A powder sensor includes a piezoelectric element, an oscillator circuit, a phase determination circuit, and a powder presence/absence determination 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 phase determination circuit determines phase of a terminal voltage of the piezoelectric element relative to phase of the output signal from the oscillator circuit. The powder presence/absence determination circuit determines that powder is absent if the phase determination circuit determines, n consecutive times (where “n” is an arbitrary integer satisfying n≧2), that the phase of the terminal voltage of the piezoelectric element, relative to the phase of the output signal from the oscillator circuit, satisfies a predetermined condition.
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 phase determination circuit that determines a phase of a terminal voltage of the piezoelectric element relative to a phase of the output signal from the oscillator circuit; and
a powder presence/absences determination circuit that determines whether powder is present or absent based on results of determination of the phase determination circuit, wherein
the phase determination circuit includes an n-stage shift register (where “n” is an integer and at least 2), the n-stage shift register stores in a first stage of the n-stage shift register a level of the terminal voltage as a binary signal of the piezoelectric element at each of predetermined level transitions of a phase judgment signal, which has a cycle that is the same as the cycle of the output signal of the oscillator circuit,
the n-stage register shifts the binary signal to a subsequent stage of the n-stage shift register,
the n-stage shift register outputs, n consecutive times, the level of the terminal voltage as binary signal of the piezoelectric element at each of the predetermined level transitions, and
the powder presence/absence determination circuit determines that powder is absent if every output signal of each stage of the n-stage shift register is at a predetermined level.
2. 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.
3. The powder sensor according to claim 1 , wherein the powder presence/absence determination circuit includes a logic gate that receives, as an input, the output signal of each stage of the n-stage shift register.
4. The powder sensor according to claim 1 , wherein the phase judgment signal is the output signal of the oscillator circuit.
5. The powder sensor according to claim 1 , wherein the phase judgment signal has a phase delayed by a predetermined angle from the phase of the output signal of the oscillator circuit.
6. The powder sensor according to claim 1 , including a converter converting the terminal voltage of the piezoelectric element to the binary signal, an inputting the binary signal to the first stage of the n-stage shift register.