DETAILED DESCRIPTION

[0025] As shown in FIG. 1, the basic configuration of the stress measuring apparatus 100 and method comprises an optical waveguide 110 affixed on an edge to the stress bearing member 120. The optical waveguide 110 transmits ultraviolet, infrared, and far infrared frequencies. Others skilled in the art may use a waveguide that transmits both visible and non-visible frequency ranges or electromagnetic radiation waves.

[0026] The stress bearing member 120 may be a beam as shown in FIG. 1 or a torque bearing shaft 130 as illustrated in FIG. 3 and will exhibit some degree of deformation stemming from an applied force. FIG. 1 shows the stress bearing element undergoing a bending moment deflecting force. FIG. 1a shows the stress bearing element undergoing a tensile or compressive deflective force. FIG. 3, the preferred embodiment, shows the torque bearing shaft 130 under going a torque driven deflecting force. The optical waveguide 110 may be a fiber optic cable 140 as illustrated in FIG. 3. FIG. 3 illustrates a preferred embodiment that uses a geometry that increases the amount of fiber optic cable 140 affixed to the surface of the torque bearing shaft 130 and aligns the fiber optic cable 140 with the principle stress vector of the torque bearing shaft 130. Using the torque bearing shaft 130 with the fiber optic cable 140 wound helically around its outside diameter, a force is applied in the form of a torque that acts to twist the torque bearing shaft 130. The torque bearing shaft 130, having torque applied to it, exhibits helical principle compressive and tensile stresses on its surface proportional to the magnitude of the torque.

[0027] The torque bearing shaft 130 requires composition of a compliant material (i.e. aluminum) for rigidity factors, as well as a diameter specification such that a smaller outside diameter facilitates more preload optical waveguide 110. Preload refers to an initial state where the optical waveguide 110 is already under a stress. The torque bearing shaft design exhibits a degree of twist over the torque range. The torque bearing shaft 130 comprises a cylindrical shape.

[0028] FIG. 4 shows an alternative embodiment where a stress measuring apparatus 100 uses a fiber optic sleeve 150 as an optical waveguide 110. Fiber optic sleeve 150 is coaxial in nature with a hollow interior permitting the torque bearing shaft 130 to be positioned at the concentric centerline. This approach promotes a freely rotating variation of the stress measuring apparatus 100 by facilitating the launching and collection of the optical signal through non-contacting means. Various physical embodiments are possible including direct deposition of the optical material to the underlying torque bearing shaft 130, or attachment mechanisms such as drawing-down a sleeve of optical material onto the torque bearing shaft 130, or affixing an optical sleeve 150 to the torque bearing shaft 130 through the use of adhesives.

[0029] The principle compressive and tensile stresses that develop along the two counter-spiraling, mutually orthogonal 45° helices are defined by the equation:

T< /highlight>=Tr/J

[0030] where T is the torque applied to the shaft 130, r is the shaft radius and J is the polar moment of inertia. Letting πr⁴/32=J for a solid cylindrical shaft and r=d/2 yields:

T< /highlight>=16T/πd³

[0031] Furthermore, the degree of twist experienced by the shaft 130 for a given torque is given by:

θ=32(LT )/(πd⁴G)

[0032] Where L is the length of the shaft 130, T is the applied torque, d is the diameter of the shaft 130 and G is the modulus of rigidity of the shaft 130. The modulus of rigidity defines the level of elasticity of the shaft material, thus, a lower G value would manifest in a shaft with a higher degree of twist for any given applied torque.

