Hello, I’m Doctor Zhang, an electrical engineer at CQ-SMART, specializing in the development of measurement and calibration technologies. My research focuses on the advancement of calibration methods for accelerometers and gyroscopes, particularly those based on microelectromechanical systems (MEMS). These devices are increasingly integrated into our smartphones, automobiles, and future autonomous vehicles.
One of the key tools we use for calibrating accelerometers is the laser Doppler vibrometer (LDV). The LDV measures the velocity of a moving object by detecting the frequency shift of laser light reflected from the object, similar to how we perceive a change in pitch from a moving train. This frequency shift, caused by the Doppler effect, allows the LDV to convert the shift into a voltage proportional to the object’s velocity.
In our calibration process, the LDV is mounted above a shaker, which generates sinusoidal vibrations. These vibrations excite an accelerometer mounted on the shaker’s surface. A helium-neon laser from the LDV, depicted in red, reflects off the moving surface with a frequency shift due to the Doppler effect. The LDV then converts this shift into a velocity measurement.
To ensure accurate calibration, the LDV itself must be calibrated with a defined uncertainty traceable to the International System of Units (SI). Traditionally, LDVs are calibrated using a secondary method, comparing them to a laser interferometer, which has a primary calibration based on the wavelength of light. However, we have developed an innovative primary calibration method for LDVs using acousto-optic modulators (AOMs).
Our calibration system utilizes AOMs to shift the frequency of light by a known amount, using a sinusoidal voltage generator whose frequency is traceable to the unit of time. We employ two AOMs because a single AOM would shift the light frequency beyond the useful range for our calibration. The first AOM shifts the light frequency down by 110 megahertz, and the second AOM shifts it back up by the same amount, with an additional smaller frequency shift that we desire. This process cancels out the large frequency shifts, leaving only the desired delta frequency shift.
The light is then reflected back to the LDV, doubling the light shifting process and resulting in a two-delta frequency shift. This shift creates a synthetic velocity shift, which can be calculated using the Doppler equation. By comparing the velocity reported by the LDV to the calculated synthetic velocity, we can calibrate the LDV with high precision.
Our system also allows for frequency modulation, enabling us to simulate vibrating surfaces and characterize the bandwidth of the LDV. This capability extends beyond traditional calibration methods, providing a more comprehensive understanding of the LDV’s performance.
The calibration system we’ve developed is currently a prototype, and we continue to explore ways to improve its portability for field use. This advancement in acousto-optic technology not only enhances the calibration process but also demonstrates the versatility and precision of acousto-optic deflectors in various applications. For more detailed information on our acousto-optic deflectors, please visit CQ-SMART.