In the development of household devices, designers today are expected to bring new levels of convenience and flexibility to consumers. Some design limitations on achieving those goals can include traditional assumptions for connecting power and establishing communications.

Wireless communications has changed the world by offering an unparalleled level of connectivity. By use of a new inductive coupling technology, the wireless concept can be extended to power systems, giving design engineers a new path for creating innovative household devices.

Wireless or inductive power solutions have been developed in the past, but in each case the limitations or implementation costs diminished the potential gains of the system. As a result, a household today might contain only a few systems utilizing inductively coupled power, devices such as cordless shavers and toothbrushes. These applications are typically low power and require a fixed spatial relationship between the charger and the device.

Access Business Group has developed a new adaptive, inductive coupling technology that overcomes many of the weaknesses inherent in conventional inductive power transfer. This technology has enabled the company to design systems that can provide power to multiple devices without requiring a fixed spatial relationship between the power supply and the device. One aspect of this technology includes the continuous and automatic tuning and adjustment of the power supply based upon dynamic load requirements. Among other benefits, this tuning permits spatial flexibility between the power supply and a load. Using this new technology, the company has also designed a novel, multiple-axis, secondary coil that allows power transfer in any orientation, providing even more design freedom.

By incorporating communications, the technology also enables the use of control loops that allow the wireless device to specify power levels or control curves for precision power control.

In the water treatment application discussed below, the use of this new adaptive inductive power transfer technology helped reduce manufacture and maintenance costs by 40 percent while improving reliability. The same inductive coupling technology could be employed in many other applications.

Technology origins

Disassembled unit shows the modular, easy-to-assemble design of the water purifier. The clear, disk-shaped part toward front right is the system controller and contains the primary coil. The controller sits on top of the treatment chamber, the large cylindrical part at far left, which contains the UV light. The projection on the top of chamber contains the secondary coil.

Over the years, Access Business Group has designed and produced millions of water treatment systems. Since these water treatment systems operate in an inherently wet environment, the reliability of the electrical connections became a key issue. Reliability concerns included corrosion, electrolysis, connector fatigue by consumer, plating, contact pressure, and general wear. When asked to address these concerns, Access determined that a wireless power system could provide a solution.

Research showed that wireless power technologies used in the past were lacking the flexibility and robustness demanded for the premium product in development by the company. Improving such technology was expected to be expensive, but deemed worthwhile, given the enhanced product attributes such improvements could deliver.

The challenge

Enlarge this picture Fig. 1. No load characterization of a primary circuit.

Disinfection of water by UV light is a function of light intensity and time of exposure. In a conventional UV-based water treatment appliance, the water chamber remains filled with water and the UV lamp is continuously lit, maximizing exposure time. To achieve differentiation from competitive products, the new water treatment machine had the design objective of treating water on demand, eliminating the need for a continuously lit lamp and standing water. The benefits to the consumer would be two-fold. The water would be cool, instead of room temperature, and the appliance would consume less energy.

The on-demand design objective, however, presented several technical challenges. One of the bigger challenges was to wirelessly transfer up to 80 W of power to start the UV lamp as close to instantaneously as possible. The design objective was to sense the flow of water and immediately start the UV lamp to instantly reach a disinfection intensity level. Achieving this objective requires submersing the UV lamp in the water that is to be treated. This design called for 40 W of power to bridge about 0.5 in. of water, metal, and plastic.

Another challenge was in determining how to provide the sequence needed to start a low-pressure, mercury lamp inductively. The sequence requires three steps:

1. Low-voltage, high-current preheat.
2. High-voltage, high-current strike voltage.
3. Drop to operational voltage and current after the gas in the lamp has ionized.

The design effort involved several complications, including:

• Variations in lamp tolerances and manufacturing methods also increased variations in the load requirements.
• The need to strike lamps every time that water flows.
• Variation in incoming water temperature, which could range from 4 DegC (39.2 DegF) to 40 DegC (104 DegF).
• Variation in ambient temperatures due to different geographic locations as impacted by local heating and air conditioning systems.

