Circuit arrangement for determining differences in capacitance

• US 5,629,629 A
• Filed: 04/04/1996
• Issued: 05/13/1997
• Est. Priority Date: 04/06/1995
• Status: Expired due to Fees

• Current Assignee: Siemens AG
• Original Assignee: Siemens AG
• Inventors: Hildebrandt, Andreas, Tielert, Reinhard
• Primary Examiner(s): Wieder, Kenneth A.
• Assistant Examiner(s): DO, DIEP N
• Application Number:
  US08/627,407
• Time in Patent Office: 404 Days
• Field of Search: 324/665, 324/672, 324/678, 324/607, 324/663, 324/679, 73/503, 73/514.32, 73/718, 73/862.52, 73/304 C, 327/63, 327/65, 340/870.37
• US Class Current: 324/679
• CPC Class Codes: G01R 27/2605 Measuring capacitance capacitive sensors G01D5/24

Abstract

In a circuit arrangement for determining differences in capacitance, two capacitors (1,2) are alternately connected to an integrator with differential amplifier (3) whose input current is positive (+I) when the first capacitor (1) is integrated or negative (-I) when the second capacitor (2) is integrated, whereby the switches (5,6,7) are synchronously switched. The differential amplifier keeps the voltages at the two capacitors the same. A clocked control device (4) controls the synchronous switching and separately measures and adds the respective charging and discharging times and, as warranted, measures and adds the output voltage of the integrator and controls the last charging cycle such that the voltage at the capacitors is subsequently equal to the initial voltage. The relative or absolute differences in capacitance are calculated from the identified sums and can be digitally output.

Claims

1. A circuit arrangement for determining a difference between a capacitance of a first capacitor and a capacitance of a second capacitor, comprising:

a differential amplifier having an inverting input and a non-inverting input and having an output, a positive current source that delivers a current with a constant current intensity and a first flow direction, a negative current source that delivers a current with constant current intensity and a second flow direction opposite to the first flow direction, a ground terminal, a control device, a first switch device and a second switch device;

said first switch device having first and second switch positions, in the first switch position the positive current source being connected to the inverting input of the differential amplifier and in the second switch position the negative current source being connected to the inverting input of the differential amplifier;

the non-inverting input of said differential amplifier connected to the ground terminal;

said second switch device configured such that in a first switch position a first terminal of the first capacitor is connected to the inverting input of the differential amplifier and a first terminal of the second capacitor is connected to the non-inverting input of the differential amplifier, and such that in a second switch position the first terminal of the first capacitor is connected to the non-inverting input of the differential amplifier and the first terminal of the second capacitor is connected to the inverting input of the differential amplifier;

second terminals of said first and second capacitors connected to the output of said differential amplifier and to an input of said control device;

said control device synchronously switching said first switch device and said second switch device;

device for determining a time between two switching events and the voltage occurring at the first and second capacitors, for adding said times and for determining values required for defining a difference in capacitance between the first and second capacitances.

2. The circuit arrangement according to claim 1, wherein the first switch device is a first switch and the second switch device is a second switch and a third switch,

wherein said first, second and third switches are respectively alternatively brought into a first switch position and into a second switch position wherein a fixed terminal of the appertaining switch is respectively connected to one of two alternating terminals,

wherein the fixed terminal of the first switch is connected to the inverting input of the differential amplifier and one of the alternating terminals of the first switch is connected to the positive current source and the other of the alternating terminals of the first switch is connected to the negative current source, wherein either

a) the fixed terminal of the second switch is connected to a terminal of one of the first and second capacitors and the fixed terminal of the third switch is connected to a terminal of the other first and second capacitors and respectively one of the alternating terminals of the second switch and of the third switch is connected to one input of the differential amplifier and the respectively other of the alternating terminals is connected to the respectively other input of the differential amplifier or

b) the fixed terminal of the second switch is connected to an input of the differential amplifier and the fixed terminal of the third switch is connected to the other input of the differential amplifier and respectively one of the alternating terminals of the second switch and of the third switch is connected to a terminal of one of the first and second capacitors and the respectively other of the alternating terminals is connected to a terminal of the other capacitor of the first and second capacitors, and

wherein the control device has a further device with which first, second and third switches for this second switch device can be synchronously switched.

3. The circuit arrangement according to claim 2, wherein the switches are individually switched.

4. The circuit arrangement according to claim 1, wherein the positive current source and the negative current source deliver current with substantially identical current magnitudes.

5. The circuit arrangement according to claim 1, wherein the first switch device is switched independently of the second switch device.

6. The circuit arrangement according to claim 1, wherein the second switch device enables a first switch position wherein the first terminals of the first and second capacitors connected to an input of the differential amplifier are both connected to the ground terminal;

wherein the second switch device is switched, synchronously with the first switch device, between said first switch position and each of two switch positions wherein the first terminals of the capacitors, connected to an input of the differential amplifier, are connected to different inputs of the differential amplifier; and

wherein the control device has a further device for adding changes in the voltage at the capacitors respectively ensuing at least for one of these switch positions and for storing a sum that is a result of the adding.

