Stepping Motor Control Switching Open-Loop Feedback for Autofocus
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Solution Overview
Problem
It is challenging to effectively feedback-control a stepping motor across the entire velocity range from low to high velocities while maintaining positioning accuracy and reducing driving vibrations and noises, especially in image pickup apparatuses where motor characteristics differ based on capture modes like still image or motion image capture, and focusing types such as autofocusing and manual focusing.
Innovation Solution
A control unit for a stepping motor that includes an encoder to detect rotational position and a controller that switches between open-loop and feedback control based on capture modes, using open-loop control during drive start and stop periods for autofocusing in still image capture and open-loop control for motion image capture, ensuring accurate and stable motor operation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If feedback control is used across the entire velocity range, then positioning accuracy is improved, but driving vibrations and noises increase
Solution Approach 1:
The velocity range is divided into multiple segments: low velocity range and high velocity range. The controller selectively applies feedback control only in the low velocity range where positioning accuracy is critical, while using open-loop control in the high velocity range to avoid vibrations and noises. This segmentation resolves the contradiction by applying control strategies locally optimized for each velocity segment.
Solution Approach 2:
The control strategy dynamically switches between open-loop and feedback control based on the current operating velocity. The controller monitors the motor velocity and transitions between control modes appropriately, making the system adaptive to different operating conditions. This dynamic adjustment allows the system to maintain positioning accuracy when needed while avoiding vibrations during high-velocity operation.
2Measurement precision
If feedback control is applied, then positioning accuracy is maintained, but device complexity increases
Solution Approach 1:
The control system is segmented into two distinct modes: open-loop control and feedback control. By dividing the control approach, the system only implements the complex feedback control mechanism where it is absolutely necessary (low velocity positioning), while using simpler open-loop control elsewhere. This reduces overall system complexity compared to implementing feedback control across the entire operating range.
Solution Approach 2:
Feedback control with encoder is applied locally only in the low velocity range where positioning accuracy is most critical. In the high velocity range, the system uses simpler open-loop control. This localized application of feedback control maintains positioning accuracy where needed while minimizing the complexity burden across the entire control system.
3Reliability
If open-loop control is used at high velocity, then stepping out is prevented, but positioning accuracy deteriorates
Solution Approach 1:
The velocity range is segmented into low velocity (where feedback control ensures positioning accuracy) and high velocity (where open-loop control prevents stepping out). This segmentation allows each control mode to operate in its optimal performance zone, resolving the contradiction between reliability and positioning accuracy.
Solution Approach 2:
The controller dynamically switches between feedback control and open-loop control based on velocity conditions. At high velocities, open-loop control is used to maintain reliability and prevent stepping out. At low velocities, feedback control is engaged to ensure positioning accuracy. This dynamic switching optimizes both reliability and positioning accuracy across different operating conditions.
4Measurement precision
If feedback control is used in still image capture, then positioning accuracy is improved, but response time increases
Solution Approach 1:
The control approach is segmented based on capture mode: feedback control is applied during the positioning phase in still image capture to ensure accuracy, while open-loop control is used during high-velocity movement phases. This segmentation optimizes both positioning accuracy and response time by using the appropriate control mode for each phase of operation.
Solution Approach 2:
The control system employs periodic switching between open-loop and feedback control modes during still image capture. Open-loop control is used for rapid positioning movements, and feedback control is periodically applied when precision positioning is required. This periodic action maintains positioning accuracy while minimizing overall response time.
Data Source
AI summary
A control unit for a stepping motor includes an encoder and a controller. The controller controls the stepping motor based on information sent from an image pickup apparatus configured to capture an object via an image pickup optical system that includes the focus lens. in a case that the image pickup apparatus performs autofocusing in capturing a still image, the controller open-loop-controls the stepping motor in at least one of a drive start period and a drive stop period of the stepping motor, and feedback-controls the stepping motor using an output of the encoder between the drive start period and the drive stop period of the stepping motor. The controller open-loop-controls the stepping motor in a case that the image pickup apparatus performs the autofocusing in capturing a motion image.