EP0466757A1 - A controller for an electrical load - Google Patents

A controller for an electrical load

Info

Publication number
EP0466757A1
EP0466757A1 EP19900905575 EP90905575A EP0466757A1 EP 0466757 A1 EP0466757 A1 EP 0466757A1 EP 19900905575 EP19900905575 EP 19900905575 EP 90905575 A EP90905575 A EP 90905575A EP 0466757 A1 EP0466757 A1 EP 0466757A1
Authority
EP
European Patent Office
Prior art keywords
output
control circuit
waveform
generator
oscillator
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP19900905575
Other languages
German (de)
French (fr)
Inventor
Derek Stanley Adams
Edward Lukaszewski
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Chloride Group Ltd
Original Assignee
Chloride Group Ltd
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Chloride Group Ltd filed Critical Chloride Group Ltd
Publication of EP0466757A1 publication Critical patent/EP0466757A1/en
Withdrawn legal-status Critical Current

Links

Classifications

    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M3/00—Conversion of dc power input into dc power output
    • H02M3/02—Conversion of dc power input into dc power output without intermediate conversion into ac
    • H02M3/04—Conversion of dc power input into dc power output without intermediate conversion into ac by static converters
    • H02M3/10—Conversion of dc power input into dc power output without intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
    • H02M3/145—Conversion of dc power input into dc power output without intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal
    • H02M3/155—Conversion of dc power input into dc power output without intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only
    • H02M3/156—Conversion of dc power input into dc power output without intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only with automatic control of output voltage or current, e.g. switching regulators
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M1/00—Details of apparatus for conversion
    • H02M1/44—Circuits or arrangements for compensating for electromagnetic interference in converters or inverters

