Laser Calibration Ensures Machine Tool and CMM Accuracy

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As shown in Fig. 1a, a commercial LDDM optical system includes four components: a frequency stabilized HeNe laser, an electro-optic assembly, a photodetector, which functions as a receiver, and a target reflector. The laser light reflected by the target is frequency-shifted by the motion of the target. The photodetector measures the phase variation caused by the frequency-shift, which corresponds to the displacement of the target. When the displacement is larger than the half- wavelength, l/2, a counter records the total phase changes as: Df total = 2p N + f , (1) where N is the number of half wavelengths, and f is the phase angle less than 2p. The total target displacement, Dz, can be expressed as [5]: c Dz = (N + f/2p) (2) 2f₀ , where f₀ is the frequency of the laser; and c is the speed of the light. If we make the laser light reflect back and forth M times between the fixed base and the target before it finally reaches the photodetector, as shown in Fig. 1b, then introducing equation (2) gives c Dz = (N + f/2p ) (3) 2f₀ M , which indicates that the multiple-reflection optics provides M-times resolution extension power for the system. Figure 2 shows the self-aligning three-dimensional multiple-reflection optical design for the LDDM system resolution extension [6]. In this design, the heterodyning detector is housed coaxially inside the frequency-stabilized laser source. Instead of a typical single reflection on the moving target, the laser beam is reflected back and forth twelve times between the fixed base and the moving target. The laser beam, which is reflected back to the heterodyning detector, is frequency-shifted by the movement of the moving target relative to the fixed base. With the same LDDM laser source and detector electronics, this optical path provides twelve-times resolution extension power for the linear displacement measurement and encoding. The resolution of the custom-made commercial LDDM system that was used during this test, was 2 nm (1 nm LSB), so that, theoretically, a 0.166 nm resolution (0.083 nm LSB) was reached by the prototype LDLE system. Fig. 2. Configuration of a self-aligning twenty-four reflection optical design. In this figure, item 1 is the frequency- stabilized laser source with heterodyning detector, items 2 – 8 are right-angle prisms, and item 9 is the end retroreflector.