Method apparatus for energy conversion

Method and apparatus for regulating a converter having transistors, in which the value of a number N.sub.1 of state variables is measured and stored, then action is carried out on a number N.sub.2 of control variables. N.sub.1 being greater than N.sub.2, the value of the control variables is calculated, at a given instant, for a number P of periods such that the product of N.sub.2 multiplied by P is greater than or equal to N.sub.1.

BACKGROUND OF THE INVENTION
 The present invention relates to the field of DC/AC conversion of
 electrical energy.
 Inverters are supplied with a DC voltage and deliver as output an AC
 voltage by virtue of one or more transistor half-bridges. The output AC
 voltage is generally subjected to filtering.
 Inverters of this type are, amongst other things, used for the electrical
 power supply of an X-ray tube.
 An X-ray tube mounted, for example, in a medical radiology instrument,
 comprises a cathode and an anode which are both enclosed in an evacuated
 leaktight casing, so as to produce electrical insulation between these two
 electrodes. The cathode produces an electron beam which is received by the
 anode on a small surface constituting a focus from which the X-rays are
 emitted.
 When a high supply voltage is applied using a generator to the terminals of
 the cathode and the anode, so that the cathode is at a negative potential
 -V and the anode is at a positive potential +V, with respect to the
 potential of the cathode, a so-called anodic current is set up in the
 circuit through the generator which produces the high supply voltage. The
 anodic current passes through the space between the cathode and the anode
 in the form of an electron beam which bombards the focus.
 The anode is in the shape of a flat disc which is supported by a shaft,
 driven in rotation by a rotor of an electric motor, the stator of which is
 arranged outside the casing, with the aim of promoting the dissipation of
 the energy. The X-ray tube is arranged in an enclosure filled with an
 insulating refrigerant.
 The characteristics of the X-rays which are emitted by the tube, in
 particular their hardness, depend on a number of parameters, including the
 value of the high voltage applied to the electrodes. This high voltage
 should be adjustable in order to obtain the desired characteristics, and
 should remain constant throughout the radiological exposure time, so as
 not to alter the operating characteristics of an X-ray receiver which
 receives the X-rays which have passed through the object which is
 undergoing examination.
 X-rays tubes for medical diagnosis operate in pulses. It is therefore
 important for the time taken to establish the high voltage, as well as the
 time taken to return from this high voltage to a zero value, to be as
 short as possible.
 A high-voltage generator for an X-ray tube generally comprises a supply
 circuit which delivers a DC voltage E starting with an AC voltage
 delivered by the mains. The voltage E is applied to the terminals of an
 inverter of the type which comprises at least one transistor half-bridge,
 each branch of the half-bridge comprising a switch S consisting of a
 transistor T and a freewheeling diode D mounted in antiparallel. The AC
 signal delivered by the inverter is applied, via a filter, to the primary
 of a step-up voltage transformer having a turns ratio k. The secondary of
 the step-up voltage transformer is connected to a rectifying and filtering
 circuit comprising at least one diode halfbridge and capacitors C.sub.f
 for filtering the voltage.
 In known fashion, the inverter comprises a transistor pair connected in
 series to the output terminals of the supply circuit. A diode is connected
 between the collector and the emitter of each transistor T, so that its
 anode is connected to the emitter of the corresponding transistor. The
 bases of the transistors are connected to a control circuit which delivers
 switching signals for the transistors. In the case of a single
 half-bridge, the two output terminals of the inverter consist of the
 common point of the two branches of the half-bridge and of a point common
 to two capacitors of the half-bridge which are mounted in parallel and, in
 the case of two half-bridges, of each point common to the two transistors
 of a half-bridge.
 The output filter of the inverter comprises, for example, a coil L.sub.r
 and a capacitor C.sub.r which are arranged in series, and a coil L.sub.p
 which is arranged in parallel with the capacitor C.sub.r. One of the
 terminals of the filter is connected to an output terminal of the
 inverter, and the other terminal is connected to a terminal of the primary
 circuit of the transformer. A filter with single resonance may also be
 used.
