Smartphone Motion Sensor Synchronization Calibration
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Solution Overview
Problem
Recent smartphone security enhancements introduce significant synchronization errors between smartphone and motion sensor time bases, resulting in an unacceptable uncertainty of over 400 ms in calculated run time, making it difficult for athletes to accurately time their runs without a second person to clock the event.
Innovation Solution
A one-time calibration routine is used to characterize the relative time drift between the smartphone's and motion sensor's time bases, allowing for the removal of synchronization errors, ensuring accurate run time calculations with a residual error of less than ±8 ms, which is within an acceptable limit.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If smartphone security enhancements are implemented, then security is improved, but synchronization error increases to over 400 ms
Solution Approach 1:
The patent performs a one-time calibration routine at the factory to characterize the relative time drift between the smartphone's time base and the motion sensor's time base. This preliminary characterization allows the system to compensate for synchronization errors in future measurements without requiring continuous user intervention or real-time calibration during actual use.
2Measurement precision
If a one-time calibration routine is used to characterize time drift, then synchronization error is reduced to within ±8 ms, but device complexity increases
Solution Approach 1:
The calibration routine is executed once at the factory during manufacturing, before the device is delivered to the user. This preliminary action characterizes the time drift between the smartphone and motion sensor time bases, allowing the system to compensate for synchronization errors in future measurements without requiring continuous user intervention or real-time calibration during actual use.
Solution Approach 2:
The system automatically uses the characterized time drift data from the one-time calibration to compensate for synchronization errors in subsequent run time calculations. The calibration data is stored and applied automatically by the system without requiring repeated user intervention, making the complex calibration process transparent to the end user.
3Measurement precision
If factory calibration is performed to eliminate time drift, then measurement accuracy is improved, but manufacturing complexity increases
Solution Approach 1:
The calibration routine is executed once at the factory during manufacturing, before the device is delivered to the user. This preliminary action characterizes the time drift between the smartphone and motion sensor time bases, allowing the system to compensate for synchronization errors in future measurements without requiring continuous user intervention or real-time calibration during actual use.
Data Source
AI summary
An athlete (1) measures sprint time by locating a smartphone (3) having a camera and crystal oscillator clock which is first activated at the finish line. The sprint end time is recorded by a photo stamp time app activated by a video trigger causing the smartphone (3) to send a RF stop event signal to the athlete's wrist mounted motion sensor (2). Before this a sensor timer or clock is started via the sprinter's start event. The sprinter's start activates the sensor's clock and saves the captured start time including time drift error. Upon the phone app selecting the run time function, a sync command sent to the sensor (2) by the app zeros out the phone and sensor timers. A one-time crystal calibration routine correcting for drift errors caused by the smartphone's operating system is activated which provides the sprint with a corrected start time.