Rex Niven
EDN
RFID is great technology, but noisy power and other aggressors can ruin performance. RFID readers using low frequency signals (ca. 130 kHz), like the industry-workhorse TIRIS RFM-007B, are of course very sensitive to noise in that frequency range. Given switched-mode power supplies often generate grunge in this range, a heavy and expensive linear supply is often needed to get maximum sensitivity.
However, the operational cycle of the RFID reader has several phases, only one of which is noise sensitive. They are: high power (transmit, 50 ms at 10 W), low power (receive, 18 ms), and idle phases (up to 33 ms depending on firmware). Noise is an issue only during the short receive phase.
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| Figure 1. | RFID with selectively disable PSU. | |
This Design Idea shuts down the switching power supply during receive and idle phases, letting the module “coast” using the stored energy in C2 (Figures 1, 2).
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| Figure 2. | Typical PSU cycle timing. | |
Buck converters like the LM2576 have a shutdown input making this operation simple to achieve.
The RFID module’s transmit control line from the MCU – /TxCT – also controls the shutdown line via U3 & Q1, or, a separate MCU output can be employed if desired.
RFID is often used on forklift trucks for location sensing, allowing automated logistics and safety features like speed limitation.
The environment in an electric forklift is extremely noisy, and difficult for RFID due to high-amperage, high-frequency modulated currents in the motor drives. If the control technique shown here were extended to synchronising the RFID cycle with a modified motor-drive cycle, loss of sensitivity due to motor-drive interference could be slashed.
