Electricity: measuring and testing – Measuring – testing – or sensing electricity – per se – Magnetic saturation
Patent
1997-09-16
2000-03-14
Strecker, Gerard
Electricity: measuring and testing
Measuring, testing, or sensing electricity, per se
Magnetic saturation
324119, 324142, 324251, G01R 2108, G01R 1900, G01R 3307
Patent
active
060377631
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The present invention relates to an electricity measurement apparatus and a method of electricity measurement, in particular for current measurement, power measurement and Watthour metering.
When current flows through a conductor, a rotational magnetic field is set up. The magnitude of this magnetic field is proportional to the density of current flowing: conductor. This value I/A is the current density J.
It is known to use sensors, such as Hall effect sensors, to measure current flow though a conductor by detecting the magnetic field. For example, using a Hall effect sensor, the sensor output signal has a voltage, VH, which is proportional to the magnetic field strength B at the sensor, and a bias current IB flowing through the sensor as follows:
The proportionality coefficient S is the sensitivity of the sensor. By keeping constant the current through the Hall sensor, a linear relationship is achieved between the detected magnetic field and output voltage.
If such a sensor is placed near a conductor, the two equations (1) and (2) above can be combined allowing current flow to be determined:
Hall sensors produce Direct Current (D.C.) offset errors, which are independent of the direction (positive or negative) with which the bias current IB is applied to the sensor. Conventionally, as shown in FIG. 3, the bias current IB is selected to be proportional to the electricity mains supply voltage. The current IB and detected magnetic field B are both Alternating Current (A.C.) signals which vary according to a sine relation. Accordingly, the output voltage VH follows a sine squared relation where D.C. offsets are components of the resultant VH signal.
Other D.C. offsets can also occur due to the electronic circuits associated with the subsequent processing operations.
The effects of D.C. offsets are minimised conventionally by using large Hall sensors, in particular so called "external" sensors not integrated with other processing circuitry on an integrated circuit such as a silicon chip. The conventional principle applied is that the larger the Hall sensor is, the larger is the Hall voltage which it provides relative to its D.C. offset error.
SUMMARY OF THE INVENTION
The present invention preferably provides an electrical measurement apparatus comprising supply means operative to supply a rectified A.C. supply signal to a sensor, the sensor being operative to detect magnetic field strength due to A.C. current flow alone a conductor and to provide an output signal dependent on detected field strength and the rectified A.C. supply signal, the output signal following a sine-squared relation with alternating positive and negative sign in each half cycle, and filtering means operative to remove a component due to a D.C. offset error from the output signal. The rectified A.C. signal is preferably a rectified A.C. bias current signal. The filtering means preferably comprises a capacitor. The A.C. mains signal is the alternating voltage signal of an electrical supply network to which the conductor is connected.
It will be seen that preferred embodiments of the invention have the advantage that components due to D.C. offset errors are cancelled. This is done in a simple and automatic manner. In consequence, further advantages arise. In particular, sensors can be made sufficiently small that multiple sensors can be integrated on a single silicon chip.
Preferably, the sensor is a Hall sensor and the supply signal to the sensor is a current signal. The supply signal is preferably proportional to the electricity mains supply voltage. The output signal from a sensor is preferably a voltage signal. The A.C. current flow along the conductor is preferably an electricity mains supply current.
The present invention also provides a method of measuring electricity flow alone a conductor including the steps of supplying a rectified A.C. supply signal to a sensor, the sensor detecting magnetic field strength due to A.C. current flow along a conductor and providing an output signal dependent on detecte
REFERENCES:
patent: 5065088 (1991-11-01), Habiro et al.
patent: 5497077 (1996-03-01), Nukui
patent: 5554927 (1996-09-01), Maruyama
International Search Report, Int'l Appn. No. PCT/GB96/00539, Int'l Filing Date Aug. 03, 1996, 3 pp.
Horstmann Timers & Controls Limited
Strecker Gerard
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