An industrial application can have thousands of embedded subsystems that need to communicate with their environment. Each of these requires its own data and power connection. Cabling is costly to install, it is a frequent source of failure and a curb on flexibility. Such applications are better served by wireless technologies.
Wireless communication in automation environments has made important advances in recent years [1]. However, wireless power supplies remain a challenge. In 2004, ABB released a series of unique wireless products - wireless proximity switches - in which both communication and power supply are wireless. Since the introduction of these devices, the “WISA” (Wireless Interface to Sensors and Actuators) technology [2] on which they were developed has been further expanded to include new products and communication profiles.
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The devices that benefit most from a wireless power supply are components of distributed control and automation systems, typically sensors and actuators – devices that often have embedded intelligence. They are generally located in remote environments, where no general power source is available, or in areas that are difficult to access. They can be amongst numerous other devices, in highly mobile assemblies, such as robots, or in highvoltage environments.
In such applications, energy efficiency is a primary concern. The power consumption of conventional wireless technologies, as used in standard -office wireless components, for example, is generally far higher than that of dedicated systems that have been designed around low-energy requirements. Low-energy devices often make savings by using hierarchical, pulsed operation modes. Most applications, such as data collection, actuation, processing and communication, are performed only at specific time points. Such tasks can be carried out using pulsed operation modes that are interspersed with energy-saving sleep phases.
1 ABB’s wireless power technologies are based on the well-known transformer principle
Sources of wireless power
Generally speaking, wireless power/energy can be either:
- Included in the system in the form of batteries, fuel cells, etc.
- Taken from the local environment in the form of light, heat, vibration, user activation, etc.
- Transmitted to the system via optical or radio frequencies, sound, etc.
The primary WPU100 units are tuned and -controlled automatically, which means that the -WISA-POWER supply system can be applied
to different applications just by altering the primary coil geometries.
Although the use of battery power is considered acceptable in the consumer world, regular battery charging or -replacement is not a practical option in typical industrial applications. In extremely remote areas, batteries, perhaps in combination with solar or geothermal power, remain the only practical option. In general industrial applications, however, where hundreds of devices that require constant, reliable power supplies run day and night, -batteries are not an option. Their energy density, around 1.2Wh/cm3, is too low. Fuel cells are somewhat better, but even their potential is little more than 2Wh/cm3, and much development is still required before they can be used in everyday industrial installations.
Environmental energy sources also fail to meet the needs of industry applications, due to their unpredictable nature – both in terms of general usability and reliability; and so they fail to meet one of the most important considerations of all. Such solutions would also incur considerable engineering and design costs for every single application.
After a thorough evaluation of the -various available options [3], it seems that the only viable, generally applicable solution is one based on long-wave radio frequencies, a form of “magnetic coupling”. ABB has a number of power supply options that use magnetic coupling . Depending on the transmission distance, a wide range of applications and power -levels can be implemented.
The power needs of distributed electronic devices, such as single sensors, and wireless I/O (input/output) devices, in discrete factory automation settings, are covered by the first generation of WISA-products . The wireless supply unit WPU100, together with a coil setup (“primary loop wires”), -provides a low-level power supply across large distances (a few meters). This is suitable for most sensors and actuators in discrete factory automation.
WISA-POWER: The "magnetic supply"
The basic principle of a magnetic field-induced power supply can be -described by the well-known “transformer” principle 2. In the case of the WPU100, the power supply feeds the primary winding 2b, a large coil, which can be arranged around a production cell, the secondary side of which corresponds to a practically -unlimited number of small receiver coils 2c. Each receiver coil is equipped with a ferrite core to increase the amount of flux collected by the coil.
2 WISA-POWER: Wireless power supply.
A power supply a feeds primary loop b with a current at 120 kHz. Sensors c within the primary loop are equipped with secondary coils. The right-hand schematic shows the equivalent circuit diagram with loose coupling
For this type of “transformer”, magnetic coupling is low. The receivable power is determined by the amplitude of the magnetic field at the location of the “receiver” (secondary) winding. However, if the primary coils are set up in a “Helmholtz-like” arrangement 3, the field (and therefore the receivable power) will be fairly constant over a large volume of space.
The design rules for the number and size of the primary coils are very simple: D = 0.7 S, where S is the smallest dimension (width or height) of one coil frame and D is the separation between the coils to provide an adequately homogeneous field amplitude inside the arrangement 3.
3 A "Helmholtz-like" arrangement of rectangular coils integrated into an industrial application. D is the separation between coils and S is the smallest dimension (width or height).
Although people will rarely work continuously in such an automated production cell, the strength of the magnetic field at all normal working positions (including within such a cell) complies with international occupational regulations and recommendations [4]. WISA-POWER works at a similar frequency (120kHz) and in -exactly the same way as many of the anti-theft and radio frequency identification systems used in shops and -supermarkets.
The fundamental challenges of wireless power distribution and the reliability of real-time-suited wireless communication have been successfully resolved.
Within this therefore limited-amplitude magnetic field, power levels can be scaled according to the needs of different applications by changing the size of the secondary coil. This allows embedded systems to be connected to wireless power by the integration of a suitable receiver coil and circuit. A good example of this can be seen in 4. Artis, a company, based in Bispingen (Germany), has used -WISA-POWER technology to create its own secondary-side electronics, adapted to the special needs of tooling sensors [5].
