What Is a Laser Doppler Speedometer?
Laser Doppler Speedometer is a device that measures the velocity of an object using the Doppler effect of laser light. A laser beam is shined onto the object to be measured, and the velocity of the moving or rotating object is measured. Advantages of Laser Doppler Speedometers include high resolution and the ability to detect the direction of rotation, such as reverse rotation.
Laser Doppler Speedometers are also called Laser Doppler Velocimeters when the object being measured is a gas or liquid. Laser Doppler Speedometers measure flow velocity by shining a laser beam on tiny droplets of water or other substances mixed into the gas or fluid flow.
Uses of Laser Doppler Speedometers
Laser Doppler Speedometers are used to measure the speed of moving objects such as belts, and rotating objects such as rolls, motors, and gears. They are also used to detect the opening and closing of elevators. Objects to be measured include paper, building materials, aluminum foil, cables, cheese (food), steel plates, and many others.
Applications of laser Doppler anemometers range from basic fluid dynamics research to industrial measurements of flow around objects such as aircraft, ships, and automobiles. Examples include wind experiments to analyze the aerodynamic characteristics of vehicles and aircraft, and the analysis of the flow of fuel, air mixtures, and exhaust gases in engines.
Principle of Laser Doppler Speedometers
Both Laser Doppler Speedometers and velocimeters utilize the Doppler effect. The Doppler effect is a phenomenon in which the frequency of a wave is shifted when the source of the wave, such as sound or light, and the device that observes or detects the wave, or both, are moving. A familiar example in our daily lives is the phenomenon in which the sirens of ambulances and fire trucks are heard with a sudden change in sound when they pass in front of our eyes.
The principle of Laser Doppler Speedometer is that when light is irradiated onto a moving object, the frequency of the reflected light is shifted, and this reflected light is detected and used to determine the speed of the object. Since the shifted frequency is smaller than the light and difficult to detect, the original light source is superimposed on the reflected light.
Features of Laser Doppler Speedometers
Other devices for measuring the velocity of fluids include pitot-static pressure tubes and hot-wire anemometers. Compared to these devices, Laser Doppler Speedometers are superior because they do not require calibration, can make non-contact measurements, and can also measure reverse flow.
Other Information on Laser Doppler Speedometers
Characteristics of Laser Light
Laser light has some advantageous features for detecting the Doppler effect. Laser is an acronym for Light Amplification by Stimulated Emission of Radiation. Translated into Japanese, it stands for Light Amplification by Stimulated Emission of Radiation. Laser light has four characteristics not found in ordinary light such as sunlight.
First, it is highly directional. High directivity means that light travels straight in one direction.
Second, it has good coherence. Coherence means that the phases of light are regularly aligned.
The third characteristic is that the wavelength range is very narrow and monochromatic light can be obtained. Finally, it has excellent convergence. Good convergence means that it is easy to focus light to a single point using a lens. When we collect sunlight with an insect-eye lens, paper and other objects burn, but because sunlight has a variety of wavelengths, not all the light is actually concentrated at a single point, even when we think we have focused the light. This is because there is a focusing error called aberration. However, laser light has excellent convergence properties that make it easy to focus light to a single point using a lens.
Measuring Gases and Fluids With Laser Doppler Speedometers
Laser Doppler Speedometers that measure the velocity of gases and liquids require the inclusion of tiny tracer particles such as water droplets, silicone oil, or titanium dioxide in the flow. To measure the velocity of the flow, a laser beam is shone onto these tracer particles, and the scattered light emitted by the particles is detected to determine the velocity of the tracer particles. Therefore, it is important that the tracer particles follow the flow sufficiently.