1. - Introduction.
In the latest years the use of devices implemented with sensors for the measurement of different parameters has extended ( temperature, humidity, acceleration, pressure, parameters…), these devices use in addition a radio frequency system to communicate that parameter to a central unit that manages it at monitorización level, alarms, activation of third elements, etc… many times these devices are placed in areas with lack of accessibility and is necessary to implement maintenance plans that replace the batteries of these devices or replace the complete device.
A clear trend in modern electronics is the consumption reduction in devices like microcontrollers than adopt formulas of partial activation of circuitry or wathdogs to achieve consumptions next to few microamperes, this low consumption makes or also reduce the size of the batteries or increase the life of the same ones.
Sometimes we have small devices with low consumption equiped with sensors placed in remote areas (bridges, damps...), machinery (elevators, motors, big fans, automotive parts,..) or even in humans (patients or sportman..), those areas or devices has some parts than can move or even trasmit vibration.
The mechanical energy created by the movement or vibration can be used and be transformed into electrical energy. If we analyzed the commented sensorial devices and the low consumption that, in numerous cases, they characterize them we can think that a device that is able to extract energy of the movement it can be sufficient to feed electronic microdevices or to help to increase the life of the batteries used like primary power supply.
To the devices that can generate electrical energy from mechannical movement or vibration we can denominate harvesting device or energy scavengers . In case they use permanent magnets together with windings to obtain the energy generation we will denominate VEPGs (Vibration to Electrical Power Generator).
PREMO group is developing technology of micro VEPGs able to replace batteries in applications with sensors and low consumption microcontroller, applications like TPMS in automotion, devices for sportsmen equipped with HBMS (Heart beat Monitoring systems, pulse monitoring) and others with industrial sensors. These micro VEPGs are able to generate between 2mW and 6mW (enought to replace small batteries type 2016 or 2032).
2. Electromotive force generated in the VEPGs.
According to the laws of electromagnetism, if we changed a magnetic flux into a wire loop we can induce an electromotive force in the ends of the same one. The magnetic flux in the loop can be caused of two different forms:
- Changing the flux density when cross the loop.
- Moving the loop in relation to the magnetic flux.
So, a movement or a vibration in a loop crossed by a magnetic flux creates a voltage induced in the terminals directly proportional to the cross area of the loop, the relative movement that makes and the speed whereupon does it, if this is valid for a single loop we can also suppose that in n-turns the induced voltage could be n times.
A permanent magnet normally generates a symmetrical field and the electromotive force generated is fully dependent on the axial component (Bz) and radial (Br) of the density of magnetic flux and it does not have dependency of tangential component (BF)
We see than the induced voltage is proportional to the axial field as as much radial, the relative speed the turn against the permanent magnet and the length that it crosses this in relation to the turns.
Development tools and simulation of enviroments, tests of VEPGs.
PREMO group is working in the development of a complete tool for the design of VEPGs systems, in this development program of this systems is collaborating with development teams the Universities of Malaga, Spain (Department of electronic Technology) and Barcelona ( CEIB Group of the UPC).
The tool include a complete system to catch information of movements and vibration, development of SW applications to process the data, tools of VEPGs simulation by finite elements, manufacture of advanced prototypes and a system of functional test that allows to a test in similar conditions to which will be the microgenerator.
When we considered to extract energy of a system that vibrates or makes a movement is helpful to know the parameters of this movement or vibration, the basic parameters of which we would extract but information to design a device harvest would be the frequency and the amplitude of the movement and in case they are periodic movements the cadence of he himself or the period in which it takes place.
A real application of this type of problems we can find it in case we want to design a generator for a system of monitoring vital parameters of a marathon runner, in this case we need to feed the pulse and temperature sensors together with the display that normally uses batteries for the feeding of the electronic circuitry.
PREMO has developed together with the University of Malaga a new system that allows on the one hand to extract and to catch the data of a movement by means of a series of sensors and by a other application of high level that allows the interpretation of the data to make decisions from location of the microdevices to maximize the energy generation.
We see in the following figure an example of this application in which we can appreciate the capacity of generation of energy in sportmens provided with a system of monitorización in different positions of the body to evaluate the capacity to generate energy to feed a measurement device of the heart beat.
With the generated data in the system we make the calculations to evaluate the prototype, previously to the construction we can go to simulation tools to make adjustments in the design and to optimize the VEPG with the following prototiping time saving , for this function Premo has the help CEIB group of the Polytechnical University of Catalonia.
Once the prototype is build we could use a tool that allows us to reproduce the vibration applying real parameters to a device that is able to reproduce movements with a variable amplitude and frequency, the generated levels of voltage also are transfered to the system that is able to assign variable loads to him to establish the levels of power given by the microgenerator. In the following figure we see the structure with which it operates the device based on the control of two motors that make the transfering of the vibration to the device
Example of use we found it in project of design of device able to feed sensor on heart rate typically used sportsmen to monitorizar exercises or races, are systems normally fed by batteries type 2032 ( 2mW of power needed to measure and transmit the datas to device display, typically implemented in a watch which it informs to the runner), the harvesting device is implemented with harvesting generator able to give this power level using the movement of the own sportsman.
PREMO has developed technology for VEPGs taking models and simulations based on real enviroments, the core technology of the company is focussed in ultrafine wire products manufacturing and the miniaturization of microgenerators fits perfectly with this core technology, smaall devices are wound with 5000-10000 turns of 20 to 60 microns wire. Powers in the order from 2 to 10 mW are widely achievables, batteries like type 2016, 2032 and similar are suitables to be removed by the VEPGs whenever the device to feed is affected by vibrations or/and movements.
The typical applications in which the VEPGs devices can be used can be from sensors of vital parameters for sportsmen, small power supplies for medical elements that need amplification, industrial systems working under vibration (sensor in motors, elevators,…), sensors of all type in automotion (batteryless sensors for maesuring of tyre pressure, measurement of parameters in motors…).
The use of these sensors with harvesting devices is specially interesting forming networks that can remotely be accessible from control units without hardly maintenance on the part of the manager of networks when not having to change batteries throughout their life utility, this is specially important in applications of movement or vibration monitoring in structures, great buildings as viaducts or measurement of cracks to high altitude (bridges, damps..).
With the expansion of these networks of sensors with harvesting devices the capacity of control of the denominated 3,0 WEB or Internet of the things will be increased, getting to be able to monitoring specific complete systems from specific webs for sensors management.
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