RF-Powered Wireless Sensors

A joint article by Powercast (Harry Ostaffe, Charlie Greene) and AVX (Bharat Rawal), titled “Power Module and Double Layer Capacitor Harvest Energy From Radio Signals“  was recently published in Power Electronics Technology.  Combining Powercast’s P2110 Powerharvester receiver with an AVX Double-Layer Capacitor (super-capacitor) results in a battery-free, wireless power supply that can be used for low power applications such as wireless sensors.  The article describes in detail the operation of the P2110 Powerharvester.

Figure 1 shows a general block diagram of the P2110 Powerharvester.  The RF energy is converted to DC and stored into an external capacitor.  For some applications a small electrolytic capacitor may be sufficient, but other cases will require more energy and therefore a larger double-layer capacitor (super-capacitor).  The voltage on the external capacitor is typically managed between the operating range of 1.25V, when the output voltage is turned on, and turned off at the low threshold of 1.05V.  The boost converted is used to provide a regulated output voltage from a range of 2-5.25V to accommodate a wide range of applications.

The timing diagram in Figure 2 provides more detail on the operation.  The INT pin provides a logic-level output to indicate when Vout is active, and the RESET pin enables an external device such as a microtontroller or external timer to turn off Vout.  The RESET function saves energy and allows the P2110 to recharge the capacitor more quickly.

Figure 3 show a system-level implementation that is typical in a wireless sensor, such as HVAC control sensors using ZigBee, WiFi, or other protocols, or industrial sensors using ISA100 or WirelessHART.

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Sensors Magazine recently published the article “RF Energy Harvesting Enables Wireless Sensor Networks” by Harry Ostaffe of Powercast.  The article is a brief introduction to RF energy harvesting: what it is, what it does, and how it enables wireless sensor networking applications.

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Powercast will be demonstrating RF energy harvesting at the IDTechEx Energy Harvesting & Storage Conference in Denver (USA) on Nov 3-4, 2009.   On display will be Powercast’s RF energy harvesting technology integrated with wireless sensors from Texas Instruments, Jennic, and EnOcean.

All of the demonstration modules are battery-free and are powered by RF energy that is converted to DC by Powercast’s P2100 Powerharvester module and stored in a supercapacitor.  Harry Ostaffe of Powercast will also be giving a presentation on Practical Applications of RF Energy Harvesting.

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Jennic recently announced collaborations with multiple energy harvesting companies for vibration, solar, thermal, and RF energy harvesting to power wireless sensor networks based on the IEEE802.15.4 standard such as ZigBee PRO and 6LoWPAN.   The companies involved included Micropelt, CYMBET, AdaptivEnergy, and Powercast.

Jennic’s wireless microcontrollers offer exceptionally low-current operation, and with advanced software based monitoring and control of the energy source, they are able to achieve the extreme efficiency demanded by systems powered by sustainable energy sources.

At Powercast, we used a 915 MHz radio transmitter and the P2100 Powerharvester(TM) module to power wirelessly, without batteries, the new JN5148 module which sent ID, voltage, and temperature readings back through the JN5139-based access point to a PC application.

Jimi Simpson, Jennic Product Marketing, explained, “Harvesting energy from sustainable sources presents designers with the ultimate power challenge: the energy supplied is not necessarily continuous and is available at relatively low levels. This means that every element of the design, from the sensor to the microcontroller, must be considered and managed to achieve the highest levels of power efficiency.”

Jennic press release (PDF)

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At the Darnell nanoPower Forum on May 18th, Powercast and CAP-XX presented a battery-free wireless power module for wireless sensors.  The module uses the Powercast P2100 Powerharvester receiver, a CAP-XX GZ 115 supercapacitor, and the Texas Instruments eZ430-RF2500 wireless board.  The modules work by receiving radio waves and converting them into DC, which is then stored in the supercap.  When a charge threshold is reached on the supercap, the output to the wireless sensor is turned on which activates the sensor.  The sensor can have zero stand-by power instead of using a sleep mode, and power can be sent on demand, o na scheduled basis, or continuously.

The joint press release by Powercast and CAP-XX contains additional details.

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Battery-free wireless sensors can be a reality using RF as the power source.  To demonstrate this concept, Powercast has a created prototype battery-free wireless sensor with the temperature sensor board in our Lifetime Power™ Evaluation and Development Kit.

A short clip of the sensor in action is shown below.

The prototype sensor has a dipole receiving antenna and is operating from power being broadcast by a nearby 900MHz band transmitter.  The sensor operates continuously when close to the transmitter and intermittently as it is moved further away.  The frequency of operation decreases the further it is moved away from the transmitter.   For many applications, having the sensor wake-up once a minute to transmit a reading is sufficient.

The LCD driver and display requires much more power than low power RF radios like 802.15.4, ZigBee , ULP WiFi, etc, so radio-enabled sensors will work at an even greater distance than this prototype.

Applications include perpetual, wireless powering of battery-free wireless sensors for commercial buildings, industrial facilities, and process monitoring, or unattended ground sensors for military or surveillance activities that can be activated on-demand.

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