INTRODUCTION
Most high-precision machines are positioning stages with Multiple degrees of freedom(DOF), which often consist of cascaded long-and short-stroke linear actuators that are supported by mechanical or air bearings. Usually, the long stroke actuator has micrometer accuracy, while the Submicron accuracy is achieved by the short-stroke actuator. To build a high-precision machine, as much disturbances as possible should be eliminated. Common sources of disturbances are vibrations, Coulomb and viscous friction in bearings, crosstalk of multiple cascaded actuators and cable slabs.
A possibility to increase throughput, while maintaining accuracy is to use parallel processing, i.e. movement and positioning in parallel within section, calibration, assembling, scanning, etc. To meet the design requirements of high accuracy while improving performance, a new design approach is necessary, especially if vacuum operation is considered, which will be required for the next generation no lithography machines. A lot of disturbance sources can be eliminated by integrating the cascaded long-and short-stroke actuator into one actuator system. Since most long-stroke movements are in a plane, this can be done by a contactless planar actuator.
The topology proposed and tested in this paper provides long-stroke contact less energy transfer (CET) in a plane with only small changes in power transfer capability.
ACTUATOR
Actuator is a mechanical device used for moving or controlling a mechanism or system. It converts electrical signals into motion.
Here we are using a linear actuator; it converts electrical signals into linear motion i.e. the movement is linear in manner along a plane.
CET TOPOLOGY
The design of the primary and secondary coil is optimized to get a coupling that is as constant as possible for a sufficiently large area. This area should be large enough to allow the secondary coil to move from one primary coil to the next one without a large reduction in coupling. If this can be achieved, the power can be transferred by one primary coil that is closest to the secondary coil. When the secondary coil moves out of range the first primary coil is turned off and the next one will be energized. To ensure a smooth energy transfer to the moving load, the position dependence of the coupling should be minimized, while keeping the coupling high enough to get a high-efficiency energy transfer.
A lot of systems use 2D spiral coils for the primary and secondary coil, since the spiral coil geometry allows relatively high coupling (upto60%) and some tolerance form is alignment of the coils. However, to allow the secondary coil to move from one primary coil to the next, the tolerance for misalignments should be increased. In the proposed system this is done by using a 3D geometry for the primary coil. This results in a fairly constant B-field around the primary coil, which accommodates good coupling in a large area. Further more, since the system is supposed to transfer power to a load moving in a plane, it is convenient to use a shape that is symmetrical in 2D for both the primary coil and the secondary coil:
a square for instance. The geometry of the primary and the secondary coils are optimized with FEM using Maxwell 3D10 Optimetrics. The resulting geometry of the coils is shown in Fig.1 and 2 and the dimensions are listed in Table 1
The drawing in Fig.3 shows one secondary coil above nine primary coils. The black square shows the area in which the center of the secondary coil can move while maintaining good coupling with the middle primary coil. The secondary coil is situated in the bottom-left corner of the area of interaction with the middle primary coil. The coupling between the primary coil and the secondary coil within that area is calculated with Maxwell 3D 10Optimetrics and measured. The results are shown in Fig.4, which show that the FEM predictions are very close to the measured values. The coupling K is fairly constant within most of the area, only on the outer edges it drops fast. However, the ripple defined by Eq.1 is 25%, which is quite small considering the large displacement of the secondary coil:
Ripple = max (k) - min (k) ·100% (1)
max (k)
Although this system is designed with square shaped coils, it is also possible to design a system with similar characteristics using rectangular coils.
STEADY-STATE ELECTRIC CIRCUIT ANALYSIS
Since the system will be used in a maglev application based on repulsive forces between coils and permanent magnets, the use of iron or ferrites is prohibited. In addition, the use of cores will limit the stroke of the system. Therefore, a coreless or air core inductive coupling is used to transfer the energy. To keep the efficiency of an air core inductive coupling high a resonant capacitor is used for both the primary and the secondary coil. Moreover, due to the position dependent coupling, a series resonant capacitor is used for both coils to ensure that the resonant frequency of the circuit does not depend on the coupling. The electric circuit of the CET system is shown in Fig.5, where V 1 is the RMS voltage of the power supply, I 1 is the RMS current supplied by the power supply, I 2 the RMS current induced in the secondary circuit. C 1and C 2 are the series resonant capacitors in the primary and secondary circuit, R 1 is the resistance of the primary coil, R2 is the resistance of the secondary coil. L 1 and L 2 are the self inductance of the primary and secondary coil, respectively. k is the inductive coupling factor between the primary and secondary coil, and R L is the resistance of the load. The load R L represents the rectifier and additional power electronics.
Simplified versions of the circuit are shown in Fig.6a and b, where Z R is the reflected load of the secondary circuit on the primary circuit and Z 1is the load seen by the power supply.