7. The circuit arrangement according to claim 6, wherein the control device has device for switching between switch positions as soon as a magnitude of the sum of changes in the voltage at the capacitors, respectively ensuing in a specific configuration of the switch positions of the first and second switch device reaches a level of a value stored by the control device.

8. A method for determining a quotient of capacitances of first and second capacitors or a quotient of a difference and a sum of the capacitances of the first and second capacitors, comprising the steps of:

providing a differential amplifier having an inverting input and a non-inverting input and having an output, a positive current source that delivers a current with a constant current intensity and a first flow direction, a negative current source that delivers a current with constant current intensity and a second flow direction opposite to the first flow direction, a ground terminal, a control device, a first switch device and a second switch device;

said first switch device having first and second switch positions, in the first switch position the positive current source being connected to the inverting input of the differential amplifier and in the second switch position the negative current source being connected to the inverting input of the differential amplifier;

the non-inverting input of said differential amplifier being connected to the ground terminal;

said second switch device being configured such that in a first switch position a first terminal of the first capacitor is connected to the inverting input of the differential amplifier and a first terminal of the second capacitor is connected to the non-inverting input of the differential amplifier, and such that in a second switch position the first terminal of the first capacitor is connected to the non-inverting input of the differential amplifier and the first terminal of the second capacitor is connected to the inverting input of the differential amplifier;

second terminals of said first and second capacitors being connected to the output of said differential amplifier and to an input of said control device;

synchronously switching said first switch device and said second switch device via said control device;

providing a device for determining a time between two switching events and the voltage occurring at the first and second capacitors, for adding said times and for determining values required for defining a difference in capacitance between the first and second capacitances;

a) permanently setting a quotient of current intensities of a current delivered by the positive current source and of a current delivered by the negative current source;

b) placing the first switch device in a first switch position; and

setting the second switch device such that one capacitor is connected to one input of the differential amplifier and the other capacitor is connected to the other input of the differential amplifier, and respectively charging or discharging the capacitor connected to the first switch device during a predetermined time period or during a time period identified by the control device from previous method steps, the differential amplifier charging the other capacitor to the same voltage;

c) synchronously switching the first and the second switch device, so that the terminals of the capacitors connected to one input of the differential amplifier are connected to the respectively other input of the differential amplifier;

d) implementing step b),

e) repeatedly implementing steps c) and d) in succession, whereby times during which step b) or step d) has been implemented are separately added for each of the first and second switch positions between which switching was carried out in step c), and whereby step d) is repeatedly implemented until the voltage at the capacitors subsequently coincides with the voltage at the capacitors at the start to at least a degree as required for a predetermined measuring precision; and

f) calculating the quotient of the capacitances or the quotient of the difference and the sum of the capacitances of the capacitors from one of known or separately identified quotients of the current magnitudes and the sums of the times added separately for the switch positions.

9. The method according to claim 8, wherein the first switch device is switched independently of the second switch device, and wherein the method for determining the quotient of the current magnitudes of the current delivered by the positive current source and of the current delivered by the negative current source, has the steps of:

aa) placing the first switch device into a first switch position and setting the second switch device such that at least one respective capacitor is connected to the first switch device;

bb) respectively charging or discharging said respective capacitor during a predetermined time or during a time identified by the control device from previous method steps;

cc) switching over only the first switch device;

dd) implementing step bb);

ee) repeatedly implementing steps cc) and dd) in succession, times during which step bb) or dd) has been implemented being added separately for each of the first and second switch positions of the first switch device and repeatedly implementing the step dd) until the voltage at the capacitor subsequently coincides with the voltage at the capacitor at the start to at least a degree required for a predetermined measuring precision; and

ff) determining a quotient of the current magnitudes of the current delivered by the positive current source and the current delivered by the negative current source as quotient of a sum of the times separately added for a respective switch position wherein the capacitor is connected to the negative current source and a sum of the times separately added for a further respective switch position wherein the capacitor is connected to the positive voltage source.

10. A method for determining a difference of capacitances of first and second capacitors or a quotient of a difference and a sum of the capacitances of the first and second capacitors, comprising the steps of:

providing a differential amplifier having an inverting input and a non-inverting input and having an output, a positive current source that delivers a current with a constant current intensity and a first flow direction, a negative current source that delivers a current with constant current intensity and a second flow direction opposite to the first flow direction, a ground terminal, a control device, a first switch device and a second switch device;

said first switch device having first and second switch positions, in the first switch position the positive current source being connected to the inverting input of the differential amplifier and in the second switch position the negative current source being connected to the inverting input of the differential amplifier;

the non-inverting input of said differential amplifier connected to the ground terminal;

