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A vibration energy harvesting system is presented, which is equipped with a smart resonance-frequency adaptation. The system is able to react to environmental frequency shifts in a completely autonomous way and can be excited in a resonant mode over a large frequency range (150 to 190 Hz). The device was programmed to readjust its resonance frequency every 22 s, and even for large adjustment steps, the recovery time is less than 10 s at an acceleration amplitude of 0,6 g. The effective output-power ranges between 30 and 45 μW, depending on the ambient vibrational frequency. The control-unit in use relies on an ultra-low-power microcontroller, equipped with a look-up table and a learning algorithm. The look-up table allows a very fast and hence energy saving frequency adjustment. With the learning algorithm the control unit is able to keep the look-up table up to date. It can therefore correct inaccurate factory settings and deal with hysteresis, temperature or ageing effects.