Capacitive Proximity Detection Using Delta-Sigma Conversion

• US 20140368222A1
• Filed: 06/11/2014
• Published: 12/18/2014
• Est. Priority Date: 06/12/2013
• Status: Active Grant

• Current Assignee: Neodrón Limited (Atlantic IP Services Limited)
• Original Assignee: Microchip Technology Incorporated
• Inventors: Curtis, Keith
• Granted Patent: US 9,587,964 B2
• Time in Patent Office: Days
• US Class Current: 324/672
• CPC Class Codes: G01D 5/24 by varying capacitance
• CPC Class Codes: G01D 5/2405 by varying dielectric
• CPC Class Codes: G01R 27/26 Measuring inductance or capacitance; Measuring quality factor, e.g. by using the resonance method; Measuring loss factor; Measuring dielectric constants ; Measuring impedance or related variables
• CPC Class Codes: G01R 27/2605 Measuring capacitance capacitive sensors G01D5/24
• CPC Class Codes: G05B 2219/37284 Capacitive 3-D proximity sensor
• CPC Class Codes: G06F 3/04186 Touch location disambiguation
• CPC Class Codes: H03K 17/955 using a capacitive detector
• CPC Class Codes: H03K 2217/960725 Charge-transfer
• CPC Class Codes: H03M 3/456 the modulator having a first order loop filter in the feedforward path
• CPC Class Codes: H03M 3/462 Details relating to the decimation process decimation filters in general H03H17/0416, H03H17/0621
• CPC Class Codes: H03M 3/494 Sampling or signal conditioning arrangements specially adapted for delta-sigma type analogue/digital conversion systems

Abstract

Capacitive proximity detection is provided by combining a delta-sigma modulator and a capacitive voltage divider circuit to create a high resolution capacitive-to-digital converter. Period and duty cycle counters provide duty cycle ratios from the delta-sigma modulator for comparison of changes in capacitance values of a capacitive sensor when an object is in proximity thereto.

Claims

1. A proximity detection system, said system comprising:

a comparator having first and second inputs and an output, wherein the second input is coupled with a reference voltage and the first input is coupled with a holding capacitor;

a capacitive sensor;

a switch for coupling the capacitive sensor alternately to a voltage and the holding capacitor;

a clock having an output;

a flip-flop having an input coupled with the output of the comparator, a clock input coupled to the output of the clock and an output;

a feedback resistor coupled between the output of the flip-flop and the first input of the comparator;

a duty cycle counter having a clock input coupled to the output of the clock, a reset input, an enable input coupled to the output of the flip-flop, and an output for providing a number of clock outputs counted, wherein the duty cycle counter only counts the clock outputs when the enable input thereof is at a first logic level; and

a circuit for controlling the switch whenever the output of the flip-flop goes from a second logic level to the first logic level;

wherein the number of clock outputs counted are used in determining proximity of an object to the capacitive sensor.

2. The proximity detection system according to claim 1, further comprising a period counter having a clock input coupled to the output of the clock, a reset/enable input and a carry output, wherein the period counter determines a measurement time period.

3. The proximity detection system according to claim 2, wherein count values in the duty cycle and period counters are used in determining a duty cycle dependent upon a capacitance value of the capacitive sensor.

4. The proximity detection system according to claim 3, wherein the duty cycle is the count value in the duty cycle counter divided by the count value in the period counter.

5. The proximity detection system according to claim 4, further comprising a digital processor and memory coupled to the clock, and the duty cycle and period counters, wherein the digital processor reads the count values from the duty cycle and period counters, and therefrom determines duty cycles related to capacitance values of the capacitive sensor.

6. The proximity detection system according to claim 5, wherein the duty cycles related to capacitance values of the capacitive sensor are stored in the digital processor memory and compared in determining whether a duty cycle change indicates a detection of an object proximate to the capacitive sensor.

7. The proximity detection system according to claim 5, wherein the carry output of the period counter generates an interrupt signal to the digital processor.

8. The proximity detection system according to claim 1, wherein the voltage coupled to the capacitive sensor is at a logic high.

9. The proximity detection system according to claim 1, wherein the voltage coupled to the capacitive sensor is at a logic low.

10. The proximity detection system according to claim 5, further comprising an output driver having an input coupled the digital processor and an output coupled to the switch wherein the voltage coupled to the capacitive sensor alternates between a logic high and a logic low for sequential duty cycle periods.

11. The proximity detection system according to claim 5, wherein a microcontroller comprises the digital processor and memory, the comparator, the clock, the flip-flop, the feedback resistor, the duty cycle counter, the period counter, the switch, and the circuit for controlling the switch.

12. The proximity detection system according to claim 1, wherein the feedback resistor is replaced with a current source and a current sink.

13. The proximity detection system according to claim 1, wherein a resistance value of the feedback resistor is programmably adjustable.

