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M2M COMUNICATIO NS UHF RFID tags ready up for analogue sensing and data sharing By Mikel Choperena RF identification (RFID) has already reached its maturity in identification solutions. Developed as an alternative for the barcodes with competitive advantages such as no-lineof sight reading allowing the simultaneous identification of multiple items while offering memory read and write capabilities, UHF RFID systems have been widely deployed in supply chain and logistics applications. The innovation in this industry comes in the way of value added services. From the software development side, new creative solutions are being implemented. As examples, golf entertainment complexes are using RFID to compute players’ scores based on which zone the balls land in. Fig. 1: Architecture of a RFID tag with four voltage From the hardware point of view, the innovations come meters. in the form of integrating more components while maintaining the battery-free nature of the tag. Sensors are being integrated into UHF passive RFID tags to share environment data along with the unique ID of the tag. Digital temperature sensors have historically been the first approach since these can be integrated into the RFID IC. They share the same manufacturing processes (CMOS) so it was a no-brainer to develop those first. External, commercial components are now the target. Battery-free sensor tags with external digital sensors have been around for some time now. For example, Farsens makes available temperature, pressure, humidity and 3-axis accelerometers from its website. The latest innovations come in the form of analogue sensors and actuators attached to a passive UHF RFID sensor tag. Latest developments include wireless and battery-free ambient light sensors, thermistors, soil moisture sensors, bi-static relays or LEDs. All these alternatives are possible because of the concept of resistance meters and voltage meters. Any resistance dependent sensor or voltage dependent sensor can be monitored with these. With all the above in mind, Farsens has developed a battery-free UHF RFID tag with a four channel voltage monitor. Both the tag and the voltage monitors run without batteries by using the energy harvested by the tag antenna, which is then converted by the RFID IC. Figure 1 shows the architecture of the QVMeter battery-free RFID sensor tag. The QVMETER-LV10 tag consists of an ANDY100 IC for energy harvesting and wireless communication, a start-up circuitry based on a voltage monitor, a microcontroller with integrated ADC (10 bits) and a signal conditioning circuitry for measuring low voltage values. The conditioning circuitry also includes all the necessary components Fig. 2: Weight error versus a reference weight with a specific QVMeter tag and load cells configuration. to read four independent wheatstone bridges. The communication with the external microcontroller is done via memory mapping. This method ensures the tags are fully EPC C1G2 compatible and avoids any compatibility issues. User memory banks (defined in the standard) are redirected to the ADC voltage values. This way, the reader only needs to request the value contained in a specific memory location. If the requested address contains the direction assigned to the ADC outputs, the answer of the tag will contain the value of the specific voltage. The main advantage of this solution is that any C1G2 network does support it, and as the EPC code is not altered, none of the advanced features of the EPC networks are damaged. Since the available energy is very limited, the focus of the product is the low power consumption of all external electronics. Specifically, the tag architecture addresses this requirement by including a startup circuit that isolates the external circuitry whenever the energy is too low (so power consumption is completely off). It implements a microcontroller firmware with a very low duty cycle, most of the time in deep sleep mode to reduce power consumption and only activated when the ADC measurements are needed. The device also limits power consumption of the signal conditioning circuit. Since there are four voltage monitors, the design allows for a complete shut off of the lines that are not being used at any given time. The signal conditioning is composed of an instrumentation amplifier and a multiplexer. The amplifier can be configured to have any gain between 1 and 1000 by setting the appropriate value of a resistor. The multiplexer is controlled by GPIOs and routes the sensor supply voltage and the differential input of the amplifier to one of the four channels. The signal conditioning circuitry can be enabled or disabled with a NMOS switch in order to reduce power consumption when no measurements are being made. The ANDY100 UHF RFID IC has an SPI master module. This module generates SPI signalling towards the microcontroller when a proper command is issued by the UHF RFID reader. Given that the RFID communication protocol specifies timing restrictions for answer, the microcontroller returns the measurement value stored in a buffer and triggers a new measurement. Thus, the answer of the tag to the reader includes the value of Mikel Choperena is Product Development Manager at Farsens - www.farsens.com 26 Electronic Engineering Times Europe November 2014 www.electronics-eetimes.com


EETE NOV 2014
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