16-05-2014, 03:54 PM
CONTACTLESS ENERGY TRANSFER
Abstract
In this paper a new topology for contactless energy transfer is proposed and tested that can transfer energy to a moving actuator using inductive coupling. The proposed topology provides long-stroke contactless energy transfer capability in a plane and a short-stroke movement of a few millimeters perpendicular to the plane. In addition, it is to lerant to small rotations. The experimental setup consists of a platform with one secondary coil, which is attached to a linear actuator and a 3-phase brushless electromotor. Underneath the platform is an array of primary coils that are each connected to a half-bridge square wave power supply. The energy transfer to the electromotor is measured while the platform is moved over the array of primary coils by the linear actuator. The secondary coil moves with a stroke of 18cm at speeds over 1m/s, while up to 33W power is transferred with 90% efficiency.
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.
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
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.
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.
Abstract
In this paper a new topology for contactless energy transfer is proposed and tested that can transfer energy to a moving actuator using inductive coupling. The proposed topology provides long-stroke contactless energy transfer capability in a plane and a short-stroke movement of a few millimeters perpendicular to the plane. In addition, it is to lerant to small rotations. The experimental setup consists of a platform with one secondary coil, which is attached to a linear actuator and a 3-phase brushless electromotor. Underneath the platform is an array of primary coils that are each connected to a half-bridge square wave power supply. The energy transfer to the electromotor is measured while the platform is moved over the array of primary coils by the linear actuator. The secondary coil moves with a stroke of 18cm at speeds over 1m/s, while up to 33W power is transferred with 90% efficiency.
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.
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
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.
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.