Page 191 - MEMS Mechanical Sensors
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180                                                                  Inertial Sensors

                                                   f s
                                                                           Digital
                                                                     1     bitstream
                                                 S/H      C(z)
                                                                     0
                                      Pick-off        Compensator  Comparator  V out
                                                                  V f


                                                                  V f


                 Figure 8.6  Closed loop accelerometer with analog force-feedback.




                    In this approach the information about the deflection of the proof mass is fed
                 into a comparator. Depending on the dynamics of the proof mass, a compensator
                 may be required to stabilize the loop. The comparator controls a range of switches,
                 which applies a feedback voltage to the electrode that the proof mass is further away
                 from in this moment; the other electrode is being grounded. This is done for a fixed
                 time interval, which is locked to the sampling frequency of the comparator. As with
                 their electronic counterpart, this electromechanical sigma-delta modulator is an
                 oversampling system; hence, the clock frequency has to be many times higher than
                 the bandwidth of the sensor. This approach has a number of advantages over analog
                 force-feedback:

                    1. No electrostatic pull-in is possible as an electrostatic feedback force is only
                       produced in one direction (i.e., pulling the proof mass to its nominal
                       position).
                    2. The output signal, taken from the comparator, is a direct digital signal in the
                       form of a pulse-density modulation (i.e., the number of high-bits in a given
                       interval is a measure of the input acceleration). The output signal can
                       interface directly to a digital signal processor (DSP), which can perform the
                       necessary lowpass filtering and further signal processing if required.
                    3. As with any sigma-delta modulator, such a sensor will produce a self-
                       sustained oscillation at the output even if no input acceleration is present. As
                       a constant signal at the output would indicate a sensor failure, this can be
                       used as a simple form of functionality test.


                    This approach has gained much popularity in recent years, with a number of
                 researchers reporting accelerometers with such a closed loop control system [7–10].



                 8.2.2  Research Prototype Micromachined Accelerometers
                 Many prototype micromachined accelerometers have been reported during the last
                 two decades. In the following sections, an overview of key devices will be given and
                 a few examples of interesting and representative devices will be described in
                 more detail. The classification used here is mainly based on the position sensing
                 mechanism.
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