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| 4 | WISA-POWER primary spot coil, integrated into a machine tool with receiver coil provided by customer and embedded electronics (wireless tool survey system DDU WiSy Courtesy of ARTIS GmbH, Bispingen, -Germany)[5] |
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| 5 | WISA-Power ring-type primary coil structure in a completely rotating cable winding machine (ABB High Voltage Cable, Karlskrona, Sweden). 156 wireless proximity switches WPS with embedded electronics rotate in a complex 2D-movement inside the machine, to ensure a failure-free production. |
Energy losses in such a system are surprisingly small and are mainly due to skin and eddy current effects in the coil or in nearby metallic objects. In typical factory automation environments, energy losses are around 15W/m3.
Resonant, medium-frequency power supply
These unconventional transformers are best operated in a ‘resonant’ mode. In this mode, the transformers’ relatively large leakage inductances are compensated for by a capacitance, which allows the WPU to stimulate the resonant circuit at relatively low voltages. The WISA WPU100 primary power supply must also be able to -accommodate:
- Changes in the environment over time, eg, caused by the movement of large, mobile metallic objects such as robots.
- Different ’load’ requirements, caused by differently sized primary coils (inductance values), and losses, caused by factors such as eddy currents in adjacent metallic objects.
- Other nearby wireless supply systems, which may couple inductively.
To accommodate these requirements, the WISA WPU100 contains a fast and highly accurate control unit, which compensates for such changes and -automatically keeps the primary system at a fixed resonance frequency of 120kHz. The WPU100 unit can adapt to inductive loads from 11–54µH and supply currents of 4–24 A.
The primary WPU100 units are tuned and controlled automatically, which means that the WISA-POWER supply system can be applied to different applications just by altering the primary coil geometries, eg, by using ring- or line-type coil structures or spot coils 4 5. This is particularly useful if wireless power is needed only in certain areas of an installation, for example in devices that move along a ring or line structure, or for bridging critical spots in a system.
Due to its unique capabilities, the WPU100 unit can also be used in -applications with customized power-receiver units adapted to specific geometrical and application needs .
Rotating field
Unidirectional magnetic fields can be shielded unintentionally by metal objects. To avoid this, two loops can be mounted orthogonally. The loops must be fed by separate power supplies, whose currents are phase-shifted by 90° with respect to each other. This creates a continuously rotating, 2-dimensional field.
Omni-directional receiver structure
To achieve sufficient power output on the receiver side, the secondary coils must also be operated in a resonant mode. Further, to make the available power independent of the receiver’s orientation with respect to the primary field vector, an orthogonal setup of three coils on a common core has been chosen. Being easily tunable to the fixed resonance frequency, this -arrangement is well suited to mass production.
The generic and modular technology of the WISA family products, which started with the wireless proximity switches, is now being extended to other devices and applications.
With this technology, the available power densities for typical “worst-case” shielding conditions in real -applications remain in the order of 1.2mW/cm3. The absolute power level can be modified with the coil size and shape.
The scalability and integration of -WISA-POWER receiver coils into products has been demonstrated in different applications. Total power consumption of the wireless proximity switch and its electronics 6 is significantly below 10mW – the new sensor pad, WSP100 7, which allows the connection and supply of up to eight sensor heads, can already provide several tens of milliwatts in “worst case” conditions of shielding. Under normal conditions, at this size, the WISA-POWER principle can provide several hundred milliwatts, and, under controlled conditions, up to 1 Watt.
6 WISA-POWER “Power Cube” integration -into the WISA communication module WSIX100 of a wireless proximity switch
7 WISA-POWER Integration of a scaled “Power- Cube” into the WISA sensor pad WSP100 to supply eight sensor heads and their real-time WISA-COM communication
Unbounded future
With the introduction of the WISA power and communication technologies, ABB has made significant advances in the technology of wireless embedded systems. The fundamental challenges of wireless power distribution and the reliability of real-time-suited wireless communication have been successfully resolved.
The generic and modular technology of the WISA family products, which started with the wireless proximity switches, is now being extended to other devices and applications. The generic WISA-POWER supply and -WISA-COM communication technologies are set to find their way into many further applications.
Guntram Scheible
ABB STOTZ-KONTAKT GmbH
Heidelberg, Germany
guntram.scheible@de.abb.com
Rolf Disselnkoetter
ABB Corporate Research
Ladenburg, Germany
rolf.disselnkoetter@de.abb.com
References
[1] Niels Aakvaag, Jan-Erik Frey: Wireless communication and sensor networks. New-breed net-working solutions for industrial automation ABB Review 2 /2006
[2] Jan-Erik Frey, Andreas Kreitz, Guntram Scheible: Wireless but connected, ABB Review 3 and
4 /2005
[3] G. Scheible: Wireless energy autonomous systems: Industrial use? Sensoren und Messysteme VDE/IEEE Conference, Ludwigsburg, Germany, March 11–12 2002.
[4] International Commission on Non-Ionizing Radiation Protection (ICNIRP): Guidelines for Limiting Exposure to Time-Varying Electric, Magnetic, and Electromagnetic Fields (up to 300 GHz). Health Physics vol 74, no 4, 494–522, 1998.
[5] Berend Denkena, Dirk Lange, Dipl.-Ing. Jan Brinkhaus: Spielraum in der Überwachung; Fachzeitschrift mav „maschinen anlagen verfahren“ Konradin Verlag Robert Kohlhammer, 2005
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