[0033] The fiber optic cable 140 may be actually fixed in relative position to the shaft 130 in order to predictably transfer stresses from the shaft 130 to the fiber optic cable clad 160 which refers to the outer surface (shown in FIG. 7) of the fiber optical cable 140. Similarly, the index of refraction of the cladding material 160 of the fiber optic cable 140 will change when the torque-imposed stresses alter its microstructure. The un-modulated (i.e., no torque signal present) transmission signal 170, preferably a photonic wave carrier, propagates along the fiber optic cable 140 according to Snell's Law. Others skilled in the art may choose to transmit electromagnetic radiation signals depending on the specific optical waveguide 110 used. Snell's Law describes the bending of light that occurs when light passes across the interface of two different materials. Referring to FIG. 5, the angle that light is refracted (i.e., bent away from a straight path) when passing across the interface between two such materials is related to the index of refraction of each material and the angle of the incidence light with respect to a line normal to the interface in accordance with the relationship:

n₁ sin Ø₁=n₂ sin Ø₂

[0034] The index of refraction, n, of a given material is defined as the ratio of the speed that light travels through that material, ν, and the speed that light travels through a vacuum, c.

n=c/ν

[0035] Thus, ν=c and n=1 for a vacuum. For any medium other than a vacuum, ν<c and n will be >1. Conversely stated, the velocity of light is greater for less dense materials manifesting in lower n values. As light slows down, it cover less distance in a given time period where n1<n2 and distance b<a. The distances a and b that light travels in a given time period, t, can be described in terms of light velocity as:

a=v₁t and b=v₂ t

[0036] or, after rearranging variables,

v₁= a/t and v₂=b/t.

[0037] Since by definition n₁=c/v₁ and n₂=c/v₂, then after substitution, n₁ and n₂ can be rewritten as:

n₁= c/[a/t] and n₂=c/[b/t].

[0038] Solving for a and b in each equation, respectively, yields:

a=ct/n₁ and b=ct/n2.

[0039] From the right triangle 180 of FIG. 5 with hypotenuse of length h and with one side of length a, it is evident from trigonometry that:

a=h sin Ø₁

[0040] or

h=a/(sin Ø₁).

[0041] In the other medium, the right triangle 180 with one side of length b shares the hypotenuse with the previously discussed right triangle and is described by:

b=h sin Ø₂

or

h=b/(sin Ø₂).

[0042] Combining the previous equations for h yields:

h=a/(sin Ø₁)=b< /italic>/(sin Ø₂)

or

a sin Ø₂=b sin Ø₁.

[0043] Finally, substituting the solutions for a and b into the previous equation produces the form:

[ct/n₁]sin Ø₂=[ct /n₂]s in Ø₁.

[0044] Canceling terms that are common to both sides simplifies the equation to:

[1/n₁] sin Ø₂=[1/ n₂] sin Ø₁

or

n₁ sin Ø₁=n₂ sin Ø₂

[0045] which is the common form of Snell's Law.

[0046] Referring to FIG. 6, in the case of the sensor apparatus 100, the speed of light is slower in the fiber optic cable's core 190, the inner surface of the fiber optic cable 140, than in the clad 160, the outer surface of the fiber optic cable 140, and the ratio between the two refractive indices are such that the light is totally internally refracted.

[0047] It should further be noted that the frequency of light in a vacuum, f_c, is related to its wavelength, λ, by the relationship:

f_c= c/λ.

[0048] The constant c is the speed of light in free space (i.e., a vacuum). In general, for propagating waves, the wavelength is:

λ=v/f_c

or

v=f_c λ.

[0049] This shows that the velocity of light, v, is directly proportional to its wavelength at a fixed frequency. In terms of the index of refraction, by considering a given frequency of light in free space and in some other medium, the equation for n becomes:

n=c/v=(f_cλ_c)/(f< sub>cλ_v )=λ_c/λ_v

or

n=λ_c/λ_v.

[0050] A lightwave of frequency f_c that is propagating through free space at velocity c yielding a wavelength λ_c is compared to a lightwave, also of frequency f_c, propagating through some medium other than free space at velocity v yielding a wavelength λ_c to produce the ratio n. Once the lightwave leaves the vacuum and enters the denser medium, its velocity slows down as its wavelength grows thereby keeping its frequency unchanged at f_c. Finally, by combining n=c/v with n=λ_c/λ_v the relationship:

c/v=λ_c/λ< sub>v

or

v=(λ_v/λ_c) c

[0051] is established.

