Acceleration Measurements in Reduced Gravity by Fiber Optic Accelerometer

Test Description

INTRODUCTION AND BACKGROUND

Over the last few years, fiber optics sensors have been applied to many fields
because of their distinct advantages, i.e. lightweight, very small sizes,
passive, low power, resistant to electromagnetic interference, high sensitivity,
environmental ruggedness, and price.  Fiber optic sensors have replaced
traditional sensors for rotation, acceleration, electro- and magnetic-field
measurement, temperature, pressure, acoustics, vibration, linear and angular
position, strain, humidity, viscosity, chemical measurements, etc. As a result
of fiber optics, sensor technology has experienced tremendous growth since its
early beginnings in the 1970s. [1]
	There are several kinds of optical accelerometers; most common are the
cantilever, compliant cylinder, shutter, and lateral.  Cantilevers
accelerometers are well tested and precise, yet are not extendible to three
dimensions. [2] Shutter accelerometers have the advantage wide operation range,
but are purely amplitude based lacking precision. [3] Compliant cylinder
accelerometers were designed for geological purposes and therefore very rugged;
however, the engineering aspects of the fiber coils limit precision. [4] We will
therefore be constructing a lateral type of optical accelerometer, as it is
precise (phase based), extendible to three dimensions, quite durable, and among
the most affordable.  Most kinds of lateral accelerometers are made for one
dimension and the seismic mass's motion is restricted completely in the other
two dimensions by mechanical means. [5] Measurements of acceleration will be
made in all three dimensions by having the optical fibers themselves restrict
the motion of the seismic mass, while still being detectors.  

Fiber Bragg Gratings

Fiber Bragg gratings are induced refractive index variations along the axis of
the fiber. The change in the index of refraction is permanently recorded using
UV laser light. This change in the core of the fiber's index of refractions acts
as an axial Bragg grating. A light beam propagating along the fiber is confined
to the core of the fiber. When the beam reaches the grating it reflects back a
narrow band of the spectrum of the beam. The fiber Bragg grating acts as a
narrow band reflective filter. The central wavelength of the reflected beam is
determined by the period of the grating and the index of refraction of the core
of the fiber. If the fiber is stretched, the grating period increases and causes
the central wavelength to change accordingly. This makes the fiber very
sensitive to any stress that is applied to it. Measuring the change in the
central wavelength of the fiber gratings can be used to determine precisely the
exerted stress on the fiber.

Proposed Method

Figure 1:  Internal View of Optical Accelerometer
	
We propose to use fiber Bragg gratings to determine acceleration by measuring
the central wavelength shift of the grating when forces are applied. The
wavelength shift is created by the tension in the fiber. Using this property of
the fiber Bragg gratings, and a known mass, we will be able to accurately
determine the acceleration on the unit in micro-g. The grating will be
pre-tensioned on the ground in order to properly calibrate the unit. Figure 1
shows the orientation of the fibers in relation to the central mass. Four
fibers, one along each of the standard XYZ axes and one at a 45º angle from the
corner will suspend the mass. The 45º placed fiber will account for rotation and
provide measurements for the vectoral loads imposed on the mass. Two fibers will
also be attached to the walls of the box to compensate for temperature and
pressure expansion/contraction of the container. They will also provide a
baseline measurement of initial conditions. 



Figure 2: Fiber Cable Connection to Containment Box

The internal fibers are connected through the casing using a threaded insert for
initial tensioning (Figure 2).  They are then connected to the LED and DAQ with
a coupler as shown in figure 3.  
	


Figure 3:  The connections to the Fiber Optical Accelerometer

We propose to use the varying gravitational environment produced by NASA's
KC-135, which will provide the maximum range of forces, in both positive and
negative regimes, on our optical accelerometer. This testing will give true
performance and capabilities of the optical methods used to measure the
acceleration. Performing this experiment on NASA's KC-135 would provide pure
acceleration, which would be difficult to reproduce in a lab environment. This
environment would also provide a suitable means of measuring micro-g
accelerations. This will provide data to determine the precision and accuracy of
micro-g level measurements.

Measuring The Tension in the Fiber

	The tension in the fiber induces a change in the fiber Bragg grating
filter's central wavelength. This change in the wavelength can be measured using
the techniques described in reference [6]. Thus the change in the fiber tension
and consequently the acceleration will result in an electronic signal that will
be measured and recorded throughout the flight using DAQ.

Pre and Post Flight Data Analysis

	For the pre-flight analysis we will design, build and calibrate the
accelerometer to find the effective spring constants of the optical fibers.  We
will then run the accelerometer at known small torques, comparable to what we
would expect from the KC-135.  This will serve as a durability test as well as
giving us a plot of rotational effects on the accelerometer.  From the plots we
will be able to determine the effective shear modulus of the optical fibers,
allowing us to computationally account for torque.  We will also take initial
temperature and check that the accelerometer's case did not come loose during
transport, keeping the case expansion effect to a minimum.  
	During the experiment the semi-continuous phase change of the Fiber
Bragg Grating will be recorded.  The phase change will give us the rough
estimate for how much the fiber has stretched.  To sharpen that estimate, we
will use the constants found in the pre-flight analysis to account for the
stretching of the fibers due to rotation.  Note that the over all effect of the
rotations will have a less than an angle squared effect, and as the angle will
be at a maximum of 5o, the end results should be quite accurate.  We will then
subtract the temperature and casing expansion from our plots and get the final
plots.