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Fig. 4: Angle error using finer sample resolution with (left) segmented linearization and (right) harmonic linearization. Both of these techniques can be readily implemented using Allegro-supplied software to calculate coefficients and program on-chip EEPROM: • Segmented linearization is a programmable feature that allows adjustment of the transfer characteristic of the angle sensor IC such that linear changes in the applied magnetic field vector angle can be output as corresponding linear angle increments by the angle sensor IC. It is performed on the data collected from one rotation of the magnet around the angle sensor IC. • Harmonic linearization applies linearization in the form of 15 correction harmonics whose phase and amplitude are determined by means of an FFT (Fast Fourier Transform) performed on the data collected from one rotation of the magnet around the angle sensor IC. Comparative tests on both techniques show that, although segmented linearization achieves shorter processing times, it is limited in its ability to correct for sinusoidal error terms. In this regard, the harmonic linearization technique performs better. In addition, the flexibility of the harmonic linearization approach - particularly the ability to change the number of correction harmonics used - allows the user to achieve the optimal trade-off between computation time and error performance. Tests have shown that angle errors of ±20° can be reduced to within ±0.3°degrees with linearization applied. Segmented linearization Figure 2 illustrates the angle output of the angle sensor both with and without segmented linearization. In order to obtain these results, an initial set of linearization coefficients has to be created. The user takes 15 samples of angle at every 1/16 interval of the full rotational range from 0 to 360̊. A segmented linearization algorithm is then applied. Figure 3 shows the angle error obtained by subtracting the reference encoder value, both with and without segmented linearization applied, while figure 4 is a “zoomed in” look at the angle error profile with segmented linearization applied. The segmented linearization implemented in the A1332 only allows for 16-segment linearization. The performance of this method could conceivably be improved by either increasing the number of segments or by making the segment length variable, so that finer segments can be used for areas with higher curvature. How¬ever, both these enhancements result in higher processing time and complexity. Harmonic linearization Harmonic linearization applies the linearization in the form of 15 harmonics whose phase and amplitude are determined by means of an FFT (Fast Fourier Transform) performed on the data collected from one rotation of the magnet around the angle sensor IC at the customer’s end-of-line facility. There is a great deal of flexibility built into the harmonic linearization function. The value of the individual harmonic amplitudes and phases are stored in 12-bit EEPROM fields for each of 15 harmonics, and the number of harmonics that need to be applied in a linearization can be specified by the user. In addition to enabling side-shaft applications, the flexibility built into this linearization method is also very useful in removing static misalignment errors at the customer’s end-of-line. Angle latency considerations Both segmented and harmonic linearization techniques are well suited for on-axis and off-axis magnetic applications. While segmented linearization divides the magnetic range into smaller sections which are linearized in a piece-wise fashion, harmonic lin- FLEXIBLE IN MEETING YOUR NEEDS Faster to a custom solution with modular sensor systems Whether for speed, position, or angle sensing – with our modular sensor systems, custom solutions for industrial applications can be implemented quickly and at low cost. Use this flexibility. www.cherryswitches.com info@cherryswitches.com Phone +49 9643 18-0 www.electronics-eetimes.com Electronic Engineering Times Europe June 2015 39


EETE JUN 2015
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