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Optical Translation Measurement (OTM™)
OTM™ sensor measures the relative motion of surfaces placed adjacent to its aperture. It includes a laser diode, detectors and optics integrated into a small transistor-style package.
An optical grating is integrated in the sensor optics and reflects part of the illuminating light as back-diffracted beams that are focused onto the detectors. The measured surface is also illuminated by a single collimated laser beam, and reflects light in all directions. Part of the light reflects back from the surface onto the detectors. The grating reflected beam serves as a local oscillator, which interferes with the reflected surface light for coherent homodyne detection by the detector.
A two-dimensional grating having three or more non-collinear diffraction orders is used for three-dimensional (3D) motion measurement. The arrangement of the detectors inside the components allows measurement of orthogonal axes. Thus, the OTM™ sensor is a very compact 3D Doppler-based motion sensor, measuring two transverse (parallel to the grating plane) and one longitudinal directions of motion.
In addition, using quadrate detector pairs for each motion axis and creating the direction signal by using different light polarization on the two detectors provides measurement of the direction of motion. The OTM™ sensor can therefore measure not only the magnitude of motion, but also its direction.
Relief profile of the chessboard grating |
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Chessboard grating |
OTM™ Characteristics:
Following are key competitive advantages of the OTM™ technology. Some of these advantages, and especially the combination of them, make OTM™ the only commercially applicable solution to several of the applications that will be described below, and indeed, to other applications not yet considered:
- Non-contact Measurement: OTM™ does not require any contact to be formed between the measured object and the encoder. Indeed, the object may touch the encoder aperture, but it may also be at a varying distance from it, while still maintaining accurate motion measurement. The effective distance for measurement may be tailored to the specifications of the required application.
- 3D motion measurement: OTM™ is capable of measuring full relative spatial movement between it and the measured object, in a single element. OTM™ can be customized to measure only two or single dimensional displacement, with or without direction detection.
- Resolution: the theoretical limit to the resolution of OTM™ is in the order of magnitude of the laser wavelength. Since the OTM™ senor normally uses near IR lasers, the relevant resolution is around 1µm. Given the simple optical set up of OTM™, the actual resolution may be very close to the theoretical limit and can be determined by the specifications of the relevant application.
- Robustness: the OTM™ technology is based on measuring reflected light from the measured object. In fact, the only quality required from the object is that it diffusively scatters light. In other words, any surface that is not perfectly light absorbing or an optically polished material, like a mirror, is measurable.
- Quality: OTM™ technology detects motion only by projecting light and reading the reflected light. It therefore is completely non-mechanical and has no moving parts. The optical component itself can be encapsulated and the laser life expectancy is measured in years of operation, resulting in a maintenance free use of the OTM™ component and a long MTBF.
- Power consumption: Some of the applications are portable applications, requiring a battery or solar power source. Such applications necessitate low power consumption. OTM™ uses a power efficient VCSEL type laser and because of its intrinsic high SNR it requires very low power (around 1mW optical power). In addition, its electronics set up is minimal; hence it requires very low operating current and voltage.
- Cost: the cost of an OTM™ component in high-volume manufacturing is very competitive with other motion encoding solutions. The optical elements are compatible with low cost molded plastic manufacturing, the electronic circuitry is very simple and the low tolerance, pick-and-place assembly, frequently used in the components industry, can be highly automated. The most significant factor in determining the unit cost of OTM™ are the volumes involved, as those reflect both the manufacturing investment and its amortization schedule and the company’s fees for use of the technology, which are derived from the business model relevant for the application in case.
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