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Filtering of Hall-Sensor Signals for Improved Operation of Brushless DC Motors This paper appears in: Energy Conversion, IEEE Transactions on Date of Publication: June 2012 Author (s): Alaeinovin, P. Volume: 27 , Issue: 2 Page(s): 547-549 Product Type: Journals & Magazines 研究生：劉適存 學號：MA120209 Department of Electrical Engineering Southern Taiwan University of Science and Technology Abstract Brushless dc motors controlled by Hall-effect sensors are used in variety of applications, wherein the Hall sensors should be placed 120 electrical degrees apart. This is difficult to achieve in practice especially in low-precision motors, which leads to unsymmetrical operation of the inverter/motor phases. To mitigate this phenomenon, an approach of filtering the Hall-sensor signals has been recently proposed. This letter extends the previous work and presents a very efficient digital implementation of such filters that can be easily included into various brushless dc motor-drive systems for restoring their operation in steady state and transients 2 Introduction The Hall-sensor-controlled BLDC motors are preferred in many applications due to their relative simplicity, low cost, and reliable operation in wide range of loading conditions and speeds. The Hall sensors must be placed exactly 120 electrical degrees apart. However, in many low-precision motors, the sensor positioning could be quite inaccurate. 3 Introduction 4 EFFICIENT ALGORITHM AND ITS IMPLEMENTATION Fig. 1 The filtering block shown in Fig. 1 accepts the original Hallsensor signals (input), and provides the switching circuitry with the modified signals (output). The purpose of filtering is to remove harmonics from the sequence τ (n). This can be achieved using averaging filters that have the following form: 5 EFFICIENT ALGORITHM AND ITS IMPLEMENTATION Several such filters have been proposed in Bibe including three-step averaging, six-step averaging, linear extrapolating-plus-averaging, and quadratic extrapolating-plusaveraging, which have the following form: 6 EFFICIENT ALGORITHM AND ITS IMPLEMENTATION A simple and effective algorithm that assumes a microcontroller implementation is described here. Any change in the original Hall signals H1, H2, and H3 triggers the input interrupt service routine (ISR) at time tin (n) as depicted in Fig. 2. The output ISR has to be invoked at a particular time tout(n) to provide the inverter with the modified (corrected) signals H1’, H2’, and H3’, which are used to switch the transistors. Denoting the most-recent calling of the input ISR by tin (n), the time of the next output ISR can be expressed as where τcorr(n) is the appropriate correction term. Fig. 2 7 EFFICIENT ALGORITHM AND ITS IMPLEMENTATION To understand how the correction time τcorr(n) can be obtained, it is instructive to consider a reference time that can be obtained by averaging the three consecutive ISR times as Then, according to Fig. 2 and (6), we have Since Tin(n−1) and Tin(n−2) in (6) refer to the previous consecutive input ISR times, they can be expressed as Combining (7)–(10), after some algebraic manipulations, the correction term can be expressed as 8 EFFICIENT ALGORITHM AND ITS IMPLEMENTATION Thereafter, it becomes possible to determineτcorr(n) for any filter (2)–(5), by substituting the relevant expression for into (11). In particular, using the same notation as in [5], the final correction terms for the four filters are as follows: 9 EFFICIENT ALGORITHM AND ITS IMPLEMENTATION Hence, as can be seen in Fig. 2, the time intervals τ (n) are readily available simply as the timer counts between the transitions of the original Hall sensors, H1, H2, and H3. Therefore, the final implementation according to (6) requires a very simple scheduling of the output ISR by the offset correction time τ corr(n) [one of (12)–(15)], which determines the timings for the corrected Hall signals H1’, H2’, and H3’. As depicted in Fig. 2, the resulting (modified) output sequence is evenly spaced by ¯τ (n), which achieves the desired goal. Fig. 2 10 CONCLUSION This letter describes a very efficient and straightforward approach for implementing the averaging of the Hall-sensor signals for improving the operation of low-precision BLDC motor drive systems. Using this approach, any of the filters can be readily implemented either in the code of existing motor controllers, or as a dongle in a low-power standalone programmable integrated circuit. 11 Thank you for your attention.