Capacitive proximity detection using delta-sigma conversion
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.
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 a capacitance value of the holding capacitor is programmably adjustable.
13. The proximity detection system according to claim 1 , wherein a resistance value of the feedback resistor is programmably adjustable.
14. 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.
15. The method according to claim 14 , 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.
16. The method according to claim 15 , further comprising the step of determining proximity of an object to the capacitive sensor from the duty cycles.
17. The method according to claim 15 , further comprising the step of determining changes in capacitance values of the capacitive sensor from the duty cycles.
18. A method for detecting an object proximate to a capacitive sensor, said method comprising the steps of:
discharging a capacitive sensor;
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.
19. The method according to claim 18 , 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.
20. The method according to claim 19 , further comprising the step of determining proximity of an object to the capacitive sensor from the duty cycles.
21. The method according to claim 19 , further comprising the step of determining changes in capacitance values of the capacitive sensor from the duty cycles.
22. 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.
23. The apparatus according to claim 22 , wherein a resistance value of the feedback resistor is programmably adjustable.
24. The apparatus according to claim 22 , 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 22 , 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 22 , wherein a capacitance value of the holding capacitor is programmably adjustable.