Appliance Design - February 2008 - (Page 33) CONTROLS & SENSORS 5 um Particulates 1 0 um 3 0 um 5 0 um CCD 1 0 0 um VCSEL Polarizers -6 -4 -2 0 2 4 6 Detectors Angle (Degrees) Fig. 2. Normalized intensity as a function of scattering angle and particle size Fig. 1. Schematic of a turbidity sensor. clothes washing machines, dishwashers, water purifiers, etc. The output of the sensor is proportional to the clarity or the turbidity of the sample, which is typically aqueous, but could be any other solution, or even a gas-based suspension. While some useful information can be gleaned from amplitude data alone, the ability to further probe the nature of the suspension would be beneficial. When a coherent (single wavelength) light and polarized source is used as the illuminator, it is possible to further analyze the suspension, extracting additional information that can be used as the basis for improved system control. In a typical washing process, the solubilization of debris such as oil, dirt, and grease is accomplished by a surfactant in the detergent. The surfactant essentially allows for these materials to be dissolved into the water. Once in the water, the surfactant forms a shell around the outside of the debris, causing it to remain in solution. The size of a typical surfactant molecule is on the order of a few nanometers, while the size of the debris, once surrounded by the surfactant, can be a thousand times larger. Therefore, one piece of potential information that can be used to enhance the washing process is knowledge of the size of the particles in the solution. When illuminated with coherent light, the angle of scattering is dependent on the size of the particles in the suspension. The theoretical basis for scattering of light from particles of an arbitrary size is well developed, generally known as the Mie theory, and is fully described in reference [3]. In general, the effects of particle size on scattering are most dramatic when the size of the particles is near to or smaller than the wavelength of the probe light, but useful information can be obtained for particles up to 100 mm in diameter. Fig. 2 is a plot of the normalized intensity as a function of the scattering angle for particle sizes ranging from 5 mm to 100 mm. The analysis here is done assuming non-absorbing spherical particle cross section. When the particles are non-spherical, then the scattering angle will depend on the shape of the particles. For example, a particle that is cylindrical will scatter differently in the two dimensions of the cylinder. This information can be used to determine the size and shape of the particles in the suspension. To fully utilize the capability of the VCSEL-based turbidity sensor, the detectors shown in Fig. 1 could be placed at specific angles to determine the state of a washing cycle, or could be replaced with a low cost CCD camera to fully analyze the washing cycle. Another benefit to using a VCSEL light source is the known state of polarization of the light “Hi-Rel” Transformers & Power Supplies UL/CSA/CE Standards & Medical UL2601 Featuring Standard & Custom SMT Products Capacitors • Varistors • Transformers • Plug-In Power Adapters 346 Monroe Ave., Kenilworth, NJ 07033 Tel: (908) 272-9262 • Fax: (908) 272-7630 www.ventronicsinc.com • e-mail: ventronics@prodigy.net For more Information Enter 113 www.applianceDESIGN.com AD02081Vent.indd 1 applianceDESIGN February 2008 33 1/14/08 10:58:15 AM http://www.ventronicsinc.com http://www.ventronicsinc.com http://www.appliancedesign.com
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