BACKGROUND OF THE INVENTION Inductive power transfer (IPT) systems operate by using an elongated primary loop or track supplied with an alternating current at VLF frequencies-typically in the range 5-50 kHz. In practical situations a multiplicity of pick-up coils are magnetically linked to this track and tuned to operate at the track frequency. In this way power can be transferred from the loop to the pick-up coils. This power at system frequency is rectified and is then available for tasks as required.

THE PROBLEM IPT in this form has a number of associated problems in that the power must be controlled and power flow or lack of power flow to one pick-up coil must not compromise power flow to the other pick-ups. This problem has been solved by our previous patent granted in a number of countries and published under the publication number WO 92/17929 where the concept of decoupling is used. Here when a pick-up is decoupled no power flows to it or from it but power flow to the other pick-up coils is not compromised. This very simple decoupling idea may be implemented in a variety of ways but it is characterised by the observation that the average power transferred is directly linearly proportional to the fraction of the time that the system is coupled for, and that the power transfer can be reduced to zero. The technique may be used with parallel or series tuned circuits and may be implemented with or without an auxiliary coil.

In practice for parallel tuned pick-up systems decoupling is most conveniently achieved using the circuit illustrated in Figure 14 of our previous patent publication WO 92/17929, the disclosure of which is incorporated herein by reference. For convenience, Figure 14 of WO 92/17929 is reproduced in this specification as Figure 1 and a brief explanation of that figure is provided further below.

The"decoupling"solution that is disclosed in our previous patent is a practical technique for IPT systems and is being implemented widely. There are however a number of

problems that it does not solve. A system with perhaps 10 1 kW pick-up coils all operating at say 50% of rated power requires an average power of but 5 kW but the power supply must be able to supply 10 kW as there is a 0. 1% chance that the actual power demand will instantaneously be 10 kW. This position is clearly much worse for a larger system where again the peak power from the power supply must be much larger than the average power demand. This situation arises as the IPT track power supply is generated by power electronics means and is necessarily power limited. In many power supply circuits there may also be a track current limit where if the resistive load on the power supply is too high the power supply will cease to operate-for example if the track inductance is part of a resonant circuit then excess load may cause the resonance to cease.

Similarly when an unpowered system is first turned on the effect is for all pick-up coils to start coupling power to charge their DC busbar capacitors and the load on the power supply is the maximum possible load. A system with 100 1 kW coils will take 100 kW on start-up but in operation at an average 500 W per pick-up the average power will be 50 kW and the standard deviation of that power flow will be 5 kW so that powers of greater than 65 kW occur less than 0.5% of the time but the power supply must be designed for 100 kW.

The power supply over-rating comes about as the supply is current limited. Conventional voltage power supplies do not suffer from this and simple fuses will allow momentary overloads without difficulty.

With an IPT system the power supply and the track represent major expense items so reducing their size is of great importance.

OBJECT OF THE INVENTION It is an object of the present invention to provide an improved IPT system and/or improved control of power drawn by IPT system pick-up loads, or to at least provide the public with a useful choice.

A further object of the invention includes providing a power supply for an IPT system, which power supply may be sized so as to be much closer to the average power requirement for the system without comprising system security.

SUMMARY OF THE INVENTION In one aspect the invention may broadly be said to consist in an inductive power distribution system including an electric power supply a primary conductive path connected to the electric power supply at least one electric device for use in conjunction with the primary conductive path, the device being capable of deriving at least some power from a magnetic field associated with the primary conductive path, the device including at least one pick-up means comprising a resonant circuit having a pick-up resonant frequency, at least one output load capable of being driven by electric power induced in the pick-up means, switch means operable to reduce the power transferred from the primary conductive path to the pick up means, control means to operate the switch means to control the transfer of power from the primary conductive path to the pick-up means, and the control means being operable to limit the power transferred from the primary conductive path.

Preferably the control means operates the switch means at a substantially high predetermined frequency.

Preferably the control means has a predetermined limit to limit the mark-space ratio for operation of the switch means to a predetermined minimum or maximum.

