Photoelastic stress sensor signal processor

• US 4,515,473 A
• Filed: 09/13/1984
• Issued: 05/07/1985
• Est. Priority Date: 09/13/1984
• Status: Expired due to Term

• Current Assignee: Science Applications International Corporation
• Original Assignee: Geo Corporation
• Inventors: Mermelstein, Marc D.
• Primary Examiner(s): Ciarlante, Anthony V.
• Application Number:
  US06/650,229
• Time in Patent Office: 236 Days
• Field of Search: 73/705, 73/800, 73/862.38, 73/862.62, 73/862.63, 73/862.64, 73/657, 250/225, 250/227, 350/371, 350/373, 350/400, 350/401, 350/406, 356/32, 356/33, 356/34, 356/35, 356/364, 356/365, 356/366, 356/368
• US Class Current: 356/33
• CPC Class Codes: G01B 11/18 using photoelastic elements
• CPC Class Codes: G01L 1/243 using means for applying force perpendicular to the fibre axis

Abstract

A signal processor is arranged to process polarized light signals obtained from a stress sensor of the type having a photoelastic element that responds to stress by causing a phase difference between components of the polarized light propagating through that element. The stress sensor provides two output beams, each of which has a different polarized component of the transmitted light. The signal processor employs a pair of photodetectors which respond to the intensities of the two polarized light beams by converting the polarized light into electrical signals. Those two electrical signals provide the inputs to a difference differentiator that provides an output proportional to the difference between the derivatives of the inputs. The two electrical output signals of the photodetectors are also applied as inputs to a multiplier whose output is related to the product of its inputs. The square root of the output of the multiplier is obtained from a square root device and is used as the divisor in a divider whose numerator is the output of the difference differentiator. The output of the divider is applied to the input of an integrator whose output is a measure of the stress imposed on the photoelastic element.

Claims

1. In a photoelastic polarimetric stress sensor of the type having

(a) a photoelastic element,

(b) means for directing polarized light into the photoelastic element,

(c) stress imposing means responsive to a phenomenon to be measured for stressing the photoelastic element,

(d) beam splitter means for splitting the polarized light passed through the photoelastic element into a plurality of beams each of which has a different polarized component of the light transmitted through the photoelastic element, and

(e) means responsive to the light intensities of the split beams for providing an output that is a measure of the stress on the photoelastic element,

the improvement wherein the means responsive to the light intensities of the split beams comprises

(i) a pair of photodetectors, each photodetector providing an electrical output signal in response to a different one of the split beams,

(ii) a difference differentiator responsive to the outputs of the pair of photodetectors for providing an output proportional to the difference between derivatives of its inputs,

(iii) a multiplier responsive to the outputs of the pair of photodetectors for providing an output related to the product of its inputs,

(iv) a square rooter responsive to the output of the multiplier for providing an output that is the square root of its input,

(v) a divider responsive to the output of the difference differentiator as the numerator and the output of the square rooter as the divisor for providing an output signal, and

(vi) an integrator responsive to the output of the divider for providing an output signal that is a measure of the imposed stress.

2. A method of processing signals provided by a photoelastic polarimetric stress sensor of the type having

(a) a photoelastic element,

(b) means for directing polarized light into the photoelastic element,

(c) stress imposing means responsive to a phenomenon to be measured for stressing the photoelastic element,

(d) beam splitter means for splitting the polarized light passed through the photoelastic element into a plurality of beams each of which has a different polarized component of the light transmitted through the photoelastic element, and

(e) a pair of photodetectors, each photodetector providing an electrical output signal in response to a different one of the split beams,

the method comprising the steps of

(i) differentiating the outputs of the photodetectors and obtaining the difference between those derivatives,

(ii) multiplying the outputs of the photodetectors and obtaining an output related to the product thereof,

(iii) obtaining the square root of the multiplied output,

(iv) dividing the difference obtained in the first step by the square root of the multiplied output, and

(v) integrating the resultant of the preceding division.