Jason Tollefson, Product Marketing Manager at Microchip Technology Inc. discusses the latest advances in remote sensors powered by RF harvesting and how this technology is offering a promising outlook to the mainstream
Sources of energy, such as light, wind, temperature, vibration, radio waves and even PH have been converted to usable energy but the challenge is how to convert the tiny amount of energy generated to perform a useful function, such as reliably powering an environmental sensor.
An examination of the power budget holds the key, together with the harvested source and the components. RF energy can be obtained from ambient energy or controlled by the use of a dedicated transmitter. Devices using RF harvesting can be untethered and work in almost any environment.
RF energy can be harvested from sources such as broadcast TV and radio stations, mobile phones and base stations, and transmitters in unlicensed bands including 915MHz, 868MHz or 2.4GHz, making it commercially viable worldwide.
RF does not depend on the time of day or require exposure to heat or wind, and can be moved freely within the range of the transmission source. Energy can be transmitted continuously, on a scheduled basis or on demand. A rechargeable battery or super capacitor can store the converted RF energy for operation during the peak periods.
A convenient and reliable power source is just the start; a proper system design is needed to maximise performance with the tiny amount of energy provided. This can be addressed by either using extremely low power components or implementing power balancing.
The trend for lower-power electronic components is fuelled by consumer demand for portable products, and has delivered new low power microcontrollers, analogue, radios and communication protocols that complement RF harvesting.
Low power consumption levels are standard on Microcontrollers. A device such as Microchip’s PIC24F with eXtreme Low Power (XLP) technology consumes only 20nA while sleeping, and can execute code with currents as low as 8µA.
Analogue components and a radio are required to complete an environmental sensor. The radio stretches the power-budget because of the protocol used and the Transmit/Receive (Tx/Rx) current. New radios have started to address the issue, and now feature receive currents as low as 3mA, which helps to reduce power consumption, but the wireless communications protocol remains the driving factor.
The power balance factor
Long execution times and bloated wireless protocols will consume the power budget when working with the tiny amounts of power generated by energy harvesting. The key reduction is to select a protocol that allows for scaling functionality.
The company’s MiWi protocol allows for minimalist implementation, with radio transmission times driven as low as 5ms. Power management through charge-based execution and state-of-charge monitoring deliver further improvement capabilities.
Power is cut off completely from the sensor system in charge-based execution. When the RF harvester has collected enough energy the device consumes zero power while replenishing the energy reservoir.
The frequency of the sensor’s execution is dependent upon the rate of charge to the reservoir. This is impacted by distance to the RF source, the receiving antenna, and obstructions such as walls, which works well when the sensor is located such that the frequency of sensor operation is fitted to the needs of the overall system.
To avoid flooding the network with unnecessary packets, the RF harvester can also use the received signal strength (RSSI) as a mechanism for controlling the rate of data transmissions.
If the RF harvester is charging a battery, a microcontroller can be used to monitor the length of the charging cycle and estimate the state of charge. It can then calculate the run-time based on what sensor operations are performed by recording the current consumed during the various parts of the sensor operation. For example, the sensor node might consume 100µA when measuring the sensor’s output, and 20mA during the radio transmission of that data.
The microcontroller can use this information to estimate the charge that will be depleted each time one of these functions is completed. The state of charge is thus found in the comparison between charging and depleting. This method can taper off the frequency of sensor transmissions, based upon state of charge and can even call for help, by transmitting a message to the RF power source to send more power.
RF harvesting is a viable option for a wide range of applications with proven technology providing a platform for prototyping new products.
Battery powered sensors can be replaced with a selection of components and power balancing. RF harvesting is a practical option providing control of the source and the ability to operate in any environment, which may also drive RF harvesting to the mainstream market.
RF harvesting is one of the first proven and reliable options; it meets environmental regulations and CSR requirements, makes economic sense and avoids being at the mercy of the sun, wind or unpredictable temperature conditions and so is most promising.
Microchip
