Published June 30, 2014

How to use RFID as the Source for Autonomous Sensors

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RFID battery-free sensors are a very good alternative to natural source energy harvesting solutions for those applications in need of a reliable wireless energy source.

While batteries can be a suitable solution for a wireless sensor network (WSN) located in easily accessible spots, experience says power supply is the biggest issue when talking about WSN. In the long run, battery maintenance is not cost effective for millions of devices.

Energy harvesting from natural sources is the optimum solution. However, the efficiency of devices commercially available is still a bit limited. On top of that, natural energy sources are generally not reliable (i.e.: photovoltaics depend on a sunny day).

As an alternative solution, RFID battery free sensors are dependent on a reliable power source: an RFID reader. These devices can be fixed or handheld (the size of a PDA) and can power up and communicate with battery free sensors located in their read range. Generally limited to around 2 meters (Personal Area Network or PAN) if using UHF technology and some centimeters when using HF technology, the information collected by the reader can then be shared via Wi-Fi, Ethernet, Bluetooth or similar means.

RFID sensor tags can be placed in hardly accessible locations to forget about wiring and battery changes forever. As examples of use cases to easily understand the system we have:

• Structural Health Monitoring in construction. Embedding RFID strain gages in beams of a bridge can give you information about its status. Maintenance staff will periodically visit the site with a handheld reader and check the sensor that’s embedded in the concrete.
• Rotor contact temperature monitoring in electrical engines. Placing a thermistor or thermocouple in contact with the rotor surface to monitor the temperature of the rotor material. The RFID sensor will continuously send information to a nearby reader and the engine will never have to be stopped in order to change batteries.

RFID sensor tags harvest the energy from the RF field created by the UHF RFID reader. Figure 1 shows the typical architecture of a battery free RFID sensor tag.

Fig. 1. Architecture of a RFID tag with external sensor.

The antenna receives the signal emitted by the reader. In order to achieve the maximum power transference from the antenna to the voltage multiplier, a matching network is required. Typically this matching network is implemented together with the antenna. The voltage multiplier rectifies the incoming signal charging the supply capacitor CSUPPLY. This capacitor is used to supply power to the rest of the tag. The analog front-end provides the signals that the rest of the tag requires to work properly, such as regulated voltages, clock and reset signals. It is also in charge of demodulating the incoming ASK signal and modulating the tag answer. The digital core communicates with the EEPROM and, when present, the external sensors or devices. It also realizes the required actions to answer the reader queries using the EPC C1G2 standard.

Power Management in RFID sensor tags

The sensors embedded in RFID tags require a lot more power than standard RFID identification tags need. For this reason, a proper power management must be implemented at the complete system level:

1. Reader output power optimization. Not only the power level is important for passive sensor solutions (generally limited by country regulation) but also the time during which the RFID reader is actually transmitting this power should be taken into account.
Reader SW tests must be run to understand what the reader capabilities are and how they fit with the specific application.

See Figure 2 and Figure 3 to compare the effectively transmitted power in comparison with the output power level by using different read modes from the reader software.

Fig. 2. Synchronous read

Fig. 3. Asynchronous read

Figure 2 shows the result obtained when doing a synchronous read in a loop. This setup has a duty cycle of around 50%. This result cannot be considered a good result since half the time the reader is not transmitting.

The same reader running in an asynchronous mode obtains a result close to 99% (Figure 3).

In both read modes the output power is 2W ERP (ETSI regulation).

Being aware of the amount of energy being transferred with every command sent from the reader is key to optimize and control system performance.

2. Tag power management. Once the energy is harvested a proper use of this energy must be made. It can get quite complex when designing RFID sensor tags but keeping it simple it is all about a balance between sensor data rate (samples per second required) and idle time.

Consider the appropriate activity in your system. Current consumption always increases when the communication port (SPI, I2C…) is active. This consumption cannot be eliminated but it can be reduced by reducing the data acquisition. So, by minimizing the active state of the communication port average current consumption is reduced.

Beware of peak current consumptions. While current consumption data for normal modes or on-states are more or less defined in the data sheets, it’s unusual to find peak current consumption information of the switch on phase when turning on these devices. And this is remarkable, because in most low power devices, the highest current consumption appears during the transient states in which the different blocks of the devices are activated.

In general introducing a start-up circuit with a capacitor is recommended.

Figure 4 shows the schematic of an R-Meter from Farsens, a battery-free RFID tag that measures the resistance of the device that is connected to it. This RFID tag is intended to be used with resistance dependent sensors such as Light Dependent Resistors (LDRs), strain gages, thermistors and so on.

Fig. 4. Schematic of an R-Meter battery-free RFID tag. (click here to zoom)

The voltage monitor and the transistor isolate the circuitry so that it does not start consuming before a certain amount of energy is stored in the capacitor.

Again, the size of the capacitor must be balanced with the time it will take to charge it.

The communication protocol for UHF RFID sensor tags should be based on the EPC C1G2, which is the standard for this frequency band. Even though the standard was not thought for sensors embedded in identification tags, sensor systems can be implemented by using standard commands in any commercial reader.

Using custom commands is considered a limiting factor since it reduces compatibility with existing readers and RFID infrastructure.