The present application is based on and claims the benefit of U.S. patent application Ser. No. 14/948,702 filed on Nov. 23, 2015 which is based on U.S. patent application Ser. No. 13/261,259 filed on Jul. 30, 2012, which is a Section 371 application based on International Application Number PCT/NZ2010/000203 filed on Oct. 12, 2010, which claims priority from New Zealand Application Serial No. 580388 filed Oct. 12, 2009 and the entire contents of each of which are herein incorporated by reference.
This invention relates to Inductive Power Transfer (IPT) and has particular, but not sole, application to the provision of an AC power source. The invention may also be used to provide a DC power source.
IPT systems are now widely used in industry and elsewhere to couple power from one reference frame to another without physical contact. An example of such a system is described in specification U.S. Pat. No. 5,293,308, the contents of which are incorporated herein by reference.
IPT technology allows large amounts of electrical energy to be transferred between two loosely coupled inductors over relatively large air gaps. An IPT system can be divided into two sections—a primary supply and one or multiple secondary pickups. The, or each, pickup receives power inductively from the primary. For an IPT system used in material handling applications, multiple secondary pickups are coupled on one long track as shown in
In order to improve power transfer capacity in the IPT system, some compensation or tuning capacitor is required in the secondary pickup. The two most common compensation topologies used in the pickup are parallel and series tuned systems as shown in
One technique is to use primary side control to achieve voltage regulation on the secondary pickup. This method sends feedback signals such as output voltage of the secondary pickup back to the primary converter via a wireless communication channel. Generally, primary side control has two possible methods of realization—frequency control or primary current control.
For applications such as material handling systems with multiple secondary pickups, control on the primary side cannot be used since regulating voltage on one pickup will affect the operation of other pickups which may be operating at different power levels. One conventional method to regulate the output voltage on the secondary side is to use a linear voltage regulator after the pickup. However, due to the tolerance of the output voltage of the pickup and the poor efficiency of the linear regulator, this topology is limited to low power applications. Another method cascades a buck converter after the series tuned pickup to regulate the output voltage with more electrical efficiency. However, this is not ideal because of the large number of components required which increase cost. In addition, the two stage (AC-DC and DC-DC) conversion process has losses in each stage which reduce efficiency. Other secondary side control techniques directly regulate power on the AC side to deliberately tune or detune the resonant tank circuit by adding extra reactance. One technique to realize a variable reactance component is to use a magnetic amplifier to produce a variable inductor. Although this may vary the AC power directly, the use of a variable inductor in the non-linear region of the B-H curve can limit the efficiency of the overall system. In addition, the variable inductor is expensive to manufacture because it has to manage the high resonant current without fully saturating.
It is an object of the invention to provide an IPT system that provides an AC power source, or to at least provide the public with a useful choice.
In one aspect the disclosed subject matter provides a method of providing a power supply from IPT pickup having a pickup coil and tuning capacitor connected in series to provide a series resonant circuit, the method including the step of varying the phase angle between the open circuit pickup coil voltage and the pickup coil inductor current to provide a controlled AC supply to an output of the pickup.
In one embodiment the AC supply at the output is rectified to provide a DC supply at a further output.
In one embodiment the phase between the pickup coil open circuit voltage and the pickup coil inductor current is varied by substantially preventing current flow in the resonant circuit for a selected time period.
In one embodiment the selected time period is varied to vary the phase angle.
In one embodiment the step of substantially preventing current flow includes detecting when the current in the resonant circuit is substantially zero and maintaining the current at substantially zero for the selected time period.
In one embodiment the current is substantially prevented from flowing by operating a switch. In one embodiment the switch comprises a bidirectional switch.
In one embodiment the method includes the step of comparing the output of the pickup with a reference, and increasing or decreasing the selected time period to change the output of the pickup toward the reference.
In another aspect the disclosed subject matter provides a controller for an IPT pickup having a pickup coil and a tuning capacitor connected in series, the controller including one or more switches to control the pickup coil inductor current to thereby vary a phase angle between the pickup coil open circuit voltage and the pickup coil inductor current.
In one embodiment the phase between the pickup coil open circuit voltage and the pickup coil inductor current is varied by operating the one or more switches at a selected time to substantially prevent current flow in the resonant circuit for a selected time period.
In another aspect the disclosed subject matter provides an IPT pickup comprising a pickup coil and a tuning capacitor connected in series to provide a series resonant circuit, and a controller to vary a phase angle between the pickup coil open circuit voltage and the pickup coil inductor current to thereby provide a controlled AC supply to an output of the pickup.