EXPERIMENTAL SETUP
An experimental setup was built to test the CET design, which consists of an array of three stationary primary coils that are fixed in a row on top of a ceramic structure. The ceramic structure is used to allow heat from the coils to be conducted to the iron base frame and at the same time to prevent eddy current losses in the iron base frame. The primary coils are made of litz wire. Each bundle of litz wire consists of 60 strands of 71 µm and the strands are wrapped together with a layer of cotton. The strand size has been chosen after examining the AC losses. The turns of the coil are fixed by glue that has been applied during the winding process. Approximately 120 turn fitted in the cross-section, resulting in a 0.3 filling factor.
Each primary coil is connected in series with a resonance capacitor. Each resonant circuit is driven by a separate half-bridge power supply that applies a square wave voltage of 191 kHz over the resonant circuit. The schematic of the half-bridge power supply is shown in Fig. 7. An overview of the primary coils and the corresponding series capacitors is shown in Table II. The secondary coil is fixed onto a ceramic plate that is bolted to the mover of a linear actuator. Again ceramic material is used for heat conduction and the minimization of eddy current losses. The linear actuator can move the secondary coil over the three primary coils. The position of the secondary coil with respect to the array of primary coils is measured by the encoder of the linear actuator. A picture of the experimental setup is shown in Fig. 8.
The secondary coil is connected in series with a resonant capacitor. The circuit is then connected to a full-bridge diode rectifier to generate a DC output. The DC output of the rectifier is connected to the load, which is an electromotor of
a CD drive running at 12 VDC as shown in Fig. 9.
All subsystems are connected to a ds1103 dSpace system running the control program at 8 kHz. This way the DC bus voltage of the primary coil power supplies is controlled as well as which of the primary coil power supplies is enabled. The position of the linear actuator is controlled using a PID controller running on the dSpace system. Depending on the position of the linear actuator the dSpace system enables the primary coil that is completely overlapped by the secondary coil.
The primary coil activation is controlled by a multi-port switch. The multi-port switch has four active coil states; state1 enables the power supply of the first primary coil, state 2 and 3 enable the power supply of the second and third primary coil, respectively. State 4 disables all power supplies and this state is used for switching from one power supply to the next. When the secondary coil moves out of range of primary coil 1 (active coil state 1), the active supply is switched off (active coil state 4) and one sample time later the second supply is switched on (active coil state 2). For one sample time none of the power supplies is active (active coil state 4), which is necessary to allow the triac in the power supply that is switched off (see Fig. 7) to block the circuit after the current in the resonant circuit is damped. There is no other control mechanism in the power electronics, and the system operates without any measurement on the secondary site, except for the position of the secondary coil
RESULTS
An electromotor of a CD drive that runs on 12 VDC is connected to the rectifier. The voltage and current from the DC bus supply as well as the voltage and current to the CD drive are measured and shown in Fig. 10 and 11. The secondary coil is moving over all three primary coils following a sinusoidal position reference, which represents a total displacement of 18 cm (i.e. the amplitude of the sine wave is 9 cm). The frequency of the sinusoidal position reference is 2 Hz, so in one second the secondary coil makes two cycles (one cycle implies moving from primary coil 1 over primary coil 2 to primary coil 3 and back). The cycle is clearly visible from the Active Coil plot in Fig. 10 and 11, which represents the state of the active coil multi-port switch. The secondary coil reaches a maximum speed of 1.1 m/s over the second primary coil. Due to this speed the secondary coil is in range of the second primary coil for only 60 ms.
By calculating the RMS values of the voltages and currents
the power from the DC bus supply Pin as well as the power to the CD drive load Pout and the efficiency η according to Eq. 14 can be calculated. This calculation includes losses in the power electronics. The values are listed in Table III.
In Fig. 12, the transient behavior is shown when the secondary coil is moving from primary coil 1 to primary coil 2. It is clearly visible that all power supplies are switched off when the active coil state has value 4. There is also some delay between the active coil state switch and the response from the power electronics, which is caused by a slow rising edge of the enable signal and by delay in the power electronics. In Fig. 10 and 11 the switching is also visible in the current waveforms, since no current is drawn from the DC bus supply and no current is available for the electromotor of the CD drive.
The ripples visible in the voltage and current waveforms
from the DC bus power supply and to the CD drive are related to the changing coupling. However, since the CD drive does not represent a purely resistive load, the ripple is somewhat smoothed by the inductance of the load. This effect is more visible when a purely resistive load will be connected to the system. In addition, the CD drive does not need much power to operate and a resistive load can be operated at higher power levels. Therefore, a 50 Ω resistive load is used at a higher power level. The same trajectory is used for the secondary coil. The measured voltage and current waveforms of the DC bus supply and the load are shown in Fig. 13 and 14 respectively. The RMS values of voltage, current and power as well as the efficiency are shown in Table IV.
