Imagine you are driving at top speed on a two-lane road. And suddenly it appears on the right
The airbag is activated byaccelerometer - a sensor that detects sudden changes in speed. Accelerometers keep rockets and planes on the correct flight path and provide navigation for self-driving cars. They are also built into mobile phones, tablets and e-readers to display images correctly when the user turns the device over.
Researchers from the National InstituteStandards and Technologies (NIST) have developed an accelerometer just a millimeter thick. It uses laser light instead of mechanical deformation to generate a signal. Scientists hope to meet the growing demand for accurate acceleration measurements in small navigation systems and other devices.
Although some other accelerometers alsorely on light, the NIST instrument's design makes the measurement process easier while providing greater accuracy. In addition, it operates over a wider frequency range and has undergone more stringent testing than similar devices.
NIST device - optomechanical accelerometerdoes not require a lengthy periodic calibration process. In fact, because the instrument uses laser light of a known frequency to measure acceleration, it could ultimately serve as a portable reference for calibrating other accelerometers currently on the market, making them more accurate.
The accelerometer will also improve inertial navigationin critical systems such as military aircraft, satellites and submarines, especially when GPS signal is not available. NIST researchers Jason Gorman, Thomas LeBrun, David Long and their colleagues described their work in the journal Optica.
Accelerometers, including the new NIST device,record changes in speed by tracking the position of a freely moving mass, called a “reference mass,” relative to a fixed reference point inside the device. The distance between the reference mass and the reference point changes only if the accelerometer slows down, accelerates, or changes direction. The same is true if you are a passenger in a car. If the car is stationary or moving at a constant speed, the distance between the person and the dashboard remains unchanged. But if the car suddenly brakes, the driver is thrown forward and the distance between the person and the dashboard decreases.
The movement of the reference mass creates a detectablesignal. The new accelerometer uses infrared light to measure the change in distance between two highly reflective surfaces that cover a small area of empty space. A control mass suspended on flexible beams one-fifth the width of a human hair supports one of the mirror surfaces. The other reflective surface, which serves as a fixed reference point for the accelerometer, consists of a fixed microconcave mirror.
Together, two reflective surfaces and a blankthe space between them forms a cavity in which infrared light of the desired wavelength resonates or reflects between the mirrors, increasing the intensity. This wavelength is determined by the distance between the two mirrors, just as the pitch of a plucked guitar depends on the distance between the instrument's fret and the bridge. If the reference mass moves in response to acceleration by changing the distance between the mirrors, the resonant wavelength also changes.
To track changes in resonant lengthresonator waves with high sensitivity, stable single-frequency laser is tied to the resonator. The scientists used an optical frequency comb to measure the length of the resonator with high accuracy. The ruler marks (comb teeth) can be thought of as a series of lasers with equally spaced wavelengths. As the test mass moves during the acceleration period, shortening or lengthening the cavity, the intensity of the reflected light changes as the wavelengths associated with the comb teeth move in and out of resonance with the cavity.
Accurate conversion of control movementmass into acceleration has been problematic in most existing optomechanical accelerometers. However, the new design of the device ensures that the dynamic relationship between reference mass displacement and acceleration is simple and easy to model using first principles of physics. Simply put, the test mass and support beams are designed to behave like a simple spring or harmonic oscillator. It vibrates at one frequency within the operating range of the accelerometer.
This simple dynamic response allowed scientistsachieve low measurement error over a wide range of acceleration frequencies - from 1 to 20 kilohertz - without the need to calibrate the device. This feature is unique in that all commercial accelerometers must be calibrated, which is time consuming and expensive. Since publishing their study in Optica, the researchers have made several improvements that should reduce the error of their device to almost 1%.
Optical-mechanical accelerometer capable ofdetect displacements of a reference mass that are less than one hundred thousandth the diameter of a hydrogen atom, detecting accelerations of up to 32 ppb ag, where g is the acceleration due to Earth's gravity. This is higher sensitivity than any accelerometer currently on the market of similar size and bandwidth.
With further improvementsNIST's optomechanical accelerometer could be used as a portable high-precision reference device to calibrate other accelerometers without having to bring them to the lab.
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In optics, a frequency comb is a lasera source whose spectrum consists of a series of discrete, equally spaced frequency lines. The frequency comb allows direct communication from RF standards to optical frequencies. Modern frequency standards such as atomic clocks operate in the microwave region of the spectrum, and a frequency comb brings the precision of such clocks to the optical part of the electromagnetic spectrum.