Manufacturing Bits: March 23

Measuring acceleration
The National Institute of Standards and Technology (NIST) has developed a new and better way to measure acceleration.

NIST has developed an optomechanical accelerometer, a technology that has more resolution and bandwidth than conventional accelerometers. Optomechanical accelerometers uses laser light of a known frequency to measure acceleration. With the technology, NIST has achieved the highest measured acceleration resolutions in the market.

An accelerometer is an electromechanical device or sensor, which measures vibration or acceleration of motion of a structure, according to Omega Engineering. By definition, acceleration is the force acting per unit mass or A = F/m, according to Omega.

Accelerometers are used in aerospace, automotive, smartphones and many other products. In smartphones, an accelerometer enables the device to detect acceleration in any direction. That way, the images on the phone rotate right-side up when it’s being flipped, according to Omega. In cars, an airbag makes use of an accelerometer, which detects sudden changes in velocity.

These devices measure either static or dynamic acceleration. “Static acceleration is the constant force acting on a body, like gravity or friction,” according to Omega. “Dynamic acceleration forces are non-uniform, and the best example is vibration or shock.”

There are various types of accelerometers. One type, called a piezoelectric accelerometer, sends an electrical signal from the sensor when it experiences a sudden acceleration, according to the company. Traditional accelerometers, however, have reached their sensitivity and bandwidth limits in many applications, according to NIST.

In contrast, NIST’s optomechanical accelerometer is said to be more precise than traditional accelerometers. Based on a Fabry–Perot microcavity in a silicon chip technology, NIST’s accelerometer is precise, field deployable, and can self-calibrate.

Accelerometers from NIST and others record changes in velocity. This is done “by tracking the position of a freely moving mass, dubbed the ‘proof mass,’ relative to a fixed reference point inside the device. The distance between the proof mass and the reference point only changes if the accelerometer slows down, speeds up or switches direction,” according to NIST. “The motion of the proof mass creates a detectable signal. The accelerometer developed by NIST researchers relies on infrared light to measure the change in distance between two highly reflective surfaces that bookend a small region of empty space.”

NIST’s technology achieved the thermodynamic limit of resolution over a frequency range greater than 13kHz. “The NIST device consists of two silicon chips, with infrared laser light entering at the bottom chip and exiting at the top. The top chip contains a proof mass suspended by silicon beams, which enables the mass to move up and down freely in response to acceleration. A mirrored coating on the proof mass and a hemispherical mirror attached to the bottom chip form an optical cavity,” explained Feng Zhou from NIST.

“The wavelength of the infrared light is chosen so that it nearly matches the resonant wavelength of the cavity, enabling the light to build in intensity as it bounces back and forth between the two mirrored surfaces many times before exiting,” Zhou said. “When the device experiences an acceleration, the proof mass moves, changing the length of the cavity and shifting the resonant wavelength. This alters the intensity of the reflected light. An optical readout converts the change in intensity into a measurement of acceleration.”

Hummingbird wing measurements
The Eindhoven University of Technology, Sorama and Stanford University have developed a new technique that measures the speed and sound of a hummingbird’s flapping wings.

Researchers hope to unravel the sound of a hummingbird’s wings and explore ways to improve products like drones, and make them quieter. “The knowledge gained in this research helps improving aircraft and drone rotors as well as laptop and vacuum cleaner fans,” according to Sorama.

In this research, scientists examined six Anna’s hummingbirds at Stanford. The birds drank sugar water from a fake flower in a flight chamber. To measure the sound, researchers observed hummingbirds using 12 high-speed cameras, 6 pressure plates and 2,176 microphones.

When it hovers in front of a flower or other object, the wings of hummingbird flap at about 40 beats per second, thereby generating sound.

“The hummingbird’s hum originates from the pressure difference between the topside and underside of the wings, which changes both in magnitude and orientation as the wings flap back and forth,” according to researchers. “These pressure differences over the wings are essential, because they furnish the net aerodynamic force that enables the hummingbird to liftoff and hover.”

“This is the reason why birds and insects make different sounds. Mosquitoes whine, bees buzz, hummingbirds hum, and larger birds ‘woosh’. Most birds are relatively quiet because they generate most of the lift only once during the wingbeat at the downstroke. Hummingbirds and insects are noisier because they do so twice per wingbeat,” said David Lentink, a professor at Stanford. “The distinctive sound of the hummingbird is perceived as pleasant because of the many ‘overtones’ created by the varying aerodynamic forces on the wing. A hummingbird wing is similar to a beautifully tuned instrument.”

Patrick Wijnings, a PhD student at Eindhoven University of Technology, added: “We developed an algorithm for this that can interpret a 3D acoustic field from the measurements, and this enabled us to determine the most probable sound field of the hummingbird. The solution to this so-called inverse problem resembles what a police facial composite artist does: using a few clues to make the most reliable drawing of the suspect. In this way, you avoid the possibility that a small distortion in the measurements changes the outcome.”