METHOD AND DEVICE FOR MEASURING CAPACITANCES

• US 20060250144A1
• Filed: 05/05/2006
• Published: 11/09/2006
• 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: Bahnmuller, Friedrich, Braun, Augustin
• Granted Patent: US 7,403,020 B2
• Time in Patent Office: Days
• US Class Current: 324/678
• CPC Class Codes: G01R 27/2605 Measuring capacitance capacitive sensors G01D5/24

Abstract

For measuring unknown capacitances, such as 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. Method for measuring the capacitance, 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, wherein charging/discharging times of a known reference capacitance and stray capacitances are measured across a resistor.

2. Method according to claim 1, wherein the measured results or the values obtainable therefrom and in particular the charging and/or discharging times are so mutually related as to eliminate the stray capacitances.

3. Method according to claim 1, wherein the charging/discharging time of the stray capacitances is measured in isolation or separately.

4. Method according to claim 3, wherein stray capacitances and optionally the comparator running time are eliminated by subtraction of the measured charging/discharging time of the isolated measurement of the stray capacitances from the measured charging/discharging time of the running time measurement across the stray capacitance and measuring/reference capacitance.

5. Method according to claim 1, wherein the charging and/or discharging times of all the capacitances present in the measuring network, measuring capacitance, reference capacitance and stray capacitances are measured.

6. Method according to claim 5, wherein the charging or discharging times of all the capacitances present in the measuring network, measuring capacitance, reference capacitance and the total capacitance of all the capacitances present are measured.

7. Method according to claim 1, wherein the determination of the passing above or below a specific voltage value necessary for determining the charging or discharging times takes place by means of a comparator.

8. Method according to claim 7, wherein the determination of the passing above or below of a specific voltage value necessary for measuring the charging or discharging times takes place using a Schmitt trigger as the comparator.

9. Method according to claim 7, wherein the connection or disconnection of a voltage supply takes place by means of a transistor switch.

10. Method according to claim 1, wherein charging of the capacitances takes place with the resistor disconnected from earth.

11. Method according to claim 1, wherein the switches are switched by a control unit, such as an analog multiplexer.

12. Method according to claim 1, wherein the time measurements are carried out by means of a TDC.

13. Method according to claim 1, wherein measurement takes place repeatedly with measuring frequencies of more than 0.5 kHz, preferably more than 10 kHz.

14. Method according to claim 1, wherein the measurements take place with measuring frequencies below 50 Hz, preferably below 20 Hz.

15. Method according to claim 1, wherein the measurements are performed with operating currents below 50 μ

A, preferably below 10 μ

A.

16. Device for measuring an unknown capacitor to be measured, with a voltage supply, 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, wherein there are switches for connecting and disconnecting the measuring capacitor and at least one reference capacitor with respect to circuit.

17. Device according to claim 16, wherein a device is provided for bringing into relationship the measured results or values measured therefrom, particularly charging and/or discharging times in such a way that the stray capacitances are eliminated.

18. Device according to claim 16, wherein the charging/discharging time of the stray capacitances can be measured in isolation or separately.

19. Device according to claim 17, wherein a device is provided for eliminating stray capacitances and optionally the comparator running time through the subtraction of the measured charging/discharging time of the isolated measurement of the stray capacitances from the measured charging/discharging time of the running time measurement across the stray capacitance and measuring capacitance or reference capacitance.

20. Device according to claim 16, wherein a measurement takes place of the charging and/or discharging times of all the capacitances present in the measuring network, measuring capacitance, reference capacitance and stray capacitances.

21. Device according to claim 20, wherein measurement takes place of the charging or discharging times of all the capacitances present in the measuring network, measuring capacitance, reference capacitance and total capacitance of all the capacitances present.

22. Device according to one of the claims 16, wherein a switch is provided for disconnecting and connecting resistor to and from the side of capacitances remote from voltage supply.

23. Circuit according to claim 16, wherein there is a comparator, preferably in the form of a Schmitt trigger, for determining the passing above or below of a specific voltage threshold.

24. Circuit according to claim 16, wherein there is a control unit for controlling the switches present.

25. Device according to claim 24, wherein the control unit is an analog multiplexer.

26. Device according to claim 16, wherein there is a time measuring device, preferably a TDC.

27. Device according to claim 16, wherein it is integrated into an integrated circuit.

28. Device according to claim 16, wherein the resistance is at least 1 kOhm, preferably more than 10 kOhm.

29. Device according to claim 16, wherein the capacitances and the resistance are connected to earth.