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Session C7: Inertial Measurement Unit (IMU)

Fibernetics Inertial Measurement Unit
Ralph Bergh, Fibernetics LLC
Location: Ballroom D
Date/Time: Thursday, Aug. 26, 11:30 a.m.

Our company’s Inertial Measurement Unit (IMU) we are presenting is navigation grade and consists of three fiber optic gyroscopes (FOGs), three Q-Flex accelerometers, a temperature compensated crystal oscillator (TCXO), and associated electronics. The accelerometers and the FOGs are solidly mounted to each other, but are vibration isolated from the external chassis to reduce unwanted, high-frequency, mechanical noise from getting into the sensors. The stiffness of the isolators is selectable during the manufacturing of the IMU and can be tailored for specific applications. The three FOGs are supplied by a single light source and a novel optical circuit that is more efficient than other standard single-source approaches. We will describe the optical circuit and its advantages in the presentation.
Overview
The IMU is designed to deliver high performance in a small package. While it is similar in size to the Honeywell HG9900 with a somewhat different form factor, the Fibernetics IMU has one-tenth the noise. Performance has driven the design, but considerations for size, weight, and power (SWAP) have also played a large role in the development of this IMU. The IMU requires a single DC voltage between 18 and 35 volts and delivers a serial digital output signal. The output signal default is designed to mimic the asynchronous output of a Honeywell IMU, but can be changed to accommodate customer requirements. Coning and sculling corrections are implemented for the 100-Hz data rate portion of the Honeywell protocol used for navigation. {Previous sentence confuses me. This is part of the Fibernetics IMU???} The Fibernetics IMU consumes about 12 Watts of power at room temperature. At temperature extremes the power consumption rises by a couple of Watts.
FOGs
Each of our FOGs includes a Sagnac interferometer operating closed-loop for superior scale-factor performance up to the tested rate of 750 degrees per second. The Sagnac interferometer consists of an integrated optic chip (IOC) and a fiber coil wound with our proprietary custom winder at Fibernetics. The three FOGs are supplied by a single lightsource and a novel optical circuit that is more efficient than other single lightsource approaches. We use three optical detectors, one for each FOG, to convert the optical power at each of the detectors to an electrical signal. The three electrical signals are amplified,digitized, and processed to provide rotation information, and to adjust the feedback signal delivered to the IOC to effect the control loop of the FOG. We will provide performance data and testing conditions in the presentation.
Electronics
Our electronics consist of four boards: one for the lightsource, one for the three detectors and preamplifiers, one for processing the signals, and one for converting the single supply voltage to the various voltages necessary to power the electronics. These boards are our own design, but they are produced and populated by other companies. We receive the populated boards and attach the optical components and wires to them. We then upload the firmware to the processing board and integrate these electronics with the FOGs and accelerometers.
Accelerometers
We use the Q-Flex accelerometers developed by Sundstrand (now Honeywell). The electrical signals from these accelerometers are digitized on the processing board and combined with the rotation signals. We anticipate being able to accommodate other accelerometers according to the preferences of our customers.
Performance
Typical FOG performance includes bias stability of less than 0.005 degrees per hour, random walk of less than 150 microdegrees per root hour, and scale factor stability of about 10 ppm. Accelerometer performance depends on the actual accelerometer selected, but we have used QFlex accelerometers with bias stability of less than 25 micro-g, and scale factor of less than 100 ppm. The Fibernetics IMU compares favorably to the Honeywell HG9900, especially in random walk, lack of vibration, and lifetime.

Applications
We have had interest in using these IMUs in undersea applications and for space and space-launch applications. We also foresee their use for aircraft navigation and for platform stabilization.



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