Definitions

  • This invention relates to controllers for electrical loads.
  • the invention is particularly, though not exclusively, applicable to electric motor chopper controllers.
  • Electric motors for example those used as traction motors in electrically propelled vehicles, are typically controlled using a variable mark-space ratio rectangular control signal. This is commonly known as "chop" control.
  • control circuit of such controllers may include a constant frequency, constant amplitude triangular or saw tooth waveform generator.
  • the output of the generator is compared with a demand signal in a comparator such that the result of the comparison is a rectangular wave in which the duty cycle is dependent on the coincidence of the amplitude of the demand signal relative to the changing level of the triangular voltage.
  • the duty cycle would be variable with the demand level.
  • the rectangular waveform may form the input to a suitable conventional power stage controlling the load.
  • the chopping frequency of such a controller is a few kilohertz. It has been found that at such chopping frequencies the controller, the electrical load and/or the power source will emit noise. Within the bandwidth of the emitted noise there is a significant audible component. This is undesirable in many applications, particularly in those in which electric vehicles are used on the public highway.
  • an electric load control circuit comprising an electrical waveform generator arranged to produce a variable period and variable amplitude output and a comparator operable to produce a chop control output signal in response to a comparison between an input signal and the output of the generator.
  • the invention can be considered as providing an electric load control circuit comprising an electrical waveform generator arranged to produce a variable period output of predetermined rising and/or falling slope and a comparator operable to produce a chop control output signal in response to a comparison between an input signal and the output of the generator.
  • the predetermined slope is conveniently a substantially constant slope. However, it may equally well be any other predetermined waveform, such as an exponential one.
  • the generator produces a triangular waveform, for example a sawtooth waveform.
  • pseudo-random signal sequence generating means are arranged to produce an input signal which may be supplied to a voltage controlled waveform generator.
  • the sequence generating means are preferably operable to vary the period of the output of a voltage controlled oscillator.
  • the output of the oscillator may be a pulse waveform of varying frequency which constitutes the input signal applied to the pseudo-random generator.
  • the pulses in the output from the oscillator are preferably of substantially constant duration.
  • variable amplitude waveform having an effectively randomly varying frequency is applied to the comparator.
  • Figure 1 is a schematic block diagram of a circuit according to the invention.
  • Figure 2 is a circuit diagram of a basic oscillator
  • Figure 3 is a circuit diagram of a modified version of the oscillator in Figure 2 for use in the invention.
  • Figure 4 is a circuit diagram of an embodiment of the invention.
  • a chopper controller for an electrical vehicle traction motor chopper controller comprises a voltage controlled oscillator (VCO) 10 which receives a modulation voltage.
  • VCO voltage controlled oscillator
  • the output of the VCO 10 is supplied to a triangular waveform generator 12 which supplies an output waveform signal of approximately constant slope to one input of a comparator 14.
  • the other input to the comparator is an integrated error signal e which is the difference between, for example, the demanded motor speed, i.e. the armature current of the electric motor and the actual motor speed.
  • the controlled parameter may, of course, be something other than motor speed, for example torque.
  • the basic oscillator constituting the VCO 10 is illustrated in Figure 2. This is an astable multivibrator arrangement well known to those skilled in the art.
  • the oscillator comprises a comparator 16 (which may be an operational amplifier functioning as a comparator) connected for both positive and negative feedback.
  • a capacitor 17 is connected between the negative supply rail and the inverting input of the comparator 16.
  • the output of the oscillator in Figure 2 is approximately a square wave and the voltage across the capacitor 17 is roughly a triangular waveform of constant amplitude.
  • the input required to the sawtooth generator 12 is a pulse train.
  • the basic circuit of Figure 2 ⁇ is modified in the present invention as illustrated in Figure 3.
  • the collector of an NPN buffer transistor TR1 is connected to the positive supply rail while the emitter of TR1 is connected to the negative supply rail via a resistor.
  • the base of the transistor TR1 is connected to the output of the comparator 16. Since, in practice, most comparators have a so-called "open collector” output, a resistor R2 is included to provide the base current for transistor TR1.
  • a diode Dl and resistor Rl are serially connected between the emitter of the transistor and the inverting input to the comparator 16. The value of resistor Rl is chosen to produce much more rapid charging of capacitor 17 than the discharging.
  • a resistor R is connected across the capacitor 17 to provide a discharge path.
