Vibration Energy Scavenging and Management for Ultra Low Power Applications

One of the award winning papers at the 2007 Low Power Electronics and Design symposium was prepared by Lu Chao, Chi-Ying Tsui and Wing-Hung Ki from The Hong Kong University of Science and Technology. The work is the design of a mechanical vibration energy scavenging and management system. Including a maximum power point tracking scheme (MPPT) to maximize the amount of energy harvested from piezoelectric conversion. This scheme has achieved an energy harvesting efficiency higher than 90%.

It has been found that for piezoelectric conversion there is an optimal output voltage point for any given magnitude and frequency of vibration. Hence to obtain maximum energy harvesting one needs to track this optimal voltage point with changing vibrations that are common to most real world applications. Furthermore, if the energy harvesting scheme is to be applied to a batteryless system, it is essential that it is capable of self-starting even after extended periods with no vibrations. Existing MPPT schemes are not designed for low power applications because they employ complex circuit components and computation-intensive control algorithms. However the proposed system overcomes these design challenges and has very low power overhead allowing it to be self-powered and capable of self-starting. To provide a self -starting system a hybrid scheme is used to rectify the piezoelectric AC input signal to a DC voltage source for powering up the circuit. Since a passive diode rectifier causes a significant reduction in the output voltage of the converter however is essential for self-starting, the scheme employs both passive and active diodes for an optimal solution. The passive diode rectifier is used for self-starting and the active diode rectifier takes over when the harvester can provide the required power. This ensures self-starting and maximum efficiency whenever possible.

The overall system consists of the energy harvester, tracking unit, tracking pulse generator, refreshing unit, control unit and a micropower buck converter. The energy harvester consists of the hybrid AC-DC converter as well as a storage capacitor. The tracking unit detects the vibration variations and generates a reference voltage for the control unit. It tracks the variations with a time-multiplexing scheme which is multiplexed with the AC-DC rectifier, reducing overhead. Also, to ensure low power the duty cycle of the tracking unit is kept to a minimum allowing for the harvester to spend most of its time supplying power to the load. Through some calculations the minimum duty cycle is 1.5T, were T is the cycle time of the vibration. The tracking pulse generator generates the tracking signal for the tracking unit. It gets this signal directly from the AC signals coming in from the piezoelectric film. Using a comparator, resistor divider, counter and combinational logic gates the pulse generator outputs a tracking pulse with a fixed duty cycle of 1/64 and a pulse width that is equal to 2T satisfying the 1.5T minimum requirement of the tracking unit. The refreshing unit is used to periodically refresh the reference voltage obtained by the tracking unit and provide this voltage to the control unit. Basically this is achieved by charging a capacitor to the new voltage reference value. The control unit uses a band-band control strategy to maintain the output voltage of the energy harvester at the optimal value. The control unit is made up of a Schmitt trigger and a voltage comparator. An output voltage of 0.9875-1.0125 V optimal is maintained by the control unit. With a supply voltage range of 2V-6.5V the total power overhead of just the control unit is 360nW- 1.17uW. Lastly, since conventional buck converters are not optimized for ultra low power applications, the system consists of an optimized micropower buck converter. The main difference of this converter is the use of a PFM control scheme instead of the conventional PWM scheme.

With a power harvesting efficiency above 90%, this vibration energy scavenging and management system has a promising future. There are many applications that I can think of that would greatly benefit from such a power supply, however I think that ultra low power wireless sensor will probably be one of the first applications to truly take advantage of this technology. This is a very practical as well as innovative paper and I strongly recommend it to anyone that is looking into piezoelectric energy harvesting.

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