Wireless Freedom

Why wireless online monitoring systems are not all the same

 

With reduced cable lengths and shorter implementation time, there is no doubt that wireless online Condition Monitoring Systems (CMS) are a cost-efficient solution. However, several concepts of data acquisition and wireless transmission exist with direct impacts on various factors such as measurement data quality, transmission range and power management. Therefore, an informed decision is necessary when implementing a wireless online CMS. This article describes the different concepts with their strengths and limitations. Two selected case studies illustrate best practices for two most different wireless applications.

Why Wireless?

When implementing a reliability-based maintenance (RbM) program to monitor production critical equipment, sooner or later the RbM-manager has to consider the purchase of an online condition monitoring system. Allocating the budget for such projects is not always easy because the installation of an online system can be time and resource consuming. Particularly, the laying the cables from the system to the vibration transducers is a critical and resource-intensive part of a project that may not be underestimated. Within the past years, wireless online monitoring systems have started to offer a valuable alternative to conventional cable-based systems, keeping their promises of shorter installation time and a quick and easy integration into already existing maintenance strategies.

 

Power management and transmission concepts

 

The absence of cables has consequences on the power management of wireless online condition monitoring systems. The majority of the systems are battery driven and therefore the energy consumption of a wireless online CMS must be kept minimal in order to ensure long battery lifetime. Therefore as it stands today, additional power consuming sensors like speed-transducers are not part of the outfit of a typical wireless system. In addition, other features like digital inputs for triggering measurements or a Modbus interface are typically not implemented in most wireless online systems. Apart from vibration sensors, temperature probes (typically via a low power consuming PT 100 sensor) are also integrated in some wireless online CMS. The restricted energy resources are a constraint and set wireless systems clearly apart from conventional online CMS. Cable-based CMS are therefore better suited for the surveillance of highly speed variable and load changing equipment, because of their additional monitoring capabilities.
Another decisive criterion is the frequency range and the communication protocol that are utilized to transmit the data wirelessly. The “WIFI” frequency band that is used by the majority of wireless systems is broadcasting on the 2,4 GHz band simply because this band is declared “free for usage” in most countries by the national authorities. However, also other devices like mobile phones or tablet pcs are using this band and the signal range of WIFI is limited. To overcome this drawback, intelligent mesh networks that require repeater type instruments need to be added to a network of wireless online CMS. But not all systems require this additional hardware.

VIBCONNECT RF wirelessly transfers data within the RF and WIFI ranges
Fig. 1: VIBCONNECT RF wirelessly transfers data within the RF and WIFI ranges

The wireless online CMS VIBCONNECT RF from PRUFTECHNIK Condition Monitoring is transmitting the data also in the WIFI range but is also coming along in versions that broadcast the data via radio frequency (RF) at 868 MHz or 916 MHz. The radio frequency offers a long range in a less crowded frequency band. Additional equipment to extend the range is not required under normal conditions. Apart from the battery powered versions, VIBCONNECT RF is also available in a 24 Volt powered version via cables
(if available at the machine) in order to guarantee a maximum flexibility and to adapt the monitoring system to existing resources. With the power supply permanently available through wires,
this architecture is suitable for more critical machines
that require more frequent measurements.



Wireless systems and sensor position

 

Choosing the best measurement location for a vibration transducer is most essential. Several guidelines [1,2] exist, which recommend for the positioning of transducers. One critical factor to obtain meaningful data with high amplitudes is the positioning of the sensor within the load zone of a machine. In many cases, this zone is directed to the bottom and no direct line of sight is available for transmitting the data wirelessly. Neighboring machines, pipes, conduits or parts of the machine foundation typically are present and attenuate or even block the wireless signal transmission. Because of this, compact wireless online CMS (magnet, transducer and battery in the same housing) cannot always be placed at the optimal measurement position of the machine. Apart from compact systems other wireless online CMS in the market separate their sensors and the transmitting unit with short wires that are already part of the system. The RF Sensor Unit of PRUFTECHNIK’s monitoring system VIBCONNECT RF offers such an architecture. It consists of a robust, compact case that contains the exchangeable batteries and connects the output of the signal electronics to the transmission antenna. The case can be placed on top of the machine in order to ensure the best signal quality. On the other side, two 2,9 meter wires connect to one sensor each. The PRUFTECHNIK-made sensor consumes very little power in order to ensure a long battery life. A combined vibration and temperature sensor also delivers bearing information (envelope analysis) in addition to machinery vibration monitoring.

