Wireless technologies in condition monitoring and remote diagnostics of electric drives; requirements and applications

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1 Wireless technologies in condition monitoring and remote diagnostics of electric drives; requirements and applications Ville Särkimäki, Risto Tiainen, Jero Ahola, Tuomo Lindh Lappeenranta University of Technology Skinnarilankatu 34 FIN-53850, Lappeenranta, Finland Tel.: +358 / (5) Fax: +358 / (5) ville.sarkimaki@lut.fi URL: Keywords «data transmission», «sensor», «measurement», «device application», «diagnostics» Abstract This paper concentrates on utilization of wireless technologies on the data transfer needs of condition monitoring, maintenance and remote diagnostics of electric drives. A short introduction to typical shortrange radio technologies is given. Requirements for wireless technologies in condition monitoring are discussed. The main focus is the introduction of different applications that the use of short-range wireless technologies makes possible. Introduction Condition monitoring of electric drives is very important aspect when reducing costs caused by unplanned shutdown of the production process. Early detection of fault also prevents additional damage to the electric drive and helps to determine repair operations for the next scheduled maintenance break. This also reduces unnecessary and costly maintenance operations. To detect faults in electric drives, we have to measure and collect data. Measurements can be, for example, temperature, vibration, torque and current. Measurement data are then transferred from the sensors to an upper hierarchy level, to be analyzed and saved as history data [1]. Data processing can be also done at different stages, even at the sensor itself. However prognosis or results have to be somehow transferred to operators or maintenance workers. One future trend is intelligent electric motor that, in addition to providing self-diagnostics information, can also work as a sensor as a part of the whole condition monitoring system. Maintenance workers also need history information and instructions while operating at factory level. So there is certainly need for data transmission and applicability of wireless technologies in these data transfer needs is discussed in this article. This paper concentrates on the applicability of wireless technologies in condition monitoring data transfer and maintenance data collection. There has been a lot of discussion that wireless technologies can reduce costs in industrial applications [2]. During last few years, short-range radio devices have become generally available. These are for example, IEEE , Bluetooth and Zigbee as well as different nonstandardized technologies. Short introduction to these technologies is given later. They are widely used in commercial applications and have started to gain foothold in industrial applications, too. In this paper, the requirements for wireless technologies in condition monitoring and remote diagnostics are discussed. Based on the benefits and requirements of the wireless technologies introduced, some wireless

2 applications for condition monitoring and remote diagnostics are presented. The applicability of different short-range technologies, in these applications, is further discussed. Industrial environment and wireless technologies In indoor environments, radio waves can have multiple routes from transmitter to receiver. Instead of just having a direct line of sight route, radio waves can reflect, diffract and scatter from different obstacles to reach the receiver. This is called multipath propagation. Typical radio propagation mechanisms in indoor communication systems are given in [3]. Making a generalization of a typical industrial environment is not straightforward or even possible, but it can be thought to be a large building, several floors high and filled with steel constructions and machinery. In such an environment, determining the coverage area of a radio link can be difficult. Industrial environments can be considered more challenging than office environments for communication. This is because of large metal structures that cause multipath propagation, and interference caused by machines and other transmitters operating on the same frequency band [4]. In addition, moving obstacles such as vehicles and humans can cause blackouts in radio transmission. Manufacturers of wireless data links give typical operating ranges for their devices. These operating ranges are usually measured in open space and in industrial environment actual operating ranges are shorter, especially if no direct line of sight between transmitter and receiver exists. Short-range radio devices are limited on transmission power and band where they operate. Because of limited transmission power ( 100mW), their operating range indoors is usually less than 100 meters. Nowadays there is a wide range of different short-range devices operating in different bands, using different modulation and coding techniques. Common to all of these technologies is that they don t need any license to be operated. Short-range radio devices are available from various manufacturers that offer their own technologies and protocols. In addition to this, there are also available some standardized technologies, for example, Bluetooth, IEEE and Zigbee. Depending on wireless technology used, power dissipation, operating range, network topology and transfer speeds are different. Bluetooth was originally designed for wire replacement between a computer and peripheral devices, but it has also been used in industrial applications where data is transferred from sensors. Technology allows point-to-point and point-to-multipoint links and also offers a solution for more complex networking topologies. However, current Bluetooth applications use primarily simple point-to-point data links [5]. Bluetooth includes several power-conserving features, but operation times of several years with batteries in condition monitoring sensor networks are unlikely [6]. Bluetooth devices are widely available, prices are relatively low, and complete modules, which reduce design time, make this technology interesting. Also, Bluetooth technology is integrated in several cellular phones, which can be used as user interface devices [7]. These things make Bluetooth suitable wireless technology for some condition monitoring and maintenance applications that are discussed later. More information about Bluetooth can be found from [5] and [7]. WLAN technologies (specifically IEEE as well as Hiperlan and others) are not intended to be used in low power sensor networks. Power and microprocessor requirements are high and the protocol is complex. However, price of these devices is quite low, they are widely available and they make direct connection to an Ethernet network easy. In addition to, this transmission speeds (up to 54 Mbps) are suitable for transmitting live video picture [8]. Although these technologies are not very well suited for condition monitoring sensor networks, they can be used, for example, to generate so called WLAN hot spots to industry floor that allow wireless access to the Internet or the factory s database for maintenance workers. Zigbee is a very promising technology specially intended for use in large sensor networks. It allows sensor networks of tens of thousands of nodes with mesh-, star- and cluster tree-topologies. Power requirements

