II. WORKING PRINCIPLE Figure 1 illustrates the principle of the optical strain sensor using a diffraction grating and a position-sensitive detector. The grating is attached to the surface of the sample, and when a monochromatic collimated beam is incident perpendicularly onto the linear grating (>40 lines/mm), a set of diffracted spots appears on a screen parallel to the grating plane. In the setup shown, a laser beam is directed perpendicularly onto a reflective diffraction grating. For high-frequency gratings, only the ±1 diffraction orders are used for strain measurement. These beams are detected by a high-resolution position-sensitive detector. When the sample deforms, the grating moves, causing the diffracted beam to shift. The displacement of the ±1 order beam along the sensor length is given by equation (1), where p is the grating’s spatial frequency, B is the diffraction angle, and l is the laser wavelength. A small deformation causes a change in the grating spacing (Δp), leading to a change in the diffraction angle (ΔB). This results in the displacement of the diffracted beam along the sensor, which can be calculated using equations (2)–(5). By combining these equations, the basic strain measurement formula is derived.

III. SENSOR SYSTEM AND MEASUREMENT METHOD

1. Sensor System Hardware Figure 2 shows the configuration of the sensor system, suitable for both laboratory and industrial applications. It includes a laser source, two position-sensitive sensors, two 633 nm bandpass filters, a converging lens, and a diffraction grating. The grating has a spatial frequency of 1200 lines/mm and is bonded to the sample surface. A He-Ne laser (632.8 nm) with a diameter of about 1 mm illuminates the grating. The position-sensitive detectors are optoelectronic devices based on single photodiodes. Key features of the system include: high spatial resolution, dual voltage signals for fast signal processing, compact size, high relative position resolution (1/5000), insensitivity to light intensity changes, wide spectral sensitivity (300–1100 nm), and a fast response time (<20 ms) for dynamic measurements. The output voltages from the sensors are converted to digital signals via an A/D converter, with a maximum sampling rate of 105 samples/s. Two 633 nm filters help eliminate background noise.

2. Adjustment Method To ensure accurate measurements, the laser beam must be perpendicular to the sample surface. Misalignment can lead to significant errors. The key adjustment step involves aligning the zero-order reflected beam with the incident beam. The grating must be firmly attached to the sample, and the specimen should be precisely positioned. Additionally, the position-sensitive sensors should be adjusted so that the ±1 order diffracted beam is centered on their detection planes.

3. Measurement Methods The main steps are as follows: 1) Prepare the sample and diffraction grating similar to a Moiré interferometer setup; 2) Determine the distance L between the sensor and the grating (100–500 mm) and input it into the software; 3) Perform an initial test to measure the average positions x10 and x20; 4) Apply load and measure the average positions x1 and x2; 5) Calculate the strain using equation (6). All calculations are performed automatically by the software.

4. Interface Software The interface is developed using LabVIEW and includes functions for data sampling, filtering, calculation, memory reading/writing, and display. The processing speed is very high, with a full cycle taking approximately 0.1 seconds. Signal processing and data acquisition are fully automated, and the strain results are continuously displayed on the PC screen in numerical and graphical formats.

IV. SYSTEM CHARACTERISTICS Several factors influence the accuracy of the sensor system, including random noise from the position-sensitive detectors, A/D converter noise, and systematic errors caused by deviations in the laser beam’s incident angle. Random noise affects the system’s sensitivity and spatial resolution. The main sources of noise in the position-sensitive detectors include light source intensity fluctuations, amplifier voltage noise, thermal noise from feedback resistors, and shot noise. The A/D converter noise variance is D²/12, where D is the digitized value. The position resolution of the system is approximately 0.3 mm after accounting for noise. Strain sensitivity varies with the distance L, as shown in Table 1. Systematic errors occur when the laser beam deviates from the normal direction of the sample, and these are analyzed mathematically. The spatial resolution is influenced by the laser beam diameter, and using a converging lens can improve it to 0.1 mm.

V. TECHNICAL PARAMETERS AND CHARACTERISTICS The sensor system has the following specifications: 1) sensitivity of 1 me; 2) variable spatial resolution (0.1–2 mm); 3) maximum strain capacity of 15%; 4) flexible measurement positions across the grating plane; 5) capability for dynamic and continuous strain measurement; 6) automated data acquisition and processing; 7) user-friendly interface; and 8) compact, lightweight design.

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Tag: sensor system, diffraction grating, sample, laser beam, position resolution

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