Method and apparatus for detecting smoke in an ion chamber

1. A method for detecting smoke, comprising the steps of:

determining a voltage on a conductive screen with a delta-sigma analog-to-digital converter, wherein the conductive screen is located between a first ion chamber and a second ion chamber, wherein the first ion chamber is open to smoke ingress and the second ion chamber is closed to smoke ingress; and

detecting a presence of smoke by:

applying a first voltage potential to the first and second ion chambers at a first polarity;

determining a first voltage on the conductive screen caused by the first voltage potential at the first polarity;

applying a second voltage potential to the first and second ion chambers at a second polarity;

determining a second voltage on the conductive screen caused by the second voltage potential at the second polarity;

determining a voltage difference between the first and the second voltages; and

detecting the presence of smoke when the voltage difference changes by a certain amount.

2. The method according to claim 1 , wherein the steps of applying the first or applying the second voltage are performed using first and second multiplexers.

3. The method according to claim 1 , wherein the presence of smoke is detected if the voltage on the conductive screen changes by the certain amount within a certain time.

4. The method according to claim 1 , where the step of determining the first voltage and the step of determining the second voltage each comprise:

comparing the voltage on the conductive screen to a reference voltage from a voltage reference with a voltage comparator;

charging a capacitance coupled between the conductive screen and the voltage reference when the voltage on the conductive screen is less than the reference voltage;

discharging the capacitance coupled between the conductive screen and the voltage reference when the voltage on the conductive screen is greater than the reference voltage;

generating a pulse width modulated signal with respect to charging and discharging of the capacitance; and

determining a duty cycle of the pulse width modulated signal.

5. The method according to claim 4 , wherein the duty cycle of the pulse width modulated signal is determined by clocked timers.

6. The method according to claim 1 , further comprising the step of compensating for temperature change with temperature information from a temperature sensor.

7. The method according to claim 1 , further comprising the step of compensating for relative humidity change with relative humidity information from a relative humidity sensor.

8. The method according to claim 1 , further comprising the step of compensating for voltage change with voltage information from a voltage sensor.

9. The method according to claim 4 , wherein the analog-to-digital digital converter is part of a microcontroller and controlled by a digital processor of the microcontroller, wherein the digital processor goes into a low power sleep mode when the cloked timers are counting.

10. The method according to claim 9 , further comprising waking up the digital processor to read count values of the clocked timers.

11. An apparatus for detecting smoke, comprising:

an ionization chamber having a radiation source and comprising first and second chambers with a conductive screen there between, wherein the first chamber is open to smoke ingress and the second chamber is closed to smoke ingress;

a voltage comparator having a first input coupled to the conductive screen and a second input coupled to a voltage reference;

a capacitor coupled between the first and second inputs of the voltage comparator;

a flip-flop having a D-input coupled to an output of the voltage comparator and a clock input coupled to a clock generator, wherein each time a clock signal is received from the clock generator a logic value at the D-input is transferred to a Q-output of the flip-flop;

a feedback resistor coupled between the Q-output of the flip-flop and the first input of the voltage comparator for charging and discharging the capacitor;

wherein when a voltage on the first input of the voltage comparator is greater than a voltage from the voltage reference the output of the voltage comparator is at a logic low and the capacitor is discharged, and when the voltage on the first input of the voltage comparator is less than the voltage from the voltage reference the output of the voltage comparator is at a logic high and the capacitor is charged;

a first counter for counting a first number of clock pulses from the clock generator when the Q-output of the flip-flop is at a logic high during a certain time period; and

a second counter for counting a second number of clock pulses from the clock generator during a certain time period;

wherein when the first number changes a certain amount within the certain time period a presence of smoke is detected within the first chamber.

12. The apparatus for detecting smoke according to claim 11 , further comprising circuits for alternately coupling the ionization chamber to voltage potentials at first or second polarities, wherein the second polarity is opposite the first polarity.

13. The apparatus for detecting smoke according to claim 12 , wherein the circuits for alternately coupling comprises voltage multiplexers.

14. The apparatus for detecting smoke according to claim 11 , wherein the ionization chamber is coupled to a microcontroller having a digital processor and memory, and the microcontroller performs all of the aforementioned functions for detecting the presence of smoke.

15. The apparatus for detecting smoke according to claim 14 , further comprising an alarm circuit coupled to the digital processor.

16. The apparatus for detecting smoke according to claim 14 , further comprising a temperature sensor coupled to the digital processor and a temperature compensation look-up table stored in the memory coupled to the digital processor and used to compensate temperature induced changes of the capacitance of the ionization chamber.

17. The apparatus for detecting smoke according to claim 14 , further comprising a humidity sensor coupled to the digital processor and a humidity compensation look-up table stored in the memory coupled to the digital processor and used to compensate humidity induced changes of the capacitance of the ionization chamber.