14. The proximity detection system according to claim 1, wherein a capacitance value of the holding capacitor is programmably adjustable.

15. A method for detecting an object proximate to a capacitive sensor, said method comprising the steps of:

charging a capacitive sensor to a first voltage;

coupling the charged capacitive sensor in parallel with a holding capacitor;

comparing a resulting voltage on the parallel connected capacitive sensor and holding capacitor to a reference voltage with a voltage comparator, wherein;

if the resulting voltage is greater than the reference voltage then partially discharge the holding capacitor to a lower voltage, and

if the resulting voltage is less than the reference voltage then partially charge the holding capacitor to a higher voltage and add one count to a duty cycle counter;

add one count to a period counter;

return to the step of charging the capacitive sensor to the first voltage.

16. The method according to claim 15, further comprising the steps of:

storing a plurality of duty cycle period counts; and

calculating duty cycles from the plurality of duty cycle counts divided by respective ones of the plurality of period counts.

17. The method according to claim 16, further comprising the step of determining changes in capacitance values of the capacitive sensor from the duty cycles.

18. The method according to claim 16, further comprising the step of determining proximity of an object to the capacitive sensor from the duty cycles.

19. A method for detecting an object proximate to a capacitive sensor, said method comprising the steps of:

discharging a capacitive;

coupling the discharged capacitive sensor in parallel with a holding capacitor;

comparing a resulting voltage on the parallel connected capacitive sensor and holding capacitor to a reference voltage with a voltage comparator, wherein;

if the resulting voltage is less than the reference voltage then partially charge the holding capacitor to a higher voltage, and

if the resulting voltage is greater than the reference voltage then partially discharge the holding capacitor to a higher voltage and add one count to a duty cycle counter;

add one count to a period counter;

return to the step of charging the capacitive sensor to the first voltage.

20. The method according to claim 19, further comprising the steps of:

storing a plurality of duty cycle period counts; and

calculating duty cycles from the plurality of duty cycle counts divided by respective ones of the plurality of period counts.

21. The method according to claim 20, further comprising the step of determining changes in capacitance values of the capacitive sensor from the duty cycles.

22. The method according to claim 20, further comprising the step of determining proximity of an object to the capacitive sensor from the duty cycles.

23. An apparatus for proximity detection, comprising:

a comparator having first and second inputs and an output, wherein the second input is coupled with a reference voltage and the first input is coupled with a holding capacitor;

a switch adapted for coupling a capacitive sensor alternately to a voltage and the holding capacitor;

a clock having an output;

a flip-flop having an input coupled with the output of the comparator, a clock input coupled to the output of the clock and an output;

a feedback resistor coupled between the output of the flip-flop and the first input of the comparator;

a duty cycle counter having a clock input coupled to the output of the clock, a reset input, an enable input coupled to the output of the flip-flop, and an output for providing a number of clock outputs counted, wherein the duty cycle counter only counts the clock outputs when the enable input thereof is at a first logic level; and

a circuit for controlling the switch whenever the output of the flip-flop goes from a second logic level to the first logic level.

24. The apparatus according to claim 23, wherein the number of clock outputs counted are used in determining proximity of an object to the capacitive sensor.

25. The apparatus according to claim 23, wherein the number of clock outputs counted are used in determining changes in capacitances values of the capacitive sensor.

26. The apparatus according to claim 23, wherein the feedback resistor is replaced with a current source and a current sink.

27. The apparatus according to claim 23, wherein a resistance value of the feedback resistor is programmably adjustable.

28. The apparatus according to claim 23, wherein a capacitance value of the holding capacitor is programmably adjustable.

29. An apparatus for proximity detection, comprising:

a voltage summer having first and second inputs and an output;

a switch adapted for coupling a capacitive sensor alternately to a voltage and the first input of the voltage summer;

a clock having an output, wherein at each clock pulse the switch briefly couples the capacitive sensor to the voltage then back to the first input of the voltage summer;

a voltage reference having a reference voltage output;

an operational amplifier having a first input coupled to an output of the voltage summer and a second input coupled to the output of the voltage reference;

an integrating capacitor coupled between the first input of the operational amplifier and an output thereof;

an analog-to-digital converter (ADC) having an input coupled to the output of the, wherein the ADC converts the operational amplifier output to a multi-bit digital representation thereof;

a digital-to-analog converter (DAC) having inputs coupled to the ADC and converting the multi-bit digital representation from the ADC into an analog feedback voltage coupled to the second input of the voltage summer;

a decimating filter having inputs coupled to the multi-bit digital representation from the ADC; and

a digital signal processor (DSP) coupled to the decimating filter, wherein the DSP determines when an object is proximate to the capacitive sensor.

30. The apparatus according to claim 29, wherein the ADC, DAC and decimating filter are synchronized together with clock pulses from the clock.