said second switch device being configured such that in a first switch position a first terminal of the first capacitor is connected to the inverting input of the differential amplifier and a first terminal of the second capacitor is connected to the non-inverting input of the differential amplifier, and such that in a second switch position the first terminal of the first capacitor is connected to the non-inverting input of the differential amplifier and the first terminal of the second capacitor is connected to the inverting input of the differential amplifier;

second terminals of said first and second capacitors being connected to the output of said differential amplifier and to an input of said control device;

synchronously switching said first switch device and said second switch device via said control device;

providing a device for determining a time between two switching events and the voltage occurring at the first and second capacitors, for adding said times and for determining values required for defining a difference in capacitance between the first and second capacitances;

the second switch device enabling a first switch position wherein the first terminals of the first and second capacitors connected to an input of the differential amplifier are both connected to the ground terminal;

the second switch device being switched, synchronously with the first switch device, between said first switch position and each of two switch positions wherein the first terminals of the capacitors, connected to an input of the differential amplifier, are connected to different inputs of the differential amplifier;

the control device having a further device for adding changes in the voltage at the capacitors respectively ensuing at least for one of these switch positions and for storing a sum that is a result of the adding;

a) permanently setting the quotient of the current intensities of a current delivered by the positive current source and a current delivered by the negative current source;

b) placing the first switch device in a first switch position and setting the second switch device to one of first and second switch positions wherein either one capacitor of the first and second capacitors is connected to one input of the differential amplifier and the other capacitor of the first and second capacitors is connected to the other input of the differential amplifier, or terminals of the first and second capacitors connected to an input of the differential amplifier are both connected to the ground terminal, respectively charging or discharging a further capacitor that is connected between the inverting input of the differential amplifier and the output of the differential amplifier and in parallel to the differential amplifier during a predetermined time or during a time identified by the control device from previous method steps, the differential amplifier charging the other capacitor or capacitors to the same voltage as the further capacitor,

c) synchronously switching the first and the second switch device, so that the capacitors proceed from a set specific switch position into another set specific switch position;

d) implementing the step b);

e) repeatedly implementing steps c) and d) in succession, times during which step b) or step d) has been implemented being separately added for each of the first and second specific switch positions and changes in the voltage at the capacitors respectively occurring during these times being separately added at least for one of the first and second specific switch positions and repeatedly implementing step d) until the voltage at the capacitors subsequently coincides with the voltage at the capacitors at the start to at least a degree required for a predetermined measuring precision, and storing by the control device a sum of changes in the voltage at the capacitors that occur in the first and second specific switch positions;

f) setting the second switch device into one of second and third specific switch positions, whereby, in the third specific switch position one capacitor of the first and second capacitor is connected to one input of the differential amplifier and the other capacitor of the first and second capacitors is connected to the other input of the differential amplifier and the terminals are thereby transposed compared to the first specific switch position;

g) implementing step b);

h) implementing step c), whereby switching is carried out between the second and third specific switch positions;

i) implementing step b);

j) repeatedly implementing steps h) and i) in succession and separately adding times during which step g) or step i) has been implemented for the second and third specific switch positions and separately adding changes in the voltage at the capacitors respectively occurring during these times at least for one of the second and third specific switch positions, and either storing by the control device a sum of the changes in the voltage at the capacitors that occurred in a respective specific switch position is likewise stored by the control device or repeatedly implementing step i) in the respective specific switch position until an amount of this sum reaches an amount of the value stored by the control device in step e), and implementing step i) overall as often and respectively until the voltage at the capacitors subsequently coincides with the voltage at the capacitors at the start to at least a degree required for a predetermined measuring precision; and

k) calculating a quotient of the difference and the sum of the capacitances of the capacitors from a known or separately identified quotients of the current magnitudes, the sums of the times separately added for the switch positions and the quotient of the sums of the changes in the voltage respectively acquired in steps e) and j) or the difference in the capacitances of the capacitors being determined from these values and a known or separately identified value of the capacitance of the further capacitor.

11. The method according to claim 10, wherein the first switch device is switched independently of the second switch device, and wherein the method for determining the quotient of the current magnitudes of the current delivered by the positive current source and of the current delivered by the negative current source, has the steps of:

aa) placing the first switch device into a first switch position and setting the second switch device such that at least one respective capacitor is connected to the first switch device;

bb) respectively charging or discharging said respective capacitor during a predetermined time or during a time identified by the control device from previous method steps;

cc) switching over only the first switch device;

dd) implementing step bb);

ee) repeatedly implementing steps cc) and dd) in succession, times during which step bb) or dd) has been implemented being added separately for each of the first and second switch positions of the first switch device and repeatedly implementing the step dd) until the voltage at the capacitor subsequently coincides with the voltage at the capacitor at the start to at least a degree required for a predetermined measuring precision; and

ff) determining a quotient of the current magnitudes of the current delivered by the positive current source and the current delivered by the negative current source as quotient of a sum of the times separately added for a respective switch position wherein the capacitor is connected to the negative current source and a sum of the times separately added for a further respective switch position wherein the capacitor is connected to the positive voltage source.