18. The apparatus for detecting smoke according to claim 14 , further comprising a voltage sensor coupled to the digital processor and a voltage compensation look-up table stored in the memory coupled to the digital processor and used to compensate voltage induced changes of the capacitance of the ionization chamber.

19. The apparatus for detecting smoke according to claim 11 , further comprising an audible alert actuated by the presence of smoke in the ionization chamber.

20. The apparatus for detecting smoke according to claim 11 , further comprising a visual alert actuated by the presence of smoke in the ionization chamber.

21. The apparatus for detecting smoke according to claim 14 , further comprising a microcontroller having a low power sleep mode, wherein the digital processor and memory of the microcontroller go into the low power sleep mode during counting by the first and second counters.

22. The apparatus for detecting smoke according to claim 21 , wherein the second counter is a sleep wake-up timer for the digital processor and memory of the microcontroller.

23. An apparatus for detecting smoke, comprising:

an ionization chamber having a radiation source and comprising first and second chambers with a conductive screen there between, wherein the first chamber is open to smoke ingress and the second chamber is closed to smoke ingress;

an analog-to-digital converter (ADC) coupled with the conductive screen;

circuits for alternately coupling the ionization chamber to voltage potentials at first or second polarities, wherein the second polarity is opposite the first polarity;

wherein the apparatus is configured to control the circuits for alternately coupling the ionization chamber to voltage potentials to apply a voltage potential having the first polarity to said ionization chamber and measure a first voltage with said ADC and subsequently to apply a voltage potential having the second polarity to said ionization chamber and measure a second voltage with said ADC,

wherein the apparatus is further configured to determine a voltage difference between the first and the second voltages and to detect the presence of smoke when the voltage difference changes by a certain amount.

24. The apparatus for detecting smoke according to claim 23 , wherein the analog-to-digital converter comprises:

a voltage comparator having a first input coupled to the conductive screen and a second input coupled to a voltage reference;

a capacitor coupled between the first and second inputs of the voltage comparator;

a flip-flop having a D-input coupled to an output of the voltage comparator and a clock input coupled to a clock generator, wherein each time a clock signal is received from the clock generator a logic value at the D-input is transferred to a Q-output of the flip-flop;

a feedback resistor coupled between the Q-output of the flip-flop and the first input of the voltage comparator for charging and discharging the capacitor;

wherein when a voltage on the first input of the voltage comparator is greater than a voltage from the voltage reference the output of the voltage comparator is at a logic low and the capacitor is discharged, and when the voltage on the first input of the voltage comparator is less than the voltage from the voltage reference the output of the voltage comparator is at a logic high and the capacitor is charged;

a first counter for counting a first number of clock pulses from the clock generator when the Q-output of the flip-flop is at a logic high during a certain time period; and

a second counter for counting a second number of clock pulses from the clock generator during a certain time period.

25. The apparatus for detecting smoke according to claim 23 , wherein the analog-to-digital converter and circuits for alternately coupling the ionization chamber to voltage potentials are part of a microcontroller comprising a digital processor, and wherein the second counter is formed by a wake-up timer of the microcontroller.

26. The apparatus for detecting smoke according to claim 23 , further comprising an alarm circuit coupled to a digital processor configured to generate an audible alert actuated by the presence of smoke in the ionization chamber.

27. The apparatus for detecting smoke according to claim 23 , further comprising a temperature sensor coupled to a digital processor and a temperature compensation look-up table stored in a memory coupled to the digital processor and used to compensate temperature induced changes of a capacitance of the ionization chamber.

28. The apparatus for detecting smoke according to claim 23 , further comprising

a humidity sensor coupled to a digital processor and a humidity compensation look-up table stored in the memory coupled to the digital processor and used to compensate humidity induced changes of the capacitance of the ionization chamber.

29. The apparatus for detecting smoke according to claim 23 , further comprising

a voltage sensor coupled to a digital processor and a voltage compensation look-up table stored in the memory coupled to the digital processor and used to compensate voltage induced changes of the capacitance of the ionization chamber.

30. The apparatus for detecting smoke according to claim 23 , further comprising an alarm circuit coupled to a digital processor configured to generate a visual alert actuated by the presence of smoke in the ionization chamber.

31. The apparatus for detecting smoke according to claim 24 , further comprising a microcontroller comprising a digital processor and memory having a low power sleep mode, the circuits for alternately coupling the ionization chamber to voltage potentials, and said ADC, wherein the digital processor and memory of the microcontroller go into the low power sleep mode during counting by the first and second counters.

32. The apparatus for detecting smoke according to claim 31 , wherein the second counter is a sleep wake-up timer for the digital processor and memory of the microcontroller.