  • the discharge path could be via a resistor connected in parallel with either Dl, or Rl and Dl in series.
  • the output of the oscillator, at the emitter of the transistor TR1 is modified to be a pulse train.
  • the waveform across the capacitor 17 is now approximately a sawtooth.
  • the control of the frequency of the oscillator in Figure 3 can be effected in two ways. Firstly, a control voltage can be applied via a resistor and, possibly, a diode in series, to point A at the non- inverting input to the comparator 16. Modulation via the point A varies the frequency of oscillation by varying the hysteresis of the astable multivibrator arrangement. In this case, the slope of the sawtooth voltage across the capacitor 17 remains substantially constant while the amplitude of the sawtooth voltage varies with the modulation.
  • modulation can also be effected via point B at the inverting input to the operational amplifier 16 again via a resistor and, possibly, a diode in series. This varies the frequency by altering the slope of the ramp voltage across the capacitor 17. In this manner the hysteresis and the voltage amplitude across the capacitor remain constant.
  • FIG 4 is a circuit diagram of one form of the chopper controller circuitry of Figure 1.
  • the modulation voltage for the VCO 10 is provided by a pseudo-random signal generator 18.
  • the arrangement of the generator will be well known to those skilled in the art.
  • the output of the comparator 16 is supplied to one input of an exclusive-OR gate AO via a diode.
  • the other input to the exclusive-OR gate AO is connected to the negative supply rail.
  • the output of the gate AO thus provides a clock input to a shift register SR which is a 4006 device configured as an 18- stage or 18-bit shift register. While it is convenient to utilise the output of the comparator as the source of the clock input pulses, any other suitable oscillator could be used if desired.
  • the decoding logic of the pseudo-random signal generator 18 comprises 3 exclusive-OR gates Al, A2 and A3.
  • Pin 13 of the shift register is connected to one input of gate Al and pins 9 and 5 are connected to the other input thereof.
  • Gate A2 in turn has one input connected to the output of gate Al and its other input connected to zero volts.
  • Pins 10 and 4 of the shift register are connected to one input of gate A3 which has its other input connected to the output of gate A2.
  • Pins 12 and 1 of the shift register are interconnected and pin 6 is connected to the output of the pseudo ⁇ random signal generator 18.
  • the output of the VCO 10 is a pulse train which is applied to the sawtooth generator 12.
  • the pulse train is varied by the input voltage from the pseudo-random signal generator 18 and is thus a pseudo-randomly occurring sequence of pulses.
  • the frequency of the pulses switches pseudo-randomly between about 1.8 kHz and about 2.2 kHz influenced directly, or indirectly, by the output of the signal generator 18.
  • the generator 12 comprises a capacitor 30 and a resistor 31 connected in series between zero volts and the negative supply rail.
  • a transistor 28 has its collector-emitter path connected across the capacitor, its collector being connected to zero volts and its emitter being connected to the junction between the capacitor 30 and the resistor 31.
  • the transistor is actuated by the output of the VCO 10 applied to its base. When this output goes high the capacitor 30 is discharged by the transistor 28. When the output goes low the voltage across the capacitor increases at a rate determined by the time constant of the capacitor 30 and the resistor 31.
  • the output of the generator 12 is connected, via a resistor, to the non-inverting input of the comparator 14.
  • the error signal is applied to the inverting input of the comparator 14.
  • a resistor 32 is connected between the output of comparator 14 and its non- inverting input to provide a small amount of positive feedback and, thus, more positive switching at the output.
  • the high and low output states of the pseudo-random signal generator 18 cause the VCO 10 to produce a pseudo-randomly occurring pulse train by varying the hysteresis of the astable oscillator arrangement.
  • the invention provides variable frequency reset pulses for the generator 12, each pulse initiating a sawtooth at the output of generator 12.
  • the output sawtooth has approximately a fixed slope and, consequently, a variable amplitude. It is this near constant slope sawtooth which is compared with the error signal e by means of the comparator in order to produce the mark- space signal.
  • the output of the comparator 14 is a variable mark-space rectangular waveform.
  • mark-space ratio varies with the frequency of the pseudo-randomly occurring oscillator pulses from the VCO 10
  • the time constant of the motor is of sufficient length for the disparity not to be disadvantageous. Indeed, this additional variation in mark-space ratio may actually contribute to noise level reduction.
  • a particular attraction of the embodiment illustrated in Figure 4 is that, when the load is a motor with separate control of armature and field circuits, one master oscillator can be used to control separate sawtooth generators for both armature and field current control. The individual sawtooth generators for armature and field control can then be located close to their associated control circuitry thus minimising the risk of interference between the two control loops.

