As one of the technologies for “Attracting Tomorrow”, TDK is working on the development of wireless power transfer technology. Just as wireless communication technology and ICT technology have brought about a telecommunications revolution, the spread of wireless power transfer will also bring about sweeping innovations in industries and social infrastructure as well as lifestyles.
Wireless power transfer (WPT), through the transmission of cordless/contactless power, is not only being used for charging batteries in mobile devices such as smartphones, and in EVs (electric vehicles), but it is also being increasingly used in the field of industrial applications. In addition to the electromagnetic induction type of wireless power transfer conforming to Qi and PMA standards, TDK has also been one of the first companies to work on the technical development of the magnetic resonance method which has been attracting attention of late.
This article presents wireless power transfer technology using one of the magnetic resonance technologies which were newly developed by TDK for various industrial applications, including robots. TDK has put together three power platforms at 1kW, 200W, and 50W, which allow for flexible system construction to meet application requirements, such as power transfer to moving parts on automatic guided vehicles (AGV) and elevators, power transfer to rotating parts such as a robotic arms and surveillance cameras, etc. Wireless power transfer for industrial applications: eliminates the need for power cables, enables safe and secured automatic charging, improves the working environment, increases production efficiency, potentially reduces cost, and also allows power transfer in harsh environments which cannot be accessed by people.
The idea of wireless transfer of power has been around for a long time. Nikola Tesla and other pioneers pursued experiments in wireless power transfer using electromagnetic waves as far back as the 1880s. Table 1 shows some representative systems of wireless power transfer.
|Electrical field coupling||Electrical field resonance||Up to a few cm|
|Magnetic field coupling||Electromagnetic induction
||Up to a few cm||Has been implemented in many fields since the 1990s. TDK is heavily engaged in this technology.|
||Up to a few 10cm||Kick-started by research papers presented at MIT in 2006, this method has been in development since the early 90’s and is now progressing worldwide. TDK is deeply involved in this technology (presented in this paper).|
|Microwave||Up to a few m|
|Laser||Up to a few m|
1) Other WPT technologies include RF and ultra sound-based technologies. 2) Transfer distances are based on the examples of common systems.
Wireless power transfer can be broadly classified into the radiative type, in which energy is transferred using a radio wave (microwave) or laser, and the non-radiative type, in which energy is transferred using an electrical field or magnetic field. The radiative type has the advantage of being able to transfer over long distances, but the energy loss due to environmental conditions, etc. is large and therefore transfer is not so efficient. Compared to this, the non-radiative type is basically designed to reduce energy loss as much as possible, and hence, yields better transfer efficiency than the radiative type, but its weakness is that the transfer distance is limited.
There are two types of non-radiative wireless power transfer systems, namely, the magnetic field coupling type and the electrical field coupling type. Of these, the magnetic field coupling type of electromagnetic induction method of wireless power transfer has been widely used since the 1990s to charge the battery of cordless telephones, electric shavers, electric toothbrushes, etc. Electric buses, which run by using the electromagnetic induction method for wireless charging of their battery, have already been implemented. Additionally, various types of charging pads and charging stands, which use the electromagnetic induction method (example: Qi and PMA standards) to charge the battery of mobile devices such as smartphones, are commercially available.
For magnetic resonance method, a major boost in interest was a result of research papers presented in 2006 and 2007 at MIT (Massachusetts Institute of Technology). With this increased attention, proof-of-concept experiments have begun which are now leading to product development worldwide. TDK is an early adopter of both electromagnet induction and magnetic resonance technologies.
|Magnetic field coupling||Electromagnetic induction
||Up to a few cm||
||Up to a few 10 cm||
The differences between the electromagnetic induction method and magnetic resonance method are explained based on the principle of the transformer used in the switching power supply (Figure 1). The transformer is a device with a structure in which a primary winding and secondary winding are wound around a magnetic core. The primary winding and secondary winding are electrically isolated, but the change in magnetic flux produced by the current flowing through the primary winding (excitation current) is passed to the secondary winding via the magnetic core, and electromotive force is generated because of the electromagnetic induction effect, causing current to flow to the secondary winding (induced current).
The electromagnetic induction method of wireless power transfer uses a system which consists of an amplifier unit, power transmission coil, power reception coil, and power reception unit. The power transmission coil unit and power reception coil unit are similar to a structure in which the transformer core is divided to create an empty space, or “air gap”. The electromagnetic method of wireless power transfer has the merit that it can be implemented at low cost because the system is simple, but as the distance between the power transmission coil and power reception coil increases, transfer efficiency drops drastically due to reduced magnetic coupling. As the distance between the coils increases, a portion of the magnetic flux becomes leakage flux, which then weakens the magnetic coupling between the coils.
