Device for measuring a measurement variable

1. An apparatus for measuring a measured variable, including:

first inductance;

a measurement inductance that is set up to detect the measured variable and that is coupled to the first inductance;

a capacitance that is interconnected with the first inductance to form a parallel resonant circuit; and

a microcontroller, connected directly to the parallel resonant circuit via at least one output port and connected directly to the measurement inductance via at least one input port, the microcontroller is configured to execute software instructions to:

excite the parallel resonant circuit into oscillation at an excitation frequency that is derived from a clock of the microcontroller, and

measure a value revealing the measured variable using the measurement inductance.

2. The apparatus as claimed in claim 1 ,

wherein

the parallel resonant circuit is connected to a first port pin and to a second port pin of the microcontroller, the second port pin being supplied with a clock that is inverted in relation to the first port pin.

3. The apparatus as claimed in claim 2 ,

wherein

the parallel resonant circuit is at least one of:

i) connected in parallel with the first port pin to a number of further first port pins that are connected to the first port pin in sync,

ii) connected in parallel with the second port pin to a number of further second port pins that are connected to the second port pin in sync.

4. The apparatus as claimed in claim 1 , wherein

the port pins to which the parallel resonant circuit is connected have a push-pull output stage or a tri-state output stage.

5. The apparatus as claimed in claim 1 , wherein

the excitation frequency differs from a resonant frequency of the parallel resonant circuit by no more than 25%.

6. The apparatus as claimed in claim 1 , wherein

the measurement inductance has a first pole connected to a port pin of the microcontroller and has a second pole connected to a potential that corresponds to at least approximately half the supply voltage of the microcontroller.

7. The apparatus as claimed in claim 6 , wherein

the potential that corresponds to at least approximately half the supply voltage of the microcontroller is produced by a voltage divider.

8. The apparatus as claimed in claim 7 , wherein the potential that corresponds to at least approximately half the supply voltage of the microcontroller is produced by a smoothing capacitor wherein the smoothing capacitor has a first pole connected to a reference-ground potential or to the supply voltage, and wherein a second pole of the smoothing capacitor is connected via a resistor to a port pin of the microcontroller, which port pin has a pulsed signal applied to it.

9. The apparatus as claimed in claim 6 , wherein

the potential that corresponds to at least approximately half the supply voltage of the microcontroller is produced by a smoothing capacitor,

wherein the smoothing capacitor has a first pole connected to a reference-ground potential or to the supply voltage,

and wherein a second pole of the smoothing capacitor is connected via a resistor to a port pin of the microcontroller, which port pin has a pulsed signal, applied to it.

10. The apparatus as claimed in claim 9 , wherein

the microcontroller is configured to first of all, when it is switched on, charge the smoothing capacitor to at least approximately half the supply voltage, while the port pins to which the parallel resonant circuit is connected have the same logic level and begin excitation of the parallel resonant circuit only afterwards.

11. The apparatus as claimed in claim 9 , wherein the pulsed signal has a prescribed duty ratio.

12. The apparatus as claimed in claim 1 , wherein

a signal generated by the measurement inductance is captured with an analog-to-digital converter of the microcontroller, aliasing being taken into consideration when determining the characteristic values of said signal.

13. The apparatus as claimed in claim 1 , wherein

a frequency component is ascertained at an evaluation frequency from a signal captured in the microcontroller,

wherein the evaluation frequency is the excitation frequency or an alias of the excitation frequency.

14. The apparatus as claimed in claim 1 , wherein

the measurement inductance is DC or magnetically coupled to the first inductance.

15. The apparatus as claimed in claim 1 , wherein

the parallel resonant circuit has a maximum Q factor obtained through maximisation of a value of Vt*Vt/V0,

where Vt denotes a ratio of coil current and supply line current given maximum deviation of the capacitance and of the first inductance from their respective values at the resonant frequency of the parallel resonant circuit, and

where V0 denotes a ratio of coil current and supply line current for respective values of capacitance and first inductance at the resonant frequency of the parallel resonant circuit.

16. The apparatus as claimed in claim 1 , wherein the measure variable is detected by at least one of:

i) altering a position of a magnetic core in the measurement inductance

ii) altering a spacing between the measurement inductance and the first inductance

iii) altering a position of:

a magnetic and conductive,

a nonmagnetic and conductive, or

a magnetic and nonconductive, element adjacent to the measurement inductance and the first inductance.

17. The apparatus as claimed in claim 1 , wherein

the apparatus has two, three or more than three measurement inductances.

18. The apparatus as claimed in claim 1 , wherein the excitation frequency differs from a resonant frequency of the parallel resonant circuit by one of:

i) no more that 20%,

ii) no more than 15%, or

iii) no more than 10%.

19. The apparatus as claimed in claim 1 , wherein a signal generated by the measurement inductance is captured with an analog-to-digital converter of the microcontroller, aliasing being taken into consideration when determining amplitude, phase, real part and/or imaginary part values of said signal, at the excitation frequency.