Taking The Errors Out of Your Encoders

By | June 29, 2015

For all their advantages in high-resolution motion control systems, sinusoidal encoders do produce measurement errors that you should take into account. These errors result from encoder signal imperfections and include gain mismatch, phase mismatch and excessive DC offsets. Left unchecked, they can ultimately reduce your system’s positioning fidelity.

Fortunately, we’ve just developed a brand new way to fix these errors on the fly using the interpolation software and circuity that processes the encoder signals. Called the auto-correcting interpolator, this new technology continuously identifies encoder errors and automatically corrects the calculated interpolated positions based on the most recently identified error characteristics.

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Over the years, there have been many attempts to fix encoder errors during the interpolation process. These attempts have had limited success because of the constantly changing, unpredictable nature of encoder errors. And in the past, analog-to-digital converters (ADC) simply didn’t offer the combination of speed and resolution required for reliable error correction—particularly in high-frequency applications.

The auto-correcting interpolator, by contrast, uses new algorithms made possible by the growing availability of fast ADCs. How fast? The auto-correcting interpolator fully processes encoder signals at 20 MHz or about 400 times faster than our previous, already-fast interpolator.

Here’s a look at what this new technology can deliver:

Dramatically Reduced Position Errors. The automatic correction functionality typically reduces the position errors from encoder signal imperfections by more than 95%.

The Highest Positioning Resolution. Dividing each line of encoder output into more than 65,000 states, the auto-correcting interpolator drastically improves the resolution you can achieve with a given sinusoidal encoder. In our lab, we’ve used the new interpolation technology to make positioning moves within 32.5 picometers, or slightly more than the radius of a helium atom. This extreme resolution has implications for all kinds of inspection systems for optics, semiconductors and other precision manufactured goods.

The Smoothest Velocity and Acceleration Signals. Of course, not every application needs all that resolution. But keep in mind that the high resolution produced by the new interpolators can be valuable even in applications that do not require very high position accuracy. The high position resolution reduces quantization noise in the feedback, especially in the estimated velocity and acceleration, permitting higher servo gains and better performance. By using the auto-correcting interpolator, you not only get better position control, but velocity and acceleration signals become cleaner and more reliable too.

We’re just scratching the surface here. The auto-correcting interpolator has other technical advantages too. You can read about them in our latest white paper, which includes a detailed description of how the interpolator works and how it stacks up to earlier technologies.

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