Figure 1 Basic principle of a transformer and wireless power transfer system using the magnetic field coupling method
The magnetic resonance method of wireless power transfer is a method which has emerged to overcome the problem of drop in efficiency due to the distance between the power transmission coil and power reception coil. Magnetic resonance is a special case of magnetic inductance.
The extent of magnetic coupling between the power transmission side and power reception side is expressed by a value known as the coupling coefficient. If the inductance of the power transmission coil and power reception coil are L1 and L2 respectively, and the mutual inductance is M, the coupling coefficient (or factor) k is expressed by the following formula.
The coupling coefficient is a value in the range of 0≦k≦1, and it is ideally equal to 1 (=100% transmission efficiency) in the absence of leakage flux. But as the distance between the coils increases, and as the distance between the center of the coils becomes larger, the leakage flux increases, as a result of which the coupling coefficient drops.
In the magnetic resonance method, a capacitor is inserted on the power transmission side as well as the power reception side to form a LC (inductor and capacitor) resonance circuit, and power is transferred by matching the resonance frequency on both sides. Its merit is that high transmission efficiency can be obtained even when the coupling coefficient is low, typically ＜0.5 (Figure 2).
Figure 2 Basic principle of the magnetic resonance method of wireless power transfer
The relationship of the distance, between the power transmission coil and power reception coil, and the transmission efficiency, in both the electromagnetic induction and the magnetic resonance methods, are shown in Figure 3. This is a comparative example of two 40 x 40cm coils which are made to face each other, and their relative position is then changed. As the distance between the coils (along the Z-axis) is gradually increased, in the electromagnetic induction method, the transmission efficiency drops to about 40% at a distance of about half the coil diameter, but in the magnetic resonance method, transmission efficiency is maintained at 90% or more.
And, if the distance between the coils is maintained as 10cm and the alignment between the two is changed (along the X-axis), when the misalignment distance between the center of the coils is 20cm, transmission efficiency drops to 40% in the electromagnetic induction method, but in the magnetic resonance method, transmission efficiency is maintained at 90%.
Figure 3 Comparison of the distance between the coils and the transmission efficiency in the electromagnetic induction method and the magnetic resonance method
Both the electromagnetic induction method and the magnetic resonance method are wireless power transfer systems which use magnetic coupling, and transfer power by using a high frequency power supply to pass the change in the high frequency magnetic field from the power transmission coil to the power reception coil. Even though it is a change in a high frequency magnetic field, it is not an electromagnetic wave. Changes in the magnetic field are released as electromagnetic waves in the far field region which represents a distance of 1/2π (roughly 1/6) from the source, while it behaves as a magnetic field in the near field region which is closer to the source, and its intensity is inversely proportional to the square of the distance. This is why the power transfer distance is small in the electromagnetic induction method.
With innovations to the circuit in the magnetic resonance method, even when the distance between the coils is half the coil diameter or more, power can be transferred with high efficiency. And, since the magnetic resonance method supports transfer over a wide power range, from levels below 1W to high level of power greater than 10kW, there are great expectations that it will become one of the mainstream methods of wireless power transfer in industrial applications.
The maximum transmission efficiency in the magnetic resonance method is expressed as a function of the product of the coupling coefficient (k) and the Quality factor (Q) of the coil (kQ product). Even if the coupling coefficient is low, high transmission efficiency can be obtained by increasing the coils’ Q. This is its greatest difference from the magnetic induction method. But, several problems need to be overcome to implement the magnetic resonance method of wireless power transfer.
The coil’s Q-value is expressed as Q=2πfL/R (where f is the resonance frequency, L is the coil inductance, and R is the coil AC resistance component). From this formula, if the inductance is increased by increasing the coil diameter, or by increasing the number of turns of the coil, theoretically, Q will be increased. However, since the resistance component also increases in this case, it is necessary to optimize the shape and size of the coils during coil design in order to balance both.
One more requirement is that it should support changes in resonance frequency. In the magnetic resonance method, maximum transmission efficiency can be obtained when the power transmission coil and power reception coil are placed at an optimum distance from each other. Unlike the electromagnetic induction method, reducing this distance may cause transmission efficiency to drop instead of increasing. This is because when there is a deviation from the optimum distance, the mutual inductance M changes, causing the coupling coefficient and the resonance frequency to change. Additionally, the stray capacitance from objects around the coils also affects the resonance frequency, resulting in an untuned, non-optimized system.
Hence, in the magnetic resonance method, a special circuit is typically required to automatically track and tune the circuit for the maximum efficiency, which makes the system more complicated than the electromagnetic induction method. There are various techniques to compensate for these fluctuations in resonance frequency, but this is the most important technical consideration in the magnetic resonance method, along with the coil design technology.
