Electric lamp and discharge devices: systems – Regulation of the control current and/or potential applied...
Patent
1994-05-19
1996-04-09
Pascal, Robert J.
Electric lamp and discharge devices: systems
Regulation of the control current and/or potential applied...
315200R, 315207, 327131, 327348, G05F 100
Patent
active
055064779
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a method of sensing the rms value of an electrical waveform and in particular but not exclusively to a method of controlling the rms voltage in a load circuit supplied with a phase chopped sinusoidal voltage waveform.
2. Description of Related Art
The precise measurement of rms (root mean square) values of non-sinusoidal waveforms has hitherto required circuitry which is relatively complex and expensive. A typical analogue circuit computes the rms value by squaring the instantaneous value of input signal, dividing this result by the previously computed rms value and averaging the result to provide an updated rms value. The required linear circuits for the squaring and division calculations result in considerable complexity and expense if satisfactory results are to be produced.
A common circuit requirement is to provide in a load circuit a constant rms supply voltage by regulating a mains supply using a phase chopping device, the phase firing angle at which the sinusoidal mains voltage is chopped being adjusted to regulate the rms power in the load circuit in order to compensate for fluctuations in the amplitude of the mains supply voltage and fluctuations in the load impedance. The present invention seeks to provide a relatively simple method of sensing the rms value of the waveform in such circuits where the shape of the waveform is known to be that of a phase chopped sinusoidal waveform.
SUMMARY OF THE INVENTION
According to the present invention there is disclosed a method of sensing the rms value of a phase chopped sinusoidal electrical waveform comprising the steps of combining an input signal representative of the waveform with an auxiliary signal in a combining circuit which produces a combining circuit output equal to whichever is the greater of the auxiliary signal and the modulus of the input signal, and averaging the combining circuit output over at least one half cycle of the waveform to produce an output signal level representative of the sensed rms value wherein the auxiliary signal is either the product of the output signal level multiplied by a predetermined factor less than unity or is the product of the predetermined factor multiplied by a desired value of the output signal level.
An advantage of such a method is that it is not necessary to provide circuits capable of performing squaring and division.
Preferably the predetermined factor is substantially equal to 78.5% throughout each cycle.
The value of 78.5% has been determined empirically as being appropriate for providing a satisfactory representation of the rms value when processing a waveform which is phase chopped. The term phase chopped is throughout the specification to be understood to mean the modification of a sinusoidal waveform in which the waveform is held constant at zero for a controlled time interval following immediately after each zero crossing point of the waveform, the time interval corresponding to a phase firing angle of a phase chopping device.
Where other forms of chopping of a sinusoidal waveform are employed for power regulation, or where the basic form of the waveform is other than sinusoidal, then other values of the predetermined factor may be empirically determined for use in accordance with the present invention.
Preferably the combining circuit comprises a rectifier producing the modulus of the input signal and also comprises a linear OR function device which combines the modulus of the input signal with the auxiliary signal in the above described manner.
The combining circuit output may conveniently be averaged by a low-pass filter.
Conveniently the auxiliary signal is provided by a control signal equal to the product of the predetermined factor multiplied by a desired value of the output signal level, the method including the step of comparing in a comparator the output signal level with the desired value of output signal level to produce a feedback signal which regulates a phase chopping device to regulate the phase fir
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Cuthbertson Brian G.
Davy Peter G.
Multiload Technology Limited
Pascal Robert J.
Philogene Haissa
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