Method and device for measuring capacitances

• US 7,403,020 B2
• Filed: 05/05/2006
• Issued: 07/22/2008
• Est. Priority Date: 05/07/2005
• Status: Active Grant

• Current Assignee: ScioSense B.V. (Ams-Osram AG)
• Original Assignee: Acam-Messelectronic GmbH (Ams-Osram AG)
• Inventors: Bohnmuller, Friedrich, Braun, Augustin
• Primary Examiner(s): Gutierrez; Diego
• Assistant Examiner(s): VALONE, THOMAS F
• Application Number:
  US11/381,786
• Publication Number:
  US 20060250144A1
• Time in Patent Office: 809 Days
• Field of Search: 324/678, 324/686, 324/658
• US Class Current: 324/678
• CPC Class Codes: G01R 27/2605 Measuring capacitance capacitive sensors G01D5/24

Abstract

For measuring unknown capacitances, which exist in capacitive sensors, such as pressure, moisture, inclination sensors and the like, the invention proposes a capacitance measurement method, in which a capacitance to be measured is charged and discharged across a resistor and the charging or discharging time up to a preset voltage value is measured and in which the charging/discharging times of a known reference capacitance and stray capacitances are measured across a resistor. The invention also proposes a circuit for measuring an unknown capacitance to be measured and having a voltage source, a resistor for charging or discharging at least the measuring capacitor, at least one switch for connecting and disconnecting the measuring capacitor with respect to the voltage supply and a time measuring device, in which switches are provided for connecting and disconnecting the measuring capacitor and at least one reference capacitor with respect to the circuit.

Claims

1. A method for measuring a capacitance, the method comprising:

measuring a time for charging or discharging a known reference capacitance C_ref and unknown stray capacitances C_par based on a comparator running time t_comp by

t₁=(C_ref+C_par)*R+t_comp  

 

(1);

measuring a time for charging or discharging a known capacitance to be measured C_meas and unknown stray capacitances C_par based on said comparator running t_comp by

t₂=(C_meas+C_par)*R+t_comp  

 

(2);

measuring a time for charging or discharging stray capacitances C_par based on said comparator running time t_comp by

t₁₂=C_par*R+t_comp  

 

(3);

determining said capacitance to be measured C_meas by eliminating said stray capacitances C_par as well as the comparator running time t_comp according to;

$\frac{\left(t_2-t_{12}\right)}{\left(t_1-t_{12}\right)}=\frac{\left(\left(\left(C_{\mathrm{meas}}+C_{\mathrm{par}}\right)*R+t_{\mathrm{comp}}\right)-\left(C_{\mathrm{par}}*R+t_{\mathrm{comp}}\right)\right)}{\left(\left(\left(C_{\mathrm{ref}}+C_{\mathrm{par}}\right)*R+t_{\mathrm{comp}}\right)-\left(C_{\mathrm{par}}*R+t_{\mathrm{comp}}\right)\right)}=\frac{\left(C_{\mathrm{meas}}*R\right)}{\left(C_{\mathrm{ref}}*R\right)}$

and accordingly

$C_{\mathrm{meas}}=\frac{C_{\mathrm{refr}}*\left(t_2-t_{12}\right)}{\left(t_1-t_{12}\right)};<br><br>\mathrm{and}$

determining a sensor parameter based on said measured capacitance C_meas.

2. A method according to claim 1, wherein passing above or below a threshold voltage value necessary for determining the charging or discharging times is determined by means of a comparator.

3. A method according to claim 2, wherein the determination of the passing above or below of said voltage threshold value necessary for measuring the charging or discharging times takes place using a Schmitt trigger as the comparator.

4. A method according to claim 2, wherein the connection or disconnection of a voltage supply is provided by means of a transistor switch.

5. A method according to claim 1, wherein charging of the capacitances takes place with the resistor disconnected from earth.

6. A method according to claim 1, wherein switches are switched by a control unit, such as an analog multiplexer.

7. A method according to claim 1, wherein measurement takes place repeatedly with measuring frequencies of more than 0.5 kHz.

8. A method according to claim 1, wherein the measurements are performed with operating currents below 50 μ

A.

9. A method according to claim 1, wherein the time measurements are carried out by a time to digital converter (TDC).

10. A method for measuring a capacitance, the method comprising:

measuring a time for charging or discharging a known reference capacitance C ref and unknown stray capacitances C_par based on a comparator running time t_comp by

t₁=(C_ref+C_par)*R+t_comp  

 

(1);

measuring a time for charging or discharging a known capacitance to be measured C_meas and unknown stray capacitances C_par based on said comparator running t_comp by

t₂=(C_meas+C_par)*R+t_comp  

 

(2);

measuring a time for charging or discharging the known reference capacitance C_ref, the unknown capacitance to be measured C_meas and the unknown stray capacitance C_par based on the comparator running time t_comp by

t′

₁₂=(C_par+C_ref+C_meas)*R+t_comp  

 

(3);

determining the capacitance to be measured C_meas by eliminating the stray capacitances C_par par as well as the comparator running time t_comp according to

