Device and method for measuring parts

A device and method for measuring moving material includes a processor and operating software associated therewith; a light source for emitting at least two polarized light beams in a manner wherein the beams cross thereby creating an interference region and generate a set of fringes; a sensor aligned relative to the interference region wherein the fringes have a predetermined orientation to the directional movement of the material and wherein the sensor is operably equipped to receive scattered light emanating from the interference region and provide a time varying signal to the processor such that the processor can manipulate and convert the signal to speed and distance and a polarizing filter operably associating a polarizing filter with one of the sensor and the emitting means in a manner to substantially preclude reflected polarized light from the interference region back to one of the sensor and the emitting source.

FIELD OF INVENTION

The present invention relates to improvements in devices and methods for measuring parts. More particularly, the invention relates to improvements in measuring moving parts to compensate for reflected and scattered light in the use of laser optics.

BACKGROUND OF THE INVENTION

There exist many Laser Doppler Velocimetry devices (LDV) (also known as laser Doppler anemometry, or LDA) to measure the speed and length of moving parts. These devices employ a technique for measuring the direction and speed of material that is processed.

The LDV crosses two beams of collimated, monochromatic, and coherent laser light in the flow of the material being measured. The two beams are usually obtained by splitting a single beam, thus ensuring coherency between the two and have the same polarity and exit the device at an angle. The two beams cross at some standoff distance from the device.

Where the beams cross (intersection) an interference pattern is created. At the beams intersection (the focal point of a laser beam), they interfere and generate a set of straight fringes.

A sensor is then aligned relative to the intersection such that the fringes are perpendicular to the directional movement of material. As material pass through the fringes, they reflect light (only from the regions of constructive interference) into a photodetector (typically an avalanche photodiode), and since the fringe spacing d is known (from calibration), the velocity can be calculated to be u=f×d where f is the frequency of the signal received at the detector.

Since the beam angle is fixed and the wavelength is constant, the distance between the fringes is known and is constant. As particles on the measurement surface move through this interference pattern, a time varying signal is created and measured by the device and converted to speed and distance. It is the light that scatters off of the light stripes of the fringe pattern that generates the signal. This signal is received by the APD (Avalanche Photo Diode).

The particles must be big enough to scatter sufficient light for signal detection (good signal to noise ratio) but small enough to follow the flow. By analyzing the Doppler-equivalent frequency of the laser light scattered (intensity modulations within the crossed-beam probe volume) by the particles within the movement, the local velocity of the material can be determined. The area of interest within the material field is sampled by a crossed-beam in a point by point manner.

While the above system works well on many surfaces, problems can arise when the surface is smooth and shiny. As the surface gets shinier the ratio of reflected light to scattered light increases. The speed information is only in the scattered light. As the reflected light increases, the APD gain decreases. It can decrease to the point where the scattered light can no longer be detected. In extreme cases, the APD can actually saturate due to too much light. In both of these cases, there is no measurement.

Another effect of too much reflected light is that light can feed back into the laser diode and cause it to mode hop. A mode hop is a wavelength change which affects the measurement accuracy. The diode can even get into a state where it is constantly mode hopping and this can result in no measurement.

SUMMARY OF INVENTION

It is an object to improve measurement of moving parts.

It is another object to improve devices that measure moving parts.

It is yet a further object to provide an improved device and method for accurately measuring moving material with enhanced scattered light detection.

It is another object to minimize negative effects from reflective light in Laser Doppler Velocimetry.

Accordingly, the instant invention is directed to a device for measuring moving material, which includes:

a processor and operating software associated therewith;

means for emitting at least two polarized light beams in a manner wherein the beams cross thereby creating an interference region and generate a set of fringes;

a sensor aligned relative to the interference region wherein the fringes have a predetermined orientation to the directional movement of the material and wherein the sensor is operably equipped to receive scattered light emanating from the interference region and to provide a time varying signal to the processor such that the processor can manipulate and convert to speed and distance; and

a polarizing filter operably disposed between the sensor and or emitting means and the interference region.

