Appliance Design - June 2008 - (Page 44)

ELECTRONICS Resources Used for Field-Oriented Control to assess water quality in the treatment cycle, are very precise and rugged, but are too expensive for appliance use. Appliance turbidity sensors employ optical techniques, where a beam of light is passed through the liquid being tested. Two optical detectors — one positioned head-on to the light source, the other at an angle of 90 Deg to the light source — measure the transmitted and scattered light photons, respectively. The greater the concentration of suspended particles in the water, the less light gets through and the more it is scattered. The turbidity of the water is determined by analyzing the ratio of the scattered light signal, divided by the transmitted light signal. Most dishwashers have removed the detector collecting scattered light information to save cost, which has resulted in reduced sensitivity. On the other hand, while this technique measures occlusion to a degree that is satisfactory for dishwasher applications, it may not be sensitive enough for clothes washer applications where the change in turbidity is relatively small. The frequency-domain analysis of transmitted light would not only provide the strength of the collected signal, but also spectral information that may yield sufficient information to enhance sensitivity for washing machines. By implementing frequency-domain analysis of turbidity information on a DSC, dishwashers may be able to optimize detergent use and reduce rinse cycles by administering proper concentrations of detergent for local water conditions. Digital sensor processing may also be able to distinguish between particulates, normal turbulence, and the presence of detergent by examining frequency domain characteristics. This may permit the detector to have dual uses: both detergent detection and normal turbidity measurement. An expensive element of turbidity detection is the material through which the liquid passes. Lower-cost material can succumb to abrasion following long-term exposure to particulates in the water flow. This causes reflected light that reduces the sensitivity of measurement. Digital sensor processing may permit lower-priced materials to be employed, by using digital techniques to improve signal selectivity — thus extending sensor life. Fig. 1. Resource utilization for an advanced sensorless FOC algorithm on a 28-pin, 128 KB Flash DSC. Flash, the dsPIC33F128MC202 DSC. (Note: dsPIC is a registered trademark of Microchip Technology.) There is richness to sensor data in the frequency domain that is not apparent in the time domain. Sensors are often remote from signal conditioning circuitry, and their signals are subject to environmentally induced noise. A low signal-to-noise ratio can adversely affect the performance of the end application. Take, for example, the thermocouple, which is a common sensor used for measuring temperature. Thermocouples are 2-wire sensing elements that use the Seebeck effect to measure the temperature at the junction of the two wires. The Seebeck effect creates a small voltage across the junction of two dissimilar metals that correlates to the temperature at the junction. Because of low signal amplitude and low current drive, thermocouple signals are highly susceptible to power-line contamination. For slowly moving temperatures, a low pass filter may satisfactorily reject power-line noise. Digital filters are preferable for filter characteristics requiring adaptation (such as sharp rejection of a 50 Hz signal versus a 60 Hz signal), sharp cutoff frequencies or stability over time, temperature and production variation. For complex filters, the traditional analog filtering approach requires more components, is subject to drift over temperature and aging, and requires component swapout for filter characteristic changes. The thermocouple example can be extrapolated to any other sensor with induced noise that degrades application performance. For sensor applications with higher rates of change, the power-line noise may fall within the spectra of the sensor signal. This may best be filtered using a notch digital filter. Fig. 2 indicates a time-domain representation of a signal buried in noise. Fig. 3 illustrates how the desired signal becomes easily discernable in the frequency domain. Another illustrative example can be found in a different class of sensor processing; one where sensor accuracy or reliability is desirable, but economically challenging. In such cases, two approaches can be taken. One is to use a lower-cost sensor and augment the lost reliability with digital sensor processing. The other is to combine sensor functions to eliminate a sensor. Alternately, it may be possible to do both. Turbidity detection can be used as a platform to illustrate plausible concepts. Turbidity sensing Turbidity detection can be used to detect the level of particulates in dish or laundry water, to determine that the load is clean enough to terminate a wash cycle early. This helps to avoid additional tub fills, which saves machine energy and time. Industrial-grade turbidimeters, used at water treatment plants Practical considerations Appliance engineers do not need to be steeped in DSP technology to take advan- 44 applianceDESIGN June 2008 www.applianceDESIGN.com http://www.appliancedesign.com

Table of Contents Feed for the Digital Edition of Appliance Design - June 2008

Appliance Design - June 2008 Contents Editorial Shipments/Forecasts News Watch 21st Annual Excellence in Design Winning Designs Combine State-of-the-art Technology with . . . Meet the Judges Design Marts Fans & Blowers Mother Nature Has Her Own Ideas About Designing for Fluid Flow . . . Using CFD Simulation Software Early in the Product Development . . . Electronics Digital Signal Controllers Enable Digital Motor Control . . . Classifieds Meetings Advertiser’s Index Association Report: AHRI

Appliance Design - June 2008

For optimal viewing of this digital publication, please enable JavaScript and then refresh the page. If you would like to try to load the digital publication without using Flash Player detection, please click here.