Patent Application
20060192548
The present invention relates to electric current sensors. More particularly, the invention relates to a sensor using a magnetic toroid with a primary conductor wrapped through the toroid as a coil and carrying the current to be measured.
There are a number of current sensors used in industrial applications. Examples are motor control, uninterruptible power supplies, variable speed drives, welding power supplies and the like. There is a trend toward smaller size and lower cost for these current sensors. A number of designs use external magnetic fields, such as, for example, U.S. Pat. No. 3,461,387 which uses two or more coils and it is a device that detects external magnetic fields, not current. The use of a saturated magnetic core has been shown in U.S. Pat. No. 5,239,264 and creates a field current in a coil. U.S. Pat. No. 5,831,432 uses a pair of magneto-impedance elements to cancel out uniform disturbance magnetic fields such as the terrestrial field.
The use of an amorphous wire has been proposed in U.S. Pat. No. 5,994,899. The amplitude of the voltage is asymmetrically varied with a variation in an externally applied magnetic field. A similar use of asymmetrical magneto-impedance is shown in PCT publication WO 02/061445 A1, which is used as a current leakage detector.
U.S. Patent Application Publication No. US 2003/0006765 A1 discloses a sensor coil on an open core, asserting higher accuracy and miniaturization. U.S. Pat. No. 6,512,370 also uses a coil on an open core.
In traditional Hall effect and MR current sensors, the core is used to concentrate flux on a sensor and to partially shield stray fields. Because these sensors have a gap, it is not possible to completely shield external stray fields.
When the loading of coils is used to sense current, the magnitude of the impedance changes with stray field, temperature, part variation and the like. Thus it is not practical to construct a current sensor that relies on an absolute value of the impedance.
In some devices, it is necessary to have some feedback to improve accuracy. This is not a good solution, however, because an additional coil would be required to provide the feedback signal.
It would be of advantage in the art if a small, inexpensive sensor could be developed that would occupy a very small space, in the order of a 10 mm square footprint.
Yet another advantage would be if a sensor could be provided that is capable of sensing both DC and AC current.
It would be another advance in the art if a feedback coil could be provided in a current sensor without adding additional cost and bulk to the sensor.
Other advantages will appear hereinafter.
It has now been discovered that the above and other advantages of the present invention may be obtained in the following manner. Specifically, the present invention provides a current sensing device that has a rapid response time, has a high precision response, is small in size, low in cost, an other important properties.
In its simplest form the present invention comprises a toroid shaped core having two windings. The first winding contains the primary current of interest. This primary current can be DC or AC. The second winding contains an AC signal that responds such that its instantaneous loading, either as impedance or admittance, corresponds to or is a function of the first or primary current. Typically, only one winding loop is necessary for the primary current of interest. The secondary winding is a plurality of loops, preferably from at least twenty windings. Devices have been made using windings of 30 turn, 100 turns, and 400 turns. The actual number of winding turns is a design variable, depending on the cost and size limitations and the degree of sensitivity and response time needed.
The toroid is formed from a magnetic material such as an amorphous core magnetic material. Preferred is a magnetic material having an hysteresis saturation point at least 50 times larger than the coercivity of the material. One such material is Metglas® 2714, available from the Metglas Inc. It is a cobalt based, ultrahigh permeability magnetic alloy. Other materials are also useful, such as at least some forms of permalloy.
The device of this invention includes an amplifier for receiving the AC signal and placing the AC signal on the plurality of windings. A resistor measures the instantaneous loading of the signal, which is representative of the primary current in the first winding. In one embodiment, the circuit places an AC voltage on the windings, and the resistor reads the current thereby produced. In another embodiment, the circuit places an AC current on the windings, and the resistor reads the voltage thereby produced.
The output of the sensor is symmetrical for positive or negative primary current about zero, and this symmetry is constant over variations in temperature, frequency, stray fields, manufacturing variations, and part to part variations, even though the magnitude of the output my change with these conditions.
For a more complete understanding of the invention, reference is hereby made to the drawings, in which:
The present invention provides for substantial improvements in small current measuring devices. Specifically, the device of this invention operates based on the way the magnetic properties of a toroid core change with current applied to turns off wire wrapped around the core. Applied current, called the primary current or current being sensed, generates a magnetic field that becomes trapped in the core. This magnetic field starts to saturate the core. Saturation changes the AC losses and inductance of the core. This change in core properties is detected as a change in impedance looking into a second coil wrapped around the core.
In prior art Hall effect and MR current sensors, the core is used to concentrate flux on a sensor and to partially shield stray fields. In the present invention, by looking at how the impedance, or inversely the admittance, in the core changes with applied current, the core itself becomes the sensor, resulting in a cost savings. Moreover, without a gap for a Hall effect o MR sensor, external stray fields are completely shielded. Further, a toroid without a gap removes the process step to cut a gap in the toroid, which reduces cost and complexity.
The preferred embodiment of the present invention is shown in
Coil 15 is a plurality of windings (400 in
Also passing through toroid core 17 is a primary current 19 which has one winding around said core 17. Current 19 is the current sought to be measured. The sensor of this invention is operable with primary currents from DC current to AC currents up to 200 Hz or more. In
Similarly,
While particular embodiments of the present invention have been illustrated and described, it is not intended to limit the invention, except as defined by the following claims.
