Sensors and Actuators A 110 (2004) 171–176
An investigation of self-powered systems for condition monitoring applications夽 E.P. James, M.J. Tudor, S.P. Beeby, N.R. Harris, P. Glynne-Jones, J.N. Ross, N.M. White∗ School of Electronics and Computer Science, University of Southampton, Southampton SO17 1BJ, UK Received 20 September 2002; received in revised form 15 October 2003; accepted 20 October 2003
Abstract Over recent years there has been a growing interest in the field of micro-systems and their applications across a wide range of areas, including sensor-based systems able to operate with full galvanic isolation. This paper details the development of a self-powered system, specifically for sensor applications that can be energised on a test rig by an electromagnetic vibration-powered generator. This enables wireless operation without the use of a battery with a finite service life. The results of two systems designed for remote sensing in condition monitoring applications are discussed. The first system uses a liquid crystal display to provide the system output; the second uses an infra-red link to transmit the data output. © 2003 Elsevier B.V. All rights reserved. Keywords: Energy harvesting; Self-powered; Wireless communications
1. Introduction Condition monitoring provides information on the health and maintenance requirements of industrial machinery and is used in a wide range of industrial applications. Parameters such as vibration, temperature, lubricant quality and power consumption can be used to monitor the mechanical status of equipment. An example application is mechanical bearings where impending failure is often indicated by a change in the frequency and amplitude of the vibrations detected in the bearing housing. As such, condition monitoring can play an important role in effective maintenance scheduling by preventing unnecessary repairs and catastrophic failures. At present, such systems must be physically wired in to provide both power and communications paths. In many applications, for example, on rotating machinery or high temperature environments, such wired connections are often inconvenient or impractical. Where this is the case, remote operation is a possible solution. The system power can be provided by, for example, a lithium battery [1] and an appropriate form of wireless communication must be employed [2,3]. Batteries, however, have a finite operational life that 夽 Presented at the Conference Eurosensors XVI, Prague, Czech Republic, 15–18 September 2002. ∗ Corresponding author. Tel.: +44-23-8059-3765; fax: +44-23-8059-2901. E-mail address:
[email protected] (N.M. White).
0924-4247/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2003.10.057
imposes its own burden on the plant maintenance schedule. Recent advances in micro-systems technology have not been matched by similar advances in battery technology and as a result the battery is often larger than the system itself. Additionally, the environmental impact imposed by the disposal of batteries can be addressed by the use of renewable energy sources such as vibration or solar power. Given that ambient vibrations are present in many of these applications, self-powering of the monitoring system by converting mechanical energy to electrical energy is an obvious way forward. Such an approach is sometimes referred to as energy harvesting and enables a remotely operated condition monitoring system to be employed, which is itself maintenance free for the lifetime of the machinery. Indeed, the power output from the generator could be used as part of the condition monitoring system and could also operate at temperature extremes where batteries may not. This paper details such a sensor system designed to operate on machines exhibiting regular vibrations. Such a self powered system relies not only on a mechanism for transducing the kinetic energy into electrical energy (the generator), but also the system electronics required to make the power supply useful. This presents many system challenges and hence this paper provides a comprehensive discussion of the relevant electronics design criteria and efficiencies of two different system designs. Finally, the suitability of different wireless communications methods, given the characteristics of the vibration based power supply, are also explored. The
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integration of the vibration generator, sensor, power regulation and communications is a novel approach for developing the next generation of intelligent sensor systems.
The wireless communication is provided by an infra-red link. The raw sensor data can be transmitted as required and, since power is not an issue at the receiver, signal conditioning can be carried out here. The two prototypes and the design criteria are discussed below.
2. System design Two prototype self-powered systems have been developed and the key blocks of such a system are listed below and shown diagrammatically in Fig. 1. • A magnet-coil generator [4,5]: This is mounted on a shaker rig to simulate different real life environments. • Conversion circuitry: This is required to step up the generator output voltage and provide rectification and regulation. • A sensor and associated circuitry: This is for monitoring the condition of the machinery; its exact details depend upon the particular application. • Wireless communication circuitry: This is required to relay the data from the SPMS to a suitable display or network location. The first prototype micro-system uses a transformer to increase the voltage output of the generator and a LCD to display the system output. This represents the simplest form of wireless communication. In practice, such an approach relies upon the signal conditioning being carried out within the system and this therefore increases the total power consumption. The second prototype uses a voltage multiplier circuit to both increase and rectify the generator’s output.
Fig. 1. Block diagram of the prototype system.
3. The vibration-based magnet-coil generator Each prototype is powered by an electromagnetic generator. Experimental work at the University of Southampton into several different types of magnet-coil generator has been carried out to determine the magnitude of power that can be achieved at operating frequencies of between 50 and 300 Hz. The generator is shown in Fig. 2 and has been well documented by Glynne-Jones et al. [4,5]. Neodymium iron boron (NdFeB) magnets are mounted on a steel beam that oscillates about a copper coil. The generator is designed to operate at its resonant frequency and therefore must be tuned to match a particular frequency of vibration present in the application environment. This is achieved by simply varying the length of the steel beam. To simulate environmental vibrations, the generator is mounted on a Goodman V.50 Mk.1 (Model 390) shaker which can produce vibrations in the range of 5 Hz–4 kHz and is limited by the resonant frequency of the shaker rig. The rig is controlled by a Hewlett Packard HP35660A Dynamic Signal Analyser driving a Cambridge Audio 65 W integrated amplifier. Fig. 3 shows a spectral analysis of the self-powered generator giving the voltage output of the generator when excited by a broad band random frequency generator. Point A represents 50 Hz mains noise, point B represents the fundamental resonant frequency at 102 Hz, and points C and D represent other torsional and harmonic modes. Since the output power increases with the third order of frequency [4],
Fig. 2. A prototype vibration based power generator.
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Fig. 4. Model of the transformer impedance matching circuit.
Fig. 3. Determination of the generators main resonant frequency.
harmonic frequencies can also potentially be used to generate useful amounts of power. A displacement of the magnet of 0.4 mm at the generator fundamental resonant frequency of 102 Hz produces approximately 0.5 Vrms and a constant power of approximately 2.5 mW with a load impedance of 100 . Given this level of power, efficiency is a key factor in the selection of each component of the system. Both prototypes use a low-power accelerometer (Analog Devices ADXL202) as the sensor. The accelerometer operates within ±2 g and has a bandwidth that can be set, using external capacitors, from 0.01 to 5 kHz. This is a realistic device for condition monitoring applications where a change in machinery vibrations can often indicate that maintenance is required. The low power consumption of the device, which functions at
