Energy harvesting makes electronic control systems independent of an external power supply by harvesting energy from their surroundings – from motion, light or differences in temperature, for example. Even people themselves can become a source of energy, by pressing a button for instance, or through heat from their bodies. Recently, the principles of energy harvesting wireless have been enshrined in the international standard ISO/IEC 14543-3-10 which provides a “Wireless Short-Packet (WSP) protocol optimized for energy harvesting -- Architecture and lower layer protocols.”
The protocol supports energy harvesting sensors and switches that do not require wires and batteries. It is the only standard specifically designed to keep the energy consumption of such sensors and switches extremely low, an order of magnitude lower than alternative approaches. Modules achieves this by transmitting multiple, very short transmissions. Wireless pulses are just 0.7 msec wide and are transmitted at a 125 kbits/sec data rate. By using the 868 MHz and 315 MHz radio frequency bands, the protocol offers fast system response and minimal interference. The range of standard-based wireless sensors is about 1000 feet in an open field and up to 100 feet inside buildings. The resulting wireless communications enable the use of small, cost-effective, and maintenance-free energy harvesters that outperform battery-powered solutions.
Various Sources of Energy
Where other wireless standards integrate physical and data link layers and the network layer is integrated in the protocol stacks, in the case of ISO/IEC 14543-3-10, the standard offers physical and data link layer as well as the network layer, with the EnOcean Alliance. This non-profit organization has more than 300 companies sharing their knowledge and experience on energy harvesting wireless applications. These application profiles sit on top of ISO/IEC 14543-3-10 and are defined in order to achieve interoperability between products from different vendors. These application level protocols are referred to as EEPs (EnOcean Equipment Profiles).
EnOcean has developed energy converters such as an electrodynamic energy generator that uses mechanical motion, or a miniaturized solar cell harvesting from indoor light. Besides harvesting energy from light or motion, electricity is produced from differences in temperature as a third power source (Fig. 1). Modules use this energy to detect environmental data and transmit it over-the-air. If needed, an additional charge capacitor ensures an adequate power reserve to bridge intervals when little or no energy can be harvested.
One of the core elements of energy harvesting wireless technology is the mechanical energy converter which has recently been introduced to market in third generation. So how does this converter generate enough energy for a wireless module to send radio signals? Let’s take a closer look at this new generation of mechanical energy converters. To be versatile, the energy converter needs to have certain properties. First, it should have a good level of efficiency so that little or no energy is wasted. A good level of efficiency also enables small forces and movements to be used effectively. Other key factors are a long lifespan, compact design and low cost.
An energy converter (Fig. 2) combines these sometimes conflicting requirements in an efficient concept: A magnetic flux is passed through two magnetically conductive laminations by a small but very strong magnet, and is enclosed in a U-shaped core. An induction coil is wrapped round this core. The magnetic parts are held in position by a plastic frame and a spring-loaded clamp. The U-shaped core leading through the coil is movable – it can take up two positions, in each of which it touches the opposite lamination – and in each end position the magnetic flux is reversed in the U-core. This design ensures maximum magnetic flux alteration through the coil with minimal movement of the core and therefore high efficiency. This science is applied to real world applications in the form of controls such as self-powered light switches, hotel keycard switches and door/window handles.
Independent of Speed
Moreover, the energy output does not depend on the speed of actuation. A mechanical energy store in the form of a leaf spring takes care of this. As the leaf spring is bent more, it stores mechanical energy until the magnetic forces can no longer hold the U-core in its position. If the spring forces exceed the holding power of approximately 3.5 newtons (N), the core flips quickly into its second position, accelerated by the spring. This generates a voltage pulse in the induction coil. The speed with which the core flips largely determines the amount of energy that can be taken from the coil – and is always constant, as the spring always accelerates the U-core in a similar way, irrespective of how quickly it was tensioned.
So each actuation produces a small electrical pulse that can be used immediately for the brief operation of electronic circuits. One particularly useful application is to combine it with a wireless transmitter as this makes it possible to have a switch that has no wires or batteries – and is therefore maintenance-free. An example is the ECO 200 by EnOcean (Fig. 3). When combined with the wireless transmitter module PTM 330 (Fig. 4), it creates a complete wireless technology system incorporating all components and functions without batteries.
Energy Harvesting System Approach
So energy harvesting wireless is not only the energy converter, but a platform strategy that features a growing number of energy converters, energy storage solutions, suitably optimized wireless transmitters, sensors, software and development tools. Such a system enables OEMs to implement their own energy-autonomous applications simply and at low cost.
An example is an application in the public transport sector: a wireless, self-powered stop button for buses from UK based company BMAC (Fig. 5). While other pushbuttons need to be connected to the driver’s stop signal of the bus via yards of cable, this stop button functions with a small battery-free microchip. When a passenger presses the stop button, the energy converter converts this small movement into electrical power and a radio signal is sent to the receiver module. This activates the stop display and the audible stop signal. The transmitted radio signal is unique to each stop button, so there is no interference with other buttons in this bus or other buses nearby. The receiver module is connected to the vehicle electronics. After installation, each stop button in the bus is programmed for its own logic circuit – for example front, middle, rear or wheelchair users. This solution saves over 100 yards of bus cabling in and reduces the need to replace defective cables.
The technology of energy harvesting wireless switches and sensors enables OEMs to implement self-powered, wireless switching solutions in buildings, smart homes, transportation, and industry. Thanks to the open ISO/IEC wireless standard and the specified equipment profiles of the EnOcean Alliance, the products are all interoperable and can be combined with industrial controls and receivers from various vendors. Devices complying with the EnOcean standard devices work seamlessly with other communication protocols such as TCP/IP— thus opening the way to implement developments such as M2M or the Internet of Things.