Accurate length change measurement by laser beam timing - uses polygonal rotating mirror array to split pulsed beam into reflected and reference beams

1. title

Non-contact measurement of relative change in length on the basis of runtime comparison measurements of laser reflections with continuous scanning of the measuring length 2. Application: - strain-controlled vibration tests even at higher test frequency and temperature - strain measurements at notches - COD measurements (fracture mechanics) - determination of yield strength and elastic modulus on ultra-brittle tools -3. State of the art 3.1 Conventional devices for measuring relative change in length require direct mechanical contact with the measuring object. This limits the possible applications.

Eg due to: Temperature influence of the sensor for measurements above and below the room temperature Violation of the test object due to cutting Necessity of a certain geometrical shape of the test section 3.2 Contactless Extensometer®, which work with normal light, are technically complex and thus extremely expensive. The measuring length must be marked by clear black and white edges. A change in the measuring length usually means a change of the recording optics.

Previously known non-contact measuring methods with laser beams (laser interferometer or principle of intensity differences) are also very expensive or limited to specific Ai application areas.

4th Neßaufgabe Non-contact measurement of the relative change in length (= elongation) of fully loaded static or dynamic material samples and construction elements.

5th Measuring principle 5.1 Preparation of the test object The test object requires a zone with different reflection characteristics compared to the surroundings.

The application of the zone is effected by slight surface roughening or by vapor deposition of a matt layer (preferred for measurements at higher temperatures).

The width of the zone must be only slightly above the laser beam diameter. The length of the zone (= measuring length) is limited only by the size and @ shape of the object to be measured, as well as by other experimental conditions (heating furnace, linkage, etc.). Measuring lengths 4 1 mm are possible.

5.2 Measuring Method A beam of a small laser focused on the test object uses a rotating polygon mirror to scan the measuring length described in 5.1. A photodetector with upstream optics converts the reflected from the D object I, aserlicht into square pulses whose width is a measure of the instantaneous absolute Meßlänge. A beam splitter directs a @ part of the deflected by the polygon mirror steel via an adjustable aperture on a second similar photodetector. This produces a constant reference signal whose width corresponds to the measuring length 10. The electronic subtraction of measuring and reference signal gives the change in length = l1 - l0. The electronic division of this difference by the reference signal de relative change in length ε, A relative movement of the object to be measured to the meter is chne meaning on the measurement result, since the resulting difference signal pairs cancel each other by opposite signed sign. The division shortens any fluctuations in the sampling frequency. Interference from extraneous light is eliminated by built-in optical moths, which are tuned to the wavelength of the laser.

6th Advantages of the method Non-contact measurement of the relative change in length with continuously adjustable measuring lengths on moving parts.

For measurements at higher temperatures, only small stomata are necessary in the heating furnace.

Use of any type of sample Low production cost for series @ mature.

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