Accurate Persistent Nodes in Passive RFID Tags
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Solution Overview
Problem
RFID tags, especially passive ones, face challenges in accurately maintaining persistent states for extended periods due to their intermittent power supply, leading to inaccurate inventory control and inefficient counting processes, particularly in dock-door scenarios where power interruptions and tag movement cause issues with state persistence.
Innovation Solution
A combination of a power capture circuit and an Ultra-Low-Power counter circuit is used to create a 'one-shot' event timer with a long-time constant, independent of power supply variations, allowing accurate control of persistent nodes for durations up to several seconds, ensuring reliable state persistence and reducing the need for repeated counting.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If an analog capacitor-based persistent node is used to maintain tag state, then the tag can remember its state through power interruptions, but the timing becomes very inaccurate taking up to minutes to revert
Solution Approach 1:
The patent replaces the mechanical/electrical analog capacitor-based timing system with a digital counter system. The digital counter uses discrete clock cycles to measure time intervals, providing precise timing control. The counter is driven by a reference clock and can accurately count for extended periods (e.g., 500 milliseconds to several seconds) without the drift and inaccuracy inherent in analog capacitor discharge methods.
2Productivity
If the reader instructs all tags to wake up for counting, then all tags can be counted, but tags that enter the field later won't be detected and the process is time-consuming
Solution Approach 1:
The patent implements periodic wake-up commands where the reader sends wake-up signals at regular intervals rather than a single bulk wake-up. This allows the reader to detect tags as they enter the field during subsequent periodic cycles. The persistent node timing ensures that tags remain in a wake state long enough to be detected in the periodic scanning process, improving both counting speed and completeness.
3Reliability
If the persistent node timing is extended to several seconds for accurate inventory control, then state persistence is improved, but power consumption increases for passive tags
Solution Approach 1:
The patent makes the persistent node circuit self-sufficient by capturing and storing energy from the RF reader signal itself. The energy capture circuitry harvests power during the brief moments when the tag is illuminated by the reader, storing it in a capacitor or rechargeable element. This stored energy then powers the digital counter and maintains the persistent state for several seconds without requiring continuous external power, thus extending timing accuracy while maintaining low power consumption for passive tags.
Data Source
AI summary
A timing circuit that can function as an accurate persistent node in an RFID tag includes a power capture circuit for capturing power from a power source, and a counter circuit that provides a count representing a progression of time. The count can then be compared to a reference value representing a time constant of the circuit.