Micromachined Electromagnetic Generator
The electromagnetic generator uses the motion of a moving magnetic field intersecting a static wound copper coil. The basic design of the device is shown in figure 1. It consists of a discrete coil located in a machined base. The magnets are located at the end of a cantilever beam along with addition mass in the form of wire eroded tungsten blocks. The inertial mass and magnets are designed to vibrate vertically with the magnetic circuit designed to maximise the flux gradient across the coil and therefore the voltage and power generated.
Figure 1 - Micromachined electromagnetic generator design
The voltage output is a key consideration and therefore coils wound from 12 micron diameter wire have been used. This gives around 2800 turns in a coil just 0.5 mm thick and with an outer diameter of 2.8 mm. This number of turns combined with the optimised magnetic circuit yields voltages in excess of 700 mVrms from vibrations of 60 milli-g at 50 Hz. The power output is in excess of 55 microwatts.
This voltage is sufficient to enable voltage multiplication and rectification using a diode based circuit the output of which is stored on a supercapacitor. The generator and power conditioning circuit have been combined with a low power micro controller, an accelerometer and a wireless transmitter to successfully demonstrate an autonomous wireless sensor powered solely from vibrations. The system transmits peak acceleration data and the operating duty cycle is automatically adjusted to match the available power. A picture of the system is shown in figure 2.
Figure 2- Electromagnetic generator powered wireless acceleration sensor
Please click here to download a video of the system in action.