distances.

Wiring costs are greater for large distributed systems. These tend to be slow process-type applications, rather than the high-speed machinery associated with assembly lines.

"The location of plant equipment means that maintenance people spend a lot of time moving and inspecting," says Symbol Technologies' Prouty. "Wireless saves time and data entry costs, as well as error-proofs the collection of data."

When it comes to control applications, many of the processes do not have very stringent real-time requirements. For instance, if you are controlling the level of a tank, you don't need a millisecond update rate for the control signals.

"The applications for wireless within process industries are centered on asset management and condition monitoring," says ABB's Frey. "The requirements for these types of applications are quite relaxed and thus easier to fulfill using standard wireless technologies.

"Since most wireless systems are battery operated, the duty cycle of the wireless nodes is a critical factor to enable a sufficient battery life," adds Frey. "Hence, today's battery-operated wireless solutions are suitable primarily for applications with a low update rate, such as asset monitoring applications."

Many applications in discrete manufacturing environments, such as assembly lines, have a more sophisticated and tightly meshed communication infrastructure due mainly to the shorter distances involved. "Therefore, the cost benefits to going wireless have not been as great," says Phoenix Contact's McCurdy. "However, we may see this change in the future as secure high-speed wireless Ethernet products become available for industrial applications. This should become an attractive cost alternative to upgrading existing data cabling."

In fact, several European manufacturers have successfully integrated wireless systems on their assembly lines. For example, Volkswagen AG (Wolfsburg, Germany) is using an industrial wireless local area network on its powertrain assembly line in Emden, Germany. Mobile screw-fitting stations that are used to assemble engine blocks and gearboxes are controlled through a wireless system.

The station is driven by a motor along a rail that serves as a waveguide for radio signals. Through a radio module, the screw-fitting station and the workpiece carrier send their positions to the system's control unit. The control unit compares the reported values, determines the advance increment needed, and radios the control commands to the screw-fitting station, which then adjusts its position accordingly. The engine block number and torque settings are automatically radioed by the control unit.

Wireless technology is also used to control robots at a Volvo assembly plant in Olofstrom, Sweden. A wireless I/O and contactless power transfer system is used in a welding cell that assembles S80, XC70 and V70 sedans. Engineers first installed a pilot system in parallel with the existing wired one, but didn't encounter any reliability problems. Communication is accomplished via a wireless I/O module that is attached to SCARA robots.

Assembly Line Challenges

Wireless technology is generally more difficult to implement in an assembly environment, primarily due to how plants are designed and controlled. For instance, operating conditions can be extremely harsh, with metal production machines, such as welding equipment, creating different types of interference.

However, Frey says numerous techniques can be used to combat these issues. "One possible way of improving the reliability is by increasing the redundancy," he points out. "We can, among other things, transmit the message along different paths, on different frequencies, several times on the same frequency, or even send it using different modulation schemes."

Most assembly lines use numerous sensors and actuators, such as inductive proximity switches and pneumatic valves. They are often controlled in real-time by a PLC. To control these devices wirelessly, engineers need to address real-time wireless communication and wireless power supply challenges.

Assembly line applications require a wireless communication system that can handle a high number of nodes-hundreds within a manufacturing cell; thousands within a plant-with minimal communication latency, such as a few milliseconds of delay.

"Most existing communication standards focus on throughput and low average latency, and simply are not built for those types of applications," warns Frey. "The emerging ZigBee standard does support a high number of nodes, but only provides real-time guarantees for a limited number of nodes. ZigBee further suffers from the fact that it is not frequency hoping, and thus susceptible to static disturbances."

Finding a reliable power supply is another important piece of the wireless puzzle that engineers must solve. "With the high update rate of manufacturing control applications, battery-powered solutions are simply not a feasible solution," Frey points out. "We need a wireless power solution in which the energy required by the wireless nodes is provided in an autonomous manner."

Frey and his colleagues have developed a system called WISA (wireless interface to sensors and actuators) that solves this problem by electromagnetic induction, in which a low-power magnetic field is generated throughout the cell. Small receiver coils within the sensors and actuators pick up this field and convert it to useable energy.

Other systems exist, such as energy scavenging, where ambient energy sources are harvested. The most common systems are based on harvesting electrical energy from light, vibration and temperature gradients. "None of these, however, are generally applicable in a manufacturing plant, and thus render themselves only to niche applications," claims Frey.

"The dream of wireless factories is contingent on reliable and low-cost wireless sensing technology," adds Phoenix Contact's McCurdy. "There is a ton of money being invested currently in this research area and it certainly provides a lot of promise.

"When I say wireless sensor, I'm not just talking about making existing sensor applications wireless," explains McCurdy. "I'm talking about creating entirely new applications enabled by wireless. [Under such a scenario], sensors can be easily and cost effectively embedded in many manufacturing processes and communicate over the air to repeaters and gateways on a company's network system."

Advantages to Going Wireless

• Eliminates expensive cable and conduit costs, such as labor and material.
• Reduces time for deployment.
• Improves assembly line flexibility, due to easier replacement and upgrade.
• Makes it easier to access and monitor difficult-to-reach places.
• Lowers maintenance costs.
• Makes it easier to monitor moving or rotating devices.
• Eliminates miscellaneous, but expensive, costs related to laying cable and conduit, such as obtaining building and safety permits.
• Allows faster system upgrades.
• Applications with high amounts of movement, such as slip rings, collector wires or drag chains, benefit from the reduced wear on cables.