Flame detector trapezoidal excitation generator output control circuit and method

1. An electronic circuit for generating a trapezoidal excitation waveform, comprising:

a controllable frequency source configured to generate a square wave having a frequency that, upon energization of the controllable frequency source, decreases from an initial frequency value to a substantially constant frequency value a time period after energization of the controllable frequency source; and

a trapezoidal waveform generator coupled to receive the square wave and operable, in response thereto, to generate a trapezoidal waveform at least when the square wave frequency attains the substantially constant frequency value, the trapezoidal waveform generator comprising:

an integrator circuit coupled to receive the square wave and operable, in response thereto, to supply an integrated waveform, the integrated waveform having peak positive and negative voltage amplitude values, and a wave shape that is a mathematical integral of the source waveform over time; and

a clipper circuit coupled to receive the integrated waveform and operable, in response thereto, to limit the peak positive and negative voltage amplitude values to predetermined positive and negative values, respectively.

2. The circuit of claim 1 , further comprising:

an amplifier circuit coupled to receive the trapezoidal waveform and operable, upon receipt thereof, to supply an amplified trapezoidal waveform.

3. The circuit of claim 2 , further comprising:

a transformer having a primary winding and one or more secondary windings, the transformer primary winding coupled to receive the amplified trapezoidal waveform.

4. The circuit of claim 1 , wherein the source waveform, at least upon attaining the constant frequency value, has a 50% duty cycle.

5. The circuit of claim 1 , wherein the trapezoidal waveform generator is further operable, in response to the source waveform, to generate a triangular waveform at least when the source waveform frequency is the initial frequency value.

6. The circuit of claim 1 , wherein the trapezoidal waveform has a substantially constant frequency value at least when the source waveform frequency is at the substantially constant frequency value.

7. The circuit of claim 1 , wherein the controllable frequency source is selected from the group consisting of a processor, a voltage controlled oscillator, and a programmable logic device.

8. The circuit of claim 1 , wherein the positive and negative voltage values are equivalent in magnitude.

9. A gas turbine engine, comprising:

a compressor having an inlet and a compressed air outlet;

a turbine having at least an inlet;

a combustor having at least an air inlet, a fuel inlet, and a combustion gas outlet, the air inlet in fluid communication with the compressed air outlet, the fuel inlet adapted to receive a flow of fuel from a fuel source, and the combustion gas outlet in fluid communication with the turbine inlet; and

a flame detector coupled to the combustor, the flame detector configured to detect the presence of a flame in the combustor and supply a signal representative of an intensity of the flame;

an excitation circuit coupled to the flame detector and operable to supply an excitation signal thereto, the excitation circuit including:

a controllable frequency source configured to generate a square wave having a frequency that, upon energization of the controllable frequency source, decreases from an initial frequency value to a substantially constant frequency value a time period after energization of the controllable frequency source; and

a trapezoidal waveform generator coupled to receive the square wave and operable, in response thereto, to generate a trapezoidal waveform at least when the square wave frequency attains the substantially constant frequency value, the trapezoidal waveform generator comprising:

an integrator circuit coupled to receive the square wave and operable, in response thereto, to supply an integrated waveform, the integrated waveform having peak positive and negative voltage amplitude values, and a wave shape that is a mathematical integral of the source waveform over time; and

a clipper circuit coupled to receive the integrated waveform and operable, in response thereto, to limit the peak positive and negative voltage amplitude values to predetermined positive and negative values, respectively.

10. A method of generating a trapezoidal waveform, comprising:

generating a square wave having a frequency that decreases from an initial frequency value to a substantially constant frequency value over a time period;

integrating the square wave to thereby generate a triangular wave having a peak voltage magnitude that increases from an initial voltage value to a substantially constant voltage value over the time period; and

limiting the peak voltage magnitude of the triangular wave to a predetermined value,

wherein the trapezoidal waveform is generated when the triangular wave peak voltage magnitude exceeds the predetermined value.

11. A flame detector, comprising:

a sensor configured to detect the presence of a flame and supply a signal representative of an intensity of the flame;

an excitation circuit coupled to the sensor and operable to supply a sensor excitation signal thereto, the excitation circuit including:

a controllable frequency source configured to generate a square wave having a frequency that, upon energization of the controllable frequency source, decreases from an initial frequency value to a substantially constant frequency value a time period after energization of the controllable frequency source; and

a trapezoidal waveform generator coupled to receive the square wave and operable, in response thereto, to generate a trapezoidal waveform at least when the square wave frequency attains the substantially constant frequency value, the trapezoidal waveform generator comprising:

an integrator circuit coupled to receive the square wave and operable, in response thereto, to supply an integrated waveform, the integrated waveform having peak positive and negative voltage amplitude values, and a wave shape that is a mathematical integral of the source waveform over time; and

a clipper circuit coupled to receive the integrated waveform and operable, in response thereto, to limit the peak positive and negative voltage amplitude values to predetermined positive and negative values, respectively.

12. The detector of claim 11 , wherein the excitation circuit further comprises:

an output amplifier coupled to receive the trapezoidal waveform and operable, upon receipt thereof, to supply an amplified trapezoidal waveform.

13. The detector of claim 12 , wherein the excitation circuit further comprises:

a transformer having a primary winding and one or more secondary windings, the transformer primary winding coupled to receive the amplified trapezoidal waveform.

14. The detector of claim 11 , wherein the source waveform, at least upon attaining the constant frequency value, has a 50% duty cycle.

15. The detector of claim 11 , wherein the trapezoidal waveform generator is further operable, in response to the source waveform, to generate a triangular waveform at least when the source waveform frequency is the initial frequency value.

16. The detector of claim 11 , wherein the positive and negative voltage values are equivalent in magnitude.

17. The detector of claim 11 , wherein the trapezoidal waveform has a substantially constant frequency value at least when the source waveform frequency is at the substantially constant frequency value.