 

Case study 1: Monitoring of cooling tower drives

 

Cooling towers exist in various architectures. Medium size units for heat rejection are typically driven by a constantly running motor that connects via a long shaft to a gearbox. Since the gearbox is located within the tower, it cannot be accessed during a route-based data acquisition with a portable system. However, the criticality of a cooling tower drive demands periodical monitoring. Online wireless CMS help significantly reduce the data collection effort on such a remote location. The Gendorf Industrial Park, Bavaria’s
largest chemical park required monitoring of cooling tower drives and cooling water pumps. The objective was to check the condition of shaft and motor bearings and fan imbalance at regular intervals.

The cooling towers in Gendorf Industrial Park. The cooling tower drives and cooling pumps are monitored wirelessly.
Fig. 2: The cooling towers in Gendorf Industrial Park. The cooling tower drives and cooling pumps are monitored wirelessly.

The contract for the on-site monitoring of the installation was awarded to a technical service provider. The service company opted for the wireless CMS VIBCONNECT RF precisely because of the advantages it offers in such an industrial landscape over comparable systems with conventional wiring, namely the easy and inexpensive installation. The monitoring was performed by two sensor units installed on each of the four fans. The sensor units send a signal to the base station several times per hour in order to provide ISO readings and diagnostic information about the machinery vibration (velocity spectra) and bearing condition (envelope spectrum). Within just a few weeks after installation, insufficient lubrication was detected in its early stage and corrected. As a result, the service life of the bearing has been lengthened and the maintenance interval has been optimized.

In some cases, a wireless solution can be a relevant
Fig. 3: In some cases, a wireless solution can be a relevant

Case study 2:
Monitoring of complex machines

MGS Tecnologia, the Brazilian sales representative for PRUFTECHNIK Condition Monitoring was invited by Robert Bosch do Brasil in order to discuss possibilities for the monitoring of spindle bearings. Machine tools cut, grind and process all kinds of material and the wear of the bearing of a spindle is a normal process, especially when the end product is made of hardened metal. The complexity of a machine tool depends strongly on the production process and even most powerful online systems come to their limits. High vibration amplitudes within seconds, highly variable conditions and extremely harsh inner environments make reliable monitoring a challenge. After a detailed inspection of the critical machine tool, the production process and the spindle architecture, the engineers from MGS Tecnologia and the maintenance staff from Robert Bosch decided to monitor the spindle with VIBCONNECT RF. Because a measurement can be triggered externally via a short pulse, coming e.g. from the PLC, the sensor unit measures exactly then, when the spindle is running for a few minutes under constant condition (monitoring mode). The low installation costs and compact size of the combined temperature and vibration sensor were decisive for Robert Bosch to opt for VIBCONNECT RF. Readings can be transferred via Modbus TCP to the instrumentation department. This advanced feature, hard to be found in other wireless monitoring systems finished to convince Robert Bosch.

Growing wireless potentials


Wireless online CMS keep pushing the technical limits and will, in the near future, be equipped with further innovative machine monitoring features. For example, independent power supply concepts like the creation of an energy source from the machine’s vibration via power harvesters are potential alternatives for autarkical online CMS. While the integration into potentially explosive environments remains a challenge for most wireless systems, an ATEX version of the VIBCONNECT RF from PRUFTECHNIK Condition Monitoring will soon be available. Surely, cable-based systems will remain the systems of choice for the majority of complex assets within the coming years but applications powered by wireless systems will definitely continue to grow in popularity.

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