[0052] As transmission signal 170 propagates through a medium, such as a fiber optic cable 140, its velocity is related directly to the wavelength of the transmission signal 170. More specifically, the longer wavelength of light, the faster it propagates. The equation for the propagation constant β also shows a decrease in propagation time with increasing wavelength:

β=2πn< /highlight>(λ)/λ

[0053] The index of refraction is more accurately specified as a function of the propagating light wavelength.

[0054] Therefore, the longer wavelength light will propagate faster than shorter wavelength light, thus, if a spectrum of light is launched into a medium, the longer wavelength will reach the receiver 300, preferably a photo receiver, first.

[0055] Referring to FIG. 7, the refractive index of a material is based on it microstructure and, as such, the index of refraction will be impacted by any microstructure changes stemming from externally imposed influences, such as torque-induced stress that affects the clad material 160 and/or the core 190 density. In the case of a fiber optic cable 140, altering the index of refraction of the clad material 160 in response to an external physical parameter creates modulation in the form of attenuation, lost modes, spectral spreading or chromatic dispersion (or combination of all conditions). Therefore, if the angle of refraction is changed significantly enough by the imparted torque-related stress variations in the index of refraction, then the modulated signal 200 or modulated light exiting the fiber cable 140 will show a measurable change, and the fiber optic cable 140 acts as a sense-element.

[0056] Cable bending affects the stress related changes in the microstructure of the fiber optic cable 140 and subsequent changes in its refractive index. Macro-bending imparts stresses into the microstructure that are analogous to those transmitted into it during torque sensing application. Furthermore, macro-bending is used to preload the cable 140 in its quiescent (no torque applied) state in order to make the influence of an imposed torque more immediate and substantial. Preloading systemically brings the cable 140 to a threshold point where additional stresses significantly impact optical carrier transmittal.

[0057] The minimum radius of curvature, minimum radius of bend or critical bending radius specifies the allowable amount of bending before the output signal is degraded such that the number of modes propagated drop by 50%. As explained earlier, the light of different frequencies travels at different velocities, refracts differently and thus, follows different paths as it propagates along the fiber optic cable 140. These paths are referred to as modes and are characterized by the frequency of light that they carry. Single mode optical cables are only capable of carrying one mode. Multi-mode cables carry more than one mode. All fiber optic cables 140 used for present invention are multi-mode type.

[0058] Referring to FIG. 6, the transmission signal 170 normally propagates light through a fiber optic cable 140 because the angle of refraction at the interface between the core 190 and clad material 160 is such that any light launched into one end of the fiber optic cable 140 at the correct angle is internally refracted back along the core 190. This is referred to as the critical angle and creates a condition identified as total internal refraction within the fiber optical cable 140. Radiation losses occur when light escapes from the total internal refraction state. Light that is incident upon the core material 190 and clad material 160 interface at an angle that is beyond the critical angle will be refracted out of the core 190 and into the clad 160 where it will be eventually dissipated.

[0059] The critical bending radius is given by:

R_c˜3n₁< highlight>²λ/[4π(n₁²−n₂²)^3/2]

[0060] Note that the critical bending radius is a function of the index of refraction of both the clad material 160 and the core material 190. It is also affected by the wavelength of the propagating transmission signal 170, although, that parameter is held constant by design. By wrapping the fiber optic cable 140 around the torque bearing shaft 130, the cable 140 is brought close to the minimum curvature of radius, in effect, preloading the cable 140 such that additional torque induced stresses will rapidly attenuate the signal.

[0061] The fiber optic cable 140 may comprise plastic. However, one skilled in the art may use a different material such as glass. Similarly, the fiber optic cable 140 comprises multimode type. However, one skilled in the art may select a different type such as a single mode.

