Electricity: measuring and testing – Impedance – admittance or other quantities representative of... – Lumped type parameters
Reexamination Certificate
2000-09-18
2002-09-17
Le, N. (Department: 2858)
Electricity: measuring and testing
Impedance, admittance or other quantities representative of...
Lumped type parameters
C324S704000, C324S705000, C324S706000
Reexamination Certificate
active
06452405
ABSTRACT:
TECHNICAL FIELD
This disclosure relates to a current sensing system calibration method and apparatus, and more particularly, to a current sensing system calibration method and apparatus for a current sensing system that is used for controlling a motor.
BACKGROUND OF THE INVENTION
Current sensors are known in the art for monitoring current signals.
FIG. 1
depicts a conventional current sensing system generally denoted by the numeral
10
. An input current
12
flows through a current sensing resistance
14
, such as a shunt resistance of a motor wiring. A first end
16
of a pair of input resistances
18
is connected to both ends of the current sensing resistance
14
. Input resistances
18
may be used to control the input impedance of the current sensing system
10
. Each of the input resistances
18
has a second end
20
connected to a filter
22
. The current
12
through current sensing resistance
14
establishes a voltage
24
at filter
22
. The filter
22
has a pair of output leads
26
connected to an amplifier
28
. The amplifier
28
generates an output
30
indicative of the current
12
.
The current sensing resistance
14
needs to have a reference value so that the voltage
24
input at filter
22
accurately reflects the current
12
. Variations in the current sensing resistance
14
from a reference value will cause variations in the voltage
24
thus producing inaccurate current measurements. Several techniques have been proposed to calibrate the current sensing system
10
to accommodate the value of the current sensing resistance
14
.
One calibration technique is to change gain resistors in amplifier
28
. Changing gain resistance in the amplifier, however, may not be practical in certain situations. For example, the gain resistor may be internal to an integrated circuit (IC). The addition of further gain resistors external to the IC would affect gain accuracy and common mode rejection due to variability of the on-chip resistances over build and temperature.
Another technique for calibration of the current sensing system
10
involves changing or trimming the current sensing resistance
14
to alter its resistive value. Trimming can be difficult because of the high power involved (80 Amps, 2 milliohms). For example, in a situation wherein a blade shunt is used as the current sensing resistance
14
, trimming would most likely damage the current sensing resistance. Resistors such as thick film shunt resistance are expensive and tend to be unsuitable for trimming.
Yet another technique for calibrating the current sensing system
10
is to use software to adjust the signal generated by amplifier
28
. One drawback to the software approach, among others, is that it hurts resolution.
SUMMARY OF THE INVENTION
Disclosed is a current sensing system comprising a current sensing resistance through which a current to be measured flows. The system further has a calibration resistance coupled to the current sensing resistance and a pair of input resistances each having a first end coupled to one of the ends of the current sensing resistance and a second end coupled to one of the ends of the calibration resistance. A voltage is created across the calibration resistance and the calibration resistance is set at a predetermined level to compensate for any deviation of the current sensing resistance from a reference value.
Also disclosed is a motor driving system that uses the current sensing system of the invention to measure a DC bus current.
Also disclosed is a method for calibrating the current sensing system.
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patent: 4922169 (1990-05-01), Freeman
patent: 5122756 (1992-06-01), Nelson
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patent: 5450015 (1995-09-01), Mastico et al.
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patent: 5939991 (1999-08-01), Deng
Anderson Edmund P.
Delphi Technologies Inc.
Hamdan Wasseem H.
Le N.
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