Three-Phase Optical Encoder Angle Interpolation With Lower Harmonic Error
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Solution Overview
Problem
Existing angle-calculation methods for optical encoders in AC servomotors suffer from limited resolution and robustness due to harmonic components in sinusoidal signals, leading to periodic errors of ±2.5% and inability to achieve high resolution.
Innovation Solution
An angle-calculation apparatus for three-phase optical encoders uses three-phase sinusoidal signals with 120-degree phase difference, employing an A/D converter, digital signal processor, and phase digitalizer to perform inverse trigonometric calculations within a relatively linear region, minimizing the impact of non-ideal sinusoidal signals and reducing interpolation errors.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional two-phase sinusoidal signals are used for angle calculation, then the system structure is simple, but the resolution is limited and periodic errors of ±2.5% occur due to harmonic components
Solution Approach 1:
The patent transitions from two-phase sinusoidal signals to three-phase sinusoidal signals with 120-degree phase difference. This dimensional change from 2D to 3D signal space enables more accurate angle calculation by providing additional independent information, reducing periodic errors from ±2.5% to ±0.6%, and allowing the use of relatively linear regions in the signal characteristics for interpolation.
2Measurement precision
If full-amplitude sinusoidal signals are used for interpolation, then the signal range is maximized, but the presence of harmonic components degrades angle calculation accuracy
Solution Approach 1:
The patent identifies and utilizes the relatively linear regions within the three-phase sinusoidal signals for interpolation operations. By locally selecting the linear portions of the waveforms rather than using the full amplitude range, the method achieves high interpolation accuracy while being insensitive to harmonic components, thereby improving both precision and robustness simultaneously.
Solution Approach 2:
The patent converts the presence of harmonic components, which are typically harmful, into a beneficial situation. By using three-phase signals and operating in relatively linear regions, the system becomes inherently more robust to harmonics, reducing periodic errors to ±0.6%. The harmonic content that would normally degrade two-phase systems actually provides additional information that reinforces the three-phase angle calculation accuracy.
3Measurement precision
If higher resolution is achieved by increasing mark number on optical encoder wheel, then the resolution improves, but manufacturing difficulty and diffraction phenomenon increase
Solution Approach 1:
The patent replaces the mechanical approach of increasing physical mark density on the encoder wheel with an electronic signal processing approach. By using three-phase sinusoidal signals and performing interpolation in the digital domain within relatively linear regions, the system achieves high resolution without requiring extremely fine mechanical manufacturing, thereby avoiding diffraction effects and manufacturing complexity.
Data Source
AI summary
An angle-calculation apparatus for three-phase optical encoder receives three-phase sinusoidal signals 120 degree phase from the optical encoder and obtains angle information for a motor rotor. The angle-calculation apparatus includes an A/D converter, a digital signal processor (DSP), a phase digitalizer and a digital counter. The A/D converter converts three-phase analog signals of the optical encoder into three-phase digital signals. The phase digitalizer converts the three-phase analog signals into digital phase signals. The digital counter generates a counting value based on the digital phase signals. The DSP performs an inverse trigonometric function calculation on a relatively linear region of the three-phase digital signals to obtain the angle information. The DSP obtains the rotation turn number and rotation direction of the motor rotor according to the counting value.