A New Approach to Wireless Power TransferJanuary 1, 2010 By: Fady Mishriki, Sensors
Supplying power wirelessly provides greater design flexibility.
In the past, product designers and engineers have faced challenges involving power: the continuity of supplied power, recharging batteries, optimizing the location of sensors, and dealing with rotating or moving joints. Although those challenges remain, new demands that arise from increased use of mobile devices and operation in dirty or wet environments mean that designers require new approaches to supplying power to equipment.
Equipment built for industrial sectors often has to be designed to operate reliably in wet and dirty environments. An example of this is robotic arms or rotating joints that must be capable of moving continuously through 360°, such as those encountered in construction machinery and wind turbines. Supplying power to these rotating parts frequently relies on the use of mechanical slip rings that suffer from performance failures, wear and tear, and intermittent connections that require frequent maintenance or limit mobility.
Alternatively, industrial assembly or process lines involve a flow of items and will use sensors to track each item at different points along the assembly process. Often sensors must be installed in difficult, hard-to-reach locations or must operate in hostile environments. Rather than hard-wiring each sensor, the devices can be powered wirelessly, simplifying sensor installation, maintenance, and replacement.
Getting Power to Hard-to-Wire Places
The conventional approach to getting power to the rotating parts of industrial equipment is to use a mechanical slip ring. Unfortunately, slip rings present reliability problems along with the associated downtime and ongoing maintenance costs.
In assembly or process lines, engineers can design around a mechanical slip ring. However, because of cabling costs, cable termination, and changing voltage or current requirements, they may look to a battery-based system. In battery-based systems the issue changes from supplying power to recharging and replacing batteries. An RF or charged capacitor system could work if the power needs were tiny, but if the sensor needs more than 50 mA, you're stuck.
Today, however, device designers have another option: wireless power transfer.
Third-Generation Wireless Power
PowerbyProxi has developed a 3G wireless power delivery system. Earlier generations of wireless power technology were based on split transformers consisting of two halves: an input side (primary) and an output side (secondary). Electrical energy applied to the primary is converted to an electromagnetic field that induces a current in the secondary, which passes the energy to a load. The essential difference between earlier generations of wireless power solutions and the one developed by PowerbyProxi is that the PowerbyProxi system offers high efficiency levels in relatively loose coupling arrangements across an air gap or through any nonmetallic substrate.
Conventional inductively coupled power transfer (ICPT) systems have typically achieved <50% levels of efficiency, even when the distance between the coils is minimized, orientation is fixed, or the coils are precisely tuned. These design constraints, coupled with heat and reliability problems due to the systems' inherent inefficiency, have offset the benefits associated with ICPT.
Figure 1. A demonstration model of the PowerbyProxi technology
PowerbyProxi takes a different approach to wireless power with its patented ProxiWave (Figure 1) technology. It uses coils to transmit and receive power between a power transmitter (PTx) and power receiver (PRx). These coils are analogous to antennas used for radio communication. A converter, powered through AC mains, a truck battery, or other source, is used to drive the PTx coil. The ProxiWave power controller regulates the power flow from the receiving PRx coil to the target electrical device or electronics, depending on their specific power requirements.
As RF antennas need to be tuned to maximize signal strength, so the ProxiWave system is tuned to maintain high-efficiency power transfer even under variable and zero load environments. At its core PowerbyProxi uses a patented tuning technology called dynamic harmonization control (DHC) which provides significant advantages over alternative solutions by dynamically varying frequency in response to environmental and load changes. DHC achieves greater power transfer efficiency and enables reduced receiver size while producing negligible electromagnetic interference even as it allows greater transmission range.
In most circumstances—for example, slip-ring applications—ProxiWave technology has achieved an industry-leading 90% efficiency level. It is particularly suited for variable load environments where the amount of power required changes on a continual basis, depending on the requirements of the machinery it is supplying.
Design Considerations: Third-Generation Wireless Power
Like other design elements, effective use of wireless power involves balancing the tradeoffs. Designers must consider four key parameters in specifying a wireless power system:
- Range and Orientation—proximity measured in inches, with optimum efficiencies achieved over distances up to 2 in.
- Power capacity—measured in watts from milliwatts to 10 kW.