Preferably the system includes communication means to enable the limit on the pick-up or selected pick-ups to be varied or changed as required.

In a further aspect the invention may broadly be said to consist in an pick-up for an inductive power transfer system, the pick-up including resonant pick-up means,

control means for controlling the power transferred from a primary conductive path of the system to the pick-up means, the control means including a limiting means to limit the maximum power transfer from the primary conductive path to the pick-up means.

Preferably the limiting means is selectively actuable.

Preferably the control means comprises a microprocessor.

Preferably the limit is substantially a predetermined limit.

Alternatively, the limit may be changed dynamically during system operation.

Preferably the pick-up includes communication means to at least receive information as to the desired power transfer limit.

Preferably the limiting means is activated upon predetermined events, such as system start up or excessive load on the supply to the primary conductive path.

In a further aspect the invention may broadly be said to consist in a control circuit for a pick-up for an inductive power transfer system, the control circuit being operable to control the transfer of power from a primary conductive path of the system to a resonant pick-up, the control circuit including amplifier means to amplify the difference between the pick-up load voltage and a reference voltage, wave form generating means, comparator means to compare the output of the amplifier means with the wave form generated by the wave form generating means to thereby provide an output signal at a substantially constant frequency having a mark-space ratio which varies dependent upon the load voltage.

Preferably the wave generator produces a triangular wave form at a substantially high frequency comparable with the track frequency.

Preferably the output signal comprises a pulse width modulated signal.

Preferably for a simple linear controller a first lower load voltage is chosen to correspond to a substantially decoupled pick-up and a second high load voltage is chosen to correspond to a fully coupled condition, and in between these voltages the pulse width modulation mark-space ratio varies in direct linear proportion.

Preferably the mark-space ratio may be restricted to limit the maximum possible power flow from the primary conductive path to the pick-up.

Preferably the maximum power flow is limited by limiting the mark-space ratio.

Preferably the maximum power flow is limited at a particular time, for example when the pick-up is switched on or when the system is switched on, or when it is otherwise desirable to limit current flow.

To those skilled in the art to which the invention relates, many changes in constructions and widely different embodiments and applications of the invention will suggest themselves without departing from the scope of the invention as defined in the appended claims. The disclosure and descriptions herein are purely illustrative and are not intended to be in any sense limiting.

DRAWING DESCRIPTION The invention consists of the foregoing and also envisages constructions of which the following gives examples only.

One presently preferred embodiment of the invention will now be described with reference to the accompanying drawings, wherein ; Figure 1 is a circuit schematic for a pick-up for an IPT system and corresponds with Figure 14 of WO 92 17929, Figure 2 is a circuit schematic for the pick-up of Figure 1, but with an alternative form of control circuit, and Figure 3 is a circuit diagram for the control circuit of Figure 2.

DETAILED DESCRIPTION As described in the introduction above, Figure 1, which is Figure 14 of our original patent published under the number WO 92/17929 shows the most convenient general form of circuit for practical decoupling of parallel tuned pick-up systems to control power flows.

Referring to that figure, the circuit is briefly explained for purposes of clarity. Pick-up coil 101 has a tuning capacitor 102 connected in parallel across that coil. In use, the pick- up coil 101 is located adjacent to a primary conductive path (not shown) so as to be capable of receiving power derived from a magnetic field associated with the primary conductive path. The voltage across coil tuning capacitor 102 is rectified by a diode bridge 103 from which it passes through inductor 104 and through diode 106 to output capacitor 107 across which load 108 is connected. Comparator 109 monitors the DC output voltage on capacitor 107 and compares this voltage against a reference voltage 111.

If the load power is less than the maximum power able to be sourced from the pick-up coil, then the capacitor voltage will increase. This will cause the comparator to turn on switch 105, thereby effectively shorting the tuning capacitor 102. Diode 106 prevents the DC output capacitor from also being shorted. The result is that the power transferred to the pick-up coil from the primary conductive path is virtually zero. Consequently, the DC voltage cross capacitor 107 will decrease until the point where the comparator will turn off the switch again. The rate at which the switching occurs is determined by the hysteresis of the comparator, the size of the capacitor 107 and the difference between the load power and the maximum coil output power.

