Remotely Powering Devices, Wirelessly
Table of Contents
With electronic devices being available in smaller and smaller sizes and the prevalence of wi-fi networks, the rise of remote monitoring devices around the home and the workplace continues to grow. Smart heating solutions now place sensors in every room, monitoring everything from temperature to occupancy to provide a highly efficient zonal heating system. Intruder detection systems no longer require thousands of meters of wiring to be installed when connecting the sensors around a building. Now, a sensor can be simply attached to any convenient location regardless of whether it is practical or possible to route a cable to it. The one downside of this explosion of wireless monitoring devices is that they still require a power source. There’s no point eliminating signal cables if the device still requires a power cable. It is estimated that more than half of the cost of setting up an Internet of Things (IoT) device is for cabling and installation. The move to wireless data transfer takes away some of this cost but being able to power a device remotely without wires would eliminate all cabling costs and reduce installation costs.
Current technology requires batteries to be installed in every device. Having rechargeable batteries is better for the environment, and from a spare parts point of view, it’s often simpler to recharge than replace. However, if we could eliminate the batteries, it takes away the time and effort expended to get that hard to reach device, find the right charger, and deal with a system with at least one sensor offline when it needs recharging.
On a more serious note, medical electronics are a growing field, and embedded devices are becoming more and more common. We started off with battery-powered pacemakers, but these have batteries that need periodic replacement. Modern Pacemaker batteries last for around ten years, after which an operation is required to replace them. If more and more medical devices are embedded into the body, there’s a real risk that battery replacement surgery could become an operating theatre’s primary use. The ideal solution is to get rid of the batteries (metaphorically, we’d maybe hang on to them as a back-up energy store) and power the devices through the air in the same manner that we provide connectivity for the information that passes to and from them.
This is where the idea of remote power sources comes in; it’s starting to come into widespread public use for applications such as wireless smartphone charging. The irony is that these devices aren’t tricky to plug into a power source, so the driver for this technology is essentially user laziness rather than a real practical need.
As an aside, from a device design point of view, it’s worth noting that if we could eliminate the need to replace or recharge batteries, it removes the need for the device to have an accessible opening in its enclosure. A device without a battery compartment could be a perfectly sealed unit, ideal for deployment in harsh environmental conditions. It would also be physically smaller and lighter, cheaper to manufacture, and potentially more reliable as dirt and dust won’t be penetrating inside every time the batteries are removed.
There are two main options for a device to remotely source the power it requires to operate. The first is to harvest energy from sources that are already present in the environment. The second is to install an energy source specifically to deliver energy to the device. Let’s take a look at these options in a little more detail.
Energy harvesting systems can collect power from both artificial and natural sources. Electrical energy can be created from movement using the piezo-active effect and ambient heat using a thermoelectric (Seebeck) effect. Power from the environment in terms of solar, wind, or waterpower is excellent outdoors but not that good in the office environment. For remote sensors located within a building, the use of ambient heat may offer a useful energy source. Pyroelectric devices can generate electrical energy from a change in material temperature, while thermoelectric devices can generate electrical energy from temperature differences across a thermocouple. Operating such devices would require access to two surfaces of different temperatures, which may not be readily available if the device is located in an area with a reasonably constant ambient temperature and no localized heat source. The operation relies on the flow of thermal energy from an area of higher temperature point to an area of lower temperature that does not achieve thermal equilibrium. While energy harvesting from naturally occurring energy sources can be successfully applied across a range of applications, the typical installation of remote monitoring devices within buildings does not lend itself to the use of this technology.
The most practical option for energy harvesting would be to exploit existing radio-frequency (RF), present in the form of television and radio transmissions, mobile telephone communications, and wi-fi. In the context of wi-fi connected remote monitoring devices, this would represent the most convenient source in terms of the highest local field strength and availability.
With wi-fi connectivity, the emitters and an electromagnetic field are already in place; the problem now becomes one of adapting it to enable energy harvesting. The main problem with energy harvesting from standard wi-fi technology is the tiny amounts of power that can be harvested, typically only a few µW/cm2. Using a large receiving antenna provides more usable energy but is at odds with the application of this technology in compact remote sensors. Therefore, this option is not currently practical with current technology. Research in this area is focused on improving efficiencies such that devices can operate with the relatively minuscule levels of power available.
With energy harvesting an unrealistic option, the alternative is to install an energy source specifically designed to deliver energy to the device. The techniques available to transfer energy to electronic devices remotely fall into two types, categorized as near-field and far-field. Near-field transfer uses non-radiative technology for transferring energy over short distances using inductive or capacitive coupling techniques. While this is the more efficient of the two technologies and the one used for smartphone charges, the short distance limitation is not useful for powering remote sensors. What we are interested in then is far-field power transfer, which uses radiant field technology for transferring energy over longer distances.
