A better way to measure acceleration

You go down the speed limit on a two-lane lane if a car on your right exits a ramp. You step on the brakes, and within a fraction of a second of the impact, an airbag inflates, saving you from serious injury or even death.

The airbag deploys thanks to an accelerometer – a sensor that detects sudden speed changes. Accelerometers keep rockets and planes on the right flight path, provide navigation for self-driving cars and rotate images so that they stay on top of the phone and tablets, among other essential tasks.

To accurately measure the increasing demand for acceleration in smaller navigation systems and other devices, researchers from the National Institute of Standards and Technology (NIST) have developed an accelerometer of only one millimeter thick that uses laser light instead of mechanical voltage to to deliver signal.

Although a few other accelerometers also rely on light, the design of the NIST instrument makes the measurement process more straightforward, providing higher accuracy. It also works over a wider range of frequencies and has been tested more closely than similar devices.

Not only is the NIST device, known as an optomechanical accelerometer, much more accurate than the best commercial accelerometers, it also does not have to undergo the time consuming process of periodic calibrations. In fact, because the instrument uses laser light at a known frequency to measure acceleration, it can eventually serve as a portable reference standard to calibrate and accelerate other accelerometers currently on the market.

The accelerometer can also improve inertial navigation in critical systems such as military aircraft, satellites and submarines, especially if a GPS signal is not available. NIST researchers Jason Gorman, Thomas LeBrun, David Long and their colleagues describe their work in the journal Optics.

The study is part of NIST on a Chip, a program that brings the institute’s cutting-edge technology and expertise directly to users in commerce, medicine, defense and academia.

Accelerometers, including the new NIST device, record changes in velocity by detecting the position of a free-moving mass, called the “proof mass”, relative to a fixed reference point within the device. The distance between the mass of evidence and the reference point changes only if the accelerometer slows down, accelerates or changes direction. The same goes if you are a passenger in a car. If the car is resting or moving at a constant speed, the distance between you and the dashboard remains the same. But if the car suddenly brakes, you are thrown forward and reduce the distance between you and the dashboard.

The motion of the mass of evidence creates an observable signal. The accelerometer developed by NIST researchers relies on infrared light to measure the change in distance between two strongly reflective surfaces that provide a small empty area of ​​space. The test mass, which hangs through flexible beams one-fifth the width of a human hair so that it can move freely, supports one of the mirror surfaces. The other reflective surface, which serves as the fixed reference point of the accelerometer, consists of an immovable microfabricated concave mirror.

Together, the two reflective surfaces and the empty space between them form a cavity in which infrared light of just the right wavelength can resonate between the mirrors, or can bounce back and forth and build in intensity. The wavelength is determined by the distance between the two mirrors, just as the pitch of a picked guitar depends on the distance between the instrument’s fret and bridge. As the mass of evidence moves in response to acceleration and the separation between the mirrors changes, so does the resonant wavelength.

To detect the changes in the resonant wavelength of the cavity with a high sensitivity, a stable single frequency laser is connected to the cavity. As described in a recent publication in Optics letters, the researchers also used an optical frequency comb – a device that can be used as a ruler to measure the wavelength of light – to measure the wavelength with high accuracy. The marks of the ruler (the teeth of the comb) can be considered as a series of lasers with equal wavelengths. When the mass of evidence moves during an acceleration period, or shortens or lengthens the cavity, the intensity of the reflected light changes as the wavelengths associated with the teeth of the comb move in and out of the resonance with the cavity.

The accurate conversion of the displacement of the mass of evidence into an acceleration is a critical step that has been problematic in most existing optomechanical accelerometers. However, the new design of the team ensures that the dynamic relationship between the displacement of the mass of evidence and the acceleration is simple and easy to model through the first principles of physics. In short, the proof mass and support beams are designed so that they act like a simple spring, or harmonic oscillator, vibrating at a single frequency in the working range of the accelerometer.

This simple dynamic response has enabled scientists to achieve low measurement uncertainty over a wide range of acceleration frequencies – 1 kilohertz to 20 kilohertz – without ever having to calibrate the device. This feature is unique because all commercial accelerometers need to be calibrated, which is time consuming and expensive. Since the publication of their study in Optics, the researchers made several improvements to reduce the uncertainty of their device to almost 1%.

The optomechanical accelerometer can detect accelerations of less than one hundred thousandths of the diameter of a hydrogen atom, and detect accelerations as small as 32 billionths of a second, where g is the acceleration due to the Earth’s gravity. This is a higher sensitivity than all accelerometers currently on the market, with the same size and bandwidth.

With further improvements, the NIST optomechanical accelerometer can be used as a portable, high-accuracy reference device to calibrate other accelerometers without bringing it into a laboratory.