It was determined that a microprocessor-controlled system would be necessary to manage the start and operation sequence of the lamp, and adapt to all the different load changes with dynamic response.

Wireless power design

Enlarge this picture Fig. 2. Load characterization by a digital system.

The first step in defining the mechanical requirements was to define the geometry of the system, which would include its spatial requirements. This would require understanding the minimum and maximum tolerance stack-ups for the mechanical system. The overall geometry and special requirements would then help determine the initial size requirements for the power-side primary coil and the load-side secondary coil.

The first design attempt of the electrical circuitry needed to power the water treatment system began with the establishment of a driver bridge circuit, given that a pure sine wave was crucial to reduce noise and maintain efficiency. Initially, the transfer of power was achieved by adjusting the DC rail voltage and controlling the frequency with a function generator, but it was quickly realized that the dynamics of the system could become overwhelming.

Design efforts then turned to a simple analog circuit that sensed current through the load and would adjust the power supply toward resonance automatically. This approach allowed the circuit to optimize the preheat phase and then automatically adjust to an open circuit, producing the strike voltage needed to ionize the lamp. Once the lamp had been ionized, the circuit would then automatically adjust to the operating impedance of the lamp and function properly for that lamp.

An important discovery in this design process was that the primary circuit needs to be sensitive to the load requirements and responses, including the mutual inductance. This means that the circuit must monitor the load from the primary side, which makes it very important to tune the secondary side to manage the power ranges. Tuning can be targeted to the operating or the strike phase of lamp operation. An air-core transformer is one factor that permits this sensitivity, which allows the circuit to adjust specifically to what the designed load needs.

One subsequent modification of this technology employed digital technology for adjusting the power supply to achieve resonance with the load. The digital system can tune for optimal peak performance. During the design process, it was noted that, when an inductively coupled circuit is separated from the load, the primary will still attempt to reach resonance. This effect can create a run-away situation, in which the primary continues to increase power output until the system is damaged or otherwise powered-down. However, the use of a digital control allows very robust system protection and monitoring of both load conditions and no-load conditions, thereby preventing a potentially damaging run-away situation.

The charts in Fig. 1 and Fig. 2 show both loaded and unloaded conditions. The capability to control and monitor these areas of operation allow many additional features and enhancements to the system. One example is the ability of this system to identify a load based upon sensed load characteristics. The system can also determine when a load has been removed, and can then automatically go into a low-power scan mode, in which the primary is occasionally powered-up for a short period of time, until a load load is placed within the primary field.

System benefits

Cutaway view of the water treatment module. The UV lamp in the center disinfects the water that flows in the chamber around it. The water is further cleaned by passing through the carbon filter that surrounds the inner chamber.

Redesigning the water treatment appliance to power the lamp by inductive coupling yielded numerous benefits:

• More user-friendly interface.
• Higher reliability.
• Improved diagnostics.
• Easier servicing.
• Easier lamp replacement.
• Improved safety.
• Simpler assembly.
• Improved system performance.
• Increased consumer appeal.

In addition to acquiring safety certification approvals for 55 different countries, the redesigned appliance also complies with the CISPR 22 standard that places limits on EMI emissions.

Opportunities

Access Business Group has been tracking the trends in wireless power and has identified additional opportunities for the new adaptive inductive power supply technology outside of the water treatment system arena. While conventional wireless power supplies are limited to low-current applications or high-cost, high-current applications, the technology developed by Access Business Group has the potential to create a new category of low-cost, high-capability applications.

The technology could bring added safety to electrical products used outdoors, such as pressure washers and moveable signage. It could help reduce or eliminate the tangle of power cords in both residential and commercial environments. And it could provide industrial designers the potential to create novel designs in lighting, consumer electronics and small appliances.

One possible concept involves a kitchen countertop that could power devices from 1 W to 1,400 W with greater than 98 percent efficiency. The system would not present increased safety issues for the user because it would only power up after detecting and recognizing a load. The countertop is just one example of how wireless power could create new opportunities for collaboration among designers of electrical products and home furnishings.


For more information, email: eCoupled.Sales@alticor.com