In one embodiment the phase between the pickup coil open circuit voltage and the pickup coil inductor current is varied by the controller substantially preventing current flow in the resonant circuit for a selected time period.
In another aspect the disclosed subject matter provides an IPT pickup comprising a pickup coil and a tuning capacitor connected in series to provide a series resonant circuit, and switch connected in series with the resonant circuit, the switch being operable to vary a phase angle between the pickup coil open circuit voltage and the pickup coil inductor current to thereby provide a controlled AC supply to an output of the pickup.
In one embodiment the switch comprises a bi-directional switch. A controller may be provided to control operation of the switch.
In yet another aspect the disclosed subject matter provides an IPT system including an IPT pickup according to any one of the preceding statements.
The invention may also be said broadly to consist in the parts, elements and features referred to or indicated in the specification of the application, individually or collectively, in any or all combinations of two or more of said parts, elements or features, and where specific integers are mentioned herein which have known equivalents in the art to which the invention relates, such known equivalents are deemed to be incorporated herein as if individually set forth.
An embodiment of the invention will be described by way of example with reference to
A new type of AC processing pickup illustrated herein exhibits excellent features such as simple circuitry, lower production cost and very high efficiency operation.
This specification discloses a new series AC processing pickup that uses an AC switch operating near ideal soft switching operating conditions to regulate the output voltage of the pickup directly. The output can be either controlled AC or DC depending on whether a rectifier is added to the AC output at the end of the resonant network.
According to one embodiment of the invention a series AC processing pickup is shown in
To illustrate the circuit's operation,
The series AC processing pickup also achieves near ideal soft switching conditions. From
Analysis
From the previous section, it can be seen that the phase shift between Voc and IL can be controlled by adjusting the phase delay ϕ. In this section, the phase delay ϕ is used in an exact analysis in the time domain to determine the characteristics of the circuit under steady state operation. The basis of the analysis method is that the conditions existing in the circuit at the end of a particular switching period must be the initial conditions for the start of the next switching period, and these conditions must be identical, allowing for steady state resonant operation.
The analysis procedure is greatly simplified based on the three following assumptions:
Assuming the resonant tank is perfectly tuned,
C2=1/(ω2L2) (1)
With reference to
A. Resonant State
During the resonant state, the inductor current may be described as:
Considering the initial condition iLr(t)|t=0=0 and
the complete solution of the above equation is:
In a similar way, considering the initial condition
the complete solution to the capacitor voltage is:
To investigate how long the circuit stays in the resonant state, iL(t)=0 can be substituted in (3), resulting in the following expression:
iLr(tz)=0 (10)
where tz is the time the circuit operates in the resonant state.
B. Discontinuous State
During the discontinuous state, the series resonant circuit becomes an open circuit and the capacitor voltage remains constant while the inductor current is zero.
Vcd(t)|t=0=Vc(tz) (11)
iLd(t)=0 (12)
Because the resonant state and the discontinuous state are repeated each half cycle (with only a polarity change), the relationship Vc(0)=−Vc(T/2) must hold. Hence, the capacitor voltage and inductor current are given by,
Fourier analysis can be performed on the inductor current waveform to compute the harmonics. The in-phase and quadrature components of both the fundamental and harmonics are given by:
It is important to determine the amount of power sourced from the primary IPT power supply for the pickup to operate. If harmonics are ignored, the real and reactive power sourced from the primary supply are given by:
C. Computation Routine
The above analytical analysis is very difficult as the solution of tz and Vc(0) are governed by (13) and (14) with θv and θi as interim variables which are associated with the auxiliary equations (7) and (9). This is in the form of transcendental equations that can only be solved using numeric solvers such as MATLAB or EXCEL. A computer program based on an iterative computation, shown in
D. Rectifier Load Modelling
The series AC processing pickup can output a controlled DC voltage by adding a rectifier with a large DC filter capacitor. The output voltage is maintained at a DC level with the high frequency AC component removed. As a result, the AC voltage at the input of the rectifier becomes a rectangular waveform with an amplitude of the DC output voltage and two diode forward voltage drops from the rectifier. If only the fundamental component is modelled and the harmonic components are ignored, the RMS value of the rectangular voltage is given by:
Assuming that Q2 of the circuit is relatively high during normal operation, the input current to the rectifier can be approximated by half sinusoids with discontinuous sections in between. Then the RMS AC current is related to the rectified DC current by:
Therefore the equivalent AC load is:
The equivalent resistor can be used in (4) in the AC analysis described in the section above to compute the operating waveforms of the series AC processing pickup with a rectifier load.