3 are lower than in the previously mentioned technologies. The protocol stack is kept simple, so it can run on a low-cost 8-bit microcontroller. However, the transmission speeds are lower (<200kbps) than in the previously mentioned technologies, but suitable for low data rate sensor networks. All of these make it a very promising standardized wireless technology for data transfer needs of sensors used in condition monitoring and remote diagnostics [9]. One interesting technology is RFID (Radio Frequency Identification). It allows wireless identification of vehicles, machines and production items. Identification is done, for example, with mobile RFID reader. RFID tags contain a microchip attached to an antenna. Tags can be divided into passive and active tags. Passive tags have no onboard power source, but are powered from the reader device by received signals. Active tags have a battery and usually larger data storage. There is a wide range of different tags operating in different frequencies (125kHz-2.4GHz). Operating ranges vary from a couple of centimeters up to 30 meters [10]. This technology is not intended for transmitting data between sensors, but it can be used in condition monitoring and maintenance in other ways. An example of this is given later in the article. Requirements There is a wide range of requirements that have to be considered when choosing wireless technologies for condition monitoring and maintenance applications. These are for example: transfer range, transfer speed, frequency band, network topology, connection to sensor, connection to Ethernet/fieldbus, power consumption, price and whether standardized technology is needed (connection to a mobile device, for example). Based on these requirements, a suitable wireless technology for each application can be chosen. In this chapter, these requirements are discussed. Wireless technologies are often promoted by stating that their use reduces the installation cost of wires and the installation times. Even though this may be true in many situations, this should not be the only reason to choose wireless technologies over wires. The transmission medium in wireless transmission is constantly changing, because of the multipath propagation and variations in environment. This makes it hard to predict coverage areas. One big question is the powering of wireless sensors. When using batteries as a power source, they have to be replaced in certain time periods, which requires additional labor. Electric motors always have power cables attached so there are other means to supply power to the sensors than using batteries. Additionally, power cables can also be used as a communication channel for condition monitoring sensors [11]. One additional requirement can be data encryption. In wireless data transmission, unwanted users can possibly connect to the transmission channel and gain access to the transmitted data. Most standardized technologies provide techniques for data encryption. In applications where data are transferred from fast moving or rotating devices the Doppler effect has to be considered. Topologies A communication network is composed of nodes, each of which has computing power and can transmit and receive messages over communication links. Basic wireless topologies can be categorized as point-topoint links, point-to-multipoint links and mesh-networks [10]. As mentioned earlier, industrial environment is harsh for wireless transmission and blackouts can happen. This is extremely bad for pointto-point and point-to-multipoint links, because no data can be transmitted if link is cut off. In meshnetworks, there exist alternative routes where data can be transmitted to the receiver. Also shorter transmission ranges are required in a mesh-network for a single hop. Typically, failures that can be monitored in electric motors develop slowly, so there is a period of time between first indications of fault before it causes failure. So measurement data from sensors related to condition monitoring are not time critical. For example, if we measure temperature once an hour from one electric motor, short breaks in transmission can be accepted and more important is to transfer data correctly.

4 Wireless technologies help us easily build up different networks. In condition monitoring, maintenance and remote diagnostics of electric drives, the following topologies can be considered. 1. Network composed of sensors attached to the same electric motor. Measurement data are transferred forward through a collector unit, which has wireless connection to the plant s Ethernet. 2. Field level network, where each individual sensor is directly connected to the network. Together these form a large mesh-network. 3. Network where sensor data are routed through a frequency converter (or a motor controller). The connection between the frequency converter and the motor is point-to-point and the connection between the frequency converters and Ethernet is point-to-multipoint. In Figure 1, these topologies are illustrated. Transmission range in these network topologies is short, typically meters. This can be accomplished using short-range radio devices. It is possible to get longer transmission ranges and better reliability by using mesh-networks as mentioned earlier. In these different concepts, it is also important to consider, where the analysis is carried out. The measurement data can be processed and analyzed in different levels of the network. For example, some form of data processing can be done at a sensor, because a microprocessor is included. This reduces the amount of data that has to be transferred. 3. Ethernet 2. T A P A G A RH M T A 1. Fig. 1: Different network topologies for the condition monitoring of electric drives