$\frac{\left(t_2-t_{12}^{\mathrm{\prime <br><br>}}\right)}{\left(t_2-t_{12}^{\mathrm{\prime <br><br>}}\right)}=\frac{\left(\left(\left(C_{\mathrm{ref}}+C_{\mathrm{par}}\right)*R+t_{\mathrm{comp}}\right)-\left(\left(C_{\mathrm{par}}+C_{\mathrm{ref}}+C_{\mathrm{meas}}\right)*R+t_{\mathrm{comp}}\right)\right)}{\left(\left(\left(C_{\mathrm{meas}}+C_{\mathrm{par}}\right)*R+t_{\mathrm{comp}}\right)-\left(\left(C_{\mathrm{par}}+C_{\mathrm{ref}}+C_{\mathrm{meas}}\right)*R+t_{\mathrm{comp}}\right)\right)}=\frac{\left(C_{\mathrm{meas}}*R\right)}{\left(C_{\mathrm{ref}}*R\right)}$

and accordingly

$C_{\mathrm{meas}}=\frac{C_{\mathrm{ref}}*\left(t_1-t_{12}^{\mathrm{\prime <br><br>}}\right)}{\left(t_2-t_{12}^{\mathrm{\prime <br><br>}}\right)}$

determining a sensor parameter based on said measured capacitance C_meas.

11. A method according to claim 10, wherein passing above or below a threshold voltage value necessary for determining the charging or discharging times is determined by means of a comparator.

12. A method according to claim 11, wherein the determination of the passing above or below of said voltage threshold value necessary for measuring the charging or discharging times takes place using a Schmitt trigger as the comparator.

13. A method according to claim 11, wherein the connection or disconnection of a voltage supply is provided by means of a transistor switch.

14. A method according to claim 10, wherein charging of the capacitances takes place with the resistor disconnected from ground.

15. A method according to claim 10, wherein switches are switched by a control unit, such as an analog multiplexer.

16. A method according to claim 10, wherein measurement takes place repeatedly with measuring frequencies of more than 0.5 kHz.

17. A method according to claim 10, wherein the measurements are performed with operating currents below 50 μ

A.

18. A method according to claim 10, wherein the time measurements are carried out by a time to digital converter (TDC).

19. A method for determining an unknown capacitance C_meas to be measured, the method comprising:

connecting a known reference capacitance C_ref connected to the unknown capacitance to be measured C_meas via connecting lines having individual stray capacitances Cs1-Cs4;

providing a power source v_cc;

connecting the reference capacitance C_ref and unknown capacitance C_meas to said power source via switches S1 to S4;

connecting the reference capacitance C_ref and unknown capacitance C_meas to a ground via transistor switches T1 to T4;

charging or discharging the capacitances and determining the charging or discharging times according to the following scheme;

Measure- ment S1 S2 T1 T2 S3 S4 T3 T4 1 T11 = (Cref + Cs1 + 1 0 0 1 0 0 1 1 Csg) * R + t_comp 2 T12 = (Cref + Cs2 + 0 1 1 0 0 0 1 1 Csg) * R + t_comp 3 T13 = (Cs1 + Cs2 + 1 1 0 0 0 0 1 1 Csg) * R + t_comp 4 T21 = (Cmess + Cs3 + 0 0 1 1 1 0 0 1 Csg) * R + t_comp 5 T22 = (Cmess + Cs4 + 0 0 1 1 0 1 1 0 Csg) * R + t_comp 6 T23 = (Cs3 + Cs4 + 0 0 1 1 1 1 0 0 Csg) * R + t_comp 7 Tsg = Csg * R + t_comp 0 0 1 1 0 0 1 1

wherein C_ref, C_meas, Cs1 to Cs4, Csg represent the respective capacitances and resistance in the respective charging or discharging circuit of each measurement determined by a position of the respective switches S1 to S4 and T1 to T4, wherein each position “

1”

indicates that the respective switch is closed and conductive, wherein the position “

0”

indicates that the respective switch is open and non-conductive, wherein t_comp represents a comparator running time;

determining the unknown capacitance to be measured C_meas by

$C_{\mathrm{meas}}=\frac{C_{\mathrm{ref}}*\left(T_{21}+T_{22}-T_{23}-T_{\mathrm{sg}}\right)}{\left(T_{11}+T_{12}-T_{13}-T_{\mathrm{sg}}\right)};<br><br>\mathrm{and}$

determining a sensor parameter based on said measured capacitance C_meas.

20. A method according to claim 19, wherein passing above or below a threshold voltage value necessary for determining the charging or discharging times is determined by means of a comparator.

21. A method according to claim 20, wherein the determination of the passing above or below of said voltage threshold value necessary for measuring the charging or discharging times takes place using a Schmitt trigger as the comparator.

22. A method according to claim 20, wherein the connection or disconnection of a voltage supply is provided by means of a transistor switch.

23. A method according to claim 19, wherein charging of the capacitances takes place with the resistor disconnected from ground.

24. A method according to claim 19, wherein switches are switched by a control unit, such as an analog multiplexer.

25. A method according to claim 19, wherein measurement takes place repeatedly with measuring frequencies of more than 0.5 kHz.

26. A method according to claim 19, wherein the measurements are performed with operating currents below 50 μ

A.

27. A method according to claim 19, wherein the time measurements are carried out by a time to digital converter (TDC).