 The rectifying circuit connected to the secondary of the step-up voltage
 transformer consists, for example, of a two-diode bridge, the point common
 to the two diodes being connected to one of the output terminals of the
 secondary of the transformer, two capacitors C.sub.f1 and C.sub.f2 being
 arranged in parallel with the diode bridge, the other terminal of the
 secondary of the transformer being connected to the point common to the
 two capacitors C.sub.f.
 The control circuit essentially comprises a comparator, a circuit for
 measuring the current I.sub.1r. at the primary of the transformer, and a
 circuit for developing the switching signals for the transistors of the
 inverter. One of the two output terminals of the comparator is connected
 to the common point of two resistors of a voltage divider, to which the DC
 supply voltage V.sub.cf of the X-ray tube is applied, and the other is
 connected to a reference voltage source. The output terminal of the
 comparator delivers a signal whose amplitude is proportional to the
 difference between the two voltages applied to the input terminals, and it
 is connected to an input terminal of the circuit for developing the
 switching signals, so as to bring about a change in the frequency of the
 control signals for the transistors. The output terminal of the circuit
 for measuring the current in the primary of the transformer is connected
 to another input terminal of the circuit for developing the switching
 signals, with the aim of detecting and avoiding certain malfunctions of
 the inverter.
 In conventional fashion, the control variable on which the control circuit
 acts is the time period T.sub.d until the transistors are turned on,
 starting from the instant when the current of the inverter reaches a zero
 value.
 The presence of a filter with double resonance makes it possible to have
 the current of the inverter change as a monotonically increasing function
 of frequency, between the parallel resonant frequency and the series
 resonant frequency, the values of which depend on the values of the
 capacitor C.sub.r of the series coil L.sub.r and of the parallel coil
 L.sub.p of the filter. It therefore seems possible to control the power
 transmitted to the X-ray tube by the operating frequency of the inverter,
 and consequently the activation delay T.sub.d. A filter with single
 resonance also makes it possible to control the power by the activation
 delay T.sub.d.
 However, known control circuits do not make it possible to set the DC
 supply voltage V.sub.cf of the X-ray tube to its desired value until too
 much time has elapsed, which results in time being wasted and by a dose of
 X-rays being received superfluously by the patient. This is because the
 efficiency of the X-rays for taking exposures is proportional to the
 voltage V.sub.cf raised to the fifth power. The dose of X-rays received
 before the desired voltage V.sub.cf is established, cannot be used for
 taking exposures.
 These control circuits leave some degree of ripple in the voltage V.sub.cf
 after it has been established. This ripple is at a frequency of 100 or 300
 Hz, depending on the supply type used: single-phase or three-phase. These
 ripples are even more problematic since the number of exposures taken per
 second may be as many as thirty and since it leads to an instability in
 the images. For a scanner, which is provided with a rotary source so as to
 obtain three-dimensional information, the image computation presupposes
 that the images are stable and consequently that the voltage is constant.
 SUMMARY OF THE INVENTION
 It is therefore desirable to overcome the drawbacks mentioned above, by
 providing a method and a device for regulation which make it possible to
 reduce the time taken to establish the voltage V.sub.cf at the start of an
 exposure.
 It is further desirable to reduce the ripple in the voltage V.sub.cf in
 steady state.
 The method for regulating a converter having transistors, in an embodiment
 of the invention, comprises steps of measuring and storing the value of a
 number N.sub.1 of state variables, then of acting on a number N.sub.2 of
 control variables. V.sub.cf being greater than N.sub.2, the value of the
 control variables is calculated, at a given instant, for a number P of
 periods such that the product of N.sub.2 multiplied by P is greater than
 or equal to N.sub.1. It is thus possible to act on all the state variables
 while retaining the possibility of establishing a hierarchy between them
 and of assigning priority to one or other of them.
