Method and control unit for demodulation

A method for demodulation comprising the steps of:

• exciting a vibrationally mounted, at least sectionally bar-shaped oscillating element for oscillating in the range of a resonance frequency of the oscillating element, wherein a temporally varying excitation signal is used for excitation, and wherein at least the temporal variation of the excitation signal is known or determined;
• detecting a modulated oscillation of the oscillating element by at least one sensor, wherein the sensor supplies a sensor measurement variable that varies over time as a function of an amplitude and a phase of the modulated oscillation of the oscillating element;
• wherein the method comprises the following:
• generating a first comparison signal by amplitude modulating a known temporally varying demodulation signal by the temporally varying sensor measurement variable;
• wherein the at least one sensor comprising a piezoresistive element and the sensor measurement variable is the electrical resistance of the piezoresistive element;
• wherein the demodulation signal is an AC voltage signal and that the first comparison signal is generated by a voltage divider;
• wherein the voltage divider is formed from a first electrical resistance and a second electrical resistance, and wherein the second electrical resistance comprises the electrical resistance of the piezoresistive element;
• wherein a difference signal is generated by a linear combination of the first comparison signal with a known temporally varying second comparison signal;
• wherein the difference signal is filtered by at least one element acting as a low-pass filter, thus generating a filtered difference signal,
• wherein the second comparison signal is generated by a further voltage divider, which is formed from a third electrical resistance and a fourth electrical resistance;
• wherein the voltage divider and the further voltage divider are preferably part of a bridge circuit, and in that the first comparison signal and the second comparison signal are subtracted from one another to generate the difference signal;
• wherein a difference signal is generated by a linear combination of the first comparison signal with a known temporally varying second comparison signal;
• wherein the difference signal is filtered by at least one element acting as a low-pass filter, thus generating a filtered difference signal, wherein the second comparison signal is generated using the demodulation signal;
• wherein the modulated oscillation of the oscillating element is additionally detected with a second sensor, including a second piezoresistive element essentially identical to the piezoresistive element;
• wherein the second sensor also supplies a sensor measurement variable that varies versus time as a function of the amplitude and the phase of the modulated oscillation of the oscillating element and wherein the senor measurement variable of the second sensor is the electrical resistance of the second piezoresistive element, in that a first phase-shifted comparison signal is generated by amplitude modulating a known temporally varying phase-shifted demodulation signal by the temporally varying sensor measurement variable of the second sensor;
• wherein the phase-shifted demodulation signal has a defined known phase shift in relation to the demodulation signal, in that the phase-shifted demodulation signal is also an AC voltage signal;
• wherein the first phase-shifted comparison signal is also generated by a voltage divider, wherein the voltage divider is formed from a first electrical resistance and a second electrical resistance and wherein the second electrical resistance includes the electrical resistance of the second piezoresistive element, in that a phase-shifted difference signal is generated by a linear combination of the first phase-shifted comparison signal with a known temporally varying second phase-shifted comparison signal, in that the second phase-shifted comparison signal is generated by the phase-shifted demodulation signal;
• and
• wherein the second comparison signal is generated by a further voltage divider, which is formed from a third electrical resistance and a fourth electrical resistance, wherein the voltage divider and the further voltage divider are part of a bridge circuit, and wherein the first phase-shifted comparison signal and the second phase-shifted comparison signal are subtracted from one another to venerate the phase shifted difference signal, in that the phase-shifted difference signal is filtered by at least one element acting as a low-pass filter, thus generating a filtered phase-shifted difference signal.