Electricity: measuring and testing – Measuring – testing – or sensing electricity – per se – Self-calibration
Reexamination Certificate
1998-05-08
2002-06-04
Metjahic, Safet (Department: 2858)
Electricity: measuring and testing
Measuring, testing, or sensing electricity, per se
Self-calibration
C324S132000
Reexamination Certificate
active
06400131
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to offset compensation of the output of a transducer. The invention is particularly, though not exclusively, applicable to offset compensation in current measurement in the control of a switched reluctance machine.
2. Description of Related Art
Many electrical and electronic systems require transducers for converting a parameter into an electrical signal. For example, electric motors and generators require some means of monitoring current for a variety of well-recognized reasons, such as measurement, control and equipment protection. A simple technique for doing this is to derive a signal indicative of the current from the voltage dropped across a series-connected resistor in accordance with Ohm's law. This is a simple technique, but it has various drawbacks. Firstly, the resistor will have a non-zero temperature coefficient of resistance. It generates heat by virtue of the current flowing through it, which will distort the reading when it is used at a temperature other than that at which it is calibrated, due to its non-zero temperature coefficient of resistance. Secondly, the resistor must be connected directly in the circuit to be monitored. This alone makes resistance current monitoring impracticable in, for example, power circuits in which the current to be monitored is in a circuit at a high potential with respect to the circuit to which the monitored current signal is to be relayed. Thirdly, connecting the resistor in the monitored circuit may distort the operation of the circuit itself to an unacceptable degree.
The problem in relatively high potential circuits has been addressed by electrically isolating the circuit being monitored from the monitoring circuit itself. However, the need for isolation raises the further problem that the potential across the isolation barrier may change very rapidly. A typical example of this is in semiconductor switching circuits in which rapid changes in voltage in the monitored circuit occur as a result of switching. The large rate of change of voltage with respect to time (dV/dt) in the monitored circuit can cause capacitive current flow, induced across the isolation boundary, creating a further opportunity for corruption of the transducer output signal.
Current transformers (CT's) are a form of transducer by which a measure of current in a conductor can be derived. They are electrically isolated from the conductor itself and they have found extensive use in the field of electrical power engineering as, for example, monitors in current regulation and protection systems.
A known CT relies on the substantial balance of magneto-motive force (MMF) between primary and secondary windings that would exist in a CT using a high permeability core. Ideally, a zero secondary circuit impedance (“burden”) would mean that this balance condition would be achieved at zero core flux. In practice, however, the non-zero burden dictates that a voltage will be dropped across the secondary winding with the result that the core flux will also be non-zero.
The core flux is proportional to the integral of the secondary voltage. In the case of an alternating waveform, the amplitude of the core flux will therefore be inversely proportional to the frequency of the monitored current. In addition, the finite permeability of a real core requires MMF to drive the flux around the core. Assuming a linear response of the magnetic material of the core, this MMF will be directly proportional to the flux. As the core flux increases, a larger MMF will be needed to support it. Thus, with decreasing frequency the CT core absorbs an increasing proportion of the primary MMF. Therefore, the secondary MMF and the output current must fall.
It has been considered that this fall-off in the lower frequency response of CT's represents an operating limit on their usefulness. A low frequency CT means both a large core and a low secondary impedance to offer a flat frequency response over a specified working frequency range. In the limit, known CT's cannot operate at dc (zero frequency) because of the non-zero secondary circuit resistance which is present in practice.
To address the problem of measuring current at low frequencies and at dc, current measuring devices have been developed that rely on the Hall effect. These are responsive to the strength of the magnetic field created by the current to be monitored. They are also often referred to in the art as “current transformers” although transformer principles are not involved.
A known current transducer based on the Hall effect uses a Hall-effect device arranged in an air gap in an otherwise toroidal core. The conductor carrying the current to be monitored is arranged to pass through the central aperture of the toroid. The Hall-effect device in the gap measures directly the flux resulting from the introduction of MMF in the core due to the current in the conductor.
While the device is relatively simply constructed, it has some disadvantages. Firstly, the response of the core material is not linear in practice. Secondly, the Hall-effect device also has a non-linear response and displays characteristics which introduce a static offset error into measurements. Furthermore, the small amplitude of the Hall voltage at the output of the device requires relatively large gain amplification which may render the monitoring circuit as a whole unacceptably prone to noise.
In general, the open-loop Hall-effect element tends to exhibit inconsistency in its output offset characteristics. That is, the output can be expressed as (k*I)+c, where c is a non-constant offset term. The value of c may vary significantly from transducer to transducer, and may also vary with time, temperature, supply voltage and other factors. This can be a significant deterrent to using what would otherwise be an attractive, low-cost solution. For example, one manufacturer offers a range of current sensors based on their Hall-effect device, but the output offset voltage of their low-cost unit varies by ±10% initially, is proportional to supply voltage and exhibits a temperature coefficient of ±0.05% per Kelvin. The initial offset can be trimmed out, but the temperature and supply-dependent offset variations may be less easy to deal with.
Feedback has been used in conjunction with a CT and a Hall-effect element. In this arrangement the problem of the secondary voltage in a CT is addressed by controlling a secondary current with an amplifier having an input which is a negative feedback signal from the Hall-effect element proportional to core flux. The secondary MMF is then independent of burden voltage and can be made to follow the MMF due to the current in the conductor closely by adjusting the product of the gain of the feedback amplifier and core permeability. With very large amplifier gain, the balance between the primary and secondary MMF's is determined only by the offset null of the Hall-effect element. Core linearity becomes largely irrelevant because the feedback action is always such as to maintain zero flux and thus to balance the MMF's. The ratio of primary to secondary current is, therefore, determined by the transformer turns ratio only.
Such transducers of the “flux-nulling” Hall-effect type have been popular in the electric machine control field (for example on switched reluctance motors and generators) because of their dc response, wide bandwidth and small size. An example of the flux-nulling sensor is one manufactured by LEM s.a. of Geneva, Switzerland. These sensors are non-invasive and electrically isolated from the monitored current. However, they are relatively expensive because they need an accurately zeroed Hall-effect element and fast responding amplifiers.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide offset compensation for a transducer or a transducer circuit that is both inexpensive and does not require the complexity of the closed loop solutions referred to above.
According to an embodiment of th
Dicke, Billig & Czaja P.A.
Metjahic Safet
Nguyen Jimmy
Switched Reluctance Drives Ltd.
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