5 Applications A typical application for wireless technologies has been wire replacement and access points that provide access to the factory s database. They can be used to easily build large sensor networks, because little new cabling and other infrastructure is needed. Also, new applications to be used in condition monitoring data transfer of electric drives and maintenance can be introduced. In this chapter some of these applications and suitable technologies are discussed. In Table I, some applications for condition monitoring data transfer and maintenance data collection are presented. In the table, SRD (short-range device) means some general short-range devices that are nonstandard. WLAN covers both IEEE and Hiperlan technologies. Bluetooth and RFID were introduced earlier in this paper. The range is given as a distance between two transceivers in a point-topoint link. In Table I, ranges are approximated based on what they could be in given application. If wireless technology allows larger mesh-networks to be composed, the network dimensions can be much larger than distances given in Table I. The data throughput depends heavily on the measurement. For example, one temperature reading is only a couple of bytes, while a single vibration measurement produces close to a hundred kilobytes. More discussion about capacities needed is given in [12]. We can conclude that the produced amount of data can be easily transferred with typical short-range radio technologies in case of vibration and temperature measurements. As mentioned earlier in this article, data to be transferred in condition monitoring applications is not time critical. If data can be stored prior to transfer, lower transfer capacity is sufficient. Capacities shown in Table I are estimated and can be achieved with preferred wireless technologies. One condition monitoring application could be transferring of live video image from the monitored device. This requires more capacity from transfer mechanism and suitable standardized wireless technologies are WLAN and Bluetooth. Low capacity of Zigbee-technology (<200 kbps) does not meet the requirements.

6 Table I: Applications for condition monitoring data transfer and maintenance data collection. Application Range Capacity Technologies Additional remarks Sensor network for condition monitoring of electric drives Up to 200 kbps Zigbee, SRD, Bluetooth Sensor node attached to a rotating device Identification of operating device or sensor wirelessly with mobile reader and saving and reading maintenance information from tag Portable and temporary measurement system Connection to history/ maintenance information in factory s network with portable device Parameterization of operating device with a portable device <10 m (maximum distance between two nodes) <1 m 1-10 kbps Zigbee, SRD -Sensor network can cover the whole industrial floor -Sensors can, for example measure temperature, vibration, humidity and current. -For example, torque measurement from shaft -No galvanic connection <10 m <10 kbps RFID -Mobile phone can work as a reader device -Tags are attached to sensors or electric drives -Tags can contain maintenance and device specific information m kbps Bluetooth, WLAN, SRD m >10 kbps WLAN, Bluetooth <10 m 1-50 kbps Bluetooth, WLAN -Easily attached wireless sensors -Data logger can be a laptop computer -Maintenance workers can get real-time information/ instructions related to electric drives -Mobile phone can be used, for example, as a frequency converter s user interface Different applications listed in Table I are chosen so that wireless transmission is preferable to wires for data transmission. In some applications listed, it is the only possible solution. For example, sensors used for condition monitoring and remote diagnostics that are attached to a rotating or moving device, and possibly operated with batteries, still need a way to transfer measured data to upper hierarchy levels for further analysis. Wires can be used for data transmission, but wireless data transmission makes things easier. Benefits are reduced installation times and cost, devices can rotate or they can be moved to other places freely as long as distances between transmitter and receiver are short enough for connection to be created. Easy and fast installation makes wireless technologies ideal solution for temporary measurement systems. Sensor can be easily attached to the device monitored and a laptop computer can be used to save the measurements. As previously mentioned, wireless technologies allow identification of devices using RFID technology. Identification can also be done by using bar codes, but RFID is more useful because it makes it possible to store data in a tag. In harsh and dirty environments bar code can become unreadable. Also, RFID tags allow data amounts of several kilobytes to be saved in a tag. During the manufacture of an electric machine, specific design information, characteristics and other related information can be saved to the tag. Later, maintenance operation history can be updated and viewed. Closely related to the previous