 In one embodiment of the invention, there is a single control variable,
 which is a time variable. In this case, a number P of periods equal to the
 number N.sub.1 of state variables will generally be chosen, which leads to
 a system of N.sub.1 equations with P unknowns, the P unknowns being the
 values of the time variable for each period.
 In one embodiment of the invention, the regulation is of the
 proportional-integral type for one of the state variables and of the
 proportional type for the other state variables.
 Advantageously, the state variables are measured each time one of the said
 state variables reaches a predetermined threshold value, for example a
 zero value.
 In one embodiment of the invention, the state variables are the current
 I.sub.1r in a series filtering inductor, the current I.sub.1p in a
 parallel filtering inductor, the voltage V.sub.cr across the terminals of
 a filtering capacitor, and the output voltage V.sub.cf of the converter.
 The device for regulating an energy conversion assembly, according to the
 invention, comprises means for discretely measuring the values of a number
 N.sub.1 of state variables, and means for controlling a number N.sub.2 of
 control variables, with the aim of obtaining, at the output of the control
 assembly, an optimum time variation of one or more of the state variables,
 it being possible for the variation of the energy conversion assembly over
 a period between two measuring instants to be estimated by knowing the
 state variables at the first measuring instant and the control variables
 during the period in question. The device comprises means for developing
 the values of the N.sub.1 control variables over a number P of measuring
 instants, N.sub.1 being greater than N.sub.2, with P such that the product
 of N.sub.2 multiplied by P is greater than or equal to N.sub.1.
 By virtue of the invention, a significant reduction, of the order of 60%,
 is obtained in the time taken to establish the voltage V.sub.cf. A very
 great reduction is thus obtained in the dose received superfluously by the
 patient during the set-up time. This also improves the quality of the
 images obtained with the radiology device, by virtue of better stability
 of the voltage V.sub.cf in steady state.
 Of course, the invention can be applied to various types of converters and
 makes it possible for their performance to be improved significantly.

DETAILED DESCRIPTION OF THE INVENTION
 In FIG. 1, a DC voltage source E supplies a half-bridge provided with two
 switches S.sub.1 and S.sub.2, each composed of a power transistor, for
 example of the insulated-gate bipolar transistor (IGBT) type, and of a
 freewheeling diode. The inductors L.sub.1 and L.sub.2 serve to assist
 switching. The capacitors C.sub.1 and C.sub.2 are mounted in series and
 filter the voltage E. The voltage E is delivered by the mains AC voltage
 which is rectified by means (not shown).
 The center output of the half-bridge formed by the switches S.sub.1 and
 S.sub.2 is connected to one terminal of the primary of a transformer
 T.sub.R, the other terminal being connected to the point common to the two
 filtering capacitors C.sub.1 and C.sub.2. A filter with double resonance
 is arranged between the point common to the two switches S.sub.1 and
 S.sub.2 and the transformer T.sub.R. This filter comprises a series
 inductor L.sub.r, a series capacitor C.sub.r and a parallel inductor
 L.sub.p which is mounted in parallel with the capacitor C.sub.r. The
 transformer T.sub.R steps up the voltage by a coefficient K. This voltage
 is then rectified by a diode half-bridge and by two filtering capacitors
 C.sub.f. Of course, a four-switch inverter and a four-diode rectifier
 could be used. The output voltage V.sub.cf of the rectifier is sent to an
 X-ray tube (not shown).
 The value of the components of the filter is chosen so as to define a
 parallel resonant frequency F.sub.p =20 kHz so as to be outside the
 audible spectrum, and a series resonant frequency F.sub.s which is greater
 than the parallel resonant frequency and is chosen in accordance with the
 frequency limitations imposed by the switching times of the transistors of
 the half-bridge of the inverter, for example 70 kHz. When the operating
 frequency, lying between the parallel resonant frequency F.sub.p and the
 series resonant frequency F.sub.s, approaches the series resonant
 frequency F.sub.s, the impedance of the filter decreases, which leads to
 an increase in the current and therefore in the power transmitted to the
 transformer. Conversely, when the operating frequency decreases and
 approaches the parallel resonant frequency F.sub.p, the impedance of the
 filter increases and the output current I.sub.o tends towards a zero
 value. It is therefore possible to control the current I.sub.1r by the
 operating frequency.