The variation in coupling is now clearly visible in the current and voltage waveforms of the load. This suggests that the power transfer can be further smoothed by measuring the coupling and changing the voltage of the DC bus supply accordingly. The results are very similar to the results of the CD drive. Higher power levels have not been tested using the linear actuator, since the capacitors in the resonant circuit cannot operate above 800 V. Operating at higher power requires new capacitors which have not been realized yet. It is expected that power transfer up to 300 W is feasible.
CONTACTLESS ENERGY TRANSFER
A Better Solution for a Mobile World
Talk to any plant engineer or production system designer and you’ll find that electrical wiring is the bane of their existence. From installing the wires, to rewiring as production lines need to be changed, to repairing damage caused by careless workers, electrical wires represent an ongoing cost and risk for downtime in manufacturing plants. Until recently, the miles of electrical wiring that snake around any manufacturing facility, hanging down from ceilings and extending across corridors between equipment, have been viewed as a necessary aspect of industrial automation. But today industry is moving toward a wireless world. Like consumers with their cell phones, laptops and PDA’s, industrial companies want wireless technologies that improve versatility, reduce costs and maintain connectivity. One of the latest developments to draw interest among engineering personnel is contactless energy transfer for powering and controlling motors. While wireless communication is now common in factories, wirelessly transferring 16kW of electricity through the air to power equipment is a relatively new phenomenon in the United States
In a typical automated manufacturing environment, where carts full of parts must be moved between the different stages of a production process, a contactless system transfers electrical energy inductively from an insulated conductor in a fixed installation to one or more mobile loads. Electromagnetic coupling is realized via an air gap, so it is not subject to wear and costly maintenance. Contactless energy transfer reduces costs in several ways: It eliminates festooning or overhanging utilities. The underground wiring is compact and poses no trip hazards. There is no carriage to run out on the shop floor. There are also no pits to be dug to drop in trailing utilities.
In addition to lower costs, a mobile system using contactless energy transfer provides greater versatility: The contactless system enables more flexible track layout with curves and switches, simple segmentation of tracks, which makes it easy to extend a track or change travel directions, and higher speeds.
APPLICATIONS
Contactless energy transfer is ideal for applications where:
• The mobile equipment has to cover long distances
• A variable, extendable track layout is required
• High speeds have to be achieved
• The energy transfer has to be maintenance free
•Additional environmental contaminants are not permitted in sensitive areas
• The operation takes place in wet and humid areas
Maintenance and ambient conditions are important factors in constructing systems for material handling and transportation applications, such as automotive assembly, storage and retrieval logistics and sorting. Typical applications that could benefit from
Contactless energy transfer includes:
• Overhead trolleys
• Conveyor trolleys
• Guided floor conveyors
• Push-skid conveyors
• Storage and retrieval units
• Pallet transportation systems
• Baggage handling
• Panel gantries
• Elevator equipment
• Amusement park rides
• Battery charging stations
By replacing a drag-chain system in a conveyor trolley that transports and sorts pallets, for example, contactless energy transfer enables pallets to transverse over longer distances. Complicated holders for drag chains are eliminated, as is downtime for repairing cable breaks and battery charging. Repairs for wear from bending or torsion are also eliminated. The wear-free power supply in a contactless system has many advantages in designing and maintaining push-skid conveyors used in automotive assembly, for example, or in storage and retrieval units in a high-bay warehouse. Because there is no conductor rail, there is no danger of introducing contaminants from system leakages and no components that are difficult to reach for maintenance. Problems with fitting the platforms into conveyor belts are also eliminated, since there’s no need for high mechanical tolerances between the line cable and pick-up.
Perhaps the biggest advantage of a system based on contactless energy transfer is higher system availability because the system is essentially maintenance free. In a manufacturing environment where change is a constant and speed is an imperative, the versatility, flexibility and reliability of contactless energy transfer systems can reduce the wear-and-tear on plant engineers as well as equipment.
CONCLUSION
A new topology for contact less energy transfer (CET) to a moving load has been proposed, built and tested. The CET topology allows for a long-stroke movement in a plane and a short-stroke movement of a few millimeters perpendicular to the plane. In addition, it is tolerant to small rotations. The power electronics consist of a half-bridge square wave power supply for each primary coil and series resonant capacitor and a full-bridge diode rectifier at the load.
Power transfer up to 33 W with resistive load of 50 Ω has been demonstrated The CET system was used to power a 3-phase brushless electromotor of a CD drive and showed stable power transfer of 3.44 W. The power was transferred at approximately 90 % efficiency, while the secondary coil was moving with speeds up to 1.1 m/s over the primary coils
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