Interference-tolerant proximity sensor system having a resonance-tracking impedance analyzer

• US 6,208,134 B1
• Filed: 09/30/1999
• Issued: 03/27/2001
• Est. Priority Date: 09/30/1999
• Status: Expired due to Fees

• Current Assignee: Honeywell Incorporated (Honeywell International Inc.)
• Original Assignee: Honeywell International Inc.
• Inventors: Demma, Nick Anthony
• Primary Examiner(s): PATIDAR, JAY M
• Application Number:
  US09/407,178
• Time in Patent Office: 544 Days
• Field of Search: 324/207.12, 324/207.15, 324/207.16, 324/207.17, 324/207.24, 324/207.25, 324/207.26, 324/236, 324/652, 324/655, 324/658, 324/675, 324/682, 331/65, 361/179-181
• US Class Current: 324/207.26
• CPC Class Codes: G01B 7/023 for measuring distance between sensor and object G01B7/082 and G01B7/102 take precedence

Abstract

A system to analyze the impedance of interference-tolerant proximity sensors. A lock-in amplifier (LIA) is implemented as a resonance-tracking oscillator. The control loop of the amplifier monitors the response of sensor in relation to the output of a local oscillator to adjust the oscillator'"'"'s frequency to resonance. The nature of the sensor acts as a pre-filter of noise for a synchronous demodulator, which demodulates the sensor'"'"'s response with the local oscillator'"'"'s driving signal. The output of the system is a filtered version of this demodulated signal. This system is effective in sensing objects in the presence of electromagnetic noise that is many times stronger than the signal produced by the local oscillator.

Claims

1. A proximity sensor system having a resonance-tracking impedance analyzer for eliminating effects of interfering signals, comprising:

a voltage controlled oscillator;

a voltage-to-current converter connected to said voltage controlled oscillator;

a resonant circuit connected to said voltage-to-current converter;

a first demodulator connected to said resonant circuit and to said voltage controlled oscillator;

a second demodulator connected to said resonant circuit and to said voltage controlled oscillator;

a first filter connected to said first demodulator;

an integrator connected to said first filter and to said voltage controlled oscillator; and

a second filter connected to said second demodulator.

2. The proximity sensor system of claim 1, wherein:

said resonant circuit is an inductive-capacitive circuit having an impedance that varies relative to proximity of a target; and

said first and second filters are low pass filters.

3. The proximity sensor system of claim 2, further comprising a threshold detector connected to said second filter.

4. An interference tolerant proximity sensor system comprising:

a voltage controlled oscillator having an input, and first and second outputs;

a voltage-to-current converter having an input connected to the first output of said voltage controlled oscillator, and having an output;

a resonant circuit having a terminal connected to the output of said voltage-to-current converter;

a first multiplier having a first input connected to the second output of said voltage controlled oscillator, a second input connected to the terminal of said resonant circuit, and having an output;

a first filter having an input connected to the output of said first multiplier, and having an output;

an integrator having an input connected to the output of said first filter and having an output connected to the input of said voltage controlled oscillator;

a second multiplier having a first input connected to the first output of said voltage controlled oscillator, a second input connected to the terminal of said resonant circuit, and having an output; and

a second filter having an input connected to the output of said second multiplier, and having an output.

5. The proximity sensor system of claim 4, wherein said first and second filter are low pass filters.

6. The proximity sensor system of claim 5, wherein said resonant circuit is an inductive-capacitive circuit having a resonant frequency.

7. The proximity sensor system of claim 6, wherein:

a first signal, from the first output of said voltage controlled oscillator and having a first frequency goes to the input of said voltage-to-current converter;

a second signal from said voltage-to-current converter goes to said resonant circuit;

the second signal, from the terminal of said resonant circuit and having the resonant frequency, goes to the second inputs of said first and second multipliers;

a third signal, from the second output of said signal voltage controlled oscillator, having the first frequency and an approximate quadrature phase relationship relative to the first signal, goes to the first input of said first multiplier;

a fourth signal, from the output of said first multiplier and having a DC magnitude representing a difference between the resonant frequency and the first frequency, goes to the input of said first filter;

a fifth signal, from the output of said first filter and having the DC magnitude, with AC components filtered out, goes to the input of said integrator; and

a sixth signal, from the output of said integrator to the input of said voltage controlled oscillator and being a fifth signal that has been integrated, is a variable DC voltage that varies according to a magnitude of the DC magnitude of the fifth signal and the variable voltage adjusts the first frequency of the first and third signals to approximate the resonant frequency.