Abstract

Régulateur pour moteur électrique comprenant un circuit oscillateur à tension de commande (10), dont la sortie est transformée en forme l'onde triangulaire par un générateur de forme d'onde (12). La sortie du générateur est appliquée à une entrée d'un comparateur (14). L'autre entrée est alimentée par un signal d'erreur (e) indiquant la différence entre la sortie demandée et la sortie réelle. Pour atténuer le bruit acoustique du régulateur, la période et l'amplitude de la sortie de l'oscillateur et, par conséquent, du générateur se forme d'onde, sont modifiées par un générateur à séquence pseudoaléatoire (18) qui est connecté à l'oscillateur. La sortie du comparateur est une onde rectangulaire de rapport marque/espace variable.Regulator for electric motor comprising a control voltage oscillator circuit (10), the output of which is transformed into a triangular wave by a waveform generator (12). The generator output is applied to an input of a comparator (14). The other input is supplied by an error signal (e) indicating the difference between the requested output and the actual output. To attenuate the acoustic noise of the regulator, the period and the amplitude of the output of the oscillator and, consequently, of the generator is wave-shaped, are modified by a pseudo-random sequence generator (18) which is connected to the 'oscillator. The comparator output is a rectangular wave with variable mark / space ratio.

Description

A CONTROLLER FOR AN ELECTRICAL LOAD
This invention relates to controllers for electrical loads. The invention is particularly, though not exclusively, applicable to electric motor chopper controllers.
Electric motors, for example those used as traction motors in electrically propelled vehicles, are typically controlled using a variable mark-space ratio rectangular control signal. This is commonly known as "chop" control.
Practically, the control circuit of such controllers may include a constant frequency, constant amplitude triangular or saw tooth waveform generator. The output of the generator is compared with a demand signal in a comparator such that the result of the comparison is a rectangular wave in which the duty cycle is dependent on the coincidence of the amplitude of the demand signal relative to the changing level of the triangular voltage. Thus the duty cycle would be variable with the demand level. The rectangular waveform may form the input to a suitable conventional power stage controlling the load.
Conveniently, the chopping frequency of such a controller is a few kilohertz. It has been found that at such chopping frequencies the controller, the electrical load and/or the power source will emit noise. Within the bandwidth of the emitted noise there is a significant audible component. This is undesirable in many applications, particularly in those in which electric vehicles are used on the public highway.
It is an object of the present invention to provide an electrical load controller in which the perceived level of the noise emitted from the system to which it is fitted is substantially less than has previously been the case, particularly in the audible frequency range.
According to the present invention there is provided an electric load control circuit comprising an electrical waveform generator arranged to produce a variable period and variable amplitude output and a comparator operable to produce a chop control output signal in response to a comparison between an input signal and the output of the generator.
Alternatively the invention can be considered as providing an electric load control circuit comprising an electrical waveform generator arranged to produce a variable period output of predetermined rising and/or falling slope and a comparator operable to produce a chop control output signal in response to a comparison between an input signal and the output of the generator. The predetermined slope is conveniently a substantially constant slope. However, it may equally well be any other predetermined waveform, such as an exponential one.
Preferably, the generator produces a triangular waveform, for example a sawtooth waveform.
Preferably, pseudo-random signal sequence generating means are arranged to produce an input signal which may be supplied to a voltage controlled waveform generator.
The sequence generating means are preferably operable to vary the period of the output of a voltage controlled oscillator. In this case, the output of the oscillator may be a pulse waveform of varying frequency which constitutes the input signal applied to the pseudo-random generator. The pulses in the output from the oscillator are preferably of substantially constant duration.
Thus, according to the invention the variable amplitude waveform, having an effectively randomly varying frequency is applied to the comparator.
A specific embodiment of the invention will now be described by way of example with reference to the accompanying drawings in which:
Figure 1 is a schematic block diagram of a circuit according to the invention;
Figure 2 is a circuit diagram of a basic oscillator;
Figure 3 is a circuit diagram of a modified version of the oscillator in Figure 2 for use in the invention; and
Figure 4 is a circuit diagram of an embodiment of the invention.
Referring to Figure 1, a chopper controller for an electrical vehicle traction motor chopper controller comprises a voltage controlled oscillator (VCO) 10 which receives a modulation voltage. The output of the VCO 10 is supplied to a triangular waveform generator 12 which supplies an output waveform signal of approximately constant slope to one input of a comparator 14. The other input to the comparator is an integrated error signal e which is the difference between, for example, the demanded motor speed, i.e. the armature current of the electric motor and the actual motor speed. The controlled parameter may, of course, be something other than motor speed, for example torque.
The basic oscillator constituting the VCO 10 is illustrated in Figure 2. This is an astable multivibrator arrangement well known to those skilled in the art. The oscillator comprises a comparator 16 (which may be an operational amplifier functioning as a comparator) connected for both positive and negative feedback. A capacitor 17 is connected between the negative supply rail and the inverting input of the comparator 16. However, the output of the oscillator in Figure 2 is approximately a square wave and the voltage across the capacitor 17 is roughly a triangular waveform of constant amplitude. The input required to the sawtooth generator 12 is a pulse train. Thus, the basic circuit of Figure 2 ^is modified in the present invention as illustrated in Figure 3. In this modified form, the collector of an NPN buffer transistor TR1 is connected to the positive supply rail while the emitter of TR1 is connected to the negative supply rail via a resistor. The base of the transistor TR1 is connected to the output of the comparator 16. Since, in practice, most comparators have a so-called "open collector" output, a resistor R2 is included to provide the base current for transistor TR1. A diode Dl and resistor Rl are serially connected between the emitter of the transistor and the inverting input to the comparator 16. The value of resistor Rl is chosen to produce much more rapid charging of capacitor 17 than the discharging.
