Industrial-Strength Wireless Networking Connects Motion Control

Purpose-built protocols and ruggedized components yield robust industrial wireless networks.

无线技术在当今世界无处不在. It's not just a question of personal or computer-based communications. In the Olympic Games, scoring for both tae kwon do and fencing has increased the accuracy and reduce inconvenience presented by wireless. RF-enabled units speed rental car return and streamline package tracking. Those advantages also hold for wireless industrial communications. In the case of factory floor production and packaging lines, wireless communications can cut costs, reduce footprint, and minimize points of failure.

尽管无线通讯佛的好处r industrial applications, it has also presented challenges. At the same time, the technology has matured over the past several years (see figure 1). “In the old days everyone talked about the issues in wireless being very much related to the issues like the resilience of the signal and the security of the signal,” says Steve Toteda chairman of WINA (Wireless Industrial Networking Alliance) and Vice President and General Manager of the wireless business unit at Cooper Industries. “I think we've moved well beyond that - the integrity of the signal is quite robust with the appropriate technology. In many cases, the system can be far more secure than the wired equivalent given the work that's been done with security and encryption.”

Latency remains a limitation, however. Wireless can’t support the data rates required for commutation commands and high speed path planning - those communications remain wired. Still, for less time-critical data transfer, wireless technology can work just fine. “Wireless technology tends not to be usable in the microsecond range but much of the wireless technology now available today from an industrial perspective is absolutely usable in the millisecond range," says Toteda, mentioning a system developed to work with Profibus. "We operate the wireless radio system in what we call continuously transmitting mode, which means that the radio is always available to transmit and the delays are simply the delays in digitizing the signal as it comes in. Those delays are in the range of 3 to 7 ms, which is really in the sweet spot of monitoring and control systems in many different applications.”

Industrial applications can be punishing, forcing systems to survive, temperature extremes, shock and vibration, corrosive chemicals, and humidity. Perhaps more damaging to wireless communications, industrial environments introduce a wide range of types of electrical interference. In addition to the overall electrical power running through mazes of cabling, machinery can generate low and high frequency harmonics. Add to that the ubiquitous cell phone and other Bluetooth devices, not to mention less obvious culprits like microwave ovens, and you have a very difficult environment.

Modifications include ruggedized packaging with specialized gasketing, materials, and coatings. Altman, for example, works with a unit for open pit mining that is robust enough to survive being dunked in a bucket of water. On the transmission front, technologies like frequency-hopping spread spectrum allow the system to switch among a defined set of frequencies in order to route itself around potential interference. In addition, the systems feature built-in fail safes and redundancies such as handshaking to ensure that commands are received.

That said, such robustness costs money. End-users need to resist the temptation to choose the cheapest solution rather than one that has been ruggedized for industrial applications. “When you talk about Wi-Fi, there's a huge range of products that may mislead the end user,” says Joseph Altman, president of MDA Controls Inc. “There is a Wi-Fi that you can buy for a few hundred dollars and it does things in a certain environment. When you go in the industry, it's got to operate at a higher level." End users need to be sure they are buying a system that can tolerate the conditions of their application, even though that might increase cost by a factor of 10 to 20 times.

Choosing the right solution
Although it would be useful to make blanket statements about the capability of wireless, the reality is that there is no single form being used for industrial motion but rather a range of solutions that vary depending on the specifics of the application.

Early on, system integrators and machine builders tended to use proven, easily integrated technologies like ZigBee. The ZigBee network architecture is a clustered-tree design in which end devices communicate through routers to form a mesh network overseen by a coordinator (see figure 2). The architecture can be somewhat complicated because new end devices can only be added through a router. The protocol calls for devices to acknowledge transmission receipt and avoid transmitting data when channels are active. The technology uses frequency hopping to avoid interference.

Zigbee is an economical and broadly available solution with large number of chipset suppliers. In the industrial realm, the more commercial-grade technologies don’t always work as envisioned, though. Although ZigBee is a good fit for many-to-one applications with infrequent data transmission, it's not necessarily suited to harsh environments.

The wireless highway addressable remote transducer (WirelessHART) standard provides an effective industrial alternative. Both ZigBee and WirelessHART are based on the IEEE 802.15.4 standard.

WirelessHART is based around a "mesh to edge" approach consisting of RF-enabled field devices that can communicate with one another, making it easy to add new nodes (see figure 3). A gateway device acts as the overall controller. Time-synchronized communications schedules communications, eliminating data conflicts. Wireless heart also takes advantage of transmission acknowledgments and frequency agility to minimize the effects of interference.

Choosing the right application
Much of engineering is about trade-offs, matching the right solution to the right problem. For Altman, the first step is determining the best-fit wireless solution - or whether wireless is the best solution at all. "It's pretty much first understanding what to do and what not to do, where wireless can go and where it can't go,” he says. After all, even if wireless can do the job, it may not be the most cost- or time-effective solution.

An ideal application for wireless motion control would be rotational machinery. Normal solutions require components like slip rings, or long cable runs that allow 360 or more degrees of wind up. Wireless technology eliminates the need.

It’s more than just a cable replacement solution to eliminate points of failure or simplify complex solutions, however. “We've moved beyond that into a realm where wireless has become an integral part of the overall communications network,” says Toteda. He points to not just motion but to sensors and sensor networks. “You now have this concept of mesh technology and interconnectedness throughout the manufacturing environment where there's no way you would have been able to wire all this in the past without ending up with a tangled web of wiring across the plant. This, then, becomes the new paradigm, this concept where you essentially build an umbrella of wireless across your manufacturing environment and then use it for a variety of foundations."

And that includes motion. "Having encoder systems that are now part of the overall wireless control system is even more critically important because it's part of the overall manufacturing environment,” he says, “so I think there's a natural evolution to more use of wireless.”

Altman points to a system that uses wireless motion at underground mining equipment. Operators use joysticks and sensors to run unmanned machines from distances as far as 1000 feet. The application is a perfect fit for wireless motion - using cable is at best cumbersome and at worst highly vulnerable to faults, but the application doesn't present the demanding speed requirements of machine-critical operations such as motor commutation and path planning. If an operator sends a command to dig and it arrives a few hundred milliseconds later it won’t impact the application. For this kind of noncritical operation, wireless enhances safety and reliability while performing as required.

The pharmaceutical industry can also benefit from wireless. Pharmaceutical manufacturing tends to take place in work cells. Depending on the processes involved, the production line may be entirely reconfigured for each new medication, so many pieces of equipment are actually portable, which makes running a wired system challenging. At the same time, manufacturing takes place in a highly regulated environment driven by the need to monitor the process in real-time, collecting data and storing it with appropriate timestamps to ensure robust, traceable operations. Given the need for flexibility, wireless can provide a strong solution for gathering and transferring that data.

Another good fit is large-vehicle engine assembly, which involves the interlocks and subsystems mounted on wirelessly controlled trolleys as they move through the manufacturing line for various steps.

Although wireless technology has progressed significantly, work remains. Today, Toteda suggests, the challenges tend to be centered around implementation choices and standards development work. “We need to make sure that standards are embraced by the multiple vendors that serve this market and that the products themselves are in swappable with one another and work with the specifications that the customers need.” Still, the technology has gained broad acceptance in the industrial community. “There's probably not a factory out there today that does not have wireless," he says.