 However, the values of the inductors L.sub.r and L.sub.p and of the
 capacitor C.sub.r of the filter are only known to within 5%. Moreover, the
 resonant frequencies are determined by the values of these components.
 Control by means of the frequency is not very effective because of this
 inaccuracy. A different approach is therefore used, involving the
 difference between the operating frequency and the actual series resonant
 frequency. Specifically, when the operating frequency tends towards the
 series resonant frequency F.sub.s, the conduction delay T.sub.d of the
 diodes of the switches tends towards zero. The zero crossing of the
 current I.sub.1r in the series inductor L.sub.r is detected, and a counter
 is triggered until the transistor of the opposite branch of the
 half-bridge is caused to switch on. It is thus possible to synchronize
 with the zero crossing of the current I.sub.1r.
 In FIGS. 2 and 3, small positive and negative threshold values of I.sub.1r
 are defined, on the basis of which the period T.sub.d after which the
 transistor is switched on is counted down.
 When the current I.sub.1r at the end of a negative half-cycle becomes
 greater than the negative threshold value, the counting of the period
 T.sub.d is triggered. At the end of the period T.sub.d the current
 I.sub.1r has become positive and the switching-on of the transistor
 T.sub.1 is triggered. Then, when the current I.sub.1r becomes less than
 the positive threshold, the counting of the period T.sub.d is again
 triggered. When the current I.sub.1r becomes negative, the diode D.sub.1
 becomes forward-biased. When the period T.sub.d has elapsed, the
 transistor T.sub.2 is switched on.
 As illustrated in FIG. 4, the sampling instants are chosen such that the
 current I.sub.1r is zero, and at this instant the value of the other state
 variables V.sub.cf, V.sub.cr and I.sub.1p are measured, V.sub.cr being the
 voltage across the terminals of the series capacitor C.sub.r, and I.sub.1p
 being the current in the parallel inductor L.sub.p. These values are then
 stored in a memory. In view of the characteristics of the system, the
 waveform of the state variables between two sampling instants depends only
 on the value of the three aforementioned state variables V.sub.cf,
 V.sub.cr, I.sub.1p and on the value of the chosen activation delay
 T.sub.d. On the basis of these data, it is possible to try to find the
 transfer function and carry out simulations over a half-cycle while
 remaining close to steady state.
 The aim of the regulation of a converter for an X-ray tube is to obtain the
 output voltage V.sub.cf while maintaining correct operation of the
 converter. However, acting on the single control variable consisting of
 the activation delay T.sub.d over one half-cycle, only makes it possible
 to regulate one of the state variables, namely V.sub.cf, which runs the
 risk of leading to undesired values of the voltage V.sub.cr and the
 current I.sub.1p.
 Regulation is therefore carried out over three half-cycles, and there are
 thus three available control variables T.sub.dk, T.sub.dk+1, T.sub.dk+2,
 which makes it possible to provide as many control variables as there are
 state variables and to reach a stable state at the end of the third
 half-cycle. Since regulating the voltage V.sub.cf takes priority over
 regulating the voltage V.sub.cr and the current I.sub.1p, provision is
 made to add an integral term to the regulation of V.sub.cf, while the
 regulation of V.sub.cr and I.sub.1p is merely proportional.
