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A closed loop circuit capable of tracking resonant frequencies for MEMS-based piezoresistive cantilever resonators is
developed in this work. The proposed closed-loop system is mainly based on a phase locked loop (PLL) circuit. In order
to lock onto the resonant frequency of the resonator, an actuation signal generated from a voltage-controlled oscillator
(VCO) is locked to the phase of the input reference signal of the cantilever sensor. In addition to the PLL component, an
instrumentation amplifier and an active low pass filter (LPF) are connected to the system for gaining the amplitude and
reducing the noise of the cantilever output signals. The LPF can transform a rectangular signal into a sinusoidal signal
with voltage amplitudes ranging from 5 to 10 V which are sufficient for a piezoactuator input (i.e., maintaining a large
output signal of the cantilever sensor). To demonstrate the functionality of the system, a self-sensing silicon cantilever
resonator with a built-in piezoresistive Wheatstone bridge is fabricated and integrated with the circuit. A piezoactuator is
utilized for actuating the cantilever into resonance. Implementation of this closed loop system is used to track the
resonant frequency of a silicon cantilever-based sensor resonating at 9.4 kHz under a cross-sensitivity test of ambient
temperature. The changes of the resonant frequency are interpreted using a frequency counter connected to the system.
From the experimental results, the temperature sensitivity and coefficient of the employed sensor are 0.3 Hz/°C and 32.8
ppm/°C, respectively. The frequency stability of the system can reach up to 0.08 Hz. The development of this system
will enable real-time nanoparticle monitoring systems and provide a miniaturization of the instrumentation modules for
cantilever-based nanoparticle detectors.
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