- System efficiency—power received as a percentage of power transmitted. This can range from 50%–90%, with lower efficiencies resulting from larger transmission distances.
- System volume—based on the amount of power required and ranges from as small as a 25-cent coin for lower power levels to the size of a phone book for kW power levels.
These parameters are all related. For example, the range of a system directly impacts its volume and power level. In that case, a system that is not volume-constrained and needs to supply a few watts of power will have a much greater range than a volume-constrained system that requires hundreds of watts. Similarly, if power needs are minimal, the designer can accept lower efficiency but at the cost of greater heat (a byproduct of inefficiency). ProxiWave technology minimizes the tradeoffs involved.
Wireless Power Design Use Cases
Wireless power technology such as that provided by PowerbyProxi can be applied in a range of situations, including those
- where freedom of movement is required;
- where wired power is not practical;
- where the limitations, drawbacks, and overhead of batteries are unacceptable;
- where dirt and water present a design impediment;
- where a nonmetallic barrier may interfere (PowerbyProxi technology transmits power through nonmetallic barriers including water, wood, and plastic as well as through air).
Wireless power technology can be applied to products for use in natural resource extraction (forestry, mining, drilling) and construction industries, in manufacturing environments, and in emerging green industries.
Applying PowerbyProxi in a Factory Situation
A manufacturing facility needs a sensor network to capture data during its manufacturing process. The factory environment is dirty. Sensors are placed on items that are in constant motion. The power required varies from 5–10 W; the maximum range is 20 cm.
Batteries are deemed inappropriate for this environment due to their management overhead and form-factor considerations. Similarly, slip rings prove impractical. The planners turn instead to ICPT in a loom configuration to power the factory-wide network of sensors on moving items. These sensors might include photoelectric, temperature, humidity, or pressure sensors; strain gauges; and accelerometers. Generally, the sensors are communicating via CANbus although Ethernet and RS-485 standards are also supported.
Specifically, the planners opted for the Proxi-Loom system, a track loop whereby sensors—with receivers mounted on them—can draw power and signal without a physical connection to the loom, just proximity to it. Given the 5–10 W power requirement they estimated a PowerbyProxi receiver the size of 4 AA batteries could achieve a transmission range of 20 cm between the Proxi-Loom track and the Proxi–Loom transmitter plugged into the electrical mains.
As part of the design, the engineers tested the PowerbyProxi technology alongside wireless sensors that used the 802.11b wireless protocol and operated at 2.4 GHz and Texas Instruments TMS320F2801 DSP to ensure system compatibility with wireless sensor nodes. They also planned for a SensiFi, G2Microsystems and Decawave platform, (all of which generally operate in the 2.4 GHz band and communicate using an 802.11-based protocol). Intel Atom and ARM processors will function alongside PowerbyProxi technology. The solution proved successful.
Benefits of the ProxiWave Approach
The ProxiWave approach delivers a number of benefits. Depending on the specific PowerbyProxi technology involved, these benefits include:
- Autonomous charging
- Ability to charge multiple devices simultaneously
- Elimination of corrosion-prone metallic contact points
- Ability to create products that can be easily installed and waterproofed
Proxi-Ring (Figure 2)
- Freedom of movement (360° continuous rotation)
- Unlimited RPM
- No need for lubrication and cooling
- Increased uptime and reliability
- Reduced total cost of ownership
- Simplicity through plug-and-play system
- Wireless CANbus data communication system
Figure 2. The Proxy-Ring 240
- Elimination of electrical terminations points for reduced cost of installation and certification
- Ability to deliver power to moving sensors/devices
- Ability to move sensors/devices without needing to rewire them
- Ability to replace faulty sensors/devices easily
- Ability to power thin/small form factors
- Overcome the size constraints of physical connectors
- Enable the migration to rechargeable batteries
- Ability to deliver unplugged power for mobility and convenience
More Power Options
PowerbyProxi's ProxiWave approach gives planners and designers new and flexible options for delivering power. They no longer need to feel bound by the constraints of wired power or the limitations and drawbacks of battery power.
By taking advantage of PowerbyProxi's range of configurations, designers can leverage the benefits of wireless power in their products and equipment. This will enable them to lower costs, simplify deployment, and reduce ongoing maintenance and operations overhead. Ultimately, it will speed time to market while resulting in better products.
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