 In other words, in order to regulate suitably a system having a number of
 state variables greater than the number of control variables, this
 regulation is carried out over a number of half-cycles greater than 1, so
 that the product of the number of half-cycles multiplied by the number of
 control variables is greater than the number of state variables, in order
 to have a number of transfer equations equal to this product and therefore
 greater than the number of state variables. It is therefore possible to
 predict the future status of the regulating system with equations of the
 type X(k+1)=A*X(k)+B*T.sub.d, in which X is the vector formed by the state
 variables. The values of the matrices A and B are determined close to a
 particular operating point in steady state.
 These constants may be calculated by simulating low-amplitude transients
 with respect to this operating point, V.sub.cf and T.sub.d being fixed.
 Simulations are successfully carried out on the voltage V.sub.cr (FIG. 5)
 starting from the following initial conditions: V.sub.crk =V.sub.crs
 +dV.sub.cr, I.sub.1pk =I.sub.1ps and V.sub.cfk =V.sub.cfs, the values
 assigned an index S corresponding to the operating point in steady state.
 The same simulation is carried out on the current I.sub.1p, starting from
 the following initial conditions: V.sub.crk =V.sub.crs, I.sub.1pk
 =I.sub.1ps and V.sub.cfk =V.sub.cfs. The curves represented in FIG. 6 are
 obtained. A last simulation is carried out while keeping the state
 variables to their value of the operating point in steady state, and by
 modifying the value of the activation delay: T.sub.dk =T.sub.ds +dT.sub.d.
 By virtue of this simulation, the transfer function of the converter close
 to a stable state is obtained, this being written as follows:
 ##EQU1##
 The value of the activation delay for a given half-cycle is therefore
 calculated from the following formula:
 ##EQU2##
 in which the dynamic response of the system in closed loop depends on the
 choice of the gains k.sub.vcr, k.sub.I1p and k.sub.vcf. These gains may be
 calculated using the Ackermann method, starting with the gains of the
 transfer function. Tables of values of these gains may be stored in memory
 for various values of current and voltage, to be extracted later during
 the operation of the converter, illustrated in FIG. 8.
 The values of the three state variables I.sub.1p, V.sub.cr and V.sub.cf are
 extracted from the power part 1 of the converter at the sampling instants.
 These values are relayed, via delay cells, to comparators forming part of
 a digital processing unit 2. The other input of the comparators is
 connected to a circuit 3 for developing reference values V.sub.cfs,
 V.sub.crs and I.sub.1ps.
 The circuit 3 develops the aforementioned reference values on the basis of
 the voltage V.sub.cfs and the desired activation delay T.sub.ds. At the
 output of the comparators, the quantities .DELTA.I.sub.1pk,
 .DELTA.V.sub.crk and .DELTA.V.sub.cfk are assigned their respective gain
 coefficients then added. The quantity .DELTA.V.sub.cfk, assigned a gain
 coefficient k.sub.1, is delivered to an integrator circuit in order to
 provide the regulation with an integral term and ensure the priority
 afforded to the regulation of the V.sub.cf. The output of the said
 integrator circuit is also connected to the circuit which adds the other
 quantities. The output of the adder circuit is connected to another adder
 circuit which receives the value of T.sub.d, from the circuit 3. The
 activation delay T.sub.dk+1 for the next half-cycle is obtained at the
 output of this other adder circuit and is delivered to a circuit (not
 shown) for controlling the transistors of the power part 1.
 By virtue of the invention, the regulation of the converter is improved
 considerably while reducing the rise time of the output voltage and the
 ripple in steady state. Depending on the gain coefficients which are
 chosen, it is, for example, possible to allow for an exponential growth in
 the voltage at the start of a voltage rise ramp, in order to profit from a
 high current at low voltage, which, in the case of an X-ray tube, makes it
 possible to reduce the doses received by the patient, and also at the end
 of a voltage rise ramp in order to avoid excessive ripples in the current.
 Of course, the invention can be applied to extremely wide-spread types of
 converters intended for supplying different kinds of electrical loads.
 Various modifications in structure and/or function and/or steps may be made
 by one skilled in the art to the disclosed embodiments without departing
 from the scope and extent of the invention.