(October 19, 2010) — Francesco Doddo, Roberto Condorelli, Roberto De Nuccio, STMicroelectronics (NYSE:STM), acknowledge the benefits and upsides of solar photovoltaic (PV) installations. However, safety and security concerns have accompanied photovoltaic installations from the start. Among the MEMS applications in the solar market are vibration analysis and anti-theft and include residential inverters, inverters mounted on poles, and solar street lighting. With PV installations on the rise globally, the authors look at various MEMS applications in this renewable energy system.

Vibration analysis for earthquake detection and wind monitoring

When solar PV is located in a seismic zone, earthquake forces must be considered; additionally, oscillations caused by the wind have to be taken in to account when the solar system is located in hurricane zones.

In the case of grid-connected systems (i.e., residential inverters, inverters mounted on poles, solar street lighting), earthquake and hurricane events increase the risks of fire, arc generation, and electrocution. At this time, every solar system should be disconnected from the grid.

In contrast, for off-grid systems such as battery-powered solar LED street lights, during an earthquake, the solar LED should be turned on with maximum power — working as emergency lights to ensure a population’s safety and avoid panic. In both cases, MEMS accelerometers are an efficient tool to monitor the solar panel vibrations due to wind or to earthquakes.

Figure 1. a) FFT analysis; and b) instantaneous accelerations plot.

Figure 1 contains photos taken using the graphical user interface (GUI) of an evaluation board that features a 3-axis ±8g smart digital accelerometer. Figures 1 and 2 show the fast-Fourier transform (FFT) calculation and the instantaneous accelerations plot along the three axes of the accelerometer [1,2]. Combining the interrupt functionalities, the sleep-to-wake and the low-power mode allows the application to have a fast and efficient vibration detection and vibration analysis.

Figure 2. A STEVAL-MKI022V1 evaluations board.

In a typical vibration analysis application, the digital accelerometer may be configured to work in low-power mode and generate an inertial wake-up interrupt signal accordingly to a programmed acceleration event along the enabled axes. Thanks to the sleep-to-wake function in conjunction with the low-power mode, the device, even if asleep, continues sensing acceleration and generating interrupt requests.

When the acceleration on one of the axes overcomes a user-programmed threshold, the sleep-to-wake function is activated, and the accelerometer is able to automatically wake-up as soon as the interrupt event has been detected, increasing the output data rate and bandwidth. With this feature, the system may be efficiently switched from low-power mode to full performance depending on user-selectable positioning and acceleration events, thus ensuring power saving and flexibility. Depending on the level of complexity requested by the application, the acceleration data can be sent to a host microcontroller (i.e., 32bit STM32L or 8bit STM8L low power ST MCUs) through the on-board SPI interface.

Tilt detection for anti-theft or safe maintenance

When solar installations are located in remote places — solar pole installations, solar farms — an anti-theft system is useful to avoid the risk of theft. MEMS accelerometers can be used for tilt detection to detect a change in the installation angle. In a similar way, tilt detection can help users understand if the solar panel has been removed from its original location (i.e., for maintenance); this information can be used to put the system in a safe condition, i.e., trough electronic switch off the line (Fig. 3).

Figure 3. a) Tilt measurement; b and c) the installation angle change during a theft or maintenance action.

The accelerometer measures the gravity vector projection on the sensing axis. The amplitude of the sensed acceleration changes as the sine of the angle α between the sensitive axis and the horizontal plane. With a 3-axis accelerometer, the user can use the Z axis to combine with the X and Y axes for tilt sensing, to improve tilt sensitivity and accuracy over 360° of rotation (Fig. 4) [3].

Figure 4. Tilt sensitivity of a 3-axis accelerometer, and b/c) an example of tilt sensing using an evaluation board that uses a 3-axis accelerometer, and the associated GUI.

References:
1. ST Microelectronics’ MEMS Website page: http://www.st.com/mems
2. MEMS Evalboards: http://www.st.com/stonline/products/families/evaluation_boards/steval-mki022v1.htm
3. Tilt measurement Application Note: http://www.st.com/stonline/products/literature/an/17289.pdf

Francesco Doddo holds a degree from Università degli Studi di Messina and is market development engineer at STMicroelectronics, Lexington, Massachusetts, USA, www.st.com. The article was co-authored by Roberto Condorelli and Roberto De Nuccio, also of STMicroelectronics.

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