[0062] Having the fiber optical cable 140 mounted such that it is nearly at its minimum radius of curvature is crucial to obtaining the highest level of variation in the signal (or highest depth of modulation) in response to the applied force to the underlying the torque bearing shaft 130. Thus, quiescent state bending occurs by wrapping the fiber optic cable 140 around the shaft 130 placing the cable 140 in a condition where it is more susceptible to the influence of any additional stressing.

[0063] The fiber optic cable 140 affixes around the torque bearing shaft 130 along its helix. As mentioned earlier, the 45° helix of a solid cylindrical shaft is where primary torsional stresses (compressive and tensile) develop as torque is applied.

[0064] A suggested embodiment uses two-part epoxy that affixes the optical fiber 140 to the torque bearing shaft 130. The two-part epoxy does not attack the fiber optic cable 140 and can be a polymercaptan, amine, nonylphenol-based agent. Attachment of fiber optic cable 140 to the torque bearing shaft 130 is not limited to epoxy based schemes. Others skilled in material bonding techniques might utilize alternative adhesion methods including, a single stage glue or heating the shaft 130 so that the fiber optic cable 140 melts directly on the shaft 130. Affixing preserves the relative position between a stress bearing element 120 and an optical waveguide 110. One skilled in the art may affix the stress bearing member 120 to the optical waveguide 110 by bonding techniques, using mechanical fasteners, component embedding or molding, or using standoffs as shown in FIG. 2 and FIGS. 8a -8c. The preferred embodiment may use a bonding technique to affix the fiber optic cable 140 helically around the torque bearing shaft 130.

[0065] A receiver 300 with photodiode for collecting the modulated signal 200 and the LED optical transmitter 310 for emitting transmission signal 170 must also operate at the same wavelength as the individual fiber optic cables 140. The fiber optic cables 140 are generally optimized for the red visible light spectrum or light having a wavelength of 650 nm. The receiver 300 should preferably not have an integral signal conditioning (i.e., no output wave shaping). Signal conditioners, such as comparators, schmitt triggered gates, clippers and filters, would strip away the desired modulation. Thus, the receiver 300 preferably may be linear in nature. One skilled in the art may use a digital receiver with other corresponding processing means.

[0066] The fiber optical cable 140 is driven digitally by a standard LED optical transmitter 310. A current source and an analog oscillator drive the LED source 310. Others skilled in the art may use a laser light source in place of LED source 310.

[0067] Referring to FIG. 8, an alternative embodiment shows a multiple helical fiber optic cables 320 wrapped around the torque bearing shaft 130 at 45°. The multiple helical fiber optic cables 320 is contiguous, appearing like a ribbon cable, in effect creating a continuous sleeve that would permit a version of the stress measuring apparatus 100 with a freely rotating torque bearing shaft 130 and with a non-contacting excitation and output signaling. This embodiment allows RPM or rotational speed measurements and angular acceleration as the signal attenuates during the transition between the contiguous multiple fiber optic cables 320. As the torque bearing shaft 130 rotates, the amplitude of the output signal or modulated signal 200 will momentarily decrease after each of the multiple helical fiber optic cables 320 passes the stationary receiver 300.

[0068] Signal conditioning compares input and output signals. In a communications system, the output should be a reproduction of the input, thus, the input signal (photonic wave carrier 170) and the output signal (modulated signal 200) should be identical. Any differences can be found by subtracting the input signal from the output signal and would have to be attributed to distortion caused by the LED optical transmitter 310, the receiver 300, or the fiber optic cable 140. If the difference signal changes when stresses are imparted into the torque bearing shaft 130, then the source of the variation would be from changes in the fiber optic cable 140. Therefore, the fiber optic cable 140 would be sensing the stresses or torque applied to the torque bearing shaft 130. Alternative signal detection methods such as a phase-lock-loop approach or spectrum analysis may be used by those skilled in the art.

[0069] The present invention has been disclosed with reference to certain embodiments, numerous modifications, alterations, and changes to the described embodiments are possible without departing from the sphere and scope of the present invention. Accordingly, it is intended that the present invention not be limited to the described embodiments and equivalents thereof.