Time to Slip Away From Slip Rings?

What Should Our Reader Know About Adding Wireless to Their Rotating Segments of Equipment in Tough, Noisy Environments?

By Control Design Staff

We've relied on slip rings for many, many years to get essential signals from rotating segments of our equipment. Anyone who uses them in difficult, hot, dirty or less-than-perfect environmental settings struggles with maintenance and realibility. There's pressure for us to provide more data and more precise and reliable data from these points. This makes both operational and support issues a problem. There are wireless alternatives, but we haven't seen much evidence of them in tough, noisy environments. What do we need to know?

—From October '13 Control Design

SEE ALSO: Wireless Makes It Simple


Slip Rings Might Slip Up
Slip rings do a great job of transmitting electrical power. However, when used for data communications some issues could arise due to excessive signal jitter (deviation from the periodic timing of the signal). Excessive vibration of the sliding contacts or the break of signal impedance beyond that tolerated by the data signal can cause poor performance. Slip rings can do a great job when they are new, but suffer poor performance as they wear or between adjustments.

Fault-tolerant wireless in the form of Bluetooth, Wi-Fi or frequency-hopping, spread-spectrum (FHSS) technology can offer an alternative for data communications. FHSS radios typically used are in the 900-mHz unlicensed band.

Another alternative that works with existing slip rings is to convert the data signal to a Symmetric High-Speed Digital Subscriber Line (SHDSL). This technology, used to improve the bandwidth of older telecommunication lines, can be used with slip rings to improve communications throughput. SHDSL uses multiple carrier frequencies. Ethernet and other native signal types use a single carrier frequency. The use of multiple carrier frequencies improves the chance that some of the signal will get through. While there is some tradeoff in bandwidth, the improvement in communications can be considerable. SHDSL modems are available for serial, Ethernet and other communication types. SHDSL modems have been used in wind turbine slip rings to improve Ethernet communications between the blade hubs and nacelle.

Daniel Sylawa,
business development — renewable energy
Phoenix Contact USA

Wireless Wins
In the fourth industrial revolution, or Industry 4.0, production systems will combine with the Internet, sensors and big data analysis to increase efficiency and deliver more insight into system health. Wireless systems will play a critical role in Industry 4.0, enabling the sharing of data across devices, machines, HMIs and the enterprise. Online wireless-monitoring systems provide access to more data than traditional approaches at a fraction of the maintenance costs. In the areas of diagnostics and predictive maintenance, robust wireless monitoring systems already have proven successful by overcoming the challenges of data throughput, EMI and environmental conditions. As an example, an industry-leading metallurgy company in Canada deployed NI's WSN systems to control and monitor machines that shape and form molten aluminum. This harsh, high-EMI environment reaches up to 70 °C, while the wireless sensor network system continues to perform optimally. WSN systems have access to 14 wireless channels, which makes it possible to coexist with other wireless devices and prevent interference. Also, its protocol helps provide reliability of data by performing multiple node-to-gateway handshakes and data checks throughout the data transfer process.

Nick Butler, group manager,
embedded systems product marketing,
National Instruments

A Vote for Wireless
Using wireless in place of slip rings has many benefits. It can be set up quickly and easily. Radio communications can be done with standard antennas or with radiating cable (leaky feeder cable). The most important feature of using wireless in place of slip rings is reliability.

Wireless is not mechanical, so it will not have a mechanical failure or does not require regular maintenance stoppages.

Other advantages include remote monitoring and remote programming possibly without stopping the machines.

Gary Enstad,
wireless strategic product marketing manager,
ProSoft Technology

[Editor's note: The following response was received by posting the problem on the LinkedIn Automation Engineers Group.]

Wireless Will Work
Consider using WirelessHART transmitters. They are battery-powered and communicate via radio. No wires for power. No wires for signal. No wires. No problem. Common examples of eliminating slip rings include rotating kiln and rotating reactors (see bit.ly/1ePJVDO). Customized WirelessHART transmitters are available to measure pressure, flow, DP level, valve position, pH, conductivity, on/off contact, radar level, vibration, temperature, multi-temperature and acoustic. Level switches are also available, as well as pneumatic on/off valve actuation. Not only do you get the PV from these devices, but also diagnostics and setup information, enabling intelligent device management. (Check www.eddl.org/DeviceManagement.)

Jonas Berge,
director, applied technology,
Emerson Process Management, Singapore

[Editor's note: The following responses came from posting the problem on the LinkedIn Business Industrial Network Group.]

Are Your Slip Rings Top Quality?
It could be that the end user is not using test-grade level of quality slip rings like Michigan Scientific provided back in my day. If the highest quality slip rings already are being used, then it might need to be re-engineered. In that case, when considering wireless in a noisy environment, they might want to explore a line-of-sight wireless communication mode like infrared for noise immunity. They may also have to do the cost comparison on a machine-by-machine basis. With high-speed rotating equipment and cutting fluid everywhere, it's a challenging project whichever way you approach it. I have no experience with wireless in the reader's particular application.

Don Fitchett,
Business Industrial Network

Evaluate This
I hate to be an advertiser for somebody, but you might check out ProxiCom at www.powerbyproxi.com/innovations/industrial/proxi-com.

I used their inductive coupled devices years back on pantogram arms in sawmills. I'm sure the technology has improved in the last 15 years.

Without knowing anything about the type of data or environment, this is a tough question to work on. Optical coupling has made leaps and bounds. Inductive coupling is slow, but more proven.

What's the "noise"? They could be able to get a demo unit for testing from one of the companies pursuing this type of technology.

Chuck Plante,
systems development engineer,
Pacific Woodtech