Shadow-Cast Encoder Apparatus Minimizing Harmonic Distortion
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Solution Overview
Problem
Incremental encoders face sub-divisional error due to imperfections in signal interpolation, particularly caused by undesirable frequencies in the fringe pattern, which affect the accuracy of position measurement.
Innovation Solution
The encoder apparatus configures the electromagnetic radiation source to minimize total harmonic distortion (THD) of the fringe pattern, ensuring it is no more than 6%, by appropriately spacing and shaping the radiation sources to cancel out specific harmonics, thereby improving the signal quality and reducing interpolation error.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional electromagnetic radiation sources (square or circular shapes) are used to illuminate the scale, then the encoder apparatus can operate, but the fringe pattern contains undesirable frequencies (harmonics) that cause sub-divisional error and reduce measurement accuracy
Solution Approach 1:
The electromagnetic radiation source is designed with non-uniform radiant power distribution along its extent parallel to the measurement direction. Specifically, the radiant power varies sinusoidally or is concentrated at specific locations (e.g., third-points for third harmonic cancellation, or optimally distributed for fifth harmonic cancellation) to produce a fringe pattern with minimized total harmonic distortion, thereby reducing sub-divisional error and improving position measurement accuracy
Solution Approach 2:
The patent changes the radiant power parameter of the electromagnetic radiation source along its extent, transitioning from uniform distribution (conventional sources) to non-uniform distribution (sinusoidal variation or concentrated at specific points). This parameter change modifies the fringe pattern's harmonic content, reducing total harmonic distortion to no more than 6% and eliminating specific harmonics (third or fifth), thus improving measurement precision
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This configuration significantly reduces sub-divisional error, enhancing the accuracy of position measurement by minimizing harmonic distortion and improving the circularity of the Lissajous pattern, leading to more precise relative position determination between the scale and readhead.
Implementation Method 1
incremental encoders can work by utilising the diffraction of light to produce at a detector a resultant field which changes with relative motion of the scale and readhead. For instance, light can be diffracted by the scale and a diffraction grating in the readhead so as to form an interference fringe at the detector
Implementation Method 2
light can be diffracted by the scale and a diffraction grating in the readhead so as to form an interference fringe at the detector
Implementation Method 3
incremental encoders to operate by the scale features selectively preventing (e.g. blocking) light from reaching the detector such that a non-imaged representation (e.g. shadow) of the scale's features is cast on the incremental detector
Data Source
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Figure 3~4
Figure 5(a)~5(c)
AI summary
A shadow-cast encoder apparatus comprising a scale and a readhead. The readhead comprises at least one electromagnetic radiation source for illuminating the scale in order to produce a shadow-cast fringe pattern at a detector configured to detect the shadow-cast fringe pattern. The encoder apparatus can be configured so as to suppress the total harmonic distortion of the fringe pattern, e.g such that the total harmonic distortion of the fringe pattern is not more than 6%.