Method and device for monitoring a process variable with vibronic sensor
A method to determine and/or monitor at least one process variable of a medium with at least one vibration-capable unit. The vibration-capable unit is excited to mechanical vibrations by means of an electrical excitation signal of an adjustable frequency; wherein the mechanical vibrations are transduced into a received electrical signal, which is characterized at least by a frequency and/or a phase and/or an amplitude. The excitation signal is generated based on the received signal; wherein the voltage values of the received signal are sampled at specified predetermined points in time, starting from the excitation signal. The real part and the imaginary part of the received signal are determined from the sampled voltage values of the received signal by means of a Goertzel algorithm; wherein at least one Goertzel coefficient—in particular the number of the sample values and/or an operating frequency and/or a sample frequency—is provided for performing the Goertzel algorithm. At least the current phase and/or the current amplitude of the received signal are calculated from the real part and the imaginary part of the received signal; wherein the frequency of the excitation signal is adjusted such that a predeterminable phase shift is present between the excitation signal and the received signal; and wherein the at least one process variable is determined.
1. A device to determine and/or monitor at least a fill level, a density, a viscosity, or a volume flow rate of a medium, comprising:
at least one mechanical vibration-capable unit;
a control unit;
an electromechanical transducer unit which is designed to excite said at least one mechanical vibration-capable unit to mechanical vibrations based on an electrical excitation signal of an adjustable frequency, and to receive the mechanical vibrations and transduce them into an electrical received signal, which electrical received signal is characterized at least by a frequency and/or a phase and/or an amplitude;
an electronic unit with a microprocessor which is designed:
to generate the electrical excitation signal based on the electrical received signal,
to sample voltage values of the electrical received signal at defined predetermined points in time,
to determine the real part and imaginary part of the electrical received signal from the sampled voltage values of the electrical received signal by means of a Goertzel algorithm,
to preset at least a number of sample values and/or a working frequency and/or a sample frequency for performing the Goertzel algorithm,
to calculate at least a current phase and/or a current amplitude of the electrical received signal from the real part and imaginary part of the electrical received signal; and
a control unit that is designed to set the frequency of the electrical excitation signal such that a predeterminable phase shift is present between the electrical excitation signal and electrical received signal; and
said electronic unit is designed to determine the fill level, density, a viscosity, or volume flow rate.
2. The device according to claim 1 , wherein, further comprising:
an adaptive switched capacitor filter.
3. The device according to claim 1 , wherein:
said electromechanical transducer unit is provided by a piezoelectric actuator or an electromagnetic actuator.
4. A method to determine and/or monitor a fill level, a density, a viscosity, or a volume flow rate of a medium with at least one vibration-capable unit, comprising the steps of:
exciting the vibration-capable unit to mechanical vibrations by means of an electrical excitation signal of an adjustable frequency;
transducing the mechanical vibrations into electrical received signal, which electrical received signal is characterized at least by a frequency and/or a phase and/or an amplitude,
generating the electrical excitation signal based on said electrical received signal;
sampling voltage values of said electrical received signal at specified predetermined points in time, starting from said electrical excitation signal;
determining the real part and imaginary part of said electrical received signal from the sampled voltage values of said electrical received signal by means of a Goertzel algorithm,
presetting at least a number of sample values and/or an operating frequency and/or a sample frequency for performing the Goertzel algorithm;
calculating at least a current phase and/or a current amplitude of said electrical received signal from said real part and imaginary part of said electrical received signal; and
adjusting the frequency of said electrical excitation signal such that a predeterminable phase shift is present between said electrical excitation signal and said electrical received signal, and
determining the fill level, a density, a viscosity, or a volume flow rate based on said received signal.
5. The method according to claim 4 , wherein:
a working frequency is set to the frequency of said electrical excitation signal.
6. The method according to claim 5 , wherein:
a whole-number multiple of a period of said electrical excitation signal is selected for the number of said sample values.
7. The method according to claim 4 , wherein:
a sampling frequency is selected as a whole-number multiple of the frequency of said electrical excitation signal.
8. The method according to claim 7 , wherein:
two or four times the frequency of said excitation signal is set as said sampling frequency.
9. The method according to claim 4 , wherein:
the Goertzel algorithm is used across multiple periods of said electrical excitation signal.
10. The method according to claim 4 , wherein:
said electrical excitation signal is a square wave signal or a sine signal.
11. The method according to claim 4 , wherein:
said predeterminable phase shift is set using the quotient from the number of said sample values and said sampling frequency by means of a time shift of said sample values in relation to said excitation signal.
12. The method according to claim 4 , wherein:
said predeterminable phase shift is 90°.
13. The method according to claim 4 , further comprising the step of:
subdividing a measurement period into at least two time intervals, wherein:
respectively a first predeterminable phase shift is set in a first time interval, and a second predeterminable phase shift is set in a second time interval.
14. The method according to claim 4 , wherein:
a function defined by the quotient of the real part and the value of the imaginary part of said received signal, or the inverse of this function for a control deviation, is used to set said predeterminable phase shift.