A method of measuring moving material, includes the steps of providing a processor and operating software associated therewith; providing a source for emitting at least two polarized light beams in a manner wherein the beams cross thereby creating an interference region and generate a set of fringes; operably disposing a sensor aligned relative to the interference region wherein the fringes have a predetermined orientation to the directional movement of the material and the sensor receives scattered light emanating from the interference region and providing a time varying signal to the processor such that the processor can manipulate and convert to speed and distance; and operably associating a polarizing filter with one of the sensor and the emitting means in a manner to substantially preclude reflected polarized light from the interference region back to one of the sensor and emitting means.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Referring now to the drawings, the device for measuring moving material of the instant invention is generally designated by the numeral10. The device10can include a processor unit12having operating software, hardware and display readout, and operative key board associated therewith. A laser diode14can be employed for emitting a light beam15which can preferably be passed through an acusto-optical modulator16acousto-optic modulator (AOM), also called a Bragg cell, which uses the acousto-optic effect to diffract and shift the frequency of light using sound waves (usually at radio-frequency). The light beam15can then be passed through an optical beam splitter18to provide two light beams15A,15B having the same polarity. The light beams15A and15B exit the beam splitter18in a non parallel manner wherein the light beams15A and15B cross thereby creating an interference region20and generating a set of fringes. This can also be referred to as the measurement region20.

A sensor22(such as an Avalanche Photo Diode) can be aligned relative to the interference region20wherein the fringes have a predetermined orientation to the directional movement of the material. The sensor22is operably equipped to receive scattered light emanating from the interference region20and to provide a time varying signal to the processor12such that the processor12can manipulate the signal and convert the signal to speed and distance. A receiving lens24is operably disposed between the sensor22and the interference region20and a polarizing filter26can be operably disposed between the interference region20and the receiving lens24.

As depicted inFIG. 2, the reflected polarized light15C does not pass back through the polarizing filter26. The scattered light17which is the light that scatters off of the light stripes of the fringe pattern of region20passes through the receiving lens24which in turn is received by the sensor22.

To reiterate, issues can arise when the material surface is smooth and shiny. As the surface gets shinier, the ratio of reflected light15C to scattered light17increases. The speed information is only in the scattered light. As the reflected light15C increases, the sensor22gain traditionally decreases to the point where the scattered light17can no longer be detected and the sensor22can actually saturate due to too much light.

To keep the sensor22from saturating and to keep light from feeding back to the laser diode14, optics can be used to block reflected light15C from entering either the sensor22or the laser diode14. To keep reflected light15C from entering the sensor22, polarizing filter26can be employed between the sensor22and interference region20. Since light coming from the laser diode14is relatively well polarized, the reflected light15C off the measurement surface stays relatively well polarized. The polarized filter26, which can either be a linear polarizer or a combination of circular and linear polarizers, is oriented in such a way to block the reflected, polarized light15C. The scattered light17which comes off the measurement surface is randomly polarized and therefore it can pass through the polarizing filter26. The scattered light17is attenuated, but enough passes through to get a measurement. In this regard, the sensor22can receive the scattered light17and send a signal to the processor unit12and convert the signal to speed and distance data.

To keep reflected light from entering the laser diode14, circular polarizers can be added to the source path of the laser diode14. A key to this principle is that the polarizing beam splitter18, already in the system, is used to keep light from going back down the same path back into the diode.

This effect is used in the construction of instant invention which permits light to initially pass through an optical isolator (e.g., receiving lens24with polarized filter26and polarized beam splitter18) but prevent such light, when reflected, from returning through the optical isolator back to the light source. Since the light is circularly polarized, when it reflects off of the surface and goes back through the circular polarizers (λ/4 plates)26B associated with the beam splitter18and mirror19in the opposite direction as seen inFIG. 3, it becomes linearly polarized perpendicular to the source. When it passes through the beam splitter18, instead of being directed toward the laser diode14, it is reflected away from the laser diode14.

By so providing, the improvements in the art are found to significantly enhance the performance of Laser Doppler Velocimetry when the material being measured is shiny. In field applications where it was previously impossible to get accurate measurements, these two improvements have allowed the Laser Doppler Velocimetry to measure accurately as it does on non-shiny surfaces.

While the present invention has been set forth above in a preferred embodiment, it is contemplated that other modifications, improvements and derivations will be readily apparent to those skilled in the art. Accordingly, the appended claims hereto should be accorded the full scope of protection of any such modifications, improvements and derivations.