Therefore, here when the switch 105 is"on"the system is decoupled and when the switch 105 is"off'the system is coupled. Preferentially switch 105 is operated at a slow frequency switching from the coupled to the decoupled state to control the average power flow to match the load. This system is preferred as the system dynamics are then unimportant and the controller as described is the simplest and lowest cost solution.

However, another option for controlling switch 105 is to operate the switch at a substantially constant frequency, but vary the mark-space ratio of the switching cycle in order to control the flow of power to the pick-up. In particular, the switching frequency may be chosen to be a frequency which is higher than the resonant frequency of the pick- up. At these much higher switching frequencies a much more sophisticated controller is needed as the system dynamics cannot be ignored. In these conditions switch 105 is operated at the much higher frequency, say at 25kHz for example. When switch 105 is

'on'the system is decoupled and no power flows to the output capacitor and vice versa.

But the'on'and'off times for the switch are so short that the pick-up itself, in the sense of the resonant circuit, does not completely decouple or couple before the switch changes state. Under these conditions the resonant circuit operates at a stable voltage somewhere between the maximum voltage and zero voltage and power flow from the primary conductive path to the pick-up coil is continuous. The same characteristics as with the controller described with reference to Figure 1 still apply; the power flow to the load is essentially linearly proportional to the percentage of time that the switch is"off' (coupled), and the system can be completely decoupled so that there is no power flow at all.

The decoupling circuit of Figure 1 is suited to high frequency switches, but the controller of Figure 1 is not suitable for this purpose. A suitable controller, in conjunction with the decoupling circuit of Figure 1, is illustrated in Figure 2 and described with reference to that figure. Referring to Figure 2, it will be seen that the operational amplifier 209 previously used as a comparator is now used as an amplifier, reference voltage 211 and resistors 210,210, 212 and 213 are still retained. In addition there is the simple addition of a wave generator 215 and a further comparator 214. The wave generator is preferably a triangular wave generator. In accordance with the high frequency example of 25kHz switch operation outlined above, the triangle wave generator 214 will be chosen to provide a triangle wave at a frequency of 25kHz. The triangle wave is then compared by comparator 214 with the output of the comparator. This produces a pulse width modulated (PWM) output which is used to operate switch 205.

As outlined above and described in more detail below, this simple circuit uses a proportional gain amplifier, but a more complex system with proportion-integral control is also easy to implement.

In Figure 3, one example of a circuit which puts the schematic of Figure 2 into effect is shown, and will now be described. Referring to Figure 3, the input to the circuit is derived from the +300 V DC to supply line 301 and a local ground corresponding to the DC output voltage of the pick-up system.

The circuit power supply at 301 is derived directly from the 300 V through dropper resistors, thus ensuring that the circuit supply line follows excursions of received supply voltage.

From this supply a regulated voltage from resistor 302 and zener diode 303 is supplied to one input of operational amplifier 304 and a reduced voltage via resistor 305 to the other input. The output of the operational amplifier will therefore rise and fall with the supply voltage. In practice the circuit operates such that if the input voltage is 288 V the output of op amp 304 is +8V and if the input voltage is 312 V the output is +2 V.

A triangle wave generator 306, operating at about 25KHz supplies one input of op amp 307, whose other input is the varying output from op amp 304. Op amp 307 amplifies the triangle wave to approximate a square wave, but its operating point is biased up and down the triangular wave. This results in a varying mark-space ratio in the output waveform providing, effectively, pulse width modulation, which acts to vary the switching period of switch 205 and thus maintain the output voltage constant.

If the DC supply is greater than 312 V then, with the circuit shown, the output will be permanently high and if it is less than 288 V DC it will be permanently low.

Operationally this means that if the supply voltage is too great the switch will turn completely on until the output voltage collapses to within range again. Similarly if the DC supply is too low the output will be permanently low and the switch will be permanently open, directing all output to the output circuit. Between 288 and 312 volts the circuit switches'on'and'off rapidly in a PWM fashion so that it couples and decouples at a 25 kHz rate providing essentially continuous control of power flow to the DC capacitor.

