A powder sensor includes a piezoelectric element, an oscillator circuit, a first square wave signal generator, a second square wave signal generator, and a phase judgment circuit. The oscillator circuit applies to the piezoelectric element an output signal having a frequency equal to or near a resonance frequency of the piezoelectric element. The first square wave signal generator generates a first square wave signal by converting a terminal voltage of the piezoelectric element. The second square wave signal generator generates a second square wave signal that transits from an initial level to a detection level when the first square wave signal rises or falls in a detection period and that does not transit outside of the detection period. The detection period is a part of each cycle of the output signal of the oscillator circuit. The phase judgment circuit determines phase of the second square wave signal.
1. A powder sensor comprising:
a piezoelectric element;
an oscillator circuit that applies to the piezoelectric element an output signal having a frequency equal to or near a resonance frequency of the piezoelectric element;
a first square wave signal generator that generates a first square wave signal by converting a terminal voltage of the piezoelectric element into the first square wave signal;
a second square wave signal generator that generates a second square wave signal that transits from an initial level to a detection level when the first square wave signal rises or falls in a period during which the output signal of the oscillator circuit is at a first level and that does not transit from the initial level to the detection level when the first square wave signal rises or falls in a period during which the output signal of the oscillator circuit is at a second level;
a clear signal generator that generates a clear signal and inputs the clear signal to the second square wave signal generator to set the second square wave signal to the initial level after the output signal of the oscillator circuit transits from the first level to the second level and before the output signal of the oscillator circuit subsequently transits from the second level to the first level;
a phase judgment signal generator that generates a phase judgment signal which makes a predetermined level transition at a predetermined phase angle after the output signal of the oscillator circuit transits from the second level to the first level; and
a phase judgment circuit that determines whether the second square wave signal has a phase lag or a phase lead relative to the phase judgment signal, wherein the second square wave signal generator comprises an edge trigger D flip-flop having a clock input receiving the first square wave signal, a D-input receiving the output signal of the oscillator circuit, and a clear input receiving the clear signal.
2. The powder sensor according to claim 1 comprising a powder presence/absence judgment circuit that determines whether powder is present or absent based on the phase lead or phase lag determined by the phase judgment circuit, wherein the powder presence/absence judgment circuit makes a determination that powder is absent if the phase judgment circuit detects, n consecutive times (where “n” is an integer and n>2), that the phase of the second square wave signal satisfies one of the phase lead or the phase lag relative to the phase judgment signal.
3. The powder sensor according to claim 2 , wherein
the phase judgment circuit comprises an n-stage shift register, and
the powder presence/absence judgment circuit comprises a logic gate that receives, as an input, output signals from the n-stage shift register.
4. The powder sensor according to claim 1 , wherein the oscillator circuit is a sweep oscillator circuit that sweeps the frequency of the output signal through a frequency range inclusive of a resonance frequency of the piezoelectric element.
5. The powder sensor according to claim 1 , wherein the phase judgment circuit comprises an edge trigger D flip-flop having a clock input receiving the phase judgment signal and a D-input receiving the second square wave signal.