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Embedded Systems December 2000 Vol13_13

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SPECTRA . in more recent designs in which the con- trollers are literally placed with the dl-ives right on the motors they control. In high power applications, a gTeat deal of cur- rent flows, and because the applications FIGURE :2 Sine and cosine overlaid -----, w· .. ·~ , ~. ,/; • 0.5 - .... .B o 0; • ! -0.5 ! ! .: . .. . - . are ovenvhelmingly PWM-oriented, there is also a great deal of noise. Each time the integrated gate bipolar transis- tors (IGSTs) switch, large amounts of current flow or cease to flow, and the - .- . . . _ eleclJical tate wiUlin the drive fluctu- ates; large current pikes result. Switch ing high voltage in close prox- imity to other (albeit unrelated) circuit- ry in Ul is manner can cause "sympathet- ic" ClUTents to flow in those circuits (Ampere's law) and trash your analog '. .. .. "S9_. .- ' . . ' .. '. '. . . . o 1t/4 2 3 o to 21t FIGURE J Basic resolver construction .- Secondary (sine) - - - - - - - - - - - - - -__ . 4 ~ -- . . . 5 . o ' • j II II il 6 - . Primary - - - - -- --- / ): .1,:-' - / ,-- / , ./ ,. - / feedback. Even differential signals can have difficulty in environments like this. This would be a difficulty whether you chose to do the conversions yourself or used an off-chip solution. With a DSP, however, it is possible to filter the input signals before using them in calculations and th e reby reduce the amount of error caused by these noise transients. And you can tai- lor that fi lter mechanism to meet your needs regarding PWM frequency, motor velocities, and other ulings that influence your corner frequencies . Of course, you must be mindful of the phase distortion that can result when Ule data rales from your feedback device approach the turn-over fi-equency (- 3dB)ofyour fi lter. You can ovel-come this by putting Ule appropliate all-pass filter in line with the plimary filter. These are useful techniques and definitely workable. In fact, a number of products on the market implement this Ulem. Of course, there might be a better way. Next month , we wi ll discuss an enhanced approach to th is technolo- gy, one that wil l incorporate the nec- essary fi ltering as an integral part of the transfe r function ; it is also easy to implement and requires less horse- powel- from your DSP. esp ring multi-Tate signal processing and wavelets, called Numerical Methods for DSP System in C. He is also the autlwr of Practical D P Modeling, Techniques, and Programming in C, published byJohn Wiley & Sons, and Numerical Methods for Em bedded Systems from M &T. Don s I!-'rnail addTess is dgm@bay"ilty.com. Don Margan is senior engineer at Ultra Stereo Labs and a consultant with 25 years of D.-1)(f- Tience in signal processing, embedd£d systems, haTliware, and sojl.waTe. Morgan mcently com- pleted a boo" about n:urnerical methods, f ea- tU' 168 DECEMBER 2000 Embedded Systems Programming

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