One option for far-field transfer is to use technology currently available that requires the electromagnetic radiation to be targeted at the receiver in a narrow beam, using microwave or laser emitters to achieve the required focused delivery. Currently, this technology can achieve successful power transfer over distances up to 50m. The problem here is that the emitter must be in the line of sight (from an electromagnetic point of view, or literally in the case of the laser) with all receivers. This doesn’t help if receivers are distributed around a multi-room building that contains structures that block or significantly attenuate the electromagnetic radiation. This problem is partially solved with the introduction of 5G technology that relies on beamforming to direct the transmitted signals to specific receiving devices. This is employed to provide a faster and more reliable connection. This has a potential application in allowing more efficient far-field power transfer for the transmitters. However, the energy requirements of implementing beamforming technology in the transmitting circuitry of a remote sensing device would seem, with current technology advancements, unrealistic for the foreseeable future. However, for pure power transfer, products using beamforming antenna arrays are in development that can deliver around 100 milliwatts at distances of 10 m, though such systems are subject to regulatory approval due to the large transmission power ratings required.
A more reliable and straightforward alternative is to extract power from a broadcast electromagnetic field, using bespoke RF emitters to create a field of sufficient strength that the remote devices can collect enough energy to perform their function and maintain an energy store to allow operation during any temporary loss of emission. Radiative far-field power transfer using a broadcast electromagnetic field isn’t a novel concept; Nikola Tesla first suggested the idea in 1901 and developed a few proofs of concept, and in 1931 Westinghouse demonstrated it in action by powering a lightbulb from a distance of around fifteen feet away, though admittedly it required 15kW of power to dimly illuminate the bulb. With an efficiency of less than 1%, the Westinghouse experiment neatly demonstrates the problem that needs to be overcome for far-field transfer power transfer to become a beneficial solution.
Current far-field systems typically transmit in the 915 MHz industrial, scientific, and medical (ISM) band. The advantage is that the use of this band only requires compliance with FCC regulations for non-interference. The received energy is converted back to usable electrical power by a receiver/rectifier. The use of Direct Sequence Spread Spectrum (DSSS) modulation for the power component of the transmitted signal to increase the transmitted signal’s overall bandwidth maximizes the energy reaching the receiver.
The advantage of using ISM over wi-fi or other radio transmitters in common usage is it allows power transfers that are an order of magnitude greater. When working with such low field densities, every increase has an enormously beneficial effect. The downside is that a system based on ISM will require a bespoke transmission system rather than being able to piggyback on any existing wi-fi infrastructure.
A governing factor is that the received power is limited by the amount of power that can be transmitted safely if the transmitter and receiving devices are located in an environment where humans will also be present. FCC approval and certification are necessary given the potential safety risks of transmitting considerable energy levels in close proximity to people. It’s not hard to imagine that there might be resistance to the deployment of such technology, especially given the reception that mobile technologies such as 4G and 5G have received in some quarters. Transmitted power can also be restricted in uninhabited industrial settings, such as where an explosive atmosphere may be present due to combustible vapors. The other factor for received power is the distance between the transmitter and receiving devices, the received power having a square-root relationship with distance. So, there will be a significant drop-off in received power when operating across large spaces.
With the careful design of receiving antenna, receiver, and rectifier, optimized for maximum efficiency, the capability to deliver sufficient energy to a low-power, duty-cycled wireless IoT sensor becomes feasible. The operation of sensor devices in duty-cycled mode is necessary with current commercially available technology due to the limited energy levels that can be collected from the environment. Realistically using pulsed power transmissions over a distance of 10 meters, the technology exists that can deliver a receive a few milliwatts for a period of a few milliseconds each minute to a sensing device. The challenge is to transfer significantly larger amounts of energy to deliver fast charge, and high energy power transfers to open up far-field power transfer to a broader range of applications.
There is a clear requirement for far-field power transfer that is partially being driven by the expanded deployment of IoT devices across homes and businesses. While systems are now available for specialist niches, more practical solutions are in development. For the designer looking to incorporate remote power capability, the technological challenges coupled with the FCC approval and certification hurdles will place significant obstacles in the way. However, it is expected that this will change in the near future. It’s worth keeping an eye on the products available from TechNovator (https://technovator.co/) and Ossia (https://www.ossia.com/cota/) for an off the shelf solution. As the market for this capability grows, availability and costs are expected to significantly decrease, making this an affordable option for low-cost remote sensor devices. It can only be a matter of time before commercially available systems become available at a price point where the requirement for powering remote devices with batteries or fixed cabling becomes unattractive.
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