Researchers at the National Institute of Standards and Technology (NIST) have developed an accelerometer just a millimeter thick. It uses laser light instead of mechanical deformation to generate a signal.
Imagine driving at top speed on a two-lane road. And suddenly a car appears on the right. After pressing the brake, an impact occurs, and within a split second after the airbag is deployed. This saves the person from serious injury or even death.
The airbag is deployed thanks to the accelerometer, a sensor that detects sudden changes in speed. Accelerometers keep missiles and planes on the correct flight path and provide navigation for unmanned vehicles. They are also built into mobile phones, tablets and e-books to correctly display the picture when the user flips the device.
Researchers at the National Institute of Standards and Technology (NIST) have developed an accelerometer just a millimeter thick. It uses laser light instead of mechanical deformation to generate a signal. In this way, the scientists hope to meet the growing demand for accurate measurement of acceleration in small navigation systems and other devices.
While some other accelerometers also rely on light, the design of the NIST instrument makes the measurement easier while providing greater accuracy. In addition, it operates over a wider frequency range and has undergone more stringent testing than comparable devices.
The NIST device is an optomechanical accelerometer that does not require a lengthy periodic calibration process. In fact, because the instrument uses laser light of a known frequency to measure acceleration, it can ultimately serve as a portable reference for calibrating other accelerometers on the market today, making them more accurate.
The accelerometer will also improve inertial navigation on mission-critical systems such as military aircraft, satellites and submarines, especially when a GPS signal is not available. NIST researchers Jason Gorman, Thomas LeBroon, David Long, and their colleagues have reported their work in Optica.
Accelerometers, including the new NIST device, record speed changes by tracking the position of a freely moving mass, called a “reference mass,” relative to a fixed reference point within 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 detectable signal. 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 from flexible beams one-fifth the width of a human hair supports one of the mirrored surfaces. Another reflective surface, which serves as a fixed reference point for the accelerometer, consists of a fixed micro-concave mirror.
Together, the two reflective surfaces and the empty space between them form a cavity in which infrared light of the desired wavelength resonates or reflects between the mirrors, increasing in 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 the resonant wavelength of the resonator with high sensitivity, a stable single frequency laser is tied to the resonator. The scientists used an optical frequency comb to measure the cavity length with high precision. The ruler marks (ridge teeth) can be thought of as a series of lasers with equally spaced wavelengths. As the test mass moves during the acceleration period, contracting or lengthening the cavity, the intensity of the reflected light changes as wavelengths associated with the ridge teeth enter and exit resonance with the cavity.
Accurate conversion of the movement of the reference mass into acceleration has been problematic in most of the 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. In simple terms, 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 scientists to achieve low measurement uncertainty 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 the publication of their study on Optica, the researchers have made several improvements that should bring their device’s uncertainty down to nearly 1%.
An optomechanical accelerometer, capable of measuring reference mass shifts of less than one hundred thousandth the diameter of a hydrogen atom, detects accelerations of up to 32 billionths of ag, where g is the acceleration due to the Earth’s gravity. This is a higher sensitivity than all accelerometers of the same size and bandwidth available on the market today.
With further enhancements, the NIST Optomechanical Accelerometer could be used as a portable high-precision reference device to calibrate other accelerometers without having to bring them to the lab.