TDK is focusing on wireless power transfer energy for three main markets, namely, ICT, Automotive, and Industrial Equipment and Energy, and is actively proceeding with development of wireless power transfer systems for mobile devices and EVs as well as industrial equipment such as automatic guided vehicles, logistics robots, mobile robots, etc. The development portfolio of wireless power transfer systems from TDK is shown below.
Installation of wireless power transfer systems in industrial equipment has the following merits.
TDK has newly developed 3 platforms for the wireless power transfer system for industrial equipment using the magnetic resonance method, with output of 1kW, 200W, and 50W respectively. The system configuration and merits of each are explained below.
|Automatic guided vehicle (AGV)||
In the automated production lines and warehouses, computer-controlled automatic guided vehicles (AGV) are heavily used for transport of goods. There are various types such as the trolley type and lift-up type, but since all of them are battery operated, frequent battery replacement (or charging) is necessary.
An automatic guided vehicle carrying a load of 100 kg can be continuously used for 8 hours. Besides the labor and time required for battery replacement, the cost of arranging for replacement batteries also needs to be borne. Also, manual battery replacement inside a clean room could adversely affect its cleanliness.
The solution to these problems is to use wireless power transfer with automatic charging, which also has the added advantages of labor and time savings. And by sequential charging when the AGV is stationary for cargo to be loaded or unloaded, a battery with a relatively small capacity can be installed, resulting in substantial reduction in cost.
The basic configuration of the 1kW system from TDK for automatic guided vehicles and logistics robots is shown in Figure 5. The power transfer system consists of an amplifier unit and Tx coil unit, while the power reception system consists of an Rx coil unit and power reception unit. The transfer distance of between the Tx coil and Rx coil which face each other is 20 - 40mm, with a tolerance of ±30mm, as a result of which highly efficient wireless power transfer can be achieved. The feature of the power reception system is that it is extremely compact, and making it ideal for smaller automatic guided vehicles.
The 200W system has been developed for applications such as an industrial robot which works while moving. For this type of industrial robot, the power cable which is laid on the floor spoils the working environment, and also results in the added problem of cable damage. Moreover, the use of power cables also restricts the movement distance.
The 200W system from TDK integrates the amplifier and coil in a compact unit, and hence it can be used in a variety of mobile robots such as the wheel type as well as the caterpillar type. TDK’s 200W system has achieved power transfer distance of 10 - 30mm tolerance of ±10mm, and 88% system efficiency. (Figure 6).
Mobile robots with wireless power transfer are suitable for inspection and monitoring of facilities, and working in harsh environments. A robot contest known as the “ARGOS CHALLENGE” was held by the French National Research Agency (ANR) from 2014 to 2017. The purpose of this contest was to encourage the development of autonomous robots to contribute to oil and gas exploration and other production activities in harsh environments, such as the frigid polar regions or barren deserts. Five teams were selected from all over the world, one of which was the robot known as Team Air-K from Japan, which used the 200W wireless power transfer system from TDK (Figure 7).
Various rotating parts
The 50W system is a wireless power transfer system developed for applications having rotating parts such as a robot arm and surveillance camera.
As shown in Figure 8, in the connection method using a power cable, there was the risk of the cable getting twisted, or entangled with the shaft, and resulting in disconnection. There were also restrictions on the angle of rotation. These problems can be solved by using a slip ring, but it results in the problem of degradation and wear of the contacts of the brush used to send power to the collector ring of the rotating part.
One solution to these problems is wireless power transfer to the rotating part, but the conventional method (Figure 8 right) leads to other problems. Since power is transferred using a high frequency magnetic field, if the shaft is metallic, eddy current is produced, and heat is generated, leading to a drop in efficiency. TDK’s 50W wireless power transfer system for rotating parts has been developed to solve a variety of such problems (Figure 9).
Its internal structure is shown in Figure 10. The structure consists of a power transmission coil unit which is housed in a cylindrical case, and it is enclosed in a larger cylindrical case which houses the power reception coil unit. The power transmission coil and power reception coil are placed on the inner wall of their respective cases. Hence, wireless power transfer is possible to the rotating parts without wrapping cable around them, and as a result, there is no restriction on the angle of rotation.
Additionally, ferrite sheets are fitted on the inside of power transmission side case as well as the outside of the power reception side case to contain the magnetic flux of the coil. The magnetic flux generated from the coils does not leak out because it circulates in a closed magnetic circuit inside the ferrite sheet. Therefore, even if the shaft is metallic, or there are metallic objects nearby, they do not cause problems such as heat generation or drop in efficiency due to eddy current. Furthermore, the design of the circuit board is simple and compact (outer diameter 75mm, height 45mm) because it is integrated in the coil. Therefore it is an optimum wireless power transfer system for rotating parts such as a robot arm and surveillance camera.
In both the electromagnetic induction and the magnetic resonance methods of wireless power transfer, transfer efficiency is greatly dependent on the characteristics of the material used in the core of the power transmission coil and power reception coil. This is because some of the magnetic flux generated by the coils results in core loss which is released as heat. TDK's core technologies originate from ferrite technology which is extensively used in the wireless power transfer systems as well.
