Wireless technologies in condition monitoring and remote diagnostics

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) – 621 6772. Fax: +358 / (5) – 621 6799. E-Mail: [email protected] URL: http://www.lut.fi

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 802.11, 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

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 802.11 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 802.11 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

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 (