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Fig. 6. Block diagram of the feedback loop.
Synoptic view is resumed in Fig. 7 with the complete
block diagram of the closed-loop system. Abbreviations
mentioned are the ones used in Fig. 4 and Fig. 6.
Henceforth, output voltage is a function of the electrical
power required to maintain a zero temperature difference
between the two detectors.
Fig. 7. Block diagram of the closed-loop system.
With this closed-loop configuration, we perform a
frequency analysis, visible in Fig. 8, of the accelerometer
presented in section 2. A sensitivity S 1 = 76 mV/g or
0.034 °C/g closed to the open-loop configuration one is
obtained while the cut-off frequency has increased by more
than a factor 15 with a value F c = 1025 Hz. In this case, a
resonance appears that would disappear with better settings
on the PID controller. Nevertheless, this bandwidth result is
the larger never obtained with a thermal accelerometer. It is
ten times larger that could be found in the literature.
Fig. 8. Closed-loop frequency response.
11-13
May 2011, Aix-en-Provence, France
V. CONCLUSION
This work investigates the behavior improvement
brought by a closed-loop configuration when using a
MEMS thermal accelerometer. This sensor was
micromachined by micro-electronics techniques. In a first
time, we studied the open-loop configuration with typical
characteristics. A closed-loop structure has been conceived
to improve bandwidth with no sensitivity reduction.
Feedback loop is directly included in this sensor
architecture by addition of two resistors close to detectors.
Modulation of the electrical power injected allows
temperature profile to remain symmetric. Experimental
measures prove that we can achieve a large bandwidth for a
system based on thermal exchanges with no modification
of thermal sensitivity.
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