As shown in Figure 11, Mn-Zn ferrites, which are heavily used in the transformer core of the switching power supply, generally show valley-shaped core loss - temperature characteristics. For consumer applications, ferrite materials best suited for the rather limited ranges are selected. In case of robots and industrial equipment, however, their use in harsh outdoor environments such as frigid temperatures and scorching heat must be taken into consideration. The Mn-Zn ferrite material PC95 was chosen as an ideal choice from among several proprietary TDK materials. This material is also being used in the transformer core of the DC-DC converters in EVs and HEVs.
PC95 has been manufactured as an extremely uniform and highly dense sintered material, through optimum materials design and strict control of the sintering temperature and environment. It is a ferrite which has extremely flat and low core loss characteristics in a wide range of temperatures. In addition to making the wireless power transfer system for industrial equipment more compact, it also contributes to reducing the power consumption.
In the magnetic resonance method of wireless power transfer, the resonant capacitor also plays an important role along with the coils. Generally, film capacitors are used as resonant capacitors. This is because film capacitors have a good balance of withstand voltage characteristics and relatively high capacitance, and are also advantageous from the price aspect. There are different kinds of film capacitors based on the differences in the dielectric materials, but for the purpose of resonance, the type with a PP (polypropylene) dielectric material is particularly suitable since it has a low tanδ (Tangent delta: dielectric dissipation factor) , and can also support large amounts of current (Table 2).
Tanδ is an indicator of the performance of the capacitor, and its reciprocal is the Q of the capacitor (quality coefficient ). The phase difference in the current of the voltage applied on the capacitors is generally 90°, but due to dielectric loss, and depending on the inductor components in the electrodes, the lag may be more than 90°. The angle δ of this lag is known as the loss angle, and when it is expressed using the trigonometric function tan, it is known as tanδ. The smaller this value, the lower the loss (heat generated), which is a measure of the excellence of the capacitor. The merit of PP is that tanδ for PP is one digit lower than tanδ for PET (polyethylene terephthalate) which is 0.3 - 1%, and it is also stable with respect to change in temperature.
◎: Excellent ○: Good △: Somewhat poor
The lower the value of tanδ (dielectric dissipation factor) , the lower the loss (heat generation), which means that the capacitor is excellent.
The merit of PP is that tanδ for PP is a point lower than the tan δ for PET which is 0.3 to 1%, and it is also stable from the point of view of change in temperature.
Trend in temperature characteristics of tanδ
In terms of miniaturization, MLCCs (multilayer ceramic chip capacitors) are also a feasible choice as resonant capacitors. MLCCs are divided into two major categories according to type of dielectric material used, namely type 1 (temperature compensation type) and type 2 (high dielectric constant type). Since the rate of change of capacitance and hysteresis loss due to temperature is small for type 1 MLCCs, and they also have excellent frequency characteristics, they are used in circuits which require high precision such as resonance circuits, temperature compensation circuits, etc.
However, in recent years, even type 1 MLCCs have been developed with characteristics approaching those of film capacitors, and there is an increasing need to use them to replace film capacitors in in-vehicle devices and industrial equipment. Of these, MLCCs with C0G temperature performance characteristics are especially suited to be used as resonant capacitors because of their extremely stringent standards in the temperature range of -55 to +125°C, where the temperature coefficient = 0ppm/°C and tolerance = ±30ppm/°C. They also support the mounting of resin electrodes and metallic terminals (under the product name MEGACAP) that have excellent resistance to board curvature.
Various core technologies based on the materials technologies, process technologies, evaluation and simulation technologies that have been developed by TDK over the years have been put to maximum use in the wireless power transfer systems. Various electronic components and devices from TDK are heavily used in wireless power transfer systems. In additions to ferrites and capacitors, these include protective elements such as varistors and thermistors, as well as current sensors, lithium polymer ion batteries, etc. (Figure 12).
The introduction of wireless power transfer systems are expected to bring about improvement in convenience, safety, and reliability, as well as labor saving and cost reduction due to automatic charging, even in the field of industrial equipment such as automatic guided vehicles and robots. At TDK, we have newly developed 3 platforms (for 1kW, 200W, and 50W for rotating parts) which can be used to build state-of-the-art wireless power transfer systems for a variety of applications.
TDK's strengths are derived from our comprehensive technical competence which allows us to propose various systems of wireless power transfer, ranging from small to large amounts of power, to suit every application, along with the optimum electronic components and devices that should be used. At TDK, we are continuing to put in efforts to enhance the potential of the magnetic resonance method of wireless power transfer. If you have any projects where the TDK technologies and products presented in this article can be used, please feel free to get in touch with us.
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