A resistor R is connected across the capacitor 17 to provide a discharge path. Alternatively, the discharge path could be via a resistor connected in parallel with either Dl, or Rl and Dl in series. Thus, the output of the oscillator, at the emitter of the transistor TR1, is modified to be a pulse train. Furthermore, the waveform across the capacitor 17 is now approximately a sawtooth.
The control of the frequency of the oscillator in Figure 3 can be effected in two ways. Firstly, a control voltage can be applied via a resistor and, possibly, a diode in series, to point A at the non- inverting input to the comparator 16. Modulation via the point A varies the frequency of oscillation by varying the hysteresis of the astable multivibrator arrangement. In this case, the slope of the sawtooth voltage across the capacitor 17 remains substantially constant while the amplitude of the sawtooth voltage varies with the modulation.
Secondly, modulation can also be effected via point B at the inverting input to the operational amplifier 16 again via a resistor and, possibly, a diode in series. This varies the frequency by altering the slope of the ramp voltage across the capacitor 17. In this manner the hysteresis and the voltage amplitude across the capacitor remain constant.
It has been found that, in certain circumstances, the modulation of the frequency is preferred via the point B, since this can be slightly easier to implement practically. It should be noted, however, that with modulation applied at point A, the circuit in Figure 3 could, by itself, form the basis of a combined oscillator and sawtooth generator, since, with suitable modulation at point A, it will itself produce a variable frequency, variable amplitude output.
Figure 4 is a circuit diagram of one form of the chopper controller circuitry of Figure 1. The modulation voltage for the VCO 10 is provided by a pseudo-random signal generator 18. The arrangement of the generator will be well known to those skilled in the art. The output of the comparator 16 is supplied to one input of an exclusive-OR gate AO via a diode. The other input to the exclusive-OR gate AO is connected to the negative supply rail. The output of the gate AO thus provides a clock input to a shift register SR which is a 4006 device configured as an 18- stage or 18-bit shift register. While it is convenient to utilise the output of the comparator as the source of the clock input pulses, any other suitable oscillator could be used if desired.
The decoding logic of the pseudo-random signal generator 18 comprises 3 exclusive-OR gates Al, A2 and A3. Pin 13 of the shift register is connected to one input of gate Al and pins 9 and 5 are connected to the other input thereof. Gate A2 in turn has one input connected to the output of gate Al and its other input connected to zero volts. Pins 10 and 4 of the shift register are connected to one input of gate A3 which has its other input connected to the output of gate A2. Pins 12 and 1 of the shift register are interconnected and pin 6 is connected to the output of the pseudo¬ random signal generator 18.
As discussed previously, the output of the VCO 10 is a pulse train which is applied to the sawtooth generator 12. The pulse train is varied by the input voltage from the pseudo-random signal generator 18 and is thus a pseudo-randomly occurring sequence of pulses. The frequency of the pulses switches pseudo-randomly between about 1.8 kHz and about 2.2 kHz influenced directly, or indirectly, by the output of the signal generator 18.
The generator 12 comprises a capacitor 30 and a resistor 31 connected in series between zero volts and the negative supply rail. A transistor 28, has its collector-emitter path connected across the capacitor, its collector being connected to zero volts and its emitter being connected to the junction between the capacitor 30 and the resistor 31. The transistor is actuated by the output of the VCO 10 applied to its base. When this output goes high the capacitor 30 is discharged by the transistor 28. When the output goes low the voltage across the capacitor increases at a rate determined by the time constant of the capacitor 30 and the resistor 31.
The output of the generator 12 is connected, via a resistor, to the non-inverting input of the comparator 14. The error signal is applied to the inverting input of the comparator 14. A resistor 32 is connected between the output of comparator 14 and its non- inverting input to provide a small amount of positive feedback and, thus, more positive switching at the output.
The high and low output states of the pseudo-random signal generator 18 cause the VCO 10 to produce a pseudo-randomly occurring pulse train by varying the hysteresis of the astable oscillator arrangement. Thus the invention provides variable frequency reset pulses for the generator 12, each pulse initiating a sawtooth at the output of generator 12. The output sawtooth has approximately a fixed slope and, consequently, a variable amplitude. It is this near constant slope sawtooth which is compared with the error signal e by means of the comparator in order to produce the mark- space signal. As a result, the output of the comparator 14 is a variable mark-space rectangular waveform.
It has been found that, although the mark-space ratio varies with the frequency of the pseudo-randomly occurring oscillator pulses from the VCO 10, the time constant of the motor is of sufficient length for the disparity not to be disadvantageous. Indeed, this additional variation in mark-space ratio may actually contribute to noise level reduction.
A particular attraction of the embodiment illustrated in Figure 4 is that, when the load is a motor with separate control of armature and field circuits, one master oscillator can be used to control separate sawtooth generators for both armature and field current control. The individual sawtooth generators for armature and field control can then be located close to their associated control circuitry thus minimising the risk of interference between the two control loops.

