Method and device for measuring capacitances
View Patent ↗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.
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
t 1 =( 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
t 2 =( 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
t 12 =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:
(
t
2
-
t
12
)
(
t
1
-
t
12
)
=
(
(
(
C
meas
+
C
par
)
*
R
+
t
comp
)
-
(
C
par
*
R
+
t
comp
)
)
(
(
(
C
ref
+
C
par
)
*
R
+
t
comp
)
-
(
C
par
*
R
+
t
comp
)
)
=
(
C
meas
*
R
)
(
C
ref
*
R
)
and accordingly
C
meas
=
C
refr
*
(
t
2
-
t
12
)
(
t
1
-
t
12
)
;
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 μA.
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
t 1 =( 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
t 2 =( 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
(
t
2
-
t
12
′
)
(
t
2
-
t
12
′
)
=
(
(
(
C
ref
+
C
par
)
*
R
+
t
comp
)
-
(
(
C
par
+
C
ref
+
C
meas
)
*
R
+
t
comp
)
)
(
(
(
C
meas
+
C
par
)
*
R
+
t
comp
)
-
(
(
C
par
+
C
ref
+
C
meas
)
*
R
+
t
comp
)
)
=
(
C
meas
*
R
)
(
C
ref
*
R
)
and accordingly
C
meas
=
C
ref
*
(
t
1
-
t
12
′
)
(
t
2
-
t
12
′
)
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 μA.
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 Cs 1 -Cs 4 ;
providing a power source v cc ;
connecting the reference capacitance C ref and unknown capacitance C meas to said power source via switches S 1 to S 4 ; connecting the reference capacitance C ref and unknown capacitance C meas to a ground via transistor switches T 1 to T 4 ;
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 , Cs 1 to Cs 4 , 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 S 1 to S 4 and T 1 to T 4 , 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
meas
=
C
ref
*
(
T
21
+
T
22
-
T
23
-
T
sg
)
(
T
11
+
T
12
-
T
13
-
T
sg
)
;
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 μA.
27. A method according to claim 19 , wherein the time measurements are carried out by a time to digital converter (TDC).