In the design of the controller for this circuit there are several issues. Energy can be stored in the tuned circuit of the pick-up, in the DC inductor, and in the output capacitor.

These stored energies and the way that they change make the controller design complex.

Here a simple proportional controller has been used. With a pick-up coil of 211 pH and a tuning capacitor of 1.2 u. F (for resonance at 10 kHz), a DC inductor of 3.1 mH, and a DC capacitor of 940 u. F the pick-up system has excellent stability with the proportional gain as outlined giving a range of zero to full power from 312 to 288 V DC.

Another example for implementation of a control circuit where switch 205 is switched at a substantially constant frequency is to use a micro-controller. In this example, the comparator which compares the DC output voltage with the reference voltage is now used as a prescaler to the microprocessor to prescale the input voltage from the DC bus bar voltage to the range where 280 to 320 volts corresponds to, for instance, 0-5 volts. The

micro-controller includes an A/D converter that measures this input voltage and outputs a PWM signal such that 280 volts corresponds to completely decoupled and 320V corresponds to fully coupled and in between the PWM mark-space ratio varies in direct linear proportion.

In situations where the power must be controlled-for example on switch on-the mark- space ratio can be restricted to say 10% thereby limiting the maximum possible power flow to 10%, or to such figure as the system designer chooses. This presettable mark- space ratio can be manually preset or it can be dynamically changed so that the maximum possible power taken by a pick-up system changes depending on where the pick-up system is in the IPT system. Such dynamic control will require a communications radio channel, infrared link or inductive link via the IPT power cable and is clearly most easily implemented with a micro-controller using the serial input channel to change the maximum power as required.

Thus the maximum possible mark-space ratio may be varied from the power supply by communicating to the pickups on the system as the power supply approaches peak rating to reduce the maximum mark-space ratio and thereby reduce the power load on the supply.

In this fashion there need be no power surge on switch on, and in the running condition the IPT power supply drives a system where the fluctuations in power are minimal and controlled. For the same 100 pick-up coils system the power required would be 50 kW and the power supply could be safely designed to have a maximum capacity of 55 kW with no fear of an overload condition. Such a power supply may well be almost 2 times lower cost than a power supply for an IPT system with conventional pick-up coils switching (coupling and decoupling) at a low frequency.

It will be seen that a micro-controller enables limits for the mark-space ratio to be easily preset, or dynamically changed, particularly if a communication interface is used.

However, it will also be seen that a communication interface does not necessarily have to be used. In situations where the pick-ups are attached to vehicles to supply power to motors or other appliances that enable the vehicles to operate or perform functions, appropriate triggers could be mounted on the track on which the primary conductive path is located, and sensors could be used on each vehicle to implement the power flow limitation. For example, the trigger could be a projection or an indentation on the track, or could instead be a small transmitting device on or adjacent to the track that provides an

appropriate signal that the pick-up can receive and then use to implement the power flow limitation.

Furthermore, the circuits of Figures 2 and 3 may also have selective power flow limitation provided. Therefore, for example with reference to Figure 3, the resistor network about amplifier 304 could include appropriate transistors or other electronic switches which switch resistors in or out to change the output signal of amplifier 304. For example, the resistor network could be switched to change the output of U1 so that the output does not exceed say 7.5 volts. In this way, the output of 307 will never be switched on continuously. This will prevent full power being transferred to the pick up coil. Of course, a variety of different maximum output voltages for 304 could be chosen, each successively lower output voltage providing a lower maximum power transfer to the pick- up coil.

VARIATIONS The switching frequency is preferably not related to the power system frequency by an integer factor to reduce the generation of harmonic switching transients.

INDUSTRIAL APPLICABILITY The invention is useful in the field of inductive power transfer to vehicles to allow the size of the inductive track to be only sufficiently large that it will cope with the mean maximum loading rather than the absolute maximum loading. It is also effective in limiting the maximum load drawn from the inductive track at startup.