Design and modelling of a vibration powered micro-generator

Sensors are being used in increasingly diverse application areas where physical connections to the outside world are difficult. Examples include medical implants, sensors in nuclear processing plants, environmental monitoring and structural monitoring. If the sensor is embedded within a structure, routine maintenance duties such as changing batteries may cause significant problems. The need for having a power supply that will last for the lifetime of the sensor is therefore evident. A popular approach is to use the ambient energy within the environment of the sensor. Possible sources include solar energy, thermal energy from temperature gradients and kinetic energy in the form of environmental vibrations. Each of these approaches offers its own advantages and disadvantages, but of the three techniques mentioned the latter has attracted the least interest to date. The general approach gives rise to the term self-powered microsystem, and a review of such devices is covered by G1ynne-Jones and White]. One of the issues for vibration-based generators is that of energy storage, as the vibrations may not necessarily be present in a continuous and uniform manner. Potential storage media include lithium batteries and super capacitors. The remainder of this paper will be dedicated to the design and modelling of two possible techniques for producing vibration-based generators. The first device is a magnet and coil arrangement where relative movement between the coil and the poles of a permanent magnet generates electricity by electromagnetic induction. The second device is based on using a piezo-electric material to generate electrical energy from vibration-induced deformations (the direct piezo-electric effect). The device is fabricated using thick-film technology, with a piezo-electric paste based on lead zirconate titanate (PZT)2.Analytical solutions have been derived for a wide range of physical effects and, where appropriate, provide a relatively simple technique for modelling microsystem characteristics. In the case of the microgenerator they have been used to calculate resonant frequencies and electrical power output. Finite element analysis (FEA) is a very powerful and flexible technique for modelling complex geometries and is widely used in the design of microsystems. Typically, the structure is divided into a finite number of elements, each with precisely defined material and physical properties, and computers are used to solve the resulting matrix equations. Many FEA packages exist; the work presented here was performed using the ubiquitous ANSYS® software. FEA is particularly useful in microsystems designs to simulate characteristics such as stress distributions, deflections, resonant frequencies and mode shapes, viscous damping effects, piezo-electric coupling and thermal distributions