Wireless condition monitoring

Wireless technology can aid preventative maintenance in plants. Jim Ralston writes.

Mechanical failure of motors, drives and other vital electromechanical equipment are among the most common reasons for production stoppages.

Recent advancements in vibration monitoring and data analysis have lead to condition monitoring systems that can accurately detect a problem before failure, thus reducing costly machine shutdowns and maximizing production output.

These systems are installed on the monitored equipment and are typically networked back to a central computer for data analysis and alarm annunciation. Because the machines may be in remote locations where network infrastructure is not available, or on moving platforms where hardwired network connectivity is not practical, wireless communications is a networking alternative that offer installation cost savings, quicker deployment and improved reliability in certain situations.

Implementation Challenges

For many industries, a condition monitoring system is justified with a simple return-on-investment (ROI) calculation.

For a relatively nominal cost, vital machines may be retrofitted with condition monitoring to reduce operating failures. But where network infrastructure is not already in place, there are additional costs to consider.

These include fibre optic cable installation, conduit engineering/installation, trenching between buildings, leasing phone lines for remote sites, and installation of festooning or slip rings for moving equipment.

These additional costs may push the ROI out beyond what management will accept.

The installation costs of cable in an industrial plant can vary greatly based upon the type of plant and physical configurations.

The actual cable cost depends on the location of the machine relative to existing network infrastructure, type of cable needed (e.g. fibre optic), conduit engineering (if needed), labour cost rates and if trenching is required.

If the machine is in a remote location several kilometres or more away, then leasing phone lines for communications is required.

Leased phone line costs usually include an initial activation/installation fee and then a monthly fee based upon speed of service. Since vibration monitoring is continuous and typically data intensive, the phone line service must support a high enough speed for continuous monitoring.

Phone line service to remote sites such as pump stations are also prone to communication failures due to poor line quality.

Wireless cellular services are sometimes an option for remote sites, but are subject to service availability and limited in speed. Cellular data subscription costs may also be expensive.

If the machine is on a moving platform (such as an overhead crane, transfer car or conveyor system), then connecting the condition monitoring system to the plant network is a particular challenge.

Depending on the speed and distance that the platform travels, traditional cabling methods such as festooning may be possible. However, festooning is subject to wear and tear, and cables may break.

For spinning platforms, slip rings with Ethernet support are available but are expensive and require periodic maintenance. Some machines move so fast that the only practical communication method is wireless RF.

Given the challenges of networking condition monitoring systems, wireless communications offer lower installation costs (shorten ROI time), eliminate phone lines and allows machines that were previously not practical to be monitored to be remotely monitored. But wireless technologies and equipment vary widely in performance and reliability in industrial installation.

Designing a successful wireless network requires an examination of current wireless usage, RF paths and environmental challenges of the industrial plant.

Industrial Wireless Technologies

The most common approach to wireless Ethernet is RF transmission in the spread spectrum bands. Globally, the 2.4 GHz and 5.8 GHz bands are available for license-free use in most countries.

Spread spectrum literally means spreading the RF energy across the entire (or wide portion of the) spectrum. This technique permits relatively high speed communications while being designed to operate in noisy environments where multiple RF systems are present.

There are two major methods of spreading RF energy: Direct Sequence and Frequency Hopping. Both methods have advantages and disadvantages for industrial wireless communications.

Direct Sequence uses a wide channel within the band to simultaneously modulate a highly encoded bit pattern.

It offers the fastest spread spectrum data rates as the wide channel permits transmission of complex modulation schemes.

Some complex modulation techniques can transfer data quickly at rates of up to 54Mbps.

Direct Sequence is the method used by all popular open Wi-Fi standards.

While the wide band modulation offers high speed, it is more vulnerable to noise problems when multiple systems are operating in close proximity.

Due to overlapping channels and the popularity of Wi-Fi systems in plants, band over crowding and RF saturation can lead to poor wireless performance.

Frequency Hopping is a very popular technique for industrial systems because it is more resistant to noise. Frequency Hopping uses many smaller channels in the spectrum and rapidly changes channels or “hops around” from channel to channel.

By incorporating error correction techniques, frequency hopping offers the best chance for successful data transmission as the transmitter will send the packet over and over again using different channels until an acknowledgement is received.

However Frequency Hopping is slower than direct sequence and has longer data latency.

Most Frequency Hopping systems are limited to 1Mbps or less. If the data rate is sufficient enough for the application, the reliability of frequency hopping is tough to beat especially if more RF systems will be added in the future.

Frequency hopping modems are proprietary, meaning that each manufacturer uses their own technique with no industrial sharing of information.

This provides security and isolation from the wireless IT system.

Due to the closed standards of frequency hopping technology, manufacturers can use unique authentication processes and sophisticated encryption techniques.

While standard wireless technology has improved security with the introduction of WPA and WPA2 standards, hackers continually target those networks.

Many industrial Wi-Fi manufacturers now include an option to hide the access point by not transmitting its SSID beacon.

This technique is effective at hiding the access point from potential hackers.

Frequency hopping also offers plant managers the ability to operate their own wireless network separate from the IT department.

Condition Monitoring Integration

Most condition monitoring systems have an Ethernet communication option for network connectivity. Ethernet is the most easily adaptable interface for wireless if two considerations are observed: bandwidth and data latency.

These considerations especially come into play when multiple remote machines are monitored.

It is important to design an RF network that effectively reaches all remote sites while maintaining adequate data rates. If the number of remote machines is high, then it may be best to install separate RF systems to maximize the performance of each system.

Machine locations and building structures will determine antenna placement and may be another reason to consider multiple RF systems.

Many industrial systems also support packet repeating to aid in RF signal propagation while also creating self-healing meshes.

Finally, it is very important for the wireless equipment to be designed specifically for industrial installations. Key specifications to examine are RF power output (higher is usually better), operating temperature, built-in diagnostics, hazardous certifications (if necessary) and, perhaps most importantly, the support staff’s level of industrial networking knowledge.

Remote condition monitoring can benefit just about every industry where electromechanical machines are vital to production.

Several applications where wireless condition monitoring is particularly effective include monitoring of pumps in wastewater treatment plants, drives used on oil/gas drilling rigs, drives on assembly lines in automotive plants and overhead cranes in hot metal mills.

One application is power plant cooling fan monitoring.

A coal fired power generation plant wanted to monitor their cooling fans located at the base of their cooling towers.

The cooling fans are mounted in very harsh areas where hot steam is always present.

When a fan fails, the tower has to be shut down to allow repairs, thus reducing the power output of the plant, sometimes during peak demand periods.

By installing the condition monitoring system, the plant would be able to schedule fan repair during non-peak shutdowns.

The condition monitoring system was relatively easy to install, but the towers lacked Ethernet network infrastructure.

The cost of pulling fibre optic cable would cost over $100,000 and take more than six months to install.

Wireless Ethernet was found to cost a fraction of fibre, with installation complete within three weeks.

Summary

Advances in vibration analysis have lead to modern condition monitoring systems that can significantly improve plant production. Unfortunately, the costs of networking these devices can be very high or impractical.

Industrial wireless technologies offer an alternative to hardwired networking and can result in lower costs and better reliability.

Care must be taken, though, to choose the best technology and wireless hardware to insure a successful system.

Jim Ralston has over ten years of experience with industrial wireless systems and is currently a wireless sales engineer for ProSoft Technology.

Jim Ralston

ProSoft Technology

jralston@prosoft-technology.com

www.prosoft-technology.com