Claims

1. An electric load control circuit comprising an electrical waveform generator arranged to produce a variable period and variable amplitude output and a comparator operable to produce a chop control output signal in response to a comparison between an input signal and the output of the generator.
2. A control circuit as claimed in Claim 1 in which the output of the waveform generator is of predetermined rising and/or falling slope.
3. A control circuit as claimed in Claim 2 in which the output of the waveform generator is of substantially constant slope.
4. A control circuit as claimed in Claim 2 in which the output of the waveform generator is a substantially exponential rising and/or falling waveform.
5. A control circuit as claimed in Claim 1 in which the waveform generator is arranged to produce a triangular waveform, for example a sawtooth waveform.
6. A control circuit as claimed in Claim 1 in which pseudo-random sequence generating means are arranged to produce an input signal to the waveform generator to vary the output thereof.
7. A control circuit as claimed in Claim 6 in which the waveform generator includes a voltage controlled oscillator, the pseudo-random sequence generating means being arranged to change the output period of the oscillator to vary the output of the waveform generating means.
8. A control circuit as claimed in Claim 7 in which the output of the oscillator is a pulse waveform.
9. A control circuit as claimed in Claim 8 in which the output pulses from the oscillator are of substantially constant duration.
10. A control circuit as claimed in Claim 8 in which the pulse waveform output of the oscillator is arranged to supply a drive pulse input to the pseudo¬ random sequence generator.
11. An electric motor controller including the circuit as claimed in Claim 1.
12. An electric motor including a control circuit as claimed in Claim 11.
EP19900905575 1989-04-07 1990-04-06 A controller for an electrical load Withdrawn EP0466757A1 (en)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
GB898907918A GB8907918D0 (en) 1989-04-07 1989-04-07 Improvements in controllers for electrical loads
GB8907918 1989-04-07

Publications (1)

Publication Number Publication Date
EP0466757A1 true EP0466757A1 (en) 1992-01-22

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EP19900905575 Withdrawn EP0466757A1 (en) 1989-04-07 1990-04-06 A controller for an electrical load

Country Status (5)

Country Link
EP (1) EP0466757A1 (en)
JP (1) JPH04504496A (en)
CA (1) CA2051402A1 (en)
GB (2) GB8907918D0 (en)
WO (1) WO1990012446A1 (en)

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Also Published As

Publication number Publication date
JPH04504496A (en) 1992-08-06
CA2051402A1 (en) 1990-10-08
GB2232835A (en) 1990-12-19
WO1990012446A1 (en) 1990-10-18
GB9007846D0 (en) 1990-06-06
GB8907918D0 (en) 1989-05-24

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