7 application is connection to the maintenance database in the factory s network with a portable device. There exist already some mobile phones, which have built-in RFID readers as well as Bluetooth and/or WLAN capabilities. With this kind of portable device, maintenance workers can get real-time information about devices just by going close to them and using RFID technology to identify the device. WLAN can be then used to download information and current condition monitoring measurements and results directly to the portable device. This type of communication where identification is combined with other interconnection technologies is called NFC (Near Field Communication) [13]. By using mobile device with wireless connection, reading of sensors as well as parameterization of them and devices can be done easily. This can bring benefit in such industrial plants where devices are in hard to reach places and going too close may cause dangerous situations. Example application An example of condition monitoring application where wireless technology gives advantage over wires could be a sensor attached to the rotating shaft of an electric motor or a generator. The sensor can measure, for example, temperature and/or torque directly from the shaft. Measured data are then transferred wirelessly to the receiver (another sensor, for example) that is attached to the motor frame. For low-cost sensor unit, containing a simple 8-bit microcontroller, preferred wireless technology is Zigbee. For temperature measurement, digital measuring element can be used, or if analog is used, microcontroller s built-in analog-to-digital converter can be used to digitize the measurement. Power requirements are low, so this kind of sensor can be operated with batteries for periods of a couple of years, depending on how often measurements are taken. In case of torque measurement, strain gages are attached to the shaft. An additional amplifier and a higher resolution A/D converter is needed to get accurate measurements. Power requirements for the measurement are comparatively high, so there is a need for an external power source. An illustrative example of a wireless sensor, which measures temperature and torque, and is attached directly to the shaft, is shown in Figure 2. Also, a vibration sensor is mounted on bearings to monitor bearing failures. All sensors can communicate wirelessly and form a network. Sensor that was presented here can be applied to detect broken rotor bars as well as damaged end rings in an induction motor. Vibration sensor Torque and temperature sensor Fig. 2: An example of an electric motor with wireless vibration sensor attached to the bearings and wireless torque and temperature sensor attached to the shaft.

8 Conclusion Utilization of wireless technologies in condition monitoring and remote diagnostics of electric drives is not straightforward. There are certain requirements, which were discussed in this paper that have to be fulfilled. These are, for example: transfer range, transfer speed, frequency band, network topology, connection to sensor, connection to Ethernet/fieldbus, power consumption and price. Also, different approaches in network topologies and applications are possible and were considered. New wireless technologies, especially short-range devices, offer new concepts and applications in condition monitoring of electric drives. In this paper, the applicability of different wireless technologies in condition monitoring data transfer and maintenance data collection applications was discussed. Data transfer speeds, range, power consumption and price of short-range devices make them suitable for applications presented. They offer new applications and in some situations they offer also cost reduction because building up a sensor network is fast and easy since no new cabling is required. The biggest benefit comes in the form of new concepts and applications for condition monitoring data transfer and maintenance data collection. More research work is needed to test the suitability of these different concepts, which were introduced. References [1] T. Lindh, On the Condition Monitoring of Induction Machines. Doctoral dissertation. Lappeenranta University of Technology, Lappeenranta ISBN [2] T. Brooks, Wireless technology for industrial sensor and control networks. Sensor for Industry, 2001, Proceedings of the First ISA/IEEE Conference, 5-7 Nov Pages: [3] J.B. Andersen, T.S. Rappaport, S. Yoshida, Propagation measurements and models for wireless communications channels. Communications Magazine, IEEE, Volume: 33, Issue: 1, Jan Pages: [4] S. Kjesbu, T. Brunsvik, Radiowave propagation in industrial environments. Industrial Electronics Society, IECON th Annual Conference of the IEEE, Volume: 4, Oct Pages: vol.4 [5] P. McDermott-Wells, What is Bluetooth?. Potentials, IEEE, Volume 23, Issue 5, Dec Jan Pages:33-35 [6] R. Shorey, B.A. Miller, The Bluetooth technology: merits and limitations. Personal Wireless Communications, 2000 IEEE International Conference on Dec Pages: [7] Bluetooth Special Interest Group. [8] Jui-Hung Yeh, Jyh-Cheng Chen, Chi-Chen Lee, WLAN standards. Potentials, IEEE Volume 22, Issue 4, Oct.-Nov Pages:16-22 [9] The Official Website of the Zigbee Alliance. [10] F.L. Lewis. Wireless sensor networks. Appeared in Smart Environments: Technologies, Protocols, and Applications. Ed. D. Cook, S. Das. New York, ISBN: [11] J. Ahola, Applicability of power-line communications to data transfer of on-line condition monitoring of electrical drives, Dissertation, Acta Universitatis Lappeenrantaensis 157, Lappeenranta University of Technology, Finland 2003, ISBN [12] R. Tiainen, V. Särkimäki, T. Lindh, J. Ahola, Estimation of the data transfer requirements of vibration and temperature measurements in induction motor condition monitoring. EPE2005, proceedings of the. Dresden Sep [13] NFC Forum.