8. The proximity sensor system of claim 7, wherein:

the first signal goes to the first input of said second multiplier;

a seventh signal goes from the output of said second multiplier to the input of said second filter and has a DC voltage component which varies with an impedance of said resonant circuit, which in turn is affected by a proximity of a target;

said second filter removes much of an AC voltage component in the seventh signal, resulting in an eighth signal at the output of said second filter; and

the eighth signal, a substantially DC voltage, has a magnitude that is at least partially determined by a distance of the target.

9. The proximity sensor system of claim 8, wherein:

the first signal approximates a triangular wave;

the second signal approximates a sinusoidal wave; and

the third signal approximates a square wave.

10. The proximity sensor system of claim 9, further comprising a threshold detector, having an input connected to the output of said second filter, wherein the a ninth signal at the output of said threshold detector is a logic high or low depending on the magnitude of the eighth signal.

11. The proximity sensor system of claim 10, wherein the logic high is indicative of the target within a certain distance of said resonant circuit.

12. A means for interference tolerant proximity sensing, comprising:

means for providing a first waveform at a first frequency;

means for providing a second waveform in a quadrature phase relationship with the first waveform;

means for converting the first waveform into a current waveform;

means for target sensing, having a resonant circuit with a resonant frequency, which has an impedance that varies relative to a distance of a target from the resonant circuit;

feeding the current waveform into the resonant circuit, wherein the resonant circuit converts the current waveform into a voltage waveform across the impedance and at the resonant frequency;

means for multiplying the first waveform and the voltage waveform to provide a first resultant waveform having AC and DC components; and

means for filtering out the AC component from the first resultant waveform to provide a first DC signal, which is indicative of the impedance of the resonant circuit.

13. The means of claim 12, further comprising:

means for multiplying the second waveform and the voltage waveform to provide a second resultant waveform having AC and DC components;

means for filtering out the AC component from the second resultant waveform to provide a second DC signal which has a magnitude that indicates a difference between the first and resonant frequencies; and

means for integrating the second DC signal and outputting a control signal for changing the first frequency to approximately equal the resonant frequency.

14. The means of claim 13, wherein:

the first waveform is triangular;

the second waveform is square;

the voltage waveform is sinusoidal; and

the resonant circuit is inductive-capacitive.

15. The means of claim 14, further comprising means for thresholding the first DC signal into a logic high or low signal.

16. The means of claim 15, wherein the logic high is indicative of the target within a certain distance of said resonant circuit.

17. A method for interference tolerant proximity sensing comprising:

generating a first waveform at a first frequency;

operating a second waveform having a quadrature phase relationship relative to the first waveform;

converting the first waveform into a current waveform;

converting the current waveform into a voltage waveform that varies in magnitude in response to an impedance in a resonant device that changes relative to distance of a target from the resonant device, wherein the voltage waveform has a resonant frequency;

multiplying the first waveform and the voltage waveform to produce a first resultant waveform having AC and DC components; and

filtering out the AC component from the first resultant waveform to produce a first DC signal that indicates an amount of impedance in the resonant circuit.

18. The method of claim 17, further comprising:

multiplying the second waveform and the voltage waveform to produce a second resultant waveform having AC and DC components;

filtering out the AC component from the second resultant waveform to produce a second DC signal that has a magnitude representing a difference between the first and resonant frequencies; and

integrating the second DC signal into a voltage control signal for changing the first frequency to approximately equal the resonant frequency.

19. The method of claim 18, wherein:

the first waveform is triangular;

the second waveform is square;

the voltage waveform is sinusoidal; and

the resonant device is an inductive-capacitive circuit.

20. The method of claim 19, further comprising

thresholding the first DC signal into a logic high or low signal.

21. The method of claim 20, wherein the logic high is indicative of the target within a certain distance of the resonant device.