Pickup Characteristics
The output current (or inductor current) characteristics of the pickup are shown in
The normalized output voltage is shown in
The output voltage-current characteristic is shown in
The normalized reflected impedance characteristic is shown in
Design
In this section, the design of a 1.2 kW series AC processing pickup according to the circuit of
A. Implementation of the AC Switch
The AC switch, shown in the embodiment of
One key requirement of practically realizing the AC switch is to allow the switches to operate on simple gate drive waveforms. In the description of circuit operation provided with respect to
B. Snubber Design
A simple RC snubber was designed to damp the high frequency resonant oscillations caused by the pickup inductance and parasitic output capacitance of the IGBT. This is required because although an IGBT with low reverse recovery charge was chosen to reduce the transient generated from the natural turn off of the body diode, when the current tries to flow in the opposite direction, the remaining voltage oscillation is still significant. This oscillation arises because the reverse recovery current of 3 A becomes the initial inductor current of an LC resonant circuit comprising the pickup inductance of 115 uH and the switch output capacitance of 130 pF. A large over shoot of more than 80% of the steady state value appears across the switch due to the resulting resonant oscillations. A snubber capacitance of 2.2 nF was connected across each switch in the usual manner.
C. Component Stress
To calculate the maximum rating conditions for the components, the phase delay ϕ has to be set slightly above zero degrees in order to observe the peak voltage across the switches and maximum RMS rating for the capacitor and inductor. The calculated peak and RMS value of the voltage and current for the capacitor, inductor and switch are listed in Table I. It can be concluded from Table I that the switches have to be rated for both 310V and 22 A at normal operation. However, an overshoot of 10% is still possible from the snubber design, so the switch rating should be greater than 350V. In practice a 500V device may be used.
D. Controller
A practical system setup with controller for the AC processing pickup is shown as a block diagram in
Experimental Results
The AC processing pickup as described above was coupled to a small section of track (
A rectifier and a 2 mF filter capacitor are added at the output of the pickup to output a controlled DC voltage. The pickup is operated with a closed-loop controller where the output voltage is set to a desired value in the microcontroller. The microcontroller is configured to maintain the desired load voltage by adjusting ϕ in accordance to the feedback of the output voltage.
An efficiency vs. output power plot is shown in
This document discloses a new IPT pickup having a series resonant circuit in which power is processed on a cycle by cycle basis in AC (i.e. without rectification being necessary) and which can produce a variable AC output voltage into a load or a rectifier filter and load combination for controlled DC. This eliminates the bulky DC inductor required in the traditional series tuned controllers that use a buck converter to produce a DC controlled output. In addition, the pickup operates under near ideal soft switching conditions which give the pickup a very high efficiency. Although this pickup reflects back VAR's back onto the primary track, the overall stress imposed on the primary power supply is relatively small. The circuit operation has been theoretically analyzed and experimental results have verified the proposed design procedure. The AC processing pickup can be controlled over a wide load range for a 1.2 kW system and a maximum efficiency of 96% was obtained.
Although certain examples and embodiments have been disclosed herein it will be understood that various modifications and additions that are within the scope and spirit of the invention will occur to those skilled in the art to which the invention relates. All such modifications and additions are intended to be included in the scope of the invention as if described specifically herein.
Number | Date | Country | Kind |
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580388 | Oct 2009 | NZ | national |
Number | Name | Date | Kind |
---|---|---|---|
5293308 | Boys et al. | Mar 1994 | A |
5701121 | Murdoch | Dec 1997 | A |
6621183 | Boys | Sep 2003 | B1 |
20070109708 | Hussman | May 2007 | A1 |
20110101790 | Budgett | May 2011 | A1 |
20110221277 | Boys | Sep 2011 | A1 |
20120217111 | Boys | Aug 2012 | A1 |
Entry |
---|
James J. Et Al: “A Variable Inductor Based Tuning Method for ICPT Pickups” , Faculty of Engineering Papers, The University of Auckland, 2005, pp. 1-6 (not numerated). |
Hu, A. P. Et Al: “A New High Frequency Current Generation Method for Inductive Power Transfer Applications” , 37th IEEE Power Electronics Specialist Conference 2006, PESC ' 06, Jun. 18-22, 2006, p. 1-6. |
International Preliminary Report on Patentability of International Application No. PCT/NZ2010/000203. |
Number | Date | Country | |
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20190006883 A1 | Jan 2019 | US |
Number | Date | Country | |
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Parent | 14948702 | Nov 2015 | US |
Child | 15946539 | US | |
Parent | 13261259 | US | |
Child | 14948702 | US |