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Energy efficiency for the Internet of Things

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An increasing number of networked devices are of late, finding their way into consumer, industrial and medical applications. Such networks often employ distributed nodes, which cannot practically be connected to the power grid. Powering these devices can be a challenge due to the cost implications of using battery or solar power, sending technicians for maintenance visits to replace batteries, or having to install one’s own power network for the IoT system.

However, by using highly energy-efficient design practices at both the hardware and software levels, the power requirements can be reduced significantly, altering the power supply paradigm to one of lower cost and higher efficiency. This is especially applicable for remote or portable devices that use RF/microcontroller chipsets.

High-power and efficient wireless network nodes can be engineered using modern RF microcontroller system-on-chip devices, activating sensors and peripheral hardware devices only when they are required, and then putting them into low-power sleep modes when not in use. Similarly, the RF transceiver can be switched into a very-low-power sleep state until the microcontroller decides that a transmission of collected sensor data is required. The microcontroller can then wake up the radio, perform the required transmission, and then revert to sleep mode.

In some cases, a burst of data transmission across the wireless network might only occur when a small, intermittent energy-harvesting power supply has accumulated enough energy in a capacitor to power a transmission. Alternatively, a low-power wireless sensor node can ‘wake-on-radio’, only taking the microcontroller out of its sleep state when a message is received over the wireless network requesting a sensor readout and only powering up the sensors and microcontroller at this time.

With most of the components of the system such as the microcontroller, radio and sensors kept offline or asleep for the maximum practical amount of time, efficiently designed wireless sensor nodes may achieve operating timescales as long as years off a single battery. Today’s typical wireless RF microcontroller system-on-chips targeted at IoT applications consume about 1-5 microwatts in their sleep state, increasing to about 0.5-1.0 mW when the microcontroller is active, and up to around 50 mW peak for brief periods of active RF transmission.

However when considering the design of energy-efficient, low-power IoT sensor networks, it can sometimes be advantageous to think not just in terms of power consumption, but in terms of the amount of energy required to perform a particular operation. For example, let’s suppose that waking up a MEMS accelerometer from sleep, performing an acceleration measurement and then going back to sleep consumes, say, 50 micro joules of energy; or that waking up an RF transceiver from sleep, transmitting a burst of 100 bytes of data and then going back to sleep consumes 500 micro joules.

If we know the specific energy consumption of each operation, then the average power consumption is simply the energy per operation multiplied by the frequency of that operation, summed over the different kinds of operations. Of course, this assumes that the continuous power consumption of each device when it is asleep is very small and can be ignored. Alternatively, if we have a certain known power budget available and a known energy budget for each sensing, computation or transmission operation, we will then know the maximum practical frequency at which a sensor node can perform sensor measurements and transmit its data.

Additionally, efficient wireless sensor nodes can take advantage of some form of energy harvesting power supply, employing energy sources such as solar cells, vibrational energy harvesters or thermoelectric generators to minimise maintenance and extend battery life, with the possibility of completely eliminating external power supplies, but only if the power consumption of the system is small enough and a capacitor is employed for energy storage.

In many applications, solar cells are the most familiar and relatively mature choice for low-power network nodes operating outdoors or under good indoor light conditions. However, there are other technologies suitable for extracting small amounts of power from the ambient environment. For example, a wireless sensor node set up to monitor bearing wear in a generator could employ a piezoelectric crystal as a vibrational energy harvester, converting motor vibration into usable energy, or a thermoelectric generator mounted on a hot exhaust could harvest a small amount of otherwise wasted energy from the thermal gradient.

Typical vibrational energy harvesters usually operate with a cantilever of piezoelectric material that is clamped at one end and tuned to resonate at the frequency of the vibration source for optimal efficiency, although an electromagnetic transducer can be used in some cases. While the electrical power available is dependent on the frequency and intensity of the vibrations, the cantilever tip mass and resonant frequency can generally be adjusted to match the machinery or system from where the energy is to be harvested.

Furthermore, energy harvesting management ICs that manage the accumulation of energy in a capacitor over a period of time can enable short bursts of relatively high power consumption, such as when a node wakes up and transmits a burst of data, and are particularly well suited to low-power wireless sensor nodes.

Even with the examples mentioned above, the energy-efficiency possibilities are significant and can be a reality. While energy use may not be a priority when designing prototypes or proof-of-concept demonstrations, one can only benefit by taking energy efficiency into account when it comes time to generate the final product.

Companies that are considering creating or modifying existing designs and are not sure about available energy-saving and generating options should ideally discuss their needs with an organisation that has the knowledge, experience and resources to make these design requirements a reality.

LX Group has a wealth of experience and expertise in the embedded hardware field, and can work with new and existing standards both in hardware and software to solve problems. 

Clients seeking a reliable implementation can partner with LX Group, which is equipped to create or tailor just about anything from a wireless temperature sensor to a complete Internet-enabled system within the required timeframe and budget. 

An award-winning electronics design company based in Sydney, Australia, LX Group specialises in embedded systems design and wireless technologies.

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