Displacement sensor employing discrete light pulse detection
Optical sensors, and methods for operating optical sensors, are disclosed. One such sensor may include: a reflector positioned a distance from a reflective diffraction grating and a light source for providing light. A first portion of the light can be reflected from the reflective diffraction grating and a second portion of the light passes through the grating to the reflector and is reflected back through the diffraction grating. The sensor may further include a detector for sensing an intensity of light in an interference pattern formed by the first portion of the light reflected from the diffraction grating and the second portion of the light reflected from the reflector. The sensor includes a controller configured to modulate an emission of the light.
1. A sensor comprising:
a reflector positioned a distance from a reflective diffraction grating;
a light source for providing light, a first portion of the light being reflected from the reflective diffraction grating and a second portion of the light passing through the grating to the reflector and being reflected back through the diffraction grating;
at least a first detector configured to sense an intensity of light in an interference pattern formed by the first portion of the light reflected from the diffraction grating and the second portion of the light reflected from the reflector and configured to generate an output signal representative of the intensity of light in the interference pattern; and
a controller configured to cause the light source to generate a sequence of discrete light pulses directed to the reflector.
2. The device of claim 1 , wherein the controller is configured to cause the light source to generate a sequence of discrete light pulses by intermittently providing power to the light source.
3. The device of claim 1 , further comprising:
processing circuitry for calculating a distance between the reflective diffraction grating and the reflective surface as a function of intensity of light in the interference pattern.
4. The device of claim 1 , wherein the controller is configured to generate the sequence of discrete light pulses at a predetermined number of pulses per unit time.
5. The device of claim 4 , wherein the number of pulses per unit time corresponds to a frequency that is at least twice the maximum frequency of a desired measurement signal.
6. The device of claim 4 , further comprising: a demodulator configured to generate an electrical signal that corresponds to a continuous representation of the intensity of light in the interference pattern.
7. The device of claim 6 , wherein the demodulator comprises a coherent detector comprising:
a band-pass filter for passing a portion of the output signal near the modulation frequency; and
a mixer configured to receive the portion of the output signal near the modulation frequency and a periodic signal at the modulation frequency or a harmonic of the modulation frequency to output a processed signal.
8. The device of claim 6 , wherein the demodulator comprises an envelope detector.
9. The device of claim 6 , wherein the demodulator comprises:
a high-pass filter for passing a portion of the output signal at a frequency higher than half of the modulation frequency;
an input for receiving a voltage bias to shift the average voltage of the signal passed through the high-pass filter to produce a shifted signal; and an envelope detector for receiving the shifted signal and producing a processed signal.
10. A method comprising:
directing a sequence of discrete light pulses to a sensor that includes a reflective surface and a diffraction grating spaced apart from the reflective surface;
sensing an intensity of a sequence of diffraction pulses wherein each of the diffraction pulses is representative of a distance between the reflective surface and the diffraction grating and generating a discrete signal representative thereof; and
generating a continuous signal representative of the distance between the reflective surface and the diffraction grating over time by filtering the discrete signal.